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TEXT-BOOK OF BOTANY

SACHS

Hontron
HENRY FROWDE

OXFOKD UNIVERSITY PRESS "WAREHOUSE
7 PATERNOSTER ROW

TEXT-BOOK

OF

BOTANY

MORPHOLOGICAL AND PHYSIOLOGICAL

BY

JULIUS SACHS

PROFESSOR OF BOTANY IN THE UNIVERSITY OF WURZBURG

EDITED, WITH AN APPENDIX,

SYDNEY H. VINES, M.A., D.Sc, F.L.S.

FELLOW AND LECTURER OF CHRIST's COLLEGE, CAMBRIDGE

SECOND EDITION

AT THE CLARENDON PRESS

MDCCCLXXXII

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AUTHOR'S PREFACE.

This Text-book of Botany is intended to introduce the student to the
present state of our knowledge of botanical science. Its purpose is not only
to describe the phenomena of plant-life which are already accurately known, but
also to indicate those theories and problems in which botanical research is at
present especially engaged; the arrangement of the material and the mode of
treatment of the separate subjects are adapted to this purpose. Detailed dis-
cussions of questions of minor importance have been avoided, as these would
only mar clearness of outline in the design ; critical remarks have been intro-
duced occasionally where they seemed necessary, in order to determine facts, or
to justify the views taken on matters of fundamental importance.

The historical development of botanical views and theories does not seem
to come within the scope of a Text-book of Botany, and would only interfere
with the unity of design of the work. It would therefore be superfluous to
quote scientific works which have only a historical interest. In the references
which will be found in the work the chief object has been to introduce the
student to those writings in which he will find a fuller discussion of those parts
of the subject which have been only touched on briefly. In some cases the
writings of others have been quoted because they represent views different from
those of the author, and because it is desirable to place the student in a posi-
tion to form a judgment for himself. Others again of the references are simply
for the purpose of citing the authorities on which reliance is placed for state-
ments that have not come within the range of the author's own observation.
The reader of this work will at least learn the names and standing of those
workers who have in recent times contributed most essentially to the science
of which it treats.

By far the greater number of the illustrations are original, many of them
the result of laborious investigation. Where they have been copied the name of
the author from whom they are borrowed is in each case given in the descrip-
tion; illustrations from other sources are used only when the objects themselves
have not been accessible, or when it seemed impossible to obtain better ones.

The Table of Contents will give sufficient indication of the plan of the
work; the Index should be consulted for references to other parts of the book
where an explanation of technical terms will be found when their meaning does
not appear in any particular passage.

EDITOR'S PREFACE.

When I undertook, about a year ago, to prepare a new English edition of this
important work, based upon the German edition of 1874, I found that it would be
necessary to make considerable alterations and additions if the work were to main-
tain in any degree its high reputation as adequately representing the attainments of
Botanical Science. It is with the object of maintaining this reputation that 1 have
ventured, not without considerable diffidence, to add to and to alter Professor Sachs'
work ; but I have been careful to distinguish my alterations and additions either by
enclosing them in brackets or by quoting my authority, so that the reader will have
no difficulty in recognising them. I cannot flatter myself, however, that I have been
altogether successful in my attempt. Complete success could only have been
attained by rewriting a considerable portion of the work, but this did not come
within my province.

I found also that nearly the whole of Book I. had already been for some time
in print, and that consequently a number of important recent discoveries had not
been noticed in it. In order to meet this difficulty I suggested to the Delegates
of the Clarendon Press that the first thirty-two pages should be revised and re-
printed, and that the additional notes necessary for the completion of Book I. should
be incorporated in an Appendix, a suggestion which met with their approval. An
opportunity was thus afforded me of adding some further notes and references on
the remainder of the work. As it also contains the Corrigenda, the Appendix has
come to be an important feature, and I therefore especially recommend it to the
notice of the reader.

I have no doubt that many errors of omission and of commission will be
detected ; for these I would beg the reader's indulgence, in so far as I am respon-
sible for them. They would have been much more numerous but for the valuable
criticisms and suggestions of many friends, among whom I may especially mention
Mr. W. T. Thiselton Dyer, Assistant Director of the Royal Gardens, Kew, Mr. D.
H. Scott, Assistant Professor of Botany in University College, London, and Mr. F.
O. Bower, Lecturer in Botany at the Normal School of Science, South Kensington.
To my friend Mr. A. E. Shipley, Scholar of this College, I am much indebted for
his kindness in assisting me in the serious labour of preparing the Index.

S. H. V.

Christ's College, Cambridge,
August^ 1882.

CONTENTS

BOOK I.
GENERAL MORPHOLOGY.

CHAPTER I.
Morphology of the Cell.

PAGE

Sect. I. Preliminary Inquiry into the Nature of the Cell i

2. Difference in the Forms of Cells 5

3. Formation of Cells 7

4. The Cell- Wall 19

5. Protoplasm and Nucleus 37

6. The Chlorophyll-Bodies and similar Protoplasmic Structures ... 45

7. Crystalloids . 49

8. Grains of Aleurone (Protein-grains) 51

9. Starch Grains 56

10. The Cell-sap '. . . .62

11. Crystals in the Cells of Plants 64

C H A P T E R II.
Morphology of Tissues.

Sect. 12. Definition 70

13. Formation of the Common Wall of Cells combined into a Tissue . . 72

14. Forms and Systems of Tissues 79

15. The Epidermal Tissue 94

16. The Fibro- vascular Bundles . . 108

17. The Fundamental Tissue .120

18. Secondary Increase in Thickness of Stems and Roots . . . .125

19. The Primary Meristem and the Apical Cell 137

X CONTENTS. i

CHAPTER III.
Morphology of the External Conformation of Plants.

PAGE

Sect. 20. Difference between Members and Organs. Metamorphosis . . .149

21. Leaves and Leaf- forming Axes 153

22. Hairs (Trichomes) 160

23. The Root 162

24. Different Origin of Equivalent Members 169

25. Different Capacity for Development of the Members of a Branch-system . 176

26. The Relative Positions of Lateral Members on a Common Axis . .187

27. Directions of Growth 202

28. Characteristic Forms of Leaves and Shoots 211

29. Reproduction; Sexual Organs; Alternation of Generations . . .222

BOOK 11.
SPECIAL MORPHOLOGY

AND OUTLINES OF CLASSIFICATION.

Group I. Thallophytes 231

Class I. Protophyta 245

„ 2. Zygosporeae 250

„ 3. Oosporeae 267

„ 4. Carposporeae 284

Group II. MuscinesB 342

Class 5. Hepaticae . . , 346

„ 6. Musci 361

Croup III. Vascular Cryptogams 385

Class 7. Equisetineae 3^2

„ 8. Filicineae 409

„ 9. Dichotomeae ^60

Group IV. Phanerogams ^36

Class 10. Gymnosperms 4^8

Sub-group. Angiosperms C34

Class II. Monocotyledons 618

„ 12. Dicotyledons 63,

CONTENTS, xi

BOOK III.
PHYSIOLOGY.

CHAPTER I.
Molecular Forces in the Plant.

FAGE

Sect. I. The Condition of Aggregation of Organised Structures .... 663

„ 2. Movement of Water in Plants 674

„ 3. Movements of Gases in Plants 691

CHAPTER II.

Chemical Processes in the Plant.

Sect. 4. The Elementary Constituents of the Food of Plants 694

„ 5. Assimilation and Metastasis ' 703

„ 6. The Respiration of Plants 721

CHAPTER III.
General Conditions of Plant-life.

Sect. 7. The Influence of Temperature on Vegetation . . . . . .725

„ 8. Action of Light on Vegetation 737

J, 9. Electricity 768

„ 10. Action of Gravitation on the Processes of Vegetation 770

CHAPTER IV.

The Mechanics of Growth.

Sect. ir. Definition 773

Various Causes of Growth 775

General Properties of the Growing Parts of Plants 778

Causes of the Condition of Tension in Plants 787

Phenomena due to the Tension of Tissues in the Growing Parts of Plants . 794

Modification of Growth caused by Pressure and Traction .... 809
Course of the Growth in Length under constant External Conditions . .815
Periodicity of Growth in Length caused by the alternation of Day and

Night 823

Effect of Temperature on Growth 828

Action of Light on Growth in Length. — Heliotropism . . . .832

Action of Gravitation on Growth in Length. — Geotropism . . . . 839

Unequal Growth 854

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xii CONTENTS.

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Sect. 23. Torsion 859

„ 24. The Twining of Climbing Plants 862

„ 25. The Twining of Tendrils 865

„ 26. Movements of growing Leaves and Floral Organs produced by Variations

of Light and of Temperature 871

CHAPTER V.

Periodic Movements of the Mature Parts of Plants and Movements
dependent on Irritation.

Sect. 27. Introduction 878

„ 28. Review of the Phenomena connected with Periodically Motile and Irritable

Parts of Plants 880

„ 29. Mechanism of the Movements caused by Contact or Concussion . . 887

„ 30. Mechanism of the Movements produced by Variations of Temperature

and of Light 894

„ 31. Mechanism of spontaneous periodic Movements 895

CHAPTER VL
The Phenomena of Sexual Reproduction.

Sect. 32. The Nature of Sexuality 896

J, 33. Influence of the origin of the Reproductive Cells on the product of

Fertilisation 904

„ 34. Hybridisation 914

CHAPTER VII.

The Origin of Species.

Sect. 35. Origin of Varieties 920

„ 36. Accumulation of new characters in the Reproduction of Varieties . . 924

„ 37. Causes of the Progressive Development of Varieties 928

J, 38. Relationship of the Morphological Nature of the Organ to its adaptation to

the Conditions of Plant-life 933

„ 39. The Theory of Descent 940

Appendix 94^

Index 963

BOOK I.

GENERAL MORPHOLOGY.

CHAPTER L

MORPHOLOGY OF THE CELL.

Sect. i. Preliminary Inquiry into the Nature of the Cell. — The sub-
stance of plants is not homogeneous, but is composed of small structures, generally
indistinguishable by the naked eye. Each of these is, at least for a time, a whole
complete in itself, being composed of solid, semi-solid, and fluid parts which differ
in their chemical nature. These structures are termed Cells. For the most part,
large numbers of them are in close contact and firmly united; and they then
form a Cellular Tissue. But in every plant which completes its term of life there
is at least one period in which certain cells or groups of cells separate at definite
points from the union, and, after isolation, begin for themselves an independent
course of life, as spores, pollen-grains, oospores, gemmae, &c.

Like the shape and size of the whole plant, the form, structure, and size of
its individual cells are subject to regular changes ; and the nature of these cannot be
ascertained from the study of a single phase, but only from the entire series of
changes which may be called the life-history of the cell. And as, moreover, each
cell fulfils its own definite part in the economy of the plant, i. e. is specially intended
for certain chemical or mechanical purposes, so also cells exhibit a diversity of
form corresponding to their different functions. These differences, however, do
not usually arise until the cells have passed through their earlier stages; the
youngest cells of a plant differ only slightly from one another.

The general morphological laws which prevail in all cells are also more clearly
evident in the young state ; the more the developing cells adapt themselves to the
special purposes which they have to fulfil, the more difficult it becomes to recognise
in them these laws, which we will now expound more in detail.

By far the largest proportion of cells in the living succulent parts of plants,
such as young roots, leaves, internodes, and fruits, are made up of three con-
centrically-disposed parts. First is an outer skin, firm and elastic, the Cell-wall,

B

MORPHOLOGY OF THE CELL.

consislinp: of a peculiar substance, which we call Cellulose (Fig. i, B, C, h). Close
to the inner side of this membrane, which forms a closed envelope, is a second
layer, also entirely continuous, the substance of which is soft and inelastic, and
which always contains albuminous matter; to this substance H. v. Mohl gave the
distinctive appellation of Protoplasm ^ In the cells now under consideration it

forms a sac enclosed by the
cell-wall, in which sac other
portions of protoplasm are
also usually present in the
form of plates and threads
(Fig. I, B, C, /). Absent
from some of the lowest or-
ganisms, but present in all the
higher plants without excep-
tion, there lies imbedded in
the protoplasm a roundish
body, the substance of which
is very similar to that of the
protoplasm, the Nucleus {A,
C, k). The cavity enclosed
by the protoplasm-sac is filled
with a watery fluid, the Cell-
sap (B, C, s). In addition to
these, there are also very com-
monly found in the interior of
the cell granular bodies, which
however may be passed over
for the present.

Cells, in the stage of deve-
lopment now described, consist
therefore of a firm membrane,
semi-solid protoplasm (includ-
ing the nucleus), and fluid cell-
sap. At first, however, the cell-
sap is wanting. If the same
cells are examined in a very
early stage of their development they are smaller (A), their cell-wall thinner, and
the protoplasm forms a continuous mass, in the middle of which lies the nucleus, at

FIG. I. — Parenchymatous cells from the median cortical layer of the root of
Fritillaria impcrialis ; longitudinal sections ( X 550). A very young cells lying-
close to the apex of the root, still without cell-sap. B cells of the same description
about 2 mm. from the apex of the root, the cell-sap s forming separate drops in the
protoplasm/; between the drops are plates of protoplasm; C cells of the same
description about 7 — 8 mm. from the apex of the root ; the two cells to the right
below are seen in a front view ; the large cell to the left below is in optical section ;
the cell to the right above is opened by the section ; the nucleus shows, under the
influence of the penetrating water, a peculiar appearance of swelling (x, y).

^ H, V. Mohl, Ueher die Saftbewegungen im Inneren der Zellen, Bot. Zeitg. 1846, p. 73. The
importance of this substance to the life of the cell was recognised at the same time by Nageli,
who, in conjunction with Schleiden, termed it ' Schleim.' (Zeitschr. fiir wissensch. Bot. von
Schleiden u. Nageli, Heft III, 1846, p. 53.) [The 'nucleus' was figured by F. Bauer in 1830 in
the stigmatic cells of Bletia TankervillicE from a sketch made in 1802. Meyen in the former year
i?idicated it in his ' Phytotomie.' It was first described by Robert Brown (see Misc. Bot. Works, vol. I.
p. 512) in 1833. Schwann in 1839 applied the term 'nucleolus' to the body previously discovered
by Schleiden (Schwann and Schleiden's Researches, p. 3). Cohn in 1850 pointed out the analogy
of the ' protoplasm ' of botanists with the ' sarcode ' of zoologists.]

PRELIMINARY INQUIRY INTO THE NATURE OF THE CELL.

3

this time relatively very large (/c). The cell-sap first appears, when the cell is grow-
ing quickly {£), in the form of drops ( Vacuoles) in the interior of the protoplasm
{B, s); at a later period these drops usually coalesce, and form a single sap-cavity
(C, s), which is enclosed by the now sac-like hollow substance of the protoplasm.

In their earliest state the cells of the wood and cork of trees show conditions
of development which correspond essentially to those represented in Fig. i. In
these cells, however, a new condition follows very soon after the appearance of the
cell-sap; the protoplasm containing the nucleus disappears, leaving the cell-cavity
filled either with air or with water. Older wood and cork thus consist of a mere
framework of cell-walls.

An important difference exists between the further behaviour of those cells
which enclose protoplasm, and of those from which it has disappeared. The former
only can grow, develope new chemical combinations, and, under certain conditions,

Fig. 2. — Sexual reproduction oi Fucus vesiculosiis ; A branclied hair bearing antheridia ; 5 antherozoids ; / an oogonium C^^ with
paraphyses / ; // the exterior membrane a of the oogonium is split, the inner membrane i protrudes, containing the oospheres ; III an
oosphere escaped, with antherozoids swarming round it ; V first division of the oospore or fertilised oosphere ; IV a young plant
resulting from the growth of the oospore {after Thuret, Ann. des Sci. Nat. 1854, vol. 11). (/9 x 330 ; all the rest X 160.)

form new cells. The latter are never capable of further development; in the case
of wood, they are of service to the plant only from their firmness, power of absorbing
water, and peculiar form; in cork, by forming protecting envelopes which surround
the living succulent cellular tissue.

Since then no further development can take place in cells which no longer
contain protoplasm, it may be concluded that the latter is the proximate cause of
growth. We shall see hereafter that the development of each cell begins with
the formation of a protoplasm-mass, and that the cell-wall is also generated from
it; but the relation of the protoplasm to cell-formation is, still more striking when
it exists for some time in the free state as a mass of definite form, which eventually
clothes itself again with a fresh cell-wall, and takes up cell-sap. We have an
excellent example of this in the reproduction of the Fucaceae. On the fertile
branches of these large marine Algse, of which we may take Fucus vesiculosus as

B 2

MORPHOLOGV OF THE CELL.

an example, large cells, the oogonia (Fig. 2, /, Og), are formed in peculiar
receptacles; the space enclosed by the cell-wall is densely filled with fine-grained
protoplasm, which is at first homogeneous, but subsequendy breaks up into eight
portions (oospheres) ; these, completely filling up the cell-cavity of the oogonium,
press against one another and become polygonal. The wall of the oogonium
consists of two layers; the outer one splits, and the inner one protrudes in the
form of a sac, which becomes distended by absorpdon of water ; in this enlarged sac

the oospheres become globular
(Fig. 2, //); when this bursts,
the oospheres, now completely
spherical, escape. By the fertilis-
ing action of other smaller proto-
plasm-masses, the antherozoids,
these balls of protoplasm or oo-
spheres are excited to further
development; on the exterior of
each fertilised oosphere or oospore
a colourless substance makes its
appearance, which hardens into
a continuous cell-wall. The newly-
formed cell now grows in two
different directions in different
modes, and produces, after further
transformations (Fig. 2, V and
IV\ a young Fucus-plant.

Sdll more clearly does the inde-
pendence of the protoplasm of
a cell show itself in the formation
of the swarm-cells^ (zoogonidia)
of Algfe and of some Fungi. In
many cases, as in Stigeodonium
insigne (Fig. 3, B, a), the proto-
plasm of a cell filled with cell-sap
contracts, expels the cell-sap, and
forms a roundish ball, which,
escaping through an opening in
the cell-wall, swims about in the
water (C). While passing out,
the protoplasm shows, by its motions and changes of form, that it is soft and
extensible ; but, once freed, it assumes a definite form. ' Usually after some hours,
the swarm-cell comes to rest ; if killed by proper means, the protoplasm contracts
(JE, F, p), and a delicate cell-wall may now be recognised, which it did not
possess at the time of its exit, when it began to swim about. When once at

FIG. 3. — Stigeoclotiiion insigne {^SXex Nageli, Pflanzenphysiol. Untersuch-
ungen, Heft \); A s. filament of the Alga consisting of one row of cells, with a
lateral branch ; f/ green protoplasm-masses (chlorophyll-bodies), imbedded in
the colourless protoplasm of each cell not shown in the drawing; B the proto-
plasm of the cells contracting and protruding through openings in the cell-
wall ; C swarm-cells still without cell-wall ; D one come to rest ; at E and F
killed ; the protoplasm /is contracted and shows the newly-formed cell-wall-;^ ;
Ha. young plant grown from the swarm-cell; G two cells of a filament in the
act of dividing ; the protoplasm of each cell {x, y) has split into two equal
parts, and contracted by addition of a reagent.

^ [For the exact meaning of this term see Book II. Chap, i, the Introduction to Thallophytes,
The term 'swarming' is applied to any apparently spontaneous motion imparted to a naked
protoplasmic body by vibratile cilia.]

DIFFERENCE IN THE FORM OF CELLS. 5

rest, it also changes its form, and increases in volume, cell-sap collecting in the
interior. The cell formed in this way now grows in a manner dependent on the
specific nature of the plant; — in our example it elongates itself (Fig. 3, D and
//), — and new changes (in this case cell-divisions) begin.

These examples — and many more might be added — show us that the proto-
plasm constitutes the cell ; the cell, in the sense defined above, is evidently only a
further development of it; the formative forces proceed from it. It has hence
become usual to consider a protoplasm-mass of this kind as a cell, and to designate
it as a naked membraneless or Primordial Cell.

The development of a swarm-tell, like that of the oosphere of Fucus, shows, —
iS does also the case of every other cell, — that the substance of the cell- wall was
il ready contained in the protoplasm in some form or other which could not be
recognised; the formation of the cell-wall must be regarded as a separation of
matter already existing in the protoplasm. In the same manner the water of the
cell-sap, although taken up from without, must nevertheless pass through the proto-
plasm ; and, while it collects inside it as cell-sap, it takes up from it soluble sub-
stances ; so far the formation of the cell-sap is also a separation of matter hitherto
contained in the protoplasm. The nucleus is probably to be regarded as a diflfaren-
tiated porfion of the protoplasm. Thus the mature cell, provided with cell-wall,
nucleus, and cell-sap, is the result of a differentiation of matter already contained in
the protoplasm. The essential point is this, — that this differentiation always leads
to the formation of concentrically disposed layers, the outer of which, the cell-wall,
is firm and elastic, the inner, the protoplasm-sac, soft and inelastic. If the cell, as
is usually the case, is at first without any sap-cavity, the protoplasm is less firm and
more watery in the centre, or a nucleus is in this case formed, which, at least in
young cells, is always more watery than the surrounding protoplasm. When at last
the cell-sap makes its appearance, the cavity of the cell is always filled with actual
fluid, in which the nucleus often takes up a central position surrounded by proto-
plasm, or, more usually, approaches the circumference of the sap-cavity, and becomes
parietal. So long as the phase of cell-development in which the cell appears as
a sap- cavity bounded by a membrane— certainly the one most commonly seen — had
alone been observed, it was correct enough to define the cell as a vesicle ; but it is
obvious that this view does not apply to many true cells, e.g. to young cells
which form component parts of a tissue (as Fig. i, A), of the true nature of
which we should get but an ill-defined conception were we to regard them as
vesicles. The term applies still less to swarm-cells and to the oospheres of
Fucus.

Sect. 2. Difference in the Form of Cells. — The development of the indi-
vidual cell by no means always results in the forms just described ; further changes
of form usually take place in the separate parts of the cell. The volume of the
entire cell generally increases for a considerable lime, with corresponding increase
of the cell- sap ; not unfrequently it amounts to a hundred or even a thousandfold
the original volume. During this increase, the form commonly changes; if it was
at first roundish or polyhedral, it may become filiform, prismatic, or tabular, or
branch into a number of arms. The cell-wall may increase very considerably in

6 MORPHOLOGy OF THE CELL.

thickness; but this thickening is usually not uniform; particular spots remain thin,
in others the thickened membrane projects internally or externally in ribbon-shaped
prominences, spines, or knobs. In the substance of the cell-wall itself differences
also manifest themselves, which give it greater firmness, elasticity, or hardness,
or, on the other hand, greater softness or pliancy. The protoplasm may, in
these processes, decrease more and more, until at last it forms an extremely thin
membrane, applied so closely to the cell-wall that it does not become visible till
contraction takes place ; after the completion of the growth of the cell-wall it may
even entirely disappear. But in many other cases the protoplasm increases with
the increase in volume of the cell ; it forms a- thick-walled sac, the substance of
which is endowed with constant motion, while filiform or ribbon-shaped strings
of protoplasm often traverse the sap-cavity. In those cells which appear externally
green, certain portions of the protoplasm become separated, and assume a green
colour; these particles, the Chlorophyll-bodies, may have the form of bands, stars,
or irregular masses ; but they usually form numerous roundish granules, and are
always parts of the collective protoplasm -mass of the cell. Sometimes, mixed with
the green colouring-matter which tinges them, are pigments of other colours, red,
blue, or yellow (as in the Florideae, Oscillatorieae, and Diatomacese) ; or the chlo-
rophyll-granules assume, through changes in their colouring-matter, other tints,
mostly yellow or red. Colouring-matters may also be dissolved in the cell-sap.
The other chemical compounds, which are formed in extremely large num|Ders in
the cell, are mostly dissolved in the cell-sap; but many of them assume definite
forms; thus arise granules of fat, drops of oil, and frequently true crystals or
crystal-like bodies. One of the commonest granular compounds present-in almost
all plants, with the exception of Fungi and some Algae and Lichens, is starch, the
grains of which often accumulate in the cell in quantities greatly exceeding all other
substances.

Cells of the most perfectly developed form are found in certain families of Algae,
the Conjugatse, Siphoneae, and Diatomaceae. Since in these cases the same cell unites
in itself all vegetative functions, and at the same time a many-sidedness presents itself
in the vital phenomena, the whole cell attains a high degree of differentiation ; the
separate parts — the cell-wall, the protof)lasm, and the bodies enclosed in it — show a
variety of structure which does not occur elsewhere concurrently in the different parts
of one and the same cell. In addition to this, the same cell has in these cases often to
go through the most diverse metamorphoses, so that besides its manifold development as
to size, it also undergoes a series of transitory changes of form. Hence these types
of Algae are of great importance for an accurate comprehension of the nature of the
cell. (See Book II, Algae.) But these cells are also remarkable in this respect, — that,
after they have attained their highest grade of development, they are still able to divide
and to multiply ; sooner or later the protoplasm can abandon the cell-wall, contract, with
all its contents (starch, oil, chlorophyll, &c.), expel the water of the cell-sap, and form
eventually a new cell.

We may pass over the innumerable intermediate forms, and turn our attention to
the other extreme, namely, to those plants of which each usually consists of thousands
or even millions of cells, as is the case with Vascular Cryptogams and Phanerogams,
and in which the different parts of the plant undergo an entirely different morpho-
logical development, and are adapted to different functions for the support of the
whole. Here we find that certain cells never attain their full development ; they
remain constantly in the immature condition which is represented in Fig. i, A; these

FORMATION OF CELLS. 7

however assist the whole by continually giving rise to new cells by division, which then,
on their part, undergo a further development. Such cells, which serve exclusively for
the purpose of producing new ones, are found at the extremities of all roots and branches,
and between the bark and wood of exogenous trees and shrubs. The cells produced in
these positions undergo a different development according to their situation, and usually
in such a way that aggregations of them into layers or strands follow simultaneously
the same mode of development. Some grow quickly in all directions, their wall remains
thin, the great bulk of their protoplasm becomes transformed into chlorophyll-granules,
they are rich in cell-sap, and serve, as we shall see hereafter, for assimilation, i. e. the
production of new organic substance which is formed out of the elements of the absorbed
nutrient material. In other parts of the same plants the cells extend greatly in length,
their diameter remains small, they form no chlorophyll ; a certain number remain suc-
culent and serve to convey assimilated substances ; other cells of the same strand thicken
their walls rapidly in many ways, their septa become absorbed, numerous cells in the
same row combine into a long tube (vessel), from which the protoplasm and the cell-sap
disappear; they serve then as organs of conduction for the plant. In their neighbour-
hood are formed the wood-cells ; these are mostly prosenchymatous, extended in length,
their wall greatly thickened, and its substance chemically changed (lignified) ; they form
collectively a firm frame-work which supports the remaining tissues, lends firmness and
elasticity to the whole, and is especially adapted for the rapid conduction of water
. through the plant. In the tissue of tubers, bulbs, and seeds most of the cells remain
thin-walled ; they become filled in the interior with albuminous substances, starch,
oil, inulin, &c., which afterwards, when new organs are being formed, serve as material
for the construction of new cells. In the same manner a considerable series of other
forms of tissue could be named, cork, the testa of seeds, the stone of stone-fruit, &c.,
which all alike attain their needful firmness and strength by a peculiar development of
their cell-walls, in order to serve as protective envelopes for other masses of cells which
are still capable of further development ; their contents disappear as soon as the cell-
wall has assumed these properties, and their purpose has thus been fulfilled.

Each of the forms of cell just spoken of, occurring in the same plant, serves
principally or even exclusively for one purpose only ; in correlation with this, either
the cell-wall, the protoplasm, the chlorophyll-granules, the cell-sap, or its granular
deposits, is specially developed. Very commonly these specialised cells lose the power
of reproduction and of multiplying by division; when they have fulfilled their function,
they disappear, or their lignified cell-vi^all alone remains. The whole plant, of which
these cells form a part, continues no less to live, since at special points it possesses cells,
which, at the proper time, again produce new masses of cells capable of fulfilling in their
turn the same functions.

Sect. 3. Formation of Cells ^ — The formation of a new cell always
commences with the re-arrangement of a protoplasm-mass round a new centre ;
the material required is always afforded by protoplasm already present, and the

^ H, von Mohl, Vermischte Schriften botanischen Inhalts Tubingen 1845, pp. 67, 84, 363
[Anatomy and Physiology of the Vegetable Cell, translated by Henfrey, London 1852]. — Schleiden
in Miiller's Archiv, 1838, p. 137 [Taylor's Scient. Mem., vol. II. pp. 281-312, and Sydenham
Society, 1847].— Unger, Bot, Zeit. 1844, p. 489; H. v. Mohl, Bot. Zeit. 1844, p. 273.— Nageli,
Zeitschrift fiir wiss. Bolanik, vol. I, 1844, P- 34j ^o\s. Ill, IV, 1846, p. 50. — A. Braun, Verjiingung in
der Natur, Freiburg 1850, p. 129 et seq. [Ray Soc. Botanical and Physiological Memoirs, 1853]. —
Hofmeister, Vergleichende Untersuchungen iiber die Embryobildung der Kryptog. u. Conif, Leipzig
1851 [Ray Soc. 1862]. — De Bary, Untersuchungen liber die Familie der Conjugaten, Leipzig 1858 —
Nageli, Pflanzenphys. Untersuch. Heft I. — Pringsheim, Jabrb. fur wiss. Botanik, vol. I, 1858, pp. i,
284, vol. II. p. I. — Hofmeister, Lehre von der Pflanzenzelle, Leipzig 1867. [Strasburger, Ueber
Ztllbildung und Zelllheilung, Jena iSSo. — id., Studien iiber Protoplasma, Jena 187O.]

8 MORPHOLOGY OF THE CELL,

newly constituted protoplasm-mass clothes itself, sooner or later, with a cell-wall.
All processes of cell-formation agree in these points ; but a description which goes
more into detail requires a distinction to be drawn between different cases in which
the process now varies in many different ways.

It must first be noted that the formation of new cells does not always result in
an increase in the actual number of cells. Three types may be distinguished in this
respect: — (i) The Renewal or Rejuvenescence of a cell; that is, the formation of a
single new cell from the whole of the protoplasm of a cell already in existence ;
(2) The Conjugation or Coalescence of two (or more) protoplasm-masses in the
formation of a new cell ; (3) The Multiplication of a cell by the formation of two
or more protoplasm-masses out of one. Each of these types shows a series of
variations and transitions into the others. The greatest diversity is exhibited in
the multiplication of cells ; two cases must be distinguished, according as part
only of the protoplasm of the mother-cell is applied to the formation of the new
cells, or as the whole mass is converted into daughter-cells. The latter, by
far the more common case, again exhibits variations, according as the protoplasm-
masses, which collect around new centres, expel water, contract and become
globular; according as the cell-wall is secreted during division or after its com-
pletion; and, finally, according to the way the cell-sap and nuclei make their
appearance.

The different processes of cell-formation are in turn brought into play throughout
the life of the plant ; — Rejuvenescence, Conjugation, Free cell-formation, and Cell-
division with contraction and rounding off, are the forms connected with repro-
duction ; while in the growth or increase in volume of an organ by the formation
of new cells, cell-division only comes into play, and almost invariably by division
of one cell into two without any considerable contraction and rounding off of the
new cells ; the multiplication of cells in growing tissue may therefore be described
as a bipartition of those already in existence. In the formation of reproductive
cells, the tendency to isolation and rounding off is most strongly displayed ; while
in cell-formation accompanying growth, the mother-cells are divided by partition-
walls in such a manner that the resulting daughter-cells resemble their mother-
cells, or are able to develope into any required form.

The more important modes of cell-division must now be examined in a few
examples.

A. Cell-formation in relation to Reproduction.

1. Cell-formation by Rejuvenescence. — A good example is afforded in the
formation of the swarm-cells of Stigeoclonium insigne (Fig. 3, p. 4). The protoplasm
of a cell of a filament contracts, and expels a portion of the water of the cell-sap ; the
arrangement of the differentiated protoplasm-mass is changed, and the chlorophyll-bodies
become indistinct ; its form alters as it escapes from its cell-wall ; from almost cylindrical,
the protoplasm-mass becomes ovoid, with a broad green and a narrower hyaline end ;
after its movement has ceased, the latter becomes the base, and the green end alone
grows when the new cell clothes itself with a cell-wall. The observations of Pringsheim
on (Edogonium also show that the direction of growth of the renewed cell is at right
angles to the original direction of growth before the renewal; for the hyaline or
radicular end of the swarm-cell, which afterwards becomes the point of attachment, is

FORMATION OF CELLS.

formed on the side (Fig. 4, A, E), not at one end of the protoplasm -mass. The arrange-
ment of the entire protoplasm of the cell is therefore entirely changed ; the trans-
verse becomes the longer diameter of the cell and of the plant arising from it. The
material remains, as far as can be seen, the same, but its arrangement is different.
This is the point of morphological importance, that every formation of a new cell
depends essentially on a fresh arrangement of protoplasm already in existence. Hence
the rejuvenescence of a cell not only may but must be regarded morphologically as the
formation of a new one.

2. Cell-formation by Conjugation. — The protoplasm of two or more cells
coalesces to form one common protoplasm-mass, which surrounds itself with a cell-wall
and becomes endowed with the other properties of a
cell. To study this process, which presents many
variations, we may take one of our commonest fila-
mentous Algae, Spirogyra longata (Figs. 5, 6). Each
filament (Fig. 5) consists of a row of similar cylin-
drical cells, each of which contains a protoplasm-sac ;
this encloses a relatively large quantity of cell-sap, in
the midst of which hangs a nucleus, enveloped in a
small mass of protoplasm, and attached to the sac
by threads of the same substance; in the sac lies a
spirally coiled chlorophyll-band, with thickenings
(chlorophyll -granules) at intervals which contain
starch-grains. The conjugation always takes place
between opposite cells of two more or less parallel
filaments. The first stage is the formation of lateral
protuberances (Fig. 5, a), which continue to grow
until they meet [b). The protoplasm of each of
the two cells concerned then contracts^, detaches
itself from the surrounding cell-wall, rounds itself
into an ellipsoidal form, and contracts still more by
expulsion of the water of the cell-sap. This may
occur simultaneously in the two conjugating cells.
Next, the cell-wall opens between the two protuber-
ances (Fig. 6, a), and one of the two ellipsoidal
protoplasm-masses forces itself into the connecting
channel thus formed, gliding slowly through it into
the other cell-cavity ; and as soon as it touches the
protoplasm-mass contained in it, they coalesce (Fig.
6, a). After complete union (Fig. 6, b) the united
body is again ellipsoidal, and scarcely larger than
one of the two from which it was formed ; during
the union a contraction has evidently taken place,
with expulsion of water. The coalescence gives the
impression of a union of two drops of fluid; but
the protoplasm is never fluid in the physical sense of

the word. The conjugated protoplasm-mass clothes itself with a cell-wall, and forms a
Zygospore, which germinates after a period of repose of some months, and then developes
a new filament of cells. With greater or smaller deviations from this plan, conjugation
takes place in a large group of Algae, the Conjugatae, among which the Diatoms must be
included, and in some Fungi, the Zygomycetes. In the latter more considerable de-
viations occur. In Spirogyra nitida it also happens (De Bary, Conjugaten, p. 6) that one

Fig. 4-—^, B escape of the swarm-cells of an
(Edo^otiium; C one free in motion; /) the same
after it has become fixed and has formed the
attaching disc ; E escape of the whole protoplasm
of a young plant of (Edogonium in the form of a
swarm-cell (X350). (After Pringsheim, Jahrb. fur
wiss. Bot. vol. I. pi. r.)

^ [Strasburger, Ueber Befruchtung und Zelltheilung, 1878. The protoplasm of one cell, the one
the protoplasm of which passes over into the other, usually contracts first.]

10

MORPHOLOGV OF THE CELL.

cell conjugates with two others, and takes up both their protoplasm-masses ; in these
cases the zygospore is the product of three cells. In the Myxomycetes the swarm-cells
(Myxo-amceba:), which are endowed with a peculiar motion, coalesce gradually in great
numbers, and finally form large, motile, naked protoplasm-masses, the Plasmodia, which
only at a subsequent period are transformed into a number of cells.

In the cases hitherto considered the uniting protoplasm-masses are of equal size ;
the process of fertilisation in many Cryptogaijis differs from them only in the fact
that the two protoplasm-masses which coalesce .are of unequal size, and otherwise
of different properties. In Book II. we shall treat in detail of the reproduction of
Cryptogams; here we need only state that the male fertilising bodies (antherozoids)
of Cryptogams are usually motile naked protoplasm-masses, that is, primordial cells ; the
female organ of these plants usually opens outwardly, and contains a primordial cell

<iw^

VlG. 6.— Spirogyra longata. A cells in the act of conjugation; at
a the protoplasm of one cell is passing over into the other; at b this has
already taken place ; the chlorophyll-band is still partially recognisable.
B the young zygospores surrounded by a cell-wall; the protoplasm con-
tains numerous drops of oil ( X 550).

Fig. 5. — Spirogyra longata. Cells of two filaments in an early stage of conjugation, showing the spirally coiled chlorophyll-bands,
in the chlorophyll-granules of which lie rings of starch-grains ; small drops of oil are also distributed through them (see Sect. 6) This
is the condition of the chlorophyll after the action of strong sunlight ; the nuclei are also to be seen in the cells, each surrounded by
protoplasm, threads of which reach the cell-wall in different places ; a and b are the pjotuberances in two different stages (X 550).

(oosphere) which is fertilised by the antherozoids. In cases which have been accurately-
observed ((Edogonium, Vaucheria) these coalesce with the former, and the new cell
results from this coalescence. As with the Conjugatae and some Fungi, the cell formed
in this manner is always a reproductive cell; with it begins the growth of a new
individual plant.

3. Free Cell-formation. — In the protoplasm of a cell new centres of formation
arise, round each of which a portion of the protoplasm collects, and forms a cell.
A portion of the protoplasm may remain and represent the protoplasm-mass of
the mother-cell which persists for a longer or shorter time. [The new centres of
formition are indicated by the previous appearance of nuclei formed by the repeated

FORMATION OF CELLS.

II

bipartition of the nucleus or nuclei of the cell.] Generally, the number of daughter-cells
which arise in this manner is considerable; as an instance may be mentioned the
formation of spores in Ascomycetes {Peziza ^), (Fig. 7). The tubular mother-cells of
the spores (asci) {a) are at first densely filled with protoplasm, and contain only one
small nucleus. [This divides into two, and this process is repeated until eight nuclei
are formed:] the protoplasm becomes frothy, and roundish drops of sap make their
appearance in it {h, c). The first stage in
the formation of the spores is the conden-
sation of the protoplasm in the upper part
of the ascus, while it remains frothy in the
lower part (<?,/). In this case eight spores
are always formed in each ascus within the
upper dense protoplasm ; /'. e. round each of
the eight nuclei an ellipsoidal protoplasm-
mass collects {d) ; each consists at first of
coarse-grained protoplasm surrounded by a
clear space ; a portion of fine-grained pro-
toplasm forms the matrix in which the
spores are imbedded. Afterwards each
spore becomes more sharply defined ; the
clear space disappears {e) ; its substance
becomes more fine-grained and clearer;
and in one of its foci is formed a vacuole,
i.e. a transparent drop of fluid. Finally,
each spore surrounds itself with a firm
membrane, the vacuole disappears, and in
the centre is formed a large strongly re-
fractive oil-drop, as well as numerous
smaller ones.

An example is atforded by the forma-
tion of the oospheres of Achlyo (Fig. 8)
of a somewhat different mode of free cell-
formation. The protoplasm collects at the
end of a hypha or of a branch of one ; the
larger end itself swells up into a globular
form (^, J3), and, after the formation of a
septum (C), becomes an independent cell
(the oogonium). [Numerous small nuclei
are present in the protoplasm (as in C).
The nuclei multiply by bipartition.] The
whole protoplasm breaks up into two,
three, four, or more parts, which very
quickly round themselves off into a per-
fectly spherical form ; [in each of these
several nuclei are uniformly distributed.]
The parts thus formed {e, e in D) contract
greatly during their separation, and their
protoplasm becomes denser by expulsion

of water; after they have become fertilised by the antheridial tubes {a, b in Z)),
they become invested with a cell-wall, [and the nuclei in each coalesce to constitute
the nucleus of the oospore].

Fig. 7. — Peziza convexula. 'A vertical section of tlie whole
plant (X about 20); h hymeniuin, i.e. the layer in which the
spore-sacs (asci) lie; .^the tissue of the Fungus enveloping the
hymenium at its edge <7 in a cup-like manner ; at the base of
the tissue 5' are delicate rhizoids, which grow between the
particles of earth. R a smaller portion of the hymenium ( x 550) ;
sh sub-hymenial layer of densely interwoven cell-filaments
(hypliae) ; «— ^asci; among them thinner sacs, the paraphyses,
in which lie red granules.

' [Strasburger, Zellbiklung und Zelltheilung, 1880; Schmitz, Sitzber. d. nicderrhein. Ges. zu
Bonn, 1879.]

12

MORPHOLOGY OF THE CELL.

This form of free cell -formation is distinguished from the preceding by the circum-
stance that the whole protoplasm (and not a part only) collects round several centres.
If the whole proto[)lasm, in its contraction, were to form only one mass, which some-
times happens, the case would be analogous to Rejuvenescence.

There occurs also in Achlya a variation of this process, when it forms its swarm-cells
or zoogonidia (Fig. 9). The protoplasm in the club-shaped swollen end of a hypha
[contains a number of small nuclei which multiply by bipartition] ; it breaks up
into a large number of small portions {A), [each including a nucleus,] which become
completely rounded off {a) after their escape from the cell in which they are produced

Fig. 8.— Oogonia and antheridia of Achlya lignicola, growing on f^lG. 9.— Cell in which are produced the

wood in water; the course of developtnent is indicated by the letters v^—/^. swarm-cells or zoogonidia of an Achlya.

a the antheridium, b its tube penetrating into the oogonium (x 550). (X 5So). A one still closed, B one allowing

the zoogonidia to escape ; beneath it a
'ateral shoot c ; a the zoogonidia just es-
caped ; b the abandoned membranes of the
zoogonidia which have already swarmed ;
e swarming zoogonidia.

(B), and then clothe themselves with a delicate membrane, which they abandon {b)
when their movement commences {e).

4. Formation of Reproductive Cells by Division of the Mother- Cell. — In
the protoplasm of a cell new nuclei are formed by division; round each of these a
portion of the protoplasm collects and becomes rounded off with a greater or less
amount of contraction, in order to form a new cell ; in this manner the entire proto-
plasm of the mother-cell is completely used up ; its cell-wall alone remains, if it possess
one, which is not always the case.

The formation of the spores of Mosses and Vascular Cryptogams, and of the pollen-
grains of Phanerogams, ahvays takes place by the division of the mother-cell into four

FORMATION OF CELLS.

13

parts, either at once or by a repeated bipartition. In the special details of cell-division,
however, some deviations occur.

[The first stage is, in all cases, the division of the nucleus of the mother-cell into
two. This may be at once followed by the simultaneous formation of a cell-wall in
the plane of the cell-plate (see infra, The Behaviour of the Nucleus during Division),
as in the development oPthe pollen-grains of most Monocotyledons, and of the micro-
spores of Isoetes ; a repetition of these processes in each of the two daughter-cells leads
to the formation of the four pollen-grains or spores. More commonly the cell-plate
between the two nuclei undergoes absorption ; the two nuclei then divide, and the
missing cell-plates are reconstituted ; cellulose walls are now simultaneously formed in
the cell-plates so as to divide the protoplasm into four parts, each containing a nucleus,
which constitute the spores or the pollen-grains.

The division of the two secondary nuclei takes place in certain cases (pollen-grains
of most Monocotyledons) in one plane, and this plane is at right angles to that of the
division of the nucleus of the mother-cell. As the result of this, the four spores or
pollen-grains formed lie in one plane and have a rounded form ; they are said to be
bilateral. In other cases (pollen-grains of Dicotyledons, spores of Equisetum) the division
of each secondary nucleus takes place in a plane which is at right angles to that of the
other and to that of the nucleus of the mother-cell; as a consequence the four spores
or pollen-grains do not lie in one plane but are arranged tetrahedrally, and have more-
over a somewhat tetrahedral form; they are said to be radial. In some plants the
spores or pollen-grains are formed sometimes in one way and sometimes in the other,
and are therefore either bilateral or radial. This is the case, for instance, in Liverworts
and Mosses: amongst Ferns, whereas in the Hymenophyllaceae and Cyatheaceae only
radial spores have as yet been observed, radial spores have been found in some and
bilateral spores in others of the genera of Polypodiaceae, and this is probably also the
case in the Schizaeaceae and Gleicheniaceae : in the Marattiaceae and in Ophioglossum
radial and bilateral spores may be produced in the same sporangium : this last condition
also obtains in Psilotum and in Lycopodium Selago and inundatum : finally, these two
modes of the development of the pollen-grains have been observed by Strasburger in
Allium Moly among Monocotyledons.

In the following paragraphs a detailed account is given of the course of development
in certain cases illustrating the two modes above-mentioned.

a. No cell-nvall is produced until after four nuclei ha've been formed by division ; the
resulting cells are arranged tetrahedrally.^

Development of the spores of Equisetum. At first the mother-cells swim in the fluid
which fills the cavity of the sporangium, in groups of two or four together (Fig. 10,
a, b). Each mother- cell consists at first of a large spherical nucleus (including nu-
cleoli), surrounded by fine-grained protoplasm, with a sharply-defined outline, but is
without a cell-wall ^. [The first indication of division is the coalescence of the granules
in the nucleus to form fibrillae ; it becomes elongated in form, assuming a spindle-shape.
An aggregation of coarse granules now makes its appearance in the equatorial plane
of the spindle, constituting the nuclear disc : this splits into two discoid halves, and
each half then travels to one pole of the nucleus and there forms a new nucleus.
A fresh aggregation of granules now appears in the equatorial plane of the spindle,
constituting the cell-plate ; this extends quite across the protoplasm of the cell. The
fibrillae connecting the two new nuclei with the cell-plate now disappear, and then the
cell-plate also. During this time the two nuclei have begun to divide in the same way
as the primary nucleus, in planes at right angles to each other and to that of the
division of the primary nucleus. Six cell-plates are now formed between the four
nuclei, and become connected with them by the formation of fibrillae. In the cell-
plates cellulose walls are now formed and the division is complete. The protoplasm
of each of the young spores rounds itself off, and its nucleus assumes a central position

' [According to Strasburger (loc. cit.) a delicate cell-wall is present,]

H

MORPHOLOGY OF THE CELL.

and a rounded form. The next step is the absorption of the cellulose walls and the
setting free of the young spores as primordial cells, which soon become clothed with
a cell-wall. This cell-wall subsequently becomes differentiated in a manner which is
described in detail on page 403.

In other cases the wall of the mother-cell presents ingrowths at points correspond-
ing to the lines along which the cellulose walls are formed, and this conveys the erroneous
impression that the wall of the mother-cell grows inwards constricting the protoplasm
and ultimately dividing it into four parts. A good example of this case is afforded by
the development of the pollen-grains of Dicotyledons. Fig. 11, taken from Tropxolum
minus, will serve to illustrate it. In a four nuclei are present, which have been formed
in the manner described above with reference to Equisetum : they are connected by

y

Fig. ri.— Mode of development of the pollen of Tro-
pcBolu)n ntiHus (X 550 and reduced).

Fig. 10 — Mode of development of the spores of Equisetum liniosttm (X 550) ; a group of four, b group of two^ mother-cells ;
c and (i mother-cells preparing for division, showing the nuclear disc ; e one with two nuclei ; y, g, and i division into four spores ;
h abnormal formation of three spores from one mother-cell.

fibrillas, and six cell- plates are present in the equatorial planes of the groups of fibrillae
which connect the nuclei. The wall of the mother-cell is thickened, especially at two
parts of its surface, but it presents no ingrowths ; but now {b) ingrowths begin to be
formed at points which correspond to the superficial ends of the six cell-plates {f,g,h,k').
Their growth does not, however, proceed so as to cause the constriction of the proto-
plasm into four masses : it is soon arrested ; the cell-walls are formed simultaneously
in the six cell-plates and are attached externally to the ingrowths of the cell-wall of
the mother-cell. Later each mass of protoplasm forms a new proper wall around
itself, which is the wall of the pollen-grain ; the thick cell-vi^alls surrounding them are
absorbed, and the four pollen-grains are set free. (See also p. 551.)

b. A cell-'^all is formed after each nuclear di'vision ; the resulting cells may be arranged
either tetrahedrally or in one plane.

This mode of cell-division closely resembles that which takes place in a growing
tissue. It may be very clearly observed in the formation of the pollen-grains of most
Monocotyledons. Fig. 12 shows the process in Funkia ovata. In / two nuclei are

FORMATION OF CELLS.

15

present which have been formed by the division of the nucleus of the mother-cell.
The next stage is that represented in II, where a lamella of cellulose, which has been
formed in the cell-plate between the two nuclei, completely divides the mother-cell.
Thickening takes place at the junction of this partition- wall with the wall of the
mother-cell, and the protoplasm of the two daughter-cells becomes rounded off. In
IF the nucleus of each daughter-cell is seen to have divided into two; sometimes this
division does not take place in one of the cells (F). A cell- wall is now formed simul-
taneously between each pair of nuclei : a proper cell-wall is subsequently formed round
each mass of protoplasm, the thick investing cell-walls undergo absorption, and the
pollen-grains are set free.]

The four cavities into which the mother-cell is divided in the development of spores
and pollen-grains were at one time desig-
nated the 'Special mother-cells' of the
pollen-grains, in which the grains them-
selves were formed. The term is however
inaccurate, because the four protoplasm-
masses are themselves the essential parts
of the new cells, becoming subsequently
enclosed in cell-walls. If the contents of
the cavities are termed special- mother-
cells, they are identical with the daughter-
cells, /'. e. the pollen-grains ; if, on the other
hand, the term is applied to the walls of
the cavities, this is not in accordance with
the present cell-theory. The term is in
fact altogether superfluous.

c. Cell-formation by Budding and Ab-
striction. Among Fungi certain kinds of
reproductive cells, conidia,^stylospores, and
basidiospores, are produced by small wart-
like protuberances on a mother-cell, which
then swell out and become rounded. The
daughter-cell thus formed retains however
at first its connection with the mother-
cell at its base, the contents of the two
being connected by a channel through this
narrow portion. A septum is eventually
formed across this channel which splits
into two lamellae, and in this manner the
separation of the spore from its mother-
cell is effected. The production of these spores therefore commences by a budding and
IS completed by division ; and the whole process may be termed Abstriction. It
occurs in a typical form in the reproduction of the yeast-fungus {Saccharomyces).
A wart- like out-growth from which several spores may be abstricted one after another
is called a Sterigma ; when the mother- cell bears several sterigmata, as in the Basi-
diomycetes, it is termed a Basidium. Intermediate processes between cell-formation
by abstriction and ordinary cell-division occur when the protuberance is broad, and
the daughter-cell therefore attached to the mother-cell by a broad base and separated
from it by a broad septum, as in the branching oi Cladophora ; or, on the other hand,
when the terminal portion of a hypha is divided by septa, and the resulting cells
become rounded off" and detached, as in the formation of the spores of Cystopus,
JEcidium, and other Fungi. [When the budding cell contains a single nucleus this
probably divides, and one of the new nuclei goes to the bud ; when the cell contains
several nuclei some of these travel into the bud.]

Fig. 12.— /—F// Successive stages in the formation of the
pollen of Funkia ovata (X 550). In VII the wall of the
daughter-cell has absorbed water till it has burst ; its proto-
plasm is forcing itself through the cleft, and is becoming
rounded off.

i6

%ORPIIOLOGr OF THE CELL.

B. Cell-formation in Growing Organs: Vegetative Cell-formation.

This mode of cell-formation consists almost invariably in the bipartition of a mother-
cell ; there is no or scarcely any perceptible rounding off or contraction of the dividing
protoplasm ; the two daughter-cells entirely occupy the place of the mother-cell.

I. The partition-^ all is formed gradually , The protoplasm of the mother-cell projects
in the plane of division as a circular protuberance, and a ridge of cellulose is formed
in it which finally developes into a complete septum.

A clear and well-studied example
ft is afforded in the stouter species of

the g^^wAS Spirogyra'^ . In order to
observe the divisions, it is necessary
to place filaments that are in active
growth in very dilute alcohol after
midnight, that they may be exam-
ined by day, the divisions taking
place only by night. Fig. 13 A
shows a living cell of a filament of
5. longata by day \ B io E the stages
of division at night ; the protoplasm
of the cells is contracted by the
alcohol.

[The phenomena of division fall
into two groups, namely those which
attend the division of the nucleus,
and those which are connected with
the ingrowth of the partition-wall.
The nucleus of a cell which is about
to divide becomes broader, assum-
ing the form of a biconcave lens,
and its nucleolus breaks up into
irregular granules, which, together
with its other granular contents,
begin to form a nuclear disc in the
equatorial plane. A delicate stria-
tion is now apparent in what is
becoming the long axis of the nu-
cleus, at right angles to the nuclear
disc, and a characteristic nuclear spindle is gradually produced. The nuclear disc splits
into two halves lying side by side, each of which travels to the corresponding pole of
the nucleus ; thus two new nuclei are constituted which are connected by fibrillae.

The first indication of the formation of the partition- wall is the accumulation of
protoplasm at about the middle of the length of the cell, so as to form a slight annular
protuberance into the sap-cavity: this takes place about the same time as the first
changes in the nucleus. Within this protuberance a rim of cellulose is formed which
grows inwards, carrying before it the spiral chlorophyll-band (Fig. 13, 5 and C) ;
soon the chlorophyll-band is cut through, and margins of the ring of protoplasm
coalesce in the middle so as to form a plate, to which the fibrillae connecting the two
nuclei, which have been formed in the meantime, become attached ; in this plate the
remainder of the cellulose wall is formed. The nucleus of each cell now travels to

Fig. ^-^.—Spirogyra longata (X 550). A a cell in the living state;
B, C cells laid in dilute alcohol during the division by night ; D, E central
portion of cells in the act of division.

^ This case was the first of all the processes of cell-formation that was accurately examined :
H. von Mohl first described it in 1835 '^^ Conferva glomerata. Mohl, Vermischte Schriften bot.
Inhalts, Tubingen 1845 : [Strasburger, Zellbildung und Zelltheilung, 1880.]

FORMATION OF CELLS.

i7

its centre, and the protoplasm surrounding it throws out pseudopodia which attach
themselves to the chlorophyll-granules in the green band and suspend the nucleus in
the sap-cavity.]

Fig. 14.— Formation of the antheridium of Nitella Jlexilis (see Book II). In B the protoplasm has been
made to contract by reagents.

2. The partition-^ all is formed all at once] that is, when first visible it appears as
a thin membrane of cellulose stretching across the whole interior of the mother-cell ;
the protoplasm-masses of the two daughter-cells lie in the two cavities of the mother-

FlC. 15.— Embryos in the embryo-sac oi Allium Cepa; the cells contain very large nuclei, each with two nucleoli. In
/ the spherical apical cell contains two nuclei (a) ; in // it has already divided [a has split up into a' and a") ; and in the
same manner the cell c (in /) has split up into c and c'.

cell thus produced. This mode of cell-division is usual and perhaps even universal in
the formation of tissues, especially in the case of more highly organised plants.

[The nucleus divides, and between the two new nuclei a cell-plate is formed in
which the cellulose wall is simultaneously produced.] (See Figs. 14 and 15.)

\The Beha'viour of the Nucleus during Division. The general rule is that the division
of a cell is preceded by that of its nucleus, but to this there are certain exceptions.
Strasburger has found that, in the mother-cells of the spores of Anthoceros and of the
macrospores of Isoetes, the protoplasm divides into four parts before the nucleus shows
any signs of division, and that in rare instances cells of Spirogyra had divided without any
division of the nucleus having taken place.

In the process of division into two the nucleus usually goes through a series of
changes which are designated by the term Karyokinesis : but in order that the descrip-
tion of the more complicated forms of karyokinesis may be intelligible, some account

c

1 8 MORPHOLOGY OF THE CELL.

must be given of the structure of the nucleus in a state of rest'. The nucleus consists
of a ground-substance which is homogenous, but sometimes presents, after careful pre-
paration and staining, a finely punctuated appearance : it is denser towards the periphery,
and thus constitutes a sort of membrane which gives to the nucleus its sharply-defined
contour. In the ground-substance a more solid substance is present either in the form
of granules, the larger of which are the nucleoli, or in the form of fibrillae united into
a network, the points of junction of the fibrillae, the nodes, being somewhat thickened.
Since, when the tissue is carefully stained with certain colouring-matters, the granules
or the network become coloured more readily than the ground-substance of the nucleus,
Flemming has designated the former chromatin and the latter achromatin.

When nuclear division is about to take place the nucleus presents a coarsely
granular appearance ; the granules then coalesce to form convoluted threads, and these,
together with the fibrillae of the nuclear network which have become free, arrange
themselves parallel to the long axis of the now elongated nucleus. At this time the
nucleus has usually lost (Strasburger mentions Spirogyra nitida and the mother-cells of
the spores of Equisetum as exceptions) its well-defined contour: it has assumed a
spindle-shape, and an aggregation of protoplasm has been formed at each pole, in which
a radial arrangement of the granules, with the pole as a centre, is usually evident. The
fibrillae of chromatin now contract so as to form a disc of greater or less thickness
{equatorial plate of Flemming, nuclear disc of Strasburger) in the equatorial plane of the
spindle, and faint striae can now be detected between the nuclear disc and each pole,
some of which apparently pass through the disc from one pole to the other. The
nuclear disc now splits into two halves lying side by side, and each half travels to the
corresponding pole along the fibrillae which cause the striation of the spindle, and each
then constitutes a new nucleus at the pole. The few fibrillae which now connect the
two nuclei are augmented by the formation of new ones, and the whole complex of
fibrillae assumes the form of a biconvex lens and extends across the cell. A row" of
granules now makes its appearance in the equatorial plane of the fibrillae consisting,
according to Strasburger, of starch or of a substance allied to starch and cellulose ^ ;
this is the cell-plate, and in this the cell-wall is simultaneously formed. If the complex
of fibrillae does not extend across the cell, so as to be in contact with both its lateral walls,
the new wall is formed in contact with one lateral wall, and the complex gradually grows
towards the other wall by the formation of new fibrillae in which additions to the new cell-
wall are deposited. The new wall is clothed on each side with a layer of protoplasm, the
nucleus assumes its normal position in the cells, and the process of division is complete.

The above description applies to the most complicated form of karyokinesis, as it
occurs in relation to the development of the reproductive cells and to the reproductive
processes of the higher plants. In the cell-division of the vegetative organs of these
plants the process is usually simpler ; thus, according to Schmitz, in the meristematic
cells of Phanerogams the nucleus becomes elongated, the ends being swollen and the
middle connecting part remaining narrow and presenting a longitudinal striation ; the
swollen ends become defined as new nuclei, and a cell-plate appears in the narrow
portion in which the cell-wall is soon formed ; the narrow portion gradually loses its
striated appearance and assumes the appearance of the cell-protoplasm. In the cell-
division of Spirogyra, as we have already seen, no cell-plate is formed. Strasburger
draws attention to the fact that in those Thallophytes (excluding the Characeae) in which
karyokinesis has been observed, the cell-plate is not formed, as in the higher plants, in a
complex of fibrillae which connect the two new nuclei, but in a bridge of protoplasm
which connects the two nuclei and extends across the cell in the plane of division.]

^ [This account is taken from the above-quoted work of Strasburger, and from those of Schmitz
(Sitzber. d. niederrhein. Ges. in Bonn, 1880) and Flemming (Arch. f. micr. Anat. vol. 18).]

* [According to Schmitz {loc. cii.) the granules in question are microso^nata, i.e. constituent
elements of the protoplasm.]

THE CELL- WALL.

19

Sect. 4. The Cell- Wall ^ — The substance of the cell-wall is secreted from
the protoplasm. In what form it is previously contained in the protoplasm is not
yet certainly known. The substance capable of forming cell-wall always consists
of a combination of water, cellulose, and incombustible materials (ash-constituents),
but may afterwards undergo further chemical changes.

By the continual secretion from the protoplasm of the substance out of which
the cell-wall is formed, and its intercalation between the micellae (see Bk. III.
Sect, i) of the cell-wall already in existence, this grows in both surface and thickness.
The mode of both processes of growth is dependent on the specific nature of the
cell, and on the function which it has to fulfil in the life of the plant; it therefore
varies almost infinitely. Generally the surface-growth first preponderates, afterwards
that in thickness. Neither the one nor the other is uniform over all points of a
cell-wall ; hence each cell during its growth also changes its form. The growth of
a cell-wall continues only so long as it is in immediate contact on its inner side
with protoplasm.

The want of uniformity in the surface-growth at different points causes cells
which are at first, for example, spherical, ovoid, or polyhedral, to become subse-
quently cylindrical, conical, tubular, tabular, bounded by waved surfaces, &c.
The want of uniformity in the growth in thickness usually brings about a sculpture
of the surface which is very characteristic. The thickened parts may project either
outwardly or inwardly. The former occurs commonly in the surface of the cell-wall
which is exposed, the latter in the partition-walls of adjoining cells. The thickenings
which project outwards may take the form of knots, humps, spines, or ridges ; but
those which project on the inside are much more various. In this case conical
protuberances occur but seldom ; annular ridges or spiral bands are much more
common ; these latter may be united in a reticulate manner, so that the
thin interstices are polygonal; or the thickened part of the wall may be broader,
and the thin parts then appear in the thick wall as fissures or roundish pits. If
the wall is very thick, the latter become channels, which pass entirely or partially
through the wall. Not unfrequently the thin portion of the wall which at first
closes such a channel on the outside becomes absorbed, and the cell-wall is then
perforated. But as, when contiguous cells are united into a tissue, the partition-
wall usually becomes thickened in the same manner on both sides, the pits and
pit-canals on the two sides meet, and the intermediate thin portion of membrane
(sooner or later) becomes absorbed ; a channel thus arises uniting two cell-cavities
(Bordered Pits, perforated septa of vessels).

During the increase of the wall both in surface and thickness by the inter-
calation of new substance between the micellae already in existence, a further
internal structure usually becomes visible, which is termed Stratificaiion and Stria-'
Hon. Both are the result of a different regularly alternating distribution of water
and solid material in the cell-wall ; at every point water is combined with cellulose,

^ H. von Mohl, Vermischte Schriften hot. Inhalts, Tubingen 1845 (numerous memoirs). —
Schacht, Lehrbuch der Anat. und Phys. der Gewachse, 1856. — Nageli, Sitzungsberichte der Miinch.
Akademie, 1864, May and July.— Hofmeister, Die Lehre von der Pflanzehzelle, Leipzig 1867.
Also numerous memoirs in the Botanische Zeitung.

C 2

20

MORPHOLOGY OF THE CELL.

but in different proportions ; portions less and more watery, denser and less dense,
alternate. Thus, in every cell-wall sufficiently thick, a system of concentric layers
becomes visible, of which the outermost and innermost are always denser, while
between them alternate more and less watery layers. The stratification is visible
on the transverse and longitudinal sections of the cell-wall, the striation on the
surface as well ; it is usually most evident there, but is in general less easily seen
than the stratification; it depends on the presence of alternately more and less dense
layers in the cell-wall, meeting its surface at an angle. Generally two such systems
of layers may be recognised mutually intersecting one another. There are thus
altogether three systems of layers present in a cell-wall, one concentric with the
surface, and two vertical or oblique to it mutually intersecting, like the cleavage-
planes of a crystal splitting in three directions (Nageli) ; and just as this cleavage
is sometimes more evident in one direction, sometimes in another, so is it also with
the stratification and striation.

Independently of this internal structure, chemical changes arise in the cell-wall
which never affect the whole mass uniformly, but usually mark out the thickened

Fig. 16.— From the transverse section of a leaf oi Camellia Japonica ; /> parenchymatous cells with chlorophyll-
granules and drops of oil ; i^ a very thin fibro-vascular bundle ; vv z. large, branched, thick-walled cell, which
intrudes its branches between the parenchymatous cells.

cell-wall into concentric layers differing from one another chemically and phy-
sically. These chemical changes, which are always attended by an alteration of
physical properties, are very various, but can conveniently be reduced to three
categories; — Suberous or Cuticular change^ Ligneous change^ and Mucilaginous
change. The first consists in the change of the outer layers of the cell-wall into
an extensible very elastic substance, which water is unable, or nearly so, to penetrate
or cause to swell, as the outer cell-wall-layer of the epidermis (cuticle) and of
pollen-grains and spores, and cork. Lignification increases the hardness of the
cell-wall, diminishes its extensibility, and renders it more easily permeable to water
without considerable swelling. The conversion into mucilage renders the cell-wall
capable of absorbing great quantities of water, so as to increase its volume and
give it a gelatinous consistence. In the dry state such cell-walls are hard, brittle,
or flexible like horn, as the cell-walls of many Algse, the so-called 'intercellular
substance' of the endosperm of Ceratonia Siliqua, linseed, and quince-mucilage.
Several of these changes may occur simultaneously in a cell-wall, so that, for

THE CELL- WALL. 21

instance, the outer layers become woody and the inner mucilaginous, as in the
wood-cells of the root of Phaseolus.

Besides these changes in the substance of the cell-wall, which are not unfre-
quently correlated with peculiar colourings, changes in its chemico-physical behaviour
also arise from the interposition between its micellae of considerable quantities
of incombustible substances, especially lime and silica. If the deposition of these
substances take place in sufficient quantities, they remain behind, after the combus-
tion of the organic groundwork of the cell-wall, as an ash-skeleton.

(a) The Surface^grcfivth causes not only an increase in the size of the cell, but also
changes in its form, when it takes place irregularly at different points of the circum-
ference. Hence cells of originally dissimilar form may become similar by unequal
growth; but it is much more common for cells originally alike in form to become
entirely unlike. This is most usually the case in multicellular organs of the higher
plants, such as leaves, stems, and roots; in their earliest state their cells are often
scarcely distinguishable from one another ; whereas in the completely developed organ
the most various forms are juxtaposed (Fig. i6). It is only rarely, as in the growth of
some spores and pollen-grains, that the surface-growth is so uniform that the original
form is nearly retained even after considerable increase in size {e.g. pollen of
Cucurbita and Althad). But even in these cases the uniformity is only temporary;
for the pollen-grains subsequently emit their pollen-tubes, or the spores germinate, in
both cases by the local growth of the inner layer of the cell- wall. This also shows
at the same time that the surface-growth of a cell-wall may be very different at
different times ; and this indeed is usually the case. From the infinite variety of the
surface-growth of cell-walls, it is convenient, for the sake of arrangement, to reduce
the different cases to classes, and to bestow names upon them ^. Thus it is usual to
distinguish between intercalary and apical growth of the cell-wall. Apical growth takes
place when the surface-growth attains a maximum at any one spot (by interposition
)f new micellae of cellulose), while its intensity decreases in all directions from it,
md at a definite distance reaches a minimum, so that this portion of the cell-wall
)rojects as a point, or appears as the rounded apex of an excrescence or of a cylindrical
[tube, as in hairs or filamentous Algae. If several points of apical growth occur in a cell
I which was originally round, it may become star-shaped ; if new points of growth are
formed behind the continuously growing end of a tubular cell, it branches, as in many
[filamentous Algae, hyphae of Fungi, Faucheria, Bryopsis. Hofmeister^ distinguishes as
peculiar form of apical growth the case in which the maximum of growth is localised
in a line instead of at a point ; this may occur as the line of intersection of two curved
irfaces. Intercalary growth of the cell- wall occurs in a typical form when the de-
)osition of new substance is localised in a zone of the cell-wall; this zone extends,
[and a fresh piece of cell-wall is by degrees intercalated between the old ones. Very
similar to this is the common occurrence of growth in the whole of the side-wall of
a cubical, tabular, or cylindrical cell, as, for example, in the cells of Spirogyra, and the
I parenchymatous cells of growing roots and stems of Phanerogams (see Fig. i). (Edo-
^gonium presents a peculiar case of intercalary surface-growth (Fig. 17). Below a
septum an annular deposit of cellulose {A, iv) is formed ; at this place the cell-wall
splits, as if by a circular cut, into two pieces, and these separate from one another,
but remain united by a zone of cell-wall {B, iv) formed by the extension of the
annulus ou. After the intercalation of this new zone, cell-division follows ; and, since

^ A good classification of the processes of growth is, of course, still more important for the
study of the mechanics of growth; but little has, however, yet been done in this direction, and we
can only give a brief abstract.

* Handbuch der physiol. Botanik, vol. I. p. 162.

22

MORPHOLOGY OF THE CELL.

this is repeated many times, the. appearance is presented which is figured at J, c (the
formation of a so-called cr.p ^).

(b) The Groivtb in Thickness of a Cell-ivall is usually strictly localised, so that the
thicker parts appear as very abrupt projections on the thinner parts of the cell-wall,
either outside or inside. The general effect produced by the sculpture depends on
whether the thick or thin portions occupy the greater extent of surface. If the thicken-
ing is strongest at certain points only, it takes the form of knobs or spines projecting
outwardly (Fig. 19) or inwardly (Fig. 18, C,D)\ if it occurs most strongly in linear
or ribbon-shaped portions of the cell-wall, projecting ridges or bands are formed
on the inner or outer side. These ridge-like projections may form reticulate figures
(Fig. 18, B, Fig. 20, /), or rings, or spiral bands, an appearance especially frequent in
certain thickened tissue-cells. If the rings or spiral bands
which project inwards are thick and firm, and the inter-
mediate portions of cell-wall thin and easily destructible,
these thickenings may become detached even within the
plant, and remain lying as isolated threads of cellulose in
channels of the tissue, as in the annular vessels in the fibro-
vascular bundles of Equisetaceae, Maize, &c. ; but the spiral
thickenings may often also be drawn out to a considerable
length as isolated fibres. Very striking examples are found
in the rachis of the inflorescence of Ricinus communis and
in the leaves of Agapanthus. If the thickening aff'ects the
whole surface of the cell-wall more completely, the small
portions which remain thin will appear as Pits of very
various outline, either roundish or fissure-like, or, when
the thickening of the cell-wall is very considerable, as
Canals^ which perforate them. Thickenings of this kind
most frequently affect the inner side of the cell-wall ; the
canals therefore run from the cavity of the cell outwards,
and are there closed by a thin membrane^. When the
cell loses its protoplasm and dies, this membrane is in
many cases destroyed, and the pit or the canal then be-
comes open, as, for instance, in Sphagnum and in many
wood-cells. The pits, especially in elongated cells, are
generally arranged in spiral rows ; but in other cases they
are peculiarly grouped (Fig. 21, A). A remarkably striking
form is the Sie've-like or Lattice-like marking which occurs
in the Sie've-tubes of fibro-vascular bundles, generally in the
septa, but also in the side-walls. In the simplest case the
thin spots (pits) are densely crowded, only separated by
thicker ridges, and polygonal in shape ; they very often
appear as sharply circumscribed groups of numerous dots.
In many cases the thin parts of such an area become absorbed, and the protoplasmic
contents of adjoining cells enter into communTcation through these narrow channels.
Sometimes the structure of these Sieue-plates {e.g. in Cucurbita Pepo) becomes, when
old, very peculiar and complicated, from further thickening and swelling of the thickened
portions ^.

Fig. 17. — Intercalary surface-growth of
CEdogonium,

^ For further details of these somewhat complicated processes see Pringsheim, Jahrbuch fiir
wissen. Bot. vol. I; Hofmeister, Handbuch der phys. Bot. vol. I. p. 154; and Nageli und Schwen-
dener, Das Mikroskop, vol. II. p. 549.

^ Sometimes strongly thickened cell-walls with branched pit-canals have a very complicated
structure, e. g. in the hard testa of Bertholletia. (See Millardet in Ann. des Sciences Nat., fifth series,
vol. VI.)

3 Compare Nageli, Ueber die Siebrohren von Cucurbita, in the Sitzungsberichte der k. bayerischen

THE CELL' WALL.

n

One form of internal thickening which is extremely common in wood-cells and
vessels, viz. the formation of Bordered Pits \ deserves a fuller exposition.

The formation of Bordered Pits takes place as follows. When the cell-wall begins
to thicken, comparatively large spaces remain thin (Fig. 23, /; Fig. 24, B, t)\ but, as
the thickening augments, it reaches even the thin spaces of the wall (Fig. 23, a-e\ Fig.
24, C-F). The outline of the thin spaces of the wall in the wood of Pinus sylvestris
appears circular on a front view ; the edge of the thickening-mass which arches over
it grows also in a circular manner, and gradually contracts the opening; thus the front
view of such a pit shows two concentric circles, the larger of which corresponds to the
original thin space (Fig. 23, /), and the smaller to the inner edge of the thickening

Fig. 18. — Cell-forms of Marchantia polymor-
^A« with thickenings projecting inwards; A half
of an elater from the sporogonium, with two spiral
bands ; A' a portion more strongly magnified ;
B a parenchymatous cell from the centre of the
thallus, with reticulate thickenings projecting
inwards ; C a slender rhizoid with thickenings
projecting inwards, arranged on a spiral con-
striction of the cell-wall ; D a thicker rhizoid,
with thicker branched projections, and the spiral
arramgement still clearer.

FIG. \().—B a young pollen-grain oK Fiinkia
ovata; the knob-like thickenfngs projecting out-
wards are still small ; in the older pollen-grain C
they are larger ; they are arranged in lines united
into a net-wtDi-k.

Fig. 20. — Ripe pollen-grain of Cichoriutn
Intybus; the almost spherical substance of the
cell-wall is furnished with ridge-like thickenings
united into a net-work ; each of these bears
thickenings which project still more, in the form
of spines arranged like a comb.

Fig. 18 *ej.— Piece of an annular vessel from the fibro-vascular bundle of the stem of Maize (X 550).
h h the thin cell-wall of the vessel, on which the boundary lines of the adjoining cells are clearly
seen ; r r the annular thickenings of the wajl of the vessel ; y the inner substance of one of the rings
laid open ; t the denser layer which extends over the inner side of the ring projecting into the cavity of
the cell.

(Fig. 23, «-tf; Fig. 24, C, D). Now since this process takes place on both sides of
a partition-wall of two cells, a lenticular space is enclosed by the two overarchings,
divided in the middle by the original thin cell-wall (Fig. 24, jP", w), each half of this
pit-cavity communicating with the cell-cavity by a circular opening; When the wood-

Akad. der Wissenschaften, Munchen 1861, On the actual perforation C>f Sieve-plates see Sachs
in Flora, 1863, p. 68, and Hanstein, Die Milchsaftgefasse, Berlin 1864, p. 23 et seq.

^ The development of these was first accurately recognised by Schacht, De maculis in plantarum
vasis, &c., Bonn i860.

24

MORPHOLOGY OF THE CELL.

cells lose their protoplasm and become filled with air and water, the thin cell-wall dis-
appears (as in Fig. 24, E). and the two pits form a single cavity, which is bounded
by the over-arching thickening-masses, and is united with the adjoining cell-cavities
by a circular opening (Fig. 24, ^, D, E). In Pinus sylvestrts the pits are large and
distant from one another, and the whole process may be easily traced step by step.

The process appears somewhat different when pits lie very near to one another,
as in Pitted Fesse/s. In this case the thickening first appears in the form of a net-work,
of which the thin parts of the cell-wall occupy the polygonal meshes, as may be very
easily seen in young Maize-roots, for instance. Fig. 25, ^, represents a portion of the
side-wall of an already mature vessel ^ of the root-tuber of the Dahlia. The ridges
which first appear on the cell-wall are indicated at a, and are left white ; they enclose
elliptical spaces pointed at both ends. As the thickening continues, the free edge of
each ridge, as it grows further inwards, spreads, and becomes arched over the thin parts
of the cell-wall. In this case, however, the overarchings do not grow uniformly, but
in such a manner that their edges form at length a narrow fissure (c, in A and JB).

Fig. 21.—^ a parenchymatous cell from the cotyledon of Phaseo/iis ntultijlorus
isolated by maceration ; i i the parts of the cell-wall where it is bounded by inter-
cellular spaces ; 1 1 cell-wall furnished with numerous simple pits, but not greatly
thickened ; the thinnest parts of the pits are figured dark. B epidermis (e) and
coUenchyma (cl) of the leaf-stalk of a Begonia; the epidermal cells are thickened
unifonnly on the outer wall, but where they adjoin the collenchyma only at the
angles where three cells meet ; these thickenings have great capacity for swelling ;
chl chlorophyll-granules ; / parenchymatous cell (X 550).

Fig. 22.— Hypodermal cell of
the underground stem of Pteris
aquilina, isolated by boiling with
potassium chlorate and nitric acid ;
it is more strongly thickened on the
left side ; the unthickened spaces
appear as branched canals (X 550).

Here also, when two similar cells adjoin, the same process takes place on both sides
of the partition-wall, and lenticular spaces are formed by the overarchings ; these are
at first bisected by the original thin cell-wall, which afterwards disappears, and the
two cell-cavities are placed in communication at each bordered pit; the canal or
bordered pit which unites them is wide in the middle, and opens into each cell by
a narrow fissure (Fig. 25, 5, G). If, on the other hand, a vessel of this kind adjoins
a parenchymatous cell which remains always full of sap and closed, the thickening
and overarching of the pit occur only on the side of the vessel (Fig. 26, T), the thin
parts of the cell-wall remaining intact ^, and the bordered pit remains closed ; from

^ For the definition of a vessel see Chap. ii.

" These thin pieces of cell-wall which close up bordered pits may, by rapid surface -growth,
form bag-like protrusions, which grow through the pores of the pits into the vessels, spread them-
selves out there, become divided by septa, and thus form a thin-walled tissue, which not unfre-
quently fills up the whole of the cavity. These formations are known under the name of ' Tiillen '
or ' Tyloses ; ' they are abundantly and easily seen, for instance, in old roots of Cuairbita, and in the

THE CELL- WALL,

25

the cell-cavity of the vessel a narrow fissure {c) opens bet^^een the expanded thickening-
masses {b) into a M^ider cavity, which is bounded on the sides by the narrow part of the
thickening-masses {a), on the outside of the primary cell-wall. These processes can
only be seen in sections of extraordinary tenuity ; but these are easily obtained if good-
sized pieces of the parts to be studied are allowed to lie for months in plenty of
absolute alcohol, taken out before the preparation is made, and the alcohol allowed to
evaporate ; in this manner pieces of sufficient hardness are obtained to cut extremely
well with a very sharp knife.

Fig. ■2j,.—Pinus sylvestris ; radial longitudinal sec-
tion through the wood of a rapidly growing branch ;
cb cambial wood-cells ; a — e older wood-cells ; 1 1" t"
bordered pits of the wood-cells, enlarging with age ;
St large pits where cells of the medullary rays touch
the wood-cells (X SSo).

Fig. 24.— Pimes sylvestris; A transverse section of mature
wood-cells (X 800) ; m middle lamella of the common partition; i
inner layer, clothing the cavity ; z intermediate layer of the cell-
wall ; t a mature pit cut through the middle ; 't' the same, but at a
thicker part of the section, the part of the cavity of the pit lying be-
neath is seen in perspective ; t" a pit cut through beneath its inner
opening ; B transverse section through the cambium (X 800) ; c
cambium ; h wood-cells still young ; between them two very young
wood-cells with pits it beginning to form ; (C — F diagrammatic).

In the walls of scalariform vessels, which are developed with peculiar beauty in
the higher Cryptogams, the bordered pits are fissure-like ; they often stretch across
the partition-wall of two adjoining cells, but are very narrow in the longitudinal
direction. In Fig. 27, A^ is shown the lower half of a vessel of this kind with fissure-
like pits, separated by thickening-masses like rungs of a ladder ; the larger white spaces

wood of Robinia pseudacacia, &c, [See Joum. of Bot. 1872, pp. 321-323, t. 126; and Reess, Bot.
Zeitg. 1868, pp. i-ii, t. I ; also infra, Book iii. Sect. 16.]

26

MORPHOLOGY OF THE CELL.

are

.„. the angles of contiguous cells. The formation of such a scalariform thickening
begins by the growth, on the thin wall which separates two vessels (C, /), of transverse
ridges {v), which unite on either side with the thickening which always lies at the
angle of a cell-wall. C shows this in front view, D in vertical section. When com-
pletely developed, the thin cell-wall (/) is absorbed {c, c, in B), the thickening-ridges
have overarched, so that only a narrow fissure (5, d) remains between their margms;
still further inwards the ridge again becomes narrower. The interior cavities of two
adjoining vessels are thus united by a number of long narrow fissures {B, s) ; the frame-
work of the ladder is formed of peculiarly-shaped rungs, which may be seen in B at
f f in section, at e in front view. Where the wall of a vessel bounds a parenchymatous
cell (£), the scalariform thickening takes place only on one side of the vessel {g), and
is absent from the other side {p). In this case also the thin original wall remains,
closing externally the space of the fissure-like pit.

The variety in the formation of pits is by no means exhausted by these examples;
but all the processes cannot be described here ; we can only indicate a few.

^Pi^pi^iiMgm

Fig. 25.— Wall of a vessel with bordered pits from the
tuberous root of the Dahlia ; A front view of the wall of a
vessel from without ; B transverse section of the same (hori-
zontal, at right angles to the paper) ; C longitudinal section
(vertical, at right angles to the paper) ; q septum ; a the
original thickening-ridges ; b the overarching part of the
thickening-masses ; c the fissure through which the cavity of
the pit communicates with the cell-cavity. At a and /3 the
corresponding front view is appended in order to make the
transverse and longitudinal sections more clear (x 800).

Fig. 26.— From the root-tuber of
the Dahlia ; P parenchymatous wood-
cell ; Fa piece of the wall of a vessel,
where it adjoins a parenchymatous
wood-cell ; a b the thickening-masses
of the wall of the vessel cut through
at right angles ; c fissure of the pit ;
d simple pits in the parenchymatous
wood-cell (X 800).

In the formation of vessels in the Dahlia (Fig. 25) the pit occupies at first a large
round space, while the edges of the overarching thickening enclose a fissure. By
a modification of this process of growth the fissure may attain a length much greater
than the diameter of the pit, which then appears, on a front view, as a roundish opening
crossed by a fissure (Fig. 26, P). It also sometimes happens that the pit-fissure changes
its direction as the thickening increases; in this case, on a front view, two fissures
appear as if they cross one another (Fig. 28, A and 5, st). But in order to be certain
that this takes place within the layers of the wall of a single cell, the cells must be
isolated by maceration. Similar appearances are also often seen if the partition-wall
of two cells is observed from the front. If the fissure inclines to the left in the one

THE CELL- WALL.

27

cell, the corresponding fissure may incline to the right in the other ; viewed one over
the other they then appear crossed ^.

In cells which form a tissue, the partition-wall is always at first a very thin lamella;
as the thickness increases, the thickening-masses project into the adjoining cell-cavities.
Generally, as we have already seen, the thickenings on either side of a partition-wall
correspond; and this is very evident in the formation of pits, inasmuch as the pit-
canals of adjoining cells meet one another. But since a cell often adjoins cells of
a very different character, different sides of the same cell may show different forms
of thickening and different descriptions of pits. The
total growth in thickness may also be very different
on different sides ; thus, for instance, epidermal cells
are mostly strongly thickened on the outer exposed
wall (cuticle); the inner wall, where they adjoin
parenchymatous cells, being either very thin or cor-
responding in form to that of the adjoining cells.

The correspondence in the growth of the thick-
enings is less evident when they have a distinctly
spiral structure, or when they occur in the form
of strong spiral bands, as in spiral vessels; if, in
this case, in each adjoining cell one or more spiral
bands wind in the same direction, they must neces-
sarily cross on the common partition-wall.

(c) Stratification and Striation of the Cell-wall^.
When the cell-walls have attained a certain thick-
ness and extent of surface, stratification and stria-
tion become more or less evident. In consequence
of stratification the cell-wall appears as if composed
of very thin membranes enclosed one within another
and fitting very closely together ; the stratification
is seen both in the transverse and the longitudinal
section of the cell-wall. The striation is generally
to be seen most plainly from the front ; it may be
observed in the form of two systems of lines (some-
times apparently more) marked on the surface. The
one system, consisting of parallel striae, is always
intersected by the other system which also consists
of parallel striae. A closer investigation shows that
the appearance of striation does not belong to the
surface only or to one layer of the cell-wall, but
that the striation rather penetrates the whole thick-
ness of the cell-wall, and that the striae are due to
layers which intersect the surface obliquely, and all
other layers concentric with it. If the striation is
very strongly marked, and nearly parallel to the
longer axis of the cell, it may be recognised in a
transverse section in the form of lines crossing the

concentric layers ; in a longitudinal section only those systems of striation are easily
seen which, viewed on the surface, cut the longer axis nearly at right angles.

Fig. 27.— Vessel from the underground stem
of Pteris aqiiiliita thickened in a scalariform
manner ; A half of a vessel, isolated by Schulse's
maceration ; H — D sections obtained from pieces
of the stem hardened in absolute alcohol ; B after
a very clean section, represented in a partly dia-
grammatic manner ; to the right, front view of
the wall of the vessels from within ; c c vertical
section of the same ; C front view of the young
wall of a vessel ; D its vertical section ; E place
where a vessel adjoins a parenchymatous cell, in
section vertical to the thickening-ridges of the
vessel (X 800).

^ A representation of a twisted pit-canal, whose outer and inner fissure (within the same cell-
wall) cross, may be seen in Nageli, Berichte der Miinchener Akademie, 1867 (July 9), t. v. fig. 45.

"^ H. von Mohl, Bot. Zeitg. 1858, pp. i, 9, — Nageli, Ueber den inneren Bau der vegetabilischen
Zellenmembran, in the Sitzungsberichte der Miinchener Akad. der Wissenschaften, 1864, May and
Jul^. — Hofmeister, Lehre von der Pflanzenzelle, p. 197. f

a8

MORPHOLOGY OF THE CELL,

Every system of stratification or striation consists of layers of visible thickness and
of different refractive powers, so that a more strongly refractive layer or stria always
alternates with a less strongly refractive one. This difference of refraction results
from a diflerent distribution of water and of solid particles in the cell-wall ; the less
strongly refractive layers contain more water and less cellulose, and are therefore
less dense ; the more strongly refractive and denser layers contain less water and more
cellulose. Hence stratification and striation of the cell-wall disappear both when water
is completely eliminated, and also when it absorbs much water; because, in the first
case, the more watery layers are reduced to the condition of the less watery ones,
in the latter case the less watery become similar to the others. On the other hand
stratification and striation become most conspicuous when, from the particular propor-
tion of water in the cell-wall, the difference between the dense and the watery layers
is greatest. In many cases this may be brought about by addition of acids or alkalies

Fig. 29. — Transverse section of a cell from
the root-tuber of the Dahlia (X 800); / the
cell-cavity; A' canals which penetrate the
stratified cell-wall ; sp a crack by which
an inner group of layers has become sepa-
rated.

FIG. 28.— Sclerenchymatous cells ol Pteris aquilina', A half of a cell isolated and rendered colourless by Schulze's
maceration ; B a piece more strongly magnified (X 55o) ; the fissure-like pits cross ; i. e. the fissure is twisted ; at/ side-
view of a fissure, appearing here as a simple canal. C transverse section of the same.

which occasion a moderate swelling. But if the dense layers are very dense, and the
others very watery, as is the case with some wood-cells {e. g. Pinus syl'vestris), the stria-
tion becomes more evident through desiccation, because this brings out the dense layers
and effaces the less dense ones.

The systems of striation and stratification of a cell-wall intersect one another, like
the cleavage-planes of a crystal splitting in three directions. But since the striation and
stratification are produced by layers of a measurable thickness, composed of alternately
denser and less dense substance, the cell-wall appears to be composed of parallelopipedal
pieces, distinguished from one another by the proportion of water contained in them.
If we for a moment disregard the stratification, and assume that we have two inter-
secting systems of striation ; then, where two dense striae intersect, the densest or least
watery places are always to be found ; where two watery ones intersect, the least dense
or most watery, and where places of greater and less density intersect, areolae of inter-

THE CELL- WALL,

29

mediate density are found. The intersecting systems of striation must form prisms

which stand vertically or obliquely upon the surface of the cell-wall. If the concentric

stratification is very strongly developed, every one of these prisms must be cut up into

more and less dense sections parallel with its base ; if the concentric stratification is

feebly developed, the prismatic structure may predominate. The peculiar internal

structure of the epispore of Rhizocarpeae (Fig. 33, p. 31)^, and the yet more various

structure of the extine of many pollen-grains, may perhaps be resolved into a further

development of this kind of process ; but our space

does not permit us to pursue this question in detail.

The layers which produce the external appearance

of striation may possess the form of closed rings, i. e.

may be similar to thin sections of the cell, or may

run in a spiral manner round the axis of the cell. A

distinction must accordingly be drawn between annular

and spiral striation ; it is often, however, very difficult

to decide which of the two is present ; sometimes both

are developed at different parts of the same cell-wall.

Sometimes one system of striation is very obscure, the

other all the more strongly marked ; or one system may

be the better developed in one layer of the cell-wall,

the other system in another layer; and this is genetically

connected with the above-mentioned twisting of the

pit-fissures. The striation is mostly clearest in cells

with broad uniform thickening-surfaces, as Falonia

utricularis, hairs of Opuntia, pith-cells of the root-tubers

of the Dahlia (in the latter case remarkably plain) ; but
[it may also be recognised when the sculpture of the
cell-wall is complicated ; e. g. in the walls of very wide
[vessels of Cucurbita Pepo provided with densely crowded
small bordered pits (after Schulze's maceration, espe-
.cially in vessels of the root, where the crossed spiral
[striation is very clear). The striation may itself give
^occasion to diflferences of elevation ; sometimes the
denser layers project a little on the inner side of the
cell- wall (Fig. 32 jB); or individual denser layers of one
system of striation alone become prominent ; thus, for
instance, a fine spiral band makes its appearance on

not unfrequently crossed by one running in the opposite
^direction. When elongated fissure-like pits are ar-
ranged in spiral lines on the cell-wall, a system of stria-
tion is generally found in a corresponding direction.

This slight sketch must suffice to explain the nature
of stratification and striation, and their relation to the

sculpture of the cell-wall; further detail would exceed the limits of this work^.
(d) Intussusception the cause of growth of the Cell-quail in surface and thickness,

Fig. 30.— Cells from the leeit of //oya car-
nosa {X800), showing- striation. The striae
are not nearly so strongly marked in nature,
but are quite as evident ; a optical longitu-
dinal section of the crossed annular stria-
tion; b front view where the annular
striations cross ; c, d front view where they
do not cross ; e a piece of cell-wall, where
only a few annular striations are to be
seen.

The

* See also Book II, Rhizocarpeae.

"^ The striation may easily be seen, even with slight magnifying power, on the large pith-cells of
the root-tubers of Dahlia, on the hairs of Opuntia, and on Valonia utricularis, but only with very high
magnifying power on isolated wood-cells of Firms, bast-fibres, &c. ; one of the examples longest
known are the bast-cells of Apocynaceae possessed of alternate dilatations and constrictions. (Mohl,
Veget. Zelle, Fig. 27.)

30

MORPHOLOGY OF THE CELL.

surface-growth of the cell- wall can be regarded only as an intercalation, between its
already existing particles, of new particles which force the old ones asunder. It is very
probable that the striation has a genetic connection with this process, similar to that
which Nageli has shown to exist betvyeen the stratification of starch-grains and their
growth. It was long thought that the growth in thickness of the cell-wall arose from
the repeated deposition of new concentric layers on its inner side ; in which case the
innermost layer would always be the youngest. This appeared to be an extremely
simple explanation of the stratification of the cell-wall ; and the chemical differentiation
of thick cell-walls appeared entirely to support this idea. But the increased powers of
the microscope revealed a fact quite fatal to the theory of apposition ; the stratification
of thickened cell-walls was shown, as we have seen, to be not a contiguity of similar, but
an alternation of dissimilar, layers. For reasons which cannot here be discussed, it must

Fig. 31.— Hypodermal eel
from the stem of Pteris aqiii-
Una, isolated by Schulze's
maceration. The wall is seen
in optical longitudinal sec-
tion; it shows an innermost
very dense layer, a central
less dense layer {the dark
streak to the right below) en-
closed by two denser layers;
these layers are penetrated
by pit-canals, which are seen
on the hinder wall in trans-
verse section.

Fig. 32.— Striation of the wood-cells of Pinus
Strobus ; A front view of a young cell ; a fissure
J runs across the still young bordered pit, cor-
responding to the spiral striation ; R sectional
view of the cell-wall with a part of the side view ;
i the middle lamella of the wall common to two
cells ; -v V the thickening-layers in contact with
it ; these are striated, the striation may be recog-
nised as a formation of layers penetrating the
whole thickness; the denser (white) layers pro-
ject in the form of little knobs. C front view of
a pit ; the striation here appears as a star-like
arrangement of less dense spots (X 800).

be concluded that these alternate deposits of more and less watery layers must be the
result not of an apposition, but of an internal differentiation of the cell-wall already
formed. The fact is decisive, that on the inner side of every cell-wall there always lies
a dense layer containing but little water; if growth in thickness took place by successive
deposition of layer.s, the innermost and youngest layer would be alternately more and
less dense, which is not the case. The growth also of such thickening-masses as project
externally, like the crests and spines of pollen-grains, &c., can only be explained by in-
tussusception, not by apposition.

Growth by intussusception can be regarded only as the diffusion of an aqueous
solution from the protoplasm between the micellae of the cell-wall. What this solution
is, cannot at present be said with certainty ; probably it contains some carbo-hydrate
which is easily transformed into cellulose. This substance then forms between the
micellae of the cell-wall new solid micellae of cellulose. The actual process of growth,
the internal structure of the cell-wall already described, and certain phenomena which it
exhibits with polarised light, as well as the swelling of the cell-wall, lead to the conclusion

I

THE CELL- WALL.

31

that it consists of solid micellae of definite form, each of which is surrounded by an en-
velope of water, and is thus separated from the adjoining micellae ; the more watery a
cell-wall-layer, the smaller, according to the principles laid down by Nageli ^, are the solid
micellas, the more numerous and the thicker their aqueous envelopes. From this it
follows that a certain quantity of water is as indispensable to the growth and to the in-
ternal organisation of the cell-wall as is cellulose itself. This water may be designated
(water of organisation^ in the same sense as we speak of ' water of crystallisation ; * and as
the latter is indispensable to the formation of many crystals, so is the former to the
structure of the cell-wall. It is moreover, as we shall see, a peculiarity of all organised
structures to contain this water of organisation, at least as long as they continue to grow,
because they all alike grow by intussusception.

From what has been said it will easily be seen that the concentric formation of
layers of a cell-wall growing by intussusception differs essentially from the repeated

FIG. 33.— Macrospores oi Pilidarta globttlifera, in optical longitudinal section; A a still unripe spore, in
which the outermost gelatinous layer of the cell-wall is still wanting, though present in the ri^e spore B ; the two
outermost layers of the cell-wall of the latter (c and d) have assumed a prismatic structure, especially in c ; in a?
stratification is feebly indicated at the same time. Seen from the surface the prisms appear like areola;. The
bounding-surfaces of the prisms are, in the corresponding cell- wall layer oi Marsilia Salvatrix, more solid and
cuticularised, by which the appearance of a honey-comb is produced. (The layers b, c, and d together form the
epispore.) (See also Book II., Rhizocarpeae.)

formation of a cell-wall round one and the same protoplasm-mass ; cell-walls enclosed
one within another may be produced in this manner; but these cannot be considered as
layers of one cell-wall ^. Such a process is very common in the formation of the pollen-
grains of Phanerogams; within each of the four cells formed by the division of the
mother-cell the protoplasm forms round itself a new cell-wall, before the mother-cell-
wall has disappeared (Fig. 34).

But the renewal of a cell-wall may also be brought about by the internal layers of
the cell-wall increasing by intussusception, while the external mass of layers undergoes no
further growth. Thus the cell-wall of spores and pollen-grains originally grows as a

' The theory of the growth of the cell-wall (as of all organised structures) by intussusception
was first originated by Nageli in his great work on Starch (1858). Compare also Sachs, Handbuch
der Experimental-physiologie der Pflanzen, § 114.

^ [A striking instance of such a multilaminated cellulose-envelope is afforded in the remarkable
organism described by Archer under the name of Chlamydomyxa, Quart. Jour. Micr. Sci. 1875,
p. 129.]

3^

MORPHOLOGY OF THE CELL.

whole, increasing by intussusception ; by subsequent internal differentiation, masses
(shells) of layers are formed, differing in their chemical and physical properties; the
outer firm cuticularised shell (exospore, extine) remains unchanged, and is thrown off as
an envelope, while an inner mass of layers (endospore, intine) begins a new growth with
the germination of the spores or the development of the pollen -tubes. A similar process
occurs with many filamentous Algae (Rivulariese and Scytonemeae), where a large number
of cell-wall-layers are successively formed one within another, while from time to time
the older masses of layers cease to increase, and are broken through by the growing
filament, which now forms new cell-wall-layers (see Niigeli und Schwendener, Das
Mikroskop, 2nd ed., p. 547). It need scarcely be mentioned that these facts do not
contradict the theory of growth by intussusception, but only correspond to particular
modifications in the life of the cell.

Fig. 34. — Pollen-mother-cell of Cucurbita Pepo; sg the externa common layers of the mother-cell undergoing
absorption; sp the so-called 'special mother-cells' consisting of masses of layers of the mother-cell-wall which surround
the young pollen-grains ; they also are afterwards absorbed ; ph the wall of the pollen-grain ; its spines grow outwards and
penetrate the 'special mother-cell;' v hemispherical deposits of cellulose on the cell-wall of the pollen-grain, from which
the pollen-tubes are afterwards formed; / the contracted protoplasm of the pollen-grain (the preparation was obtained
by section of an anther which had lain for some months in absolute alcohol) ( x 550).

(e) Differentiation of the Cell-<zvall into Systems of layers {Shells) ivith different chemical
and physical properties.

Very young and thin cell-walls, while still in rapid growth, as also many older
ones, are composed throughout their whole thickness of what has been termed pure
cellulose ; i. e. they are easily permeable by water, only slightly extensible or capable
of swelling, very elastic, colourless, and soluble in sulphuric acid; with iodine and
sulphuric acid they assume an intense blue colour, as also with Schultz's solution, rarely
with solution of iodine alone (as the asci of Lichens). Together with these common
properties, they may, according to the nature of the cell, possess many other peculiar
reactions. Among older fully developed cells, succulent thin-walled parenchymatous
cells of the higher plants behave for the most part in this manner, many thick-walled
cells of Algse, and — with the exception of the blue colour produced by iodine and
sulphuric acid, and by Schultz's solution — most hyphae of Fungi and Lichens.

With more strongly thickened cell-walls (rarely with moderately thin ones, as some
cork-cells), whole masses of layers behave in a different manner chemically and physically ;
in consequence of this the cell-wall is at once seen to consist of two or more Shells ^, each

^ It is desirable to employ the expression 'layers' (Schichten) only in the sense mentioned
in paragraph (d), where it implies a regularly alternating difference in the proportion of water,

THE CELL-WALL.

^3

of which may again exhibit numerous layers and the striation already described. In the
case of exposed cells which require protection (as pollen-grains or spores), or of those
which themselves serve as a protection to other tissues (as cork), an outermost shell
(of greater or less thickness) of each cell-wall is transformed into cork or cuticle.
When the cells are destined to form a firm frame-work (as in wood-cells), the outer
masses of layers become lignified ; in other cases, on the other hand, the outer layers,
rarely the inner ones, are transformed into mucilage. Usually an inner layer of the cell-wall
remains unchanged in all three cases, and gives the above-mentioned cellulose
reactions, while the suberised and
lignified shells of the cell-wall
may, after previous treatment
with alkalies or with nitric acid,
also exhibit these reactions; the
layers which are transformed into
mucilage are the most refractory.
Some of the morphological
relations here treated of find
their explanation only when we
study the formation of tissues;
but I cannot here discuss the
chemical behaviour of the cell-
wall ; the reactions must properly
be regarded not as chemical tests,

>ut only as the means of recog-
nising a morphological differen-

iation. The description of some

Lamples will be sufficient to guide

le beginner.
The pollen of Thunbergia alata

\. 36) shows that the different

jvelopment of two shells of a

ill-wall may go so far that the
juticularised shell, the Extine, be-
comes actually separated from
the non-cuticularised shell, the
Intine, which still possesses the
power of growth ; by this means
it becomes broken up in most
cases by fissures previously formed
into one or two spiral bands.
This can be artificially induced
by laying these pollen-grains in
concentrated sulphuric acid or potash solution ; the extine then assumes a very beautiful
red colour, while the intine in the first case dissolves, in the second case swells a
little and remains colourless. In the germination also of many spores {e.g. Spirogyra,
Mosses, &c.) the cuticularised exospore becomes completely separated and stripped
from the endospore, which still continues to develop; both shells, however — cor-
responding to the extine and intine of the pollen-grain — consist, in their actual
development, of systems of layers of a single cell-wall possessing different chemico-
physical properties.

FIG. 35. — A pollen-grain of Cucttrbita Pepo, which has emitted a pollen-
tube (sf) into a papilla of the stigma. The cell-wall of the pollen-grain consists
of a cuticularised extine (<■), and an intine capable of growth (?) ; the latter is
greatly thickened at certain places {B i) ; on each thickening -mass the extine
forms a roundish lid (d) ; when the pollen-grain is commencing to emit its
pollen-tubes, the thick parts of the intine swell, and thus tilt up and lift off the
lid-like piece of the extine j one or two of these thickening-masses form pollen-
tubes (X 550).

as in the striae; another term must be employed for the structures now under consideration, and
the expression * shells' (Schalen) appears to answer the purpose.

D

34

MORPHOLOGY OF THE CELL.

In the epidermal cells, the cuticularisation either affects a shell of the outer wall,
or it attacks the side-walls, as may be well seen, for instance, on the under-side of
the leaf-veins of the holly. If a very thin transverse section (Fig. 37, J) is treated with
Schultz's solution, and submitted to a very high magnifying power (800), each cell-wall
of the epidermis appears to be composed of two shells, of which the inner one, which
is softer and more capable of swelling (c), becomes dark blue, while the outer shell
does not. But this latter shows itself to be further composed of two chemically different
layers, an inner (h), which assumes a yellow colour and penetrates laterally between ,
the cells {b'), and an outer one which remains colourless (a), and extends continuously |
over the cells (the so-called true Cuticle). Between these two may be observed yet
another boundary-zone, which, when the microscope is focussed to it, passes over the
field of view like a shadow. The inner shell which assumes the blue colour, as well
as the outer cuticularised substance, are each composed of a system of layers. In

the latter moreover the radial stria-
tion is more evident, as is shown
in Fig. 37, ^, «, ^ ; these radial lines
are not, as was formerly thought,
pores, but are the transverse sec-
tions of layers which, in a front
view of the cuticle (Fig. 37, B, j),
appear as striae, and, following the
veins of the leaf lengthways, pass
over the septa of the cells (q).

An example of strongly lignified
cell-wa'.ls split up into three shells
occurs in the dark-brown-walled
sclerenchymatous cells which com-
pose the firm bands between the
fibro-vascular bundles in the stem
of Pteris aquilina (Fig. 38). The
very thick wall between two cells
contains a hard, dark-brown lamella
{a) in the centre of the double cell-
wall; this is followed on each side
by a fight-brown, more horny shell
{b) ; and this encloses a third shell
likewise fight-brown. By boifing in
nitric acid with potassium chlorate
the first {a) is dissolved, and the
cells are thereby isolated (see Fig.
28, p. 28) ; the two other shells of the cell-wall {b and c) remain unchanged by the
maceration, except that they lose their colour; and hence the shell c is shown to
be composed of different layers, some more and some less watery (Fig. 28, C, c). The
three shells also show a different behaviour on treatment with concentrated sulphuric
acid; a becomes a dark reddish brown and does not swell, or only slightly; b swells
in the radial direction and becomes thicker ; while c swells in the radial, tangential,
and longitudinal directions (see Fig. 38, C, c, and D, c) ; in transverse sections c breaks
away from b and curves in a vermiform manner (C) ; in longitudinal sections it is bent
in a wavy manner (D).

In true wood-cells, e.g. in Pinus syl'vestris (Fig. 24, A, p. 25), three shells are likewise
generally to be distinguished ; one in the centre of the double cell-wall {A, m), next
a thicker one {%), and then an innermost (z) ; the two first turn yellow on treatment with
solution of iodine or iodine and sulphuric acid, the innermost blue with the latter
reagent ; z and i are dissolved by concentrated sulphuric acid, while the central lamella

Fig. 36.— Pollen of Thunbtrgia alata (X 530). / and // placed in
concentrated sulphuric acid; IV, V, VII after solution of the intine ;
sometimes the fissures of the extine run so that isolated pieces of it fall
off, corresponding to the lids of the extine of other pollen grains, e.g. of
Cucurbita ; III in Schultz's solution, section ; VI in strong solution of
potash, e extine, i intine. The fissures of the extine clearly arise from
subsequent internal differentiation, in the same manner as the elaters are
formed from the so-called 'special mother-cells' of the spores oi IZqiiise-
turn, (See Book II., Equisetaceae.)

THE CELL- WALL.

35

m remains. Here also the possibility of isolating the cell depends on the circumstance
that the central lamella m may be dissolved by boiling in nitric acid with potassium
chlorate; and thus the walls of the isolated cells consist only of the two inner shells.
In many wood-cells (the ^ Libriform Fibres^ of Sanio) the inner thickening-layers
form a shell of cartilaginous or gelatinous consistence, as in the wood of many
Papiiionaceae.

When the outermost layers of the walls of cells which are combined into tissues
become gelatinous or mucilaginous, their boundary-line disappears; and the cells,
enclosed by the inner shell which is not mucilaginous, appear to be imbedded in a
homogeneous jelly; this appearance gave rise to the theory of 'Intercellular Sub-
stance,' to which we shall recur. This behaviour occurs in the tissue of some Fucaceae,
md also in the endosperm of Ceratonia Siliqua (Fig. 39) ; cc are the outer layers of the

/5 «

Fig. 37.— Epidermis of the central vein of the leaf of
the holly ; A transverse section ; B superficial appearance
(front view).

I

Fig. 38.— Structure of the sclerenchyma in the stem
of Pteris aqttilina (X 550). A a fresh thin transverse
section ; B the longitudinal wall between two cells, fresh
(a curved pit-canal at the lower end) ; C transverse sec-
tion in concentrated sulphuric acid ; D longitudinal sec-
tion of the wall in sulphuric acid; a the central lamella of
the wall ; b second shell ; c third or innermost shell of the
cell-wall ; p pore-canals ; / cavity of the cell

wall of the cells <?, which have become entirely converted into mucilage and rendered
indistinguishable, their innermost system of layers {b) appearing as a strongly refractive
shell. In the dry state the mucilaginous mass is almost horny; it swells up strongly
in water with potash solution ; with iodine and sulphuric acid it does not become
coloured, but the sharply defined inner shell b turns blue.

In isolated cells numerous cell-wall-layers may also form a mucilaginous shell
which is most beautifully develop^'d in the spores of Pilularia (Fig. 33, p. 32) and
Marsilea. In the sporocarp of these plants arc certain masses of parenchyma, the
cell-walls of which become mucilaginous on the inner side ; when dry the mucilaginous
masses are firm and horny, but can absorb so much water that they increase in bulk
several hundred-fold, and burst the wall of the sporocarp (Book II., Rhizocarpeae).
A similar transformation into mucilage of inner layers of cell-wall, while an outer,
thin, and cuticularised shell resists, occurs with linseed and quince-seed. The inner
thickening-masses of the epidermis of the seed, transformed into mucilage, absorb the
surrounding water, swell up violentlyj and, bursting the cuticle which is incapable

D 2

3<5

MORPHOLOGY OF THE CELL.

of swelling, appear, in the presence of a small quantity of water, as a hyaline layer
enveloping tiie seed ; with more copious addition of water they become more and more
diluted into thin mucilage. A similar process occurs in some other seeds, as those
of Teesdalia nudicauUs and Plantago Psyllium, in the seed-hairs of Ruellia, and the
pericarp of Salvia. Gum-tragacanth consists of the cells of the pith and medullary
rays of Astragalus creticus, A.lragacanthayTund other species, transformed into mucilage^
When the walls of these cells become mucilaginous, and swell up on copious addition
of water, they force themselves through cracks in the stem as viscid masses, and dry
up on the outside into a horny substance capable of swelling in water. Vegetable
mucilage can, however, arise in other ways ^.

(f) Incombustible Deposits occur in every cell-wall. The presence of lime and silica
can be directly proved, and it can scarcely be doubted that potash, soda, magnesia,
iron, sulphuric acid, &c., also occur in small quantities ; the lime-salts and silica
increase with age. The deposition may take place in two ways. Usually only ex-
tremely small particles of incombustible substance are deposited regularly between

the molecules of the organised substance of
the cell-wall ; and this may be recognised by
the ash remaining behind after ignition in
the form of the organised cell-wall (as a
skeleton). But lime-salts may also be con-
tained in the cell-wall in the form of nu-
merous very small crystals; they then lie
imbedded in the substance of the cell-
wall itself, sometimes in the form of
growths which project into the cell-cavity
{Cyjtoliths).

Skeletons composed of a substance^ soluble
in weak acids (generally thought to be lime)
are obtained by combustion of very thin
layers of tissue on glass or platinum-foil ;
they occur so generally that it is unneces-
sary to adduce examples ; from entire vascular cells I obtained, in the case of Cucurbita
Pepo, beautiful lime-skeletons. Silica-skeletons are obtained most abundantly from epi-
dermal cells and from Diatoms ; but silicified cell-walls occur also in the interior of
tissues, as in the leaves of Ficus Sycomorus, Fagus sylvatica, Quercus suber, Deutzia scabra,
Phragmites communis, Ceratonia Siliqua, Magnolia grandijlora, &c., according to Mohl*.
The silicification does not generally affect the whole thickness of the cell-wall, but
only an outer shell ; as, for instance, in the case of epidermal cells, the cuticularised
portion only. In order to obtain fine skeletons, it is necessary previously to soak the
removed epidermis or thin sections of it in nitric or hydrochloric acid, and then to
burn them on platinum-foil. I have found another method much more convenient.
I place larger pieces of the tissue {e.g. of leaves of grass, stems of Equisetum, &c.) on
platinum-foil in a large drop of concentrated sulphuric acid, and heat over the flame ;

Fig. 39.— Section of'the endosperm of Ceratonia
Siliqua.

^ [H. von Mohl, Bot. Zeitg. 1857, p. 33 ; Pharmaceut. Journ. Jan. 1859.]

2 Compare further, Frank, Ueber die anatomische Bedeutung und die Entstehung der vegeta-
bilischen Schleime, in Jahrb. fiir wissen. Bot. vol. V. 1866.

^ The salts found in the ash are partly products of combustion. Carbonates may be pro-
duced by the combustion of salts of vegetable acids. Since a strong red heat is necessary
for complete combustion, easily volatile chlorides (potassium or sodium chloride) may disappear
from the ash.

* H. von Mohl, Ueber das Kieselskelet lebender Pflanzenzellen, in Bot. Zeitg. 1861, no. 30 et
seq. — Rosanoff, Bot. Zeitg. 1871, nos. 44, 45.

THE CELL- WALL. ^^

the mass immediately turns black, and a violent formation of gas follows ; the heat
must be continued until only the pure white ash remains. This is soon effected by this
means, whereas otherwise the reduction to ash is generally very tedious, and often
does not afford an entirely colourless skeleton ^

Sect. 5. Protoplasm and Nucleus^. — Now that the significance of proto-
plasm as the peculiar living essence of the cell has been sufficiently brought out,
we need only add what is absolutely essential, both as respects its chemical and
physical nature and its structure and movements. Protoplasm consists of a com-
bination of (apparently different) albuminous substances (proteids) with water and
small quantities of incombustible materials (ash). In most cases it also contains,
as may be concluded on physiological grounds, considerable quantities of other
organic compounds, belonging probably to the series of carbo-hydrates and oils.
These admixtures are distributed through its mass in an invisible form ; but it
not unfrequently includes granules of starch and drops of oil, which at a
subsequent period may either entirely disappear or may increase in bulk. Very
commonly the rapidly increasing protoplasm, in itself colourless and hyaline, is
rendered turbid by numerous small granules, consisting, probably, of minute drops
of oil. The protoplasm, as it is gencEally met with, ought therefore to be con-
sidered as true protoplasm with varying admixtures of different formative materials
[Meiaplasm of Hanstein). The consistence of protoplasm varies greatly at different
times and under different circumstances, even in the same protoplasm-mass. It
commonly appears soft, plastic, tough, inelastic, and very extensible; in other
cases it is more gelatinous, sometimes stiff, brittle (in the embryos of seeds before
germination); but very commonly it gives the impression of being a fluid. All
these properties depend essentially on the quantity of water it has absorbed. But,
however great may be the quantity of water, and its consequent similarity to a
fluid, the protoplasm is nevertheless never a fluid ; even the mucilaginous or gelatinous
conditions of other bodies can only be very superficially compared with it. For
the living and fife-giving protoplasm is endowed with internal forces, and, as the
result of this, with an internal and external variability which is wanting in every
other known structure ; its active molecular forces cannot, in short, be compared
with those of any other substance ^ The capacity which protoplasm has, in
consequence of the forces which become manifested in it, of assuming varying
definite external forms, as well as its capacity of secreting substances of different
chemical and physical properties according to definite laws, is the immediate cause
of cell-formation and of every process of organic Hfe.

^ On the crystals sometimes deposited in the cell-wall, see Sect. 11.

2 H, von Mohl, Bot. Zeitg. 1844, p. 273, and 1855, p. 689 ; [Ann. des Sci. Nat. 1857, vol. VII.
P- 253].— Unger, Anatomie und Physiologic der Pflanzen, p. 274, 1855. — Nageli, Pflanzenphysiol.
Untersuchungen, Heft I. Zurich. — Briicke, Wiener akad. Berichte, p. 408 et seq., 1861.— Max
Schultze, Ueber das Protoplasma der Rhizopoden und Pflanzenzellen, Leipzig 1863. — De Bary,
Die Mycetozoen, Leipzig 1864. — Hofmeister, Die Lehre von der Pflanzenzelle, Leipzig 1867. —
Hanstein, Sitzungsberichte der niederrheinischen Gesellschaft in Bonn, Dec. 19, 1870.

3 For further details on this point, see Book III ; also my Handbuch der Expcrimental-
Physiologie der Pflanzen, § 116, Leipzig 1865.

38 MORPHOLOGY OF THE CELL.

The protoplasm of plants, when in a state of vital activity, is generally very
watery; on one hand it exhibits an internal differentiation of its substance into
layers and portions differing in their consistence and chemical nature ; on the other
hand it assumes definite outlines, and becomes bounded by surfaces of determinate,
and mostly very variable, form.

The internal differentiation of protoplasm usually manifests itself by the
formation of an external hyaline, firmer, but very thin layer, surrounding the inner
mass, with which it remains in the most intimate contact. Every portion of a
protoplasm-mass immediately surrounds itself, when it becomes isolated, with
such a skin. In the interior a quantity of fluid sap, which permeates its
substance throughout, becomes separated in the form of drops ( Vacuoles) ; when
the protoplasm is contained in a growing cell these vacuoles increase as the
cell grows, and the protoplasm-mass becomes a sac filled with watery sap.
One of the most common internal differentiations of the young protoplasm-
mass, while constituting itself a separate individual, is the formation of the
Nucleus, The substance of the nucleus is at first indistinguishable from the
rest of the protoplasm, and its formation is essentially the accumulation of
certain particles of protoplasm round a centre, which is also usually the centre
of the whole protoplasm-mass. Once formed, the nucleus — whose chemical
nature, as far as observation goes, is altogether similar to that of the protoplasm
— becomes more sharply defined; it may itself become enveloped in a skin,
and vacuoles and granules (the Nucleoli) may become separated in it. But the
nucleus always remains a part of the protoplasm-mass ; it is always imbedded in
it; very commonly it becomes again absorbed, after a short existence, in the
protoplasm, e.g. in cells which divide frequently, see p. 14; in the elongated cells
of the Characese the nucleus disappears altogether when the circulating motion of
the protoplasm begins. Another very common differentiation of the substance
of the protoplasm consists in portions of it becoming separated in a definite
form, and assuming a green colour, forming the Chlorophyll-bodies, which, like the
nucleus, not only arise out of the protoplasm, but always remain portions of the
protoplasm-mass. But since these require more minute investigation, the next
section will be devoted to them.

The external configuration of the protoplasm as a mass of definite form can
be reduced to two cases : — either all its most minute particles group themselves
concentrically round a common centre; or an internal motion takes place, which
causes the protoplasm-mass to become elongated in some one direction, and
disturbs the centripetal arrangement. The former occurs commonly in the for-
mation of new cells, the latter during their growth.

The movements of the minute particles of protoplasm which determine its
grouping and configuration during the formation and growth of cells, are generally
so slow as not to be visible even when subjected to a very high magnifying power.
Much quicker movements, even appearing rapid under a very high magnifying
power, occur in cells either before their growth, as in swarm-cells, or when it
is nearly completed. Merely having regard to external appearance, the following
kinds of movements of this nature may be distinguished : —

(A) Movements of naked protoplasm-masses, (i) Swimming of swarm-cells

PROTOPLASM AND NUCLEUS. 39

and antherozoids. This is characterised by the naked protoplasm-mass — swarm-cell
or antherozoid — not changing its external form, while motile vibratile cilia, which
are probably slender threads of protoplasm, cause rotation round the longer axis,
and at the same time a progressive motion in the water. (2) Amoehoid movement ; —
consisting of rapid changes in the external shape of naked protoplasmic structures,
Myxoamoebae and plasmodia, which, while under water or in moist air on a
firm support, creep about as if flowing, extending, and contracting; while within
both the principal mass and the appendages which proceed from it a ' streaming '
motion occurs.

(B) Movements of the protoplasm within the cell-wall. These commence
after the protoplasm- mass of the cell has formed a larger sap- cavity, and continue
commonly after the growth of the cell has ceased until the end of its life.
(3) Those movements are distinguished as Circulation where threads and bands,
proceeding from the parietal protoplasm, run to that portion which envelopes the
nucleus, and often stretch completely across the sap-cavity. A distinction is drawn
between movements c-f larger portions of protoplasm, and the 'streaming'
movement of the substance of which they are composed ; the former consist in the
accumulation or diminution of the parietal layer, in movements in different directions
of the mass which contains the nucleus, and, dependent on this, in different groupings
of the threads. Within these structures themselves currents often occur, which
are apparent from the movement of the enclosed granules, and are often in
opposite directions within the same slender thread. In the cells of lower and higher
plants which contain much protoplasm and sap but only a small quantity of granular
contents, the circulation is a widely distributed phenomenon, especially visible in
the hairs. (4) The term Rotation is applied to those cases where the whole
mass of protoplasm enclosing a cell-cavity circulates as a thick current complete
in itself, and carries along with it the grains and granules contained in it. This
occurs in some water-plants, Characeae, Vallisnen'a, root-hairs of Hydrocharis, &c.

(a) The protoplasm exists in two conditions, which may be distinguished as the
living and the dead ; the former passes over into the latter by the most various chemical
and mechanical processes; the reactions of living protoplasm towards chemical reagents
are essentially different from those of dead protoplasm ; but this of course can only
be perceived when the reagents do not at the same moment cause death. Solutions
of different colouring matters, as aqueous solutions of the colours of flowers and the
juices of fruits, especially also weak acetic solution of carmine, have no power of
colouring living protoplasm ^ ; but if it has been previously killed, or if it has lost
its vital properties by long-continued action of these reagents, it absorbs a re-
latively larger quantity of colouring material than of the solvent, and the whole
substance assumes a much more intense colour than the reagent. Solutions of iodine
in water, alcohol, potassium iodide, or glycerin, act in a similar manner; they all cause
a yellow or brown colouring of the protoplasm, which is more intense than that of
the solution itself. If protoplasm is first treated with nitric acid, the excess of
acid removed by water, and potash solution added, it assumes a deep yellow colour ;
saturated with a solution of cupric sulphate and then treated with potash, it becomes

* In consequence of this the protoplasm and nucleus are colourless even when the sap is coloured
in living cells; in other cases, on the other hand, the protoplasm is tinged by a colouring matter soluble
in water which is not present in the cell-sap, as in Floridea; and the flowers of Compositge.

40 MORPHOLOGY OF THE CELL.

of a beautiful dark violet. Protoplasm containing but little water treated with a
large quantity of concentrated sulphuric acid assumes a beautiful rose-red colour,
without at first changing its form; subsequently this colour and the form disappear
together, the protoplasm dissolving. Dilute potash solution (sometimes also ammonia
solution) dissolves protoplasm, or at least destroys its form, and makes it homogeneously
transparent. If, on the other hand, cells with protoplasm of characteristic form are
placed in concentrated potash solution, the form remains for weeks, but disappears
immediately on addition of water. All these reactions are collectively characteristic of
true albuminoids, as casein, fibrin, and albumen;^ and we are therefore justified in
assuming that substances of this kind are always contained in protoplasm. If the
protoplasm-sac in cells with much sap is very thin, it acquires a greater power of
resistance to the solvents. In another respect also protoplasm behaves like albuminoids.
By heating very watery protoplasm to above 50° C, it is killed and becomes turbid
and stiff, as if coagulated ; alcohol and dilute mineral acids act in the same manner.
The nucleus behaves towards all colouring substances, solvents, and coagulating agents,
in the same manner as living watery protoplasm, or it shows itself even more sensitive,
especially in young cells ; in older cells however it is acted on less easily.

At the base of all protoplasmic structures there probably lies a substance which is
colourless, homogeneous, and not visibly granular, to which alone the name Protoplasm
ought perhaps to ba applied, or which ought at all events to be distinguished as the
basis of protoplasm. The fine granules which are so often mingled with it. and
whic'.i some consider an essential ingredient, are probably finely divided assimilated
foDl-materials, which undergo a further chemical metamorphosis into protoplasm; every
intermediate form occurs from these more or less fine granules to the largest which
may be clearly recognised as oil and starch. Homogeneous protoplasm destitute of
granules is found in the cotyledons of dormant embryos of Helianthus, and in the first
leaves of Phaseolus ; out of it chlorophyll is subsequently formed, and it contains but
very little water; but the extremely watery protoplasm which rotates in the cells of
Vallisneria is also not itself granular; nothing but nucleus and chlorophyll-granules
can ba recognised in it. In the development of the spores oi Equ'isetum (Fig. 10, p. 14)
the finer granules separate repeatedly from the homogeneous protoplasm, and afterwards
become again distributed through it. But in some cases the protoplasm is so loaded
with granular and coloured materials, that the colourless hyaline basis can no longer be
distinguished ; as, for instance, in the oospheres of Fucus (Fig. 2, p. 3), the zygospores
of Spirogyra (Fig. 6, p. 10), and in many spores and pollen-grains \ In the reservoirs
for reserve-materials contained in dry seeds {e.g. the cotyledons of peas and beans),
the protoplasm itself is often contracted into small roundish grains, among which lie
the starch-grains.

(b) Skin, Vacuoles, Mo'vement'^ . Naked protoplasm-bodies, as the plasmodia of the

^ Hanstein gives to the substances mingled with the true protoplasm, and which undergo many
transformations, the collective name of Metaplasm. (Bot. Zeitg. 1868, p. 710.)

" [The recent observations of Strasburger differ in some respects from what is stated in the
text. Strasburger recognises a differentiation of protoplasm into two layers, which may be termed
Ectoplasm and Endoplasm respectively In vegetable cells the former is hyaline, while the
chlorophyll-grains are imbedded in the latter. The ectoplasm does not however consist of the
mere hyaline basis of the protoplasm, nor is it identical with the skin which is formed on a free
surface when exposed, but is the result of a true process of differentiation. In numerous cases,
though not universally, Strasburger has found that the ectoplasm presents the radial striation
referred to in the text, and sometimes also a striation parallel with the surface. In the swarm-cells
of V.iucheria treated with a one per cent, solution of osmic acid, he found that this striation depended
on th:i presence in the ectoplasm of small rods of denser, imbedded in more watery protoplasm.
Extern illy and internally these rods are in contact with a delicate continuous layer of protoplasm,
and the cilia, which are more slender than the rods and about twice as long, arise from them. In
the earliest stage the cilia are small processess of the ectoplasm corresponding in position to the rods

PROTOPLASM AND NUCLEUS.

41

Myxomycetes, some swarm-cells, e.g. of Faucheria, allow the skin to be recognised,
under sufficient magnifying power, as a hyaline edging; in the swarm-cells of Vaucheria
it is evidently striated radially when seen in section, just as some cell-walls are ;
Hofmeister (Handbuch, vol. L p. 25) found the same appearance in the plasmodia of
Mthalium. Probably this skin is nothing but the pure basis of the protoplasm itself
free from granules, of which the whole body is formed, but which is masked in
the interior by grains and granules. It follows that in the amoeboid movements of
Plasmodia the new extensions are always at first formed of the skin alone ; it is only

Fig. 40. — B — G protoplasm from an injured filament of Vaucheria terrestris, slowly emerging in water, in different successive
conditions, at intervals of about five minutes; h the wall of the ruptured filament; i the protoplasm which still remains in the
filament ; a in B, C, D, and F, a ball of protoplasm detaching itself, forming vacuoles, then dissolving (in F)\ b a. branchlet of the
protoplasm from which the mass b' k detached, this mass isolated in D, dissolved in P; c and c' behave in a similar manner;
G shows the further changes of the part c" laF. A a freshly escaped mass of protoplasm, rounded off into a sphere, the chlorophyll-
granules lying all together in the inside; a hyaline layer of protoplasm envelopes the whole as a skin.

when they increase in size that the interior granular substance makes its appearance
in them. This is more clearly the case in the masses of protoplasm that escape into
water from the injured filaments of Faucheria, which often instantly become rounded
into globular bodies, but not unfrequently show the amoeboid movement of plasmodia

a later stage ; they are somewhat longer, the full extremity of each being terminated by a knob ;
'as development proceeds the cilia become longer and the knobs become smaller in proportion to the
increase in length, until the final hair-like form is reached. As long as the swarm-cell is in contact
with the mother-cell-walls, the cilia are closely adpressed to the surface of the ectoplasm, with their
apices directed forwards. The secretion of a cellulose envelope is closely and even inseparably con-
nected with the presence of an ectoplasmic layer. See Strasburger, Studieu iiber Protoplasma, Jena
1876; Vines in Quart. Journ. Micr. Sci. 1877, pp. 124-132.]

42

MORPHOLOGy OF THE CELL.

for as much as half-an-hour or an hour (Fig. 40). This interpretation of the skin
is not at all opposed to the fact that it is denser than the inner and more watery
substance. That the cohesion in each protoplasm-mass decreases from without inwards
is shown by the greater mobility of the inner portion, especially with plasmodia, and
also by the formation of vacuoles, which clearly depends on the collection of part of
the water present in the protoplasm round internal points in drops, presupposing
that the cohesion is overcome at these points. The view that the hyaline homogeneous
basis itself forms on each exposed surface of the protoplasm the skin destitute of
granules, entirely agrees with the supposition that not only every vacuole in a pro-
toplasm-mass, but also every thread of protoplasm which traverses the sap-cavity, and
finally the sap-cavity itself, is also bounded by a skin, even if it be so thin that it cannot
be seen when strongly magnified ^

If the protoplasm is not enclosed in a cell- wall, the vacuoles are usually small and
not numerous. If, on the other hand, a cell-wall is formed, and if the cell grows

rapidly, this is always accompanied by an
increase in number and size of the vacuoles
(Fig. T, p. 2). This not unfrequently leads
to a frothy condition of the protoplasm where
the vacuoles are separated only by thin lamellae
of that substance (Fig. 41, ^); but in other
cases the inner protoplasm-mass of a cell
breaks up into smaller portions, each of which
encloses a large vacuole surrounded by a thin
skin of protoplasm (Fig. 41, 5, b). These
are the ' Sap-'vesicles ' which are so com-
mon, and which sometimes enclose granules
of chlorophyll and other substances, and thus
resemble cells ; they are not uncommon in the
flesh of berry-like fruits, and in tissues with
mucilaginous juices. If the rapidly growing
cell does not form new protoplasm as the size
of the cell and the amount of sap increase,
the quantity of protoplasm decreases ; and not
unfrequently it forms a thin sac not im-
mediately visible, lying between the cell-wall
which it lines and the cell-sap which it in-
vests, and becoming visible only by means of
reagents that remove the water and separate
the protoplasm -sac — the Primordial Utricle of
MohP — from the cell wall by contraction (Fig. 41, C,p). The significance of this thin
protoplasm- sac, and its production owing to the increase in number and size of the
vacuoles in an originally continuous protoplasm-mass, will no longer be doubtful to
the reader after all that has been said in Sects, i, 2, and 3, and by comparison of
Fig. I with fig 41.

In young cells, where the protoplasm still forms a thick layer or a net- work per-
meated by vacuoles, its substance — with the exception perhaps of the outermost layer
lying on the cell-wall — appears to be always engaged in a 'streaming' movement, which
is however usually very slow. In many mature and large cells, which do not serve
for the storing up of assimilated materials, and where the protoplasm-mass is sufficiently
nourished, and does not, as the cell increases in size, contract to a mere thin skin,

Fig. 41. - Kornis of tlie protoplasm contained in cells
^1 and B of maize ; ^ cells from the first leaf-sheath of
a jjenninating plant ; B from its first internode ; Cfrom the
tuber of the Jerusalem artichoke, after action of iodine
and dilute sulphuric acid ; /t cell-wall ; A nucleus ; / pro-
toplasm (primordial utricle).

^ See Hanstein, Die Bewegungserscheinung des Zellkerns, u. s. w. in Sitzungsberichte der nieder-
rheinischen Gesellschaft zu Bonn, Dec, 19, 1870, p. 224.

[H. von Mohl, Bot. Zcitg. 1844, p. 273.]

I

PROTOPLASM AND NUCLEUS.

43

this condition is permanent. If the whole protoplasm-mass withdraws to the cell-
wall, enclosing a single large vacuole (the Sap-cwvity of the cell), all the particles of
protoplasm, flowing in one direction, may form a continuous broad current encircling
the cell (rotation), the direction of which is always such as to describe the longest
course round the cell-cavity (Nageli). Examples occur in Characeae, and in many
other submerged water-plants, as Fallisneria, Ceratophyllum, Hydrilla, and root-hairs of
Hydrocharis ; the globular nucleus, when present (in Characeae it soon disappears), is
carried along with the current. The protoplasm-mass which encloses a large sap-cavity
may, however, possess a net-work of ridge-like prominences, the substance of which
flows in diff"erent directions; the nucleus may then either remain at rest and form
the centre of movement, or be carried along with the current. Cases of this kind
occur tolerably frequently in the hairs of land-plants, as in the stinging hairs of the

Fig. 42.—^ stellate hair on the calyx of the young flower-bud of the hollyhock ; thicker ridges of protoplasm project into the
sap-cavity of each cell; these are in 'streaming' motion (indicated by the arrows). B epidermis (ej>) with the basal portion of
a mature stellate hair, showing the structure of the wall (X 550).

stinging-nettle, and the stellate hairs of the hollyhock. But threads of protoplasm
which exhibit these currents may also penetrate the sap-cavity of the cell ; not un-
frequently {e.g. Spirogyra, hairs of Cucurbita) the nucleus then lies in the centre,
enveloped by a mass of protoplasm, the threads uniting it with the layer which
clothes the cell- wall. These threads, stretching across the sap-cavity, arise from the
thin lamellae of protoplasm which in young quickly-growing cells separate adjoining
vacuoles ; when these finally flow together into a single sap-cavity, the thicker parts of the
lamellae (Fig. i, B, p. 2) remain as threads, forming a more or less irregular net-work,
which at first corresponds to the coalescent vacuoles, but which undergoes further
changes of form as the cell continues to grow, and in consequence of the internal
movements of the whole protoplasm. New threads also make their appearance ; ridge-
like portions arise from the peripheral protoplasm, or even on the thicker threads, and

44 MORPHOLOGY OF THE CELL.

finally become detached, leaving the two ends of the new thread united with the rest
of the protoplasm ; they do not grow up as branches with one free extremity.
(Hanstein, A c. p. 221.) Some of the threads also disappear, the two ends remaining in
connexion with the rest of the protoplasm and coalescing with it. The threads form,
with the central mass of protoplasm which contains the nucleus and the layer which
clothes the cell-wall, a connected system, portions of which may change their position
with respect to one another.

Besides these displacements — in consequence of which the parietal protoplasm accu-
mulates or diminishes at any one spot, and the mass of protoplasm in the cell-cavity
which contains the nucleus wanders about, and alters accordingly the grouping and form
of the threads — under high magnifying power another form of movement comes into
view, which is undoubtedly connected with the former, although the exact mode is
unknown. In the parietal protoplasm and in the mass which contains the nucleus,
but most distinctly in the threads, the minute granules — generally of chlorophyll-
are to be seen in a ^streaming' motion, which, under high magnifying power,
may even appear very rapid. It must not, however, be overlooked that when
the cell is magnified, say five hundred times, the rapidity of the motion is also ap-
parently increased five hundred-fold. Within even a very slender thread, the granules
near one another not unfrequently flow in opposite directions. Chlorophyll-granules
often appear to be in motion on the surface of slender threads ; it may nevertheless
be assumed with certainty that they also are enclosed in the substance of the thread ;
but, being very prominent, are covered by only a very thin lamella of it.

Those movements of protoplasm which produce changes in the internal grouping
of the protoplasm of the cell may be compared to the displacements of the mass
which, in the case of naked Amoebae, change the external contour, and cause its
creeping motion. In the case of circulating protoplasm, the firm cell-wall hinders the
change of contour, as well as the change of place of the whole mass ; but the large
sap-cavity allows of internal changes of position of larger or smaller portions. The
' streaming ' movement, which is visible by means of the imbedded granules, occurs
in the creeping naked protoplasm of the Amoebae as well as in that enclosed in a
cell-wall.

(c) The Nucleus. That the nucleus, which is never absent from Muscineae and Vascular
plants, but more often from Thallophytes, is a product of difl:erentiation of the proto-
plasm, is sufficiently evident, not only from its chemical behaviour [njide supra, under a),
but also from its participation in the processes of cell-formation (see Sect. 3). On
the other hand, it must be borne in mind that, once formed, it constitutes a character-
istically organised portion of the cell, which, to a certain extent, has a mode of
development of its own. At first the nucleus is always a homogeneous roundish pro-
toplasm-mass; subsequently its surface becomes firmer without taking the form of a
special skin ; in the interior there appear usually two or three (sometimes more) large
granules, called Nucleoli, which, however, are often wanting. The nucleus has, at the
time of its origin, generally already attained its permanent size, or nearly so ; its growth
is never proportional to that of the cell; while in young tissue-cells (Fig. r, p. 2) it
usually occupies a large portion of the cell-cavity, in mature cells its mass is progressively
smaller in proportion to that of the whole cell. Its further development consists
in its obtaining a firmer outer layer, and in the formation of small vacuoles and
nucleoli ; only rarely does it grow for a longer time ; its substance may become
frothy from the further formation of vacuoles, and sometimes it exhibits a circulation
in the interior of the firmer enveloping layer, as in a cell ^ The nucleus always
remains enclosed in the substance of the protoplasm ; if this latter forms vacuoles, or
developes the circulation already described, the nucleus remains enveloped in a coating

^ In young hairs of Hyoscyajnus fiiger, according to A Weiss in the Sitzungsberichte der kais,
Akademie der Wissenschaften zu Wien, vol. LIV, July 1866.

PROTOPLASM AND NUCLEUS. 45

or mass of protoplasm which is connected with the parietal protoplasm-sac by the
lamellae lying between the vacuoles or by the threads. The nucleus apparently follows
passively the movements of the protoplasm in which it is enveloped ; it also undergoes
changes of form under the pressure and traction of the moving mass beneath the
eye of the observer. 'During the movement/ says Hanstein (A c. p. 226), 'the
bands of protoplasm are very tightly stretched, so that the envelope of the nucleus is
drawn out into sharp angles. It looks as if the nucleus (with its envelope) were
towed about like a ferry-boat by ropes. But since during this towing the bands
alter their direction and form, it is evident that the envelope of the nucleus must
also change its form. But not only the envelope, but also the nucleus itself, does
this; this latter is never spherical or of any regular form during its movement, but is
irregularly elongated, and usually in the direction of its motion at the time.' This
change in the form of the nucleus may also be recognised from the displacement of
the nucleoli within its mass.

Sect. 6. The Chlorophyll-bodies and similar Protoplasmic Structures \

-Chlorophyll, the green colouring matter so generally distributed through the

jgetable kingdom, is always united to definite portions of the protoplasm-mass

►f the cells in which it is found ; these green- coloured portions of protoplasm

lay, in contradistinction to the colouring matter itself by which they are tinged, be

designated Chlorophyll-bodies. Every chlorophyll-body consists then of at least

vo substances, the colouring matter and its protoplasmic vehicle ; if the former

removed by alcohol, ether, chloroform, benzin, or essential or fatty oils, the latter

Remains behind colourless. The colouring matter contained in each chlorophyll-

)ody is itself only extremely small in quantity; after its removal the protoplasmic

)asis retains not only its form but also its previous volume. The latter is always a

Continuous soft substance containing extremely small vacuoles, in which the colouring

latter is generally distributed universally, though not always uniformly.

Chlorophyll-bodies arise in the young cells by the separation of the pro-
toplasm into portions which remain colourless and others which become green and
sharply defined. This may be due to very small particles of a somewhat different
lature either originally existing or being produced in the previously homogeneous
)rotoplasm, and which collect to form distinct masses. The chlorophyll-bodies
,'hich arise in this manner always remain imbedded in the colourless protoplasm
In a similar manner to the nucleus ; they are never in immediate contact with
the cell-sap. Their chemical and physical properties distinctly show that their
iclourless basis is a substance altogether similar to protoplasm. The chlorophyll-
)dies consequently always behave as integral parts of the protoplasm ; and this
especially evident in the division of cells, conjugation, the formation of swarm-
Jells, &c. But the chlorophyll-bodies, when once formed, grow; and if they
)ossess roundish forms they may increase by division. Both processes appear always
to depend on the growth of the protoplasm-mass in which they are imbedded.

' H. von Mohl, Bot. Zeitg. nos. 6 and 7, 1855; [Ann. des Sci. Nat. vol. VI, 1856, p. 139.]

-A. Gris, Ann. des Sci. Nat. 4th Ser. vol. VII. 1857, p. 179.— Sachs, Flora, 1862, p. 129;

[1863, p. 193. Sachs, Handbuch der Exper. Physiol, der Pflanzen, § 87, Leipzig 1865. — Hofmeister,

^ie Lehre von der Pflanzenzelle, § 41, Leipzig 1867. — Kraus, Jahrb. fiir wissensch. Bot. VIII.

fl87i, p. 131. [Ditto, Zur Kenntniss der Chlorophyllfarbstoffe u. ihrer Verwaudten. Stuttgart 1872.

-For Sorby's researches on chlorophyll see Book II. chap. 3. sect. 8.]

46 MORPHOLOGY OF THE CELL.

It is only in the Algae that the forms of the chlorophyll-bodies show much
variety. In them it is frequently the case that the whole protoplasm-mass, with
the exception of an outermost layer or a little more than this, either appears
homogeneously green (as many swarm-cells, Palmellacese, gonidia of Lichens) ; or
the chlorophyll- bodies assume stellate forms (e.g. Zygnema cruciaium. Fig. 43),

or they form several lamellae with a stellate
transverse section when the cell is cut across

W/AV^hd^_

(as in Closlerium, Sec), or straight or spiral
bands {e.g. Spirogyrd). But in most Algae,
and in all Muscineae and Vascular plants, the
chlorophyll-bodies are rounded or polygonal

l-'u;. 43.— A cell of Zyjpiema crucicxtuni, with two * ^ i. j -^

stellate chiorophyii-botiies which are suspended in the masscs collcctcd arouud a ccutrc, and avc

interior of the cell ; they are united by a colourless bridge

of protoplasm in which lies a nucleus ; the rays which form tcrmcd ChJorophylls^ramcIes. Gcncrallv a

the union with the primordial utricle are nearly colourless . . "*

in the middle. In each of the two chlorophyll-bodies largC nUmbcr arC COntaincd iu OUC CCll ; some-
lies a large starch-grain (X 550). ^ r I'll

times, however, only a few relatively large
ones {e.g. Selaginelia), and in one of the Hepaticae of simplest structure {Anthoceros)
only a single chlorophyll-granule exists in each cell, enclosing the nucleus ; this
therefore, when the cells divide, itself also divides in a corresponding manner.

With extremely few exceptions Starch-grains arise in the homogeneous
substance of the chlorophyll-bodies, and, where these have special forms, are
distributed in definite places (see, e.g., Fig. 5, p. 10); they are produced, in larger
or smaller numbers, in the interior of ordinary chlorophyll-granules. They are
at first visible as points, gradually increase in size, and finally may so completely
fill up the space of the chlorophyll-granule that its green substance is represented
only by a fine coating on the mature starch-grain; even this disappears under
certain circumstances (as in old yellow leaves of Pisum sativum or Nicoiiand), and
the starch-contents then lie in the cell, which now contains no protoplasm, in the
place of the chlorophyll-granules. Sometimes drops of oil also form in the interior
of the chlorophyll-body (<?. g. in the bands of Spirog)>ra) ; and here and there
granular contents of an unknown nature are observed. All these structures which
arise in the chlorophyll-bodies are, however, not constant portions of them ; their
appearance and disappearance depend entirely on light, temperature, and other
circumstances; the appearance of the chlorophyll-bodies themselves is also bound
up with these conditions of life, to a description of which we shall recur in Book III,
where it will be shown that chlorophyll is one of the most important elementary struc-
tures, and that its contents are the products of its assimilation. The consideration
of these and other purely physiological properties of chlorophyll must be deferred
till then. Sooner or later, in the normal course of things, the chlorophyll-bodies
are again absorbed ; this occurs in the most conspicuous manner at the time when
the leaves of the higher plants are preparing for their fall; for instance, in the
case of most of our native trees and shrubs, in the autumn. The whole proto-
plasm-mass— and with it the chlorophyll-granules from the cells of the leaves about
to fall — is then absorbed and transferred to the perennial persistent structures.
The phenomena which accompany this process vary greatly; but finally there
remain in the cells filled with water and often with acicular crystals, a number
of yellow glittering granules which bear no resemblance to chlorophyll; if the falling

THE CHL OR OPHYLL- B ODIES.

47

leaves are red, this depends on a substance dissolved in the sap ; but in this case
also the yellow granules are to be found.

The presence of chlorophyll in tissues is not always to be recognised by the
naked eye. Sometimes the cells that possess chlorophyll contain a red sap; in
other cases the green tissue of the leaves is covered by an epidermis with red sap,
as in young plants of Airiplex hortensis ; in this case, if the coloured epidermis
be removed, the green tissue may be recognised. But in Algae and Lichens we
find that the chlorophyll-body of the cell itself contains, in addition to the green
colouring matter, a red, blue, or yellow substance soluble in water; the fresh

^^^H( 550). A in the middle, B at the margin ; ch the chlorophyll-granules ; visible
^^^K them are the small starch-grains; eu the lower epidermis; eo the upper
^^^■udermis ; / an air-conducting intercellular space ; sp stomata.

m

Fig. 45.— Chlorophyll-granules of Ftinaria hygrovtetrica {X 55°)- ^ cells of a mature leaf, seen from the surface ; the chlorophyll-
granules He in a parietal layer of protoplasm, in which the nucleus is also imbedded ; they contain starch-grains (left white). B single
chlorophyll-granule containing starch ; a a young one, 5 an older one, 5' and *" granules in the act of division ; c, d, e old chlorophyll-
granules, the starch-grains of which take up the spaci of the chlorophyll ; y a young chlorophyll-granule swollen up in water ; i.'- the
same after longer action of the water; the chlorophyll is destroyed, the starch-grains which it contained remaining behind.

chlorophyll-body appears then, by the admixture of the chlorophyll contained in
it with these substances, verdigris-green {Oscillatoria^ Feliigera canina, &c.), a fine red
(Florideae), brown {Fucus, Laminaria saccharina), or buff (Diatomacese). (See Book
11., Algee.)

From this are to be distinguished those cases in which the originally green
chlorophyll-granules assume a red or yellow colour from the alteration of their
colouring material, a phenomenon which, from its physiological bearings, I have
termed Degradation of chlorophyll. Thus the green bodies in the walls of the

48 MORPHOLOGY OF THE CELL.

antheridia of Mosses and Characeae become, at the time of fertilisation, of a
beautiful red; in ripening fruits {Lycium barbarum, Solanuin pseudo-capsicum, Sec),
the change of colour from green to yellow and red depends also on a similar loss
of colour of the chlorophyll-granules, accompanied by a breaking up into angular
forms with two or three points (Kraus, /. c). Nearly related to the chlorophyll-
granules are the vehicles of the yellow colouring materials to which many petals
owe their colour {e.g. Cucurbita). The occasional blue {Tillandsia anicena) or brown
and violet bodies {Orchis Morio) are much further removed from this type, although
they also have a basis similar to protoplasm, which is tinged by a colouring material,
in these cases soluble in water. .

(a) The Substance of the Chlorophyll-bodies is, irrespectively of the contents referred
to, destitute of those fine granules which are so generally distributed through colourless
motile protoplasm. In spite of their sharply defined form they are very soft and
greasy when crushed; when they come into contact with pure water, vacuoles are
formed, which at last burst through the green substance as hyaline bladders. Young
chlorophyll-granules may thus become converted into delicate bladders, in which the
starch-grains remain ; old grains have much greater consistence. After extraction of
the green colouring matter out of true chlorophyll-bodies, e. g. the bands of
Spirogyra or granules of Allium Cepa, the remaining colourless basis possesses greater
power of resistance, is coagulated, and shows all the reactions of protoplasm already
mentioned.

(b) The Origin of the Chlorophyll-bodies has at present only been directly observed
in the granular forms; it can to some extent be compared with the process of free
cell-formation. Round centres of formation within the protoplasm small portions of
it collect in defined masses; if the centres of formation are at a considerable dis-
tance from one another, the chlorophyll-granules become round (as in the hairs
of Cucurbita)', but if they are large and lie close to one another, they are at
first polygonal, as if flattened by pressure. The process then resembles the formation
of numerous small swarm-cells in a single cell of Achlya (Fig. 9, A, p. 13); only that
in this latter case colourless protoplasm always continues to lie between the green
portion, as in the parietal chlorophyll of the leaves of Phanerogams. If a mass of
protoplasm collects round the central nucleus during the formation of chlorophyll,
the granules are often formed in its neighbourhood ; they may then revolve with the
protoplasm in the cell, or afterwards assume definite positions. In the filamentous
Algae with apical growth {e.g. Faucheria, Bryopsis), they are produced in the colourless
protoplasm-mass of the growing end of the filament, and then remain closely
applied to the wall. In ripe spores of Osmunda regalis the chlorophyll surrounds the
nucleus in the form of amorphous cloudy masses, which, however, separate on germi-
nation as ovoid granules, at first weakly defined, afterwards more sharply (Kny). In
those cells of young leaves of Phanerogams which contain chlorophyll (cotyledons
of the sunflower, primordial leaves of Phaseolus, buds of the tubers of the Jerusalem
artichoke, &c.) a definite layer of hyaline protoplasm devoid of granules is to be observed,
close to the cell-wall, in which the chlorophyll-granules are subsequently formed ; here the
appearance is sometimes presented as if the mass were cut up into polyhedral pieces.
The formation of the chlorophyll-granules is not always contemporaneous with that
of its colouring matter ; they may be at first colourless (as in Faucheria or Bryopsis,
according to Hofmeister) or yellow (in the case of leaves of Monocotyledons or
Dicotyledons imperfectly exposed to light or in process of development), and be-
come green at a subsequent period; in the cotyledons of Coniferae the green colour
appears contemporaneously with their origin even in the dark when the temperature
is sufficiently high, as also in Ferns. The chlorophyll-granules, after assuming their
green colour, grow by intussusception to many times their original size; if they are

THE CHLOROPHYLL-BODIES. . 49

Irietal, their growth in length and breadth is generally proportional to that of the
11-wall and protoplasm in which they lie. But if the growth of the cell is very
considerable, the growing parietal chlorophyll-granules divide ; this takes place by bipar-
I tition,, in a direction at right angles to the longest diameter, into two secondary granules
i usually equal in size. • If it contained small starch-grains before the division, these
arrange themselves round the centres of the newly formed granules. These processes
are inferred from the increase of the number of granules on the one hand, and from
the frequent occurrence of constricted hour-glass- shaped forms on the other. After
this bipartition had been discovered by Nageli in Nitella, Bryopsis, Valoniay and in the
prothallia of Ferns, it was subsequently noticed in all the families of Cryptogams which
form chlorophyll ; among Phanerogams also it appears widely distributed ; it was dis-
covered by Sanio in Peperomia and Ficaria, subsequently by Kny in Ceratophyllum^
Myriophyllum, Anacharis, Utricularia, Sambucus, Impatiens, &c. In cells of the prothal-
lium of Osmunda exposed to feeble light and containing but little chlorophyll, Kny
states that moniliform rows of chlorophyll-granules are produced by repeated bipartition,
which, like the chains of cells of Nostoc, continue to elongate by intercalary divisions ;
branching takes place also, in a manner similar to that which occurs in Nostoc, some
of the chlorophyll-granules increasing in size transversely, and producing branch-rows
by division.

(c) With reference to the Internal Structure of the Chlorophyll-bodies, scarcely any-
thing more can be said than that their outer layer often appears denser, and that the
proportion of water increases towards the interior, the cohesion decreasing, as is apparent
from the formation of vacuoles. A differentiation into intersecting layers of different
density has only been once observed, by Rosanoff, in old chlorophyll-granules of Bryopsis
plumosa.

Sect. 7. Crystalloids^. — A portion of the protoplasmic substance of a cell
sometimes assumes crystal-like forms ; bodies are produced which, bounded by plane
surfaces and sharp edges and angles, possess an illusory resemblance to true crystals,
even in their behaviour to polarised light ; but they are essentially distinguished from
crystals by the action of external influences, and at the same time present significant
resemblances to organised parts of cells. It is therefore legitimate to distinguish
them by the term Crystalloids'^ proposed by Nageli. They are usually colourless,
but sometimes act as vehicles of colouring matters (not green), which may be
removed from them. Their substance exhibits all the more essential reactions
of protoplasm, its power of coagulation and of taking up colouring matters, the
yellow reaction ^\\\h potash after treatment by nitric acid, as well as that with iodine.
The solubility of diflferent crystalloids is very different, as is generally the case with
proteids. They are capable of imbibing water, and swell up enormously under
the influence of certain solutions ; their outer layer possesses greater power of
resistance than the inner more watery mass. Those crystalloids which have been

^ Hartig, Bot. Zeitg. 1856, p. 262. — Radlkofer, Ueber die Krystalle proteinartiger Korper
pflanzlichen und thierischen Ursprungs, Leipzig 1859, — Maschke, Bot. Zeitg. 1859, p. 409. — Cohn,
Ueber Proteinkrystalle in den Kartoffeln, in the thirty-seventh Jahresbericht der schlesischen Gesell-
schaft fiir vaterland. Cultur, 1858, Breslau. — Nageli, Sitzungsberichte der k. bayer. Akademie der
Wissenschaften, 1862, p. 233. — Cramer, Das Rhodospermin, in the seventh volume of the Viertel-
jahrsschrift der naturforsch. Gesellschaft in Ziirich.— J. Klein, Flora, 1871, No. ii. — Kraus, in
Jahrb. fUr wissensch. Bot. vol. VIII. p. 426.

^ [The term ' crystalloid' is, in another portion of this work, used in a different sense, to express
any substance capable of crystallisation ; see Book III. Chap. i. Sect, i.]

50 MORPHOLOGY OF THE CELL. |

most carefully examined consist of a mixture of two ingredients of different
solubility; the two are so combined that when the more soluble is slowly removed,
the less soluble remains as a skeleton (Nageli).

Their form varies greatly in different plants ; they appear as cubes, tetf ahedra,
octohedra, rhombohedra, and in other forms ; usually, however, their crystallographic
characters cannot be exactly defined, a consequence of their small size and of the
inconstancy of their angles.

In the rapidly growing organs of flowering plants they are known only in
LathrcEa squaviaria^ ; more commonly they are produced in cells where large
quantities of reserve-materials are collected which are only turned to use at a later
period. The crystalloids themselves appear to be a form of protoplasmic structure
especially adapted for a dormant condition (as in potato-tubers and many oily
seeds) ; they are seldom found in cells which contain sap (potato-tubers), but more
often in cells which do not contain it, and especially in oily seeds. Crystalloids
containing colouring matters are found in petals and fruits. Sometimes they are
formed only after the action of alcohol or a solution of sodium chloride on the
plants externally or internally (Rhodospermin).

The crystalloids of potato-tubers are imbedded in the protoplasm; those that
are widely distributed in the tissues of Lalhrcea squamaria are contained in great
numbers in the interior of the nucleus; those found in oily seeds are generally
enclosed in aleurone-grains.

The crystalloids discovered by Gohn in the tubers of the potato are convenient for
observation; they are found very abundantly in some kinds, in others less frequently,
in the parenchymatous cells which contain but little starch beneath the skin but
tolerably deep in this tissue, lying enclosed in the protoplasm. Generally they are in
the form of perfect cubes (less often of derivative forms, as tetrahedra). Those found
by Radlkofer in the nucleus of the cells of LathrcEa squamaria lie together in great
quantities ; they have the form of thin rectangular plates ; sometimes they have rhombic
or trapezoid forms ; Radlkofer thinks it most probable that they belong to the rhombic
system. In these cases they may be seen in sections without further preparation, and
their relation to their surroundings is clear. The case is different with the crystalloids
of oily seeds enclosed in aleurone-grains ; I shall recur to their properties, and will only
mention that from the brazil-nut they are obtained in quantities by washing the crushed
oily parenchyma by oil or ether, the crystalloids settling down in the form of a fine
powder; in sections but little can be made out. They were carefully investigated in
the isolated state by Nageli ; according to him they appear rhombohedral, octohedral,
or tabular; but it is uncertain whether they belong to the hexagonal or the klino-
rhombic system. Dried and then placed in water, they alter their angles about 2° or 3°;
in potash solution they swell strongly and alter their angles 15° or 16°. By weak acids
and dilute glycerin a substance is extracted, and a weak skeleton with firmer skin
remains behind. The crystalloids in the cells of the endosperm of Ricinus communis
are, like all crystalloids, insoluble in water, and are easily seen when thin sections of the
tissue are laid in water, which destroys the substance surrounding the crystalloid, and
sets it free. They frequently take the form of octohedra or tetrahedra, less often of
rhombohedra; but the system is not certainly determined. The crystalloids which
contain colouring matters were first detected by Nageli in an imperfect form in the

^ [According to Prillieux, the brown colour of Neottia nidus-avis is due to brown crystalloids
which assume a green colour when the plant is immersed in alcohol or boiling water ; see Ann. des
Sci, Nat., 5th ser., vol. XIX. p. 108.]

CRYSTALLOIDS, 5I

petals of Fiola tricolor and Orchis, and better developed in the dried fruits of Solarium
americanum ; in the latter case they form in the large parenchymatous cells clusters of
a deep violet colour; the separate crystalloids are thin rhombic plates, often with
truncated angles, &c. According to Nageli the crystalline form is the rhombic prism in
a very abbreviated tabular shape ; the six-sided plates are composed of six simple ones.
In pure water they remain unchanged ; alcohol extracts the colouring matter, as also do
dilute acids ; both leave, after long treatment, a very slight skeleton which is capable of
swelling, while the whole crystalloid does not swell ; Nageli states that the crystalloid
consists of a very small quantity of albuminous and a large quantity of another substance,
with some colouring matter.

Crystalloids of albuminous substance have also been found in red marine Algae
(Florideae) and in one Fungus. Cramer observed the first case of this kind ; in speci-
mens of Bornetia secundijlora which had lain a long while in solution of sodium chloride,
as well as in specimens in alcohol of Callithamnion caudatum and seminudum^ he found
hexagonal plates and prisms with all the properties of crystalloids, and coloured red by
the colouring matters of the Algae. They were found in the vegetative cells as well
as in the spores. In sodium chloride preparations of Bornetia octohedral crystalloids
were found also, apparently belonging to the klino-rhombic system ; they were colour-
less. In living plants of the same Alga, Cohn also discovered colourless octohedral
crystalloids which absorb the red colouring matter expelled from the pigment-grains.
Within and without the cells of Ceramium rubrum preserved in sea-water with glycerin,
klino-rhombic prisms formed, coloured red by the exi>elled pigment ; they are clearly,
like the hexagonal crystalloids observed by Cramer, produced only after death, while the

I R colourless octohedra are to be found in the living cells. Finally, in dried specimens of
■other Florideae, Griffithsia barbata, G. neapolitana, Gongroceras pellucidum, and Callithamnion
mjeminudum, Klein observed colourless crystalloids of a different form. These bodies
■may all be comprised in the name first given by Cramer, — Rhodospermin. In the
fcporangiophores of Pilobolus Klein also found colourless octohedra of tolerably regular
Bstructure with the properties of crystalloids.
Sect. 8. Aleurone- Grains ^ — The reservoirs for reserve material contained
I in ripe seeds, t. e. in the endosperm or the cotyledons of the embryo, always contain
considerable quantities of proteids, together with starch or oily matter. If they
contain much starch, as in Grasses, Phaseolus, Vicia, the oak, horse-chestnut,
Spanish chestnut, &c., the proteid, which only contains very Httle oily matter,
occupies the interstices; it then consists of small or even minute granules, as
shown in Fig. 46. In oily seeds, on the other hand, in place of the starch-grains
are found granular roundish or angular structures (Fig. 47), sometimes not dissimilar

* These structures were discovered by Hartig (Bot. Zeitg. 1855, p. 881, and described in detail
but imperfectly {ibid. 1856, p. 257; [Ann. des Sci. Nat., 1856, vol. VI. p. 325]); further researches
were undertaken by Holle (Neues Jahrb. der Pharmacie, vol. X. 1858) and Maschke (Bot. Zeitg.
1859). All these observations left undecided the relationship of the grains to the surrounding
matrix ; it appeared to be assumed that in oily seeds the matrix consists of oil only. In the first
and second editions of this book I opposed this view, and pointed out that the matrix in the cells
of oily seeds consists of a mixture of oil and proteids, or rather, of a very oily protoplasm; on
the other hand I fell into the error, partly in consequence of the use of diluted ether, of considering
the aleurone-grains themselves as a compound of proteids and oil. This error has been refuted
by Dr. Pfeffer's recent researches, commenced in the Wlirzburg laboratory, where I had the oppor-
tunity of seeing numerous preparations which were decisive as to the principal question. Dr. Pfeffer
had the kindness to communicate to me a detailed account of his labours ; what I have said above
follows his views tolerably closely. See Pfeffer, in Pringsheim's Jahrb. fur wissens. Bot. vol. VIII.
p. 429.

E 2

52

MORPHOLOGY OF THE CELL,

to Starch-grains in their appearance, and surrounded by a more or less homo-
geneous matrix, which, as closer investigation shows, consists, according to the
oiliness of the seed, of more or less oil combined with proteids. The grains
themselves, on the other hand, consist, independently of certain enclosed matters,
of proteids.

In the aleurone-grains the proteid itself must be distinguished from the
enclosed substances. The latter are either crystals of calcium oxalate, or non-
crystalline, roundish, or clustered granules, known as Globoids. These are a double
calcium and magnesium phosphate, in which the latter base is greatly in excess.

Fig. 46. — Cells shown in a very thin section througli a
cotyledon in a ripe seed of Pisutn sativutn; the large
concentrically stratified grains st are starch-grains (cut
through) ; the small granules a are aleurone, consisting
principally of legumin with a little oily matter; ?the inter-
cellular spaces.

Fig. 47.— Cells from the cotyledon in a ripe seed oi Lupinus varius ; A in an alcoholic solution of iodine ; B after destruction
of the aleurone-grains by sulphuric acid ; z the cell-wall ; p the protoplasmic matrix, containing but little oily matter ; y the
aleurone-grains ; a drops of oil expelled from the matrix by the action of the sulphuric acid ; m empty spaces from which the
aleurone-grains have been dissolved (x 800).

The whole proteid is sometimes amorphous, and in that case is not doubly
refractive; or the greater part, with the enclosed substances already named, is
developed into a crystalloid (Sect. 7), and this, surrounded by a thin amorphous
envelope, constitutes the aleurone-grain. (Fig. 48.)

The proteids are all insoluble in water; neither alcohol nor water extracts
anything from them. The grains which contain no crystalloids dissolve in water
entirely {Pceonia)^ partially [Lupinus), or not at all [Cynoglossum). But all dissolve
completely in water containing only a trace of potash. With careful treatment there
always remains behind a membrane surrounding the grain, which behaves like

ALEUR ONE- GR A INS.

S^

coagulated albumen ; but it may be a yet unknown proteid. With aleurone-grains
containing crystalloids, after careful solution, a similar membrane remains, but the
crystalloid itself also leaves behind one of the same kind ; this occurs also in the
solution of globoids in acetic or hydrochloric acid, and reminds one of the similar
behaviour of true crystals of calcium oxalate.

The crystaUine enclosures of calcium oxalate occur as clusters, distinct crystals,
and needles, but are not commonly met with.
The globoids, on the other hand, are never absent
lom aleurone-grains; when they are accom-
panied by crystals, it is almost always the case
that the grains of one cell contain only globoids,
those of another only crystals, as in Silyhum
marianuvi, and all Umbelliferse that have been
examined. There occur however exceptions ;
in the grape-vine a globoid forms itself round
a crystal or a cjuster of crystals. The globoids
are soluble in all inorganic acids, and in acetic.-
oxalic, and tartaric acid, but not in dilute
potash.

The globoids, like the crystals, may occur
in an aleurone-grain singly or many together;
in the latter case they are small, and even too
minute to be measured, but are then present
in enormous numbers in one grain, e.g. Lu-
pinus luteus, L. polyphyllus, Delphinium Requienii, &c. Large globoids surrounding
crystals occur singly, the largest in the grape-vine. Pfeffer found crystals accom-
panying crystalloids only in jEthusa Cynapium. The enclosed substances are usually
absent from very small aleurone-grains.

In some seeds there is' in each cell one aleurone-grain distinguished from the
others by its size (' Solitar ' of Hartig), both when crystalloids are present and
when they are absent {Elaeis, Myristica, Vitis, Lupinus luteus)\ it may be dis-
tinguished by its enclosed substances. Thus in Lupifiiis luteus it contains a tabular
rystalloid ; the other surrounding grains only small and numerous globoids. In
nlybum a cluster of crystals lies in one large grain, in the others a number of
keedle-shaped crystals. In other cases the enclosed substances are similar, as is
le case with the globoids, which are merely larger in the large grain.

The crystalloids are tolerably widely distributed in aleurone-grains, although
le greater number of seeds are destitute of them. They are not, however, cha-
icteristic of natural families, but may be present or absent in members of the
ime family; thus among palms, Sabal Adansonii is without, Elaeis guineensis has
rystalloids ; in the same manner all Umbelliferae which have been investigated want
^hem except ^thusa Cynapium. In other cases all seeds of the same family appear
contain crystalloids, as in the Euphorbiacese, among which Ricinus supplied the
rst example of fine crystalloids in the aleurone-grains.

The matrix which surrounds the grains of aleurone in oily seeds is, as has
Jen mentioned, always a mixture of oily matter and proteids, but the proportion

Fig. 48. — Cells from the emlosperm of Ricinns
commtcnis (X 800). A fresh, in undiluted glycerin,^ in
dilute glycerin, C warmed in glycerin, D after treat-
ment with an alcoholic solution of iodine, the aleu-
rone-grains are destroyed by sulphuric acid, the
proteid remaining behind as a net-worlc. In the
aleurone-grains the globoid may be recognised, and
in (B, C] the crystalloid.

^4 MORPHOLOGY OF THE CELL,

of the latter may be very small. Thus even in the castor-oil-plant and brazil-nut,
where the matrix appears to consist entirely of oily matter, the albuminous constituent
is quite discernible, as is shown in Fig. 48, D ; Pfeffer succeeded most readily
by extracting with an alcoholic solution of corrosive sublimate, and then colouring
with anilin-blue dissolved in water. The matrix may be considered as the proto-
plasm-mass of the cell, in which the water is replaced on drying by oil. But in
addition it contains not only insoluble proteids, but other substances soluble in
water rendered alkaline by potash. This composition of the matrix, together with
the solubility of the amorphous mass of the aleurone-grains in water, are the cause
of the complete loss of form which the cell-contents of oily seeds immediately
undergo in water, as shown in sections under the microscope. In order to recognise
their structure it is necessary to place fresh sections in undiluted glycerin, alcoholic
solution of corrosive sublimate, oil, or concentrated sulphuric acid.

The oily matter may also separate from the matrix in crystals, as Pfeffer has
observed in the brazil-nut, Elaeis guineensis, and the nutmeg.

To the above may be added, from Pfeffer's communication, some explanations con-
cerning the more difficult points.

(a) The substance of the aleurone-grains always consists, to by far the greater extent,
of proteids, with which very small quantities of other vegetable substances are usually
or always mixed which are difficult of detection. All aleurone-grains are absolutely
insoluble in alcohol, ether, benzol, or chloroform ^ ; these reagents would dissolve oil
(alcohol dissolves also glucose), if it were present, and would consequently alter the
appearance of the grain. Some are insoluble in water {e.g. those oiCynoglossum officinale) ;
those soluble in water ^ yield with corrosive sublimate in absolute alcohol a mercury-
compound insoluble in water. Gum, pectinaceous substances, cane-sugar, and dextrin
do not, under this treatment, yield an insoluble compound. Of all widely distributed
vegetable substances, only proteids behave in this manner towards corrosive sublimate.
Boiling the mercury-compound with water reproduces a proteid insoluble in dilute acids
and alkalies.

(b) In proving that the aleurone-grains of oily seeds contain no oil, we have already
seen that it must be present in the matrix. The doubt which arises from the first
glance at sections of oily seeds, whether the great mass of oily matter can find space
in the interstices between the grains, can be settled by calculation ; for if spheres
(the grains may be so regarded) are placed in an equal number of cubes forming
part of one great cube, 47-6 p. c. of the cavity remains unoccupied ; and if the
spheres are distant from one another only about one-third of their radius, 697 p. c.
of the cavity is left, and this is more than is sufficient in oily seeds to take up the
oily matter.

Immediate proof can be given of the existence of oil in seeds which contain it by
the appearance presented by dry sections; if benzol is added, the matrix disappears,
while small quantities of proteids remain. With alcoholic tincture of alkanet the matrix
becomes of a deep blood-red colour if it contains a considerable amount of oil ; but if
the oily constituents of the seed are very small, the evidence cannot be obtained in
this manner.

If the oil is extracted from the sections by alcohol, and the aleurone-grains then
removed by potash solution, a net-work remains behind in which the grains are replaced

' That I formerly considered them soluble in ether, was the result, as Pfeffer showed, of the
containing a small quantity of water.
2 On the causes of the solubiUty in water see Pfeffer's treatise already cited.

ALEURONE-GRAINS. 55

by cavities ; with acetic acid and iodine the net-work assumes a yellow-brown colour
(Fig. 47, B\ 48 D). In most seeds this net-work is very beautiful and comparable to
a parenchymatous tissue ; in extremely oily seeds it often breaks up into fragments, the
nucleus lying in it like a shrivelled ball. The threads of the net-work are composed of
the insoluble proteids of the matrix and of the enveloping membranes of the aleurone-
grains; although the net-work may exist without the latter if the grains have fallen
from their places.

(c) The Crystalloids of the aleurone-grains are insoluble in water ; they may therefore
easily be isolated by treatment of sections with water, the amorphous portions of the
grains dissolving, and any residue of cell-contents being destroyed ; they then show all
the reactions and the different forms of the crystalloids mentioned in Sect. 7. But
that they consist of two proteids and grow by intussusception Pfeifer thinks very
doubtful.

(d) If sections of the endosperm of the peony are treated with alcohol containing a
small quantity of sulphuric acid, and if, after washing, they are placed in water, the
substance of the aleurone-grains (not containing crystalloids) is seen to be distinctly
stratified ; but only a few firm and soft layers occur, the inner part of the mass being
amorphous. Pfeifer's work should also be consulted here.

(e) The De'velopment of the aleurone-grains is thus described by Pfeflfer. — Their
formation does not commence till the seeds* have attained their last condition of
ripeness and the funiculus begins to dry up ; in the turbid emulsion which now fills
the cells, the enclosed substances, especially the globoids, are already formed, but
not fully developed. As the seed loses water, the formation of mucilaginous proteid-
masses commences, mostly surrounding enclosed substances ; these mucilaginous bodies,
usually nearly globular, grow, their mutual distance thus decreases, and at last their
differentiation is complete; the aleurone-grains, still consisting of mucilaginous sub-
stance, are distinctly separated from the still turbid matrix, which becomes clearer
and clearer as the seed dries. The previously spherical or ellipsoidal grains become
more or less polyhedral, especially in oily seeds which have but little matrix {e.g.
Lupinus).

While the formation of the aleurone-grains is beginning, the protoplasm of the cell is
only to be detected with difficulty in the turbid cell-contents; yet, on removing the
oily matter by alcohol, it may be shown that it is present in the normal form ; some-
times in the copious matrix of some seeds the dried threads of protoplasm may
still be seen. In Lupinus luteus the crystal of calcium oxalate, which is after-
wards enclosed by the largest grain, is already present before the formation of the
aleurone-grains. Pfeffer was able to follow the development of the grains with remark-
able ease in the peony ; in this case the seed, even when it has attained its full size, is
entirely filled with large starch-grains, which become changed into oil only when fully
ripe. The starch is not always, however, completely changed into oily matter. If the
starch-grains in the seeds of the peony were not completely transformed, and the inter-
mediate mass, almost devoid of oily matter but very rich in proteids, formed very
small aleurone-grains, we should have what does actually occur in Phaseolus and in other
seeds extremely rich in starch. There are, however, also seeds in which aleurone- and
starch-grains occur in nearly equal quantities, but then always associated with oily
matter.

No conclusion as to the manner of growth can be deduced from the turbid
condition of the cell-contents and the softness of the growing aleurone-grains.
Nevertheless it can mostly be affirmed with regard to ripe grains, that they are softer
towards the inside, and that, consequently, on the application of very dilute reagents,
they dissolve from within outwards. Different facts appear, nevertheless, to show that
no growth takes place by intussusception, as is the case with starch-grains. The
origin of aleurone-grains is simply a dissociation, which arises from loss of water
by the seed; and, on germination, the original cell-contents are again more or less

^6 MORPHOLOGY OF THE CELL.

completely reconstituted by the union of the matrix with the substance of the aleurone-
grains.

Pfeffer followed out the formation of the crystalloids in Ric'mus and Euphorbia sege-
turn; they arise nearly simultaneously with the globoids, at a rather early period, and
both grow gradually, while the turbidity of the cell-contents at first somewhat increases.
They lie, at an early stage, close to one another, and completely surrounded by the
turbid mass; the vacuoles which Gris figures (Recherches sur la germination, PI. I, Figs.
10-13) are the result of the disorganisation of the cell-contents. The crystalloids are
from the first sharp-edged; and, as soon as their size permits their shape to be
recognised, it agrees with the mature form. The envelopment of crystalloid and
globoid by an amorphous coating does not begin till the crystalloids are mature and
the seed has begun to dry.

On germination the crystalloids dissolve as well from without as from within, till
after the amorphous envelope has first disappeared ; their external membranes for a time
persist, but gradually become invisible. The globoids also dissolve (no doubt in conse-
quence of the acid reaction which the tissue assumes), and in the case of old seeds from
the outside inwards. The aleurone-grains destitute of a crystalloid swell up, and resume,
on the germination of the seed, the form which they possessed in ripe but undried
seeds ; they begin to mix gradually with the substance of the matrix ; their solution
can sometimes be followed from without inwards ; but they often coalesce as mucila-
ginous masses. These changes occur with the first signs of germination in the embryo ;
formation of starch takes place simultaneously in the contents of the cells.

Sect. 9. Starch- Grains ^ — Plants which grow under favourable circumstances
produce by assimilation a larger quantity of formative organisable substance than
they require or can employ at the time for the growth of the cells. These mate-
rials are stored up in some form or other in the cells themselves, and only come
into use later. It has already been show^n how this takes place with albuminous
protoplasm-forming materials and with oily substances. Another substance, in the
highest degree organisable, Starch, is formed beforehand and stored up in far larger
quantities in anticipation of future use. Starch always appears in an organised
form as soHd grains having a concentrically stratified structure, which arise at
first as minute dots in the protoplasm, and continue to grow while lying in it ; if,
at a subsequent period, they reach . the cell-sap and cease to remain in contact
with the protoplasm which nourishes them, their growth ceases ^. Every starch-grain
consists of starch, water, and of very small quantities of mineral substances (ash).
Starch itself is a carbo-hydrate of the same percentage composition as cellulose, to
which it bears the greatest resemblance of all known substances in chemical and
morphological properties. Starch, however, occurs in each grain in two modi-
fications : — Granulose, more easily soluble, and assuming a beautiful blue colour
with iodine in the presence of water, and Farinose, easily soluble, and more resembling

^ Nageli, Die Staikekorner, in Pflanzenphys. Untersuchungen, Heft II, and Sitzungsber. der k.
bayer. Akad. der Wissenschaften, 1863. — Sachs, Handbuch der Exp. Physiol., Leipzig 1865, § 107.
The account given here is chiefly derived from Nageli's work.

2 According to Hofmeister, the starch-grains in the latex of Euphorbia appear to form an
exception; nothing however is known about their development; the latex always contains pro-
toplasm-forming substances, prottids, which perhaps here also take part in the production of
the starch-grains.

STARCH-GRAINS.

57

cellulose in its reactions \ At every point of a starch-grain both constituents occur
together ; if the granulose is extracted, the farinose remains behind as a skeleton ;
this skeleton presents the internal organisation of the whole grain, but is less
dense or poorer in substance, and its weight amounts to only from 2 to 6 p. c.
of the whole grain. Since the granulose greatly preponderates and is present at
every point, the starch-grain shows the blue granulose-colouring with iodine
throughout its whole extent.

The starch-grains have always rounded forms organised around an internal
centre of formation ; when young and small the grains appear to be always spherical ;
but since their growth is scarcely ever uniform, their form changes into ovoid, lenti-
cular, rounded polyhedral, &c.

The internal organisation of the starch-grain depends essentially on the dif-
ferent distribution of water in it {water of organisaiion). Every point of the grain
contains water in addition to granulose and farinose. Most usually the amount
of water increases from without inwards, and attains its maximum at a fixed point
in the interior. With the increase in the proportion of water, the cohesion and
density decrease, as also the index of refraction. This change in the proportion of
water is not, however, constant, but intermittent. -To the outermost least watery
layer succeeds a sharply defined watery layer, to this again a less watery one, and
so on, until the innermost less watery denser layer surrounds finally a very watery
part, the nucleus. All the layers of a grain are disposed round this nucleus as their
common centre, but every layer is not continuously developed round the whole
nucleus; in small spherical grains with few layers this is always the case, but
when their number increases, it does so most in the direction of most vigorous
growth, which is continuous in a straight or curved line with the direction of least
vigorous growth. This line is called the axis of the grain, and always passes
through the nucleus.

The growth of the grains of starch is accomplished exclusively by Intussus-
ception; new particles become intercalated between those already existing both in
a radial and tangential direction, by which means the proportion of water at
particular places is at the same time changed. The youngest visible globular
starch-grains consist of denser less watery substance; in this is formed subse-
quently the central watery nucleus; in the latter a central part may become
denser; and in this, when the increase in size has advanced sufficiently, a softer
nucleus may again arise. It may however also happen, after a softer nucleus
surrounded by a dense layer has arisen by diff"erentiation of the originally dense
grain, that in the dense layer a new soft one may arise, and it may thus become
split into two dense layers, the inner of which encloses the soft nucleus. The
layers increase in thickness and circumference by intercalation. When a layer

i^ [The most recent researches seem to show that the supposed distinction between granulose
d farinose is one of mechanical or molecular condition only. The coloration of starch by iodine
pears not to depend on the formation of a definite chemical compound, but to be the consequence
the mechanical interposition of the iodine between the molecules of starch (see Miller's Chemistry,

58 MORPHOLOGV OF THE CELL.

iTas attained a definite thickness, it becomes differenliated by further growth into
three layers. If it is a dense layer, watery substance becomes intercalated in its
middle, and the dense layer splits into two layers separated by a less dense one.
But when a watery layer becomes sufficiently thick, its middle lamella may be-
come denser, and a new dense layer be thus formed between two less dense
layers. This process of splitting of the layers depends on their increase in thick-
ness; and since this itself is the most vigorous where the layers are intersected
by the longer branch of the axis of growth, the splittings, /. e. the new formations of
layers, take place there most abundantly, and least so on the opposite side of the
nucleus, where they may even entirely cease. The layers of the more quickly
growing side of the grain become, as they pass to the slowly growing side,
gradually thinner, and finally disappear. Lenticular grains {e.g. in the endosperm of
wheat) have a lenticular nucleus ; their layers grow most quickly in the direction of
the radii of a great circle concentric with it, and commonly split, the nucleus re-
maining central. If, on the other hand, the growth takes place in one direction
[e. g. in the ovoid grains of the potato-tuber) the nucleus becomes eccentric, is
further and further removed from the centre' of gravity of the grain, and is in
this case globular. In some ellipsoidal (in the cotyledons of peas and beans) or
elongated grains, the nucleus is extended in the direction of the longest axis.

It is very common for two nuclei to form in a small young grain ; round each
of them layers are formed, and the growth is strongest in the line of union. The
distance of the nuclei from one another becomes continually greater ; thus a tension
arises in the few layers which are common to both ; this leads to the formation
of an inner fissure, which lies at right angles to the line of union of the two
nuclei; it is continued towards the outside, and the grain breaks up into two
half-grains which may nevertheless adhere to one another. If this division occurs
more often, compound grains arise, consisting of numerous secondary grains, the
number of which may amount even to thousands (e.g. in the endosperm of Spinacia
and Avena).

Compound grains of from two to ten secondary grains, with a mulberry-like
appearance, are extremely common in the parenchyma of quickly growing plants,
e. g. in seedlings of Phaseolus and stem of Cucurbita. Grains of this description
are different in their origin from compound grains of the kind which occur in
chlorophyll; in this latter case a number of small grains exist from the first,
which only touch and adhere to one another in consequence of increase of size.
(See Fig. 45, p. 47.)

Partially compound starch-grains result when new nuclei with their surround-
ing masses of layers are formed after the grain has already formed several layers.
The secondary grains appear therefore to be inclosed within the layers of the
mother-grain. In this case also tension arises from the unequal growth of the
common layers and of those belonging to each secondary grain, leading at
length to the formation of fissures ; but these do not usually extend to the outside ;
the secondary grains remain united.

(a) The growth of starch-grains by intussusception must be inferred from the fol-
lowing considerations:— Supposing that the formation of layers occurs by deposition,

STARCH-GRAINS.

59

grains would be found the outermost layer of which would be a watery one; this,
however, never occurs; the outermost layer is always the densest and least watery.
According to this supposition the nucleus would also possess the properties of the
youngest grains, whereas the nucleus is always soft, the youngest grains dense. The
theory of deposition could only explain the formation of partially compound grains
if we suppose that the common layers had been subsequently deposited round two or
more previously isolated grains ; but the com-
mon layers would have a different form, and
the fissures in the interior of such grains
remain unexplained. The theory of deposi-
tion, finall)', is incompetent to explain why,
in the secondary grains, the strongest growth
always takes place in the line joining their
nuclei (Fig. 49). The older hypothesis of a
deposition of new layers from within pre-
supposes that the starch-grains were at first
lollow vesicles, which has never been ob-
served; on this hypothesis, moreover, it cannot
)e explained how the phenomena arise which
)ccur in the formation of secondary grains;
md this hypothesis must moreover suppose
[growth by intussusception to explain the
superficial extension of the layers. The hy-
fpothesis of growth by intussusception affords
:the simplest explanation of all the pheno-
• mena; and, after Nageli's researches, may be
'Considered as a fully established fact. The
laterial which penetrates into the grain, and
there becomes deposited in the form of new
I particles of starch, is, of course, in solution ;
[but its chemical nature is not yet certainly
[known; dissolved starch can never be proved
'to exist in the plant, at least in those cells
where active formation and growth of starch-
^grains has been observed. It is, however,

probable that a solution of sugar contained in the protoplasm is the material out of
[Which particles of starch are formed by chemical and physical changes. The starch
is easily changed into sugar by different agencies. From various facts (e.g. the pro-
[duction of radial fissures on drying), it must be concluded that the molecules of starch
lave not only a definite order of deposition in the direction of the radii, but are also
[arranged tangenitially in a definite manner in each layer. A corresponding stratified
J^structure with radial striation, and the consequent formation of areolae, has, however,
been observed only occasionally and imperfectly.

Growth by intussusception depends on the permeability of all parts of the grain to
swater and aqueous solutions. This can only be explained by supposing that the
^substance of starch is not continuous, but consists of distinct molecules, each of which
[Possesses the power of attracting water, and surrounds itself with an aqueous enve-
ilope; the molecules of starch are separated from one another by these aqueous
[envelopes ; the smaller the molecules in a given portion of a starch-grain, the thicker
[are these envelopes and the more watery the particular portion. From this it results,
I on purely mechanical principles, that, as the molecules increase in size, the aqueous
^envelopes become thinner, and the molecules approach nearer one another. The
eatery layers therefore consist of small molecules which are separated by thick
iqueous envelopes, the denser less watery layers of larger molecules with thinner

(X

Fig. 49.— Starch-g^rains from the tuber of a potato
00). A an older simple grain ; £ a partially compound
grain ; C, D perfectly compound grains ; // an older grain,
the nucleus of which has divided ; a a very young grain,
b an older grain ; c a still older grain with divided nucleus.

6o MORPHOLCGV OF THE CELL.

envelopes. The internal organisation depends, in these cases, on a definite relation
of the water and the molecules of starch; the stratification of a starch-grain dis-
appears, like that of a cell-wall, when the water is removed from it [e. g. by evapo-
ration or action of absolute alcohol, &c.), because the more watery layers then
become similar to the less watery ones, and the difference of refractive power in
the two ceases. In the same manner the stratification also disappears when the
substance of the grain is rendered capable by chemical means (as dilute potash solu-
tion) of absorbing large quantities of water ; the denser layers absorb relatively more
water ; they thus become similar to the more watery layers, and it is no longer possible
to distinguish between them.

Besides the differentiation in the proportion of water which is recognised in the
form of stratification, there is also in every grain an increase from without inwards
in the amount of water. This is partly ascertained by the refraction, partly by the
regular decrease of cohesion from without inwards. If the water is removed from
fresh starch-grains, fissures are formed which cross the layers at right angles; in the
interior a cavity is formed from which the fissures radiate ; these become narrower
outwardly, and are widest in the centre. From this it follows that on drying the
greatest loss of water occurs in the interior, and that this regularly decreases towards
the outside ; but it also follows that the cohesion of the layers is less in the tangential
direction (at right angles to the fissures) than in the radial. This points to the con-
clusion that within every layer the loss of water is greater in the tangential direction
than in the radial.

If the water be removed from a fresh starch-grain or from one which has taken
up as much as it can, it contracts; the molecules contained in it approach one another
as their envelopes of water become thinner. A similar change takes place if the
granulose is removed from a grain ; the farinose-skeleton of the grain which remains
is, although saturated with water, much smaller than the original grain. This possibly
results from the fact that the molecules, now consisting only of farinose, possess less
attraction for water, and, having thinner envelopes, approach nearer; the cause may
however also be that the number of molecules has diminished.

(b) The Extraction of the Granulose of starch-grains, leaving behind a skeleton of
farinose, can be brought about in very different ways: — i. By maceration in saliva at
an elevated temperature; in the starch of Canna indica the extraction, according to
H. von Mohl, is slow at 35°-40° C, but is accomplished in a few hours at 5o°-55° C. ; a
lower temperature suffices for wheat-starch, a higher is required for that of the potato ;
Nageli gives in general 40°-47° C. 2. According to Melsens a similar extraction may also
be effected by organic acids, diastase, and pepsin. 3. According to Nageli it can be
accomplished also by very slow action of hydrochloric or sulphuric acid which has been
so diluted with water that it does not cause the starch-grains to swell. 4. According to
Franz Schulze, the granulose is extracted in from two to four days by a saturated
solution of sodium chloride containing i p. c. of hydrochloric acid, at a temperature of
60° C; the residuum, which perfectly exhibits the organisation of the starch-grain,
amounted, according to Dragendorfl", to 57 p. c. in potato-starch, 2*3 p. c. in wheat-
starch. These skeletons are not coloured by iodine (Nageli's preparation with sul-
phuric acid after fifteen months' extraction), or they become copper-red, and in
places where the extraction was not perfect, bluish. They do not swell in boiling
water, i. e. do not form paste. At 70° C. the whole of the starch-grain, according to
Mohl, is dissolved in saliva; the skeleton produced at 40°-55° G. is, however, not affected
by saliva at 70°.

Within the living cell the starch may be dissolved in very different ways ; probably
solution occurs mostly under the influence of protoplasm, or by the assistance of
nitrogenous combinations in the cell-sap. Sometimes the solution begins, as in the
extractions mentioned above, with the removal of the granulose, the farinose remaining
behind; but this often takes place only partially; the extraction proceeds in places

STARCH-GRAINS.

61

I from without inwards ; the extracted places are coloured copper-red by dilute iodine,
Ihe remaining mass blue; then the grain breaks up into pieces, which are finally
completely dissolved (as in the endosperm of germinating wheat, Fig. 50, B). In
other cases the solution begins also in particular spots of the circumference; the
whole substance, however, gradually dissolves; holes are formed, and finally the
grain breaks up into pieces (as in the maize. Fig. 50, A). In the cotyledons of
germinating beans, the solution of the
ellipsoidal grains begins from within; but
before they break up into pieces, the
granulose is often so completely extracted
that they assume with iodine a copper-red
and in parts a bluish colour; afterwards
the whole is dissolved. In germinating po-
tatoes and the rhizome of Canna lanuginosa^
on the other hand, the solution of the
grains advances from without inwards, re-
moving layer after layer. Probably this
takes place when saliva is employed,
whether the solvent acting slowly first ex-
tracts the granulose, or attacking it ener-
getically dissolves the whole substance.
Observations on embryos of the same
species, germinating at different tempera-
tures, would possibly show corresponding
'differences.

(c) Solubility, Snivelling. If starch-grains
lare crushed in cold water, a small portion
»f the granulose is dissolved ; addition
>f iodine occasions precipitation of fine-
'grained blue pellicles^. Starch -grains
ground with fine sand yield an actual
solution of granulose to cold water. Other
fluids, as dilute acids, do not cause a solu-
tion of the starch, but rather a trans-
formation into other substances (dextrin,
dextrose), which then dissolve.

Water of at least 55° G. causes the
swelling and conversion into paste of larger
[more watery starch -grains; in smaller
[denser ones this begins, according to
Nageli, at 65°. After heating in the dry
state to about 200° C, subsequent moisten-
ing causes swelling; but the substance is by this means chemically changed; it is
transformed into dextrin. In the production of paste, the interior watery parts swell
first, the outermost layer scarcely at all ; it bursts and remains for a long time re-
cognisable by iodine as a pellicle, even after the breaking up of the inner parts into
small particles. A similar effect is caused by weak cold potash or soda solution ; the
volume of a grain may thus be increased one hundred and twenty-five fold, and so
much fluid be absorbed that the swollen grain contains only from 2 to 0-5 per cent, of
solid starch.

Fig. 50. — yi a cell of the endosperm of the maize, filled with
crowded and therefore polyhedral starch-grains : between the
grains lie thin plates of dried-up fine-grained protoplasm ; small
cavities and fissures are formed in the interior of the grahis by
drying ; a^£- starch-grains from the endosperm of a germi-
nating seed of maize ; B lenticular starch-grains from the
endosperm of a germinating seed of wheat ; the commencing
of the action of the solvent is shown by the more evident ap-
pearance of stratification (X 800).

^ On the actual solubility of starch, see the remarks in my Handbuch der Experimental
lysiologie, p. 410.

62 MORPHOLOGY OF THE CELL,

I

Sect. io. The Cell-sap. — The term Cell-sap may be understood in a wider
or in a narrower sense. In the former it would express the collective mass of all
the fluids by which the cell-wall, the protoplasm-body, and all other organised
structures of the cell are saturated, and would also include the fluids contained in
tke vacuoles of the protoplasm ; in a narrower sense the latter only is ordinarily
designated as cell-sap. In any case there are grounds for considering the com-
position of the cell-sap as very variable, according as it has been imbibed by the
protoplasm, the chlorophyll, the cell-wall, or the starch-grains of one and the same
cell, or occurs as vacuole-fluid. The latter may in general represent the reservoir
out of which the organised absorbent parts of the cell supply their needs, but in
which, on the other hand, the superfluous soluble products of assimilation and
metabolism ^ and the food-materials that have been absorbed, also for a time collect.
One constituent of the cell-sap, water, is always common to the vacuole-fluid and to
the fluid which saturates the organised structures. The share taken by the water of
the cell-sap in the entire building-up of the cell has already been entered into suffi-
ciently in detail. Its function in the cell is a very manifold one ; it is at once the
general solvent and the agent of transport of the food-materials within the cell ; the
water itself enters in many ways into the chemical constitution of the substances
produced in the plant ; its elements are essential for the production of assimilated
substances ; for the formation of organised structures, the cell-wall, the protoplasmic
structures, and the starch-grains, it is indispensable (water of organisation); the
growth of the whole cell depends immediately on the absorption of water, and on the
accumulation of the cell-sap as vacuole-fluid (see Figs, i, 41, 42, pp. 2, 42, 43). The
increase in size of rapidl}^ growing cells is nearly proportional to the accumulation
of the sap in them. The hydrostatic pressure which the vacuole-fluid exercises on
the primordial utricle and cell-wall is a factor in determining the form of the cell.

The substances dissolved in the water of the cell-sap — whether salts absorbed from
without, or compounds produced in the plant itself by assimilation and metabolism —
are, as such, not immediately the subject of morphological observation, to which we are
for the time confining ourselves. But the cell-sap sometimes contains substances the
presence of which in the cell in characteristic forms can be proved by simple reactions,
or which occur in nature in the form of definite structures, as drops or granules. Among
the most important of the former is Inulin ^. This substance, nearly related in com-
position to starch and sugar, occurs in the cell-sap of many Compositae^. In sap

^ [In the first edition of this translation the term ' Stoffwechsel,' which includes as a general
term any transformations which are effected in the products of assimilation, was translated
' metastasis.' In the Hterature of animal physiology the same idea has been rendered by the terms
metabolism and metabolic (see Mayne's Expository Lexicon, i860; Foster's Text-book of Physio-
logy). For the sake of uniformity these terms will be adopted in the following pages. The
products of metabolic transformations have been conveniently termed by Foster ' metabolites.']

2 Sachs, Bot. Zeitg. p. 77, 1864. — Prantl, Das Inulin, ein Beitrag zur Pflanzen-Physiologie ;
Preisschrift, Munich 1870. — Dragendorff, Materialien zu einer Monographic des Inulins, Petersburg
1870.

3 [Kraus, Bot. Zeitg. 1875, P- i7i. shows that, in addition to Compositse, inulin is found also in
the Campanulacese, Goodeniaceae, Lobeliacese, and Stylideoe ; and in these orders not only in the
underground organs, but also in the stems and in the cells of the leaves which contain chlorophyll.
Its sokibility in water appears to vary]

THE CELL- SAP.

63

obtained by pressure or boiling, it precipitates spontaneously after some time in the form
of a white fine-grained precipitate. From solutions it crystallises in the form of so-
called Sphere-crystals (Fig. 51 A), consisting of crystalline elements disposed in a radiate
manner. Within the cells it may be made visible as a finely granular precipitate by
drying or by rapid removal of water by alcohol (Fig. 51, F). It is abundantly pre-
cipitated in the cells on dipping thin sections of the tissue into alcohol, in the form of
smaller sphere-crystals which become readily visible on addition of water (Fig. 51, B).
They are obtained much larger by laying entire specimens oi Acetahularia or large pieces
of tissue containing inulin (tubers and stems of the dahlia or Jerusalem artichoke) for
a longer time in alcohol or
glycerin; in the latter case a
sphere-crystal very commonly
includes several cells of the
tissue (Fig. 5 1, £), a proof that
the crystalline arrangement is
not necessarily destroyed by

fthe cell-walls. Similar forms

[(as in Fig. 51, JS) are formed
^hen tissues containing inulin

[freeze ; and they do not again

[become dissolved in the cell-

[sap on thawing. Since the

^sphere - crystals consist of

f doubly refractive crystalline

i elements arranged radially,

'they show, with polarised

[light, the characteristic cross.

|They are not' capable of
swelling, are slowly dissolved
in a large quantity of cold

[water, rapidly in a small qu an-
ility of warm water of from-

■ 5o°-55° G. ; in nitric or hydro-

[chloric acid or potash solu-
tion they dissolve easily, the
solution always commencing
from without; by boiling in
very dilute sulphuric or hydro-

|<;hloric acid the inulin is im-

Imediately transformed into

Iglucose. Solutions of iodine

[in alcohol or water penetrate

finto the fine crevices of the

[sphere-crystals, but produce

'no special colour. Inulin-structures are easily and certainly recognised by these re-
actions. If masses of tissue containing much inulin (tubers of Inula Helenium and
Jerusalem artichoke, roots of dandelion and of other Gompositae) are examined in the
air-dried state, the parenchymatous cells are found to be filled with angular, irregular,
shining, colourless fragments, which are seen in polarised light to be crystalline, and

:inay be recognised as inulin by the reactions above-named.

If the ovaries and unripe fruits of the orange or citron are laid for some time in

alcohol, concretions are found m their tissues, which completely resemble in form the

sphere-crystals of inulin ; but the chemical reactions and the degree of solubility show

[that they do not consist of this substance. Pfeffer has examined these structures more

Fig. 51. — Sphere-crystals of inulin. A from an aqueous solution laid
aside for 21 months ; at a the action of nitric acid is commencing. B cells of
the root-tuber of the dahlia ; a thin section was placed for 24 hours in alcohol
of 90 p. c, and was then dipped in water. C two cells with half sphere-crystals
having their common centre in the middle of the separating cell-wall ; from an
internode 8 mm. thick at the apex of an older plant of the Jerusalem artichoke
which had remained for some time in alcohol. D fragment of a sphere-crystal.
E a large sphere-crystal including several cells, from a larger piece of the
stem-tuber of the artichoke, after lying for a longer time in alcohol. F inulin
after evaporation of the water from a thin section from the same {X 500; E
not so much).

64 MORPHOLOGY OF THE CELL.

minutely, and has ascertained (as Lebreton also thought, see Husemann, Die Pflanzen-
stoffe, p. 709) that they consist of Hesperidin. It is remarkable that only some indi-
viduals of the species named yield sphere-crystals, as for instance those in the botanical
garden at Wiirzburg ; a tree in the garden at Marburg yielded them in its unripe fruits
in 1 87 1, but not since. Sphere-crystals of organic structure (combustible), but of
otherwise unknown constitution, have also been described by Kraus and Russow; the
former found them in the epidermis of the leaves and stem of Cocculus laurifolius, on
treatment of the fresh cells with alcohol, glycerin, or even water. Russow ^ found, in
the living cells of the petiole and mesophyll of Marattia cicutcpfolia and Angiopteris erecta,
sphere-crystals which enclosed a small crystal or other minute body as a central nucleus.
Where the living cells did not contain these structures, he obtained them by treatment
with alcohol ; they left behind, on combustion, a considerable quantity of ash. Russow
also found similar bodies in the cortex of Selaginella Martensii and in tropical orchids,
when the plants had lain for some time in alcohol. He states that all these sphere-
crystals have the property of staining with carmine.

In the cell-sap of the Hepaticae there occur vesicles or nodules of a peculiar
appearance. In a letter PfefFer states that they are formed in the very young leaves
of Jungermannieae [Alicularia scolaris, Radula complanata, &c.) by the coalescence or
grouping together of minute drops of oil, which are first formed in the cell-sap, not
in the protoplasm, and must be regarded as products of excretion which have no
further purpose in assisting growth ; as is the case also with the fatty oils which are
stored up as reserve-materials. A membrane-like envelope surrounds these drops of
oil, whose substance consists, in addition to oil, of water and small quantities of
proteids. Of a similar nature are the bodies found in the thallus of the Mar-
chantiese, which, in the case of Lunularia, also contain tannin.

Among other organised constituents of the cell-sap must be mentioned the spherical
drops or granules containing tannin and surrounded by a thin membrane which are
found in particular cells of the cortex of many plants rich in tannin, as Salix, Betula,
Alnus, Quercusj Sec. (see Nageli u. Schwendener, Das Mikroskop, p. 492). They are still
more conspicuous in the motile parts of the leaves of the sensitive plant, where
Pfeffer^ has investigated them with care. Here they consist of a thin but tolerably
firm membrane, enclosing a concentrated solution which contains a large quantity of
tannin. The strongly refractive contents of the spherical bodies are coloured blue by
solutions of iron, and form a reddish-brown mass with potassium bichromate. If the
contents are extracted with water containing alkali, acid, or alcohol, the membrane
is left, and is perhaps what has been termed a 'pellicle-precipitate,' consisting of a
combination of tannin with a proteid^. PfefFer states that spherical bodies of
this nature are found in particular cells of the cortex of the stem and petiole of
the sensitive plant, as well as in the petioles of Oxalis stricta and Acetosella; ac-
cording to Meyen they occur also in Desmodium gyrans ; and, according to Unger, in
Glycyrrhiz.a.

Sect. ii. Crystals in the Cells of Plants'*. — The crystal-like forms de-
scribed in Sect. 7, in which proteids are sometimes found, though always mixed
with other organic compounds, are not common phenomena, and must not be
placed in the same category as the very abundant true crystals of lime salts now

^ Untersuchungen liber die Leitbiindelkryptogamen, Petersburg, 1872, p. 109.

^ Physiol, Untersuch., pt. I, Ueber Reizbarkeit, Leipzig 1873, p. 13 et seq.

^ See Book III. Chap. i. Sect, i, Traube's artificial cells.

* Sanio, Monatsber, der Berl. Akad., April 1857, p. 254. — Hanstein, ibid. Nov. 17, 1859. —
Holzner, Flora, 1864, pp. 273, 556, and 1867, p. 499. — Hilgers, Jahrbuch fiir wiss. Bot. vol. VI,
1867, p. 285. — Rosanoff, Bot. Zeitg. 1865 and 1867. — Solms-Laubach, Bot. Zeitg. 1871, nos. 31-33.
— Pfitzer, Flora, 1872, p. 97.

CRFSTALS IN THE CELLS OF PLANTS. 63

«

to be described ; both from a morphological and physiological point of view the
difference is very great.

Calcium carbonate occurs, v^^here it has hitherto been observed in plants, not
in the form of large crystals with clearly defined faces, but in finely granular
deposits whose crystalline nature is recognised only by their behaviour to polarised
light (illuminating in a dark field of view by a crossed Nicol); while their
solubility in weak acids with evolution of bubbles of gas characterises them as
calcium carbonate. It occurs, according to De Bary, in the form of roundish
grains in the plasmodium of Physarum. The calcium carbonate imbedded in
the cell-walls of many marine Algse, Acetalularia, Corallina, Melobesia, &c., seems
to be still more finely divided, their structure becoming in consequence stony and
brittle. It occurs in an excessively fine state of division, in the form of molecules
.invisible even under a magnifying power of 800, in the structures known as
Cystoliihs, club-shaped outgrowths of the walls of certain cells projecting into
;^e cavity, found in Urticaceae and Acanthaceae {vide infra).
BB, All other crystals found in plants and hitherto accurately examined are shown,
^By their form where this is recognisable, and by their reactions, especially by their
r jsolubility in acetic acid, and their solubility in hydrochloric acid without evolution
|H| bubbles, to consist of Calcium oxalate. This salt is widely distributed, especially
^H the tissue of the Crustaceous Lichens, most Fungi, and Phanerogams, and in
^He form of very small granules of crystalline structure, of clusters, of bundles of
nieedles {Raphides), or often of large, beautiful individuals with perfectly formed
crystalline flices.

In Fungi and Lichens the crystalline granules are commonly small, and are
not deposited in the interior of the cells, but on the outside of the cell-walls,
and frequently in such large numbers that the hyphal tissue becomes opaque and
brittle in consequence. In some Lichens, as in Psorosma lentigerum, according to
De .Bary, minute granules of calcium oxalate are deposited in the cell-walls of
the dense cortical tissue. It is only exceptionally that crystalline deposits occur
in the interior of the cells of Fungi, as, for example, in the form of radiate
spheres (sphere-crystals) in the swellings of some of the hyphse of the mycelium of
Phallus caninus.

Little or nothing is known of the occurrence of calcium oxalate in most Algae,
in Muscineae, and in Vascular Cryptogams ; but it is found very abundantly in the
tissues of most Phanerogams. In Dicotyledons it often occurs in the form of
large beautifully perfect Crystals in the cavities of cells {e.g. in the mesophyll and
petiole of Begonia, and the stem and root of Phaseolus). Clusters of crystals
are, however, much more common in this class, and are especially abundant in
•the bark of many trees, in the rhizome of Rheum, &c. They are deposited in a
protoplasmic nucleus {e.g. in the cotyledons of Cardiospermum Halicacabum), the
separate crystals being completely formed only in the exposed part. Sometimes
also (as in the hairs of Cucurbita) small and perfectly developed crystals are seen
in the circulating protoplasm.

In Monocotyledons, especially those allied to the Liliaceae and Aroideae, the
crystals of calcium oxalate occur mostly in the form of bundles of long very slender
^Jjeedles, forming the so-called Raphides, which lie parallel to one another, and

66

MORPHOLOGY OF THE CELL.

usually more or less completely fill up the cells, which are mostly elongated. Needles
of this kind arc formed also in great quantities when the leaves of many woody
plants change their colour and lose water by evaporation in the autumn, although
absent during the period of growth.

Where the crystals lie in the cavity of the cell — and this is usually the case with
Angiosperms — they are commonly, perhaps always, coated by a thin membrane,
which remains after solution of the calcium oxalate, and must probably be con-
sidered as a coating of protoplasm. This is also the case, according to Payen,
even with raphides, and, according to the accurate observations of others, also in
the larger single crystals aAd clusters.

In Angiosperms calcium oxalate occurs apparently only rarely deposited in the
substance of the cell-wall; Solms-Laubach (/.<:.) cites different species of Mesem-
bryanihemum {M. rhombeum, tigrinum lacerum, stramineum,
Lemanni) and Sempervivum calcareum, in which fine granules
or (in the last case) larger angular fragments of crystalline
calcium oxalate are scattered through certain layers of the
outer wall of the epidermal cells of leaves. Among Mono-
cotyledons, Pfeffer has observed well-developed crystals in
the thickened cuticle, and in cells which lie deeper in
the tissue, of Draccena rejiexa, arborea, Draco, and umbra-
culi/era.

The occurrence of crystals of calcium oxalate in the sub-
stance of the cell-walls is, on the other hand, according to
Solms-Laubach, of common occurrence in Gymnosperms.
They generally consist of numerous small granules of un-
recognisable shape; not unfrequently, however, they are
well-developed crystals. In the bast- tissue of all parts of
the stem deposits of this kind are found in the Cupressinese,
Podocarpus^ Taxus, Cephalotaxus, and Ephedra ; they are
absent, on the other hand, from Phyllocladus trichomanoides,
Salisburia adianti/olia, Dammar a australis, and from all Abie-
tinese that have been examined. The small angular granules
or larger individual crystals are usually deposited in the soft
middle lamella of the walls between the bast-cells. Calcium
oxalate occurs still more widely deposited in the cell- wall
of the cortical parenchyma of the branches and leaves of
Gymnosperms, with the possible exception of some Abie-
tinese; here also the middle lamella of a common cell- wall
i^ the place where the crystals are formed, as also in the
bundles of thick-walled hypodermal cells {e.g. Ephedra).
The thick-walled often branched prosenchymatous cells
abundantly scattered through the parenchymatous tissue of Gymnosperms, the so-
called * Spicular cells,' not unfrequently contain crystals deposited in the outer layers
of their cell-walls ; these occur in unusually large numbers and great perfection in
Welwitschia mirabilis (Fig. 52). If the crystals are dissolved in hydrochloric acid,
the empty cavities in the substance of the cell-wall retain completely the form of

Fig. 52.— Half of a spicular cell
of IVelwitschia tnirabili's, with a
great number of crystals of cal-
cium oxalate imbedded in the
outer layer of the very thick cell-
wall.

CRYSTALS IN THE CELLS OF PLANTS. 6y

the crystals, so that the unpractised observer thinks that he still sees them. Fine
granules are abundantly scattered through the thickened cuticle of Gymnosperms
(Welwi/sc/im, Taxus baccala, Ephedra, &c.) ; or, in other cases, well-developed small
crystals {Bioia ori'entalis, Libocedrus Doniana, Cephalotaxus Fortunei, &c.).

Closely related to these deposits in the cell-wall itself are the clusters of
crystals discovered by Rosanoff (Bot. Zeitg. 1865, 1867) in the pith of Kerria
japonica and Ricinus communis^ and in the petiole of some Aroideae {Anthurhim,
Philodendron, and Poihos), which, lying in the cavity of the cell, are attached to
the cell-wall by simple or branched threads of cellulose, and are even covered with
a cellulose membrane. Pfeffer has shown that the large and beautifully developed
crystals which occur in the leaves and branches of Citrus vulgaris, as well as in the
bark of Salix aurita, Celtis auslralis, llhamnus Frangula, Acer opulifolium, the
Lombardy poplar, beech, and oriental plane, are also enclosed in a cellulose
membrane which is often quite thick, and united in its growth, in one or more
spots, with the cell-wall.

The Crystalline forms in which the calcium oxalate occurs in the cells of plants
are extremely numerous, a result of the circumstance that this salt crystallises in
two different systems, according as it is combined with six or with two equivalents
of water. The calcium oxalate containing six equivalents of water of crystallisation

(r;^ Jr tc^ 06 + 6 aq.) crystallises in the quadratic system, the fundamental form

being an obtuse quadrate-octahedron (the shape of a letter-envelope) ; combinations
of the quadratic prism with the obtuse octahedron are met with in abundance. The
raphides, however, belong, as respects their behaviour in polarised light, according to
Holzner, to the klino-rhombic system, in which calcium oxalate crystallises with two

equivalents of water of crystallisation (p^ q[ Q Og-h 2 aq.Y The fundamental form

of the numerous combinations belonging to this class is a hendyohedron ; it produces
derivative forms which are very similar to calcspar (as, for instance, in the deposits
in the cell-wall), and others very similar to calcium sulphate. The clusters of
crystals (sphere-crystals) may consist of individuals of one or the other system ^.

a. As respects its physiological significance, calcium oxalate is a metabolic product
which is of no further use to the plant, an excretion similar to the volatile oils, resin,
and other substances which are often contained in glands.

When the crystals remain so small that their volume appears inconsiderable in pro-
portion to that of the cell itself, this latter may possess protoplasm capable of motion,
nucleus, chlorophyll, and starch, as in the case of the hairs of Cucurbita or the mesophyll
of Begonia. When, on the other hand, a crystal or a cluster or a bundle of raphides,
or finally a mass of s-mall crystals, nearly fills up a cell, no other organised constituent is
usually present. The cells which contain raphides have loosened walls which easily
swell, and the bundles of raphides are generally surrounded by a thick gummy mucilage.
Such cells, which serve as receptacles for crystals, may be compared to simple glands
which contain volatile oils and similar substances.

^ [Vesque has succeeded in reproducing artificially the crystalline forms in which calcium
oxalate makes its appearance in vegetable tissues. Raphides are produced, according to his experi-
ments, in the presence of glucose and also of dextrin. See Ann. des Sc. Nat,, 5th ser., vol. XIX.
p. 310.]

F 2

68

MORPHOLOGF OF THE CELL.

b. The crystals of calcium oxalate which are imbedded in the middle lamella of
a common cell-wall, such as those found in the soft bast of Goniferae, have evidently
been formed in the very spot where they occur. Pfitzer has shown, on the other
hand, that the crystals found in Citrus 'vulgaris are formed in the cell-cavity, and
become enclosed at a subsequent period by cellulose secreted from the surrounding
protoplasm, which then coalesces in its growth, at one or more spots, with the cell-
wall. It is highly probable that the same is the case in other instances.

c. Cystoliths are at present known only in the Urticaceae, Cannabineae, Moraceae, and
Acanthaceae {Justiciar Adhatoda). In the three first-named families they occur in
isolated but numerous cells belonging to the epidermal system, especially of the leaves,

either in the larger epidermal cells (spe-
cies of Ficus), which are often elongated
from a swollen spherical base into short
bristles (the hop, fig, Broussonetia, &c.), or
in more deeply buried hypodermal cells
of the upper side of the leaf, as in Ficus
elastica (Fig. 53). In the Acanthaceae
they occur in large numbers in isolated
cells of the cortical parenchyma which
are also somewhat enlarged. In all these
cases the cystolith fills up the cell-cavity
almost completely, no other cell-contents
being discoverable. The mature cysto-
lith resembles in appearance a bunch of
grapes with its stalk, the stalk being
attached, in Broussonetia— where two cys-
toliths usually occur in a cell — to its
side- wall. The body of the cystolith is
hard and brittle, the stalk flexible. In a
dark field of view between the crossed
Nicols, I have found the cystoliths of Ficus
elastica not luminous even in small frag-
ments ; they do not polarise light ; the
calcium carbonate cannot therefore be
deposited in a crystalline form. Nothing
of a crystalline character can be detected
in the body itself (see Hofmeister, Lehre
von der Pflanzenzelle, p. 180). If the
object is treated with acetic acid, bub-
bles of carbonic acid gas are developed
in the neighbourhood of the cystolith,
while the previously opaque substance of the concretion becomes gradually trans-
parent from without inwards. Finally, there remains behind an insignificant skeleton
of an organic matrix, in which the calcium carbonate was evidently deposited in the
finest state of division. No cavities are to be seen out of which crystals can have
disappeared; the matrix is perfectly homogeneous. Neither is there any reason for
assuming that the lime was deposited between the layers of the matrix ; since the outer
portion of the mass, which contains an especially large quantity of lime, is quite un-
stratified. There is, on the other hand, a central nucleus, in direct connexion with
the stalk, which is much denser than the external portion, and manifests an evident
transverse stratification, -as well as radiating fibres of a denser substance, which are
obviously a faint indication of an intersecting striation. On addition of Schultz's solution
the stratified and striated nucleus of the matrix assumes a beautiful dark blue colour, the
outer portion only a light blue ; the former consists of denser, the latter of more watery

Fig. S3.— Cystolith c c in a hypodermal cell of Ficus elastica ;
h h cells near the upper surface of the leaf; e epidermis ; ch the
inner tissue of the leaf containing chlorophyll.

CRYSTALS IN THE CELLS OF PLANTS, 69

cellulose, between the molecules of which those of the lime have been deposited. The
:ystoliths originate (in Ficus elastica on Schacht's authority, in Broussonetia from my
)wn observation) as wart-like outgrowths from the inner side of the cell-wall, which
len swell up into a club-shaped form at their free end, and become impregnated with
Ime. After the lime has been dissolved and solution of iodine added, it is seen that
the surface of the cystolith is coated with a thin protoplasmic membrane in which the
[original sculpture of the whole can still be perfectly made out.

[M^Nab gives (Journal of Botany, new series, vol. I. p. 33) for the composition of the
potassium chlorate solution : three grains of potassium chlorate dissolved in two drachms of
nitric acid of sp. gr. iio. The preparation of ' Schultz's solution' is thus described by_ Schacht
(The Microscope and its application to vegetable anatomy and physiology, translated by F. Currey,
p. 43) : Zinc is dissolved in hydrochloric acid ; the solution is allowed to evaporate under contact
with metallic zinc, until it attains the thickness of a syrup ; the syrup is then saturated with
potassium iodide, the iodine added, and the solution, when necessar}', diluted with water. For the
'iodine-solution' the same authority recommends one grain of iodine and three grains of potassium
iodide in one ounce of distilled water.]

CHAPTER II.

MORPHOLOGY OF TISSUES.

Sect. 12. Definition. — In the widest sense of the term, every aggregate of
cells which obeys a common law of growth (usually however not uniform in its
action) may be termed a Tissue. Aggregates of this kind may originate in different
ways. The cells may be at first isolated ; subsequently during their growth they
may come into contact, and so completely coalesce at the surfaces of their walls
that the boundary between them becomes indistinguishable. This happens, for

example, in the sister-cells
which have arisen by divi-
sion in the mother-cells of
Pediasirum, Ccelastrum, and
Hydrodiciyon ; the sister-
cells within the mother- cell
have a 'creeping' motion
which lasts for a consider-
able time before they be-
come united into a plate
{Pediasirwri) or into a sac-
like hollow net {Hydrodtc-
tyon), and continue to in-
crease as a tissue. In the
same manner the sister-
cells which arise in the em-
bryo-sac of Phanerogams
by free-cell-formation unite
with one another and with
the wall of the embryo-sac itself, continuing then to develop as a continuous tissue
(the endosperm) and to increase by division.

In Fungi and Lichens tissues originate by the juxtaposition and apical growth
of slender filaments consisting of rows of cells (hyphae), and of different orders
of branchlets from them; each filament has its own growth, increasing the
number of its cells by division, and branches copiously; but this takes place in
such a manner that the different hyphae undergo a similar development at definite

Fig. e^/^—Pediastrum granulatmn (after A. Braun, X 400). A a plate consisting of
united cells ; at g the innermost layer of a cell-wall is protruding ; it encloses the
daughter-cells resulting from division of the green protoplasm ; at t are various states of
division of the cells; sp the fissures in the already empty cell- walls ; R the inner lamella
of the mother-cell-wall which has entirely escaped (greatly enlarged) ; b contains the
daughter-cells g, these are in active 'creeping' motion; C the same family of cells
4i hours after its birth, 4 hours after the small cells have come to rest; these have
arranged themselves into a plate, which is already beginning to develop into one
similar to A.

DEFINITION.

n

spots on the whole body of the Fungus or Lichen, so as to form surfaces, threads,
hollow structures, &c., which possess a common growth, and yet consist of sino-le
elementary structures developing individually (Fig. 55).

With the exception, however, of the instances named, and of some similar
ones, the formation in the vegetable kingdom of multicellular bodies obeying a
common law of growth always arises from the cells which originate by bipartition
from common mother-cells remaining in connexion; the cells are in these cases

Fig. 56.— Epidermis (e) and subjacent cortica) parenchyma
of the hypocotyledonary segment of the stem of the sunflower,
which thickens quickly after completion of germination ; the
darker, thicker cell-walls are the original ones, the thinner
transverse ones those most recently formed. The strong tan-
gential growth even of the epidermal cells together with their
cuticle is of special interest in this process.

Fig. 55.— Part of a longitudinal section of a Gasteromycete (Crucibulum vulffare), showing the course of the hyphse : their interstices
are filled with a watery jelly, which has probably resulted from the conversion into mucilage of the outer cell-wall layers of the
hyphae. (For further details of the internal organisation, see Book II, Fungi. The drawing is partially diagrammatic, inasmuch as
the hyphae are shown too thick for the small magnifying of the whole (about 25), and not so numerous as in nature.)

at least originally, so united that they appear like chambers in a mass which
continues to grow as a whole (Fig. 56).

The two first-named kinds may be distinguished as false tissues from the latter or
true form; but there is no sharp boundary-line between them. In many cases, for
example, the endosperm is only in its rudimentary state a false tissue, due to the
coalescence of isolated cells; in its further development by cell-division it becomes
a true tissue {e.g. Ricinus, Sec). The cortex of many Algae and of the genus Chara
is formed by the coalescence of isolated filaments ; but the result cannot be distinguished
from true tissues. Nageli and Schwendener (Das Mikroskop, vol. II. p. 563 et teq.) may

7^

MORPHOLOGy OF TISSUES.

be consulted further on the growth of Jicrochaticum pul'vereum, Stypopodium atomnrium^
Delesseria Hypoglossunt, and the leaves of Mosses ^

Sect. 13
Tissued— If

Formation of the Common Wall of Cells combined into a

the cell-wall between two adjoining cells is thin, it appears, even
when very highly magnified, single ; and some-
times this is also the case when it has already
attained a considerable thickness, as in succulent
parenchymatous cells. Usually it is only when the
wall has become moderately thick that it can be
seen that the one side of the partition-wall belongs
to one, the other to the adjoining cell. If strati-
fication and differentiation into layers occur in a
sufficiently thickened wall between two cells, a
middle lamella always becomes discernible (Fig.
57, m), on either side of which the cellulose is
superposed in the form of layers and shells gene-
rally symmetrically distributed, so that those on
one side appear to belong exclusively to the
one adjoining cell, those on the other side to the
other cell (Fig. 57, i). The impression may thus
be given as if the layers which are concentrically
deposited round each cell-cavity formed the wall
belonging to it alone, while the middle lamella
belonged to a common matrix in which the cells
are imbedded; or as if it were excreted from the
neighbouring cells. Both views were actually held
for a considerable time, and the middle lamella
was then termed 'Intercellular Substance.' If the
older fragments of tissue represented in Fig. 57 are
compared with the younger condition of the same,
the thought at first suggests itself that the middle
lamellae may be the original thin walls, on which
the thickening-layers have been deposited on both
sides; this view has also found its defenders,
by whom the middle lamella was distinguished
as the * Primary Cell-wall/ The remaining thick-
ness is then correspondingly described as ' Se-
condary,' or if it is differentiated into two shells, as * Secondary ' and * Tertiary
Cell-wall.'

Fig. 57.— Transverse section through thick-
ened cells with evident formation of middle
lamellae (w) ; i the whole of the cellulose super-
posed on this middle lamella; / the cavity of the
cell, from which the contents have been removed.
A from the cortical tissue of the stem oi Lycopo-
dium ChamiEcyparissiis ; B wood-cells from the
inner part of the wood of a young fibro-vascular
bundle of the sunflower ; C wood of Finns syl-
■vestris, si a medullary ray (x 800).

^ On the formation of the cortex of Ceramiaceae see Nageli, Die neueren Algensysteme
(Neuenburg 1847), and Nageli und Cramer, Pflanzenphysiologische Untersuchungen.

^ H. V. Mohl, Vermischte Schriften botanischen Inhalts. Tubingen 1845, p. 311 et seq. —
Ditto, Die vegetabilische Zelle, p. 196. — Wigand, Intercellularsubstanz und Cuticula. Braunschweig
1850. — Schacht, Lehrbuch der Anatomic und Physiologic der Gcwachse, 1856, vol. I. p. 108. —
Miiller, Jahrb. fiir wiss. Bot. vol. V. 1867, p. 387. — Hofmeister, Lehre von der Pflanzenzelle. Leipzig
1867, § 31.

FORMATION OF THE COMMON WALL OF CELLS.

n

In lignified tissues the middle lamella is generally thin but strongly refractive,
and is formed of dense substance not capable of swelling. When the rest of the
substance of the cell-wall has been dissolved in concentrated sulphuric acid, it
remains (in fine transverse sections) as a delicate net-work; if, on the other hand,
the cells are isolated by boiling in potash or nitric acid, this middle lamella is
dissolved, while the rest of the cell-wall is preserved, as in all wood-cells and
very many bast-cells. In other cases, as has already been mentioned in Sect. 4,
the middle layers of the partition-wall of adjoining cells are, on the contrary,
converted into mucilage ; the layer of cellulose immediately surrounding each
cell-cavity is dense, and the whole appears as if the cell-wall were imbedded in
a mucilaginous refractive matrix (the so-called 'intercellular substance'); this occurs
in many Fucacese and in the endosperm of Ceratonia Siliqua (Fig. 39, p. 36). On
a fine transverse section through the cambial tissue of a branch of Pinus syl-
vesiris, the two phenomena here described may be seen side by side; the wood-
cells show the thin dense middle lamella; the young bast-cells appear deposited
in a soft mucilaginous substance, which is especially thick between the radial rows
of cells, and is interspersed with fine strongly refractive granules (crystals); but
both forms arise out of the same young tissue (the cambium), the walls of which
are simple thin lamellae, between which the cell-cavities themselves appear as so
many compartments. Objects of this kind are well adapted to prove the correct-
ness of the supposition that in general the formation of denser or softer middle
lamellae depends only on a differentiation of the substance of the partition-walls
during their thickening, a view which explains in a perfectly simple manner all
the phenomena belonging to it, and altogether accords with growth by intus-
susception.

The thin perfectly homogeneous lamella of cellulose which bounds the young
cells never exhibits a separation into two lamellae; the boundary-line between the
two cells is never marked by a
fissure dividing the partition-wall.
Nevertheless such a spHtting of
the still very thin lamella often
takes place when the surface-
growth is more rapid, as in
the formadon of the intercellu-
lar space, in the large-celled
succulent tissue (parenchyma) of
vascular plants, in the formation
of stomata, &c. Fig. 58 shows
some fully grown parenchyma-
tous cells from the stem of the fig. 58.— Transverse section through the succulent parenchyma of the

stem of the maize ; ^r^" common partition-wall of each pair of cells : si inter-
maiZe m transverse section; the ceUular space caused by the spatting ofthewal)(X 530).

cells were at first bounded by ,

perfectly flat walls, which met nearly at right angles; as the size increased, a
tendency arose towards a rounding off of the polyhedral forms, the unequal
growth clearly leading to tensions which are only neutralised by the destruction
of the cohesion in the substance of the cell-wall on the line where one wall meets

74

MORPHOLOGY OF TISSUES.

the other. Thus a fissure arises which, in consequence of the mode of its origin,
assumes the form of a triangular prism with concave sides (Fig. 58, z). It is
filled with air, and becomes one of those intercellular spaces which very usually
form in the parenchyma a continuous system of narrow channels. Not unfrequently
the portions of the wall which bound the intercellular space grow rapidly, and thus
it increases in size; the cells assume irregular star-shaped outlines in transverse
section, touching one another only at small portions of the surface, as in the
parenchyma on the under side of the leaves of many Dicotyledons, and the
stem o[ /uncus effusus. In the faces of the cell, where no other wall intersects
them, splittings of the homogeneous lamella may also occur locally; sometimes
these are limited to narrowly circumscribed places, and produce flattened cavities
in the homogeneous partition-wall. In other cases the splitting into two lamellae
takes place in such a manner that only isolated roundish places remain
unaffected by it; the separated lamellae continue to grow rapidly by intercalary

Fig. 59.— Two rows of cells running in a radial direc-
tion (/, //, /// and I, 2, 3) from the cortical parenchyma of
the root of Sagittaria sagittifolia in transverse section ;
a the protrusions, / the intercellular spaces between them
(X about 350).

Fig. 60.— From a transverse section of the leaf of
Pinus Pinaster ; h half of a resin-passage, to the left
parenchymatous cells containing chlorophyll with
folds (/) of the cell-wall; t pit-like formations {the
contents of the cells contracted by glycerin, and con-
taining drops of oil) (X8oo).

growth, and bag-shaped protrusions of adjoining cells are formed which are
separated by the fragments of the originally unsplit cell- wall (Fig. 59). In other
cases there follows on the partial splitting of the partition-wall a local growth of
one or both of the two lamella (or of only one), so that a fold arises which intrudes
into the cell-cavity, as shown in Fig. 60,/ Finally, in some species of the genus
Spirogyra, the septum between each pair of cells splits into two lamellae, each of
which grows as a protrusion into the interior of the adjoining cell, and, when
the adjacent cells separate, becomes turned inside out somewhat like the finger
of a glove previously folded in. When the walls of cells forming a tissue split
everywhere into two lamellae (the separation proceeding always from the inter-
cellular spaces) and become rounded off, a complete dissolution of the tissue
takes place into a mere mass of isolated cells. This occurs in the flesh of many

FORMATION OF THE COMMON WALL OF CELLS. yt

succulent fruits (e.g. Symphoricarpus in winter), and can sometimes be artificially
brought about by continued boiling in water (as in potato-tubers).

The origin of the partition- walls in tissue- cells which increase by bipartition
by no means requires the supposition that they were originally composed of two
lamellae. In this case one would be led, by a consideration of the properties
of tissues where numerous divisions follow one another and intercellular spaces
ifterwards arise, to extremely complicated hypotheses, which, moreover, are not
(n harmony with growth by intussusception. Even where the union of the cells
(nto a tissue arises from the amalgamation of originally separate cells (not sister-
:ells), the union of the cell-walls is so intimate that no boundary-line can any
mger be perceived; and the formation of a middle lamella proves also in such
ises ^ (as does the formation of a middle lamella generally) that the hypothetical
)undary-surface does not exist, and that the splitting of the homogeneous lamella
a consequence of different growth on its two sides. Both the manner in which
le splittings of the thin homogeneous partition-walls arise, and also the formation
)f the middle lamella of thick walls, contradict the supposition of an originally
louble partition-wall in tissue-cells ^.

The splitting of the partition-wall and the growth of its now separated lamellae
[ead to a variety of configurations in the interior of tissues, which may be col-
jctively included in the conception of the Intercellular Space. To this belong
jspecially the large air-conducting channels in the tissue of many water and marsh-
"plants (Nymphaeaceae, Iridese, Marsileacese, &c.), and the formation of the cavity
between the wall and the spore-sac in the sporogonium of Mosses^. Peculiar
processes of growth of the adjoining cells are not unfrequently connected with
the origii* of intercellular spaces, of which the following are examples: — the for-
mation of ordinary stomata, of the peculiar stomata of the Marchantieae, and of
resin and gum-passages {vide infra).

But the behaviour of the partition-wall of two cells contributes in quite a dif-
ferent manner to the production of air- or sap- conducting channels, which, like
the air- or sap- conducting intercellular spaces, may form a continuous system
throughout the plant. This happens by the partial or entire absorption of the
partition-walls of adjoining cells, by which the cavities of long rows of cells of a
tissue become connected. Unger has appropriately designated this a Coalescence
of Cells. Vessels of this kind {Trache'ides of Sanio) are formed in the xylem
of the fibro-vascular bundles, from which the protoplasm and cell-sap have dis-
appeared; they serve for conducting air. In the sieve-tubes in the bast-portion
of the fibro-vascular bundles, on the other hand, the watery mucilaginous
contents of the cells are not replaced by air ; the communication established

^ For examples see Hofmeister, Handbuch, vol. I. pp. 262, 263.

^ I may remind the reader of the cleavage of crystals as an analogous case; the cleavage-
surfaces are determined by the molecular structure, but there is a wide difference between them
and true fissures, however fine.

^ The wide air-canals in the stem of Equisetacese, Grasses, species of Allium, Umbelliferse, and
Composite, arise, on the other hand, from the cessation of the growth of inner masses of tissue and
their drying and splitting, while the surrounding tissues continue to grow.

76 MORPHOLOGY OF TISSUES.

between the cells of one row serves rather for a more rapid movement of the
contents over greater distances. Laticiferous Vessels must also be regarded as
composed of coalesced cells; they are the result of very early and complete
absorption of the partition-walls of adjoining cells belonging to straight or much
branched rows in different systems of tissues.

Here however it is only necessary to point out the contrast between vessels
produced by the coalescence of cells and intercellular spaces; a more minute
consideration will come better in describing the systems of tissue.

(a) ^Intercellular Substance' and ^Primary Cell-wall.' The hypothesis implied by
these terms could only be entertained so long as it was supposed that the original
thin lamella between two adjoining tissue-cells was double, and so long as it was
believed that the stratification of the cell-wall was brought about by the deposition of
new layers. The expression that the original partition -wall between two tissue-cells
is double can only be understood in two senses: — either it means that the lamella
consists of molecular layers, and that two of these contain between them the ideal
boundary-surface of the two adjoining cells, or that there is an actual interruption
of molecular connexion. The last supposition does not rest upon observation ;
it is besides contradicted by the detection of weak boundary-lines between
layers which nevertheless are molecularly united, and have no cleft between them.
Thus in the layers of thick cell-walls and of starch-grains there are no clefts, and
yet the boundary-lines between the layers may be seen. If the first alternative is
assumed to be correct, the question with reference to the intercellular substance
depends on a mere verbal controversy ; for if the original homogenous partition-wall
is held together everywhere by molecular forces, and the supposed boundary-surface
is no interruption of the molecular structure, then the deposition of a special inter-
cellular substance at the same place is nothing but a process of ordinary growth by
intussusception. The fact that the boundary-line between cells previously separate
disappears by subsequent coalescence proves that the outer molecular layers of cell-
walls may enter into molecular union. If in such cases a middle lamella is after-
wards differentiated, this is the most striking evidence against the explanation of
it as primary cell-wall. If an attempt is made to follow step by step the be-
haviour of developing woody tissue on the theory of the primary cell-wall, one
is immediately involved in difficulties which do not arise on the supposition that
the middle lamella is simply the result of subsequent differentiation.

(b) Examples of the formation of Intercellular Spaces. The origin of these spaces
is very often connected, as has been mentioned, with a peculiar development of
the separating cells, quite different from that of the rest of the tissue ; so that the
intercellular space together with its surrounding cells constitutes a peculiar form of
tissue or an organ for a definite purpose. The observation of some cases of this
kind is well calculated to show the beginner how, even in tissue-formation, processes
which are morphologically similar or equivalent lead to entirely different physiological
results. This subject will be treated in a more general and detailed manner in the
third Chapter, and in Book III.

(i) The cleft of the Stomata of the epidermis belongs to the category of Intercellular
Spaces, and its origin is peculiarly calculated to afford an insight into the mode of
formation of an intercellular space. I have chosen the stomata on the leaves of
Hyacinthus orientalis as an example. Figs. 61-64 are transverse sections perpendicular
to the surface of the leaf; ^^ in all of them are the epidermal cells, />/> the parenchyma
of the leaf. The stoma, S, is formed of a rather small epidermal cell, which divides
into two equal sister-cells by a wall vertical to the leaf; in Fig. 61, 5, this has just

FORMATION OF THE COMMON WALL OF CELLS.

77

taken place ; the partition-wall is formed ', and appears as a very thin simple lamella,
which soon thickens, and especially where it meets the wall of the mother-cell (Fig.
62, J). The thickening-mass appears at first quite homogeneous; afterwards an
indication of stratification is to be observed, and the first trace of a separation into
two (Fig. 62, J5). In Fig. 63, t, the splitting is already completed; the growth of
the separated lamellae now proceeds in a peculiar manner, so that a cleft arises

Figs. 61-63.— Development ot the stomata in the leaf oi Hyacinthics orientalts, seen in vertical section (X !

which is narrower in the middle, wider without and within, and which connects the
intercellular space / (the stoma) with the external air (Fig. 64). It is worth mention
that before the division of the mother-cell an obvious cuticle has already over-
spread it together with the adjoining epidermal cells. This cuticle is easily
recognised in the condition B, Fig. 62, while still continuous; by the sphtting
of the partition-wall into two lamellae it finally becomes ruptured (Fig. 63), and
by the cuticularising of the outermost layer
of the now separated lamellae it is afterwards
continued over the surfaces of the cleft (Fig.
64). If the process of the formation of the
stoma is followed in a front view, it is seen
that the splitting of the partition- wall does
not extend throughout, but that a portion
still remains undivided at each end where
it adjoins the original mother-cell-well. The
two cells which enclose the cleft, or Guard-
cells^ are not only distinguished from the
other epidermal cells by this peculiar mode ^^' ^'

of division and of growth ; they also differ from them in containing chlorophyll and
starch.

(2) In the family of Marchantieae belonging to the Hepaticae, the origin and
structure of the stomata (Fig. 65, 5, sp) is much more complicated; of this we must
speak hereafter. Here it need only be pointed out that even before the formation
of the stoma the epidermal cells become detached from those lying beneath over
rhomboidal areas which are marked off from one another by walls formed of cells
which are not detached (Fig. 65, jB, ss). These large hypodermal chambers, each
of which opens to the outside in its middle by a stoma, are destined to enclose the
chlorophyll-containing tissue of these plants. The layer of cells which forms the

^ I was unable to detect nuclei immediately before and for a considerable time after the
division. [Strasburger has however , succeeded with preparations preserved in alcohol in distin-
guishing, in the development of stomata in Iris pumila, the nucleus of the mother-cell and the
successive stages of its division; see Ueber Zellbildung und Zelltheilung. p. 115, t. v. figs. 38, 39;
French translation, pp. 1 14-1 16.] •

78

MORPHOLCGV OF TISSUES,

bottom of each chamber, after repeated divisions vertical to the surface, sends out
protrusions upwards into the cavity ; th,es2 grow in a manner similar to many fila-

FlG. 65.— Transverse section through tlie horizontal thallus oi Marchautia polyfnorpha ; A central part, furnished
on the under side with the leaf-like appendages b, and the rhizoids h (x 30) ; B marginal part of the thallus, more highly
magnified; / colourless reticulately thickened parenchyma; o epidermis of the upper side; chl the cells containing
chlorophyll ; sp stoma ; s partition-walls between the hypodermal chambers ; u lower epidermis with dark-coloured
cell-walls.

Fig. 66.— Sap-conducting intercellular passages in the young stem of ivy, in transverse section {X 800) ; A,B,C show
young passages at^, placed at the boundary of the cambium c and the soft bast ivb ; h the xylem ; ^ in £» and E larger
and older passages, lying at the boundary of the bast b and the cortical parenchyma rp.

mentous Algae, divide, branch, and form chlorophyll, while the whole of the rest of
the tissue of these plants is devoid of it.

FORMATION OF THE COMMON WALL OF CELLS. 79

(3) The origin of Resin-, Gum-, and Latex-passages depends also on the formation
of intercellular spaces with a peculiar development of the cells which bound them.
As I shall recur again to these structures, it is sufficient here to refer to one example.
Fig. 66 represents passages of this kind in the transverse section of young portions of
the stem of the ivy. Conditions such as B, C show clearly that the intercellular
space arises by the parting of four or five cells; and that these latter, distinguished
by their turbid granular contents, increase by division. The formation of the much
wider passages D, E must also be referred to a subsequent increase and growth
of the cells which surround the passage. By the growth of the cells which bound

II the intercellular passage, as well as by the manner of their division, by their con-
I tents, and by the circumstance that they excrete a peculiar sap into the passage,
■a structure of this kind becomes a differentiated part of the tissue, which is
sharply marked off from its environment, and has a physiological function of its
own.
Sect. 14. Forms and Systems of Tissues. — The entire mass of the cell-
ssue which forms the body of a plant may be uniform or not; in the first case
the cells are all similar to one another, and their modes of union everywhere the
same. This case is rare in the vegetable kingdom; and it is only the simplest
organisms that are constructed in this manner. Since in a homogeneous undifferen-
tiated tissue all the cells are alike, their union into a whole is physiologically and
morphologically of very subordinate importance, because each cell represents the
character of the whole tissue. Hence it not unfrequently happens in these cases
that the cells become actually isolated, and continue their life singly; and such
individuals are termed Unicellular Plants. Only a little higher are those which
consist of an unbranched row of perfectly uniform cells, or of an aggregation
of such into a plate or mass. When numerous and densely crowded cells
form a mass of tissue, then it is usually the case that different layers of tissue
develop differently; the plant then consists of differentiated tissues. In general
their arrangement is determined by the fact that the whole mass of tissue has
a tendency to become definitely bounded on the outside, so that there arises
a differentiation of outer layers of tissue from the inner mass. But in the
interior of the mass enclosed by this Epidermal Tissue fresh differentiations arise
in the higher plants; string-like arrangements of cells are formed, separated
from one another and from the epidermis by Fundamental Tissue; these strings of
tissue, the Vascular, Fibrous, or Fibro- Vascular Bundles, usually follow in their
longitudinal course the direction of the most vigorous growth which immediately
precedes their differentiation. Not only the epidermal layer, but also the vascular
bundles and fundamental tissue, are however usually themselves differentiated;
the epidermal tissue into layers of different nature; the bundles also exhibit
differentiation and generally in a still higher degree. In this manner arise in the
higher plants, instead of different layers, Syste?7is of Tissues. We thus usually
find an Epidermal System, a Fascicular System, and the System of Funda-
mental Tissue lying between them (Fig. 67). But whenever a differentiation of
tissues of this kind occurs in a plant, it only takes place progressively; originally
the whole mass of a growing portion of the plant (stem, leaf, or root) consists
of a uniform tissue out of which, by diverse development of its layers, these
tissue-systems have their origin. This tissue of the youngest parts of plants

8o

MORPHOLOGY OF TISSUES.

which is not yet dilTerentiated may be termed Primary Tissue, or, since its cells
are always capable of division, Primary Meristem ^

In the present section a separate paragraph is devoted to each tissue-system,
and a description will first be given of the various forms of cells and tissues which

are constituents of all systems. Those
forms which are peculiar to one or
another system will be discussed in
their proper place.

(a) In reference to external form,
the following cell-combinations may
be enumerated : —

(i) The term Ttssue may be ap-
plied par excellence to aggregations of
similar cells which, without any well-
defined external form, consist, in what-
ever direction the section be made, of
numbers of cells. Illustrations of such
tissues occur in the greater number
of the larger Fungi ; the fundamental
tissue of thick stems of Ferns ' and
Monocotyledons is a parenchymatous
tissue penetrated by other forms of
tissue in the shape of strings. In
Dicotyledons the pith especially comes
under the same denomination ; it may
also be found in the purest form in the
flesh of succulent fruits. In stone-
fruits, such as the peach, plum, cocoa-
nut, &c., the stone consists of a
sclerenchymatous tissue. It may also
occur that forms of tissue which
differ morphologically may be aggre-
gated into a mass of tissue uniform
in its physiological characters, as in
the secondary wood of trees and the succulent tissue of tubers, such as the potato,
dahlia, &c.

(2) A row or Filament of Cells is composed of similar cells placed singly side by
side or in rows ; but they are usually connected genetically. Isolated filaments of
this character occur in the hyphge of P\mgi and in many Algae, where they have
arisen by transverse partition of an apical cell or by intercalary transverse division.
In the higher plants we have abundant illustrations: — in the epidermaF hairs, vessels^
laticiferous vessels, sieve-tubes, tannin-receptacles (as in the phloem of Phaseolus)^ &c.
wi.ich are found in the interior of tissues.

(3) A simple Layer of Cells results where similar cells are so united in one plane
that the entire layer is only a single cell in thickness. Among Cryptogams it is not

Fig. 67. — Transverse section of the stem of SelagUiella inceqicali-
folia. The outer layers of cortical have thick dark-coloured cell-walls;
the thinner-walled fundamental tissue envelopes three fibro-vascular
bundles, separated from it by large intercellular spaces I {X 800).

* It may not be superfluous to remark that the pith and cortex are neither forms nor
systems of tissue, but are altogether indefinite and undefinable; we speak, for example, of
cortex in Thallophytes in quite a different sense to what we do in Vascular Plants ; the cortex of
Monocotyledons is something different from that of Conifers and Dicotyledons; in the latter the
cortex has quite a different signification in young and in older parts of stems. The same is the
case with the pith,

FORMS AND SYSTEMS OF TISSUES.

81

FIG. 68.— Transverse section of a part of the petiole of Nuphar advena ;
i large intercellular spaces bounded by simple layers of cells ; s s stellate id'.oblasts ;
g a fibro-vascular bundle.

rare for a whole organ to consist of a single layer of cells, as the leaves of Junger-
manniese, or even the entire plant, as among Algae in Vl'va and Rytiphlcea. In the
higher plants the epidermis usually consists of a single layer, and this is not unfre-
quently the composition of the vascular bundle-sheath (always in the case of young
roots); in water and marsh-
plants the fundamental tissue
often resolves itself into simple
layers which enclose large in-
tercellular spaces, as in Nuphar,
Fig. 68, Sal'vinia, Musa, &c.
It is not uncommon for masses
of tissue to be composed of a
number of simple layers of
cells, as occurs frequently in
the secondary wood of trees
and the primary cortex of
branches and roots; or seve-
ral simple layers alike among
themselves are in close juxta-
position, as in the epidermis
of the leaves of Begonia and
Ficus elastic a.

(4) A String or Bundle
of Cells is an elongated mass
of tissue, the transverse sec-
tion of which consists of a
number of cells. Many of

the lower plants, such as some simple Fungi and Floridese, consist of such strings

of cells. The fundamental tissue of the higher plants is sometimes traversed by

bundles of peculiar cells; such are the

brown sclerenchymatous strings in the

stem of Pteris aquilina and of Tree-ferns.

The true bast of Dicotyledons not unfre-

quently forms bundles in the soft bast.

In all vascular plants the bast-like and

wood-forming elements are united into

bundles, the Fibro-vascular Bundles, which

form true tissue-systems, and traverse the

fundamental tissue.

(5) Groups of Cells are roundish aggre-
gations of similar cells. In the lower Algae,
as the Ghroococcaceae, groups of this nature
arise, each from a single mother-cell, and
carry on an independent life as Cell families.
In the fundamental tissue of the higher
plants groups of peculiar cells are often
formed, strikingly diiferent from those that
surround them, as for example the groups
of laticiferous cells illustrated in Fig. 69,
or the groups of sclerenchymatous cells in
the soft flesh of pears. True (compound)

glands are formed by the dissolution of such groups of cells {'vide infra).

(6) In the cases already named a number of similar cells are always united into
a whole; but it also frequently happens that a single cell acquires a character

G

Fig. 69.— Vertical section of a leaf of Psoralea hirta;
w. a group of laticiferous cells imbedded in the chlorophyll-
tissue/ ; e e the upper and under epidermis.

8a MORPHOLOGY OF TISSUES.

diflerent from that of the surrounding ones, owing to a difference either in its form
or its contents; as for instance the stellate hair formed in the interior of the tissue
of NymphaeacejE (Fig. 68), and the Lithocysts in which the cystoliths (Fig. 53, P- ^^)
are formed, &c. Cells marked by such a striking peculiarity I propose to call
Jdioblasts; the special forms will be described further on. I'he term Eremoblast, on
the other hand, may be given to all those forms of cells which detach themselves
altogether from their sister-cells, and continue an independent life ; such as the spores
of Cryptogams, antherozoids, pollen-grains, some gemmae of Hepaticae, &c. The entire
individuals of some lower plants are eremoblasts, as most Desmidieae, Bacillarieae,
and Siphoneae {Unicellular Plants). There is this in common between idioblasts and
eremoblasts, that the individual cells usually attain a high degree of development; but
with this difference, that in the former the development is commonly in one direction
only, while in the latter the individual cell may undergo the most varied differentiation, '
constituting in itself an organism endowed with independent life.

(b) Nageli classifies all the various forms of tissue under two primary heads : —
Generating Tissue or Meristem, and Permanent Tissue. This classification is by no means
parallel wdth one into younger and older cells ; for, even when the former consists of
younger, the latter of older cells, there are also other additional and important
differences.

(i) Meristem consists of cells which increase slowly in size, and divide repeatedly
in such a manner that some of the resulting cells continue to divide, while others
pass over into permanent tissue. The cells which are still capable of division
(merismatic) are mostly much smaller than the permanent cells which are developed
from them, and are more or less similar to one another; they are easily recognised
by their smooth thin walls, the quantity of protoplasm they contain, and the absence
of any coarsely granular deposits. Meristem may be again divided into the two fol-
lowing kinds : —

{a) Primary Meristem comprises the whole cellular tissue of very young organs or
parts of organs, as the apices of roots and stems, the youngest leaves, and the embryo ;
from it are subsequently developed the later forms of tissue. We shall hereafter
discuss this form of tissue more in detail.

{b) Secondary Meristem occurs in those organs or parts of organs which have
advanced beyond the condition of primary meristem, and therefore contain differentiated
forms of permanent tissue, among which the secondary meristem is usually found in
the form of thin layers, and furnishes, from its power of forming cells, the material
for the production of new permanent tissue in addition to that already in existence.
Various forms of secondary meristem will be hereafter described under the names of
Cambium, Thickening-ring, and Phellogen or Cork-cambium.

It not unfrequently happens that the large cells of a permanent tissue which have
already attained a high degree of development, with moderately thick walls, a large
quantity of cell-sap, ^but a relatively small proportion of protoplasm, and which
contain chlorophyll-granules or other coarsely granular deposits, begin to grow
and to divide afresh ; as, for example, in the primary cortex of annual shoots and
stems when they increase rapidly in diameter. The cortical cells are compelled, by
the increase in size of the wood, to stretch in a tangential direction ; each breaks up
by repeated radial division into a number of segments (as shown in Fig. 56, p. 71);
and these, as soon as they are formed, behave like mature permanent cells. Among
plants of very simple structure such a division of more mature cells takes place in
Conjugatae, especially in Spirogyra (Fig. 13, p. 17). A tissue in this condition has
been termed Older Meristem, in contrast to Younger Meristem, in which all the
dividing cells are still in their youngest condition.

(2) Permanent Tissue is the result of a further development of those cells of the

FORMS AND SVSTEMS OF TISSUES. Set

primary and of the various forms of secondary meristem which no longer divide, but
grow vigorously, and finally attain a definite development, in which they are of service
to the plant by the rigidity or some other property of their cell-walls, or by the
chemical activity of their contents. The most various forms of permanent tissue result
from similar cells of the same primary or secondary meristem.

On attaining their permanent condition many forms of permanent tissue lose the
whole of their living contents, especially their protoplasm, of which dry granular vestiges
are sometimes left behind. Such forms of tissue may be termed Kenenchyma^, and
include, for example, cork-tissue, and the tracheal elements of the wood-vessels and
vessel-like wood-cells. In contrast to this is the Succulent tissue, the cells of which,
during the life of the organ, remain filled with chemical products of the vital activity
of the plant, not with air or water. These succulent tissues may again be divided
into two groups: — In the first the cells still contain protoplasm in an active vital
condition, and are therefore able, under favourable conditions, to grow and divide
afresh — i,e. to pass over into older meristem, phellogen, &c., or, on being wounded
to form a callus or cork-tissue— as, for example, in chlorophyll-containing tissue,
the succulent parenchyma of cortex, or of tubers, &c. In the second kind the cells
pass into a condition of permanent quiescence, as in all those cases in which the
protoplasm becomes unrecognisable, or, remains behind as a doubtful residue; and
where the cells are filled, not with chlorophyll, starch, sugar, inulin, fatty oil, aleurone,
or other reserve-materials, but with excrementitious products of various kinds, as
volatile oils, resin, gum, cystoliths, clusters of crystals, &c. Cells of this description,
and tissues composed of them, are apparently never capable of any further develop-
ment; they are incapable, for example, of forming a healing cork-tissue over wounded
surfaces. There is however no sharp line of demarcation between these diff'erent
forms of tissue distinguished by their contents ; it is only the extreme cases that can
be thus characterised.

We also find the greatest variety of intermediate links, if we consider the various
kinds of tissue in reference to the thickness and consistency of the cell- wall.
Starting from ordinary succulent parenchyma with thin but firm and elastic walls
composed of nearly pure cellulose, we see how, on the one hand, the thin cell-walls
become converted into cork (periderm), while on the other hand the cell-walls of
other kinds of tissue thicken and become lignified or converted into mucilage, or
as hard as stone. The nature of lignification and conversion into mucilage has
already been pointed out; the collenchymatous development of certain hypodermal
tissues will be spoken of hereafter ; here it is necessary only to refer to the fact
that layers, strings, or groups of cells are frequently distinguished by the extraordinary
hardness and thickness of their cell-walls. Such tissues, which may arise in all systems
— as for instance the ' Stone-cells ' (scleroblasts) in the flesh of pears and in the bark
of jnany trees, the dark-brown strings in the stem of Tree-ferns, &c. — may be in-
cluded under the collective term Sclerenchyma.

If we now consider tissues in reference to the form and the mode of combination
of their cells, it is of course evident that the latter must depend on the former ; but
that, on the other hand, the position of cells already existing must influence the
growth, and therefore the form, of those which are developing. If we for the moment
leave idioblasts out of account, and consider merely the mode of association of similar
cells, we find that the old distinction into Prosenchyma and Parenchyma can still be
maintained. By the former is meant a grouping together of elongated fusiform pointed
and usually thick-walled cells, whose ends are dove-tailed between one another without
intercellular spaces. A prosenchymatous arrangement of this kind is well seen in

* [This term, which has not hitherto been employed, is proposed as the equivalent of the
German ' Leerzellengewebe.']

84 MORPHOLOGY OF TISSUES.

the fascicular^elcments of the wood and of true bast; but the fundamental tissue
is also not unfrequently prosenchymatous. When, on the contrary, the cells are
roundish or polyhedral, leave wide spaces between them, and touch one another by
broad faces, the tissue is said to be parenchymatous. In elongated organs, like roots,
internodes, &c., the cells are usually elongated in the direction of the axis of growth ;
but are truncated at both ends, and arranged in parallel rows with broad septa. In
green leaves, on the contrary, there are usually two kinds of parenchymatous tissue:
—the so-called Palisade-parenchyma beneath the upper epidermis, with cells elongated
in a direction vertical to the surface of the leaf, but densely crowded together side
by side; and the Spongy Parenchyma of M'hich the under half of the leaf consists,
with roundish cells which either leave between them comparatively large intercellular
spaces, or are furnished with outgrowths and branches with which they touch the
adjoining cells, so that the tissue becomes still more spongy. The parenchyma attains
the highest degree of looseness when the cells form a number of many-rayed stars
which are in contact only by the ends of the rays, as in the stems of many rushes,
the petiole of Musa, &c. The tissue of the larger Fungi (Fig. 55) P- 70 can be properly
termed neither prosenchymatous nor parenchymatous ; but consists of a number of
Hyphce^ i.e. long slender branched filaments growing at the apex, and dividing trans-
versely ; it is best to term it a Hyphal Tissue. When the hyphae are densely interwoven,
and their cells short and broad, a parenchymatous appearance is presented in transverse
or longitudinal sections, and such a tissue has been called Pseudo-parenchyma.

it has already been mentioned that it is not unusual for individual cells in a tissue
otherwise homogeneous to become developed in a manner strikingly different from
their neighbours ; to such cells I have applied the term Idioblast. They may differ
from the surrounding tissue in three different ways: — (i) Their form is the same,
but they are distinguished by their contents ; as, for example, by a coloured cell-sap,
or by containing a volatile oil, resin, gum, or other similar substance, in which case
they are termed Simple Glands ; or they may contain groups of crystals, bundles
of raphides, or cystoliths, in which case I have termed them Lithocysts. (2) In cells
which contain a coloured sap, simple glands, and lithocysts, the cell-wall remains thin,
while in other cases it becomes so thickened that the cavity of the cell is reduced
to a narrow canal or a small central hollow ; the thickened cell-wall manifests strati-
fication and pore-canals, and is usually very hard. Idioblasts of this kind may be
included in the general term Stone-pells or Scleroblasts. They are seldom isolated,
but more often associated in groups or layers, and then form the tissue already de-
scribed as Sclerenchyma. (3) In the two kinds of idioblasts already described a
tendency is exhibited to attain a larger size than that of the surrounding cells;
scleroblasts are especially characterised by outgrowths. In the third group this takes
place to a remarkable extent, this kind of idioblast being distinguished not only by
its contents and the form of its cell-wall, but especially by its great increase in
size and vigorous growth and branching. This is illustrated to a moderate extent in
Fig. 16 (p. 21), more strikingly in the Spicular Cells of Gymnosperms (Fig. 52, p. 66),
which usually contain a number of crystals in their thick wall, are considerable in
size, and generally branch extensively. To this class belong also the internal
'stellate hairs' of the petiole oi Nuphar (Fig. 68, p. 81); and closely allied to these are
the structures termed by Van Tieghem^ 'polls' in the fundamental tissue of the
Monsterineae, a group of Aroideae. Fig. 70 represents a longitudinal section through
the petiole of Monstera deliciosa ; a cell s lying in the middle parenchymatous row
has branched right and left and put out two arms extending upwards and downwards
into the intercellular spaces of the parenchyma, one of which has again put out a small

^ Van Tieghem, Structure des Aroidees ; in Ann. des Sci. Nat. 1866, vol. VI. See also Otto
Buch, Ueber Sclerenchymzellen. Breslau 1870.

FORMH AND SySTEMS OF TISSUES.

85

lateral branch. The wall of this cell has become so thick that its cavity is reduced
to a narrow canal. These cells are very abundant in the tissue of Monsterineae,
and present the appearance, when the petiole is broken across or cut with a blunt
knife, of tough, slender hairs projecting out of the tissue. For idioblasts of this kind
I propose the term Trichoblast, in order to express their resemblance to many epi-
dermal trichomes. Those now referred to have such thick walls that their contents
are of very little importance from a physiological point of view, if indeed they do
not altogether disappear and become replaced by air. But in other cases they
contain latex, which flows out in greater or less abundance when the plant is wounded.
Such structures, the peculiarities cf which were first
recognised by David ^ and compared by him to the
trichoblasts in the petiole of Monsterineae, and termed
Latictferous Cells, were previously confounded with
true laticiferous vessels which result from a coales-
cence of cells (see p. 86). The receptacles for latex in
Euphorbiaceae, Moreae, Apocynaceae, and Asclepiadeae
are very long cells which are closed on all sides, often
much branched at the ends, but not communicating
with one another ; they are formed at an early period,
near the apex of the stem in the young fundamental
tissue of the primary cortex, or of the pith when the
side of the vascular bundles next the pith contains
phloem {Hoya carnosa) ; in other cases laticiferous cells
belonging to the cortex put out branches through the
ring of wood into the pith (Euphorbiaceae, Moreae).
The laticiferous cells of the leaves are, in the case of
Euphorbia, only prolongations of those of the inter-
nodes. The extraordinary length of the laticiferous
cells, especially in Hoya carnosa, makes it difficult to
recognise their true nature ; but it is easily explained
by their early formation near the apex of the stem,
which necessitates their keeping pace with the growth
in length of the stem as well as that of the whole
surface of the leaf. These laticiferous trichoblasts are
most easily seen in Euphorbia splendens, since they are
readily isolated in consequence of their thick firm cell-
wall, and can be distinguished with certainty from any
other form of tissue, by the peculiarity in the form
of the starch-grains as well as the coagulated latex
which they contain. The laticiferous cells represented
in Fig. 71 (slightly magnified and reduced about \ in
length) have been obtained by allowing the ends of
branches oi Euphorbia splendens to decay in water until
the tissue has become very soft ; lumps of the soft
mass were then dissected with needles under the microscope, and the pulpy tissue
washed away as completely as possible, until the long laticiferous cells were ex-
posed, and allowed their closed ends to be fully examined. The structure of the
laticiferous cells in the leaves can, according to David, be determined with much
certainty by first extracting with alcohol, and then rendering them transparent by
boiling in potash. In Ficus elastica the laticiferous cells are thin-walled and more

Fig. 70. — From a longitudinal section
through the petiole of Monstera reli-
giosa; d d parenchymatous cells; j j a tri-
choblast.

' G. David, Ueber die Milchzellen der Euphorbiaceen, Moreen, Apocyneen, u. Asclepiadeen
Dissertation. Breslau 1872.

86

MORPHOLOGY OF TISSUES.

diflicult to follow; still more so in the oleander, where also the contents are more

limpid.

A Coalescence of Cells arises from the tissue-like union
of similar cells, until their contents completely coalesce,
the partition-walls becoming partially or wholly ab-
sorbed. In this manner are formed those filiform ag-
gregates of intercommunicating cells, constituting tubes
filled with air or sap, which are known as Fessels, such as
Wood-vessels, Bast-vessels or Sieve-tubes, and Latici-
ferous Vessels. But roundish groups of cells may also
coalesce by the absorption of their walls and the forma-
tion of a single large cavity filled with sap ; these are
included under the general term Glands (or may be
specially distinguished as Compound Glands). Just as the
distinction between idioblasts and true tissue-cells is
only a progressive one, depending on the augmentation
of certain characters, so a coalescence is only an extreme
case of the ordinary behaviour of adjacent cells; the
contents of these mingle to a certain extent through
their partition-walls by diffusion (osmose). Hence we
frequently meet with tissues consisting of peculiar cells,
which behave physiologically as if they had coalesced,
although it is questionable whether the cavities of the
cells are actually in communication one with another.

True Latic'iferous Vessels are composed of coalescent
cells containing latex and endowed with the same pro-
perties as we have already described as belonging to the
laticiferous cells of Euphorbiaceae, Moreae, Apocynaceae,
and Asclepiadeae. As far as can be judged from obser-
vations which are not yet brought to a conclusion, these
vessels originate from rows of cells in the young tissue,
and especially in the fibro-vascular bundles, coalescing
at an early stage by the complete disappearance of
their transverse septa; long tubes (as shown in Fig.
72) being thus formed filled with latex, which usually
anastomose with one another laterally, and traverse the
whole plant in the form of a continuous system of
tubes ^.

The Cichoriacese, Campanulaceae, and Lobeliaceae
possess very perfectly developed laticiferous vessels
belonging to the fibro-vascular bundles, which they
accompany throughout the whole plant in the form
of reticulately anastomosing tubes, imbedded, in the
case of Gichoriaceae in the outer, in that of the
two other orders in the inner phloem-layer. Their
form is best recognised by boiling sections of these
plants for some minutes in dilute potash solution ; the
anastomosing tubes are then clearly recognised in the
transparent tissue (Fig. 72), and it is easy to expose them entirely in large pieces. In

Fig. 7t.— Laticiferous cells from the end of a
branch of Euphorbia spUndens, exposed by
maceration ; A the whole of one and part of
another laticiferous cell (slightly magnified) ; B a
piece of one containing starch-grains of peculiar
form (strt ngly magnified).

^ What follows is founded mainly on Hanstein's researches reported in his Preisschrift, Die
Milchsaftgefasse und die verwandten Organe der Rinde. Berlin 1864. See also Dippel, Entstehung
der Milchsaftgefasse und deren Stellung in Gefassbiindelsystem,
fiir wiss. Bot. vol. V. p. 31.

Rotterdam 1865. — Vogel in Jahrb.

FORMS AND SYSTEMS OF TISSUES.

87

Papayaceae {Carica and Vasconcelld) the laticiferous vessels, on the other hand, run
through the xylem-portion of the fibro-vascular bundles; they — i.e. the cells by the
coalescence of which they are formed— are repeatedly produced in layers from the
cambium with the other elements of the xylem ; pitted and reticulately thickened
vessels alternate with them. The branches of the laticiferous vessels envelope these
in all directions, and are sometimes firmly fixed to them superficially; but hori-
zontal branches of these tubes also penetrate the medullary rays, and tenninate,
towards the primary cortex, in scat-
tered ramifications or recurrent knots,
as also in the pith if the stem is hollow.
As in the other families, a copious
anastomosis of laticiferous vessels is de-
veloped in the horizontal partition-walls
which the medullary tissue forms at the
origin of each petiole in the hollow of
the stem, penetrating the horizontal
partition-wall in countless ramifications
and in several layers one over another,
and connecting the vessels belonging to
the medullary rays with these of the
whole wood-cylinder. In Papaveraceae
{Cbelidonium, Papa-ver, Sanguinaria) the
laticiferous vessels are also very per-
fectly developed; they are not here,
however, as in the families just named,
united into ribbon-shaped groups, but
run mostly at a greater distance from
one another, dispersed through the
phloem and the surrounding paren-
chyma; single ones appear also in the
pith, but do not penetrate into the
xylem. Lateral outgrowths and cross-
anastomoses are seldom found in the
stem, but abundantly in the leaves, and
especially in the carpels, in which close-
meshed reticulations are formed, ac-
cording to Unger, in the parenchyma-
tous fundamental tissue; similarly also
in the cortex of the root. In this
family, especially in the parenchyma
of the root of Sanguinaria canadensis^
the origin of the laticiferous vessels
from the coalescence of rows of cells
may, according to Hanstein, be proved ;
owing to their imperfect union the re-
sulting tubes appear moniliform. In Aroideae laticiferous vessels united into a net-work
occur in the fibro-vascular bundles and the fundamental tissue; but some genera, as
Caladium and Arum, also exhibit the peculiarity of laticiferous tubes running within
the xylem, which, from their position, and to a certain extent also from their struc-
ture, must be regarded as metamorphosed spiral vessels. Simple broad tubes similar
to these also traverse the fundamental tissue. In the genus ylcer the sieve-tubes are
transformed into laticiferous vessels, as may be seen from their position in the phloem
and the structure of their wall.

True laticiferous vessels scarcely occur among Monocotyledons. The peculiar and

Fig. T2.—A tanjfential long-itudinal section through the phloem
of the root of Scorzonera hispanica ; a number of laticiferous
vessels, anastomosing laterally with one another, traverse the
parenchymatous tissue ; B a small piece of a laticiferous vessel with
the adjoining parenchymatous cells, more strongly magnified.

88

MORPHOLOGY OF TISSUES.

abundant latex in the bulb-scales of Allium Cepa is found in broad elongated rows of
cells, whose broad septa exhibit sieve- or latticed structure, but whose actual per-
foration is not quite certain (Fig. 73). Where two tubes of this kind lie side
by side, the longitudinal walls also show a pitted structure similar to that of im-
mature sieve-tubes. These rows of cells traverse the bulb-scales, at whose base they
anastomose, as well as the leaves and scapes, in long nearly parallel rows, which are
generally separated from the epidermis by from one to three layers of cells. Similar
rows are formed by the Utricular Vessels ^ of Amaryllideae, as Narcissus, Leucojum,
and Galanthus ; they resemble, moreover, laticiferous vessels in this, that the septa

of the rows of cells become partially, sometimes
entirely, absorbed ; but their sap is not milky, and
contains numerous needle-like crystals of calcium
oxalate (raphides). Allied to these are numerous
other structures in Monocotyledons which bear
scarcely any other resemblance to laticiferous ves-
sels. In some genera of Liliaceae, as Scilla, Ornitho-
galum, and Muscari, the utricular vessels often consist
of short interrupted rows of cells, and in the bulbs
themselves larger isolated parenchymatous cells, re-
sembling the former in containing raphides. That
cells containing raphides may, however, unite into
tubes, which resemble morphologically laticiferous
vessels, is shown in Gommelynaceae. Here rows of
cells which are early distinguished from those which
surround them by containing raphides arise in the
young parenchyma of the fundamental tissue of
the internodes and leaves. While their neigh-
bours continue to divide, they do not, but re-
main unaltered, and their septa are, according to
Hanstein, absorbed as the entire organ grows and
the cells in consequence elongate. In this man-
ner long continuous tubes, filled with raphides
of enormous length, are formed from the rows
of cells of the fundamental tissue which contain
crystals ^.

As in Monocotyledons we find transitional forms between the imperfect laticiferous
vessels of bulb-scales and simple lithocysts which do not contain latex but only
raphides; so, on the other hand, Sie've-iubes result from a true coalescence of cells
which usually, it is true, contain mucilaginous proteids, but sometimes also latex, as
in Acer according to Hanstein, and in Gonvolvulaceae according to Vogel, /. c.^

While true laticiferous vessels are confined to a few natural orders. Sieve-tubes, on
the contrary, are apparently an'essential constituent of the phloem (bast-portion) of the
fibro-vascular bundles. They occur nowhere else, and their morphological structure
will therefore be spoken of under the fibro-vascular bundles; we must here speak

Fig. 73. — Longitudinal section through a bulb-
scaie of Allium Cepa; e the epidermis; c the
cuticle;)/ parenchyma; sg the latex of the utri-
cular vessel coagulated" by potash solution ; q q
its septum; the longitudinal wall which separates
the utricular vessel from one lying behind it
exhibits a pitted structure.

^ [These vessels were termed 'vesicular' in the 1st edition of this translation. The present
rendering, corresponding to the French ' vaisseaux utriculeux,' is a more correct rendering of
' Schlauchgefasse,' and expresses the fact that they are composed of rows of cells (utricles) the cavities
of which have not coalesced by the absorption of their septa.]

^ See Hanstein, /. c. ; also in Monatsber, der Berliner Akad. 1859,

' la addition to the forms of cells already named, latex occurs also in the intercellular passages
of many plants, as Rhus, Alisma Plajitago, &c., and occasionally in the vessels of the wood, as
in Carica, some Convolvulacege, &c. On this subject see Trecul, Compt. rend. vol. LXI, 1S65. —
Van Tieghem, Ann. des Sci. Nat., 5th ser. vol. VI, 1866. — David, I.e. p. 57.

FORMS AND SFSTEMS OF TISSUES.

89

)f them as a special form of coalescence of cells. They make their appearance in the
roung phloem in the form of long tubes arranged in rows, with thin walls and trans-
verse or oblique septa, on which a net-work of thickening-ridges is soon observed
tnclosing thinner areolae. At a later period these latter appear to be actually perforated

^hile the thickening-ridges between them often swell up enormously. In this con-
htion the septum, perforated by a number of pores, is termed a Sie've-plate ; it is

sually broader than the diameter of the tube, which therefore appears dilated at its

?pta, the sieve-plates, and hence acquires a very characteristic form (Fig. 74). Sieve-
)lates of simpler structure are also usually formed in the side- walls where two sieve-
^ubes come into contact. In their early stage the sieve-tubes usually contain a tough
llbuminous mucilage very little affected by various solvents, which accumulates on

)th sides of the plate, and fills up the pores. The peculiar configuration of the
peve-plates, and the difficulty of obtaining longitudinal sections of them, render the
observation of these characters extremely difficult ;

lis is especially the case with the perforation of
^he sieve-plates, which can, however, be proved by

method first employed by myself ^ It is suffi-
:ient to saturate thin longitudinal sections of the
)hloem with iodine-solution until the contents of
the sieve-tubes begin to turn brown, and then to
idd concentrated sulphuric acid ; this dissolves the
:ell-walls and the substance of the sieve-plates, and
lothing is left but the mucilaginous contents coloured

deep brown. The accumulations of protoplasm
)n each side of the sieve-plate are now seen to be
inited by slender threads of the same substance (Fig.
'4, p), which evidently previously filled the perfora-
tions or Sie've-Pores ; and their continuity proves
that the pores actually constituted a connection
)etween two neighbouring tubes. Mohl gave to the
sieve-tubes discovered by Hartig the term Latticed
veils'^, since neither he nor subsequent observers
^ere able to ascertain the actual perforation; but

[anstein succeeded in determining it by means of
Jchulze's solution. It is not even yet by any means
certain whether all the rows of cells in the phloem
7hich have in recent times been called sieve-tubes

lave perforated sieve-plates, and are therefore the result of actual coalescence. The
:ells of the parenchymatous fundamental tissue also not unfrequently exhibit a sieve-
plate-like structure on their walls (see Fig. 21, p. 24), as, for example, in the pinnae
of Cycadeae, the bark of Ceropegia aphylla (Asclepiadeae), &c. ; but with respect to the
latter Borscow unhesitatingly asserts^ that they are not perforated, the pores being
still closed by thin membranes. It is an interesting fact that the laticiferous cells of
Ceropegia, as well as (according to David, /. c. p. 57) those of Euphorbiaceae, are connected

Fig. 74. — Places where sieve-tubes unite,
showing; the perforation of the septa after solu-
tion of the cell-wall by sulphuric acid. A and B
from the petiole of Ciicicrbita ; C from the
stem of the dahlia. In A the cell-wall h h' is not
yet completely absorbed; s' the protoplasmic
mucilage, o and 71 accumulation of it on the upper
and under side of the septum (sieve-plate) ; / the
threads of protoplasm which unite these accumu-
lations and pass through the pores of the sieve-
plates.

^ Mohl, Bot. Zeit. 1855, p. 873, [Ann. des Sci. Nat. 1856, vol. V. pp. 141-159.] — Nageli, Sitz-
ungsber. der k. bayer. Akad. der Wissen. 1861, — Sachs, Flora, 1863, P- 68, — Hanstein, Die
Milchsaftgefasse, Berlin 1864, P- 23 et &eq.

^ [Hartig termed the sieve-tubes Siebrohren, which has been rendered ' cribriform vessels ' by
some English, and ' tubes cribreux ' by French writers. Mohl preferred, for the reason stated in the
text, to call them ' Gitterzellen,' which has been variously rendered ' cellulse clathratoe,' ' cellules
treillisees ' or ' grillagees,' ' latticed cells ' or ' clathrate cells.']

^ Borscow, Jahrb. flir wiss, Bot. vol. VII, p. 348.

90

MORPHOLOGY OF TISSUES.

with the parenchymatous cells which adjoin them by such lattice plates, which,
although they do not bring the cells into communication, must nevertheless facilitate
the interchange of certain constituents by diffusion.

An illustration of the coalescence of cells is furnished by the vessels of the wood
(mostly filled with air) especially those which have bordered pits, as well as the wood-
cells with bordered pits {Tracht'ides), of which sufficient has already been said (P^igs. 23-
27, pp. 25-27). In the vessels of the secondary wood of Angiosperms formed from short
cells with large cavities, the transverse or oblique septa commonly disappear altogether,
so that the entire row of cells forms a completely continuous tube. But frequently,

as in Helianthus^ Sonchus, Cirsium, &c.*, the
septa are only partially absorbed, thick ridges
remaining, which have a reticulate or lat-
ticed form, or, when the septa are very ob-
lique, even scalariform. When true wood-vessels
form air-conducting tubes in this manner, the
separate parts having previously been closed
cells, the tubes are also in communication with
one another laterally through the open bordered
pits already described. In tracheides which are
arranged in a prosenchymatous manner, as those
of Conifers (Fig. 23, p. 25) and Ferns (Fig. 27,
p. 27), this lateral communication is the only one,
since in the cells pointed at both ends there are
no true septa which could be broken through. It
is, on the other hand, doubtful whether the cells
out of which annular and spiral vessels are formed
(see Fig. 18, p. 23) are always in communica-
tion with one another, especially when the spiral
cells remain short, as in the ultimate branches
of the vascular bundles in the veins of leaves,
where they are often considerably enlarged.
Preparations in which the structure can be very
easily observed may be obtained by boiling very
young leaves for some time in potash solution
and then placing them in glycerin. Where,
however, the spiral vessels are formed at an
early period in these organs, attaining subse-
quently a considerable length, so that the coils
of the spiral thread which w^ere at first very
close become vi'idely separated (Fig. 75), it may
be assumed that the thin membrane which sepa-
rates the contiguous ends of the spiral prosen-
chymatous cells becomes ruptured, and thus the
cells are placed in communication with one another for considerable lengths. For these
and other reasons, it is convenient not to limit the definition of a vessel to cases
in which the component cells actually coalesce into a tube. Here, as elsewhere in
the different forms of tissue, we find transitional structures ; and the definition of terms
must not be founded on a single characteristic selected arbitrarily, but on a general
consideration of all the morphological and physiological characters.

The forms of coalescence now described possess the common physiological function of
providing a means for the transport of food-materials, and of promoting and accelerating

■f

Fig. 73.— From a very young fibro-vascular bundle
of a young petiole oi Scrophularia aqiiatica; part
of a spiral vessel surrounded by procambium ; two
spirally thickened cells are in prosenchymatous appo-
sition ; by the elongation of the petiole the coils of
the spiral band, now lying close to one another, are
drawn apart ; the spiral band becomes detached from
the thin wall which is common to the vessel and to
the adjoining cells, and in this way a spiral band is
formed capable of unwinding.

See E. Tangl in Sitzungsber. der kais. Akad. der Wiss. Vienna, May 1871.

FORMS AND SYSTEMS OF TISSUES.

9^

this transport to the more distant organs through the continuous tubes which they
form. This is shown, among other evidence, by the course of these tubes, which is
almost invariably in the direction of growth, and therefore enables them to place
those organs in which food-material is produced in connection with those which
require it. This is unquestionably the case with the sieve-tubes, which serve for
the transport of the difficultly diffusible proteids, and secondarily also of the carbo-
hydrates^. It is also true with respect to the laticiferous vessels, in so far as they
contain proteids, oils, and carbohydrates. This function of the laticiferous vessels
is not disproved by the fact^ that they usually also contain secretions that are not
serviceable. The purpose, finally, of the wood-vessels is to form channels filled with
air within the close woody tissue, replacing the air- conducting intercellular spaces of the
succulent parenchyma.

A totally different physiological function must, on the other hand, be assigned to the
last form of coalescence of cells to be described, the Compound Glands, That these have
no use connected with the transport of food-material is shown by their round form,
which renders them quite unserviceable for placing different parts of the plant in com-
munication. The same conclusion is indicated by the fact that the substances which
accumulate in glands do not in any way contribute to growth, but must be regarded as
excrementitious, or as secondary products of metabolism •^.

The popular usage of the term Gland is extremely indefinite, including not only
single cells with peculiar contents, but also certain external organs like the nectaries
of flowers and the colleters or glandular hairs of many leaf-buds. In this extended
signification it is impossible to give an exact definition to the term. In order to
get a good definition, we must exclude in the first place the bodies hitherto known
as Unicellular Glands, which must be associated with lithocysts and gum-cells under the
designation of Idioblasts, as the term has already been defined (p. 84). We may now
define a Gland^ as a group of cells sharply differentiated from those that surround
them, whose intervening septa become absorbed, so that a single cavity is formed,
which is often surrounded by special layers of tissue, and filled with excrementitious
products, especially volatile oils. This definition excludes certain closely related forms
of tissue, such as the nectaries and colleters"'^ already mentioned, which, however, in
order to indicate their affinity, may be designated Gland-like bodies, in contradistinction
to true glands.

Good examiples of glands in this sense are furnished by the large receptacles for
volatile oil which occur abundantly in the rind of various species of Citrus. They
may be recognised, even i'h the young ovary of the flower, as roundish groups of
cells, distinguished by containing a turbid protoplasm and small drops of oil. The
walls of these cells soon begin to swell, and the individual cells can be separated
by pressure. The walls then deliquesce, and a large globular cavity is formed,

* See Sachs, Flora, 1863, p. 50.— Briosi, Bot. Zeit. 1873, nos. 20-22. [Briosi detected the
presence of extremely finely-divided starch in the sieve-tubes of a large number of plants. Experi-
ments lead him to think that by compression the starch particles may be made to pass through the
perforated partition from one cell to another.]

^ Sachs, Experimental- Physiologic, p. 386.

3 [See also Meyen, Ueber die Secretionsorgane der Pflanzen, Berlin 1837.— J. B. Martinet,
Organes de secretion des vegetaux : Ann. des Sci. Nat. 5th ser. vol. XIV, 1872. It has been
suggested that the contents of glands and similar secreting organs are not really excrementitious, but
that they serve to protect the plant by preventing the consumption of the leaves &c. by insects
and other animals.]

* [The term is here used in a somewhat more restricted sense tTian is usual in English botanical
works, in consequence of the etymological meaning of the corresponding German term * Driise,*
in which the idea of something compound is implied.]

' See Sect. 15, under the head of Epidermis, p. loi.

92

MORPHOLOGY OF TISSUES.

filled with watery protoplasm with large drops of a volatile oil floating in it. The
layers of cells which surround the cavity form an envelope, which difVcrentiates it
sharply from the surrounding tissue. The origin of two different forms of gland
in Dictamnus Fraxinella is illustrated in the accompanying figures, taken from Rauter^
Fig. 76 represents the development of a gland on the upper side of the leaf, the contents
of which are the source of the powerful odour of the plant. These leaf-glands of
Dictamnus originate from only two cells, one of which belongs to the young epidermis,
the other to the subjacent parenchymatous layer; the former divides again into two
layers of cells, the outer of which {d) forms a continuation of the epidermis, while
the inner one (r) contributes to the formation of the tissue of the gland, the principal

Fig. 76.— Gland from the upper side of the leaf oi Dictam-
nus Fraxinella (after Rauter). A and B early stages of de-
velopment, C mature gland; d the covering layer, forming
a continuation of the epidermis ; c and p mother-cells of the
gland-tissue ; o a large drop of volatile oil.

Fig. 77.— Gland and hair from the inflorescence of
Dictamnus Fraxinella (after Rauter). A and B early
stages of development ; C mature gland, with the hair h
at its apex.

part of which originates by divisions of the two mother-cells of the gland {p p) ; the
enveloping layer of the gland is here but slightly developed, as is shown in Fig. 76, C.
On the flower-stalks, bracts, and sepals of the same plant are formed large sessile or
shortly-stalked glands of somewhat ovoid form, bearing at their apex a single hair
(Fig. 77, h). These always arise, as Rauter has shown, from a single cell of the young
epidermis, which divides first vertically, then tangentially (Fig. 77, A)\ thus two layers
are formed, the outer of which is a continuation of the epidermis, while the inner
produces, by further divisions, the tissue of the gland (J5). In the further course of

' Rauter, Zur Entwickelungsgescliichte einiger Trichomgebilde. Wien 187 1.

FORMS AND SFSTEMS OF TISSUES.

93

development the whole substance of the gland now becomes, as it were, forced outwards
above the surface of the organ (C) ; and when, finally, the secreting tissue is absorbed,
a cavity is formed filled with mucilage and drops of volatile oil, and surrounded
only by the continuation of the epidermis. Similar to glands in their origin are the
gum-passages and gummy swellings of diseased stone-fruit. Gregorieff found the seat
of the formation of the gum in them to ba principally the soft bast of the fibro-vascular
bundles which traverse the fruit-pulp ; the cell-walls become absorbed after they have
swelled up, and cavities with undefined boundaries filled with gum are thus formed,
which sometimes exude their contents externally through the flesh of thi fruit when
the production of gum is excessive.

While the origin of the structures now described is the coalescence of cells previously
separated by partition-walls, the canal-like Receptacles for Secretions are formed, in many
plants, by cells, previously in contact by their partition-walls, separating from one.
another and leaving an intercellular space (in the manner represented in Fig. 66, p. 78),
into which the secretion flows from the surrounding cells. According to the nature of
the secretion, we thus get Resin-passages, as in most Coniferae and Terebinthaccce,
Gum-passages in Gycadeae, passages with a mixture of gum and resin in Umbelliferae
and Araliaceae ; Latex -pas sages in Rbus and in Alisma Plantago beneath the epidermis
of the petiole in front of the fibro-vascular bundles ; and passages with diff'erent vola-
tile oils, often coloured, as in the Compositae. Similar Secretion-canals ^ occur also in
Clusiaceae (the contents of the canals of Garcinia Morella yielding gamboge) and in
Pittosporeae ; among Monocotyledons, in addition to AUsma Plantago^ we find them
in Aroideae. Among Ferns, they are stated to have been detected in Marattia and
Angiopteris, and by Hegelmaier in some species of Lycopodium, as L. inundatum, alo-
pecuroides, and annotinum. Plants which possess secretion-canals have, as a rule, no
laticiferous vessels ; but they both occur in some Gichoriaceae as Scolymus, Gynaraceae
as Cirsium and Lappa, and in some Aroideae as Philodendron. They are in that case
distributed through different tissue-systems. Thus in Philodendron the laticiferous
vessels are found in the phloem of the fibro-vascular bundles, the secretion-canals in
the fundamental tissue ; while in Scolymus and Cirsium the laticiferous vessels run
through the phloem, the secretion-canals through the fundamental tissue of the
cortex^. Where there are only secretion-canals, they may belong exclusively to the
fundamental tissue of the primary cortex {Tagetes patula, according to Van Tieghem),
or exclusively to the phloem (the stem of Pittcsporum Tobira, according to Van
Tieghem), or to both systems (Umbelliferae) ; in Goniferae they occur in the pith
and primary cortex, and also in the phlo?m and xylem. Where they are found in
the secondary phloem formed out of the cambium, the secretion-canals may be
produced repeatedly with other elements of the soft bast in concentric layers, as in
Cussonia, Umbelliferae, &c.

The simplest forms of secretion-canals are produced by three, or more commonly
fouf, longitudinal rows of secreting cells separating from one another far enough to
form a narrow intercellular passage which they fill with their secretion, as, for example,
in the roots of Gompositae, where definite groups of canals of this nature are formed
in the double vascular bundle-sheath. If the tissue which surrounds the secretion-
canal attains a vigorous development in breadth and thickness, the intercellular pas-
sages, which were at first narrow, become considerably broader (Fig. 66, p. 78), while
the secreting cells which surround them also enlarge, divide in radial and tangential
directions (in reference to the centre of the canal), and thus form round the canal

^ See especially Van Tieghem, Les canaux secreteurs des plantes, Ann. des Sci. Nat., 5th ser.
vol. XVI, 1872.— Mliller in Jahrb. flir wiss. Bot. vol. V. p. 387.— Thomas, ibid. vol. IV. pp.
48-60.

^ Van Tieghem, in the French translation of the 3id ed. of this work, p. 158.

94

MORPHOLOGY OF TISSUES.

a mass of tissue of characteristic form one or more layers in thickness, the cells
of which are distinguished by containing a turbid fluid with drops of oil and resin
floating in it, and by their walls being not lignified and usually thin. If the tissue
which surrounds the secretion-canal does not increase in size, the formation of the
canal or intercellular passage is sometimes altogether suppressed; the secreting cells
retain their secreticm, and form gland-like groups of cells instead (Fig. 78, A, D).

Secretion-canals resemble laticiferous vessels in this respect, that they follow, as
a rule, the direction of growth of the organ, even when they do not lie in the fibro-
vascular bundles, and also that they penetrate into all parts of the plant, although
particular organs, especially the fruit and flower, are destitute of them. When
any part of the plant, especially the stem, which is rich in them, is wounded, and the

Fig. 78.— Transverse section of resin-passagfes (g) at the base of a first year's branch oi Finns sylvesiris {x 5So). A^ B, C
passages lying in the periphery of the pith (j^ spiral vessels of a fibro-vascular bundle) ; at A the formation of a passage has not
taken place, but the cells destined for its formation are there, their walls h.iving become softer; D wood-cells (h) enclosing
a group of resin-cells, not forming a passage (st a medullary ray) ; E part of the wood containing a resin-passage (^) ; next it
wood-cells containing starch (am), forming a zone passing from one passage to another.

canal therefore broken, the contents, forced by the pressure which masses of tissue
exercise on one another, escape, in the same manner as latex, collect round the
wound, and finally become hard. In this manner are obtained the various kinds of
resin, as that of Coniferae, mastic, sandarach, &c., in the same way as dried latex such
as euphorbium, lactucarium, opium, &c.

Sect. 15. The Epidermal Tissue. — A differentiation into epidermal tissue
and inner fundamental tissue can evidently only arise in those plants and parts
of plants which consist of masses of tissue. In general the contrast of the two

THE EPIDERMAL TISSUE.

9^

is the plainer the more the part of the plant is exposed to air and light, under-
ground and submerged parts showing it in a smaller degree; in those destined
to a longer term of life the formation of epidermis is usually also more perfect.
The difference between epidermis and fundamental tissue can only be estab-
lished by the outer layers of cells, whose morphological character is otherwise
similar, becoming distinguished by the thickness and firmness of their cell-walls,
and by having smaller cell-cavities than those which lie deeper. In this case
there is usually no sharp boundary-line between the two tissues ; the charac-
teristics gradually increase the more nearly the cell-layers approach the surface.
This is usually the case, among Algae, with the Fucacese and larger Floridese,
with many Lichens and the fructifications of Fungi ; even in the stem of Mosses
the formation of epidermis is often indicated only in this manner. The con-
trast between epidermal and inner tissue becomics most marked when, besides
a sharp boundary between the two, a different morphological development dis-
tinguishes the two kinds of tissue. In the sporogonia of Mosses and in all
Vascular Plants at least one outer layer of cells may be distinguished in this sense
as epidermal tissue, and is termed the Epidermis. In true roots and many root-like
underground stems, as also in many submerged plants, the epidermis is only
slighily different from the subjacent tissue ; but in most parts of stems and leaves it
shows an altogether peculiar development of its cells, giving rise to stomata and
trichomes of the most various kinds. In many leaves and parts of stems, the
epidermis, after it has already become a recognisable tissue (during or after the
bud- condition) undergoes cell- division tolerably late, by which it becomes divided
into two or more layers. From this epidermis formed of several layers of cells
(Pfitzer, /. c. p. 53) those layers of tissue may be conveniently distinguished as
Hypoderma which lie beneath the simple, rarely beneath the multilamellar epidermis,
and perform the physiological function of strengthening the epidermal tissue, without
however belonging to it genetically, while they are strikingly distinct from the deeper
lying fundamental tissue, although genetically a part of it. This hypoderma consists
chiefly of layers of thick-walled sclerenchymatous cells, sometimes even of bast- like
fibres. In Phanerogams, especially Dicotyledons, the hypoderma is mostly developed
as Collenchyma, the cell-walls being strongly thickened, and in a high degree capable
of swelling at the longitudinal angles where three or four of them meet (Fig. 21,
B, p. 24).

In those parts of plants which live long and which increase greatly in thickness,
the epidermal system attains a further development in the production of Cork. This
originates in the epidermis itself or in the subjacent layers of tissue by subsequent
cell-division, occurring often very late, and by the suberisation of the newly-formed
cells. The formation of cork often continues for a very long period, or is renewed
after interruption ; and when this occurs uniformly over the whole circumference,
there arises a stratified cork-envelope, the Periderm, replacing the epidermis, which

Ienerally perishes, and surpassing it as a means of protection. But not unfre-
uently the formation of cork penetrates much deeper; lamellae of cork arise deep
ithin the stem as it increases in thickness; parts of the fundamental tissue and
f the fibro-vascular bundles, or of the tissue which afterwards proceeds from
them, become, as it were, cut out by lamellae of cork. Since everything which

96

MORPHOLOGY OF TISSUES.

lies outside such a structure dies and dries up, a peripheral layer of dried tissue
collects, which is very various in its form and origin. This structure, abundant in
Coniferae and in many dicotyledonous trees, is the Bark, the most complicated
epidermal structure in the vegetable kingdom.

(a) The Epidermal Formation of Ihallophytes is chiefly confined to the cells of the
fundamental tissue becoming smaller and firmer the nearer they lie to the surface;
the cell-walls very generally become darker, as in the outer layers of the cortical
tissue of many Lichens, and the outer layers of the peridia in Gasteromycetes and
Pyrenomycetes ; in the pileus of many Hymenomycetes the epidermal layer may
be detached in large pieces (Fig. 79). From the small difference between cortex
and fundamental tissue in these Thallophytes, it may appear doubtful whether the

Fig. 80.— Transverse section of the stem of Pryum
rosenni (X90): iv rhizoids developed from single
cells of the outermost layer.

Fig. 79.— Fructification of Boletus flavidus in longitudinal section slightly magnified ; st stipes ; htt pileus, hy hymenium ; v velum ;
h cavity beneath the hym'enium ; /"prolongation of the hymenial layer on the stipes ; ht the separable yellow epidermal layer of the
pileus.

outer layer should be termed cortex or epidermis; when the cortical tissue is
moderately thick, this layer can usually be distinguished from it. With Thallophytes, as
with higher plants, the outermost layer of cells displays a tendency to the formation
of hairs.

The Muscineae (Hepaticae, Sphagnaceae, Musci) exhibit a great variety with reference
to the epidermal formation. While in many other Hepaticae we have scarcely any
indications of one, in the family of Marchantieae (Fig. 65, p. 78) an epidermis perfectly
developed and provided with stomata suddenly makes its appearance. In Mosses the
epidermal formation on the leafy stem is limited to this, — that the cells towards
the surface become narrower, their walls becoming thicker and assuming a deeper
red colour; the outermost layer often produces numerous long rhizoids (Fig. 80),
In Bog-mosses (Sphagnaceae), on the other hand, a single outermost layer of cells
of the stem, or from two to four such, assume an entirely different character. These
cells (Fig. 81, £>) have thin colourless walls; their cavities are much larger than those

THE EPIDERMAL TISSUE,

97

of the inner tissue; the walls sometimes show slender thickening-bands running in
a spiral manner, and open externally by large orifices, being also in communication
with one another by similar ones (/). In the mature state they contain nothing
but air or water, which rises in them by capillarity. Within this epidermal tissue the
stem is similar to that of Mosses; the cells become towards the surface gradually
narrower, thicker-walled, and of a darker colour. A similar epidermal layer, and with
similar hygroscopic properties, occurs in the aerial roots of Orchids and of some
Aroidege.

Like the other forms of tissue, the epidermis also attains a greater perfection in the
sporogonia of Mosses ; the variously differentiated internal tissue of the sporangium

Fig. 81.— Transverse section of the stem of Sphagw
cymbifolium (X 900) ; x inner cells with colourless soft walls ;
r cortical cells, becoming gradually narrower and thicker-
walled towards the surface ; e e the epidermal layer ;
/ orifices through which adjacent cells communicate with
one another.

I

Fig. 82. — Part of a radial longitudinal section through the sporangium of Funaria hygrotnetrica (X 300) ; e epidermis ; the thick
black line on the outside is the cuticle. (For further explanation of the Fig. see Book II.)

is surrounded by a highly developed true epidermis, sometimes provided with stomata
(Fig. 82).

(b) The Epidermis^. In Vascular Plants the epidermal tissue consists usually only
of a single superficial layer of cells, the true Epidermis. In its origin it always consists of
a single layer ; but this sometimes splits into two or more by divisions parallel to the

^ H. von Mohl, Vermischte Schriften hot. Inhalts. Tubingen 1845, p. 260— F. Cohn, De
. Cuticula. Vratislaviae 1850.— Leitgeb, Denkschriften der Wiener Akad. 1865, vol. XXIV. p. 253. —

Nicolai, Schriften der phys.-okonom. Gesells. Konigsberg, 1865, p. 73. — Thomas, Jahrb. fiir wiss.
' Bot. vol. IV. p. 33.— Kraus, ibid., vol. IV. p. 305, and vol. V. p. 83. — Pfitzer, ibid., vol. VII. p. 561,

and vol. VIII. p. 17.— De Bary, Bot. Zeitg. 1871, nos. 9-11 and 34-37.

98

MORPHOLOGY OF TISSUES.

surface, during or after the bud-condition of the organ in question. In such cases
the outermost may be distinguished as the Epidermis proper from those which lie
beneath, or the Strengthening-layers; these latter generally consist of large thin-walled
cells with contents as clear as water, for which reason Pfitzer terms them Aqueous
Tissue. Epidermis of this kind consisting of several layers occurs in the leaves of
most species of Ficuj, in the stems and leaves of many Piperaceae, and in the leaves
of Begonia. In the roots also of some species oi Crinum the epidermis, at first simple,
splits into several layers; but this is much more striking in the aerial roots of Orchids
and Aroidea!, where these cell-layers afterwards lose their succulent contents and sur-
round the substance of the root as an air-containing envelope to the root {Felamen).

The Eypoderma is genetically distinct from the strengthening-layers which result
by division from the originally simple epidermal layer, since it arises from the layers of
the fundamental tissue which are covered by the true and simple epidermis. The cells
of the hypoderma may also become developed as aqueous tissue like that mentioned
above, and often to an enormous thickness; this occurs in many Bromeliaceae and
some species of Tradescontia. The hypoderma more often exists in the form of
layers of very thick-walled often sclerenchymatous cells, whose origin has been proved
to be from the fundamental tissue, not from the epidermis, at least in the case of
Ephedra and E/egia, and is very probably so in other cases. While this scleren-
chymatous hypoderma is especially frequent in Vascular Cryptogams (e.g. Equisetaceae
and Ferns) and in the leaves of Gymnosperms, a third form, the Collencbyma, occurs
very abundantly in the petioles and succulent stems of Angiosperms, especially of
Dicotyledons; its usually narrow but long cells are strikingly distinguished by the
thickening-masses often forming longitudinal ridges at the angles projecting internally,
and swelling greatly with water or more powerful reagents (Fig. 2r, B, p. 24). That
the collenchyma originates from the fundamental tissue, and thus not from the epi-
dermis, has been actually observed only in Euonymus latifolius^ Peperomia, Nerium, and
Ilex, but is probable also in other cases.

When the term Epidermis is hereafter used without further remark, the ordinary
simple layer, or the outermost when the epidermal tissue consists of several layers,
is always to be understood.

The cells of the epidermis, as also those of the strengthening-layers and of the
hypoderma, are in close contact on all sides ; the only intercellular spaces are those
between the guard-cells of the stomata, through which the large cavities in the funda-
mental tissue communicate with the external air. This close approximation of its
cells is sometimes the only distinguishing mark of the epidermis, as in the submerged
Hydrilleae, Ceratophyllum, &c. ; in other cases the formation of hairs helps to distinguish
it, as in most roots, where the cells of the epidermis are otherwise similar to those
of the fundamental tissue in contents and in the nature of their wall. But usually
in the stem and foliar organs the epidermis is destitute of chlorophyll, starch, and
granular contents generally, while in Ferns and in the water-plants mentioned above,
as well as in other cases, the epidermal cells contain chlorophyll-granules. Not unfre-
quently the otherwise colourless cell-sap is in them tinged by a red substance.

In organs which grow chiefly in length, as roots, long internodes, and the leaves
of Monocotyledons, the epidermal cells are usually elongated longitudinally; in leaves
with a broad surface they are mostly broadly tabular; in both cases the side-walls
are often undulated, so that the adjoining cells interlock with one another.

The outermost lamella of the epidermal cells is always cuticularised, and usually to
such an extent that cellulose cannot be detected in it, or only with difficulty. This
true Cuticle extends uninterruptedly over the surface, and is strongly contrasted
with the subjacent layers of the cell-wall. With preparations of iodine, with or
without addition of sulphuric acid, the cuticle is coloured yellow or yellow-brown ;
it is insoluble in concentrated sulphuric acid, but soluble in boiling caustic potash.
In submerged organs and roots it is very thin, difficult to be seen immediately, but

THE EPIDERMAL TISSUE.

99

rendered visible by iodine and sulphuric acid. The trua cuticle is much thicker in
aerial stems and leaves; it may be obtained in them even in large lamellae by decay
or solution of the subjacent cells in concentrated sulphuric acid. In many cases, and
especially in stout leaves and internodes, the outer cell-wall layers of the epidermal cells
which lie beneath the cuticle are strongly, often enormously, thickened ; while the inner
Malls remain thin, the lateral walls are usually strongly thickened towards the surface,
becoming suddenly thinner towards the inside. The thick portions of the wall are
usually differentiated into at least two shells; an innermost thin shell, immediately
surrounding the cell-cavity, shows the reactions of pure cellulose, while the layers
of the cell-wall lying between it and the cuticle are more or less cuticularised, and
the more so the nearer they lie to the cuticle. Not unfrequently these cuticularised
layers extend downwards into the thick part of the lateral walls, in which case the
middle lamella sometimes behaves like the true cuticle, with which it is in contact
on the outside^ Like the cuticle of isolated cells (pollen-grains, spores), that of the
epidermis has also a tendency to form projections, ridges, &c., but they almost always
remain very insignificant, and are best seen on a superficial view; as, for example, in
many dehcate petals (see Sect. 4 (e), p. 33).

According to the recent researches of De Bary, particles of Wax, which cannot be

seen on section, but which exude in the form of drops when warmed to about 100° C,

are deposited in the substance of the cuticular layers of the epidermis. This deposit of

wax (often associated with resin) is one of the contrivances which protect the aerial parts

of plants from becoming moistened with water. But very frequently the wax extends in

an unexplained manner over the cuticle, and becomes deposited there in different forms,

:onstituting the so-called ' bloom ' on fruits and some leaves, or as a continuous shining

fcoating, which is again formed on young organs after being wiped off, and in ripe fruits

[of Benincasa cerifera (the wax-cucumber) appears again long after maturity. De Bary

listinguishes four principal forms of this wax-coating. The bloom or gloss which is

iasily wiped off consists of small particles of two forms: — (i) Quantities of delicate

linute rods or needles, as in the white-dusted Eucalypti, Acacige, many Grasses, &c. ;

)r of granules collected into several layers, as in Kle'mia Jicoides and Ricinus communis ;

[these are aggregated wax-coatings. (2) Simple granular coatings consisting of grains

[isolated or touching one another in one layer; this is the most common form, e.g. in

Iris pallida, the onion and cabbage, &c. (3) Coatings of minute rods consisting

lof long, slender, rod-shaped particles, bent above or even curl-shaped, and standing

[vertically upon the cuticle, e.g. Heliconia farinosa and other Musaceae, Gannaceae,

ISaccharum, Benincasa cerifera, leaves of Cotyledon orbicularis. (^) Membrane-like layers

'of wax or incrustations ; (a) as a gritty glazing in various species of Sempervi'vum,

Euphorbia Caput-Medusce,lhuja occidentalis ; (b) as thin scales, in Cereus alatus, Opuntia,

Portulaca oleracea, the yew ; (c) as thick continuous incrustations of wax, which

sometimes permit a finer internal structure to be recognised, similar to the striation

and stratification of the cell-wall: Euphorbia canariensis, fruits of species of Myrica,

stems of Panicum turgidum. On the stem of the Peruvian wax-palms, especially of

Ceroxylon andicola, these incrustations attain a thickness of 5 mm. ; those on the stem

of Chamcrdorea Schiedeana are thinner, but of similar structure. According to Wiesner

(Bot. Zeitg. 1 87 1, p. 771), these flakes of wax consist of doubly refractive four-sided

prisms standing perpendicularly close to one another.

Hairs ^ are products of the epidermis; they originate from the growth of single
epidermal cells, and are present in most plants in large numbers ; when they are
wanting in any part of a plant, it is termed glabrous. Their form is subject to

^ A. Weiss, Die Pflanzenhaare, in vols. IV and V of the Bot. Untersuchungen aus dem phys.
Laborat. by Karsten, 1867.— J. Hanstein, Bot. Zeitg. 1868, p. 697 e/ sty —Ranter, Zur Entwickel-
ungsgeschichte einiger Trichomgebilde. Wien 1871. [See also J. B. Martinet: Organes de secretion
des vegetaux, Ann. des Sci. Nat., 5th series, vol. XIV, 1871.]

H 2

lOO MORPHOLOGY OF TISSUES.

extraordinary variation. The first indication of the formation of hairs occurs in the
papillose protuberances of the epidermis of many petals, to which their velvety ap-
pearance is due. Among the simplest forms are also the Root-hairs which grow from
the epidermis of true roots or underground stems, as Pteris aquilina, Equisetaceae, &c. ;
they are thin-walled protuberances of the epidermal cells which lengthen by growth
at the apex, or only branch exceptionally, as occurs sometimes in the turnip. In
Vascular Cryptogams their wall readily acquires a brown-red colour; their length
of life is usually short, and when they die all trace of them disappears. The structure
is similar of the woolly hairs which appear on the leaves and internodes of vascular
plants while still in the bud, especially Dicotyledons. On the unfolding of these organs
they commonly fall off and disappear, as in the horse-chestnut. Rhododendron, and
Jralia papyrifera, where they form a felt easily wiped off from the newly unfolded
leaves ; in other cases they remain as a woolly coating, especially on the under-sides of
leaves. In Prickles the wall is mostly thicker, silicified, and hard; they are shorter
than the woolly hairs, pointed at the apex, and are usually separated by a septum
from the mother-cell. When the free outer wall of unicellular hairs exhibits
greater apical and surface-growth at two or more spots, branched forms result
with a continuous cavity. The papillose bulging of an epidermal cell may become
separated by a septum; the hair then consists of a basal cell and a free hair-
cell, as in Anemia fraxinifolia ; but the separated papilla may also become seg-
mented by the formation of more or less numerous septa when the hair grows
considerably in length, and thus arise segmented hairs, as e.g. on the filaments of
Tradescantia. Sometimes the segments form lateral shoots, and thus arise tree-like
blanched structures with whorled or alternate branches, e.g. in Verhascum Thapsus
and Nicandra physaloides. If longitudinal divisions occur in the segment-cells of the
hair, or if the hair continues to grow by an apical cell which forms segments on
two sides, flatly expanded hairs are the result. To this form belong, for example, the
so-called Palea of Ferns which sometimes entirely cover the younger leaves. Finally,
the divisions in the young hair may be so arranged that it forms at length a tissue,
which on its part may again assume different forms, e.g. the pappus-like hairs of
Hieracium aurantiacum and Azalea indica, the capitate hairs of Korrea and Ribes
sanguineum.

The papilla which projects above the epidermis and is separated by a septum often
becomes divided by vertical and radial walls, and expands in a disc-like manner, so
that the head consists of a radially arranged disc of numerous cells ; thus arise Peltate
Hairs, such as those of Elceagnus, Hippuris, and Pinguicula. Tufts of hairs arise when
the mother-cell of the hair which belongs to the epidermis divides into several cells
lying close to one another ; each of these then grows independently into a hair, as is
shown in Fig. 83, which is supplemented by Fig. 42, p. 43.

Not unfrequently a luxuriant growth of the parenchyma takes place beneath the
hair, and subsequently also in the epidermis ; the hair itself is then borne on a conical
promTnence or protuberance of the leaf or stem, into which its lower part is often
deeply implanted ; as, for instance, in the stinging hairs of the stinging-nettle.
Thus also the prickles (climbing hairs) on the six projecting angles of the stem of the
hop are inserted at their base into a protuberant mass of tissue, while the upper
part grows into two opposite sharp points. Such double-pointed unicellular hairs
occur also on the under-side of the leaf of Malpighia urens ; they are from five to
six mm. long, fusiform, very thick-walled, and are attached to the epidermis (without
any protuberance) by their central part. In this case they easily become detached,
and remain sticking in the skin of the hand which touches the leaf. (For further details
on the Morphology of Hairs, see Sect. 21.)

It is very common for hairs to be secreting organs. Such are the Stinging Hairs
already mentioned of Urticaceae, many Loasaceae, &c., as well as the short hairs of
some Urticaceae which contain cystoliths. But the most remarkable examples are

THE EPIDERMAL TISSUE.

lOI

Glandular Hairs. These consist of a stalk and a terminal head which is either com-
posed of a single cell filled with resin or a volatile oil, or constitutes a true Gland,
made up of a number of cells which have coalesced, so that nothing remains but the
external cuticularised cell-wall in the form of a hollow vesicle containing the secretion.
The oily, viscid, odoriferous secretion not unfrequently penetrates through the cell-wall,
and raises the cuticle in the form of a bladder, collecting beneath it as a clear fluid,
while the cells which produce it partially or entirely disappear, as in Salvia, Cannabis,
and Humulus, in the latter case on the perianth of the female flowers. We are indebted
to a careful work by Hanstein ^ for an accurate knowledge of the glandular hairs
on the leaf-buds of many trees, shrubs, and herbs. The parts of the bud are coated
by a gummy substance, or one composed of gum- mucilage and drops of balsam, which
he calls Blastocolla, while the glandular hairs which produce them he terms Colleters.

Fig. 83. — Development of the hairs on the calyx of a flower-bud of the hollyhock (X 300) ; A luk woolly hairs on the
inner surface; b and c glandular hairs in different stages of development; at a in ^ rudiment of a glandular hair;
ep always signifies the (still young) epidermis. The figures a.\r\. A,^ in C, and 7 in /? show the first stages of develop-
ment of the stellate hairs (or rather tufts of hairs), the subsequent condition of which may be compared in Fig. 42 (p. 43) ;
at ^ « is the hair in longitudinal section; /3 and 7 show the appearance seen from above ; the cells are rich in proto-
plasm ; the formation of vacuoles (t/) in the protoplasm is beginning in y.

These shortly-stalked multicellular hairs springing from an epidermal cell may expand
towards the apex in a strap-shaped manner (Rumex), or may bear cells arranged in
a fan-like manner on a kind of mid-rib {Cunonia, Coffea), or may form spherical or club-
shaped knobs {Ribes sanguineum, Syringa 'vulgaris) ; in Platanus acerifoUa branched rows
of cells occur, the roundish terminal cells of which are glandular. The colleters attain
their full development at a very early period in the bud, when the foliar structures
and portion of the stem out of which they spring are still very young and consist
of tissue which is scarcely diff"erentiated. They are borne especially on the en-

^ Ueber die Organe der Harz- nnd Schleimabsonderung in den Laubknospen, Bot. Zeitg. iS68, no.
43 et seq. The very instructive illustrations to this paper should be consulted.— See also Martinet, /. c.

I02

MORPHOLOGY OF TISSUES.

vcloping scales of leaf-buds {Aesculus), on stipules which precede the leaves in de-
velopment {Cunonia, Viola, Prunus), on ochreae (Polygonaceae), or on young leaves
themselves {Ribes, Syringa). The secretion of the coUeters is a watery mucilage in
the Polygonaceae ; in the rest it is mixed with drops of balsam or resin. The gum-
mucilage always arises from the conversion of a layer of cellulose lying beneath Jhe
cuticle of the colleter, the substance of which swells on addition of water, and raises
the cuticle in p'aces into small bladders (Rumex), or detaches it continuously from
the hair as a larger bladder; finally the cuticle bursts, and the mucilage escapes and
flows over the bud; the uninjured inner layer of cell-wall can, on its part, form
a cuticle, beneath which a layer of cellulose again separates, and the process is repeated.
Where balsam is also excreted, it may be recognised even in the cells of the hair;
but it appears outside the cell-wall in dro s as a deposit in the mucilage, or forms
the basis of the secretion. Frequently also the young epidermis itself between the
colletcrs participates in these processes (Polygonaceae, Cunonia) ; and the blastocolla is
even produced exclusively from the epidermis; thus arises, for instance, the greenish
balsam on the bud-scales and foliage-leaves of poplars'.

FIG. 84. — Development of the stomata oi Pteris flabellata (seen from the surface) ; A very young epidermal cells;
B nearly mature ; v cell formed by the preliminary division ; A s mother-cell of the two gfuard-cells s svs\ B.

The Stomata^ are never found on the epidermis of true roots; on the other
hand they are usually present on underground stems and leaves ; according to Borodin
they are occasionally found even on submerged parts; but they are formed in the
largest numbers on the aerial internodes and foliage-leaves, though not altogether
absent from the petals and carpels; they even occur in the interior of the cavity of

^ [Some reference should here be made to the remarkable discovery by F. Darwin (Quart.
Journ. Micr. Sci. 1877, p. 245) of the protrusion of protoplasmic filaments from the glands within
the cup formed by the connate bases of the leaves of Dipsacus sylves(ris, which he believes to have a
function connected with the absorption of nitrogenous matter for the nutrition of the plant]

2 H, von Mohl, Verm. Schriften hot. Inhalts. Tubingen 1845, pp. 245, 252, — Ditto, Bot. Zeitg.
185^), p. 701. — A. Weiss, Jahrb. fur wass. Bot. vol. IV, 1865, p. 125. — Czech, Bot. Zeitg. 1865,
p, loi. — Strasburger, Jahrb. fiir wiss. Bot. vol. V. 1866, p. 297. — E. Pfitzer, ibid., vol. VII, 1870,
p. 5;^2. — Rauter, Mittheil. der naturwiss. Vereins fiir Steiermark, vol. II. Heft 2, 1870. —
Borodin, Bot. Zeitg. 1870, p. 841. — Hildebrand, ibid., p. i. — Ditto, Einige Beobachtungen aus
dem Gebiete der Pflanzenanatomie. Bonn 1861. — Prantl, Ergebnisse der neuern Untersuchungen
iiber Spaltcffnimgen, Flora 1872.

THE EPIDERMAL TISSUE.

103

le ovary {e.g. in Ricinus). They are most numerous where an active interchange
>f gases takes place between the plant and the external air ; for, considered physiologi-
cally, they are nothing but the mouths of the intercellular spaces of the inner tissue

/hich open externally between the epidermal cells ; this is however always preceded
)y a peculiar development in a young cell of the epidermis. Since the stomata do not
irise till a late stage in the development of the internodes and leaves, or even after their
expansion, their arrangement is partially dependent on the already elongated form of the
epidermal cells ; if these are greatly elongated in one direction and arranged in rows
in Equisetum and the stem and leaves of many Monocotyledons and Pinus), the

tomata are also arranged in longitudinal rows, the cleft lying in the direction of
khe axis of growth, the guard-cells right and left ; if the epidermal cells are irregular
)n a superficial view, curved, &c., the position of the stomata is more undefined and
ipparently irregular. The number of the stomata is generally extraordinarily great
the epidermis of organs containing chlorophyll. In 54 species A. Weiss counted on
me square mm. from r to 100 stomata, in 38 species 100-200, in 39 species 200-300, in 9
Species 400-500, and in 3 species 600-700 stomata. The origin of stomata is always

le formation of a mother-cell, first of all by division of a young epidermal cell, which
sometimes preceded by several preparatory divisions in it ; this mother-cell becomes

Fig. 85. — Development of tlie stomata in the leaf of Sedmn purpitrasceiis. A very young stomata ; B one ueai'ly
mature ; e e epidermal cells ; the numbers indicate the successive order of the preparatory divisions.

then more and more rounded off, and the Guard-cells of the stoma are produced from
It by division. The variety of these processes up to the point when the cleft itself
ippears can hardly be explained in a few w^ords; I prefer therefore to describe some
examples more minutely. One of the simplest is afforded by the development of the

tomata on the leaf of Hyadnthus orient alts, which has already been represented in

vertical section in Figs. 61-64 (p. 77). The preparation for the formation of the stoma

here very simple. A nearly cubical piece of a long epidermal cell is separated by

septum, and this is the mother-cell of the stoma. It is divided by a longitudinal

^all {i.e. by one parallel to the axis of growth of the leaf and at right angles to its
surface) into two equal cells, which round themselves off as they grow. How the
splitting of the partition-wall takes place has already been depicted in Figs, 61-64, and
;an now easily be understood by the help of the surface-view in Figs. 84-86. In

Iquisetum Umosum a similar appearance to that represented in Fig. 61 shows itself
immediately after the first formation of the mother-cells of the stomata; but the

lother-cell undergoes in these cases three divisions, first one obliquely to the right,

len one obliquely to the left, finally the middle cell is bisected by a wall at right
ingles to the surface. Four cells thus arise in one plane, of which the two outer ones

row more rapidly, while the inner ones are forced downwards and beneath them;

04

MORPHOLOGY OF TISSUES.

the stoma then appears, when perfect, as if it had been formed according to the
Hyacinthus type, in which each guard-cell has been again divided into an upper and
a lower cell. But, according to Strasburger, this is not the case ; the two pairs of j
guard-cells lie originally in one plane, and, strictly speaking, it is only the middle cell, |
—which is divided by a vertical wall, and the splitting of which forms the cleft,—
that is to be considered as the mother-cell of the stoma; the two oblique divisions
by which the two lateral cells are formed that afterwards lie uppermost must be
regarded merely as a preparation for the formation of the mother-cell. Preparatory
divisions of this kind occur in many Dicotyledons ; one of the young epidermal cells
becomes the primary mother-cell of the stoma, and is divided successively in dif-
ferent directions by walls at right angles to the surface; finally (Fig. 85) we have
a cell surrounded by several cells formed in this manner, which afterwards forms the
two guard-cells (as in Crassulaceae, Begoniaceae, Gruciferae, Violaceae, Asperifolieae,
Solanaceae, Papilionaceae). In other plants, on the contrary, especially Monocotyledons,
after the formation of the mother-cell of the stoma which results from the division

Fig. 86.— Development of the stomata in a leaf of Commelyna coeUsits; A very young stoniata ; B nearly mature ;
ss'va. A and B the mother-cells of the stoma in C; s s xa C the gfuard-cells ; A and B show the formation of the
neighbouring cells.

of a young epidermal cell, divisions also take place in the adjoining epidermal cells, so
that the stoma is surrounded by a pair or by two decussate pairs or by some other
arrangement of neighbouring cells (Fig. 86) ; as in Aloe socotrina, Gramineaj, Juncaceae,
Cyperaceae, Alismaceae, Marantaceae, Proteaceae, Coniferae, Pothos crasshier'via, F'tcus
elastica, Iradescantia %ebrina. The origin of the mother-cell of the stoma in Planta-
gineae, CEnothereae, Sileneae, Centradenia, and many Ferns deserves special study in
reference to the mode of cell-division. In these cases the mother-celP is cut out on
one side from the young but already tolerably large epidermal cell by a wall bent in
a U-shape, the convexity of which faces the cavity of the epidermal cell, while the ends

^ Strasburger calls them 'special mother-cells,' I think it, however, better entirely to abandon
this expression, the more so as its first introduction in the formation of pollen depended on an
obsolete view of the formation of the cell-wall (compare our description, pp. 33, 34).

THE EPIDERMAL TISSUE.

105

are applied to one of its side-walls (Fig. 84). Not unfrequently, especially with Ferns,
e.g. Asplenium biilbiferum, Pteris cretica, Cibotium Schiedei, &c., preparatory cells are
cut out in this manner from the epidermal cell before the formation of the mother-
cell, out of which the guard-cells are then formed by simple longitudinal division.

In consequence of the U-shape of the division-wall which separates the mother-
cell of the stoma from the epidermal cell, the former is half, or more than half, en-
closed by the latter when looked at from above. In some Ferns and Sileneae the wall of
the mother-cell of the stoma is from the first so strongly curved that it touches the epi-
dermal cell only in a narrow band ; in Anemia 'villosa it touches it only at one spot, the
partition-wall seen from above appearing like a circle. In Anemia densa ^.nd fraxinifolia
the side-wall of the epidermal cell does not anywhere touch the wall of the mother-cell
of the stoma \ When first formed this cell has the form of a hollow cyhnder, or,
more exactly, of a truncated cone, the base and truncated end of which are portions
of the upper and lower wall of the epidermal cell ; out of the latter a cell is thus
cut out like a piece out of a cork by a corkborer ; this piece is the mother-cell of the

Fig. 87. — Superficial view of a stoma of Anetnia fraxini-
folia with the epidermal cell completely surrounding it ; e epi-
dermis, jj guard-cells ; cl chlorophyll-granules.

Fig. 88.— Transverse section of a leaf of Piuw;
Pinaster (X 800) ; s guard-cells of the stoma; p
its cleft ; tj entrance ; I air-cavity ; c cuticularised
layers of the epidennis ; a middle lamella, i inner
thickening-layers of the cells beneath the epidermis ;
g parenchyma of the leaf containing chlorophyll.

stoma; and thus arises the remarkable arrangement represented in Fig. 87, where, as
may be seen, the two guard-cells are entirely enclosed within a single epidermal
cell. Similar, but more complicated, is, according to Rauter, the structure in Nipho-
bolus Lingua.

By further growth of the guard-cells and of the epidermal cells which surround
them, different relative positions of the former to the surface may be brought about ;
the guard-cells may, when mature, lie in one plane with those of the epidermis, or may
be deeply depressed and apparently belong to a deeper layer of cells (Fig. 88) ; some-
times they are, on the contrary, elevated above the surface of the epidermis.

The stomata of Marchantieae may shortly be mentioned here in connexion with
what has already been said on Fig. 65, p. 78. After the formation of the air-cavities,
which are filled with outgrowths containing chlorophyll (Fig. 89, A, chl), one cell of the

' Stiasburger, in Jahrb. fiir wiss. Bot. VII. p. 393 ; also Rauter, /.

io6

MORPHOLOGy OF TISSUES.

epidermis lying above the centre is divided by several bipartitions into four, six (Mar-
chantia, Fegatella), or several {Rehouillia) cells, which are arranged radially about a point
where their walls unite. Here the cells separate from one another, and the cleft (Fig,
89, B and C, po) is surrounded by four, six, or more guard-cells {si). Each of these cells
is finally divided by walls parallel to the surface into from 4 to 8 cells lying one above
another, and the stoma becomes a canal surrounded by 4, 8, or more rows of cells.

V\0.^q.—Marcha7ttiapolymorpha. Part of a young- fructification ; A vertical section, o epidernns, .S" partition-
wall between the air-cavities with their chlorophyll-cells chl\ g large parenchymatous cell ; sp stoma ; B and C young
stoHiata seen from above ; po cleft ; si guard-cells ( x S5o)-

(c) Cork, and Epidermal Formations formed from it'^ (Periderm, Lenticels, Bark\
When succulent organs of the higher plants, no longer in the bud-condition, are injured,
the wound generally becomes closed up by cork-tissue; i.e. new cells arise near the
wounded surface by repeated division of those which are yet sound, and these, forming
a firm skin, separate the inner living tissue from the outermost injured layers of cells.
The walls of this tissue resist the most various agents; similar to the cuticular layers
of the epidermis in their physical properties, flexible and elastic, permeable only with
difficulty by air and water, they for the most part soon lose their contents and
become filled with air. They are arranged in rows lying at right angles to the sur-
face, of parallelopipedal form, and constitute a close tissue without intercellular spaces.
These are the general distinguishing features of cork-tissue. It not merely forms on
wounded surfaces, but arises in much greater mass where succulent organs require an
effectual protection, as on potato-tubers, or where the epidermis is unable to keep
up with the increase of circumference when growth in thickness continues for a long
period. In these cases, which occur but seldom in Monocotyledons {e.g. stem of
DraccEna), but generally in stems and roots of Conifers and Dicotyledons when
several years old, the cork-tissue is formed even before the destruction of the epider-
mis; and when this splits under the action of the weather and falls off, the new
envelope formed by the cork is already present. The cork-tissue is the result of
repeated bipartition of the cells by partition-walls, rarely in the epidermis itself, more
often in the subjacent tissue. These partition-walls lie parallel to the surface of the
organ ; where the increase of the circumference necessitates it, vertical divisions also

' H. von Mohl, Vermischte Schriften hot. Inhalts. Tiibingen 1845, pp. 221, 233. — Hanstein,
Untersuch. iiber den Bau u. die Entwickelung der Baumrinde. Berlin 1853. — Sanio, in Jahrb. fiir
wiss. Bot., vol. II. p. 39. — Merklin, Melanges biol. du Bulletin de TAcad. Imp. des sciences de
St. Petersbourg, vol. IV. Feb. 26, 1S64.

THE EPIDERMAL TISSUE.

107

take place, by which the number of the rows of cells is increased. Of the two newly
formed cells of each radial row (i. e. vertical to the surface of the organ) one remains
thin-walled, rich in protoplasm, and capable of division ; the other becomes suberised and
permanent. Thus arises, usually parallel to the surface of the organ, a layer of cells
capable of division, which continues to form new cork-cells, the Cork-cambium or layer
of Phellogen. In general this is the innermost layer of the whole cork-tissue, so
that the cork increases centrifugally, and new layers of cork are constantly formed
out of the phellogen inside those already in existence. But, according to Sanio, it
also happens, when cork is, beginning to be produced, that the formation of per-
manent cells proceeds centripetally, or an alternation of centripetal and centrifugal cell-
formation takes place in the young cork-tissue. But sooner or later the centrifugal
formation of cork with phellogen on the inner side always commences, a result of
the circumstance that the tissues lying on the outside of completely suberised layers
of cells die sooner or later. Usually the
formation of cork begins first at single
places of the periphery of lignified branches;
but the phellogen gradually forms a con-
tinuous layer, from which new layers of
cork are developed centrifugally. When
in this manner a layer of cork arises,
increasing progressively from the inside,
it is termed Periderm. The develop-
ment and configuration of the cork-
cells may change periodically during the
formation of periderm; alternate layers of
narrow thick-walled and broad thin-walled
cork-cells are formed; the periderm then
appears stratified, like wood with annual
rings, as in the periderm of the cork-oak,
birch, &c. In some cases the phellogen
gives rise not only to cork-cells, by which
the periderm increases in thickness, but
parenchymatous cells are also formed con-
taining chlorophyll ; but it is only daughter-
cells of the phellogen lying on the inner side
(facing the wood) that undergo this meta-
morphosis. In this manner the green cor-
tical tissue of some dicotyledonous plants
becomes thickened by the layers of tissue
proceeding from the phellogen, which
Sanio terms the suberous cortical layer or
Phelloderm. This occurs, for example, in

branches two years old or more of Salix purpurea and alba^ the beech, &c. In
such cases the phellogen lies between the periderm and the phelloderm, the outer
daughter-cells producing cork-cells, the inner phelloderm (Fig. 90). The layers of
periderm which first undergo suberisation sometimes bear a very close resemblance to
true epidermis, as, for instance, in branches one year old (August) of the Scotch fir,
where, while the epidermis still remains, the cork-cambium is formed in the cortical
parenchyma, and at first presents the appearance as if a second epidermis were formed
with cells greatly thickened on the outside.

As the epidermis is at first replaced by the periderm, so this again is afterwards
replaced by the formation of bark when the increase in thickness continues long and
vigorous. In large trees, as oaks and poplars, the boughs are covered with epidermis in
their first year, when several years old with periderm, the older branches and the stem

sdi=S' '

Fig. 90. — Formation of cork in a branch oi Ribes nigmtn.
one year old ; part of a transverse section; e epidermis, h hair,
b bast-cells, pr cortical parenchyma distorted by the increase
in thickness of the branch ; K the total product of the phel-
logen c ; k the cork-cells arranged radially in rows formed from
c in centrifugal order, pd phelloderm (parenchyma containing
chlorophyll formed centripetally from c) (X 550).

lo8 MJRPHOLOGY OF TISSUES,

with bark '. The formation of bark depends on the repeated production of new layers
of phellogen in the succulent cortical tissues of Conifers and Dicotyledons which con-i
tinue to grow centrifugally. Layers of cells which can extend through the niost;
different tissues of the cortex are changed into cork-cambium, which ceases to bej
active after the production of thicker or thinner layers of cork. These layers of
cork cut out, so to speak, from the cortex, scaly or annular pieces of the surface ;|
everything which lies outside them becomes dried up; and since this process is con- i
stantly repeated on the outside of the stem, and the new layers of cork continually I
intrench further on the growing cortical tissue, a mass of dried up portions of tissue,
constantly increasing in thickness, becomes separated from the living part of the
cortex ; and this is the Bark. The process is very clear in the bark of the oriental
plane which detaches itself in large scales, and almost as clear in old stems of the Scotch
fir. Since the bark does not follow the increase in thickness of the stem, it splits in
longitudinal crevices from the surface inwards, as in the oak, according to the direction
of weakest cohesion ; in other cases it peels off in the form of horizontal annular bands
from the stem (ring-bark), as in the cherry.

Lenticeh are a peculiarity of cork-forming Dicotyledons. They appear before the
formation of periderm in branches during their first year, as long as the cortex is still
covered with uninjured epidermis, and are visible as roundish bodies. At the end of
the first or in the following summer, the epidermis splits above the lenticel in the
direction of its length ; the lenticel becomes changed into a more or less projecting
wart, which is often divided by a central furrow into two lip-like ridges ; its surface is
generally brown, its substance to a certain depth dry, brittle, and cork-like. With the
further increase in thickness of the branch, the lenticels become extended in a direc-
tion transverse to the branch, and present the appearance of transverse streaks ;
when afterwards cork or bark is formed, the splitting of the cortex commences with
these, and they become indistinguishable (as in the silver poplar, apple, and birch) ; by
the scaling off of the bark they are of course removed. According to Unger, the
lenticels arise only at those portions of the cortex where stomata occur in the epidermis ;
according to Mohl the inner cortical parenchyma projects in a wart-like manner
through the outer, and forms a cork-tissue, which, on the formation of periderm,
coalesces with the cork of that tissue ; as occurs, for example, in young potato-tubers.
The formation of cork on the lenticel continues for a number of years, until the cortex
which afterwards grows from within dies off on the outside, periderm or bark-
formations becoming interposed between the lenticels and the living part of the cortex.
In many trees, as Cratoegus, Pyrus, Salix, Populus, where the formation of periderm
begins from single spots, and becomes further extended, the lenticels are, according to
Mohl, the points of departure ^

Sect. i6. The Fibro- vascular Bundles l — The tissue of the higher
Cryptogams and of Phanerogams is traversed by filiform or string-Hke masses of

* A considerable increase of thickness is not always associated with the formation of periderm,
as, for example, in the sunflower and other annual stems. In Viscum the epidermis always remains
capable of development, and its thick cuticular layers render the protection of periderm super-
fluous. The formation of bark is also not a necessary consequence of vigorous increase of thickness;
the copper-beech and the cork-oak, for example, form nothing but periderm.

2 [For a detailed investigation of the development of lenticels, see E. Stahl, Entwickelungs-
geschichte und Anatomic der Lenticellen, Bot. Zeitg., 1873,]

3 H. von Mohl, Vermischte Schriften, pp. 108, 129, 195, 268, 272, 285. — Ditto, Bot. Zeitg.
^855, p. 873. — Schacht, Lehrb. der Anat. u. Phys. der Gewachse, pp. 216, 307-354. — Nageli,
Beitrage zur wiss. Bot. Leipzig 1858, Heft i.— Sanio, Bot. Zeitg. 1863, no. 12 et sey.— Nageli, Das
Dickenwachsthum des Stammes u. die Anordnung der Gefasstrange bei den Sapindaceen. Miinchen
1864. — Caractere et formation du liege dans les dicotyledons, in Rauwenhoff 's Archives Neerlandaises,
vol. V. 1870, [For the recent literature see Vines in Quart. Jour. Micr. Sci. 1876, pp. 388-3(^8.]

THE FIBRO-VASCULAR BUNDLES.

109

tissue, which in some cases develop by increase in thickness in such a manner
that they lose externally the form of strings and present that of large masses,
retaining, however, internally their characteristic structure. These are the Vascular
or Fihro-vascular Bundles. Very often they can be completely isolated with ease
from the rest of the tissue of the plant. If, for instance, the petiole of Plantago
major is broken across, they hang out from the parenchyma as tolerably thick,
extensible, elastic threads. In Pteris aquilina it is possible, by scraping off the
mucilaginous parenchyma, after removing the hard epidermal tissue of the under-
ground stem, to expose them as strap-shaped or filiform very firm light yellowish
bands (Fig. 91), In older leaves of trees, dry pericarps (as Datura), stems of
Cactus, &c., the fibro -vascular bundles are left, through the decay of the parenchyma
which surrounds them, as a skeleton retaining more or less the original form.
Beautiful and instructive skeletons of this nature
are afforded by the stems of Tree-ferns, Dra-
ccEfia, Yucca, maize, &c., when their parenchyma
has been destroyed by gradual decay, and
only the epidermal tissue and the firm bundles
in the interior remain ; and the student would
do well in any case to make for himself pre-
parations of this kind, or to examine them in
collections ; they are extremely useful for a clear
comprehension of structure. This is, however,
the case only with lignified fibro-vascular bundles
which run isolated between soft parenchyma ;
in some plants, on the contrary, the tissue of
die bundles is even softer and more delicate
than that which surrounds them {e.g. Cerato-
phyllwu, Myriophyllum, Hydrillese, and other
water-plants) ; and in these cases they cannot of
course be isolated. But in the older lignified
stems and roots of Conifers and Dicotyledons,
the fibro-vascular bundles are so densely crowd-
ed, and so extended by the further development
of their tissue, that at last very little or even

nothing is left of the original fundamental tissue which separated them, and such
stems consist almost entirely of fibro-vascular masses.

Each separate fibro-vascular bundle consists, when it is sufficiently developed, of
several different forms of tissue, and must therefore itself be considered as a tissue-
system ; but different bundles, often in very large number, unite in most plants to
form a system of a higher order. At present however we shall consider only the
separate bundle.

The fibro-vascular bundle consists at first of similar cells fitting together
without intercellular spaces ' ; this form of tissue in the young undifferentiated

FIG. ^x.— Pteris aqiiiliiia. A transverse section
of the underground stem (natural size) ; r lirown hard
epidermal tissue ; / soft mucilag nous parenchyma
containing starch ; pr dark-walled sclerenchyma, form-
ing two broad bands traversing the stem ; a^ fibro-
vascular bundles running outside these bands of
sclerenchyma ; ig others running within them. B the
fibro-vascular bundle represented in A, isolated by
scraping off the parenchyma; it exhibits divisions
and anastomoses; the dotted lines it sliow the outluie
of the stem s , of its branches s^ -and st" , and of a
petiole b.

^ The young cells of the fibro-vascular masses are not always elongated and prosenchymatous ;
in the roots of maize the young vascular ctlls which no longer divide, as well as the adjoining ones,
are tabular or cubical.

no

MORPHOLOGY OF TISSUES.

bundle may be termed Procamhium'^. As it grows older, some of its rows of
cells change into permanent cells of definite form (vessels, bast-fibres, &c.); and
from these starting-points the transformation of the procambium- cells into per-
manent cells in the transverse section of the bundle advances until the cells have
all undergone this change; or an inner layer of the bundle remains capable of

Fig 92.— Transverse section of a closed fibro-vascular bundle in the stem of maize ( X 550) ; tlie fibro-vascular bundle
consists of the xylem-portion^'-^, J, r,l, and the phloem-portion w t' z'; the thick-walled tissue on the outside is the bundle-
sheath belonging- to the fundamental tissue ; // the thin-walled parenchyma of the fundamental tissue ; a outer side, /inner
side (facing the axis of the stem) ; g g twi> large pitted vessels ; J spirally thickened vessel ; r isolated ring of an annular
vessel ; / air-containing cavity, caused by splitting resulting from growth ; v v the cambiform or latticed cellular tissue which
has last passed over into permanent tissue ; between it and the vessel s lie reticulately thickened vessels with bordered pits.

further development, and is then called Camhiuin. In advanced age there are thus
bundles devoid of and bundles containing cambium; the former may be termed
closed^ the latter open'^. As soon as a procambium -bundle has become transformed

^ Nageli calls the tissue of the young fibro-vascular bundles simply Cambium, and applies
the same term to the tissue, capable of further development, of the bundles whicli increase in thick-
ness, which nevertheless ought to be distinguished from them. Sanio terms the latter only
Cambium, a restriction which I adopt. (Sanio in Bot. Zeitg. 1863, p. 362.)

^ This distinction was first made by Schleiden ; but he incorrectly ascribed to Dicotyledons in
general only open bundles ; his distinction of simultaneous and successive cannot be sustained ; all
bundles become differentiated successively in transverse section. Schleiden's simultaneous bundles
of the higher Cryptogams belong to the closed description.

THE FIBRO-VASCULAR BUNDLES. Ill

into a closed fibro-vascular bundle, all further growth ceases, as in Cryptogams,
Monocotyledons, and some Dicotyledons. The open fibro-vascular bundle, on
the other hand, continues to produce new layers of permanent tissue on both
sides of its cambium, and thus the portion of the stem or root concerned con-
tinually increases in thickness, as occurs in woody Dicotyledons and Conifers;
the foliar organs, however, of these plants possess closed bundles, or, if they are
open, the activity of their cambium soon ceases.

The different forms of tissue of a differentiated fibro-vascular bundle may be
classified into two groups, which Nageli calls the Phloem- (Bast) and Xylem- (Wood)
portion of the bundle. They are separated by the cambium, if there is any. In
each of the two constituents of the bundle, the phloem and the xylem, three
forms of tissue are especially to be distinguished: -(i) Vascular cell-unions (the
wood-vessels of the. xylem, the sieve-tubes of the phloem) ; (2) Prosenchymatous
Tissue (the wood-fibres in the xylem, the bast-fibres in the phloem); and
(3) Parenchymatous Tissue (the wood-parenchyma in the xylem, the bast-paren-
chyma in the phloem). The phloem consists of succulent, generally thin-walled
cells ; only the bast-cells, which are often absent, but very frequently massively
developed, are usually greatly thickened (mostly however not lignified but flexible).
The thin-walled succulent cells are either parenchymatous, or they are cambiform
or latticed-cells, or finally sieve-tubes. The xylem-portion of the fibro-vascular
bundle has mostly a strong tendency to thicken its cell-walls, which become hard
and lignified; in vessels and wood-cells with bordered pits the contents disappear,
and they henceforth contain air. Lignified parenchyma is also abundant, but in
some cases no lignifying takes place ; the whole bundle is then soft and succulent,
sometimes traversed only by single thinner strings of lignified vessels and wood-cells,
as in the roots of the radish, tubers of the potato, &c. The elements of the
fibro-vascular bundles, as far as they consist exclusively of procambium, are mainly
prosenchymatous, or at least elongated in the direction of the axis of growth of the
bundle. In open bundles there arise also in the cambium, with the increase of their
thickness, radial rows and layers of horizontally extended cells, by which the later-
formed xylem- and phloem-layers of the bundle become broken up in a fan-like
manner. These horizontal elements mostly assume the character of parenchymatous
cells, and may be generally designated as rays; within the xylem they are called
Xylem-rays, within the phloem Phloem-rays.

The position of the layers of phloem and xylem in the transverse section of a
bundle varies according to the class to which the plant belongs and the organ in
which they are found ; in the open bundle of the stem of Dicotyledons and
Conifers the former lie towards the circumference ^ the xylem facing the axis of
the organ ; between the two lies the cambium-layer (Fig. 93). But a layer of
phloem is sometimes found in addition on the axial side of the xylem, so that
the bundle possesses two phloem-layers, a peripheral, and an axial-layer, e.g. in
Cucurbitaceae, Solanaceae, and Apocynacese. In the closed bundles of Dicotyledons
there occur considerable deviations from the typical position of the tissues ; among

^ See, however, what is said in Book II., at the end of the section on Dicotyledons, on the
formation of tissue in that class.

112

MORPHOLOGV OF TISSUES.

Monocotyledons these are still more conspicuous, especially if the bundle-sheath
of lignified prosenchyma, which often occurs in them, is taken into account (see
Fig. 92). Among Ferns, Lycopodiaceae (with isolated bundles^), and Rhizocarpeae,
the xylem lies in the centre of the bundle, while the phloem forms a soft succulent
sheath round it (Fig. 67, p. 80, and Fig. 94).

According to the relative position of the two principal constituents of fibro-
vascular bundles, they may be classified under two varieties: — (i) Collateral bundles,
in which the phlo6m and xylem run parallel to one another, like the two halves of a

Fig. 93.— Part of a transverse section from the. mature hypocotyledonary portion of the stem (tigellum) o( Ricmtcs com-
■mtmis; r parenchyma of the primary cortex, m of the pith, both belonging- to the fundamental tissue ; between r and b is the
simple bundle-sheath containing starch-grains, also belonging to the fundamental tissue. The fibro-vasculnr bundle consists
of the phloem b, y, the xylem g; t, and the cambium c c, and is therefore an open bundle. The cambium of the bundle
cc\s also continued into the fundamental tissue that lies between it and the neighbouring bundles in the form of interfascicular
cambium cb, formed by subsequent divisions of large parenchymatous cells (secondary meristem). In the phloem are bast-
fibres b b, the soft-bast jt'jv (partly parenchyma and partly sieve-tubes); in the xylem narrow pitted vessels it, broad pitted
vessels g-g^, and between them wood-fibres.

rod bisected lengthwise, as occurs in Phanerogams and Equisetaceae ; and (2)
Concentric, when the xylem is completely enclosed within a layer of phloem, as
in Ferns, the simple bundles of Lycopodiaceae ^, &c. The latter kind are always

^ The bundle in the stem of Lycopodium Chamcecyparissus, Sec, is clearly a union of several
fibro-vascular bundles, such as occurs in the axial cylinder of roots (see Book II,, Lycopodiacese).

^ See Russow,VergleichendeUntersuchungen liber die Leitbiindelkiyptogamen. Petersburg 1872,
p. 159 et seq. The terminology , employed in this paper does not seem to me happy, with the
exception of the term ' collateral bundles.'

THE FIBRO-VASCULAR BUNDLES.

113

closed; while the former may be either closed or open. {Vide infra under the
Fibro-vascular system of roots.)

Every one of its cell-forms may at one time or other be absent from a fibro-
vascular bundle ; bundles may occur without wood-cells, without vessels (very rarely),
without bast-fibres, &c. ; it is only the soft bast (the succulent thin-walled cells of the
phloem) that is scarcely ever absent. All these variations may occur in the same
fibro-vascular bundle in diiferent parts of its length, when this is considerable. The
terminations of the bundles which traverse the stem of Phanerogams are usually
found in the leaves; there, as their thickness decreases, they lose all the elements
of the xylem except one or two spiral vessels, and finally these also ; the extreme
ends of these bundles which traverse

the mesophyll of the leaves often
consist only of long narrow thin-
Wcilled cambiform cells.

If the fibro - vascular bundle is
formed at the very earliest period
within an organ which afterwards
grows rapidly in length, then the
elements which were formed before
the increase in length (the inner-
most vessels and the outermost bast-
cells) are the longest, since they par-
ticipate in the whole increase in
length of the organ ; the elements
developed during the elongation are
shorter; and those are shortest of
all which arise after the increase in
length of the whole organ has been
completed ; this occurs in particular
with the open bundles of Dicotyle-
dons and Conifers.

The development of the elements
of a bundle always begins at single
points in the transverse section,
and extends from them in different
directions; and thus the permanent
cells which arise successively acquire different mature forms. In the open bundles
in the stem of Dicotyledons and Gymnosperms the development usually begins
with the thickening of single bast-cells on the peripheral side of the bundle;
somewhat later single spiral vessels (or annular vessels) arise next the pith;
and while the development of the phloem proceeds centripetally — forming succes-
sively and often alternately bast-fibres, sieve-tubes and parenchyma — annular or
spiral vessels with reticulate thickenings (or both forms), and eventually vessels
with bordered pits alternating with wood-fibres and wood-parenchyma, arise
centrifugally in the xylem (Fig. 95). In Coniferge only prosenchymatous cells with
bordered pits (together with xylem-rays) are subsequently produced, so long as

Fig. 94. — A fourth of a transverse section of one of the lar^e fibro-
vascular bundles in the stem of Ptn-is aqicilina, with a portion of the
surrounding' parenchyma, P, this is filled with starch (in winter) ; s spiral
vessel in the focus of the elliptical transverse section of the bundle, sur-
rounded by thin-walled wood-cells containing starch ; .e £■ the vessels
thickened in a scalariform manner, the structure of which is explained in
Fig^. 29 (p. 28) ; J/ wide sieve-tubes ; between them and the xylem lies, in
the winter, a layer of cells containing starch ; b bast-cells, with thick soft
wall ; sff the bundle-sheath ; between b and sg- is a layer of cells contain-
ing starch.

114

MORPHOLOGY OF TISSUES.

the stem or root grows. In Dicotyledons, on the contrary, after the first year,
a combination of vessels and wood-prosenchyma, often mixed with wood-paren-
chyma, is annually formed'. In trees with annual rings in the wood a periodicity
may be remarked in the development of the cells of the xylem ; and on this depends
its stratification into annual layers. Not unfrequently the phloem-portion also shows
a similar stratification. In the closed bundles of Monocotyledons the order of
development in the first year is similar to that already described. In Fig. 92, for
example, the annular vessel r is first formed in the xylem-portion, then the spiral
vessel Sy then, advancing right and left, the pitted vessels gg, and in the middle
(advancing radially) the narrow pitted vessels. It sometimes occurs {e. g. in
CalodracoTiy according to Nageli) that the formation of vessels advancing right
and left encloses the procambium, which afterwards passes over into latticed cells.

Fig. 95. — Longitudinal section of a fibro-vascular bundle of Riciiius, the transverse section being shown in Fig. 93 ;
r cortical parenchyma; gs bundle-sheath; m parenchyma of the pith; b bast-fibres; /phloem-parenchyma; c cambium, the
row of cells between c and / develops afterwards into a sieve-tube. In the xylem-portion of the bundle the elements are
developed from s successively to t' ; s the first narrow and very long spiral vessel, s' wide spiral vessel, both with a spiral
band which can be unrolled ; / vessel thickened partly in a scalariform, partly in a reticulate manner ; h h' wood-cells ; t pitted
vessel, at q the absorbed septum ; h" h'" wood-cells ; t' pitted vessel, still young ; the pits at first show the outer border ;
afterwards the formation of the inner orifice commences ; a.t £ 1 1' in the wall of the vessel are observed the boundary-lines of
the adjoining cells which have been removed.

In the petiole of Pteris aquilina the development of the xylem begins in the
procambium-bundles, by the formation of some narrow spiral vessels in the foci
of their elliptical section ; scalariform vessels are then formed in the direction of
the longer axis of the ellipse, first centrifugally then centripetally, until a compact
woody mass is produced, elongated in transverse section; round this the pro-
cambium which is still left is transformed into latticed cells, sieve-tubes, and
cambiform tissue, and partly (at the circumference) into bast-fibres (Figs. 94, 96).
The same is the case with most concentric bundles of Cryptogams.

^he Fibro-'vascular System of Roots. Bundles of the kind now described traverse
the stem and branches usually in large numbers (sometimes, as in Palms, the number
is enormous), bending at their upper end into the leaves, where they ramify copiously in

I

THE FIBRO-VASCULAR BUNDLES.

115

the lamina, constituting its venation. Every bundle is completely surrounded by
fundamental tissue both in the stem and in the leaf, and is therefore isolated from
the rest ; the only connection between different bundles takes place at their lower
ends within the stem. The arrangement of the fibro-vascular system in the root is
strikingly different, if we compare with it in the case of stems only the original
structures produced by differentiation from the primary meristem, and not the
thickening-tissues which subsequently arise from the secondary meristem or cambium
(see Sect. 18). The root is usually cylindrical and very slender; and its transverse
section, both in Cryptogams and Phanerogams, shows, beneath the epidermis, a thick
layer of parenchymatous fundamental tissue, surrounding a cylindrical bundle which
traverses the whole length of the root. This bundle may be termed the axial cylinder
or Plerome; it is always sharply separated from the cortical parenchyma by an innermost
layer of the latter, the Bundle-sheath or Plerome-sheath. In most stems also a similar
sheath separates the cortex
from an internal cylinder
of tissue^ containing the
fibro-vascular bundles (Fig.
93, p. 112). It may easily
be recognised by the cha-
racter of the longitudinal
partitions of its cells in a
radial section, which, in
consequence of their pecu-
liar folding, appear, on
transverse section, as if
marked with a black dot
(Fig. 96, s)'.

Within this bundle-
sheath is usually found in
thick roots a large number
of ribbon- shaped vascular
bundles arranged in a ring.
In each vascular bundle the
oldest but smallest vessels
lie on the outside next the
sheath(Fig. 96, />,/>) ; from
them the formation of ves-
sels advances centripetally,
so that the later-formed
vessels which lie nearer the centre are always larger and broader. Between any two
groups of vessels there always lies a bundle of phloem {ph), which not unfrequently
has true bast-fibres on its outer margin. The rest of the axial cylinder consists of paren-
chymatous tissue.

In slenderer roots the number of xylem- and phloem-bundles in the axial cylinder
is commonly reduced to two or three, and in that case the former usually meet
in the axis, so as to form either a broad band which divides the cylinder in two, or
a three-rayed star of vessels. In thicker roots, on the contrary, the inner edges of
the vascular bundles do not usually reach the centre of the cylinder, which in these
cases consists of a mass of parenchymatous tissue or pith, m. Roots which, like most

Fig. 96. — Transverse section of root of Acorns Calamus, showing the axial cylinder
and the surrounding cortical tissue; s bundle-sheath or plerome-sheath; pp the oldest
narrow peripheral vessels ; g- large broader vessels which are always younger ; ph phloem-
bundle ; m central parenchymatous tissue or pith ; pc pericambium.

' See also the end of Sect. 19, and Van Tieghem, Memoire sur les canaux secreteurs; Paris,
1872 ; foot-note, p. 17.

'^ [This is well shown by Van Tieghem, Recherches sur la symt^trie de structure des plantes vas-
culaires, in Ann. des Sci. Nat. 5th ser, 1871, vol. xiii. pi. 3-8.]

I 2

Il6 , MORPHOLOGY OF TISSUES.

primary roots of seedling Dicotyledons and Conifers, are thick in their upper part,
gradually decreasing in thickness below as they lengthen, have no pith in their lower
part, so that the vascular .bundles, separated above by this pith, are in contact in the
lower parts of the root, presenting a star-shaped appearance on transverse section.

The outermost portion of the axial cylinder consists (except, according to Nageli
and Leitgeb, in the case of Equisetum) of a simple layer of cells, which they have termed
Pericambium. It must lie, as will be seen from what has been said, on the inside of the
plerome- or bundle-sheath (Fig. g6,pc). The importance of the pericambium in the
branching of the root and in its increase in thickness will be discussed further on
in Sects. i8 and 23.

In the present state of our knowledge of tissues it is still doubtful whether the axial
cylinder of roots should be regarded as a single fibro-vascular bundle, or as a coalescence
of a number of such bundles, corresponding to the bundles of xylem and phloem. Van
Tieghem (/. c.) adopts the latter view, laying special stress on the fact that in most stems
also a plerome-sheath (which he terms ' membrane protectrice ') separates all the fibro-
vascular bundles together with the pith from the cortex. On this view it is only
necess.iry that the bundles in the stem should be closely crowded together, and their
phloem-portion placed at the side of instead of in front of the xylem-portion, in order
to get an axial cylinder like that of roots. This view is supported by the fact that
in some stems (as in those of Lycopodium, Fig. 100, B. p. 122, and some water-plants like
Hippuris, Hydrilla, &c.) there is an actual axial cylinder similar to that of roots.
Further investigation is, however, required to show how far one is justified in regarding
the entire axial cylinder of roots, like that of the stems just referred to, as the result
of a longitudinal coalescence of true fibro-vascular bundles.

Forms of Cells. I have at present indicated only the relative positions of the
separate forms of tissue in the fibro-vascular bundle; some remarks will follow on the
forms of their cells ; but here also, in consequence of the numerous special modes of
development, reference must be made to the special morphology of separate classes
of plants in Book II. The cell-forms of the fibro-vascular bundles attain their most
perfect and varied development in Dicotyledons ; the forms which occur in them may
therefore be employed as a basis for the critical examination of those of other classes of
plants.

The Xylem-portion of the fibro-vascular bundle of Dicotyledons is composed of
numerous cell-forms, which may be referred, according to Sanio's careful researches,
to three types. He distinguishes (i) Vascular, (2) Prosenchymatous, and (3) Paren-
chymatous.

To the Vascular forms belong the Ducts (pitted or dotted wood-vessels) and the
vascular wood-cells or Trache'ides. This form of cells is characterised by their walls
forming open orifices where two cells of the same kind meet, so that their contents soon
disappear and air takes their place ; the thickenings show a tendency towards the
formation of spiral bands, reticulations, and bordered pits. True Vessels (Figs. 25, 94, pp.
26, 113) arise when the septa of similar cells arranged in longitudinal rows are entirely or
partially absorbed ; and thus long tubes originate, filled with air, and distinguished from the
adjoining wood-cells principally by their greater breadth \ The septa may be horizontal
or more or less oblique ; and in general the mode of their perforation corresponds ;
horizontal walls are often entirely absorbed, or are pierced by large round openings. The
more oblique the septum, the more do the perforations take the form of narrower or
broader parallel fissures ; and the thickening-bands of the septum which remain present
more or less the appearance of rungs of a ladder, while they often combine into a net-
work. These scalariform septa are found, according to Sanio, not only in reticulately
thickened vessels and those with bordered pits, as was previously supposed, but also in
spiral vessels {e.g. in Casuarina, Olea, Vitis), where coils of the spiral band pass imme-

^ See, however, p. 90.

THE FIBRO-VASCULAR BUNDLES.

IJ7

diately into the scalariform markings. The detachment of the spiral band of the first-
formed spiral vessels in stems and petioles which are growing rapidly, appears to
depend solely on the separation of the band frorri the thin quickly-growing wall
common to the vessel and to the adjoining cells. If the band were unrolled
owing to the absorption of this wall, the adjoining cells would necessarily be opened.
If the septa of the separate vascular cells are very oblique, the cells assume a
prosenchymatous appearance, and the more this is the case the more does the vessel
appear completely continuous. In the xylem of Ferns this is often carried to so
great an extent that, after
the cells have been isolated
by maceration, it would be
easy to believe that we
have not cells united into
vessels, but fusiform pros-
enchyma (Fig. 27, p. 27);
but in this case also all kinds
of transitions occur to the
typical scalariform septal
Vessels with prosenchyma-
tous constituents form the
immediate passage to the
vascular wood-cells (Tra-
cheides). If the form of
the cells is such that there
is no difference between
the longitudinal wall and
the septum — which is pos-
sible only in decided pros-
enchyma — then the perfor-
ations between cells which
lie above and cells which lie
beside one another are no
longer different in form ;
rows of cells no longer
give rise to continuous
tubes, but whole masses of
cells (bundles, &c.) are con- p \
nected with one another by
means of open bordered
pits. This occurs in an
especially marked manner
in the tracheides in the
wood of Goniferae {nj'ide
Figs. 23, 24, p. 25). There
is no other difference between these and true vessels ; for vessels with open bordered

Fig. 97. — Tangential longitudinal section through the secondary wood of
AilaiUus glandiUosa; g g vessels ; st xylem-rays cut through transversely ; / vvood-
parenchyina ; t tracheides ; (/"libriform fibres.

' See Dippel in the Amtlichen Bericht der 39. Vers, der Naturforscher u. Aerzte. 1865 (Giessen),
PI. 3, Figs. 7-9. Dippel's observations on Cryptogams and the v^'hole description of the formation
of vessels here given, their passage into tracheides, and especially the fact that the air-conducting
vascular forms have open bordered pits, and are thus in communication even when the paren-
chymatous constituents of a vessel are united not by large openings, but by narrow fissures, &c. (and
are hence not closed cells, as Caspary thinks), compel us to reject Caspary's hypothesis of the
absence of vessels in Cryptogams and many Phanerogams. (See Caspary, Monatsberichte der k.
Akademie der Wissenschaften in Berlin, 1862, p. 448 [Nat. Hist. Rev., 1863, pp. 364-367].)

ii8

MORPHOLOGY OF TISSUES.

pits (ducts) behave in reference to the side-walls exactly like tracheides (Fig. 25, p. 26).
The separate elements of the vessels of Ferns composed of prosenchymatous cells (Fig.
ay, p. 27) may be correctly designated tracheides.

The Prosenchymatous cell-forms of the xylem are always fusiform, very thick-walled
in comparison with their diameter, with usually simple, but sometimes bordered pits,
the pits small, always without a spiral band ; during the repose of vegetation they
contain starch. Next to the middle lamella of their partition-walls there often lies
an unlignified gelatinous thickening-mass which is coloured violet-red by Schultz's
solution, and resembles many bast-fibres. These cells are generally much lunger
than the vascular forms. Sanio distinguishes two forms ;— the simple {Hbriform) and

3=f?^^

Fig. 9S. — 1 ransverse section through the phloem of
a fibro-vascular bundle in the stem of Cucurbita Pepo
( X 550) ; si the septa of the young sieve-tubes with areolae,
the sieve-pores being not yet developed ; / / phloem-
parenchyma ; cc cambium. The bast-fibres are here want-
ing, the whole of the phloem consisting of soft bast.
(Respecting the sieve-tubes see Fig. 74.)

FlC. 99 —Longitudinal section through the phloem of a
fibro-vascular bundle of Ciimrbita Pepo ; three sieve-tubes are
visible, whose septa ^ q are not yet perforated ; the protoplasm
si and ps contained in the sieve-tubes is contracted ; si a
young sieve-plate in the side-wall ; at x and / sieve-pores will be
formed later; z narrow parenchymatous cells between the
sieve-tubes.

the septate fibres ; the latter are distinguished from the former by their cavity
being partitioned by several thin septa, while the common wall of the whole fibre is
thick. These prosenchymatous cell-forms are found in the wood of dicotyledonous
trees and shrubs in the most various intermixture with the vascular elements and the
other forms to be named immediately. Whether libriform fibres occur in Cryptogams
is at least doubtful.

The Parenchymatous cell-forms of the xylem are widely distributed, and especially
abundant when the woody substance of the fibro-vascular bundles attains a considerable
thickness. They arise by transverse division of the cambium-cells before their thicken-
ing commences. The sister-cells show this origin chiefly by the mode in which they are

f

THE FIBRO-VASCULAR BUNDLES. II9

arranged;, when completely developed they are thin- walled, with simple closed pits.
Their contents in winter consist of starch, often associated with chlorophyll, tannin,
and crystals of calcium oxalate. It also happens sometimes that the cambium-cells on
the xylem-side of the bundle become transformed, without transverse division, into
parenchymatous, thin-walled, simply pitted, elongated, succulent cells, which must
also be considered as parenchymatous forms of wood-cells ^ To this last type are also
to be referred the parenchymatous elements in the xylem-portion of the closed fibro-
vascular bundles of Monocotyledons and Cryptogams ; but these thin-walled, mostly
elongated cells do not in this case originate in the cambium (since this, according
to the terms in customary use, is absent from the closed bundles), but immediately
from the procambium of the bundle (Fig. 95, p. 114, near S). Sometimes the wood-
parenchyma derived from the cambium of Dicotyledons (parenchyma of the xylem)
is more strongly developed, while only a few vessels and tracheides are formed :
this occurs in the thick napiform roots of the radish, carrot, beet, and dahlia, and
in potato-tubers. The apparent pith of these organs corresponds, in its origin,
to the wood of a dicotyledonous tree; but the elements of the xylem are not, or
only slightly, lignified ; the succulent contents and the thin soft cell-walls scarcely
give this xylem the appearance of a homologue of the ordinary wood, although there
can be no doubt about the homology.

The Phloem-portion of the fibro-vascular bundles shows, when fully dev'lopfd,
similar cell-forms to the xylem-portion ; the sieve-tubes correspond to the vessels,
the true bast-cells to the libriform fibres, the bast- or phloem-parenchyma to the
wood-parenchyma. When the bast-parenchyma consists of long, narrow, very thin-
walled cells, it has been termed by Nageli Cambiform tissue.

Parenchyma, cambiform tissue, and sieve-tubes may be included in the term Scft-
bast, in opposition to the true bast which is sometimes entirely absent (as in Cucurbita),
but in other cases is very abundantly developed (as in the Jerusalem artichoke, lime,
&c.), and consists of elongated, prosenchymatous, flexible, tough, firm cells, usually with
strongly thickened walls. In Dicotyledons they are generally arranged in bundles, fre-
quently forming layers alternating with soft-bast, as in the grape-vine ; but sometimes,
especially in the later portions of the phloem, which are formed from the cambium,
they occur also as separate fibres, as in the stem and tuber of the potato. The
middle lamella of the partition-wall of two fibres is generally lignified or cuticularised
(resistant and coloured yellow by iodine) when they are closely crowded ; but in
other cases it forms a mucilaginous ' intercellular substance ' in which the cells (in
transverse section) appear imbedded {e.g. the laburnum according to Sanio, Goniferae
and especially in Cryptogams, see Fig. 95, b, p. 1x4). The true bast-fibres of the phloem,
like the libriform fibres of the xylem, may become partitioned by subsequent septa
(as in the vine, occidental plane, horse-chestnut. Pelargonium roseum, Tamarix gallica,
according to Sanio, /. c. p. iii). As the libriform fibres of the xylem are often found
branched after isolation by maceration, so also are the bast-fibres ^.

The forms of cells now described are the ordinary and essential constituents of the
fibro-vascular bundles ; various other forms of tissue occur, however, occasionally, as

' Sanio applies to these cells the term 'Ersatzzellen,'

^ It may not be supei-fluous to point out that many writers very inconveniently also designate
certain cell-forms of the fundamental tissue as bast, when they are thick-walled, elongated, pointed at
the ends, or even branched. In that case the libriform fibres of the xylem must also be called bast ;
and it is evident that the term vi'ould then have no exact scientific meaning. The term bast-cells
has, till recently, been given to hyi^odermal prosencliyma, to the bundle-sheaths of Grasses,
Aroideoe, and Palms, as well as to the cells which we have, in Sect. 14, called trichohlasts, even
when they contain latex, as the laticiferous cells of Euphorbiaceoe. This practice is greatly to
be deprecated, as it must import great uncertainty into a correct interpretation of the different
fonns and systems of tissue.

I20 MORPHOLOGV OF TISSUES.

will have been seen from Sect. 14; and this happens especially in the phloem. Thus
lithocysts, cells containing pigments, receptacles for oil, and other idioblasts like the
rows of cells in Phaseolus M'hich contain tannin; the true laticiferous vessels of/]icho-
riaceae, Gampanulacese, and Lobeliaceae, belong to the phloem, while those of Papa-
yaceae and Aroideae belong to the whole fibro-vascular bundle. In may even occur
that vessels belonging to the xylem contain latex, as in Ipomceaj Jrgemone, Gomphocarpus,
Euphorbia, Carica, Lactuca, and in Gampanulaceae (David, /. c). In the same manner
it has already been shown, in Sect. 14, that the secretion-canals — i.e. intercellular
spaces containing oil, resin, or gum — may occur either in the phloem, or in the
xylem, or in both.

Sect. 17. The Fundamental Tissue. — By this name I designate the tissue
of a plant or of an organ which still remains after the formation and development
of the epidermal tissue and the fibro-vascular bundles. It consists very com-
monly of thin-walled succulent parenchyma filled with assimilated food-materials ;
but not unfrequently it is thick-walled ; sometimes portions assume the form of
strings of strongly lignified prosenchymatous cells. The most various forms of
cells may arise in the fundamental tissue, as in the epidermal system and the fibro-
vascular bundles; a portion may persist in a condition capable of division, while
the surrounding portion passes over into permanent tissue ; or special layers of
the fundamental tissue, long after it has been transformed into permanent tissue,
may again become subject to cell-division, and a generating tissue thus be pro-
duced, out of which originate, not only new fundamental tissue, but also fibro-
vascular bundles {e-g. in Aloineae).

In Thallophytes and many Muscineae the whole mass of tissue, with the
exception of the outermost layer which is often developed as epidermal tissue,
may be considered as fundamental; but in these cases, in consequence of the
absence of fibro-vascular bundles, this distinction has but little practical value.
In Mosses, which have string-like cell-groups in the stem, it may appear doubtful
whether these are to be considered as peculiar forms of fundamental tissue, or
as very rudimentary fibro-vascular bundles. In Vascular plants, on the other hand,
the independence of the fundamental tissue, in contradistinction to the epidermal
system and fibro-vascular bundles, is at once apparent; it fills up the interstices
of the fibro-vascular bundles within the space enclosed by the epidermal tissues.
Where the fibro-vascular bundles are closed and do not increase in thickness
(as in many Ferns), it is frequently the one which occupies the greatest
space; where, on the other hand, closely crowded fibro-vascular bundles produce
large masses of xylem and phloem by the development of cambium (as in the
stems and roots of Conifers and Dicotyledons), the fundamental tissue becomes
less important. The fibro-vascular bundles in stems are usually so arranged as
to separate the fundamental tissue into an inner medullary portion the Ptlh, sur-
rounded by the bundles, and an outer cortical layer or Cortex enveloping them.
Since the bundles are not in contact laterally, or only partially so, there still remain
between them portions of the fundamental tissue which connect the pith with the
cortex, and which are termed Medullary Rays. If the fibro-vascular bundles of
an organ form a solid axial cylinder, as occurs in some stems and in all roots,
the fundamental tissue takes the form of cortex only.

p

THE FUNDAMENTAL TISSUE. 121

(a) The whole course of my description of tissue-systems necessitates the in-
troduction of the idea of a Fundamental Tissue. It has, in fact, long been required ;
it was often necessary, in anatomical descriptions of tissues which are neither epi-
dermal nor fibro-vascular, to distinguish them by some common term. Many writers
employ the term Parenchyma in this sense, in opposition to the fibro-vascular bundles
and the epidermis ; but this usage is not scientific ; the fibro-vascular bundles often
contain parenchyma, and, 'vice 'versa, the fundamental tissue is not always paren-
chymatous but sometimes distinctly prosenchymatous. We have, moreover, to deal
here not with forms of cells, but with the contrast of different systems of tissue,
each of which may contain the most various cell-forms. I must compare some-
what more closely my description and use of tei*ms with those of Nageli. It
might be supposed that Nageli's Protenchyma is synonymous with my fundamental
tissue ; but this is not the case ; the protenchyma of Nageli is a much more
comprehensive idea; everything which I call fundamental tissue is protenchyma,
but all protenchyma is not fundamental tissue. Nageli^ says, for example, that he
would call the primary meristem and all tissues which arise immediately from it
(/. e. only through the medium of secondary meristem, but not of cambium) Prot-
enchyma (or Proten) ; the cambium, on the other hand, and everything which
directly or indirectly originates from it, Epenchyma (or Epen). When Nageli gave
these definitions, he was treating of fibro-vascular bundles; and it is intelligible that
he on this occasion included everything which does not belong to the fibro-vascular
bundles under one common name (Proten). But our business is to give a uniform
description of the various differentiations of plant-tissues; and there is no reason
for bringing only into prominence the contrast between fibro-vascular and non-
fibro-vascular tissues (Epenchyma and Protenchyma), and for considering all other
differentiations as less important ; the protenchyma of Nageli therefore includes, accord-
ing to my use of terms, three kinds of tissue each of equal value with his epenchyma.
The primary meristem is not more opposed to the fibro-vascular (epenchyma) than
to the epidermal and fundamental tissues ; for the three systems of tissue equally
arise by differentiation from it. The term Proten, after distinguishing from it the
primary meristem, might be applied collectively to the epidermal and fundamental
tissues ; but I see no reason for bringing into prominence this contrast alone ; the
differentiation between epidermal and fundamental tissues is as essential as that between
fibro-vascular bundles and fundamental tissue. From all this it follows that epidermal
tissue, fibro-vascular bundles, and fundamental tissue are conceptions of equal value ;
in each we find the most various forms of cells ; and secondary meristem may also
arise in each. In the fibro-vascular bundles the cambium is of this nature; the whole
of the young epidermis is a generating tissue in as accurate a sense as the cambium ;
if this latter forms vessels, xylem, phloem, &c., the former produces hairs, stomata,
prickles, &c. ; the phellogen, belonging to the epidermal system, arises still more de-
cidedly as a generating tissue ; finally, even in the fundamental tissue a portion may
persist for a considerable time as generating tissue, or may subsequently produce such
a tissue, e.g. the meristem of the stems of Draccena, which brings about its increase
in thickness, and thus forms new fibro-vascular bundles'^.

(b) Examples. The relationship of the three systems of tissue may be observed
very readily, and undisturbed by subsequent formations, in the foliage-leaves of Ferns
and of most Phanerogams ; in these the fundamental tissue is generally the prevailing
system, and is developed into different cell-forms. Isolated fibro-vascular bundles,
separated by the fundamental tissue, traverse the petiole, and are distributed through the

^ Beitrage zur wissenschaftlichen Botanik, Heft i, p. 4.

2 Since the publication of the ist edition of this work, the classification of tissues here proposed
has been generally adopted, especially by younger botanists ; as also in the main, with some
deviations in particular points, by Russow, Unters. liber die Leilbiindelkryptogamen, Petersburg 1873.

122

MORPHOLOGY OF TISSUES.

lamina; in the petiole they are generally surrounded by a thin-walled parenchymatous
fundamental tissue with wide elongated cells; this also forms sheath-like envelopes
around the stronger bundles of the lamina, which are conspicuous on its under-side
as the Feins ; but the finer branches, and the finest of all, run through the so-
called Mesophyllf i. e. a peculiar form of the fundamental tissue distinguished

by containing chlorophyll. Not
unfrequently single cells of
the fundamental tissue of the
lamina assume the form of idio-
blasts, e.g. the larger stellate
cells in the leaf of Camellia ja-
ponica, the erect rod-like cells
upon which the stomata of the
leaves of Hakea are, as it were,
supported. All these forms of
tissue are enveloped by the
epidermis, and frequently also
by hypodermal tissue. In the
carpels of Phanerogams there
occurs commonly a more mani-
fold difl'erentiation of the funda-
mental tissue ; I will instance only
the formation of the so-called
' stones ' of Amygdalese. The
stone is the inner layer of the
fundamental tissue of the same
foliar structure of which the outer
layers form the succulent flesh of
the fruit ; the former is scleren-
chymatous, the latter parenchy-
matous and succulent, both being
traversed by fibro-vascular bun-
dles. Equally clear is the struc-
ture in the stems of Ferns, among
which Tree-ferns and Ptei-ij
aquilina are of special interest,
because the fundamental tissue
occurs in them in two quite
different forms. Its preponder-
ating mass consists, e. g. in Pteris
aquilina (Fig. 91, p. 109) of a
thin-walled colourless mucila-
ginous parenchyma, and con-
taining starch in winter ; parallel
to the fibro-vascular bundles
there are also threads or
bands of prosenchymatous thick-
walled dark brown scleren-
chyma; these have nothing in
common with the fibro-vascular bundles, but are only a peculiar form of the funda-
mental tissue, a prosenchymatous form of which often occurs also elsewhere in
Cryptogams. The tendency to a prosenchymatous development of the cells of the
fundamental tissue is also well seen in the stems of Lycopodiaceae. In Selaginella
denticulata (Fig. .100 A) the axial fibro-vascular bundle is surrounded by a very

FIG. 100. — A transverse section of the stem oi Sela^ineUa dentimlata; the
fibro-vascular bundle is not yet fully developed ; the vessels are already ligiii-
fied on both sides, but not yet in the centre; / air-conducting intercellular
spaces in the parenchyma enveloping the bundle ; towards b is the tissue
corresponding to the bundle which bends outwards to the leaf. B transverse
section of the mature stem oi Lycopodium Chantacyparisstis, the axial cylin-
der consists of densely crowded and coalescent fibro-vascular bundles; the
four xylem-portions are quite separated, forming four bands on the transverse
section, between and round which are found the narrower cells of the phloem.
The phloem portions of the four bundles have coalesced ; between each pair
of xylem-bundles is seen a row of wider cells, the latticed cells or sieve-tubes ;
the narrow cells lying on the right and left edge of each xylem-portion are
spiral vessels (also in A). In the thick-walled prosenchymatous fundamental
tissue which envelopes the axial cylinder is seen the dark transverse section
of a slender fibro-vascular bundle which bends outwards to a leaf; it consists
almost exclusively of long spiral vessels (x about 90).

THE FUNDAMENTAL TISSUE.

123

loose parenchyma with large intercellular spaces ; this innermost portion of the funda-
mental tissue is enveloped by a thin-walled tissue without interstices, which is seen
on longitudinal section to be prosenchymatous ; the cells are pointed at both ends,
and penetrate to a considerable distance between one another ; towards the circum-
ference they become gradually narrower and more pointed ; the outermost are dark-
walled, and form the epidermal system which gradually passes over into this fundamental
tissue. In Lycopodium Chamcpcyparissus (B) the axial cylinder, which consists of several
fibro-vascular bundles, is surrounded by a thick layer of greatly thickened prosenchyma ;
in the young stem the cells are similar to those of Selaginella ; but here also the
prosenchymatous cells of the fundamental tissue undergo an enormous thickening. This
prosenchymatous fundamental tissue is in its turn enveloped by a layer of tissue, the cells
of which are thin-walled and not prosenchymatous ; this layer is a descending continua-
tion of the fundamental tissue of the leaves, which envelopes the stem everywhere,
and is itself covered by a clearly developed epidermis.

(c) The Forms of Cells and Tissues in the system of the fundamental tissue have not
yet undergone a comparative and comprehensive investigation \ like those of the fibro-
vascular bundles. Out of the very
scattered material I select the fol-
lowing for the information of the
student.

Irrespectively of many altogether
special phenomena, it is chiefly in
connexion with the true epidermal
tissue on the one hand and the fibro-
vascular bundles on the other hand
that the differentiation of the funda-
mental tissue has to be considered.
Certain forms of this tissue occur as
strengthenings, or at least as accom-
paniments of the epidermal tissue,
and have already been described as
Hypoderma; other masses of tissue
accompany the separate fibro-vascular bundles as partially or entirely closed envelopes or
sheaths; these I term generally Bundle-sheaths or Vascular Bundle- sheaths. In the
same manner the whole remaining internal space of the organ concerned is commonly
filled up by other forms of tissue, which do not, as for the most part the two former
do, occur in the form, of layers, but in masses; these I will designate simple Inter-
mediate Tissue. Each of these combinations may be composed of very different forms
of tissue.

(a) The Hypoderma appears sometimes as a thin-walled succulent watery tissue,
as in the leaves of Tradescantia and Bromeliacese ; in the stems and petioles of Dicotyle-
dons it commonly consists of collenchyma, the cells of which are extended longitudinajly,
narrow, and thickened in the angles by a mass capable of great swelling; or the
hypodermal fundamental tissue is developed in a sclerenchymatous manner, as in the
stem of Pteris aquilina ; or it occurs in the form of thick-walled but flexible fibres,
forming either layers and bundles as in the stem of Equisetaceae and leaves of Goniferae
(Fig. 102), or in long isolated fibres, similar to true bast-fibres, e.g. leaves of Cycadese.
In all these cases the hypodermal cells are extended longitudinally; but when layers
capable of great resistance are also required, the cells often extend in a direction
vertical to the surface of the organ, and, increasing greatly in thickness, form layers of

Fig. ioi. — Transverse section through the undergroiina bicui of
Pteris aquilina; h root-hairs ; beneath the epidermis are strongly thick-
ened brown-walled cells ; q one lying deeper and less strongly thickened ;
a part of the wall is seen in front ; se cells of the deeper layers containing
starch, forming the passage to the inner colourless parenchyma of the
fundamental tissue.

^ Since the pivblication of the 3rd edition of this work, a close investigation of the fundamental
tissue has been undertaken by Russow (/. c.) ; he has however occupied himself chiefly with the
various forms of the bundle-sheaths or ' Critenchyma.'

134

MORPHOLOGV OF TISSUES.

closely packed prisms, as in the pericarp of Marsilea and Pilularia, and the testa of the
seeds of Papilionaceac. Isolated cells of the same kind are sometimes found in the
hypoderma, as accompaniments of the stomata and air-cavities, e.g. in leaves oi Hakea.

0) The Bundle-sheaths consist, in many Monocotyledons (as Palms, Grasses,
Aroideae, &c.) S of elongated very thick-walled cells belonging to the fundamental tissue
which is in close contact with each fibro-vascular bundle, either as a continuous sheath
composed of several layers (Fig. 92, p. no), or only as a partial investment. In Ferns
and allied Cryptogams, on the other hand, a single layer formed of peculiar cells encloses
each separate bundle as a cylinder (Fig. 95, p. 114) ; and the same is the case in a few
Phanerogams, as in the petiole of Menyanthes trifoliata, Hydrocleis Humboldtiy &c. A
layer of fundamental tissue of similar structure envelopes, as we have already seen on
p. 1 15, the axial fibro-vascular cylinder of all roots and of many stems {Lycopodium, Hydril-
leae, Hippuris), and in most stems of Phanerogams with isolated vascular bundles (not
forming an axial cylinder) separates the cortex from the internal tissue which encloses
the vascular bundles and the pith. In Dicotyledons with the bundles arranged in a ring
this layer (Fig. 93, p. 112) surrounds the mass of tissue enclosed by the cortex in such a
manner that the separate fibro-vascular bundles are only in contact with it at their
phloem-portion. If the axial mass of tissue of roots and stems is called plerome, then

O--^

Fig. 102.— Transverse section of tlie acicular leaf of
Pinns Pinaster {X about 50) ; e epidermis ; es hypodennal
bundles of prosenchyma ; sp stomata; h resin-passages;
g b colourless inner tissue enclosing two fibro-vascular
bundles.

Fig. 103.— The left-iiand corner of the previous figure
(x8oo); c outer cuticularised layer of the epidermal cells;
i inner non-cuticularised layer ; c' very strongly thickened
outer wall of the epidermal cells situated at the corner; g i'
hypodermal cells ; g central lamella ; if stratified thickening
mass : / parenchyma containing chlorophyll ; pr its contents
contracted.

this layer, which separates it from the cortex, may conveniently be called the Plerome-
sheath. In the rhizome of many Monocotyledons, as Aroideae, Zingiberaceae, /r/V,
Veratrum, it may be seen with the naked eye. The unilamellar plerome- or bundle-
sheath consists of cells which usually become lignified at an early period, and strongly
resist solution in sulphuric acid ; the radial side-walls and the upper and under septa are
distinguished by a peculiar folding, which, in transverse section, gives the appearance
of a thickening of the walls, or of a black dot. The inner walls which face the bundle,
as well as the radial side-walls, often become greatly thickened, especially in Ferns,
where the thickened walls frequently assume also a deep brown-red colour.

(y) The Intermediate Tissue usually consists of thin-walled succulent parenchyma with
intercellular spaces which are absent from all other forms of tissue ; in the stem, how-
ever, of Lycopodiaceae and of some other Cryptogams it consists of prosenchyma, and
this is then either thin-walled as in Selaginelleae, or thick-walled as in Lycopodieae.

^ Caspary, Jahrb. fiir wiss. Bot, vols. I and IV, p. loi et seq. — Sanio, Bot. Zeitg. 1865, p, 176
et seq. — Pfitzer, Jahrb. fiir wiss. Bot. vol. IV. p. 297. — Van Tieghem, Canaiix secreteurs, in Ann.
des Sci. Nat. 1872, vol. XVI.

THE FUNDAMENTAL TISSUE. 125

When it is parenchymatous, it may be termed simply Fundamental Parenchyma. Two
principal forms of this may be distinguished, which are nevertheless united by transi-
tional forms, "z-'/'x. the colourless parenchyma which occurs in the interior of large
succulent stems and tubers, and in all roots and succulent fruits, and the parenchyma
containing chlorophyll which forms the superficial layers beneath the epidermal tissues
of stems and fruits. In foliage-leaves, when thin and delicate, it fills up the space
between the upper and lower epidermis ; if they are very thick, as in Aloe, it
forms only the superficial layers, while the inner mass of tissue consists of colourless
parenchyma.

The hypodeimal layers, bundle-sheaths, and interm.ediate tissue are the ordinary
and essential constituents of the fundamental system ; but in addition forms of cells
and tissues developed in a peculiar manner occur, and much more frequently than in
the fibro-vascular bundles. Of the former kind are the majority of the cells described
as idioblasts in Sect. 14, isolated cells containing a pigment, tannin, a volatile oil, clusters
of crystals, &c., or large utricular vessels, or isolated scleroblasts, or the branched cells
comprised under the term trichoblasts, such as spicular cells, the hairs in the interior of
Nuphar, in the root of Pilularia, in the petiole and stem of Monsterineae, &c., or finally,
the laticiferous cells of Euphorbiaceae, Moreae, Asclepiadeae, and Apocynaceae. True
laticiferous vessels are, on the other hand, less often found in the fundamental tissue ;
but in the cortex of many Liliaceae they are replaced by utricular vessels (see
Sect. 14). Among the more complicated forms of tissue which occasionally enter into
the composition of the fundamental tissue may be named true (compound) glands, or
more frequently secretion-canals containing gum, resin, a volatile oil, or even latex, as
in AUsma and Rhus. Of very common occurrence are, moreover, groups or layers of
scleroblasts (especially in the cortex of many woody plants and the juicy flesh of pears),
and layers, bundles, or bands of brown-walled sclerenchyma (in Pteris aquil'ina and Tree-
ferns). Attention has already been called to the sclerenchyma of which the stone of
stone-fruit (drupes) consists as a form of fundamental tissue ; the natural contrast to this
is the pulp or flesh of berries and of many stone-fruits.

Sect. 18. The Secondary Increase in Thickness of Stems and Roots ^ —

During the period when the younger portions of stems and branches are still
increasing in length, they are also increasing in girth, the primary meristem
becoming differentiated into other tissues which grow not only in the direction
parallel to the axis of growth, but also in the radial and tangential directions.
At an early period roots attain, immediately behind the growing point, the size which
they retain^until they have ceased growing in length.

This increase in diameter of stems, which accompanies, or even for a
short time outlasts, the growth in length, is frequently occasioned mainly by the
tangential extension of the outer layers of tissue, while that of the pith does not
keep pace with it. The pith will then split and the stem become hollow ; and
this is often carried to such an extent that the substance of the cylinder itself

^ Nageli, Ueber das Wachsthum des Stammes u der Wurzel, in Beitrage zur wiss. Bot. Leipzig
1858, Heft I. — Sanio, Bot. Zeitg. 1865, p. 165, etseq. — Millardet, Sur I'anatomie et le developpement
du corps ligneux dans les genres Yticca et Dracazna, in Mem. de la Societe Imper. des Sci. Nat. de
Cherbourg, vol. XI, 1865. — On abnormal formations of wood in Dicotyledons see Criiger, Bot.
Zeitg. 1850 and 1851. — Nageli, Dickenwachsthum des Stengels u. s. w. bei den Sapindaceen. Munich
1864. — Eichler, Ueber Menispermaceen, in Denkschrift der k. bayer. bot. Gesellschaft zu Regensburg,
1864, vol. V. — Sanio, Bot. Zeitg. 1864, p. 193 et seq. — Askenasy, Botanische morphologische
Studien, Dissertation, Frankfort-a-M. 1872.— On the increase in thickness of roots see Van Tieghem,
Recherches sur la syinetrie, &c., Ann. des Sci. Nat,, 5th ser., vol. XIII.

126 MORPHOLOGY OF TISSUES.

becomes very thin. In this mode are produced the hollow stems of Equisetaceae,
Grasses, many species o( A//mm, and many exogenous plants, such as Umbclliferae,
DipsacuSy Taraxacum^ &c. Septate hollow stems have a diaphragm of firm tissue,
traversed by fibro-vascular bundles, at the insertion of each leaf.

In existing Cryptogams ^ and in most Monocotyledons the root and every
part of the stem retain the diameter which they had attained during their growth
in length ; even at a great age no further increase in thickness takes place. When,
in these plants, the stems of older specimens greatly exceed in diameter those of
seedlings, this is a consequence of the circumstance that the apex of the stem which
is enclosed within the leaf-bud increases in diameter as it lengthens, so that thicker
parts of the stem are constantly emerging from the bud, until a stationary condition
is at length reached, when the stem no longer increases in girth. When this occurs,
as in Palms, Ferns, thick-stemmed Grasses, Aroidese, &c., the stem may attain a very
considerable thickness while still very young, in and below the bud, without having
any power, at a later period, to increase further in size. In the same manner the
roots, when they first emerge from the stem, are thicker the higher they stand
on it ; and in Pandanics it would appear as if roots as thick as the arm were formed
without any secondary increase in thickness.

Very different processes are, on the other hand, the cause of the con-
siderable diameters of the stems and roots of Gymnosperms, Dicotyledons, and
arborescent Liliaceae. After its growth in length is completed every part is
slender, usually only a few millimetres in thickness, rarely so much as i-or 2
centimetres ; but in the course of months and years those parts become much
thicker ; the stem of a seedling Cistus only 2 or 3 mm. in diameter may attain,
after two or three months, a thickness of 2 or 3 cms. ; while in the case of the oak
the increase from the same original thickness may amount to from 40 to 60 cms.
Since the girth increases with the age of each section, the oldest, and therefore the
lowermost sections of the erect stem are the broadest, the successive diameters
decreasing gradually to the summit of the tree, as is well illustrated in the slender
conical stem of pines. In them the stem of the mature plant is a cone standing
on its base, while in Monocotyledons and Cryptogams the cone stands on its apex,
as is well seen in the large climbing Aroidese. In Dicotyledons and Gymnosperms
every part of the stem is at first slender, always becoming thicker at a later period ;
in Monocotyledons and Cryptogams, on the contrary, every part retains the thick-
ness which it had acquired at the close of its growth in length ; the increase
in thickness of the entire stem takes place at the upper end from the growth of
the bud.

This increase in girth which commences only after the close of the growth of
length of the organ, and which then generally lasts during the whole of its life,
may be termed, in contradistinction to that arising from growth of the bud, the
secondary increase in thickness. It is always the result of an inner layer of tissue
remaining in a condition capable of division (merismatic), and continually producing

^ In the Lycopodiaceoe of the Carboniferous period Williamson has recently demonstrated
a secondary increase in thickness. An indication of this is found in the stem of Isoetes. (See
Book II.)

»

SECONDARY INCREASE IN THICKNESS OF STEMS AND ROOTS. 127

new layers concentrically one on another. The mode in which the meristem itself
is formed, and in which the secondary layers of tissue are produced from it, differs
greatly according to the nature of the plant. The numerous special modes of
increase in thickness may be classified under three types, viz.: —

1. Type of the Arborescent Liliacese. The innermost primary cortical layer
produces a meristem, in which new closed fibro-vascular bundles continue to arise,
which anastomose into a network, while the tissue between the bundles developes as
secondary fundamental tissue.

2. Type of normal Gymnosperms and Dicotyledons. The vascular bundles of
the stem are open and arranged in a ring ; the generating tissue which lies between
the phloem and the xylem of each bundle is also continued through the medullary
rays, /. e. those parts of the fundamental tissue which lie between any two adjoining
bundles. Thus arises a continuous ring of meristem, which, according to the old
use of terms, and to distinguish it from the ring of meristem in the preceding type,
is commonly called the Cambium-ring. While in the preceding type new vascular
bundles arise only in the ring of meristem, the meristem- or cambium-ring in
this type crosses the primary vascular bundles, which have their phloem lying
on the outer side, their xylem on the inner side of the cambium. The increase
in thickness consists in new secondary xylem being continually formed out of the
cambium-ring on its inner side, new phloem or secondary cortex on its outer
side.

3. The type of Roots (of Gymnosperms and Dicotyledons). In the axial fibro-
vascular cylinder or plerome-bundle there lie, as has been mentioned, alternate
groups of vessels (xylem) and phloem-bundles side by side; on the inner side of
each of the latter arises a cambium-layer, which produces secondary xylem on its
inner side, phloem on its outer side ; on the outer side of the primary groups of
xylem meristem is also formed, which either produces only secondary funda-
mental tissue, or combines with the cambium-layers already mentioned into a
complete cambium-ring, out of which xylem is again formed on the inside, phloem
on the outside.

Further details may now be given regarding each of these three types, with
an example ; the nomenclature of the more important deviations, especially for the
second type, will be deferred till the end of this section.

(i) The Type of Arborescent LiliacecB is represented in the genera Draccena, Aleiris
(^Calodracon), Yucca, Aloe, Lomatophyllum, and Beaucarnea^. Specimens of the old
stems of these plants are often found in botanical collections so decayed that within
the thin layer of periderm the whole of the parenchymatous fundamental tissue has
completely disappeared, while the fibro-vascular bundles are preserved entire. If
one of these stems is split lengthwise, it is seen that completely isolated bundles
run down the middle, as is the case in all Monocotyledons. Each bundle begins
below at the periphery of the stem; higher up, it bends towards the centre of
the stem, and then again outwards, finally entering a leaf at its upper end. The

^ A fourth type may be furnished by the mode of the increase in thickness in the primeval
Lycopodiacece {vide supra) ; but scarcely anything certain is at present known about it.

128

MORPHOLOGV OF TISSUES.

course of these bundles necessitates that they must cross one another, and form
a loose mass consisting of slender isolated strings, surrounded by a more or less
thick layer of denser woody substance. This woody substance forms a cylinder
which, in decayed stems, is altogether separated from the layer of periderm, and
loosely enveloped by it. The isolated strings in the interior are the primary
vascular bundles which have been formed during the growth in length (properly only
their lower ends or Leaf-traces, since the upper ends bend outwards into the
leaves). The woody cylinder which envelopes them all consists, on the contrary, of

secondary fibro-vascular bundles formed by
the increase in thickness, which are closely
crowded and anastomose copiously with
one another both in the tangential and
radial directions, and thus form, accord-
ing to circumstances, a more or less com-
pact or spongy mass, the true nature of
which it is easy to recognise in Aloe
and Beaucarnea. The course of develop-
ment of these stems is as follows. The
isolated fibro-vascular bundles (which in
old specimens are found in the interior)
are formed in the primary meristem of the
apex of the stem, while the whole remaining
tissue between them passes over into pri-
mary fundamental tissue ; but after con-
siderable time (in Aletris fragrans it takes
place about 4 or 5 cm., in Draccena reflexa
as much as 17 to 20 cm. below the apex
of the stem) a fresh formation of (second-
ary) meristem begins in one of the cell-
layers of the fundamental tissue which
immediately surround the outermost fibro-
vascular bundles. The permanent cells
concerned in it divide repeatedly by tangen-
tial and subsequently sometimes by radial
walls ; and there arises (seen in transverse
section) a ring of meristem (Fig. 104, x),
the cells of which are arranged in radial
rows. In this meristem new fibro-vascular bundles are produced ; one, two, or more
adjoining cells (on the transverse section) dividing repeatedly by longitudinal walls
in various positions. Out of the procambium-bundles which arise in this manner
the fibro-vascular bundles proceed immediately, the procambium-cells being trans-
formed into fibro-vascular tissue^; the intermediate meristem passes over likewise

Fig. 104.. — Part of the transverse section of a stem oi Dra-
caua (probably reflexa) about 13 mm. thick and 1 metre high,
about 20 cm. below the summit, e epidermis ; k cork (peri-
derm) ; r cortical portion of the fundamental tissue ; /' transverse
section of a fibro-vascular bundle, bendinar out to a leaf; m the
primary fundamental tissue (pith) ; g the primary vascular
bundles ; x the ring of meristem in which very young fibro-
vascular bundles are to be seen, while the older ones g have
already partially or entirely passed out of it, its lower part
becoming transformed into fundamental tissue st arranged
in radial rows. '

^ It appears, however, that the thick-walled lignified cells on the outside of such a bundle
do not belong to it, but to the secondary fundamental tissue, and therefore represent only a
sclerenchymatous bundle-sheath, while the bundles enveloped by them are themselves very slender.

SECONDARY INCREASE IN THICKNESS OF STEMS AND ROOTS.

T29

into permanent tissue, and indeed into thick-walled parenchyma, which now forms
the secondary fundamental tissue between the secondary fibro- vascular bundles.
Since the cells of the thickening-ring which face inwards pass over in centrifugal
succession into permanent tissue, while the outermost divide repeatedly, the whole
ring continually moves centrifugally as it increases in diameter, and leaves behind
new bundles and parenchymatous cells. In Fucca Millardet found the origin of
the ring of meristem (thickening-ring) as litde as 3 mm. below the apex of the
stem ; in Calodracon {Cordyline) Jacquini, the meristem-ring is derived immediately,
according to Nageli, from the primary meristem of the apex of the stem, this layer
remaining in a condition capable of
division while the primary vascular bun-
dles and fundamental tissue are being
differentiated out of the primary meri-
stem.

(2) The Type of tiormal Gymnosperms
and Dicotyledons may be made clear by
a reference to Fig. 105, which — with
the exceptions of a few points of sub-
ordinate importance, such as the substi-
tution of six fibro-vascular bundles for
eight — represents in a simple diagram-
matic manner the phenomena connected
with the growth in thickness of the
hypocotyledonary portion of the stem
(tigellum) of Ricinus conununis. We
may commence with the period when, in
the seedling stem, the fibro-vascular bun-
dles— which are prolongations down-
wards of those bundles which bend out-
wards above into the first leaves or
cotyledons — have become clearly differ-
entiated. They lie, when seen in trans-
verse section (Fig. 105 ^4), in a ring, and
run parallel to one another and to the
surface of the stem. The ring of fibro-
vascular bundles divides the primary
fundamental tissue into pith {M) and
cortex {R\ which, however, still retain their connection by broad bands of funda-
mental tissue lying between the bundles, the Medullary Rays. Each of the bundles
consists of an outer phloem-portion (/>) add an inner xylem-portion {x\ between
which lies a layer of cambium. The next 'change consists in the bands of cam-
bium belonging to the bundles uniting into a' continuous ring (Fig. 105, B\
meristem being formed between each pair of adjacent bundles by divisions in
the corresponding layer of the medullary rays, as is more exactly shown in Fig. 93
(p. 112), which relates to this stage of development. Although there is no essential
difference between this portion of the cambium-ring and that which lies in the

K

FIG. 105.— Diagrammatic representation of the secondary increase
in thickness of a normal dicotyledonous stem.

130 MORPHOLOGY OF TISSUES.

bundles themselves, they may conveniently be distinguished, the latter as Fascicular
Cambium (B, /c), the former as Interfascicular Cambium {B, ic). It may further
be mentioned that three small groups of bast-fibres b b b lie in the phloem-portion
of each bundle.

The activity of the whole continuous cambium-ring now commences. The
ring consists of rows of cells arranged radially; in each of these rows the cells which
lie on the inner side are the origin of the secondary xylem, those which lie on the
outer side of the secondary phloem ; while a middle layer of the cambial cells always
remains capable of division, and thus maintains the continued formation of secondary
xylem and phloem. In this manner the seedling stem increases considerably, in
the course of a few weeks, both in diameter and rigidity, in consequence of the
formation of a cylinder of secondary wood (xylem) surrounded by a layer of
secondary cortex (phloem), understanding by the latter term all the layers of phloem
developed out of the cambium-ring. Fig. 105, C is the transverse section of the
tigellum of the seedling plant which has already increased considerably in thick-
ness; by the deposition of secondary phloem, the primary phloem (i?) has been
compelled to grow tangentially ; its cells, as well as those of the epidermis,
stretch in that direction, and become divided by radial longitudinal walls. At
this time it is still possible to refer back the various parts of the secondary
tissue to their origin ; the original xylem-portions of the bundles (x), which
were developed before the increase in thickness, can still be recognised; they
appear as projections of the woody substance into the pith, and are collectively
comprised under the term Medullary Sheath x x x. It is somewhat more difficult
to make out the original phloem-portions of the bundles ; but we are assisted by the
position of the groups of true bast-fibres b b b already mentioned ; they still remain
unchanged, but are pushed much further asunder, because the intermediate soft bast
has extended in the tangential direction, in consequence of the pressure of the
secondary tissue from within. The radial diameter of each phloem-portion p
has also increased, secondary layers, formed out of the fascicular cambium, having
been added to the primary phloem from within. The primary xylem-portion x
of each bundle is separated from its phloem-portion / by a thick layer of
secondary xylem fh, which has been developed from the fascicular cambium.
These portions may be distinguished as Fascicular Xylem from the Interfascicular
Xylem (Fig. 105, C, ifli) formed out of the interfascicular cambium; a layer oi Inter-
fascicular Phloem ifp on the outside corresponds to each of these latter. The entire
mass of secondary xylem consists therefore of fascicular and interfascicular xylem,
the entire mass of secondary phloem similarly of fascicular and interfascicular
portions. The primary xylem is wanting on the inner or medullary side of the
interfascicular xylem, the primary phloem in the interfascicular phloem.

With reference to the elementary constituents of the secondary or thickening-
tissues, it is first to be noted that in this respect the fascicular and interfascicular
formations are alike. The whole mass of secondary xylem, with the exception
of the medullary rays or ' silver- grain,' consists, in Conifers of tracheides with bor-
dered pits (Fig. 23, p. 25); in Dicotyledons of wood-prosenchyma, wood-parenchyma,
and pitted vessels formed from short cells. In the secondary xylem no annular,
spiral, or reticulated vessels are formed ; these arise only in the original xylem-portions

SECONDARY INCREASE IN THICKNESS OF STEMS AND ROOTS. 13I

of the bundles, and are therefore subsequently found only in the medullary sheath (in
Gymnosperms also) ; and since these were formed during the growth in length,
they are considerably longer than the secondary elements. The secondary phloem
consists of phloem-parenchyma, sieve-tubes, and sometimes of true bast-fibres ;
but the latter are often wanting. The mode in which these elements combine to
form the secondary tissues varies greatly in different plants, and is, at present, of
subordinate importance.

The elements of the secondary xylem of which we have now spoken, as well as
those of the secondary phloem, are, like those of the primary xylem and phloem,
elongated in the direction of the axis of growth. But elements also occur in the
thickening tissue placed horizontally {z. e. at right angles to the axis of growth) and
radially, out of which the radiate tissue is composed. In Fig. 105, C (p. 129) the
secondary xylem and phloem are represented as crossed in a radial direction by dark
lines, some of which pass through all the secondary layers, while others begin only
in the secondary xylem and end in the secondary phloem ; the former are first formed,
the latter subsequently, and constantly in increasing numbers. Each of these dark
lines in the figure represents a ray of parenchymatous cells placed horizontally; each
of the rays runs, as will be seen, uninterruptedly from the xylem through the cambium
into the secondary phloem ; as long as it runs through the xylem it is called a
Xylem-ray ^; its continuation into the secondary cortex is a Phloem-ray. These rays
split up, as it were, the secondary tissue in the longitudinal and radial directions into
sections which have a wedge-shaped form when cut through horizontally, and which
increase in number as the cambium-ring increases in size. Each separate ray does
not, however, by any means extend through the whole length (in the direction of
growth) of the secondary tissue ; but has generally only an inconsiderable height.
If a thick stem is split longitudinally, the rays have the appearance, in many close
woods, of glistening bands (the ' Silver-grain '), traversing the prosenchymatous
woody tissue in a radial direction ; in a tangential section they have the appearance
of wedges driven into the mass of the wood. Each ray is sharp-edged above and
below {i. e. in the direction of the axis of growth), thin, but usually thickened
in the middle (in reference to its height), and sometimes composed of a number
of layers of cells, as is shown in Fig. 97, p. 117. This and the position of the
rays causes the elements of the secondary xylem and phloem which are elongated
longitudinally to be more or less bent in different directions. If the rays were
imagined to be altogether removed, the entire thickening-tissue would then consist of
bundles penetrated by empty meshes, and anastomosing tangentially above and
below them. A very good idea of this structure may be obtained by examining a
piece of ordinary lime-bast, or stems, such as the cabbage, in which the soft
medullary rays have decayed.

Just as the elements of the secondary xylem and phloem which are elongated

^ The term Medullary Ray should be avoided in reference to these ; since most of the rays are
neither connected with the pith, nor have any of its properties. [It is difficult however to substitute
any other expression, although the term is used in a somewhat conventional sense. It is convenient
to distinguish as Primary Medullary Rays those which are connected with the pith, and as Secondary
Medullary Rays those which only commence in the secondary xylem.]

K 2

132 MORPHOLOGY OF TISSUES.

longitudinally are formed out of longitudinally elongated cambial cells, so the rays
are formed out of cambial cells lying in rows and elongated in the radial direction.
In the xylem the cells of the rays are usually lignified, and sometimes have very
hard walls, as in the copper-beech, where they alone remain after the decay of the
wood, and then have completely the appearance of constituents of the wood. At
other times they continue thin-walled, unlignified, and different from the true
woody tissue. The phloem-rays are usually thin-walled, parenchymatous, and at
their outer end, where they pass through the primary phloem, they are frequently
compelled, by the increase in size of the stem, to extend tangentially, when they
become divided by radial longitudinal walls ; the phloem-bundles which lie between
them becoming thus pushed further and further apart (Fig. 105, C).

All the most essential points have now been spoken of in reference to the theory
of the increase in thickness of the stems of Gymnosperms and Dicotyledons. But
the formation of secondary xylem and phloem from the cambium, which we have
at present followed only in the early period of growth, continues, in perennial
plants, throughout their whole growth ; the wood and the secondary phloem are
therefore constantly increasing in thickness ; but the increase of the xylem is
generally considerably greater than that of the phloem. At an earlier or later period
in the thickening of the stem periderm is formed in the primary cortex, which
sometimes, as in the beech, copper-beech, birch, and cork-oak, follows the increase
in size of the stem, and surrounds it as a continuous envelope of cork. But usually
Bark is formed ; t. e. lamellae of cork cut out flakes oF 1;he primary, and subse-
quently also of the secondary phloem, which then dry up, and, accumulating on the
surface, as in the pine, oak, &c., form the bark, or fall ofl" periodically, as in the
plane. The whole of the primary cortex (phloem) is then replaced by bark ; with
the exception of the pith and medullary sheath, the stem consists then entirely of
masses of secondary tissue which have all originated in the cambium ; but even of
the secondary phloem only the inner younger layer usually retains its vitality, the
outer layers uniting sooner or later, in the production of bark.

In tropical woody plants when several years old, the additions to the wood
formed in each successive year are not generally distinguishable on a transverse
or longitudinal section ; the entire mass of the wood is homogeneous. In woody
plants, on the contrary, that grow in a climate in which the periods of growth
are interrupted by a cold or wet season, as is the case with us, the annual additions
to the wood may be recognised as sharply separated concentric layers, known as
Annual Rings. Their sharp separation from one another is caused by the Vernal
Wood being of a looser texture than the Autumnal Wood. Every annual ring
consists, therefore, on the inside of looser, on the outside of denser wood which
pass into one another insensibly without any sharp line of demarcation; while,
on the contrary, the dense autumnal wood of the preceding ring is very sharply
separated from the looser vernal wood of the succeeding ring. In Coniferse the
distinction between the looser vernal and the denser autumnal wood consists only
in the trachei'des of the former having larger transverse diameters, while in the
later wood, and especially that formed at the end of the period of growth, they
are narrower. The same mass of vernal wood includes therefore a larger amount
of cell-cavity, and is consequently looser, than the autumnal wood. The walls

SECONDARY INCREASE IN THICKNESS OF STEMS AND ROOTS. 1 33

of the later-formed wood-cells being thicker tends to the same result. In Dico-
tyledons, in addition to this difference, the vernal wood contains a larger number
of large pitted vessels than that formed in the autumn, where, indeed, they may be
altogether wanting.

The cause of the formation of annual rings lies, as I was the first to suggest,
and as has been proved experimentally by de Vries, in the pressure, changing with
the time of year, which the cortex, and especially the bark, exercises on the
cambium and the young wood. This will be shown more in detail in Book III.
Sect. 16.

In most woods, but not all, whether the annual rings are conspicuous or not,
there is a distinction, when the stem has attained a sufficient thickness, between the
so-called Duramen and Alburnum. The Duramen consists of the older and inner
layers of wood, and is of a dark-brown, yellow, red, or even black colour, and of a
firmer harder texture ; the Alburnum, consisting of the last annual additions, forms
a light-coloured or white, softer and more spongy tissue round this nucleus. The
inner layers of alburnum are gradually transformed into duramen, the cambium
depositing new layers of wood on the outside, and the cell-walls assuming a darker
colour, from saturation with resin, colouring substances, &c. The distinction
between alburnum and duramen is very clear and well-marked in the oak, walnut,
cherry, elm, 'acacia' [Robmia Pseud-Acacia), lignum-vitse {Guaiacum officinale), log-
wood {Hcematoxylon ca?}ipechianu?ii), brazil-wood [Cccsalpiftia echinala), &c. On the
other hand no duramen is found in the silver fir, Scotch fir, lime, or Pauloivnia
imperialis, &c.

(3) The Secondary Increase in Thickness of Roots in Gymnosperms and Dicoty-
ledons differs from that which takes place in the stem only in the first production of
the cambium-ring ; and this difference consists merely in the fact that in roots
the phloem does not lie outside the xylem-bundles (in the radial direction), but is
disposed alternately with it on the periphery of the axial fibro-vascular cylinder.
When the cambium is once formed, it behaves in precisely the same manner as
it does in the stem.

We may first investigate the process in a root which is from the first moder-
ately thick, and has a true pith enclosed within its axial fibro-vascular cylinder
or plerome, as in many primary roots, at least in their upper thicker portion.
Fig. 10^ represents a transverse section through the upper part of a root of the
scarlet-runner {Phaseolus muliiflorus) at the time when the cambium has been formed
and has commenced its work; s is the plerome- or bundle-sheath belonging to the
fundamental tissue, which immediately encloses the pericambium or outermost layer
of the plerome p c. Within this may be observed the four primary xylem-bundles,
consisting of vessels of which the oldest and smallest p lie on the outside, while the
younger and larger g g are next to the pith M. In the interval between each
pair of xylem-bundles lies a broad phloem-bundle b, b, with true bast-fibres.
The parts now described have been formed during the growth in length ; this
has however, at the time when the section was taken, ceased for three or four
days in this part of the root and increase in thickness has taken its place. The
cells lying on the inner side of each phloem-bundle first divide repeatedly by
tangential longitudinal walls ; a layer of cambium {c) is thus formed behind each

134

MORPHOLOGY OF TISSUES.

i

phloem-bundle, consisting of radial rows of cells. Some of the innermost cambial
cells g^g'^ have already enlarged considerably, to form the first vessels of the secondary
xylem; these are arranged, in P/iaseo/us, in an annular zone continuous with the
inner youngest vessels of the primary xylem-bundles, a structure which is seen
nowhere else. Between and in front of these vessels other cambial cells develope
into prosenchymatous and parenchymatous wood-cells. Since therefore the cam-
bium-layer behind each phloem-bundle is continually producing vessels, wood-paren-
chyma, and wood-prosenchyma on its inner side, a four-rayed cross of woody tissue
is formed, as is shown in Fig. icy (less strongly magnified). The four arms of this
cross correspond in their position — as will be seen from what has been said, and
from a comparison with Fig. io6 — to the four primary phloem bundles d, h, b, b.

Fig. io6. — Transverse section of the primary root of a seedling of Phaseolus miilHfloyns, from the upper thicker
part, at the time when the first leaves of the seedling have emerged above the ground and the first lateral roots have
already been formed. (The pericambium pc has been drawn too thick-walled.)

On the outer side of each cambium-layer, but within the primary phloem, a zone of
new true bast-fibres (Fig. 107 <5') has already been formed in the secondary phloem.
Between the four arms of the secondary xylem and secondary phloem belonging
to it, lie four broad rays of tissue, consisting of large parenchymatous cells elongated
in the horizontal and radial directions. These four bands lie on the same radii as
the four primary xylem-bundles (Fig. 106, />) ; in front of each of these latter a
meristem-layer has been formed, which however has not produced xylem and
phloem like the cambium behind the phloem-bundles, but only radiate parenchyma
to tlie extent required by the growth of the adjacent masses of secondary xylem
and phloem. Fig. 107 further shows the tissue which surrounds the primary and
secondary phloem in the act of division ; radial rows of cells have been produced
here also, and form a layer of cork or periderm k on the outside of the cortex.

SECONDARY INCREASE IN THICKNESS OF STEMS AND ROOTS. 1 35

Precisely similar to the growth in thickness in Phaseolus is that of the primary
roots containing pith of seedlings of Cucurbita Fepo, Convolvulus tricolor, Cereus,
Clusia, &c. ; only that in these cases the secondary xylem does not coalesce with
the primary xylem, but remains quite distinct, so that the alternation of the primary
and secondary elements of the xylem is at once evident \ In slender roots which
have no pith, and where the primary xylem-bundles meet in the centre, cambium
is also formed on the inner side of the primary phloem, and the secondary fibro-
vascular masses form, therefore, in this case also two, three, or more groups,
which originate in the intervals between the primary xylem-bundles, as is very
clearly seen in Tropceolum, but project much further outwardly. When there are two
of these primary xylem-bundles, as in Beta, Tropceolu?n, Taxus, and Umbelliferae,
they form a vascular band dividing the axial cylinder in half; when three or more,
as in Pisum, a three- or four-rayed star.

In the cases hitherto considered, the secondary fibro-vascular tissues (consisting
of xylem and the phloem belonging to it) remain separated into two, three, four,
or more masses, nothing but parenchymatous fun-
damental tissue being formed between them and
in front of the primary xylem, as in Phaseolus. In
other cases, on the contrary, true cambium is formed
in front of the primary xylem, producing xylem on
the inside, phloem on the outside \ and thus is
formed a compact cylinder of secondary xylem.,
surrounded by a continuous layer of secondary
phloem, as in Taxus, Pinus, Beta, &c., and as is
shown, in the case of the stem, in Fig. 105.

The secondary xylem of roots very frequently
consists for the most part of succulent wood-
parenchyma, in which the few vessels, surrounded fig. lOj.-Xransverse section of the upper
i^ J ^ ' part of the primary root of an older plant of

by a few lignified cells, are collected into isolated PhaseoiusmHMjiorus^bbbbpnm^rypUoom-

J o ' bundles; I?'* bast-fibres in the secondary phloem;

groups. This occurs especially in the cultivated >& pericambium.
beet-root, the cultivated carrot, and in Althcea officinalis, Rheum rhaponticum, Atropa
Belladonna^ Cocculus palmatus. Inula Helenium, &c. The secondary phloem of roots
also has a tendency towards an abundant formation of parenchyma, with a diminished
development of bast. The secondary tissues of roots so completely resemble, on the
other hand, those of stems in the formation of xylem- and phloem-rays (medullary
rays) and in other respects, that it is not always possible, especially when the
wood of the root is not strongly lignified, to distinguish it from the wood of the
stem without examining the central axis, when its real nature may always be
determined from the primary xylem-bundles and the occasional absence of the
pith.

From the great numbers of species belonging to the classes Gymnosperms and
Dicotyledons, and from their extremely different adaptations to various vital conditions,
it is not surprising that in the processes connected with the increase in size of the
stem there should be a great variety of deviations from the normal types described

Very instructive figures of this are given by Van Tieghem, /. c.

136 MORPHOLOGY OF TISSUES.

above. An exhaustive description of these deviations^ would far exceed our space,
and would lead us to great difficulties, since there is at present no portion of the
field of botany so unworked as the anatomy of plants properly so called. What
follows will only serve to direct the attention of the student to some of the most
striking deviations from the processes described under the head of the second type, to
which some reference will again be made at the conclusion of the account of the
structure of Dicotyledons in Book II.

(a) The variation is only slight when the increase in thickness is not uniformly con-
centric, but is altogether suspended on one side, as in the roots of Polygala Senega, or
is much stronger on one side than elsewhere, as in the root of Ononis spinosa and in
many stems.

(b) The cambium is sometimes formed only in the fibro-vascular bundles, which in
consequence alone produce secondary xylem and phloem, while the bridging over of the
medullary rays by cambium does not take place, and interfascicular secondary fibro-
vascular masses are therefore not formed. The fundamental parenchyma which lies
between the bundles increases like them slowly in thickness by the extension of its
cells and their occasional division, as for example in the stem of Cucurbita, where
the bundles are also singular in possessing a layer of phloem on the inside of the
xylem, a peculiarity belonging also to Solanaceae and Apocynaceae. But in old stems
this ends in an over-bridging of the medullary rays by meristem.

(c) The processes deviate much more widely from the normal type in those cases
where the activity of the original cambium soon ceases, and new bundles are produced
out of a meristem in the surrounding cortex ; these bundles first increasing in thickness
by cambium, but then ceasing to grow, when a new ring of bundles is formed in a new
meristem ; so that at length the wood consists, not of a compact mass, but of concentric
layers of isolated fibro-vascular bundles. This process presents a certain resemblance
to what takes place in the Aloinese (Type i). The rather frequent instances of this
mode may, according to Niigeli and Eichler, be classified into two groups, according as
the ring of meristem which produces the new bundles is formed in the primary or
secondary phloem.

Each successive ring of bundles originates in the primary cortex in Gnetum (Gymno-
sperms), in Menispermaceae, and in Rhynchosia scandens among Leguminosae. Each of
the secondary bundles has on its outer side a border of phloem, which in Gnetum even
forms true bast.

The new rings of fibro-vascular bundles originate successively in the secondary
cortex, according to Eichler in Dilleniaceae, many Leguminosae {e.g. Bauhinia, Caulo-
tretus), some Polygalaceae {Securidaca and Comesperma), Ampelideae {Cissus according to
Criiger), and Phytolaccaceae.

(d) 'The stem of Malpighiaceae and Bignoniaceae ' (Eichler I.e.) 'at a later period
often agrees so far with the foregoing, that the wood appears as if broken up by
layers of a parenchyma resembling cortical tissue into a number of isolated portions,
of which only the outer ones are merismatic. The history of development is,
however, quite different. It does not consist, as in the previous cases, of an actual
new formation of vascular bundles outside those already in existence, but in a subse-
quent vigorous cell-multiphcation within the medullary rays and the layers of wood-
parenchyma which intersect the fibro-vascular bundles. All the portions of wood
which are by this means more or less driven apart and displaced are therefore only
portions of the primary ring of fibro-vascular bundles ; it is obvious that only the outer
ones are capable of increase, since cambium is found in them only.' The wood
however does not always break up longitudinally into pieces that are actually separated ;
sometimes it is only, as seen on transverse section, divided into lobes, or shows deep
indentations, which appear from without as deep furrows, running usually in a spiral

^ [Oliver has collected the bibliography of the stem in Dicotyledons in the Nat. Hist. Review
1862, pp. 298-329, and 1863, pp. 251-258.]

SECONDARY INCREASE IN THICKNESS OF STEMS AND ROOTS. 1 37

direction. These lobes, separated by radial lamellae of tissue resembling cortex,
may again be divided in the same manner into smaller lobes; as occurs in some
Malpighiaceae like Heteropterys and Bannisteria. In some species of Bignonia the wood,
as seen in transverse section, forms a four-armed cross, the arms of which are
separated by very broad soft masses of tissue resembling medullary rays, which pass
outwards into the cortex surrounding the stem (see Fig. 107). In this order Tecoma
radicans has also the peculiarity, noticed by Sanio, that a new ring of wood is
formed in the pith, altogether separated from the medullary sheath of the ordinary
ring ; this new ring has on its inner side a secondary layer of phloem, and is increased
by cambium.

(e) Some of the most peculiar modes in which the wood is formed are found in many
climbing Sapindaceae. The abnormalities here described are, in fact, met with chiefly
among lianes, i.e. woody cHmbing and twining plants growing in tropical forests;
although there are typical liane-stems which do not climb. In the abnormal Sapindaceae,
especially the genus Serjania, a transverse section shows three, four, five, or more portions
of wood completely separated from one another, of which the central one is usually
much the most strongly developed. Each of these masses of wood presents the
structure of a normal dicotyledonous stem ; medullary rays proceed from its central pith
towards its circumference ; each is surrounded by a narrow secondary cortex, by which
they are, however, united into a whole. Looked at from without, the more slender
outer portions of wood have the appearance of cushions, which project below out of
the central larger one, and coalesce again with it above. The separate masses, which
on a transverse section seems to be completely independent, are therefore only pro-
jections or outgrowths of the originally homogenous stem. According to Nageli this
abnormality is caused, at the very origin of the fibro-vascular bundles, by their not
being arranged in a ring, so that a single cambium-ring cannot unite them all ^.

(f) In the cases hitherto described the deviation from the typical processes is
caused essentially by abnormalities in the origin of the cambium-ring, or by its sub-
sequent behaviour, or finally by repeated fresh formation of fibro-vascular bundles in a
secondary meristem. The case is diff'erent in Piperaceae, Begoniaceae, Nyctagineae, and
Amaranthaceae. An al?normality is here brought about at a very early period in the
growth of length of the stem, of such a nature that, in addition to the leaf-trace-bundles
which bend above into the leaves, vascular bundles also arise in the stem which do
not pass out into the leaves, i.e. 'cauline bundles.' On a transverse section even
of a very young intern ode a number of fibro-vascular bundles are seen dispersed
through the fundamental tissue ; and the alternative now presents itself whether the
leaf-trace- or the cauline bundles shall be united by cambium and hence become
thicker, or whether the formation of cambium shall be altogether suppressed. Sanio
states that the latter occurs among Piperaceae, in the genus Peperomia, where the leaf-
trace-bundles which constitute an outer circle are not united by a ring of cambium,
any more than the inner ones which do not bend out into the leaves. In Cha^vica,
on the contrary, this takes place in the outer ring of leaf-trace-bundles, while in the
pith which is enclosed by them the cauline bundles remain isolated and without any
secondary increase in thickness. The structure is the same in Begoniaceae. That of
Nyctagineae, on the contrary, to a certain extent varies from this; the thick leaf-
trace-bundles, ascending through the middle of the fundamental tissue, remain isolated,
while a wider outer ring of cauline bundles forms a ring of wood by the activity of its
cambium.

Sect. 19. The Primary Meristem and the Apical Cell^. — At the growing
ends of shoots, leaves, and roots, the forms of tissue hitherto described do not

^ Nageli (/. c.) describes the very complicated processes more fully.

2 Nageli, Die neueren Algensysteme. Neuenburg 1847.— Cramer in Pflanzenphysiol. Unter-
suchungen, Heft III. p. 21, Zurich. — Pringsheim, Jahrb. fiir wissen. Bot. vol. III. p. 484. — Kny,

138 MORPHOLOGY OF TISSUES.

exist; here is found a homogenous tissue, the cells of which are all capable of
division, rich in protoplasm, with thin and smooth walls, and containing no coarse
granules. This tissue is termed Primary Men's iem ; it is a meristem because all
the cells are capable of division, and primary because it presents the condition out
of which the different permanent forms of tissue are successively formed by differen-
tiation (' Proto-meristem ' might perhaps, therefore, be a better term). If the general
structure of the plant is simple, as in Algae and Characeae, the cell-forms arising
from the primary meristem only differ slightly from one another. If the plant belongs
to a higher type, as in Vascular Cryptogams and Phanerogams, layers of tissue of
a different character first originate from the undifferentiated primary meristem which
proceeds from the growing apex, and in these the different cell-forms of the
epidermal and fundamental tissues, as well as of the fibro-vascular bundles, finally
arise by further development, at a still greater distance from the primary meristem.
The differentiation takes place so gradually, and at such different periods in the
various layers of the tissue, that it is impossible at any one time to assign a definite
lower limit to the primary meristem. As growth proceeds at the end of shoots, leaves,
and roots, lower portions of the primary meristem become gradually transformed
into permanent tissue; but the primary, meristem is always again renewed by the
production of new cells close to the apex. Nevertheless whole organs, the apical
growth of which soon ceases, may at first consist entirely of primary meristem,
which finally passes over altogether into permanent tissue, so that no primary
meristem is left. Examples of this are furnished by the development of the
sporogonium of Muscineae, of the sporangia of Ferns, and even of most leaves
and fruits of Phanerogams.

The terminal portion of an organ with permanent apical growth, consisting
entirely of primary meristem, is termed the Growing Point or ' Punctum Vege-
tationis;' not unfrequently (but by no means always) it* projects as a conical
elongation, and is in this case distinguished as the Vegetative Cone or Cone of
Growth.

The production and renewal of the primary meristem commence with the
cells lying at the apex of the growing point ; and, by the manner in which this
happens, two extreme cases may be distinguished, which are however united by
transitional forms. In the one case, the usual one with Cryptogams, though not
without exception, the whole of the cells of the primary meristem trace their origin
back to a single mother-cell lying at the apex of the growing point and called
the Apical Cell. In some Cryptogams, on the other hand, and in Phanerogams,
there is no single apical cell of this character; even when a cell lies at the

ditto, vol. IV. p. 64. — Hanstein, ditto, vol. IV. p. 238. — Geyler, ditto, vol, IV. p. 481. — Miiller,
ditto, vol. V. p. 247. — Rees, ditto, vol. VI. p. 209. — Nageli und Leitgeb, in Beitrage zur wissen.
Bot., Heft IV. Miinchen 1867. — J. Hanstein, Die Scheitelzellgruppe im Vegetationspunkt der
Phanerogamen (in the Festschrift der niederrh. Ges. fUr Natur- und Heilkunde, Bonn ; and
Monatsiibersicht of the same Society, July 5, 1869). — Hofmeister, Bot. Zeitg. 1870, p. 441. —
Leitgeb, Sitzungsb. der Wiener Akad. 1868 and 1869, and Bot. Zeitg. 1871, nos. 3 and 34. —
Reinke in Hanstein's Botan. Untersuchungen, Bonn 1871, Heft III. — Russow, Vergleich. Untersuch.
der Leitbiindelkryptogamen, in Mem. de I'Acad. Imp. de St. Petersbourg, 7th ser., vol. XIX. no. i,
Petersburg 1872. — Warming, Recherches sur la ramification des Phanerogames, in Vidensk. Selsk.,
Skr. 5, Rakke 10, B. i, Copenhagen 1872 (Danish, with French abstract).

THE PRIMARY ME RI STEM AND THE APICAL CELL.

139

apex, it is not, as in the former case, distinguished by its greater size ; and, what
is of more importance, it cannot be recognised as the single original mother-
cell of all the cells of the primary meristem, nor even of a definite layer. We
may distinguish, therefore, between the Growing Point with and without an Apical
Cell.

(a) Growing Point with an Apical Cell. The formation of the primary
meristem out of the apical cell may be brought about, as will be shown presently,
in different ways, but it generally results from the
constant repeated division of the apical cell into two
unequal daughter-cells. One of the two daughter-cells
(the Apical Cell) remains from the first similar to the
mother-cell, and includes the apex; it is immediately
enlarged by growth till it equals the previous apical
cell in size, and then again divides, and so on. This
process produces the appearance as if the apical cell
always remained intact ; and this has been assumed in
ordinary language, although the apical cell existing at
any time is only a daughter-cell of the preceding one.
The other daughter-cell, on the other hand, appears
from the first like a piece cut off from the back or side
of the apical cell, generally in the form of a disc or
angular plate, and is hence called the Segment^. In
the simplest case the segment may remain undivided;
and then the whole tissue which is produced from the
apical cell has the form of a simple row of cells, as in
some Algae, and in Fungus-hyphae and hairs. But
generally the segment divides into two cells, each of
which again breaks up into two, and this process is
mostly repeated many times in the daughter-cells, until
a more or less extensive mass of tissue is produced from
the segment. The aggregate of such masses of tissue
constitutes the primary meristem. A very simple case of
this kind is shown in Fig. 108, where the apical cell (j),
here very large, growing straight out from its base, is
divided by septa (/", /*), and thus forms the segments
which lie in a row one over another. But each of these

Fig. 108.— a branch of the thallome of
Stypocaulon scoparium with two branchlets
X and y, and the rudiment of a third branch-
let z (after Geyler) ; all the" lines indicate
cell-walls.

^ The portions of wall which enclose a segment-cell differ in their nature and origin, and
behave differently in their subsequent growth. Each segment possesses two walls which were
originally division-walls of the apical cell ; they are generally parallel to one another, and are called
the 'Principal walls' of the segment; the older faces the base, the younger the apex of the organ.
Another portion of the wall of the segment is a part of the outer wall of the apical cell ; it may be
termed the 'Outer wall' of the segment. Where the segments arise as transverse discs of an apical
cell, as in Stypocaulon and Characese, the outer wall is cylindrical ; when the segmentation takes
place on two or three sides the process is very complicated; the segments have in this case,
besides the two principal walls and the outer wall, side-walls which intersect at an acute angle.
The side-walls are portions of the principal walls of older adjoining segments, and are successively
cut off by the youngest septum of the apical cell, which is at the same time the youngest
principal wall.

140

MORPHOLOGY OF TISSUES.

last is again immediately broken up by a septum {11", IP) into two disc-shaped
cells, and in each of these numerous small cells arise by the formation of vertical
and afterwards horizontal walls (as may be seen in the figure further back from
the apex); and it is easily seen how the whole branch is built up of portions
of tissue, each of which originated from a single segment. The same takes place

Fig. 109.— Apical region of a shoot oi Metzgeriafurcata in the act of bifurcation, looked at from tlie surface {after Kny).
The shoots consist of a single layer of cells (ff), which is however penetrated by a mid-rib n n', three to six layers in
thiclcness.

in the lateral branchlets x, y, which in this case arise originally from lateral
protuberances of the apical cell. These processes are remarkably clearly seen
in Stypocaulon, in the first place because only one row of segments is formed
lying one over another, and in the second place because the segments themselves
are transformed into portions of tissue without at the same time growing, as is

Fig. 110.— Diagrammatic representation of the segmentation of the apical cell, and of the first divisions in the segment of
Metxgeriafurcata (after Kny). A apex seen from the surface ; B the same in vertical longitudinal section ; C an apex in the
act of bifurcation ; a new apical cell is formed in the third-youngest segment.

usually the case. Distortions often occur from the growth of the segments, which
render difficult an investigation of the processes of division.

Figs. 109 and no show us a case in which the apical cell is divided alternately
right and left by oblique walls, so as to produce two rows of segments attached
to one another in a zigzag manner within and behind, but separated to some

THE PRIMARY MERISTEM AND THE APICAL CELL. 141

distance in front; in the angle which the two youngest segments enclose lies
the apical cell s. Fig. 109 shows the end of a shoot of Metzgeria furcata in
the act of bifurcation; each fork ends in an apical cell j; the segments and
the masses of tissue which are formed from them are drawn just as they appear
to the eye under the microscope on a superficial view of the flat ribbon-shaped
shoot. From the course of the cell-walls and the resulting grouping of cells
round the apical cell, the diagram Fig. wo, A is deduced, in which the dis-
tortions of the cell-walls occasioned by growth are neglected, and hence the
genetic relationships represented more clearly. For further elucidation Fig. no, B
is added, which also represents diagrammatically the longitudinal section of
the apical region, at right angles to the broad surface of the ribbon-shaped
shoot. This longitudinal section bisects, behind the apical cell, the mid-rib
(Fig. 109, n, n), which consists of several layers of cells, while the lateral ex-
pansions of the shoot are only one layer in thickness. The origin of the tissue
is now clear from the diagrammatic Fig. no, A and B, if it is observed in the
first place that the portions of the surface indicated by ?n, n, 0, p, and q are the
segments of the apical cell s which were formed successively in the same order,
so that ni represents the oldest, q the youngest segment. From each segment a
small piece is first cut off behind by a wall oblique to the axis of the shoot ;
from the zigzag row of these posterior sections arises the mid-rib of the shoot, which
[attains a thickness of several layers of cells, each division first of all splitting
:up by a wall parallel to the surface of the shoot into two cells lying one over
another, and each of these cells on its part again dividing in the same manner.
Divisions at right angles to the surface of the shoot (Fig. no, B) are then also
formed in the uppermost and undermost of the cells produced in this way; an
outer small-celled layer is formed on the mid-rib covering its upper and under
side, and surrounding an inner bundle which consists of longer cells. While
the posterior sections of the segment produce the tissue of the mid-rib, the tissue
of the flat lateral portion (Fig. 10^,/,/') proceeds from the anterior sections which
face the margin of the shoot ; and this tissue is only one cell-layer in thickness,
no division taking place in it parallel to the surface of the shoot. All the divisions
in these marginal sections of the segment are, on the contrary, at right angles to
the surface, and are produced by the marginal section first of all breaking up
into two cells lying side by side (see Fig. no, A, 0), each of which then forms
several shorter cells by repeated bi-partition, and these may again undergo further
division according to the activity of the growth. In general the first divisions
only of the segment are constant; the further course of cell-multiplication is,
according to the minute investigations of Kny, subject to many deviations. Since
the tissue produced from the marginal sections becomes prominent during growth,
it results that the apical cell, with the youngest segments, lies in a depression of
the margin of the shoot; and we have here a simple example of the depression
of the growing point in the tissue which grows more luxuriantly around it, such
as often occurs to a much greater extent in Fucacese, Ferns, and Phanerogams.
The differentiation of the tissue of the shoot of Meizgeria furcata does not
attain a high degree ; the cells of the margin and of the mid-rib, when mature,
differ only slightly from one another; but this differentiation is brought about

142 MORPHOLOGY OF TISSUES.

very early, even in the first division of the segment, so that the marginal tissue
and the youngest extremity of the mid-rib can be traced close up to the apical
cell. Fig. no, C, finally, affords an opportunity of learning the mode of formation
of a new apical cell out of a cell of the meristem, a case which occurs often enough
in Muscineae and the higher Cryptogams. While the thallome of Stypocaulon
(Fig. 1 08) shows how the apical cell of the lateral shoot grows immediately from
that of the principal shoot as a lateral protuberance, which is then cut off by a
wall, in Meizgeria furcata, as is shown by the researches of Hofmeister, Kny,
and Miiller, it appears that the origin of a new apical cell may be brought about
in a different manner. Fig. no, C shows the case described by Kny; in the
third-youngest segment 0^ which is formed from the apical cell s, the customary
separation into a mid-rib-cell and a margin-cell has first taken place; the latter
then divides, as usual, into two cells lying side by side ; but the new apical cell is
constituted by the appearance of a curved wall in one of these margin-cells of
the second order; and this wall intersects the dividing wall of the margin-cell
previously formed, thus cutting out a wedge-shaped piece z, which assumes at
once the function of the apical cell of a new shoot ^

In Equisetacese and many Ferns, the axis of the shoot terminates in a
comparatively very large apical cell, which is bounded by four walls — an outer one,
overarching the apex and spherically triangular, and three converging obliquely
below and within, which form at the same time the upper principal walls of the
youngest segments (Fig. in. A, D); the apical cell has hence the form of a
segment of a sphere, or of a three-sided pyramid with its spherical base upper-
most. The three plane principal walls of the apical cell are of different ages.
The next division-wall arises in the apical cell, and is parallel to the oldest wall;
a segment is formed bounded by two triangular principal walls, an arched outer
wall, and two nearly oblong side-walls^; after the apical cell has again grown to
its original size, a second division follows parallel to the next-younger principal
wall, which is followed again, after fresh renewal of the apical cell, by a division
parallel to the youngest principal wall. Three segments are now formed, placed
somewhat like the steps of a winding staircase ; each is in contact with a principal
wall of the apical cell; and in this manner the divisions are repeated; and since
each segment takes in a third of a circuit of the winding staircase, the segments
out of which the stem is built up all lie in three rows parallel to the axis,
each embracing a third of the diameter of the stem. In Fig. in, B and C, the
segments are numbered /, //, ///, &c., according to the order of their formation,
and are represented as they appear when the apex of the stem is seen from above
(not in transverse section), or as if the arched surface of the apex were spread
out flat. If the segments are followed according to the order of their numbering,
the path described is an ascending spiral, because each segment lies higher than
the older ones, as is shown in Fig. in, D, where, however, only two rows of

^ We shall recur, in Chap. Ill, to this case of spurious dichotomy.

^ These side-walls are pieces of the principal walls of the adjoining segments, as is seen,
in B and C.

THE PRIMARY MERISTEM AND THE APICAL CELL.

43

segments are to be seen from without. The formation of tissue begins by each
segment dividing into two, soon after its production, by a partition parallel to the
principal walls; the new half-segments are indicated in B, C, and JDhy i, i. Since
in each of these two half-segments the further processes are almost exactly the same, it
is necessary to keep in view only one of them. Each half-segment becomes divided
first of all by a curved vertical wall, which meets internally a side-wall, externally the
outer wall of the segment at its middle (Fig. iii, £). Since three segments com-
pose a transverse section of the stem, and each half-segment divides into two cells, the

Fig. ht.— Apical region of the stem of an Eqiiisetum; A longitudinal section of an underground very strong bud of
/•". maximtim, in September (X 550) ; B view of the apex from above (both from nature) ; C, D, E the same of E. ar-vense (after
Cramer). C diagrammatic ground-plan of the apical cell and of the youngest segment ; D external view of the apex of
a slender stem; E transverse section through this at D, /; .S" is in all cases the apical cell, /, //, ///, &c. the segments;
1, 2, 3, &c. the division-walls iu the segments in the order of their formation ; x, y, b, bs in A the first rudiments of leaves.

section of the stem now appears as if composed of six cells or sextants, whose walls
are placed nearly radially, forming a six-rayed star, as is shown in the transverse
section Fig. in, E. Hence the walls by which this division is brought about are
called sextant-walls ; in C and D they are indicated by the figure 2. Each of the
sextant-cells is still further broken up by vertical walls into an outer larger and
an inner smaller cell (Fig. in, E); and thus the foundation is laid of the two
layers of tissue into which the primary meristem separates, viz. into an outer and
an inner layer, as is clearly shown in Fig. in, ^. In the outer layer divisions

144

MORPHOLOGY OF TISSUES.

parallel to the principal walls and in the vertical radial direction at first preponderate;
in the inner layer the divisions are less numerous, so that the cells become more
uniform in diameter. This inner mass of tissue, arising from the inner sections
of the sextants, is the pith which splits as the stem developes, dries up, and thus
causes the hollowness of the stem ; from the outer layer of the primary meristem
are also formed further from the apex the cortex, the fibro-vascular bundles, and
later the epidermis ^ The external organs of Equisetum are also derived from the
outermost layer of the primary meristem, as has already been shown in Fig. in, ^,
where the protuberances x, j>, d, bs represent the rudiments of leaves. To these
processes I shall recur hereafter ; here it need only be mentioned that each set of
three consecutive segments undergoes at an early period a small vertical displace-
ment, so that at least their outer surfaces form a horizontal zone which then
bulges out and is the origin of a leaf-sheath.

As a final example of the formation of the primary meristem from an apical
cell, we may now consider the processes that take place at the growing end of a

Fig. 112.— Apical region of a Fern-root ; A longitudinal section through the end of the root of Pterts hastata;
^transverse section through the apical cell and adjacent segments of the root of u4s^/e>iium FtVix-J'cemma {after
Nageli and Leitgeb).

Fern-root, with which the greater number of roots of Cryptogams agree in the main.
Fig. 112, A represents an axial longitudinal section through a Fern-root, with the
apex uppermost. From the apical cell v arises not merely the tissue of the sub-
stance of the root o, c, but also the root-cap k, /, m, n, a mass of tissue which
covers like a helmet the growing point of every root. The apical cell in this case
resembles that of the stem of Equisetaceae and of many other Cryptogams, in so
far as it presents a three-sided pyramidal segment of a sphere; this form is
sufficiently seen by comparing the longitudinal section A with the transverse
section B. Here also three straight rows of segments are formed by successive
divisions of the apical cell, which are numbered according to their order in age,
/, //, ///, &c., in Fig. B ; and here also a spiral is described by the line con-
necting the centres of the consecutive segments. The great difference between
the apices of roots and the growing stem of Cryptogams is however, that in the
former the apical cell not only produces these segments which build up the

Compare Book II, Class Equisetacese, under the formation of their tissue.

I

THE PRIMARY MERISTEM AND THE APICAL CELL. I45

t.

tissue of the root \ but other segments also which build up the Root-cap. These
latter are cut off from the apical cell by septa in such a manner that they cover
them like a cap ; and every such segment belonging to a root-cap is hence termed
simply a Cap-cell. According to the investigations of Nageli and Leitgeb, it
appears to be the rule that whenever three segments have been formed (from the
substance of the root), a new cap-cell arises ; but this rule is not always strictly
adhered to.

The cap-cell increases quickly in breadth ; and its transverse section, originally
herically triangular, becomes circular. It is simultaneously divided into two equal
alves by a vertical wall (parallel to the axis of the root); in each of these halves
a wall again arises at right angles to the former; and thus four quadrant-cells are
formed. Each quadrant again breaks up into two cells (octants), the further divisions
varying in different species. In the successive layers of the cap the quadrants are
not superimposed but alternate ; /. e. the quadrant-walls of one layer deviate from
those of the preceding and following ones by about 45°.

The growth in length of the substance of the root, in so far as it is occasioned
by divisions of the apical cell, proceeds, as has already been indicated, in such a
manner that the septa which arise in spiral succession are parallel to the sides
of the apical cell. Each segment-cell is bounded by five walls, as at the apex
of the stem of Equisetum, — two principal triangular walls, two oblong side-walls,
and one somewhat convex outer wall, in contact with a root-cap. The first wall
which arises in each segment-cell stands at< right angles to the principal walls, and
is, with reference to the whole root, a radial longitudinal wall. Two cells arise
in this manner side by side, unequal in form and size, the septum meeting internally
a side-wall, but externally the middle of the outer wall. In this manner the trans-
verse section of the root, composed at first of three segment-cells, breaks up into six
cells or sextants (compare the processes described above in the stem of Equiseium) ;
three of these sextants reach to the centre of the section; but the three which
alternate with them do not. The sextant- walls are seen in Fig. 112, B, in the
segments IV, V, VI, VII, as hues dividing the outer wall in half; in a deeper
transverse section they would form, together with the three side-walls of the three
segments, a six-rayed star, similar to that in Fig. iii, E^. Each sextant-cell is next
divided again by a wall parallel to the surface of the root into an inner and an outer
cell ; in the transverse section of the root {i. e. in the corresponding transverse section
beneath the apex), twelve cells can therefore be recognised at this stage, of which
the six outer ones form a peripheral layer, and the six inner ones a central nucleus.
The longitudinal section, Fig. 112, A, shows this wall at c c, and it may thus be seen
how the mass of the substance of the root is broken up by it into an outer layer 0 c
and an inner thick bundle c c c c. Out of the former arises by further division a
tissue which becomes differentiated further backwards into epidermis 0 and cortex
(between 0 and c) ; the axial bundle c c c c^oyi the other hand, which is the result
of further longitudinal divisions of the inner sections of the sextants, forms the
procambium- cylinder of the root, in which the vascular bundles arise. In this

* They are bounded by thicker lines in the longitudinal section A,
2 Compare Book II, Equisetaceae, diagram of root, Fig. 284.

L

146 MORPHOLOGY OF TISSUES.

case also the ultimate separation of the mass of tissue into two distinct portions
is determined by the first divisions of the youngest segments ; but a comparison of
the corresponding process in the stem of Equiseium shows that the mass of tissue
which is formed from the central portions of the sextant has quite a different
signification from what it has there ; and the same is the case with the peripheral
layer. A further discussion of the origin of the forms of tissue of the root out of
these portions of the primary meristem will be entered into when treating of Ferns
and Equisetacese.

In conclusion, it may be remarked that the segments of the apical cell, where
they arise in two or three rows, have at first a position oblique to the ideal axis of
the organ, and enclose an angle open towards the apical cell ; but, in consequence
of growth, the position of the segments generally changes so that they come to lie
gradually more transversely, and finally, at a certain distance from the apical cell,
the principal walls lie at right angles to the axis of the organ. The process is not
clearly shown in Figs, in and 112; but more evidently in examples to be brought
forward later {e.g. Fig. 116, p. 153).

(b) Growing Point without an Apical Cell. This occurs universally in Phanero-
gams ; the apical region of growing shoots, leaves, and roots consisting of a primary
meristem, the cells of which are very small in proportion to the size of the entire
growing point, and very numerous. It has not yet been demonstrated whether
even the cells next the apex can be traced back to a single primary mother-
cell, although sometimes undoubtedly one cell lying at the apex is distinguished
by a somewhat greater size and by its form. In many shoots the surface of the
apex seen from above shows an arrangement of the superficial rows of cells
which to a certain extent points to this one cell as their common primary mother-
cell; but even if this were the case, which is by no means proved, it is, on the
other hand, altogether impossible to connect genetically the inner layers of cells
also with this cell. The peculiar significance of the apical cell of Cryptogams lies
in the fact that all the cells of the primary meristem furnish evidence of descent
from it in diff"erent degrees.

But as in Cryptogams the first divisions of the segment-cells determine certain
layers of the primary meristem which subsequently pass into the differentiated tissue-
systems further backwards from the apex, so also in Phanerogams a definite
arrangement of the cells is brought about in the primary meristem of the growing
point, of such a kind that the various layers of the primary meristem, when
followed further backwards, have a genetic relation with the epidermal tissue, the
cortex, and the fibro-vascular bundles, and may be recognised as the first rudiments
of them. The outermost layers run uninterruptedly over the apex of the growing
point, overarching an inner mass of tissue of the primary meristem, which latter,
on its part, sometimes runs out beneath the apex into a single cell (in Hippuris
and Anacharis canadensis, according to Sanio), but usually terminates in a somewhat
subordinate group of cells.

While in Cryptogams with an apical cell an evident cell of this kind is formed
where a lateral outgrowth (shoot, leaf, or root) is about to be developed on the
growing point, in Phanerogams, on the other hand, a whole group of cells,
including inner and outer layers, makes its appearance at the spot in question, so

THE PRIMARY MERISTEM AND THE APICAL CELL.

147

that even at the first commencement of an organ no one predominating apical cell
can be recognised (Fig. 113). After Sanio^ had investigated these processes in
Phanerogams, Hanstein ^ studied them in a more general and detailed manner, and
has recently shown that even in the embyro of Phanerogams the first divisions take
place in such a manner as to negative from the first the existence of an apical cell ;
while, on the other hand, a differentiation into an outer layer and an inner nucleus
of tissue early manifests itself^.

The outermost layer of the primary meristem which covers the growing point
together with its apex is the immediate continuation of the epidermis of the older
part which lies further backwards ; it may therefore be termed the Primordial
Epidermis ; Hanstein has however already applied to it the name Dermatogen.
It is distinguished by the circumstance that divisions occur in it exclusively at right
angles to the surface ; it is only at
a subsequent period that tangential
divisions also sometimes occur, when
the epidermis becomes divided into
several layers.

Beneath the Primordial Epidermis
one or more layers are generally
found which also cover the apex
continuously, and out of which the
cortex originates further backwards
from the apex (Fig. 122, rr, p. 163);
they represent therefore the Pri-
mordial Cortex ; Hanstein calls this
layer of the primary meristem the
Periblem. Enclosed and overarched
by this is a nucleus of tissue,
which may be followed out as an

immediate continuation of the fibro-vascular bundles, and of the pith enclosed
or traversed by them. The layer of tissue in which the first fibro-vascular
bundles originate, termed by Sanio the Thickening-ring, thus corresponds to the
outer layer of this inner tissue-nucleus (which Hanstein terms Plerome), when a
pith is formed ^ If no pith is formed, as in many roots and some stems {e.g,
Hippuris, Anacharis, Sic), the whole of the plerome is developed into procambium,
and this into an axial fibro-vascular cylinder, which is then traversed by two or more
vascular bundles and bast-bundles.

The origin of the root-cap in Phanerogams may be considered, according to
the recent investigations of Hanstein and Reinke, simply as a luxuriant growth of
the primordial epidermis or dermatogen, localised at the apex in such a manner that

Fig. 113. — Longitudinal section through the apical region of the stem
of an embryo of Phaseolus inultiflorus ; ss apex ; fb parts of the two
first leaves ; k k their axillary buds.

^ Sanio, in Bot. Zeitg. 1865, p. 184 et seq.

2 Hanstein, Die Scheitelzellgruppe im Vegetationspunct der Phanerogamen. Bonn 1868.

3 Hanstein, Monatsber. der niederrh. Gesell., July 5, 1859. For further details see the general
characteristics of Phanerogams in Book H.

* Compare however Russow, I.e., pp. 177, 183, from which it appears questionable whether this
first differentiation of the primary meristem invariably takes place in this way.

L 2

148

MORPHOLOGY OF TISSUES.

the part of the dermatogen which covers the apex of the root divides periodically
by tangential walls. Thus the dermatogen splits at the apex into two layers of cells,
the outermost of which developes into a many-celled cap, the Root-cap, while the
inner layer at first again performs the functions of dermatogen, until a new splitting of the
layer at the apex causes the formation of a new stratum, which again, on its part, as

Fig. 114.- Longitudinal section through the apical region of the young root of the sunflower (after Reinke) ; A A the root-
cap ; b b (figured dark) the dermatogen ; // the plerome ; its inner (dark) layer ir tt the pericambium; between ir and b lies the
periblem ; i i the primary mother-cells, the source of the periblem and plerome.

in Cryptogams, becomes separated by tangential divisions into several layers, as is
exemplified in Fig. 114.

According to the description here given, which can only serve as an introduction to
what follows for the student in a few examples, it might almost appear as if the processes
in the growing point of Phanerogams were essentially different from those in Cryptogams,
a hypothesis which I however do not accept. On the one hand the careful investigations
of Nageli and Leitgeb in Lycopodiaceae (/. c.) on this point prove that in this family the
significance of the apical cell in the production of the primary meristem is different from
that in other Cryptogams, and approximates to what occurs in Phanerogams; and
that, on the other hand, the apical cell of Cryptogams may, equally with the apical cell-
, group of Phanerogams, be considered the starting-point of the first difFerentation of the
layers of tissue.

CHAPTER III.

MORPHOLOGY OF MEMBERS.

Sect. 20. Distinction between Members and Organs ^. Metamorphosis.

The parts of plants which are ordinarily termed their Organs — very various in
their form, and serving different physiological purposes — may be considered scien-
tifically from two diflferent points of view. The question may be asked at the
outset, How far are these parts adapted, by their form and structure, to perform
their physiological work ? In this case they are regarded from one side only, as
instruments or organs, and this mode of regarding them belongs to physiology. In
the other case these relationships may be completely put aside, and the question
may be kept out of consideration what functions the parts of the plant have to fulfil,
and the only point kept in view may be where and how they arise, that is in what
manner the origin and growth of one member are related in space and time to
those of another. This mode of regarding them is the morphological one. It is
obvious that it is as one-sided as the physiological ; but investigation and description
require, here as everywhere else in science, abstractions of this kind ; and they are
not only not hurtful, but even of the greatest assistance to investigation, if the
investigator is only clearly conscious that they are abstractions.

In this chapter we shall concern ourselves exclusively with the morphological
consideration of the parts of a plant. But before we proceed to a more minute
investigation, it will be useful to get a somewhat more exact comprehension of the
relationship between the physiological and the morphological view.

Morphological investigation has led to the result that the infinite variety of the
parts of plants, which in their mature state are adapted to functions altogether
different, may nevertheless be referred to a few Original forms ^ if regard is paid
to their development, their mutual positions, the relative time of their formation,
and their earliest states ; that, for instance, the thick scales of a bulb, the mem-
branous appendages of many tubers, the parts of the calyx and corolla, the stamens
and carpels, many tendrils and spines, &c., altogether resemble, in these respects,

^ Nageli und Schwendener, Das Mikroskop. Leipzig 1867, p. 599.— Hofmeister, Allgemeine
Morphologic der Gewachse. Leipzig 1868, Sect, i, 2. — Hanstein, Botanische Abhandlungen aus
dem Gebietc der Morphologic u. Physiologic. Bonn 1S70, Heft I. p. 85.

150 MORPHOLOGY OF MEMBERS.

the green organs which have been termed simply leaves (foliage-leaves). All
these structures are therefore also called Leaves {Phyllomes) ; and this designa-
tion is frequently justified by the fact that many of these organs actually become
transformed, under peculiar conditions, into green leaves. Since the green organs
which are termed leaves in popular language (foliage-leaves) may be considered
as leaves par excellence, the other structures, which are also considered to be
foliar, are termed metamorphosed leaves. The same is also the case with those
parts to which the leaves are attached, and on which they grow as lateral ap-
pendages. They sometimes have the form of cylindrical or prismatic, slender,
greatly elongated stems, sometimes of thick roundish tubers, or are often hard
and lignified (trunks) ; in other cases they are soft and flexible, either embracing
other firm bodies (bines), or firmly attached "to them, as in the ivy; they may
also occur as sharp spines or tendrils (grape-vine). All this is connected with
the mode of life of the plant, and with the functions of the structures under
consideration. But if the one characteristic only is kept in view, that they all bear
leaves which arise below their growdng apices, an agreement is found as important
as complete, which may for the time be altogether abstracted from the physio-
logical functions and the corresponding structure ; and when once this abstraction
is made, the agreement may be denoted by applying a common name to all those
parts which bear leaves ; they may be termed Stem-structures {Caulomes) or simply
Axes. In the same sense therefore in which, for example, the tendril of a pea is a
leaf, the tuber of a potato is also a stem or axial structure ; and just as the tendril of
a pea is termed a metamorphosed leaf, so the tuber of a potato may also be called a
metamorphosed stem.

The same is the case with hairs as with leaves and stems; the distinguishing
characters of root-hairs, woolly hairs, prickles, glandular hairs, &c., is that they all
originate as outgrowths of epidermal cells. If we now go a step further, we may term
all appendages of other parts which originate as outgrowths of epidermal cells,
whatever their form and function, Hairs {Trichomes). Thus the so-called paleae and
the sporangia of Ferns are trichomes ; or, if the ordinary filiform hairs are considered
the original form, they are then metamorphosed hairs. It does not necessarily follow
that hairs grow from a true epidermis ; it is held sufficient if they arise from single
superficial cells ; and thus the number of the external appendages termed trichomes
is still further increased.

As in the case of stems, leaves, and hairs, we may speak also of metamorphosed
roots ; they are usually filiform, long, and slender, but sometimes thick and tuberous ;
usually they grow beneath the ground, but also sometimes above ground, and even in
an upward direction. Nevertheless, under all circumstances roots maintain so striking
a similarity to their typical forms, that the term metamorphosed is but seldom ap-
plied to them.

This mode of investigation, applied to Vascular Cryptogams and Phanerogams,
has shown that all the organs of these plants may be referred to one of these
morphological categories; every organ is either Stem (Axis), Root, Leaf, or Hair.
The Muscineae have no roots in a morphological sense, although they possess
organs which completely fulfil the functions of roots ; on the other hand most have
leaves which grow on stems (axes). In Algae, Fungi, and Lichens, the plant has

DISTINCTION BETWEEN MEMBERS AND ORGANS. I51

generally appendages which may be termed hairs ; but there are never any roots
in the morphological sense, and the term leaf, as understood in higher plants, can no
longer be rightly applied even in those cases where the external form of the mature
parts is similar to the foliage-leaves of higher plants, e.g. Laminaria digitata, &c.
It is now agreed to apply to those vegetable structures in which the morphological
distinction of stem and leaves cannot be carried out in the present state of our
knowledge (and which have never any true roots), the morphological term Thallus
or Thallome. In contradistinction to Thallophytes, all plants in which leaves
can be morphologically distinguished might be termed Phyllophytes ; the name
Corviophytes has, however, been given in preference to them. From what has
been said it will be seen that the thallophyte is only distinguished from a cormophyte
by the lateral outgrowths which occur somewhere or other on it not presenting
sufficient morphological distinctions from the part which bears them, to permit us to
term them leaves in the same sense as in the more highly differentiated plants.
But as the morphological distinctions of stem and leaf are not yet sufficiently
established even in higher plants, it is impossible to draw a sharp boundary
between Thallophytes and Cormophytes, and indeed it is certain that one does not
exist.

If we now accept the terms Thallome, Stem (Caulome), Leaf (Phyllome), and
Hair (Trichome)\ in the senses indicated, it can no longer be said that the leaf is the
organ for this or that function ; for leaves may undertake all possible functions ; and
the same remark applies also to the other parts. It is therefore on all accounts
inexpedient simply to apply the term Organs to thallomes, stems, leaves, and hairs,
for many of them have in fact no function at all. In order to avoid this mode of
expression, which is confusing and foreign to morphology, it is obviously best to
speak in this sense not of Organs, but of Members. The term Member is used when
we speak of a part of a structure in reference to its form or position, and not to any
special purpose it may serve. Thus, from a morphological point of view, stems,
leaves, hairs, roots', thallus-branches, are simply members of the plant-form ; but a
particular leaf^ a particular portion of the stem, &c., may be an organ for this or that
function, which it is the province of physiology to investigate.

The morphological nature of a member is best recognised in its earliest
stages of development, and by its relative position in the series of processes of
growth ; the morphological definitions depend therefore essentially on the history
of development.

The older a member becomes, the more obvious becomes its adaptation to a
definite function, the more completely is its morphological character often lost. In
their earliest states the members to which the same morphological term is applied
{e.g. all the leaves of a plant) are extremely similar to one another; at a subsequent
period all those distinctions aj-ise which correspond to their different functions.
We can now arrive at a definition of Metamorphosis which may be used in a
scientific manner : — Metamorphosis is the varied development of members of the
same morphological value resulting from their adaptation to definite functions.

\

^ See Nageli und Schwendener, Das Mikioskop, vol. II. p. 591.

152

MORPHOLOGY OF MEMBERS.

(a) The conceptions of Stem, Leaf, Root, Trichome, as at present employed in botany,
result from the examination of highly developed plants, the different members of which
actually present considerable diversities, or display considerable differentiation ; but if the
attempt is made to apply these conceptions in the same manner to the less differentiated
Hepaticae, Algae, Lichens and Fungi, many difficulties arise, depending principally
on the fact that the members of the thallome sometimes display striking resemblances
to leaves, hairs, stems, and even roots, while wanting others of their characteristics.
Transitions occur from the members of Thallophytes which are but slightly differen-
tiated morphologically to the highly differentiated members of Gormophytes. In the
members which we term stem, leaf, root, hair, it is clear that those differences are
only augmented which 'also occur, though in a lesser degree, in the more homoge-
neous ramifications of the thallome, especially of the higher Algae; absolute distinctions
between thallomes and leaf-bearing axes are not to be found. It is therefore a matter
of convenience where the boundary-line is drawn.

Fig. 115. — Longitudinal section through the apical region of three primary shoots of Charafragilis ; /the apical cell, in
which segments are formed by septa ; each segment being further divided by a curved septum into a lower cell which does
not further divide and which developes into an internode g s" s"'t and an upper cell which produces a node ?« m' and the
leaves. Each node-cell produces a whorl of leaves of different ages. (For a more exact description, see Book II, Characese.)

(b) The expressions Thallome, Caulome, Phyllome, Trichome, Root, designate, as has
been said, general ideas, from the definition of which are eliminated all those properties
of the members which adapt them only for definite functions, while a few characters
only, drawn from their origin and mutual position, are kept in view. Parts which
are physiologically entirely different may therefore be morphologically equfvalent^
and, vice 'versa, physiologically equivalent organs may fall morphologically under quite
different conceptions. The statement, for example, that the sporangia of Ferns are
trichomes, means only that they originate, like all hairs, from epidermal cells ; in this
characteristic hairs and the sporangia of Ferns are morphologically equivalent. On the
other hand the underground hairs of Mosses and the true roots of vascular plants are
physiologically equivalent ; both serve for the absorption of nourishment and the fixing of
the plant in the ground, although the former are morphologically trichomes, the latter
roots.

DISTINCTION BETWEEN MEMBERS AND ORGANS.

^53

(c) General ideas, like those considered here and in the sequel, depend always on
abstractions ; they therefore necessarily want the practical clearness of the particular
ideas from which they have been abstracted. How far the abstraction may be carried is
more or less arbitrary ; and the only correction for this lies in the scientific usefulness
of the idea. Those ideas are the most useful which, from the greater precision of
the definition, and from their greater clearness, include the greatest possible number
of particular cases ; for in this manner is that complete general comprehension of the
phenomena most easily obtained which must precede a closer examination of them. The
definitions in the following paragraphs are given from this point of view.

Sect. 21. Leaves and Leaf-bearing Axes^ — The members of the plant
which are called Leaves (Phyllomes) in Characeae, Muscineae, Vascular Cryptogams,
and Phanerogams, are related to
the axis or stem from which they
are derived in the manner de-
scribed in the following para-
graphs.

(i) T/ie Leaves ahvays origi-
te below the groiving apex of the
ieni as lateral outgrowths, either
singly, or several at the same
height, i.e. at an equal distance
from the apex ; in the latter case
they form a whorl, the single
leaves of which may differ in age,
as in Chara and Salvinia, and in
the whorls of many flowers.

(2) So long as the apex of
the shoot continues growing in a
straight line, and the portion of
the shoot which produces leaves
lengthens, the leaves arise in
acropetal order ; i. e. the nearer
the leaves are to the apex, the

younger they are. In this case leaves are never produced further from the apex
than those already in existence. It is only when, as not unfrequently happens
with the flowers of Phanerogams, the growth in length of the shoot ceases or
becomes weaker at the apex, while, at the same time, active growth continues in
a transverse zone or place beneath the apex, that new leaves can arise between
those already in existence ^.

(3) The Leaves always originate frofn the Primary Merisiem of the Growing

Fig. 116.— Longitudinal section through the apical region of a stem of
Foiitinali'! antipyretica, an aquatic Moss (after Leitgeb) ; v the apical cell
of the slioot, producing three rows of segments which are at first oblique
and afterwards placed transversely (distinguished by a stronger outline).
Each segment is first of all divided by the septum a into an inner and
an outer cell ; the former produces a part of the inner tissue of the stem,
the latter the cortex of the stem and a leaf. Leaf-forming shoots arise
beneath certain leaves, a triangular apical cell ^ being formed from an outer
cell of the segment, which then, like v, produces three rows of segments ;
and each segment here also forms a leaf. (For a more exact description see
Book II, Mosses.)

^ Nageli u. Schwendener, Das Mikroskop. Leipzig 1869, p. 599 et seq. — Hofmeister, AUge-
meine Morphologic der Gewebe. - Leipzig 1868, Sect. 2. — Pringsheim, Jahrb. fiir wissen. Bot.
vol. III. p. 484. — Ditto on Utricularia, in Monatsber. der Berliner Akad., Feb. 1869, — Hanstein, Bot.
Abhandlungcn, Bonn 1870, Heft I. — Leitgeb, Botan, Zeitg. 18 71, no. 3.— Warming, Recherches sur
la ramification des Phanerogames. Copenhagen, 1872, p. vi.

^ Since phenomena of this kind are confined to the flowers and inflorescence of Phanerogams,
their consideration may for the time be postponed.

154

MORPHOLOGY OF MEMBERS.

Point, never from those parts of the stem which already consist of fully differentiated
tissues. In Characeae, Muscineae, &c., before or during the first divisions of their
segments, the leaves become visible close beneath the growing point as protuber-
ances, the outer portion constituting an apical cell, from the segments of which a
leaf is built up. In Vascular Cryptogams a many-celled cone of growth often over-
tops the youngest rudiment of a leaf, as in strong Equiseiwn buds, Salvima, many
Ferns and Selaginelleae. In Phanerogams (Figs. 117, 118, 119) this is general^ ; in
them the rudiment of the leaf does not begin with an apical cell projecting from
the cone of growth, as in Cryptogams, but a rounded or broad cushion is formed,
which from its very first origin consists of numerous small merismatic cells.

(4) The Leaves are always Exogenous Formations, i. e. the rudiment of the
leaf never has its origin exclusively in the interior of the tissue of the stem, and
is never covered by layers of tissue of the stem which take no part in its formation,

Fig. 117. — Terminal region of two primary shoots of maize.
Apex of the very small-celled cone of growth, out of which the
leaves b, b' , b', b'" arise as multicellular protuberances, which
soon embrace the stem, and envelope it and the younger leaves
like a sheath. In the axil of the third youngest leaf b" the young-

est rudiment of a branchlet is visible as a roundish protuberance. leaves ; r cortex ; m pith.

Fig. 118.— Longitudinal section through the apical region of the
primary stem of the sunflower, immediately before the formation of
the flowers ; j apex of the broad growing point ; b b the youngest

as is the case with roots and many endogenous shoots. In Cryptogams it is usually
a single superficial cell {i. e. superficial before the differentiation of the epidermis)
which forms the foliar protuberance. In Phanerogams a mass of tissue bulges
out as the rudiment of the leaf, and consists of a luxuriant growth of the periblem
covered by dermatogen (Sect. 19, Fig. 113, p. 147). By this means the leaf is at
once distinguished from the hair even in its most rudimentary state. The hair is an
outgrowth of the epidermis; but since in Phanerogams the primordial epidermis
or dermatogen covers the whole of the growing point above the leaves, hairs may
also spring up higher in position than the youngest leaves, from single cells

^ [Warming however remarks (Ramification des Phanerogames, p, iii) that the growing point
may have the most various forms, from that of a rather acute cone, as in Graminese, Amaranthus,
and Plantago, to that of a cup-shaped depression, e. g. Digitalis, and that the form may differ even in
species belonging to the same genus ; thus in Digitalis Intea it is convex, in D. parvijlora con-
cave.]

LEAVES AND LEAF-BEARING AXES.

-^5^

belonging to the dermatogen, as in Utricularia according to Pringsheim. But in
Cryptogams the dermatogen becomes differentiated only after the formation of the
leaf; and hence the hairs are always at a greater distance from the apex than the
youngest leaves (Fig. ii6); the superficial cell of the stem, which in Cryptogams
becomes the apical cell of a new leaf, is not an epidermal cell, since its origin dates
long before the differentiation of the tissue into epidermis and periblem.

(5) The Tissue of the mature Leaf is continuous in its formation with that of the
Stem. It is impossible, histologically, to find a boundary line between the stem
and the base of the leaf, although such a boundary line must be assumed theo-
retically. If the surface of the stem is imagined to be continued through the
base of the leaf, the transverse section thus caused is called the Insertion of the
Leaf

The continuity of the tissue is especially observable in vascular plants, where
the well-developed leaves^ consist, like the stem, of epidermal and fundamental
tissues and fibro-vascular bundles. The
cortical layers of the stem bend out with-
out interruption into the leaf, and consti-
tute its fundamental tissue; in the same
manner the epidermis passes over from
the stem into the leaf; the fibro-vascular
bundles of the leaves have, in Phanero-

FiG. 119.— Longitudinal section through the apical region
of an upright shoot oi Hippiiris vulgaris; s apex of the stem ;
b, b, b the verticillate leaves ; A /6 the buds in their axils, which
all develope into flowers ; g g the first vessels (the dark parts
of the tissue indicate the inner cortex with its intercellular
spaces).

gams and many Cryptogams, the appear-
ance of being the upper ends of the * com-
mon' bundles which ascend in the stem
(Fig. 119); and where this is not the
case, as in Lycopodiacese, the basal por-
tions of the foliar bundles and the fibro-
vascular mass of the stem are nevertheless
in continuity.

The main cause of the continuity of
tissue between stem and leaf is that the
leaf arises from the cone of growth of the stem, where it still consists entirely
of primary meristem; in vascular plants the young leaf appears as a luxuriant
development of its outer layers (the dermatogen and inner layers of periblem, see
Sect. 19). And as vascular bundles (at first in the form of procambium) become
differentiated in the central tissue of the stem or plerome, similar bundles also
appear in the tissue of the growing leaf, in such a manner that the two are in
connection with one another. This connection may be such that the foliar
bundles appear as the upper prolongations of those of the stem; thus arise the
' common ' bundles of Phanerogams, the portion that nms through the stem
being termed the Leaf trace (see Sect. 18). But in some Vascular Cryptogams,
as Lycopodiaceae and Equisetaceae, the procambium bundles which are differentiated
in the tissue of the young leaf are so connected with the young fibro-vascular

^ Leaves which wither early, or which persist as small scales, like all the leaves of Psilotnm, and
many small leaf-scales of Phanerogams, have no fibro-vascular bundles.

1^6 MORPHOLOGY OF MEMBERS.

bundles of the stem, that although they are actually continuous, their independent
origin is still easily seen in radial longitudinal sections through the apices of young
stems. In both cases the development of the first vessels usually commences in
the region where the. bundle that bends out into the leaf unites with that of the
stem. In addition to the Common Bundles formed in this way, fibro-vascular bundles
may also arise in the stem, as has already been described in Sect. i8, which belong
to it alone, and are therefore termed Cauline Bufidles. Cauline bundles may be
formed at an early period of the growth of the stem, as in Piperaceae, Nycta-
gineae, &c., or only as a consequence of increase in thickness, when the leaves have
already long been developed, or have even fallen off, as in Aloineae, Menispermaceae,
&c. (see Sect. i8).

(6) The Leaves usually grow more rapidly in length than the parent shoot above
their insertion (Figs. ii6, 117, 118). If the leaves are formed rapidly one after
another, they envelope the end of the shoot, and thus form a Bud, in the centre
of which lies the leaf-forming growing point. This production of a bud depends
also on the more rapid growth of the outer or under side of the leaves in their
young state, by which they become concave on the inner (afterwards the upper)
side, and adpressed to the stem. It is only by the extension of their tissue that
the leaves ultimately turn outwards in the order of their age, and thus escape
from the bud. If the portion of the stem between the insertions of the leaves
undergoes at the same time a considerable extension, the leaves then become
placed at a distance from one another, and a shoot results with elongated inter-
nodes. In such cases the section of the stem in which the leaf-insertion lies
usually developes in a different manner from the intermediate portions ; these zones
are termed the Nodes, the intermediate portions the Internodes, as in Characese, Equi-
setaceae, and Grasses. If the stem remains entirely undeveloped between the nodes,
it possesses no proper exposed surface, but is entirely covered by the leaf-insertions,
as in Nephrodium Filix-mas ; but more commonly this is only apparently so from
the internodes being very short, as in many palm-stems. The internodes may be
present immediately after the first formation of the leaves, when the consecutive
leaves or leaf-whorls appear at considerable distances in height from one another,
as in Chara ^ and Zea (Fig. 117); or they may originate only after further develop-
ment of the stem-tissue, as in Mosses (Fig. 116) and Equisetaceae, where every
segment of the apical cell of the stem forms a rudiment of a leaf, so that the
leaf-rudiments follow immediately one after another; and it is only by further
cell-formation, growth, and differentiation that the lower portions of the segment
become developed into the exposed portions of the surface of the stem, as is
clearly shown in Fig. 116. The formation of a bud in the way described above
does not take place when on the one hand the leaves are developed very slowly
one after another, or on the other hand when the stem grows rapidly in length
between the youngest leaf-rudiments or even before the appearance of the youngest,
so that there is always only one slightly developed leaf near the apex, as in the
underground creeping shoots of Pteris aquilina (see Book II, Ferns).

^ I consider in Chara, as in Muscinese, and in fact universally, that the cortex belongs originally
to the stem, and not to the leaf.

IBel

LEAVES AND LEAF-BEARING AXES. 157

(7) Every Leaf assumes a form different to that of the Stem which produces it, and
to that of its lateral Shoots. This is usually so conspicuous that no further descrip-
tion is needed. Nevertheless one point must be mentioned which often causes
difficulty to the student. It not unfrequently occurs that lateral shoots of certain
plants present a great similarity in form and physiological properties to the foliage-
leaves of other plants, as the flat lateral shoots (phylloclades) which bear the flowers
in Ruscus, Xylophylla, Muhlenheckia platyclada, &c. ; but the course of development

hows that these apparent leaves must, from their position, be lateral shoots, them-
Ives producing leaves ; and the leaves of these plants are usually of quite a dif-
ferent form from these leaf- like branches. The phrase ' leaf- like' has in these cases
usually no distinct morphological, but only a popular meaning; and what will be
said under paragraph (8) may be applied here. The branches or leaf-bearing lateral
shoots arise in very difl"erent ways in diff'erent plants; but very commonly they
have this in common with leaves, — that they originate also as lateral and exogenous
outgrowths in the primary meristem of the growing point ; that they are formed,
like the leaves, in acropetal succession; and that the diff'erentiation of their tissue
proceeds continuously with that of the primary shoot. They are distinguished,
however, from the leaves of the same plant by their place of origin, by their
much slower growth — at least at first (later they may overtake the leaves), — and
by their relations in point of symmetry, of which we shall speak hereafter. The
leading fact, however, is that the lateral shoot repeats in itself, by the formation
of leaves, all the relations hitherto named between leaf and stem, and is there-
fore a repetition of the primary shoot, although in other physiological characters
it may diff"er greatly from it.

(8) The morphological conceptions of Stem and Leaf are correlative ; one cannot
be conceived without the other ; Stem (Caulome) is merely that which bears Leaves ;
Leaf (Phyllome) is only that which is produced on an axial structure in the manner
described in paragraphs (1-7) ^ All the distinguishing characters which are ap-

' plicable to the definition of Caulome and Phyllome express only mutual relationships
of one to the other ; nothing is implied as to the positive properties of either. If
we compare together all the structures which we call leaves without reference to
the stems to which they belong, we shall be unable to find a single characteristic
which is common to them all and which is wanting in all stems ^ But that which
is common to all leaves is their relation to the stem. Hence the ideas Phyllome
and Caulome cannot be obtained by comparing together the positive properties

I of leaves or the positive properties of stems, or by la}dng stress on the points
which they have in common and on those wherein they differ; but these ideas
^ There are, for instance, thallomes strikingly similar to certain leaf-forms, as those of Lami-
naria, Delesseria, &c. ; they are, however, not leaves, since they are not formed on a stem as
lateral structures.

2 [Warming (Ramification des Phanerogames, p. xvii) remarks that while it is impossible to find

constant characters for separating phyllomes from caulomes, they spring from the peripheral tissue

at slightly different depths. Phyllomes originate in the superficial layers of the periblem, from the

first to the third; feebly developed foliar organs, such as bracts, even in the first layer alone.

1^ Caulomes scarcely ever originate in the first layer, but usually in the third or fourth. Warming

^K attributes this to the necessity for the largest structure to have the deepest origin.]

I

l^H MORPHOLOGV OF MEMBERS.

are obtained by observing leaves exclusively in their relation to the stem which
produces them, and stems in relation to the leaves produced from them. In other
words, the expressions Stem and Leaf denote only certain relationships of the parts
of a whole — the Shoot ; the greater the differentiation, the more clearly are Stem
and Leaf distinguished. The measure of the difference is usually arbitrary; but
if we confine ourselves to those plants to which the term leaf is applied in ordinary
language, the distinction of leaves from stem depends on the relationships named
in paragraphs (1-7); and in this sense certain lateral outgrowths in some Algae
may be termed Leaves, and the axial structures which produce them Stems {e.g.
Characeae, Sargassum). But when the difference between the outgrowths and the
axial structures which produce them is less, one or more of the relationships named
in paragraphs (1-7) disappear, and it becomes doubtful whether the expressions
Leaf and Stem ought still to be used ; and when finally the similarity preponderates,
the whole shoot is no longer called a Leafy Stem, but a Thallome. A branched
thallome has the same relation to a leaf-bearing stem as a slightly differentiated to a
highly differentiated whole.

The external differentiation of the members of the shoot into Stem and Leaf
is to a certain extent independent of the internal differentiation which brings
about the different forms of tissue and the cell-divisions, as is shown in the
comparison of Muscinese and Characeae with Phanerogams. The internal seg-
mentation may be reduced to a minimum of cell-divisions, or may altogether dis-
appear; in the latter case the single cell represents a shoot, the lateral outgrowths
of which behave asjeaves and the axial part as stem, as, for example, in Caulerpa
amongst Algae. What has already been said as to the continuity of the tissue
of stem and leaf and their common origin from the primary meristem, must
here be understood in an extended sense. In place of the primary meristem we
have the growing point of a single cell continuing its growth, and instead of
the differentiation of tissue the development of the older part of the cell-wall
and of its contents. Caulerpa consists of a single cell, which grows like a
creeping stem and puts out lateral leaf-like protuberances and tubular hairs which
even perform the function of roots, the w^hole enclosing a continuous cell-cavity
without partition- walls \

(a) The leaves, like the shoots, grow at first at the apex, i. e. at the end opposite
the place of their origin. This apical growth continues indefinitely in many thallomes
and leaf-bearing axes until checked by some external cause ; this is especially the case
in the primary shoots of Fucacese, pleurocarpous Mosses, Characeae, the rhizomes of
Eqiiisetacege, Ferns, and the primary stems of Coniferae and of many Angiosperms. If
the primary shoots themselves bear organs of reproduction, the apical growth generally
ceases with their development, as in many acrocarpous Mosses, the fertile stems of
Equisetaceae, the haulms of grasses which bear the inflorescence, and in all cases in
Angiosperms where a primary shoot ends in a flower. The lateral shoots are usually
of limited growth ; the growth frequently ceases without any external cause, more
especially when they bear reproductive organs, or become transformed into spines, or
are very different in their shape from the primary shoot, as the horizontal lateral
branchlets of many Coniferae, the leaf-like shoots (phylloclades) of Phyllocladus, Xylo-
phylla, Ruscus, &c.

^ See Nageli, Zeitschrift fiir wissenschaftliche Botanik, and Neuere Algensysteme.

LEAVES AND LEAF-BEARING AXES. 1 59

In by far the greater number of leaves the apical growth ceases early, the apex
itself becoming transformed into permanent tissue. In Ferns, however, the apical growth
of the leaves usually continues, and in many genera is even unlimited, the apex of the
leaf always remaining capable of development, and not becoming transformed into
permanent tissue, as in Nephrolepis ; in Gleichenia, Mertensia, Lygodium, and Guarea, the
growth of the apex of the leaf is, as in many shoots, periodically interrupted, and again
renewed in each period of growth.

(b) Besides the apical growth, there always exists, however, both in stems and in
leaves, an interstitial growth, by which the parts produced by the apical growth increase
in size and become further developed. The development of the internodes of the stem
depends almost exclusively on this interstitial growth, as indeed is shown by the crowded
position of the leaves and the shortness of the internodes in the bud ; it is generally at
first very rapid, and the increase in size occasioned by it is often very considerable ; but
it usually soon ceases, and the tissues become differentiated into unchanging permanent
forms. Not unfrequently, however, a basal zone of the internodes (as in Grasses, Equi-
setum hyemale, &c.), and in many cases the base of the leaf also, remains for a long time
in a condition capable of development, while the parts nearer to the apex, long since
transformed into permanent tissue, have attained their full growth. In this manner
a secondary basal increase in length, often continuing for a long time, is occasioned
in parts which have long ceased to grow above ; this occurs in a peculiarly marked
manner in the long leaves of many Monocotyledons (Grasses, Liliaceae, &c.) which
are sheath-like in their lower part, and to a smaller degree in many Dicotyledons
{e.g. Umbelliferas). Where, as in Ferns, and in a lower degree in many pinnate leaves
of Dicotyledons, the apical growth long remains active, the basal interstitial growth
usually soon ceases, and, 'vice 'versa, continues the longer the earlier the apical growth
comes to an end. Two extreme cases may therefore be distinguished in leaves, although
closely connected by intermediate forms ; the predominantly apical and the predomi-
nantly basal growth.

If the interstitial growth continues at one part of the surface of the leaf, and attains
there a maximum which then decreases, a bag -like projection of the surface of the
leaf is formed, which is termed a Spur, such as occurs in many petals, as Aquilegia,
Dicentra, &c.

(c) Before the tissues which are differentiated from the condition of primary
meristem assume their definite forms, a rapid growth usually takes place in their cells,
which is no longer accompanied by cell-division ; the size of the cells is not unfrequently
increased by this means ten or even a hundred-fold and more. This process, which is
mainly dependent on the rapid increase of the watery sap, may be termed Extension, in
contradistinction to the growth of the younger cells which is connected with their
divisions, and which always precedes the extension. On this extension depends the
rapid unfolding of the parts of the bud, which had long before assumed their main out-
lines, but had remained small. The buds very often remain a long time in a condition
of rest, until a rapid unfolding of the leaves and internodes already formed suddenly
takes place; as, for instance, in the germination of many seeds, and in the persistent
buds of many trees (horse-chestnut), bulbs (tulip), and corms (crocus, &c.), formed in
the summer and germinating in the spring after long rest in winter.

(d) The Axis of gro^vth or of length of a member (as will further be shown in a special
paragraph) is an imaginary line passing from the centre of the base to the apex. The
entire growth both of leaves and of stems is usually most rapid in the direction of this
line ; they are therefore for the most part longer than they are broad or thick. In
stems the growth is most often nearly equal along all diameters ; they assume therefore
cylindrical, prismatic, or bulbous rounded forms. It is, however, sometimes the case that
the growth in length advances much more slowly than that in diameter ; and then the
stem becomes tabular or flat, as in many bulbs, the corm of the crocus, and especially in
Iso'ites. It is only in the lateral shoots of higher plants which have a very limited

l6o MORPHOLOGY OF MEMBERS,

growth that the internodes expand in a plane which also includes the axis of length, and
thus become leaf-like, as in Ruscus, Xylophylla, &c.

In leaves the principal growth is usually in a plane which cuts the stem transversely,
and is mostly symmetrical right and left of a plane which includes the axes of length
both of the leaf and the stem ; the common form of leaves is therefore that of thin
plates symmetrically divided in half in the direction of their length. There occur,
however, cylindrical and roundish tuber-like leaves, in which the growth has been nearly
equally rapid in all diameters at right angles to the axis of the leaf, as in Mesembryan-
themum ech'matum ^.

Sect. 22. Hair (Trichome) '^ is the term given in the higher plants to
those outgrowths which arise only from the epidermis, i.e. from the layer of cells
which always remains the outermost in roots, stems, and leaves, whether these
outgrowths assume the form of simple tubular protuberances, rows or plates of cells,
or masses of tissue, or have the physiological character of woolly envelopes of the
young leaves, root-like absorbing organs as in Muscinese, glands, prickles, or spo-
rangia as in Ferns ^.

Hairs may originate from the primary meristem of the growing point, or from
young leaves and lateral shoots, if an external layer of cells has already been
differentiated as dermatogen, as in Phanerogams; but they may originate also in
much older parts the tissue-systems of which have already become further differ-
entiated, and which exhibit interstitial growth, because in such cases the epidermis
produces new cells, for example stomata, and long remains capable of cell- division.

When hairs spring from the growing point, they are usually formed after the
leaves, i. e. further from the apex than the youngest leaves ; but it also occurs in
Phanerogams that they are developed above the youngest leaves and nearer to the
apex, the outermost layer of cells of the growing point having in this case already
become differentiated as dermatogen, as in Utricularia according to Pringsheim.
In Muscinese and Vascular Cryptogams also, where the leaves become visible long
before the differentiation of the external layers of tissue, the hairs do not appear on
the surface of the stem till a later period and further from the apex.

If the hairs arise near the apex of a growing point or on a zone of interstitial
basal growth, as do the sporangia of Hymenophyllacese, they may be arranged
according to a definite law, which is not the case with hairs that spring from older
organs, or at least not evidently so.

Hairs are always strikingly different in their form from the leaves and lateral
shoots of the same plant, although they sometimes bear a certain resemblance to
these organs in other plants. The development in size of a single hair is usually
extremely small compared to that of the member which produces it ; even the mass
of all the hairs of a leaf, root, or stem is generally quite inconsiderable compared
to the weight of the organ.

^ [The leaf is also frequently unsym metrical, i.e. the growth has not been equally vigorous
of the two halves separated by the axial plane, as in the lime, Begonia, &c.]

2 Rauter, Zur Entwickelungsgeschichte einiger Trichomgebilde. Vienna 18 71, p.. 33. — Compare
also Sects. 15 and 19 (b). — Warming, Sur la difference entre les trichomes et les epiblastemes, d'un
ordre plus eleve (extract from the Videnskabelige Meddelelser de la societe d'Hist. Nat. de
Copenhague, nos. 10-12, 1872.

3 [Hairs may develope into adventitious buds, as in Begonia; see Caruel, Trasformazione di
peli in gemme, Nuov. Giorn. Bot. Ital,, July 1875.]

HAIR (tRICHOME). i6i

(a) The woolly and glandular hairs on buds are distinguished by a remarkably
ipid growth ; they are often perfectly formed long before the parts of the bud unfold,

mt then they generally die off; the persistent hairs which remain during the life of
the leaves are formed much more slowly, and are marked by a great variety of form,
'he root-hairs are formed at a considerable distance from the growing point of the
root, often from i to 2 cm. from the apex, and mostly die off after a few days or
Iweeks, so that the older parts of the roots of even annual plants are destitute of living
lairs. The existence of these hairs is connected with the activity of the roots in the
"ound.
The root-hairs which spring from the stems of Mosses are marked by a very long
Continued apical growth, and often by repeated branching. They consist of cells divided
ito rows by oblique septa, and, viewed physiologically, replace the root-system of
vascular plants. These root-hairs of Muscineae are remarkably endowed with generative
)ower, and behave in many respects like the Protonema, a means of propagation pecu-
liar to Muscineae; like it, they produce gemmae, which, when exposed to light, grow
into leafy stems. If the root-hairs themselves are exposed to the air {e.g. by turning
[up a sod) they put out rows of cells containing chlorophyll, on which also gemmae
[are produced.

(b) Thallophytes, when they consist of a mass of tissue, also form true hairs, like
[Cormophytes ; but when the thallome consists only of one layer of cells, or, like Caulerpa
land others, is unicellular, one can no longer speak of an external layer corresponding to
|the epidermis ; and its hair-like outgrowths cannot therefore be considered as trichomes
pn the same sense as those of the higher plants. Nevertheless it is customary to speak in

5uch cases also of hairs, when the outgrowths are long and slender, destitute of chloro-
Fphyll, and otherwise dissimilar to the thallus which produces them. On the other hand
structures occur in highly organised plants which are closely analogous to many forms of
hairs in their physiological, and partly also in their morphological properties, but which
[differ from true hairs in not originating from single epidermal cells, but consist of
[outgrowths of the tissue which lies beneath the epidermis, remaining however covered
)y a continuation of it. Examples of such structures, which may perhaps be dis-
tinguished by the term Emergences, are afforded, according to Rauter, by the prickles^
and glandular hairs of roses, and perhaps also of the various species of Rubus. Closely
^related to these are probably the warts, tubercles, and knobs on the surface of many
•uits (according to Warming, for example, on the fruit of Datura Stramonium, and,
iccording to my own observations, on that of Ricinus). To the same category belong
Pthe ' beards ' of many petals (according to Warming, e. g. those of Menyanthes trifoliata) ;
the 'tentacles' on the leaves of Drosera, the sharp hairs beneath the calyx oi ^grimcnia
Eupatorium, the pappus of Compositae, &c. Larger emergences of this nature may even
be penetrated by branches of the vascular bundles from the organs which produce them,
(as in Drosera, Datura, &c. They resemble the leaves and branches of Phanerogams in
[their origin and mode of formation, while they agree with hairs in the late period at
[which they are produced, their occurrence on stems and leaves, and their frequently irre-
jular distribution both as respects one another and the organ on which they grow. The
classification adopted by Warming (A c. p. 27), i>/2. including emergences under the term
trichome, and dividing this class of structures into two sub-classes, hairs and emergences,
iems to me, if not false, at all events inconvenient ; because it becomes impossible to
five any exact definition to the term trichome. The fact that emergences constitute a
transition between trichomes, in the stricter sense of the term, and leaves or secondary
ixes, does not justify including them under the former term ; they might as well be
treated as branches of leaves or of stems. If the occurrence of transitional structures
were held to prevent our distinguishing certain groups of members sharply from one
another, then the distinction must be abandoned between phyliome and caulome, or

^ On spines, which must not be confounded with prickles, see Sect. 28.

M

J 62,

MORPHOLOGY OF MEMBERS.

between caulome and root, since transitional structures occur also in these cases. The
occurrence of these transitional structures in nature is in itself a reason for framing exact
definitions. Definitions are not in themselves objects; but are means for arranging
objects, and for enabling us to understand them.

Sect. 23. The term Root^ is applied, in botanical morphology, in contrast
to its use in popular language, only to such outgrowths of the substance of the
plant as are clothed at their growing apex with the Root-cap already described in
Sect. 19. Roots do not form leaves or other exogenous foliar structures; their
epidermal cells, on the contrary, generally develope into long tubular appendages,
the Root-hairs, The apex of every root which is just beginning to be formed lies
beneath the surface of the organ from which it proceeds^; it is then usually

Fig. 120. — Longitudinal section through the
young primary root of the embryo of Marsilea
salvatrix; lus the apical cell, wh', wh", ivh"'
the still simple layers of the root-cap ; x, y the
last segments of the substance of the root ; i i
intercellular s aces.

Fig. 121. — Longitudinal section through a somewhat older primary root o( Marsi/ea salvatrix ; wj the apical cell ; wklx
•wh% the first, wh^-'rivh^ the second, ivh^ the third layer of the root-cap, each layer now consists of two divisions ; 'x y the
youngest segments of the substance of the root ; o epidermis ; gf fibro-vascular bundle ; h the part of the root-cap which
extends furthest back.

covered with thick layers of tissue, which it breaks through in its further growth.
Hence roots are always endogenous formations, by which character they are dis-
tinguished from all trichomes and leaves, and from most lateral shoots.

^ Nageli und Leitgeb in Nageli's Beitragen zur wissen. Bot., Heft IV, 1867. — Hofmeister,
Morpliologie der Gewebe. Leipzig 1868, Sect. 5. — Hanstein, Botan. Abhandlungen. Bonn 1870,
Heft I. — Dodel, Jahrb. fiir wiss. Bot., vol. VII. p. 149 et s^g.— Reinke, Wachsthumsgeschichte der
Phanerogamenwurzel, in Hanstein's Botan. Untersuchungen, Heft III. Bonn 1871. — Van Tieghem,
Recherches sur la symetrie de la structure des plantes vasculaires, Fasc. i, La racine, Paris 187 1 ;
(also in Ann. des Sci. Nat., 5th ser., vol. XIII, 187 1.)

^ I choose this expression because it appears also to fit the primary root of the embryo of
Vascular Cryptogams. %

ROOT. j5^

Roots occur only in those plants the tissue of which is traversed by fibro-
vascular bundles, and they themselves therefore always contain fibro-vascular
bundles ; but these latter differ from those of the stem and leaves in the first
vessels being formed near the circumference of the bundle, while the later
ones are always formed further inside, and hence centripetally in reference to
the diameter of the root. The phlofe'm-bundles lie in the intervals between the

FIG. 122.— Longitudinal section thfough the apex of a root of maize ; a a outer and oldef layers of the root-cap ! i' z Itiner
and younger layers; j apex ; wz^ytlie plerome ; w becomes the pith, ^ a vessel, ^xylem ; ;f 7- the cortex which is produced
from the periblem at the apex; ee epidermis, continued into the dermatogen at the apex; vv thickened outer wall of the
epidermis (cuticle) ; (tlie origin of the root-cap from the dermatogen is not evident here; the figure was drawn lohg before this
discovery).

primary vascular bundles at the circumference of the fibro-vasculaf cylinder (see
Sects. 16. 18.)

Although roots are commonly present in vascular plants, t. e, the higher
Cryptogams and all Phanerogams, there occur even in these groups particular species
from which they are entirely absent. Thus among Rhizocarpese the genus Salviniaf

M 2

1 54 MORPHOLOGY OF MEMBERS.

among Lycopodiacese the genus Pst7o/um, among Orchideae Epipogum Gmelini and
Corallorhiza mnafa, are destitute of roots; the little Lemna {Wolffia) arrhiza does
not form roots, and is at the same time destitute of vascular bundles.

With reference to the place of their formation roots are remarkably variable.
A root is usually present even in the young embryo which proceeds from the
fertilised ovule (but not in Orchideae) ; it appears at the posterior end of the em-
bryonal stem, and may be termed the Primary Root, whether it remains weakly
and soon dies, as in Cryptogams and Monocotyledons \ or whether it continues
to grow more vigorously, like the rest, as in many Dicotyledons. But besides the
primary roots, there are usually formed in addition a large number of Secondary
Roots, or simply Roots; since they are enormously more numerous than the
primary roots, and of much greater importance to the plant, a special name is
superfluous wher^ the contrast to the primary root is not of importance. They
arise in the interior of the primary or secondary roots, and on stems and
petioles. The primary root with its secondary roots, or any root with its lateral
roots, may be termed a Root-syslem. With the exception of many Dicotyledons
with a persistent strongly developed primary root, the majority of roots spring
from stems, especially when these latter creep, float, climb, or form bulbs or tubers.
In Tree-ferns the stem is often densely covered throughout its whole length with
a felt of delicate roots. In Ferns with densely crowded leaves in which no portion
of the surface of the stem is left bare, the roots spring exclusively from the
petioles, as, for example, in Nephrodium Filix-mas, Asplenium Filix-foemina, Cera-
topteris thalidroides, &c. ; sometimes the fronds put out roots as in Mertensia 2.
When the stem possesses clearly developed nodes and internodes, the roots usually
spring from the former ; thus, for example, exclusively from the nodes in Equise-
taceae, and most commonly so in Grasses.

Roots owe their origin either to the primary meristem, or to partially diff"erentiated
masses of tissue, or finally to a secondary meristem enclosed between layers com-
pletely diff'erentiated. The primary roots of embryos arise from quite undiff"erentiated
primary meristem.; the lateral roots of Cryptogams, as Nageli and Leitgeb have
shown, originate near the growing point of roots, where the differentiation of their
tissues first begins; and with Phanerogams the same is the case. But stems may
also produce roots near their growing point, where the differentiation of the primary
meristem first commences; this occurs in the case of the creeping stems of Rhizo-

^ [On the primary root of Monocotyledons, which disappears at an early period, see Falkenberg,
Vergleichende Untersuchungen iiber den Bau der vegetationsorgane der Monocotyledonen. Stutt-
gart 1876.]

^ A leaf of Phaseolus multijlorus cut off at the pulvinus and placed in water developed from the
callus an abundant root-system, and remained living for some months. [The leaves oi Ficus elastica
behave in the same way.] According to Van Tieghem, the cotyledons of the sunflower, scarlet
runner, Cucurhita maxima, Mirabilis Jalappa, &c., when laid on damp moss in a temperature of from
22° to 25° C, produce in a few days a number of roots ; and this takes place even if the cotyledons
are cut into small pieces, the roots then proceeding from the sections of the vascular bundles. I have
myself seen a seedling of Cucurbita covered up too thickly with earth put forth long roots from its
cotyledons. See further Dodel, Jahrb. fur wiss. Bot. vol. VIII. p. 177.

ROOT.

165

m

B

hE

carps and in Pleris aquilina. Roots are formed out of a secondary meristem much
further backwards- from the growing point, where
the tissue is already completely differentiated, in
older portions of stems, and especially when
mutilated, or when kept dark and damp.

The order of development of the secondary
roots is, according to Nageli and Leitgeb, dis-
tinctly acropetal in the primary roots of Crypto-
gams, where they arise near the apex; new roots
are probably never formed in these plants be-
tween those already in existence. The same is
: probably always the case where roots are pro-
duced in the primary meristem or near the grow-
ing point of the stem, as in Pilularia, Marsilea,
■Cereus, &c. But even where their origin is
further from the apex, as with the lateral roots
from the primary root of Phanerogams and
from many stems, such as the maize, they gener-
ally appear in acropetal order ; but owing to
subsequent disturbance roots may arise adven-
titiously, /. e. in abnormal positions, especially on
older primary roots of Dicotyledons.

Secondary roots usually make their appear-
ance on the exterior of the fibro-vascular bun-
dles ; the fibro-vascular bundle of the secondary
root is then placed at right angles, or nearly so,
to those of the mother-root; the cortex is then
only incompletely continuous with that of the
latter, the epidermis not at all so. The case
is different in the primary roots of embryos,
which are formed early and mostly so near the
surface of the embryo that a complete con-
tinuity is possible in all the tissue-systems be-
tween stem and primary root ; but in Grasses
and some other Phanerogams the first root
arises so deep in the interior of the substance
of the embryo that it is covered, in the fully
developed embryo of the ripe seed, by a thick
layer of tissue (Fig. 124, ws), which is ruptured
on germination (Fig. 123, ivs), and is known by
the name of Root-sheath or Coleorhiza. Similar
formations occur also in the first lateral roots of
seedlings of Allium Cepa, and occasionally else-
where. But in other cases the secondary roots
which are formed deeper in the tissue simply split
the layers of tissue which cover them, and project from a two-lipped chink

Fig. 123.— Germination of maize in the order /, //,
///; A and B the embryo separated from /, in A
seen in front, in B from the side ; lu the primary root ;
■ws its root-sheath ; -w', -w", w'" secondary roots ;
e the part of the seed filled with endosperm ; k the
plumule ; sc scutellum of the embryo ; r r its ope:i
margins; * *' *'' the first leaves of the seedling
(natural size).

66

MORPHOLOGY OF MEMBERS.

The typical form of roots is filiform and cylindrical ; their section is usually
circular when not altered by external pressure. ' It is only when roots undergo

a secondary increase in thickness, and serve
as reservoirs for reserve-material, as in
many Dicotyledons and some Monocoty-
ledons, that the original filiform shape is
changed into the fusiform or into tuberous
swellings, as in the turnip, the tuberous
roots of the dahlia, Bryonia, Asphodelus, &c.
Roots rarely form chlorophyll, and
even then, as in Menyanthes, only in small
quantities ; usually they are quite colour-
less, not only when they grow in the
ground, but also in water or air.

A secondary basal growth appears never
to occur in roots, as it does in many
leaves and internodes when the regions near
the apex have already been transformed
into permanent tissue. Interstitial growth
behind the apex often continues, however,
for a long time (in Lycopodiaceae accord-
ing to Nageli and Leitgeb) ; the extension
of the tissue commences immediately behind the terminal part of the root formed
of primary meristem, an arrangement by which the elongation of the roots in the
ground is essentially assisted.

(a) The primary root of the embryo of most Phanerogams gives the impression of
being entirely exogenous, as if its apex were the actual posterior termination of the
embryonal stem ; but its first origin is endogenous ; for the posterior end of the embryo
is originally attached to the ' pro-embryo' or suspensor in Phanerogams, and the primary
root is, at its first origin, covered by this\ There was formerly some doubt as to
the endogenous origin of the primary root of Ferns and Rhizocarps ; but when it is
observed that the root is not constituted as such until the apical cell has thrown off the
first layer of the root-cap, it is evident that in this case also the apex of the new root
lies from the first inside the tissue of the embryo ^.

(b) The origin of lateral roots in a mother-root is always on the outside of its
axial fibro-vascular or plerome-cylinder ; and the points where the new formation com-
mences is — with a few exceptions among Phanerogams — on the outside of the vascular
bundles, so that each bundle corresponds to a longitudinal row of secondary roots.
There are however some differences between the phenomena in Cryptogams (Ferns,
Marsileaceae, and Equisetaceae '^j and Phanerogams, 'z^/z., that in the former the roots
originate from the innermost cortical layer or plerome-sheath which surrounds the

Fig. 124. — Longitudinal section through a grain of Inaiae
(X about 6); c pericarp; n remains of the stigma; fs base of
the grain ; eg hard yellowish part of the endosperm ; ew whiter
less dense part of the endosperm ; sc scutellum of the embryo ;
ss its point ; e its skin ; k plumule ; w (below) the primary
root ; ws its root-sheath; -w (above) secondary roots springing
from the first internode of the embryonal stem st.

^ A more exact account of this, according to Hanstein's researches on the formation of the
embryo, will be given in Book II, on the Characteristics of Phanerogams.

2 Compare the drawings of the embryos of Ferns and Rhizocarps in Book II,

3 In Lycopodiacece (and according to Van Tieghem, Ophioglossaceae) no lateral roots are
formed in the mother-roots, the roots branching dichotomously, and the growing point which is
enveloped by the root-cap splitting into two growing points, each of which forms. its own root-cap
(see Fig, 138, p. 182),

ROOT,

167

fibro-vascular bundle, while in Phanerogams they proceed from the pericambium which
is enveloped by the plerome-sheath (see Sects. 16, 18). In the Cryptogams named
above the new roots originate each from a single primary mother-cell, and there are
always particular cells of the plerome-sheath which give rise to the rudiment of a root,
while in Phanerogams, on the contrary, several of the pericambial cells take part in
the production of each secondary root. Another difference consists in this, that the
plane of symmetry of the secondary root is, in Cryptogams, at right angles to that of
the mother-root, while in Phanerogams (according to Van Tieghem) the two coincide,
at least when the mother-root and lateral roots each contain only two vascular
bundles.

In Ferns, Marsileaceae, and Equisetaceae, where the root developes with an apical
cell which becomes seg-
mented on three sides,
and contributes the cap-
cells to form the root-
cap (Sect. 19), the for-
mation of the lateral
roots commences with
cell-divisions, by v/hich
a three-sided pyramidal
cell is formed with its
base outwards, which be-
haves as the mother-cell
oftheyoungroot. These
mother-cells of the la-
teral roots lie in the
plerome-sheath of the
axial bundle, in front of
its groups of vessels,
and are therefore sepa-
rated from the outer-
most of these vessels
by the pericambium.
Further transforma-
tions take place sub-
sequently in the peri-
cambium, in conse-
quence of which the
fibro-vascular cylinder
of the lateral root co-
alesces with that of the
mother-root. This does
not take place, however,
in Equisetaceae, where there is no pericambium.

In Phanerogams it is also the general rule, as has already been mentioned, for the
lateral roots to originate outside the vascular bundles of the mother-root. An excep-
tion to this is however, according to Van Tieghem, afforded by Grasses, since thes2
have no pericambium exterior to the vascular bundles; the new roots originate there-
fore on the outside of the phloem-bundles which lie between the vascular bundles and
exterior to which pericambium occurs. The phenomena are also different in Umbelli-
ferae and Araliaceae ; a secretion-canal lies here in the pericambium outside each fibro-
vascular bundle ; and the lateral roots are therefore formed midway between each pair
of bundles, and therefore outside the phloem-bundle.

In Phanerogams, according to Reinke, the commencement of a lateral/root is

Fig. 123.— Mode of formation of the lateral roots from a mother-root of Trapa ttatans
(after Reinke). A the pericambium (tt), bounded by the plerome-sheath r, splits into der-
matogen [d) and an inner layer n, which in B is already again divided. C young secondary
root enclosed in the tissue of the mother-root ; R r cortex of the latter ; tt the pericam-
bium of the mother-root from which the secondary root has been formed; It the first
layer of its root-cap, d its dermatogen. D secondary root in a further stage of develop-
ment, enclosed only by the plerome-sheath r of the mother-root ; / > its periblem,
within which is the plerome ;« m.

l68 MORPHOLOGY OF MEMBERS.

indicated by the splitting of several cells of the pcricambium of the mother-root by
tangential walls, so that it is divided into two layers (Fig. 125, ^). The outer layer
is immediately constituted into dermatogen (d), which afterwards forms the layers of
the root-cap by tangential divisions ; since each outer layer of cells which results from
the successive layers of the dermatogen constitutes a layer of the root-cap (C^). The
inner layer of cells (^, « «), which faces the vessels of the vascular bundle of the
mother-root, then also splits again into two layers (B) ; and further longitudinal and
transverse divisions follow, by which the primary meristem of the young root is formed.
This soon divides into periblem and plerome, as may be clearly seen in D, where p p \s
the periblem, and m m the basal portion of the plerome, by which a union is effected
with the fibro-vascular cylinder of the mother-root. While the young root lengthens
somewhat obliquely to the axis of the mother-root and downwards, it compresses the
cortical tissue (i)) ; the plerome-sheath {A-D, r) resists disorganisation longest, and, at
least at first, follows the growth of the young root, surrounding it with a sheath until
it is destroyed. Finally the young root lengthens and its apex protrudes through the
cortical tissue of the mother-root.

(c) In stems lateral roots arise either from the interfascicular cambium {e.g. in Im-
patiens paw'iflora immediately above the soil in the primary stem), or from the outermost
phloem-layer of the fibro-vascular bundles, which is more commonly the case. These
layers of tissue then behave like the pericambium of a primary root, as in Veronica Becca-
bunga, Lysimachia nummularia, or the ivy, according to Reinke.

(d) While the formation of the root-cap, as has already been shown in Sect. 19,
is proceeding at the apex of the root, its outermost layers pass over into per-
manent tissue; the cells retain simple forms, but their walls become thicker, and
in the outermost cell-layers of the cap swell up, become gelatinous, and thus cause
the apex of the root to appear viscid ; finally they die and become detached. In
aerial and underground roots the root-cap is closely attached to the substance of
the root by its oldest layers, which generally extend backwards ; in the roots of Lem-
naceae, Stratiotes, and some other plants, which float on the water, it forms a loose
sheath which envelopes the substance of the root high up, and is only fixed below to its
apex.

(e) Roots are generally clearly distinguished, by the characteristics mentioned
above, from leaf-bearing shoots ; there occur, however, a few transitional forms
which show that roots can become directly transformed into leafy shoots, as in
Neottia Nidus-a'vis, where (according to Reichenbach, Irmisch, Prillieux, and Hof-
meister) older lateral roots of the stem throw off their root-caps and form leaves
beneath the apex. On the other hand, leaf-bearing shoots cease to produce leaves, as
in many Hymenophyllaceae, and, according to Mettenius, form root-hairs, and assume
the habit of true roots (whether they actually form a root-cap is doubtful) ; in these
species true roots are wanting. In Psilotum triquetrum Nageli and Leitgeb have shown
that the apparent roots are only underground shoots, on which more or less evident
traces of leaf-formation may be recognised ; they resemble true roots in function and
in the mode of formation of their tissue, but have no root-cap, and, when they come
above ground, grow in the manner of ordinary leafy shoots. In Selaginelleae also, the
same investigators have shown the presence of leafless shoots (rhizophores) which
grow downwards, and do not form root-caps until they touch the ground (see Book II,
Lycopodiaceae).

We thus see that transitional structures between roots and leafy shoots are found
even in highly differentiated plants. But even in Algae the thallus is often fixed to
its substratum by organs of attachment, which may be compared with roots in their
habit and in many functional properties ; and this occurs not only in the case of the
large Fucaceae and Laminarieae, but even in the unicellular Vaucheria and Caulerpa.

In confirmation of the Theory of Descent referred to at the conclusion of this work
(see Bpok III. Chap. 7), it is of great importance to know that members differing

ROOT. i6g

Ito the greatest extent morphologically and physiologically are connected by transi-
ftional forms, and that, especially in the branched thallomes of Algae, the rudiments
are to be found of all the differentiations of the higher plants. Distinctions which, in
[the ramifications of the Alga-thallus, are only of a weak, undefined, and rudimentary
[character, increase more and more in the higher plants; points which can be sharply
■defined in the latter become indistinguishable when we are considering the more simple
Thallophytes. The more the attempt is made to establish exact definitions for single
forms, the more does one become convinced that all definition, all limitation, is arbi-,
trary, and that Nature presents gradual transitions from the indistinguishable step by
[step to the distinct, and finally to the opposite.

members of a plant spring out of one another ; the members produced may there-
fore be similar (homogeneous), or dissimilar (heterogeneous) to the member which
produced them. In the former case the formation of new members is ordinarily
termed Branching ; in the latter it is regarded as the production of a new member.
A root, for instance, branches in the production of new roots, a stem in that of new
stems, a thallome in that of new thallomes ; in the same sense the production by a
leaf of lateral leaf-structures must also be considered a case of branching. On the
other hand the stem produces also leaves, roots, and hairs; leaves not unfrequently
produce leaf-bearing shoots, sometimes roots, generally hairs; leaf- forming buds may
also arise from roots. But since members which are morphologically dissimilar —
stem, leaf, root, trichome — do not differ absolutely, but only in degree, the difference
between branching and the production of new members, between homogeneous
and heterogeneous growth, must be regarded not as an opposition, but only
as a gradually increasing differentiation of the members which grow out of one
another.

(2) New members may originate either by Lateral Budding or by Dichotomy.
Lateral budding occurs when the producing member, after its previous increase in
length at the apex, forms outgrowths below it, which are from the very first weaker
than the portion of the axial structure which lies above them. Dichotomy, on the
other hand (rarely Polytomy), is caused by the cessation of the previous increase in
length of a member at its apex, and by two (or more) new apices arising side by
side at the apical surface, which, at least at first, are equally strong, and develope in
diverging directions. Lateral budding may either form structures which are similar or
dissimilar to the axial structure ; and thus leaves, roots, hairs, or branches arise by
lateral budding from the stem ; leaflets, lobes, hairs, sometimes leaf-bearing shoots,
or even roots, from the leaf. Dichotomy, on the contrary, never produces struc-
tures which are dissimilar to the producing structure ; the divisions of a root
produced by dichotomy are both roots, those of a leaf-bearing shoot both leaf-
bearing shoots, those of a leaf both foliar structures ; dichotomy hence always falls
under the conception of branching in the above-named narrower sense.

1 Compare the Hterature mentioned in the previous sections, and in addition, H. von Mohl,
Linnaea, 1837, p. 487.— Trecul in Ann. des Sci. Nat. 1847, vol. VIII. p. 268.— Peter- Petershausen,
Beitrage zur Entwickelungsgeschichte der Brutknospen. Hameln 1869.— Braun and Magnus, Ver-
handlungen des Bot. Vereins der Provinz Brandenburg, 1871 (on Crt///o/)s/s).— [Warming, Ramification
des Phanerogames ; Danish with French abstract. Copenhagen 1872.]

170 MORPHOLOGY OF MEMBERS.

Dichotomous branching is very common among Thallophytes, especially Algae
and the lower Hepatica^ ; among Phanerogams it occurs only exceptionally ; among
Vascular Cryptogams it appears to occur in Ferns {e.g. the leaves of Platy cerium
alcicorne) ; but it is the only mode of branching in all shoots and roots of Sela-
ginelleae, Lycopodieae, and in the roots of Isoeteae ^

(3) The origin of lateral members, whether similar or dissimilar to the pro-
ducing member, is either exogenous or endogenous. The former term is applied
when they are formed by lateral outgrowth of a superficial cell or of a mass of
cells which includes the outer layers of tissue, as in the case of all leaves and
hairs and most normal leaf-bearing shoots. A member is of endogenous origin
when it is covered, even when in a rudimentary condition, by a layer of the tissue
of the producing member which does not take part in the new formation, as in
all roots, all lateral shoots of Equisetaceae, and in adventitious buds.

(4) Lateral members of any kind are almost always formed in considerable
numbers on the axial structure which produces them, and even repeatedly one
after another, because the producing structure continues to increase in length,
and the conditions for similar equivalent outgrowths are repeated. Thus the
stem, so long as it continues to grow at the apex, produces leaves, hairs, often
even roots, and generally lateral shoots in great numbers, one after another;
roots usually form in succession many lateral roots, branching leaves usually several
segments. If the apical growth ceases early, the number of the lateral members is
also limited; thus the short primary stem of Welwitschia mirabilis produces only
two leaves. When the increase in length of the stem is very slow, the formation of
lateral shoots from it is sometimes altogether suppressed, as in Isoeies, Boirychhim,
and Ophioglossum.

(5) An axial structure may produce either several equivalent lateral members
at the same level, or only one; in the second case the members formed in
succession are termed solitary, in the first case a Whorl or Verticil. Leaves
often occur in whorls, branches less frequently, roots occasionally (in the primary
roots of Phanerogams). In the same whorl the members may arise either simul-
taneously, as the petals and stamens of many flowers, or the foliage-leaves of
many Phanerogams ; or successively, as in Characese and Salvinieae. A whorl is
a true one when the zone on which the lateral members are inserted is trans-
verse from the first, as occurs in both the last-named plants and in many flowers;
Spurious Whorls, on the other hand, are formed by displacement and unequal
growth in the axis, as in Equisetaceae, where the leaves, roots, and branches arise
from transverse zones which are themselves formed by displacement of three segments
of the stem '-.

(6) Similar and equivalent lateral members usually arise on their common
axial structure in acropetal or basifugal order, i.e. the younger a member is the

^ For further details of lateral branching and dichotomy see the conclusion of this section and
Sect. 25.

^ The three segments, which together form the periphery of the stem, stand at first at different
heights, but arrange themselves, as Rees has shown, in a transverse zone, which developes ex-
ternally a circular protuberance, the rudiment of the leaves (see Book II, Equisetacese).

DIFFERENT ORIGIN OF EQUIVALENT MEMBERS.

171

nearer it is to the apex. The lateral members which are formed from and suffi-
ciently near the growing apex of an axial structure are apparently always acropetal ;
but the order is disturbed when lengthening at the apex ceases and new forma-
tions occur in the primary meristem below it, as in many flowers and in the
abnormal inflorescence represented in Fig. 126. The lateral members formed at
a greater distance from the growing apex of the axial structure are sometimes,
but not always, acropetal. Since branching and the formation of lateral members
out of the growing point occur in nearly all plants^ and, by their regular repetition
at definite points of the growing axis, determine the external form of the plant,
they may be considered as normal, in opposition to the ad'veniitious production of
members which takes place at the older parts of the axial structure at a distance
from the apex and without definite order. Such new formations are equally adven-
titious even when they are of
great importance to the plant
from a physiological point of view.
Adventitious shoots are generally
formed internally by the side of
the fibro-vascular bundles of the
branch, leaf, or root ; but it does
not follow from this that all en-
dogenous shoots are adventitious.
All the shoots of Equisetaceae are
endogenous in their origin; but
they are not adventitious, since
they are produced in the primary
meristem below the apex of the
mother-shoot, and in a perfectly
definite order. It is equally in-
correct to call all roots adventi-
tious although they arise in the
interior of the stem, leaves, or
roots. They are adventitious

only when they occur in older parts, and even then not always; when they
arise close to the growing point of a' mother-root or a stem, they are arranged
in strictly acropetal order, and are for that reason not adventitious. When a
member has a basal zone of growth, and produces lateral members from it, they may
be arranged in basipetal order, as the sporangia on the columella of Hymenophyl-
laceae, according to Mettenius, or the segments of the leaves of Myriophyllum.

(7) When in the higher plants a new individual is formed desdned for per-
manent and independent growth, a leaf-bearing axis is first constituted, /. e. a
shoot on which roots, hairs, and lateral shoots subsequently arise. In all vascular
plants this first shoot arises immediately out of the sexually-produced embryo ; and
the externally undifferentiated embryo must therefore be considered as itself a
primary axis\ In Muscineae, on the other hand, the sexually-produced embryo is

Fig. 126 —Median longitudinal section through a young inflorescence of
the sunflower, the broad axis of which has been injured at the apex s, and has
in consequence ceased growing. The zone z z commenced instead an inter-
calary growth, and behaved as an apical region taking the place of the
summit a s a; the bracts and flowers of this apical portion have in conse-
quence been formed from above downwards in a centrifugal direction ; while
before the injury to the apex they were produced at n n in the normal
manner acropetally. The relative position of the bracts and flowers is also the
reverse in the abnormal part of the inflorescence of what we find normally.

^ Compare what will be found under Rhizocarpese and Angiosperms in Book II.

172

MORPHOLOGY OF MEMBERS.

transformed into the sporogonium, a structure without leaves, roots, or branches,
the sole function of which is the production of spores. A new Moss-plant is,
on the contrary, constituted by the production of a leaf-bearing shoot from
a branch of the alga-like Protonema, which branches, strikes root (by root-
hairs), and is independently nourished. The shoot first produced, from which
are developed the rest, is termed the Primary Shoot ; it is often more strongly
developed than its lateral shoots, as in most Ferns, Cycadeae, Coniferae, Palms,
and Amentiferae. The primary shoot produces Lateral Shoots of the first order
or Secondary Shoots, these again lateral shoots of the second order, and so on.
Nevertheless it often happens that lateral shoots of any order take root and

become detached from the primary shoot;
they then assume all its peculiarities, and
may equally be considered as primary
shoots. But it also happens that the
primary shoot itself is arrested at an
early period, while new orders of shoots
proceed from it which gradually become
more vigorous, as in many bulbous and
tuberous plants. Shoots which become
detached from the mother-plant when but
slightly developed, continue to grow by
independent nourishment, and repeat the
peculiarities of the primary shoot, are
called GemmcB or Bulbils ; they are often
adventitious shoots; but bulbils may also
be shoots of normal origin, as in many
species o^ Allium.

Fig. i^T.—Asplenitan decussatum ; middle part of a
mature leaf; its mid-rib st bears the pinnae / /; at the base
of one of these is formed the bud K, which has also already
put out a root (natural size).

Now that we have already spoken of
the origin of leaves, hairs, and roots, and
entered sufficiently into detail on the more important points (Sects. 20, 21, 22), it only
remains to go a little further into the various modes of origin of leaf-bearing shoots.

(a) In many Ferns leafy shoots arise from Lewves, and especially when the stem
branches but little or not at all, as in Nephrodium Filix-mas, Asplenium Filix-fcRmina,
Pteris aquilina, &c. In these species the buds spring singly out of the lower parts of the
petiole at a greater or less height above its insertion. In other species it is usually
the lamina which produces numerous buds, generally in the axils of the pinnae, as in
Asplenium decussatum (Fig. 127), A. Bellangeri, A. caudatum, Ceratopteris thalictroides, or
on the surface of the leaf itself, as in Asplenium furcatum, &c. In all these cases the
buds produced on the leaves are exogenous in their origin, and those on the petioles
of the first-named species arise, while the leaves are still very young, out of single
superficial cells ^ These shoots take root while still in connexion with the mother-leaf,
but sooner or later become detached ; in Nephrodium Filix-mas and Pteris aquilina often
only after some years, when they have already acquired considerable strength, and the
base of the mother-leaf has died off and decayed.

In Phanerogams buds also occur on leaves, although much more rarely. The best

^ Hoftneister, Beitrage zur Kenntniss der Gefass-Kryptogamen, vol. II. Leipzig 1857.

DIFFERENT ORIGIN OF EQUIVALENT MEMBERS,

173

known are those which are formed abundantly in the indentations of the leaves of

Bryophyllum calycinum ; according to Hofmeister^ they arise before the complete un-

ifolding of the leaf as small masses of primitive parenchyma in the deepest parts of the

incisions of the leaf. In the aquatic Utricularia 'vulgaris weak shoots arise, according

to Pringsheim^, mostly in the neighbourhood of the axils of the divisions of the leaf; in

)th cases these shoots are of exogenous origin. Nothing is known of the develop-

lent of the buds produced on the leaves

)f Atherurus ternatus or Hyacinthus Pouzohii

[(Doll, Flora von Baden, p. 348).

(b) Adventitious shoots springing from
[Roots are always endogenous; they arise,
[according to Hofmeister, in the neighbour-

lood of the fibro-vascular bundles or in the
:ambium, as in Ophioglossum, Epipactis mi-
trophylla, Linaria "vulgaris, Cirsium ar'vense,
the aspen, and apple.

(c) Adventitious Buds arise moreover
iin an endogenous manner under peculiar
^circumstances from older detached leaves

or pieces of stem and root, especially when
;kept damp and in darkness. On this de-
^pends the propagation of many plants in

gardens, as of Begonias from leaves, Marat-

tias from their thick stipules, &c. Adven-
[titious buds also sometimes appear in con-
fsiderable quantity in old stems of woody
; plants ; this occurs on the callus formed

between the bark and the wood, when

the stem is cut off above the root. The

branchlets which break out in old stems

of Dicotyledons and Monocotyledons are,

however, often not true adventitious shoots,

but old dormant * eyes' which have been

left behind, having been formed at an

earlier period as normal exogenous axillary

buds, when the stem itself was still in the

bud-condition ; they had become enveloped

by the bark as the stem increased in thick-
ness, and carried on a feeble existence,

until placed in a condition for active

growth by a favourable accident, as the
, removal of the stem above them (Hartig).

(d) In the genus Iso'e'tes the leaf-bearing shoot arises exclusively from the fertilised
germ-cell or embryo, and forms neither normal lateral buds out of the stem nor any
from the leaves or roots, nor any kind of adventitious buds.

(e) The Normal Formation of Lateral Shoots from the primary meristem of the growing
point of the primary axis is endogenous in Equisetaceae ^. With the exception of the

-primary axis which is developed out of the embryo, all the lateral shoots are here of

Fig. i2B.—Eguis€tu>n arvetise; longitudinal section through
an underground bud in March ; ss the apical cell of the stem ;
b—^b its leaves ; K K' two endogenous lateral buds exposed by
the section. The youngest rudiments of buds are to be found,
however, at b", amd they have probably begun to be formed
even at a greater height (x 50).

J69.

* Hofmeister, Allgemeine Morphologie, p. 423.

^ Pringsheim, Zur Morphologie der Utricularien ; in Monatsb. der k. Akad. der Wissen. Berlin

[Some doubt is, however, now thrown on this exception ; see Book II, Equisetaccoe.]

174

MORPHOLOGY OF MEMBERS,

endogenous origin (Fig. 128, K}C)\ they are developed out of a cell in the interior of
the tissue of the stem near to the growing point, and afterwards break through the base
of the older leaf-sheaths. In some Jungermanniese the normal terminal branching of
the stem takes place partially or entirely by endogenous formation of shoots ^

With these exceptions all normal lateral branches produced at the cone of growth
of the bud or in its neighbourhood are, like the leaves, exogenous.

(f) The lateral branches which arise normally below the growing apex of a mother-
shoot are always produced in acropetal order, like the leaves, with which they exhibit
various relationships as to position, age, and number.

(n) The numerical relationship of the lateral branches to the leaves formed ou
the same axis is variable. If the number is unequal, a greater number of leaves;
than of branchlets usually arises on the same axis; in Muscineae, Ferns, Rhizocarpeae.
Cycadeae, and Coaiferae a much larger number, A branchlet may arise when a de-
finite number of leaves has been formed, as in many Muscineae and some Ferns, or

Fig. 129. — Longitudinal section through the apical
region of a branch of Cletnatis apiifoUa; s apex of the
stem ; b b leaves ; ^^ the first traces of spiral vessels,
bending out uninterruptedly from the stem into the leaves.

FIG. i:-,o.-'^n\h of Muscari botryoides ; one of the
lower bulb-scales is thrown back, in order to show the
numerous buds standing side by side in its axil.

the formation of a branchlet results when the increase in length of the primary axis
and the formation of its leaves ceases for a time and is subsequently renewed, as
in Abies. When the leaves stand in whorls, the number of the lateral branches
may be equal to that of the members of the whorl, as in Equisetaceae, or smaller, as
in Characeae. It is unusual for the number of branchlets to be larger than that of
the leaves, but this occurs in some Angiosperms, where two or more lateral buds often
arise side by side above a leaf (Fig. 130), or one above another, as in Aristolochia Sipho,
Gleditschia, &c. In most Angiosperms the number of the lateral branchlets (with the
exception of the flower-shoots) is, at first, the same as that of the leaves ; but usually
only a much smaller number continue to develope.

(/3) The relationship in position and origin of leaves and branches is constant in each
species and often in a whole cla.ss of plants. The lateral branches arise below the leaves
(according to the acute investigations of Leitgeb 2) probably in all Mosses, as well as in

» [See Leitgeb, Bot. Zeit., 1R72.]

^ Leitgeb, Beitrage zur Entwickelungsgeschichte der Pflanzenorgane, in Sitzungsber. der kaij

DIFFERENT ORIGIN OF EQUIVALENT MEMBERS,

175

the Hepaticae Radula and Lejeunia; the branch springs (as shown in Fig. 116, z, p. 153)
out of the lower part of a segment of the stem, the upper part of which has developed
into a leaf. In Fontinalis this occurs below the median line of the leaf, in Sphagnum
laterally below one half. According to the same observer, the lateral branches arise
in place of a half-leaf beside the remaining half in many Jungermanniese, as Frullaniay
Madotheca, Mastigobryum, Jungermannia tricbophylla \ If each tooth in the leaf-sheath
of an Equisetum be considered as a leaf, the buds originate at the side of the leaves and
between them, for they break through the leaf-sheaths between the teeth. In Gharaceae
and Angiosperms the normal lateral branchlets spring from the acute angle which the
leaf forms with the stem (Figs. 129, 131). Usually only one is formed above the middle
of the insertion of the leaf, or two or three one above another; sometimes several
are formed side by side, as in the bulbs of Muscari (Fig. 130), and the flowers in the
axils of the bracts of Musa. Such branchlets are called Jxillary Shoots; in Angio-
sperms the branching is, with a few doubtful exceptions, always axillary'^

The axillary shoot is usually so situated that it is attached both to the Subtending leaf
in the axil of which it grows and to the primary axis, and is therefore in direct con-

FlG. 131. — Apical region of a primary axis
oi Dictatnnus Fraxinella, seen from above;
J apex of the primary axis ; b b b the young
leaves ; k k their axillary buds, the two young-
est leaves have not yet axillary buds.

Fig. 132.— Young inflorescence of Isaiis
taurica, seen from above ; s apex of the
axis of the inflorescence ; the flower buds
appear beneath it (in whorls of four) ; the
youngest are still simple leafless elevations.

nection with both. But it is not unusual for the lateral shoot to advance along the
primary axis and thus lose its connection with the subtending leaf ^, or contrariwise to
appear as if attached to the base of the leaf away from the axis. Examples of both are
furnished by the sporangia of Lycopodium and Selaginella. The advance of the axillary
shoot along the base of the subtending leaf is not uncommon in the inflorescence of
Phanerogams, where the flower-bud springs from the base of the bract, as in Hippuris
(Fig. 119, p. 155), Amorpha, Salix nigricans, Sedum Fabaria, &c. But, on the other
hand, the subtending leaf may advance on its axillary shoot, when it originates later than

Lkad. der Wissen. zu Wien, vol. LVII, 1868, and vol. LIX, 1869 ; and Bot. Zeitg. 1871, no. 34. See
Iso more in detail, Book II, Muscinese.

^ Leitgeb, Bot. Zeitg. 1871, p. 563 ; see also Book II, Hepaticre.

^ [The relationship of leaf and lateral shoot is intelligible in the Muscine<£, where the two struc-
ires are derived from the same ultimate segment of the apical cell. In Phanerogams Warming
'considers (/. c. p. xxiii) the leaf and its axillary bud (which are always united at their base) to no
less form a whole, and to constitute a double organ whose parts have a different morphological
value and are sometimes equally developed, while in some cases one is developed at the expense of
the other.]

^ [This occurs not unfrequently in the flowering shoots or inflorescences of Phanerogams, when
such shoots are termed extra-axillary. '\

176 MORPHOLOGY OF MEMBERS.

the latter ; in this manner is formed the bract on the flower-bud in Anthemis, Sisymbrium^
and Umbellifcrai. When in addition to this it also happens that after the formation of
the subtending leaf the basal portion common to it and to the bud lengthens, then the
former is elevated on the axis of its axillary shoot, and the shoot appears to have no
subtending leaf, because this latter is placed upon it, and constitutes its first leaf. This
occurs in Tbesium ebracteatum, Samolus Falerandi, Spireea, Ruta, Tilia (in this case the
bract and the entire inflorescence), Borragineae, Solanaceae, and Crassulaceae ^.

(y) In reference to the relative time of production of the leaves and lateral branches
on a primary axis, the general rule is that axillary buds are formed later than their
subtending leaves. This is the case in Gharaceae, Hepaticae, Equisetaceae, and, with the
exception of some inflorescences, in Phanerogams. In the region of growth the sub-
tending leaf attains a considerable size, even in the latter, before its axillary shoot is
formed ; and in the meantime new and younger leaves are produced above the youngest
bud. In those inflorescences, on the contrary, where the formation of leaves is more or
less suppressed, the bud is often formed immediately after the bract, or at the same time,
or even earlier than it ; and in the two last cases the bract has often the appearance of
being a product of the bud (see conclusion of par. /3). It may even happen that when
the flower-buds are formed very rapidly, the production of bracts is altogether sup-
pressed, as in most Gruciferae (Fig. 132)^.

(b) The fact that lateral shoots arise far most frequently at a greater distance from
the apex of the stem than the youngest leaves, distinguishes them sufficiently from
dichotomous branchings, which must always of necessity arise above the youngest leaf.
But even when the leaves are visible only later than the corresponding buds, as in the
inflorescence of Grasses, or is even completely suppressed, as in most Gruciferae, it is
still impossible to confound lateral with dichotomous branching, if, as in these cases, the
cone of growth greatly overtops the youngest lateral bud, and continues to grow in a
straight line (P'igs. 117, 119, pp. 154, 155). Still more conspicuous is the distinction
between lateral branching and dichotomy when the primary axis ends in a broad flat
apical surface, as in the young capitula of Compositae. Here the lateral shoots (the
flowers) are so small in comparison to the mother-shoot, and are from the first placed at
so great a distance from its apex, and so uniformly on all sides of it, that the mother-
shoot must be regarded as the independent centre of all new formations. The idea
of dichotomy supposes, on the contrary, that the mother-shoot ceases as such, and that
two branches, at first at least equally strong, continue to grow in length in diverging
directions in its place.

If it is desired to include lateral branching from the growing point and dichotomy of
the apex under one common term, in order to distinguish them from the adventitious
formation of branches from older portions of the stem, leaves, or roots, the expression
terminal Branching commends itself.

Sect. 25. Different capacity for Development of the members of a
Branch-system^. — Systems of similar members originate by branching; out of
a root a root-system originates, out of a shoot a shoot-system ; when a leaf
branches, we get a pinnate, digitate, lobed, or incised leaf, &c. We may therefore
examine the more important relationships of form of such a system, taking into
account for the time only the relative size and capacity for development of the

^ See Warming, Recherches sur la ramification des Phanerogames. Copenhagen 1872, p. xx.

2 [On the suppression of the bracts in Gruciferae, see Masters, Journ. Linn. Soc. 1875, vol.
XIV. p. 39I-]

3 Nageli und Schwendener, Das Mikroskop, p. 599. — Hofmeister, Allgemeine Morphologic der
Gewebe, Sect. 7. — Kaufmann, Bot. Zeitg. 1869, p. 886. — Kraus, Medic.-Phys. Soc. in Erlangen, Dec.
5, 1870. — Warming, /. c.

I

DEVELOPMENT OF THE MEMBERS OF A BkANCH-SVSTEM.

177

ranches of the various orders. We may here leave adventitious branchings en-
tirely out of consideration; for it is evident that in respect to the phenomena
now under consideration they play no essential part in the building up of the
whole plant. We have therefore to do only with the branchings which arise af
the end of a growing shoot, leaf, or root, i. e. with terminal branchings. These
may be referred (as has already been shown in Sect. 24, div. 2) to two principal
forms, dependent on the origin of the branching by dichotomy or by lateral
branching; branch-systems of the first kind may be called simply Dichotomies, of
the second kind Monopodia.

A Dichotomous Branch-system, according to the definition given in Sect. 24, is
the result of the cessation of the growth at the apex in the original direction,
and its continuation in two new direc-
tions at newly constituted apical points,
as is very clearly shown in Fig. 133 ^
We may term the newly formed branches
Bifurcations, and the member which pro-
duces them the Base of the bifurcation.
Every base can only bifurcate once; but
every branch may again become the base
of a new bifurcation 2.

A Monopodium arises when the gener-
ating structure, following the direction of
its previous growth, continues to grow
at its apex, while lateral structures of a
like kind are produced beneath it in acro-
petal succession, their longitudinal axes
being placed obliquely or transversely to
that of the generating member. The
generating member, since it continues to grow during the branching, may form
numerous lateral members; for all these it is the common base; hence the name
Monopodium (Figs. 119, 123, 132). Every lateral branch may again branch in
the same manner, and thus itself become a monopodium of the second order.

Fig. 133.— Dichotomy of the X^^tCAms Qi Dictyota dichotoma
(after Nageli) ; the order of development is according to the
letters A—E; the letters t—z indicate the segmentations of
the apical cell before it dichotomises ; i is the division-wall by
which the dichotomy commences ; 2 — 6 the segments of the
new apical cells.

^ Since we have to give here a narrower application to the term Direction of Growth, it will
be necessary to compare with this Sect. 27.

^ In Cryptogams with apical cells it may be thought that dichotomy must necessarily be brought
about by longitudinal division of the apical cell. When the segments arise by transverse division
this is actually the case, as is shown in Fig. 133 ; but when the segmentation of the apical cell takes
place in two or three rows, this would necessitate that the dichotomising wall should bisect its
inferior angle, and thus have a position which is apparently universally avoided in cell-division.
It is nevertheless possible that a true dichotomy may take place without this. Suppose the old,
apical cell, immediately after the formation of a new one by its side, were to change the direction
of its longitudinal growth, so that both apices diverge from the previous direction of growth ; the
old apical cell then represents the apex of a new direction of growth. From this it seems to me
that we are able to arrive at the distinction between a dichotomy and monopodium. Mutatis
mjitandis this is also true of Phanerogams which have no apical cell. It is necessary here again
to point out that the occurrence of transitional forms between dichotomies and monopodia does
not prevent our giving an exact definition of these terms ; it is only, in fact, by this very means that
transitional forms can be recognised as such.

N

178

Morphology of members.

Just as the dichotomy may consist of numerous bifurcations, so may a mono-
podium consist of several orders of monopodial branching.

These definitions refer only to the bud-condition of the branch -system. Not
unfrequently, both in dichotomous and in monopodial systems, the original character
is maintained in their further growth; the two bifurcations develope, in the case
of dichotomy, with equal strength and branch uniformly ; in the case of a mono-
podium the primary axis continues to grow more strongly than all the secondary
axes, and branches more copiously. But it is very commonly the case that in
a dichotomous system some of the bifurcations are weaker, or that in a mono-
podial system some of the lateral axes, soon after their formation, grow more

strongly and branch more copiously than
the primary axis. In such cases the ori-
ginal character of the branch-system be-
comes less and less evident as it developes;
and it may happen that systems originally
dichotomous have subsequently the appear-
ance of monopodia, and vice versd. It is
hence impossible to infer from a mature
system whether it originated in dichotomy
or in lateral branching. It will therefore
be desirable to make a simple classification
of the most important changes which a
branch-system undergoes during the de-
velopment of its members.

(i) The Developvient of Dichotoinous
Systems may take place either in a bifurcate
or a sympodial manner; I call a system
bifurcate when at each fork the two
branches develope with equal strength, as
in Fig. 134, A. The dichotomous system is
developed sympodially when at each bifurcation one branch developes more strongly
than the other ; in this case the base of each successive bifurcation forms apparently
a primary shoot, on which the weaker branches appear as lateral shoots (Fig. 134, B,
C). The apparent primary shoot, which in fact consists of the bases of consecutive
bifurcations, may on this account be termed a Pseud-axis or Sympodium. Thus in B
(Fig. 134) the sympodium is composed of the left-hand branches /,/,/; in C of the
alternate left and right-hand branches Ir^lr. Whether the case represented in B,
which, on account of its similarity to certain monopodial systems, may be termed a
Helicoid (bostrychoid) Dkhoiomy, actitally occurs is doubtful (it probably does how-
ever in the leaf oi Adiantuin pedaium). On the other hand the development repre-
sented in Fig. 134, C is common in shoots of Selaginelleae, and, on account of its
resemblance to some monopodial systems, may be termed a Scorpioid (cicinal)
Dichotomy \

(2) The development of Monopodial Systems may take place in a racemose

Fig. 134.— Diagram of the various modes of develop-
pient of a dichotomy ; A one developed in a bifurcate
nianaer ; B » helicoid ; C a scorpioid dichotomy.

^ On Dichotomous Inflorescences see l^ook II, Phanerogams.

DEVELOPMENT OF THE MEMBERS OF A BRANCH-SYSTEM.

179

II'
Hi

or cymose manner; and the cymose development may be either apparently
dichotomous (or even apparently polytomous) or sympodial.

(a) A racemose system occurs when the monopodial mother-shoot continues
to develope more strongly than all the lateral shoots, and when the lateral shoots
of each successive order behave in the same manner in respect to their mother-
shoot. This occurs very clearly, for instance, in the stems of most Conifers
(especially Pinus, Araucana, &c.) and in the compound leaves of Umbellifers.

(b) The cymose development of a monopodial system, or a Cyme, depends on
the fact that each lateral shoot begins from an early period to grow more strongly,
and in consequence of this, also branches more copiously than the mother-shoot,
the growth of which then usually soon ceases. Two principal forms of Cyme
may be distinguished, according as a pseud- axis (sympodium) is formed or not.

(a) When two, three, or more lateral shoots arise beneath the growing end of
each shoot, which develope
in different directions more
strongly than their mother-
shoot, the growth of which
soon ceases, a false Dicho-
tomy (or Trichotomy, or
Polytomy) arises. Fig. 135
represents the formation of
a false dichotomy; the shoot
/ produces the shoots // ,
//'', originally weaker, but
soon growing more strong-
ly, while the growth of /

eases; the same takes place

ith T//", and ///'^ False

ichotomies of this kind, which occur abundantly in the inflorescences of Phanero-
gams, are termed by Schimper Dichasia, But instead of two lateral branches
growing out in opposite directions, three or more shoots standing in a true
or spurious whorl may develope more strongly than their mother-shoot, and
thus arises an umbellate system, such as is developed in a typical manner in the
inflorescences of our native Euphorbias; a system of this kind may be called a
Cymose Umbel.

(/3) The sympodial development of an originally monopodial system occurs
when one lateral shoot always developes with greater vigour than its mother-
shoot, as is shown in Fig. 136, A, where the lateral shoot 2, 2 grows more
strongly than the part 2,1 of its mother-shoot, and so on. 'Usually the portions
of all the shoots which lie below their lateral branches develope more strongly
than the terminal portions, as is indicated in the figure by the thicker lines ; the
terminal portions (indicated by thin lines) often die off early; the thicker basal
portions of the different ramifications which proceed from one another then com-
monly place themselves in a straight line, and have the appearance of a connected
whole ; like a primary shoot to which the terminal portions of each separate order
of shoots are attached as if they were lateral branches. The apparent primary

N 2

Fig. 135.— Diagram of a false dichotomy (dichasium) ; the numerals Indicate the order
of development of the shoots of the system.

3 8o

MORPHOLOGY OF MEMBERS,

shoot of the system is called the Sympodium or Pseud-axis. It consists, in
Fig. 136, B, of the pieces between i and 2, 2 and 3, 3 and 4, 4 and 5 ; the weaker
terminal portions of the respective branches i, 2, 3, &c. are bent sideways. A
comparison of Fig. 136 C with A shows that between a sympodially developed and
a spurious dichotomous system the only point of difference is that in the latter each
branch produces two stronger lateral branches. If in C one of the branches is
imagined to be suppressed alternately left and right, the form A results, which is
then easily transformed into B.

Sympodial systems occur in two different forms, according as the lateral
shoots, the basal portions of which form the pseud-axis, arise always on the
same side or on different sides of it.

Fig. 136.— Cymose branching's represented diagrammatically ; v4, 5 scorpioid (cicinal) cynje ; Cdichasium; Z> helicoid (bostry-
choid) cyme ; the numerals indicate the order of succession of the lateral shoots which spring from one another.

If the sympodial ramification takes place always on the same side — e. g. always
to the right, as in Fig. 128, Z>, or always to the left — the whole system is called a
Helicoid Cyme ^ or Bostryx ; if, on the other hand, each branch which continues
the system arises alternately right and left, as in Fig. 136, A^ B, the system is
a Scorpioid Cyme or Cicinus. If in these cases we have to do with leafy shoots
where the leaves are arranged spirally, a more exact definition of the terms right and
left becomes needful. It is then necessary to imagine a median plane drawn through

* [Some difficulty will perhaps be felt with regard to Fig. D, which stands for a helicoid cyme
in the text, but which is also identical with the scorpioid cyme of descriptive botany, and corresponds
to the specific name ' scorpioides'' given by Linnaeus to several plants in which it occurs. The term
scorpioid was introduced by A. P. De CandoUe (Organographie, vol. I. p. 415), to express a unilateral
cyme the undeveloped portion of which is usually rolled up. This is the characteristic inflorescence
of Borraginese, amongst which Myosotis has long been distinguished as ' scorpion-grass ' on this
account. Bravais (Ann. des Sci. Nat. 2nd ser. vol. VII. p. 197) distinguished the helicoid cyme,
which he defined as having the successive flowers ranged in a spiral round the pseud-axis. He
amended De Candolle's definition of the scorpioid cyme by pointing out that the flowers are in two
rows parallel to the pseud-axis,]

DEVELOPMENT OF THE MEMBERS OF A BRANCH-SYSTEM.

l8l

the axis of growth of each shoot and through that of its immediate mother-shoot;
then, in the helicoid cyme each following median plane always stands right or
left of the preceding one, following the course of the leaf-spiral ; in the scorpioid
cyme, on the other hand, the consecutive median planes stand alternately right and
left.

(a) In Thallophytes and the Thalloid Hepaticae, dichotomy is very common, but
monopodial branchings also occur developed in the most various ways. The dichotomous
branching is unusually clear and generally bifurcate among Algae, especially in Dictyoteae
and species of Fucus (in particular F. serratus). In some there occurs a tendency towards
a sympodial development of the bifurcations, but usually only at a late period ; so
that the dichotomous branching can be clearly recognised at the ends of the branches
even with the naked eye. The same is the case, among Hepaticae, in Anthoceroteae,
Riccieae, Marchantieae, and in Met%geria (Fig, 137), where a flat expansion of the thallus
or thalloid stem arises between

the young bifurcations, first of
all ' as a protuberance (/' y"),
which however cannot be con-
sidered as a continuation of
the shoot, since it has no apical
cell or mid-rib ; subsequently
this protuberance disappears,
asin/'"\

Distinctly monopodial (la-
teral) ramifications are parti-
cularly clear in filamentous
Algae, when the apical cell re-
mains unbranched, and lateral
branches grow only out of the
individual cells (segments of
the filament) ; as in Cladophora,
Lejolisia, &c. It occurs how-
ever sometimes that lateral
branches proceed out of the
apical cell itself, as is espe-
cially shown in Stypocaulon (Fig. 108, p. 139). In other cases the branching of the
apical cell is dichotomous, as in Coleocheete soluta (see Book 11, Algae).

(b) In the roots of Ferns, Equisetaceae, and Rhizocarpeae, as well as in those of
Conifers, Monocotyledons, and Dicotyledons, the branching is always, as far as is known,
at first monopodial, and even at a later period the primary root generally remains stronger
than its lateral roots ; these root-systems are therefore developed in a racemose manner
(Fig. 123, p. 165); this is seen very beautifully in the root-systems which proceed from
the primary roots of Dicotyledons when they are allowed to germinate and grow in water.
Dichotomy of roots occurs only in Lycopodiaceae, and probably in Gycadeae, where they
appear at a later period as systems of bifurcations. According to the most recent
researches of Nageli and Leitgeb, it is still altogether doubtful whether the branching
depends, even in Lycopodiaceae, on true dichotomy'^; but the root-branches of Lycopo-

FlG. 137.— Flat thallus of Metzfreria fnrcata branching dichotoniously
(X about 15) ; m vi mid-rib consisting of several layers, branching dichoto-
mously at ^; ss apical points of the branches; yythe wing-like expansions
of the thallus, consisting of one layer of cells ; /' f" the wings between the
mid-ribs of younger branches. (The left-hand figure is seen from below, the
right-hand one from above.)

^ For the above-mentioned reasons I share Kny's view that the branching is in this case
dichotomous. (See Hofmeister, Allgemeine Morphologic, p. 433.)

^ Compare Nageli's Beitrage ziir wissen. Botanik, Heft IV, 1867. I would lay less stress on the
relation of dichotomies to the apical cell, because the latter has scarcely the same decided signifi-
cation in Lycopodiaceae as in Ferns, Equisetacese, and other Cryptogams ; and the apical growth
apparently approaches nearer to that of Phanerogams.

l82

MORPHOLOGY OF MEMBERS.

diaceae always arise so near to the apex, and they assume at so early a period the
character of dichotomies developed in a bifurcate manner, that, until further inves-
tigation proves the contrary, they must be considered as such. It is scarcely necessary
to mention in conclusion that when roots branch dichotomously the bifurcations are at
first covered by the original root-cap, as is shown in Fig. 138.

(c) Lea-ves. Bifurcations resulting apparently from true dichotomy occur in the leaves
of some Ferns, e.g. Platycerium alcicorne^ ; and, according to an older statement of
Hofmeister, it appears that the branching of Fern-leaves generally commences dicho-
tomously, although mature leaves mostly resemble a monopodium. On a mid-rib
forming a continuation of the petiole are placed numerous secondary mid-ribs with
secondary laciniae (pinnae). Since these branches are apparently always alternate and
not opposite, and the terminal lobes of the leaves are frequently developed as equally
strong bifurcations, leaves of this kind may be considered, according to Hofmeister's
hypothesis, as dichotomies developed in a sympodial (and indeed a scorpioid) manner,
the mid-rib representing the sympodium, and the apparent lateral branchlets the weaker

Fig. 138. — Dichotomy of the root of Isoeies l^Kiistris (after
Hofmeister) (x 400); 1 1' the apical cells of the branches; wh
the old root-cap formed before the bifurcation ; wA2 the two
root-caps of the branches, sti 1 covered by the former one ; e
epidermis ; / parenchyma ; tyfibro-vascular bundle of the root.

Fig. 139. — Part of a male Rov/er oi Rtcinus cotnrnunis
cut through lengthways ;yy the basal portions of the com-
poundly branched stamens ; a their anthers.

branches (as in Fig. 134, C, p. 178) ; a process which is repeated in the segments of the leaf
itself when the leaf is doubly or many times pinnate. A similar interpretation may per-
haps be permitted of the simply pinnate leaves of Cycadeae. The repeated branching
of the stamens in the male flowers of Ricinus appears, according to Payer ^, to proceed
from dichotomy, and to a certain extent even from polytomy, commencing at an early
period. The separate stamens appear as roundish protuberances on the floral axis, and
each of these immediately forms two or more similar protuberances on its surface, and
on these the same process is again repeated. When mature, the stamens (Fig. 139)
appear as if divided dichotomously or trichotomously upon long stalks, the branches
being developed somewhat irregularly.

* The petiole of Adiantum pedatum divides above into two equally strong branches, each of
which forms a helicoid cyme of ramifications arising probably by dichotomy ; the weaker branches
of the helicoid cyme stand upright and their numerous pinnce form a scorpioid cyme produced
by further dichotomy. This is one of the most beautiful forms of leaves, the history of the
development of which would be of unusual interest.

^ Payer, Organogenic de la fleur, pi. 108, cor.finned by Warrhing /. -.

DEVELOPMENT OF THE MEMBERS OF A BRANCH-SYSTEM.

iSi

On an originally monopodial branching depends, on the other hand, the form of
the pinnate, lobed, divided, and toothed foliage-leaves of Angiosperms^. The leaf
appears on the cone of growth as a roundish protuberance which quickly broadens into
a shell-like form (Fig. 140, ^, b), and grows vigorously at its apex. Beneath the apex
protuberances arise at the right and left in acropetal order; these also grow in
the same manner at their apex (/), and produce again lateral protuberances of the
second order (0) ; which, according to the extent to which the surface of the leaf is
developed, become lobes of a simple leaf or distinctly separated leaflets.

When two rows of lateral branches arise successively on the median axis of the leaf,
they generally remain weaker than it, and their lateral branches are also less numerous
and weaker ; the development of such an originally monopodial branch-system of kaves

Fig. 140.— Development of the pinnate leaves of
Unibelliferse ; A, B oi Pastinaca sativit; C of LcVi-
stictint officinale; A apical region of the primary
stem ; its cone of growth is seen at j, its youngest
leaf at b; b' youngest leaf but one With the pinnation
commencing; C. bs apex of tlie leaf ; y, y"', /" ' leaf-
branches of the first order ; ^ of the second order.

FIG. 141. —Leaves of Amorphophallu bulbosUs ; A with a simple, B with
a tlireefold branching of the lamina;.

is therefore racemose. But the development may also be cymose, atid rii^y even- lead to
the formation of sympodia, especially when only one branch arises right and left on the
primary leaf. This is the case, for instance, in the leaves of Helleborus^ Rubusy and of
several Aroideae, as Sauromaium and Amorphophallus . Fig. 141, ^ represents a weakly
leaf of the last-named plant with only one branch on each side ; but when the leaves
attain a more vigorous development, as shown at jB, each lateral lobe, 2 2, forms on its
outer side again a lobe of the third order, 3 3, which again produces a similar one of the
fourth order, 4 4, and so on. According to the general definitions given above, the first
branch of the leaf, i, forms with 22a dichasium; but each branch of the dichasium

* This was first shown in detail by Nageli (Pflanzenphys. Untersuch. von Nageli und Cramer,
Heft 11) in the leaves of Aralia spinosa, — See Eichler, Zur Entwickelungsgeschichte des Blattes
(Dissertation, Marburg, 1861).

i84

MORPHOLOGY OF MEMBERS.

dcvclopes further only on one side, the new branches always arising either only on the left
or only on the right side, 3 from 2, and 4 from 3 ; every lateral branch thus produces a
sympodial system, and in fact a helicoid cyme.

If now the basal pieces 2, 3, 4, combined in a sympodial manner on both lateral shoots,
are imagined to be much shortened, so that the bases of the lobes 2, 3, 4 come close to
the base of the lamina i, then all the lobes of the leaf will appear to spring from one
point, and the leaf is called digitate. It would appear, however, that such leaves may also
arise by the formation from the broad end of the young leaf itself, first of a middle lobe,
and then of new lateral lobes right and left from above downwards, as in Lupinus,
according to Payer's drawings (Organogenie de la fleur, pi. 104). If the lobes remain
completely United or have the appearance of a continuous lamella, we have a peltate
leaf \ It is impossible to go more into the detail of these processes without numerous
illustrations which cannot be given here. Fig. 142 will explain, in conclusion, the
origin of the quadripartite lamina of the leaf of Marsilea Drummondi, according to

Hanstein (Jahrb. fur.
wissen. Bot. vol. IV).
The leaf has its origin
in a cell of the cone
of growth of the stem,
which, becoming the
apical cell of the leaf,
produces two rows of
segments from which
the right and left halves
are formed. Thus a
broad cone first arises,
growing at its apex, and
bent towards the stem
{A, B) ; when this,
which is the future
petiole, has attained a
certain height, it in-
creases in breadth right
and left. Beneath the
still growing apex, Z), ^j,
a protuberance {stb) arises on both sides ; and while the latter (destitute of an apical
cell) becomes still more arched (C, stb), the apical growth of the leaf ceases (C, bs)^ its apical
cell disappears, and soon two equally strong outgrowths arise near the apical point, which,
like the earlier lateral ones, increase vigorously and grow out into broad lobes of the leaf.
Thus arises a quadripartite lamina at the end of the petiole, the lateral lobes of which
have resulted from lateral branching, but the middle ones by dichotomy. The four
lobes remain, as they grow, narrow at their base, becoming much broader at the free
margin ; and, since the part of the leaf from which they originated remains short and
narrow, they appear, in the mature leaf, to spring from a single point, the end of the
petiole.

(d) Branch-system of Leaf-bearing Shoots. The branching of the stem of Lyco-
podiaceae is dichotomous. In Psilotum triquetrum all the branches develope uniformly;
and this is the most regularly developed dichotomy found among vascular plants.
In Lycopodieae the development is much more irregular, but the bifurcation is
always evident throughout; in Selaginelleae, on the other hand, it is generally to be

Fig. 142. — Development of the leaf of Marsilea Drn^nmondi (after Hanstein). A, C, D
seen from the inner surface; B longitudinal section vertical to ^ ; bs apex of the leaf;
g—z the segments of the apical cell ; stb lateral lobes of the lamina in their earliest state.

^ Compare further Trecnl, Formation des feuilles, in Ann. des Sci. Nat. vol. xx. 1853; and
Payer, I.e. p. 403; also Entwickelung der Blattgestalten, Jena 1846.

DEVELOPMENT OF THE MEMBERS OF A BRANCH-SYSTEM. 1 85

recognised only on the youngest branches, since the bifurcations are developed sym-
podially, and in fact as scorpioid cymes. This often happens (as in Selaglnella
Jiabellata) in such a manner that the entire outline of a branch consisting of nume-
rous bifurcations assumes a form similar to that of a multipinnate Fern-leaf. The
student who desires to obtain a clear idea of the different modes of development of
a system produced from a dichotomous origin, and especially of the formation of
sympodial forms out of dichotomies, could find no better object of study than the
Selaginelleae which are cultivated in all hot-houses. On the branching of the stem of
Ferns and Rhizocarps, reference should be made to the description of the respective
classes in Book II.

The branching always originates monopodially in the stems of Gharaceae, Equiseta-
ceae, and Coniferse, and here also its future development is always racemose. The
branch-systems of Mosses also always originate monopodially, but are sometime developed
sympodially (as the ' innovations' of Acrocarpous Mosses beneath the sexual organs). It
is often very irregular, but is sometimes of such a nature that much-branched systems
of shoots develope racemosely and assume defined outlines, like those of multipinnate
leaves, as in Hylocomium, Thuidium, &c.

The branching of Monocotyledons and Dicotyledons is always originally monopodial,
but the mode of development of the system is extraordinarily variable ; on the same plant,
and even on the same branch-system, different forms, both racemose and cymose, may
arise. The peculiarities of the different forms of development are usually very con-
spicuous in inflorescences, and are of many different kinds ; and since the attention of
botanists has been turned for a long time in this direction, they are not only copiously
employed in the description of plants, but also furnished with names, which are here
lused, in a more general sense. A more special description of those branch-systems
which, in the case of Flowering Plants, are called Inflorescences, will follow in the general
■ consideration of Angiosperms in Book II ; here it is only necessary to mention that the
forms distinguished as spikes, racemes, and panicles are examples of the racemose
^development, while those termed dichasia, cymose umbels (in Euphorbia), and
;scorpioid and helicoid cymes, are examples of the cymose development of branch^
[systems which are at first monopodial.

Every other form of vegetative branching of Flowering Plants may be regarded from
the same point of view. The formation of sympodia is not unfrequently brought
'about by arrest of the growth of the terminal portion or bud of the shoot, while the
nearest lateral bud developes more vigorously, and appears like a continuation of the
mother-shoot, as in Robinia, Corylus, Cercis, and many other plants; in the lime the
rprimary stem itself is a sympodium formed in this manner. If the flower-bearing shoots
1 above ground die annually, while the underground portions remain in a living condition,
underground sympodia sometimes arise composed of the comparatively short but thick
; basal portions of numerous larger shoots which have long since died off. This is the case,
for instance, in Po/ygonatum mult'iflorum, the underground stem of which is known under
the name of Solomon's Seal. In Fig. 143 is represented the anterior portion of one of
; these underground stems, those produced during eight previous years having been re-
moved. The stem denoted by b 1866 is the lower portion of the upright aerial shoot
bearing leaves and lateral flowers, which was in existence in that year ; but this shoot is
.itself only the terminal part, its much thicker basal portion is denoted in the diagram B
(as seen from above) by « + 2 ; the slenderer terminal part dies off in the autumn, and at
h, b, beneath the numbers 1864 and 1865, are shown the scars which remain behind
after the death of the similar earlier terminal parts. The portion of the sympodium
[here represented thus consists of the three basal portions n, n+ i, n + 2, oi three shoots,
each of which unfolded its aerial portion bearing leaves and flowers in the year indicated.
In the same manner the bud « + 3 will now develope further ; it springs from the axil
of the leaf, the scar or insertion of which is denoted by 9". The basal portion of the
shoot which proceeds from it will add a new piece to the sympodium, its terminal part

i86

MORPHOLOGV OF MEMBERS.

will grow upwards, develope leaves and flowers, and then die off. Just as « + 3 sprang
from a leaf-axil as a lateral shoot of n + 2, so did this also spring from « + i. Each
of these shoots produced on its basal portion nine membranous colourless scale-like
leaves^ which are still partially retained in « + 3, while in n, n + i, and n + 2, only their
scars are to be seen; the numbers 1-9 indicate these in each year's growth. The new
lateral shoot arises each year in the axil of the ninth and last scale-leaf, and the suc-
ceeding leaves are foliage-leaves on slender elongated internodes, while the internodes
of the basal portion between the membranous scale-leaves are thick and short. The
leaves are in two rows on the basal parts, alternately right and left, as may be seen by
their scars ; if the position of the ninth leaf of the segment « is called left, then that
of the segment « + i is right, that of the segment n + 2 left ; the shoots which continue
the sympodium are thus again alternately right and left ; and hence the sympodium is in
this case a scorpioid cyme.

It is evident that the processes of growth would remain precisely the same, if, at the

close of each period
of growth, after the
bud for the next
year had attained
sufficient vigour, the
whole shoot, includ-
ing its basal portion,
had died off and
decayed ; then, of
course, no sympo-
dium would be
formed, but the
development of the
underground buds
would nevertheless
be sympodial. This
occurs, for instance,
in our native tu-
berous species of
Ophrys, but with
the difference that
if a sympodium were
actually formed, it
would be a helicoid cyme. The processes in Colchkum are similar, but somewhat more
complicated.

The explanation of processes of growth of this nature requires much space, as is
shown by the above example ; I must refer therefore to the labours of Irmisch men-
tioned below ^. Where the leaves are clearly developed in Monocotyledons and Dicoty-
ledons—and it is only in a few forms of inflorescence that this is not the case — it is
almost always easy lo understand the true nature of a branch-system, even without
microscopic examination ; because, with but few exceptions, the branching is axillary ;
the position of the leaves then makes it sufficiently clear which is mother-shoot and

Fig. n^.—Polygonatum multijlorum ; the anterior piece of a much longer rhizome, consist-
ing of four annual growths. ^ seen in profile, B from above ; all the adventitious roots have been
cut off, their position being indicated by the roundish scars. The numbers 1864, 1865, 1866
denote the years in which the respective pieces of the sympodium have been produced.

^ [Niederblatter or ' Cataphyllary leaves ' of Henfrey ; Braun's Rejuvenescence in Nature ; in
Ray Soc, Botanical and Physiological Memoirs, 1853, p, 4.]

2 Irmisch, Knollen und Zwiebelgewachse. Berlin 1850.— Ditto, Biologic und Morphologie
der Orchideen. Leipzig 1853. — Ditto, Beitrage zur Morphologie der Pflanzen. Halle 1854, 1856. —
See also his papers in the Bot^nische Zeitung and the Regensburg ' Flora.' [Henfrey, Bot. Gaz.
r85o, 1851.]

DEVELOPMENT OF THE MEMBERS OF A BRANCH-SYSTEM.

187

Fig. 144.— Apical region of a shoot of Coriaria mrrtifolta ; A in
transverse section, B in longitudinal section; s apex of the stem; bb

in decussate whorls of twos ; k axillary bud ;

which lateral shoot when we have sympodial pseud-axes. Sometimes, however, distor-
tions occur {e.g. in Solanacese) which might lead to erroneous conclusions if reference
were not made to the earliest stages of development.

Sect. 26. The Relative Positions of Lateral Members on a Common
Axis\ — In order to bring the facts which we have now to consider into a clear
and simple arrangement, it is necessary, in the first place, to explain the use of a
few technical expressions and geometrical modeg of representation.

By the term Axial Structure or Axis is to be understood, in future, when the
contrary is not expressly stated, any member that continues to grow at its apex
and produces lateral members ; for example, a mother-root with its lateral roots,
a stem with its leaves ^ the mid-rib
of a leaf with its leaflets, pinnae,
or lobes, or a thallus-shoot with its
lateral outgrowths.
^ If two or more similar lateral
members proceed in different direc-
tions from the same transverse zone
of an axis, they constitute a Whorl.
A true whorl results when the zone
of the axis which produces it is
always at right angles to the axis
(Fig. 136); a Spurious or Pseudo-
Whorl when the zone is the result
of unequal development of the axis,
or when lateral members which
were formed at the same level
have become so far separated by
subsequent unequal elongation of
the axis, that they appear, in the
mature state, distributed into different
zones. Simultaneous Whorls are those
whose members are formed simul-
taneously (Fig. 144). Whorls are
successive when the members at the
same zone grow in succession either
right and left, as is shown in Fig 145, and as occurs in the true leaf-whorls of

^ Roper, Linneea, 1827, p. 84. — Schimper-Braun, Flora, 1835, pp. 145, 737, 748. — Bravais,
1. des Sci. Nat., vol. VII. 1837, pp. 4^» I93- — Wichura, Flora, 1844, p. 161. — Sendtner, Flora,
1847, pp. 201, 217. — Brongniart, Flora, 1849, p. 25. — Braun, Jahrb. fur wissen. Bot. vol. I. 1858, p.
307._Irmisch, Flora, 1851, pp. 81, 497. — Hanstein, Flora, 1857, p. 407.— Schimper,ditto, p. 680. —
Buchenau, Flora, i860, p. 448. — Stenzel, Flora, i860, p. 45. — Numerous papers by Wydler, e. g.
Linnoea, 1843, p. 153; Flora, 1844, 1850, 1851, 1857, 1859, i860, 1863, and elsewhere. — Hofmeister,
Allgemeine Morphologie der Gewebe, §§ 8, 9. [Haughton, Manual of Geology. — Ellis, Mathe-
matical Tracts.— A. Dickson, Trans. Royal Soc. Edinb. vol. XXVI. p. 505.— Chauncey Wright,
Mtm. Araer. Acad. vol. IX. p. 379. — H. Airy, Proceedings Royal Society, vol. XXI. p. 176. —
Beal, American Naturalist, 1873, vol. VII. p. 449.]

'^ [The term Phyllotaxis is used in works on descriptive botany to denote the mode of arrange-
ment of leaves, and especially of the foliage-leaves on the stem.]

leaves in pairs, t.
g youngest vessel.

Fig 145.— Development of the flower of the mignonette (after Payer) ;
A a younger, B an older bud ; from the latter the anterior sepals s have
been removed, the posterior ones left; // petals; j/ stamens, the pos-
terior ones already large, the anterior ones not yet even in a rudimentary
state ; c the rudiment of the pistil.

i88

MORPHOLOGY OF MEMBERS.

Chara ; or in a different order, as in the true leaf-whorls of Salvinia {vide infra),
and in the three- or five-parted calyces of most Phanerogams.

The lateral members are, on the other hand, isolated or scattered when each
member stands on a different zone of the axis. If the surface of an axial structure
(which sometimes is quite imaginary, as in Nephrodium Fitix-mas, &c.) is supposed
to be continued through the base of each lateral member, the section forms its Plane
of Insertion. An imaginary point in this is considered its organic centre, but does
not usually correspond to its geometrical centre ; this point may be termed the Point
0/ Insertion (see Sect. 27). A plane which bisects a lateral member symmetrically,
or divides it into two similar halves, and contains the axis of growth of the lateral
member as well as that of the axial member, passes through the point of insertion,
and is called the Median Plane of the lateral member in question. If members are
so arranged at different heights on an axis that their median planes coincide, they
form a straight row or Orthostichy ; generally there are two, three, or more orthostichies
on an axial structure, and the members are then said to be recti-serial. If there are

Fig. 146.— Diagrams of a shoot with the leaves arranged
with a uniform divergence of ^.

Fig. 147. — Diagram of the flower-stalk of Paris quadri-
folia; //whorl of the large foliage-leaves beneath the flower;
ap outer, ip inner perianth-whorl ; aa outer, ia inner stamens ;
in the centre is the rudiment of the pistil consisting of four
carpellary leaves.

no orthostichies, /. e. if the median planes of all the members intersect one another
on an axis without coinciding, their arrangement is solitary.

The angle which the median planes of two members of the same axis
enclose is their Divergence ; it is expressed either in degrees or as a fraction of
the circumference of the axis, which is then supposed to be a circle, although in
fact this is not usually the case. In Order to represent the divergences clearly, they
may be drawn on a horizontal section of the vertical axial structure, in the manner
represented in Figs. 146 and 147. The transverse sections of the axial structure
which bear the lateral members — in this case leaves — are denoted by concentric
circles, the outermost circle corresponding to the lowest, the innermost to the
highest transverse section. On these circles, which thus represent the relative
ages from without inwards according to their succession in the acropetal develop-
ment of the axis, the positions of the members are denoted by dots, or the forms
of the planes of insertion themselves may be approximately indicated, as in the
figures. On such a projection or diagram the median planes of the members

RELATIVE POSITIONS OF LATERAL MEMBERS.

189

appear as radial lines, indicated in Fig. 146 by I-V. Since in this case several
members stand upon each median plane, they are arranged in orthostichies ; and
these again are so placed that they divide the circumference into five equal parts.
But if the members are considered in reference to their age, as indicated by the
figures i-ii, it is seen that the divergence between i and 2 is f, as also is that
between 2 and 3, between 3 and 4, and so on. The divergences are therefore all
equal, or the members have in this case the constant divergence f. In Fig. 147
the members are arranged in a quaternary whorl; on each circle or section there
stand in this case four similar members with the divergence J ; but the successive
whorls are so placed that the median planes of one whorl exactly bisect the
angle of divergence of the preceding and following whorls ; the whorls are here
alternate, and all the members are arranged in eight orthostichies. If, on the other

Fig. 148. — Diagram of a weakly plant of Euphorbia helioscopia ; c c the cotyledons ; /. / the first, i— lo the later foliage*
leaves ; numbers 6—10 form one whorl ; at ^ / in the centre is the terminal flower of the primary shoot, B II the terminal
flower of one of the five axillaryshoots, ///, ///, /// the leaves of three axillary shoots of the second order.

hand, two whorls stand one over the other in such a manner that their members fall
into the same median planes or cover one another, they are said to be superposed.
Thus, for instance, the staminal whorl is superposed to that of the corolla in
Primula; and in the primary roots oi Phaseolus, Tropceolum, Cucurbita, and other
Dicotyledons, superposed whorls of lateral roots not unfrequently occur. When
alternate whorls have only two members, they are said to be decussate, as in Fig. 144,
a very common arrangement with leaves.

If it is required to represent by a horizontal projection not merely the diver-
gences on an axis but those on an axial system, such as a system of leaf-bearing
shoots, it may be done on the same principle, as is shown in Fig. 148. Each

190 MORPHOLOGY OF MEMBERS.

system of concentric circles comprises the members — in this case leaves — on an
axis ; the lateral axes — here secondary shoots — are interposed between the insertion
of the respective leaves and their primary axis.

If the axial members are greatly shortened, the view (from above) of an
axis, with its lateral members, often itself supplies the diagram ; as, for instance, in
the leaf-rosettes of Crassulaceae, and in most flowers. In other cases a transverse
section through the bud enables the observer to examine the divergence of the
leaves ; but in many other cases the relative positions are more obscure, and can
only be ascertained by careful examination. In addition to the study of the history
of development, particular methods, depending on geometrical principles, are often
necessary in order to represent the relative positions correctly and at the same
time clearly.

There are also circumstances in which it is desirable, instead of representing
the relative positions on a horizontal projection, to project them on the unrolled
surface of the axial structure, considered as a cylinder the surface of which is
flattened out. The transverse sections of the axis lying one over another are
denoted on this surface by straight horizontal lines on which the positions of the
members are drawn.

Among the diff"erent arbitrary constructions which may be attempted on paper,
for the purpose of comparing the relative positions of the members on an axis,
or of reducing them to short geometrical or arithmetical expressions, the following
has been employed. A line is imagined proceeding from any one of the older
members in such a direction that, passing round the axis towards the right or
the left, it includes the points of insertion of all the successive lateral members
in the order of their age; the horizontal projection of this line is called the
Genetic Spiral; in reality it is a spiraP running round the stem more or less
regularly. The importance of this construction has been very much overrated,
and it has been employed where it is not only inapplicable to the elucidation of
the history of development, but even where it has not even a geometrical meaning,
and no longer assists a conception of the relative positions, but even makes it
more difficult and complicated.

When we are dealing with solitary leaves or shoots, standing out from the axis
in three, four, five, eight, or more directions, and when the divergences are not too
variable, the construction of the genetic spiral is of excellent service for a ready
understanding of the position of the leaves (Fig. 149) ; and a more exact knowledge
of the peculiar properties of this ideal line may, under these circumstances, be of
great use in morphology. In some cases it may be applied with advantage even
to the relative position of whorls ; but in a large number other constructions appear
much more natural, since they aff"ord an easier explanation of the relative positions,
and are more in accordance with the phenomena of growth. The construction
of the desired genetic spiral is altogether impossible where the leaves are formed

* If the spiral winds from right to left, the right edge of the leaves (as you ascend) is called the
kathodic, the left edge the anodic ; the reverse in the spiral of an opposite direction seen from
without.

RELATIVE POSITIONS OF LATERAL MEMBERS.

191

in simultaneous whorls \ as the petals, stamens, and carpels of most flowers; or

even in successive whorls where the members are formed in advancing order right

and left, as in Characeae and the flowers of

Reseda (Fig. 145). In the successive whorls of

Salvinia naians the construction of a genetic

spiral would be equally impossible. Fig. 150, B

shows the diagram of the stem of this plant

with three consecutive three-leaved whorls ; in

each of these the leaf w is formed first, then

the leaf Z^ and finally

the leaf Z,

If

attempt be made
must pass from w
again
to Z„

an

it

to construct the spiral,
over Z„ across to Z,, then

Fig. 149. — Transverse section through the convolu-
tion of the ieaf-sheaths i — 6 oi Sabal ttmbraculifera ;
in the centre is a young leaf-blade. The arrangement
of the leaves is a ? divergence. If the numbers i — 6
are united by a line, the genetic spiral is obtained.

'1'

in the same direction over w across
,2, the figure thus formed is a circle, in
which the divergences of successive leaves vary
greatly. If we now pass to the next whorl, the
line proceeds in a spiral direction to the next
leafw; but then, to retain the genetic succes-
sion in the second whorl, the line must be continued in an opposite direction ;
and this is repeated with every new whorl. It is evident that no clear conception
can be obtained in this forced manner, and the whole construction appears alto-
gether superfluous, since it is _
required by no feature in the
history of development. The
stem of this plant is construct-
ed, as Pringsheim has shown,
of two rows of segments {G,
RJ, K, &c., in Fig. 150, A),
which arise alternately right
and left from the apical cell.
Even before the production of
the leaves each segment under-
goes various divisions, and in
this manner the stem is built
up of transverse disks which
are in alternate succession
nodes and internodes. Each
nodal disk consists of the
interior half of an older
segment and the posterior
half of a segment next younger in age, as shown in the figure. An internode
is formed of a whole segment of one row and of two half-segments of the other

Fig. x^o.—A the cone of growth of the stem of Salvinia natans, regarded
diagrammatically and looked at from above; xx projection of the plane which
divides it vertically into a right and left half; the segments are indicated by
stronger outlines, their divisions by thinner lines ; the succession of the segments
is denoted by the letters F—P; B diagram of the stem with three whorls of leaves,
its ventral side indicated by v v ; w the first-formed floating leaf; L\ the aerial leaf
formed next ; Li, the second aerial leaf of the same whorl formed last of all between
the two first (after Pringsheim) ,

* Many writers employ even in such cases the conceptions borrowed from a spiral arrangement,
considering arbitrarily as of successive origin the members of the whorl which arise simultaneously;
but this is not in harmony with a true scientific method.

19^ MORPHOLOGY OF MEMBERS.

row. Nodal cells occupying clearly-defined positions produce the leaves in the
order stated. This development furnishes no evidence that the leaves are formed
in spiral succession ; the bilateral structure of the stem shows rather that a
spiral construction is in this case altogether inadmissible. The same may be
shown to be the case in Marsilea, where the creeping stem bears on its upper
side two rows of leaves, while the under side forms roots; the leaves borne on
the upper side may in this case be united in the order of their age by a zigzag

Fig. 151.— Diagram of a flower-stalk oi Fritillaria imperialis, showing the divergences of the first twenty-four foliage-leaves ;
the relative lengths of the internodes are indicated by the larger or smaller distances between the circles.

line broken right and left, which does not anywhere touch the leafless under side
of the stem, and corresponds in its course to the bilateral structure of the stem.
The spiral construction appears also to be meaningless in all those cases where it
is indifferent whether the spiral be carried right or left. This is the case where the
members are placed in two rows, with a' constant divergence of \, and are thus
arranged alternately in two orthostichies lying exactly opposite to one another, as is
the case with the branchings of many thallomes {e.g, Stypocaulon, Fig. 108, p. 139),
the leaves of Grasses, the lateral shoots of the lime, elm, hazel, &c. In all these

RELA TIVE POSITIONS OF LA TERAL MEMBERS,

193

-cases of decidedly bilateral construction the genetic spiral might be imagined just as
well, and with the same divergence, ascending right or left, by which of course it
loses its importance for any morphological conclusion, as much as if one supposed
it to change its direction from leaf to leaf.

It is principally in upright axes with solitary leaves arranged in three, four,
five, or more directions, that the spiral construction appears conformable to
nature, and agrees with the symmetrical relationships of plants, of which more
will be said hereafter. The spiral construction proves to be opposed to nature
in bilateral structures, especially in creeping or climbing stems, and in lateral
branches.

In those cases in which the spiral construction may be employed naturally
to elucidate the relative positions of the members, two cases may be distinguished,
^according as the divergences, on the one hand, are very unequal and change
.abruptly, or, on the other hand, are nearly or quite equal to one another or
.only change gradually. In the first case the members appear to be arranged
irregularly and without order, as the foliage-leaves on the stem of Fritillaria
■iviperialis (Fig. 151), the flowers on the rachis of the raceme of Triglochin
palustre or of many Dicotyledons. When the change of divergence on the same
axis is abrupt, it may also ap-
pear more natural to represent
.the phyllotaxis by two homodromal
spirals instead of one, as in many
species of aloe, where the shoots
commence with leaves arranged in
two rows, and then pass over into
complicated divergences which lead

■finally to rosettes of leaves radiat- fig. 152.— Transverse section of a shoat of ^/,?i'.S-?rr«.

ing on all sides. This occurs,

e.g. in Aloe €iliaris^ latifolia, brachyphylla. Lingua, nigricans, and Serra. Fig.
152 shows the transverse section of a shoot of the last-named species; the
first six leaves are arranged alternately in two rows with a constant diver-
gence \\ at the 7th leaf this arrangement is suddenly changed; instead of
being placed over 5, its position is between 5 and 6; but the 8th leaf exhibits
.the divergence \ from the 7th; the 9th again changes the divergence, instead of
^being placed over 7, it is between 7 and 6; the loth leaf again diverges about \
.from the 9th; and so on. The leaves 7-15 are evidently arranged in pairs,
the pairs being 7, 8; 9, 10; 11, 12; 13, 14; each pair consists of two alternate
4i.e. not opposite) leaves, the divergence of which is \\ but the pairs them-
, selves diverge from one another by smaller fractions. If it is desired to unite all
the leaves from i to 15 by a genetic spiral, an abrupt alteration of the divergence
would occur in it. The relative positions are shown, however, more simply
and clearly if, keeping in view the bilateral origin of the shoot, two spirals are
constructed, each of which commences from one of the original orthostichies, and,
r50 to speak, continues it in a spiral curve ; the one contains all the leaves with an
'even number, the other those with an uneven number; the two are homodromal,
^running in the same direction round the stem. The bilateral origin of the shoot

o

194

MORPHOLOGY OF MEMBERS,

may be followed in this manner up to the terminal rosette of leaves. Similar
phyllotaxes appear to occur in Dracccna and in some Aroidea^ ; and at first sight
present the appearance as if the leaves were placed in two rows which have become
changed into spirals by the torsion of the stem.

If we now turn to those cases which clearly gave rise to the erroneous
hypothesis that the primary law of phyllotaxis is a universal spiral arrangement,
we find the leaves placed singly, and their divergences almost or quite equal or
gradually passing over into some other value, thus corresponding to the second
case named above of spiral arrangement. In these cases the spiral construction
affords a simple expression of the law of phyllotaxis; the only thing required
is to name the constant angle of divergence ; — according as this is J, j, -g^, f , |,
&c., the phyllotaxis is termed simply one of \, \, \, and so on. It is usual in
such cases for the divergence not to remain constant for all the lateral members
of an axis; shoots which form numerous leaves mostly begin with more simple
arrangements, as ^, and then pass over into more complicated ones, an arrange-
^ ment being considered more complicated

when the numerator and denominator
of the fraction of divergence are larger.
When the divergences between lateral
members placed solitarily with a spiral
arrangement are equal, they must also
stand in straight rows, the number
of which is expressed by the denom-
inator of the angle of divergence. If,
for instance, the divergence is a constant
one of I, as in Fig. 153, there are eight
orthostichies, the 9th member standing
on the same median plane as the ist,
the loth as the 2nd, the nth as the
3rd, and so on. In a f phyllotaxis, in
the same manner, the 6th member stands over the ist, the 7th over the 2nd, and so
on. In some cases the orthostichies are very obvious, as, for instance, in cacti with
prominent angles to the stem, the angles corresponding to the orthostichies of the
spirally arranged leaves, which, however, in this case mostly remain undeveloped.
In verticillate leaves also the straight rows are mostly conspicuous if the shoot be
looked at from above, as, for instance, in the decussate two-leaved whorls of Eu-
phorbia Lathy r is ^ and the cactus -like E. canarihtsis.

When the members of a spiral phyllotaxis with a constant angle of divergence
stand sufficiently close to one another, other spiral arrangements are easily seen,
one of which may be followed to the right and the other to the left, and more or less
completely concealing the genetic spiral. These rows are called Parastichies, and
are particularly clear in fir-cones, the leaf-rosettes of Crassulacese, the flower-heads
of the sunflower and other Compositse, and the spadices of Aroideae. They may be
seen in ev-ery spiral phyllotaxis with a constant divergence, and can always be made
clear in the diagram, or when the arrangement is represented on an unrolled
cylindrical surface. The consideration of these constructions leads to definite

Fig. 153.— Diagram of a shoot in which the leaves have
3 constant phyllotaxis of S.

RELATIVE POSITIONS OF LATERAL MEMBERS, 1 95

geometrical rules, by means of which the genetic spiral can be easily deduced
from the parastichies \

It is evident that the constructions hitherto mentioned can only be more -or
less convenient aids to an understanding of the actual principles of the arrangement
of leaves. But in order to obtain, with their assistance, a deeper insight into the
processes of growth themselves of which these principles are the result, it is
necessary to follow the development, and in every single case to ask the question,
what circumstances are the cause of a new member being formed just in this place
and nowhere else. It may be well, therefore, to bring forward here some of the
points which must be considered in reference to this view.

(i) The first point is always to determine with certainty the order of succession
in which the lateral members are formed.

(2) Attention/must be paid not only to the lateral divergence, but also to the
longitudinal distance at which a new member is formed at the growing point above
the members last preceding it. The longitudinal distances of the youngest
lateral structures of a growing point from one another are usually very small ;
there is often no space to be distinguished between them, i. e. between the
planes of insertion of the youngest members. This circumstance may, on the
one hand, assist in the determination of the place where the next member must be
produced ; but, on the other hand, may give occasion, as the development of the
axis proceeds with its crowded lateral members, to compression and distortion, by
which the original arrangement is altered.

(3) By the increase in length of the common axis, members which were at
first closely crowded become placed at a considerable distance from one another,
while others, in consequence of slower growth, remain closely packed; so that a
different distribution occurs in different parts of the 'stem, as in the leaf-rosettes and
flower- stalks of Crassulaceae, Agave, Aloe, &c. In the same manner the angle of
divergence frequently becomes changed by the more rapid increase in thickness of
the axial structure on one side than on the other; and still more commonly by
torsion round its own axis of growth. By such torsions lateral members, arranged
at first exactly in straight rows, become displaced so that the orthostichies appear as
if wound spirally round the axis. This occurs, for instance, according to Nageli
and Leitgeb, in the root-systems of Ferns, Equisetaceae, and Rhizocarpeae, as well as
in the three-rowed phyllotaxis of the Moss Fontinalis antipyretica, according to
Leitgeb. But the most striking example is furnished by the stem of the screw-pine,
Pandanus uHlis. In the bud, the numerous leaves, already strongly developed,
stand, as is shown by the transverse section, in three perfectly straight lines with
the phyllotaxis \ ; but, as the development of the stem advances, it undergoes so
severe a torsion that the three orthostichies are transformed into three strongly
curved spiral lines running round the stem (see Fig. 154). In these and similat
cases the change in the relative positions caused by the torsion of the actual
structure can be easily and certainly determined. But when the structures are so
arranged at the apex of the axial structure that the angle of divergence cannot

* As the treatment of the subject is only of value to those who are practically concerned with
phyllotaxis, I must refer to the detailed description in Hofmeister's Allgemeine Morphologic, § 9.

O 2

ig6

MORPHOLOGY OF MEMBERS.

be accurately estimated by an apical view from above, it must remain uncertain
whether the position of the mature members is unchanged, or has been altered
by lateral displacement and torsion 'of the axis. A displacement, for instance,
of about '9° would be sufficient to suiter the divergence from f to |, a similar
displacement of 1-3° would change the divergence from ^^ to ^j. When the phyl-
lotaxis is very complicated and the number of the longitudinal rows very large,
extremely smaH and almost inappreciable distortions are sufficient to destroy the
original arrangement, and to bring into existence altogether different systems of

parastichies. This observation is of in-
terest so far as it makes it seem doubtful
whether certain complicated phyllotaxes
are always due to the original arrange-
ment of the members \

(4) It must be observed whether the
position of newly-formed members or the
subsequent change shows any relation to
the direction of the force of gravitation, of
the light which falls upon them, or of any
pressure acting from without^. The effect
of the force of gravitation is that primary
shoots which are in the main upright put
forth leaves spreading on all sides ; while
such as have a decidedly horizontal growth,
in which a rooting under side is contrasted
with an upper side, usually show an ar-
rangement of leaves on the latter in two
rows, or one which is divided into two
equal halves by a plane cutting the stem
longitudinally, as Salvmia, Marsilea, Poly-
podium aureum, Pteris aquih'na, &c. When
vertical primary shoots with leaves in
several rows bear secondary horizontal
branches with leaves in two rows, this
relationship is less clear, as in the cherry-
laurel, sweet chestnut, hazel, &c., because
an influence independent of gravitation
must in these cases be presumed to be
exercised by the primary upon the lateral
axis, as is shown by the position of the
leaves in the lateral buds before unfolding (see Fig. 155, p. 208).

(5) It must further be observed whether the first appearance of lateral

Fig. 154 —Diagframmatic representation of the orthostichies
bf a i phyllotaxis in Pandantis utilis ; A before, B after the
torsion of the stem. Each of the orthostichies /, //, /// is indi-
cated by a double line ; the genetic spiral is simple ; where
it crosses the orthostichy, the leaf-insertions are indicated by
figures.

' [See Airy, Proc. Royal :Soc. /. c]

' Hofmeister (Allgemeine Morphologic, §§23, 24) has collected a series of facts which show
relationships of this kind ; but, both with reference to the individual facts and to the interpretation
which he gives, I am decidedly of a different opinion, the reasons for which would carry me loo far.
(Fit/ffi«/ra, Sect. 27.)

RELATIVE POSITIONS OF LATERAL MEMBERS, I97

members is preceded by circumstances connected with their development which assist
in determining their place of origin. 0-f this nature, for instance, is the connexion
between the points of origin of lateral roots and the fibro-vascular bundles, the
course of which determines the arrangement of the roots in rows; and this in
turn determines the lateral roots being arranged spirally or in whorls. Here
the arrangement in longitudinal rows is clearly the general and primary one; the
divergences and longitudinal distances are a secondary effect determined by special
accessory circumstances. The point of origin of a lateral shoot is, on the other
hand, in general primarily determined by its relation to the nearest leaf, since it must
be formed beneath, beside, or above its median plane ; forces of secondary import-
ance then determine whether lateral shoots are formed in connexion with each leaf
or only with particular leaves of an axis, and so forth. The phyllotaxis of the lateral
shoot may differ from that of its primary shoot, because the growth^ of the latter
assists in influencing it ; as, for instance, in theccase of lateral shoots with a distichous
phyllotaxis on primary shoots with an arrangement in. several rows. Under this
heading falls also the bilateral branching of leaves, whether -the stem itself be bilateral
or multilateral. The dimensions of the growing point; and the thickness of the axial
structure derived from it may also determine the number of the rows of lateral
structures; thus thick mother- roots usually produce three or more rows of seconc^ary
roots, while more slender primary roots produce only two rows or at all events a
smaller number. Thus, for instance, the roots of Cryptogams (according to Nageli
and Leitgeb), the thick primary roots of the-, maize, oak, pea, scarlet-runner, &c.,
form three, fourj five, six, or more orthostichies of lateral roots, which, on their part,
are much slenderer and produce fewer* orthostichies. The same is not unfrequently
the case with the phyllotaxis of stems. When the size of the grov/ing point increases,
the leaves are arranged in a larger number of rows, as in the vigorous seedlings of
many Dicotyledons, in Palms, Nephrodium Filix-mas, &g. This is most strikingly
exhibited in the many-rowed flower-heads of the sunflower on the four-rowed foliage-
stem, the size of the growing point undergoing a sudden and great increase at the
period when the flower-head is being formed (Fig. 126, p. 171). But, vice versS^
the number of the rows of leaves diminishes when the size of the growing end of the
stem decreases in consequence of vigorous growth in lengthy this is seen, for
instance, in the few-rowed long and slender peduncles which proceed from the
many-rayed leaf-rosettes of s^QCiQS-^'Oi Alo€\ Echeverm, &c. If the insertion of the
leaves or shoots takes up, at an early stage, a large part of the periphery at the
-growing point, only a few rows of leaves are formed; if the insertion-planes are
relatively small, the number of rows- on the axis increases. This is- illustrated by the
many rows of small flowers in "the spadicesof Aroideae or the racemes of Trifolium,
while the leaves of' the same plants are in few rows, their insertions embracing the
stem or being-even broaden Hofmeister \ to whom we owe the introduction of this
point of view in the theory of phyllotaxis, states the general rule in' the following

B' Allgemeine Morphologic, § 11, where particular cases are discussed in detail. This treatise is
ond question the most important that has hitherto been written on phyllotaxis; nevertheless, in
my account, which necessary limits have confined almost to a mere sketch, I differ from Hofmeister's
views even in some points of primary importance.

jgS MORPHOLOGY OF MEMBERS,

words : — New lateral members have their origin above the centres of the ividest gaps
which are left at the circumference of the growing point between the insertions of the
nearest older members of the same kind. The rule is illustrated by the case of alterna-
ting whorls (especially of pairs crossing one another at right angles, or 'decussating '),
or by that of distichous leaves with a base which grows early in breadth, in
Phanerogams where the growing point consists of small cells. Where, on the other
hand, we have decidedly bilateral horizontal axes, as in Pieris aquilina, Salvinia, and
Marsilea, or definite relations of the phyllotaxis to the segmentation of an apical
cell as in Mosses, or distinctly successive formation of the members of the same
whorl, as in Char a, Salvinia, the flowers of Reseda, kc, the mechanical importance
of the rule is, in my opinion, subordinate to the other causes which then have the
greatest influence in determining the position of the new members. Independently
of the points of view referred to in paragraphs 1-4, the genetic relationships indicated
in this paragraph show that it is scarcely possible to find a single rule which will
govern all cases of phyllotaxis. Causes which belong to altogether different cate-
gories must, according to circumstances, exercise the greatest influence in determining
the point at which a new member is formed.

(6) I consider it a circumstance of primary importance that the same or very
similar kinds of phyllotaxis may be brought into existence by very different com-
binations of causes, and arrangements apparently very different by very similar
combinations of causes. Among the causes here referred to I include the anterior
development of the axis and of its lateral members, the influence of the primary on
the secondary axes, the effect of pressure, gravitation, light, and similar conditions.
This position becomes evident when it is observed that the same or similar diver-
gences of leaves or lateral shoots may occur everywhere, in unicellular plants, in
multicellular plants with a distinct apical cell, and in those in which the growing
point consists of a small-celled tissue without any definite relation to the segmentation
of an apical cell, as in Phanerogams. The mechanics of growth must undoubtedly
be different when the lateral bi-anches of the single cell of Vaucheria are formed in
two rows, and the leaves of a Fissidens or of a Grass are produced in the same or
a similar position, in which case the cell-walls of the primary meristem represent a
multiplicity of causes of growth and of hindrances to it. The similar arrangement
of the outgrowths under such different circumstances does not prove that the cir-
cumstances themselves are indifferent, but only that altogether different combinations
of causes may lead to very similar relationships of position. In Muscinese and
Vascular Cryptogams the relation of the formation of leaves to the segmentation
of the apical cell is the more obvious the nearer the leaves originate to the apex.
It is most obvious of all in Mosses, where each segment grows out into a leaf-
forming protuberance as soon as it is formed, and before further cell- division
takes place. Here the immediate controlling cause of the position of the leaves is
that of the leaf-forming ' segments ' themselves ; when these latter are formed in two
alternating longitudinal rows, as in Fissidens ^, two rows or orthostichies of alternating
leaves arise with the divergence \. When the segmentation of the apical cell is into

* Lorentz, Moosstudien. Leipzig 1864.

RELATIVE POSITIONS OF LATERAL MEMBERS, I99

three rows, so that each new division-wall of the apical cell is parallel to the last
division-wall but two, as in Foniinalis, three rows of leaves result, arranged spirally
with the constant divergence \. Whem the apical cell is a three-sided pyramid,
but the new walls which are formed in it are not parallel to those already in existence^
but oblique, so that, for example, all the segments are broader on the anodic than
on the kathodic side, then the segments no longer lie in three straight rows, but
either three spirals or one only can be recognised encircling the axis 3 and since each
< segment' in this case {e.g. in Pdlylnchum, Calharinea, and Sphagnum'^) developes
into a leaf, the leaves are formed in spiral phyllotaxes, with divergences depending
on the obliquity of the principal walls of the segments to one another'^. These
phenomena show clearly that when each segment produces a leaf, the phyllotaxis
depends on the manner in which the new principal walls of the segments arise ; and
since the direction taken by the segmentation of the apical cell depends again on
causes of which we are at present ignorant, the phyllotaxis must also finally be
referred to these unknown causes. In certain cases a reason may be given why, when
the mode of segmentation of the apical cell is the same, the positions at which the leaves
are formed are nevertheless variable. The segments of the apical cell, both in
Fontinalis and in Equisetuniy lie in three straight rows ; but in Fontinalis the solitary
leaves stand in straight rows and are arranged spirally with the constant divergence
J, while in EquisetunL, on the contrary, alternating whorls of leaves arise which have
grown together in the form of a sheath ; because here, as Rees has shown ^, the three
segments of each cycle, arranged originally in a spiral manner, are finally placed, in
consequence of the growth not being uniform, on the same zone. From this a cir-
cular projection next grows out, on which the sheath-teeth are formed. From the
want of uniformity in the growth of the segments, the causes of which are at present
unknown, still further differences, as compared with FontinaliSy are introduced, in
consequence of which the development of the whorls themselves becomes alternate
instead of superposed, as might be the case. If the processes which take place in
Marsilea^ as Hanstein has described them *, are compared with this, it is seen that the
segmentation of the apical cell of the stem, agrees in the main with that of Fontinalis
and Equisetum ; it is in three rows with a divergence \. As in Fontinalis, the leaves
originate by a curving outwards of the segment-cells ; but the leaves are in this case
not arranged in three rows as in Fontinalis, nor in whorls as in Equisetum, but in two
rows. The immediate cause of this must be sought in the fact that the stem, together
with the growing point, lies in a horizontal position ; it has an upper and an under
side. The segments of the apical cell form two rows on the upper and one on the under
side ; but the former produce leaves, the latter roots. The horizontal position of the
stem and its bilateral development are here perhaps the cause why the upper side

* Compare the admirable description by Leitgeb in the case of Sphagnum, in the Sitzungsber*
der kais. Akad, der Wissenschaften, Wien, March 1869.

^ See Hofmeisler, Allg, Morph. p. 494; and Miiller, Eine algemeine morphologische Studie,
Bot. Zeitg. 1869, t. IX. fig. 24. In such cases the behaviour of the apical cell may be represented by
imagining it to rotate on its axis, as I expressed it in my first edition. The description there given
does not however uomt appear to me suited to the beginner.

^ Rees, Jahrb. fiir wissen. Bot. vol. VI. p. 216.

* Hanstein, in Jahrb. fiir wissen. Bot. vol. IV. p. 252.

200 MORPHOLOGY OF MEMBERS.

only produces leaves ; and since its segments lie in two rows, there are two rows of
leaves, which we may imagine united by a zigzag line. But a further cause of the
difference, as compared with Fontinalis and Equtseium, arises from the fact that in
'Marsilea it is not every segment of the two rows on the upper side that forms a leaf;
according to Hanstein, certain segments remain sterile, and these form the internodes
which are at first wanting in Fontinalis and Equisetum, and are only formed at a
later period by further differentiation and intercalary growth. In Pteris aquilina and
in Salvinia the segments of the apical cell of the stem are also formed, as in Fissidens,
in two rows ; but the phyllotaxis is in all these cases very different. The effect of the
difference of growth is first of all shown in the decidedly horizontal position of the
stem of these plants, and also in the circumstance that the segments themselves grow
vigorously in thickness and length, and divide before the formation of the leaves
commences ; it is not from the segment- cells which are already in existence that the
leaves originate, but from certain cells resulting from their division at a distance
from the apex of the stem. This is common to Pteris and Salvinia; but in
the divisions of the segments and in the whole growth of the stem considerable
•differences between the two occur. Pteris aquilina forms on the upper side of its
thick underground horizontal shoots two alternating rows of leaves, while Salvinia
forms alternating whorls on its slender floating shoots, the members of the whorls
showing a very peculiar order of succession corresponding to the bilateral arrange-
ment and the horizontal growth of the axis.

The genetic forces which have an evident influence on the phyllotaxis of Cryp-
togams through the segmentation of the apical cell and the further behaviour of the
segments, are wanting in Phanerogams, where the leaves spring from a small-celled
cone of growth the tissue of which behaves like an almost homogeneous plastic mass.
The immediate causes which determine the spot where a leaf or shoot is to arise
can no longer be referred here, step by step, to the behaviour of an apical cell ;
they lie rather in the position of leaves already in existence, in their increase in
breadth, in the form a^id size of the cone of growth, in its inclination to the vertical,
in its relation to the size of the mother-shoot, &c. — conditions which, as has already
been mentioned under paragraph 5, have been treated in detail by Hofmeister. The
rule there enunciated, that lateral shoots arise above the centres of the widest
intervals between the youngest contiguous shoots, gives an efficient cause for the
determination of the place of origin of new members, and may be applied also to the
first leaves of lateral shoots, which generally show a definite relationship to the
subtending leaf. In Monocotyledons, for instance, the first leaf of an axillary shoot
usually stands on its posterior side, /. e. next the mother-axis ; while in Dicoty-
ledons the axillary shoot generally begins with two leaves, which stand right and
left of the median plane of the subtending leaf, and thus fall in the space between
it and the primary axis which is least exposed to pressure.

As has now been shown in this brief introduction, the investigations of
phyllotaxis cannot at present do more than ascertain in each separate case the
phenomena preceding and accompanying the origin of a member, as well as those
forces which, from their direction, exercise an influence on the point of origin,
and then lay down more general laws as the result of comparison in a sufficient
number of cases. In these as in -all other investigations into organisms, we are

RELATIVE POSITIONS OF LATERAL MEMBERS. 201

always however met at the very outset by a consideration of great importance which
imposes itself upon us, I mean the /ou^ ensemble of properties which define the
character of the natural group, class, or order. By recognising a plant as a member
of a particular class, e.g. Muscineae, Filices, Equisetacese, Rhizocarpeae, Phanerogamia,
&c., an aggregate of properties is ascribed to it, which must be taken into account as
such. If we pay special regard to the point of view opened out by the Theory of
Descent, we must recognise in the law of heredity and the physiological adaptation
of organs, the difficulty or even impossibility of demonstrating the causes of any
morphological phenomenon in any other manner than genetically. Organic forms
are not the result of combinations of forces and materials given once for all,
and always again reproduced in exactly the same manner, as in the case of
a crystal which is first dissolved and then re-crystallised, but of combinations
which repeat themselves hereditarily and which at the same time undergo change.
To understand these it is necessary to refer to the past, and not merely to the im-
mediate present.

I Abundant opportunity will be afforded in the description of the various classes in
3ook II. for a more exact observation of particular relations of position ; but what has
low been said is sufficient as a preliminary. Some additional remarks on the Spiral Theory
n the doctrine of phyllotaxis may however find a place here. It has already been
ihown that the construction required and employed in this theory is not in all cases
jossible, being sometimes arbitrary and without relation to development, and at other
imes simply meaningless; and that, finally, only those cases admit of the application
. of this theory without violence, where the shoot forms three or more rows of leaves
distributed singly and uniformly in all directions. The history of development often
points to quite different constructions, even in those cases in which the spiral is still
geometrically possible. But even where the connection of the leaves in order of suc-
cession in age by a spiral running round the stem in the same direction is possible and
even apparently useful, there is not in the phenomena connected with development
any sufficient reason for the hypothesis that the growth of the generating axis itself
actually follows a spiral ^.

Closely connected with the spiral theory, which must be carefully distinguished from
the doctrine of phyllotaxis, is another very peculiar law connected with the angles of
divergence. It was thought, namely, that a kind of natural law was found when it was
discovered that some of the most commonly occurring constant divergences i, ^, f, f, 3^,
and some of the less common ones, as ^/i, ^|, |;j, xV\, &c.^, may be represented as succes-
sive convergents of the continued fraction

I

2 + I

I + 1
I . . . .
Were it possible to combine all kinds of phyllotaxis without exception in this manner
. into one single continued fraction, we should actually have a kind of natural law, in which
, there would be no relation of cause and effect, and which would hence stand out as an

^ See on this point Hofmeister, Bot. Zeitg. 1867, nos. 5, 6, 7, and Allgemeine Morphologic,
p. 481.

^ It m«st be observed in reference to this that it remains uncertain whether such complicate4
flivergences are ever so formed originally, or whether they are not always consequences of compli-
cated displacements, in consequence of which the direct observation of the growing point does
not give in these cases a certain conclusion.

202 MORPHOLOGY OF MEMBERS,

inexplicable curiosity. This is not howeyer exactly the case. There are many phyl-
lotaxes which cannot be expressed by this continued fraction ; and in order to carry
out the method, jiew continued fractions have to be constructed, e.g,

I I

I^T_ or **^— Sec;

I + I I + I

I. . .. I.. ..

of which indeed only one or two convergents are for the most part met with as actual
angles of divergence. And since it is possible immediately to construct a new continued
fraction for every phyllotaxis which cannot be arranged under those already in existence,
it is of course possible to represent by this method all varieties of phyllotaxis; but it
follows at the same time that the method itself thus loses all deeper significance. If
those divergences only occurred on one and the same axis or on one system of axes
which can be represented by convergents of one and the same continued fraction, or
if the different values of one particular continued fraction occurred exclusively in a
genus, family, or order, the method would even in that case be of some value. But this
is not the case. Since moreover no actual relationship of the method to the history
of development, to the classification of plants, or to the mechanics of growth, has been
established, in spite of numberless observations, it seems to me absolutely impossible to
imagine what value the method can have for a deeper insight into the laws of phyllotaxis.
But even as a mnemonic assistance it appears to me not only superfluous, but even dis-
advantageous, since the use of it diverts the attention from relationships which are of
real importance ^

Sect. 27. Directions of Growth^. — (i) In every thallus, branch, stem, leaf,
hair, and root, it is easy to distinguish between two opposite ends, the jBase and the
Apex. The base is the place where the member originated and began to grow;
the apex is the extremity in the direction of the growth. The direction from the

^ [Chauncey Wright (Memoirs of A'mer. Acad. vol. ix. p. 3.89) has painted out an interesting
property of the series |, ^, §, f which includes all the more common arrangements of phyllo-
taxis. If the spiral line passing through successive leaves be traced the long way round, we obtain
the complementary series ^, f , -g, | the terms of which are successive convergents of the con-
tinued fraction i + 1

i + i

I + &c.

If we put this = K then K = — —

or K^=i-K

.-. i:K=K:i-K
or ^ is the ratio of the extreme and mean proportion ; and generally

^" = ^"-^_^"-'.
K is therefore the angular divergence which would effect * the most thorough and rapid distribution
of the leaves round the stem, each new or higher leaf falling over the angular space between the two
older ones which are nearest in direction, so as to divide it in the same ratio, K, in which the first
two or any two successive ones divide the circumference. Now | and all successive fractions differ
inappreciably from K.' Practically, therefore, all terms of the series above the. third fulfil the
condition of that leaf-distribution which is theoretically the most efficient by distributing the leaves
most completely to the action of the surrounding atmosphere.]

2 H. von Mohl, Ueber die Symmetric der Pflanzen, in his Vermischte Schriften. 1846. —
Wichura, Flora, 1844, pp. 161 et seq.—Hoimehter, Allgemeine Morphologic, §§ i, 23, 24. — Pfeffer,
Arbeiten des botan. Instituts in Wurzburg, 1871, p. 77.

DIRECTIONS OF GROWTH. JJOJ

base to the apex is the longitudinal direction of the member under consideration ;
and a section made in this direction is called a longitudinal section. The transverse
direction and section of the member are at right angles to the longitudinal ones.

(2) In every transverse section of a member there is a point about which the
internal structure and external contour are so arranged that it must be considered
as its Organic Centre. Every line drawn from this point towards any point of the
circumference is a radius; every portion of the transverse section has one side
facing the circumference and one facing the centre, these being usually developed
in a different manner from the sides that face the radii, and hence easily distin-
guishable from them. These relationships are recognised with ease in the transverse
section of woody stems and of all roots, but can be easily made out in other
cases also, even in unicellular plants and hairs. The organic centre of the trans-
verse section- does not usually coincide with the geometrical centre, as is easily
seen in the transverse sections of most petioles and horizontal branches with an
' eccentric' pith.

(3) If a line be imagined uniting the organic centres of all the transverse
sections of a member, this is the Longitudinal Axis or Axis of Growth of the
member. The axis of growth may be a straight or a crooked line ; in the younger
parts (nearer the apex) it may be crooked, and again straight in those which are
further developed (further from the apex), as in Salvinia and Utricularia ; or the
reverse. A plane which passes through the member in such a manner as tO' contain
the axis is called an Axial Longitudinal Section. If the axis be curved in a plane,
this plane coincides with the axial longitudinal section ; if the axis is straight, the
number of possible axial longitudinal sections is very large or even infinite.

Growth in the direction of the longitudinal axis is generally quicker, and also
generally lasts longer, than in the transverse directions, as is clearly seen in most
stems (haulms, flower-stalks, scapes, palm-stems),, in long leaves, in all roots, and
in most hairs and thallomes. This characteristic cannot however be used in the
general definition; for there are cases in which it appears doubtful whether the
growth in the direction of the longitudinal axis is more intense or more prolonged
than in the radial directions; as, e.g. in the stem oi Isoetes, and the prothallium of
some Polypodiacese, But the characteristic is superfluous for the determination of
the longitudinal axis ; its direction can always be recognised by the position of the
base and apex of a member ; and the point where it cuts the transverse section (the
organic centre) can be found without anything else being known about the relation-
ships of growth. It is always possible, without even knowing the duration or
intensity of the growth, to decide which is the longitudinal and which the transverse
section of a member ; this can indeed be determined from a very small fraction of
; in a Mammilaria, a Melocactus, or a Cereus, it is just as easy to determine the
mgitudinal axis of growth in early youth, when these cacti are often as thick as
ley are long, as it is later when they are much longer than thick. This is also the
ise in the abbreviated axis of bulbs, in many tubers and corms (as the crocus),
id in fruits, like those of many gourds, whose diameter is much greater than their
!ngth.

The growth of roots and stems in the direction of their longitudinal axis is
generally unlimited, that of leaves and hairs mostly limited, although these rela-

ii54 MORPHOLOGY OF MEMBERS.

tionships are sometimes reversed. When the growth is unlimited, its products"
along the axis are usually constantly repeated, the segments formed one after
another are similar, the lateral members that spring from them (branches, leaves^
lateral roots, &c.) are uniform, or they exhibit in their development a repeated
alternation, as, e.g. in Moss-stems, rhizomes oi Equisetum, primary stems of Coni-
fers, &c. When, on the contrary, the growth along the axis is limited and definite,
the resulting segments are dissimilar, and their outgrowths exhibit progressive
changes (metamorphosis). This occurs in most leaves, the basal portions of which
are usually strikingly different in form from the parts nearer the apex; it occurs
also in the stems of Angiosperms with terminal flowers> which commence, for
instance, with the formation of radical leaves, proceed to that of foliage-leaves, and
then, through the bracts, pass over into the production of floral leaves,^ closing with
that of carpellary leaves.

Axial growth is always limited when true dichotomy occurs at the apex; on
the other hand, bifurcations repeat and continue the mode of development of their
common basal portion (as' in Fucus or Selaginella), although individual branches
may terminate their growth without dichotomy by producing fruit.

(4) If an axial longitudinal section is imagined to pass through a member,
the conformation right and left may be similar, like the right and left halves
of the human body. If the two halves are so similar that the one is a reflected
image of the other, they are symmetrical^ and the dividing plane between them
is called a plane of symmetry. In this strictest sense symmetry is very rarely
found in plants (most nearly in many flowers and stems with decussating whorls);
and accordingly the term is constantly employed in a laxer sense. Two, three, four,,
or a larger number of symmetrically dividing planes often pass through a member
(a branch or root), all of which intersect in the axis of growth. Such members
are called polysymmeirical ;■ so-called 'regular' flowers, stems- with alternating
whorls, and most roots, are polysymmetrical. If, on the contrary, it is possible
to imagine only one symmetrically dividing plane, as in the flowers of. Labiatae
and Papilionaceae ^, in stems with opposite pairs of leaves, where the median
plane of the two rows of leaves is at the same time the plane of- symmetry, in
the thalloid shoots oi Mar-chaniia, and in most leaves, the object is monosymmetrical,
or simply symmetrical. Monosymmetry is however only a particular case of the
ordinary bilateral structure, which consists in the processes of growth being
similar to the right and left of an axial longitudinal section, although the
two halves of the member do not lie exactly opposite to one another like
reflected images. Thus, for example, the oblique leaves of Begonia are not
symmetrical, although bilateral; the one half to the right- of the mid-rib of the
lamina is larger and of somewhat different shape to the other half to the left of
the mid-rib; and the same is the case with the elm. A branch with alternating
leaves in two rows is also simply bilateral without being monosymmetrical ;
if it is divided at right angles to the common median plane of all the leaves,
the two halves bear each one row of leaves ; but the one is not the reflected image

^ A. Braun calls monosymmetrical flowers zygomorphic, an expression which is also elsewhere
interchangeable with monosymmetrical.

r DIRECTIONS OF GROWTH. 205

of the other, since the leaves of the two rows spring from different heights. Where
a true monosymmetrical structure occurs, it may be considered a particular case of
the bilateral; the latter, therefore, being the more common, is the more important
phenomenon.

There is the same relationship between polysymmetry and multilateral arrange-
ment as between monosymmetry and bilateral arrangement ; polysymmetry must be
considered only as a particular case of the multilateral structure. This latter always
occurs where several pairs of halves can be produced by axial longitudinal sections,
so that the two halves of €ach pair are very similar to one another, but not exactly
alike, like an object and its reflected imag'C. Thus the short stems of Sempervivum,
the leaf- rosettes of ionium, and fir-cones with their scales, can be easily halved by
numerous longitudinal sections, but the halves thus formed are never symmetrical^
because the leaves and scales are arranged spirally, and a spiral can never be divided
symmetrically ; but in so far as the spirally arranged leaves stand in three, four, five,
eight, thirteen, &c. orthostichies, the shoot itself may be said to be tri-, quadri-,
quinqui- octo-, trideci- lateral, &c.

The most common distinction is between bilateral and multilateral structures ;
in both cases the lateral arrangement may rise into symmetry, the former into
monosymmetry, the latter into polysymmetry. The extremes are seen on the one
side in roots with a circular transverse section, on the other side in most leaves and
leaf- like shoots with only two symmetrical halves. If, however, in the case of roots
regard is paid to the number of their fibro-vascular bundies, the apparently infinite
number of their planes of symmetry may usually be reduced to two, three, four;
Or five.

To obtain a convenient mode of expression for relationships of this kind, each
longitudinal section which produces two similar halves may be termed a principal
section or principal plane ; and if the two halves are symmetrical it is a symmetrical
section or plane. Thus bilateral structures have one principal section, multilateral
structures two or more principal sections.

(5) Lateral arrangement and relationships of symmetry may be looked at from
two important points of view, according as the members of a plant are compared
with one another, or are considered in reference to their relation with the external
world, with gravitation, light, or the pressure of external objects.

If the members of a plant are compared with one another, it is seen, for ex-
ample, that the principal sections of all the leaves, though on opposite sides of the
stem, may lie in one plane, in which case the shoot itself is bilateral ; or they may lie
in two planes crossing one Another at right angles, when the shoot is quadrilateral,
as, for instance, when it bears decussate whorls of two members, a case which, in
reference to other relationships, is very near to that of bilateral arrangements, and
may be termed a double bilateral arrangement. In these cases the principal sections
of the leaves are also at the same time principal sections of the stem. \xi Salvinia^
Marsilea, Polypodium aureum, and Pterts aquilina^ on the contrary, the principal
sections of the leaves, forming two rectilineal series, lie right and left of the
single principal section of the bilateral stem, an arrangement which is in these
cases dependent on the horizontal growth.

The relationship of lateral arrangement and symmetry to the external envi-

206 MORPHOLOGY OF MEMBERS,

ronment of the plant is shown, for example, in the fact that multilateral shoots
usually grow upright, while l;>ilateral shoots generally lie horizontal, the principal
section being then vertical. Many bilateral shoots cling on one side to a horizontal,
oblique, or vertical support, as the Marchantieae, Jungermannieae, the ivy, &c. ;
and in that case the principal section is at right angles to the support. Bilateral
Structures, leaves or whole shoots and systems of shoots, generally form their two
sides to which the principal section is perpendicular very differently in respect
to the external world; so that, in addition to a right and left half (right and left
of the principal section), there is also a clear distinction between an upper and
under side, an attached and free side, a dark and light side; and it is in this
respect that the dependence of lateral arrangement on external conditions is most
clearly evident.

In each special case it must, however, be left for more exact observation to
settle how far the position of the principal sections of the members of a plant are
determined by internal relationships of growth, or by external influences ^ a question
wh'ich can seldom be satisfactorily answered when not decided by experiment. To
this end the researches on Marchantia poly?jidrpha, begun by Mirbel in 1835 and
carried on with great success by Dr. Pfeffer in 1870 (/. c), are of peculiar interest.
Dr. Pfeffer shows that the two flat sides of the gemmae of this liverwort are
identical ; i. e. each of the two sides has the power of forming root-hairs when
turned downwards. Bilateral arrangement and differentiation of the upper and
under sides are first developed in the fiat shoot which springs from the gemma.
The side of the shoot exposed to light, whatever its position, becomes under all
circumstances the upper side which forms stomata, the dark side becomes the under
side which produces root-hairs and leaf-like processes. Even after the lateral
shoots have been formed, the two sides of the gemma itself are still alike. Similar
relationships may also prevail in the germinating spores of creeping Jungermannieae
and in the formation of the prothallium of Ferns ; but on this point more exact
researches are still wanting. In Ferns only thus much is known, that (according
to Wigand) when the light is stronger from one side, the principal section places
itself in the direction of the strongest illumination, while the apex of the axis of
growth is turned towards the shade.

What has now been said will serve as a definition only of the most important ideas,
and an illustration of the points of view from which observations must be made. The
results obtained by them cannot be given in detail ; and since a definite theory has not
yet been elaborated, a more detailed description must deal with numerous peculiari-
ties and critical comparisons, for which we have no space. A few important facts may,
nevertheless, be briefly mentioned.

(i) In reference to the Direction of the Axis of Growth, it appears to be the general
rule that the origin of a new individual coincides with the commencement of a new
direction of growth. This is very strikingly the case in the swarmspores of (Edogonium
(Fig. 4, p. 9), the longitudinal axis of which is transverse to that of the filament which
produces them, and becomes the longitudinal axis of the new plant ; and the same is the
case with the origin of new filaments of Nostoc and Rivularia (see Book II, Algae). In
many Cryptogams, researches have not yet been made on this point, or it would carry
us too far to mention them. It may be mentioned, merely by way of example, that the

^ Compare Hofmeister, Allgemeine Morphologic, §§ 23, 24.

DIRECTIONS OF GROWTH, 20J

axis of growth of the embryo of Ferns and Rhizocarps is distinctly transverse to the axis
of the archegonium. In Phanerogams the direction of growth of the embryonal stem is
opposed to that of the ovule ; the apex of the young stem is formed in a direction
different to that of the ovule, and it continues its growth in this direction. The forma-
tion of the sporocarp of Mosses forms an exception to this manner of growth, if it is
considered as a new individual, but this appears very questionable ; it grows in the same
direction as the archegonium, and even in the direction of the axis of the stem when the
archegonium is apical {i.e. in Acrocarpous Mosses).

A second remark relates to the fixation of the base of the axis of growth. In all
lateral members and bifurcations the base is the fixed point at which the branching or
new formation begins; and in the new formation of an axis of growth from swarm-
spores and fertilised oospheres, the growth in a definite direction does not begin until
some one cell has become fixed. This occurs in all swarmspores, which do not begin
to grow into tubes and filaments until their hyaline end, the anterior one in the
swarming, has become fixed, even if only on the surface of the water. The ger-
minating spore also of Ferns and Equisetaceae puts out at an early stage a root-hair
which fixes it to the support (the macrospore of Rhizocarps and Selaginelleae does not
require this in consequence of its weight). In a similar manner also the longitudinal
growth of the embryo of Phanerogams does not begin until it has become attached at
its posterior end to the apex of the embryo-sac. The embryo of Vascular Cryptogams
produced by a sexual process fixes itself laterally by the portion called the foot into the
tissue of the prolhallium.

It is only in some Algae of the simplest structure that there is no attachment of
a point of the newly constituted plant to an external object (for which purpose any
portion of the generating body may serve). In this case the opposition of base and
apex disappears; growth may then produce a uniform arrangement in different and
even opposite directions ; simple threads result in which an anterior and posterior end
can no longer be distinguished, as in some Desmidieae and Diatoms, or spherical families
of cells, like Glceocapsa.

But when once a fixed point is established as a base, the increase of length takes
place uniformly in one direction only from it ; i. e. whatever grows in this direction is a
member of a morphologically definite character. This does not however prevent the
setting up of a new growth in the opposite direction ; but the member which is formed
in this direction is of a different nature morphologically ; as occurs, for example, in the
embryos of Phanerogams, in which, according to J. Hanstein's recent researches, the
origin of the primary root is in fact such that its longitudinal axis must be considered
as the prolongation of that of the stem in a posterior direction.

(2) With reference to the Relations of Symmetry, the most important point is that dicho-
tomous branching is frequently repeated in one and the same plane in thallomes (as in
Fucaceae and Metzgeria), stems {Marchantia, Selaginella ^), and leaves (in some Ferns).
A different development then generally t^kes place on the two sides of the plane of
dichotomy, one side of the shoot clinging closely to the ground or to upright bodies (as
in Hepaticae), or one side turning to the light, the other side to the shade (Selaginelleae) ;
in such cases the shoots have their greatest diameter in the plane of dichotomy.
Where no such different development of the two sides occurs, as in Lycopodium (espe-
cially L. Selago according to Cramer), consecutive bifurcations may take place in
different planes ; and this is also the case with the roots of Lycopodiaceae (see Nageli
and Leitgeb, and Pfeffer, I.e. p. 97).

As has already been mentioned, it is usually impossible, without experiment, to
determine whether the position of the principal section of bilateral shoots and leaves

I is directly brought about by external conditions, such as pressure, gravitation, and

I

* In the roots of Selaginella, however, the successive bifurcations lie in planes at right angles.
' This subject has been treated by Hofmeister (AUgemeine Morph. §§ 23, 74) from another

ao8

MORPHOLOGY OF MEMBERS.

mother-shoot, and at the same time to the direction of gravitation, h'ght, and pressure
(the latter in clinging or climbing plants, such as ivy, Jungermannieae, &c.). It is
therefore probable that internal and external causes generally cooperate to determine
the direction of the longitudinal axis of a member when first formed, as well as of its
lateral shoots \ Further development may show new relations to the original axis and
to external influences. The horizontal lateral shoots of numerous woody Dicotyledons
with alternate leaves in two rows have the principal section vertical, their rows of leaves
right and left. The axillary buds of these leaves which remain dormant through

the winter show an altogether different disposition of
their parts ; the axis of the bud is parallel to that of
the mother-shoot ; it bears its leaves in two rows, one
facing the sky and the other the earth (Fig. 155); the
mid-ribs of the folded leaves are always turned out-
wards, away from the mother-axis; the principal axis
of the whole bilateral shoot (the bud) is horizontal.
But when the bud unfolds in the spring, a torsion
takes place of such a nature that the principal section
assumes a vertical position, the prominent mid-ribs of
the leaves turn downwards, while their faces turn up-
wards ; and thus the lateral shoot of a horizontal mother-
shoot acquires the same position as its parent. The
fact that the two rows of leaves within the lateral
bud arise on the upper and under side, and consequently
both in a vertical plane, might be referred to the im-
mediate influence of gravitation ; but this view is
opposed by the fact, among others, that the position of
the terminal bud^ of the hori^iontal mother-shoot is
usually from the first different. In Cercis and Cory /us,
for example, the terminal bud stands on the under
side of the horizontal end of the branch, and its leaves
right and left of the vertical principal section of the
bud* The position of a terminal bud may be easily
represented by turning Fig. 155 so that the parent axis
a lies above, the subtending leaf If beneath th€ bud (the apparently terminal bud thus
becoming axillary), and the vertical line t becomes horizontal. This difference, which
exists from the very first in the position of the lateral and terminal buds in horizontal
bilateral mother-shoots, excludes the immediate influence of gravitation ; but by a useful
adaptation, even in the bud all the parts are so arranged that by a single twist of the axis
during unfolding, they assume those positions which are most favourable for the functions
of the leaves, and by which their faces are turned towards the light. In the terminal
buds of such shoots this twisting is not necessary. Whether it is gravitation or the
influence of light on growth which occasions this torsion of the axis of the bud is not

FIG. 155.— Lateral bud of a horizontal branch
of Cercis canadensis (in December), in vertical
transverse section; i — 7 the' consecutive leaves
with their pairs of stipules indicated by the same
' numbers. The outer bud-scales have been re-
moved, the two inner ones indicated by 3, 3. In
"the centre is the growing- point of the bud. i5 the
position of the leaf in whose axil the bud grows ;
a axis of the mother-shoot; v direction of gravi-
tation.

point of view ; but on consulting the facts themselves I find much that is not in agreement with his
statements, and in their interpretation I arrive at essentially different conclusions, the reasons for
which cannot be explained here in detail.

^ In order to determine this question experimentally, the apparatus which is described in Book
III. Chap. 3, Sect. lo, should be used, a drum which holds the plant, and which rotates slowly
round a horizontal axis.

^. It is for our present purpose the same whether the bud at the end of the horizontal shoot be
its true terminal bud, or a lateral bud the development of which is induced by the abortion of a
terminal bud, as in Cercis and Corylus, In reference to the position of the terminal bud it is also
indifferent that the position of the lateral buds is sometimes not quite horizontal, but a little
oblique upwards and outwards, as in Coryhis, Celtis, Sec.

DIRECTIONS OF GROWTH. 209

the question now under discussion. But the most important result is that the principal
sections of the axillary buds of a bilateral mother-shoot may have greatly varying posi-
tions, and that, in consequence, the arrangement of the parts of the bud is independent
of gravitation ; on the other hand, there is a definite relationship to the mother-axis
in the arrangement of the parts of the bud. The axillary bud of such a shoot may
arise either laterally or on the under or upper side ^ ; in all cases its leaves turn their
prominent mid-ribs outwards, away from the mother-shoot, so that the principal section
of the bud is always at the same time an axial longitudinal section of the mother-shoot.

We are led to the same conclusion by the study of two- or three-year-old seed-
lings of Thuja and other Gupressineae. The leaves of the primary stem are arranged
below in alternating whorls of fours, and consequently in eight longitudinal rows;
higher up the whorls are alternate and of three leaves, and the leaves are in six rows.
The axillary shoots, the number of which is very small in proportion to the leaves,
appear, both in the eight-rowed and in the six-rowed region of the stem, to be generally
in two rows, so that in reference to its branching the primary stem is bilateral (other
positions of the branches occur, however, higher up, especially later). These lateral
shoots of the first order begin at once with alternating whorls of two leaves, or a
decussate arrangement; and the first pair always stands right and left of the subtend-
ing-leaf. Every such lateral shoot of the first order usually produces a bilateral
branch-system, which spreads out in one plane. This plane is usually horizontal in
seedlings of Thuja gigantea, T. Lobbii, &c., and the principal section therefore vertical.
But this is not without exception; lateral branch-systems are formed here and there
spreading in a vertical plane, the principal section of which is therefore horizontal ; and
this is sometimes repeated in some of the lateral shoots of the second order. I found,
however, on a strong seedling of Cupressus Lanusoniana seventeen lateral systems of
shoots (standing in two opposite rows on the primary stem) all spreading in a vertical
plane, while only one lower system spread horizontally. These differences in the
position of the principal section of lateral branch-systems are not however brought
about by torsions, which would easily be recognised from the phyllotaxis; they are
original, and permanent. Where a lateral shoot of the first order branches hori-
zontally, its branches are produced only from the axils of leaves that stand right and
left; where it branches vertically, only from the axils of leaves that stand above and
below. Now since the principal sections of these lateral branch-systems have altogether
different inclinations to the horizon, it is scarcely possible to suppose that gravitation
(or light) has any immediate influence on the origin of the lateral branches of the
second order. The vertical position of the principal section of lateral shoots of the
first order is much more constant in Araucaria excelsa ; and here, as horticultural
experience shows, we have a phenomenon inherent to lateral arrangement; — lateral
shoots, planted verticcUly as cuttings, take root and continue to grow , vertically, but
produce only lateral shoots in two rows ; the branch which has once been produced as
a lateral shoot does not change, when placed vertically, into a many-sided primary stem^.

I may add a few remarks on the species of the genus Begonia, which show that the
relation of the bilateral arrangement of the parts to external influences may be entirely
different in very closely allied forms, while it is constant for members of the same plant.
The leaves of Begonias are alternate in two rows; on thick stems the two rows
approximate on one side of the stem, and the other side of the stem appears naked ;
the shoot is thus not only bilateral, but it has a leaf-bearing anterior and a naked
posterior side which are very unlike. The blade of the leaves is very unsymmetrical,
one half being much larger. The larger halves of all the leaves are turned towards
the posterior side of the stem ; and this can be used to distinguish the posterior and

* Axillary shoots are formed on the upper side of the mother-shoot near its base in Cercis, and
bear inflorescences.

^ [This is not without exception; see Goeppert, Acta Acad. Nat. Cur. 1868, p. 34, t. i.]

P

2ro MORPHOLOGY OF MEMBERS.

anterior sides even in slender-stemmed species, as B. undulata and incarnata^ although
in these cases the leaves are not approximated on the anterior side, but follow exactly
the divergence \. It is well to remark at the outset that the leaf-stalks of Begonias are
moderately heliotropic, while the axes of the shoot are scarcely so. In thick axes
heliotropism appears to be entirely wanting ; in slender axes {E, undulata and incarnata)
it is always very slight. Some species with moderately thick stems, as B. Verschaffeltii
and manicata, grow upright when the light comes from one side ; very thick-stemmed
species bend without reference to the direction of the light, while slender-stemmed species
allow their weak branches to hang down without pointing in any one definite direction.
If attention be now paid to the direction in which the stems bend, the plane of
curvature is found always to coincide with the principal section of the shoot which
divides it into two similar halves, each possessing a row of leaves. A definite relation-
ship is also manifested between the tendency to bend and the relative thickness and
length of the internodes. If the thickness of the internodes be represented by i, then,
in the upright stems of Begonia nitens, Mohringi, and stnuata, their respective lengths
are 9, 3*2, and 2 ; in the slightly curved B. manicata it is i or less; but in procumbent
and strongly-curved stems as low as 07 (J5. hydrocotylifolia), 0*4 {B. pruinata), and 0*2
{B. ricinifolia). In the slender-stemmed upright species the rows of leaves are
diametrically opposite to one another ; in the species with slightly-curved thicker stems
they approach on the anterior side ; in the very thick-stemmed species which are bent
downwards the insertions of the leaves are placed entirely on the anterior side^ In
the thick-stemmed species the stem curves downwards with the anterior side concave,
or lies procumbent ; but in this case it is always the leafless posterior side which
faces downwards and puts out adventitious roots {B. ricinifolia and macrophylla). In
species on the other hand which have tall stems and slender internodes, the branches
hang down, and in this case it is the posterior side which becomes convex and faces
upwards {B. undulata and incarnata). Or, in other words, if we look at the buds, in
the slender-stemmed forms all the larger halves of the leaves when first formed turn
upwards, while in the thick-stemmed forms they turn downwards. The want of sym-
metry of the leaves is thus, when the position of the bud is inclined, opposed to
gravitation, and when the stem is upright has no relationship with it. In species with
short internodes and thick stems only a few lateral shoots are developed, in those
with slender stems a great many. Such a relation constantly occurs in other cases
(Cactaceae, Palms, Ferns, and to an extreme extent in Isoetes). The bilateral arrange-
ment of the lateral branches is connected with that of the parent axis in the following
way : — in all species the posterior side of the lateral branch, and hence the larger halves
of the leaves, faces the parent axis ; the principal section of the former in the slender-
stemmed species is therefore at right angles to that of the latter. In thick-stemmed
species, where the axillary shoots are approximated in front, the principal section of the
lateral branch makes an acute angle with that of the parent axis. As development pro-
gresses, the branches of slender-stemmed species retain nearly their original position ;
in thick-stemmed species where the anterior and posterior sides differ greatly, the lateral
branch twists in such a manner that its posterior side faces in the same direction as that
of the parent axis.

I have no precise information as to the mode of life of different species of
Begonia, but suppose that those species in which the anterior and posterior sides are
differentiated, and which do not cling to the ground, may have the power of climbing, like
the ivy ; although observations which I have had made for this purpose in the botanical
garden at Wiirzburg have not yet led to any satisfactory result, partly because the
plants were already too old, partly because the access of light was possibly too small on

* The absolute measures of thickness run almost parallel to the above-named relative ones ; the
relatively thickest internodes are also usually absolutely the thickest, and these stems show the most
decided tendency to a horizontal growth.

CHARACTERISTIC FORMS OF LEAVES AND SHOOTS. 21 1

the anterior side. The facts stated above do not negative the hypothesis that with
stronger access of light from one side the stems of Begonias may possibly be aphelio-
tropic. It appears moreover from Martins ('Flora brasiliensis,' fasc. XXVII. p. 394)
that at least some Begonias cling to rocks and the stems of trees ^.

Sect. 28. Characteristic Forms of Leaves and Shoots. — The peculi-
arities of thallomes, leaves, axes, and roots which are common to whole classes,
orders, or families, are the subject of special morphology and systematic botany;
on the other hand it is the province of physiology to study the special organi-
sation by which the members of a plant become adapted to perform definite
functions. There are, however, some pecuHarities of growth which recur in dif-
ferent divisions of the vegetable kingdom, or present themselves in striking con-
trast to the ordinary phenomena, and which are for this reason well adapted to
bring into prominence the value of general morphological ideas. Peculiarities
of this kind are termed characteristic^ and they must be briefly mentioned here,
chiefly in order to explain some scientific terms which will be used in Book II.
We may limit our remarks to leaves and leaf-bearing shoots, since the forms
of the thallus will be treated in sufficient detail in the chapter on Thallophytes,
and those of roots present only slight characteristic difl"erences, to which refe-
rence has already been made; the characteristic forms of hairs has already been
sufficiently alluded to.

(i) Forms of Leaves. When fully developed, leaves are usually flatly extended
plates of tissue, the extension being generally in directions right and left perpendi-
cularly to the median plane or principal section, so that the surface of the leaf lies
transversely (at right angles or obliquely) to the longitudinal axis of the stem. This
is generally quite true for the base of flat leaves ; but the upper part of the surface
of the leaf is sometimes itself extended in the direction of the median plane, so
that the plane of extension coincides with an axial longitudinal section of the stem,
as in the genera Ixia, Iris, &c. But sometimes the leaves are not flat, but conical
or polyhedral ; conical with almost circular transverse section in Characeae and
Pilularia, polyhedral in some species of Mesembryanthemum and Alo'e\

The outHne of leaves is either simple or segmented ; the former is the case
when no definitely separated parts can be distinguished in the leaf; a leaf is
segmented when it consists of pieces of various shapes, which are more or less
separated from one another. Leaves which are not flat are usually simple, as
I also are those which are flat but small, their length and breadth being inconsider-
able relatively to the stem, and not exceeding a few millimetres or centimetres in
absolute measurement. Larger leaves are usually distinctly segmented, and in
general the degree of segmentation increases with the increase of size ; the small
simple leaves of Mosses, for instance, may be contrasted with the large segmented
leaves of Ferns, the small simple leaves of Lycopodiaceae and Coniferae with the
large compound leaves of Cycadeae, the small simple leaves of Linaceae with the large
much-divided leaves of the nearly-aUied Geraniaceae, &c. The segmentation of leaves
usually consists in the distinction of a basal portion which generally remains narrow,
cylindrical, or prismatic, and of an upper portion which is flatly extended ; the

* On hcliolropism^see Book III. Chap. 3. Sect.
P 2

212 MORPHOLOGY OF MEMBERS.

former being called the leaf-stalk or petiole, the latter the blade or lamina. Or the
lower portion of the leaf has the form of a sheath enclosing the stem and
younger leaves like a hollow cylinder. If the upper part is flatly expanded the leaf
then consists of a sheath or vagina and a blade ; it sometimes also happens that a
stalk intervenes between the sheath-liKe basal portion and the lamina, as in Palms and
some Aroidea3 and Umbelliferse. Segmentation into sheath, petiole, and blade may
be distinguished as longitudinal, from lateral segmentation, which consists of actual
branching, as in pinnate, deeply lobed, or compound leaves, or of a rudimentary
branching, as in indented, toothed, and sinuate leaves. Leaves are termed compound
in which the individual lateral pieces of the lamina are completely separated at their
base ; while those forms are termed lohed in which the lateral branches are only more
or less projecting portions which unite at their base. If the individual branches of a
branched leaf are sharply separated, each branch forms independently, so to speak, a
leaf, and is hence distinguished as a leaflet. The pinnation, like the formation of
lobes, may be repeated. If the branches are obviously arranged in two rows the
leaf is said to be pinnate if it is a compound leaf; pinnately -lobed, pinnatisect, or
pinnatifid if the divisions are incomplete ; dentate, serrate, or crenate if the lateral
projections are very small relatively to the lamina. If, on the contrary, the branches
or lobes of the lamina are aggregated at the end of the petiole, and radiate
from it, the leaf is said to be digitate, palmately lobed, &c. It is termed peltate
when the lamina is attached not by a portion of its margin, but at a point
on its under surface (as in TropcEolum, Nelumbium, &c.). These are only a few of
the more important forms ; the student will find in every text-book a number of
other distinctions and terms employed in the special description of plants ^

As occasional appendages, which indicate a still further segmentation of leaves,
must be mentioned stipules, ligular structures, and hood-like outgrowths.

Stipules may be considered as lateral branches of leaves which arise at their
very point of insertion ; they stand in pairs right and left of the base of the leaf,
either entirely distinct from it [free) or united to it in growth {adnate) ; each single
stipule is usually bilaterally unsymmetrical, and its shape is the reflected image
of the other. Stipules are not formed until after the origin of the leaf, but then
grow much more rapidly, and attain their final development at an earlier period;
hence they play an important part in the position of the parts in the bud. In
vernation they either extend by their inner margins (those facing the median plane of
the leaf) over the back of the leaf and cover it outside either partially or entirely,
or they extend in front of the leaf (on the side facing the stem) right and
left, and thus cover the parts of the bud next youngest in age. In one or the
other of these modes chambersare not unfrequently formed by the stipules, in which
the formation of the leaves is completed, and from which they expand and unfold;
the stipules then either also remain and unfold, or die and drop off".

The term ligule is applied to a membranous outgrowth on the inner side of
the leaf of Grasses at the point where the flat lamina bends out at an angle from
the sheath; it stands transversely to the median plane of the leaf. Similar out-
growths are also found elsewhere, as on the petals of Lychnis and Narcissus (where

^ [Gray's Botanical Text-book, Part I, Structural Botany, 1879, Si^es perhaps the best English
exposition of the current terminology of Flowering plants.]

CHARACTERISTIC FORMS OF LEAVES AND SHOOTS.

213

they form the so-called Corona), on the leaves of Allium, &c., and may be included
in the general term of ligular structures. Outgrowths sometimes occur from the
posterior (outer) side of leaves, as, for instance, the large hood-like appendages
of the stamens in Asclepiadeae.

It is only in some Muscineae that the tissue of the leaf consists throughout of
'single layer of cells. Usually, especially in large leaves, the tissue is composed
of several layers, and, in vascular plants, is distinguished into epidermis, paren-
chymatous fundamental tissue, and fibro-vascular bundles. The fundamental tissue
is termed mesophyll ; the system of the fibro-vascular bundles running into the
leaf forms the so-called venation. In the leaves of many Mosses which otherwise
consist of only one layer, there runs in the middle from the base towards the
apex a bundle of several layers, called the mid-rib ; in leaves of more com-
plicated structure there is also usually a mid-rib which runs from the base to the
apex of the lamina, and divides it more or less symmetrically into two halves.
The same occurs in every lateral leaflet or in every branch or lobe of the lamina ;
from the mid-rib spring the lateral veins which run to the margin of the leaf. In
larger leaves, especially those of Dicotyledons, the fibro-vascular bundles which
traverse the mid-rib and its stronger branches are enclosed in a thick paren-
chymatous layer of tissue, the cells of which differ from those of the mesophyll.
Usually these veins project on the under side of the leaf, and the larger the
whole lamina the more strongly are they constructed (especially the mid-rib).
The finer veins, on the contrary, consist of single fibro-vascular bundles, often
branching extensively, which run through the mesophyll of the lamina itself.
The kind of venation varies in different classes of vascular plants, and is often
very characteristic of large groups. This will be explained more in detail in the
proper place.

In Characeae, Muscineae, and Vascular Cryptogams, all the leaves of a plant
are usually similar, being either simple or segmented in the same manner, although
the segmentation, especially in Ferns and Rhizocarps, is simpler in young than in
the large leaves of mature plants. But it also happens, even in Cryptogams, that
leaves of very different forms are found on the same plant. Thus some Muscineae
form colourless minute leaves on the underground creeping shoots, and in the neigh-
bourhood of the organs of reproduction they often produce leaves of a different
shape from those on the rest of the upright parts. In the same manner among
Ferns the leaves on the underground shoots (stolons) of Struthiopteris germanica
are represented by thin membranous scales, which are replaced on the upright end
of the stolon by large green pinnate leaves. In Salvinia, among Rhizocarps, each
"'whorl consists of two simple roundish leaves which rise into the air, and one that
hangs down into the water and consists of filiform branches. Even in Coniferae and
' Cycadeae variation in the leaves of the individual plant is much more common ;
while in Monocotyledons and Dicotyledons the shapes of leaves become extra-
ordinarily variable, not only on the same plant but often on the same axis.

The two most common forms of leaves are the scale- or 'cataphyllary' leaves *
and the foliage-leaves.

* [Henfrey, in his translation of Braun's ' Rejuvenescence in Nature' (Ray Soc, Botanical and
Physiological Memoirs, 1853), first proposed to render the terms Hochblatt, Nicdcrblall, and

a 14 MORPHOLOGY OF MEMBERS.

The foliage-leaves'^ are always distinguished by their green colour, owing to
their containing chlorophyll (which however is sometimes concealed by red sap);
and it is these which, in popular language, are exclusively called leaves. Usually
they are the largest foliar organs of the plant, lasting the longest, and distinguished
by the greater degree of segmentation of the outline, as well as by the more perfect
development of their tissue. As the chief store-houses of chlorophyll they are the
most important organs of assimilation, and are always destined to be expanded to the
light even when they are formed on underground growing points (as in Sabal, Pieris
aquilina, &c.). When small they are usually produced in great numbers on a shoot ;
as they increase in size their number and the rapidity of their growth diminishes in
proportion. In this respect the numerous small leaves of Mosses may be compared
with the few large leaves of Ferns, the numerous small leaves of Conifers with the
few large ones of Cycads, &c.

Scale- or Cataphyllary leaves are usually produced on underground shoots,
and remain buried in the earth, although they also frequently occur above ground,
especially as an envelope to the winter-buds of woody plants (as the Horse -Chesnut,
Oak, &c.). In the genus Pimis the primary and strong lateral axes form leaves
of this kind only ; the acicular foliage-leaves appear in tufts on small axillary
shoots; in Cycqs scale-leaves alternate regularly on the stem with large foliage-
leaves, Seedlings (as of the Oak) and the lateral shoots from underground axes
often begin with scales and only advance at a later period to the production of
foliage-leaves {e. g. Struthiopteris, jEgopodiutn, Orchis, Polygonatum, &c.). In
parasites and plants which live on decaying vegetable matter (saprophytes) and
are destitute of chlorophyll {e.g. Monotropa, Neottia, Corallorhiza, Orobanche, &c.)
the scales are the only foliar structures of the vegetative parts, the foliage-leaves being
absent. Even in those plants whose foliage-leaves are much segmented the scales
remain simple ; they are distinguished by a broad base, usually diminutive length,
the absence of prominent veins, and by forming no chlorophyll or only very little.
They are colourless, yellowish, reddish, or often brown ; their texture is, according
to circumstances, fleshy, succulent (as in some bulbs), membranous, or tough like
leather.

In Phanerogams, especially in Monocotyledons and Dicotyledons, several other
forms of leaves make their appearance as a preliminary to fertilisation — bracts,
sepals, petals, stamens, and carpels. The thick seed-leaves or cotyledons will be
spoken of in detail as a peculiarity of these classes.

From the point of view of the Theory of Descent we are justified in considering
all other forms of leaves as subsequent metamorphoses of foliage-leaves ; and these
latter are therefore regarded as the original typical leaves. When they lost their
original function — the assimilation of food-materials — and served other purposes,
they assumed at the same time other forms and structure. The same is meant
when certain tendrils and spines are said to be metamorphosed leaves. Leaf-tendrils

Laubblatt by ' hypsophyll,' 'cataphyll,' and 'euphyll.' The two first of these are useful arlditions
to botanical terminology ; the last, however, does not seem to be required, being precisely equivalent
to the term foliage-leaf, which is already in general use (hypsophyll = bract)].

^ Compare the characteristics of the formations of leaves in A. Braun, Verjiingung in der
Natur, Freiburg 1849-50, p. 66. [Ray Soc, Bot. and Phys. Mem. 1853, p. 62.]

CHARACTERISTIC FORMS OF LEAVES AND SHOOTS, 11^

are leaves or parts of leaves which have become filiform, and possess the power
of winding round slender bodies and thus of serving as climbing organs (as in
Vtda, Glorwsa, Smilax aspera, &c.). Leaf-spines are leaves which have developed
into long, conical, pointed, woody bodies; they take the place of foliage-leaves
{Berberis) or stipules {Xanthtum spinosum^ some species of Acacia). These two
kinds of metamorphosis occur almost exclusively in Flowering plants (Angio-
sperms), the morphological and physiological perfection of which, in comparison
with Cryptogams and Gymnosperms, is especially due to the capability of their leaves
to assume the most various forms.

(2) Forms of Shoots. The axis of leaf-bearing shoots is, when sufficiently
developed, usually columnar, with a cylindrical or prismatic surface. If the growth
in length is very small in proportion to that in thickness, the short column forms
a plate, as in the bulbs of Allium Cepa, and Isoetes ; if the growth in length is
somewhat greater, with at the same time considerable increase of thickness,
rounded or elongated masses are produced (as the tuber of the Potato and Arti-
choke, the aerial stems of Mammillaria and Euphorbia meloformis) ; when the
growth in length greatly preponderates we have stems, scapes, and filiform struc-
tures of various kinds. Very commonly the same shoot shows differences of this
kind in successive stages of its longitudinal growth; thus the stem of the Onion,
which is at first broad and tabular, afterwards rises as a high naked scape, the
end of which in its turn remains short, and thus produces the umbellate in-
florescence ; and in the same manner the thick tuber of the Potato is only the
swollen end of a slender filiform shoot. Among the numerous deviations from
the columnar form of the axis the conical is of peculiar interest. The conical
stem is of two kinds; it may be slender at the base, increasing in thickness with
further growth in length, so that each portion of the axis is thick in proportion
to its youth, and the upright stem resembles a cone placed upon its point; the
growing apex then lies on the surface which is turned uppermost, or rises above
it as an upright cone. This form occurs in the stems of Tree-ferns, Palms, in
Maize and in many Aroideae ; it depends on the absence of a secondary growth
in thickness, while, with the age of the plant, the young tissue of the stem becomes
constantly larger in circumference immediately beneath its apex; when this in-
crease ceases, the circumference of the later increment of length remains the
same, and the inverted conical stem continues to grow in the form of a cyhnder.
The second form of conical stem is caused by a long-continued secondary growth
in thickness together with the small circumference of the shoot at the growing point ;
this occurs in Conifers and many dicotyledonous trees, the older stems of which
are thick below but slender above, and thus resemble a slender cone placed on
its base.

The habit of a leafy axis or of a segment of one is usually in close relation
to the number, size, and formation of its leaves. If the internodes are very short
but the leaves small and numerous, the surface of that portion of the axis is nowhere
exposed, and the leaves only are seen, as in species of Thuja and Cupressus^ and
some Mosses {Thuidiwn)-, in such cases whole branch-systems frequently have the
appearance of multipinnate leaves. If the closely packed leaves are large, they form
a rosette enveloping the end of the stem, while the older parts of the stem are

ai6

MORPHOLOGY OF MEMBERS.

clothed with the remains of the leaves, or are naked, as in Tree-Ferns, procumbent
species of Aspidium, Palms, species of Aloe, &c.

If a comparison is made between the amount of development in bulk which
takes place in the leaves and in the axis of a shoot, we find , as extremes on one
side, for example, the Cacti {Cereus, Mammillaria, Echinocadus, &c.) with gigantic
axes and entirely abortive leaves, on the other side the Crassulaceae with fleshy
crowded leaves and comparatively weak stems; or on one side the underground
tubers of the Potato with scarcely visible scales, and on the other side the bulbs
of Liliacese with fleshy scales which entirely envelope the short stem.

In reference to the formation of leaves M'hich appear on the shoots, it must
first be noted whether the same axis always produces only similar leaves or such
as gradually vary in form. The first is the case, for example, in most Muscineae,
Ferns, Lycopodiacese, Rhizocarps, all Equisetaceae, and most Conifers ; the latter,
on the other hand, occurs commonly in shrubby Dicotyledons. In Monocotyledons
and Dicotyledons (to a certain extent even in Conifers) it not unfrequently happens

Fig. 156.— Rhizome of Pteris aquilina; /, //, /// the underground creeping axes; jj the apex of one of them ; 1—6 the
basal parts of the leaf-stalks ; 7 a young leaf; b a decayed leaf-stalk, the basal portion of which is still living and bears a bud
Ilia; the hairy threads are roots which arise behind the growing apex of the stem.

that the different forms of leaves are distributed over different generations of shoots ;
certain shoots produce, for example, little or nothing but foliage-leaves, others
produce only bracts with or without flowers {e. g. Begonia). In such cases the
shoots may be designated, according to their leaves, scaly shoots, leafy shoots,
bract-axes, flowers, peduncles, &c. On this point further details will be given in
Book 11.

It is of very common occurrence with Cryptogams and Angiosperms (not
with Gymnosperms) for a persistent primary axis or branch-system to continue to
grow underground, and to send up only at intervals long foliage-leaves or shoots,
which subsequently disappear in their turn and are replaced by others. When
such axes or branch-systems lie horizontally or obliquely in the ground, and
produce lateral roots, they are called rhizomes (Fig. 156), (as in Iris, Polygonatum,
Pteris aquilina and many other Ferns). Frequently they die at the posterior and
continue to grow at the anterior end. Underground tubers and bulbs are more
transitory structures, usually lasting only for one period of vegetation ; the former

CHARACTERISTIC FORMS OF LEAVES AND SHOOTS.

2iy

are characterised by the preponderance of the axial mass with a very small amount
of leaves, the latter, on the contrary, by the preponderance of leaves closely packed
round a short stem. If the lower parts of a plant produce slender lateral shoots
with small scales growing upon or beneath the earth, and after rooting at a con-
siderable distance from the mother-stock produce foliage-shoots or shoots stronger
than themselves, they are called stolons, as, for instance, in jEgopodtum Podagraria,
Fragarm, Siruihiopteris germanica, and in Mnium and Caiharinea among Mosses.

The greatest degree of variation from the ordinary forms of shoots is displayed
by the flat leaf-like axes and branch-systems, and by the stem-tendrils and spiny
shoots which occur frequently in Angiosperms. Leaf-like axes {phylloclades)
are found in those Phanerogams in which large green foliage-leaves are wanting,
and they replace them physiologically; their axial structure is of considerable
superficial extent, and they produce and expose to the light large quantities of
chlorophyll; they generally bear only very small membranous scale-leaves. Ex-
amples may be found in Phyllocladus among Conifers, Ruscus among Mono-
cotyledons, and among Dicotyledons in Muhlenbeckia platyclada (Polygonaceae),
' Xylophylla (Euphorbiacese), Carmichaelia (Papilionacese), and in Opuntia hrasiliensis
and Rhipsalis crispaia (Cactaceae), &c.

Stem-tendrils, like leaf-tendrils, are long, slender, filiform structures, which have
the power of winding spirally round slender bodies in a horizontal or oblique position
with which they come laterally into contact, and thus serve as climbing organs ; they
spring laterally from shoots which have not the form of tendrils, and are distinguished
by the absence of foliage-leaves, their power of forming leaves being mostly limited
to very minute membranous scales. They are usually easily distinguished from leaf-
tendrils by their origin, position, and by the production of leaves ; cases, however,
occur where the morphological nature of a tendril is doubtful, as, for instance,
in Cucurbitacese ^ Evident examples of stem-tendrils are to be met with in
Vilis, Ampelopsis, and Passiflora. Shoots which bear strongly developed foliage-
leaves on long slender internodes, and which have the power of winding in an
ascending manner round upright supports, are not considered tendrils, but are
called twining or climbing stems'^; a distinction is drawn between tendril-
climbers (as Vitis) and stem-climbers (as Phaseolus, Humulus, Convolvulus, &c.). In
Cuscuta, where the primary shoot and all the lateral shoots, except the inflorescences,
twine in the manner of tendrils and of climbing stems, and where foliage -leaves are
also entirely suppressed, the pecuHarities of tendrils and of climbing stems are to a
certain extent united. A distinction similar to that between stem-tendrils and climb-
ing stems is also possible in leaves; the foliage-leaves of Lygodium exhibit con-
tinuous growth in length, and behave completely like climbing stems, the rachis
of the leaf corresponding to a climbing axis, and the leaflets to its foliage-leaves ^

The axial shoots of many Angiosperms have, like the leaves, the power of

^ According to Warming these are also metamorphosed branches.

^ Compare H. von Mohl, Ueber den Ban und das Wmden der Ranken und Schlingpflanzen.
Tubingen 1827. [See also Darwin, On the Movements and Habits of Climbing Plants, London
1875.]

^ Compare Book II, Ferns, and Book III, on the Physiological Signification of Tendrils and
Climbing Stems.

ziH

MORPHOLOGY OF MEMBERS.

forming spines, becoming transformed into conical, pointed, hardened bodies.
This may take place either by the whole shoot or even a whole branch-system
becoming spiny, with suppression of the foliage-leaves, as in the branched spines
of Gleditschia ferox, or by the shoot first producing foliage-leaves, growing in the
ordinary manner, and finally finishing its growth in length by a spiny point, as
in the lower axillary shoots of Gleditschia iriacanthos, Prunus spinosa, and many
others.

Among Phanerogams, especially Monocotyledons and Dicotyledons, displacements of
the leaves and lateral shoots (as well as roots), and adhesions of members, constantly occur,
which, as development advances, are in apparent contradiction to the typical laws of
growth and local position which are the ordinary ones in these classes ; and it appears

Fig. 157. — Diagram of the adhesion of leaves
with the axial parts of their axillary shoots (after
Nageli and Schwendener ; Das Mikroskop).

Fig. \^.—Hermintnnt Monorchis (after T. Irinisch : Biologie
und Morphologie der Orchideen. Leipzig- 1853) ; / the lower part
of a flowering shoot (natural size) ; //, /// the part containing the
bud (magnified).

impossible to apply even the most general rules of growth which we have now been con-
sidering. It would be difficult for even a clever beginner to explain by the principles which
have been regarded in this chapter as most universal, the structure, for instance, of the ex-
panded flower of an Orchis, Rose, Lamium, Salvia, or of many other plants, of a partially or
wholly ripe fig, or the phyllotaxis in the inflorescences of Borragineae and Solanaceae and
many others. But the history of development shows that even such cases may be ranged
under these laws, and that peculiarities of structures of this kind only arise at a later
period, or in such a manner that they confirm general rules. The deviations from these
laws are caused by the cessation of the growth of particular parts at an early period, while
others undergo a great advance ; or they are caused by the adhesion of parts originally
distinct. Although it is quite impossible to give general rules for the explanation

CHARACTERISTIC FORMS OF LEAVES AND SHOOTS. ' 219

of abnormal formations, yet the causes which most commonly produce these results
may be mentioned ; these may be included under the heads displacement, adhesion, and
abortion. Very commonly the two first act in unison, and in many flowers combine with
abortion to produce complex organs difficult to explain. It belongs to the most beautiful
problems of morphology to refer such apparent exceptions to more general laws of
development ; and the determination of natural affinity, the definition of the typical
characters of whole classes, orders, and families, depends upon it. Since, however, these
complicated phenomena belong almost exclusively to Angiosperms, and in them occur to
much the largest extent in the flowers and inflorescences, the best place for a more
detailed description will be when the characteristics of this class are under consideration.
Some explanation may, however, be given here, by means of a few examples, of the use
of the terms displacement, adhesion, and abortion.

Fig. 157 represents diagrammatically a branch-system developed sympodially and
proceeding from an axillary shoot; i, i being the first shoot with its two leaves i* and
i'^ ; in the axil of the leaf i^ is developed the shoot 2, 2, with its two leaves 2% 2^ ; in
the axil of its leaf (2^) again arises the lateral shoot 3, 3, with its leaves 3% 3^, and so on.
The parts of the stem of the shoots i, 2, 3, 4, which proceed from one another, form a
straight pseud-axis or sympodium with the peculiarity that the mother-leaf in whose axis
the lateral shoot developes adheres to it, and is carried up by it for some distance. If
we call the globular ends i, 2, 3, 4 of the figure flowers, the whole is well adapted to
represent diagrammatically the inflorescence of some Solanaceae. If the leaves 1% 2% 3%
4^ are supposed to be removed, the diagram might stand for the primary branch of the
inflorescence of Sedum. If, on the other hand, a lateral shoot is supposed to be formed
in each case in the axil of the leaves i*, 2% 3% 4* in the same manner as on the other
side with displacement of the mother-leaf, this would repeat diagrammatically in a simple
manner the branching and phyllotaxis of Datura ^.

Still more complicated are the phenomena in Fig. 158, where /represents the lower
part of a flowering plant of Herminium Monorchis. t t \s the surface of the ground, and
what lies below this is therefore underground : 5 is a swollen spherical root, above
which rises the leaf-bearing shoot, which produces in its lower part slender lateral roots,
iv, nu, eiv, as well as a sheath-like scale ^ b, and two foliage-leaves c, d, and continues
higher as a slender scape j4, bearing a raceme of flowers at its summit. Turning our
attention exclusively to the structure H; we find it to be a shoot which contains the bud
for the next year ; for the whole plant A, Bm I dies off" after flowering, a similar plant
being produced the next year from the bud contained in H. H is therefore an axillary
shoot of the scale b, an earlier condition being represented in Fig. ///, where M repre-
sents the base of the leaf I? cut through its median plane ; ^ is a fibro-vascular bundle
running from the primary axis to the bud m; ^/ is the first leaf of this bud u which
is placed with its back to the mother-axis and forms a diminutive sheath enclosing
the succeeding leaves of the bud « ; B^ is the young tuberous root with its root-sheath v.
In order to understand the displacement which has already taken place, the whole lower
part between M and 'v must be imagined shortened to such an extent that B"^ would be
somewhere near the letter g ; and the bud u must be supposed at the same time moved
backwards towards 0. By this means the normal position of the parts of H under con-
sideration is restored, and it is intelligible that the channel /, inclosed by the base of the
leaf bl, is a consequence of the oblique direction outwards of the growth of the tissue
lying between 0 and u, that the root-sheath 'v must be regarded as a part of the surface
of the primary axis above M, and that in consequence B"^ has been formed in the
tissue of the mother-axis beneath the bud u, and laterally on the fibro-vascular bundle g.
In the normal position of the bud and root, the axis of growth of the latter would form
almost a right angle with that of the bud, whereas by the displacement one forms a

' [See Payer, Elements de Botanique, p. 117.]

* A first scale in the axil of which the bud k stands is no longer to be seen.

'Z^O

MORPHOLOGY OF MEMBERS.

prolongation of the other. The growth of the mass of tissue lying between g and u further
continues in the same direction, and the whole lateral shoot assumes the form repre-
sented in H (Fig. I) ; the still further change of position of the parts which takes place
in consequence is explained by Fig. //, where ^n represents the bud u in ///, bl the
still more elongated sheath of the leaf bl in ///; the channel / is the cavity inclosed by
the leaf /^/increased in breadth, and which, were there no displacement, would be entirely
filled up by the bud u (or kn) ^.

In order to make the following displacement, which occurs very commonly, more
intelligible, reference should be first made to Fig. ii8, p. 154. This shows how the
tissue beneath the apex expands laterally by early growth, so that the surface of the
growing point, which would otherwise be elevated in a cone, becomes almost level ; and
the apical point thus comes to lie in the middle of a plane instead of at the point of
a cone. In the Sunflower this state of things remains nearly unchanged as the capitulum

Fig. 159.— Development of the fig of Fi'cus carica (after
Payer : Organogenie de la fleur).

Fig. 160,— Development of the flower oi Rosa alpina (after Tayer
Organogenie de la fleur) .

developes; but the abnormal growth increases in many cases to such an extent that
the apical point eventually lies at the base of a deep hollow, the walls of which result
from older masses of tissue, which properly lie beneath the apex, growing upwards and
overarching the apex itself. This occurs, for instance, in the development of the fig,
which, as shown in Fig. 159, is a metamorphosed branch, the apex of which is at /» still
nearly level, at // has already been outstripped by a circular leaf-bearing cushion, and
at IIP' is depressed in the form of an urn. The apical point of this shoot lies in this
case in the deepest part of the hollow, the inner side of which is properly only the

^ [See also J. H. Fabre, De la germination des Ophrydees et de la nature de leurs tubercules,
Ann. des sc. nat. 1856, vol. V.]

CHARACTERISTIC FORMS OF LEAVES AND SHOOTS.

121

prolongation of the outside of the fig, and bears in consequence a large number of flowers
(exogenous lateral shoots). In the nearly related genus Dorstenia the fig remains open ;
the margins of the tabular part of the axis which bears the small flowers do not arch over
and unite.

On a process very similar to the formation of the common fig depends the origin of
perigynous flowers and of inferior ovaries. Fig. i6o represents this in the perigynous
flower of a Rose. / shows the very young shoot which is to develope into a flower, seen
half from above and from the outside ; the end of the shoot is thickly swollen ; it has
already produced the five sepals / /, and the five petals alternating with them are visible
as little knobs, c, between which the apical region of the floral axis appears broad and
flat. While the sepals grow quickly, the zone of the tissue of the axis out of which
they spring becomes elevated in the form of a circular wall x in //, which afterwards
contracts the opening above as seen in IF\ an urn-shaped structure is thus formed
which is known under the name of a hip, and is distinguished when ripe by its red
or yellow colour and its sweet pulpy tissue. Here also the apical point lies in the
middle of the bottom of the hollow, and the inner surface of the wall of the urn is
a portion of the outer side of the floral axis which has been turned in. To this cor-
responds the acropetal succession of the leaves (which, however, is only adhered to in a
general way). It is clear that if the apical point lies ?ity (in 7/), the order of succession
of the leaves (in this case stamens st and carpels k) from above downwards must be
termed acropetal.

If an additional proof of what has just been said were wanted, it would be fur-
nished by the history of development of the flowers of Geum\ a genus very nearly
related to the Rose (Fig. i6i). That part of the floral
axis which bears the sepals /, the corolla c, and the stamens
a a, is elevated in the form of a circular wall yy\ but
the apical region which in Rosa entirely ceases to elongate,
becomes here again elevated as a conical body x, bearing
at its summit the apical point of the floral axis. The order
of succession of the foliar structures is again acropetal,
and in consequence the stamens a are formed on the
inner side of the axis yy from above downwards, the
carpels which succeed them on x from below upwards.
In Geum and other Dryadese the urn yy spreads out at
the time of fertilisation, its margin grows so vigorously in
size that it expands in the form of a flat plate, and after
the expansion its inner surface becomes the outer surface,
in the middle of which the gynophore x rises like a cone,
and in Fragaria afterwards swells out, becomes fleshy, and
forms the strawberry (a pseudocarp like the hip).

It will be seen that the formation of the fig, the hip, and
[that of the subsequently flat receptacle of Geum depends
?on a displacement which is caused by vigorous growth of
rmasses of tissue that arise in the form of zones beneath the
fgrowing point. There is in these cases no such thing as
adhesion of foliar structures (as is usually stated in works

[on descriptive botany). The so-called coherent corolla and calyx of gamopetalous or
sympetalous and gamosepalous or synsepalous flowers are also not the result of cohe-
fcsion; the petals or sepals are on the contrary formed as a whorl of separate pro-
[tuberances on the broad end of the young flower-stalk. That a gamopetalous corolla or
[gamosepalous calyx subsequently has the appearance of a bell having at its margin only
[as many teeth as the leaves of which it is considered to consist, does not depend on
flateral cohesion of the margins of the leaves, but on the fact that the whole annular zone
|of the young receptacle which bears the corolla or calyx grows up ; the bell-shaped part

Fig. i6i.— Longitudinal section of a
young flower of Geum rivale.

222 MORPHOLOGY OF MEMBERS.

therefore never consisted of distinct leaves, but is the common basal piece which is
developed from the floral axis as a whole, and which shows at its margin the original still
separate leaves as teeth of the bell. The reverse is the case in the leaf-sheaths of
Equisetum, where an annular wall originally projects round the axis, from which the
separate leaf-teeth afterwards grow out. In this case also the sheath cannot be con-
sidered as formed by the cohesion of previously distinct pieces, but the separate teeth
of the sheath must rather be considered as branches of a single annular rudimentary leaf.
A similar explanation applies to the bundles of stamens which are generally termed
coherent (monadelphous, polyadelphous, &c.) stamens. As many protuberances are
formed originally as there are bundles of filaments to be produced; and these pro-
tuberances must be considered as the original staminal leaves which subsequently
produce by branching a larger or smaller number of stalked anthers (as e.g. in Hypericum,
Callithamnus, &c.). Cohesions of parts originally distinct are rare ; examples are fur-
nished by the connate inferior ovaries of two opposite flowers of an inflorescence in
Lonicera alpigena, the fruits of Benthamia fragifera which cohere into a large pseudo-
berry, and the cohesion of the two stigmata in the flower of Asclepias to each other and
to the anthers. The anthers of Compositae are not truly coherent, but only glued
together by their sides.

Much more common than actual cohesion is the abortion of members already
formed. Thus, for instance, the paripinnate leaves of some Leguminosae^ originate as
imparipinnate leaves ; the terminal leaflet which is finally aborted is at first in the bud
even larger than the lateral leaflets ; but, as development progresses, it is so retarded
that in the mature leaf it overtops the origin of the uppermost lateral leaflets only as a
minute point. In the same manner the whole (branched) leaf-blades of many Acacias
are also abortive, and are replaced by the petiole (phyllode), which is then expanded
in its median plane. Still more complete is the abortion of the leaves from the axils of
which spring the branches of the panicles of Grasses ; and in this class whole flowers
are often aborted. In diclinous Phanerogams the unisexuality of the flowers
usually depends on the abortion of the stamens in .the female, of the carpels in the
male flowers. Sometimes only one of several stamens is aborted, as in Gesneraceae
{e. g. Columnea, where it is transformed into a small nectary) ; and the same occurs
with the carpellary leaves {e.g. in Terebinthaceae). In all these cases the structure which
is afterwards abortive is actually present in the bud or even later, but its further
growth ceases. The comparison, however, of nearly related plants shows that very
commonly certain members are wanting in the flower the presence of which might
be expected from the position and number of the others and from their presence
in nearly related forms, although in such cases even the earliest condition of the
bud does not exhibit a trace of the absent member. Since from the point of view
of the Theory of Descent it must be assumed that nearly related plants are descended
from a common ancestral form, the absent member may in such cases also be sup-
posed to be aborted, only the arrest of development which has once taken place at an
early period is so complete and has become so hereditary, that even its first rudiment
is suppressed. The true theory of the structure of many flowers, and the reference
of diff"erent forms of flowers to common types, often depends on the restoration of
aborted members of this kind; but to this we shall recur in detail in Book II, when
treating of Phanerogams.

Sect. 29. Reproduction; Sexual Organs ; Alternation of Generations.

Reproduction, or the production of new individuals, is generally brought about
by particular portions of an individual becoming detached, which then have the
power, first of all of producing new organs of nutrition, and afterwards of continuing

^ Hofmeister, Allgemeine Morphologic, p. 546.

ALTERNATION OF GENERATIONS. 22^

their growth in such a manner that all the vital phenomena of the mother-plant
are one by one reproduced in it. Since the same individual can form a number
of reproductive organs, either simultaneously or successively, so there is also, in
the reproductive process, at least the possibility of a multiplication of individuals,
inasmuch as, under favourable vital conditions, a number of descendants of the
same mother-plant actually come into existence. But since all those portions of
the surface of the globe which it is possible for plants to inhabit are already covered
with vegetation, it is in general only possible for such a proportion of these
descendants to arrive at full maturity, that the number of individuals in existence
remains the same from year to year. We shall see in the third book what an
important bearing this fact has on the struggle for existence and on the conse-
quent production of new vegetable forms. At present we have to consider only
the most important morphological phenomena connected with the organs of
reproduction.

The parts which become separated for the purpose of reproduction are very
various in their nature. Among Cryptogams they consist most commonly of single
cells, — spores, gonidia (or conidia\ oospheres, anther ozoids : less often they are bodies
consisting of a small number of cells united into a tissue, like the gemmce or bulbils
of the Marchantieae. In the more highly organised plants it frequently occurs that
shoots, t. e. portions of the axis bearing leaves, become detached of their own accord
in the bud- condition, then put out roots, and continue an independent growth ; buds
of this kind occur, for example, in some Mosses, in many Ferns, in Lilium
hulUferum, several species oi Allium, &c. Very often almost any part of the plant,
such as detached pieces of leaves, stems, roots, &c., may become organs of reproduc-
tion, that is, they are able, under favourable conditions, to put out adventitious buds,
and thus develope into new plants. In Phanerogams, finally, the normal reproductive
bodies are the seeds, in which, even before separation from the mother-plant, a
new individual has already advanced to a lower or higher stage of development,
so that when the seed germinates nothing more is necessary than an increase in
size of the parts that are already formed, — root, stem, and leaves.

In some cases the organs of reproduction appear, as it were, accidentally;
but we will not here consider these cases, but rather turn our attention to those
in which the formation of these organs is a necessary part of the life-history, and
is essential to the complete development of the plant. These normal but still
very various reproductive organs may be divided first of all into two groups, the
sexual and the asexual.

Reproduction is said to be asexual when the part of the plant which becomes
detached is able, without the assistance of any other organ, to produce a new
individual. Of this nature are the spores of the Hymenomycetous Fungi and of
Ferns, the gemmae of Hepaticse, and most zoogonidia of Algae.

Reproduction is, on the contrary, sexual when two organs, developed expressly
for this purpose, co-operate to produce a body out of which, either directly or after
some further processes, one or more new individuals arise. Notwithstanding the
great variety in the form of the organs of sexual reproduction in the vegetable
kingdom, and the complicated nature of the processes which often, especially in
the higher plants, precede the act of sexual union, the essential feature of this

224 MORPHOLOGY OF MEMBERS.

act is always that two cells of the simplest possible kind combine, either completely
coalescing with one another, or at least their contents becoming partially inter-
mingled so as to afford a starting-point for a fresh development. It is the
essential characteristic of sexual reproductive cells that each is incapable of
further development by itself, this being the result of the combined action of
two such cells.

It is only in the lowest forms of vegetable life, in some Algae and Fungi,
that the two cells which take part in the act of sexual union are alike or at least
very similar in size, form, and physical properties. In this case their union is
called conjugation^ and the cell capable of germination which results from the
union a zygospore. In all other cases the two uniting cells are strikingly different
in size, form, and physical properties. In these cases one of the two cells,
the male cell, conveys to the other only a very small quantity of material by
means of which it produces an effect upon it ; this other cell, the female cell,
contains by far the largest proportion of the material which takes part in the
development incited by the act of union. With the exception of a few complicated
cases among Algae and Fungi, which will be particularly described, the relation-
ship between the two sexual reproductive cells is still more clearly indicated
by the fact that the male cell is motile, carrying to the other cell the fertilising
material. The motion of the male cell is however of two kinds ; it may either be
spontaneous, as in most Cryptogams, when the cell is termed an anther ozoid^ ;
or, as in Phanerogams, the male cell, then called a pollen-grain, becomes detached
from the parent-plant, and conveys the fertilising material to the female cell first
of all by the aid of external forces, then by its own growth. The female cell, on
the other hand, which is fertilised by means of the male cell, remains at rest at
the place where it was formed, or at most, as in the Fucaceae, is carried about
passively; with the exception of the cases above referred to it is always a naked
primordial cell, and is termed the odsphere, or germinal vesicle. After fertilisa-
tion, it secretes a cell- wall, and is then termed an oospore; from this, either at
once, or after a period of rest {resting spore), the young plant is subsequently
developed. Some not inconsiderable deviations from this plan will have to be
pointed out in the class Carposporeae of Thallophytes ; but even in these cases
the essential condition of fertilisation remains, that the male cell only incites to de-
velopment, while the development itself proceeds entirely from the female organ.

Very considerable variety is shown in the morphological characters of the
sexual organs, if we take a comparative view of the whole vegetable kingdom ;
but in the larger groups of plants we find the morphology of these organs to agree
completely in all essential points, even when the anatomy of the vegetative organs,
the habit and mode of life of the plants that compose them, vary greatly. When
considering, in Book II, the distinguishing characteristics of the different classes
of plants, our attention will be specially directed to the morphology of the sexual
organs, and it will be sufficient here, as an introduction to what follows, to define
the most general terms connected with these organs.

' [The term 'anthcrozoid' was first proposed by Derbes and Solier, Ann. des sc. nat. 1850, vol.
XIV. p. 263.]

225

R ALTERNATION OF GENERATIONS.

In some plants we find only one kind of reproduction, either only the
cual, as in some of the simplest Algae and Fungi, or only the sexual, as in
the Conjugatae.

But in most plants reproductive organs of both kinds, sexual and asexual,
are produced either simultaneously or successively. The two kinds of reproduction
may then occur in the same individual, as in Vaucheria or Eurotium^ or may be
distributed on different individuals. In both cases the entire process of development
may be divided into two sharply separated stages: — At the termination of one
stage sexual organs are formed : by their union the second stage of develop-
ment is rendered possible, and this closes with the production of asexual spores.
Such a course of development is termed, from the analogy of certain processes
in the animal kingdom, an Alternation of Generations, a term which is especially
applicable in those cases in which, in one or both of the two stages of develop-
ment, multiplication of the individual also takes place by gonidia or gemmae,
so that each of the two stages is complete in itself as a sexual or asexual
generation.

Since we have here to do with phenomena which are foreign to ordinary life,
and hence somewhat difficult to understand, we will illustrate the nature of alternation
of generations by a few simple examples.

An alternation of generations is very evident in Ferns. The plant which we call
in common language the Fern is merely the second stage in the process of develop-
ment of the plant, or the asexual generation [sporophore]. It consists of a stem which
forms true leaves and roots; on the leaves are produced small capsules or sporangia,
in which the spores are produced without any sexual process. But each of these
spores does not, on germination, again produce a Fern,, but a minute plant of
extremely simple structure which nourishes itself independently as a leaf-like thallus
with root-hairs. This little plant, termed a prothallium, may, under certain circum-
stances, reproduce itself by gemmae ; and thus from a single spore an entire genera-
tion of prothallia will arise, which also behave as independent plants, although
each prothallium is only the first stage of development, the sexual generation
[oophore], of a Fern. For, finally, the prothallia produce sexual organs of repro-
duction; and from the oosphere of the female organ is produced an embryo
which developes into a Fern with true roots and leaves. In this stage also
the Fern is capable of immediate multiplication by the production of bulbils
from which Fern-plants are directly developed, but the normal development closes
with the production of spores. Precisely the same processes as in Ferns take
place also in the Equisetaceae and Ophioglossaceae. In the Selaginelleae the pro-
thallium is formed inside the spore, and this class therefore establishes a transition
to Phanerogams, where the prothallium is altogether rudimentary, and is found in a
spore-like structure, the embryo-sac, within the ovule; so that the alternation of
generations, so evident in Ferns, can be recognised here only by the most careful
comparison with the most highly developed Cryptogams. This will be explained
more in detail in Book II.

In the Muscineae the alternation of generations is no less clear than in Ferns,
although it assumes an entirely different form. A Moss, in the state in which we
ordinarily see it, consists of a stem provided with numerous leaves and root-hairs^

Q

226 MORPHOLOGY OF MEMBERS.

but this leafy plant docs not, as in the case of the F(^rn, produce spores, but can be
reproduced by bulbils of different kinds. At length however, like the prothallium
of Ferns, it produces sexual organs, and an embryo is the result of the fertilisation
of the oosphere : this is not connected organically with the Moss-plant, but remains
attached to it, deriving its nourishment from it, and finally developes into a capsule
supported on a long stalk, the Sporogomu?n, in the interior of which are produced
numbers of spores. A number of these stalked capsules, i. e. entire generations,
may arise on the same Moss-plant either simultaneously or successively. The
course of development of a Moss is therefore divided into two sharply separated
stages, VIZ. the formation of a leafy stem which produces sexual organs (oophore),
and the production of stalked capsules out of the fertilised oospheres of the female
organs. In Muscinese the second or asexual generation (sporophore), the sporo-
gonium, has no power of directly producing its like from itself, as is possible in
the case of Ferns, by bulbils; its only function is to produce spores^; and when
the spore germinates it gives rise first of all to a Protonema, which sometimes con-
tinues to grow for a long while, and can reproduce itself by gemmse, until at lengh
Mo 3s- stems with true leaves again appear on it, which also are capable of multi-
plication by means of bulbils.

Even in Thallophytes we meet with various forms of an alternation of gene-
rations^. It is well shown in certain Fungi of the class Ascomycetes which have
been closely investigated, as, for example, in the common mould, Penicilliwn
glaucum, the ordinary form of which is only the first generation or stage of
development in its life. During this stage, the first or sexual generation, the
so-called Mycelium, developes on special branches a number of cells {conidia),
by which the Fungus is continually propagated in this form. But when the
excessive development of these conidia is prevented by exclusion of the air,
sexual organs arise, as Brefeld has shown, on the luxuriant mycelium, and in
consequence of their union a tuberous body is formed of a totally different
nature, within which spores are finally produced in extremely numerous sacs
{asci) of peculiar form; and these, when they germinate, again produce the
mycelium with its penicillate conidiophores. The mycelium of this Fungus
(and strictly speaking of all Fungi) corresponds therefore to the first stage of
development, the prothallium, of Ferns, or to the leafy Moss-plant; and all three may
be considered as the Sexual Generation [oophore], since their normal development
ends with the formation of sexual organs. In all three cases, this sexual gene-
ration (prothallium, Moss-plant, mycelium) may propagate itself by gemmae or by
conidia before it produces the sexual organs. The small tubers which are the result
of fertilisation in Penicillium correspond to the second stage of development, viz.
the sporogonium of Mosses, and the mature Fern-plant^; in all three cases the
product of this second generation [sporophore] is a large number of spores, by

^ [The researches of Pringsheim and Stahl however have shown that this linaitation can no

longer be maintained. See Book II. Group 2.]

^ [This explanation is now no longer generally accepted ; see Journ. of Botany, 1879.]

^ If these small tubers in Penicillium are termed the fructification, then in the same

sense the sporogonium of Mosses is a fructification; and the Fern is also the fructification of

the prothallium.

ALTERNATION OF GENERATIONS. 22 J

which the whole process of development is again repeated. And as in Ferns, in
addition to the spores, bulbils are also sometimes developed on the mature plant,
so penicilliate conidiophores are also occasionally produced on the tuber of
PenicilUum. For the purpose of including this second stage of development in
the various classes of plants under a common name, it may be termed the Spore-
forming or Asexual Generaiion [sporophore], excluding from the idea of true
spores the conidia of Penicilliwn^ as well as the gonidia of Thallophytes.

If now, in the three examples cited of alternation of generations, we compare
the organisation of the first or sexual with that of the second or spore-forming
generation, it is seen that the latter, the result of an act of sexual union, is more
highly and perfectly developed than the former, and therefore in this respect
also represents the true close of the process of development. Thus in PenicilUum
"we find the first generation developed in the form of a so-called mycelium,
consisting of slender segmented branched filaments, while the second generation
consists of a compact tissue of complicated structure. In Mosses again the
first generation commences with a protonema, consisting, like a mycelium, of
branched segmented filaments of cells; but here this generation advances to a
higher development, since the protonema produces the leaf-bearing Moss-plant,
the histological structure of which is still however very simple in comparison with
the much more complete differentiation of the sporogonium. Still more strikingly
are these characteristics seen in Ferns, where the first generation or prothallium
consists of a plate of tissue which shows scarcely any external differentiation,
while the second generation or true Fern is a very highly organised plant, dif-
ferentiated externally into root, stem, and leaves, the tissue itself being also
differentiated into three well-marked systems, the epidermal system, the fibro-
vascular bundles, and the fundamental tissue.

Starting then from Algse and Fungi, and proceeding through the classes of
Muscineae, Filices, and Equisetaceae to the Lycopodiaceae, and finally to the Phane-
rogams, it is seen that in the alternation of generations, the first generation
(oophore) continually recedes in importance and independence, while the develop-
ment of the second generation (sporophore) continually advances ; so that at length
in Phanerogams the former is no longer a plant with independent power of growth,
but takes the form of a special mass of tissue, the so-called Endosperm in the repro-
ductive apparatus of the latter, filling up along with the embryo the cavity of the
seed-coats. In contrast to this, at the starting-point of the series (Algae and Fungi),
the first or sexual generation is alone developed as a plant with independent growth ;
the second (asexual) generation appearing on it as its fructification or spore-fruit,
represented, in its simplest form, by a single spore resulting from fertilisation, as
will be illustrated in the introducdon to the Thallophytes.

Designating the course of development which we have sketched out as
alternation of generations, each of the two stages may be termed an Alternating
Generation. Each may, as we have seen, be propagated directly by gemmae, or
by conidia, those developed by the first generation again producing individuals
of the same kind ; and in the same manner bodies of the same nature produced
by the second generation will reproduce it. But this mode of reproduction may be
wanting in either of the two alternating generations.

Q 2

228 MORPHOLOGY OF MEMBERS.

If we now regard the two alternating generations as two stages of development
of the same plant, each of which is necessary to supplement the other, it is seen
that, in the first place, the entire course of development of a plant commences
twice with a simple cell :— the first time the development begins with the spore
to form the first (sexual) generation, the second time with the oosphere in the
female organ to produce the second (spore-forming or asexual) generation. Secondly,
we find that, in addition to these two beginnings from spore and oosphere, which
are united to one another by the complete course of development, a subsidiary
mode of development may also occur, each of the two generations having the
power of propagating itself directly. For the purpose of distinguishing them
from the true spore? with which the development of the second generation closes,
we term all those reproductive organs which immediately propagate the same
generation either GemmcB or Gonidia, A Spore, in our sense of the term, arises
only from the second generation, and gives rise, on germination, to the first
generation; a bulbil or gonidium, on the contrary, may arise from either of
the alternating generations and reproduce it. The same facts may be ex-
pressed in the following manner: — Sexual cells (oospheres) and true | spores
indicate the turning-points in the alternation of generations; they are not organs
for direct reproduction, for each of them always produces something different from
that from which it immediately sprung ; the spore of the Fern, for example, gives
rise to a prothallium, the oosphere of the prothallium to a Fern; the spore of
the tuber of Penicillium does not again give rise to a tuber, but to a fila-
mentous mycelium, on which the tuber again arises as the result of fertilisation
of the female cell. Bulbils and gonidia are, on the contrary, organs for direct
reproduction, by means of which the same stage in the process of development
is again repeated; the bulbil, for example, which arises on the leaf of a Fern,
does not produce a prothallium, but a Fern; in the same manner the conidia
formed on the branches of the mycelium of Penicillium do not, on development,
give rise to the tuber, but to a mycelium like that on which they were borne.

The alternation of generations, as we have now described it in a few examples
where it is peculiarly well exhibited, does not occur in those classes of Thallophytes
which have the simplest structure ; its first indications are met with where an act of
sexual union is first detected, until at length, in the more highly developed plants,
the alternation is manifested with perfect sharpness.

a. The idea of an alternation of generations is extended by some botanists con-
siderably beyond the limits to which we have here confined it. It has been proposed,
for instance, to apply the term to the case of Phanerogams in which lateral branches
with foliage-leaves spring from a rhizome clothed only with scales, and from these
other branches which develope into flowers; and to others of a like nature. It is
clear, however, that the cases in question have a totally different significance in the
history of development to that of the alternation of generations, using the term in
the sense indicated above ; they might be included, by way of distinction, under the
common phrase Alternation of Axes. This phenomenon is one which is very incon-
stant even within limited groups of plants, while, on the other hand, true alternation
of generations prevails over almost the entire vegetable kingdom; and the mode in
which it runs through particular groups of plants is one of the weightiest arguments
in favour of the natural system. This will be further elucidated in Book II.

ALTERNATION OF GENERATIONS.

229

b. The problem which is met by the theory of alternation of generations in the form
:m which it has now been presented, is to refer the most important stages in the
[history of development of all plants to a single scheme which is illustrated most clearly
in the cases of Muscineae and Filices, where Hofmeister first discovered this alternation
in 185 1. The same botanist was also the first to explain the development of the seed
[in Gymnosperms by the alternation of generations in Lycopodiaceae, and hence to
[compare it with the same phenomenon in Filices and Muscineae. At the present time
[our knowledge of the development of Thallophytes has made so much progress that
[it is possible to determine what are its main features, and to compare them with those
[of Muscineae and vascular plants. This comparison, which has only been briefly
[indicated above, will be followed out more in detail in the sequel, and will lead to
[the result that Thallophytes may also be included in the scheme under which the
)ther classes are comprised, the first stage of development closing with the formation
[of sexual organs, from which proceeds the second generation, essentially different
[from the first, and closing with the production of true spores. It will therefore show
[that the development of all plants which possess sexual organs may be divided into
two stages which correspond in all essential points to the two generations in the life-
[history of a Fern ; and that there is, therefore, in the whole vegetable kingdom,
[only one type of alternation of generations so far as it is brought about by sexual
: organs.

BOOK II.

SPECIAL MORPHOLOGY

AND

OUTLINES OF CLASSIFICATION.

GROUP I.
THALLOPHYTES.

In this group are comprised Algae, Fungi, and Lichens, the term being applied
to them because their vegetative body is usually a Thallus, i.e. exhibits no differ-
entiation into stem, leaf, and root, or, if at all, only in a very rudimentary
degree. There occur however in various groups of Thallophytes transitions from
the simplest forms, which display no external differentiation, to others which show
some indication of it ; in the most highly developed representatives of some
groups the external differentiation is carried so far that the terms leaf and stem
are as applicable to them as to the higher plants. A true root, in the sense in
which the term is applied to vascular plants, is however never found, though root-
like organs are commonly present which are termed Rhizoids; these are however
always distinguishable by the absence of a root-cap and by the branching not being
endogenous.

Like the external, the internal differentiation of Thallophytes also begins at
the lowest stages, ascending by numberless transitional steps to a more perfect
development of cells and tissues ; but even in the most perfectly developed forms we
do not meet with any sharp differentiation into tliose different systems which we
know among the higher plants as epidermal tissue, fundamental tissue, and fibro-
vascular bundles. Even where the thallus consists of large masses of tissue, as in
Fungi, it is still strikingly homogeneous.

Thallophytes nevertheless present a great variety of examples of the mode in
which morphological differentiation proceeds from the simplest organic forms to

232

THALLOPHYTES.

i

Others which are both externally and internally more and more complex. In the
simplest stage the whole vegetative body consists of a single small cell of a
roundish form, the cell-wall of which is thin and smooth, and within which proto-
plasm, chlorophyll, and cell-sap are only imperfectly separated. Advancing from
this, progressive development is first displayed in the perfection of the single cell,
which increases in size, and often attains dimensions unknown elsewhere in the
vegetable kingdom, the differentiation being either chiefly in the contents, or in the
external form, i. e. in branching. The growth of the cells may, on the other hand,
be accompanied by cell-division, the thallus becoming multicellular, so that from a i
single cell there arises either a row of cells or a segmented filament, a simple plate
of cells, or finally a massive tissue growing on all sides. Each of these processes
further presents a great variety of modifications.

In the simpler Thallophytes a tendency prevails for a larger or smaller portion
of their existence to be passed in the condition of motile primordial cells, which
bear more or less resemblance to the simplest Infusoria, and were in fact until
recently confounded with them. In some cases cells which are already clothed
with a cell-wall, or assemblages of such cells, remain for a considerable time in a
motile condition, swimming freely in the water. But these motile conditions are
always interrupted by long periods of rest, during which growth and increase in
size usually take place. In many of the more highly developed Thallophytes
this power of motility is however limited to the male * swarming ' fertilising
elements, the antherozoids ; and in many cases is not displayed even here.

Like the structure of the vegetative body, the mode of reproduction of
Thallophytes also exhibits great variety, commencing with the simplest kinds,
and progressing finally to modes of reproduction almost as complicated as those
which are met with in the highest plants. In the simplest cases reproduction
appears to be coincident with ordinary cell-multiplication ; the cell which con-
stitutes the vegetative body grows and divides, each derivative cell then carrying
on an independent life and repeating the process. In the more highly developed
forms the unicellular or multicellular thallus continues to grow for a longer time, and
becomes differentiated externally and internally, until at length at some one spot
reproductive cells are produced. In most Thallophytes both kinds of reproduction,
sexual and non-sexual, occur; and in the higher forms an evident alternation of
generations is manifested. The reproductive organ which becomes separated from
the mother-plant is almost always a single cell, which however varies greatly in its
origin, significance, and capacity for development. The nomenclature which gives to
all these reproductive cells the name ' Spores ' is a very unfortunate one, obscuring
an insight into the course of development of the different forms, and rendering dif-
ficult the comparison of members of one group of Thallophytes with those of another.
The erroneous theory of so-called ' Pleomorphy among Fungi was the result of
a defective perception of the true nature of the different kinds of reproductive organs
on which the common name of Spore had been bestowed. What we have now
to say on this point for the purpose of counteracting the prevalent confusion
of terms is founded on the view explained in Sect. 29 of Book I, on alternation
of generations and on the relationship to it of the different reproductive organs.
I begin by designating as Spores the reproductive cells which are produced in the

INTRODUCTION.

^^33

I

^^Hporangia of Ferns and capsules of Mosses. These are obviously the result of a
vegetative process, excited by the act of sexual union, in consequence of which
they arise on the second (non-sexual) generation [sporophore] which springs from
l^^be fertilised oosphere of the first (sexual) generation [oophore]. Let us now
^^^ansfer these conceptions to the most highly developed Thallophytes which ex-
hibit an evident alternation of generations, as the Ascomycetes. We have already
n, in the Section referred to, that the ascospores of Penicillium are the result
a vegetative process brought into action by the sexual organs of the mycelium,
d which has for its result the formation of the tuberous fructification which
nstitutes the second generation. The ascospores of Penicillium therefore cor-
pond to the spores of a Moss or a Fern. If now we suppose the result of
;e union of the sexual organs to be a very inconsiderable vegetative structure,
d the second generation consequently to be merely rudimentary and a simple
pendage to the first, the spores themselves would then seem to be an almost
immediate result of fertilisation, as occurs for instance in the Nemaliese (Fig. 164,
C, p. 237). If we were further to imagine that the act of fertilisation did not
result in the production of any vegetative structure, or the second generation to
be altogether suppressed, the fertilised oosphere would then itself become a spore,
as in the Coleochaetese, (Edogonieae, and Vaucheria. In this case the spore is
an equivalent for the whole of the second generation ; it stands for the entire
fructification of the Ascomycetes, the entire spore-capsule of a Moss, «&c. Precisely
the same is true for the zygospore which results from conjugation. The zygospore
(as for example in the Mucorini), or the oospore (as in Vaucheria), represents there-
fore in a morphological sense the entire second generation of these plants. This
conclusion, which might easily be proved more in detail, may be briefly summed up
in the statement that the Spore is either an immediate product of fertilisation
(zygospore, oospore), or of a process of growth which is induced by fertilisation ; and
this vegetative growth may either be inconsiderable, as in the Nemalieae and Erysiphese,
or it may be considerable, and it then gives rise to the second generation in which
the spores are produced, as in Penicillium and other Ascomycetes. This explanation
shows at once how in Thallophytes the second generation is a gradually increasing
structure developed in consequence of the act of fertilisation. But for the purpose
of a scientific nomenclature the term Spore (if used in the same sense as in
Muscineae and Vascular Cryptogams) must be applied in Thallophytes only to
those reproductive cells which are the result of an act of impregnation, whether
direct, or indirect through the production of a vegetative body which constitutes
a second generation and closes the entire course of development of the plant. All
other unicellular and non-sexual organs of reproduction we shall not term spores,
but gonidia or cojiidia.

We may now proceed to a further description of the various kinds of sexual
organs found among Thallophytes, and of the true spores which result from their
union with or without an alternation of generations. The following three principal
forms or types may be distinguished \

* More minute evidence of the statements here made will be found in the sequel in the
description of Algae and Fungi. The facts stated are derived from the writings of Pringsheim,

234

THALLOPHYTES.

1. Conjugation and Production of Zygospores. Two cells of similar if not
always of precisely the same nature coalesce, and produce a reproductive cell
termed a Zygospore^ which germinates after a shorter or longer period of rest, and
then gives rise either to spores or at once to a plant of the same kind as that in
which the conjugation took place. An alternation of generations is exhibited only
in so far as the zygospore constitutes the entire second (non-sexual) generation.

The process of the formation of zygospores has a very different appearance
according to the nature of the conjugating cells. The simplest case is presented

in the conjugation of zoogonidia
A r, discovered by Pringsheim (Fig.

162, A). These bodies during
the process of swarming come
into contact in pairs by their
hyaline anterior ends, and then
gradually coalesce into a primor-
dial cell, which subsequently be-
comes invested with a cell-wall,
and then grows, producing again
motile cells, and each of these
gives rise to a plant of the ori-
ginal kind. These Zoospores which

Fig. 162.— Various forms of conjugation and the production of zygospores; rCSUlt frOm thc ZVG'OSDOrC maV
A conjugation of the zoogonidia of Panciorina ; H formation of zygospores / o x^ /

in Piptocephalis (after Pringsheim and Brefeld). The numbers indicate in '^g COnsldcrcd aS trUC SDOrCS in
each case the successive stages of development. ' ^

the same sense as those of the
Muscineae; for the zygospore is homologous with the spore-capsule of Mosses,
and represents a rudimentary alternate generation. The conjugation of Spirogyra,
as illustrated in Fig. 6, p. 10, is somewhat more complicated. The conjugating
cells are here surrounded by a firm cell-wall ; they put out protuberances opposite
to one another, which unite to form a canal, through which the contents of
one cell pass over into the other, and coalesce with its contents; the resulting
protoplasmic body invests itself with a cell-wall, and becomes a zygospore, which
again produces a Spirogyra filament by direct germination. The formation of
zygospores in the Zygomycetes is represented in Fig. 162, B. Here the two cells
which coalesce after having grown towards each other are perfectly alike and immotile ;
and it is only a portion of the coalesced contents which becomes separated by a
partition-wall, and produces the thick-walled zygospore which germinates after a
periodjof rest.

2. The Formation of Oospores in Oogonia. The two reproductive cells are
here essentially different ; the female cell ox Oosphere is always a naked immotile
primordial cell developed within an older cell which is termed the Oogonium.
The male cells, the Antherozoids, the mother-cells of which are called Antheridia^
are very small, and are endowed with motion by means of vibratile cilia; they
swarm round the oosphere, and cause its impregnation by the coalescence of their

De Bary, Thuret, Nageli, Janczewski, Brefeld, and others ; though a different signification to
^that of the authors is sometimes applied to them.

INTRODUCTION.

^35

substance with it. The size of the antherozoids is so inconsiderable that they
scarcely add appreciably to the mass of the oosphere, but yet produce a change in
it, one consequence of which is that it becomes invested with a firm cell-wall, and
then constitutes the Oospore.

The oospore may germinate immediately and give rise to a plant resembling the
mother-plant, as in Fucus, or only after a certain period of rest like the zygospores,
and this is the usual case. But here again the oospore may on germination give
rise directly to a plant resembling the mother-plant, as in Vaucheria and some
Saprolegnieae ; or it may, after remaining dormant, produce out of its contents a
larger or smaller number of zoospores, each of which finally gives rise to a plant
like the mother-plant, as in Sphceroplea, (Edogonium, and Cystopus. In this process
a rudimentary alternation of generations can again be detected:— an oospore which
breaks up into zoospores may be compared to the sporogonium of a Moss in

Fig. 163.— Examples of the production of oospores; A in CEdogonium ; B va. Saprolegiiia
(after Pringsheim) ; og the oogonium ; o oosphere; a antheridium ; ni small male plant or dwarf
male ; s antherozoid.

/hich all the parts except the spores are suppressed. If we were to imagine the
fertilised oosphere in the archegonium of a Moss as itself producing the mother-
cells of the spores \ we should have something similar to one of these oospores.
In this case therefore the oospore is properly a many-spored fructification in
the same sense as the Moss-capsule ; the zoospores produced from it are true
spores in the sense of those of Muscinese and Ferns, and we have consequently
the first indication of the alternation of generations which attains its highest
development in those classes. The new plants which result from the direct
germination of the oospores, or through the medium of zoospores, have the power,
in most cases, of propagating non-sexually by the formation of gonidia, until at
length individuals arise which produce antheridia and oogonia. This non-sexual
reproduction iriay be compared to that of the Marchantiese by gemmae produced
on their vegetative body, until finally antheridia and archegonia are developed.

^ That such an analogy is not altogether fanciful is shown by Riccia, a genus of Hepaticse,
the extremely simple sporocarp of which may well be compared to the oospore of an CEdogonium.
Pringsheim and De Bary have already pointed out this analogy (see De Bary, Die Familie der
Conjugaten, Leipzig 1858, p. 60).

%ofi THALLOPHYTES.

But on the other hand the formation of oospores exhibits a certain resemblance
to the process of conjugation. It is distinguished from that of the Pandorineae in
this point of importance only, that the two coalescing sexual cells are not alike,
so that the fertilisation of Vaucheria and (Edogonium may be considered as a
higher form of conjugation from a morphological point of view. But the mode
of formation of many oospores displays also a greater or less resemblance to the
mode of fertilisation which we shall describe as a third type ; and in this respect
the Saprolegniese in particular present a similarity to certain Ascomycetes.

3. Formaiion of Carpospores in Carpogonia. This type resembles the second
in the fact that the two sexual organs contribute in very different degrees to the
production of the fertilised body, the male organ only inciting to change, while
the whole of the further development of the plant proceeds from the female organ,
the result being the production of the Sporocarp.

The female organ, which may consist either of one cell or of more, may be
designated by the general term Carpogomum. The male organs vary greatly
according to the group to which the plant belongs; they may be swarming or
passively motile antherozoids, or tubular PoUinodia ; and fertilisation may be
effected by the entrance of 'the antherozoids, as in the case of oospores, or by a
kind of conjugation, the sexual cells coalescing by means of openings in the cell-
walls of both, or finally by simple apposition and probably diffusion of a fertilising
substance. The product of fertilisation is sometimes a single cell germinating
directly or through the medium of zoospores ; but more generally a multicellular
body results, from which spores are finally produced. An alternation of generations
may here also be recognised, rudimentary or more fully developed according as the
structure of the fructification is simpler or more complicated. In the simplest
cases the sporocarp appears only as an appendage of inconsiderable size to the
plant; in the other extreme the fructification is able to continue an independent
growth for a considerable time, and thus constitutes a second alternating genera-
tion. These phenomena will be described more in detail in the special description
of the Carposporese. One essential difference between sporocarps and oospores
consists in this, that in the production of the former certain cells also take part
which were not immediately concerned in the act of impregnation; and that, with
the exception of the simplest cases, the portion of the fructification which produces
the spores is surrounded by a sterile envelope which serves merely for protection
or also for further nourishment. Fig. 164 illustrates some of the most different
forms of sporocarps.

In ColeochcBte {A) the female organ or carpogonium (hitherto described as the
oogonium) consists of a single cell w which runs out upwards into a long narrow
canal opening at the apex. Fertilisation is effected by small roundish swarming
antherozoids m, and as a consequence the portion of the protoplasm (oosphere)
which occupies the basal part of the cell becomes invested with a firm cell-wall. So
far the phenomena are the same as in the formation of the oospores of Vaucheria
or (Edogonium^ the only important difference consisting in the long canal formed by
the cell-wall. A more essential deviation is now manifested in that the body which
previously had the appearance of an oospore grows considerably after fertilisation,
and in the fact that the effect of fertilisation shows itself also in the growth of the

INTRODUCTION.

^^31

cells which adjoin the female organ, so that this latter becomes surrounded by
an envelope h. A sporocarp is thus formed, the fertilised oosphere of which
produces out of its contents after a certain period of rest a mass of tissue, all
the cells of which produce zoospores, and each of these gives rise to a plant of

Fig. 164.— Various forms of carpogfonia, and of sporocarps resulting from them ; iv the female organ before fertilisation ;
ttt the male organ; y the entire sporocarp; h its envelope; cs the spores; A Coleochate; B Characea ; C Netnalion;
D Lejolisia ; E Podosphcera ; F Ascobolus (after various authorities).

ie same kind. The sporocarp of the Coleochaeteae combines the most essential
characters of an oospore with those of the sporocarp of the Florideae and of some
Fungi. As respects an alternation of generations, the oosphere surrounded by its
envelope, together with the tissue which subsequently fills it up and which produces

238 THALLOPHYTES.

the zoospores, must be regarded as the second generation, while the zoospores
may be compared homologically with the spores of Mosses.

In Nemalion (C) the carpogonium w consists also of a single cell which is wide
below and narrow and elongated above. This elongation, termed the Trichogyne^
is a closed tube; the male fertilising cells attach themselves to it, empty their
contents into it, and thus incite a further development of the basal part of the
female cell, which now increases in size, and divides into a number of cells which
grow out into densely crowded branches. A spore is formed at the end of each
of these branches; and the whole assemblage of spores together with its short
pedicel constitute the sporocarp, which in this case has no envelope.

In the true Floridese, of which Nemalion may be considered the simplest form,
the carpogonium w consists, even before impregnation, of a number of cells {D) ;
a lateral row of cells bears at its apex a closed hair-like prolongation, the tri-
chogyne, and is hence termed the Trichophore. The trichogyne receives the fer-
tilising substance from the male cells which become attached to it ; but neither the
trichogyne itself nor the trichophore is thereby excited to any further development,
the sporocarp resulting from the other cells of the carpogonium which lie beside the
trichophore. The fertilisation therefore takes effect at a distance from the spot to
which the male cell has attached itself. Certain cells of the carpogonium grow,
divide, and finally produce the stalked spores, the pericarp or envelope of the fruit
arising, as the result of branching beneath the carpogonium.

The sporocarp of the Characese {B\ which has hitherto been without any
analogy, becomes intelligible if we compare it on the one hand with that of the
Coleochaetese, on the other with that of the Florideae. The carpogonium w
consists of a large ovoid cell which is borne on certain small round basal cells
(Braun's ' Wendezellen '). These basal cells take no part in the development
brought about by fertilisation, their behaviour being similar to that of the
trichophore of the Florideae. The large cell is fertilised by filiform antherozoids,
and itself forms the single carpospore in the sporocarp, the envelope of which
has been completely developed before fertilisation ; and it behaves also in other
respects in a similar manner to that of the Coleochgeteae, Florideae, and Erysipheae.
That the large cell which becomes the carpospore does not possess any hair-like
receptive organ or trichogyne is a point of very subordinate importance, since in
the carpogonium of the Ascomycetes this organ is sometimes present, sometimes
absent \

One of the simplest cases of the formation of the fructification in the
Ascomycetes is afforded by Podosphcera {^); and we here get the transition to
an evident alternation of generations. The carpogonium 7v consists of a single
cell, and is fertilised by another tubular cell, the pollinodium. The result of fer-
tilisation is that the female cell grows, and divides into two cells, of which the
upper one forms in its interior several spores (Ascospores), and is hence termed
the Ascm. Beneath the pedicel- cell of the ascus shoot out filaments which form
the envelope of the fructification y.

The processes are somewhat more complicated in Ascoholiis, another Asco-

See De Bary, Btitrage zur Morphologic u. Physiologic cler Pilze, vol. III. p.

INTRODUCTION,

339

mycete, of which a diagrammatic section is given in F. w is the carpogonium
consisting of several cells, which is fertilised by the tubular branched pollinodium ;
the result is that a number of filaments shoot out from a central cell of the"
carpogonium, which then form sacs at the apices of their branches, and in these
a number of carpospores. The envelope of the sporocarp is in this case very
massive, and consists of cellular filaments which shoot out beneath the carpogonium ;
and it finally forms a compact pseudo-parenchyma in which the carpogonium is en-
closed together with the ascogenous filaments and the asci which proceed from them.
The mycelium which produces the carpogonia in both the Fungi now described is
inconsiderable in comparison to the large sporocarp which results from the fer-
tilisation of the carpogonium ; the sporocarp itself in many cases continues to grow
for some time independently of the mycelium, and therefore constitutes a second
(non-sexual) generation of these Fungi. If the mycelium were large and vigorous,
and the sporocarp which springs from it small, as is the case in the Florideae and
Characeae, the sporocarp would in these cases also have the appearance of being
only an appendage of the sexual generation, and an alternation of generations
would scarcely •be suggested, although this does actually occur, since the entire
course of development of such a Fungus can be divided into two sharply-defined
phases, of which the first is the mycelium with its sexual organs, while the second
is the sporocarp with its independent power of growth.

Besides the true spores produced directly or indirectly by the act of fertilisation,
which complete the course of development of the plant by a rudimentary or an
evident alternation of generations, there is commonly among Thallophytes an
extremely productive propagation by gonidia which are not brought into existence
either directly or indirectly by any act of fertilisation, and which have therefore
nothing to do with the alternation of generations.

The Gonidia^ often arise on the thallus by the whole of the contents of
certain cells of the thallus dividing, and thus producing one or more gonidia
which become detached from the plant. But in other cases special supports or
receptacles are formed on the thallus, the sole function of which is to produce
gonidia, either by the abstriction of the ends of special branches {Siylogonidm, as
in Ptpfocephalis, Pemcillium, &c.), or by free cell-formation in the interior of large
cells (Undogom'dia, as in the Saprolegnieae, Mucorini, and Vaucherid). In many
cases, especially among Fungi, propagation is effected almost exclusively by such
gonidia, the normal completion of the development by actual fertilisation being
attained only under specially favourable conditions. This is the reason why the
sexual organs of many Thallophytes are at present entirely unknown, while their
gonidia are perfectly familiar. It is therefore often a very difficult matter to
assert with respect to a Thallophyte, that it never produces sexual organs; since
even in the case of the common Mould-fungus {Penicilliuni) and in many Algae,
the gonidia of which have long been known, it is only quite recently that the sexual
organs and the alternate generation resulting from them have been discovered. Even
in the majority of the large marine Algae, the Phseosporeae, and the numberless

^ Among Fungi the small gonidia which become detached in great numbers are also calltd
Conidia, from the Greek Kovia^ dust.

240 THALLOPHYTES.

large Fungi (Basidiomycetes), the sexual organs arc still unknown ; although in
the latter case the analogy of the Ascomycetes renders it very probable that the
Fungi known as Hymenomycetes and Gasteromycetes are only the fructification
which is the result of the union of sexual organs on the mycelium. The spores
which are produced on these Fungi must therefore be treated as true spores in
our sense of the term, and their mode of formation as something quite different
from that of the gonidia of the Mould-fungi.

It is very common among Algae, and occurs also in some Fungi which grow
in water or on a moist substratum, for the gonidia when they escape from the mother-
cell to be naked, i.e. without any cell-wall, and motile; after their escape they
have for some minutes or even hours the power of swimming about, at the same
time rotating on their axis (swarming). The anterior end is hyahne, destitute of
granules or colouring matter ; and in some Algse a minute red dot lies at one side
behind the hyaline part j the cause of the motion is the vibration of certain very fine
threads, the Vibraiile Cilia. Usually two of these cilia are attached to the hyaline
anterior end, or one in front, the other at the side ; but sometimes there is only
one, while in others the hyaline anterior end is encircled by a *^ense circlet of
numerous cilia ; or finally the entire surface of the zoogonidium is covered with
short cilia. During swarming a cell-wall of cellulose begins to be secreted; the
zoogonidium then comes to rest, attaches itself to some solid body by its anterior
end, the cilia disappear, and germination commences, the end which was posterior
during swarming becoming the growing point and hence the anterior end of the
young plant. It has already been mentioned that in some cases swarming cells
conjugate, and these must then of course be regarded, not as gonidia, but as sexual
organs which bear only a deceptive resemblance to zoogonidia; at any rate
there are reasons for believing that the motile cells of some Algae which have
hitherto been regarded simply as gonidia, are capable of conjugation and are
therefore sexual organs.

Motile cells of the kind now described may make their appearance at any stage
in the course of development ; it is not uncommon, as we have seen, for the entire
contents of an oospore or even of a carpospore (as in ColeochcBie) to be transformed
into motile cells which can then germinate ; even in the so-called conidia of the
Peronosporeae the whole of the contents may break up into motile cells. In other
cases again these bodies are produced in special branches of the thallus, and not
unfrequently any vegetative cell of the thallus may allow its whole contents to
escape in the form of motile cells. These motile cells have hitherto been all
known as Swarm-spores or Zoospores ; but, according to the definition of the term
Spore which we have now adopted, we must term the asexually produced motile
cells Zoogonidia^ and designate the receptacles in which they sometimes arise in
large numbers, not zoosporangia, but Zoogonidia-receptacles. It is moreover
obviously of secondary importance whether the gonidia simply become detached, as
in most Fungi, or whether they take the form of motile cells. The difference is
evidently dependent on the mode of life of the plant ; the presence or absence of
the power of swarming is not one of morphological, but only of physiological
importance ; just as, in the seeds and fruits of Phanerogams, some have a power
of transportation by means of a special floating-apparatus, while others simply fall

INTR OD UC TION. 24 £

off. In the genus Vaucheria we find all stages of transition from motile cells to
gonidia which simply fall off. •

The Classification of Thallophytes has been till quite recently based essentially
on characteristics relating merely to the mode of Hfe, according to which they have
been divided into three classes, — Algae, Fungi, and Lichens; the Characese have
been sometimes included under Algae, sometimes altogether separated from Thallo-
phytes. But since more accurate investigations have enabled us not only to elucidate
the morphological significance of the growth of these plants, but also to discover the
sexual organs in the main divisions, and in many cases to follow the whole course of
development, this classification can no longer be maintained, since it depends
essentially only on differences in the external appearance and mode of life, while on
the other hand it is seen that a totally different classification is necessitated by their
morphological characteristics. The admirable labours of Schwendener, for example,
have shown that Lichens, hitherto considered as a separate class, must not only be
included among Fungi, but must be regarded as a section of a particular order, the
Ascomycetes. Since we have become more accurately acquainted with the sexual
organs of the Coleochoeteae and FlorideoD, it can scarcely be doubted that these Algae
have a close affinity to Characeae in the structure of their fructification. We can
therefore now distinguish only two classes of Thallophytes, — Algae and Fungi. But
it has long been admitted that it is impossible to draw any satisfactory boundary-line
between these two classes ; several writers have indeed frequently pointed out that
some families of Fungi must be closely associated with certain families of Algae. It has
been recognised more and more clearly that these .two classes are separated only
by a single distinguishing character; if the two old-established groups are to be
retained, the only distinction between the two (and the one adopted in the earlier
editions of this book) is to place under the head of Algae all those Thallophytes
which contain chlorophyll, under the head of Fungi all those which do not. But
this separation is altogether artificial, and could only be tolerated so long as want of
an accurate knowledge of the morphology of these plants compelled us to admit
a classification having no foundation in morphology. In the present state of our
knowledge, in which at least the morphological foundation for a scientific classi-
fication of Algae and Fungi may be laid down, it is not only permissible but
incumbent, in the interest of progress, at least to attempt a morphological classi-
fication of Thallophytes.

The first point to note is that the presence or absence of chlorophyll can be
no sufficient reason for separating plants which are nearly related to one another
morphologically, and which agree in their structure, their sexual organs, and their
alternation of generations. In Phanerogams this principle is thoroughly admitted.
If all Flowering-plants which do not contain chlorophyll were formed into one
class in contradistinction to those which do contain it, the Rafflesiaceae, Balanopho-
raceae, Corallorhiza, Cuscuta, Orobanche^ Monotropa^ &c. would have, in spite of
the differences in their organic structure, to be combined into one class, and
removed from their true relationship. No one however disputes that Cuscuta
belongs to the Convolvulaceae, Orobanche- to the Labiatiflorae, Monotropa to the
Pyrolaceae, and Corallorhiza to the Orchideae. These affinities are inferred, among
Phanerogams, chiefly from the structure of the flowers and the embryo, and no one

242 THALLOPHVTES.

attaches the least importance to the fact that the want of chlorophyll and the pe-^
culiar mode of life of these plants gives them so different an appearance from
that of their nearest allies. It is one of the most beautiful results of a truly
scientific morphology and classification that, among Phanerogams, the remarkable
habit of parasites and saprophytes is regarded as an altogether secondary matter.
But the same principle should also be applied in determining the systematic
relationships of Thallophytes ; — habit and mode of life, the presence or absence
of chlorophyll should also be treated as characters of altogether subordinate
importance. All Thallophytes which are destitute qf chlorophyll — z'. ^. all those
which have hitherto been termed Fungi — must necessarily agree with one another
more or less in their habit and mode of life, because they are all adapted to absorb
organic carbonaceous nutriment from their environments. If they obtain it from
living bodies, we have parasitism developed in its various forms ; if they have
the capacity of consuming dead organic remains, the habit and mode of life of
the plant must vary accordingly. Algae, in the sense in which the term has hitherto
been used, are able themselves to produce carbonaceous food-materials out of
carbon dioxide by assimilation; they are not therefore usually either parasites or sa-
prophytes, but can maintain an independent life; they are howe\er compelled, by
the peculiarities of their organisation, to live in water or in damp places. Their
dependence on assimilation requires that Algae should inhabit localities where there
is free access of light, while Fungi are not absolutely dependent on light for
their supply of food.

But all these facts are of altogether secondary importance in determining
degrees of affinity in the compilation of a natural system of classification of
Thallophytes. This object can be attained only by a comparison of such mor-
phological characteristics as a thorough knowledge of development reveals. The
determining considerations of a morphological nature are in Thallophytes, still
more than in other groups of plants, dependent on the question whether they
possess sexual organs, and, when this is the case, how these are formed, how the
act of fertilisation is effected, and especially what is the nature of that structure
which results either directly or indirectly from it, in one word, how the act of
fertilisation affects the entire course of development.

We have already described the more important forms of the organs of
fertilisation in Thallophytes, and the origin of an alternation of generations con-
sequent upon it. If now the plants which agree in these characters are compared
with one another, it is seen that the remaining morphological facts also suggest
a close affinity. The structural peculiarities connected with sexuality may there-
fore be regarded as the guiding characteristics, by which we are directed to
relationships within the group. With our present still very imperfect knowledge of
Thallophytes it is however not surprising if, in a classification founded on these
principles, forms are nevertheless occasionally found placed near each other which
appear to have but little affinity. This is unavoidable, because the intermediate
transitional forms are unknown; and it must moreover be observed that in Thal-
lophytes of a simple structure the morphological characteristics are more easily
concealed by physiological adaptations and by changes in habit than in the higher
plants.

INTRODUCTION. , . 343

The classification here adopted does not therefore make any pretension to^
be one which will endure for all time ; it rather claims to be in accordance with ,
the present state of our knowledge, and to bring into proximity those forms which
agree in the most important features of their development. In a certain sense
this classification may still be called an artificial one ; but it is natural in so far as
it attempts to bring into prominence actual affinities and not merely differences and
resemblances of external habits

It is, as every one knows, easier to make objections to a system than to lay
down clear principles on which one should be established; and we will therefore
only add a few explanatory remarks respecting the following table. In the present
state of our knowledge, we seem compelled, first of all, to establish a class of
Thallophytes in which not only is sexual reproduction unknown, but in which
there is no near affinity to any sexual forms ; this class will include only the
simplest and most minute of all plants, and is therefore formed into the first
class under the name Protophytes. But in a large number of Thallophytes in
which we are unacquainted with the sexual organs, there is an obvious close affinity
with well-known forms, with which therefore they may be associated in classification.
Finally, there are other Thallophytes in which the sexual organs are still unknown,
but in which no distinct affinity is exhibited with the Protophytes or with other
well-known forms. These plants are altogether omitted from our classification,
since it is not my purpose to present an index of all existing forms, but only to
show the affinities of those that are best known 2.

Each of the four classes here proposed starts with very simple forms, and
attains, through diverging lines, very diff"erent degrees of development. The, closest
affinities are therefore found by comparing the simplest primary members of each
class, especially those of the 2nd, 3rd, and 4th classes ; and the widest diff"erences
by comparing the most perfect forms of the diff"erent classes. In this respect
therefore these proposed classes resemble the recognised divisions in the groups
of Muscinese, Vascular Cryptogams, and Phanerogams.

In order not to depart too widely from the classification still current, and
to facilitate a general view, I shall, as will be seen from the following table, treat
separately the forms which contain chlorophyll (so-called Algae) from those destitute
of chlorophyll (so-called Fungi) within each class ^

' Cohn, who was the first to give up the division of Thallophytes into Algte and Fungi, has
not been, in my opinion, so happy in the classes which he has proposed. He does not start
from any definite principle, but employs as his typical characters sometimes points of great,
sometimes those of secondary morphological importance, as is shown by the names of the classes : —
Schizosporeoe, Zygosporese, Basidiosporeoe, Ascosporese, Tetrasporese, Zoosporese, and Oosporeoe
(see Hedwigia, 1872, p. 18).

2 [Reference for some criticism and additional details on this classification of the Thallophytes
may be made to Quart. Journ. Micr. Sc. 1875, pp. 295-326, and pp. 396-401 : see also de Bary, Bot.
Zeitg. 1881. For a Classification of Fungi, see de Bary, Beitr. IV. 1881.]

^ Since this classification and the following account of Thallophytes has been ready for the
press, 1 have had the opportunity of seeing a letter addressed to Dr. Brefeld by Prof. Fischer
(Oct. 29th, 1873), in which the following classification is proposed : —

THALLOPHYTES.

Myxomycetes. Fungi. Algae.

R 2

244

THALLOPHYTES.

THALLO'PHYTES.
Class I. Protophyta.

Containing ChlorophylL
Cyanophyceae.
Palmellaceae (in part).

Not containing Chlorophyll.
Schizomycetes.
Saccharomycetes.

Class II. Zygosporese.

Conjugating cells motile.

Myxomycetes.

Conjugating cells stationary.
Conjugat3e (including Diatomaceae). Zygomycetes.

Pandorinese.
(Hydrodictyeae).

Class III. Oosporege.

Sphaeroplea.
Vaucheria .
Volvocineae.
(Edogonieae.
Fucoideae.

Coleochaeteae.
Florideae.
Char ace ae.

{Coeloblastce).

( Saprolegnieae.
( Peronosporeae.

Class IV, Carposporeae.

Ascomycetes (including Lichens).
JEcidiomycetes (Uredineae).
Basidiomycetes.

Saccharorayces.

Zygomycetes.

Class I. No sexual reproduction.

Phycochromacese,

Class II. Reproduction by conjugation.

Diatomaceoe, Conjugatoe.

Class III. Beproduction by oospores, the result of fertilisation.

PeronosporeEe. Palmellacese, Siphoneoe.

Saprolegnieae.

Confervacese, Fucacece, Coleochoeteae,
Characete (?).

Class TV. A compound fructification resulting from fertilisation
(alternation of generations).

Ascomycetes. Floridese.

Basidiomycetes.

Prof. Fischer still treats Algoe and Fungi as two entirely distinct series developed in parallel
rows ; while I suppose that in each class Fungi have diverged as ramifications from various types of
Algae : there are other important differences between our systems in the position of the Myxomy-
cetes, Coleochgeteas, and Characeae. But, with the exception of these differences, the main point is
the agreement of our views with respect to the establishment of four classes which serve equally for
Fungi as for Algae.

PROTOPHYTA, ^45

CLASS I.

PROTOPHYTA.

In this class are comprised the most simple and minute of all plants, whether
they contain chlorophyll, and therefore have been regarded as belonging to the
Algae, or whether they contain no chlorophyll, as the Yeast-fungi and the so-called
Schizomycetes (Bacteria, etc.).

i Those Protophytes which contain chlorophyll live chiefly in water, or at least
1 damp localities, sometimes as pseudo-parasites, and their green colouring matter is
ften mixed with a blue one which is soluble in water. Those which are
estitute of chlorophyll are either true parasites or inhabit the moist surfaces of
rganic bodies, or are found in fluids which contain organic substances in solution
■cm which they derive their nutriment, and which they decompose, causing
putrefaction or fermentation.

The structure of Protophytes is always extremely simple, and in the
simplest of all the cells are so small that they can be seen only under a high
magnifying power. In the smallest a distinction can scarcely be detected be-
tween cell-wall and cell-contents; and when this can be done the contents are
homogeneous, or minutely granular. The cell-wall has a tendency to deliquesce
into a soft jelly in which the cells remain imbedded either regularly or irregularly;
but sometimes it only swells up, and is then manifestly stratified.

In the simplest forms the cells are isolated ; the two halves of a divided mother-
cell increase till they attain its size, and then again divide; the derivative cells
separate, and carry on an independent life. In the more highly developed forms
the derivative cells remain united, and the final result is either simple and often
extremely slender rows of cells, thin lamellae, cell-division taking place in one plane,
or agglomerations in consequence of cell-division taking place in all directions,
is only in the most perfectly developed individual that the multicellular body
,s a determinate external form.

The species which contain chlorophyll are in general larger than those which
not, and the structure of their cells more perfect ; the largest and most perfectly
veloped among the latter being Yeast-cells. Even in this, the lowest stage in the
igetable kingdom, the want of chlorophyll is seen to be usually associated with
degradation of structure. All the cells in the individual are usually exactly
ke; it is only in the higher forms that a few larger cells of a diff'erent colour
;ermed Heterocysts — are intercalated among the otherwise similar cells of a
,ment.

In most cases there is no distinct base and apex, and therefore no definite
ection of growth; but in a few of the highest members of the class a base

246 THALLOPHYTES.

and apex of growth can be distinguished, and a kind of branching makes its
appearance.

Although zoogonidia, in the sense in which the term is used in the higher
Thallophytes, do not occur (with the exception of some Palmellaceae which perhaps
do not belong to this class), many Protophytes are nevertheless endowed with a
power of motion by means of which they swim about ; spirally-wound multicellular
filaments turn on their axis; or the filaments themselves bend backwards and
forwards; or some other kind of motion occurs.

No sexual organs have yet been observed, and in most cases there are no
non-sexual organs of reproduction, the multiplication of individuals being effected
by the separation of the ordinary vegetative cells \ In other words, organs for
nutrition and reproduction are not differentiated; it is only in the most highly
developed forms that cells of peculiar form are produced for the sole purpose of
reproduction.

The class has hitherto been divided into three groups distinguished by their
colour; viz. i. those containing pure chlorophyll, Palmellaceae ; 2. those in which
the chlorophyll is mixed with a blue pigment, and which therefore appear of a li^ht
green or bluish green colour, Cyanophyceae ; and 3. those in which there is no
chlorophyll, Schizomycetes and Yeast. While limiting the class of Protophyta
to these three groups, it is nevertheless possible that some of the forms included
in it are not independent species, but merely stages in the development of other
higher Thallophytes, which have a perpetual power of reproducing themselves.
Thus it has already been determined^ that the genus PleurococcuSy hitherto placed
among Palmellacese, is merely a stage of development of Chlamydomonas which
belongs to Pandorineae, a class of Zygosporese; and it is not improbable that
the whole group of Palmellaceae, and perhaps also some Chroococcaceae, are
of the same nature, and must at some time be eliminated from the class of
Protophyta.

FORMS CONTAINING CHLOROPHYLL.

A. CYANOPHYCEiE. These organisms are of a bluish, emerald, or brownish green,
or some similar colour, due to a mixture of true chlorophyll and phycocyanin ; this
pigment becomes diffused out of dead or ruptured cells, and thus produces the blue
stain on the paper on which Oscillatorieae are dried. From crushed specimens
treated with cold water phycocyanin is extracted as a beautiful blue solution,
blood-red in reflected light ^. When the crushed plants are treated with strong alcohol
after the extraction of the blue pigment, a green solution is obtained which contains
true chlorophyll, and probably a special yellow pigment, phycoxanthin*.

I. The Chroococcace89 exist as isolated roundish cells or in roundish families, the
cells of which are imbedded either in an amorphous mucilage or in the swollen walls
of their mother-cells. They occur as gelatinous growths in damp places. Several
genera are distinguished, with numerous species : — e.g. Chroococcus and GlcEocapsa

* [Gonidia have been discovered in Glceocapsa by Bornet (Ann. sci. nat., sen V. XVII; in
Nostoc by Janczewski (ib. XIX), and in Bacillus by Cohn (Beit, zur Biol. d. Pflzn. I).]

^ Cienkowski, Bot. Zeit. 1865, no. 3 ; and Rostafinski, Bot. Zeit. 1871, p. 786.

3 Cohn, in Schulze's Archiv fiir mikrosk. Anatomic, vol. III. p. 12. — Askenasy, Bot. Zeit. 1S67.

* Millaidet and Kraus, Comptes Rendus, vol. LXVI. p. 505.

PROTOPHVTA.

247

Fig. 165.— Mode of cell-division ;
GlceoMpsa,

(Fig. 165), which divide in all directions, the latter imbedded in a stratified jelly;
GlcEothece, imbedded in a stratified jelly but dividing in one direction only; and
Merismopedia, the cells of which divide cross-wise in a single plane.

2. The NostoeaeeaB form lumps of mucilage or gelatinous pellicles which float
in water or lie on damp earth or among moss. In the jelly are serpentine moniliform
rows of roundish cells, a few larger cells, termed Hetero-

cyjts, the contents of which are of a different colour, being
interposed at intervals. The filaments increase in length
by the division of the individual cells, thus constantly
adding to the coils which lie in the jelly that they ex-
crete. New colonies are, according to Thuret^, formed
in the following manner : — The jelly of the old colony
becomes softened by water, the portions of the threads
lying between the heterocysts become detached, separate
from the jelly, and straighten themselves, while the hetero-
cysts -themselves remain in the jelly. After they have
entered the water, the old portions of the threads become
endowed with motion like the Oscillatorieae, and their exit
is probably caused by this movement''^. The roundish
cells of the filaments grow transversely, i.e. at right

angles to the axis of the filament, and then divide, the division-planes being parallel
to the axis of the filament, which now consists of a series of short articulate threads,
the axis of whose growth is at right angles to its own. The threads which are thus
formed increase in length and join, placing their terminal cells in contact [alternately
above and below in each successive thread], and thus unite into a single curved
A^oj^or-filament. Individual cells, apparently without any definite law, become hetero-
cysts. In the meantime the gelatinous envelope is developed, and the new colony,
which is at first microscopic, attains the size of a walnut^.

3. The OscillatorieaB consist of rigid cylindrical filaments of various thickness,
often extremely slender, divided into disc-like cells by very delicate transverse septa.
The filaments are not straight, but somewhat coiled in the form of a very oblique
spiral ; they revolve on their axis, and become matted, when large numbers grow
together (in water or on moist earth), into balls or pellicles. When a lump is placed
in water or on wet paper, Nageli has shown that it assumes a star-like arrangement
in consequence of these movements.

4. The Rivularieae * form soft greenish blue lumps of jelly which swim about in
stagnant water or grow attached ; in the first case they are spherical, in the second
hemispherical, the smallest about ^ millimetre in diameter, the largest the size of a
hazel-nut.

^ Thuret, Observations sur la reproduction de quelques Nostochinees, Mem. de la soc, imp.
des sci, nat. de Cherbourg, vol. V. 1857. [Ann. and Mag. Nat. Hist. 1858, vol. II.]

^ The^e motile threads of Nostoc were seen by Janczewski to enter the young stomata on the
lower side of the thallus of Anthoceros IcBvis, where they further develope into round balls. Such
colonies of Nostoc have been known for a long time in cavities and in the tissues of different Hepa-
licse {Blasia, Pellia, Diplolcena, Aneura, Riccia), but have generally been considered endogenous
gemmx of these species, until Janczewski proved their true nature. Nosioc also estabhshes itself in
the large porous cells of the leaves of Sphagnum. The entrance of Nostoc into the parenchyma
of the stem of a dicotyledonous plant, Gunnera, is brought about, according to Reinke, in a different
manner ; the deeper-lying parenchymatous cells of the outer part of the stem, themselves covered by
layers of parenchyma, are densely filled v/ith colonies of the Alga. (Bot. Zeitg. 1872, pp. 59
and 74.) [See also Ann. des Sc. Nat. 1872, p. 306, and Quart. Journ. Micr. Sc. 1873, p. 369.]

^ [Archer has described the occurrence of 'spores' in Nostoc paludosum which were always
placed singly between the heterocysts. Quart. Journ, Micr. Sc. 1872, p. 367.]

* De Bary, Flora, 1863, P* 653.

348 THALLOPHYTES.

jelly arranged radially; at the free end the filament runs out into a long hyaline
hair, while at the central end is a large heterocyst which gives the whole filament the
form of a riding-whip. The filament increases in length by the transverse division of
its cells. Reproduction is effected by the cell which lies next to a basal heterocyst
becoming thicker, increasing considerably in length and assuming a cylindrical form ;
its contents become denser, and invested by a firm membrane. When the whole
of the rest of the colony perishes these Resting-spores only remain. They subse-
quently germinate, the contents dividing into from 4 to 12 shorter cylindrical pieces,
each of which again divides repeatedly, until more than 100 cells are formed which
become rounded off and the filament moniliform. During this lengthening the mem-
brane of the germinating cell ruptures, the upper end of the filament projects, the
lower portion subsequently creeps out of the sheath, and the terminal cells become
pointed. The filament, now free, breaks up into several pieces, which become
closely packed together into a tuft or ball. Each filament now lengthens at one
end into a segmented hair, while the cell at the other end becomes a heterocyst.
Such a tuft which springs from a germinating cell forms a young mass of Ri'vularia,
the filaments becoming enveloped in jelly. The multiplication of the filaments of
a growing mass takes place by a kind of branching ; /. e. one of the lower cells becomes
a new heterocyst; the portion of the filament that lies between it and the old
heterocyst developes into an independent filament alongside the parent-filament.

5. The SeytonemesB form branched filaments enclosed in thick gelatinous enve-
lopes, which — at all events in their older portions — consist also of several rows of cells.
To this family belong Scytonema, Sirosiphon^ &c.^

B. The PALMELLACEiE contain pure chlorophyll. The cells live singly or remain
in families imbedded in mucilage ; they resemble the Gyanophycese in many ways. Thus
Gloeocystis, belonging to this family, has the appearance of a pure-green Glmocapsa;
Tetraspora forms Nostoc-VikQ lumps of jelly, but is propagated by zoogonidia; in
Occardium the filaments are arranged radially in jelly, as in Ri'vularia. The delicate
green growths on damp walls, stems of trees, &c. consist of cells either isolated or
grouped into families which are known under the names Protococcus, Palmella, Cysto-
coccus, &c. ; Palmella cruenta forming blood-red incrustations. Probably, as has already
been said, all these forms are only stages in the development of higher Algae, which
attain their further normal development only under favourable conditions of growth.

FORMS NOT CONTAINING GLOROPHYLL.

G. The ScHizoMYCETES^ (Fig. 166) live in fluids which contain organic substances
(albuminoids) liable to putrefaction, from which they obtain their nutriment, and of the
putrefaction of which they are the cause. The greater number consist of extremely
small cells without any differentiation into cell-wall and cell-contents, so that in some
cases their organic nature can only be determined by indirect methods^. Where
they occur, an enormous number of individuals are usually imbedded in a gelatinous

^ Various forms of the Algse described under A. and B. occur again in the bodies described as
gonidia of Lichens. (See Fig. 222.)

* Cohn, Untersuchungen iiber Eacterien, Beitrage zur Biologic, 1872, Hft, 2. p. 127 ; 1876, Bd.
ii. Hft. 2, [Quart. Journ. Micr. Sc. 1873, p. 156; 1877, p. 81 ; 1879, pp. 356-404.]

^ Since the smaller Schizomycetes, usually called Bacteria, are found also on the slimy surface
of living bodies, on wounds, &c., they have recently, from a medical point of view, been regarded as
the cause of diseases. A copious literature, generally deficient in even an elementary acquaintance
with scientific botany, treats of the Schizomycetes from this point of view ; but it can hardly be
doubted that observers have frequently mistaken the mere products of decomposition of organic
substances, and every crystalline precipitates of an inorganic character, for Bacteria.

PROTOPHFTA.

249

Fig. 166. — Schizoraycetes : i Sarcina ; 2 Bacterium ;
3 Vibrio; ^Spirillum (after Colin).

mucilage, and this is especially true of the most minute forms, the investigation of
which is hence rendered extremely difficult.

The forms and vital phenomena of the Schizomycetes, as far as they are accurately
known, recall various species of Chroococcaceae and Oscillatorieae ; but they are in
general much smaller than the corresponding
forms belonging to the chlorophyllaceous
series. Thus, for example, Sarcina (Fig.
166, i), which grows in the human stomach,
corresponds to Merismopedia, the small cells
dividing cross-wise and remaining for a time
united into tetrads. In the remaining
Schizomycetes, which are commonly known
as Bacteria, growth takes place only in the
direction of length, and the cells formed
by repeated transverse division either sepa-
rate or remain united into filaments. Gohn
divides them into four groups: — i. Sphaaro-
bacteria, with extremely small roundish
cells which become detached, corresponding
to the most minute forms of the Chroococ-
caceae and Palmellaceae. They grow on the surface of moist dead organic bodies
forming gelatinous growths, often of an intense yellow-green, blue, or violet colour;
these pigments which are contained in the protoplasm are sometimes soluble, sometimes
insoluble in water. 2. Bacteria proper, in which the cells, when separate, are long,
rod-like, very minute, and able to swim about in the fluid. Fluids that contain
albuminoids putrefy and become milk-white from the multiplication of these ordinary
Bacteria; they correspond in form to the genus Synechococcus among Chroococcaceae
which forms bluish-green coatings on rocks ^. 3. Filobacteria, in which the slender
cells remain united into threads, which are either straight, forming the genus Bacillus of
Cohn, or curved and bent, when they are Vibrio (Fig. 166, 3). They do not excrete
a gelatinous envelope and resemble small Oscillatorieae. If the Schizomycetes are
constituted into a distinct group, then Beggiatoa, with contractile filaments, which has
been hitherto referred to the Oscillatorieae, must be included in it. 4. Spirobacteria,
^"which form spirally curved filaments sometimes of considerable size in comparison to the
preceding ones. Cohn distinguishes the genera Spii^illum (Fig. 166, 4) and Spirochcete,
which recall Spirulina among the Oscillatorieae.

D. The Saccharomycetes, of which the genus Saccharomyces is the only one that
[is accurately known, consist of small round cells which live isolated, and resemble in
form some Chroococcaceae and Palmellaceae ; their organisation is nevertheless capable
[of a more accurate investigation than that of the Schizomycetes, which they also usually
jreatly exceed in size. The genus Saccharomyces^ which causes the alcoholic fermentation
in saccharine fluids, consists of separate cells of an ellipsoidal form with smooth and thin
[walls, the protoplasm in which can be clearly recognised as such and encloses one or
jmore vacuoles. When growing in a solution capable of fermentation these cells
[multiply very rapidly; not however by the ordinary mode of division, but by budding
and abstriction. At some point or other of the yeast-cell a small protuberance makes
its appearance, which increases to the size of the mother-cell ; the very narrow point of
[union then gives way, and the two cells then carry on an independent life, and again
repeat the process. Whether under some circumstances the cells can also grow out

* [E. R. Lankester (Quart. Journ. Micr. Sc. 1873, p. 408) believes, from the investigation of a
ich-colouied Bacterium, that the series of forms distinguished by Cohn cannot be maintained as
listinct. Lister (ibid. 1873, p. 393) believes that he has demonstrated the origin of Bacteria from
a Fungus, a species of Dematiutn.']

♦250 THA LL OPHVTES.

into tubes and assume hypha-likc forms, as asserted by Cienkowski ', appears to be
uncertain. Reess^, however, has discovered that when yeast-cells are grown on the
surface of cut pieces of potato, turnips, &c., they attain a larger size, and the pro-
toplasm contained in them breaks up into from two to four roundish endogenous
gonidia, which, when placed in a saccharine fluid, at once again produce yeast-cells
by budding and abstriction. Reess considers these endogonidia of Saccharomyces to be
ascospores, and the yeast-fungus therefore to be an Ascomycetc. Brefeld, however,
makes the forcible objection to this view, — that if a yeast-cell which forms gonidia is
considered an ascus, and the gonidia ascospores, it must be shown that the supposed
ascus is developed from an ascogonium, i.e. from a female sexual organ, as in the
Ascomycetes. Such an origin has, however, never been proved, and is extremely
improbable.

The view advocated by Pasteur ^, and since his time very popular, but never enter-
tained by me, that the yeast-fungus can live in fluids which do not contain any oxygen
diffused through them, and that they obtain the oxygen necessary for their respiration
by the decomposition of chemical compounds, and especially by that of sugar into alcohol,
carbon dioxide, and other products, has been shown to be altogether without foundation
by the recent researches of Brefeld * carried out in the botanical institute of Wiirz-
burg. Yeast-cells, like all other vegetable cells, require for their growth oxygen
either free or diffused through the fluid ^. The Fungi of fermentation afford no
exception to this general law, and are only distinguished by the fact that they are
able to make use of even the most minyte quantities of oxygen diffused in the fluid.
If the saccharine fluid is altogether free from diffused oxygen and from other
nutrient substances, fermentation still takes place, but the yeast-cells do not grow, but
pass into a dormant condition, perishing after a time; dead yeast-cells do not cause
fermentation. It follows from this that the decomposition of the sugar into alcohol and
carbon dioxide is caused by the living yeast-fungus, but has nothing to do with its
respiration, growth, or nutrition ^\

CLASS II.
ZYGOSPORES.

According to the principles laid down in the Introduction to the group, this
class comprises plants which have hitherto been placed among Algae on the one
hand and among Fungi on the other hand,— all those, in fact, in which sexual

^ [Bull. Acad, imp. St. Petersb. 1872, vol. XVII ; Quart. Joum. Micr. Sc. 1875, pp. 145-149.]

2 Reess, Botanische Untersuchungen liber des Alkoholgahrugspilze, Leipzig 1870. [Quart. Joum.
Micr. Sc. 1875, PP- 142, 143.]

2 [Comptes lendus, 1872, pp. 784-790; Quart. Journ. Micr. Sc. 1873, p. 351. — See also Mayer,
Lebrbuch der Gahrungs Chemie, 1876; Schiitzenberger, Les Fermentations, 1876.]

* Vortrag, July 26th, 1873, in the Physik.-medic. Gesellschaft of Wiirzburg.

5 The nature and the necessity of respiration in plants was first pointed out by me in my
Handbuch der Experimental-physiologic, pp. 263-264; (see infra. Book III. Chap. 2. Sect. 6).

^ On other Fermentation-fungi see Van Tieghem, Ann. Scientif. de I'ecole normale, vol. I. 1S64,
and Ann. des Sci. Nat. 5® ser. vol. VIII. 1868, as well as the French translation of this work,
p. 352.

ZYGOSPORES. 251

reproduction takes place by means of conjugation, the essential characteristics
of this process being that the two cells which take part in it are alike, and
produce, by the coalescence of their protoplasmic contents, a cell of peculiar
form, the Zygospore, which usually remains for a time dormant, and is then termed
a resting-spore. Conjugation is the simplest form of sexual reproduction, and
the morphological characters of the plants belonging to this class are, as might
be expected, much simpler than of those which constitute the succeeding classes.
The mere fact of sexuality does nevertheless show an advance on the mode
of reproduction of the Protophyta, and the Zygosporeae manifest in consequence a
higher degree of organisation, and present the transition from the non-sexually
propagated Protophytes to those forms of Thallophytes in which reproduction is
sexual in the strict sense of the term.

Like that of the vegetative organs, the form of the organs of conjugation
varies greatly in the different sections of this class; the zygospore is sometimes
produced by the conjugation of naked zoogonidia, sometimes of highly developed
cells belonging to the thallus, sometimes of special branches which do not occur
elsewhere in the thallus. One of the most remarkable phenomena connected with
sexuality is the formation of Auxospores in the Diatomaceae, which, as far as our
present knowledge goes, takes place in some cases by actual conjugation, but in
others, as Schmitz has shown, by the simple approximation of two cells without
any coalescence or actual contact, an interchange of substance taking place pro-
bably by diffusion.

The plants comprised in this class differ greatly in the structure of their
vegetative body; and we are at present acquainted with but few intermediate
transitional forms connecting the various sections belonging to it. This evi-
dently arises from the fact that it is only of late years that the process Of
conjugation, previously known only in the Conjugatse, has been studied in the
Pandorineae, Zygomycetes, and other families. It may be expected that the further
investigation of the zoogonidia of a large number of Algse will show them to be
conjugating sexual organs; and it is even possible that among plants which are
very nearly allied some produce zoogonidia which actually conjugate, while in others
the corresponding cells do not usually conjugate, but proceed to a further develop-
ment, or, in other words, are propagated parthenogenetically. Some such phe-
nomenon is indicated in the formation of the auxospores of the Diatomaceae already
mentioned; and, on the other hand, we find forms, like the Hydrodictyese^ in which
no conjugation of the zoogonidia has hitherto been observed, although they are
nearly connected, in their morphological characters, with the Pandorineae in, which
this mode of reproduction does occur. I do not therefore hesitate in assigning the
Hydrodictyeae a place among the Zygosporeae. The question is more difficult
whether, in addition to the Zygomycetes, the Chytridineae and Myxomycetes should
also be included in this class. In the Chytridineae it is probable that some of
the zoogonidia conjugate, although this has not hitherto been observed^. In the

' [The conjugation of microzoogonidia was observed by Suppanetz in 1873, Rostafinski, Mem.
Sec. Sc. Nat. de Cherbourg, 1875, vol. XIX, p. 152.]

^ [Novakowski has discovered sexual reproduction in Polyphagus EnglencB. Cohn, Bcitr. 3. Biol.
1876, Bd. II. See also infra, p. 264.]

252 THALLOPHYTES.

Myxomycetes a conjugation of zoogonidia does take place if the term is used
in its most extended sense ; for there is absolutely no reason why the coalescence
of the Myxoamoeba^i should not be regarded as a form of conjugation, and the
production of plasmodia as analogous to that of zygospores. What has hitherto
prevented botanists from recognising this analogy is merely the habit of life of
the Myxomycetes, especially the peculiarity of not forming cells, and the cir-
cumstance that conjugation takes place between thousands of zoogonidia. If the
coalescence of the myxoamoebae into a plasmodium is compared to the conjugation
of the zoogonidia of the Pandorineae, then the fructification which results from the
Plasmodium must be regarded as a large zygospore, the contents of which break up
into a number of spores, like the large resting-spore of Synchitrium. They have at
first, it is true, no power of motion, but this is quite a secondary consideration if
regard is had to the corresponding processes in the Pandorinese.

With reference to the structure of the thallus, all the plants included in this
class may be termed unicellular, the thallus consisting of a single independent cell,
an eremoblast ; or, when it is multicellular, the separate cells are nevertheless essen-
tially similar, so that their union is scarcely necessary physiologically for the existence
of the whole. Such unions of equivalent cells, the product of a single mother-
cell, sometimes take place only after the individual cells have passed through
a kind of swarming phase ; when they come to rest, a number of cells unite
to form the so-called Ccenobium, and continue their development as a body with
definite form. This formation of coenobia, as it occurs in Hydrodiciyon and Pedi-
antrum, exhibits a certain analogy, not only with the process of conjugation generally,
but with the formation of plasmodia in the Myxomycetes in particular, except that
a Plasmodium, if regarded as a ccenobium consisting of masses of naked protoplasm,
no longer manifests any independence of the separate parts.

The formation of a tissue in the ordinary sense of the term occurs only in a few
Algse, as Ulothrix and some Conjugatae, in so far that the divisions take place in
one direction only, and the cells thus produced remain more or less firmly united,
forming filaments in which all the cells are still perfectly equivalent, each one of them
representing therefore the entire plant. The predominant unicellular character is
manifested also in individual cells being capable of a high development, especially
in the Conjugatae, Diatomaceae, and Zygomycetes. This tendency is manifested
in the former in the contents of the cells and in the peculiar sculpture of the
cell-wall, in the Zygomycetes in the extremely complicated branching, while the
cell-contents remain simple.

The greater number of forms belonging to the class, especially those which
contain chlorophyll, the Pandorineae, Hydrodictyeae, Desmidieae, and Diatomaceae,
exhibit in their thallus no distinction between base and apex; they form either
spherical or tabular coenobia or rows of cells, all of which multiply by division
without any distinction between base and apex; but in some Diatomaceae and at
all events in the germination of the zygospores of the Mesocarpeae and Zygnemeae
adhesion takes place to a substratum, and hence a kind of contrast arises between

^ See Brefeld, Ueber Dictyostelium mucoroides in Abhandl. des Senkenb. Gesellsch., vol. VII.
1869, p. 20.

ZYGOSPORES. 2^^

base and apex. This occurs in a much higher degree in the Zygomycetes, and
even in the Myxomycetes, which, growing on a substratum, send up their gonidio-
phores and sporocarps into the air.

A non-sexual reproduction takes place universally, but in different ways. In
the Pandorineae, Desmidiese, and Diatomaceae every cell-division may be regarded
as a vegetative multiplication, since every single cell constitutes an individual; but
in the Hydrodictyeae and in Ulothrix peculiar zoogonidia are formed, different
from the ordinary vegetative cells. Propagation by gonidia takes place however
in the most perfect form in the Zygomycetes, where, before the formation of the
zygospores, receptacles are produced on long stalks which develope endogonidia
in vesicular swellings, the so-called sporangia, or conidia (stylogonidia) on branched
stalks. In many Zygomycetes the much-branched cellular filament which forms
the mycelium may break up under unfavourable vital conditions into a number of
spherical cells or gonidia, each of which may subsequently reproduce the plant.

Many Zygosporeae recall the Protophyta in this respect, that they have also con-
ditions, during their true vegetative period, in which they have the power of motion ;
this occurs to an especially high degree in the Pandorineae and Myxomycetes, much
less so in some Conjugates and Diatomaceae.

FORMS CONTAINING CHLOROPHYLL.

A. Conjugation takes place between motile cells [Zoosporece).

I. The Pandorine^^ consist of cells which are either isolated or united into
coenobia by gelatinous envelopes; the coenobia are either spherical, as in Stephano-
\.sph(Bra, ellipsoid, as in Pandorina, or are square plates, as in Gonium. In these states,
[although surrounded by a cell- wall, they have still a power of motion, each cell pos-
[sessing two long cilia which protrude through the cell-wall. The isolated cells of
fChlamydomonas and Chlamydococcus swim about in this manner like ordinary zoogonidia;
[in the coenobia, on the contrary, the cilia of all the individual cells project through the
[common envelope, and, by their united action, set the whole ccenobium in a twisting
[and rolling motion. The course of life of these plants may be illustrated in two
[iexamples.

The genus Chlamydomonas consists of isolated zoogonidia which multiply in the

regetative state by hi- or quadri-partition. But in the sexual reproduction the zoogonidia
[divide into eight motile daughter-cells each, provided with four cilia, differing from
[one another in size but smaller than the mother-cells. According to Rostafinski these

:ells conjugate in precisely the same way as Pringsheim has described in the case of

'^andorina {'vide infra) ; the zygospores thus formed come to rest and continue to grow
for some weeks ; if then dried and again placed in water they divide repeatedly, and
Form resting motionless families of cells identical with the genus Pleurococcus formerly

)laced under Palmellaceae.

^ Cohn, Ueber Chlamydococcus und Chlamydomonas, Berichte der schles. Ges. 1856. [Ray Soc,
Bot. and Physiol. Mem. 1852.] — Cohn und Wichura, Ueber Stephanosphcera pluvialis. Nova Acta
Acad, nat. curios, vol. XXVI. p. i. [Quart. Journ. Micr. Sc. 1858, p. 131; Archer, ib., 1865,
pp. 1 16-185.] — Pringsheim, Ueber Paarung der Schwarmsporen, Monatsber. der Berliner Akad., Oct.
1869. [Ann. des Sc. Nat. 1869, vol. XII.]— De Bary, Bot. Zeitg. 1858, p. 73.— Rostafinski, Bot. Zeit.
1871, p. 787.

254

TIIALLOPHYTES.

In Pandorina Morum (Fig. 167) the complete course of development was followed
by Pringsheim, this being the first instance that had been observed of the conjugation of
zoogonidia. Pandorina is one of the commonest of the Pandorineae. The sixteen cells
of a ccEnobium (Fig. 167, /) are closely crowded together, and surrounded by a thin
gelatinous envelope out of which the long cilia protrude. The non-sexual multipli-
cation results from each of the sixteen cells breaking up into sixteen smaller cells ;
the sixteen daughter-families (77) become free by the absorption of the gelatinous
envelope of the mother-family ; each daughter-family, again surrounded by a gelatinoug
envelope, grows to the original size of the mother-family. The sexual reproduction

Fig. 167.— Development o{ PandortHa Morum (after Pringsheim) ; / a swarming family; //one
divided into sixteen daugliter-families ; /// a sexual family, the separate cells emerging from the
gelatinous envelope ; /y, V conjugation of the zoogonidia ; VI a zygospore just formed ; yil a
mature zygospore; VIII transformation of the contents of a zygospore into a large zoospore;
IXixee zoospore ; ^ young family produced from the last.

is brought about in exactly the same way, but the gelatinous envelopes of the young
families become softened, and the separate cells are thus freed and each swims about by
itself (777) ; these free zoogonidia are of very variable size, rounded and green at the
posterior end, pointed, hyaline, and furnished with a red corpuscle in front, where they
bear the two cilia. Among the crowd of these zoogonidia may be seen some which
approach in pairs as if they were seeking one another. When they meet, their points
come in contact, and they coalesce into a body at first hour-glass-shaped (7^), but
gradually contracting into a ball (T); in this ball the two corpuscles and the four
cilia at the enlarged hyaline spot are still to be seen for a time ; but these all soon

ZYGOSPORES.

^55

disappear. Some minutes after the commencement of conjugation the resulting
zygospore is a spherical cell {VI), which remains at rest for some time enclosed in
its cell-wall, its green colour passing over into a brick-red. If the dried-up zygospores,
which have now greatly increased in size, are placed in water, germination begins
after twenty-four hours ; the outer shell of the cell-wall breaks up, an inner membrane
protrudes and now contains one, two, or three large zoospores which finally escape
{Fill, IX), surround themselves, after a short period of swarming, with a gelatinous
envelope, and break up by successive divisions into sixteen primordial cells which
now again form a ccenobium like Fig. i.

A further illustration of the course of development in the Pandorineae is furnished
hy StephanosphoBra plwvialis, one of the rarest and most beautiful of the family (Fig. i68),
occurring rarely in the rain-water which collects in the hollow of large stones.
The process of vegetative reproduction is the same as in Pandorina; but, according
to Cohn and Wichura, the succession of generations of this kind is interrupted by
the cells belonging to a family dividing repeatedly (Fig. i68, XII) into zoogonidia
which ultimately become free {XIII), and probably produce resting zygospores by

Fig. i68.— Successive stages in the reproduction oi Stepkanosphara plumalis (after Cohn and Wichura).

conjugation after the manner of Pandorina. It is stated by the observers above-named
ithat stationary immotile balls (/) accumulate at the bottom of the water which, as they
[grow, assume a red colour. After these resting-cells, which are probably zygospores,
[have lain for some time dry and then again been moistened, they germinate, the contents
)reaking up into from 4 to 8 zoospores {II— V) which invest themselves with a cell-
rail, and each gives rise, in a single day by successive division {VII— IX), to an
[eight-cell coenobium {X, XI), which again in the next night gives birth to eight
finotile families \

2. The nearest allies to the Pandorineae are probably the Hydrodictye^, as is
[shown by their formation of coenobia, and still more by the whole course of their de-
[velopment, which Pringsheim^ first described in detail. Although the conjugation of
the zoogonidia has in this case also not been actually observed, they offer a striking

* [Archer has described, /. c, pp. 7, 8, a remarkable amoeboid phase which the primordial cells
\Siephanosphcera undergo.]

* Pringsheim, Mon. der konigl. Akad. der Wiss. zu Berlin. Dec. 13, i860. [Ann, des Sc.
It. i860, vol. XIV; Quart. Journ. MIcr. Sc. 1S62, pp. 54, 104. See also A. Braun, Verjungung,

46 ; Ray Soc. Bot. and Physiol. Mem. 1853, pp. 137, 190, see ante, p. 251.]

256

THALLOPHYTES.

resemblance in their mode of origin and subsequent development to those of the
Pandorineae. When the ccenobia multiply, a large number of zoogonidia are formed
in each mother-cell, within which they move about for some time ; when they come
to rest they congregate in some definite arrangement, and then continue their develop-
ment unitedly, eventually again multiplying in the same way, as for instance in Pediastrum
(Fig. 169). In Hydrodictyon utriculatum, which occurs occasionally in ditches, the mature

Fig. Tb^.— Pediastrum gratiidatttm (after A. Braun, x 400). A a plate consisting of
united cells ; at ^ the innermost layer of a cell-wall is protriKling- ; it encloses tlie
daughter-cells resulting from division of the green protoplasm ; at t are varions states of
division of the cells; sp the fissures in the already empty cell-walls ; B the inner lamella
of the mother-cell-wall which has entirely escaped (greatly enlarged) ; b contains the
zoogonidia g, these are in active ' creeping ' motion ; C the same family of cells
4^ hours after its birth, 4 hours after the zoogonidia have come to rest ; these have
arranged themselves into a plate, which is already beginning to develope into one
similar to A.

plant or coenobium consists of a sac-like net several centimetres long, which is composed
of a great number of cylindrical cells united at their ends so as to form a four- or six-
cornered mesh. The ordinary mode of reproduction consists in the green contents
of one of the cells of the net breaking up into from 7,000 to 20,000 zoogonidia which
move about with a trembling motion within the wall of the mother-cell, come to rest in
the course of half-an-hour, and then arrange themselves in such a way that, by their
elongation, they again form a net of the original kind which is set free by the absorption
of the wall of the mother-cell, and attains, in the course of three or four weeks, the size
of the mother-plant. In other cells of the mature net the green contents break up
into from 30,000 to 100,000 microzoogonidia which at once leave the mother-cell and
swarm about for some hours. The hypothesis has not yet however been confirmed
that conjugation then takes place between these zoogonidia^ ; but when they come to
rest they are spherical, invested with a firm cell-wall, and may retain their vitality for
months when dried up if protected from light. After remaining several months at rest
these resting-spores begin to grow slowly, and after they have attained a considerable
size their contents break up into two or four large zoospores which come to rest
after a few minutes, and assume a peculiar angular form when they have reached a
considerable size, putting out horn-like appendages. In each of these so-called poly-
hedra the green parietal protoplasm again breaks up into zoogonidia which move about
for 20 or 40 minutes within a sac which protrudes out of the polyhedron. When come
to rest they arrange themselves into a sac-like net consisting of from 200 to 300
cells, but in other respects resembling one of the ordinary ones. In some of the
polyhedra smaller and more numerous zoogonidia are formed, but these also unite
into a net.

[See note supra, y>. 251,]

ZYGOSPORES.

'^Sl

3. Ulothrichace^. Differing in many respects from the Pandorineae and Hydrodic-'
tyeae is the genus Ulothrix, an Alga consisting of segmented filaments composed of cells
which are all alike. It is mentioned in this connection only because it is characterised
by the conjugation of equivalent zoogonidia. Cramer states^ that the contents of
some of the individual cells of a filament break up into two, four, or eight zoogonidia
which immediately germinate and produce new filaments. Other of the cells, on the
other hand, give birth to 16 or 32 microzoogonidia which, after escaping, conjugate
exactly like those of Pandorina. Nothing is known of the development of the zygo-
spores. It is questionable whether Ulothrix is not more nearly related to Sphceroplea
among the Oosporeae, and whether the conjugation of the zoogonidia is not to be
regarded as a simpler case of the formation of oospores which occurs in the latter 2.

B. Conjugation takes place betiveen stationary cells.

(a) The CoNjUGATiE ^ consist of cells with a limited power of growth, which
lultiply to an unlimited extent by bipartition ; the cells thus produced either live
mtirely independently, or remain united in filaments. The chlorophyll-bodies form
jither parietal bands, axile plates, or radiate bodies arranged in pairs. Conjugation
takes place between ordinary vegetative cells, the contents coalescing in a variety of
'ways, the resulting zygospore becoming invested with a cell-wall, germinating after
a period of repose, and presenting essential differences in form from that of the
vegetative cells. There are no distinct gonidia, the ordinary vegetative cells per-
forming the functions of reproductive organs. De Bary divides the Conjugatae into
three families : —

(i) The MesocarpeeB consist of cylindrical segmented filaments with an axile plate
of chlorophyll ; filaments which lie parallel to one another put out conjugative processes,
or two cells of contiguous filaments come into contact by knee-like projections ; the parts
of the walls which are in contact become absorbed and a broad canal is formed in
which the protoplasm of the two conjugating cells collects; the canal then becomes
shut off by two or four transverse septa and constitutes the zygospore. This mode of
production of zygospores is clearly analogous to similar processes in the Zygomycetes.
On germination the zygospore produces at once a segmented filament, the end that
remains in the spore forming its base and the exposed end its apex. This contrast is
not, however, permanent ; all the cells subsequently multiply by transverse division. To
this family belong the genera Mesocarpus, Craterospermum, and Staurospermum.

(2) The ZygneraeaB consist also of cylindrical segmented filaments with the
chlorophyll arranged in straight or spiral parietal bands or in stars placed in pairs.
Conjugation takes place between two parallel filaments; the individual cells put
out opposite conjugative protuberances (see Figs. 5 and 6, p. 10) which eventually
touch one another, when the absorption of the cell-walls at the point of contact
forms a narrow canal. Since a number of cells of two filaments usually conjugate
at the same time, the whole forms a ladder-like structure in which the rungs are
represented by the canals. After the formation of the conjugating canal the proto-

^ Cramer, Naturfor, Gesellsch. in Zurich, March 21st, 1870.

^ [A. Dodel (Jahrb. fiir wiss. Bot. 1876, vol, X. Heft 4) has described in detail the germination
of the zygospores of Ulothrix. The whole course of development presents a striking analogy to
that of Hydrodictyon. Dodel has, like Areschoug, observed occasional conjugation between the
microzoogonidia ; if these do not conjugate, they then propagate themselves non-sexually like the
macrozoogonidia.]

^ De Bary, Untersuchungen iiber die Familie der Conjugaten, 1858. [See also Hassall, Hist.
Brit. Freshwater Algoe, 1845; Wittrock on Mesocarpese ; Quart. Journ. Micr. Sc. 1873, p. 123.
Besides the modes of conjugation described in the text, contiguous cells of the &ame filament conju-
gate by lateral processes both in MesocarpecB and Zygnemece.]

S

258

THALLOPHYTES.

plasmic bodies of the two cells contract, one of them passes through the canal into the
other, and the two coalesce into a round zygospore invested with a thick cell-wall
consisting of several layers which lies within the much larger mother-cell, and which
germinates only after a period of rest (Fig. 170). In the Zygnemeae there is also at first
a contrast in the young plant between base and apex, which afterwards disappears, all
the cells behaving exactly alike during growth. Among the genera are Zygnema,
Spirogyray Mougeotia, Sirogonium, and Zygogonium.

(3) The DesmidiesB ^ consist of cells which live isolated or less often in rows which
easily break up and are imbedded in mucilage. The cells are either cylindrical or
fusiform, sometimes furnished with horn-like appendages {Scenedesmus) ; in other cases
they have a circular or elliptical outline, and are divided by a deep constriction into two
symmetrical halves. Even where there is no such constriction, the chlorophyll-body is
divided symmetrically in the interior of the cell, or the symmetry is indicated by the
so-called chlorophyll-vesicles [amylum-bodies] ^ and the distribution of the starch-grains.
In accordance with this symmetrical structure the vegetative propagation of the cells
(individuals) is effected by the formation of a septum in the plane of symmetry or
within the constriction which splits into two lamellae and divides the cell into two halves.
Growth at the point of separation then produces a segment which restores the symmetry.
The mode of formation of the zygospores is similar to that in the Zygnemeae ; but in the

Fig. 170. — Germination of Spiroffyra jtigalis (after Pringsheini, Flora, 1852, no. 30 ; / a resting zygospore ; // commence-
ment of its germination ; /// the young plant further developed from a zygospore, which had been enclosed in the cell C, the
conjugating canal c being still visible; e outer cell-wall of the spore; y yellowish brown layer of the cell-wall ; g the third and
innermost layer of the cell-wall of the spore, which forms the germinating filament ; w 71/ the first septa of the germinating
filament, the posterior end d growing into a narrow appendage.

simplest forms, as Cylindrocystis and Mesotceniurn, where the conjugating individuals are
of very simple form, conjugation appears to be nothing but a coalescence, altogether
similar to that of the zoogonidia of Pandorina. The zygospore either germinates
directly or divides into two or four daughter-cells, each of which repeats the vegetative
mode of reproduction already described. In the last-named case we may consider, as
in Pandorina, the zygospore to be a fructification which produces several spores in the
sense in which the term is used in the Muscineae; and we may here again have an
indication of an alternation of generations.

These processes may now be explained more in detail in the case of Cosmarium
Botrytis (Fig. 171), as described by De Bary [I.e.). The cells live isolated, and are
symmetrically bisected by a deep constriction (Fig. 171, X), and are also compressed

* [See also Ralfs, British Desmidieoe, 1848. — Archer in Pritchard's Infusoria.]
2 [Strasburger has shown (Zelltheilung iind Zellbildung, 2*® Aufl. p. 83) that the structure
existing in the chlorophyll-bodies of Zygnema and Spirogyra are true chlorophyll-granules in which
numerous starch-grains are placed concentrically so as to form a layer at the surface. These struc-
tures are identical with those mentioned above as chlorophyll-vesicles and amylum-bodies.]

ZYGOSPORES.

259

at right angles to the plane of constriction (/, a) ; in each half-cell are two amylum-
bodies and eight chlorophyll-plates, which curve and, running from two centres to the
wall, converge in pairs. The multiplication of the cells by division is brought about
by the narrowest part of the constriction elongating a little, when the thicker outer
layer of the cell-wall opens by a circular fissure ; the two halves of the cell hence appear
separated from one another and united by a short canal, the wall of which is a con-
tinuation of the inner layer of the walls of the half-cells. A septum soon appears in
the canal which unites them, by which the cell is divided into two daughter-cells, each
of which is a half of the mother-cell. The septum, at first simple, splits into two
lamellae, which immediately become convex towards one another {IX, h), and each
daughter-cell now possesses a small rounded outgrowth which grows gradually and
assumes the form of a half-cell, so that each daughter-cell now again consists of two
symmetrical halves {X). While the wall is undergoing this growth, the chlorophyll-
body of the old half-cell grows into the newly-formed half. The two amylum-bodies
in the old half-cells elongate, become constricted, and each divides into two; of
these four, two pass over into the new growing half-cell, and all again arrange them-

FlG. XTi.—Cosmarium Botrytis (after De Bary. I— III X 390, IF— A' X 190).

selves in the original symmetrical manner. Conjugation takes place between cells
lying in pairs in a crossed position enclosed in soft jelly (Fig. 171, /). Each of the two
cells puts out from its centre a conjugating protuberance (7, c) which meets the other ;
these protuberances are formed of a delicate membrane which is a continuation of the
inner layer of the cell, the firm outer layer of which is split (7, c). Each of the
two protuberances swells up into a hemispherical bladder, they come into contact,
the separating wall disappears, and the contents unite in the broad canal thus
formed; the protoplasm becomes loosened from the cell-wall, and contracts into a
spherical form. The united protoplasmic body appears as if surrounded by a delicate
gelatinous wall (77, /) by the side of which lie the empty cell-walls (77, e, b). The
zygospore now becomes rounded into a ball ; its wall forms, as it matures, three layers,
an outer and an inner colourless layer of cellulose, and a middle firmer brown layer.
This stratified cell-wall grows out at several points into spiny protuberances which
are at first hollow and afterwards solid, each of them again producing at its end a few
smaller teeth (777). The starch-grains of the conjugating cells become transformed into
oil in the zygospore. Germination commences by the protrusion of the colourless inner
layer through a wide split in the outer layer {IF), the thin-walled ball thus set free

26o THALLOPHVTES.

considerably exceeding the zygospore itself in size. In the contents of this sphere {F)
may be recognised two chlorophyll - masses surrounded by oily protoplasm, which
might have been distinguished even before their escape from the external layer of the
zygospore. The contents now contract and become surrounded by a new wall {V)
from which the older wall detaches itself as a delicate vesicle. After some time the
protoplasm becomes constricted by a circular furrow, and splits into two hemispheres,
each of which contains one of the two chlorophyll-masses {FI). Each hemisphere
remains for a time naked, and again constricts itself; but this time the constriction
does not advance to the centre; the hemisphere changes its form in other respects
also, and each assumes the form of a symmetrically divided Cosmarium-ceW {FII), which
surrounds itself with a wall of its own. The planes of constriction of the two cells
derived from the zygospore cut the dividing plane of the zygospore itself at right angles ;
they themselves also lie crossed in the mother-cell at right angles to one another. The
contents in each now arrange themselves in the manner above described ; the mother-
cell-wall is absorbed and the new cells separate from one another. All these processes
of germination are completed in one or two days. The new cells, whose outer wall is
smooth, now divide in the usual manner, but the newly-grown halves increase in size
and become rough on the outside {Fill — X) ; the two daughter-cells of each of the
two cells produced from the zygospore have dissimilar halves, and the four cells pro-
duced by their further bipartition are therefore of two different forms : two have their
halves equal and two unequal; the latter constantly produce by division one with
equal and one with unequal halves.

(b) The DiATOMACE^ ^ (Bacillarieae), a group extremely rich in species, follow natu-
rally after the Desmidieae ; in particular they are allied to the Gonjugatae by processes
of development which correspond to the conjugation of the latter, or at least bear
a certain resemblance to it ^. They bear a special resemblance to the Desmidieae in the
form of their cells, in the manner of division, and in the mode of completion of the
daughter-cells. Like the Desmidieae, the similar cells of the Diatomaceae may be united
into rows, or may live entirely isolated. The tendency of Diatoms to secrete a soft
jelly in which they live socially is found also in Desmids, although less strongly dis-
played. In the same manner the movements of Diatoms are not altogether dissimilar
to those of Desmids, and even the silicification of the cell-wall, which is very strong
in the former, is found, though to a smaller extent, in Closterium and other Desmids ;
and the fine sculpturing of the silicious shell also finds an analogue, although in a coarser
form, in the cell-wall of some Desmids. The Diatoms are the only Algae, except the
Gonjugatae, in which chlorophyll occurs in plates and bands, but in some forms it
is also found in granules, and the green colouring matter is concealed, as in the
chlorophyll-granules of the Fucaceae, by a buff-coloured substance, Diatomin or Phy-
coxanthin^ One of the most prominent peculiarities of Diatoms consists in their
silicified cell-wall being composed of two separated halves or valves of unequal age, of
which the older one partially envelopes the younger like the lid of a box. When
the cell begins to divide, the valves separate from one another, and after the division
of the contents into two daughter-cells, each of them forms a new layer at the plane
of division which is adjusted by its turned-in margin (the girdle) to the girdle of the

* Luders, Ueber Organisation, Theilung und Copulation der Diatomeen, Bot. Zeitg. 1862. —
Millardet and Kraus discuss their colouring-matter in Compt. rend. vol. LXVI. p. 505 — Askenasy
in Bot. Zeit. 1869,. p. 799. — Pfitzer, in Hanstein's Bot. Abhandl., Heft 11, 1871, gives the most im-
portant contribution to our knowledge of the Bacillarieae. [Quart. Journ. Micr. Sc, 1872, 1873.] —
Borscow, die Siiswasser-bacillariaceen Russlands, 1873.

^ [Thwaites first discovered the conjugation of the Diatomacese, Ann. Nat, Hist. 1847,
vol. XX; see also Carter, Ann. and Mag. Nat. Hist. 1856. — Schmitz, Quart. Journ. Micr. Sc,
1873, p. 14.5. — Smith, Synopsis of British Diatomacese]

3 [Pinnularia viridis and Synedia splendens are green. Navicula fusiformis var, ostrearia is cobalt
blue and communicates a green colour to the oysters which feed on it ; Nature, vol. XVI. p. 397.]

ZYGOSPORES. 261

old valve of the mother-cell; this latter extends, like the lid of a box, over the newly-
formed valve; and the two valves of the two daughter-cells lie next one another.
Since, according to Pfitzer, the silicious valves, which also contain some organic matter,
do not grow, it is clear that the new cells must constantly become smaller from
generation to generation. When they have thus attained a certain minimum size,
large cells, the Auxospores, are suddenly formed ; the contents of the small cells,
leaving the silicious valves which fall apart, increase either simply by growth or by
both growth and conjugation. After this the auxospores surround themselves with
new valves. Since the large auxospores are of somewhat different shape to their
smaller mother-cells and primary mother-cells, the first result of their division must
necessarily be cells of a different form and with unequal halves, as in the Desmidieae.
The formation of the auxospores has been more exactly followed out only in a few
cases. It would appear that they are formed in very different ways, from two or from
one mother-cell, singly or in pairs, and with or without conjugation ; they are alike
only in so far that their size greatly exceeds that of the mother-cell. It has already
been mentioned that, according to the recent researches of Schmitz in the case of
Cocconema Cistula, the auxospores are produced by two cells laying themselves parallel
side by side and allowing their contents to escape by the opening of the two valves ; the
cell-contents then commence growing rapidly without coming actually into contact,
and a pair of auxospores is thus formed. Diatoms are found in enormous numbers at
the bottom both of the sea and of fresh water, and attached to the submerged parts of
other plants. Besides the ordinary rotation of protoplasm in their interior, they also
exhibit a creeping motion by means of which they crawl over hard bodies or push small
granules along their surface. This occurs only in a line drawn along the length of
the cell- wall, in which Schultze^ supposes crevices or holes through which the pro-
toplasm protrudes; and this, although not yet actually seen, probably occasions the
creeping motion.

FORMS NOT CONTAINING CHLOROPHYLL.

A. Conjugation takes place between motile cells (myxoamoebse).

The Myxomycetes^ differ so greatly in external appearance from the rest of the
Thallophytes that they have been by some altogether separated by the vegetable
kingdom. But if attention be directed not so much to the peculiarities of their external
appearance as to those points in each stage of their development w hich are important from
a morphological point of view, and especially to the fact that the formation of a cell-wall
is a point only of secondary importance in the lower Thallophytes generally, it becomes
clear that the Myxomycetes are allied in all essential points to the Pandorineae ; and
even if their mode of life is so remarkably different from that of this group of plants, the
chief cause of this is that they do not, like them, live in water, but in the interstices of
moist substrata, in consequence of which their motion is necessarily of a very different
kind from that of the Pandorineae. In place of swimming and rotating, we have in the
Myxomycetes a creeping of amoeboid masses of protoplasm such as alone is practicable
on the substratum on which they grow. It is impossible, in the space we have here at
command, to go into these relationships more in detail ; it must suffice to trace the
main points in the history of their development, and to point out, in passing, the most
important analogies.

' [See Pop. Sci. Rev. 1866, p. 395— Engelmann, Bot. Zeit. 1S79, p. 49.]

^ De Bary, Die Mycetozoen, 1864. [Ann. Nat. Hist, i860, vol. V. p. 233.] — Cienkowski in
Jahrb. fiir wiss. Bot. vol. III. pp. 325, 400. — Brefeld, Ueber Dictyoslelium mucoroides, 1869, Abhandl.
der Senkenberg. Gesellsch., vol. VII.— Rostafinsky, Versuch eines Systems der Mycetozoen, 1873.—
Brefeld, Ueber Mucor racemostis uiid Hefe, Flora 1873 (latter part of the article).

262

THALLOPHVTES.

The Myxomycetes generally first make themselves visible at the time when they
emerge from their porous substratum, and form their comparatively large fructification.
The largest of these are the sulphur-yellow bodies which appear in summer on tan-
heaps, and which are known under the name of ' Flowers of Tan ' {jEthalium septicum) ;
those of Lycogala, which are found on the stumps of trees, are of the size and form of
hazel-nuts. In most other Myxomycetes the fructification is a small stalked capsule,
containing, like those already mentioned, an enormous number of minute roundish,
thick-walled spores. "When these capsules burst, other structures often make their
appearance, to which the term Capillitium has been given : — capillary tubes or threads,
often united together in a reticulate or lattice-like manner, to the origin of which we
shall recur presently (Fig. 172, C). In the simplest form at present known, Dictyostelium
mucoroides discovered by Brefeld, not only is the capillitium wanting, but also the outer
wall of the fructification, which consists only of a stalk composed of parenchymatous

FIG. \T2.—A Plasmodium of Dtdymium leucopus (after Cienkowski, x 350) ; B a fructification of Arcyria incarnaia still
closed; C after rupture of the wall/ and extrusion of the capillitium cp (after De Bary, x 20).

cells and of a roundish mass of spores. It is instructive to dwell on this simplest form
of the Myxomycetes in order to arrive at an understanding of their systematic
position. The spores of Dictyostelium will develope on the stage of the microscope
in an aqueous decoction of rabbit's dung, and produce ripe fructifications after some
days beneath the eye of the observer. When the spores germinate, the entire pro-
toplasm of each of them escapes from the ruptured wall, creeps about with an
amoeboid motion, absorbs nutriment, and grows. It cannot be doubted that these
amoeboid bodies must be regarded as zoogonidia, only differing from ordinary ones
in their mode of motion. After they have increased considerably in size in the course
of several days, they divide, and multiply repeatedly in this way, a process which may
unquestionably be compared directly with the vegetative propagation of Cklamydomonasy
and indirectly also with that of Pandorina. The nucleus of these bodies subsequently
disappears, their motion becomes more sluggish, and conjugation commences. They

ZFGOSPOREM.

363

creep about in troops, come into close contact with one another, and finally coalesce
into large lumps. As sodn as one of these lumps is formed, the rest collect from all
sides round it as a centre, coalesce with it, and increase in this manner the mass of
protoplasm which then becomes more and more rounded off. There is every reason
to believe that this collective union of zoogonidia is a conjugation and therefore a
sexual act, in the same sense as the conjugation of the zoogonidia of the Pando-
rineae ; and the large mass of protoplasm formed in this way, which is called a Plasmo-
dium, must therefore be treated as the analogue of the zygospore. The only difference
is that in this case the zygospore does not become invested with a cell-wall nor go
through a period of rest, but at once undergoes further development, becoming
transformed into a stalked fructification which produces a large number of spores.
In accordance with their mode of origin these spores may be compared not merely
with the zoospores developed from the zygospore of Pandorina, but also with the
ascopores of the Ascomycetes, and even with the spores of the Muscineae. The
formation of this fructification out of the roundish plasmodium of Dictyostelium com-
mences with the production in its centre by free cell-formation of a number of cells
each surrounded by a cell-wall of cellulose, which unite into a parenchymatous tissue
forming in the interior of the Plasmo-
dium a column or stalk standing erect
on the substratum. As this column con-
tinues to grow in height, the rest of the
protoplasm which surrounds it creeps
up it, and collects at its summit into
a round lump, the entire substance of
which now breaks up into a number of
spores. This example furnishes the
simplest case of the course of devel©p-
ment of a Myxomycete. In most other
instances it is much more complicated,
the development being more complete,
and a reduction to this plan becoming

constantly more difficult. But the first ^IG. i^^.-Physarum album {after Cienskowski). 1 spore ;

^ . 2 escape of its contents ; 3 the contents when free ; 4, 5 the same in

stage of the development is essentially the form of a zoogomdium provided with a ciUum; 6,7 after loss of

4.U^ ^„.^^ :„ «ll l\/T ^.«.,^^4.^„ TT^^U the cilium ; 9— ii coalescence of the amoeboid bodies ; 12 a small

the same in all Myxomycetes. Each piasmod.um.
spore gives birth to from one to eight

amoeboid bodies, which grow and multiply by repeated division, subsequently coalescing
with one another in large numbers and producing plasmodia. The plasmodia of other
Myxomycetes, however, do not at once produce fructifications, but maintain for a
longer period an independent life, creeping about in the moist interstices of their
substratum; as, for example, the yellow plasmodia inside a tan-heap, which at length
come to the surface, and then coalesce into the large bodies which are known as
* flowers of tan.' Other plasmodia creep about for a time on rotten wood or among
decaying leaves, at length in the same way reaching the surface, and then usually
producing simultaneously a number of fructifications. Fig. 172 ^ may give an idea of
the mode in which net-like structures are produced by these motions of the plasmodia.
The substance of the plasmodium is thin and granular in the interior and bounded on
the outside by a homogeneous pellicle ; it is constantly changing its form ; protuberances
arise at various spots which move onwards with a flowing and creeping motion, ramify,
and anastomose with one another, while the substance flows into them from behind,
and in this manner enables the entire structure to creep gradually forwards. Imme-
diately before the period when the fructification is produced, a tendency is manifested
to creep up erect bodies, so that finally the fructification is frequently found on
plants, stems, or leaves at a considerable distance from the original nutrient sub-
stratum. At this period the plasmodium collects at certain spots, and forms either

264 THALLOPHVTES.

a broad cake as in the * flowers of tan,' or weak ascending outgrowths which gradually
assume the form of the mature fructification, usually a stalked spherical or club-
shaped body, or a spiral tube ^, the growth of which is generally completed in a few
hours. It has already been mentioned that the ripe fructification is usually sur-
rounded by a firm wall, and that it often contains the so-called capillitium, in the
interstices of which lie the numerous spores. Neither the wall of the fructification
nor the capillitium is composed of cellulose, nor is the fruit-stalk, which is usually
hollow; it must rather be supposed that the substance of the Plasmodium, after it
has already assumed the external form of the fructification, becomes differentiated
into two distinct substances, one of which becomes hardened in various ways into
pellicles, tubes, and solid threads, and thus forms the stalk, the wall of the fructifica-
tion, and the capillitium, while the rest of the protoplasm, which has the capacity
of further development, breaks up into small round portions which become invested
with cell-walls and thus form the spores. The substance of the wall and of the
capillitium corresponds, therefore, as Brefeld has pointed out, to the mass capable
of swelling which fills up the space between the conidia in the conidiophore of the
Mucorini. In the process of free cell-formation also by which the ascospores of the"
Ascomycetes are produced, there often remains within the ascus a considerable portion
of its contents which is obviously not adapted to enter into the composition of a spore
capable of germination.

In the differentiation of the protoplasm of the Plasmodium into spores and into
those portions (capillitium and wall of the fructification) which take no part in the
further development, other portions of the contents which are useless for purposes of
reproduction also become eliminated, and especially lime, which is often excreted in
large quantities in the form of fine granules of calcium carbonate, and the yellow sub-
stance which coats in loose flakes the fructification of the ' flowers of tan.'

It must be added in conclusion that under favourable vital conditions both the
separate zoogonidia and the young or old plasmodia pass into a resting-state, the former
simply becoming invested by a cell-wall, the latter becoming transformed into masses
of cells'^.

B. Conjugation takes place bet'ween stationary cells.

In the Zygomycetes^ the vegetative body, which, as in all Thallophytes des-
titute of chlorophyll (Fungi), is termed the mycelium, consists of a ramified hollow
tubular cell (Fig. 174 B, m), in which septa only appear when it is fully mature and
ready for sexual or non-sexual propagation. The ramifications of this mycelium all
originate from a single germinating filament which is developed from a non-sexual
reproductive cell (conidium), and may, in the course of a few days, cover with a

^ The term ^thalium is given by Rostafinski to those large fructifications produced by the co-
alescence of several simple ones, and which are therefore syncarps, as in the case of ' flowers of tan.'

2 On the Chytridinese, which are perhaps allied to the Myxomycetes, see Braun, Abhandl. der
Berliner Akad. 1856, p. 22. — De Bary and Woronin, Berichte der naturf. Gesellsch. in Freiburg, 1863,
vol. III. Heft 2. — Woronin in Bot. Zeit. 1868, p. 81. [Sorokin (Bot. Zeit. 1874) has discovered in
Zygochytrium a process of conjugation resembling that of Mucorini, and in Tetrachytrium a con-
jugation of zoogonidia. Pfitzer (Monatsb. d. Acad, d, wiss., Berlin, 1872) includes Chytridtacece with
all the CoeloblastcB destitute of chlorophyll in one group, the Phycomycetes. Sorokin suggests the
name Siphomycetes for this, of which Amcehidium (Cienkowski, Bot. Zeit. 1861) is to be regarded as
the simplest form.]

3 Brefeld, Botanische Untersuchungen liber Schimmelpilze, Heft i, 1872. — Van Tieghem, Re-
cherches sur les Mucorinees, Ann. des Sc. Nat., ^^ ser., 1873, vol. XVII, and in the French translation
of this work, p. 336 et seq. [Quart. Journ, Micr. Soc. 1871, pp. 49-76. — Ann. des Sc. Nat.,
6® ser,, vol. I, 1875. p. i,]

ZVGOSPOREM.

26s

weft an area of several square centimetres. The mycelial filaments grow on organic
substrata, such as fruits, bread, glue, or even on saccharine fluids or dung, absorbing
their nutriment from them ; many species even grow parasitically on others nearly
allied, exhausting the contents of their cells through peculiar feeding organs, the Haus-
toria (see Fig. 175 h).

The non-sexual propagation of the mycelium may continue through an endless
number of generations, until the conditions are favourable for the formation of con-
jugating organs, i.e. for sexual reproduction. The conidia arise in two different
ways: — In the family of Mucorini stout branches grow from the mycelium erect

I

Fig. 174.—^ Mycelium three days old of Phyconiyces nitetts grown in a drop of
gelatine with a decoction of plums ; the finest ramifications are omitted ; g the conidio-
phore ; A a conidiophore of Mucor Mucedo in optical longitudinal section ; C a germi-
nating zygospore of Mucor Mucedo z ; the germinating filament /6 is here putting out a
lateral conidiophore^. D Free conjugating branches 6 b, the extremities of which a a
have not yet coalesced, but are separated by a septum ; the zygospore results from the
coalescence. of the cells a a. {A,C, D after Brefeld ; B from nature.)

into the air, reaching a height of several centimetres, and at length swelling up at
the summit into a sphere (Fig. 174 B, g). Within this sphere are formed a number of
round endogonidia, which become free by the rupture of the wall, germinate at once,
and reproduce the mycelium. In a second family, the Piptocephalidae, the conidia
are also produced on erect stalks, but these stalks branch copiously near their summit,
and form numerous conidia (stylogonidia) by abstriction at the extremity, which
produce a mycelium just as in the previous case. Besides these normal non-
sexual reproductive organs, the mycelium also frequently produces gonidia of

266

THALLOPHVTES.

a different kind, the filaments breaking up by the formation of septa into short
segments, which then become rounded off, and are able, under favourable conditions,
to produce new mycelium. This explains the production of the so-called iWwcor-yeast
(from Mucor racemosus'^), which bears a striking resemblance to true yeast {Saccharo-
myces) ; the mycelial gonidia can multiply by a yeast-like budding when placed in
an unsuitable nutrient fluid.

The mycelium has the power, under special circumstances, of reaching the mor-
phological completion of its development by the formation of sexual reproductive organs.
Thicker branchlets with a club-shaped extremity are produced on neighbouring filaments

Fig. \^<,.—P^ptocephalis Freseniana (after Brefeld). M a piece of the
mycelium of Mucor Mucedo from which the mycelium of Piptocephaiis tn m
derives its nutriment ; h the haustoria of Piptocephaiis which penetrate the
mycelial filaments of Mucor; c a conidiophore ; ss the two conjugating
branches of the mycelium which give rise to the zygospore Z.

of the mycehum ; the apices of these branchlets come into contact, and coalesce by the
disappearance of the cell-walls at the point where they touch (Fig. 174 D and 175 Z;
see also 162 B) after a septum has been formed in each filament on each side of
the point of conjugation. The protoplasm collects in the space which has been
thus marked off; and a zygospore of comparatively large size is produced by the
growth either of the entire space thus separated (Fig. 174 C) or of its central part
only (Fig. 175 Z) ; the thick outer wall of the zygospore being usually of a dark
colour and furnished with protuberances or spiny outgrowths. The zygospore ger-

^ Brefeld, Flora, 1873, no. 25.

OOSPORES. 267

minates only after a long period of rest, and does not then give rise at once to a
new mycelium of the ordinary kind; but the inner layer of the cell-wall bursts the
outer layer and protrudes through it in the form of a tube (Fig. 174 C), which
immediately developes into a conidiophore that breaks up into a number of conidia
each of which developes into a new mycelium. This behaviour is evidently analogous
to the germination of the zygospores of Pandorina, and to that of the oospores of
(Edogonium and Cystopus hereafter to be described, which do not however produce
conidiophores, but a number of zoospores which give birth to new individuals.
The zygospore may therefore be considered as a sporocarp, forming, together with
its conidiophores, a second generation, only that the carpospores are here precisely
like the ordinary non-sexual conidia.

In our present state of knowledge the. Zygomycetes may be divided into two
famiUes : —

1. The Mucorini, in which the conidia are formed, by free cell-formation, in
the interior of spherical receptacles. In the genus Mucor the conidia are set free by
the bursting of the fragile wall of the receptacle, while in Pilobolus this remains

lintact, but when ripe becomes detached at its base and, together with the conidia,
[is thrown to a distance by its elasticity. Mucor Mucedo is one of the commonest
[moulds, being found on fruits, bread, dung, and even in the interior of nuts and
[apples into which the mycelium penetrates. Mucor stolonifer covers in a short space
of time large pieces of the same substrata, the mycelium putting out long stolon-like
(branches which attach tjiemselves by their extremity, and produce conidiophores with
[black heads. The mycelium can even penetrate through the shell of fresh-laid eggs,
and form conidiophores within them. Phycomyces nitens (Fig. 174 B) is distinguished by
its conidiophores, which are ten or fifteen centimetres in height and of a violet colour.
Ihamnidium bears an ordinary large conidiophore at the summit of each of the long
stalks, and below these whorls of small branches with very small receptacles containing
only a few conidia. Pilobolus almost always makes its appearance when fresh horse-dung
is covered by a bell-glass^.

2. The Piptocephalidae bear a number of stylogonidia on their conidiophores which
are much branched towards the summit. The two genera described by Brefeld,
Chcetocladium and Piptocephalis, are parasitic upon Mucor Mucedo^ the latter being
represented in Fig. 175.

I

CLASS III.

OOSPORES.

To this class belong all those Thallophytes, whether containing chloro-
phyll or not, which are reproduced sexually by means of oogonia. An Oogo-
nium is a cell distinguished by its size and shape; the contents either form
a naked primordial cell — the Germ-cell or Oosphere — by simple contraction and
rounding off within the oogonium, which opens later, or divide into two
or more portions, which similarly become oospheres. The fertilisation of
these oospheres is effected by means of motile Antherozoids produced within

^ [On Pilobolus crystalUnus see Cohn, Nova Acta Acad, Nat. Curios, vol. XV. pt. I. p. 370. —
KJeiii in Pringshciin s Jahrb. flir wiss. Bot. vol. VIII.]

268 THALLOPHYTES.

Anlheridia, which penetrate into the oogonia and fertilise the oospheres; or a
kind of conjugation takes place between the antheridium and oogonium, as
Pringsheim has shown to be the case in the Saprolegnieae. With the exception
of the extremely simple Sphcei-oplea, the antheridial cell is always smaller than
the oogonium and different in form ; the motile antherozoids, which are pro-
duced either by simple contraction of the contents, or more commonly by division
into a great number of portions, are always much smaller than the oosphere,
and generally many hundred or thousand times. This great difference in size is
the essential difference between this mode of sexual union and conjugation, be-
sides the fact that the immotile oosphere passively awaits fertilisation by the
antherozoids which swim around it. After fertilisation the Oospore which results
from the oosphere behaves precisely as a zygospore; it becomes invested by a
firm cell-wall, and (except in Fucaceae) must undergo a period of rest before
germinating. Germination is in most cases indirect ; i. e. the contents of the
oospore do not at once develope into a new plant, but divide into a smaller
or larger number of cells which escape in the form of naked zoospores, each of
which grows into a new plant. In such cases therefore the oospore, as we have
repeatedly seen in the Zygosporeae, may be regarded as a very simple sporocarp
or as a second generation. The oosphere is the analogue of the oosphere in
the archegonium of Mosses; the ripe oospore, with its contents which break up
into zoospores, is the very simple equivalent of the moss-capsule, as has been
pointed out by Pringsheim, to whose researches we owe almost all that is here
described. But cases occur, as in the Zygosppreae, in which the oospore germi-
nates directly, as in the Fucacese, and there is no period of rest.

The vegetative body of the Oosporeae may consist of undifferentiated cells, as
in Sphceroplea, in this respect resembling the Conjugatae, the filiform thallus present-
ing no distinction of base and apex. In one large group (Coeloblastae) the thallus
consists, until the formation of fructification, of a single tubular cell, which often
branches copiously, as in the Zygomycetes; in a further grade of development
the thallus consists of branched and segmented filaments composed of cells of
different kinds, and the plant, which is fixed to a substratum, manifests a well--
marked contrast between base and apex. Finally we find the Fucaceae, in which
the thallus is very massive, and forms an actual tissue in which differentiation
may be recognised into epidermal layers and fundamental tissue.

There is no non-sexual multiplication by gonidia, either in the simplest form
belonging to the class, Sphceropka, or in the most highly developed, the Fucaceae.
The thallus of the other intermediate forms, on the contrary, produces abundance
of gonidia, by means of which propagation may take place through many genera-
tions, in the same manner as Marchantia is propagated by bulbils. The gonidia
are produced either singly or in numbers as endogonidia in the interior of cells,
escaping in the form of zoogonidia, or as stylogonidia by abstriction at the extremity
of special branches ; they are then immotile, as in the Peronosporeae, though their
contents may become transformed into motile zoogonidia.

In a- systematic classification of the Oosporeae, the forms which do not contain
chlorophyll may be arranged as a special section of those that do, the genetic
relationship being here unquestionable.

OOSPOREM. 269

A. Sphjeroplea atjnuHna^ is at present the sole representative of the most simply
organised of the Oosporeae. It consists, when fully developed, of cylindrical filaments
divided by septa into very long cells, the green protoplasm enclosing rows of large
vacuoles, and thus forming girdle-like rings. When in the vegetative state the cells
are all alike; it is only when the process of sexual reproduction is commencing that
a difference makes its appearance, some of the cells forming nothing but antherozoids,
the rest nothing but oospheres. The latter are produced in large numbers within
each cell, the contents breaking up, after various previous changes, into several globular
portions, each of which is characterised by a hyaline speck. The antherozoids are
formed in extraordinarily large numbers by the breaking up of the entire contents of a
cell which had previously assumed a yellowish brown colour. In both kinds of sexual
cells a number of holes are formed by the absorption of portions of the wall ; the
antherozoids escape through these orifices, forcing themselves in quantities into the cells
in which the oospheres lie. The antheridia and oogonia therefore resemble one another
in external form, while the antherozoids and oospheres are very unlike; the latter
are elongated and club-shaped, with two cilia at the p Dinted end. The oospore or
fertilised oosphere clothes itself with a thick warty cell-wall; its contents become
brick-red. Its further development commences after a resting stage; the red
contents divide by successive bipartition into a large number of primordial cells
which escape from the oospore, are provided with two cilia, swarm about, and
germinate after a period of rest. The germinating cell, which is at first shortly
fusiform, then grows in both directions into a capillary filament, so that the anterior
and posterior ends are exactly alike, and there is no distinction between base and apex.
After the filament has grown to a considerable size, transverse divisions take place and
the filament is then composed of a number of equivalent cells.

B.— CCELOBLASTiE.

In this section are included for the present all those Oosporese in which the thallus

consists of a single tubular cell, without a nucleus'^, more or less branching, and usually

ixed to one spot. It is only when reproduction is commencing that branches of special

form are separated by septa. The Goeloblastae are therefore not, accurately speaking,

inicellular ; it is only the vegetative body that is so. In all cases which have been accu-

frately observed a non-sexual reproduction takes place by means either of stylogonidia

[or of motile endogonidia. The oogonia and antheridia are usually terminal cells of short

[lateral branches, and are very different in form ; the former are more or less spherical,

the latter tubular ; the two being mostly placed close beside one another.

FORMS CONTAINING CHLOROPHYLL.

I, Vaueheria^ consists of a single elongated cell, branched in various ways, some-
times as much as 30 centimetres long, containing no nucleus, and developing on damp
sarth or in water. The fixed end is hyaline and branched in a wavy manner ; the free
)art contains within the thin cell-wall a layer of protoplasm rich in chlorophyll-granules
Fand drops of oil, and enclosing the large sap-cavity. This part of the thallus forms one
[or more main filaments which branch behind their growing point ; only in F. tuberosa is

^ Cohn, Ann. des Sci. Nat., 4* ser., 1856, p. 187.

'■' [According to Schmitz (Sitzber. d. niederrhein. Ges. in Bonn, 1879) ^ g^^^^ number of nuclei
"are present in Vaucheria and its allies.]

3 Pringsheim, Ueber Befruchtung und Keimung der Algen, 1855; Jahrbuch fiir wissen. Bot.
Bd. II. p. 470.— Schenk, Wurzburger Verhandl. Bd. VIII. p. 235.— Walz, Jahrbuch fur wissen.
Bot. Bd. V. p. 127.— Woronin, Bot. Zeit. 1869, nos. 9, 10. [Stahl, Bot. Zeit. 1879.]

270

THALLOPHYTES.

the branching dichotomoiis; though commencing monopodially, the lateral branches
often develope sympodially. At the commencement of the period of growth in the
spring the plants arise from the oospores which have remained dormant through the
winter, and are at first propagated through several generations in a purely non-sexual
manner. This is effected either simply by the abstriction of the ends of branches, or by
very large zoogonidia which escape from the interior of a cell, and are covered by very
short cilia. Between these two forms the various species of Vaucheria show intermediate
steps. In V. tuberosa, for example, branches swell up to a considerable size, become
detached at the base, and put out at once one or more germinating tubes. In
V. geminata the end of a branch swells up to an oval shape, its contents become
separated by a septum, contract, and form a new cell-wall, and the gonidium thus
formed either becomes free by the decomposition of the mother-cell-wall, or falls off
along with it, germinating after some days. The gonidia of F. hamata are formed in
the same way, but are thrown out with a jerk, remain at rest, and germinate
during the next night. In other species, as V. sessilis, sericea, and piloboloides, the

Fig. 176. — Vaucheria sessilis (X about 30).

contents of a branch become separated and contract, and force themselves out as
a zoogonidium through a crevice at the extremity. The motion of the zoogonidia lasts,
in V. sericea^ only for ^ to i^ minutes, but in other cases for hours. The rotation
begins, in V. sessilis, as I have distinctly seen, during their escape ; and if the opening
of the mother-cell is too small, the zoogonidium splits into two pieces ; each becomes
rounded off and again constitutes an entire zoogonidium; the outer one swims away,
while the inner one remains rotating within the mother-cell. The formation of the
zoogonidia begins in the night, as is the case with most Algae and Fungi; they escape in
the morning, and their germination commences during the day or the next night.
They put out on germination either only one or two tubes (Fig. 176, C, Z)), or form at
the same time a root-like organ of attachment (-B, F, wS). The oogonia and antheridia
originate as lateral protuberances from a filament which contains chlorophyll (Fig. 177,^,
5), sometimes even on the germinating tube of a zoogonidium. All the species of Vaucheria
are monoecious, and the two kinds of sexual organs are mostly found very near together.
The antheridia (Fig. 177, ha) are the terminal cells of slenderer branches, the contents

OOSPORES.

2,yi

of which contain but little chlorophyll but produce a large number of very small
antherozoids (Fig. 177, D) ; these escape from the antheridial cell which bursts at
the apex. In several species the, antheridia are curved like horns ; in others they are
straight {F. sericea) or curved sacs {V. pachyderma)] in the V. synandra^ Aiscoxered by
Woronin, from two to seven small horns arise on the large ovoid terminal cell of a two-
celled branch. The oogonia arise as thick protuberances (Fig. 177, A, B, og) filled
with oil and chlorophyll. They swell up into an obliquely ovoid form, and finally the
dense contents are separated by a septum (F^ osp). The green and coarsely granular
"mass collects in the centre of the oogonium, while colourless protoplasm accumulates
at its apex, at which spot it opens; at this moment the whole contents contract
and form the oosphere. In some species a colourless drop of mucilage is expelled
from the mouth. After the entrance of the antherozoids the oosphere clothes itself with
a thick cell-wall ; its contents become red or brown, and the oospore now commences
its period of rest. The formation of the oogonia and antheridia begins in the evening.

•\<rr

K

^^^^H Fig. fj-j. — Vaucheria sessilis ; A,B origin of the antheridium a on tlie branch b, and oogonium og; C an open oogonium

^^^^H expelling a drop of mucilage si ; D antherozoids ; E the antherozoids collected at the mouth of the oogonium ; /=", a an empty
^^^B antheridium ; osp the oospore in the oogonium (A, B, E, F from nature, C, D after Pringsheim).

^Hand is completed the next morning ; fertilisation is accomplished between ro and 4
^^Kin the day^.

^^m Closely allied with Vaucheria in the form of their thallus — which is usually a
^^Rmuch-branched filament undivided by septa — are a series of other genera formerly
^^■comprised in the class SiPHONEiE, the mode of sexual reproduction of which is how-
^^Hever at present unknown, if indeed they have any. It cannot therefore yet be affirmed
^^Bwith certainty whether the Siphoneae constitute a natural group. The mode in which
^^Hthe gonidia are formed is at least not opposed to this idea. The following forms may be
^•especially mentioned : — In Botrydium^ the young plant is a spherical cell, from which

^^B ^ (Stahl has described a peculiar encysted form of Vaucheria geminata : this has been previously
^oescribed by Kiitzing as a distinct plant under the name Gongrosira dichoioma. (Bot. Zeit., 1879.)]
^ Braun, Verjiingung in der Natur, p. 136 [Ray Soc, Rostafinski and Woronin have detected

(Bot. Zeit. 1877) the conjugation of zoogonidia in B. gratndatum. The former considers the Botry-

diacecE allied to PandorinecE and Hydrodiciyece.']

272 THALLOPHVTES.

a hyaline prolongation is subsequently formed which branches like a root and penetrates
the earth, while the upper part swells up into an ovoid vesicle, in which the proto-
plasm forms a parietal layer containing chlorophyll. From this arise, after growth
is completed, a number of zoogonidia which are set free by the wall of the mother-
cell becoming gelatinous and deliquescing. This is evidently a more simple mode of
growth than that of Vaucheria, A higher degree of branching is found in Bryopsis, which
is also unicellular. This genus also forms on one side root-like organs of attachment, on
the other upright much-branched stems (several centimetres in height) with unlimited
growth at the apex ; small branches with limited apical growth are formed on them in
two rows or spirally, which clothe the stem like leaves, and after they have become
shut off from it by septa, fall off; in them numerous zoogonidia are formed^. The
branching of a single large cell is carried still further in the genus Caulerpa, which
forms creeping stems growing at the apex with descending branched rhizoids and ascending
leaf-like branches 2. The growth of a unicellular thallus takes place in still another
manner in Aeetabularia. Here the plant, two or three centimetres high, has the form
of a slender Hymenomycetous fungus, the stem of which developes a rhizoid below and
bears a pileus above, consisting of a disc of closely crowded rays, which are themselves
radial branches of the stem. The stem ends above in the form of a boss ; at the base of
the radial branches surrounding the boss stands an umbellate whorl of branched seg-
mented hairs. In the rays of the pileus are formed the non-sexual zoogonidia ^. Finally
Udotea cyathiformis must be mentioned here. This species forms a stalked leaf-like
thallus, the stalk \ inch, and the thallus from \ inch to 2 inches long and broad, its
thickness from t^o to ^V I'ne. When cut transversely it seems to consist of a cellular
tissue, but in reality the thallus is composed of a great number of branched filaments,
which, forming a cortical and medullary layer, are all ramifications of a single cell ^.

FORMS NOT CONTAINING CHLOROPHYLL.

2. The SaprolegniesB ^ are colourless parasites usually found attached to animal
or vegetable organisms in water, especially dead insects, forming dense radiating tufts.
The individual plants are long undivided filaments, penetrating deeply into the substratum
by means of root-like branches, and branching more or less in the surrounding water,
sometimes in an arborescent manner. The zoogonidia are formed in the branches
after the contents have been separated by a septum ; occasionally a number of such
septa are formed, and they are then produced in each cell. The zoogonidia are
formed simply by the simultaneous division of the contents into a very large number
of portions (Fig. 178, A)\ the cell opens at the apex and the gonidia are expelled;
these at once swarm about in the water and become dispersed, or at first accumulate
in a resting state in front of the opening; each gonidium then becomes immediately
invested with a firm cell-wall, but after a short time it abandons it and then begins

^ Pringsheim, on Bryopsis, in the Monatsber, der Berlin. Akad. May 1871.

^ Nageli in Zeitschrift fvir wiss. Bot. 1844, Bd. I. p. 134.

2 [Woronin, Ann. des Sc. Nat. 4° ser., vol. XVI. p. 200. De Bary and Strasburger have
detected (Bot. Zeit. 1877) the conjugation of zoogonidia.]

* Nageli, Die neueren Algensysteme, p. 177, [The remarkable fossil plant from Canada of
Devonian age, Prototaxites Logani, v^^as probably an enormous Siphonaceous Alga ; see W. T.
Thiselton Dyer, Journ. of Bot. 1871, p. 252 ; Carruthers, Monthly Micros. Joum. 1872, p. 160.]

^ Pringsheim in Jahrb. fiir wiss. Bot. vol. I. p. 285 [Ann. d. Sc. Nat. 1859, torn. XI. p. 349] ;
vol. II. p. 205 ; vol. IX. p. 191. — De Bary, ditto, vol. II. p. 169. — Hildebrand, ditto, vol. VI. p. 249. —
Leitgeb, ditto, vol, VII. p. 257.— Cornu in Ann. de Sci. Nat., 5th sen, vol. XV. p. 328. — Schenk,
Bot. Zeit. 1859, p. 348. — Pfitzer in Monatsber. d. Berlin. Akad. May 1872. — [De Bary und Woronin,
Beit, zur Morphol. u. Physiol, d. Pilze, IV, 1881.]

I

OOSPORES,

^n

arming about (Fig. 178, B). It also sometimes occurs that the gonidia clothe them-
Ives with a delicate cell-wall while still within the mother-cell, forming a kind of
parenchyma, and then escape from the mother-cell through numerous holes in its wall.
These various modes of formation of the gonidia, by means of which species and even
genera have hitherto been distinguished, can, as Pringsheim has shown, take place
simultaneously on the same plant, as in Saprolegnia and Achlya. In Saprolegnia,
when the gonidia have escaped from the terminal cell of a branch, the septum
bulges out and developes into a new gonidial receptacle which takes the place of the
one that is now empty; in Achlya a lateral branch beneath the septum becomes the
new gonidial receptacle. In the very small and simple genus Pythkim, which is parasitic,

~IG. 178.— Two gonidial receptacles of
chlya; A one still closed; B with the

fonidia escaping; a gonidium just es-
?ed and still in a state of rest, c after'com-

icing to swarm, having abandoned their

•walls b.

Fig. 179. — Oogonia and antheridia oi Achlya lignicola, growing on
wood in water ; the course of development is indicated by the letters A —F.
a the antheridium, b its tube penetrating into the oogonium (X 550).

laments emerge which open at their apex and allow the protoplasm to escape in a ball
which then breaks up into a number of zoogonidia. After coming to rest the zoo-
gonidia of the Saprolegnieae give rise to new plants, which, when they have access to
a fresh substratum, such as a dead fly, produce in succession several generations of
non-sexual gonidia-forming individuals.

It is only towards the close of the period of growth that sexual individuals make
their appearance. The extremities of the filaments then swell up into a globular
form (Fig. 179), a septum is formed beneath the swelling, which constitutes the oogo-
nium, and its protoplasmic contents either simply contract to form the oosphere, as
in Pythium monospermum and Aphanomyces, or the contents divide into two or more

T

274 THA LL OPHYTES.

oospheres, considerable contraction taking place. At the same time the antheridia
are also formed, appearing as much slenderer lateral branches, beneath the oogonia
but sometimes above their basal wall. These antheridial branches are usually much
curved; their upper portion is separated by a septum, and the terminal cell thus formed;
the antheridium usually applies itself firmly to the surface of the wall of the oogonium.
The process of fertilisation itself, which had previously been only imperfectly known,
presents, according to the most recent observations of Pringsheim, several remarkable
peculiarities which throw considerable light on the genetic relationships of the Sapro-
legnieae, and on the essential nature of the various modes in which it takes place.
Even before the contents of the oogonium have broken up into oospheres, a larger
or smaller number of spots of a lighter colour (Fig. 179, C) may be observed on the
oogonium ; at these spots Pringsheim states that the inner layer of the cell-wall protrudes
in the form of a wart to form the conjugating organ by which fertilisation is subsequently
effected : in some species of Saprolegnia and Achlya these warts remain covered by the
outer layer of the cell-wall of the oogonium. As soon as an antheridial cell becomes
closely applied to the oogonium, a protuberance of the inner layer of the cell-wall
of the antheridium first of all penetrates into the outer layer of the cell-wall of
the oogonium, and meets the wart already described belonging to the inner layer;
this latter is then absorbed at the point of contact, and the protuberance of the
antheridium grows into a narrow tube which penetrates deeply into the oogonium, its
extremity burying itself in the midst of the oospheres. These processes are only par-
tially represented in Fig. 179, which was drawn long before the discovery of the actual
phenomena. According to Pringsheim the extremity of the fertilising tube opens in the
interior of the oogonium ; extremely minute bodies are repeatedly expelled in jerks
and after long intervals, which are seen among the oospheres, and which are probably
antherozoids or bodies with a similar function. But the substance expelled from a single
fertilising tube may serve for the impregnation of several oosphores in an oogonium ; and
it is the more probable that this takes place since the number of these tubes which pene-
trate into an oogonium is usually different from that of the fertilised oospheres. With
regard to the bright spots in the wall of the oogonium, through which, as we have
seen, the fertilising tubes enter, it is evident that a process takes place here similar in
its nature to conjugation, since not only does the antheridium grow into the oogonium,
but the inner layer of the wall of the oogonium also, so to speak, grows towards the
antheridium by the formation of the wart already described, in order to unite with
it, although no actual coalescence of the contents takes place at that time, the wart
being subsequently penetrated by the fertilising antheridial tube. But in some
Saprolegnieae the formation of this conjugating wart proceeds further ; it penetrates
through the outer layer of the wall of the oogonium, opens at the surface, and
each wart thus becomes an orifice through which the antheridial tubes penetrate :
sometimes, according to Pringsheim, the conjugating warts become raised above the
spherical surface of the oogonium in the form of more or less elevated protuberances,
or even grow into tolerably long tubes, the apices of which are occasionally met with in
.conjugation with an antheridial tube. These cases, although rare, manifest still more
strikingly a certain resemblance to the conjugation of the Zygomycetes, and perhaps
a still closer analogy to the process of fertilisation in some Ascomycetes, such as
Pe%i%a confluens ; or a comparison might even be drawn between the trichogyne of
Nemalion and the protuberances of the oogonium. The similarity of the process of
fertilisation in the Saprolegnieae to that already described in the case of Vaucheria is
sufficiently clear ; that spot in the oogonium of Vaucheria which subsequently opens and
exudes a drop of mucilage may obviously be compared with the conjugating wart in those
oogonia which contain only a single oosphere ; only that in Vaucheria no fertilising
tube is usually developed from the antheridium, since the antherozoids are larger, and
remain motile for a considerable period, finding their own way into the open oogonium.
Pringsheim has already pointed out the corresponding analogy with the CEdogonieae.

OOSPORES.

^15

Two other remarkable facts have still to be mentioned: — in the first place that
determined by Pringsheim, that parthenogenesis is not an uncommon phenomenon in
Saprolegnia and Achlya. Some or all of the oogonia on a plant may not be fertilised
at all, the formation of antheridia on them being altogether suppressed ; and yet the
oospores are perfectly developed and are capable of germinating. We shall refer
more in detail in Book III. Sect. 30 to this and other instances of parthenogenesis in the
vegetable kingdom ; here it need only be mentioned that the forms hitherto considered
as dioecious— no antheridial branches being found on the oogonia — are not distinct
species, but only parthenogenetic forms of moncecious species. In contrast to the
development of oospores without fertilisation is the occurrence of antheridia the tubes
of which do not penetrate into oogonia, but open and expel the fertilising particles into
the surrounding water. In these, as well as in those tubes which pierce the oogonia,
Pringsheim observed that the expulsion does not take place all at once, but in repeated
jerks and after considerable intervals.

The oospores or fertilised oospheres clothe themselves with a thick firm cell-wall
and remain dormant in the oogonium for months. Their germination takes place
subsequently in two different ways : — either the oospore puts out a germinating filament
which at once developes into a branched plant in which zoogonidia are subsequently
formed ; or it puts out a short filament which opens at its apex and allows the whole
of its contents to escape in the form of zoogonidia. Sometimes after the com-
mencement of the formation of a filament the contents break up into gonidia, or
the entire contents, surrounded by an inner cell-wall, escape from the outer envelope,
and then germinate. In their mode of germination the Saprolegnieae approach on one
hand the Mucorini, on the other hand the Peronosporeae.

3. The Peronosporeee ^ are all parasitic on the succulent parenchymatous tissue
of living Phanerogams (Dicotyledons), the irregularly-branched mycelium spreading over
large areas in the intercellular spaces, and putting out at a number of places appendages
(Haustoria) which penetrate into the interior of the cells of the host, and enable it
to absorb the cell-contents as food. As in the preceding families, the entire vegetative
body or mycelium consists of a single tubular cell not divided by septa. Multiplica-
tion again takes place at the commencement of the period of growth exclusively in the
non-sexual mode by the production of gonidia. These are abstricted and fall off from
the end of branches, and are therefore stylogonidia, or, as they are usually termed,
conidia. In the genus Peronospora, which comprises a large number of species, long
slender branches of the mycelium emerge into the air through the stomata of the host
for the purpose of the production of conidia, then branch in a somewhat arborescent
manner, and form at the end of each branch a comparatively large ellipsoidal conidium.
In Cystopus a large number of short club-shaped branches (Fig. 180, B) are formed
densely crowded together on the mycelium beneath the epidermis of the host, and
each of these produces at its extremity a series of conidia, until finally they burst
the epidermis and escape in the form of a fine dust. The conidia germinate in
different ways. In Peronospora there are some species, as P. infest ans and nivea^
in which the contents of the conidia, after falling off and reaching a drop of
rain or dew, break up into a large number of swarming zoogonidia and escape
(Fig. 180, C, jD) ; in others, as P.pygmcea, the whole of the protoplasm escapes out of
the conidium and forms a roundish cell which at once puts out a germinating filament.
In a third and fourth section of the genus the conidium puts out at once a germinating
filament which emerges either at a definite spot {P. gangliformis) or indifferently at any
spot {P. parasitica^ calotheca, Alsinearum, &c.). In Cystopus either all the conidia are
alike, i.e. when they reach a drop of water they produce swarming zoogonidia

* De Bary, Ann. des Sci. Nat., 4^ sen, i860, vol. XIII ; 1863, vol. XX. [On Phytophthora {Pe-
ronospora) infestans, see De Bary, Joum. of Bot. 1876, pp. 105-126, and 149-154: see also Bot. Ztg.

1 881.]

T 2

37^

THALLOPHVTES.

(C. candidus)y or the terminal conidium of each chain gives rise to a germinating filament,
if it is capable of germinating, while the other cells of the conidial chain produce
zoogonidia (C. Portulacoc). After the swarming is finished the zoogonidia become firmly
attached to the cuticle of the host, invest themselves with a thin cell-wall, and, in the
case of Peronospora infestans, put out a delicate germinating filament directly into an
epidermal cell, piercing through its outer wall. After it has entered the cell (Fig.
1 80, H) this filament absorbs the whole of the protoplasm of the zoogonidium, and,
after piercing also the inner wall of the epidermal cell, reaches an intercellular space,
where the mycelium developes. The zoogonidia of Cystopus are firmly attached near the
stomata of their host, and push their germinating filaments into their orifices (Fig. 180, G),

"^rrrt

Fig. iZo.— Cystopus candidus. A branch of the mycelium growmg at the apex t with haustoria h between the paren-
chymatous cells of Lepidium sativum; B conidia-bearing branch ofjthe mycelium; C, D, E formation of zoogonidia
from the conidia; F zoogonidia germinating; G a zoogonidium germinating on a stoma; H zoogonidium oi Peronospora
infestans penetrating through the epidermis of a potato-stem (after De Bary, X 400).

and thus find their way at once into the intercellular spaces. When the mycelium has
once obtained a footing in the parenchyma of the host, it continues to grow in it, and
finally often spreads through the whole plant, putting out its conidia-bearing branches
at various places in the stem, leaves, or inflorescence. In this manner the (unicellular)
mycelium of P. infestans can even hibernate in the tubers of the potato, to undergo
further development in the shoots in the following spring. The sexual organs of
the Peronosporeae are developed in the interior of the tissue of their host. Spherically
dilated ends of branches of the mycelium shape themselves into oogonia (Fig. i8r, ^, og)j
in each of which an oosphere is formed out of a portion of the protoplasm {B, os).
From another branch of the mycelium a branchlet grows towards the oogonium,

OOSPORES.

277

swells, and becomes closely attached to it ; and the thicker part becoming separated
by a septum (just as takes place with the oogonium itself), developes into an anthe-
ridium. As soon as the oosphere is formed, a fine branch of the antheridium (jB, an)
reaches it, penetrating the membrane of the oogonium. After fertilisation the oosphere
becomes surrounded by a coat which thickens and forms an external rough dark-brown
exospore and an inner endospore. These oospores remain dormant through the
winter and then germinate ; in the case of Peronospora Valerianellce they form a myce-
lium on moist ground; those of Cystopus, however, produce zoospores; the endo-
spore (i) forces itself like a bladder out of the ruptured exospore (Fig. 181, F), and
then bursting, the zoospores (G) are set free, which behave in exactly the same
manner as the zoogonidia produced from the conidia of this genus.

Fig. \%i.— Cystopus candidiis. A mycelium with young: oogonia ; B oogonium og with oosphere os and antheridium «« ;
C ripe oogonium ; D ripe oospore ; E, F, G formation of zoospores from oospores ; i endospore (after De Bary, x 400).

The genus Empusa, in which no sexual organs have as yet been discovered \ is

probably related to the Saprolegnieae and to the Peronosporeae. Empusa musccB is the

parasite which proves fatal, more especially in the autumn, to house-flies. If these

^^ insects remain attached to the window-panes they become surrounded by a powdery

|H| substance which consists of extruded conidia. These conidia are capable of forming

^B 1 [Brefeld, Untersuch. iiber die Entwickelung der Empusa Mtiscce uud E. radicans, 1871.
I^Wowakowski and Brefeld have both recently published observations on Empusa musc<2 (Entomoph-
^mkora) in the Bet. Zeit. for 1877. While Nowakowski states that he has observed zygospores,
■Wfirefeld believes that the resting spores arise asexually. Nowakowski thinks the Entomophthorea
allied with PiptocepkalideiE.]

278 THALLOPHVTES.

secondary conidia, and of projecting them to some distance. If one of these happens
to strike a fly upon the white area of the under surface of its body, it sends a hypha
through the skin, and there forms toruloid cells, which multiply by germination and
are disseminated in the blood throughout the body of the fly. These finally grow into
hyphae which penetrate the skin, each forming a conidium, which is cast off with con-
siderable force.

C— VOLVOCINEiE *.

[This group includes two genera, Vol'vox and Eudorina, which resemble Pandorina in
many respects, and were formerly included with it in one group. Like Pandorina^ these
plants are motile, and consist of a number of ciliated cells aggregated into a coenobium.
In Eudorina, which so much resembles Pandorina as to be sometimes mistaken for it, the
number of cells is 16 or 32; in Vol'vox it is very great, amounting even to thousands.
The cells are so arranged as to form the wall of a hollow sphere. The distinction
between these plants and Pandorina is that in them certain cells of the coenobium
develope into antheridia and oogonia. In Eudorina four of the cells become antheridia,
and the remainder oogonia : in Vol'vox a certain number only of the cells take on the
function of reproductive organs. The antherozoids formed in the antheridia escape and
make their way through the wall of an oogonium to the oosphere which it contains, and
fertilise it. The oosphere then surrounds itself with a firm double cell-wall, and
assumes a red colour: it becomes an oospore.

Non-sexual reproduction is effected by the repeated division in Vol'vox of certain
cells (gonidia) of the coenobium, and in Eudorina of all the cells. In the former, the
young coenobia which are thus formed escape into the cavity of the mother-coenobium,
where they remain until they are set free by its death and disintegration : in the latter,
they are at once set free.

Sexual and non-sexual reproduction do not go on simultaneously in the same
coenobium of Fblvox, and in one species (Vol'vox minor, Stein) the male and female
organs are not produced in the same coenobium, whereas this is the case in Vol'vox
Globator, L.

The germination of the oospore has been observed by Kirchner in Vol'vox minor. The
protoplasmic contents divide into two, and this is repeated until about 500 small cells
have been produced ; these are arranged like those of the adult coenobium. The wall of
the oospore gradually deUquesces, and the young coenobium is set free.]

D. — CEdogonie^e.

The CEdogoniesB ^ include at present only the two genera (Edogonium and Bulbo-
crcete, the few species of which are common in fresh water, fixed by an organ of attach-
ment at the lower end to solid bodies, mostly the submerged parts of other plants.
The thallus consists of unbranched {(Edogonium) or branched {Bulbochsete) rows of
cells, which multiply by intercalary growth, while the terminal cells elongate into hyaline
bristles. The longitudinal growth of the cylindrical cells begins with the formation of
an annular cushion of cellulose inside the cell, close beneath its upper septum ; the
cell-wall ruptures at this place circularly; the ring of cellulose then stretches, and a
broad transverse zone is thus intercalated in the wall of the cell. The process is
constantly repeated immediately beneath the older very short upper piece of the cell,

^ [On Volvox, see Cohn, Beitr. z. Biol. d. Pflzn. I. 3, and Kirchner, ibid. II. i : on Eudorina see
Carter, Ann. of Nat. Hist., ser. III. vols. 2 and 3.]

^ Pringsheim, Morphologie der Qidogonieen, Jahrb. fiir wissen. Bot. vol. I. De Bary, Ueb.
(Edogonium u. Bulbochcete, 1854. — Juranyi, Jahrb. d. wiss. Bot. IX. [Ann. des Sci. Nat. 1856,
vol. V. p. 251.— Carter, Ann. and Mag. Nat. Hist. 1858, vol. I. pp. 29-39.]

OOSPORES.

%n

so that these pieces, forming small projections, give to the upper end of the cell the

1^^ appearance of consisting of caps placed one over another, while the lower end of the
I^B cells appears to be enclosed in a long sheath (the lower old piece of cell- wall). This
^* lower part of an elongated cell is always separated by a septum from the upper cap-
bearing piece (see Fig. 17, p. 22). In Bulbochgete the growth of all the shoots, even of
the first which proceed from the spores, as far as it depends on cell-multiplication,
is limited to the division of their basal cell ; it follows therefore that the cells of each
shoot must be considered to be the basal cells of the lateral branches which stand upon

FIG. 182. — Development of -the zoogonidia of FIG. 183.— ^ CEdogonium ciliatum (X 250) middle part of a sexual filament

(Edogonhitn (after Pringsheim). A, B their origin with an antheridium (tn) at the upper end, and two fertilised oogonia (og) and
from an older filament ; C free zoogonidium in a dwarf male plant m ; B oogonium at the moment of fertilisation ; o the
motion ; D commencement of its germination ; oosphere, z the antherozoid in the act of forcing its way in, nt dwarf male
E a zoogonidium formed out of the entire contents plant ; C ripe oospore ; D piece of the male filament of O. gemelliparttni,
of a germinating plant (X350). z antherozoids. £ branch of a hybernated plant of Bulbochate intermedia,

with one oogonium still containing a spore, another in the act of allowing it to
escape ; in the lower part an empty oogonium. F the four zoospores result-
ing from an oospore; G zopspores from an oospore come to rest (after
Pringsheim).

them. The cells contain chlorophyll-grains and a nucleus in a parietal layer of proto-
plasm. The zoogonidia, as well as the oogonia and antheridia, are formed from cells of the
filaments, which only become enlarged and assume a more or less spherical form when
they give rise to oogonia. From the oospores which have remained at rest for a con-
siderable period several (usually four) zoospores are immediately formed, which produce
asexual, i. e. zoogonidia-forming plants, from which again similar ones proceed, until the
series of them is closed by a sexual generation (with formation of oospores) ; but the
sexual plants produce zoogonidia as well. The sexual plants are either monoecious or
dioecious ; in many species the female plant produces peculiar zoogonidia (Androgonidia),

a8o THALLOPHVTES.

out of which proceed very small male plants (dwarf males). Several generative cycles
or only one may be completed in a vegetative period. The zoogonidium is formed in
an ordinary cell of the filament (sometimes even in the first cell, Fig. 182, E) by the
contraction of its whole protoplasmic substance ; it becomes free from the mother-
cell, the cell-wall splitting by a transverse slit into two very unequal halves (as in the
division of the cells) (Fig. 182, yl,B,E). It is at first surrounded by a hyaline mem-
brane, which however it also breaks through. At its hyaline end — the anterior end
during the swarming — it is encircled by a circlet of numerous cilia. This end lies
laterally in the mother-cell, and, after the movement ends, becomes the lower attached
end which grows out into a rhizoid ; the direction of growth of the new plant is thus
at right angles to that of the mother-cell. The antherozoids are very similar in form to
the zoogonidia, but much smaller (Fig. 183, D, «); their motion, due to a circlet of
cilia, is also similar. The mother-cells of the antherozoids are cells of the filament,
but shorter and not so rich in chlorophyll as the vegetative cells; they lie either
singly or in groups (sometimes as many as twelve) above one another. In most species
each antheridium-cell divides into two equal special mother-cells, each of which produces
an antherozoid ; they escape by the splitting of the mother-cell (as in the case of the
zoogonidia) (Fig. 183, D). The androgonidia from which the dwarf male plants arise
are produced from mother-cells similar to those which give birth to the antherozoids
(without formation of special mother-cells). After swarming they fix themselves to a
definite part of the female plant, on or near the oogonium, and after germination produce
at once the antheridium-cells, and in them the antherozoids (Fig. 183, ui, B, m, m). The
oogonium is always developed from the upper daughter-cell of a vegetative cell of the
filament which has just divided, and immediately after the division swells up into a
; spherical or ovoid form. In Bulbochaete the oogonium is always the lowest cell of a
fertile branch. This is not opposed to the law of growth above-mentioned, inasmuch as
the mother-cell of a branch fulfils at the same time the function of its basal cell ; the
oogonium of Bulbochsete is never the first cell of a branch, since this is always developed
as a bristle. The oogonium becomes at first more completely filled with contents
than the remaining cells ; immediately before fertilisation the protoplasm contracts and
forms, as in Faucheria, the oosphere, in the interior of which the chlorophyll granules
are densely crowded. The part of the oosphere which faces the opening of the
oogonium consists simply of hyaline protoplasm. The opening of the oogonium is
produced in a variety of ways. In some species of CEdogonium and all of Bulbochsete its
wall has an oval hole in its side, out of which the colourless part of the oosphere
protrudes in the form of papillae, and takes up the antherozoids. In some species of
CEdogonium (Fig. 183, A, B), on the other hand, the oogonium-cell splits, as when the
zoogonidia are escaping ; and the otherwise straight row of cells of the filament thus
appears as if broken at this spot. In the lateral fissure appears some colourless muci-
lage, which the observer can actually see take the form of an open beak-like canal (Fig.
183, B, z), through which the antherozoid enters. The antherozoid coalesces with the
hyaline part of the protoplasm of the oosphere and disappears^. Immediately after
fertilisation the oosphere surrounds itself with a membrane, which afterwards, like its
contents, assumes a brown colour ; but in Bulbochsete the contents of the oospore thus
formed is of a beautiful red. The oospore remains enclosed in the membrane of the
oogonium, which separates from the neighbouring cells of the filament and falls to the
ground, where the oospore passes its period of rest. When it awakes to new activity,
the oospore does not itself grow into a new plant ; but in Bulbochxte, where this process
has been observed, its contents divide into four zoospores, which escape together with
the inner skin of the oospore, and after this latter is dissolved, swim about. After
becoming stationary each grows into a new plant.

* In O. diplandrum, discovered by Juranyi, the large antherozoids creep in an amoeboid manner
over the oogonium until they reach the canal, which they slowly enter.

OOSPORES.

381

I The ConfervaeesB, like the (Edogonieae, consist of rcJws of cells or segmented fila
ments, which either remain unbranched, as in Chxtomorpha, or become branched, as in
g

\Cladophora, Rhizoclonium, Stigeoclonium (Fig. 3), Draparnaldia, Chxtophora^ . With re-
erence to their reproduction, it is only known that macro- and micro-zoogonidia are
formed in the cells of the filaments [Chxtomorpha, Cladophora), the sexual significance
of which is still unknown^; and that in the other above-mentioned plants resting-spores
are formed in certain cells of the filaments. Pringsheim suggests that they are probably
equivalent to oospores, but that they are produced parthenogenetically. ,

The Fucace® comprise
^genera of large marine Algae,

E. FuCOIDEiE.

in the narrow limitation proposed by Thuret^, a few
the thallomes of which, often many feet long, have a

Fig. \Z\.—Fucus platycarptis (after Thuret) ; A end of one of the larger branches (natural size) ; yy fertile branchlets ;
B transverse section of a conceptacle ; d the surrounding epidermal tissue ; a the hairs projecting from the mouth ; b hairs
in the interior ; c oogonia, e antheridia (cf. Fig. 2, p. 3).

greenish-brown colour and a cartilaginous consistency. They are fixed to stones or
other bodies by a branched attachment-disc. The thallomes branch dichotomously,
and the further development is also frequently forked, but in other cases sympodial, as
in Fig. 184. The ramifications, irrespectively of later displacements, all lie in one
plane.

^ [The TJlvacese are probably allied to the Confervacece. In them the cells are arranged so
as to form a delicate membrane.]

2 [According to Areschoug (Nov. Act. reg. soc. sci. Upsal. ser. 3. vol. IX) the microzoogonidia
of Cladophora conjugate in pairs.] ,

^ G. Thuret, Ann. des Sci. Nat. II. 1854, p. 197.

28:j THALLOPHVTES.

The tissue consists at the surface of small closely-crowded cells ; in the interior it is
laxer, and the elongated cells are often connected into articulated threads. The cell- walls
often consist of two clearly distinct layers, an inner thin, firm, compact layer, and an
outer gelatinous one, capable of swelling greatly in water, which fills up the interstices of
the cells, and has the appearance of a more or less structureless * intercellular substance ;'
it is clearly the cause of the slimy character which the Fucacese assume after lying for
some time in fresh-water. The granular cell-contents have been but little investigated ;
they appear to be mostly brown, but contain chlorophyll which is concealed by other
colouring materials; from dead plants cold fresh-water extracts a bufF-coloured sub-
stance^. The tissue often becomes hollowed out internally into large cavities containing
air which are forced outwards and serve as swimming bladders. The thallus has not,
as far as I know, been further minutely examined ^ ; the outer conformation especially
has been but little investigated from a morphological point of view. (Gf. Nageli, Neuere
Algensysteme.)

The mode of sexual reproduction is far better known through the labours of Thuret
and Pringsheim. The antheridia and oogonia are formed in spherical hollows (Con-
ceptacles) which make their appearance in large numbers and densely crowded at the
ends of the longer forked branches or of lateral shoots of peculiar form. These con-
ceptacles are not formed in the interior of the tissue, but are depressions of the sur-
face which become walled in by the surrounding tissue and so overgrown that at
length only a narrow channel remains, opening outwards. The layer of cells which
clothes the hollow is thus a continuation of the external epidermal layer of the thallus ;
and since the filaments which produce the antheridia and oogonia sprout from it, these
latter are, morphologically, trichomes. Some species are monoecious, i.e. both kinds
of sexual organs are developed in the same conceptacle, as in Fucus platycarpus (Fig.
184) ; others are dioecious, the conceptacles of one plant containing only oogonia, those
of another plant only antheridia {e.g. Fucus 'vesicu/osus, serratus, and nodosus, Himanthalia
lorea). A number of hairs which grow in the conceptacles among the sexual organs are
long, slender, articulated, but unbranched, and project in F. platycarpus out of the mouth
of the receptacle in the form of tufts (Fig. 184, B). The antheridia are produced as
lateral ramifications of branched hairs. Each antheridium consists of a thin-walled oval
cell, the protoplasm of which splits up into numerous small antherozoids ; these are
pointed at one end, and each is furnished with two motile cilia ; in the interior they
contain a red spot. The formation of the oogonium begins with the papillose swelling of
a parietal cell of the conceptacle ; the papilla is separated off by a septum, and divides,
as it grows in length, into two cells, a lower, the pedicel-cell, and an upper, which
forms the oogonium ; this swells up into a spherical or ellipsoidal form and becomes
filled with dark protoplasm. The protoplasm of the oogonium remains undivided in
some genera {Pycnophycus, Himanthalia, Cystoseira, Halidrys), and the whole contents
of the oogonium thus form one oosphere ; in others [Pel'vetia) it divides it into two, four
{0%othallia 'vulgaris), or eight {Fucus). Fertilisation takes place outside the concep-

* In a recent paper (Comptes Rendus de I'Acad. des Sci. Feb. 22, 1869) Millardet showed that
from quickly-dried and pulverised Fucaceae an olive-green extract is obtained by alcohol, which,
shaken up with double its volume of benzine and then allowed to settle, produces an upper green
layer of benzine containing the chlorophyll, while the lower alcoholic layer is yellow and contains
phycoxanthine. Thin sections of the thallus, completely extracted with alcohol, contain also a
reddish-brown substance which in fresh cells adheres to the chlorophyll-grains, and can be extracted
by cold water, more easily when the dried Fucus has been previously pulverised. Millardet calls
this reddish-brown substance phycophoeine. (Compare further the interesting treatise of Rosanoff,
Observations sur les fonctions et les proprietes des pigments de diverses Algues, in Memoires de la
Societe des Sci. Nat. de Cherbourg, vol. XIII. 1867 ; and Askenasy, Bot. Zeitg. no. 47, 1869.) [^^^
also Sorby, Proc. Roy. Soc. 1873, vol. XXI, pp. 445, 454, 461.]

2 [See Rostafinski, Beit. z. Kenntniss der Tange, 1876: also Bower, Quart. Journ. Mic. Sci.
vol, 20, new series.]

OOSPORES.

283

icles. The oospheres are expelled, surrounded by an inner membrane of the oogonium,
md escape through the opening of the conceptacle; the antheridia at the same time
)ecome detached, and collect in numbers before the mouth of the conceptacle when the
fertile branches are lying out of the water in moist air. When they again come into
rontact with the sea-water, the antheridia open and allow the antherozoids to escape,
the oospheres at the same time escaping from the envelope which still surrounds them, and

?hich is then seen to consist of two separated layers (Fig. 185, II). The antherozoids

)llect in numbers around the oospheres, become firmly attached to them, and when

leir number is sufficiently great, their movement becomes so energetic that they impart
the very large oosphere a rotatory motion which lasts for about half an hour.

''hether the antherozoids force themselves into the oosphere Thuret leaves undecided ;
)ut analogy with the processes observed by Pringsheim in Faucheria and (Edogonium

:arcely admits of a doubt that one or several of them mingle their substance with that
)f the naked ball of protoplasm. A short time after these processes are completed, the

jrtilised oosphere surrounds itself with a cell-wall, fixes itself to some body or other,
ind begins, without any period of rest, to germinate, and, lengthening at the same time.

Fig. i^e^.—Fucus vesiculosus (after Thuret); A a branched hair bearing antheridia; 5 antherozoids ; / an oogonium,
C!^ after the contents have divided into eight portions (oospheres), surrounded by simple hairs (/) ; // commencement of
the escape of the oosphere ; the membrane {ft) has burst ; the inner membrane i is ready to open (the two together
constitute an inner layer of the cell-wall of the oogonium) ; /// oosphere surrounded by antherozoids ; IV, V, germination of
the oospore (B x 330, all the rest X 160).

undergoes first of all a transverse division followed by numerous other divisions. The
mass of tissue thus formed puts out from the part on which it rests a root-like hyaline
organ of attachment, while the thick free end forms the growing apex (Fig. 185, IV).

There are numerous marine Algae, included in the group of PhfiBosporesB, which
resemble the Fucaceae as well in the structure of their vegetative organs as in the
presence of a colouring-matter mingled with their chlorophyll. To this group belong
the often enormous Laminarieae (Macrocystis, Laminaria, Lessonia, etc.), as also the
smaller Ectocarpeae, Sphacelarieae, Ghordarieae, and Dictyoteae.

The Phaeosporeae are reproduced non-sexually by zoogonidia, a mode of reproduc-
tion which does not occur among the Fucaceae. In some, bodies which appear to be
antheridia have been detected, but no oogonia ^

^ [Goebel has observed (Bot. Zeitg. 1878) the conjugation of zoogonidia ia two species of
Ectocarpus: see also Berthold, in Mittheil. d. Zool. Stat. Neapel, II. 1881.]

384 THALLOPHYTES.

CLASS IV.

CARPOSPORE^.

Under the name of Carposporeae I include in a single class the Coleochaeteae,
Characeae, and Floridese on the one hand, all of which contain chlorophyll, and on
the other true Fungi, namely the Ascomycetes, Basidiomycetes, and ^icidiomycetes.
The remarkable differences of their habit and mode of life offer no real obstacle to
such an arrangement, any more than do the similar differences which exist between
the Lemnacese and the Palms to their being both included in the class of Monocoty-
ledons. In this case, as in that, it is the peculiarities which accompany the forma-
tion of the reproductive organs and become especially prominent in the product of
fertilisation that indicate the existence of a relationship between plants which are at
first sight so different.

All the plants belonging to this class are characterised by the formation of
a spore-fruit {sporocarf) as the result of the fertilisation of the female reproductive
organ, and differ herein from all those which have hitherto been considered. This
spore-fruit consists, except in certain cases in which it is rudimentary, of two distinct
parts, a fertile part, which is directly derived from the female organ and which pro-
duces at a later period one or more commonly numerous true spores, and an
investing part, which encloses the spores. Occasionally this fruit attains a very con-
siderable size, as, for instance, in the Truffle : in other forms it may remain com-
paratively small. In all cases, however, fertilisation not only causes a further
development of the female cell, as in the Zygosporese and Oosporeae, but certain
processes of growth are initiated which may lead to comparatively insignificant
results (as in Characeae), but which are usually of an extensive character, so that
a sporocarp is produced consisting of a considerable mass of tissue. In those cases
in which the sporocarp remains relatively small, and in which it continues to derive
its nourishment from the parent-plant until the spores are ripe (as in Characeae,
Florideae, Coleochaeteae, and in some Fungi), it seems to be a mere appendage, like
an apple on a tree : but in those cases in which it attains a considerable size, grows
for a length of time and finally produces numerous spores, after that the parent-
plant has decayed, an alternation of generations becomes apparent. Such is the
case among the true Fungi, especially in the Ascomycetes. In these, the fructifica-
tion continues to grow for some time, and presents the appearance of being a
perfectly independent plant. In fact, it is usually but erroneously regarded as being
the whole Fungus, whereas it is only the product of a process of sexual reproduction
which has taken place on the vegetative body, the mycelium. In such cases the
process of the life-history of the plant is similar to that of a Fern ; the insignificant
mycelium corresponds to the prothallium, and the well-developed sporocarp of the
Fungus to the spore-bearing Fern. If we consider from this point of view the
various ways in which the fruit is formed among the Carposporeae, we shall find that

CARPOSPOREM. 385

all possible intermediate stages exist between two extreme forms, the simplest being
that form in which the sporocarp produces only a single spore or a single mother-
cell in which spores are formed, the most complex being that in which the sporocarp
exists as an independent plant and produces countless spores.

The sporocarp is derived from a female organ which we will term the Carpo-
gonium. It is only in the simplest case that the carpogonium is a single cell, and
then it sometimes resembles very closely the oogonium of Class III, especially
when, as in the Coleochseteae, fertilisation is effected by means of motile anthero-
zoids, or when the unicellular carpogonium is fertilised by a tube growing from the
male organ {Sordaria, Podosphcerd), just as is the case in the Saprolegniese. In the
majority of Cases, however, the carpogonium is multicellular before fertilisation, and
its cells contribute in different ways to its further development; some absorb the
fertilising substance, whilst others give rise to that part of the fructification which
produces the spores. This division of labour is very evident in the Ceramieae and
other Florideae, as also in many Ascomycetes (e. g. Ascobolus furfuraceus). It
occurs both in unicellular and in multicellular carpogonia that a more or less
elongated tubular projection arises from the carpogonium, the function of which is
the absorption of the fertilising substance. This organ, which takes no part in the
subsequent development of the fruit, is termed the Trtchogyne, a name given by
Thuret and Bornet to this organ in the Floridese. Like the style on the ovary
of Phanerogams, the trichogyne of the Carposporeae may be sometimes well-
developed, and sometimes entirely absent. For instance, in Characeae and in many
Ascomycetes {Sordaria, Erysiphe) it is wanting ; it is only imperfectly developed in
Peziza confluens, and it is well- developed in Coleochaeteae and Florideae. The male
reproductive organ occurs in very various forms in the different groups of Carpo-
sporeae, this variety being evidently dependent upon the varying form of the carpo-
gonium and the habitat of the plant. In Coleochaeteae and Characeae only is
fertilisation effected by motile antherozoids ; in Florideae it is effected by cells which
are conveyed passively, and in most Fungi by tubular outgrowths.

Should it be suggested as an argument against the existence of a relationship
between the true Fungi and the green plants which are included within this class,
that the difference of habit between them is very great, it might be replied that the
tissue of many Fungi presents striking resemblances to that of many Florideae. The
hyphal tissue of many gelatinous Fungi finds its analogue in the gelatinous tissue of
many Florideae. The rows of cells too, of which the mycelial filaments (hyphae)
of Fungi consist, differ only in habit from the branched rows of cells of which the
thallus of many Coleochaeteae and of very many Florideae consists.

It must be remembered that, in order to detect the relationships existing
between different groups of plants, the simplest and not the highest forms are those
which must be compared. If this be done in this case, it becomes evident that the
simplest Florideae are connected on the one hand with the Coleochaeteae and
Characeae, on the other with the simplest Ascomycetes. Each of these series of
forms, however, becomes developed into higher forms in some particular direction,
and so if the most perfect Ascomycetes be compared with the most highly-developed
Florideae and Coleochseteae, only a very slight similarity between them will be
detected.

286

THALLOPHYTES,

FORMS CONTAINING CHLOROPHYLL.

These are all submerged water-plants, the vegetative and reproductive organs
of which have a well-marked tendency to clothe themselves with a peculiar cortex.
This is especially remarkable in the genus Chara, and it will be described in detail
hereafter when that genus is under consideration. It is also very evident in the
Ceramiaceae, and but rudimentary in the Coleochaetese, where it is confined to the
fruit. Side by side with forms possessing this cortex, there are others, very nearly
related, which do not possess it.

In all the plants belonging to this group the fruit is small in proportion to the
thallus which bears it, and the alternation of generations which finds its expression

in the formation of the fruit is therefore
not very clearly marked.

A. The Coleoch^te^.

The carpogonium is unicellular with a long
trichogyne opening at its apex. Fertilisation
is effected by antherozoids which are formed
either in special small branches or in the cells
of a filament which have undergone division.
In the basal portion of the fertilised carpogo-
nium there is a cell which grows considerably,
and becomes invested by outgrowths derived
from neighbouring cells. In the next period
of vegetation it gives rise to numerous carpo-
spores in the form of zoospores.

The Coleochaetese^ are small (about 1-2
mm.) fresh- water Algae, of a bright green
colour and constructed of branched rows of
cells, attached in standing or slowly-running
water to the submerged parts of other plants
{e.g. Equisetum), and forming circular closely-
attached or cushion-like discs. Their chloro-
phyll assumes the form of parietal plates or
of large granules. The name of the genus
Ccleochsete (sheath-hair) is due to the circum-
stance that certain cells of the thallus bear
lateral colourless bristles fixed in narrow
sheaths (Fig. 186, A, h). If the phenomena
of growth of the diflferent species are com-
pared, two extreme cases are seen, connected
by transitional forms. The one extreme is
formed by C. di'vergens, which', as it developes
from the spore, produces first of all creeping
irregularly-branched articulated threads; from these spring ascending articulated
branches which are also irregularly branched ; the whole thallus does not assume
any definite form. In C. pul'vinata, on the contrary, the thallus forms a hemi-

FiG. 186.—^ an asexual plant of Coleocheete soluta (x 250);
B a piece of a similar disc ; the letters a-g indicate the suc-
cessive dichotomous branchings of the terminal cells (after
Pringsheim).

Prii\gsheim in Jahrbuch fur wissenschaftliche Botanik, vol. II. p.

CARPOSPOREM. 287

spherical cushion ; the cellular filaments which are the result of germination branch
somewhat irregularly in one plane, but form something like a disc ; from them rise
up ascending articulated branches, which again branch and form the cushion. In the
following species no ascending branches are formed, but those which cling to their
support form a more or less regular disc. In C. irregularis this takes place by irregular
ramifications which lie in one plane gradually filling up all the interstices, till an almost
uninterrupted layer of cells is obtained. In C. soluta (Fig. 186), on the other hand,
a dichotomous ramification commences in the two first daughter-cells of the germinating
spore, with corresponding cell-division of such a nature that even at a very early
period a closed disc of radial forked branches is formed, which either lie loosely or are
closely crowded side by side. While in the species already named the branches arise
laterally from cells, but never from the terminal cell of a branch, in C. soluta we have the
first instance of dichotomy as well as regular disc-shaped centrifugal growth, a condition
which attains the highest development in C. scutata. In this species the cells which
result from germination remain from the first united laterally and do not form isolated
branches ; the circular disc, when once formed, continues to grow by increase of its
circumference, the marginal cells dividing by radial and tangential walls. This mode
of growth may be explained in this way, that the first branches are united laterally
and grow with equal rapidity in a radial direction, and then become divided by septa
(in this case tangential) ; while the broadening of the terminal cell of each radial row
corresponds with the succeeding radial division of a dichotomy. The law which prevails
in the species previously mentioned, that only the terminal cell of a branch is divided by
transverse septa, is exemplified in C. scutata by the marginal cells only of the disc being
divided by tangential walls.

The Reproduction of the Goleochaeteae is brought about by asexual zoogonidia and
by resting oospores produced sexually. The oospores do not at once produce new
plants, but several zoospores ; and the following alternation of generations takes
place: — The zoospores, which arise in the early part of the year at the commence-
ment of vegetation from a sporocarp of the previous year, produce only asexual
plants, or, in other words, only such as can form zoogonidia. Only after a series of
asexual generations varying in length does a sexual generation arise, which may be
either monoecious or dioecious according to the species. Fertilisation produces one
oospore in the carpogonia, which clothe themselves with a peculiar layer of cortical
cells; and this oospore itself again developes into a parenchymatous reproductive
body, from the cells of which the zoospores proceed in the next period of vegetation
(Pringsheim). The zoogonidia (Fig. 187, D) may arise in all the vegetative cells of the
Coleochaeteae ; in C. pul'vinata especially from the terminal cells of the branches ; they
are always formed from the entire contents of the mother-cell, and escape through
a round hole in its cell-wall.

The carpogonium is always the terminal cell of a branch, and hence in C. scutata
the terminal-cell of a radial row (Nageli). The peculiar mode of its development is
subject, according to the growth of the plant, to some, though subordinate, modifi-
cations. One species, C. pul'vinata (Fig. 187), may first of all be examined somewhat
more closely. The terminal cell of a branch swells up and at the same time elongates
into a narrow sac (Fig. 187, A, og, to the left), which then opens {og", to the right) and
exudes a colourless mucilage. The protoplasm of the swollen part, which contains
chlorophyll, forms the oosphere in which a nucleus is visible. The antheridia are formed
at the same time in adjoining cells, two or three protuberances {A, an) growing out,
which become separated by septa ; each of the cells thus formed, which have somewhat
the shape of a flask, is an antheridium ; its entire contents form an antherozoid (2) of
oval shape with two cilia which is endowed with motion like a zoogonidium ; its entrance
into the oogonium has not yet been observed. The effect of fertilisation is seen in
that the contents of the carpogonium become surrounded with a proper membrane and
form the oospore. This now grows considerably, and at the same time the formation of

288

THALLOPHVTES.

the cortical layer (r) of the carpogonium commences ; out of the cells that support it
proceed branches {A, og") which cling closely to it. These again form branches which
also cling closely and divide transversely ; the branchlets of other branches also ramify
( J3) ; and only the neck of the carpogonium does not become covered with the cortical
layer. All this happens between May and July; later, the contents of the remaining cells
of the plant disappear, and the walls of the cortical layer of the carpogonium assume
a deep dark-brown colour. The further development of the oospore within the carpo-
gonium now covered with its cortical layer begins only in the next spring; a paren-
chymatous tissue is formed by successive bipartitions ; the cortical layer splits and is

Fig. 187. — A part of fertile thallus of Coleochate fulvinata (X350); i? ripe carpogonium enclosed in its cortical layer;
C germinating sporocarp, in the cells of which the zoospores are formed ; D zoospores (B—D X 280, after Pringsheini).

thrown off (Fig. 187, C) ; and from each cell arises a zoospore, and from this again an
asexual plant. C. scutata (the most abnormal species) deviates from these processes
only so far that in it the carpogonia provided with their cortical layer lie on the surface
of the disc, and the antheridia are the result of divisions of disc-cells into fours.

Pringsheim {loc, cit.) has already pointed out various relationships existing between
the Coleochsetese, the Florideae, and the Gharaceae.

B. The Floride^.

The carpogonium is either unicellular or composed of several cells, and it is provided
with a permanently-closed trichogyne. If the carpogonium is multicellular, the tri-
chogyne is borne by a lateral row of cells, which is termed the trichophore. Fertilisation
is effected by non-motile rounded antherozoids which become attached to the trichogyne.
As a consequence of fertilisation the basal portion of the carpogonium, which does not
form the trichophore, forms a great number of spores by budding, each spore being the
terminal cell of a short branch. The mass of spores is usually surrounded by an invest-
ment and thus a cystocarp is formed.

The Florideae^ are a group of Algae of extraordinarily variable form, belonging.

^ Nageli und Cramer, Pflanzenphys. Unters. Zurich, Heft I. 1855; Heft IV. 1857. — Thuret,
Ann. des Sci. Nat. 1855, Recherches sur la fecondation, &c. — Pringsheim, Ueber die Befruchtg. u.
Keimung der Algen, Berlin 1855. — [Quart. Journ. Micr. Sc. 1856, vol. IV. pp. 63, 124.] — Nageli,

I

CARPOSPOREm. oH

9

with few exceptions {Batrachospermum, Hildenbrandtia'^), to the sea. In the normal
condition they are of a red or violet colour ; the green colour of their chlorophyll is
concealed by a red pigment^, soluble in cold water.

The Thallus of the Florideae consists, in the simplest forms, of branched rows of cells,
which elongate by apical growth and transverse division of their apical cell. An appa-
rent formation of tissue occurs in many Geramiaceae (G. Cramer, Physiolog. u. system.
Untersuch. liber die Geramiaceen, Zurich, 1863) from the branches growing closely
adpressed to their mother-axes, and thus surrounding them with a cortex, reminding one
of the formation of the cortex in Char a. In other Florideae the thallus is a flat expan-
sion of cells, but often consisting of several layers ; in some (as Hypoglossum and Deles-
seria) it assumes the contour of stalked leaves, even the venation being represented; in
others {e.g. Sphxrococcus and Gelidium) it consists of filiform or narrow strap-shaped
masses of tissue, which ramify copiously {e.g. Plocamium, Sec). In all these cases Nageli
asserts (Neuere Algensysteme, p. 248) that apical growth takes place from an apical cell
(in Peissomelia possibly from several). In the simpler forms the segments of the apical
cell are formed in one row by transverse divisions, in others in two or three rows by
oblique walls. One group which comprises a large number of species, the Melobesiaceae
(RosanofF, Mem. de la Soc. Imp. des Sci. Nat. de Gherbourg, vol. XII. 1866), forms disc-
like thallomes, which grow centrifugally at the circumference and are closely attached
to the substance on which they grow, which generally consists of larger Algae; they
resemble Coleochxte scutata in their size and mode of life, but their thallome generally
consists of several layers, and the cell-wall is encrusted with lime.

The asexual organs of reproduction are gonidia: since four are usually formed in
a mother-cell, they are termed Tetragonidia, but sometimes only one, or two, or eight
are formed. They do not occur in the Nemalieae. When the thallus consists of rows of
cells, the tetragonidia are produced in the apical cell of lateral branches ; in the rest (with
the exception of the Phyllophoraceae, according to Nageli) they lie imbedded in the tissue
of the thallome, often in branches of peculiar shape, in great numbers.

The sexual organs, antheridia and carpogonia, are produced on other plants of the
same species; the sexual plants are frequently dioecious.

Sitzungsb. der k. bayer. Akad. der Wissen.— Eornet and Thuret, Ann. des Sci. Nat. 5th series,

vol. VII. 1867. — Solms-Laubach, Bot. Zeitg. nos. 21, 22, 1867. [See also Agardh, Florideernes

Morphologie, 1880; Kny, Ueb. Axillarknospen bei Florideen, 1873; Thuret, Etudes phycologiques,

1878; Bornet et Thuret, Notes Algologiques, 1876-80; Janczewski, Le developpement des cysto-

carpes dans les Floridees, Cherbourg, 1876; Berthold, Zur Kennt. d. Bangiaceen (Mitth. d. Zool.

Stat. z. Neapel, 1880) ; Sohns-Laubach, Corallineen (Fauna u. Flora d. Golfes von Neapel, 1881).]

^ [Also Lemaneacese, Sirodot, Ann. des Sci. Nat. 5th ser. 1872, vol. XVI, and Bangia^

^ Rosanoff extracted the red colouring matter by cold water, and examined it accurately. In

] transmitted light it is carmine-red, in reflected reddish-yellow ,- the grains of chlorophyll also show

j this fluorescence, and when the red colouring matter (the phycoerythrine) has escaped from them in

! consequence of injury to the cells they are green ; the whole plant also remains green when the red

colouring matter has been extracted by water or destroyed by heat. (Rosanoff in Compt. Rend.

April 9, 1866.) Besides the chlorophyll-granules coloured red by phycoerythrine, Cohn found in

,■ Bornetta colourless crystalloids of an albuminous substance which are coloured a beautiful red by the

Jj colouring matter that escapes from the chlorophyll-granules when the cells are injured or killed.

] (Schultze's Arch, fiir mikr. Anat. Ill p. 24.) Cramer had previously observed crystalloids of this

'j kind in Borneiia vihich had been preserved in a solution of sodium chloride, and had accurately

H described them; according to him they are partly hexagonal, partly octahedral (Rhodospermin).

I, (Vierteljahrschr. der naturf. Ges. in Zurich, vol. VII.) Julius Klein (Flora, no. 11, 1871) found

colourless crystalloids in Griffithsia harhata and neapolitana, Gongoceras pellucidum, and Cnllithamnion

seininudiim ; and states that the red crystalloids v/hich are also found outside the cell-cavity only

, appear after treatment with sodium chloride, alcohol, or glycerine, since their colourless matrix takes

;,|'up the diffusible red colouring matter of the Florideae. On Phycoerythrine see Askenasy, Bot. Zeitg.

no. 30, 1867. [Sorby, Monthly Mic. Journ. vol. VI. 1871, p. 124. Van Tieghem has detected starch

in the Floridex, Compt. Rend. 1865.]

U

290

THALLOPIirTES.

The ylntheridia are either single cells at the end of long articulated branches, when,
as in Batrachospermum, each produces only one antherozoid, or the mother-cells of the
antherozoids are congregated together in large numbers on a common axis as the
terminal members of a very short branching-system (as in Ceramiaceae). In Nitophyllum
they densely cover certain portions of the surface of the thallus which consists of a single
layer of cells ; in the Melobesiaceae they are produced in cavities which are formed by
the overarching of the surrounding tissue. The roundish antherozoids have no cilia and
do not swarm, but are moved along passively by the water; some of them are thus
brought into contact with the trichogyne ; they adhere to it, and, in consequence of
the absorption of the cell-walls at the points of contact, their contents pass into it.
The trichogyne remains otherwise permanently closed.

According to the structure of the Carpogonium, three types may be distinguished :
(i) In the Nemalieae, to which Bat rachospermum belongs^, the entire female organ
consists, as in the Coleochaeteae, of a single cell, which is prolonged upwards into a

Fig. i98.—Ne7vah'on multifidttm ; I a branch with carpogonium c and antherozoids j/ ; //,
/// commencement of the formation of the sporocarp ; IV, V development of the cluster of
carpospores ; t the trichogyne ; c the carpogonium or sporocarp (after Thuret and Bornet).

trichogyne (Fig. i88, /, /). After fertilisation the basal portion of the carpogonium
becomes multicellular in consequence of divisions having taken place (Fig. i88, //, c).
The cells thus formed bulge outwards and give rise to a dense aggregation of short
branches {IV, F, c), the terminal segments of which are the carpospores. This simple
sporocarp acquires in Batrachospermum a loose investment by the outgrowth of pro-
longations from the cells beneath the carpogonium.

(2) In the Ceramieae, Spermothamniese, Wrangelieae, &c., the carpogonium is a
multicellular structure before fertilisation, which has arisen from the terminal cell of
a short branch. A lateral row of the cells bears the trichogyne, and is termed the tri-
chophore (Fig. 189, A,f). This structure undergoes no further developement after the
carpogonium has been fertilised. Certain other cells, however, lying in the neighbour-

^ As to Lemanea, which probably belongs to this group, see Sirodot, Ann. des Sci. Nat.
5® serie, vol. XVI. 1872. [Sirodot (Compt. Rend., 1873 and 1880) has found that the spores of
Batrachospermum produce a Chantransia from which again the Batrachospermum is developed.]

CARPOSPOREM.

291

hood of the trichophore are stimulated by the fertilisation to fresh growth, and develope
numerous closely-packed spores, each upon a short stalk (Fig. 189, B, g, h). In Lejol'isia

Fig. xi^.—Herpothamtiion hermaphroditum. A a branch with a carpogoniumyand an antheridium an.
B the mature cystocarp after fertilisation (after Nageli).

(Fig. 190) the carpogonium is also multicellular, and it is from its central cell that the
spores are developed, whilst the outer cells grow out into filaments forming a closed

FIG. \<^.—Lejolisia mediterranea (after Bornet, X about 150). A a small piece of a creeping filament with a root hair and
an upright branch, its lowest cell bearing a branchlet with tetragonidia (tt). B a sexual (monoecious) plant ; w a root-hair of
the creeping stem, its apical cell situated at s, and its upright branches bearing the sexual organs ; a a antheridia, the axial
row of cells being however not indicated ; tg trichogyne by the side of the apex t of the fertile branch ; h the envelope
of the cystocarp ; sp a spore escaped from the cystocarp ; C an empty cystocarp, its envelope consisting of rows of cells.

U 2

292 THALLOPHYTES.

investment, which subsequently opens at the apex. The trichogyne and the trichophore
can be observed lying externally to it (Fig. 190, tg). It is evident from these examples
that neither the trichogyne nor even the cells of the trichophore undergo any further
development as a consequence of fertilisation, but that it is in cells adjacint to them that
the consequences of fertilisation are manifested, in their growth, branching, division, and
final formation of spores. The formation of an investment is also a consequence of
fertilisation. The fruits of Florideae are usually termed Cystocarps.

(3) The most complicated and most extraordinary process of fertilisation was found
to occur by Thuret and Bornet in the genus Dudresnaya. Here the cystocarps are
formed upon branches other than those which bear the trichophore. After that the
long trichogyne, which is coiled at its base, has been fertilised, tubular branches spring
from beneath it, which grow towards the true fertile carpogonial branches. Each of
these latter has a spherical apical cell to which the outgrowth from the trichophore applies
itself, and at the point of contact the cell-walls become absorbed. The apical cell of the
carpogonial branch which has thus been fertilised becomes distended and filled with
protoplasm ; it becomes isolated by the formation of cell-walls, and then gives rise to the
cystocarp. These tubular outgrowths convey the fertilising eff^ect from a single
trichogyne to numerous carpogonial branches, and thus one act of fertilisation suffices
for the developement of several cystocarps on diiferent branches^.

C. The Charace^^^

The carpogonium consists of one relatively large cell and several smaller ones. The
latter are known as ' Wendungszellen,' and probably represent a very rudimentary
trichophore, the trichogyne of which is undeveloped. Fertilisation is effected by means
of filiform antherozoids which are formed in very remarkable antheridia. The carpogo-
nium is invested before fertilisation by five spirally-wound cells which arise from its
stalk-cell. As a consequence of fertilisation the large cell of the carpogonium becomes
a resting spore, producing, by its germination, a pro-embryo from which the sexual
plant springs as a lateral shoot. No gonidia are formed.

The Gharaceae are submerged aquatic plants, rooting in the ground and growing
erect, attaining a height of from tV metre to a metre, and containing abundance
of chlorophyll. They are very slender, forming stems and leaves only ^ to 2 mm.
in thickness. With an alga-like habit, they possess a delicate structure, though some-
times attaining greater firmness from the deposition of lime on their surface. They
live gregariously, mostly in crowded tufts, at the bottom of fresh-water ponds, ditches,
and streams; they may grow in deep or in shallow, in stagnant or in quickly-flowing
water ; and are either annual or perennial.

In the greater number of species, which are distributed over all quarters of the
globe, there prevails nevertheless so great a uniformity that they may all be arranged
into two genera.

' [This mode of fertilisation has been detected by Thuret and Bornet in Polyides rotwidns, also
by Berthold in Halymenia Floresia and nlvoidea, Nemastoma dichotonia and cervicornis, Grateloupia
Consentinii, Jilicina, and dichotoma, and by Sclimitz in the Squamariej.e : see Falkenberg, Die Algen,
1881.]

^ A. Braun, Ueber die Richtungsverhaltnisse der Saftstrome in den Zellen der Charen, in Monats-
berichte der Berliner Akad. der Wiss. 1852 and 1853. — Pringsheim, Ueber die nacktfussigen Vorkeime
der Charen, in Jahrb. f. wissen. Bot. 1864, vol. III. — Nageli Die Rotationsstromung der Charen, in
his Beitragen zur wissen. Bot. i860, vol. II. p. 61. — Thuret, Sur les antheridies des cryptogames,
Ann. des Sci. Nat. 1851, vol. XVI. p. 19. — Montague, Multiplication des charagnes par division,
ditto, 1852, vol. XVIII. p. 65.— Goppert u. Cohn, Ueber die Rotation in Nitella Jiexilis, Bot. Zeitg.
1849. — De Bary, Ueber die Befruchtung der Charen, Monatsber. der Berliner Akad. May 1871.
[For additional Bibliography, see Lindley, Vegetable Kingdom, 3rd edit. p. 28 : also Journal of
Botany, 1878.]

CARPOSPOREM.

293

From the carpospore of the fruit of Cbara^ the sexual leaf-forming plant is not
immediately developed, but a Pro-embryo precedes it, which attains only small dimen-
sions and consists of a single row of cells with limited apical growth. The stem of
the Leaf-bearing Sexual Plant springs from a cell which lies at some distance from
the apex of the pro-embryo and grows in a direction nearly at right angles to that of
its axis. The unlimited apical growth of the plant depends on an apical cell (Fig. 192,
C, t) from which segments are cut off by transverse septa. Each segment immediately
divides again by a transverse septum into two superposed
cells, the lower one of which {g) always grows without
further division into a long internode (frequently 5 to 6 cm.
in length) ; the upper one scarcely lengthens, but is first
divided in half by a vertical wall, and each half then
divides by further successive septa so as to form a whorl
of peripheral cells {b). From the node thus constituted
the leaves are developed, each from a peripheral cell,
and the normal lateral branches, which always originate
from the axil of the first or of the two first leaves of
the whorl. The leaves of such a whorl, from 4 to 10
in number, repeat in a modified manner the develop-
ment of the stem, but their apical growth is limited :
after the formation of a definite number of segments,
the apical cell ceases to divide and grows into the
terminal cell of the leaf which is usually pointed (Fig.
192,^, b"). From these leaves lateral leaflets may arise in
a similar manner to that in which the leaves themselves
have been formed from the stem ; and the leaflets may
again in turn produce others of a higher order. The
successive whorls of a stem alternate, and in such a
manner that the oldest leaves of the whorl, in the axils
of which the branches stand, are arranged on a spiral
line winding round the stem. Each internode also
usually undergoes a subsequent torsion in the same
direction. The lateral branches, of which in Chara one
is always developed in the axil of the oldest, in Nitella
one in the axil of each of the two oldest leaves of the
whorl, repeat the primary stem in all respects (Fig.
203). It has already been mentioned that the leaves
undergo a segmentation similar to that of the stem; they
also consist at first of very short internodes which are
afterwards greatly elongated (Fig. 192, 5, -y), and are
separated by inconspicuoi^ transverse plates or nodes.
From these the leaflets arise in whorls the members of
which are formed in succession, but they are directly
superposed one above another, and do not alternate
like the whorls of primary leaves (Fig. 193, C-E, /3).
(the basal node), by which it is united with the stem-node, and so is each leaflet
with its primary leaf. These basal nodes are the points of origin of the formation
of the cortex which, in the genus Chara, covers the internodes of the stem, but

FIG. igi.— Chara /ra£-ilis ; sp germinat-
ing- spore; i d q pi together form the pro-
embryo (pi is segmented, which is not
clearly indicated in the drawing) ; at d are
the rhizoids iu ; iv' the so-called primary
root ; s the first leaves (not a whorl) of the
second generation or leaf-bearing plant
(after Pringsheim, X about 4).

Each leaf begins with a node

^ This has not yet been observed in Nitella. [See De Bary, Zur Keimungsgeschichte der
Charen, Bot. Zeitg, 1875. The carpospore is first divided by a wall at right angles to its long axis
into a small upper and a large lower cell. The upper cell is then divided by a wall at right angles
to the first into two equal cells : from one of these the pro-embryo is developed, from the other the
• primary root.']

294

THALLOPHVTES.

is wanting in Nitella. From the basal node of each leaf one distinct cortical lobe
runs downwards, and one upwards^ (Fig. 192, r, r, r", and Fig. 194). In the
middle of each internode therefore as many descending cortical lobes as there are

Fig. 192.— Longitudinal section througli a bud of Chara fragilis ; in A the contents of the cells have been left out ; in
B the fine-grained substance is protoplasm, the larger granules are chlorophyll ; the formation of vacuoles is shown ; in C the
contents of the cells have been contracted by iodine solution (x 500).

leaves in the whorl meet with the cortical lobes that ascend from the whorl next
below. The number of the latter is, however, smaller, because the leaf in the axil

A"^

Fig. 193.— Leaves of Chara frag ill's; a terminal cell, * penultimate cell of a leaf ; z internodal cell ; 7u cells of the leaf-
node ; y" mother-cell of a leaflet and of its basal node : from it arise tj and tc (the uniting celt), br the basal node which
produces four simple cortical lobes and j8 the leaflet. A and C in longitudinal section, B an entire young leaf, external
view, with the ' stipule' j and its descending cortical lobe sr ; D external view of the middle part of an older leaf, though
still young ; E transverse section of a leaf-node, of the same age as D.

of which the lateral branch arises does not form an ascending lobe. The cortical
lobes are in close contact laterally, and form a closed envelope round the inter-

"^ The first internode of every branch and leaf becomes covered with a cortex derived only from
the descending cortical lobes of the next node above.

CARPOSPORE^.

295

node, the ascending and descending lobes dove-tailing in a prosenchymatous manner.

The formation of the cortex takes place so early that the elongating internode is

;overed by it from the first, the lobes keeping pace with its extension in length and

lickness. Each lobe continues to grow, like the stem, by means of an apical cell, which

)ecomes segmented by transverse septa ; out of each of the segments cortical internodal

md nodal cells are formed by repeated divisions. The latter divide, by successive septa,

[into an inner cell (Fig. 194, Z), c), in contact with the internode of the stem, and three

)uter cells, the middle one (/) of which commonly grows into the form of a spine or

:nob, resembling a leaf. The outer lateral cells {n n) of the cortical node, on the other

land, following the elongation of the internode itself, grow into longer tubes, so that

;ach cortical lobe consists of three parallel rows of cells, the middle row however

:ontaining alternately short and long (internodal and nodal) cells. The cortex of the

leaves is derived from the leaflets, and its formation is much simpler (Fig. 193, C-E, br).

'rom the basal nodes of Chara other foliar structures also arise, both on the inner and

)uter side of the base of the leaf (Fig. 192, 6), which Braun calls Stipules; they are

ilways unicellular, and are sometimes very short, sometimes elongated.

The nodes are the parts from which all the lateral members of the Characeae
)riginate. The root-like structures or Rhi%oids spring from the outer cells of the

Fig. 194. — Development of the cortex of the stem of Chara fragilis ; A a very young internode of the stem with the
cortical lobes r still consisting of one cell ; R—D its further development ; r r signifies in all the figures the cortical lobes
that ascend from the lower, r' r' those that descend from the upper leaves ; v v the apical cell of each cortical lobe ; ^^ its
internodal cells ; n nt n the commencement of the formation of the node ; D c the central cell of a cortical node ; .S" signifies
in all the figures the unicellular ' stipules' which spring in pairs from the base of the leaves.

)wer nodes of the primary shoot, and consist of long hyaline tubes growing obliquely

lownwards, and elongating only at their apex. They are formed by the outgrowth

)f flat cells at the circumference of the node, and are therefore attached to it by

broad base; but the bases of the stouter rhizoids themselves divide still further,

[giving rise, especially at the upper margin, to small flat cells from which slender

jrhizoids are developed. The rhizoids are segmented by only a few septa which lie

[far behind the growing apex, and have at first an oblique position. The two ad-

f joining cells abut upon one another like two human feet placed sole to sole. The

(branching always proceeds only from the lower end of the upper cell (Fig. 195, B)\

|a swelling is here formed which becomes cut off by a wall, and by further division

)roduces several cells which grow into branches; these therefore stand on one side

[like a tuft. The tubular cells composing the rhizoids attain a length of from several

lillimetres to more than two centimetres, with a thickness of from tV to tV mm.

The Vegeiati've Reproduction of Characeae always proceeds at the nodes, and has
three modifications: — (i) Tuberous formations called Bulbils (starch-stars) which occur
in Chara stelligera. They are isolated underground nodes with greatly abbreviated

2(^6

THALLOPHVTES.

whorls of leaves of beautiful regularity; their cells are densely filled with starch
and other formative materials ; new plants are produced from shoots laterally
developed. (2) The Branches ficith naked base of Pringsheim. These are formed
on old nodes which have survived the winter or on cut nodes of Chara in the
axils not only of the oldest but also of the younger leaves of a whorl, and are in
fact only slightly different from the normal branches, the greatest difference being in
the partial or entire absence of a cortex on the lower internode and on the first
whorl of leaves. The cortical lobes which descend from the first node of the branch

Fig. 193.— Rhizoids of Chara fraf^ilis ; A end
in process of development ; B a 'joint,' the lower
part of the upper cell is branching (after Pring-
sheim, X 240). The arrows indicate the direction
of the currents of protoplasm.

Fig. igd.— Chara fragilis, A an entire pro-embryonic branch;
I the lowermost colourless cell below the root-node; rf root-produc-
ing leafless node ; q the long cell proceeding from the middle cell of
the bud-rudiment ; pt apex of the pro-embryo ; g the pseudo-whorl
of leaves, "v the bud of the leaf-bearing plant ; B upper part of
a young pro-embryonic branch ; i, d, q as before, b apex of the
pro-embryo ; /, //, /// the young leaflets of the transitional node,
V the bud of the leafy stem ; C still younger pro-embryonic branch ;
i, d, q, b as before ; /, //, /// the cells out of which the transi-
tional nodes arise, v apical cell of the stem-bud (after Pringsheim,
B X 170).

often become detached from the internode and grow free, curling upwards, while the
leaves of the lowermost whorl often do not form nodes. (3) The Pro-embryonic
Branches. These spring, together with the last, from the nodes of the stem, but
are essentially different from the branches, and have a similar structure to the pro-
embryos which proceed from the spores. Like the last, they have only been observed
in Chara fragilis (by Pringsheim). A cell of the node protrudes and grows into a
tube, and its apex becomes separated by a septum. In this growing terminal cell

CARPOSPORE.E.

297

further divisions take place, till the ' apex of the pro-embryo ' which proceeds from
kt consists of a row of from three to six cells. Beneath the apex of the pro-embryo
|(Fig. 196, C, a, b) the tube swells, and the distended part becomes separated by
fa septum as a cell, which Pringsheim calls the * bud-rudiment,' (Fig. 196, C, including
the parts from "v to d). This cell is now divided by two oblique walls into three
;ells, the middle one of which (q) lengthens into a tube (like an internode), while the
ipper and lower ones remain short. Out of the lower cell is afterwards formed a
'oot-producing leafless node (Fig. 196. d, and Fig. 191, d), while the upper one, which
[lies between the apex of the pro-embryo a b and the elongated cell q becomes the
[axis of the new generation. It becomes arched on one side outwards, and divides in
succession into the cells J, //, ///, and ij. Each of the cells /, //, and /// becomes
f transformed by divisions into a disc of cells or transitional node, three of which thus
i stand over one another with-
[out intermediate internodes.
JTheir lateral cells grow right
ind left, and form imperfect
leaves of different lengths. The
:ell which lies outermost (Fig.
[96, C, n}) now begins to un-
lergo a series of divisions,
:orresponding to those of a
normal leaf-bearing shoot. It
is, in fact, the apical cell of
the sexual leaf-bearing plant
which arises from the pro-
embryo. The displacement
indicated in Fig. 196, C, sub-
sequently causes the apex of
the pro-embryo to be pushed
to one side; and since this
apex has the appearance of a
simple leaf uncovered by cor-
tex, the further development
of the lateral leaves which
spring from the cells /, //,
and 7J/, brings about an ap-
pearance as if these different
leaves together formed a whorl ;
and the bud of the lateral
shoot thus comes to stand
apparently in the centre of
this pseudo-whorl (Fig. 196,
A), If the structure which

springs from the germinating spore is now compared with the pro-embryonic branch,
the perfect homology cannot fail to be observed which Pringsheim pointed out in
the parts that will be found indicated by the same letters in Figs. 191 and 196;
but the pro-embryo of the spore has in addition a small node at the opening of the
spore from which a rhizoid, sometimes called the primary root of Chara, springs
(Fig. 191, w').

The Antheridia and Carpogonia are always borne by the leaves. An antheridium
is in all cases the metamorphosed terminal segment of a leaf or of a leaflet, and the
carpogonium, in the monoecious species, arises close beside the antheridium from the
basal node of the same leaflet [Chara) or from the last node of the leaf bearing a
terminal antheridium {Nitella) ; hence in the monoecious Niteliae the carpogonia are

Fig. \()-].— Chara fragilts. A middle part of a leaf b with an antlicridium
a and a carpogonium 5, c its crown ; |3 sterile lateral leaflet ; |3' large leaflets
by the side of the carpogonium ; ^" the bracteoles, spinging from the basal
node of the carpogonium ( X about 50). B a young antheridium a with a still
younger carpogonium SK; w the nodal cell of the leaf, u the uniting-cell
between it and the basal node of the antheridium ; / cavity of the internode
of the leaf; br cortical cells of the leaf ( X 350) {cf. Fig. 203).

298

THALLOPHVTES.

placed below, in the Charx above or by the side of the antheridium. In the dioecious
species of course no such relation of position can exist, but the morphological signi-
ficance of these organs and the place of their origin are the same. We will now
consider the structure of these organs when they are fully developed.

The Antheridia (Globules) are globular bodies ^ to i mm. in diameter, at first green,
then red. The wall consists of eight flat cells, four of which, situated around the distal
pole of the ball, are triangular, while the four situated around the base are quad-
rangular and become narrower below ; each of these cells forms a segment of the
shell of the ball, and are called Shields. When unripe their inner cell-wall is covered
with green chlorophyll-granules, which, in the ripe state, are of a red colour. Since

FIG. x^.—Nitella flexilis. A fertile branch (natural
size) ; i internode, b leaves ; B upper part of a fertile leaf
b with the node K ; on the node are two lateral leaves n b,
and two very younjj carpogonia S; a an antheridium ;
C older leaf with two leaflets, a ripe antheridium a, and
two unripe carpogonia S; Da half-ripe carpogonium more
strongly magnified.

Fig. 198. — Xitella Jlexilis. A an almost ripe antheridium at the end of the primai-y leaf, by it.s side two lateral leaflets, i
neutral lines ; the arrows indicate the direction of the currents of protoplasm ; B a manubrium with its capitulum, secondary capitula,
and the whip-shaped filaments, in which the antherozoids arise ; C end of one of the young filaments ; D middle part of an older
one ; E of one still older : F ripe antheridial filament with antherozoids G (C—G X 550).

the outer wall is destitute of these granules, the outside of the ball appears clear
and transparent (Fig. 197, A). From the lateral walls several folds of the cell-wall
penetrate towards the middle of each shield, which gives them the appearance of being
lobed in a radiate manner. From the middle of the inner face of each shield a cylin-
drical cell projects inwards, nearly to the centre of the hollow globule; this cell
is called the Manubrium; at the central end of each of the eight manubria is a
roundish hyaline cell, the Capitulum. The flask-shaped cell which supports the antheri-
dium also penetrates into the interior between the four lower shields ; and these twenty-
five cells form the framework of the antheridium. Each capitulum bears usually
six smaller cells {secondary capitula), and from each of these grow four long slender

CARPOSPOREM.

299

whip-shaped filaments, which, being coiled round and round, fill up the interior of the
globule (Fig. 198, B). Each of these filaments (the number of which amounts to about
200) consists of a row of small disc-shaped cells (Fig. 198, D, E, F), numbering from
100 to 200. In each of these 20,000 to 40,000 cells is formed an antherozoid, a slender
spiral thread, thickened behind, and bearing at its pointed end two long fine cilia
(Fig. 198, G). When perfectly ripe, the eight shields fall apart, their spherical
curvature becoming diminished; the antherozoids leave their mother-cells and move
about in the water. This breaking up appears generally to happen in the morning,
and the antherozoids are in motion for some hours, till evening.

The mature Carpogonium (Nucule), when ready for fertilisation, is a longer or
shorter prolate spheroid ; it is placed upon a short pedicel, visible externally only in
Nitella, and consists of an axial row of cells, closely surrounded by five tubular cells
which are coiled round it spirally. The whole must be considered as a metamorphosed
branch. The lowest cell corresponds to the lower internode of a branch ; it bears a
short central nodal cell, around which the five enveloping tubes spring like a whorl of
leaves. Above the nodal cell rises the peculiarly developed apical cell of the branch, very
large as compared to the other parts, and ovoid. At its base, immediately above the
nodal cell, an inconspicuous hyaline cell is separated at an early stage in Cbara; in Nitella
a somewhat disc-shaped group of similar cells takes its place, which have been termed
by Braun ' Wendungszellen,' and which I consider as forming a very rudimentary
trichophore. The large apical cell of the carpogonium is filled with a number of
drops of oil and grains of starch as well as with protoplasm ; it contains pure hyaline
protoplasm only in its apical region (the apical papilla). The enveloping tubes, which
contain a quantity of chlorophyll, project above the apical papilla and bear the Cronvn,
consisting in Cbara of five larger, in Nitella of five pairs of smaller cells, which have
already been separated at an early stage from the enveloping tubes by septa. Above the
apical papilla and beneath the crown, which forms a compact lid, the five enveloping
tubes form the neck which encloses a narrow cavity, the apical cavity; above the
papilla this cavity is of an obconical figure narrowing upwards, the five segments
of the neck projecting and forming a kind of diaphragm, through the central very
narrow opening of which the union with the upper roomy part of the apical cavity
is effected. This is closed above by the crown ; but, at the time of fertilisation, it opens
externally by five clefts between the coronal cells ; and through these clefts the anther-
ozoids penetrate into the apical space filled with hyaline mucilage, to find their way into
the apical papilla of the oosphere, where the cell-wall is apparently absent. After
fertilisation the chlorophyll-granules of the envelope become reddish-yellow, the wall of
the tubes which lie next the oosphere increases in thickness, becomes lignified, and
assumes a black colour ; and thus the oosphere, now transformed into a carpospore,
becomes surrounded by a hard black shell with which it falls off, to germinate in the
next autumn or spring.

With regard to the various processes of development, I will here describe only those
of the antheridia and carpogonia.

Antheridia. The order of development of the cells has already been exhaustively
described by A. Braun in the case of 'Nitella syncarpa and Chara Baueri ; it agrees with
that of Nitella Jlexilis and Chara fragilis. In Nitella the terminal cell of the leaf becomes
the antheridium ; the oldest leaf of a whorl first forms its antheridium, the others follow
according to their age ; the antheridia are recognisable even in the earliest state of
the whorl of leaves. In Fig. 200, A, is shown a longitudinal section through the apex
of a branch, t being its apical cell ; its last-formed segment has already been divided
by a septum into a nodal mother-cell AT and an internodal cell lying beneath it; beneath
this lies the node with the last whorl of leaves ; h is its youngest leaf, bK the basal node
of the oldest leaf which already consists of the segments 7, //, 77/ ; a is the terminal cell
of this leaf which is becoming transformed into the antheridium. While the antheridium
is becoming developed, the leaf also undergoes still further changes which must be first

300

THALLOPHYTES.

considered. The segment /// becomes the first internode of the leaf, II becomes a node
from which are developed the lateral leaflets nb'inC and D. The cell / divides into two

Fig. 200.— Development of the antheridia oi Nitellajlexilis. In B, C, and D the protoplasm has been
contracted by glycerine.

(C, I), the lower of which remains short, while the upper grows into a flask-shaped cell
(Fig. 200, D,f, and Fig. 201,/).

The globular mother-cell of the antheridium (Fig. 200, ^, a) first of all divides into
two hemispheres by a vertical wall passing through the axis of the leaf ; each of these
is divided into two segments by a vertical wall at right angles to the first ; in each of the

four quadrants a third division takes place hori-
zontally and at right angles to the two last walls ;
and the antheridium now consists of four lower
and four upper octants of a sphere. Contrac-
tion by glycerine clearly shows that each of
these divisions of the protoplasmic body is com-
pletely effected before the appearance of the
cellulose-wall (Fig. 200, B) ; the second division
even takes place before the wall has arisen
between the two first-formed halves; and the
four quadrants may be made to contract without
any wall being visible between them. In Fig.
200, 5, the third division has also taken place,
the second vertical wall is already formed, and
the two quadrants there visible are already
divided ; but no horizontal wall has yet appeared.
In Fig. 200, A, a, are shown the eight octants
in perspective together with their nuclei. Each
octant now breaks up first of all into an outer
and an inner cell (Fig. 200, C) ; the latter is
again divided in all the eight octants (D), so
that each octant now consists of an inner,
a middle, and an outer cell {D, i, m, e). Up
to this time the globe remains solid, and all
the cells lie close to one another ; but now
commences an unequal growth, and with this
the formation of intercellular spaces (Fig. 201). The eight outer cells {e) are the young
shields, the side-walls of which show even at an earlier period the radial infolding
already mentioned; they grow more strongly in a tangential direction than the inner

Fig. 201.— Antheridium of Nittllaflexilis in a further
stage of development (x about 500).

CARPOSPOREM.

3OT

cells, the outside of the globe increasing more rapidly than the inside : the middle cells
(m), which form the manubria, remain attached to the centres of the shields, but are
separated from one another by the tangential growth of the shields; they grow slowly in
the radial direction : the innermost cell i of each octant is rounded off and becomes the
capitulum.

The cellyin Fig. 200, D, now also grows quickly, and forces itself between the four
lower shields into the interior of the globe ; it becomes the flask-shaped cell, upon the
apex of which rest the eight capitula. In Fig. 201 this condition of the antheridium
is shown in longitudinal section; here the walls of the capitula bound the intercellular
spaces which have now been formed and are filled with fluid ; they put out branches (c)
which become septate, and again ramify ; and these branches elongate by apical growth
and also become septate. Their ^asal cells swell up into a roundish shape, and form
the secondary capitula, upon which stand the whip-shaped filaments, consisting of the
discoid cells which are the mother-cells of the antherozoids. (Compare Fig. 201 with
Fig. 198, B.)

The antheridia of Chara fragilis are produced by metamorphosis of those leaflets
which form the innermost row on a leaf, and in fact, as is shown in Fig. 203, the
development advances downwards on the primary leaf. The succession of cells and the
mode of growth show no noteworthy differences from those of Nitella ; the flask-shaped
pedicel is here placed on a small cell wedged in between the cortical cells, which is the
central cell of the basal node of the leaflet : Braun asserts that this cell is present also in
sterile leaves, but I have not succeeded in finding it.

Antherozoids. The whip-shaped filaments in which the antherozoids arise do not
grow merely at their apex, but have also an intercalary growth. This is shown by the
elongated cells in the middle of young filaments, each with two nuclei, between which no
division-wall has yet been formed (Fig. 198, C). The longer the filaments become, the
more numerous are their divisions, until at length the individual cells have the appearance
of rather narrow transverse discs. The further development of the contents of these
mother-cells of the antherozoids progresses backwards from the end of the filament ;
the antherozoids are formed in basipetal order in each filament. At first the nucleus
of each mother-cell lies in its centre, later it places itself in contact with one septum ;
the nucleus then disappears, and its substance becomes mixed with that of the pro-
toplasm, which now forms a central discoid mass in the mother-cell, surrounded by
a hyaline fluid (Fig, 198, E). From this is formed the antherozoid, and, when it is
mature, there is no granular protoplasm left over in the cell ^. The antherozoids begin
|to rotate even while within their cefl, and escape out of it after the rupture of the
^antheridium ; the filiform antherozoid has in Nitella 2 or 3, in Chara 3 or 4 coils; the
^posterior thicker end contains a few glistening granules.

The De'velopment of the Carpogonium has already been described in detail by A. Braun ;
[I have also studied it in Nitella ft enilis and Chara fragilis. In Nitella fexilis it springs
from the node of the leaf beneath the antheridium (Fig. 199, B and C) ; its origin is
.much later than that of the latter. Fig. 202, J, represents a very young carpogonium ;
^the lowest cell of the pedicel [b] bears the small nodal cell with the five rudiments of
Ithe enveloping tubes {h), (two only are shown here in longitudinal section). Above the
jnodal cell lies the apical cell (j) of the branch, for such is the nature of the carpo-
^gonium. B represents a further stage of development, in which the first of the cells,
[designated by A. Braun the ' Wendungszellen,' has already made its appearance, and
■two septa have also appeared in the upper part of each enveloping tube; these
[upper short cells are raised up by the intercalary growth of the tubes, above the apical
11, and form the crown ^ in C and D. The lowest of the cells of the crown each
, forms a prolongation projecting inwards and downwards, as shown in Fig. 202 C and
JD, so that the whole carpogonium resembles a ' lobster-pot.' The spiral torsion of

' Compare the opposite view of Schacht, Die Spermatozoiden im Pflanzenreich, 1864, p, 30.
[The account given ia the text is confirmed by Strasburger, Zellbildung u, Zelltheilung, 1880.]

3oa

THALLOPHVTES.

the enveloping tubes does not begin till a later period; the coils become gradually
flatter while the apical cell of the branch increases considerably in size and developes
into the oosphere (Fig. 199). The development and fertilisation of the carpogonium
of Chara has recently been described in detail by De Bary in the case of C. f(ttida.
Here also it consists, from an early stage of its development, of an axial row of three
cells, and five others consisting each of two cells which form an envelope round it.
The lowermost cell of the axial row is the nodal cell, the second remains small and
colourless, and corresponds to the first * Wendungszelle * in Nitella. It becomes in this
case also, as De Bary's drawings show, separated by a somewhat oblique septum at
the base of the apical cell (now the third of the axial row). Originally almost hemi-
spherical, the apical cell grows first of all into the form of a narrow cylinder, and
then becomes ovoid ; it is provided, until it attains its full size, with a thin very
delicate cell-wall. Drops of fat and starch grains accumulate in its protoplasm : its
apex however remains free from these, and forms a transparent finely granular terminal
papilla, the receptive portion. The protoplasm of the apical cell of the carpogonium
has therefore become transformed into an oosphere. The five enveloping tubes are
from the first in close contact with the apical cell ; after each has become divided by

Fig. 202. Development of the carpogonium of Nitella Jlexihs (x about 300) ;
X ' Wendungszellen.'

a septum about half way up, the uppermost of the cells thus separated also become
closely united with one another above the oosphere. This closing of the envelope
takes place at least in Cbara fcetida, before the ' Wendungszelle ' has separated from
the oosphere. The five upper cells of the envelope are at first as long as the five
lower ones, and the septa which separate them lie about half way up the oosphere.
As it now increases in size, the five lower ones become elongated into long tubes, which
are at first straight but afterwards wind spirally round the oosphere. The five upper
cells form the crown, which is elevated some distance above the apex of the oosphere.
Between the crown and the apex of the oosphere the enveloping tubes grow inwards
and increase in breadth, so that together they form, above the apical papilla of the
oosphere, a thick diaphragm open only in the middle, by which a narrow space lying
below the crown is separated from a still narrower one above the oosphere. The cells
of the crown form a closed cover above the upper space ; the upper and under space are
united through the narrow opening in the diaphragm. De Bary found a similar struc-
ture in Nitella. As soon as the carpogonium attains its full size, the small space above
the diaphragm enlarges, while the tubes between the diaphragm and the crown grow
longer. This part of the envelope, which only attains its full size at a late period,
De Bary calls the Neck. The tubes now separate laterally from one another, forming
five clefts below the crown and above the diaphragm. Through these clefts the

CARPOSPOREjE.

3^3

antherozoids force their way in great numbers into the apical space, which is filled
with a hyaline mucilage. That one or more of them even find their way into the
oosphere is rendered the more certain by the fact that .about this time its papilla
is protected by a very weak cell-wall or has none at all, as is shown by the small
pressure required to expel its contents into the apical space.

A. Braun's account of the morphological value of the carpogonium of Chara is fully
confirmed by our Fig. 203, ^. It is necessary to explain, in the first place, that this
is the lower part of a young fertile leaf of Chara fragilis, together with the contiguous
piece of the stem and an axillary bud, represented in longitudinal section ; m is half of
the nodal cell of the stem, i its upper, i' its lower internode, sr a descending,^ an ascend-
ing cortical lobe ; j/ the cortical lobe of the lower internode which descends from the

Fig. ■20-^.— Chara fragilis, A lower pare of a fertile leaf, a lateral bud springfing from its axil ; B lower part of a sterile
leaf without an axillary shoot (in longitudinal section).

leaf, rK a node of it ; /'" the first internode of the axillary bud which rests upon the
cell n that unites the nodal cell m with the basal node of the leaf. The leaf shows its
three lower internodes, 2, 2, 2, still rather short ; they eventually attain from 6 to 8
times this length. Between them are the nodal cells <ii;, w ; 1;, 1; are the cells which
unite the leaf-node with the basal node of the leaflets j3 on the outer side of the leaf ; a
the corresponding cells on the inner side of the leaf ; br the cortical lobes of the leaf,
two of which go upwards and two downwards from each leaflet /3; the lowermost
internode of the leaf is however covered only by descending lobes ; by the side of one
of them stands the stipule s\ x, x are the cortical lobes which descend on the inside
of the internodes of the leaf, where the leaflets are transformed into antheridia, «, a ; the
ascending cortical lobes of the leaf are here absent, because one carpogonium always

304

THALLOPHFTES,

springs from the basal node of each leaflet. (Compare with this Fig. 197, yl and B.) In
reference to the origin of the carpogonium, A. Braun says (/.c. p. 69) that just as a branch
springs from the basal node of a leaf, so does the carpogonium from that of a leaflet (in
Cbara fragilis from the basal node of an antheridium which stands in the place of a
leaflet). As in the leaf which subtends a branch the ascending cortical lobes are wanting,
so also in the leaflet which bears the carpogonium the cells forming the ascending
portion of the cortex are also wanting. As it is the first leaf of the whorl on the stem
that produces a branch in its axis, so it is also from the first (inner) leaflet of the whorl
on the leaf that the carpogonium originates. The basal node of the antheridium in
C. fragilis has, according to A. Braun, not four peripheral cells, as in sterile leaflets, but
five ; an upper odd one which is first formed, two lateral ones which follow, and two
lower ones which are formed last of all. Of these five cells only the two lower ones are
developed into cells which form the cortex (of the leaves), the upper one, wanting in the
sterile basal nodes, is the mother-cell of the carpogonium ; but the two lateral ones
are developed into leaflets which stand laterally between the antheridium and carpogo-
nium {cf. Fig. 197, jS") ; these latter Braun calls Bracteoles. The mother-cell of the
carpogonium now grows out of the axil of the antheridium, and divides itself by a
septum into an upper outer terminal cell and a segment which in its turn is broken
up into two discs by a wall parallel to the previous one (Fig. 203, A, SK). The
lower cell does not divide any further, it forms the concealed pedicel of the car-
pogonium, and corresponds to the first internode of a branch ; but the upper one has
the character of a nodal cell ; it is divided by tangential walls into a zone of five outer
and one inner cell [SK') ; the former are the rudiments of the enveloping tubes, which
are therefore morphologically leaves.

The Gharaceae are distinguished by the size of their cells, and by the simple relations
of the individual cells to the structure of the whole body. All the young cells contain
nuclei, which at first always lie in the centre of the protoplasm that fills the whole cell ;
each bipartition of a cell is preceded by that of the nucleus and the formation of two
new nuclei. As the cells grow, vacuoles form in the protoplasm which finally coalesce
into a single large vacuole (the sap-cavity). The protoplasm, now clothing the wall as
a thick layer, commences its rotatory motion which always follows the longest direc-
tion of the cell. Chlorophyll-granules are now formed : with the growth of the whole
cell they also grow and multiply by repeated bipartition; they adhere to the inner
side of the outermost thin stationary layer of protoplasm, and take no part in the
rotation of the layers which lie further inwards ; the nucleus, which is elongated and
granular, also lies in this layer \ The rotating protoplasm becomes differentiated, as
the ceil grows, into a portion which is very watery and others which are denser,
the former looks like hyaline cell-sap in which the latter float in the form of roundish
larger or smaller lumps. Since these denser masses are passively swept along by the
rotating clear protoplasm, as may be seen from their tumbling over one another,
the appearance is presented as if the cell-sap caused the rotating motion. Together
with the denser masses of protoplasm of less regular form, there are also a number of
bodies of globular shape covered with delicate spines, consisting also of protoplasm.
The current, as Nageli has shown, is most rapid next the stationary parietal layer, and
becomes gradually slower towards the interior; hence the spheres and globules which
swim in the thin rotating protoplasm tumble over one another, because they become
immersed at different spots in layers of different rapidity. Dependent on the direction
of the current, the chlorophyll-granules are arranged in longitudinal rows on the stationary
layer, and are deposited so thickly that they form a stratum ; they are absent only at the
neutral lines (Fig. 198, J, i). These neutral lines mark the position where the ascending

* [In old internodal cells there are numerous nuclei which have been produced from the primary
nucleus by division and ' fragmenlation.' See Schmitz, Sitzber. d. niederrhein. Ges. in Bonn, 1879;
Unters. ueb. die Zellkerne der Thallophytcn. Strasburger, loc. cit. Johow, Bot. Ztitg. 1881.]

CARPOSPOREJE. 305

and descending portions of the rotating protoplasm of a cell run side by side in opposite
directions and neutralise each other, and where therefore there is no motion. The
direction of the rotatory motion in each cell stands in a regular relation to that of all the
other cells of the plant, and hence to its morphological structure, as has been shown by
A. Braun.

FORMS WITHOUT CHLOROPHYLL.

To this second principal group of the class Carposporese belong all those plants,
the fructifications of which have long been known as Fungi or Mushrooms, but
which are now known under the names of Ascomycetes, ^cidiomycetes, and
Basidiomycetes. As a matter of fact a process of sexual reproduction has been
actually observed as yet in only a few out of the very numerous genera of these
plants, and these all belong to a single sub-division, namely that of the Ascomycetes.
Among the Basidiomycetes mere traces of such a process have as yet been detected,
and among the ^cidiomycetes not even these have been observed. Nevertheless it
is permissible to assume, until further information is obtained, not only that such
a process actually takes place, but also that it agrees in its principal points with that
observed in the Ascomycetes ; at any rate such an assumption seems to be imme-
diately suggested by the very similar course of development which obtains in these
I three groups ^ We are, in fact, somewhat in the same position with respect to the
sexual reproduction of tl^se plants as were the botanists of the last century with
respect to that of Phanerogams ; they had observed the process of fertilisation in
a few cases only, but they did not hesitate to assert, arguing from analogy based
jupon the similarity of the parts of the flowers and of the fruits developed therefrom,
the sexuality of all Phanerogams.

Accepting the facts established by Tulasne, De Bary, and their followers with
reference to the Ascomycetes, we may describe the life-history of one of these Fungi
as follows. From the true spore (carpospore) there is developed a vegetative body,
the mycelium, consisting of much-branched multicellular filaments, the hyphae,
which covers the surface or penetrates into the interior of the substratum upon
which it grows ; it has often but a short term of existence, but it may occasionally
continue to grow, as it appears, for years. In many cases this mycelium is capable
of producing non-sexual reproductive cells which nearly always occur as conidia, and
are usually developed upon special branches of the mycelium, the conidiophores, in
large numbers by abstriction. These conidia, which correspond to the zoogonidia
of Algae and to the tetragonidia of the Florideae, produce new mycelia on germina-
tion. Usually a mycelium reproduces itself thus asexually for many generations,
and consequently many Fungi are only known in this stage of their life-history. In
all cases, however, in which the life-history of a Fungus has been continuously traced,
it has been observed that, under certain favourable conditions, the mycelium finally

I

^ [From the researches of Brefeld (Unters. ueb. Schimmelpilze, III, IV) it appears that there is
no ground for assuming the existence of sexual reproduction in the Basidiomycetes. He is of opinion
also that no sexual process takes place in the Ascomycetes, for, though they still possess sexual
organs, these organs (except in Lichens) seem to have lost their function.]

x

3o6 THALLOPHVTES.

developes sexual organs, and that, as the result of fertilisation, a structure is produced
which is quite different from the mycelium, a fructification, which, except in the
simplest forms of these Fungi, consists of an aggregation of numerous hyphae, and
which presents great varieties of form. These fructifications are in many species
small in proportion to the mycelium, and appear to be merely fruits developed
upon it ; in other cases, however, they continue to grow vigorously for some time,
and attain a considerable size, obtaining their nourishment independently. Under
these circumstances they appear to be independent plants, or, according to our
modes of expression, to be alternate generations destined to produce true spores
in usually very large numbers. The spores thus formed within a fructification (car-
pospores) are in these cases, as also among the Algse, extremely different if not in
their size at any rate in their form and other properties, from the conidia produced
asexually on the mycelium. When the fructification is of considerable size it is
commonly regarded as being the whole Fungus, just as a Horse-tail or a Fern
is thought to be the entire plant, although the insignificant prothallium is an essen-
tial phase of the life-cycle of each of the latter. The mycelium, like the prothallium,
is only the first stage of development, or, as it may be termed, the sexual generation
(oophore), whilst the fructification corresponds to the fully-developed Horse-tail or
Fern (sporophore). In those cases in which the fructification remains comparatively
small and is nourished'by the mycelium until iriaturity, a considerable similarity of
habit becomes apparent between the Fungus and a Moss, for the sexually produced
fructification of a Moss also derives its nourishment from the vegetative body of
the first generation.

Like that of the Florideae, the Characeae, and the Coleochaeteje, the fructification
of a Fungus consists of two essentially distinct parts, namely, of a sterile portion,
which is usually relatively large, and in some of the larger fructifications is by far the
larger, and of a fertile portion in which, sooner or later, spores are formed. In the
simpler forms the sterile tissue is merely an investing membrane which surrounds
the spore-producing portion, but in larger and more complex fructifications, like those
of Penicillium and Tuber, the sterile tissue is a compact mass into which the hyphae
which are to produce the spores penetrate, and within which they obtain nourish-
ment and further ramify. A still ilfgher degree of independence is attained by the
sterile portion when the fertile hyphae contained within it do not immediately give
rise to spores but undergo a period of inactivity. Under these circumstances the
fructification is, during the period of rest which may extend over weeks or months,
simply a mass of tissue, which undergoes further development only when, under
favourable circumstances, the contained fertile hyphae produce spores. An inactive
fructification of this nature is termed, at the suggestion of Brefeld who carefully
studied these phenomena in Penicillium, a sclerotiuin.

When the formation of spores commences in the fructification, the fertile hyphae
may either grow towards the exterior and form the spores at the surface, when the
fructification is said to be gymnocarpous, or they form the spores quite in the interior
of the mass of sterile tissue, the outer layer of which then usually constitutes a firm
cortex, the peridium, when the fructification is said to be angiocarpous. When
numerous fertile hyphae form a coherent layer on the surface of the fructification,
such a layer is termed a hymenimn ; if, however, there is developed within the peridium

CARPOSPOREM. 307

of an angiocarpous fructification a peculiarly formed mass of tissue in which very
large numbers of spores are produced, such a mass is termed a nucleus or gleba.

The sexual organs, so far as they are known among the Ascomycetes, consist
of a carpogonium as the female, and of a so-called pollinodium (antheridium) as the
male. The two organs may differ but little in size and shape, and in Gymnoascus
they are quite similar ; more commonly, however, the carpogonium is larger and is
multicellular, whereas the pollinodium is a thin, usually branched, tubular cell. In all
cases the carpogonium differs from the pollinodium in that from it alone the fertile
hyphse take origin from which the spores are finally developed, the sterile tissue
being derived from the hyphse bearing the carpogonium or even from neighbouring
cells.

Fertilisation is never affected by means of antherozoids ^ but by the close appli-
cation of the pollinodium in its whole length to the carpogonium, or merely of the
apex of the pollinodium to the anterior portion of the carpogonium. Occasionally
this portion of the carpogonium is prolonged into a narrow tube like the trichogyne
of Nemalion. In the majority of the observed cases there is no direct exchange of
protoplasmic substance between the pollinodium and the carpogonium in the process
of fertilisation : the two organs remain closed and the fertilising matter passes from
the pollinodium into the carpogonium apparently by diffusion. The fertile hyphse
of the fructification arise usually not from that part of the carpogonium which has
been in direct contact with the pollinodium, but from nearer its base. Here again an
analogy with the formation of the fructification of Florideae presents itself.

Histologically considered, the mycelium and the fructification of these Fungi
consist of hyphae. The hypha? are multicellular, usually much-branched, filaments
which grow at their apices, and are generally very long and thin. In many cases,
even in the larger fructifications, as for instance in the Mushrooms, it is easy to
make out the individual hyphse, but in other cases, although the tissue really consists
of hyphae, a so-called pseudo-parenchyma is formed by the close aggregation of their
short thick segments. In angiocarpous Fungi (Tuberaceae, Gasteromycetes) this
pseudo-parenchyma is usually differentiated into well-defined concentric layers.
Commonly, but not universally, the cell-walls of the pseudo-parenchyma are not
coloured blue when treated either with iodine alone, or with iodine and sulphuric
acid, but in certain cases this colouration is produced by the action of iodine alone
(Asci of Lichens). Starch, as also chlorophyll, is absent from all Fungi, and this is
the more remarkable since starch is found in Phanerogams which possess no
chlorophyll.

Fungi grow exclusively upon organic substrata. Many grow in earth which is
rich in humus or some other organic matter, others are parasitic upon and within
animals and plants; this parasitism may present itself in the most varied manner,
the most remarkable case being perhaps that which, as we shall hereafter see, occurs
among Lichens. The mycelium usually buries itself in the nutrient substratum and
can scarcely be separated from it, whereas the fructification comes to the surface.

As I do not propose to give an exact systematic account of the innumerable

^ [In Lichens the carpogonium is fertilised by non-motile antherozoids, termed spermatia,
which resemble those of the Florideae,]

X 2

3o8 THALLOPHVTES.

species of Fungi, I will simply endeavour to illustrate the characteristics of the three
principal groups above-mentioned by giving a complete account of such forms
as have been accurately described by reliable observers, laying especial stress upon
the more important features of their life-history.

A. The Ascomycetes.

In the fructification of the Fungi belonging to this group there arise from the
ends of the fertile ascogenous hyphae cells of a club-shaped or spherical form (asci),
from the protoplasm of which numerous spores {ascospores) are developed by free-cell-
formation ; usually a definite number of these spores, either four or eight, occurs
in each ascus. The fructifications are either open {apothecia) or more or less com-
pletely closed {pertihecta).

The spores always possess a firm cuticularised external membrane, the exo-
spore, the surface of which usually presents asperities of different kinds : the inner
membrane (endospore) of the spore forms, when the exospore has become ruptured
in germination, the first hypha (or more than one) from which the mycelium takes
its origin.

The mycelium produces in many instances conidiophores upon which the
conidia are developed by abstriction. The conidia generally have a smooth surface
and a very thin external membrane. In many genera they do not occur, although
nearly-related genera possess them in abundance : for instance, they are absent in
Tuber and present in Penicillium, In addition to the conidiophores there occur
beside the fructification or even upon it, certain peculiar receptacles in which larger
or smaller conidia {Stylogonidia in Pycnidta, Spermatia in Sper?nogom'a) are developed,
which, ever since the publication of the important mycological works of Tulasne,
have been regarded as non-sexual reproductive cells of the Ascomycete upon which
they exist. Since De Bary has shown that in many cases the pycnidia belong to
other Fungi which are parasitic upon those in question, there has been some justifi-
cation for the assumption that the so-called spermogonia also represent distinct
genera of Fungi ; and this assumption gains in probability when it is considered that
it deprives the doctrine of the pleomorphism of Fungi of its last remaining support.

(i) Gymnoascus ^ is one of the simplest of the Ascomycetes. It is a small Fungus
growing upon horse- or sheep-dung, the mycelium of which developes numerous sexual
organs. Here the poUinodium and the carpogonium are completely similar before
fertihsation, but after it has taken place the carpogonium divides so as to form a row
of cells which grow out into short branched filaments bearing at their ends dense
masses of asci each containing eight spores. As the investment is quite rudimentary
the fertile portion of the fructification is in this case naked, and resembles, in this
respect, that of the simplest Florideae. {Nemalion).

(2) Discomycetes ^. In order to illustrate as fully as possible the formation of

^ Baranetzky, Bot. Zeitg. 1872, no. 10.

' De Bary, Ueber die Ffuchtentwickelung der Ascomyceten, Leipzig, 1863, p. 11. — De Bary
tind Woronin, Beitrage zur Morphologic u. Physiologie der Pilze, 2nd series, pp. i and 82, Frankfort
1866. — Tulasne, Annales des Sci. Nat. 5th series, vol. VI, p. 247. 1866. — Glinka- Janczewski, Bot.
Zeitg. 1871, no. 18, [Quart. Journ. Micr. Sci. 1878, p. 438. — Brefeld, Unters. lib. Schimmel-
pilze IV.]

CARPOSPOREM,

309

Fig. 204.— Diagrammatic section of the fructification
of Ascobolus furfuraceus (after Janczewski) ; nt myce-
lium ; c carpogonium ; / pollinodium ; s ascogenous fila-
ments ; a the asci; rp the sterile tissue from wfiich the
paraphyses h are developed.

a completely developed fructification, I select as an example Ascobolus furfuraceus^
a Discomycete described by Janczewski. Fig. 204 represents a vertical section of the
entire fructification of this Fungus, whilst still
in connection with a portion of the mycelium,
somewhat diagrammatically drawn for the sake
of clearness. The carpogonium c and the
pollinodium / arise from branches of the my-
celium. The former consists of a row of thick
short cells, and is considerably curved; the
delicate branches of the latter become closely
applied to the anterior portion of the carpo-
gonium. In consequence of fertilisation, one
of the central cells of the carpogonium (which
is designated the ascogonium) grows more
vigorously than the others, assumes a some-
what spherical form, and developes by gem-
mation numerous filaments from which, at a
later period, the asci are developed. In the
meanwhile there has been formed from the
hyphae bearing the sexual organs a mass of
filaments which completely invests the carpo-
gonium, and which forms the large sterile por-
tion of the fructification. Its hyphae are so
aggregated as to form a pseudo-parenchyma,

r in Fig. 204 being the cortical layer, 2i\i^ pp the internal portion, in which the sterile
hyphae are diagrammatically indicated. The ascogenous filaments which have sprung
from the ascogonium continue to grow forming a layer ss within the fructification, the
subhymenial layer, and send upwards thick club-shaped branches, the asci, within which
the spores are developed. In this way the hymenium aa \'& formed, and it is com-
pleted by the upgrowth between the asci of parallel branches, the so-called paraphyses,
from the sterile portion. Finally the cortex r gives way at the apex, the hymenium
comes to lie at the surface and expands in the manner represented in Fig. 205, in
order that the spores may readily escape from the asci. In Pe%'i%a confuens, the
species in which the sexual reproduction of the Ascomycetes was first discovered by
De Bary in 1863, the process is as follows, according to De Bary's and Tulasne's
exhaustive researches: — The mycelium of P. confluens grows on the ground; branches
arise at particular points of its hyphae which are directed upwards and again branch
abundantly ; at the end of the branchlets the organs of conjugation or fertilisation
are produced in large numbers close together, forming rosettes. The terminal cells
of the stronger branchlets swell up into ovoid vesicles (Fig. 206, a), which put out
a usually crooked prolongation (/*). From another cell of the same branch lying
beneath this carpogonium grows a club-shaped branchlet, the pollinodium, the apex
of which (z) unites with the prolongation just mentioned. After this has taken place,
a number of fine hyphae {h) shoot out of the filament which bears these organs, and
these surround the rosette of the organ of conjugation, enclosing it in a dense felt.
This felt forms the body of the fructification ; upon its upper side densely crowded
hyphae immediately rise up to form the hymenial layer ; finally the fructification becomes
an apothecium, which possesses somewhat the form represented in Fig. 205, and pro-
duces the ascospores in its asci. Woronin observed similar phenomena in P. granulosa
and scutellata. In these species branches consisting of three or more cells arise from
the mycelium; the terminal cell swells out into a globular or ovoid form, without,
however, putting out a prolongation ; from the cell lying beneath it arise two or
more slender filaments which attach themselves closely to the former. The conju-
gating apparatus now becomes densely enveloped in numerous hyphae which 01 iginate

3IO

THALLOPHYTES.

beneath it ; from them is developed the wall of the apothecium. In Ascobolus pulcherrimus
the carpogonium consists of a vermiform body, which Tulasne calls the Scolecite. It is
a branch of the mycelium, consisting of a row of short cells which are much broader
than those of the mycelium. The adjacent threads put out small branches, pollinodia,
the terminal cells of which attach themselves firmly to the anterior part of the scolecite.
It is subsequently covered over, together with this fertilising organ, by branched hyphae

Fig. 205. — Peziza convexula ; A vertical section of the
whole plant (X about 20) ; h hymenium or layer in which lie the
asci ; 5 the hyphal tissue surrounding the hymenium like a
cup at its margin g; at its base fine filaments proceed from
the tissue, which penetrate into the soil ; B a small part of
the hymenium (X about 500) ; sh subhymenial layer of densely
interwoven hyphse ; a—f asci, with intermediate slender para-
physes, in which are red granules.

Fig. 206.— Conjugating apparatus o^ Peziza coff
fluens (atter Tulasne, very strongly magnified) ; in R
is shown the commencement of the formation of hy-
phte h, in consequence of fertilisation, from which the
apothecium is developed.

which spring from the neighbouring mycelium ; and a ball is thus formed in the middle
of which lies the scolecite ; and this finally grows into the apothecium. In all these
cases the origin of the ascogenous filaments from the carpogonium has not as yet been
observed, but according to analogy there can be no doubt that such is the case.

In this group of the Discomycetes there are some individuals of which the mycelium
forms conidia, and the unripe fructification is in inactive sclerotium. Pe%i%a Fuckeliana ^

[On Peziza Fuckeliana and Sderotionim see Pirotta, Nuov, Giorn. Bot. Ital. i88r.]

CARPOSPOREJE.

311

has been carefully investigated by De Bary with a view to the elucidation of these
points. The mycelium of this Fungus is found in the autumn growing upon the wet
dead leaves of the Vine. From it there arise erect segmented filaments, several milli-
meters in height, which ramify frequently towards their upper ends, numerous oval
conidia being developed on each branch, which are capable of immediate germination
and give rise to new mycelia. This stage of development of this Peziza was formerly
regarded as representing a distinct Fungus, known as Botrytis cinerea. Later on the
sclerotia are formed upon the mycelium, and although their origin from sexual organs
has not been actually observed, yet after the observations of Brefeld upon Penkillium
it cannot be doubted that such is the case. The . sclerotia appear as callosities of
various form, with a diameter of from ^ to i millimeter, in the tissue of the leaf which
is infested with the Fungus, and persist after that this tissue has undergone decay.
They consist of a dense felt of hyphae with a black cortex. If they be placed upon
moist earth soon after their formation, a great number of conidiophores are developed
from them. If, however, the sclerotia have remained inactive for some months, they
produce, when placed upon moist earth, small stalked cups, the flattened cavity of
which bears a hymenium in which ascospores are formed (Fig. 205) ; this fructification
is the Pe%i%a Fuckeliana.

In addition to other genera which have small fructifications, this group includes also
the Helvellaceae, to which the genera Morchella (Morel), Hel'vella, Spathularia, and
Geoglossum belong. In these the fructification is borne on a stalk, and is either hat-
shaped or club-shaped, and attains a considerable size. The hymenium covers the
greater part of the surface of the fructification.

(3) The ErysiphesB ^ form spherical perithecia upon the surface of the sub-
stratum which they inhabit, but they remain so small that they can scarcely be seen
with the naked eye, whereas the mycelium attains a considerable size. The investment
of the fructification is a delicate hollow sphere consisting of pseudo-parenchyma, sur-
rounding the few asci which spring from the carpogonium.

The very numerous species of the genus Erysiphe (Mildew) occur upon the surface
of the leaves and green stems of Dicotyledons, and less frequently on those of Mono-
cotyledons also^. The ramified mycelial filaments extend over the epidermis, crossing
and re-crossing one another, and throw out haustoria at numerous points which pene-
trate into the cells of the epidermis. The mycelia are reproduced by means of conidia
which are abstricted in rows at the upper end of the erect unbranched conidiophores.
These reproductive organs, formerly termed Oidium, are the only ones at present known
in many species, as, for instance, in Erysiphe {Oidium) Tuckeri, the Fungus which pro-
duces disease in grapes. In many other species, however, the sexually developed fruits
may easily be found. Usually numerous filaments grow out from the cortical portion
of these fruits, which either attach themselves, like the mycelial filaments, to the
substratum, or remain quite free forming a delicate fringe. Both fruits and conidia
may be developed upon the same mycelium.

The simplest mode of the formation of the fruit occurs in the sub-genus Sphxro-
theca, Fig. 207. The carpogonia and pollinodia are developed together at the points
at which the mycelial filaments cross one another, and are in contact from their first
appearance. They are both small lateral branches; the one from which the carpo-
gonium c is to be formed assumes an ovoid shape, and is then shut off by a septum ;
the one which is to become the pollinodium p curves over the apex of the carpogonium,
and a septum is formed in its curved portion. After fertilisation, filaments spring from
beneath the basal wall of the carpogonium, as also from the pollinodium (IV h), which
closely invest the carpogonium and grow up and come together over its apex ; they

^ Tulasne, Selecta fungorum carpologia, I. Paris i860. — De Bary und Woronin, Beitiage ].mx
Morphol. und Physiol, der Pilze, 3rd series. Frankfort 1870.

^ Perhaps Sphcerotheca pannosa (Rosarum) also penetrates into the tissues of its host.

312

THALLOPHVTES.

become multicellular in consequence of the formation of transverse septa within them,
and, lying closely side by side, they give rise to a pseudo-parenchyma. From the
internal surface of this investment, as it increases in size, short filaments are given
off which fill up the space between it and the carpogonium which has not as yet
undergone much change (V h). The still unicellular carpogonium now begins to
grow vigorously; it is divided by a transverse septum into two cells, an upper and
a lower, and may be regarded as a simple form of ascogenous filament, the apical
cell of which is directly converted into an ascus (V a). The apical cell, by its rapid
growth, soon occupies the whole of the cavity of the fructification, and eight spores
are produced by free-cell-formation in its protoplasm. Slight pressure upon the fruit
causes the extrusion of the ascus (II a). In other Erysipheae, the perithecia of
which contain several asci, as E. Umbelliferarum, communis^ lamprocarpa^ etc., the car-
pogonium is also originally unicellular, but it grows within the investment into a long,
thick, curved filament, which is divided into segments by numerous septa. Many of
these segments throw out lateral branches which bear the asci.

Fig. 207. — / Conidiophore ; //ripe perithecium oi Erysiphe (Spfuerotheca) pa7t7iosa (after Tulasne); ///carpo-
gonium and pollinodium ; IV the same after fertilisation ; V the young perithecium of Podosphara, Castagnei (after '
De Bary) ; c carpogonium ; p pollinodium ; h wall of the perithecium ; a the single ascus.

These Erysipheae which have numerous asci afford a transition to the EurotiesB^ in
which the carpogonium, even before fertilisation, elongates considerably and becomes
spirally wound.

The life-history of Eurotium repens and that of Eurotium Aspergillus glaucus have
been also recounted in detail by De Bary. Both species are found on the most
various decaying or dead organic bodies, and are especially abundant on preserved fruit.
The Fungus makes its appearance as a delicate flocculent white mycehum overspreading
the surface, from which the upright conidiophores soon rise in large numbers. These
swell in the upper part into a globular form, and on the upper half of the globe
there arise a number of peg-shaped projections, densely crowded and arranged radially,
the sterigmata, each of which produces gradually a long chain of greenish conidia ;
so that finally the head of the receptacle is covered by a thick layer of them. During
this formation of conidia, the sexual organs appear on the same mycelium. The female
organ, the carpogonium, is the corkscrew-like end of a branch of the mycelium (Fig. 208,
A, as), the coils of which become gradually closer, until, when actually in contact, they
form a hollow spiral {B, C). During this process about as many septa are formed as
there are turns of the helix (i.e. 5 or 6). From the lowest coil of the carpogonium two

* [Wilhelm, Die Pilzgattung Aspergillus, 1877.]

CARPOSPOREM.

313

slender branches now shoot out at opposite points, and grow upwards on the outside
of the helix ; one of these developes more quickly, reaches the uppermost coil, and
becomes closely attached to it by its apex (B, p). This branch is the poUinodium.
Conjugation takes place between its apex and that of the carpogonium, the cell-walls
being absorbed at the point of contact, and the protoplasmic contents of the two cells
commingle. Soon afterwards new filaments sprout out from the lower part of the
poUinodium and of the carpogonium, which increase in number, cling closely to the spiral
(C), and finally entirely envelope it. From these filaments a layer of polygonal cells (D)
is formed by numerous transverse divisions, which envelopes the carpogonium. The
cells of the enveloping layer grow inwards as papillae which become septate {E). While
the enveloping layer is increasing in size, the cavity of the perithecium, which is thus
enlarged, is filled up by the papillae, and they finally insert themselves between the coils

Fig. 208.— Development oi Eurotiuvi repens (after De Bary). A small portion of a mycelium, with the conidia-bearing
hyphae c and young' ascogonium (or carpogonium) as; B the spiral ascogonium as with the poUinodium /; C the same,
beginning to be surrounded by the threads out of which the wall of the perithecium is formed ; D a perithecium ; E, F
section of young perithecia, w parietal cells, y pseudo-parenchyma, as ascogonium ; G an ascus ; H an ascospore.

of the carpogonium which have now become looser. These papillae become divided
by septa into numerous cells of similar diameter, so that at last the space between
the enveloping layer and the coils of the carpogonium is filled by a pseudo-parenchyma
{F). During these processes a large number of septa arise in the carpogonium, and
soon there shoot from its cells numerous commencements of branches, which penetrate
on all sides between the cells of the pseudo-parenchyma, become septate, and ramify.
Their last ramifications are the asci (G), which therefore owe their origin to the
fertilised carpogonium. These internal changes are accompanied by a considerable
increase in size of the whole perithecium. During the development of the asci
the pseudo-parenchyma becomes looser, its cells round themselves off, become capable
of swelling, lose their fatty contents, and finally disappear ; in the ripe perithecium

3^4

THALLOPHVTES.

it is replciced by the asci. The increase in size of the cells of the parietal layer keeps
pace with that of the whole perithecium, and they become covered with a sulphur-
yellow coating which attains a considerable thickness and consists probably of a resinous
or fatty substance. Finally these cells also collapse and dry up ; the eight-spored asci
also break up, till finally the perithecium consists only of the brittle yellow coating and
of the mass of spores enclosed by it, which are set free by gentle pressure. In a
similar manner to the perithecium, the mycelium also becomes covered by a coating,
in this case of a chestnut colour, on which the perithecia are now individually visible
to the naked eye as yellow granules. The ripe spores have the form of biconvex lenses
(H) ; when germinating the endospore which puts out the germinating filament swells
up violently and splits the exospore into two halves. The mycelium which proceeds
from the ascospores produces, like that which arises from the conidia, at first conidio-
phores and afterwards perithecia ; but a proper alternation of sexual and non-sexual
generations does not occur here.

(4) The TuberaeeaB (Truffles) form subterranean tuberous fructifications, which
may be as large as a clenched fist. They are usually provided with a firm thick
cortex of pseudo-parenchyma, and consist internally of a dense felt of hyphae in
which the ascogenous filaments ramify. The asci, within which the spores are pro-
duced, are imbedded in this tissue, and are arranged in groups or layers of various
forms which, in a section, present the appearance of chambers or of a dark veining.
Until recently the development of these fructifications had not been traced, and their
morphological structure was but imperfectly understood. The discovery made by
Brefeld that the commonest of all Moulds, Penicillium glaucum, is merely the conidia-
bearing mycelium of a small Truffle, has, however, thrown much light upon the
morphology of this group of Fungi.

The mycelium of Penicillium glaucum grows upon nearly all organic substrata, even
upon liquids, forming a dense felt. From it erect filaments ai ise which form at their
upper ends pencils of branches, and at the extremities of these long rows of greenish
conidia are developed. These conidia are everywhere dispersed in the air, and it is for
this reason that this Fungus is of such universal occurrence.

Like the Truffles, Penicillium only produces its fructification when deprived of
air and of light, under circumstances in fact which are unfavourable to the develop-
ment of the conidiophores. The fructifications are of a yellowish colour and attain the
size of a small pin's head. On this account they were overlooked until Brefeld suc-
ceeded in producing them artificially. * The mycelium ^ must be cultivated upon a
substratum which affbrds it abundant nourishment and enables it to attain, without any
interruption, its most complete vegetative development. This is reached as a rule
in from seven to ten days after the spores have been sown. The access of the atmo-
spheric oxygen must now be diminished by proper means, and the exhaustive formation
of conidiophores will thereby be prevented. Since these conditions are not commonly
fulfilled in nature, it is easy to understand why it is that only the asexual form of
Penicillium has hitherto been known.

' The sexual organs of Penicillium agree in all essentials with those of Eurotium
described by De Bary. They consist of a spirally-wound ascogonium (carpogonium),
the female organ, and of a poUinodium, the male organ.

* After the fertilisation of the ascogonium, a process of development commences
which difi'ers very materially from anything of the kind as yet described among
the Ascomycetes. In this case also the fertilised ascogonium becomes invested by
filaments which arise, evidently in consequence of fertilisation, from beneath the
ascogonium, but here the ascogonium itself at once begins to grow and its branches
extend among the surrounding filaments. When the growing ascogonium is enclosed
by eight or more (8-15) layers of filaments, no new layers are formed, but those which

' The following is taken literally from Brefeld's preliminary account in Flora, 1873, no, 21.

CARPOSPOREM. ' 315

already exist become further developed. This development consists in the division
of the filaments into very numerous cells, which expand and thus form a coherent tissue.
The further extension of the ascogenous filaments is first diminished and then arrested
by the gradually increasing coherence of the investing cells, and they are seen, in
a median section, as thick hyphae running concentrically. After this formation of
tissue has taken place an expansion of the cells, which is not uniform throughout,
takes place, so that they attain six or eight times their previous size, and then their
walls become much thickened. This thickening begins simultaneously at two points,
internally, in the ascogenous hyphae, and externally, in a zone which is separated from
the periphery by a few layers of cells.

* The fructification, now free from the mycelium, is of the size and colour of a
grain of coarse yellow sand. It is a sclerotium, consisting of from two to four peri-
pheral layers of cells elongated tangentially, of a yellowish-brown colour, and internally
of large cells arranged radially which become smaller towards the interior. Between
these run the firm ascogenous hyphae appearing like much-branched passages in the
tissue.

* The sclerotia may be preserved in the dry state for as long as three months without
losing their vitality. If they are placed upon moist blotting-paper, a further development
of the ascogenous hyphae takes place within six or seven weeks. They again acquire the
appearance of living hyphae, and become divided into numerous cells, each cell being
capable of producing a branch which, at its first appearance, divides into a thick and
a thin filament. The thick filaments are concerned in the development of the spores,
whereas the thin filaments, which are but slightly branched and without septa, cause the
absorption of the surrounding tissue and supply the thick ones with nutriment. The
thick filaments form numerous closely-placed lateral branches immediately behind their
apices, a septum being formed between each pair. These branches form a series of asci,
each of which contains eight spores.

' The result of further development is that the whole internal sterile tissue is absorbed,
the brown external layers alone remaining; the ripe asci together with the hyphae bearing
them and the nutrient filaments also disappear, so that finally, in six or eight months,
the sclerotium, although it has not altered in external appearance, has become converted
into a vesicle filled with a dense mass of countless spores of a bright yellow colour.

' When seen under favourable circumstances each ascospore gives rise to a mycelium
quite similar to that developed from a conidium, which bears the characteristic conidio-
phores, each one of which can be genetically traced through the filaments of the mycelium
to the individual spores.

* If, in consequence of desiccation, of a too advanced maturity, or for any other
reason, the sclerotia lose their capacity for growth, that is, if the ascogenous filaments
within them have lost their vitality, certain cells of their tissue may still be capable of
germinating. The hyphae springing from them come to the surface through fissures in
the sclerotium, and proceed to form the ordinary conidiophores. In this process the
physiological difference between the ascogenous filaments and the tissue surrounding
them, which amounts to a perfect contrast, becomes more definitely manifest.'

The similarity of structure presented both by the ripe and the unripe fructifications
of PenicilUum with young and mature Truffles makes it at once evident that Penicillium
belongs to the Tuberaceae, and suggests that the formation of the fruit of the other
members of this group takes place in the way so fully described with reference to
Penicillium. Tulasne's ^ figures, especially that of Elaphomyces Leveillei, represent the
ascogenous filaments within the sterile tissue of the Truffles, and Brefeld observed them
again in Tuber rufum. The well-known yellow bands of hyphae correspond to the asco-

* Tulasne, Fungi hypogsei, Paris 1862. [See also Reess, Parasitismus von Elaphomyces gramt'
tus, Sitzber. d. phys. med. Soc. zu Erlangen, 1880.]

3i6

THALLOPHVTES.

genous filaments; they give rise to asci and at the same time absorb the surrounding
sterile tissue.

The dark colour of the interior of Truffles is due to the numerous dark-coloured spores,
and the marbled appearance with light and dark veins is the result of the distribution of
the bands of sporiferous filaments in the colourless sterile tissue. The latter contains
air between its hyphae and therefore appears white when seen by reflected light. The
spores of Truffles are formed in club-shaped or spherical asci of considerable
size, by a process of free-cell formation. They are invested by an exospore which is
covered with asperities, or has a rugose surface.

It is as yet unknown whether or not the mycelium of the Truffles, like that of
PeniciUium, lives exposed to the air and forms conidia at any period of its existence.

(5) The Pyrenomycetes^ usually produce their asci, which generally contain eight
spores and are of an elongated club-shape, within small flask-shaped or roundish recep-
tacles, which are termed perithecia. The wall of a free isolated perithecium (as in
Sphseria, Sordaria, and others) consists of a firm pseudoparenchymatous tissue of a dark
colour. The perithecium contains at first a delicate transparent tissue free from air,
which is afterwards absorbed by the asci and paraphyses. These spring from a hymenial
layer which clothes either the whole of the wall of the perithecium, or only its basal
portion. The perithecia are either open from the first (as in Sphceria typhina, Sordaria),
or they are originally closed and afterwards form an orifice clothed with hairs through
which the spores can escape {Xylaria).

In a number of forms (Sphg&riae, simpUces such as Pleospora, Sordaria) the free
perithecia originate singly or in groups upon the inconspicuous filamentous mycelium
which usually inhabits dead plants, but occurs also on living ones. It is certain from
Woronin's observations upon Sphaeria Lemannex and Sordaria that in these cases each
perithecium is the result of a sexual act, and therefore represents an entire fruit. In
other Pyrenomycetes, however, a so-called stroma is first formed from the mycelium.
This is a cushion-shaped, mushroom-like, cup-shaped, or arborescent structure, consist-
ing of a dense mass of apparently homogeneous tissue, in which numerous perithecia are
developed. It remains uncertain whether, in such cases, the stroma is merely a peculiar
form of the mycelium within which the sexual organs are subsequently developed and
which bears a corresponding number of perithecia, or whether the entire stroma is the
result of one act of fertilisation and is therefore to be regarded as a single fructification
the asci of which are produced in numerous perithecia. Of these alternatives the latter
is the more probable, for in Claviceps the stroma itself is derived from a scelerotium,
which is doubtless the product of a sexual process.

The asexual reproductive cells or conidia are developed, among the Pyrenomycetes,
not merely from the mycelium, but more especially from the stroma, and (as in Penicil-
Hum) even from the wall of the perithecium. They are formed on longer or shorter
hyphal branches usually in considerable numbers, and occasionally larger and smaller
conidia occur in the same species. It has already been pointed out that the receptacles
known as spermogonia and pycnidia"^, which also form larger and smaller conidia, are
probably parasites, and do not form part of the cycle of life of the plant which they
infest.

I select as an example for more detailed description the Fungus which produces the
Ergot, — Clawceps purpurea^. Its development begins with the formation of a filamentous

* Tulasne, Selecta fungorum carpologia, Paris 1860-65. — Woronin und De Bary, Beitrage zur
Morph. u. Physiol, der Pilze, Frankfurt 1870. — Fuisting, Bot. Zeitg. 1868, p. 179. [Gilkinet, Rech.
morph. sur les Pyrenomycetes, I, Sordariees, 1874.]

^ [Bauke, Beitr. z. Kennt. der Pycniden, Nov. Act. Leop-Carol. Akad. 1876, has shown that,
in certain cases at least, the pycnidium with its stylogonidia is a definite part of the life-history of
these Fungi.]

^ Tulasue, Anuales dcs Sci. Nat. vol. XX. p. 5. — Kiihn, Mittheilungen des landw. Inst, in Halle,
vol. I. 1S63.

CARPOSPOREM.

3'^7

mycelium, which attaches itself to the surface of the ovary of Grasses, especially of Rye,
while still enclosed between the paleae, covers it with a thick weft, and partially pene-
trates into its tissue, while the apex and often other parts of the ovary remain exempt
from its attacks. The ovary becomes replaced by a soft white mycelial tissue which
retains nearly its original form, the style being not unfrequently still borne on its summit.
The surface of the tissue of the Fungus is marked by a number of deep furrows and
forms a large number of conidia on basidia arranged radially, imbedded in a mucilaginous
substance which exudes between the paleae. In this condition the Fungus had been at
one time considered a distinct genus, and described under the name of Sphacelia. The
conidia can germinate at once and immediately again detach conidia, which, according to
Kiihn, again produce a sphacelia in other Grasses. The mycelium of the sphacelia
forms, when the production of conidia has reached its height, a thick felt of firmer hyphge
at the base of the ovary, which is at first still surrounded by the looser tissue of the
sphacelia. This is the commencement of the sclerotium or Ergot; its surface soon assumes

Fig. "Kfj.—Claviceps purpurea. A a sclerotium forming stromata c/ (Ergot) ; B longitudinal section of upper part of a
stroma, cp the perithecia ; C a perithecium with the surrounding tissue (greatly magnified) ; cp its orifice, hy hyphse of
the pileus, sh epidermal layer of the pileus ; D an ascus ruptured and allowing the spores to escape (after Tulasne).

a dark-violet colour, and grows to a horn-shaped body, often as much as an inch in
length. In the meantime the sphacelia ceases to grow, its tissue dies and is ruptured
beneath by the sclerotium, and carried upwards on its summit, where it is placed like a
cap and afterwards falls off. The hard ripe sclerotium now remains till the autumn, or
usually till the next spring in a dormant state; the formation of the stroma begins when
the sclerotium is lying on the damp ground. The stromata arise beneath the skin, a
number of closely-packed branches being formed at definite points from the medullary
hyphae; the bundle breaks through the skin, and grows up into the stroma which consists
of a long stalk and a globular head. In the latter a large number of flask-shaped perithecia
(Fig. 209, JB and C, cp) appear, which do not possess a clearly-defined wall. Each
perithecium is filled from the bottom by a number of asci, in each of which several
slender filiform spores are produced. These spores swell up in damp situations, and
put out germinating filaments at several points. When they reach the young flowers of

3i8

THALLOPHFTES,

Rye, or of other nearly allied grasses, Kiihn states that the sphacelia arises from them,
and the cycle of development is thus completed.

(6) Lichens^ From the researches of Schwendener'^, there can no longer be any
doubt that the Lichens are true Fungi belonging to the Ascomycetes (Discomycetes
and Pyrenomycetes), but distinguished by a singular parasitism. Their hosts are Algae,
which grow normally in damp places but not actually in water, and belong, moreover,
to very various groups (rarely Gonfervaceae, frequently Chroococcaceae and Nostocaceae,
more often Palmellaceae, sometimes Chroolepideae). The Fungi themselves (Lichen-
forming Fungi) are not found in any other form than as parasites on Algas ; while the
Algae which are attacked by them, and which, when combined with the Fungus, are
called Gonidia, are known in the free condition without the Fungus. When the species
attacked by the Lichen-fungus is a filamentous Alga, and the development of the
hyphal tissue is only moderate (as in Ephebe and Coenogonium), the true state of the
case is at once clear; and as Lichens of this kind have become better known, the
suspicion has frequently arisen that they are in fact only Algae infested by Fungi.
In the Collemaceae also attention has frequently been drawn to the identity of the
gonidia with the moniliform filaments of Nostocaceae ; but in this case the nourishing Alga
usually undergoes considerable changes of habit, at least in its external contour, from
the influence of the parasitic Fungus, like Euphorbia Cyparissias from its parasitic
jEcidium. But the greater number of Lichen-fungi prefer as hosts the Chroococcaceae
and Palmellaceae which grow as stains and incrustations on damp ground, the bark
of trees, and stones. The separate cells and groups of cells of these Algae become so
involved by the tissue of the Fungus, that they are at last only interspersed here and
there in the dense hyphal tissue, or appear in it as a special layer (the gonidial layer).
The growth and multiplication of these Algae, which thus become entirely enclosed by
their parasites, is not hindered, but their development is disturbed in other ways. When,
however, they are freed from their enclosing Fungus-tissue, their normal development
proceeds, and in a few cases even the formation of zoogonidia takes place in them,

* Tulasne, Memoire pour servir a I'histoire organographique et physiologique des Lichens
(Annales des Sci. Nat. 3rd series, vol. XVII). — Schwendener, Untersuchungen iiber den Flechten-
thallus (in Nageli's Beitrage zur w^issensch. Botanik. i860 and 1862. — Ditto, Laub- u. Gallertflechten
(Nageli's Beitrage zur wissensch. Botanik. 1868). — Ditto, Flora, 1872, nos. 11-15. — Stahl, Beit. z.
Entwickel.-Gesch. der Flechten, 1877. [Quart. Journ. Micr. Sc. 1873, p. 235, and 1878, pp.

144.438-]

"^ [The views of Schwendener have been corroborated by Bornet in an elaborate memoir pub-
lished in the Ann. des Sci. Nat. 1873, vol. XVII. He also put them to a synthetical test by sowing
the spores of Parmelia pariethia upon Protococcus. About the fifteenth day the hyphse were well
developed and ramified. Wherever they met isolated cells of Protococcus or groups of them,
they attached themselves either directly or by means of a lateral branch. They did this to the
Protococcus only, neglecting altogether the other bodies which were mixed with it. Similar results
were obtained when the spores of Biatora muscorum were sown upon Protococcus. Spores of
Parmelia sown separately ramified much less and developed no chlorophyll ; Protococcus, on the
other hand, during the same period remained unchanged and put out no hyphse. Tulasne, however,
sowed the spores of Lichens and believed that he twice detected the formation of gonidia upon the
hyphse (Ann. des Sci. Nat. 1852, XVII, pp. 96-98). De Bary indeed described the green gonidium
as originating by the expansion of a short lateral branch of the hypha into a globular cell, which
is shut off by a septum and assumes a green colour ; once formed, it increases independently by
division, and a number of the gonidia eventually lie without stipites in the interstices of 'the
Lichen-tissue (Morph. u. Phys. der Pilze. pp. 258, 263-265). Berkeley also believes that the
gonidia originate from the hyphse. having had ' a good opportunity of ascertaining their development
from the threads of the mycelium in specimens developed within the vessels of pine wood * (Introd.
to Crypt, Bot. p. 373). For a careful resume of all the recent literature of the subject by Archer,
see Quart. Journ. Micr. Sc, 1873, p, 217. In this country Bentham has criticised Schwendener's
view (Address to Lin, Soc. May 23, 1873), and Thwaites and Berkeley have also expressed their
dissent (Gard. Chron. 1873, p. 1341).]

CARPOSPOREM.

31^

a fact first observed by Famintzin and Baranetzky, but incorrectly explained. It is to
Schwendener's knowledge of the facts, the result of researches extending over many
years, that the correct interpretation is due in these cases of the relationship borne by the
Lichen-forming Fungus to the gonidia, i. e. to the Alga which it attacks\

After these preliminary remarks the following description will be intelligible to the
beginner. It is transferred, with but slight alterations, from the first edition of this
book. We will consider first the Lichen as a whole, as it comes under observation, the
nourishing Alga being distinguished as an elemental form of the thallus under the name
Gonidia ; and we will afterwards discuss the question of their algal nature more in detail.

The Thallus of Lichens is commonly developed in the form of incrustations which
cover stones and the bark of trees, or penetrate between the lamellae of the epidermis of
woody plants, and then expose only the fructifications above the surface. These Crus-
taceous Lichens, as they are termed, have become so completely united in their growth
to their substratum, at least on the under side, that they cannot be detached completely
from it without injury to the thallus (Fig. 210, A^ B,C). The crustaceous Lichen-thallus

Fig. 211.— a piece of the foliaceous thallus o{Pei-
tigera horizonialis ; a the apothecia ; r the rhizines
(natural size).

Fig. 2to. — A, B Graphis elegans, a crustaceous Lichen
growing on the bark of the Holly ; A natural size,'/? slightly
magnified ; C Pertusaria Wtdfeiii, another crustaceous
Lichen (slightly magnified)

Fig. ■2\'2.—CoUe7na pidposictn, a gelatinous Lichen
(slightly magnified).

passes over, through various gradations, into that of the Foliaceous Lichens ; the latter
forms flake-like expansions often curled, which can be completely detached from the
ground, stones, moss, bark, &c. which support them, since they are attached to it only
in places by a few organs of attachment, the Rhi%ines. The foliaceous thallus often
attains considerable dimensions, in the large species of Peltigera and Sticta as much as
a foot in diameter, and from ^ to i mm. in thickness, and then generally assumes a
circular form; at the growing margin it forms rounded indented lobes (Fig. 211 and
Fig. 213, 5). A third form of the Lichen-thallus, also united with the previous one by
transitional forms, is shown in the Fruticose Lichens, which are attached only at one spot
and with a narrow base, and rise from it in the form of small much-branched shrubs.
The branches of the thallus are either flat and ligulate, like ths lobes of many foliaceous
Lichens, or slender and cylindrical (Fig. 213, A). In Cladonia and Stereocaulon we have not
so much a transition from the foliaceous to the fruticose thallus as a combination of the

* A few additional historical notes will be found at the end of this section.

320

THALLOPHYTES.

two, a foliaceous expansion of small size being first fornned, the cup-shaped or fruticosely-
branched thallus afterwards rising from this.

Fig. 213. — A Usnea barbata, a fruticose Lichen (natural size) ; B SHcta pulmojtacea, a foliaceous Lichen (natural size)
seen from beneath ; a apothecia.ythe attaching disc of ^, by which the Lichen becomes attached to the bark of a tree.

The thallus of Lichens can be dried, so as to be pulverised, without losing its vitality.

When saturated with water it has generally a leathery consistence, is tough, elastic,

^ ^^^ _^ - ^_ and flexible ; but a large number of

w^^^m.

Fig. 214. — Transverse section through the foliaceous thallus of Sticta
fidiginosa (X 500) ; o cortical or epidermal layer of the upper side ; u of
the under side ; r r rhizines or attaching fibres, springing from the
epidermal layer and' therefore trichomes ; m the medullary layer, the
hyphse of which are seen cut, some transversely, some longitudinally.
The upper and under cortical layers also consist of hyph?e, which how-
ever are much thicker, consist of shorter cells, and are united without
interstices, forming a pseudo-parenchyma ; g the gonidia (tlieir light-
green masses of protoplasm are coloured dark) ; each gelatinous en-
velope encloses several gonidia produced by division.

genera, which are remarkable also
in other ways, are slimy and gela-
tinous in this condition. These
Gelatinous Lichens, as they are
termed, form cushion-like masses
with an undulated surface, and in
their growth are sometimes more
like the fruticose, sometimes more
like the foliaceous Lichens. A typical
form is shown in Collema, Fig. 212.
The disposition of the gonidia and
hyphae in a thallus may be such that
these two structures appear about
equally mingled (as in Fig. 215),
and the thallus is in this case called
homoiomerous ; or the gonidia are
crowded into one layer (as in Fig.
214), by which the hyphal tissue is
at the same time separated accord-
ing to circumstances into an outer
and inner or an upper and under
layer ; the thallus-tissue is then
stratified, and such Lichens are
termed heteromerous (Figs. 214 and

217).

The mode of growth, branching,
and external structure of the Lichen-

CARPOSPOREM.

321

fhallus may either be determined by the gonidia, the hyphae being concerned only in a
secondary degree in its construction, or it may happen that the hyphae determine the

ft

Fig. 215.— Vertical section of the gelatinous thallus oi Leptogiiim scottnum (x 500); an epidermal layer clothes the
interior tissue, which consists mainly of amorphous and colourleis jelly in which lie the coiled chains of gonidia ; some of tlie
larger cells of the chains are colourless ; between them run the fine hyphae.

form and mode of growth, while the gonidia have only a secondary share in the forma-
tion of tissue. The former is the case in only a few Lichens; the latter is much

Fig. ziT.—Usnea barbata. A longitudinal section of a slender
branch, soaked in potash solution ; B transverse section of an older
thallus-steni with the basal portion of an adventitious (or soredial)
branch sa (X 300) : s apex of the branch, r the cortex, x the axial
medullary bundle of hyphae, m the loose medullary tissue, s t'^e
gonidial layer.

Fig. 216.— a branch of the thallus of
Ephebe pubescens (X 550).

the more common, and is that of the typical Lichens, especially of those that are
heteromerous. In some homoiomerous gelatinous Lichens (as Fig. 215) it appears

32 2 TIIALLOPHVTES.

doubtful whether the change in the external form proceeds more from the gonidia
or from the hyphic. This relationship, which, although both morphologically and
physiologically important, has not hitherto had sufficient attention paid to it by lichen-
ologists, will be made sufficiently clear by an examination of Figs. 216 and 217. In
Fig. 216 is shown the longitudinal section of a branch of Ephebe pubescens ; the large
gonidia are left dark, and the very fine hyphae are indicated at h. The branch increases
at the apex by longitudinal growth and by transverse division of a gonidium {gs),
which is here the apical cell of the branch. The cells produced from the apical
gonidium afterwards divide parallel to the longer axis of the branch ; still later divisions
are formed in' different directions, and thus groups of gonidia arise at some considerable
distance from the apex of the branch. The delicate hyphae are represented in our figure
as reaching to the apical gonidium; in other cases they come to a termination at a
considerable distance beneath it. Even in this case it is only a few single hyphae
which follow the longitudinal growth of the branch ; these grow within the gelatinous
envelope which is evidently derived from the gonidia. At a considerable distance from
the apex of the branch the hyphae first put forth lateral branches which penetrate be-
tween the single or grouped gonidia, forcing their way through the deliquescent mass of
their gelatinous cell-walls. Thus the whole form of the branch, its growth both in
length and thickness, is determined by the gonidia ; the hyphae, from their small number
and their fineness, produce scarcely any essential alteration either in the external form
or the internal structure of the branch. This is clearly shown also in the origin of
the lateral branches of the thallus. One cf the exterior gonidia lengthens in a direc-
tion at right angles to the axis of the parent-branch, and becomes the apical cell of
the lateral branch, producing at the same time new cells by transverse divisions, as
is shown in Fig. 216, a. Branches of the adjacent hyphae turn in the same direction,
and behave, in relation to the new apical cell, in the manner described above with
respect to those of the primary branch.

In a manner similar to Ephebe pubescens^ Vsnea barbata, a fruticose Lichen, also forms
\ a much-branched fruticose thallus. The branches of the thallus here also elongate by
apical growth (cf. Fig. 217, A); but this is not brought about, as in Ephebe, by the
gonidia, nor by a single apical cell. Each of the hyphae at the end of the branch, which
are nearly parallel and approximate at the apex, elongates by the apical growth of its
terminal cell, and thus they produce in common the apical growth of the branch ; this is
followed further backwards by an intercalary growth, the result of the intercalary
elongation of the hyphae and of the formation of new hyphae in different directions.
The hyphae lie so close together near the apex that they form a compact mass without
interstices ; it is only at some distance from it that the hyphal tissue is differentiated
into a very d^nse cortex of fibres interwoven on all sides, an axial bundle of densely-
crowded threads running in the direction of length, and a looser layer (the medullary
layer) furnished with air- containing interstices. The point below the apex where this
differentiation of the hyphal tissue begins is also that of the point of commencement of
the gonidial layer, which consists of small roundish green cells, collected in small groups
in consequence of multiplication by division ; and these groups themselves form a layer
between the medullary and cortical layers {cf. Fig. 217, B, the transverse section).
Below the growing apex of the branch of the thallus there are only single gonidia,
by the division of which the cells of the gonidial layer are produced. ~ It is evident
therefore that in Usnea barbata the growth in length and thickness and the internal
differentiation of the tissue depend entirely on the hyphae, and that the gonidia behave
like foreign bodies in the hyphal tissue ; the formation of new branches proceeds also
from the hyphae and not from the gonidia. The branching may be dichotomous ; and
in this case the apical cells of the hyphae converge towards two nearly adjacent points,
and then continue to grow in corresponding directions, so that the two equal branches
form an acute angle. Adventitious branches arise laterally below the apex of the thallus,
the cortical fibres forming at a particular point a new apex and subsequently growing

CARPOSPOREJE.

3^3

outwards. Gonidia are also to be found behind the new apex, while the base of the
branch sends out medullary fibres and an axial bundle into the primary branch, so
that the homologous forms of tissue of the two are continuous. The growth of Usnea
may be compared, irrespectively of subordinate points, to that of the so-called stroma
of the Xylariae ; the formation of the gonidia is a subordinate element in the structure
of the whole.

In some crustaceous Lichens the thallus possesses in general no defined contour, and
no external differentiation takes place; the thallus appears as a somewhat irregular
aggregation of masses of gonidia traversed by hyphae. In other crustaceous Lichens (as
Sporastatia Morio, Rhizocarpon subconcentricum, Aspicilia calcarea, &c.) the thallus forms
lobed discs which increase by centrifugal growth at the margin; the growing margin
consists altogether of hyphal tissue, in which, further inwards, masses of gonidia appear
at a few isolated spots and gradually spread ; the cortical tissue is indented at the circum-
ference of the spots where the gonidia are formed. Isolated scaly pieces of a true
Lichen-thallus thus arise on a fibrous substratum called the hypothallm ^

'^he Formation of the Spores of Lichens takes place in receptacles termed Apothecia,
when they are similar to those of the Discomycetes, or Perithecia^ when they are similar to
those of some Pyrenomycetes. They are formed in the interior of the tissue of the thallus,

I Fig. 218.— Vertical section of the apotheciuin o'i Anaptychia ciliaris (X about 50); h the liymenium. jv sub-hynienial

layer and excipuluin ; all the rest belongs to the thallus ; m its medullary layer, r its cortex, g its gonidia ; at ^ ^ the thallus
forms a cup-shaped rim round the apothecium.
md only appear above its surface at a later period, and then, in the one case they expand
heir hymenial layer to the air (Gymnocarpous Lichens), and in the other, they allow the
pores to escape through an orifice (Angiocarpous Lichens). In all Lichens without
!xception the receptacle and all its essential parts take origin exclusively from the hyphal
issue ; it is the Fungus alone that produces the receptacles ; the nourishing Algae, /. e. the
;onidia, take no part whatever in it ; or only in a secondary manner in so far as the
hallus-tissue together with its gonidia grows like a wall round the apothecium and to a
certain extent envelopes it (as shown in Fig. 218), or grows luxuriantly beneath the
receptacle and raises it upon a kind of stalk above the surrounding thallus. The only
exception to this endogenous origin of the receptacle occurs in Ccenogonium and similar
forms, where it is impossible, because the hyphae form only a very thin layer round tke
filamentous Alga which performs the part of gonidia'^. These forms serve to show with
especial clearness, as we know from Schwendener's researches, that the receptacle of
Lichens belongs exclusively to the hyphal tissue.

The investigation of the development of the apothecium is attended with great
difficulty, and more than one point is still obscured It originates, in heteromerous

^ See Schwendener, Flora, 1865, no. 26.

^ [See Archer, On Apothecia in some Algae, Quart. Journ, Micr. Sci. 1875.]
^ What follows is taken from De Bary's account of his own researches, and from those of
Schwendener and Fuisting. [See Stahl, loc. czV.]

Y 2

324 THALLOPHVTES.

Lichens, beneath the cortical layer, in the lower part of the gonidial zone, or, in some
crustaceous Lichens, in the deepest part of the thallus in immediate contact with the
substratum; in homoiomerous gelatinous Lichens and in Ephebe it arises beneath the
surface of the thallus. The commencement of the apothecium is, in heteromerous
Lichens, a very small roundish ball of confused interwoven hyphae, on the outer side
of which a tuft of very delicate hyphae — the first paraphyses — rises at a very early
period. The most external hyphal investment of this ball, and therefore surround-
ing the tuft of paraphyses and opening above (outwards), is termed by lichenologists
the Excipulum. The further growth of the rudiment of the apothecium is now
occasioned by the increase in size of the excipulum by the formation of new fibres,
while new paraphyses are intercalated among those already formed and outside the
tuft, the extension of the apothecium being the immediate result of the fresh forma-
tion of these bodies. Growth is. first completed in the centre of the apothecium ;
at the outside it continues longer, often even after the appearance of the apothecium
above the surface of the thallus. The mother-cells of the spores, the asci, are formed,
according to Schwendener and Fuisting, in a peculiar manner. ' Even in the young ball,
and among the first rudiments of the paraphyses, thicker hyphae are to be seen inter-
woven among the rest, rich in protoplasm, undivided by septa, and with numerous
ramifications ; the upright ends of the branches of these hyphae which penetrate between
the ends of the paraphyses develope into club-shaped asci ; they may hence be termed
ascogenous hyphoe. They are very readily distinguished from the paraphyses by their
membrane being coloured blue by iodine after treatment with potash-solution, while
that of the paraphyses remains colourless. They disappear at a very early period from
the lower part of the rudiment of the apothecium, and remain only in one narrow layer
which runs parallel to the upper surface of the apothecium, and extends below the
lower ends of the ripe asci. In this layer they ramify in a centrifugal direction in
proportion as the margin of the excipulum grows, and send out new asci among the new
paraphyses. The first asci appear in the centre of the apothecium ; and Schwendener
states that no genetic connection exists between the ascogenous hyphae and those from
which the paraphyses are derived; the two form separate systems but interwoven
into one another^. The layer in which the ascogenous hyphae run is called the Sub-
hymenial Layer; the hymenium itself consists of the paraphyses and the asci taken
together. The term Hypothecium is given to the mass of fibres which lies beneath the sub-
bymenial layer, and is often strongly developed through subsequent growth ; it consists
of hyphae the branches of which end in the hymenium as paraphyses, and of the remains
of the primary ball ; when mature, it can scarcely be distinguished from the excipulum.
The growing apothecium bulges more and more, and finally breaks through the
layer of thallus which covers it; the hymenium and the margin of the excipulum
appear above the surface of the thallus, or the part of the thallus which surrounds
the excipulum rises and grows with it forming a bowl-like rim. Among the medullary
hyphae which surround the apothecium a number of gonidia subsequently appear in
many Lichens, so that a gonidial layer runs beneath the apothecium. In Peltigera and
Solorina even the young apothecium is expanded flat, its paraphyses project vertically
towards the surface of the thallus, and the layer of thallus which covers them is finally
'lifted like a thin veil. In Bceomyces, Calycium, &c. the basal portion of the hypothecium
is developed into a long stalk which supports the apothecium.

' From the newly-discovered processes in the formation of the reproductive organs of the
Pyrenomycetes and Discomycetes, especially from the most recent statements of Janczewski on Asco-
bolus furfuraceu& {cf. p. 309), it may be assumed that the tubular hyphae of the sub-hymenial
layer arise from a yet undiscovered ascogonium or scolecite ; and that thus the apothecium of
Lichens is the result of a sexual process in a similar manner to the perithecia of the Pyrenomy-
cetes and the apothecia of Peziza and Ascobolus. [This has been discovered by Stahl. See the end
of this section.]

1

CARPOSPOREJE.

3^5

The perithecium of Angiocarpous Lichens is so similar in its mode of development
and in its mature state to that of the Xylariae, that there is no need to give an exact
description of it.

The club-shaped asci of Lichens are similar in every essential point to those of the
Pyrenomycetes and Discomycetes; their wall is often very thick and capable of swelling;
the spores (Fig. 219) arise simultaneously, as in those Fungi, by free-cell-formation,
while a considerable portion of the protoplasm often remains unused in their production.
The normal number of spores is eight, although sometimes only 1-2 (in Umbilicaria and
Megalospora), 2 or 3 or from 4 to 6 (in several Pertusariae) ; in Bactrospora, Acarospora^
and Sarcogyne on the other hand their number amounts to some hundreds in one ascus.

Fig. 2r9.— Vertical section of a small portion of the apothecium oi Anaptychia cilia) is (X550); *n the medullary
layer of the thallus ; y the hypothecium, together with the sub-hymenial layer ; / the paraphyses of the hymenium, their
upper ends of a brown colour ; among them are the asci in various stages of development ; in i are the young spores not yet
septate, in 2—4 the spores more fully developed ; the protoplasm in which the spores are imbedded is contracted by the
drying up of the Lichen before the preparation was made.

The structure of the spores is very various, but in general similar to that of the Asco-
mycetes; very commonly they are septate and multicellular; the exospore is usually smooth
and often variously coloured.

The spores are set at liberty by moisture penetrating the hymenium; they are
suspended in the fluid which fills the ascus, and are expelled together with 'the fluid by
the rupture of its apex. This expulsion is probably caused by the lateral pressure of
the swollen paraphyses and the property of swelling possessed by the membrane of the
ascus itself.

The germination of the spores of Lichens takes place by the endospore of each
spore-cell putting out a filament which ramifies and extends over the damp substratum

326

THALLOPHYTES.

on which the spore is placed. The mode of germination of the very large spores of
some genera, Megalospora, Ochrolechia, and Pertusaria, differs from that of all the rest.
They are simple, not septate, and densely filled with drops of oil (Fig. 220, A, B). Each
spore puts out from different parts of its circumference a great number, even as many as
a hundred, germinating filaments. The formation of each begins with the appearance
in the endospore of a cavity widening from within outwards, which becomes sur-
rounded by a very delicate membrane and grows outwards in the form of a filament
(Fig. 220, A,B).

Besides the apothecia with ascospores capable of germination, Spermogonia are also
generally present in Lichens, as in Ascomycetes ; they generally occur on the same

FIG. 220.— Lichen-spores germinating ; A longitudinal section of a spore oi Peitusarui commtcnis &iter lying 34 hours
in glycerine, s the rudiments of the germinating filaments ; B spore oi Pertusaria leioplaca with a number of germinating
filaments (after De Bary, x 390) ; C germinating septate spores of Solorina saccata (after Tulasne).

thallus as the apothecia. They are cavities in the thallus which are globular, flask-
shaped, or sinuous, and densely clothed and almost filled with sterigmata ; from these
sterigmata the spermatia are detached in very large numbers, and escape through a
fine orifice in the spermogonium. Sometimes also receptacles are found in which larger
bodies, more like spores, are detached from the sterigmata ; receptacles of this kind are
called Pycnidia, as in the Pyrenomycetes.

Besides the spores, most Lichens also possess organs termed Soredia, by which they
are very extensively reproduced. They are single gonidial cells or groups of gonidia
which, surrounded by a weft of hyphge, are pushed out of the thallus, and are able,
without any further process, to grow into a new Lichen-thallus. The soredia are pro-
duced from the thallus in the non-gelatinous Lichens, as a fine powder, forming sometimes

CARPOSPOREm.

r-1

dense pulvinate masses (as in Usnea, Ramalina, E'vernia, Pbyscia, Parmelia^ Pertusaria, &c.).
In the heteromerous thallus the soredia appear in the gonidial layer; single gonidia, or
sometimes several together becoming woven over by branches of hyphae which cling

Fig, 221. — A — D soredia of Usiiea barbata; A a simple sorediuiii, consisting of a gonidium covered with a web of
hyphae ; B a soredium, in which tlie gonidiuin has multiplied by division ; C a group of simple soredia, resulting from the
penetration of the hyphw between the gonidia ; D, E germinating soredia ; the liypliai are forming an apex of growth, and
the gonidia are multiplying ; i7-c soredia oi Physcia parietina; « with an envelope of pseudo-parenchyma ; * the envelope
producing rhizines ; c a young thallus formed from a soredium (after Schwendener, X 500).

closely to them and form an envelope of fibres. The gonidia divide repeatedly, and each
daughter-cell is again woven over. This process is often repeated, the soredia accu-
mulate in great numbers in the gonidial layer, and finally rupture the cortex. After

Fig. 222.— Examples of various Algre which act as the gonidia of Lichens (after Bornet) ; h in all cases is the hyjilia of
the Fungus, ajid^rthe gonidium. A, germinating spore s oi Physcia parietina, the hypha of which has attached itself to
Protvcocctis viridis; R, a filament ofScytonema invested by the hyphae oi Stereocajtloii ramnlosum; C, taken from the thallus
of the \Jichen, Pkysjna chalaf^amtm ; a branch of the hypha is penetrating into a cell of the Nostoc-filament (gonidium) ;
D, taken from the thallus of Synalissa symphorea, the gonidia are the Alga Glceocapsa; E, from the thallus oi Cladoma
Jttrcata, the gonidium {Protococcus) is invested by hyphae.

escaping in this manner, the soredia can still further multiply outside the thallus; but
under favourable conditions either a single soredium or a mass of them grows out at once
into a new thallus (Fig. 221). Schwendener states that in Vsnea barbata this may occur

32S THALLOPHYTES.

while the sorcdia are still included in the mother-thallus ; soredial branches, as they are
termed, are thus produced.

We may now turn lo the consideration of the other elemental form out of which, in
addition to the Fungus-hyphae, the thallus of Lichens is constructed, the Gonidia. It has
already been suggested that these are nothing but Algae which are attacked and sur-
rounded in their growth by Ascomycetes, and serve as hosts to them, the capability of
assimilating inorganic materials being wanting on the part of their parasites.

Passing over the views of the older lichenologists, which will be found collated in the
writings, cited below, of Baranetzky and of Schwendener, it may be pointed out here that
De Bary (Handbuch der physiol. Bot. vol. ii. p. 291) arrived at the following alternative
conclusions with respect to the gelatinous Lichens, such as Ephebe and similar forms ; 'that
either these Lichens are the completely developed fructifying states of plants the
incompletely developed forms of which have hitherto been placed among Algae, as
Nostocaceae and Chroococcaceae, or the Nostocaceae and Chroococcaceae are typical Algae
which assume the form of Collemae and Ephebae &c., in consequence of the penetration
into them of certain parasitic Ascomycetes the mycelium of which extends throughout the
growing thallus and often becomes attached to the cells filled with phycochrome
{Plectospora, Omphalaria). In the latter case the plants in question might be termed
Pseudolichens.' From the close of this quotation it appears that the writer does not
apply the latter alternative to the heteromerous Lichens at any rate. Soon afterwards
Famintzin and Baranetzky, and then the latter alone, published researches upon the
further changes which the gonidia of Lichens undergo when they are set free by the
decomposition of the hyphal tissue in water ^. Baranetzky comes to the conclusion that
* the gonidia of the heteromerous chlorophyll-containing Lichens {Physcia, E'vernia, Cla-
donia), as well as the heteromerous forms containing phycochrome {Peltigera) and of the
gelatinous Lichens (Co/lema), are capable of carrying on an entirely independent life out-
side the lichen-thallus. When set free, the lichen-gonidia appear to extend their cycle of
life; thus, for instance, the independently vegetating gonidia of Physcia, E'vernia, and Cla-
donia produce zoogonidia.' He also found that all the cells of the spherical n^asses com-
posed of the gonidia of Peltigera undergo a transformation so as to become extremely like
the interstitial cells of a Nostoc, and he did not doubt that this was their permanent
condition. 'Some, perhaps many,of the forms hitherto described asAlgae must be considered
as independently vegetating lichen-gonidia, for the present at any rate; such are Cystococcus,
Polycoccus and Nostoc.^ The researches of Schwendener carried on, in part earlier, in part
simultaneously and later, in the most careful manner, led to the opposite conclusion, that
the gonidia are in fact Algae which are more or less disturbed in their manner of life by
the Fungus which is parasitic upon them. He first definitely stated and explained this
view in his treatise 'Ueber die Algentypen der Flechtengonidien,' (Basel, 1869), as
applying to all Lichens. In this memorable work, which assigned to the Lichens for the
future their true systematic position among the Ascomycetes, he gives an account of those
genera of Algae which were to that time known as the hosts of lichen-fungi, that is, as
playing the part of gonidia.

I. Bluish-green Algae. (Nostochineae.)
Name of group of Alga. Lichen in nuhich they occur as gcnidia,

(i) Sirosiphoneae . . Ephebe, Spilonema, Polychidium.

(2) Rivularieae . . Thamnidium, Lichina, Racoblenna.

(3) Scylonemeae . . Heppia, Porccyphus.

(4) Nostocaceae . . Collema,Lempholemma,Leptogium,Pannaria, Peltigera.

(5) Chroococcaceae . . Omphalaria, Euchylium, Phylliscium.

* Mem. de I'Acad. Imp. des Sci. de St. Petersbourg, 7th series, vol. XT. no. 9 and Melanges
biologiques tires du Bulletin de I'Acad. Imp. de St. Petersbourg, vol. VI. 1867. — [Ann. des Sci. Nat.
5th series, 1867, vol. VIII. pp. 137-144.] — Also Itzigssohn, Bot. Zeitg, 1868, [and Woronin, Ann. Sci,
Nat. XVI, 1872].

I

CARPOSPORE.E. 329

II. Green Algae.

(6) Confervaceae {Cladophora) . . Ccenogonium, Cystocoleus.

(7) Chroolepideae .... Graphideae, Verrucarieae, Roccella.

(8) Palmellaceae .... Many fruticose and foliaceous Lichens.

Cystococcus humicola . . Physcia, Cladonia, Evernia, Usnea, Bryopogon,

and ylnaptychia.
Pleurococcus .... Endocarpon and various crustaceous

Lichens.
Protococcus .... Cladonia, Physcia.
Stichccoccus . ... Sphccromphale, Polyblastia.

(9) Coleochaeteae [Phyllactidium^ Kiltz.) Opegrapha filicina.

The inconceivable opposition offered to Schwendener's theory by lichenologists
must surely be overcome by a recent publication of Bornet's\ After careful investi-
gation of sixty genera of Lichens, he comes to the conclusion that each lichen-gonidium
can be referred to some species of Algae, and that the relations of the hyphae to the
gonidia are of such a kind as to exclude any possibility of assuming the existence of
a genetic connection between them ; that they find, in fact, their only satisfactory
explanation in the theory of parasitism. Bornet shows that not only does the Alga
which is the host of the Fungus become modified in consequence of the cohabitation, but
that the Fungus itself often undergoes some change. He describes more accurately than
had previously been done how the hyphae of the Fungus attach themselves to the algal
cells, and even penetrate into them, in order to absorb their contents, as the occurrence
of the empty cell-walls of gonidia in the lichen-thallus suggests. He did not content
himself with seeking in nature material which would show the Algae being attacked by
the lichen-fungus and the gradual formation of the thallus, but he sow^ed spores of Lichen-
fungi upon Algae "^ in order to be able to observe the manner in which the Fungus avails
itself of the Alga. Of more especial interest is his proof of the fact that the same Alga
may serve for very different Fungi; for example, Chroolepus umbrinum supports no less
than thirteen genera belonging to five families of Lichens. Although many Lichen-fungi
require to have particular Algae as their hosts, a condition which occurs also in other cases of
parasitism, it also happens that the same Lichen-fungus can avail itself of various forms
of Algae as its gonidia. The Alga which has been attacked by the Fungus and has
become surrounded by its hyphae is not always hindered in its growth, but in many
cases is actually stimulated to more active vegetation. For further important details
I must refer the reader to the work itself.

All reliable observations thus lead to the conclusion that a lichen-thallus is a mycelium
which is nourished as a parasite by an Alga. The fructification of the Lichen, the
apothecium or perithecium, belongs exclusively to the mycelium.

[The most conclusive evidence in favour of the truth of Schwendener's theory has
been brought to light by the researches of StahP. In the first place, he succeeded, by
cultivating the spores and hymenial gonidia of Endocarpon pusillum, in producing arti-
ficially a Lichen-thallus which bore perithecia and spermogonia. In the second place,
he discovered that carpogonia are present in the thallus of certain Lichens which he
investigated. The carpogonium of Collema microphylliim, for instance, is a hyphal
filament which forms closely appressed coils at some distance below the surface of the
thallus, and is then prolonged straight to the surface beyond which it projects. The
carpogonium thus consists of two parts, the coiled portion, which Stahl terms the asco-
gonium, and the straight portion, which he calls the trichogyne. Spermatia, derived from

^ Bornet, Recherches sur les Gonidies des Lichens, Ann. des Sci. Nat. t. XVII. 1873.

^ Compare also Reess, Monatsber. der Berl. Akad. 1871 ; and Schwendener, Flora, 1872; also
Treub, Bot. Zeitg. 1873.

2 [Beitrage zur Entwickelungs-geschichte der Flechten, I, IT, 1877: also de Bary, Die Ev-
scheinung der Symbiose, 1879.]

330 THA LL OPHYTES.

the spcrmogonia, are disseminated over the surface of the thallus by the agency of water,
and are thus brought into contact with the projecting cell of each trichogyne, and to this
they adhere. The contents of one at least of the spermatia pass into the trichogyne in
consequence of the absorption of the cell-walls at the point of contact, and thus the
carpogonium is fertilised. It will be seen that this mode of fertilisation is the same as
that which has been described in the case of the Florideae. In consequence of fertilisa-
tion the cells of the ascogonium, as well as those of the adjacent hyphae, are stimulated
to growth, and the result is the formation of an apothecium ; from the ascogonium are
produced the asci, and from the adjacent hyphae the paraphyses and the wall. It is clearly
shown that the apothecium is derived solely from the fungal constituent of the Lichen.
These results, besides throwing light upon the nature of Lichens, give a clue to the
significance of the spermogonia and spermatia in other Ascomycetes and in the iEcidio-
mycetes.]

B. The ^cidiomycetes \
{UredinecE.)

If, in characterising this group, attention is confined, as in the case of the
preceding groups, to those forms whose development is completely known, two
extreme cases, as regard the conditions of reproduction and the alternation of
generations, present themselves. In the simplest case the mycelium produces
a fructification, the so-called JEcidium, which consists, in its mature condition, of
a cup-shaped investment {peridmm) and of a hymenium occupying its basal part;
from the basidia of the hymenium spores are formed by abstriction. The spores
thus produced {cBcidiospores) at once germinate, and each one developes a short fila-
ment consisting of but few segments, the growth of which soon ceases, and bearing
upon short delicate branches smaller reproductive cells, the sporidia, which may be
included under the term conidia in the sense in which that term has hitherto been
used. The hypha bearing them is to be regarded as a promycelium. The sporidia,
on germination, throw out hyphae which penetrate into the epidermal cells of the
host, and give rise to a mycelium which, in its turn, forms aecidium-fruits. In this
case, which is found represented by Endophyllum Sempervivi, there occurs a simple
alternation of generations, the alternating generations being the mycelium and the
fructification (secidium), with the slight variation that the aecidiospores give rise to
the mycelium not directly but indirectly by means of the promycelium and its
sporidia. The other extreme case is represented by jEcidium Berbertdis, ^cidiuni
Leguminosarum, and others. Here new mycelia are directly formed by the secidio-
spores, without the intercalation of a promycelium. They do not, however, give rise
to aecidium-fruits but develope conidia (the so-called Uredospores) upon basidia closely
packed so as to form a kind of cushion, by means of which numerous generations
of mycelia are produced during the period of vegetation. It is not until later that
reproductive cells of another kind, the Teleutospores, are produced in these genera-
tions to which the name Uredo has been given. These germinate in the following

^ Tulasne, Ann. des Sci. Nat. 3rd ser, vol. VII; 4th ser. vol. II. — De Bary, Ann. des
Sci. Nat. 4th ser. vol. XX, and Monatsber. d. Berl. Acad. 1865. — Oersted, Bot. Zeit. 1865. p. 291. —
Reess, Die Rostpilzformen der devitschen Coniferen, Halle 1869 (Abh. der naturf. Gesellsch. Bd. XI).
• — Oersted's System der Pilze, Lichenen, iind Algen, translated into German by Grisebach and Rcinke,
Leipzig 1873, p. 19. [Schroter, Entwick, einig. Rostpilze, in Cohn's Beitrage, I, 1875.]

CARPOSPORE.E.

331

spring and form promycelia, from the sporidia of which the myceHa which bear the
secidium-fruits are developed.

On comparing this second case with the first it becomes evident that here
several generations of mycelia are intercalated between the formation of the secidio-
spores and the formation of the promycelium. These generations give rise to
peculiar reproductive cells, the uredospores and the teleutospores.

A sexual act has not as yet been observed even in these well-known forms of
jEcidiomycetes. If, however, we adhere to the rule that in the Thallophytes, as in
Cryptogams generally, the most complex form of development is the result of a
sexual act, and if we assume that an act of this kind does actually take place in this
case, we cannot but regard the aecidium-fruit as the sexually-produced generation ^
The secidiiim-fruit will then correspond to the fructification of the Ascomycetes, the
[ecidiospores to the ascospores, and the uredospores, teleutospores, and sporidia to
various forms of conidia. Should these probable assumptions be substantiated by
future discoveries, it becomes at once evident that the nomenclature of the genera
must be based not upon the forms bearing teleutospores, but upon those bearing
aecidia, for instance the genus now known as Puccinia will have to be reconstituted
under the name oi JScidium^ the genus Gymnosporangium under that oi Roestelia, &c.,
just as among the Ascomycetes not the conidia but the sporocarps aff'ord the basis
for their systematic arrangement.

That the uredospores and teleutospores are merely forms of conidia is demon-
strated by the fact that they are present in some genera and species and absent in
others, resembling in this respect the conidia of the Ascomycetes. They are both
absent in E?tdophyllum, the uredospores are absent in Roestelia^ and both are present
in JEcidium Berber idis and in JEcidium Leguminosarum.

The view which is here maintained is exclusively founded upon the well-known
forms. There remains a much larger number of forms of which the life-history is
only imperfectly traced. In a series of forms, for instance, the aecidium-fruits only
are known {jEcidium elatinum, Pint, abietinum'^, &c.), and it is still uncertain whether
or not they reproduce themselves by the aecidiospores alone : in others only the
teleutospores are known {Chrysomyxa, Puccinia Dianihi, compactd) : in others again
only the uredospores {CcEoma pinitorquuni): uredospores and teleutospores, without
gecidium-fruits, are known in Melampsora and Coleosporium. The last-named cases
recall Penicillium and Eiiroiium in which, formerly, only the conidia were known
and not the true fructifications, but they differ from them in that they possess two
kinds of conidia. It appears that, like Penicilliimi and other Ascomycetes, certain
^cidiomycetes can reproduce themselves for many generations solely by means of
their conidia (uredospores and teleutospores) without attaining the completion of
their development in the formation of a true fructification (aecidium).

The formation of the fructification of the ^cidiomycetes, like that of many
Ascomycetes, is accompanied by the development of peculiar receptacles, the

^ To this view I drew attenti»n in the first edition of this book (1868). Oersted, loc. cit., and
Brefeld also support it. [See also Stahl, Bot. Zeitg. 1874.]

^ [The life-history of JEc. ahielinum has since been traced by De Bary (Bot. Zeitg. 1879)". The
mycelium bearing uredo- and teleutospores infests the leaves of Rhododendron ferrugineum and
hirsutuin.']

33^

THALLOPHVTES.

spermogonia, the significance of which in both cases is obscure. They make
their appearance usually just before the aecidium-fruits and close beside them upon
the leaves of the host. They extrude small conidia (spermatia) formed by abstric-
tion, the significance of which in the life-history of the ^cidiomycetes is completely
unknown. It has been suggested that they are to be regarded as parasites, although
the extraordinary constancy of their appearance certainly militates against this view.

The iEcidiomycetes exclusively inhabit living Phanerogams, mostly their stems and
leaves, but also the living cortex of trees (Goniferae). The extension of the mycelium
in the intercellular passages of the host does not usually produce much injury, though in
some cases the host becomes deformed, as for instance, the formation of 'witches-
brooms' in Firs by the growth of Ailcidium elatinum. Occasionally the mycelium is
confined to certain circumscribed areas of its host {^cidium Leguminosarum and others),
but more commonly it extends throughout its tissues [JEcidium EuphorbicB cyparissiai^
Endophyllum Sempewi'vi). The fructifications, as also the conidia, are developed beneath
the epidermis of the host, and penetrate through it to the surface only when they are
mature.

Some of the best-known forms possessing conidia avail themselves of the same p'ant
as a host throughout their whole life ; for example, jEcidium Leguminosarum and Trago-
pogonls : in others the various reproductive forms are developed upon diflferent hosts,
for example, the aecidium-fruits of JEcidium Berberidis occur only on the leaves of
Berberis ruulgaris, whilst the uredospores and the teleutospores are formed only
upon Grasses. Similarly the large aecidium-fruits of Roestelia cancellata occur only
upon the leaves of Pomaceae, the teleutospores only upon those of species oi Juniper us.
Such forms as these are said to be heteroecious (metoecious), to distinguish them from
those above-mentioned which inhabit the same host throughout their whole life
(autoecious).

The sporidia developed from the promycelium (whether this is derived from aecidio-
spores or from teleutospores) give oft' hyphae which pierce the walls of the epidermal
cells and penetrate into the interior of the host, whilst the hyphae derived from the
aecidiospores and from the uredospores grow upon the epidermis of the host until
they reach a stoma through which they enter the intercellular passages. To this rule
Puccinia Dianthi offers an exception, in that the promycelium derived from the teleuto-
spores forms sporidia which send their hyphae through the stomata.

Both the uredospores and the teleutospores protrude their hyphae from certain
definite portions of their surface at which the cuticularised external membrane (the
exospore) is either wanting or is very thin. Each uredospore presents from three to
six such areas lying in its equator, and each teleutospore has one in each cell. The
teleutospores may be single, as in Uromyces, or in pairs, as in Puccinia, or three may be
aggregated together, as in Triphragmium, or even four, as in Phragmidium. They
germinate, usually after a considerable period of rest, in the spring, but occasionally
immediately after their formation {Roestelia, Puccinia Dianthi).

In order to illustrate their life-history, I select as an example the Fungus the
uredospores of which produce the 'rust' of Wheat, the A^cidium Berberidis, hitherto
known as Puccinia Graminis.

On the leaves of Berberis 'vulgaris are found in the spring yellowish swollen spots,
where dense masses of mycelial filaments are interposed between the parenchyma-cells
(Fig. 221, ji and I, the felted mycelium, lying between the cells, being indicated by
dots). In these swollen spots are found two kinds of fructification, the Spermogonia,
which are produced somewhat earlier, and the A^cidia. T*he spermogonia (Fig. 223, /, sp)
are urn-shaped receptacles surrounded by a layer of mycelium as an envelope; hair-like
threads which clothe the cavity protrude in the form of a brush from the opening of
the spermogonium, penetrating the epidermis of the leaf; the bottom of the spermo-

CARPOSPOREM.

3,3,3

gonium is covered with short mycelial branches, from the ends of which are detached
numerous very small spore-like bodies, the Spermatia. It has already been stated that
their significance in the development of the Fungus is unknown. The aecidium-fruits
lie at first beneath the epidermis of the leaf, where they form a tuberous parenchymatous
body {A), also surrounded by an envelope of fine mycelial filaments. When mature the
aecidium breaks through the epidermis of the leaf and forms an open cup, the wall of
which (the peridium, p) consists of a layer of hexagonal cells arranged in rows, which
are produced at the bottom of the cup from basidium-like mycelial branches. The
bottom of the cup is occupied by a hymenium, the hyphae of which have their apices

Fig. 223. — Puccinia Grattwns. A part of a vertical section of a leaf oi Herberts vulgaris with a young secidium-fruit ;
/ section of leaf of Herberts with spermogonia sp and aecidium-fruits a ; p their peridium ; at ^ is the natural thickness of the
leaf which is enormously thickened between x and y; II & mass of teleutospores on a leaf of Couch-grass ; e the ruptured
epidermis; b the hypodermal fibres; t teleutospores; /// part of a mass of uredospores ur with one teleutospore t;
sh sub-hymenial hyphae {A and / from nature ; // and /// after De Bary).

directed outwards and are continually detaching new conidia-like spores, which, originally
of a polyhedral form in consequence of pressure from opposite sides, afterwards become
rounded, and separate from one another at the opening of the cup (/, a). The peridium
itself has the appearance of a peripheral layer of similar spores ; its cells however remain
united, and, like the spores, contain red granules. The aecidiospores produced upon
the leaves of Berberis only develope a mycelium when their germination takes place
upon the surface of a leaf or stem of Grass (as Wheat or Rye). The germinating fila-
ments then penetrate through the stomata, and the mycelium produced in the paren-
chyma of the Grass generates within 6 or 10 days the uredospores (///, «r), which are

334

THALLOPHVTES.

formed in cushion-like masses of mycelium upon densely-crowded branches (basidia)
directed outwards immediately beneath the epidermis. They contain red granules and
are perceptible with the naked eye as narrow long red projections upon the leaves and
stems of Grasses. These uredospores are dispersed after the rupture of the epidermis,
and germinate after some hours upon the surface of the Grasses (P'ig. 224, D) : in these
they form new mycelia which, in 6 or 10 days, bear uredospores again. While
the Fungus is multiplying in this manner for several generations on Grasses during
the summer in its uredo-form, the production of a new form of spores begins in the
older uredo-fruits ; the long two-celled teleutospores begin to be formed near the
roundish uredospores (Fig. 223, ///, /). The formation of uredospores in the uredo-

FlG. 22i. — Pucctttta Gramuiis. A germinating- teleutospore t, the promycelium of which forms the sporidia sp;
B a promycelium (after Tulasne) ; C a piece of the epidermis of the lower surface of the leaf of Berberis vulgaris with a
germinating sporidium sp ; i its germinating filament penetrating the epidermis ; D a germinating uredospore 14 hours after
dissemination (after De Bary, /. c).

fruits then entirely ceases, and teleutospores only are produced (Fig. 223, 77), and with
them the period of vegetation closes. The teleutospores persist on the grass-haulms
through the winter, and do not germinate till the spring ; they emit from their two cells
short septate germinating filaments (Fig. 224, A, B), the promycelia, at the ends of
which, on slender branches, the sporidia are produced. These sporidia develope a new
mycelium only when they germinate on the surface of the leaves of the Barberry; their
mode of germination differs from that of the other forms of spores, their germinating
filaments penetrating, as in the Peronosporeae, into and through the epidermis-cell (Fig.
224, C, sp and /), and thus reaching the parenchyma. They there form a mycelium

CARPOSPOREm. 2,^^

which produces the swelling of the leaf constituting the first stage which we considered,
a mycelium which bears spermogonia and aecidium-fruits.

The genus Roestelia possesses no uredospores. Its aecidium-fruits which make their
appearance in July and August upon the leaves, petioles, and fruits of the Pomaceae
{Pyrus, Cydonia, Sorbus) resemble long-necked flasks and may become as much as eight
millimetres long ; they open either at their apices or laterally by means of slits. The
chains of spores present a peculiarity which occurs also in other instances, that between
any two spores there lies a sterile cell which subsequently decays. The teleutospore-
fruits belonging to Roestelia (formerly known as Gymno sporangium) appear upon species
of Juniperus in the spring as spherical, conical, clavate, tongue-shaped, or palmate gela-
tinous masses of a yellow or brown colour. They consist of closely-placed basidia
arising from the mycelium which extends beneath the epidermis of the leaves and in
the cortex of the branches, and bearing the teleutospores. The teleutospores resemble
those of jEcidium Berberidis, and like them produce promycelia on germination, the
sporidia of which reproduce Roestelia with aecidium-fruits upon the leaves of Pomaceae.

Under the name of HypodermiesB De Bary unites the Uredineae with the Ustila-
gineae, which, however, do not seem to be very closely related to them, l^he
UstilaginesB ^ (Smuts) are parasitic in the tissues of Phanerogams, especially of Grasses,
in which their mycelium ramifies without at first effecting any injury. It is only when
the fructification, consisting of spherical dark-coloured conidia, is formed that the
vegetable organ in which it occurs becomes deformed : this usually swells up into
a vesicle, the whole of the internal tissue being absorbed and replaced by a black
powder, the conidia of the Fungus. Maize seeds are in this way converted by the
Ustilago Maidis into vesicles of the size of a nut, which, on bursting, liberate the
powdery conidia; Oats attacked by the smut are entirely filled with the conidia of
Tilletia caries. The germinating conidia produce a small promycelium which bears
sporidia : the hyphae developed from the sporidia penetrate into the sprouting grain
and the mycelium continues to grow until it produces conidia in the ears.

C. The Basidiomycetes ^

Although this division includes the largest and most beautiful of the Fungi, yet
it is just here that our knowledge of their life-history is most imperfect. All that

^s certainly known is that the basidiospores developed upon the large fructifications
consisting of masses of hyphse, germinate, forming mycelia, and that at a later

Iperiod these mycelia bear fructifications. A development of sexual organs, by
leans of which the formation of the fruit could take place, has not as yet been

[observed upon the mycehum ; still, a consideration of our knowledge with regard to

Jthe Ascomycetes, more especially the Discomycetes, makes it at least probable that
the spore-producing fructification is to be regarded as a true fruit which owes its

f^origin to the as yet undiscovered sexual organs existing upon the mycelium^.

[However this may be, the whole process of development naturally divides itself in

^ [Tulasne, Memoires sur les Ustilaginees ; Ann. Sci. Nat., ser. 3, VII, ser. 4, II. — De Bary,
[Unters, ueb. die Brandpilze, Berlin 1853. — Fischer von Waldheim, Aper9U systematique des Ustila-
^ginees, Paris 1877. — De Bary, Protomyces microsporus und seine Verwandten, Bot. Zeitg. 1874.]

^ See De Bary. Morphol. u. Physiol, der Pilze, Flechten, und Myxomyceten, Leipzig 1866.

^ [From the researches of Brefeld (Basidiomycetes, 1877) it appears that these plants have no
fsexual reproduction. Their large fructifications are comparable to the asexual conidia-bearing
fructifications of the Ascomycetes, The so-called 'basidiospores' are, like the 'uredospores' and
teleutospores ' of the iFcidiomycetes, merely conidia.]

^^6 THA LL OPHVTES.

this case also into two principal stages, the first being that of a filamentous mycelium
derived from the spores, the second, that of a fructification of solid tissue pro-
ducing numerous spores. No other organs of reproduction are known to recur with
certainty in the Basidiomycetes (see De Bary, loc. ciL).

The external form and the internal structure of these fructifications vary to
a very great extent, but the formation of the spores takes place upon one common
plan. Certain branches of the fertile hyphae become swollen so as to be club-
shaped, and constitute the cells which bear the spores, the Basidia. Each basidium
produces simultaneously two or more, usually four, basidiospores (or even eight).
They are formed by the outgrowth of the wall of the basidium into delicate papillae
which become spherically or ovally dilated at their free ends : each of these dilata-
tions becomes invested by a firm membrane, and is a spore which remains for a time
upon the pedicle but at length falls off.

The basidia are developed simultaneously in large numbers and are usually
closely arranged parallel to each other ; in this way the hymenia are formed, which,
among the Hymenomycetes, contain, as in the Discomycetes, sterile cells (para-
physes) among the fertile ones (basidia). If the hymenial layer clothes its free
external surface, the fructification is said to be gymnocarpous ; if, however, it lines
cavities within its tissue, the fructification is said to be angiocarpous.

The majority of Basidiomycetes grow upon humus, or soil containing decaying
vegetable matter; some develope their mycelium in old wood, as in the cortex
of living tree-trunks ; the smaller forms use fallen leaves and decaying branches as
their substrata. More rarely they occur as true parasites upon living vegetable

The following account will suffice to draw attention to some of the most widely
differing forms which are of morphological importance.

(i) The simplest form of fructification is found in Exobasidium Vaecinii^, the
mycelium of which is parasitic in the leaves and stems of Vaccinium Vttis idcea. On
the surface of the organs which it has attacked, the mycelium directly bears a hyme-
nium consisting of closely-packed basidia, each bearing four spores.

(2) The gelatinous Fungi, TremellineaB, which grow upon dead wood or upon the
trunks of old living trees, produce fructifications of a gelatinous consistency and of
irregular form, usually occurring as thick rugose incrustations. The delicate hyphae
run in the gelatinous mass and form the hymenia at the surface. The formation of the'
spores takes place in a more complicated manner than in the other Basidiomycetes ^.

(3) Among the Hym.eiiom.ycetes the best known and most abundant species ar^
those commonly known as Mushrooms. The structure which is usually called the Fungus
is the fructification which springs from a mycelium vegetating in the ground, or on wood
or some other substance. Usualhy, but not always, the cap [pileus) is stalked ; on its under--
surface the hymenial layer lies upon projections of the substance of the pileus of various^
forms. In the genus Agarlcus these projections consist of numerous lamellae attached
vertically and running radially from the summit of the stalk to the margin of the pileus ;
in Cyclomyces the lamellae form concentric circles ; in Polyporus and Doedaka they ana-
stomose in a reticulate manner ; in Boletus they form closely crowded vertical tubes ; in
Fistulina the tubes stand alone; in Uydnum the lower side of the pileus is covered with soft

* Woroniii, Bericht der naturf. Gesellsch. in Freiburg, vol. IV. 1867.

^ See Tulasne, Ann. des Sci. Nat., 3rd series, vol. XIX. [Also, ih. ser. 5, XV, and Brefeld,
loc. c//.]

CARPOSPOREM.

!>

Z31

dependent spines like icicles, the surface of which bears the hymenium, &c. In many-
cases the fructification is naked ; in others the lower side of the pileus is covered with a
membrane which is afterwards ruptured [njelum partiale), or the pileus and stalk are both
enveloped in such a membrane {'velum uni'versale) ; or finally, in a few species {Amanita)
both are found. This formation of a velum is connected with the entire growth of
the whole fructification ; the naked pilei are essentially gymnocarpous, those covered by
a velum indicate a transition to the angiocarpous fructifications of the Gasteromycetes.
Jlgaricus 'variecolor is to a certain extent an intermediate form between those with naked
pileus and those furnished with a universal velum. The fructification in this species
.arises as a slender cone on the mycelium (Fig. 225, /, a, b), consisting of parallel

Fig. 225. — Agaricus variecolor. /mycelium m, with yoimg
fructifications a b (natural size) ; c longitudinal section of one of
the latter (magnified) ; //an older fructification, with commence-
linent of the formation of the pileus ; ///the same in longitudinal
section ; IV s. more mature pileus, v the velum. The lines in the
Bctions indicate the course of the hyphae.

. ■. .'li.

Fig. 226. — Agaricus campeslris (natural size).

fhyphae growing at the apex (/, c) ; an outer layer of hyphae is present at an early stage
'Surrounding the whole body as a loose envelope; afterwards the direction of growth
alters, the branches of the hyphae turn outwards beneath the apex (77, 777) and thus form
^the pileus {IV), the margin of which continues to grow centrifugally ; the lamellae are
formed on its under-surface : as the distance of the margin of the pileus from the stalk
increases, the loose peripheral layer of hyphae becomes stretched {IV, 1;), and forms a
rudimentary universal velum. An example of the formation of a stalked pileus with a partial
velum is afforded by the common mushroom {Agaricus campestris). Fig. 226 shows at ^ a
small piece of the greatly extended reticulately anastomosing mycelium {m), from which
spring a number of fructifications ; these are at first solid pear-shaped bodies composed
of young hyphae all similar to one another. At an early stage the tissue of hyphae gives
way beneath the apex, leaving an annular air-cavity (77, /), the upper wall of which forms

z

33^

THALLOPHYTES.

the under-side of the pileus ; and from this the radial hymenial lamellae grow downwards
(///, /), filling up the air-cavity. The hyphae run from the base of the whole fructification
to the margin of the pileus, forming the outer wall of the air-cavity; the tissue lying in
the centre elongates into the stalk {IV, st), while the distance from it of the margin of
the pileus constantly increases ; the hyphae which lie beneath the air-cavity that con-
tains the lamellae become stretched in consequence, and separate from the stem from
below upwards, forming a membrane {V, v), running from the upper part of the stalk
beneath the lamellae to the margin of the pileus, into which their hyphae are continued.
When at length the pileus extends horizontally from the elongation of the tissues, the

membrane (velum) becomes detached
from its margin, and hangs from the
stem like a ruffle {annulus). (Compare
also Fig. 79, p. 96, Boletus Jlanjidus .)

The hymenium, as has already been
mentioned, covers the surface of the
lamelliform, peg-shaped, or tubular
projections of the under-side of the
pileus. A transverse section of the
latter across the hymenium gives, in all
three cases, nearly the same figure, as
is seen in Fig. 227, drawn from Agaricus
campestris. A shows a piece of the disc
of the pileus cut transversely, h the
substance of the pileus, /the lamellae;
in 5 a piece of a lamella is more strongly
magnified , to show the course of the
hyphae. The substance of the lamella,
called the Trama (t), consists of rows
of long cells, which diverge from the
centre right and left to the outside,
where the cells of the hyphae are short
and round, and form the sub-hymenial
layer {sb in JB and C). From these short
cells spring the club-shaped cells (q),
densely crowded and at right angles to
the surface of the lamella, forming
together the hymenial layer {B, by).
Many of these remain sterile, and are
called Parapbyses, others produce the
spores and are the Basidia, Each basi-
dium produces in this species only two,
in other Hymenomycetes usually four
spores. The basidium first of all puts
out as many slender branches {/) as
there are spores to be formed ; each of these branches swells at the end, the swelling
increases and becomes a spore (j", s'"), which falls from the stalk on which it was
placed, leaving it behind {s"").

On the formation of the tissue of this group only one further remark need be made ;
that in the fructification of some Agaricinae {e.g. Lactarius) some of the much-branched
hyphae are transformed into laticiferous vessels, from which large quantities of latex flow
out when they are injured.

(4) The Gasteromycetes ^ agree with the previous group in the mode of formation

Fig. 227. — Agaricus campestris; structure of the hymenium; A, B
slightly magnified ; C a part of 5 (X 35o)' The protoplasm is indicated by
fine dots.

* [De Bary, Morphol. und Physiol, d. Pilze, 1866. — Brefeld, Basidiomycetes, p. 175.]

CARPOSPOREJE.

339

of their spores (eight spores are often produced on a basidium) ; but their fructifications
are always angiocarpous. 'The hymenia are formed in the interior of the fructification,
which is at first usually spherical, or at any rate does not present externally any distinc-
tion of parts. The spores are disseminated by means of remarkable differentiations of
the different layers, the growth of particular masses of tissue, or the simple bursting
of the outer layer (the peridium). The nature of these processes, which are extremely
various in their external appearance, may be understood from two examples. The first
example, Crucibulum 'vulgare^, is selected from the beautiful NidularieaB. The my-
celium forms a small white crust of branched hyphae, which extend over the surface of
wood. In the middle of the crust the filaments are interwoven into a roundish body, the
rudiment of the fructification ; this grows by the intercalation of new branches of the
hyphae, and gradually assumes a cylindrical form. The outer hyphae form at an early
stage yellowish-brown branches, which are again branched and directed outwards, form-
ing a dense covering of hair. While the spherical fructification is becoming changed

Fig. ■22%.— Crucibulum -vtUgare ; A, B, C
in longitudinal section (slightly magnified) ;
D the entire plant nearly mature (natural
size).

Fig. -zi^.— Crucibulum. vulgare; longitudinal section through the
upper part of a young fructification (X about the same as Fig. 228, B).
The section is seen by transmitted light ; the dark parts in the interior
are those where air occurs between the hyphae; at the light parts a
transparent mucilaginous substance free from air has formed between
the hyphae. The light parts of this figure are dark in the previous ones.

into a cylinder, a large number of brown threads shoot out from it (Fig. 228, C, rf),
which form a firmly-woven layer, the outer peridium, and on the outside of this a dense
mass of radially projecting hairs. The walls of the hyphae of this part assume a dark
colour, but the inner tissue remains colourless (Fig. 228, A)\ its apex increases in
breadth, the hairs separate from one another, and the outer peridium ceases to exist at
the apex (Fig. 229, «/>). In the meantime the differentiation of the tissue commences in
the interior of the Fungus, which is at first formed of densely-woven much-branched
hyphae, enclosing amongst them a considerable quantity of air which gives the whole
a white appearance. Certain portions of the air-containing tissue become mucilaginous
and freed from air; between the threads is formed in some places a hygroscopic
transparent jelly, while in others none is produced. The conversion into mucilage
begins first below the surface of the white medulla (Fig. 228, A)^ and its outer layer is
thus transformed into an inner peridium which is a colourless sac projecting beyond the

^ Compare Sachs in Bot. Zeitg. 1855.
Eidam, in Cohn's Beitrage, vol. 2, 1877.]

[See also Tulasne, Annales des Sci. Nat. 1844, vol. I;

340

THALLOPHVTES.

dark outer peridium, and composed chiefly of branches of hyphae running longitudinally
upwards (Figs. 229 and 230, ip). While this differentiation is proceeding from below
upwards, small mucilaginous areolae form at certain points in a deep layer of the white
air-containing medulla, also proceeding from below upwards, like all the succeeding
differentiations (Fig. 228, 5, and Fig. 229). The formation of mucilage advances at
the same time from the inner peridium inwards, and leaves round each of the muci-
laginous areolae a border of air-containing tissue (Fig. 229), which afterwards developes,
by the dense interweaving of its branched hyphae, into a firm envelope consisting of two
layers, in which the mucilaginous areola lies. Each of these areolae becomes a hyme-
nial chamber. While the centre of the Fungus is becoming changed into mucilage,

FIG. 230. — Criicibiihitn vulffare; longitudinal section through the
upper part of the right side of the mature fructification, showing the
course of the filaments ; for the sake of clearness the number of filaments
has been reduced and their thickness increased.

Fig. 231.— Longitudinal section of a nearly ripe
fructification of Phallus impiidicus immediately be-
fore the elongation of the stalk (J the natural size) ;
a outer layer of the peridium ; g its gelatinous layer ;
i inner peridium ; j^the stalk of the pileus t not yet
elongated, covered by the white honeycomb-like
ridges ; sp the dark -green mass of spores (gleba) ;
h hollow cavity of the stalk, filled with watery jelly ;
n the cup in which the base of the stalk remains after
its elongation ; x the place where the inner peridium
becomes detached by the elongation of the stem ;
m mycelial filament.

the chambers grow into lenticular bodies ; a mucilaginous point has appeared at an
early stage on the lower and outer part of each chamber, and forms its umbilicus.
From it a denser bundle of threads runs downwards to the peridium, the umbilical
bundle (Fig. 229, «, and Fig. 230, ns)\ this is itself surrounded by a conical bag (/)
which surrounds the bundle like a loose sheath. This sheath eventually becomes
mucilaginous; the bundle runs upwards into the mucilaginous depression of the
umbilicus, where it is resolved into its threads which are now more loosely connected.
The mucilaginous tissue in the interior of each chamber disappears, leaving a len-
ticular space similar in form to the chamber itself; and from the inner layers of the

CARPOSPOREJE.

341

hyphae of the chamber branches now arise which are directed inwards and form the
hymen ium. Each chamber is therefore clothed on its inner surface by a hymenial
layer formed of paraphyses and basidia; each of the basidia produces four spores
on short stalks. As the Fungus matures, the upper part of the peridium becomes
stretched and flat, forming the Epiphragm; it afterwards ruptures and disappears, and
the Fungus thus opens into a cup. The mucilage which surrounds the chambers dries
up, and the chambers now lie free in the cup formed by the peridium, held by their
umbilical bundles, which, when moistened, may be drawn out into long threads. If we
imagine the chambers more numerous and more closely packed and with less dense walls,
we obtain an explanation of the roundish cell-like loculi which occur in the fructification
of other Gasteromycetes (as Octwuiania, Scleroderma, &c.).

Still more remarkable are the changes produced in the Phalloide89 ^ by internal
differentiation of the tissues; but of these only the most important points can be
illustrated in the case of Phallus impudicus. Here also the young fructification
formed on the underground perennial mycelium is at first a homogeneous convolution
of hyphal filaments, in which differentiation begins and advances during growth.
When the body has attaineid the size and form of a hen's or even a goose's egg,
a longitudinal section gives the appearance represented in Fig. 231. The tissue
consists at this time of different portions which may be classified into four groups —
(i) The peridium, composed of an outer firm, thick, white membrane («), of an inner
white, firm, but thin membrane (z), and of an intermediate thick layer of mucilaginous
hyphae {g), (the gelatinous layer) : (2) The spore-forming apparatus or Gleba {sp)y
bounded on the outside by the inner peridium (/'), on the inside by a firm thick layer
(/) from which walls project outwards united in a honeycomb manner dividing the
gleba into a number of chambers. In these chambers the fertile branches of the hyphae
are found in great numbers, and on their basidia are formed four or more spores ; so
that, when ripe, the dark-green gleba appears to consist almost entirely of spores:
(3) The stalk {st), formed of air-containing tissue hollowed into a large number of
very narrow chambers; its axial portion is transformed into a deliquescent jelly, and
the canal thus formed is open above in some individuals, in others it is closed by
the inner peridium : (4) The Cup (n) forms a low broad column of firmer tissue,
the outer part running upwards into the inner peridium, and sending up at the same
time a layer which becomes softer between the stalk and the inner membrane of
the gleba (t) ; the base of the cup is continuous with the outer firm peridium. In
this state the spores ripen ; but for the purpose of their dissemination a great elon-
gation of the stalk (st) takes place ; the peridium is ruptured at the apex, the gleba
becomes detached from the inner peridium, this latter splitting at x, and the mem-
brane t becoming detached below. The gleba is by this means raised up high above the
peridium on the apex of the stalk, while the stalk attains the height of from 6 to 12
inches. This elongation is brought about by the expansion of its chambers, which
give the mature stalk the appearance of a coarsely porous sponge ; it increases in
thickness in proportion to its increase in length. The spores now drop oflf the gleba
in masses, the sporiferous hyphae deliquescing into thick tenacious mucilage ; till at last
nothing remains of the gleba but the membrane (t) with its honeycombed walls, which
depends like a frill from the apex of the stalk, and is called the pileus. The peculiari-
ties in the details of these processes exhibit the greatest variety in different species of the
Phalloideae.

^ [De Bary, Beitr. zur Morphol. u. Physiol, der Pilze, I. 1864.]

342 MUSCINEM,

GROUP II.
MUSCINE^.

The Liverworts (Hepaticse) and Mosses (Musci), which are comprised under
the term Muscineae, are distinguished by a sharply-defined Alternation of Generations.
From the germinating spore is developed either immediately a sexual generation
rich in chlorophyll and self-supporting (as in most Hepaticae), or a confervoid
thallus is first formed {Protonevia), out of which the sexual generation grows as
a lateral shoot (as in some Hepaticae and all Mosses). In the female sexual organ
of this first generation there arises, as a new generation — the result of fertilisation —
a structure of an entirely different form, which is destined exclusively for the
asexual production of spores. Without being organically united to the previous
generation, this structure is nevertheless nourished by it, and appears, when ob-
served externally, simply as its fruit, like the smaller fructifications of the Thallo-
phytes. Since however it is an organism of an altogether peculiar kind, it may
be desirable to give it a special name, which shall at once exclude any false analogy*;
I propose therefore to call it the Sporogonium.

The Sexual Generation (Oophore) of Muscineae which is produced directly from
the spore or with the intervention of a protonema, is either a flat leafless thallus,
as in many Hepaticae, or a slender leafy stem, often much branched. In both cases,
which are united by gradual transitional forms ^, a number of root-hairs are usually
formed, which fix the thallus or the stem to the substratum. In some cases this
vegetative body scarcely attains a length of i mm., but in others as much as
from lo to 30 cm. or even more, and ramifies copiously. In some of the smallest
forms its term of life is limited to only a few weeks or months ; in most it may
be said to be unlimited, since the thallus or the leaf-bearing stem continually grows
at its apex or by a process of renewal (Innovation), while the oldest parts die off"
behind. In this manner the branches become finally independent plants ; and this,
as well as the multiplication by gemmae, stolons, detached buds, the transformation
of hairs into protonema (in Mosses), &c., serves not only to increase enormously
the number of individuals formed by the asexual method, but is also the immediate
cause of the social or cespitose mode of growth of these plants. Many Mosses
in particular, even those which only rarely fructify, may in this manner form dense
masses extending over considerable areas (as Sphagnum, Hypnum, Mnium, &c.).

^ It is incorrect, for instance, to regard the ' fruit ' of a Moss as the morphological equivalent of
the sporangium of a Rhizocarp or of the fruit of a Phanerogam.

' From the great similarity of the true leafless thallus of some Hepaticse to the thalloid stems
of others furnished with leaves on the under side, it will be convenient to use the term ' thalloid
forms' for both ; the term including both a true thallus (e. g. Anthoceros) and also a thalloid stem
(as in Marchantia).

INTRODUCTION. 343

The sexual organs are Anther idia and Archegonia. The mature antheridium is
a body with a longer or shorter stalk, of a spherical, ellipsoidal, or club-shaped form,
the outer layer of its cells forming a sac-like wall, while each of the small and very
numerous crowded cells enclosed within it developes an antherozoid. The anther-
ozoids are freed by the rupture of the wall of the antheridium at the apex ; they
are spirally coiled threads thicker at the posterior and tapering to a fine point at
the anterior end, at which are placed two long fine cilia, the vibrations of which
cause their motion. The female organs, which since the time of Bischoff have been
called archegonia, are, when in a condition capable of being fertilised, flask-shaped
bodies bulging from a narrow base and prolonged into a long neck. The wall
of the ventral portion encloses the central cell, the inferior and larger part of which
forms the oosphere. Above this begins a row of cells which passes through the
neck in an axial direction, and is continued as far as the cells which form the
so-called ^Stigma.' The cells of this axial row become broken up before fertili-
sation, and transformed into mucilage which finally swells up and forces apart the
four stigmatic cells. In this manner an open canal is formed, which leads down
as far as the oosphere, and enables the antherozoids to enter it.

The great diversity in the origin of the sexual organs of Muscinese is of
great importance. In the thalloid Hepaticae these organs arise behind the
growing apex from the superficial cells of the thallus or of the prostrate thalloid
stem, or on specially metamorphosed branches (as in the Marchantiese) ; in the
foliose Jungermanniese and in the Mosses not only the antheridia but also the
archegonia may be formed from the apical cell of the shoot or from segments
of it; in this case they may take the place of leaves, or of lateral shoots, or
even of hairs. Thus the antheridia appear as metamorphosed trichomes in the
axils of the leaves of Radula, as metamorphosed shoots in Sphagnum, as apical
structures and also as metamorphosed leaves in Fontinalis. In the same manner
the first archegonium of the fertile shoots of Andrecea and Radula arises from the
apical cell, the later ones from its last segments; and this is probably the case in
Sphagnum.

The antheridia and archegonia are usually produced in great numbers in close
proximity; in the thalloid forms of the Hepaticae they are generally enveloped
by later outgrowths of the thallus ; in the foliose Jungermannieae and in Mosses
several archegonia are commonly surrounded by an investment formed of leaves
which is termed the PerichcBtium ; in Mosses the antheridia (with sometimes some
archegonia) are usually borne in this manner also, while the antheridia of the
Jungermannieae and of Sphagnum stand alone. Very commonly, especially in the
foliose kinds, Paraphyses, i.e. articulated threads or narrow leaf-like plates of cells,
are developed by the side of the sexual organs. Besides the perichaetium, there
is also often in Hepaticae (but not in Mosses) a so-called Pertgynium, which grows
as an annular wall at the base of the archegonia, and finally surrounds them as an
open sac.

The Asexual Generation (Sporophore), the Sporogonium, arises in the archego-
nium from the fertilised oosphere (oospore). It first developes by repeated cell-
divisions into an ovoid embryo, growing at the end turned towards the neck of
the archegonium, that is, the apex. Its final form is very different in different

344 MUSCINE^.

sections. In its lowest type (in Riccid) it is a globe, the outer cell-layer forming
the wall, while all the inner cells become spores. In all other cases the sporo-
gonium becomes differentiated externally into a stalk, which may be short or
long and slender, termed the Seta, and which penetrates into the bottom of the
archegonium and even into the underlying tissue, its base often becoming dilated,
forming the Foot, and a Capsule (Urn or Theca) turned towards the neck of the
archegonium, in which the spores arise. Together with the spores, long cells
thickened by spiral bands, the Elaters, are also produced in most Hepaticae. The
internal differentiation of the spore-capsule is, in addition to this, very varied, and
attains a very high degree of complexity, especially in the Mosses.

While the sporogonium is developing, the ventral portion of the archegonium
also continues to grow ; its cells rapidly increase in number, and it thus becomes
broader, enclosing the young sporogonium, and, in this condition, is termed the
Calyptra. Its behaviour supplies distinctive characters for the larger groups. In
the lowest Hepaticse {Riccia) the sporogonium remains always enclosed in the
calyptra ; in the higher Hepaticae it protrudes only after the ripening of the spores,
its seta elongating suddenly, and the capsule protruding from the ruptured calyptra
for the purpose of disseminating the spores, the calyptra surrounding the base of
the seta as a cup-like membranous structure. In the typical Mosses, on the
other hand, the young sporogonium first assumes the form of a greatly elongated
fusiform body, which, even before the development of the capsule, exerts a strong
upward pressure upon the calyptra, which becomes ruptured at its base, and
is raised up by the young sporogonium in various forms ; the seta penetrates
deep down into the tissue of the stem, by which it is surrounded as a sheath
( Vaginula).

The spores of the Muscineae arise in fours ; the mother-cells — which had
previously been united into a tissue with the surrounding cell-layers, but had
become isolated even before the formation of the spores — show a rudimentary
division into two previous to complete division into four. The number of the
mother-cells and the place where they are produced in the sporogonium depends
essentially on the internal differentiation of the latter. The ripe spores show a
thin cuticle (the exospore) provided with small excrescences, which is ruptured on
germination by the inner layer of the cell-wall (the endospore). Their contents con-
sist, in addition to colourless protoplasm, of chlorophyll-granules, starch, and oil.

The Differentiation of the Tissues of Muscineae is very various, and more con-
siderable than in the Algae, but less so than in the Vascular Cryptogams. Fibro-
vascular bundles are not found ; only in the stem and leaf-veins of the more perfect
Mosses is an axial bundle of elongated cells differentiated, which may be con-
sidered as a slight indication of a fibro-vascular system. The Marchantieae, on
the other hand, show on the upper side of their thalloid stems, and the Mosses on
their thecae, a distinctly differentiated epidermis, which usually also forms stomata.
The cell-walls of the Muscineae are generally firm, often thick, tough, and elastic,
and in this case frequently of a brown, bright red, or violet colour. The tendency
towards the formation of jelly and mucilage, so general in the Thallophytes, is not
found in the Muscineae, with the exception of certain processes in the mother-cells
• of the spores. Various forms of thickening are not uncommon, especially in the

INTRODUCTION.

345

jpore-capsule, in the spiral bands of the elaters of Hepaticae, and in the formation
)f the epidermis and peristome of the thecae of Mosses.

Classification of MuscineoB. The sexual generation is developed from the spore,
generally after the previous formation of a protonema. It is the longest-lived of the
two generations, and constitutes the self-supporting vegetative body of these plants,
presenting either a flat dichotomously branched thallus, or a thalloid stem, or a filiform
stalk furnished with from two to four rows of leaves. True fibro-vascular bundles
are not developed. The archegonia and antheridia are, except in the simplest thalloid
forms, stalked multicellular bodies, usually free, but sometimes buried in neighbouring
masses of tissue from the subsequent growth of these latter. The central cell of the
ventral part of the archegonium undergoes division, a ventral canal-cell being cut off,
and the remainder of its protoplasm constitutes the oosphere by rejuvenescence. The
antherozoids are spirally coiled threads with two cilia on the anterior pointed end.

The asexual generation or sporogonium arises from the fertilised oosphere within the
actively growing ventral part of the archegonium, which becomes developed into the
calyptra. The sporogonium is nourished by the sexual plant; it has therefore no
independent existence, and appears externally as an appendage to it. It is usually a
stalked capsule, in which usually a number of cells are developed into the mother-
cells of the spores ; and from these the spores are formed by division into four after
bipartition has commenced but has not been completed.

(i) Hepaticce. The sexual generation arises either directly from the spore or with
the intervention of a small inconsiderable protonema. It is developed as a flat dicho-
tomously branched thallus or a thalloid stem, or finally as a filiform stalk furnished with
two or three rows of leaves. This vegetative body is usually broadly expanded and
clings closely to the ground or to some other substratum ; even when the stems grow
erect there is still an evident tendency towards the distinction of an upper (dorsal) and
an under (ventral) surface. The mode of growth is hence^ always distinctly bilateral.
The asexual generation or sporogonium remains surrounded by the calyptra until the
spores are ripe ; the calyptra is usually at length ruptured at the apex, and remains at
the base of the sporogonium as an open sheath, while the free spore-capsule projects
above its apex, to allow the escape of the spores. The mother-cells of the spores arise
either from the whole of the cells except those of the single layer which forms the wall
of the capsule, or the intermediate cells commonly become developed into elaters: a
columella is present only in the Anthoceroteae.

(2) Musci. The sexual generation is developed from the spore with the intervention
of a protonema consisting of branched rows of cells and often vegetating for a con-
siderable time independently, even when it has already produced leafy stems by lateral
budding. The vegetative body is here always a cormophyte, a filiform stem furnished
with leaves in two, three, or four rows, usually without any definitely indicated bilateral
structure, and generally branched in a monopodial, never in a dichotomous manner.
The asexual generation or sporogonium is only at first formed in the calyptra ; after-
wards this is usually ruptured below (at the vaginula), and raised up by the apex of
the sporogonium, which it covers like a cap. The capsule, which is now first developed,
produces the spores from an inner layer of tissue, while a large central mass of tissue
remains sterile and forms the columella. The wall of the capsule is covered by a
distinctly differentiated epidermis, the upper part of which usually becomes detached
from the lower part (the Urn) in the form of a lid, in order to allow of the escape of
the spores.

34^ MUSCINEJE.

CLASS V.
H E PATIC^i.

(i) The Sexual Generation (Oophore) is developed, in some genera, directly
from the germinating spore, its first divisions resulting in the formation of a cellular
lamina or a mass of tissue which fixes itself by root-hairs and produces the thallus
by growth at its apex, as in Anihoceros and Pellia. In other cases the body which
results from the divisions of the spore first forms a narrow ribbon-like lamina of
cells, the apical cell of which becomes subsequently the apical cell of a stem, and
its segments form leaves, as in Jungermannia bicuspidata (according to Hofmeister).
Or again, the bud of a leafy stem springs immediately from the spore {Frullania
dilatatd). In other cases, on the other hand, a protonema is formed ; the endospore
which grows out into the form of a tube produces a short articulated filament on
which the rudiments of the thallus are formed as lateral shoots, in a manner similar
to the leaf-buds of Mosses on the protonema {e.g. Aneura palmaia^ Marchantia).
In Radula the spore produces first of all a flat plate of cells, from which the first
bud of the leafy stem springs laterally (Hofmeister), a process which finds its ana-
logue among Mosses.

The vegetative body of Hepaticae is always formed in a distinctly bilateral
manner; its free side, turned towards the light, is differently organised from that
which faces and often clings closely to the substratum and is not exposed to light.

In the greater number of families and genera the vegetative body is a broad,
flat or curled plate of tissue, varying in length from a few millimetres to several
centimetres ; and is either a true thallus without any formation of leaves, as in
Anthoceros, Meizgeria, and Aneura, or lamelliform outgrowths arise on the under
or shady side, which at the same time produces root-hairs ; and these outgrowths
may be looked on as leaves. For the sake of having a common expression for these
forms extremely similar in habit, they may be comprised under the term Thalloid'^,

^ Mirbel, Ueber Marchantia, in the Mem. de I'Acad. des Sci. de I'lnst. de France, vol. XIII,
1835.— G. W. Bischoffjin Nova Acta Acad. Leopold. Carol. 1835, vol. XVII. pt. 2.— C. M. Gottsche,
ibid., vol. XX. pt. i. — Gottsche, Lindenberg u. Esenbeck, Synopsis Hepaticarum, NUrnberg, 1844. —
Hofmeister, Vergleich Untersuchungen, 1851. — [On the Germination, Development, and Fructifica-
tion of the Higher Cryptogamia : Ray Society, 1862.] — Kny, Entwickelung der laubigen Lebermoose ;
Jahrb. fiir wiss. Bot. vol. IV. p. 66, and Entwickelung der Riccien, ibid., vol. V. p. 359. — Thuret, in
Annal. des Sci. Nat. 1 851, vol. XVI (Antheridia).— Strasburger, Geschlechtsorgane u. Befruchtung bei
Marchantia; Jahrb. fiir wiss. Bot. vol. VII. p. 409: [also Befruchtung und Zelltheilung, 1878].
— Leitgeb, Wachsthumsgeschichte der Radula cotnplanata ; Sitzungsber. der Wiener Acad. 1871,
vol. LXIII. — Ibid., Bot. Zeitg. 1871, no. 34, and 1872, no. 3. — A portion of what is said about the
apical growth of Jungermanniese is derived from communications by letter from Leitgeb. — Janc-
zewski, Bot. Zeitg. 1872. — [Leitgeb, Unters. ueb. Lebermoose, 1874-78. Goebel, Zur vergl. Anat.
der Marchantieen, Arb. d. bot. Inst, in Wiirzburg, II. 3, 1880.]

^ [The term 'thalloid' is here, as on p. 342, preferred to the one in more general use,
' frondose.']

ifEPATICM.

347

in contrast to the Foliose Hepaticae belonging to the family of Jungermannieae, the
vegetative body of which consists of a small slender filiform stem, bearing distinctly
differentiated leaves {Jungermannia, Radula, Mastigobryum^ Frullania^Lophocolea, «&c.).
Between the thalloid and foliose forms of this family are some which present various
stages of transition (as Fossombronia and Blasia).

The leaves of all Hepaticae are simple plates of cells, in which even the
mid-rib usual in the leaves of Mosses is always wanting.

In most of the thalloid forms the growing apical region of each shoot
(Fig. 232, j) lies in an anterior depression, produced by the more rapid growth in
length and breadth of the cells which are derived right and left from the seg-
ments of the apical cell. In the Anthoceroteae, Riccieae, and Marchanticae, there
is a group of apical cells, and the terminal branching is truly dichotomous. In
the thalloid Jungermannieae, there is a single apical cell; the terminal branches
originate from the youngest segments of this cell, and, from their position in the

Fig. i-yi.—Metzgeriaftircata; the right-hand figure seen from the upper, the left-hand figure frotn the under
side ; tn the mid-rib ; s, s', s" the apical region ; ^y wing-Hke expansion formed of a single layer oicaWs;/'/"/'"
its mode of development after branching (X about 10).

depression and their active growth, push aside the apex of the primary shoot,
and form with it a fork (false dichotomy). In the angle between the two bifur-
cations the permanent tissue increases more rapidly, and forms, so long as the two
forks are still very short, a projection (Fig. 232,/"', y") which overtops and separates
the apical regions, but which, when the forks are longer, is in turn overtaken by them,
and now appears as an indented angle of the older fork {/). The filiform stem
of the foliose Jungermannieae, on the other hand, ends in a bud as a more or less
prominent vegetative cone, with a strongly arched apical cell. In this case also the
lateral branches spring from individual mother-cells, which, however, do not origi-
nate from the youngest segments of the apical cell, but lie, even at their first
formation, some distance below the apex; the branching is therefore, at its
commencement, distinctly monopodial.

We shall speak, under the separate sections, of the form of the apical cell,
which forms two, three, or four rows of segments; as well as of the origin of the

348

MUSCINE^.

leaves and lateral shoots, since Leitgeb's researches show that great morphological
differences occur in the different genera. For the same reason very little of a
general character can be said, in addition to what has been mentioned above, on
the habit and anatomical nature of the vegetative body, which must therefore be
considered under the separate families.

The Asexual Propagation of Hepaticse is often brought about by the dying off
of the thallus or stem from behind, the branches thus losing their connexion and
becoming independent. Adventitious branches, arising in the thalloid forms from
cells of the older marginal parts, become detached in a similar manner. The
propagation by gemmcE is very common and characteristic ; not unfrequently a
number of cells of the margin of the leaf of foliose Jungermannieae {e.g. in Mado-
thecd) simply detach themselves as gemmae ; in Blasta, on the other hand, as well as
in Marchantia and Lunulana, peculiar cupules are formed on the upper side of the
flat shoots exposed to the light, which are fiask-shaped in Blasia^ broadly cup-shaped

Fig. 233. — Marchantia folymorpha ; A, B young
shoots ; C the two shoots which result from a gemma,
with cupules ; -v v the depressed apical region : D a
piece of the epidermis seen from above : sj> stomata
on the rhomboid plates (A—C X slightly; D more
strongly).

FIG. 234.— Development of the gemmae
Marchantia,

of

in Marchantia, crescent-shaped and deficient on one side in Lunularia. From the
bottom of these cupules shoot out hair-like papillae, the apical cells of which be-
come transformed into a mass of considerable size constituting the gemma. (See
Figs. 233, 234.) From the two depressions which lie right and left on the margin
of the lenticular gemma (Fig. 234, F7) spring the first flat shoots (Fig. 233, B,, C),
when the gemmae have fallen out of the cupule and lie exposed to light on damp
ground.

The Sexual Organs are developed, in the thalloid forms, on the upper side
exposed to light; in Anthoceros in the tissue of the thallus itself (endogenous) ; in
the other thalloid forms from cells which project like papillae and are of definite
origin in reference to the segments of the apical cell. In the Marchantieae branches
of a very peculiar shape, which have a tendency to shoot upright from the flat
stem, are formed, producing the antheridia on the upper, the archegonia on the
under side ; the male and female receptacles may be distributed either monoeciously

349

If HEP A TIC M.

ir dioeciously. There is a general tendency in the thalloid Hepaticae for the
exual organs to be depressed into hollows by overarchings of the surrounding tissue,
often opening externally by only a narrow mouth. An example of this is given in
Fig. 235.

In the foliose Jungermannieae the origin of the antheridia and archegonia is
very various, and they are also enveloped in different ways. Further reference will
be made to this in describing the different families.

The antheridium consists, in the mature state, of a pedicel surmounted by
a globular or ellipsoid body; in those which are imbedded in the tissue the former
is usually short, in the free forms it is long, and com-
posed of from one to four rows' of cells. The body
of the antheridium consists of a wall formed of a single
layer of cells containing chlorophyll ; the whole of the
space enclosed by it is densely filled by the mother-
cells of the antherozoids ; their escape is occasioned
by the access of water and separation of the cells of
the wall at the apex ; sometimes, as in Fossombronia,
these cells even fall away from one another. The
small mother-cells of the antherozoids which escape
in great numbers, separate in the water; the anthero-
zoids become free, and have the appearance of slender
threads curved spirally from one to three times, and fig. 235.-Anterior margin of the youn?

.JJ..1 .. j'ili 1 n antheridial disc of Marchantia polymor-

provided at the anterior end with two long very tine pna; a the growing margin; a, a. a the

.,. 1 c ^ ' \ ,-\ • .!_ i. '1.1 antheridia in different stages of develop-

Cllia, by means 01 which they move m the water with ment : ^Z the stomata above the air-cavltles

. ,. ,. TT 11 o J /v ^1 i between the antheridia (after Hofmeister,

a rotating motion. Usually they drag after them at X300).

the posterior end a small delicate vesicle, the origin of

which Strasburger traces to the central vacuole in the protoplasm of the mother-cell,

in the periphery of which the antherozoid has been formed.

The succession of cell-divisions in the formation of the antheridia has been
shown by the researches of recent observers to present great diversities in the
different genera; they agree, however, in the antheridium always making its first
appearance as a papilliform swelling of a cell from which it is separated by a
septum. This papilla thus detached again divides into a lower and an upper cell,
the former of which produces the pedicel, the latter the body of the antheridium
(parietal layer and mother-cells of the antherozoids).

The succession of cell-divisions in the formation of the archegonia, from the
observations of Janczewski*, Leitgeb, Kny, and Strasburger, appears to be essentially
the same in the different families, even, mutatis mutandis^ among the Anthoceroteae.
It is certain that the archegonium, like the antheridium, makes its first appearance
as a simple papilla, which, in the case of the first archegonium of a receptacle
of Radula^ is itself the apical cell of the shoot. This papilla is shut off by a septum,
and, in Riccia^ is at once the mother-cell of the whole archegonium : in the other
Hepaticae it is divided by a second septum into two cells, the lower one of which

* [Janczewski has made a series of comparative researches into the development of the arche-
gonium of Muscinese, Bot. Zeitg. 1872, p. 869 et seq.]

350

MUSCINEM.

produces the pedicel, the upper one the archegonium itself. The lower cell
undergoes numerous transverse and longitudinal divisions into several rows of cells.
In the mother-cell of the archegonium there arise three longitudinal walls, by which
three outer cells are formed ; these, on their part, enclosing an internal cell which
overtops them. The three outer cells are divided by radial longitudinal walls and
thus form five or six investing cells ; the median cell is divided by a transverse
septum into an upper (stigmatic) and a lower cell. After the whole structure has
increased somewhat in length, it comes to consist of two tiers in consequence of the

Fig. 236.— Later stages in the development of the archegonia and origin of the sporogonium of Marchantia polymorpha ;
/, //, young archegonia; ///, IV, after absorption of the axial row of cells of the neck; Kwhen ready for fertilisation;
VI-VIII the cells of the mouth of the neck x relaxed after fertilisation ; the fertilised oosphere /"shows its first divisions. In
these figures I-V si is the ventral canal-cell which is last converted into mucilage, e the unfertilised oosphere ; // in V-VII
the perigynium in process of development; /JT the unripe sporogonium in the ventral portion of the archegonium which has
developed into the calyptra ; a neck of the archegonium ; y wall of the sporogonium ; st its stalk (foot) ; inside the sporogonium
are the young elaters arranged in rays, among them the spores. (I-VIII X 300, IX about 30.)

division of each of the six investing cells, like the internal cell, by a transverse
septum. The lower tier forms the ventral portion, the upper the neck of the
archegonium. The internal cell of the ventral portion, the central cell, increases
considerably in size and is divided by a transverse septum into a large inferior cell,
the oosphere, and a small upper cell, the ventral canal-cell. Meanwhile the upper tier
of cells, the neck of the archegonium, elongates; its axial cell dividing into four,
eight, or sixteen long narrow cells, the canal-cells of the neck. Further transverse

HEP A TIC^.

351

and longitudinal walls are formed in the external cells of the ventral portion, and
a wall consisting of one or two layers of cells is produced ; similarly, the wall of the
neck, which consists of five or six longitudinal rows of cells, is formed by the
transverse division of the peripheral cells of the upper tier. The primary stigmatic
cell divides into the five or six stigmatic cells of the neck. The cell originally
constituting the pedicel has also undergone both longitudinal and transverse divi-
sions. Whilst the oosphere is being formed, the walls of the canal-cells of the neck
and the transverse septum beneath the ventral canal-cell become converted into
mucilage, the swelling up of which forces the protoplasm of the canal-cells out
through the opened apex of the neck. (See Figs 236 and 256.)

(2) The Asexual Generation, the Sporogonium, arises and is entirely formed
within the growing ventral portion of the archegonium, which from this time is
termed the Calypira. The sporogonium does not anywhere unite in its growth
with the surrounding tissue of the vegetative structure of the sexual generation,
even when its seta penetrates into it.

The external form and internal structure of the sporogonium are very different
in the diff"erent groups. In the Anthoceroteae it is when mature an elongated two-
valved pod projecting from the thallus. In the Riccieae it is a thin-walled ball
entirely filled with spores, and, together with the calyptra, depressed in the thallus.
In the Marchantieae it is a shortly-stalked ball enclosing elaters as well as spores,
and, after it has broken through the calyptra, bursting irregularly or opening by a
circular fissure and detaching an operculum. In the Jungermannieae it also ripens
within the calyptra, but breaks through it and appears as a ball borne upon a
long slender stalk; the wall consists, as in the Marchantieae and Riccieae, when
ripe, of a single layer of cells, but separates cross-wise into four lobes, to which
the elaters remain attached. The elaters are, as in the Marchantieae, long fusiform
cells, the delicate colourless outer layer of which is thickened within by from one
to three brown spiral bands.

The sporogonium also originates in diff"erent ways. [The fertilised oosphere
is always first divided into two cells by a wall {basal wall) which, in the lower forms,
is inclined at an acute angle to the long axis of the archegonium, but in the Junger-
mannieae is at right angles to it. In Riccia the capsule is developed from the whole of
the oospore : in the Marchantieae and Anthoceroteae the short seta of the sporogonium
is developed from the lower or posterior {hypobasal cell) of these two cells, the
capsule being developed only from the upper or anterior {epibasal) cell : in the
Jungermannieae the capsule and the seta are developed from the upper (epibasal)
cell, the product of the development of the lower {hypobasal) cell being a small
filamentous appendage upon the dilated base (foot) of the seta. This first division is
followed by two others at right angles to it and to each other, so that the embryo
now consists of eight cells {octants). The subsequent divisions take place more
or less irregularly, but it appears that in the Jungermannieae the four epibasal
octants behave Hke apical cells, segments being continually cut off" horizontally,
but doubtless intercalary divisions also occur \] When the young sporogonium has

* [On the embryology of the Hepaticse, see Hofmeister, loc. ciL; Kienitz-Gerloff, Bot. Zeitg.
'874-5; Leitgeb, Entw. d. Kapsel von Anthoceros, Sitzber. d. Wien. Akad. 1876, and loc. cii.J

3.5^ MUSCINEM.

in this manner attained its destined height, and partially even at an earlier period,
a number of divisions of different kinds take place by which the structure is com-
pleted. The wall of the sporogonium becomes differentiated from the tissue from
which the mother-cells of the spores are to arise ; if elaters are formed they originate
from the same tissue, the cells -ceasing to divide transversely at an earlier period
and remaining long, while the intermediate cells become rounded off and give
rise to the mother-cells of the spores (Hofmeister).

The mode of division into four of the mother-cells of the spores also
varies. Those of Anihoceros form at first two, and afterwards four, new nuclei
which are arranged tetrahedrally, the protoplasm dividing before the nucleus ; cell-
walls are then formed, and thus the mother-cell breaks up into four spores \ In
Pellia and Frullania^ on the other hand, the division of the mother-cells commences
by four protuberances arranged tetrahedrally, which at length are cut off by cell-
walls; each contains a nucleus, and they form as many spores; in Pellia the spores
immediately again divide several times, and thus give rise to a young plant.

The Hepaticae are usualy divided into five families, viz.: —

1. Anthoceroteae,

2. Riccieae,

3. Monocleae,

4. Marchantieae,

5. Jungermannieae,

of which the first four include only thalloid forms, the fifth both thalloid and foliose
genera.

I. AnthocerotesB. Anthoceros IcB^vis said punctatus, which grow in summer on loamy
ground, develope a perfectly leafless flat ribbon-like thallus, its irregularly developed
ramifications forming a circular disc ; the regularity of the dichotomous branching is
disturbed by the adventitious shoots, which proceed from the margin of the thallus, and,
in A.punctatus, also from the upper surface. The thallus consists of several layers of
cells, and the apical cells of the branches which lie in the anterior depressions are
divided by walls inclined alternately upwards and downwards (Fig. 237, C). In each
of the cells of the thallus, the upper layer of which does not become differentiated
into an epidermis, only one chlorophyll-granule is formed, surrounding the nucleus.
On the under side of the thallus, Janczewski states that stomata are formed close behind
the growing margin, through which filaments of Nostoc frequently penetrate, forming
roundish balls in the tissue of the thallus (Fig. 237, B), which were at one time considered
to be endogenous gemmae'^. The antheridia and archegonia arise apparently without
any defilnite arrangement in the interior of the upper side of the thallus. The formation
of the antheridia commences by a circular group of cells of the outer layer separating
from the subjacent tissue and thus producing a broad intercellular space, several of the
lower bounding cells of which, after some vertical divisions, rise up in the form of
papillae, and form the antheridia (Fig. 237, 5, ««). It is only when the chlorophyll-
granules in the walls of the antheridia have assumed a yellow colour and the anther-
ozoids are mature that the roof of the cavity is ruptured, the antheridia opening
at their apex and allowing the antherozoids to escape. In the Riccieae and Mar-
chantieae the archegonia, which are at first free, become gradually surrounded by masses
of tissue, but in Anthoceros they are enclosed from the first. One of the superior

^ [On the development of the spores of Pellia and Anthoceros see Strasburger, Zellbildung und
Zelltheilung, 3rd ed. p. 156.]

2 [See Waldner, Ueb. die iVos/oc-Colonieen bei Blasia, Sitzber. d. Wien. Akad. 1878,}

HEP A TICM.

Z^Z

segmental cells, near to the growing-point of the thallus, divides into an outer and
an inner cell ; the outer (superior), which bulges slightly, is the mother-cell of the
archegonium and is further divided (as is the case in other Liverworts) by three longi-
tudinal walls vertical to the surface of the thallus, into one internal and three external
cells. The latter produce the six primary investing cells M^hich, at a later period,
give rise to as many rows of cells which are enclosed on all sides by the tissue of
the thallus. The internal cell is divided by a transverse septum, into two cells, the
lower of which becomes the central-cell, and by division gives rise to the oosphere and
the overlying ventral canal-cell ; the upper undergoes transverse divisions in consequence
of which a row of neck canal-cells is formed, as also the primary stigmatic cell which
subsequently divides cross-wise to form the stigmatic cells. The variations from the
mode of formation of the archegonium obtaining in the other Liverworts are thus seen
to be slight, and scarcely justify the formation of the Anthocerotese into a distinct class.
Fig. 237 C dates from a time at which the above-mentioned details were unknown, but
it suffices nevertheless to give some idea of the important points in the process.

After fertilisation the oospore is divided in the manner described above. Whilst
the developing sporogonium is gradually becoming a multicellular body dilated inferiorly

Fig. "z-^j.—Anthoceros Icevts (after Hofmeister) ; A a branciicd thailus ; .5 longitudinal section of a shoot (X4o); a»
antheridia beneath the layer of superficial cells ; C longitudinal section through the apical part of a shoot ; nr rudiments of
archegonia {X 500) ; D nr fertilised archegonium in the longitudinal section of a shoot, with an embryo consisting of
two cells; E multicellular embryo: A' in .ff a colony oi Nostoc settled in the tissue of the thallus.

(Fig. 237, E)f the cells of the surrounding tissue of the thallus undergo numerous
divisions and form an upwardly projecting involucre which is broken through at a later
period by the elongating sporogonium. Differentiation now takes place in the homo-
geneous tissue of the sporogonium ; a central cylinder of from twelve to sixteen rows
of axially elongated cells is marked out, forming the columella, whilst the cells of the
layer immediately adjacent to it undergo division by horizontal walls and give rise to the
mother-cells of the spores and elaters ; the external four or five layers of cells form the
wall of the future ' pod.' Those cells of the layer investing the columella which are to
form elaters undergo one or more vertical divisions. The elaters are here transversely
directed rows of cells in which no spiral bands are formed. The mother-cells of the
spores round themselves off, become gradually isolated, beginning from the apex of the
sporogonium and proceeding basipetally, increase in size, and then divide into four, giving
rise to the tetrahedral spores. The sporogonium elongates and becomes a pod-shaped
structure of from fifteen to twenty millimetres in height, the brown wall of which,
provided with an epidermis bearing stomata, splits from above downwards into two
valves.

354

MUSCINE^.

2. The I'iimily MonocleflB appears, according to the 'Synopsis Hepaticarum,' to
contain transitional forms between the Anthoceroteae and the Jungermannieae. The
long sporogonium has a longitudinal dehiscence and no columella; and the sexual
generation is either thalloid or foliose.

3. The RicciesB form a flat dichotomously branched thalloid stem, floating in
water or rooting in the ground, the apical cells of which, lying close to one another

Fig. i-^.—Anthoceros Itrvis ; sg the young sporogonium, the letters point to the foot ; L the invohicre ;
c—c the columella ^after Hofmeister, X 130) .

in the anterior depressions of the branches, are stated by Kny to become multiplied by
vertical longitudinal partitions, and segmented by walls inclined upwards and down-

FlG. "iy^.—Riccia glauca; A vertical longitudinal section through the apical region; j apex, b leaves, a young an-
theridium, a' older antheridium already surrounded by involucral tissue w ; B rudiment of an antheridium a already
overarched ; C young antheridium a in longitudinal section (after Hofmeister, x 500).

wards. On the upper side a distinct epidermis is differentiated, but without stomata,
and beneath this lies the green tissue often provided with air-cavities, which is derived
from the upper segments of the apical cells ; the under side is provided with a single
longitudinal row of transverse lamellae, which, resulting immediately from the lower
segments of the apical cells, must be considered as leaves. Afterwards they split length-

HE PA TICM.

zss

wise and form two rows; between them arise a number of root-hairs with conical thick-
enings projecting inwards.

The archegonia and antheridia are formed on the upper side from young epidermal
cells which grow into papillae, and are overarched, in consequence of their mode of
development, by the surrounding tissue (Fig. 239). This involucre sometimes forms
an elevated neck above the sessile antheridia. The archegonia project, at the time of
fertilisation, above the epidermis ; subsequently they are arched over, and develope from
their fertilised oosphere the globular sporogonium with a wall consisting of a single layer
of cells, and entirely filled with spores, without elaters. The spores are set free by the
decay of the surrounding tissue.

4. The Marehantie83 have all a thalloid stem extended flat upon the ground ;
it is ribbon-like, dichotomously branched, possesses a mid-rib, and is always composed of

Fig. lifi.—Riccia glauca; A apical region in vertical longitudinal
section ; ar archegonium ; c oosphere (X 560) ; B the unripe sporogonium
sg surrounded by the calyptra, which still bears the neck of the arche-
gonium ar (X300, after Hofmeister).

FIG. 240 bis. —Cell-forms of Marcha7ttia
polymorpha with thickenings; A an elater
(one-half) from the sporogonium, with two spiral
bands ; A' a portion more strongly magnified ;
B a parenchyma-cell from the centre of the thal-
lus, with thickenings projecting inwards in a
reticulate manner ; C a slender root-hair with
thickenings projecting inwards, these are ar-
ranged on a spiral constriction of the cell-wall ; at
D a thicker-root-hair, with thicker branched pro-
jections, and spiral arrangement still more evi-
dent.

several layers ; the under side produces a number of hairs with conical thickenings
projecting inwards placed upon a spiral constriction of the internal cavity (Fig. 240, bis,
C), and also two rows of leaf-like lamellae, like the Riccieae. The upper side is covered
by a very distinctly differentiated epidermis, penetrated by large stomata ^ of peculiar
form. Each of these stands, in Marchantia, Lunularia, &c., in the centre of a rhombic
plate ; these plates are parts of the epidermis which overarch large air-cavities, from the

I bottom of which the cells containing chlorophyll spring in a conferva-like manner, while
the rest of the tissue is destitute of chlorophyll and consists of long horizontal cells

^ These stomata are formed (see Fig. 89) by the simple separation from one another of four or
)re epidermal cells which afterwards are divided by walls parallel to the surface of the thallus.
iitgeb.)

A a 2

^5^

MUSCINEJE.

The sexual organs of the Marchantieae are borne on monoecious or dioecious recep-
tacles ^. The antheridia, although springing from cells of the epidermis, are, as in Ricciay
depressed in the upper side of the thalloid stem, and overarched by the surrounding
tissue ; they occur in larger or smaller numbers close together upon receptacles, which
are discoid or shield-shaped sessile or stalked branches that have undergone a peculiar
transformation. The archegonia are only in the Targionieae inserted at the apex
of an ordinary shoot ; in the other families they are produced on a metamorphosed
branch, which rises like a stalk and developes in different ways at its summit; it
bears the archegonia on its outer or lower side. With the variation in the form of the
part which bears the archegonia is connected an equally varied mode of envelopment
of the archegonia by involucres. Since it is impossible to describe these structures in
a short space, we may take Marchantia polymorpha, the species most perfectly endowed
in this respect, as an example. The explanation of the figures 241-243 will suffice
to illustrate at least the most essential points.

The sporogonium of the Marchantieae, usually shortly stalked, contains elaters which
radiate from the bottom towards the circumference {cf. Fig. 236). It bursts either at

Fig. 241. — Mai-cnantia polymurpha; A a horizontal branch t with two ascending branches which bear male recep-
tacles hit; K vertical section through an incompletely developed male receptacle hti and the part of the thalloid stem
a from which it springs ; b b leaves ; h root-hairs in a channel of the receptacle ; o o openings of the hollows in which
the antheridia a are placed ; C a nearly ripe antheridium ; st its pedicel ; iu the wall ; D two anthernzoids (these last
X 800).

the apex with numerous teeth, or is four-lobed, or the upper part becomes detached as
an operculum. The peculiar gemmae and their cupules have already been described.

5. The Jungerm.annie8B. In this family occur forms of which the vegetative body
is a true flat leafless thallus, as Metzgeria and Aneura, as well as transitional forms whose
flat thalloid stem forms leaves on the under surface {Diplolcena), or whose stem, as in
Blasia, elliptical in section in its early stage, becomes broad and leaf-like when older, and
produces leaves on both surfaces. Closely allied to these is a genus ' with a less dilated
stem, though still always greatly flattened on the upper side, and bearing leaves only
above, {Fossombronia ?). The greater number of the genera, however, the foliose Junger-
mannieae, form a slender filiform stem, with numerous sessile leaves with broad inser-
tions but distinctly diflferentiated ; these leaves commonly occurring only in two rows
situated on the upper side, as in Radula, some species of Jungermannia, Lejeunia, and
Plagiochila. Typically, however, there are three rows of leaves, one being developed on
the under or shaded side (hence termed Amphigastria), the other two rows on the upper
side {Fridlania, Madotheca, Mastigobryum). In the flagelliform branches the leaves
remain very small, and are sometimes almost invisible.

* [Leitgeb, Die Inflorescenzen der Marchantiaceen ; Sitzber. d. Wien, Akad. t88o.]

HEP A TICM.

^51

The bilateral structure is distinctly manifested not only by the thalloid forms, which
mostly cling closely to the substratum, in that the sexual organs are formed only on the
upper side or the one exposed to the light, and rhizoids and leaves on the under or
shaded side ; but in the foliose forms also this tendency is clearly shown, whether they
cling closely to the substratum or rise from it obliquely. This bilateral structure is
manifested not only in the different mode of the formation of the leaves on the two
sides, and in the expansion of the ramifications in a single plane, but is also deter-
mined, both in the foliose and in the thalloid forms, by the growth of the apical region
of the shoot. Even the youngest segments of the apical cell exhibit it, as is shown

Fig. 242. — Female receptacle of Marchatitia J>oly-
morpha seen laterally from below ; st stalk with two
channels ; sr the radiate outgrowths of the disc ; fc the
intermediate perichistium ; /sporogonia (X about 6).

Fig. ■z^'i.—Marchantia polymorpha; A vertical section through a female receptacle hu; bb leaves ; h root-hairs in its
channel ; g large cells between the air-cavities of the upper side ; B horizontal section of half an older receptacle and of its
stalk St; chl the chlorophyll-bearing tissue of the disc, below which are large hyaline cells ; a unfertilised archegonia; // peri-
gynia of fertilised archegonia ; C vertical longitudinal section through the receptacle ; a two archegonia ; pc perichcetium {pc
in Fig. 242).

in the different organisation of the upper and under sides, and in the similarity (though
not symmetrical) of the right and left sides of the shoot.

Enough has already been said on the position of the apical region in an anterior
depression in the thalloid forms, as well as on the termination of the filiform stem
in the leaf-bud of the foliose genera. The form of the apical cell, and its segmen-
tation in the thallus of Meizgeria, have been represented in detail in Fig. no ; in jineura
and Fossombronia it is also two-sided. In Blasia, on the other hand, Leitgeb states
that it is four-sided, and forms four rows of segments, a dorsal, a ventral, a right, and a
left row. ' This may be most easily represented by supposing a wedge-shaped apical cell

358

MUSCINEM.

forming segments by walls inclined alternately upwards and downwards (towards the
dorsal and ventral surfaces), as well as lateral segments from which the leaves proceed ;
a leaf is produced from the dorsal part of a lateral segment, a kind of leaf-tube from its
central part, and a second leaf from its ventral part, though this last is more often
absent * (Leitgeb, in lit.).

In the Jungermannieae with filiform stem and leaves arranged in two or three
rows, the stem ends in a three-sided apical cell which forms three rows of segments
in spiral succession ; two rows being dorsal and lateral, while the third row forms the
under or ventral side of the stem. The successive septa of each row of segments are
parallel to one another, and the segments themselves are in straight rows, the rows being
parallel to one another and to the axis of growth of the stem \ In the species with
leaves arranged in two rows, a leaf springs from each of the dorso-lateral segments ;
when the leaves are arranged in three rows each segment of the ventral side also
produces a leaf, which is however smaller and of simpler structure and is also inserted
transversely, while the insertion of the dorsal rows of leaves is oblique to the axis of the
stem, so that the lines of insertion of each pair form an acute angle. Before a lateral
segment has developed a papilla from which the leaf is formed, it divides by a longi-
tudinal wall into an upper and a lower half facing dorsally and ventrally, each of which

Fig. 244. — Diagram of the branching of those
Jungermannieae in which lateral shoots take the
place of the ventral lobe of the dorsal leaves;
B, ventral surface (after Leitgeb).

Fig. 245. — Inflorescence of Raeiitla cotn-
pianata ; ar archegonium ; an antheridium ;
b leaf (after Hofnieister).

now forms a leaf -papilla. Hence it arises that the leaves of Jungermannieae are to a
certain extent bisected or two-lobed; in the simpler leaves this is usually shown by
a more or less deep incision of the anterior margin ; but even when the leaves are quad-
ripartite, as in Tricbocolea, the primitive double origin can still be recognised. The lower
lobe of the leaf is usually smaller, of peculiar form, and hollowed out.

The branching of the growing end of the shoot in the case of Metzgeria has already
been represented in Fig. no. According to Leitgeb it takes place in a similar manner
also in the other thalloid forms with a two-sided apical cell, viz. in Jneiira and Fossom-
bronia. The very variable relation of the branching to the leaves discovered by Leitgeb'-^
is especially remarkable. In Metzgeria and Aneura no leaves, but only branches, are
formed out of the segments ; in Fossombronia the lateral shoot springs from the segment
in place of a whole leaf; on the other hand, in the greater number of Jungermannieae
with filiform leafy stem and three-sided apical cell, the lateral shoot springs from the
segment in place of the lower or ventral lobe of the leaves of the dorsal side, so that in
these cases the branch may be considered as a metamorphosed half-leaf. Fig." 244 will

^ Compare in reference to this v^^hat follows with respect to Mosses.

^ What follows is partially derived from I^eitgeb's letters. [See his Unteisuchungen, III.]

HEPA TICM. 359

serve to explain this remarkable process, where the apical view of a branching shoot is
represented diagrammatically : I, II ... FI are the segments of the apical cell S of the
primary shoot ; II, F being segments of the ventral, /, ///, IF, FI of the dorsal side.
The two segments / and /// are already divided by a longitudinal wall each into two
halves respectively dorsal and ventral ; and in the latter the apical cell s of each lateral
shoot has already been constituted by the formation of the walls i, 2, 3, while the dorsal
half of each of these segments has developed into half a leaf. The other segments which
do not form shoots develope normal two-lobed leaves. This is the process that occurs in
Frullania, Madotheca, Mastigobryum, Lepido%ia, Trichocolea, and Jungermannia trichophylla.
A third type of branching occurs finally in Radula and Lejeunia, where the formation of
leaves is not disturbed by the branching, the branches springing from behind the leaves at
their base, and from the same segments.

Besides these modes of ramification of the segments of definite position, Leitgeb has
recently discovered an endogenous formation of shoots, which are sometimes fertile
branches, from the ventral segments provided with amphigastria, e.g. in Mastigobryum,
Lepidozia, and Calypogeia ; or they are formed without the production of a ventral row
of leaves, as in Jungermannia bicuspidata and other Jungermannieae with leaves in two
rows. In those especially which belong to the section Trichomanoideae the fertile
branches are developed thus, and break out from the older parts of the stem as adven-
titious shoots ; probably, however, their mother-cells always originate regularly in
acropetal succession in the primary meristem of the vegetative cone, as in Mastigobryum
and Lepido%ia, but their development is deferred. Finally according to Leitgeb, the
whole branching of many Jungermannieae appears to depend exclusively on the pro-
duction of branches in this manner.

The reproductive organs are distributed monoeciously or dioeciously, and are formed,
in the thalloid genera, on the dorsal side of the shoot ^, in the foliose Jungermanniege at
the end of primary shoots or of special small fertile branches, which commonly have the
above-described adventitious origin on the ventral side. The antheridia are usually in
the axils of the leaves, singly or in groups. The archegonia appear generally in large
numbers at the summit of the shoot, either on those which bear antheridia below, or
on special branches, which in the Geocalyceae are hollowed out in such a manner that
the archegonia are sunk in a deep pitcher-shaped hollow, an arrangement which may be
compared, to a certain extent, with the structure of a fig. This occurs in an especially
striking manner in Calypogeia. Where this peculiar enveloping of the archegonia does
not occur, they are concealed by the nearest leaves (the perichaetium) ; and a perigynium
is usually formed in addition, which grows round the archegonia as a special membranous
envelope. The development of these organs has been accurately described by Leitgeb
in the case of Radula complanata (Fig. 245). The primary and lateral shoots both bear,
as a rule, both kinds of reproductive organs; such a shoot is always at first purely
vegetative, but forms after a time antheridia, and finishes with the archegonia. Less
often, however, it again recurs, after the production of antheridia, to a vegetative
development. The antheridia of Radula are metamorphosed trichomes; they stand
singly in the axils of the leaves, and are completely enclosed in the hollow formed by
the very concave lower lobe of the leaf. They arise from the club-shaped protuberance
of a cell belonging to the cortex of the stem and lying before the leaf at its base.
The archegonia of Radula always stand at the end of the primary or of a lateral shoot,
from three to ten together, surrounded by a perigynium, which is again enveloped by a
perichaetium of two leaves. The archegonia together with the perigynium are de-
veloped from the apical cell of the shoot and from its three youngest segments. The
archegonia arise from the apical cell itself, and from the upper parts of its lateral

^ In Metzgeria furcata the antheridia and archegonia make their appearance dioeciously on the
concave dorsal surface of adventitious branches which arise from the ventral surface of the mid-rib
and are so curved as to enclose the sexual organs. (Leitgeb.)

360

MUSCINEjE.

segments; the lower parts together with the ventral segment are employed in the
formation of the perigynium. The further development of the archegonia and antheridia
has already been described.

In the species examined by Hofmeister the fertilised oosphere is first divided by a
transverse septum, /. e. at right angles to the axis of the archegonium. Only the upper
of the two cells, the one towards the neck of the archegonium, becomes further divided,
and it gives rise to four apical cells arranged as octants of a sphere, as described
above.

The basal portion of the growing archegonium becomes swollen out and penetrates
down into the tissue of the stem, being nourished and firmly enclosed by it (the vaginula).

As soon as the young sporogonium consists
of a number of cells, its wall becomes differ-
entiated from the inner tissue which is to
form the spores and elaters. In Frullania it
is a single circular disc of cells lying trans-
versely beneath the dome of the young sporo-
gonium from which the vertical elaters, and
by further divisions, the mother-cells of the
spores arise, a process which reminds one of
what occurs in Sphagnum. In most true
Jungermannieae there is, on the other hand,
a column of tissue consisting of vertical rows
of cells (surrounded by the wall of the sporo-
gonium consisting of two layers), out of which
the elaters and spores are formed. The elaters
lie, in this case, horizontally, and radiate from
the ideal longitudinal axis to the wall of the
sporogoninm (Fig. 246). In Pellia the inner
fertile tissue forms, after the differentiation of
the wall of the sporogonium, a hemisphere,
from the cells of which arise the spores and
the elaters radiating from below upwards, in a
similar manner to what occurs in the Mar-
chantieae.
By a rapid extension of the hitherto short seta, the calyptra is ruptured at the
apex, and the globular sporogonium with the already fipe spores is raised up on it.
Whilst the spores are ripening, the inner layer of the wall of the sporogonium becomes
absorbed; the single layer which still remains is ruptured at the apex, and splits into four
(rarely mof-e) longitudinal valves, which, flying asunder in the form of a star, carry with
them at the same time the elaters, by which the spores are dispersed. The elaters, when
mature, are long fusiform thin-walled cells, round the interior of which run from one to
three brown spiral bands.

Fig. "zAd.—Jtmg

bicuspidata ; longitudinal

section of the unripe sporogonium s^, surrounded by tlie
calyptra ar ; ar' archegonia which have remained unfer-
tilised ; / base of the perigonium ; st stem ; b leaf : jjr points
to the dilated base of the seta, which still bears the hypo-
basal cell (after Hofmeister).

MVSCI. 361

CLASS VI.
MUSCP.

The spore produces a conferva-like thallus, the Protonema, from which the
leaf-bearing Moss arises by lateral branching with differentiation into stem and
leaf. On this plant the sexual organs are formed; from the fertilised oosphere
proceeds the sporogonium, in which the spores are formed from a small portion
of the inner tissue.

The Protonema arises, in the typical Mosses, as a tubular bulging of the
endospore, which elongates indefinitely by apical growth and becomes septate, the
septa being oblique. The cells do not undergo any intercalary divisions, but form
branches immediately behind the septa; these branches also become septate, and
usually show a limited apical growth ; they may, in turn, produce ramifications
of a higher order. The part of the endospore which lies opposite the germinating
filament may develope into a hyaline rhizoid, which penetrates into the ground.
The cell-walls of the protonema-filaments are at first colourless, but as the primary
axes lie upon the ground or even penetrate into it, their cell-walls assume a brown
colour, while the cells above ground develope abundance of chlorophyll-granules ;
and the protonema is hence nourished independently by assimilation ; it not only
attains a considerable size in some genera, covering a surface of from one to several
square inches like turf with its densely matted filaments, but its term of life may be
regarded as unlimited. In most Mosses it altogether disappears after it has
produced the leafy stems as lateral buds; but where these latter remain very small
and have only a short term of life, as in the Phascaceae, Pottia, Physcomitrium, &c.,
the protonema still remains vigorous after it has produced the leafy plants, and
when the sporogonium has already been developed upon them. In such cases
all three stages of the cycle of development are present simultaneously in genetic
connexion. The Sphagnaceae, Andreasaceae, and Tetraphideae differ from the typical

* W. P. Schimper, Recherches anat. et pliysiol. sur les Mousses (Strassburg 1848).— Lantzius-
Beninga, Beitrage zur Kentniss des Baues der ausgewachsenen Mooskapsel, insbesondere des Peri-
stoms (with beautiful illustrations) in Nova Acta Acad. Leopold. 1847. — Hofmeister, Vergleich.
Untersuch. 185 1. [On the Germination, Development, and Fructification of the Higher Cryptogamia,
Ray Soc. 1862,] — Hofmeister, in Berichte der Kon. Sachs. Gesellsch. der Wissens. 1854,— Ditto,
Entvvickelung des Stengels der beblatterten Muscineen (Jahrb. fiir wissens. Bot. vol. IH). — linger,
Ueber den anat. Bau des Moosstammes (Sitzungsber. der Kais. Akad. der Wissens. Vienna, vol.
XLIII. p. 497). — Karl Mllller, Deutschlands Moose (Halle 1853). — Lorentz, Moosstudien (Leipzig
1804).— Ditto, Grundlinien zu einer Vergleich. Anat. der Laubmoose (Jahrb. fiir wissen. Bot. vol. VI,
and Flora 1867). — Leitgeb, Wachsthum des Stammchens von Fontinalis andpyretica u. von Sphagnum;
sowie Entwickelung der Antheridien derselben (in Sitzungsber. der Kais. Akad. der Wissens. Vienna
1868 and 1869). — Nageli, Pflanzenphysiol. Untersuchungen, Heft I, p. 15. — Julius Kiihn, Entwickel-
ungsgeschichte der Andreseaceen (Leipzig 1870), (Mittheilungen aus dem Gesammtgebiet der Botanik
von Sahenk u. Luerssen, vol. I). — Janczewski, Ueber Entwickelung der Archegonien, Bot. Zeilg,
1872.

362

MUSCINEM.

Mosses both in the structure of the sporogonia, and in the mode of formation of
the protonema. The spores of the Sphagnaceae produce, at least when they grow
upon a firm substratum, a, flatly expanded plate of tissue, which branches at the
margin, and produces from its surface the leafy stems. In AndrccEa, according
to the investigations of Kiihn, the contents of the spore divide, while still within
the closed exospore, into four or more cells, and a tissue is thus formed similar to
that produced in the spores of some Hepaticae (as Radula and FruUanid) ^ : finally,
from one to three peripheral cells grow into filaments which extend over the
hard stony substratum. The branches of the protonema may now develope further
in three different ways; longitudinal as well as transverse divisions arise, and
irregularly branched cellular ribbons are formed; or, divisions also taking place
in addition parallel to the surface of these ribbons so that they come to be several
layers thick, the protonema developed in this manner as a mass of tissue becomes

Fig. 247. — h'unaria hygrotnetrica ; A germinating spores; v vacuole; iv root-hair; J exospore; B part of a developed
protonema, about three weeks after germination ; h a procumbent primary shoot with brown wall and oblique septa, out
of which arise the ascending branches with limited growth ; K rudiment of a leaf-bearing axis with root-hair w (^ x 550 ;
B about 90).

erect and branches in an arborescent manner ; finally, in the third form, the leaf-like
branches of the protonema are plates of tissue of simple definite outline. Closely
allied to this last form is the flat protonema of Tetraphis and Tetradontium, which,
as will be further shown in a following illustration, arises at the end of longer and
slenderer filaments'^.

The buds which develope into the Moss-stems apparently never arise at the
end of one of the principal protonemal filaments, but as lateral branches upon
them. The idea suggested by me that the protonema, as also the equivalent
rhizoids of the Bryineae, represent a very rudimentary much elongated Moss-stem,
just as the branches with naked base of Chara are merely simple forms of its
stem, has been proved correct by the observations of Schuch (i 870-1871) made

* In true Mosses also (as Bartramia, Leucohryum, Mnium, and Hypmtm) the first septum of the
protonema is formed, according to Kiihn, even within the spore.
^ Compare Berggren, Bot. Zeitg. 1871.

MUSCI.

3^^

in the botanical laboratory at Wurzburg, and concluded b}^ MuUer (1873). The
principal filaments of the protonema and the large rhizoids have a very much
elongated apical cell in which (and ' never in its segments) oblique septa are
formed which are regularly inclined (spirally) in three or more directions, just in
the same manner as the principal walls of the segments of the trilateral apical
cell of the Moss-stem. These walls do not, however, intersect, as in the stem,
since the segments (cells of the filament) are so long. Each segment is capable
of forming a protuberance immediately behind its anterior wall, which is shut off
by a wall corresponding to the ' foliar wall ' formed in the stem-segments. After
this protuberance has become elongated a wall is formed within it corresponding to
the 'basal wall' formed in the segments of the stem. In this way the protuber-
ance comes to consist of two cells; the one directed towards the growing apex
of the filament corresponds to the mother-cell of the leaf, and the other, lying
behind the preceding, developes a lateral branch, just as is the case in the stem ^
Other unimportant divisions of these cells need not be mentioned here. Usually

Fig. 248.— Production of rhizoids from the protonema oi Mnium hornunt, with leaf-forming buds K ;
■w TV the root-hairs of an inverted sod, from which shoot protonema-filaments « « (X go),

the anterior cell does actually give rise to a foliar organ, the posterior to a bud,
but often one or both simply develope into rhizoids. The position of the walls
of these cells is precisely similar to that of the corresponding cells of the stem ;
the protonema differs from the Moss-stem simply in the distance of one segment
from the other, and in the suppression of those further divisions by which the
tissue of the stem is produced from its segments. When a stem is developed
from the protonema, it originates as a bud from the posterior of the two cells of
the lateral protuberance. Its mode of origin is usually this, that the cell at first
elongates into a filament by the formation of segments the primary walls of which
do not intersect; after this segments are cut off by walls which do intersect,
and from them foliar outgrowths, and later true leaves, are formed. From this
it is evident that the formation of a Moss-stem from the protonema essentially
depends upon the more rapid formation of segments one after another ; a Moss-
stem is, so to speak, a protonemal filament with very short segments, forming

See Fig. 116, the walls c, h.

364 MUSCINEM.

mother-cells of leaves which at once grow out into expanded leaves instead of
into filamentous structures. This interpretation is confirmed by all possible tran-
sitional forms, and a comparison of the ordinary characteristics of the stem and
of the protonemal filament at once demonstrates its truths Whether or not the
Andreaeaceae, Sphagneae, &c. resemble the Bryineae in this feature, and the extent
of such a resemblance, must be decided by future researches.

The apical cell of the stem is two-sided in Schistostega and Fissidens, and
produces two straight rows of alternating segments; in the rest of the Mosses it
is a three-sided pyramid, with the basal surface turned upwards (Fig. 116). Each
segment of the apical cell arches outwards and upwards as a broad papilla; this
is cut off by a longitudinal wall (which Leitgeb calls a foliar wall), and developes,
by further divisions, into a leaf, while the lower inner part of the segment produces,
by further divisions, part of the inner tissue of the stem. Since each segment
forms a leaf, the phyllotaxis is determined by the position of the consecutive
segments. In Fissidens two straight rows of alternate leaves are thus formed ; in
Fontinalis three straight rows with the divergence J, the segments themselves lying
here in three straight rows with the ^ arrangement, because each newly formed
primary wall is parallel to the last but three (both belonging to one segment).
In Polytrichum, Sphagnum^ Andrecea, &c., on the other hand, each new primary
wall encroaches on the ascending side with regard to the leaf-spiral; the primary
walls of each segment are therefore not parallel; the segments themselves do not
lie, even when first formed (without the assistance of any torsion of the stem), in
three straight rows, but in three parallel spiral Hnes winding round the axis of
the stem one above another ; and the consecutive segments and their leaves diverge
at an angle which, from what has been said, must be greater than \ ; the phyllo-
taxis is f , I, and so on ^.

The primary meristem of the stem, situated beneath the pundum vegelationis,
passes over into permanent tissue and usually becomes differentiated into an inner
and a peripheral mass of tissue, which are not generally sharply defined ; the cell-
walls of the peripheral and especially of the outermost layers are usually strongly
thickened and of a bright red or yellowish red colour; the cells of the inner funda-
mental tissue have broader cavities and thinner walls more slightly or not at all
coloured. In some Moss-stems this differentiation goes no further than into an
outer skin consisting of several layers and a thin-walled fundamental tissue {e.g.
Gymnosto?7ium rupestre, Leucobryiim glaucum, Hedwigia ciliala, Barhula aloides, Hylo-
comium splendetis, &c., according to Lorentz) ; while in many other species a central
bundle of very thin- walled and very narrow cells is formed in addition {Gri7?imia,

^ I must content myself with the above brief account, for Herr Schuch has not yet published his
observations w^hich I had the opportunity of following in all their details, Herr Miiller is at present
(1874) working at this subject, but is not yet in a position to publish his results. Figs. 247 and 248
were drawn at a time (1866) when the views expressed above were unformed. (See Arb. d. bot. Inst,
in Wurzburg, Heft IV. 1874.)

'^ If the position of each fourth division of the apical cell is kept in view, it gives the impression
as if the apical cell rotated slowly on its axis, producing, at the same time, leaf-foiming segments.
(Compare on this subject the work of Leitgeb mentioned above, Loientz's work, Hofmeister's Mor-
phologic, p. 194, and Miiller, Bot. Zeitg. i86y, pi. Vlll.)

MUSCI. , '^6^

Funan'a, Bartramia, Mnium, Bryum, and others) ^ In PoIyirichu?n, Atrichum, and
Dawsonia alone do decided thickenings of the cell-walls take place in the central
bundle in such a manner that each of several groups of originally thin-walled cells
becomes surrounded by a thick wall and they
together form the bundle. In Polytrichum
vnmune there are found similar thinner extra-
axial bundles. Sometimes bundles of thin-
walled cells run from the base of the leaf- veins
obliquely downwards through the tissue of
the stem as far as the central bundle, which
Lorentz regards as foliar bundles {e.g. in
Splachnum luteum, Voiiia nivalis^ &c.). If it
is borne in mind that in some vascular plants
fibro-vascular bundles of the most simple
structure occur, and if the similarity of the
cambiform cells of true fibro-vascular bundles
to the tissue of the central and foliar bundles ^'^- '''■;::S^':^::S^!St^''''"^""'
in Mosses is considered, these latter may with-
out doubt be held to be fibro-vascular bundles of the simplest kind.

As has already been mentioned, the leaf originates from the broad papillose
bulging of a segment of the apical cell of the stem : this is cut off by a wall. The
lower (basal) part is concerned in the formation of the outer layers of tissue of
the stem, whereas the apical part of the papilla constitutes the apical cell of the
leaf; it forms two rows of segments by walls perpendicular to the surface
of the leaf and inclined to the right and left. The number of the segments to
be formed, in other words, the terminal growth of the leaf, is limited, and the
formation of tissue from the cells thus formed advances downwards, ceasing finally
at the base. The whole of the tissue of the leaf is sometimes (as in Fontinalis)
a simple layer of cells; but very commonly a vein, i.e, a more or less broad bundle,
is formed from the base towards the apex, dividing the unilamellar lamina into
right and left halves, and consisting itself of several layers of cells. The vein is
sometimes composed of uniform elongated cells, but more often various forms of
tissue become differentiated in it, among which bundles of narrow thin-walled cells
similar to the central bundle of the stem frequently occur, and these are sometimes
continued to it through the external tissue of the stem as foliar bundles {cf. Lorentz,
/. f.). The shape of the leaves of Mosses varies from almost circular through broadly
lanceolate forms to the acicular ; they are always sessile and broad at their insertion ;
usually densely crowded ; only on the stolons of some species, the pedicels of the
cupules of the gemmae of Aulacomnion and Tetr aphis, as well as at the base of
some leafy shoots, do they remain small and remote (cataphyllary leaves). In the
neighbourhood of the reproductive organs they usually form dense rosettes or buds,
and then not unfrequently assume special forms and colours. In Racopilum, Hypo-
pierygium, and Cyathophorum, there are two kinds of leaves, a row of larger upon

' It is stated by Lorentz that the seta of the sporogonium is always provided with a central
bundle of this kind.

366 MUSCINEM,

one side, and a row of smaller leaves upon the other side of the stem. The
leaves are not branched, but entire or toothed, rarely slit. In some kinds peculiar
outgrowths are formed upon the inner or upper surface of the leaves ; in Barbula
abides articulated capitate hairs. The lamina, which in other cases expands
right and left from the median plane, is, in Fissidens, expanded in the median
plane itself, proceeding from an almost sheathing base. The tissue of the leaf is,
with the exception of the central vein, usually homogeneous and composed of
cells containing chlorophyll, which sometimes project above the surface as mamillse ;
in the Sphagnaceae and Leucobryum the tissue is differentiated into cells containing
air, and others which contain sap and chlorophyll, arranged in a definite manner.

The mode of branching of the stem of Mosses is apparently never dichotomous,
but also probably never axillary, although connected with the leaves. Even when
the branching is copious the number of lateral shoots is nevertheless usually much
smaller than that of the leaves ; in many cases the lateral branches are definitely
limited in their growth, leading sometimes to the formation of definite ramified
systems similar to pinnate leaves {Thuidium, Hylocomiuni). When the primary
shoot produces reproductive organs at the summit, a lateral shoot situated beneath
it not unfrequently displays a more vigorous growth, continuing the vegetative
system; and by such innovations sympodia are formed. It sometimes happens
that stolons, that is shoots either destitute of or furnished with very small leaves,
creep on or beneath the surface of the ground, elevating themselves at a later period
as erect leafy shoots. The mode of branching is very various, and is closely con-
nected with the mode of life. The morphological origin of the lateral shoots has
been carefully investigated by Leitgeb in the case of Fontinalis and Sphagnum^ and
admirably described. Since these two genera belong to very different sections, the
results obtained in this case may be considered as of general application to the
whole class. They agree in the fact that the mother-cell (which is at the same
time the apical cell) of a branch originates beneath a leaf from the same segment
as the leaf (Fig. 116). In Foniinalis the branch arises beneath the median line
of the leaf; but in Sphagnum beneath its cathodal half. In consequence of the
further development of the mother-shoot, the lateral shoot in Sphagnum appears at
a later period to stand by the side of the margin of an older leaf; and this is
probably the explanation of the earlier statement of Mettenius that in Neckera
complanata, Hypnum tn'queirum, Racomitrium canescens, and others, the lateral shoots
stand by the side of the margins of the leaves. When the shoot arises beneath
the median line of a leaf, and the leaves are arranged in straight rows, the further
growth of the stem may cause it to seem as if the shoot originated above the
median line of an older leaf, in other words as if it were axillary. Leitgeb states
that articulated hairs arise in the genera named in the axils of the leaves, or perhaps
more correctly at the base of the upper surface of the leaves.

The dimensions attained by the leaf-bearing axes and axial systems of Mosses
show a wide range. In the Phascacese, Buxbaumiay and others, the simple stem
is scarcely i mm. in height ; in the largest species of Hypnum and Polytrichum
it is not unfrequently 2,' 3, or more decimetres in length, and, if belonging to
more than one axis, even longer, owing to the formation of innovations and sympodia
{Sphagnum). The thickness of the stem is less variable ; yV mm. in the smallest,

MUSCI. , ^6"]

it scarcely exceeds i mm. in the thickest forms. Its dense tissue, coloured exter-
nally, is however very firm, often stiff, always very elastic, and capable of offering
long resistance to decay.

The Root-hairs (Rhizoids) play an extremely important part in the economy
of Mosses. It is only in the otherwise very abnormal section of the Sphagnacese
that they are very sparsely and poorly developed; in most other forms they occur
in large numbers at least at the base of the stem, often clothing it completely with a
dense reddish-brown felt. Morphologically the rhizoids are not sharply distinguished
from the protonema ^ ; and it will be seen further on that they, like it, are capable
of forming new leafy stems. They arise as tubular protuberances from the superficial
cells of the stem, elongate by apical growth, and are segmented by oblique septa ;
at the growing end the wall is hyaline, and particles of earth become attached
to it in the ground; subsequently these fall off; the wall becomes thicker and
brown, as is also the case with the aerial root-hairs. The cells contain a
considerable quantity of protoplasm and drops of oil (Fig. 250, B). In many
Mosses the root-hairs branch very copiously in the ground ; they often form a dense
inextricable felt ; a felt of this kind may even arise above ground as a dense turf,
and may serve as a soil for future generations. In Airichum and other Polytri-
chaceae, the stouter rhizoids coil round one another like the threads of a rope, the
branches which proceed from them doing the same, and only the last and finest
ramifications remain free.

The Vegetative Reproduction of Mosses is more copious and varied than is the
case in any other section of the vegetable kingdom. It presents the peculiarity that
the production of a new leaf-bearing stem is always preceded by the formation of
a protonema, even when the propagation takes place by gemmae. Exceptions are
afforded only by the few cases in which leaf-buds become detached and commence
immediately to grow.

In describing the different cases in detail, the first point that must be brought
prominently forward is that both the protonema which proceeds from the spore
itself and the leafy stems which spring from it are capable of reproduction of
different kinds. The original protonema is so far an organ of reproduction that it
may produce upon its branches a smaller or larger number of leafy stems in
succession or simultaneously ; sometimes the individual cells of the protonema-
branch separate from one another after they have become rounded off into a
spherical form, acquire thicker walls, and become for a time inactive (as in Funaria
hygrometrica), forming, probably, at a later period again protonema- filaments. A
secondary protonema may be formed from any root-hair when exposed to light in
a moist atmosphere {cf. Fig. 247 and Fig. 250, A,p). In some species {Mnium,
Bryum, Barbuia, &c.) it is sufficient to keep a turf of Moss damp for some days
and turned downwards, in order to produce hundreds of new plants in this manner.
Some apparently annual species, e.g. of Phascum, Funaria, and Pottia, persist
perennially by means of their root-hairs ; the plants disappear completely from the

^ The rhizoids appear to be distinguished from the protonema only by the absence of chlo-
rophyll and by their tendency to grow downwards ; the protonema developes certain branches as
rhizoids ; and the rhizoids may, on their part, develope single branches as a protonema growing
upwards and containing chlorophyll ; see p. 362.

368

MUSCINF.JE.

surface of the ground from the lime that the spores become ripe till the next
autumn, when the root-hairs again produce a new protonema, and upon this new
stems arise.

Similar "outgrowths from the roots occur also, according to Schimper, in
the felted protonema of some species of Polytrichum (P. nanum and aloides) on
the slopes of hollow roads, and on that of Schistoslega osmundacea in dark hollows.
The root-hairs may also immediately produce leaf-buds, and behave, in this respect,
exactly like the protonema. When the buds arise on underground ramifications
of the root-hairs (Fig. 250, B) they remain in a dormant state, as small microscopic

Fig. •2<,o.—A young plant of a Barhula (m) with the root-hairs h, to the gfrowingf ends of which particles of earth have
become attached; at / a superficial root-hair is putting out -branches containing chlorophyll, in other words a protonema;
at ^ a tuberous bud (bulbil) is growing from an underground branch of the root-hairs ; R this bud more strongly magnified
(^ X 20 ; i? X 300).

tuberous bodies {bulbils) filled with reserve food-material, until they chance to
reach the surface of the ground, when they undergo further development {e.g.
Barbula muralis, Grimmia pulvinaia^ Funaria hygrometrica, Trichostomum rigidum^
Atrichum). The aerial root-hairs may, however, not only produce a protonema
containing chlorophyll, but also leaf-buds without its intervention ; and Schimper
cites the remarkable fact that in Dicranum undulalum annual male plants are
formed in this manner on the tufts of perennial female plants, and fertilise the
latter.

Even the leaves of many Mosses produce a protonema, their cells simply
growing, and the tubes thus formed becoming segmented. This occurs in Oriho-

MUSCI,

3^9

trichum Lyelli and ohtusifolium ; in O. phyllanthum tufts of club-shaped protonemal
filaments with short cells arise at the apex of the leaves ; and the same phenomenon
occurs in Grimmia trichophylla^ Syrrhopodon, and Calymperes. In Oncophorus
glaucus a dense felt of interlacing protonema-filaments is formed at the summit
of the plant where the reproductive organs are produced, which arrests its further
growth, and hence produces at a later period new clumps of young plants. In
Buxbaumta, especially B. aphylla, the marginal cells of the leaves form a pro-
tonema enveloping them as well as the stem with its filaments. Lastly, even

Fig. z^i.—Tetraphis pellucida ; A a plant producing gem-
ma (natural size) ; B the same, magnified ; y the cup in which
the gemmae are collected ; C longitudinal section through the
summit of the plant, b the leaves of the cup, K the gemmae in
various stages of development ; the older ones are forced off
their stalks by the later growth of the younger ones, and forced
over the side of the cup ; D a mature gemma (X 500), consisting
at the margin of one, in the centre of several layers of cells.

Fig. 252. — Tetraphis pelhicida; A, b a. gemma, detached
from its stalk at a, the protonema-filament *• >» has been formed
by the growth of a marginal cell of the gemma, and the flat
structure / as a lateral outgrowth from the protonema ; this has
also put out root-hairs w, w', w'' (X 100) ; B,p a flat protonema
from the base of which a leaf-bud K and root-hairs w, w' have
sprung ; the base of the protonema often puts out a number of
new flat protonemata before a leaf-bud is formed.

detached leaves, if kept moist, may emit a protonema, as for instance those of
Funaria hygrometrica ^.

GemmcB^ which, like those of the Marchantiese, are stalked fusiform or lenti-
cular cellular bodies, occur in Aulacomnion androgynum at the summit of a leafless
elongation of the leafy stem [Pseudopodtum) ; in Tetraphis pellucida enveloped by an
elegant cup composed of several leaves, out of which they subsequently fall. These

^ [It has been shown by Pringsheim (Jahrb. fur, wiss. Bot. XI) and by Stahl (Bot, Zeit. 1876)
that the sterile cells of the sporogonium and of the seta may give rise to protonema.]

B b

37^^

MUSCINEjE.

latter then put forth protonemal filaments, which produce first of all a flat mem-
branous protonema; and upon this finally new leaf-buds arise (Figs. 251, 252).

Finally the deciduous branch-buds of Bryum annotinum may also be considered
as organs of reproduction ; as also, according to Schimper, may the branches of
Conomitrium julianum and Ciiididotus aqualicus, which likewise have the power of
detaching themselves.

The Sexual Organs of Mosses usually occur in considerable numbers at the
end of a leafy axis \ surrounded by enveloping leaves often of peculiar shape,
and mixed with paraphyses, and the whole group of organs may, for the sake of

FIG. 253.— Longitudinal section of the summit of a very small male plant of Funaria hygrometrica ; a a young, * a nearly
ripe antheridium ; c paraphyses ; d leaves cut through the mid-rib ; e leaves cut through the lamina (X 300).

brevity, be called a ' Receptacle.' The receptacle of Mosses either terminates the
growth of a primary axis (Acrocarpous Mosses), or the axis is indeterminate, and
the receptacle is placed at the end of an axis of the second or third order (Pleuro-
carpous Mosses). Within a receptacle either both antheridia and archegonia are
produced (bisexual receptacles), or it contains only one kind of sexual organ, and
the receptacles may then be either monoecious or dicecious. Sometimes the male
receptacles appear on smaller plants with a shorter duration of life (as Funaria hygro-
metrica, Dicranum undulatum, &c.). In external appearance the bisexual are similar

^ The male branches of Sphagnum form an exception {vide infra).

MUSCI.

37^

to the female receptacles, while the habit of the male receptacles is altogether
different. In the former the archegonia and antheridia occur either close to one
another at the summit of the stem in the centre of the envelope {PerichcBtium),
either in two groups, or separated by peculiar enveloping leaves, and the antheridia
stand in the axils of these arranged in a spiral, surrounding the central group of
archegonia. The form of the perichsetium is, in the female and bisexual receptacles,
that of an elongated almost closed bud, formed by several turns of the leaf-spiral. Its
leaves are similar to the foliage-leaves, and become smaller towards the interior, but
grow all the more vigorously after fertilisation. The male perichaetium consists of
broader firmer leaves, and is of three different forms ; usually it is bud-shaped, and
resembles that of the female receptacle, but is shorter and thicker, its leaves often
coloured red, and decreasing in size towards the outside ; receptacles of this type are
always lateral. Secondly, the male perichaetia are sometimes shaped like capitula, and
are, on the contrary, always terminal on a stouter shoot and globular ; their leaves are
broad, sheathing at the base, thinner and recurved at the upper part ; they become
smaller towards the interior, and leave the centre of the receptacle, with the anthe-
ridia, free ; these receptacles are sometimes borne on a naked pedicel, a prolonga-
tion of the stem {Splachnuvi, Tayloria). Finally, the male perichaetia are sometimes
discoid and consist of leaves which are very different from the foliage-leaves ; they are
broader and shorter, expanded horizontally at
the upper part, delicate and of a pale green,
orange, or purple colour ; they are always smaller
the nearer the leaf-spiral approaches the centre ;
the antheridia stand in their axils [Mnium, Poly-
trichum, Pogonatum^ Dawsonid). The para-
physes stand between or by the side of the sexual
organs ; in the female receptacle they are always
articulated filaments ; in the male, fiUform or
spathulate, and consisting, in the upper part,
of several rows of cells.

The Antheridia are, when mature, stalked

ics with a wall consisting of a single layer of

ills containing chlorophyll - granules, which

)wever, in the ripe state, assume a red or
fellow colour. In the Sphagnaceae and in Bux-

lumia the antheridia are nearly spherical, but
all other Mosses of an elongated club shape.
tn the Sphagnaceae they open in the same
manner as in the Hepaticae ; in the other orders
by a slit across the apex, through which the
antherozoids still enclosed in their mother-cells
are discharged as a thick mucilaginous jelly.
The interstitial mucilage dissolves in water, and
the antherozoids escape from their mother-cells and swim about free.

The careful investigations of Leitgeb show that the morphological significance
of the antheridia is very various. In Sphagnum the mother-cell of the antheridium

B b 2

FIG. 254.
ridium bursting,

hygrometrica ; A an anthe-
the antherozoids (x 350); B the

antherozoids more strongly magnified, b in the
mother-cell; c free antherozoid of Polytrichtim
(X800).

372 . MUSCINEX.

arises in exactly the place in which a branch would otherwise be formed,
/'. e, from the segment of the axis which lies beneath the cathodal half of the
leaf; the antheridia may in this case be considered as metamorphosed branches.
In Foniinalis, on the other hand, their morphological significance varies within
the same receptacle; the one first formed is the immediate prolongation of
the axis of the shoot, arising from its apical cell; the succeeding ones are
developed from its last normal segments, and therefore resemble leaves in
their origin and position ; the last antheridia, finally, exhibit the morphological
characters of trichomes, both in their variable number, their development as cells
of the epidermis, and the want of definiteness in their place of origin. According
to Kiihn, AndrecEa behaves in precisely the same way as Fontinalis. The mother-cell
of the antheridium of Fontinalis is constituted as an apical cell forming two
alternating rows of segments; in forming the oldest and terminal antheridium
the apical cell changes from a triseriate to a biseriate segmentation. These seg-
ments are next divided by tangential walls in such a manner that the transverse
section (which meets two segments) of the young organ shows four outer and
two inner cells; the wall of the antheridium, one cell in thickness, arises from
the former by further division; the small-celled tissue which produces the anther-
ozoids from the latter. Andrecea behaves also very similarly in these respects; the
primary mother-cell of the antheridium appears as a papilla and is cut off by a
septum; the lower cell produces a cushion-like support; the upper cell is again
divided by a septum into a lower cell from the divisions of which the tissue of
the stalk is formed, and an upper cell out of which the body of the antheridium
arises ; the formation of the latter takes place in the same manner as in Fontinalis.
In Sphagnum the long stalk originates by transverse divisions of the growing
papilla which produces the antheridium, the segments dividing again in a cruciform
manner. The terminal cell then swells, and becomes divided by oblique walls of
somewhat irregular position ; a tissue is thus formed, which, at a subsequent period,
consists also of a wall formed of a single layer of cells and an inner very small-celled
tissue which produces the antherozoids.

The Archegoniu?n consists when mature of a massive, moderately long base,
which supports a roundish ovoid ventral portion ; above this rises a long thin neck,
generally twisted on its axis. The wall of the ventral portion, which consists, even
before fertilisation, of a double layer of cells, passes up continuously into the wall of
the neck consisting of a single layer of cells formed of from 4 to 6 rows (Fig. 256).
Together they enclose an axial row of cells, the lowest of which, ovoid and lying
in the ventral portion, produces the oosphere and the ventral canal-cell, the upper
cells being the canal-cells of the neck. These and the ventral canal-cell become
mucilaginous before fertilisation. This mucilage forces the four uppermost cells
(stigmatic cells) of the neck apart, and thus opens the canal of the neck, allowing
the antherozoids to penetrate to the oosphere. Fig. 256, B^ shows the row of
canal- cells at the period when disorganisation is beginning, and when the stigmatic
cells of the neck are still closed. In reference to the morphological significance
of the archegonia, Leitgeb has already shown that at least the first archegonium
of Sphag?ium arises immediately from the apical cell of the female shoot; more
recently Kiihn found that in Andrecea the first is formed from the apical cell, the

MUSCI.

373

succeeding ones from its last segments, in the same manner as the antheridia of
the same genus, and those of Radula and Fontinalis. According to preparations
which Schuch obtained in the laboratory at Wiirzburg, the first archegonium
arises also in typical Mosses from the apical cell of the shoot.

The order of succession of the cells in the development of the archegonium
has been studied by Kiihn in AtidrecBa, and by Janczewski in the Phascaceae, Bryinese,
and in Sphagnum. As in the Liverworts so here also, the whole archegonium is
derived from an outgrowth of a superficial cell of the punctum vegetationis. This
is divided by a transverse wall [nim, Fig. 255 yi) so as to form a lower cell
(corresponding to the pedicel of the Liverworts) and an upper external cell, in

Fig. 255. — First stage of development of the archego-
nium oi Ancire^a (after Kiihn); A terminal archegonium
arising from the apical cell of the shoot ; b b the youngest
leaves ; B after the formation of the central cell and stig-
matic cell; C transverse section of the j'oung ventral
portion.

'?lC,.rz<,(i.—Fiinaria hygrometrica ; A longitudinal section of the summit of a weak female plant (Xioo), a archegonia,
* leaves; B an archegonium (X 550), b ventral portion with the oosphere, h neck, nt mouth still closed; the canal-cells are
beginning to be converted into mucilage (the preparation had lain three days in glycerine) ; C the part near the mouth of the
neck of a fertilised archegonium, with dark red cell-walls.

which, as in the corresponding cell of the antheridium, two oblique walls inclined
in opposite directions appear. The two oblique cells thus cut off give rise, at a
later period, to the tissue of the lower part of the ventral portion of the archegonium,
which is here more developed' than in the Liverworts (Fig. 256, B). The upper
cell undergoes the same divisions as it does in the Liverworts; the mode of
formation of the ventral wall and of the central cell is the same in this group as
in that; but the formation of the neck is here quite different. Whereas in the
Liverworts the first transverse division of the internal cell produces an upper cell
which at once represents the * stigma' of the archegonium, the cell thus formed

374

MUSCINEM.

in this group acts like an apical cell, giving rise by a series of longitudinal divisions
to several tiers of cells, each tier consisting of three external cells and an internal
canal-cell and in other respects resembling the single tier of neck-cells formed in
the Liverworts. In this way a long and subsequently twisted neck is formed,
consisting of six external rows of cells investing the central row of canal-cells.
Below, the cells forming the neck become continuous with the wall of the ventral
portion of the archegonium, which consists usually of two (four in Sphagnuni) layers

of cells. The central cell, which makes
its appearance here earlier than in the
Liverworts, becomes divided by a trans-
verse wall into an upper cell, the ventral
canal-cell, and a lower cell, the proto-
plasm of which contracts and forms the
oosphere (Fig. 256, E). The conversion
into mucilage of the canal-cells and the
opening of the neck take place in the
same manner as in the Liverworts.

The Sporogomm?!, which results from
the fertilised oosphere, attains, in Sphag-
num, almost perfect development within
the actively growing ventral portion of the
archegonium, which becomes transformed
into the calyptra; but in all other Mosses
the calyptra is torn away from the vagi-
nula at its base, by the elongation of the
sporogonium, usually long before the de-
velopment of the spore-capsule, and (ex-
cept in Archidiwn and its allies) is raised
up as a cap. The neck of the arche-
gonium, the walls of which assume a deep
red-brov.'n colour, still for some time
crowns the apex of the calyptra. The
sporogonium of all Mosses consists of a
stalk (the Seta), and the spore-capsule
{Theca or Urn); but the former is very
short in Sphagnum, Andrecea, and Archi-
dium, longer in most other genera, and with its base planted in the tissue of
the stem, which, after fertilisation, grows luxuriantly beneath and around the
archegonium, forming a sheathlike investment, the Vaginula. The unfertilised
archegonia may frequently be seen on the exterior slope of the vaginula, since
only one archegonium is usually fertilised in the same receptacle, or it is only
in the one first fertilised that an embryo is developed. The capsule has in
all Mosses a wall consisting of several layers of cells and a distinct epidermis which
sometimes possesses stomata*; the whole of the inner tissue is never used up in

^ The stomata upon the capsules of Mosses are peculiar, as Schimper has shown, in that the
mother-cell of a stoma is not divided into two guard- cells, for the dividing wall does not extend

Fig. 257. — Fwtaria hygronietrica ; A origin of the sporo-
gonium ff in the ventral portion b b oi the archegonium
(longitudinal section x 500) ; B, C different further stages of
development of the sporogoniumyand of the calyptra c; /t neck
of the archegonium (x about 40).

MUSCI.

?>15

le formation of spores, even when, as in Archidium, it is subsequently supplanted
)y them ; a large part of the central tissue remains as the so-called Columella, and
[t is at the circumference of this that the mother-cells of the spores are formed,
'he structure of the mature capsule, and especially the contrivances for dispersing
le spores, are, however, so different in the various principal sections of Mosses
that it will be better to consider them more closely separately, and the more so
jcause by this means we shall at the same time arrive at the distinctive characters
)f the larger natural systematic groups.

^In the mode of origin of the sporogonium there is, as might be expected,
;ss variety. [The oospore is first of all clothed with a cell-wall, continues to grow
considerably, and is then divided by a horizontal or slightly oblique wall {basal
mil). The lower {hypobasal) of these two cells undergoes only one or two divisions,
id, as in Jungermanniese, contributes but litde to the formation of the embryo,
'he upper {epibasal) cell gives rise to the capsule and the seta : a two-sided apical
Jell is formed in it, by means of two oblique divisions.] Hofmeister asserts that in
'iryum argenteum the upper cell (that facing the neck of the archegonium) is divided
mce or twice by horizontal septa before the first oblique division, while in Phascum
id Andrecea this oblique septum is formed immediately after the first horizontal one.
'he apical cell now forms two rows of segments by partition-walls inclined alter-
lately, and these segments are next divided by radial vertical walls, followed by
irther numerous transverse divisions. By this process the young sporogonium is
ransformed into a multicellular body which is usually fusiform, the lower end not
'participating in the growth in length. A swelling of this lower end, such as usually
occurs in Hepaticse, takes place also in certain cases such as Sphagnum, Archidium,
and Phascum. The apex of the sporogonium now becomes inactive, and beneath it
the capsule is formed as a spherical, ovoid, cylindrical, or frequently unsymmetrical
swelling which originates, in the typical Mosses, only after the elongation of the
fusiform or cylindrical sporogonium, and after the raising up of the calyptra. The
internal differentiation of this mass of tissue, at first homogeneous, gives rise to
the various tissues which compose the capsule of Mosses, and especially to the
mother-cells of the spores which first of all become isolated and then divide so
as to form four spores. The contents of the mother-cell begin to divide into

completely across it ; it forms simply a trabecula between the outer and the inner wall, which splits
into two lamella. Between these is the opening of the stoma.

^ [The embryology of Mosses is treated of in the following works. Hofmeister, On the higher
Cryptogamia, Ray Soc. 1862. — Schimper, Rech. anat. et physiol. sur les Mousses, 1848. — Kiihn,
Entwick. d. Andreoeaceen, Bot. Mittheil. von Schenk und Luerssen, 1874 — Vouck, Entwick. d. Sporo-
goniums von Orthotrichum, Sitzber. d, Wien, Akad. 1876 — Leitgeb, Das Sporogon von Archidium,
ibid. 1879. — Kienitz-Gerloff, Bot. Zeitg. 1878. It appears that the tissue of the sporogonium presents,
at an early stage, a differentiation into an amphitheciutn, from which the wall of the capsule and of
the spore-sac are derived, and a central endothecium, corresponding to the future columella and
sporogenous layer. Leitgeb has however pointed out that in the Sphagnacege the sporogenous layer
is probably derived from the amphithecium. It is of interest to note also that, as Goebel has
pointed out, the spores of certain Vascular Cryptogams are developed, like those of Mosses, from a
layer of cells {archesporium), {vide irifra, p. 388). The development of the spores from a layer of cells
occurs also in the developing sporogonia of some Liverworts: it is distinct in Anthoceros, less so in
the Jungermanniese, still less in the Marchantiese, and does not take place in Riccia.']

'M^

MUSCINEM.

two, but this bipartition is usually not completed, the division into four taking
place at once. The preparation for the formation of spores takes place simulta-
neously everywhere within the same capsule. The ripe spores are roundish or
tetrahedral, surrounded by a thin finely granulated exospore, which is of a yellowish,
brownish, or purple colour. Besides protoplasm, they contain chlorophyll and
oil. In Archidium, where only sixteen are formed in each capsule, they are
about \ mm. in size, in the highly developed Dawsonia scarcely -^-^ mm. (Schimper).
When kept dry the spores often retain their power of germination for a long time,
but when moist they frequently germinate after a few days, those of Sphagnum
after two or three months.

The time necessary for the formation of the sporogonium varies greatly in the
different species, but is usually very long in comparison with the small size of the

Fig. 259. — Sphagituvi acutifolhtnt ; the flat protonema/r with a young leafy
stem m (after Schimper, X about 20).

Fig. ■zej^.—Sphag7ium acutifolium ; ^ a large
spore, seen from the apex ; B a small spore ; C a
protonema n n' resulting from the spore s ; pr
rudiments of young plants (after Schimper).

body concerned. The Pottieae blossom in summer, and ripen their spores in the
winter ; the Funarieae are perennially in blossom, and have constantly sporogonia in
all stages of development, each occupying for its completion probably 2 to 3 months ;
Phascum cuspidaium developes in the autumn from its perennial underground proto-
nema, and ripens its spores in a few weeks before the winter. The bog Hypna, on
the other hand (ZT. gi'ganteum, cordifolium, cuspidatum, miens, &c.), blossom in August
and September, and ripen their spores in July of the next year ; they often require
ten months for the development of their sporogonia. H. cupressiforme bears in
autumn at the same time sexual organs and ripe spores, and hence requires one year.

MUSCI.

?>11

The same length of time is required by Philonotis, and by some species of Bryum
and some of Polytrichum which blossom in May and June \

Mosses may be distributed naturally into four parallel orders : —

1. Sphagnaceae,

2. Andreaeaceae,

3. Phascaceae,

4. Bryaceae (True Mosses).

Of these the first includes a single genus, the second and third only a few ; the fourth all
the remaining extremely numerous genera. The first three groups recall, in many
respects, the Hepaticae ; even the series of true Mosses commences with some genera
which still resemble that class; the lowest forms of all the groups exhibit many
resemblances which are wanting in the most highly developed. We have therefore four
diverging series.

I. The Sphagnaceae^ include only the single genus Sphagnum. When the spores
germinate in water, a branched protonema is developed, on which the leaf-buds imme-
diately appear laterally (Fig. 258, C).
On a solid substratum, on the other
hand, the short protonema forms first
of all a branching flat protonemal
expansion (Fig. 259), on which (as
in Tetr aphis) the leaf-buds appear.
The leafy stems produce root-hairs
only in the young state. The abun-
dant protonema of true Mosses is
entirely wanting. The stem, as it
increases in strength, produces later-
ally, by the side of every fourth leaf,
a branch, which, even at the very
earliest period, is again much divided;
tufts of branches arranged regularly
thus arise which form a compact mass
at the summit of the stem, but lower
down are more distant from each
other. The separate branches de-
velope in diff'erent ways ; one is pro-
duced each year beneath the summit
after the ripening of the fruit, and
developes in a similar manner to the
primary stem, growing up along with
the prolongation of the latter, so that
each year a false dichotomy takes
place on the stem. These innovations
afterwards become separated by the
slow decay of the plant advancing
from below, and constitute indepen-
dent plants. Some of the branches of

each tuft, however, turn downwards, become long, slender, and finely pointed, and
are closely applied to the primary stem, forming a dense envelope around it ; while other

Fig. -ifxi.— sphagnum acutifolium ; part of the stem below the
apex ; a a the* male branches, b leaves of the primary stem ; ch peri-
chsetial branch with old still closed sporogonia (after Schimper,
XS-6).

* Klinggraff, Bot. Zeitg. i860, p. 344.

^ W. P. Schimper, Versuch einer Entwickelungsgeschichte der Torfmoose, Stuttgart 1858 (with
lany beautiful plates). [Russow, Beitr. z. Kenntn. d. Torfmoose, 1865. — Leitgeb, Wachsthum des
Jtammes und Entwickelung der Antheridien bei Sphagnum, Sitzber. d. Wien. Akad. 1869.]

37^

MUSCINEJE,

branches of each tuft turn outwards and upwards. The leaves spring from the stem and
the branches from a broad base, and are usually arranged with a divergence of ^; they are
tongue-shaped or apiculate, and, with the exception of the first on the young stem, are
composed of two kinds of cells arranged regularly. The young leaf necessarily consists
of homogenous tissue; but as the development progresses the cells of the veinless lamina
become differentiated into large broad cells about the shape of a long lozenge, and
into narrow tubular cells, running between the former, bounding them, and connected
with one another into a network ; they are, as it were, squeezed in among the larger
ones. The larger cells lose the whole of their contents, and hence appear colourless;
their walls show irregular narrow spiral bands with the turns some distance apart, as well
as large dots, each of which has a thickened edge, while the part of the cell- wall which
closes the dot is absorbed. Large, usually circular holes, are thus formed in the cell-wall
of the colourless cells. The intermediate tubular narrow cells retain their contents,

;,r.S»>*^

Fig. ■2(>\.—SJ>hagnuni acniifoliunt ; A a portion of the surface of
the leaf seen from above, cl the tubular cells containing chlorophyll,
/"the spiral bands, / the holes in the large empty cells ; B transverse
section of a leaf, cl the cells that contain chlorophyll. Is the large
empty cells.

Fig. ofi'i.— Sphagnum acntifoliutn ; A a male
branch, with the leaves partially removed in order
to shovK the antheridia a; Ban open antheridium
(very highly magnified) ; C a free motile anthero-
zoid (after Schimper).

form chlorophyll-granules, and thus constitute the functional tissue of the leaf, the
entire area of which is, however, smaller than that of the colourless tissue (Fig. 261).
The stems consist of three layers of tissue, the innermost of which is an axial cylinder of
thin-walled, colourless, elongated, 'parenchymatous cells ; it is enveloped by a layer of
thick-walled, dotted, firm (lignified ?), prosenchymatous cells, with their walls coloured
brown. The epidermal tissue of the stem, finally, consists of from i to 4 layers of very
broad thin-walled empty cells, which, in S. cymbifolium, possess spiral thickenings and
round holes similar to those of the leaves {cf. Fig. 81). These colourless cells, both those
of the leaves and of the epidermal layer of the stem and of the branches, serve as a
capillary apparatus for the plant, through which the water of the bogs in which it grows
is raised up and carried to the upper parts ; hence it results that the Sphagna^ which
always grow erect, are penetrated with water to their very summits like a sponge, even
when their tufts stand high above the surface of the water.

Musci. ' 379

The archegonia and antheridia of Sphagnum arise on the fascicled branches, as long
as they are still near the summit of the primary stem and belong to the terminal tuft.
The time of bearing them is mostly in autumn and winter, but is not exclusively confined

■ to these periods. The antheridia and archegonia are always distributed on different

[branches, sometimes even on different plants, and in this case the male and female plants
form large distinct patches. When the primary stem does not continue to grow during
the development of the sporogonia in consequence of dry weather, these are to be found

[on the branches of the terminal tuft at a later period ; but when the supply of water is

[great and vigorous increase of length takes place, the fertile branches become separated
from one another, and are subsequently found lower down on the stem ; the sporogonia
and older male branches are thus removed to a distance from the summit, although

[at the time of their development they stand near it. The branches which bear the
antheridia are generally conspicuous externally by their imbricated leaves forming beauti-
ful densely crowded orthostichies or spiral parastichies ; the leaves are generally yellow,

[bright red, or especially dark green, and can hence be easily recognised (Fig/ 260, a a).

iThe antheridia stand, on the mature shoot, h}' the side of the leaves ; they are never

[terminal, and are found only in the middle

■part of the male branch, one standing beside
each leaf; the male branch may therefore

[continue to grow at the summit, and be-

|come an ordinary flagellate branch. This
position of the antheridia, and still more
their roundish form and long pedicel, causes
the Sphagnaceae to resemble some Junger-
mannieae; the mode in which they open
(Fig. 262) recalls the Hepaticae even more
than the true Mosses. The archegonia
arise at the blunt end of the female branch,
the upper leaves of which form a bud-like
envelope; but the young perichaetial leaves
are still contained within this at the time
of fertilisation, although they afterwards

.become further developed. The archegonia

[are exactly like those of the rest of the
Mosses; several of them are usually fertilised
in one perichaetium, but only one perfects
its sporogonium. The development of it
occurs within the perichaetium ; the summit
of the branch then begins to elongate ; it

tgrows out into a long naked receptacle, and

televates the sporogonium contained in its

|Calyptra high above the perichaetium. This

tso-called Pseudopodium must not, therefore,
be confounded with the seta of other Mosses
(see Andreaeaceae). At Fig. 263, J5, is shown
in longitudinal section the nearly ripe spo-
rogonium developed within the calyptra.

' Its lower part forms a thick base imbedded

Jn the end of the pseudopodium which is transformed into the vaginula. The spore-
mother-cells are formed from a cap-shaped layer of cells beneath the apex of the
spherical theca; the part of the inner tissue which is found beneath it forms a low
nearly hemispherical column, which is in this case also termed the columella, although
it is distinguished from the columella of true Mosses by not reaching to the apex of the
theca. The mode of the formation of the spores from the mother-cells resembles that

tifoliujn; A lon^itudina

Fig. 263.—^, B, Sphagtiunt
section of the female flower, ar archegonia, ch perich?etial
leaves still young-, y the last perichsetial leaves ; B longi-
tudinal section of the sporogonium sg, the broad base of
which jg-' remains in the vaginula v, while the capsule is sur-
rounded by the calyptra c, upon this is the neck of the arche-
goniumrtr,/j the pseudopodium; C Sphag;}uim squarrosum,
ripe sporogonium sg with its lid d and ruptured calyptra c, qs
the elongated pseudopodium growing from the perichaetium ck
(after Schimper).

380

MUSCJNEM.

of true Mosses ; but there occur, besides the ordinary (large) spores, also smaller spores
in special smaller sporogonia, which owe their origin to a further division of the mother-
cells (cf. Fig. 258, B). The theca opens by the detachment as a lid of the upper seg-
ment of the wall of the spherical capsule, which is sometimes more strongly convex.
The calyptra, which closely surrounds the growing sporogonium as a fine envelope, is
ruptured irregularly.

2. The AndresBacesB ^ are small cespitose Mosses which are very leafy and much
branched ; their very shortly stalked theca is elevated, as in Sphagnum, above the
perichaetium on a leafless pseudopodium. The long apiculate theca raises up the
calyptra in the form of a pointed cap, as in the true Mosses, while the short seta
remains buried in the vaginula. The body of the young sporogonium becomes differen-
tiated into a parietal tissue consisting of several layers which surrounds the simple layer
of the spore-mother-cells without any intermediate cavity, and a central mass of tissue,
the columella : in the same manner as in the Sphagnaceae, the layer of cells which
produces the spores is bell-shaped and closed above, the columella terminating beneath

Fig. 264. — Archidtunt phascoides ', A longitudinal section
of the young sporogonium, showing the mother-cell m of the
spores ; B longitudinal section through the young sporogonium
with its calyptra and vaginula.yfoot of the sporogonium, -w wall
of the theca, i intercellular space, c columella, h hollow out of
■which the spore-mother-cells have fallen, v vaginula, st stem,
b leaves, a neck of the archegonium. After Hofmeister (Xzoo).

Fig. Q&^.—Archidmm phascoides;
longitudinal section through a nearly
ripe sporogonium, w its wall, sp its
spores, V the vaginula, b leaves of the
stem, St base (foot) of the sporogonium.
After Hofmeister (X 200).

it. The ripe theca does not opeil by an operculum, but by four longitudinal slits at
the sides ; four valves are thus formed united at the apex and at the base, which are
closed in damp, but open in dry weather.

3. The PhascaceflB are small Mosses, the short stems remaining attached to the
protonema until the spores are ripe ; they may be considered as the lowest form of the
following group, to which the genus Phascum forms the transition. They are, however,
all distinguished by their theca not opening by an operculum, but allowing the escape
of the spores only by its decay. While in the genera Phascum and Ephemerum"^ the
internal differentiation of the theca corresponds essentially to that of true Mosses,
although more simple, the genus Archidium displays a more considerable deviation,
and as an interesting transitional form may be examined a little more closely ^. The

^ J. Kiihn, Zur Entwickelungsgeschichte der Andreaeaceen. Leipzig 1870.
^ J. Miiller, in Jahrbuch fiir. wiss. Bot. 1867, vol. VI. p. 237.

^ Hofmeister, in Bericht der konigl. Sachsich. Gesellsch. der Wiss. 1854, April 22. [See also
Leitgeb, Das Sporogon von Archidium, Sitzber. d. Wien. Akad. 1879. He shows that the mother-

MUSCI.

381

very short seta of the sporogonium swells, as in Sphagnum and Hepatkse ; the
roundish theca ruptures the calyptra laterally, without raising it up as a cap. Archidium
agrees with the true Mosses in the formation in the theca of an intercellular space
running parallel to its lateral surface, which separates the wall from the inner mass
of tissue. The latter appears as a column continuous at the base and apex with the
wall of the theca. But while in the true Mosses a layer of cells parallel to this inter-
cellular space produces the spore -mother -cells, it is here only a single cell lying
eccentrically in the inner mass of tissue that becomes the primary mother-cell of all
the spores (Fig. 264, A). It grows considerably, and supplants the other cells, until
it lies free in the hollow of the theca; it then divides into four cells, each of which
produces four spores. The wall of the primary mother-cell remains entire, while the
sixteen spores grow, and fill up the whole of the theca, the inner cell-layer of which
is also absorbed (Fig. 265).

4. In the BryacesB or True Mosses the sporogonium is always stalked, and the
seta is usually of considerable length. The seta is cylindrical, obtusely pointed

Fig. ■zfA.—Ftmaria hygrometrica ; A a young leafy plants
■with the calyptra c; B a. plant g- with the nearly ripe sporogo-
nium, s its seta, y the theca, c the calyptra ; C longitudinal sec-
tion of the theca bisecting it symmetrically ; d operculum,
a annulus, / peristome, c c' columella, h air-cavity, s the pri-
mary mother-cells of the spores.

FIG. 267.— Mouth of the theca of Fonttnalis
antipyretica ; ap outer oeristome, i inner peri-
stome. After Schimper^ 50).

below, and firmly implanted in the vaginula; the theca always opens by the de-
tachment of its upper part as a lid {Operculum) ; the operculum is either simply and
smoothly detached from the lower part of the theca, or a layer of epidermal cells
termed the Jmiulus is thrown off in consequence of the swelling of their inner walls,
and the operculum in this way separated from the theca. Most commonly, after the
operculum has fallen off, the margin of the theca appears furnished with appendages
of very regular and elegant form arranged in one or two rows; the separate append-
ages are termed leeth or Cilia, the whole together the Peristome; if the peristome is
wanting, the theca is said to be gymnostomous. The theca is at first a solid homo-
geneous mass of tissue; the differentiation of its interior begins with the formation of
an annular intercellular space which separates off the wall of the theca consisting of
several layers of cells; but the wall remains attached above and below to the colu-

ells of the spores are not derived from a single primary cell. A variable number of spore-mother-
ells (1-7) are developed independently, each of which gives rise to four spores. They are arranged
juite irregularly. -

382

MUSCINEJE.

mella. The intercellular space is traversed by rows of cells which stretch across from
the wall of the theca to the inner mass of tissue ; they resemble most nearly proto-
nemal filaments, or those of Algae, but have been formed by simple differentiation of the
tissue of the theca. They contain chlorophyll-granules like the inner cell-layers of
the wall. The outer layer of the wall of the theca is developed into a very character-
istic epidermis strongly cuticularised externally. The third or fourth layer of cells of the
inner mass of tissue, which is therefore separated from the annular air-cavity by two or
three layers of cells (forming the spore-sac), produces the mother-cells of the spores.
The cells of this layer are first of all distinguished by being densely filled with proto-

FlG. 268. — Punarta hy^^rometrica ;
transverse section through the spore-
sac ; A, sti the primary mother-cells ;
B, sm the spore-mother-cells not yet
isolated ; a outer side, i inner side of
the spore-sac_{x 550).

Fig. 269.— Development of the spores of Fimaria hygrometrica observed in
very dilute glycerine ; A mother-cells, at a still united, at b and c the separation has
commenced; ^isolated mother-cells clothed with cell-walls; at y expelling the
protoplasmic contents ; C mother-cells with indication of the commencement of
the bipartition of the contents ; D the contents have divided into four masses of
protoplasm, still surrounded by the prnnary cell-wall, but they themselves are
naked ; E the spores enveloped by cell-walls ; F ripe spores (X 550).

Fig. 270.— Various states of division of the mother-cells of the spores oiFtmaria
hygrometrica, observed in water, the progress of development indicated by the
letters a—i.

plasm, in which lies a large central nucleus, and are attached without interstices to the
surrounding tissue in a parenchymatous manner. From their division proceed the spore-
mother-cells, which become isolated by the deliquescence of the cell-walls, and then float
in the fluid contained in the spore-sac, till they form the spores by repeated division.
The Spore-sac is the term given to those layers of cells by which the large air-cavity is
separated from the spore-mother-cells. It seems convenient to consider the layers
which bound the spore-cavity on the axial side (Fig. 268, i) also as a part of the
spore-sac; its cells contain on both sides starch-forming chlorophyll-granules. The
inner large-celled tissue, which contains but little chlorophyll, and is thus surrounded

MUSCI. • 383

on all sides by the spore-sac, is distinguished as the Columella. The spore-sac is ruptured
by the casting off of the operculum, but the columella remains dried up, and in Poly-
trichum there remains also a layer of cells, the Epiphragm, attached to the points of the
teeth of the peristome, and covering the opening of the theca.

We must now examine somewhat more closely the origin of the peristome. In
those genera which, like Gymnostomum, do not form a peristome, the parenchyma
which lies immediately beneath the operculum is homogeneous and thin- walled; when
the theca is ripe, it contracts and dries up within the operculum, which is formed
essentially only of the epidermis ; or it remains attached to the columella and forms
a thickening at its summit, which projects over the opening of the theca; or again
it forms a kind of diaphragm, which closes the mouth of the theca after the casting
off of the operculum {Hymenostomum). The transition to the genera provided with
a true peristome is furnished by Tetraphis. In this genus the firm epidermis of the
upper conical part of the theca falls off as the operculum, while the whole of the tissue
immediately beneath the operculum, the two outer layers of which are thick-walled,
splits across into four valves. These are also termed by systematists a peristome,
although their origin and structure are widely different from that of the true peristome
in other genera. For, except in the Polytrichaceae, neither the teeth nor the cilia con-
sist of cellular tissue, but only of thickened and hardened parts of the walls of a layer of
cells, which is separated by some layers of thin-walled cells from the epidermis which
forms the operculum ; the latter layers, as well as the dehcate parts of the former,
become ruptured and disappear, while the thickened parts of the wall remain after
[the casting off of the operculum. This will be rendered clear by an example. Fig.
271 represents a part of the longitudinal section which bisects the theca of Funaria
hygrometrica symmetrically, corresponding to the part in Fig. 266, C, designated a]
e e \s the reddish brown epidermis strongly thickened on the outside ; at the part where
it bulges its cells are of a peculiar shape, forming the ring or annulus ; se is the tissue
lying between the epidermis of the theca and the air-cavity h ; the large-celled tissue p
is the prolongation of the columella into the cavity of the operculum ; at S are seen
the uppermost spore-mother-cells ; directly above the air-cavity h rises the layer of cells
which forms the peristome ; its walls («), which face outwards, are strongly thickened,
and of a bright red colour ; the thickening is continued also partially along the septa ;
the longitudinal walls which lie on the axial side of the same layer of cells (i) are also
coloured, but less strongly thickened. In Fig. 272 is shown further a part of the
transverse section through the basal part of the operculum ; r r are the epidermal cells
placed immediately above the annulus, forming the lower edge of the operculum; a
and i the thickened parts of the layer of cells concentric with the operculum, which
form the peristome. A section near the apex of the operculum would show, instead
of the broad thickening-masses i, i', i" , only the middle part of the inner wall, but
more strongly thickened. If now it is supposed that when the theca is ripe the annulus
and the operculum fall off, the cells/ and those which lie between a and e (Fig. 271)
disappear, and that the thin pieces of wall between a, a', a" , and between z, i', i"y in Fig.
272, are also destroyed, then the thick red pieces of wall alone remain, forming sixteen
pairs of tooth-like lobes pointed above, crowning the edge of the theca in two concentric
circles. The outer row are termed Teeth, the inner row Cilia. The thickened cells at
/, Fig. 271, unite the base of the teeth with the edge of the theca. According as
the layer of cells which forms the peristome consists, in transverse section, of a larger
or smaller number, and according as one or two thickened walls are formed within each
one of these cells, the number of teeth and ciHa varies ; it is always however a multiple
of four, generally 16 or 32. In many cases the thickening at i is wanting; the peri-
stome is then simple, and formed only of the teeth of the outer row. The thickenings
at a are very commonly much stronger than is the case in Funaria, and the teeth there-
fore stouter. The thickened parts of the wall may also partially or entirely coalesce
laterally with one another ; and then the parts of the peristome either above or below

3«4

MUSCINE2E.

form a membrane ; in this case the teeth appear split from one another above, and the
endostome (the inner peristome) is composed of a lattice-work of longitudinal or trans-
verse ridges instead of cilia (Fig. 267). A great variety is met with here, which may

Fig. 2ji.—Ft(naritt hygrometrtca ; part of a longitudinal
section of an unripe theca.

Pig. ■2T2,—Fiaiaria hygrotnetrica ; part of a transverse section
through the operculum.

Fig. 273.-^4 longitudinal section of the theca oi Poly-
trichtim J>ili/erum. (after Lantzius-Beninga, X is) ; B the
transverse section (X about 5) ; w wall of the theca, cu
operculum, c c columella, / peristome, ep epiphragm, a a
annulus, i i the air-cavities penetrated by alga-like cel-
lular filaments, s spore-sac, containing the primary mother-
cells of the spores, st the seta, the upper part of which
forms the apophysis ap.

easily be understood by the beginner when he has obtained a clear idea of the principle.
The inner and outer sides of the teeth of the peristome are hygroscopic to a different

VASCULAR CRYPTOGAMS. 385

degree ; hence, as the amount of moisture in the air varies they bend inwards or out-
wards, or sometimes in a spiral whorl, as in Barbula,

The genus Polytrichum, to which the largest and most highly developed Mosses
belong, differs from the other genera in several points in the structure of its theca.
The teeth of the peristome are composed not simply of single pieces of membrane, but
of bundles of thickened prosenchymatous cells ; these bundles are horseshoe-shaped ;
the branches of two adjoining bundles directed upwards form together one of the 32-64
teeth. A layer of cells uniting the points of the teeth (Fig. 273, ep) remains, after the
casting off of the operculum and the drying up of the adjoining cells, as an epiphragm
stretched across the theca. The spore-sac is, in some species {e.g. P.piliferum), separated
from the columella by an air-cavity, which is penetrated, like the outer air-cavity, by
conferva-like rows of cells. In most species the seta is swollen beneath the theca,
forming the Apophysis, a phenomenon which is repeated in a somewhat different manner
in the genus Splachnum, where this part is sometimes expanded transversely as a flat
disc.

GROUP III.

VASCULAR CRYPTOGAMS.

Under this term are included in one group the Ferns, Equisetaceae, Ophio-
glosseae, Rhizocarpeae, Lycopodiaceae, Selaginelleae, and Isoetese. As in the
Muscineae, the life-history of the plant is divided into two generations which are
extremely different both morphologically and physiologically. From the spore
proceeds first of all a sexual generation; from its fertilised archegonium is pro-
duced in the second place a new plant, which does not form sexual organs, but in
*their place a number of spores. In the Ferns, Equisetaceae, and Lycopodiaceae these
spores are all aUke ; the Rhizocarpeae, Selaginelleae, and Isoeteae, on the contrary,
produce two kinds of spores, large and small, Macrospores and Microspores.

The Sexual Generation [Oophore] which is developed from th^ spore always
preserves, in Vascular Cryptogams, the form of a thallus ; it never attains, as in the
more highly developed Mosses, to a differentiation into stem and leaf, but remains
small and dehcate, and closes its life with the commencement of the development of
the second generation. It appears, therefore, externally as a mere precursor of further
development, as a transitional structure between the germinating spore and the
variously differentiated second generation. Hence the name Prothallm?n has been
given to this first or sexual generation of Vascular Cryptogams.

In the Ferns and Equisetaceae the prothallium resembles the thallus of the
lowest Hepaticae. The prothallia sometimes continue to grow for a consider-
able time ; they contain a large amount of chlorophyll, and form numerous root-
hairs. After they have thus attained sufficient vigour by independent nourishment,
they produce archegonia and antheridia, usually in considerable numbers. A
tendency to become dioecious is sometimes manifested in these prothallia, although
they proceed from similar spores; both kinds of sexual organs being, however,

c c

386 VASCULAR CRYPTOGAMS.

often produced on the same prothallium. In the Rhizocarpeae, Selaginelleaj, and
Isoeteaj, on the other hand, the separation of the sexes is already prefigured by the
two kinds of spores, the Macrospores being female, in so far as they develope
a small prothallium, which produces exclusively archegonia, or sometimes only
a single one ; the Microspores being male, inasmuch as they develope a still smaller
prothallium bearing exclusively antheridia. The female prothallium of the Rhizo-
carpeae is a small appendage of the macrospore, formed in its interior but afterwards
developed externally although nourished by it; in Selaginelleae and Isoeteae the
prothallium is developed in the spore itself, filling it up with a mass of tissue, the
archegonia becoming exposed only by the splitting of the cell-wall of the spore.
The male prothallium consists, in these groups of plants, of a single sterile or
vegetative cell, and of a larger or smaller number of cells in which antherozoids
are developed.

The Archegonia of Vascular Cryptogams, like those of the Muscineae, are
bodies, consisting of a ventral part which encloses the oosphere, and of a neck,
usually short and composed of four longitudinal rows of cells. The two groups
differ in the fact that in Vascular Cryptogams the tissue of the wall of the ventral
part is formed from the prothallium itself; and the ventral part of the archegonium
is therefore enclosed in the tissue of the sexual generation, the neck only projecting
beyond it. The archegonium originates from a superficial cell of the prothallium
which is divided by a tangential wall into two, an external and an internal. The
former forms, by intersecting longitudinal and subsequent transverse divisions, the
four rows of cells of the rather short neck ; the latter grows outwards between the
neck-cells into a projection which becomes separated, forming the neck-cell, and
another segment is cut off from the large inferior cell (the central cell, Janczewski)
to form the ventral canal-cell. Thus there arises from the original internal cell an
axial row of three cells, the lowest of which is the oosphere. The two neck-cells
become converted into mucilage as in the Muscineae. The mucilage thus produced
in the neck finally swells up considerably, forces apart the four apical cells of the*
neck, and is expelled ; an open canal is thus formed, leading from without to the
oosphere ; the expelled mucilage appears to play an important part in the conduc-
tion of the ' swarming' antherozoids to the opening of the neck. Fertilisation is
always effected by means of water, which determines the opening of the antheridia
and archegonia, and serves as a vehicle for the antherozoids. The advance of these
latter as far as the oosphere, and even their entrance into and coalescence with its
protoplasm, has been directly observed in the different groups.

The Antherozoids^ are, like those of the Muscineae, spirally coiled threads usually
with a number of fine cilia on the anterior coils. In the cases hitherto observed they
arise from the peripheral part of the protoplasm of their small mother-cells, a central
vesicle of protoplasm, containing starch-grains, being left over, which, adhering to a
posterior coil of the antherozoid, is often dragged along by it, but is detached before

^ [On the development of the antherozoids of the Vascular Cryptogams, see Strasburger, Zell-
bildung und Zelltheilung, 3rd, ed. 1880, The formation of the antherozoid is, in all cases, preceded
by the disappearance of the nucleus of the mother-cell, its substance becoming diffused throughout
the protoplasm.]

INTRODUCTION. 387

its entry into the archegonium. The mother-cells of the antherozoids arise in the
antheridia, which in Ferns and Equisetacese project free from the prothallium as
roundish masses of tissue, but in the Ophioglossese and Lycopodium are imbedded
in the prothallium. Among Rhizocarpeae, Salvinia forms a very simple antheridium
which projects from the microspore, while the Marsiliaceae and Selaginelle^B pro-
duce their antherozoids within the microspore itself, after a few- celled mass of
tissue has been formed in it which must be considered as a rudimentary
prothallium.

The Asexual Generation [Sporophore] which produces spores, arises from the
oospore or fertilised oosphere in the archegonium. In Ferns, Equisetaceae, and
Rhizocarpeae, its earliest divisions, the rudiments of the first root, the first leaf, and
the apex of the stem can be recognised, while at the same time a lateral outgrowth
of its tissue, called the Foot^ commences at the bottom of the ventral part of the
archegonium, and draws from the prothallium the first nourishment for the young
plant. The ventral part of the archegonium at first grows vigorously (except appa-
rently in the Selaginelleas), enveloping the embryo, until this latter finally protrudes
free, leaving however, for some time, the foot still attached to it as a nutritive organ.
This process offers an unquestionable analogy to the formation of the calyptra of
the Muscineae. While, however, the spore-producing generation of the Muscineae
remains a mere appendage of the sexual plant, appearing in a certain sense as its
fruit, the corresponding generation of Vascular Cryptogams developes, on the con-
trary, into a conspicuous, highly organised, independent plant, which frees itself at a
very early period from the prothallium, and obtains its own nourishment. It is this
asexual generation which is called, in ordinary language, simply the Fern, Equisetuniy
&c. ; it always consists of a leafy stem, usually producing a number of true roots ;
roots may, however, occasionally be entirely absent, as in some Hymenophyllaceae,
and in Psilotum and Salvinia. In many cases, especially in Ferns, Equisetaceae, and
(especially the extinct) Lycopodiaceae, the spore-producing generation attains great
dimensions with an unlimited term of life ; only a few species are (like Salvinia)
annual, or very small, resembling Mosses in habit, as Azolla and some Selaginelleae.

The Leaves are either simple, unsegmented, or variously branched (FiUcineae).
There does not, however, occur so great a variety due to metamorphosis in the
forms assumed by the leaves in the same plant as in Phanerogams.

The Roots usually arise in acropetal succession on the stem (or on the leaf-
stalk in some Ferns), and branch monopodially or dichotomously ; they always
remain nearly uniform in size, the first root never attaining the dimensions of a
tap-root, as in many Phanerogams. The lateral roots do not arise, as in Phanero-
gams, from the pericambium, but from the innermost cortical layer of the main
root.

The Differentiation of the Systems of Tissue attains a high degree of perfection
for the first time in this group of plants. The epidermis, fundamental tissue, and
fibro-vascular bundles are always clearly distinct, and are composed of cells of
various forms. The fibro-vascular bundles are closed ; the phloem usually sur-
rounds the xylem of each separate bundle like a sheath.

The Branching of the Stem is very different in the different classes of Vascular
Cryptogams ; it is essentially monopodial, but it is often apparently dichotomous :

c c 2

c^ScS VASCULAR CRYPTOGAMS.

axillary branching, in the sense in which the term is applied to Phanerogams,
probably does not occur here.

The Development of the Sporangia^ is in most cases evidently a function of
ordinary or of specially modified leaves ; it is only when they arise singly upon the
upper surface of leaves, as in Lycopodium, that they come to stand in the axils of the
leaves: in Selaginella they arise from the stem, and in Psilotum and Tmesipteris
from lateral branches. In the earliest stage of their development they are in the
Filicinese outgrowths of single epidermic cells : in the other Vascular Cryptogams
they are developed from a group of cells. [The mother-cells of the spores are
derived from one or more cells which lie at first immediately beneath the epidermis
of the sporangium (hypodermal), and which Goebel terms the archesporium. The
archesporium becomes invested by one or more layers of cells, derived either from
the division of its own cells or of the cells forming part of the sporangium, to which
the name of tapetum is given. As the sporangium ripens, the cells of the tapetum
usually become absorbed, and the mother-cells of the spores become isolated from
their combination into a tissue. From each mother-cell four spores are developed :
this takes place in two ways ; either the cell is divided into two by a cell-wall after
the nucleus has divided, and this process is repeated in each of the two daughter-
cells, or the nucleus divides into two and each of these again into two before any
cell-wall is formed. The latter is by far the more general mode. The distinction
between macrospores and microspores in the Rhizocarpese and Selaginellese is
manifested only after the division into four of the mother-cells which were pre-
viously alike.]

It is clear from what has now been said that the sporangium of Vascular
Cryptogams is equivalent, from a physiological but not from a morphological point
of view, to the sporogonium of Mosses. This latter forms by itself the whole of
the asexual generation of Mosses ; while the sporangium of Vascular Cryptogams

^ [On the Comparative Development of the Sporangia see Goebel, Bot. Zeitg. 1 880-81. He
classifies the Vascular Cryptogams according to the mode of development of their sporangia, as
follov/s : —

Leptosporangiata. The sporangium is developed from a single epidermal cell. The arche-
sporium is a single cell, and the tapetum is derived from it.
Filices ; Rhizocarpese.

Eusporangiata. The sporangium is developed from several cells, which, except in the case of
Tsoeies, are all superficial.

a. Archesporium unicellular ; tapetum derived not from it, but from the tissue of the sporangium.
Ophioglossese ; Marattiacese ; Equisetacese.

b. Archesporium unicellular ; tapetum derived from it.
Psilotum; Tmesipteris {?).

c. Archesporium unicellular ; tapetum derived partly from it, and partly from the tissue of the
sporangium.

Selaginella.

d. Archesporium multicellular (a row^ of cells) ; tapetum formed not from it but from the tissue
of the sporangium.

Lycopodium.

e. Archesporium multicellular (a layer of cells) ; tapetum derived from it.

Isoetes.
The Phanerogams also belong to the Eusporangiata, see infra.]

INTRODUCTION.

389

a relatively small outgrowth borne on either the leaf or the stem of the asexual
aeration which consists of stem, leaf, and root.

The proof that what is termed the Moss-fruit, i.e. the sporogonium, is, from its
position in the alternation of generations, the equivalent of the entire leafy and rooting
spore-producing plant of Vascular Cryptogams, was brought forward by Hofmeister
as long ago as 1851 (Vergleichende Untersuchungen, p. 139 i). In connection with the
relationships pointed out by him between the Selaginelleae and Isoeteae on the one hand
and the Goniferae on the other, this discovery is one of the most fertile in results that
has ever been made in the domain of morphology and classification. The researches
of Pringsheim and Hanstein on the development of Rhizocarps, carried out with great
acuteness and deep penetration, those of Nageli and Leitgeb on the roots of Vascular
K^ryptogams, and of Cramer on the apical growth of the stem of Equisetacese and Lyco-
)odiaceae, to which numerous more recent observations may be added, have not only
:ontributed to a more accurate knowledge of this group of plants, but have especially
cleared up the fundamental morphological facts. Since the appearance of the first
jdition of this book, our knowledge of the alternation of generations has been enriched
>y Millardet's discovery of the male prothallium in Isoeteae and Selaginellese ; and the
ibours of Millardet, Strasburger, Kny, and especially of Janczewski, have resulted in
more complete acquaintance with the development of the sexual organs and of the
>rocess of fertilisation itself in its details.

Taxonomy. Our ideas as to the mutual relationships existing between the various
livisions of Vascular Cryptogams are at present very variable, they are, in fact, in a
State of transition. The division into Isosporeae and Heterosporeae suggested by me
fn the first edition and retained in the third edition of this book seemed to be fully
justified as long as it could be assumed that in the Lycopodieae two forms of pro-
thallium were developed as in the Selaginelleae and Isoeteae. This assumption has
)een proved untenable by Fankhauser's discovery of the monoecious prothallium of
.ycopodium. Still a separation of the isosporous Lycopodieae on this account from the
leterosporous Selaginelleae and Isoeteae would be unjustifiable. Besides, recent researches
lave shown that the Rhizocarpeae are much more closely related to the true Ferns than
to the heterosporous Selaginelleae and Isoeteae. As a consequence, the division of Vas-
:ular Cryptogams into Isosporeae and Heterosporeae must be given up as being purely
irtificial, and we are led to assume that the differentiation of primarily similar spores
into microspores and macrospores has taken place in two distinct groups of these
)lants; for the first time, in a developmental series which begins with the true Ferns,
for the second, in a series which begins with the Lycopodieae. This assumption is
supported by the fact that the differentiation of two kinds of spores takes place in
the Rhizocarpeae in a manner very different from that in which it takes place in the
lelaginelleae and Isoeteae. Further, since I pointed out in the first edition (1868) that
the sporangia of some forms arise as multicellular bodies, whereas others are developed
From single cells, younger botanists (especially Luerssen and Russow) have extended
ly observations and have laid stress upon the distinction of Trlchosporangia from Phyllo-
[Caulo-) sporangia as a point of great systematic importance.

If, in accordance with the present state of our knowledge, I give up my former
[classification, I must also, on the same grounds, decline to accept the classifications
suggested by Luerssen ^ and by Russow ^, for they are based on isolated characteristics,

* [On the Germination, Development and Fructification of the Higher Cryptogamia, and on
the Fructification of the Goniferae, by W. Hofmeister; translated by F. Gurrey ; Ray Soc. 1862,
P- 434-]

^ Luerssen, in den Mitth. aus dem Gesammtgebiet der Botanik von Schenk und Luerssen,
Bd. I. p. 107. -

^ Russow, Vergleichende Untersuchungen, St. Petersburg 1872.

390

VASCULAR CRYPTOGAMS.

upon which too much importance is laid, and therefore run counter to natural affinities
and cause forms which are widely different to be placed side by side.

The following new classification lays no claim to be regarded as definitely indicating
for all time the relationships existing among Vascular Cryptogams, but I believe that
it corresponds more closely than any previous arrangement with the present state of
our knowledge on the one hand, and with those affinities which may be said to be
self-evident on the other. It appears certain that the three Classes here formed,
the Equisetinese, the Filicineae, and the Dichotomeae, represent three distinct and
separate types. Great difficulties are ofl'ered to the systematic sub-division of the
Filicineae, for certain groups of them, especially the Osmundaceae, the Schizaeaceae and
the Gleicheniaceae, have not yet been thoroughly investigated from a morphological
point of view.

Systematic revieiv of the Vascular Cryptogams.

CLASS VII.

Equisetine^.

The spores are of one kind and give rise to prothallia which vegetate independently,
and which are usually dioecious, the female prothallia being the larger, the male the smaller.
The second generation consists of a copiously branched stem, with well-defined inter-
nodes, and bearing relatively small sheathing whorls of leaves. The branches arise in
whorls and in strict acropetal order from the nodes of the stem. A root may arise
below each branch; its ramification is monopodial. The sporangia are borne upon
metamorphosed peltate leaves which form a terminal spike : they originate as multi-
cellular protuberances (emergences), from five to ten in number upon each scale. The
mother-cells of the spores, it appears, are derived from a unicellular archesporium.
Both stem and root increase in length by means of a large apical cell which gives rise
to three rows of segments. The fibro-vascular bundles of the stem are arranged in
a circle ; their xylem is rudimentary, resembling that of the bundles of Monocotyledons.
The axial fibro-vascular cylinder of the root has no pericambium.
Family. (i) Equisetacese.

CLASS VIIL

FlLICINE^.

The majority of these plants possess spores of one kind only, from which inde-
pendent monoecious prothallia are developed : but the Rhizocarpeae have macrospores
and microspores forming rudimentary prothallia which never become free from the
spore. The second generation is a stem bearing numerous well-developed leaves which
are usually much branched. The stem either does not branch, or it does so but rarely,
and it bears numerous roots. The sporangia are borne on ordinary leaves, or on leaves
which have been specially modified : they are usually aggregated into small groups (Sori).
The sporangia of the Ophioglosseae and Marattiaceae are from the first multicellular, and
those of the Filices are developed from single superficial cells : the archesporium is
unicellular. An apical cell is present in the stem and in the root: it forms, in the
stem, two or three rows of segments, in the root usually three rows. The fibro-vascular
bundles are generally very well developed, and the central xylem, which consists for
the most part of scalariform tracheides, is usually surrounded by the phloem.

Order \. StipulatSB. The spores, so far as is at present known, are of one kind
and give rise to independent monoecious prothallia. The second generation is a simple,

INTRODUCTION.

391

usually unbranched, erect or oblique stem of a somewhat tubercular form, bearing leaves
arranged close together in a spiral manner. The leaves are relatively large, are usually
much branched, and usually bear stipules at the base of their petioles. The sporangia
are borne either upon the under surface of ordinary leaves, or in spicate or paniculate
fructifications which are metamorphosed leaf-segments. They are derived from groups
of cells which form projections upon the surface of the leaves. The mother-cells of
the spores are found in large numbers. A single apical cell is present in the stem
and root.

Families. (i) Ophioglosseae.

(2) Marattiaceae.

(3) Osmundaceae.

(4) Schizaeaceae.

Order II. Filices. The spores are of one kind and develope independent monoe-
cious prothallia. The second generation is either an erect unbranched stem, or a stem
which is more or less horizontal and bilaterally symmetrical and which branches occa-
sionally. The leaves do not possess stipules, are circinate in their vernation, and bear
upon their laminae, which may or may not be more or less specially modified, very
numerous sporangia usually arranged in sori covered by indusia. The sporangia arise
from distinct epidermal cells, and usually sixteen spore-mother-cells are formed in
each. The sporangia are opened by means of the so-called ring (annulus). Both
stem and root possess a single apical cell. The ground-tissue tends to become
converted into brown sclerenchyma, which especially supports the sheaths of the
fibro-vascular bundles.

Families. (i) Gleicheniaceae (?), (Osmundaceae, Schizaeaceae?).

(2) Hymenophyllaceae.

(3) Cyatheaceae.

(4) Polypodiaceae.

Order III. RhizocarpesB. The sporangia are of two kinds, those of the one kind
containing macrospores, those of the other microspores. Within the macrospore a
small prothallium is formed which does not become separated from the spore ; within
the microspore the mother-cells of the antherozoids are formed from a very rudi-
mentary prothallium. The second generation is a bilaterally symmetrical, horizontal,
regularly branched stem, which bears leaves upon its dorsal surface in two or more
rows, and roots upon its ventral surface. {Sal'vinia is rootless.) The sporangia arise
from individual superficial cells of the placentae and are enclosed in sporocarps which
may be uni- or multilocular, and are formed by metamorphosed leaves or segments of
leaves. The placenta of each loculus of the sporocarp bears a sorus of sporangia,
and there are sixteen mother-cells of the spores in each sporangium. The microspores
are formed in great numbers in each microsporangium (4 x 16), but only one macrospore
in the macrosporangium comes to maturity. The stem grows by means of a two- or
a three-sided apical cell ; the root has always a three-sided apical cell.

Families. (i) Salviniaceae.
(2) Marsiliaceae.

CLASS IX.

DiCHOTOMEiE.

The prothallia are developed either from spores of one kind only and are then
independent and monoecious, or they are developed from spores of two kinds (macro-
and microspores), and then remain within the spore until the period of fertilisation.

392

VA SCULAR CR YP TO GA MS.

The second generation is a simple or repeatedly branched stem, usually possessing roots
and always bearing simple, unsegmented, comparatively small, but very numerous leaves,
which are traversed by only a single fibro-vascular bundle. All the branchings of the
stem and of the roots present the appearance of having originated dichotomously.
The sporangia are borne singly upon the upper surface of the base of the leaves, or
in their axils, or even in an extra-axillary position upon the stem. They originate as
masses of cells derived in part (Isoetes) from internal tissues, covered by the epidermis
which forms their walls. The archesporium is multicellular in some cases, unicellular
in others.

Order I. Lycopodiaceas. The prothallia, which are developed from spores of one
kind only, are capable of independent growth and are monoecious. The roots branch
dichotomously in alternate intersecting planes, and neither stem nor root possesses
a single apical cell. The leaves have no ligula. The fibro-vascular cylinder of the
stem consists of numerous xylem-bundles, each of which is surrounded by phloem.
Families. (i) Lycopodieae.

(2) Psiloteae.

(3) Phylloglosseae.

Order II. LigulatSB. The spores are of two kinds. Each macrospore forms a
rather large internal female prothallium, the archegonia of which only become exposed
when the wall of the spore is ruptured. Within each microspore a rudimentary pro-
thallium completely filling it is formed, certain cells of which give rise to the mother-
cells of the antherozoids. The second generation is of very different habit in the two
families. The leaves are always provided with a ligula borne above their base, and
below this lies the sporangium which contains either numerous microspores or four or
more macrospores.

Families. (i) Selaginelleae.
(2) Isoeteae.

CLASS VII.

EQUISETINE^

The Sexual Generation (Oophore). The spores of the Equisetaceae, so soon as
they have attained the ripe condition (they retain their power of germination only

^ G. W. Bischoff, Die kryptogamischen Gewachse (Niirnberg 1828). — W. Hofmeister, Vergl.
Unters. (1851). — Ditto, Ueber die Keimung der Equiseten (Abh. der konigl. Sachs. Gesell. d. Wiss.
1855, vol. IV. p. 168). — Ditto, Ueber Sporenentwickelung der Equiseten (Jahrb. fiir wiss. Bot. vol. III.
p. 283). — [Germination, Development, and Fructification of the Higher Cryptogamia (Ray Society),
pp. 267-306]. — Thuret (in Ann. des Sci. Nat. 1851, vol. XVI. p. 31). — Sanio, Ueber Epidermis
und Spaltoffnungen des Equis. (Linnaea, vol. XXIX. Heft 4).— C. Cramer, Langenwachsthum und
Gewebebildung bei E. arveme und sylvadcum (Pflanzenphys Unters. von Nageli und Cramer, 1855,
vol. III). — Duval-Jouve, Hist. Nat. des Equisetum (Paris 1864). — H. Schacht, Die Spermatozoidtn
im Pflanzenreich (Braunschweig 1864). — Max Reess, Entwickelungsgeschichte der Stammspitze von
Equisetum (Jahrb. fiir wiss. Bot. 1867, vol. VI. p. 209). — Milde, Monographia-Equisetoium, in Nova
Acta Acad. Leop. Carolinoe, 1867, vol. XXXV. — Nageli und Leitgeb, Entstehung und Wachslhum

EQUISETINEjE.

393

for a few days), show, when sown in water or on damp soil, the preparatory
phases of germination after a few hours. In the course of some days the pro-
thallium becomes developed into a multicellular plate, the further growth of which
then proceeds very slowly. The spore, which contains a nucleus and chlorophyll-
granules, increases in size as soon as germination commences, becomes pear-shaped,
and divides into two cells, one of which is smaller with scarcely any except colourless
contents, and soon developes into a long hyahne
root-hair (Fig. 274, /, //, III, w), while the
anterior and larger cell includes all the chloro-
phyll-granules of the spore which multiply by
division. This cell produces by further divisions
the primary plate of the prothallium, which in-
creases by apical growth and soon branches
(///- VI). The process of multiplication of the
cells is therefore apparently extremely irregular;
even the very first divisions vary ; sometimes
t:he first wall in the primary apical cell which

contains chlorophyll is but little inclined with

ispect to the longitudinal axis of the young plant
fn E. Telmateia 'it sometimes coincides with it) ;
other cases, on the contrary, this cell developes

ito a longish tube, the apical part of v^hich is
^ut off by a transverse septum (occasionally in

r. arvense). The further growth is brought about
one or more apical cells dividing by trans-
;rse septa, and longitudinal walls are subse-
quently formed in the segments in an order very

lifficult to determine. Ramification takes place

)y the bulging out of lateral cells, which then

Continue their growth in a similar manner. The

jhlorophyll-granules in the cells also increase con-

inuously by division. The young prothallia are,
E. lebnateia, usually narrow and ligulate, and

Consist of but a single layer of cells. The older

>rothallia are, both in this and in other species,

branched in an irregularly lobed manner ; one of

le lobes takes, sooner or later, the lead in growth, becomes thicker and fleshy,
)nsisting of several layers of cells, and puts forth root-hairs from its under side.

The prothallia of the Equisetaceae are, in general, dioecious. The male pro-
lallia remain smaller, attaining a length of a few millimetres, and produce archegonia

mly in exceptional cases on shoots of later origin (Hofmeister). The female

Fig. 274. — First stage of development of the
protha liuni of Equisetutn Tehnaieia ; tv the first
root-hair; t rudiment of the prothalliuin. The
order of development follows the numbers/— K/
(X about 220).

ler Wurzeln (Beitr. zur wissen. Bot. von Nageli, Heft IV. Miinchen i867>.— Pfitzer, Ueber die
jchutzscheide (Jahrb. fur wissen. Bot. vol. VI. p. 297).— Russow, Vergl. Unteis. iiber die Leit-

indelkrypt. Petersburg 1872, p. 41. — Janczewski, iiber die Archegonien, Bot. Zeit. 1872, p 420.
''an Tieghem, on the roots, in Ann. des sci. nat fth series, vol. XIII.— -[Sadebeck, Ueb. die Entwicke-

ing der Prothallien der Schachteliialme, Bot. Zeit. 1877.]

394

VASCULAR CRYPTOGAMS.

prothallia are larger (as much as ^ inch) ; Hofmeister compares them to the thallus
of Anthoceros punctatus, Duval-Jouve to a curled endive-leaf. Duval-Jouve states
that the antheridia appear about five weeks after germination, the archegonia much
later. These statements refer especially to JE. arvense, limosum, and palustre ;
according to the same writer, the prothallia of E. Telmaleia and sylvaticum are
broader and less branched; those of E. ramosissimum and variegatum slenderer
and more elongated.

The Antheridia^ arise at the end or margin of the larger lobes of the male

Fig. 275. — A male prothallium oi Equisetum arvense WxXkv
the first antheridia a (after Hofmeister, X 200) ; B—E anthero-
zoids oiE. Telmateia (after Schacht).

Fig. 276.— Lobe of a highly developed female prothallium oi Eguisettim arvetise cut through vertically (after Hofmeister,
X about 60) ; a « a two abortive and one fertilised archegonium, h root-hairs.

prothallium. The apical cells of the enveloping layer of the antheridium contain
but little or no chlorophyll; they separate from one another on the addition of
water (like those of Hepaticae), to allow the escape of the antherozoids, which are
still enclosed in vesicles and number from loo to 150. The hindermost and
thickest of the two or three coils of the antherozoid, which is larger in this class
than in all other Cryptogams, bears an appendage on the inner side which

* [Sadebeck, Ueb, die Antheridien-Entwickelung der Schachtelhalme Sitzber. d. Gesellsch.
naturfor. Freunde zu Berlin, 1875.]

EQUISETINE.E. 395

Hofmeister terms an undulating Float, Schacht a thin-walled vesicle of proto-
plasm, and which contain granules of starch and sap (compare with Ferns and
Isoeteae).

The Archegonia are developed from single superficial cells of the anterior
margin of the thick and fleshy lobes of the female prothallium. As the tissue
of the prothallium beneath them continues its growth, the archegonia come, as
in Pellia, to stand on its upper surface. The mother-cell of the archegonium,
after it has become much curved, divides by a wall parallel to the surface of the
prothallium. From the outer of the two daughter-cells is formed the neck, con-
sisting, at a subsequent period, of four parallel rows each of three cells. Of these,
the four upper cells become very long ; the four middle ones remain shorter ; the
four lower ones scarcely elongate at all, and contribute by their multiplication,
like the cells of the prothallium which surround the central cell, to the formation
^of the wall of the ventral part of the archegonium, which consists of one or two
lyers. The other daughter-cell, which is sunk in the tissue of the prothallium,
jlongates whilst the wall of the neck is being formed and projects between
^he four rows of cells constituting it. This projection is then cut off by a septum
From the lower large portion of the cell. Of the two cells thus formed, the
former is the single canal-cell of the neck, and the latter is the central cell of
the archegonium. The central cell is divided again into two, the upper being
the ventral canal-cell, the lower contracting and forming the oosphere. In these
>rocesses the archegonium of Equisetum resembles that of Ferns, the only dif-
jrence being that in the former the canal- cell does not occupy the whole length
>f the neck (Janczewski). The four upper long cells of the neck curve radially
mtwards, when the canal of the neck is being formed, like a four-armed anchor,
[mmediately after fertilisation the canal of the neck closes, the oosphere, and which
ias now become the oospore, enlarges, and the cells of the wall of the ventral
)art of the archegonium which surrounds it begin rapidly to multiply.

Development of the Asexual Generation (Sporophore) of Equisetum. The
formation of the embryo from the oospore is the result of divisions, the first of which
inclined to the axis of the archegonium, and is followed, according to Hof-
leister, in each of the two cells by a division-wall placed perpendicularly to the
irst. The embryo appears to be composed of four cells arranged like the quarters
>f a sphere. The same author states that the foot arises from the lower quarter,
le rudiment of the first shoot from one of the lateral ones, turning upwards
Immediately afterwards and producing as the rudiment of the first leaf a pro-
jecting girdle, which then grows out into three teeth (Fig. 277 B). The apical
;ell of the first root arises from an inner cell of the tissued

* [On the Embryology of Equisetum see Sadebeck, Jahrb. f. wiss. Bot. XI. 1878, and also in
Jchenk's Handbuch, III. The embryology of Equisetum closely resembles that of Ferns (see infra,
). 426). The oospore divides by successive bipartitions into eight segments : of the four octants

totming the upper (epibasal) half of the embryo, one gives rise to the stem, two give rise to
le first leaf (cotyledon), and one gives rise to the second leaf: of the four octants forming the

lower (hypobasal) half of the embryo, two give rise to the foot, one disappears, and the remaining
)ne, which is diametrically opposite to the stem-octant, gives rise to the root.]

39<5

VASCULAR CRVPTOGAMS.

Fig. 277. — Development of tlie embryo of h'qutsetum ar-
vense (after Hofmeister) ; A archegonium cut through vertically
with the embryoy (X200) ; B embryo further developed and
isolated, b rudiment of a leaf, s apex of the first shoot (X 200) ;
C vertical section of a lobe of a prothallium / /, with a young
plant, -w its first root, b b' its leaf -sheaths (X 10).

The first leaf-bearing shoot grows upwards, and forms from ten to fifteen
internodes with leaf-sheaths ending in three teeth. It soon produces at its
base a new stronger shoot with four-toothed sheaths (as in E. arvense, pratense,
and variegaium, according to Hofmeister), which in turn gives origin to new

generations of shoots, developing con-
stantly thicker stems and sheaths with a
larger number of teeth. Sometimes the
third or one of the succeeding shoots
penetrates downwards into the ground,
forming the fiist perennial rhizome, which
again produces from year to year new
underground rhizomes and ascending leafy
shoots.

In order to facilitate the understanding
of the Mode of Growth of the Stem and
Leaves, it is necessary to glance in the first
place at their structure in the mature state.
Every axis of an Equisetum consists of a
series of joints (internodes) usually hollow
and closed at their base by a thin septum.
Each internode passes upwards into a
leaf-sheath embracing the next internode,
the sheath being split at its upper margin
into three, four, or usually a larger number of teeth. From each tooth of the
sheath a fibro-vascular bundle runs vertically downwards into the internode as far as
the next node, parallel with the other bundles of the same internode ; at the lower
end each bundle splits into two short diverging limbs, by which it unites with the
two neighbouring bundles of the next lower internode, where they descend into
it from their sheath-teeth. The joints of the stem and their leaf-sheaths therefore
alternate ; and since in each joint the arrangement of bundles, leaf-teeth, projecting
longitudinal ridges, and depressions or furrows, is exactly repeated in the transverse
section, the different parts of a joint always correspond to the intervals between
the homologous parts of the next upper and next lower joint. If the internode
has projecting longitudinal ridges on its surface, one of these always runs down-
wards from the apex of each leaf-tooth parallel with the others as far as the
base of the internode; between each pair of leaf-teeth commences a furrow or
channel, which also continues as far as the base of the internode. The projecting
ridges lie on the same radii as the fibro-vascular bundles, each of which contains
an air-canal; the depressions or furrows lie on the same radii as the lacu^iae of
the cortical tissue (which are sometimes wanting), and alternate with the fibro-
vascular bundles. The branches and roots spring exclusively from within the
base of the leaf-sheath; and as this forms a whorl, the branches and roots are
also verticillate. A root may arise beneath the bud of each branch ; both break
through the leaf-sheath at its base. All the joints of the axis agree in these respects,
however they may be modified as underground rhizomes, tubers, ascending stems,
leafy branches, or sporangiferous axes.

EQUISETINEM.

397

The end of the stem, enveloped by a large number of younger leaf-sheaths,
terminates in a large apical cell, the upper wall of which is arched in a spherical
manner, while the three infero-lateral walls are almost plane. The apical cell
has therefore the form of an inverted triangular pyramid, the upturned basal
surface of which is a nearly equilateral spherical triangle. The segments are cut
off by walls which are parallel to the oblique sides of the apical cell, that is, to
the youngest primary walls of the segment; the segments, disposed in a spiral ^
arrangement, lie in three vertical rows. Each segment has the form of a triangular
plate with triangular upper and under walls, rectangular lateral walls lying right
and left, and an outer rectangular wall which is curved. Each segment is first
divided — as was shown by Cramer and Reess and confirmed by myself — by a wall
parallel to the upper and under surfaces into two equal plates lying one above

Fig. 278. — Eqicisetum Telmateia ; A piece of an upright stem (natural size), i i' internodes, h its central cavity,
/lacunae of the cortex, S leaf-sheath, z its apex, a a' a" the lower internodes of young shoots ; B longitudinal section
of a rhizome (X about 2), k septum (diaphragm) between the cavities hk, g fibro-vascular bundle, / lacunae of the cortex,
^leaf-sheath; C transverse section of a rhizome (X about 2), ^ and / as before ; Z) union of the fibro-vascular bundle of
an upper and lower internode iif. A!" the node.

mother, and consequently each half the height of the undivided segment. Each
lalf-segment is then again halved, in the most usual case, by a vertical nearly radial
^all. The segment now consists of four cells, two of which lie one above the other

id reach as far as the centre, but the other two do not because the vertical wall is
lot accurately radial but intersects one of the lateral walls of the segment (the
modal wall), (Fig. 279, E'). Divisions now take place without any strict rule in
|jhe four cells of each segment parallel to the primary and the lateral walls; and

mgential divisions also soon make their appearance, by which the segment is
Split up into inner and outer cells, in which further divisions afterwards take place.

'he former produce the pith, which is soon destroyed as far as the septum at
the base of each internode by the expansion of the stem ; the latter produce the

iaves and the entire tissue of the hollow internodes. The segments are, as has

398

VASCULAR CRYPTOGAMS.

been mentioned, disposed originally in a spiral with ^ arrangement ; and since
each segment without exception (as in Mosses) produces a leaf or what corresponds
to a part of a leaf-sheath, the leaves of Equisetum must also be arranged in a
spiral. This does, in fact, sometimes occur when the growth is abnormal ; but
when the growth is normal, a small displacement takes place at a very early
period, of such a nature that the three segments which form a cycle always
come to be arranged into a disc transverse to the stem, their outer surfaces thus

Fig. 279.-/4 longitudinal section of the end of a stem in an underground bud oi Eqtcisettini Tehnateia ; S apical cell, xy first
indication of the girdle from which the leaves are subsequently formed, b b <\. more advanced and distinctly marked foliar
girdle, bs the apical cells of a strongly projecting foliar girdle, rr rudiment of the cortical tissue of the internodes, ^.^ rows
of cells from which the leaf-tissue and its fibro-vascular bundle proceed, ?« the lower layers of cells of the segment which take
no part in the formation of leaves (from nature) ; B horizontal projection of the apical view of the end of a stem oiE. Tehnateia;
s apical cell, I — /^the successive segments, the older ones still further divided ; C horizontal projection of the apical view of
K. arvense; D optical longitudinal section of the end of a very slender stem ; E transverse section of the end of a stem after the
formation of the vertical and first tangential walls. (C, A -£'. after Cramer ; the Roman numerals indicate the segments, the
Arabic numerals the walls formed in them in the order of their succession ; the letters the primary walls of the segments.)

forming an annular zone or girdle. According to Reess, to whom this observation
is due, the three segments of each cycle are formed in rapid succession, while
a longer time elapses between the formation of the last segment of the preceding
and that of the first of the succeeding cycle. Thus by the unequal growth of
the segments in longitudinal direction each cycle of segments or turn of the
spiral produces a whorl, which therefore, strictly speaking, is a pseudo-whorl,
because resulting from subsequent displacement. Each whorl of segments now

EQIJISETINEM.

399

forms a leaf-sheath, and the corresponding internode or joint of the stem. The
above-mentioned divisions take place in the three segments during their arrange-
ment into a transverse disc, each segment becoming converted into a mass of cells
consisting of from four to six layers. As soon as the transverse zone is formed,
the formation of the leaves commences by the growth of the outer cells of the
segments. They form an annular ridge ; one of the upper transverse cell-layers of
the whorl of segments projects out-
wardly, forms the apex (the circular
apical line) of the ridge (Figs, 279, 280,
bs), and those of its cells which lie
most externally (the apical cells) divide
by walls inclined alternately towards
and from the axis. The circular apical
line becomes more and more elevated,
and thus the annular ridge becomes a
sheath enveloping the end of the stem.
JThis same layer, of which the outer-

lost cells form the apical line of the
mular ridge, produces in the interior

>f the sheath a meristem in which the

ibro-vascular bundles of the leaf-sheaths

rise. The lower transverse cell-layers

>f the whorl of segments grow only

lightly outwards and upwards, become

livided by vertical and afterwards rapidly

^y transverse walls, to produce the
5sue of the internode, which passes

radually into that of the leaf A ver-
tical layer of this tissue forming a hollow
cylinder (Fig. 280, v v) is distinguished
by numerous vertical divisions; it forms
a ring of meristem (procambium, thick-
ening-ring in Sanio's sense), in which

the vertically descending fibrO-VaSCUlar fig. 280.— Left half of a radial longitudinal section beneath the

J, r 1_ • J r 1 s^p&x of an vtvAergronnA hud oi Egutsetum Telmaieia m S&^ismher ;

bundles 01 the mternOae are lOrmed. /viewer part of the vegetative cone. ^''iJ",^"' leaves, *j their apical
rp, 1 ji r 1 1 • cells, r* r" r'" the cortical tissue of the corresponding internodes ;

1 heSe bundles lOrm the prolongations ^ ^ pith, v v v thickening ring, g g layer of cells from which the

fibro-vascular bundles of the leaf-tooth arises.

or those 01 the leaf-teeth, which they

meet, as shown in Fig. 281,^/, at an obtuse angle, and coalesce to form curved
* common' bundles. The layers of cells which lie outside the ring of meristem
that gives rise to the bundles produce the cortex of the internode, in which air-
conducting canals soon arise. Even at an early period the first rudiments of the
sheath-teeth appear as protuberances at regularly distributed points, each of them
ending in one or two apical cells (Fig. 282) ^

^ On the original number and subsequent increase of the sheath-teeth, &c., compare Hofmeister
and Reess, /. c.

400

VASCULAR CR VP TOGA MS.

The Branching of the Equisetaceae depends exclusively on the formation of
lateral buds. Each of these is formed from a superficial cell lying just above the
youngest foliar girdle, and this takes place at points alternating with the sheath-teeth,
long before the differentiation of the fibro-vascular bundles. [It was thought
that these buds were of endogenous origin \ but the researches of Janczewski and
of Famintzin have shown that this is not the case^] Hofmeister was the first to
show that each bud proceeds from a single cell of the tissue ; and although I have
myself never seen it in a unicellular condition, I have found rudimentary branches
composed of only two or four cells; and these showed that even the first three
divisions of the mother-cell of the branch are inclined in three directions in such

Fig. 282. — External view of three teeth of a young
leaf-sheath of Equisetwni Telntateia.

Fig. 281.— The same as Fig. 280, but at a. greater distance from the apex, showing a further advance of the differentiation of
leaf-sheath and internode ; r r cortex of the upper, r' ■/ r' cortex of the lower internode, e e the inner, e' e' the outer epidermis
of the leaf-sheath, ^^ the foliar portion of the fibro-vascular bundle,^' ^'^ its descending portion belonging to the internode ;
the first annular vessel is formed at their point of meeting.

a manner that a trilaterally pyramidal apical cell is produced; and the first three
divisions thus form the first three segments. Lateral buds of the rhizome of
E. Telmateia and E. arvense, late in the autumn or early in the spring, usually
show in longitudinal section all the stages of development of. the buds. After
they have formed several foliar girdles and their apex is covered by a firm envelope
of leaf-sheaths, they break through the base of the parent leaf-sheaths. They
may also remain dormant for a long period, as is shown by the circumstance
that buds break out when the underground nodes of ascending stems are exposed

^ See also the account of the Jungermanniese, p. 359.

^ [Janczewski. Rech. sur le dev. des bourgeons dans les Preles ; Mem, Soc. de Sci. Nat. de Cher-
bourg, XX. 1876-77.— Famintzin, Bull, de I'Acad. Imp. d. St. Petersbourg, XXII. 1876.]

EQVISETTNEJE.

401

to the light. It may be assumed that there is always as large a number of buds
in a rudimentary condition as there are sheath-teeth. On the erect leafy stems
of B. Telmateia, E. arvense, and other species, they all attain complete development,
and produce the numerous slender green leafy shoots of these species; in other
species the development of the branches is more sparing; some, as E. hiemale,
usually form no aerial lateral shoots at all except when the terminal bud of the
stem is injured, and then the node next below produces a shoot. Branches do
not usually make their appearance on rhizomes in the form of complete whorls,
but in twos or threes, but on the other
hand they are more vigorous ; they be-
come either new rhizomes or ascending
stems. Since, in the cases first mentioned,
the buds arise like the leaves in strict
acropetal succession, it may be assumed
that where the production of shoots is only
induced at a later period by accidental
circumstances, the buds have up to that
time remained dormant in the interior.

The Roofs arise in whorls, each im-
mediately below a bud ; but they may also
often be suppressed, and may be deve-
loped, according to Duval- Jouve, even on
aerial nodes, by humidity and darkness.
Their development has been studied by
Nageli and Leitgeb (/. c.) ; in its earliest
stages, which are represented diagram-
matically in Fig. 284, it resembles essen-
tially that of Ferns. The cortex is differ-
entiated into an inner and an outer layer ;
the former forms air-conducting intercel-
lular spaces, at first arranged, like the cells
themselves, in radial and concentric rows,
and afterwards combining by the rupture
of the cells into a large air-cavity sur-
rounding the central fibro-vascular cylinder.
As the fibro-vascular cylinder of the root
developes (seen in transverse section),
each of the three primary cells which

alone of the six reach the centre is first of all divided by a tangential wall, so that
the rudiment of the vascular bundle now consists of three inner and six outer cells.
The six outer cells produce a cambial tissue in which the formation of vessels
begins, commencing from two or three points of the circumference and advancing
towards the interior. Last of all one of the three inner cells forms a broad central
vessel; and phloem is produced in the circumference of the vascular cylinder. In
the other vascular Cryptogams the innermost layer of the cortical tissue forms
the bundle- sheath {Pleromscheide, Schutzscheide), the radial walls of the cells

Dd

Fig. 283. — Longitudinal section tlirougli an underground
bud oi Eqiiisetuni arvense; ss apical cell of the stem, li — gb the
leaves; K K' two buds; the horizontal lines across the stem
indicate the position of the diaphragms.

402

VASCULAR CRYPTOGAMS.

having the characteristic folding, but in the Equisetaceae it is the last layer but
one of the cortical tissue which presents this appearance, whilst the innermost
layer which directly abuts upon the axial cylinder seems to supply the place of
the pericambium which does not exist in the roots of these plants. This innermost
layer differs from the pericambium of the roots of other Vascular Cryptogams in
that the lateral roots take origin from it, so that here also, as in all other Vascular
Cryptogams, the lateral roots arise from the innermost layer of the cortical tissue.
As a pericambium is wanting here, the commencing roots arise in immediate
proximity to the external vessels of the axial cylinder. The cells, each one of
which gives rise to a lateral root, are formed in strictly acropetal succession in
the innermost cortical layer on the outer side of the primary xylem-vessels ^

The Sporangia of Equisetaceae are outgrowths of peculiarly metamorphosed
leaves which are generally formed in numerous whorls at the summit of ordinary

<-' .3z / r t

Fig. 284. — Diagram of the succession of cell-divisions in the apex of the root of Equisetum hiemale (after Nageli and
Leitgeb), (this diagram will serve also in the main for Ferns and for Marsih'a). A longitudinal section ; B transverse section at
the lower end oi A ; h h h the primary walls, s s s the sextant walls of the segments, indicated in A by the figs. I— XVI,
k I m n p the layers of the root-cap, all the further divisions being omitted ; c c in the interior of the root indicates the walls by
which the rudimentary fibro-vascular cylinder (procambium) is divided from the cortex of the root, e the boundary-wall between
the epidermis o and the cortex (epidermal wall), r r boundary-wall between the outer and inner cortex (cortical wall), i, 2, 3, the
successive tangential walls by which the inner cortex is divided into several layers, the radial divisions being omitted.

shoots or of those specially destined for this purpose. Above the last sterile leaf-
sheath of the fertile axis an imperfectly developed leaf-sheath is first of all produced
(Fig. 285,^), a structure corresponding in some degree to the bracts of Phanerogams.
The development of this structure is sometimes more, sometimes less leaf-like ; foliar
girdles are formed above it in acropetal succession beneath the growing end of
the shoot, projecting however but slightly, as in the ordinary formation of leaves
of Equisetum. A large number of protuberances project from each of these girdles,
corresponding to the teeth of the ordinary leaf-sheaths ; and thus several whorls of
hemispherical projections are formed lying closely one over another, which, in-
creasing more rapidly in size at their outer part, press against one another, and
thus become hexagonal, the successive whorls alternating; while the basal (inner)

^ [See Van Tieghem, La Racine, Paris 187 1.]

EQUISETINEM.

403

portion of each protuberance remains slender, and forms the pedicel of the hex-
agonal peltate scale. The outer surface of these scales is tangential to the axis
of the spike ; on its inner side, facing the axis, arise the sporangia, five or ten in
number on each scale. In the early stages of development each single sporangium
has the appearance of a small blunt multicellular wart; [an axial row of the cells
grows more vigorously than the rest, and it is the terminal hypodermal cell of
this row which constitutes the archesporium.
The epidermal layer divides by walls parallel to
the surface, so that four layers of cells invest the
archesporium, of which the two outer form the
wall of the sporangium, and the two inner the
tapetum]. By repeated divisions the archesporial
cell produces the spore-mother-cells which be-
come isolated, while of the exterior cell-layers
which at first envelope them only the outermost
finally remains as the wall of the sporangium.
The mother-cells of the spores, connected to-
gether in groups of fours or eights, float freely
in a fluid which fills the sporangium and is inter-
spersed with granules. The processes that take
place in the mother-cells up to the time of the
formation of the spores have already been de-
scribed in detail in Chap. I (see Fig. 10, p. 14)^
It was there shown how the division into four
of the mother-cells is preceded by an indication
of a division into two, in a manner analogous
to the corresponding process ili Ferns. The
ripe sporangium opens by a longitudinal slit
on the side which faces the pedicel of the peltate
scale. The very thin-walled cells of the wall
have previously formed spiral thickening ridges
on the dorsal, annular ones on the ventral side
of the sporangium, arising, according to Duval-
Jouve, in the case of E. hmosum, with extra-
ordinary rapidity immediately before the de-
hiscence. The development of the spores of

Equisetum, after they have made their appearance as naked primordial cells by
the division into four of their mother-cells, shows the peculiarity of a successive
formation of distinct coats. Each spore forms first of all an outer non-cuticularised
coat capable of swelling, which, splitting subsequently into two spiral bands, forms
the so-called elalers, 3. second and third coat soon afterwards making their appear-
ance within it. All three lie at first closely one upon another like successive layers
of a single coat; but when the spore is placed in water, the outer one, even at
this period, swells up strongly and becomes detached from the others (Fig. 286,

Fig. 285.— E^w'setum Te/ma/em; A upper part of
a fertile stem with the lower half of the spike (natural
size), b leaf-sheath, a the annular 'bract,':*; the pedi-
cels of peltate scales which have been cut off, y
transverse section of the rachis of the spike ; B
peltate scales in various positions (slightly magni-
fied) ; St the pedicel, s the peltate scale, sg the spo-
rangia.

[On the development of the spores see Strasburger, Zellbildung und Zelltheilung, 3rd ed., p. 155,]

D d 2

404

VA SC ULAR CRYPTOGA MS .

B). The three coats may be easily distinguished even in the quite fresh spore
when placed in distilled water (A), (in the case of J^. Umosiini), the outer one
(i) being colourless, the second (2) light blue, and the third (3) yellowish. As
the development advances, the outer coat is separated like a loose investment from
the body of the spore (C, d, e), and at the same time its division into elaters is
first indicated. The optical longitudinal section shows that the spiral thickening-
bands of this coat are separated only by very narrow spaces of thin membrane (Z),
E)\ these at length entirely disappear, and, when the surrounding air is dry, the
thicker parts separate from one another as spiral bands, forming when unrolled
a four-armed cross ; they are united by their centre, and attached there to the
second coat. It is probably this spot which may be recognised even in the
unripe spore in the form of an umbilical thickening {n in A and B). In the
fully developed elaters an external very thin cuticularised layer may be distin-
guished. They are extremely hygroscopic ; when the air is damp they are rolled
round the spore, but when dry are again unrolled. When this alternation takes

Fig. 286.— Development of the spores of Jiqiiisetttm limosum (x8oo) ; A unripe spore with three coats just placed
in water; S the same after two or three minutes in water, the outer coat having become separated, a large vacuole
IS seen by the side of the nucleus ; C commencement of the formation of the elaters on the outer coat <> (=i in Figs.
A and B); D, E the same stage of development in optical section after lying twelve hours in glycerine, e the outer coat ;
2, 3, the inner coats separated from one another ; F the outer coat split into spiral elaters, coloured a beautiful blue by
Schultz's solution.

place rapidly (as when lightly breathed on under the microscope), the spores are set
in active motion by the bendings of the elaters. If spores, the outer coat of which
has not yet become split up into elaters. but which already show the corresponding
differentiations {D, E), are allowed to lie for some time in glycerine, the spore
contracts considerably, surrounded by its inner coat, while the second cuticu-
larised coat raises itself from the former in folds. The inner coat is differentiated into
a granular cuticularised exospore, and an endospore of cellulose.

Very little need be said about the Classification of Equisetaceae, as all existing forms
are so nearly related to one another that they may be included in a single genus, Equi-
setum. Even the Equisetaceae of earlier geological periods, the Calomites^ show, in the
little that is still discernible of their organisation, the closest agreement with existing
forms.

The Hahit of the Equisetaceae is, like their morphological structure, of a very
characteristic kind. In all the plant is perennial by means of creeping underground
rhizomes, from which ascending aerial shoots rise annually, mostly lasting only for one

EQUISETINEm,

405

period of vegetation, less often for several years. The sporangiferous spikes appear
either at the summit of these axes, which are at the same time the organs of assimi-
lation, or on special fertile shoots which, when destitute of chlorophyll and unbranched,
die after the dissemination of the spores [E. awense and Telmateid), or throw off
the terminal spike and act as vegetative shoots (.£. syl'vaticum and pratense). The
fertile axes are developed from the underground internodes of the erect vegetative
axes ; they remain during the summer, in which the latter are unfolded, in the bud-
condition beneath the ground, but during this period either develope their sporangi-
ferous spikes so far that in the next spring nothing is necessary except elongation and
the dissemination of the spores {E. ar'vense, pratense, Telmateia, &c.), or the spikes attain
their full development only in the spring after the elongation of the axes which bear
them {E. limosum). The habit of the aerial shoots is determined especially by the
number and length of the verticillate usually very slender lateral branches ; in some,
as E. hiemale, trachyodon, ramosissimum, and variegatum, they are generally entirely
wanting ; in others, as E. palustre and limosum^ they are few ; in others again, as E. ar-
'vense, Telmateia, and syl'vaticum, they are developed in large numbers. The height
of the leafy stem is in our native species mostly from i to 3 feet; in E. Udmateiay
where the ascending axis of the sterile shoots is colourless and destitute of chloro-
phyll, it attains a height of 4 or 5 feet and a thickness of about ^ inch ; while the green
slender leafy branches are even in this case scarcely § line thick. The tallest stems
are produced by E. giganteum in South America, as much as 26 feet high, but only about
the thickness of the thumb, and are kept in an upright position by neighbouring plants.
The Calam'ttes were as lofty, and as much as i foot thick. The rhizomes mostly creep
at a depth of from 2 to 4 feet beneath the ground, and extend over areas 10 to 50 feet
in diameter; but are also found at a much greater depth. They prefer damp, gravelly,
or loamy soil, their thickness varying from i to 2 lines to as much as ^ inch or more.
The surface of the internodes of the rhizome is, in some species, as E. Telmateia and
syl'vaticum, covered with a felt of brown root-hairs, which also clothe the leaf-sheaths of
the underground part of ascending stems, a peculiarity which reminds one of Ferns.
In some species, as E. limosum and palustre, the surface is smooth and shining, while in
others it is dull. The ridges and furrows of the aerial stems are usually but little de-
veloped on the underground stems ; sometimes the rhizomes are twisted. The central
canal of the internodes is sometimes wanting in the rhizomes ; but the lacunae of the
fibro-vascular bundles (carinal canals) and those of the cortical parenchyma (vallecular
canals) are always present ; the air which the tissues require and which is not found
in the usually very compact soil is carried by these canals from the surface to the
underground organs. As in the case of the spikes, the formation of the branches of
the leafy stems has already taken place entirely or at least to a great extent in the
preceding year in the underground bud, so that in the spring the internodes of the
ascending axis have only to extend and the slender lateral branches to unfold, as may be
seen with especial ease in E. Telmateia. All the more important cell-formations and the
processes of morphological differentiation thus take place underground ; the aerial un-
folding has for its main purpose only the dispersion of the spores and assimilation by the
leafy shoots, by the exposure of the cortex, which contains chlorophyll, to light. The
rapid growth of the upright stems in the spring is especially brought about by the simple
elongation of the internodal cells already formed, although permanent intercalary growth
of the internodes sometimes also takes place, and especially at their base within the
sheaths. The tissues often remain there for a long time in the young state, and in E.
hiemale the shorter internodes grow out of their leaf-sheaths after passing through the
winter, and they are then lighter in colour ; the shorter they were before the winter,
the more they elongate afterwards.

Special Organs for Fegetati-ve Propagation, like those of Mosses, are not found in the
Equisetaceai any more than in Ferns ; but every part of the rhizome, and the under-
ground nodes of ascending stems, are adapted for the production of new stems. ^ In

406 VASCULAR CRYPTOGAMS.

some species some of the underground shoots swell up into ovoid {E. ar-vense) or pear-
shaped {E. Telmateia) tubers about the size of a hazel-nut ; Duval- Jouve states that
these occur also in E. palustre, jyi'vatlcum, and littoraley but in other species {E. pratense,
limosum, romosissimum, 'variegatum, and hiemale) they have not yet been observed. The
tubers are produced by the rapid increase in thickness of an internode at the end of
which is situated the terminal bud ; this may repeatedly form tuberous internodes so that
the tubers become moniliform, or they may develope simply as a rhizome, or sometimes
a central internode of a rhizome is developed in a tuberous manner. The parenchyma
of these tubers is filled with starch and other food-materials ; they may apparently long
remain dormant and form new stems under favourable circumstances.

Among the Forms of Tissue of the Equisetaceae ^ the epidermal system and the funda-
mental tissue are in particular developed in a great variety of ways. The fibro-vascular
bundles, which in Ferns are so thick and so highly organised, especially in their xylem-
portion, appear to be less developed in the Equisetaceae ; they are slender, the lignification
of the xylem-portion very slight (as in many water and marsh plants) ; the firmness of
their structure is chiefly due to the epidermal system with its highly developed epidermis,
and to the hypodermal fibres. What follows has special reference to the internodes ;
the leaf-sheaths are usually similarly constituted in their lower and central parts ; at the
teeth the tissue is simpler and somewhat different.

The Epidermal Cells are mostly elongated in the direction of the axis, and are
arranged in longitudinal rows separated by transverse or slightly oblique walls; the
boundary-walls of the adjoining cells are often undulating. The epidermis of the
underground internodes is almost always destitute of stomata, and consists of cells with
either thick or thin walls, usually brown, which, in some species, as E. Telmateia and
arvense, develope into delicate root-hairs. The epidermis of the deciduous sporangi-
ferous stems of the species just named is similar to that of the rhizome and without
stomata; and the same is the case with the upright colourless sterile stem of E. Tel-
mateia. On all the aerial internodes which contain chlorophyll, on the leaf-sheaths and
on the outer surface of the peltate scales, the epidermis possesses numerous stomata
which always lie in the channels, never on the ridges, and are arranged in longitudinal
rows either single or lying close to one another. On the ridges the epidermal cells are
long, in the channels between the stomata shorter. All the cells, even those of the
stomata, have their outer walls strongly silicified, and exhibit very often on their outer
surface protuberances of various forms, which are also and indeed peculiarly strongly
silicified. These protuberances resemble fine granules, bosses, rosettes, rings, transverse
bands, teeth, and spines; on the guard-cells they usually occur in the form of ridges,
running at right angles to the orifice. The guard-cells are generally partially covered
by the neighbouring epidermal cells. The mature stoma appears to be formed of two
pairs of guard-cells lying one over another; Strasburger asserts that these four cells
arise from one epidermal cell, and lie at first side by side at the same level. Only
at a later period the two inner ones (the true guard-cells) become pressed inwards and
overreached by the two outer ones which grow more rapidly. Bundles or layers of firm
thick-walled cells (Hypodermal Tissue) are of common occurrence beneath the epi-
dermis of rhizomes, of upright stems, and of their leafy shoots (with the exception
of the deciduous sporangiferous stems). In the rhizomes they form a continuous
stratum of brown-walled sclerenchyma consisting of several layers ; in the aerial inter-
nodes they are colourless and are developed with especial prominence in the projecting
ridges.

The Fundamental Tissue of the internodes consists in the main of a colourless thin-
walled parenchyma occurring only in the rhizomes, the deciduous sporangiferous stems,
and the colourless sterile axes of E. Telmateia. The green colouring of the other shoots

^ [For further details on this subject, see De Bary, Vergleichende Anatomic der Vegetations-
organe der Phanerogamen und Fame, 1877.]

EQUISETINEJE. ^oj

is caused by layers consisting of from i to 3 strata of parenchyma containing chlorophyll
(the cells lying transversely). This green tissue lies especially beneath the furrows, cor-
responding to the stomata, and appears in a transverse section as ribbon-shaped masses
concave outwardly; in the slender leafy branches, where the ridges sometimes cause
the transverse section to have a stellate outline (e.g. E. ar-vense), the tissue containing
chlorophyll is in excess. The vallecular canals, which correspond to the furrows, arise
in the fundamental tissue by separation and partially by rupture of the cells; they may-
be absent from the slender leafy branches.

The Fibro-'vascular Bundles are arranged, in a transverse section of the internode,
as in Dicotyledons, in a circle, each corresponding to a ridge of the surface, between
the cortical canals but somewhat nearer the centre. In the axis of the sporangiferous
stems, where the diaphragms are wanting, they run in the same manner, and bend out
singly into the pedicels of the peltate scales (as into the sheath-teeth). The bundles of
a shoot are all parallel to one another; each bundle is the result of the coalescence
of two portions ; one of these belongs to the leaf-sheath and developes in the median
line of one of its teeth from below upwards ; the other portion developes in the internode
itself from above downwards. At the angle where the two portions meet, the forma-
tion of tissue begins in both, and thence advances in opposite directions; the lower
end of each bundle unites by two lateral commissures with the two next alternate
bundles of the next lower internode (Fig. 278); the Equisetaceae have therefore only
* common ' bundles. In transverse sections these bundles resemble those of Mono-
cotyledons, especially of Grasses ; the first-formed annular, spiral, or reticulated vessels
belonging to the inner side, together with the thin-walled cells which separate them,
are subsequently destroyed, and a canal (carinal) remains in their place traversing the
whole length of the fibro-vascular bundle on its inner side. Right and left of this
lie on the outside a few not very broad vessels thickened reticulately ; external to
the canal lies the phloem-part of the bundle, formed of a few wide sieve-tubes
and narrow cambiform cells, and at the circumference of a few thick-walled narrow
bast-like cells. A bundle-sheath, as it is termed, sometimes surrounds each bundle
{E. limosum), but generally runs continuously outside the circle of all the bundles, as in
most Phanerogams.

[Professor W. C. Williamson is led by a study of the internal organisation of CalaniHes and
^alatnodendra^ to the conclusion that in England at least we have but one group of these fossil
'plants. "When young their vascular zone, separating a medullary from a cortical parenchyma,
was scarcely more than a thin ring of longitudinal canals, each of which had a few vessels at its
outer border. In this state the structure of the plant presented a close resemblance to that of a
recent Equisetum. But as the plant grew in size, new vessels were added to the exterior of the pre-
existing bundles, so that each of the latter became the starting point of a woody wedge which con-
tinued to grow peripherally until it assumed large dimensions. In some specimens these wedges
measure fully two inches between the canal marking their medullary angle and their peripheral or
cortical base. Each wedge is composed of vertical radiating laminge of barred or reticulated vessels
separated by cellular rays. The medullary portion became fistular, as in the recent Equisetacese,
at an early age, and when the fistular cavities became filled with sand or mud, the very thin layer
of medullary cells which remained did not prevent the sand from moulding. itself against the inner
angles of the vertical woody wedges, which thus produced the longitudinal grooves so characteristic
of the casts commonly seen in collections. In such specimens most of the vegetable elements dis-
appeared during fossilisation, and what remained, in the shape of a thin film of coal, moulded itself
upon the medullar)^, cast, and gave to the specimens the appearance of having had corresponding

* [On Fossil Equisetacees, see Williamson, Mem. Lit. Phil. Soc. Manch, 3rd ser, vol, IV.
pp. 155-183; Ditto, Trans. Roy. Soc. vol. CLXI. pp. 477-510: also vol. CLXIX. Part 2, 1878.—
Coemans, Journ. Bot. 1869, pp. 337-340. — Dawson, Ann. Nat. Hist. 4th ser. vol. IV. pp. 272-273. — •
Grand'Eury, Ann. Nat. Hist, 4th ser. vol. IV. pp. 124 128 ; Compt. Rend. vol. LXVIIL— M^Nab,
Journ. of Bot. 1873, pp. 72-80.]

408 VASCULAR CRVPTOGAMS.

grooves upon tlieir outer bark surfaces. No single example of a specimen of which the internal
organisation is preserved — and we now possess these in great numbers — sustains this latter con-
clusion. Wherever the true bark is preserved it exhibits an outline indicating a smooth surface,
longitudinal internodal flutings and transverse nodal constrictions being alike absent.

In its young state the bnrk consisted of undifferentiated cellular tissue, and we now have
evidence that a hypodermal layer of prosenchyma was developed in it which ultimately attained to
an enormous thickness.

The woody wedges extended vertically through each internode without interruption, but at the
node each wedge split into two halves, each half coalescing with the contiguous half of the wedge
nearest to it to form one of the wedges of the next internode. At each node in young stems
numerous small cellulo-vascular bundles passed outwards through the vascular zone to supply some
peripheral organs ; probably verticillate leaves and branches. But most of the latter became
abortive; the branches of the larger stems being few, and unsymmetrically disposed. Stems and
branches were thickened simultaneously as in ordinary exogenous trees. Besides these diver-
ticula, in most of the Catamites, immediately below each node, there passed outwards, through
the cells of the large primary medullary rays, so conspicuous in all young shoots, and near the
medullary surface of matured vascular axes, an exceedingly regular verticil of primary canals, with
circular or oblong sections, from the central fislular cavity through the woody zone to the bark.
One of these canals occupied the uppermost end of each of the large cellular rays which separated
the vascular wedges of each internode. In the common fossilised casts these canals are indicated
by a very regular verticil of small round or oblong impressions, which some writers have erro-
neously associated with roots, and others with vascular bundles going to leaves or branches. But
they never contained any vascular tissues whatever. Of the leaves of Calamites we have but
little knowledge, although some have identified them with those of Asterophyllites and Spheno-
phyllum.

Professor Williamson has only obtained one example of a fruit which he can with confidence
identify with Calamites (On a new form of Calamitean Strobilus, Williamson, in Mem. Lit. Phil. Soc.
Manch. 3rd ser. IV. p. 248). It is a strobilus the structure of the axis of which corresponds most
closely with that of a young Calamitean shoot. At each node it has a curiously perforated
transverse disk fringed with numerous peripheral bracts. From the upper surface of each disk
there projects vertically upwards a ring of slender sporangiophores, around each of which were
clustered three or four sporangia full of spores. These sporangia are so compactly compressed
that a transverse section of this fruit presents the appearance of a compact mass of spores, amongst
which the outlines of the sporangia are traceable with difficulty. Whilst he has failed to find any
true stem in which the outer surface of the bark was fluted, that of the internodes of this fruit was
undoubtedly so. The flutings of the fruit-bark do not, like those seen in the carbonaceous film
covering the common casts, correspond in number and position with those caused by the woody
wedges, since two vascular bundles are located in each projecting ridge of the axis of the former
structure, instead of one as in the latter.

Mr. Carruthers believes the fruits figured by Mr. Binney, Professor Schimper, and himself,
under the several names of Calamodendron commune, Calamostachys Binney ana, 'OJid Volkmantiia
Binneyi (Journ. of Bot. 1867, pp. 349-356), to belong to Calamites; and he further regards the
spores as having been furnished with elaters similar to those of Equisetum. Professor Williamson is
unable to agree with either of these conclusions. The numerous specimens which his cabinet now
contains make it absolutely certain that the supposed elaters are merely fragments of the torn
mother-cells of the spores, and it is his impression that these fruits, now known to be heterosporous,
have closer affinities with the Lycopodiaceie than with the Equisetaceee ; they certainly are not the
Strobili of Calamites.']

FILICINEM. • 409

CLASS VIII.
FILICINE^.

The plants included in the group of the Filicineae are distinguished from the

Squisetacese and from the Dichotomeae by the various and complete development

^hich their leaves attain. In proportion to the stem, the leaves are always of con-

Iderable size, and in their anatomical structure, as well as in their external form,

ley manifest a higher differentiation than do those of the two groups above

lentioned. In these two groups the whole external form of the plant depends upon

le formation and branching of the stem, and the most important physiological

mctions are performed by this organ; in the Filicineae the stem is essentially an

jrgan for bearing leaves and roots ; its growth is slow, frequently its development is

imperfect that no internodes are formed, whereas the leaves are endowed with an

:tive apical growth which continues for a considerable time and in some cases is

mlimited. Further, the stem, in the Filicineae, has but little tendency to branch ; in

?hole families it remains simple, and not unfrequently the formation of buds is

Provided for by the leaves in which a strong tendency to branch is manifested, and

"which present, in consequence, the most varied forms of pinnate and palmate

segmentation and of dichotomous branching. In the Equisetacese and Dichotomese

it generally happens that the stem takes part in the formation of the fructification ;

in the Equisetaceae it is always and in the Dichotomeae it is usually an apical

spike which is terminal upon the branch bearing it. In the Filicineae this is never

the case ; the function of reproduction is discharged solely by the leaves, the stem

taking no part whatever in it. The leaves bear very numerous sporangia (the

number varying with the size of the leaf), whereas the peltate scales of the Equise-

taceae bear but a few, and the fertile leaves of the Dichotomeae only one. The mode

of development of the sporangia upon the leaves of the Filicineae is not uniform ;

in the Stipulatae each sporangium arises from a group of epidermal cells, in the

Filices and Rhizocarpeae from a single epidermal cell \

It appears, moreover, that the Rhizocarpeae, which were formerly separated
from the Ferns on account of the mode in which their fructification is developed,
present (more especially the Salviniaceae) a sufficient number of resemblances to the
true Ferns to justify us in regarding them as a branch of the ancestral tree from
which the Hymenophyllaceae and Polypodiaceae have sprung.

It is not easy to give a brief account of the relationships existing between the
groups contained within this class, for a whole series of Ferns, the Osmundaceae,
Schizaeaceae, and Gleicheniaceae have been as yet but imperfectly investigated from a
morphological stand-point. Our present knowledge (1874) of these plants is very
superficial, it suffices merely for the diagnoses of systematists. For those who
can obtain the necessary material and who possess the requisite morphological

' [See Goebel, loc. citJ]

4IO VASCULAR CRVPTOGAMS.

education a field for work is here open. We must content ourselves now with briefly
stating all that is actually known, and with merely mentioning the less known forms
when occasion demands.

Order I. Stipulat^e.

Under this name, which is based upon the peculiar formation of stipules common
to the members of the two groups, I include the Ophioglosseae and the Marat-
tiaceae, for the relationship existing between them is manifested in several important
particulars. In both groups the prothallia are moncecious and capable of inde-
pendent growth ; but there is a tendency to dioecism in the Marattiaceae. That the
prothallia of the Ophioglosseae are subterranean, whereas those of the Marattiaceae
are not so, is a difference of physiological and not of morphological importance. The
stem of the second generation in both families is characterised by its very slight
growth in length, the usual absence of internodes and of any branching, the whole of
its surface being occupied by the insertions of the leaves, and by the development of
roots acropetally immediately behind its apex. The absence or rudimentary deve-
lopment of bundle-sheaths and of brown schlerenchyma in the ground-tissue of the
stem and of the leaves distinguishes these plants from the true Ferns. The Ophio-
glosseae diverge most widely from the Ferns in that their sporangia are imbedded in
the tissue of the leaf. The Marattiaceae present an intermediate condition in that
their sporangia are quite external to the leaf and are attached by a narrow base.

Although it is not improbable, it is still an open question whether or not the
Osmundaceae are nearly related to these two families. Their petioles bear at their
bases lateral membranous wing-like appendages which may fairly be termed stipules,
but which are certainly very different from those of the Marattiaceae and Ophio-
glosseae. Further, the stem of the Osmundaceae, which is thickly covered with roots,
is not erect like that of the other two groups, and it is uncertain whether the
numerous lateral branches arise from it or from the petioles of the leaves. The
fructification seems to indicate a relationship, for it recalls the paniculate fructifi-
cation of the Botrychiae, but the fertile segments of the leaves have no mesophyll.
In this respect the Schizaeaceae resemble the Osmundaceae, but in other features,
more especially in the want of stipules, they differ from the Stipulatae.

Although the connection of the Ophioglosseae with the Marattiaceae is tolerably
evident, it may be advisable to give a separate account of each family.

Family I. OphioglossesB^ The Sexual Generation (Oophore). The pro-
thallium is at present known only in Ophioglossum pedunculosum and Botrychiu7n
Lunaria, In both cases it is developed underground. It is destitute of chloro-
phyll, and foniis a parenchymatous mass of tissue which, according to Mettenius,
has at first, in the species first-named, the form of a small round tuber, out

* Mettenius, Filices horti botanici Lipsiensis. Leipzig 1856, p. 119. — Hofmeister, Abhandlungen
der konigl. Sachs. Gesellsch. der Wissens. 1857, p. 657. — [On the Germination, Development and
Fructification of the Higher Cryptogams, Ray Soc. 1862, pp. 307-317.] — Russow, Vergleich. Unters.
St. Petersburg, 1872, pp. 117 ff. - [HoUe, Ueb. Bau und EntwickeUing der Vegetationsorgane der
Ophioglossen, Bot. Zeitg. 1875.]

FILICINEM.

411

of which is subsequently developed a cylindrical vermiform shoot, which grows
erect underground, is rarely and slightly branched, and elongates by means of
a single apical cell. When the apex appears above ground and becomes green,
it forms lobes and ceases to grow. The tissue of this prothallium is differen-
tiated into an axial bundle of elongated, and a cortex of shorter parenchymatous
cells, and the surface is clothed with root-hairs. With a transverse diameter
of \ to i\ Hues, it attains a length of from 2 lines to 2 inches. The pro-
thallium of Botrychium Lunaria is, according to Hofmeister, an ovoid mass of
firm cellular tissue, the greatest diameter of which does not exceed \ line, and
is often much less (Fig. 287, A\ It is light brown externally, yellowish white
internally, and provided on all sides with sparse moderately long root-hairs.
These prothallia are monoecious; each one produces a number of antheridia and
archegonia, which are distributed with tolerable uniformity over the whole of its
upper surface, with the exception, in O. pedunculosum, of the small primary tuber ;
in Botrychium it is the upper side which chiefly bears antheridia.

The Antheridia are cavities in the tissue of the prothallium covered externally
by a few layers of cells, and in Ophioglossum only slightly projecting beyond the

Fig. iZ-j.—Roirychittm Lunaria ; A longitudinal section of protballium (X 5°). c-c an archegonium, an an antheri-
diuui, w root hairs; B longitudinal section of the lower part of a young plant dug up in September (Xao) ; st stem,
b b' b'' leaves (after Hofmeister).

surface. In this genus the mother-cells of the antherozoids originate by repeated
divisions from one or two cells of the inner tissue (covered externally by one or
two layers of cells) ; they form a mass of tissue of roundish form, and, as in
Botrychium, give rise to the antherozoids, which are similar in form to those of the
Polypodiacege, but larger ; they escape through a narrow opening in the cover of the
antheridium.

The Archegonia are apparently developed in a similar manner to those of other
Vascular Cryptogams. Mettenius saw in Ophioglossum instances in which they
consisted of two cells, a superficial cell and one lying below it; this latter, he
considered, became the central cell, the former producing the neck of the arche-
gonium by dividing into four cells arranged crosswise, which then produce, by
further divisions, four vertical rows each consisting of two or more cells, and thus
form the neck. The wall of the ventral part which surrounds the central cell
is formed by divisions of the cells of the prothallium which surround it; the
ventral part is therefore completely imbedded, and only the neck, which is usually
very short, projects above the surface. Mettenius asserts that in Ophioglossum a

412

VASCUI^AR CRYPTOGAMS.

prolongation of the oosphere (probably a canal-cell, as in Ferns and Rhizocarps),
penetrates into the lower part of the neck.

The Asexual Generation (Sporophore). The first divisions of the oospore are not
known ; but the mode of formation of the embryo differs, as may be concluded from
more advanced stages, from that of Ferns. Mettenius states that in Ophioglosswn
pedunculosum the end of the embryo which faces the apex of the prothallium
developes into the first leaf, while the opposite end produces the first root. Unlike
what occurs in Ferns, the concave upper side of the first leaf faces the neck of

FIG. 288. — A Ophioglosstivi vulgaturn ; B Botrychiwn Lunaria (both natural size) ; iu roots, st stem, bs leaf-stalk,
X point where the leaf branches, the sterile lamina b separating from the fertile branchy:

the archegonium ; the rudiment of the stem (which IVIettenius terms the * primary
rudiment of the embryo') lies nevertheless on the side of the embryo which faces the
base of the archegonium. Hofmeister, on the other hand, makes the following state-
ment with regard to Botrychium : — ' The position of the embryo with respect to the
prothallium differs widely from that which occurs in the Polypodiacese and Rhizo-
carpese ; Botrychium approaches in this respect those Vascular Cryptogams the
prothallium of which, like that of Ophioglossacese, is destitute of chlorophyll [Isoetes,
Selaginella). The punctum vegetationis of the embryo lies near the apical point of

FILICINEM.

413

the central cell of the archegonium ; the first roots arise beneath it, near the base of
the archegonium' (/. c. p. 308).

The processes of growth of the mature plant have not yet been ascertained with
as much certainty as in other Vascular Cryptogams. In Ophioglossum vulgatum and
Botrychium Lunaria the erect stem, buried deep in the earth and growing very
slowly in length, branches but rarely. Even the comparatively thick roots rarely
branch, and it is not known whether the branching is then monopodial or dicho-
tomous. [According to Holle, the roots have a
trilaterally pyramidal apical cell. They do not
branch in Ophioglossum^ and in Botrychium their
branches are probably produced laterally and not
by dichotomy.] The flattened apex of the stem,
surrounded by the insertions of the leaves, is buried
deeply in the leaf-sheaths, and shows, in Ophioglos-
sum vulgatum, according to Hofmeister, a three-
sided pyramidal apical cell as seen from above.
The leaves have a sheathing base, and each is
completely enclosed in the next older one, as shown
in Fig. 289 in the case of Botrychium Lunaria. In
Ophioglossum the relative positions of the parts at
the end of the stem are still more complicated, from
the fact that the rudimentary leaves, while com-
pletely enclosed one within another, produce stipular
structures which grow together so completely that
each leaf appears as if enclosed in a kind of
chamber formed by the cohesion of the stipular
parts of leaves of different ages, recalling a similar
arrangement in Marattia. These cohesions how-
ever leave an opening at the apex of each chamber ;
the apex of the stem is therefore exposed to the
air through a narrow canal (Hofmeister).

As soon as the plant has attained a certain
age, each leaf bears a fructification, which forms a
branch springing from the axial side of the leaf.
In the genus Ophioglossum both the outer sterile
and the fertile branch of the leaf are unbranched
or only lobed ; in the Brasilian O. palmatum the
lamina is dichotomously lobed, and its margin bears
on each side, as it joins the petiole, numerous fertile
lobes or spikes of sporangia. In the genus Botry-
chium both are branched and in parallel planes

(Fig. 288, A and B). The earlier hypothesis of a cohesion of the two leaf-stalks af
a fertile and of a sterile leaf is at once negatived by the history of development
(Fig. 289); the history of development rather indicates, as Hofmeister first showed,
that the fructification originates on the inner side of the leaf.. In the mature state
the fertile leaf-branch either separates from the sterile (green) one at the base or at

FIG. 289.— Longitudinal section through the
lower part of a mature plant oi Botrychium I.tt-
naria. st stem, ^^ fibro^vascular bundles, -w a
young root, b apex of tlie stem, b b' b" b'" the
four leaves already formed, b'" the one unfolded
during the present year : b' shows the first indi-
cation of the branching of the leaf; in b" this has
advanced further ; tn is the median line of the
sterile lamina, having already its lobes right and
left which are not shown ;ythe fertile lamina with
the young branches, on which the sporangia will
be produced (X about lo).

414

VASCULAR CRYPTOGAMS.

the middle of the lamina {0. pcnduluni), or the two branches of the leaf appear as if
separated deep down to their origin {0. Bergianum)^ or, finally, the fertile branch
springs from the middle of the leaf-stalk {Botrychium rutafolium and dissecium).

The Sporangia of the Ophioglossaceae are so essentially different from those of
Ferns and Rhizocarps that these plants cannot, for this reason, be arranged in
either of these classes. They arise from several epidermal cells. The wall of the
sporangium consists of several layers of cells, its outer limit being formed by the
epidermis of the leaf itself. The mother-cells of the spores in Botrychium Lunaria
and probably also in Ophioglossum are derived, according
to Goebel {loc. cii), from a single central cell (archesporium)
which is invested by peculiar cells (forming the tapetum)
developed by the division of the surrounding cells of the
sporangium. A longitudinal section through the unripe
so-called spike of O. vulgatum (Fig. 290) shows that the
outer layer of the wall of the sporangium is a continuous
prolongation of the epidermis provided with stomata and
covering the whole of the fertile branch of the leaf. At
the places where the lateral transverse line of dehiscence
subsequently appears in each sporangium, these epidermal
cells are elongated radially, and the whole layer exhibits
an indentation at first scarcely perceptible. The spherical
cavities which contain the masses of spores are imbedded
in the tissue of the organ, and are therefore entirely sur-
rounded by its parenchyma, there being several layers of
it on the outer side where the transverse fissure subsequently
arises. The middle part of the mesophyll is penetrated by
fibro-vascular bundles which anastomose with one another
into long meshes, and send out a bundle transversely between
each pair of sporangial cavities. The course of development
is the same in Bot7ychium, if the separate sporangiferous
branches of the panicle are compared with the spike of
Ophioglossum. The sporangia are similarly placed on them
in two rows and alternate ; only they project further because
the tissue between each pair of sporangia is but slightly
developed. Four spores are formed from each mother-cell.
The mother-cell, after an indication of a division into two,
divides into four segments, each surrounded by a delicate
cell-wall. The protoplasm of each of these special mother-
cells becomes invested by a new wail, the true wall of the spore, and the primary
walls become absorbed, so that the spores become free. In specimens of both
genera preserved in spirit, the young spores, still connected together in fours, are
found imbedded in a colourless, granular, coagulated mass of jelly, which in the
living plant clearly corresponds to the fluid in which the spores of other Vascular
Cryptogams float before they are ripe. The spores are tetrahedral ; in Botry-
chium they are provided, even in a very early state, with knob-like projections on
the cuticularised exospore.

Fig. 290.— Longitudinal section
through the upper part of a spike
of Ophioglossum vulgatum ; s
its free apex, sp the sporangial
cavities, r the part where they
burst transversely; g the fibro-
vascular bundles (X about lo).

FILICINEJE.

415

Among the Forms of Tissue of the Ophioglossaceae, the prevailing one is parenchy-
matous fundamental tissue ; there is no sclerenchyma. It consists, especially in the leaf-
stalk, of long, almost cylindrical, thin-walled succulent cells with straight septa and large
intercellular spaces; in the lamina the latter are, in O. 'vulgatum, very large, and the tissue
spongy. In O. 'vulgatum and B. Lunaria, the epidermal tissue nowhere possesses special
hypodermal layers; a well developed epidermis with numerous stomata on the upper and
under side of the leaves immediately covers the outer layers of the fundamental tissue :
at the periphery of the stem layers of cork are formed. The fibro-vascular bundles of O.
'vulgatum form, according to Hofmeister, a hollow cylindrical network in the stem, on
which the leaves are arranged spirally, with a f phyllotaxis ; each of the meshes of this
network corresponds to a leaf, and gives off to it the foliar bundles from its superior angle :
the lower end of each foliar bundle terminates in a root. The leaf-stalk is penetrated by
from 5 to 8 slender fibro-vascular bundles, which, in transverse section, are arranged in a
circle, and between which the fundamental tissue forms wide lacunae. Each of these
bundles has on its axial side a strong fascicle of narrow reticulately thickened vessels, a
broad fascicle of soft bast (phloem) lying on their peripheral side. In the sterile lamina
the slender bundles branch copiously and anastomose into a network ; they run into the
mesophyll which contains chlorophyll, without forming projecting veins. The slender
stem of B. Lunaria has the same structure as that of Ophicglossum ; its vascular bundles
appear to be only the lower ends of the foliar bundles (Fig, 289), which are arranged in a
circle in the stem and form a hollow fibro-vascular cylinder consisting of numerous
xylem-bundles surrounded by a common investment of phloem. In each leaf-stalk,
which has a conical cavity below obliterated above, arise two broad ligulate bundles,
which split above, below where the leaf divides into the fertile and sterile laminae, into
four narrower bundles. Each of these latter consists of a broad axial fascicle of tra-
cheides thickened in a scalariform or reticulated manner, which is enveloped by a thick
layer of phloem. This layer shows an inner stratum of narrow cambiform cells, while
the outside is formed of soft thick-walled bast-like prosenchyma (as in Pteris and other
Ferns). In the lobes of the sterile lamina the bundles repeatedly split dichotomously, and
run through the mesophyll without forming projecting veins.

The ground-tissue either forms no sheath round the fibro-vascular bundles of the
leaves {Ophioglossum) or it forms a sheath consisting of collenchyma {Botrychium) ; the
usual bundle-sheath of cells with sinuous walls appears to be wanting. According to
Russow, the fibro-vascular cylinder formed in the stem of Botrychium by the lower
portions of the foliar bundles is surrounded by a sheath of this kind (Plerome-sheath).
He also believes that the fibro-vascular bundles in the stem of Botrychium undergo a
slight subsequent growth in thickness. In the petiole of Ophioglossum I find, as Russow
describes, that the thin fibro-vascular bundles have collateral phloem and xylem and that
the central xylem of Botrychium is surrounded on all sides by phloem.

[According to Russow, a formation of cork takes place at the surface of the rhizome
of Ophioglosseae ; in this, so far as is known at present, they are unique among Vascular
Cryptogams.]

Habit and Mode of Life. The number of leaves which appear each year is small, and
constant in the species; thus O. 'vulgatum and B. Lunaria unfold only a single leaf
annually, B. rutcefolium two, a sterile and a fertile one ; O. pedunculosum from 2 to 4
(Mettenius). The extremely slow development of the leaves is remarkable; in B.
Lunaria each leaf requires four years, of which the three first are passed underground ;
in the second year the two branches (the sterile and fertile laminae) are formed, and
further developed in the third; in the fourth year they for the first time rise above
ground (Fig. 289), the process reminding one of the slow formation of the leaves of
Pteris aquilina ; the same occurs in O. 'vulgatum. In both genera the formation of the
sporangia begins a full year before they ripen.

Vegetati've Reproduction takes place in Ophioglossum by means of adventitious buds from
the roots. O. pedunculosum is so far monocarpous that, after the production of fertile

41 6 VASCULAR CRYPTOGAMS.

leaves, it as a rule dies down, but maintains a perennial existence by means of the root-
buds (Hofmeister). Most species are only, reckoning from the base of the stem to the
apex of the leaf, 5 or 6 inches high ; a few attain the height of a foot ; B. lanug'mosum of
the East Indies is stated by Milde to be 3 feet high; the leaf is three or four times
pinnate, and the stem contains from 10 to 17 fibre- vascular bundles.

Family 2. Marattiacese^ i.ThQ Sexual Generation {Oo\)hoTe). [The spores,
which are of two forms, reniform and nearly spherical, germinate much in the -same
way as those of the Polypodiaceae. In Angiopteris the first root-hair is developed
at an early period, but in Marattia it does not make its appearance until some
time after the commencement of germination, when the prothallium is already
multicellular. The prothallium, like that of the true Ferns, is somewhat cordate
and forms a flattened expansion upon the surface of the soil, but it is more fleshy
and it is dark green in colour. Sometimes it grows by means of an apical cell,
but this is not always formed. The antheridia are developed on either the under
or the upper surface of the prothallium, from single superficial cells. The arche-
gonia are developed in the same manner as those of the Polypodiaceae, and more
especially on the lower surface. The antheridia are not developed until some
months after germination begins, and the archegonia still later.]

2. The Asexual Generation (Sporophore) when mature resembles a Fern in
habit. The mode of its development is still unknown. It consists of a usually
erect, short, thick, tubercular stem which bears large, closely-packed, spirally ar-
ranged leaves, with long petioles, the lamina being usually pinnatifid, but sometimes
palmatifid. The resemblance to the true Ferns is rendered more striking by
the circinate vernation of the leaves, and by their gradual unrolling from below
upwards.

The Stem of Marattia^ Angiopteris^ and Dancca recalls on the whole the mode
of growth of the stem of the Ophioglosseae. It grows erect, but does not attain
any considerable height. It is a tubercular mass, partially imbedded in the earth,
and it is so completely covered with leaves that no portion of its surface is freely
exposed. In some species it is small, but in the large Marattieae and in Angiop-
teris evecta it may be from one to two feet high and broad in proportion. The
stem of Kaulfussia assamica is a subterranean, creeping, bilateral rhizome, according
to de Vriese, which bears leaves upon its upper and roots upon its under surface.
It appears that the stem of the Marattiaceae (except, according to Holle, in the
case of DancBa irifoliatd) never branches. The lower older portion of the stem
is covered by the basal parts of the older petioles, bearing the stipules, from which
the upper parts of the petioles, which at this point are provided with a large articular
swelling, have become detached, leaving a smooth cicatrix encircled by the stipule
(Fig. 291, w). At the upper part of the stem, the still living leaves form a large
rosette, in the centre of which lies a bud consisting of numerous young leaves of

^ De Vriese et Halting, Monog. des Maratt. Leide et Dusseldorf. 1853. — Liirssen, Mittheilg.
aus dem Gesammtgehiet der Bot,, Bd. I. Heft. 3. 1872; id. Bot. Zeit. 1872, p. 768, and 1873, p.
625. — Russow, Vergl, Unters. 1872, p. 105. Some information derived from drawings and letters
communicated by Prof Tschisliakoff has been embodied in the text. [Holle, Die Vegetationsorgane
der Marattiaceen, Bot. Zeit. 1876. Jonkman, Entwickelung des Prothalliums der Marattiaceen, Bot.
Zeit. 1878.]

FILICINEM.

417

different ages (^, nb). The young leaves have a circinate vernation, and are com-
pletely invested by the stipules until the time when the petiole begins to elongate
and the lamina to unroll itself. Each pair of stipules belonging to a petiole forms,
as is shown in Fig. 291, ^ and B, an anterior and a posterior chamber, which are
separated by a longitudinal wall (commissure). In the posterior chamber lies the
rolled-up leaf, to which the stipules actually belong, the two posterior wings of the
stipules extending round it. The chamber formed by the anterior wings of the
stipules encloses the group of young leaves. This is the arrangement in Angiopkris,
and it appears from herbarium specimens to obtain also in Dancea, and from

Fig. 291 B. — Basal portion of
a petiole st with the stipule cut
through obliquely ; v the anterior
and h the posterior wing; at the
junction of tlie anterior and pos-
terior wings the stipules are con-
nected by a coijimissure c (natural
size).

Fig. 291 .<4.— Vertical section of the stem of a young An^iopteris evecta: above are the youngest leaves {b) still completely
surrounded by the stipules nb ; si petiole of an unfolded leaf with its stipule nb ; n, in every case the cicatrix on the basal
portion of the petioleyy, from which the upper portions have separated ; c c the commissures of the stipules in vertical section ;
■w TV roots (natural size).

drawings in Marattia. Harting's representation of the stipules is quite erroneous.
These peculiar stipules remain fresh and succulent not only during the life of the
leaves but also after they have fallen, and adventitious buds may originate from
them.

The roots arise, as is shown in Fig. 291, A^ in the tissue of the stem
immediately below the growing-point. One arises apparently at the base of each
young leaf They grow obliquely through the succulent parenchyma of the stem
and of the older basal portions of the leaves, and finally reach the surface at a

E e

4i8

VASCULAR CRYPTOGAMS.

lower level between them or through the cicatrix of a leaf. They are not so
numerous as those of most true Ferns, and they differ from them in their light
colour, their more delicate structure and their greater thickness, peculiarities which
they share with those of Ophioglosseae. They ramify considerably in the soil,
apparently in a monopodial manner.

The leaves which in the smaller species attain a height of from one to two
feet, in the largest {Angt'opferis) of from five to ten feet, have a long firm petiole,
channelled on its inner surface, which bears the compound lamina which is either
pinnate or bi-pinnate, or palmate as in Kaul/ussia. The primary petiole is attached
to the basal portion by means of an articular swelling, and the secondary petioles
are connected with it, the leaflets with their rachis, in the same manner, just as is
the case in the Leguminosae.

The Marattiacese differ from the glabrous Ophioglosseae in that they are
hirsute, but not nearly so much so as the true Ferns.

-A under surface of the upper part of a leaflet of Angiopteris cciudata, with sori j j. B some teeth of the margin of
the leaf of Marattia sp. with sori s s. C half a sorus with opened sporangia (chambers).

The Sporangia of the Marattiacese are developed in considerable number on
the underside of ordinary leaves which have undergone no further modification.
Like those of the majority of true Ferns they are borne upon the veins of the
leaves, and are usually arranged in two rows forming sori, which either cover the
veins running from the midrib to the margin of the leaflet throughout their whole
length {Danced), or only for a short distance near the margin {^Angiopteris, Marattia) ;
in Kaulfussia they are placed upon the delicate anastomosing branches of the
veins. The sorus is borne upon a cushion-like outgrowth of the tissue of the
vein, the placenta. In Angiopteris alone are the individual sporangia free from
each other ; they are ovoid and sessile, and when mature they open by a lon-
gitudinal slit on their internal surface. If the sporangia of each row of the
sorus be imagined to have become coherent, and the two rows to have become
attached by their adjacent surfaces or to have completely coalesced, a structure
results which actually occurs in Marattia (Fig. 292, B, C). This structure might
well be regarded as a sporangium with numerous chambers arranged in two rows

FILICINEM.

419-

if the history of its development alone decided the question, and then it would
offer no analogy to the same structure in Angiopteris. But analogy clearly indicates
that in Alarattia we have not to do with a multicellular sporangium but with a
sorus, the individual sporangia of which have become united. Like those of
Angiopteris^ each of these sporangia opens by a longitudinal slit upon its inner
surface. It is of but little importance for this interpretation that the apparently
multicellular sporangium, which we regard as a coalesced sorus, is borne in
Eiipodium {Mar. Kaulfussii) on a stalk of considerable height, for the sorus in
many of the true Ferns {Cyathea, Thyrsopteris) is also stalked. It can scarcely
be doubted therefore that the multicellular fructification of Marattia is a coalesced
sorus, and the same holds good also for Kaulfussia and Daticea. In Kaulfussia
the sporangia of a sorus (from eight to twenty in number) are arranged in a circle
and are united to form a many-chambered ring. Each opens on its inner side
by a longitudinal slit. This arrangement is even more striking in Dancea, where
the united sporangia form two long rows covering the vein bearing them throughout
its whole length, and where each chamber (sporangium) opens at its apex. The
sorus is usually surrounded by flattened lobed hairs forming a kind of indusium,
which, in DancBa, appears like a kind of cup in which the sorus lies. Luerssen's
argument that these outgrowths of the epidermis are not to be regarded as an
indusium because they occur elsewhere upon the leaves and are therefore merely
hairs, is not valid, for the indusium of the true Ferns is a hair-like outgrowth, and
must be regarded as a trichome. As in the Ferns, so in the Marattiaceae, the
indusium does not occur in all species.

The development of the sori has been studied by Luerssen and by Goebel
in Marattia, and by these observers and by Tschistiakoff in Angiopteris. In both
cases the placenta arises as a cushion-like protuberance from the fertile vein of
the epidermis and the subjacent tissue. In Angiopteris two separate rows of
papillae make their appearance upon the receptacle, each of which consists from
the first of a group of cells derived from a group of the superficial cells of the
placenta. Each papilla becomes one of the free sporangia of the sorus. In
very young sporangia Tschistiakoff was able to detect an internal cell (arche-
sporium) surrounded by two or three layers of cells which gave rise by repeated
division to a group of spore-mother-cells. In Marattia two parallel swellings
appear on the placenta, which soon become separated by a deep and narrow fold.
In each of these swellings a row of cell-groups, the mother-cells of the spores,
are differentiated, which have been formed by the division of the archesporium.
Each of these groups corresponds to a sporangium, the walls of adjacent sporangia
coalescing from the first. The inner surfaces of the two parallel swellings approach
each other more and more closely as development proceeds, but they separate
widely when the spores are ripe, so that the multilocular fructification splits longi-
tudinally into two halves, and the loculi of each half open by vertical slits upon
their inner surfaces.

The development of the spores, four from each mother-cell, differs but little
from that of the Ophioglossese and the Ferns. It is important to note that in
the Marattiaceae the wall of the mature sporangium consists of several layers of
cells, whereas in the Ferns it consists only of one.

420 VASCULAR CRYPTOGAMS.

Hhtology. As a peculiarity of the epidermal tissue the very large, widely-open
stoinata of the leaves of Kaulfussia may be mentioned. They are developed in the usual
way, but they soon become remarkable on account of the extraordinary size of the
aperture and of the arrangement of the guard-cells in a narrow ring, surrounded by
two or three rings of epidermal cells (Luerssen).

In the intercellular spaces of the parenchymatous ground-tissue of the leaves Luerssen
found outgrowths from the walls of the surrounding cells. Where the spaces were small
these outgrowths assumed the form of bosses or pegs, but where the spaces were
large they were long thin filaments. They are quite solid and consist of cuticularised
cell-membrane. The large intercellular spaces are quite filled with a felt-work of these
filaments. Luerssen found this to be the case in Kaulfussia^ Danoea, Angiopteris^
Marattia.

In the ground-tissue of the leaves bands and bundles of sclerenchyma are differen-
tiated, but it is not so hard or so darkly-coloured as that of Ferns. In the articular swellings
collenchyma is developed. Elongated cells containing tannin are to be found in all parts
of the ground-tissue, and gum-ducts are scattered throughout the thin-walled paren-
chyma. Reference has been made on page 64 to the Sphaerocrystals.

In the stem of Angiopteris, which I have investigated, there is no sclerenchyma. It
consists for the most part of large thin-walled parenchymatous cells, amongst which are
scattered very numerous cells containing tannin, as well as gum-ducts. The contents of
the latter cover a piece of the stem when placed in water with a thick layer of
gelatinous mucilage.

The fibro-vascular bundles of the leaves and of the stem resemble those of the Ferns.
The central xylem consisting of wide scalariform tracheides is surrounded by a layer of
phloem. In the leaf the bundles (of Angiopteris) are usually flattened, in the stem they
have a circular outline. The usual bundle-sheath, consisting of a single layer of cells with
a peculiar folding on their adjacent walls, which is especially constant in the Ferns, is
absent in Marattia and Angiopteris from the fibro-vascular bundles both of the leaf and of
the stem, but it is present in Dancea. In the root it is present, and consists of large cells.
Harting has described the roots which traverse the parenchyma of the stem (Fig. 291
A, q.v) as fibro-vascular bundles, and has figured them on Plate VII. figs. 3 and 4 of his
Monograph of the Marattiaceae. He did not investigate the structure of the real fibro-
vascular bundles at all. It is necessary to draw attention to this mistake because Russow,
relying upon Harting, describes the fibro-vascular bundles of the stem as possessing an
external sheath [Schutzscheide), and states that this structure occurs only in the roots
which traverse the stem. It is difficult to imagine how Russow could have overlooked this
obvious mistake of Harting's. It is by no means easy (in Angiopteris) to obtain a
transverse section of one of the fibro-vascular bundles of the stem, for they are very
irregularly curved and are everywhere covered with roots which traverse the network
formed by the bundles. As I had only one stem at my disposal I was unable to satisfy
myself as to the true form of the fibro-vascular system, but it appears that Harting's
figure is not very true to nature. The numerous bundles which bend outwards into each
leaf are formed by the division in the lower part of the petiole of the few bundles which
spring from the fibro-vascular network of the stem (Fig. 291 A).

According to Holle, the stem of Marattia grows by means of a four-sided apical cell.
In the stouter roots, according to Harting and Russow, the place of the apical cell is
taken by a layer of very large cells. In the slender roots of Marattia and Angiopteris
Holle has found a four-sided apical cell.

FILICINEM. 421

Order II. Filices\

The Sexual Generation (Oophore) or Prothallium of Ferns is a thalloid body-
containing chlorophyll and obtaining its nourishment independently; its development
presents striking resemblances to that of the simpler Hepaticae, and to a certain
extent even to the formation of the protonema of some Mosses. It produces simple
tubular unarticulated root-hairs, and finally antheridia and archegonia. Its develop-
ment and the duration of its life may embrace a considerable space of time, espe-
cially when the archegonia are not fertilised.

When the spores germinate, which usually does not take place till a con-
siderable time" after dissemination (but in Osmunda after only a few days), the cuti-
cularised exospore, generally provided with ridges, bosses, spines, or granulations,
spHts along its edges ; the endospore, which now protrudes and is not unfrequendy
already divided by septa, produces the prothallium, either immediately, as in
Osmunda, or after the preliminary formation of a filamentous protonema, which
presents in Hymenophyllaceae certain resemblances to that of the Andreseaceae and
of Tctraphis among Mosses. The development of the prothallium has been more
exactly investigated only in the Hymenophyllacese, the Polypodiacese, and also in
Osmunda and Aneimia'^ ; and the considerable differences which have thus been
established necessitate separate descriptions.

In the Hymenophyllaceae the contents of the spore are divided, even before
germination, into three cells meeting in the centre ; in some species of Trichomanes
small cells are cut off at three points of the circumference, while a large central

^ H. von Mohl, Ueber den Ban des Stammes der Baumfarne (Verm. Schriften, p. 108). — Hof-
meister, Ueber Entwickelung und Bau der Vegetationsorgane der Fame (Abhandlungen der konigl.
Sachs. Gesells. der Wissen. 1857, vol. V). — Ditto, Ueber die Verzweigung der Fame (Jahrb. fiir
wissen. Bot, vol. III. p. 278). — Mettenius, Filices Hort. Bot. Lipsiensis (Leipzig 1856). — Ditto, Ueber
die Hymenophyllaceen (Abhandlungen der konigl. Sachs. Ges. der Wissen. 1864, vol. VII;. — Wigand,
Botanische Untersuchungen (Braunschweig 1854). — [On the Germination, Development, and Fruc-
tification of the Higher Cryptogamia, &c. Ray Society, 1862, pp. 128-266.] — Dippel, Ueber den
Bau der Fibrovasalstrange, in the Berichte deutscher Naturforscher u. Aerzte in Giessen, 1865,
p. 142. — Reess, Entwickelung des Polypodiaceensporangiums (Jahrb. fur wissen, Bot. 1866, vol. V.
p. 5). — Leszczyc-Suminski, Zur Entwickelungsgeschichte der Farnkrauter, 1848. — Strasburger,
Befruchtung der Farnkrauter (Jahrb. fiir wissen. Bot. 1869, vol. VII. p. 390). — Kny, Ueber
Entwickelung des Prothalliums und der Geschlechtsorgane, in the Sitzungsberichte der Gesellschaft
naturforschender Freunde in Berlin, Jan. 21 and Nov. 17, 1868. — Kny, Ueber Bau und Entwickelung
des Farnantheridiums (Monatsberichte der kais. Akad. der Wissen. Berlin, May 1869). — Kny,
Beitrage zur Entwickelungsgeschichte der Farnkrauter (Jahrb, fiir wissen. Bot. vol. VII. p. i). — •
Russow, Vergl. Unters. Petersburg 1872. — Janczewski, Ueber die Archegonien. Bot. Zeit. 1872, p.
418. — [On the development of the prothallium, see also Bauke, Keimungsgeschiche der Schizseaceen,
Jahrb. f. wiss. Bot. XI ; Burck, Develop, du prothalle des Aneimia, Arch. Neerlandaises, X ; Bauke,
Entwick. d. Prothalliums bei den Cyatheaceen, Jahrb. f. wiss. Bot. X ; Goebel, Entwick. d. Prothal-
liums von Gymno gramme leptophylla, Bot. Zeit. 1877; Janczewski et Rostafinski, Le prothalle de
V Hymenophyllum Tunbridgense, Mem. soc. nat. d. sci. natur. de Cherbourg, XIX ; Prantl, Die
Hymenophyllaceen, 1875.]

^ Although the Osmundacese, Schizseacese, and Gleicheniaceec probably constitute a group apart
from the Polypodiacese and Hymenophyllacese, I introduce here what little is known concerning them,
for our knowledge is not sufficient to permit of any but an imperfect account of them being given.
Where it is not expressly stated to be otherwise, the descriptions given above refer to the Polypodiacese
and Cyatheacese.

422 VASCULAR CRYPTOGAMS.

cell remains undivided. The cells develope into germinating filaments, bursting; the
exospore in three directions ; these filaments then grow at their apex, and become
segmented by septa ; only one of them however generally attains a more decided
development, the others soon assuming the form of hairs. In Hymenophyllum Tun-
bn'dgense this one frequently devel6pes at once into a cellular plate ; but in other
species it forms a much-branched conferva-like protonema, on which flat prothallia
2 to 6 lines in length and J to i ^ in breadth are formed as lateral shoots. Each cell
of the filament may give rise to a branch which is given off behind the anterior
septum, and is at once separated by another septum. Some of these branches
continue to grow indefinitely like the mother-shoot, others end in becoming hairs ;
a larger number are transformed into flat prothallia, but most develope into
root-hairs. Here and there the rudiment of a filamentous branch becomes con-
verted into an antheridium, or even into an archegonium. At the apex of the
flat prothallia spherical cells arise in Trichomanes incisum on marginal flask-shaped
cells : these must probably be considered as organs of propagation ; but the mar-
ginal cells of the flat prothallia may develope into root-hairs and new protonemal
filaments, and also into new flat shoots. The root-hairs are mostly short, with
brown walls, and produce at their end lobed attaching-discs or branching tubes.

In the Polypodiaceae and Schiza^acese the endospore developes into a short
articulated filamentous protonema, at the end of which, even at an early stage, a
more or less considerable increase in breadth takes place ; a plate of tissue is thus
formed consisting at first of only one layer, which soon assumes a broadly cordate
or even reniform shape, and has its growing apex situated in an anterior depression.
Its apical cell forms two rows of segments right and left, by walls which are per-
pendicular to the surface, and from their further divisions the flat tissue is produced.
The power of rejuvenescence of the apical cell is, however, limited ; it ends in
the formation of a septum by which a new apical cell is formed, which then divides
by longitudinal walls, and thus forms a row of apical cells lying side by side
which occupies the bottom of the depression of the prothallium-disc, in the same
manner as in the thallus of Pellia. The root-hairs are all lateral structures,
springing in large numbers from the under-side of the posterior part of the pro-
thallium; among them are the antheridia, which in this case are only rarely
marginal. The archegonia are also produced on the under-side, but on a cushion
behind the anterior depression formed of several layers; in Ceratopteris several
cushions are formed bearing archegonia.

Osmimda (examined minutely by Kny, and compared with the preceding, /. c.)
is distinguished in the first place from the Polypodiaceae and Schizaeaceae by the
absence of the protonematous filament. The endospore undergoes divisions at the
very commencement of germination, which form a plate of tissue of which a pos-
terior cell is converted, as in Equisetaceae, into the first root-hair. The succeeding
root-hairs arise from marginal cells and on the under-side of superficial cells of the
prothallium, the apical growth of which follows a similar course to that of Polypo-
diaceae. The mid-rib consisting of several layers is characteristic of Osmimda,
penetrating the ribbon-like prothallium from the posterior end to the apex, and
producing a large number of archegonia on both sides. The antheridia spring partly
from the margin, partly from the lower surface with the exception of the mid-rib.

FILICINEM.

w

Like many thalloid Hepaticae, the prothallia of Ferns also produce adventitious
loots from single marginal cells ^; this happens with especial profusion in Osmunda,
rhere the adventitious shoots become detached, and play the part of vegetative
>rgans of reproduction.

The prothallia show a tendency to be dioecious, which is manifested in the
ict that all the spores from a sporangium sometimes produce prothallia bearing
^ntheridia only (as in Osmunda regaJis) ; while in other cases the archegonia
Appear later and in smaller numbers, and are fertilised by the antheridia of younger
prothallia.

The Antheridia are, speaking morphologically, trichomes ; they are produced in

le same manner as the root-hairs, as outgrowths of the marginal or superficial cells

^f the prothallia ; in the Hymenophyllacese they are also produced on the protonemal

ilaments. The projection is usually separated from the mother-cell by a septum,

ind swells up spherically at once or after the formation of a pedicel. In some

:ases the mother-cells of the antherozoids are formed at once in this globular cell ;

)ut it usually undergoes still further divisions^, in consequence of which the wall of

le antheridium consists of a single layer of cells surrounding the central cell.

''ithin the cells of this layer chlorophyll-granules are formed towards their inner wall,

rhile the central cell of the antheridium divides further into the mother-cells of the

mtherozoids, which, however, are not numerous. The dehiscence of the ripe anthe-

idium is the consequence of a rapid absorption of water in the parietal cells, which

swell up violently and compress the contents of the central cell till the antheridium is

ruptured at the apex. The antherozoid-cells thus escape, and out of each of them

is set free an antherozoid coiled spirally three or four times. The finer anterior end

)f each antherozoid is provided with a number of cilia ; the thicker posterior end

>ften drags with it a vesicle furnished with colourless granules, which subsequently

Falls off and remains at rest, while the filament alone continues in motion. Stras-

mrger states that this vesicle is formed from a central part of the contents of the

lother-cell, the parietal protoplasm of which forms the filament and its cilia. The

vesicle is hence properly not a part of the antherozoid; it is only attached to it,

ind swells up strongly in water by endosmose, as is shown in Fig. 293.

The Archegonium arises from a single superficial cell of the prothallium, which
is at first only slightly arched and is divided by two walls parallel to the upper

* [On the formation of gemmae by Fern-prothallia, see Cramer, Ueb. geschlechtslose Vermehrung
ies Farn-pro thallium, Basel 1881.]

^ These divisions take place in a very remarkable manner. In the hemispherical mother-cell of
the antheridium of Aneimia hirta an arched vi'all arises, by which it is divided into an inner hemi-
spherical cell, and an outer one which covers the former like a bell ; the latter is then split up by a
transverse annular wall into an upper lid-like and a lower hollow cylindrical cell. The same thing
;curs in Ceratopteris ; in other cases, as in Asplenium alatum, a funnel-shaped wall is formed in
le hemispherical mother- cell of the antheridium, the wide end of the funnel being directed towards
le upper surface of the mother-cell, and the upper part of the mother-cell is cut off by a transverse
;ptum as a covering cell ; two, or even three, funnel-shaped walls may be formed in succession, so
lat the parietal layer of the antheridium consists of two or three superposed cells forming its
Jircumference and a covering cell (as in Fig. 293). The mode of formation of the antheridium-wall
quite different in Osmunda, where it consists below of two or three cells, upon which rest several
>f the upper cells which result from the division of the covering cell (Kny, /. c). — [See also Stras-
nirger, Theilungsvorgange in den Antheridicn der Fame; Zcllbildung und Zeltthcilung, 3rd edition.]

4H

VASCULAR CRYPTOGAMS.

surface. The lowest of the three cells thus formed, which Janczewski calls the
basal cell (Fig. 294, below e), subsequently divides in the same manner as the cells
of the surrounding tissue, and thus contributes to the formation of the ventral wall
of the archegonium which is completely embedded in the tissue of the prothallium.
The most external of the three primary cells gives rise to the wall of the neck of the
archegonium (Fig. 294, A, hh), by dividing crosswise into four cells from which the

Fig. 293.— Antheridia of Adiantum. Capillus-Veneris (X SSo), in longitudinal optical section ; / not yet ripe ;
// the antherozoids alrt ady mature ; /// the antheridium burst, the parietal cells greatly swollen radially, the
antherozoids mostly escaped ; / prothallium, a antheridium, s antherozoid, b the vesicle containing starch-
grains.

four rows of cells of which the neck of the archegonium consists are produced by
oblique divisions. The anterior wall of the neck (that is, the wall which is directed
towards the apex of the prothallium) grows more rapidly than the opposite wall and

fi A.

Fig. 294.— Young archegonia of Pteris serrtilata (after Strasburger) ; e the central cell, h h the neck, k the canal-cell.

becomes convex. Accordingly, the number of cells in the anterior row is larger
than that of the posterior row, in the former it is usually six, in the latter, four.

From the middle one of the three primary cells the central cell and the canal-
cell of the neck are derived, that is, the axial row of cells of the archegonium.
Whilst the wall of the neck is being formed, this middle cell becomes pointed above
and penetrates between the cells of the neck (Fig. 2^4, A); the pointed portion is

I

FILICINEM.

435

then cut off by a transverse septum and forms the single canal-cell of the neck,
which elongates with the growth of the neck and fills its cavity. According to
Strasburger a tendency to division, which however does not actually take place, is
indicated (Fig. 294, B) by the appearance in the neck-cell of several nuclei, a view
which is opposed by Janczewski. According to the latter observer the large central-
cell divides into an upper small cell, the ventral canal-cell (Fig. 295, ^, j), and into a
lower much larger cell, the oosphere, which subsequently rounds itself off. The
walls of the canal- cells swell up, become mucilaginous, and finally the watery muci-
lage together with the protoplasm of the canal-cells is forced out of the opened neck.
The antherozoids are retained by this mucilage and collect in large numbers before
the archegonium ; a number force themselves into the canal of the neck, often finally
stopping it up ; a few reach the oosphere, force themselves into and disappear in it.

Fig. 295. — Archegonia oi Adianturn Capilltis-Vene.ris (X 800); A, B, C, E in longitudinal optical section ; D in transverse
optical section ; A, B, C before, E after fertilisation ; k neck of the archegonium, st mass of mucilage, e oosphere ; E, e the
two-celled embryo (observed after lying one day in glycerine).

The entrance takes place at a lighter spot of the oosphere facing the neck, which is
termed the Receptive Spot^ (compare the oogonia of Algae). After fertilisation the
neck closes.

The Asexual Generation (Sporophore) or Fern (as it is popularly termed) is
developed from the oospore or fertilised oosphere of the archegonium ^. At first the
surrounding tissue of the prothallium keeps pace with the increase of the embryo, so

* Strasburger states that the act of fertilisation may be observed especially clearly in Cera-
topteris ; the forcible entrance of the antherozoids as far as the oosphere had previously been seen
by Hofmeister.

^ [From the researches of Farlow (Quart. Jour. Micr. Sci. 1874) and of De Bary (Bot. Zeit. 1878)
the sporophore is frequently developed from the oophore in some Ferns {Pteris cretica, Aspidium
falcaium) without the intervention of sexual organs. The young Fern is produced as a bud from
certain cells of the prothallium. De Bary terms this mode of development Apogamy.]

426 VASCULAR CRYPTOGAMS,

that this latter remains for some time enclosed in a protuberance springing from
the under surface, until the first leaf and root break through. The first processes
of division of the oospore are, as Hofmeister has shown in the case of Pteris aquilina
and Aspidium Fih'x-mas, not entirely alike in different Ferns\ It is certain, how-
ever, that the first division-wall (called the basal wall) of the oospore is transverse
to the longitudinal axis of the prothallium, and inclined to it obliquely ; as shown
in Fig. 295, E, its inclination is the same as that of the neck of the archegonium.
It is also certain that each of the two daughter-cells is at once divided again by a
wall in the plane of the prothallium (called the transverse wait), so that the embryo
now consists of four cells placed as quadrants of a sphere, and these are further
divided by a wall parallel to the long axis of the prothallium (called the median wall).
In Fig. 296 these first transverse divisions are indicated by thicker lines, the embryo
being seen in longitudinal section. The explanation of the figure points out the
interpretation which Hofmeister gives to the first four cells of Pieris aquilina, which
the reader may compare with the corresponding development of Salvinia and
Marsilia ; but it must not be forgotten that the embryo of the Fern lies, so to
speak, on its back. Although it is impossible in this place to go into a more

Fig. 296 Vertical longitudinal section of the embryo oi Pteris aqitiliiia (after Hofmeister, Entwickelung und

Bau der Vegetationsorgane der Fame, p. 607) ; the thicker lines are sections of the first two division-walls by
•which the embryo is divided into four cells (the continuous thick line represents the basal wall). The lower anterior
cell forms the leaf b ; from the upper anterior is derived the stem st ; from the lower posterior cell is produced
the root, stv being its apical cell and wA its root-cap ; the foot y is formed from the upper and posterior of the first
four cells.

minute description, it is still necessary at least to point out that a close resem-
blance exists between the embryo of Ferns and that of Rhizocarps.

[The embryo now consists of eight cells. Of the four octants which lie in front
of the basal wall (the epihasal half of the embryo) the two upper (z*. e. those nearest
the neck of the archegonium) give rise the one to the growing point of the stem, the
other to trichomes, from the two lower octants the first leaf (cotyledon) is developed.
Of the four octants which lie behind the basal wall (forming the hypobasal half of the
embryo), the upper two form the foot ; and of the lower two, the one which is diame-
trically opposite to that which forms the stem gives rise to the root, and the other is
gradually suppressed.]

* [On the embryology of Ferns, see Kny, Keimung und Entwickel. von Ceratopteris, Bot. Zeit.
1874, and Die Entwickelung der Parkeriaceen, Nov. Act. Acad. Leop.-Carol. 1875.— Vouk, Die
Entwick. des Embryo bei Asplenium Shepherdi, Sitzber. d. "Wien Akad. 1877. — Leitgeb, ibid. 1878,
Zur Embryologie der Fame. — Kienitz-Gerloff, Entwickelung des Embryo bei Pieris serrulata, Bot.
Zeitg. 1878. — A good summary is given by Sadebeck in Schenk's Handbuch, vol. I. Compare also
the accounts given of the embryology of Eqnisetum, Marsilia, Salvinia, and Selaginella, as also of
Muscinese: further, Goebel, zur Embryologie der Archegoniaten, Arb. d. bot. Inst, in Wiirzburg,
II. 3, 1880.]

FILICINEM.

427

Hhe fool is an organ by which the embryo attaches itself to the tissue of the
prothallium, in order to draw nourishment from it while the first roots and leaves are
being formed. The first parts of the stem and the roots and leaves, which are now
developed in succession from the embryo, are very small, and remain so ; those
which are formed later are gradually larger. The leaves become constantly more
complex in form, and the structure of the stem more intricate as the new portions
formed by its growth in length increase in diameter. The first parts of the stem,
like the first leaf-stalks, contain each only one axial fibro-vascular bundle ; the later
ones a larger number when both stem and leaf-stalk have attained a considerable
thickness. In this manner the Fern continues to gain strength, not by subsequent
increase of size of the embryonic structures, but by each successive part attaining
a more considerable size and development than the preceding ones ; until at length
a kind of stationary condition is arrived at in which the newly-formed organs are
nearly similar to the preceding ones. The following observations refer especially to
this mature condition of Ferns.

i

i^lG. 297. — Adiantutn Capillus-Veneris ; vertical longitudinal
section through the prothallium // and the young Fern E;
h root-hairs, a archegonia of the prothallium, b the first leaf,
tv the first root of the young plant (X about 10).

Fig. 298. — AdiaiitHtn CapiUtis-Venerts ; the pro-
thallium // seen from below with the young Fern
attached to it ; b its first leaf; -w' 7u" its first and
second roots ; h root-hairs of the prothallium (x about
30) •

The mature Fern is, in some Hymenophyllacese, a small delicate plant, not
much exceeding in dimensions the larger Muscineae; in other sections the fully
grown plants attain the size of considerable shrubs; some species, natives of the
Tropics and of the Southern Hemisphere, assume even a palm-like habit, and are
called Tree-ferns. The stem creeps on or beneath the ground (as in Polypodium
and Pieris aqm'lmd), or climbs up rocks and stems; in some it ascends obliquely
{e.g. Aspidium Filtx-mas) ; in Tree-ferns it rises up vertically in the form of a
column. The roots are usually very numerous ; in Tree-ferns the stem is often
entirely covered by a dense mantle of them. They arise on the stem in acro-
petal succession; sometimes close to the growing apex of the stem (as in P fen's
aquilina). When the internodes remain very short, and the stem is entirely covered
with the bases of the leaves, the roots arise, as in Aspidium Filix-mas, from the leaf-
stalks. In many Hymenophyllaceae which have no true roots, branches of the stem
assume a root-like structure. In creeping and climbing species the leaves are sepa-
rated by distinct internodes which are sometimes very long; in thick, ascending,

428

VASCULAR CRYPTOGAMS.

and vertical stems, the intemodes are usually undeveloped, and the leaves so crowded
that no free portion of the stem remains uncovered, or only a very inconsiderable
one ^ The leaves of Ferns are usually characterised by a circinate vernation, and
they only unroll in the last stage of their growth ; the mid-rib and the lateral veins
are curved from behind forwards. The forms of the leaves are among the most
perfect in the whole vegetable kingdom ; they manifest an enormous variety in
their outline, the lamina being usually deeply lobed, branched, or pinnate. In com-
parison with the stem and the slender roots they are mostly very large, and some-
times attain extraordinary dimensions, even a length of from 6 to lo feet (as in
Pteris aquilina and Cihotium). They are always stalked, and continue their growth
at the apex for a long time ; the leaf-stalks and the lower parts of the lamina are
often completely unfolded while the apex is still growing (as in Nephrolepis). This
apical growth is not unfrequently interrupted periodically {vide infra) ; in Lygodium

Fig. -i.^^.— Pteris aquilina, a part of the underground stem with leaves and bases of the leaf-stalks (reduced about one-
half) ; / older portion of the stem bearing the two bifurcations // and //', jj the apex of the weaker branch //; beside it the
youngest leaf-rudiment 8 ; 1-7 the leaves of this branch, one being developed in each year ; 1-5 the leaves of earlier years, which
have already died off at some distance from the stem ; 6 the leaf of the present year with unfolded lamina, the stalk having been
cut off; 7 the young leaf for next year ; at the apex of the stalk is the lamina still very small and entirely clothed with hairs. The
leaf-stalk / bears a bud /// a, which has developed a leaf* that has already died off. The more slender filaments are roots. All
the parts shown in the figure are subterranean.

the leaf-stalk or the rachis resembles a twining stem with long-continued growth, the
pinnae presenting the appearance of leaves. The amount of metamorphosis of the
leaves is, notwithstanding, very inconsiderable ; on the same plant the same forms
of leaves, mostly foliage-leaves, are constantly repeated ; scale-like leaves occur on
underground stolons {e.g. in Siruthiopteris germanicd), and in many cases the fertile
leaves (those which bear sporangia) assume special forms. Such differences as
occur in most Phanerogams are not found in the development of the leaves of one
plant ; Plaiycerium alcicorne must, however, be mentioned, as having the foliage-
leaves alternately developed as broad plates closely applied to the supporting surface
and as long dichotomously branched ribbon-shaped erect leaves.

Among the various forms of trichomes of Ferns those termed Ra?7ienia are

* From the change of form and of size presented by the older cicatrices Brongniart concluded
that the stems of the Tree-ferns continue to grow in length (and in thickness?) for some time after
the leaves have fallen off.

FILICINEM.

429

especially striking, from their great numbers and from being frequently flat and leaf-
like ; the younger leaves are generally entirely covered and concealed by them.

After these preliminary particulars, we may now turn to a consideration of the
mode of growth of the separate organs.

The growing end of the stem sometimes far outruns the point of attachment of
the youngest leaves, and then appears naked, as in Polypodium vulgare, P. sporodo-
carpu?n, and other creeping Ferns, as well as in Pteris aquilina, where, according to
Hofmeister, it frequently attains in old plants a length of several inches without
bearing leaves. Mettenius states that in many Hymenophyllaceae leafless prolon-
gations of the axis of this kind have been taken for roots. In other cases, on the
contrary, especially in Ferns with an erect growth, the increase in length of the
stem is much slower, its apex remaining enclosed in a leaf-bud. The stem generally
ends in a flat apex ; sometimes, as in Pteris, it is even imbedded in a funnel-shaped
elevation of the older tissues (Fig. 301, E). The apex of the stem is always occupied
by a clearly distinguishable apical cell, which is either divided by walls alternately
inclined, and then resembles, when viewed from above, the transverse section of
a biconvex lens ; or it is a three-sided
pyramid, with a convex anterior surface
and three oblique lateral surfaces, which
intersect behind. The outlines of the
segments, which are in the first case in
two, in the second case in three rows,
or arranged with more complicated
divergences, soon disappear in conse-
quence of numerous cell- divisions and
of the displacement caused by the
growth of the masses of tissue and
leaf-stalks surrounding the apex. The
apical cell, for instance, of Pteris aqui-
lina^ is wedge-shaped, the segments
on the horizontal stem forming a right

and a left row ; the edges of the apical cell face upwards and downwards
(Fig. 300). The same is also the case, according to Hofmeister, in Niphoholus
chinensis and rupestris, Polypodiiini aureum and punctulatum, and Platycerium
alcicorne. In Polypodi^vi vulgare he states that it is sometimes wedge-shaped,
sometimes pyramidal with three faces ; the last-named form occurs also in Aspidium
Filix-mas, &c. As a rule it may for the present be assumed that creeping stems
with a bilateral development have a wedge-shaped apical cell, upright or ascending
stems with radiating rosettes of leaves one that is a three-sided pyramid.

The further relationships of the segments of the apical cell of the stem to the
origin of the leaves and to the building up of the tissue of the stem itself are
still but litde known in detail. It cannot be doubted that each leaf results from
a single segment only, and that this segment-cell is devoted from an early period
to the formadon of the leaf, but it appears doubtful whether the segments always
form leaves, and if not what number of sterile segments intervenes between those
from which a leaf is developed.

Fig. 300.— Apical view of the end of the stem of Pteris aqui-
lina; y the apical cell of the stem ; x the apical cell of the youngest
leaf; h h hairs which cover the apical region surrounded by a
cushion of tissue.

430 VASCULAR CRYPTOGAMS.

The phyllotaxis of Ferns sometimes corresponds to the rectilinear arrangement
of the segments of the apical cell. Thus the distichous arrangement of the leaves
of Pteris aqutlina, Niphobolus rupesiris, and of some species of Polypodium, corre-
sponds to the biseriate segmentation of the apical cell of the stem. But where the
phyllotaxis is complicated and spiral, and the apical cell a three-sided pyramid, as
occurs in Aspidium Filtx-mas, the same processes may take place as in those Mosses
which have their leaves arranged in many rows with a trilateral apical cell, such
as Polytrichum ^

The Terminal Branching of the stem is considered by Hofmeister to be
dichotomous in all Ferns '^. The branches arise very near the end of the stem, and
are, at least at first, equivalent to the primary stem, so that the branching is a
bifurcation. That the branches are independent of the leaves is inferred by this
writer from the fact that the ends of the stem of Pteris aquilifia, which are leafless
and often several inches long, regularly bifurcate. These branches are, in this
and in many other cases, not axillary ; and where, in other Ferns, they appear
axillary, we must assume, with Hofmeister, that the bifurcation has taken place
immediately in front of a youngest leaf, and that the limb of the fork which stands
before the leaf developes to a smaller, while the other (the prolongation of the primary
stem) does so to a greater extent. The branching at the end of the stem does not
necessarily take place in the same plane as the insertion of the leaf immediately pre-
ceding ; when it does, the branch stands laterally on the stem beside the leaf. To this
class belongs, according to Mettenius's description, the extra-axillary branching of those
Hymenophyllacese which have their leaves in two rows. That which distinguishes
Ferns from Phanerogams with axillary branching, especially Angiosperms, is the
rarity of terminal branching. While in the latter every leaf-axil, at least in the
vegetative region, bears a bud, even the apparently axillary branches of creeping
Ferns with long internodes occur mostly only at great distances, being wanting in
a number of intermediate leaves. In those Ferns where the growth of the stem is
slow and the apical region of considerable size, especially in erect species like Aspi-
dium Filix-mas and the Tree-ferns, terminal branching of the stem is reduced to
a minimum, or is entirely absent, or occurs only in abnormal cases.

The formation of new shoots from the bases of leaf-stalks must be distinguished
from the normal terminal branching of the stem. These have nothing to do
genetically with the stem, any more than the formation of adventitious shoots from
the lamina of the leaves {ii)ide infra).

The Development of the Leaf^ is exclusively basifugal and apical, the further
growth being also basifugal. The leaf-stalk is first formed; at its apex the lamina
begins subsequently to show itself; its lowest parts are formed first, its higher parts
in basifugal succession. The extraordinary slowness of this growth is very re-

^ See Hofmeister, AUgemeine Morphologie, p. 509; and Bot. Zeitg. 1870, p. 441.

^ [According to Mettenius (^Ueb. Seitenknospen bei Farnen, Abhdl. d. kgl. sachs. Ges. d. Wiss.
V. 1861, this is not the case. Mettenius' view has been recently confirmed by Prantl, who has
found that the branching in many Ferns, such as Cyitopteris montana, Phegopteris vulgaris, Dryopteris
calcarea, is effected by lateral buds (Flora, 1875). Ceratopleris appears to be the only Fern which
does not produce them (Kny, loc. cit.)]

2 [See Sadebeck, Ueb, d. Entwick. d. Farnblattes ; Verhandl. d. bot. Ver. d. prov. Branden-
burg, 1874.]

FILICINEJE.

431

larkable, finding its parallel only among the Ophioglossaceae. In old plants of

^kris aquilina the formation of the leaf commences fully two years before its un-

)lding: — at the commencement of the second year only the leaf-stalk is as yet

existence, about one inch high. Up to this period its growth has been effected

Jy a single apical cell which is divided by oblique walls in alternating directions;

is now carried on by a number of marginal cells which divide. The pinnae are

'derived from the segments of the single apical cell; the veins are formed by the

repeated divisions of the marginal cells (Sadebeck). In the summer of the second

year the lamina arises for the first time at the apex of this rod-like body, and may be

found hidden in the form of a minute disc beneath the long hairs. It immediately

bends downwards at its apex, and hangs down like an apron from the apex of

the stalk (Fig. 301, B, C, D). Its growth now proceeds underground, so that it

does not begin to unfold till the third spring, when it is raised above ground

by the elongation of the leaf-stalk. The whole of the leaves of a rosette of Aspidium

Fig. 301. — Pteris aquilina; A the end of a stem st, the apex lying at ss ; by its side at b is the rudiment of a leaf,
bs the stalk of a leaf in the second year, at h its lamina enveloped by hairs, K a bud at the back of the leaf-stalk,
w roots ; B a young leaf in the second year, bs its stalk, / its small lamina with the hairs removed ; C longitudinal section
of a similar leaf, connected with the transverse section of the stem st, bs and /, as m. B ; D the lamina of a leaf in the
second year seen in front, i. e. on the upper side (X about 5) ; the first segments have begun to be formed ; E horizontal
longitudinal section of a branching of the stem, ss s's the two apices, a a brown epidermal tissue, b b brown sclerenchyma,
^ fibro-vascular bundles. (A, B, C natural si2e.)

Filix-mas have been in course of formation two years before their unfolding;
the leaf-stalk is in this case also formed in the first year, and the first formation
of the lamina takes place on the oldest leaves of the young rosette.

The basifugal apical growth of the lamina of Fern-leaves is however most
conspicuous when it continually advances for a considerable time without attaining
a definite conclusion while the lower parts of the lamina have long been fully
developed, as in Nephrolepis. The periodical interruption of the apical growth
of the lamina already mentioned occurs in many species of Gleichenia and Merknsia,
where the development of the leaves remains stationary above the first pair of
pinnae, and when the pinnation is compound this is often repeated in the several
orders of branching, so that the apex, forming apparently a bud in the fork, either
remains altogether undeveloped, or is developed in a succeeding period of vegetation,
and then only incompletely. This intermittent development of the leaves may

43*

VASCULAR CRYPTOGAMS.

apparently extend over many years (see 13raun, 'Rejuvenescence,* p. 123). Ac-
cording to Mettenius, the lamina of some Hymenophyllaceae is capable of unlimited
development, and is annually renewed. In Lygodium the primary branches of the
lamina remain also in a bud-like condition at the end after the formation of each
pair of pinnae of the second order, while the rachis of the leaf grows without
limit and resembles a twining stem.

Fig. ■yyz.—Aspidium Filix-mas ; A longitudinal section through the end of a stem, "v the apical part of the stem st,
b b leaf-stalks, b' a young leaf still rolled up, the rest enveloped in long hairs, i^ fibro-vascular bundles ; B a leaf-stalk of
the same plant broken off, bearing at /(; a bud with several leaves, w a root of this bud ; C a similar leaf-stalk cut through
lengthways, bearing a root at lu and a bud at h; D end of a stem with the leaf-stalks cut off with the exception of the
youngest leaves of the terminal bud, in order to show the arrangement of the leaves, the spaces between the stalks b b
are filled with numerous roots which themselves all spring out of the stalks ; E end of a stem the cortex of which has
been peeled off in order to show the net-work of fibro-vascular bundles g; F a mesh of this net-work slightly magnified,
showing the basal portions of the bundles which pass into the leaves.

The branching of the lamina of Fern-leaves is not unfrequently forked in the
mature state, as in Platycerium, Schizcea, &c. ; but Hofmeister refers also the pinnate
forms to dichotomous branching at the commencement, which becomes sympodial
with further development, a right and left limb of the bifurcation being alternately
weaker in its growth, and forming the lateral pinnse ; while the branches, the growth
of which is favoured, form the rachis of the leaf or of the branch of a leaf \

^ It must be observed here also that Hofmeister applies the term ' dichotomy ' in a much wider
sense than is usually done. New examinations of a large number of species are greatly to be desired.

filicineje: 433

The Formation of Adventitious Buds'^ which do not result from the terminal
branching of the stem, is, in Ferns, connected with the leaves. These buds make
their appearance on the leaf-stalk or on the lamina itself. The shoots of Pteris
aquilina which spring from the leaf-stalk (Fig. 301) stand at the back of the
individual leaf-stalks near the base; in Aspidium Filix-mas (Fig. 302) they arise
at a moderate height above the insertion, usually on one of the lateral edges of the
leaf-stalk. In both cases Hofmeister states that they are formed on the young
leaf-stalk even before the first appearance of its lamina, and before the differentiation
of its tissue. A single superficial cell of the leaf-stalk is the mother-cell of the new
shoot; and as the surrounding tissue of the leaf-stalk grows like a wall around them,
they may, as in Pteris, be placed in a deep depression, where they sometimes remain
dormant for a long period. Even when the leaf has long died away, the leaf-stalk
still remains succulent up to the bud, and filled with food-materials; and in
Aspidium Filix-mas vigorous stems are not unfrequently found with a number of
leaves at their posterior end still attached to the leaf-stalk of an older stem. In
some cases, as in Struthiopteris germanica, the buds produced on the leaf-stalks
develope into long underground stolons furnished with scale-leaves, which become
erect at the end and unfold a crown of foliage-leaves above ground. In Nephro-
lepis undulata they swell at the end into tubers. Adventitious buds spring from the
lamina, especially in many species of Asplenium ; in A. furcatum^ e.g., often in large
numbers from the middle of the upper surface of the pinnae; in A. decussatum from
the base of the pinnae (or axillary on the mid-rib?). Ceratopteris thalictroides not unfre-
quently produces buds in the axils of all the divisions of the leaves, which, especially
when the detached leaf is laid upon damp ground, develope rapidly, and grow into
vigorous plants. According to Hofmeister, these buds also spring from superficial
cells of the leaf. The long pendulous leaves of some Ferns touch the ground with
their apices, root, and sometimes also put out new shoots from these points {e.g.
Chrysodium flagelliferum, Woodivardia radicans, &c.).

The Roots. During its growth the stem is usually constantly forming new roots
in acropetal succession, which, in the creeping species, become at once fixed to the
substratum. [As a rule they are developed quite endogenously, but in Ceratopteris
thalictroides, according to Kny, they arise from cells immediately beneath the
epidermis. According to Conwentz, a distinct relation can usually be observed
between the number of the roots and that of the leaves.] In Pteris aquilina the
new roots appear close behind the apex, and, both in this species and in Aspidium
Filix-mas, they also proceed from the adventitious buds of the leaf-stalk while
still very young. It has already been mentioned that, in the last-named species,
when the mature stem is completely covered by leaf-stalks, all the roots spring
from them and not from the stem. In Tree-ferns especially the lower part of
the erect stem is entirely covered by slender roots, which grow downwards, forming
an envelope several inches thick before they penetrate the soil, and thus give a

both in reference to the formation of leaves and to the terminal branching of the stem. [According

I to Sadebeck, the pinnse are developed as lateral outgrowths, but the branching of the veins is the
result of the dichotomy of the marginal cells ; see supra.']
^ [On the development of these adventitious buds, see Heiniicher, Sitzber. d. Wien. Akad. 1878
and 1881.]

434 VASCULAR CRYPTOGAMS.

broad base to the stem although it is there really much more slender; but in the
upper part there are also a great many roots. In small plants they are very slender;
on large plants they attain a diameter of from i to 3 mm. ; they are cylindrical,
generally covered with a number of root-hairs which form a kind of felt, and are
of a brown or black colour. The history of the growth of Fern-roots has been
studied by Nageli and Leitgeb\ The apical cell is a three-sided pyramid, with
a convex equilateral base. The segments or layers of the root-cap, detached by
convex septa parallel to the base, first divide into four cells placed crosswise,
and the walls which effect this division alternate in successive layers by about
45°; each of the four cells of a layer then splits up into two external and one
internal (central one), so that the layer is now formed of four internal cells
arranged in a cross, and of eight external cells. Further divisions may then
follow; the central cells of the layer grow more quickly in an axial direction,
and may become divided by transverse septa, by which the layer is made to
consist of two or more strata in the middle. The formation of a layer of the
root-cap is generally followed by that of three root-segments before a further
new layer is formed; these segments, corresponding to the faces of the three-
sided apical cell, lie in three straight longitudinal rows. Each of these tri-
angular tabular segments includes a third of the circumference of the root, and
is first divided by a radial longitudinal wall into two unequal portions. The trans-
verse section of the root now shows six cells, three of which meet in the centre,
while the other three do not reach quite so far. Each of these six cells is then
divided by a tangential wall (parallel to the surface) into an inner and an outer
cell; the inner ones form the fibro-vascular bundle, while the six outer cells form
the rudiment of the cortex. If the root becomes thick, the six cortical cells
divide by radial walls: if it remains slender, this division does not take place.
The six or twelve cortical cells are now divided by a tangential longitudinal wall,
and the fibro-vascular bundle is enclosed by two layers of cells, the outer of
which forms the epidermis, the inner the fundamental tissue of the cortex (see
Book I, p. 144).

The roots of Ferns, like those of Equisetaceae, branch monopodially, and
the lateral roots arise on the outer side of the primordial fibro-vascular bundles
in acropetal succession, usually in two rows, but occasionally in three or four.
The cells from which the lateral roots spring belong to the most internal layer
of the cortical parenchyma (bundle-sheath), and are opposite to the xylem-bundles :
they are separated from the fibro-vascular bundles of the parent root by the
pericambium. The rudiments of the roots make their appearance near to the
growing-point before the vessels are formed. Adventitious lateral roots, arising
behind those which already exist, do not occur. The three-sided pyramidal apical
cell is formed in the mother-cell of a lateral root by three oblique septa, and then
the first root-cap is produced from it. If two primordial fibro-vascular bundles
are developed in the lateral root, they He right and left with reference to the

^ Sitzungsber. der bayr. Akad. der Wiss. Dec. 15, 1865. Compare with what follows the
diagram of a root given under the Equisetacege, which serves in the main also for Ferns and
Rhizocarps; also p. 144. [See also Conwentz, Beit, z, Kennt. des Stammskelets einheimischer
Fame, Bot. Zeit. 1875.]

FILICINE^ 435

parent root. The cortex of the parent root is simply broken through, no root-sheath
being formed.

The Trichomes of Ferns assume a great variety of forms. True root-hairs,
simple unarticulated tubes, arise, not only on the roots themselves, but also on
underground stems and on the bases of leaf-stalks (as in
Pieris aquilina and Hymenophyllaceae). On aerial creeping
stems and on the leaf-stalks the numerous usually brownish
or dark-brown flat multicellular hairs, the Palece or Ramenia,
occur, soon becoming dry, often entirely enclosing the buds,
and attaining a length of from i to 6 cm. (as in Poly-
podium, Cihoiium, &c.). Long strong bristles are sometimes
found on the lamina (in Acrosiichum crinitum), and very often
fine, delicate, articulated hairs. They are formed from single
superficial cells at the growdng-point.

The Sporangia of Ferns are small rounded capsules, which
are borne on long stalks in the Polypodiacese and Cyatheaceae, fig. sos.-under-side

1 1 • 1 M • 1 -r-> ' rT-<i ^^ r ^ °^ ^ lacinia of a leaf of

but which are sessile m other Terns. 1 he wall of the sporan- Aspidium Futx-mts.

. ' r 1 .11 /• 11 . ^''^'^ eiglit indusia i

gium, when mature, consists of but a single layer of cells. A (X2).

ring of cells belonging to the wall of the capsule and running

across it transversely or obliquely or lengthwise is generally developed in a peculiar

manner, and is then termed the Annulus. By its contraction when dried up the

capsule bursts at right angles to jthe plane of the annulus. Sometimes, instead

of the annulas, a terminal or lateral group of the cells of the wall of the capsule

is developed in a similar manner.

The sporangia are generally combined into groups, each group being termed a
Sorus; the sorus contains either a small definite number or a large indefinite
number of sporangia, and among them also very commonly some slender articu-
lated hairs, the Paraphyses. The whole sorus is very generally covered by an
excrescence of the epidermis, the true Indusium^ : in other cases the indusium
consists of an outgrowth of the tissue of the leaf itself, and is then composed of
several layers, and even has stomata ; or the covering of the sorus is simply the.
result of the margin of the leaf being recurved or rolled over it : in these cases the
indusium is said to ht false. In Lygodium each separate sporangium is covered by a
pocket-shaped growth of the tissue of the leaf like a bract. Sori are not usually
formed upon all the leaves of the mature plant; sometimes groups of fertile and
sterile leaves alternate in regular succession, as in Struthiopteris ger??iattica. In some
cases the sori are uniformly distributed over the whole of the lamina, in others they
are connected with definite portions of it. The fertile leaves may be in other
respects hke the sterile ones, or they may be strikingly diff"crent from them ; and
this difference is not unfrequently occasioned by the partial or entire failure of deve-
lopment of the mesophyll between and near the fertile veins ; the fertile leaf, or the
fertile part of the leaf, then appears like a spike or panicle furnished with sporangia
{e, g. Osmunda, Anemia). The sporangia generally arise from the epidermis of the

^ [On the development of the indusium, see Prantl,' Die Hymenophyllaceen, 1875, and die
Schizseaceen, 1881 ; Burck, Over de ontwikkelingsgeschidenis van het indusium der Varens,
Haarlem 1874.]

F f 2

43^

VASCULAR CRYPTOGAMS.

veins of the leaf, and especially on the under side of the lamina ; but in the Acro-
stichacese they spring both from the veins and from the mesophyll ; in Olfersia they
cover both surfaces of the leaf at the sides of the mid-rib, or in Acrostichum only the
under side. When, as is usually the case, the veins are the only parts that bear the
sporangia, the fertile veins may be like the sterile ones, or they may undergo a variety
of changes at the spots where they bear the sori ; they may be swollen into a cushion
(forming a placenta), or they may project beyond the margin of the leaf, as in the
Hymenophyllaceae. The sorus may be seated on the end of a vein, which then
frequently puts out two branches in the angle of which is placed the sorus, or it may
be formed upon the vein and behind its end ; or the sorus may run for a consider-
able distance by the side of the vein. Sometimes the fertile veins run close to the
margin of the leaf, in other cases close to the mid-rib of the lamina.

The Development of the Sporangium} is accurately known only in the Poly-
podiaceae ; it arises there from a papillose outgrowth of one of the epidermal -cells

Fig. yi^.—Asptdium Filix-inas. A transverse section of a leaf with a sorus consisting of the sporangia s and the
indusiuin i i; right and left in the mesophyll of the leaf are two small fibro-vascular bundles, the sheath of which shows
the dark brown thickenings on the walls that face inwards. B a young sporangium, its annulus standing vertically to
the plane of the paper, r its apical cell ; in the interior four cells are seen resulting from the division of the central cell;
C lateral view of a nearly ripe sporangium, r r its annulus, d the stalked gland peculiar to this species ; within the sporan-
gium are seen the young spores already formed.

from which the sorus originates. Reess has shown that before the formation of
the sporangium the epidermal cell concerned has been already divided cross-wise ;
the papilla is cut off by a septum, another septum arising, after further elongation,
in the mother-cell of the sporangium thus formed ; the lower cell forms the pedicel,
the upper cell the capsule of the sporangium. The pedicel is usually transformed.

' When the first sporangia are ripening, all stages of development of the younger ones may
Be found in the same sorus side by side. [The following are the more important works on the
subject : Reess, Zur Entwickelungsgeschichte des Polypodiaceen-Sporangiums, Jahrb. f. wiss. Bot.
V, and Bot. Zeit. 1867. — Tschistiakoff, Die Sporangien und Sporen der Polypodiaceen, Nuov. Giorn.
bot. Ital. VI. — Russow, Vergl. Unters. Petersburg 1872. — Fischer von Waldheim, Ueb. die Ent-
wickelung der Farnsporen, Jahrb. f. wiss. Bot. IV. — Kny, Ueb. Ceratopteris thalictroides, Bot, Zeit.
1874. — Prantl, Die Hymenophyllaceen, 1875, and Die Schizaeaceen, 188 1.]

FILICINEM.

437-

by intercalary transverse divisions and longitudinal walls, into three rows of cells;
the nearly hemispherical mother-cell of the capsule is next transformed, by four
successive oblique divisions, into four plano-convex parietal cells and a tetrahedral
inner cell (archesporium) ; in the former further divisions follow perpendicular to
the surface, while the inner cell again forms
four tabular segments which are parallel to
the outer parietal cells and which constitute
the tapetum. These inner parietal cells also
divide perpendicularly to the surface of the
capsule, and may form two layers. The cells
of the outer parietal layer from which the
annulus is to be formed are further divided
by parallel walls perpendicular to the surface
of the sporangium and to the median line of
the annulus, until the prescribed number of
cells of the annulus is reached ; these cells
then project above the surface of the capsule.
While the tetrahedral central cell is now pro-
ducing the mother-cells of the spores by suc-
cessive bipartitions, the cells of the tapetum
are absorbed, and the cavity of the sporan-
gium is considerably enlarged by this means
and by the superficial growth of the outer
parietal layer ; so that the mass of mother-
cells (according to Russow there are usually
sixteen) floats entirely free in the fluid that
fills the sporangium (Fig. 304).

But little is known concerning the de-
velopment of the sporangia in other families
of Ferns. According to Russow and Prantl
it takes place in Alsophila (Cyatheaceae) in
the same way as in the Polypodiaceae, whilst
in Aneimia and Mohria (Schizseaceae), accord-
ing to their figures and descriptions, each sporangium arises, before the differen-
tiation of the epidermis, from a single cell. Probably the sporangia of the
Osmundacese originate in the same way as those of the Schizseaceae. In both
families a much larger number of spore- mother-cells is formed in the sporangium
than is the case in the Polypodiaceae, and this fact also recalls the Ophioglosseae
and the Marattiaceae.

Each spore-mother- cell is, in Aspidium Filix-mas (Fig. 306, /), provided with
an evident nucleus; in consequence of its division, two new large clear nuclei
arise (///), between which an evident line of separation is sometimes to be seen.
After the division of these nuclei, four new smaller nuclei appear (/F), the
mother-cell splitting up into four spore-cells (F), the relative position of which
varies (as is shown in Figs. F/, VII, and VIII). The spore now becomes clothed
with its cell-wall, which is differentiated into an endospore consisting of cellulose

Fig. 305.— Development of the sporangium of Asple-
nium Trichonianes ; the order of succession according to
the letters a-i; in i the annulus r is shown ; the other
figures are seen in optical longitudinal section, and the an-
nulus is perpendicular to the plane of the paper (X 55o)-

438

VA SCULAR CRVPTOGA MS.

and a cuticularised brown exospore furnished with ridges {IX), and chlorophyll
is formed within the spore.

In various other Polypodiaceae, according to Russow, the course of the de-
velopment of the spores is in so far different, that the mother-cell, as occurs also
in the formation of the pollen in Phanerogams, divides into four thick-walled
cells, the so-called special mother-cells; the protoplasm of each of these then
forms around itself a permanent coat, and the walls of the mother-cell undergo
absorption. Spores of the shape indicated in Fig. 306 are said to be bilateral
in contradistinction to those which have been formed from a mother-cell in
which the four nuclei were placed tetrahedrally, and which have therefore a
rounded tetrahedral form. In the Hymenophyllaceae, Osmundaceae, and Cyatheaceae
the latter only occur, in the other families sometimes the one kind and some-
times the other.

The spores of many Polypodiaceae are distinguished by the long period
during which they retain their power of germination, and by the slowness of this
process; those of Hymenophyllaceae often begin to germinate while still in the
sporangium.

Fig. 306. — Development of the spores of Aspidiicr,i Filix-,

{X 550).

(a) Histology^. With reference to the Epidermis, attention has been directed on p. 105
to the peculiar mode of development of the stomata in many cases. It may also be
mentioned that the epidermal cells usually contain chlorophyll-granules.

The Fundamental Tissue of the stem and of the leaf-stalks consists, in some species
(as Polypodium aureum and 'uulgare, and Aspidium Filix-mas), entirely of thin-walled
parenchyma; in others (as Gleichenia, species of Pteris, and Tree-ferns), string-like,
ribbon-shaped, or filiform portions of the fundamental tissue become differentiated, the
cells of which undergo great thickening, and become brown-walled, hard, and prosen-
chymatous, forming sclerenchyma. In the stem of Pteris aquilina (Fig. 307, A) two
thick bands of sclerenchyma of this description {pr) lie between the inner and outer
fibro-vascular bundles, and fine threads of sclerenchyma appear on the transverse section
of the colourless parenchyma as dark points. In other cases (as in Polypodium 'vaccinii-
folium and in Tree-ferns), dark layers of sclerenchyma, the nature of which was in
these cases first correctly recognised by H. von Mohl, form sheaths round the fibro-
vascular bundles, to which the erect stem more especially owes its firmness. The outer
layer of the fundamental tissue of thicker stems and leaf-stalks lying beneath the epi-
dermis is often dark brown and sclerenchymatous, forming a hard firm sheath, as again,
for instance, in Pteris aquilina (Fig. 307, A, r) and Tree-ferns. In order to facilitate,
in spite of this firm coat, the communication of the outer air with the inner parenchyma

^ [For further details see de Bary, Vergleichende Anatomic der Phanerogamen und Fame, 1877.]

FILICINEm.

439

which is rich in assimilated food-materials, it is, in Pteris aquilina, interrupted along
two lateral lines, where the colourless parenchyma rises to the surface. In Tree-ferns,
on the other hand, according to H. von Mohl, depressed cavities appear on the enlarged
base of the rachis of the leaf, where the sclerenchyma is replaced by a loose and pul-
verulent tissue.

It may be mentioned here in addition, as an isolated histological peculiarity, that in
Aspidium Filix-mas, according to Schacht, roundish stalked glands occur in the funda-
mental tissue of the stem, which I have also noticed in the green parenchyma of the
leaves, and on the pedicels of the sporangia of the same Fern (Fig. 304, C, d).

The lamina of the leaf consists in Hymenophyllaceae only of a single layer of cells,
as in Mosses ; in all other Ferns it is formed of several layers. Between the upper and
under epidermis lies a spongy parenchyma containing chlorophyll, the mesophyll, pene-
trated by the fibro-vascular bundles which form the venation of the leaf. The course

FIG. -^OT.— Pteris aquilina ; A transverse section
of the stem, r its brown sheath (the layer of scleren-
chyma beneath the epidermis), / the soft colourless
parenchyma of the fundamental tissue ; 4'' inner fibro-
vascular bundles ; ag upper broad outer bundle ; B
the separated upper fibro-vascular bundles of the
stem st, and of its branches st' and st", b bundles of
the leaf-stalk, u u outline of the stem (natural size).

Fig. 308.— a quarter of tlie transverse section of a fibro-vascular
bundle from the stem of Pteris aquilina, with the adjacent parenchyma
P containing starch, sg- the bundle-sheath, b the layer of bast-fibres,
sp the large sieve-tubes, gg the large vessels of the xylem thickened in a
scalarifonu manner, 5 a spiral vessel surrounded by cells containing
starch (x 300).

of the veins is very various ; sometimes they run branching dichotomously at acute
angles, or spreading like a fan upwards and sideways, without anastomosing and without
forming a mid-rib ; more often the undivided lamina, or a division of the lobed, incised,
or pinnate leaf, is penetrated by a distinct median vein though but slightly projecting,
from which spring more slender branches, which themselves again ramify dichotomously
or apparently monopodially, and run to the margins. The finer veins frequently anasto-
mose like those of the leaves of most Dicotyledons, and divide the surface into areolae
of characteristic appearance.

The Fibro-'vascular Bundles of Ferns are closed; they consist of a mass of xylem,
completely enveloped by a layer of phloem. Besides a few narrow spiral vessels, lying
at certain definite points in the transverse section, the xylem consists of vessels with
bordered pits which usually resemble transverse clefts (scalariform vessels), their ends

440 VASCULAR CRYPTOGAMS.

being mostly obliquely truncated, or fusiform and pointed. Between the vessels lie
narrow thin-walled cells, which contain starch in winter. The phloem, in addition to
cells similar to those last named, contains wide sieve-tubes or latticed cells, and at the
circumference narrow, bast-like, thick-walled fibres. The whole bundle is usually en-
closed by a distinct sheath of narrower cells (vascular bundle sheath) ; the latter often,
but not always, is invested by a layer of brown sclerenchymatous cells, the walls of
which are very much thickened either, as in Platycerium, on that side which is next
the bundle, or, as in Blechnum brasiliense, on that side which is most distant from it.
This layer is easily mistaken for the bundle-sheath itself. A single layer or several
layers of cells may often be found at the periphery of the phloem lying just inside the
true bundle-sheath. Russow regards this structure as belonging, like the bundle-sheath,
to the ground tissue, and he terms it the phloem-sheath. Such a phloem-sheath is repre-
sented in Fig. 308 as a layer of cells containing starch lying between sg and b.

The fibro-vascular bundles are single and axial in very slender filiform stems, as in
those of Hymenophyllaceae, and in the young plants of larger species. When the stems
of the latter become thicker with increase of growth, a network of anastomosing bundles
is formed in place of the axial bundle, presenting, in typical cases, a wide-meshed hollow
cylinder, by which the fundamental tissue of the stem is separated into an outer cortical
layer and an inner medullary portion (Fig. 302, A and E). Not unfrequently, however,
isolated bundles also arise in addition ; thus in Pteris aquilina two strong broad cauline
bundles are formed within the medullary portion (Fig. 302, A, ig), and in Tree-ferns
a number of filiform bundles are scattered through it which enter into the leaf-stalk
through the meshes of the primary bundle. The primary bundles which form the cylin-
drical network already mentioned are mostly ribbon-shaped, broad, and, in the case of
Tree-ferns, commonly have their margins curved outwards, so that they with their
thick, firm, brown sheaths of sclerenchyma occupy most of the circumference of the
stem. From these margins spring the more slender filiform bundles which enter the
leaf-stalk, and are more numerous in proportion to its thickness. These may also
coalesce laterally into plates of different forms, or may run separately side by side. The
leaf-stalk always corresponds to an opening of the meshes of the cylinder of the primary
bundle. The thick bundles which run through the stem appear to be all cauline. Hof-
meister found in Pteris aquilina ^ that they exhibit the same distribution on the leafless
elongated ends of the stem as on its leafy parts, a proof that the distribution does not
depend on the leaves, as in Phanerogams. The end of the bundle may even be followed
up to near the apical cell of the stem, in places where the nearest leaf-stalks have not
yet begun to form bundles.

(b) Taxonomy. The Ferns may be classified as follows: —

Family i. Osmundaeeas. In Osmunda the fructification is paniculate, the sporangia
being borne on the laciniae of leaves the mesophyll of which is not developed. In Todea
the fertile leaves resemble the sterile ones. The shortly-stalked, unsymmetrically-
rounded sporangia are furnished on one side of the apex with a group of peculiarly-
formed cells, and they split open longitudinally on the other side. The stem, which
is densely covered with roots, throws out lateral shoots resembling itself.

Family 2. Schiza3ace89 ^. Except in Mohria, where the sporangia lie on the under
surface of the leaf near the margin which is incurved over them, the laciniae bearing
the sporangia are arranged in spikes or panicles. In Scbizcea and Lygodium the sporangia
are arranged in two rows upon the under surface of a very much contracted lacinia,
each sporangium of Lygodium being invested by a sac-like indusium. In Aneimia the
two lowest branches of the lamina have no mesophyll, and form stalked panicles on the

^ I found a stem of Pteris aquilina in which the two internal cauline bundles had coalesced
laterally so as to form a hollow cylinder enclosing one part of the parenchymatous ground-tissue as
a medulla.

^ [Prand, Die Schizseaceen, 1881.]

FILICINEM. 44 T

ultimate branches of which the sporangia are developed as if they were metamorphosed
lacinige. The ovoid or pear-shaped sporangia are sessile, the apex of each being occu-
pied by a cap-like zone of peculiarly-formed cells. The dehiscence is longitudinal. The
stem (also in Lygodium) does not branch much, and is but feebly developed, A single
fibro-vascular bundle traverses the petiole. The leaves of Lygodium resemble twining
stems.

Family 3. GleieheniaceaB. The sessile sporangia are borne upon the dorsal surface
of ordinary leaves. They form sori of usually three or four sporangia, and no indusium
is developed. The sporangium has a complete transverse annulus, and longitudinal
dehiscence. The stem is a thin, creeping rhizome. The lamina of the leaf is remark-
able for its innovation.

Family 4. HymenophyllaceaB ^. The sporangia have an oblique or transverse
complete annulus; and therefore burst with a longitudinal slit; they are formed on
a prolongation of the fertile vein (the Columella), projecting beyond the margin of the
leaf, which is surrounded by a cup-shaped indusium. The mesophyll of the leaves
usually consists of a single layer of cells, and is then necessarily destitute of stomata,
which do however occur in Loxsoma on the leaf, which then consists of several layers.
The stem is generally creeping and mostly very slender, and furnished with an axial
fibro-vascular bundle. True roots are not present in all the species; where they
are absent, the stem itself is clothed with root-hairs: a large number of species
of Trichomanes are described by Mettenius as rootless, and in these cases branches
of the stem assume a deceptive root-like appearance. The development of the axes
precedes by a long space that of the leaves ; several internodes have usually completely
ended their growth while the leaves belonging to them are still very small ; and these
apparently (or actually?) leafless shoots often branch further to a great extent. The
formation of the tissue of these families shows also many peculiarities, concerning which
reference must be made to Mettenius (Hymenophyllaceae, /. c). The fertile end of the
veins of the leaf projecting beyond its margin, the columella, elongates by intercalary
growth, and the newly-formed sporangia are, in a corresponding manner, produced in
basipetal succession. They are arranged in a spiral line on the columella. The almost
sessile sporangia are biconvex, and are attached to the columella by one of their convex
surfaces. The annulus projecting in the form of a cushion which separates the two
convexities is usually oblique, and divides the circumference into two unequal portions.
In Loxsoma the sporangia are pear-shaped and distinctly stalked. Paraphyses occur only
in a few species of Hymenophyllum.

Family 5. CyatheacesB. The sporangia are shortly stalked and have a complete,
oblique, eccentric annulus. They are borne upon a strongly-developed placenta
forming a closely-packed sorus, which is either naked or invested by an indusium, which
may be cup-shaped or completely encloses it. The genera Cibotium, Balantium, Alsophila,
Hemitelia, and Cyathea include the so-called Tree-ferns, with a lofty, erect, unbranched
stem, often thickly covered with roots, bearing at its apex a rosette of large usually
compoundly pinnate leaves.

Family 6. PolypodiaceaB. The sporangia are borne in great numbers on the under
surface of usually unmodified leaves. They have a vertical incomplete ring, and they
dehisce transversely. The following subdivisions of this family, which contains the largest
number of species of any, may be distinguished : —

(a) Acrostichex. The sori cover the surface and veins of the under side or of both
sides, or are placed upon a thickened placenta which stands on the vein. There is
no indusium. {Jcrostichum, Polybotrya.)

(b) Polypodiedd. The sori are rounded or linear, and terminal or lateral on the veins.
They are naked. The leal-stalk is either articulated to the stem {Pclypodium), or is not
{Phegopteris.)

^ [Prantl, Die Hymenophyllaceen, 1875.]

442 VASCULAR CRYPTOGAMS.

(c) AsplenieaR. The sori are unilateral on the course of the veins, and are covered
by a lateral indusium, or rarely without any ; or they extend at their apex over the
back of the veins, and are covered by an indusium springing from it ; or they occupy
special anastomosing branches of the veins, and are unilateral and covered by an indusium
free on the side of the vein. {Asplenium, Scolopendrium.)

(d) Aspidiex. The sori are dorsal on the veins, covered with an indusium, or ter-
minal and without indusium. {Aspidium.)

(e) Dwvalliex. The sori are terminal on a vein or at a fork, and are furnished with
an indusium ; or are placed on an intramarginal anastomosing bend of the veins, and
covered with a cup-shaped indusium, free at the outer margin. {Daruallia, Nephrolepis.)

(f) Pteridex. The sori are continuous along the margin of the leaf, and are covered
by a false indusium. {PteriSf Adiantum^ Blechnum.)

Order III. RHIZ0CARPE^^

The Sexual Generation (Oophore) of Rhizocarps is developed from spores
of two different kinds ; the smaller spores {niicrospores) produce antherozoids,
and are therefore male ; the larger spores {macrospores), which exceed the smaller
kind several hundred times in size, produce a small prothallium, which never
separates from them, and forms one or several archegonia; the macrospores may
therefore be considered to be female.

The development of the antherozoids is preceded by the formation of a very
rudimentary Male Prothallium. In the genus Salvinia the microspores lie imbedded
in a mass of granular hardened mucilage (as they do also in Azolla, in which plant
their germination is not known), which fills up the whole of the microsporangium ;
they do not escape, but the endospore of each of them grows out into a tube
which pierces the mucilage and the wall of the sporangium and forms a septum at
its curved end (Fig. 309, A and B). The terminal cell of the tube thus produced
is again divided by an oblique wall, after which the protoplasm contracts in the two
cells (which Pringsheim together calls the antheridium), and splits up by repeated
bipartition into four roundish primordial cells, each of which forms an antherozoid.
In addition a small portion of the contents remains inactive in each of the two
cells. The antheridial cells burst by transverse slits to allow the escape of the
antherozoids. The spirally- coiled antherozoid is still enclosed for a time in its

•^ G. W. Bischoff, Die Rhizocarpeen u. Lycopodiaceen (Niirnberg 1828). — Hofmeister, Vergleich.
Untersuch. 1851, p, 103. — [On the Germination, Development, and Fructification of the Higher
Cryptogams, Ray Soc. 1862, pp. 318-335.] — Ditto, Ueber die Keimung der Salvinia natans (Abhand.
der konigl. Sachs, Gesellsch. der Wissensch. 1857, p. 665). — Pringsheim, Zur Morphologie der Sal-
vinia natans (Jahrb. fiir wissensch. Bot. vol. HI. 1863). — J. Hanstein, Ueber eine neuholliindische
Marsilia (Monatsber. der Berliner Akad. 1862, Ann. des Sci. Nat, 4th series, vol. XX, 1863, PP- ^49-
166). — Ditto, Befruchtung u. Entwickelung der GdXX.\xng Marsilia (Jahrb. fiir wissensch. Bot. vol. IV,
1865). — Ditto, Pilularise globuliferse generatio, cum Marsilia comparata (Bonn 1866), — Nageli u.
Leitgeb, Ueber Entstehung u. Wachsthum der Wurzeln bei den Gefasskryptogamen (Berichte der
bayer. Akad. der Wissensch. 1866, Dec. 15, and Nageli's Beitiage zur wissensch, Bot, vol. IV. 1867).
— Millardet, Le Prothallium male des Cryptogames vasculaires (Strasbourg 1869), — A. Braun,
Ueber Marsilia u, Pilularia (Monatsber, der konigl. Akad, der Wissensch, Berlin, Aug, 1870). —
E. Russow, Histologic u. Entwickelung der Sporenfrucht von Marsilia (Dorpat 187 1). — Strasburger,
Ueber Azolla (Jena 1873). — Juranyi, Uber die Entwickelung der Sporangien und Sporen von Salvinia
natans (Berlin 1873), — [Arcangeli, Sulla Pilidaria globulifera e suUa Salvinia natans; Nuov. Giorn,
Bot. Ital. 1876.]

FILICINEM.

443

mother-cell (Fig. 309, D). In Marsilia and Pilularia the antherozoids are pro-
duced in much the same manner; the protoplasmic contents of the microspore
divide into three cells : when the exospore bursts, the endospore protrudes containing
these three cells, of which one is sterile (rudimentary prothallium) and the other
two produce antherozoids: within each of these two cells divisions take place so
that a number of tetrahedrally-arranged primordial cells are formed which become
surrounded with thin cell-walls, and are the mother-cells of the antherozoids. As
in Ferns, we find also in Rhizocarps only a portion of the contents of the
mother-cell applied to the formation of the antherozoid. According to Millardet,

Fig. 309. — Saliiinia natans ; A an entire microspo-
rangium, the tubes of the microspores st breaking through
(X about 100) ; B one of these tubes st protruding from the
envelope of the microsporangium h (x about 200), a the
antheridium still closed ; C tube with the empty anthe-
ridium ; D antherozoids (XSoo) (after Pringsheim).

Fig. T,io.— Marsilia Salvatrix; A macrospore sp with its epispore st and the apical papilla projecting into its funnel: in
the papilla is a broad yellowish drop, sg the ruptured wall of the macrosporangiuni (X about 30). B a microspore burst after
the escape of the antherozoids ; ex the exospore, dl the protruding endospore containing granules, z, z the spiral anthero-
zoids, jyj' their vesicles containing starch-grains. The epispore of the microspore is no longer in existence (XSSo)- (The
exospore does not show the regular arrangement of the protuberances, which is indicated erroneously in the figure.)

this portion assumes the form of a roundish turbid mass consisting of protoplasm
and starch-granules, which, during the formation of the antherozoids, becomes
gradually clearer, and, when the latter escape from the mother-cell, forms a
vesicle consisting of the unused protoplasm and the starch-granules lying in it. In
Pilularia, where the antherozoid is a thread coiled four or five times, this vesicle
remains attached to the mother-cell. In Marsilia, on the contrary, it adheres to
the posterior coils of the corkscrew-like antherozoid, which is coiled 12 or 13 times ;
and is often carried about with it for a considerable time by its swarming motion,

444

VASCULAR CRYPTOGAMS,

but finally becomes detached. When the antherozoids are formed in their mother-
cells, the exospore bursts at the apex, the endospore swells up as a hyaline bladder,
which finally bursts and allows the escape of the antherozoids (Fig. 310, B).

The Female Prothallium is formed within the apical papilla of the macrospore
from a small part of its protoplasm, and only partially emerges at a later period
from the spore-cavity, but remains united with the latter, closing it by its basal
surface, for the purpose of using up the food-materials (starch-grains, fatty oil,
and albuminous substances) which are stored up there. The separate stages in
the first formation of the prothallium are still in many respects not clear ; but it is
certain that it arises from a collection of protoplasm in the cavity of the papilla ;
this protoplasm immediately breaks up into several cells, which, according to
Hanstein in Marsilia and Pilularia^ and according to Juranyi in Salvinia, become
clothed only at a later period with cell-walls and thus form a tissue. The further

Fig. 311. — Salvmia 7iatans (after Pringsheim). A longitudinal section through the macrospore, prothallium, and embryo
in the median line of the prothallium (X about 70), a wall of the sporangium, b epispore, c proper wall of the spore, e its
prolongation, rfthe diaphragm mentioned above which separates the prothallium from the spore-cavity, /r the prothallium
already broken through by the embryo,/, // its two first leaves, s the scutiform leaf; B an older seedling with the spore sp
and prothallium /r (X 20), a the caulicle, b the scutiform leaf, /, // first and second single leaves, L, L' aerial leaves of the
first whorl, tu submerged leaf of the first whorl.

processes seem to me, according to the statements of Pringsheim, Hanstein, and
Hofmeister, compared with my own observations on Marsilia Salvairix, to be
briefly these : — the tissue of the prothallium is for a certain time completely enclosed
in the apical papilla of the macrospore, covered above by the epidermal layers
of the apex of the spore itself, and shut off from the spore-cavity below by a
wall of cellulose which is stretched across like a diaphragm and is attached at
the circumference to the endospore. By the further growth of the prothallium
the epidermal layers of the papilla are ruptured" abote, the dorsal part of the
prothallium projects into the funnel-shaped cavity which is left by the absence
of the thick episporial layer of the macrospore ; subsequently the diaphragm

^ According to Aicangeli {loc. cit.) the prothallium is formed in Pihdaria by cell-division.]

FILICINEM.

445

arches convexly, and the prothallium is thus pushed further outwards. This
is the present state of our knowledge with respect to the position of the prothallium
in the macrospore. (Compare the explanations of the figures further on.)

The prothallium of Salvinia natans attains a much more considerable size than
that of the two other genera already mentioned ; it contains abundance of chlo-
rophyll, and forms a number (which may even be large) of archegonia in definite
positions. After it has broken through the membrane of the papilla, it appears,
seen from above, as three-sided between the three torn lobes of the epispore;
one of these sides is anterior ; the two posterior sides meet behind at an acute
angle; a line from this angle to the centre of the anterior side runs above the
elevated saddle-shaped back of the prothallium, and forms its median line. The
anterior side projects above the back, and, where it meets the two posterior

Fig. 312. — Development of the archegonia of
Salvinia natans (after Pringsheim, X 150).

Fig. 313. — Salvinia natans ; median longitudinal section through the prothallium and young embryo ; y4 after the first
three divisions of the oospore, / and // are separated by the basal wall ; / the hypobasal segment divided by the wall jc into
the cells a and *; // the epibasal segment.divided by the wall 2 (transverse wall) ; cd axis of growth ; B embryo in a further
stage of development, rrr first stage of the foot, s apical cell of the scutiform leaf. III— VI the succeeding segments,
V apical cell of the stem ; mm A and B the closing cells of the archegonium (after Pringsheim).

sides, the two angles grow subsequently into long wing-like prolongations
hanging down by the sides of the macrospore. The first archegonium makes
its appearance on the median line of the elevated back immediately behind the
growing anterior side of the prothallium; two other archegonia then invariably
appear right and left of the first, so that they stand in a transverse row parallel
to the anterior side. If one of these archegonia is fertilised there is an end of
the growth of the prothallium; but if this does not happen, the prothallium continues
to grow on its anterior side, and from i to 3 new transverse rows of archegonia
are produced, each of which contains from 3 to 7. The long oosphere of each
archegonium lies obliquely in the tissue of the prothallium, so that the outer (neck)

44^

VASCULAR CRYPTOGAMS.

end is directed backwards, its inner deeper end facin^^ the anterior margin. At this
latter point lies at a subsequent period the apical cell of the embryonal stem. Young
archegonia have the apex of their central cell covered with four superficial cells
arranged in the form of a cross ; in each of these latter a vv.iU arises inclined from
without inwards and downwards, followed in each inner cell by another similar
partition (Fig. ^12 I, a, b, c). By the succeeding growth these cells are transformed
into four rows, each consisting of three segments lying one above another, forming
the neck (//, ///), the lower of which are termed * closing cells,' the upper pair
the 'stigmatic cells' (///, h). In the meantime a new cell arises at the apex of
the central cell, which, with its conical point, forces itself between the closing
cells (/, d, HI, d), and forms the canal-cell, first discovered by Pringsheim; ac-
cording to Janczewski a small segment is cut off from the upper part of the
central cell to form the ventral canal-cell, so that here, as in the other Vascular
Cryptogams, two canal-cells are developed. The two canal-cells become transformed

Fig. 3i4.—Af(irst'h'a Salvatrix; A ft\i\\e. prothallium projecting through the ruptured membrane r of the spore; j/the
mucilaginous epispore which forms the funnel, with a number of antherozoids ; B vertical section of a prothallium pt with
an archegonium a and oosphere o ; C, D, E young embryos, s apex of the stem, b leaf, iv root, /"foot {^B—E after Hanstein).

into mucilage, which escapes from the canal laid open by the throwing off of the
stigmatic cells. The large remainder of the central cell (/, //, ///, e) becomes
the oosphere. After fertilisation has been accomplished, the canal again closes
by the lateral approximation of the ' closing cells.'

The prothallium of Marsilia and Pilularia projects as a hemispherical mass of
tissue from the apical papilla of the macrospore, after it has ruptured the walls
of the spore at that place (Fig. 314, A, B), and remains buried at the bottom
of the funnel formed by the epispore of the macrospore. Even at an early period,
before the rupture, Hanstein asserts that the large central cell may be recognised in
it, surrounded, in its entire circumference, at least at first, by a single layer of cells,
so that the prothallium bears originally only a single archegonium. The central
cell is here also covered by four cells arranged crosswise, which form at the same
time the apex of the whole prothallium. By a similar process to that which occurs

filicinem:

447

in Sahinia, they form the free neck-portion (which in Marsilia projects only
slightly, in Pilularia very much) and the 'closing cells' of the archegonium. Above
the central cell, the protoplasm of which contracts, a small canal-cell is visible,
according to Hanstein, penetrating between the ' closing cells,' and behaving as
in Salvinia. Hanstein was unable to recognise any further cell-formation within
the central cell, and he concluded that the whole of its protoplasmic body was
converted into the oosphere; Janczewski, however, found here also the ventral
canal-cell which occurs in other Vascular Cryp-
togams, as a small mass of protoplasm cut off
from the central cell. After fertilisation the
layer of tissue of the prothallium surrounding
the central cell becomes double ; a few chloro-
phyll granules arise in it, and the outer cells
grow in Marsilia Salvatrix (Fig. 3 1 5) into long
root-hairs, which are especially luxuriant when
no fertilisation takes place. In the case of
Marsilia Salvatrix the antherozoids collect in
large numbers at the time of impregnation in
the funnel above the prothallium, and force
themselves into the neck of the archegonium.
Development of the Asexual Generatiojt.
The first processes of division by which, in
Salvinia, the oospore is transformed, after fer-
tilisation, into the embryo, have been most ac-
curately described by Pringsheim. l.^he first
division is effected by a wall (basal wall) which
separates the posterior (hypobasal) half of the
oospore, above which is the mouth of the
archegonium, from the anterior (epibasal) half,
which is usually larger; this wall is nearly per-
pendicular to the median line of the prothallium.
The two cells are next divided by walls (trans-
verse) nearly at right angles to the previous
one. If the angle enclosed by these two walls
is bisected by a straight line (Fig. 313, A, c, d),
this line represents the axis of growth of the
stem. This is followed by walls (median) at
right angles to the two former, and thus the
embryo comes to consist of eight cells, octants

of a sphere. Of the four epibasal octants, the two upper give rise to the first leaf
(cotyledon), which, on account of its peculiar form, is known as the ' scutif(7rm leaf:'
one of the two lower gives rise to hairs, and the other to the apical cell of the stem
which now lies in front and below (^, v); in this latter walls are now formed
inclined alternately upwards and downwards, and by this means the two rows of
segments are formed out of which the structure of the stem of Salvinia is gradually
developed. In Fig. 313 B are shown, at ///, IV, V, and VI, these segment-cells

Fig. 315.— Longitudinal section through the spore
prothallium and embryo of Marsilia Salvatrix
(X about 60) ; atn starch-grains of the spore, i inner
coat of the spore burst above into lobes, ex the
exospore consisting of prisms, c the cavity beneath
the arched diaphragm on which is the basal layer of
the prothallium. pt the prothallium, -wh its root-hairs,
a the archegonium, f the foot of the embryo, -w its
root, s the apex of its stem, b its first leaf by which the
prothallium becomes extended, si the mucilaginous
epispore which at first forms the funnel above the
papilla, and which still envelopes the prothallium fifty
hours after the dissemination of the spores.

44^ VASCULAR CRYPTOGAMS.

undergoing still further division. The four hypobasal octants give rise to the foot.
Thus no root is developed at this period, nor is one developed subsequently;
Salvinia is absolutely roodess. In order to understand the subsequent processes
of growth, Fig. 311 must be compared with Fig. 313. The growing embryo
bursts the prothallium; from r r r, in Fig. 313 Z?, arises the foot (caulicle) of
the young plant (Fig. 311 a)\ from s, Fig. 313 B^ is formed the scutiform leaf
(Fig. 3ii.j5, /^), by the growth of which the terminal bud of the stem becomes
directed downwards (Fig. 311 A, v). The epibasal part of the embryo faces the
anterior side, its hypobasal part the posterior side of the prothallium ; its axis of
growth lies in the same plane with the median line of the latter.

The first divisions of the embryo of Marsilia Salvatrix agree in all essential
points, according to Hanstein's observations and my own, with those of Salvinia; and
Hanstein states that this is also the case with Pilularia; but in both these genera the
rudiment of the first root is visible at an early period. The stem in these genera
also creeps or floats in a horizontal direction from the first, as in Salvinia, and
forms a number of roots in acropetal succession. Fig. 314 shows the first divisions
of the embryo of Marsilia Salvatrix. The oospore is divided by a nearly vertical
basal wall into an anterior (epibasal) larger and a posterior (hypobasal) smaller
cell : these are divided, as in Salvinia and in Ferns, by a transverse and by a
median wall, so that the embryo consists of eight cells. Of the four epibasal
octants, one of the two upper ones becomes the apical cell of the stem, whilst
the other gives rise to the second leaf (cotyledon) ; from the lower two the first
leaf (cotyledon) is developed. Of the four hypobasal octants, the lower one, which
is diametrically opposite to the apical cell of the stem, gives rise to the primary
root, the other octant at the same level becoming suppressed : the two upper
ones give rise to the foot (Fig. 314 -£', /"). The union between embryo and
prothallium is brought about by the foot. The apical cell of the stem, Fig. 314
E, s, thus lies, after the formation of the first three walls, between the anterior
margins of the first leaf and of the foot. In the stage represented in Fig. 315
this origin of the first leaf, first root, and foot, may still be recognised from the
arrangement of the cells.

The further growth of the three genera, otherwise very different in their habit,
to which we must add Azolla, although its development has not yet been investigated,
agrees in maintaining the bilateral structure already manifested in the embryo in
connection with the decidedly horizontal growth, although, as we shall see, the
position of the apical cell and of its segments varies. In contrast with Muscineae
and Equisetaceae, but in accordance with Ferns, a leaf is not produced in the Rhizo-
carpeae from every segment of the stem ; certain of the segments remain sterile, and
these then go to the formation of internodes. The leaves grow, as in Ferns and
Ophioglossaceae, basifugally by means of an apical cell which forms two rows of
alternating segments. Before the development has assumed a constant course, an
increase of vigour of the young plant takes place, which is shown in the enlarge-
ment of the leaves and the greater perfection of their forms, as well as in a change
of their relative positions. But in order to make this clear, it is necessary to
observe separately Salvinia on the one hand, and the Marsiliaceae {Marsilia and
Pilularia) on the other.

FILICINE^.

449

The embryo of Salvima, as long as it is enclosed in the prothallium, forms, as
we have seen, the segments of its apical cell alternately above and below; but when
the apex of the stem is exposed in consequence of its elongation, a torsion takes
place to the extent of about 90°, so that the two rows of alternate segments of the
apical cell lie right and left, a peculiarity which has also been observed by Hof-
meister in Pteris aquilina. The first* leaf is the scutiform leaf mentioned above,
which is placed medio-dorsaily ; then follow a second and third aerial leaf standing
singly, after which the definite verticillate arrangement of the leaves at length
commences at the fourth node ; each whorl thereafter consists of a submerged leaf
springing on the ventral side (right or left), which at once branches, and forms a
tuft of long filaments hanging down into the water ; while two other leaves have
quite fiat laminae and spring from the dorsal side, touching the water only with their
under surface (Fig. 319). These three-leaved whorls alternate, and thus form two
rows of ventral submerged, and four rows of dorsal aerial leaves. Their succession

Fig. 315 a.— A the vegetative cone of the stem oi Salvinia nutans, represented diagrammatically and looked at from
above; xx projection of the plane which divides it vertically into a right and left half; the segments are indicated by
stronger outlines, their divisions by weaker lines ; the succession of the segments is denoted by the letters F—P;
B diagram of the stem with three whorls of leaves, its ventral side indicated by w; w the first-formed submerged leaf;
L\ the aerial leaf formed next ; 7.2 the second atrial leaf of the same whorl formed last of all between the two first (after
Pringsheim).

in age in the whorl, and the position of the whorls (antidromal among themselves),
are indicated in Fig. 315 «. The node of the stem which produces a whorl of
leaves is, as was shown by Pringsheim, formed of a transverse disc of the long
vegetative cone, which in its length (or height) corresponds to a half-segment, while
each internode corresponds to the whole height of a segment. Each nodal disc, as
well as each internode, consists of cells of the right and left row of segments of
different ages ; in Fig. 315 a an internode is formed of the segment H on the right
side, of the anterior half of the older segment, G, and of the posterior half of the
younger segment, J, on the left side ; the next internode is the product of the whole
of the left segment, Z, and of the two halves of K and M lying to the right ; the
intermediate nodal disc which forms the leaves w, Z^, L^ consists, on the other
hand, of the anterior half of the left older segment J and of the posterior half of the
right younger segment K\ in the preceding and succeeding node the relationships

Gg

450

VASCULAR CRYPTOGAMS.

are the same, right and left being transposed. In each whorl the submerged leaf
is the oldest, the one further from it of the two aerial leaves the second ; the nearer
atrial leaf is the last formed. Each leaf arises from a cell of definite position,
which becomes arched outwards (Fig. 316, ^, Z^, L^, and, becoming the apical cell
of the leaf, forms a row of segments on each side.

In the genus Azolla, which has been studied by Strasburger, the apical cell
of the horizontal floating stem which curves upward near its growing end gives
rise to a right and to a left row of segments, each of which is divided by a lateral
longitudinal wall into a dorsal and a ventral half. Each of these halves is divided
by a transverse wall into an acroscopic and a basiscopic portion, and each of these
four cells is further divided^ into two by an oblique or a vertical longitudinal wall.
The stem then consists (disregarding the subsequent divisions) of eight longitudinal
rows of cells which have been formed from two rows of segments. The two
dorsal rows remain sterile and form neither leaves nor buds. The two rows of
leaves lie right and left on the dorsal aspect, and from the neighbouring rows

Fig. 316.— Apex of the horizontal floating- stem oi Salvinia (after Pringsheim) ; A ventral side, 5 left side, C transverse
section of the long vegetative cone, ^S apical cell of the stem, y its last septum, -w submerged leaf, Z its lateral teeth,
L L aerial leaves, h h hairs.

of the ventral half, a little in front of or a little behind the leaves, arise the branches
of the stem. Finally, the two inferior ventral rows bear the roots, each of which
arises close to a bud and grows by means of a three-sided apical cell. If the
leaves marked L^ in Fig. 315 « be regarded as the only*ones present, the arrange-
ment of the leaves in Azolla is approximately represented, with reference to which
the buds and roots are placed in the manner above described. However, the
arrangement of the leaves differs from this diagram in so far that, in Azolla,
the leaves of the one row all arise from one cell of the acroscopic part of the
segment, whereas those of the other all arise from one cell of the basiscopic
part, in consequence of the position of the first leaf which always arises on the
inner side of a branch and is directed towards the parent stem. Between any
two leaves, which are placed alternately and in two rows, is an internode of the
length of half a segment, one side of the internode being formed from the basiscopic,
the other from the acroscopic half of a segment.

In Marsilia the apical cell of the embryo is so placed that dorsal and ventral

FILICINE^.

451

segments in two rows are at first formed from it by walls inclined upwards and
downwards ; the dorsal median leaf also proceeds from the first dorsal segment.
But a different arrangement is soon produced as the plant increases in strength;
the apical cell of the stem forms segments arranged in three rows with a J diver-
gence, and in such a manner that one row of segments comes to lie below (ven-
trally), while the two other rows form the dorsal side of the stem. The ventral side
of the stem forms roots in strictly aero petal succession, as in Azolla, the youngest
being found near the apex of the stem. On the dorsal side of the stem the

Fig. 317.— Anterior part of the stem oi Marsilia
Salvatrix with leaves (reduced one-half) ; A' terminal
bud. b b leaves.y/sporocarps springing from the leaf-
stalks at X.

Fig. -^i^.—Pzlularta globitlt/era ; A natural size, B end
of a shoot magnified, s terminal bud of the stem, deleaves,
IV roots, /'sporocarps, A' lateral bud.

leaves arise in two alternating rows, some of the dorsal segments remaining at
the same time sterile and serving for the formation of internodes. The first leaf
of the young plant, lying in the median line and without a lamina, is followed,
in the biseriate arrangement which now results, by a number of young leaves with
a short stalk and a lamina at first entire but afterwards divided into two and
four lobes ; normal leaves, circinate in their vernation, are then for the first time
formed with a long stalk and a quaternate lamina. In the processes which have
now been described, Pilularia agrees, according to Hanstein's observations, with

G g 2

452 VASCULAR CRYPTOGAMS,

Marsilia, except that all the leaves remain destitute of a lamina (Fig. 318); they
are long, conical, filiform, and at first rolled up spirally forwards.

The Branching of Rhizocarps is similar to that of Ferns. Pringsheim states
that in Salvmia terminal branching never occurs ; new shoots arise, on the contrary,
exclusively from the basal part of the submerged leaves, each leaf of this description
forming a shoot on the side which faces the nearer aerial leaf; every branch pro-
duces at once a trimerous whorl of leaves. Strasburger, however, considers it
possible that these branches arise, as in Azolla (see ante\ from the stem itself
close to the leaves. The branching of the Marsiliaceae has been termed by Hanstein
axillary, a designation with which, however, I am unable to agree. The lateral
shoots have altogether the appearance of springing from the stem itself near to
the leaves, but at a later period they appear to be lateral and not axillary to the
leaves. As to their first origin, which has not yet been accurately ascertained, it
appears to be most natural to refer to the relative positions of these organs in
Azolla, where the lateral branches of the stem arise from cells lying anteriorly or
posteriorly or even dorsally to the origins of the leaves.

The Growth of the roots of the Marsiliaceae and their monopodial branching
agree with that of Ferns and Equisetaceae in all important points. It has already
been mentioned that among the Salviniaceae, Salvmia itself is quite destitute of
roots, and that in Azolla the two ventral rows of cells of the stem give rise to
roots by the side of the lateral buds. The apical cell of the stem of these plants
forms segments in two rows, whereas that of the root is a three-sided pyramid
like that of the Marsiliaceae, Ferns, and Equisetaceae. A root-sheath is formed,
according to Strasburger, from the cells of the stem which lie over the endo-
genous mother-cell of the root of Azolla, which keeps pace with the growth of
the root and invests it during the whole of its existence. It is still more remarkable
that the root-cap of Azolla is formed from one single cell. From this cell two
layers of cells are derived which grow with the growth of the root and completely
invest it on all sides.

The Sporocarps. The formation of the fructification shows even more evi-
dently than the structure of the vegetative organs, that the two families of the
Rhizocarpeae present considerable differences in certain particulars, which demand
separate consideration, as well as an agreement in essential characteristics.

The SalviniacecB^ including the two genera Salvinia and Azolla, occupy with
respect to their fructification an intermediate position between the Ferns and the
Marsiliaceae. Their sporangia are enclosed in unilocular capsules, two or more
of which occur on the segments of the leaves (Fig. 319^, B). In Salvinia
these capsules are borne on the basal segments of the submerged leaves ; in Azolla
it is the external, downwardly directed segment of the deeply bisected leaf, and
moreover of the first leaf of each branch, which bears them. The apex of the
segment of the leaf which is to form the sporocarp grows out into a columella
(placenta) upon which the sporangia are developed, and at the same time an
annular wall arises round the base of the columella, and continues to grow until
it closes over its apex, thus forming the wall of the sporocarp by which the sorus
is completely enclosed. The sporocarp of the Salviniaceae closely resembles the
sorus of the Hymenophyllaceae, but differs from it in that the indusium of the

FILICINEM.

453

latter is cup-shaped and therefore open at the apex, whereas the wall of the former
completely encloses the sorus, as does the indusium of Cyathea. The sporocarp
of the Salviniaceae is then a sorus, the term being used in the sense in which it is
applied to the fructification of Ferns, but here the indusium is much more fully
developed, consisting of two layers of cells, the walls of which, toward the upper
part of the capsule, become lignified in Azolla.

Each sporocarp contains either microsporangia or macrosporangia, but both
kinds of sporocarp occur in the same plant and even upon the same leaf, so

Fig. ^Tg.—Salvtm'a nutans -, A transverse section of a stem bearing a whorl of leaves, I aerial leaves, iu submerged leaf with
several teeth, ysporocarps on it (natural size) ; B longitudinal section through three fertile teeth of a submerged leaf, a a sporo-
carp with macrosporangia, i i two sporocarps with microsporangia ; C transverse section of a sporocarp with microsporangia
■mi; D transverse section of an aerial leaf, hu hairs of the under side, ho hairs of the upper side, ep epidermis, I air-cavities, the
dark ones show the vertical walls of the tissue in the background (B—D X lo) ; E cells of a lamella of tissue in the leaf; F one of
the cells after contraction of the contents in glycerine.

that the plant is monoecious. The microsporangia are contained in Azolla^ as
in Salvinia, in large numbers within the sporocarp; in Azolla only a single
macrosporangium is formed in a sporocarp, whereas in Salvinia several macro-
sporangia are formed. All the spores derived from the mother-cells (i6) in a
microsporangium reach maturity, but only one of the (4x16) spores of a macro-
sporangium ever becomes ripe, so that in Azolla, where there is but one macro-
sporangium in a sporocarp, a single macrospore is enclosed by the wall of the

454 VASCULAR CRYPTOGAMS.

sporocarp and by that of the sporangium (which subsequently disappears ?). The
sporangia are capsules borne on long stalks, the wall of which, when mature,
consists of a single layer of cells. The macrosporangium is shortly stalked and
probably arises from the apex of the columella.

The development of the sporangia in Salvinia has recently been investigated
by Juranyi. The columella is covered with a layer of radially elongated cells, each
of which grows out into a papilla and forms the rudiment of a sporangium. The
papilla is divided by a transverse septum into a lower andean upper cell. The lower
one undergoes repeated transverse divisions and forms the long, segmented stalk,
which comes to consist of several rows of cells by means of longitudinal divisions in
the case of the macrosporangia only; the upper one assumes a hemispherical form and
gives rise to the body of the sporangium by means of divisions which are similar to
those occurring in the Polypodiaceae (see Fig. 305). A wall is formed consisting
of a single layer of cells, within which is a single tetrahedral cell, the archesporium.
From this, four flat segments are cut off which form a layer of cells containing
much protoplasm surrounding the tetrahedral central cell. The central cell, by
repeated division, gives rise to the sixteen mother-cells of the spores, and meanwhile
the layer investing it has undergone division into two layers which together form the
tapetum. Each of the sixteen mother-cells of the spores then divides into four
tetrahedrally placed spore-cells. Up to this point the processes are the same in
both micro- and macrosporangia. In the microsporangia all the sixty-four cells
develope ; they become separate and are scattered irregularly in the cavity of the
sporangium. The tapetum becomes disorganised and forms a frothy mucilage,
which subsequently hardens, in which the spores are enclosed. In the macro-
sporangia only one of the sixty-four rudimentary spores continues to develope ;
it becomes enormously enlarged so that finally it nearly fills the cavity of the
sporangium. A large nucleus lies at that end of the macrospore which is directed
towards the apex of the sporangium. During the growth of the macrospore the
tapetum, and subsequently all the other undeveloped spores, undergo disorganisation,
becoming converted into a frothy plasma which is in contact with the outer coat of
the macrospore, and is especially accumulated at its apex. It is this frothy plasma
which becomes hardened and forms a thick investment for the mature macrospore,
termed the Epispore (Fig. 311, A), which splits even during its formation into three
lobes over the apex of the spore, so as to permit of the subsequent outgrowth
of the prothallium.

Strasburger had already demonstrated the existence of this hardened frothy
mucilage in both kinds of sporangia of Azolla, but in this plant it assumes a far
more conspicuous appearance. In the microsporangia the mucilage looks like a
large-celled tissue, and forms from two to eight separate clumps [Massulce), each of
which encloses a number of microspores. In some species [A. filiculoides, Caro-
liniand) these massulae have their surfaces covered with hair-like appendages,
barbed at their free ends {Glochidia), by means of which they attach themselves,
when they have escaped from the sporangia, to the floating macrospores. The
macrosporangium of Azolla contains a single spheroidal macrospore, which does not
nearly fill its cavity, the whole surface of which is completely covered with a thick rugose
layer of this hardened frothy mucilage. It projects from the apex of the spore and runs

FILICINF.^.

455

out beyond it into a pencil of delicate filaments ; three (or three times three) large
masses of it are attached at this point. The frothy mucilage with its cavities full
of air thus forms a float for the macrospore, and bears on its surface the upper
part of the ruptured sporangium.

The sporocarps of the Marsiliacese are much more complex and more firmly
constructed than those of the preceding family. The sporocarp of Pilularia is
a shortly-stalked spheroidal capsule, standing apparently by the side of a leaf
towards its ventral aspect. Its morphological significance is not yet fully under-
stood. The capsule has a very thick hard
wall, consisting of several layers of cells,
and contains, besides a considerable quan-
tity of soft succulent parenchyma, hollow
chambers which extend from base to apex.
Pilularia globuUfera (Fig. 320) has four
such chambers, P. minuta two, and P.
americana three. The external wall of
each chamber bears a thickening along its
median line of the inner surface throughout
its whole length, and in this thickening
runs a fibrovascular bundle. On this
receptacle (placenta) the stalked sporangia
are borne, forming a sorus which consists
at its lower part mostly of macrosporangia,
at its upper of microsporangia. Probably
each chamber has, at an early stage of
development, an aperture at its apex, but

it is not clear here, any more than in Alarsilia, how far a comparison of the
delicate tissue which invests the sorus to an indusium is justifiable, a comparison
which is made by many botanists.

The sporocarps of the various species of Marsilia are usually somewhat
bean-shaped capsules with very hard walls, borne on long or short stalks, which
arise either upon the ventral aspect of the petioles of ordinary foliage -leaves (Fig.
317) or at their bases. Their stalks may be simple, bearing but one sporocarp, or
forked, bearing many sporocarps. From a petiole several usually arise together.
The stalk is continued along the posterior edge of the sporocarp (Fig. 325), giving
off" lateral branches right and left, which dichotomise and run towards the ventral
^^^<^. The ripe capsule is bilaterally symmetrical and contains two rows of chambers
each of which extends from the ventral to the dorsal margin (Fig. 321, A^ B), In
the very young fruit, according to Russow, each chamber communicates with the
exterior by means of a narrow canal opening on the ventral aspect. A thickening
(placenta) runs along the external wall of each chamber, bearing the macrosporangia
on its central ridge, and the microsporangia on its flanks ; thus each chamber
contains a sorus consisting of two kinds of sporangia. When the sporocarp bursts
it becomes evident (Fig. 325) that the soft internal tissue forms, as in Pilularia,
a completely closed investment for each sorus. When mature the microsporangia
contain numerous (4x16) spores, the macrosporangia but a single one.

Fig. 320. — Transverse section of the sporocarp oi Pilularia
globuli/era below the middle, where the macrosporangia and
microsporangia ma and mi are intermingled ; g: the fibro-
vascular bundles, h hairs, e epidermis of the outer surface.

I

45^

VASCULAR CRYPTOGAMS.

The development of the sporangium begins with the outgrowth of one of the
superficial cells of the placenta which bears the sorus. The subsequent divisions are
the same as those above described with reference to Salvinia, so that here also the
sporangium is soon elevated on a stalk and consists of a wall of a single layer
of cells and of a tetrahedral central cell or archesporium (Fig. 322, I-IIT). From
this a tapetum is cut off by four septa parallel to its sides, which, as in the
Salviniaceae and in the true Ferns, surrounds the cell from which the spores are
subsequently derived (Fig. 322, /F, V). The stalk of the sporangium of the
Marsiliaceae, in accordance with the foregoing description, consists at first of three
rows of cells, but these rows are afterwards multiplied by longitudinal divisions.
As the young capsule of the sporangium gradually increases in size, the cells of the
wall undergo radial division and the cells of the tapetum both radial and tangential.
The central cell by repeated bipartition forms the sixteen mother-cells of the spores,

Fig. 32t,— a very young sporocarp oi Marsilia data (after Russow); A a median longitudinal section ; B a transversa
section ; C part of a longitudinal section at right angles to A. ff fibro-vascular bundles, s s sori, sk canals of the sori,
fna macrosporangia, w»i microsporangia (compare Fig. 325).

each of which developes four tetrahedrally arranged spores in the usual way.
During this process the cells of the tapetum gradually undergo disorganisation, and
the cavity of the sporangium becomes filled with a granular plasma in which lie
the mother-cells and the tetrads of spores and from which the remarkable epispore is
subsequently formed. Up to this point the course of development in both kinds of
sporangia is the same, but differences now become apparent. All the spores of the
sixteen tetrads formed in the microsporangia reach maturity; each of the four spores
within a mother-cell ^ surrounds itself with a permanent coat, and then the wall of the
mother-cell becomes absorbed. In the macrosporangia one of the young spores of
each of the sixteen tetrads grows more vigorously than the other three ; finally all the
tetrads but one cease to develope, and the largest cell of this one — the future

^ Russow raises the objection, I. c. p. 62, that I make no mention of special mother-cells ii
describing the development of the spores. His mistake finds its correction in § 3 of Book I.

FILICINEJE,

457

macrospore — increases very much in size whilst the other three gradually shrivel.
Figures 323 and 324 illustrate the development of the macrospore of Pilularia
globult/era, after drawings made by me in 1866. They show the young macrospore
in /, //, III, still in connexion with its three sister-cells which are invested by the
cell-wall of the m.other-cell which has already become mucilaginous (7). The four
cells are attached to each other by means of rigid spine-like projections, that of the
macrospore being the most strongly developed. At a later period the macrospore is

Fig. 322. — Development of the sporangium of /"z'/w/arza
globulifera, all the figures in optical longitudinal section ;
JV c primary mother-cell of the spores invested by the
tapetum ; sm mother-cells of the spores (X 5So).

Fig. 323.— Development of the macrospore of Pilularia
globulifera ; x the abortive sister-cells, nt the macrospore, A' its
nucleus, a the inner, b the outer coat.

of very considerable size; its aborted sister-cells are attached to it laterally (Fig.
324, x), and its firm coat has become brown and is invested by a layer of mucilage
(Fig. 323, IV, b) which often appears to be folded. It forms a papilla (Fig. 324, b')
at the apex which shrivels when the spore ripens. A layer of a soft substance,
presenting a distinctly prismatic structure, makes its appearance (Fig. 324, C) on the
outside of the mucilaginous layer, and becomes in its turn invested by a third layer
which is thicker than itself of a less distinctly organised structure. Both these layers
leave the apex of the spore uncovered and form a funnel through which the

45«

VASCULAR CRYPTOGAMS.

antherozoids penetrate after germination (comp. Fig. 314)- The macrospores of
Marsilia possess a similar epispore, the development of which has been very fully but
not very intelligibly described and figured by Russow. According to him the muci-
laginous investment of the abortive sister-cells passes over to the macrospore and
forms the first layer of its epispore ; then a protoplasmic vesicle encloses the whole
macrospore, and within this the thick prismatic layers of the epispore are developed.
Russow's account confirms the view which I had already expressed that these
investing layers are deposited from without, a view which is rendered even more
probable by the facts kndwn concerning these processes in the Salviniaceae, but
it makes the whole process somewhat obscure, and I have no material at present for
a fresh study of the formation of the epispore in the Marsiliaceae.

The ejection of the macro- and microspores from the very firm sporocarps is
associated with some remarkable processes, for a knowledge of which we are
indebted to Hanstein. The ripe fruits of Pilularia glohulifera lie either in moist

Fig. 324.— Further development of the macrospore c^i Pilularia glolmlif era; h cavity of the spore, a the first inner coat, /' the
second, c the third, d the fourth coat ; the second, third, and fourth together form the epispore {X 80).

earth or upon it ; the wall splits at the apex into four valves, and a hyaline
viscid mucilage, evidently derived from the tissue separating the chambers, escapes
and forms a drop upon the surface of the soil which continues to increase in size for
some days. In this mucilage the microspores and the macrospores are carried out,
and they germinate in it. When fertilisation has taken place the drop of mucilage
becomes fluid, and leaves the fertilised macrospores on the moist ground to which
they are attached by the root-hairs of their prothallia until the first root of the
embryo penetrates it. Fig. 325 represents the most important of the corresponding
processes in the case of Marsilia Salvairix. If the sporocarp, which is as hard as
a stone, be slightly injured along its ventral margin and be then placed in water, the
water gains access to the interior and causes the tissue forming the walls of the
chambers to swell-up, and the result is that the testa splits along its ventral side into
two valves. It is shown in Fig. 325, B^ that a hyaline ring protrudes, which lay pre-
viously in the angle along the ventral edge, and carries with it the chambers

FILICINE^.

459

containing the sori which are less capable of swelling-up. As the ring gradually
expands the attachments of the chambers to the dorsal margin of the sporocarp
become ruptured and they are drawn completely out of the testa. As a rule the
attachment of the ring is broken through at one end, it then straightens itself and
bears the chambers, which are still closed sacs, in two longitudinal rows, there being
now a considerable interval between each pair of chambers, whereas they were closely
packed together whilst still within the testa. These processes are completed in a few
hours, both kinds of spores are set free, and, if the temperature be favourable,
fertilisation takes place within from twelve to
eighteen hours after placing the sporocarp in
water.

(a) Histology'^. The development of the tissues
of the Rhizocarps agrees in all essential points
with that of the true Ferns. The growth of the
stem, of the root, and to a certain extent of the
leaves, by means of an apical cell, is as evident
here as in the Characeae and Equisetaceae, and
has been thoroughly studied. The epidermis
presents several peculiarities, more especially as
to the stomata. The fundamental tissue is re-
markable, as is usually the case with water or
marsh plants, for its large intercellular spaces.
For information as to the occurrence of scleren-
chyma in the leaves and in the testae, the me-
moirs of Braun and of Russow must be consulted.
The fibrovascular bundles, more especially those
of the Marsiliaceae, very closely resemble those
of the true Ferns in their composition. There
is a central xylem entirely surrounded by phloem,
and this again is invested by a single layer of
cells with folded lateral walls forming the bundle-
sheath. A single bundle traverses stem, root, and
leaf. In Marsilia, the bundle branches in the leaf,
forming a dichotomous venation. A transverse
section of the stem of the Marsiliaceae shows the
fibrovascular mass forming a ring surrounding a
central mass of fundamental tissue. The fibro-
vascular mass consists evidently of several bundles
which have coalesced ; that this is so is indicated
by the fact that the phloem of its inner side is
bounded by a bundle-sheath just as the outer

side is. In Pter'is aquilina it often happens that the two wide cauline bundles of the
stem unite laterally so as to form a tube enclosing a medulla.

(b) Classification. It is already evident from what has been already said, that the
Rhizocarpeae belong to two clearly-defined families, of which the one, the Salviniaceae
is closely allied to the true Ferns, whereas in the other, the Marsiliaceae, the especial
characteristics of the order find their most complete expression.

Family i. SalviniaeesB. Plants floating horizontally on the surface of water. The

Fig. ^zi,.— Marsilia Salvatrix; A a sporocarp
(natural size), st the upper part of its pedicel; A' a
sporocarp which has burst in water and is protruding
its gelatinous ring (after Hanstein) ; C the gelatinous
ring .^ ruptured and extended, sr compartments of
the sporocarp ; sch testa of the sporocarp ; D a com-
partment of an unripe sporocarp with its sporangia ;
E one from a ripe sporocarp ; mi microsporangia,
ma macrosporangia.

^ [For further details on this subject see De Bary, Vergleichende Anatomic der Vegetations-
organe, 1877.]

460 VASCULAR CRYPTOGAMS.

apical cell of the stem forms two rows of segments, a right and a left. Sori either male
or female : each sorus contained in a unilocular sporocarp. Spores invested by hardened
frothy mucilage (Massulae, Epispore). The prothallium derived from the macrospore
is well developed and bears several archegonia. Azolla has roots, Salnjinia has none.

Family 2. MarsiliacesB, Plants creeping on moist earth, or floating to some extent
in water. The three-sided apical cell of the stem forms two dorso-lateral rows, and
one ventral row of segments. Each sorus includes both macro- and microsporangia,
and two or more sori are contained in the multilocular sporocarp. The spores are in-
vested by hardened mucilage — epispore — which presents a radially prismatic structure,
and is to some extent capable of swelling-up. The prothallium of the macrospore bears
a single archegonium. This family includes the genera Marsilia and Pilularia.

CLASS IX.

DlCHOTOME^ ^.

Hitherto the Lycopodieae, Psilotum, the Selaginellae, the Isoetese, etc., have
been included together in the group of the Lycopodiaceae, and rightly so, for
these genera exhibit not only in their habit, but also in their morphology, a degree of
relationship which makes it impossible to separate any one of them from the others
with the view of erecting it into a new class or of including it in one of the other
two classes of Vascular Cryptogams. Recent investigations have, however, shown
that this class includes two well-defined subdivisions which must be kept distinct.
To one of these two orders belong the genus Lycopodium and its immediate allies,
and it must therefore necessarily take the name of Lycopodiaceae; consequently
a new name has to be found for the whole class, and I select that of Dichotome^,
because it brings into prominence one of the most obvious of the characteristics
of these plants, that the branching presents the appearance of being the result of
dichotomy, although, as a matter of fact, the branching of the stem is monopodia!.
It must be remembered, however, that these two modes of branching gradually pass
one into the other. These plants are as remarkable among vascular plants for their
evident tendency to branch dichotomously as are the Equisetaceae for their whorls of
leaves. That dichotomy is indeed a typical peculiarity of these plants is proved by
the fact that their roots are the only ones at present known which branch
dichotomously.

The dichotomous habit of the branching is by no means the only characteristic
common to the members of this class, as a comparison with the Equisetaceae and
Filicineae at once shows. They possess but Htde in comm^on with the Equisetaceae
except perhaps the relatively slight development of the leaves, although that is
brought about in this class in a very different way, for the similarity in the mode of
development of the sporangia in the two classes does not merit consideration since,
as we have seen in the Filicineae, this is a characteristic which is critical only for the

^ [Inasmuch as dichotomous branching is not universal even among the Lycopodieae, many
authors prefer to designate this group of plants as Lycopodinse.]

FILTCINEJE. 461

smaller subdivisions. It is easier to find points of connection between this group
and the Filicineae, such, for instance, as the similarity in the composition of the
fibrovascular bundles, and certain modifications of the fundamental tissue which
occur in the two classes, but the external conformation is the most important point
to be considered as an indication of relationship. In the Filicineae the well-developed
leaves form the principal morphological feature, whereas in the Dichotomese they are,
though numerous, of very simple structure and of small size ; in the one class the
external conformation depends upon the stem, in the other upon the leaves. It is of
great importance to note that each leaf, in the Dichotomeae, produces a single
sporangium, whereas each leaf, in the Equisetaceae and Filicineae, bears several
sporangia. In this group the sporangium arises typically near to the base of the
leaf, and it is of but little consequence that in some cases it arises further back
from the axil of the leaf or even from the stem ; this is merely an example of an
occurrence common among Phanerogams that organs of the same morphological
value (in their case the branches) arise at one time at the base of the leaf, at another
in its axil, or even from the stem above the axil. Although it has already been
pointed out that the mode of development of the sporangia is insufficient to define
the principal subdivisions of the Vascular Cryptogams, it must be remarked here that,
in contrast to the various modes occurring in the Filicineae, the mode of develop-
ment of the sporangia is constant, even when the other morphological characteristics
are widely diff'erent; here the sporangium is always developed from a group of cells.
A short definition of the two orders — I. Lycopodiaceae, II. Ligulatae— is given at
the close of the introduction to the Vascular Cryptogams.

Order I. Lycopodiace^ \

I. The Sexual generation (Oophore). The conditions which are necessary for
the germination of the spores of Lycopodium, Psiloium, etc., are as yet unknown. In
spite of the very numerous attempts made to cultivate them only one observer,
De Bary, has succeeded in obtaining the early stages of the development of the
prothallium of Lycopodium inundatum. The endospore protruded as a spheroidal
vesicle from the exospore which had split into three valves : the vesicle was then
divided by a transverse septum into an inner basal cell, which underwent no subse-
quent alteration, and an outer cell which grew like an apical cell and formed two
rows of segments. Each segment was then subdivided by a tangential wall into an

^ Bischoff, Die cryptog. Gewachse. Niirnberg 1828. — Spring, Monographic de la famille des
Lycopod. (Mem. de I'Acad. roy. Beige, 1842 et 1849). — Cramer, liber Lycop. Selago in Nageli und
Cramer, Pfl. phys. Unters. Heft 3, 1855. — ^^ Bary, iiber die Keimung der Lycop. in Ber. d. naturf.
Ges. zu Freiburg-i-Br. and Ann. Sci. Nat. 1858. Heft IV. — Nageli und Leitgeb, iiber die Wurzeln,
in Nageli's Beit, zur wiss. Bot. Heft 4, 1867, — Payer, Botanique cryptogamique. Paris 1868. —
Hegelmaier, Bot, Zeit. 1872. — Russow, Vergl. Unters. Petersburg 1872. — Mettenius, \xher Phyllo-
glosstim, Bot. Zeit. 1867.— Juranyi, \xh&x Psilohim. Bot. Zeitg. 1871. — Fankhauser, Bot. Zeitg. 1873. —
Strasburg-er, Bot. Zeitg. 1873. [Arcangeli, Studii sul Lycopodium Selago, 1874.— Prantl, Bemerk. lib.
d. Verwandtschaftsverhaltnisse der Gefasskryptogamen ; Verb. d. phys.-med. Ges. zu Wiirzburg,
1875.— Braun, Blattstellung und Verzweigung bei Lycopodium, Sitzber. d. bot. Ver. d. Prov. Branden-
burg, 1874. — Hegelmaier, Zur Kenntniss einiger Lycopodinen, Bot. Zeitg. 1874. — Bruchmann, Ueb.
Anlage u. Wachsthum der Wurzeln von Lycopodium, Jenaische Zeitschrift, 1874.]

k

46a

VASCULAR CRYPTOGAMS.

inner and an outer cell, so that at this stage the young prothallium consisted of four
short cells forming an axial row terminated above by the apical cell, below by the
basal cell, and enclosed laterally by two rows of cells. It was impossible to trace
the development any further. Fifteen years later (1872) Fankhauser found in
Switzerland completely developed prothallia oi Lycopodium annotinum among some
Mosses, one of which was still connected with the young plant of the second
generation (Fig. 326). These prothallia, which had grown in the absence of light,
were irregularly lobed masses of cells of a pale yellow colour, furnished sparingly
with small root-hairs. On the upper surface were numerous antheridia which are
ovoid cavities in the tissue of the prothallium covered by a single layer of cells and
filled with the very numerous mother-cells of the anthero-
zoids. The form of the antherozoids themselves was not
very clearly made out. These prothallia had no longer
any archegonia, but they bore young plants : hence it ap-
pears that Lycopodium produces only one kind of spores, a
conclusion which is quite in accordance with the results of
direct observation, and that the prothallia are monoecious,
a peculiarity which at once sharply distinguishes the Lyco-
podiceae from the Isoeteas and Selaginellese, as does also
the very considerable size attained by the prothallium and
its complete independence of the spore. Probably these
conditions are the same in those genera which possess but
one kind of spores, Psilotum, Tmesipteris, Phylloglossum.
The prothallia of Lycopodium evidently bear several arche-
gonia, for Fankhauser found on them young plants in
various stages of development. From the attachment of the
young plant to the prothallium, it appears that the arche-
gonia lie upon its upper surface in the grooves between the
lobes.

2. The Asexual generation (Sporophore). From what has
already been said it is evident that nothing is known con-
cerning the development of the embryo. The young plants
which Fankhauser found were attached to the prothallium
by means of a small projection, about the size of a pin's
head inserted into its tissue. This projection arises at the
side of the base of the stem and of the first root ; it evidently corresponds to the foot
of Ferns.

The habit of the mature plants is very various in the different genera. Some
species of Lycopodium have an erect stem and erect branches (Z. Selago), in which
case the roots arising from the lower part of the stem often grow downwards within
it and issue in a tuft from its base (Z. Phlegmaria, ulicifolium, &c.). Very frequently
the main stem and its largest branches creep upon the earth, sending out roots here
and there into the soil, only certain branches, more especially those which bear the
sporangia, being erect. In these forms there is a tendency towards bilateral
organisation, which finds expression more particularly in the structure of the axial
fibrovascular mass. In all species the stem is thickly covered with small, narrow,

FIG. -i-id.— Lycopodium a7ino-
tinum, after Fankhauser; p the
prothallium, I the young plant ;
•w its root (natural size).

FILICINEM.

4^3

sometimes elongated leaves. The differences of habit are chiefly due to the greater
or less development of one or -other of the branches of the bifurcations. The
sporangia are borne in L. Selago in the axils of ordinary foliage-leaves, but usually
they are borne in the axils of leaves which are peculiar in shape and colour, and
which form terminal spikes on special fertile branches which are frequently curiously
modified.

In Psilotum the stem is thin, much branched, and developed throughout in
a bifurcate manner, so that the plant presents the appearance of a straggling
shrub. It possesses no true roots, but a number of underground branches dis-
charge the functions of roots. The leaves are few, and are but small pointed scales
even upon that part of the stem which is above ground. The sporangia are
borne three or four together on small, short, lateral shoots of the branches, and
do not form a definite fructification. Similar arrangements occur in T?nesipteris,
but here the leaves are much larger. With these genera is associated Phyllo-
glossujn, a small Australian plant only a few centimetres high, which differs from
them considerably. It consists of a stem arising from a small tubej, and bearing
at its lower part a rosette of a few long leaves and one or more lateral roots; it
is prolonged above this as a thin scape and terminates in a spike of small leaves
bearing the sporangia. The plant is propagated by means of adventitious shoots
consisting of a tuber with a rudimentary leafless bud : in this respect it resembles
our native Ophrydese.

The development of the vegetative organs is only thus completely known with
reference to native Lycopodieae; our knowledge of the other genera is very frag-
mentary. We are indebted for the most valuable information to the labours of
Hegelmaier, of Leitgeb, and of Nageli. According to Cramer, Pfeffer, and Hegel-^'
maier, and this is corroborated by my own observations, the growing end of the stem
of Lycopodium does not possess a single apical cell ; this is true also of the leaf and
of the root. The growing point of the stem consists of a small-celled primary
meristem which does not in all cases present a distinct differentiation of dermatogen
and periblem, but there is a distinct axial plerome which extends to close below the
apex. In L. Selago the apex is flat, but in L. complaiiatum^ clavatum, annotmum,
alpinum, &c., it projects beyond the youngest leaves in the form of a dome. Just as
in Phanerogams, the leaves and the rudimentary branches are not developed from
single cells of the growing-point, but from groups of cells which include both the
superficial and deep layers of the primary tissue.

The branching of the stem of Lycopodium may be generally described as being
dichotomous, although, according to Hegelmaier, the first development of a branch
begins in many cases at the side of the growing apex of the stem. In all cases,
however, this takes place above the youngest leaves without any reference to their
position, and herein it differs from the branching of Phanerogams. According to
Cramer two new growing points of equal activity are formed side by side upon the
flat apical surface of Z. Selago, and they in turn develope dichotomously, and
Hegelmaier found the same to be the case in the vegetative shoots of heterophyllous
species (Z. complanalum, chamcecyparissus). In these cases then true dichotomy
occurs. On the other hand, the rudiment of the branch appears as a lateral pro-
tuberance from the projecting growing point of the vegetative shoots of Z. clavatum

I

464 VASCULAR CRYPTOGAMS.

and annoitnum and of the creeping shoots of L. inundatum; yet it arises in such
a manner that the branching maj' be regarded as being closely allied to dichotomy.
The bifurcation of the fertile branch of L. alpinum presents another form of
dichotomy (false dichotomy), for here the growing-point is extended by the develop-
ment of two new growing points, one on the right and the other on the left side of
it; the central one then ceases its activity, whereas that of the two lateral ones
commences, so that the apex of the parent shoot becomes indistinguishable. Those
Lycopodieae which have their leaves arranged in four rows, and which resemble the
Selaginelleae (Z. complanaium, chamcEcyparissus)^ branch in one plane only, which
coincides with that of^the larger leaves; whereas those species which have their
leaves arranged spirally or in whorls, branch in various intersecting planes. Hegel-
maier recognised the bulbils which occur in several species (Z. Selago, lucidum,
reflexuni) as being peculiarly modified branches. They arise on the shoot in the
place of leaves. They fall off of themselves, and are provided with a few leaves and
a rudimentary root.

Since the leaves are closely placed by the side of and above each other from the
first, so as to cover the whole surface of the stem, there are no internodes (as also in
Ophioglossum, Marattia, Aspidium, Isoeies) ; and not only so, but the outer cortical
layer of the stem is genetically connected with the tissue of the bases of the leaves.
The leaves become separated the one from the other by subsequent intercalary
growth, and in many cases the base of the leaf becomes sharply defined from the
stem.

The first rudiments of the leaves of Lycopodiese appear as multicellular lateral
protuberances of considerable breadth upon the growing apex of the stem. They
grow at first apically, but this soon terminates in the formation of a hair-like
prolongation ; all further growth is intercalary at the base of the leaf. The size and
form of the leaves are very different in the various species, but they are always
simple unbranched, and not stalked but sessile with a narrow base. Occasionally
their surfaces, with the exception of the free apices, are closely applied to the stem
(as is the case in Thuja), but more commonly the leaves are quite free. They are
acicular, or at any rate of small width, and, as in all Dichotomeae, a mid-rib only and
no lateral veins is present.

The phyllotaxis is sometimes verticillate, sometimes spiral, and both kinds may
occur on the same plant. The whorls may consist of pairs of leaves which
decussate, or of three, four, or more, and are arranged in creeping stems on zones
the planes of which are oblique to the long axis of the stem. The number of leaves
in a whorl varies even on the same shoot. According to Hegelmaier, the whorls are
true ones ; the leaves composing them are developed simultaneously and at the same
level at the puncium vegetaiionis. Where the phyllotaxis is spiral it is so from the
first, and the divergence undergoes no important modification. The small and very
various divergences of the leaves are remarkable, as Braun pointed out ; he found in
Z. clavatuvi the following divergences, f , y^j, ^^^ y^g-, y2_^ as well as whorls consisting
of from four to eight leaves ; in Z. annotinum they were f , f , and whorls of four or
five leaves; in Z. inundatum f and whorls of five leaves (Bot. Zeit. 1872, p. 815).

The much bifurcated, multiangular, thin stem of Psilotum grows by means of
a three-sided apical cell, which, according to Nageli and Leitgeb, forms (in the

FILICINEJE. 465

subterranean shoots) three spiral rows of segments, the segmental walls advancing
in the anodal direction, as in many Mosses. The small widely separated leaves,
which have no fibro-vascular bundles, are borne upon the angles of the stem and bear'
no apparent relation to its branches.

Psilotum tri'quetrum is a plant perfectly destitute of roots, forming however a
number of underground shoots which serve the purpose of roots and are extremely
similar to them. On the shoots of the rhizome which approach the surface of the
ground may be detected with a lens minute leaves of a whitish colour and acicular
shape ; the deeper root-like shoots ha^e a blunter end, on which no trace of leaves
can be detected, even with the lens. While the anatomical structure of the super-
ficial shoots corresponds to that of the true stem of these plants, in these deeper
shoots the vascular bundles are united into an axial group, as in true roots. The
shoots which bear visible rudiments of leaves may turn upwards, become green and
transformed into ordinary foliage-shoots, while the root-like shoots, which, are more
slender, may also turn upwards, become thicker, and assume the appearance of
the ordinary superficial rhizome-shoots. In this point therefore they differ at once
from true roots, but still more in the absence of a root-cap. They terminate in an
apical cell, which forms oblique segments alternating in different directions. The
most important point, however, is that these shoots really possess rudiments of leaves
which consist of only a few cells and do not project above the surface, but remain
concealed in the tissue. They are best recognised in longitudinal section, when
they are seen to consist of an apical cell and from two to five cells with the
characteristic arrangement of leaf-cells. Similar rudimentary leaves consisting of
but few cells occur also on the ordinary rhizome-shoots, where, however, they do
not undergo further development, especially when the end of the shoot appears
above ground. The root-Uke shoots branch like the ordinary ones ; a cell is /;ut
off by an oblique wall from one of the youngest segments, and forms the apical
cell of the new shoot.

The other genera all possess true roots. In the Lycopodieae with creeping
or climbing stems they arise singly and dichotomise in rectangularly intersecting
planes in the soil. It has already been mentioned that in the Lycopodieae witn
erect stems, such as in Z. Selago^ Phlegviaria, ulicifolium, the roots issue in a tuft
from the base of the stem which is somewhat tubercular. These roots originate high
up in the stem, as high as five centimetres and even above the first bifurcation
according to Strasburger ; they develope at the periphery of the axial fibro-vascular
mass, but they are peculiar in that they grow down through the fundamental tissue of
the stem and even dichotomise there. (Compare with Angiopteris, p. 417.)

The Sporangia, in the genus Lycopodiuvi, occur singly on the bases of the
leaves or in their axils. As in all Dichotomeae, they are here larger than in the
Ferns. They are borne on short broad stalks, and the capsule is somewhat reniform,.
its longer axis lying transversely to that of the leaf. They open by means of a slif
running in this direction over the apex, two valves being formed which remained
united at the base. The contained spheroidal or tetrahedral rather small spores are
numerous ; they all have the same shape and are provided with a sculptured exospore.
After I had pointed out in the first edition of this book (1868) that the sporangia of
the Lycopodieae originate as multicellular protuberances of the tissue pf the leaves,.

Hh •

466

VASCULAR CRYPTOGAMS.

Hegelmaier and Russow have confirmed and extended the observation. They are
developed from a small group of epidermal cells, as Goebel has shown, and, in con-
sequence of repeated divisions, they soon appear as flat projections occupying the
whole breadth of the base of the leaf, consisting of a group of internal cells covered
by an external layer. Tangential divisions take place in the epidermal cells by means
of which the wall becomes two-layered, and the upper or external portion of the
tapetum is formed, the tapetum being completed by similar divisions taking place in

Fig. 327. — A dichotomoiisly branched fertile shoot oi Lycopoditun Chaniiecyparissus, in longfitudinal section, shghtly magnified;
yy the axial fibrovascular mass, bb leaves, ss the young sporangia.

more deeply placed cells : the archesporium probably consists of a transverse row of
cells; these undergo division and form a rounded mass of spore-mother-cells.
These cells become isolated, their walls undergo considerable thickening, and, after an
indicated division into two they form four chambers (the so-called ' special mother-
cells '), within each of which the contained protoplasm surrounds itself with a
permanent spore-wall. It is not until the projections, spines, &c., have been
developed upon this wall that the walls of the chambers of the mother-cells are
absorbed.

filicineM. 467

The nature of the sporangia of Psilotum and of Tmesipleris is in many respects
obscure. In Psilotum the short branches which bear the apparently trilocular
sporangia^ arise as lateral papillae on the growing point, and, according to Juranyi,
are provided with a three-sided apical cell like the vegetative branches. A fibro-
vascular bundle runs from that of the parent shoot to the papilla, but does not
extend beyond the half of its height. The two small leaves of this fertile branch,
which were formerly regarded as being the segments of a single leaf, arise separately
from the papilla and coalesce at a later period. Even at a tolerably advanced stage
the papilla still consists of undifferentiated tissue which, as in the case of the anthers
of Phanerogams, forms parietal layers and three groups of spore-mother-cells ; in
this way three loculi are formed, which are separated by longitudinal walls and an
axial mass of tissue, and which project considerably on the exterior. I regard these
three loculi as so many sporangia which are formed in the apical part of the fertile
shoot to which the axial fibro-vascular bundle extends. In Tmesipteris the spo-
rangium, which is apparently divided into two loculi by a transverse septum, is borne
upon a small lateral branch which bears a leaf to the right and to the left.

Fig 328.— Transverse section of tlie stern of J.ycoj>odiU7n Chamaxyparissiis (x iSo).

Histology'^. The Epidermis of the leaves of L. annotinum, cla'vatum, and Selago,
is provided with stomata on both surfaces; the stomata are often arranged in small
groups. The leaves of the heterophyllous species which have them arranged in four
rows, possess stomata on their inner surface; stomata occur on the outer surfaces
of those portions of the leaves which adhere to the stem and which are directed
towards the earth. The epidermis of the root is sometimes strongly cuticularised, as
in L. cla'vatum.

The Fundamental Tissue of the stem consists of cells which are sometimes thin-walled
throughout, as in L. inundatum, but usually the inner layers have thick walls, and are

^ [This has been cleared up by Goebel (Beitr. z. vergl. Entwick. der Sporangien, Bot. Zeit.
1881). He finds that the 'trilocular sporangium' oi Psilotum is really a group of three sporangia,
each of which contains primarily a unicellular archesporium, from which a two-layered tapetum is
subsequently derived, as also the mother-cells of the spores. The ' loculi ' of the apparently bilocular
'sporangia oi Tmesipteris are, like those oi Psilotum, which they closely resemble in their development,
really distinct sporangia.]

^ [For further details see De Bary's Vergleichende Anatomic, 1877.]

H h 2

468 VASCULAR CRYPTOGAMS,

prosenchymatous, or even sclerenchymatous, though they never become brown like
those of the Ferns (Fig. 328).

The axial fibro-vascular cylinder is sharply defined from the ground tissue by a well-
developed bundle-sheath consisting of from one to three layers of cells. In the leaves
of the heterophyllous species air-cavities exist, and they are found also in the stem
of L. inundatum. In this species Hegelmaier found gum-canals in the stem and leaves
(one in the mid-rib), which are formed by the gradual separation of cells from each
other. The bordering cells project into the canal like varicose hairs. In L. annotinum
such canals occur only in the fertile branch.

The Fibro-'vascular Bundles of the Lycopodieae are very characteristic. In the stem
and in the root there is a large axial cylinder which has usually a circular outline.
In this (Fig. 328) lie bands of xylem which are either isolated or united in various
ways so as to form figures which would be symmetrically bisected by an axial longi-
tudinal section. Transverse sections taken at different heights show the xylem arranged
in different patterns, for the bands anastomose in their course. These bands of xylem
consist, like those of Ferns, of tracheides which are pointed at the ends, which are
wider in the centre than at the edges of the bundle, and which, when they are narrow,
have round pits, when broad, the pits are fissure-like. Small spiral vessels (proto-xylem
cells) are to be found at the edges of the bands of xylem. In creeping and oblique stems
the concavity of the bands is always directed upwards. These bands are embedded in
a mass of small-celled phloem, in which rows of wider cells lying between the xylem
bundles occur. Although, according to Hegelmaier, these cells possess no sieve-plates,
they may be regarded as the representatives of the sieve-tubes.

The ' proto-phloem cells' (bast-fibres) lie toward the exterior between the ends
of the xylem bundles. The arrangement of the elements recalls that of the axial
cylinder of roots. Within the bundle-sheath are several layers of rather large cells
which invest the peripheral phloem; to these Hegelmaier has given the name of
* phloem-sheath,' and probably they truly correspond to the layer occurring in Ferns
to which the same name has been given. I maintain the view which I formerly ex-
pressed that the axial cylinder of the stem of the Lycopodieae consists of several
fibro-vascular bundles which have coalesced ; for Hegelmaier's argument against it, that
the bands of xylem are not isolated throughout their whole extent, is by no means
conclusive, and moreover the resemblance between the axial cylinder of the stem and
that of the root supports my view. Each leaf contains a single thin fibro-vascular bundle
of very simple structure, which runs very obliquely from the base of the leaf through
the cortex, to become connected lower down with the margin of one of the xylem
bundles of the stem. The leaves of Psilotum contain no fibro-vascular bundles. Ac-
cording to Russow the xylem of the axial fibro-vascular cylinder of the stem forms
an angular hollow cylinder at the projecting angles of which are groups of narrow spiral
vessels.

The axial cylinder is cauline : it can be followed in the procambial condition to just
beneath the apex of the stem. The rows of spiral cells are first formed within the
xylem bands, with which the similar elements in the bundles of the leaves become con-
nected (Fig. 327) long before the development of the tracheides.

Order H. Ligulat^^

I. The Sexual Generation (Oophore). Like the Rhizocarpese among the
Filicineae^ the Ligulatae, including the genera Selaginella and Isoeles, are distinguished

'-%

^ Hofmeister, Vergleich. Unters. 1851. — (Germination, Development, and Fructification of the
Higher Cryptogamia, Ray Soc.) — Hofmeister, Entwick. der Isoetes lacustris, in Abhand. d. Konigl.
Sachs, der Wiss. IV. 1855. — Nageli und Leitgeb, liber Entstehung imd Wachsthum der Wurzeln, in

FILICINEM,

46g

among the Dichotomeae in that they possess spores of two kinds, the macrospores
and the microspores. As in the Rhizocarpeae so here the carrying back of sexual
differentiation to the development of the spores is associated with this pecuHarity,
that the spores in germinating seem to attain their object, the formation of re-
productive organs, as directly as possible, for the prothallium is not a plant capable
of independent growth but a development of tissue within the spore. The mode
in which this is brought about in the Ligulatae differs in essential points from
that obtaining among the Rhizocarpeae.

The Microspores of Isoetes and Selaginella do not produce the mother-cells of
the antherozoids immediately from their contents, as was formerly thought. To the
treatise of Millardet mentioned in the foot-note we owe our knowledge of the fact,
so important in connection with the relationship of the higher Cryptogams to the
Gymnosperms, that at the period when the microspores are ripe, their contents are
transformed into a mass of tissue consisting of but few cells. One of these cells

Fig. 329. — Germination of the microspores of Isoetes lacustris (after Millardet). A and C microspores seen on the right
side, B and D on the ventral face ; A and B show the formation of the antheridium, S S its dorsal cells, ^ fi its ventral cells,
C and D the formation of the antherozoids, /S and S have disappeared : v is the vegetative cell (prothallium of Millardet) ;
«— ^/"development of the antherozoids (A-^D and a—d X 580, e andy X 700).

remains sterile, and may be considered a rudimentary prothallium ; while from the
others originate the mother- cells of the antherozoids, and these may therefore be
looked on as a rudimentary antheridium.

The microspore of Isoetes lacustris breaks up, after hibernation, into a very
small sterile cell and a large one comprising the whole of the rest of the contents
(Fig. 329 A — C). The former {v\ cut off by a firm wall of cellulose, does not
undergo any further considerable changes ; the latter, on the other hand, splits up
into four primordial cells without cell-walls, of which the two ventral ones produce

Nageli's Beitr. z. Wiss. Bot. IV. 1867. — A. Braun, iiber /soeV^s, in Monatsber. d. Berl. Akad. 1863. — •
Milde, Filices Europse et Atlantidis, Leipzig 1867.— Millardet, le prothallium male des crypt, vase.
Strasburg 1869.— Pfeffer, Entw. des Keims der Gattung Selaginella in Hanstein's Bot. Abhand. IV.
1871.— Janczewski, Bot. Zeitg. 1872, p. 441. — Tschistiakoff, liber Sporenentwickelung von Isoetes, in
Nuovo Giornale bot. Ital. 1873.— Russow, Vergl. Unters. Petersburg 1872.— [Braun, Ueb. Blatt-
stellung und Verzweigung bei Selaginella, Sitzber. d. bot. Ver. d. Prov. Brandenburg, 1874. — Hegel-
maier, Zur Kennt. einiger Lycopodinen, Bot. Zeitg. 1874. — Treub, Recherches sur les organes de la
vegetation du Sdaginella Martensii, 1877.]

47© VASCULAR CRYPTOGAMS.

each two antherozoid-mother-cells, and therefore four in all. Pfeffer has confirmed
the statements of Millardet that in Selaginella, long before the spores escape from
the sporangium, a small sterile cell is first of all separated by a firm wall, while the
other large cell breaks up into a number (6 to 8) of primordial cells (Fig. 331 A — D).
He found, however, their arrangement different in Selaginella Mariensii and caulescens
from that which Millardet described in the case of S. Kraussiana, a variation which
seems immaterial when compared with similar differences in the antheridium
of Ferns. The essential difference between the results of the two observers con-
sists in this : — that, according to Millardet, only two of the primordial cells produce
the mother-cells 'of the antherozoids, which then, increasing in number, cause the
absorption of the rest of the primordial cells, and fill up the spore; while Pfeffer found,
in his species, that all the primordial cells underwent further division, and con-
tributed to the formation of the antherozoids. As to the mode of development
of the antherozoids they were both in accordance. In Isoetes the antherozoids
are long and slender, attenuated, and splitting up at both ends into a tuft of long
slender cilia; in Selaginella they are shorter, thick behind, finely drawn out in
front, and divided there into two long fine cilia. In the perfectly mature condition

Fig. j,y:i.—Isoetes lacustris (after Hofmeister) ; A inacrospore, two weeks after its escape from the sporangium, rendered
transparent by glycerine (X 60) ; B longitudinal section of the prothalliuin four weeks after the escape of the macrospore,
a archegonium (X 40)-

the antherozoids are rolled up into an elongated helix or into a short spiral. The
mode of their formation in the mother-cells is the same in both genera, and agrees
in essential points with that of Ferns. A cell-nucleus is not present at the time
when the antherozoid is first formed; the contents of the cell are perfectly homo-
geneous ; the antherozoid originates from a shining scarcely granular mass of
protoplasm which encloses a vacuole, the cilia at one end being formed first, and
the spiral body becoming differentiated from before backwards by a kind of split-
ting of the protoplasm. The antherozoid is originally curved spirally round the
central vacuole ; this latter, surrounded by a fine membrane, not unfrequently
remains attached to the posterior end of the antherozoid after it has escaped, and
is carried along by it. The movement does not last longer than five minutes in
the antherozoids of Isoetes^ in Selaginella from one-half to three-quarters of an
hour. From the commencement of germination till the complete maturity of the
antherozoids there is, in Isoetes, an interval of about three weeks ; the same period
from the dissemination of the spores is necessary in Selaginella.

The Macrospores produce the female prothallium, which is an endogenous struc-
ture in a still higher degree even than is the case in the Rhizocarps. In this respect

FILICINE^.

471

and in the mode of its development, it shows a still greater resemblance to the tissue
that fills up the embryo-sac of Gymnosperms, and even of Angiosperms. In Isoetes
the cavity begins to be filled with cellular tissue a few weeks after the escape of the
macrospores from the decaying macrosporangium ; the cells of this tissue are all at
first naked (without cell-wall); they appear to become enclosed in firm cell-walls
only when the whole cavity of the endospore is filled with them (Fig. 330). In
the meantime the endospore thickens, becomes differentiated into layers, and
assumes a finely granular appearance, phenomena which, as Hofmeister insists, are
exhibited in like manner in the embryo-sacs of Coniferae. The spherical pro-

FlG. 331.— Germination ai Selaginella (after Pfeffer) ; /—///, S. Martejisii, A—D, S. catilescens ; I longitudinal section
of a macrospore filled with the prothallium and * endosperm,' d the diaphragm, e e' two embryos in process of formation ;
// a young archegonium not yet open ; /// an archegonium with the oospore fertilised and divided once ; A a microspore
showing the primordial cells ; B C different views of these divisions ; D the mother-cells of the antherozoids in the perfect
antheridium ; v, vegetative cell.

thallium now swells up, the three convergent edges of the exospore burst length-
wise and thus form a three-rayed fissure, where. the prothallium is covered only
by the membranous endospore; this also peels off, and softens, finally exposing
the corresponding part of the prothallium. At its apex appears the first arche-
gonium ; if this is not fertilised, several others are subsequently formed at its side.
In Selagmella, even when the macrOspores are still lying in the sporangium, the
apical region is found to be clothed with a small-celled meniscus-shaped mass of
tissue which is probably formed, during the ripening of the spores, by the division

472 VASCULAR CRYPTOGAMS.

of an accumulation of protoplasm. This tissue afterwards produces the archegonia,
and is therefore the true prothallium ; but a few weeks after the dissemination free
cell-formation begins beneath it in the spore-cavity, finally filling up the whole cavity,
and forming a large-celled tissue, which Pfeffer, supported by considerations with
which I also agree, compares to the endosperm of Angiosperms, and, following
this analogy, calls by the same name. At the period of fertilisation and of the
formation of the embryo, the macrospores of Selaginella contain, therefore, both
a prothallium and an endosperm. The formation of the archegonia begins even
before the rupture of the exospore, which occurs in this genus in the same manner
as in Isoeles, The first archegonium originates at the apex of the prothallium ; the
others arise, whether the first is fertilised or not, in centrifugal succession on the
exposed parts of the prothallium.

In both genera the archegonium originates by division of a superficial cell
parallel to the surfa-ce ; the upper of the two new cells divides into four cells placed
crosswise, each of which splits by an oblique division into two, one lying over the
other; in this way the neck is formed, consisting of four rows, each of two cells
in Selaginella, and of four cells in Isoeles. The lower of the first two cells sends
out a narrow prolongation between the neck-cells, which becomes the canal-cell
of the neck (Fig. 331 II). The lower larger portion, the central-cell according
to Janczewski, then has a small portion of its protoplasm cut off which corresponds
to the ventral canal-cell of the other Archegoniata, the remainder constituting the
oosphere. The two canal-cells become mucilaginous and are extended from the
opened neck so as to permit the access of the antherozoids to the oosphere.

2. The Asexual Generation (Sporophore). The Development of the Embryo.
The first division of the oospore (formation of the basal wall) differs from that of
Ferns and Rhizocarps, taking place perpendicularly to the axis of the archegonium.
According to Hofmeister, each of the two cells first formed is divided in Isoeles
in a plane at right angles to that of the first division, the relation of which to
the first root, the first leaf, the stem, and the foot of the embryo, requires yet
further elucidation ^ The formation of the embryo of Selaginella has recently
been investigated in detail by Pfeffer. From an elongation of the upper half
(hypobasal) of the oospore is formed the Suspensor, a body which is wanting in all
other Cryptogams, but universally present in Phanerogams, and through which Sela-
ginella consequently approaches flowering plants. The suspensor seldom remains

^ [According to Bruchmann (Jenaische Zeitschrift, 1874) the upper (epibasal) cell grows
rapidly towards the neck of the archegonium and produces the apex of the stem and the first leaf
(cotyledon) ; at the lower part of the anterior surface of the cotyledon a cell grows out and gives rise
to the 'ligula,'and it is the cells lying immediately at the base of this organ which constitute the
growing point of the stem. The lower (hypobasal) of the two primary cells grows slowly downwards
into the spore, forming the foot; from the superficial cells of that part which is diametrically
opposite to the growing point of the stem the primary root is developed. From the more recent
researches of Kienitz-Gerloff (Bot. Zeitg. 1881) it appears that the first divisions of the oospore
resemble those in the Filicinese (see supra, p. 426). The two anterior superior (epibasal) octants
give rise to the cotyledon, the two posterior (hypobasal) superior octants to the root, and the four
inferior to the foot. The ligula is developed from one- of the cotyledonary octants ; from the root-
octants a cotyledonary sheath is also developed. He does not agree with Bruchmann that the root
has an exogenous origin.]

FILICINEM.

473

a simple cell ; a smaller or larger number of divisions usually takes place in its
lower part (Fig. 332, A — D). The embryo itself originates from the lower (epibasal)
half of the oospore. By the elongation of the suspensor and the compression and
absorption of the surrounding cells, the embryo is forced into the endosperm, in
which it now undergoes further development, as in Phanerogams. In the mother-
cell of the embryo (epibasal cell) two segments^ are in the meantime formed by
the wall II (transverse wall); out of each proceeds an embryo-leaf (cotyledon),
and a longitudinal half of the hypocotyledonary segment of the stem. The foot and
root originate besides from the right segment, and in the left (in the figure) segment
is formed the two-sided apical cell of the stem (Fig. 332, A, B). While the two
segments are becoming transformed by a number of cell-divisions into masses of
cells, of which an inner mass very soon separates itself as the procambium of the
axial bundle and a peripheral mass as dermatogen and periblem, a swelling is
produced laterally beneath the first leaf, forming the foot 2; by its increase the

Fig. 332.— Development of the embryo of Selazinella Martensu (after Pfeffer) \ A, H lower part of the suspensor with
the first much-divided sejjments of the embryo, and the apical cell s of the future stem ; bb the first leaves ; C apical view of
the same ; D the apex seen from above in the act of forming two new apical cells, right and left ; /, //, /// the primary
walls of the primary apical cell ; /'— K//' the longitudinal walls by which the two new apical cells are formed. (/ basal
wall ; // transverse wall.)

Stem is forced over to the other side (that of the younger segment); so that the
apex comes to lie horizontally, and afterwards is even directed upwards (Fig. 331 /);
and finally the bud, with its first leaves, the cotyledons, grows out upright from
the apical part of the macrospore when the embryo begins to increase in length.
The first root is formed a considerable time afterwards between the foot and
the suspensor. It is lateral, and its apical cell is formed from an inner cell of
the right segment; but the first layer of its root-cap originates from the splitting
into two layers of the overlying dermatogen ; the later layers of the root-cap arise
from the apical cell of the root itself.

It has already been mentioned that in Pteris and Salvinia the position of the
apical cell of the growing stem is placed at an angle of about 90° with respect to

^ [There is reason to believe that four octants are formed from the epibasal cell as in the other
"Vascular Cryptogams,]

^ [On the propriety of regarding this organ as the morphological equivalent of the foot of the
other Vascular Cryptogams, see Quart, Journ. Micr. Sci. 1878.]

474

VASCULAR CRYPTOGAMS.

that of the embryo. Something of the same kind occurs in Selaginella ; the apical
cell, which lies between the rudiments of the first two leaves, is divided by walls in
such a manner that a four-sided apical cell is formed (Fig. 332 C, D), the segments of
which arise in decussate pairs. In the fifth or sixth segment a second four-sided
apical cell is now formed by a curved wall with the convexity turned towards the
primary apical cell, so that a longitudinal section through the two apical cells cuts
at right angles the common median line of the first leaves, and that of the original
two-sided apical cell. Each of the two four-sided apical cells now developes into
a branch ; but neither of the branches continues to grow in the direction of the
hypocotyledonary axis ; the branching therefore takes place immediately above
the first leaves or cotyledons. The four-sided apical cells of the two rudimentary
branches are soon transformed into two-sided apical cells each forming two rows
of segments.

Fig. 333.— Longitudinal section oilsoetes lacusiris at right angles to the furrows of the stem ten months old (after Hofmeister),
5' stem, b^—l^ leaves, >-i— ;-io roots (X 30) ; the ligula of the two developed leaves is shaded.

The first formation of all the organs and the first branching always take
place before the protrusion of the embryo from the spore.

External Differentiation, The Stem is distinguished in Isoetes^ as has already been
mentioned, by its extraordinarily small growth in length, with which is connected, in
this as in other cases (Ophioglossese, Marattiaceae, and many Ferns), an absence of
branching ; no internodes are formed, the leaves with broad bases of insertion con-
stituting a thick rosette, without leaving between them any surface of the stem bare.
The upper region of the stem, which is furnished with leaves^ has the form of a
shallow funnel, depressed in the centre or apex (Fig. 333). The long-continued
increase in thickness, which distinguishes the stem of Isoetes from that of all other
Cryptogams^, is brought about by an internal layer of meristem, surrounding the

^ Compare what has been said already about Bofrychium and what follows about Lepidodendron.

FILICINE.E.

475

central vascular body, and continually producing new layers of parenchyma on
the outside. This takes place especially in two or three directions, so that two or
three corresponding masses of tissue are formed, slowly dying off on the outside
between which lie as many deep furrows meeting on the under surface of the stem.
From these a large number of roots are produced in rows in acropetal succession.

In the Selaginelleae the stem remains slender, but lengthens rapidly, branching
profusely, and forms distinct internodes. The end of the stem rises above the
youngest leaves as a slender cone. In Selaginella a tendency prevails to sympodial
scorpioid development of the branches which not unfrequently leads to the system of
abundantly branched shoots developed bilaterally in one plane attaining a definite
outline, and a corresponding resemblance to a compoundly pinnate leaf. In con-
sequence of the small size of the leaves in this
genus, the general habit is mainly dependent on
the development of the systems of branches.
The main shoots which result from the sympo-
dial development of the branches may creep like
rhizomes, may grow obliquely upwards, may
climb, or may form the stems of arborescent
and fruticose plants. In all cases the repeated
branchings take place in one plane, for the
bilateral symmetry which is so marked in the
position of the branches and in the phyllotaxis
already exists in the growing point.

The Leaves are always simple, unbranched,
penetrated by only a single fibro^ vascular bundle,
terminating in a simple point, and ending, in
Selaginella^ in a fine awn. The largest leaves
occur in Isoeles, where they attain a length of
from 4 to 60 cm. They are in this case divided
into a basal part or sheath, and an upper part
or lamina. The sheath does not entirely em-
brace the stem, but rises in a somewhat trian-
gular form from a very broad insertion, and is
acuminate ; it is convex behind and concave in
front, where there is a large depression, the Fovea,

containing the sporangium ; the margin of this depression rises in the form of a thin
membranous outgrowth, which in many species lies above the sporangium and
envelopes it, the Velum. Above the fovea and separated from it by the * saddle,'
lies a smaller depression, the Foveola, the lower margin of which forms a lip, the
Labium, while from its bottom an apiculate membranous structure, the Ligule (or
Lingula), with a cordate base, is prolonged beyond the foveola (Fig. 334, A). The
lamina of the leaf, confaining chlorophyll, into which the sheath passes above, is
narrow and thick, almost cylindrical, but flattened in front, and penetrated by four
wide air-canals, which are divided by septa. This form is exhibited by the fertile
leaves of all the species of Isoeles ; a rosette of such leaves is produced annually ;
but between each pair of annual whorls is formed a whorl of imperfect leaves, which

Fig. 334. — A longitudinal section through the base
of a leaf of Iso'ites lacustris with its microsporangium
■)ni still unripe ; B longitudinal section of the lower
part of a young sporangium (X 300) (after Hofmeister).

476

VASCULAR CRYPTOGAMS,

consist, in /. lacus/n's, of only a small lamina, but in the terrestrial species are
destitute even of this, and are simply cataphyllary leaves (phyllades).

The leaves of Selaginella are never more than a few millimetres in length, and
are usually cordate at the base with a narrow insertion, acuminate, and from lanceo-
late to ovate in form. In the greater number of species the sterile leaves are of two
different sizes, the ventral leaves attached to the under or shaded side of the
obliquely ascending stem are much larger than the dorsal leaves on the upper side
exposed to the light (Fig. 335, A). Both kinds taken together form four longi-
tudinal rows {vide infra). On its upper side and near the base each leaf bears a

ligule ; the point of attachment of the sporan-
gium is below this on the fertile leaves. The
fertile leaves form a quadrangular terminal spike,
are uniform in size, and usually of somewhat
different form from the sterile ones.

PhyUolaxis. In Isoetes the rosettes are ar-
ranged spirally, with the divergences f, yV> yt*
^f, the fractions becoming more complicated
the larger the number of leaves that are an-
nually formed. In the species of Selaginella
which have their leaves arranged in four rows,
each dorsal and ventral leaf form together a
pair, whose median plane, however, does not
intersect that of the next pair at right angles
but obliquely, an arrangement which is often
clearly seen on old shoots of S. Kraussiana,

The Apical Growth of the stem takes place
by means of an apical cell ^ That of Isoetes
lacustris is, according to Hofmeister, two-edged
when the stem has two furrows ; in the species
with three furrows it is a three- sided pyramid.
In young plants the leaves stand accordingly
in the first case in two, in the second case in
three rows; but later the phyllotaxis becomes
more complicated and spiral, indicating perhaps
that in the older stem the primary walls of the
segments advance in the anodal direction, as is
the case in those Mosses which have a three-
sided apical cell and a complicated phyllotaxis. In those species of Selaginella which
have the leaves in four rows, the apical cell of the stem is, according to Pfeffer, two-

Fig. 335. — Selaginella incequali/olia ; A fertile
branch (one-half natural size) ; B apex in longitudinal
section bearing microsporangia on the left, macro-
sporangia on the right (magnified).

^ [Hegelmaier (Bot. Zeitg. 1874) was unable to find an apical cell in Isoetes velata or in
7. Diirieui. He considers that, since in certain Selaginellese (S. arborescens, Pervillei, Lyallii ; Russow,
Vergl. Unters.) there is not a single apical cell, but a group of dividing cells, and in S. Wallichii
(Strasburger, Bot. Zeitg. 1873) there are two apical cells, whereas in the other Selaginellese there'is
a single apical cell, it is possible that similar differences may exist in the genus Isoetes; some species
having a single apical cell, and others a group.]

FILICINEM,

All

sided^ (Fig. 336, A,B). The two rows of segments here form an elevated vegetative
cone, at the base of which the rudiments of the leaves first appear at the height of the
fourth or fifth segment. The two edges of the apical cell are directed upwards and
downwards (on the obliquely ascending shoot). The relationship of the leaves to
the segments has not yet been entirely made out. The two leaves of each pair arise
obliquely, one above, the other below, and alternately right and left where the pairs
cross obliquely, by the outgrowth of zones of cells, each of which embraces about a
fourth of the circumference of the stem. Divisions then take place in these cells
which are directed obliquely upwards and downwards, and a row of apical cells is
thus formed, by means of which the growth of the leaf is continued (Fig. 336, A).
The branching of the shoot is effected by a second two-edged apical cell being
formed from the youngest segment (Fig. 335, C, B), a wall being developed in it
which is convex to the existing apical cell and which intersects the primary wall
below. The two shoots which are thus formed grow right and left of the previous

Fig. 336. —Apex of the stem oi Selaginella MarteMsu (d^har Pfeffer) ; A longitudinal section of the end of the stem with
the first rudiment of the leaves ; B apex of the stem seen from above ; C formation of an apical cell seen from the side ;
D the same seen from below. The primary walls of the segments are denoted by darker lines ; the segments themselves
are numbered with Roman figures.

direction of growth, and all the successive branchings take place in one and the
same plane.

The Roots. All the species of Selaginella possess true roots ; but in some,
as -5". Mariensii and Kraussiana, they arise on a structure which Nageli calls the
Rhizophore, and which has no root-cap. In S. Kraussiana the rhizophores spring
from the dorsal side of the stem, nearly at the base of the weaker of each pair of
branches, curl themselves round it, and then grow downwards; it is only rarely
in this species that two of these organs arise near one another. S. Martensu\ on the
other hand, forms at each branching two rhizophores, one on the dorsal and one on
the ventral side (the plane which passes through them is perpendicular to the plane
of branching), but usually only the ventral one undergoes further development, while
the dorsal generally remains in the form of a small protuberance. The rhizophores

' [Treub has shown (Recherches etc. sur le Selaginella Martensii) that the form of the apical cell
is very variable ; it is sometimes two-sided and sometimes three-sided in branches of the same plant.]

47^ VASCULAR CRYPTOGAMS.

arise very near the puncium vegetationis^ probably at the same time as the branches ;
unlike the roots, they are exogenous structures, which, when young, possess a
distinct apical cell. This is probably two-sided (it is four-sided in 6". Marlensit),
but soon ceases to form new segments, the further growth being effected by
intercalary division of the segments and elongation of the cells which proceed from
them. After the cessation of the apical growth, the end of the still very short
rhizophore swells up into a spherical form; its cell-walls become thicker, and in
the interior of the swelling the first rudiments of the true roots originate, which
however do not break through until the rhizophore has attained such a length by
intercalary growth that its swollen end penetrates into the ground. The cells of this
terminal part become disorganised and deliquesce into a homogeneous mucilage,
through which the true roots penetrate 'into the ground. The rhizophores. as Pfeffer
has shown (in S. Marlensu, incEqualifoUa, and levigala), are often transformed into
true leafy shoots, which at first show some deviations from the normal structure in
their leaves, but afterwards continue to grow as normal shoots, and even produce
sporangiferous spikes.

In Selagmella cuspidata, and some other species, there are no rhizophores, but
roots spring immediately from the places nearest the ground where the stem
branches, and, like the rhizophores of ^9. Martensii, they branch even before they
reach the ground. These roots are also formed very early, near the puiictum vege-
tattonis, probably at the same time as the branches of the stem. The roots which
spring immediately from the stem, as well as those which proceed from the
rhizophores, branch in such a manner that the planes of the successive branchings
cross one another at right angles. The branchings of the roots follow one another
very quickly, and at the end of the mother-root are densely crowded ; the apical cell
is difficult to detect, but is probably, like those of the stem and of the rhizophore,
two-sided (four-sided in S. Martensii). It soon ceases to form segments; the
increase of length of each branch of the root takes place therefore almost exclusively
by intercalary growth. Similar phenomena are observable in the roots which
proceed from the furrows of the stem of Isoeles, and which branch (by true
dichotomy) three or four times in planes at right angles to one another. Nageli
and Leitgeb failed to find in them any apical cell distinguished by its form or size,
although they considered the existence of a two-edged apical cell probable ^ (See
Fig. 138, after Hofmeister.) In Isoetes the plane of the first dichotomy is parallel
to the axis of the stem, in Selaginella {cuspidata and levigatd) the plane of branching
is at right angles to it.

The Sporangia of the Ligulatae are of considerable size in proportion to the leaf,
and are borne on short thick stalks. Each fertile leaf bears a single sporangium
which always lies below the ligula either on the leaf itself {Isoetes), or in its axil, or
even on the stem {Selaginella).

The sporangia of Isoetes are sessile in the fovea of the leaf-sheath, to which
they are attached by a narrow base (Fig. 334, A). They are unquestionably
products of the leaves ; the outer leaves of the fertile rosettes produce only macro-

^ [According to Bruchmann, there is not a single apical cell, but a meristem resembling that of
some Phanerogams.]

TILICINEM.

479

sporangia, the inner ones only microsporangia, the former containing a large number
of macrospores. Both kinds of sporangia are imperfectly chambered by bands of
tissue {trabeculcB) which cross from the ventral to the dorsal side. The sporangia do
not dehisce, but the spores escape by the decay of the wall.

In the SelaginellecB the sporangia are shortly stalked roundish capsules. The
macrosporangia contain usually four, less often two or eight macrospores. In the
division of Articulatae the lowermost sporangium only of a spike produces macro-
spores ; in the other divisions there are several macrosporangia. The sporangia do
not take origin, as Hofmeister's older accounts would seem to show, from single cells

Fig. 337.— Development of the sporangia and spores of Selaginella inaqualifolia ; the order of succession is indicated
by the letters A—D; A and B serve for all the sporangia, C and D for the microsporangia only ; E division of the mother-
cells of the microspores, h four nearly ripe spores ; in ^, C and D, a, b are the two layers of the wall of the sporangium,
c is the tapetum, d the primary mother-cells of the spores (A, B and £ X Soo ; C and D X 200).

of the epidermis, but, as in Lycopodium^ from a group of such cells (according to
GoebeP).

The sporangia arise on the growing-point of the stem immediately above
the base of the corresponding leaves, but this by no means justifies us in regarding
them as cauline organs as Russow does. Like those of Lycopodium^ they at first
appear as flattened protuberances which become more or less spheroidal at a later
period and finally clavate. At a later period the sporangia appear to be inserted
in the axil of the leaves or on their base. The fibro- vascular bundle of the leaf runs

[Beit. z. Vergl. Entwick. d. Sporangien; Pot. Zeitg. 1881.]

I.

480

VASCULAR CRYPTOGAMS.

beneath the sporangium without giving off a branch to it. By repeated divisions of
the primary cells a mass of tissue is formed which is differentiated into an outer layer
of cells, the wall of the sporangium, and an internal group of cells, one of which is
the archesporium ; the tapetum'(Fig. 337, <r) is formed toward the free surface of the
sporangium, from cells which are cut off from the archesporium, and it is completed
toward the base by cells which ape cut off by tangential walls from the cells which
surround the archesporium. The cells forming the wall also undergo division by
walls parallel to the surface, and thus the wall of the sporangium comes to consist of
two layers (Fig. 337, «, ^). The mother-cells of the spores are produced by repeated
divisions of the archesporial cells. These cells soon become isolated and round
themselves off, and, in the case of the microsporangia, they all divide, after an indicated
division into two, into four tetrahedrally-placed spores which retain their relative
positions until they reach maturity (Fig. 337, E, g^ h). In the macrosporangia, on
the other hand, one of the mother-cells grows more strongly than the rest ; it divides

and gives rise to the four macrospores, all the
other mother-cells remaining undivided but
continuing to exist (at least in Selaginella
incEqualifolid) for a considerable time. The
macrospores are arranged, in consequence
of the mode of division of the mother-cells,
as the corners of a tetrahedron, an arrange-
ment which persists until they are set free.
Very commonly weakly macrospores are to be
found in otherwise normal spikes of sporangia.
The tapetum persists until the spores are ripe,
whilst in the case of Ferns it is absorbed
during the formation of the spores.

[In the case of Isoeles, it has been shown
FIG. 338- A nearly ripe macrosporangium of Seia- by Tchistiakoff, by Hcgelmaier, aud by Goe-
fs"roUnS:rd'£1Sr'''°"^''^'''""''^'"^^ bel, that the sporangium arises from a group

of cells at the base of the leaf, this group
including cells belonging to the three superficial layers of their tissue. In con-
sequence of cell-division and growth the sporangium soon appears as a swelling
in the fovea. As in the Selaginellese, the fibro-vascular bundle of the leaf runs
beneath the sporangium without giving off a branch to it. The most deeply-placed
cells of the group form the short thick stalk of the. sporangium; the superficial layer
forms its wall; the intermediate layer constitutes the archesporium, from which the
mother-cells of the spores as well as the trabeculae are derived. In the micro-
sporangium the archesporial cells elongate and are divided by walls parallel to the
free surface of the sporangium, and thus rows of cells are formed. Of these rows
some undergo no further change, and these form the trabeculae. In the others
either single cells or groups of cells increase in size and become divided by both
transverse and longitudinal walls ; these divisions produce a tapetal layer at an early
stage which surrounds each group of spore-mother-cells. The differentiation of the
macrosporangia proceeds in much the same manner, but here the sporogenous
cells of the archesporium only undergo such divisions as are necessary for the

FILICINEM. 481

formation of the tapetum, the remaining cells being spore-mother-cells. The tapetal
cells then divide by transverse and longitudinal walls, so that the spore-mother-cells
come to lie deeply within the tissue of the sporangium.

In the macrosporangium of Isoetes each spore-mother-cell divides to form four
macrospores ; its nucleus divides into two and each of these again into two, before
any intervening cell-wall is formed. There is this peculiarity about the mode of the
division, that the protoplasm, as in the delevopment of the spores of Aiilhoceros,
begins to divide before the nucleus. The spore-mother-cells of the microsporangium
divide in a different manner, the only other known instance of the kind occurring in
the pollen-mother-cells of Monocotyledons \ In them the nucleus divides into two,
and this is followed by the formation of a cellulose wall between the two cells : the
nucleus of each of these then divides, and a wall is formed between the resulting
cells. It is in this way that the four ' special ' mother-cells of the microspores
are produced.]

Fig. 339.— a transverse section of the stem ol Selagiiiella denticiilata, the central vessels of the bundle not yet lignified;
b air-cavity surrounding a bundle which is being given off to a leaf.

Histology'^. In the Selaginelleae, to which group the following remarks more
especially apply, the epidermis of the stem consists of long prosenchymatous cells
between which no stomata occur. The cells of the epidermis have often beautifully
sinuous lateral walls, and, like those of the Ferns, they contain chlorophyll which
occurs in these cells as well as in the cells of the fundamental tissue of the leaf in the
form of large granules, only a few of which are to be found in each cell (Fig. 44). The
leaves usually possess stomata on the under surface only, but they occur on both surfaces
of the small leaves of S. pubescens. In several species (such as S. stenophylla and Martensii)
single epidermic cells occur with walls so thickened that the lumen is almost occluded.
(Russow). In most of the species the epidermis of the upper differs from that of the
under surface, in others {S. Gakotti, Kraussiana) the epidermis of the two surfaces is
of the same nature.

The Fundamental Tissue of the stem consists, as \r\ Lycopodium, of elongated cells
with septa which are either oblique or transverse: these cells retain, however, their
thin walls and large cavities, in contrast to what is usually the case in the Lycopodieae,
the hypodermal layers only becoming thick-walled (Fig. 340). It appears that the cells
of the fundamental tissue, and consequently those of the other tissues also, are capable

' [Strasburger, Zellbildung und Zelltheilung, 3rd ed., 1880, p. 167.]
^ [For further details see De Bary, Vergleichende Anatomic, 1877.]

I i

482 VASCULAR CRYPTOGAMS.

of long-continued growth both in length and in circumference, which accounts for the
intervals between the leaves of old stems and for considerable thickness of the stems
themselves, a fact which is worthy of investigation not only with reference to these
plants, but also to the Lycopodieae and many Ferns, It is a striking peculiarity of the
Selaginelleae that the ground-tissue (as also in the stem of Mosses) presents none of
the usual small intercellular spaces, a result probably of the prosenchymatous arrange-
ment of the cells. This is compensated for by the development of a large air-cavity,
which everywhere surrounds each fibro-vascular bundle of the stem (Figs. 340 and 341).
This cavity is traversed by transverse rows of cells forming trabeculae attached to the
bundle : if the cells are somewhat rounded, the bundle appears to be surrounded by
a loose spongy parenchyma (Fig. 339), which is sharply defined from the firm compact
ground-tissue. The ground-tissue of the leaf is a loose spongy parenchyma containing

Fig. 340.— Tran sverse section of the stem oi Selagiitella inaquali/olia (X igo).

chlorophyll ; in small species with thin leaves this tissue is developed only round the
single fibro-vascular bundle traversing the leaf, so that at the margins the epidermis of
the upper and that of the under surface come into contact.

The Fibro-'vasciilar Bundles, one or more of which traverse the stem, are cauline,
like those of the Lycopodieae. They can be traced in the form of procambium beyond
the youngest leaves up into the apex of the stem to close beneath the apical cell. The
separate bundles coming from the leaves become united with the cauline bundles in
these plants, as in the Lycopodieae, only at a later period. In their composition the
fibro-vascular bundles resemble those of the true Ferns. They have usually an elon-
gated elliptical form. The xylem is central, consisting for the most part of scalariform
tracheides, and it is surrounded by the thin-walled phloem (Figs. 339, 340). The very
narrow spiral vessels (Fig. 341) which are the primary elements of the xylem lie at the
ends of the long axis of the bundle, and it is from these two points that the development

FILICINEX.

483

and lignification of the wider tracheides proceed (Fig. 339). The layer of phloem which
invests the xylem is itself surrounded by two or three layers of parenchymatous cells,
which Russow compares to the phloem-sheath of the Ferns, but which must at any
rate be regarded as forming a bundle-sheath belonging to the ground-tissue, investing
the bundle within the above-mentioned air-cavity. A well-defined sheathing layer com-
posed of cells with folded lateral walls is not to be found in the stem or in the leaves.
In the latter, the fibro-vascular bundles are delicate and of simple composition : the
xylem consists of spiral and reticulated tracheides, and it is invested by a scanty phloem.
To this brief description must be added a few words with respect to Isoetes. The
short stem of the mature plant contains an axial woody body which can scarcely be
termed a bundle, consisting of short tracheides loosely united, with spiral or reticulated

Fig. ■3,i,\.—SelagineUa inaqnalifoUa ; longitudinal section through the right side of the axis of a spike 5, the base
of the leaf*, the ligule n, and the sporangium j/ ; ^ point where the cauline and foliar fibro-vascular bundles unite;
/ air-conducting intercellular spaces ; X series of cells traversing the spaces.

thickening bands, and of delicate parenchymatous cells, which is invested by a layer
of clear, shortly prismatic cells with large but delicate pits on their walls : these Russow
considers to represent the phloem. The bundles appear to be built upon the collateral
type, the phloem forming a continuous ring external to the xylem. The fibro-vascular
bundles proceed, one into each of the very numerous leaves (Fig. 333) and into the
roots. The stem of losetes probably does not possess any cauline fibro-vascular bundle ^

^ [Hegelmaier (Bot. Zeitg. 1874) ^.nd Bruchmann (Jenaische Zeitschrift, 1874) are of opinion
that the apical portion of the fibro-vascular body is really a cauline bundle. De Bary, however {loc.
cit.), does not accept this view.]

112

484 VASCULAR CRYPTOGAMS,

at all ; it would appear rather, from the position of the vessels, that the axial fibro-
vascular body consists only of the lower (inner) commencements of the foliar bundles,
which are here densely crowded. In the same manner the basal disc-like woody
body may consist only of the densely crowded commencements of the bundles of the
roots. If this view is correct, the class of Dichotomeae presents two extremes, one
in Psilotum, where the foliar development is small, and where there are, according to
Nageli, no foliar bundles, but the elongated stem forms a fibro-vascular bundle belong-
ing to it only ; the other in ho'e'tes, where the short stem possesses no cauline fibro-
vascular bundle, and only the strongly developed leaves have one each. The structure
of the leaves of Isoetes varies according as the species grow submerged in water, in
marshes, or on dry ground. In the first case they are long and conical, penetrated by
four air-cavities divided by septa into channels, with a weak fibro-vascular bundle in
the axis of the organ, and the epidermis destitute of stomata ; in the second case they
are similar, but provided with stomata and strands of hypodermal fibres ; in the third
case the epidermis is also provided with stomata, and the basal portions of the dead
leaves (phyllopodes) form a firm black coat of mail round the stem. The ground-
tissue is not separated from the single fibro-vascular bundle traversing the leaf by a
bundle-sheath ; according to Russow it forms sclerenchyma under the epidermis which
is usually colourless, in Isoetes Hystrix, and dark brown sclerenchymatous strands which
constitute most of the sheathing portion of the leaf.

Subsequent Continuous Gro^ivth in Thickness of the Stem. Outside the layer of clear
tissue (phloem) which surrounds the central woody mass of the stem of Isoetes is a
layer of meristematic cells by the activity of which the stem grows in thickness : it
forms phloem-cells internally, thus adding to the fibro-vascular mass, and cortical paren-
chymatous tissue on its outer side. The cortical tissue is formed much more rapidly
than the fibro-vascular, and thus, in an old stem, the cortex is the preponderating tissue.
This meristematic layer, is evidently not analogous to the cambium of Dicotyledons and
Conifers inasmuch as it forms fibro-vascular tissue on one side only, whereas in these
groups of plants the cambium forms fibro-vascular tissues on both surfaces, xylem in-
ternally and phloem externally. It is rather to be compared to the thickening-ring of
Draccena and other arborescent Liliaceae in which a continuous growth in thickness
of the stem occurs. This view is supported by the fact that isolated bundles are
occasionally formed by this meristematic layer in the stem of Isoetes. Thus Russow
says that he found 'lying round the central woody mass of the stem of a robust
specimen of Isoetes lacustris, but separated from it by five or six layers of cells repre-
senting soft-bast, xylem bundles (consisting like the central xylem of shortly fusiform
cells with irregular spiral thickenings) invested both on the outside and on the inside
by tabular cells; between these, the bundles radiating from the central woody mass
to the older dead leaves are disposed.' No connexion could be traced between these
bundles and either leaves or roots.

A similar but much more considerable growth in thickness by means of a layer of
meristem surrounding the axial fibro-vascular bundle has been recently shown by Pro-
fessor Williamson to have occurred in the extinct Lepidodendra which are so commonly
present in the Coal Measures, and which are evidently closely allied to the Selaginelleae.
In these plants, however, if I rightly interpret Professor Williamson's account, it appears
that a phellogen layer also existed at the periphery of the stem in correlation with the
considerable growth of thickness resulting from the activity of the internal layer of
meristem. These facts, taken in connexion with the probability of a growth in thickness
of the stem of Botrychium, seem to indicate that this growth is generally wanting in the
existing Vascular Cryptogams because they are less highly developed than their remote
ancestors.

[Professor W. C. Williamson has contributed the following note on the Carboniferous Lyco-
podiacesB :— * The large and varied group of the Lycopodiaceous plants of the Coal Measures exhibits
so many modifications that it is difficult to give a brief statement of their characteristic features. But

FILICINEM. 485

so far as the Lepidodendroid and Sigillarian forms are concerned, our British forms all exhibit one
type of internal organisation. In the very young state each twig has a central bundle of scalariform
vessels surrounded by a "bark," which usually exhibits an inner parenchymatous layer surrounded by
a more prosenchymatous one, which is again invested by a second but more unequal parenchyma.
This prosenchyma, as in Calamites, increases steadily in thickness as the growth of the stem
advances, until it appears to constitute the chief tissue of the bark. Bundles of vessels given
off by the central vascular axis proceed to each of the leaves. As the twig enlarges, the
central axis almost invariably expands into a vascular cylinder, its interior becoming occupied
by a cellular parenchyma of large size, and which now occupies the position and exhibits the
appearance of a true medulla. The parenchyma of the leaves appears to be an extension of
the outermost parenchyma of the bark. The above remarks appear to represent the common
history of all the Lepidodendroid plants up to a certain stage of their growth. Beyond this stage
their histories vary somewhat in the different groups. In some forms, e.g. those to which the
Halonioe belong, the branches attain considerable dimensions without undergoing any great change
in their internal organisation ; but in others a new development of vascular tissue invests the central
cylinder at a period which seems to have varied in different species. This new growth takes place
in successive layers, which are arranged in vertical laminae disposed m radiating planes separated by
tracts of muriform parenchyma ; successive additions are made to the outer margins of the woody
wedges previously formed through the agency of a pseudo-cambial layer of the innermost ' bark.'
These exogenous growths continued until the woody zone attained to a great thickness in the larger
trunks. These exogenous layers took no part in supplying the leaves with vessels. The foliar
bundles invariably pass through them on their way from their source in the inner non-radiated
vascular cylinder to the leaves. It being now admitted that Stigmaria was the general form of root
of Lepidodendroid and Sigillarian types it is necessary to correlate its tissues with those of the
aerial stem. It contains a "medulla" surrounded by a cylinder composed of radiating vascular
laminoe separated by cellular rays, and enclosed in a thick " bark." Large vascular bundles are given
off from the vascular wedges to supply the rootlets. Thus the structure of the root differs from that
of the aerial stem in two ways. (1) The inner vascular cylinder of the latter, characterised by the
non-radiating arrangement of its vessels, by the absence of "cellular rays," and by the numerous
foliar bundles which it gives off to the leaves, is altogether wanting in the former. (2) On the other
hand, the exogenous zone of the stem is prolonged into the roots, retaining all its more important
features. These however are modified in tv/o ways — ist, in the absence of small passages for the
transmission of foliar bundles of vessels; and, 2nd, in their replacement by much larger spaces
having a lenticular section, and through which large vascular bundles, directly derived by enlarging
from the exogenous laminae themselves, pass outwards to the succulent rootlets. The rootlets of
Stigmaria ficoides, which equally belong to Sigillaria and to Lepidodendron, have a very remarkable
internal organisation, identical with that which is characteristic of the roots of recent Lycopods,
a fact which affords additional confirmation of the close affinity of the Sigillarice and the Lepi-
dodendra. That Lepidostrobi are the fruits of Lepidodendroid plants is certain. Equally so is it
that many of the former produced microspores in the upper sporangia of each cone, and macro-
spores in those occupying its basal end. The incalculable myriads of these macrospores found in
many coals render it probable that a very large number of the Lepidostrobi possessed both kinds
of spores ; indeed it is far from certain that any of them did otherwise. In the great majority
of cases the sporangia of these fruits are shrivelled and empty, the spores having been shed ; and
this renders it impossible to say what their original character was ^.']

^ [For the literature of the Carboniferous Lycopodiacese see Brongniart, Archives du Mus.
d'Hist. Nat. vol. I, and Journ. Bot. vol. VII. pp. 3-8.-^King, Edin. New. Phil. Journ. vol. XXXVI.
— Hooker, Mem. Geol. Surv. vol. II. — Carruthers, Monthly Mic. Journ. vol. I. pp. 1 77-1 81 and
225-227 ; Quart. Journ. Geol. Soc. vol. XXV. pp. 248-254.— Williamson, Phil. Trans, vol. CLXII.
pp. 197-240, and Phil. Trans, vol. CLXXII. Part II, 1881.— Thiselton Dyer, Quart. Journ. Mic. So.
1873, pp. 152-156.]

4^6 PHANEROGAMS.

GROUP IV.

PHANEROGAMS.

The Alternation of Generations in Phanerogams is concealed in the formation
of the Seed, which, at least in its earliest stage, consists of three parts: — (i) The
Tes/a, which is a part of the mother-plant; (2) The Endosperm^; and (3) The
Embryo, which is the product of the development of the fertilised oosphere.

In Vascular Cryptogams we have already seen the sexual generation which
results directly from the spore, the prothallium, losing more and more of its character
as an independent plant. In the Ferns, Equisetaceae, and Ophioglossaceas it grows
independently of the spore, often for a considerable period ; in the Rhizocarpeae and
Ligulatae, where male and female spores are formed, it arises in the interior of the
spore, the female prothallium still protruding, in the former group, out of the cavity
of the macrospore, but remaining united with it; while in Isoetes it fills up the
interior of the macrospore as a mass of tissue which only bursts the cell-wall of
the spore in order to render the archegonia accessible to the antherozoids. In the
Cycadeae and Coniferae this metamorphosis is carried one step further; the pro-
thallium ^, which is now known as the Endosperm, remains during its whole existence
enclosed in the macrospore or E?jibryo-sac ; it produces before fertilisation arche-
gonium-Hke structures, the ^ Corpuscula,' in which the oospheres arise. The pro-
cesses which take place in the embryo-sac of Monocotyledons and Dicotyledons
appear somewhat different, and bear a greater resemblance to what takes place in
the macrospore of Selagtnella. In this genus, besides the prothallium which pro-
duces the archegonia, there arises subsequently, by free cell-formation, another tissue
which fills up the rest of the space of the macrospore ; to this tissue the endosperm
of Monocotyledons and Dicotyledons, which is formed by free cell- formation only
after fertilisation, appears to correspond '\ If, therefore, the embryo-sac is the

^ The only reason why the ripe seeds of many Dicotyledons do not contain any endosperm is
because it has already been absorbed and supplanted by the rapidly growing embryo before the seeds
become ripe ; while in others this absorption happens only on germination after the ripening of
the seeds, i. e. on the unfolding of the embryo ; more rarely the formation of endosperm is from
the first rudimentary.

^ The analogy of the endosperm with the prothallium of the higher Cryptogams was first shown
by Hofmeister (Vergleich. Untersuch. 1851), [Germination, Development, and PVuctification of the
Higher Cryptogamia, Ray Soc. 1862, p. 438].

^ Compare Pfeffer in Hanstein's Botanical Dissertations, Heft. IV. p. 24. The ' Antipodal
Cells' in the embryo-sac of Angiosperms may probably be considered as a rudiment of the true
prothallium. [According to Strasburger (Angiospermen und Gymnospermen, 1879) not only the
antipodal cells and the egg-apparatus, but also the endosperm of the embryo -sac of Angiosperms,
represent the prothallium (endosperm) of the Gymnosperms and of the Vascular Cryptogams. This
view leaves the ' endosperm ' of Selagtnella without any representative in other groups of plants.
Goebel has however expressed the opinion (Bot. Zeitg. 1880) that the endosperm of Selaginella
corresponds to the antipodal cells of Angiosperms.]

INTR ODUCriON. 487

representative of the macrospore, that part of the ovule in which the embryo-sac
arises (the nucellus) must be considered the equivalent of the macrosporangium.
But, as in the formation of the ovules of Monocotyledons and Dicotyledons, certain
processes of development (the formation of the archegonia or ' corpuscula '), being
no longer necessary, are suppressed, and the oosphere is immediately produced
within the embryo-sac as the analogue of the macrospore, so also the production
of the embryo-sac immediately from the tissue of the nucellus of the ovule is more
direct. Its production is due to the increase in size of an inner cell of the nucellus
which here represents the sporangium. But while even in the most highly developed
Cryptogams the macrospore still becomes detached from the mother-plant, and the
full development of the prothallium takes place only after the dissemination of the
spores, so that the embryo always arises in structures distinct from those of the
mother-plant, the embryo-sac (or macrospore) of all Phanerogams remains, on
the contrary, enclosed in the ovule, the endosperm in the embryo- sac, and the
embryo in the endosperm. In this manner arises that structure peculiar to Phane-
rogams, the Seed, the testa of which, the product of the envelopes of the ovule,
closely invests both endosperm and embryo. The whole becomes separated from
the mother-plant after the embryo has attained a certain very variable degree of
development. Germination consists in the further development of the embryo at
the expense of the endosperm.

If, on the other hand, the microspores of Selagiftella and Isoetes are compared
with the pollen-grains of Phanerogams, a series of analogies is again seen which be-
comes intelligible on comparing the intermediate phenomena presented by Gymno-
sperms. Indications of the male prothallium and antheridium are indicated, as
Millardet and Pfeffer have shown, by certain cell-divisions which may also be recog-
nised in a simpler form in the pollen-grain of Gymnosperms and in a still simpler
form in those of Angiosperms. Like the microspores, the pollen-grains contain the
male fertilising substance, which, passing into the oosphere, 'causes it to develope the
embryo; but a great difference is displayed in the mode in which the fertihsing
substance is conveyed. In Cryptogams the fertilising substance takes the form of
antherozoids endowed with motion and adapted to force themselves, with the
assistance of water, into the oosphere through the open neck of the archegonium.
In Phanerogams, where the oosphere is enclosed in the embryo-sac and ovule, and
in Angiosperms by the wall of the ovary in addition, such a conveyance of the
fertilising substance would not serve the purpose intended; the pollen-grains are
therefore themselves conveyed to the ovule by foreign agencies, such as the wind,
mechanical contrivances in the flowers, and especially insects; and then germinating
like spores, they emit their pollen-tubes, which, penetrating through the tissue of the
ovule, finally reach the embryo-sac, and transmit the fertilising substance to the
oosphere. The analogy of pollen-grains to spores becomes still more evident when
we examine the mode of origin of both. The mass of tissue in which the pollen is
formed, the pollen-sac, shows, not only in its morphological but also in its anatomical
relationships, a striking resemblance to the sporangium of Vascular Cryptogams.
As in the latter the spore-mother-cells are formed by the isolation of cells previously
combined, so also are the mother-cells of the pollen ; and as the former themselves
usually produce the spores by division into four, after previous indication of a

488 PHANEROGAMS.

bipartition, the pollen-cells are produced from their mother-cells in a similar manner.
Moreover, in the points here indicated Gymnosperms' again appear as a connecting
link between Cryptogams and Angiosperms ; the pollen-sacs of Cycadeae and of
some Coniferse closely resembling, in form and position, the sporangia of some
Vascular Cryptogams.

The general result of these observations is that the Phanerogam, with its
pollen-grains and its embryo-sacs, is equivalent to the spore-producing (asexual)
generation (Sporophore) of the Vascular Cryptogams. But as in Vascular Cryp-
togams the sexual differentiation first makes its appearance (in Ferns and Equise-
taceas) in the prothallium only, and next (in Rhizocarpese and Ligulatge) in the
spores themselves, so, in Phanerogams, this process is carried back a step further,
the sexual differentiation arises still earlier, being manifested not only in the forma-
tion of embryo-sac and pollen-grains, but also in the difference between ovule and
pollen-sac, and between the leaves bearing them (carpels and stamens), and even
earlier in the distinction between male and female flowers, and last of all in the
dioecious condition of the plants themselves^. [The sexual generation (Oophore)
is represented in the pollen-grains (microspores) by the formation of cells within
them which correspond to a male prothallium, and in the embryo-sac (macrospore)
by the formation of the egg-apparatus, antipodal cells, and endosperm, which together
correspond to a female prothallium. A distinct alternation of generations can there-
fore be traced in the life-history of a Phanerogam.]

The fertilised oosphere of Phanerogams produces a Suspensor, growing towards
the base of the embryo-sac and dividing, a structure which we have already met with
in Selaginella, on the apex of which there is a mass of tissue at first almost globular,
which is the embryo. The development of the embryo usually proceeds, even
before the maturity of the seed, to such an extent that the first leaves, the primary
axis, and the first root, can be clearly distinguished. It is only in parasites and
saprophytes devoid of chlorophyll that the embryo usually remains rudimentary until
the dissemination of the seeds without discernible external differentiation; while
in those Phanerogams which contain chlorophyll the embryo not unfrequently
attains a very considerable size and external differentiation (as in Pinus, Zea,
^sculus, Quercus, Fagus, Phaseolus, &c.) Independendy of any curving of the
embryo, the primary apex of its stem always lies originally pointing towards the
bottom of the embryo-sac (the base, chalaza^ of the ovule) ; the first root (primary
root) coincides with a posterior prolongation of the primary stem ; it faces the apex
(micropylar end) of the embryo-sac, and is of distinctly endogenous origin, inasmuch
as its first rudiment at the posterior end of the embryo is covered by the nearest
cells of the suspensor.

The apical cell of the punctum vegetatiom's, which is easily recognised in many Algae,
in Characese, Muscineae, Ferns, Equisetaceae, and Rhizocarpeae, as the primary mother-
cell of the tissue, has already, as we have seen, been replaced by a small-celled
primary meristem in the Lycopodiacese. The apical growth of the axes, leaves, and
roots of Phanerogams also can no longer be referred to the activity of a single apical
cell from which the whole primary meristem has proceeded. Even in those cases

^ Compare what is said on Dichogamy in Book III.

INTRODUCTION. 489

where a single cell (not, however, of preponderating size) occupies the apex, and the
arrangement of the superficial cells of the punctum vegetationis appears to point to it
as the primary mother-cell, it is nevertheless by no means to be assumed that all the
cells, and especially the internal mass of the primary meristem, have proceeded from
it. The primary meristem of the punctum vegetationis consists of a large number of
usually very small cells, more or less evidently disposed in concentric layers ; an
outer single layer, the dennatogen, may be recognised in Angiosperms as the imme-
diate continuation of the epidermis of the older parts, and is continuous even over
the apex of the punctum vegetationis. Beneath it lies a second meristematic tissue,
[the periblem'], consisting usually of a few layers of cells, which covers the apex and
passes lower down into the cortex ; this envelopes a third inner mass of tissue, [the
plerome^ terminating beneath the apex as a single cell ^ {Hippuris^ &c.) or as a group
of cells ; and out of it proceeds either an axial fibro-vascular body (in roots, and in
the stems of water-plants), or the descending limb of the fibro-vascular bundles. In
harmony with this the root-cap does not proceed, as in Cryptogams, from transverse
divisions of an apical cell, but arises, on the contrary, in Gymnosperms from a
luxuriant growth of the layers of periblem of the root and from their splitting away
towards the apex, and in Angiosperms from a similar process in the dermatogen,
or from a special meristematic layer the calyptrogen ^. Even the first rudiments of
lateral structures, leaves, shoots, and roots, cannot be traced back in Phanerogams
to a single cell in the same sense as in Cryptogams. They are first observable as
protuberances consisting of a few or a larger number of small cells ; the protuberance
which is to form a shoot or a leaf shows, even when it first begins to swell, an inner
mass of tissue which is connected with the periblem of the generating vegetative
cone, and is covered over by a continuation of the dermatogen.

The normal Mode of Branching at the growing end of the shoot, leaves, and
roots, is, with few exceptions, monopodial ; the generating axis continues to grow
as such, and produces lateral members (shoots, lateral leaf-branchings, lateral roots)
beneath its apex. Some cymose inflorescences appear however to be the result of
dichotomous branching, and it is possible that in the Cycadeae also the branching
of the stem and leaves may be dichotomous. The monopodial branching of the
axes is usually axillary ; i. e. the new rudiments of shoots appear above the median
plane of very young (but not necessarily the youngest) leaves, in the angle which
they form with the shoot, or somewhat above it. In Gymnosperms every axil of
a leaf does not usually produce a shoot; sometimes (in Cycadeae), the branching
of the stem, as in many Filicineae, is reduced to a minimum. In Angiosperms, on
the contrary, it is the rule that every axil of a foliage-leaf {i. e. one not belonging to
the flower) produces a lateral shoot (sometimes even several side by side or one
above another); but commonly the axillary buds, once formed, are inactive, or
develope only at later periods of vegetation. In addition to the above-mentioned

^ As in so many other respects, here also Isoetes shows an affinity to Phanerogams, as is
evident from Nageli and Schwendener's researches on the apical growth of roots. (Compare Nageli's
Beitragen, 1867, Heft. IV. p. 136.)

2 See Hanstein, Bot. Abhandl. Heft I, and Reinke, Gottinger Nachr. 1871, p. 533. [Janczewski,
L'accroissement terminal des Racines, Mem. soc. nat. de Sci. de Cherbourg, 1874, and Ann. d. Sci.
nat. ser. 5, t. XX.]

49© PHANEROGAMS.

cases of apparent dichotomy, there are in Angiosperms only a few cases of actual or
apparent extra-axillary branching, which will be mentioned when discussing the
characteristic features of this class.

Phanerogams are distinguished from Cryptogams by an extraordinarily varied
and complete metamorphosis of members bearing the same name ; and this is con-
nected with the almost infinite variety in the mode of life, and the more marked
differentiation of the physiological functions of these plants; and the same is the
case with the differentiation of tissues, which in Phanerogams greatly exceeds even
that of Ferns. In these respects also Gymnosperms assume an intermediate
position between Cryptogams and the rest of Phanerogams.

What has now been said will serve to explain on one hand the distinction between
Vascular Cryptogams and Phanerogams, on the other hand the points in which they
agree, and the affinity of the two groups in their main outlines. In order, however, to
facilitate the comprehension by the student of the characteristics of the separate classes
of Phanerogams which are now to be described, we must in the first place keep in
view a few of their peculiarities, which have at present only been briefly touched upon,
and attempt to settle the nomenclature, which has become to some extent obsolete
and out of harmony with the most recent theories.

The Floiver, in the broadest sense of the term, is composed of modified foliar organs
and of an axis which bears them. [The most highly modified leaves of the flower are
the stamens and the carpels : these so-called ' sexual organs ' are really spore-bearing
organs, comparable to the spore-producing leaves of the Vascular Cryptogams.] When
the leaves which stand immediately beneath the sexual organs on the same axis differ
from the rest of the leaves of the plant in their arrangement, form, colour, or structure,
and are physiologically connected with fertilisation and its results, they are considered as
belonging to the flower, and are termed collectively the Floral Leaves or Perianth,
The separate flowers are distinguished from the Inflorescence by including, together
with their sexual organs and perianth, only one axis, while the inflorescence is an axial
system with more than one flower ^ Roper has termed the tout ensemble of the male
sexual organs of a flower the Andrcecium, that of the female organs the GyncBceum,
When a flower contains sexual organs of both kinds it is called hermaphrodite or bisexual;
if it contains only male or only female sexual organs, and is therefore unisexual^ it is termed
diclinous; when flowers of both sexes occur on the same individual plant, the species
is moncEcious, when on different individuals it is dioecious. Usually the apical growth
of the floral axis ceases as soon as the sexual organs make their appearance, and fre-
quently even earlier ; the apex of the floral axis is then concealed, and is often deeply
depressed in the centre of the flower ; but in abnormal cases (and normally in Cycas) the
apical growth of the floral axis re-commences, again produces leaves, and sometimes
even a new flower, and a Proliferous Flo'^ver is thus produced. The sexual organs
and perianth of a flower are usually crowded (arranged in rosettes either spirally or
in whorls) ; the part of the floral axis which bears them remains very short, no inter-
nodes being in general distinguishable in it ; and it not unfrequently expands into the
form of a club or disc, or becomes hollow, and this part of the floral axis is called the
Torus or Receptacle. In Coniferae and Cycadeae (occasionally also in Angiosperms), it
is however sometimes elongated to such an extent that the sexual organs appear loosely
arranged along an axis in the form of a spike. Beneath the receptacle the axis is mostly

* In some cases it is hov^^ever difficult to distinguish between a flower and an inflorescence;
as in some Coniferse, and especially in Euphorbia. {On the latter, see Warming in Flora, 1870,
no. 25 ; Schmitz, do. 1871, nos. 27, 28; and Hieronymus, Bot. Zeitg. 1872, no. 12.) [E. Waiming,
Er Koppen hos Vortemaelken en Blomst eller en Blomsteistand, Kobenhavn 1871.]

INTRODUCTION. 49 1

elongated and more slender, either entirely naked or bearing one or two small leaves or
Bracteoles. This part of the axis is the Peduncle ; if it is very short, the flower is said to
be sessile. No shoots usually arise from the axils of the floral leaves, even when they
are produced in all the other leaf-axils of the plant ; there occur, however, abnormal
cases (which are not very uncommon) of axillary branching or prolification even within
the flower.

The male spores or Pollen-grains are equivalent to the microspores of the higher
Cryptogams, and arise in receptacles corresponding to the sporangia in those plants,
which may be termed in general Pol/en-sacs. These are at first solid masses of tissue
in which, as in the sporangia, an inner mass of cells becomes differentiated into the
mother-cells of the pollen-grains (at first by more vigorous growth of the single cells),
while the surrounding layers of tissue become developed into the wall of the pollen-sac.
[The mother-cells of the pollen-grains are derived from one or more hypodermal cells
constituting the arcbesporium, which is invested by a layer of peculiar cells, the tapetum :
the tapetal cells are derived either entirely from the archesporium, or entirely from
the tissue of the anther, or partly from the archesporium and partly from the tissue
of the anther ^] It has already been mentioned that the mother-cells of the pollen
become separated and detached from the tissue (though this rule is subject to ex-
ceptions), and then produce the pollen-cells by division into four after actual bipartition
or at least an indication of it. A special description of these processes will be given under
the heading of the separate classes ; at present we must however premise a few facts
relative to the morphological nature of the pollen-sac. Like the sporangia of most
Vascular Cryptogams, the pollen-sacs of Phanerogams are usually products of the leaves,
which however mostly undergo in this case a striking metamorphosis, remaining much
smaller than all the other leaves. A leaf which bears pollen-sacs may be termed a
Staminal Leaf or Stamen ; the most recent researches have, however, shown cases in
which the pollen-sacs arise on the elongated floral axis itself, as Magnus has illustrated
in the case of Naias, Kaufmann in Casuarina, and Rohrbach in Tj/pba ; in these cases it
is still doubtful whether the pollen-sacs may not be the only surviving portions of
otherwise completely abortive staminal leaves ^. In the Cycadeae the pollen-sacs grow
singly or in groups on the under side of the relatively large stamens, often in large
numbers, resembling in position the sporangia on Fern-leaves. In the Coniferae the
stamens have still more lost the appearance of ordinary leaves ; they remain small, and
form several or only two relatively large pollen-sacs on the under side of the lamina
which is still distinctly developed. In Angiosperms the stamen is usually reduced to a
slender weak and often very long stalk called the Filament, bearing two pairs of pollen-
sacs at its upper end or on both sides beneath the apex, which are included as a whole
under the term Anther; the anther therefore usually consists of two longitudinal halves,
united and at the same time separated by a part of the filament termed the Connectinje.
The two pollen-sacs of each half of the anther are contiguous throughout their length,
and frequently both halves of the anther are in close apposition. The separate pollen-
sacs then appear as compartments of the anther, which is in this case quadrilocular, in
contrast to those anthers (of rare occurrence) in which each half contains only a single
pollen-sac, and which are therefore bilocular.

The female spore or Embryo-sac, the analogue of the macrospore, is usually derived
from a hypodermal cell of the nucellus of the ovule, which must be regarded as the
archesporium, the ovule itself corresponding to the macrosporangium of the hetero-
sporous Vascular Cryptogams ^. The nucellus is a small-celled mass of tissue of usually

^ [Warming, Unters. iib. Pollenbildende Phyllome und Caulome, in Hanstein's Bot. Abhandl. II.
1870: also Goebel, loc. cit.']

^ [For instances of the production of pollen-grains in abnormal positions, even in ovaries or in
the ovules themselves, see Masters, Vegetable Teratology, Ray Soc. London 1869, pp. 182-188.]

^ [See Strasburger, Angiospermen und Gymnospermen, 1879 ; and Goebel, loc citJ]

I.

49» PHANEROGAMS.

ovoid form, and enclosed, with a few exceptions, in one or two envelopes, each of
which consists of several layers of tissue. These envelopes or Integuments grow round
the young nucellus from its base (the chalaza), and form at its apex — where they ap-
proach and often greatly overtop it — a canal-like entrance, the Micropyle or Foramen^
through which the pollen-tube forces its way, in order to reach the apex of the embryo-
sac. Very commonly the nucellus, enclosed in its integuments, is seated on a stalk, the
Funiculus ; but this is sometimes wanting, and the ovule is then said to be sessile ; the
point of attachment of the ovule to the funiculus is termed the hilum. The funiculus
is, with a few exceptions (Orchideae), penetrated by an axial fibro-vascular bundle which
usually ceases at the base of the nucellus. The external form of the ovule when in a state
for fertilisation is very various. Independently of outgrowths of various kinds at the
funiculus and the integuments, the direction of the nucellus (together with its coats),
with respect to the funiculus, is of especial importance. The ovule is orthotropous when
the nucellus lies in the same straight line as the hilum and chalaza, which coincide
in position, and the apex of the nucellus is the apex of the entire ovule. Much more
frequently the ovule is anatropous, i.e. the apex of the nucellus, and therefore the
micropyle which projects beyond it, lies close to the hilum, the chalaza being at the
opposite end, and the funiculus runs along the side of the nucellus, so that the ovule
appears as if sharply curved at its base ; the integuments (or at least the outer one) have
united in growth with the ascending funiculus, which, so far as this union is complete,
is termed the Raphe; the nucellus itself being in this case straight. Much less com-
mon is the campy lotropous ovule, where the nucellus itself (together with its coats) is
curved; its apical part, and therefore its micropyle, chalaza and hilum, lie close to-
gether at its base. These are, however, only the most striking forms, whJth are
united by transitional states. The place from which the ovules spring is called the
Placenta, and belongs to the axis of the flower, or more commonly to the carpels
themselves. The placentae often do not show any peculiar phenomena of growth ; but
more commonly they project like cushions, and may thus assume the appearance of
special organs, finally becoming detached from the surrounding tissue. While after
fertilisation, both the endosperm and the embryo are undergoing simultaneous develop-
ment in the embryo-sac, the former most commonly increases considerably in size, and
supplants the surrounding layers of tissue of the nucellus (sometimes even of the inner
integument) ; and the tissue of the integument which is not displaced, or usually only
certain definite layers of it, becomes then developed into the T:esta. If a portion of
the tissue of the nucellus, filled with food-materials, remains unchanged until the seed
is ripe, it is distinguished as the Perisperm ; its food-materials, although lying outside the
embryo-sac, are consumed by the embryo during germination ; and the perisperm may
then act physiologically as the representative of the endosperm \ The seeds of
Piperaceae, Zingiberaceae, and some Nymphaeaceae contain both endosperm and peri-
sperm. Sometimes the ovule, during the period of its development into a seed, is
enveloped from below by a new coating, which usually itself surrounds the tough testa as
a soft mantle, and is termed the ^ril. Of this nature is the red pulp which surrounds
the hard-shelled seed of the Yew ; and the origin is the same of the so-called ' mace ' of
the nutmeg, the seed of Myristica fragrans.

If we now turn our attention to the morphological nature of those structures from
which the ovule immediately springs, we find a considerable variety. Only rarely does
the orthotropous ovule appear as the prolongation or terminal structure of the floral
axis itself, so that the nucellus forms directly the vegetative cone of the latter, as in
Taxus and the Polygonaceae. It is more usual for the ovule to grow laterally on the
floral axis, thus corresponding in position to a leaf, as in Juniperus, Primulaceae, and

^ [The endosperm and perisperm are generally both included in English text-books under the
term 'albumen,' a term which should by all means be avoided, as conveying the idea of a definite
chemical composition, whereas that of the endosperm varies greatly.]

INTRODUCTION.

493

Compositae. But the most common case is where the ovules spring from undoubted
leaves— the carpels — and usually from their margin, like pinnae from the leaf (this is very
clear, e.g. in C^cas), more rarely from their upper (or inner) surface (as in Butomus,
Akebia^ Nymphoea, &c.). If the ordinary morphological definitions are applied to these
relationships, we should have in the first-named case ovules of an axial nature, or they
would be metamorphosed caulomes^; where they spring laterally from the axis, they
would have to be considered as metamorphosed entire leaves ; and where they proceed
laterally from the margins of carpellary leaves, as metamorphosed pinnae. For those
ovules which spring from the surface of carpels there is no clear analogy with any
purely vegetative structures [i.e. with any that do not subserve the purpose of fertilisa-
tion) ; though in this case we may be reminded of the sporangia of Lycopodlum. The
ovules, finally, of some Gupressineae, which appear to have an axillary position on the
carpels, have not yet been sufficiently investigated with respect to their true relationships.
In some cases the morphological interpretation is supported by malformations which not
unfrequently occur. Cramer, to whom we are indebted for an admirable investigation
of this question, has shown that the ovules of Primulaceae and Compositae, which arise
laterally beneath the apex of the axis of the flower, become gradually transformed into
entire leaves of the ordinary form ; and that in the same manner the ovules of Delphi-
nium, Melilotus, and Daucus, which spring laterally from the margins of the carpellary
leaves, may become developed into ordinary parts of the lamina, as laciniae or leaflets.
It appears on the other hand significant that nothing of the kind has yet been
observed in those ovules which have been interpreted above as metamorphosed por-
tions of the axis. The development not only of normal, but still more plainly that of
abnormal ovules, shows further that a morphological distinction exists between the
nucellus on the one hand and the funiculus together with the integuments on the other
hand. In those anatropous ovules which may be regarded as metamorphosed leaves or
parts of leaves, the nucellus makes its appearance as a new lateral structure inserted on
the rudiment of the ovule, and when this latter becomes developed in a leaf-like manner
it appears as an outgrowth of the surface of the leaf. This fact, the morphological
importance of which was first insisted on by Cramer, is however not universal, as is
especially shown in the development of the ovules of Orchideae, the nucellus of which
unquestionably corresponds to the apex of the entire ovule, although it becomes
anatropous by subsequent curvature ; still less possible does it appear to consider
the nucellus of the orthotropous ovule of Taxuj and the Polygonaceae as a lateral
formation, since it is obviously an elongation of the apex of the floral axis (see Angio-
sperms) '^.

The Carpellary Leagues are the foliar structures of the flower which stand in the
closest genetic and functional relationship to the ovules. They either produce and
bear the ovules, or are constructed so as to enclose them in a chamber, the O'vary, and to
form the apparatus for the reception of the pollen, or Stigma. The varying morphological
significance of the carpellary leaves is clearly seen by a comparison of the genera Cycas
and Juniperus. In Cycas the carpels resemble the ordinary leaves of the plant, and the
ovules are produced on theii* margins and remain ' entirely exposed ; in Juniperus the
ovules spring from the floral axis itself, corresponding, even in their position, to a whorl

* Cramer, Bildungsabweichungen bei einigen wichtigeren Pflanzen-familien, u. die morpholo-
gische Bedeutung des Pflanzeneies (Zurich 1864), is inclined to consider all ovules as metamorphosed
leaves or parts of leaves. To this view I have already expressed some hesitation in the first edition
of this book ; the description here given, which differs from the earlier one, is derived as much as
possible from direct observation.

^ [In view of the very great variety of position in the development of the sporangia (including
pollen-sacs and ovules) it will be on the whole simpler and more satisfactory, as Goebel has
suggested, not to attempt to assign them to the categories of phyllome and caulome, but to regard
them as organs having a morphological value of their own.]

494 ^^^ NEROGA MS.

of leaves, but the preceding whorl of carpellary leaves swell up after fertilisation, and
envelope the seeds in a pulpy mass, the berry-like fruit of these plants. In Primulaceae
the ovules spring from the elongated floral axis itself, and thus correspond in their
position to entire leaves ; they are however enclosed, even at the period of their
formation, by an ovary, consisting of the carpels and an elongated style bearing the
stigma. In most other Dicotyledons and Monocotyledons the ovules are seated on
the revolute margins of the carpels which have grown together into an ovary, and
which therefore in these cases both produce and enclose the ovules. But notwithstanding
these very considerable morphological differences, the carpellary leaves are always alike
physiologically in being excited by fertilisation to further development during the
maturing of the seeds, and in taking a certain share in their future history.

Pollination and Fertilisation. By Pollination is meant the conveyance of the pollen
from the anthers to the stigma of Angiosperms or to the nucellus of Gymnosperms.
The pollen is detained there by a viscid substance, or often by hairs, and the emission is
thus brought about of the pollen-tube which in Gymnosperms penetrates at once the
tissue of the nucellus, but in Angiosperms grows downwards through the tissue of the
stigma and the frequently very long style in order to reach the ovules ; it then forces
itself into the micropyle and advances as far as the embryo-sac. It is only when it
reaches the embryo-sac (in Gymnosperms however it penetrates still more deeply) that
fertilisation of the oosphere results. A considerable time, occasionally even months,
often elapses, between pollination and fertilisation ; but commonly only a few days
or hours.

Pollination is rarely effected by the wind alone, those plants in which this is the case
are said to be anemophilous ; in this case large quantities of pollen are produced in order
to secure the result, as in many Goniferae. In a few cases the pollen is thrown on to the
stigma by the bursting of the anthers (e.g. in some Urticaceae) ; but the means usually
employed is that of insects, and the plants in which this is the case are said to be
entomophilous. For this purpose special and often very complicated contrivances are met
with to allure insects and attract them to visit the flowers ; and at the same time the
object is accomplished of always conveying, where possible, the pollen to the stigma of
a different flower to that which produced it (even when they are hermaphrodite).
In reference to this object the parts of the flower also assume definite forms and
positions, which will be followed out further in Book III. Here it need only be
mentioned that insects are especially attracted to visit flowers by the nectar
secreted in them ; this usually sweet juice is generally produced deep down among
the foliar structures of the -flower, and the form of the parts is generally so contrived
that the insect, while it is obtaining the nectar, must place its body in certain definite
positions by which it at one time brushes the pollen out of the anthers, at another time
attaches it to the stigma of another flower. The diversity in the forms of flowers
depends especially on these relationships, a comparatively simple plan of structure
underlying them all. The organs which secrete the nectar, the Nectaries, are
therefore of extreme importance in the life-history of most Phanerogams; they are,
nevertheless, usually very inconspicuous, and, — which is very significant with respect to
the relationship of morphology with physiology,— notwithstanding their enormous
physiological importance, they are attached to no definite part of the flower in a
morphological sense ; almost every part is able to perform the function of a nectary.
This term therefore does not denote a morphological but a purely physiological idea.
The nectary is usually only a small spot at the base of the carpels (as in Nicotiana), or of
the stamens (as in Rheum), or of the petals {e.g. Fritillaria) which, without becoming
more prominent, produces the nectar; but frequently it is in the form of glandular
protuberances of the floral axis between the insertion of the stamens and petals (as
in Cruciferae and Fumariaceae). A particular organ, e.g. a petal, is often transformed,
for the purpose of secreting and storing up the nectar, into a hollow receptacle, forming
a spur-like protuberance {e.g. Viola) ; or all the perianth-leaves become developed into

INTRODUCTION. 495

hollow or pitcher-like nectaries (as in Helleborus), or they assume the most wonderful
forms, like the petals of Aconitum'^.

Even before fertilisation, pollination is usually followed by striking changes in the
parts of the flower, particularly in the gynaeceum, and especially when the parts con-
cerned are delicate; thus the stigmas, style, and corolla wither, the ovary swells up
(as in Gagea and Puschkinia), and the like. The most striking result of pollination
is shown in many Orchideae, where the ovules are only formed as a consequence of this
process.

[The process of Fertilisation is essentially this, that protoplasmic and nuclear substance
passes from the pollen-tube into the oosphere ; the protoplasmic substance coalesces
with that of the oosphere, and the nuclear substance {male pronucleus) with that of the
oosphere {female pronucleus) to form the definitive nucleus of the oospore.]

Those changes however which are excited by fertilisation are still more energetic
and varied than those which are consequent on pollination ; the oospore developes into
the embryo; the endosperm — formed previously in Gymnosperms— is completed in Angio-
sperms only subsequently to fertilisation ; the ovules grow along with the ovary, their
layers of tissue are differentiated, become lignified, pulpy, dry, &c. The increase in size
of the ovary, which is frequently enormous (in Cucurbita, Cocus, &c. several thousand
times in volume), shows in a striking manner that the results of fertilisation extend
to the rest of the plant, in so far as it affords the materials of nourishment. Striking
changes in form, structure, and size take place after fertilisation, especially in the
carpels, placentae, and seeds ; but very frequently similar changes result also in other
parts. Thus, e.g., it is the receptacle that constitutes the fleshy swelling which is called
the Strawberry, on the surface of which are seated the small true fruits ; in the Mulberry
it is the perianth of the flowers that swells up to form the succulent coating of the fruit ;
in Taxus it is a cup-shaped outgrowth of the axis beneath the ovule (the aril) that
surrounds the naked seed with a red fleshy coating, &c. Popular usage includes under
the term Fruit all those parts which exhibit a striking change as the result of ferti-
lisation, especially when they separate as a whole from the rest of the plant; in ordinary
language the Strawberry, as well as the seed of the Yew surrounded by its aril, the Fig,
and the Mulberry, are all fruits. Botanical terminology limits the idea of Fruit within
narrower boundaries, which, however, are not yet sharply defined. In the most exact
use of botanical terms, the whole of the gynaeceum which ripens in consequence of
fertilisation may be termed the Fruit. When the gynaeceum consists of coherent
carpels or of an inferior ovary, the flower produces a single entire fruit ; if the carpels
do not cohere, each forms a part of the fruit, or a fruitlet. This limitation of the term
is often, however, inconvenient ; and it would seem preferable to give it a definition
which will vary in the different sections.

The point to be most clearly borne in mind by the student is that the fruit is
not a new plant-structure. All the parts of the fruit which are morphologically
determinable, originate and assume their morphological character before fertilisation ;
the result of fertilisation is merely a physiological change in the parts. The only new
parts in a morphological sense are the embryo and the endosperm, which are pro-
duced in the ovule.

The Inflorescence. When a shoot which has previously formed a large number of
foliage-leaves terminates in a flower, the flower is said to be terminal ; if, on the other
hand, a lateral shoot developes at once into a flower, with one or at most a few bracteoles
beneath it, the flower is termed lateral. Sometimes the first primary axis which proceeds
from the embryo terminates in a flower ; but more often the axis continues to grow,
or its growth comes to an end, without forming a flower, and it is only lateral shoots of
the first, second, or a higher order that terminate in flowers. In the first case the

* [On this subject see MUller, Befruchtung der Blumen durch Insekten, 1873 ; and Sir John
Lubbock, British Wild Flowers in relation to Insects ; also Book III. of this work,]

49^ PHANEROGAMS,

plant may be termed, in reference to the formation of its flowers, uniaxial, in the other
cases bi-, tri-axial, &c. "When a plant produces only terminal flowers, or when the
lateral flowers spring from the axils of single large foliage-leaves, they are said to be
solitary. When, on the other hand, the flowering branchlets are densely crowded,
and the leaves within this region of ramification are smaller and of a different form
and colour from the others, or are entirely absent, an Inflorescence arises, in the nar-
rower sense of the term, usually sharply differentiated from the vegetative region of
the plant, and not unfrequently assuming very peculiar forms which require a special
terminology. This occurs however only rarely among Gymnosperms, the formation
of multifloral inflorescences of peculiar form being characteristic of the more highly
developed difi'erentiation of Angiosperms ; and it will therefore be convenient to defer a
more detailed classification and definition of inflorescences until we are treating of
that class.

With reference also to the Histology ^, one point only need be mentioned here, in
which Gymnosperms and Angiosperms agree. The Fibro-'vascular Bundles of Phane-
rogams exhibit the characteristic peculiarity that every bundle which bends outwards to
a leaf is only the upper limb of a bundle which runs downwards into the stem ; in other
words, we have here * common ' bundles, each of which has one lirhb that ascends and
bends out into the leaf, and another which descends and runs down into the stem ; the
latter is called by Hanstein the 'leaf-trace.' In the most simple cases {e.g. in most
Coniferse) only one bundle bends out into each leaf; but when the insertion of the leaf
is broad, or the leaf is large and strongly developed, a larger number of bundles pass
from the stem into the leaf, in which they ramify when the lamina is broad ; the leaf-
traces may consist therefore of one or more bundles. The bundles are usually thicker
at the spot where they pass from the stem into the leaf than lower down in their course.
Each bundle of this kind may pass downwards through only one internode or through
several ; in the latter case an internode with several leaves standing above it contains
the lower parts of bundles which bend outwards above into leaves of different height
and different age. The descending foliar bundle seldom has its lower extremity free ; it
is usually attached laterally to the middle or upper part of a lower (or older) bundle.
This may take place by the bundle splitting below into two branches which anastomose
with the lower bundles; or the thin ends of the descending bundles may intercalate
themselves between the upper parts of older foliar bundles ; or each bundle may bend
right or left and become finally joined laterally to a lower bundle. In this manner the
foliar bundles, originally isolated, are united laterally in the stem into a connected
system ; and this, when copiously developed, gives the impression of having arisen by
branching, whereas it arises in fact from the coalescence of separate portions originally
distinct.

Besides the descending limbs of the common bundles, others may however occur in
the stem of Phanerogams ; first of all net-works (as in Grasses) or girdle-like reticula-
tions (as in Rubiaceae or Sambucus) are frequently formed in the nodes of the stem
by horizontal bundles. Furthermore, longitudinal bundles may become differentiated in
the stem, which have nothing to do with the leaves ; and the mode of formation of
these * cauline bundles ' may vary greatly. They originate either at an early period in
the primary meristem of the stem, immediately after the foliar bundles and in the pith
(as in Begoniaceae, Piperacese, and Gycadeae), or only at a much later period in the
outer layers of the stem when this has continued to increase in thickness, outside the
foliar bundles (as in Menispermaceae, Aloineae, and Draccena).

The further development of the foliar bundles varies in Monocotyledons on the one
hand and in Gymnosperms and Dicotyledons on the other. In the former they are
closed ; in the latter a layer of formative cambium remains, which, in stems that increase
rapidly in thickness and become woody, usually prolongs itself across the medullary

[For further details see De Bary, Vergleichende Anatomie der Phanerogamen und Fame, 1877.]

INTRODUCTION.

497

rays so as to form a perfect ring (the cambium-ring), and then produces regularly new
layers of phloem on the outside and of xylem on the inside. In the primary roots and
the stouter lateral roots of Gymnosperms and Dicotyledons an increase of thickness
also takes place by the subsequent formation of a closed cambium-ring, which, like that
of the stem, is not found in Cryptogams, and commonly leads to the formation of strong
persistent root-systems, which are more often replaced physiologically in Monocoty-
ledons by rhizomes, tubers, and bulbs. With the persistent increase in thickness is
connected, finally, the active and extensive production of cork, a process foreign both
to Cryptogams and to Monocotyledons. It will be more convenient, however, to defer
the special discussion of these points also until we are treating of the characteristics of
the separate classes.

SYSTEMATIC REVIEW OF PHANEROGAMS.

The distinguishing characteristic of Phanerogams, as contrasted with Cryptogams,
lies in the formation of the Seed. This organ is developed from the ovule, which, in
its essential part the nucellus, produces the Embryo-sac, and in this the Endosperm and
the Oosphere. The latter is fertilised by the Pollen-tube, an outgrowth of the Pollen-
grain, and, after, produces the Embryo borne on a Suspensor. The phanerogamic
plant which is differentiated into Stem, Leaves, Roots, and Hairs, corresponds to the
spore-forming (asexual) (Sporophore) generation of Vascular Cryptogams ; the Embryo-
sac to the Macrospore ; the Pollen-grain to the Microspore ; the Endosperm is equivalent
to the female Prothallium ; and the Seed unites in itself, at least for a time, the two
generations, the Prothallium (Endosperm), together with the young plant of the second
generation, the Embryo.

Flowering Plants may be classified as follows : —

I. Phanerogams without an Ovary.

The ovules are not enclosed before fertilisation in a structure (the Ovary) resulting
from a cohesion of carpellary leaves. The endosperm arises before fertilisation, and
forms archegonia {i.e. * corpuscula '), in which the oospheres originate. The contents
of the pollen-grains are divided before the formation of the pollen-tube, corresponding
to divisions taking place in the microspores of Selag'mella.

I. Gymnosperms. The first leaves produced from the embryo are arranged
in whorls of two or more.

A. CycadecB. Branching of the stem very rare, or entirely suppressed;

leaves large, branched.

B. ConifercB. Axillary branching copious, but not from all the leaf-axils ;

leaves small, not branched.

C. Gnetacece. Mode of growth very various ; flowers similar in many

respects to those of Angiosperms.

II. Phanerogams with an Ovary.

The ovules are produced in the interior of a structure (the Ovary) formed by the
cohesion of carpellary leaves (often only of one carpel, the margins of which have
become coherent), bearing at its summit the stigma upon which the pollen-grains
germinate. The endosperm is formed after fertilisation at the same time as the
embryo, both remaining rudimentary in some cases. A division of the contents of the
pollen-grain is indicated. The branching is almost always axillary and from the axils of
all the foliage-leaves.

Kk

498 PHANEROGAMS.

2. Monocotyledons. The first leaves produced from the embryo are alternate;

endosperm usually large ; embryo small.

3. Dicotyledons. The first leaves of the embryo form a whorl of two (or are

opposite); endosperm very often rudimentary, often entirely absorbed by
the embryo before the ripening of the seeds.

CLASS X.

GYMNOSPERMS

This class embraces, in the orders Cycadeae, Conifeise, and Gnetaceae, plants
of strikingly different habit, but evidently closely allied in their morphological
structure, in the peculiarities of the mode of formation of their tissue, and espe-
cially in their sexual reproduction. On these grounds they take up an interme-
diate position between Vascular Cryptogams and Angiosperms, while they approach
Dicotyledons among the latter, especially in their anatomical structure.

The Pollen-grains suggest a homology with the microspores of Selaginella,
their contents undergoing before pollination one or more divisions into cells which
resemble a very rudimentary male prothallium. One of these cells (the largest) grows
into the pollen-tube when the pollen-grain has reached the nucellus of the ovule.
The pollen-sacs are always outgrowths from the under side of structures unquestion-
ably foHar (staminal leaves), and bear a striking resemblance in many cases to the
sporangia of some Vascular Cryptogams. They are produced either in larger or
smaller numbers or in pairs on a staminal leaf, without cohering in their growth.

The Ovule^ which is almost always orthotropous, and usually provided with only
one integument, either appears to be the metamorphosed end of the floral axis itself,
or it originates laterally beneath its apex (or is apparently axillary), or it grows from
the upper surface or margins of the carpels. These never cohere so as to form a true
ovary before fertilisation, although during the ripening of the seeds they often in-
crease considerably in size, close together, and conceal the seeds, usually separating
again when they are mature in order to allow them to fall out ; the cases are, however,
not rare in which the seeds remain quite naked from first to last. The embryo-sac
is formed beneath the apex of the ovule, which consists of small- celled tissue and
remains enclosed until fertilisation by a thick layer of the tissue of the nucellus.
Sometimes the formation of several embryo-sacs commences in one nucellus, but
only one of them attains its full development. The Endosperm arises by free
cell-formation long before fertilisation in the embryo-sac, which is distinguished by
its firm wall ; but the cells soon become combined into a tissue and increase by
division. Within this mass of tissue, corresponding to the endogenous prothallium
of Selaginella^ arise the Archegonia (or Corpuscula^) in larger or smaller numbers.

^ [The central cells of the archegonia of Gymnosperms were discovered by Robert Brown in
1834. He called them corpuscula or embryoniferous areolae (Miscellaneous Botanical Works,

OVMNOSPERMS. 499

Strasburger states that each of these bodies is formed from an endosperm-cell
lying at the apex of the embryo-sac, which increases considerably in size and
produces the neck and central cell of the archegonium by division. According
to the same authority a small upper portion of the central cell beneath the neck
is even separated as the canal-cell. There is litde doubt that, as Strasburger asserts,
the whole of the central cell is to be considered as the oosphere, although Hofmeister
thought that a number of oospheres arose in it by free cell-formation^. After the
pollen-tube has penetrated the tissue of the nucellus and forced itself as far as the
archegonium (corpusculum), where its fertilising material is conveyed to the oosphere,
the Suspensor is formed by division of a cell which is developed in the lower part of
the oosphere. The cells of the suspensor are at first small, but the middle or upper
ones elongate very much, and, pushing the lower ones before them, penetrate into a
softened part of the endosperm. Sometimes the suspensors which are produced side
by side separate ; each bears at its apex a small-celled rudiment of an embryo. On
this account, and also because several archegonia are often fertilised in one endo-
sperm, the unripe seed contains several rudimentary embryos, of which, however, only
one usually increases greatly in size, the others withering away.

During the development of the embryo, the endosperm becomes filled with
nutrient materials and increases greatly in size ; the embryo-sac which encloses it
grows at the same time, and finally entirely absorbs the surrounding tissue of
the nucellus ; the integument, or an inner layer of it, becomes developed into a
hard shell, while frequently (in naked seeds) its outer mass of tissue becomes
fleshy and pulpy and gives the seed the appearance of a drupaceous fruit {e. g.
Cjycas, Salisburia). The effect of fertilisation not unfrequently extends also to the
carpels or other parts of the flower, which grow considerably, forming fleshy or
woody coatings to the seeds, or cushions beneath them.

The ripe Seed is always filled with the endosperm, in which the embryo lies
and is distinctly differentiated into stem, leaves, and root. It fills up an axial
cavity of the endosperm, is always straight, its radicle being turned towards
the micropyle, its plumule towards the base of the seed. The first leaves which
the embryonal stem produces stand in a whorl, consisting generally of two opposite,
but not unfrequently of three, four, six, nine, or more members. At the period of
germination the radicle first protrudes through the split testa ; the bud which is
formed between the Cotyledons or first leaves at the apex of the stem is forced out by-
their elongation, the cotyledons still remaining concealed in the seed, and remaining
in it until its food-materials have been completely consumed by the embryo. Some-
times they remain concealed there as organs which have become useless ; but in
Coniferae they are drawn out by the elongation of the embryonal stem and brought
above the surface of the ground, where they unfold as the first foliage-leaves. The
cotyledons of Coniferse become gr^en even within the seed in complete darkness, the

vol. I. pp. 567 and 570). The structure of the neck of the archegonium was made out by Hofmeister,
who applied to it the term rosette (On the Higher Cryptogamia, p. 411). Archegonium and
corpusculum do not seem exactly synonymous, since the latter, properly speaking, is only equivalent
to the central cell of the former. Henfrey termed the central cells 'secondary embryo-sacs'
(Elementary Couri-e, 2nd edition, p. 608).]

^ More will be said on this subject under Coniferae.

K k 2

,'500 PHANEROGAMS.

formation of chlorophyll taking place, as in Ferns, without the assistance of light.
It is not known whether the same thing occurs also in the Cycadeae and Gnetaceae.
The young plant, freed from the seed, consists of an erect stem, passing below
insensibly into the vertically descending tap-root, from which numerous secondary
roots soon proceed in acropetal order, usually forming finally a well-developed
root-system. The embryonal stem grows vertically upwards, and is usually not only
unlimited in its growth, but is much stouter than all the lateral shoots, even when
these are formed in abundance, as is the case with Coniferae. In the remarkable
Gnetaceous Wehvilschta, however, the apical growth altogether ceases at a very early
period, and even the production of new leafy shoots is suppressed, as is usually the
case also in Cycadeae.

An apical cell does not exist either at the ends of the shoots or at the apices of
the roots of Gymnosperms. In this respect they resemble the other Phanerogams,
but they differ from them in that the primary meristem of the Punctuni vegetaiionis of
the stem shows either no differentiation (Cycadeae, Abietineae), or only an indistinct
differentiation, of Dermatogen (young epidermis) and Periblem (young cortex). At
the apex of the root the well-defined axial fibro-vascular mass (Plerome) is covered by
a continuation of the cortical tissue (Periblem). Layers of cells belonging to this
tissue, which cover the apex, become thickened and split off, thus forming the root-
cap. The root-cap therefore is not derived here, as in most Angiosperms, from the
active growth and splitting of the young epidermis (Dermatogen), or from a proper
meristematic layer (Calyptrogen).

The Flowers are usually developed on small lateral shoots, often of a high order
of ramification ; terminal flowers occur on the primary stem only in the Cycadeae
(and in them not exclusively). They are always diclinous; the plants themselves
monoecious or dioecious. The male flower consists of a slender axis usually greatly
elongated, on which the staminal leaves are arranged in large numbers usually
spirally or in whorls. The female flowers are remarkably different in their external
appearance, and usually very unlike those of Angiosperms. A kind of perianth of
rather delicate leaves occurs only in Gnetaceae ; in Coniferae and Cycadeae it is
wanting or is replaced by scales. But what makes the female flowers peculiarly
strange, independently of the absence of an ovary, is the elongation of the floral
axis, on which the foliar structures are placed not in concentric circles as in Angio-
sperms, but in a distinctly ascending spiral arrangement, or in alternating whorls
when they are numerous. When only a few ovules are produced on a naked
or small-leaved inflorescence, as in Podocarpus and Salisburi'a, the last trace of
resemblance in habit to the flowers of Angiosperms ceases. But to clearly under-
stand the matter it is only necessary to retain distinctly in mind the definition of
a flower, viz. an axis bearing members which are modified for the production of
spores.

On the histology of the Gymnosperms see the remarks at the conclusion of the
description of the whole class.

CrCADE^.

501

A. CYCADE^i.

The Embryo^ enclosed in the large endosperm, possesses two opposite unequal
cotyledonary^ leaves, which lie with their inner surfaces face to face, cohering
towards their apices. The tendency of
the subsequent foliage-leaves to branch is
sometimes displayed even in these cotyle-
dons, a rudimentary lamina being formed
on the larger one, with an indication of
pinnae (as in Zamia, Fig. 342 B'). The
seed germinates when laid in moist earth,
but only after a considerable interval ; the
testa splits at the posterior end and allows
the emission of the primary root, which
at first grows vigorously downwards, but
sometimes assumes afterwards a tuberous
form or produces a system of rather thick
fibrous roots. According to Fig. 342 C
borrowed from Schacht, and a more recent
statement by Reinke, the branching of the
primary root is laterally monopodial; Mi-
quel, however, asserts the existence of
bifurcations of the more slender roots in
older plants of Cycas glauca and Encepha-
lartos. According to Reinke and Stras-
burger, only those lateral roots which
emerge from the ground branch dichoto-
mously, probably as a pathological pheno-
menon. By the elongation of the coty-
ledons which remain in the endosperm

Fig. 342.— Gerniiuation of Zamia spiralis (after Schacht.
reduced). B conimencenient of gerniiuation, ct the cotyledons
coherent above their elongated base, one of them having at
its apex (magnified at B') an indication of a pinnate lamina ;
C seedling six months old ; sa seed, ?£/ tfte primary root, * the
first pinnate leaf, Jfx rudiments of the adventitious roots whkh
afterwards grow upwards.

^ Miquel, Monographia Cycadearum, 1842. [Ditto, On the Sexual Organs of the Cycadacese ;
Journ. of Bot., March and April 1869]. — Karsten, Organogr. Betracht. iiber Zamia muricata, Berlin
1857. — Mohl, Bau des Cycadeenstammes (Vermischt. Schrift. p. 195). — Mettenius, Beitrage zur
Anatomic der Cycadeen (Abhandl. der konigl. Sachs. Gesellsch. der Wissensch. vol. VII, 1861). —
[W. C. Williamson, Contributions towards the history of Zamia gigas, Trans, l.inn. Soc. vol. XXVI,
1870. — Carruthers on Fossil Cycadean Stems from the Secondary Rocks of Britain, ibid.'] — On the
structure of the pollen see Schacht, Jahrb. fiir wissensch. Bot. vol. II. p. 142 et seq. — Kraus, Ueber
den Bau der Cycadeenfiedern (Jahrb. fiir wissensch. Bot. vol. IV). — Reinke in Nachrichten der konigl.
Gesellsch. der Wissensch. in Gottingen, 1871, p. 532.— De Bary, Bot. Zeitg. 1870, p. 574. — Juranyi,
Bau u. Entwickelung des Pollens bei Ceratozamia (Jahrb. fiir. wissensch. Bot. vol. VIII. p. 382). —
Ueber Wachsthum und Verzweigung der Wurzeln, Reinke, Morphol. Abhandl. 1873. — [Warming,
Undersogelser og Betragtninger over Cycaderne, and, Bidrag til Cycadeernes Naturhistorie, in the K. D.
Vidensk. Selsk. Forhandl. 1877 and 1879; also Bot. Zeitg. 1878.]

^ Van Tieghem found in three seedlings, hybrids of Ceratozamia longifolia ? with Cer. Mexicana i,
only a single sheathing cotyledon, in another two very unequal cotyledons. Two embryos of Zamia
spiralis had two unequal cotyledons, another had three, and another had only one. (See Van Tieghem
in the French edition.)

502 PHANEROGAMS.

and absorb their nourishment from it, their basal parts and the intermediate plumule
are pushed out of the seed. The portion of the axis which bears the cotyledons,
as well as that which developes above them, remains very short, but a consider-
able lateral increase of size takes place beneath the apex, due to a large develop-
ment of parenchymatous tissue. The stem thus acquires the form of a roundish
tuber, which it retains even at a later period in some species ; but in most it
lengthens in the course of years into an erect tolerably stout colunm which some-
times attains a height of some metres. This slow increase in height, together with
the considerable increase in thickness of the growing end, is correlated with the
absence of a tendency to branch as in other similar cases (Isoe/es, Ophioglossum,
Aspidium Filix-mas, &c.). The stem of Cycadeae usually remains perfectly simple,
although old stems sometimes divide into branches of equal stoutness. But when
several flowers are formed at the summit, this evidently depends on branching ; and,
as far as one is able to judge from drawings, it is probable that this branching
is dichotomous. In old or sickly plants small bulbous or tuberous gemmae are not
unfrequendy found at the base of the stem under or above ground, the morphological
nature of which is still doubtful ; in Miquel's opinion it is not impossible that they
spring from old leaf-scales, and have therefore nothing to do with the branching of
the stem.

The whole of the surface of the stem is furnished with leaves arranged
spirally ; no internodes can be distinguished. The leaves are of two kinds ; dry,
brown, hairy, sessile, leathery scales of comparatively small size, and large, stalked,
pinnate or pinnatifid foliage-leaves. The scales and the foliage-leaves alternate
periodically ; a rosette of large foliage-leaves is produced annually or biennially, and
among these the terminal bud of the stem is enveloped with scales, under protection
of which the new whorl of foliage-leaves is slowly formed. This alternation begins
at once on germination in Cycas and other genera, a number of scale-leaves follow-
ing the leaf-like cotyledons, and enveloping the bud of the seedling ; after these a
pinnate though small foliage-leaf is then usually developed, which is again followed
by scales. It is only as the strength of the plant increases after several years'
growth that the foliage -leaves are produced in whorls constantly increasing in size,
and forming, after the older ones have died off, the palm-like crown of leaves, the
scales which stand above them enclosing at the same time the apical bud of the
stem. In this bud the foliage-leaves are so far formed beforehand, that when they
at length burst the bud they only have to unfold, this process then occupying only a
very short time, while one or two years elapse before the unfolding of the next
rosette of leaves. The leaves which proceed from the bud are in Cycas and other
genera circinate like those of Ferns ; in others the rachis of the leaf only is rolled
up ; in others, finally, as Dion^ the growth of the leaf is straight, its lateral leaflets
being also straight before expansion \ The unfolding is, as in Ferns, basifugal, and,
probably in consequence of this, there is also a permanent apical growth and a basi-
fugal development of leaflets. The leaflets are usually simple, and generally stand
alternately on the rachis, which is often i to 2 metres long. The mode in which

* [This statement is not quite exact. In Zamia and Encephalartos the leaves are not circinate in
vernation ; and even in Cycas it is only the leaflets and not the rachis that is so.]

CrCADEM.

5^3

the lamina terminates above, points to a dichotomdus branching of the leaf, the rachis
of which may therefore be considered as a sympodium composed of the basal
portions of the successive bifurcations, while the lateral leaflets represent the bifur-
cations of th€ lamina of the leaf, the growth of which is arrested and flattened.
The whole leaf would therefore be a dichotomous cymose branch-system. Re-
searches into the history of its development are however wanting, as in the case of
the branching of the stem and root.

The Flowers of the Cycadeae are always dioecious, and the plants are therefore
either male or female. Both kinds of flowers appear at the summit of the stem,
either singly, as in Cjycas, as terminal flowers of the primary stem, or in pairs or
larger numbers as in Zamia muricaia and Macrozamia spiralis^ where they may
perhaps be regarded as metamorphosed bifurcations of the stem\ The flower con-
sists of a strong conical elongated axis, sometimes supported on a naked peduncle,
but densely covered in other parts by a large number of staminal and carpellary
leaves arranged spirally.

In Cycas the female flower is a rosette of foHage-leaves which have under-
gone but slight metamorphosis (Fig. 343), the apex of the stem developing again
first of all scale-leaves, and then new whorls of foliage-leaves ; the stem, therefore,
grows through the female flower, thus furnishing an instance of proHfication. The
separate carpels are, indeed, much smaller than the ordinary foliage-leaves, but are
essentially of the same structure; the lower pinnae are replaced by ovules, which
attain, even before fertilisation, the magnitude of a moderate-sized ripe plum, the
fertilised seed acquiring the dimensions and the appearance of a moderate-sized
ripe apple, and hanging quite naked on the carpel. Whether the male flower of
Cycas also exhibits proHfication I do not know, and it seems improbable ; the very
numerous staminal leaves are much smaller, 7 to 8 cm. long, and undivided; they
expand considerably from a narrow base and terminate in an apiculus. They are
furnished on the under side with a number of densely-crowded pollen-sacs ; the
whole flower is from 30 to 40 cm. long.

The male and female flowers of the remaining genera of Cycadese resemble
fir-cones externally. The comparatively slender floral axis rises as a rachis on a
short .naked peduncle, and on this are seated the numerous staminal or carpellary
leaves (Fig, 344). The axis terminates with a naked apex which undergoes no
further development (Fig. 344 D). The stamens are, indeed, but small in comparison
to the foliage-leaves of the same plant, but are, nevertheless, the largest which
occur anywhere among Phanerogams. In Macroza?nia, as in Cycas, they are from
6 to 8 cm. long, and as much as 3 cm. broad ; they spring, with rather a narrow
base, from the floral axis, and expand into a kind of lamina, terminating in an
apiculus {Macrozamia) or in two curved points {Ceratozamia), or the lower part of
the stamen is thinner and stalk-like and bears a peltate expansion {Zamia), They
are also distinguished from the stamens of most other flowering plants by their

' The hypothesis that the male flower of Cyca& Rumphii is one, the leaf-bud by which the stem
is prolonged the other bifurcation of the dichotomising apex of the stem, is not supported by
De Bary's recent researches. [According to Warming (Joe. cit.) all the flowers are probably terminal;
possibly the male flower (in Ceratozamia longifolia) is produced on a branch of a dichotomy of the
stem : it is certainly not borne on a lateral branch.]

504

PHANEROGAMS.

persistence, becoming lignified and often very hard. The numerous pollen-sacs on
the under side of the stamens are usually collected into small groups numbering
from two to five, like the sori of Ferns, these again forming larger groups on
the right and left side of the leaf. The pollen-sacs are globular or ellipsoidal,
usually about i mm. in size, and are attached with a narrow base to the under
side of the stamen ; Karsten states that in Zamia spiralis they are even stalked.
They dehisce longitudinally, and are in all respects much more like the sporangia

Fig. 343.— a carpel of Cycas revoliUa (reduced about h.)\/ piimse of the leaf-like carpel ; sk ovules replacing the"
lower pinnce ; sk' an ovule further developed.

of Ferns than the pollen- sacs of other Phanerogams, from which they also differ
in the firmness and hardness of their wall. The mode of development of the
pollen-sacs and pollen-grains of Cycadeae was till lately unknown ; it has only
quite recently been observed by Juranyi in Ceratozamia longt/olia. The pollen-
sacs are formed on the under side of the stamens in the form of small papillae,
probably consisting from the first of several cells over which the epidermis of
ths surface of the leaf is continuous. The inner tissue is next differentiated

C VGA DEM.

505

(as in the sporangia of Lycopodiaceag, Equisetaceae, and Ophioglossaceae) into an
outer layer of smaller cells enclosing a larger-celled tissue ; the cells of the latter
continue to grow and divide in all directions, and the mother-cells of the pollen
are finally isolated, but densely crowded together, as in Dicotyledons. The mode
of division of the mother-cells is nevertheless more like that of Monocotyledons in
this respect, that they first of all divide into two daughter-cells, each of which
again undergoes bipartition. The first division- wall is partially formed, .^s in Dico-
tyledons, by the slow growth of an annular ridge of cellulose, formed in the depres-

FlG. ^^^.—Zainia nmricata (after Karsten). A a male flower (natural size) ; B transverse section of one; C one
of its stamens with the pollen-sacs x and the peltate expansion s (seen from below) ; D the upper part of a female
flower (natural size); E transverse section of one, s the peltate scale bearing the ovules s k; ^longitudinal section of
a ripe seed ; e endosperm, c cotyledons, x the folded suspensor.

sion produced by the previous constriction of the protoplasm of the mother-cell ;
but in each of the two daughter-cells the second partition appears to be formed
simultaneously, as in Monocotyledons. The four young pollen-cells are now freed
by the rapid absorption of the cell-wall which surrounds and separates them. The
pollen-grains, when free from their mother-cells, are unicellular and spherical; but,
during their further growth, the contents, enclosed by an extine and intine, divide
into two cells, a smaller and a larger one, each possessing a nucleus. The smaller
of these two cells, lying on one side against the intine of the pollen-grain, becomes
arched on the opposite side, and projects in the form of a papilla into the larger

5o6

PHANEROGAMS.

one. This smaller cell now again undergoes a transverse division parallel to the
first, and this is sometimes followed by a second ; a two- or three-celled body is
thus formed, attached on one side to the intine, and projecting into the cavity
of the larger cell, as in Abietineae, which, moreover, further resembles Ceratozamia
in the fact that, as in the Coniferae, the large cell, formed by the first division of the
pollen-grain, developes into the pollen-tube, the mass of small cells remaining
inactive in the pollen-grain. In Cycas Rumphu, Encephalartos, and Zamia, the
pollen-grain also splits up, according to De Bary, into a larger and a smaller cell,
the latter also in this case again dividing once, and the larger cell developing into the
pollen-tube. The spot where the intine which developes into the pollen-tube breaks
through the extine lies exactly opposite the mass of small cells (the secondary
cells of the pollen-grain) ; the extine is in this place thinner, and in the dry
pollen-grain deeply folded in, so that the transverse section of the dry pollen-grain
is kidney-shaped. During the absorption of water which
precedes the formation of the pollen-tube the pollen-grain
again assumes a spherical form.

The carpellary leaves are arranged spirally or in apparent
verticils, closely crowded on the axis of the female flower.
Those of Cycas have already been described ; in Zamia,
Encephalartos, Macrozamia, and Ceratozamia, the carpels are
much smaller, and each bears only two ovules, attached
right and left to a peltate expansion which terminates a
slender pedicel (Fig. 344). The ovule is always orthotro-
pous, and consists of a large nucellus and a thick integu-
ment the inner layer of which (in contrast to that of other
Phanerogams) is penetrated by a number of fibro-vascular
bundles. The micropyle is a slender tube, formed by the
prolongation of the contracted margin of the integument
beyond the summit of the nucellus. According to De Bary's
researches a second inner integument appears to exist in
the case of Cycas revoluta. [In the nucellus a group of
cells can be readily distinguished at an early stage, which
Warming considers to be homologous with the mother-
cells of the spores in the sporangia of the Vascular Crypto-
gams; from one of these the embryo-sac is formed. The wall of the embryo-
sac becomes thickened, and its cavity becomes filled with endosperm. From
certain superficial cells of the endosperm the archegonia (corpuscula) are formed.
The neck of the archegonium consists of two cells. From the large central cell
a canal-cell is cut off, leaving the remainder as the oosphere. After fertilisation,
each oospore gives rise to a single suspensor ; the embryo is not developed
at its apex until after the seed has been sown. The embryo of Ceratozamia has
only one cotyledon ; Cycas and Macrozamia have two ; Van Tieghem has, however,
found two cotyledons in some cases in Ceratozamia Mexicana, and three in Zamia
spiralis^

In consequence of the form and position of the carpels, the ovules are covered
and concealed before and after fertilisation, except in Cycas; at the period of polli-

Fl<;. 345- — CeriuostuHta iongi-
folia (after Juranyi) ; A pollen-
grain containing a group of three
(vegetative) cells y; B develop-
ment of the pollen-tube from the
large cell of the grain ; e the ex-
tine, bs the pollen-tube covered
by the intine.

^" ai

CONIFERJE.

507

ation, which is apparently brought about by insects, the carpels separate from one
another, and the micropyle excretes a fluid to which the pollen-grains adhere. The
outer layer of the testa is usually fleshy, the inner one hard, and the seed therefore
resembles a plum, with its surface often brightly coloured.

B. CONIFER^i.

Germination. The endosperm surrounds the embryo in the form of a thick-
walled sac open at the radicular end ; the embryo lies straight in the central cavity of
the endosperm ; its axis is continuous behind with the rudiment of the primary root,
and bears at its anterior end a whorl of two or more cotyledonary leaves, between
which it terminates in a roundish apex (Fig. 346 /). The Taxineae and most
Cupressineae and Araucariese have two opposite cotyledons, although in some
Cupressineae there are from three to nine, and in some Araucarieae whorls of four
cotyledons ; while among the Abietineae there are rarely so few as two, more often
four or even as many as fifteen. To refer this larger number of cotyledons to the
division of two opposite ones, as Duchartre proposes, is entirely opposed to the other
processes of leaf-formation in these plants, especially to the common occurrence of
whorls consisting of several leaves on the growing axis of seedlings.

When placed in damp soil the endosperm swells up, bursts the testa at the
radicular end of the embryo, which is then pushed out by the elongation of the axis,
and grows into a strong descending tap-root, from which lateral roots proceed, suc-
ceeding one another rapidly in acropetal succession, and subsequently branching.
This is the commencement of the root-system of Conifers, which is frequently
strongly developed and persistent. After the emergence of the root, the coty-
ledons elongate in their turn, push out their bases from the seed and the end of
the axis that lies between them, but they themselves remain in the endosperm until
it has been absorbed. In Araucaria (sub-genus Colymbea) and in Salisburia the
hypocotyledonary portion of the axis remains short, and the cotyledons remain

' For the structure of the flowers, see R. Brown, On the Plurality and Development of the
Embryos in the Seeds of Coniferae : Misc. Bot. Works, London, 1866, vol. I. pp. 567-576. — H. von
Mohl, Vermischt. Schrift. pp. 25 and 49. — Schacht, Lehrb. der Anat. u. Phys. vol. II. p. 433. — Eichler
in Flora, 1863, p. 530, [and Nat. Hist. Rev. 1864, pp. 270-290; Flora, 1873, and Trans. Bot. Soc.
Edin. 1873, pp. 535-541. — Dickson, Trans. Bot. Soc. Edin. VI. p. 420; New Phil. Journ. 1861, pp. 198,
199. — J. D. Hooker, On the Ovary oi Siphonodon in Trans. Liim. Soc. XXII. pp. 137, 138. — Caspary
in Ann. des Sci. Nat. 4th series, vol. XIV. p. 200, and Flora, 1862, p. 377. — Brongniart, Bull. Bot, Soc.
France, XVIII. p. 141. — Van Tieghem, Ann. des Sci. Nat. 5th series, vol, X.] For the fertilisation,
Hofmeister in Vtrgl. Unters.-i85i [On the Germination, Development, and Fructification of the
Higher Cryptogams, Ray Soc, pp. 400-433.] — Strasburger, Die Befruchtung der Coniferen, Jena
1869. For the pollen, Schacht in Jahr. f. wiss. Bot, vol, II. p. 142, — Strasburger, Ueber die Bestau-
bung der Gymnospermen, Jenaische Zeitschr. vol. VI. Also in addition : [Zuccarini, Morphology of
the Coniferoe, Ray Soc, Rep, and Pap, on Bot, 1845,] — Pfitzer, Ueber den Embryo der Coniferen,
Niederrhein. Ges, fiir Natur. u. Heilk. Aug. 7, 1871. -Reinke, Ueber das Spitzenwachsthum der
Gymnosperm-Wurzeln, Gottinger Nachr. 187 1, p. 530. — [Strasburger, Die Coniferen u, die Gnetaceen ;
eine morphologische Studie, Jena 1872. — Eichler, Sind die Coniferen gymnosperm oder nicht? Flora
1873, and Bluihendiagramme, I. 1875. — Strasburger, Die Angiospermen und die Gymnospermen,
1879,]

508 PHANEROGAMS,

contained in the seed ; in most Conifers, on the contrary, this portion becomes
greatly elongated, making a sharp bend in an upward direction, pierces the soil,
and draws the cotyledons with it. As soon as these are exposed to light, the
hypocotyledonary portion straightens itself, the whorl of cotyledons expand, and,
having become green while still underground, act as the first foliage-leaves of

Fig. ^^d.—Pztius Pittea ; /longitudinal section through the middle of the seed, jv the micropylar end; //conuiiencement
of germination, emergence of the root ; /// completion of germination, after the endosperm lias been absorbed (the seed
lay at too small a distance below the surface, and was therefore raised up by the cotyledons when the stem began to grow) ;
A shows the ruptured testa s, B the endosperm e, one half of the testa having been removed, C longitudinal section of the
endosperm and embryo, D transverse section at the commencement of germination; c the cotyledons, ru the primary root,
X the embryo-sac pushed out by it (ruptured in B), he hypocotyledonary portion of the axis, w' secondary roots, r red mem-
brane within the hard testa.

the seedling, the apex of its axis having in the meantime formed a bud with
new leaves (Fig. 346).

Mode of Growth and External Differentiation. The terminal bud of the stem
of the seedling grows more rapidly, though frequently interrupted, than the lateral
shoots which arise subsequently. The primary stem is thus a direct prolongation of
the axis of the embryo; it never ends in a flower, but grows indefinitely at the

CONIFERM.

509

summit, becoming thickened to a corresponding extent by the activity of a cambium-
ring, and thus becomes a slender cone attaining a height of 100, 200, or even
more feet ^, and a diameter at the base of 2 or 3 or as much as 20 feet. On this
highly-developed primary axis the lateral axes of the first order are produced;
often periodically in rosettes at the apex (pseudo-whorls) or distributed irregularly
and branching again in the same manner. Each primary axis usually grows more
vigorously than its secondary axes; and hence the collective form of the system of
branching, as long as the primary axis continues to grow vigorously, is that of a
panicle of conical or pyramidal form. While in Cycadeae the branching is almost
entirely suppressed, the peculiar form and beauty of Conifers depends chiefly on the
branching, the more so as the leaves are almost always small and inconspicuous,
serving only, as far as the outward appearance of the plant is concerned, as a cloth-
ing to the system of branching. The branching is always axillary ; but Conifers
differ from Angiosperms in not producing buds in nearly all the leaf-axils; in
Araucaria and some species of Taxus, Abies, and other genera, it is chiefly or
exclusively the youngest leaf-axils of a year's growth which produce branches, and
these grow vigorously. In Juniperus communis, indeed, buds occur in most of the
leaf-axils, but only a few develope. In Pinus sylvestris and its allies shoots are formed
only in the axils of the cataphyllary leaves which are borne exclusively by the
primary stem and the permanent woody branches, remaining however very short,
and producing two, three, or more acicular foliage-leaves, from the axils of which no
lateral shoots are produced. In Larix, Cedrus, and Salisburia, buds are formed in
the axils of a considerable number — but not nearly all — of the foliage-leaves, a few
growing rapidly, and serving for the development of the branch-system, while others
remain very short, and form annually a new rosette of leaves without lateral buds. In
Thuja and Cupressus also, which are distinguished by their copious branching, the
number of small leaves is still very much larger than that of the axillary shoots.
Many Conifers exhibit a very regular arrangement of those branches of different
orders which arrive at their full development, the symmetry of the whole tree being
at the same time increased by their difference in size. The branches of the first
order on the upright primary stem are frequently formed in a pseudo-whorl of several
members at the conclusion of each period of vegetation, the same process being
frequently repeated on the branches themselves {e.g. Pi?ius sylvestris, Araucaria
hrasiliensis, and especially Phyllocladus trichomanoides, and many others) ; more
commonly a tendency to bilateral ramification appears on the horizontal branches
of the first order (as in Abies pectinata); and not unfrequently besides these strong
branches from which the framework of the tree is constructed, smaller ones are
also formed between them (e.g. in Abies excelsd). In many cases the arrangement
and growth of the branches are more irregular; the greatest deviation from this
type being shown in the Cupressinese, especially Cupressus, Thuja, and Lihocedrus,
in which the tendency to bilateral ramification ^ is seen even on the primary stem,
which is more perfectly developed on the lateral shoots. Branch-systems of three or

' [The trunk o{ Sequoia {Wellingtonia) gigantea of California attains the height of 400 feet.]
* In many species also oi Abies and Piniis there is an evident tendency to bilateral development

in the horizontal lateral shoots, the spirally arranged leaves inclining over to the right and left, and

thus forming two omb-like rows.

510 PHANEROGAMS.

four orders of shoots are developed in one plane in such a manner that a system
of this kind assumes a definite contour and somewhat the appearance of a pinnate
leaf. In Taxodium the foliage-leaves are formed in two rows on slender branches
a few inches in length ; in T. dislichum these fall off in the autumn together with
their leaves, thus presenting a still greater resemblance to pinnate leaves. Finally
Phyllocladus produces on all its verticillate shoots only small colourless scale-like
leaves, from the axils of which, but beneath the terminal bud, whorls of shoots spring
with limited power of growth, developing their bilateral side-shoots in the form of
flat-lobed foliage-leaves. These remarks, incomplete as they are, may suffice to
turn the attention of the student to the phenomena of the branching, which are
moreover easily observed.

The Leaves (with the exception of the floral leaves) are either all foliage-leaves
containing chlorophyll, as in Araucarta, Juniperus^ Thuja^ &c. ; or all colourless or
brownish scales, as in Phyllacladus, where the foliage-leaves are replaced by
leaflike shoots ; or, finally, scales and foliage-leaves are very frequently formed at
the same time, and even on the same shoots (as in Abies), where the scales only
serve the purpose of protecting the buds ; or the two forms are distributed on
different axes, as in the true Pines (Pinus), the permanent woody shoots of which
produce only membranous scales, the axils of which develope sterile foliage-shoots
which afterwards die off. The foliage-leaves of Conifers are mostly small, of simple
structure, and scarcely ever compound; they are smallest and at the same time most
numerous in the Cupressineae, where they form a dense covering to the axes of the
branchlets (as Thuja, Cupressus, &c.) ; in most of the Abietineae and in Taxus and
Juniperus, they are larger, more sharply separated from the axis, narrow and com-
paratively thick, usually angular and prismatic (acicular); intermediate forms between
these acicular leaves and the broad expanded leaves of Thuja occur in Araucaria
excelsa, &c. In Podocarpus and Dammara the leaves are flat and broader, and in
Salishuria they are stalked and even two-lobed, with a deeply emarginate apex as
if from dichotomous division. Not unfrequently, especially in the Cupressinese,
the foliage-leaves of the elongated primary axis are different in form from those
higher up the same axis and from those on the lateral shoots ; in Thuja, Juniperus
virginiana, Cupressus, Sec, the former are acicular, patent, and of considerable size,
the latter very small and closely appressed to the axis; the youthful foliage some-
times recurs on isolated branches of adult plants. The axis of the shoot within
the bud is so densely covered with the bases of leaves that no free portion of the
surface of the axis is visible between them. When the axis has attained a con-
siderable length on the unfolding of the bud, the bases of the leaves generally grow
at the same time in length and breadth, so that they entirely cover the surface of
the enlarged shoot also, clothing it wiih a green cortex, in which the parts belonging
to the separate leaves can be distinctly recognised. This is especially clear in
Araucaria and many species of Pinus, but is very common also in other genera;
in Thuja, Cupressus, Libocedrus, &c., the axis of the shoot is also completely
covered with these leaf-cushions; but the free parts of the leaves are very small
and often project only as short points or projections. The phyllotaxis is spiral
in the Abietineae, Taxineae, Araucarieae, Podocarpus, &c. ; the Cupressineae bear
whorls which^ above the cotyledons, contain generally from three to five leaves, but

CONIFERM. 511

usually fewer at a greater height on the primary axis. The secondary axes usually
begin at once with decussate pairs, which, in bilateral shoots, are alternately larger
and smaller (as in Callitris and Libocedrus) ; in Juniperus and Frenela the whorls
on the secondary axes also consist of from three to five leaves, and are alternate; the
pairs of leaves of Dammara stand at an acute angle to one another. The foliage-
leaves of most Conifers are very persistent, and may live for several years, their leaf-
cushions keeping pace in growth for a long time with the increase in size of the axis ;
in Larix and Salisburia the leaves alone are deciduous in autumn, in Taxodium
distichum the axes that bear them are also deciduous.

The Flowers of Coniferae are always diclinous; either monoecious, as in the
Abietinege and Thuja, or dioecious, as in Taxus, Salisburia, and Juniperus communis ;
the male are usually much more numerous than the female flowers. They are never
terminal on the primary stem, differing in this respect from those of Cycadeae ; even
the larger woody branches bear only rarely, as in Abies excelsa, terminal (in this
case only female) flowers. Usually the flowers are produced at the apex of small
foliage-shoots of the last order, or in the leaf-axils of the stronger foliage-shoots.
In Thuja, for instance, male and female flowers appear at the end of small short
green shoots of the bilateral system of branches ; in Taxus and Juniperus, on the
other hand, in the axils of foliage-leaves of larger shoots; in Abies pedinata they are
found on the under side of shoots of a higher order at the summit of older trees,
both kinds in the axils of foliage-leayes, the female flowers singly, the male in
larger numbers. The flowers of Pinus sylvestris and allied species appear in the
place of the undeveloped branches (tufts of leaves) which stand in the axils of the
scales of growing woody shoots, the male flowers usually in groups forming an
inflorescence the primary axis of which is the mother-shoot, the female flowers
generally more scattered. In Salisburia the flowers appear exclusively on the
short lateral branches which annually form new rosettes of leaves, and they are
situated in the axils of the foliage-leaves or of the inner bud scales (Fig. 347,
A and B).

, The part of the floral axis immediately beneath the organs of reproduction is
densely covered with scales or foliage-leaves in the female plant of Taxus, Juniperus,
&c. (Figs. 348, 349) ; it is developed as a naked stalk in the Abietineae, Salis-
buria, the male plant of Taxus, Podocarpus, &c. (Fig. 347 A, E). The flowers
of Coniferae resemble those of Cycadeae in the peculiarity that the axis elongates
even at the part that produces the organs of reproduction ; if these are numerous, the
whole flower presents the appearance of a long cone, resembling externally a catkin ;
and this term is indeed given to it in the superficial language of many systematists.
In Angiosperms the flowering shoot usually undergoes a very peculiar development
at its summit, the portion of the axis which bears the flower (the receptacle)
remaining very short and broad, and the floral leaves and organs of reproduction
being formed in positions which diff"er greatly from those of the foliage-leaves ;
in Coniferae the distinction between a floral and a foliage- shoot is much less, and
this is especially conspicuous in the arrangement of the leaves; if those of the
foliage-branches are arranged spirally, so also are usually those of the flowers,
as, ^.^., in the Abietineae; if, on the contrary, as in the Cupressineae, they occur
in alternating whorls, the staminal and carpellary leaves are arranged in the same

5^»

PHANEROGAMS.

way. In Jimiperus co^jununis even the ovules, here the representatives of whole
leaves, are arranged in alternating whorls. But, occasionally, as in Taxus, greater
differences are to be observed in the phyllotaxis of the flowering shoot as compared
with that of the foliage-shoots.

The Male Flowers always consist of a distinctly elongated axis provided with
staminal leaves, and ending above in a naked apex (Fig. 349 A). The stamens are
mostly more delicate and of a different colour from the foliage-leaves, and are usually
divided into a slender pedicel and a peltate lamina bearing the pollen-sacs on its
under side, as in Taxus, the Cupressineae, and Abietineae (Fig, 348 B^ 349 A, B,
350 A). The flat expansion at the end of the pedicel may, however, be entirely
absent, as in Salishuria (Fig. 347 C), where it is reduced to a small knob on which
the pollen-sacs hang. That the parts which bear the pollen-sacs in Coniferae are
beyond doubt metamorphosed leaves, is evident not only from their form, but still

FiC. -i^T.—Salisbiiria adiajitt/olia (natural size). A a short secondary foliage-shoot with female flowers, on the naked axes
of which are placed the ovules sk ; B a male flower ; C part of one magnified, a the pollen-sacs ; D longitudinal section of
an ovule magnified ; E a ripe seed with an abortive one by its side on the floral axis.

more from their arrangement, which has already been spoken of. If the staminal
leaves of the Cycadeae show a resemblance in more than habit to the sporangiferous
leaves of Ferns, those of Coniferae may perhaps be compared to the peltate scales that
bear the sporangia of Equisetaceae ; and not unfrequently, as in Tax us, Juniperus,
&c,, the resemblance of the male flowers to the inflorescence of Equiseiwn is as
striking in external appearance as in the actual agreement between them from a
morphological point of view. The pollen-sacs usually hang, with a narrow base, on
the under side of their support, and do not cohere in their growth ; their number is
usually much smaller than in Cycadeae, but much more variable than in Angio-
sperms ; in the Yew the peltate part of the staminal leaf bears from three to eight,
in the Juniper and most Cupressineae three roundish pollen-sacs (Figs. 348, 349).
Those of Pinus, Abies, and their allies lie in pairs parallel or placed obliquely to
one another, right and left of the pedicel, which here resembles the connective of

CONIFERM.

513

Angi(?sperms; in Araucana and Dammar a, on the other hand, the long sausage-
shaped pollen-sacs hang in larger numbers free beneath the peltate limb. The
wall of the pollen- sacs is usually delicate, and finally dehisces longitudinally to
allow the escape of the pollen-grains, which are produced in extraordinarily large
numbers, since they have usually to be carried by the wind to the female organs
of the same or of another tree. The pollen-grains which happen to fall on the
opening of the micropyle of the ovules are retained by an exuding drop of fluid,
which about this time fills the canal of the micropyle, but afterwards dries up.

Fir,. 348. — 7 axus baccata ; A male flower (mag^nified),
a the pollen-sacs ; B a stamen seen from below with open
pollen-sacs ; C piece of a foliage-shoot with leaf (^, from the
axil of which springs the female flower, s its envelope of
scales, sk the terminal ovule ; D longitudinal section of the
same (magnified), i integument, kk nucellus of the ovule,
X a rudimentary axillary ovule ; E longitudinal section
through a more mature ovule before fertilisation, j integu-
ment, kk nucellus, e endosperm, m aril, s upper scales of
the envelope.

Fig. ■^Ag.—yuntperns communis; A longitudinal
section of a male flower, B (upper figure) a stamen seen
from the front and the outside, (lower figure) seen from
the back of the axis; Clongitudinal section of a female
flower; a the pollen-sacs, j the peltate lamina of the
stamen, b lower leaves of the floral axis, c carpels,
sk ovules, kk nucellus, i the integument (A and C X
about 12).

and thus draws the captured pollen-grains to the nucellus, where they immediately
emit their pollen-tubes into its spongy tissue. In the Cupressineae, Taxineae, and
Podocarpese this contrivance is sufficient, since the micropyles project outwardly;
in the Abietineae, where they are more concealed among the scales and bracts,
these themselves form, at the time of pollination, canals and channels for this
purpose, through which the pollen-grains arrive at the micropyles filled with fluid
((/". Strasburger, /. <:.). The large number and lightness of the pollen-grains enables
them to be carried great distances by the wind; in the true Pines and the Podo-

l1

5H

PHANEROGAMS.

tarpeae their capacity for transport is increased by the vesicular hollow protrusions
of the extine, as represented in Fig. 351, 7V, V, hi. [The pollen-sacs (microspo-
rangia) of the Coniferae resemble the sporangia of the Vascular Cryptogams in
the mode of their development. A section through the pollen-sac of one of the
Cupressineae, for example, shows that it resembles a sporangium of Lycopodium :

Fk;. j^o.—Adtes pectinata ; A a male flower, b the delicate bud-scales forming a perianth, a the stamens ; B a pollen-grain
(after Schacht), e its extine, forming the two large vesicular protrusions bl.

in the centre is a group of sporogenous cells surrounded by a layer of flattened
tabular cells, the tapetum, and externally is the wall of the sporangium. From
Goebel's^ researches it appears that the archesporium, in Biota orientalis at any
rate, is a hypodermal cell, the terminal cell of one of the axial rows of cells of

Fig. 351.— y^ pollen-grain of Thuja orientalis before its escape from the pollen-sac, / fresh, //, /// a''ter lying in
water, the extine e having been stripped off by the swelling of the intine i; B pollen-grain o( Piiius Pinaster before its
escape, e the extine with its vesicular protuberances bl.

which the young pollen-sac consists. Possibly the archesporium consists in this
case, as in Lycopodium, of a transverse row of such hypodermal cells. The cells
of the tapetum are derived here, as in Selaginella, partly from the archesporium
and paitly from the tissue of the wall of the pollen-sac. In Pinus the pollen-sac

^ [Goebel, Vergl. Entwick. d. Sporangien, Bot. Zeitg., 1881.]

CONIFFRM. • ' S^5

is developed from a group of superficial cells : these multiply by repeated division,
so that a protuberance is formed consisting of a number of rows of cells invested
by a parietal layer: the terminal cell of the lowest of these rows becomes the
archesporium, and it is therefore, in this case also, a hypodermal cell. The cells
of the tapetum are derived entirely from the parietal cells of the pollen-sac, just
as is the case in the sporangium of Lycopodhim. Goebel draws attention to the
fact that in the majority of the Cupressineae the pollen-sacs are protected by an
outgrowth of the staminal leaf which he considers to be analogous to the indusium
of Ferns.]

A formation of cells, recalling the rudimentary development of a prothallium
in the microspores of Selaginella and Isoetes, takes place in the pollen-grains of
the Coniferae before their dissemination, in the same manner as in those of the
Cycadeae (compare Fig. 3^). The process is a very simple one in Taxus
Podocarpus, the Cupressineae, Araucaria and in the true Pines. The contents of
the pollen-grain are divided by a septum so as to form a large and a small cell,
the latter undergoing no subsequent change (Fig. 351). In the other Abietineae
the septum becomes arched into the cavity of the larger cell, and a second septum
is formed in the smaller cell, so that a two-celled body is formed which is situated,
as in the Cycadeae, at the posterior end of the grain. At this point the extine
presents a split which was formerly regarded by Schacht as being the basal cell
of the internal group, but which, according to Strasburger, arises in the same
way as the vesicular expansions of the extine found in the grains of many species
of Pinus. Strasburger has shown that in all cases it is the large cell cf the
pollen-grain which grows out into the pollen-tube, as is the case in Cycadeae
(Fig. 345). If we pursue the comparison with the highest Cryptogams which
has been already initiated, we must regard the pollen-tube as representing the
antheridium to some extent, and the internal group of cells, formed before the
development of the pollen-tube, as corresponding to the vegetative cell in the
microspore of the Ligulatae, for, like it, it undergoes no subsequent change. A
peculiarity which distinguishes the pollen-grain of Conifers from that of Angiosperms
lies in the rupture and final stripping off of the extine by th^ swelling of the
intine (Fig. 351, /, //, ///). Even in this apparently insignificant fact a resem-
blance is again seen to the microspores of Cryptogams, and especially to those of
Marsiliaceae, in which the swelling endospore protrudes through the exospore.

The structure of the Female Flowers is very different in the different sections of
Coniferae, and in some cases the homology of the separate parts is still doubtful.
The position of the ovules, as far as can be judged from advanced stages of develop-
ment, is, in particular, very variable, and with this is again connected the fact that
different opinions may be entertained as to the part which should be called the
carpel. The following description of these structures, a full discussion of which is
not permitted by our limited space, is drawn immediately from the observation of
advanced stages of development ; it is possible, however, that the direct observations
of the most rudimentary stage will cause an alteration in some points ^

The female flowers of Taxus spring from the axils of foliage -leaves belonging

Compare Strasburger and Eichler, loc. cit.

l1 2

51 6 PHANEROGAMS.

to elongated woody shoots. They have the form of short branches covered with
decussate scale-like bracts (Fig. 348, C, D); the axis of the shoot ends in an
apparently terminal ovule, the nucellus of which has the appearance of being
the vegetative cone of the axis, but axillary ovules also occur. In Salishuria the
female flowers spring from the axils of foliage-leaves belonging to short lateral
branches which annually produce new rosettes of leaves (Fig. 347 A)\ the single
flower consists of a stalk-like elongated axis which bears immediately beneath its
apex two or more rarely three lateral ovules. Neither in this genus nor in Taxus
are there any foliar structures close to the ovules which either from their position
or from any other circumstance can be regarded as carpels. In the genus Podo-
carpus small flowering shoots are developed, springing in P. chinensis (according
to Braun) from the axils of foliage-leaves, in P. chilina from the axils of very
small scale-leaves at the end of elongated leafy shoots ; they consist of an axial
structure slender and stalk-like below, club-shaped above, and bearing three pairs
of very small decussate scales. The floral axis terminates between the upper
pair; the ovules, in this case anatropous, with their micropyle turned downwards
and towards the floral axis, spring from the axils of this pair ; one ovule however
is usually abortive, and the flower becomes one-seeded. In Phyllocladus the lower
lateral branchlets of the leaf-like flattened shoots are transformed into female
flowers which are raised upon a pedicel and are swollen above into the form of
a club, the large ovules standing (according to a drawing of Decaisne's^) in the
axils of small leaves. In these two genera the small scales from the axils of
which the ovules spring may be regarded as carpels, if it is thought necessary to
assume the existence of these organs.

The ovules of Juniperus communis (Fig. 349, C) stand in whorls of threes
beneath the naked extremity of the floral axis, the flower springing as a little
shoot from the axil of a foliage-leaf, and its axis bearing whorls of three leaves.
The ovules apparently alternate with the three leaves of the upper whorl, and
hence must, from their position, be themselves considered as metamorphosed
leaves; these leaves of the upper whorl swell after fertilisation, grow together
and become fleshy, forming the pulp of the juniper-berry in which the ripe seeds
are entirely enclosed ; they may therefore be termed carpels. In the other Cupres-
sineae the flower consists of decussate whorls of two or three leaves, which grow
considerably after fertilisation and attain a considerable size, enveloping the seed
and forming a pericarp which may therefore correctly be said to be formed of
carpels. In Sabina the pericarp is fleshy and berry-like, as in Juniperus; in the
other genera, on the other hand {Thuja, Cupressus, Calliiris and Taxodium),
the carpels become woody and assume the form of stalked peltate scales, or of
valves separating from one another longitudinally {Freneld) ; these are closely
approximate during the development of the seed, but afterwards open to allow
the ripe seeds to fall out. The erect ovules of Cupressineae sometimes appear
to stand in the axils of the carpels; but it is clear in other cases that they
spring from the carpels themselves, either low down near their point of insertion

^ [See Le Maout and Decaisne's Descriptive and Analytical Botany, edited by Dr. Hooker,
London. 1873, p. 747.]

CONIFERM. 517

or at a greater height. In Sabina and Callitris quadrivalvis (Fig. 352) only two
decussate pairs of carpels separate like a star at the time of flowering ; in Sabina
the ovules stand in pairs in the axils of the two lower carpels, right and left of
their median line, some of them being frequently abortive ; in Callitris quadri^
valvis a pair occurs on each of the lower carpels and a pair higher up ; but this
position can only be explained by further investigation of the history of their
development. In Thuja and Cupressus there are three or four decussate pairs of
carpels, in Taxodium a still larger number; in Thuja and Taxodium two erect
ovules are situated at the base of each of the central pairs of carpels, springing
from the right and left of their median line \ in Cupressus there are a considerable
number at the base of each carpel. In Arceuthos drupacea and Frenela verrucosa
the fruits (in the collection at Wurzburg) consist of alternating whorls of three
carpels, opening, in the last species, after the seeds become ripe, like a six-lobed
capsule. Each carpel is swollen on its inner side into a thick placenta ascending
from the base to the apex, and bearing numerous winged seeds which stand in
transverse rows of threes ; there are from four to six of these rows on each carpel,
the whole inner side therefore bearing seeds nearly up to the apex.

So far as the relative positions of the parts of the flower can be explained

Fig. zS^-— Callitris quadrivalvis ; A female flower (magnified) ; d d two pairs of decussate leaves (carpels) m the
axils of which are six ovules (Ks) ; j5 vertical longitudinal section of an ovule through its broader diameter ; AT^the
nucellus still without an embryo-sac ; i the tubular elongated integument with the micropyle nt,

without going back to their earliest stage, a great diversity is thus shown in the
two families of Taxineae and Cupressinese ; the ovule is terminal in Taxus^ lateral
beneath the summit of the axis in Salisburia, carpellary leaves appearing to be
entirely absent. In Podocarpus and Phyllocladus they are indicated indeed, as small
scales, the ovules springing from their axils ; but they are small and do not at any
time constitute a pericarp. A structure of this kind, in the form of a berry or of a
chambered woody fruit, is indeed formed after fertilisation in the Cupressineae, the
carpels either becoming fleshy and growing together (as in Juniperus and Sabina),
or becoming woody and closing in laterally by their peltate expansions (as in
Cupressus, Thuja, and Callitris), or presenting the appearance of the lobes of a
unilocular capsule {e.g. Frenela); but the carpels are in these cases at first entirely
open. In Juniperus communis the ovules form a whorl alternating with the carpels ;
in the other genera they stand in pairs or in larger numbers at their base, or cover
the whole of their inner side (as Frenela).

In the Abietine^ the well-known cones are the female flowers (or rather fruits).
The cone is a metamorphosed shoot, its axis bearing a number of crowded woody
scales arranged spirally, the ovules arising on them rarely singly, usually in pairs,
occasionally in larger numbers. In the Abietineae i^Abies, Picea, Larix, Cedrus, and

5i8

PHANEROGAMS.

Pinus) the seminiferous scales (Fig. 353, A, B, s) appear to be axillary striiclures
in the axils of bracts (c) which sprinyj from the axis of the cone ; but the exami-
nation of very young cones of Adi'es peclinata shows that the seminiferous scale
itself arises as a protuberance at the base of the bract (r), and is therefore not
axillary. While the bract afterwards grows very little or not at all, this protuber-
ance increases greatly, and produces on its upper surface two ovules which are

attached to it by one side with the micropylc
towards the axis of the cone. The seminiferous
scale of these genera must therefore be con-
sidered as a greatly developed placenta growing
out of a carpel (Fig. 353 A, B, c) which is very
small or even abortive ^ According to this view
the whole cone is a single flower with a number
of small open carpels (hilherLo considered as
bracts), which are far outstripped in their growth
by their seminiferous placentae (the scales). In
the other Abietineae also, the female flowers of
which 1 have had no opportunity of examining,
it may be concluded from the descriptions that
the cone is a single flower with numerous semi-
niferous scales arranged spirally, not springing
from the axils of leaves, but growing imme-
diately out of the axis of the cone, and therefore
themselves leaves and of a carpellary nature.
Eichler (/. c.) says, in reference to Dammara,
Cunninghamia, Arthrotaxis, and Sequoia: — 'The
scales of a cone are in these genera all of one
kind; they consist simply of open carpels; and,
in order not to introduce confusion into the
definition of a flower, the whole of what is found
on the axis, in other words the whole cone, must
be considered a single flower; and this is also
necessary in the case of the Araucarieas, the
Cupressineae, and the male 'catkins' of all Coni-
ferae^' In Araucaria each scale (or carpel)
bears only a single ovule, which, according to
Eichler, is so enveloped by it that the only

Fig. 353.— ^^jfj- pectinata (after Schacht). A a
leaf detached from the female floral axis seen from
above, with the seminiferous scale j bearing the
ovules sk (magnified) ; B upper part of the female
flower (or cone) in the mature state ; sp floral axis or
axis of the cone, fits leaves, j the largely developed
seminiferous scales ; C a ripe seminiferous scale
with the two seeds sa, f the wing of the seed
(reduced).

^ Biaun, Caspary, and Eichler consider the seminiferous scale in Pinus, and Larix as itself a
flower; i. e. as a short axis which has coalesced with its two carpels and stands in the axil of the bract
{c in our figure). In that case the cone of these genera, in contradistinction to that of the other
Coniferse and of Cycadese, would be an inflorescence {cf. Caspary in Ann. des Sci. Nat. 4th series,
vol. XIV, p. 200, and Flora, 1862, p. 377) ; but this view I have already contested more in detail in
my first edition, p. 427. It is impossible to consider the seminiferous scale o{ Phms and Abies itself
as a single carpel. In opposition also to the most recent views of Mohl (Bot. Zeitg. 187 1, p. 22, and
of Strasbuiger), I cannot bring myself to consider the seminiferous scale of the true Abietineae as a
coherent structure formed of two leaves of an undeveloped branch.

^ Eichler thinks that an exception must be made in favour of Podocarpus and Cephalolaxus.

CONIFERS. 519

opening left is that of the micropyle which faces the axis of the cone ; in Cun-
ninghamia there are three ovules, in Arihrotaxis from three to five, in Sequoia
from five to seven, in Sciadopitys as many as seven or eight on one scale, and
their micropyle here also faces the axis of the cone. In Dairimara the scale
bears, according to Endlicher ^ only one ovule which, like those of Sequoia and
Sciadopitys, are inserted near the apex and hangs down free^.

^ [Van Tieghem has been led by studying the distribution of the bundles in the different parts of
the female bud of Coniferte to the opinion — different from that expressed by Sachs— that the female
flower throughout this group of plants is in every case constructed after a single fundamental type
which has undergone various secondary modifications. He has given in a note to his French trans-
lation of the present work the following abstract of the conclusions which are worked out in greater
detail in his paper already cited in the Annales des Sciences Naturelles.

Neither the axis of the female bud nor its leaves or bracts of the first order ever bear ovules. It
is always upon structures arising from the axils of these bracts that the ovules make their appearance.
This establishes a fundamental distinction between Cycadese and Coniferse. In the former group it
is always the leaves of the female bud of the first order that produce the ovules directly. While
therefore we may regard the female bud in Cycadese as well as the male as contributing a single
flower, this does not hold good in the case of Coniferse. We may if we please regard the male bud
of Coniferse as a single flower, but the female bud is an inflorescence. The structure which bears
the ovule in Coniferse is always a foliar organ — the first and only leaf of an axis which undergoes no
further development. This leaf, which is more or less largely developed beyond the circumscription
of the ovule or ovules which it bears, is an open carpel and in itself constitutes the whole female
flower. It is always inverted, that is to say, it arises upon the suppressed axis which bears it with
its ventral face opposite to and united with the ventral face of the primary bract. When the ovules
do not terminate the carpel, it is upon its structurally dorsal — but in respect of position upper — face
that they arise, just as it is upon its structurally dorsal — but in respect of position lower — face that
the pollen-sacs arise upon ths stamen.

This is the general type. It remains to consider the principal secondary modifications which are
superinduced upon it in the different genera.

The axillary branch, which is reduced to its first leaf, is most frequently of the first generation
in respect to the axis of the female bud ; but it is also sometimes of the second (Tnxus) and may
even be of the third order {Torreya). The carpel itself is either entirely distinct from the parent
bract (the Pinese, Taxinese) or the two leaves are united together by their ventral surfaces and are
only separate towards their summit (Cupressinese, Sequoiese, Araucariese). This difference merely
depends upon a different localisation of the intercalary growth of the two leaves ; it is a difference
the same in kind as that which separates a dialypetalous corolla from a gamopetalous one. Whether
free or united with the bract, the carpellary leaf bears its ovules sometimes towards its base
(Cupressineoe), sometimes towards its middle (Pinese), sometimes towards its summit (Araucariese) ;
each represents a lobe, more or less developed, of the dorsal face of the carpel.

In the TaxinecE the ovules terminate the carpellary leaf; they result in this case from the trans-
formation of its who.e entire limb, whether each half of the limb forms an ovule {Salisburia, Cepha-
lotaxus), or whether the entire limb has only produced a single one {Podocarpus, Phyllocladns, Taxris,
Torreya, &c.). In this case it is evidently only the petiole of the ovuliferous leaf which represents the
carpel ; if the petiole is long (Salisburia) the carpel is obviously developed ; but if it remains very
short {Cephalotaxus, Podocarpus, Phyllocladns, Taxus, Torreya, &c.) the carpel is alrnost absent — in
otl.er words, the carpellary leaf is reduced to a sessile limb completely converted into a single ovule
{Podocarpus',^ Taxus, &e.) or into two ovules {Cephalotaxus). The number of the ovules which each
carpellary leaf bears, as well as the number of carpellary leaves themselves, that is to say, of the
female flowers which enter into the composition of the inflorescence, both vary, and may even be
simultaneously reduced to unity, which is the ordinary case in Taxus.]

2 [For a review of the literature of the question whether the ovules of Coniferx are really naked
or whether there is a true ovary, see Eichler, ' Sind die Coniferen gymnosperm oder nicht ? ' in ' Flora'
for 1873, translated in Trans. Bot. Soc. Edin. 1873, PP- 535-541- ^^- Eichler here, in opposition to
the contrary view of Strasburger, sums up the whole argument strongly in favour of the opinion that

520 PHANEROGAMS.

The Ovules (macrosporangia), as we have already seen, are in the Podocarpeae
anatropous and furnished with two integuments ; in the rest of Coniferae they are
orthotropous and possess only one integument ; in the Cupressineae and Taxineae
they are erect, in the Abietineae inverted, with the micropyle towards the base of
the scale, to which the ovules are usually attached on one side. In these cases
there is no funiculus, and the ovule consists only of the small-celled nucellus and
one integument, which usually projects above it and forms a comparatively wide
and long micropylar canal, through which the pollen-grains reach the apex of
the nucellus, which is sometimes depressed (see Figs. 347, 348, 349, 352). Lateral
outgrowths of the integument not unfrequently cause the ovule, and afterwards
the seed, to appear winged on both sides, as in Callitris quadrivalvis (Fig. 352),
Frenela, &c. The wing-like appendage of the seed of Pinus and Abies, on the
other hand, is the result of the detaching of a plate of tissue from the seminiferous
scale, which remains attached lo the ripe seed.

[The Development 0/ the Ovule and 0/ the Embryo-sac. The development of the
ovule begins in Taxus (according to Strasburger, Angiospermen und Gymno-
spermen) with the division, parallel to the surface (periclinal), of a group of hypo-
dermal cells at the punctum vegetationis of a lateral branch, and this is followed by
similar divisions in the overlying epidermis. The rudiment of the ovule is sur-
rounded by a projecting ring of tissue, also developed by the division of hypodermal
cells, which is the first indication of the integument. The ovule elongates in
consequence of growth and repeated cell-divisions, and consists internally of a
number of longitudinal rows of cells, which have all been derived from the hypo-
dermal layer, invested by several layers of cells which have been derived from the
epidermal layer. The apical cells of these internal rows are distinguished by their
size and by the granularity of their protoplasm ; they constitute the archesporium :
they are surrounded by tapetal cells, but it is not clear whether the tapetum is
derived from the archesporium or from the surrounding cells of the nucellus. One,
but sometimes more, of the archesporial cells, usually the central one, divides so as
to form three cells lying one above the other, and it is the lowest of these which
enlarges and becomes the embryo-sac. The development of the ovule in Ginkgo
hiloba and in Podocarpus chinensis follows essentially the same course, as does also
that of Thuja occidenialis. It will be observed that in these plants several embryo-
sacs are frequently indicated at first, but only one attains perfect development.

In the Abietinese [Larix europcEa, Pinus sylvestris and Pumilio) the multipli-
cation of cells by the repeated division of the hypodermal and epidermal cells is
much less considerable. The archesporium is unicellular; as it elongates a small
cell is cut off from it towards the free surface of the ovule, which is a tapetal cell ;
the lower cell now divides into two, and of these two the upper divides again into
two ; the lowest of these three cells developes into the embryo-sac. Only one
embryo-sac is ever indicated in the group.

A section of the young ovule of one of the Cupressineae, before the embryo-
sac is developed, presents, as Goebel has pointed out, a considerable similarity

the Conifers are really gymnospermous. In his most recent work on the subject (Die Angiospermen
und die Gymnospermen) Strasburger accepts the view of the gymnospermous nature of the ovule.]

CONIFERJE.

521

to a section of a young sporangium of Lycopodiwn : the archesporium consists
in both cases of a group of cells bounded by a tapetal layer. There is reason,
inasmuch as their mode of development is the same, to regard these cells in the
Cupressineae as being, like the archesporial cells of Lycopodi'ufn, spore-mother-cells ;
but whereas in Lycopodium each of these mother-cells produces four spores, in
the Cupressineae only one of them persists and it produces only one spore, the
embryo-sac. The embryo-sac is then the equivalent of the macrospore of the
heterosporous Vascular Cryptogams.

In the Abietinese, the archesporium has become much reduced as compared
with that of the Cupressineae. The Cupressineae resemble the higher Vascular
Cryptogams in the mode of development of the ovule and embryo-sac, whereas the
Abietineae approach the Angiosperms.]

Develop7neni of the Endosperm (Prothallium). The first stage in the development
of the endosperm is the division of the nucleus of the embryo-sac ; each of the two
nuclei divides again, and this process is repeated until a number of nuclei have been
formed which lie in the peripheral protoplasm. Around each nucleus the protoplasm
becomes marked off by an ectoplasmic layer, cell-walls are formed, and thus one
or two layers of endosperm-cells are formed in the embryo-sac in contact with its
wall. These cells grow in the radial direction, and divide in such a manner that
the embryo-sac is filled with parenchymatous tissue. In those Coniferae in which
the seeds take two years to ripen, as Pinus sylvestris and Juniperus communis^
the endosperm formed in the first summer is again absorbed in the spring, the
protoplasm of the primary endosperm-cells is set free by the deliquescence of their
cell-walls, and forms by division a number of new cells which, in May of the second
year, again fill with parenchymatous tissue the embryo-sac now considerably in-
creased in size.

The archegonia (corpuscula) are developed, according to Strasburger's obser-
vations, from superficial cells of the prothallium (endosperm) in the same manner
as the archegonia of the highest Cryptogams. The mother-cell increases in size
and is divided by a septum parallel to the surface of the investing embryo-sac ;
a large inner (lower) cell is thus formed, the central cell of the archegonium, and
an upper small one, lying next the embryo-sac from which the neck of the
archegonium is formed^. In Abies canadensis this neck remains simple and uni-
cellular, and elongates considerably with the increase in size of the surrounding
endosperm ; but usually the original cell which constitutes the neck divides into
several cells which either lie only in one plane (Figs. 354 A, d, 355 /, d\ the
' stigmatic cells,' or form several layers lying one over another (as in Abies excelsa
and Pinus Pinaster). Seen from above the neck appears to form a four-celled,
or, in Abies excelsa, even an eight-celled rosette. The homology of the ' corpus-
culum' with the archegonium of Vascular Cryptogams, already established by the
earlier investigations of Hofmeister, is carried a step further by Strasburger, who
discovered the formation also of a canal-cell. He considers that the part of the
protoplasmic contents of the large central cell which lies immediately beneath the

^ Hofmeister (Vergleichende Untersuchungen, p. 129) gives a somewhat different account of the
origin of the archegonium [Germination, &c., p. 410].

522

PHANEROGAMS.

neck is separated from the rest by division, and a small cell is thus formed shortly
before fertilisation {t. e. before the access of the pollen-tube to the endosperm) ;
this cell being clearly equivalent to the canal-cell so often mentioned in Vascular
Cryptogams which is afterwards converted into mucilage ^ In Abies canadensis and
excelsa and Pinus Larix this canal-cell is, according to Strasburger, very evident ;
while in the Cupressineae {Thuja, Juniperus, and Callilris) its demarcation from the
rest of the contents of the central cell is only slight. As in those Vascular Crypto-

FIG. 354. — Taxiis canadensis (after Hofmeister). A longitudinal section through the upper end of the endosperm <?«
and the lower end of the pollen-tube/, c c the archegonia, d their stigniatic cells, the left archegoniuin is fertilised, x rudi-
ment of the embryo (June 5), (X 300). £ part of the endosperm with an archegonium, the embryo of which v is already
further developed, p the pollen-tube (June 10) (X 200) ; C longitudinal section of a nucellus (June 15), kk nucellus, e e endo-
sperm,/ pollen-tube, v v embryos proceeding from two archegonia (X 50).

gams where the ventral part of the archegonium is plunged in the tissue of the
pro thallium, the neighbouring cells become transformed by further divisions into a
parietal layer surrounding the oosphere, so the same thing takes place also in the
endosperm of Coniferae. In the Abietineae each archegonium is separated from an
adjacent one by at least one, often by a large number of layers of cells : those of the
Cupressineae, on the other hand (Fig. 355, cp), are in lateral contact. The arche-

^ In Y'lgi
indicated.

354 and 355, which are transferred from the first edition, the caial-cell is not

CONIFERM.

523

gonia of Taxus are short ; in those of the Abietinese the oosphere is elongated ;
in the Cupressineae it becomes angular from the pressure of the adjacent cells.
The number of the archegonia which are formed in the endosperm beneath the
apex of the embryo-sac is very various ; Hofmeister and Strasburger stale that in
the Abietineae it is from three to five, in the Cupressineae from five to fifteen
(according to Schacht it may even be thirty), in Taxus baccata from five to eight.
The continuous growth of the surrounding endosperm causes the formation of
funnel-shaped depressions above the archegonia, which in some Abietineae are but

F'<^- 2SS-—y>"i'perus communis (after HofmeisteH- / three archegonia cp close beside one another, in two of them
the rudimentary embryo ei\s seen at the lower end of tlie oosphere, rf neck-cells, / pollen-tube (July z8) (X 300). // a
similar section, ^^ the endosperm, v v the embryos : /// lower end of one of a suspensor with the rudiment of the embryo
eb; /K longitudinal section of the nucellus kk, e the endosperm, e' portion of the endosperm that is bn>keu up,/ pollen-
tube, cp the archegonia, v the suspensors (beginning of August) (X 80).

shallow, in Pinus Pmaskr, P. Sirobus, &c., deep and narrow. In these species each
of the funnel-shaped depressions leads down only to the neck of one archegonium ;
in the Cupressineae {Calliiris, Thuja, and Juniperus), where they lie closely crowded
together, the cluster of them is walled round by the endosperm, and a funnel is
formed common to them all, which still remains closed by the cell-wall of the
embryo-sac.

Fertilisation. The pollination of the ovules takes place before the archegonia
are formed in the .endosperm ; the pollen-grains, having reached the apex of the

L

5^4 PHANEROGAMS.

nucellus, put out their tubes at first only for a short distance into its tissue ; their
growth is then for a time suspended. After the archegonia are completely de-
veloped, the pollen-tubes begin to grow again into the endosperm in order to
reach them'. This interruption of their growth lasts, in those Coniferse whose
seeds ripen in a single year, for only a few weeks or a month; when the seeds
take two years to ripen, as in Juniperus sibirica and communis, and Pinus sylvestris
and P. Strobus, it lasts until June of the next year. Whilst the pollen-tubes pene-
trate through the loose portion of the tissue of the nucellus, their width gradually
increases at their lower end, their wall becoming at the same time thicker ; until at
length they meet the wall of the embryo-sac which has now become soft, break
through it, penetrate into the funnel of the endosperm mentioned above, and attach
themselves firmly to the cells of the neck of the archegonia. In the Abietineae and
Taxinese each pollen-tube fertilises only one archegonium ; and several tubes there-
fore penetrate into the funnel at the same time ; in the Cupressineae on the contrary
one pollen-tube suffices for the fertilisation of the whole group of archegonia beneath
the broad funnel of the endosperm. The tube entirely fills up the funnel and applies
itself to the necks of the whole group of archegonia; short narrow protuberances
from the wide pollen- tube now grow into the separate necks of the archegonia,
forcing the neck-cells from one another and destroying them, and at length reaching
the oosphere. The same process takes place in the Abietineae and Taxineae; the
pollen-tube, after widening, becomes narrower and enters the neck of only one
archegonium, and penetrates finally as far as the oosphere. A thin spot may be
observed at the extremity of this protuberance of the thick-walled pollen-tube, which
obviously facilitates the escape of the fertilising substance ; and this is probably
assisted by the pressure exerted by the tissue which lies above on the part of the
pollen-tube outside the archegonium. Hofmeister states that a few free primordial
cells (Fig. 355, /) are sometimes formed in the end of the pollen-tube, which he was
inclined to consider as rudimentary indications of mother-cells of antherozoids
(corresponding somewhat to those in Salvinia), and Strasburger has detected the
existence of bodies of this kind in Juniperus and Pinus. [The process of fertilisation
takes place, according to Strasburger^, as follows. \xi Juniperus virginiana he ob-
served that when the apex of the pollen-tube reached the archegonia, the primordial
cells in the end of the tube arrange themselves so that they lie over the archegonia.
These cells now undergo absorption, the more anterior ones disappearing first. He
frequently detected in the oosphere of Picea^ at the time of fertilisation, two nuclei.
One of these is the nucleus of the oosphere, and may be termed the ' female pro-
nucleus ;' the other appears to have passed into the oosphere from the pollen-tube,
and is the 'male pronucleus' {spermakern). These two nuclei coalesce to form the
definitive nucleus of the oospore. It seems that, in addition to the nuclear sub-
stance, a certain amount of protoplasm also passes into the oosphere from the
pollen-tube, and fuses with the protoplasm of the oosphere. The pollen-tube of
the Abietineae contains a considerable number of starch-grains, and these too are

* In Salishtiria {GingT<o hiloba) fertilisation does not take place until October, when the seed is ripe
and has already fallen off. The embryo is developed within the seed during the winter months. (See
Strasburger, Die Coniferen und Gnetaceen, 1872, p. 291.)

2 [Ueber Befruchtung und Zelltheilung, 18/8.]

conifer:^. ^ 525

conveyed, probably in a soluble form, into the oosphere. In the Cupressineae, the
pollen-tube contains no starch-grains, but they appear abundantly in the nucleus of
the oospore. The pollen-tube remains completely closed during the whole process.
The first manifest result of fertilisation in the oospore is the division of its nucleus\
In Pinus and Picea the nucleus usually travels to the lower end of the oospore ; it
then divides into two, and each of these divides again into two, and thus four nuclei
are formed which lie in one horizontal plane. Around each of them protoplasm
becomes aggregated, and thus a formation of cells takes place. It is from these cells
that the embryo or embryos are developed. Exactly the same takes place mjuniperus.
In Ginkgo {Sah'sburm) the nucleus divides, and this is repeated until a large number
of nuclei have been formed; a process of free cell- formation is then initiated round
these nuclei ; the cells become separated by cellulose walls, multiply by division, and
together constitute the embryo.]

The mode of origin of the embryo is very different in the various families, but in
nearly all of them certain cells form a suspensor, and, by their great elongation, carry
the rudimentary embryo, which lies at the apex of the suspensor, through the base of
the archegonium out into the tissue of the endosperm where the embryo developes
into a young plant ^. In the Cupressineae the cell from which the embryo is de-
veloped (Fig. 355) becomes divided so as to give rise to three superposed cells, of
which, in Thuja occidentalism only the upper two become divided into four by longi-
tudinal walls, whilst in the lowest the rudiment of the embryo is formed by means of
oblique septa : the embryo is then extruded from the archegonium by the elongation
of the upper cells. In this case an oospore gives rise to a single embryo, growing
at first by means of an apical cell from which two rows of segments are cut off but
which it afterwards loses. In Juniperus, on the other hand, the lowermost of these
three cells becomes divided into four by intersecting longitudinal walls, which are
forced out by the elongation of the cells above them : these four cells become
rounded off and they separate, each giving rise to a rudimentary embryo. In this
case one oospore gives rise to four embryos, only one of which developes into a
plant. The first development of the suspensor in the Abietinese is somewhat
different. The four cells lying side by side in one plane become divided by trans-
verse septa so as to form three tiers ; the cells of the second tier grow out into long
coiled filaments, whereas those of the uppermost tier remain in the oospore as a
rosette : the four cells of the lowest tier, which have been forced into the endosperm
by the elongation of those above them, are repeatedly divided and thus contribute to
the elongation of the suspensor ; the four rows of cells then separate, each bearing
a terminal cell from which the rudiment of the embryo is developed in such a
manner that the possibility of the existence of an active apical cell is excluded^.
Thus in the Abietineae also four rudimentary embryos are developed from a single

^ [Strasburger, Zellbildung und Zelltheilung, 3rd edition, p. 46.]

"^ [Strasburger has observed (Angiospermen u, Gymnospermen, p. 149) that in Cephalotaxus For-
tunei and in Araucaria hradliana certain cells at the apex of the embryo are thrown off when it has
made its way into the endosperm.]

^ [Skrobiszewski has pointed out (Bull, de la Soc. Imp. des Natural, de Moscow, 1873), and his
observations have been confirmed by Strasburger (Die Angiospermen und die Gymnospermen), that
the embryo of Pinus Slrobus, unlike that of the other Abietinese, grows in length by means of a two-
sided apical cell.]

526 PIIA NER 0 GA MS.

oospore; but Picra imJgaris in this group resembles /?wiperus in the preceding, in
that only a single embryo is developed. In Taxus haccafa the rudiment of the
embryo consists of two or three tiers of cells, the upper of which elongate to form
the suspensor : the lower tier consists of from four to six cells, one only of which
gives rise to the embryo : the suspensors do not become isolated (Fig. 354). In
Salisbun'a, according to Strasburger, no suspensor is formed : the cells which are
formed in the oospore, after fertilisation, form a coherent mass of tissue, which
increases by cell-division, and this constitutes the embryo.

It appears, therefore, that several embryos can be produced from one oospore ;
the number within a single endosperm being increased by the simultaneous fertilisation
of severa larchegonia. Polyembryony, which is rare among Angiosperms (see infra),
is thus the typical condition among Conifers and generally among Gymnosperms,
but only in the very earliest stage ; for usually only one of the rudiments developes
into a vigorous embryo, such as has already been described. During its development
the endosperm also continues to grow vigorously; its cells become filled with reserves
of food-material (fat and albuminoids) ; the embryo-sac which surrounds it grows at
the same time, and finally supplants the tissue of the nucellus, the tissue of the in-
tegument hardening at the same time into the testa. In Salisburia, however, an
outer strong layer of tissue forms the pulpy envelope which causes the seed to
resemble a drupe. The elongated cells of the suspensor usually disappear during
these processes, but according to Schacht are permanent in Larix.

During the period that the seeds are ripening, the carpels and the placentae
also continue to grow and to undergo changes in texture. In Taxus a red aril,
which afterwards becomes pulpy, grows round the ripening seed (Fig. 348 m)\ in
Podocarpus the part of the floral axis that bears the scales and the seeds, and which
was already considerably swollen, becomes fleshy; in Juniperus and Sahina the
carpels themselves form the blue 'berry' which envelopes the seeds: in most other
Cupressineae the carpels grow, close up laterally and become woody ; and the same
occurs in those Abietineae which are without bracts (in respect to Cunninghamia,
vide supra) ; while in Pinus, Abies, C^drus, and Larix, it is the placental scales
which after fertilisation grow vigorously, outstripping in their growth the true carpels
(bracts), become woody, and form the mature cone. In all these cases (except
Podocarpus, Salisburia, and Taxus), the seed is closely and firmly enclosed during
ripening by the carpels or placental scales ; it ripens within the fruit, the parts of
which do not again separate or become detached (as in Abies pec tinata) in order to
allow of the escape of the seeds until they are completely ripe.

So long as we are still in doubt as to the nature of the female flowers of various
genera, the systematic arrangement of the Goniferae can only be considered as provi-
sional; Endlicher (Synopsis Coniferarum, 1847) distinguishes the following families: —

Family r. Cupressineaa. Leaves, including those of the flowers, opposite or
verticillate (in Division e single); flowers monoecious or dioecious; stamens terminating
in a shield-like expansion bearing pollen-sacs in twos or threes or larger numbers;
female flower consisting of alternate whorls of carpels, bearing at their base or on their
inner surface two or a larger number of erect ovules (in Juniperus communis the ovules
are alternate with the three carpels on the floral axis) ; embryo with two, rarely three
or nine cotyledons.

GNETACEJE. ^1"]

(a) Juniperinetp. Fruit berry-like {Juniperiis, Sabiha).

(b) Act'mostrcbe(v. Carpels united into valves ; afterwards separating as a four- or

six-rayed star {Widdringtonia, Frenela, Actinostrobus^ Callttris, Libocedrus).

(c) Thujopside(e. Carpels partially overlapping one another {Biota, Thuja, Thujopsis.)

(d) Cupressinece vercB. Carpels peltate and polygonal in front (Cupressus, Ch^-

moEcyparis).

(e) Taxodineee. Carpels peltate or overlapping; leaves alternate (Taxodium Gly-

ptojtrobus, Cryptomeria),

Family 2. AbietineaB. Leaves usually acicular and arranged spirally, singly, or in
twos, threes, or rosettes on special short shoots ; flowers monoecious, rarely dioecious :
stamens numerous, with two or more long pollen-sacs ; female flower consisting of
a number of scale-like placentae arranged spirally, which are either themselves carpels or
are lignified outgrowths of small carpels ; micropyle of the ovule turned towards the base
of the placenta ; embryo with from two to fifteen cotyledons.

(a) Abiet'meoB 'vera. Seeds in pairs on a scale-like placenta which springs from a

small open carpellary leaf {Pinus, Abies, Tsuga, Larix, Cedrus).

(b) Araucariece. Seed single on the carpel, and enveloped by it (Araucaria).

(c) Cunninghamieee. Seeds single or numerous on a carpel [Dammara, Cunning-

hamia, Arthrotaxis, Sequoia, Sciadopitys).

Fam'ly 3. PodocarpeaB. Leaves acicular or broader, and arranged spirally ; flowers
monoecious or dioecious ; stamens short, with two roundish pollen-sacs ; female flower
consisting of an axis swollen above with small scale-leaves, from the axils of which (?)
the ovules spring ; embryo dicotyledonous.
Podocarpus {Dacrydium, Microcachrys).

Family 4. TaxineaB. Leaves arranged spirally, acicular or often of considerable
breadth ; in Phyllocladus there are no foliage-leaves, these being replaced by leaf-like
branches ; flowers always dioecious ; stamens of various forms, bearing two, three, four,
or eight pendent pollen-sacs ; female flowers always consisting of a naked axis or
of one furnished with small leaves, bearing the erect ovules terminally or laterally ;
ripe seed enclosed in a fleshy aril or with the outer layer of the testa fleshy ; embryo
dicotyledonous.

Phyllocladus, Salisburia, Cephalotaxus, Torreya, Taxus.

C. GNETACE^^

This order includes three genera which differ strikingly in habit. The Ephedrae
are shrubs with no foliage-leaves and with long, slender, cylindrical green-barked
branches ; at the joints of the stem are two opposite minute leaves which grow
together into a bidentate sheath, and from their axils the lateral branches spring.
In Gnetum the leaves are also opposite on the jointed axes, but large and
stalked, with a broad lanceolate lamina and feather-veined venation. Thirdly,
Welwitschia mirahilis, so remarkable a plant in many other ways, possesses only two
foliage-leaves of immense size. They are extended on the ground and become
divided into strips as they become old ; the stem remains short, rising only slightly

^ See Strasburger, Die Coniferen und Gnetaceen, Jena, 1872,

5^8 PHANEROGAMS.

above the ground, and is broad above with a furrow across the top, while it is
tuberous below, and passes into the tap-root \

The Flowers of Gnetaceae are unisexual, and are arranged in dioecious
{Ephedra) or monoecious inflorescences ; the inflorescence has a well-defined form,
and in Ephedra and Gnetum springs from the axils of the opposite leaves. The
male flower of these genera consists of a small bifid perianth, in the middle of
which rises a staminal column, which in Gnetum is bifurcate above and bears two
bilocular anthers, in Ephedra a larger number crowded into a head. The female
flower oi Gnetum (Eichler, in Flora 1863, p. 463), like that oi Ephedra, also possesses
a perianth, flask-shaped in the former, obscurely trigonous in the latter genus ; it
envelopes a central ovule possessing in the case of Ephedra one integument, in
that of Gnetum two, the inner of which is elongated like a styled The more exact
morphology of these flowers is still doubtful. In Gnetum the inflorescence, which
springs from the axil of the foliage- leaves, consists of a jointed axis with verticillate
leaves, in the axils of which the flowers, male and female, are agglomerated. The
inflorescence of Welwitschia'^ is a dichotomously branched cyme nearly a foot in

^ For a full description of this remarkable plant see J. D. Hooker in Trans. Linn. Soc.
vol. XXIV.

"^ [According to Beccari (Delia Organogenia dei fiori feminei del Gnetum Gnemon, Nuovo
Giorn. bot. ital. IX, 1877) the female flowers are lateral axes, the nucellus being the end of the axis.
Strasburger (Angiospermen und Gymnospermen) regards the two investments of the nucellus in
Ephedra and the three in Gnetum as integuments.]

3 [According to Professor W. R. M^Nab, 'The cones of Welwitschia consist of numerous
opposite and decussate bracts, with a sessile flower in the axil of each of the bracts. The perfect
flowers in the male cone consist of two outer perianth leaves (calyx) placed right and left, two inner
ones (corolla) placed anteriorly and posteriorly, six stamens united below, and two carpels anterior
and posterior, the conical end of the axis projecting as a rudimentary axile ovule surrounded by the
two carpels. The outer parts of the perianth are first developed appearing as two shoulders at the
very base of the young floral branch. The flower next in age has the floral axis more elongated, the
outer parts of the perianth larger, and a distinct swelling is visible above the outer parts. These
swellings are anterior and posterior, and much larger than the outer parts. Above the inner parts of
the perianth the axis is expanded, and contracts near the rounded apex. The expanded portions are
superposed on the outer lateral parts of the perianth, and are the two primordial staminal cushions.
These cushions are semilunar, and in the earlier stages show no trace of division into three. At this
stage the parts of the perianth rapidly enlarge and cover in the central parts of the flower. A pro-
jection now forms anteriorly and posteriorly, the first indication of the two carpels. The next stage
shows the two staminal cushions each forming three elevations, the central one larger than the two
lateral ones.- The six stamens are thus produced by the branching of two primordial stamens. In
the next stage the carpels elongate and cover in the punctum vegetationis, ultimately developing
the peculiar style and stigma-like process. The axis elongates slowly and forms a conical pro-
jection which is undoubtedly a rudimentary axile ovule, but it never shows any appearance of
an embryo-sac.'

In the female flowers, which are produced in different cones from the male flowers, the develop-
ment is very different. A very short stalk is developed in the female, which is wanting in the male ;
then two shoulders are developed exactly like the two outer parts of the perianth in the male flower,
to which Dr. Hooker considered them to be equivalent. Judging from the construction of the male
flower. Professor M'^Nab was disposed to accept this view; but with hesitation, as he could not
account for the stalk-like process. Strasburger however concludes that they are carpels, and in
that M'^Nab quite concurs. Above the carpels the axis elongates slightly, and a ring is formed
surrounding the punctum vegetationis. This ring is the ovular integument. Comparing the two
flowers, it will be seen that in the male there are four series of parts, in the female the three outer
series are wanting and only the carpels remain. But in the male flower the carpels are anterior and

GNETACEM. 5^9

height, rising above the insertion of the two enormous leaves on the periphery of the
broad apex of the stem. The branches of the inflorescence are terete and jointed,
spring from the axils of the bracts, and bear upright longish cylindrical cones ; these
are furnished with from seventy to ninety broadly ovate scale-leaves standing closely
one above another in four rows, a single flower being seated in each axil, male and
female in different cones. The male flowers are pseudo-hermaphrodite, and possess
a perianth consisting of two pairs of decussate leaves ; the lower ones are entirely
free, sickle-shaped and pointed, the upper ones broadly spathulate and coherent at
their base into a compressed tube. Within this tube are six stamens, monadelphous
at the base, with cylindrical filaments and terminal spherical bilocular anthers, which
dehisce above the apex with a three-armed fissure ; the pollen-grains are simple (?)
and elliptic. The centre of the flower encloses a single erect orthotropous sessile
ovule with broad base, and with no other investment than a simple integument,
which is drawn out into a style-like tube with a margin expanded in a discoid
manner ; the nucellus, however, has no embryo-sac, or is sterile. In the female
flowers the perianth is tubular, greatly compressed, somewhat winged, and altogether
undivided; there is no indication of any male organ; the ovule (in this case of
course possessing an embryo-sac) is entirely enclosed in the perianth, and is similar
in its external form to that of the male flower, but with this diff'erence, that the elon-
gated point of the integument is only simply slit, not expanded into the form of a
plated When ripe the cone is about two inches long and of a scarlet colour; the
scales are persistent ; the perianth enlarges considerably and becomes broadly
winged ; its cavity is narrowed above into a narrow canal, through which the apex
of the integument passes. The seed is of the same form as the unfertilised ovule,
and contains abundant endosperm, in the axis of which lies the dicotyledonous
embryo ; the embryo is thick at the radicular end, and is there attached to the very
long spirally-coiled suspensor. The formation of endosperm commences in the
embryo-sac before fertilisation ; archegonia without necks, consisting, that is, only of
oospheres, are formed which grow out of the embryo-sac to the number of from
twenty to sixty, and penetrate into the canal-like cavity of the nucellus, there they
are fertilised by the pollen-tubes which have grown to meet them. [After fertilisation,
the oospore elongates into a tube, a portion of which is cut off" by a wall near its
lower end. This cell at once divides into four, placed crosswise. These cells mul-
tiply by division, so that a group of cells is formed which, for the most part, constitute
the embryo ; the marginal cells of the group grow out into long tubes, the so-called
* embryonal tubes.' Although from two to eight archegonia are fertiUsed only one
embryo is developed.

The development of the ovule and of the embryo-sac has been studied by
Strasburger (Angiospermen und Gymnospermen) in Gnetum Gnemon. Groups of

posterior, while in the female they are lateral. This is to be explained by the fact that the carpels
are here the first leaves of a branch, and that it is very rare (except in grasses) that the first leaves of
a shoot are anterior or posterior, and not lateral. The ovular integument of the female flower is
wanting in the male. While therefore the male flower is complex, the female is remarkably simple.

For further details see Transactions of the Linnean Society, 1873, vol. XXVIII. pp. 507-512.
On the general Morphology and Histology of Wehviischia see Bower, Quart. Journ. Micr. Sci. 1881.

^ [Strasburger {loc. cit.) now regards the ' perianth ' as an outer integument.]

M m

530 PJIA NER OGA MS.

epidermal cells, situated in the ' cupule' formed by the bracts, begin to divide, and
then divisions take place in the hypodermal cells ; by this means a number of
protuberances are formed, usually six or eight, in a whorl, which are the rudimentary
ovules. Around each of these the external integument begins to grow up : when the
external integument covers about two-thirds of the nucellus, the middle integument
begins to be formed, and this is immediately followed by the appearance of the
innermost integument. The nucellus elongates above the insertion of the external
integument, and in consequence the cells of which it consists can be distinctly seen
to be arranged in longitudinal rows. The terminal cell of one or more of these
rows, which is therefore hypodermal, elongates, and thus the archesporium is consti-
tuted. These cells now undergo divisions, cells being cut off from them towards
the free surface of the nucellus ; these cells thus cut off form part of the tapetum,
the larger cells beneath them being the mother-cells of the embryo-sacs. Of these,
one only gives rise to an embryo-sac, the others becoming obliterated. The forma-
tion of the endosperm begins wnth the division of the nucleus of the embryo-sac :
a number of nuclei are formed by repeated division, and around these free cell-
formation takes place.

The development of the ovule of Ephedra is much the same as that above
described in Gnelum. The endosperm of Ephedra produces from three to five
archegonia, with an elongated oosphere, a distinct canal-cell, and a long neck con-
sisting of rows of cells. After fertilisation the nucleus of the oospore divides, and
the two new nuclei separate and travel towards the opposite ends of the cell. Here
they undergo division, and this is repeated until usually eight nuclei have been
formed. Around these a process of free cell-formation now takes place, and each of
the cells thus formed becomes enclosed in a cellulose wall. Each of these cells,
which are quite distinct from each other, now grows out into a tube which escapes
from the archegonium and penetrates into the endosperm : a small portion of the
tube is now cut off by a transverse septum near the apex. It is from this cell
that the embryo is developed. It divides into two by a transverse wall parallel
to the first, and these two cells grow and divide in various directions ; sometimes a
two-sided apical cell is formed by means of which the embryo grows.]

A small cell is formed in the pollen-grain of Ephedra as in that of the Cupres-
sinese.

The Histology of Gymnosperms. From the abundant though still unsifted material
I will only adduce a few particulars as a contribution to the special characteristics
of this section.

The Fibro-'vascular Bundles ^ are similar in their structure to those of Dicotyledons.
There is a system of bundles common to the stem and leaves; the portions which

^ Mohl, Bau des Cycadeenstammes (Verm. Schr. p. 195).— Kraus, Bau der Cycadeen-Fiedern
(Jahrb. f. wiss. Bot. vol. IV. p. 329).— Geyler, Ueber Gefassbundelverlauf bei Coniferen (ditto,
vol. VI p. 68). — Thomas, Vergl. Anat. des Conifer-BIattes (ditto, vol. IV. p. 43). — Mohl, Ueber die
grossen getiipfelten Rohren von Ephedra (Verm. Schr, p. 269). — J. D. Hooker, On Welwitschia
(Trans. Linn. Soc. vol. XXIV). — Dippel, Histologie der Coniferen (Bot. Zeitg. 1862 and 1863). —
Rossmann, Bau des Holzes (Frankfurt-a-M. 1863). — Mohl, Bot. Zeitg. 1871, — [Bertrand, Anat. comp.
des tiges et des feuilles chez les Gnetacees et les Coniferes, Ann. Sci. Nat., ser. 5. XX. — Bower, On
Welwitschia, Quart. Journ. Micr. Sci, 1881.— De Bary, Vergleichende Anatomieder Vegetationsorgane,
1877.]

ONETACEM.

531

descend into the stem forming a circle, where a closed cambium-ring is produced by
the formation of interfascicular cambium. The ascending portion, which curves out into
the leaf itself, assumes in Cycadeae more or less the character of a closed bundle, while
in the leaves of many Coniferae it at least retains the appearance of an open bundle. [In
most cases the growth in thickness of the stem is brought about by the division of the cells
of the cambium-layer, but in Cycas and Encephalartos the activity of the cambium-layer
is of limited duration. The subsequent growth in thickness of the stem in these two
genera is effected by the production of successive new cambium-rings, which make their
appearance just outside the bast of the older vascular bundles.] No exclusively cauline
bundles are produced in the stem of Coniferae or oi Ephedra ; but in some Cycadeae acces-
sory-bundles arise in the older stem which are independent of the common bundles and of
those formed by the cambium-layers. Thus in the tissue of the pith of Encephalartos slender
isolated bundles occur ; while in Cycas a system of thicker bundles is developed in the cortex
which may form there in old age one or more apparent rings of wood. As far as we can
judge from Hooker's description, bundles occur in the bark of Wehvitschia which owe their
origin to a layer of meristem enveloping the whole stem \ The Coniferae, as has been
mentioned, possess only common bundles, the descending portions passing through a
number of internodes, and then joining others lower down either unilaterally or on both
sides by splitting into two arms and turning to both sides. The leaves of Coniferae,
when narrow, contain only one fibro-vascular bundle from the stem, which then usually
splits into two halves running parallel to one another (Fig. 102); when the leaves are
broader, two {Salisburia, Ephedra) or even three bundles occur ; when the leaf forms
a flat broad lamina, as in Salisburia and Dammara^ the bundles ramify in it, but without
forming a net- work ; in Salisburia they repeatedly branch dichotomously. In Coniferae
these bundles seldom form prominent veins, but run through the middle of the tissues of
the leaf. In the two gigantic leaves of Welivitschia there are a number of bundles, the
parallel ramifications of which run into the middle layer of tissue. In the large pinnate
leaves of Cycadeae there are also several bundles which curve nearly horizontally within
the cortical parenchyma, and split into a number of stout bundles in the leaf-stalk when
it is thick ; these bundles exhibit a beautiful arrangement when seen in transverse sec-
tion (in Cycas re^oluta, e,g. in the form of an inverted Q). They run parallel in the
rachis of the pinnate leaf, and give off branches into the pinnae, where they either run
parallel in the middle layer of tissue (as in Dion) or dichotomise {e.g. Encephalartos) ;
while in Cycas they form a mid-rib projecting beneath. The course of the bundles in
the leaf therefore shows a decided resemblance to that of many Ferns.

The substance of the wood of the stem is foVmed from the descending bundles,
which are at lirst completely isolated, but soon coalesce into a closed ring by portions of
cambium which cross the medullary rays. The primary wood or xylem, termed the
Medullary Sheath, which consists of the xylem-portions of the descending limbs of the
common bundles, contains, in all Gymnosperms, as in Dicotyledons, long narrow vessels
with annular or spiral thickening-bands, while further outwards occur scalariform or
reticulately thickened vessels. The secondary wood produced from the cambium-ring
after the cessation of growth in length consists, in Cycadeae and Coniferae, of long
tracheides grown one into another in a prosenchymatous manner {cf. p. 25) with a few
large bordered pits, which are usually circular, at least when the wood is mature. Every
possible stage of transition occurs between these tracheides and the spiral vessels
of the medullary sheath. The secondary wood of Cycadeae ajid Coniferae is distin-
guished from that of Dicotyledons by the striking peculiarity that it is composed only
of this prosenchymatous form of cells ^); and that the wide dotted vessels composed of
short cells are wanting which penetrate the dense narrow-celled masses of the wood of
Dicotyledons. In the younger stems of Cycadeae the tracheides with broad bordered

^ [De Bary compares the structure of the stem of Welwitschia to that of Chenopodiaceae and
Amarantacese, etc. (see infra) ; that of Gnetnm to that of Phytolacca, Polyga!e?e, Dilleniaceoe, etc.]
^ Wood-parenchyma is not formed, or only in small quantity.

M rn 2

^;^2 PHANEROGAMS,

pits and hence with a more or less scalariform wall are very much like the long prosen-
chymatous vessels of Vascular Cryptogams ; and this resemblance extends even to
the tracheidcs of Goniferae, so far as they are distinctly prosenchymatous, although the
smaller number and round form of the bordered pits shows a more marked difference.
The bordered pits of Coniferae are usually developed only on the wall which faces the
medullary rays, in one or two rows, but in Araucaria in larger numbers and densely
crowded. In the structure of the secondary wood, as in that of their flowers and
in their habit, Gnetaceae approach Dicotyledons; in Ephedra broad vessels occur in
it together with the usual tracheides in the inner part of the ring of wood, but their
component ceUs are separated by oblique septa, and are therefore still prosenchymatous,
and are penetrated by several roundish holes ; their lateral walls show bordered pits like
the tracheides, and furnish a striking evidence that the true vessels in the secondary
wood of Dicotyledons are connected by intermediate forms with the vessels of Vascular
Cryptogams formed from prosenchymatous cells. In the wood of Welwhschia tracheides
with doubly bordered pits are also present.

The medullary rays of the secondary wood of Coniferae are very narrow, often only
one cell in breadth ; the cells are strongly lignified, and their lateral faces in contact with
the adjoining tracheides are provided with closed bordered pits. In Cycadeae the rays are
broader, and their tissue bears a closer resemblance to the parenchyma of the pith and
cortex ; their number and width cause the whole substance of the wood to appear spongy,
and its prosenchymatous cells are seen to be strongly curved in different directions in
tangential sections. The phloem-portion of the fibro-vascular bundles of Gymnosperms
resembles that of Dicotyledons ; it is mostly composed of true strongly-thickened bast-
fibres, cambiform cells, latticed cells, and parenchymatous cells ; while in Coniferae they
are formed in alternate layers. Usually the soft bast predominates.

The Fundamental Tissue of the stem of Gymnosperms is separated by the ring of wood
into pith and primary cortex. Both are very strongly developed in Cycadeae, especially
the pith, and consist of true parenchyma, while the woody portion is considerably
smaller. In Wehvitschia the parenchymatous tissues appear also to predominate ; but
the greater part of their substance is the product of the activity of the meristem-layer of
the stem already mentioned. A large number of so-called spicular cells occur dispersed
in all the organs of this remarkable plant, they are fusiform or branched and greatly
thickened ; and a number of beautifully developed crystals are found imbedded close to
one another in their cell-wall. Similar structures also occur in Coniferae (p. 66).

The parenchymatous fundamental tissue of Coniferae decreases greatly with the
increase in age of the stem (and of the root). With the exception of the pith, which is
here small, the stem consists exclusively of the products of the cambium-ring, since the
primary cortex, and afterwards also the outer layers of the secondary cortex which
always have a subsequent growth, are used up in the formation of cork. In the stem of
Cycadeae, the increase of which in thickness is inconsiderable, the formation of cork is
also very small; in fVelivitschia it appears to be entirely wanting (?).

Intercellular Passages are widely distributed in Gymnosperms ; their structure is that
which has been explained generally at pp. 78 and 94. In Cycadeee they are found in all
the organs in large numbers, and contain gum, which exudes from incisions in thick
viscid drops ; in Coniferae they contain oil of turpentine and resin. In this latter order
they occur in the pith of the stem, in the whole substance of the wood, and in the
primary and secondary cortex, as well as distributed through the leaves ; always
following the direction in length of the organs, like the gum-passages of Cycadeae.
In many Conifers with short leaves roundish resin-glands also occur in them (as in
Callitris, Thuja, and Cupressus, according to Thomas) ; in Taxus the resin-canals are
entirely wanting ^.

^ [Van Tieghem (Ann. des. Sci. Nat. 1872) distinguishes the six following modifications of the
distribution of the secretory organs in Coniferae: — r. No canals in the root nor stem: Taxus. 2. No
canals in the root ; canals in the cortical parenchyma of the stem : Cryptomeria, Taxodinm, Podo-

GNETACEm.

^3^

The Leaves of Cycadeae and Goniferae are covered by a firm epidermis, usually
strongly cuticularised, and furnished with numerous stomata, each with two guard-cells.
In the Gycadege the guard-cells are more or less deeply depressed, and the stomata occur
only on the under side of the lamina, and are either irregularly scattered, or arranged in
rows between the veins (Kraus). In the leaves of Conifers the guard-cells are also,
according to Hildebrand (Bot. Zeit. 1869, p. 149), always depressed in the epidermis;
and the stoma has hence always a vestibule. In Goniferae the stomata are developed
either on both or only on one side of the leaf ; when the leaf is broad, as in Dammara
and Salisburia, they are irregularly scattered ; when the leaves are acicular they mostly
lie in longitudinal rows ; and in the large leaves of Wehvitschia they are also arranged in
rows. The firm texture of the leaves of Gycadeae and Goniferae is due to a hypodermal
layer, often strongly developed, consisting of strongly-thickened, generally long, fibre-like
cells lying parallel to the surface ; in the leaf of Welivitschia
this hypoderma consists of spongy succulent tissue penetrated
by bundles of fibres, which acquires its hardness from a mass
of spicular cells. The chlorophyll-tissue of the leaves lies
beneath this layer, and is developed on the upper side of the
leaves of Gycadeae and of the broader leaves of Goniferae as
the so-called Pallisade-tissue ; /. e. its cells are elongated in a
direction vertical to the surface of the leaf and are densely
packed together. In Pinus, Larix, and Cedrus the cells which
contain chlorophyll exhibit the infoldings of the cell-wall
which have been already mentioned at p. 74. The middle
layer of the tissue of the leaf, in which also the fibro-vascular
bundles run, has usually a peculiar development in Gymno-
sperms ; in Gycadeae and Podocarpeae it consists of cells elon-
gated in a direction transverse to the axis of the leaf and to
the bundles, but parallel to the surface of the leaf, leaving
large intercellular spaces (Transfusion-Tissue of Mohl). In
the acicular leaves of the Abietineae the fibro-vascular bundle,
split into two, is enveloped by a colourless tissue, which is

sharply diflferentiated from the surrounding chlorophyll-tissue (Fig. 102). It is paren-
chymatous, and is distinguished by the large number of bordered pits which the walls
of the cells bear (Fig. 356)^

FIG. 3S^.—Pinus Pinaster;
two cells of the colourless paren-
chyma surrounding the fibro-vas-
cular bundle of the leaf; t i the
bordered-pits cut across, f the
same seen from the surface.

carpus, Dacrydium, Torreya, Tstiga, Cumiinghajnia. 3. No canals in the root ; canals in the cortical
parenchyma and in the pith of the stem : Salisburia. 4. A secretory canal in the root ; canals in the
cortical parenchyma of the stem: Cedrus, Abies, Pseudolarix. 5. Canals in the wood of the fibro-
vascular bundles of the root and stem ; canals in the cortical parenchyma of the stem : Pinus, Larix,
Picea, Psetidotsuga. 6. Canals in the liber of the fibro-vascular bundles of the root and of the stem ;
canals in the cortical parenchyma of the stem : Araucaria, Widdringtonia, Thuja, Cupressus, Biota,
In Cycadeae the canals are found disseminated through the cortical parenchyma of the stem ; the
pith of Cycas appears destitute of them. In their distribution they resemble therefore that which
occurs in the second class of Coniferse.]

^ For further details see Mohl, Bot. Zeit. 187 1, Nos. i, 2.

h

534

PHANEROGAMS.

A NGI OS PERM SI.

Monocotyledons and Dicotyledons are distinguished from Gymnosperms by
the following characters : — their ovules are formed within a receptacle, the Ovary ;
the endosperm originates in the embryo-sac only after fertilisation, — characteristics,
the importance of which has already been shown in the general introduction to
Phanerogams. Concurrently with these distinctions there are however a number
of other peculiarities in these plants taken as a whole which distinguish them from

Fig. -^^T.—Akebia quinata; A part of an inflorescence, v female, O male flowers ; B a male flower cut through length-
wise, c its sterile carpels ; C horizontal section of a female flower (magnified) ; D horizontal section of a male flower ;
E gynjeceum of the female flower with the sterile stamens a; /^ an ovary cut through horizontally ; G an ovule ; H horizontal
section of an anther ; « (in 5 and C) the outer, a' the inner stamens, c (in E) the carpels ; / (in ^ and C) the perianth.

all other vascular plants ; and this is especially the case with the structure of the
flowers and the fruit, the normal morphological relations undergoing such peculiar
combinations and changes that a more detailed description of them must precede
the special description of the two classes which they include.

The Flower as a whole ^. The flower of Angiosperms is rarely terminal, t. e. the
primary stem, which is a prolongation of the axis of the embryo, rarely terminates

^ From ay^eTov, a receptacle, capsule, ovary, and avfpfxa, seed.

^ The most important and comprehensive work on the flowers of Angiosperms is Payer's Traite
d'Organogenie de la Fleur (Paris 1857), with 154 plates. Also Van Tieghem, Rech. sur la structure
du pistil (Mem. des savants etrangers, XXI. 1871), and his notes in the French edition. [Eilcher,
Bliithendiagramme, and Gray, Structural Botany.]

ANGIOSPERMS.

535

in a flower, making the plant uniaxial. When this is the case a sympodial or
cymose inflorescence is usually developed, new axes with terminal flowers arising
beneath the first flower; but it is more common for only axes of the second,
third, or a higher order to terminate in a flower, so that the plant may in this
respect be termed bi-, tri-, or multi-axial.

While in Gymnosperms the flowers are typically unisexual or diclinous, herma-
phroditism largely prevails among Angiosperms, although monoecious and dioecious
species, genera, and families are not uncommon. The male flowers are sometimes
essentially difl"erent in structure from the female flowers (as in Cupuliferse and
Cannabinese), but in most cases the unisexuality arises merely from the partial or
entire abortion either of the androecium or the gynaeceum, the flower being in other
respects constructed on the same type (Fig. 357, A); and in such cases it also
frequently happens that hermaphrodite flowers are developed in addition to the
male and female (polygamous species, as the Ash, Acer, Saponaria ocymoides, &c.).
But even in the greater number of cases where the male and female organs are
completely developed in hermaphrodite flowers and functionally perfect, fertilisation

Fig. 358 .—Asaritfit arttadense; A the flower cut through lengthwise, / the perianth ; B horizontal section of the flower
above the ovary ; C horizontal section of the sex-locular ovary ; D a stamen with its lateral anther-lobes a.

takes place by the conveyance of the pollen of one flower to the gynaeceum of
other flowers or even of other individuals of the same species, because either polli-
nation within the same flower is impossible in consequence of special contrivances
(such as dichogamy), or because the pollen is potent only in the fertilisation of
ovules of another flower (as in Orchideae, Cotydalis, &c.). To these phenomena
we shall recur more in detail in the Third Book, when speaking of the physiology
of sexual reproduction.

While in Gymnosperms the floral axis is usually elongated to such an extent
that the sexual organs, especially if numerous, are evidently arranged one above
another in alternate whorls or in spirals, — in Angiosperms, on the contrary, . the
floral axis which bears the floral envelopes and sexual organs is so abbreviated
that space can only be found for the various foliar structures by a corresponding
expansion or increase in size of the receptacle or torus ; this receptacle swells even
before and during the formation of the floral leaves in a club-shaped manner, and is
not unfrequently expanded flat like a plate or even hollowed out like a cup in such a
manner that the ape^ of the axis is placed at the bottom of the hollow (p. 220), while

L

5?>^

PHANEROGAMS.

the cup thus formed encloses the carpels (as in perigynous flowers), or even takes
part in the formation of the ovary, which is then inferior (Fig. 358). The result of
this abbreviation of the axis is that the separate parts do not usually stand one above
another, but rather in concentric whorls, or in scarcely ascending spirals, for which
reason the explanation of the relative positions expressed by a diagram in the sense
explained on p. 188 appears the most obvious. This abbreviation of the axis is also
obviously the immediate cause of the numerous cohesions and displacements which
are nowhere met with so frequently as in the flowers of Angiosperms. The small
development of the floral axis in length depends on the early cessation of its apical
growth; the acropetal or centripetal order of succession of the floral leaves may
therefore be disturbed ^ by the production of intercalary zones of growth, although
even in these cases the disturbance of the ordinary regularity remains inconsiderable.
The acropetal order of succession is however even here in most cases strictly carried
out, and the apical growth of the floral axis not unfrequently continues long enough

hAJ.

Fig. se,g.—CheHOJ'ocitum Quinoa; /—/^development of the flower (in longitudinal section), /the calyx furnished
with glandular hairs h, a anthers, k, k carpels, sk ovule, x apex of the floral axis, V horizontal section of an anther
with four pollen-sacs on the connective on (strongly magnified).

to allow the foliar structures to arrange themselves in evident whorls placed one over
another or in spirals {e.g. Magnolia, Ranunculaceae, Nymphaeaceae). Occasionally
also particular portions of the axis are greatly elongated within the flower, as the
portion between calyx and corolla in Lychnis (Fig. 361), in Passiflora that between
corolla and stamens, in Labiatse that between stamens and ovary.

The flower of Angiosperms, like that of Gymnosperms, is a metamorphosed
shoot, a leaf-bearing axis ; but this section of the vegetable kingdom is especially
characterised by the high degree of metamorphosis which the floral shoot has
undergone, and by the very peculiar characteristics and the different arrangement
of the foliar structures as contrasted with those of the purely vegetative shoots.
As far as external appearance goes, the flower of Angiosperms is an altogether

^ The cases adduced by Hofmeister (Allgemeine Morphologic, § 10) of the absence of strict
acropetal succession in the foliar structures all belong to this category.

ANGIOSPERMS.

537

peculiar structure, sharply differentiated as a whole from the rest of the organism.
This peculiar appearance is due not only to the special properties of its axis,
but especially to the presence of the floral envelopes, and most of all to the
circumstance that the foliar structures of the flower are arranged, with rare
exceptions, in the form of whorls, even when the leaves of the vegetative shoots
are alternate or distichous, or disposed in other similar arrangements. Each of
the distinct appendicular organs of the flower, viz. the perianth, androecium, and
gynaeceum, is usually represented by several members arranged in concentric
circles or in a spiral ; so that one or more perianth-whorls are immediately succeeded
within by one or more whorls of stamens, and these by the gynaeceum in the
centre of the flower. One or other of these whorls may however be absent, or
each of the separate whorls may be represented by only a single member, as

Fig. 360.— Htp^urts vulgaris; A piece of an erect stem, the flowers standing in the axils of the whorl of leaves (which have
been cut off) ; B horizontal section of a female flower above the ovary, p perianth, cp carpel ; C horizontal section of the anther ;
J— IV longitudinal section of flowers in various stages of development, a anther, f filament, g style, « stigma, / perianth,
fk the inferior ovary, sk the pendulous and anatropous ovule.

in Hippuris (Fig. 360), where only one stamen and one carpel are contained within
a scantily developed perianth. It is only rarely that the whole flower is reduced
to a single sexual organ, as the female flowers of Piperaceae, or the male and
female flowers of some Aroideae; it is much more commonly the case that the
flower is composed of successive whorls of members disposed from without inwards
(or from below upwards), consisting of the same or multiples of the same number *,
radiating from the centre on all sides like a rosette, an arrangement which is

* [To this peculiarity of structure the term ' symmetrical ' is generally applied in English text-
books ; in the present v/ork however this word is used in a very different sense, namely in reference to
any structure (foliar or floral) which can be divided into two similar halves, or the parts of which are
radially disposed around a central point; see p. 204.]

558 PHANEROGAMS.

frequently partially obscured at a subsequent period by bilateral development and
abortion.

'The Floral Envelope or Perianth is only rarely entirely wanting, as in the
Piperaceae and many Aroideoe ; more often it is simple, i. e. it consists of only
one whorl of two, three, four, five, or rarely a larger number of leaves (as in
Figs. 357, 358) ; in this case the perianth is frequently inconspicuous and composed
of small green leaves, as in the Chenopodiaceae and Urticaceae, but is sometimes
large, of delicate structure and brightly coloured (petaloid), as in Aristolochia,
Mtrabtlis, &c. But in both classes of Angiosperms (Monocotyledons and Dicoty-
ledons) the perianth is usually composed of two alternating whorls consisting of the
same number of leaves, two, three, four, five, or rarely more. In most Dicotyledons
and many Monocotyledons the form and structure of these two whorls is very
different ; the outer whorl or Calyx consisting of stouter, green, usually smaller leaves
{Sepals), while the inner whorl or Corolla is more delicate, and is formed of white or
bright-coloured, usually larger leaves (Petals). It is however more convenient, for
the sake of brevity, as Payer has already suggested, to designate the inner whorl as
corolla, the outer whorl as calyx, even in those cases where the structure of the two
is the same ^ ; and this is the more necessary since the contrast of structure
referred to is frequently wanting, both whorls being either sepaloid, as in Jun-
caceae, or both petaloid, as in Lilium ; in Helleborus, Aconiium, and some other
species, the outer whorl or calyx alone is petaloid, the inner whorl or corolla
being transformed into nectaries. In some Dicotyledons the perianth does not
consist of alternating whorls, but of a smaller or larger number of turns of spirally
arranged leaves, the number of which is then usually large or indefinite ; the outer
or lower leaves of this spiral arrangement may in this case also be sepaloid, the inner
ones alone petaloid {e.g. Opuntia), or they may all be petaloid (as in Epiphyllum
and Trollius), or a gradual transition takes place from the sepaloid through the
petaloid to the staminal structure (as in Nymphcea).

But besides the usual sepaloid and petaloid form and structure of the perianth-
leaves, there occur other considerable deviations from the ordinary foliar structure.
Thus, for example, the (imperfect) perianth of Grasses consists of very small delicate
colourless membranous scales (the Lodicules), that of some Cyperaceae is replaced by
hair-like bristles, the SetcB ; in the place of the calyx of Compositae a crown of
hairs, the Pappus, surrounds the corolla ; and it has already been mentioned that the
petals of Aconitum, Helleborus, &.c. are transformed into nectaries of a peculiar
form.

Whether the perianth consist of one or two whorls, the leaves of the same
whorl have very commonly the appearance of being coherent or of coalescing with
one another, forming a cup, bell, tube, and so forth, the number of the coherent
sepals or petals being determined by that of the marginal teeth. Coherent perianth-
whorls are produced, after the formation of the distinct foliar structures at the cir-
cumference of the receptacle, by the common zone of insertion of these distinct
structures being raised up by intercalary growth as an annular wall, and forming, as

^ The substantives calyx and corolla then designate the position of the whorl, the adjectives
sepaloid and petaloid the nature of the part.

ANGIOSPERMS.

539

it continues to develope, the part common to the whole whorl of floral leaves.
The coherent tubular or campanulate part does not therefore consist of originally
free portions which cohere subsequently by their edges, but from the very first it
forms a whole which is intruded, so to speak, at the base of the perianth-leaves ;
the originally free leaves eventually forming the marginal teeth of the common
basal portion. Applying the term Sepal to a calycine, Petal to a corolline leaf,
a calyx consisting of coherent leaves is gamosepalous or synsepalous, a corolla con-
sisting of coherent leaves gamopelalous or sympetalous ; if the leaves of the perianth-
whorl are not coherent, but free, this is expressed by the terms eleutherosepalous
or aposepalous, and eleutheropetalous or apopetalous^. When there is only one
perianth-whorl, and it is desired to state whether it consists of coherent or of free
leaves, the terms gamophyllous or symphyllous and eleutherophyllous or apophyllous
may be used. It sometimes happens moreover that two perianth-whorls coalesce
into one, so that, for example, two alternating trimerous whorls have united into a
six-toothed tube (as in Hyactnthus, Muscari, &c.).

Fig. 361. — Longitudinal section through the flower oi Lychnis flos-Jovis ; y the elongated portion of the axis
(anthophore) between calyx and corolla ; x ligule of the petals or corona.

If the leaves of the outer and inner whorls are free (not coherent), and if
the distinction between calyx and corolla is clearly marked, then, in addition to
the structural distinctions already named, other differences of form are also usually
to be observed. The sepals have generally a broader base, are sessile, usually of
very simple outline and pointed at the apex ; the petals have mostly a narrower
base, their upper portion is often very broad, and a distinction is not unfrequently
apparent of claw {unguis) and blade {lamina), and the lamina is often divided or
otherwise segmented. At the point where the lamina bends back from the unguis,
ligular structures are often formed on the inner or upper side, which, when treating
the flower as a whole, are comprised under the term Corona, as in Lychnis
(Fig. 361), Saponaria, Nerium, Hydrophylleae, &c. When the corolla itself is

^ The terms ' polysepalous ' and ' polypetalous ' are objectionable, since these terras do not
express the contrast correctly ; still more so are ' monosepalous ' and ' monopetalous,' as applied to
the coherent whorls, because they have no reference to the true nature of the phenomenon.

540 PHANEROGAMS,

gamopetalous, the parts of the corona also coalesce, as in Narcissus, where it is
very large.

The whole form of the perianth, especially when its structure is decidedly
petaloid and its dimensions considerable, always stands in a definite relation to pol-
lination by the aid of insects [or birds] : and large, brilliantly coloured, odoriferous
flowers only occur where the fertilisation is brought about by this means. The
purpose of these properties is to attract insects to visit the flowers ; and the in-
finitely varied and often wonderful form of the perianth is especially adapted
to compel certain positions of the body and certain movements on the part of
insects of a definite size and species when searching for the nectar, by which the
conveyance of pollen from flower to flower is unintentionally accomplished by
them. We shall recur in detail to these physiological questions in the Third Book.
The radial or bilateral symmetry of the perianth is usually associated with that of the
other parts of the flower, and will therefore be discussed in connection with it.

Besides the perianth in the narrower sense which we have hitherto considered,
there are often additional envelopes to the separate flowers. In the Malvaceae and
some other plants the true calyx appears to be surrounded by a second calyx
{Epicalyx or Calyculus)^ the morphological significance of which, however, varies.
In Malope irifida, for example, the three parts of the epicalyx represent a sub-floral
bract with its two stipules; in Kitaibelia vzti/olia, the six-parted epicalyx consists
(according to Payer) of two such sub-floral leaves with their four stipules. Again,
the epicalyx may be purely illusory from the production of stipular structures by
the true sepals, as in Rosa and Potentilla. In Dianthus Caryophyllus and some
other species a kind of epicalyx results from two decussate pairs of small bracts
which are found immediately beneath the calyx ; in the terminal flowers of Anemone
a whorl of bracts stands at a short distance below the flower, which takes the
form in the nearly allied Eranthis hyemalis of a kind of epicalyx ^ The epicalyx of
the small flowers of Dipsacaceae is of special interest, each of them being surrounded,
within the crowded inflorescence, by a membranous tube, which here forms the
epicalyx. Sometimes, after the perianth and sexual organs have begun to be formed,
an outgrowth of the flower-stalk, at first annular, is formed below the flower, growing
up afterwards in the form of a cup or saucer, and bearing scaly or spiny protuberances.
A structure of this kind is called a Cupule ; and the cup in which the acorn of the
various species of Oak is seated is of this nature ^ In this case the cupule surrounds
only one flower, in the Sweet- Chestnut and Beech on the other hand it encloses a
small inflorescence. This spiny cupule afterwards splits from above, separating into
lobes, to allow the escape of the fruit which has ripened within it. When an
inflorescence is surrounded by a peculiarly developed whorl or rosette of leaves, as in
Umbelliferse and Compositse, this is called an Involucre; when a single sheathing leaf
envelopes an inflorescence springing from its axis, it is a Spathe. Both involucre and
spathe may assume a petaloid structure, the former, for example, in Cornus florida^
the latter in Aroidese.

^ [The garden Clematis known as ' Lucie Lemoine ' possesses a well-marked seven-leaved
involucre which has evidently originated from the growth of the axis above the outermost whorl
of the multiplied petaloid sepals.]

^ On the development of the acorn-cup see Hofmeister, Allgemeine Morphologic, p. 465.

ANGIOSPERMS. 54 1

The Andrcecium is composed of the assemblage of the male sexual organs of a
flower. Each separate organ is called a Stamen^ and consists of the Anfher and its
stalk the Filament, which is usually filiform, but sometimes expanded like a leaf.
The anther consists of two longitudinal halves (anther-lobes) placed on the upper
part of the filament right and left of its median line ; and the portion of the filament
which bears the lobes of the anthers is distinguished as the Connective,

The lateral position of the stamens on the floral axis (the receptacle) is quite
unmistakeable in all hermaphrodite and in most exclusively male flowers. Their
lateral position, their exogenous origin from the primary meristem next the punctum
vegetationis of the floral axis, their acropetal order of development, and the frequent
monstrosities in which the stamens assume more or less the nature of petals, or even
of foliage-leaves^, place it beyond doubt that they must be considered morpho-
logically as foliar structures, and make it convenient to term them Staminal Leaves ;
the filament, together with the connective, being considered as the leaf, of which
the two anther-lobes are appendages. From a morphological point of view it is
therefore indifferent whether the filament greatly preponderates in size, or is incon-
siderable as compared to that of the anther. Certain cases have, however, become
known in which the anther appears itself to be a product of the floral axis, and the
stalk, which corresponds to the filament, is the floral axis itself, but doubts suggest
themselves as to the accuracy of these observations and as to the correctness of their
interpretation. According to Magnus ^, the vegetative cone of the male floral axis
of Naias becomes transformed into a quadrilocular anther by the formation of
pollen-mother-cells in four peripheral longitudinal strips of its tissue. Kaufmann
had previously described a somewhat similar process in the case of the anther of
Casuarina ; and, according to Rohrbach^, the apex of the floral axis of Typha
either itself developes into the anther, or it first of all branches and then forms an
anther on each branch. Schenk asserts in a letter, that this latter statement is
erroneous ; according to his observations the stamens are developed like those of
the Compositae on the margin of the shallow depression at the apex of the parent
axis. The question as to the nature of the organs bearing the anthers in the
Euphorbiese, whether they are modified branches (caulomes) or leaves, is discussed
in a considerable literature which does not, however, lead to any decision"^. Even if,
as Warming states, the single anther of Cyclanthera is developed at the apex of the
floral axis, this central organ is not necessarily a caulome any more than the axillary
sporangia of many Lycopodieae. The true significance of such cases as these
cannot be arrived at from a study of development alone, but comparisons must be
instituted, as also in those cases in which complete abortion of certain parts of the
flower occurs, with nearly related forms, that is, the ' phylogenetic method ' must be
followed. These remarks apply also to the above-mentioned peculiarities of the
anther in Naias and Casuarina ^.

* [On ' phyllody ' and ' petalody ' of stamens see Masters, Vegetable Teratology, Ray Soc. 1 869,
pp. 253-256, and 285-296.]

2 Magnus, Bot. Zeitg. 1869, p. 771.

2 Rohrbach, in Sitzungsber. der Gesellsch. naturf. Freunde in Berlin, Nov. 16, 1869.

* Warming, in Hanstein's Bot. Abhandl. Bd. II.

'•> [See also Magnus, Beitr. z. Kennt. d. Gatt. Naias, Berlin, 1870. — Strasburger, Die Coniferen

542 PHANEROGAMS.

But, besides, the morphological homology of the separate parts of the ordinary
stamens is not yet altogether determined, more precise investigations into the history
of development being still wanting in this direction. Cassini and Roper consider
the two anther-lobes as the swollen lateral halves of the lamina of the stamen ; their
loculi would therefore in that case be mere excavations in the tissue of the leaf;
the pollen-mother-cells become differentiated inside the young tissue of the leaf,
According to this view the furrow between the two pollen-sacs of an anther-lobe (see
Fig. 357, ZT) would correspond to the margin of the staminal leaf; but this cannot be
the case\ at least not always, according to Mohl's observations. When the stamens
become transformed into petals (by the so-called ' doubling ' of the flower) as in the
Rose, Poppy, Nigella damascena, Sec, it may be observed with certainty that the
anterior and posterior loculi do not stand opposite one another, which would be
the case if one belonged to the upper, the other to the under side of the staminal
leaf; but that both are formed on the upper surface, the anterior loculus nearer the
median line of the leaf, the posterior one nearer its margin. It is further observable
that in such cases the two pollen-sacs of an anther-lobe do not always stand close to
one another, but that they are frequently separated by a tolerably broad piece of the
leaf, and that this intermediate piece contracts in the normal state into the par-
tition-wall between the two pollen-sacs. The greater stress must be laid on these
observations of Mohl, because in Uiem the abnormal development only shows more
plainly what can often enough be seen in a horizontal section of the anther and
connective of normal stamens, viz. that the pollen-sacs of an anther-lobe evidently
belong to one side of the stamen ; it appears, however, that they must in some cases
be referred to the under (Fig. 357, C, H), in others to the upper side (Fig. 360, C).
The origin of the pollen-mother-celis and the development of the wall of the separate
pollen-sacs calls to mind so vividly in all essential features the corresponding pheno-
mena in the sporangium of Lycopodiacese and even of Equisetacese, that it may be

und Gnetaccen, 1872. — Hieronymus, Zur Deutung sogen. axiler Antheren, Bot. Zeit. 1872, and
Beitr. z. Kennt. d. Centrolepidaceen, Halle 1873. — Reuther, Beitr. z. Kntwick. d, Bluthe, Bot, Zeilg.
1876.- — Engler, Beitr. z. Kennt. der Antherenbildung, Jahrb. f. wiss. Bot. X. 1876. — Celakovsky,
Teiatologische Beitiage zur morphol. Deutung des Staubgefa ses, Jahrb. f. wiss. Bot. XI. 1878.

With reference to Typha, Magnus finds that the apparently axial stamen consists really of three
coherent lateral stamens. In Naias, Casuarina and Cyclanthera, the stamen is undoubtedly axial.
It would appear, therefore, that stamens are not always phyllomes. It is still possible, however, that
they may be phyllomes in these cases. Celakovsky goes so far as to regard the stamen of Naias as
a terminal leaf, a quite impossible morphological conception (Flora, 1874). The researches of
Hieronymus tend to show that the statement made above on p. 491 may be near the truth, namely,
that in these cases the pollen-sacs may be the surviving portions of otherwise abortive staminal leaves.
In Brtzula, one of the Centrolepidacese, he finds a single axial stamen ; in Alepyrum and Centrolepii he
finds that the stamen is developed from one longitudinal half of the growing-point, and that the other
half, the persistent growing-point, is forced on one side by the growth of the stamen, so that the
stamen lies in the same straight line as the long axis of the stem. He finds this to be the case also
in a Grass, Festuca pseudo-viyvrus ; in the nearly-allied F. geniculata, which usually has three lateral
stamens, it sometimes happens that only one stamen is present, and this is then developed in the
manner described above. On the other hand, these facts may Ije used to prove that a stamen may be
sometimes a caulome and sometimes a phyllome. It must be borne in mind that the ideas of
caulome and ph}llome are relative and not absolute.']

^ H. V. Mohl, \^ermischte Schriften, p. 42,

ANGIOSPERMS.

543

assumed, until more exact observations bring something different to light, that each
pollen-sac {i.e. each loculus with its wall) corresponds to a sporangium, and hence
also to a single pollen-sac of Cycadeae and Cupressineae ; and that therefore the
anther usually consists of four pollen-sacs springing side by side from the anterior or
posterior side of a staminal leaf, the sacs lying in pairs so close to one another
right and left of the connective, that they coalesce more or less laterally to form one
anther-lobe. But before we pass on to the consideration of the pollen-sacs and
their contents, we must again recur to the discussion of the entire stamen and
androecium.

The stalk of the anther (the filament with its connective) is either simple or
segmented. The simple filament may be filiform (Fig. 359) or expanded into the
form of a leaf (Fig. 358), sometimes even very broad, as in Asclepiadeae and
Apocynacese; or it may be broad below (Fig. 363,/) or above; it generally termi-
nates between the two anther-lobes, but is not unfrequently prolonged above
them (Fig. 358, B) as a projection, or in the form of a long appendage as in the
Oleander. If the upper part of the stalk, the connective, is broad, the two anther-

FlG. 362. — Stamen o{ Mahonia Aqiti-
Jfllinm ; B with the anther open (by re-
curved valves).

Fig. 363. — Stamen oi Arbiitits hybrida,
antlier open (by pores) ; x appendage.

Fig. 364. — Stamens of Centj-adenia
rosea ; A a larger fertile one, B a smaller
sterile one of the same flower.

lobes are distinctly separated (Figs. 358, 362); if it is narrow, they lie close to one
another. The articulation of the stalk is very commonly the result of the con-
nective being sharply separated from the filament by a deep constriction; the
connection of the two is then maintained by so thin a piece that the anther, together
with the connective which unites the anther-lobes, swings very lightly as a whole on
the filament (versatile anther). The point of connection may be at the lower end, at
the centre (Fig. 363), or at the upper part of the connective; sometimes the detached
connective attains a considerable size, and forms appendages beyond the anther
(Fig. 364, A, x), or it is developed between the two lobes like a cross-bar, so that
the filament and connective form a T, as in the Lime, and to a much greater extent
in Salvia, where the transversely extended connective bears an anther-lobe on one
arm only, while the other is sterile and is adapted for a different purpose. Whether
the anther-lobes are parallel depends on the mode of their connection with the
connective ; if they are so, they are usually attached to the connective for their
whole length ; or in other cases they are separated above, or free below and
coherent above, in which case they may become placed at such a distance from one

544

PHANEROGAMS.

another that the two lobes lie in one line above the apex of the filament, as in many
Labiataj. Not un frequently the filament also has appendages ; as, for example, the
membranous expansions or appendages right and left below in Allium which
resemble stipules, or a hood -shaped outgrowth behind as in Asclepiadeae, or ligular
structures in front as in Alysswfi t?ionlanum, or conical prolongations beneath on
one side as in Crambe, or on both as in Mahonia (Fig. 362, x).

A phenomenon of great importance from a morphological point of view is the
branching of stamens which occurs in many Dicotyledons, a peculiarity of structure
which was erroneously confounded by the older botanists with their cohesion,
although the two are fundamentally distinct. Sometimes the branching of stamens
takes place, like that of foliage-leaves, bilaterally in one plane, right and left of the
median line, so that the branched stamen has a pinnate appearance, as in Calo-
thamnus (Fig. ^d^, si), where each division bears an anther. In other cases the
branching takes place in a kind of polytomy, as in Ricinus (Fig. 366), where the

Fig. 365.— Long^itudinal section of the flower of Calo-
thamnus ;ythe ovary, j calyx,/ petals,^ style, j-/ branched
stamens.

Fig. 366.— Part of a male flower of Ricinus communis
cut through lengthways ; yy the basal portions of the
compoundly-branched stamens ; a the anthers.

separate stamens arise in the form of simple protuberances from the receptacle, each
one repeatedly producing new protuberances, which at length develope by inter-
calary growth into a corapoundly and repeatedly branched filament; the ends of
the branches all bearing anthers. In the Hypericinese \ three or five large broad
protuberances (Fig. 367, II-V, a) spring from the periphery of the floral axis after
the formation of the corolla, on each of which smaller roundish knobs are produced
in basipetal succession from its apex ; these latter become the filaments, each of
which eventually bears an anther, and are connected at their base with the primordial
protuberance of which they are branches. A horizontal section through the flower-
bud before the opening of the flower shows, especially in Hypericum calycinum, the
numerous filaments which spring from one original protuberance densely crowded
into one bundle. In this and many similar cases the common primordial basal

^ [For further details see Molly, Unters. lib. die Bliithenentwickelung der Hypericineen und
Loasaceen, Diss, Inaug. Bonn, 1S75.]

ANGIOSPERMS.

545

portion of the stamen remains very short, while the secondary filaments lengthen
considerably and subsequently present the appearance of a tuft springing from the
receptacle, the true nature of which can only be ascertained by the history of its
development. If, on the contrary, the primordial basal portion lengthens, as in

Fig. 367.— Development of the flower of Hypericum perforatum ; I young flower-bud in the axil of the bract B, with its
two bracteoles bb, s the sepals, / first indication of the petals ; // middle part of a somewhat older bud, y rudiment of the
ovary, a, a, a the three stamens with the rudiments of their branches arising as protuberances ; /// a flower-bud of nearly
the same age as in //, but seen from the side, s a sepal, a a the stamens.y the ovary ; /y and F flower-buds in further
stages of development, the letters indicating the same as in /, //, and /// ; i, 2, 3 ovary in various stages of development cut
through horizontally.

Calolhamnus, the whole stamen is easily recognised as branched even in the mature
condition.

Of no less importance for understanding the entire plan of structure of a
flower, and especially the relations of number and position which actually occur,

Fig. 368.— Development of the androecium of Cucurbtta Pepo (after Payer) ; in all the figures the simple stamen is to the
right, behind and to the left two double ones. The anthers grow vigorously in length and form vermiform coils.

is the cohesion of stamens which grow side by side in a whorl. In Cucurbita,
for example, there are, in the earliest stage, five stamens, but at a later period only
three are found, two of which are, however, broader than the third ; these are each

N n

546

PHANEROGAMS.

the result of the lateral coalescence of two stamens. In this case the filaments
become combined into a central column, on which (as is shown in Fi^. 368, ///) the
pollen-sacs grow more rapidly in length than the filaments, forming vermiform coils.
The relationships are much more complicated and more difficult to understand
when cohesion and branching of the stamens occur simultaneously, as in Mal-
vaceae. In Althcp.a rosea, for instance, the filaments form a membranous closed
tube which completely envelopes the gynaeceum; springing from this tube are five
vertical and parallel double rows of long filaments, each of which (Fig. 369, B)
again splits into two arms (/), and each of these arms bears a single anther-lobe.
The history of development and a comparison with allied forms shows that the tube
is formed by the lateral coalescence of five stamens ; but the coherent margins produce
double rows of lateral ramifications, in other words, of filaments, which then again
split into two arms. A horizontal section of the young staminal tube (Fig. 369, A)
shows plainly these double rows of split filaments ; the part {v) which lies between
two of these must be considered as the substance of a stamen, the margins of which

Fig. 369 — Althaa rosea; A horizontal section through the young andrcecium ; K a piece of the tube of a mature
androecium with several stamens ; h cavity of the tube, v substance of the tube, a anthers, t the spot where the
filament divides, /"the spot where two filaments spring from the tube (A much more strongly magnified than B).

each bear right and left a simple row of filaments as lacinias or branches ^ In
the Lime, where the five primordial stamens also branch at the margins, and form
anthers on their branches, the stamens remain free, but in other respects the
phenomena are altogether similar [cf. Payer, /. <r.)

The stamens not unfrequently suffer conspicuous displacements by the inter-
calary growth of the tissue of the receptacle in the region of their insertion ; and
such displacements are also ordinarily included under the term cohesion (or adhe-
sion)^. Thus the stamens often adhere to the calyx or corolla; and then, when

' The strangeness of this conception will disappear if the structure is recalled of a unilocular
ovary with numerous carpels coherent at the margins, e. g. Viola, where the ovules arise in double
rows on the lines of junction (^the placentae). What takes place in one case in the inside in reference
to the ovules takes place in the other case on the outside in the formation of the filaments.

^ [It has come to be the usage in English works on descriptive botany to apply the term
' cohesion ' to the apparent union of organs of the same kind, ' adhesion ' to the apparent union of
organs of a different kind.]

ANGIOSPERMS.

547

mature, the filaments appear as if they sprang from the inside of the perianth ;
the earhest stages of development show, however, that the perianth-leaves and the
stamens spring in succession and separately from the receptacle ; it is not till a later
period that intercalary growth begins at the part of the receptacle from which both
spring ; in this manner a lamella grows up which structurally forms the basal portion
of the perianth-leaf, and which at the same time bears the stamen, so that the
appearance is presented as if the stamen sprang from the centre of its inner surface.
This is shown in Fig. 370, ^, where /> is a perianth-leaf and a an anther sessile upon
it ; the two stand at first distinct on the young receptacle one over the other ; the
portion of leaf lying beneath a and p is not formed till a much later period by
intercalary growth, and pushes up at the same time the true perianth-leaf ^ and
the stamen a. This kind of adhesion is especially frequent in those flowers whose
petals have also become coherent laterally into a tube, such as Compositse, Labiatae,
Valerianacese, &c. On the other hand, the stamen may also become adherent
in various ways to the gynaeceum. In Sterculia Balanghas (Fig. 371) this structure

Fig. 370. — VXawe^x o{ Manolesia glabraia ; 4 before open-
inff ; B open ; C the gyneeceum, gp the gynophore ; D hori-
zontal section of the ovary ; E fruit ripening on its pedicel.

Fig. 371. — Flower oi Sterculia Balanghas;
A, gs the gynophore, y ovary, n stigma ; B hori-
zontal section of the ovary.

is only apparent, depending simply on the small stamens, which are placed close
beneath the ovary, becoming raised up together with it by the elongation of a
part of the receptacle ; from their small size they appear like a mere appendage of
the large ovary ; the part which bears both the organs, the Gynophore^ is therefore
in this case an internode of the floral axis. Much more complicated is the history
of the formation of the true Gynostemium (column) which is formed above an
inferior ovary, as in the Aristolochiaceae, and especially in the Orchideae, where
these adhesions and displacements of the parts of the flower are also combined with
abortion of certain members. Since these relationships will be explained in the
sequel, the examination of Fig. 372 will suffice for the present, where the flower of
Cypripedium is represented from the side (^), from behind (-5), and from front (C),
after removal of the perianth {pp). f is the inferior ovary, gs the gynostemium,
resulting from the adhesion of three stamens — two of which {ad) are fertile, while the
third (.9) forms a sterile staminode — with the carpel, the anterior part of which bears
the stigma {n). In this case the gynostemium consists entirely of coherent foliar
structures, or of the basal portions of the staminal and carpellary leaves, both of

N n 2

548

PHANEROGAMS.

which spring from the upper margin of the hollowed-out receptacle which constitutes
the inferior ovary \

The size and form of the stamens frequently varies within one and the same
flower; thus, for instance, in the Cruciferse there are two shorter and four longer
(tetradynamous), in the Labiatae two larger and two shorter (didynamous) stamens ;
in Cen/radem'a, as was shown in Fig. 364, A, B, they are not only of different size,
but are also differently segmented. A correct conception of the history of develop-
ment and a comparison of the relationships of number and position in nearly allied
plants enable one to apply the term stamen even to structures which have no anther
and therefore want the ordinary physiological character of stamens. Thus, for
example, in Geranium there are two whorls of fertile stamens, while in the nearly
related genus Erodium those of one whorl are without anthers. Such sterile stamens
or Staminodes generally undergo further metamorphosis, by which they become unlike
the fertile ones and not unfrequently petaloid, as the innermost staminal leaves of
Aquilegia ; or assume very peculiar forms, as in Cypripedium (Fig. 372, s). In

Fia 372.— Flower of Cypripedium Calceoliis after removal of the perianth.

some Gesneraceae a glandular structure or nectary is found in place of the poste-
rior stamen (compare the drawing of Columnea, Fig. 416). Metamorphoses of
this kind may be considered as the first steps to a condition of abortion, the final
stage of which is the production of a vacancy at the spot where the stamen should
be, as in the Labiatas, an order closely allied to the Gesneraceae, where, in the place
of this staliiinode there is no structure whatever ; instead of the five stamens to which
the plan of construction of the flower points, there are only four, even the rudiment
of the fifth, the posterior one, being suppressed, as is seen in Fig. 373 2. Phenomena
of this kind altogether justify the hypothesis of abortion in those cases in which
the absent organ does not disappear in the course of development, but never comes

. * Compare the account of the development and significance of the flowers of Orchidese in the
sequel.

^ [Peyritsch however (Sitzungsb. der k. Akad. der Wissen. zu Wien, 1872) infers, from the
constant reversion to fours in the peloric flowers of Labiatce, and from other considerations, that the
original type of the flower is tetramerous.]

ANGIOSPERMS.

549

into existence at all, if the hypothesis of the suppression of the part is confirmed
by a comparison of the relationships of number and position in nearly-allied plants.
The hypothesis of an abortion of this kind was, however, for the first time placed
on a firm basis by the theory of descent.

The number of stamens in a flower is only rarely so few as one or two ; like
the perianth-leaves they are usually numerous, and they are then arranged in the
form of rosettes, either spirally or in whorls. If the arrangement of the perianth-
leaves is spiral, that of the stamens is usually the same, and the number of the
latter is then very commonly large and indefinite, as in Nymphcea, Magnolia, Ranun-
culusy Helleborus, &c. ; but in this case they are sometimes also few in number and
definite.

Much more often, however, the stamens are arranged in one or more whorls,
those in one whorl being then usually equal in number and alternate with those in

Fig. 373.— Various stages of development of the flower of Lamitun album ; I, II, III very young buds
seen from above ; in / the rudiments of the sepals s are formed, in // those of the petals/, in /// those of the
stamens st and of the carpels c; /K horizontal section of an older bud, s tube of the gamosepalous calyx, / that
of the gamopetalous corolla ; a anthers, n stigmas ; y upper lip of the corolla with the epipetalous stamens ;
yi entire mature flower seen from the side.

the other whorls, and with the perianth-leaves [symmetrical flowers of English text-
books]. There are, however, numerous deviations from this rule [unsymmetrical
flowers of English text-books] occasioned frequently by the abortion of particular
members or of whole whorls, or by their multiplication, or by the superposition of
consecutive whorls; and not unfrequently in the place of a single stamen two or
even more will arise side by side {dedoubkment). These phenomena, which are
often difficult to make out, are nevertheless of great value in the determination of
natural affinities, and will be still further examined in the sequel.

Development of the Pollen and of the Anther-waW^ . The description given in
this place will apply only to the ordinary cases in which the pollen is formed in

^ Nageli, Zur Entwickelungsgeschichte des Pollens, Zurich, 1842, — Hoftneister, Neue Beitrage
zur Kenntniss der Embryobildung der Phanerogamen, II. Monocotyledonen. — Warming, UntefS, Ub,
pollenbildende Phyllome und Caulome, in Hanstein's Bot. Abhdl. Bd. II. 1873. — [Goebel, Beit. z.
Entwickgesch. d. Sporangien, Bot. Zeit. 1881.]

550

PHANEROGAMS.

separate grains in the four loculi of the anther, and falls out of the anther after it
has opened ; some of the more important exceptions will be mentioned hereafter.

Immediately after the perianth-leaves, or their innermost whorl, first become
visible on the receptacle as roundish protuberances, the rudiments of the stamens
make their appearance in a similar manner, but usually obtain a considerable start
in growth of the corolla, which not unfrequently remains for a considerable time in
a very rudimentary condition. These bodies, which consist of homogeneous primary
meristem, very soon show the outlines of the two anther-lobes united by the con-
nective; the filament is still very short, subsequently it also grows slowly, and it
is only just before the expansion of the flower that it elongates very rapidly by
vigorous intercalary growth. When the four pollen-sacs make their appearance
externally on the young anthers as longitudinal protuberances, a layer of cells
becomes differentiated within them from the hitherto homogeneous tissue. The
young anther consists at first of a small-celled proto-meristem in which a fibro-
vascular bundle becomes differentiated lying in the axis of the connective; the
external peripheral layer forms the dermatogen or the young epidermis. According
to Warming's comprehensive researches, it is usually only that layer which imme-
diately underlies the epidermis (the most external layer of the periblem) which gives
rise to the mother-cells of the pollen and to layers of the wall of the pollen-sac
surrounding them. [These cells constitute the archesporium of Goebel. It will
be seen that in the pollen-sac, as in the sporangia of the Vascular Cryptogams,
the archesporium is derived from hypodermal cells.] Within each of the before-
mentioned four longitudinal protuberances, the layer of cells (archesporium) imme-
diately underlying the epidermis splits into two, the inner of which gives rise to
the mother-cells of the pollen. These cells soon become conspicuous on account
of their large size, and, on a transverse section of the anther, they are seen to
form a multicellular layer, concave internally, in each of the four protuberances.
It does occur that the transverse section of one of these protuberances shows
only a single mother-cell, so that here one row only of these cells is present
(Compositge, Malvaceae). Less frequently the mother-cells are isolated (as in
Mimosese). The mother-cells of the pollen divide but rarely until the formation
of the tetrads begins; hence the number of mother-cells primarily formed is only
slightly increased: but the simple row or layer of primary mother-cells may
form several layers or a cylindrical mass of mother-cells as the result of divisions
in all directions. That layer of cells which, as mentioned above, was formed by
the longitudinal splitting of the layer (archesporium) from which the mother-cells are
derived, lies between them and the epidermis. It divides, according to Warming,
into usually three layers, in which radial, horizontal, and tangential divisions succes-
sively occur. The innermost of these three layers (Fig. 374, A ep, Fig. 377, Bn),
which is completed by a corresponding layer on the inner side of the group of
mother-cells, developes into a peculiar epithelium (the tapetum), investing the
mother- cells of the pollen on all sides and lining the cavity of the pollen-sac ; it
corresponds to the tapetum in the sporangia of the Vascular Cryptogams, and,
like it, it is finally absorbed in the process of the development of the pollen.
The same fate overtakes the next outer layer of cells. The most external of
the three above-mentioned layers, which lies immediately beneath the epidermis.

ANGIOSPERMS.

55^

forms the layer of fibrous cells which cause the dehiscence of the anther (Fig. 382,
Gft); to this we shall have to refer hereafter.

The mother-cells of the pollen are at first large and their walls thin (Fig. 374,
A, sm); but these increase considerably in thickness, though generally not uniformly
(Figs. 375, 378, A), the thickening matter being usually distinctly stratified. In
many Monocotyledons the mother-cells now become completely separated, the
pollen-sac becomes broader, and the cells float singly or in connected groups in
a granular fluid which fills up its cavity, as is shown in Fig. 374, B, a phenomenon

Fig. 3y4-—f uH^ia cordata ; A transverse section through a
young pollen-sac before the isolation of the mother-cells svt,
ep the tapetum which clothes the anther-lobe, 7u wall of the
pollen-sac ; B the anther-lobe after isolation of the mother-cells
sm; ep the tapetum (X 500).

Fig. 375.— Mode of formation of the pollen of Fitukta
ovata (X 550). In VII the wall of the daughter-cell has
absorbed water till it has burst ; its protoplasm is forcing
itself out through the fissure, and is lying before it
rounded off into a spherical form.

which calls strongly to mind the formation of the spores of Vascular Cryptogams.
In other cases, however, as for instance in many Dicotyledons [TropcBolum, Alihcea^
&c.), the very thick-walled mother-cells do not become isolated; they completely
fill up the pollen-sac, but may become separated after the rupture of the anther-
wall in water. In most Monocotyledons the large central nucleus divides, and
two fresh nuclei make their appearance ; this is followed by the division of the
protoplasm, and by the simultaneous formation of a cell-wall in the plane of
division. The same process is repeated in each of the two cells, and thus the

L

55^

PHANEROGAMS,

four 'special' mother-cells of the pollen are formed (Fig. 375). Thus the mode
of development of the pollen-grains in Monocotyledons resembles that of the
microspores of Isoetes. It sometimes happens that only one of the two secondary
cells divides, and then only three pollen-grains are formed, two of them being of
normal size, the third considerably larger : or again, neither of the two cells may
divide, or only imperfectly, and this leads to modifications of the size and shape

Fig. 376.-5 a young pollen-cell oiFunkia ovata; the thickenings which project outwardly are still small, in the
older pollen-cell C they are larger ; they are arranged in lines connected into a net-work.

of the resulting pollen-grains. In other Monocotyledons {Asphodelus albus and
luteus) the development of the pollen proceeds in the manner to be described
below as being characteristic of Dicotyledons \ A second process is especially
characteristic of Dicotyledons, in which, after the division of the nucleus of the
mother-cell, the two secondary nuclei divide in planes at right angles to that of the
first division and to each other; the four nuclei are thus arranged, as it were, in
the corners of a tetrahedron. The protoplasm becomes then constricted into four

Fig. 377.—^ pollen-sac oi Althaa rosea seen from the side, m the archesporium ; B transverse section of an anther-
lobe showing the two pollen-sacs, m the mother-cells of the pollen, in ^ still united into a tissue, in B already divided
each into four pollen-cells, n the tapetum of the pollen-sac. Each anther-lobe, consisting of two pollen-sacs, is here
borne on a long branch of the filament.

lobes, each nucleus forming the centre of one of the lobes, by the ingrowth of
the thick wall of the mother-cell ; a simultaneous formation of cell-walls now takes
place in the planes of division, the walls being attached to the margin of the
ingrowths of the wall of the mother-cell, and thus the four masses of protoplasm
which have become rounded off during the division become four distinct cells
(Fig. 378, A-E).

The mass of cellulose round each of the daughter-cells of the

^ [This account of the development of the pollen is taken from Strasburger (Zellbildung und
Zelllheilung, 3rd edition, 1880.]

ANGIOSPERMS.

55.3

tetrad now becomes differentiated into concentric systems of layers, and these are
enveloped by layers which are common to the whole tetrad (Figs. 378 ^, 379).
If the tetrads have lain for some time in water, the layers usually burst, and the
protoplasmic contents of the young pollen-cells are forced out through the fissure,
and become rounded off into a sphere (Figs. 375 VII; 378 I^, G). Soon after
the conversion of the mother- cells of the pollen into a tetrad, each protoplasmic
mass becomes clothed with a new cell-wall, at first very thin, which is continuous
with the inner layers of the wall of the mother-cell, as is shown by its becoming
detached from them when caused to contract by alcohol. This is the true cell-wall
of the pollen, which now increases greatly in thickness, and becomes differentiated

Fig. yji.—AltfuEa rosea ; A—E division of the mother-cells of the pollen into four ; F and G a tetrad, the
walls of whose special mother-cells have burst under the influence of water, and have allowed the protoplasmic body
of the young pollen-cells to escape ; H a mature pollen-grain seen from without, magnified to the same extent
(cf. Fig. 11).

into an outer cuticularised layer and an inner one of pure cellulose, the Extine and
the Inline. The former becomes covered on the outside with spines (Fig. 379, pJi)^
warts (Fig. 376), ridges, combs, &c. ; while the latter frequently forms considerable
thickenings which project inwards at particular spots (Fig. 379, v)^ and at a later
period are employed to form the pollen-tube. During these processes the layers
forming the envelope of the tetrad become slowly absorbed, their substance is
converted into mucilage, and they at length entirely lose their form; their dis-
organisation may commence either on the inner (as in Fig. 375, VII, x) or outer
side (Fig. 379, sg) of the wall of the mother-cell. In consequence of this absorption

554

PHANEROGAMS.

the young pollen-cells become free, separate, and float in the granular fluid which
fills up the cavity of the anther; and within this they now attain their definite
development and size. The fluid being thus used up, the mature pollen-grains finally
fill up the cavity of the anther in the form of a powdery mass.

[The ripe pollen-grain of Angiosperms has been found in many cases to contain
two nuclei ^ It appears that when the pollen-grains have become isolated from each
other, the nucleus of the grain undergoes division into two, one larger, the other
smaller. The smaller nucleus travels to the wall of the grain and becomes invested
by protoplasm, thus constituting a primordial cell, which, in some cases, is cut off
from the rest of the grain by a wall of cellulose : the larger nucleus remains as the
nucleus of the larger cell of the pollen-grain. The smaller nucleus may divide once
or twice, thus giving rise to a group of cells; the large nucleus does not divide: the
form of the nuclei varies very much. These processes resemble those which have

Fig. 379.— Mother-cell of the pollen oiCucurbita Pepo; sg the outer common layers of the mother-cell itl the act
of being absorbed ; sp the so-called ' special mother-cells,' consisting of masses of layers of the mother-cell which
surround the young pollen-cells ; they also are afterwards absorbed ; ph the wall of the pollen-cell ; its spines grow
outwards and penetrate the special mother-cell ; v hemispherical mass of cellulose on the inside of the pollen cell-
wall, from which the pollen-tube is afterwards formed ; / the protoplasm contracted ( < 550). (The preparation was
obtained by making a section of an anther which had lain for some months in absolute alcohol.)

been described as taking place in the pollen-grains of the Gymnosperms : the small
cell (or the cells derived from it) evidently corresponds to the * vegetative ' cells in the
grains of Gymnosperms and in the microspores of the heterosporous Vascular
Cryptogams.] The pollen-tube is formed from the large cell : it is developed as a
protuberance of the intine, which perforates the extine at certain definite spots that
have usually been prepared beforehand. The spots where this perforation takes place
are often more than one, or even very numerous (Fig. 380 a, 381 o)\ yet, notwith-
standing the possibility of the formation of this number of pollen-tubes from one grain,
only one usually grows to an extent sufficient to effect impregnation. Independently
of the structure of the extine itself which has already been mentioned, the external

^ [Hartig (in Karsten's Botan. Untersuch. III. 1866) was the first to observe two nuclei in a
pollen-grain : he found them in the grains of Tradescantia, Campa7mla, CEnothera, Lilium, Clematis,
Allium, etc. His observations have been extended by Strasburger (Ueber Befruchtung und Zelltheilung,
1878) and by Elfving (Jenaische Zeitschrift, 1877, and Quart. Journ. Micr. Sci., XX. 1880.)]

ANGIOSPERMS.

555

form and sculpture of the outer coat of pollen-grains depends chiefly on the number
of the spots at which the perforation may take place, on the mode in which these are
arranged, and on the circumstance whether the extine is at these spots merely thinner
and the intine projects in the form of a wart (Fig. 380), or whether roundish pieces
of the extine become detached in the form of a lid, as in Cucurbitacese and Passiflora
(Fio- 35> P- 33)' ^^ whether it splits into bands by spiral fissures, as in Thunbergia
(Fig. 36, p. 34), &c.^ At the points of perforation the intine is generally thicker, often
forming hemispherical protuberances which furnish the first material for the formation
of the pollen-tube (Fig. 381, /), or the extine only forms thinner longitudinal striae
which fold inwards when the pollen-grain becomes dry (as in Gladiolus, Yucca^

^d^^.

riG. 380. — Transverse section of a pollen-grain of Ej>ilo-
btum angustifoHum: a the points where the intine i pro-
trudes, the intine being there thicker and tlie extine e thinner
(Xsoo).

Fig. 381.— Pollen-grain of Althaa rosea: A a piece of the
extine seen from without ; B the half of a very thin section
through the middle of the pollen-grain, st large spines, Ks
small spine of the extine, o perforations through the extine e,
i the intine, / the protoplasm of the pollen-grain contracted
(X800).

Helleborus, &c.). Very commonly however the intine is uniformly and continuously
thickened, as in Canna, Sirelitzia, Musa, Persea, &c. ; and in this case, according to
Schacht, no definite spots are prepared beforehand where the perforation is to take
place. The number of these pecuHarly organised points of perforation is definite in
each species, often in whole genera and families ; there is only one in most Mono-
cotyledons and a few Dicotyledons, two in Ficus, Justicia^ «&c., three in the Onagrarieae,
Proteaceae, Cupuliferae, Geraniacese, Compositae, and Boragineae ; four to six in
Impatiens, Asirapaa, Alnus, and Carpinus, while the number is large in Convolvulacese,

^ For more minute details see Schacht, Jahrb. fiir wissensch. Bot. II. p. 109, and Luerssen,
ibid. VII. p. 34. — [Fritzsche, Beitrage zur Kenntniss des Pollen. Berlin, 1832. — Mohl, Beitrage zur
Anatomic u. Physiologic der Gewachse, ist Heft, Bern, 1834. — Edgeworth, Pollen, London, 1877.]

55^ PHANEROGAMS.

Malvaceae, Alsinese, &c. (see Schacht, /. c). The extine is rarely smooth, more often
marked on the outside by the sculpture to which reference has already been made.
When it is very thick, layers of different structure and texture may frequently be
detected, and differentiations sometimes occur in a radial direction, penetrating the
thickness of the extine (Fig. 381), and giving it in some cases the appearance of con-
sisting of rod-shaped prismatic pieces or of honeycomb-like lamellae, &c., peculiarities
of structure recalling those of the epispore of Marsiliaceae. The contents of the ripe
pollen-grain, the Fovilla'^ of the older botanists, usually consists of a dense coarse-
grained protoplasm in which grains of starch and drops of oil may be recognised.
When the grain bursts in water, the fovilla escapes in masses connected by mucilage
and often in long vermiform threads. The surface of the extine is commonly found
coated with a yellow oil, or of some other colour, often in evident drops, which
renders the pollen viscid and adapted to be carried by insects from flower to flower ;
in only a comparatively few cases is it quite dry and powdery, as in Urticaceae
and many Grasses, where it is projected with violence from the anthers or simply
falls out.

At the time when the pollen-grains are nearly mature, and the flower-bud is
preparing to open, the wall of the pollen-sacs undergoes a further development ^.
The outer layer of cells or epidermis always remains smooth-walled (see Fig. 382,
P- 558) ; th^ inner layers or endothecium are also smooth if the anther does not
dehisce. If on the other hand it opens by recurved valves (Fig. 362 U), the cells of
the inner layers of these valves only are provided with thickening-bands (or are
fibrous) ; while, when the pollen-sacs dehisce longitudinally, the whole of their endo-
thecium contains fibrous cells. There is usually only one such layer, sometimes
several; in Agave americana as many as from eight to twelve. The thickening-bands
of the fibrous cells which project inwards are usually wanting on their outer wall; on
the side walls they are generally vertical to the surface of the pollen-sac; on the inner
wall they run transversely and are united in a reticulate or stellate manner. Since the
epidermal cells contract more strongly when the ripe anther-walls dry up than those
of the endothecium which are provided with thickening-bands, they exert a force
which has a tendency to make the anther-wall concave externally and to give way at
its weakest point. The modes in which the anthers open are very various, and are
always intimately connected with the other contrivances which are met with in the
flower for the purpose of pollination with or without the agency of insects. Sometimes
only a short fissure (pore) is formed at the apex of each anther-lobe, as in Solanum^
EricacecE (Fig. 363), &c., through which the pollen of both the contiguous pollen-sacs
escapes ; but more commonly the wall gives way in the furrow between the two sacs
(the suture) along its whole length, the tissue which separates them becoming at the
same time more or less destroyed, and thus both pollen-sacs dehisce at the same
time by the longitudinal fissure (Fig. 382). It is this phenomenon that has given rise
to the erroneous description of these anthers as being bilocular ; but if nomenclature
is to have a scientific basis, they must be termed quadrilocular, in contrast to the

^ [On the constitution of the ' amyloid corpuscles ' in the fovilla of pollen see Saccardo, Nuovo
Giornale Botanico Italiano, 1872, p. 241.]

^ Compare H. v. Mohl, Vermischte Schriften, p. 62. — Purl<yne, De cellulis antherarum fibrosis,
Vratis. 1830.

ANGIOSPERMS.

557

really bilocular anthers of Asclepiadeae and the octilocular ones of many Mimoseas.
Sometimes again the anther-lobes open at the apex by a pore which results simply
from the destruction of a small portion of tissue at this spot (Hofmeister). In other
respects we still want a detailed and comparative investigation of these processes,
which are very various and of great physiological importance; only the additional
remark need be made here, that it is very important from a systematic point of view
whether the anthers open inwards towards the gynaeceum (introrse), or outwards
(extrorse), the difference depending on the position of the suture and hence on that of
the pollen-sacs on the inner or outer side of the filament.

In several families of Monocotyledons and Dicotyledons more or less con-
siderable deviations ^ occur from the course of development of the pollen and
from its final structure which has been here described. Naias and Zostera deviate
only to this extent, that no thickening of the wall of the mother-cells takes place,
and that the pollen-cells themselves are very thin-walled, acquiring in Zostera a very
strange appearance from assuming, instead of the ordinary rounded form, that of long
thin tubes lying parallel to one another in the anther. The deviations are more
considerable in the formation of compound pollen-grains. The origin of these is
either that only the four daughter-cells (pollen-cells) of one mother-cell remain
more or less closely united, like the pollen-tetrads (four-fold grains) of some
Orchideae, Fourcroya, Typha.^ Anona, Rhododendron, &c.; or the whole product of
one primary mother-cell remains unseparated and forms a mass of pollen consisting
of eight, twelve, sixteen, thirty-two, or sixty-four connected pollen-cells, as in many
Mimosese and Acaciese^. In these cases the cuticle or extine is more strongly
developed on the outer surface of the daughter-cells lying at the circumference of
the mass, and covers the whole as a continuous skin ; while only thin ridges of the
cuticle project from this skin inwards between the separate cells. In the various
sections of Orchideae every gradation occurs from the ordinary separate pollen-grains
of Cypripedium, through the four-fold grains of Neotiia, to the Ophrydeae, where all
the pollen-grains which are formed from each primary mother-cell remain united,
and thus a number of pollen-masses lie in one pollen-sac ; and finally to the Pollinia
of the Cerorchideae, where all the pollen-grains of a pollen-sac remain united into
a cellular mass. In this case, as in the Asclepiadeae with only bilocular anthers,
where the grains of each pollen-sac are firmly united by a waxy substance, it is
obvious that the pollen cannot be dispersed, nor can the pollen-masses fall out spon-
taneously from the anthers; but the flower is provided with very peculiar con-
trivances by means of which insects in search of honey extract from the pollen-sac
the pollinia or the masses of pollen which are glued together, and again get rid of
them on to the stigmas of other flowers of the same species (see Book III on Sexual
Reproduction).

The Female Sexual Organs or Gynceceum ^ (Pistil) of the flowers of Angiosperms

* In reference to what follows compare Hofmeister, Neue Beitrage, pt. II. (Abhand der konig.
Sachs. Gesellsch. VII); also Reichenbach, De poUinis Orchidearum genesi, Leipzig 1852; and-
Rosanoff, Ueber den Pollen der Mimosen (Jahrb. fiir wissensch. Bot. VI. p. 441).

2 In many Mimosece the anther is, according to Rosanoff, octilocular, two pairs of small loculi
being formed in each anther-lobe; the pollen-cells of each pollen-sac remain united into a mass.

^ Compare with this Payer's view (Organogenic de la fleur, p, 725). which differs in some
essential points.

5o«

PHANEROGAMS.

consist of one or more closed chambers in which the ovules are formed ; the lower,
hollow, swollen part of each separate seed- chamber which encloses the ovules is
called the Ovary ; the place or the mass of tissue from which the ovules spring
directly into the ovary is a Placen/a. Above the ovary the seed-vessel narrows
into one or more thin stalk-like structures or Styles, which bear the Sltgmas ; these
are glandular swellings or expansions of various forms which retain the pollen that
is carried to them, and by means of the moisture which is excreted from them induce
the emission of the pollen -tubes.

Fig. ■iZ'2—Bntonttis iimbellatiis : A flower (natural size) ; B the gyn;uceum (magnified), the perianth and stamens removed,
n the stigmas; C horizontal section through three of tlie monocarpellary ovaries, each carpel bearing on its inside a number of
ovules ; D a young ovule ; h an ovule immediately before fertilisation, i i the integuments, K the nucellus, KSxhe. raphe, etn the
embryo-sac ; F horizontal section through the stigmatic portion of a carpel (strongly magnified), pollen-grains attached to the
stigmatic hairs ; G horizontal section of a quadrilocular anther, but the valves z are so separated at h that it then appears
bilocular; A'^ part of an anther-lobe (corresponding to /S in G),y the point where it has become detached from the connective,
tfthe epidermis, jr the fibrous layer of cells (endothecium) ; / diagram of the entire flower; the perianth // consists of two
alternate whorls of three leaves, as also does the androecium, but the stamens of the outer whorl /"are double, those of the inner
whoriy simple and thicker ; the gynaeceum also consists of two whorls of three carpels, an outer c and an inner whorl c' ; there
are therefore six alternate whorls of three, the members of the first staminal whorl being doubled.

The Gynaeceum is ahvays the final structure of the flower. When the floral
axis has attained a sufficient length, the gynaeceum is formed at its apex ; if the axis
is flat, disc-like, or expanded, it stands in the centre of the flower ; if it is hollowed
out or cup-shaped, the gynaeceum is placed at the bottom of the hollow, in the centre
of which lies the apical point of the floral axis. In the diagram of the flower,
Figs. 382 /, and 384 B, where each outer circle represeiits a lower transverse section,
and each inner circle a higher one, the gynaeceum necessarily appears always as
the innermost central structure of the flower, the longitudinal displacements on the
floral axis being neglected in the construction of the diagram.

ANGIOSPERMS.

559

When the axial part of the flower, the Receptacle or Torus, is so elevated in the
centre that the base of the gynseceum lies evidently above the stamens, or at least in
the middle of the androecium, the perianth and the androecium, or even the
whole flower, is said to be hypogynoiis (Fig. 382). When, on the contrary, the
receptacle is hollowed out like a cup or saucer, bearing the perianth and stamens on
its annular margin, while the gynaeceum springs from the bottom (Fig. 384, A), the
flower is said to be perigynous. It is obvious that intermediate forms are possible
between extreme cases of hypogynous and perigynous flowers ; and these are in fact
common, especially among Rosiflorae. In both these forms of flower the gynasceum

Fig. 384. —Flower of Eletagnus
fitsca : A lonjjitudinal section, d disc ;
B diagram.

Fig. 383.— Longitudinal section through the inferior owary of Eryng-ium catnpestre; I sepals, c petals, y filament, gr style,
h disc, KK nucellus of the ovule, i integument.

is free, the receptacle taking no part in the formation of the wall of the ovary,
although this appears to be the case externally in some perigynous flowers,
as Pyrus and Rosa. The flower finally is epigynous when it possesses an actually
inferior ovary. This latter is distinguished from the ovary which is buried in the
receptacle of perigynous flowers by its wall being formed of the receptacle itself
hollowed out into the form of a cup or even of a long tube. The carpels, which in
the case of the free superior ovary form its whole wall, spring in the inferior ovary
(like the perianth and the androecium) from the margin of the hollow receptacle,
and only close up the cavity above, where they are prolonged into the style and

560 PHANEROGAMS.

bear the stigmas (Fig. 383). Intermediate forms are also not uncommon between
the superior ovary of hypogynous and the inferior ovary of epigynous flowers ; the
ovary may, for example, be composed in its lower half of the receptacle, in its upper
part of the coherent carpels; transitional forms of this kind are found especially
among Saxifragacese. When the gynaeceum of a flower consists of a single ovary
only one fruit is formed, and the flower is said to be monocarpous (Figs. 383, 384),
in contradistinction to the polycarpous flowers, the gynseceum of which consists of
several isolated ovaries from which the same or a smaller number of fruits are
developed (Fig. 382).

It will be easier to understand the diff'erent forms of the gynaeceum if the more
important ones are considered separately ; and for this purpose the following classi-
fication may be made : —

I. Gynseceum Superior ; flower hypogynous or perigynous.

A. Ovules attached to the carpels.

a. Ovary monocarpellary ;

(a) flower with one ovary,

(jS) flower with two or more ovaries.
h. Ovary polycarpellary ;

iy) ovary unilocular,

(S) ovary multilocular.

B. Ovules attached to the floral axis ;

(e) ovule solitary, terminal,
{() ovules one or more, lateral.

II. Gynseceum Inferior ; flower epigynous.

C. Ovules attached to the carpels ;

{r)) ovary unilocular,
{6) ovary multilocular ;

D. Ovules attached to the floral axis ;

(t) ovule solitary, terminal,
(k) ovules one or more, lateral.

The Superior GyncEceum is constructed essentially of peculiar foliar organs,
the carpellary leaves or carpels. These usually produce the ovules, which generally
spring from the margins of the carpels, as in Fig. 385, but frequently also from the
whole inner surface, as in Fig. 357 i^, and Fig. 382 C The ovary is monocarpellary
(simple) when it consists of only a single carpel, the margins of which are coherent,
so that the mid-rib runs along its back, and the ovules, when they are marginal,
form a double row opposite to it. The inflexed margins of the carpellary leaf may
swell up into thick placentae (as in Fig. 386) and produce a larger number of rows
of ovules. The number of ovules is, on the other hand, not unfrequently reduced
to two (as in A mygdalus)^. In monocarpous flowers there is only one such car-
pellary leaf, as in Figs. 384, 385 ; in polycarpous flowers there may be two, three.

' With reference to the occurrence of a single ovule standing in the axil of the carpel (as in
Rammaihs), see infra.

ANGIOSPERMS.

561

or more, or even a very large number : if the number is two, three, or five, they
usually stand in a whorl ; if four, six, or ten, they are generally arranged in two
alternating whorls (see Fig. 382, B, I). When the number of monocarpellary
ovaries in a flower is considerable, as in Ranunculacese, Magnolia^ &c., the part
of the axis which bears them is commonly elongated (to a very considerable ex-
tent for example in Myosurus), and their arrangement is then spiral. The mono-
carpellary ovary is originally always unilocular, though it may subsequently
become multilocular from the production of ridges by the luxuriant growth of
the inside of the carpel, which divide the cavity longitudinally into compartments,
as in Astragalus, or transversely, as in Cassia fistula. Ovaries of this kind may be
distinguished as monocarpellary with spurious loculi, but ought not to be called
polycarpellary.

A polycarpellary (compound) ovary is always the result of the union of all the
carpels of a flower, the number being usually two, three, four, or five, arranged
in one whorl, the floral axis terminating in the midst of them. If the separate

FIG. Tfic^.—Phaseohcsvuloaris; A horuontal section through the flower-bud, /calyx-tube, ccoroUa.y filaments of the outer,
a anthers of the inner staminal whorl, K carpel ; B longitudinal section of the carpel, with the ovules SK and stigma n ; C, D, E
horizontal sections of carpels of different ages, -^A'the parietal (marginal) ovules, g; mid-rib of the carpel.

carpels remain open, and cohere in such a manner that the right margin of one
unites with the left margin of another, the result is a unilocular polycarpellary
ovary. The placentation is in this case parietal when the coherent margins project
only slightly inwards, as in Reseda, Viola, &c. But if the coherent margins of the
carpels project further inwards, the cavity of the ovary becomes imperfectly multi-
locular, the chambers being connected with one another in the centre, as in Papaver,
where the imperfect dissepiments are covered on both sides by a number of ovules.
A bi- or multilocular polycarpellary ovary results when the margins of the carpels
project inwardly so far that they meet or cohere either at or near the axis of
the ovary, the elongation of the floral axis in the centre frequently contributing
to this result. The mode of cohesion of the carpels in multilocular ovaries may
vary greatly in other respects, according as it takes place along the whole length
of their inflexed margins, or only below, while the upper parts resemble
a whorl of monocarpellary ovaries (Figs. 386-389). Since the margins of the
carpels which meet in the centre become developed into the placentse, the ovules
make their appearance in the central angles of the loculi (axile placentation), as

o o

562

PHANEROGAMS.

is seen in Fig. 388 ; but very commonly the margins of the carpels which turn
in as far as the centre then split into two lamellae which are bent back and swell
out into placentae in the middle of the loculi, as is shown in Fig. 387. It is clear
that in this case the two placentae within each loculus correspond to the margins of
the same carpel which forms the outer wall of the loculus.

Fig. 386.— Gyn.-ieceiim of Saxifra£a cordifoHa ; A longitudinal section, g style, n stigma ; B horizontal section
at different heights, / placenta.

Spurious dissepiments may arise in polycarpellary as in monocarpellary ovaries;
if the polycarpellary ovary consists of two loculi, it may thus become quadrilocular,
or five original loculi may become divided into ten. The first case is universal in
Labiatae and Boragineae. Fig. 390 shows that the ovary is formed of two coherent
carpels, the margins of which {I-IV) projecting inwards form a right and a left
placenta (//) ; on each of these placentae which correspond to the margins of the

B

Fig. 387. — Gynaeceuin ot I'yroia wnbellata ; A longitudinal section, s sepals, ■/> petals, st filaments,/" ovary, it stigma,
d nectar-glands ; B horizontal section through the ovary, y the wall, //placentae.

carpels a posterior and an anterior ovule are produced, but an outgrowth from
the mid-rib of the carpel (/F, F/, x) inserts itself between the two ovules be-
longing to each loculus, dividing it into two one-seeded lobes. Since at a sub-
sequent period the outer part of the wall of each of the four lobes bulges strongly
outwards and upwards {E), the separation of the bicarpellary ovary into four
separate parts becomes still more distinct ; and finally they completely separate as

ANGIOSPERMS.

5H

one-seeded lobes of the fruit; while in Boragineae the separation is still more
complete. The division of the five loculi of the ovary of Linwn into ten by spurious
dissepiments is not so perfect, the projections from the centres of the carpels not
reaching the central axis of the ovary.

Fig. ■^.—Dictammcs hraxi7ielia; A young flower-bud, with rudiments of sepals j; bolder flower-bud, with rudiments of
petals/ ; C still older state, with rudiments of the five stamens a, five more stamens a' arise between them, of which three are
already visible ; b the bract, f a liracteole ; D—H development of the ovary //fe, sk ovules, ^p gyiiophore, g style.

Before passing to the consideration of ovaries in which the ovules are borne by
the floral axis {i. e. with axial placentation), it should be mentioned that there are
cases in which the present state of our knowledge does not enable us to decide with

Fig. 389.— Ripe fruit of riciammis Fraxiiiella ; the anterior carpel has been removed and the two lateral
ones opened ; g gfj'nobasic style (natural size).

certainty whether the ovules arise from the axis or from the margins of the cirpels
which have become united to it; and these doubtful cases are possibly more
numerous than is generally thought. Payer's observations on Cerastium and Mala-
chium show that in Caryophylleae the expanded apex of the floral axis becomes

002

5^4

PHANEROGAMS.

considerably elevated even before the formation of the carpels ; the carpels are then
seen in a whorl, and are attached by means of their coherent margins to the elevated
axis ; each forms what may be described as a pocket attached to the axis. As the
axis becomes elongated, the margins of the carpels form radial dissepiments sepa-
rating the pockets, which widen into loculi ; and the carpels finally rise above the
apex of the axis. In Cerasiiwn and other genera the dissepiments also rise above
it as free lamellae which do not meet in the centre, so that the ovary is quinque-
locular below, while in the upper part it remains unilocular. The ovules are
produced in two parallel rows on the axial face of each loculus, this face being
apparently formed from the axis itself. In some genera of Caryophyllese it seems
probable that the placentae are axial, while in others they would appear rather to be
carpellary.

1[ M

Fig. 390.— /—^V/ stages of development of the ovary oi Phlomis ptcngens (a Labiate), ^in longitudinal, the rest in trans-
verse section; A a gynaeceum seen from without ready for fertilisation; B the same in longitudinal section, the lines u u,
o (? correspond to the transverse sections VI and VII; pi the placenta, x the spurious dissepiment, /"loculi, sk ovules, c wall
of the carpel, t disc, g style, n stigma.

Among Superior Ovaries with axial Flacentation, those of Typha, Naias, and
Piperaceae^ require especial mention. In these cases the very simple female flower
consists (with the exception of the perianth of Typha, which is represented by hairs) of
nothing but a small lateral shoot transformed into an ovary with a central ovule ^.
The apex of the axis of this shoot itself developes into the terminal nucellus of the
ovule, round which an annular zone grows up from below, overarches it, closes up
above, and thus forms the wall of the ovary. In Typha only one style and stigma

^ Magnus, Zur Morphologic der Gattung iVnms (Bot. Zeit. 1869, p. 772), — Rohrbach, Ueber
Typha (in Sitzungsber. der Gesells. naturf. Freunde Berlin, Nov, 16, 1869). — Hanstein u, Schmitz,
Ueber Entwickelung der Piperaceenbliithen (Bot. Zeit, 1870, p, 38),

^ As in the case of the 'axial anthers,' so here also some uncertainty still exists. In a letter
to me Schenk distinctly denies the axial nature of the ovule in Typha; he states that it is lateral,
that it appears as a small protuberance on the wall of tlie ovary, a position which it retains until
maturity.

ANGIOSPERMS.

5<^5

surmount the ovary, which may therefore be considered to be composed of a single
carpel which rises up from the floral axis as an annular zone. In Piperacese
however the stigma, which is sessile on the apex of the ovary, is often placed
obliquely or divided into several lobes ; and this, like the two or four styles
which surmount the ovary of Naias^, indicates that the ovary is not composed of
one but of several carpels, which first make their appearance, like the leaf-sheaths
of Equiseium^ as an unbroken ring, which only at a later period becomes resolved
at its upper margin into teeth. This hypothesis appears the more admissible
since, in other Angiosperms where a comparison with nearly allied forms justifies

Fig. 391. — Longitudinal section of the flower ol Rheiitn
undulatunt ; jleaf of the outer,/ of the inner perianth- whorl ;
a a a three of the nine anthers.y ovary, « stigma, ,t,4nucellus
of the ovary, dr glandular tissue at the base of the filaments
forming the nectaries.

Fig. ■y)2.—AnagciUis arvenszs ; A longitudinal section of
a young flower-bud, / sepals, c corolla, a anthers ; A' carpel ;
S apex of the floral axis ; B the gynaeceum further developed,
the stigma « being now formed, and the ovules on the central
placenta 5 ; C the gynaeceum ready for fertilisation, / pollen-
grams on the stigma n,gr style, ^central placenta, 5"A'ovules;
D unripe fruit, the placenta 5 has become fleshy and swollen
so as to fill up the spaces between the ovules.

US in inferring a number of coherent carpels, these carpels originate as an un*
divided annular zone which developes into the ovary, style, and stigma ; as, for
instance, in Primulaceae (Fig. 392) (free central placentation). In Polygonaceae, on
the other hand, where the ovary also forms eventually a closed cavity containing the
central ovule (Fig. 391), the cohesion of two or three carpels to form the ovary may
not only be recognised from the corresponding number of the styles and stigmas ;
but separate carpels appear at first distinct on the floral axis, and only amalgamate
in the course of their growth, their zone of insertion becoming elevated as a ring.
Since the wall of the ovary does not in any of these cases form placentae from

^ I am unable to understand why Magnus calls the wall of the ovary 'perianth.'

^66 PHANEROGAMS.

the number and position of which the number and position of the carpels might
otherwise be more easily determined, we are thrown back on the direct observation
of the first stages of development and on the numbers of the styles and stigmas.
Failing this, the solution of the question depends on morphological relationships
which are still by no means made out with sufficient certainty, notwithstanding the
numerous researches which have been made on the development of the flower.

Besides the number of the carpels which have coalesced to form the ovary, it
is a question of interest whether in any particular case the ovules have been pro-
duced laterally on the floral axis or as its terminal structure. In the cases of
Piperacese, Polygonacese, Naias, Typha, &c., where only a single ovule springs from
the base of the ovary, it is evident that this must be the terminal structure of
the floral axis ; and the investigations of Hanstein and Schmitz, Magnus, Rohrbach,
and Payer, have proved in addition that not only the ovule as a whole, but the
nucellus itself, must be considered as a terminal structured It must not, however,
be inferred from this that every ovule which springs- from the base of the cavity of
the ovary necessarily forms the apex of the floral axis ; for it is conceivable that the
axis itself may have ceased to grow, but has produced an ovule at the side of its
apex, a case which we shall meet with further on in the inferior ovary of Compositse.
In a few cases the floral axis rises free within the spacious cavity of the ovary and
produces ovules laterally, as occurs in Primulaceae (Fig. 392) and Amaranthaceae
(in Celosi'a, according to Payer).

The Inferior Ovary of epigynous flowers results from the retardation or com-
plete suppression of the apical growth of the young floral axis, its peripheral tissue
rising as an annular zone, and producing on its free margin the perianth, stamens,
and carpels (Figs. 393, 394). The hollow structure which is thus formed, and
which is at first open above, is afterwards covered over by the carpellary walls
which close in above it ; the apex of the floral axis lies at the bottom of the elongated
cup-shaped or tubular cavity. Notwithstanding this striking displacement of the
axial parts, the structure of the inferior ovary resembles that of the free polycarpellary
ovary in almost all respects ; it may also be either unilocular or multilocular — if
unilocular, the placentation may be basilar, lateral, or parietal. When the placentation
is basilar, the ovule sometimes appears as if it were the terminal structure of the
apex of the axis ; as for instance the erect ovule of Juglandeas. In Compositae,
on the other hand, the position of the single anatropous ovule is not terminal
but lateral; the apex of the floral axis may often be clearly made out as a small
elevation beside the funiculus, and in abnormal cases it undergoes further develop-
ment into a leaf-bearing shoot ^. In Samolus the apex of the axis rises within the
unilocular inferior ovary as in the superior ovary of other Primulaceae (Fig. 392), and
bears a number of lateral ovules. If the placentae of the unilocular inferior ovary
are parietal, they form on the wall two, three, four, five or more ridges from above
downwards or from below upwards, and bear two or a larger number of rows of
ovules (as in Opmifia or Orchideas). These placentae, which project more or less

^ [See infra, p. 574.]

^ Cramer, Bildungsabweichurigen und morphologische Bedeutung des Pflanzen-Eies (Zurich 1864).
— Kohne, Die Bllithenentwickelung der Compositen, Berlin 1869, — Buchenau, Bot. Zeit. 1832,
No. 18 et seq.

ANGIOSPERMS.

567

into the interior, may be regarded as the prolongations of the margins of the carpels
downwards on the inside of the ovary. A similar explanation may be given of the
longitudinal dissepiments of the multilocular inferior ovary ; the same differences
occur in them as those which have already ]?een described in the case of the
superior ovary; for they may either meet in the middle and bear the ovules in
the axile angles of the loculi (Fig. 358), or they may split into two lamellae, bend
back, and bear the ovules in the middle of the cavity of the loculus (as in Cucur-
bitaceae). Usually two, three, or more carpels share in the formation of the upper
part of the inferior ovary, their elongated margins being prolonged inwards and
developing downwards into the parietal placentae or the dissepiments of the multi-
locular ovary. In such cases the inferior ovary must be termed polycarpellary, like

F'G. 393.— /-AV/ stages of development of the flower of
Helianthus annutts; I calyx, c corolla, y filaments, a anthers,
X basal portion which afterwards developes into the lower
part of the tube of the corolla which bears the epipetalous
stamens, fK the inferior ovary, SK the ovule, A carpel,
S^r style.

Fig. 394. — ^/—Z? stages of development of the flower of
Calanthe veratrifolia (after Payer) ; A and C seen from above,
B and D in longitudinal section, s sepals, / petals, // the
petal which developes into the labellum, (r/the single fertile
anther, ae and at abortive anthers of the outer and inner
whorl; in B as are the sterile stamens, in D ep one of the
three carpels.

the superior ovary of similar structure. Examples of a monocarpellary inferior ovary
appear to be very rare ; Hippuris (Fig. 360) affords one ; its inferior ovary consists
of a single carpel, and contains a solitary anatropous pendulous ovule.

The Style is a prolongation of the carpel above the ovary ; in monocarpellary
ovaries there is therefore only one style (Figs. 382, 384), which may however be
branched ; when the ovary is polycarpellary, the style consists of as many parts as
there are carpellary leaves ; these parts may be free for the whole distance above
the ovary (Fig. 386), or coherent for a certain distance above it, separating only at
a greater height; or, finally, they may cohere for their whole length (Figs. 388 G^
390). Although the style arises from the apex of the young carpel, it may subse-
quendy stand on the axile side of the monocarpellary ovary, the carpel becoming
considerably bulged outwards by the more rapid growth of the dorsal side of the

568

PHANEROGAMS.

ovary (as in Fragaria and Alchemilla). If this occurs with each of the carpels of a
polycarpellary ovary, the ovary itself appears to be depressed in the middle, and the
style rises from the depression (Figs. 387, 388). In Labiatae and Boragineae this
peculiarity is especially conspicuous, the four lobes of the bilocular ovary forming
strong protuberances (Fig. 390, A, B\ so that the style finally appears to spring
from between four parts of the ovary which seem to have scarcely any connection
with one another, and is hence termed a gynobasic style.

The style may be hollow, that is, it may be penetrated by a channel consisting
of a narrow elongation of the cavity of the ovary, as in Butonius (Fig. 382, B, F),
where it opens on the hairy surface of the stigma; or in Viola (Fig. 395), where
the channel is broad, and opens above into the spherical cavity of the stigma ;
or in Agave and Fourcroya, where the style is hollow throughout its whole length

and open to the stigma, the simple channel dividing
below into three tubes which run into the loculi of
the ovary, a phenomenon which occurs also in other
Liliaceae\ In other cases it is at first hollow, as in
Anagallis (Fig. 392, B), but becomes afterwards filled
up by the growth of the tissue. There is usually no
channel to be detected in the style when the pistil is
ready for fertilisation, or at least not in its upper part ;
in the place of this its centre is occupied by a mass
of loose tissue, the ' conducting tissue,' through which
the pollen-tubes grow till they reach the cavity of the
ovary. The external form of the style is usually
cylindrical, filiform, or columnar, sometimes prismatic
or ribbon-shaped ; in the Iridese it generally attains
a considerable size ; in Crocus it is very long, tripartite
above, each division being deeply hollowed out like
a cup ; while the genus Iris is distinguished by its
three free broad petaloid coloured styles. Sometimes
the portion of the style which belongs to each carpel
branches, as in Euphorbiaceae, where a tripartite style,
each arm of which bifurcates, corresponds to the three
carpels. The style frequently remains very short, and then has the appearance of
being a mere constriction between the ovary and stigma, as in Viiis.

The Stigma '^^ in the narrower sense of the term, is the part of the style which
is destined for the reception of the pollen. When pollination takes place it is
covered with a viscid secretion, and usually with delicate hairs or short papillae,
constituting a glandular structure which is sometimes merely a peculiarly developed
portion of the surface of the style, sometimes a special organ of very variable ap-
pearance attached to it. The form of the stigma always has an intimate connection
with the mode of conveyance of the pollen by insects or otherwise, and can be
understood and explained only when these facts are taken into consideration. A few

Fig. 395. — Lonsritudinal section througli
the ^ynfficeuni of Viola tricolor ; SA' the
anatropous ovules, ,^/i channel of the style,
o its opening : in the hollow of the stigma
which is filled with the stigniatic secretion
are pollen-grains which are putting out their
pollen-tubes.

* Zuccarini, Nova Acta Ac. Leopold, XVI. pt. II. p. 665.

^ [Behrens, Untersuch. ueb. d. anat. Bau des Griffels und der Narben, Gottingen 1875.]

ANGIOSPERMS. 569

specially interesting cases will be described in Book III; it is sufficient now to
mention that the surface of the stigma forms the exit of the open channel of the
style when there is one ; if this channel is closed or entirely absent, the stigma
has the appearance of a superficiai glandular structure upon or beneath the apex
of the style or of its arms. If these arms are long and slender, and covered with
long hairs, the stigma has the form of a pencil or tuft of hairs or feathers, as in
Grasses; in Solanaceae and Cruciferae the moist surface of the stigma covers a
knob-like indented thickening at the end of the style ; in Papaver it forms a many-
rayed star on the lobed style. Sometimes the stigmatic portion of the style is
greatly swollen, as in the Asclepiadeae, where the two monocarpellary and distinct
ovaries cohere by the stigmas ; the true stigmatic surface into which the pollen-tubes
penetrate lies in this case concealed on the under side of the stigma \

The Nectaries ^. Wherever pollination is effected by insects, glandular organs
are found in the flowers which secrete odoriferous and sapid (generally sweet)
juices, or contain them within their delicate cellular tissue from which they are
easily sucked out. These juices are included under the term Nectar, the organs,
which produce them being the Nectaries. The position, form, and morphological
significance of the nectaries are very various, and always stand in immediate
relation to the special contrivances for the pollination of the flower by means of
insects. The nectaries are often nothing but glandular portions of tissue on the
foliar or axial parts of the flower ; very often they project in the form of cushions
of more delicate tissue, or take the form of stalked or sessile protuberances ; or
whole foliar structures of the perianth, of the androecium, or even of the gynaeceum,
are transformed into peculiar structures for the secretion and accumulation of
the nectar. Since it is quite impossible to treat these organs morphologically in
general terms, a few examples may serve to show the student where he will
have to look for the nectaries in difl'erent flowers. In Fritillaria imperialis the
nectaries are shallow excavations on the inner side of the perianth-leaves near their
base, large clear drops of nectar exuding from them ; in Elceagnus fusca a glan-
dular annular cushion on the gamophyllous perianth (Fig. 384 ^) ; in Rheum slight
glandular protuberances at the base of the stamens (Fig. 391 dr)\ in Nicotiana
an annular callosity at the base of the superior ovary ; in the Umbelliferae a fleshy
cushion surrounding the bases of the styles united above the i-nferior ovary (Fig.
383 hh, p. 559); in Compositse they are also at the base of the style (Fig. 393).
In Citrus, Cobcea scaiidens, Labiatae, and Ericaceae, the nectary appears as a develop-
ment of the floral axis or receptacle in the form of an annular zone beneath th^
ovary (Figs. 387 d, 390^, /), &c.; in . Cruciferae and Fagopyriim in the form of
four or six roundish or club-shaped outgrowths or warts between the filaments, &c.
An abortive stamen is converted into a nectary in the Gesneraceae; in Cucumis Melo
(the Melon) the whole androecium is replaced in the female and the gynaeceum in
the male flowers by a similar organ. As a rule the nectaries occur deep down
among the other parts of the flower ; and when they secrete nectar, it collects at the

^ On the position of the lobes of the stigma in relation to the placentae in different plants, see
Robert Brown, Misc. Bot. Works, Ray Soc. 1867, vol. I. pp. 553-563.

■•^ [Behrens, Die Nectarien der BlUthen, Flora, 1879. — Bonnier, Les nectaires, Ann. d. sci. nat,
ser. 6. t. VII.l

570

PHANEROGAMS.

bottom of the flowers, as in Nicotiana and I.abiatae. Frequently, however, special
hollow receptacles are constructed for this purpose, as is especially the case with the
bag-like appendages of the perianth-leaves (Fig. 396), usually called Spurs. In
Viola only one of the perianth-leaves forms a hollow spur, into which the ap-
pendages of two stamens are prolonged and secrete the nectar. The cup-shaped
stalked petals of Hellcborus and the slipper-shaped petals of Nigella secrete at the
bottom of their cavity the nectar which gathers there.

The Ovule (macrosporangium) of Angiosperms usually consists of a clearly
developed, sometimes even very long stalk or Funiculus (as in Opuniia and Plum-
bagineae) — which, however, is sometimes entirely wanting, as in Grasses — and one
or two integuments which enclose the nucellus. [The general rule is that the
ovules of the gamopetalous Dicotyledons have one integument, and that the ovules
of the apetalous and polypetalous Dicotyledons as also those of the Monocotyledons
have two integuments. Exceptions occur, however : thus, among gamopetalous
Dicotyledons the Primulaceae, Myrsineae, Plumbaginaceae, and Cucurbitacese have
two integuments ; and among apetalous and polypetalous Dicotyledons the following

Fig. 396.— Flowers with spurred sepals (A) and petals {B, C) ; A Kiscutella htspida, B Epimediitm grandiflorur

C AquiUiiia canadensis.

have only one integument, the Loaseae, Pittosporeae, Umbelliferae, Callilriche,
Empetrum, Hippuris, and Escallonia (Warming).] A third envelope, the Aril, is
frequently formed subsequently (as in Myristica, Euonymus, Asphodelus lulea, Aloe
subluberculata, &c. When the ovule is the terminal structure of the floral axis,
and has a short funiculus, it is orthotropous, as in Piperaceae and Polygonacese ;
the campylotropous form, i.e. where the nucellus together with its integuments
is itself curved, is comparatively rare, but occurs in Grasses, Fluviales, Caryo-
phylleae, &c. The usual form of the ovule of Angiosperms is the anatropous ;
the nucellus together with its integuments is inverted, so that the micropyle faces
the point of origin of the funiculus from the placenta (hilum) (Figs. 382,^, 383);
in this case the funiculus runs up the side of the ovule, coalesces with it, and is
termed the Raphe. The micropyle is frequently, especially in Monocotyledons,
formed by the inner integument only of the nucellus; but sometimes, especially
among Dicotyledons, the outer integument grows also above the opening of the
inner one, and the channel of the micropyle is then formed at its outer part (the
Exosiome) by the outer, at its inner part (the Endoslome) by the inner integument.

ANGIOSPERMS.

57^

When there are two or three integuments, the innermost (the Primine of Mirbel)
is generally formed first, then the outer one (the Secundine), and finally, usually at
a much later period, the Aril ; the order of development is therefore basipetal in
reference to the axis of the ovule. The transverse zone from which the single
or the two true integuments spring is termed the Chalaza (more correctly the base
of the ovule).

The integuments are usually only a few layers of cells in thickness, and have
the appearance, especially when they enclose a large nucellus, of thin membranes

Fig. 397.— /—K// stages of development of the ovule of Orchis militaris (X 55") ; I— VI seen from the side in longitudinal
section, VII from the front, the funiculus being behind, VIII a horizontal section of /; x x the axial row of cells, the upper one
of which is the archesporium, ^.ythe funiculus, ii the inner, ai the outer integument, K the nucellus^ es the micropyle, h an
intercellular space : in VII the embryo-sac e has completely replaced the tissue of the nucellus.

(Fig. 382, E). But when only one integument is developed, the nucellus usually
remains very small, while the integument becomes thick and solid, extending far
beyond the nucellus, and forming, before fertilisation, the principal mass of the ovule,
as in Hippuris (Fig. 360), Umbelliferae (Fig. 383), and Compositae (Fig. 393).

There is still much doubt about the history of development of the separate
parts of the ovule ^ ; the following may be stated as certain or at least probable.
In the formation of the erect orthotropous ovule the apex of the floral axis rises
within the ovary as a roundish or conical ovoid protuberance which forms the

[For an account of the development of the ovule see infra, p. 576.

^n

PHANEROGAMS.

nucellus; an annular wall grows up first, and finally envelopes the nucellus and
extends beyond it as an integument. If a second outer integument is formed in
addition, this arises in a similar manner, and grows up around the first (as in
Piperacese, Polygonaceae, &c.). The anatropous ovule may be at first a straight or
slightly curved projection of tissue (as in Fig. 397, /), but it immediately becomes
evidently curved at the spot where the first or the single integument springs from it
(Fig. 397, //, ///, IV^', the apical part enclosed by the integument then forms the
nucellus, while the subjacent basal part becomes the funiculus. As the integuments
arise, the curvature becomes gradually stronger, and the nucellus becomes inverted
even before the outer integument has entirely developed. This latter is therefore
not formed on the side next to the raphe, but clothes all the free part of the
ovule, right and left of the raphe (Fig. 397, F, VI, VII). Cramer was the first to
point out that anatropous ovules may originate in another way (and this is probably

Fig. 398. — Ftinkia cordata; A horizontal section of the young superior trilocular ovary, two ovules 5A' are seen in each
loculus, growing from the revolute margins of the carpels, ^ a fibro-vascular bundle surrounded by light-coloured parenchyma ;
B and C two successive states of the young ovule in longitudinal section, KK tissue of the nucellus, zV inner, ia outer integument,
e embryo-sac {A slightly, B, C very highly magnified).

the most common case), the ovule developing as a secondary lateral projection
beneath the apex of the young conical funiculus, and curving backwards subse-
quently towards the base of the latter. This inversion takes place while the single
or the inner integument is enveloping the nucellus from the summit of the funi-
culus ; the second integument, if there be one, then similarly clothes the free part
(see Fig. 398, B, C). Kohne ^ has indeed thrown some doubt on the actual lateral
origin of the nucellus, not only in Compositse, but also in Solanum, Hedera^ Fuchsia,
Begonia, &c. I have, however, had the opportunity of observing a number of
different stages of development in this respect, and not only of convincing myself
that the funiculus arises laterally with respect to the apex of the floral axis, but also
that the nucellus, when first visible, stands laterally also below the apex of the funi-
culus. It is possible that the observation of peculiarly favourable cases will remove

^ Kohne, Ueber die BlUthenentwickelung bei den Compositen. Berlin 1866, [Kohne's view is
supported by Ilaenlein's observations (Beit. z. Entwickelungsgeschichte der Compositenbluthe,
Scheiik's Mittheilungen, II. 1875).]

ANGIOSPERMS. 573

the last remaining doubt on this points Cramer has shown in a number of
other instances that all stages of the metamorphosis of ovules occur when the
flower is developed in a monstrous condition, leading also to the conclusion that
the nucellus is a lateral development on the funiculus of the ovule. Malformations
of Delphinium elatuni, where the ovules spring from the margins of the carpels, show
that the carpel is transformed into a flat open pinnate leaf, the lobes of which are
the metamorphosed ovules. The nucellus here springs from the upper or inner
side of the lobe of the leaf which represents the transformed funiculus together
with the integument. In Meliloius, Primula chinensis, and Umbelliferae, Cramer
found the same to be the case ^. Relying on this and other facts, and on the
hypothesis that the ovule is never a terminal structure of the floral axis, Cramer^
adopted the view that the ovule is either a metamorphosed leaf or part of a leaf
(a tooth or outgrowth of the upper surface). The ovule of Primulaceae and Com-
positse he considered to be a whole leaf, and he supposed that closer observation
would show the same to be the case in other flowers also, especially in those
where the flower is said to possess a solitary ' reputed terminal ovule,' as Urtica
(and Taxus), and perhaps also the Dipsacaceae and others. The nucellus would in
this case be a new formation on the surface of the ovular leaf, the funiculus would
correspond to the base of this leaf, and the integuments to its upper part, which is
folded once or twice in the form of a cup or hood round the nucellus. On the
other hand he would consider as only portions of the leaf (teeth or outgrowths of
the upper surface) all those ovules which spring singly or in numbers from the
margin or upper surface of carpellary leaves, as those of Cycadeae, Abietineae (?),
Liliaceae, Umbelliferae, Ranunculaceae, Resedaceae, Cruciferae, Leguminosae, &c. In
these cases the nucellus would be a new formation on the surface of the lobe, the
funiculus would correspond to its base, and the integuments to its upper part folded
once or twice round the nucellus in the form of a cup. Only in those few plants in
which the ovule has no integument would the naked nucellus or entire ovule corre-
spond to this lobe of the carpellary leaf. In the first edition of this book I expressed
my agreement with Cramer's view, but with a reservation with respect to Orchideae,
being especially influenced by the importance which I then attached to the morpho-
logical equivalency of the nucellus in all Phanerogams. Further reflection has,
however, deprived this reason of its importance; and I am the more induced to
ascribe diff'erent morphological significations to the ovules, according to their mode
of origin and their position, because (as has been shown by Magnus, Rohrbach,
Hanstein, and Schmitz *) in Piperaceae, Typhaceae, and Naiadeae the ovule is
actually the terminal structure of the floral axis, and in Naias this terminal ovule
is also anatropous. In these statements I not only find the confirmation of my

' Schenk writes, ' that which appears plausible enough in the Compositoe is certainly not the
case in other families ; the ovule is not a lateral branch of the primitive rudiment, but this itself
developes into the ovule.'

^ Compare also H. von Mohl, Vermischte Schriften, pi. I. figs. 27-29.

^ Cramer, Bildungsabweichungen bei einigen wichtigeren Pflanzenfamilien und die morpho-
logiscbe Bedeutung des Pflanzeneies (Zurich 1869, p. 120), where the literature of this subject has
been carefully treated.

* These researches have been already quoted; [see also Eichler, Helosideen, Bot. Zeitg. 1868,
and Stiasburger, Coniferen und Gnetaceen, 1872.]

574 PHANEROGAMS.

own observations on Chenopodiaceae and Polygonaceae, but they also warrant
the assumption that the ovules previously described by Payer as terminal are
really so. Since, however, it is not my object here to enter into a detailed proof
of theoretical matters, it will be sufficient for the present to summarise the various
phenomena.

With respect to position, the following classes may first of all be distinguished: —

A. Ovules produced on the Carpels and springing from the carpellary leaves ;
and either

1. Marginal, from the reflexed margins of the carpels (Figs. 385, 386,

387, 390) ; or,

2. Superficial, from the whole of the inner surface of the reflexed halves
of carpellary leaves, always apparently with the exception of the
mid-rib of the carpellary'leaf (Fig. 357, 382).

3. Axillary or basal, arising from the base of the upper surface of the

carpel or in the axil of the carpel {Ranuticulus, Seduui, ZanichelHa
according to Warming^).

' See Warming, Rech. sur la ramification des Phanerogames, Kopenhagen i''72, p 22. Tab. XI,
fig. I -TO. Axillary ovules are no more to be regarded as buds (caulomes) than are the axillary
sporangia of Lycopoditim.

[The question of the morphological significance of the placenta and of the ovule is one which
has been much discussed of late. With regard to the placenta, Schleiden, starting with the concep-
tion of the ovule as being a bud, considered the placenta to be necessarily an axial structure,
inasmuch as only axial structures normally bear buds (Princip'es of Scientific Botany. 1849,
pp. 382 ff.), a view which was adopted and developed more especially by French botanists (see
Payer, Organogenic). According to a second view, the placenta is a portion of the carpel itself,
usually of its margin, that is, that it is always borne by a leaf. This view has been revived of late
years by Van Tieghem (Rech. sur la structure du pistil, 1871), by Celakovsky (Ueb. Placenten und
Hemmungsbildungen der Carpelle, Sitzber. der k bohm. Ges., Prag, 1875 ; Vergl. Darstellung der
Placenten, ihid. 1876), by Braun (Bemerk. ueb. Placentenbildung, Sitzber. d bot. Ver. d. prov. Brand.
1874), and Eichler (Bliithendiagramme, II. 1878) has now accepted it. In spite of all that has been
written in support of these two theories, it cannot be admitted that either of them satisfactorily
explains every possible case : if the former must evidently be forced when it is applied to a case of
parietal placentation, this is equally the case with the latter when it is applied to free-central placen-
tation, as in the Primulacese. Both these attempts at generalisation seem to be too arbitrary. In
consequence a third view has been promulgated, more especially by Huisgen (Untersuch. ueb. die
Entwickelung der Placenten, Bonn 1873) and formerly held by Eichler (Bliithendiagramme, I. 1875),
that the nature of the placenta is not the same in all ca.es. In the Primulacese, for instance, it
belongs to the floral axis, and, according to Huisgen, this is also the case in certain instances of
axile placentation, as in the Solanaceae. Lobeliacese, Ericaceae, Malvaceae, and Hypericaceae, the
placenta in these orders being a prolongation of the stem ; it also belongs to the axis in such forms
as the Piperaceae, in so far as any placenta can be said to exist in them at all : in the Violace^ and
Leguminosse and in Monocotyledons the placenta is a development of the carpels : finally, in Ciuci-
ferae and Resedaceae, and, according to Barcianu, in the Onagraceae (Ueb, die Bliithenbildung der
Onagraceen, Schenk's Mittheilungen, II. 1875), the placenta is an independent organ, probably a
phyllome, a view which was held by Treviranus (Physiologic, II, 1838).

Now with regard to the ovule. Schleiden, Braun, and most of the older botanists regarded
the ovule as being a bud, but many regarded it as a leaf or part of a leaf (for the early history of the
subject see Braun, Polyembryonie und Keimung von Ccelehogyne, i860), a view which, as stated
above in the text, has been more recently revived by Cramer ; this view has been further developed
by Celakovsky (Ueb. die morphol. Bedeutung der Samenknospen, Flora, 1874; Zur Discussion
ueber das Eichen, Bot. Zeitg. 1875, and Vergriinungsgeschichte der Eichen von Alliaria officinalis,
ibid. 1875, und von Trifolinm repens, ibid. 1877), and so far modified that, according to him, the

ANGIOSPERMS. ^"jr^

B. Ovules produced on the Axis and springing from the prolongation of the
floral axis within the ovary, the carpels themselves being sterile; these may
be either —

4. Lateral, when they stand beside or below the apex of the floral axis,

which either rises as a columella and bears a number of ovules (as
in Fig. 392), or is arrested in its development, so that the single
ovule formed appears terminal (as in Fig. 393); or,

5. Terminal, when the apex of the floral axis itself becomes the nucellus

(as in Fig. 391, and in Piperacese, Naias, Typha, &c.).

To which of these classes the ovules belong in any given plant must be decided
in each separate case ; the position on the margin of the carpels is by far the most
common among Angiosperms, both the superficial and the axial position belonging
only to single families or genera. If these facts are compared with what occurs in
Gymnosperms, the ovules of Cycadeae must be classed with the marginal carpellary,
those of many Cupressineae with the superficial description ; while those of Taxus
are axial and terminal, and those of Salisburia lateral.

When the position of the ovules is given, so also is some information as to their
morphological significance : the terminal ovules may be regarded as the terminal
portion of the axis, the lateral as equivalents of whole leaves, the marginal as branches
of leaves (lacinise, pinnae, or lobes) ; the superficial ovules may be included in the
category of such foliar outgrowths as we have already found to occur in the form of

ovule is a metamorphosed segment of a carpellary leaf, a definition which Eichler has now accepted
(Bliithendiagramme, 11). A third view, the one stated in the text, is now held by many, that, as in
the case of the placenta, the morphological value or 'dignity' of the ovule is not always the same.

Of these views, the one which aj)pears to be the most true to nature is the one which allows
the greatest latitude : but it is not always possible to refer an ovule to one of the categories,
caulome and phyllome, for its position does not necesmrily indicate its morphological significance.
Thus, a lateral ovule, as in Composite and Primulacese, might be either a leaf or a bud ; its probable
leaf-nature in these cases depends entirely on teratological evidence, which is of very doubtful value,
for an organ in a monstrous condition does not necessarily assume its primitive archetypical form.
Again, an organ borne by a leaf does not necessarily represent some typical part of the leaf, witness
the adventitious buds which are developed on leaves in many cases, and the ovules which cover the
surface of the carpels in Nuphar luteum and Brasenia peltata (Strasburger, Angiospeimen und Gymno-
spermen, p. 57).

The difficulties met with in endeavouring to regard the ovule as a caulome or a phyllome may
be got over by regarding it as an 'emergence' (Strasburger, loc. cit.), borne sometimes on an axial,
sometimes on a foliar member. This view, to be completely satisfactory, ought to be applicable
also to pollen-sacs and to the sporangia of the Vascular Cryptogams. This cannot be quite accu-
rately done, for an emergence (p. 162 ante) is described as being developed not only from the
epidermal layer but also from the subjacent cells of the organ bearing it ; and we know that the
sporangia of the Vascular Cryptogams (except those of Isoetes) and the pollen-sacs of Pinus are
derived from one or more epidermal cells. Goebel (Bot. Zeitg. 1881) has expressed the opinion that
a sporangium (ovule, pollen-sac) is simply a sporangium, an organ srn generis as much as a stem or
a leaf.

A few words may be added here with reference to the morphological significance of the integu-
ments of the ovule. The view has been often expressed that the integuments of the ovule are
homologous with the indusium of Ferns; this is opposed by Strasburger (Angiospermen und
Gymnospermen) and by Goebel (Bot. Zeitg. 1881) on the ground that the integuments arise from
the ovule itself, whereas the indusium is an outgrowth of the leaf bearing the sporangium. These
organs are analogous, but not homologous.]

[)!'

PHANEROGAMS.

sporangia in Lycopodiuin. These explanations are so far confirmed by the occurrence
of malformations, that the lateral axial and the marginal carpellary ovules are often
enough transformed into foliar structures of ordinary form, while this appears never
to occur with terminal or superficial ovules.

These remarks have at present been confined to the ovule as a whole, although
reference has already been made to the theory of Cramer on the various morpho- •
logical relationships of the nucellus and of the other parts, the funiculus and the
integuments. Malformations \ which in this respect are even more instructive than
the normal development, led Cramer to the conclusion that when the ovule appears
to be the equivalent of a lateral branch or of the whole of a leaf, the funiculus and
the integuments together correspond to the foliar structure in each case ; the nucellus
arises from it as a lateral outgrowth, while the integuments correspond to the hood-
shaped lamina of the leaf, growing over the nucellus.

The ovules are sometimes rudimentary ; those of Balanophoreae and Santa-
laceae have no integument ; the nucellus is naked, and in some species is itself
composed of only a few cells. In Loranthaceae the development does not even
proceed so far as the formation of a distinctly differentiated ovule ; the growth of
the apex of the floral axis ceases so soon as the carpels begin to be formed ; and the
cohesion of these is such that it is scarcely possible to speak of a cavity of the ovary;
the formation of the embryo-sac in the axial part of the tissue of the inferior ovary is
the only indication that this spot corresponds to the ovule ; and since more than one
embryo-sac is formed, it still remains doubtful whether this mass of tissue must be
regarded as the equivalent of one or of several ovules^

The Develop7nent of the Ovule and of the Embryo-sac^ . [The first indication of
the development of the ovule is the division by a wall parallel to the surface
(periclinal) of one or more cells lying immediately beneath the epidermis of the
placenta^ ; in the case of ovules which have a simple structure when mature, as those

^ [See Masters, Vegetable Teratology, 1869; and Peyiitsch, Zur Teratologic der Ovula, Fest-
schrift d. k. k. Zool.-Bot. Ges. Wien, 1876.]

^ Hofmeister, Neiie Beitrage, I (Abhandl. der kon. sachs. Gesellsch. der Wissensch. VI). [The
remarkable position of the ovule in Hydnora iProtopanche) americana, immersed in the placental
tissue, is comparable with that of the sporangium in Isoetes (Fig. 334). It is not possible to
say at present if the embryo-sac of Hydnora belongs, like that of the Loranthacese, to an ovule
which arises as a prominence on the placenta and is subsequently overgrown by it, or if it is
developed from a cell of the placental tissue, the ovule not becoming differentiated from the placenta.
On Hydnora see De Bary, Abhandl. der naturf. Gesellsch. zu Halle, vol. X ; Hooker, Journ. Linn.
Soc. vol. XIV. p. 182.]

2 Hofmeister, Neue Beitrage zur Kenntniss der Embryobildung der Phanerogamen, Abhandl. d.
sachs. Ges. d, Wiss, VI, VII, 1859. — [Strasburger, Die Coniferen und Gnetaceen, p. 409; id. Ueb.
Befruchtung und Zelltheilung ; id. Ueb. Zellbildung und Zelltheilung, 3rd ed. ; id. Die Angiospermen
und die Gymnospermen. — Warming, De I'ovule, Ann, d. vSci. Nat. 1878. — Vesque, Dev. du sac
embryonnaire des Phanerogames, Ann. d. Sci. Nat. ser. 6, VI, 1878; id. Neue Untersuchungen, Bot.
Zeitg. 1879. — Treub et Mellink, Notice sur le dev. du sac embryonnaire, Arch. Neerlandaises, XV. —
Fischer, Zur Kenntniss der Embryosacentwickelung, Jenaisch. Zeitschr. XIV, 1880. — Marshall
Ward, Embryo-sac of Gymnadenia conopsea. Quart. Journ. Micr. Sci. XX, 1880; id. Journ. Linn.
Soc. XVII, 1880.]

* [In the fourth German edition, Prof. Sachs, relying upon the observations of Hofmeister,
regards the ovules of Orchidese as being trichomes, inasmuch as they are stated by Hofmeister to be
developed from single epidermal cells of the placenta. Hofmeister's observations have been shown
to be erroneous by Strasburger (Coniferen und Gnetaceen) and by Warming (Joe. cit.). The ovules

ANGIOSPERMS. 577

of Orchids, only a single cell is thus divided, in other cases several. By repeated
divisions parallel to the first the cells are multiplied, and a protuberance is formed
consisting in Orchids (Fig. 397) of a single row of cells, in other cases of a number
of rows, invested by the epidermis ; this protuberance is the nucellus.

. The general course of the development of the embryo-sac is as follows : the
terminal cell of the row in simple ovules (Orchids, Monotropa), the terminal cell of
the axial row in more complex ovules, a hypodermal cell therefore \ becomes distin-
guished by its size and by the granularity of its protoplasm. This cell elongates with
the growth of the nucellus, and a segment is cut off from it towards its upper
(micropylar) end by a transverse wall, and this may be followed by the cutting off of
a second segment in a similar manner ; only one such segment is cut off in Tritonia
aurea, Anther icum ramosum, Triglochin palustre, Luzula pilosa, Tradescaniia vi'rgim'ca,
Chenopodium fceiidum, Helianthemum Rhodax, etc., two in many Rosaceae : when
only one segment is cut off, it usually divides into two by a wall parallel to the long
axis of the nucellus (anticlinal), and whether one or two segments have been primarily
cut off, they may undergo division by transverse walls. The large remaining cell now
usually is divided by a transverse wall into two of nearly equal size, and one or both
of these may be divided in a similar manner. The walls which are formed in connexion
with these divisions are remarkable for their thickness and their glistening appearance.
The result of these divisions is the formation of a row of three or four cells lying
in the long axis of the nucellus ; it is usually the lowest cell of this axial row which
enlarges and becomes the embryo-sac, causing by its growth the absorption of
the others.

Before going into further detail it will be well to become acquainted with the
terminology which is to be used in describing these phenomena, and this may be best
done by comparing them with those which accompany the development of the
sporangia in the Vascular Cryptogams. It has been already pointed out that the ovule
corresponds to a sporangium, and we see, from the facts stated in the preceding
paragraph, that in its first development it resembles the sporangia of Isoetes in that it
is derived not from the epidermis only but also from subjacent cells, and that, as in
the sporangia of the majority of Vascular Cryptogams, there appears within it at an
early stage a hypodermal cell which is readily distinguishable from the cells surround-
ing it : to this cell the term archesporium may be applied as well here as in speaking
of the Vascular Cryptogams, Similarly we may call the cell or cells which are cut
off from the archesporium toward its micropylar end tapetal cells, the tapetum being
completed in these plants by cells of the nucellus, a condition which recalls that in
Selaginella. The further divisions of the archesporium are comparable to those which
take place in the sporangia of the Vascular Cryptogams and which result in the forma-
tion of the mother-cells of the spores : the axial row of cells is then a row of spore-
mother-cells, and, inasmuch as one of these developes into the embryo-sac, the

of Orchids are developed in the manner described above in the text. Warming also states that in
many cases the first cell-divisions make their appearance in the layer next but one to the epidermis
(Ribes, Viola, Ficaria, Geum, Lamium, Symphyttim, Verbascum), or even in a deeper layer {Malva,
Pisum).']

^ [In Carex prcecox, according to Fischer, the archesporium is derived from a more deeply
placed cell of the nucellus.]

pp

578 PHANEROGAMS.

embryo-sac in the ovule of an Angiosperm is equivalent to one of the spore-mother-
cells in the sporangium of a Vascular Cryptogam.

Among the more important deviations from the above described mode of the
development of the embryo -sac, the following may be mentioned. In Tulipa Ges-
neriana and in Lilhmi bulbi/erum, according to Treub and Mellink, the archesporium
undergoes no division, but simply enlarges and becomes the embryo-sac. In a
number of cases described by Strasburger (Angiospermen und Gymnospermen) and
Fischer {Myosurus minimus^ Senecio vulgaris^ Lamium maculatam. Delphinium, tridac-
tylon and villosum, Sisyrinchium iridifoliuvi, Orchis pallens, Gymnadenia conopsea,
Monotropa Hypopitys, and several Grasses) no tapetal cells are formed. In Alisma
Plan/ago, Allium Jislulosuni^ Chenopodium foetidum, and Sabulina longi/olia, the arche-
sporium only divides once, that is, it produces a row of only two cells, the lower of
which becomes the embryo-sac.

It has been not unfrequently observed that the terminal cell of more than one
of the rows of cells of which the nucellus primarily consists assumes the characters
of an archesporial cell : in such cases the archesporium is multicellular, a condition
which is the normal one in Isoetes among Vascular Cryptogams. This appears to
be commonly the case among the Rosacese. In Fragaria vesca Strasburger found
(and his observations have been confirmed by those of Fischer on Cydonia japonica,
Geum slrictum, Sanguisorba pratejisis^ Rubus ccBsius, and Agrimonia Eupatorid)
several archesporial cells forming a hypodermal layer or row. Each of these
cells behaves in the manner already described: one or two tapetal cells are cut
off, and then the cell undergoes division so as to form a row consisting of three
or four cells. The lowest cell of each of these rows now begins to develope into
an embryo sac, but the cell of the axial row developes more rapidly than the
others, causing their absorption; it is this cell which constitutes the embryo-sac.
In Rosa livida Strasburger has observed that the uppermost cells of the rows,
which usually consist of four cells, but sometimes of five or even six, develope and
enlarge, and the second cell of the row often does the same. In the process of
development some of these commencing embryo-sacs become absorbed, as do also
the tapetal cells and the layers of cells which have been formed at the apex of the
nucellus by the repeated division of the epidermal layer; one of them generally
extends to the integument and becomes the largest embryo-sac, but the mature
ovule contains several embryo-sacs. Tulasne has pointed out ^ that in the Cruciferac
{Cheiranthus Cheiri) several embryo-sacs are developed, but only one of them is
persistent. These phenomena recall the fact that in Taxus, Ginkgo, Thuja and
Gnetum, among Gymnosperms, several embryo-sacs are at first formed.] The
multiplicity of embryo-sacs in the ovary of Viscum cannot be included under this
head, for the absence of the differentiation of the ovule makes it uncertain whether
the mass of tissue in the ovary, to which we have already alluded, is to be regarded
as the equivalent of one or of several ovules.

[In a great number of cases the epidermis of the nucellus remains a single layer
of cells, but not unfrequently two layers are formed at the apex of the nucellus by
the periclinal division of the primary epidermis (dermatogen). In some cases several

[Etudes d'embryogenie vegetale, Ann. d. Sci. Nat. XII, 1849.]

ANGIOSPERMS,

579

layers of cells are thus formed constituting a sort of cap at the apex of the nucellus :
this is indicated in Hippuris, and it is well seen in Delphinium, Helianthemum, and in
the Rosaceae ; to the last case reference has been made in the preceding paragraph.
In Geum urbanum, Iris Pseudacorus, and Agrostemma the whole free surface of the
nucellus is covered by a number of layers of cells which have been formed in this
manner (Warming).

The integuments are developed as outgrowths of the nucellus. In those cases
in which the ovule has only a single integument, the greater part of it is developed
from the epidermis of the nucellus. When the ovule has two integuments, the
inner oiiie is developed principally from the epidermis, the outer principally from
the subjacent cells ^ Cell-division and growth take place iff a zone encircling the
nucellus, and thus the integument grows up, at first as a ring, but later as a con-
tinuous membrane investing it. Accordingly as this zone is narrow or broad, the
integument will consist (in thickness) of one or more layers of cells. As a general
rule the inner integument in those cases in which two are present is developed
first ; exceptions to it appear to occur, according to Warming, in Euphorbia^ Cuphea,
Mahernia glabrata.

In some cases three integuments appear to be present {Asphodelus luteus and
creticuSy Reseda luted), but it seems probable that the external one is to be regarded
as an arillus. On the other hand it appears, in some cases in which two integuments
are developed, as if only one were present {Viola, Ficus, Convallaria, Orchis, Tro-
pcEolum, Delphinium)', this is due to the very intimate connexion of the two in-
teguments.]

The further behaviour of the embryo-sac of Angiosperms differs in many ways
from that of Gymnosperms. In Gymnosperms it remains surrounded by a thick
layer of the tissue of the nucellus till after fertilisation has taken place ; it is com-
paratively small, and is surmounted by a strongly developed nuclear protuberance.
In Angiosperms, on the other hand, the embryo-sac has grown considerably even
before fertilisation ; it usually supplants the surrounding tissue of the nucellus so far
that it remains enveloped by only a thin layer of it, or is even in actual contact with
the inner surface of the inner integument, as in Orchideae (Fig. 397, VII). In such
cases the tissue of the apex of the ovule often still remains entire (as in Aroideae),
but frequently the apex of the embryo-sac bursts through it, and projects into the
micropyle (as in Crocus and Labiatae), or even grows out beyond it as a long tube
{e.g. Santaluni). The middle and lower part of the sac also frequently extends con-
siderably; in many gamopetalous Dicotyledons it puts out vermiform appendages
which penetrate into and destroy the tissue of the integument, as in Rhinanthus,
Lathrcea, and some Labiatae. While this process of growth is proceeding, the proto-
plasm which at first fills up the whole sac becomes full of vacuoles ; a large sap-
cavity arises surrounded by a parietal mass of protoplasm, which accumulates
especially in the apical prominence and at the bottom of the embryo-sac, while

^ [According to Warming the integuments are developed in some cases from the dermatogen
alone ; as in Orchis, Primula chinensis, Centradenia floribunda, Lysimachia verticillata, Linnea perenne
(Strasburger), and Begonia heracleifolia, among plants which have two integuments, and in Peperomia
and Monotropa among those which have only one integument. This mode of development of the
integuments is comparatively rare.]

P p 2

580 PHANEROGAMS.

threads, in which currents are visible, radiate to the walls from the protoplasm
which envelopes the nucleus.

[During the growth of the embryo-sac its nucleus divides, and the two new
nuclei travel to the opposite ends of the sac, a large central vacuole being formed.
Each of these nuclei divides into two, and each of these again into two, so that there
are four nuclei at each end of the embryo-sac. One nucleus from each end now
travels towards the centre of the sac, where they meet and coalesce to form the
definitive nucleus of the embryo-sac; these two nuclei may be termed the polar
nuclei. Round the three nuclei at the two ends of the sac a process of free cell-
formation now takes place, so that there are three cells at each end ofvthe sac.
The cells at its lower (chalazal) end soon become surrounded with cell-walls, and
constitute the antipodal cells : the cells at its upper (micropylar) end remain naked,
and constitute the egg-apparatus. Two of the cells of the egg-apparatus lie nearer
the apex of the sac than the third ; they are somewhat elongated superiorly, and
the nucleus lies in this elongated portion, the rounded inferior portion containing
a large vacuole : these cells have been termed by Strasburger the Synergidcs. In
many cases the elongated superior ends of these cells presents a longitudinal
striation (as in Gladiolus, Crocus, Santalum, Zea, Polygonum, etc.), first observed by
Schacht*, and termed by him the Filiform Apparatus. The third cell, which lies
at rather a lower level than the other two, is the oosphere : it is more or less rounded
in form, and its nucleus lies towards its lower end. All three are usually attached to
the wall of the embryo-sac. It is important to note that the nuclei of the synergidae
are sister-nuclei; the nucleus of the oosphere is the sister-nucleus of the polar
nucleus which coalesces with the polar nucleus from the lower end to form the
definitive nucleus of the embryo-sac.

The following are some of the principal deviations from the series of pheno-
mena which have been described above. Sometimes, as in Ornithogalum nutans,
only one of the synergidae is present. It may be that this is due to the omission of
one of the divisions of the nuclei at the apex of the embryo-sac ; but, inasmuch
as two are generally to be found in preparations of early stages and only one in
preparations of later stages of development, it is possible that two are primarily
formed but that one soon undergoes absorption. In Sinningia, according to Stras-
burger, only one synergida is present in some cases, and very rarely both are absent.
In exceptional instances two oopheres have been found in the embryo-sac, in
Sinningia by Strasburger, and in Gomphrena by Fischer. This peculiar abnormality
appears to be the rule in Santalum album. Strasburger explains it by supposing
either that one of the three nuclei of the egg-apparatus has undergone division,
so that four cells are formed instead of three, or that the upper polar nucleus
becomes the nucleus of the second oosphere; in the latter case, the definitive
nucleus of the embryo-sac would be constituted by the lower polar nucleus alone.
The antipodal cells are very imperfectly diff"erentiated in the Orchids, and do not
become clothed with cell-walls : it is this fact, doubtless, which caused Flofmeister to
assert that the antipodal cells are frequently wanting in Orchids. In some cases,

^ Jahrb. fiir wiss. Bot. I and IV, [Schacht states that in Santalum album the two synergidae are
separated by a septum which is formed in the apical portion of the embryo-sac ; this observation has
been confirmed by Strasburger (Zelltheilung und Befruchtung).]

ANGIOSPERMS.

;8i

such as Allionia and Delphinium^ they are very large and well-developed. In the
Gramineae, Fischer has found that they divide and give rise to a considerable mass
of cells, a fact which Strasburger has ascertained with regard to Ornitkogalum,
though here it is exceptional. In some instances the two polar nuclei meet, not
in the centre, but towards the upper end of the embryo-sac; in this case the
upper nucleus is stationary, and the lower nucleus has to travel nearly the whole
length of the embryo-sac. This occurs, according to Fischer, in Elodea and in
many Gramineae, and apparently also in Allium fisiulosum. The coalescence of the
two polar nuclei usually takes place before fertilisation, but in Alisma and in Allium
fisiulosum it does not take place until the pollen-tube has reached the embryo-sac
or even until fertilisation has been actually effected.

As regards the fate and the function of these various cells which are formed in
the embryo-sac, the oosphere is the one which undergoes fertilisation and developes
into the embryo, the others being transitory structures. The synergidae appear to

Fig. j,^.~FuHkta cordata; A apex of the embryo-sac e, covered with a layer of cells belonging to the nucellus KK,x owe.
of the synergidae, 6 the ciosphere with its nucleus ; B, C oospore before, A £ after the first division ; I^ the spherical suspensor
with the two-celled rudimentary embryo (X Soo).

cause the disintegration and in some cases at least the absorption of that part of the
wall of the embryo-sac with which they are in contact, and besides this, they have
a further function in the process of fertilisation to which reference is made below.
In. some cases {Crocus vernus, Torenia asialica, Sanlalum album) their pointed ends
become covered with a cap of a homogeneous substance which gives the reactions
of cellulose : by this means they replace that part of the wall of the embryo-sac
of which they have caused the absorption. After fertilisation the synergidae undergo
absorption. In many cases the antipodal cells soon undergo absorption, but in
some they persist, and may be seen at the base of the endosperm in the fertilised
ovule.

We will now proceed to discuss these phenomena from a morphological point
of view. We have seen above that the cell which developes into the embryo-sac is
the equivalent of one of the mother-cells of the spores in the sporangium of a Vas-
cular Cryptogam, and to this we may add that it is equivalent to one of the mother-
cells of the pollen in the pollen-sac of the stamen. With this as the basis of their

5^2 PHANEROGAMS.

reasoning Warming and Vesque consider that the processes of cell-formation which
go on in the embryo-sac correspond to the formation of spores or pollen-grains
from their mother-cell, the four nuclei at each end of the embryo-sac representing
a tetrad of spores. In order that this view may be perfectly consistent it is obviously
necessary to assume that the embryo-sac is formed by the fusion of two cells equiva-
lent to spore-mother-cells, inasmuch as two tetrads of nuclei are formed within it,
and further, that the cells which are formed round these nuclei are each of them
equivalent to a spore.

The view which is more generally held, and which is due to Strasburger, is that
the cell which forms the embryo-sac is a spore-mother-cell which does not undergo
any division into special spore-mother-cells, but which developes without dividing
into a single spore, the embryo-sac. The processes of cell-formation which go on
in the embryo-sac are not therefore to be compared to those which accompany the
formation of spores or of pollen-grains from their mother-cell, but they are to be
compared to those which accompany the germination of a spore or of a pollen-
grain. The six cells which are formed in the embryo-sac, three at the chalazal and
three at the micropylar end, represent a rudimentary prothallium; they are com-
parable to the endosperm of Gymnosperms and to the prothallium in the macrospore
of the heterosporous Vascular Cryptogams \ These cells, which we may term
primary endosperm-cells ^ are, as we have seen, subsequently differentiated into the
antipodal cells and the egg-apparatus; thus three of them are purely vegetative,
whilst the other three are concerned in the sexual reproduction of the plant, one
of them becoming the oosphere and the other two the synergidae. In the Vascular
Cryptogams we saw that the archegonium was developed from a single superficial
cell of the prothallium ; in the Gymnosperms we saw that the archegonium was
developed from a single superficial cell of the endosperm, that it was a less com-
plex organ than in the Vascular Cryptogams, and that it had undergone a reduction,
a condition which we found most evident in Welwitschia in which the mature arche-
gonium consists of only a single cell ; in the Angiosperms this reduction is carried
still further, the archegonium being represented only by the oosphere, which is one
of the primary endosperm-cells. It was thought at one time that the Filiform
Apparatus of Schacht represented the canal-cell of the archegonium which we have
found to be present in the higher Cryptogams and in most Gymnosperms, but this
cannot be the case inasmuch as this apparatus is, as stated above, simply the striated
ends of the synergidae ; still less can the synergidae be canal-cells, for they are the
product of a nuclear division and cell-formation in which the oosphere is not
directly concerned; moreover their function is entirely peculiar. The completion
of the prothallium takes place when what we may term the secondary endosperm
is formed ; this is what is commonly termed the endosperm, and its formation in
the embryo-sac does not commence until the fertilisation of the oosphere has been
effected.]

Fertilisation'^. The pollen-grains which germinate on the stigma send out

^ [Allusion has already been made (on p. 486) to Goebel's view that the antipodal cells of
Angiosperms correspond to the ' endosperm ' of Selaginella.']

^ Besides the works of Hofmeister already quoted, see his historical account in Flora, 1857,
p. 125, where the literature is collected. [Also Strasburger, Ueb. Befruchtung und Zelltheilung. —

ANGIOSPERMS. 583

their tubes through the channel of the style where there is one, or more usually
through the loose conducting tissue in its interior, down to the cavity of the ovary.
Frequently both in erect basilar (Fig. 391) and in pendulous anatropous ovules the
micropyle lies so close to the base of the style that the descending pollen-tube
can enter it at once : but more often the pollen-tubes have to undergo further
growth after their entrance into the cavity of the ovary before they reach the micro-
pyles of the ovules; and they are then guided in the right direction by various
contrivances. We frequently find papillose projections of the placentae or other
parts of the wall of the ovary, to which the pollen-tubes attach themselves; in our
species of Euphorbia a tuft of hairs conducts them from the base of the style to
the neighbouring micropyle ; in the Plumbagineae, the conducting tissue of the style
forms a conical descending outgrowth, which conducts the pollen-tube into the
micropyle ; and so forth. [The conducting tissue is also secretory, and it appears
that, as Amici originally suggested (and this view has been recently confirmed by
Dalmer), the pollen-tube obtains the materials necessary for its growth from the
secretion.]

[We have already seen that two cells (at least) are formed in the pollen-grain of
Angiosperms, and that they are sometimes separated by a cellulose wall, but more
commonly by an ectoplasmic layer {hautschichi) of protoplasm. The pollen-tube is
formed from the larger of these two cells. It sometimes happens that the smaller
(vegetative) cell of the pollen-grain is unaffected by the formation of the tube, but
more commonly the layer separating the two cells is absorbed, and the two nuclei
travel, together with protoplasm, into the growing tube, the nucleus of the larger
cell frequently going first.]

Since every ovule requires one pollen tube for its fertilisation, the number of
tubes which enter the ovary depends, speaking generally, on the number of the ovules
contained in it ; the number of pollen-tubes is however usually larger than that of the
ovules; where these latter are very numerous, the number of pollen-tubes is therefore
also very large, as in Orchideae, where they may be detected in the ovary even by the
naked eye as a shining white silky bundle.

The time that intervenes between pollination and the entrance of the pollen-
tube into the micropyle depends not only on the length of the style, which is often
very considerable (as in Zea and Crocus)^ but also on the specific characters of the
plants. Thus, according to Hofmeister, while the pollen-tubes of Crocus vernus
only require from twenty-four to seventy -two hours to penetrate the style which is
from 5 to 10 cm. in length, those of Arum maculatum take at least five days, although
the distance they have to go over is scarcely more than 2 or 3 mm., and those of
Orchideae require ten days or even several weeks or months, during which time the
ovules first become developed in the ovary, or even are not formed till then.

The pollen-tube is usually very slender and thin-walled as long as it is increasing
quickly in length ; after entering the micropyle its wall generally thickens rapidly and
often considerably, chiefly, as would seem, by swelling, so that its apical portion
communicates with the rest of it by only a narrow channel, or is entirely cut off.
Hofmeister compares it, in this condition, to a thermometer-tube (as e.g. in Lilium^

Elfving, Jenaische Zeitschrift, 1879, and Quart. Journ. Micr. Sci. XX, 1880. — Dalmer, Jen. Zeilsch.
1880.— Capus, Anat. du tissu conducteur, Ann. d. Sci. Nat. ser. 6. t. VII,]

584 PHA NEROGA MS,

Cactus, and Malvd) ; while sometimes the cavity of the tube becomes wider (as in
Oenothera and Cucurbitacese). Its contents consist of granular protoplasm, usually
mixed with a number of starch-grains (the Fovilla).

Within the micropyle the pollen-tube either comes immediately into contact with
the naked apex of the embryo-sac, or, as in Watsonia and Sanialum, with the pro-
jecting striated ends of the synergidae; but very commonly a portion of the tissue of
the apex of the nucellus still remains through which it has to make its way to the
embryo-sac. The wall of the embryo-sac is often weak at the apex, and is frequently
inflexed by the advancing end of the pollen-tube, or even perforated.

[Strasburger has carefully studied^ the process of fertilisation in Torenia astatica,
Gloxinia hybrida, various Orchids, Monotropa, and Pyrola. He finds that when the
pollen-tube first (jomes into contact with the synergidae or with the embryo-sac, one
or both of the nuclei can be distinguished in it near its apex. These very soon
disappear, and its contents present a highly refractive, finely granular appearance.
The appearance of one or both of the synergidae now begins to change ; its nucleus
and its vacuole disappear, and its protoplasm becomes uniformly and highly granular,
closely resembling in appearance the contents of the pollen-tube as described
above; it loses its form, becoming irregular, and it is closely attached to the oosphere;
portions of it may break off and fasten on to the oosphere here and there. The
oosphere now becomes granular, and two nuclei can be detected in it; one of these is
the nucleus of the oosphere (the female pronucleus), the substance of the other (male
pronucleus) has doubtless been derived, through the synergidae, from the pollen-tube.
These two nuclei meet and coalesce, constituting the nucleus of the oospore; this
nucleus is often seen to contain two nucleoli, the nucleoli of the male and female
pronuclei, which coalesce somewhat later : the oosphere now surrounds itself with a
cellulose wall, and with this its conversion into the oospore is complete.]

The further results of this process can usually be observed after a short time in
the behaviour of the nucleus of the embryo-sac and of that of the oospore. It
frequently however occurs that a considerable time elapses after the entrance of the
pollen-tube before the commencement of the development which is induced by it ;
several days or even weeks in many woody plants, as Ulmus, Quercus, Fagus,Juglans,
Citrus, JEsculus, Acer, Cornus, Robinia, &c. ; almost a year in the American Oaks, the
seeds of which take two years to ripen ; in Colchicum autumnale the pollen-tube enters
the embryo-sac at the latest at the beginning of November, but it is not till May in the
next year that the formation of the embryo begins. (Hofmeister.)

Even the advance of the pollen-tube through the conducting tissue of the style
and into the cavity of the ovary often causes extensive changes in the flower ; if the
perianth is delicate it usually loses at this time its freshness, fades, and afterwards
entirely falls off ; among Liliaceae it is common for the ovary to commence growing
actively even before the fertilisation of the ovules (Hofmeister); in Orchideae not
only is the active growth of the ovary, which often lasts for a considerable time,
occasioned by pollination, but the ovules themselves are by it rendered capable of
fertilisation ; in some cases even their production is thus induced from the placenta
which would otherwise remain sterile. (Hildebrand : see also Book III on the
Sexual Process.)

^ [Befruchtung und Zelltheilung, 1878, p. 52.]

ANGIOSPERMS.

585

Results of Fertilisation in the Embryo-sac ; Formation of the Endosperm ^ and
Embryo. [The first result of fertilisation seen in the embryo-sac is the division of its
nucleus. The action of the pollen-tube on the oosphere is apparent about the
same time : the oospore increases in size, elongating in the direction of the long axis
of the nucellus. The formation of the endosperm very commonly begins before the
division of the oospore, at the latest during the formation of the suspensor. The two
nuclei which are formed by the division of the nucleus of the embryo-sac divide again,
and this process is repeated until a number of nuclei are formed, lying in the proto-
plasm which lines the wall of the embryo-sac, the number of nuclei becoming greater
as the embryo-sac increases in size. When the embryo-sac has ceased to grow the
protoplasm becomes aggregated around the nuclei, and the area of protoplasm sur-
rounding each nucleus is marked out by an ectoplasmic layer, and in this layer a
cellulose membrane is formed which is attached externally to the wall of the embryo-
sac, and is free at its inner margin ; thus the cells are separated from each other.
The cells now increase in size, become vacuolated, and a cellulose wall is formed
on their internal surfaces. These processes of cell-formation begin usually at the
chalazal end of the embryo-sac and gradually extend to the micropylar.

Fig. 400. — Viola tricolor; A longitudinal section through the anatropous ovule after fertilisation, // the placenta, w cushion
on the raphe, a outer, i inner integument, / the pollen-tube which has entered the micropyle, e the embryo-sac containing the
embryo (to the left) and a number of young endosperm cells ; B and C the apices of two embryo-sacs e with the embryo eb
attached to it ; the suspensor in B is two-celled.

In many cases the development of endosperm does not proceed beyond this
point, for the growing oospore comes into contact with the parietal layer of cells and
prevents any further cell-multiplication : in other cases the cells of the parietal layer
multiply by division, and thus fill up the embryo-sac] If the sac increases greatly in
size, as, for instance, in Ricinus and in the large-seeded Papilionaceae, the filling up
with endosperm does not take place till later, and the centre of the sac is filled in the
unripe seed with a clear vacuole-fluid. In the embryo-sac of the Cocoa-nut, which
grows to an enormous size, this fluid— the cocoa-nut-milk— remains until the seed is
fully ripe, the tissue of the endosperm forming a layer only some millimetres in
thickness, which lines the inside of the testa. The very narrow elongated embryo-
sacs of plants with small seeds, as Pistia and Arum, are filled up by a single
longitudinal row of endosperm cells. In a large number of dicotyledonous plants (as
Loranthacege, Orobancheae, Labiatae, Campanulaceae, «&c.), with long narrow tubular

^ [Hofmeister, Neue Beitrage, Abhand. d. k. sachs. Ges. d. Wiss. VI. — Strasburger, Befruchtung
und Zelltheilung ; id. Zellbildung und Zelllheilung, 3rd ed. ; id. Angiospermen und Gymnospermen.
— Hegelmaier, Vergl. Unters. ueb. Entwick, dikotyledoner Keime, 1878; id. Zur Embryogenie und
Endospermentwickelung von Lupinus, Bot. Zeitg. 1880. — Darapsky, Der Embryosackkern und das
Endosperm, Bot. Zeitg. 187^.]

586 PHANEROGAMS.

embryo-sacs, the space of the embryo-sac is first of all divided by two transverse
septa, further transverse divisions succeeding in all or some of the cells thus formed,
followed by longitudinal ones ; the tissue of the endosperm is thus formed, and in this
case often fills up only certain parts of the embryo-sac ; or the sac is divided by a
septum into two daughter-cells, the upper of which contains the rudimentary embryo,
and produces endosperm in small quantities by free cell-formation {e.g. Nymphcea^
Nuphar, Ceraiophyllum, Anthurium'^). In a few families only the formation of endo-
sperm is rudimentary, and limited to the temporary appearance of a few free cells or
nuclei, as in TropcBolum, Trapa, Naiadeae, Alismaceae, Potamogetoneae, Orchideae ;
in Canna even this rudimentary production of endosperm appears to be suppressed.

During the first formation of the endosperm, the embryo-sac usually increases in
size, and thus displaces the tissue of the nucellus which still to a certain extent
surrounds it ; only in a few cases is the nucellus still partially or entirely preserved ;
it becomes filled with food-materials, like the endosperm, and replaces this latter
as a reservoir of reserve-materials for the embryo. In most of the Scitamineae
{e.g. Canna), this tissue, the Perisperm, is very strongly developed, while the endosperm
is altogether wanting ; in the Piperaceae and in many of the Nymphaeaceae there is a
small endosperm in the ripe seed, lying in a hollow of the much larger perisperm.

Fig. ^01.— Viola tricolor, posteriot part of the embryo-sac, e its cell-wall, 5 the cavity of the cell, A", A'young
endosperm-cells which have been produced in the protoplasm /r.

While the endosperm surrounded by the embryo-sac increases in size, the Testa
is formed from the development of the integuments which accompanies that of the
endosperm; but in Crinum capense and some other Amaryllideae the growing endo-
sperm is stated by Hofmeister to burst the testa and even the wall of the ovary ; its
cells produce chlorophyll, and the tissue remains succulent and forms intercellular
spaces (which does not occur in other cases). In Ricinus a similar growth takes
place when the ripe seed germinates in moist earth, bursting the testa (according to
von Mohl); and the endosperm, previously ovoid and from 8 to lo mm. long, is
transformed into a flat broad sac 20 to 25 mm. in length, which surrounds the
growing cotyledons until they have absorbed all the food materials from it.

In Monocotyledons and many Dicotyledons the embryo remains small and is
either enveloped by the endosperm or lies by its side (as in Grasses) ; the cells of
the endosperm, which are in close contact without intercellular spaces, become filled,
until the seed is ripe, with a protoplasmic substance and fatty matter or starch or
both, in which case they remain thin- walled ; it then appears as the mealy (full of
starch) or fatty portion of the ripe seed, the embryo being found by its side or
within it; but it is often horny in consequence of a considerable thickening of
its cell-walls which have the power of swelling {e.g. the Date and other Palms,
Umbelliferae, Coffea, &c.) If this thickening has taken place to a very great extent,

* For further details on this point see the account of the Dicotyledons.

ANGIOSPERMS. 587

the endosperm may fill up the testa as a hard mass, forming, for instance, the
* vegetable ivory ' in the Phytelephas. In these cases the thickened walls of the
endosperm-cells, which are absorbed during germination together with their proto-
plasmic and fatty contents, serve for the first nourishment of the embryo. The
ripe endosperm, when copiously developed, has usually the form of the entire
ripe seed, being uniformly covered by its testa; its external form is therefore
generally simple, often round ; although considerable deviations from this frequently
occur, especially among Dicotyledons. Thus, for instance, the substance known as
the ' coffee-berry ' consists, with the exception of the minute embryo which is con-
cealed in it, entirely of the horny endosperm; but this, as a transverse section shows,
is a plate folded inwards at its margins. The marbled (ruminated) endosperm which
forms the nutmeg (the seed of MyrisHca fragrans) and the areca-nut (the seed
of the Areca-palm) owes its appearance to the circumstance that an inner dark
layer of the testa grows in the form of radiating lamellae between narrow fold-
like protuberances of the light-coloured endosperm. The ripe endosperm is either
a perfectly solid mass of tissue, or it possesses an inner cavity, as in Sirychnos
Nux-vomica, where, like the seed itself, it is broad and flat. This is clearly the
result of the endosperm which grows inwards from the periphery of the embryo-sac,
leaving a free central space, which, as has already been mentioned, is very large
and filled with fluid in the case of the cocoa-nut. In these cases the endosperm
is therefore a hollow thick-walled sac, enclosing a roundish or flattened cavity.

In a large number of families of Dicotyledons, the first leaves of the embryo,
the Cotyledons, grow, before the seeds are ripe, to so considerable a size that they
displace the endosperm which was previously present, and finally fill up the whole
space enclosed by the embryo-sac and the testa ; while the axial part of the embryo,
and the bud (plumule) that lies between the bases of the cotyledons, attain even
in these cases only inconsiderable dimensions. In these thick fleshy or foliaceous
cotyledons (which are then usually folded) the reserve of food-material accumulates,
consisting of protoplasmic substance, of starch and fatty matter, which is in other
cases stored up in the endosperm, and is made use of during the development
of the seedling. This storing of the cotyledons with so large a quantity of
food-materials appears to take place by its transference from the endosperm ; and
hence the diff"erence between those seeds which in the ripe condition contain no
endosperm [* exalbuminous '] and those which do contain it [' albuminous '] con-
sists essentially only in the fact that the food-material of the endosperm has passed
over in the former case before germination into the embryo ; while in the latter case
this only takes place during the process of germination. The presence or absence
of the endosperm in ripe seeds is more or less constant within large groups of
forms, and is therefore of value in classification. Of the better-known families, for
example, the Compositae, Cucurbitaceae, Papilionaceae, Cupuliferse (the Oak and
Beech), &c. are destitute of endosperm. Sometimes also the embryo increases in
size to such an extent that the endosperm appears as a thin skin surrounding it.

We must now recur to the oospore in order to follow the formation of the
Embryo. The cell-wall of the oospore coalesces superiorly with the wall of the
embryo-sac at its apical swelHng, its free end being turned towards the base of
the ovule; it then lengthens, and undergoes one or more transverse divisions.

^88 PHANEROGAMS.

[A row of cells is thus formed, from the lowermost of which, usually a spheroidal
cell which we may term the embryo-cell, the greater part of the embryo is in most
cases formed; but in many cases (in many Papaveracese and Caryophyllaceae, in
Asclepias, Cuscuta, Nicotiana, Viola, and others, amongst Dicotyledons, and com-
monly among Monocotyledons) the two end cells of the row form the chief part
of the embryo. Hanstein considered that the presence of two embryo-cells was
characteristic of Monocotyledons, but Hegelmaier has found that a single embryo-
cell occurs in this group {e.g. Orniihogalum natans), and that two embryo-cells
frequently occur in Dicotyledons, as mentioned above. The upper cells of the
row form the Suspensor. In some cases {Glaucium) the cells of the suspensor
undergo longitudinal divisions, and it consists consequently of several rows of cells.
Usually the lowest cell of the suspensor, the hypophysis, contributes to the forma-
tion of the embryo. In the Gramineae the cells of the suspensor divide and form
a multicellular appendage at the radicular end of the embryo {Keimanhang, Han-
stein) ; when the primary root begins to elongate this mass of cells is split off and
it forms a sort of sheath {coleorhiza) to the young root.] The suspensor usually
remains short (Fig. 400) ; sometimes, as in Funkia, its basal cell swells up into a
globular form (Fig. 399); in other cases (as, according to Hofmeister, in Loranlhus)
the oospore lengthens before division, and penetrates to the considerably enlarged
base of the long tubular embryo-sac. In those Dicotyledons where the endosperm
is formed only at certain lower parts of the embryo-sac by division, a similar elon-
gation of the oospore is usual, although not to so great an extent {e.g. Pedicularis,
Catalpa, Labiatae). In the embryo-cell a longitudinal or only slightly oblique
division-wall first of all makes its appearance, indicating the commencement of the
formation of the embryo (see also Fig. 15, p. 18). As this is followed by rapidly
repeated divisions, a spherical or ovoid mass of small-celled tissue is produced, from
which the first foliar structures, the cotyledons, subsequently arise, while the rudi-
ment of the first root may be observed in the differentiation of the tissue at the
boundary-line of the suspensor and embryo. The first cells of the embryo are
not unfrequently disposed as if they had resulted from oblique divisions of an
apical cell in two or three directions (Fig. 400 C), a supposition which is com-
pletely supported by the oblique position of the first septum in the embryo-cell;
in Rheum I also found the apex of young embryos to present an appearance which
suggested the existence of a three-faced apical cell. According to Hanstein's new
and prolonged researches, the process is, nevertheless, different ; he asserts that the
first longitudinal wall, even when it stands obliquely to the last transverse wall, is
still in the median plane of the body of the embryo which is being formed, and
is frequently at right angles to the last transverse wall, and therefore in the axis
of growth of the suspensor \ The formation of this median longitudinal wall in

^ The description in the text is taken from Hanstein's preliminary publications (Monatsberichte
der niederrhein. Gesellsch. fUr Natur- und Heilkunde, July 15 and August 2, 1869), as well as from
more detailed communications in letters. Professor Hanstein has also had the kindness to allow me
the sight of a number of drawings ; and, with his permission, the figs. 402-405 are copied from
them. I have also had the opportunity, in the summer of 1869, of seeing preparations of Hanstein's
similar to Fig. 403. Compare also Hanstein, Botanische Abhandlungen, Heft I, for a more detailed
description of the development of the embryo in Monocotyledons and Dicotyledons. [See also
Quart. Joum. Micr. Soc. 1873, p. 51. The following are some of the more important contributions

ANGIOSPERMS.

589

the embryo-cell completely excludes the possibility of a bi- or pluri-seriate seg-
mentation of the apical cell. We learn from Han stein that the mode of formation
of the embryo of Monocotyledons may be seen remarkably clearly in Alisma. In
Fig. 402, //, are shown, above the suspensor v, two other cells a {a is formed
by the division of the cell v in I) and c lying one over the other, the last of which
is already divided by a longitudinal and a transverse wall into four cells arranged
like quadrants of a sphere. A comparison of the stages //-— V shows that the
further development advances first of all in a basipetal direction. A cell w or k,
the result of intercalary division, which arises at the end of the suspensor next to
the embryo a c already formed, is especially to be noted. It is from this that the
root is subsequently developed. Hanstein calls it and the tissue which proceeds
from it the hypophysis ^ Before the body of the embryo undergoes any external

Fig. 402. — Formation of the embryo of Monocotyledons (Alisma) (after drawings by Hanstein) ; / — VIII various
stages of development ; v the suspensor, /* the hypophysis, w the region in which the radicle is formed, / the region in
whicn the plumule is formed, c cotyledon, b first leaf (VII and VIII much less magnified than the rest ; the dermatogen
is shaded). / consists of the rudimentary suspensor v, and of the embryo- cell.

differentiation, its tissue differentiates into a single peripheral layer (shaded in the
drawing), and a tissue internal to this ; the former is the primary epidermis or

to the recent literature of the subject: Fleischer, Beitr. zur Embryologie der Monokotylen und
Dicotylen, Flora, 1874. — Hegelmaier, Zur Entwick. monokotyledoner Keime, Bot. Zeit. 1874; id.
Vergl. Untersuch. lib. Entwick. dikotyledoner Keime, mit Beriicksichtigung der pseudo-monokotylen,
Stuttgart, 1878. — Solms-Laubach, Ueb. monocotyle Embryonen mit Scheitelbiirtigem Vegetations-
punkt, Bot. Zeitg. 1878. — Treub, Sur I'embryogenie de quelques Orchidees, Amsterdam, 1879. —
Westermaier, Die ersten Zelltheilungen im Embryo von Capsella Bursa pastoris. Flora, 1876. —
Famintzin, Embryologische Studien, Mem. de I'Acad. Imp. de St. Petersbourg, ser. 7, t. XXVI,
1879.]

^ [According to Famintzin, the cell w or h is derived, not from the suspensor, but from the
division of the cell a : this being the case, it is tlie cell a which must be regarded as the hypophysis.
If this is so, the hypophysis, in Alisma, contributes more largely to the formation of the embryo
than in other instances. Possibly this explanation may be applied to all cases in which two
embryo-cells are said to be present.]

59°

PHANEROGAMS.

dermatogen, which continues to grow only in extent and divides only in a radial
direction; the figures IV — VI show that the dermatogen is marked off from the
primary cells of the embryo by tangential divisions proceeding towards the base.
The inner mass of tissue soon undergoes further differentiation ; an axial string of
tissue is produced by divisions, especially longitudinal, forming the plerome or tissue
which subsequently produces the fibro-vascular bundles ; the primary meristem lying
between the plerome and the dermatogen, and which undergoes copious transverse
divisions, is the periblem, /. e. the primary cortical tissue. At the same time that
this differentiation of tissue is first indicated in the upper part a c of the embryo,
it begins also in the hypophysis k. The lower layer of the hypophysis takes no
part in the formation of the dermatogen, while from its upper layer (in VI) is
formed a prolongation of the dermatogen and of the periblem of the body of the
embryo, from which, as will be explained further on, the root is developed as a
posterior appendage of the embryo. Hanstein designates the apical part c of the
embryo the cotyledon, at the base of which d the apex of the stem is afterwards
formed laterally : the cotyledon is apparently developed from the cells c in //, the
apex of the stem and its hypocotyledonary portion (hypocotyl) together with the
upper part of the primary root from the cell a in //. But if this apical part is
really the cotyledon, which seems to me to be not yet sufficiently established, the
cotyledon cannot possibly be a foliar structure (phyllome), even if (as in Allium)
it subsequently assumes altogether the appearance of a foliage-leaf.

The different stages in the development of the embryo from the oospore are
much more clearly seen in Dicotyledons than in Monocotyledons, the Grasses in
particular among the latter presenting difficulties. Hanstein has singled out Capsella
Bursa-pastoris for detailed description. Fig. 403 shows first of all that the mass
of the embryo is developed from the spherical apical cell of the suspensor v^ and
in what manner this takes place; here also a cell h of the suspensor forms the
hypophysis which contributes to the formation of the primary root (radicle). The
spherical primary embryo-cell divides first by a longitudinal wall i — i (in / — IV)\
[each of the two cells thus formed is then divided into two by a longitudinal wall at
right angles to the first, so that the embryo now consists of four cells which are
quadrants of a sphere ;] this is followed in each of the quadrants by a transverse
division 2 — 2, so that the body of the embryo now consists of eight cells (octants
of a sphere), each of which next undergoes a tangential division, by which eight
outer cells are formed as the rudiment of the dermatogen, and eight inner central
cells (//). While the first only multiply by radial divisions, the inner mass of tissue
grows in all directions, resulting at an early period in its differentiation into plerome
(///, /F, F", shaded in the drawing) and periblem. The mass of tissue which is
produced from the primary embryo-cell thus increases rapidly by the multiplication
of its cells, and two large protuberances (F, <: r), the first leaves or cotyledons, soon
make their appearance one on each side of the apex {$) ; the apex of the stem
exists for the present only as the end of the longitudinal axis of the embryo ; an
elevated mass of tissue, the vegetative cone of the stem, is not formed till later, deeply
enclosed between the cotyledons. The posterior or basal end of the axis of the
embryo after the differentiation of its primary meristem into dermatogen, periblem,
and plerome (//, ///, IV), is, so to speak, open, as long as this differentiation

ANGIOSPERMS.

591

has not also taken place in the hypophysis (h); but finally it takes place in it
also and in such a way (as is shown in Fig. 403, V) that the upper of its two
cells breaks up into two layers {h'), the outer of which becomes continuous with
the dermatogen of the axis, while the inner layer forms a prolongation of the
internal axial tissue. The lower cell of the hypophysis {h) divides cross-wise
{V b, seen from below) and may be regarded as a transitional structure between
suspensor and root (appendage of the root) or as the first layer of the root-cap.
The root-cap is formed in this case simply by a luxuriant growth of the dermatogen.
The dermatogen, which elsewhere remains simple, and passes over into permanent

Fig. 403.— Development of the embryo of Capsella Btirsa-pastorts (after Hanstein) ; I— VI various stages of
development, F* apex of the root seen from below; i, r, 2, 2, the first divisions of the embryo-cell, h h the hypophysis,
■V the suspensor, c the cotyledons, s apex of the axis, lu root (the dermatogen and plerome are shaded).

tissue in forming the epidermis, increases in thickness, on the contrary, where it
covers the punctum vegetationis of the root, and undergoes repeated periclinal divi-
sions (parallel to the surface). Of the two layers which are successively formed
on each of these occasions, the outer becomes a layer of the root-cap (Fig. 404
wh, and Fig. 405, 2) ; the inner remains as dermatogen and again undergoes the
same process. This dermatogen which covers the vegetative cone of the root
behaves therefore like a layer of phellogen, with this difference, that the cells
produced from cork-cambium become at once permanent cells, while those of
the root- cap remain still capable of division ; so that each layer split off as it

59*

PHANEROGAMS.

were from the dermatogen forms a cap consisting of several layers of cells, its
growth being most active in the centre, and diminishing towards the periphery.
The splitting of the dermatogen into two lamella? usually progresses from the
centre towards the periphery of the apex of the root ; in the secondary roots of
Trapa, Hanstein and Reinke state that the reverse is the case\

Lateral roots not unfrequently arise in the embryo even before the ripening of
the seed, in addition to the primary root which we have hitherto alone considered ;
as, for instance, in many Grasses and some Dicotyledons {e. g. Impa/t'ens, according
to Hanstein and Reinke, Cucurbiia from my own observations). In Trapa natans
the primary root soon becomes abortive, lateral roots arising at an early period from
the hypocotyledonary portion of the axis.

Hanstein and Reinke state that the lateral roots of Angiosperms have their
origin in the pericambium, in Nageli's sense of the term ^. Their development was

d P^ ?f

Fig. 404. — Diagrammatic representation of the
formation of the primary root in Monocotyledons and
its connection with the stem (after Hanstein) ; v sus-
pensor, h hypophysis, iv w line of separation of the root
and stem, ivh layer of the root-cap, d dermatogen,
pb periblem, pi plerome.

Fig. 405. — Diagrammatic representation of the
same in the case of Dicotyledons (after Hanstein);
I, 2, the first layers of the root-cap, p periblem, a? derma-
togen,// plerome.

found in several plants to harmonise with this. In Trapa natans^ for example, it is
as follows : — A group of cells of the pericambium, which consists of only one layer,
divides radially; the newly-formed cells elongate in the same direction, and then
divide tangentially ; the outer of the two layers produces the dermatogen, the inner
the body of the root. The dermatogen, pushed outwards by the development of the
body of the root, produces the root-cap in the way already mentioned ; the tissue of
the body of the root itself which is covered by it becomes differentiated into plerome
and periblem. The same process takes place in PisHa, and probably also in
Grasses. Hanstein and Reinke do not find 'anywhere an apical cell which ori-
ginates the growth, as in Cryptogams, but a group of cells which obey a common
law of growth.'

The variation in the size of the embryo in the ripe seed of Angiosperms has

^ [See the account of the apical growths of roots and of the development of lateral roots which
is given in the Appendix.]

^ Compare what was said on Fig. 125, p. 167.

ANGIOSPERMS. 593

already been mentioned when speaking of the endosperm. The external differ-
entiation sometimes goes no further than the rudiment of the root (radicle) at the
posterior end of the stem of the embryo, and the cotyledons {e.g. in Cucurbiia,
Helianlhus, Allium Cepa, &c.), between which lies the naked punclum vegetationis.
But frequently this latter undergoes further growth before the seed is ripe, and
produces additional foliar structures (as in Grasses, Phaseolus^ Faba^ Quercus, Amyg-
dalusj &c.), which are then included, in the ordinary nomenclature, under the term
Plumule^ but do not unfold until the germination of the seed. The systems of
tissue are usually sufficiently clearly differentiated as such at the period of maturity
of the seed ; but the different forms of permanent tissue do not become developed
till later, during germination. A striking exception to this advanced development
of the young plant within the ripening seed is afforded by parasites and ' saprophytes
destitute of chlorophyll, but especially by Orchideae. In them the embryo remains
until the seed is ripe as a small round body consisting sometimes of only a few
cells, without any external differentiation into stem, leaves, and root; this takes
place only after germination, and even then sometimes quite imperfectly.

Polyembryony and Parthenogenesis^. In a few cases polyembryony, that is the
presence of more than one embryo in a single seed, has been found to occur in
Angiosperms, but it is brought about in a way which is very different from that
in which, as we have seen, it is caused in Gymnosperms. It was thought by
Hofmeister that, in the cases which he investigated {Funkia ccerulea, Scabiosa,
Cilrus), a number of oospheres were formed in the parietal protoplasm of the embryo-
sac, and that these were fertilised, but that of the large number of rudimentary
embryos thus formed, which is very considerable especially in Citrus, only a few
become fully developed. [This subject has been carefully investigated by Stras-
burger, and he has found that these embryos are not formed from oospheres, but
are developed as outgrowths from the cells of the nucellus which bound the embryo-
sac. In some cases {Funkia^ Nothoscordum /ragrans, Citrus) it appeared as if this
adventitious development of embryos were dependent upon the fertilisation of the
oosphere ; a development of an embryo from the oospore, in addition to the ad-
ventitious development of embryos from the nucellus, was only observed in Citrus.

Coelebogyne ilicifolia has long been known as a plant with polyembryonic seeds,
and it has been observed that these fertile seeds are produced without pollination
of the female flower. It was concluded that this plant was an instance of partheno-
genesis, that is, of the development of an embryo from an unfertilised oospherfe.
Strasburger has found that its numerous embryos are developed adventitiously in the
manner described above : it is therefore not parthenogenetic. ]

Development of the Seed and Fruit. While the endosperm and embryo are
becoming perfectly formed in the embryo-sac, growth proceeds not only in the
ovule but also in the wall of the ovary that encloses it. Since the testa is formed

^ [Hofmeister, Die Lehre von der Pflanzenzelle, 1867, P- "4- — Braun, Ueber Parthenogenesis
bei Pflanzen, Berlin 1857; id., Ueber Polyembryonic und Keimung von Coelebogyne, Berlin i860. —
Braun und Hanstein, Die Parthenogenesis der Coelebogyne ilicifolia, in Hanstein's Botanische Abhand-
lungen, III, 1877.— Strasburger, Ueber Befruchtung und Zelltheilung, 1878. See also the section on
Parthenogenesis in Book III.]

Q q

594 PHANEROGAMS.

at the expense of the whole or part of the cellular layers of the ovular integuments \
and presents extreme diversities in its structure, the ovule, together with its contents
which have resulted from fcrlilisation, becomes the Seed. The wall of the ovary,
the placentae, and the dissepiments, not only increase in dimensions, but undergo
the most various changes of external form and still more of internal structure.
Together with the seeds they constitute the I&my. The transformed wall of the
ovary now takes the name of Pericarp ; if an outer epidermal layer is specially
diiferentiated it is called the Epicarp, and the inner portion the Endocarp ; while a
third layer, the Mesocarp,' frequently lies between these two. A number of typical
kinds of fruit are distinguished according to the original form of the ovary and the
structure of its tissue when ripe, the nomenclature of which will be given in the
sequel. But sometimes the long series of deep-seated changes induced by fertilisa-
tion extends also to parts which do not belong to the ovary, and even to some
which have never belonged to the flower. But as they are part of the fruit from
a physiological point of view, and are usually associated with it as a whole, while
sharply differentiated from the rest of the plant, a structure of this kind (such as
the Fig, Strawberry, and Mulberry) may be termed a Fseudocarp.

At a certain period either the fruit together with its seeds becomes detached
from the rest of the plant, or the seeds alone separate from the dehiscent fruit ; and
this is the period of maturity. In many species the whole plant dies down when the
fruit is ripe, and a plant of this description is termed monocarpic (bearing fruit only
once). Monocarpic plants may be distinguished into those which fructify in the
first period of vegetation {annual plants), those which do not till the second year
{biennial plants), and finally not till after several or a large number of periods of
vegetation (monocarpic perennial plants, as Agave americand). Most Angiosperms
are however polycarpic ; i. e. the vital power of the individual is not exhausted by the
ripening of the fruit ; the plant continues to grow and periodically fructifies afresh,
or is polycarpic and perennial.

I. The Inflorescence''-'. It is comparatively rare for the flowers of Angiosperms to
arise singly at the summit of the primary shoot or in the axils of the leaves ; peculiarly
developed branch-systems are much ' more commonly produced at the end of the
primary shoot or in the axils of its foliage-leaves, which usually bear a considerable
number of flowers and are distinguished by their collective form from the rest of the
vegetative body ; in polycarpic plants these may even be thrown off after the ripening
of the fruit. Such a system of branching is termed an Inflorescence, The habit of the
inflorescence does not depend merely oi;i the number, form, and size of the flowers which
it bears, but also on the length and thickness of the branches of different orders, as well
as on the degree of development of the leaves from the axils of which the branches
spring. These leaves are generally much simpler in form and smaller than the foliage-

^ [From the very extensive recent literature on the structure and development of the seed-coats
the following may be cited : Lohde, Ueb. d. Entwickelungsgeschichte und den Bau einiger Samen-
schalen, Schenk's Mittheilungen, II, i875.--Sempolowski, Beitr. z. Kennt. des Baues der Samenschale,
Leipsig, 1874, — Hegelmaier, Ueb. Bau und Entwickelung einiger Cuticulargebilde, Jahrb. f. wiss.
Bot. IX, 1874. — Chatin, Le dev, de I'ovule et de la graine, Ann. de Sci. Nat. ser. 5, t. XIX, 1874. —
von Hohnel, Morphol. Unters. iib. die Samenschalen der Cucurbitaceen, Sitzber. d. Wien. Akad.
LXXIII, 1876. — Haberlandt, Entv^^ick. und Bau der Samenschale von Phaseohis, ibid. LXXV, 1877.]

•^ [See also Eichler, Bliithendiagramme, I, and Asa Gray, Structural Botany, 1880.]

ANGIOSPERMS. 595

leaves; frequently coloured {i.e, not green) or altogether colourless. They are dis-
tinguished as Hypsophyllary Leaves or Bracts; and in this term are frequently included
the small leaves which spring Irom the pedicels and which often have no axillary shoots
{Bracteoles). Leaves of this kind are sometimes entirely absent from the inflorescence
or from certain parts of it ; the ultimate floral axes or pedicels of the flowers are then
not axillary, as in Aroideae, Gruciferae, &c.

A large number of different forms of inflorescence may arise by the combination
in different ways of the determining characters already mentioned. Each form is
constant in the same species, and is often characteristic of a whole genus or family ;
hence the form of the inflorescence often not only determines the habit of the plant,
but is also of value to its systematic classification.

The most convenient basis for the classification of the forms of inflorescence is the
mode of branching. This is less variable than the other features, and can be referred
to a few types ; it also affords distinctive characters for the principal groups, which
might then be further sub-divided according to the length and thickness of the separate
axes and other points.

With reference to the mode of branching, the first point to observe is that every
inflorescence originates from the normal terminal branching of a growing axis ; the
mode of branching is always monopodia! in Angiosperms with the exception of the cases
mentioned under Division 14; i.e. the branches arise laterally beneath the apex of
the growing mother-shoot. If the leaves on this shoot (the bracts) are conspicuously
developed, the lateral axes arise in their axils ; if they are inconspicuous or abortive, the
lateral axes of the inflorescence are not indeed axillary, but their mode of branching and
growth remain the same as if the bracts were present ; and it is usual, in framing the
divisions, not to lay great stress on this circumstance (see p. 176). But the presence of
bracts is of great practical value, since it assists in the recognition of the true mode of
branching even in the mature inflorescence, inasmuch as the axillary shoot is always
lateral. When the bracts are absent it is often difficult to distinguish a lateral
from a primary axis, since the former often grows as vigorously as the latter, or even
more so. In Section 24 of the chapter on General Morphology (p. 169 et seq.) the
principles have been laid down according to which the various systems of branching
may be generally classified ; these will serve also in every respect for inflorescences,
and form the basis of the characters of the larger groups in the following classifica-
tion. Of the great number of separate forms of inflorescence only the more common
ones, a nomenclature for which is already provided in systematic botany, will be
enumerated ^

A. Racemose (monopodial). Centripetal, or Indefinite Inflorescences, in the

widest sense of the terms, result from the primary axis or rachis of the branching system
producing a larger or smaller number of lateral shoots in acropetal succession; the
capacity for development of each lateral shoot being smaller, or at least not greater,
than that of the portion of the primary axis which lies above it.

a. Spicate hiflorescences arise when the lateral axes of the first order do not branch
and are all floral axes ; the rachis terminates with or without a flower,
(a) Spicate Inflorescences with elongated rachis : —

1. The Spike: Flowers sessile; rachis slender (as in some Grasses).

2. TheSpadix: Flowers sessile ; rachis thick and fleshy, usually enveloped

in a large spathe ; bracts generally undeveloped (Aroideae).

3. The Raceme: Flowers distinctly stalked {e.g. Gruciferae, without bracts ;

Berberis, Menyanthes^ Campanula^ rachis terminating in a flower).

^ Compare the dissimilar descriptions in Ascherson's Flora of the Province Brandenburg,
Berlin, 1864, and in Hofmeister's Allgemeine Morphologie, § 7.

Q q 2

596 PHA NER OGA MS.

(3) Spicate inflorescences with abbreviated rachis : —

4. The Capitiilum : Rachis conical or tubular, or even hollowed out like a

cup ; flowers sessile ; bracts frequently absent (Gompositae, Dipsa-
caceae).

5. The Simple Umbel : Flowers stalked and springing from a very short

rachis {e.g. the Ivy).

b. Panicled Inflorescences arise when the lateral axes of the first order again branch
and produce axes of the second and higher orders ; every axis or only those
of the last order may terminate in a flower ; the capacity for development
usually decreases from below upwards both on the lateral and on the pri-
mary axis.

(a) Panicled Inflorescences with elongated axes: —

6. The true Panicle : Axes and pedicels elongated {Crambe, Grape-vine).

7. The Compound Panicle made up of Spikes : The elongated lateral axes

bear sessile flowers {Veratrum, Spircsa Aruncus, the ' ears ' of Wheat,
Rye, &c.).
O) Panicled Inflorescences with abbreviated axes : —

8. Compact spike-like Panicle : The very short lateral axes are arranged

on an elongated primary rachis (the 'ears' of Barley, Alopecurus,
&c.).

9. Tbe Compound Umbel : The very short rachis bears a densely compact

umbel of secondary (partial) umbels usually with long stalks (cf.
No. 5) ; if the compound umbel is surrounded by a whorl of leaves
this is called the In'volucre ; a similar whorl surrounding the secondary
umbel is an Involucel (secondary involucre) ; one or both may be
absent ; (most Umbelliferae).

B. Cymose, Centrifugal, or Definite Inflorescences result from the primary
axis branching beneath the first flower in such a manner that each lateral axis itself
terminates in a flower, after producing one or more lateral axes of a second order which
in their turn terminate in flowers and continue the system in this manner ; the develop-
ment of each lateral shoot is stronger than that of the primary axis beyond the point of
origin (see Figs. 134-136, pp. 178-180).

a. Cymose Inflorescences without a Pseud-axis : Two or more lateral axes are de-
veloped beneath each flower, terminating in flowers ; lateral axes of a higher
order continuing the system in the same manner.

10. The Anthela : An indefinite number of lateral axes are produced on

each axis, and overtopping the primary axis develope in such a
manner that the entire inflorescence does not acquire any definite
shape {e.g. Juncus lamprocarpus^ tenuis, alpinus, and Gerardi, Lunula
tiemorosa, &c.^). The an.hela of these genera, as well as of Scirpus
and Cyperus, exhibits a number of different transitional forms to the
panicle and even to the spike, and on the other hand to the formation
of cymose inflorescences with pseud-axes, e.g. in Juncus bufonius.
The inflorescence of Spiraea Ulmaria is included in this form by
myself and others.

11. The Cymose Umbel : A whorl of three or more equal axes springs from

the primary one, secondary whorls of lateral axes being again pro-
duced from it, and the process being then again repeated (see
Fig. 148). The whole system resembles a true umbel in habit ;

* Compare the careful description by Buchenau in Jahrb. flir wissensch. Bot. IV, p. 393 et !^eq»
and PI. 28-30.

ANGIOSPERMS, 597

very good examples are afforded' by several species of Eufborbia,
especially E. Lathyris and helioscopia. This form of cyme is not
essentially distinct from the next, and in the highest orders of
brapching commonly passes into it ; in Periploca grceca, for example,
even in the first ramification.

12. The Dichasium : Each primary axis terminating in a flower produces

a pair of opposite or nearly opposite lateral axes, which in their turn
produce pairs of the second order, and so on. The whole system
appears as if composed of bifurcations, especially after the older
flowers have fallen off; as in Euphorbia, many Sileneae, some Labiatae,
&c. The dichasium easily passes, in the first or a succeeding order
of lateral axes, into a sympodial mode of development.

b. Cymose Inflorescences nvith a Pseud-axis {Sympodial Inflorescences'). Each axis
which terminates in a' flower bears only one lateral axis of the next order.
The basal portions of the consecutive orders of axes may lie more or less in
a straight line, and may become thicker than the flower-stalk (above the
branching). A pseud-axis or sympodium may thus become either straight or
curved first in one direction and then in another, the flowers appearing to be
produced on it as lateral shoots (see Fig. 136, A, B, D, p. 180). If the sym-
podium is clearly developed, it resembles a spike or raceme, from which
however it is easily distinguished when bracts are present by their being
apparently opposite to the flowers (as in Helianthemum) ; but displacement
not unfrequently causes it to assume a different form (as in Sedum).

13. The Unilateral Helicoid Cyme (Bostryx) is a sympodial cyme in which

the median plane of each of the successive axes which constitute the
system is always situated on the same side, whether right or left,
with respect to the preceding one (see Fig. 136, D) ; as for instance,
in the primary branches of the inflorescence of Hemerocallis fulva
and fla'va, and in the partial inflorescences of Hypericum perforatum
which are themselves arranged in a panicle. (Hofmeister.)

14. The Unilateral Scorpioid Cyme (Cicinnus) is one in which the successive

axes arise alternately to the right and left of the preceding one
(Fig. 136 A)i as in Helianthemum, Drosera, Tradescantia, and Scilla
'bifolia. (Hofmeister.) The inflorescence of Eche'veria belongs also
to this kind of originally monopodial sympodium ; the mature cyme
has a pseud-axis on which the flowers are placed opposite the
leaves. While the summit of each successive axis is converted into
a flower, a lateral axis arises in the axil of the subtending leaf. This
lateral axis developes further, forms a new leaf in a plane nearly at
right angles to the last, and becomes transformed into a flower,
while a lateral axis appears in the axil of its leaf which continues
the development ; the leaf which arises on this axis is in the same
plane as the last but one. (Kraus.)

The inflorescences of the Boragineae and Solanaceae differ both in their mode of
development and in their external appearance from the plan described in B b, Kaufmann
has already stated^ that the inflorescence of some Boragineae is the result of repeated

^ Kaufmann, Bot. Zeitg. 1869, p. 886 [and Nouv. Mem. de la Soc. Imp. des Nat. de Moscow,
XIII, p. 248]. [Kaufmatm's observations have been confirmed by Warming (Ramification des
Phanerogames, 1872), by Pedersen (Bot. Tidskrift, 1873), and by Kraus (Bot. Zeitg. 1871) in so
far as bracteate scorpioid cymes are concerned. (See also Wydler, Zur Morph. d. dichotomen
BlUthenstande, Jahrb. f. wiss. Bot. XI, 1878). Warming considers that dichotomy also occurs in
naked scorpioid cymes, but Kraus states that these are monopodial, and Warming admits that lateral

598 PHANEROGAMS.

dichotomy of the apex of an axillary bud ; and Kraiis has also shown that the leafless
inflorescence of Heliotroplum and Myosotis is a monopodium, at all events when luxu-
riant. A thick and flattened vegetative cone developes two alternate rows of flowers
on its upper side; on this side the longitudinal growth of the primary axis is at first
stronger ; and the younger part of the inflorescence is consequently rolled with its apex
downwards in a circinate manner. An inflorescence which is formed in this manner,
as will be seen from what has already been said, cannot properly be described as a
scorpioid cyme, but corresponds rather to a raceme or spike which bears flowers only
on one side of its rachis. The leafy scorpioid cymes of Anchusa, Cerinthe, Borago,
and Hyoscyamus are, on the contrary, the result of dichotomous branching; a leaf
which stands on the primary axis ending in a flower bears in its axil a vegetative cone
which is at first hemispherical ; this becomes broader and dichotomises in a direction
parallel to the surface of the leaf ; one of the bifurcations becomes a flower, the other
bears a new leaf at right angles to the last, and forms a dichotomy above it as before.
The planes of dichotomy therefore cross one another at right angles ; and this is the
reason why the leaves always stand between the sympodial axis and the flower. Lateral
displacements of the leaves begin at the second division and continue afterwards.

According to Kraus it is doubtful whether the sympodial inflorescences of Ompha-
lodes and Solanum nigrum are the result of dichotomous or of lateral branching. On
the side of the primary axis which becomes a flower a leafless lateral axis arises which
continues to branch, and the right and left lateral axes of which are alternately trans-
formed into flowers. Kraus entertains a similar doubt respecting weak inflorescences
of Myosotis and Keliotr opium {'vide supra).

It will be seen from what has now been said, that within an inflorescence which
consists of several orders of axes there may be produced not only diff'erent forms
of one section, but forms belonging to both sections (A and B), mixed inflorescences
being thus formed. Thus, for example, a panicle may form dichasia in its last ramifi-
cations (as in some species of Silene) ; a dichasium may bear capitula (e.g. Silphium)^
or even in its first branches or in those of a higher order may pass into a helicoid
or scorpioid cyme (as in Garyophylleae, Malvaceae, Solanaceae, Linacege, Cynanchum^
Gagea, Hemerocallis, &c.). The mode of branching of the inflorescence is in most
cases diflTerent from that of the vegetative stem. Not unfrequently it passes abruptly
from one to the other, but often through intermediate modes of branching.

In the older systems of nomenclature a number of other terms are given to various
forms of inflorescence, such as glomerulus, corymb, &c. ; but they all designate merely
the habit or external form of the system, and must be referred, in a scientific description,
to one or other of the above forms or to combinations of them.

2. With regard to the Change in the Mode of Branching accompanying the transition
from the vegetative to the floral region of a shoot. Warming gives some very valuable
information in his Recherches sur la ramification des Phanerogames (Kopenhagen 1872),
from which it appears that the numerous cases of extra-axillary branching in in-
florescences can be referred to axillary branching as the typical mode. He lays it down
that the axillary branch with the corresponding leaf are to be regarded as a whole, one

branching Usually takes place in weak inflorescences. Goebel (Arb. d. Bot. Inst, in Wurzburg, II,
1880) finds not only that Kraus' account of the development ot the inflorescence of Myosotis and of
Heliotropum is accurate, but that it applies also to that oi Symphytum officinale, A jichusa, Cerinthe, Borago,
Cynoglossum, Echium vulgare, Lithospermum arvense, and Caryolopha sempervirens ; the development of
the flowers is lateral also in Hyo^cynmtis niger, Khigia Notoniana, and in Helianthernum. Goebel's con-
clusions have been combated by Celakovsky (Flora, 1880). It must be borne in mind that dichotomous
and lateral branching are not absolutely distinct forms, but that they are connected by intermediate
conditions, so that they gradually merge one into the other. (See also Henslow, On the Origin of
the so-called Scorpioid Cyme, Trans. Linn. Soc, 1880; and the History of the Scorpioid Cyme,
Journ. of Botany, 1881.)]

ANGIOSPERMS. 599

part of which — the leaf, — or the other part — the branch, — may be developed earlier
than the other or simultaneously with it, or more or less completely than it. It is
evident in the vegetative region that the subtending leaf always arises first, and de-
velopes more actively, to begin M'ith at any rate, than the corresponding branch which
only becomes apparent when one or more young leaves have arisen above the leaf
in question. (Figs. 129, 131.) In many inflorescences the development of the leaf
precedes that of the axillary branch by a much shorter interval, as in the spikes and
racemes of Amorpha^ Salix, Rudbeckia, Lupinus, Veronica, Digitalis, Orchis, Delphinium.
In the development of other inflorescences the axillary branches are formed immediately
after their subtending leaves, so that no rudimentary leaf intervenes between the apex
of the shoot and the youngest axillary branch {Plantago, Orchis, Epipactis). Sometimes
leaf and branch arise simultaneously, as in the Gramineae, Cytisus, TrifoHum, Orchis,
Plajitagc, Ribes. Or again, the axillary branch is formed first, before its subtending
leaf, in which case the leaf attains only a slight development, its presence being merely
indicated as in Sisymbrium, Brassica and other Cruciferae, Umbelliferae, Anthemis,
Valeriana, Asclepiadeae, Bryonia, Cucumis. Or the subtending leaf may not make its
appearance at all, and no bracts are developed, as in many Cruciferae (Fig. 132),
Gompositae, Gramineae, Umbelliferae, Papilionaceae, Gucurbitaceae, Asperifolieae, Solaneae,
Hydrophylleae, Saxifrageae, Potamogetoneae. In all these inflorescences the youngest
buds are nearer to the apex of the parent shoot than any foliar organs in so far as these
have been developed, but the branching must not on this account be regarded as
dichotomous. A dichotomy of the parent shoot only takes place when a vigorous
branch is developed so near to the apex that a continuation of the direction of growth of
the shoot is rendered impossible, its apex apparently dividing into two or more apices.
According to Warming this is the case in Hydrocharis, Vallisneria, the Asclepiadeae, the
scorpioid cymes of the Solaneae, Asperifolieae, Hydrophylleae, Gistaceae, and many
Gucurbitaceae. This tendency to dichotomise shown by plants the vegetative parts of
which branch in a lateral axillary manner is doubtless connected with the suppression
of the development of leaves in the inflorescences, and this is confirmed by the fact that
the tendrils of Vitis and Cucurbita, on which the development of leaves is rudimentary,
exhibit the same tendency. ^

The axillary branches of the vegetative region are usually so placed that they arise
both from the basis of the leaf and from the tissue of the stem ; but it sometimes
happens that the branch is entirely transferred to the stem and becomes isolated from the
leaf. In the floral region, on the other hand, it not unfrequently happens that the axillary
branch (the inflorescence) arises solely from the leaf, as in Hippuris (Fig. 119), Amorpha,
Salix nigricans. If, however, the subtending leaf (bract) is developed later than the axil-
lary branch (inflorescence), it may arise from it, so that the leaf has no direct connection
with the parent shoot, but appears to be the first lowest leaf of the lateral branch ; this
is the case, according to Warming, in Anthemis, Sisymbrium, Umbelliferae, and to a slight
degree in Papilionaceae, Orchideae, Valerianeae, and others. These relations are usually
evident at the earliest stage of development, but frequently the subtending leaf is found
upon the axillary branch in the mature condition, as in Thesium ebracteatum, Samolus
Valerandi, Boragineae, Solaneae, Grassulaceae, Spiroea, Loranthaceae, Ipomcea bona nox,
Agtt've americana, Ruta, Paliurus, Tilia (in which this applies to the large bract of the
inflorescence), and others.

3. Number and Relative Position of the Parts of the Floiver'^. Just as the forms of
branching of the inflorescence are usually diff"erent from those of the vegetative stem,
the arrangement of the leaves of Angiosperms is also usually diff'erent on the shoot which
constitutes the flower from that on other parts of the same plant. The cessation of
the apical growth of the receptacle, its great increase in breadth, or even hollowing out,
before and during the time when the perianth and the sexual organs are being formed,

[See Eichler,' BlUthendiagramme.]

600 PHANEROGAMS.

influences their order of succession and their divergence from one another. But since,
notwithstanding the extraordinary variation of the other relations of form, the true
position of the floral leaves varies but little— though it may often be difficult to determine
— the knowledge of this position is often of great importance in the determination of the
affinities of the species, and hence for purposes of classification. This is especially
the case if we at the same time take into account the abortion of individual members
which is here of so common occurrence, the multiplication of the parts which take place
under certain circumstances, and their branching and cohesion.

In order to fiicilitate a description of these relationships, it is necessary to explain
certain terms and methods of description.

In the first place it is important to denote the position of all the parts of a flower
with respect to the mother-axis of the floral shoot. For this purpose the side of the
flower which faces the mother-axis is termed the posterior, that which is most remote
from it the anterior side. If a plane be imagined to divide the flower longitudinally from
front to back, and to include the primary axis of the flower as well as that of the
mother-shoot, this is the median plane of the flower, dividing it into a right and a left
half. Floral leaves, as well as ovules and placentae, which are bisected longitudinally
by the median plane, are said to have a median position, either posterior or anterior.
If another plane is now imagined at right angles to the first, and also including the axis
of the flower, it may be termed the lateral plane; this plane divides the flower into a
posterior and an anterior half, and parts which are longitudinally bisected by it are
precisely lateral. The two planes which bisect the right angle between the median
and the lateral planes may be called diagonal planes , and the parts which are bisected by
them be said to have a diagonal position. Flowers usually have some of their floral
organs placed exactly posteriorly or anteriorly, not so commonly exactly right and
left or exactly diagonally; but usually other additional terms must be used, such as
obliquely posterior or obliquely anterior.

If next the position of the parts of the flower with respect to one another be ex-
amined, their arrangement, as has already been mentioned, is either spiral or -verticillate.

Flowers with a spiral arrangement of their parts are comparatively rare, and appar-
ently occur only in certain orders of Dicotyledons (Ranunculaceae, Nymphaeacese,
Magnoliaceae, and Galycanthaceae). Braun has termed such flowers acyclic, when the
transition from one foliar series to another, as from calyx to corolla or from corolla
to stamens, does not coincide with a definite number of turns of the spiral (as
Nymphaeaceae and Helleborus odorus) ; hemicyclic when it does so coincide. This latter term
may also be employed when some of the foliar structures are actually cyclic (verticillate),
others spiral, as in Ranunculus, where the calyx and corolla form two alternating whorls,
followed by the stamens and carpels arranged spirally. Parts which have a spiral
arrangement sometimes occur in definite numbers, more often in larger indefinite
numbers.

When on the other hand the parts of the flowers are arranged in whorls, the number
of the whorls, as well as that of the members of each whorl, is constant in the same
species, and within larger or smaller circles of affinity ^ When the number of members

* [The number of whorls in a flower may vary very widely, from one {Carex) to fifteen or sixteen
(Aquilegia). In some cases the calyx, corolla, andioecium, and gynseceum each consists of a single
whorl, so that the flower has four whorls. More commonly, however, one or other of these series
consists of more than one whorl. This is most frequently the case in the androecium, so much so in
fact that it is customary to regard the typical flower as containing two whorls of stamens : in an
isomerous flower, if the stamens are in a single whorl it is said to be isostemonous, if in two whorls
diplostemonous, and so on. The calyx often consists of more than one whorl (Menispermaceae,
Berberidacese), and in most tetramerous flowers (Cruciferae, Onagraceae) it is composed of two
decussating dimoerous whorls. More rarely the corolla consists of more than one whorl ; instances
of this occur in the Fumariacese, Eerberidaceae, Papaveraceoe, Menispermacese. A gynseceum of two,

ANGIOSPERMS. 6oi

of each whorl is the same, and those belonging to the different whorls are placed one over
another so as to form orthostichies, I adopt Payer's expression of superposed (instead
of the ordinary one of 'opposite'). When the stamens are superposed on the calyx or
corolla, they are termed respectively antisepalous and antipetalous ; if the members of a
whorl fall between the median lines of those of the next whorl above or below, the
whorls are alternate. When the number of members is the same in each whorl, they are
Said to be isomerous, when this is not the case heteromerous ; and Braun calls those flowers
eucyclic in which the members of all the whorls are equal in number and alternate. It
also happens however that members of the same kind arise subsequently between those
of a whorl already formed ; as, for instance, five later stamens between the five earlier
ones m Dictamnus Frax'inella (Fig. 414), and probably in many eucyclic flowers with ten
stamens. Members subsequently introduced in this manner into a whorl may be called
interposed. (For further details, 'vide infra.)

The consideration of the number of the parts of the flower cannot be separated from
that of their relative position. But before entering more minutely upon this subject, the
construction of the Floral Diagram must be described.

The Floral Diagram is constructed differently according to the purpose it is intended
to serve. Some treat it as a somewhat free drawing of an actual transverse section of
the flower, and indicate on it not merely the number and position, but approximately
the form, size, aestivation, cohesion, &c. of its parts. This purpose is however clearly
best attained by preparing as accurate drawings as possible of actual transverse sections
of the flower-bud, which will then also contain much that would be superfluous for
observations of a certain kind. But if it is merely required to represent the number and
position of the parts of the flower in such a manner as to render as easy as possible the
comparison in this respect of a number of flowers, it is best to disregard all other
peculiarities, and to adopt one and the same plan for all diagrams, and that as simple as
possible, so as to represent nothing but the variations in the relationships of number and
position. This is the only purpose kept in view in the diagrams given in the remainder
of this work, of which Figs. 406-408 may serve for the present as examples. They are
constructed according to the rule already given on p. 188 ; the dot above the diagram
always represents the position of the mother-axis of the flower ; and the lower is there-
fore the anterior part. Although mere dots would be sufficient to indicate perfectly the
number and position of the parts of the flower, different signs have nevertheless been

rarely more, whorls is found in many Butomaceoe and Alismacese ; usually when the number of the
carpels is great they are arranged spirally. When the members of a series (calyx, corolla, etc.) are
in one whorl, the series is said to be monocyclic ; if in more than one, di-, tricyclic, etc. ; if in many,
polycyclic.

In isostemonous flowers it frequently occurs that the stamens are antipetalous, as in Ampelidese,
RhamnacejE, Plumbaginacese, Primulaceae. This is usually ascribed to the abortion of an exterior
whorl of antisepalous stamens, an assumption which is based either on the presence of a whorl of
staminodes in the place of the missing stamens, or on the presence of two whorls of stamens in
allied Orders.

It not unfrequently happens in a diplostemonous flower that the Stamens of the outer whorl are
antipetalous {Limnanthes, Ruta, Dictamnus (Fig, 414), Pyrola, Monotropa, Chrysosplenium, Epilobium,
CEnothera, Fuchsia, Geraniacese, Zygophyllacege, Crassulaceoe, Ericaceae, Rhodoracese, etc.) : when
this is the case the androecium is said to be obdiplostemonous ; the carpels are superposed on the
stamens of the outer whorl, and therefore also on the petals. The^most satisfactory explanation of
obdiplostemony is that given by Celakovsky (Flora, 1875), though, as we shall see below (note, p.
606), it cannot be applied in all cases : according to him the staminal whorls arise in regular acropetal
succession, the antisepalous stamens being developed first, as in direct diplostemony; the antipetalous
stamens are developed internally to the others but become gradually displaced outwards, so that they
appear either to lie in the same whorl as the antisepalous stamens or externally to them. This is the
real meaning of the 'interposition' mentioned above. (For further details on these points see Gray's
Structural Botany, and Eichler's Bluthendiagramme.)]

6o2

PHANEROGAMS,

chosen for the various separate organs, in order to render the explanation more readily-
visible to the eye. The leaves of the perianth are represented by arcs of a circle, a kind
of mid-rib being indicated on each of the outer whorl of these, or calyx, merely in order
to distinguish them at a glance from the inner whorl. The sign chosen for the stamens
resembles a transverse section of an anther, but without reference to the position of the
pollen-sacs or of their mode of dehiscence whether inwardly or outwardly. When the
stamens are branched, this is indicated by the signs being grouped, as in Fig. 408, where
the five groups correspond to five branched staminal leaves. The gynaeceum is treated
as a simplified transverse section of the ovary, since it is thus most easily distinguished
from the other parts; the marks within the loculi of the ovary indicate the ovules,
which however are only represented in those cases where their actual position can be
expressed by so simple a plan. The size, form, and cohesion of the separate parts are
not taken into account at all. The construction of these diagrams is based partly on
careful investigations of my own, but chiefly on the studies of Payer in the history of
development (Organogenic de la fleur), as well as on the descriptions of other authors
(Doll, Eichler^, and Braun).

I draw a distinction between empirical and theoretical diagrams. The empirical dia-
gram only represents the relative number and position of the parts, just as a careful
observation shows them in the flower ; but if the diagram also indicates the places
where members are suppressed — -which can only be determined by the history of
development and by comparison with allied species, especially if it points out relationships

Fig. 406 —Diagram of the flower of
L iliacece.

FIG. 407.— ^Diagram of the flowel- of
Celasiriis (after Payer).

FJg. 408.'— Diagl-atn of the flower of
Hypericum calyctnum.

which are entirely the result of theoretical considerations — I call it a theoretical diagram.
If the comparison of a number of diagrams shows that, although empirically diff'erent,
they nevertheless yield the same theoretical diagram, this common theoretical diagram
may be termed the type or typical diagram according to which they are all constructed.
I consider the careful determination of such types an important problem, the solution of
which may be extremely useful in the classification of Angiosperms. When the type has
once been ascertained, the theoretical diagrams which correspond to it may be treated
as derivative forms from which particular members have disappeared, or where they have
been replaced by a number of members. From the stand*point of the theory of descent
the type corresponds to a form still in existence or that has already disappeared, from
which the species to which the derivative diagrams belong have arisen by degeneration
{i. e. by abortion ^) or by multiplication of the parts.

A few examples will explain this. The flower of Grasses, which is seated among the
paleae, .may be deduced, as is shown in Fig. 409, on the theory of the abortion of certain

* [Eichler, Bliithendiagramme, I, II, 1875-8.]

^ The construction of the diagram itself shows that the theory of abortion is justified even
where the earliest state of the flower-bud gives no indication of the absent member, if the number and
position of the parts present point distinctly to such a hypothesis. If the idea of abortion in this
sense is not admitted, neither can the increase in number of individual parts, or their replacement by
several, be allowed. It is only the theory of descent that gives a rational explanation of either fact,
and that a very clear one.

ANGIOSPERMS.

603

parts from the typical flower represented in Fig. 406, which is itself the typical diagram
of Liliaceae. ^ is the diagram of Bambusa, which only deviates from the type in
the absence of the outer perianth-whorl which is indicated by dots. But in most other
Grasses {B) the posterior leaf of the inner perianth-whorl (this whorl appearing generally
only in the form of small colourless scales), the whole of the inner whorl of stamens, and
the anterior carpel, are also wanting. In Nardus again (C), the anterior carpel only is
present (as far as the pistil is concerned) ; all the absent parts are represented by dots,
and the diagram is therefore so far a theoretical one. If the dots are removed, we get
the empirical diagram ; the number and position of the carpels is here determined from
those of the stigmas ^

FIG. 409.— L iagrain of the flower of a Grass ; A Bambusa; B of most Grasses ; C oi Nardus (from Doll, Flora von Baden,

vol. I. pp. 105, 133).

The flowers of Orchideae can also be derived, like those of Gramlneae, from the type
represented in Fig. 406, the empirical diagram of Liliaceae, although their external form
is so remarkably different. While in Grasses the perianth is especially degenerated or
even partially abortive, in Orchids both whorls are developed in a petaloid, and like the
whole flower, in a zygomorphic or monosymmetrical manner. Of the androecium, which
consists typically of two alternating whorls, each of three stamens, only a single stamen
is completely developed in most Orchids (Fig. 410, A)^ viz. the anterior one of the outer
whorl, the others being abortive. Indications of these are however sometimes found
in the young bud, as in Calanthe veratrifolia (according to Payer, cf. Fig. 394), where at

FIG. 410.— Diagram of tlie flower of Orchideae ; A the ordinafy structure ; B that of Cypripedium (see Figs. 372 and 41J
the dots indicate stamens which are altogether abortive, the shaded figures rudimentary stamens which become abortive
transformed into staminodes.

least the two anterior ones of the inner whorl (but not the posterior one) appear as
small elevations which soon disappear. In Cypripedium, on the contrary, a large
staminodium (see Fig. 372) takes the place of the anterior stamen which is else-
where fertile ; while the two anterior and lateral anthers of the inner whorl are fally
developed and fertile (Fig. 410, 5). In Ophrydeaj two small staminodes are found
beside the gynostemium {cf. Fig. 418, D^st) in the place of the two fertile stamens of
Cypripedium ; while in Uropedium all three of the inner whorl are completely developed.
(Doll.) The carpels which, by adhesion with the androecium form the gynostemium, are
developed unequally, a diflference which however is usually not discernible in inferior
ovaries, and is therefore not indicated in the diagram. The student who desires to

1 Compare further, Doll, Beitrage, in the Jahresbericht des Mannheimer Vereins fiir Naturkunde,
1870, where an actual pentacyclic trimerous flower oi Streptoch<Bte is described.

604 PHANEROGAMS.

investigate these relationships for himself must observe that the long inferior ovary of
most Orchids undergoes a torsion {resupination) at the time of the opening of the flower,
which causes the posterior side of the flower to assume an anterior position ; but trans-
verse sections even of advanced buds show clearly the true position of the parts of the
flower in relation to their mother-axis.

The flowers of most Monocotyledons, like those of Orchids and Grasses, can be de-
rived from a type which is actually seen in Liliaceae, and which represents a flower
consisting of five alternating whorls, each with three members, of which the two outer
ones constitute the perianth, the two next the androecium, and the last the gynaeceum ;
although the latter may sometimes be represented by two whorls. Occasionally instead
of abortion an increase of number takes place in particular whorls, by the formation
of one member instead of two (as in Butomus, Fig. 382).

Increase in the typical number of the members of a whorl may arise in different
ways, as the following examples will show. According to the detailed researches of

Fig. 411. — Diagram of the flower of Fumariacerc (after Eichlcr).

Eichler^, the flowers of Fumariaceae may be referred to a type in which there are six
decussate pairs of members (Fig. 411), viz.

two median sepals,

two lower lateral (exterior) petals,

two upper median (interior) petals,

two lateral stamens,

two median (always abortive) stamens,

two lateral carpels.

The two lateral stamens are however represented in some genera (as Dicentra and
Corydalis) by-two groups, each consisting of three stamens, an inner one with an entire
quadrilocular anther, and two lateral stamens each with a bilocular anther, a struc-
ture which Eichler explains on the hypothesis that the lateral stamens are only stipular
structures, and therefore branches from the base of the middle one. In Hypecoum
Eichler assumes a cohesion of each pair of opposite stipular stamens so as to form an
apparent whorl of four stamens.

Eichler also deduces the flowers of Cruciferse and Cleomeae (a section of Gappa-
rideae) from a type represented by Fig. 412 ^, which is also the empirical diagram for
Cleome drosercpfolia, and for certain species of Lepidium, Senebiera, and Capsella, This
typical flower consists of

two lower median sepals,

two upper lateral sepals,

four diagonal petals in one whorl,

two lower lateral stamens,

two upper median stamens,

two lateral carpels.

^ Eichler, Ueber den Bliithenbau der Fumariaceen, Cruciferen, und einiger Capparideen, in
(Regensburg) Flora, 1865, nos. 28-35, and 1869, p. i. — Peyritsch, Ueber Bildungsabweichungen der
Cruciferenbluthen, Jahrb. fur wiss. Bot. vol. VIII. p. 117.

ANGIOSPERMS.

605

Deviations from this type are produced by the formation of two or more stamens in
place of each of the upper (inner) ones; in the Gruciferae usually two (Fig. 413), in the
Gleomeae sometimes two, sometimes more (Fig. 412 B). Such a replacement of one
stamen by two or more is termed by Payer Dedoublement'^, by Eichler and others Colla-
teral Chorisis, and must apparently be considered as a branching of very early origin.
This view is confirmed in this case by the fact that in the Crucifer Atelanthera the

Fig. 412— Diagram of the flower of Capparideae ; A Cleome di-o-
serafolia, B Polanisia graveoUits (after Eichler).

Fig. 413.— Diagram of the flower of
Gruciferae.

median stamens are only split and the two halves of each provided with half-anthers,
while in Crambe each of the four inner stamens puts out a lateral sterile branch, which
may be explained as the commencement of a further multiplication of the stamens
such as actually occurs in the Crucifer Megacarpeea and in many Gleomeae. Even if
the way in which increase of the typical dimerous number of the inner whorl of stamens
has been brought about be still obscure, it appears certain that the inconstancy of
the number of the members of the staminal whorl proves that in Gruciferae and Gle-
omeae a deviation has arisen in this part of the flower from the typical dimerous number,
while the other whorls have remained unchanged. The only 'deviation which occurs in
the gynaeceum of the Grucifers is in the genera Tetrapoma and Holargidiunty where,
besides the two lateral carpels, two median ones are also .produced, thus forming a
four-lobed ovary '^.

An essentially different kind of increase in the typical number of the members of a
floral whorl may be caused by the formation in the still very young bud of new
members of the same kind between those already in existence and on the same zone

Fig. 414.— Diagram of the flower oi Dictamnus Fraxinella {ff. Fig. 38 ).

of the receptacle ; /. e. by what we have already described as the Interposition of new
members. This I found to occur, for example, in Dictamnus Fraxinella (Fig. 388), and
is represented in the diagram. Fig. 414, by the stamens of later origin being shaded not so
dark as those of earlier origin. It may, I think, be inferred from Payer's descriptions

^ [The theory of an original dimerous symmetry in the flowers of Gruciferae has been pushed
still further by Meschaeff (Bull. Soc. Imp. Nat. Mosc), who regards the four petals as also the result
of a lateral dedoublement of a single pair (see Bentham, Ann. Address Linn. Soc. 1873).]

2 [Holargidium is a section of Draba. According to Bentham and Hooker the four carpels of
Tetrapoma are an abnormality not constant under cultivation. The same authors also mention the
occasional occurrence of a similar abnormality in Brassica and Nasturtium.']

6o6 PHANEROGAMS.

and drawings that the same process occurs in the nearly related genus Ruta, and in the
families Oxalideae, Zygophyllaceae, and Geraniaceae included in the same circle of affinity;
viz. that in these cases also five stamens are interposed between those already in
existence^ If the five interposed stamens are supposed to be removed, there remains
in these f^imilies a regular pentamerous flower with four alternating whorls each
consisting of five members, such as is found in the nearly related Linaceae and Bal-
samineae'^.

Floral Formulcc. The diagram may, under certain circumstances, be substituted, at
least partially, by a formula composed of letters and numbers. In a floral formula of
this kind the relative positions of the parts cannot indeed always be represented with
accuracy ; but it has the advantage that it can be expressed by ordinary printer's type,
and, what is perhaps of greater importance, is capable of a wider generalisation, since
the numerical coefficients may be replaced by letters.

The construction and application of these formulae will easily be made intelligible by
a few examples^.

The formula SsPiSts+^C^ corresponds to the diagram of the Liliaceae, Fig. 406, and
signifies that each of the two perianth-whorls -the outer whorl or sepals S, and the inner
whorl or petals P — consists of three members, the andrcecium of two whorls each of three
stamens St, and the gynaeceum of three carpels C. The diagram shows in addition that
these trimerous whorls alternate without interruption ; but since this is the usual case
with flowers, it need not be specially indicated. The formula S^P^ •S'^s^+s Q+s gives the
relative positions of the parts of the flower of Butomus umbellatus (Fig. 382). It
is distinguished from the previous one by the gynaeceum consisting of two whorls of
three carpels each, and the andrcecium having the typical three stamens of the outer
whorl each replaced by two stamens, which is expressed by the symbol i^. The
formula S^P^St^^-^C^ corresponds to the diagram of the flower oi Bambusa, Fig. 409 J,
and differs from that of Liliaceae only in the suppression of the outer perianth-whorl,
represented by S^. The numerical relations of the parts of the flower of Orchideae,
Fig. 410 A, might be expressed by the formula S^P.^Sti+QC^, the symbol Sti 0 indicating
that all the members of the inner staminal whorl are abortive, while on the other hand
in the outer whorl the two posterior ones are suppressed, the anterior outer stamen
being perfectly developed ; the two dots over the number i' are meant to indicate
that the absent members are the posterior ones ; were the anterior ones deficient the
dots would be placed beneath the number, as in the formula S^P^^St^+QC^ which cor-
responds to the ordinary flower of Grasses represented by the diagram Fig. 409 B. The
formula S-i^P-^St^+iC.^ expresses the whorls consisting of decussate pairs which form the
flower oi Maianthemum bifoUum ; the formula .S4 P4 5^4 +4 C4 or S^ P^ ^^5+5 Q the flowers of
Paris quadrifolia, in which all the whorls are either tetramerous or pentamerous. These

^ [These are all cases of obdiplostemony. In the case of Dictamnus and of Ruta this is to be
explained by Celakovsky's theory of displacement. In the Oxalideae and Geraniaceae Frank has
found (Jahrb. f. wiss. Bot. X), in opposition to Payer, that the antipetalous stamens are developed first.
It is therefore difficult to give a satisfactory account of the obdiplostemony in these orders. Eichler
regards it as due to constant deviation from the normal acropetal development of the whorls of the
flower.]

2 Doll (Flora von Baden, vol. III. pp. 1175, 1177) and others suppose that a whorl has
become abortive between the corolla and ovary in Rutaceae and Oxalideae, a hypothesis which is
not supported by the history of development, and which is superfluous on our hypothesis. To
assume abortion merely because certain whorls do not alternate seems to me to be going too far.
Besides, the ten stamens of Epacrideae and Rhodoraceae cannot belong to two but only to one whorl
in which five are of earlier origin, and five have been interposed. (Compare Payer, Organogenie de
la fleur, pi. 118.)

^ Grisebach (Grundriss der systematischen Botanik ; Gottingen, 1854) has denoted the relative
numbers of the parts of flowers in a different manner, placing the numbers of the members of a
whorl simply one after another, and indicating cohesions by strokes.

ANGIOSPERMS. 607

and most other formulae for the flowers of Monocotyledons may now be combined into
a general expression 5„ P„ Stn+n Cni+n)i which signifies that the flowers belonging to this
type are usually constructed of five alternating whorls each with the same number of
members, two of which are developed in the form of perianth-whorls, two as staminal
whorls, and generally only one as a carpellary whorl ; the bracket ( + «) at the end of
the formula indicating that a second carpellary whorl sometimes occurs in addition.
The general number n may, as the examples which have been adduced show, have the
value 2, 3, 4, or 5 ; 3 is the most common. If a considerable increase of the number
of members takes place in a whorl, and if this number, as is then usually the case, is
variable, this is expressed by the symbol oo ; thus the formula for Alisma Plantago is
S^ /*3 5/3+3 ^"^ '

As has already been mentioned, no further indication is given of the position of the
whorls when they alternate ; when a departure from this rule occurs, this can be more
or less accurately expressed by special symbols. Thus, for example, the formula for
the flower of Gruciferae, Fig. 413, might be represented by Sc^+^P^^Stc^^.^ Cc^[+^),ihe
symbol Px4 signifying that the decussate pairs of sepals are followed by a corolla con-
sisting of one whorl of four petals, which are however arranged diagonally to the sepals.
In order to express the superposition of two consecutive whorls, a vertical stroke might
be placed after the number of the first whorl ; thus ^5 P^ \ St^ C^ might represent the
formula for Hypericum calycinum (Fig. 408), | St-^ indicating that the androecium con-
sists of five branched (s'^) stamens which are superposed on the petals. If, finally, it is
desired to signify that members of a second whorl are interposed at the same level
between those of one already in existence, the number of the new members may be
placed simply beside those of the original whorl ; thus the formula ^5 P5 5/5.5 Cg would
correspond to the diagram Fig. 414.

In the formulae already given no cohesions of any kind have been indicated; they
can however under certain circumstances easily be expressed by special symbols.
Thus, in the formula for Convolvulus S^ Pg 5/g Cg, the sign Pg indicates a gamopetalous
corolla of five petals, C2 a syncarpous ovary of two carpels. In the formula for the
flowers of Papilionaceae again ^g P^ 5/5+4+1 Cj, the expression 5/544+1 signifies that the five
stamens of the outer and four of those of the pinner whorl have united into a tube, while
the posterior stamen of the inner whorl remains free'.

The mode of writing the formulae must vary according to the object which one has
in view ; the greater the number of relationships it is intended to express, the more
complicated will they become ; and care must be taken that they do not lose their
clearness by being overladen by too many signs.

The examples of formulae which have hitherto been adduced all illustrate cyclic
flowers; those parts of flowers which are arranged spirally may be denoted by the
symbol -^ placed before them, and the angle of divergence may also be affixed to
their number. Thus, for example, the relative numbers and positions of the parts of
the flower of Aconitum, according to Braun's investigations, may be expressed by the
formula 5^2/ bP^zi sSf^si ^ C^ , which indicates that all the foliar structures

15 Is /2I °° 3'

of this flower are arranged spirally, and that the calyx consists of five sepals with the
divergence ^/g, the corolla of eight petals with the divergence ^s? ^^^ ^^^ androeciuhi of
an indefinite number of stamens with the divergence 721' It would however be sufficient
in this case, since the spiral arrangement runs through the whole flower, to place the
symbol only once before the whole formula, thus -^ 52/ 5P3/ 8 5/8/ ao C

In flowers with a cyclic arrangement of their parts a statement of the angle of
divergence is generally unnecessary, since the members of each whorl usually arise
simultaneously, and are arranged so as to divide the circle into equal parts. When
they do not arise simultaneously but successively in the circle with a definite angle

' See also Rohrbach, Bot. Zeitg. 1870, pp. 816 ei seq.

6o8 PHANEROGAMS.

of divergence, as in most trimerous or pentamerous calyces, this can be indicated by
placing the angle of divergence after the number of the members ; thus the formula for
Linaceae would be S^^'^j^^P^^St^.C^. If, on the other hand, the members of a whorl are
formed in succession from front to back, this may be shown by an arrow pointing
upwards t, as in the formula for Papilionaceae S^] P^\ St^\ +5! C^. If they are formed
in succession from back to front, the arrow may be made to point downwards j, as
in the formula for Reseda 5„i P„l Stp[ + q[ C,-, where the number of the parts is expressed
by letters instead of figures in consequence of its variability \

4. Order of De'velopment of the Parts of the Flower. The foliar structures arise on the
axis of the floral shoot, as on other axes, in ticropetal order below the growing apex. It
is however not uncommon in the formation of flowers for the apical growth of the axis to
cease altogether or to become extremely slow, while the receptacle continues to increase
in breadth, and to develope transverse zones of intercalary growth. When this is the
case the acropetal order of development is disturbed, and new whorls may become
interposed between those already in existence. But even within the same floral whorl
the individual members may be formed in a very different order of succession, according
as the zone of the receptacle which bears the floral leaves is developed in a uniform
manner all round (as in polysymmetrical flowers) or more rapidly on the anterior or
the posterior side (which is especially the case in monosymmetrical or zygomorphic
flowers).

In flowers with a spiral arrangement of their parts^, disturbances of the acropetal
order of development are of less importance the more numerous the parts with a spiral
arrangement, and the longer the apical growth of the floral axis continues. Those mem-
bers which have a spiral arrangement arise one after the other in ascending order ; the
angle of divergence may either be constant or may change. Thus, according to Payer,
in Ranunculaceae and Magnoliaceae the perianth-leaves and stamens arise in a con-
tinuous spiral, but each turn of the spiral consists of a larger number of stamens
than of perianth-leaves ; thus, e. g., in Helleborus odorus, where all the organs of the
flower are arranged spirally, the corolline turn includes only thirteen petals, while
each turn of stamens numbers twenty-one. According to Braun the turns of the
calyx of Delphinium Consolida have a ^5 arrangement^; the divergence then under-
goes a small change, but without materially deviating from ^5 ; the first turn with this
altered arrangement is the corolla ; the three following ones are the stamens, and the
spiral terminates with a single carpel. In the section Garidella of Nigella the first of the
turns with a ^5 angle of divergence is the calyx and the second the corolla; then follows
a slight change in the angle to ^g? the stamens forming one or two turns with this
arrangement; and the spiral closes with three or four carpels. In the section Delphi-
nellum of Delphinium the calyx constitutes a turn with ^s) the corolla one with ^g
angle of divergence ; then follow two or three turns of stamens with the angle very
near %, the spiral closing with three carpels. In the section Staphisagria of the same
genus, and in Aconitum, the calyx forms a turn with ^5, the corolla one with % angle ;
the stamens stand in one or two turns with the divergence ^21 or '734 > concluding
with three, five, or rarely a larger number of carpels. It must be noted in reference to
these arrangements that the members of successive turns stand in orthostichies when
the angle of divergence remains constant ; but that the orthostichies pass into oblique
rows when the divergence undergoes a small change.

The first thing to observe in cyclic flowers (/. <?. those in which the parts are arranged
in whorls) is the order of formation of the whorls with respect to one another, and then

* See Payer, Organogenic de la fleur ; also the next paragraph.

2 Compare Payer, Organogenie de la fleur, pp. 707 etseq.; and Braun, Jahrb. flir wissensch. Bot. I,
Ueber den Bliithenbau der Gattung Delphinium.

^ Compare with this what is said below respecting sepals and petals which are formed with the
angle of divergence Y3 and Yg.

ANGIOSPERMS. 609

the order in which the members of each whorl are themselves formed; -although the
two are in fact closely connected. A disturbance of the acropetal order of succession in
the formation of the whorls occurs when the carpels have begun to be formed before all
the stamens which stand below them have been produced, as in Rubus, Potentilla, and
Rosa\ or when the calyx is not formed until after the andrcecium (as in Hypericum
calycinum according to Hofmeister), or when the calyx is not observable until after
the corolla has become considerably developed or even after the formation of the
stamens and carpels, as in Compositae, Dipsacaceae, Valerianacese, and Rubiaceae.

One of the most remarkable deviations from the general rule of the order of develop-
ment of the floral whorls occurs in Primulaceae, where five protuberances (primordia)
appear on the receptacle above the calyx, each of which grows up into a stamen, while
on the posterior or lower side of the base of each primordial stamen a lobe of the corolla
subsequently appears^. Pfeffer, who has observed this order of development (Jahrb. fiir
wissensch. Bot. vol. VII. p. 194), considers that the same probably also happens in the
pentandrous Hypericineae and in Plumbagineae ; he therefore explains the corolla-
lobes as posterior outgrowths of the stamens (a posterior ligular structure), such as, for
instance, occur on the stamens of Asclepiadeae in the form of hood-shaped nectaries,
where a true corolla is also present. The flowers of Primulaceae would therefore be
strictly apetalous in the morphological sense of the word, since their corolla is not a true
floral whorl, but only an outgrowth of the staminal whorl. In other families of Dicoty-
ledons, on the other hand, superposed corollas and androecia arise separately and in
acropetal order ; as, for instance, in Ampelideae, probably also in Rhamnaceae, Santalaceae,
and Chenopodiaceae.

The individual members of a floral whorl may arise in succession from front to
back or the reverse, especially when the flowers themselves are subsequently developed
zygomorphically. Thus, for instance, in Papilionaceae the anterior median sepal is
formed first, then simultaneously one to the right and one to the left, and finally the
two posterior ones ; but before these last arise the two anterior petals appear, followed
by the two lateral and finally the posterior one ; and the andrcecium, consisting of two
alternating whorls of five stamens each, is formed in the same manner from front to
back\ In the Resedaceae, on the contrary {Reseda and Astrocarpus), Payer states that
the petals, stamens, and carpels are developed from behind forwards on both sides {cf.
Fig. 145, p. 187).

When the calyx consists of pairs of sepals, those of each pair are formed, as Payer
has shown, simultaneously ; but if the calyx consists of three or five sepals, they are
usually formed one after another, and with the angle of divergence in the one case Ya? ^^
the other ^5 ; but the succeeding whorls, the petals, stamens and carpels, usually arise
as simultaneous whorls, with the exceptions already named and others still to be
spoken of.

It is well to draw attention here to the circumstance that it does not follow from the
order of succession advancing from one point, with a definite angle of divergence, say
Vs <^'' Vs) that the arrangement is a spiral one^; it may just as well in such cases be a
whorl. The nature of the arrangement depends on the circumstance whether the foliar
structures in question are formed at the same height or not, i, e. at an equal distance
from the centre of the flower ; if this is the case, we have a whorl ; but if the members

^ Compare Hofmeister, Allgemeine Morphologic, pp. 463 et seq., where Payer's observations on
this point will also be found.

^ [According to Frank (Ueb. d. Entwick. einig. Bluthen, mit bes. Beriicksichtigung der Theorie
der Interponirung, Jahrb. f. wiss. Bot. X. 1876), the stamens and petals of the Primulaceae arise
independently, but they fuse with the stamens during their development, and subsequently become free
again.]

'■^ On the nearly related Coesalpinese see Rohrbach, Bot. Zeitg. 1870, p. 826.

* Compare the successive true whorls of Chara and Salvinia, pp. 187, 191, 293, 449.

R r

6rO PHANEROGAMS.

arise in acropetal order at different heights, i.e. approaching the centre of the flower
with each step in the divergence, the arrangement is a spiral one. The last appears to
be actually the case in many calyces ; but it is doubtful whether it ever occurs where the
angle of divergence of the sepals is '/s or '^/^

We must now refer again to the cases already mentioned, where new members of a
whorl are formed between those already in existence and at the same height^. In the
Oxalideae, Geraniaceae, Rutaceae, and Zygophyllaceae, an entire whorl of five stamens is
thus interposed between those already present; according to Payer, in Peganum Harmala,
a whorl of ten stamens is even formed in this manner, arising, not in pairs between the
first five, but lower down at the bases of the petals; whether the later formed stamens
arise on the same level with the first, or lower down, is obviously regulated according to
the space afforded by the changes of form of the growing receptacle. A still further
departure from the ordinary process occurs in the Acerineae, Hippocastaneae, and Sapin-
daceae, where Payer asserts that a whorl of five stamens is first of all formed alternating
with the corolla, in which an imperfect whorl of two or four stamens is subsequently
interposed at the same height, as is shown by his illustrations. In Tropaolum, on the

Fig. 415. — Flower oi Ileracleu7tt piibescens with zygoinorphic corolla.

other hand, according to Payer and Rohrbach^, three stamens first of all appear after
the formation of the petals, and then between them five others, the distance of which
from the centre of the flower is however rather greater than that of the three earlier
ones.

5. Symmetry of the Flonver. If the observations which will be found on pp. 187 et seq.
under the head of General Morphology are now applied to the floral shoot, it is seen
that true symmetry and distinctly bilateral structure occur here far more commonly

* Compare also on this point Pfeffer, Jahrb. fiir wiss. Bot. vol. VIII, p. 205,
^ Rohrbach (Bot. Zeitg. 1869, Nos. 50, 51) however gives a different explanation to these obser-
vations from that mentioned here. The equal or greater distance at which the later stamens arise
from the centre of the flower is a distinct proof that one cannot in this case suppose that the parts
are produced in a spiral arrangement advancing from without inwards, [See note on p. 601.
Assuming the correctness of Payer's observations, these are instances of incomplete obdiplostemony.
According to Buchenau (Morph. Bemerk, lib. einige Acerineen, Bot, Zeitg. 1861), all the stamens are
developed simultaneously in the Acerineoe,]

ANGIOSPERMS.

6ii

than on the vegetative shoots. In contrast to the lax mode of expression used by many
botanists, I understand by Symmetrical Structures those which may be divided into two
halves, each of which is an exact reflex image of the other. If a flower can be divided
in this manner by only one plane, I call it simply symmetrical or monosymmetrkal ; if, on
the contrary, it can be symmetrically divided by two or more planes, it is, as the case
may be, doubly or poly symmetrical. The happy expression zygomorphic already used by
Braun may be applied equally to monosymmetrical flowers and to those polysymmetrical
ones in which the median section produces halves of quite a different form from
those caused by lateral section {e.g. Dicentra). I apply the term regular to a poly-

FlG. 416.— Zygomorphic flower of Columnea Schiedeana : A entire flower after removal of two sepals ; B androecium ;
Cgynaeceum; Z) the coherent anthers magnified and seen from behind; £■ horizontal section of the ovary ; /=■ diagram ;
a anthers, n stigma, g style,y-6 ovary, d the staminode developed into a nectary,// the lateral oblique placentae.

symmetrical flower only when the symmetrical halves produced by any one section are
exactly like or very similar to those produced by any other section ; or — which comes
to the same thing— when two, three, or more longitudinal sections divide a flower into
four, six, or more equal or similar portions.

In exactly defining the symmetrical relations of a flower, the relative positions of the
parts, as represented by the diagram, must first of all be distinguished from the entire
form of the flower, such as is realised in the development of the organs.

If attention is paid first of all only to the relative positions of the parts, it is clear
that they can never be distributed symmetrically in flowers with a truly spiral structure ;

R r 2

6ia

PHANEROGAMS.

while in hemicyclic flowers those members at least which are arranged in whorls may
possibly be distributed symmetrically. If, on the contrary, the parts are all arranged in
whorls, they are usually distributed monosymmetrically or polysymmetrically on the
receptacle. Thus, for example, the diagram Fig. 406 can be divided symmetri-
cally and regularly by three planes. Fig. 407 by four, and Fig. 408 by five planes.
The diagrams Fig. 409 B and C, as well as Fig. 410, can, on the contrary, be symmetri-
cally halved by only one plane, which is at the same time the median plane. The diagram
Fig. 411 can be divided by the median plane into two symmetrical halves which are
unlike those produced by the lateral section ; this diagram is, like those in Figs. 409 B, C
and 410, zygomorphic, but is doubly while these are only singly symmetricaP.

The symmetry of mature unfolded flowers is indeed usually connected genetically
with the relations of symmetry of the diagram (which represents only the position and
number of the parts); as will be made clear by a comparison of Figs. 416 and 418 with
Fig. 410 -<^. But inasmuch as the entire form of the mature flower is essentially deter-

FlG. 417. — Zygomorphic flower of Polygala gratidiflorn : A entire flower seen from the, side after removal of one
sepal k ; B flower divided symmetrically without the gynaeceum ; C the gynaeceum magnified ; D horizontal section
of the ovary ; E median longitudinal section of the ovary ; F horizontal section of the flower ; k calyx, c corolla,
st staminal tube, cp gynophore, /" ovary, g- style, « stigma, sk ovules, xx the tube formed by the adhesion of the
petals and stamens.

mined by the shape, size, torsion, and curvature of the separate parts, these circum-
stances also exert a preponderating influence on the relations of symmetry of the open
flower, and to such a degree that even flowers which have their parts arranged spirally
may become monosymmetrically zygomorphic in reference to their entire form, as is
the case to a high degree, for example, in Aconitum and Delphinium. It must however
be observed that the zygomorphism of the flower is here brought about principally or
entirely by the calyx and corolla, the spiral arrangement of which may perhaps still be
doubtful, but which always occupy so narrow a zone on the receptacle that their position
may be considered practically to be verticillate. If, on the other hand, the floral axis is
sufficiently elongated to show that the arrangement is a distinctly ascending spiral one,
as in the perianth and androecium of Nymphoea and the androecium and gynaeceum of
Magnolia, the subsequent development of the organs appears also not to show any zygo-
morphic nor indeed generally any kind of actually symmetrical arrangement.

^ The beginner may make these relations more evident to himself by placing a small mirror with
a smooth edge vertically upon the paper so as to bisect the diagram.

ANGIOSPERMS.

613

The zygomorphic and monosymmetrical form occurs, on the contrary, very com-
monly in those flowers the parts of which are arranged in whorls. A very distinctly
zygomorphic arrangement is not imfrequently united with a partial or entire abortion of
particular members, as, e.g., in Columnea, Fig. 416, and other genera of Gesneraceae,
where the posterior stamen is transformed into a small nectary; while in Labiatae
it is entirely wanting. This abortion is
carried still further in Orchideae, where,
of the six typical stamens, only the
median anterior one of the outer whorl
or the two lateral anterior ones of the
inner whorl are developed (see Fig.
410). The ultimate monosymmetrical
arrangement is sometimes to a certain
extent indicated by the order of their
formation, even in the rudimentary con-
dition of the parts of the flower, when
their origin is not simultaneous in the
whorl, and does not progress with a
definite angle of divergence, but is so
arranged that the development com-
mences with one anterior or one pos-
terior member, and then advances
simultaneously right and left from the
median line towards the opposite side
of the whorl. Examples have already
been given of this arrangement in
Papilionaceae in the one case and Re-
sedaceae in the other.

In the zygomorphic flowers of Fu-
mariaceae, the diagram (Fig. 411) is, as
we have already pointed out, symme-
trically divisible in different ways by
two planes. The anterior and posterior
halves, symmetrically similar to one
another, are unlike the right and left
halves, which again are symmetrically
alike. This is the arrangement of the
parts in the mature flower of Dicentra ;
in Fumaria and Corydalis the right side
is developed differently from the left,
one producing a spur, the other not;
while the anterior and posterior sides
remain symmetrical. In this case there-
fore the plane of symmetry coincides
with a lateral section. In the zygomor-
phic flowers of some Solanaceae the plane of symmetry and the median plane intersect
at an acute angle. But by far the greater number of zygomorphic monosymmetrical
flowers are so constructed that the median plane coincides with a longitudinal section
which divides the flower symmetrically ; as for instance in Labiatae, Papilionaceae,
Orchideae, Scitamineae, Lobeliaceae, Compositae, Delphinium, and Aconitum'^. The zygo-
morphic development is especially prevalent in the lateral flowers of spicate, racemose,

Fig. 418. — Zygomorphic flowfer of Orchis in&cuMta : A bud
divided symmetrically through the middle ; B transverse section
of the bud ; C horizontal section of the ovary ; D entire mature
flower, one of the lateral perianth-leaves having been removed ;
X mother-axis of the flower ; b bract, s outer, / inner perianth-
leaves — the posterior one I becomes the labellum, a the single
stamen, st staminodes, gs gynostemium, // poUinium, h its viscid
disc (retinaculum), sp spur of the labellum, y" the inferior ovary,
twisted in D (resupinate) (compare the diagram Fig. 410).

* In observations of this kind attention must be paid to torsions, such as occur in the ovary of
Orchidece, the flower-stalk of Fumariaceae, the Laburnum, &c.

6 1 4 PHA NEROGA MS.

or paniculate inflorescences ; but is found also in those that are cymose and that have
all the flowers terminal (Labiatae and Echium). It seems as though the vigorous
development of the principal rachis of the entire inflorescence — whether the final
ramifications are cymose or not — often determines a zygomorphic development of
flowers, as is shown in Labiatae, Scitamineae, and JEsculus. The formation of a vigorous
pseud-axis appears to exercise a similar influence in the case of sympodial inflorescences
(as in Echium).

6. The Fruit of Angiosperms is the mature ovary which contains the ripe seeds and
has undergone physiological changes as the result of fertilisation. The style and stigmas
are frequently deciduous (as in Cucurbita, Grasses, &c.). Some of the ovules not un-
frequently disappear, and the number of seeds is thus less than that of the ovules.
When all the ovules of one or more loculi of a multilocular ovary disappear in the
process of ripening, only the fertile loculus continues to grow; the others become
partially or entirely suppressed, and can be recognised only with difficulty or not at all.
A multilocular ovary may thus produce a unilocular, and often a one-seeded fruit. Thus
from the trilocular ovary of the Oak, each loculus of which contains two ovules, results
a unilocular one-seeded fruit, the acorn. A less complete disappearance of two or four
loculi together with their ovules occurs in the tri- or quinqui-locular ovaiy of the Lime,
the fruit usually containing only one seed.

Parts of the flower again which do not belong to the gynaeceum, or even not to the
flower, undergo changes resulting from fertilisation. The entire structure which is thus
formed may be termed a Pseudocarp, and may be composed of a single fruit or of a
number of true fruits together with the surrounding parts which have undergone
peculiar development. Thus, for example, the strawberry is a pseudocarp, the axial
part (or receptacle) of the flower swelling out and becoming fleshy, and bearing on its
surface the true small fruits. In the 'hip' of the Rose the hollow urn-shaped flower-
stalk (a^ain the receptacle) encloses the separate ripe fruits in the form of a red or
yellow succulent envelope. The apple is also in the same sense a pseudocarp ; and the
mulberry results from a whole spike of flowers, the perianth-leaves of each separate
flower swelling and becoming fleshy and enclosing the small dry fruit. In the fig the
hollowed- out stalk of the whole inflorescence forms the pseudocarp, bearing the fruits
inside.

Starting from the definition that a fruit is always the product of a single ripe ovary,
it follows that several fruits may arise from one flower, whenever, namely, there is more
than one monocarpellary ovary in the flower ; in other words, when the flower is poly-
carpellary and apocarpous; each carpel therefore produces 7i simple fruit. The simple
fruits, taken together, may be termed an aggregate fruit, but it would be much better to
apply to it the term Syncarp. Thus, for example, the small fruits resulting from the
flower of Ranunculus or Clematis or the larger ones from the flower of Pceonia or Helle-
borus, form together a syncarp. Of a similar character is the blackberry, consisting
of a number of drupe-like fruits, the product of a single flower. The fleshy receptacle
of the Rose-hip again encloses a syncarp, but the separate fruits constitutmg it are in
this case dry and not fleshy. The syncarp must not be confounded with the pseudocarp
resulting from an entire inflorescence, as in the cases of the mulberry and fig already
named, or the pine-apple, or Benthamia fragifera.

The single multilocular ovary of a flower may undergo transformation so as to pro-
duce two or more parts, each containing seeds, and appearing like simple fruits, and
hence termed Mericarps, while the whole fruit is called a Schi%ocarp. This separation
may take place at a very early period in the process of the formation of the fruit; as in
IropcBolumy where each loculus, enclosing a single seed, becomes rounded and at length
entirely separated from the others as a closed mericarp ; and in Boragineae and
Labiatae, where each of the two carpels produces two one-seeded chambers, all four
becoming at length completely separated, and surrounding the style as distinct mericarps
(here called CarceruU) ; or the separation only takes place by the splitting and rupture

ANGIOSPERMS. 6 1 5

of certain plates of tissue in the fully ripe fruit (as in Umbelliferae and Jeer), then termed
a Cremocarp, where the fruit breaks up into two one-seeded halves or mericarps by the
splitting of the dissepiment or ' carpophore ' along its length. The quinquilocular fruit
of Geranium splits up in the same manner into five one-seeded mericarps.

True single fruits are in general unilocular or multilocular, according as the ovary
was divided or not. But the unilocular ovary may produce a multilocular fruit by
spurious dissepiments, i. e. such as cannot be considered as the reflexed margins of the
carpels ; and the loculi of such a fruit may lie either one above another or side by side.
The compartments, for example, of the legume (lomentum) of some Papilionaceae and
of Cassia fistula lie one over another, while the two spurious loculi of the legume of
Astragalus lie side by side. A multilocular ovary may, n)ice 'versa, produce a unilocular
fruit by the suppression of one or more loculi, as in the Oak and Lime. A classification
of fruits into monocarpellary and polycarpellary cannot therefore be carried out as it
can be in ovaries ; the terms having now a different application.

The wall of the ovary becomes the wall of the fruit or Pericarp, If sufficiently thick,
it can generally be divided into two or three layers, the tissue of which is developed
differently ; the outer one, often nothing but the epidermis, is then called the Epicarp,
and the inner one the Endocarp. If another one lies between these, it is called the
Mesocarp, or when it possesses a fleshy character, the Sarcocarp.

Using the nomenclature which has now been described, we may classify all true
fruits into two principal sections, and each of these again into subdivisions, according to
whether the pericarp consists, when the fruit is ripe, of succulent fleshy layers or not,
and whether the fruit dehisces in order to allow the escape of the seeds which become
detached from the placentae, or not ; viz.

A. Dry Fruits. Pericarp woody or tough and leathery, the cell-sap having
disappeared from its cells.

a. Dry Indehiscent Fruits. The pericarp does not split open, but encloses the

seed till germination; the testa is thin and membranous, and but little
developed.

(o) One-seeded dry indehiscent fruits.

1. The Nut or Glans : the dry pericarp is thick and hard, and consists of

lignified sclerenchymatous tissue; ^.^. the Hazel-nut.

2. The Caryopsis or Achenium: the dry pericarp is thin, tough, and

leathery, in close contact with the seed, and separable or not from
the testa; as the fruit of Gompositae, Grasses, the Sweet-Chestnut.

(/3) Bi- or multilocular dry indehiscent fruits.

3. These are mostly Schi%ocarps splitting up into Mericarps, each of

which resembles a nut or achenium; e.g. Umbelliferae, Geraniaceae.
When the mericarp is winged, as in Acer, it is called a Samara.

b. Dry Dehiscent Fruits or Capsules in the more general sense. When the

fruit is perfectly ripe, the pericarp bursts or splits to allow the escape of the
seeds, which are themselves clothed with a strongly developed usually hard or
tough testa. They generally contain more than one seed.

(a) Capsules with longitudinal dehiscence : —

4. The Follicle consists of a single carpel which splits along the ventral

suture or coherent margins of the carpels which bear the seeds ; as
in Pceonia, Aquilegia, and IlUcium anisatum ; in Asclepias the thick
, placenta also becomes detached.

5. The Legume consists also of a single carpel, which however splits not

only along the ventral but also along the dorsal suture, and thus
separates into two halves; Phaseolus, Pisum.

6l6 PHANEROGAMS.

6. The SUiqua consists of two carpels which form a bilocuhir fruit with a

longitudinal (spurious) dissepiment ; the two halves of the pericarp
separate from the dissepiment and the placentae which remain behind
{re-plum) ; Ouciferae.

7. The Capsule (in the narrower sense of the term) results from a poly-

carpellary unilocular or multilocular ovary, and splits longitudinally
into two or more lobes and valves, which separate from one another
only partially from the apex downwards (as in Cerastium), or entirely
to the base. If, in a multilocular fruit, the fissures cause the dis-
sepiments themselves to split, the dehiscence is septicidal (as in
Colchicum) ; if, on the contrary, the fissure is in the middle between
each pair of dissepiments (/. e. along the dorsal suture of the carpels),
the dehiscence is loculicidal (as in Tulipa and Hibiscus) ; if again a
part or the whole of each dissepiment remains attached to a central
column (which in the latter case is winged), from which the valves
become detached, the dehiscence is septifragal (as in Datura)^. If
the capsule results from a unilocular polycarpellary ovary, the
separation of the valves may take place either at the sutures cor-
responding to the septicidal dehiscence (as in Gentiana), or in the
middle between them, corresponding to the loculicidal dehiscence
(as in Fiola).

(/3) Capsules with transverse dehiscence : —

8. The Pyxidium opens by the separation of an upper part of the pericarp

which falls off like a lid, while the lower part remains attached
to the flower-stalk in the form of an urn {e.g. Plantago, Hyoscy-
amus, Anagallis).

(y) Capsules opening by pores : —

9. The term Pore-capsule might be given to those in which openings of

small size result from small valves becoming detached at certain
points of the pericarp ; the small seeds being shaken out by the
wind through these openings {e.g. Papanjer, Antirrhinum).

B. Succulent Fruits. The tissue of the pericarp or certain layers of it remain
succulent until the fruit is ripe, or assume a fleshy pulpy texture.

c. Succulent Indehiscent Fruits. The succulent pericarp does not burst, and
the seeds therefore do not escape.

10. The Drupe or Stone-fruit. A mesocarp of fleshy texture and usually

considerable thickness lies within a thin epicarp ; the endocarp
forms a thick hard layer (the stone, called also the putamen) which
usually encloses only one seed with a membranous testa (the Plum,
Cherry, Peach, &c.).

11. 1 he Berry. The rest of the tissue of the pericarp is developed in the

form of a succulent pulp within a more or less tough or hard
epicarp, the seeds being imbedded in the pulp and surrounded by a
firm or even hard testa. The berry is distinguished in general from
the drupe by the absence of a hard endocarp, and usually contains
more than one seed (as the Currant, Gourd, Pomegranate, Potato-
berry), but sometimes only one (as the Date). Closely resembling
the berry is the fruit of the various species of Citrus., sometimes
called Hesperidium, the pericarp of which consists of a leathery

* [Septifragal dehiscence may take place either septicidally (as in Rhododendron, Kalniia) or
loculicidally {Datura).] *

ANGIOSPERMS. 617

outer layer and a pithy inner layer ; at a very early period multi-
cellular protuberances are developed from the innermost layer of
tissue of the wall of the multilocular ovary, which gradually fill up
the cavity of the loculi of the fruit with isolated but closely
crowded succulent lobes of tissue, and form in this case the pulp.
d. Succulent Dehiscent Fruits. The succulent but not fleshy pericarp splits and
allows the escape of the seeds which have usually a strongly developed testa.

12. The term Succulent Capsule might be given to those fruits the

succulent pericarp of which opens by dividing into lobes, and allows
the seeds to escape (as in the Horse-Chestnut and Balsam).

13. The fruit of the Walnut corresponds again to the drupe; the outer

succulent layer bursts, a stony endocarp surrounding the thin-
skinned seed. It might be called a Dehiscent Drupe.

14. The fruit of Nuphar bears more resemblance to a berry, but differs in

the bursting of the outer firm layer of the pericarp; it may be
termed a Dehiscent Berry] in iV. ad-vena this exposes an inner
coating of each loculus of the fruit, which floats for some time on
the water like a bag filled with seeds.

The enumeration here given includes only the more common forms of fruits ; there
are a number of others which cannot be placed exactly in any of the above categories,
but to which no special name has been given \

The Ripe Seed depends, as respects its external nature, on the development of the
pericarp. The testa is in general thicker, firmer, and harder in proportion to the
softness of the pericarp, especially when this latter bursts to allow the dispersion of
the seeds. When, on the contrary, the pericarp is tough or woody, and encloses the
seeds until they germinate, as in the caryopsis, nut, drupe, and schizocarp, the testa
remains thin and soft, as also when the endosperm is strongly developed and very
hard and encloses a small embryo, as in the Date and Phytelephas. The testa of
the seeds of dehiscent fruits is usually covered by a distinctly differentiated epi-
dermis ; and it depends on the configuration of this epidermis whether the seed has a
smooth appearance (as in the Pea and Bean), or displays a variety of sculpturing, such as
pits, warts, bands, and so forth (as in Hyoscyamus, Datura, Papa-ver, Nigella, &c.). The
epidermal cells of the seed not unfrequently grow into hairs; cotton consists, for
example, of the long woolly hairs which clothe the seed of Gossypium; in some cases
only a pencil-like tuft of long hairs is developed, as in jisclepias syriaca. The epidermal
cells of some seeds, as the Plax, Quince, Plantago Psyllium, arenaria, and Cynops, contain
layers of cellulose which have become converted into mucilage, swell up strongly with
water, become separated, and envelope the seeds when moist in a layer of mucilage.
Pericarps which are indehiscent and which contain small seeds not unfrequently assume
a character closely resembling that of the testa of the seeds of dehiscent fruits; and
this is especially the case with the achenium and caryopsis, which are hence popularly
called seeds. The corona of hairs which serves as an apparatus for the dissemination of
many seeds through the air is frequently developed in the caryopsis as an appendage of
the pericarp (as the pappus of Compositae, which properly replaces the superior calyx).
The wings answering the same purpose which are formed during the development of
the testa, of some seeds in dehiscent fruits (seen in an especially beautiful manner in
Bignonia) recur again on the pericarp of indehiscent fruits (as in Acer). The muci-
laginous epidermis spoken of above of the seeds of dehiscent fruits recurs in the epi-
dermis of the carcerulus of Sal-via and other Labiatae, &c. These and a number of
other facts show that all that is essentially required in the development both of the

^ [For other recent attempts to classify fruits, see Dickson, Brit. Assoc. Rep. 1871, also Nature,
vol. IV, p. 347, and Journ. of Bot. 1871, p. 310; McNab, Nature, vol. IV. p. 475; Masters,
Nature, vol. V. p. 6 ; and Gray, Structural Botany.]

6 1 8 PHA NER OGA MS.

pericarp and the testa is to furnish means for the dissemination of the seeds in various
ways; structures which are morphologically very different thus attaining the same
physiological development, while those which are morphologically similar attain the
most various physiological development. A detailed enumeration is therefore more in
the province of physiology and biology than in that of morphology and classification.
(See Book III.)

To complete the subject of nomenclature, it only remains to remark that the part
of the seed where it has become detached from the funiculus — usually easily distin-
guished after falling out — is termed the Hilum or umbilicus. The micropyle is often
also to be recognised, lying, in anatropous and campylotropous seeds, close beside the
hilum (as in Faba, Phaseolus, and Corydalh), generally as a wart pitted in the middle.
When outgrowths occur on the seed, either along the raphe, as in Chelidonium majus,
Asarufn, Viola, &c., or as a cushion covering the micropyle as in Euphorbia, they are
variously called Ci^est, Strophiole, or Caruncle. The j4ril which envelopes the base of the
ripe seed or the entire seed as a fleshy succulent mantle and is easily removed from the
true firm testa has already been described in detail.

CLASS XI.

MONOCOTYLEDONS.

The Seed of Monocotyledons usually contains a strongly developed endosperm
and a comparatively small embryo ; and this is exhibited in an especially striking
manner in large seeds, such as those of Cocos, Phoenix, Phyielephas, Crinum, &c.
In the Naiadese, Juncaginese, Alismaceae, and Orchideae, the endosperm is wanting
from the first ; and in the Scitaminese, where it is usually wanting, it is replaced by a
copious perisperm.

The Embryo is usually cylindrical, fusiform, and sometimes considerably
elongated, and is then also curved spirally {e.g. in Potamogeton and Zanichellia) ;
its form is not unfrequently that of an erect or inverted cone, in consequence of
a considerable thickening of the upper end of the cotyledon. The axis of the
embryo is generally very short and small in comparison to the cotyledon; in
the Helobise on the contrary the axial portion of the embryo forms the greater
part of it. At the posterior end of the axis is the rudiment of the primary root,
in addition to which two or more lateral roots also originate in Grasses, which, like
the primary one, are surrounded by a root-sheath (Fig. 124, see also p. 588). The
embryo of Grasses is also distinguished by the presence of the Scutellum, an
outgrowth of the axis beneath the cotyledon, which envelopes the whole of the
embryo like a mantle, and forms a thick peltate plate on the posterior side where
it is in contact with the endosperm \ In the Orchideae, Apostasiaceae, and Bur-

^ [Van Tieghem (Ann. des Sci. Nat, 5th series, vol. XV, 1872) gives a useful summary of the
various views which have been held with respect to the homology of the parts of the embryo of
Grasses, He regards the scutellum as the cotyledon, and what Sachs considers the cotyledon as only
its strongly developed ligule. According to Hegelmaier (Bot. Zeitg. 1874) the cotyledon of Grasses
consists of two parts ; the one forms the scutellum, the other forms a sheath round the plumule.]

MONOCOTVLEDONS.

619

manniacege, the parts of the embryo of the ripe seed are not differentiated; it
consists of a round mass of tissue on which the plumule is developed only during
germination.

Germma/i'on^ either begins at once with the lengthening of the roots — their

Fig. 419.— Germination of Phcenix dacfyli/cra : I transverse section
of the dormant seed ; III— IV different stages of germination {IV the
natural size) ; A transverse section of the se&d at xx in IV; B transverse
section at xy, C at jz z; e the horny endosperm, j the slieath of the
cotyledon, st its stalk, c its apex developed into an organ of absorption
vk'hich gradually consumes the endosperm and at length occupies its
place, 7u the primary root, 7v' secondary roots, b' b" the leaves which
succeed the cotyledon, b" becomes the first foliage-leaf: in B and C its
folded lamina is seen cut across.

Fig. 420.— Plant QiPolygoiiatum multifloriiin
in its second year ; B its stem magnified, iv the
unbranched primary root, w' lateral roots spring-
ing from the stem st, I foliage-leaf of the second
year, k bud, c the scar where the cotyledon was
attached, i and 2 scars of the first sheath-leaves
which precede the foliage-leaf I. /, // the suc-
ceeding sheath or cataphyllary leaves of the bud
in B. (C/. Fig. 143)

protrusion causing in Grasses the rupture of the root-sheath which envelopes them,
and which remains attached to the axis of the embryo as the coleorhiza (Fig. 123).
— or, as is more commonly the case, the lower part of the cotyledon lengthens,

See Sachs, Bot. Zeitg. 1862 and 1863.

620 PHANEROGAMS.

and pushes the end of the root, together with the plumule which is enveloped
by the sheath of the cotyledon, out of the seed (Fig. 419), while its upper part
remains in the endosperm as an organ of absorption, until the endosperm is
consumed. In Grasses, however, the whole of the plumule projects from the seed,
the scutellum only remaining behind in it, in order to convey to the embryo the
food-material contained in the endosperm.

The growth of the primary root of Monocotyledons soon ceases even when
it is very strongly developed during germination, as in Palms, Liliacese, Zea, &c.;
lateral roots are produced in its place, springing from the axis, which are stronger
the higher up they are produced on it. No such permanent root-system is
developed from the primary root of Monocotyledons as is found in Gymnosperms
and in many Dicotyledons; sometimes no roots at all are produced, as in some
Orchidaceous saprophytes destitute of chlorophyll (as Epipogiurn and Corallorhiza),
which never possess any roots.

The plumule of the embryo is usually completely enclosed in a single
sheath-like structure, the first leaf or cotyledon, which developes either into a
sheath-like cataphyllary leaf or at once into the first green foliage-leaf of the
young plant (as in Allium). Within the cotyledon there is generally a second
and sometimes (in Grasses) a third and fourth leaf, which protrude on germination
out of the sheath of the cotyledon, increasing by intercalary growth at their base ;
these and the leaves which are formed subsequently are larger the later they are
formed on the growing axis. The axis usually remains very short during germi-
nation without forming any distinct internodes {Allitmi, Palms, &c.), or it lengthens
more rapidly and becomes segmented into evident internodes (Zea and other
Grasses).

The increase in strength of the plant may take place by the powerful growth
of the axis of the embryo itself, so that this at length forms the primary stem
of the mature plant bearing the organs of reproduction, as for instance in most
Palms, Aloes, Zea, &c. If the axis of the embryo remains short while it increases
in strength, it may grow considerably in thickness and form a tuber (Fig. 420),
or, if the bases of the leaves become thick (as in Allium Cepa), a bulb. If the
axis of the embryo itself developes into the primary stem, whether into an upright
one or a creeping rhizome, it first of all takes the form of an inverted cone, which
is elongated or abbreviated according to the length of the internodes. This
peculiarity, which belongs to most Monocotyledons in common with Ferns, depends
on the absence of any subsequent increase in thickness; the portions of the stem
first formed retain their size, while each successive portion is larger ; the diameter
of the stem is therefore so much larger the nearer it is taken to the apex. As
long as this growth proceeds, the stem continues to grow stronger ; but sooner
or later there comes a time when every portion of the stem acquires the same
thickness as the previous one ; the stem then becomes cylindrical, or, if it is
compressed like some rhizomes, still with a uniform breadth. The lateral shoots
exhibit the same peculiarity when they spring low down from the primary stem
(as in Aloe, Sec). But the primary shoot which springs from the embryo not
unfrequendy disappears after producing lateral shoots which grow more vigorously
than it, and these again transfer the further growth to new shoots, which now

MONOCO TVLEDONS.

621

produce from generation to generation thicker axes, larger leaves, and stouter
roots, until at length a condition again results in which each successive generation
of shoots produces others of equal strength. If the portions of the axes of the
shoots beneath the points where the shoots of the next order arise are persistent,
sympodia arise (as represented in Fig. 143); but frequently each shoot entirely
disappears after producing one of the next order, as for instance in our native
tuberous Orchids (Fig. 158), or in the Crown-Imperial (Fig. 421), or Autumn
Crocus (Fig. 422) \

The normal Mode of Branching of Monocotyledons is always monopodial
and usually axillary ^ ; a bud is generally formed in the axil of each leaf, but often
does not unfold, so that the number of branches visible is often less than that
of the leaves (as in Agave, Aloe, Draccena, Palms, many Grasses, &c.). But some-
times several buds are formed in the axil of a leaf, and if the insertion of the
leaf is broad these are placed side by side, as occurs in many bulbs (Fig. 130).

Fig. 421. — Bulb of Fritillaria imperialis in November : A longitudinal section of the whole bulb reduced, z z the
coalescent lower portions of the bulb-scales, bb their free upper portions; the scales enclose a cavity /which contains the
decayed flower-stem : next year's bud is formed in the axil of the innermost scale ; its first leaves will form the new bulb, while
its axis will develope into the flower-stem ; the root w springs from the axis of this bud. B longitudinal section of the apical
region of next year's bud, s apex of the stem, bb' b" youngest leaves.

In Musa a number of flowers even stand side by side in the axil of a bract,
and in Musa Ensete two rows one over the other. In the Spadiciflorae the bracts
are often absent ^ and the ebracteate flowers stand on the rachis of the inflorescence,
but are distinctly lateral in their origin. This is also the explanation of the
branching of Lemna, which does not in general form any foliage-leaves, but
the vegetative body of the plant consists of disc-like or swollen portions of the
axis containing chlorophyll which branch laterally out of one another, and are
connected together only by slender stalks, or soon separate. The plane of rami-
fication coincides with the surface of the water on which they float; each shoot

^ Further details of the great variety of modifications of these processes of growth will be found
in Irmisch, Knollen und Zwiebelgewachse (Berlin 1850), and Biologic und Morphologic der
Orchideen (Leipzig 1853).

^ According to Magnus (Bot, Zeitg, 1869, p, 770) the flower of Naias occupies exactly the place
of the first leaf of a branch; but it appears from p. 771 as though the flower and the shoot that
i)ears it were the bifurcations of a dichotomy.

^ Compare under Dicotyledons, p. 638.

622

PHANEROGAMS.

produces only one or a pair of opposite lateral shoots, and the branching is therefore
distinctly cymose, sympodial, or, as in Lemna irisulca^ dichasial.

Besides the formation of shoots by the branching of the axis, adventitious
shoots also sometimes occur on leaves which perform the function of gemmae ;
as for instance on the margins of the leaves of Hyacitilhus Pouzolsii and some
Orchids (Doll, Flora, p. 348)^ The large gemmae which appear very regularly
at the point of junction of the leaf-stalk and lamina, and at the base of the lamina
of Atherurus krnatus, are especially striking. The small bulbs on the stem of

Fig. 422.— The underground part of a flowering plant of Colchicum autiannale : A seen in front and from without, k
the conn, s' , s" cataphyllary leaves embracing the flower-stalk, 7uk its base from which proceed the roots 7v ; B longitudinal
section, hhs. brown skin which envelopes all the underground parts of the plant, st the flower- and leaf-stalk of the
previous year which has died down, its swollen basal portion fc only remaining as a reservoir of food-materials for the
new plant now in flower. The new plant is a lateral shoot from the base of the corm fc, consisting of the axis from the base
of which proceed the roots ■»', and the middle part of which (k') swells up in the next year into a corm, the old corm k
disappearing ; the axis bears the sheath-leaves s, s', s" and the foliage-leaves I', I" ; the flowers b. b' are placed in the
axils of the uppermost foliage-leaves, the axis itself terminating amongst the flowers. The foliage leaves are still small
at the time of flowering ; in the next spring they emerge from the ground together with the fruits ; the portion of the
axis k then swells up into the new corm, on which the axillary bud k'' developes into the new flowering plant, while the
sheath of the lowermost foliage-leaf is changed into the brown enveloping skin.

^ [On the buds developed on the leaves of Malaxis which exhibit a striking resemblance to the
ovules of Orchideoe, see Dickie, Journ. Linn. Soc. vol. xiv. pp. i and 180. Dr. Dickie considers the
structure of these buds to favour the theory that the ovule is homologous to a bud, the nucellus-like
body of the bud corresponding to an axis. See also Henslow on Malaxis, Mag. Nat. Hist. vol. I.
1829, pp. 441, 442.]

MONOCOTYLEDONS.

623

Lilium hulhiferum are, on the other hand, normal axillary shoots, and probably
the same is the case with those on the infloresence of some species of Allium.
Adventitious buds are stated by Hofmeister to occur on the roots of EpipacHs
microphylla.

The Leaves of Monocotyledons are seldom verticillate, though this occurs in
the foliage-leaves of Elodea and the bracts of Alisma ; they are very commonly

Fig. 423.— Cyot7«- verftus: A the bulbous stem seen from above, B seen from below, C from the side and cut
throuj^h lengthwise ;yyy the circular line of scars of the cataphyllary leaves, k k the corms which grow in their axils ;
h the base of the decayed flower- and leaf-stem, by its side (lik in C) next year's bud, from which a new corm and
flower-stem will be produced ; D longitudinal section through this bud, n n its cataphyllary leaves, 1 1 foliage-leaves,
h bract, / perianth, a anthers, k a bud in the axil of a foliage-leaf.

arranged alternately in two rows, as in Graminese, Iridese, Phormium^ Clivta, Typha^
&c. This arrangement either prevails over the whole shoot together with its
secondary shoots, or occurs only at first, and then passes into spiral arrangements,
which very commonly lead to the formation of rosettes radiating on all sides, as
in Aloe (see Fig. 152, p. 193), Agave, Palms, &c. The arrangement with the angle

Fig. 424.— Bud in the inside of a bulb of Alltunt Cepa, the scales having been removed ; st the short flat base of the
stem on which the bulb-scales are inserted ; ImA lamina, sh the sheath of the foliage-leaves still short ; in B the outer
leaves have been removed, and an axillary bud k" has made its appearance in addition to the terminal bud k'.

of divergence Yg is much rarer, but occurs in some species of Aloe^ Carex, Pan-
daftus, &c. Spiral arrangements with a smaller divergence than V3 ^-^so occur
sometimes; as e.g. in Musa (in M. rubra the angle is, according to Braun, 7^ in
the foliage-leaves, "/n i^^ the bracts), and Coslus (where the angle of the foliage-
leaves is from y^ to V5), &c. The axillary shoots of Monocotyledons mostly begin
with a leaf in close contact with the primary axis and with its back turned towards

624

PHANEROGAMS,

it, and usually bicarinate. Of this character must be considered, for instance, the
upper pale of the flower of Grasses, which is itself an axillary shoot of the lower
pale. When the phyllotaxis of successive orders of ^oots is alternate in two rows,
the result of this arrangement is that a whole system of shoots is bilateral, or may
be divided by a plane which bisects the leaves (as in Po/amogeion, Typha, &c.).

The mode of insertion of the cataphyllary and foliage-leaves, and very
often that of the hypsophyllary leaves (as for instance that of the spathe which
is of common occurrence), is entirely or at least generally amplexicaul, and the
lower part of the leaf is in consequence sheathing ; and this is evidently con-
nected with the want of stipules, which are so frequent
among Dicotyledons. The cataphyllary and many of the
hypsophyllary leaves are usually reduced to this sheathing
part, which generally passes immediately into the green
lamina in the case of the foliage-leaves ; but in Scitami-
nese, Palmaceae, Aroideae, and some others, a long and
comparatively slender stalk developes between the sheath
and the lamina. When the leaf-stalk is absent, and the
lamina sharply marked off from the sheath, a Ligule is
not unfrequently present at the point where the two meet,
as in Grasses and Allium (Fig. 425).

The lamina is generally entire and of a very simple
form, commonly long and narrow (ligulate), rarely roundish
and disc-shaped {e.g. Hydrocharis), or cordate or sagittate
(as in Sagittaria and some Aroideae). Branching of the
lamina is a rather rare exception among Monocotyledons;
and then takes the form either of lobes from a broad
common base or less often of deep divisions, as in some
Aroideae {e.g. Amorphophallus, Fig. 141, Atherurus and
Sauromatuni). The division of the compound and pinnate
leaves of Palms is not due to a branching occurring at an
early stage, but to a splitting which takes place on unfold-
ing, and is caused by the drying up of certain strips of.
tissue within the lamina, which is at first sharply folded up.
The formation of the tendrils of Smilax appears, on the
other hand, to depend on actual branching of the leaf-stalk.

The Venation of the foliage-leaves differs from that of
most Dicotyledons, in that the weaker veins do not gener-
ally project on the under side of the leaf, but run through the mesophyll ; in the
smaller leaves there is even no projecting mid-rib. The mid-rib is, on the other
hand, strongly developed in the large stalked leaves of the Spadiciflorae and Scita-
mineae, and is permeated by a number of fibro-vascular bundles. When the leaf is
ligulate and its insertion broad, the fibro-vascular bundles run nearly parallel to one
another ; in broader leaves without a conspicuous mid-rib they describe curves from
the median line to the margins (as in Convallarid). But when a strong mid-rib
occurs in a broad lamina, as in Musa &c., the fibro-vascular bundles which run
through it give off laterally smaller thin bundles which run parallel to one another

Fig. 425.— a leaf of Allium
Cepa divided lengthwise ; z the
thickened base of the sheath,
which persists as a bulb-scale
after the upper part of the leaf
has died down, j the membranous
part of the sheath, / the hollow
lamina, h cavity of the lamina, i'
inner side of the lamina, x ligule.

MONOCOTYLEDONS .

625

in large numbers to the margin of tlie leaf. These parallel transverse nerves are
sometimes united into a lattice-like network by short straight anastomosings (as in
Ali'sma, Cos/us, and Ouzn'randra, the mesophyll being absent within the meshes of
the latter). It is only rarely (as in some Aroideae) that projecting lateral veins
are given off from the mid-rib, a finer reticulated venation springing from them.

The Flower of Monocotyledons usually consists of five alternating isomerous
whorls ; viz. an outer and an inner perianth-whorl, an outer and an inner whorl
of stamens, and a carpellary whorl, which is succeeded by a second carpellary whorl

FIC. 426.— Diagram oi Scirpus (Cyperacese).

Fig. 427.— Diagram of Iridere.

Fig. 428.— Diagram of Musaceae.

only in Alismacese and Juncagineae. The most common typical flora formula is
therefore S^Pn^ln^n^ni^-n)- It is only in the Hydrocharideae and a few other
isolated cases that the number of whorls of stamens is larger. Where in other
cases, as Bulomus, an increase of the typical number of stamens occurs, this takes
place by dedouhlement without any increase of the number of whorls (Fig. 431 A).

The number of members in each whorl is two {S^ P^ 8/^+2 C^), in only a very
few cases scattered through the most different families {e.g. in Maianthemiim and some
Enantioblastae ; it is sometimes four or five (occasionally in Paris quadrifolia and

FIG. 429.— Diagram of Zingiberaceae ; A Hedychmm (after Le Maout
and Decaisne), B Alpinia (after Payer).

Fig. 430. — Diagram of Catma (Musaceae), after Payer.

in some Orontiacese) ; but the usual nymber of members in each whorl is three,
and the typical formula therefore S^ P^ St^^^ ^(+3)- In the large section of Lilii-
florae, in some Spadiciflorae, and in many Enantioblastae, Juncagineae, and Alis-
maceae ^, this typical floral formula is at once obtained empirically ; in most others
particular members or whorls are wanting, but the abortion of these is generally
at once evident from the position of those that are present. In the Scitamineae
with only one or even with only half an anther (Fig. 429, 430) the rest of the
members of the androecium are present or only partially deficient, but are trans-

^ The dimerous flower of Po^rtmoo-e/o« {S^P^St^^ ^C^ (see Hegelmaier, Bot. Zeitg. 1870, p. 287)
differs from the typical formula only to this extent, that the four carpels arise simultaneously, and
are placed diagonally to the preceding pairs.

s s

626

PHANEROGAMS.

formed into petaloid staminodes. It has already been pointed out how the flowers
of Gramineae and Orchideae can be traced back to the trimerous pentacyclic type ;
the theoretical diagrams here given (Figs. 426 — 433) will answer the same purpose
for some of the other more important families.

If the pentacyclic flower with the formula S^Pn^L^n^ni+n) is considered as
the typical one for Monocotyledons, it will be seen that the great majority of
families the number of whose parts deviates from this type do this only by
the suppression of single members or of whole whorls, the typical position of
those that still remain with respect to one another not being disturbed. The

Fig. 431. — Diagram of Alisniacese ; A Bntomus, B Alisnia.

FIG. 432.— Diagram of Triglochm (Juncagineae).

variety in the forms of flowers in this class is therefore brought about almost
entirely by abortion ^ ; and it is not uncommon for abortion to be carried to such
an extent in Monocotyledons that nothing is left at last of the whole flower but
a single naked ovary or a single stamen, as happens frequently in Aroideae. In
these cases a similar explanation of the relationships of the parts of the flower is
rendered possible and even evident by the occurrence of flowers with the actual
typical structure, and by a complete series of transitions caused by partial abortion.
It is especially in small closely crowded flowers, as those of Spadiciflorae, Glumi-
florae, &c., that so great a reduction of the typical number of members is observed ;

Fig. 433.— Diagram oi Cymiiostachys (Aroideje), after Payer.

while in larger and more isolated flowers the number of members in each whorl
is usually complete or even excessive (as Butomus and Hydrocharis), and deviations
usually result from petals (or petaloid staminodes) being formed in the place of
fertile stamens (e.g. Scitamineae). With reference to the abortion which is often
carried to so great an extent in small flowers, it may in certain cases even be
doubtful whether in an assemblage of stamens and carpels we have a single flower
or an inflorescence consisting of several flowers reduced to a very simple state by
abortion, as for example in Lemna.

* Compare what was said on Abortion at p. 222 and in the Introduction to Angiosperms.

MONOCOTYLEDONS, 62J

When both the perianth-whorls are well developed, they are usually similar in
structure; in large flowers they are generally delicate and petaloid and either
brightly coloured or not (Liliaceae, Orchidese, &c.); in small flowers on the con-
'trary they are firm, dry, and membranous, as in Juncaceae, Eriocaulonese, &c.
Sometimes however the outer perianth-whorl is green and sepaloid, the inner whorl
larger, delicate, and petaloid {Canna, AUsma, Tradescantid)\ in the very small and
closely crowded flowers of the Glumiflorae, the perianth-leaves, when present, take
the form of hairs (the setae of Cyperaceae) (Fig. 426), or of small membranous scales
(the lodicules of Grasses).

The Stamens generally consist of a filiform filament and a quadrilocular anther ;
though variations frequently occur, especially in the form of the filament and
connective. Among the most striking deviations from the ordinary type are the
petaloid staminodes of Cannaceae and Zingiberaceae. It has already been pointed
out (pp. 491, 541), that the foliar nature of the stamens is subject to an exception
in the Naiadeae (at least in Naias) according to the researches of Magnus. The
stamens of Monocotyledons scarcely ever branch, as is often the case in Dicoty-
ledons ; and this corresponds to the customary absence of branching in the other
foliar structures also. If the diagram of the flower of Canna (Fig. 430), drawn
according to Payer's description, is correct \ the petaloid staminodes are branched.

The GyncBceum has usually a trilocular ovary ; less often it is tricarpellary but
unilocular ; in both cases it may be either superior or inferior, but the latter occurs
only in plants with large flowers [Hydrocharis, Iridese, Amaryllideae, Scitamineae,
Orchideae, &c.). The formation of three or more monocarpellary ovaries is limited
to the alliance of the Juncagineae and Alismaceae, in which the ordinary number
of members and of whorls of the gynaeceum is also exceeded, reminding one of
the Polycarpae among Dicotyledons.

Adhesion and displacement are not so common in the flower of Monocoty-
ledons, and usually not so complicated as among Dicotyledons; among the most
striking phenomena of this nature are the formation of the gynostemium of Orchids ;
the cohesion of the six similar perianth-leaves into a tube in Hyacinthus, Con-
vallarta, Colchtcum, &c. ; and the epipetalous and episepalous position of the
stamens in the same plants and in some others. Adhesion of the stamens to the
calyx or corolla occurs much less constantly in particular families among Mono-
cotyledons than among Dicotyledons.

Terminal flowers :to a leafy primary shoot occur very rarely among Mono-
cotyledons {e.g. in Tulipa)) terminal inflorescences are more common. The flower
acquires a tendency to zygomorphism, especially as it increases in size ; but this is
often only feebly indicated, and attains its highest development in Scitamineae and
Orchideae.

The Ovules of Monocotyledons usually spring from the margins of the carpels,
rarely from their inner surface (as in Butomus) ; the single orthotropous ovules of
Naias (according to Magnus) and Typha (Rohrbach) arise by the transformation of

^ According to Eichler's masterly description of the flower of Canna (Bot. Zeitg. 1873) the
relations of the androecium are not quite those expressed in the diagram Fig. 430. Eichler gives the
formula as being ^3 P3 ^/{o) + (o.i2.i) Q.

S S 2

628 PHANEROGAMS.

the end of the floral axis itself (see pp. 492, 571); in Lemna and in sonne Aroideae
one or more ovules stand at the bottom of the cavity of the unilocular ovary. The
prevailing form of the ovule is anatropous ; but in Scitamineae, Gramineae, and some
other orders, campylotropous ovules occur ; in the Enantioblastae and a few Aroideae
they are orthotropous, either erect or pendulous. The nucellus is almost without
exception enclosed in two envelopes {Crinum however forms an exception).

The Embryo-sac ^ generally remains surrounded by one layer of the tissue of the
nucellus till the time of fertilisation; the apex is sometimes destroyed so that the
embryo-sac projects (as in Hemerocallis, Crocus^ Gladiolus, &c.) ; but, on the other
hand, the apex not unfrequently remains as a cap of tissue covering the top of
the embryo-sac (as in some Aroideae and Liliaceae). In Orchideae the growing
embryo-sac completely destroys the layer of tissue that envelopes it together with
the apex of the nucellus; and this happens after fertilisation in all the other
Monocotyledons that possess an endosperm, and in this case the embryo-sac some-
times advances even to the inner integument and destroys it {Allium odorans,
Ophrydeae).

In the greater number of Monocotyledons a copious development of endo-
sperm-cells in the parietal protoplasm follows quickly after fertilisation. They
soon unite into a layer of tissue and divide tangentially, until at length the embryo-
sac is filled with radial rows of cells the result of division. Narrow embryo-sacs
are filled up by the growth of the first endosperm-cells ; but sometimes the cells
formed by free cell-formation in the parietal layer of protoplasm constitute at first
a loose mass which fills up the embryo-sac and only closes up into a tissue at
a later period {e. g. Leucojum, Gaged). The narrow embryo-sac of Pistia is filled
up by a row of broad disc-shaped cells which lie in it like transverse compartments
and are perhaps the result of division of the sac itself. In some Aroideae only
a part of the embryo-sac is filled with endosperm, the rest remaining empty.

The endosperm still continues to grow after it has filled up the embryo-sac,
the seed which it fills increasing also in size. It has already been mentioned how
considerable this growth is in Crinum (p. 586).

In all those Monocotyledons which form an endosperm (albuminous), it becomes
closed up into a continuous tissue enveloping the embryo before this has completed
its growth. By the growth of the embryo a part of the endosperm which surrounds
it is again forced aside ; and on this displacement depends the lateral position of
the embryo in Grasses by the side of the endosperm, and the absence of this latter
in some Aroideae. But in all the other Monocotyledons which have no endosperm
(exalbuminous), Naiadeae, Potamogetoneae, Juncagineae, Alismaceae, Cannaceae, and
Orchideae, its formation is altogether suppressed, or transitory preparations for it
only take place.

On the first origin of the embryo reference must be made to what was said in
the Introduction to Angiosperms (p. 589) ; there are many points which are still
doubtful in the formation of the plumule, scutellum (in Grasses), and root, from the
original small-celled mass of tissue of the embryo.

^ See Hofmeister, Neue Beitrage (Abhandl. der konigl. Sachs. Gesellscli. der Wissensch.
vol, VII) ; also mpra, p. 576.

MONOCOTYLEDONS. 629

With respect to their Histology '^^ Monocotyledons differ from Dicotyledons and
Gymnosperms chiefly in the course of the fibro-vascular bundles in the stem, and in
the want of a true cambium-layer. A number of common bundles (/. e. those com-
mon to the stem and leaves) enter the stem side by side from the broad insertions
of the leaves, pass obliquely downwards into it, and then again bend outwards as
they descend, approaching gradually the surface of the stem. The common bundle
is usually thickest and most perfectly developed at the curved portion which lies
deepest in the stem, while the limb which bends upwards into the leaf becomes thinner
and simpler upwards, and the descending limb of the bundle behaves similarly down-
wards. Hence a transverse section of the stem which cuts through the different
descending limbs at different heights in their course shows bundles of different structure
and of various sizes. A radial longitudinal section through the bud or through mature
stems with short internodes (as Palm-stems, thick rhizomes, bulbs, &c.) shows how the
bundles which descend from different leaves, the curves of which lie at different
heights, cross one another radially, some of them bending inwards where others are
already turning outwards. In elongated internodes, as for instance those of the stalks
of Grasses and of some Palm-stems (like Calamus), the long scapes of Allium, &c., the
bundles run nearly parallel to one another and to the surface ; the curves and inter-
sections of the bundles may be easily distinguished at the apex of such stems, and localise
themselves in the transverse plates or nodes which do not elongate between each pair
of internodes. The nodes are not unfrequently traversed by a network of horizontal
bundles ; this is very conspicuous in the Maize.

The course of the fibro-vascular bundles which has now been described renders
impossible the separation of the fundamental tissue of the stem into pith and cortex
in the sense in which this occurs in Conifers and Dicotyledons. The parenchymatous
fundamental tissue fills up homogeneously the spaces between the bundles which are
generally numerous ; but a separation takes place not unfrequently into an outer peri-
pheral layer and an inner region, a layer of tissue being formed between the two
the cells of which are thickened and lignified in a peculiar way (as for instance in
most thickish rhizomes, in the hollow scape of Allium, &c.).

In consequence of their not being parallel, and of their scattered distribution in the
transverse section of the stem, the descending bundles of Monocotyledons have not the
power of coalescing into a closed sheath by connecting bands of cambium (interfasci-
cular cambium), as is the case in other Phanerogams. In correlation with this the layer
of cambium between the phloem and xylem is also absent ; the fibro-vascular bundles
are closed. When a portion of the stem ceases to grow in length, the whole of the
tissue of the bundles becomes transformed into permanent tissue (see e.g. Fig. 92,
p. no) ; and there is in consequence usually no subsequent increase in thickness; each
portion of the stem, when once formed, maintains the thickness which it had already
attained within the bud near the apex of the stem. But in Draccena, Aloe, and Yucca, a
renewed increase of thickness begins afterwards at a considerable distance from the
apex of the stem, which may even continue for centuries and may cause a considerable
though slow increase in its circumference. But this subsequent growth in thickness
takes place in a way quite different from that which occurs in Gymnosperms and Dico-
tyledons ; — a layer of the fundamental tissue parallel to the surface of the stem becomes
transformed into meristem which continually produces new closed fibro-vascular bundles,
and between them parenchymatous fundamental tissue (Fig. 104). A more or less
evidently stratified network of slender anastomosing bundles is thus formed, the posi-
tion and connection of which is easily recognised on stems which have been exposed
to the weather, and in which the parenchyma which fills up the interstices has

^ Von Mohl, Bau des Palmenstammes, in his Vermischte Schriften, p. 129. — Nageli, Beitrage
zur wissensch. Bot. Heft. I. — Millardet, Memoires de la Soc. Imp. des Sci. Nat. de Cherbourg,
vol. XI, 1865. — [De Bary, Vergleichende Anatomic der Vegetationsorgane, 1877.]

630 PHANEROGAMS.

decayed. This network of closely-placed closed fibro- vascular bundles now forms a
kind of secondary wood which surrounds like a hollow cylinder the space in which
the original fibro-vascular bundles of the stem run isolated and loose in the form of
long threads. This thickening ring of the arborescent Monocotyledons resembles
the secondary woody mass of Conifers and Dicotyledons in the fact that it belongs
altogether to the stem and has no genetic connection with the leaves, differing in this
from the original common bundles. An exception to the ordinary structure of Mono-
cotyledons occurs in submerged water-plants {Hydrilla and Potamogeton), in which,
according to Sanio (Bot. Zeitg. 1864, p. 223, and 1865, p. 184), an axial cauline bundle
in the stem lengthens continuously, while the foliar bundles do not unite with it till a
later period, a peculiarity which recurs in some dicotyledonous water-plants, and re-
minds one of the corresponding processes in Selaginella.

The Systematic Classification ^ of the sub-sections of Monocotyledons here adopted is
that of A. Braun (in Ascherson's Flora of the province Brandenburg, Berlin 1864) ; but
with the variation that the order Helobiae there given is broken up into a series of
orders. In the short diagnoses of the orders only a few of the characters are specified
which are most important from a systematic point of view ; the figures placed within
brackets refer to those attached to the families belonging to the order in which the
characters named are present or absent. A complete account might have been given of
the characters of the separate families of Monocotyledons ; but since a similar treatment
of the class of Dicotyledons would have far exceeded our limits, the mere enumeration of
the families must, for the sake of uniformity, suffice.

SERIES I.— Helobi^.

Water-plants ; seed with little or no endosperm, but a strongly developed hypo-
cotyledonary axis to the embryo. The number of parts of the flower usually varies
from the ordinary type of Monocotyledons.

Order i. CentrospermaB (so named from the central position of the seed in
(i) and in Naias). Flowers imperfect, very simple, usually without a perianth; in
(i) consisting of two stamens and a unilocular ovary (containing from i to 6 basilar
ovules) surrounded by a sheath (perianth or spathe) ; ovary in (2) unilocular, usually
one-seeded ; seed with but little endosperm. The Lemnaceae consist of small
branched leafless floating vegetating bodies, generally with true pendent roots; the
Naiadeae are slender branched long-leaved submerged plants; this family is not
definable systematically, and should be split up into several. (The Lemnaceae
should perhaps be united to the Aroideae.)
Families: i. Lemnaceae.
2. Naiadeae.

^ [The systematic classification adopted in this book is not one v^hich the reader will find
followed in any standard English work, either as respects Monocotyledons or Die tyledons. The
work now generally adopted as containing the most satisfactory system of distribution of the vegetable
kingdom into classes, orders, and genera, is Bentham and Hooker's Genera Plantarum (London, 1862-
1873)^ which is however at present only completed so far as to include the Gamopetalse with
inferior ovary. In Dr. Hooker's edition of Le Maout and Decaisne's Traite Generale de Botanique
(London, 1873) will be found the outlines of this classification completed as far as relates to the
classes and orders. De Candolle's Prodromus Systematis Naturahs Vegetabilium in 1 7 vols. (Paris
1818-1873) contains a description of every known species of Dicotyledons; Walpers' 'Repertorium*
and 'Annales ' serving as supplements to the earlier volumes, which are far less complete than the
later ones. For an admirable epitome and illustrations of the character of each of the natural
orders see also Oliver, Illustrations of the Principal Natural Orders o-f the Vegetable Kingdom ;
London, 1874.]

MONOCO rVLEDONS. 63 1

Order 2. Polycarpae. Flowers pentacyclic or hexacyclic (2, 3); whorls in (i)
dimerous and decussate, with four monocarpellary ovaries placed diagonally; in
(2, 3) trimerous, or with a larger number of stamens and carpels (see p. 626) ; the
gynaeceum consists of three or more monocarpellary ovaries, which are one- or
more-seeded ; endosperm absent. Perennial floating water- or upright bog-plants,
with large lattice-veined or long narrow (2) leaves.
Families: i. Potamogetoneae.

2. Juncagineae.

3. Alismaceae.

Order 3. Hydrocharideaa. Flowers dioecious or polygamous, with trimerous
whorls, and perianth consisting of both calyx and corolla ; male flowers of from one
to four whorls of fertile stamens and within these several whorls of staminodes ;
female flowers with an inferior tripartite or six-chambered (3) many-seeded ovary ;
endosperm absent. Perennial submerged or floating water-plants with spiral or
verticillate (r) leaves.

Family i. Hydrocharideae ; with the subsections —

1. Hydrilleae.

2. Vallisnerieae.

3. Stratioteae.

SERIES II.— MiCRANTH^.

Land- or bog-plants; the individual flowers usually very small and inconspicuous,
but collected in large numbers in the inflorescence^ and almost always referable to the
dimerous or trimerous pentacyclic type.

Order 4. SpadiciflorsB. Inflorescence a spadix or panicle with thick branches
(4), generally enveloped in a large sometimes petaloid (i) spathe ; bracts small of
altogether absent ; perianth never petaloid, usually inconspicuous or altogether
abortive (1-3) ; sexual organs generally diclinous by abortion; fruit always superior
and often very large (2, 4) ; the seed mostly large or of an immense size and with
a very large endosperm ; embryo small, straight. Mostly large strong plants with
the stem strongly developed, chiefly above ground, and a great number of large
foliage-leaves; in (i, 3, 4) they have a broad branched or apparently pinnate or
compound lamina, a leaf-stalk and sheath, in (2) they are sessile, very long and
narrow.

Families: i. Aroideae.

2. PandanaceaSk

3. Cyclantheae.

4. Palmaceae.

Order 5. Glumiflorss. Inflorescence spicate or panicled, without a spathe;
flowers very small and inconspicuous, usually concealed among thickly-placed dry
hypsophyllary leaves (glumes or pales) (2, 3); perianth absent, or replaced by hair-
like structures or scales ; fruit superior, small, one-seeded, dry and indehiscent
^ (a caryopsis) ; embryo in (i) long and in the axis of the endosperm, in (2) by its
side and very small, in (3) also by the side of the endosperm, but considerably
developed and provided with a scutellum. Plants with persistent underground
elongated rhizomes, and upright foliage-leaves in two or three (2) rows; (i) should
perhaps rather be included in the fourth order.
Families: i. Typhaceae,

2. Cyperaceae.

3. Gramineae.

632 PHANEROGAMS.

Order 6. EnantioblastSB. Flowers in crowded (4) cymose inflorescences,
inconspicuous (i, 2), or conspicuous (3, 4), pentacyclic, and usually trimerous (in
(r, 2) often dimerous) ; perianth-whorls glumaceous in (i, 2), developed into calyx
and corolla in (3, 4) ; fruit a superior bi- or trilocular capsule with loculicidal
dehiscence ; ovule orthotropous, and the embryo (I^XdcrTrj) therefore opposite (epav-
Tios) the base of the seed (hilum). Plants with grass-like (1-3) or succulent habit (4).
Families: i. Restiaceae.

2. Eriocauloneae.

3. Xyrideae.

4. Commelynaceae.

SERIES III.— COROLLIFLORiE.

Both the perianth-whorls conspicuous, usually large and petaloid ; the two staminal
whorls completely developed or partially wanting by abortion, and then replaced by
staminodes; one carpellary whorl; the five whorls, with few exceptions, trimerous.

Order 7. LiliijElGrsB. Inflorescence very various, racemose or cymose; the
large flowers sometimes single. Flowers pentacyclic and trimerous, except a few
cases where they are dimerous, tetramerous, or even pentamerous; in (3) the
inner staminal whorl is wanting; perianth-whorls similar, in (i) inconspicuous
and membranous, but usually petaloid (2, 3, 5-8) and often large ; sometimes all
the six leaves are coherent into a tube (6 and elsewhere), often with epipetalous
and episepalous stamens; ovary superior in (i, 2), inferior in the other families,
usually forming a trilocular capsule or berry ; embryo surrounded by endosperm.
Plants of very various habit ; with strong woody stems increasing in thickness in
yi/oe, Tucca, and Draccena (2) ; more often with underground rhizomes, corms, or
bulbs, from which spring leafy annual shoots; leaves mostly long and narrow, in (4)
with a broad lamina and slender stalk.
Families: r. Juncaceae.

2. Liliaceae.

3. Iridese.

4. Dioscoreae.

5. Taccaceae.

6. Haemodoraceae.

7. Pontederiaceae.

Order 8. AnanasinesB. Flowers consisting of the typical five trimerous whorls;
outer perianth-whorl developed into calyx, inner one into corolla; ovary trilocular
and many-seeded, superior or inferior ; embryo by the side of the endosperm ;
leaves long, often very narrow.

Family: i. Bromeliaceae.

Order 9. Scitamineas. Floral whorls trimerous and zygomorphic ; both
perianth-whorls or only the inner one (3, 3) petaloid ; of the stamens the pos-
terior one of the inner whorl is abortive in (i), this alone being fertile in (2, 3)
(in 3 with only half an anther), while the rest are changed into petaloid staminodes
(see Figs. 428-430) ; fruit inferior, trilocular, a berry or capsule ; endosperm usually
absent, but replaced by a copious perisperm. Usually handsome, often very large
(i) leafy shrubby plants springing from a persistent rhizome, with large leaves,
generally divided into a broad lamina, leaf-stalk, and sheath.
Families: i. Musaceae.

2. Zingiberaceae.

3, Cannaceae.

DICOTYLEDONS. 633

Order 10. Gynandrae. The entire flower zygomorphic in origin and de-
velopment; by the torsion of the long inferior ovary (i) the anterior side of the
mature flower usually becomes posterior; both of the trimerous perianth-whorls
petaloid, the posterior leaf of the inner one (the labellum) generally provided
with a spur ; of the six typical stamens of the two whorls only the anterior ones
are eventually developed, and in (i) (with the exception of Cypripedium) the
anterior one of the outer whorl is alone fertile and has large anthers, the two
anterior ones of the inner whorl forming small staminodes ; but in Cypripedium it
is these latter that are fertile, the anterior one of the outer whorl forming a large
staminode ; in (2) the same occurs, or the three anterior ones are fertile ; filaments
of the fertile and sterile stamens coherent with the three styles into a gynostemium ;
pollen in single grains, tetrads, masses, or pollinia ; ovary inferior and unilocular
with parietal placentation (i) or trilocular with axile placentation (2) ; ovules
anatropous; seeds very numerous, very small, without endosperm, and with the
embryo undiff"erentiated. Small herbs or larger shrubby plants ; the tropical
Orchideae often epiphytal and furnished with peculiar aerial roots; our native
species perennial with underground rhizomes or tubers ; some Orchideae are
saprophytes destitute of chlorophyll, and a few have even no roots {Epipogium^
Corallorhiza).

Families: i. Orchideae.

2. Apostasiaceae.

The BurmanniacesB with cymose inflorescence, three or six fertile epipetalous
stamens, free tripartite style, and uni- or tri-locular inferior ovary, are allied to the
Gynandrae by their small seeds without endosperm and their undifferentiated
embryo ; and in this order, which consists for the most part of small plants, there
are some saprophytes destitute of chlorophyll.

CLASS XIL

DICOTYLEDONS.

The ripe Seed of Dicotyledons contains either a large endosperm and a small
embryo (as in Euphorbiaceae, Coffea, Myrtstica, Umbelliferae, Ampelideae, Polygon-
aceas, Csesalpinese, &c.) ; or the embryo is comparatively large, and the endosperm
occupies but a small space {e.g. Plumbagineae, Labiatse, Asclepiadeae, &c.); or,
thirdly, the endosperm is entirely wanting, and the embryo fills up the whole of the
space enclosed by the testa, and thus, when ripe, often attains a very considerable
size {e.g. ^s cuius, Juglans, Cucurbita, Tropceolum, CupuHferae, Leguminosae, &c.);
though in small seeds it still remains of moderate dimensions (as in Cruciferae, Com-
positae, Rosiflorae, &c.). The absence of endosperm generally results from its
absorption by the rapid growth of the embryo before the ripening of the seed;
only in a very few cases is it rudimentary from the first {Tropceolum, Trapd). In
most of the Nymphaeaceae and in the Piperacese the embryo and the endosperm
which surrounds it both remain small, the rest of the space within the testa being
occupied by perisperm.

^34

PHANEROGAMS.

The Embryo generally attains but very small dimensions in the small- seeded
parasites and saprophytes destitute of chlorophyll, and remains without differentiation
until the time of ripening of the seed ; in Monotropa it never consists of more than
two cells, and even in Pyrola secunda, which possesses chlorophyll, only of from eight
to sixteen (Hofmeister). The ripe seeds of Orobanche, Balanophora, Rafflesiaceae,
&c. contain a very small undifferentiated embryo in the form of a roundish mass of
tissue ; the embryo of Cuscuta is of moderate size and length, but the formation of
leaves and roots on the filiform stem ^ is suppressed. The Mistletoe (Loranthaceae),
on the other hand, parasitic but containing chlorophyll, produces an embryo which
is not only large but well-developed.

If the embryo of the ripe seed is differentiated, as is generally the case, it
consists of an axis and two opposite primary leaves (cotyledons) between which the
axis terminates as a naked vegetative cone {Cucurbiia)^ or bears a bud which some-
times consists of several leaves {Vicia Faba^ Fig. 436, Phaseolus, Quercus^ &c.).
Instead of the two opposite cotyledons, a whorl of three is not unfrequently formed
in those plants which normally possess only two ^ {Phaseolus, Amygdalus^ Quercus,
&c.). The opposite cotyledons are usually alike in form and vigour; in Trapa
however one remains much smaller than the other ; and cases even occur in which

Fig. i^'i.^.—Chunonanthnsfragrans: A horizontal section of the nearly ripe fruit ; B lottgituditial section of the
same, /"the thin pericarp, e remains of the endosperm, c cotyledons ; C the embryo removed from the seed, showing
the cotyledons rolled round one another, the radicular end below.

only one has been formed, as in Ranunculus Fi'cana^, where it remains below in
the form of a sheath, and in Bulbocapnos, a section of Corydalis^. The two coty-
ledons generally form by far the larger part of the ripe embryo, so that the axis
has the appearance only of a small fusiform appendage between them ; and this
structure is especially striking when the embryo attains a very considerable absolute
size in those seeds which possess no endosperm, and the cotyledons swell up into
two thick fleshy bodies (as in jEscuIus^ Castanea, Quercus, Fig. 438, Amygdalus,
Vicia Faba, Phaseolus, the Brazil-nut, &c.) ; but more often the cotyledons remain
thin like shortly stalked foliage-leaves of simple form (as in Cruciferae, Euphor-
biaceae, and Tili'a, the last with a three- to five-lobed lamina). Most often they

* According to Uloth (Flora, i860, p. 265) the root- cap is also absent. On parasites see
especially Solms-Laubach in Jahrb. fiir wissensch, Bot. vol. VI. pp. 599 et seq. [Uloth's statement is
confirmed by Koch (Ueb, die Entwick. der Cuscuteen, Hanstein's Bot. Abhdl. II. 1874).]

* Numerous additional instances are given in the Bot. Zeitg. 1869, p. 875. [Masters, Vegetable
Teratology, Ray Soc. 1869, p. 370.]

^ Irmisch, Beitrage zur vergleichenden Morphologic der Pflanzen, Halle 1854, p. 12.

* [To these instances of what is termed a ' pseudo-monocotyledonous' development may be
added Carum Bulbocastanum (see Hegelmaier, Entwick. dicot. Keime, 1878).]

DICOTYLEDONS.

^35

lie with their inner faces flat against one another (Figs. 435, 436); but are not
unfrequently folded or wrinkled and curved backwards and forwards (as in Theobroma
with thick, Acer and Convolvulaceae, &c. with thin cotyledons) ; less often they are
rolled spirally round one another (Fig. 434).

The axis of the embryo beneath the cotyledons is generally elongated and
fusiform, and when of this shape is described in works on descriptive botany as the
Radicle. This fusiform body consists however in its upper and usually larger part of
the hypocotyledonary portion of the stem, and only the lower posterior terminal
piece, which is often very short, is the rudiment of the primary root (Fig. 437). The
rudiments of the secondary roots can sometimes be distinguished in the tissue of the
primary root (in Cucurbita, and according to Reinke in Impatiens).

Fig. A-^^.—Rictnus commums ; I longitudinal section of
the ripe seed ; // germinating seed with the cotyledons still
in the endosperm (shown more distinctly in A and />),
s testa, e endosperm, c cotyledon, he hypocotyledonary
portion of the stem, w primary root, -w' secondary root,
X the caruncle, an appendage of the seed characteristic of
Euphorbiaceae.

\r.s

FtG. 436. — Vicia Faba: A seed with one of the coty-
ledons removed, c the remaining cotyledon, -w radicle,
kn plumule, j testa ; B germinating seed, j testa, / a por-
tion of the testa torn away, n hilum, st petiole of one of
the cotyledons, k curved portion of the axis above the
cotyledons, he the very short hypocotyledonary portion of
the axis, h the primary root, ws its apex, kn bud in the
axil of one of the cotyledons.

Germination generally takes place — after the testa, or in dry indehiscent fruits
the pericarp, has burst from the swelling of the endosperm or of the cotyledons
themselves — by the elongation of the hypocotyledonary portion of the axis to such
an extent as to push the radicle out of the seed, the root then beginning to grow
rapidly and generally attaining a considerable length and forming secondary roots
in acropetal succession, while the cotyledons and plumule still remain in the seed
(Figs. 435, 436, 437). Thick fleshy cotyledons usually remain in the seed during
germination, finally perishing after their food-material has been consumed (as in
Phaseolus muUiflorus, Vicia Faba, Fig. 436, Quercus, Fig. 438). In this case the
petioles of the cotyledons lengthen so much that the plumule which is concealed

6^6

PHANEROGAMS,

between them is pushed out (Fig. 438), and now grows upright so that the seed
and cotyledons together have the appearance of being a lateral appendage of the
axis of the embryo. But usually the cotyledons are destined for further develop-
ment, especially when they are thin, and form the first foliage-leaves of the plant.
In order to liberate them and the plumule which lies between them from the seed,
the hypocotyledonary portion of the axis increases considerably in length, making

Fig. 437. — Longitudinal section of the axis of the
embryo in the ripe seed of Phaseolus fniMiflorus ,
parallel to the cotyledons {X about 30) ; ss apex of the
stem, ws of the root, ct cushion at the insertion of
the cotyledons, zthe first internode,/^ the petioles of
the first foliage leaves, v,v,f\}s\& procambium of the
fibro-vascular bundles.

Fig. 438. — Qiiercus Robiir ; I longitudinal section of the embryo (mag-
nified) after removal of the anterior half of both cotyledons c, c; the
hypocotyledonary portion of the axis he, the primary root iu, and plumule
b are concealed between the lower portion of the thick cotyledons ; st
petiole of the cotyledons ; // seed at the time when germination is com-
mencing (natural size), the pericarp and one cotyledon have been removed,
the hypocotyledonary portion of the axis and the radicle have elongated ;
/// further stage of germination, the plumule having emerged from the
testa sh and pericarp s by the elongation of the petiole of the cotyledons
st, w primary root, iu' secondary roots.

first of all a curve which is convex on the upper side (Fig. 435), because the coty-
ledons still remain in the seed while the lower end of the stem is attached by the
root to the ground. Ultimately, by a final lengthening of the hypocotyledonary
portion, the upper part of the axis together with the cotyledons is drawn out of
the seed in a pendent position. The axis now straightens as it continues to grow,
and the cotyledons expand in the air, the plumule developing more completely and

DICOTYLEDONS.

637

\

pushing up between them. The cotyledons which thus become exposed to the
light usually increase rapidly in size, and constitute the first green leaves of the
plant, which are of simple form {e.g. Cruciferae, Acer, Cucurbila, Convolvulaceae,
Euphorbiaceae, &c.). If the seed contains an endosperm, the cotyledons do not
emerge till after it has been absorbed (Fig. 435)-
Many transitional forms occur between the dif-
ferent modes of germination now described;
peculiar phenomena sometimes appearing which
are caused by special vital conditions. In Trapa,
for example, the primary root is from the first
rudimentary, and remains altogether undeve-
loped ; the hypocotyledonary portion of the
stem lengthens considerably, curves upwards,
and protrudes a great number of lateral roots
in rows which fix the plant into the ground \

The further development of the young
plant may take place by the rapid enlargement
of the primary axis of the embryo. While the
axis is growing, generally in an upright direction,
the shoot which developes from the plumule
becomes the primary stem of the plant, length-
ening at the summit, and usually producing
weaker lateral shoots {e.g. Helianihus, Vici'a,
Fopu/us, Impah'ens, &c.). When the main stem
is perennial, it sooner or later ceases to develope
further at the apex, or the lateral shoots nearest
to the apex become equally strong. An arbor-
escent head is thus formed, the main stem or
trunk becoming denuded by the dying off of
the lower branches, or the main stem continues
to grow erect as a sympodium (as in Ricinus,
the Lime, &c.); or lateral shoots are formed
at an early period at the base of the primary
stem which grow as strongly, and thus give rise
to a shrubby plant. When the axis of the em-
bryo grows vigorously, the primary root generally
also grows vigorously in a downward direction^;
and a Tap-root is thus formed, from which, as long
as it increases in length, the lateral roots spring
in great numbers in acropetal succession. When the growth in length of the tap-

FlG. 439.— Almond-seed germinating, one of the
cotyledons c' c" being split ; the letters as in Fig. 438,
i the first inteniode strongly developed.

* [See De Candolle, Organ ographie Vegetale, PL 55,]

^ One of the most remarkable exceptions is afforded by the genus Cuscuta, which has no
primary root, the posterior end of the axis penetrating into the ground on germination, but soon
dying off when the upper filiform portion of the stem has embraced the plant on which it becomes
parasitic, and has fixed itself on to it by its short suckers ; the plant afterwards grows vigorously
and branches.

638 PHANEROGAMS.

root ceases, adventitious roots become intercalated among the lateral roots already
formed, and like them, grow vigorously, and may themselves produce lateral" roots
of higher orders. A strong root-system is thus produced with the primary root of
the embryo for its centre, which endures as long as the stem itself. By the subse-
quent increase in thickness the primary stem (as well as its branches) assumes the
form of a slender upright cone, the base of which rests on the base of the inverted
cone formed by the primary root which has also increased in thickness. While
these processes, which are here described in their main outlines, take place almost
invariably among Conifers, a number of deviations occur, on the other hand,
among Dicotyledons similar to those which have been spoken of under the head
of Monocotyledons. The primary axis may die soon after germination or at the
end of the first period of vegetation, the primary root often perishing as well, while
the axillary shoots of the cotyledons or of subsequent leaves continue the life of the
individual. Thus, for example, in the Dahlia, a strong adventitious root is given out
laterally from the hypocotyledonary portion of the axis at the close of the first period
of vegetation of the young plant, and swells into a tuber; the primary root-system
and the portion of the axis above the cotyledons disappear, and there remain only
for the continuance of the life of the plant the new tuberous root, the hypocoty-
ledonary portion of the axis, and the axillary buds of the cotyledons. The process
is still more striking in Ranunculus Ficaria, where, after the development of the
primary root, a tuberous lateral root is produced below the primary axis of the
embryo, sheathed by a coleorhiza, and maintains its existence together with the
axis, while the primary root and the first leaves perish. Among the numerous
cases belonging to this category may be mentioned also Physalis Alkekengi, Mentha
arvensis, Bryonia alba, Polygonum amphibium, and Lysimachia vulgaris ^ The pro-
duction of bulbs also occurs among Dicotyledons (as in species of Oxalis), though
not so commonly as among Monocotyledons ; of more common occurrence are
tubers or swellings of underground branches, stolons, or rhizomes of greater or less
thickness. The greater number of Dicotyledons have perennial underground roots
or stems which send up periodically leafy and flowering shoots that die at the end
of each period of vegetation. In all such cases, w"here the primary root-system
of the seedling perishes, new roots are repeatedly developed from the stem ; and
the power possessed by most Dicotyledons of producing adventitious roots from the
stem, especially when kept moist and dark, enables them to be reproduced to almost
any extent from branches and portions of branches. Some species climb, like the
Ivy, by roots put out regularly from the weak stem which requires a support ; others
send out runners to a distance, on which the bud forms a new plant, as in the Straw-
berry, the stem which is thus formed putting out roots. The order of succession of
new roots from the stem is in general acropetai, but they do not usually make
their appearance except at a considerable distance behind the growing bud ; many
Cactaceae however not unfrequently produce them close below it.

T^^^ Mode of Branching. The normal monopodial branching is axillary; the
lateral shoots are produced in the angle which the median line of the leaf forms

* The above is taken from Irmisch's detailed descriptions in his Beitrage zur vergleichenden
Morphologic der Pflanzen, Halle, 1854, 1856; Bot. Zeitg. 1861 ; and elsewhere.

DICOTYLEDONS. 639

with the internode. On a vegetative shoot at least one lateral shoot is produced
in the axil of each leaf, although only a few of the axillary buds unfold. Sometimes
other axillary buds are produced in rows above the original one ; as, for instance,
above the axils of the foliage-leaves in Aristolochia Stpho, Glediischia, Lonicera, &c.^,
above the axils of the cotyledons in Juglans regia, and that of the larger cotyledon
in Trapa. In woody plants the axillary buds destined to live through the winter
are not unfrequently so completely surrounded by the base of the leaf-stalk that
they are not visible until the leaf has fallen off, as in Rhus typhtnum, Virgilia lutea,
Platanus, &c., and are then called Inirapetlolar Buds. Besides the ordinary axillary
branching, some cases are known among Dicotyledons of lateral and monopodial
but extra-axillary branching. To this description belong the tendrils of Vitis and
Ampelopsis which are produced (according to Nageli and Schwendener) beneath the
punclum vegetationis of the mother-shoot, opposite to the youngest leaf and somewhat
later than it. In Asclepias syriaca and some other plants a lateral vegetative branch
stands beneath the terminal inflorescence between the insertions of the foliage-leaves,
which themselves also produce shoots in their axils. According to Pringsheim^
lateral shoots arise on the concave side of the long spirally-curved vegetative cone
of Utricularia vulgaris which he considers to be extra-axillary branches, while
normal shoots are formed in the axils of the leaves which stand in two rows on
the convex side of the shoot or by their side. It appears to me however certain
that these extra-axillary structures on the concave side of the mother-shoot are leaves
of peculiar form ^, since inflorescences are produced in their axils.

The suppression of the bracts of the inflorescence, which is not uncommon,
cannot be placed in the same category as the cases just mentioned of extra-axillary
branching, where large leaves in the axils of which buds are also formed exist
near the extra-axillary lateral branches. Here, on the contrary, as for instance in
Cruciferse and the capitulum of many Compositae, the formation of leaves on the
branching axis of the inflorescence is itself entirely suppressed ; there are no leaves
in the axils of which the branches could stand. The branches are however produced
as if the leaves were actually there. With reference to the changes in the mode of
branching met with in passing from the vegetative to the floral region and to the
frequent transference of the bract on to the branch axillary to it, the remarks on
p. 598 may be consulted.

Adventitious buds are rare in Dicotyledons, as they are in Phanerogams
generally. Those which are commonly formed with an exogenous origin in the
indentations of the margins of the leaves of Bryophyllum calycinum are well known,
and serve to propagate the plant. They sometimes occur (according to Peter-
hausen*) in Begonia coriacea in the form of small bulbs on the peltate surface of

* See Guillard, Bull. Soc. Bot. de France, vol. IV, 1857, p. 239 (quoted by Duchartre, Elements
de Botanique, p. 408).

^ Zur Morphologie der Utricularien, Monatsber. der konigl. Akab. der Wissench. Feb. 1869.

3 This of course depends on what is considered leaf and what shoot ; this is not however a
matter of simple observation, but rather of conventional conceptions convenient for a special
purpose.

* Beitrage zur Entwickelung der Brutknospen (Hameln 1869), where various examples are also
given of axillary buds of Dicotyledons which form deciduous gemmae; as in Polygonum viviparum,

640 PHANEROGAMS.

the leaf where the principal veins radiate '. On the adventitious buds on the leaves
of Utricularia, Pringsheim's treatise already quoted may be consulted. Adventitious
buds more often spring from roots, e.g. in Anemone japonica, Linaria vulgaris ^
Cirsium arvense^ and Populus tremula, according to Irmisch^. The shoots which
spring from the bark of the older stems of trees must not at once be set down as
the development of adventitious buds ; since the numerous dormant buds of woody
plants may long remain buried and yet retain their vitality.

The Leaves of Dicotyledons exhibit a greater variety both in their position and
their form than those of all other classes of plants put together. The ordinary
phyllotaxis of seedlings begins with a whorl of two cotyledons, and continues
either in decussate pairs or passes into a distichous arrangement or into whorls
consisting of larger numbers or spiral arrangements with the most various angles
of divergence. More simple arrangements, especially that of decussate pairs, are
generally constant in whole families, the more complicated arrangements usually less
constant. Axillary branches usually begin with a pair of leaves which are either
opposite or alternate, and stand right and left of the median line of the mother-leaf.

It is quite impossible to give in a short space even a general account of the
forms of leaves, even apart from cataphyllary leaves (scales on underground
stems and those which envelope persistent buds), hypsophyllary leaves or bracts,
and floral leaves; only a few of those forms of foliage-leaves can be mentioned
here which are peculiar to or characteristic of Dicotyledons. The foliage leaves
are usually divided into a slender leaf-stalk {peliole) and a flat blade {lamina) ; the
lamina is very commonly branched, /, e. lobed, pinnate, compound, or incised ; and
even where it forms a single plate (simple leaf) the tendency to branching is gene-
rally indicated by indentations, teeth, or incisions in the margin. The branching
of the lamina has usually a distinctly monopodial origin, but its development may
continue in a cymose manner, a helicoid succession of lateral lobes being formed
on each side right and left of the centre of the leaf (as in Rubus, Helleborus,
&c., see Fig. 141). The sheathing amplexicaul base is not common in Dicotyledons
(but occurs in Umbelliferae) ; and the occurrence of Stipules in its place is more
common. The cohesion of opposite leaves into a single plate pierced by the stem
is not uncommon (' perfoliate ' leaves, as in Lamium amplexicaule, Dipsacus Fullonum,
Lonicera Caprifolium, species of Silphium, Eucalyptus, &c.) ; as well as the downward
prolongation of the lamina of the leaves (' decurrent leaves '), which distinguishes the
' winged ' stem of Verbascum, Onopordon, &c. The not uncommon ' peltate ' leaf also
scarcely occurs in so marked a manner in any other class {TropcBolum, Victoria regia,
&c.). The power of Dicotyledons to develope from their foliage-leaves organs of
the most diverse functions adapted to the most various conditions of life is seen in
a very striking manner in the common occurrence of leaf-tendrils and leaf-spines,
and still more in the formation of the ascidia or ' pitchers ' of Nepenthes, Cephalotus,
Sarracenia, &c.

Saxifraga granulata, Detitaria bulbifera, Rafmncidus Ficaria, &c. [Berge, Ueb. Bryophyllnm calycimim,
Zurich 1877.]

* The common method of propagating Begonias is by cutting or tearing the leaf, which, if then
placed on moist soil, produces buds on the edges.]

^ [Irmisch, Bot. Gaz. III. pp. 146 and 160.]

DICOTYLEDONS. 64 1

The Venation of the fohage-leaves (with the exception of the thick leaves of
succulent plants) is distinguished by the numerous veins which project on the
under side, and by their curvilinear anastomoses by means of fibro-vascular
bundles running through the mesophyll itself. The mid-rib, which usually divides
the leaf into two symmetrical but sometimes into very unsymmetrical halves, gives
off lateral veins right and left; one, two, or three strong nerves, similar to the
mid-rib, often springing in addition from the base of the lamina right and left
of the median line. The whole system of the projecting veins of a foliage-leaf
behaves like a monopodial branch-system developed in one plane, the interstices
being filled up by the green mesophyll in which lie the anastomoses combined
into a small-meshed network. Within the meshes still finer bundles are usually
formed which end blindly in the mesophyll. In membranous cataphyllary and
hypsophyllary leaves and the perianth-leaves of the flowers the projecting veins
do not usually occur; the venation is more simple and more like that of Mono-
cotyledons ^

The Flower ^. In the great majority of Dicotyledons the parts of the flower are
arranged in whorls, or the flowers are cyclic ; only in a comparatively small number
of families (Ranunculaceae, Magnoliaceae, Calycanthaceae, Nymphaeaceae, and Nelum-
biaceae) are all or some of them arranged spirally {acyclic or hemicyclic ^).

In Cyclic Floweis the whorls are usually pentamerous, less often tetramerous,
both numbers occurring in nearly-related plants. Dimerous or trimerous or combi-
nations of dimerous and tetramerous whorls are much less common than penta-
merous, and are usually characteristic of smaller groups in the natural system.

When the floral whorls are tetramerous or pentamerous, they are generally four
in number, and are developed as Calyx, Corolla, Androecium and Gynaeceum. In
dimerous or trimerous flowers the number of the whorls is much more variable, and
then it is not uncommon for each series of organs to be made up of two or three
whorls ; while in the previous case the multiplication of the whorls is almost entirely
confined to the androecium.

The corolla is frequently absent, and the flowers are then said to be apelalous.
When the calyx and corolla are both present the number of their parts (sepals and
petals) is almost always the same {Papaver is an exception) ; but this is not the case
with the number of the whorls. In Cruciferae, for example, the calyx consists of two
decussate whorls of two sepals each, the corolla of one whorl of four petals. When
the perianth and androecium are both present (whether the former consist of calyx
only or of both calyx and corolla), the number of their parts is usually the same, that
is, the flower is isostemonous, but the stamens are often more, rarely fewer in number
than the parts of the perianth, and the flower is then anisostemonous. When the

' [The structure of the leaf compared with that of the stem has been worked out by Casimir De
Candolle, Archives des Sciences, 1868; the 'Student' for the same year contains an abridged
translation of his paper.]

^ The floral diagrams given here are drawn partly from my own investigations, but chiefly from
the researches of Payer into the history of development, assisted by Doll's Flora of Baden. The
figures placed beneath the diagrams are intended to indicate the number and cohesion of the carpels
as well as the placentation in those plants the diagram of which is otherwise the same. [See also
Eichler, Bluthendiagramme ; Gray, Stmctural Botany.]

^ Compare pp. 600 and 608.

T t

642

PHANEROGAMS.

flower is tetramerous or pentamerous the number of carpels is usually less ; when the
flower is dimerous or trimerous, or when the parts are arranged spirally, the number
of carpels is not unfrequently larger.

It will be seen from this brief outline that the relations of number and position
in the parts of the flowers of Dicotyledons are very various, and cannot be referred,
as is the case with Monocotyledons with but few exceptions, to a single type. Even
the establishment of diff"erent types for the larger groups is attended with great
uncertainty, since the knowledge of development necessary in order to refer par-
ticular forms of flowers to general formulae is often wanting. The too universal
application of the spiral theory of phyllotaxis in the case of cyclic flowers has
often increased the difficulty, and has even occasioned doubts which would not
have arisen without the theory.

For the great majority of Dicotyledons the floral formula may be given
'^n^n'^^n{+n+...)^n{-m)' This formula holds good for most pentamerous flowers
and for those which are truly tetramerous (or octamerous as Michauxia) ; so that n
is in these cases 5 or 4 (or 8 as the case may be). In the androecium an indefinite
number of (alternating) whorls -S'/„(+„^ ^ must be assumed in order to include

Fig. 440.— Diagram of Caprifoliaceae; A Ley-
cesteria, a Lo?iicera, b Symphoricarpus .

Fig. 441.— Diagram of Par-
nassia (Saxifragacese).

Fig. 442. — Diagram of Campanulaceae ;
A Campanula, a Lobelia.

the large number of flowers in which the androecium consists of more than one
whorl (as e.g. Fig. 451). The mode of expressing the gynaeceum C,j(_„j) is intended
to show that very commonly the number of carpels is fewer than 5 or 4 (or 8 as the
case may be) ; m may be of any value from o to n. In the majority of gamopetalous
orders and elsewhere there are very commonly only two carpels ; and in this case
they stand in a median line posterior and anterior ; but on the hypothesis that the
typical gynaeceum consists of five alternating carpels and has been reduced to two by
abortion, one must stand in the median position in front, the other obliquely behind.
A similar difficulty is also presented when the gynaeceum consists of three or of
only one carpel. It would carry us too far to detail the reasons which nevertheless
determine me to retain the formula above given for the gynaeceum of flowers of this
description ; it need only be mentioned that species or genera with the typical five
carpels occur in the most diverse families and orders where a smaller number is the
normal one.

The diagrams Figs. 440-450 represent a selection of cases which can be
reduced (if no further reference is made to the consideration mentioned above) to
the general formula which here assumes the simpler expression -S",, P^^ St^ C'h(_,„).

DICOTYLEDONS.

643

A comparison with nearly-allied forms leaves little room for doubt that the vacant
spaces indicated by dots in the three outer whorls correspond to abortive members
in the sense already frequently indicated, even when the absence of these members
is so complete that the earliest stages of development of the flower give no indi-

I-'IG. 443.— Diagram of Valei-ianaceo;; A Valeriana,
' B Ceutnaitthiis.

Fig. 444.— Diagram of Cu-
curbitaceae.

Fig. 445.— Diagram of Cora-
positae.

Fig. 446.— Diagram of some Rubiac re." FiG. 447.— Diagram of Plantagine.ie. FiG. 448.— Diagram of Oleaceae.

Fig. 449.— Diagram of Menispermacefe.

Fig. 450.— Diagiam oi Ctnnamcmum (Laiiracea;).

Fig. 451.— Diagram oi Aquilegia (Ranunculacese).

cation of them. The same is the case also when the number of carpels is less than
the typical one. Other cases however occur, as in the case of Rhus (Fig. 452),
where certain members, in this case two out of the three carpels, disappear in the
course of development. Crozophora tindoria (Fig. 453) is especially instructive in

T t 2

644

PHANEROGAMS.

regard to the relationships here suggested, the flowers becoming diclinous from
the stamens in the female flowers developing as sterile staminodes, which may be
considered as the first step towards abortion, while in the male flowers the three
carpels are replaced by as many fertile stamens (Payer).

Fig. 452.— Diagram oi Rhus (Anacardiaceae).

FIG. 453.— Diagram of Crozophora (Euphorbiaceae), a female,
b male flower.

Reference was made in the Introduction to Angiosperms (pp. 601, 610) to the
interposition of a whorl of stamens between the members of a previously formed
staminal whorl; and it was mentioned that the interposed whorl has sometimes not

Fig. 454.— Diagram, of pentamerous Ericaceae and Epacridese.

Fig. 455.— Diagram oi .-Esachis (Hippocastanene).

the full number of members. These phenomena occur in various large groups of
Dicotyledons \ In Fig. 454 the five stamens of the decandrous flower of the group
of Bicornes which are interposed as a whorl of full number within the first whorl

Fig. 456.— Diagram of Primulaceae.

Fk;. 457.— Diagram of Vitis (Ampelideae).

are indicated by the lighter colour. The same is the case with the larger number of
Gruinales, among which however the Balsaminese possess only the typical five
stamens ; the Lineae and the genus Erodium have five additional rudimentary
stamens interposed between them ; while in Peganum Harmala and Monsonia the

^ Payer's figures show that the interposed whorl, although of later origin, is sometimes exterior
to the typical whorl. The main point is that the position and number of the other parts of the
flower are exactly as if there were no interposed whorl.

DICOTYLEDONS. 645

number of stamens in the interposed and outer whorl is doubled. The order
^sculineae is of special interest in this connection, since in some of its families
(Acerineae and Hippocastaneae, Fig. 455) the interposed staminal whorl remains
incomplete, so that the total number of stamens is not a multiple of the typical
fundamental number (five). Among pentamerous flowers Lythrarieae, Crassulacese,
and Papilionacese may be mentioned in addition, and among tetramerous ones
(Enotherese, in which a complete staminal whorl is interposed.

One of the most remarkable deviations from the ordinary structure takes the
form in not a few families of Dicotyledons of the simple staminal whorl being
superposed on the corolline whorl, as shown in Figs. 456, 457, and as occurs also
in the Rhamnaceae, Celastrineae, the pentandrous Hypericineae, and Tilia. Pfeffer ^
has shown that the two superposed whorls of Ampelideae arise independently of one
another and in acropetal order, while on the other hand in Primulace^e they first
appear in the form of five projections each of which forms a stamen, and from each
of which a petal subsequently grows outwards. In these cases we have no sufficient
ground for the hypothesis that an alternating whorl has been suppressed between
the two superposed ones ; although in other cases this supposition is justified, or at
least is very probable. Thus in the order Caryophyllineae, families, genera and

Fig. 458.— Diagrram oi Scleranthus FiG. 459. Diagram of Phytolacca FIG. 460.— Diagram of Celosia

(Paronychiacere). (Phytolaccaceae). (Amaranthaceae).

species occur in which the corolla is absent and the stamens are superposed on the
sepals ; and since in the same natural group species also occur with a corolla, it may
be assumed that where the corolla is absent this is the result of abortion. The
diagram of these plants (Figs. 458, 459) is complicated still further by the tendency
which they exhibit to a de'doublement of the stamens and even of the carpels.

When a flower has more stamens than sepals or petals, this may be the result,
as has already been mentioned, on the one hand of an increase in the number of
staminal whorls (as in Fig. 451), or on the other hand, of the interposition of
a perfect or imperfect whorl among the typical ones, or of de'doublevient of the
stamens (as in Fig. 458). These cases must be clearly distinguished from those in
which a larger number of stamens results from the branching of the original ones,
a phenomenon which is found in diff"erent sections of Dicotyledons, and is some-
times constant in whole families (see p. 544). Thus, for instance, in Dilleniaceae
(Fig. 461), Aurantiacese (Fig. 462), and Tiliaceae (Fig. 463), each symbol which
indicates a group of anthers corresponds to a single original stamen. In this case
the number of original stamens is the same as that of the petals and sepals;

' Pfeffer, Bot. Zeitg. 1870, p 143; and Jahrb. fiir wissensch. Bot. vol. VIII. p. 194: (see
supra, p. 609).

646 PHA NEROGA MS.

but sometimes it is less (as in Hypericum perforalum with three staminal bundles
in the pentamerous flower); so that an increase in the number of stamens is
united with a decrease of the typical number of staminal leaves.

The branching of carpels is much less common than that of stamens. It
occurs very clearly in Malvaceae, where the typical number of carpels is five,
and they are often developed as such (as in Hibiscus). In some genera however
(as Malva, Malope, and Althced) five original rudiments of carpels first of all make
their appearance in the form of a low cushion. Each of these forms very early
a larger number of outgrowths lying side by side, and each of these produces a style
and a one-seeded compartment of the peculiarly-shaped gynaeceum ^

This short sketch will be sufficient to show what variations are possible in
the numbers and positions of the parts that may be included under the expression
S^Pn'^UUn^" ) Cn (-m)j which, as has already been said, is especially characteristic
of flowers with pentamerous or truly tetramerous whorls. True tetramerous flowers
are allied not only to those that are octamerous (like Michauxia), but also to
those with dimerous whorls, among which (Enotherese may be especially men-
tioned. Of genera belonging to this family, Epilobium, for example, is constructed
on the formula ^92^2 ^X4 '^^4.4 Q> Circcea on that of S^ P^ Sl^ Q '■> ^^^ Trapa, with

Fig. 461.— Diagram of CandolUa FIG. 462.— Diagram of Citrus FiG. 463.— Diagram of Tiliaceaj.

(Dilleniaceae). (Aurantiaceje).

the formula S^^^P^^^St^C^, must also be included here. Although in Epilobium
and Trapa the calyx really consists of two whorls, this pseudo-whorl formed of
two decussate pairs is followed by the other whorls exactly as if it were a true
tetramerous whorl. But other dimerous and tetramerous flowers exhibit a more
considerable deviation from the type, inasmuch as the two dimerous perianth-whorls
which develope as if they were a tetramerous calyx or corolla are followed by a
staminal whorl which is superposed on the pseudo-whorl consisting of two decussate
pairs, as in Urtica and other genera of the order, and in Proteaceae with the
formula ^2+2 ^^^^4 <^i (Fig- 37o)-

Among the dimerous and trimerous flowers of the orders Polycarpae and
Cruciflorse, where they are the most perfectly developed, a tendency prevails for
more than one whorl to go to the formation of the calyx, the corolla, the androecium,
and even the gynaeceum, a tendency which may be expressed by the formula
»S'^(+p+...)^p(+iJ+---)'^^p(+Pt--")^i>(fz)4-"); for example,

Fumariaceae, S^P^j^.^St^^.,. C^.

Berberideae,

Epimedivm, S^^^ P2+2 ^4+2 Q'
* See Payer, Organogenie de la fleur, PI. 6-8.

I

I

DICOTYLEDONS. 647

Berberis, 6*3+3 ^3+s ^^4+3 ^'i,
Podophyllum, S, P,^i St,\, C,.
Cruciferse, ^'2+2 Pxi 'S'4+2' C^^^^y

A large number of examples of this general formula are afforded by the
family Menispermaceae, in which the whorls are sometimes dimerous, sometimes
trimerous, while sometimes whorls of each description occur in one flower; and
where almost every one of the organs may disappear by abortion ^

In addition to the trimerous flowers already mentioned, there are also some
which come under the first-mentioned general formula '5'„/'„ ^/„(+„) Q(_„,) ; as,
for example, Rheum with the formula S^ P^ St^^^ Cy Other trimerous flowers
again appear to belong to a third type, as Asarum with the formula .^3 St^^^ C^.

When the number of staminal whorls is considerably increased, it not unfre-
quently happens that the number of stamens in each whorl also undergoes change,
and complicated alternations arise. Flowers the structure of which is otherwise
altogether different resemble one another in this respect, as is shown by the
Papaveraceae on the one hand (Fig. 464), and by the Cistineae and some Rosacese
on the other hand.

Fig. 464.— Diagraiti of PapavefacecE ; A Ckelidintitan, a PapWver.

The reduction of the flower to a simpler condition is often carried so far
in many Dicotyledons (as in Monocotyledons) that each individual flower consists
only either of an ovary with one or several stamens, or, when the arrangement
is diclinous, even only of a single ovary or of a single or several stamens; the
perianth being either entirely absent (as in Salix and Piperaceae) or reduced to a
cup-like structure {Populus, the female flower of Cannabineae, &c.) or to hair-like
scales among the sexual organs which represent the flower {e.g. Platanus). Flowers
of this kind are generally very small and densely crowded in large numbers in
the inflorescence (such as capitula, spikes, or catkins). In some cases it may even
be doubtful whether we have an inflorescence or a single flower, as in the genus
Euphorbia ^.

The development of the separate parts and the entire form of the flower
in the mature state is so various that it is scarcely possible to state any general

' Eichler, Ueber die Meilispermaceen, Denkschrift der k. bayer. Ges., Regensburg 1^4. — •
Payer, Organogenic de la fleur, PI. 45-49. — Eichler, Flora, 1865, Nos. 2-8 et seq.

^ See Payer, I.e. p. 529; [also foot-note to p. 490. It is now generally admitted that the
Cyathiutn of Euphorbia is an inflorescence, a view which was first enunciated by Robert Brown in
opposition to Linnseus, who regarded it as a single flower. It appears that the stamens are here
axial (Warming, Ueb. poUenbildende Caulome und Phyllome, in Hanstein's Bet. Abhdl. 11, 1873)].

648 PHANEROGAMS.

facts concerning them. The perigynous structure of the flower is peculiar to
Dicotyledons, as is also the occurrence of hollowed axes of the inflorescence, like
the fig and similar structures, and the cupule, which occur in some families, and are
dependent on similar processes of growth.

The Ovules exhibit, in the diff'erent divisions of Dicotyledons, all those varieties
of structure which have already been mentioned in the introduction. Very commonly,
especially among the Gamopetalae, the nucellus is covered by only one integument,
which is then often very thick before fertilisation. But on the other hand the
third integument or aril is much more common than among Monocotyledons.
When there are two integuments, the outer one — difl'ering again in this respect
from most Monocotyledons — takes part in the formation of the micropyle, enveloping
the exostome or entrance to it. In some parasites the ovules are rudimentary,
and in many Balanophoraceas are reduced to a naked few-celled nucellus; while
in Loranthaceae they are coherent with the tissue of the floral axis in the inferior
ovary.

The behaviour of the Embryo-sac^ before and after fertilisation is similar
in most Dicotyledons to that which occurs in Monocotyledons. The endosperm
usually originates by free cell-formation, and is transformed by repeated divisions
of the first cells which are formed in this manner into a more or less dense tissue,
which fills up the embryo-sac either before or after the formation of the multi-
cellular rudiment of the embryo. But in a very considerable number of families
belonging to altogether diff'erent groups the embryo-sac exhibits on the one hand
striking phenomena of growth, elongating considerably before impregnation into
a long tube, and emitting after impregnation one or more vermiform protrusions
which penetrate into and destroy the tissue of the nucellus and of the integu-
ments, or even protrude altogether out of the ovule (as in Pedicularis, LathrcBa^
and Thesiuui). On the other hand, in those plants in which the endosperm
originates by cell-division we learn from Hofmeister that the following variations
occur: — 'The whole of ihe cavity of the embryo-sac behaves like the first cell
of the endosperm in Asarineae, Aristolochiaceae, Balanophoracese, Pyroleae, and
Monotropeae ; the first division of the sac is the result of a partition-wall which
divides it into two nearly equal halves, each of which encloses a cell-nucleus
and again divides at least once into daughter-cells. In other cases the first
cell of the endosperm includes the upper end of the embryo-sac ; the embryo-sac,
immediately after fertilisation, appears to be divided by a transverse septum into
two halves, the upper one of which developes into the endosperm by a series of
bipartitions \ while no such bipartition of the lower one occurs in Vtscum, Thestutfi,
Lathrcea, Rhmanlhus, Afazus, Melampyrum., or Globularia. The first cell of the
endosperm fills up the middle part of the embryo-sac in Veronica., Nemophila,
Pedicularis, Plantago, Campanula, Loasa, and Labiatae ; its lower end in Loranlhus,
Acanthus, Catalpa, Hebenstreitia, Verbena, and Vaccinium.'' In NymphcEa, Nuphar,
and Ceratophyllum, the upper end of the embryo-sac is cut off" from the rest
of the space by a septum soon after fertilisation, and the further development of

^ Hofmeister, Jahrb. fiir wiss. Bot. vol. I. p. 185 ; and Abhandl. der kon. Sachs. Ges, der Wiss.
vol. VI. p. f^ > ^i;ee supra, p. 579).

DICOTYLEDONS. 649

the daughter-cells or endosperm takes place only in the lipper part which also
encloses the oospore. This mode of formation of the endosperm differs however
from that which occurs in the plants mentioned above, in taking place in the upper
half of the embryo-sac by free cell-formation.

In the very large majority of true parasites (except Cuscutd) and saprophytes,
the endosperm is formed by cell-division ; in Cuscuta however by free cell-formation.
Hofmeister states that only slight indications of the formation of endosperm are
to be found in Tropaiolum and Trapa.

The mode of formation of the Embryo of Dicotyledons, as it has now been
elucidated by Hanstein's recent researches, has already been explained in the
introduction to Angiosperms (see Fig. 403). It need now only be stated in
addition that in parasites destitute of chlorophyll and in some saprophytes the
seeds become ripe before the embryo has emerged from the condition of a
roundish mass of tissue still without exlernal differentiation of parts {e.g. in
Monotropa^ Pyrola, Orobanche, Balanophoracese, and Raflflesiaceae).

With reference to the Histology \ I will confine my remarks here to a description
of the behaviour of the fibro-vascular bundles and of the mode in which the stem
increases in thickness.

With the exception of a few water-plants of simple structure, in which a purely
cauline fibro-vascular cylinder runs through the stem and increases in length at its
summit, the foliar bundles originating from it later (in Hippuris, Aldro'vanda, Cerato-
phyllum, and to a certain extent also Trapa, according to Sanio), it is the general rule
that * common ' bundles are first formed, the ascending branches of which enter the
stronger foliage-leaves generally in large numbers, and then pursue their course as isolated
bundles in the leaf-stalk and mid-rib, giving ofl' the secondary bundles which constitute
the venation of the lamina '^. The branches which descend into the stem mostly run
downwards through several internodes, become first interposed between the upper parts
of the older bundles, and sometimes (Fig. 465) first split and then coalesce laterally
with the older bundles lower down. Sometimes (as in Iberis) every bundle is twisted
in the stem and in the same direction, so that the bundles which have coalesced
sympodially, belonging to leaves of different heights on the stem, ascend spirally within
the cortex. But most commonly they run parallel to the axis of the stem, until they
anastomose with older bundles lower down. The bundles do not bend deeply into the
inner tissue of the stem, but turn downwards and run parallel to one another at the
same distance below the surface, so that they lie in one layer, which presents the
appearance of a ring on transverse section separating the fundamental tissue into pith
and primary cortex. The portions of the fundamental tissue which lie between the
fibro-vascular bundles connect the pith with the primary cortex, and form the primary
Medullary Rays. If there is no subsequent increase in thickness no further change takes

1 Hanstein, Jahrb. fiir wiss. Bot. vol. I. p. 233 et seq. ; and for the girdle-shaped combinations
of vascular bundles, Abh. der Berl. Akad. 1857, 8. — Nageli, Beitrage zur wiss. Bot. Leipzig, Heft I,
1858 ; and Dickenwachslhum und Anordnung der Gefassstrange bei den Sapindaceen, Munchen 1864.
— Sanio, Bot. Zeitg. 1864, p. 193 et seq., and 1865, p 165 et seq. — Eichler, Denkschrift der kon. bayer.
Bot. Gesells. vol. V. Heft I. p. 20, Regensburg 1864. — [De Bary, Vergleichende Anatomic der
Vegetationsorgane der Phanerogamen und Fame, 1877.]

^ When several fibro-vascular bundles enter a leaf-stalk, they are generally widely separated
by the fundamental tissue ; but sometimes, as in the Fig, the bundles are arranged in a circle on
transverse section, and form a closed hollow cylinder which divides the fundamental tissue of the
leaf-stalk into pith and cortex. Isolated fibro-vascular bundles also run into the pith of the leaf-stalk
in the Fig, as occurs also in the stems of some Dicotyledons.

6jO

PHANEROGAMS.

place. But usually, even in annual stems (as Hel'tanthus and Brassica) and invariably in
woody stems and branches, several years old, the subsequent increase in thickness
begins after the elongation of the internodes. A layer of cambium is formed in each
bundle between the phloem, which is external, and the xylem which is turned towards
the axis of the stem ; the cambium layers of the bundles, which are at first separated by
the medullary rays and lie side by side in a ring, unite into a closed mantle of cambium,
an interfascicular cambium being formed by divisions in the intermediate cells of the
medullary rays, which bridge over the spaces between the layers of the cambium
of the separate fibro- vascular bundles (see Fig. 93). The Camhium-ring thus formed
produces on the outside layers of phloem, on the inside layers of xylem, while it is at
the same time itself constantly increasing in circumference. All the tissue formed in the
fibro-vascular bundles from the cambium-ring on the outside may be termed Secondary
Bast, all the xylem formed on the inside Secondary Wood, in opposition to the Primary
Bast and the Primary Wood, which consist of the isolated bundles of phloem and
xylem of the fohar bundles which were already in existence before the formation of the
cambium-ring. While the wood which is produced from this cambium-ring forms a
hollow cylinder, the primary woody bundles project from the inside of the ring into the
pith as ridges, and often cause it to present on transverse sectic^n the appearance of

FlC 465. — The course of the bundles in two iuternodes of Satnbiiciis Hbulits: they lie in a cylinder which ia here
flattened out ; each internode bears two opposite leaves, and each leaf receives from the stem a middle bundle h h and
two strong lateral bundles j' s' ; the descending limbs of the bundles split and interpose between the lower bundles ;
there are in addition weaker bundles s" s" united by horizontal branches, from which bundles n n ascend into the
stipules. (After Hanstein.)

a star. The whole of these primary xylem-bundles are included in the term Medullary
Sheath ; and in the same sense one may adopt Nageli's term of Cortical Sheath to express
the whole of the primary bast-bundles at the periphery of the bundles. The growth in
length of the medullary and cortical sheaths accompanies that of the internodes which
takes place before the formation of the cambium-ring, and they therefore generally consist
of very long elements ; — the medullary sheath of very long annular, spiral, and reticulated
vessels intermixed with long woody fibres ; the cortical sheath containing bundles of long
bast-fibres which become widely separated from one another by the increase in circum-
ference of the stem, and which are often strongly thickened but long and flexible; in
addition to these, long cambiform cells and elongated bast-vessels (latticed and sieve-tubes)
occur in it. The structural elements of the secondary cortex are, like those of the
secondary wood, shorter ; in the secondary wood there are no annular or spiral vessels,
these being altogether replaced by shorter and broader vessels with bordered pits, sur-
rounded by wood-fibres intermixed with woody parenchyma (see p. r 16). The secondary
cortex forms either a number of layers of thick-walled as well as thin-walled bast-fibres,
and partially parenchymatous masses of phloem, or these last only, or the most various
combinations of both. Finally the primary cortex and the epidermis are both generally
supplanted by the formation of periderm and bark ; although they may sometimes
undergo a considerable growth in thickness by increasing in diameter at the same time

L

DICOTYLEDONS. 65 1

that longitudinal divisions are formed (as in Fiscum, Helianthus annuus, &c.). The
masses of xylem and phloem formed by the activity of the cambium-ring are pene-
trated lengthwise in radial direction by secondary medullary rays consisting of hori-
zontal cells which in the wood are not always lignified, and in the secondary cortex
are generally soft and parenchymatous. In the one case they are called xylem-
rays, in the other phloem-rays, and always have the power of taking up assimilated
food-materials. In proportion as the cambium-ring increases in size, the number of
these rays increases ; and the later layers of wood are always traversed by a larger
number of rays. They are one or more layers of cells in thickness, and form thin
vertical plates wedge-shaped at their upper and lower edges, which have the appearance
in a longitudinal section of ribbon-like structures (the 'silver-grain'). In a tangential
section the fibro-vascular bundles which run through the length of the stem are seen to
form a network of elongated meshes, through which the rays pass (especially clearly seen
in decaying cabbage-stumps). The medullary rays, like the fibro-vascular bundles, are
added to by means of the cambium-ring outwards and inwards ; and as the ring increases
in thickness, it produces new rays between the old ones.

[The following tabular account of the structure of the secondary wood (xylem) in
Dicotyledons is taken from De Bary (Vergleichende Anatomic) : —

1. Wood consisting only of tracheides with bordered pits : —

Wintereae [Drimys Winteri, lasmannla aromatica ; also Trochodendron ara-
lioides): (Conifers^).

2. Wood consisting of vessels, tracheides, parenchyma, and intermediate cells

{ersat^fasern^) : —

a. With no intermediate cells ; Ilex aquifolium, Staphylea pinnata, Rosa

canina, Cratagus monogyna, Pyrus communis^ Spircca opulifolia, Ca-
mellia, &C.

b. With no parenchyma ; Porlieria.

c. With both parenchyma and intermediate cells; Jasmlnum renjolutum,

Kerr'ta, Potentilla fruticosa, Casuarina eqwsetifolia and torulosa, Aristo-
lochia Sipho, &c.

3. Wood consisting of vessels, tracheides, fibres, parenchyma, and intermediate

cells : —

a. With no intermediate cells; fibres unseptate; e.g. Sambricus nigra

and racefnosa, Acer platanoides, pseudoplatanus, and campestris.

b. With both parenchyma and intermediate cells ; fibres unseptate ;

Berberis vulgaris, Mahonia ; [Ephedra).

c. With no intermediate cells ; fibres septate and unseptate ; Punica,

Euonymus latifolius and europceus, Celastrus scandens, Fitis 'vinifera,
Fuchsia globosa, Centradenia grandifolia, Hedera Helix, &c.

d. With all four kinds of cells ; MUhlenbeckia complexa, Ficus (?).

4. Wood consisting of vessels, tracheides, fibres, parenchyma, and intermediate

cells. This is the most common, and may be taken as the typical struc-
ture.—

a. With no intermediate cells ; Sparmannia africana, Calycanthus, Rhani'

nus cathartic a, Ribes rubrum, Quercus, Castanea, Carpinus sp.,

Amygdaleae, Melaleuca, Callistemon sp., &c.

^ [According to Sanio, Taxiis haccata has v^^ood of this composition, but Hartig and Kraus
state that some parenchyma is also present ; this parenchyma is better developed in the other
Conifers.]

2 [See &upra, p. T19. These cells were first distinguished by Sanio, who termed them 'ersatz-
fasem' (replacing- fibres), because they frequently take the place of the wood-parenchyma. The
term 'intermediate' used above refers to the fact that they are intermediate in form between
prosenchymatous and parenchymatous cells.]

6 ^^2 PHA NER OGA MS.

b. With no parenchyma ; Caragana arhorescens,

c. With both kinds of cells; most foliage-trees and shrubs, e.g. Snlix,

Populus sp., Lir'iodendron, Magnolia acuminata, Alnus glutinosa, Bftula
alba, Juglans regia, Nerium, Tilia, Hakea sua'veoltns, Ailanthus,
Robinia, Gleditschia sp., Ulex europceus, &c.

5. Wood consisting of vessels, fibres, parenchyma, and intermediate cells: —

a. With no parenchyma; Viscum album.

b. With no intermediate cells ; Avicennia.

c. With both kinds of cells ; Fraxinus excelsior, Ornus, Citrus medicuy

Platanus, &c.

6. Wood consisting of vessels, fibres, and parenchyma: —

Cheiranthus Cheiri, Begonia. Also many Grassulaceae and Caryophyllaceae.

7. W^ood consisting of vessels, fibres, parenchyma, and true woody fibres^ (?) : —

Coleus Macraei, Eugenia australis, Hydrangea hortensis.

8. Wood consisting of vessels, tracheides, woody fibres, septate fibres, parenchyma,

and intermediate cells : —

Ceratonia siliqua, Bignonia capreolata ; it is however still doubtful if true
woody fibres are present.]

When the increase in thickness of a stem ceases periodically and is renewed
with each new period of vegetation, as in our woody plants, a layer of wood is
formed during each period of growth (and usually also a secondary cortical layer),
which is sharply marked off from those of the preceding and of the following year,
and is called an Annual Ring of the wood. These annual rings are usually distinctly
visible to the naked eye, because the mass of wood formed in the early part of each
period of vegetation has usually a different appearance from that formed in the autumn,
the latter being denser, the former less dense and generally with a greater number of
vessels. The wood formed in the spring consists also of wider cells than that produced
in the autumn, and the radial diameter of the cells is usually greater. The cells formed
in the autumn appear compressed radially and broad in the tangential direction ; their
cavities are smaller, and hence, other things being equal, the thickness of their wall is
greater. A given quantity of wood produced in the autumn is therefore denser than
a like volume formed in the spring 2. While Dicotyledons differ so widely from Mono-

^ [The distinction which is drawn between 'fibres' and 'true woody fibres' is in their contents :
the former nearly always contain starch, in some cases {Spircea salicifolia, young wood of Vitis
vinifera and Centradenia grandifolia) chlorophyll, and in others {Syringa vulgaris) tannin; the latter
contain air and water, and sometimes a mere residue of protoplasm. These two kinds of fibres
usually occupy the same relative position in a fibro^vascular bundle, and it is doubtful if they are
ever both present in the same bundle: the 'fibres ' gradually become converted into 'woody fibres.']
^ The cause of this difference is not yet known ; but I suppose that it depends simply on the
difference in pressure to which the cambium and the wood are subjected from the surrounding cortex.
This pressure is less in the spring, and constantly increases till the autumn. I have no direct
measurements of this, but conclude it from the fact that the longitudinal fissures in the bark
become wider in F'ebruary and March, as may be clearly seen in the Oak, Maple, Poplar, Walnut, &c.
I cannot here explain the cause of this ; but in any case the bark, the longitudinal fissures of which
have become wider in winter, must exert less pressure on the cambium in the spring, and the cells of
the wood must therefore be able to extend more easily in a radial direction. The pressure which
the bark exerts on the cambium must continually increase by the thickening of the ring of wood
internally and the drying up of the bark in summer externally, and must affect the radial growth of
the young cells of the autumnal M^ood. Further investigations which I am proposing to make will
determine whether my theory is correct.' — This hypothesis, which I brought forward in the first
edition, has recently been fully confirmed by the researches of H. de Vries. (See Flora, 1872, no. 16,
and sect. 15 of Book III of this work.)

DICOTYLEDONS. 6

13

cotyledons in this mode of increase of their stems in thickness, they agree almost entirely
in this respect with Gymnosperms, except that in these latter there are no pitted vessels
in the secondary wood. In this respect however, according to von Mohl, Ephedra
indicates a transition to Dicotyledons. The organisation of Dicotyledons shows also
in some sense a higher stage of development in the greater varieties of the forms of
cells of which the xylem and phloem are composed.

A remarkable deviation from these normal processes is exhibited by the Sapindaceae.
In some plants of this order the stem has the ordinary structure ; but in others a
transverse section shows several smaller woody cylinders of various sizes outside the
usual one and lying in the secondary cortex. Each of these increases in thickness, like
the normal ones, by a cambium-layer which surrounds it, Nageli supposes the cause
of this structure to be that the primary fibro-vascular bundles of the stem do not lie in
a circle on the transverse section, but in groups more towards the outside or inside.
When the connecting bands of cambium are formed in the fundamental tissue, the
isolated bundles become united on the transverse section, according to their grouping,
into one (as in Paullinia) or several {e.g. Serjania) closed rings.

The cause of a large number of deviations of different kinds from the normal
structure of the stem in Dicotyledons which occur in various families, is the formation
of other cauline bundles of later origin in the stem besides the common bundles, either
within the primary pith or outside the ring in which the common bundles lie. We owe
to Nageli a more exact knowledge of these cases, and more especially to the very
exhaustive labours of Sanio, which form for the most part the basis, in addition to
my own observations, of the following short sketch, without going in detail into special
cases ^. I must refrain, in particular, from giving a detailed account of the behaviour of
Sanio's thickening-ring or of Nageli's meristem-ring, as this would involve considerable
prolixity.

These phenomena may be classified into two groups, according as the secondary
(cauline) bundles originate within or without the circle of the primary (common)
bundles. Sanio calls the former the endogenous, the latter the exogenous mode of
origin.

First Group. The secondary bundles are formed outside the primary bundles
(exogenous).

a. The primary (common) bundles lie near the axis of the stem, and remain more or
less isolated, while the secondary (cauline) bundles are formed by a closed cambium-ring
external to the primary bundles, which continues to grow on the outside (originally a
'thickening-ring' in Sanio's sense). Examples are furnished hy Mirabilis, Amaranthus,
Jltriplex, Chenopodium album, and probably by all the Nyctagineae and Mesembryan-
themaceae.

b. The primary (common) bundles lie in a ring on the transverse section and continue
their growth by means of a closed cambium-ring, which however soon disappears. A
new cambium-ring is then formed outside the one which has disappeared, and another
one again outside this one when it has in turn disappeared. Several circles of fibro-
vascular bundles are thus formed, continually increasing in number. In many Meni-
spermaceae {e.g. Cocculus), the new outer circle of vascular bundles together with its
cambium-ring is developed from a ring of meristem which lies in the primary cortex
and therefore outside the primary bast, — a phenomenon which is repeated in the cortex
as its growth proceeds (Nageli). In Phytolacca, on the other hand, and, according to
Eichler, also in Dilleniaceae, Wistaria, Bauhinia, Polygaleae {Securidaca and Comesperma),
Cissus, and Phytocrene, the successive circles of bundles originate in the secondary bast.

Second Group. The secondary bundles arise early after the primary bundles further
inwards or nearer the axis of the stem (endogenous).

^ [Oliver has collected the bibliography of the structure of the stem of Dicotyledons in the Nat.
Hist. Rev. 1862, pp. 298-329, and^i863, pp. 251-258.]

654 ^^^^ NEROGA MS.

o. Both the primary and the secondary bundles remain isolated ; they are not
united by a closed cambium-ring, but anastomose with one another, as in Cucurhita,
Nympha^aceac, and Papaver. In the Gucurbitaceae and Piperaceae the more internal
bundles are arranged in a ring, but in the Nymphaeaceae they are arranged irregularly,
so that the transverse section of the stem bears a greater or less resemblance to that
of a Monocotyledon.

b. The primary bundles lie in a ring on the transverse section, and are united by
a cambium-ring ; the secondary bundles arise at an early period in the pith and remain
isolated and scattered on the transverse section ; they anastomose with one another and
with the primary bundles in the nodes of the stem. Examples are furnished, according
to Sanio, by Begoniaceae, Aralia, and some Umbelliferae.

The Cell-forms of the phloem and xylem of Dicotyledons have already been described
in general terms (see p. ii6 et seq.). Only two peculiar phenomena need be mentioned
here. In Gucurbitaceae, some Solanaceae, and Nerium (and in a certain sense also in
Tecoma radicans^), a phloem-tissue is found not only on the outside but also on the inside
of the fibro-vascular bundles, which is developed with especial strength in Gucurbitaceae.
The isolated fibro-vascular bundles in the pith which are enclosed by the ring of wood
sometimes show an abnormal arrangement of their phloem and xylem. Thus, according
to Sanio, Aralia racemosa has, within the normal circle which grows by means of a
cambium-layer, an endogenous circle of closed fibro-vascular bundles in which the xylem
is peripheral and the phloem central as regards the stem. The isolated bundles in the
pith oi Phytolacca dioica on the other hand consist, according to Nageli, on a transverse
section, of a hollow woody cylinder which surrounds the phloem on all sides and is
penetrated by xylem-rays. The isolated fibro-vascular bundles of the pith in the rachis
of the inflorescence of Ricinus communis also consist of a thin axial bundle of phloem (?),
surrounded by a sheath of cells (xylem ?) arranged in rays.

A layer of collenchyma is very common in Dicotyledons beneath the epidermis of
the internodes and leaf-stalk.

The Classificat'tGn of Dicotyledons ^ has now been carried out so completely that the
smaller groups which are called Families ^, and which usually comprise genera very
nearly related to one another, have been united into larger groups or orders ; so that
at present only a few families remain unplaced. The greater number of the orders can
also be again arranged into larger groups which are clearly connected by actual relation-
ship. Systematists have not however up to the present time agreed as to how many of
these cycles of affinity should be established, so as to make the primary division of the
whole class of Dicotyledons in accord with the requirements of scientific classification.
The grouping of all Dicotyledons into three sections, Apetalae, Gamopetalae, and Eleu-
theropetalae, proposed by De GandoUe and Endlicher"^, ic now abandoned by most,
although still much in use for practical purposes. A. Braun^ placed among the Eleu-
theropetalae the greater number of plants previously classed among Apetalae ; and
Hanstein^ has now distributed among them the remainder, so that the whole class
consists of only two sub-classes, Gamopetalae and Eleutheropetalae. This classifica-

* [A cambium-ring is formed internally to the primary bundles in the stem of this plant ; see
Nageli, Beitrage, I : also Sanio, Bot. Zeitg. 1864, p. 228.]

^ [See note to p. 630.]

^ Le Maout and Decaisne's Traite general de Botanique, descriptive et analytique, is strongly to
be recommended for a study of the diagnosis of the families [translated by Mrs. Hooker; London

1873]. ^

* Endlicher, Genera plantarum secundum ordines naturales disposita, Vindobonae, 1836-1840;
and Enchiridion botanicum, Lipsiae — Vienn^e, 1841.

•'"' A. Braun, Uebersicht des naturlichen Systems, in Ascherson's Flora der Provinz Brandenburg,
1864.

•^ Hanstein, Uebersicht des natlirlichen Pflanzensystems, Bonn 1867. In the first edition of this

DICO TYLEDONS. 6 55

tion however assigns far too great an importance to this particular point of structure,
considering that on the one hand flowers occur among the Eleutheropetalse which
differ greatly from one another not only in this but also in every other respect ; while
on the other hand the most intimate relationship exists between particular sections of
Eleutheropetalae and of Gamopetalae. I therefore think it convenient, while retaining
the largest sub-divisions of the class, to employ also other characters in the classifi-
cation ; and to make use of the character drawn from the cohesion or non-cohesion
of the petals in the subdivision of the largest group, that provided with two perianth-
whorls. In the following classification Dicotyledons are split up into five divisions of
equal systematic and morphological value, which should rather be arranged parallel to
one another than in a single linear series. This classification has also, I think, a practical
advantage ; since the extraordinarily large number of families and orders can be more
easily kept in the memory when they are at once arranged in several comprehensive
groups of equal value.

DICOTYLEDONS.

I. JuliflorsB :

A. Piperineae,

B. Urticineae,
G. Amentiferae.

1 1 . Mon oclilamydesB :

A. Serpentarieae,

B. Rhizantheae.

III. AphanocyclaB :

h. Hydropeltidineae,

B. Polycarpae,

C. Cruciflorae.

IV. TetraeyclfiB :

(a) GamopeialcB :

A. Anisocarpae,

B. Isocarpae.
(/3) EleutheropetalcB :

C. Eucyclae,

D. Centrospermae,

E. Discophorae.

V. Perigynas :

A. Calyciflorae,

B. Corolliflorae.

The sections designated by capital letters correspond partly to single orders, partly to
whole series of orders in the system referred to above.

I. JULIFLORiE.

Flowers very small or inconspicuous, crowded in dense inflorescences — spikes, capi-
tula, or less often panicles — which are often of very peculiar form ; naked or with a
simple sepaloid perianth, and usually diclinous; the male and female flowers often
different. Leaves simple.

A. PiperineoB. Flowers very small, in dense spikes subtended by bracts, without

book I followed this work with but little deviation. Compare also Grisebach, Gnindriss der
systematischen Botanik.

656 PIIA NEK OGA MS.

a perianth. The small embryo lies, surrounded by the endosperm, in a hollow
of the copious perisperm. Herbs or shrubs, often with verticillate leaves.
Families: i. Piperaceae,

2. Saurureae,

3. Ghlorantheae.

B. Urticineoi. Perianth simple, sepaloid, three- to five-partite, sometimes absent;
stamens superposed on the segments of the perianth ; flowers hermaphrodite or
diclinous, and then the male and female flowers difl'erent (3), usually in densely
crowded inflorescences, the female flowers in spikes, umbels, capitula (2) or some-
times panicles (3), not unfrequently developing into peculiar pseudocarps (as the
Mulberry, Fig, Bread-fruit, and Dorstenia). Fruit usually unilocular, rarely bilocular ;
ovules one or rarely two in each loculus ; seed usually with endosperm. Large
shrubs or trees ^ ; leaves stalked, usually stipulate.

Families: i. Urticaceae,
Urticeae,
Moreae,
Artocarpeae,

2. Platanaceae,

3. Gannabineae,

4. Uhnaceae (including Celtideae).

C. AmentifercB. Flowers diclinous, epigynous, in compact panicles (false spikes);
the female few-flowered inflorescence in (2) surrounded by a cupule. Fruit dry,
indehiscent, one-seeded ; seed without endosperm. Trees with deciduous stipules.

Families: i. Betulaceae,
2. Cupuliferae.

II. MONOCHLAMYDEiE.

Flowers large and conspicuous and consisting of a simple more or less petaloid,
usually gamophyllous perianth, one or more staminal whorls, and a polycarpellary ovary ;
carpels equal in number to or double that of the segments of the perianth. The number
of members of the whorls is derived from the typical numbers two, three, four, or five,
and generally increases in the inner whorls. Ovary generally inferior and surmounted
by a short thick columnar style, to which in the hermaphrodite flowers the stamens are
usually partially or entirely adherent. Flowers often diclinous. Seeds numerous.

A. Serpent arie(B. Creeping or climbing plants with slender stems and large
simple leaves; floral whorls dimerous and tetramerous (i) or trimerous and hex-
amerous; perianth-leaves free (i) or coherent into a tube; ovary of four or six
loculi ; embryo small but diflferentiated.

Families: i. Nepentheae,

2. Aristolochiaceae,

3. Asarineae.

B. Rhi%anthe(B. Root-parasites without chlorophyll or foliage-leaves, generally
with stunted vegetative organs and very large solitary flowers or small flowers in
a dense inflorescence (i); whorls dimerous to octamerous (i), trimerous (2), or
pentamerous and decamerous (3); ovary with one or eight (i) loculi; the placentae
and anthers of very peculiar form ; a very great number of small seeds with rudi-
mentary embryo.

Families: i. Cytineae,

2. Hydnoreae,

3. Rafflesiaceae.

[The Urtice?e include a number of herl^aceous genera.]

DICOTYLEDONS. 657

III. APHANOCYCLiE.

Flowers hemicyclic or cyclic, or the parts arranged spirally ; the members of each
whorl usually free, not coherent with one another, or only in the gynseceum ; perianth
generally distinctly differentiated into calyx and corolla ; the numbers of the parts in the
four whorls very variable ; stamens usually more in number than perianth-leaves ; carpels
forming generally one, several, or a large number of monocarpellary ovaries ; the ovary
is usually superior and, when polycarpellary, is uni- or multi-locular. Ovules springing
occasionally in all the groups from the inner surface of the carpels.

A. HydropeltidinecB. Water-plants with solitary lateral and usually large flowers,
the perianth-leaves and stamens variable in number and arranged spirally ; ovaries
several and monocarpellary (r, 2), or one only polycarpellary and multilocular ; embryo
small, usually surrounded by a small endosperm in a hollow of the perisperm.

Families: i. Nelumbiaceae,

2. Cabombeae,

3. Nymphseaceae.

B. Polycarpce. Parts of the flowers arranged spirally or in whorls, when in
whorls usually dimerous or trimerous, each series generally consisting of more than
one whorl, rarely in four pentamerous whorls (2); gynaeceum consisting of one,
several, or a larger number of monocarpellary ovaries (trimerous and unilocular in 8),
which are one- or many-seeded; embryo small; endosperm none (8), abundant, or
very large (9).

Families: i. Ranunculaceae,

2. Dilleniaceae,

3. Schizandreae,

4. Anonaceae,

5. Magnoliaceae,

6. Berberideae,

7. Menispermaceae,

8. Laurineae,

9. Myristicaceae.

C. Cruct/lorce. Perianth-whorls dimerous; in (3) and (4) corolla of four petals
placed diagonally ; staminal whorls two or more, each consisting of two stamens or
divisible by two ; ovary polycarpellary, unilocular, or (spuriously) bilocular, or multi-
locular; seed with (i, 2) or without endosperm.

Families: i. Papaveraceae,

2. Fumariaceae,

3. Cruciferae,

4. Capparideae.

IV. TETRACYCLiE.

Parts of the flower always arranged strictly in whorls ; the typical number of whorls
is four, the calyx, corolla, androecium, and gynaeceum each consisting of a single whorl;
whorls generally pentamerous, rarely tetramerous (very rarely dimerous or octamerous) ;
any one of the whorls may be entirely wanting, or individual members may be abortive ;
this occurs most often with the stamens and carpels. Increase in number of the stamens
usually takes place by the interposition of one perfect or imperfect whorl between the
members of the typical whorl or a little outside it, or by doubling of the members, or by
branching of the original staminal leaves ; increase in number of the staminal whorls
themselves is rare. All the whorls usually alternate, but the stamens are not unfrequently
superposed on the petals. A tendency prevails in all the sections to a diminution of the
number of carpels below that of the members of the perianth-whorls ; very commonly

u u

658 PHANEROGAMS.

there are only two, one anterior and one posterior. Ovary almost always single and
polycarpellary, inferior or superior, unilocular or multilocular.

I. GamopetalflB or SympetalsB.
The petals united at the base into a tube or cup ; corolla never wanting.

A. Anisocarpm, The number of whorls or of members of the whorls is never
larger than the typical number ; calyx or some of the stamens sometimes abortive ;
carpels usually only two, one anterior and one posterior, or three and united into a
single ovary ^.

a. HypogynsB.

Order 1. Tubiflorae.
Families: i. Convolvulaceae (including Cuscuteae),
3. Polemoniaceae,

3. Hydrophyllaceae,

4. Boraginese,

5. Solanaceae.

Order 2. Labiatiflorse.

Families: i. Scrophulariaceae,

2. Bignoniaceae,

3. Acanthaceae,

4. Gesneraceae,

5. Orobancheae,

6. Ramondieae,

7. Selaginese,

8. Globulariaceae,

9. Plantagineae,

10. Verbenaceae,

11. Labiatae.

Order 3. Diandrse.

Families :

I. Oleaceae,

2. Jasminiaceae.

Order 4. Contortae.

Families :

I. Gentianaceae,

,

2. Loganiaceae,

3. Strychnaceae,

4. Apocynaceae,

5. Asclepiadeae,

b. EpigjmsB.

Order 5. AggregateB.
Families: i. Rubiaceae,

2. Caprifoliaceae,

3. Valerianaceae,

4. Dipsacaceae.

Order 6. Synandree.
Families: i. Gucurbitaceae,

2. Campanulaceae,

3. Lobeliaceae,

The orders are arranged mainly after Braun and Hanstein,

DICOTYLEDONS. 659

4. Goodeniaceae,

5. Stylidieae,

6. Calycereae,

7. Compositae.

B. IsocarpcE, Carpels equal in number to the sepals and petals, usually five,
rarely four, and coherent into a generally superior ovary (except Order i, Family i,
where there are only two median carpels) ; diminution of the number of stamens
does not occur (except in Order i, Family i); in Orders 2 and 3, on the other
hand, a perfect staminal whorl is usually interposed; in Order i the stamens are
superposed on the petals, and a number of seeds spring from an elevated axial
placenta in the unilocular ovary; in Orders 2 and 3 the ovary is multilocular and
many-seeded.

Order 1. Prim.uline8B.
Families; i. Lentibulariaceae,

2. Plumbagineae,

3. Primulaceae,

4. Myrsinaceae.

Order 2. Diospyrinesa.
Families: i. Sapotaceae,

2. Ebenaceae (including Styracaceae).

Order 3. Bicornes.
Families: i. Epacrideae,

2. Pyrolaceae,

3. Monotropeae,

4. Rhodoraceae,

5. Ericaceae,

6. Vaccinieae.

II. Eleutheropetalse or DialypetalsB.

Petals free, sometimes wanting.

G. Eucyclce. Gorolla very rarely wanting; stamens very commonly twice or
three times as many as petals by the interposition of a perfect or even double
(Orders 6, 7) whorl, or by the interposition of an imperfect whorl differing in number
from the corolla (Order 5) ; in the isostemonous flowers the stamens are sometimes
superposed on the petals (Order 4), or the original stamens branch (especially in
Orders 2,3, and 8) ; the number of carpels often the same as that of the sepals
and petals (Orders 7, 8), but commonly less — two, three, or four ; ovary unilocular
with parietal placentae in Order i, in the others multilocular; seed generally without
endosperm.

Order 1. Parietales.
Families : i . Resedaceae,

2. Violaceae,

3. Frankeniaceae,

4. Loasaceae,

5. Turneraceae,

6. Papayaceae,

7. Passifloraceae,

8. Bixaceae,

9. Samydaceae,
10. Gistineae.

u u 2

666 PHANEROGAMS.

Order 2. Quttiferse.
Families: i. Salicineae,

2. Tamariscineae,*

3. Reaumuriaceae,

4. Hypericineae,

5. Glusiaceae,

6. Marcgraviaceae,

7. Ternstroemiaceae,

8. Chlaenaceae,

9. Dipterocarpeae.

Order 3. Hesperidse.

Families: i. Aurantiaceae,

2. Meliaceae (including Cedreleae),

3. Humiriaceae,

4. Erythroxylaceae.

Order 4. FrangulinsB.

Families: i. Ampelideae,

2. Rhamnaceae,

3. Celastrineae,

4. Staphyleaceae,

5. Aquifoliaceae,

6. Hippocrateaceae,

7. Pittosporeae.

Order 5. JEsculinsB.

Families

: I.

Malpighiaceae,

2.

Sapindaceae,

a. Acerineae,

b. Sapindeae,

c. Hippocastaneae

3.

Tropaeolaceae,

4.

Polygalaceae.

Order 6. TerebinthinsB.

Families

: I.

Terebinthaceae,

a. Anacardiaceae,

b. Burseraceae,

c. Amyrideae,

2.

Rutaceae,

a. Ruteae,

b. Diosmeae,

c. Xanthoxyleae,

d. Simarubeae,

3.

Ochnaceae.

Order 7. ,Gruinales.

Families

: 1.

Balsamineae,

2.

Limnanthaceae,

3-

Linaceae,

4-

Oxalideae,

5-

Geraniaceae,

6.

Zygophyllaceae.

DICOTYLEDONS. 66 1

Order 8. ColumniferfiB.
Families: i. Sterculiaceae,

2. Byttneriaceae,

3. Tiliaceae,

4. Malvaceae.

Order 9. Tricoccas \
Families: i. Euphorbiaceae,

a. Euphorbieae,

b. Acalypheae;
2. Phyllanthaceae,

a. Phyllantheae,

b. Buxineae.

D. Centrospermce. Corolla usually wanting (except in Fam. 6) ; stamens fewer
or more often more numerous than the sepals, in the last case generally double as
many (4 or 6) ; ovary usually superior and unilocular, with one or more basal often
campylotropous ovules, less often multilocular with central placentation.

Order 1. Caryophyllineae.
Families: i. Nyctagineae,

2. Chenopodiaceae,

3. Amaranthaceae,

4. Phytolaccaceae,

5. Portulacaceae,

6. Garyophylleae :

a. Paronychieae,

b. Sclerantheae,

c. Alsineae,

d. Sileneae.

E. Discophorce. Ovary inferior (Order i) or half inferior or even superior, and
then (Order 2, Family 5) carpels distinct; carpels as many as or fewer than
sepals and petals (often two) ; when the ovary is inferior or half inferior a necta-
riferous disc usually occurs between the styles and the stamens; stamens equal
in number to sepals and petals (Order i) or twice as many, or even a still larger
number; calyx-limb usually obsolete in Order i ; seed generally with copious
endosperm.

Order 1. UmbelliflorsB.
Families: 1. Umbeiliferae,

2. Araliaceae,

3. Cornaceae.

Order 2. SaxifraginesB ^.
Families: i. Saxifragaceae (including Hydrangeae,
Escallonieap, and Gunoniaceae),

2. Grossulariaceae (?),

3. Philadelpheae (?),

4. Francoaceae (?),

5. Crassulaceae (?).

V. Perigyn^.
Flower displaying a tendency towards the perigynous structure. An annular body
is elevated from the floral axis bearing the perianth and the stamens, and enveloping
the gynaeceum as a cup-, saucer-, or urn-Uke receptacle ; or it becomes adherent in its

^ The position of this order is doubtful.

* The position of the families marked (?) is doubtful.

662

PHANEROGAMS,

growth to the carpels (B, Order 2, Family 2). In a few families which are placed here
provisionally (B, Order 3, Families 4-6) the ovary is truly inferior.

A. Calycijlorm. Perianth simple, either sepaloid or petaloid and usually tetra-
merous; the tubular receptacle is generally of the same nature, and in Family 3 is
even quadripartite, corresponding to the four perianth-leaves and to the four stamens
superposed on them (see Fig. 370) ; stamens fewer than, as many as, or twice as
many as the perianth-leaves ; ovary monocarpellary, rarely bilocular, with one or a
few seeds ; seed with little or no endosperm.

Order 1. ThymelsBineaB.
Families: i. Thymelaeaceae,

2. Elaeagnaceae,

3. Proteaceae.

B. CorolliJiorcE . Calyx, corolla, and androecium placed on a flat (Order i) or
cup-shaped receptacle, or on one hollowed out into a deep urn-shape (Order 2 and
in part 3), which is often (Order 2) thick and succulent (as in the Apple, Rose-hip,
&c.) ; sepals distinct or coherent (Order i); petals always distinct (corolla dialy-
petalous) ; the two perianth-whorls usually pentamerous, sometimes tetramerous ;
stamens as many as or twice as many as (Order i) sepals and petals, or a much
larger number (Order 2), in Order 3, Family 3, commonly branched ; gynaeceum
composed of one (Order i, and in part 2) or several or a large number of mono-
carpellary ovaries; or (in Order 3) ovary polycarpellary, and sometimes inferior
(Families 4-6).

Order 1. LeguminosaB.
Families: i. Mimoseae,

2. Swartzieae,

3. Caesalpineae,

4. Papilionaceae.

Order 2. Bosiflorae.

Families: i. Calycanthacese,

2. Pomeae,

3. Rosaceae,

4. Sanguisorbeac,

5. Dryadeae,

6. Spirseeae,

7. Amygdaleae,

8. Chrysobalaneae.

Order 3. Myrtiflorae.
Families: i, Lythrarieae,

2. Melastomaceae,

3. Myrtaceae,

4. Combretaceae,

5. (Enothereae,

6. Haloragideae.

Families of unknonvn or -very doubtful affinity.
Hippurideae. Polygonaceae.

Balanophorze.

Santalaceae.

Callitrichaceae.

Begoniaceae.

Elatineae.

Casuarineae.

Loranthaceae.

Ceratophyllaceae.

Podostemoneae.

Empetraceae.

Mesembryanthemeae.

Tetragonieae.

Cactaceae.

Myricaceae.

Juglandeae.

' The position of these families here is very doubtful.

BOOK III.

PHYSIOLOGY.

CHAPTER I.
MOLECULAR FORCES IN THE PLANT.

Sect. i. — The Condition of Aggregation of organised structures i.

Cell-walls, starch-grains, and protoplasmic structures consist, in their natural con-
dition, at every point that can be seen even under the microscope, of a combination
of solid material with water. If these organised structures are placed in a sub-
stance capable of removing water, a part of their aqueous contents is withdrawn ;
while, on the other hand, if they are in contact with aqueous solutions possessing
certain chemical properties and of a proper temperature, they absorb more water.
The volume alters with the change in the proportion of water ; loss of water causes
diminution, absorption of water a corresponding augmentation of volume. Since
the absorption of water occasions a considerable elevation of temperature (air-dry
starch rises 2° or 3° C. when mixed with water of the same temperature), it must
be supposed that the water becomes denser as it is absorbed^. ' Within certain limits
these variations in the proportion of water may occur without occasioning any per-
manent change in the intimate structure ; but if, with a higher temperature and in
the presence of chemical reagents, the proportion falls below a certain minimum
or exceeds a certain maximum, permanent changes of the intimate structure take
place which can no longer be reversed ; and the internal organisation of the body
becomes partially or entirely destroyed.

' See Sachs, Handbuch der Experimental-Physiologie, p. 398 et seq. — Nageli u. Schwendener,
Das Mikroscop, p. 422 et seq. ; compare also Book I of this work, p. 28 et seq. — Cramer, Naturforsch.
Gesells. in Ziirich, Nov. 8, 1869.

2 Jungk, in Pogg. Ann. 1865, vol. 125. p. 292 et seq.

664 MOLECULAR FORCES IN THE PLANT. ,

These facts, in connection with a number of other phenomena, first led Nageli
to the hypothesis that organised bodies consist of isolated particles or MicellcE^
between which the water penetrates, and which are solid and relatively unchange-
able, and invisible even with the most powerful microscopes. Every micella of a
saturated organised body is, on this hypothesis, surrounded by layers of water by
which the adjacent micellae are completely separated from one another. These
micellae may be supposed to be of various sizes, and it is evident a priori that, if
the thickness of the aqueous envelope is the same, larger micellae will form a
denser, smaller micellae a less dense substance ; and it may therefore be concluded
conversely that the layers and lamellae of organised bodies of different densities,
especially those of the cell-wall and of starch-grains, are composed of micellae of
different sizes ; and the difference in the proportion of water in such cases leads to
the hypothesis that the densest substance consists of micellae which are several
thousand times larger than those of the more watery substance. As the micellae
increase in size, the density of the whole substance is moreover increased by the
smaller distance that intervenes between them, so that larger micellae are separated
from one another by thinner layers of water. The changes in volume of organised
bodies due to the removal of water or its absorption depend, according to this view,
on the fact that when swelling takes place the micellae are forced further apart by
the water which penetrates between them ; while, on the other hand, when water is
removed they approach one another in proportion as the water is withdrawn from
their interstices.

The forces which are concerned in these processes in the interior of an organ-
ised body may be divided into three kinds : — (i) the Cohesion within each separate
micella impermeable to water, which is itself an aggregate of molecules and atoms ;
(2) the Attraction of the adjacent micellae for one another, in consequence of which
they tend mutually to approach; and (3) the Attraction of the surfaces of the micella
for the absorbed water, which counteracts the mutual attraction of the neighbouring
micellae.

In starch-grains, cell-walls, and to a certain extent in crystalloids^, the absorbed
water is not deposited uniformly in all directions ; the micellae are, on the contrary,

^ [The term Micella was applied by Nageli to the aggregates of molecules of which organised
bodies consist in the second edition of his work on the Microscope (1877). Pfefifer, in his Osmotische
Untersuchungen, published in the same year, applies the general term tagma to all aggregates of
molecules, and the term syn'agma to bodies which are built up of tagmata. In his Theorie der
Gahrung (1879), Nageli gives the following definitions of the terms which he suggests for describing
the constitution of matter: — Atom, the ultimate particle of a chemical element : Molecule, an aggregate
of atoms, and hence the ultimate particle of a chemical compound: Pleoti, an aggregate of molecules,
which, like the molecule, can be neither increased nor diminished without a change in its chemical
nature ; examples of this are afforded by compound salts {e. g. alum), and by salts which contain
water of crystallisation {hydropleon) : Micella, like the pleon, an aggregate of molecules, but differing
from the pleon in that it consists of a much larger number of molecules, and in that increase or
decrease of size does not affect its chemical constitution; in this latter respect it behaves like a
crystal. The micellse may combine to form a Micellar Aggregate {Micellverband), and this may be
so large that it is readily visible ; the crystalloids are examples of large micellar aggregates. It will
be noted that the Atom, Molecule, and Plepn are chemical ideas, whereas the Micella and the
Micellar Aggregate are purely physical.]

^ [See Book I. p. 49, on Crystalloids.]

f

CONDITION OF AGGREGATION OF ORGANISED STRUCTURES. 66^

forced further from one another in certain directions, as is clearly seen upon the
change of form of the whole, from the formation of fissures, &c. One of the most
remarkable effects of the tensions thus caused in the interior of the body is the fact
that when swelling takes place particular diniensions may even decrease ; thus, for
example, the layers of stratification of bast-fibres become very considerably shorter
when they swell up under the influence of dilute sulphuric acid, the coils of the spiral
striation becoming closer and larger in circumference. Crystalloids change their
angles several degrees when they swell. These phenomena are explicable only on
the supposition that the micellar forces in the interior of organised substances vary
in intensity in different directions ; and this again is conceivable only on the hypo-
thesis that the form of the micellae is not spherical. Nageli and Schwendener
obtained a deeper insight into these laws by a very careful observation of the phe-
nomena produced by polarised light in cell-walls, starch-grains, and crystalloids \
They inferred from these facts a crystalline structure of the individual micellae, and
that the crystals are doubly refractive and have two optical axes which are so
arranged, at least in the greater number, that one axis of elasticity within each
micella of starch-grains and cell-walls is placed radially, but the two other axes
of elasticity tangentially. In crystalloids the micellae are probably arranged as in
true crystals, but separated also by layers of water parallel to the faces or Hues of
cleavage.

The behaviour of chlorophyll-granules and of colourless protoplasm towards
polarised light, as well as under the addition and removal of water, is at present but
little known ; and a more definite idea of the form of their micellae is therefore not
yet possible.

The solid micellae of one and the same organised body which are separated
by aqueous envelopes always vary in their chemical nature ; so that at every visible
point micellae which possess chemically different properties lie by the side of and
among one another separated by layers of water. In starch-grains, cell-walls, and
crystalloids this fact is inferred from the circumstance that certain substances are
extracted by the application of certain solvents, while other substances remain behind,
constituting what is called the skeleton. This skeleton is of course less dense than
the original substance ; and it is evident that the extraction has taken place at all
visible points, without the external form or internal structure having undergone any
essential change. Thus, for example, a skeleton of cellulose remains behind when
the lignin has been extracted from wood-fibres by maceration in nitric acid and
potassic chlorate ; and again, a skeleton of silica remains behind with all the optical
properties of the cell-wall w^hen the organic substance has been burnt away. In the
same manner a grain of starch leaves behind a skeleton containing very litde solid
material when the granulose has been extracted by saliva or some other reagent.
From crystalloids also a skeleton in this sense of the term containing very little solid
ynatter can be obtained by the solution of a part of their substance. The properties
of these skeletons show that the micellae which remain behind after solution of the
rest still occupy essentially the same position and are endowed with the same fojces

^ Hofmeister (Handbuch der phys. Bot. vol. I. p. 348) has arrived at altogether different
conclusions, with which I cannot agree.

666 MOLECULAR FORCES IN THE PLANT,

as before ; it is therefore probable that the extracted substance lay previously
between these micellae without being contained in them. This view is also more or
less probable in the case of chlorophyll-granules and protoplasm ; in the former the
fundamental protoplasmic substance remains behind as a very solid skeleton when
the green colouring matter is extracted by ether, alcohol, oil, &c. Very different
substances are certainly combined in the protoplasm ; and when a naked primordial
cell secretes a cell-wall, it may be assumed that the micellae which form the cell-wall
v^'ere previously distributed between those of the protoplasm, and only change their
position and their chemical nature when they are secreted in the formation of the
cell-walP ; the protoplasm which remains behind retaining essentially its original
properties. The same is the case when grains of starch or chlorophyll-granules
are formed in the protoplasm. A fundamental substance is clearly present in the
protoplasm which always retains the essential properties of protoplasm ; but various
other substances penetrate between its micellae which are afterwards excreted. This
is especially observable in the formation of zygospores and swarmspores.

The nutrition and growth of organised structures takes place, as has already
been shown in Book I, by intussusception ; the nutrient solution penetrates between
the micellae already in existence, and either occasions by apposition an enlarge-
ment of the individual micellae, or new micellae of small size are produced in the
spaces filled with water, which then increase by the apposition of new matter, or the
increase takes place in both ways at different points. The increase in surface of the
cell-wall, starch-grain, &c. is therefore brought about by the micellae being forced
apart from within. Connected with the growth of the micellae already in existence
and with the formation of new ones is a continual disturbance of the osmotic equi-
librium between the surrounding fluid (the cell-sap in the widest sense of the term,
see p. 62) and that within the body, which has the effect of constantly drawing fresh
particles from the surrounding fiuid to the interior of the body which is undergoing
augmentation.

Chemical processes in the interior of the growing body are also always con-
nected with these processes of growth. The nutrient fluid which penetrates from
without contains in fact the material for the formation of micellae of a definite
chemical nature; but this material is chemically different from the micellae which
it produces. Thus starch-grains are nourished by a fluid which clearly does not
contain any starch in solution ; and again the cell-wall grows by the absorption of
substances out of the protoplasm which are not dissolved cellulose. The colouring
matter of the chlorophyll arises in the interior of the chlorophyll-granule ; and the
substances by which the protoplasm is nourished by intussusception are clearly
only produced in the interior of the protoplasm, as is shown in particular by naked
Plasmodia and by unicellular Algae and Fungi. Growth by intussusception is
therefore connected not only with a continual disturbance of the molecular equi-
librium, but also with chemical processes in the interior of the growing structure.
Chemical compounds of the most various kinds meet between the micellae of an

^ [According to Schmitz (Sitzber. d. Niederrhein. Ges. f. Natur. und Heilkimde, Bonn, 1880),
the cell-wall is formed, at least in the cases which he observed, by the actual conversion of a
peripheral layer of protoplasm into cellulose.]

CONDITION OF AGGREGATION OF ORGANISED STRUCTURES, 66y

organised body, so that they act upon and decompose one another. It is certain
that all growth continues only so long as the growing parts of the cell are exposed
to atmospheric air ; the oxygen of the air has an oxidising effect on the chemical
compounds contained in the organised structure ; with every act of growth carbon
dioxide is produced and evolved. The equilibrium of the chemical forces is also
continually disturbed by the necessary production of heat; and this may also be
accompanied by electrical action. The movements of the atoms and molecules
within a growing organised body represent a definite amount of work, and the equi-
valent forces are set free by chemical changes. The essence of organisation and
of life lies in this : — that organised structures are capable of a constant internal
change ; and that, as long as they are in contact with water and with oxygenated
air, only a portion of their forces remains in equilibrium even in their interior, and
determines the form or framework of the whole; while new forces are constantly
being set free by chemical changes between and in the molecules, which forces in
their turn occasion further changes. This depends essentially on the peculiarity of
micellar structure, which permits dissolved and gaseous (absorbed) substances to
penetrate from without into every point of the interior, and to be again conveyed
outwards.

This internal instability attains its highest degree in chlorophyll-granules and pro-
toplasm. In the former chemical processes take place with great energy and activity
under the influence of light, such as the formation of the green colouring matter and
of starch ; and when deprived of light other chemical changes at once ensue, which
terminate only with the complete destruction of the entire chlorophyll-granule. The
remarkable properties of protoplasm, which we have already examined from different
sides in discussing the structure of the cell, attain their climax in its spontaneous
automatic power of motion, and in its capacity of assuming different forms and
changing both its shape and its internal state, and therefore of bringing into action
internal forces, even when corresponding impulses from without cannot be observed.
It is impossible to enter here in detail into the explanation of these remarkable facts;
but they will be understood, at least generally and to a certain extent, if it is borne
in mind that neither the chemical nor the molecular forces are ever in equilibrium in
the protoplasm ; that the most various elementary substances are present in it in the
most various combinations ; that fresh impulses to the disturbance of the internal
equilibrium are constantly being given by the chemical action of the oxygen of the
air ; and that energy is continually being set free at the expense of the substance of
the protoplasm itself, which must lead to the most complex actions in a substance of
so complicated a structure. Every impulse from without, even when imperceptible,
must call forth a complicated play of internal movements, of which we are able to
perceive only the ultimate effect in an external change of form.

The destruction of the micellar structure of organised bodies may take place in
many different ways, and affords an insight into many physiological processes.

The most important forces by which the micellar condition of organic substances
is permanently altered are changes in temperature, chemical reagents, and substances
which have a powerful attraction for water. But these agencies do not in general
cause destruction until they have exceeded a definite degree of intensity ; while dif-
ferent changes of temperature and different states of concentration of the reagents

668

MOLECULAR FORCES IN THE PLANT.

not unfrequently give rise to phenomena differing not only in degree but even in kind.
The effect of most external influences depends moreover to a great extent on the
chemical nature of the substance which forms the material and micellar framework of
an organised body. Cell-walls ^ and starch-grains for instance differ from crystalloids,
chlorophyll-granules, and protoplasm, since the former consist mainly of carbo-hydrates
insoluble in water, the latter chiefly of albuminoids.

The following are some of the more obvious phenomena selected from the great
mass of existing observations, which are, however, still incomplete.

(a) Temperature does not usually cause any striking
or permanent change or destruction of organisation till
it exceeds 50°, or sometimes even 60° C, even when
the substance affected is completely saturated with
water. Air-dry organised bodies can generally bear
much higher temperatures without injury. Thus, for
example, the denser portions of a starch-grain which
is saturated with water are not converted into paste
below 65° C, while the more watery portions undergo
this change at 55° G. (Nageli), the capacity for absorb-
ing water and in consequence the volume then in-
creasing enormously. Payen gives the increase in
volume of starch in water at 60° G. as 142 p. c, at 70°
to 72° G. as 1255 p. c, the starch originally containing,
according to Nageli, only from 40 to 70 p. c. water.
Air-dry starch must be heated to nearly 200° G. before
its power of absorbing water materially increases ; but
it is then changed chemically and converted into dex-
trine. The corresponding action of temperature on
cellulose is not yet accurately known, but it is certainly
different from that on starch. Like albuminoids, proto-
plasmic structures consisting for the most part of these
substances are, when saturated, coagulated by a tem-
perature of from 50° to 60° G,, while when air-dry they
can stand much higher temperatures without their
micellar structure being destroyed ^. The remarkable
difference in the action of temperature on saturated
starch on the one hand and on saturated protoplasm
on the other hand must not be overlooked. In the
former case the power of absorbing water is enor-
mously increased ; its structure becomes looser and
more easily susceptible to chemical action ; while the
coagulation of protoplasm diminishes its power of ab-
sorbing water and the mobility of its micellae, and
increases its power of resisting chemical action. This
difference is also manifest when the change of micellar
structure is caused by acids ; and in this respect normal
cellulose behaves in a similar manner to starch.
(b) Acids (especially sulphuric acid) when greatly diluted cause starch-grains and
cellulose at the ordinary temperature to swell up much more violently than pure water,
without however destroying their organisation ; and the previous condition returns when
the acid is washed out. If, on the other hand, the acids are more highly concentrated,

FIG. 466.— Trichoblasts from a leaf of
Hoya carnosa (see Fig. 30, p. 29) ; a and b
after the commencement of the action of
iodine and dilute sulphuric acid; c, when
the swelling in dilute sulphuric aeid has
proceeded further, a. and /3 in a are the
outermost layer not capable of swelling,
and coloured dark-blue, which breaks up
somewhat irregularly in these cases, but in
c more regularly, into a spiral band, while
the inner layers swell between them, and
are coloured light-blue by iodine ; 7 in c
is the cavity of the cell ; e and >) are con-
strictions at points where the outer layer is
especially firm ; at 5 the greatly swollen
substance is beginning to become disor-
ganised (X 800).

^ The cell-wall I suppose here and in the sequel to be neither cuticularised, lignifiied, nor
converted into mucilage.

See Sachs, Handbuch der Experimental-Physiologic, pp. 63 et seq.

CONDITION OF AGGREGATION OF ORGANISED STRUCTURES, 669

a violent swelling takes place in cellulose and starch-grains, and they pass into a pasty
state. Protoplasmic substances, on the contrary, coagulate, as they do under the in-
fluence of higher temperatures. Concentrated sulphuric acid finally completely destroys
the micellar structure of both with a smaller or larger amount of chemical change, and
they deliquesce.

(c) Solution of Potash acts on starch-grains like sulphuric acid, especially in causing
them to swell up. Its action on protoplasmic substances is on the other hand very
different from that of acids ; if the solution is dilute they swell up strongly or deliquesce,
and this is especially the case with protoplasm and the nucleus of very young cells (the
nuclei of older cells often resist the action strongly). But in a highly concentrated
solution of potash protoplasmic structures often retain their form and apparently their
structure ; they neither coagulate nor deliquesce. The fundamental destruction of their
micellar structure which has nevertheless taken place is evident from the fact that they
immediately deliquesce if water is added copiously.

(d) Mechanical Influences. Organised structures bear without injury mechanical forces
such as pressure, impact, or slight traction ; they are either sufficiently elastic, like starch-
grains and cell-walls, again to bring into equilibrium the changes which are thus caused
in their internal tension and external form ; or they are inelastic like protoplasm and
chlorophyll-granules, and can then equalise small passive changes of form in another way.
But stronger forces cause disruptions of the micellae which cannot be again effaced.
The micellar structure of the separated portions may however still be perfectly retained,
as is shown by fragments of starch-grains and cell-walls. This is still more evident
in motile protoplasm, where the separated portions of the previously continuous sub-
stance behave like so many individuals, and have the power of independent motion ;
as, for example, separated portions of plasmodia, the detached halves of the rotating
protoplasm in the root-hairs of Hydrocharis when contracted by a solution of sugar, &c.
In the same manner two or more separated portions of protoplasm may unite into a
whole, as in the formation of large plasmodia and of zygospores, the fertilisation of
oogonia, &c. The only purely mechanical mode in which complete destruction of an
organic structure can be accomplished is by crushing ; /. e. by complete disseverance
of its micellae and their subsequent promiscuous intermixture. In this case a chemical
change usually directly follows the mechanical destruction of the micellar structure of
the protoplasmic substance. In some cell-walls the mere interruption of continuity by
a cut causes striking changes in the adjoining and the more distant parts ; thus, accord-
ing to Nageli, cell- walls of Schi%omeris that have been cut through become shorter and
thicker to a remarkable extent.

{e) Changes in the micellar structure of organised structures caused by injurious
influences determining their death are often accompanied by striking changes in their
relations to diffusion. With respect to starch and cellulose but little is known in this
respect ; but the phenomena connected with protoplasm, including the nucleus, are very
remarkable \ Normal living protoplasm does not, for example, absorb any colouring
material from the surrounding solution ; but as soon as it has been killed by heat or by
a chemical reagent, the dissolved colouring material not merely penetrates into it, but
accumulates in it to such an extent that the dead protoplasm appears of a much deeper
colour than the surrounding solution of the colouring substance. Starch and cellulose,
on the contrary, even in a fresh unchanged condition, absorb from a solution of iodine
a comparatively much larger quantity of iodine than of the solvent, and become of a
much deeper colour than the surrounding solution ; the colour is also different, usually
blue, while the surrounding solution is yellowish brown. The protoplasm which fills the
cells and has been killed in any manner, by frost, heat, or chemical agents, is more

* Nageli, Pflanzenphysiologische Untersuchungen, vol. I. p. 3 et s^g-.— Hugo de Vries, Sur la
permeabilite du protoplasm des betteraves, Arch. Neerland. vol. VI, 1871 : [also id., Unters. ueb.
die Mechanischen Ursachen der Zellstreckung, i87jf. — PfefTer, Osmotische Untersuchungen.]

670 MOLECULAR FORCES IN THE PLANT,

permeable (whether cellulose is so also is not known) ; it allows the cell-sap, which in
living and growing cells is always subject to high pressure, to filter out as if it had
become porous. This is well seen when coloured cells or tissue are frozen or heated
above 50° C. ; they then allow their coloured contents to diffuse out, which they do
not do when living.

(/) The true nature of the change which the micellar structure of moist organ-
ised bodies undergoes by heating above 50° or 60° C, or when they are made to swell
up strongly by treatment with acids or alkalies, is considered by Nageli to lie in the
destruction of the crystalline micellae. In the case of starch-grains and cell-walls this
view is supported by a few facts which have hitherto not been explained in any other
manner. The increase of the power of absorbing water under such conditions is then
explained on the hypothesis that the number of particles which attract water is increased
and their size diminished by the destruction of the micellae ; and this must necessarily
be connected with an increase in the proportion of water and a corresponding increase
in volume. It is especially noteworthy that the denser layers of starch-grains and cell-
walls become under these circumstances homogeneous with the least dense and most
watery layers. But since the denser layers probably consist of large, the less dense
layers of small micellae, the explanation may lie in the fact that the large micellae
of the dense substance are broken up into a number of small micellae, and thus
become similar to those of the less dense substance. The same explanation may be
given of the fact that when the organised structure is changed by undergoing strong
swelling, the optical properties of starch and cellulose also undergo change ; their pre-
vious action on polarised light disappearing altogether. This is also explained if we
suppose that under the action of these agents the micellae which produce the optical
effect lose their form, and that their fragments are irregularly intermixed.

How far these views can be applied also to protoplasmic structures and their coagu-
lation remains at present uncertain.

(g) The disorganisation of the micellar structure of organised bodies may take place
gradually ; and when it has exceeded a certain limit, a new substance is produced from
the originally organised material, the molecular condition of which has, since the time
of Graham, been termed colloidal. From the similarity which, according to Nageli and
Schwendener, exists between organised and crystalline bodies, it is not surprising that
there are also mineral substances which, like silica, are usually crystalline, but become
under certain circumstances colloidal ^. Organised bodies absorb water and other fluids,
increasing at the same time in volume up to a certain maximum at which they are
saturated ; crystalline bodies dissolve in a definite minimum of water and produce a
saturated solution which can be diluted ad libitum. Colloidal bodies show in this respect
intermediate properties ; they can be mixed with water in all proportions without any
minimum or maximum. Solvents cause in organised and crystalline bodies a sudden
passage from the solid to the fluid condition. Colloidal bodies pass from the solid to
the fluid condition, when they are soluble, through all stages of softening ; in a certain
state when they contain but little water they are hard, then tenacious, then viscous and
scarcely fluid, finally when mixed with abundance of water perfectly fluid. Even in
the fluid state they may be mucilaginous, adhering strongly to organised, less strongly to
crystalline substances ; and even when greatly diluted they diffuse very slowly, and some
of them appear unable to penetrate organic membranes such as cell-walls. On drying
they afford a homogeneous substance which differs greatly in its capacity for swelling and
in its optical properties from crystals and from organised bodies. In contradistinction to
these latter, colloidal bodies may be considered amorphous internally as well as externally.
Colloidal bodies occur abundantly in plants as products of the decomposition of organised
bodies, and under certain circumstances they supply material for the production of new
organised bodies. Thus gum-bassorin and perhaps also gum-arabic, as well as the

^ See, among other authorities, Graham, Phil. Trans. 1862; Journ. Chem. Soc. 1862.

CONDITION OF AGGREGATION OF ORGANISED STRUCTURES. 6yi

mucilage of quince and linseed, result from the decomposition of cell-walls; perhaps also
the formation of the substance of the cuticle must be included in this category. Viscin
is the product of decomposed cellulose ; the origin of colloidal pectin and caoutchouc is
still unknown ; but none of these substances are of any further use to the plant.

(b) Traube's Artificial Cells ^ Among the most important of the phenomena belonging
to the growth of the plant are those connected with the cell-wall; and everything
which contributes to a more exact knowledge of its development must always be
welcome. The researches of Traube, of which an abstract is here given, are of great
interest from this point of view ; even though it may not always be possible to transfer
all the properties of his artificial cells to the real plant.

Starting from Graham's observation that dissolved colloids cannot diffuse through
colloidal membranes, and from the empirical fact that precipitates of colloidal substances
are usually themselves colloidal, Traube found that a drop of a colloid A placed in a
solution of a colloid B must become surrounded by a pellicle. If ^ is also more con-
centrated (or rather if its attraction for water is greater) the cell must become turgid,
i.e. the precipitated pellicle must become stretched by the additional water that is
absorbed; and the micellae of the pellicle thus become separated to such an extent
that a fresh precipitate takes place between them which occasions increase in the
superficies of the pellicle. For a more exact study Traube chiefly employed cells the
pellicle of which consisted of a precipitate, of gelatine tannate. For this purpose the
tendency of the gelatine to coagulate was destroyed by boiling for thirty-six hours.
A large drop of this so-called 3 gelatine of the consistency of syrup was taken up by a
glass rod, allowed to dry for some hours in the air, and then plunged into a flask half
filled with a solution of tannic acid, into the cork of which the rod was fixed. The
portion of gelatine which undergoes solution on the outside of the drop immediately
forms a completely closed pellicle with the surrounding solution of tannin ; and the
water which penetrates through it constantly dissolves the gelatine within. In a dilute
solution of tannin of o"8 to i'8 p.c. a tense pellicle which is not iridescent and is there-
fore thick is formed; in a concentrated solution of from 3*5 to 6 p.c. (in which therefore
there is a smaller difference between the concentration of the two fluids) a thin flaccid
iridescent pellicle is formed^. Traube found that the cells which are at first thick-walled
go through various stages of development ; they remain spherical so long as the nucleus
of gelatine is not completely dissolved ; a turbidity then sets in from above downwards
owing to the solution of a part of the pellicle in the solution of gelatine which is more
dilute in its upper part ; the pellicle at the same time begins to collapse and to become
iridescent ; and finally the contents become clear and tension is again set up. After
the lapse of some weeks the cell still allows gelatine to escape when torn. The greater
the difference in the concentration of the two fluids, the firmer and more tense is the
pellicle; i.e. the greater the intensity of the endosmotic attraction the greater is the
number of layers of atoms which coagulate so as to produce the pellicle, and therefore
the thicker it is.

With reference to the properties of the pellicle, Traube shows that all pellicles
hitherto employed in experiments on diffusion have perforations^, while the precipitated

' Traube, Experimente zur Theorie der Zellbildung u. Endosmose, in Arch, fiir Anat., Phys.,
u. wissensch. Medicin, von Reichert u. Du Bois, 1867, p. 87 et seq. : [also Pfeffer, Osmot. Unters.]

"^ Only pellicles of gelatine behave in this way; all others are iridescent when tense.

^ It is easy to convince oneself of the presence of actual perforations in pig-bladder, ox-bladder,
the pericardium, amnion, collodion-membrane, or parchment, with which experiments on diffusion
have hitherto usually been made, by stretching them over a wide glass tube, pouring in a column of
water from 20 to 40 cm. high, and repeatedly drying the free surface of the membrane with filtering
paper. Water is then almost always seen to ooze out at particular spots ; a piece of membrane
2 or 3 cm. square is seldom water-tight. The perforations are still more evident if the tube is filled
with a concentrated solution of common salt and the membrane dipped in water. Instead of a

672 MOLECULAR FORCES IN THE PLANT.

pellicles have only micellar interstices; and indeed these latter are, according to
him, smaller than the molecules of the precipitate of which the pellicle is composed.
But in spite of the greater density, the endosmose is quicker than with all other
membranes, because they are thinner. The pellicle becomes firmer (stiffer ?) when lead
acetate or copper sulphate is added to the /3 gelatine. As soon as the micellae of the
stretched pellicle have become so far separated by the pressure of the cell-contents
which have increased in quantity by the action of endosmose that their interstices allow
the passage of the two substances from which the pellicle is formed, these substances
must obviously again at once mutually react upon one another at those points, and must
cause the production of new micellae of pellicle, which are deposited between those
already in existence. Growth therefore takes place by intussusception, and is caused by
the stretching of the pellicle, which stretching is on its part occasioned by endosmose.
That the growth takes place not only by stretching but also by deposition Traube
proved by replacing the tannic acid by water. As soon as this was done {i.e. as soon
as the formation of new molecules of the precipitate in the pellicle was prevented, the
endosmose still continuing) the growth ceased.

As long as the concentration of the contents of the artificial cell is everywhere the
same, the pellicle remains everywhere equally thick, and the cell retains its spherical
form. But when the contents become diluted, a denser solution is formed in the lower
part of the cell, a more watery solution in the upper piirt. The pellicle becomes
in consequence thinner above, because the difference of concentration is smaller there,
and therefore more extensible ; hence the pellicle becomes more strongly stretched above
and increases more rapidly in superficies, and protuberances directed outwards are not
unfrequently formed. This may be expressed shortly by saying that endosmose takes
place principally in the lower part of the cell, growth in the upper part. The difference
however in the concentration in the interior of the cell which causes this is the con-
sequence of the water which penetrates by endosmose not mixing at once uniformly
with all parts of the interior solution, so that layers of different specific gravity lie
one over another.

Further experiments showed that growing pellicle-precipitates having the form of
cell-walls are produced also by mixing colloids with crystalloids^; e.g. tannic acid with
copper and lead acetates, gelatinous silica with the same substances or with copper
chloride, or finally crystalloids with one another, as potassium ferro-cyanide with copper
acetate or chloride. Traube came to the conclusion that every precipitate the inter-
stices of which are smaller than the molecules of its components must assume the form of
a pellicle when the solutions of its components come into contact with one another.
Since the pellicle-precipitates, as has already been mentioned, contain micellar interstices
but no perforations, they are peculiarly well adapted for the study of endosmotic pro-
cesses. They behave in this respect very differently from other membranes, being
themselves often perfectly impermeable to the most diffusible substances, but allowing
other chemical compounds to pass through them ; and every kind of pellicle has in this
respect its own peculiarities. Independently of the fact that every pellicle-precipitate
is impermeable to the fluids from which it is itself produced, the 3 gelatine tannate is,
moreover, impermeable for example also to potassium ferro-cyanide, but permeable to
ammonium chloride, barium nitrate, or water. The pellicle of copper ferro-cyanide
which is formed round a drop of copper chloride in potassium ferro-cyanide is imper-
meable to barium chloride, calcium chloride, potassium sulphate, ammonium sulphate,

diffusion-current equal over the whole surface of the membrane, separate threads of the solution of
salt are seen to sink down into the water. These experiments show how little dependence is to
be placed on the researches hitherto made on diffusion with membranes.

^ [The term 'crystalloid' is here used in the sense in which it was first employed by Graham,
to indicate those substances— as opposed to 'colloids' — which may be susceptible of crystallisation,
and which are endowed with the power of diffusion through a porous septum.]

CONDITION OF AGGREGATION OF ORGANISED STRUCTURES. 673

or barium nitrate, but permeable to potassium chloride or water. Traube considers
that in the permeability of the pellicle-precipitates we have a means of determining the
relative size of the molecules of different solutions, since only those molecules can pass
through the pellicle which are smaller than its micellar interstices and therefore smaller
than the molecules of the solutions which produce it.

If a small quantity of ammonium sulphate is added to a solution of /3 gelatine, and a
small quantity of barium chloride to one of tannic acid, and the two mixtures thus
obtained are themselves mixed, a pellicle is formed of gelatine tannate, and in it a
precipitate of barium sulphate which diminishes the size of the interstices; the two
solutions which cause the deposit can no longer diffuse ; but the incrusted pellicle is still
permeable to the smaller molecules of ammonium chloride and water.

Traube maintains that there is no such thing as an endosmotic equivalent in the
sense of the older theory. Endosmose is independent of any interchange, since it
results entirely from the attraction of the soluble substance for the solvent ; and this
attraction is invariable at any given temperature and may be termed Endosmotic Force.
The endosmotic force of grape-sugar, for instance, is very great, that of gelatinous
substances very small.

To these researches, which are of extreme importance in reference to vegetable
physiology, and of which we shall make much use in the sequel, though with a cautious
selection, Traube has added observations on the growth of the pellicle-precipitates of
copper ferro-cyanide, the main results of which however I have been unable to confirm
after a number of experiments.

If a drop of a very concentrated solution of copper chloride is dropped into a
dilute solution of potassium ferrocyanide, it immediately becomes coated with a thin
brownish pellicle of copper ferrocyanide which exhibits peculiar phenomena. It is
more convenient to place small pieces of copper chloride in the ferrocyanide solution,
where a green drop is immediately formed at the expense of the water of the solu-
tion, producing the pellicle on its surface, and still enclosing the solid copper chloride
which dissolves gradually from the permeation of the water. These cells manifest active
growth and a variety of differences not easy to explain and dependent on secondary
circumstances. Some have very thin pellicles, are roundish, and exhibit a slight tendency
to grow upwards ; they usually form a number of small wart-like outgrowths and attain
very considerable dimensions (from i to 2 cm. in diameter) ; they appear to be formed
chiefly by the solution of large pieces of the copper chloride. Others have thick reddish
brown pellicles, grow quickly upwards in the form of irregular cylinders, rarely branch,
and attain a diameter of from 2 to 4 mm. and often a height of several centimetres.
Combinations of the two forms also occur which sometimes form a kind of horizontal
tuberous rhizome-like structure from which long stalk-like outgrowths arise upwards,
and root-like protuberances downwards.

It is impossible, in the space at our disposal here, to give a detailed description
of these phenomena ; one only may be specially mentioned : — that these pellicles of
copper ferrocyanide do not grow, as Traube supposes, by intussusception, but in quite
a different way (by eruption). When a brown pellicle has been formed round the green
drops, water penetrates quickly from without through it to the copper chloride ;
the pellicle becomes rapidly stretched, and, as may be clearly seen, at length ruptured.
The green solution immediately escapes through the fissure, but becomes at once coated
with a pellicular precipitate which appears either as an intercalated piece of the previous
one, or as an excrescence or branch of it, a process which is repeated as long as any
copper chloride remains inside the cell. We cannot therefore in this case conclude
that deposition of fresh micellae of the pellicle takes place between those already in
existence. These cells cannot, so to speak, be injured; if they are pricked, then at
the moment when the point which pricks them is withdrawn an outgrowth follows
immediately, which is easily to be explained from what has been said. In conse-
quence of the rapid flowing in of water through the perforation, the dissolved or the

X X

674 MOLECULAR FORCES IN THE PLANT.

still solid copper chloride has no titne to form a homogeneous solution ; a stratifica-
tion arises which begins in the lower part of the cell with a very concentrated
solution, and passes in the upper part into almost pure water when the cell has already
grown to some height. Since the dilute upper fluid is lighter than the surrounding
solution, it exerts an upward pressure upon the membrane — just as a cork held down
under water attempts to rise— till it is ruptured below or at the apex (in the second
form of cell). But the lighter fluid, when on the point of ascending, becomes at once
surrounded by a pellicle which remains attached to the walls of the fissure of the old
one ; and thus apical growth takes place in cells of this description in the form of
eruptions, just like the formation of branches and excrescences in the round ones. If
the fluid in the upper part of the cell is pure water, large pieces of the pellicle break oiT
and rise up into the surrounding solution like air-balloons open below. If the copper
chloride is entirely consumed in the formation of the pellicle, the opening caused by
the tearing cfF of the upper cap does not close, or the whole cell ascends like an air-
balloon. If rapidly growing cells of the second form are placed in a horizontal posi-
tion, an outgrowth takes place at the extreme apex as the least solid point, which is
directed vertically upwards, and then grows in this direction like the former apex of the
cell. This process, even though it calls to mind distantly the bending upwards of grow-
ing stems which are placed horizontally (geotropism), bears in fact not the least actual
resemblance to this phenomenon, as will be shown in Chap. IV; and this is at once
evident if it is remembered that in these cells there is no such thing as growth by
intussusception.

Sect. 2. Movement of Water in Plants^ The growth of the cells of plants
is always connected with the absorption of water, not only as regards the in-
crease of size of the vacuole, but the growth of the cell-wall and of other organ-
ised structures is also accompanied by the intercalation of particles of water be-
tween the solid micellae. Water must therefore be conducted to the growing cells
and tissues; and when the organs which absorb the water lie at a distance from
those which require it for their growth, the movement which results is necessarily
considerable. Water is in the same manner required by the organs of assimilation,
since it furnishes the hydrogen required for organic compounds. The reservoirs
of food-material in which the assimilated compounds are for a time accumulated
also require water for the purpose of again dissolving these substances, in order that
they may be carried as formative materials to the leaves and the growing apices of
roots and stems. All these movements of water, which are necessarily connected
with nutrition and growth, proceed slowly Hke growth itself; their direction is in
general determined by the relative positions of the organs which absorb the water
from without and of those which make use of it.

In plants which grow under water or beneath the ground where no loss of
water takes place or only to a very considerable extent, there is no need for
these processes. The case is nearly the same also with some land-plants which
are almost completely protected by a pecuHar organisation from loss of water by
evaporation (transpiration) when it has once been absorbed, as the Cactus-like
Euphorbias, Stapehas, &c., which are by this means enabled to live in the most
arid localities. But the great majority of plants have foliage with a very large

^ See Sachs, Handbuch der Experimental-Physiologic, the section on the movement of water,
p. 196, where the literature up to 1865 is mentioned; the most important of the more recent
publications are quoted in the sequel.

MOVEMENT OF WATER' IN PLANTS, 675

superficial development; when the leaves are also delicate, as in most plants with
a rapid growth, a very considerable portion of the water of their cell-sap is
removed by transpiration within a short time, so that in the course of a single
period of vegetation the quantity of water which has been withdrawn by tran-
spiration may exceed many times the weight and volume of the plant itself. It is
easy to understand that this is possible only when the loss is compensated by the
absorption of corresponding quantities of water through the roots, and that the
water withdrawn from the leaves is replaced in this way. As long as the tissue
of plants in which transpiration takes place remains turgid, the supply must nearly
equal the loss; so long therefore as transpiration proceeds continuously from the
leaves or other surfaces, a constant current of water exists from the roots to the
leaves. When transpiration ceases, as in very moist air, or when the leaves are
wetted by dew or rain or after the falling of the leaves, the current of water also
ceases as soon as the tissues which have become somewhat flaccid are again tur-
gescent. Since transpiration is accelerated by a high temperature of the air, by its
dryness, and above all by sunshine, and as these conditions are constantly changing,
the rapidity of the current of water is also subject to continual change.

The current of water occasioned by transpiration has, as will be seen, no
immediate connection with the processes of growth and nutrition ; the Horse-
Chestnut and other trees and shrubs which put out in spring only a definite number
of leaves, and during the summer do not any further increase their foliage, transpire
the most rapidly during this time; and at this time also the current of water is
most considerable in them. In winter both growth and transpiration, and with the
latter the amount of water also in the tissues, remain stationary ; when the buds are
put out, the water is first of all only set in motion to the extent required by the
increase of the growing organs; but as the development of the organs increases their
surface, the amount of evaporation again rises, and the current begins afresh.

While the movement of water required for purposes of growth and nutrition
must take place in the most different forms of tissue — in the parenchyma and even in
the primary meristem of buds and of the apices of roots — it is nevertheless certain
that the current of water caused by transpiration passes exclusively through the woody
portion of the fibro-vascular bundles ; all the rest of the tissue may be destroyed
at any place without the current of water ceasing, if only the wood remains entire.
In Conifers and Dicotyledons which have a compact wood, one main current passes
through the root and stem, dividing in the branches and leaves into constantly
narrower channels; while in Ferns and Monocotyledons the current of water
passes, even in the primary stem, through isolated narrower channels corresponding
to the course of the isolated woody bundles. That the lignified elements of the
xylem of the fibro-vascular bundles determine the channel of the current, is seen
not only from direct observation, but also from the fact that the formation of
wood is the more considerable, the greater the evaporation and the stronger
the current of water in a plant. In submerged and underground parts of
plants from which no transpiration takes place the xylem remains entirely or
nearly unlignified; in Dicotyledons and Conifers, where the transpiring surface
increases with age, the channel taken by the current is also annually widened by
.the increase of the wood. The crown of leaves of Palm-trees remains after a

x x 2

(i^d

MOLECULAR FORCES IN THE PLANT.

certain time of nearly the same size, and the stem and the channels of the
current (woody bundles) which traverse it consequently retain their diameter
unchanged.

The movements of water caused by growth as well as those induced by
evaporation have this in common, that their direction is towards the places where
they are required. If growth or transpiration begins at a certain time at a
definite spot, the nearest portions of the tissue give up their water first of all,
then the more distant ones, the organs at the greatest distance, generally the
roots, absorbing water from without. The movement therefore propagates itself

continually further and further from the point to which
it tends, and finally over the whole plant to the root.
The kind of motion may therefore — without consider-
ing for the moment its actual causes — be described
as a process of suction. This is especially evident
in leafy stems and branches which, having been cut
off and placed with their cut surface in water, suck
up as much water through their woody bundles as
is required for transpiration and for the unfolding of
fresh leaves, unassisted in this case by any pressure
from below.

Another kind of motion of water in the plant,
depending not on suction but on pressure from below,
is caused by the roots, and is altogether independent
of the use of the water for the purpose of growth or
of transpiration. If the woody stem of a land-plant
is cut through above the root, the root being attached
to the ground in the ordinary manner, and if the
ground is damp and sufficiently warm, water exudes
from the transverse section of the stem either at once
or after some time, the current continuing for days,
and the quantity of water which flows out amounting
sometimes to many times the volume of the root.
This current of water, which rises through the wood
and especially in the vessels, can only be induced by
a pressure existing in the lower parts of the root. If
a manometer of a proper form is fixed in the section
(Fig. 467), it shows that even in smaller plants with but little wood (as Tobacco,
Maize, the Stinging Netde, &c.) the water which exudes stands at a pressure
which holds in equilibrium a column of mercury several centimetres in height ;
while in some woody plants, as for instance the Vine, this pressure may amount
to 76 cm. (or one atmospheric pressure).

In many plants of small height this root-pressure is observable from the fact
that water exudes at particular points of the leaves in the form of drops, pro-
vided that the internal supply of w^ater is nowhere diminished by powerful transpira-
tion, and the pressure thus_ removed. Thus drops of water appear abundantly
and repeatedly on the margins and apices of the leaves of many Grasses (especially

Fig. 467.— Apparatus for observing the
force with which water escapes under root-
pressure from the transverse section of a
stem r. The glass tube R is first of all
firmly fastened to the stem, and the tube r
then fixed into it by the cork A. H is com-
pletely filled with water, the upper cork i
then fixed in it, and mercury poured into
the tube r so as to stand from the first
higher at g' than at g, the level ^' rising
above ^ according to the intensity of the
root-pressure. Tiie apparatus is much
more convenient to handle than that hither-
to in use.

MOVEMENT OF WATER IN PLANTS, 6y'J

Striking in the Maize), Aroideae, Alchemilla^^ &c., when transpiration is diminished
by the absence of light and the cooling of the air, and the activity of the roots
increased by warm damp earth. Even in unicellular plants, or those which
consist only of rows of cells, as the Mucorini (e. g. Pilobolus crystallinus), Pem'cillium
glaucum, and the large Fungi (as Merulius lacrymans), the water is forced out in
drops from the upper part, it having been absorbed by the lower parts which
perform the function of roots and press it upwards.

Fluid however not unfrequently appears in drops in places where there can
b'e no pressure directed upwards from the root. Thus the nectaries of flowers,
as those of Fritillaria imperialism and the glands in the pitchers of Nepenlhes ^, &c.,
exude drops of liquid even when the stem is cut off from the root and merely
placed in water. In this case the forces which cause the pressure must arise in
the upper masses of tissue, perhaps even in the organ itself, for the water is
conveyed to the cut stem not by pressure but by suction.

The phenomenon known as the ' bleeding ' of wood cut in the winter must not
be confounded with this. This bleeding occurs when the cut branch or piece of
stem, previously cold and saturated with water, is rapidly warmed ; the air which
is enclosed with the water in the cells and vessels of the wood expands, and forces
the water out where it can find an opening. If the piece of wood is again cooled,
the air contracts, and the water in contact with the section is again sucked in. It
is evident that these expansions and contractions of air in the wood must also take
place when the woody substance of the tree is uninjured; and hence currents are set
up from the parts which are becoming warmer to those which are becoming cooler,
and tensions are brought about. All this however happens only so long as air as
well as water is found in the cavities of the wood, as is the case in the winter
and spring before the leaves unfold and transpiration begins.

Although the movements of water in plants have been copiously investigated and
discussed for nearly 200 years, it is nevertheless still impossible to give a satisfactory
and deductive account of the mode of operation of these movements in detail ^ This

^ According to Duchartre, De la Rue, and Rosanoff, the exudation usually takes place through
stomata, which are either developed in a peculiar manner, or are very large (water-pores), or
possess the ordinary form. De Bary remarks in connection with this : — ' If water is forced
into the wood of a branch of a plant adapted to the purpose, e. g. Fuchsia globosa, by the moderate
pressure of a column of mercury, drops of water at once exude from the large stomata ' (Bot. Zeitg.
1869, No. 52, p, 882). [This subject has been further investigated by Moll (Med. d. k. Akad. v.
Wet. XV. 2, 1880 : see also Nature, vol. XXII, 1880.]

^ [The liquid contained in the pitcher-like organs of Sarracenia, Nepenthes, Cephalolus, &c. is
not pure water. Dr. Volcker (Ann. and Mag. of Nat. Hist. vol. IV. p. 128, and Phil'. Mag. vol.
XXXV. p, 192) states that it is generally clear and colourless, rarely yellowish, and reddens litmus.
The proportion of residue left on evaporation varies from 0*27 to 0*92 p. c. This residue consists
of 38*61 p. c. organic matter, chiefly malic acid with a little citric acid, 5002 p. c. potassium
chloride, 6*36 p. c. soda, 2*59 p. c, lime, and 2*59 p. c, magnesia. Dr. Buckton (Nature, vol.
HI. p, 34) found that the liquid contained in the pitcher-like labellum of Coryanthes consists of
98*5 1 p. c. water and volatile oils, and 1-49 p. c. non-volatile residue. It is clear and somewhat
glutinous in consistence, with a high refractive power, and a sp. gr. 1-062; neutral to test-paper;
on evaporation it becomes milky, finally yielding a transparent gum insoluble in alcohol. See
paragraph {e) in the appendix to this section.]

3 Although Dr. Miiller, in the second part of his 'Botanische Untersuchungen' (Heidelberg 1872),
assumes that he has actually accomplished this, those only will believe it who are entirely ignorant
of vegetable physiology.

678 MOLECULAR FORCES IN THE PLANT.

much appears certain, that the ultimate force concerned is diffusion (in the broadest
sense of the term). But since in the living plant this force acts under conditions widely
different from those in operation in artificial apparatus, we are compelled on all essential
points to draw our conclusions as to the internal processes from the careful study of
the phenomena in the plants themselves. Our space will however only permit us to
refer to these in general terms. The main result of the investigations hitherto made
is to maintain the distinction between the different causes of motion in the fluids of the
plant to which we have already alluded, until a more thorough knowledge justifies some
other interpretation. What follows is less for the purpose of explaining the phenomena
than of illustrating by examples what has already been said.

(a) The slow movement of water caused merely by Growth and Assimilation is
seen in its simplest form in unicellular Fungi and Algae and in those in which the cells
are arranged in rows and plates, and in germinating spores and pollen-grains ; since in
these cases the growing and assimilating cells absorb the water which they require im-
mediately from their moist environment. That this is caused by the imbibing power
of the cell-wall and of the protoplasm as well as by endosmose {i. e. the attraction of
the dissolved substances within the cell for water), is certain, although we have not
yet sufficient knowledge of the exact mode in which these processes go on. On the
other hand, in plants which consist of masses of tissue the young growing parts
withdraw the water of vegetation from the older mature parts, and these latter become
in consequence empty if they receive no fresh supply from without. This is seen
clearly when tubers, bulbs, trunks of trees which have been cut down, &c.^ put out
buds in ordinary moderately dry air, and thus gradually lose the water they have
contained ^.

{b) Transpiration ^ — /'. e. the evaporation of water from cells and masses of tissue — is
produced and modified by external and internal conditions and causes. Among ex-
ternal causes those must first be noted which produce evaporation from moist surfaces,
such as the relative temperature and dryness of the air and that of the transpiring
tissue itself. Evaporation will generally increase as the temperature of the surrounding
air rises and its degree of saturation consequently decreases ; and this must for our
purpose be considered the most direct measure of the greater or less tendency to
evaporation. It must not however be expected that the amount of evaporation from
plants is simply in proportion to any one of these conditions. It is still doubtful whether
light, i.e. radiation as such, independently of the elevation of temperature caused
by it, influences transpiration ^. The stomata of most plants open more widely in light
than in the dark*; that is, the openings which allow of the escape of the aqueous
vapour formed in the interior of the tissue become larger, and this must have the effect
of promoting further evaporation within. It is not yet decided whether light acts on
the stomata as such, or by means of the heat which accompanies it, or the chemical
changes which it causes.

Among the conditions connected with the organisation of the plant itself which de-
termine the amount of transpiration must be noticed the nature of the cortical tissue, the
size and number of the intercellular spaces, and the character of the substances dissolved
in the cell-sap. When the cortical tissue is a continuous and thick layer of periderm as
in many woody branches, potato-tubers, &c., or a thick layer of bark as in older trunks
of trees, the evaporation of water from the succulent tissues which lie beneath is rendered

^ For further details see Nageli, Berichte der kon. bayer. Akad. 1861 ; Botanische Mittheilungen,
vol, I. p. 40.

^ Sachs, Experimental-Physiologie, p. 22 r. — Miiller, Jahrb. fiir wiss. Bot. vol. VII, 1868.—
Baranetzky, Bot. Zeitg., 1872, Nos. 5-7.— [See also Vesque, Ann. Sci. Nat. 1877, and Burgerstein,
Ueb, den Einflluss aeusserer Bedingungen auf die Transpiration, Wien, 1876.]

^ Deherain's researches (Ann. des Sci. Nat. 1869, pi. XII. p. i) do not decide the question.

* Vou Mohl, Bot. Zeitg. 1836, p. 697.

f

MOVEMENT OF WATER IN PLANTS. 679

difficult in the extreme^. The cuticularised outer wall of the epidermis of young
leaves and internodes is less efficacious in this respect ; if it is very thin as in many
quickly-growing leaves, especially those of water-plants, or altogether imperceptible as
in roots, these parts dry up very quickly in ordinary air. In contradistinction to this
the evaporation is very small from hard evergreen leaves, Cactus-stems, &c., which are
covered by a thick cuticular coating. It may be assumed that in plants provided with
a thick cuticle transpiration takes place principally through the stomata, and is therefore
dependent on their smaller or larger number and size. The evaporation does not in
this case proceed from the surface of the organ (or only to an imperceptible extent)
but in its interior, 'viz. at the places where the cells of the parenchyma bound the inter-
cellular spaces. These spaces may be supposed to be always at least nearly saturated
with aqueous vapour ; but the vapour will escape through the stomata with every increase
of its tension or decrease of the tension of the vapour without, and will thus give rise to
the production of more vapour in the inside. The production of vapour in the inter-
cellular spaces is moreover the more abundant the larger they are themselves, or in
other words the larger the superficies of cell-wall which bounds them. This circum-
stance, and the much larger number of stomata on the under side of the leaves, are
clearly the reason why evaporation is generally so much more copious from it than from
the upper side. Since water containing any substance in solution evaporates more slowly
than pure water, and the more slowly the more concentrated and denser the solution,
this must also be considered among the conditions which limit the transpiration of water
from the sap of plants. It must not however be forgotten that evaporation takes place
only on the external surfaces of the cell-walls of tissues, which on their part remove the
water by imbibition from the cell-sap.

The conditions now named which regulate transpiration are combined in the most
various ways, and not only cause different plants to show different amounts of transpira-
tion, but also the amount to be very different in the same plant at different times.
A definite statement cannot however be made of the total amount of transpiration, /. e.
of the quantity of water required by a plant during its period of vegetation, although
certain very variable limits can always be assigned to each species in this respect.
Two plants of the same species may, as any one may see, thrive equally well if one
grows in damp soil and dry air, the other in dry soil and damp air, the former thus
using up a large, the latter a small amount of water. In general the conditions of
transpiration which have been mentioned exhibit periodic variations related to the
meteorological distinction of day and night ; the temperature, the moisture of the air,
and light, are usually favourable to evaporation by day, unfavourable by night ; but
under certain circumstances this condition may even be reversed.

(c) Currents of Water in the Wood, Superficial cells or those which bound intercel-
lular spaces and lose water directly by evaporation would very soon collapse and dry up
if they were not able again to replace that which they have lost. This can only take
place by the flow of water from the adjoining cellular tissue from which no evaporation
occurs ; but when this tissue is placed in the same condition as the former, it must also
compensate its loss from more distant layers of tissue, and these again from those which
are connected with the conducting organs or woody bundles which convey the water
from the roots. The question here presents itself whether this movement of water
within the succulent tissue (especially in the parenchyma of the leaves) is caused by
endosmose from cell to cell, or whether it does not occur at least principally along
the cell- walls, these latter forming the channels of communication between the woody
bundles and the surfaces where the evaporation takes place, the contents of the cells
being only incidentally concerned in the process.

The chief evidence of the fact that the rapid currents of water in the roots, stem,
and branches caused by transpiration take place only in the wood, /. e. in the lignified

[It may be effected in the summer by means of the lenticels; see ante, p. 108.]

680 MOLECULAR FORCES IN THE PLANT.

xylein, 1ms already been stated. It can be demonstrated in a more conspicuous manner
by placing a cut stem or branch with its cut surface in a coloured solution ^ while the
leaves are transpiring. If the stem or branch is cut through at various heights after
a few hours, or according to circumstances after a longer period, the colouring of the
wood will show how high the solution has been sucked up in it, and will be seen only in
the woody bundles and not in the cortex or pith. If branches with pure white flowers
are employed in this experiment, such as a white-flowered Iris or Deutzia, and if they
are placed in a dark aqueous solution of aniline, the white petals are found, after from
ten to fifteen hours, to be permeated by dark blue veins corresponding to the fine woody
bundles of the venation. This beautiful appearance however soon vanishes, the poisonous
colouring material subsequently killing the adjoining layers of parenchyma, and colouring
the spaces between the veins blue by diffusion, and the corolla consequently becomes
flaccid ^

The difference in the amount of transpiration under different external conditions
must also correspond to a difference in the rapidity of the current of the water in the
wood. In rainy weather, when there is no evaporation or but very little from the
leaves, the movement of the water in the stem will be very slow ; but when the
transpiration increases with sunshine and wind, the current of water in the woody
bundles is also accelerated. Under the hypothesis that the water moves only in the
woody substance of the walls of the wood-cells themselves and not in their cavities, I
have calculated the rapidity of the ascending current of water in a branch of the Silver
Poplar in which there was strong transpiration, and obtained a rate of 23 cm. per hour.
M'^Nab placed branches of Prunus Laurocerasus^ from which transpiration was taking

^ I must take this opportunity of making the remark that I still entertain, and in a high degree,
the doubt previously expressed, whether it is not a purely pathological phenomenon that is produced
in this manner.

"^ [This is a method of experimentation which has been practised by numerous observers since
the commencement of the last century, when it was apparently first tried by Magnol. Sarrabat
(otherwise Delabaisse) coloured the veins of the flowers of the tuberose (Polyanfhes tuberosd) and
Snapdragon {Antirrhinum majtis) by watering the plants with the juice of the berries of Phytolacca.
(Dissert, sur la circul. de la Seve, Bordeaux, 1733.)

Van Tieghem (in the French edition of this work, p. 791) quotes Reichel as having plunged the
roots of a flowering plant of Dali/ra Stramonium into a decoction of the wood of Fernambouc ; the
liquid followed the course of the vessels, and after eight days veined the corolla with red, and made
its appearance also in the stamens, the walls of the fruit, and even in the style. (De vasis plantarum
spiralibus, Leipzig, 1748.) For other old authorities see De Candolle, Phys. Veg. i. 82.

De Saussure found that the stem of a bean became coloured by a decoction of Brazil-wood ; and
this was one of the facts upon which he based the conclusion that organic matters were capable of
being taken up by the roots of plants (Ann, des Chem. u. Phys, xlii. p. 275). Biot noticed that the
red colouring matter of Phytolacca was absorbed by white hyacinths when poured upon the soil in
which they were grown ; after two or three days, however, the red colour disappeared from the
flowers, (Comptes Rendus, 1837, i. 12.) linger also made the same experiment (Botanical Letters,
p. 38). Hallier immersed the ends of cuttings of plants in solution of indigo or black cherry juice.
(Phytopathologie, 1868, p. 67). Persoz states (Introd. a I'etude de la Chimie moleculaire, p. 553)
that plants of Impatiens parvijiora, the roots of which are immersed in a solution of sulphindigotic
acid, absorb that fluid in a reduced or colourless state due to the action of the roots upon it ; in the
petals it again undergoes oxidation and becomes blue. The experiments of Herbert Spencer (Prin-
ciples of Biology, i. p. 538) may also be referred to.]

2 M'^Nab, Transactions of the Botanical Society of Edinburgh, 1871. [Dr. Pfitzer has suggested
that the result may be arrived at by the much simpler mode of allowing the plant grown in a pot to
become so flaccid from want of water that the leaves droop perceptibly, and then, after supplying the
root with water, to observe the length of time that elapses before the leaves at various heights from
the ground recover their normal position. Pfitzer found by this means a much more rapid rate of
ascent indicated than that stated by M*"Nab ; and believes that there is a serious source of error
in M^'Nab's expeiiments, from the saline solution not rising so fast as pure water: (also Trans. Roy.

MOVEMENT OF WATER IN PLANTS, 68 1

place in a solution of lithium citrate, and then examined the ashes of successive inter-
nodes by the spectroscope. The solution was found to rise from 42 to 46 cm. in one
hour. But neither method of calculation is exact or probably of much value.

The current of water in the wood which replaces the loss occasioned in the leaves
by transpiration is not caused by osmose, since at the time when the transpiration is
strongest and therefore the current in the wood quickest, the cavities of the conducting
wood-cells do not contain sap but air, or at the most are only partially filled with fluid.
If the rising of the water took place by endosmose from cell to cell, the cells would
necessarily possess closed cell-walls and be full of sap, the concentration of which would
constantly increase from below upwards in the wood. But the conducting cells are at
this time not closed, but partially or altogether (as in Coniferge) connected with one
another by open bordered pits ^. In the spring, before strong transpiration sets in, ai;d
therefore at a time when the water in the wood is comparatively at rest, the wood-cells
also, it is true, contain sap, flowing in quantities out of their communicating cell-cavities
when holes are bored in the trunks (as in the Birch, Maple, &c.). But this sap does
not, as is proved by analysis ^, show a concentration increasing from below upwards.
The fact ^ also that water rises in cut leafy stems placed with their upper end in water
though planted and rooted, and flows therefore in a direction opposite to the ordinary
one in the stem, shows that endosmose depending on a definite distribution of the
concentration of the sap cannot be the cause of the current of water. Since vessels
and wood-cells communicating with one another through their open pores form narrow
cavities which sometimes become wider as they proceed, sometimes narrower, the woody
substance may be represented by a bundle of narrow glass tubes alternately bulging and
contracting, in which the water which fills them rises by capillary attraction. But how
little efficacious a contrivance of this kind would be is seen at once from the width of
the capillary tubes, which is much too great to raise water to a height of 100 feet or
more. It must also be pointed out that in the summer, when the current of water
is strongest, it is principally air and not fluid that is conveyed through the cavities of
the cells.

Since it is evident from what has been said that the movement of the water takes
place in the woody substance and not in the cell-cavities filled with water, there remain
only two hypotheses; 'viz. (i) that the movement takes place in the water contained

Irish Acad. vol. XXV, 1874). Sachs has foxind that salts of lithium do travel along cell-walls as fast

as the water in which they are dissolved. By supplying the roots of plants with a solution of a salt

of lithium, he has obtained the following rates at which it travelled in the root and stem : —

Plants with roots in water. Rate per hour.

SalixfragiUs .... 85*0 cm.

Zea Mais . . . , . 36*0 „

Plants with roots in earth.

Nicotiana Tnbacum . . . I180 ,,

Albizzia lophantha . . , 154'0 >»

Mtisa Sapientntn .... 99*7 »

Helianthus annuus . . . . 630 ,,

Vitis vinifera .... 98'0 „

In all these cases the plants were under such conditions as to promote transpiration to the utmost.

(Sachs, Beitr. z. Kennt. d. aufsteigenden Saftstroms in transpirirenden Pflanzen, Arb. d. hot. Inst, in

Wurzburg, II. i, 1878.]

^ [Sachs has found that in Abies pectinata the bordered pits of the spring-wood are closed
(Porositat des Holzes).]

^ The older statements of Unger are referred to in my ' Experimental -Physiologic ; ' others will
be found in Schroder, Jahrb. fUr wiss. Bot. vol. VII. p. 266 el seq.

^ The conduction is however by no means so considerable in the reversed as in the ordinary
direction, as Baranetzky found in the laboratory at Wurzburg ; but this may be connected with other
peculiarities of the organisation.

682 MOLECULAR FORCES IN THE PLANT.

in the lignified cell-walls (or in other words imbibed by them) ; and (2) that it is caused
by a very thin stratum of water which overspreads the inner surface of the wood-cells
and vessels ^ In both cases it must be assumed that the transpiration in the tissue of
the leaves causes the upper parts of the wood to contain less water, and therefore
to draw up the water from the parts which lie lower. The woody bundles of the
roots are surrounded by succulent parenchyma, from which they remove the water;
and these again absorb it from the soil by endosmose. It may however be imagined
that this movement in the substance of the cell-walls (the contents not participating in
. it) extends as far as the surface of the parenchyma of the root, where the water con-
tained in the soil is absorbed. The question whether the attraction of the cell-walls
for water, — putting aside the question whether it moves in their substance or only on
their surface,— is sufficiently powerful to sustain the weight of a column of water of
the height of 100 or even 300 feet or more attained by some trees, may be answered
without hesitation in the affirmative, since we have to do here with molecular forces
in relation to which the action of gravity altogether disappears. But it is another
question whether the rapidity of the molecular movements of water of this nature is
sufficient to cover the requirements of the foliage of a tree which amounts on a hot day
to hundreds of pounds '^. *

The hypothesis finally that the water necessary to supply the loss by transpiration
is forced up into the stem as far as the leaves by root-pressure must be abandoned,
since this could only operate in the cavities of the wood ; and these are always empty
in energetically transpiring plants. In the case of tall trees the pressure would also not
be sufficient ; and if I at one time assumed that this might be a cooperative cause at
least in shrubs and annual plants, I must retract this after my observations made in
the year 1870; since these show that the root-stock of such plants as the Sun-flower,
Gourd, &c., is even subject to a negative pressure when they are transpiring strongly ;
i.e. does not press water up, but greedily sucks it in at a cut surface above the ground
{■vide infra) ^.

The insufficiency of all attempts hitherto made to explain the transpiration-current
in the wood is especially noticeable from the fact that it is only under certain in-
ternal conditions which cannot be more accurately ascertained that wood is capable
of conducting water with the force and rapidity required by the transpiration from
the leaves. Woody but air-dry branches with a lower cut surface placed in water
are never able to raise up as much water as is necessary to compensate the evapo-
ration even from an upper cut surface ; while the same branch in a fresh state
conducts water fast enough to replace the much greater amount of transpiration from
the numerous leaves. A change is thus caused in wood simply by drying up which
deprives it of the power of conducting water rapidly. The natural alteration which
takes place in wood, by which it is transformed as it increases in age into ' duramen ' —
the cell-walls becoming harder and of a deeper colour — also deprives it of this power.
If a tree is deprived not only of the bark but also of the 'alburnum' (the light-coloured
younger wood on the outside), in an annular zone, the foliage of the tree, according to
the statement of different writers, dries up, because the water is not conducted suffi-
ciently rapidly through the duramen.

^ This hypothesis follows from the discoveries of Quincke on capillarity, and has been commu-
nicated to me by him, [In consequence of subsequent researches, Sachs is now of opinion that the
transpiration -water travels only iri the cell-walls of the wood (Ueb. die Porosilat des Holzes, Arb. d.
bot. Inst, in Wiirzburg, II. 2, 1879).]

2 See Nageli u. Schwendener, Das Mikroskop, p. 365 et seq.

3 [This negative pressure is due to the fact that, in consequence of active transpiration, the air
which is contained in the vessels of the wood is more rarified, i. e. is at a lower pressure, than that of
the atmosphere. Consequently, when the stem of an actively transpiring plant is cut through under
water or mercury, the liquid is violently injected into the cavities of the vessels by the atmospheric
pressure. (See von Ilohnel, in Mittheil. d, k. k. Landwirth-ch. Laborat, in Wien, 1877.)]

MOVEMENT OF WATER IN PLANTS.

683

Among the most remarkable of the phenomena related to this is the fact that the
younger terminal portions of the stems of large-leaved plants partially lose the power
of conducting water when cut off in air. If the cut leafy end of the stem of Helianthus
annuus, H. tuberosus, Aristolochia Sipho, &c. be placed with the cut section in water, the
suction is not sufficient to compensate the evaporation from the leaves, which therefore
wither after a shorter or longer time ^. As I have already shown in the second edition
of this book, the withered shoot may in a short time be revived by forcing in water by
means of the contrivance represented in Fig. 468. I did not discover till afterwards that
the shoot remains turgid even when the pressure is reduced to zero, and even when
the mercury is raised up by the suction of the shoot in the same arm of the tube {q),
when therefore a force acts on the section of the shoot in the opposite direction.
This shows that the forcing in of water is only necessary at first, but that the revived
shoot has itself sufficient power of suction to raise up a column of mercury several
centimetres in height, and thus to replace the loss by transpiration from the leaves.
Thus much was known about the phenomenon of the withering of cut shoots placed in

Fig. 468.— Apparatus for showinjj the revival of withered shoots by forcing water into them. The U-shaped glass
tube is first filled with water, and the perforated stopper of caoutchouc k in which the stalk of the plant is inserted
is then fixed in. When the shoot is withered, as represented by a, mercury is poured into the other arm of the tube,
so as to stand at q' some 8 or locm. above q, and the shoot then revives, as represented by b, even when the level q
becomes subsequently higher than q'.

water, when Dr. Hugo de Vries^ took up the further investigation of it in the laboratory
of the Wiirzburg Institute. The results obtained by him I will now quote: —

* If rapidly-growing shoots of large-leaved plants are cut off at their lower part
which has become completely lignified, and are placed with the cut surface in water,
they remain for some time perfectly fresh. But if they are cut through at the younger
parts of their stem and are then placed in water, they soon begin to wither, and the
more rapidly and completely the younger and less lignified the part where the section is
made. This withering can be easily prevented by making the section under water, and
taking care that the cut surface does not come into contact with the air, the conduction

^ [Von Hohnel has shown (Bot. Zeitg. 1879) that the rapid loss of conductivity for water shown
by branches which have been cut off and placed in water is due to the fact that the contents of the
injured cells escape and form a layer on the cut surface ; this becomes infested with Bacteria, and
these form a membrane (zoogloea) over the surface which prevents the absorption of water.]

^ [Arb. d. bot. Inst, in Wiirzburg, 1. 3, 1873; Ueb. das Welken abgeschnittener Sprosse.]

684 MOLECULAR FORCES IN THE PLANT.

of water through the stem thus suffering no interruption. If care is taken that while
the section is being made in the air the leaves and upper parts of the stem lose
only a very small quantity of water by transpiration, withering does not begin till later
and inci eases only slowly after the cut surface is placed in water and the leaves again
transpire.'

It results from these experiments that the cause of withering is the interruption in
the conduction of water from below ; and this interruption produces withering not only
from the conduction of the water ceasing for a short time, but chiefly also from the
power of conducting water in the stem being diminished by the loss of water above the
cut surface, which loss cannot be restored simply by placing the cut surface in contact
with water.

If the cut surface does not remain too long in contact with the air, the diminution
of the capacity for conduction takes place in only a short piece of the stem above the
cut. When placing in water ends of shoots which have begun to wither after being cut
off, it is only necessary to remove by a new cut a sufficiently long piece above the first
cut, but this time bej^eath the water, for the shoot to revive. In the case of shoots
20 centimetres or more in length which at this distance from the apex are not ligni-
fied, the removal of a piece 6 cm. long is usually sufficient to revive the withered shoot
(e.g. in Helianthus tuber osus, Sambucus nigra, Xanthium echinatum, &c.). This experiment
proves beyond question that the change, whatever its nature may be, takes place only
in this relatively short piece above the cut. That it consists in a diminution of the
power of conducting water is shown by the following experiment : — When a sufficient
number of the lowest and largest leaves have been removed from a stem of Helianthus
tuberosus cut off in the air and placed in water, and which has begun to wither, the
leaves that are left and the terminal bud will after some time begin to revive even
without again cutting the stem. The water which is required for the transpiration of
a great number of leaves can therefore no longer be conducted through the stem after
it has been cut off in air, although that which is wanted for the transpiration of a few
leaves can be.

The cause of this phenomenon is therefore a diminution in the power of conducting
water in a short piece above the cut surface of the stem. This is evidently occasioned
by the loss of water from the cells caused by the suction of the higher parts not being
compensated by absorption from below. All circum.stances which favour this loss of
water increase also the loss of power of conducting it, and cause the shoot which is
placed in water to wither more rapidly and completely. It must therefore be assumed
that the conducting power of the cells depends on the quantity of water they contain.
The probability of this hypothesis is increased by the fact that by artificially increasing
the amount of water in the cells of this piece its conducting capacity can also be in-
creased, as is proved by forcing in water from below. If the modified portion is dipped
in water of from 35° to 40° C, the withered shoots soon revive, and if then placed in
water of 20° C, remain fresh for days (as in the case of the Elder), or at least wither
more slowly {e.g. the Artichoke).

{d) Water retained in the nvood by Capillary Attraction. If the capillarity of the cavities
in the wood must be considered as without any immediate action on the currents of
water, this force must nevertheless be taken into account with respect to other processes
connected indirectly with the movement of water in the plant. In winter and after
long-continued rain in summer a large quantity of water is found in the cavities of the
wood together with bubbles of air which occupy the wider spaces. It is not known how
this water has reached the higher parts of the trees, though it is possibly by the forma-
tion of dew as the temperature varies ; it is however to a great extent retained by
capillarity. A part of the water flows out in many cases through holes bored in the
stem if they are not placed too high, as in the Birch, Maple, Vine, &c. It may be sup-
posed that the water which flows out has been forced up by the root-pressure which
must also be taken into account ; though how far up this pressure extends is not yet

MOVEMENT OF WATER' IN PLANTS. 685

ascertained. The water which is present in the cavities when there is feeble transpira-
tion and which will not flow out of them is clearly retained by capillarity, assisted by
the air in the cell-cavities ; for Montgolfier and Jamin have shown that in capillary
spaces which contain water and air, the water is not easily set in motion. This explains
also the phenomenon already mentioned, that water escapes when pieces of wood which
have been cut off in cold weather are warmed, because the air expands and forces out
the water. Subsequent cooling causes on the contrary water to be sucked in at the cut
surface, because the air contracts, and the pressure of the external air forces in water
from without.

{e) The ascent of <water from the root into the stem *. The most important features of
this phenomenon have already been briefly mentioned. It is to be observed in the
open air in plants of the most diff"erent kind, if they possess vigorous root-systems and
well-developed wood ; as, for instance, in the Birch, Maple, and Vine, and among annual
plants, in the Sunflower, Dahlia^ Ricmtis^ Tobacco, Gourd, Maize, Stinging Nettle, &c. In
order to study the phenomenon accurately, it is best to grow the plants for some time
previously in large flower-pots until they have developed a strong root-system. Land-
plants such as Maize grown in water and artificially fed by nutrient substances are
also well adapted for the investigation. If the stem of such a plant is cut across smoothly
5 or 6 cm. from the ground, and a glass tube fixed to the stump by means of an
india-rubber tube, the result will be seen as follows. If the plant was in a condition
to transpire freely before it was cut, the cut surface of the root-stump remains at first
quite dry, and if water is poured into the glass tube it is at once sucked up 2. The
woody substance of the root-stump has evidently been exhausted by transpiration before
the operation, and contains but very little water ; not only are its cavities empty, but
even the cell-walls of the wood may not be saturated. After a shorter or longer time
however the exudation of water at the cut surface begins— rising higher and higher
in the tube — and continues from six to ten days if the plant is properly treated, be-
coming during the earlier part of the time continually more copious, attaining a maxi-
mum, and finally diminishing until it ceases with the death of the root-stock. If the
cut section is repeatedly dried with blotting paper during the time that the water is
flowing, it is clearly seen that the water exudes from the woody tissue — in Monocoty-
ledons from the xylem of the separate bundles — and that it comes principally from the
openings of the larger vessels. That the water which flows out had previously been
absorbed by the roots out of the ground, and not merely from the store in the root-
stock, is at once evident from the fact that the quantity which exudes at the cut section
is after a few days greater in volume than the whole of the stock. Under the conditions
here described, the water which flows out contains only traces of organic substances
in solution ; but the presence of mineral constituents can be easily proved, especially
lime, sulphuric acid, phosphoric acid, and chlorine, which the plant has absorbed out of
the ground. The water which flows in the spring from holes bored in trees such as the
Birch and Maple, contains however considerable quantities of sugar and albuminous sub-
stances ; since the longer stagnation in the cavities of the wood gives it the opportunity
of absorbing these substances out of the closed living cells of the wood and out of the
surrounding parenchyma, a result which cannot be expected, or only in a smaller degree,
in the case of the rapid flow from the smaller root-stocks of quickly-growing plants.

In order to determine the quantity of the outflow, a narrow burette may be used
instead of the tube, in which the amount can be read off hourly in cubic centimetres
when the outflow is at all considerable. The root-pressure which acts upon the cut
surface is however then considerably altered. In order to avoid this, a tube of the

^ See in particular Hofmeister, On the tension and the quantity and rapidity of the flow of the
juices of living plants ; Flora, 1862, p. 97.

"^ This fact is sufficient to prove that the root-pressure has no share in the ascent of the water at
the time when transpiration is active.

686 MOLECULAR FORCES IN THE PLANT.

form shown in Fig. 467, R is fixed to the stump, and to it is attached a narrow tube
instead of the manometer; the free end of this tube is bent downwards into a graduated
burette. If the tubes are from the first filled with water, as much runs into the
burette as flows out from the cut section, and the pressure therefore remains constant.
From this experiment it has been ascertained that the flow of water varies from day to
day, from one time of the day to another, and even from hour to hour ; but the causes
of these variations in the outflow, which must depend on the activity of the roots, are
not yet known ; it would even seem as if a periodicity were established independently of
the temperature and of the moisture of the ground ^

The measurement of the highest pressure at which the outflow can still take place at
the cut surface can be eff"ected by the apparatus figured in Fig. 467, where it is expressed
by the diff'erence of level of the mercury in the two arms of the tube, or by q — q'.
This will however only afford a measurement of the pressure which the outflowing \a ater
may be able to overcome at the cut surface ; but it has obviously had also to overcome
other resistances of unknown magnitude in the interior of the root-stock. With respect
to this point 1 was interested in ascertaining how great is the difference in the outflov/ if
one of two equal root-stocks has no pressure to overcome at the cut surface, the other
a considerable but constant pressure. If, in Fig. 469, a indicates the cut stem of a
Sunflower or similar plant grown in a pot, c, d, e the tube which is attached to it by the
india-rubber tube b, and / a glass tube bent downwards, which (not as in the figure)
reaches beyond the rim of the pot and terminates in a burette, while the opening of /
hes exactly on the level of the cut surface of the stem; then, when the tube c, d, e,f
has been filled with water, we have an apparatus for observing the outflow when the
pressure at the cut surface is at zero. A second root-stock from a plant of exactly
the same age and vigour and grown in a pot of the same size is provided with the
apparatus figured in Fig. 469, where the tube / through which the outflow takes
place reaches the vessel h through the cork g. This vessel contains water in its upper,
mercury in its lower part. A tube k rises from the cork ; to a certain height and is bent
round at the free end 0 where it dips into a graduated tube. If the apparatus is so
contrived that, for example, the opening for the outflow 0 stands about 15 cm. above the
level n, then the column of mercury 0 n exercises a pressure of 1 5 cms. on the water h,
and through it on the cut surface at b. When the water begins to flow out from the cut
surface at b, the quantity of water in h will be increased, and an equal volume of mer-
cury will flow out at 0. The mercury collects in the burette, and its level enables
the quantity of water which has flowed from the cut surface to be read off" from hour
to hour, and to be compared in the other apparatus where there is no pressure. After
a long period of observation, the level n falls sensibly and the pressure 0 n augments a
little. But it is easy to bring it again to the original amount if a fresh quantity of mercury
is poured in every twelve hours.

^ Very detailed observations on this point were made by Baranetzky in the Wiirzburg laboratory,
in the summer of 1872. For this purpose he availed himself of the autographic auxanometer described
hereafter. The sap flowed from the root-stock into one limb of a long narrow U-shaped tube, in the other
limb of which was a float bearing an index which marked the changes of level upon the smoked paper
of the rotating cylinder. This method gives very accurate results in so far as the quantity of the sap
is concerned. Considerable variations of temperature (10° in 24 hours) affect the flow in such a way
that every rise increases, every fall diminishes it. If the variations of temperature are small, a daily
periodicity which is independent of such variations can be detected, a maximum and a minimum being
attained daily. According to Baranetzky, the time of their occurrence depends upon the periodical
exposure of the plant to light during the time preceding the experiment. I am unable to assure
myself of the correctness of this conclusion from Baranetzky's experiments. I do not deny the
possibility of it, but I reserve my opinion until further experiments have been made. (See
Baranetzky, Bot. Zeitg, 1873, No. 5, and Die Periodicitat des Blutens, Abhandl. d. naturfor. Ges. zu
Halle, XIII, 1873.)

MOVEMENT OF WATER IN PLANTS.

687

I observed in this manner in the summer of 1870 for five days two equally strong
root-stocks of the Sunflower^; and the result was that the difference of the outflow was
but small, although the amount of pressure in one case was zero, in the other case
17 cm. of mercury. In the first thirty-three hours the outflow where there was no
pressure at the cut surface amounted to 26*45 cubic cm.; when the pressure was 17 cm.
of mercury it was 20*9 cubic cm. A sudden change in the pressure of the mercury
of I or 2 cm. also caused no considerable alteration in the rapidity of the outflow.

Our object now is to form some idea as to the cause of this powerful ascent of water
in the wood of the root-stock, and to explain how it happens that the water absorbed
at the surfaces of the roots not only passes into the cavities of the wood, but is pressed
upwards with so great a force as to be able to overcome a considerable resistance at the

Fig. 469.— Apparatus for measuring the outflow under a considerable and constant pressure. The cork /has a lateral incision
in order to allow of the escape of the air when the mercury is dropped in.

cut surface ; for it is obvious that the water which flows out above must have been
absorbed below at the surfaces of the roots. This absorption can only be induced by the
endosmotic action of the parenchymatous cells of the cortex of the root. If we suppose
that this endosmotic force is very considerable, these cells will swell greatly ; and as
much water will filter through the cell-walls into the cavities of the wood as is ab-
sorbed from without by endosmose. The parenchymatous cells which are gorged by
endosmose drive into the vessels the water which presses into them in consequence of
the endosmose, with such force that in flowing out above from the vessels it is still able
to overcome a considerable pressure. It follows from this explanation that the pressure
which acts at the cut surface must, in accordance with the laws of hydrostatics, be

I cannot here describe the whole series of minute observations.

688 MOLECULAR FORCES IN THE PLANT.

exerted also against the inside of the vessels which receive the water from the turgid
parenchymatous cells. But the water which enters them has also to overcome the
resistance to filtration exercised by the cell-walls. The endosmose of the corkical cells
of the root must overcome these resistances. Although we do not know the magnitude
of the endosmotic force, yet we have ground for supposing that it is much greater than
that given by Dutrochet's experiments on animal membranes; and this explanation would
therefore be very probable. But a difficulty occurs in answering the question why the
turgescent cortical cells of the root expel their water only inwards into the woody tissues
and not also through their outer walls. We may however here be helped by the suppo-
sition that the micellar structure of the cell-walls is different on the outer and inner
sides of the cells, and that those facing the exterior of the root are best adapted to allow
endosmose, while those facing the interior of the root are best adapted for permitting
filtration under high endosmotic pressure. It must however be observed that this
supposition is at present only a hypothesis for the purpose of explaining to a certain
extent the processes which take place in the root. The exudation of drops of water
from the upper cell of the P^ungus Pilobolus crystallinus, from the root-hairs of a Mar-
chantla grown in damp air, &c., shows moreover that cells distended by endosmotic
tension can in fact exude water at certain spots. It is difficult to give any other ex-
planation of the exudation of nectar in flowers ; the excreting cells must evidently absorb
the water or the sap with great force on one side, and then exude it on the other side.
That in this case pressure from the root does not directly cooperate is shown by the
fact that this exudation of nectar, which is often very copious, as in the flowers of Fritil-
laria imperial is and in the pitchers of iV^/^'^'w/Z'^j^, takes place even when cut flowers or
pitchers are simply placed in water. In this respect these exudations of fluid differ from
the exudation of drops on the leaves of many plants, which only takes place when they are
still in connection with the root, and which is clearly caused by the forcing power of the
root (as in Aroideae, &c.). It also happens however sometimes that drops of water are
exuded from cut surfaces of the tissue, while another cut surface of the organ absorbs
water. This I found, for instance, to be the case with pieces of the young stems of
different Grasses, cut off from 6 to lo cm. in length, which were placed with the lower
end in damp soil ; the free upper end then repeatedly and continuously exuded drops of
water in darkness and in an atmosphere saturated with moisture. Here the paren-
chymatous cells of the lower cut surface clearly acted as the cortical cells of the root,
absorbed by endosmotic action, and probably pressed the water thus absorbed into the
vessels, from which it then escaped to the upper cut surface.

(y) Tbe combined action of transpiration, conduction, and absorption of water by the
roots takes place under ordinary and favourable conditions in such a manner that nearly
as much water is absorbed through the roots and conducted upwards through the
wood as is transpired from the leaves. As long as this equilibrium lasts, the plant is
turgid and tense in all its parts ; and conversely, it may be concluded from the unaltered
turgidity and tenseness of the leaves and internodes that the conduction of water is
sufficient to compensate the transpiration by the leaves. Hence, under these conditions,
the quantity of water transpired may be taken as the measure of the absorption of the
root (or of a cut surface), and conversely the absorption observed as the measure of the

^ Compare Wunschmann's dissertation, 'Ueber die Gattung Nepenthes' (Berlin, 1872), where my
Handbook only, and not the above taken from the third edition of this work, is quoted. It is
questionable if there is any ground for distinguishing between 'excretions' and the sap which escapes
from a root-stock in so far as the mechanism of the excretion is concerned, as the older botanists and
Wunschmann do. The facts above mentioned render it improbable. They tend to show, on the
contrary, that in other vegetable organs, as well as in roots, hydrostatic pressures may be set up
which tend to force the fluids out of the tissues. It is a matter of merely secondary importance that
'excretions,' such as nectar and the fluid contained in the pitchers oi Nepenthes, aie of higher concen-
tration than the sap which escapes from a root-stock.

MOVEMENT OF WATER IN PLANTS, 689

transpiration from the leaves. Since however the tissues can be more or less turgid
without its being immediately perceptible, transpiration and absorption are not usually
exactly equal. But for most observations the small occasional difference may be
neglected so long as no actually perceptible amount of flaccidity, /. e. of withering,
caused by the collapse of the cells, takes place when the transpiration is stronger and
the absorption weaker ; or so long as, in the opposite case, no exudation of drops of water
results on the leaves of rooted plants. It is only when longer observations are made on
growing plants that the comparatively small quantities of water have to be taken into
account which are needed for the increase in size of growing organs.

Without going more minutely into the various cases which present themselves ^
it need only be pointed out in addition that withering is the consequence of the
quantity of water transpired being greater than that absorbed through the roots or
through a cut surface of the stem. This only occurs in general when the amount of
transpiration is very considerable, or when the ground is very dry, or when in cut shoots
the power of the stem to conduct water has ceased. The exudation of drops of water
already mentioned is, on the other hand, the consequence of a smaller quantity of
water evaporating from the leaves than is absorbed by the roots and forced up into
the upper organs. If a branch of a Potato-plant, a leaf of an Aroid, a cut stem of Maize,
or the like, is fixed in the cork k in Fig. 468, and if, when the transpiration is weak a
pressure of mercury of 10 or 12 cm. is allowed to act for some time, drops of water
appear at the same spots on the apices or margins of the leaves, where they would
appear in plants with roots in the evening or night or in damp weather. In the
same manner the exudation of drops from plants with roots can be produced or
increased by warming the ground and covering the leaves with a bell-glass in order
to hinder evaporation^.

The pressure due to the root which is so conspicuous in stems when cut across and
when the amount of evaporation is very small, can scarcely be of any considerable use
in promoting the current of water in the wood caused by strong transpiration. The
fact already mentioned that strongly transpiring plants suck up water at the cut surface
of their stems immediately after the upper part has been cut off, shows that the pro-
pelling force of the root does not act sufficiently quickly to protect even the vessels of
the root-stock of strongly transpiring plants from complete exhaustion ; that is, although
the force which drives the water into the root-stock is great, as we have seen, it acts
too slowly to be taken into account when the transpiration is rapid.

The same conclusion is reached if the quantity of water which exudes in the same
time from the cut stem of a plant above the root is compared with that which is
absorbed at the lower cut surface by the upper part of the same plant. The absorption
of the upper part is always much more considerable in amount than the outflow from
the root-stock, even when the withering of the upper part indicates that the capacity
of its wood for conduction has diminished, and that it absorbs less than it would absorb
in the normal condition. Thus, for example, the water absorbed by the cut leafy top of
a Tobacco-plant amounted in five days to 200 cubic cm., while the root-stock exuded
only 157 cubic cm. In the same manner in Cucurbita Pepo (when much withered) the
amount absorbed was 14 cubic cm., the exudation from the root-stock only 11 '4 cubic
cm. The withered upper part of a Sunflower absorbed in a few days 95 cubic cm.,
while the root-stock exuded only 52*9 cubic cm. The result is also the same when
the relative amounts which extend over a shorter time are compared'.

* See Rauwenhoff, Phytophysiologische Bijdraden in Verslagen en Mededeelingen der kon.
Akad. van Wetens., Afdeeling Natuurkunde, 2^^ Reeks, Deel III, 1868, where however the indis-
pensable thermometric observations are wanting.

2 The exudation of drops on the margins of the leaves of plants, the roots of which are sur-
rounded by damp warm earth, their foliage rising into moist air, is a very common phenomenon,
as I know from the experience of many years.

^ For a more complete account see Arb. d. Bot. Inst, in Wiirzburg, I. 3, 1873.

Y y

690 MOLECULAR FORCES IN THE PLANT.

It follows from these facts that, with the exception of times when the amount of
transpiration is small or when drops of water exude from the leaves, no root-pressure at
all exists when the plant is uninjured ; and that this pressure can only be detected when
transpiration and absorption have ceased or when they are very small. The exhaustion
of the root-stock of a strongly transpiring plant (as after it has been cut off) proves
rather that a plant with roots behaves in exactly the same way as a cut shoot. Just as
the latter absorbs water from a receiver, so the wood of the root-stock which has lost
water in consequence of transpiration above absorbs water from the cortical cells of the
root which obtain it by endosmose. From all this it still remains in doubt whether in
such cases the contents of the cortical cells of the root must not be left altogether out
of consideration, since it is possible that the conduction of water by the cell-walls alone,
reaches as far as the surface of the roots.

(g) The parts of land-plants which are covered with a cuticle and in which transpira-
tion takes place appear to have no power of absorbing in any considerable quantity the
water by which they are moistened, such as the rain and dew which is deposited on the
leaves. As long as the tissues and leaves of uninjured plants with roots become turgid
and are supplied with water from below, any considerable absorption through the sur-
faces of the leaves themselves, even when they are thoroughly wetted^, is not to be
expected, since it is not easy to see where the water can go in cells that are already
gorged'-^. But even when a rooted plant has withered, it is still doubtful whether the
revival which takes place when its leaves are wetted depends on the absorption of water
by the leaves, since it is not impossible for an upward pressure to take place subse-
quently. Greatly withered shoots do not become turgid when placed in water, or do so
only very slowly unless the cut surface is immersed, and even in this case there is
doubt as to the absorption of water through the surfaces of the leaves.

In harmony with this Duchartre found also^ that rooted plants {Hortensia, Helianthus
annuus)y which withered in the evening in consequence of the dryness of the earth
in the pot, did not recover or become turgid if copiously moistened by dew during a
whole night, the pots in which the roots spread being provided with a closed cover.
Epidendral Orchids, Tillandsias, &c. behave in the same way in this respect ; they
also absorb neither water nor aqueous vapour through their leaves, nor even in any
considerable quantity through the roots. The water which they require for their
transpiration and growth must be conveyed to them in the form of rain or dew which
moistens the root-envelope (velamen) or wounded surfaces*.

When land-plants wither on a hot day and revive again in the evening, this is the
result of diminished transpiration with the decrease of heat and increase of the moisture
in the air in the evening, the activity of the roots continuing — not of any absorption of
aqueous vapour or dew through the leaves. Rain again revives withered plants not by
penetrating the leaves, but by moistening them and thus hindering further transpiration,
and conveying water to the roots, which they then conduct to the leaves.

A simple experiment will afford much instruction to the student in these matters.
The pot in which a leafy plant is growing is enclosed in a glass or metal vessel provided

^ On this subject see my Experimental-Physiologie, p. 159,

^ Duchartre has neglected this obvious reflection in his researches (Bulletin de la Soc. Bot. de
France, Feb. 24, i860) ; in other respects also these experiments are very defective.

3 Duchartre, I.e. 1857, pp. 940-946.

* Duchartre, Experiences sur la vegetation des plantes epiphytes (Soc. Imp. et centrale d'horti-
culture, Jan, 1856, p. 67 ; and Comptes Rendus, 1868, vol. LXVII, p. 775). [This subject has been
recently investigated by Detmer (Theorie des Wurzeldrucks, 1877), by Boussingault (Les fonctions
physiques des feuilles, Agronomic, VI, 1878), and by Henslow (On the Absorption of Rain and Dew-
by the green parts of plants, Journ. Linn. Soc. XVII, 1879). It appears that leaves, like all other
parts of plants, will absorb water if they are immersed in it long enough, but there is no evidence
that the absorption of water, either as vapour or as liquid, is in any sense one of their functions,]

MOVEMENTS OF GASES IN PLANTS. 69 1

above with a lid in two portions, and surrounding the stem so as completely to cover
the earth in the pot. If the soil is dry the plant withers. If a bell-glass is placed over
it the plant revives, and again withers if it is removed. This shows that the withering
is the result of increased, the revival the result of diminished evaporation from the
leaves when the roots convey but very little water to the plant. If cut shoots are
allowed to wither and are then suspended in air nearly saturated with aqueous vapour,
the leaves and younger internodes again revive, although the whole shoot continues
to lose weight from evaporation. This phenomenon results from the water passing
from the older parts of the stem to the younger withered parts, as must be concluded
from Prillieux's experiments \

Sect. 3. — Movements of Gases in Plants^. All growing cells of a plant, or
all that are otherwise in a condition of vital activity, are continually absorbing atmo-
spheric oxygen and giving back in its place a nearly equal volume of carbon dioxide.
The cells which contain chlorophyll have in addition the property, under the influ-
ence of sunlight, of absorbing carbon dioxide from without, exhaling at the same
time a nearly equal volume of oxygen mixed with some nitrogen. In proportion to
the activity of the chemical processes which take place within the cells, the movements
of gases occasioned by them vary greatly in rapidity. The formation of carbon
dioxide at the expense of the atmospheric oxygen takes place continuously and
in all the cells ; but the quantities concerned are small in proportion to the large
amount of carbon dioxide which is decomposed in the green tissues, and in ex-
change for which equal volumes of oxygen are exhaled. Some idea of the activity
of this last-named process is obtained by reflecting that about one-half the (dry)
weight of the plants consists of carbon which has been obtained by the decompo-
sition of atmospheric carbon dioxide in tissues containing chlorophyll under the
influence of light.

Oxygen and nitrogen are permanent gases, as also is carbon dioxide within the
limits of the temperature of vegetation, and indeed far below it. Aqueous vapour,
on the contrary, is only produced from water within these limits, and under certain
conditions even returns to the liquid state. In other respects aqueous vapour be-
haves just like oxygen and nitrogen in reference to the processes to be considered
here.

When the gases with which we have to do are traversing closed cell-walls,
diflusing themselves through the cell-sap, or permeating or escaping from the
protoplasm, chlorophyll-granules, &c., their motion is a molecular one of diff"usion.
When they fill in their elastic condition the intercellular spaces, vessels, cells destitute
of sap, or the large air-cavities among the tissues, it is a movement of the whole
mass depending exclusively on expansive force. The movements of diff"usion tend
to bring about conditions of equilibrium which depend on the coefficient of ab-
sorption of the gas by a particular cell-fluid, on the composition of the cell-wall,
&c. on temperature, and on the pressure of the air. But these conditions are
continually varying; and the equilibrium which is aimed at is being still more
continually disturbed by chemical changes on which depend the metamorphosis

* Prillieux, Comptes Rendus, 1870, vol. II. p. 80.

'^ Sachs, Handbuch der Experimental-Physiologic, p. 243. — Miiller, Jahrb. fUr wiss. Bot.
vol. VII. p. 145.

Y y 2

692 MOLECULAR FORCES IN THE PLANT.

of substances in the plant, assimilation, and growth ; so that a state of rest can very
seldom occur. The ordinary condition of the gases which are diffused through the
cells of plants is that of movement.

But even the masses of gas found in the cavities of plants are not generally at
rest. By the setting free or absorption of carbon dioxide or oxygen in the cells, the
equilibrium is disturbed also in the neighbouring cavities; and changes in the
pressure of the air or in temperature also exert an influence. The flexions again
of the stem and leaf-stalk produced by the wind cause compressions and dilatations of
the gases which fill the cavities, and these again give rise to currents of gas in the
interior. The rapidity of the movement in the cavities varies greatly in proportion
to their size ; within the very narrow intercellular spaces of ordinary parenchyma the
motion is slow and inconsiderable even under considerable pressure, as contrasted
with the rapid currents which are possible in the large intercellular spaces of most
foliage-leaves and similar organs, or in the wide air-canals of hollow stems, or in the
lacunae of the tissue of water-plants.

In attempting to collect the most common phenomena into a more definite arrange-
ment from this general point of view, the following appear to be the more important
points.

(a) Unicellular plants, as well as those which consist merely of filaments or plates of
cells such as occur in Algae, Fungi, and Mosses, are in immediate contact with the air or
with the surrounding water which contains gas in solution. The only essential condition
here is that the gases shall be able to enter and escape from the cells by diffusion. If,
for example, a cell of this kind containing chlorophyll is placed in sunlight, the carbon
dioxide absorbed by it is decomposed ; a fresh supply of the gas is therefore continually
penetrating into it from without, because it is prevented from saturating the cell-sap ;
oxygen, on the contrary, is being constantly disengaged, the cell-sap receives more than
it can contain, and gives off the excess by outward diffusion. Under these conditions
therefore two molecular currents are set up in opposite directions which permeate the
cell- wall, the protoplasm, and the cell-sap ; and since carbonised products are formed in
the cell at the expense of the decomposed carbon dioxide, this decomposition is the
simultaneous cause of fresh quantities of the gas perpetually diffusing into the cell. The
quicker the decomposition of the carbon dioxide, the more quickly is it replaced. The
conditions are similar in cells containing chlorophyll when in darkness and in cells
destitute of chlorophyll, though the chemical process is different; they absorb oxygen
and produce carbon dioxide; only the process is much slower and less active. The
cell acts as a centre of attraction for the gas which is decomposed in it, and as a
centre of repulsion for the gas which is produced in it. This rule holds good also for
the individual cells of a tissue, only that in this case the processes are more complicated,
inasmuch as the diffusion currents of the gases do not take place between the cells and
an unlimited external volume of gas, but between cells and cells on the one hand,
between cells and internal air-cavities of limited size on the other hand.

{b) Among plants consisting of complicated aggregates of cells, submerged Water-
plants are of peculiar interest, because their intercellular spaces do not open outwardly
through numerous stomata, but communicate with large cavities which are formed in the
interior of the tissues by the disjunction of cells or by the rupture of their walls. The
underground stems of Equtsetum and of many bog-plants show a similar structure.
Uninjured plants of this kind are closed and air-tight outwardly; the gases which
collect in the cavities can originate only from the surrounding tissues, which absorb
oxygen, nitrogen, and carbon dioxide by diffusion from the surrounding water. These
gases cannot simply diffuse through the surrounding tissues, but they undergo change
within them, and when once collected in the spaces they are still further influenced by

MOVEMENTS OF GASES IN PLANTS. 693

the chemical processes that go on in the surrounding tissues. A submerged water-plant,
for example, which contains chlorophyll, absorbs carbon dioxide from without under the
influence of sunlight ; and at least a portion of the disengaged oxygen collects in the
cavities. When it becomes dark this process ceases; the collected oxygen is now
absorbed by the fluids of the tissue and gradually transformed into carbon dioxide,
which can again difl^use back into the cavities, but partially also through the layers
of tissue into the surrounding water. This, as well as the different coefficients of
diff"usion of the gases, causes the air contained in the cavities to have an altogether
diff'erent composition from that in solution in the surrounding water, and this composition
to be subject to continual change. But it is not only the chemical composition of the
gas in the cavities that is altered in this way; the pressure is also subject to variation.
When the oxygen which is liberated from the green tissues collects rapidly in the cavities
under the influence of bright light, the gas is then subject to high pressure, and escapes
with force, injuring the surrounding layers of tissue. The greater rapidity of diffusion
of carbon dioxide, and its slower production in the tissue in darkness, do not, on the.
other hand, allow an increase of tension of the gas to arise easily in the cavities of the
plant when kept in the dark.

The nitrogen of the atmosphere takes a more subordinate and secondary part in all
these processes. It is indeed never absent from the air contained in the cavities, but is
generally present in large quantities in it, together with oxygen and carbon dioxide. It
is not however subject to such rapid and considerable variations, being neither used up
nor disengaged in the changes connected with the assimilation of food in the tissues.

(c) Land-plants differ from water-plants in that their internal cavities, when present ^
communicate directly with the atmosphere through the stomata. The anatomical con-
ditions show at once that these organs are only the channels of exit from the intercellular
spaces which are in connection with one another through the whole plant ; and we know
from experiment that these are in their turn in complete connection here and there
with the cavities of the vessels and with the wood-cells'^. The large air-cavities which
are abundant even in land-plants (in hollow stems, leaves, fruits, &c.), the woody tubes
(or vessels) and wood-cells, and the usually extremely narrow capillary intercellular spaces
of the parenchyma, form therefore a system of cavities full of air and in communication
with one another, which are all closed below at the root, but w^hich open outwardly
above in the leaves, internodes, &c., through numberless extremely narrow capillary
openings.

What was said in paragraph b on the changes which take place in the air contained
in the cavities of water-plants, applies in general also to that of land-plants ; but the
equalising of the difference in the pressure at the various parts of a large plant is
facilitated by the occurrence of vessels, that of the difference between the internal and
external air by the stomata. This equalisation however proceeds in general extremely
slowly, because the stomata, in consequence of their small diameter, can allow only small
volumes of gas to pass through them in a short time. Notwithstanding their unin-
terrupted connection, there may therefore be considerable differences of pressure and
great variations in the composition of the internal and external gas, as in water-plants.
It must also not be forgotten that those layers of tissue in which a rapid interchange of
gases is proceeding are covered with an epidermis containing a greater number of

' Large Fungi and Algce have indeed no stomata ; but their internal air (among the hyphse) is
certainly in communication at least in places with the surrounding air by cavities among the super-
ficial hyphse. The stems of Mosses possess neither internal cavities nor stomata, while their spore-
capsules possess Loth.

^ [The fact to which allusion has been made on p. 682, that namely the air in the vessels of an
actively transpiring plant is at a lower pressure than that of the atmosphere, proves that the cavities
of the vessels do not communicate with the intercellular spaces. If they did so communicate, such a
difference of pressure could not possibly arise.]

694 MOLECULAR FORCES IN THE PLANT.

stomata than those which require a less active interchange in consequence of slower
growth and assimilation. In addition to this, organs with a thin cuticle are better
adapted to bring about interchange of gas by diffusion than those whose epidermis
is provided with a thicker cuticle which hinders the diffusion-current. This is clearly
the reason why roots require no stomata, since, in consequence of their slow increase in
size and their thin-walled slightly cuticularised epidermis, they can accomplish the
interchange of oxygen and carbon dioxide by diffusion alone; while the leaves, in
consequence of their thick cuticle, require a large number of stomata in order rapidly to
interchange large volumes of carbon dioxide with as large volumes of oxygen in sunshine.
Even flowers and rapidly growing parasites which contain no chlorophyll possess stomata,
though in smaller numbers, because they absorb a quantity of oxygen and exhale carbon
dioxide. When the epidermis is replaced in the older parts of stems and roots by cork-
periderm, the parts are not only externally impervious to air (with the exception of
occasional fissures) in the ordinary sense, but even the interchange of gas by external
diffusion practically ceases. But this case occurs only in those parts of plants where the
fibro-vascular bundles form air-conducting vessels and usually also air-conducting wood-
cells, by means of which an interchange of gas is brought about internally with that
contained in the parenchyma enveloped by the cork. This is especially the case with
woody Dicotyledons and Conifers.

These considerations apply also to a great extent to aqueous vapour. The evapora-
tion of the water of vegetation, resulting, as we have seen in the previous paragraph,
in the production of currents in the plant, is almost entirely prevented by cork-periderm
and bark, and at least very much hindered by cuticularised epidermal cells. Since the
parts of plants exposed to the air are covered with one or other of these epidermal
structures, evaporation can in general only take place to a subsidiary extent from their
surface ; the greater part of the aqueous vapour which these parts of the plant lose is
evidently given off from the moist cell-walls in the interior of the tissue where they
adjoin intercellular spaces and larger air-cavities. If these spaces are saturated with
aqueous vapour, evaporation ceases ; but if the external air is comparatively dry the
vapour escapes through the stomata, and evaporation into the intercellular spaces re-
commences. If the transpiring tissue is heated, as by sunshine, the formation of vapour
proceeds more rapidly in the interior, and the greater tension of the vapour causes its
more rapid passage through the intercellular spaces and stomata.

The surfaces of the organs of plants which are constantly in contact with water
cannot exhale aqueous vapour through such fine openings as the stomata under the
existing conditions of temperature ; stomata are therefore wanting in submerged plants,
or occur only occasionally. The leaves, for instance, of Water-Lilies, which float on
the water, are especially instructive in this respect; on the side in contact with the
water they have no stomata or very few, on the upper side exposed to the air a large
number. This is the more striking since leaves entirely exposed to the air have generally
a larger number of stomata on the under than on the upper side, where they are some-
times entirely wanting.

CHAPTER II.

CHEMICAL PROCESSES IN THE PLANT.

Sect. 4. — The Elementary Constituents of the Food of Plants ^ If we

dry at a temperature of 100° or 1 10° C. any fresh vegetable structure so as to expel all
the water which it contains, a friable residue will be left which no longer loses weight.
In ripe seeds this usually amounts to about f of the weight ; in seedlings after the
supply of reserve-material has been consumed, to generally less than ^q, increasing
in subsequent stages of vegetation to i or ^; in submerged water-plants and
Fungi it often amounts to less than yV? and sometimes even to only Jg-. These
proportions, which are only roughly estimated, vary within wide limits according to
the nature and age of the plant and of the particular organ.

If the dried residue of the plant is further exposed to a red heat in the
presence of oxygen, by far the greater part of it is consumed and disappears in
the form of products of combustion, chiefly carbon dioxide and aqueous vapour.
The residue which now remains behind, usually a fine white powder, is the Ask,
constituting generally only a small percentage of the dried substance, a proportion
which is again subject to great variations with the specific nature of the plant and
the kind and age of the particular organ.

Chemical analysis of the combustible part of the dried substance shows that it
consists in all plants of Carbon, Hydrogen, Oxygen, Nitrogen, and Sulphur ; the
latter remains behind after combustion in the form of sulphuric acid in combination
with the bases of the ash.

In the ash are invariably found in addition Potassium, Calcium, Magnesium,
Iron, and Phosphorus, and generally Sodium (Lithium?), Manganese, Silicon, and
Chlorine ; in marine plants also Iodine and Bromine. With these constituents
there are sometimes associated, in rare cases and under special circumstances,
very small quantities of Aluminium, Copper, Zinc, Cobalt, Nickel, Strontium, and
Barium. The presence of very small quantities of Fluorine in plants is also
inferred from the presence of calcium fluoride in the bones of animals which obtain
the whole of their food directly or indirectly from plants.

^ For a preliminary acquaintance with the very copious literature, my Handbuch der Experi-
mental-Physiologic (Sects. 5 and 6) will be sufficient. A study of Th. de Saussure's Recherches
chimiques sur la vegetation, Paris 1804, is also indispensable to any one who wishes to form an
independent judgment for himself. A detailed description of the theory of nutrition is contained in,
among other works, Mayer's Lehrbruch der Agriculturchemie, 1876. A variety of fundamental
researches will also be found in Boussingault's Agronomic et Physiologic vegetale. E. Wolffs
Aschenanalyse fiir landwirthschaftliche Prod. &c., Berlin 1871, is also very valuable; as well as
his Vegetationsversuche in wasserigen Losungen ihrer Nahrstoffe (Hohenheimer Jubilaumsschrift,
1862).

6g6 CHEMICAL PROCESSES IN THE PLANT.

It is self-evident that we have only to consider as elementary food-substances
those which are indispensably necessary for the process of nutrition ; while those the
presence of which in the plant is proved by analysis, but which may also be absent
without its nutrition being impaired, may be considered as accidental admixtures.

Of the first importance among the indispensable food-materials are the ele-
ments of the combustible substance which are present in all plants without
exception, viz. Carbon, Hydrogen, Oxygen, Nitrogen, and Sulphur ; because they are
included in the chemical formula of cellulose or of the albuminoids which con-
stitute protoplasm, and because therefore without these substances the plant-cell
itself could not exist. It may be inferred also from the invariable presence of
Potassium, Calcium, Magnesium, Iron, and Phosphorus in plants, that they are
indispensable constituents of their food, and still more from the fact established
by actual experiment, that the nutrition and growth of all plants hitherto ex-
amined for this purpose is impossible or abnormal if any one of these elements is
wanting. In the case of Sodium, Manganese, and Silicon this has not yet been
proved ; it would appear rather that they may be dispensed with in the chemical
process of nutrition. That Chlorine is necessary for the perfect nutrition of
Polygonum Fagopyrum has been shown by Nobbed Whether Iodine and Bromine
play the part of true food-materials in the marine plants in which they are found
has not yet been ascertained ; and these two elements may, from the mode of their
occurrence, be for the present neglected as unimportant in this respect.

In the more general considerations as to the nutrition of plants we have there-
fore chiefly to do with the following elements : —

Carbon, Hydrogen, Oxygen, Nitrogen, Sulphur;
Potassium, Calcium, Magnesium, Iron ;
Phosphorus, Chlorine;
to which are to be added, under certain circumstances, Sodium and Silicon.

The physiological importance of these elementary substances is however very
different. Those placed in the first line compose, as already mentioned, the greater
part of the substance of the plant ; they mainly form the organised and organisable
part of the plant and of every individual cell ; their importance therefore lies in the
fact that they furnish the chief materials for the construction of the plant. The
constituents of the ash, on the other hand, are of less importance in this respect,
if only in consequence of their much smaller quantity; they appear to promote
chemical decompositions and combinations in plants, in consequence of which the
far more abundant combustible principles are constructed out of the first-named five
elements.

Carbon is a necessary constituent of every organic compound in varying proportion;
usually about one-half the weight of the entire dried substance of the plant consists
of this element. If the large quantity of vegetable matter which is annually produced
is taken into account, the fact becomes the more remarkable that this enormous
quantity of carbon is derived from the carbon dioxide of the atmosphere of which
it forms on the average only about 0*04 per cent. It is only the cells which contain
chlorophyll — and these only under the influence of sunlight — that have the power of

Landwirthschaftliche Versuchsstationen, vol. YII, 1865.

ELEMENTARY CONSTITUENTS OF THE FOOD OF PLANTS. 697

decomposing the carbon dioxide taken up by them, and at the same time of setting
free an equal volume of oxygen, in order to produce organic compounds out of
the elements of carbon dioxide and water, or in other words to assimilate. It is
very probable that under these circumstances carbon dioxide loses only one-half its
oxygen, while the other half of the oxygen which is exhaled is derived from the
decomposition of water.

The fact is unquestionable — partly established by direct researches on vege-
tation, partly inferred from the circumstances under which many plants live in a
natural condition — that most plants which contain chlorophyll (e. g. our cereal crops,
Beans, Tobacco, Sunflower, many saxicolous Lichens, Algae, and other water plants)
obtain the entire quantity of their carbon by the decomposition of atmospheric
carbon dioxide^ and require for their nutrition no other compound of carbon from
without. But there are also plants which possess no chlorophyll and in which there-
fore the means of decomposing carbon dioxide is wanting ; these must absorb the
carbon necessary for their constitution in the form of other compounds. But since
plants destitute of chlorophyll are either parasites or saprophytes, they absorb their
carbon in the form of organic compounds which have been produced by other
plants that contain chlorophyll. Parasites draw these products of assimilation
directly from their hosts, while saprophytes (as Neottia Nidus-avis, Epipogium
Gmelini, Corallorhiza innata^ Monotropa, many Fungi, &c.) make use for the
same purpose of the materials of other plants which are already in a state of
decomposition. Even the food of Fungi which are parasitic in and on animals
is derived from the products of assimilation of plants containing chlorophyll,
inasmuch as the whole animal kingdom is dependent on them for its nutrition.
The compound of carbon originally present on the earth is the dioxide, and the
only abundantly active cause of its decomposition and of the combination of
carbon with the elements of water is the cell containing chlorophyll. Hence all
compounds of carbon of this kind, whether found in plants or in animals or in
the products of their decomposition, are derived directly or indirectly from the
organs of plants which contain chlorophyll.

Hydrogen is present, equally with carbon, in every organic compound; in
consequence however of the smallness of its combining equivalent, it falls far
below it as a percentage constituent of the weight of the dried substance of
plants. As has already been mentioned, the hydrogen of the plant is probably
derived from the decomposition of water in cells containing chlorophyll in the
presence of sunlight. It probably enters into combination with the carbonic
oxide (CO) simultaneously presented to it by the reduction of the carbon dioxide ^

* [From the researches of Moll it appears that the roots take no part in supplying the plant
with carbonic dioxide (Moll, Die Herkunft des KohlenstofFs der Pflanzen, Arb. d. bot. Inst, in
Wiirzburg, II. i, 1878).]

2 [The abstract of Adolph Baeyer's paper on the Chemistry of Vegetable Life in Journ, Chem.
Soc. 1871, pp. 331-341, should be consulted. It is shown to be probable that chlorophyll fixes
carbon oxide just as haemoglobin does. When sunlight falls upon chlorophyll which is surrounded
by carbon dioxide, that compound seems to suffer the same dissociation as at high temperatures,
oxygen is liberated and carbon oxide remains combined with the chlorophyll. The simplest reduction
of carbonic oxide is to formic aldehyde; it need only take up hydrogen, CO + H2 = COH2, and under
the influence of the cell-contents, just as by the action of alkalies (which Butlerow has shown to be
the case), the aldehyde is transformed into sugar.]

698 CHEMICAL PROCESSES IN THE PLANT.

Only a very small portion of the hydrogen contained in the nitrogenous vegetable
substances can be supplied to the plant in the form of ammonia.

Oxygen is always present in organic compounds in smaller quantities than
would be sufficient to oxidise the hydrogen and carbon present in them into
water and carbon dioxide, because organic compounds are produced from carbon
dioxide and water with the elimination of a part of their oxygen. The proportion
of oxygen in vegetable substances is moreover very variable; and some even contain
none at all of this element. But the total quantity of oxygen forms, next to carbon,
the largest proportion of the weight of the dried substance. Oxygen is intro-
duced into the plant in the form of water, carbon dioxide, and oxy-salts, in larger
quantities than any other element ; while extraordinarily large quantities of oxygen
are set free into the air by the process of assimilation in the green organs. All the
other organs of the plant also absorb atmospheric oxygen, and thus slowly re-
produce carbon dioxide and water at the expense of the assimilated substances.
Together with the process of deoxidation which is very active in the cells containing
chlorophyll, another process of oxidation is proceeding comparable to that of the
respiration of animals, but not generally very active, by which a part of the assimi-
lated substance is again decomposed.

Nitrogen^ an essential constituent of the albuminoids which form protoplasm, of
vegetable alkaloids, and of asparagine,, always forms only a small fraction of the
weight of the dried substance of plants, — often less than i, seldom more than
3 p. c. The nitrogen contained in the chemical compounds just mentioned is
obtained from compounds of ammonia and nitric acid ^ ; parasites and saprophytes
perhaps also absorb organic nitrogen-compounds from without. It is on the other
hand certain from a great number of experiments on vegetation, especially those of
Boussingault, that plants have no power of using the free nitrogen of the atmosphere
for the production of their nitrogenous compounds ^. If plants are artificially sup-
plied with all other food-materials, but it is rendered impossible for them to absorb
ammonia or compounds of nitric acid as their source of nitrogen, no increase takes
place of the albuminoids or of the nitrogenous substances generally, although the
nitrogen of the atmosphere is at the command of the plant in such great quantities,
filling up the intercellular spaces and diffusing through the fluids of the tissue ^

Sulphur, a constituent of albuminoids, of allyl, and of the essential oil of
mustard, is taken up in the form of soluble salts of sulphuric acid, and chiefly

* [Although plants containing chlorophyll usually absorb their nitrogen in the form of ammonia
or nitrates, they can nevertheless absorb it also in the form of organic compounds such as urea,
leucin, tyrosin, creatin, hippuric acid, and similar bodies; they cannot absorb it in the form of
alkaloids.]

^ [The important researches of Lawes, Gilbert, and Pugh on the sources of the nitrogen of
vegetation (Phil. Trans. 1861, pt. 2, and Journ. Chem. Soc. 1863, p. 100) should be carefully studied
on this point,]

^ [Adolf Mayer has recently carried out a number of experiments to determine whether the
aerial parts of plants have the power of absorbing ammonia or not. Preventing access of ammonia
through the roots, he subjected the leaves to the influence of ammonium carbonate both in the
gaseous and dissolved state, and found that it was absorbed in appreciable quantities, although
the plants did not appear to thrive when access of ammonia through the roots was entirely
prevented. Similar results have also been obtained by T. Schloesing (see Comptes Rendus, vol.
LXXVIII. p. 1700).]

ELEMENTARY CONSTITUENTS OF THE FOOD OF PLANTS. 699

(or perhaps always) of calcium sulphate. This salt is probably, as Holzner first
pointed out \ decomposed by the oxaHc acid which is formed in the plant itself, and
the insoluble calcium oxalate is thus formed, while the sulphuric acid parts with its
sulphur to form the organic compounds which have been mentioned.

Iron"^ (often accompanied by very variable quantities of Manganese) is indis-
pensable for the production of the green colouring substance of chlorophyll, as is
shown by experiments on vegetation ; and since the green organs which contain
chlorophyll form organic substances out of water and carbon dioxide, the
importance of this element for the life of the plant is very evident, although
extraordinarily small quantities of it are sufficient for this purpose. It may be taken
up by the plant in the form of the chloride or sulphate or of some other
compound. If larger quantities of solutions of iron become distributed through
the tissues, the cells quickly die. Although small quantities of iron are essential
for producing the green colour of chlorophyll, it is nevertheless uncertain whether
the green colouring substance itself contains iron as an integral constituent of its
chemical formula^.

Potassium is as essential for the assimilating activity of chlorophyll as iron for
its production. Nobbe^ has recently shown that if food-materials otherwise com-
plete but possessing no potassium are supplied to plants (as Buckwheat), they
behave as if they were absorbing only pure water instead of the solution of food-
material. They do not assimilate and show no increase in weight, because no
starch can be formed in the grains of chlorophyll without the assistance of
potassium. The chloride is the most efficacious form in which potassium can
be offered to Buckwheat ; the nitrate comes next to it. If the potassium is offered
only in the form of sulphate or phosphate, a very evident sickliness is apparent
sooner or later, which results from the starch which is formed in the grains of chloro-
phyll not passing into the growing organs and thus becoming available for purposes
of vegetation. Sodium and Lithium cannot replace potassium physiologically, be-
cause the former is simply useless to the plant, while the presence of the latter in
the cell-sap is injurious to the tissues.

Phosphorus^ Chlorine, Sodium, Calcium, and Magnesium have, as far as is yet
known, no definite relation to special physiological purposes. The constant occur-
rence however of compounds of phosphoric acid in company with albuminoids, as
well as of potassium salts in organs containing starch and sugar, points towards
definite relations which they may possess to those chemical processes that imme-
diately precede the processes of construction in plants. A large part of the calcium
taken up by plants is, as has been mentioned, precipitated by oxalic acid, and
remains inactive. The importance of calcium must therefore be sought partly in
its serving as a vehicle for sulphuric and phosphoric acid in the absorption of

^ Holzner, Ueber die Bedeutung des oxalsauren Kalkes, Flora, 1867. — Hilgers, Jahrb. fiir wiss.
Bot. vol. VI. p. I.

^ For special proof of the importance of iron see my Handbuch der Experimental-Physiologic,
p. 142.

3 [On the chemical composition of chlorophyll, see infraP^

* Nobbe, Schroder, and Erdmann, Ueber die Organische Leistung des Kaliums in der Pflanze ;
Chemnitz, 1871.

700 CHEMICAL PROCESSES IN THE PLANT.

food-material, and partly in its fixing the oxalic acid which is poisonous to the
plant, and rendering it harmless. The elements just named are taken up by the
plant when they are offered to it in the form of phosphates, sulphates, nitrates, or
chlorides.

Silicon finally is taken up by a very large number of plants in the form of a
very dilute aqueous solution of silicic acid ; by some in larger quantities than all the
other constituents of the ash. By far the larger part of the silicic acid passes into
the insoluble state within the cell-walls, and remains behind after the destruction
of its organic substance together with calcium (magnesium and potassium?) as a
skeleton possessing the structure of the cell-wall. In land-plants it accumulates
chiefly, though not exclusively, in the tissues exposed to evaporation, and espe-
cially in the cuticularised walls of the epidermis. In Diatoms, the cell-wall of which
is very strongly silicified, this arrangement of course does not exist. Since it is
possible to cause, by artificial feeding, plants which usually contain abundance of
silica (like Maize) to grow almost entirely without it, and without any obvious
departure from their normal structure, silicic acid appears to be of very subor-
dinate importance for the chemical and organic processes; and its deposition in
the cell-walls does not take place to any great extent until they are already fully
developed.

The compounds serving as food-material must be subject within the tissues to pro-
gressive changes of position in addition to and in consequence of their chemical trans-
formations. The equilibrium of diffusion is disturbed by the decomposition of a salt ;
immediately round the spot where this takes place the fluid of the tissue contains fewer
molecules of the compound; and the more distant molecules of the same salt in a
state of solution move therefore towards the spot where they are wanted. Every cell
therefore which decomposes any particular salt acts as a centre of attraction upon the
fluids of the tissue surrounding it, and the salt in question is drawn towards this
centre. But this process does not affect any other salt dissolved in the same fluid.
If, for example, calcium sulphate is decomposed in a cell and crystals of calcium oxalate
formed, this itself supplies a cause for the more distant molecules of sulphate to be
drawn towards that cell ; but it affords no reason for the molecules of potassium nitrate
which are also present to move in the same direction. Every substance dissolved in
the cell-sap is set in motion only in so far as the equilibrium of diffusion and the uniform
distribution of its own molecules is disturbed. It follows therefore clearly that there
can be in general no such thing as a continuous uniform motion of a so-called * nutritive
sap.' It is only when a number of compounds which supply food-material are taken up
at one spot such as the root, and are decomposed at another spot such as the buds or
green leaves, that the direction of movement is nearly the same for all ; but even in this
case the rapidity with which the molecules of each particular salt move will vary, because
this depends on the rapidity of consumption at the point towards which the movement
is directed, and on the special rate of diffusion of each compound. It is only when some
external pressure drives the whole of the cell-sap in one direction that the motion of
different substances is uniform, provided that the fluid moves in open channels such as
the laticiferous vessels or sieve-tubes ; but if the pressure causes filtration through closed
cell-walls, then the molecules of different salts are urged forward with a different speed,
because the rapidity of filtration of different solutions varies with their composition and
degree of concentration.

The same principles hold good also for the absorption of combinations of food-
material from without into the absorbing organ. It has already been shown in the
previous paragraph how the decomposition of carbon dioxide in a cell containing chloro-

ELEMENTARY CONSTITUENTS OF THE FOOD OF PLANTS. 701

phyll induces new quantities of the dioxide at once to enter this cell, whether the gas
be at the time dissolved in water or present in the atmosphere. If no carbon dioxide
were decomposed in the cell, its contents would become saturated with the gas in
proportion to the pressure and the temperature, and every cause for further motion
would be removed. But the decomposition is constantly providing more space for the
entrance of fresh molecules of carbon dioxide ; and this gas, although present in such
small quantities in the atmosphere, collects here and supplies the material for the pro-
duction of compact masses of carbon-compounds.

A water-plant acts in the same manner on the salts dissolved in the surrounding
water. The external water and the internal cell-sap are in continuous connection
through the fluid saturating the cell-walls. If the chemical processes within the plant
are supposed to be at rest, an equilibrium of diffusion will tend to become established
between the external and internal fluid according to the prevailing conditions. But
the chemical processes in the interior are continually disturbing this equilibrium, and
molecules of the salts which are being decomposed are continually streaming from
without to the places in the interior where they are to be used. If molecules of
calcium phosphate are even very sparingly distributed through the surrounding water,
a dense accumulation will gradually arise in the plant, not of calcium phosphate, but of
some other compounds of phosphoric acid and of calcium, because the molecular equi-
librium is being continually disturbed by the separation of the phosphoric acid from
the calcium, that is, by the chemical change. If the calcium phosphate remained
as such within the plant, the movement would cease so soon as the equilibrium of
diffusion was established. It will be at once clear from a consideration of these
facts that the accumulation of certain substances in the interior of plants depends in
the first place on whether the compound of them which is present in the surrounding
water is decomposed in the plant; that moreover the constituents of the different
compounds must accumulate in the plant in different quantities according to the extent
to which these compounds are decomposed ; and that finally the relative quantities of
the substances in question within the plant need not bear any resemblance to those of
the substances present in the surrounding water. Substances which are present in the
water in the form of extremely dilute solutions occur in the plant in great quantities ;
while others which are abundant in the water are much less so in the plant. Thus,
for instance, marine plants take up a much larger quantity of potassium and a smaller
quantity of sodium than corresponds to the composition of sea-water ; again, species
of Fucus collect considerable quantities of iodine which is present in sea-water only
in extremely small quantities. Since moreover different plants decompose the same
compounds with different degrees of rapidity, it is obvious that different plants which
draw their food-materials from the same water must exhibit an entirely different com-
position of their ash.

The processes are more complicated when a land-plant has to take up the saline
compounds of its food-material from the soil which contains but little water. By
far the greater number of land-plants thrive in soil which usually contains a quantity
of water much below its full capacity of absorption, its pores being almost entirely
filled with air. The small quantity of water present adheres completely to the minute
particles of soil, and for this reason does not flow away ; and this adherent water
covers the surface of the particles of earth in the form of a fine stratum. The roots
can only absorb this water when they are in the closest contact with the particles
of soil ; hence plants freshly planted wither even in moderately moist ground until a
sufficiently large number of particles of earth become attached by means of new root-
hairs to the newly-formed rootlets. At these points of intimate connection between
the root-hairs and the soil the adhering water of the latter is directly continuous with
the cell-sap of the root by means of the water saturating the cell-walls of the root-
hairs. In this manner it is possible for the root to absorb the water of the soil; as
this water enters at the points of contact, the equilibrium of the strata of water that

702 CHEMICAL PROCESSES IN THE PLANT.

cover contiguous particles of earth is disturbed, and the water of the soil retained by-
capillary attraction is set in motion towards these points of contact. This process
spreads centrifugally from every root, and thus gradually makes the most distant
parts of the soil subserve the nutrition of the plant. If salts, such as calcium
sulphate, are present in solution in the enveloping strata of water, these salts follow
the movements of the water, and finally enter at the points of contact with the root-
hairs.

But a large portion of the food-material, especially compounds of ammonia, po-
tassium, and phosphoric acid, occur in the ground in a fixed condition, or, as it is
generally termed, absorbed; they cannot be extracted from the soil even by very large
quantities of water ; the roots nevertheless take them up out of it with ease. It may
be supposed in these cases that these food-materials occur as an extremely fine coating
over the particles of soil, and can therefore only be taken up at the points of contact
of the root-hairs with these particles; and they are there rendered soluble by the
carbon dioxide exhaled by the roots. This action of the root is limited to the points
of contact ; only those particles of substance which come directly into contact with
the root-hairs are dissolved and absorbed. But since the number and length of the
roots is very considerable in all growing land-plants, and since also they are continually
lengthening and forming new root-hairs, the root-system comes gradually into contact
with innumerable particles of earth, and can thus take up the necessary quantity of the
substance in question. This power of the roots of taking up, by means of the acid sap
which permeates the walls of even their superficial cells, substances which are insoluble
in pure water, presents itself in an extremely evident manner, as I was the first to show,
when polished plates of marble, dolomite, or osteolite (calcium phosphate) are covered
with sand to the depth of a few inches, and seeds are then sown in the sand. The roots
which strike downwards soon meet the polished surface of the mineral and grow upon
and in close contact with it. After a few days an impression of the root-system is found
corroded in rough lines on the smooth surface ; every root has dissolved at the points of
contact a small portion of the mineral by means of the acid water which permeates its
outer cell-walls ^.

In taking up those constituents of the soil which are insoluble in pure water, the
solution is therefore first of all accomplished by the plant itself ; and it is at the point
where solution takes place at the surface of the root that absorption inwards is also
effected by endosmose. But in spite of this complication the same principles hold
good for the absorption of material from the soil as have been explained in the case
of absorption from a solution. Here also it is the consumption, the decomposition
of the compounds in the plant, that regulates the absorption of the material. The
quantitative composition of the ash has therefore no resemblance to that of the soil ;
and the ash of plants of different kinds growing side by side and deriving their nutri-
ment from the same soil may be altogether different 2. But the composition of the
soil is important to the plant in a secondary degree ; since plants of the same kind, if
they grow for example on a soil rich in lime, will take up a greater quantity of lime
than if the soil contained but little of it. This is obviously not in contradiction to the
principle laid down, but only shows that the decomposition of a salt in the plant will
take place more largely the more easily it is enabled to take it up.

For a more detailed account see the Handbook of Experimental Physiology, 1865, p.

^ [Messrs. Lawes and Gilbert's long series of experiments on this subject are of especial value,
(See Journ. Roy. Agric. Soc. vol. VIII. p. 496 et seq., 1847 ; Journ. Chem, Soc. vol. X. p. i, 1857 ;
Report Brit. Assoc 1861 and 1867.) Their latest publication, 'Report of Experiments on the growth
of Barley for twenty years in succession on the same land' (Journ, Roy. Agric. Soc, second series,
vol. IX) contains much information as to the power possessed by plants of extracting different
substances from the soil,]

I

ASSIMILATION AND METASTASIS.

703

Sect. 5. — Assimilation and Metastasis (Stoffwechsel) \ The food-materials
absorbed by the plant are, with a few exceptions, compounds of oxygen containing
the highest possible proportion of that element. The assimilated substances, on
the contrary, which form the greater part of the dried substance contain but
little oxygen, some even none at all. It follows from this that assimilation must
be a process of deoxidation. The transformation of food-materials containing a
large proportion into the substance of plants containing but little oxygen must
necessarily be accompanied by elimination of that element ; and since we already
know that this takes place only in cells containing chlorophyll and under the in-
fluence of sunlight, we have at once the locality, the conditions, and the time of
the assimilation thus determined. No organs which are destitute of chlorophyll
can assimilate ; and in the dark or when the amount of light is small, even those
assimilating organs which contain chlorophyll lose the power of producing organic
substances out of water and carbon dioxide with the assistance of other food-
materials, — a process to which we shall henceforward exclusively apply the term
Asshm'latwn.

The products of assimilation of the cells containing chlorophyll may undergo
various kinds of chemical metamorphosis either in these cells themselves or after
passing into other organs ; and the aggregate of these processes may be distin-
guished from assimilation as Metastasis. It is important to bear clearly in mind the
difference between these two processes, both in respect to their external conditions
and to their results, the following being the chief points : — (i) Assimilation takes
place only in those organs that contain chlorophyll ; metastasis in all alike. (2)
Assimilation occurs only under the influence of light ; metastasis equally well in the
dark. (3) Assimilation is necessarily accompanied by the elimination of a large
quantity of oxygen; metastasis is usually connected with the absorption of small
quantities of oxygen and the exhalation of small quantities of carbon dioxide.
(4) Assimilation increases the dry weight of a plant ; metastasis only alters the
nature of the assimilated materials, and these usually suffer a diminution of their
mass, the destruction of a part of the assimilated organic compounds being neces-
sarily associated with the inhalation of oxygen and exhalation of carbon dioxide
necessary for metastasis. (5) The increase in weight of a plant which contains
chlorophyll depends on the accession of assimilated substance in the organs that
contain the chlorophyll being greater during the time that they are exposed to
light than the loss in the dry weight connected with the exhalation of carbon
dioxide accompanying metastasis in all the organs and at all times of vegetation.
(6) Organs containing no chlorophyll and plants entirely destitute of it (parasites and
saprophytes) do not assimilate but absorb substances already assimilated ; no pro-
cess takes place in them except metastasis; and since this is associated with in-
halation of oxygen and exhalation of carbon dioxide, they decrease the entire store
of assimilated substances.

Growth, i. e. the formation and enlargement of cells, always takes place at
the expense of substances already assimilated; and these therefore must be subject
to continual chemical change.

^ See Sachs, Handbuch der ExperimentalPhysiologie, the section on the Transformation
of Food-material.

704 CHEMICAL PROCESSES IN THE PLANT.

Growth is only possible as a result of assimilation; but the two processes
do not usually concur either in time or locality. The assimilated substances may
remain in the plant for a longer or shorter time without becoming employed in
the growth of cell-walls or in the production of protoplasmic substances
(protoplasm or chlorophyll-granules) ; and in this case they are termed Reserve-
materials. Every cell, tissue, or organ in which assimilated substances are stored
up for subsequent use is called a Reservoir of Reserve-material. The assimilating
cell may itself serve as a reservoir for reserve-material (as unicellular Algae or the
leaves of evergreen plants) ; but usually a physiological division of labour is
effected in the plant of such a nature as to transfer the products of assimi-
lation from the organs that contain chlorophyll to other organs or masses of
tissue which serve as reservoirs of the reserve-material and give it up to the
parts destined for the formation of new organs (buds, the rudiments of roots, or
cambium). In Mosses, Vascular Cryptogams, and woody Phanerogams, the tissue
of the stem is usually also the reservoir for this purpose ; in perennial herbs and
shrubs it is more often the persistent bulbs, tubers, and rhizomes that perform
this function. The spores of Cryptogams which have the power of germination
always contain a small quantity of reserve-material, at the expense of which the
first processes of germination take place ; in Rhizocarpeae and Ligulatae the whole
of the prothallium and embryo is produced in this manner. The seeds of Phanero-
gams remove much greater quantities of reserve-material from the mother-plant,
which are accumulated either in the endosperm or in the cotyledons ; the greater
the quantity of this reserve-material the more numerous and the larger are the stems,
roots, and leaves which the seedling can produce before it begins to assimilate.
The minute seedlings, for instance, of Nicotiana and Campanula may be contrasted
with the strong ones of the Bean, Almond, Oak, &c. Since no assimilation takes
place in the dark, it is only necessary to allow seeds, tubers, bulbs, rhizomes, &c.
to germinate and develope in the dark in order to form an idea of the number
and size of the organs which can be formed from the reserve-material.

Since the organs of assimilation which contain chlorophyll are usually at a
distance from the reservoirs of reserve-material and from the growing buds and
roots, the products of assimilation have to be conveyed to the localities where
they are required and where they are temporarily deposited. Growth and the
deposition of reserve-material are therefore necessarily associated with corre-
sponding movements of the products of assimilation and of those undergoing
metastasis.

All these statements may be proved without any more accurate knowledge of
the substances themselves which are produced by assimilation in the cells that
contain chlorophyll and which undergo metastasis. But before entering on this
question, we may first of all discuss the other : — whether all the products of me-
tastasis are immediately applicable to the building up of new organs ; and if not,
what substances furnish the material for the production of cell-walls, protoplasm,
and chlorophyll -granules. Among the extraordinarily large number of the pro-
ducts of metastasis which are proved by chemical analysis to exist in various plants,
there is a comparatively small number of substances the behaviour of which in
the growth of the organs and whose universal distribution through the vegetable

ASSIMILATION AND METASTASIS. 705

kingdom clearly show that they furnish the material for the growth of cell-walls
and of other organised structures. These substances may be termed, without
reference to their chemical nature, Formative Materials. Starch, the different kinds
of sugar, inulin, and the fats must be considered the formative materials of the
cell-wall ; the albuminoids the formative materials of protoplasm and of the chlo-
rophyll-granules.

Among the remaining products of metastasis are some which stand in genetic
relation to the production of sugar; the glucosides, to which also belong certain
tannin-substances. Asparagin is formed at the expense of the albuminoids con-
tained in the reservoirs of reserve-materials, and is afterwards again used in the
formation of albuminoids in the young organs.

All those organic compounds may be termed Degradation-Products which are
produced by subsequent change in the substance of the organised structures of
plants, and which have no further use in the building up of new cell-walls or proto-
plasmic structures. Thus bassorin is a degradation-product of cell-walls, as also is
the mucilage of quince and linseed ; the substances which cause lignification, suber-
isation, or cuticularisation are also probably the result of a partial degradation of
the cellulose of the cell-walls. A residue of the protoplasm of older parenchy-
matous cells often remains until they entirely die away, and may also be considered
a degradation-product. In the same manner a small residue of the chlorophyll-
granules of leaves which die in the autumn remains over in the form of minute yellow
granules which have no further use. The red and yellow granules also which
cause the colour of ripe fruits and of the antheridia of Characeae and Mosses result
from the degradation of chlorophyll-granules, and have no further physiologico-
chemical use.

Those substances may be termed Secondary Products of Metastasis which are
formed during this process, but have no further use in the building up of new
cells, remaining inactive at the place where they are produced. Thus in the
germination of many seeds (the Date, Ricinus, Phaseolus, Faba^ &c.) tannin-like
compounds are formed in particular cells, and in many cases red colouring sub-
stances which, without undergoing any perceptible change, remain in these cells,
while the rest of the substances of the seedling go through the most various chemical
transformations and changes of place in the course of its growth. The same is the
case with the essential oils in the glands of leaves, of caoutchouc in the laticiferous
vessels, of resin and resin-forming substances in the resin-passages, and of the
gummy compounds contained in the gum-passages of many plants. In this category
may also be included the greater number of vegetable acids and many alkaloids.
No interpretation has yet been given of the function of these substances in the
internal economy of the plant; in the case of calcium oxalate Holzner's theory
has already been mentioned that it is formed as a secondary product when the
sulphuric acid combined with the calcium is replaced by oxalic acid ; and that the
free sulphuric acid then undergoes various further decompositions, while the base
of the salt remains unused and inactive in combination with the oxalic acid pro-
duced as a secondary product, as calcium oxalate in the crystalline form. Among
colouring substances no relation to the chemical processes which proceed in the
plant has been traced except in the case of the green colouring substance of chloro-

7o6 CHEMICAL PROCESSES JN THE PLANT.

phyll ; it is only in the presence of this substance that elimination of oxygen,
and therefore assimilation, can take place. In the case of a long series of other
substances, many colouring matters, acids, alkaloids, wax, tannin, pectinaceous sub-
stances, &c., no relation to the other processes of metastasis is known, nor any
physiological signification which they possess in the life of the plant.

In some cases substances which have ceased to take part in the processes of
growth and of metastasis are nevertheless important or even indispensable for other
purposes of vegetation. Of this class are the saccharine juices secreted by nec-
taries, which are of service to the plant only so far as they attract insects which thus
bring about the conveyance of the pollen to the stigma. For a similar purpose a
portion of the tissue of the anthers of Orchids is transformed into a viscid glutinous
substance by which the pollinia become attached to the proboscis of insects. Thus
again the sapid and nutritious substances which constitute the pericarps of some
fruits are of no direct use for the growth of the seeds, but cause their dissemi-
nation by animals which feed on the fruits and thus disperse the seeds.

We must now again turn, after this preliminary explanation of the various parts
■played by the products of metastasis in the life of the plant, to the most important
group of organic compounds, those which have been distinguished above as form-
ative materials.

The determination whether any chemical compound belongs to the class of
formative materials of the cell-wall and protoplasmic substances depends on its
behaviour during growth, on its chemical composition, on its appearance and dis-
appearance in growing cells and tissues, and on its chemical relations to other
substances, especially to cellulose and to protoplasmic substances. Spores, seeds,
bulbs, tubers, rhizomes, the persistent parts of woody plants, and other reservoirs
of reserve-material, always contain chemical compounds belonging to two different
groups. On the one hand nitrogenous substances are always present in the form
of albuminoids (often several different ones as in the grains of cereals) which
scarcely differ chemically from protoplasm, and when contained in the succulent
reservoirs of reserve-materials preserve even the form of protoplasm. From this
similarity, and still more when the migration and other relations of these sub-
stances are kept in view, the conclusion must be drawn that we have in them
the material for the formation of protoplasm in the newly-formed organs. On
the other hand all these reservoirs of reserve-material contain one or more non-
nitrogenous substances belonging to the series of carbo-hydrates and oils. In
seeds and spores there is generally a great deal of oily matter and little or no
starch ; but many seeds contain on the other hand a great deal of starch with but
little oily matter. In tubers, many bulbs, rhizomes, and stems, there is usually much
starch stored up with but little oily matter ; while in some tubers (as the Dahlia, Arti-
choke, &c.), the starch is replaced by inulin; in the bulbs of Allium Cepa by*a sub-
stance resembling grape-sugar ; in the root of the Beet by crystallisable cane-sugar.
Small admixtures of oily matter appear to be never absent, and in some cases,
especially in many seeds, this alone is present without any carbo-hydrate (as the
Almond, Gourd, Castor-oil plant, &c.).

Together with albuminoids, carbo-hydrates, and oils, a variety of other com-
pounds may also occur in the reservoirs of reserve-material ; but the limitation of

ASSIMILATION AND METASTASIS. 707

substances of this kind to particular species of plants shows that they are not
of the same significance as the former. They may be of great importance for
the growth of the species; but more accurate knowledge is still wanted in all
cases.

Since seeds, tubers, and other parts of plants that are filled with reserve-material
can be made to unfold buds, to put out roots, and even to form flowers and the
rudiments of fruits by supplying them with pure water and oxygenated air when the
conditions for assimilation (chlorophyll and sunlight) are absent, it follows that the
substances stored up in these reservoirs furnish the material for the growth of the
new leaves, roots, and flowers. The reservoirs are therefore emptied in proportion
as the growth of the new organs progresses ; and when finally they become com-
pletely empty, all further growth ceases, if sunlight and chlorophyll do not cooperate
to produce new formative material by assimilation. It is moreover easy to follow the
reserve-materials by means of micro-chemical reactions in their course from the
reservoirs through the conducting tissues to the growing organs, and to recognise
their relation to the growth of particular tissues. A close study leads first of all to
the conclusion that the albuminoids contained in the reservoirs of reserve-material
reappear as such in the protoplasm of the newly-formed organs, having, independ-
ently of temporary qualitative changes, only altered their position. On the other
hand it shows that the oily matter and the carbo-hydrates which had accumulated in
the reservoirs finally entirely disappear as such or leave only a small residue (oil) ;
while in their place a mass of new cell-walls is formed which were not in existence
before; and the material for the construction of these can only have been derived,
under the given conditions, from the carbo-hydrates, or, when these are absent, from
the oily matter which has now disappeared. If we thus come to the conclusion that
starch, sugar, inulin, and oil are the substances from which are formed the cell-walls
of plants, at all events in so far as they are nourished from a reservoir of reserve-
material, it by no means follows from this that the whole of the store is used up
entirely in the production of cellulose ; on the contrary a variety of other substances
are formed during growth, such as vegetable acids, tannin, colouring-matters, &c.,
which are probably also derived from the same non-nitrogenous reserve-materials.
A part of the non-nitrogenous substance is also entirely destroyed and converted into
carbon dioxide and water, a process which may cause a loss of 40 or even 50 per
cent, of the weight of the organic substance of those seeds which germinate in the
dark.

If the reserve-materials stored up in different seeds, tubers, bulbs, &c. are
compared, it is seen that starch, the various kinds of sugar, inulin, and oil, are
of the same physiological value with regard to their most important purpose, viz.
the formation of new organs ; inasmuch as these substances can replace one another.
Thus the cell-walls of the embryo of Allium Cepa are formed at the expense of
the oily matter of the endosperm ; but the cell-walls of the leaves and roots which
grow from the bulbs evidently obtain their formative material from the glucose-like
substance which fills the bulb-scales in a state of solution. In the Beet cane-
sugar is stored up for the same purpose, inulin in the tubers of the Dahlia, and
starch in the tubers of the Potato, the bulbs of the Tulip, &c. ; and these are
subsequently consumed. But in most seeds all these carbo-hydrates are replaced

z z 2

7o8 CHEMICAL PROCESSES IN THE PLANT.

by oily matter ; and it cannot be doubted that this furnishes the material for the
growth of the cell-walls when the new organs are being formed.

To the series of these substances of the same physiological value belongs
finally cellulose itself, which may also be deposited in considerable quantities as a
reserve-material, as in the endosperm of the Date, the greater part of the hard kernel
of which consists of cellulose in the form of the pitted thickening masses of the
cell-walls. These are dissolved during germination, and the products of their solu-
tion conveyed to the growing parts of the embryo, where they finally supply the
material for the growth of the new cell-walls.

If on the other hand the substances which occur in dormant seeds, bulbs,
tubers, and other reservoirs of reserve-material, are compared with those which are
found in the conducting tissues and growing organs of seedlings and young roots —
which we already know must necessarily be produced from the former, because there
is no other material which can produce them — it is seen that these reserve-materials
must undergo repeated Metamorphosis while they are being conveyed to the growing
organs and are being consumed in the process of growth, and before the permanent
form of cellulose has been attained.* Thus sugar and starch are found temporarily
in all oily seeds during germination, and are often accumulated in great quantities,
disappearing when germination is completed. In proportion as they are formed the
amount of the original oil decreases ; and in proportion as they again disappear the
quantity of cellulose in the cell-walls increases. In other cases starch is conveyed
from reservoirs of reserve-material to the growing organs, sugar being at the same
time formed^; and fine-grained starch is again temporarily formed in the growing
tissues themselves, disappearing once more with the growth of the cell-walls. This
temporary formation of starch in the growing . tissues themselves is an extremely
common phenomenon, whether the reservoirs of reserve-material were filled with
oily matter, inulin, sugar, starch, or cellulose. This transitory starch appears in the
cells of the parenchyma and epidermis of young organs (only rarely in those of the
fibro-vascular bundles) after they have become differentiated from the primary meri-
stem ; and disappears when the final elongation of the organs is completed, generally
becoming transformed into sugar (glucose), which in its turn speedily disappears.

Transitory metamorphoses also take place when the albuminoids stored up in
the reservoirs of reserve-materials are being transported and consumed^; although
these metamorphoses cannot be followed by micro-chemical observations, as in the

^ [The conversion of starch into sugar is effected by means of unorganised ferments ; some of
these have long been knovi^n, such as Emulsin (in Almonds), Diastase (in Barley), Myrosin (in Black
Mustard seeds). More recently they have been detected in various plants and parts of plants by
Kossmann (Journ. Pharm. Chem. (4) 22) and by Krauch (Landwirthsch. Versuchsstat, 23). Von
Gorup-Besanez has found ferments of this kind in the germinating seeds of Vetches, Hemp and Flax,
which have also a peptic action (see infrd)?^

^ [The first stage in the metamorphosis of the reserve-proteids is, doubtless, their conversion
into peptones, into proteid substances, that is, which are readily soluble in water and which diffuse
rapidly. This is effected by the action of unorganised ferments. These bodies have been found in
germinating seeds (v. Gorup-Besanez and Will) and in the secretion of ' carnivorous ' plants such as
Nepenthes, Drosera, and Darlingtonia. Recently a very active peptic ferment has been found in the
green fruits of Carica Papaya (Wiirtz and Bouchet, Le Papain, Comptes rendus, torn. 89, 90, and 91).'

A good resume of our knowledge on the action and distribution of unorganised ferments in plants
is given in the second edition (1882) of Husemann's Pflanzenstoffe, I. p. 537.]

ASSIMILATION AND METASTASIS. 709

case of the oils and carbo-hydrates. Thus a portion of the casein in the cotyledons
of Leguminosae passes over into albumin during germination ; the insoluble proteids
in the endosperm of Wheat are dissolved and carried up into the seedling plant.
The albuminoids contained in seeds appear to be subject during germination to Still
more complete decompositions. The asparagin which occurs temporarily in parts
of the embryo can only be formed by partial decomposition of the albuminoids ^.
It appears however that these products of the decomposition of the albuminoids
under the influence of the energetic Oxidation which takes place in the germinating
seed are used in the formation of albuminoids in the growing parts of the embryo.

The preceding remarks refer to the processes of growth which are associated
with the consumption of the substances stored up in the reservoirs of reserve-
material. If those plants are now examined in a similar manner whose reserve food-
material has been consumed, whose green leaves have begun to assimilate under the
influence of light, and which are forming the substan'=^^ necessary for the growth
of their buds, roots, &c., the same substances are found similarly distributed through
the conducting tissues of the internodes and the petioles and veins of the leaves as
far as the buds and apices of the roots, and subject to the same metamorphoses as
in the seedlings. It follows that the assimilating organs which contain chlorophyll
perform the same function for the growing parts of the mature plant that the reser-
voirs of reserve-material do for the seedling ; but with this difference, that the former
produce the formative materials afresh, while in the latter they are not formed but
only stored up.

The organic compounds originally formed in the cells containing chlorophyll
by the decomposition of carbon dioxide and water under the influence of light are
generally carbo-hydrates. The most common of these is starch ; sugar occurs less
often j oily matter perhaps occasionally. It has been shown (p. 46) that the starch
which so commonly occurs in the chlorophyll-granules of plants that vegetate under
normal conditions can only be produced when the plant is subject to the well-
known conditions of assimilation, i. e. when it decomposes carbon dioxide and water
under the influence of sunlight. Seedlings which have completely exhausted their
supply of reserve-materials by growth in the dark, and are afterwards exposed to the
action of light, do not till then develope their chlorophyll. The first grains of starch
which are found a little later in the plant are those enclosed in the chlorophyll-
granules, and are at first small, but gradually grow larger. It is only afterwards that
starch is found also in the conducting tissues of the internodes and leaf-stalks up to
the buds, which then begin to grow anew. It has been shown further that this
starch which is formed in the chlorophyll-granules disappears in the dark ; i. e. be-
comes dissolved and transferred to the conducting tissues. In Allium Cepa the
chlorophyll forms no starch ; but a substance similar to grape-sugar is found in
large quantities in the green leaves, and is distributed through all the tissues of the
plant : it is still uncertain whether or not mannite is formed in a similar manner in
the leaves of the Olive. Where drops of oil are found in the chlorophyll, they appear

^ According to Hosseus, ammonia is also formed during germination ; and Borscow maintains
that ammonia is set free during the vegetation of Fungi (Melanges biol. tires du Bullet, de I'Acad,
imp. des Sci. Nat. Petersbourg, vol. VII, 1868). This is however denied by Wolf and Zimmermann
(Bot. Zeitg. 1871, nos. 18, 19). For a further account of Asparagin see the appendix to this section.

7IO CHEMICAL PROCESSES IN THE PLANT.

to be formed at the expense of the starch which has been produced there ; this con-
clusion being derived especially from the observation of what takes place in Spirogyra
and Cereus^.

The result of tracing by micro-chemical observation the products of assimi-
lation in the conducting tissues leads once more to the conclusion that the starch
which is formed in the cells containing chlorophyll is subject to a variety of
chemical metamorphoses before it reaches the growing tissues and the reservoirs of
reserve-material. Even during the period of vegetation the substances which are
conducted to the young parenchyma of growing parts, as soon as this has been
differentiated from the primary tissue, give rise to the formation of fine-grained
starch which accumulates there temporarily, and disappears with the final and rapid
increase in size of the cells. Starch and other substances are then produced afresh
by assimilation in the fully developed leaves ; and starch and the products of its
transformation again appear in the conducting tissues, not to be consumed there,
but only to be conducted to the still younger parts. The metamorphoses of the
formative materials which are conveyed from the assimilating organs to the re-
servoirs of reserve-material, generally show a reversed order of succession to that
which takes place during germination ; the starch produced in the leaves is trans-
formed in the leaf-stalks of growing Beet into glucose, from which crystallisable
cane-sugar is formed in the swollen tuberous roots ; in the Artichoke the starch is
converted into inulin which is conducted through the stem to the underground
tubers ; in the Potato, the mature leaves of which form starch, a substance similar
to glucose is chiefly found in the conducting tissues, which is conveyed to the
growing tubers, and there evidently forms the material from which the large
masses of starch are formed. In ripening fruits and seeds a large quantity of
glucose is generally found which disappears from the seeds when they become ripe,
starch being formed in these reservoirs of reserve-material ; in Ricinus the oil of the
endosperm is evidently formed at the expense of the saccharine substance which is
conveyed to the seed ; in the embryo of the same plant, as well as in that of Cruci-
fers, fine-grained starch is formed temporarily, which disappears when the seeds are
ripe, and is replaced by oily matter.

Whether the albuminoids also are first formed in the assimilating cells which
contain chlorophyll and whether they can be formed only in them is still an unde-
cided point. It is certain that they are formed in the chlorophyll-containing cells of
Alg3e ; but it cannot be concluded from this that they can only be produced in the
corresponding cells of plants with differentiated tissues ; at all events experiments
on the artificial production of the yeast-fungus show that it is able to form out
of sugar and an ammonium-salt (with the assistance of the constituents of the
ash) not only cellulose but also albuminoids, as may be inferred from the increase

• ^ [Briosi states (Bot. Zeitg. 1873) that starch-grains are never found in the chlorophyll-granules
oi Musa and Strelitzia, but that drops of oil are present instead. His observations have been shown
to be erroneous by von Holle and Godlewski (Blora, 1877). Pringsheim (Ueb. Lichtwirkung und
Chlorophyllfunction, Jahrb. f. wiss. Bot. XII, 1880) enumerates a number of plants in which oil
is present, and not starch, in the chlorophyll-granules ; Vaucheria sessilis, Selaginella, Cycas, Stratiotes
aloides, Lilhun Martagon, Olea europcza. Begonia. He is of opinion that the first product of
assimilation is a waxy substance to which he gives the name of Hypochlorin.']

A SSI MIL A TION A ND ME TA S TA SIS. J I \

of the protoplasm in the rapidly multiplying cells. If the colourless cells of yeast
are able to do this, it may be inferred, until the contrary is proved, that those cells
of other plants which do not contain chlorophyll can also produce albuminoids, if
only a carbo-hydrate or oil (or both) is conveyed to them from the leaves, and an
ammonium-salt or a nitrate from the roots. That the formation of albuminoids
probably takes place in this way within the conducting tissues of internodes and
petioles may be concluded from the deposition of calcium oxalate in these tissues ;
since in the formation of this salt sulphuric acid becomes separated from the calcium,
and its sulphur enters into the chemical formula of albuminoids \

When the cells of the leaves become emptied of their contents at the close
of the period of vegetation, and the deciduous parts die off, not only the last portion
of starch which was formed in the latter, but also the material of the chlorophyll-
granules, is itself absorbed and conveyed through the leaf-stalks to the reservoirs of
reserve-material ; all the serviceable substances contained in the leaves become in-
corporated in the permanent organs. The leaves change colour ; a small quantity
of very small shining yellow granules usually remain behind in the cells of the
mesophyll as a residue of the absorbed chlorophyll-granules ; and the leaves which
are emptied in the autumn are therefore yellow. If they are red this is in con-
sequence of a red sap which fills the cells in addition to the chlorophyll-granules^.
Enormous quantities of crystals of calcium oxalate often remain behind in the
deciduous leaves; the constituents of the ash which are serviceable to the plant,
especially phosphoric acid and potash, are conveyed with the starch and the proto-
plasmic substances to the persisting parts ; so that the falling leaves thus consist
only of a skeleton of cell-walls and of the subsidiary products, of metastasis which
are of no value to the plant.

The direction of the Transport of the assimilated substances in the plant is
determined by the fact that it must take place from the assimilating organs to
the growing parts and to the reservoirs of reserve-material; while at the com-
mencement of every new period of vegetation its direction must be from these
reservoirs to the growing organs ; and since new organs are usually formed above
as well as below these reservoirs and the assimilating leaves, it is obvious that
the movements of the assimilated substances must take place at the same time in
opposite directions.

The Conducting Tissue for the transport of the formative materials consists,
in plants with differentiated systems of tissue, of the parenchyma and the thin-
walled cells of the phloem of the fibro-vascular bundles. By the parenchyma of
the fundamental tissue, which always has an acid reaction, are conveyed the
carbo-hydrates and oils; by the soft bast, the albuminoids which have an alkaline
reaction. Small starch-grains often occur, as Briosi has recently shown, in the pro-
toplasm of the sieve-tubes; I had already pointed out that this accompanied the
absorption from the leaves in the autumn as well as very rapid growth ^ Where

' See Sachs, Handbuch der Experimental-Physiologie, p. 345.

^ [On the colouring matter of the leaves in autumn, see Sorby, Quart. Journ. of Science, 1871,
p. 64; and 1873, p. 215.]

^ Briosi, Bot. Zeitg. 1873. It is by no means certain that the occurrence of small quantities of
starch in the sieve-lubes demonstrated by Briosi, and the possibility of their passaj;e through the

712 CHEMICAL PROCESSES IN THE PLANT.

there are laticiferous vessels, they furnish an open communication between all the
organs of the plant ; they contain albuminoids, carbo-hydrates, and oils, as well as
the secondary products of metastasis, as caoutchouc and poisonous substances, the
occurrence of which does not affect the significance here attributed to the laticiferous
vessels ; the occurrence of products of decomposition in the blood does not prevent
us from regarding the blood-vessels as organs which serve to transport nutritious sub-
stances ^

The mode of motion of the assimilated substances is usually molecular; t'.e.
it is a movement of diffusion, especially where the transport takes place through
closed cells. The pressure caused by the tension and turgescence of the tissues
has in addition a tendency to propel the fluids in the direction of least resistance,
which is also that in which they are consumed. In the system of communicating
sieve-tubes and laticiferous vessels the movement of the substances is necessarily
one of the entire mass, caused by inequalities of pressure, and by the distortions
and curvatures which the wind produces.

As far as concerns the movements of diffusion, it is a general rule that
every cell which decomposes any substance, renders it insoluble, or uses it for its
growth, acts upon the dissolved molecules of this substance in the neighbourhood
as a centre of attraction ; the molecules stream to the parts where they are wanted
because the molecular equilibrium of the solution is disturbed by its consumption.
On the other hand every cell which produces a new soluble compound acts on
the dissolved molecules as a centre of repulsion, because the continually increasing
concentration occasions at the point of production a streaming of the molecules
away from it towards the point of less concentration, the concentration continually
decreasing towards the points where the substances are consumed. When the move-
ment of diffusion is caused by the production and consumption of definite compounds
of this nature, the proximate cause of the molecular movement of the dissolved sub-
stances must be the chemical processes involved in their metamorphoses. These
metamorphoses take place, as we have seen, not only at the points where the sub-
stances are consumed in the process of growth, but also in the conducting tissues ;
and this production of transitory compounds must therefore favour movement
towards the points of deposition and of growth. The formation of insoluble
starch is in this sense a fact of peculiar importance. If for instance the starch
produced in the leaves of the Potato is required to be transported to the tubers,
it must necessarily be conveyed in a soluble form, and we find such a substance
in the conducting tissues of the stem, namely, glucose. But if this glucose had to
undergo no further change in the tubers, a solution of glucose of constantly in-
creasing concentration would be uniformly distributed through the conducting

pores of the sieves, warrants the view that the sieve-tubes are conducting organs for starch in the
same sense that they are for albuminoids. These small quantities of starch may pass into the sieve-
tubes from the neighbouring parenchyma, to be used there, in young organs, as plastic material, or, in
older organs, to take part in the formation of albuminoids. It may be that these substances are
formed in the sieve-tubes out of carbo-hydrates and nitrogenous compounds, calcic sulphate being
decomposed and a formation of crystals of calcic oxalate taking place in the cells surrounding
the phloem.

^ Sec also Faivre, Sur le latex du murier blanc ; Ann. d. Sci. Nat. ser. V. t. lo.

ASSIMILATION AND METASTASIS. 713

tissues and the tubers ; and the accumulation of the whole of the reserve-material
in the tubers would be impossible. The glucose is used up in the tubers in the
formation of starch-grains ; and a fresh quantity therefore continually streams in
that direction; the whole mass of the material produced in the leaves is therefore
gradually transferred to these reservoirs of reserve-material. The starch is first
transformed into glucose, and then back into starch ; and it is in this chemical
process that the vehicle for the movement consists. Starch is even produced tem-
porarily in the conducting parenchyma, but of course cannot be transported as such
from cell to cell ; its movement being effected by the solution of grains in one cell,
the product of solution diffusing into the adjoining cell, and being there employed
in the formation of starch-grains which are then again dissolved, and so on. When
again cane-sugar is formed in the tuberous roots of the Beet, the movement towards
the root of the glucose which is produced from the starch assimilated in the chloro-
phyll is brought about in this way, — every particle of glucose undergoes chemical
transformation when it reaches the root, and the molecular equilibrium of the
solution of glucose is thus disturbed ; the root acting as a centre of attraction on
the glucose in the leaf-stalks. But the continual formation of the solution of
glucose in the leaves at the expense of the starch causes in them an increase
of concentration and a streaming of molecules towards the root, where the con-
centration of the solution of glucose is continually decreasing, while that of the
solution of cane-sugar increases. The same is evidently the interpretation of
the formation of inulin in the tuberous roots of the Dahlia and the tubers of the
Artichoke, and of that of oil in ripening seeds at the expense of the sugar which is
conveyed to them.

I infer the co-operation in the movement towards the parts where the substances
are chemically altered, of the pressure exercised on the cell-sap by the tension of the
tissues, even where we have to do with closed cells, from the fact that considerable
quantities of the cell-sap appear on the surface of a transverse section of succulent
organs, both from the parenchyma and from the cambiform cells, and this is clearly
forced up by internal pressure. Since the tension and turgescence of the tissue are
always less in the buds and apices of the roots than in the older parts, there must
always be a tendency for the filtration of the sap towards the latter, which must act
in the same way as diffusion.

That the contents of the perforated sieve-tubes and laticiferous vessels are
also subject to considerable pressure from the surrounding tissue is shown by
the extent to which these fluids flow out when the organ is cut through. The
fluid which is subject to pressure will have a tendency to escape from these tubes
to parts of the plant where the lateral pressure is less, which is the case in the
buds and apices of the roots. The flexions and distortions occasioned in the
organ by the wind will at the same time cause the fluid contents of the sieve-
tubes and laticiferous vessels to be pressed away from the older bent parts towards
the buds where the tension is less.

The statements here compressed into a very brief space rest on a series of detailed
micro-chemical and experimental researches which I have described in the Botanische
Zeitung, 1859 and 1862-1865; Pringsheim's Jahrbiicher fiir wissenschaftliche Botanik,
vol. III. p. 183 et jeg.; Flora, 1862, pp. 129 and 289, and 1863, pp. 33 and 193; and have

714 CHEMICAL PROCESSES IN THE PLANT,

presented in a connected form in the section on the Transformation of Food-materials in
my Handbook of Experimental Physiology \ The reader will there find the reasons for
the views here given ; and a few examples will now be sufficient to render somewhat
clearer the general statements with regard to metastasis and the migration of the assim-
ilated substances. In the outset it must be stated that by grape-sugar or simply sugar
I understand a substance soluble in the cell-sap, easily reducing copper oxide, and
readily soluble in strong alcohol, although it may not always exactly correspond to the
grape-sugar of chemists, a point which is of but little importance for our present
purpose.

I. The parenchyma of the bulb-scales of the Tulip — i.e. the four or five thick
colourless leaves which serve as reservoirs of reserve material — contains, as long as the
plant is dormant, in addition to considerable quantities of albuminous substances, a very
large quantity of coarse-grained starch. The presence of sugar cannot be determined
at this time by micro-chemical processes. As soon as the bud of the leaf- and flower-
stem which is concealed within the bulb, but which had already been formed with all
the parts of the flower during the previous summer, begins to elongate in February,
and roots make their appearance from the base of the bulb, small quantities of sugar
are found with the starch in the parenchyma of the bulb-scales. The whole of the
parenchyma and of the epidermis of the leafy stem, of the young foliage-leaves, of
the perianth, of the stamens, and of the carpels, becomes filled with fine-grained starch,
the substance of which has already been derived from the bulb-scales, where the starch-
grains have become transformed into sugar, which diff'uses into the growing organs, and
there, as far as it is not directly consumed, again supplies material for the formation of
starch-grains.

Together with its consumption in the growth, at first slow, of the cell-walls, this
temporary re-formation of starch at the expense of that contained in the bulb-scales
continues at first in the young internodes, leaves, and flowers. The cells enlarge and
become continually more filled up with fine-grained starch till the time when the bud
comes above ground (Fig. 470). Then follows the rapid extension of the stem ; the
leaves expand, and the flower unfolds. With the considerable and rapid increase in
size of the cells accompanying this unfolding, the fine-grained starch disappears in all
these parts, sugar being temporarily produced which furnishes the material for the
growth of the cell-wall. When all the parts above ground are fully unfolded, the cells,
although much larger, are now devoid of starch. The corresponding loss which the
bulb-scales have experienced up to this time is clearly seen from the decrease of their
starch-grains ; they may be found in all stages of absorption. The turgescence of the
bulb-scales at the same time decreases, and they become wrinkled ; but the formation
of sugar in them still continues at the expense of the starch, even when the parts above
ground have already done growing. The starch stored up in the bulb-scales finds in
fact still another use ; while the flower-stalk is extending, the bud in the axil of the
uppermost bulb-scale begins to develope rapidly (it had already been formed in the
previous summer) ; its cataphyllary leaves swell and become filled with starch ; and
the residue of the starch not consumed in the growth of the flower-stalk is transported
from the scales of the mother-bulb through its base into the young bulb (Fig. 470, 2).
These scales become gradually entirely emptied of starch, and while the green foliage-
leaves exposed to light are assimilating and contributing their share to the growth of
the new bulb, they finally wither and dry up from the simultaneous loss of water and
of assimilated matters. The scales of the mother-bulb form thin brown membranes

^ The researches of Schroder (Jahrb. fiir wiss. Bot. vol, VIL p. 261), Soraiier, Siewert, Roestell,
&c. (collected in Hoffmann and Peters' Annual Report on the Progress of Agricultural Chemistry
for 1868 and 1869, Berlin 1871) contain fresh confirmations of the account here given. [See also De
Vries, On the Germination and Growth of Seeds, Tubers, and Roots of cultivated Plants (Clover,
Potato, Beetroot) in Landwirthsch. Jahrbiicher, vols. 6-8, 1877-79.]

ASSIMILATION AND METASTASIS.

715

which serve to protect the new bulb ; the inflorescence subsequently dies down. The
reserve-materials which accumulate in the daughter-bulbs are partly derived from those
of the mother-bulb ; but are completed by the products of assimilation of the green
leaves of the flower-stalk. When the flower-stalk has also died down, nothing remains
of the whole plant but the bud which has developed into a new bulb. For a time it
does not put out any new organs, but is apparently dormant ; but in the interior the
end of the stem continues to grow slowly, and produces new rudiments of leaves and the
flower-bud for the next year ; when the process now described is repeated.

So far we have only pointed out the rela-
tion of the starch and of the sugar produced
from it to the growth of the plant ; there are
formed however along with it, and probably
likewise at the expense of these carbo-hydrates,
other substances, such as the colouring matter
of flowers, the oil in the pollen-grains, &c.
The albuminoids at first contained in the bulb-
scales become transported to a distance from
them, and furnish the material for the forma-
tion of the protoplasm in the young cells of
the growing flower-stalk ; a large part is evi-
dently employed in producing the chlorophyll-
granules in the foliage-leaves as they become
green. Their function is now to produce at
least as much formative material by assimila-
tion as is required to build up the transitory
inflorescence and to supply the bulb.

2. The ripe seed oi Ricinus communis con-
tains a very small embryo in the middle of
a very large endosperm ; neither contains
starch, sugar, nor any other carbo-hydrate,
if we exclude the very small amount in weight
of the cellulose of the thin cell-walls. The
reserve food-material consists of a great quan-
tity of oil (as much as 60 per cent.) and albu-
minoids, the admixture and composition of
which have already been described on p. 53.
The very small quantity of these substances
contained in the embryo would only suffice
for the first and very inconsiderable develop-
ment of the seedling ; its enormous increase
in size during germination must therefore
be attributed almost entirely to the sub-
stances deposited in the endosperm. The
endosperm of Ricinus enlarges very consider-
ably, as Mohl first showed, during germina-
tion, and the material required for its growth
must therefore be diverted from the embryo,
in the endosperm, with their surfaces in

Fig. 470. — Longitudinal section through a germinating
bulb of Tiilipa pracox : k the brown enveloping membrane,
k the flattened stem which forms the base of the bulb and
bears the bulb-scales sh; si the elongated part of the stem
which bears the foliage-leaves /' /', and terminates in the
flower ; c the ovary, / perianth, a anthers ; 2 a lateral bulb
in the axil of the youngest bud-scale, which developes into
tlie bud of next year's bulb ; w the roots which spring from
the fibro-vascular bundles of the base of the bulb.

The two thin broad cotyledons remain
contact with one another, long after the
root and the hypocotyledonary part of the stem have emerged from the seed ; they
are in contact by their backs with the tissue of the endosperm which surrounds
them on all sides, and absorb the reserve-materials from it, while they keep pace
slowly with its enlargement. When the parts of the seedling have increased very
considerably and the root has developed a number of lateral roots, the hypocoty-
ledonary portion of the stem elongates so that the cotyledons are drawn out of the

7i6

CHEMICAL PROCESSES IN THE PLANT.

endosperm which is then completely emptied and reduced to a thin membranous sac.
They now rise above the ground, become expanded to the light where they continue
to grow rapidly and become green, and serve from this period as the first assimilating
organs.

In this case, as in the germination of all oily seeds, sugar and starch are produced
here in the parenchyma of every growing part, disappearing from them only when the
growth of the masses of tissue concerned has been completed. Since the endosperm
grows also independently, starch and sugar are, in accordance with the general rule,
temporarily produced in it. The cotyledons apparently absorb the oil as such out of
the endosperm, whence it is distributed into the parenchyma of the hypocotyledonary
portion of the stem and of the root, serving in the growing tissues as material for the
formation of starch and sugar, which on their part are only precursors in the pro-
duction of cellulose. In these processes of growth tannin is also formed which is of
no further use, but remains in isolated cells, where it collects apparently unchanged

Fig. 4ji.—Ru-tm(s communist I longitudinal section of the ripe seed; //germinating seed with the cotyledons still
In the endosperm (shown more distinctly in A and B), s testa, e endosperm, c cotyledon, he hypocotyledonary portion
of the stem, -w primary root, -w' secondary roots, x the caruncle.

until germination is completed. It can scarcely be doubted that the material for the
formation of this tannin is also derived from the oil of the endosperm, although perhaps
only after a series of metamorphoses. The absorption of oxygen, which is an essential
accompaniment of every process of growth and especially of germination, has in this
case, as in that of all oily seeds, an additional significance, inasmuch as the formation of
carbo-hydrates at the expense of the oil involves the appropriation of oxygen.

Since the metamorphoses of material proceed pari passu with the growth of the
separate parts, the distribution of the products of metastasis through the tissues is
continually changing, and can only be understood by a consideration of all the sur-
rounding circumstances. The micro-chemical investigation of seedlings in the state
represented in Fig. 471 //, gives, for instance, the following result: — in the endo-
sperm is found a great deal of oil and a little starch, with sugar at the outside; the
epidermis and parenchyma of the slowly growing cotyledons are filled with drops of
oil ; a large number of the epidermal cells contain tannin ; starch-granules are found

ASSIMILATION AND METASTASIS. 717

only in the parenchyma of the leaf- veins; the parenchyma of the hypocotyledonary
portion of the stem, which is at present growing the most rapidly, contains only
comparatively little oil but much starch and sugar ; and a number of the cells of the
epidermis and parenchyma are filled with tannin. The primary root has first of all
completed its growth in length and thickness (after germination it begins afresh) ; in
its lower part it contains neither starch nor sugar (the former is present in the root-
cap) ; in its upper part from which the lateral roots spring and in the lateral roots
themselves sugar is still present, which is conveyed into the growing apices of the
latter. When the hypocotyledonary portion of the stem has subsequently become
vertical and has temporarily ceased to grow, the oil, starch, and sugar have almost
entirely disappeared from it, and in their place the cell-walls have become thick, and
the vessels and first cells of the wood and bast are already thickened. After the stem
of the young plant has become upright, the cotyledons expand and grow rapidly, and the
remainder of the oil which they had taken up from the endosperm now also disap-
pears from them together with the starch and sugar. The seedling has now entered
on a state in which the non-nitrogenous reserve-materials are consumed ; a framework
of large and solid cell-walls is produced in their place ; and a quantity of tannin remains
behind in some of the cells as a secondary product, as well as various other substances
not present in the seed.

The albuminoids which form so peculiar and intimate a mixture with the oil in the
ripe seed, and which are partially contained in the aleurone-grains of the endosperm in
the form of crystalloids, are, during the processes which have been described, transferred
to the embryo, where they produce the protoplasm. During the whole of the period
of germination the cells of the fibro-vascular bundles are found to be densely filled with
albuminous substances, subsequently only those of the phloem; these substances are
evidently in motion toward the apices of the roots where new cells are continually
being formed. Every young rudiment of a lateral root behaves to reagents as an
accumulation of albuminous substance on the side of the fibro-vascular bundles of the
primary root. But a very considerable portion of this material remains in the upper
part of the stem of the seedling where new leaves are formed, and a still larger portion
in the cotyledons themselves, where it furnishes the material for the formation of the
numerous chlorophyll-granules.

After the consumption of the reserve-material at the end of the period of germi-
nation, the cells — with the exception of the youngest parts of the buds and the apices
of the roots — are destitute of any formative material; although it has grown to a
large size and contains a great quantity of water, the dried weight of the plant is
very small and even less than that of the seed, because a portion of the substance
has been destroyed in the process of respiration. But active organs have been formed
from the earlier inactive store of material ; the roots absorb water and dissolved food-
material ; the green cotyledons begin to assimilate ; they produce starch in their chlo-
rophyll ; and the same substance is subsequently found also in the parenchyma of the
petioles and in the stem as far as the bud, the young leaves of which grow from the
products of the assimilation of the chlorophyll. At first the unfolding of new leaves
and the increase in length and thickness of the stem and roots are very slow ; but the
capacity for work possessed by the plant increases with every freshly developed leaf and
every new absorbing root ; on each successive day it can produce a larger quantity
of formative material than on any preceding one, and thus the rate of growth also
increases.

-If a Castor-oil plant is examined at the time when vegetation is most active, when
the green leaves supply the material for metastasis in all the organs, starch is found
in their chlorophyll-granules and distributes itself from them through the parenchyma
of the veins and petioles downwards into the stem as far as the root, and upwards
to the young leaves which are not yet in a condition to assimilate. The excess which
is not immediately required for the purposes of growth becomes deposited in the pith

7l8 CHEMICAL PROCESSES IN THE PLANT.

and medullary rays, where (this is not the case in the chlorophyll) it is always accom-
panied by sugar ; and it is evidently this latter substance which brings about the diffusion
from cell to cell, and at the same time furnishes the material for the formation of new
starch-grains. The sugar is the migratory product which takes part in the diffusion ;
the starch-grains ai e the temporarily stationary product.

The distribution of starch and sugar shows moreover that they move from the
primary stem through the rachis of the inflorescence and the pedicels into the paren-
chymatous tissues, and penetrate into the young tissue of the flower, the growing fruit,
and the ovules, there to be employed in the production of cellulose. The distributed
starch collects more abundantly, especially in the immediate neighbourhood of those
layers of cells which afterwards form the hard endocarp and the solid testa of the seed,
in consequence of its being required here in greater quantity, disappearing also from
them after the complete development of these layers of tissue.

The sugar and starch are conveyed through the funiculus to the ovules; they are
distributed through the integuments and the parts surrounding the nucellus ; and a large
quantity of sugar enters the growing endosperm, which supplies the material for the
formation of the oil which gradually accumulates, while fresh supplies of sugar are
constantly entering from without. In the growing embryo the cells are filled at a
certain period with fine-grained starch, which then entirely disappears and is replaced
by oil. All this indicates that the oil of the ripe seed of Ricinus is produced from the
starch and sugar which were transported to it from the assimilating organs during the
period of ripening ; and even the hard woody pericarp and the testa obtain their for-
mative material from those substances. The albuminoids which collect also in the
young leaves and from which the chlorophyll -granules are formed, as well as that
portion of these substances which accumulates in the seed as reserve food-material,
are transported from the stem by the sieve-tubes and the cambiform cells of the fibro-
vascular bundles.

3. In the Leguminosae^ a very important part in the transport of the reserve proteid
substances is played by Asparagin. To demonstrate this, moderately thin sections are
placed in alcohol, and the saturation assisted by shaking. This mode is however applic-
able only when the asparagin is abundant ; when it is present in small quantities it can
still be demonstrated by placing a thin cover-glass on the sections, and running in under-
neath a little absolute alcohol. In this case the asparagin crystallises out round the
section ; while in the former case it is precipitated in the ceils in the form of crystals.
These can easily be recognised ; they are comparatively large, and cannot be mistaken
for other crystals which are formed in all plants on treatment with alcohol, even where
no asparagin is present, since these — which belong to various salts, among others to
nitrates — always remain very small and have an entirely different appearance.

Luptnus luteus is a good object for examination, and possesses the great advantage
that we have in its case an analytical investigation of Beyer's^ in which the organic
constituents and especially the asparagin have been quantitatively determined in the root,
hypocotyledonary portion of the stem, and cotyledons, at two stages of germination, the
last shortly before the cotyledons have thrown off the testa.

The migration of the non-nitrogenous reserve-materials takes place in the usual
manner. Starch is first of all formed in the hypocotyledonary portion and root, then
disappears and remains only in the endoderm, the rest being transformed into sugar.

*. What follows is taken from a letter from Dr. Pfeffer. (Compare Book I. Sect. 8. p. 51 ; also
Jahrb. fiir wiss. Bot. VIII. p. 429 et seq.)

[It must not be thought, however, that this substance is confined to this group of plants, for it
is very widely distributed, nor is it the only substance of this kind, for leucine and tyrosine have also
been detected in germinating seeds.]

" Landwirthschaftliclie Versuchsstationen, vol. IX.

ASSIMILATION AND METASTASIS. yxg

Asparagin is first found in the hypocotyledonary portion and root when they are about
lo mm, long, but then rapidly increases in quantity while these parts elongate ; and it is
now found also in the petiole of the cotyledons, and in the cotyledons themselves before
they have become green and thrown off their testa, especially in their lower part. The
conditions remain the same during the whole of the time that the reserve albuminous
substances are being consumed. Asparagin is now found in large quantities in the petiole
of the cotyledons, almost to the extent of a saturated solution (i part dissolves in 58 parts
of water at 13° C), as well as in the hypocotyledonary portion and in the stem as soon as
it begins to grow. The asparagin extends in the root and stem towards the puncta
'vegetationis almost exactly as far as the sugar, becoming finally, like the latter, less
abundant. Beneath the cotyledons it is wanting in the pith, while in the stem it is as
abundant there as in the cortical tissue ; it is never found in the vascular bundles. The
asparagin also extends into the petiole of young leaves as far as the base of the unfolding
pinnae, as well as into the lateral roots. As long as asparagin is formed out of the albu-
minous substances in the cotyledons, it may also be found in the plant distributed as has
been described ; but when the cotyledons have been entirely emptied, the asparagin also
disappears ; but this does not happen in the case of Lupinus luteus until several leaves have
completely unfolded.

The process is quite analogous in letragonobolus purpureus and Medicago tuber-
culata ; in Ficia sati'va and Pisum sati'vum the presence of asparagin in the cotyledons
themselves cannot be proved with certainty, but is found at their base and usually also
in their petiole, although these plants produce decidedly less of it than Lupinus luteus.
Since moreover chemical analysis has established the production of great quantities of
asparagin on germination in the case of a large number of other species of the order, we
may regard this substance as the form of transport for the albuminous substances
characteristic of all Papilionaceae. Albuminous substances are moreover found in
these plants also in the thin-walled elongated cells of the vascular bundles; and it is
quite possible that they are at the same time also transported by these structures. It is
evident that the source of the asparagin must be the albuminous substances, because the
absolute amount of nitrogen remains the same during germination ; and the nitrogen of
seeds is all or nearly all contained in their albuminous ingredients.

The following numbers show the percentage composition of asparagin, and the
composition of an amount of legumin, containing an equivalent quantity of nitrogen.

isparagin.

Legumin.

Difference.

C==36-4

G = 64-9

+ 28-5

H= 6-1

H= 8'8

+ 27

N = 2r2

N = 2r2

o'o

0 = 36-4

0 = 30-6

- 5-8

Asparagin contains less Carbon and Hydrogen but more Oxygen than Legumin and
other proteids. Consequently if the whole of the Nitrogen of Legumin is used in the
formation of asparagin, a considerable quantity of Carbon and Hydrogen must be given
off and a certain amount of Oxygen absorbed. Exactly the opposite will take place upon
the conversion of asparagin into proteid. Pfeffer points out (Monatsber. d. Berl. Akad.,
1873) that this regeneration of proteid from the asparagin formed during germination
depends in so far upon the action of light as this is necessary for the decomposition of
carbonic acid in cells containing chlorophyll. Asparagin remains in a plant exposed to
light if carbonic acid be not supplied to it.

In TropfEolum majus asparagin occurs in the earliest stages of germination only; it
disappears at a later period whether germination is taking place in light or in darkness.
In this case the asparagin is converted into proteid before the non-nitrogenous reserve-
materials are exhausted; hence the regeneration can take place without assimilation.
The formation of asparagin during germination takes place in light or in darkness : it is

720 CHEMICAL PROCESSES IN THE PLANT.

only the regeneration of the proteids which is dependent, as in the Papilionaceae, in the
manner above mentioned upon light, that is, upon assimilation ^

The existence of asparagin has also been proved in the leaves and stems of some
plants (see Husemann, Pflanzenstoffe) ; and its presence in the underground perennial
parts of Stigmaphyllon jatrophoefolium almost gives the impression of its being there also a
reserve-material.

The absorption of assimilated substances into the plant from without takes
place in seedlings, the reserve-materials of which are contained in the endosperm,
in parasites'^, and in saprophytes which contain no chlorophyll. Seedlings, which
have been most studied in this respect, show how the reserve-materials of the endo-
sperm may pass into the absorbing organs (in this case almost always foliar struc-
tures) without there being any actual continuity of the absorbing organ with the
endosperm; they only lie in close apposition, and can be separated without any
injury^ (as in Ricinus^ Fig. 471). It cannot be doubted that the metamorphoses
which take place in the nutrient endosperm are brought about by the absorbing
organ, that is by the embryo itself; the behaviour of the endosperm of the germi-
nating Date, which is absorbed by the delicate tissue of the absorbing organ belong-
ing to the cotyledon, shows clearly that the hard thickening-layers of the cell-walls
of the endosperm are first of all transformed into sugar under the influence of this
organ, and then absorbed. A substance evidently passes out of the absorbing organ
into the endosperm which causes this metamorphosis of the cellulose. The oil and
albuminoids of the endosperm are at the same time taken up into the embryo,
where all the conducting parts of the parenchyma are filled with sugar and starch
as long as the endosperm is not entirely absorbed. In the same manner also
in Grasses substances possibly pass out of the embryo into the endosperm, and
there bring about the chemical metamorphosis and solution of the starch and albu-
minoids before they are absorbed by the scutellum which is applied to the surface
of the endosperm. It is possible however that in this case there may be some
means in the endosperm itself of bringing about the solution of the starch and
gluten in the presence of water independently of any chemical action of the
embryo.

The absorbing roots of parasites penetrate into the tissue of the host, and
often grow into it in the most intimate manner. It is certain that the exciting
cause of the transport of the products of assimilation from the host to the parasite
resides in the latter; the parasite acts on the conducting masses of tissue of the
host like a growing bud of the host itself; the food-materials penetrate into it
because it consumes and changes them.

The influence exerted by the absorbing organ of the embryo on the substances

^ [See Schulze, Ueb. Zersetzung und Neubildung von Eiweissstoffen in Lupinenkeimlingen,
Landw. Jahrb. VII, 1878.]

2 Parasites which contain chlorophyll, like the Loranthacese, can themselves assimilate, and
only require therefore to draw water and mineral substances from their host (see Pitra in Bot. Zeitg.
1861, p. 3). Those parasites which are apparently destitute of chlorophyll (like Orobanche), and
saprophytes (as Neottia), contain, according to Weisner (Bot. Zeitg. 1871, p. 37), traces of chlorophyll,
which however can hardly be taken into account in assimilation.

^ For further details see the accounts given by me in Bot. Zeitg. 1862 and 1863, of the
germination of different plants.

RESPIRATION OF PLANTS. "JQ,!

in the endosperm, dissolving and chemically changing them, points to the way in
which the absorption of food-material is effected by saprophytes which possess no
chlorophyll, their absorbing organs probably first causing the solution and Chemical
transformatron of the decaying organic constituents of the humus. The decaying
foliage in which Monotropa, Epipogium ^ and Corallorhiza grow, does not give up
to water the serviceable materials which are still present in it, any more than the
cellulose of the endosperm of the Date, or the starch of the endosperm of Grasses,
or the oil of the seed of Ria'nus, can be extracted by water ; but these saprophytes
nevertheless obtain their nutriment from them. The fact that the roots of plants of
this kind are so few in number and so diminutive in length, as in Neottta, or are
entirely wanting, as in Epipogium and Corallorhiza^ is very remarkable in connection
with this. These plants are concealed in the nutrient substratum till the time of
flowering, and may act upon it by their whole surface ; and it is important to note
that the absorbing surface of seedlings is very small in proportion to the great
amount of work done, as is also the case with the absorbing roots of Cuscuta^
Orobanche, &c.

Sect. 6. — The Respiration of Plants^ consists, as in animals, in the
continual absorption of atmospheric oxygen into the tissues, where it causes
oxidation of the assimilated substances and other chemical changes resulting from
this. The formation and exhalation of carbon dioxide — the carbon resulting
from the decomposition of organic compounds — may always be directly observed ;
the production of water at the expense of the organic substance in consequence
of the process of respiration is inferred from a comparison of the analysis of
germinating seeds with the composition of those which have not yet germinated.
Experiments on vegetation show that growth and the metastasis in the tissues
necessarily connected with it only take place so long as oxygen can penetrate
from without into the plant. In an atmosphere devoid of oxygen no growth
takes place ; and if the plant remains for any time in such an atmosphere it
finally perishes. The more energetic the growth and the chemical changes in
the tissues, the larger is the quantity of oxygen absorbed and of carbon dioxide
exhaled; hence it is especially in quickly germinating seeds and in unfolding
leaf- and flower-buds that energetic respiration has been observed; such organs
consume in a short time many times their own volume of oxygen in the pro-
duction of carbon dioxide. But in all the other organs also — in every indi-
vidual cell — respiration is constantly going on ; and it is not merely the chemical
changes connected with growth that are dependent on the presence of free
oxygen in the tissues; the movements of the protoplasm also cease if the sur-
rounding air is deprived of this gas; and the power of motion possessed by
periodically motile and irritable organs is lost if oxygen is withheld from them;

* See Reinke, Flora, 1873, No. 10-14.

^ The special references for what is said on this subject will be found in my work on Expe-
rimental Physiology, sect, 9, On the action of atmospheric oxygen. Of more recent works may
be mentioned especially, Borscow, On the behaviour of plants in nitrogen (Melanges biologiques
tires du Bulletin de I'Acad. Imp. des Sci. Nat. de St. Petersbourg, vol. VI, 1867); also Wiesner,
Sitzungsber. der Wiener Akad. vol. LXVIII, 1871 ; Bert, Comptes Rendus, 16 Juin, 1873. [See also
Wortmann, Arb. d. bot. Inst, in Wiirzburg, Bk. II, 1880; Pfeffer, Das Wesen und die Bedeutung der
Athmung in der Pflanze, Landwirth. Jahrb. VII, 1878.]

3 A

722 CHEMICAL PROCESSES IN THE PLANT.

but if this happens only for a short time the motility returns when the oxygen is
again restored.

The respiration of plants is, like that of animals, associated with a loss of
assimilated substance, this loss being always a great deal smaller in assimilating
plants than the gain of substance by the activity under the influence of light of
the cells which contain chlorophyll ; but when, as in the germination of seeds, an
energetic growth is combined with powerful respiration, no new products of as-
similation replacing the loss, the loss in weight of the growing plant may be very
considerable. Seeds which germinate in the dark may in this way lose almost
one-half of their dry weight, and it would seem that this loss is occasioned ex-
clusively by the decomposition of the non-nitrogenous reserve material^ and its
combustion into carbon dioxide and water. If the non-nitrogenous reserve-material
consists of oil, i.e. of a substance containing very little oxygen, a portion of the
inhaled oxygen remains in the germinating plant, carbo-hydrates containing a large
quantity of oxygen such as starch and sugar being formed at the expense of
the oil.

The loss of assimilated substance caused by respiration would appear purpose-
less if we had only to do with the accumulation of assimilated products ; but these
are themselves produced only for the purposes of growth and of all the changes
connected with life ; the whole life of the plant consists in complicated movements
of molecules and atoms ; and the forces necessary for these movements are set
free by respiration. The oxygen, while decomposing part of the assimilated sub-
stance, sets up important chemical changes in the remaining portion, which on their
part give rise to diffusion-currents, and these bring into contact substances which
again act chemically on one another, and so on. The dependence on respiration of
the movements in protoplasm and motile leaves is very evident, since, as has been
mentioned, they lose their motility when oxygen is withheld from them. These
considerations lead to the conclusion that the respiration of plants has the same
essential significance as that of animals ; the chemical equilibrium of the substances
is being continually disturbed by it, and the internal movements maintained which
make up the life of the plant. Respiration is, it is true, a source of loss of sub-
stance; but it is also in addition the perpetual source from which flow the forces
necessary to the internal movements ^.

^ [According to Borodin (Ueb. die physiol. Bedeutung des Asparagins im Pflanzenreiche, Bot,
Zeitg. 1878) this is not the case. In the process of respiration the nitrogenous substances constituting
the protoplasm become oxidised, and of this oxidation asparagin is one of the products. The non-
nitrogenous materials are used up in supplying plastic material to the protoplasm.

Asparagin is regarded, from this point of view, as a nitrogenous waste- product (metabolite), and
it therefore corresponds physiologically to the urea formed in the animal body, a comparison which
was long ago suggested by Boussingault.]

^ [M. Corenwinder, from a series of observations on the Maple and Lilac, has confirmed the
view to a certain extent held by Mohl, that the process of respiration is always going on in a plant
even when concealed by the greater activity of the decomposition of the carbon dioxide by the
parts containing chlorophyll. He distinguishes two periods in the vegetative season of the plant : —
the first period, when nitrogenous constituents predominate, is that during which respiration is most
active ; the second, when the proportion of carbonaceous substance is relatively larger, is the period
when respiration is comparatively feeble, the carbon dioxide evolved being again almost entirely
taken up by the chlorophyll, decomposed, and the carbon fixed in the process of assimilation. He

RESPIRATION OF PLANTS.

7^3

i

The combination to form carbon dioxide of the inhaled oxygen with a portion
of the carbon of the assimilated substance is, like all combustion, accompanied by
the production of a corresponding amount of heat; but this only rarely leads to
a sensible increase of temperature of the masses of tissue, because respiration, and
in consequence the production of heat, is not in general very copious, while the
circumstances are very favourable to the loss of heat by the plant. In this respect
also plants may be compared to cold-blooded animals. When an amount of heat
is set free in the cells by the process of respiration, it first of all distributes itself
over the large mass of water which permeates the cells and the adjoining tissue. In
the case of a water-plant the least excess of temperature is at once equalised by the
surrounding water; while in the case of a land-plant evaporation has a powerful
cooling effect on the aerial parts, quite independently of the action of the radi-
ation of heat which is favoured by the large superficial development of most
plants, and especially by their hairiness. With these causes of a rapid loss of
heat, it is not surprising that the parts of a plant which are expanded in the
air are even colder than it, although their respiration is continually producing small
quantities of heat. But if the causes of the loss of heat are removed, it is possible
to observe with the thermometer the increase of temperature caused by respiration.
This can be done by accumulating rapidly germinating seeds, as is shown in the
considerable elevation of temperature of grains of Barley in the manufacture of malt ;
and this elevation can also be proved in the case of other germinating seeds, or
growing bulbs and tubers. The proof is more difficult in plants with green leaves.

In some flowers and inflorescences the production of carbon dioxide which
accompanies the inhalation of oxygen is very energetic, the radiation of the heat
produced being at the same time diminished by the small superficial extent of the
organ and by protecting envelopes ; and in such cases a very considerable elevation
of temperature of the masses of tissue has been observed. The best illustration of
this is the spadix of Aroidese at the time of fertilisation, where (especially in warm
air) an excess of temperature of from 4° to 5° or even of io°C. or more has been
detected. Less considerable elevations of temperature have also been observed in
the separate flowers of Cucurbita^ Bignonia radicans, Victoria regia, &c.

In the few cases in which up to the present time the development of light or
Phosphorescence has been observed in living plants, this phenomenon is also dependent
on the respiration of oxygen. In Agaricus olearius (of Provence) this has been
definitely proved by Fabre. This Fungus emits light only so long as it is alive, and
ceases to do so at once when it is deprived of oxygen ; the respiration is in this case
also very copious. Besides this Fungus, Agaricus igneus (of Amboyna), A. nodi-
lucens (of Manilla), A. Gardneri (of Brazil), and the Rhizomorphs are known to
emit light spontaneously; the statements with respect to the light emitted from
various flowers are of extremely doubtful value \

found that the proportion of nitrogenous matter in leaves gradually diminishes, while that of carbon-
aceous matter increases, between autumn and spring. (See Revue scientifique, Aug, i, 1874.)]

1 [For a collection of recorded instances of phosphorescence in plants see Hardvi^icke's Science
Gossip, 1871, p. 121.] See my Experimental Physiology, and Schmitz (Linnaea, 1843, p, 523) and
Bischoff (Flora, 1824, II. 426) on the phosphorescence of Rhizomorphs.

3 A 2

724

CHEMICAL PROCESSES IN THE PLANT.

The apparatus described in my Handbook of Experimental Physiology, p. 271, may
be easily employed, with the necessary modifications, for the observation of the pro-
duction of carbon dioxide and the elevation of temperature of germinating seeds. The
following experiment is also adapted for the demonstration of these points in a lecture.
One-third of a glass cylinder of 2 litres capacity is filled with soaked peas or some
other seeds or with flowers in the act of unfolding {e.g. small flower-heads of Com-
positae, as Matricaria or Pyrethrum)^ and closed with a well-fitting glass stopper. If
the vessel is opened carefully after several hours, the air contained will be found to
extinguish a burning taper let down into it, as if it had been filled with carbon dioxide.

In order to observe the development of heat also in small quantities of seeds and
even in single flowers of larger size, I use various forms of the apparatus represented
in Fig. 472. The flask/ contains a strong solution of potash or soda / which absorbs the
carbon dioxide set free from the plants. In the opening of the flask is placed a funnel r,

containing a small filter-paper perforated with
a needle. The funnel is filled with soaked
seeds or with cut flower-buds in the act of
opening; and a bell-glass g is now placed
over it, through the tube of which a thermo-
meter graduated to tenths of degrees is let in
so that the bulb is surrounded on all sides by
the plants. A loose plug of cotton-wool ^v
closes the tube. In order to compare the
temperature, a similar apparatus is placed
close beside, in which the seeds or flowers as
the case may be are or are not replaced by
pieces of moist paper or green leaves. It
is convenient to place both apparatuses in a
large glass case in order still more completely
to shield them from slow changes of temper-
ature in the air of the room. As the isolation
is not complete, the access of fresh oxygen-
ated air to the plants is not hindered, and the
continuance of respiration is therefore not
prevented; the arrangement is on the other
hand sufficient to reduce to a minimum the
loss of heat by radiation and evaporation.
The thermometers of both apparatuses, pre-
viously compared, must be frequently read
off" in order to detect the variations of tem-
perature. If the bulbs are small enough, the
elevation of temperature in the funnel may
be observed even with single flowers. In order to reduce still further the amount of
evaporation and radiation, it is convenient, before the bell-glass g is placed over, to
cover the funnel with a perforated glass plate, the thermometer being inserted through
its perforation.

* It is possible under favourable circumstances to observe by means of this contrivance
a rise of temperature of 1-5° G. with 100 or 200 peas, while the roots are developing;
the anthers of a flower of the Gourd caused a rise of about o"8° G. in a tolerably large
thermometer with the bulb of which they were in contact on only one side. A single
capitulunf of Onopordon Acanthium produced an elevation of 0*72° G. ; the stamens of a
single flower of Nymphcsa stellata one of about o-6° G. The temperature of a number of
flower-buds of Anthemis chrysoleuca heaped round the thermometer rose as they unfolded
about 1-6° G.

It will be readily understood that flowers must not be used for these experiments as

Fig. 472. — Apparatus for observing the rise of temperature
in flowers and germinating seeds.

INFLUENCE OF TEMPERATURE ON VEGETATION. 725

soon as they have been gathered ; but that it is necessary to wait for some hours till they
have acquired the temperature of the room. (Further details will be given elsewhere.)
M^Nab found that a large specimen of Lycoperdon giganteum produced a rise of temper-
ature of 1*2° (F. or C..?). Bot. Zeitg. 1873, P- 560.

CHAPTER HI.

GENERAL CONDITIONS OF PLANT-LIFE.

Sect. 7. The Influence of Temperature on Vegetation^ can only be
investigated scientifically by observing the influence of definite and different degrees
of temperature on the separate vital phenomena of plants, z. e. on the various pro-
cesses of assimilation and metastasis, of diffusion, of growth, of the variations in the
turgidity of the cells and tension of the tissues, of the movements of protoplasm
and irritable organs and of those endowed with periodic motion, &c.

The determination of the facts which have here to be investigated depends on
an accurate determination of the temperature of the plant in any given case, or rather
on that of the part of the plant in question on which the experiment is to be made.
This is often attended with great difficulties, and is sometimes almost impossible.
Independently of the changes of temperature, usually inconsiderable, caused by
respiration in the interior of the plant, the temperature of each cell depends on
its position in the mass of tissue and on the variations of the surrounding tem-
perature. A constant interchange of heat is going on between the plant and
its surrounding medium by conduction and radiation which essentially determines
the temperature of any part of a plant at any particular time.

In reference to the conduction of heat, it must be mentioned in the first place
that all parts of plants are bad conductors ; the differences of temperature between
them and the air, earth, or water that is in contact with them become only very
slowly adjusted in this way. The conductivity for heat is probably also always
different in different directions ; that in the longitudinal direction in dry wood bears
the proportion to that in the transverse direction of ^.^. 1-25 : i in the Acacia, Box,
and Cypress, of i-8 : i in the Lime, Alder, and Pine.

The radiation of heat is on the other hand a very frequent and rapid cause of
changes of temperature in most parts of plants ; the chief effect of these changes
being to bring about differences between the temperature of the surrounding medium
and that of the plant, especially when the parts of the plant are of small size but
have a large hairy surface, as is the case with many leaves and internodes. It
must be noted in this connection that the radiating power of a body is equal to its

* For more detailed proofs see my Handbook of Experimental Physiology, p. 48 et seq.

>j26 GENERAL CONDITIONS OF PLANT-LIFE,

absorptive power; and that radiation depends not merely on the temperature, but also
on the diathermancy of the surrounding medium.

In the aerial parts of plants, transpiration is an energetic additional cause of loss
of temperature ; inasmuch as water in the act of evaporation withdraws from the
plant the amount of heat necessary for its vaporisation, and hence makes it colder.

In investigations of the influence of temperature on the various processes of
vegetation, the conditions noticed above must always be carefully considered. It
may be assumed in general that the result of their united action is that small water-
plants and the underground parts of plants have usually nearly the same temperature
as that of the surrounding medium when this temperature is not subject to too great
variations ; but that on the other hand leaves and slender stems exposed to air are
generally colder than the air ; while the thick stems of woody plants are sometimes
warmer, sometimes colder, in consequence of their slow conducting power. How
greatly the temperature of parts of plants of considerable superficial extent may be
depressed by radiation below that of the air is shown by the fact that a thermometer
placed on the grass and exposed to radiation indicates on clear nights a temper-
ature several degrees lower than one placed in the air. If the latter is only a few
degrees above the freezing-point, the temperature of the leaves of plants may in
this manner fall below zero and they will suffer the effects of frost. The formation
of dew on summer nights, and of the hoar-frost which is deposited in such large
quantities on plants especially in the late autumn, are striking proofs of the effect
of radiation in lowering their temperature. The relation of the temperature of
plants to that of their surrounding medium is however very complicated when we
have to do with solid bodies like trunks of trees, because the different powers of
conduction in the longitudinal and transverse directions of the wood, and other
causes, then cooperate with the action of radiation and of absorption of heat
through the bark. In general, as has been shown by Krutsch's beautiful experi-
ments, the trunk is cooler during the day than the surrounding air, but warmer
in the evening and night K

With respect to the changes of volume in masses of tissue and in individual
cells as the temperature varies, nothing is known with certainty except as regards
dry wood. The numbers given by Caspary as the coefficients of the expansion of
wood caused by heat depend on untrustworthy observations and on a complete mis-
understanding of the phenomena which take place in the objects observed ^. When
leaf-stalks and the branches of trees become curved at temperatures far below the
freezing-point, this is obviously not altogether, if at all, caused by the different
layers of tissue having different coefficients of heat-expansion; but is mainly
a consequence of the fact that the, water of vegetation freezes, while the cell-
walls lose water and in consequence contract more or less according to their state

* [According to Becquerel, trees warm surrounding layers of air during the day and a good
part of the night ; they begin to cool them as soon as they have attained the same temperature.
The maximum temperature is reached by the air two or three hours after midday ; in the tree
it is reached after sunset, in summer towards 9 p. m. See Memoire sur les forets et leur influence
climateiique : Mem. de I'lnst. vol. XXXV, pp. 460-470.]

2 Proceedings of the International Horticultural Exhibition and Botanical Congress held in
London, 1866, p. 116.

INFLUENCE OF TEMPERATURE ON VEGETATION.

727

Wood.

In radial direction.

Box-wood .

. 00000614

Fir . . .

. 0-0000584

Oak . . .

. 0-0000544

Poplar . .

, 0-0000365

Maple . .

. 0-0000484

Pine . . .

. 00000341

25

16.

12 .

9

8

6

of imbibition and of lignification. The phenomenon depends therefore in the first
instance on a change in the state of imbibition and turgidity produced by different
temperatures. Villari has carefully measured ^ the coefficients of heat-expansion for
different dry woods. Like the expansion caused by the absorption of water, that
caused by heat is much less in the direction of the fibres than in the radial direc-
tion across the fibres ; but with the difference that the coefficients of expansion for
absorption are reckoned by hundredths in the radial and thousandths in the longi-
tudinal direction, those for heat by hundred-thousandths and millionths ; so that
the alterations of the dimensions of dry wood in the two directions caused by
changes of temperature are about 1000 times smaller than those caused by the
absorption of water. The following table is from Villari for temperatures between
2° and 34° C. :—

Coefficients of heat -expansion for i°C.

In longitudinal direction. Proportion.

0-00000257
00000037 I
0-00000492
0-00000385
O-OOOCO638
0-00000511

Since these numbers only hold good for dry wood, while wood as a constituent
of the living plant can be observed only in the moist state, they cannot be applied
directly to the explanation of the physiological phenomena due to changes of
temperature; but they are nevertheless of great interest, since they give us an
insight into the molecular structure of wood, especially as to its elasticity in different
directions.

Something more is known as to the influence of different degrees of temperature
on the vital phenomena of plants. On this subject the important fact must first be
noted that the exercise of every function is restricted to certain definite limits of
temperature within which alone it can take place ; t. e. all functions are brought into
play only when the temperature of the plant, or of the particular part of the plant,
rises to a certain height above the freezing-point of the sap, and cease when a
definite maximum of temperature is attained, which can apparently never be per-
manently higher than 50° C.^ Hence the fife of the plant, i.e. the course of its
vital processes, appears to be confined in general within the limits zero and 50° C.
It must however be noted that the same functions may have very different limits
between 0° and 50° C. in different plants; as is also the case with different functions
in the same plant. A few examples will serve to explain this.

Since the cell-fluids, consisting of aqueous solutions often in a state of high
concentration, do not usually freeze at zero, it is always possible for certain pro-
cesses of growth to take place when the temperature of the surrounding air is as
low as this, although this fact has not yet been sufficiently established. Uloth (Flora,

^ Poggendorfifs Annalen, 1868, vol. 133. p. 412.

^ Sachs, Ueber die obere Temperaturgrenze der Vegetation, Flora, 1864, p. 5; Krasan, Beitr. z,
Kenntniss des Wachsthums, Sitzber. d, Wien. Akad. 1873.

728 GENERAL CONDITIONS OF PLANT-LIFE.

1 87 1, no. 12) observed the remarkable fact that seeds of Acer plaianoides and of
Wheat which had fallen between pieces of ice in an ice-house germinated there
and pushed a number of roots several inches deep into the fissureless pieces of ice.
From this observation he concluded that these seeds had the power of germi-
nating at or even below the freezing-point of water; and that the penetration of
the roots into the ice is caused by the development of warmth in the seed and by
the pressure of the growing roots. It seems to me however that another expla-
nation is possible. The ice was evidently surrounded by warmer substances, such
as the walls of the house, which emitted to it rays of heat. Now it is a well-known
fact that rays of heat, when they strike upon bubbles of air or bodies firmly frozen
into a piece of ice, warm them and melt the surrounding ice. In this way not
only the seeds but also their roots were warmed by the radiation of heat which
passed through the ice, and thus the particles of ice in contact with them were
melted. This experiment gives us therefore no certain knowledge of the actual
temperature of the germinating seeds. The statements of different observers as to
the highest temperature of the water in which some of the lower Algae grow vary
greatly ; and Kegel's assertion is perhaps the most probable that water must be
below 40° C. for plants to grow in it. I have convinced myself that a considerable
number of plants are killed by an immersion for only ten minutes in water of 45° or
46° C, while flowering plants endure for a longer period an air-temperature of 48°
or 49° C; but at 51° C. lose their vitality after from ten to thirty minutes (any
possible injury by drying up being of course prevented) ^ As to the high tem-
peratures which the spores of Fungi can endure without losing their power of ger-
mination, very different statements, some of them altogether incredible, have been
made, according to which temperatures of more than 100°, even as high as 200° C,
would seem not to be injurious. Of ninety-four experiments which were made by
Tarnowsky with all possible precautions ^ the result was that the spores of Peni-
cilliutn glaucum and Rhizopus nigricans exposed for from one to two hours to air
of a temperature between 70° and 80" C. germinated only very rarely, while a
temperature of 82° or 84° C. altogether killed them. Spores heated in their proper
nutrient fluids entirely lose their power of germination at 54° or 55° C.^

The growth of parts of the embryo at the expense of the reserve-materials
begins, as my experiments show*, in the case of Wheat and Barley even below 5°C.;

^ H, de Vries, Materiaux pour la connaissance de I'influence de la temperature, in Archives
Neerlandaises, vol. V, 1870, arrived at the same results from a number of experiments on Crypto-
gamia and flowering water and land-plants. According to Schmitz (Linnsea, 1 843) Sphceria carpophila
is killed in ten minutes by water of 35°-38° R. (43-5°-47-5° C).

^ One of the most important of these precautions is to prevent with certainty the entrance of
spores after the temperature has been raised in the apparatus to the required point.

According to Wiesner (Sitzber. d. Wien. Akad. 1873) spores of PenicilUtim glaucum sown on
lemon pulp will not germinate below 1*5° C. or above 43° C. Any further development is confined
to narrower limits. The most favourable temperature is from 22° to 26° C.

^ For further details see pt. Ill of the Proceedings of the Botanical Institute of Wiirzburg.

* Sachs, Abhangigkeit der Keimung von der Temperatur, Jahrb, fiir wissensch. Bot. vol. II. p. 338,
i860. — A. De Candolle in Bibliotheque universelle de Geneve, 1865, vol. XXIV. p. 243^/5^7. —
Koppen, Warme und Pflanzenwachsthum, eine Dissertation, Moscow 1870. — According to Kerner
(Nat. wiss. Verein Janobrock, 1872) most plants, especially alpine plants, can germinate below 2° C.
See also further under chap. IV.

INFLUENCE OF TEMPERATURE ON VEGETATION, 729

in Phaseolus multiflorus and Zea Mais at 9-4° C. ; in Cucurbita Pepo at \y*f Q. But
when the reserve-materials of the seed have been consumed, a higher temperature is
apparently always necessary to enable growth to proceed at the expense of freshly
assimilated material. The highest temperatures at which my observations indicate
that germination can take place were about 42° C. in the case of Phaseolus multiflorus ^
Zea Mais, and Cucurbita Pepo ; in Wheat, Barley, and Peas, about 37° or 38° C.

The lowest temperature at which the chlorophyll- granules turn green was
determined for Phaseolus multiflorus and Zea Mais at above 6°, and probably
below i5°C.; for Brassica Napus above 6°C. ; for Pinus pinea between 7° and
ii°C. The highest temperature at which leaves already formed and still yellow
turn green was for the first-named plants above 33° ; for Allium Cepa above 36° C.

The exhalation of oxygen and the corresponding assimilation begin, according
to Cloez and Gratiolet, in the case of Potamogeton between 10° and i5°C. ; in
Vallisneria above 6° C. In many Mosses, Algae, and Lichens, assimilation may
possibly take place at still lower temperatures ; according to Boussingault (Compt.
Rend. vol. 68. p. 410), carbon dioxide is decomposed by the leaves of the Larch
at 0-5° to 2'5°C., and by those of Meadow-grasses at 1-5° to 3*5° C. Heinrich
found the minimum temperature at which bubbles of gas were given off by Hottonia
palustris to be 2-7° C. The upper limit of temperature for this function has not
been ascertained, except for Hottonia palustris, in which case Heinrich found it
to be 50^-56" C.

The irritability and periodical movement of the leaves of Mifnosa do not
begin till the temperature of the surrounding air exceeds 15° C; the periodical
movements of the lateral leaflets of the leaf of Desmodium gyrans only at tem-
peratures above 2 2°C. The upper hmit of temperature for the sensitiveness of the
leaves of Mimosa depends on the continuance of the warmth ; in air of 40° C.
they become rigid within an hour; at 45° C. within half an hour; at 48° to 50°
within a few minutes, but may again become sensitive when the temperature falls.
A temperature of 52° C. causes permanent loss of the power of motion and death.

The lower limit of temperature for the motility of the protoplasm in Nitella
syncarpa is stated by Nageli to be zero ; for the hairs of Cucurbita my observations
place it at a temperature of 10° or ii°C. The upper limit is 37° C. in the case of
Nitella syncarpa according to Nageli ; in the hairs of Cucurbita, when immersed in
water of 46° or 47° C, the current is arrested within two minutes; in the air exposure
to a temperature of 49° or 50° C. for ten minutes does not stop the current. The
current in the hairs on the filaments of Tradescantia ceases within three minutes in air
at 49° C, beginning again when the temperature is reduced.

The absorption of water through the roots is also confined to certain limits of
temperature. Thus I found that the roots of the Tobacco-plant and Gourd no longer
absorb sufficient water to replace a small loss by evaporation in a moist soil of
from 3° to 5° C. ; the heating of the soil to from 1 2° to 1 8° C. suffices to raise
their activity to the needful extent. The roots of the Turnip and Cabbage on the
contrary absorb a sufficient quantity of water from soil reduced nearly to the freezing-
point to replace a moderate loss by transpiration.

A second result of the observations hitherto made may be stated as follows : —
The functions of a plant are assisted and accelerated in their intensity when the

730 GENERAL CONDITIONS OF PLANT'-LIFE.

temperature rises above the lower limit for that function ; on reaching a definite
higher degree a maximum of intensity is attained ; the activity then decreases
with a further increase of temperature, until it entirely ceases at the upper limit.
There is therefore no proportionality between a rise in the temperature and in
the intensity of the function. Thus, according to my observations, the rate of
growth of the roots of a seedling of Zea Mai's attains its maximum at 27-2° C, of
the Pea, Wheat, and Barley at 228° C; while an increase of the temperature of the
soil beyond these points causes in each case a decrease in the rapidity of growth ^

The irritability of the leaves of Mimosa is rather sluggish between 16° and
i8°C., and appears to reach its maximum at 30° C. The periodically motile lateral
leaflets of the leaf of Desmodium gyrans oscillate, according to Kabsch, in from
eighty-five to ninety seconds at 35° C, in from 180 to 190 seconds between 28°
and 3o''C.; at lower temperatures the oscillations are imperfect, and at 23° or 24° C.
they become almost imperceptible.

The rapidity of the movement of the protoplasm in Nitella syncarpa attains its
maximum, according to Nageli, at 37° C; at a higher temperature the movement
ceases. In the hairs of Cucurbita, Solanum Lycopersicum, and Tradescantia, as well as
in the leaves of Valltsnerta, I found the motion of the protoplasm slow between 1 2°
and i6°C., very rapid between 30° and 40°, slower again between 40° and 50° C.

Very great and rapid variations of temperature between zero and 50° C. have
been shown by experiments made by De Vries on a number of different growing
plants not to be attended with danger to Hfe, inasmuch as no injury could be
detected either at the time or afterwards. It does not however follow from
this that considerable changes of temperature are without effect. It would appear
rather that when a plant is generally exposed to a favourable temperature, its
functions are carried on the more energetically the more constant this favourable
temperature remains. This is shown by ordinary experience in horticulture, and
still more by the experiments of Hofmeister (Pflanzenzelle, p. 53) and De Vries
(/. c.) on the movement of protoplasm, and of Koppen (/. c .) on the growth of roots.
The influence of sudden variations of temperature in producing an injurious effect
on the plant is however very complicated, and has not yet been thoroughly investi-
gated. I have shown that any rapid increase or decrease of temperature is accom-
panied by an increase or decrease of the rapidity of growth ; although, according to
Koppen, the increase of growth during a long period is less when the temperature
is variable than when it is constant, the mean temperature being the same in both
cases, a conclusion which the more recent experiments of Pedersen^ render
questionable.

If the upper and lower limits mentioned above are exceeded, the functions of the
plant may, according to circumstances, simply come to rest, again to become active on
the return of a favourable temperature, or permanent changes are brought about, re-
sulting in injury and finally in the destruction of the cells. '

^ Further details on this subject will be found in my treatise already named, and in De Vries
and Koppen (/. c). Compare also what is said in chap. IV, on the influence of temperature on the
rapidity of growth.

2 [Haben Temperaturschwankungen als solche einen ungiinstigen Einfluss auf das Wachsthum ?
Arb. d. bot. Inst, in Wiirzburg, I. 4, 1874.]

INFLUENCE OF TEMPERATURE ON VEGETATION. 73 1

Cells killed by too high a temperature or by freezing show in general the same
changes as if they had been killed by poison, electricity, &c. ; the protoplasm becomes
stationary, turgidity ceases because the resistance of the cell-walls together with that
of the protoplasm diminishes, and allows the sap to filter out; the tissues become
flaccid ; secondary chemical changes of the sap produce the same dark colour as in
expressed juices ; and rapid evaporation soon causes a complete drying up of the
dead tissue.

The injury resulting from too high or too low a temperature may, under certain
circumstances, be indirect and slow in its manifestation ; this will be the case when a
particular function is too highly excited or too much depressed, and thus the harmo-
nious co-operation of the various vital processes is disturbed. Thus growth may be so
excited by too high a temperature that assimilation, especially when the light is deficient,
is not sufficient to supply the necessary formative material ; and the transpiration of the
leaves may in addition be so much increased that the activity of the roots is insufficient
to replace the loss. On the other hand, too low a ground temperature may so depress
the activity of the roots that even small losses by transpiration from the leaves can no
longer be replaced. We shall refer in the sequel to the injuries caused immediately to
the cells by too high a temperature and by the freezing and thawing of the tissues.

1. The destruction of the life of cells by too high a temperature depends, like freezing,
on their containing water. While succulent tissues are killed below or at 50° C, air-dry
seeds of Pisum sati'vum can resist a temperature of over 70° G. for an hour without
losing their power of germination ; of grains of Wheat and Maize heated to 65° for an
hour, 25 p.c. germinated in one case. Peas soaked in water for an hour and exposed
to a temperature of 54° or 55° G. were all killed; Rye, Barley, Wheat, and Maize at
53° or 54° G. Spores of Fungi showed similar phenomena, as is seen from Tarnowsky's
experiments. The cause of death appears to be the coagulation of the albuminoids
of which the protoplasm is composed, and this again depends on their containing water
and on other circumstances, since these render a diff'erent temperature necessary for
coagulation in diff'erent cases. The disorganisation of the cell-wall is perceptible only
at higher temperatures; and that of starch, which only takes place between 55° and
60° G., need not be taken into consideration here, since cells which contain no starch
are also killed by a rise of temperature above 50° C}

2. Freezing, or the destruction of cells by the solidifying of the water contained in
them into ice and by the subsequent thawing of the latter, depends also mainly on the
quantity of water in the cells. Air-dry seeds appear to be able to withstand any
degree of cold without injury to their power of germination ; the winter-buds of woody
plants the cells of which contain a great quantity of assimilated substances but only a
small quantity of water, stand the cold of winter and frequent rapid thawing; while
the young leaves at the time of their unfolding in the spring succumb to a slight night-
frost. An at least equally important condition lies however in the specific organisation
of the plant ; varieties of the same species frequently differing in their power of resist-
ance to cold and thawing. Some plants, like Mosses, Hepaticae, Lichens, some Fungi
of a leathery texture, the Mistletoe, &c., appear in particular never to freeze ; Pfitzer
states that the Naviculeae freeze between - 10° and - 20° G. and continue to live after
thawing ; while many flowering plants from a southern climate are killed by rapid
changes of temperature near the freezing-point^. Schmitz (Linnaea, 1843) observed that
an Agaricus fascicularis which had been frozen stiff" grew after thawing.

* The statements of Wiesner (Sitzungsber. der Wien. Akad. 1871, Oct., vol. LXIV. pp. 14, 15)
I am unable to understand. A variety of recent statements as to the high temperatures which the
spores of Fungi are said to be able to resist without losing their power of germination are so
incredible and require such critical sifting that I pass them by altogether.

^ On the minimum of temperature which vegetation can in general bear see Goppert, Bot.
Zeitg., 187 1, no:s. 4 and 5 : [also Bot, Zeitg. 1875.]

732 GENERAL CONDITIONS OF PLANT-LIFE.

Whether the tissue of a plant can be killed simply by the solidifying of the water
contained in its cells into crystals of ice is uncertain ; while on the other hand it is un-
; questionable that in a great number of plants death is caused only by the mode in which
the thawing takes place. The same tissue which reiains its vitality if thawed slowly
after the freezing of the water of its cell-sap, becomes disorganised if thawed rapidly
after exposure to the same degree of cold. Death is therefore caused in these plants
not by the freezing but by the thawing \

When ice is formed in the tissues of a plant, two points must be taken into con-
sideration. The water, when about to freeze, is on the one hand contained in a mixed
solution, the cell-sap ; on the other hand it is retained by the force of cohesion as water
of imbibition in the micellar interstices of the cell-wall and of the protoplasm. Now it
is an established fact in physics that a solution when freezing separates into pure water
which solidifies into ice and a concentrated solution with a lower freezing-point^. When
therefore a portion of the cell-sap-water freezes, the remainder of the cell-sap becomes
more concentrated ; and chemical changes may possibly be induced, as RiidorfF has
shown, by new combinations actually arising in a freezing solution. How far this
circumstance m.ust be considered in the destruction of cells by freezing and thawing is
not yet decided.

What takes place in the freezing of a saturated organised body capable of swelling up
is somewhat similar to that which occurs in a freezing solution. In this case also, when
the temperature falls to a certain point, only a portion of the water freezes ; the rest
remains as water of imbibition between the micellae of the body, which contracts, while
the freezing portion of the water of imbibition separates to form ice-crystals. This
phenomenon happens in a striking manner in starch-paste ; a homogeneous mass before
freezing, it has the appearance after thawing of a spongy coarsely porous structure, the
water running off clear from its large cavities. The behaviour of coagulated albumen on
thawing is exactly the same. In these cases a permanent change has clearly been
brought about by the freezing of a portion of the imbibed water ; the molecules of the
substance which group themselves into a network containing but little water when ice is
formed in paste or coagulated albumen, on thawing no longer combine with the portions
of the water which separated from them on freezing into a homogeneous whole ; the
thawed paste is in fact no longer paste.

When living succulent tissue freezes, a portion only of the water separates and
freezes as pure water, the rest remaining as water of imbibition in the protoplasm and
the cell-walls, at least as long as the temperature does not sink very low. In leaves
and succulent stems frozen at a temperature between - 5° and - 10° C. it is easily seen
that only a portion of the water is present in the form of crystals of ice ; another portion
permeates the cell-walls which are not rigid but .still flexible. If the congelation takes
place slowly, the water assumes on the surface of the succulent tissue the form of a
coating of ice consisting of densely crowded small crystals. These crystals stand at
right angles to the surface of the tissue, and increase by growth at their base. A
very large portion of the water of a tissue may in this way take the form of a coating
of ice, while the tissue, becoming less watery, contracts in proportion^, and loses its

^ The correctness of this statement is supported by a careful series of observations which I
communicated to the konigl. sachs. Gesellsch, der Wissensch. i860, On the formation of crystals,
&c., and which will be found also in the Landwirthschaftliche Versuchsstationen, i860, Heft. V.
p. 167, and in my Handbook of Experimental Physiology. I do not find that Goppert's objections
(Bot. Zeitg,, 1871, no. 24) affect my results; to his experiment on Calanthe veratrifolia quite a
different explanation can be given from that suggested by him.

2 Riidorff, Pogg. Ann. 1861, vol. CXIV. p. 63 ; and 1862, vol. CXVI. p. 55.

3 When this contraction operates unequally on different sides of a leaf or branch, it is easy to see
that curvatures must result which are indeed actually frequently observed. The splitting of the
trunks of trees in consequence of frost is probably only the result of changes of this nature.

I

INFLUENCE OF TEMPERATURE ON VEGETATION. 733

turgidity. This phenomenon is seen with remarkable clearness in the large leaf-stalks
of Cynara Scolymus when they freeze slowly. The succulent parenchyma separates
from the epidermis, which surrounds the former like a loose sack ; the parenchyma itself
splits apart in the interior so that each fibro-vascular bundle is enclosed in an envelope
of parenchyma. Fig. 473 shows how the coatings of ice project from the masses of
parenchyma. From pieces of the leaf-stalk which weighed 396 grammes I have collected
99 gr. of ice, which, when evaporated to dryness after thawing, left only slight traces
(about o-i p. c.) of solid substance. I have often observed similar phenomena in other
plants; the formation of ice is however not so regular as here. In the cavities of the
ruptured tissue (as in the succulent stems of the Cabbage) small irregular flakes of ice are
formed ; sometimes the ice splits the epidermis and projects in the form of combs above
the surface of succulent stems (Gaspary). I have already shown elsewhere ^ that when
sections of succulent parts of plants (such as the Beet) are protected from evaporation
and allowed to freeze slowly, continuous coatings of ice are produced on the surfaces of
the section, consisting of prisms growing at the base. The formation and growth of
these ice-crystals may be explained in this way. The temperature of the tissue falls to
a certain point, thereby causing the freezing of an extremely thin stratum of water
which overspreads the outside of the uninjured cell-walls. A new very thin stratum of
water then immediately passes out of the cell- wall to its surface and also freezes,

Fig. 473— Transverse section of a slowly frozen leaf-stalk of Cynara Scolymus; e the detached epidermis ; g the
parenchyma in which lie the transverse sections of the iibro-vascular bundles (left white). It forms a tough but pliant
mass, which is ruptured during the process of freezing ; a peripheral layer has become separated from the inner parts
which surround the bundles ; the surface of each portion of the parenchyma is covered with a crust of ice ATA' con-
sisting of densely crowded prisms (the cavities of the ruptured tissue are left black in the figure).

thickening the stratum of ice already formed; and thus it goes on. The cell-wall
is constantly absorbing cell-sap-water from within, and at the same time allows the
outermost molecular stratum of its water of imbibition to freeze. The first thin layers
of ice on the exterior of the uninjured cells form polygonal plates in contact with one
another; each plate becomes a prism by growth on its lower side; and the closely crowded
prisms form a coating of ice which easily crumbles. These processes cause tbe cell-sap
to become a more and more concentrated solution, while the cell- wall and the protoplasm
contain a gradually diminishing quantity of water. It can now be to a certain extent
understood why a rapid thawing kills the cells, while a slow thawing does not ; for if the
thawing take place slowly, the ice-crystals melt at their base where they touch the cell ;
the water as it becomes fluid is at once absorbed into the cell ; and the original con-
ditions of the cell-sap, cell-wall, and protoplasm may be re-established, if they have not
been permanently impaired during the freezing. If on the contrary the coating of ice

* Sachs, Formation of Crystals in the Freezing, and change of the Cell-walls in the Thawing of
Succulent Parts of Plants (Bericht der kon. sachs. Ges. der Wiss, i860). I have already mentioned
in the first edition of this work the formation of crystals in the interior of frozen plants described
above, and applied it to the explanation of freezing. Prillieux (Ann. des Sci. Nat. vol. XII. p. 128)
afterwards, in 1869, also described similar phenomena in a variety of plants.

734

GENERAL CONDITIONS OF PLANT-LIFE.

melt off very quickly, a portion of the water runs into the interstices of the tissue
before it can be absorbed ; the original normal degree of concentration of the cell-sap
and degree of imbibition of the cell-wall and protoplasm cannot be re-established in the
cells; and this, depending upon the chemical nature of the substances dissolved in the
cell-sap and upon the conditions of the micellar structure of the protoplasm and of the
cell- wall, may be fatal. It is evident, on the view here taken, that the danger of freezing
increases with the amount of water in the tissue ; for the less watery the tissue the more
concentrated is the cell-sap and the larger is the proportion of water retained by the
force of imbibition ; only a small portion of the water can therefore form ice-crystals,
and when they thaw the injurious effects are not so great.

We can now also understand why some plants are killed by being thawed too quickly
when they have been frozen by very severe cold, while freezing by a moderate amount
of cold is not injurious to them ; for the lower the temperature falls the larger is the
proportion of the cell-sap and water of imbibition that is converted into ice ; the dis-
turbance of the degree of concentration of the sap and of the imbibition of the cell-
wall is always greater with the increase of the cold ; and therefore the restoration of
the normal condition on thawing more difficult. That the splitting asunder of whole
masses of tissue during freezing such as has been described has but little effect on the
continuance of the life of the organ after thawing, is shown by the fact that even the
leaf-stalks of the Artichoke, the frozen state of which is represented in Fig. 473, remain
uninjured till the following summer if thawed slowly. These internal rupturings have
as little to do with the sudden destruction of the life of the cells from cold as the splitting
of the trunks of trees caused by frost, which, when the temperature falls very low, is
produced by the contraction of the bark and outer layers of wood, the crevices again
closing when the temperature rises.

The idea that growing plants, especially those which require a high temperature for
their growth, can be directly killed by the cooling of their tissues for a short time nearly
to the freezing-point is shown by H. de Vries' experiments (/. c.) to be fallacious. The
older observations of Bierkander and Hardy that some plants of this description (e.g.
Cucurbitaceae, Impatiens, the Potato, Bixa Orellana, Crescentia Cujete, Sec.) freeze when
exposed to the air at low temperatures above the freezing-point, may nevertheless be
explained if it is recollected that the temperature of their tissues may fall below the
freezing-point from radiation, even when that of the air is 2° or 3° or even 5° C. above
it. But there is another way in which low temperatures above zero are injurious to
plants from southern climates, 'vt%. when the soil about the roots remains for a consider-
able time at this low temperature while the leaves continue to transpire. In this case
the absorption of water through the roots becomes so slow that they are no longer able
to replace the loss caused by evaporation from the leaves, which in consequence wither,
and at length altogether dry up. It is then sufficient to warm the soil about the roots,
in order to revive the withered leaves ; as I found in the case of plants of Nicotiana,
Cucurbita, and Pbaseolus grown in pots\ In England the branches of a Vine which
were made to grow into a hothouse, while the roots stood in the ground outside, withered
in winter, evidently only from the low temperature of the ground ; for when this was
watered with warm water, the branches in the hothouse recovered.

3. Among the changes caused in plants by long-continued depression of temperature,
one of the most striking is the change in colour of leaves which persist through the
winter, originally observed by MohP, and recently more minutely studied by Kraus^.

^ Sachs, in Landwirthschaftliche Versuchsstationen, 1865, Heft V. p, 195.

2 Mohl, Vermisdite Schriften ; Tubingen, 1845, p. 375.

3 Kraus, Observations on the winter colouring of evergreen plants; in the Sitzungsber. der
phys.-med. Societat zu Erlangen, Dec. 19, 1871, and March 11, 1872; also Bot. Zeitg.. 1874.
£Batalin has shown (Bot. Zeitg., 1874), and his observations have been confirmed by Askenasy
{Bot. Zeitg, 1875), that this change of colour is due rather to the influence of light than to that
of cold.]

INFLUENCE OF TEMPERATURE ON VEGETATION. 7^5'

This change is of two kinds; the leaves either merely lose their colour and become
brownish, yellowish, or rusty brown, as in Taxus, Jbtes^ Pinus, Juniperus, and Buxus ; or
turn a decided red on the upper surface, as in Sedum, Sempervi'vuin, Ledum, Mahonia,
Faccinium, &c. The loss of colour of the first group depends, according to Kraus, on
a change in the chlorophyll-granules, which lose their form and definition, a cloudy
mass of protoplasm of a reddish brown or brownish yellow colour being formed, while
the nucleus of the cell remains colourless. These changes are usually more complete
in the 'pallisade cells' on the upper side than in the parenchyma which lies deeper.
A spectroscopic examination shows that of the two pigments, a mixture of which forms,
according to Kraus, the colouring substance of chlorophyll, the golden-yellow one
remains unchanged, while the spectrum of the bluish-green substance undergoes a
slight change.

The winter-leaves of the second group, which are coloured red or purplish-brown on
the upper side, owe this colour to a rounded hyaline strongly refractive mass lying in
the upper part of the pallisade-cells, which appears of a beautiful carmine-red where
the leaves are red, but elsewhere of a pale-yellow, and consists' mainly of tannin. The
chlorophyll-granules, intact and of a beautiful green, are all crowded together in the
inner end of these cells. In the spongy parenchyma of the mesophyll a colourless or red
mass of tannin occurs in the centre of each cell, while the chlorophyll-granules, also
intact, are collected in roundish or irregular lumps, sometimes in one place, sometimes in
several, but always on the sides towards the adjoining cells. In these cases the colouring
matter of the chlorophyll is unchanged with regard to either of its constituent pigments.
The red-colouring matter is soluble in water, and cannot be distinguished by spectrum-
analysis from the red colouring substances of flowers.

In all leaves which persist through the winter, and in the green parts of bark, Kraus
found that the chlorophyll-granules had removed from the walls to the interior of the
cell, and had collected there in lumps (see Sect. 8). When the weather has become
sufficiently warm in the spring, the normal condition is restored ; the red colouring sub-
stance disappears, and the chlorophyll-granules again take up their normal position on
the cell-walls. Kraus shows that the winter change of the leaves depends on the fall of
the temperature, since it is restored to the normal state by a simple rise in the tempera-
ture, whether in the dark or the light. By taking cut branches of Box into a warm
room when the cold was severe and placing them in water, he found that the proto-
plasm of the cells, which had become homogeneous after one or two days, collected on
the walls, and then divided into grains (as in the formation of chlorophyll-granules in
the dark); the red colouring matter being changed first to a yellowish-green and
finally to pure green. After the lapse of three, five, or at most eight days, the walls of
the cells became lined with bright green sharply-defined chlorophyll-granules. In
Thuja the process required two to three weeks (with me however only a few days).
The restoration is therefore rather a slow process ; while, according to Kraus, a single
frosty night suffices to bring about the change in the form and colour of the chloro-
phyll-granules in the case of Buxus, Sabina, and Thuja. That light has no share in the
restoration of the normal condition of the chlorophyll is shown by the fact that it
takes place also in branches which are kept in a dark room. On the other hand, the fact
that the parts protected by being covered by other leaves show no change of colour
would seem to indicate that the whole phenomenon has less to do with the low tempera-
ture of the air than with the cooling produced by radiation.

4. Convenient contrivances for observing the action of particular higher or lower
temperatures on plants or parts of plants of considerable size are easily arranged^.
It is more difficult to expose microscopic objects to a particular higher or lower
temperature in such a manner that it can easily and certainly be observed, and that
the temperature of the object is also that indicated by the thermometer, or nearly so.

* See Sachs, Handb. der Exp.-Phys. pp. 64, 66.

n^

GENERAL CONDITIONS OF PLANT-LIFE.

All these requirements are fulfilled by the very cheap heating apparatus for the micro-
scope represented in Fig. 474. Since I have not only made great use myself of this
apparatus for three years, but have also recommended it to others, a description is the
more in place here as it is well adapted for demonstrations in lecture-rooms.

The size of the heating apparatus must vary with that of the microscope ; mine is
constructed for one of Hartnack's ordinary instruments. The box is nearly cubical, and
has double walls of sheet-zinc at the bottom and sides, enclosing a space 25 mm. thick,
which is filled with water through the hole /. It is quite open above ; but in the front

Fig. 474.— Heating apparatus for the microscope.

side-wall is an opening f, which is closed by a glass plate well fitted but not other-
wise fixed. This window is sufficiently large, and is so placed that it allows enough light
to fall on the mirror of the microscope which stands in the box. The height of the
box is so arranged that the upper rim of the double wall is on a level with the arm b of
the microscope. The opening of the box is closed by a thick cardboard cover d d, in
which an opening is cut exactly to fit the arm b. By the side of the tube of the micro-
scope a round hole is cut in the cover through which a closely fitted small thermometer
/ is passed, so that its bulb hangs near the object. The box is painted on the inside
with black varnish, and a piece of cardboard moistened with water lies beneath the foot

ACTION OF LIGHT ON VEGETATION.

737

of the microscope in order to prevent its moving and to keep the air within moist.
The focus is easily adjusted to the object by means of the fine adjustment s which pro-
jects above the cover; two openings in the side, one of which is shown at o, enable
the slide bearing the object to be moved, when necessary, by a pair of forceps. It is
still more convenient to fix the slide on a wire which goes through a cork fitted to the
opening o.

If observations are required at a high temperature, the water in the box is heated
by a spirit-lamp placed underneath. When the temperature has reached nearly the
desired point, the spirit lamp is replaced by an oil-lamp with a floating light ; the tem-
perature will after a time become constant. In order to obtain higher or lower constant
temperatures, one, two or three floating night-lights are placed in the lamp. If care is
taken that the combustion be uniform, the temperature in the box remains for several
hours so constant that it will vary only about i° C. This constancy of temperature
ensures that the temperature of the object itself is that indicated by the thermometer.

It is easy by means of this heating apparatus to observe and demonstrate the influence
of temperature on protoplasm-currents. To take observations at low temperatures it
is sufficient to enlarge the hole /, in order from time to time to place pieces of ice in the
cold water ^.

Sect. 8. — Action of Light on Vegetation^. A. General. The entire life
of the plant depends on the action of light on the cells that contain chlorophyll, this
being the essential condition under which new organic compounds are formed out
of the elements of carbon dioxide and water. The amount of oxygen evolved in
this process is nearly *the same as that required for the combustion of the substance
of the plant ; and the amount of work equivalent to the heat produced by this com-
bustion gives a measure of the amount of work performed by light in the chloro-
phyll-containing cells of the plant.

After a certain quantity of assimilated substance has been produced under the
influence of Hght, a long series of vegetative processes may be carried on at its
expense without any further direct action of light. The growth of new organs and
the metastasis connected with it kept up in the organs by means of respiration is
entirely or to a certain extent independent of Hght, and can even be carried on in
absolute darkness. This is the case in the germination of seeds, bulbs, and tubers,
the development of buds from woody branches and underground rhizomes, &c.
Even leafy plants which have accumulated a sufficient quantity of reserve-material in
the light put out shoots and even flowers and fruits when placed in the dark.

As the parts of chlorophyll-containing plants which are underground or other-
wise excluded from light are nourished by the products of assimilation produced in
the light, so also parasites and saprophytes destitute of chlorophyll live, as has
already been explained, on the work performed by plants that contain chlorophyll,
and are therefore dependent indirectly on light, even though the whole of their
development may be completed in darkness, as in the Truffle ; in other instances
they only emerge to unfold in the air the flowers already formed underground, and to

* [For further arrangements for maintaining a constant temperature under the microscope, see
Strieker and Burdon-Sanderson, Quart. Journ. Micr. Sci. 1870; Schafer, ibid. 1874.]

2 A. P. De Candolle, Physiologic vegetale, 1832.— Sachs, Ueber den Einfluss des Tages-lichtes
auf Neubildung u. Entfaltung verschiedener Pflanzenorgane ; Bot. Zeitg. 1863, Supplement. — Sachs,
Wirkung des Lichtes auf die Bliithenbildung u. Vermittlung der Laubblatter; Bot. Zeitg. 1865,
p. 117. — Sachs, Handb. der Exp.-Phys. 1865, p. i.

3 B

y^S GENERAL CONDITIONS OF PLANT-LIFE.

disseminate their seeds, as is the case with Limodorum aborlivum, Epipogium, Coral-
lorhiza, Monotropa^ Lathrcea, Orobanche, &c. Even many plants which do contain
chlorophyll and which live on inorganic food complete their growth and the pro-
cesses connected with it in complete darkness, only putting forth their green leaves
at certain times for the purpose of again accumulating beneath the ground fresh
formative material. This is the case with the Autumn Crocus, Tulip, Crown Imperial,
terrestrial Orchids, and many others, and especially with plants which form bulbs,
tubers, and rhizomes. If the growing end of a stem of a green-leaved plant (e. g.
Cucurbita, TropcBolum, IpomcBa, or Hedera) is secluded from all light while the
green leaves remain exposed to it, the buds develope in the dark ; leaves and flowers
are produced, which latter attain their full size and beauty of colour, are capable
of fertilisation, and produce fruits and even fertile seeds at the expense of the
substance assimilated in the light in the green leaves and carried to them by the
stem.

These and a number of other facts show that growth, /. e. the processes by
which the form of the plant is attained, and metastasis are not necessarily dependent,
or only to a subordinate extent, on the influence of light, if only the necessary
quantity of assimilated material has previously been accumulated.

This is a general statement of the case. If however the various separate
processes of vegetation are observed — the behaviour of protoplasm, the formation,
arrangement, activity, and destruction of chlorophyll, the growth of the younger
and older parts, the movements resulting from the tension of the tissues, &c.- —
a long series of very varied facts presents itself which require detailed consideration,
because the rays of different refrangibility which are mingled in white daylight
afl"ect vegetation in a manner altogether diff"erent; certain functions are induced
only by the highly refrangible rays, others only or chiefly by those of lower refran-
gibility. These efl"ects moreover vary not only with the temperature but also with
the intensity of the particular rays. Finally it must be observed that light afl"ects
plants only when its rays penetrate into their organs ; this however modifies their
intensity and to a certain extent also their refrangibility. In every investigation of
the action of light these points must therefore be kept in view. The following
summarises what is at present known as to the general facts.

(i) Action of rays of different refrangibility. The rays of different refrangibility
commingled in white sunlight which appear as variously coloured bands in the
spectrum vary in their physiological action on the processes of vegetation. Chemical
changes, so far as they are in the main dependent on light, are produced chiefly
or solely by rays of medium or low refrangibility {viz. the red, orange, yellow, or
green). This is the case for instance with the production of the green colour of
chlorophyll, the decomposition of carbon dioxide, and the formation in chloro-
phyll of starch or sugar.

On the other hand the rays of high refrangibility (the blue or violet, as well
as the invisible ultra-violet rays) are the principal or the only ones which produce
mechanical changes so far as these are dependent on light. It is these rays which
influence the rapidity of growth, alter the movements of the protoplasm, compel
swarm-spores to adopt a definite direction in their motion, and change the tension
of the tissues of the motile organs of many leaves and hence affect their position.

ACTION OF LIGHT ON VEGETATION. 739

These two laws, the result of careful observation, are only in apparent
contradiction to the division of the rays of light which is current in chemistry
and physics into those called chemically active, including the highly refrangible
blue, violet, and ultra-violet, and the chemically inactive, or at least less active,
including the less refrangible red, orange, and yellow, and partly also the green
rays. This division has long been familiar ; silver- salts, nitrogen chloride, and
other inorganic compounds, are powerfully acted on by the former, scarcely at
all by the latter. But when it was shown that the organico-chemical processes
in plants were caused mainly or solely by the latter kind of rays, it was seen that
this classification into chemical and non-chemical rays resulted from an imperfect
induction, and that the correct statement of the fact is rather that there are chemical
processes (generally dependent on light) which are related to rays of particular
refiangibility. As far as concerns the mechanical effect on the plant of the highly
refrangible rays, it is at present uncertain whether they are not ultimately due to
chemical changes. In any case the action is visible to the observer only in the
form of mechanical effect (movements, tensions, &c.); and this is in harmony
with the classification given above.

If sunlight is made to pass through sufficiently thick strata of solutions of
potassium bi-chromate and ammoniacal copper oxide ^ the first only permits the
passage of light consisting of the less refrangible half of the spectrum (red, orange,
yellow, and some green), while the blue solution allows, in addition to some green,
only the blue, violet, and ultra-violet rays to pass through. The sunlight is therefore
in each case halved by absorption in such a way that the spectrum beneath the
orange solution extends from the red to the green, that beneath the blue solution
from the green to the ultra-violet. If the light after passing through one or other of
these fluids is directed on plants capable of decomposing carbon dioxide and of curving
heliotropically, and pieces of very sensitive photographic paper are at the same time
exposed by their side, it is seen that the less refrangible rays of light (transmitted
through the potassium bichromate) effect the decomposition of carbon dioxide and
the colouration and decolouration of the chlorophyll almost as energetically as white
daylight, while they produce only a very slight effect on the photographic paper.
The growth of seedlings, on the other hand, proceeds in this light exactly as in the
dark, although the leaves turn green. Conversely the light which has passed through
the ammoniacal copper oxide has very little effect in decomposing carbon dioxide,
although the action on photographic paper is very vigorous. The growth of seed-
lings is on the other hand the same as in white light ; and the mechanical process of
heliotropic curvature is very manifest. A number of more recent observations have
confirmed and extended the results previously obtained^

(2) Variatio?i in the action 0/ light on plants in proportion to its intensity^. That

^ Sachs, Bot. Zeitg, 1864, p. le^z *' seq., where the labours of previous observers are referred to
in detail.

^ I have replied, in the second part of the * Arbeiten dee botan. Inst, in Wurzburg,' 1872,
to the objections urged by Prillieux to this statement, which rest on an entire confusion of the
ideas Intensity of Light (objective), Brightness (subjective), Refrangibility (an objective), and Colour
(a subjective) property of light.

^ With respect to the distinction which must here be borne in mind between the objective

3 B 2

740 GENERAL CONDITIONS OF PLANT- LIFE.

the action of light on plants varies with its intensity, as that of temperature with its
elevation, does not admit of a doubt, and is obvious in all physiological observations.
There can scarcely be said, however, to be any exact investigations on this point ;
and the great obstacle to iheir accomplishment is that we have at present no method
of measuring the intensity of rays of light of any particular refrangibility in terms of
a fixed unit which can be applied to plants. As far as concerns the highly refran-
gible rays, t. e. those which have the greatest mechanical effect, we are compelled to
adopt the photo-chemical method of Bunsen and Roscoe^ which however gives no
information respecting the different intensity of the red, orange, and yellow light,
and can only be applied with great difficulty to experiments on vegetation. In the
photometry of the less refrangible rays, on the contrary, we must always have
recourse, according to the ordinary method, to the sensitiveness of the eye, t. e. to
brightness, which cannot be considered in itself to be an actual objective measure of
the intensity of the light, though it may be assumed under certain circumstances
that increase or diminution of subjective brightness corresponds to increase or
diminution of objective intensity. In describing the relation between the intensity
of light and vegetation, we have therefore at present, with a few exceptions, to
employ the ordinary expressions dark, dull, bright, dazzlingly bright, &c., and to
assume that they correspond to certain objective intensities. There is one case in
which this relation between the subjective sensitiveness of the eye and the action
upon vegetation of the light which causes it can be very strikingly proved ; Pfeffer
has shown that the curve of the subjective sensitiveness of the eye for the colours of
the solar spectrum coincides exactly with the curve expressing the power of different
regions of the spectrum in decomposing carbon dioxide ^. This coincidence must
however at present be considered purely accidental^, and cannot be extended to
other phenomena. If the sunlight or diffused daylight which reaches the observer
were always of the same intensity, it would be easy to regulate artificially, according
to definite gradations, the intensity of the light that acts on the plant. But since the
light of incandescent bodies (such as the Drummond's light*) contains the same
rays as sunlight and acts similarly on the functions of plants, constant sources
of light of a definite intensity can in this way be arranged, which will admit of
gradual adjustment, in order to study the influence on vegetation of light of different
intensities.

If we now turn to the observations on record, those of Wolkoff are the only
ones in which actual measurements have been made. With the assistance of
the photometric method contrived by Bunsen and Roscoe^, he showed first of
all that changes in the intensity of the highly refrangible light do not stand in
any appreciable relation to the exhalation of gas by water-plants. This is an

intensity of light and its brightness to the eye, see the paper quoted above and the literature there
referred to.

^ See the admirable paper by Wolkoff in the Jahrb. fiir wiss. Bot. vol. V. p. i.

^ Pfeffer in Sitzungsber. der Ges. zur Beforderung der ges. Naturwiss. fiAr Marburg, 1872,
May 16.

^ vSee note on p. 747.

* See Herve Mangon, Comp. rend. 1861, p. 243. — Prillieux, ibid. 1869, p. 408.

^ Bunsen and Roscoe, Fogg. Ann. vol. 108.

ACTION OF LIGHT ON VEGETATION. 74 1

additional proof that these rays play only an extremely small part in this process, so
small indeed that in the experiments the actual effect might be concealed by other
causes (see p. 744). He next used as the source of light a dull glass plate
illuminated by daylight, at different distances from which he exposed the plants
{Ceratophylluin^ Potamogeton, Ranunculus fluiians) in a dark room; and he ascer-
tained that the exhalation of gas was, within certain limits, nearly proportional to
the intensity of the light ^ There is probably however some particular intensity of
the efficient rays at which a maximum of gas is exhaled, and above which the
rapidity of the process again decreases and the plant suffers injury ; but whether
this maximum intensity of light is attained or exceeded by the sunlight as it falls
on the surface of the earth cannot at present be determined. In reference to
the smallest degree of intensity of light at which exhalation of gas can still take
place, we have only the statement of Boussingault that a leaf of Oleander ceased
to exhale oxygen after sunset ^.

The green colour of the chlorophyll of Monocotyledons and Dicotyledons is
not produced in the dark, as may be seen by enclosing plants in closely shutting
'boxes of wood or metal, or in a dark cellar. The colouration begins however
when the amount of light is barely sufficient to read a book by ; and when it in-
creases to the ordinary brightness of a sunny summer day, the rapidity of the
change increases, and the colour becomes a deeper green than that produced
when plants are placed for a longer time in places not so strongly illuminated.
Famintzin nevertheless showed^, in the case of Lepidium sativum and Zea Mais,
that bleached seedlings become green more slowly in direct sunlight than in dif-
fused daylight.

The small intensity of light which suffices for the formation of chlorophyll is
not sufficient for assimilation or for the formation of starch in the chlorophyll-
granules. Plants (such as Dahlia, Faha, Phaseolus, Cucurbita, &c.) which rapidly
become green in the normal condition of full daylight, as well as in the diffused
light of the back of a room, still form no starch in their chlorophyll-granules. They
do however produce starch when placed in a window where, at the most, they enjoy
but half the direct sunlight and diffused daylight ; but, in harmony with this, the
assimilation of these plants is much less active . in the window than in full daylight m
the open air*. The following experiment gives a somewhat more precise result.
Four plants of Tropceolum viajus grown from seed in the back of a room, all gave,
when dried at iio°C., a smaller weight than the seed; they had not assimilated,
and died after consuming the reserve-material, although in the shade of the room
they all produced green leaves. Four other plants of the same species which
germinated at the same time grew for three months, exposed for only seven hours
each day to the diffused light of a west window in the forenoon ; they formed
nearly 5 grammes of dry substance. Four other plants which were exposed in a west
window from i p.m. till the following morning, and therefore to the afternoon sun-
shine, produced also only 5 grammes ; while four other plants which stood in the

' See also Pfeffer, Arbeiten des botan. Inst, in Wiirzburg, Heft I. p. 41.

^ Comp. rend. vol. 68. p. 410.

' F'amintzin, Melanges biologiques; Petersbourg, vol. VI. p. 94, 1866.

* Sachs, Bot. Zeitg. 1862, No 47 ; and 1864, p. 289 et seq.

742 GENERAL CONDITIONS OF PLANT-LIFE.

window during the same time day and night produced nearly twenty grammes of
dry substance \ It is a necessary conclusion from the increase in weight of these
plants, that in the diffused daylight of the window of a room carbon dioxide is
decomposed by the cells which contain chlorophyll, and that this does not take
place with great activity. The same conclusion is drawn from the observation that
Valltsneria spiralis and Elodea canadensis give off bubbles of gas when the light falls
on them for only a rather short time from the northern sky on a clear day, although
the exhalation is much more rapid in direct sunlight. In the case of most plants
which grow in full daylight, especially our cultivated plants, the increase of weight
by assimilation is greatly diminished when they are grown in a window. Within
a room itself they usually become exhausted by their own growth in consequence
of the defective assimilation, which is not sufficient to replace the material con-
sumed in growth and in respiration ; and the plant ultimately dies. Many Mosses
on the other hand, and wood-plants of various kinds which grow in the deep shade
(as the Wood- Sorrel), are killed by constant exposure to broad daylight; but
whether in these cases it is the intensity of the light or the transpiration that is too
great, and which of the two is the direct cause of injury, is unknown. Stems which '
attain an enormous length in complete darkness remain perceptibly shorter in the
shade of a room ; in a window their growth is still less, and least of all in the open
.air in full daylight. The reverse is the case with the leaves of Dicotyledons and
Ferns ; in the dark they are often very small ; in deep shade they are considerably
larger, and still more so in a light window ; in this position they even appear in
many plants {Phaseolus, Begonia, &c.) to attain their maximum of superficial de-
velopment, remaining smaller in the open air^.

(3) Penetration of the rays of light into the plant. In order to determine the
dependence on light of certain phenomena of vegetation, it is of special interest to
know the depth to which rays of a given refrangibility can penetrate any tissue
of a plant, and the intensity with which the different elements of daylight act on
particular internal layers. With the exception of the underground parts of plants,
stems enveloped in bark, young organs enclosed in leaf-buds, and the like, which
are in complete darkness, the assimilating and growing organs are penetrated by
light. The deeper the light penetrates, the more does it lose in intensity by ab-
sorption, reflexion, and dispersion. This loss however affects the different elements
of white light in very different degrees, as was shown by my investigations made
in 1859^, at present the only ones on this subject. The rays of greatest re-
frangibility are in general almost entirely absorbed by the superficial layers of
tissue, while the red light penetrates most deeply. Of successive layers of an

^ Sachs, Exp.-Phys. p. 21. It must however be observed that the shorter the duration of the
light in these cases, the longer was the time of their exposure to the dark in which they again lost a
portion of the assimilated substance by respiration.

"^ The statement made by Famintzin (Mel. biol. vol. VI. p. 73, 1866) that the motile Algae,
Chlatnydomonas pulvisculus, Euglena viridis, and Oscillatoria insignis turn both from direct sunlight
and deep shade to a light of medium intensity, is contradicted by Schmidt (quoted infra), who found
that they always turn to light of greater intensity, and even to direct sunlight. The method' of
observation of both authors was however very imperfect. [See also p. 752.]

' Sachs, Ueber die Durchleuchtung der Pflanzentheile ; Sitzungsber, der Wien. Akad. i860,
vol. 43; and Handb. der Exp.-Phys. p. 6.

ACTION OF LIGHT ON VEGETATION. 743

apple, gourd, succulent stems, &c. only the outermost receives the light that falls
on it unchanged (independently of the reflexion from the surface); each deeper
layer is penetrated by light less intense than the preceding one, and of a different
composition. This change in the light which penetrates the tissue is principally
caused by colouring materials, especially chlorophyll, which have an absorptive
power for particular groups of rays, allowing others to pass through, and producing
in addition rays by fluorescence which were not contained in the incident light.
But the relations of these changes of light in the tissues to the changes which
the light causes are not yet accurately known; not even in reference to chloro-
phyll, to which we shall again recur. What we have now said is intended only
to draw the attention of the student to the subject; more exact investigations
must be made in working out the different questions which arise.

B. Special, (i) Chemical Action of Light on Planls. {a) Formation 0/ Chloro-
phyll\ In the formation of the chlorophyll-granules the protoplasm becomes
differentiated into a colourless continuous part which forms the proper motile pro-
toplasmic body of the cell, and into smaller distinct green portions which remain
imbedded in the former, the chlorophyll-granules. This process, as far as concerns
the differentiation, is independent of light, at least in flowering plants, whjre the
chlorophyll-granules are formed in the cells of the leaves even in the dark. The
chemical process, on the contrary, by which the green colour is produced has
a complicated dependence on light. If, for instance, the temperature is sufficiently
high, the green colouring substance is formed in the cotyledons of Conifers and
in the leaves of Ferns in complete darkness as well as under the influence of light 2.
In Monocotyledons and Dicotyledons, on the contrary, the chlorophyll-granules
which are formed in the dark remain yellow ^, until they are exposed to light even of
small intensity, when they become green if only the temperature is sufficiendy high ;
and the nearer, as I have shown, the temperature approaches a definite maximum
(25 to 30° C.) the quicker does the chlorophyll of Angiosperms become green in the
light. Provided therefore that the temperature is favourable, the chlorophyll in
the cotyledons of Conifers and the leaves of Ferns does not require light in order
to assume its green colour ; while that in Angiosperms does require it ; and in both
cases the change does not take place at a low temperature (see p. 729). It may be
added here that the subterranean protonema of Mosses contains chlorophyll, though
but in small quantity.

It may be concluded from such observations as have been made that all the
visible parts of the solar spectrum have the power of turning the etiolated chlorophyll-

^ Sachs, Bot, Zeitg. 1862, p. 365, and Exp.-Phys. pp. 10 and 318. — Sachs, Flora, 1862, p. 213,
and 1864, no. 32,— Mohl, Bot. Zeitg. 1861, p. 238. — Bohm, Sitzungsber. der Wiener Akad. vol. TI.
Compare also Book I. sect. 6 of this work.

^ P. Schmidt (Ueber einige Wirkungen des Lichts auf Pflanzen ; Dissertation, Breslau 1870,
p. 22) believes that these facts can be at least partially combated; but his experiments only prove
that the chlorophyll which is formed in the dark is again destroyed by long exposure to dark at a
high temperature (337° C), as is also the case with other plants.

^ [Elfving has. found (Arb. d. bot. Inst, in Wiirzburg, II. 3, 1880) that exposure to light at a
temperature which is not sufficiently high to produce chlorophyll leads to an increased formation of
the yellow colouring matter (etiolin) in etiolated seedlings.

There is reason to believe that chlorophyll is derived from etiolin (see Wiesner, Eiitstehung des
Chlorophylls, Wien, 1877) ]

744 GENERAL CONDITIONS OF PLANT-LIFE.

granules of Angiosperms green; but that the yellow rays and those nearest to
them on each side are the most powerful ; and that this is also the case with the
exhalation of oxygen from cells containing chlorophyll \

(<$) T/ie Decompositio7i of carbon dioxide in cells containing chlorophyll, on
which depends the assimilation of plants, and which is perceptible externally by
the exhalation of a volume of oxygen nearly equal to that of the carbon dioxide
absorbed, is brought about at a favourable temperature (see p. 729) by rays of light.
In submerged water-plants the gas (always mixed with a larger or smaller quantity
of nitrogen) escapes in the form of bubbles from wounds, especially transverse cuts
of the stem; and it has been shown by Pfeffer and myself that when their size
is constant the rapidity of these bubbles, i.e. the number of them formed in a unit of
time, may even be used to give an exact measurement. In observations on land-
plants it is on the other hand necessary to expose the leaves to light together with
air containing carbon dioxide in glass vessels of a suitable size and form, and to
measure the quantity of gas by a eudiometer.

The smallest intensity of light necessary for the evolution of oxygen is —
judged by the subjective measure of its brightness to our eye — rather considerable
(see p. 742). This evolution is always taking place with considerable energy in
diffused daylight, even when the rays reach the plant only from a small portion of
the sky ; but it is much stronger in direct sunlight.

The specific effect on the evolution of oxygen of the variously refrangible
elements of sunlight, in other words of the different coloured bands of the solar
spectrum, has been carefully investigated by Draper and very recently again by
Pfeffer ^. The observations were made partly with the solar spectrum, partly with
solutions of different colours which transmitted light of a particular refrangibility.
The amount of gas exhaled was measured partly by the eudiometer, partly by the
number of bubbles. Pfeffer points out ' that each portion of the spectrum exercises
a specific quantitative influence on the power of assimilation ; and that this remains
unchanged whether the particular rays act separately on the parts of plants that
contain chlorophyll, or combined with some or with all the other rays of the
spectrum.'

The following additional result was also obtained from Draper's and Pfeffer's
observations, and from mine already quoted : — ' Only those rays of the spectrum
which are visible to our eye have the power of decomposing carbon dioxide; and
indeed those which appear brightest to the eye, the yellow rays, are alone as

^ See in particular Guillemin, Ann. des Sci. Nat. 1857, vol. VII. p. 160. [According to Wiesner
(Unters. ueb. d. Beziehungen des Lichtes zum Chlorophyll, Sitzber. d. Wien. Alcad., vol. 69,
1874; also Bot. Zeitg. 1874), etiolated plants become green much more rapidly in blue than in
yellow (intense) light. He attributes this to the more active decomposition of the chlorophyll in the
yellow light. This view is supported by the observation of Guillemin (Ann. d, Sci. Nat., 1854)
and of Famintzin (Melanges biologiques, Acad. Imp. de St. Petersbourg, vol. 6, 1866) that the
leaves of etiolated plants become green more rapidly in diffuse daylight than in sunshine.]

'^ Draper, Annales de chimie et de physique, 1844, p. 2\\et seq. — Pfeffer, Arbeiten des Botan-
ischen Instituts in Wiirzburg, Heft I. p. 48, where reference is also made to the whole of the rest
of the literature.'— Pfeffer, Sitzungsber. der Gesellsch. zur Beforderung der gesammt. Naturwiss. zu
Marburg, 1872, May 16; and Bot. Zeitg. 1872, no. 23 et seq., where the paper by Miiller, Botanische
Untersuchungen, Heft I, Heidelberg 1 871, is also discussed. [For an account of Draper's researches
into the relations existing between plants and light, see his Scientific Memoirs, London, 1878.]

ACTION OF LIGHT ON VEGETATION,

745

Gas e\

-olved.

Experiment I

Experiment II.

033

O-O

20-00

24*75

36-00

4375

O'lO

4-IO

O'O

1*00

o-o

O'O

O'O

O'O

efficacious in this process as all the others put together. The most refrangible
rays of the visible spectrum which act most energetically on silver chloride, &c.,
play a very subordinate part in the process of assimilation.'

Draper placed glass tubes filled with water saturated with carbon dioxide in
which he had placed green parts of plants, in the different coloured portions of a
solar spectrum. Seven of these tubes were exposed simultaneously in the same
spectrum. The following table gives the result of two experiments of this kind : —

Part of the Spectrum.

Dark-red

Red-orange

Yellow-green

Green-blue

Blue

Indigo .

Violet ,

Pfeffer experimented chiefly on leaves of the Cherry-Laurel and Oleander, which
were placed in air containing carbon dioxide (shut off by mercury) in suitable glass
vessels, and received the sunlight through coloured solutions (tested by the spectro-
scope). The following was the result of sixty-four experiments: — If the amount
of gas evolved in light which has passed through a stratum of water of standard
thickness is represented by 100, the numbers here given are the corresponding
quantities of carbon dioxide decomposed in light which has passed through equal

thicknesses of the solutions named.

»

^ , ^ , , , Amount of carbon dioxide

Colour of light. , J

° decomposed.

Red, orange, yellow, green 88-6

Green, blue, violet 7*6

Red, orange-green, blue, violet 53*9

Red, orange-blue, violet 38*9

Red, orange 32*1

Red-orange, yellow, green 15-9

Quite dark (14*1 carbon di-
oxide produced).

Solution.

Potassium bichromate

Ammoniacal copper oxide

Orcin

Aniline-violet

Aniline-red

Chlorophyll

Iodine solution

From a comparison of these numbers Pfeffer deduced the following values for
the decomposing power of the different regions of the spectrum, the action of white
light being again placed at 100: —

For Red-orange
Yellow
Green .
Blue- violet

32'I

46*1

15-0

7-6

100 8

and from these is deduced the first statement of Pfeffer given above.

746

GENERAL CONDITIONS OF PLANT-LIFE,

If these values are erected as ordinates upon the solar spectrum, taking its
corresponding parts as abscissae, the result, as shown in Fig. 475, is that the curve of
the different powers of light for causing evolution of gas corresponds in the main
with the curve of subjective brightness of the same regions of the spectrum ; but
does not coincide with the curve of heating power.

Pfeffer's experiments had shown that the method first employed by me for
determining the intensity of the action of light on water-plants, viz. counting the
number of the bubbles of gas given off in a unit of time, gave nearly the same
results as actual ineasurement of the gas, the result being in fact somewhat too
great, and inexact in inverse proportion to the amount of gas given off. He then
applied this method to determine the amount of oxygen given off from a small
water-plant {Elodea canadensis) when exposed to a portion 1 3 mm. in breadth of a
very intense solar spectrum 2 3 cm. long. In this experiment he had the advantage

Asiintildti

m

f^

\

*'^.,

\^ Brightness

*^v^

A

'■.\

"'^v.

/ 1

\\ - Heat

^ ^v

/ 1

\\

^^^^

/ 1

\\

^*

/ 1

\\

^»^

/ /

\\

*^ /

/

\\

/

\\

\\

/ "■»,

\ ,

/

\\

/

^^^^^ V

/

^>^ \

i\

/

/

"^-

■^r^^---^^

/

\., ^"^'"-'■-^

/ /

\""""*-^ ""T'^^^

/ /

/ y

x^ ~"~-, !

1 y

""■"---L".-.-:;--

i

1

1

1

~

1

A. B. C.

D.

E. I. r.

a.

H.

Fig. 473.— Graphic representation of the efficacy of rays of different refrangibility in causing the evolution of oxygen, com-
pared with their brightness and heating power. The solar spectrum A—H serves as a base, on which lines to represent the
three different effects are erected as ordinates ; the three curves are thus obtained which represent asbimilation, brightness,
and heat.

of being able to determine the amount of gas given off by the same plant in all
the regions of the spectrum in successive very short spaces of time, and of thus
avoiding various errors of observation which inevitably accompany eudiometric ob-
servations, or at least are very difficult to get rid of. A number of observations
conducted in this manner gave the following result as the mean capacities for
decomposition possessed by the different regions of the solar spectrum, yellow
being placed at 100: —

Red

xvcu

Orange
Yellow .
Green

^5 4

63-0

. lOO-Q

. 37'2

Blue

Indigo

Violet

22*1

. 13-5
71

ACTION OF LIGHT ON VEGETATION, 747

If allowance is made for the small error mentioned above incident to the
method of counting the number of bubbles, we find that the curve of capacity for
exhahng oxygen agrees still more exactly with the curve of brightness than is
represented in Fig. 475, which was drawn from only a few data obtained with
difficulty.

Since a comparison of ^he curve of brightness with that of the evolution of
oxygen, otherwise convenient, has turned the attention of observers in a wrong
path, and has led to many erroneous theories, it will be convenient to state the
only relation between the two with which we have to do here, in precise
terms : — The evolution of oxygen caused by chlorophyll is a function of the
length of the waves of light ; only those wave-lengths which are not greater than
0*0006866 mm. and not less than 0*0003968 mm. being able to produce this effect.
Starting from the two extremes, the capacity of light for causing evolution of
oxygen rises till it reaches its maximum at a wave-length of o"ooo5889 mm. Or,
starting with the medium wave-lengths of the coloured region of the spectrum
measured in hundred-thousandths of millimetres, the evolution of oxygen is effected
by waves of light of a minimum length of 39 ; it increases with the increase of wave-
length until the latter reaches about 59 ; it then diminishes if the wave-length con-
tinues to increase until it entirely ceases when the wave-length is 68. It will be at
once seen that we have here a similar phenomenon to that of the relation of vegetation
to temperature ; for we found (see p. 729) that this function also rises with the rise of
temperature, attains a maximum at a definite temperature, and again decreases as
the temperature rises still higher ^

Godlewski^ obtained the following results by a long series of eudiometric ex-
periments as to the influence of the percentage of carbonic acid in the air upon the
extent of the decomposition of this gas and upon the corresponding evolution of
oxygen. An increase of the amount of carbonic acid present in the air, up to a
certain limit (optimum), increases the evolution of oxygen, an increase beyond this
limit diminishes it. The limit is different for different plants ; for Glyceria speciabilis
on bright days it was from 8-100/0; for Typha laiifolia from 5-7% ; for the Oleander
probably rather lower. The increase of the evolution of oxygen consequent upon
an increased amount of carbonic acid being in the air is much greater than the
diminution produced when the optimum is exceeded by an equal amount. The
greater the intensity of light, the more is the evolution of oxygen promoted by an
increase of the carbonic acid up to the optimum, and the less is it diminished by
excess. It follows that the influence of light upon the evolution of oxygen is the
greater the more carbonic acid is contained in the air.

(c) Formation of Starch in Chlorophyll-granules^. The yellow chlorophyll (etiolin)-
granules formed in the dark are small ; after turning green on exposure to light they
become considerably larger, corresponding to the increase in size of the cells in which

1 The same law of dependence is also evidently applicable to the sensitiveness of the eye to
brightness ; and this is the cause of the curve of the brightness of light running nearly parallel to
that of the evolution of oxygen.

2 Godlewski, Arb. d. Bot. Instituts in Wlirzburg, Heft 3, 1873.

3 Sachs, Ueber die Auflosung und Wiederbildung des Amylums in den Chlorophyll-kornern bei
wechselnder Beleuchtung : Bot. Zeitg. 1864, p. 289.

748 GENERAL CONDITIONS OF PLANT-LIFE.

they are contained. It is only after they have assumed their green colour and under
the continued action of more intense light, in other words under conditions favour-
able to assimilation, that the formation commences of the starch which is enclosed
within the chlorophyll-granules (see p. 46). When cells whose chlorophyll has pro-
duced starch after exposure to light are placed in the dark, the starch is absorbed
and disappears completely from the chlorophyll-granules, and does so the quicker
the higher the temperature. If light is again allowed access, starch is again
formed in the same chlorophyll-granules ; and the formation of starch is there-
fore a function of chlorophyll-granules exposed to light, its absorption a function
of chlorophyll-granules not exposed to light. If complete or partial darkness is
continued for a length of time, the chlorophyll-granule is usually itself destroyed ;
it first loses its form, is then absorbed, and finally disappears from the cells together
with the colourless protoplasm ; in the case of leaves of rapidly growing Angio-
sperms this takes place after a few days when the temperature is high. Cactus-
stems with slow growth and the shoots of Selaginella on the contrary remain green
for months in the dark.

The absorption and re-formation of starch in the chlorophyll-granules — a pro-
cess which I was the first to demonstrate in the leaves of Phanerogams— can be seen
more readily in Algae of simple structure like Spirogjyra, which may therefore serve
for purposes of investigation. I had already shown that the formation of starch in
chlorophyll-granules depends on conditions which favour assimilation, and that the
principal feature of this process, the evolution of oxygen, proceeds vigorously in light
transmitted through potassium bichromate, and consists therefore of red, orange,
yellow, and to a certain extent green rays ; while the more strongly refrangible half
of the spectrum, consisting of green, blue, violet, and ultra-violet rays, obtained by
passing the light through ammoniacal copper oxide, has only a very slight effect.
The conclusion at once followed from this, that the formation of starch must take
place in the set of rays first named to the same extent that it does in full sunlight,
but only to a very small extent in the latter set. This was confirmed by Famintzin's
experiments ^ in which he found that in Spirogyra the formation of starch in the
chlorophyll-granules took place only in the mixed yellow light (that had passed
through potassium bichromate), and not in the mixed blue light (that had passed
through ammoniacal copper oxide) in which the starch already formed even disap-
pears. Since however a small exhalation of oxygen takes place even in the mixed
blue light, it must be supposed that a small production of starch occurs in it. Kraus's
experiments^ with Spirogyra, Funaria, and Elodea, confirm this. He also found that
in plants of Spirogyra which had lost their starch from exposure to dark, the forma-
tion of this substance in the chlorophyll-granules recommenced in five minutes in
direct sunlight, in two hours in diffused daylight. In Funaria the formation of
starch recommenced in the same manner within two hours in direct sunlight, within
six hours in diffused daylight; and similar results were obtained with leaves of
Elodea, Lepidium, and Betula^.

^ Famintzin, Action of Light on Spirogyra; Melanges biologiques, Petersburg 1865, Dec; and
1867, p. 277.

^ Kraus, Jahrb. fiir wissensch. Bot. vol. VII. p. 51 r.

' [From the observations of Weber (Ueb. specifische Assimilationsenergie, Arb. d. bot. Inst, in

ACTION OF LIGHT ON VEGETATION. 749

In accordance with the theory propounded by me that the starch formed in the
chlorophyll-granules under the influence of light is the first product of assimilation
produced by the decomposition of carbon dioxide, Godlewski has found (Flora,
1873, p. 383), as the result of experiments as simple as ingenious, that in an
atmosphere devoid of carbon dioxide no starch is produced in the chlorophyll-granules
even in the light; that the starch contained in them disappears when the carbon
dioxide is removed from the surrounding atmosphere, not only in the dark, but even
in bright light. It may be inferred from this that the starch which is at any time
found in the chlorophyll-granules is only the excess of the whole product of assimi-
lation which has not yet been taken up. Of especial importance is his observation,
which agrees with his eudiometrical experiments, that an increase in the proportion
of carbon dioxide in the atmosphere to 8 p. c. in a bright light increases the rapidity
of the formation of starch four or five fold, while in a diffused light the action is
much less. A very large quantity of carbon dioxide in the atmosphere, on the con-
trary, retards the formation of starch in inverse proportion to the intensity of the
light. Godlewski's experiments, made on the cotyledons of seedlings of Raphanus
sattvus, are opposed to the statement of Bohm (Sitzungsber. der Wien. Akad.
March 6, 1873), that the starch contained in the chlorophyll -granules is not a pro-
duct of assimilation, a view which has already been sufficiently refuted by my earlier
investigations.

(2) Mechanical Action of Light on Plants, (d) The influence of light on the
movement of protoplasm varies according to the nature of the motion. Those
movements which are the cause of the formation of new cells are not in general
directly dependent on light (see p. 752) ; since they take place, in the great majority
of cases, in partial or complete darkness. The ' streaming' motion of the proto-
plasm in older cells, or rotation and circulation, also goes on in continuous dark-
ness as well as in alternate daylight and night ; and even in the hairs of etiolated
shoots which are developed in darkness^. It has not been ascertained whether
in these cases the rapidity and direction of the movement, the mode of distri-
bution of the currents, and the accumulation of the protoplasm at particular spots,
are influenced by the direction of the rays of light. An influence of this kind is
apparently exercised by light on the plasmodia of ^thaliu?ji^. As long as the
Plasmodia are still in motion and not ripe for the production of spores, they appear
on the surface of the tan when it is dark; but in the fight, as in a sunny window,
they again conceal themselves in the dark parts of the tan, — a process which the

Wurzburg, II. 2, 1879) ^^ appears that equal areas of the leaf-surface of different plants produce
different amounts of organic substance in a given time, the conditions being the same. He obtained
the following numerical proportion for the energy of assimilation : —

Tropceolum mnjus ..... 4.466.

Phaseohis muldflorus . . . . 3-215.

Ricinus communis ..... 5.292.

Helianlhus annuus . . . . . 6 •559']

* Sachs, Bot. Zeitg,, 1863, Supplement.

* [This subject has been investigated by Baranetzky (Mem. d. 1. soc. nat. d. sci. nat. de Cherbourg,
XIX, 1876): he found that the plasmodia, whilst still young, always avoided light. Schleicher has
found on the contrary (Strasburger, Wirkung des Lichtes und der Warme auf Schwarmsporen, Jen.
Zeitschr. XII, 1878) that the young plasmodia seek the light when its intensity is small: older
Plasmodia seek the light even when it is very intense.]

^50 GENERAL CONDITIONS OF PLANT-LIFE,

plant may be made to repeat two or three times in a day. It is not till the
Plasmodium has collected into a thick firm mass, and is preparing for the produc-
tion of spores, that it comes to the surface in places exposed to light, but appa-
rently only in the night or early morning.

The protoplasm which envelopes the chlorophyll-granules in the green leaves
of Mosses and Phanerogams and in the prothallia of Ferns is induced, by the
varying intensity of the light, to accumulate to a greater or less degree at different
parts of the cell-walls, carrying the chlorophyll-granules along with it, and thus
altering their distribution in the cell. It is still uncertain whether in this case the
light affects the protoplasm only, the chlorophyll-granules being carried passively
along with it ; or whether the influence of the light is not first of all on the latter,
which then give the impulse to the protoplasm. In either case it appears certain
that the chlorophyll-granules do not of themselves possess any power of free
motion, but are carried about by the motile protoplasm. Famintzin and Borodin^
found that under the influence of continued partial darkness the chlorophyll-granules
in various Mosses and in the prothallia of Ferns collect on the side-walls of the
cells (those at right angles to the surface of the organ) ; and that when these parts
are exposed to light they leave them and distribute themselves over the parts of the
cell-walls which are parallel to the surface of the organ. Prillieux^ and Schmidt
have confirmed these statements. The view which I adopted long ago (see the first
and second editions of this work), that these changes of position in the chlorophyll-
granules are caused by the protoplasm, is confirmed by Frank's recent researches'.
He shows that when the light falls only from one side, the protoplasm and the
chlorophyll-granules collect mostly on those parts of the ; cell-walls on which the
strongest rays fall, if the cells are sufficiently large to allow the light to be so
arranged and these changes to take place in the position of their contents (as in
the prothallia of Ferns and leaves of SagUtaria). Frank brought under a general
point of view the changes in position of the chlorophyll-granules described by
Famintzin and Borodin; he shows that the protoplasm in these cells is capable,
according to circumstances, of adopting two different modes of distribution. In one
mode, which he calls Epistrophei the protoplasm and chlorophyll-granules collect
on the free cell- walls, i. e. those which do not immediately adjoin other cells ; for
instance, next the surface in the superficial cells of organs consisting of several layers
(the leaves of SagUtaria^ Vallisneria, and Eloded) ; on the upper and under walls in
organs consisting of only one layer of cells (leaves of Mosses, prothallia of Ferns) ;
and in internal cells on the parts that bound the intercellular spaces. This is
the position assumed in the normal conditions of vegetation and the mature state
of the cells, but before they become too old. The second mode, or Apostrophe^
takes place under unfavourable external conditions ; as for instance in small
fragments of tissue, when respiration is defective, turgidity diminished, the tem-
perature too low, the cells too old, or — what is of most interest here — when light
is cut off for a considerable time. Under these circumstances the protoplasm

^ Bohm, Sitzungsber. der Wien. Akad. 1857, p. 510. — Famintzin, Jahrb. fiir wissensch, Bot.
vol. IV. p. 49. — Borodin, Melanges biologiques; Petersburg, vol. VI, 1867.
"^ Prillieux, Compt. rend. 1870, vol. LXX. p. 60. — Schmidt, /. c.
^ Frank, Bot. Zeitg. 1872, Nos. 14, 15; and Jahrb. fiir wissensch. Bot. vol, VIII. p. 216 et icq.

ACTION OF LIGHT ON VEGETATION. 751

and chlorophyll-granules collect chiefly on the walls that are not free, i. e. on those
adjacent to other cells. The occurrence of apostrophe under direct sunlight which
Borodin asserts* (in various Phanerogams, as Lemna, Callitriche^ and Stellaria), is
denied by Frank, who maintains that what takes place in these cases is rather a
collection of the protoplasm at the spots where the light is strongest, which may
happen to be at the sides^.

It is evidently these aggregations of chlorophyll-granules on the side-walls of
the cells caused by sunlight which were observed by Borodin that produced the
phenomenon pointed out by Marquard and more exactly described by myself,
VIZ. that green leaves (e.g. those o^ Zea, Pelargonium^ Oxalis, Nicoiiana, Sec.) when
exposed to sunlight assumed a bright green colour in a shorter time than in
diffused light or in deep shadow. This can be made very evident by shading
particular parts by pressing closely on them a strip of lead or tinfoil ; if this
strip is removed after five or ten minutes, the parts that were shaded show a
dull green, those exposed to the sun a bright green colour. It is obvious that the
tissue will appear to the eye a deeper green in proportion as the green granules are
distributed uniformly over the surfaces facing the eye, a less deep green in propor-
tion as they collect on the side-walls. Borodin's observations directly confirm this
hypothesis. This alteration in the grouping of the chlorophyll-granules which
accompanies a change in the intensity of the light is caused only by the highly
refrangible rays; the less refrangible rays (the bright and red ones) have the same
eff"ect as darkness ^ It results therefore, as I showed in 1859, that if a strip of
blue glass is laid on a leaf exposed to sunshine, it will produce no change of colour,
while one will be caused by a strip of red glass.

Since these movements of the chlorophyll-granules are produced by the
colourless protoplasm in which they are imbedded, it might be expected that the
protoplasm of hairs which contain no chlorophyll or only a small quantity would
be similarly influenced by the colour and intensity of the light. But the state-
ments of Borscow and Luerssen^ which might be interpreted in this direction at
least to some extent have not been confirmed by the observations of Reinke^.

The swarming of zoogonidia is also connected with protoplasmic movements.
Their motile organs, the cilia, are supposed to be slender threads of protoplasm, by
the vibration of which both the rotatory and the advancing movement of the zoogo-
nidia is caused. The axis of rotation becomes subsequently the axis of growth ; the
anterior end in the advancing motion (where the zoogonidium is usually narrower,

■■ Borodin, Melanges biol., Petersburg 1869, vol. VII. p. 50.

^ [From Stahl's investigations it appears that apostrophe is produced by direct sunlight (Bot.
Zeitg, 1880). He finds that exposure to diffuse daylight produces epistrophe, that is, the position of
the chlorophyll-granules in which the greatest area of their surface is exposed to the incident rays,
M'hereas sunlight produces apostrophe, that is, the position in which the least possible area of their
surface is exposed to the incident rays. In the one case they present their flat surfaces, in the other
their edges to the incident rays.]

3 Sachs, Berichte der math.-physik. Klasse der k. sachs. Ges. der Wiss. 1859.

* Borodin, l.c, Frank, Bot. Zeitg. 1871, p. 238.

5 Borscow, Melanges biol., Petersburg 1867, vol. VI. p. 312. — Luerssen, Ueber den Einfluss des
rothen u. blauen Lichts u. s. w., Di!^sertation, Bremen, 1868.

^ Reinke, Bot. Zeitg. 1871, Nos. 46, 47. •

752 GENERAL CONDITIONS OF PLANT-LIFE.

hyaline, and provided with cilia) becomes the base of the germinating plant when
the zoogonidium has come to rest. These movements of zoogonidia and the very
similar ones of the Pandorinese are affected by light to this extent, that when the
light comes from one side they either tend towards or away from the source of light,
this depending apparently partly on the species and partly on the age of the indi-
vidual. Cohn states that here also the less refrangible rays have the same effect as
darkness, while the direction of the motion is determined by the blue and the more
highly refrangible rays ^

{e) Cell- Division and Growth'^. The first formation and early growth of the
new organs in the higher plants consisting of masses of tissue is accompanied by
a great number of cell-divisions, which usually take place in complete darkness ; as
for example, in the roots of land- and marsh-plants, the buds on underground
rhizomes, and leaves and flowers which are produced within the dense envelopes
of the bud. Cell-formation of the same kind may however take place under the
influence of light which may even be intense, as is shown by the growth of the roots
of land-plants in water exposed to light, or that of the aerial roots of Aroidese
(which are highly transparent at their cell-forming apex). The formation of
stomata and hairs which is the result of cell- division may take place either in
the light or in complete darkness within the bud, without any essential difference
being observable in the two cases. In the same manner the cambium of the
trunks of trees is covered by completely opaque envelopes, such as bark; while
that of many annual stems (as Impatiens) is exposed to the light which penetrates
the thin succulent cortex. Similar phenomena are presented in the formation and
ripening of ovules within transparent or completely opaque ovaries. They are most
obvious when shoots or even flowers which under ordinary circumstances are de-
veloped in the light are made to grow in complete darkness from bulbs, tubers, or
seeds. The small variations from the normal condition which occur in such cases
do not affect the early development of the organs; but their later growth which
does not depend on cell-division is necessarily interfered with, as well as the
development of chlorophyll. An obvious and necessary condition of these processes
of growth, whether in the dark or the light, is the presence of a supply of assimi-
lated reserve-materials, at the expense of which the formation of new cells can take
place. In the case of the buds of the higher plants their reservoirs of reserve-

^ Cohn, Schles. Ges. flir vaterl. Cultur, Oct. 19, 1865. The facts have however recently been
questioned by Schmidt. [See Sachs, Ueb. Emulsionsfiguren, Flora, 1876; Strasburger, Wirkung des
Lichts und der Warme auf Schwarmsporen, Jen. Zeitschr. XII, 1878; Stahl, Ueb. den Einfluss des
Lichts auf die Bewegung der Schwarmsporen, Bot. Zeitg. 1878, and Verh. d. phys.-med. Gesellsch. in
Wurzburg, 1879. It appears that the zoogonidia place themselves so that their long axes coincide
with the direction of the incident rays. They move either towards the source of light or away from
it, the direction of their movement being dependent upon a number of conditions, such as the
intensity of the light, the relative temperature of different portions of the water in which the
zoogonidia are, the age of the zoogonidia, and the amount of oxygen in the water. Zoogonidia which
exhibit these phenomena are said, by Strasburger, to be phototactic. Some zoogonidia (such as those
of Saprolegnia) do not appear to be affected by light.]

2 Sachs, Ueber den Einfluss des Tageslichtes auf Neubildung u. Entfaltung verschiedener
Pflanzen-organe, Bot. Zeitg. 1863, Supplement. If I here consider cell-division and growth as
essentially mechanical processes, this does not imply that chemical changes do not also accompany
every process of growth.

ACTION OF LIGHT ON VEGETATION. 753

I material are the bulbs, tubers, rhizomes, parts of the stem, cotyledons, and endo-
sperm; after the complete exhaustion of these growth ceases in the dark but
continues in the light, because the assimilating organs can then produce new
material. This relation of growth which is connected with cell-division to assimi-
lation is especially clear in Algae of simple structure (as Spirogyra, Vaucheria,
Hydrodictyon, Ulothrix, &c.), which assimilate in the day-time under the influence
of light, while cell-division proceeds exclusively or at least chiefly at night. The
swarm-spores are also formed in the night, but swarm only with access of daylight.
In some Fungi also, as Pilobolus cryslalltnus, the splitting up of the protoplasm
in the sporangium into a number of spores takes place only in the night, the spores
being thrown out on access of light. While therefore in the larger and more
highly organised plants assimilation and the construction of new cells out ,of the
assimilated substances is carried on in different parts but at the same time, in small
transparent plants in which the parts where these functions are effected are not
surrounded by opaque envelopes they take place at different times. We have here a
case of division of physiological labour which shows us that the cells which have to
do with chemical work (assimilation) cannot at the same time perform the mecha-
nical labour of cell-division ; the two kinds of labour are distributed in the higher
plants in space ; in very simple plants in time. Provided there is a supply of
assimilated reserve-material, cell-division can therefore take place either in the
light or the dark. Whether there are special cases in which light promotes or
hinders cell-division is not known with certainty. We might suppose we have such
a case when Fern-spores and the gemmae of Marchantia ^ germinate in the light
but not in the dark; but Borodin has shown that the less refrangible rays are
alone active in this process of growth, mixed blue light (passed through ammoniacal
copper oxide) acting like complete darkness. But since the less refrangible rays, as
we have seen, have exactly the same effect on growth as the absence of light, but
on the other hand are the efficient agent in assimilation, it may be supposed that
these spores and gemmae do not contain certain substances necessary for germi-
nation which must therefore be produced by assimilation. On the other hand it
has not yet been explained on what depends the formation in long-continued dark-
ness from many stems (as those of Cactus^ TropcEolum^ Hedera, &c.) of roots which
are not produced under the ordinary amount of light. Whether the degree of
humidity is an element in this is uncertain but not improbable.

When the young organs emerge from the bud-condition, an active growth
commences which is chiefly occasioned by the absorption of water into the cells
and by a corresponding superficial extension of the cell-walls, cell-division still
taking place only occasionally or not at all. This process of expansion takes
place, in the case of aerial stems and foliar structures, in the daylight which
penetrates deep into the transparent succulent tissues. In order to estimate the
amount of its influence on these processes, it is best to grow seedlings or shoots
of the same species of plant in continuous complete darkness, and others under
an alternation of day and night, especially in the height of summer. Independently

^ Borodin, Melanges biol., Tetersburg 1867, vol. VI; Pfeffer, Arbeilen des bot, Inst, in
WUrzburg, vol. I, 1871, p. 80.

3 c

754 GENERAL CONDITIONS OF PLANT-LIFE.

of the fact that the chlorophyll (with the excej)tions already named) does not assume
its green colour in the dark but remains yellow, difterences of form which are often
very striking are exhibited by plants grown in the dark, and constitute the bleached
or etiolated condition. The internodes of etiolated plants are in general much
longer than those of plants of normal growth; and the long narrow leaves of
Monocotyledons are subject to the same change. On the other hand the leaves
of Dicotyledons and Ferns usually (but not always) remain very small and do not
completely outgrow their bud-condition, or exhibit peculiar abnormalities in their
expansion. These peculiarities will be explained more in detail in Chap. IV. It is
not necessary however to contrast etiolated plants with those of the normal green
colour, in order to establish the influence of light on their growth. If plants of the
same species are compared when grown in more or less deep shade with others
grown in full daylight, these differences are still very conspicuous, varying ac-
cording to the intensity of the light. Different species are however affected to
a different extent by etiolation; the internodes of climbing plants, which are very
long even under normal conditions, become much longer still in the dark; and
some leaves of Dicotyledons, as for instance those of the Beet, become tolerably
large under the same circumstances^ while on the other hand the abnormally
elongated internodes of etiolated potato-plants put out leaves of only a very small
size \ It is remarkable that etiolation, as I have already shown 2, does not extend to
the flowers^. As long as sufficient quantities of assimilated material have been
previously accumulated, or are produced by green leaves exposed to the light, flowers
are developed even in continuous deep darkness which are of normal size, form,
and colour, with perfect pollen and fertile ovules, ripening their fruits and producing
seeds capable of germination. The calyx however, which is ordinarily green,
remains yellow or colourless. In order to observe this it is only necessary to
allow tulip-bulbs, the rhizomes of Iris, or the like planted in a pot, to put up shoots
in complete darkness, when perfectly normal flowers are obtained with completely
etiolated leaves. Or a growing bud on a stem of Cucurbita, Tropceolum, IpoirKsa,
&c., with several leaves, is made to pass through a small hole into a dark box,
the leaves which remain outside being exposed to as strong light as possible.
The bud developes in the dark a long colourless shoot with small yellow leaves and
a number of flowers, which, except in the colour of the calyx, are in every respect
normal *. The extremely singular appearance of these abnormal shoots with normal
flowers shows in a striking manner the difference in the influence of light on the
growth of different organs of the same plant.

* [For a discussion of Etiolation, see Godlewski, Zur Kenntniss der Ursachen der Formandeiung
etiolirter Pflanzen, Bot. Zeitg, 1879, where the literature of the subject is quoted,]

2 Sachs, in Bot. Zeitg. 1863, Supplement; and 1S65, p. 117.

3 [An exception to this rule is afforded by the coloured kinds of lilac which are forced during
the months of February and March by the market-gardeners of Paris, at a temperature of from
33° to 35° C, and in almost complete darkness. The flowers expanded under these conditions
are completely white. See Duchartre, Journ. de la Soc. Imp. et cent, d'hort. de France, i860,
pp. 272-280.

* Sometimes however abnormal flowers appear in the dark as well as the normal ones. See
Sachs, l.xp.-Phys p, 35.

ACTION OF LIGHT ON VEGETATION. 755

The retarding effect of light on the growth of the shoot is evident even in
a short time ; and, as I have already briefly shown ^ a periodical oscillation in the
rapidity of growth is caused by the alternation of day and night (when the tem-
perature is nearly constant). This variation is shown by the growing internode
exhibiting a maximum of hourly growth towards sunrise, decreasing gradually
from the advent of daylight till mid-day or afternoon, when it reaches its
minimum, and increasing from this time till morning, when it again attains its
maximum.

Prantl ^ has shown that a similar periodicity exists in the growth of leaves when
day and night alternate normally. The fact that the leaves of the same plants
{Cucurh'/a, Ferdinafida, Nicoliana), when they become etiolated by remaining in
continuous darkness, are much smaller, is in apparent contradiction to this. But
in such a case we have not to do with leaves which are healthy and which are
periodically assimilating, but with sickly leaves which contain no chlorophyll. These
small yellow leaves, developed in darkness, are not exposed to the favourable
influence of light upon which assimilation and its effect upon growth depend in
normal leaves.

One of the best-known phenomena occasioned in plants by light is the fact that
growing stems and leaf-stalks, when the amount of light which they receive is very
different on different sides, bend or become concave towards the side exposed to
the most intense light. This curvature is caused by the slower growth in length
of the illuminated than of the shaded side ; and parts of plants which show this
behaviour to light are called heliotropic^. From the fact of heliotropic curvature
towards the side which receives the most light, it is obvious that the plant would
grow more quickly if shaded on all sides than if the light were more intense.
The observation that leaves, some roots. Fungi, filamentous Algae (like Vau-
cheria), &c., curve heliotropically, indicates that their growth is retarded by light.
That the chlorophyll has no share in causing this heliotropism is shown by the
fact that organs which contain none, like some roots, or Fungi, as the perithecia
of Sordaria fimiseda (according to Woronin), the stipes of the pileus of Clavi-
ceps (according to Duchartre*), and colourless etiolated stems, bend towards a
stronger light. Since most heliotropic parts of plants are highly transparent, the
light which falls on one side must penetrate more or less to the other side, on
which also some light falls ; it follows therefore that even inconsiderable differences
in the intensity of the Hght which falls on the two sides must cause heliotropic
curvature; i.e. difference in the rate of growth ^ If plants which show heliotropic
properties are grown in a box which receives light from one side that has passed
in one case through a solution of potassium bichromate, in another case through

^ Sachs i Heft II of the Arbeiten des Bot. Inst, in Wiirzburg. 1872.

2 Compare infra. Chap. IV. Sect. 20 ; also Arb. d. bot. Inst. Wiirzburg, Heft III.

^ Further details on heliotropism will be given in Chap. IV.

* Diichartre, Compt. rend. 1870; vol. LXX. p. 779.

5 It must however be noted that in the case of parts containing chlorophyll the light in pene-
trating the tissues loses its more refrangible rays which are the only ones that produce the effect ; as
lias been already shown, only the less refrangible rays pass through the superficial layers (see
p. 742).

3 C 2

756 GENERAL CONDITIONS OF PLANT- LIFE.

one of ammoniacal copper oxide, the internodes of the first remain quite straight
and lengthen considerably as if they were in the dark, while those exposed to the
mixed blue light grow less and at the same time bend strongly towards the light.
It follows from this that only rays of high refrangibility, the blue, violet, and
ultra-violet, cause the curvature by retarding growth ^

In addition to the large number of the parts of plants which, when illumi-
nated unequally, bend so as to make the more strongly illuminated side concave,
there are a much smaller number which bend in the opposite direction, i. e. become
concave on the shaded side. In order to distinguish between them the former are
termed positively^ the latter negatively heliotropic *.

Both positive and negative heliotropism occur not only in organs containing
chlorophyll, but also in those that are colourless; among the former in the green
tendrils of Vitis and Ampelopsis^ ; among the latter in the colourless root-hairs
of Marchantia *, the aerial roots of Aroidese, Orchideae, and Chlorophytum Gayanum,
and the rootlets of some Dicotyledons, as Brassica Napus and Sinapis alba^.
From the statement that positive heliotropism depends on a retardation of the
growth of the organ exposed to the stronger light, it might be inferred that negative
heliotropism is occasioned conversely by a more vigorous growth of the side
exposed to the stronger light. This conclusion would be confirmed by a superficial
examination of the phenomena; but if the attendant circumstances are observed
more closely, some considerations arise which I shall examine in detail in Chap.
IV. It need only be mentioned here that according to a theory started by Wolkoff,
two different explanations are possible : — Very transparent organs, like the apices
of the roots of Aroidese and of Chlorophytum, refract the light which falls upon
them in such a manner that the shaded side of the organ may actually be more
strongly illuminated than the other; and its negative heliotropism is then only
a special case of positive heliotropism. But in other cases, as in the Ivy and
Tropceolum majus^ the internodes are positively heliotropic when young, but
negatively when old before growth ceases ; and Wolkoff supposes that the curvature
which is in these cases convex on the illuminated side is caused by the more
vigorous assimilation and consequent longer duration of growth. It depends therefore
upon nutrition which only affects the mechanism of growth in a secondary degree.

{/) Action of Light on the tensibn of the tissue of the contractile organs of leave's

' See Sachs, Bot. Zeitg. 1865, On the action of coloured light on plants, where the literature is
also quoted. I consider experiments with absorbent fluids more decisive than those with the spec-
trum ; in this latter Guillemin states that not only do all the rays act heliotropically, but that there
is even a lateral curvature towards the blue end of the spectruni. When the light is sufficiently
strong the spectrum is certainly never free from diffused white light, which will cause heliotropism
even when its intensity is very small. [Wiesner has found (Heliotropische Erscheinungen, noticed
in detail in Chap. IV) that although the yellow rays do not give rise to heliotropic curvatures,
they exercise, nevertheless, a retarding influence on growth.]

^ [Darwin (Movements of Plants) uses the terms ' heliotropic ' and • apheliotropic ' instead of
• positively ' and ' negatively heliotropic' He considers that the heliotropic movements are modified
forms of circumnutation (see infra).']

3 Knight, Phil. Trans. 181 2, Pt. I. p. 314.

* Pfeffer, Arbeiten des bot. Inst, in Wiirzburg, 1871, Heft I. Div. 2.

^ For the literature on this subject see Sachs, Exp.-Phys, p. 41.

ACTION OF LIGHT ON VEGETATION. 757

endowed with motio7i^. The leaf-blades of Leguminosae, Oxalidese, Maranlaceae,
Marsiliaceae, &c. are borne on modified petioles which serve as contractile organs,
bending upwards or downwards under various external and internal influences,
and thus giving a variety of positions to the leaf-blades. If these plants are
placed in permanent darkness, the curvatures due to internal changes alternate
upwards and downwards. Light exercises an immediate influence on these peri-
odically contractile organs ; any increase of its intensity tends to give the blade an
expanded position, such as it occupies in the day-time ; any diminution tends to
cause it to assume a closed position upwards or downwards such as it has in the
night. This effect, which I formerly termed ' the paratonic action of light,' is not
the cause of the periodic movements; but rather counteracts the periodicity caused
by the internal forces. In most leaves endowed with periodic movements the para-
tonic influence of light is so strong that it neutralises them, and induces in their
place a periodicity dependent on the alternation of day and night. In the lateral
leaflets of the leaves of Desmodiuni gyrans on the contrary the internal causes of the
rapid periodic oscillations are so powerful as to overcome the paratonic action of
light; and these leaflets move upwards and downwards when the temperature is
high even in spite of changes in the amount of light. My earlier researches ^ show
that it is only the more refrangible rays that produce a paratonic effect, while red
rays act like darkness.

The influence of light on the position of the contractile organs is not however
only of this direct character; the motile condition is also indirectly dependent on
it. Both the periodic and paratonic movement, as well as that {Mimosa) due to
mechanical irritation— in fact, the power of movement — is lost when the leaves
have remained in the dark for a considerable time, such as a whole day ; in other
words, they become rigid by long exposure to darkness. From this rigid condition
they do not immediately recover when again exposed to light ; the exposure to light
must continue for a considerable time, some hours or even days, before the motile
condition which I have termed ' Phototonus ' is restored. It is only in this condition
that the leaves are motile and sensitive to changes in the intensity of the light
or to mechanical irritation. The paratonic curvatures of fully developed contractile
organs caused by the action of light are distinguished from the heliotropic curvings
of growing organs by the fact that, firstly, they are connected with phototonus,
while the latter are not; and secondly, that they always take place in a plane
determined by the bilateral structure, while the plane of heliotropic curvature depends
only on the direction of the rays of light.

'\The Chemistry of Chlorophyll. Gautier (Comptes Rendus, 1879, and Bot. Zeitg. 1880)
and Hoppe-Seyler (Ber. deut. chem. Ges. 1879, and Bot. Zeitg. 1879) have succeeded in
obtaining green crystals on the evaporation of an alcoholic solution of chlorophyll.
Gautier considers these crystals to consist of chlorophyll, but Hoppe-Seyler is of opinion
that they are a modification of chlorophyll, to which he gives the name of chlorophyllan.
The following are the analyses : —

^ See Sachs, Ueber voriibergehende Starrezustande, &c., Flora, 1863.— Further details will be
given in Chap. IV. [Also Darwin, Movements of Plants, 1880.]

^ Sachs, Ucber die Bewegungsorgane von Phaseolus und Oxalis, Bot. Zeit. 1857, p. Sii et tcq.

y^^S GENERAL CONDITIONS OF PLANT-LIFE,

Gander.

Hoppe-SeyUr

c.

73*97

73*34

H.

9-80

9*72

N.

4*15

5-68

O.

10*33

9*54

Ash

175

p.

Mg.

1-38
o*34

lOO'OO

lOO'OO

From this percentage Gautier deduces the formula Cjg H22 Nj O3
relation to that of Bilirubin (Gjg H|g Ng O3). It is of interest to note that iron was not
found in either case. The spectrum of Hoppe-Seyler's chlorophyllan is the same as
that of chlorophyll.]

Optical Properties of Chlorophyll. If parts of plants that contain chlorophyll are
repeatedly boiled in water and then quickly dried at a temperature not too high and
pulverised, a substance is obtained which is easily examined and can be preserved for
a long time unchanged. From this powder the green colouring matter can be ex-
tracted by alcohol, ether, or oil. The green solution is speedily changed by the action
of light in proportion to its intensity, the less refrangible rays of the spectrum acting
most actively and rapidly. It then assumes a dirty brownish yellow-green colour, the
green colouring matter having become modified or lost its colour.

If sunlight that has passed through a stratum of the pure green solution not too
thick or too dark is decomposed by a prism, an extremely characteristic spectrum is
obtained in which rays of very various refrangibility appear to have been more strongly
absorbed the darker the solution or the thicker the stratum. This chlorophyll-spectrum
has been the subject of much research ; the most recent and comprehensive being that
of Kraus, from whose description I borrow the following ^ : —

The spectrum of an unchanged alcoholic solution of chlorophyll shows seven
absorption-bands, four of which are narrow (Fig. 476 A, I, II, III, IF), and are situated
in the less refrangible half; while three (F, VI, FII) are broad and are situated in
the more refrangible half. The latter, distinguishable as distinct bands only in very
dilute solutions, coalesce, even in the solutions of medium concentration which are
ordinarily examined, into a single continuous absorption-band occupying the whole of
the more refrangible half of the spectrum.

The bands /, //, ///, and IF are situated in the red, orange, yellow, and yellow-green.
The deep black band I, sharply defined on both sides, lies between Fraunhofer's lines
jB and C; the three others, shaded off on both sides, diminish in strength in the order of
their numbers. Between these bands the illumination is dim, and progressively in the
order of the numbers ; /. e. is less dim between II and /// than between / and II, &c.
To the left of / the light is undiminished.

The bands F, FI, and FII in the more refrangible half of the spectrum are shaded
on both sides; F\s situated to the right of Fraunhofer's line F', FI, which is dark in the
middle, to the left of and on the line G ; FII may be regarded as the total absorption of
the violet end. This spectrum has been found in all observations made on the most
difl^erent plants ; Mono- and Dicotyledons, Ferns, Mosses, and Algae.

The Spectrum of liuing leagues agrees with that of the solution in its main character-
istics ^ The bands I— Fare, according to Kraus, easily made out in all ordinary leaves

' Kraus, Sitzungsb. der phys.-med. Soc. in Eriangen, June 7 and July 10, 1871. See also
Askenasy, Bot. Zeit. 1867, p. 225 ; Gerland und Rauwenhoff, Archives neerlandaises, vol. VI, 1871 ;
and Gerland, Pogg. Ann. 1871, p. 585. [Kraus, Zur Kenntniss der Chlorophyllfaibstoffe u. ihrer
Verwandten : Stuttgart, 1872. For reference to Mr. Sorby's papers see Sect, 8 a.]

^ For further evidence of this very remarkable fact see Gerland und Rauwenhoff, /. c, p. 604.
It is not easy to understand how certain physicists can maintain the contrary. [Pringsheim points

ACTION OF LIGHT ON VEGETATION.

759

of Dicotyledons, Monocotyledons, and Ferns. But this spectrum differs constantly from
that of the solution in all its bands being always nearer the red end ; a point which was
determined by Kraus by the use of Browning's micro-spectroscopic apparatus. This
difference in position of the absorption-bands of the spectrum is, as he shows, an
illustration of the universal rule that the absorption-bands approach nearer to the
red end in proportion to the specific gravity of the solvent of the colouring substance.
It follows from this that the green colouring matter is distributed in such a manner
in the colourless matrix of the chlorophyll-granules that it must be considered in a state
of- solution. In no case can the colouring matter of chlorophyll in living cells be in a
solid state, or equivalent to the residue left behind when the solution is evaporated.

If an alcoholic solution of chlorophyll is agitated with any quantity of benzol (say
double its volume) two very sharply separated strata are formed after the fluid comes
to rest, a lower alcoholic stratum of a pure yellow colour, and an upper blue-green
stratum of benzol. Kraus considers this process to be a dialytic one ; there are, ac-

I E a IV

Fig. 476.— Absorption-spectra of the colouring matter of chlorophyll (after Kraus). A the spectrum of the alcoholic
extract of green leaves ; B that of the blue-green constituent soluble in benzol ; C that of the yellow constituent. The
absorption-bands of A and B are indicated in the less refranjiible (left-hand) portion as they would be produced by a
more concentrated, in the more refrangible (right-hand) portion of the spectrum as they would be produced by a less
concentrated solution; the letters B—G indicate the well-known Fraunhofer lines of the spectrum ; the figs. [—Vll Kraus's
absorption-bands in succession from the red to the violet end ; the spectra are divided into twenty equal parts

cording to him, two colouring substances in the ordinary chlorophyll-solution, a blue-
green and a yellow one, soluble in very different degrees in alcohol and benzol.

Kraus therefore holds the spectrum of chlorophyll to be a combination-spectrum,
/. e. that it arises from the superposition of the two spectra of the blue-green and
the yellow colouring-matter. The blue-green substance gives the four narrow absorp-
tion-bands in the less refrangible half of the spectrum (Fig. 476, 5), and part of the
band T/ which is situated at G in the more refrangible half. The band r(Fig. 476 C)
results from the yellow colouring matter which has absorption-bands in the more re-
frangible half of the spectrum. The band VI of the chlorophyll-spectrum is the result
of partial superposition of corresponding bands in the spectra of the yellow and the
blue-green substances, which however do not perfectly coincide. Both colouring sub-
stances alike produce the absorption-band VII at the violet end.

According to Kraus' statements it is possible to decompose chlorophyll into two

out (Ueb. Lichtwirkiing und Chlorophyllfunction, Jahrb. f. wiss. Bot. XII, 1881) that the chlorophyll-
spectrum is the ifame in leaves which are assimilating as in those which are not.]

760 GENERAL CONDITIONS OF PLANT-LIFE,

distinct colouring matters by a simple dialytic process. Conrad has shown, however
(P^lora, 1872, p. 396), that if a solution of chlorophyll in absolute alcohol be treated with
benzol, a separation of the green and the yellow never occurs. This only takes place
when dilute alcohol, of a strength less than 65 per cent., is used. Conrad points out
forcibly that Kraus used dilute alcohol, which may be at once*' inferred from the fact
that he extracted the boiled leaves with alcohol without having previously dried them.
According to Conrad, it is very doubtful if this decomposition of the chlorophyll is
simply a dialytic phenomenon. More probably a decomposition had previously been
effected by the water, a suggestion which is supported by the fact that solutions of
chlorophyll in dilute and in absolute alcohol when evaporated give in the former case,
but not in the latter, a residue containing a yellow colouring-matter soluble in water ^.

The yellow colouring matter is soluble in alcohol, ether, and chloroform, but not
in water. On addition of hydrochloric or sulphuric acid (as Micheli had already shown)
it becomes first emerald-green, then verdigris-green, and finally indigo-blue ; the
spectrum of the yellow substance which has in this manner become green shows al-
together different absorption-phenomena to those of chlorophyll. The spectrum of the
yellow ingredient of chlorophyll is identical (Kraus) with that of most yellow flowers (as
Ranunculus, Mimulus, Gentiana lutea, Brassica, Taraxacum, Matricaria, &c.), and agrees
with it also in the reactions just named, as also does that of the yellow colouring
substance of fruits and seeds {Euonymus, Solanum Pseudccapsicum, &c.). This yellow
substance is, like chlorophyll, combined with protoplasm. The substance present in
the cells in the liquid form, as for instance in the flowers of the Dahlia, is different ;
it is soluble in water, and does not give a spectrum consisting of bands, but a continuous
absorption of the blue and the violet. The colouring substance of some orange flowers,
e.g. Eschscholtzia, also soluble in alcohol, is again different, possessing a fourth band in
the blue-green to the left of the three bands of the ordinary yellow substance. The
colouring matters of bright-coloured lower organisms which are soluble in alcohol are
not identical with either of the two which constitute chlorophyll, but are related to them.

According to Kraus, the yellow substance of etiolated leaves also exactly resembles
the yellow constituent of chlorophyll.

The Fluorescence of the colouring-matter of chlorophyll is seen from the fact that
a sufficiently dark concentrated solution appears dark-red by reflected but green by
transmitted light. The fluorescence is much more decided if the pencil of converging
rays of the sun is made to fall on the green fluid through a condensing lens. If the
solar spectrum is thrown upon the surface of a solution of chlorophyll^, it may be
ascertained which rays of the sunlight cause the fluorescence ; the red begins a little
to the left of the line B of the solar spectrum, and stretches, although varying in inten-
sity, over the violet end. On the dark-red ground are seen seven intensely red bands,
each corresponding exactly both in position and in strength to an absorption-band
in the spectrum of chlorophyll. If the fluorescence caused by the solution of chloro-
phyll is itself observed through a prism, it is seen to consist only of red rays, the
refrangibility of which coincides with the strongest absorption-band of chlorophyll
between B and C Every ray produces by fluorescence only such as correspond in
their refrangibility to the absorption-band /. Whether the chlorophyll contained in
living cells is subject to the same fluorescence is not certain, from the imperfect

^ [Pringsheim (Ueb. d. Asorptionsspectra der Chlorophyllfarbstoffe, Monatsber. d. k. Akad, d.
Wis?, zu Berlin, 1874) confirms Conrad's observations. He shows that the separation into two
layers depends upon the fact that benzol will not mix with weak alcohol, but will do so with strong.
He also points out that under any circumstances some of the benzol is retained in solution in the
alcohol, and further that benzol will dissolve more chlorophyll than alcohol and will therefore
become more deeply coloured. He maintains that the yellow colour of the alcohol is due to the
presence of some chlorophyll in it, as shown by the spectroscope, though some yellow colouring-
matter (etiolin or xanthophyll) is also present.]

^ Hagenbach, Pogg. Ann. vol. 141. p. 245 ; Lommel, ih. vol. 143. p. 572.

ACTION OF LIGHT ON VEGETATION. 761

observations at present made ; but it is probable, from the absorption-phenomena and
their connection with fluorescence.

The question whether the absorption-bands of the spectrum of the colouring-
matter of chlorophyll have any causal connection with the function of the chlorophyll-
granules in decomposing carbon dioxide has recently been answered by Lommel in the
affirmative, on purely theoretical grounds, in support of which he brings forward the
following statements ^ : —

* The most efficacious rays in promoting assimilation in plants are those which
are most strongly absorbed by chlorophyll, and which at the same time possess a
high mechanical intensity (heat-action) ; these are the red rays between B and C
But a glance at the carefully prepared tables given at pp. 745-6, shows that this
theoretical reasoning is incorrect. If Lommel's hypothesis were correct, the evo-
lution of oxygen would be seen, on observing the solar spectrum, to attain its
maximum between B and C^, which however, as Pfeffer has shown, is by no means
the case. The second of Lommel's statements is : — ' The yellow rays can produce
only a small effect notwithstanding their considerable mechanical intensity, because
they are absorbed only to a small extent ; and the same is the case with the orange
and green rays.' This statement is again entirely opposed to observation ; for it
is these very rays that are the most efficacious in promoting evolution of oxygen.
Lommel says indeed (/. c. p. 584) that Uhis inference is incorrect;' it is however no
inference, but the result of actual observation. That the light which has passed through
a solution of chlorophyll causes only an inconsiderable evolution of oxygen is easily
explained when it is recollected that even the yellow is considerably weakened in
the spectrum of chlorophyll. But according to Lommel's theory there ought to be
no evolution of oxygen at all when light has passed through a solution of this kind
if it shows the absorption-bands very dark, since those rays which according to him
are alone efficacious are wanting.

There is however no need for this direct contradiction; for a correct estimate
of known facts leads to the conclusion that it cannot be .those rays which are ab-
sorbed by the colouring matter of chlorophyll that cause the evolution of oxygen;
for the rays absorbed in such a solution are the same as those absorbed in a green
leaf. In the former there is however no evolution of oxygen (and apparently also no
oxidation) ; and there is nothing to justify the supposition that the same rays which
are absorbed by chlorophyll in solution without causing evolution of oxygen should cause
it in the living leaf. It must certainly be right to suppose, as a necessary result of the
principle of the conservation of energy^, that the rays which are efficacious in causing
evolution of oxygen must be absorbed, inasmuch as they perform chemical work; but

^ Lommel, Pogg. Ann. vol, 143. p. 581 e( seq.

^ Miiller (Botan. Beobachtungen, Heidelberg 1871, Heft I) has adduced a great array of
figures in support of this conclusion. But any one who knows how such observations should be
made knows also what value is to be attached to these. See also Pfeffer, Bot. Zeit. 1872, No. 23
et seq.

^ See also what I said on this subject long ago in my Experimental Physiology, p. 287.
[Pringsheim has made a series of researches on Chlorophyll (Monatsber. d. k. Akad. zu Berlin,
1874-1881 ; Jahrb. f. wiss. Bot. XII, 1881 ; see also Nature, vols. XXI, XXIII, and Quart. Journ.
Micr. Sci. 1882), and has come to the following conclusions with respect to its function: (i) that
the rays absorbed by it are not those which promote assimilation ; (2) that, on the contrary, these
rays promote the respiration of the protoplasm ; (3) that it is the protoplasm of the chlorophyll-
granule, and not the chlorophyll, which decomposes the absorbed carbonic acid and forms organic
compounds; (4) that the energy for this purpose is derived from light, but it has not yet been
ascertained which rays are absorbed by the protoplasm ; (5) that the function of the chlorophyll
in the process is protective, that is, that it absorbs the rays which would promote respiration in
the chlorophyll-granule and this renders it possible for the synthetical processes to take place
within it.]

'j6% GENERAL CONDITIONS OF PLANT-LIFE.

observation shows that it is not the rays absorbed by the green colouring matter that
perform this woric either in the solution or in the living plant *.

The Relation of Cell-division to Light has, as I have already explained, been completely
misunderstood by Famintzin. In my paper ' On the influence of daylight on the
formation and unfolding of various organs of plants' (Bot. Zeit. 1863, Supplement)
I described in detail a long series of phenomena which show that the fresh formation
of parts connected with cell-division is in general independent of light as long as
there is a supply of reserve food-material to support growth. The main results were
again collected in my 'Handbook of Experimental Physiology,' p. 31, referring also
to that paper. Notwithstanding this, Famintzin ^ commences his paper quoted above
(three years later than one, and five than the other of my works) with the words :
'The action of light on cell-division has not yet been carefully examined by any
one. All that I have been able to find on this subject is limited to a remark of
A. Braun's on Spirogyra and a statement of Sachs relating to cell-division in general.'
He then quotes a passage from Braun cited also by me, and continues : — ' Basing his
remarks on these statements, Sachs expresses himself as follows,' and then quotes
some passages from my Handbook, p. 31, no reference being made to the earlier
paper or its conclusions. He then maintains that his own observations lead to entirely
different results ; but it is easy to show that they rather lead to the same as mine.
At the end of his memoir (p. 28) he says: — 'The cell-division of Spirogyra is not
prevented by light, as has hitherto been supposed, but on the contrary is promoted
by it ' (which is incorrect). According to Famintzin's observations, this acceleration of
cell-division by light depends on the fact that light induces the assimilation of food-
material ; which is obviously a different question from that argued by me and opposed
by him ; since, presupposing the presence of a supply of food-material, I only argued
the question whether light exerts any influence on the physical fact of cell-division.

' The cell-division of Spirogyra,' continues Famintzin, ' has been proved to be de-
pendent on light to the same extent as the formation of starch ; but the relationship
in the former case diff'ers from that in the latter in the following respect : — the formation
of starch is induced by a very brief exposure to light (about half an hour) and requires
that its action be direct; starch is formed only under the influence of light; in its
absence the formation at once ceases. Cell-division, on the other hand, is induced
only after light has acted for some hours ; it then commences in the cells whether
these are exposed to light for a longer time or are removed into the dark.' This
shows therefore that when food-materials are formed cell-division takes place in the
light as in the dark ; a fact which I had proved five years before by a greater number
of observations.

Better in more than one respect is Batalin's treatise ' On the action of light on
the development of leaves' (1871)^ Starting from the facts discovered by himself
and by Kraus that cells have the same size in small etiolated leaves as in large
leaves of the same species grown in light, he concludes with justice that the number
of cells is larger in the normal than in the etiolated leaf, and that the size of leaves
is proportional to the number of cells in them. But from this he draws the following
erroneous conclusion : — ' The leaf grows so long as it produces new cells ; and the
growth of the leaf does not depend on the increase in size of the cells.' It should
rather be, — * The growth of the leaf depends firstly and directly solely on the increase
in size of the cells, and is proportional to this ; but the cells, when they have grown
larger, divide so that they are actually of about the same size in the small etiolated
as in the large green leaf.' He continues : — ' Leaves do not grow in the dark because

* Gerland (/. c. p. 609) has also arrived at a similar conclusion.

^ Famintzin, Melanges phys. et chim., Petersbourg iSdS, vol. VII, On the action of light on
the cell-division of Spirogyra.

'^ Batalin, Bot. Zeit. 1871, p. 670.

ACTION OF LIGHT ON VEGETATION, J 6^

their cells cannot divide without the assistance of h'ght;' while the exact converse is
the fact, — they do not divide because they do not grow. This error prevails throughout
the whole treatise, which in other respects contains a number of instructive observa-
tions. Moreover, Prantl's measurements show that even in small etiolated leaves
{Phaseolus) numerous cell-divisions take place ^

It must be observed in addition that the very small growth of leaves in the dark
is not a universal phenomenon even amongst Dicotyledons. The leaves produced
from the tuberous roots of the Dahlia and Beet grown in the dark, and even those
of Phaseolus^ attain very considerable dimensions, and sometimes, especially when the
temperature is high, almost the size of those developed in the lights

Contrinjances for obserinng plants in light of different colours (or of different refrangi-
bility). In order to allow light of different degrees of refrangibility to act upon plants,
three methods may be adopted: — (i) The use of the spectrum; (2) The removal of
particular rays by absorbent media (glass or fluids) ; and (3) Coloured flames.

(i) If a ray of light is decomposed by passing it through a prism, it is possible
to expose small plants or parts of plants to the action of narrow zones of the spec-
trum ; and hence to allow light of approximately equal refrangibility to act upon
them. Draper, Gardner^, Guillemin, and Pfeff'er have worked in this manner. In
using the spectrum it must however be observed that the intensity of the light in
its different parts is less than that of the light that passes through the slit in pro-
portion to the breadth of each part. If the spectrum at the distance from the
prism where the observation is made is, for instance, 200 mm. long, but the slit only
I mm. broad, the mean intensity of light of the whole spectrum is only ^200 of that
which passes through the slit, even if no light is otherwise lost, which is seldom the
case. Only a small luminous intensity must therefore be expected in the spectrum.
In order to obviate this difficulty, it is necessary that very intense light pass through
the slit, which may be eff'ected by the use of condensing lenses. If, as is usually the
case, sunlight is employed, the ray to be decomposed must be kept in a fixed position
by a heliostat, or at least by a moveable mirror.

(2) Absorbent media. The defects which have been mentioned in observations
with the spectrum, as well as the considerable cost of a heliostat, are avoided when
coloured light is obtained by means of absorbent media. For this purpose discs of
coloured glass or strata of fluids enclosed between colourless glass plates may be
used. These last possess the advantage that almost any required amount of space
may be illuminated by the light in question, and that the transmitted light only
loses so much in intensity as is due to the small amount of absorption of the
transmitted rays by the coloured medium. It is a mistake, though a very common
one, to think that observations made with coloured screens are less exact than those
made with the spectrum ; in general it is just the reverse ; and which method should
have the preference must be decided in each particular case.

The use of absorbent media is always subject to the disadvantage that they do
not generally transmit light of a single colour, but several different kinds of rays.
This disadvantage is especially the case with coloured glass plates; and, with the
exception of the deep red ruby and the very dark blue cobalt glass, there are scarcely
any kinds which answer our purpose. It is more practicable to obtain coloured fluids
of the desired quality, although here also the number that can be used is small.
The two which have been already mentioned are particularly useful, inx. a saturated
solution of potassium bichromate, and a dark solution of ammoniacal copper oxide ; by
means of these, with the right concentration and thickness of the stratum, experiments

1 Arb. des hot. Instit. Wiirzburg, 1873, Heft III. p. 384.
"^ See infra. Sect. 20.

^ Gardner, Froriep'a Nolizen, 1844, vol. 30. No. ii. — Guillemin, Ann. des Sci. Nat. 1857, vol.
VII. p. 160.

764 GENERAL CONDITIONS OF PLANT^LIPE.

can be contrived so as to split white daylight exactly into two halves, the first
solution transmitting the less refrangible rays from the red to the green, the blue
solution all the more refrangible rays from the green to the ultra-violet. Those
fluids also are of great use which transmit the whole spectrum with the exception
of a few groups of rays as sharply limited as possible. If certain phenomena occur
when plants are exposed to light transmitted through these solutions, it is. certain
that they are not caused by rays of that particular refrangibility which are absent,
and -vice -versa. It is obvious that absorbent media are of use in experiments only
when the spectrum of the light that passes through them is accurately known.
Glass plates are employed as windows in dark boxes closed on all sides in which plants
are placed ; coloured fluids can also be employed for the same purpose by placing
them in glass vessels with parallel sides and using these as a window. When it is not
necessary to allow light to fall in parallel rays upon the plant, the most convenient
use of coloured fluids is to fill with them the space between the two walls of a
double glass bell which is then placed like an ordinary bell-glass over the plants to
be observed ^

For microscopic observations in coloured light I employ boxes like that represented
in Fig. 474; only that instead of the colourless plate of glass, a double window is used,
the space between the two panes being filled with coloured fluids.

(3) Coloured Flames— i.e. the light of bodies in a finely divided state heated to
incandescence in a flame which is itself non-luminous — have not hitherto been employed
for accurate observations on plants. I know only of one statement by Wolkoff - ;
that etiolated seedlings of Lepidium satimum became green when placed for seven or
eight hours at eight inches distance from a non-luminous gas-flame in which sodium
carbonate had volatilised and become incandescent. This light, as is well known,
consists only of rays which correspond to Fraunhofer's line D. The red light of
the flame of lithium or the blue light of that of indium &c. may be employed in
the same manner as this yellow flame, if sufficient intensity and the necessary perma-
nence can be attained with these flames.

[The foregoing account would be incomplete without some statement of the results
attained on this subject by Mr. H. C. Sorby. The following is a brief abstract, sup-
plied by him, of investigations which will be found reported in detail in his published
papers^ : —

Vegetable colouring-matters may be divided into two principal classes, fundamental
and accidental. The fundamental are those which are essential to the healthy growth of
the plant ; and by carefully studying the position of the absorption-bands in living leaves
these substances are often found in a free and solid state, even when they are soluble
in water, or could easily combine with the closely associated oils or wax. When set
free by boiling in water or by decomposition, they dissolve according to their properties
in this respect in water, or combine with oil or wax if these be present. The petals and
other portions of the organs of reproduction often contain some of the fundamental
colouring-matters of the leaves, but frequently others are developed.

Accidental colouring-matters are those which may be present or absent without
apparently interfering with the healthy growth of the individual plant, and are often so
conspicuous as to make mere colour of very little importance if it depend upon them,
and not on the difference in the kind or relative proportion of the fundamental colouring-

^ Such double-walled bell-jars can be obtained from Warmbrunn and Quilitz, Berlin.

2 Wolkoff, Jahrb. fiir wiss. Bot. 1866, vol. V. p. 11.

3 [Proceedings of the Royal Society, vol. XV. 1867, p. 433. — Quarterly Journal of Microscopical
Science, vol. IX. 1869, p. 358; vol. XI. 1871, p. 215. — Monthly Microscopical Journal, vol. III.
1870, p. 229; vol. VI. 1871, p. 124. — Proceedings of the Royal Society, vol. XXI. 1873, p. 442.
Researches on the spectrum-analysis of the green colouring-matter of plants are given by Chautard
in Ann. de Chim. et de Phybique, Sept. 1874.]

ACTION OF LIGHT ON VEGETATION.

'65

r

matters. These non-essential substances are far more common in the petals than in the
leaves, and if of any use to the plant, are only indirectly advantageous, as, for instance,
in attracting insects. It is doubtful to which of these two divisions certain substances
should be referred, and perhaps some may not be essential for the healthy performance
of vital functions, but merely necessary products ; and some may be essential to one
plant and not to others.

It has been found convenient to arrange the colouring-matters of plants in the
following groups, which are as it were of generic value, and include several different
species.

Chlorophyll group. — The green substance described as chlorophyll by many writers
must often have contained two perfectly distinct green substances, and the product of
the action of acids on one of them, mixed with one, and in some cases with three,
different species of xanthophyll, and one or two of lichnoxanthine. These two green
substances are l?lue chlorophyll and yellow chlorophyll ^ Blue chlorophyll dissolved in
alcohol is of a splendid blue-green colour, the whole of the green part of the spectrum
and a considerable part of the contiguous blue being readily transmitted. Tellonv
chlorophyll absorbs the whole of the blue and the blue end of the green, so that the
general colour is a bright yellow-green. Chlorofucine is of a clear yellow-green colour.

Red end
Blue chlorophyll ....

Yellow chlorophyll . .

Blue end.

Chlorofucine

Fig. 476 iJ.— Spectra of the chlorophyll group compared.

It has many properties in common with the above-named two kinds of chlorophyll,
being, like both of them, highly fluorescent and easily decomposed into another modifi-
cation by acids. All three are insoluble in water and soluble in absolute alcohol, but not
always in carbon bisulphide.

The difference between their spectra will be better understood by means of the
figure, 476 b, which represents the absorption-bands as seen in solutions diluted so as
to show those at the blue end, and only the darkest and most characteristic of those
in the red.

Xanthophyll group, — This group includes a number of yellow or orange-coloured
substances, insoluble in water but soluble in carbon bisulphide, giving spectra with two
more or less well-marked absorption-bands in different positions, according to the
particular species. They are not fluorescent, and when dissolved in absolute alcohol,
after addition of a little hydrochloric acid, they all gradually become colourless, but two
of them are first changed into a blue substance. Nearly all green leaves contain three
perfectly distinct fundamental species, which Mr. Sorby has named orange xanthophyll^
xanthophyll, Sindyellow xanthophyll. The spectrum given in Fig. 476, copied from Kraus,

^ [The spectrum given by Kraus (Fig. 476 B) is due to a mixture of these with some of
the products of the action of acids. See Pringsheim, Ueb. natiirliche Chlorophyllmodificationen,
Monatsber. d. k. Akad. d. wiss. zu Berlin, 1876.]

^(^6

GENERAL CONDITIONS OF PLANT-LIFE.

must have been clue to a mixture of the latter two. Olive Algae contain another
fundamental species, fucoxanthine. In many Fungi, and in the petals of flowers, occur
other more orange-coloured species, of which that in Pe%i%a aurantiaca is a good
example. Sorby adopted the name proposed by Kraus^ for a still more red orange-
coloured species; but what Kraus describes as phycoxanthine must have been a mixture
of this substance with fucoxanthine and lichnoxanthine. The difference between the
spectra of some of the above-named species will be better understood by means of the
following figure (476 r), which represents those of the solutions in carbon bisulphide.

When these various substances are dissolved in benzol, their absorption-bands are all
equally raised towards the blue end, so that we appear to have a remarkable series of
very closely related substances.

Lichnoxanthine group. — The colouring-matters belonging to this division are insoluble
in water, soluble in absolute alcohol, and sometimes also in carbon bisulphide. They all
give spectra without bands, and absorb more or less from the blue end. Some are
yellow, and others so red that they may be called lichnoerythrines. Lichnoxanthine

Red end.

Bine end.

Phycoxanthine

liiilr "^' '''

1 i

1

: J|| ,1 ■

1 1 \

1

ilit sM

Peziza xanthine . .

Orange xanthophyll

Xanthophyll

Yellow xanthopliyll . .

Fig. 476 c— Spectra of the xanthophyll group compared.

occurs in both the highest and lowest classes of plants, but the whole group is more
especially developed in Lichens and Fungi. It is not yet possible to say what part they
play in the economy of plants, and in some cases they are probably only products of the
oxidisation of chlorophyll and resins, from which they may be prepared artificially.

We now come to a number of different groups, soluble in water but insoluble in
carbon bisulphide.

Phycocyan and Phycoerythrine groups. — There are at least five distinct colouring-
matters included in these two groups, which differ from one another in many well-marked
particulars. The phycocyans are highly fluorescent, but the phycoerythrines little if at
all. They give remarkable spectra with one main absorption-band. Some are con-
nected with albuminous substances in much the same manner as the haemoglobin of
blood, being like it decomposed at exactly the same temperature as that at which
albumen coagulates, whilst the others appear to be associated with some different but
related substance. They are especially characteristic of red Algae, but also occur in a
few Lichens.

* Chlorophyllfarbstoffe, p. 109,

ACTION OF LIGHT ON VEGETATION. 767

Erythrophyll group. — The colouring- matters belonging to this group are very
numerous, and their production often depends upon obscure and accidental causes,
easily modified by slight variations in the internal or external conditions. They may be
divided into three well-marked sub-groups, according as they are changed by the action
of sodium sulphite. They are soluble in water, and are usually, if not always, dissolved
in the juices of the plant, and disseminated in cells of various kinds. A greater number
of different species occur in the petals than in the leaves. They are usually indicative
of low constructive energy, but yet are not merely products of chemical decomposition.

Chrysotann'in group. — Much remains to be learned with respect to these more or less
pale yellow or even colourless substances, and the part they play in plant-life. The most
striking fact connected with them is that when oxidised they give rise to the various
brown substances which are the cause of many of the characteristic tints of autumnal
foliage. These changes are mainly, if not entirely, due to chemical action, and can
easily be imitated artificially.

Exposure to a greater or less degree of light may produce a great quantitative or
even qualitative difference in the colouring matters. Rudimentary petals and rudi-
mentary leaves correspond closely, but subsequently development takes place in two
different directions; and very often when the petals of the more highly developed
varieties are only partially grown, the constituent colouring-matters are both qualitatively
and quantitatively the same as those in some other variety, as though this were due
simply to a natural arrest of development. By growing almost in the dark flowers
coloured by more or less of the orange species of the xanthophyll group, the petals are
obtained of the full size, but only yellow and corresponding exactly to the normally
yellow variety; and there is this remarkable peculiarity, that the relative proportion
between the different colouring-matters approximates more or less closely to what is
obtained by exposing to light a solution of those found in the normal petals ; that is
to say, absence of light tends to prevent the formation in the petals of those more
orange-coloured substances which are the most readily decomposed by exposure to
light when they are dissolved out from the petals.]^

^ [The occasional occurrence of ' chlorophylloid green colouring-matters ' in the tissues of
animals is a matter of considerable significance. Mr. E. R. Lankester has obligingly drawn up the
following list of such cases. Those marked with an asterisk have been observed by him with the
spectroscope for the first time : — Infusoria ; Stentor Mulleri and others. Foraminifera. Radiolaria ;
Rhaphidiophrys viridis, Heterophrys myriapoda (Quart. Journ. Micr. Sc. 1 869). Ccelenterata ; * Spon-
gilla fluviatilis (Journ. Anat. and Phys. 1869), * Hydra viridis, Anthea cereus var. smaragdina (chloro-
fucine). Vermes; Mesostomum viride (Planarise), ^ Bone Ilia viridis (in the skin), *^ Chcetopierus
Valenciennesii (in the walls of the alimentary canal). Crustacea; ^Idoiea viridis (Isopoda). The
chlorophylloid substance is not present in the same physical or chemical condition in all these cases.
In Rhaphidiophrys, Heterophrys, Spongilla, and Hydra, it is localised in granules imbedded in the
protoplasm ; this is also the case in Bonellia, but the granules are finer. In Idotea it is not in
granules but diffused in the chitino -calcareous integument. In all cases the chlorophylloid substance
agrees in having a strong absorption-band in the red— a little to the right or left ; and, except in
Idotea, in being soluble in alcohol ; and in having strong red fluorescence and in finally losing its
colour when dissolved. In Bonellia, Chcetopterjis, and Spongilla, the absorption-spectrum presents
differences in other respects in each case, and the green tint is itself different— being black olive-
green in Chcetopterns, bluer but equally dark in Bonellia, and apple-green in Spongilla and Idotea.
In Spongilla the green colour is not developed if the animal grows in the dark. But like etiolated
vegetable tissues, Spongilla, when immersed in strong sulphuric acid, gradually developes a strong
leaf-green colour, fully as intense as that of the naturally green specimens (Quart. Journ. Micr. Sc.
1874, p. 400). Bonellia, on the other hand, always lives in a dark hole excavated by it in calcareous
rock, and Chcetopterns lives in a thick opaque tube. Geddes has found that a green Planarian {Con-
voluta Schultzii) decomposes carbonic acid under the influence of light, oxygen being evolved and
starch formed in the cells.]

768 GENERAL CONDITIONS -OF PLANT-LIFE.

Sect. 9. — Electricity'. The chemical processes within the cells of a plant,
the molecular movements connected with the growth of the cell-wall and protoplasm,
and the internal changes on which the activity of the protoplasm depends — whether
exhibited in the formation of new cells or in movements of rotation — are probably
connected with disturbances of the electrical equilibrium, although no actual em-
pirical proof of this has yet been obtained. The fluids with different chemical pro-
perties in adjoining cells, the diffusion of salts and of assimilated compounds from
cell to cell, and their decomposition, must also bring electromotive forces into
play ; but even this has not yet been observed directly. Even the electrical currents
which must no doubt be set up by the evolution of oxygen from cells containing
chlorophyll, by the formation of carbon dioxide in growing organs (as in seed-
lings), and by the transpiration of land-plants — although investigated by a few
physicists— has not yet been actually established or accurately determined. Accord-
ing to Buff's careful observations, which have been confirmed by Jiirgensen and
Heidenhain, the internal tissue of land-plants is always electro-negative to its
strongly cuticularised surface ; the surface of roots, saturated with sap (like a trans-
verse section of the tissue), is also electro-negative to the surface of the stems and
leaves. If a plant or a cut part of a plant is placed, with the necessary precautions,
in the circuit of a very sensitive galvanometer, a current passes from the external
surface to the cut surface or to the surface of the root ; this is in consequence of
the contact of the cell-sap of the surface of the root or of a cut surface with the pure
water employed to complete the circuit. The alkaline fluids of the thin-walled
phloem of the fibro-vascular bundles are surrounded by the acid fluids of the paren-
chyma, and diffusion-currents doubtless exist between them. These must certainly
produce electromotive effects, but hitherto no investigations have been made on this
subject ^

The leaves and branches of plants present a large surface to the air; and
the tissue of the whole plant is permeated with electrolytic fluids. These con-
ditions appear to adapt plants to be the medium for equalising electrical differences
between the earth and air by means of currents traversing the plant. Since the
electrical tension of the air is generally different from that of the earth, and the
relationship of the two is constantly varying with changes of weather, it may be
assumed that in all probability constant electrical interchanges are going on through
the agency of plants^. Whether these have a favourable effect on the processes of

^ Villari, Pogg. Ann. 1868, vol. 133. p. 425. — Jiirgensen, Studien des phys. Inst, zu Breslau, 1861 ;
Heft I. p. 38 et seq. — Heidenhain, ditto 1863, Heft 2. p. 65. — Briicke, Sitzungsb. der Wien. Akad.
1862. vol. 46. p. I. — Max Schultze, Das Protoplasma der Rhizopoden; Leipzig 1863, p. 44. — Kiihne,
Untersuchungen liber das Protoplasma, 1864, p. 96. — Cohn, Jahresber. der schles. Ges. fiir vater-
landische Cultur, 1861 ; Heft i. p. 24. — Kabsch, Bot. Zeit. 1861, p. 358. — Riess, Pogg. Ann. vol. 69.
p. 288. — Buff, Ann. der Chem u. Pharm. 1854, vol. 89. p. 80 et seq. — [J. Ranke, Untersuchungen
iiber Pflanzenelektricitat, Akad. der "Wissen. Miinchen, Math.-Phys. Klasse, July 6, 1872. — Kunkel,
ueb. elektromotorische Wirkungen an unverletzten Pflanzentheilen, and Ueb. einige Eigenthiimlich-
keiten des electrischen Leitungsvermogens lebender Pflanzentheile, Arb. d. bot. Inst, in Wiirzburg,
II. I, 2, 1878-9.

^ Sachs, Ueber saure, alkalinische, und neutrale Reaction der Safte lebender Pflanzen ; Bot. Zeit.
1862, No, 33.

^ [Becquerel thought that the evaporation from leaves forms an upward current of vapour which

ELECTRICITY.

769

vegetation has at present, like the whole subject, not been investigated scientifically.
The destructive discharges of atmospheric electricity which are effected through
trees by means of flashes of lightning \ at least show that smaller differences of
electrical equilibrium between the air and the earth may also be equalised by means
of plants 2.

The researches on the action of the electric stimulus on the movements of
protoplasm and of leaves the motion of which is caused by tension of the tissues
have not at present led to any important result from a physiological point of
view, although distinguished observers have paid attention to this subject. It can
only be said in a general way that very weak constant currents or induction-shocks
(for a short time) produce no perceptible effect ; that sufficiently strong electromotive
force produces effects on the protoplasm and in the contractile tissues similar to
those produced by a high temperature and by mechanical means ; and that finally,
when the strength of the current is still further increased, the protoplasm is killed
and the motility of the leaves permanently destroyed, but sometimes in the latter
case without causing death.

Jiirgensen allowed the current from a battery of small Grove's elements, the force
of which was regulated by a rheochord, to act under the microscope on the tissue of a
leaf of Vallisneria spiralis. A constant current from one element produced no perceptible
action ; two or four elements caused a retardation of the protoplasmic movement, and
when continued for a longer time completely stopped it. When the current was
interrupted, the movement, if it had only been retarded, was restored to its original
rapidity after the lapse of a short time ; if it had entirely ceased, it was not recom-
menced even if the current was at once stopped. When the movement is thus arrested,

acted as a conductor to electricity. In this way, by destroying the necessary electrical conditions,
he thought forests tended to dissipate hail-clouds. Mem. de I'lnst. vol. XXXV. pp. 806, 807.]

^ [The disruptive effect of lightning upon trees is probably due to the sudden conversion of
moisture into steam. See Osborne Reynolds, Proc. Phil. Soc. Manch. 1874, P- ^5-]

^ [Edwin Smith (Chemical News, Dec. 17, 1869) has detected constant currents of electricity
passing in certain directions in plants, as follows : — In a cut piece of leaf-stalk (Rhubarb) from the
end nearest the root to the end nearest the blade of the leaf; from the outer side of the leaf-stalk
nearest the cuticle to the inner axis ; from the lower end of the flower-stalk (Pseony) to the bract or
petal ; from the upper to the under surface of the leaf; in the stem (Hawthorn) from the cambium to
the outer cuticle ; in the root (several plants) from the outside to the axis, and from the root-stock
towards the apex ; in the hollow stems of monocotyledonous plants (Grass) from the inner to the
outer surface ; in the Potato from the centre to the outside ; but in the Lemon, Pear, Gooseberry,
and Turnip from the outside to the centre ; in a living plant ( TropcBolum) from the plant itself to the
soil. Kunkel {loc. cit.) obtained similar results with the leaves and stems of a number of plants. He
found that the direction of the current depended upon the relative moistness of the points in contact
with the electrodes, any point being always positive with respect to another which is relatively dry.
The currents which are observed appear therefore to be due to the travelling of water in the tissues.
Phenomena of the same kind have been observed by Quincke in diaphragms.

Dr. Burdon-Sanderson has made a remarkable series of observations on the electric currents in
DiotKBa muscipula (see Report of British Association for 1873 ; also Nature, vols. VHI, X, and XV,
and Proc. Roy. Soc. vol. XXI. p. 495). By the aid of Thomson's galvanometer he has shown that
these currents are subject, in all respects in which they have been as yet investigated, to the same
laws as those of animal muscle and nerve. Further, The Electrical Disturbance which accompanies
the Excitation of the Stigma ofMimulus luteus. Nature, XVI, 1879. See also Munk, Die elektrischen
und Bewegungs-Erscheinungen am Blatte der DioncBa imiscipula, Arch. f. Anat. u. Physiol. Du Bois-
Reymond, 1876.]

3 D

770 GENERAL CONDITIONS OF PLANT-LIFE.

the chlorophyll-granules which are carried along by the very watery protoplasm accu-
mulate at various spots. A current from thirty elements causes permanent cessation of
the movement even if the connection is only momentary. Induced currents act like
constant ones ; but the number of induction-shocks which pass through the cells in a
unit of time appears to have no considerable influence on the action.

The changes of form of protoplasm under the influence of a sufficiently strong elec-
tric current are, according to the observations of Heidenhain, Briicke, Max Schultze,
and Kiihne, similar to those caused by a high temperature near the extreme limit or
beyond it. From those of Kiihne it appears to result that protoplasm is a very bad
conductor of electricity, and that the excitement caused by a current at particular
spots in the protoplasm is not easily transferred to other spots.

Cohn, Kabsch, and others, state that weak induction-currents act on the sensitive
parts of the leaves of Mimosa, the stamens of Beriberis, Mahonia, and Centaurea Scabiosa,
and the gynostemium of Stylidium graminifolium like concussion or contact, the parts
moving as if under the influence of these agencies. According to Kabsch, stronger
induction-currents, which permeate the whole plant, destroy the sensitiveness of the
gynostemium of Stylidium even for mechanical excitation ; but after half an hour the
sensitiveness again returns. The statement of Kabsch is noteworthy that the move-
ment of the leaflets of Desmodium gyrans are permanently prevented by stronger in-
duction-currents, which however do not kill them.

Sect. io. — Action of G-ravitation on the Processes of Vegetation^

Since the attraction of the earth acts uninterruptedly on all parts of the plant, the
entire vegetable organisation must be so contrived that the weight of the separate
parts of the plant is serviceable, or at least not injurious, to the various purposes of
the life of the plant.

In observing these relationships the first thing Is to distinguish between those
contrivances which have for their object to bring the weight of the parts of the plant
into harmony with the purposes of its life — gravitation itself not taking any direct
recognisable part in the attainment of these objects — and those phenomena of
vegetation on the other hand which are brought into existence by the direct influence
of gravitation on the mechanism of growth.

To the first of these groups belongs the fact that the branches and foliage of
upright stems are distributed nearly equally on all sides, and that in larger plants
the firmness and elasticity of the masses of tissue in the stem is promoted by the
formation of wood, or is brought about by other means, as for instance in the trunk
of Musa. But since it is very common in the organic world for the same purpose
to be attained by very diflferent means, slender delicate stems with but little wood
can protect themselves from sinking down and can expose- their foliage to the light
by twining round firm supports, or by climbing with the help of tendrils, hooks,
spines, &c. The same purpose is evidently served by the various floating con-
trivances of water-plants and those of fruits and seeds; in all these cases the structure
is obviously adapted to make the weight of the part of the plant serviceable or at
least not injurious to its life ; although it cannot be maintained that gravitation takes
any part in the formation of wood, in the sensitiveness of tendrils, or in the produc-
tion of a floating apparatus. The only explanation of these arrangements lies in

^ These statements are intended in the first place to draw the attention of students to the pro-
cesses of vegetation which are especially influenced by gravitation. Its action on the mechanism of
growth will be fully described in Chap, IV, where also the literature is quoted.

ACTION OF GRAVITATION ON VEGETATION.

771

Darwin's Theory of Descent ; viz. that, under the influence of long-continued natural
selection, only those structures are finally able to maintain their existence which,
while sufficient for the other requirements of life, are so arranged that the weight of
the part is not injurious or is even useful. It must not be inferred from this, nor
does observation render it probable, that gravitation takes any direct part in these
phenomena.

Gravitation however exerts a direct influence on the growth of young parts of
plants as soon as the longitudinal axis of the growing organ is inclined obliquely
to the perpendicular and therefore to the action of gravitation. In this case the
growth in length of the oblique organ is diff'erent on the upper and under sides,
and the more so the more nearly horizontal the axis of growth. According to
the nature of the organ and its purpose in the economy of the plant, either the
upper side grows more strongly than the under side, or the reverse. A curvature
concave either downwards or upwards is thus caused by the influence of gravita-
tion and growth, and this curvature increases until the free-growing end is directed

.

^^^^^ lE^

^^Tl

m\

H

■

-^ =~ z— *==-——

— ;

=7^

^M

^^m

^

s

~^—~~

^

Fig. 477.— a seedling of yicia Faba, the root and stem of which were straight, which was so fixed that the apex of the
root lay nearly horizontally upon the surface of the mercury (black in the figure), by means of a pin to the cork k\ nn is the
layer of water on the mercury. The figure shows the seedling 24 hours after its fixture; the root has bent sharply down-
wards at its growing portion, so that its apex penetrates the mercury ; the resistance experienced by it in so doing is shown
in the form of the part of the root lying just behind the downward curve. The stem has curved upward from its base. The
position of the bud is independent of the action of gravitation ; it is a phenomenon of nutation.

vertically either downwards or upwards ; the former, for example, in primary roots,
the latter in many primary stems. In lateral branches, leaves, and secondary roots,
similar phenomena occur, though not so markedly. Internal processes of vegetation,
the weight of the upper parts, or the influence of light, act in opposition to that of
gravitation, so that conditions of equilibrium arise which cause the organs to stand
horizontally or obliquely to the perpendicular.

Thus the vertical direction of primary roots and stems, and the oblique direction
of their lateral branches, are determined solely by gravitation, or at any rate to
some extent, so long as these parts are still growing; when they subsequently
become lignified or cease to grow, they maintain the position once acquired. If
therefore a growing plant rooting in the ground (inside a pot) is placed horizon-
tally, the mature parts remain in this position; but the apex of the primary root
turns downwards, and the growing internodes of the end of the stem turn upwards,
the leaves, branches, and secondary roots also bend until they make about the same
angle with the horizon that they did before the change in their position. The parts

3 D 2

772

GENERAL CONDITIONS OF PLANT-LIFE.

which were actually growing when the change was made are shown by the curvatures
caused by the influence of gravitation.

Although we must defer till the fourth chapter the consideration of the internal
changes which accompany these curvatures, the proofs that they are really caused by
gravitation may be presented in the two following forms : — /•-

(i) Individuals of the same species have everywhere on the earth's surface the
same position with respect to the horizon, and therefore also with respect to the
earth's radius. Upright stems therefore, such as those of Pines, grow in South
America in totally different directions from what they do with us ; if their axes of
growth were elongated downwards, they would intersect in the centre of the earth, and
coincide with its radii. It follows therefore that their direction of growth must be
determined by a force which stands in a perfectly definite relation to the position of
the earth's centre of gravity. But there is only one such force, viz. gravitation or the
attraction of the mass of the earth. The same argument holds for horizontal or
oblique branches, leaves, and roots, since these form a constant angle with the
primary stem.

(2) Gravitation differs from other forces in acting independently of the
chemical or other properties of the body, being regulated only by its mass ; but
the same property is also possessed by centrifugal force. If, as Knight^ first
showed, a growing seedling is made to rotate with a rapidity sufficient to bring
centrifugal force into play, this force acts on the different parts like gravitation ;
z. e. the parts which would otherwise be influenced by gravitation (as the primary
root) now follow the direction of the centrifugal force and grow outwards from
the centre of rotation, while the stem, which would otherwise grow upwards con-
trary to the direction of gravitation, now assumes a 'direction towards the centre
of rotation, i. e. in a direction opposite to that of the acting force. This law is
strikingly illustrated when seedlings, the roots and stems of which had previously
grown in one straight line, are fixed upon a rotating disc (protected from
evaporation by a bell-glass) in such a manner that the axis of growth has a
tangential direction. The mature parts maintain this direction during the rotation,
while those which are still growing bend so that the apices of the roots point
outwards and the apices of the stem inwards (towards the centre of rotation). If
the rotation takes place in a horizontal plane, gravitation acts, in addition to
centrifugal force, on the growing parts, and the direction of the stem and root
becomes oblique. But when the rotation is very rapid, it is possible to increase
the centrifugal force to such an extent that the axis of growth remains nearly
horizontal. If, on the contrary, the seedlings are fixed to a disc rotating in a ver-
tical plane, each side of the growing part is in turn directed for a short time
upwards, downwards, to the right, and to the left. The action of gravitation there-
fore affects all sides equally ; i. e. the growth of the organ is practically inde-
pendent of gravitation. Centrifugal force is therefore the only force that acts on
the growing parts ; and the root takes an outward radial direction even when the
disc is not rapidly turned, the stem an inward radial direction. If however the disc
is made to turn very slowly in a vertical plane (round a horizontal axis), so that

^ Knight, Phil. Trans. 1806, part I. p. 99.

MECHANICS OF GROWTH, yy^'

there is in fact no centrifugal force (as by intermittent turns, one revolution in ten
to twenty minutes with a radius of from 5 to 10 cm.), I have shown ^ that the organs
then grow neither in the direction of gravitation nor in that of the centrifugal force,
but just in those directions in which they had happened to be placed when fixed in
the vessel. Under such conditions parts which normally grow straight often curve
in a plane quite independently of external forces, and this can only be due to
internal causes of growth which are distributed unequally round the axis of
growth. Thus, for example, primary roots and stems of germinating seeds (Fada,
Fi'sum, Fagopyrum, Brassicd) will not lie in a straight line, but their respective axes
of growth will intersect at any angle up to a right angle, the anterior side of the
base of the stem growing more rapidly than the posterior side, and thus causing a
curvature. It is clear that the direction of the secondary roots which spring from
the primary root, as well as that of the leaves on the stem, is also, under these
conditions, affected only by internal causes of growth. It is only in this way that
we can explain the directions and forms assumed by parts of plants when unin-
fluenced by gravitation, centrifugal force, or heliotropic curvatures, which could not
occur in these experiments.

CHAPTER IV.

THE MECH^ICS OF GROWTH.

Sect, i i. Definition. The growth of crystals consists in an increase of their
volume by the apposition of homogeneous particles in definite directions. In plants
the process which we call growth is much more complicated ; and the term is
employed in different senses, according as we are speaking of the growth of a grain
of starch or of a chlorophyll-granule, of part of a cell-wall, of a whole cell, or of a
multicellular organ. The common point in all these processes is that they depend
at last on the intercalation of new micellae between those already in existence, in
other words on intussusception, as has already been explained in the first section
of Book III. But even in structures so simple as grains of starch or parts of
cell-walls, we are met with insurmountable difficulties when we attempt to explain
the mechanical process of growth in all its details ; and the present state of our
knowledge by no means enables us to propound a connected theory of the
growth of the entire cell or of a multicellular organ. We are in fact at present
able only to follow empirically the processes of growth in detail, their causes
and results. After this we may attempt to form definite ideas of the separate

1 Wurzburger Med.-Phys. Gesellschaft, March 16, 1872: [also, Ueb. Ausschliessung der geo-
tropischen und heliotropis ,hen Kriimmungen wahrend des Wachsens, Arb. d. bot. Inst, in Wiirzburg,
II. 2, 1879. Sachs calls the apparatus used for this purpose a Clinostat.']

774 MECHANICS OF GROWTH.

processes, taking for granted at the outset the purely formal phenomena of mor-
phology, and regarding as the ultimate object of the enquiry the obtaining an
insight into the mechanism of growth. If the solution of this difficult problem
must be deferred to a distant future, it at any rate lies within the scope of this
work to ■ collect together the ascertained phenomena. But even here we meet
with the difficulty that no one has as yet undertaken to limit the term Growth to
a definitely circumscribed idea. The term is however always employed in the case
of plants and animals to designate permanent changes in form or volume or both,
brought about by internal causes, themselves the result of organisadon, and in their
turn excited' and maintained by definite external causes, as heat, light, gravitation,
the supply of food-materials, water, &c. Changes in the form or volume of parts
of plants that remain quite passive to external forces, and in which changes no
organising process cooperates, ought not to be included in the term Growth. Thus,
for example, there is no growth when the form or length of an internode or root
is altered by simple stretching, pressure, twisting, or bending (it may be by the
hands). It is quite possible however that under certain circumstances internal
changes might be brought about by external influences to which the part of the
plant is at first altogether passive, but which, combined with organising processes,
cause true growth or changes of growth. By organising processes I understand those
internal changes which fulfil the two following conditions : — firstly, they are caused
by the specific organisation of the part of the plant, which is of such a nature that
any external influence can only effect changes in accordance with it; secondly,
they result in a permanent change of the organised part which is not at once
reversed by opposite external influences. If, for example, the elevation of the
temperature above the inferior limit (see Sect. 7) has caused an increase in volume
of the embryonic structures already saturated with water, the parts will not contract
to their previous volume when the temperature again falls below this point, but will
retain the increase acquired during the higher temperature ; in other words the
process is not reversed, it only ceases. Microscopic as well as other kinds of ex-
amination also show that the internal organisation has undergone" permanent change
varying with the specific properties of the plant. If on the contrary a stem is
allowed to wither from want of water, it becomes shorter and ceases to grow ; when
it again absorbs water it becomes longer and thicker and begins to grow. The
contraction on withering and the lengthening on the absorption of water are mere
physical phenomena ; but the lengthening and thickening of a part resulting from
continued turgescence may actually depend upon growth, the organisation of the
plant being altered permanently and to an amount varying with the species by the
operation of the turgescence. It is again the result of permanent and specific
change of organisation when a tendril, in consequence of the light pressure of the
body to which it clings, lengthens less on the side in contact, more on the opposite
exposed side; the curvature thus caused does not disappear if the pressure has
lasted long enough ; the whole phenomenon is therefore one of growth. When, on
the contrary, the motile organ of a Mimosa-\t2X bends downwards in consequence
of irritation, and afterwards again bends upwards, this is, it is true, caused by the
peculiar organisation of the plant; but the movement induces no change in the
organisation itself, and its eifects are not permanent, the leaf soon returning to

I

I

VARIOUS CAUSES OF GROWTH. 775

its original condition. The sensitiveness of the leaves of Mimosa does not there-
fore depend on a change of growth caused by the irritation ; while the power of
tendrils to curl round supports depends, it is true, on sensitiveness, but of such
a character as to cause a change in the processes of growth.

If increase in volume is included in the idea of growth, as is the case in
ordinary language, the rigorously scientific use of the word would require special
care ; for if we simply say that a plant or a part of a plant of considerable size
grows, this may be accompanied actually by a decrease of the whole volume. Thus,
for example, when bulbs sprout or seeds germinate in the air, the whole does not
grow, but only the younger parts develope at the expense of the older, which in
addition give off aqueous vapour and carbon dioxide. It is therefore necessary to
distinguish accurately the growing parts from those connected with them which do
not grow.

There are however changes of form in the parts of plants which are not asso-
ciated with increase, and which may even be attended with decrease in volume, but
which nevertheless depend on a permanent and irreversible change of organisation.
Thus, for instance, the pith, after removal from the internodes, increases in length
for days even while it loses water by evaporation in air that is not saturated. It
would scarcely seem convenient to exclude these and similar phenomena from the
idea of growth; and it is therefore necessary to distinguish between growth with
and growth without increase in volume ; in the latter case growth consists in a mere
change of form which again depends on an alteration of position of the smallest
particles. Every case of increase in volume of a grain of starch or of a cell must
not be regarded as growth, inasmuch as it may be caused by absorption, and
may be reversed by loss of water ; nor is it necessary that growth in a single cell
should be associated with increase in volume, since particular parts of the cell may
furnish material for the increase of other parts. In this case the cell considered as a
whole only changes its form ; and if this change is caused by internal organising
forces, it must be considered as a kind of growth. Those changes in the form and
volume of cells must, on the other hand, be excluded from the idea of growth which
occur only occasionally and admit of being completely reversed, as is the case with
the contractile organs of sensitive and periodically motile leaves.

An error which is constantly made by those who are unacquainted with physiology
is to confuse the ideas Growth and Nutrition, or to consider them identical. It is no
doubt true that all growth must be associated with the conveyance of food-materials
to the growing parts ; but these food-materials are usually withdrawn from older parts
where they were previously inactive ; the whole organism, consisting of both growing
and non*growing parts (for instance a bulb suspended and putting out leaves in the air),
is not nourished as such from without. The growth of certain parts is therefore no indi-
cation of nutrition of the whofe. Still less necessary is the connection between growth
and nutrition from without ; the special organs of nutrition, the green leaves, do not
grow after they are mature, although they carry on the process of nutrition. The two
processes may coincide both in place and time, i.e. in the same cell; but may also be
separated in both space and time ; and this is indeed usually the case, as has been suffi-
ciently shown in Sect. 5.

Sect. 12. Various Causes of Growth. Growth, like all vital processes, takes
place only when certain favourable external conditions coexist. These are the presence

776 MECHANICS OF GROWTH.

of assimilated food-material, water, oxygen, and a sufficiently high temperature.
Under these conditions individual cells or masses of tissue may grow, provided that
their organisation permits it. But independently of these conditions there are others,
as we have seen in the last chapter, which, without absolutely causing or arresting
growth, nevertheless influence it; as light, gravitation, and pressure. The first-
named may be called the necessary, the last the secondary conditions of growth.
In all growth all the necessary conditions must concur while the secondary conditions
intervene only in certain cases, and exert their modifying influence very differently
on the corresponding parts of different plants.

The conditions spoken of as Necessary and Secondary depend upon the en-
vironment of the plant, and act upon it from without. They may therefore be
described as External Condi/tons or causes of growth, in contradistinction to the
Jnierfial Conditions dependent on the organisation of the plant. The existence of
the latter conditions is most strikingly manifested in the fact that all parts of plants
are able to grow only during a certain time ; when this time — the period of youth
and development — is past, they no longer grow, even when all the favourable
conditions concur. This shows that the internal organisation undergoes changes,
which at length render the continuance of growth impossible. But even in organs
which are still growing a certain independence of external circumstances may be
perceived ; an Oak-leaf invariably grows differently from an Elm-leaf, an Oak-fruit
from an Oak-root. The differences of these processes of growth is at once manifest
in the difference of form and of the other properties of the organ ; and no com-
bination of external circumstances has the power of giving to a root, by change in
its growth, the form of a leaf or to an Oak-leaf the structure of an Elm-leaf. There
are also certain internal conditions of growth which do not decide, like the age of an
organ and the necessary external conditions, whether growth shall take place, or at
what rate; but determine how it shall proceed, and what specific and determinate
organisation shall be attained by it. This latter circumstance depends only on the
parent plants, or in other words on the species or variety to which it belongs.
Descent determines the specific character of the growth; all the other conditions
determine only whether growth shall take place at all, and with what rapidity and
energy. The innate internal conditions that regulate the nature of the growth of
the plant, when once present cannot again be destroyed or reversed ; while the ex-
ternal conditions may be at one time brought into action, at another time set aside.
The internal and external conditions of growth may therefore be distinguished as
the historical and the physical ; but those properties of a plant which have been
obtained historically are generally termed hereditary. This expression is not open
to objection unless heredity be considered, as has recently been done by many,
as a kind of natural force requiring no further analysis. For in distinguishing
hereditary conditions of growth — i.e. those that have been acquired historically —
from physical ones, it is not meant that the former do not also owe their existence
to physical causes, but only that besides the accidental concurrence of physical
conditions, it is also necessary to take into account certain characters which the
plant has acquired when in the embryonic condition (in the broadest sense of
the term) in the form of definite specialities of organisation through the influence
of its parents.

VARIOUS CAUSES OF GROWTH. 777

These suggestions must suffice here. The extremely difficult question which
has been raised may be illustrated by far-fetched and elaborate explanations, but
cannot be satisfactorily answered.

The external or physical causes of growth are the only ones that can be
submitted to direct experimental investigation; the internal hereditary causes must
be considered simply as something that exists and that is in the main unalterable ;
for if it were possible to change some of the mechanical and chemical properties
of a tissue by means of external influences, this could not affect the true kernel
of the hereditary characteristics ; and again conversely, changes in these hereditary
peculiarities, or variations, are never brought about by direct external influences,
but only by unknown internal changes. Since therefore the specific peculiarities
in the organisation of a plant are something in its nature that is entirely unknown,
any investigation of the processes of growth must rest satisfied with showing the
mode in which they are always associated with constant internal conditions, and
what visible changes are produced in the processes of growth by physical influences.
We cannot therefore be astonished if in the action of known external causes —
light, gravitation, &c. — on plants, effects are produced which appear altogether
strange to one accustomed to examine purely physical processes; but this aston-
ishment disappears when it is borne in mind that the specific organisation of a
plant itself represents a complexity of causes which we cannot analyse, and there-
fore are unable to estimate. It is in the constant recognition of this unknown
factor — which causes physiological eifects to turn out so entirely different from
purely physical ones — that the difference between physiology and physics consists.
The most striking mode however in which the aggregate of conditions of growth
manifests itself in the inherited organisation, is when the same external causes pro-
duce entirely opposite effects on plants belonging to different species and even on
different parts of the same plant.

To understand correctly the phenomena of vegetation, it is also necessary to
distinguish between the direct and indirect action of external conditions on growth.
For since growth is always dependent primarily on the presence of assimilated
food-materials, light, temperature, or other external conditions may indirectly in-
fluence growth by affecting the formation and transport of the food-materials.
But it is also possible and even probable that the mechanical process of intus-
susception itself on which growth is directly dependent may be modified by those
and other causes the influence of which on growth is therefore in that case a direct
one. The growth of one part may also be indirectly promoted or retarded by the
growth or the removal of another part.

The unknown factor which exists in the inherited properties of organisms is by
no means without analogy in inorganic nature. Chemists and physicists have also to
assume peculiar properties of elementary substances. The aggregate of properties by.
which a particle of iron is absolutely distinguished from a particle of oxygen is as
unknown and much more invariable than the aggregate of physiological causes which
distinguish the inherited properties of an Oak from those of a Pine.

So far as the definition given above of historical properties concerns the inherited
specific peculiarities of plants, the expression is not a metaphor from the point of
view of the Theory of Descent, but must be taken in its literal signification. The
specific properties which determine qualitatively the growth of each organ have

778 lifECHANICS OF GROWTH.

sprung up successively in the course of time, /'. e. in a series of generations. The
chief evidence in favour of this view will be given in the last chapter of this work.
It need only be mentioned now that this theory of the genesis of specific properties
indicates the only possibility of arriving at an understanding of them in accordance
with the laws of causality. At the present time this is possible only in the most
general outline.

The use here made of the terms * historical' and * physical' may also be illustrated
from another subject in the following manner. The nature of the geological form-
ations of which the crust of the earth consists can be understood only from a
historical point of view, because it is only at particular spots and at particular times
that the conditions have concurred which produced, for example, the Chalk or the Old
Red Sandstone. The formation of these rocks was dependent on chemical and
physical processes, which must however have been preceded by other physical
changes in the crust of the earth, in order that these rocks should be formed exactly
at particular spots and particular periods. A crystal of sodiuni chloride can, on the
contrary, be produced at any time, for the necessary conditions may be artificially
brought together. Pseudomorphism of crystals can again be explained only from a
historical point of view, although it is certain that the chemical and physical properties
of the substances are alone concerned in the process.

We see therefore — and this is the object of these remarks— that the historical
explanation of a natural phenomenon does not exclude its explanation from a physical
point of view, but on the contrary includes it where we have to do with natural
phenomena; and this principle is equally applicable to those properties of vegetable
species which have been acquired hereditarily or historically, even when the application
is practically much more difficult than in the case of inorganic nature.

Sect. 13. General Properties of tlie Growing Parts of Plants \ From
the consideration of this subject the true crystals which are found in cells may be
entirely excluded, since they do not differ in their general properties from those
which occur elsewhere. The organised elementary structures on the contrary, the
protoplasm, the nucleus, chlorophyll-granules, starch-grains, and the cell-walls, ex-
hibit properties which distinguish them from all unorganised bodies.

These organised bodies are, in the first place, all capable of swelling ; i. e.
they have the power of absorbing water or aqueous solutions between their solid
particles with such force that the particles are forced apart; the whole structure
increases in size, and can thus exercise considerable pressure on the surrounding
parts. If water is by any means withdrawn from the body which has thus swollen
up, its particles again approach one another, and with such force that considerable
strains may be exerted on the adjoining parts connected with it ; as, for example, is
shown in the bursting of dry capsules. The swelling and desiccation of organised
parts may therefore cause change of form in the surrounding parts, i.e. in other
organised parts. This power of swelling is of still greater importance, since it is
this process that renders possible the interchange of sap between the individual cells
as well as between whole masses of tissue. In order that growth by intussusception
may take place, the dissolved food-materials must be able to enter by imbibition be-
tween the particles of the growing structure, and the chemical processes must take
place there which construct from the dissolved food-materials solid particles to be
intercalated between those already in existence, and in consequence of which the
organic mass alters its volume and form (see Book III. Sect. i).

^ See Nageli u. Schwendener, Das Mikroskop, p. 540 et seq.

GENERAL PROPERTIES OF GROWING PARTS OF PLANTS. 779

A second general property of the organised parts of plants is that they c/iange
iheir form when the external conditions remain perfectly unaltered, internal changes
being the only efficient cause. Almost every process of growth is associated with
change of form. These facts may be more briefly described by ascribing to
organised structures endowed with the power of growth internal forces or plastic
tendencies, if it is clearly understood that the term is only used to express a still
unresolved aggregate of causes. As a result of these internal forces, organised
structures have the power of overcoming resistance. Thus, for example, plasmodia
which are constantly altering their form are able, notwithstanding their gelatinous
and very soft nature, to overcome their own weight, and to creep up solid bodies.
In the same manner the growth of the wood takes place with such force as to over-
come the very considerable pressure of the surrounding bark.

But although the internal causes of these plastic tendencies are able to over-
come certain obstacles, it is on the other hand certain that growth is also influenced
by external forces, such as pressure, traction, stretching, bending, &c., which are
able to alter the form of solid bodies. The observations which have been made on
this subject will be collected in the following sections; but it is in the first place
necessary to define certain terms which will frequently be employed.

Like unorganised solid bodies, those which are organised oppose a greater
or less resistance to the external forces which tend to alter their form ; and are
hence divided mto hard and soft bodies. A hard body is one which off"ers con-
siderable resistance, like many lignified or silicified cell-walls; a soft body is one
which offers very litde resistance, like protoplasm, chlorophyll-granules, or swollen
cell-walls which have ceased growing, as gum-tragacanth. Structures which become
disintegrated under pressure and traction rather than undergo any considerable
change of form are brittle^ like grains of starch or crystalloids of aleurone. If, on
the contrary, they are capable of undergoing considerable changes of form, whether
this take place by pressure or traction, they are extensible. It is clear ih^X flexibility
depends to a certain extent on extensibility, since the side of the bent part which
becomes concave is compressed, the convex side stretched. All these properties are
relative, and the same body may exhibit different phenomena according to the
nature of the external forces which act upon it. Thus, for example, under a
sudden blow the apex of a root behaves like a brittle body, and breaks easily, while
it is flexible if slowly bent.

If the form of an extensible body has been changed by pressure, traction, or
bending, and if, when then left to itself, it retains the form to which it has been
forced, it is called inelastic ; if, on the other hand, it resumes its original form, it
is elastic. If the changes of form produced by external causes are small, they are
usually completely reversed when the body is left to itself, and within these limits
the body is perfectly elastic ; but if the change of form exceeds certain limits
dependent on the nature of the body and the length of time during which the force
has been acting, it does not again assume exactly its previous form. The greatest
amount of change which yet permits a complete restoration of the original form
determines the Limit of Elasticity of the body ; when this is exceeded, the stretched
substance partially retains the form which it has been made to assume, and the less
complete the return to its primitive shape the more imperfect is its elasticity. It

780 MECHANICS OF GROWTH.

would appear as if all bodies were imperfecdy elastic to any long-continued stretch-
ing or alteration of form, and as if there were no limit of elasticity in the case
of very long-continued but weak external influence. In all these points organised
bodies, especially the growing parts of plants, exhibit the same phenomena as unor-
ganised bodies. It must however be remembered that the terms explained above
have reference only to effects visible externally ; the internal changes which bring
about the same external effect may be very different in different bodies. Rigidity,
i. €. resistance to bending, depends, for example, evidently on very different internal
conditions in the case of a woody cylinder and of a succulent stem or root consist-
ing mainly of parenchyma. This is at once experimentally proved by the woody
cylinder becoming less flexible and even brittle from loss of water, while the
flexibility of succulent parenchyma is thereby increased. This is readily understood
on recollecting that the flexibility of the woody cylinder depends on that of the walls
of the wood- cells, which are not closed cavities, and therefore cannot become turgid,
while the flexibility of parenchymatous tissue depends on the change of form of the
closed turgescent cells, the extensibility and elasticity of the cell-walls taking only
a subordinate part. Changes of form take place however more easily the less the
turgidity of the cells ; a parenchymatous tissue may be compared to an aggre-
gation of bladders each of which is full of water; if they are all turgid with water,
each bladder is tense and rigid, as also is the whole ; if, on the contrary, they
contain only enough water to fill without distending them, each separate bladder is
flaccid, as also is the whole, which can therefore be bent in any direction. A mass
of parenchyma may therefore be stiff and rigid even if its cell-walls are thin and
very flexible, if only they are firm enough not to give way from the pressure of the
water which stretches them, or to allow it to filter through. The flexibility and elas-
ticity of the moist cell-wall cannot however be compared directly with these pro-
perties in a perfectly dry cell-wall or a strip of metal, as Nageli and Schwendener
{I.e. p. 399) have already shown. 'If we consider first of all,' they say, 'a frag-
ment of moist cell-wall, say a lamella of the thallus of Caulerpa, a bast-fibre
thickened so that the cell-cavity has disappeared, a spiral vessel, and so forth, it is
proved by their behaviour to polarised light that stretchings, bendings, and other
similar forces do not perceptibly change the arrangement of the atoms in the crys-
talline micellae, but that only the distance of the micellae themselves from one
another is increased or diminished. On the other hand it is known that water
is retained in the moist cell-walls with great force; and microscopic examination
has shown that it cannot be forced out by bending or by compression of the part.
No other hypothesis is therefore possible, except that the amount of water in a tense
cell-wall is the same as in one in a neutral condition. The particles of water are
therefore merely displaced by external forces, but are not forced out ; they move,
for example, with the bending of the part from the concave to the convex side, but
afterwards fill up as completely as before the micellar interstices of the substance ;
and, since the sum of their tensions is but slightly altered, also occupy nearly the
same space. If the same reasoning is applied to tissues without intercellular spaces
and filled with sap, it is perfectly obvious that the cell-walls are not susceptible
of change of volume any more than in the previous case. The same is the case
also with the fluid contained in the cells. The only question now remaining is

m

GENERAL PROPERTIES OF GROWING PARTS OF PLANTS. 781

hether the changes of tension which are caused by external forces modify the
rmeability of the cell-walls at least in places. If this were the case, then when
a tissue is compressed — since the hydrostatic pressure (turgidity) is in no case
decreased by it, but the resistance of the cell-wall weakened ^ — a part of the cell-
fluid must obviously be forced out, until the hydrostatic pressure has again reached
an equilibrium with the diminished resistance of the cell-walls. In the same
manner the effect of traction on a tissue must be to cause an influx of water
into it, or, if this is prevented, the formation of an empty spaced If, on the
other hand, the changes of tension which occur in plants have no perceptible
influence on permeability, the tissues simply possess the properties of moist cell-
walls ; in any condition of tension ^ they always occupy the same space *.'

In order to understand many of the phenomena now to be described, it is
necessary to have a clear conception of the changes which a cell filled with sap
undergoes in reference to its turgidity when it is compressed or stretched or
simply bent by external forces. By Turgidity we understand the hydrostatic pres-
sure which the water absorbed by endosmose exercises equally on all sides on the
cell-wall, and which reacts on the contents in consequence of the elasticity of the
cell- wall; so that in a turgid cell, while the cell-wall is stretched, the contents
are compressed. A clear conception of this state of mutual tension of the cell-
wall and cell-contents may be obtained by closing a short wide glass tube at one
end with a firm fresh bladder free from holes, pouring in a concentrated solution of
sugar or gum, and finally closing also the other end with a thick bladder. This
artificial cell, placed in water, absorbs it by endosmose with great force ; the pieces
of bladder which were previously stiff and tense arch into a hemispherical form
and offer great resistance to pressure. If a hole is punctured by a fine needle in
the bladder, a jet of fluid several feet in height springs from it. The force which
drives out the fluid with such violence is the elasticity of the stretched bladder ;
but the cause which brings this elasticity into play is the endosmotic attraction for
water of the fluid contained in the cell.

If we suppose in the case of a vegetable cell enclosed on all sides a degree of
turgidity sufficient to stretch the cell-wall perceptibly, but leaving it still capable
of further tension without bursting, and if this cell-wall is supposed to be extensible
and elastic — as is especially the nature of growing and non-lignified cell-walls — the
question presents itself : — What changes does the turgidity of the cell undergo when
it is stretched or compressed by external forces or otherwise altered in form ? This
question can be sufficiently answered for our purpose by the simple contrivance
represented in Fig. 478. AT is a wide and thick india-rubber tube to which the glass
tube iS, closed at g^ acts as a stopper. After filling K with water, the glass-tube i?,
open below at 0, is fixed in and firmly fastened, the level of the water standing

^ These words are not clearly intelligible. Turgidity or the tension of the cell-wall is always
increased, as we shall see directly, by pressure from without on a turgid cell; its resistance to
filtration may in this manner be at length entirely overcome.

^ Of course only when the cell-wall does not become folded.

^ By tension is here clearly meant bending, stretching, or pressure from external forces.

* The discussion given on p. 354 of the work quoted with respect to the alteration . of the
mi cellar structure of cell-walls by violent mechanical and chemical forces is of no importance for our
present purpose.

783

MECHANICS OF GROWTH,

R

somewhere about n in the thin drawn-out upper end of the tube. In order to give
to the india-rubber tube, which here represents the cell-wall, a sufficient tension from
the outset, it is convenient to make the thin end of the tube H from 20 to 30 cm.
long, and to raise the level n in proportion. The wide part of 7? is fixed in a
holder, so that the cell hangs down. A condition of equilibrium is thus established
between the elasticity of the india-rubber tube and the hydrostatic pressure which
can be compared with the turgidity of the vegetable cell ; and in this condition the
water-level stands at «. If the tube S is now pulled downwards, the elastic tube
is lengthened and at the same time made narrower, but the amount of space enclosed
by it is increased, as may be seen by the falling of the
water-level n in the narrow glass tube. If on the other
hand the glass tube -S" is pushed up and the india-rubber
tube thus compressed without any bending or creasing
taking place in X, the space enclosed by the tube K is
diminished, as is shown by the rising of the water-level n.
The same thing takes place when the tube J^ is bent in
any way, or when it is compressed on any side ^

It is evident that if the upper glass tube R were closed
at n so as to prevent a rise or fall of the water-level, any
change which previously caused a rise of the level would
now occasion an increase of the hydrostatic pressure, and
vice versa. It may therefore be stated that in a closed
and turgid cell any pressure acting from without or any
curvature increases the turgidity, while any stretching of
the cell diminishes it. If we imagine a straight succulent
stem or a growing root to be bent, the cells on the con-
vex side will be stretched, those on the concave side com-
pressed, and the turgidity will be diminished in proportion
in the former and increased in the latter. This result is
very clearly confirmed if a very succulent rapidly growing
internode of the Grape-Vine is slowly but firmly bent till
it describes about a semicircle. It will be observed that
during the bending a number of small drops of water
escape in rows from the epidermis on the concave com-
pressed and shortened side. It is indifferent whether they
escape through fissures or are forced out through the cell-
walls; in either case they show that the cells display a
higher degree of turgidity on the concave compressed side than when the internode
was straight.

In the present state of our knowledge, if we would keep clear of uncertain
speculations, the considerations now given must be considered as by no means
complete ; but they are sufficient to draw the attention to conditions which must be
taken into account as existing in the interior of the growing parts of plants when
they are subject to pressure, traction, bending, and so forth, from external forces.

\

Fig. 478.— Apparatus for illustrating
the change in the turgidity of cells
caused by external distension or com-
pression.

See also Pfeffer, Physiol. Unters. 1873, p. 121.

i

GENERAL PROPERTIES OF GROWING PARTS OF PLANTS. 783

ut if these internal changes are for the time left out of account, the purely external
ffect of the forces already mentioned is deserving of greater attention than it has
itherto received ^ It would be of essential service, for instance, to ascertain at
what point a growing internode, root, leaf, &c. possesses the greatest extensibility,
flexibility, and elasticity, and whether this point coincides or not with that of the
most vigorous growth, and how perfect is the elasticity of the part ; and so forth.
We shall see that even somewhat crude observations in this direction afford results
which enable us to remove old errors and avoid new ones.

Compared with the extensibility of mature internodes and parts of internodes,
that of rapidly^ growing parts is very considerable, but their elasticity, on the con-
trary, is very imperfect. But the greater the development of the wood of a growing
part, the greater is its elasticity and the less its extensibility. In young non-lignified
roots, on the contrary, the resistance to bending is greater in the youngest than in
the older parts, especially those whose growth in length has long been completed.
The extremities of roots, very young leaves, and the ends of stems still enclosed
in the bud, are generally brittle under a blow or pressure, but pliable and plastic to
long-continued action of this kind, a condition that gives place during growth to
an increasing resistance to sudden blows, which is in the first place due to in-
crease of extensibility, afterwards to increase of elasticity.

In rapidly growing stems, leaves, and roots, the Hmit of elasticity is easily over-
stepped even by momentary flexion ; and they always retain afterwards a slight
though distinct curvature. It is often even possible, especially with roots and slender
internodes, to give them any desired form by repeated bending with the fingers in
different directions, like a thread of wax or a red-hot iron wire, without the power
of growth being at all injured by the process. This effect is attained with greater
certainty by exerting on the growing structure a flexion which is prolonged although
small in amount. Thus the pedicels of many flowers are bent downwards by their
weight, and retain this curvature even when the weight is removed, until a new con-
dition of growth imparts greater elasticity and firmness to the tissues : under the
influence of gravitation they then grow more rapidly on the lower side, become
upright, and raise up the still greater weight of the fruit; as is strikingly seen
in Fritillaria wipenalis, Anemone praiensis, and many other plants with pendent
flowers and erect fruits. In other cases again the curvature, which was at first
due merely to external causes, becomes permanent and fixed in the tissue itself
by the processes of growth, as in the fruit-stalks of Solanum Dulcamara.

One of the most striking phenomena of this class is that a lateral blow below a
growing internode causes it to assume a curvature in the direction assumed by the
internode at the moment of impact. The same thing occurs when the upper part of
a shoot is taken in the hand and a curvature imparted to it similar to that caused by
the blow. The upper part acquires in consequence a pendent position, which may
however be again neutralised by subsequent growth.

There has been as yet no exact or detailed investigation of the elasticity of
growing shoots, roots, and leaves; and the enquiry is, as I have convinced myself,
attended with considerable difficulty. Observations sufficient to enable us to study

See A. P. De Candolle, Physiologie Vegetale, vol. I. p. 11.

784

MECHANICS OF GROWTH.

some of the phenomena of vegetation to be described in this chapter can however
be made with the simplest methods and apparatus.

(a) Extensibility of gronuing Internodes. The upper and lower end of an internode

• of a freshly cut fragment of a stem were marked with Indian ink. The shoot was

held above and below the marks, laid on a micrometer graduated to millimetres, and

stretched as strongly as possible without breaking ^ The result is shown in the

annexed table: —

Name.

1. Cimici/uga racemosa

2. Sambucus nigra

The next older internode
A still older internode

3. Aristolochia Sipho

The next older internode

4. Aristolochia Sipho

The next older internode

5. Aristolochia Sipho

The next older internode

Imperfect as was the method of observation, these figures nevertheless show (i) that
growing internodes are highly extensible, (2) that extensibility decreases with age,
(3) that elasticity increases with age.

{b) Elasticity to flexion of gro^'ing Internodes. Internodes of fresh turgescent shoots
were cut off, and bent on a card on which concentric circles were drawn; the axis
of the internode was made to coincide as nearly as possible with one of the circles ;
the radius of this circle is recorded in the following table as the radius of cur'vature.
The internode was then left to itself, and its permanent curvature determined in the
same manner. The branch was then bent on the other side, and so on, as shown
by the table. The internode was finally laid with its concave side on the measuring
rod and pressed straight on to it.

Original length
of internode.

Amount of temporary
elongation.

Amount of perma-
nent elongation.

296 mm.

6-8 p.c.

3-5 p.c.

26

i8-o

5*4

65

3'i

I'l

115

•8

•0

102*5

4*4

I'O

242

2'2

'4

33*5

10-4

1*5

252-5

1-8

•4

71*5

6-3

3*5

226

2-6

•8

Length of the

Radius of

Radius of cur-

Thickness of

Name.

curvature

vature when

the middle part

J internode.

when bent.

left to itself.

of the internode.

Valeriana officinalis ; stalk of

young inflorescence.

Before bending .

200 mm.

cm.

cm.

6 mm.

I. Bent . . . .

—

4

13

2. Bent in opposite direct

—

4

21

3. Bent as in (i)

—

4

23

4. Bent as in (2)

—

4

24

,

Straightened

201-5

—

Cimicifuga racemosa. Before

bending

165

—

—

5

I. Bent . . . .

5

19

2. Bent in opposite direct.

5

22

Straightened

165-5

—

—

Heracleiim sibiricum ; stalk ol

umbel. Before bending

165-5

—

—

5

I. Bent . . . .

5

18

2. Bent in opposite direct.

5

23

* This somewhat primitive method of stretching, which of course does not furnish an exact
measure of the extensibility of different internodes, was employed because stretching by means of
weights necessitates fastening the shoot, which is attended with great inconveniences.

GENERAL PROPERTIES OF GROWING PARTS OF PLANTS.

785

Length of the
internode.

Radius of

Radius of cur-

Thickness of

Name.

curvature

vature when

the middle part

when bent.

left to itself.

of the internode.

3. Bent as in (i)

5

25

4. Bent as in (2)

5

22

Straightened

167*0

—

Fitis -vinifera; young internode

Before bending .

47*5

—

—

5*8

I. Bent .

2

4

2. Bent in opposite direct

2

6

3. Bent as in (i)

2

6

4. Bent as in (2)

2

9

Straightened

47*5

—

nth 'vinifera ; older internode

Before bending .

. 133-8

—

—

7

I. Bent .

4

8

2. Bent in opposite direct

4

17

3. Bent as in (i)

4

II

4, Bent as in (2)

4

25

Straightened

133-0

—

— -

These examples, selected from a long series of observations, show: — (i) that
growing internodes are very flexible, (2) that after bending they do not altogether
recover their straightness, or that the elasticity of curvature is imperfect; (3) that
repeated bendings constantly in opposite directions leave progressively smaller curva-
tures^; (4) that one vigorous bending, and to a still greater extent repeated ones
in opposite directions, leave the internode flaccid, or deprive it of its rigidity (of
which no special account is taken in the table) ; and (5) in the case of the three first
examples, that an internode bent first in one and then in another direction lengthens
slightly, while in the case of the two last there was no lengthening, but in one even
a perceptible contraction.

(c) Change of length of the conca've and con-vex sides of a bent internode. Here
again, as in paragraph ^, the bending was done by the hands, and measured by the
radius of curvature on a card on which concentric circles were drawn. The original
length, as well as those of the sides which remain concave and convex after the object
is left to itself, were measured by means of a carefully applied strip of card divided
into millimetres. In order to get a great diff'erence between the concave and convex
sides, very thick internodes were selected, and their thickness measured in the
middle.

Name.

Silphium perfoliatum
13*2 mm. thick.

Before bending

Bent . . . .

Bent in opposite direct. .

Straightened .
Ligularia macrophylla
7*5 mm. thick.

Before bending

Bent . . . .

Bent again

Bent in opposite direct. .

length of
internode.

185 mm.

Radius of
curvature
when bent.

Radius of
curvature
when left
to itselt

Contraction

of the

concave side.

Lengthening

of the
convex side.

14 cm.

26 cm.

I mm.

2 mm

14

30

I

1*5

185

199

17

13
30

3*5

3*5

•5

4

4*5

1*5

* The curvature is less the greater the radius of curvature.
3 E

7 86 MECHANICS OF GROWTH.

These observations show, as was to be expected, that the permanent curvature
of an internode is connected with a permanent contraction of the concave and
lengthening of the convex side.

(</) As to the Distribution of Extensibility in growing shoots, the observations of
de Vries^ lead to this result, that in growing strongly turgescent shoots a maximum
of extensibility and of flexibility exists immediately below the terminal bud. From
this point they diminish as the distance from it increases, and therefore also with
the age of the parts. This statement holds good independently of the age of the
growing shoots.

{i) Sudden cur'vature of grooving shoots from a blo<w or concussion. If upright growing
shoots'^ are suddenly and violently struck below at a point where growth has ceased,
the curvature thus caused advances upwards in the form of a wave, so that immediately
after the blow has been given to the lower part the apex of the shoot is strongly b^nt,
the concavity of the curvature lying on the side from which the blow was received.
The elasticity of the bent part causes the apex to spring back immediately; but when,
as we have seen, the elasticity is very imperfect, the shoot retains a part of its curvature.
As soon as the shoot has come to rest after some oscillations, it may be observed that
below the apex, where the shoot is most flexible to an ordinary passive curvature, a
permanent curvature is established, the apex bending over, and always on that side
fro n which the blow was received. In many cases this phenomenon is produced by
a single blow from a stick, as e.g. in Fagopyrum, Lythrum, and Senecioy flower-stalks
of Digitalis J Cimicifuga, Aconitum, &c. ; in more rigid stems which are less flexible and
more elastic at the corresponding part, the bending over of the apex does not take
place till after three or four or even from twenty to fifty blows have been given to the
lower woody part ; the amount of curvature also varies in diff"erent plants. If shoots
are cut off" low down so that a woody piece which has ceased to grow can be
taken in the hand, and the shoot made to oscillate rapidly backwards and forwards,
it assumes, when it comes to ripst, a distinct curvature below the apex in the region
of greatest flexibility. The plane of curvature coincides with that in which the
oscillations take place, and the apex may bend to either side; but the permanent
curvature will always be concave' on the side on which the oscillations were
strongest. If finally a rooting shoot or one firmly held in the hand receives
repeated lateral blows at its summit, that is, above the most flexible part, a perma-
nent curvature is produced in this region, but it is in this case convex to the side
from which the blows came.

In all the cases which I have described the position of the permanent curvature
is the same as that of the " strongest curvature, even if acquired only momentarily
by the shoot. The appearance is precisely the same as if the shoot were taken in
the hand and then strongly bent once, or as if it were repeatedly bent backwards
and forwards, but more strongly in one direction. Mere concussions which produce
no strong flexion of the shoot cause no permanent curvature; if shoots are
enclosed in glass tubes and violent impulses repeatedly imparted to them by jerking
the tubes upwards or swinging them from side to side no change is visible when
the shoots are removed from the tube.

If the part of a shoot susceptible of curvature is marked with ink in equidistant
divisions, . and is then made to oscillate by blows below this part, the convex side
of the permanent curvature is found to have become longer, the concave side

^ Ueber die Dehnbarkeit wachsender Sprosse, Arb. des Bot. Inst, in Wiirzburg, Bd. I, 1874.

^ The phenomenon here described was first observed and studied by Hofmeister (Jahrb. fiir
wissensch. Bot. vol. II, i860); and a few important corrections of his description were given by
Prillieux (Ann. des Sci. Nat. ser. 5, vol. IX). The statements here made, which confirm the previous
observations in all essential points, while differing from them in a few others, are entirely based on
my own observations.

"general properties of growing parts of plants. 787

shorter, precisely in accordance with the phenomena described in paragraphs ^ and c ^
For the measurements in the following table as thick shoots as possible were used,
since they give considerable differences in length between the convex and concave
sides even when the curvature is slight. The measurements were made with strips
of card graduated in millimetres, and which I applied closely to the concave and
convex sides.

Name.

Original

length.

Silphium perfoliatum .

152 mm,

do. do. .

120

Macleya cordata .

87-5

do. do. .

104

Polygonum Fagopyrum

63

Helianthus tuberosus .

98

Valeriana exaltata

150

do. do. .

no

Vitis vinifera

149

Approximate
radius of
curvature.

Lengthening

of the
convex side.

Contraction

of the
concave side.

18

cm.

3*4 p.c.
17

o*o p.c.
•6

7

2*3

17

24
8

•5

2*1

i'5
1-6

—

2-0

i'4

32

•8

7

—

7

2*1

i-IO

1*3

2*0

The permanent curvature which remains after violent oscillations of a shoot, or
the Cur'vature of Concuss ion^ is the result of a lengthening of the convex and a
simultaneous shortening of the concave side. A proof is thus afforded that the
whole phenomenon is dependent on the very imperfect elasticity and the great
flexibility of the region that is capable of flexion^. A shoot bent in this way shows
the same changes as one that is simply bent between the hands. This result
would not be at all altered were it found, in harmony with what was said in para-
graph b, that the concave side was also sometimes slightly lengthened, since it is
stretched by the recoil of the oscillations; and this elongation is not always entirely
neutralised. Prillieux has compared this curvature to that of a lead-wire fixed to
an elastic support, when the support was struck ; he was unable however to see
the reason why the older and younger parts of the shoot did not exhibit the phe-
nomenon. In the older parts this depends on their more perfect elasticity, in the
younger on their smaller flexibility, and on the circumstance that they are not
strongly bent, but are only thrown backwards and forwards by the oscillations of
the lower and more flexible parts.

The subsequent neutralisation of the curvature by growth must depend first of
all on the increase of turgidity in the concave and its diminution in the convex
side, and on the growth being consequently promoted in the former. This may
be assisted also by the secondary effect of elasticity, in consequence of which the
stretched epidermis of the convex side shortens, while the compressed tissues of
the concave side expand.

Sect, i 4. — Causes of the condition of Tension in Plants. The elasticity
of the organised parts of plants results in tension chiefly from the operation of three
causes; viz. (i) the turgidity, in other words the hydrostatic pressure of the conter*
of the cell on the cell-wall; (2) the swelling and contraction of the cell-wal^'

aiated the length
* According to Hofmeister all the sides of the shoot become longer. ^f^Q QoxiCz.\e side, which
of the curve which he took for an arc of a circle ; and Prillieux measured o^^ ^^.^ neutral axis, cannot
he found to be always shorter ; the contraction of the whole shooj^j^/j^ according to Hofmeister,
however be inferred from that of the concave side. The thicke-^^^.^^^. ^^^^^ ^^ demonstrated, in
should take place, if the shoot becomes longer on all sidj^^ ^^^^^ .^^ i^ ^^^h cases,
consequence of the extremely small change in diamete-^ .^ ^^ 0„ the Bending of the

^ Compare the different description given bv^"""''" '^ tRco

Succulent Parts of Plants, in the Berichte der i^"- ^achs. Ges. der VYiss., 1059-

3 E 2

788 MECHANICS OF GROWTH.

they imbibe or lose water ; and (3) the changes in volume and form caused by the
growth of the cells.

I . Turgidiiy, The force by which water is drawn by endosmotic attraction *
to the cell from the parts that surround it is not merely sufficient to fill the space
enclosed by the cell-wall, but also to enlarge it, the increasing amount of sap dis-
tending the cell-wall until its elasticity is brought into equilibrium with the endos-
motic absorption. In this condition the cell-wall is stretched to its full capacity,
or the cell is turgid. If the cell loses a portion of its water by transpiration or by
neighbouring cells withdrawing it, the tension of the cell-wall is decreased and the
volume of the cell diminished. The hydrostatic pressure produced by the endosmotic
action of the cell-wall acts from within and is the same at all points within the
small cell-cavity; but this does not prevent different points of the cell-wall stretch-
ing and contracting in different degrees as the turgidity increases, in consequence of
local variations in extensibility. Hence not only may the volume but also the form
of the cell be changed by turgidity. The greater the tension between the cell-
wall and its contents, in other words the greater its turgidity, the greater is the
resistance offered by the cell to external forces which tend to alter its form by
pressure, but the more readily does it burst in consequence. If the cell loses so
much water that the space enclosed by the flaccid cell-wall is no longer filled, it may
become folded inwards by the external pressure of the air or of the surrounding
water, and in this case the cell is said to collapse ; if the cell-wall is thick, firm, and
inflexible, a tension of an opposite character to turgidity takes place in the cell.
Since turgidity is nothing but the mutual tension of the cell-wall and contents, or
a state of equilibrium between endosmotic absorption and the elasticity of the
cell-wall, it is evident that only closed cells, i. e. such as have no orifices, can be
turgid The micellar interstices through which the water set in motion by endos-
mose forces its way into the cells are essentially different from pores ; the former are
so small that their diameter is completely under the control of molecular forces,
while even the smallest pore withdraws at least the middle portion of its space
from the influence of the molecular action of the substance that bounds it. Micro-
scopic openings, like the pores of bordered pits, are orifices of this latter kind, and
are excessively large compared with the micellar interstices through which endosmose
acts. Cells with pits penetrating the cell-wall cannot therefore be turgid, because
any tension however small between cell-wall and contents is at once neutralised by
the superfluous sap becoming pressed out through the orifices. It is indeed possible
for water to be forced out in this way even through closed cell -walls, but only when
'r-^ turgidity is very great, and the hydrostatic pressure of the cell-sap on the per-
. ' g-^^nse cell-wall is sufficient to force out the water through the micellar inter-
" '^ resistance offered by the cell-wall to this may be called resistance to

^ [It appears probable . , ..,,., • , „ r n n t 1

are or can be turgid are the s.ut'' °'"S''"''' ^""^^^ ^^^""^ ^""^ P'^^^"^ ''' ^^^ ''^^^"'^P °^^^ ""^^^^ ^^'""^
der Pflanzensauren fur den Turf?o7f "^^^"^^ '^^""^^ endosmosis (see de Vries, Ueb. die Bedeutung
y^T-frr T«^,^ TT^K J- • Tr •• ' ' Zellcn, Bot. Zeitg. 1870). De Vries is of opinion (Bot.

Zeitg. 1879, Ueb. die inneren Vorgange .. , „, , ,Z i ■ u ir ^ n

tViof r,v^,„fi, ,-1 ^-u ^ 1 , "' den Wacnsthumskrummungen mehrzelliger Organe)

that growth in length depends upon the cont He j ^- r .• 1 ^- ^7 • ^i,

cell-sap of the growing cells.] ^"^ P^o^^^^^o^ ^f actively osmotic substances in the

^ That the water which filters throueh under i. • i. ^ n ^u u

iKju^ii uuuer ^^^Yi circumstances actually passes through

CAUSES OF THE CONDITION OF TENSION IN PLANTS. 789

filtration. It is very different in amount in cells of different kinds, and on it the
degree of turgidity depends, when the intensity of the endosmotic force of the sap
and the elasticity of the cell-wall are constant.

What follows with respect to the turgidity of the individual cell is equally true
in general of masses of tissue; only that a much greater variety of phenomena
may arise in this case according to circumstances. If, for example, a number of
similar layers of tissue are united into a system, a curvature of the system may
take place when one layer loses water by evaporation and thus becomes shorter, or
when it absorbs more water than another layer and thus becomes longer. For
instance, the primary roots of seedlings which have become partially flaccid bv
evaporation and perceptibly shorter, quickly bend upwards concavely if placed with
one side on water; if placed entirely in water they become straight and lono-er.
Curvatures arise in the same manner when layers of different tissues, united with
one another, are subjected to variations of turgidity. Stems of the Dandelion for
instance split lengthwise and placed in water roll up in a spiral manner, the outside
being concave, because the medullary parenchyma absorbs much more water, and
consequently, from the extensibility of its cell-walls, expands more than the epidermis
or the cortex, which absorb water more slowly, and whose cell-walls are besides not
so extensible \

As a single cell, with increasing, turgidity, opposes greater resistance to forces
which tend to change its form, so also a mass of tissue becomes more rigid when all
its cells are more strongly turgid, and vice versa. If, for example, a cylinder of pith is
cut out from a growing internode, it is flaccid and flexible ; but if it is placed for a
quarter or half an hour in water, it not only becomes considerably longer, but also
very rigid and even brittle in consequence of all its cells becoming rapidly filled with
water. This effect is still more visible when the pith is surrounded by other less
extensible tissues, as in an uninjured internode. If this internode has become
flaccid from transpiration, and it is placed in water, the pith very soon begins to
become turgid and to expand ; but since it is surrounded by other tissues of
different properties, it must stretch them in order to lengthen itself; this is only pos-
sible however until the elasticity of these, layers is in equilibrium with the tendency of
the pith to expand. In this case the elongation of the whole caused by the turgidity
of the pith is much less than that of the pith alone would be ; but on the other hand
there is now a violent tension between the pith and the surrounding tissues, in
consequence of which the whole internode appears very rigid or but slightly flexible.
The whole internode may be compared to a cell the contents of which are
represented by the pith, its cell-wall by the surrounding tissues. If the pith loses
water the whole becomes smaller, the passively stretched tissues contracting elas-
tically; and since the tension is thus decreased, the whole becomes more flaccid;
the reverse when the change is in the opposite direction.

micellar interstices is clear from the fact that the amount of soluble substances contained in the
water is altered by the filtration.

' [De Vries has shown that the turgidity of cells may be diminished by placing them in
solutions of neutral salts (KNO3, Na CI) of 4-6 per cent. ; water is withdrawn from tlie cells, and
they consequently become smaller; if they are then placed in distilled water they regain their
original size (Ueb. d. mechanischen Ursachen der Zellstreckimg, 1877).]

790 MECHANICS OF GROWTH.

2. ImhihiUon is the term given, as we have already seen, to the capacity' of
organised structures to absorb water between their micellae with such force that
they are thereby driven apart, their cohesion being partially or entirely overcome, and
the whole thus increasing in volume. Loss of water, on the other hand, as by
evaporation, causes approximation of the micellae and a corresponding decrease in
volume of the whole. Both distension and contraction take place with such force as
to overcome external resistances of considerable magnitude. While in closed and
thin-walled cells the changes in form and volume are chiefly caused by turgidity,
in very thick- walled cells on the contrary with a small cavity (as many bast-fibres and
collenchymatous cells) they are brought about mainly by imbibition and desiccation
of the cell-wall, and especially when it is to a high degree capable of swelling, in
other words is in a state to absorb or give off large quantities of water. In cells
with open pores, where there can be no hydrostatic pressure or turgidity, as in
wood-cells and vessels with bordered pits, imbibition and the desiccation of the per-
forated cell- wall are the only means of changing the size and form of the cell.

If, as is usually the case with thick cell-walls, the different concentric layers of
cellulose have different degrees of capacity for imbibition and swelling (see Book I.
Sect. 4), tensions are caused between these layers by the absorption or loss of water,
which may even end in the layers becoming detached from one another; as, for
example, occurs in transverse sections of thick-walled bast-cells and in starch-grains.
But it is not only the quantity of water absorbed and given off that varies in the
different layers of a cell-wall, but also the direction in which the water is princi-
pally absorbed or allowed to escape between the micellae. Tensions are thus
caused which may lead to the production of torsions and oblique fissures, to the
rolling or unrolling of spiral bands of the cell-wall, and to a change in the obliquity
of the spirals ^.

All these changes, which are necessarily associated with the tensions of layers
that have become convex and concave, take place also in masses of tissue and
organs the cells of which have lost their contents and consequently their turgidity,
while their cell-walls have become capable of imbibition, or, as it is generally termed,
hygroscopic. The layers of cell- walls and the thin-walled masses of tissue which
in the living state contain most water, contract most strongly after death and from
desiccation; with change of form they become concave, or are ruptured by the
contraction of the intermediate lignified tissue. Without entering at present into a
detailed consideration of these extremely various phenomena, which, though often
of extreme importance in the life of the plant, do not influence growth, it need
only be mentioned that on them depend the bursting of most sporangia, anthers,
and capsular fruits, the remarkable movements of the awns of various species of
Avena and Er odium, as well as those of the Rose of Jericho {Anastatica Mero-
chuniica) and of the so-called asthygrometer ^. Of direct importance on the other
hand, as respects the mechanical laws of growth, are the changes in volume of the

^ See Nageli u. Schwendener, Das Mikroskop, p. 427 et seq. (1877).

^ Compare Cramer, in Nageli u. Cramer's Pflanzen-physiologische Untersuchungen, 1855, Heft 3.
p. 28 et seq. ; and Sachs, Experimental-Pliysiologie, p. 429.

^ Compare Cramer's statements in Wolff's treatise, Die sogenannte Asthygrometer ; ZUrich,
1867.

CAUSES OF THE CONDITION OF TENSION IN PLANTS, 791

wood and bark of trees which accompany the variation in the quantity of water
they contain, and the very powerful tension between them thus caused in woody
plants, to which I shall again recur in detail. The attention of the student need
now only be called to one point, viz. that when wood distends on imbibition or
contracts on desiccation, this is caused entirely by the alteration in form and
volume of the cell-walls, since turgidity cannot take place in wood as it does in a
tissue consisting of closed cells. The distension and contraction of wood when it
absorbs or loses water are very different in different directions, strongest in the
tangential, weaker in the radial, weakest of all in the longitudinal direction \ This
is the cause, for instance, of the longitudinal splits in woody stems when they become
dry, which close again when water is absorbed ; and the changes of dimension due
to these phenomena take place with extraordinary force.

3. Growth itself must cause states of tension in the layers of a cell-wall or of
the tissue of which an organ is composed, if the layers, although firmly united to
one another, grow unequally. It is however much more difficult to understand
the modifications of tension due to growth than those due to turgidity and imbi-
bition, as the former cannot be altered artificially without a material change being
caused also in the latter. Since the growth of every organised structure, such as
a cell-wall, can only proceed so long as it is permeated with water, and since
moreover the growth of the entire cell requires it to be in a turgid condition, and
this condition itself has an influence on growth, it is extremely difficult to decide
how far each of these phenomena is the cause of the other. If by growth we under-
stand, according to the definition already given, only permanent and irreversible
changes of organisation, affecting in the first place the micellar structure of the
organism, it may be assumed, in accordance with the present state of our know-
ledge, that growth is always preceded by imbibition and turgidity, and that it is the

^ The measurements of Laves given below illustrate these relative changes of dimension. (See
Sachs, Experimental-Physiologic, p. 431.)

In the direction of In the direction of In the direction of the

the axis. the radius. circumference.

Maple 0.072 3-35 6.59

Birch 0-222 3*86 6.59

Oak 0.400 3.90 7.55

Fir 0-076 2.41 6-18

The change in volume of wood was investigated by Weisbach {I. c, p 432).

Water absorbed by loo parts Distension of lOo parts by j

by weight of dry wood. volume of dry wood.

Maple 87 parts 9-4

do. 87 7-1

Birch 97 7'0

do. 91 S'S

Oak 60 ■ 7-2

do. 91 7«8

Fir 94 5.7

do. 130 5-1

In comparing the change in volume with the amount of water absorbed, it must be borne in mind
that the numbers in which the latter is expressed do not give merely the amount of water imbibed
by the cell'-walls, which alone causes the distension, but also that retained in the cavities by capillary
attraction. It may therefore happen that there appears a smaller increase in volume when a larger
quantity of water is absorbed.

792 MECHANICS OF GROWTH.

modification of the miccllar forces caused by these conditions which render possible
the intercalation of new solid particles among those already in existence. If, for
example, a cell-wall is stretched by turgidity, the distance of its micellae increased,
and possibly a different arrangement of them brought about, this state may be re-
versed on the cessation of the turgidity, by the elasticity of the cell-wall. But if,
during the condition of tension, growth takes place by the intercalation of new
solid micellae, the tension of the cell-wall is altered and in general diminished. If
now the turgidity ceases as before, a new condition of equilibrium occurs in the cell-
wall ; a permanent change has been effected by growth, which was rendered possible
by hydrostatic pressure and imbibition.

The share taken by growth in the tension of the tissues amounts to this : new
solid micellae are intercalated, and the tension due to imbibition and turgidity is
thereby partially neutralised. This is however only momentary; for after the inter-
calation of new micellae the turgidity again increases, the degree of imbibition is
modified, new tensions are again caused, which on their part are partially neutralised
by the intercalation of fresh solid micellae. It is probably near the truth to suppose
that the limit of the elasticity of the growing cell-walls is constantly nearly reached
by turgidity and imbibition as well as by the secondary tensions produced by them,
and that on the other hand the tension is constantly being diminished by the inter-
calation of new micellae. Growth may therefore be described as a constant over-
stepping of the limit of elasticity of the growing cell-wall which is constantly
neutralised by the intercalation of additional solid micellae.

It will of course be understood that in the brief description now given we do
not mean to state a theory of growth, but only to indicate in general terms the
mechanical effect exercised by growth on the tension of tissues, and conversely.
It would be easy to deduce the explanation in particular cases. If, for example,
a cell-wall is imagined distended by turgescence or by traction exerted by the sur-
rounding tissue, the intercalation of solid particles in the layers of cellulose already
present may take place to a greater or less extent, causing a differentiation in their
extensibility, elasticity, and power of imbibition, and thus leading to mutual tensions
of the layers, as may be seen almost invariably in thin transverse sections of the
cells of plants, and especially in the outer walls of those of the epidermis. But
these differences in the mode of intercalation in the different layers of the same
passively distended cell-wall may depend on a variety of circumstances; as, for
instance, on the degree of proximity of the layers to the protoplasm, on whether
they are in contact externally with the air, &c. But growth by intercalation may
also vary according to the nature of the tissue of which the cell forms a part, or the
chemical properties of the cell-contents, and according as the cells are passively
distended or compressed by other cells. All these considerations are however merely
hypothetical, and simply indicate the nature of the relations between growth by
intercalation and the tensions caused directly by imbibition and turgidity. It may
in any case be regarded as certain that intercalation is only possible as the result
of imbibition and turgidity ; but that these properties, as well as extensibility and
elasticity, must, or at least may be, in their turn modified by it. The volume of the
growing part increases; and since this takes place in different degrees in different
layers of the same cell-wall, and in different layers of the tissue of the same organ,
tensions varying in degree must be produced betv^^een these different layers.

It may not be superfluous to add some explanatory observations relative to what
we understand by Tension.

CAUSES OF THE CONDITION OF TENSION IN PLANTS. 793

Corresponding to every tension is an opposite tension. If a tissue which has a
tendency to become distended is prevented from doing so by its connection with
surrounding tissues, both are in a state of tension, the one negative, the other
positive. The tissues which are passively distended may be said to be in a state of
negati've tension, those which are compressed or hindered in their distension to be in
a state of positi've tension. In a turgid cell, the cell-wall is therefore in a state of
negative, the contents in a state of positive tension.

As long as there is no movement or change of form, the two opposing tensions
must be equal ; /. e. the work which the part in a state of positive tension would perform
is equal to the work which would be performed by means of its elasticity by the part in
a state of negative tension if the two were disconnected; or the elastic forces set in
action must perform the same amount of work in two layers with opposite tensions
and in equilibrium with one another. If, for example, a steel cylinder 1000 mm. long
is supposed to be placed in an india-rubber tube 500 mm. long and closed below, and
if the tube is stretched so that it can be fastened above the upper end of the steel
cylinder, we have a system in a state of tension, the india-rubber negative, the steel
positive ; and since the system is at rest, the opposing tensions must be equal ; /. e. all
the particles of the india-rubber tend to contract with the same force as that with
which those of the steel, which are now compressed, tend to separate from one another.

This example shows at the same time that the amount or intensity of the tension
can by no means be measured by the changes in dimension which the layers ex-
perience at the moment when they are set free from it. Let us, for example,
suppose, in our system of steel and india-rubber, that the steel cylinder is shortened
o'l mm. out of 1000 by the india-rubber, while the india-rubber tube must be
stretched 500 mm. out of 1000 in order to produce an equilibrium. If the tube is
now opened above, it at once contracts 500 mm. (supposing it to be perfectly elastic),
while the steel cylinder elongates only o'imm. ; the change of dimension is therefore
5000 times greater in the case of the india-rubber than in that of the steel, although
the actual tension of the two was the same. But the alteration of dimension
indicates only the amount of stretching to which the india-rubber, and of compres-
sion to which the steel was subjected. If therefore the layers of the tissue of an mter-
node are separated from one another, the alterations of dimension which then ensue
depend on the extensibility and compressibility of the layers as well as on the
amount of tension. There is only one case in which the amount of tension can
be inferred from the changes in dimension of the tissues when freed from a state of
tension, t;/2. when their extensibility and compressibility are the same, and when
perfect elasticity also exists in both. But the case is quite different with growing
internodes ; the extensibility of the tissues when in a state of tension is constantly
changing in consequence of growth. In a young internode the epidermis and wood
are very extensible; if they are separated from the pith this latter Only lengthens
slightly, because it was only slightly compressed, but the epidermis and the wood
contract very considerably because they are very extensible and were stretched by
the pith. On the other hand the alterations of dimension in layers of an older though
not mature internode will be the reverse. The pith, when freed from the tension,
elongates considerably, but the wood contracts only slightly, because its extensibility
is now but small and it was but slightly stretched by the pith ; the pith on the contrary
being very compressible, was prevented from lengthening by the resistance of the wood.
The intensity of the tension cannot by any means be determined in either case from the
changes of dimension ; these only show that there are tensions, and indicate also what
parts are extensible and compressible, and which are in a state of positive and negative
tension^. It may be laid down as a rule that when the separation of two tissues causes

^ In his treatise On the Tension of the Tissue of the Stem and its Results (Bot. Zeitg. 1867,
No. 109) Kraus has employed the differences of length between the entire internode and its isolated

794 MECHANICS OF GROWTH.

one of them to contract or expand, while the length of the other apparently does
not change, both layers were nevertheless in a state of tension, only the one which
remained unchanged in length was but slightly extensible or compressible, while the
other possessed these properties in a higher degree. When, on the other hand, an
internode consists of very extensible cortex and very compressible pith, both will
alter very considerably in length when separated; and yet the tension is not neces-
sarily as great as in another internode where the cortex is less extensible and the
pith less compressible, and where both undergo smaller alterations of length when
separated. Similarly in our system of steel and india-rubber, if the steel is supposed
to be replaced by a cylinder of india-rubber, this cylinder would be very strongly
compressed by the tube of india-rubber which in its turn would be stretched by
it; and when the system was broken up a smaller contraction would take place of
the tube but a much greater elongation of the cylinder than in the case of the steel,
even if the tension put into action had been the same in amount as in the system of
steel and india-rubber.

Sect. 15. — Phenomena due to the Tension of Tissues in the growing
parts of Plants^. A. Tension of different layers of a cell-wall. By cutting as large
pieces as possible out of the walls of Hving cells and placing them in water, it is
possible to demonstrate the existence of tensions in them ; it is found that if the
cell-wall consists of layers of which the outer ones have a less and the inner ones a
greater capacity of imbibition, the piece of cell-wall will bend so that the outer side
becomes concave, the inner side convex. If the greater part of the water of imbibition
is withdrawn from the piece of cell-wall by placing it in a solution of sugar or in
alcohol or thick glycerine, the bending diminishes or even changes into the oppo-
site direction, the inner side becoming concave ; this direction being again reversed
by again placing the object in water. Narrow strips which may be cut at right
angles to the surface out of pollen-grains of Cucurhita or Althcea or the cells of
the internodes of Niiella are well adapted for this experiment.

The concave curvature outwards evidently depends on the inner layers of the

layers of tissue as a general measure of the intensity of the tension ; but this, it will be seen from
what has here been said, is inaccurate. If, for example, the wood and pith of an old internode are
isolated, the contraction of the former is scarcely perceptible, while the latter elongates considerably ;
the pith of the internode was therefore, according to this method, in a state of great tension, while
the wood was not ; although the degree of tension of the two was really the same, differing only in
sign (positive and negative). On p. 112 (/. c), Kraus gives a correct account of the behaviour of the
layers of tissue of growing internodes.

^ The phenomena here described were first observed, although somewhat superficially, by
Dutrochet (Mem. pour servir k I'hist. des veget. et des anim. 1837, vol. II). Hofmeister, in his
treatise On the Bending of Succulent Parts of Plants (Berichte der kon. sachs. Gesells. der Wissensch.
1859), made some important corrections of the theory. On the Direction of the Parts of Plants
caused by Gravitation, see ibid, i860; on the Mechanics of the Movements due to the Stimulation of
Parts of Plants, Flora, 1862, No ^2 et seq. A connected account of the phenomena was given in my
Experimental-Physiologic, p. 465 et seq. Very minute investigations were published by Kraus in
Bot. Zeitg. 1867, No. 14 et seq., where the transverse tension of wood caused by the increase of its
diameter was also for the first time described, Nageli and Schwendener also contributed to the
development of the theory in their ' Mikroskop,' p. 396 et seq. Still these phenomena require a much
more exhaustive examination than has yet been given them ; the account here given will only serve
to introduce the student to facts which are easy of observation. In explaining the processes in the
interior I differ greatly from the views of Hofmeister (Lehre von der Pflanzenzelle, p. 272 et seq.).
The difference in our views is so complete that it would be useless to point out particular points of
difference.

PHENOMENA DUE TO THE TENSION OF TISSUES. 795

cell-wall absorbing more water in the direction parallel to their surface than the
outer layers, and thus stretching more and becoming the convex side of the system.
When water is withdrawn the opposite result must ensue. Let us suppose the cell
to be closed and entire and not at all or scarcely turgid, i. e. with no hydrostatic
pressure between cell-wall and cell-contents : the inner face of the cell-wall will then
be in contact with the cell-sap, and will absorb more water than the outside ; a
tension will therefore be produced, the inner layers of the cell-wall having a tendency
to stretch, and being partially prevented from so doing by the outer layers. This
tension of its layers will impart to the cell-wall a certain stiffness and rigidity which
is quite unconnected with turgidity. But since in the normal state, and especially
when they are growing, cells are always turgid, the whole system of tissues will be
distended independently of this.

If narrow strips are cut out of large succulent cells, or very thin slices of tissue
are made so as not to contain any perfect cells, a concave outward curvature is
obtained at the moment of making the section. This is at once explained by recol-
lecting that the outer layer, especially when cuticularised, was in a state of passive
tension before the section was made; while the inner layer, which was in an
absorbent condition, was swelled up from contact with the cell-sap. At the moment
of division this inner layer retains its water of imbibition ; but the outer layer, which
was in a state of greater tension, obeys its elasticity, and in consequence of its
contraction becomes the concave, the inner the convex surface of the section. It
is clear however that these phenomena must also occur when water is removed or
absorbed. It is only in this way that it seems to me possible for the cell-walls to
take any part in the tension of the tissues, a part which however must always be
subordinate in the closed living cell to the influence of turgidity, since this stretches
both the inner and outer layers, and every change in the degree of turgidity must
cause contraction or distension of the entire cell- wall.

It is a question not without importance in what relation the imbibition and
swelling of the cell-wall stand to the turgidity of the whole cell. If we imagine a
single turgid cell, and suppose that from any cause the cell- wall (whether the layers
are in a state of tension or not) is able to absorb more water from its contents
than it had before, the question arises whether the turgidity is thus increased or
diminished. By the increased amount of water absorbed from the contents by the
cell-wall, they must be diminished, as also must the hydrostatic pressure on the
cell-wall, and the more so when the size of the cell is increased by the imbibition.
But since the cell-wall may also increase in thickness, the pressure on the contents
may be supposed to increase from this cause. If however we take the simplest and
least favourable case, viz. that the size of the cell remains unaltered but the thickness
of the wall increases, and therefore that it distends inwardly, this will nevertheless
not cause any increased pressure between cell-wall and contents, because the water
which was the sole cause of the thickening of the cell-wall and diminution of the
cell-cavity was withdrawn from the cavity. The swelling of the cell-wall can at the
most diminish the size of the cell-cavity ^ by the volume occupied by the water with-

^ When an amount of water v penetrates into an organised body, and increases its volume, the
increase of volume can never be greater than v, but at the most as large. The evolution of heat

79^5 MECHANICS OF GROWTH,

drawn from it. No increase of turgidity can therefore take place in this case, and
still less when the cell also increases in size. The same argument of course applies
also to a multicellular mass of tissue. But the case is different when the water with-
drawn from the cell-contents by the cell-wall is replaced by means of endosmose,
and the turgidity thus again increased ; in this case in proportion as water is absorbed
by the cell-wall the turgidity and volume of the whole cell must also increase.

B. Mu/uai Tension of the layers of tissue of an organ, (i) Tension in the direction
of length; i.e. parallel to the axis of growth of the organ. In the internodes of
upright stems some idea may be obtained, if not of the intensity of the tension, at
least of its kind (whether negative or positive), and of its variation in the different
layers of tissue, by measuring the length of the internodes, and then separating the
layers of tissue by a sharp knife, and comparing their length with that of the entire
internode. It is obvious that the length of the entire internode is the result of
the mutual tensions of its layers, some being, in this experiment, shorter and some
longer than the entire internode; and it results from what has already been said
about opposite tensions that if any particular layers have not changed in length
after being separated, this does not prove that they were not distended or com-
pressed when forming a part of the system, but only that they opposed a strong
resistance to the tensioil then in existence, which resistance rendered the alteration
of their length imperceptibly small. But the opposite is also possible ; viz. that a
layer of tissue when separated will show no perceptible contraction because it was so
extremely extensible and inelastic that it yielded with extremely little resistance to
the traction of the layers which were in a state of positive tension, the limit of its
elasticity being continually overstepped.

If this method is applied to rapidly growing internodes, it is generally found
that isolated strips of the epidermis, of the cortex, or of the wood (xylem), are shorter
than the entire internode, while the isolated pith is considerably longer ; the former
therefore were in a state of negative, the latter was in one of positive tension.
All the isolated layers are flaccid, while the entire internode was rigid from the
mutual tension.

If a median longitudinal lamella bounded by two strips of epidermis is cut out
of a growing internode with its xylem still unlignified, and if its tissues are then
isolated so as to lie side by side, then, indicating the epidermis by E, the cortical
layer by C, the xylem by X, the pith by P, the respective lengths after isolation may
be stated as follows : —

E<C<X<P>X>C>E.
It is at once evident from this that every layer was before the separation in a state
of negative tension towards the next one inside, of positive tension towards the
next one outside. The epidermis alone was in a state of passive tension ; the pith
alone was passively compressed, or rather prevented from extending.

The extensibility and elasticity of tissues are altered during the growth of an
internode, as may be seen by comparing internodes of various ages ; the exten-
sibility of the wood decreases rapidly, that of the epidermis and cortex more slowly,

during imbibition indicates that a decrease of volume is taking place, and therefore that although v
is the amount of water absorbed by imbibition, the increase of volume is only v — d.

PHENOMENA DUE TO THE TENSION OF TISSUES, ygy

as may be inferred from the decreasing rapidity with which these tissues contract on
their isolation, and from the thickening of the cell-walls \ The pith from internodes
of different ages shows on isolation at first an increasing, afterwards a decreasing
amount of elongation. If the tendency of the pith to expand remained the same
at all ages, it would, when isolated, elongate more in older than in younger inter-
nodes, in consequence of the increasing resistance of the tissues which are in a
state of passive tension ; but when the growth in length has ceased, or soon after,
the pith loses its tendency to expand, as may^be concluded from the fact that on
isolation from such internodes it elongates less, and finally not at all^, although the
resistance of the wood has greatly increased; were the pith now as elastic as before,
it would expand more rapidly when freed from the very great resistance of the
wood.

The following table will now be understood ; the length of the entire internode
being always placed at loo, and the amount of contraction indicated by negative, of
expansion by positive percentages.

Number of the intemode, Change of length of the isolated tissue
counting from the youngest. in percentage of the entire internode.

icoiiana Tabacum

I-

- IV

Cortex.

-5*9

Xylem.

-1-5

Pith.
+ 2-9

V-

-VII

-31

— I'l

+ 3-5

VIII-

- IX

-35

-i'5

+ 09

X—

- XI

-0-5

-0-5

+ 2-4

do.

I-

• II

— 2-2

+ 2-3

III-

■ IV

— 1*2

+ 4'2

V-

-VII

— I'O

-1- 2-8

VIII-

-IX

-1-8

+ 2-7

imbucus nigra

I
II

— 2-6

— 2-0

-2-6

-2-8

+ 4-0
+ 5'5

III

-i'5

— o*o

+ 1-5

do.

I

-0-6

+ 3-7

II

-1-6

+ 5-1

III

— oo

+ 0-9

do.

I

-I'S

+ 6-5

II

-1*5

+ IO-I

III

-0-6

+ 2-3

These numbers, taken from my Handbook of Experimental Physiology, may
be supplemented by some others, calculated from the statements of Kraus^ (/. c.
Table i).

1 The decrease in the extensibility of the epidermis was determined by Kraus {I.e., tables,
p. 9), by attaching weights to strips of epidermis.

^ The relation between the tension of tissues and the state of growth of the internode (i. e.
the phase of its greatest period of growth) requires fresh and detailed investigation. Kraus's
Table III (Bot. Zeitg. 1867) shows that the greatest difference of length between cortex and pith
does not always occur at the time of the greatest growth ; and that even after growth has ceased,
tensions may still continue. It must however be remarked that the method by which these num-
bers have been obtained is liable to considerable suspicion.

3 Kraus has only given the absolute numbers ; but a correct notion can be obtained only by
comparing them with the length of the intemode.

798 MECHANICS OF GROWTH.

node

imber of the inter- _.

, counting from the ^^^"^^ ^^ ^^"^^^ f ^^^ !^°^.^*^^ ^'^^
vouneest ^" percentage of the entire mtemode.

Nicotiana Tabacum

Ill- IV

Epidermis.

-2-9

Cortex.

Xylem.

-1-4

Pith.

+ 3-5

V— VI

-2-9

-I'S

-0-8

+ 2-7

VII— IX

-2-7

— 21

— CO

+ 3-4

X— XII

-14

~o-5

— o-o

+ 3-4

XIII— XV ..

-1-05

— O'O

-0-8 (?)

+ 4'o

Vitis vinifera

I

-3'i

-1-6

+ 6-0

II

-1-7

— 0-0

+ 8-7

III

-2-5

(?) -i-o(?)

+ 7-1

IV

— O'O

— 0-0

+ 60

V

— o-o

— 0-0

+ 2-7

Samhucus nigra

I

31

— O'O

+ 0-0

II

-i'5

— 10

+ 6-4

III

-1-6

+ 6-5

IV

-1-6

+ o-3(?)

+ 6-1

V

— 0-2

+ 0-2(?)

+ 0-7

isus

VI

I— IV

-4*3

.-05

-0-5

+ 0-I

Helianihus tuhen

-17

+ 6-8

V— VI

~i-7

-0-0

+ 6-6

VI— VII

-0-9

-0-4

+ 4-4

VIII

-0-5

-0-0

4-3-2

IX— XI

— o-o

+ 0-9 (?)

4-2-0

It is easy to establish the existence of similar contractions of the outer tissues
and elongations of the parenchyma in the case of growing leaf-stalks, as those of
Beta^ Rheum, Philodendron, &c.

If a growing internode or a leaf-stalk is split by two longitudinal sections at
right angles to one another, the parts will bend concavely outwards, evidently in
consequence of the lengthening of the pith and contraction of the outer tissue. This
phenomenon is seen most clearly if a thin longitudinal slice is taken from the middle
of the internode, laid flat, and the pith then halved lengthwise ; as the knife advances
the two halves will bend concavely outwards. If, instead of cutting the section in
two, thin strips of tissue are cut proceeding from without inwards, first one including
the epidermis, next one including the cortical tissue, and finally one including the
wood, they will all bend concavely outwards, because the adjacent layers are all in a
state of negative tension on the outside, of positive tension on the inside, and when
separated, the outer side always becomes shorter, the inner side longer.

That this bending is caused by simultaneous contraction of the outside and
lengthening of the inside is at once clear from the measurements already given, but
may also be observed directly, as will be seen from the following table. Longi-
tudinal slices of considerable thickness were cut from the middle of growing inter-
nodes, laid flat, and the pith then halved by a longitudinal cut; the radius of the
curvature which each half at once assumed was determined, and the length of the
convex inner and the concave outer side measured by means of a strip of card
graduated in millimetres.

PHENOMENA DUE TO THE TENSION OF TISSUES.

799

Silphium perfoliatum.

Left half

Right half
Silphium perfoliatum.

Left half

Right half
Macleya cordata.

Hollow

Length of
the entire
internode.

69*5 mm.
69-5

190
190

Radius of
curvature of
the segment.

4 cm.
4

134*5

3—4
3—4

5-6

Shorten-
ing of the
concave
outer (epider-
mis) side.

2*8 p.C.
2-4

2-8
2-6

074

Lengthen-
ing of the

convex

inner (pith)

side.

9'3p.c.
93

9*5
10-8

71

Semidiameter

of the

internode.

3 mm.
3

35
45

3*3

As we have already seen from the measurements of the layers when entirely
isolated, it was also evident from the curvature of the two halves of the longitudinal
slice that the contraction of the epidermis is less than the elongation of the pith.
Since a slice is somewhat longer than the entire internode, if the length of the
slice were taken as =100, the proportionate contraction of the outside would be
greater, the lengthening of the inside less.

A rapid rate of growth, united with a certain amount of physical differentiation
of the different layers of tissue, such as occurs in erect leafy shoots, stout leaf-stalks?
and tendrils, appears generally to be favourable to the production of the tensions in
tissues of which we have been speaking, as they are not found in stems of very
slow growth, like stout rhizomes, the thick stolons of Yucca and Draccena, &c.
That the existence of tension has more to do with a physical differentiation in the
elasticity and extensibility of the layers than with a morphological one, is shown by
the fact that very considerable tensions are found even between the outer and inner
layers of the hyphal tissue of the stipes of the larger Hymenomycetous Fungi, which
are morphologically similar. Within the growing apical region of roots, on the con-
trary, where we have a combination of two layers of tissue sharply differentiated
morphologically, viz. an axial fibro-vascular cylinder surrounded by a parenchymatous
cortex, we do not find any considerable tension when the part is split by two longi-
tudinal cuts at right angles to one another, or when the layers are completely isolated.
But since it is easy to prove that the cortex of the root grows more rapidly and for
a longer time than the axial cylinder^, it may be assumed that in an uninjured grow-
ing root there is nevertheless a small tension between them, positive in the case
of the cortex, negative in that of the axial bundle ; but it is only rarely that this
tension becomes strong enough to be perceptible by the parts bending inwards when
cut lengthwise ; probably because the axial cylinder, which still consists of pro-
cambial tissue, is so extensible that it yields almost without resistance to the traction
of the cortex. The case is diflferent in the older parts of the root behind the
growing end (which does not exceed 10 mm. in length). If this portion is split, the
parts generally gape concavely outwards, although much less so than the growing
part of erect stems. The curvature is however considerable in the atrial roots

* The halves of roots split lengthwise continue to grow for days, and bend concavely on the cut
surface.

8C0 MECHANICS OF GROWTH.

of Aroideae, where the opposite curvature which takes place at the apex is also
sometimes well-marked.

The description now given of the states of tension in the case of stems is
also applicable to all expanded internodes and leaf-stalks. Within the bud itself, and
especially at the punctum vegeiaiionis , there appears to be no tension of the tissues, or
only one as slight as in the apices of roots. It is only when the epidermis is be-
coming cuticularised and the walls of the bast-cells are beginning to thicken that
the tensions become perceptible.

The individual parts of fully mature organs, especially leaves, not un frequently
retain the tensions acquired during growth, which are in such cases often particu-
larly strong. This is the case, for instance, in the contractile organs of the sensitive
and periodically motile leaves of Papilionaceae, Mimoseae, Oxalidese, &c., to which we
shall recur. While in these cases the true leaf-stalks and the internodes from which
they spring have long become rigid, and no longer show any considerable tension of
the tissues, an extraordinary elongation of the parenchymatous cortex occurs in
the contractile organs, if they are separated from the solid axial fibro- vascular
bundles; and considerable flexion results when these organs are split lengthwise.
The opposite to this occurs in the nodes of the stems of Grasses, i. e. in the annular
thickenings at the base of the leaf-sheaths ; no perceptible tension is observable in
these. If a median longitudinal section is made and divided into its inner and
outer layers, they exhibit none of the curvatures which are so striking in portions of
young internodes. This flaccidity of the tissue, or at least the insignificance of the
tension, must depend on the concurrence of two causes ; on the one hand on the
cessation of the growth of the parenchyma in the node (although it remains in a
state capable of growing, and under certain circumstances begins to grow again),
and on the other hand on the extensibility of the fibro-vascular bundles which do
not become lignified within the node, or not till a late period when the cells of the
same bundles, where they lie in the leaf-sheath and the internode, have long become
lignified and rigid. While, therefore, the parenchyma of the node continues to
grow, it stretches the unresisting fibro-vascular bundles, and when its growth ceases
no perceptible tension remains. In the contractile organs of sensitive and periodi-
cally motile leaves, on the contrary, the axial fibro-vascular bundle becomes elastic
and resistant before the growth of the surrounding parenchyma has ceased; and
when this is the case a tension remains which is further increased by the extra-
ordinary capacity of the parenchyma for becoming turgid.

If we now attempt to give an account of the causes which render the tension
at first (when in the bud) imperceptible in the internodes of erect rapidly-growing
stems, and make it subsequently increase and finally altogether disappear when the
internodes are fully mature, we find that we must content ourselves with probable
conjectures rather than with fully demonstrated propositions.

The origin of tension between the layers must in any case be referred mainly
to differences in the growth of the cell-walls of such a nature that the intercalation of
fresh material takes place less rapidly in those of one layer than in those of another ;
and it is especially manifest in those cases in which the cell-walls subsequently undergo
thickening. From the first of these causes the layers which lengthen more slowly are
placed in a state of passive tension by those that grow more rapidly; while the second

PHENOMENA DUE TO THE TENSION OF TISSUES. 8oi

cause diminishes their extensibility to an increasing extent, especially when, as in
the xylem of the fibro-vascular bundles, the cell-walls become lignified, which renders
them capable of resisting extension. The more quickly, on the other hand, the thin
cell-walls in the pith and parenchyma generally increase in size (especially in length)
by superficial growth, the stronger becomes the tension of the passively stretched
layers of tissue. To this must be added the peculiar power of the medullary
cells to absorb water from the older parts with great force and rapidity, and thus
to maintain themselves in a state of the highest turgidity. This distends the pilh
independently of the superficial growth of its cell-walls, and besides influencing the
more slowly growing layers of tissue, also contributes to increase the superficial
growth of the cell-walls of the pith. If the woody bundles then become lignified as
the tissues become more developed internally, and the resistance of the epidermis,
which is constantly becoming more cuticularised, becomes too great, these tissues
oppose an insuperable resistance to the further distension of the pith by growth and
turgidity, and no further elongation of the internode is possible. The tendency of
the pith to expand ceases; its cells lose their turgidity, they give off their water
to adjacent tissues, and become filled with air.

According to this view, which has been fully established in the main, the actual
motive power of growth in internodes emerging from the bud-condition is the pith,
and the thin-walled parenchyma generally. It is only the force thus exercised that
causes the other tissues to increase in length as long as they are sufficiently
extensible. The extraordinary absorbent power possessed by the pith enables it
when growing to withdraw the water from the surrounding layers of tissue, and
•thus prevents the cells from becoming more strongly turgid, neutralising by this
means one of the causes of the superficial growth of the cell-walls. It must also
be remembered, as has already been shown (Fig. 478), that the turgidity of the
cells of the stretched layers is even diminished, while that of the compressed cells
(in the pith) is increased by the tension; and we consequently have here another
cause of differences in the superficial growth of the cell-walls. Finally, it must
be borne in mind that the internodes, at least of land-plants, are exposed to
transpiration as soon as they emerge from the bud ; but this cause of diminished
turgidity will affect chiefly the epidermal cells and the subjacent layers, least of
all the pith.

The great importance which is here attached to turgidity as a cause of growth
is justified by the fact that the growth of the internodes is at once stopped by its
decrease, t. e. by the withering of the shoot ; while it is promoted by its increase,
i. e. the growth of the shoot in water or damp air.

The first and most efficient cause of the tension of tissues in a growing inter-
node is therefore the different capacity for turgidity of the different tissues; this
depending partly on the nature of their fluids, partly on the structure of their cell-
walls, and partly on their relative position in the internode. A more secondary place
must be assigned to the swelling of the cell-walls caused by imbibition ; since it may
be assumed that even when the turgidity of the cell is slight, the cell-wall still obtains
sufficient water to satisfy its capacity for imbibition. If it were directly dependent on
this, all the layers of tissue would grow equally rapidly, even when the turgidity was
small, or had entirely disappeared. I rather hold the state of the case to be that

3 F

8o2 MECHANICS OF GROWTH.

when the cell-wall is passively distended by turgidity or by the tension of the sur-
rounding layers of tissue, it is only enabled to deposit fresh substance in the direc-
tion of its surface when perfectly saturated ; this does not however imply that other
causes do not cooperate in promoting the intercalation.

The importance of turgidity as a cause of growth may be very strikingly illus-
trated in the case of isolated cylinders of pith, as we shall show presently.

When, in consequence of their separation, the tissues which were in a state
of passive tension become suddenly shorter, and the pith which was in a state of
positive tension suddenly longer, this process must be connected with a corresponding
change in the form of the cells ^ ; the cells which contract must at the same time
become wider in diameter, while those of the pith which lengthen must on the con-
trary become narrower. It is impossible however to measure directly these changes
of diameter, which are so small that ordinary methods are inapplicable.

It is, however, a necessary consequence of what has been said that the passive
lengthening of the epidermal cells, &c. in a growing internode makes them narrower;
the } oung epidermis must therefore be too narrow, besides being too short for the
inner masses of tissue. Similarly the pith, being prevented from elongating in the
growing internode by the surrounding layers, must in consequence have a tendency
to enlarge transversely; besides being too long for the elongated tissues, it will
also be too thick for them, and must have a tendency to force them apart. It
follows therefore from the longitudinal tension which has been observed in the
layers of tissue of a growing organ, that a transverse tension must also exist in it
of such a nature that the outer layers are in a state of passive tension, while the
medullary cells which are prevented from lengthening have a tendency to dilate
transversely.

If thick transverse sections^ from growing stems are cut radially, they gape
open, evidently because the epidermis contracts in the peripheral direction, having
been previously of too small circumference for the inner tissue, in other words, in
a state of passive tension. The tendency of the medullary cells which are pre-
vented from lengthening to become broader transversely does not appear, on the
other hand, to be always hindered by the surrounding wood and cortical tissue, but
often to be even promoted by them ; so that these layers of tissue which surround
the pith grow more rapidly in the peripheral direction than does the pith itself, and
therefore exercise a radial traction upon it. A striking proof of this phenomenon is
afforded by the frequent formation of cavities in stems and leaf-stalks at the time
and place where the growth in length is most rapid. The increase in thickness of
the pith is not sufficient to fill up the space which is enclosed by the surrounding
tissues, and which increases in size ; its cells separate in the longitudinal direction,
and the woody cylinder remains clothed on the inside by a layer of pith, the longi-
tudinal tension of which still continues. The existence of an outward traction upon

^ Any considerable change in the volume of the medullary cells when isolated must not indeed
be expected, when it is recollected that neither the water contained in the cells nor the cell-walls
permeated with water alter their volume under the forces exerted in this case. An alteration in the
volume of the entire pith could at most arise from a change in the size of the intercellular spaces in
consequence of the change in form of the cells.

^ Sachs, Experimental-Physiologic, p. 471.

PHENOMENA DUE TO THE TENSION OF TISSUES. 803

the pith can also be demonstrated in the case of internodes with solid cylinders of
pith which are growing and at the same time increasing rapidly in diameter {e.g.
Nicoiiana, Silphium perfoliatum), by dividing a fresh transverse section (laid on
glass) through the centre. The two cut surfaces of the pith now become curved
outwardly and separate from one another, while the cortical parts of the segment
still touch. This is an indication of the outward traction of the pith, and of the
tendency of the cortical envelope to dilate peripherally.

These statements rest however at present on but a small number of observa-
tions, and better results may be expected from their repetition. It may nevertheless
be assumed that in young internodes, before the fibro-vascular system has begun to
become lignified, the pith exerts an outward pressure. This is accompanied later,
when the tangential growth of the wood and cortex is more rapid, by an outward
traction, which at length becomes so strong as to exceed the tendency of the pith
to dilate transversely. The pith is therefore now actually in a state of passive
tension transversely (and at the same time compressed longitudinally), until at length
the cells in the centre of the pith become detached from one another, and a hollow
is formed, if the whole does not lose its sap and become dried up, as for example
in the Elder. Kraus observed ^ that the medullary cells of an internode are longer
when it is growing than when mature ; but this is to be attributed, in accordance
with what has been said, to the loss by the cells of the pith of their power of
elongating when isolated. In the internode they are certainly not at first longer,
and are afterwards actually shorter ; the difference is only observable on isolation,
and indicates that these cells at length lose the property of changing their form when
isolated, or in other words become rigid.

The views here brought forward respecting the tension of the tissues of growing
internodes and leaf-stalks are, I think, supported by the fact that the sudden and
very considerable lengthening of the pith at the moment of its separation from the
surrounding layers of tissue is followed by a slow lengthening which lasts for some
days, while, on the contrary, the cortex and epidermis, which are in a state of passive
tension, scarcely experience afterwards any perceptible contraction (but, according to
Kraus, do not become longer even when placed in water). This subsequent length-
ening of the isolated pith takes place with extreme force when it absorbs water,
as Kraus has already shown ; but the lengthening also continues in dry air when
the pith even loses small quantities of its water, a point which had been previously
overlooked.

The isolated cylinder of pith of a growing internode is very flaccid, flexible, and
extensible ; but if placed in water it soon becomes tense, rigid, and elastic, longer
and apparently also thicker. The lengthening may amount in a few hours to as
much as 40 p. c, or even more. These phenomena are explained if we suppose
the contents of the medullary cells to be very strongly endosmotic 2, by which they
become in a high degree turgid, and thus not only increase considerably in size.

* Bot. Zeitg. 1867, p. 112.

"^ Notwithstanding this powerful endosmose, the amount of solid substance dissolved in the
cell-sap of the parenchyma is very small, as is shown by the fact that in cylinders of pith of this
kind I found the dry weight only from 2 to 5 p. c, a considerable portion of which belonged to the
cell-walls and protoplasm.

3 F 2

8o4 MECHANICS OF GROWTH.

but also become more rigid. The considerable increase in size presupposes, how-
ever, from the rapidity with which it takes place, great extensibility in the cell-
walls. Isolated prisms of pith exposed to the air become shorter even than the
length they possessed in the internode^; the cell-walls which were previously in
a state of tension evidently contract elastically, as the turgidity diminishes from loss
of water.

But if care is taken that isolated cylinders of pith do not absorb any water,
while at the same time they can only lose a very small quantity of it, by enclosing
them in a glass tube containing about i litre of dry air, they nevertheless continue to
lengthen perceptibly for some days, although not so considerably as when they
absorb water; and this lengthening affects chiefly the older parts, while the
youngest parts sometimes contract. The whole cyhnder becomes dry and rigid
on the surface. Out of a large number of observations the following may be chosen
to elucidate this point.

A prism of pith from a part of a shoot of Senecio umbrosus 235*5 mm. long,
lengthened about 57 p. c. on isolation, and weighed 5*3 grammes. It was divided
into three parts by marks of Indian ink; their lengths being : — i. (the oldest) 100 mm.,
ii. 100 mm., iii. (the youngest piece) 49 mm. The prism of pith was now fixed in
a dry glass tube, which was then corked at both ends. After fourteen hours the
parts had lengthened as follows : — part i. about 4*5 mm., part ii. about 6*5 mm., part
iii. about 2 mm. or 4*1 p. c, while the pith had lost 0*15 grm. of water. After re-
maining for twenty-six hours more in the glass tube the following further changes
had taken place ; part i. had again lengthened about 2*5 mm., part ii. about 0-5 mm.,
while the length of part iii. had diminished about 0*5 mm. No further loss of
water had taken place, because the glass tube had become covered with moisture.
The pith was now placed in water, and after six hours the following increase of
length had taken place: — in part i. about 18 mm. or 16*8 p. c, in part ii. about
23 mm. or 2i"6p. c, in part iii. about 11 mm. or 2i'6p. c. (as compared with the
length before placing in water). The pith had also become considerably thicker,
having absorbed 6 grammes of water. The estimation of the dry weight showed that
the pith contained only 0*22 grm. of solid substance; this was combined, when the
pith was isolated, with 5*08 grm. of water ; it subsequently lost 0*15 grm., but by the
end of the experiment had again absorbed 6 grm. At first therefore the pith con-
tained 4-23 p. c, at last only 1-97 p. c. of solid substance. Experiments of this
kind show that the pith of the youngest internodes loses its water most easily by
evaporation, as is shown by its decrease in length. Kraus was led by other ex-
periments to the same conclusions; and he also showed — not in contradiction, as
he thought, but in harmony with these results {I.e. p. 123) — that the older pith of
growing internodes attracts water more powerfully and expands more than the
younger.

If the question is now asked how the lengthening of the pith can take place in
spite of the loss of water (though this may be small), it must first of all be noted that
its surface becomes remarkably dry under the circumstances described. It is scarcely
possible to attribute this significant desiccation of the surface to the small loss of

* Kraus, /. c, Tables, p. 29.

PHENOMENA DUE TO THE TENSION OF TISSUES. 805

water indicated by the weight ; it is probably rather caused by the inner cells of the
pith withdrawing water from the outer cells, and thus lengthening; but the outer
cells would become shorter if they were not stretched by the inner ones. That this
is actually the case is shown by the rigidity of the pith under these circumstances,
caused by the tension that subsists between the dry outer layer and the moister
inner mass. If the prism of pith is divided lengthwise, the parts curve outwards ;
and sometimes the outer surface becomes even strongly concave. If the inner cells
of the pith are able to withdraw water from the outer ones, it may be inferred that
the outer cells are also able to withdraw it from the surrounding wood and especially
from the peripheral tissues, preventing these from becoming strongly turgid ; their
growth being thus retarded in favour of that of the pith, by which they are now
placed in a state of passive tension. It is noteworthy that the medullary cells which
contain a minimum quantity of dissolved substances nevertheless absorb water suffi-
ciently powerfully to abstract it from the surrounding tissues which must evidently
contain a much greater quantity of dissolved substances.

It is now clear from the observations which have been described, why portions
of shoots cut lengthwise in half or in four and placed in water curve outward to
such a remarkable extent ; and why a curvature, which may be small but continues
to increase for some time, takes place when such pieces are placed in a closed glass
tube in dry air.

(2) Transverse tension caused by subsequent growth in thickness of the wood'^. It
has already been shown that transverse tensions also arise during growth which are
caused by the longitudinal tension; a more exact knowledge of these is still a
desideratum. With the commencement of the increase in thickness of the stem by
means of the cambium-ring, a new cause of tension arises, acting in both a radial
and peripheral direction ; and this transverse tension generally continues as long as
the cambium-ring remains active. The layers of tissue formed from the cambium-
ring have at first a tendency to expand in the tangential direction to an extent
greater than the space enclosed by the epidermis and the primary cortex permits.
These outer tissues therefore become stretched in the peripheral direction; and,
since they are elastic and have a tendency to contract, they exert a pressure in the
radial direction on the cambium and the tissue formed from it. It happens however
also that the rings of wood produced on the inside of the cambium grow more
strongly in the tangential direction than the phloem produced on the outside, which
is therefore passively stretched. A tension is hence set up in the transverse diameter
of the stem during its increase in thickness of such a kind that each layer is stretched
peripherally on its outside and compressed radially on its inside ; in other words, is
in a state of negative tension on its outside, of positive tension on its inside. If the
separate layers of a transverse segment — epidermis, primary cortex, phloem, and
xylem — are separated, and their peripheral length compared, we get the following
expression for the transverse tension : —

E < C < Ph < X.
As the increase in thickness proceeds the transverse tension increases, as is shown

* [See Detlefsen, Ueb. DiGkenwachsthum cylindrischer Organe, Arb. d. bot. Inst, in Wurzburg,
II. I, 1878.]

8o6 MECHANICS OF GROWTH.

by Kraus's very complete experiments ; /. e. if the rings of tissue in a transverse
section of the stem or in a woody branch are separated from one another, by
dividing it longitudinally and then separating the rings, they contract the more the
nearer they lie to the circumference, and the contraction is the more considerable,
compared with the original circumference of the whole, the older the internode from
which the section is taken. The traction upon the cells of the epidermis and of the
primary cortex caused by the transverse tension is easily observed by the microscope
in the transverse section, if young internodes of plants which increase rapidly in
thickness, as Helianthus^ Ricinus, or JRibes, are compared with those which have
already been forming wood for some weeks or months. The form of the cells
shows that they have been stretched in the peripheral direction (see Fig. 56), and
have in consequence grown rapidly in that direction; the cells which have been
thus altered in form are divided by radial septa. But at length the epidermis and
primary cortex are no longer able to obey the peripheral traction ; longitudinal
fissures occur in the cortical tissue, generally after the commencement of the forma-
tion of cork. When the periderm and cork have been formed on the older parts of
stems, these secondary epidermal tissues undergo a continuous strain in the peri-
pheral direction, and exert in turn a radial pressure on the living phloem, cambium,
and xylem. The first result of this pressure exerted by the growing inner tissues
is the splitting of the layers of bark, especially longitudinally. The form of the
fissures depends, however, on the course of the bundles of bast w^hich take part
in the formation of the bark, and on other relations of the tissues to one another.
If a stem does not in its growth take the form of a cylinder or slender cone but
of a spherical tuber, as in Beaucarnea and Tesiudinaria, the layers of periderm split
apart in the form of tolerably regular polygons which cover the spherical surface
of the stem like shields. These examples show at the same time that in those
Monocotyledons also which grow in thickness tensions are produced by the sub-
sequent increase of the stem in thickness similar to those caused by the activity
of a true cambium-ring ; for in this case it is replaced by a thickening-ring, in which
new layers of fibro-vascular bundles and intermediate parenchyma are constantly
being produced. (See Fig. 104.)

It is evident that before the bark splits or fissures already in existence become
wider and penetrate inwards, the transverse tension must attain a certain intensity,
which, from the great firmness of the bark, cannot be inconsiderable. At the
moment when the splitting takes place at least a portion of the tension must, how-
ever, be destroyed. This is clearly the reason why the transverse tension attains
its maximum (measured in the way described above), as Kraus has pointed out,
above the part of the stem where the scaling-ofi" of the bark begins. But even in
annual stems which increase rapidly in thickness, as Helianlhus, Dahlia, Sec, the
transverse tension does not progressively increase from the apex to the root, but
attains its maximum at an intermediate height, below which it diminishes. An
explanation of this phenomenon is afforded by the fact that the limit of the elas-
ticity of the bark is gradually exceeded by the long- continued pressure to which
it is subject from within, and that the cell-walls which are strained grow at the same
time by intussusception, and thus a portion of their tension becomes neutralised.

While we may consider the turgidity of the pith and its enormous endosmotic

K

PHENOMENA DUE TO THE TENSION OF TISSUES. 807

power as the principal cause of the longitudinal tension of growing internodes and
leaf-stalks before they become lignified, it is on the other hand probable that the
imbibition and swelling of the cell-walls are the chief cause of the transverse tension,
he wood, where the transverse tension chiefly originates, is, when mature, scarcely
adapted for any distension by turgidity ; while at all events in cells or vessels with
bordered pits it is altogether impossible. Closed wood-cells, when turgidity is
possible in them, cannot however expand greatly; since their own wall and the
woody substance which surround them are far too inextensible to stretch to any
considerable extent under the influence of hydrostatic pressure. It has, on the other
hand, been already shown (Sect. 13) what considerable alterations of dimension the
wood experiences especially in the peripheral and radial direction simply by imbi-
bition \ Every layer of wood freshly formed on the inside of the cambium-ring has
a tendency to grow wider in the peripheral direction, as long as the supply of water
is sufficient to cause a decided svvelling of the cell-walls. The cambial tissue is by
this means stretched tangentially, and the enlargement of its cells thus caused is
increased by turgidity; and from the thinness of their walls it may be assumed
that it is their turgidity that protects them from becoming destroyed by compression
between the wood and the bark. The elements of the secondary phloem — the bast-
cells and the phloem-parenchyma — can scarcely experience any great change of
dimensions owing to the swelling of their cell-walls ; the former are indeed thick-
walled, but they are not so arranged as to form a layer which increases in size from
this cause. The cells of the latter have such thin walls that their swelling produces
but little expansion, and experience teaches that they do not increase much in size
in consequence of turgidity. Finally, the periderm and the bark dry up and contract,
if not to any great extent, yet with considerable force.

The experience of every year shows that the fissures in the bark — especially
of thick trunks at the end of winter in February and March — become deeper and
wider, evidendy in consequence of the great swelling of the wood, which at this
time contains the greatest quantity of water; while the bark had time to dry up
and contract during the dry weather in winter. If the fissures increase in width
by the strong tension thus produced — which can be easily seen when fresh — the
damp weather in spring causes the bark to swell ; the tension between it and the
wood becomes much less, and the production of wood now begins afresh in the
cambium. While the wood is becoming thicker during the summer, the bark dries
up and shrinks, and the tension between the outside and inside again increases,
to cease once more in the following spring. Not only does an annual period of
transverse tension thus arise, but this is also the cause, as we shall see presently,
of the difference between the spring and autumn layers of wood.

The statements made in this section may be briefly summed up as follows; — The
tissues, at first homogeneous, become first of all differentiated in such a manner that
chemico-physical differences are set up, in consequence of which certain layers, espe-
cially the pith, absorb the water in the tissues more strongly than the others, and
consequently grow more rapidly ; and the layers which are less turgid and grow more
slowly are exposed to a passive traction which promotes their growth. After growth

* [Nevertheless the amount of water of imbibition which a single lignified cell-wall can take up
is small. (See Sachs, Ueb. d. Porositat des Holzes, Arb. d. bot. Inst, in Wurzburg, II. 2, 1879.)]

8o8 MECHANICS OF GROWTH.

in length has ceased it is principally the stronger imbibition and swelling of the wood
that presses the surrounding layers of tissue outwards and promotes their peripheral
growth.

The intensity of the longitudinal and transverse tensions consequently depends mainly
on the addition of water to the turgescent pith and the swelling wood ; any decrease
of the turgidity of the pith must cause it to contract, and hence the whole shoot to
become shorter and flaccid. This is in complete accord with observation, since withered
shoots, /'. e. such as have lost water by transpiration, have not only become shorter but
also flaccid. Any diminution of the amount of water absorbed by the wood must in the
same manner diminish the transverse tension and the diameter of the shoot. A small
loss of water in the peripheral tissue when in a state of passive tension does not on
the other hand usually cause directly any considerable increase in its tendency to con-
tract ; since the increase in its size due to turgidity and imbibition are generally much
less considerable than in the pith and wood.

If now there are circumstances which cause a daily periodic change in the quantity
of water contained in the tissues, the result will be also a periodic increase and decrease
in the intensity of the longitudinal and transverse tensions. Such a daily periodicity of
the tension has been actually discovered by Kraus (/. c. p. 122), who has observed that
the longitudinal tension estimated by the difference in length of the pith and the bark,
as well as the transverse tension estimated by the contraction of the bark when detached
from woody stems, decrease, under the normal conditions of life, from early morning
till midday or early in the afternoon, when they reach their minimum, and then again
increase, attaining their maximum early the next morning. Millardet determined this
periodicity in quite a different way; and since the objects on which he experimented
permitted an exact measurement, he detected in addition an increase, usually small,
of the tension in the afternoon. Notwithstanding the statements of Kraus — which
are partly opposed to this conclusion, but on the whole confirm it — I am inclined to
attribute this periodicity chiefly or altogether to the variation in the amount of water
contained in the tissues of the plant at different periods of the day. When transpiration
is greatly diminished during the night, the quantity of water in the plant must in-
crease, and with this the tension; and conversely the increase of transpiration during
the early part of the day must diminish the tension. Space does not permit me to
give in detail the opposing statements of other observers ; but this will be done in
part further on. Here I need only point out that the periodicity, especially of the
longitudinal tension, may possibly be also directly dependent on light, independently
of the heat which accompanies the light and of the increase of transpiration caused
by it (although this cannot be proved by Kraus's experiments, /. c. p. 125). As far as
concerns a daily periodicity independent of temperature, light, and the amount of water
contained in the tissues, I could only admit it when any other explanation of the
phenomena was shown to be impossible. At present this is not the case. From the
intimate dependence and correlation of growth and tension, from the fact discovered
by me^ that the daily periodicity of growth in length coincides in every particular
with the daily periodicity of tension observed by Millardet and Kraus, and that it is
caused simply by changes in temperature and light, I consider it very probable that
the daily periodicity of tension is also dependent on these agencies. On the one hand
they influence growth and through it the tension, while on the other hand they affect
the amount of water contained in the tissues by modifying transpiration and its
conduction from the roots. Like all other periodic phenomena of vegetable life,
that of tension requires a very careful investigation of its external causes before we
resort to the last expedient of assuming internal periodic changes, of which no explana-
tion can be given in the present state of our knowledge^.

^ Arbeiten des Bot. Inst, in Wurzburg, 1872, I, Heft 2. p. 168.

* [A daily periodicity of thickness in the trunks of trees has been detected by Kaiser (Ueb. die

MODIFICATION OF GROWTH CAUSED BY PRESSURE AND TRACTION. 809

Sect. 16. — Modification of Growth caused by Pressure and Traction.

Cells or whole masses of tissue may be subjected to pressure and traction in very
different ways. On the one hand these forces may result, in a perfectly normal
manner, from the tension of the tissues ; on the other hand, external and more
accidental circumstances may cause single cells or masses of tissue to be com-
pressed or stretched in particular places by solid bodies, or tissues to become
accidentally freed from the pressure and traction to which they are normally subject.
The numerous phenomena which indicate or prove that growth is altered in this
way have however at present been exactly investigated from this point of view in
only a fev/ cases. The following will therefore only serve to draw attention to a
subject further discoveries in which will doubtless contribute largely to the establish-
ment of a mechanical theory of growth.

I. Every cell-wall is subject to Pressure from within, by which it is distended,
so long as the cell is turgid. But since the daily experience of microscopists
teaches us that all growing cells are turgid ; and that no cell which is unable to be-
come turgid in consequence of openings in its cell-walls has any power of growth ;
and that withered internodes, leaves, and roots do not grow, while these organs
grow more rapidly the more strongly turgid they are, it may be inferred that turgidity
is an essential condition of the growth of the cell-walP. This appears to a certain
extent intelligible if Nageli's theory of growth and Traube's experiments on artificial
cells described in Sect, i of Book III are accepted. It may then be assumed that
the interstices between the solid particles of the cell-wall which are occupied by
water increase slightly in consequence of the distension of the cell-wall caused by
the hydrostatic pressure of the sap ; and that space is thus obtained for the interca-
lation of fresh particles of solid substance ; the distension caused by turgidity then
begins afresh and produces the same effect.

The distension which takes place at any particular spot of the cell-wall and the
consequent intercalation of fresh solid substance depend however chiefly on the
internal properties of the cell-wall itself. Not only do different parts of the cell-
wall differ in their extensibility, but they may even vary at the same spot in this
respect in the longitudinal and in the tangential or the oblique direction, as may
be seen from the swelling of the cell-wall. But that there is actually such a general
difference in the extensibility in different directions is at once shown by the fact
that growing cells assume the most various forms, — cylindrical, stellate, &c. ; while,
if the extensibility of the cell-wall were the same in all directions, the cells must all
become spherical as the result of turgidity, or polyhedral under that of mutual
pressure. This little is nearly all that we know at present with reference to exten-
sibility, turgidity, and growth by intussusception. It must be borne in mind that

tagliche Periodicitat der Dickendimensionen der Baumstamme, Diss. Jnaug., Halle 1879). See also
Kraus, Ueb. Wasservertheilung in der Pflanze, III, Die tagliche Schwellungsperiode der Pflanzen,
Halle 1 881.]

' [De Vries has shown (Mechan. Ursachen d. Zellstreckung) that when a growing intemode is
placed in a 5-10 per cent, solution of a neutral salt, its cells lose their turgidity, and that growth
ceases; when the salt solution is washed out with distilled water, the cells regain their turgidity and
growth is resumed. Sorauer (Bot. Zeitg. 1878) has pointed out that individuals grown in dry air
are much smaller than others of the same species grown in moist air. These facts, and many others
which might be mentioned, show the dependence of growth upon turgidity.]

8 10 MECHANICS OF GROWTH,

the rapidity of the growth of cells is in proportion to the thinness and therefore
the extensibility of their walls. The growth in thickness of the cell-wall usually
begins when the increase of the cell in volume begins to diminish or has altogether
ceased.

If then the distension of the cell-wall caused by turgidity is the origin of its
superficial growth, something similar must also occur when the cell-wall is stretched
in some other way by external forces, the turgidity being less. This is the case
with the epidermis and cortex of shoots as a result of the tension of the tissues.
Since in long internodes and leaves these cells usually grow principally in the longi-
tudinal direction, while in broad leaf-blades they assume the form of polygonal
plates, this may be referred in the first case partly to the traction to which they are
subject being chiefly in the longitudinal direction, in the second case to its being in
all directions parallel with the surface ^ It has already been stated that the cells of
the primary cortex of shoots which are increasing rapidly in thickness are not merely
stretched but also grow rapidly in the tangential direction '^.

2. Pressure from without on the cell -wall which is distended by turgidity occmx?,
in a very simple form when the apices of growing cells come into contact with solid
bodies ; as the root-hairs of land-plants with the particles of the soil ^ The very thin
and extensible cell-walls are in close contact with the irregular surface of the par-
ticles, just as when an elastic bladder filled with water is pressed externally by an
angular body, only that they retain, after the pressure is removed, the form which
has thus been given them, evidently in consequence of the intercalation of fresh
particles of solid matter which perpetuates the form at first acquired only by dis-
tension. The reverse takes place when the external pressure on the cell-wall is
removed. A very simple instance of this is afforded by the formation of the so-
called 'Tiillen' in vessels ^ They are formed where the thin non-lignified wall
of a cell of the wood-parenchyma, still capable of growth, adjoins the pits of a
vessel. The portion of wall which is stretched over the opening is forced through
it by the pressure of the sap of the cell and swells out in the form of a papilla into
the cavity of the vessel. As long as the vessel contained sap and was in a turgid
state, its turgidity was in equilibrium with that of the adjoining cell ; but as soon as
the cell-sap of the vessel was absorbed, the portion of cell-wall which covers the
bordered pit was subject to pressure on one side only, and was therefore forced in
the opposite direction. These phenomena can be produced artificially by the
removal of the pressure to which the cells are subject from the adjacent tissues;
thus, for example, the cambium swells up on the cut surface of woody branches
when placed in moist sand or air, in the form of a cushion between the bark and
the wood. This * Callus,' as it is termed, results from the growth of the uninjured
cambial and adjoining cortical cells next the cut, where their growth was previously

^ For further details on the possible influence of tension on the formation of stomata, see
Pfitzer, Jahrb. fiir wiss. Bot. vol. VII. p. 542.

"^ On the connection of the radial and peripheral arrangement of rows of cells in a transverse
segment with the increase in diameter, see the lucid description of Nageli in his Dickenwachsthum
des Stengels bei den Sapindaceen, Munich 1864, p. 13 ^/ seq,: [also Detlefsen {loc. cit.).']

2 Sachs, Experimental-Physiologic, p. 186.

* See Book I, p. 26 [and references in foot-note. These growths are frequently termed 'tyloses.']

MODIFICATION OF GROWTH CAUSED BY PRESSURE AND TRACTION. 8ll

prevented by the cells which have now been removed. When once projecting
beyond the cut, they grow more rapidly than before in a lateral direction in conse-
quence of the turgidity, and become divided by transverse and longitudinal walls.

The further development of such a callus where branches have been cut off
leads to the well-known overgrowth on the stumps. In internodes of seedlings of
Phaseolus which had accidentally become hollow, I found the medullary cells which
surrounded the cavity to have grown into it in the form of spherical or club-shaped
papillae; divisions ensued, and nuclei were formed in the cells thus produced. The
medullary cells which exhibited this active growth on the free surfaces of their walls
would have retained their polyhedral form had the pith remained solid, because
every surface of the cell- wall would have been exposed to the pressure of the two
adjoining cells ; but in consequence of the formation of the hollow, the pressure was
removed on one side, and the turgidity, being no longer neutralised, caused the cell-
wall to swell out, and induced in it an active superficial growth ^ These phenomena
and others of the same kind show that it is often sufficient merely to remove the
pressure to which tissues or individual cells are subject in order to bring about an
active growth of the free surfaces of their cell-walls. The first cause at least of
the new growth is the distension of the free surfaces of the cell-walls in conse-
quence of the turgidity of their cells which was previously neutralised by that of the
adjoining cells. But that a very small pressure from without is sufficient to prevent
the growth of softer tissues at the points of contact is seen in the case of many large
Fungi which develope among the vegetable mould of woods, and enclose in the
margin of their pileus light loosely lying leaves, pieces of stick, and the like. The
small pressure from without clearly prevents in these cases the superficial growth of
the walls of the cells with which these bodies are in contact, while the adjoining cells
extend laterally and enclose them.

But the most remarkable illustration of this law is seen in the effect produced by
a slight pressure on the growth of tendrils, the longitudinal growth of the cells being
thus greatly hindered and sometimes even stopped, while the cells of the opposite
free side elongate rapidly, as is seen even at the first glance without measurement by
making a longitudinal section of a tendril curling round a slender support. In what
way the slight pressure which acts in a radial direction, and is generally combined
with friction, exerts an influence on the longitudinal growth is however entirely
unknown. Very similar phenomena are exhibited by the primary and secondary
roots of seedlings (as Zea, Faba, and Pisuni). If they are allowed to grow in a damp
locality, and the growing parts are made to press on one side some solid body as
a pin or another root, the root bends like a tendril round the body with which it is
in contact, this side growing more slowly than the opposite one. It is evidently in
consequence of a similar influence of pressure on growth that the aerial roots of
Aroidese and Orchideae become closely attached to solid bodies, following exactly
their inequalities. But even unicellular tubes, such as the hyphae of Fungi and
pollen-tubes (Fig. 479), are induced by contact with a solid body to grow closely
applied to it. In this simplest case, where the hydrostatic pressure is uniform over
the cell and distends the cell-wall, it does not admit of a doubt that the pressure

Prantl succeeded in artificially inducing similar phenomena in the tubers of Dahlia.

8i2

MECHANICS OF GROWTH,

from without impedes the growth of the cell, independently of turgidity, while the
growth proceeds unhindered on the side which is not in contact.

But the mechanical processes by which pressure on an organ in the radial
direction impedes its growth on that side are unknown. The solution of the question
must depend in the first place on whether the pressure acts on the cell-wall directly
or in some way or other through the protoplasm ^

But in contrast to the phenomena which have now been described, external
pressure also sometimes causes growth at places where otherwise there would be
none. Thus Pfeffer has shown ^ that certain hyaline superficial cells on both of
the flat sides of the gemmae of Marchantia possess the power of growing out into
tubular root-hairs when they remain in contact for some time with a moist solid
body ; while contact with water produces no effect of the kind. These cells usually
develope into root-hairs only when their outer surface is directed downwards, while
those on the upper side, not being in contact with a solid body, do not grow out.
This, as we shall see presently, is an eff'ect of gravitation, which is however over-
come by the action of the slight continuous contact, since this causes the cells on
the upper side of the gemmae also to grow out into root-hairs. The 'haustoria'

Fig. 479.— Growth of the polleti-tube of Ca7npanula rapunculoides : Kp the pollen-grain ;
ps the pollen-tube closely applied to the stigmatic hair nh.

of Cuscuia and Cassytha and the adhesive discs on the tendrils of the Virginian
Creeper are only formed, as was shown by v. Mohl, on the continuous contact of
the surfaces of the tissue with a solid body ; and this has been confirmed by recent
experiments of Pfeff'er's (/. c. p. 96)^ In these cases a growth combined with cell-
division and differentiation of tissue is caused by contact or slight pressure on a
part of the organ, and would not take place without this pressure. The haustoria
and adhesive discs thus formed are altogether indispensable for the life of the
plant; for Cuscuia is nourished exclusively by the haustoria which penetrate into
the tissue of the host ; and it is by the formation of adhesive discs on the tendrils
that the Virginian Creeper is enabled to climb up walls. If the tendrils do not meet
with any solid body to which they can attach themselves by means of these discs.

^ If the relation between protoplasm and the growth of the cell-wall were better known, stress
might be laid on the fact that even a very slight pressure on the cell-wall disturbs the movement of
the protoplasm, and may even cause it to become detached from the cell- wall fsee Hofmeister,
Lehre von der Pflanzenzelle, p. 51).

"^ Arbeiten des Bot. Inst, in Wiirzburg, Heft I. p. 22.

3 [See also Darwin, On the Movements and Habits of Climbing Plants, London 1875, p. 84
et scy.]

MODIFICATION OF GROWTH fiAUSED BY PRESSURE AND TRACTION. 813

they dry up and fall oflf, while those which have formed discs increase in thickness
and become woody.

I. The retarding effect on growth of an external pressure on the cells is very evident
in the formation of the annual rings in wood. In the earlier editions of this work
I called attention to the fact that the larger radial diameter of the wood- cells in the
portion of the rings formed in the spring, and their smaller radial diameter in the por-
tion formed in the autumn, might possibly depend on a difference in the pressure from
the surrounding bark to which the cambium and the wood are subject, this pressure
being less, as we have shown, in the spring, and constantly increasing during the
summer. This hypothesis has been fully confirmed by H. de Vries's investigations ^
In branches two or three years old he increased the pressure of the bark in the spring
by firmly winding string round them at particular places. The experiment showed in
all cases, firstly, that the absolute thickness of the annual ring was less beneath the liga-
ture than the mean thickness of the same annual ring at some distance above or below
that spot. In several instances the difference was so considerable that the spot where
the experiment was made appeared of considerably less diameter even to the naked eye,
and this effect was increased by the formation of cushions of wood immediately above
and below the ligature. Secondly, the absolute thickness of the ' autumnal layer ' of
wood (up to the middle of August, when the increase in diameter of the tree on which
the observations were made ceased) was always greater, and generally considerably so,
at the spot where the experiment was made, than the normal thickness. In the trees
examined {Jeer Pseudo-platanus, Salix cinerea, Populus alba, Panjia) the autumnal wood
formed at this spot consisted of fibres flattened radially, between which were a smaller
number of vessels than in the normal wood ; its composition was therefore the same as
that of the normal ' autumnal wood.' The normal autumnal wood of Ailanthus glandu-
losa consists almost entirely of wood-parenchyma-celis flattened radially; while the
autumnal wood formed beneath a ligature made in May consisted of a thick layer of
flattened fibres, between which a few vessels could be seen. These results show that
when the pressure is increased, the formation of the autumnal wood begins at a time
when, under normal pressure, a large-celled woody tissue is still being formed.

A diminution of pressure is obtained by making radial longitudinal incisions into
the bast-tissue. The strips of bast contract somewhat tangentially, since their tension
ceases. Near the incisions the pressure of the bast upon the wood is entirely removed ;
but in the middle between two adjacent incisions a considerable pressure always remains.
The fresh portions of tissue which are formed next to the wounds differ to the greatest
extent in their composition from the ordinary structure of the wood. A layer of wood
of the ordinary structure is formed, on the other hand, in the portions of the cambium
at the greatest distance from the incisions, and afterwards also on the outside of the
abnormal portions of tissue. But it is only the tissue consisting of wood formed under
artificially diminished pressure that we have at present to consider. The incisions
were mostly 2 to 3 cm. long, and were made in the periphery of two- to three-year-old
branches at distances of from 4 to 6 cm. in the middle of June and the middle of July,
and therefore after the formation of the normal autumnal wood had already begun.
The effect of the decrease of pressure was first of all shown, after the branches had
been cut off in the middle of August, by a considerably greater increase in thickness
at the spots than above or below them. On the transverse sections the thickness of
the annual ring was greatest near the incision and decreased gradually from there to
the middle points between two incisions. The layer of wood formed after the com-
mencement of the experiment was often more than twice as thick at the former as
at the latter spots. For a more exact investigation only those pieces were used in
which a layer of distinctly flattened fibres of autumnal wood had been formed before

» H. de Vries, Flora, 1872, No. 16.

8 14 MECHANICS OF GROWTH.

the incision was made. But in all cases (the trees already named) the wood outside
this layer of autumnal wood — and therefore all that was formed after the decrease of
pressure — consisted of fibres which were not at all flattened radially, but had the same
diameter, or even one somewhat greater, than those in the middle of the normal
annual ring ; it contained also as many vessels, or even more, than the normal wood.
At the time therefore when autumnal wood is being formed in the normal parts of the
branches, a woody tissue is produced, if the pressure is artificially diminished, agreeing
in its structure with the ordinary wood formed in the middle part of the annual
ring. For the normal production of autumnal wood it seems therefore necessary for
the bark and the bast to exercise a considerably greater pressure on the cambium and
the young wood.

These results explain the older experiments of Knight in 1801. He fastened young
apple-trees with a stem of about one inch diameter so that the lower part, about three
feet long, was immoveable, while the upper part with the foliage could bend under
the pressure of the wind. During the period of vegetation the upper moveable part of
the stem increased considerably in diameter, the lower fixed part only slightly. This
is easily explained if we bear in mind that the swaying of the upper parts of the stem
in different directions by the wind must always stretch the bark on the convex side,
and therefore eventually relax it; it must thus become looser, and therefore the
pressure of the bark at these points is always somewhat less than at the lower and
immoveable parts of the tree. This explanation is completely confirmed by the fact
that in one of the trees which could be swayed by the wind only in a northerly and
southerly direction, the diameter of the stem increased so much in this direction as
to bear the proportion of 13 to 11 as compared with the diameter in the easterly
and westerly direction. It is obvious that this explanation is much more probable
than that given by Knight himself, who thought the movement of the sap in the wood
was promoted by the swaying of the stem caused by the wind.

The great assistance to the increase in diameter of trees afforded by the diminution
of the pressure of the bark on the cambium has been long employed in horticulture.
The bark of young trees is split from above downwards in summer ; cushions of wood
are formed at the edges of the incisions, which soon close up the wounds. The use
of this process is that from the more rapid increase of the wood in thickness, the
conduction of water to the leaves becomes more copious and the loss by transpiration
is more easily replaced. The development of the buds and hence the formation of
the organs of assimilation will be promoted by the increase of turgidity in the young
branches.

2. If the roots of a plant which is transpiring through its leaves can obtain but a small
quantity of water from the soil, the turgidity of the tissues in the growing organs will
be less than it would be if the roots were surrounded by more moist earth. The
immediate result of this will be, that the cells, and therefore also the organs which
consist of them (leaves, internodes), will grow more rapidly when the supply of water
is increased: in this way the assimilating leaf-surface will be increased, and, as a
secondary result, there will be increased assimilation. This, in conjunction with an
adequate supply of water, will produce a more vigorous growth of the whole plant,
which will thus attain a more considerable size and a greater dry-weight than a plant
which cannot obtain a sufficient supply of water. These far-reaching eff'ects of the
turgidity produced by an abundant supply of water are very prominent in a series
of experiments made by Sorauer (Bot. Zeitg. 1873) upon Barley-plants. These plants
were grown so that all the conditions, except the amount of water in the soil, were
the same in each case. In one case the amount of water present in the soil was 10 per
cent., in others 20, 40, 60 per cent, of the amount requisite for complete saturation.
It appeared the more moist the soil the more fully were the leaves developed, that
is, the greater the turgidity of the tissues. With the increase of breadth of the leaves,
the number of the fibro-vascular bundles in them increased ; and hot only was the

GROWTH UNDER CONSTANT EXTERNAL CONDITIONS. 815

number of the vessels increased, but also that of the cells, so that the more vigorous
grovi'th of the cells had induced more numerous divisions. The dry-weight also in-
creased with the moisture of the soil : but if the latter exceeded 60 per cent, of the
quantity of water necessary for complete saturation, the dry-weight began to diminish.
It appears that in this particular also, as in the case of heat and light, there is an
optimum, below which every increase is favourable but above which it is injurious.

Sect. 17. Course of the Growth in Length under Constant External
Conditions \ It has already been explained in the morphological portion of this
work that the organs of a plant do not grow simultaneously and uniformly at all
points ; but that roots and stems always increase slowly in size at the apex, as leaves
also do at least at first. The growing-cells multiply by cell-divisions which take place
regularly, but do not as a whole exceed a certain size, which is always small.
Below this punctum vegetatioms, consisting of primary meristem, not only does the
differentiation of the homogeneous tissue into layers of different kinds begin, but
also a more rapid increase in size of the cells, which do not now divide so often as
before. In the parts of the organ which lie further from the punctum vegetaiiojiis
cell-division ceases altogether (but at different periods in the different layers of
tissue), while the growth of the cells still actively continues, until at length, when
they have attained their ultimate form and size, the growth of the whole ceases.
The cells are then several hundred or even thousand times larger than at the time
of their formation beneath the punctum vegetatioms. When the growth of stems,
leaves, and roots has reached a sufficiently advanced stage of development, we are
able therefore to divide their tissue into three regions : — (i) the punctum vegetatioms,
where new cells are chiefly formed, and increase in size is slow ; (2) the portion
where the main part of the increase in size takes place, but where there is no longer
any cell-division or only to a subordinate extent ; this is the elongating portion of
the organ ; and (3) the portions which no longer grow, at least in length, i. e. the
mature portions of the organ. When growth entirely ceases at the punctum vege-
tatioms, as is usually the case with leaves, all the cells continue to enlarge until the
whole is mature. If the stem produces a number of closely crowded leaves, as it
usually does, at its growing end, the whole of the region in which the chief part
of the cell-division takes place is clothed with young leaves, which also themselves
consist of cells undergoing division. But as soon as the leaves enter the second
stage of development and begin to lengthen, they incline outwards ; and when
the stem is growing rapidly in length and forming evident internodes (which is
by no means always the case) the lengthening begins at those points where it bears
the leaves, which also begin to lengthen at the same time ; the older mature leaves
are generally found on mature internodes. If the internodes are clearly marked
off from one another, as is especially the case when the leaves are verticillate or
sheathing at their base, each internode forms a more or less individualised whole

^ Ohlert, Langenwachsthum der Wurzel, Linnsea 1837, ^ol. XL p. 615. — Miinter, Bot. Zeitg.
1843, p. 125, and Linnsea, 1841, vol. XV. p. 209. — Griesebach in Wiegmann's Archiv. 1843, p. 267. —
Sachs, Jahrb. fiir wissensch. Bot. i860, vol. IL p. 339, — Miiller, Bot. Zeitg. 1869, No. 24. — Sachs,
Arbeit, des Bot. Inst, in Wurzburg, 1872, Heft IL p. 102 ; ditto. Heft HI, 1873, and Flora 1873,
No. 21.— Askenasy, Flora 1873, No 15, [and Verhandl. d. nat.-med. Vereins zu Heidelberg, N. F.,
Bd. II, 1878, Ueb. eine neue Methode um die Vertheilung der Wachsthumsintensitat zu bestinimen;
Strehl, Unters. ueb. das Langenwachsthum der Wurzel, Diss. Inaug., Leipzig 1874.]

8l6 MECHANICS OF GROWTH.

as soon as it emerges from the bud, and different stages of growth may be distin-
guished in it, advancing from below upwards. This may take place in two different
ways, according as the uppermost or lowermost part of an internode remains in an
undeveloped condition, the other end being completely mature. This zone which
continues for some time in an undeveloped state — cell-division taking place actively
in it — is commonly found at the upper end of the internode (as in Phaseolus)^ and less
frequently at the lower end, and this usually when it is enveloped by closely appressed
leaf-sheaths or when it is in a bulb, as e. g. in Equisetaceae (especially E. hyemale),
Umbelliferae, the bulbous Liliaceae, the haulms of Grasses, &c. If the internodes are
not sharply defined, as in stems with small leaves and the floral axes of Dicotyledons,
the various states of growth which have been described pass insensibly into one
another on the stem ; and this is always the case with roots. If leaves when once
expanded continue to grow for some time, the process is the same as in stems or
branches ; while the lower portion of the leaf-stalk is fully mature, the upper parts
present successively younger or less developed states. The formation of cells finally
ceases at the apex and all the parts then become fully mature. This is strikingly
the case in Ferns, less so in the pinnate leaves of Papilionacese or the incised leaves
of Araliaceae. But very often the activity of the puncium vegeiationis of the leaves
lasts for only a short time and its tissue completes its growth while cell-divisions
still continue at the base of the leaf, and all the transitional states of growth are to
be found between the base and the apex. This occurs, for instance, in the long
leaves which grow from the bulbs of Liliacese and allied Monocotyledons. When
a cell-producing zone of this kind occurs at the base of an internode or of a leaf,
with more mature tissue lying above it, the whole organ behaves as if this zone were
a puncium vegeiationis ; the states of growth succeeding one another in the reverse
order. Such a zone, intercalated between mature portions of tissue, may be called
an Iniercaiary Vegeiative Zone. The growth of the internode or leaf may be termed
basipetal, in contrast to the acropetal development where the puncium vegeiaiionis
lies at the apex of the internode or leaf.

According as the conditions of growth — temperature, the supply of water, and
illumination — are favourable, these phenomena proceed more or less rapidly and
uniformly. Every young cell formed at the puncium vegeiaiionis grows and matures
more rapidly the more favourable these conditions are. But if the organs are
observed under the most constant possible conditions as they emerge from the
bud, it is seen that their growth, both in length and thickness, dependent on the
gradual development of the cells, does not advance by any means uniformly. The
growing portion of a root, internode, or leaf does not lengthen to an equal amount
in equal consecutive intervals of time ; and the same is the case with stems con-
sisting of a number of internodes, and with each zone, however small, of a growing
organ. It is seen in fact that the growth of each part begins at first slowly,
becomes gradually more rapid, and finally attains a maximum of rapidity, after
which the growth becomes again slower, and finally ceases when the organ is
fully mature.

If successive equal intervals of time are represented by T^, T^... T„, and the
increments during these intervals by I^, I2, ... I„, then it may be stated as a general
rule that —

GROWTH UNDER CONSTANT EXTERNAL CONDITIONS. 817

for

T.

T,

T3

T.

Te

Te

T,

we

shall have Ij

<I.

<

la

<I.

>!=

>I.>

zero.

This rule holds good for the separate zones of roots, internodes, and leaves, as well
as for the entire organs from their first formation to the time of their full maturity.
This course of growth I have termed Tke Grand Period^, or Grand Curve of Growth;
since it is at once evident that if the values Ij, \ .... I„ are drawn as ordinates with
the intervals of time as abscissae, a curve will be obtained which, starting from the
axis of abscissae, reaches a maximum of elevation, and returns again to the axis.
The following examples will render this more clear.

Koppen^ found the following increase of length attained in periods of twenty-
four hours with a nearly uniform mean temperature : —

Roots ^

of Lupinus albus.

Increase in

length.

Mean temperature.

First three days :

per

diem

10

mm.

i7-2°C.

Fourth day

18

i6-6

Fifth day

44

171

Sixth day

32-6

16-9

Seventh day

279

171

Eighth day

28

164

In an internode of the flowering stem of Fritillaria imperialis I found the fol-
lowing increase in length in each period of twenty-four hours * : —

Mean temperature.

March 20 2*0 mm. io-6°C.

21 5*3 io*5

22 6-1 11*4

23 68 12-2

24 9'3 7-5 mm. 13-4

25 13-4 125 139

26 12*2 12*5 14*6

27 8-5 / 11-5 15-0

28 io'6 14*2 14-3

29 io'3 126 12*4

30
31

Norma

plant

Etiolated plant

in the

light.

in the dark.

2-0 mm.

5*3

6-1

6-8

9'3

7*5 mm.

13-4

125

12*2

12*5

8-S

' 11-5

io'6

14-2

io'3

12-6

6-3

i5'9

4'7

1 6-6

5-8

18-2

44

i5'5

120

II'2

April I 5*8 18-2 107

2 44 i.*^'.^ 10*2

^ ' Grand period,' in contrast to the small periodic oscillations of growth which, if represented
graphically, would appear as smaller elevations and depressions on the grand curve.

2 Koppen, /. c. p. 48, I have calculated the daily growth from the lengths given in his tables.

^ That is, the root together with the hypocotyledonary portion of the stem.

* A few irregularities in the course of the growth are explained by the temporary acceleration
of the growth from the watering of the ground. Compare the curve in pi. i of the Arbeiten des bot.
Inst, in WUrzburg, vol. I, Heft II. p. 129.

3 G

8i8

MECHANICS OF GROWTH.

Normal plant

Etiolated plant ^^^

in the

light.

in the dark.

i.<l.\.cl,

April 3 3*8 mm.

14-0 mm.

4 2-0

13-8

5 1-2

11-9

6 o'7

8-8

7 o-o

4-4

8

21

9

06

lO

O'O

An internode of Humulus Lupulus gave —

Increase in length

Mean

in 24 hours.

temperature.

April 2 2

i9"o mm.

14-9" C.

23

25-0

14-5

24

260

i4'3

25

172

139

26

4-8

14-1

Mean temperature.

9-4^ C.
106
107

II'O
II'O
1 I"2

125

Harting found that a Hop-stem, consisting of a number of internodes, which was
492 millimetres long on May 15th had attained by the end of August a length of
7-263 metres, this growth being distributed as follows over the different months : —

0*492 metres in April.
2-230 May.

2-722 June.

1-767 July.

0052 August.

These observations and a number of others show that the grand period of
growth manifests itself even when the course of the changes of temperature acts in
opposition to it ; /. e. when the temperature rises while the rapidity of growth de-
creases owing to internal causes, and vice versa. The course of growth may no doubt
be so modified by great changes of temperature that the curve of the grand period
can no longer be recognised in the measurements.

In order to determine the grand period of growth in a piece of a growing root,
internode, or leaf-stalk, it is sufficient to mark a zone of the organ at the part where
elongation is beginning by two lines of Indian ink, and to measure the daily (or
half daily) growth of this piece until it ceases.

By applying this method to the primary root of Vicia Faha, the temperature
varying each day between 18° and 21*5° C, I found the following increase to take
place in each period of twenty- four hours in a piece originally i mm. long situated
immediately above the punclum vegetationis : —

I St day

1-8 mm.

2nd

37

3rd

17-5

4th

1 6-5

GROWTH UNDER CONSTANT EXTERNAL CONDITIONS, 819

5th day

17-0 mm

6th

14-5

7th

7-0

8th

o-o

In the same way I found that a piece at first 3-5 mm. long of the first inter-
node of Phaseolus multiflorus beneath the first pair of foliage-leaves, with a daily
variation of temperature between 12-75° and 1375° C, showed the following in-
crease : —

i

I St day

1-2 mm

2nd

1-5

3rd

2-5

4th

55

6th

7-0

6th

9-0

7th

140

8th

lo-o

9th

7-0

loth

2-0

\

Since every organ that is growing in length consists of zones of different ages,
which are produced in succession from the primary meristem of the punctum vegeta-
iionis (or of an intercalary vegetative zone), the successive zones of an internode
or a root indicated by ink-marks must show different increments of growth in
equal times. While the zone nearest the puncium vegetationis is beginning to grow,
the next one has already entered on a later phase of its grand period, while one
at a greater distance would have attained the maximum of its rapidity of growth,
and a still further one would have ceased to grow. In other words, a number of
zones below the cell-producing punctum vegetationis are in the ascending phase,
while those lying further backwards are in the descending phase of their grand
period ; or again, each zone is in a later phase of its period of growth the greater its
distance from the punctum vegetationis. If the successive zones of a growing organ
are indicated by the figures I, II, III, &c., and the increments of growth observed
at the same time in each of them by I^, I2, I3, &c. ; then we have the following
relationship : —

I II III 1 IV V VI VII VIII
\<\<\< \>\> \, > \,> zero.

There is therefore in the organ a region of maximum rapidity of growth. Thus,
for example, I found in the first internode oi Phaseolus multiJlorus,yv\i\Q\i was divided
into twelve zones, each 3*5 mm. long, in the first forty hours : —

(uppermost)

Zone.

Increment.

I St

2*o mm.

2nd

2-5

3rd

4-5

4th

6-6

5th

5*5

3 G 2

820 MECHANICS OF GROWTH.

Zone.

Increment.

6th

3*o mm.

7lh

1-8

8th

lO

9th

i-o

lolh

0-5

nth

o'5

1 2th

o'5

(lowest)

The maximum rapidity of growth lay therefore in the fourth zone, which was
originally situated at a distance of about 10*5 mm. from the upper end of the inter-
node.

As it is usual for several contiguous internodes of stems to be growing at the
same time, and the maximum rapidity of growth occurs, according to circumstances,
in the second, third, fourth, or fifth internode beneath the bud, the region of most
rapid growth is at a considerable distance from the apex of the stem, and especially
when the internodes attain a considerable length and several are growing at the same
time. In roots, on the other hand, the maximum rapidity of growth occurs much
nearer the puitctum vegetationis^ usually at a distance of only a few millimetres ;
and the portion of the root beneath its apex in which the chief part of the growth
takes place is consequently only a few millimetres long, while in stems with long
internodes it is often many centimetres in length. If therefore a root and a sterrt
with long internodes are divided into zones of equal lengths, e. g. i mm., com-
mencing from the puncium vegelatiom's, the law of growth, as expressed by the
general formula given above, is the same in both cases, but with this difference,
that in the stem the number of zones that are increasing in length at the same time
is much greater than in the root, in consequence of the fact that in the last case
each zone completes its period of growth more quickly^; its curve is shorter and
more abrupt.

Thus, for example, in a primary root of Vkm Faba which grew in damp air
and which was divided, starting from the pundum vegeiaiionis, into zones each i mm.
in length; I found the following increments of growth in the first twenty-four hours
at a temperature of 20*5° C. : —

Zone. Increment,

loth o'l mm.

9th 0'2

8th 0-3

7th 0-5

6th 1-3

5th 1-6

4th 3-5

3rd 8*2

2nd 5*8

apex 1-5

* It by no means however follows from this that the root grows more rapidly, /. e. attains in the
same time a greater length than the stem.

GROWTH UNDER CONSTANT EXTERNAL CONDITIONS. Szi

In this case, therefore, the third zone, where the maximum increase of growth took
place, was at first at a distance of only 2 mm. from the apex.

It is clear that if an organ is divided into zones of small length, each zone
will in general contain a larger number of cells the nearer it is to the punclum
vegelaiionis, since the cells are longer the further they are from the apex. But from
the point where growth ceases the number of cells in the successive zones of an
organ of uniform structure will be the same. If therefore the zones are again
designated by the numbers I, II, III, &c., the number of cells in them by N^, N^,
N3 ... N", then we have : —

I II III IV V VI VII VIII

N, > N, > N3 > N, > N, > N, > N, = N3.

But the difference in the number of cells in the zones is very far from being
the cause of the difference in the rapidity of growth that prevails in them ; as is
seen at once if it is recollected that the number continually decreases from the apex
throughout the growing region, while the rapidity of growth first increases and
then decreases. This may be expressed by the following formula : — •

I II III IV V VI VII VIII

N, > N, > N3 > N, > N, > N, > N, = N3.
Ii < I2 < I3 < I4 < I5 > le > I7 > zero.

If it were possible to divide in the same manner a filament of Vaucheria, a
root-hair of Marchantia, or a similar unicellular organ, into small zones, it can
scarcely be doubted (as we may conclude from other circumstances dependent on
growth) that we should find the same law to regulate the distribution of the rate of
growth in individual cells endowed with a power of apical growth. Since the same
law applies to roots and stems — whether zones i or 2 millimetres or, in the case of
stems, I or 2 centimetres in length are observed — it is to be expected that this
formula would hold good also if zones of only a tenth or hundredth, or even
thousandth of a millimetre could be marked out and measured. In other words,
we should find that the law of the grand period holds good for each single minute
area of the surface of the wall of a young cell.

If the power of any particular zone to attain a definite length is called its
Energy of Growth, then a zone which up to the time when its growth ceases reaches
a length of 10 mm. would have a smaller energy than one which continues to grow
until it has reached a length of 100 mm. Thus, for example, the successive interr
nodes of most stems each of which was at one period i mm. long, differ very
greatly in length when mature ; the internodes first formed are short, the next
longer, and finally we have one the longest of all, followed again towards the apex
by shorter ones. If we designate the energy of growth of the internodes I, II, III,
&c., by Ep Eg, E3, &c., we get the series —

I II III IV V VI VII VIII

E, < E, < E3 < E, > E, > E, > E, > E3.

With this increase and decrease in the energy of growth of the various inter-
nodes of a stem is usually associated a similar relationship between the size of their

822 MECHANICS OF GROWTH.

leaves, the lower ones forming smaller, the upper ones larger leaves, and then a
largest of all (or whorl of largest leaves), usually followed again by smaller ones^.
The secondary roots also which spring from the same primary root show similar
relationships, the first attaining a smaller length than those that follow, and these
being again followed by a graduated succession of shorter ones. The same is the
case also with the lateral branches of the stems of annual plants, as well as of trees,
especially when the order of development is distinctly monopodial.

It seems probable that an investigation of the zones of a root, stem, or leaf
would also show that the energy of growth of successive zones first increases, then
reaches a maximum, and finally decreases. The cells in the zone in which the
maximum energy of growth prevails would also be the largest, while their number
would be least. This hypothesis is in harmony with Sanio's measurements^ of the
wood- cells of Pinus sylvestris; for he found that the final constant size of the wood-
cells of the stem varies, increasing gradually from below upwards, till it attains a
maximum at a definite height, and then again decreases towards the apex. The
same is the case with the branches.

If it be permissible to ascribe a special energy of growth to each separate
zone of an organ, it becomes possible to understand how it is that, as is actually
the case, every zone has its separate period of growth, and that a grand period for
the whole organ itself may be determined. The maxima of rapidity of growth
attained in the successive zones first rise and then fall ; the duration of growth also
of the zones probably at first increases and afterwards diminishes. Consequently
the measurements of the whole organ represent the sum at first of only few and
small partial increments, later of more numerous and larger ones, until finally
the sum of the partial increments diminishes, because the number of zones growing
at any one time and the energy of their growth alike diminish. Further investi-
gation will show whether this hypothesis, which is at least an approximate one, is
correct.

If the increments of length of an internode, stem, or leaf, in short intervals of
time such as half-an-hour or an hour, are compared, it is usually found that they do
not increase and then decrease regularly, but irregularly, the growth being sometimes
greater, sometimes smaller. If the grand curve of growth is constructed directly
from them, it does not assume the form of a continuous curve, but shows a number
of small zigzags, which however disappear, if, for example, the interval is extended
from one to three hours or more. These phenomena I call irregular variations
of growth ^ They appear to result from the plant being subjected to continual
small variations of temperature, air, light, and moisture of the soil, which alter the
turgidity, and therefore the extensibility and elasticity of the growing cells. I come

^ This phenomenon has not at present been sufficiently investigated. In many stems, especially
creeping ones, when the leaves have reached a certain size, this size remains constant in a long series
of leaves before any decrease occurs.

^ Jahrb. fiir wissensch. Bot. 1872, vol. VII. p. 402. By a 'constant' size of the wood-cells I
understand that which they possess in the later annual rings ; in the inner annual rings they
gradually increase, until in the following ones they attain a constant size.

^ For further details see Reinke, Verhandl. des bot. Vereins fiir die Provinz Brandenburg,
Jahrg. VII; and Sachs, Arbeit des bot. Inst, in Wiirzburg, Heft II. p. 103. [See also Drude, Die
stossvveisen Wachsthumsacnderungen in der Blattentwickelung von Victoria regia, Halle i88t.]

DAILF PERIODICITY OF GROWTH IN LENGTH. 823

to this conclusion from observing that irregular variations of growth become less
the more the plant is protected from variations in the surrounding conditions. Partial
irregular neutralisations of the tension of the tissues may also cooperate to produce
this result.

Sect. 18. — Periodicity of Growth in length caused by the alternation
of day and night. The alternation of day and night implies varying combina-
tions of the conditions of plant-life, especially of those that affect growth. Day
and night are distinguished not only by the presence and absence of sunshine, but
also by a consequent higher and lower temperature, which again causes variations
in the moisture of the air. Independently of special meteorological phenomena,
the temperature falls daily with the diminishing elevation of the sun till sunrise
the next day, that of the air rapidly, that of the ground more slowly ; at sunset the
fall is sudden, as is the rise at sunrise. In general the atmosphere approaches a
state of saturation as the temperature falls, z. e. the hygrometric difference decreases,
as it increases with the rising temperature. But these general daily alternations act
in a variety of ways, and even in opposite directions on the growth of plants ; the
increasing intensity of the light after sunrise retards growth, while the increasing
temperature promotes it, as long as the other conditions remain the same ; but the
increase of the hygrometric difference caused by the increasing temperature of the
air occasions also an increase of transpiration, which effects a diminution of the
turgidity of the tissues, and this again retards growth.

It is important to ascertain which of these variable causes exercises the greatest
influence on growth ; and it will depend on this whether the growth of the plant
is most rapid by day or by night. On a cloudy but warm and damp day the weak
light has only a slightly retarding effect, but the temperature and the great amount
of moisture greatly promote growth ; under these circumstances the growth may be
greater than in the succeeding night (equal periods of time being compared),
when the total absence of light promotes growth, but the lower temperature is less
favourable to it. But the proportion may be reversed ; the plant may grow more
slowly by day than by night when the difference in the temperature and moisture
of the air during each is but small and very bright days intervene between dark
nights, the intense light retarding growth by day more than the depression of the
temperature by night.

The greatest variety of combinations may be imagined in this respect; and from
the extreme changeableness of the weather the plant will, according to circumstances,
sometimes grow more quickly by day, sometimes by night, without exhibiting any
exactly recurrent periodicity. The numerous observations which have been made
in this direction do not therefore point to any general law^ It has however
been ascertained that, especially when long periods of time such as entire days are

^ These will be found described by me in detail in the Arbeiten des bot. Inst, in WUrzburg,
1872, p. 170. [Baranetzky (Die fagliche Periodicitat im Langenwachsthum der Stengel, Mem. de
I'Acad. imp. de St. Petersbourg, XXVII, 1879) finds that there is a daily periodicity of the growth
of stems which is independent of the direct influence of any external conditions: (see also Bot. Zeitg.

1877).]

824 MECHANICS OF GROWTH.

compared, all the other conditions of growth are outweighed by the effects of the
variations of temperature, so that in general the rapidity of growth increases with a
rising and decreases with a falling temperature. The result of a number of measure-
ments made by RauwenhofF during several months in the most various weather was
that the mean growth was greater in twelve hours of the day than in twelve hours of
the night; vt'z.

By day. By night.

in Bryonia 59*0 p. c. 4i-op.c.

Wistaria 57-8 42*2

Viiis 55-1 44-9

Cucurhiia 567 43*3

do. 57*2 42-8

Dasylirion 55-3 447

A similar tabular statement shows that the favourable influence of a higher
temperature by day outweighs the retarding influence of daylight. Rauwenhoff^s
measurements show accordingly that the mean growth during six hours of the fore-
noon is less than that during six hours of the afternoon ; since, while the average
amount of light is the same, the temperature is higher in the afternoon than in the
forenoon. If the afternoon growth is placed at loo, then the morning growth is —

in Bryonia

86

Wistaria

71

Vitis

67

Cucurhita

79

do.

81

If however we calculate from Rauwenhoff''s measurements the daily and nightly
and the morning and afternoon values for shorter periods in which the changes of
the weather do not neutralise one another, it will be found that the growth by night
sometimes exceeds that by day, and that the afternoon is not always more favourable
than the morning.

It is clear from what has been said that it is impossible to determine from
observations in the open air, where the variations of temperature, light, and moisture
are very great and are combined in a great variety of ways, in what manner each
separate condition of growth affects the plant, and whether the alternation of day and
night causes a similar alternation of growth, or whether there exist in the plant itself
causes of daily periodicity independently of external changes. In order to decide
this question, it is necessary first of all to make the observations independent of the
accidents of weather, which is only possible by carrying them on in well-closed
rooms where the temperature can be kept constant or made to vary, and where the
amount of light can be increased or decreased, and the moisture regulated in the air
and in the soil of the flower-pot. Under these circumstances it is possible to study
the action of an increasing or decreasing amount of light upon a plant exposed to
constant conditions of humidity and temperature, and therefore exhibiting a con-
stant degree of turgidity; it is sufficient to measure and compare the increments of
growth during short periods of time.

I

DAILV PERIODICITY OF GROWTH IN LENGTH. 825

A long series of observations of this kind on internodes has given me the fol-
lowing results ^ : —

(i) The more exactly a constant temperature is maintained, darkness being
constant and the amount of moisture being also constant, the more uniform is
the course of growth at different periods of the day. There does not appear to be
any daily periodicity of growth independent of external influences. The irregular
variations of growth mentioned above were however observed.

(2) If great variations of temperature are allowed to act on a plant growing
in darkness and with a constant amount of moisture, to such an extent that the tem-
perature of the air round the plant alters some degrees C. from hour to hour,
the rate of growth of the internodes rises and falls with the rising and falling
temperature. If the hourly increments are taken as ordinates, and the intervals of
time as abscissae, the curve of growth follows all the elevations and depressions of
the curve of temperature, without however any actual proportion being observable
between the growth and the temperature; the curves do not run parallel but are
only of the same description.

(3) If care is taken that during the period of observation the temperature
undergoes only slight and gradual changes, while (the moisture being sufficiently
uniform) the amount of light changes in the ordinary manner, increasing from morn-
ing till midday and decreasing from midday till evening to complete darkness at
night, it will be found that the increments of growth are always greater from even-
ing till sunrise, diminishing suddenly after sunrise, and then more slowly till evening.
The alternation of day and night causes therefore under these circumstances a
periodical rising and falling of the curve of growth of such a nature that a maxi-
mum occurs in the morning at sunrise and a minimum before sunset. A second
rising of the curve of growth usually takes place also in the afternoon ; but this, as
I have shown, is a consequence of the higher temperature in the afternoon which
overcomes the influence of light. The retarding influence of light is therefore
strong enough to overbalance the favourable influence of the slight elevation of
temperature in the forenoon, but not sufficient to overcome that of the stronger
elevation of temperature in the afternoon.

The fact is of great interest that when a plant has been exposed to light during
the day, its curve of growth after sunset, or if placed in the dark in the evening, does
not immediately rise abruptly ; i. e. that the most rapid growth which is independent
of light is not at once attained when it is suddenly placed in the dark ; but that — as
is shown by the curve rising slowly till morning — the growth which has been retarded
during the day only becomes gradually more rapid in the course of some hours,
until the light to which the plant is again exposed in the morning causes a fresh
retardation of growth, which again increases from hour to hour till the slowest rate
is attained in the evening, if the temperature remains constant. In other words, the
two internal conditions of the plant which correspond to darkness on the one hand
and to daylight on the other hand pass over only gradually into one another. Light

^ Sachs, Arbeit, des bot. Inst. Wurzburg, 1872, vol. I, p. 168 et seq. The plants observed were
chiefly Fritillaria imperialis, Ilmnuhs Lupvlus, Dahlia variabilis, Polemonium repians, and Richardia
cethiopica.

826

MECHANICS OF GROWTH.

requires a considerable time in order to overcome the nocturnal, darkness a con-
siderable time to overcome the diurnal condition of growth of the plant. If this
were not the case, the curve of growth would at once rise abruptly in the evening
when the room is suddenly darkened, would then continue at the same elevation till
morning, would fall abruptly when light is again let in, and would then continue at
the same height till the evening. But this does not correspond to the observed
phenomena.

In order to study more closely the changes of growth occasioned by internal causes,
or the dependence of these changes on external conditions, it is necessary to measure

Fig. 480.— Arc-indicator, or apparatus for measuring- the growth in length of a plant
during short periods of time.

the increments in short spaces of time such as an hour or two or three hours. In the
case of internodes or leaves of large plants which are growing very rapidly, as the flower-
stems of Jgave or the leaves of Musaceae, this can be done with a certain degree of
exactness by simple measurement with a measuring-rod. But for the purpose of more
exact observations it is more convenient to make use of smaller plants which do not
grow so rapidly, the growth during an hour not amounting to more than a millimetre, or
even less. In such cases a simple measuring-rod is not sufficiently exact ; and I have
employed in its place three different methods. In each of them a thin but strong thread
of silk is fixed to the upper end of the stem or internode of the plant growing in a

DAILV PERIODICITY OF GROWTH IN LENGTH,

827

pot, the thread passing vertically over an easily moveable pulley and moving an index
fixed to the free end of the thread or to the pulley,

I. The Thread-indicator is a simple contrivance in which the free end of the thread

Fig. 48r.— Autographic Auxanometer, for recording the growth in length during short periods of an internode of a
growing plant/"; s s' tlie lines scored by the index z on the blackened paper/ fixed to the cylinder C which is made to
rotate eccentrically by means of the clock-work D.

which hangs down from the pulley and is kept tight by a weight of a few grammes
carries a horizontal needle which moves freely over a graduated scale as the end of the
thread which is fixed to the plant rises with its growth.

828 MECHANICS OF GROWTH.

2. In the Arc-indicator ihQ thread c/(Fig. 480) fixed to the plant a is carried over
the pulley d and fixed to a pin which is attached to a second pulley^. An index z mg,de
of a straight and firm straw is fastened to this second pulley in the radial direction, its
free end pointing to a graduated scale on the arc of a circle m n. The equilibrium of the
index is secured by the small weight / which tends to turn the pulley in the opposite
direction with a force which keeps the thread cf\r\ a state of tension. As the internode
below the hook h lengthens, the weight / sinks, and a pieCe of the thread c f oi equal
length is rolled off the pulley g, thus raising the index on the arc. If the index is, for
example, ten times as long as the radius of the pulley, the portion of the arc which it
will pass over represents ten times the increase in length of the internode. But since it
is not usually required to know the absolute amount of the increase but only the relative
amount in different times, it is sufficient merely to read off and compare the movements
of the index on the graduated scale. By this instrument we are able to measure very
small increments of growth; but, like the first process, it has the disadvantage that the
observer must watch it during the whole time, which renders the investigation very
difficult, especially at night.

3. The Autographic Auxanometer gets rid of this difficulty. It consists of a simpler
form of the instrument already described. The thready fastened to the plant s6ts directly
in motion the pulley which carries the index «, being fixed to it by a pin at r. The
tension of the thread caused by the index itself is still further increased by the weight g.
By this contrivance the point of the index falls as the stem grows below the point to
which the thread is fastened. By means of the clock-work D the cylinder C fixed upon
the vertical axis a is made to rotate slowly, the rotation being arranged by adjusting the
length of the pendulum / so that a revolution is completed in exactly an hour. The
cylinder is however fixed eccentrically on the axis «, so that during the rotation one side
describes a larger circle than the other side. On the former side is fastened a piece of
smoked paper pp. When the index is properly adjusted, its point touches the paper
and describes on it a white line s s' during the rotation of the cylinder. But after the
rotation has continued for some time the index is no longer in contact with the paper
owing to the eccentricity of the cylinder, but becomes so again afterwards when it
inscribes another line lower down. The distances between the lines described on the
cylinder evidently depend on the rapidity of growth of the plants When, in consequence
of this growth, the index has, after say twenty-four hours, reached the lower margin of
the paper j^/>, the clock-work is stopped, the paper removed and replaced by a fresh
piece, the index being again set by raising the pulley, and the observation repeated.
The lines on the blackened paper are fixed by a varnish of collodion and dried, and the
distances between them are proportional to the hourly growths of the internode. It is
clear that the apparatus not only magnifies the increments, but also records them in the
absence of the observer, which is very convenient, especially for observing the nocturnal
growth. It is however necessary even in this case for the observer to note the tempera-
ture and the hygrometric conditions, at least between morning and evening. Fig. 481
shows in addition a tin vessel 5, consisting of two halves united by a hinge, which may
be used for shutting out the light from the plant, even after the thread has been attached
to it. At E the thermometer / is placed in a similar vessel near the plant.

Sect. 19. — Effect of Temperature on Growth^. It has already been shown

^ See Arbeiten des Wlirzburg. hot. Inst., Heft II. [Wiesner, Ueb. eine neue Construction des
selbstregistrirenden Auxanometers, Flora 1876.]

2 F. Burkhardt in Verhandl. der naturf. Ges. in Basel, 185S, vol. II. i, p. 67. — Sachs, Jahrb. fiir
wissensch. Bot. i860, Heft 11. p. 338.— Alph. DeCandolle in Biblioth. univ. et rev. Suisse, Nov. 1866.
— H. de Vries, in Archiv. neerlandaises, 1870, vol. V. — Koppen, Warme und Pflanzen-Wachsthum,
Dissertation, Moskow 1870. [See also Haberlandt, Landw. Versuchsstationen, XVII, 1874; J"^^'
Cohn's Beitrage, II, 1877; Uloth, Flora, 1*^71 and 1875; von Hohnel, in Haberlandt's Wiss. prakt.
Unters. II, 1877.]

EFFECT OF TEMPERATURE ON GROWTH, 829

in Sect. 7 that the life of a plant generally and its growth in particular is carried on
only within certain limits of temperature (in general between zero and 50° C), and
that each function has apparently in every plant its inferior and superior limits;
so that, for example, the lowest temperature at which a plant of Wheat can grow is
different from the lowest at which a Gourd can grow, &c. It has also been shown
that growth, like other phenomena, is more active the higher the (constant) tem-
perature above the inferior limit, but that there is a certain temperature at which
growth reaches its maximum activity, and above which any further rise of temper-
ature causes a diminution of its rapidity. There is not, in the mathematical sense
of the term, any proportion between the rapidity of growth and the height of the
temperature, and the more accurately the relation between the two has been investi-
gated, the more difficult is it to express this relation by any mathematical formula.
It cannot, on the other hand, be doubted that it is of the utmost importance for
any future theory of the mechanical laws of growth to ascertain the extent to which
growth depends on temperature, at least in a few particular cases.

The difficulties of investigations of this kind are how^ever much greater than is
generally thought; and the results obtained hitherto, valuable as they are, go no
further than what is stated above, and give us no deeper insight into the way in
which that particular mode of motion of the molecules which we call heat is con-
nected with that mode of motion which causes growth.

Restricting ourselves to the results at present obtained, it will be seen that they
have a great practical value in addition to their theoretical significance. A know-
ledge of the cardinal points of temperature, viz. its superior and inferior limits and
the particular temperature at which the maximum of action takes place, is indis-
pensable to investigations of various kinds, in order to get at a correct interpretation
of the phenomena. On this account a few of the more trustworthy observations
may be given here.

In order to determine the cardinal points of temperature to which allusion has been
made, observations are of value only when conducted at nearly constant temperatures ;
the means deduced from very variable temperatures may, as I have shown, lead to
very erroneous conclusions. It is however by no means easy to maintain a sufficiently
constant temperature for a whole day even by artificial heating or cooling. Special
difficulty is met with in the determination of the inferior limit or specific zero, since the
observation must extend over a considerable time — in the case of germination, several
weeks — to be certain that growth does not take place. It would be possible, by means
of the apparatus already described, to determine in the course of a few hours whether
growth still takes place in an internode at a very high or at a very low temperature, and
at what temperature it is the most rapid, if it were not extremely difficult to regulate the
temperature of the plant in the apparatus with sufficient exactness. The auxanometer
will however be very useful even in this case. The observations on this point hitherto
made, at least those which have any physiological value, have been on germinating seeds,
as the temperature and moisture of the soil in which they grow can be more easily
regulated than of the air in the case of internodes. Special facilities are offered by the
roots of seedlings, as they do not emerge from the soil, and are more easily measured,
from their simpler and more regular form. The following figures refer only to the roots
of seedlings, the hypocotyledonary portion of the stem being also, in the case of Dicoty-
ledons, included in the root. That exactly the same figures are not always obtained by
different observers is the result of differences in the mode of observation, the amount of
water, the nature of the soil, the inaccuracy of thermometers, &c.

830

MECHANICS OF GROWTH,

The first point to determine is, whether germination — /. e. the growth of the embryo
at the expense of the reserve materials in the seed — talces place only at certain tempera-
tures, and at what temperature it takes place most quickly. Observations of my own
gave the following results :-

Inferior limit.

Most rapid growth.

Superior limit.

Triticum vulgare

5°G.

287°

G.

42-5° G.

Hordeum imlgare

5

287

377

Cucurbita Pepo

137

337

46-2

Phaseolus multiflorus

9'5

337

46-2

Zea Mais

9*5

337

46-2

This table shows, if the ascertained temperatures are correct, that grains of Wheat
cannot germinate below 5° C, or seeds of the Gourd below 137°, &c., however long they
may lie in moist earth ; and that they no longer germinate, but quickly perish at tempera-
tures above those named in the third column ; while at the temperatures named in the
second column germination takes place in a shorter time than at either higher or lower
temperatures. It may however be taken for granted, from the great difficulty of obtain-
ing these numbers, that the result of further observations will not be identical, though
probably approximate. It is clear that many series of experiments will be necessary
in order to determine each of the cardinal points. The following figures, obtained by
Koppen, agree moderately well with mine, as far as they relate to the same plants : —

Inferior limit Most rapid growth.

Triticum 'vulgare

7*5° G. 297° G.

Zea Mais

9-6 32-4

Lupinus albus

7-5 28-0

Pisum sativum

67 26*6

ig figures were obtained by H. de Vries : —

Most rapid growth. Superior limit.

Phaseolus 'vulgaris

31-5° G. above 42-5

Helianthus annuus

31*5 below 42*5

Brassica Napus

31*5 » 42-5

Cannabis sati'va

31*5 above 42-5

Cucumis Melo

37'5

Sinapis alba

27*4 above 37*2

Lepidium sativum

27*4 below 37*2

Linum usitatissimum

27-4

above 3 7 '2

The following results^, obtained byAlphonse de CandoUe, are moderately trustworthy
as far as relates to the inferior limit, but hardly so much so with respect to the superior
limit and the temperature of most rapid growth, as may be concluded from various
statements made by the observer.

Inferior limit.

Most rapid growth.

Superior limit.

Sinapis alba

o-o° G.

21° C.

28° G.

Lepidium sativum

1-8

21

28

Linum usitatissimum

1-8

21

28

Collomia coccinea

5-0

17

about 28

Nigella sativa

57

above 21 (?)

„ 28

Iberis amara

57

Trifolium repens

57

21-25

below 28

Zea Mais

9-0

21-28

about 35 2

Sesamum orientale

13-0

25-28

below 45

* I take the figures from the table of curves in De Candolle's treatise, with the assistance of
the text.

' De Candolle remarks that the seeds of Maize, Melon, and Sesamum become brown, the first

EFFECT OF TEMPERATURE ON GROWTH. 83 1

When De CandoIIe's inferior limits are below 5° C, they are most probably correct ;
his superior limits and temperatures of most rapid growth are, on the other hand, for
the most part certainly too low.

More accurate information is afforded by the figures which give the lengths attained
by roots in equal periods of time at different temperatures, and express therefore the
rate of the growth of the roots of seedlings at different constant temperatures. These
numbers increase from the inferior limit to the temperature of most rapid growth, and
fall again from it to the superior limit.

In Zea Mais, for example, I found —

in 2 X 48 hours
48
48
48
48
48

Koppen obtained the following growth in length of roots in periods of forty-eight
hours : —

Temperature.

17-1° G.
262

Length attained by the root

2*5 mm.
24'5

33-2
34*o

39-0
550

38-2

25-2

42*5

5'9

Temperature.

Lupinus

nidus.

Pisum sativum.

Zea Mais.

14-1° c.

9-1 mm.

5'o°mm.

180

11-6

8-3

IT mm.

»3-5

3I-0

30*0

IO-8

26-6

541

53-9

29*6

28-5

50-I

40-4

26-5

30-2

43-8

38-5

64-6

33'5

142

23-0

69-5

36-5

12-6

8-7

207

fhe following

are De Vries' results, also in

periods of forty-

eight hours : —

Temperature.

Cucumis Melo.

Sinapis

alba.

Lepidiunt sativum.

Linum usitatissimum.

15*1° G.

3-8

tnm.

5*9 mm.

rimm.

21-6

24-9

38-9

20-5

27-4

i8-2mm.

520

71-9

44-8

30*6

27-1

44-1

44-6

39'9

33'9

38-6

30-2

26*9

28-1

37*2

70-3

lO'O

o*o

9*2

The assertion made by Koppen, in support of which he brings forward an array of
figures, that similar parts of plants grow at different rates at the same mean temperature,
whether the mean temperature is constant or whether it varies above and below the
mean, and further that the rapidity of growth is diminished by the variations of the
temperature even when the variations take place below the optimum, was inserted in the
third edition of this book. This assertion, however, has not been confirmed by the care-
ful observations made in different ways by Pedersen in the laboratory at Wiirzburg'. We
shall see in Sect. 26 that variations of temperature act as stimuli which affect the rapidity
of growth of many foliage and floral leaves in a remarkable manner. A thorough inves-
tigation of the subject from this point of view is much to be desired.

as if burnt at 40° C, a phenomenon which has not been noticed by others. These ' burnt ' seeds
however germinated afterwards at a lower temperature.

^ Haben Temperaturschwankungen als solche einen ungiinstigen Einfluss auf dass Wachsthum?
Arb. d. bot. Inst, in Wurzburg, Bd. I, 1874.

832 MECHANICS OF GROWTH.

Sect. 26.— Action of Light on Growth in Length ^ — Heliotropism ^.

Since we shall now pay exclusive attention to the questions whether and in what
way light promotes or retards quantitatively the superficial growth of the cell-wall,
we may for the time leave entirely out of consideration those cases where it changes

* A. P. De CandoUe, Physiologic vegetale, Paris 1832, vol. III. p. 1079. — Sachs, Bot. Zcitg.
1863, Supplement, and 1865, P- n 7-— Ditto, Experimental-Physiologic, Sect 15. — Hofmeistcr, Lchre
von der Pflanzenzelle, Sect. 36. — Kraus, Jahrb. fUr wisscnsch. Bot. vol. VII. p. 209 et seq. — Batalin,
Bot. Zeitg. 1871, No. 40.

2 [H. Muller (Thurgau), Ucb. Heliotropismus, Flora, 1876; Wiesner, Die Heliotropischen
Erscheinungen, Denkschr. d. k. k. Akad. in Wien, 1878, 1880; Darwin, The Movements of Plants,
1880.

A brief account of Wiesner's conclusions may be found useful ; they are as follows : —

I. Lijluence of the Intensity of Light.

1. The maximum of heliotropic effect is produced by a certain intensity of light : increase or
decrease of intensity diminishes the heliotropic effect until it is no longer produced. The optimum
intensity varies in different plants.

2. The upper limit of intensity is either greater or less than that degree of intensity at which
the parts of plants in question can grow at all : this depends upon the relative s^sitiveness of the
plants.

3. Hence it appears that sunlight may absolutely arrest growth : young stems are protected by
their strong negative geotropism from the action of sunlight.

4. The degree of intensity at which heliotropism ceases corresponds, doubtless, to the intensity
at which the plant no longer reacts by growth ; an intensity which affects the plant no more than
complete darkness.

II. Influence of Refrangihility .

1. Not only do the rays of high refrangihility possess heliotropic power, but those also of lower
refrangihility: it is possessed by all rays from the ultra-red to the ultra-violet except the yellow rays.

2. The most marked effects are produced by the rays at the junction of the violet and ultra-
violet : from these to the green the heliotropic effect gradually diminishes ; in the yellow it is zero :
it recommences in the orange and gradually increases until it attains a second maximum (small) in
the ultra-red.

If the parts are not very sensitive, the effect is diminished in each of the colours in proportion
to their heliotropic power, so that the orange, red, green, ultra-red, blue, etc. become inert in suc-
cession.

3. The heliotropic effect is not proportional to the mechanical intensity (thermic power) of
the rays.

4. Negatively heliotropic organs exhibit the same phenomena.

III. Concomitant action of Heliotropism and Geotropism.
In strongly heliotropic organs, geotropism does not interfere with the exhibition of heliotropism
provided that the light is intense.

IV. Presence of Oxygen.
No heliotropic phenomena occur in the absence of oxygen.

V. Photomechanical Induction.

1. Both heliotropism and geotropism are exhibited after the removal of the organ under experi-
ment from the action of light or of gravity respectively : this effect is an induced effect.

2. Successive exposures to the action of light or of gravity produce their effects distinctly ; there
is no summation.

VI. Relation of Heliotropism to Turgidity.

In many cases positive heliotropic curvature does not take place in the zone of most rapid
growth, where the turgidity is greatest, but in a zone below it, where the turgidity is less.

Etiolated organs become more sensitive to the heliotropic action of light after they have been
exposed on all sides to diffuse light, probably because the turgidity of the growing cells is thereby
diminished.]

»

ACTION OF LIGHT ON GROWTH IN LENGTH. 833

or may possibly change qualitatively the physiological and morphological nature
of the newly-formed organs.

The dependence of growth on light has already been spoken of in general terms
in Sect. 8 ; and it was there especially insisted on that, in order to avoid serious
misconceptions, this must be distinctly separated from the question of the part
taken by light in assimilation. Here also we are concerned only with the processes
of growth itself, since we always start from the point at which the cells or organs
concerned have already obtained a sufficient quantity, or even excess, of formative
materials.

It has been already stated that the various parts of the flower grow as readily
in permanent darkness as in light. Most internodes, on the contrary, as has been
explained in Sect. 18, grow more slowly when exposed to light on all sides, and
remain shorter than when growing in the dark ; when the light reaches them from
one side only, they curve concavely towards the source of light. Other organs how-
ever, as root-hairs, tendrils, and some internodes, become longer on the side exposed
to light than ou that left in the dark. We have seen also that the leaves of Ferns
and Dicotyledons soon cease growing in the dark and remain small. These observ-
ations show clearly enough that different cells and organs are differendy affected
by light as respects their growth. Since the light itself remains the same and ther^
is a supply of formative materials, any explanation of these differences must aim at
showing how it is that the inherited organisation of the plant in each case is affected
just in one particular way and no otherwise by the oscillations of the ether. It is
however at present quite impossible to give such an explanation ^, since far too little
is yet known of the phenomena themselves; the ascertained facts cannot yet even be
reduced to a general law, especially in consequence of the obscurity which involves
the behaviour of leaves (see t'n/ra) and of negatively heliotropic organs under the
action of light. If these difficulties, which were referred to in Sect. 8, were solved,
the organs of plants might be divided in respect of their behaviour towards light into
three kinds : — (i) those the growth of whose cells is in general independent of light;
as petals, stamens, fruits, and seeds ; (2) those whose growth is retarded by light ;
the positively heliotropic organs which become abnormally elongated by absence of
light; and (3) those whose growth is promoted by light. To this last category
would belong negatively heliotropic organs if we could be certain that negatively
heliotropic organs grew more slowly in darkness than in light. The observations of
Schmitz^ on Rhizomorphs show, however, that this is not usually the case, for,
although they are negatively heliotropic, they grow, like positively heliotropic organs,
more quickly in darkness than in light.

The question in what manner light affects the mechanism of the growth of
the cell-wall can therefore, in the present state of our knowledge, have a definite

* If Muller, in the second part of his Botanische Untersuchungen (Heidelberg 1872), gives the
impression of having achieved this with but little difficulty, this only shows how far he is from a
true method of investigation.

2 Schmitz, Linnsea, 1843, p. 513. [Similar results have been obtained by Miiller (Thurgau)
with the negatively heliotropic roots of Chlorophytum and of Monstera Lennei (Ueb. Heliotropismus,
Flora, 1876), and by F. Darwin with those of Sinapis alba (Ueb. das Wachsthum negativ helio-
tropischer Wurzeln, Arb. d. bot. Inst, in Wiirzburg, II, 1880).]

3 H

834 MECHANICS OF GROWTH.

meaning only in reference to positively heliotropic organs ; inasmuch as it is in these
cases certain that the growth of the cell -wall in the direction of the axis of growth of
the organ is retarded and limited by light. But even in this case the question cannot
at present be answered, since several others must first be solved. It must first of
all be decided whether light acts in this manner on the cell-wall only when its
plane of incidence is inclined to the axis of growth. A similar problem, as we shall
see, is presented in the action of gravitation on growth. The various phenoinena of
positive heliotropism allow in fact of the supposition that rays of light which pene-
trate the cell-wall in a direction parallel to the axis of growth of the organ do not
hinder growth, while they do so more strongly the more nearly vertical they are to
it, whether the organ be multicellular or a simple filament. Light therefore acts more
intensely the more nearly the transverse vibrations of the ether are parallel to the
surface of the cell-wall. But the solution of these questions would by no means
explain the action of light on the growth of the cell-wall ; in the first place we must
know whether light acts directly on the cell-wall, or if the eflfect is produced by
means of the protoplasm, or by chemical changes in the cell-sap. But since we
know that the cell-wall only grows so long as it is in contact on the inside with
living protoplasm, and that the protoplasm itself is set in motion by light, in con-
sequence of which it accumulates at particular parts of the cell-wall (see Sect. 8) ;
and since this, like the growth of the cell-wall, is caused by the highly refrangible
rays — the hypothesis must not at once be set aside. The question may moreover
be asked whether light does not influence the growth of the cell-wall by means
of chemical effects which it brings about in the cell-sap or the protoplasm, which
however cannot be referred to assimilation, since they take place even in cells
destitute of chlorophyll, as for instance in the positively heliotropic neck of the
perithecium of Sordaria fimiseda, the stipes of Ciavkeps, and in many roots of seed-
lings; and since the leaves of Dicotyledons exhibit relations to light i^ide infra)
which indicate a chemical action on assimilated substances, but not on the process
of assimilation itself.

So long as we take into account multicellular organs alone and merely contrast
green and etiolated plants, great weight might be allowed to the hypothesis of a
change in the turgidity caused by light (brought about by some chemical alteration
in the cell-sap and the consequent change in diosmose^). But the fact that even
unicellular tubes like those of Vaucheria and the internodal cells of Nitella are
positively heliotropic, forbids this hypothesis, since in these cases the side exposed
to light grows more slowly than the other, although all the parts of the cell- wall are
subject to the same hydrostatic pressure from the sap.

The examples already given of positive heliotropism in submerged unicellular
filaments, as well as the heliotropic curvings of multicellular internodes under water,
show at once that they have nothing to do with a more rapid transpiration in-
duced by light or its results.

The suggestion would appear on the contrary to be worth more attention that
the reason why light retards the superficial growth of positively heliotropic cells
is because it first of all promotes increase of thickness, and therefore diminishes the

* See Dutrochet, Memoires pour servir, etc., Paris 1837, vol. II. p. 60 et seq. [This question is
discussed in the following small print.]

ACTION OF LIGHT ON GROWTH IN LENGTH. 835

extensibility of the cell-wall under the influence of the pressure of the sap on
the side exposed to the strongest light. This hypothesis would be confirmed by
Kraus's observations, according to which the cuticularisation of the epidermis as well
as the thickening of the walls of the cortical and bast-cells is in fact imperfect in
etiolated internodes, and the extensibility of these cell-walls consequently increased
by the want of light. This explanation would apply not only in the case of the
shaded side of a multicellular internode which curves towards the light, but also
in that of a Vauchen'a-iuhe or internode of Nitella ; since it may be supposed
that the wall is in the first place more strongly thickened on the side exposed
to light and hence becomes less extensible, and therefore yields less to the pressure
of the sap, and, in consequence, grows more slowly. We have at present no
observations on heliotropic unicellular filaments.

If it be proved, as the recent researches of Wolkoff give ground for believing,
that the negative heliotropism of organs which contain chlorophyll depends as
litde as that of roots on the stronger power of assimilation possessed by the
side exposed to the source of light, it must be assumed that all the actions
which have been mentioned as possible in one direction may take place also in an
opposite direction ; and this will show the great difficulty of the investigation.

A complete account of the mode in which growth depends on light is scarcely
possible at present ; what has now been said will call the attention of the reader to the
most important questions involved in the investigation. It may be desirable however to
collect some of the more important facts at present known, and to add some critical
remarks.

(a) Organs nuhose gronvth is retarded by light. To take first the case of those inter-
nodes (including, according to Hofmeister, the unicellular ones of Nitella) which, when
the light is unequal on the two sides, curve so that the side facing the source of light is
concave while the other side is convex, or in other words are positively heliotropic.
These exhibit a periodicity in their longitudinal growth corresponding to the alternation
of day and night, when the temperature is sufficiently constant. The growth is more
rapid from evening to morning, and less so from morning to evening. Both these facts
are consistent with the phenomenon that the same internodes grow longer, and often
considerably so, in permanent darkness than they would under normal conditions.
These three results lead naturally to the conclusion that it is the direct action of light
(and only in fact of its more refrangible rays, see Sect. 8) which retards the growth of
these internodes. In the case also of positively heliotropic roots (as those of Zea Maisj
Lemna, Cucurbita, Pistia, &c.), it may be supposed that if exposed to daylight they
would exhibit the same alternation as internodes ; but this is not yet fully established.
WolkofF has, on the other hand, already shown in the case of some roots, grown in
water behind a transparent glass plate, that they grow more quickly in permanent
darkness than under the alternation of day and night. Twelve primary roots of seed-
lings of Pisum sativum gave, for example, the following results : —

Day. Successive increments.

In the dark. In diffuse light.

161 mm.

153
210

»I3

78

I St

195 mm.

2nd

239

3rd

250

4th

126

5th

113

In the 5 days

923 mm.

3 H 2

715 mm.

836 MECHANICS OF GROWTH.

The increments of growth of primary roots of seedlings of Vic'ui Faba were aft
follows: —

In the dark. In diffuse light.

In 5 roots as 309 to 272

II 743 612

9 612 416

In these cases a tendency of the roots was observed, though not a very decided one,
to positive heliotropic curvature. The difference in the rapidity of growth would no
doubt have been greater if the increments in the same time had been compared during
the day only.

The long narrow leaves of many Monocotyledons exhibit the same phenomena as
internodes and roots, becoming considerably longer in permanent darkness than under
normal conditions, and showing positive heliotropic curvature when the light from the
two sides is unequal. The plane of curvature may coincide with the plane of the leaf,
so that one margin may be considerably longer than the other, and the whole leaf there-
fore unsymmetrical. I have observed this very evidently in a plant of Friiillarla impe-
rialis grown in a window ; those leaves only which sprang exactly from the side of the
stem exposed to light being symmetrical like those growing in the open air. We have
at present no observations on the daily periodicity in these leaves caused by light.

Observation of the broad netted-veined leaves of Dicotyledons is much more difficult.
From the fact that in the dark they remain smaller, and often very much so, than
under normal conditions, it might be concluded that their superficial growth presents
exactly opposite phenomena to those of internodes and the long leaves of Monocoty-
ledons. But Batalin has shown that it is sufficient to expose etiolated plants now and
then to light — the time not being long enough for them to become green — for their
growth in the dark to be afterwards considerably promoted. This leads to the suppo-
sition that light causes in etiolated leaves a chemical change which is not connected with
assimilation, by which they are enabled to grow further in the dark. In any case this
phenomenon shows that there is no real contradiction between the growth of these
leaves and that of internodes, and that the reason why they become larger under the
normal conditions of light than in permanent darkness is not because light has a directly
favourable influence on the growth of the cells of these leaves. The recent experi-
ments of Prantl ^ rather favour the hypothesis that green — and therefore healthy and
normal— leaves exhibit the same diurnal periodicity of growth as positively heliotropic
internodes. He succeeded, by a number of measurements both in breadth and length
of the leaves of Cucurbita Pepo and Nicotiana labacum, taken at intervals of three
hours, in constructing curves of growth, which, in spite of adverse fluctuations of temper-
ature, rose from evening to morning, attained a maximum after sunrise, and then fell
during the day till evening ; exactly what I showed to be the case with positively helio-
tropic internodes. If this general law is established, it results that the broad netted-
veined leaves of Dicotyledons grow more quickly in the dark than in the light, and are
therefore hindered in their growth by light. But when such leaves remain nevertheless
smaller in permanent darkness because they cease growing earlier, this must be inter-
preted as an unhealthy condition depending on the suspension of certain processes of
metastasis which must precede growth and which are induced by light. In conformity
with this hypothesis we must suppose that in leaves which unfold under the alternate
influence of day and night, growth is directly hindered by light ; but that at the same

^ Arbeit, des bot. Inst, in Wlirzburg, Vol. I, p 382. [See also Stebler, Ueb, Blattwachsthum,
Jahrb. f. wiss. Bot. XI, 1877. From his observations Stebler draws the erroneous conclusion that
light promotes the growth of leaves (of Monocotyledons). For a criticism and further observations
see Vines, The Influence of Light upon the Growth of Leaves, Arb. d. bot. Inst, in Wiirzburg, II,

1878.]

ACTION OF LIGHT ON GROWTH IN LENGTH. 837

time certain chemical changes take place which in general make growth possible, and
enable it to continue in the succeeding darkness, if it does not last too long. That this
has nothing to do with assimilation is shown by Batalin's experiments with leaves
destitute of chlorophyll.

If we now enquire what are the mechanical changes which light causes in the organs
we have been considering, and by which their growth is retarded, it is to be regretted
that no attempt has yet been made to study them in unicellular organs which exhibit
positive heliotropism, as Vaucheria-inhes and internodes of Nitella, since they present
the most simple case from a mechanical point of view. In the case of the internodes
of Phanerogams which consist of tense layers of tissue, Kraus found in the etiolated
state a smaller tension between the medullary and cortical layers, and that the cell-
walls of the layers of tissue placed in a state of passive tension by the pith were less
thickened, lignified, and cuticularised. It follows that these last are more extensible than
in the normal internode, and therefore offer less resistance to the tendency of the pith
to elongate. If we suppose that in unicellular tubes light also increases the cuticularisation
and thickening of the cell-wall, the wall will offer greater resistance to the pressure of the
cell-sap, will become less stretched, and will therefore grow more slowly ^

But little can be inferred as to the mechanical influence of light on growth from the
changes in the tension of the tissues on the convex and concave sides of internodes with
positive heliotropic curvature. If such an internode is split lengthwise so that the side
exposed to light is separated from the other side, the former becomes more concave,
while the latter becomes less convex or even somewhat concave towards the shaded side.
In other words, the tension between the outer and inner layers is greater on the concave
side exposed to light than on the convex shaded side. But the same phenomenon
occurs also in internodes with an upward geotropic curvature, and with negatively
heliotropic internodes, as well as with twining tendrils ; and could not in fact be
otherwise.

(b) Of Negat'vvely heliotropic organs'^ only a comparatively small number are at present
known. Among those which contain chlorophyll may be named the hypocotyledonary
portion of the stem of the seedling of the Mistletoe, the older nearly mature internodes
of the Ivy and TropcBolum majus, and the basal portions of the tendrils of the Vine, Vir-
ginian Creeper, and Bignonia capreolata. I pass over at present the doubtful negative

^ [With reference to the action of light upon grovi^th, it is now universally admitted that the
effects either of retardation or of curvature which it produces are the expression of modification of
the turgidity of the growing cells. Three suggestions have been made as to the way in which this
modification is brought about ; (i) by a change in the elasticity of the cell-wall, (2) by a change in
the osmotic properties of the cell-sap, (3) by a change in the permeability of the protoplasm. Some
evidence in favour of the first of these is given above in the text, and it is further supported by
Pfeffer (Physiologic, II. 145, i83i) and by Wiesner (Heliotropische Erscheinungen, II. p. 5). De
Vries states (innere Vorgiinge bei den Wachsthumskriimmungen) that * external and internal causes
produce curvatures in growing multicellular organs because they promote the formation, in the cells
of one side of the organ, of substances which are osmotically active.' For the arguments in favour
of the third suggestion see Vines, The Influence of Light on the Growth of Unicellular Organs, Arb.
d. bot. Inst, in Wiirzburg, II, 1878.

It is impossible to enter here upon a detailed criticism of these various views. The following
remarks must suffice. With reference to No. 1 , it is difficult to understand how it can be satisfac-
torily applied to explain the action of light upon unicellular organs, seeing that the effects are so
rapidly produced, nor is it clear how both positive and negative heliotropism can be explained by
means of it. No. 2 clearly cannot account for positive and negative heliotropism in the case of
•unicellular organs, and its application to the case of multicellular organs is not obvious. As to No. 3,
we know at least that light does act upon the protoplasm of zoogonidia, chlorophyll-granules, etc.,
and there seems to be no reason why it should not act directly upon that of growing cells.]

2 Knight, Phil. Trans. 1812, p. 314.— Dutrochet, Memoires, &c., vol. II. p. 6 et s^y — Durand
and Payer's statements.— Compare Sachs, Exper.-Phys., p. 41.

838 MECHANICS OF GROWTH.

heliotropism, as 1 think, of the thallus of Marchantia and the prothallia of Ferns, as
well as of other decidedly bilateral organs. Among organs which are not green must be
' especially mentioned the negatively heliotropic aerial roots of Aroideae and epidendral
Orchids ^ ; but, beyond all others, the roots of Chlorophytum guayanum, which are ex-
tremely sensitive to light coming from one side. Negative heliotropism has, in addition,
been stated to occur in the roots of seedlings of Cichoriaceae, Cruciferae, &c., and has
recently been certainly determined by WolkofF in the case of Brasska Napus and Sinap'ts
alha. Among unicellular organs destitute of chlorophyll the only ones known at present
with certainty to be negatively heliotropic are the root-hairs of Marchantia (Pfeffer).

The observation that a number of organs destitute of chlorophyll and endowed with
negative heliotropism, and in particular the highly sensitive roots of Chlorophytum, are
very transparent, led Wolkoff to the hypothesis that the rays of light may be refracted
by their cylindrico-conical shape, so as to produce a more intense illumination of the
tissue on the side removed from the source of light than on that exposed to it ; and that
therefore the concave curvature on the former side is in fact a form of positive helio-
tropism. The apices of roots, when separated by a transverse section, if illuminated
from one side and viewed from above, exhibit exactly the optical conditions which are
assumed by this hypothesis. It must however not be forgotten that the apices of roots
which are by no means negatively but at an earlier period even positively heliotropic,
like those of Vicia Faba, manifest the same phenomenon, though perhaps to a, lesser
degree. Whether, on the other hand, it is possible to suppose a similar refraction of
light in the case of the very thin-walled negatively heliotropic root-hairs of Marchantia,
is still in doubt. Further researches must show whether Wolkoff 's suggestion is tenable
or not. The Rhizomorphs would probably afford good material for observations on
this subject, since, according to the researches of Schmitz, they are distinctly negatively
heliotropic, and yet they grow more slowly in light than in darkness.

In the cases of the older internodes of the Ivy, which are only very slightly trans-
parent, the older and lower parts of tendrils, &c., the existence of an active focal line
on the shaded side cannot be admitted, because this would evidently imply that it
included more intense blue and violet light than, from the fact that the tissue which is
penetrated by the light contains chlorophyll, it is probable it does. The negatively
heliotropic curvature takes place however, at least in the Ivy as well as in the roots of
Chlorophytum, only in highly refrangible light (after passing through an ammoniacal
solution of copper oacide), not in yellow light (which has passed through potassium
bichromate). If, as Wolkoff at one time supposed, the more vigorous nourishment,
i.e. accumulation of assimilated substances, were the cause of the more rapid growth
on the side exposed to light in this class of negatively heliotropic -organs, they ought to
curve much more strongly in the less refrangible (red, orange, or yellow) than in the
more refrangible rays. This hypothesis would moreover fail to explain why the same
internodes which when young showed decided positive heliotropism, at a later period
when their growth has almost ceased manifest the opposite behaviour towards light.

The experiments which Wolkoff is now (1873) carrying on in the botanical laboratory
at Wiirzburg, and which are not yet completed, lead at present to the conclusion that
there are two kinds of negatively heliotropic organs. In one kind are included roots, in
which the negatively heliotropic curvature takes place near the apex at the spot where
growth is most rapid ; to the other kind belong internodes where the negatively helio-
tropic curvature takes place only at the older parts whose growth is completed, while
the young quickly-growing parts manifest positive heliotropism. In these latter cases
the additional peculiarity occurs that the older parts, after being exposed to light on
one side, will continue for some time to curve in the dark so that the side previously
exposed to light becomes still more convex. This is a property which appears to be
wanting in organs of the first kind as well as in those that are positively heliotropic.

According to a great number of observations of my own and statements of others.

ACTION OF GRAVITATION ON GROWTH IN LENGTH. 839

It is evident that we are here confronted with an unsolved problem ; and when all
the facts have been taken into consideration, the theory that there are two kinds of
cells, the growth of one of which (positively heliotropic) is retarded by light, whilst
that of the other kind (negatively heliotropic) is promoted by it, may be the simplest
and most in accordance with facts. This difference is the less remarkable since in the
behaviour of growing cells with respect to gravitation we find a precisely similar differ-
ence, but much more strongly marked ^

Sect. 21. — Action of Gravitation on G-rowth in Length: — G-eotropism^.

It has already been shown in Sect. 10 that, when the access of light is equal on all
sides or when heliotropism is prevented by the exclusion of light, gravitation is the
cause of certain organs turning downwards, others upwards, and others again in
a direction oblique to the horizon. At present we shall speak only of those which
take a direction directly upwards or downwards, since other causes co-operate to
bring about an oblique growth.

Just as organs, according to their internal nature, grow either more rapidly or
less rapidly on the side which faces the source of light than on the other side, so also
gravitation effects, in accordance with the nature of the organs, either an acceleration
or a retardation of growth on the side which faces the earth. Those organs which
are thus retarded in their growth are called positively geolropic^ those which are
accelerated negatively geoiropic organs. Positively geotropic organs consequently
become concave on the under side, and direct their growing apex downwards if
their axis of growth is brought into a horizontal or oblique direction ; negatively
geotropic organs, on the contrary, become convex on the under side under similar
conditions, and elevate their growing apex until it stands erect.

It has not yet been ascertained whether positively geotropic organs would mani-
fest a different rapidity of growth if entirely withdrawn from the influence of gravi-
tation (like positively heliotropic organs when withdrawn from the influence of light)
from that displayed when gravitation acts in a direction parallel to the axis of
growth^. It would seem however as if gravitation only affected the rapidity of

^ SchmitK, Linnsea, 1843, p. 513 et seq. If, as can scarcely be doubted, Schmitz's statements
with regard to Rhizomorphs are confirmed, it results that no certain inference can be drawn as to
the positive heliotropism of an organ from the fact that its growth is more rapid in the dark. We
could scarcely have a better proof of the necessity for a fresh and more accurate investigation of all
the phenomena of heliotropism. [Schmitz's observations have been confinned, in other cases, by
Miiller and F. Darwin (see ante).']

^ Knight, Phil. Trans. 1806, vol. I. pp. 99-108.— Johnson, Edinburgh Phil. Journ. 1828, p. 312.
— Dutrochet, Ann. des Sci. Nat. 1833, p. 413. — Wigand, Botan. Untersuch. Braunschweig 1854,
p. 133.— Hofmeister, Jahrb. fiir wissensch. Bot. vol. III. p. 77.— Ditto, Bot. Zeitg. i8f)8, Nos. 16, 17,
and 1869, Nos, 3-6.— Frank, Beitrage zur Pflanzen-Phys. Leipzig 1868, p. i.— MuUer, Bot. Zeitg.
1869 and 1871.— Spescheneff, Bot. Zeitg. 1870, p. 65.— Ciesielski, Untersuch. iiber die Abwarts-
kriimmung der Wiirzeln, Breslau 1871. — Sachs, Arbeit, des bot. Inst, in Wurzburg 1872,. Heft 2.
Abh. 4 and 5.— Ditto, Exper.-Phys., p. 505.— Ditto, Flora, 1873, No. 21. [Darwin (Movements of
Plants) terms what are here termed positive and negative geotropism, ' geotropism ' and • apogeo-
tropism ' respectively. He considers that both positive and negative geotropism are modified forms
of circumnutation {see infra),']

^ [Elfving has found (Beit.z. Kennt. d. physiol. Einwirkung der Schwerkraft, itelsingfors 1880)
that when the sporangiophores oi Phy corny ces, which are negatively geotropic, are grown in an inverted
^position, their growth is not so rapid as it is under ordinary conditions ; that is, that they grow less
rapidly in the direction of the action of gravity than in the opposite direction.]

I

840 MECHANICS OF GROWTH,

growth when its direction cuts that of the axis of growth at an angle, and the more
so the nearer the angle approaches a right angle.

The positive or negative character of geotropism depends as little as that of
heliotropism on the morphological nature of the organ. Not only, for example, are
all the primary roots of the seedlings of Phanerogams positively geotropic, and most
secondary roots which spring from underground stems, as tubers, bulbs, or rhizomes;
but also many leafy lateral shoots, especially those which are destined to produce
rhizomes or to form new bulbs {e.g. Tulipa, Physalis, Polygonum, &c.), and even
foliar structures, like the cotyledonary sheaths of Allium, Phoenix, and many other
Monocotyledons. Among positively geotropic organs must also be included the
lamellae and tubes of the hymenium of Hymenomycetous Fungi. All axes which grow
upright (and are not bilateral), petioles, and the stipites of many Hymenomycetous
Fungi, exhibit, on the other hand, decidedly negative geotropism.

The geotropism, like the heliotropism, of different organs varies in all degrees.
It is, for example, manifested very strongly in the primary roots and upright
primary stems of seedlings; much less strongly in the secondary roots and in
lateral branches of erect stems, &c. It appears to be the general rule that when
lateral shoots of the same kind spring from a vertical and therefore decidedly
geotropic organ, the branches of the first order are less geotropic, and the further
ramifications still less so the higher the order to which they belong ; the exceptions
to this rule may be caused by special circumstances. This gradation is very obvious
in roots. From the primary root or a strong root springing from the stem with
well-marked positive geotropism, proceed secondary roots of the first order which
exhibit the phenomenon much less decidedly ; and from these again secondary roots
of the second order which apparently are not at all geotropic, and therefore grow in all
directions as they may chance to originate. Geotropism, like heliotropism, does not
depend on the organ containing or not containing chlorophyll, nor on whether it
consists of masses of tissue or of a simple row of cells or of a single cell. To this
last category belong, for example, the positively geotropic radical hyphae of the Mu-
corini and the negatively geotropic sporangiophores of the same family and of
numerous other Mould-fungi. In the same manner the rhizoids of Char a display
positive, the stems negative geotropism, both consisting of unicellular segments, the
former destitute of chlorophyll, the latter green. Whether and how strongly an
organ is positively or negatively heliotropic or geotropic depends altogether on its
importance in the economy of the plant, and hence on its physiological functions.

From the remarkable fact that there are organs endowed with positive and
negative heliotropism and geotropism, and from many similarities exhibited by the
two phenomena, the question presents itself whether all positively heliotropic organs
must not possess one description of geotropism either positive or negative, or vice
versa; in other words, whether the two properties do not stand in some definite
relation to one another. This does not however appear to be the case. Of primary
roots, all of which are positively geotropic, some display positive, others negative
heliotropism ; and again, the aerial roots of Chlorophytum, Aroideae, and Orchideae
display very distinct negative heliotropism, but are scarcely at all geotropic. According
to Schmitz the same is the case with the Rhizomorphs. There appears therefore to
be no necessary connection between the two phenomena.

ACTION OF GRAVITATION ON GROWTH IN LENGTH. 84 1

It is clear that organs which are both heliotropic and geotropic, and on which,
since they lie obliquely to the horizon, the light falls from above or from below, are
subject to changes in their growth dependent both on light and on gravitation.
Thus, for example, the bending upwards of a branch placed horizontally on which
the light falls from above may be caused at the same time by positive heliotropism
and by negative geotropism. An erect stem, on the other hand, which turns helio-
tropically towards a source of light at the side and thus makes a curvature which is
concave below, will have a tendency to become erect in consequence of its negative
geotropism, and would do so if the light falling on it were removed. Stems there-
fore which in the evening were bent by positive heliotropism, will stand upright in
the morning. These considerations are evidently of the first importance in making
observations on the two phenomena.

We have already seen that no clear idea has yet been obtained of the mode in
which light acts in influencing the growth of heliotropic organs. As litUe are we at
present in a condition to affirm how the acceleration or retardation of the growth of the
cell-walls results from the action of gravitation \ The hypotheses and considerations
there stated may be repeated here mutatis 7nutandis. Particular stress must be laid
on the fact that movements are induced in protoplasm by the action of gravitation
just as by the action of light. Thus Rosanoff" showed^ that the plasmodia o{ JEtha-
Hum sepiicum are negatively geotropic, creeping, under the influence of gravitation,
over steep moist walls, and turning, under the action of centrifugal force, towards
the centre of rotation ; they take therefore those directions which would be
least expected from their apparently fluid condition. The question suggests itself
whether there is not also protoplasm which behaves in this respect in an exactly
opposite manner ; and from the dependence of the growth of the cell-wall on the
activity and probably also on the disposition of the protoplasm in the cell, the hypo-
thesis must not be altogether set aside that all geotropic phenomena are in the first
place caused by the protoplasm taking up definite positions in the cells under the
[influence of gravitation, and thus accelerating or retarding the growth of the cell- walls
|fc>n the under sides. Since nothing is known on this subject, we must direct our
fattention solely to the growth of the cell-walls, leaving it undecided whether the effect
)f gravitation be direct or indirect.

In order to state clearly the problem how gravitation acts on the growth of the
cell-waU^ we may consider as the simplest example a unicellular filament, such as we
find in Vaucheria, the posterior end of which developes as a positively geotropic
root, the anterior end as a negatively geotropic stem. Fig. 482 A may represent this,
assuming that the whole filament grew at first in a vertical direction either upwards or
downwards, but was then placed in a horizontal position, as shown by the light outr
lines S and W. After some time the radical end would show a downward curvature,

* [See note on p. 837.]

2 Rosanoff, De I'influence d'attraction terrestre sur la direction des plasmodia des Myxomycetes
(Memoires de la Societe imperiale des sciences de Cherbourg, vol. XIV). [According to Strasburger
(Wirk. des Lichts und der Warme auf Schwarmsporen, Jenaisch. Zeitschr., XII, 1878) this apparent
negative geotropism of the plasmodia is due simply to the fact that they tended to travel against the
direction of the stream of water by which they were kept moist during the experiments.]

3 Duchartre's assertions on geotropism in his Observations sur le retournement des Champignons
(Compt. rend. 1870, vol. LXX. p. 781) show that he has not clearly comprehended the question. ^

842 MECHANICS OF GROWTH.

like W\ the part ^S" on the contrary the upward curvature, as S'. It is self-evident
that each of these curvatures can only result from the growth, equal on all sides
when the organ is erect, having now become unequal on the upper and under sides,
the convex growing in both cases more quickly than the concave side.

If we now apply the results of my experiments on internodes and nodes of
Grasses which curve upwards to the simple tube, the growth is found to be more
rapid on the convex under side, less rapid on the upper side of the upwardly curved
part, than when it grew erect. It may be assumed, from Ciesielski's measurements of
roots, that when the filament curves downwards the growth has been more rapid on
the convex upper side, less rapid on the concave under side, than if the curved part
had grown onwards in a vertical direction. In other words, when the filament is
placed in a horizontal position the growth is accelerated on the upper side of the
positively geotropic part and on the under side of the negatively geotropic part, but
always retarded on the opposite sides.

If therefore we assume that in Fig. 482 ^ the two side walls of a transverse disc
of the part -S of the filament when in an upright position had lengthened in a definite
lime to the equal lengths 0 0 and u u, it would have remained straight ; but if the

Fig. 482.— Diagram for illustratinji geotropic upward and downward curvature.

tube had been placed horizontally during this time, the lower side would have attained
the greater length u u, the upper side the shorter length 0' o\ and the piece must in
consequence become curved. Exactly the opposite would be observed, as shown in
Fig. 482 C, if the growing piece belonged to the part PFof the filament.

If now the unicellular filament A were supposed divided by transverse and lon-
gitudinal divisions into a tissue consisting of a number of layers of cells ; or if, what
amounts to the same thing, a stem of a seedling were supposed to be substituted for
the part 6* of the filament, and a root for the part PF, the same phenomena would
occur, as experiments have shown, in every cell of the growing part, as those pre-
viously observed in the filament. In the part S every cell would grow more rapidly
on the under side, less rapidly on the upper side than if the part were upright, the
reverse in the part W. We should find that in -S both the upper and under sides of
any cell {i. e. upper and under in relation to the radius of the earth) are longer than
those of the cells situated above it, the reverse in W) in other words, that every indi-
vidual cell of a part which shows geotropic curvature behaves in the same way as if
the part previously straight were held firmly by the two ends and then bent. This
will be made clearer to the student if in the portion of the curved part included in

ACTION OF GRAVITATION ON GROWTH IN LENGTH. 843

A lines are drawn parallel both to the straight and the curved outlines, and the septa
of the cells are then indicated in the straight piece simply by parallel lines crossing
the first at right angles, in the curved part by lines corresponding to the radii of
curvature. The cells exposed by longitudinal sections through nodes of Grasses
and roots endowed with geotropic curvature exhibit this phenomenon, although
with many irregularities.

Now that the facts connected with the geotropism of the cell-wall have thus
been made clear, we may proceed to the question, why or by what effect of gravita-
tion these differences are occasioned in the growth on the upper and under sides of
every cell of a geotropic organ when placed in a horizontal position. We have
at present however no answer to this question, any more than in the case of helio-
tropism, the same diagram availing, mutatis mutandis, for the two phenomena.

The view brought forward by Hofmeister, and for some time adopted by me,
that positive geotropism occurs only in those organs and in those parts of organs
in which there is no tension in the tissues, while the organs in which there is strong
tension are negatively geotropic, rested on imperfect induction. On the one hand,
the parts of the roots of seedlings which curve downwards (as I have shown else-
where) are not entirely without tension between the cortex and the axial fibro-
vascular cylinder ; while, on the other hand, in the nodes of Grasses, although they
display a high degree of negative geotropism, there is no or very little such tension.
Even in the negatively geotropic contractile organs of the petioles of Phaseolus^ the
tension between the cortex and the axial bundle is of a similar character to that
which occurs in positively geotropic roots, but extremely intense. If therefore the
tension of tissues and the alteration effected in it by the influence of gravitation
cannot be considered as the cause of the upward curvature, it may still be admitted
that it is useful to upright organs by increasing their rigidity and elasticity, thus
making them more readily assume the erect position ; while this would be quite
unnecessary in those that grow downwards.

A good illustration of the part played by rigidity and elasticity in producing the
erect position of negatively geotropic organs is afforded by the pendent pedicels of
many flowers and flower-buds, in which the tendency to curve upwards is altogether
obscured, the weight of the flower being sufficient to bend the pedicel downwards.
If in such cases the flower-buds are cut off, the pedicel becomes erect ^ from the
stronger growth of the under side, as e.g. in Clematis integri/olia, Papaver pilosum
and dubium, Geum rivale, and Anemone pratensis. The tension in the tissue of such
pedicels is not sufficient to give them the rigidity needful to overcome the weight
of the flower by their geotropic curvature upwards; this weight, on the contrary,
overcomes the tendency of the pedicel to curve convexly on the lower side, which
tendency comes into play when the weight is removed. The same is the case in very
long but not very rigid shoots, as those of the Weeping Willow, Weeping Ash, &c.

If a number of organs grow in a horizontal or oblique direction without curving
either upwards or downwards, this may result from their not being geotropic'' and

» Sachs, Experimental-Physiologic, p. 105.

* See De Vries, in Arbeiten des Bot. Inst. Wurzburg, Heft II. p. 229.

^ [Elfving (Ueb. einige horizontal-wachsende Rhizome, Arb. d. bot. Inst, in Wurzburg, II 3,
1880) has found that the rhizomes of certain plants {Heliocharis palustris, Sparganium ramosum, Scirpus

I

844

MECHANICS OF GROWTH.

growing straight forward in the direction of their first origin, just as rootlets of a
high order grow downwards from the under side of their parent root, upwards from
the upper side, horizontally from the vertical sides, or continue to grow straight and
oblique according to the direction of the primary root. To this must be referred,
among other phenomena, the striking one described by me that plants which grow in
uniformly moist soil emit a large number of fine roots out of it with their apices pointing
upwards ; these are rootlets of the first or second order which spring from the upper
side of horizontal or oblique parent roots and grow straight upwards without being
geotropic. If the air is able to enter the ground freely, its surface is often dry, and
the fine roots which are directed upwards die off, as I have ascertained by growing
plants in glass vessels filled with earth.

Fig. 483.— Apparatus to illustrate the mode in which the geotropism of the roots h i k m of seedlings ^^^ is overcome
when they come into relation with a moist surface (hydrotropism).

But even geotropic organs may grow obliquely or horizontally when other
causes oppose or counterbalance their geotropism. One of the most common of
these causes is the bilateral organisation which makes an organ grow more strongly
on one side from internal causes. Since I shall recur to this subject in the next
section, only a single example need be given here. In the case of seedlings, rootlets
of the first order not unfrequently appear above the surface of the soil obliquely
when it is uniformly moist ; and I have convinced myself that this is the result in
cases which have been observed {e.g. Vicia Faha) of a stronger growth of their lower
side altogether independent of geotropism, in consequence of which they always

maritimns), which are not bilaterally organised, tend to maintain a horizontal direction of growth ;
they are therefore not geotropic]

ACTION OF GRAVITATION ON GROWTH IN LENGTH. 845

grow in a flat curve concave upwards. But external causes may also act in oppo-
sition to geotropism even when this is very strongly developed. Thus Knight and
Johnson have shown, as I have recently described more in detail, that primary roots
with strong positive geotropism, as well as secondary rootlets, when growing in
moderately damp air, deviate from their vertical or oblique direction when there is a
moist surface near them\ Under these circumstances a curvature concave to the
moist surface takes place at the region below the apex where there would otherwise
be a downward curvature, the apex being by this means conducted towards the moist
surface so that it may penetrate into the moister soil or grow in contact with it.
The apparatus represented in Fig. 483 is well adapted to exhibit this phenomenon.
It consists of a zinc frame a a covered below with wide-meshed network, thus form-
ing a sieve hanging obliquely and filled with moist sawdust//! The seeds ggg
germinate in the sawdust, their roots penetrating at first vertically downwards into it.
When the apex of a root escapes through the network into air, which is not too
dry, it turns towards the moister surface h-m, its geotropism being thus evidently
overcome.

The foregoing account is intended to give the reader a general idea of the various
debatable points which are especially to be remembered in the study of Geotropism
and to which frequent reference is made in the literature of the subject. Until recently
there were no complete observations or measurements of the growth which necessarily
accompanies geotropic curvature, or as to the true form of the curvature and its
relation to time and other conditions, which might give some clue as to the nature
of the internal changes which effect externally the upward or downward curvature.
I have endeavoured to supply these in the papers mentioned at the beginning of this
section. The observations were made upon organs of which the geotropism was well-
marked, such as erect growing stems, the nodes of grass-haulms, and downward-growing
tap-roots.

I. The upavard curvature of stems <which normally gro^ erect'^. My observations were
made for the most part on the thick, firm, long internodes of scapes which attain a con-
siderable height in a short time, the smooth surface of which can be marked with
Indian ink and allows of accurate measurement of the portions thus indicated. The
measurements of straight shoots as well as of the convex and concave sides of curved
ones were made by means of flexible measures of stout paper upon which the scale
was printed.

In order to be in a position to form an opinion as to the phenomena connected with
the upward curvature of stems or internodes placed horizontally, the distribution of
growth in these organs must first be understood. A general account of this was given
in Sect. 17. At first the whole internode, or a shoot consisting of several internodes,
is undergoing elongation. At a later period growth ceases at the base of the shoot, and
only a certain number of internodes lying below the terminal bud (this bud is not
taken into consideration here) constitute the region of the shoot which is growing and
which is capable of making a geotropic curvature. In the case of single internodes,
the region in which growth is to continue may lie near either to the base or to the
apex ; apical growth is the usual, basal growth the more uncommon case. It is remark-
able that similar internodes of closely allied plants behave differently in this respect :
thus in the scapes of Allium atropurpureum there is apical growth, whereas in that of

^ [This exhibition of sensitiveness to moisture has been termed ' Hydrotropism.' (See Darwin,
Movements of Plants, p. 180).]
* Sachs, Flora, 1873, No. 21.

40

„ (

in one internode

30

"

of the scape.

50

79 '

I

35

(3 internodes).

15

(4-5 internodes).

20

(numerous internodes),

25

(4 internodes).

40

(3-4 internodes).

846 MECHANICS OF GROWTH.

Jllium Porrum and of Allium Cepa the growth is basal. The position and the form
of the geotropic curvature is therefore different in the two cases.

The length of the growing region, when fully developed parts already exist, is
greatest at a certain time, after which it diminishes as the stem gradually approaches
its complete development, and finally disappears. The following measurements were
made whilst growth was still active and the growing region of considerable extent.
The length of the growing region behind the bud was in

Fritillaria imperialis 7-9 Ctm. \

Allium Porrum about

Allium Cepa

Allium atropurpureum

Cephalaria procera

Polygonum Sieholdi

Asparagus asper

Valeriana Phu

Dipsacus Fullonum

The measurement of portions of equal length (of one or of five ctm.) shows that
the growth of each such portion is greater the more distant the portion measured is
from the terminal bud, or, in the case of basal growth, from the base. At a certain
distance from the apex (or the base) the maximum occurs; beyond this distance the
growth of each portion diminishes until it altogether ceases at the limit of the growing
region. The form of the geotropic curvature and its modifications essentially depend
upon these conditions. In these particulars stems which consist of numerous internodes
but without well-defined nodes {Asparagus) resemble long single internodes, such as
the scapes of the various species of Allium. If however the stem is distinctly articulate,
a curve of fractional growth may be obtained from each internode, which rises as we
pass from its lower end until a maximum is reached, and then sinks as we pass towards
its upper end. The node itself ceases to grow in length at an early period. As the
result of this, the geotropic curvature of the whole stem is interrupted at the nodes,
and the quickly-growing central portions of the internodes describe sharper curves.
With the exception of this peculiarity, an articulate stem behaves generally in the
manner above described with reference to a long scape consisting of a single internode.
Finally, it is to be noted that each transverse zone of a growing stem grows at first
slowly, then more rapidly until a maximum rapidity is attained, and then more slowly
until growth ceases altogether. This also determines the form of the geotropic
curvature.

Those portions of a stem which have ceased to grow and which are incapable of
renewed growth in consequence of a change of position (see what is said below about
the nodes of the haulms of Grasses) will not assume an erect position when placed
horizontally or obliquely. Only those portions of a stem laid horizontally or obliquely
take part in the assumption of an erect position which are growing (as in the down-
ward curvature of the root), and this in proportion to the phase of their growth, their
thickness, rigidity, etc. This curvature is a consequence of a modification of the growth
in length of the stem produced by its abnormal position, of such a nature that the
growth of the under side is more rapid and that of the upper side less rapid than that
of the stem in the erect position. In quickly-growing parts the upper side evidently
increases in length when the geotropic curvature is taking place, but in older more
slowly growing parts the length of the upper side does not increase, and it may even
become a little shorter if the curvature is very sharp, whilst the lower side elongates
considerably. These statements can be easily verified by direct measurement of thick
firm stems before and after the curvature has taken place. It is obvious that the
convex side of a curved stem must be longer than the concave, but the question as
to whether or not the growth of the concave side is slower and that of the convex

ACTION OF GRAVITATION ON GROWTH IN LENGTH. 847

side more rapid than is the case when both these parts are in the normal erect
position, is answered in the affirmative not only by the direct measurement just men-
tioned, but also by some previous results which I obtained in another way, namely, hy
taking a number of similar shoots, splitting up some of them at once into strips of
tissue which were measured, and by splitting up and measuring the others in the same
way after continued growth, some in a vertical, some in a horizontal direction, finally
comparing these measurements with each other \ It became apparent in all cases
that of any two similar strips of tissue, the one belonging to the lower convex side had
grown more rapidly, the one belonging to the upper concave side less rapidly than
the corresponding strips of tissue in an erect shoot within the same time. As a con-
sequence of this, the difference in length between the cortex and the pith belonging
to the upper (concave) half of the upwardly curved shoot is increased, and that
between the cortex and pith of the lower (convex) half is diminished, so that the
upward curvature causes an increase of the tension between the tissues of the upper
half and a diminution of the tension between those of the lower half. This may be
illustrated by the following example. Twelve pieces of stem of Sida napcBa cut off
above and below, the leaves having been removed, each consisting of six or seven
internodes and 300 mm. in length, were taken : of these, four were at once split up
into strips of tissue, four were laid horizontally in damp sand in a box, and four were
placed nearly erect upon moist sand in a cylinder. The two following tables give
the mean measurements of the strips of tissue belonging to four pieces of stem : —

Lengths of the strips of tissue in Millimetres.

At the commencement.
(Growing erect.)

Concave cortex 298*0
„ pith 308-8

Convex pith 308-8
„ cortex 298-0

(Cu

After
rved upwards.)

310-5

337'5
342-9
328-2

20 hours.

(Standing obliquely.)

318-8

341*5
342-0
319-6

Differences of the lengths of Pith and Cortex.

Of the concave side

10*8

27-0

22-7

„ convex „

10-8

14-7

22-4

Increments of length in 20

Curved upw

Of the concave cortex 12-5

„ pith 28-7

„ convex pith 34'!

„ „ cortex 3o'2

lours.

ards.

Standing obliquely.
20-8

32-7

33-2

21-6

If a shoot which has lain for some time {}i—'2 hours) in a horizontal position and
has begun to show the first traces of an upward curvature be placed vertically, or be
moved so that the plane of curvature becomes horizontal, the commencing curvature
increases ; hence it appears that the action of gravitation has a persistent effect which
may continue as long as three hours, and may produce considerable curvature. In the
second of the two cases the curvature lies in a horizontal plane, and simultaneously
with its increase an elevation of the free apex occurs in consequence of the geotropism
induced by the new position. The persistent effect manifests itself even when the
shoot is strongly curved upwards.

My observations afford the following information as to the form of the curvature
of a shoot which is assuming the erect position under the action of gravity.

On experimental as well as on theoretical grounds it appears that the curvature
(with a few exceptions) is not and cannot be a segment of a circle: this is only the

» Sachs, Arb. d. bot. Inst. Wlirzburg, 1872, Heft II. p. 194.

848 MECHANICS OF GROWTH.

case where the curvature is greatest, that is where its radius is smallest; above and
below this the curvature is less, and therefore the radii are larger.

It appears also that from the commencement to the termination of the process the
form of the curvature is always altering, the maximum of curvature being attained by
parts which were at first not curved at all or only slightly so, and parts which were pre-
viously strongly curved becoming straight.

The following paragraphs serve to explain the foregoing. We assume, for the sake
of simplicity (excluding other possible cases), that the horizontally-placed shoot is
rooted, or that its base which has ceased to grow (and which can absorb water) is
fixed, whilst its apex can move freely. To make it more intelligible, let us consider
the whole growing region, the region, that is, which takes part in the upward curva-
ture, as divided into three parts, an apical, a middle, and a basal portion, which we
may assume to be of equal lengths.

Since the form of the curvature of the whole curved portion is determined by the
degree of curvature of each transverse zone, it is essential to know upon what
conditions the curvature of each zone depends. The following are the determining
conditions : —

1. The rate of growth.

2. The thickness.

3. The deviation from the vertical.

4. The time during which any zone lies in any given direction inclined to

the vertical.

5. The persistent eifect.

6. The rigidity and elasticity.

The curvature is greater, ceteris paribus, in any given short period of time, the more
rapid the rate of growth in length, and the more nearly the deviation from the vertical
approaches the horizontal; on the other hand, geotropism is slower the thicker the
curving region is. Further, the curvature increases, that is the radius of curvature
becomes smaller, the longer the time during which the curving region is inclined at
an angle to the vertical. Moreover each transverse zone tends, according to what
was said above, to curve more strongly than is due to its inclination to the vertical
and to the length of time during which it is in that position ; that is, each transverse
zone which has been exposed for a certain time to the action of geotropism under-
goes in consequence of its persistent effect a subsequent curvature, which is in excess
of that produced by the other conditions. Finally, as regards rigidity and elasticity,
it is clear that each transverse zone of a shoot lying horizontally must, by reason
of the flexibility of the shoot, tend to bend downwards, that is, in opposition to the
geotropic curvature, and this tendency will be greater the greater the weight which
the shoot has to bear at its growing end and the more distant the section is from
that end. It must be further borne in mind that the flexibility alters with age and
that it diminishes as the thickness increases.

If the growing region of a horizontally -placed internode or stem were of the same
thickness throughout, and if the rate of growth of all transverse zones were uniform
and the flexibility so slight that it might be neglected (as is the case in short, thick
stems), the curvature, at its first appearance, would have the form of a segment of a
circle of large radius. Of these conditions, however, one at least, viz. the uniform
rate of growth of all transverse zones, is never fulfilled, and since the region of most
rapid growth is also that of greatest curvature, it is impossible that the curvature should
be, even at the commencement, a segment of a circle.

Taking now the usual case, in which we have a shoot growing at its apex, of a conical
form, growth being more active near the apex than near the base, the curvature re-
sulting from a horizontal position will first be manifested by the apical portion, for its
growth is the most rapid, it is the thinnest portion, and it has the least weight to raise ;
at a later period a less sharp curvature of the middle portion will be observed, and

ACTION OF GRAVITATION ON GROWTH IN LENGTH, 849

still later a very gentle curvature of the basal portion of the growing region, for
the rate of growth diminishes from the apex towards the base whereas the thickness
increases, and the further from the apex any portion is the greater the weight which it
has to raise when curving. In consequence of the continued action of gravitation, or in
consequence of its persistent effect, the curvature rapidly increases, but more rapidly in
the apical than in the other portions.

As the result of this, the apical portion, and then the middle portion, becomes more and
more nearly erect, and the inclination to the vertical of the shoot is less the nearer these
portions are to the apex. For instance, a line drawn tangentially to the apical portion
will very nearly coincide with the vertical, whereas a tangent to the centre of the middle
portion will be inclined to it at an angle of about 45°, and a tangent to the centre of the
basal portion deviates not more than perhaps 5-10° from the horizontal. Consequently
the apical portion will not be affected any longer by the action of gravitation, whereas
the middle portion will continue to curve considerably, for its growth is still tolerably
rapid and it is in a position which is favourable for curvature : the basal portion grows
but slowly, but its position is very favourable for curvature. In consequence of the
continuing curvature of the middle and basal portions the now erect apical portion
becomes bent over out of the vertical, and this is increased by the persistent effect
of the action of gravitation. Thin very rapidly-growing stems acquire this form of cur-
vature in from 3 to 5 hours, thicker ones in from 1 2 to 15 hours, and very thick ones in
from 24 to 30 hours.

After this condition has been attained a remarkable change of the form of the
curvature begins. Whilst the apical portion which is erect or has curved even beyond
the vertical is straightening itself in consequence of the more rapid growth of its
concave side, the basal portion continues to curve slowly upward by reason of its still
nearly horizontal position. In consequence of this the middle portion is passively
elevated, in addition to its own active curvature, so that it comes to assume, like the
apical portion, a position which is unfavourable to its geotropism, and like it, it begins
to straighten itself (at least in its anterior part). Finally, the whole anterior part
(including the apical and middle portions) stands erect, whilst the mature portion, lying
behind the basal portion, is horizontal, the two being connected by the sharply-curved
basal portion of the growing region.

It appears, therefore, that the greatest curvature occurs first in the thin quickly-
growing apical portion, then in the thicker middle portion which grows less rapidly, and
finally in the still thicker slowly-growing basal portion.

If, on the other hand, we consider a scape of Allium Cepa or of Allium P or rum in which
the growth is basal, the first effect of being placed in a horizontal position is that the
greatest curvature is exhibited by that part of the basal region which is growing most
rapidly, the mature apical portion remaining straight and being passively elevated. The
curvature of the basal portion takes place but slowly, for it is very thick and it has to
support the overhanging weight of the anterior portion. In this case also the apical
portion may be elevated beyond the vertical, since the transverse sections of the basal
portion which lie behind the region of greatest curvature continue to curve slowly and
the position of the whole of the scape which lies in front of them is passively altered.

If a conical shoot with apical growth, the growth being more active toward the
apex, be placed in such a position that the apex is directed downwards in a direction
which deviates but little from the vertical, all the parts are at first in a position which is
very unfavourable for geotropism, since gravitation acts upon the shoot at a very acute
angle. The time which will elapse before the first appearance of curvature must there-
fore be greater than when the shoot is lying horizontally. It must be borne in mind
that, as the curvature proceeds, the parts which are affected by it come to occupy a more
favourable position for geotropism, for they approach the horizontal more and more
closely ; the action of gravitation will therefore increase as the curvature increases.
The apical portion comes, at length, to occupy a horizontal position ; it commences to

3 I

850 MECHANICS OF GROWTH.

elevate itself, and in consequence of the persistent effect of gravitation and of the
curvature of the middle and basal portions, it may pass beyond the vertical ; finally it
stands erect. The middle portion remains, sharply curved; the basal portion is but
slightly curved, for its growth ceases before it is possible for it, in consequence of its
unfavourable position, to undergo any great curvature.

Growth and Curvature without Absorption of Heater. If shoots consisting of a growing
part and of a part which has ceased to grow be cut off and placed erect (the apex being
uppermost) in a dry glass cylinder which is then closed in order to prevent excessive
evaporation, they continue to grow for a considerable time without any absorption of
water, and at the same time they lose a portion of their water by evaporation into the
closed space ^. It might be assumed that the water requisite for the elongation of the
growing portion was derived from the part which had ceased to grow. If, however, the
growing region alone be cut off and the terminal bud removed, and then marks be made
on the shoot, it becomes evident that all the segments of the shoot grow without absorb-
ing any water. The elongation is certainly less than usual, but it is distinct.

If shoots consisting of a part which is growing and of a part which has ceased to grow
be cut off and placed horizontally in a closed space and protected from excessive evapora-
tion, a curvature occurs in the growing region which may result in the elevation of the
apex into an erect position. In this case the water which is necessary for the more
rapid growth of the under side of the shoot might be absorbed from the posterior
fully-developed parts. If, however, only the growing region of the shoot be cut off, or
a single internode, the upward curvature will still take place, and, in this case, throughout
the whole piece. Accompanying this process we have (i) a loss of weight due to the
evaporation of water into the unsaturated atmosphere, (2) an elongation of the convex
lower surface, corresponding to the upward curvature, and (3) no elongation, or a very
slight one, but more generally a contraction, of the upper concave side.

Curvature of split shoots. If the growing region of a shoot is split into two symme-
trical halves which remain connected posteriorly by a portion of the shoot which has
completed its growth, they will curve concavely outwards in consequence of the
tension of their tissues. If, whilst thus curved, the epidermis of the two concave
surfaces and the two convex cut-surfaces of the pith be measured, and if then the
shoot be placed in such a way that the epidermis of one surface is directed downwards
and that of the other upwards, each half will exhibit geotropism. The growth of the
pith of the upper half will be accelerated, whereas that of the cortex of the same half
will be retarded or the cortex may even become shorter ; in the lower half, the growth
of the pith will be retarded and that of the cortex accelerated. The following were the
increments of growth in 24 hours observed in Sylphium connatum : —

epidermis (above) . . . - I'o m.m.

+ 107 „

TT 1 -i. J- 1 t- ir ( epidermis (above)

Upper longitudinal half \ r r --u /u i \
^^ ° ( surface of pith (below)

Lower longitudinal half P"':f''°fP'"?(='J"'™) " " + '■" "
( epidermis (below) . . . + 2-0 „

The same takes place, only in a more marked manner, in the case of the haulms of
Grasses, which are more adapted for observations of this kind, for, when they are split,
the two halves do not curve outwards.

If a longitudinal slice be taken from the middle of the stem of some Dicotyledon
which is not hollow but which has a thick pith {e. g. Senecio Doria), by paring away the
wood symmetrically on each side, it is possible to place it horizontally in two ways,
(a) that in which the cut surfaces are vertical, and {&) that in which the cut surfaces are

^ It is to be remembered that many shoots, such as those of Fritillaria imperialis, are much
disturbed in their growth if the apex be cut off; and they hardly grow at all if they are cut off at the
base. As a consequence the curvature of such shoots is very slight or entirely absent.

ACTION OF GRAVITATION ON GROWTH IN LENGTH. 851

horizontal. In the position (a) the vertical arrangement of the tissues will be as
follows : —

Cortex,

Pith,

Cortex :

in this position the slice will always curve upwards. In the position (6) the different
tissues lie side by side in a horizontal plane, thus.

Cortex, Pith, Cortex,

and nearly the whole of both the upper and under surfaces will be occupied by the
section of the pith. In this position it often happens that no geotropic curvature is
exhibited, but accurate observation is rendered difficult by the unfavourable conditions.

If a prism of pith be cut out of the growing region of a solid shoot, without any other
tissues being attached to it, and if this be placed for five or ten minutes in water so that
it becomes rigid and turgid, and be then laid horizontally in moist air or in water, one
end being fixed and the other free, no upward curvature takes place.

2. Upcward Curvature of Grass-haulms. In the case of the stems considered in the
preceding paragraphs, the whole region of growth, which is of considerable length, is
geotropic ; hence the curvature is gradual and therefore also of considerable length,
and every portion of the stem which has completed its growth in the erect position has
become incapable of curvature. In the haulms of Grasses, on the other hand, the
capacity for curvature is concentrated at the nodes, the long internodes remaining
straight. Hence, a haulm possessing several nodes, if laid horizontally, will exhibit after
a short time a number of sudden angular curvatures at the nodes, between which lie the
straight internodes. If the oldest internode be fixed in a horizontal position, the third
or fourth internode will have assumed an erect position in from one to three days. It is
on this that the upgrowth of * layered ' Wheat depends.

It is an especial peculiarity of the haulms of Grasses that they retain for a con-
siderable time the property of becoming erect, that is, that their under surfaces will
grow rapidly if they lie horizontally, after that they have ceased to grow while in the
normal vertical position. An abnormal position does not only aff^ect the growth of
the nodes of a haulm, but even causes it to recommence after it has already ceased ;
in this particular the motile organs of periodically motile leaves, e.g. those oi Phaseolus^
resemble the Grass- haulm.

These nodes which are thus capable of curvature are, as is well known, the basal
portions of the leaf-sheaths, which surround the base of the internode as a more or
less well-developed annular swelling of considerable thickness but of delicate succulent
structure. My observations were made upon Triticum, Dactylis, Glyceria spectabilisy
Andropogon niger and Zea Mais, in which plants the nodes are sufficiently large to
admit of tolerably accurate measurements. Portions of the haulms were cut off in
such a way that there was a node in the middle of each, connected with an internode
above and below ; the lower cut end was fixed laterally into moist sand in such
a way that the whole piece was horizontal : it was then put in a closed metal box,
the atmosphere of which was damp. The free internode was found to become
erect after two, three, or four days, according to the thickness of the node : sometimes
it became vertical, but more commonly oblique. It was easy to see that the lower
surface had el9ngated considerably. If the piece of haulm be now turned in such a
way that the convex surface of the node is uppermost, the concave side begins to
grow very vigorously; the node becomes straight, and appears uniformly elongated
on all sides.

The convex lower surface of a sharply bent node appears smooth, transparent
and glistening, whereas the concave upper surface is dark, opaque, and rough in
consequence of transverse folds formed by its epidermis and parenchyma. A deep
indentation may often be observed in addition, so that it appears as if the node had

3 I 2

No. I.

Upper surface

Lower „

No. 2.

Upper surface

Lower „

No. 3.

Upper surface

Lower „

852 MECHANICS OF GROWTH.

been artificially bent until it was dislocated. These changes are produced by a con-
siderable shortening of the upper surface which accompanies the very vigorous growth
of the lower surface. That this is the case is shown by the following measurements
made on the nodes of Maize, the thickness of which in the plane of curvature was
from 10-12 mm.

Length of the Node.

Before curvature. After curvature!,

4*3 mm. 2*5 mm.

4*1 » 9*o „

4'o 5> 3*o »

5-0 „ iro „

5*o )} 4'5 5,

5'o » 12-5 „

Median longitudinal sections through the curved nodes showed that the cells of
the epidermis and of the subjacent tissue of the underside had undergone a cor-
responding elongation, but that they had not undergone division, whilst those of the
upper surface had not grown and had become so compressed by the curvature that
the above-mentioned folds of the tissue had been produced.

3. The downward cur'vature of the tap-roots of seedlings'^ was studied especially in
a large-seeded variety of Vicia Faba, in Peas, Acorns, and Horse-Chestnuts. They
were placed horizontally either in moist air, or in water, or in damp earth. In the
last case, the seedlings were in a box having oblique walls of glass or of talc, which
permitted the observation of the roots during their growth and curvature.

The statement that it is only those portions which are still growing that are
capable of curvature holds good also for roots. I showed, in opposition to earlier
views, that it is not one part only of the growing region but the whole of it, as in
the case of stems, which exhibits geotropism (Fig. 484). Since the whole growing
region (as was shown in Section 17) is only from 8 to 10 mm. long, and in many roots
even shorter, and since the curvature can only be considerable in the middle zones,
the curvature appears, especially after a considerable time, sudden and sharp with a
very small radius, a condition which is of considerable mechanical advantage to the
penetration of the roots into firm soil. Since the considerations which were stated
above with reference to stems may be generally applied with propriety to the curvature
of roots, the form of the curvature appears to be in this case, as in the former, only
at first that of the segment of a circle of large radius; but this is merely apparent,
for, since the apex of the root is directed downwards in consequence of the curvature,
the younger transverse zones are brought into a position which is unfavourable to
geotropism, whilst the oldest soon cease to grow and can therefore curve no further.
It is the zones which are in the middle phase of growth which undergo the greatest
curvature, for these not only grow rapidly and for a considerable time, but they have
also this advantage, that they do not at once come to occupy, in consequence of their
curvature, a position which is unfavourable to geotropism, as is the case with the
youngest zones. A more detailed account of the conditions which determine the form
of the curvature of roots which are either horizontal, oblique, or erect, will be found
in my paper which is here quoted.

The measurement of the growth of the upper and under sides of roots is much
more difficult than in the case of stems and of the nodes of Grasses. I found that
the growth of the upper side was as vigorous, or even more so, as it was when the
root retained its normal position and form. The lower surface, however, is con-
siderably hindered in its growth, and. it appears from Ciesielski's statements, that

^ The curvature took place in six days.

2 Sachs, Arb. d. hot. Inst, Wurzburg, 1873, Heft III.

ACTION OF GRAVITATION ON GROWTH IN LENGTH,

853

folds are found upon the concave under surface in consequence of compression, just
as in the nodes of Grasses. When the root is curving geotropically, all the cells
within the curving portion usually grow, but their growth is slower the nearer they
are to the lower surface which is becoming concave. In passing from the convex
surface, where the cells are fully developed and contain much sap, to the concave surface,
where the cells present the appearance of young undeveloped cells containing much
protoplasm, all intermediate forms may be met with. Since, therefore, the development
of the cells of the under side is very considerably impeded, it is possible for those
of the upper side to undergo a more or less excessive elongation. Some observations,
as yet incomplete, seem to indicate that the retardation of
the growth in length of the under side is accompanied by a .

more vigorous growth of the cells in a radial direction, and
the acceleration of that of the upper surface by a less vigorous
radial growth.

If thick primary roots be split and be treated in the manner
described with reference to stems, the same phenomena
(though in exactly the opposite direction) are generally pro-
duced : this shows that geotropism is not merely a property
of the root as a whole, but also of each of its constituent
parts. These observations are, however, very difficult to carry
out. A persistent effect of the commencing geotropic action,
which was so well-marked in the case of stems, is stated
to occur in roots also by Ciesielski and Frank. I have not yet
succeeded in detecting it, but I will not reject the fact, for
other methods will perhaps afford more satisfactory results.

4. The chief result of the observations which I have made
hitherto is, I believe, this, that the phenomena of upward
geotropic curvature are essentially the same, though taking
place in the opposite direction, as those of downward geotropic
curvature, and that therefore the mechanical explanation of the
one will include that of the other. This necessarily implies
the incorrectness of the older explanations offered by Knight
and by Hofmeister.

Knight, the discoverer of the fact that it is gravitation
which induces geotropic curvature, believed the upward cur-
vature of the stem to be due to an accumulation of nutrient
materials towards the lower surface, which would induce
more vigorous growth. Hofmeister, who regarded the state
of tension of the tissues to be the most important factor in
producing the curvatures of parts of plants, considered that
the action of gravitation in producing an upward curvature
was to increase the extensibility of the passively stretched
tissues of the lower side. I pointed out that the growth of
the under surface of an organ capable of curving upwards
was accelerated, and that of the upper surface retarded : I

did not at the time express an opinion as to whether these modifications of growth
were due to an altered distribution of plastic material or to a change in the extensibility
of the passive layers of tissue.

Knight explained the downward curvature of primary roots in a somewhat obscure
manner by referring it to the softness and flexibility of the growing apex, a view
which was adopted by Hofmeister in a more precise and logically complete form,
and one which I for a time accepted. It was assumed that the tissue of the growing
root was comparable to soft dough, and that the unsupported end tended to curve
downwards under the influence of its own weight. I considered that the weight of

Fig. 484.— The growing and
curving end of the primary root of
Vtcia Faba placed horizontally in
loose earth behind a thin lamina
of talc. The root was marked out
into five portions, each two milli-
metres in length, beginning from
the punctiim vegetationis, A tri-
angular index of paper was fixed
to the talc so as to correspond to
the mark 0 on the root ; by means
of this the change of position of
the marks could be detected. A
at the commencement of the ex-
periment, B after one hour, C at
the end of two hours, D after seven
hours, E after twenty-three hours. .

854 MECHANICS OF GROWTH.

the free apex exercised a traction upon the growing cell-walls of the curving portion
of the upper surface, in consequence of which the growth, the intussusception of this
side, was accelerated, whilst exactly the opposite took place on the other side, and
I believe that Hofmeister held the same opinion. Frank did not make a new sug-
gestion when he pointed out that the downward curvature of the apex of the root
was a phenomenon of growth, that is, of the more rapid growth of the upper surface :
we held that view already. The point was to determine why the growth of the
upper surface of the apex of a root when placed in a horizontal position is more
vigorous than that of the under surface. Frank was right in asserting that the theory
of Knight and Hofmeister was untenable, for, as Johnson had already shown, the
apex of a root will curve downwards when its weight is counterbalanced and even
when the counterbalancing weight is greater, and also because when the apex of a
root is placed upon a firm horizontal support or upon the surface of mercury^ it
exhibits the same phenomena of growth which effect the downward curvature. The
explanation of Frank and the more recent ones of Miiller are inadequate to answer
the crucial questions.

Sect. 22. — Unequal Growth^. Our observations have hitherto had reference
almost exclusively to the growth of multilateral or polysymmetrical organs, such
as erect stems and descending roots. Organs of this kind offer the simplest example
of growth taking place equally on all sides. But they form only a small minority,
since not only a large number of primary stems like those of Hepaticae, Rhizocarpeae,
and Selaginelleae, but also by far the greater number of lateral branches of erect
stems, and all leaves, display a decidedly bilateral organisation, ?>. two sides of their
axis of growth exhibit different properties. With this bilateral organisation is also

^ See Fig. 477, and Arb. d, hot. Inst. Wiiizburg, Heft HI. p. 448 el seq.

^ A. B. Frank, Die natiirliche wagerechte Richtung von Pflanzentheilen (Leipzig, 1870). The
views propounded in Frank's treatise are opposed by H. de Vries in the second Heft of the Pro-
ceedings of the Wiirzburg Bot. Inst. 1871, p. 223 et seq. — See also Hofmeister, Allgemeine Mor-
phologie der Gewachse, Leipzig 1868, Sect. 23, 24. [It has been noticed by many observers that
the position of members of plants, such as leaves and the thallus of Liverworts, which have a dis-
tinctly bilateral structure (that is, which are dorsiventral), is at right angles to the direction of
incidence of the rays of light and to the action of gravity. Frank considers that this is due to
certain properties with which these members are endowed and by virtue of which their reaction to
these forces is different from that of polysymmetrical members, such as stems and roots, w^hich tend
to place themselves in the line of incidence of the rays of light and of the action of gravity. This
peculiar form of heliotropism and of geotropism he terms transverse. In the note on p. 843 it is
stated that Elfving has found instances of what appears to be transverse geotropism in certain
rhizomes, which do not however possess a bilateral structure. Frank's views are adopted by Darwin
(Movements of Plants) ; he uses the terms diaheliotropism and diageotropism.

De Vries concludes that the position of these dorsiventral members is the expression of the
resultant action of several forces, such as negative heliotropism, negative geotropism, hyponasty,
epinasty, and does not admit that these members react differently to polysymmetrical members when
exposed to the action of light or of gravity. Sachs (Ueb. orthotrope und plagiotrope Pflanzentheile,
Arb. d. bot. Inst, in Wiirzburg, II. 2, 1879) comes to much the same conclusions. He divides all
parts of plants into two classes, according to the position which they assume under the influence of
ordinary external conditions. He terms those parts orthotropic which assume a vertical position
and those parts plagiotropic which assume a position inclined to the vertical. All sides of the former
react similarly to light and to gravity, whereas in the latter one longitudinal half reacts differently
to the other longitudinal half. In the former the polysymmetrical structure is correlated with a
polysymmetrical organisation ; in the latter the bilateral structure is correlated with a bilateral
organisation. In some cases, however, members with polysymmetrical structure are bilaterally
organised, as is shown by the fact that they are plagiotropic]

UNEQUAL GROWTH, %^^

isually connected a difference in the growth of the two dissimilar sides, which causes
[curvatures and hence changes in the position of the apex. The two dissimilar sides
;0f bilateral organs must also be acted on differently by external agencies which affect
[growth, such as light, gravitation, and pressure. We do not attempt here to solve the
question of the causes which produce the bilateral structure in any particular case ;
[it need only be shown incidentally that this structure of lateral organs (as we have
already seen in Book I. Sect. 27) is probably always brought about by internal
causes, and is independent of the action of external circumstances. This is in
[general evident from the fact that the median plane of bilateral appendicular organs
[has always a perfectly definite geometrical relation to the axial structure which bears
them, and that moreover in the dark and under the influence of slow rotation round
[a horizontal axis, which eliminates the effect of gravitation, the bilateral structure and
relation to the axis remain unchanged.

But before we proceed to the consideration of the growth of bilateral organs, it
mst be premised that even in multilateral erect stems and vertically descending
roots growth does not always proceed equally and with equal rapidity on all sides of
le longitudinal axis ; it is much more common for first one side and then another
>f the organ to grow more rapidly than the rest, curvatures being thus caused the
convexity of which always indicates the side that is at the time growing most rapidly,
[f another side then grows more rapidly, it becomes convex, and the curvature
:hanges its direction. Curvatures of this kind caused by the unequal growth of
[different sides of an organ may be called Nutations^ and in so far as they are pro-
duced entirely by internal causes they may be said to be spontaneous. They occur
lost commonly and evidently when growth is very rapid, and consequently in
organs of considerable length, and are produced under the influence of a high
temperature either in darkness or when the amount of light is very small.

When two opposite sides of an organ grow alternately more and less rapidly,

^curvatures are caused first on one side and then on the other ; it will, for example,

)end first to the left, then become erect, and then bend to the right side ; as occurs,

,g. in the long flower-scapes oi Allium Porrum, which finally take an erect position

^hen their growth is ended. It is much more common for the apices of erect stems

Fabove the curved growing part to move round in a circle or ellipse, the region of

most active growth moving gradually, as it were, round the axis : it lies, for instance,

at one time towards the north, then towards the west, south, and east in succession

until it comes again to lie towards the north. This kind of nutation may be termed

a Revolving Nutation^. Since the apex of the stem is constantly rising higher

during the nutation owing to the elongation of the part below it, its revolving motion

does not take place in a plane, but describes an ascending spiral line. This form of

nutation occurs in many flower-stalks before the unfolding of the flowers, as in those

^ [Darwin is of opinion (Movements of Plants) that all nutation is revolving nutation, or, as he
terms it, circumnutation. He regards all the movements connected with growth (heliotropism,
geotropism, hyponasty, epinasty) as well as those of mature parts (spontaneous or induced move-
ments) as being modified forms of circumnutation.

The unequal growth is of course the expression of an unequal turgidity of different parts of the
growing organ (see de Vries, Ueb. die inneren Vorgange bei den Wachsthumskriimmungen mehr-
zelliger Organe, Bot. Zeitg. 1879.]

I

856 MECHANICS OF GROWTH.

of Brassica Napus, where the movement ceases when growth is completed, and the
stem finally becomes erect. It is very general in climbing stems and in almost
all erect stems that bear tendrils ; but bilateral tendrils also revolve at the time when
they are about to take hold of a support \

In bilateral appendicular organs nutation does not usually take the form of
a revolving motion, or only to a subordinate extent, as in tendrils. The outer or
dorsal side more often grows more rapidly so that the organ is curved concavely to
the primary axis, and the inner side afterwards begins to grow more quickly, so that
the organ finally becomes straight, or even concave on the dorsal side. This is the
case in all strongly developed foliage-leaves, very strikingly in those of Ferns, which
are at first rolled up towards the axis, and then unroll, often bending over back-
wards, becoming finally straight. The same phenomenon occurs in the tendrils of
CucurbitacecB, which are also at first rolled up inwards, then become straight, and
are finally rolled up outwards. Other tendrils are at first straight or only slightly

Fig. 485.— Nutation of the filaments oi Dictamntis Fraxinella; the filaments of the stamens whose anthers have not
yet opened are bent downwards ; those with anthers already burst are bent upwards.

concave inwards, like leaves in vernation, but are afterwards rolled backwards. Move-
ments of nutation are very common and^ easily observed in stamens with long fila-
ments, as TropcEolum majus, Diciamnus Fraxinella (Fig. 485), Parnassia palustris ^j
&c., and in long styles like those of Nigella saliva^ &c. They occur at the time of
the maturity of the sexual organs, and serve to place the stigmas and anthers in
the positions adapted for the conveyance of pollen by insects from one flower
to another ^ Most lateral shoots behave in the same manner as ordinary leaves,
growing at first only quickly enough on the outer side to become appressed to the
primary axis in vernation, afterwards more rapidly on the inner side, by which they
become straight and diverge at a greater or smaller angle from the primary shoot.

^ See Sect. 25, On the Twining of Tendrils.

^ [On the stamens of Parnassia, where there is not properly any movement of nutation, see Gris
Comp. rend. Nov. 2, 1868 ; and A. W. Bennett, Journ. Linn. Soc. vol. XI. p. 24, 1869.]
^ Vide infra under Fertilisation, Chap. VI.

UNEQUAL GROWTH. 857

These movements of nutation of bilateral appendicular organs take place mostly
in one plane which coincides with the median plane of the organ. As long as the
organ grows most rapidly on the dorsal side, it may be termed, after de Vries,
hyponasHc ; afterwards, when it grows most rapidly on the inner or upper side,
epinastic. Since in the later stages of development of an organ growth ceases at
certain places — while at different distances from these places it presents different
stages of growth, until it finally ceases everywhere — it is clear that in the same organ,
together with areas where growth is completed and nutation no longer takes place,
lothers occur with hyponastic and others again with epinastic growth, until at length
lutation and growth alike cease altogether, as in Fern-leaves.

Seedlings of Dicotyledons afford a remarkable illustration of bilateral structures
;hich nutate in one plane ; although their stem and primary root become afterwards
multilateral and grow vertically upwards and downwards. The stem terminates in
fft pendent or nodding bud ; and the curvature, which is generally very great, exhibits
(itself also in germination when it takes place out of the ground in a vessel that
rotates slowly round a horizontal axis ; it is a true curvature of nutation inde-
pendent of light and gravitation. But the older portions of the stem become straight
as they develope from the curved portion ; and in proportion as the stem increases
in length, the straight part which bears the nodding bud also lengthens. When
germination takes place in a feeble light, or better in a slowly rotating vessel, a
more rapid growth occurs of the side of the older portion of the stem which was
at first concave, causing it to become convex; and hence the older and younger
parts of the stem form together a letter S, as in Phaseolus, Vicia Faba, Polygonum
\Fagopyrum, Cruciferae, &c. But the primary roots of dicotyledonous seedlings also
manifest a tendency to a bilateral organisation ; since, when they develope under slow
rotation round a horizontal axis, they seldom continue to grow straight, but curve
Concavely either in front or behind, sometimes even becoming rolled up. These and
other instances of nutation are not clearly seen when the development takes place
under normal conditions, because the growth of the stem of the seedling is retarded
by light, and the curvature both of stem and root prevented by geotropism.

A knowledge of the different capacity for growth possessed by the anterior and
posterior sides of bilateral organs lies at the root of an understanding of the fact that
leaves, lateral shoots, and many secondary roots, although they are heliotropic and
geotropic, yet assume definite positions with respect to the horizon, but without
growing vertically upwards or downwards. When multilateral (orthotropic) primary
stems and roots grow vertically, the essential cause is their growth being uniform on
all sides of the axis of growth ; the different sides of the organ are in equilibrium
with one another. Every deviation from the vertical position, to the right, left, front,
or back, is counterbalanced by geotropism ; the growing part curves until the free
apex stands erect, in which position the action of gravitation is again equal on all
sides. In the same manner light acts equally strongly on all sides of such organs.
If therefore one side is exposed to stronger light, a heliotropic curvature takes place
which finally brings the free part into a position in which all sides receive equally
Strong light on all sides, and therefore grow uniformly without any further curvature.
The case is different with bilateral organs the anterior and posterior sides of which
possess independently different capacities for growth (plagiotropic), and which there-

858 MECHANICS OF GROWTH.

fore exhibit a tendency for the more rapidly growing side to become convex. If the
growth is very strongly hyponastic or epinastic, the curvature thus caused may take
place in spile of the opposing action of light and gravitation, supposing the organs
to be actually heliotropic or geotropic. Organs which grow horizontally or obliquely
to the horizon must not be assumed to be on that account wanting in heliotropism
or geotropism ; still less is it necessary to suppose in these cases any special or
altogether abnormal relations to light and gravitation ^ It is sufficient, as de Vries
has clearly shown, to suppose that light and gravitation act in the ordinary way on
the growth of bilateral organs, in order to explain their directions of growth, if only
it is borne in mind that their heliotropism and geotropism cooperate with their
hyponastic and epinastic properties, and thus bring about positions of the organs
which must be considered as the resultants of these different forces. The weight of
the overhanging part must however also be taken into account, its tendency being
always to change the lateral direction of the organ into a more horizontal or even
pendulous one; and this must occur more decidedly the less the elasticity of the
organ. When large leaves assume oblique or horizontal positions, it is because
their epinasty tends to make them concave downwards as they unfold, while their
positive heliotropism tends to make them concave upwards. The result is con-
sequently a more or less flat expansion of the leaf, the position of which depends
on the relation of the weight of the lamina to the flexibility of the petiole and
mid-rib. The same phenomena are observable in horizontal or oblique lateral
shoots, in which however the hyponasty of the axis often counterbalances the
greater mass of the pendent parts (as in Prunus Avium, Uhnus campesln's, Corylus
Avellanay Picea nigra, &c.). As soon as the position resulting from these forces is
attained, it becomes permanent, from the mature parts becoming lignified, rigid, and
hard, and thus in a condition to maintain the weight of the pendent parts.

If leaves which are unfolding or still growing have their under side turned
upwards or towards the light, very strong curvatures take place, generally combined
with torsions, by which the lamina finally resumes more or less completely its
normal position; and the impression is given as if the under side were more
sensitive to the influence of light, and the upper side to that of gravitation than
the reverse. But this hypothesis is superfluous if it is borne in mind that in this
case epinasty works concurrently with heliotropism and geotropism, and hence
much stronger curvatures must take place than in the normal position where the
former acts in opposition to the two latter forces.

The results here described are derived from the experiments of de Vries, which have
been already quoted. For the following I am also indebted to him.

(a) Leaves. If a strongly developed mid-rib is separated from a leaf in active growth,
it curls up concavely on the under side, showing that a tension exists between it and
the lamina. De Vries found this to be the case in nearly two hundred species, with
only a few exceptions. This curvature docs not take place equally strongly at all ages ;
in leaves which have but just emerged from the bud it does not occur at all; it
increases with age, and attains its maximum when the leaf is nearly fully grown, then
decreases, and altogether disappears when the leaf has reached full maturity. This
tendency to curve is at first apparent along the whole length of the mid-rib ; it dis-
appears first of all at the base, the part capable of curvature becoming constantly

* [See note 2 on p. 854 with reference to diaheliotropism and diageotropism.]

TORSION, 859

smaller and nearer to the apex. If mid-ribs of leaves are separated in this last
stage of growth and fixed upright in a damp and dark place (e.g. in wet sand in a
spacious closed zinc box), they will continue to grow for some time ; and since growth is
more vigorous on the inner (anterior or upper) side, they will curve concavely on the
posterior (or under) side, the curvature being however partially counteracted by geo-
tropism. If separated mid-ribs of leaves are fixed horizontally in wet sand, so that the
median plane lies horizontal, the epinastic curvature will take place without hindrance
in a horizontal direction ; but a geotropic curvature will at the same time ensue in a
vertical plane, so that the two kinds combine to produce an obliquely ascending position.
If, on the other hand, two similar mid-ribs are separated and fixed horizontally in wet
sand, with the posterior side in one case below, in the other case above, geotropism will
act in the former in opposition to epinasty, while in the latter the two will cooperate ;
and the consequence will be that in the former case the epinastic curvature will be more
or less neutralised, while in the latter a strong curvature will take place upwards, the two
forces acting in unison.

Phenomena of the same kind are produced by a combination of epinasty with helio-
tropism, if the separated mid-rib is fixed vertically in wet sand in a closed vessel into
which light is admitted from one side through a glass plate. Heliotropism is generally
but not always exhibited, and is then always positive ; but in all the cases hitherto ob-
served is too weak to overcome epinasty. It will be seen from what we have said that
all these movements of the mid-rib will be much less considerable when it is still in
connexion with the lamina. Petioles show in general the same phenomena as mid-
ribs, but their motions which result from heliotropism, geotropism, and epinasty are
unimpeded.

(b) Bilateral secondary shoots, such as branches of an inflorescence, horizontal or erect
leafy branches, and stolons, were experimented on in a similar manner. It was thus proved
that the branches of the inflorescence of Isatis tinctoria, Archangelica officinalis, Crambe
cordifolia, and all others that have been observed, the horizontal branches of Pyrus Malus^
Asperugo procumbens, &c., as well as the runners of Fragaria, Potentilla reptans, Ajuga
reptans, &c., are epinastic. When fixed horizontally in wet sand, they all curl upwards,
whether the side that normally faces downwards (the posterior side) be placed below
or above, but in the latter case more strongly, because geotropism and epinasty then
cooperate. In some species (as Tilia and Philadelphus) a branch, when stripped of
leaves and placed in its normal position, did not curl upwards, while one placed in a
reverse position did so, proving that there was in these cases an equilibrium between
geotropism and epinasty. The horizontal branches of Prunus A-vium, Ulmus campestrisy
Corylus Auellana, and some other plants were found on the other hand to be hyponastic ;
when laid horizontally in their natural position they curved upwards, but downwards
if reversed, because their hyponasty was stronger than their geotropism.

Similar experiments to those made on petioles with respect to heliotropism
showed in many cases the absence of this phenomenon, especially in the case of stolons ;
and that in other cases it was always positive, but too feeble to overcome the influence
of their epinasty. In the case of branches, especially such as are long and slender,
more account must be taken of weight in modifying the direction of growth than in
that of leaves. The removal of the leaves {e.g. in Corylus) is in this case followed by a
sudden curving upward, the result of elasticity ; but this is subsequently intensified by
geotropism and in many cases (as in Abies) also by hyponasty.

It may be left to the ingenuity of the student to ascertain the conditions determining
the direction of an organ in any particular case, from the points of view stated above.

Sect. 23. — Torsion ^ Organs of any considerable length very commonly ex-

* H. de Vries in the second Heft of the Proceedings of the Wurzburg Botanic Institute, 1871,
p. 272. — Wichura in Flora, 1852, No. 3, and Jahrbuch fUr wissensch. Bot. vol. II, i860. — Braun in
Bot, Zeitg. 1870, p. 158.

86o MECHANICS OF GROWTH.

hibit torsions about their axis of growth ; the striations on the surface of the organ
are not parallel to its axis of growth, but run round it in the form of more or less
oblique spiral lines, as if the organ were fastened at one end, and then twisted at the
other. Torsions of this kind occur in the unicellular internodes of Nitella ; they are
common in the elongated multicellular internodes of the erect stems of Dicoty-
ledons, universal in climbing internodes ; the setae of Mosses are generally very
strongly twisted. Even in flat leaves, as Wichura has shown, torsions of the lamina
occur very commonly; they behave like strips of paper fastened at one end and
twisted by the other round their median line. These torsions are particularly con-
spicuous in the leaves of many Grasses, o^ Allium ursinum^ species oi Alstroemerta,
&c., causing the under side of the lamina to lie uppermost towards the apex^.

Since the striae on a twisted organ run spirally round the axis, they must exceed
the axis in length ; if therefore the torsion is the result of growth, the growth of the
outer layers of cylindrical, conical, or prismatic organs (internodes, roots, &c.) must
be more rapid or must last longer than that of the inner layers ; and in twisted
leaves there must be the same difference as respects the growth of the mid-rib in
comparison to that of the margins. The fact that at the time of most rapid growth
the inner layers generally grow more rapidly than the outer ones (Sect. 13), thus
preventing the possibility of torsion, the additional fact that torsion does not gener-
ally take place until growth is ceasing, and lastly, the circumstance that etiolated
internodes, which in a normal state do not exhibit torsion, usually manifest this
phenomenon at the close of their growth, lead to the conclusion that torsion is the
result of growth continuing in the outer layers after it has ceased or begun to cease
in the inner layers. In twisted leaves, especially those oi Ahtroemeria, the torsion
however begins earlier. If the growth of the outer layers, besides being greater,
were also exactly parallel to the axis, and if the resistance to the strain thus caused
of the outer against the inner layers were exactly in the direction of the axis, there
would be no torsion, but only a longitudinal tension between them, which would
be directly opposed to the tension of the layers already described. It is however
evident that this would be possible only if all the parts were arranged with mathe-
matical precision ; but that any irregularity, however small, must give a lateral direc-
tion to the strain in the outer layers, and thus cause a torsion 2.

Torsions are also very often the result of an increase in diameter or are made
more evident as the formation of wood advances, as is often seen in the bark of old
stems of Dicotyledons and Conifers, and more clearly in the oblique course of the
fibro-vascular bundles. It may be concluded with probability that the phenomenon
is the result of the small but powerful increase in length of the young wood-cells ;
if these did not increase at all in length no torsion would take place.

^ [Similar torsions occur in petals as Cyclamen, fruits as Ailanthus malaharica, and not unfre-
quently in pedicels or inferior ovaries as Orchidese, causing the anterior part of the flower to become
apparently posterior, and vice versa.l

^ This can easily be made clear to the student in the following way. If an india-rubber tube
is strongly stretched, and another tube only a little wider is drawn over it, and the first is then
released, it contracts and is then too short for the outer tube. If the two tubes were perfectly
uniform in structure in the longitudinal and transverse directions, the only result would be
a longitudinal tension ; but torsion takes place also because a transverse is combined with the
longitudinal tension.

TORSION. 86 1

The examples of torsion we have been considering so far are produced solely
Jby internal causes; the direction in which the striae run round the axis is usually
;onstant in the same species ; but other instances of torsion frequently occur which
jsult from external and accidental circumstances. It is evident that when any
weight is attached to the side of an organ growing in a horizontal or oblique
irection, such as an internode, leaf, or tendril, the tendency will be to produce a
twisting of the organ round its axis. If the organ which is twisted in this manner is
^ery elastic, the torsion will disappear when the weight is removed ; but if it is only
?ery imperfectly elastic, the torsion will remain permanently, as in a twisted thread
►f wax ; and this will be the case if the organ is in a growing state. This does
in fact occur in growing internodes, petioles, the mid-ribs of leaves, &c. If an
organ of this kind is fixed horizontally in wet sand, after a pin slightly weighted on
one side, as by a drop of sealing-wax, has been passed horizontally through its
summit, the small twisting force is sufficient, as de Vries has shown, to cause a
permanent torsion in the growing part. The same result will of course ensue if a
leaf or branch instead of a pin is attached to the side of the organ. Branches which
grow horizontally and bear decussate pairs of leaves usually exhibit alternate tor-
sions of their internodes to the right and left, so that the leaves all stand in two
rows along the branch instead of in four. De Vries has shown that this is occasioned
by the unequal twisting force of the leaves of each pair. If the young leaves
are cut away no torsion results ; if only one of each pair is removed, the torsion is
determined by the weight of the remaining leaf.

Torsions of this kind also occur frequently when leafy shoots rise in consequence
of geotropism from a horizontal position, and are caused by the unequal distribution
of the weight of the leaves, and by their various geotropic and heliotropic curvatures
twisting the stem as it becomes erect. Very clear instances are furnished by long
petioles as those of Cucurbita, when the branch from which they spring is fixed in
a reverse position. The effect of geotropism alone or combined with heliotropism
would be simply to cause the petiole to curl upwards in a vertical plane ; but the
weight of the lamina is scarcely ever equally distributed on the two sides of the plane
of curvature ; one side is more heavily weighted, and causes the plane of curvature
of the petiole to bend obliquely to that side, and other parts of the petiole to be
.thus exposed to the influence of gravitation and heliotropism. Complicated curva-
tures and torsions of the petiole and of the lamina itself are caused in this way, the
final result being again to reverse the lamina, so as to bring its proper upper side
uppermost and expose it to the light as much as possible.

It will be seen therefore that a distinction must be drawn between two kinds of
torsion; firstly, that of erect organs; and secondly, that of organs which grow in a hori-
zontal or oblique position. In the former case the torsion results from internal con-
ditions of growth, and especially from the outer layers growing more rapidly than the
inner ones ; the arrangement of the internal parts — in the internodes of higher plants
probably the course of the fibro-vascular bundles — determines the direction of the torsion.

Torsions of the second kind are caused in quite a different way. The outer layers

of the growing organ are in a state of passive tension, and there is no internal tendency

. to torsion ; but the weight of the parts attached to it causes a torsion of the growing

organ, which is rendered permanent by growth and by the very imperfect elasticity

of the organ.

S62 MECHANICS OF GROWTH.

Sect. 24.— The Twining of Climbing Plants^ The stems of climbing plants,
composed of long internodes, have the power of twining spirally round upright
slender supports ; and the long petioles of the Fern Lygodium possess the same
property. This twining is a consequence of unequal growth, of a revolving nutation.
It is not caused, as Mohl held, by an irritation exercised by the support on the
growing internodes, and is therefore essentially distinct from the twining of tendrils
round supports, which depends on the irritation caused by constant and permanent
pressure'^.

Only a few plants twine to the right {i. e. from right to left as one looks at the
support round which the plant twines), following the course of the sun or of the
hands of a watch ; among these are the Hop, Tamus elephantipes^ Polygonum scandens^
and the Honeysuckle ; the greater number twine to the left, as Aristolochia Sipho^
Thunbergi'a/ragrans, Jasminium gracile, Convolvulus Sepi'um, Ipomcea purpurea, Asck'
pias carnosa, Menispermum canadense, Phaseolus, &c.

The first internodes of twining stems, whether they are primary stems as in
Phaseolus, lateral shoots from rhizomes as in Convolvulus, or from aerial organs as
in Aristolochia, do not twine but grow erect without any support. The succeeding
internodes of the same shoot twine ; they first of all elongate considerably, while
their leaves grow only slowly. The long young internodes incline to one side in
consequence of their weight, and in this position revolving nutation begins; the
overhanging part curves and executes a movement which causes the terminal bud to
describe a circle or ellipse. This circular motion is caused entirely by the curving
of nutation. If a black line is painted along the convex side of an internode of a
plant that twines to the right, like the Hop while the bud is pointing to the south, then,
when the bud points to the north it will be found on the concave side ; when to the
west or east, on the lateral surface between the convex and concave sides. Usually
two or three of the younger internodes are in a state of revolving nutation at the
same time ; and, since they are in different stages of growth, the curvature of the
older internode does not generally coincide with that of the younger one ; the whole
does not therefore form a simple arc, but often an elongated letter S, with the
different parts lying in different planes. As new internodes develope from the bud,
they begin to revolve, while the third or fourth internode ceases to do so, becomes
erect, and manifests another form of movement, becoming twisted, until its growth
ceases ^

^ [L. Palm, Ueber das Winden der Pflanzen : Preisschrift, Stuttgart 1827. — Mohl, Ueber den
Bau und das Winden der Ranken und Schlingpflanzen, Tubingen 1827. — Dutrochet, Comptes
rendus, 1844, vol. XIX, and Ann. des Sci. Nat. 3rd ser. vol. II. — Darwin, On the Movements and
Habits of Climbing Plants, London 1875).

^ Darwin has already attempted to show that Mohl's view of the irritability of climbing inter-
nodes is untenable, without however bringing forward any convincing proof. But this proof has
been afforded by H. de Vries in a series of investigations carried on in the Wiirzburg laboratory,
published in the third part of vol. I of the Proceedings of the Wiirzburg Bot. Inst. (1873). The
description here given of the mechanical principles is based principally on his results.

[See also Darwin, Movements and Habits of Climbing Plants, 1875, and Movements of
Plants, 1880.]

3 Torsion is therefore not the cause of the revolution of the apex of the shoot, as is seen at once
from the fact that the number of revolutions of torsion in the same time is different from that of the
revolutions of nutation.

TWINING OF CLIMBING PLANTS. 86^

The direction of revolving nutation and of torsion is, in all climbing plants, the

ime as that in which they twine round their support^ If a point in the terminal
region exhibiting nutation is prevented from moving by some external cause, as by
)eing fixed, the revolving movement of the free part will continue for some time,

)ut the free part will then grow in a spiral ascending in the direction of nutation.

'he revolving movement of nutation thus combines with the induced torsion of the
lower parts which are already coiled spirally ; but this torsion is opposed in its direc-
tion to the revolving nutation, and therefore also to the torsion previously mentioned

rhich exists in the lower portion of the free part. This latter torsion is probably

:casioned by the weight of the free overhanging apex of the shoot; at all events
causes the concave side of the part in a state of revolving nutation to face from

lat time the axis of the spiral which has been formed.

The" most common case in which revolving nutation is hindered in this way is
when the apex of a shoot comes, in consequence of this motion, into contact with
m erect support. If the support is not too thick, it forms the axis of the spiral
curvatures which the climbing stem makes round it ; when the support is very
slender, the stem winds in such large coils that they do not touch the support at all,
)r only accidentally at a few places.

But revolving nutation can also be artificially interfered with in various other

mys ; as, for example, by placing a support on the posterior side of the shoot as
respects its revolution, and fastening it by means of gum to the apex of the shoot,

;hich would otherwise become detached from it. The first spiral coil is in this case
formed in precisely the same manner as if the support were in its normal position,
)ut the support stands outside the coil which does not therefore embrace it. Spiral

)ils of this kind, not embracing any support, are frequently produced when the
stem rises above its support.

The youngest coils of a twining stem are not usually in contact with its support ;
they are wide and flat ; while the older coils are in close contact with it, and are
narrower and more oblique. This shows that the close clinging of climbing stems
to their support is a subsequent result, the coils being at first looser and wider, and
becoming afterwards narrower and more oblique. This fact, which is of great im-
portance in the interpretation of the phenomena of climbing plants, was placed
beyond doubt by de Vries, who caused the summits of climbing plants to coil in
this manner without having any support in the middle. In this case also the coils
were at first wider and flatter, and became narrower and more oblique with increasing
age, until at length the piece became quite erect, a revolution of torsion being all
that represented each spiral revolution. It is not improbable that geotropism is the
cause of the coils — at first flat and sometimes almost horizontal — becoming after-
wards more oblique. It is clear that the stronger the force with which the coils
become narrower and more oblique, the more closely must they cling to their
support. If there is a support in the axis of the coils, the younger parts of the
summit will be constantly prevented by it from performing their normal revolution
of nutation, and the apex will therefore continue to grow in a spiral, and will climb
continually further up the support, the older coils always becoming more oblique

What follows is from de Vries.

864 MECHANICS OF GROWTH.

and clinging to the support. If the support is removed soon after a few loose coils
have been formed round it, the shoot will retain its spiral form for a time, but will
then straighten itself and recommence the revolution at its apex.

A revolution of torsion of the twining internodes must, on purely mechanical
grounds, accompany every revolution of twining; but torsions of the parts which
have already coiled also occur, especially with round rough irregular supports ; their
direction is sometimes to the right, sometimes to the left.

During the course of the twining the leaves must sometimes stand on the out-
side, sometimes on the inside of the coils ^ ; in the latter case the leaf-stalk will be
pressed against the support on which it slips laterally under the pressure of the
contracting coil, dragging the internode sideways with it, and thus causing a local
torsion.

What has now been said includes almost all that we at present know on the
mechanism of the twining of climbing stems. A few remarks, borrowed from Darwin,
may be added.

The revolution of the free overhanging apex is often strikingly uniform in the same
plant under the same external conditions (as e. g. in the Hop, Micania, Phaseolus, &c,).

The following table of Darwin's gives some idea of the time required, under favour-
able conditions, for a revolution : —

Scyphanthus elegans

I hour 17 min,

Akebia quinata

I » 30 „

Con'vol'vulus sepium

I M 42 „

Phaseolus 'vulgaris

I » 55 »

Adhatoda [cydoncefolia)

48 „

The direction of the twining is usually constant in the same species ; but it does
sometimes happen, as in Solanum Dulcamara and Loasa aurantiaca, that different indi-
viduals twine in opposite directions. Darwin found, in these two species and in Scyphan-
thus elegans and Hibbertia dentata, that the same stem will sometimes twine first in one
and then in the other direction.

The positive heliotropism of twining internodes is generally feeble ; a powerful
heliotropism would obviously be only a hindrance to the twining and especially to the
revolution, by which an effort, so to speak, is made to reach the support. Heliotropism
is however shown by the fact that when the light falls from one side only, revolution
takes place more quickly towards the source of light than away from it ; as e.g. in
Ipomcea jucunda, Lonicera brachypoda, Phaseolus, and Humulus.

It may be concluded from what has been said on the mechanism of twining that
there is for every species a certain maximum of thickness of the support at which the
twining is possible. The support must not be much thicker than the diameter of the
coils which the shoot can make without a support ; if the support is too thick, the apex
of the shoot attempts to make coils by its side, and these eventually become effaced.
Darwin (/. c. p. 22) acknowledges his ignorance of the cause why the climbing plant
cannot twine round supports which are too thick ; de Vries's experiments however seem
to give a sufficient explanation.

The movements of twining internodes are more energetic the more favourable the
external conditions of growth, and the more rapid the growth itself; they are therefore
vigorous when food is abundant, temperature high, and the plants contain abundance of

' I may take this opportunity of remarking that, according to Dutrochet, the genetic spiral of
the phyllotaxis takes the same direction in climbing plants which have their leaves arranged spirally
as the twining ; and therefore also the same as the spontaneous torsion and the revolving nutation
of the same plants.

TWINING OF TENDRILS.

865

sap. The direct action of light is not necessary for twining, since even etiolated plants
(as Ipomcea purpurea and Phaseolus multiflorus) cling closely to their support in the dark.
The assertion of Duchartre that Dioscorea Batatas does not twine in the dark reduces
itself, according to de Vries's more recent observations, to the fact that while normal
green shoots continue to climb in the dark, they cease rotating and twining when they
become etiolated.

^^comprised all filiform or at least slender long and narrow parts of plants which
possess the property of curving round slender solid supports with which they come
in contact during their growth, clinging to them in consequence, and thus at length
fixing the plant to them. Tendrils are therefore at once distinguished from climbing
internodes by their irritability to contact or pressure.

Organs of the most various morphological description may assume this physi-
ological property. Sometimes tendrils are metamorphosed branches, as in Vih'Sf
Ampelopsi's, Passiflora, and Cardiospermum Halicacabum, where they may be con-

»_ sidered more accurately as metamorphosed flower-stalks or inflorescences. In

Fig. 486.— Mode of climbing of Tropceolum minus. The long petiole a of the leaf / is sensitive to
long-continued contact, and has clung round a support and round the stem of the plant itself j/ so as to
fix this stem firmly to the support ; z the shoot from the axil of the leaf.

^MCuscuia the whole stem may be regarded as a tendril rather than as a climbing
stem. In other cases, as in Clematis^ Tropceolum (Fig. 486), Maurandia, Lopho-
spermum, Solatium jasminoides, &c., the petioles may serve as tendrils. In Fu-
maria officinalis and Corydalis claviculata the whole of the finely-divided leaf is
sensitive to contact, and its separate parts have the power of twining round slender
bodies. In Gloriosa Planiii and Flagellaria indica the mid-rib protruding beyond
the leaf serves as a tendril. In many Bignoniacese, in Cobcea scandens, in Pisum,
&c. the anterior (upper) part of the pinnate leaf is transformed into slender filiform
tendrils inclined forwards, while the basal part of the leaf is rigid and divided
into leaflets ; sometimes, as in Lathyrus Aphaca^ the whole of the leaf is replaced
by a fiUform tendril. The morphological character of the tendrils of Cucurbi-
taceae is still doubtful, though they must probably be regarded as metamorphosed
branches.

' ^ See the literature quoted in the preceding section, and de Vries, Arb. d. hot. Inst, in
Wiirzburg, I.

3 K

^66 MECHANICS OF GROWTH.

The distinguishing properties of tendrils are more perfectly developed the more
exclusively they serve as organs of attachment for the sole purpose of climbing, the
less therefore they partake of the normal character of leaves or parts of the stem ; in
other words, the more perfectly the metamorphosis is carried out. To this category
belong especially the simple or branched filiform tendrils of the Cucurbitaceae,
Ampelidege, and Passifloreae. A typically developed tendril of this kind is repre-
sented in the mature state in Fig. 487, after it has seized hold of a support by its
apex and then coiled up. What is said here refers especially to true tendrils of
this description.

The characteristic properties of tendrils are developed when they have com-
pletely emerged from the bud-condition, and have attained about three-fourths of
their ultimate size. In this state they are stretched straight; the apex of the shoot
which bears them usually revolves, the tendril itself exhibiting the same phenomenon,
curving along its whole length (with the exception usually of the oblique basal
portion and the hooked apex) in such a manner that the upper side, the right side,
the under -side, and the left side become in turn convex. No torsion takes place.
During this revolution the tendril is rapidly growing in length and is sensitive to
contact; z.e. any contact of greater or less intensity on the sensitive side causes
a concave curvature first of all at the point of contact, from which the curvature
extends upwards and downwards. ' If the contact is only temporary, the tendril
again straightens itself. The degree of sensitiveness ^ is very different in different
species ; in Passiflora gracilis a pressure of i milligram is sufficient to cause curva-
ture in a very short time (25 sec.) ; in other species a pressure of 3 or 4 milligrams
is required and the curvature does not take place so soon (30 sec. in Sicyos)\ the
tendrils of other species curve, when slightly rubbed, in a few minutes ; in the case
of Dicenira thalictrifolia in half an hour ; in Smilax only after more than an hour ;
in Ampelopsis still more slowly.

The curvature on the side which has been touched increases for some time,
then remains stationary, and finally (often after some hours) the tendril again
straightens itself, in which state it is once more sensitive. A tendril the apex of
which is slightly curved is sensitive only on the concave under surface ; others, as
those of Cobcea and Ctssus discolor, are sensitive on all sides; in Mutisia and
Clematis the under and lateral surfaces are sensitive, but not the upper surface.

While the revolving nutation and sensitiveness last the tendril attains its full size
in a few days; the revolving motion then ceases, and with it the sensitiveness; and
further changes then follow, differing in different species. In some the tendrils
remain straight after they have completely developed and become motionless ; in
others they become abortive and fall off, as e.g. Bignonia, Vilis, and Ampelopsis. It is
more common for the tendrils to roll up from the apex slowly to the base, when
growth has ceased with the concave side undermost, so that they at length form
a spiral (as in Cardiospermum and Mutisia) or more often a helix narrowing conically
upwards (as in Cucurbitaceae, Passifloreae, &c.), in which state they then dry up and
become woody.

^ This and what follows is from Darwin, Movements and Habits of Climbing Plants (1875),
p. 171 et seq.

TWINING OF TENDRILS.

867

These processes must however be considered as abnormal, the tendrils having
failed of performing their purpose of coming into contact, by means of their revolving
nutation, with a support during the period that they, are sensitive and still in a grow-
ing state. If this contact takes place on the sensitive side, a curvature arises at the
spot, and the tendril clings to the support; fresh sensitive spots are thus constantly
brought into contact with it, and the free apex twines firmly round the support
in a larger or smaller number of coils (Fig. 487). The nearer the spot where
contact first takes place to the base of the
tendril the larger are the number of revolu-
tions round the support, and the stronger the
attachment; though even a small number of
coils is sufficient to attach it with con-
siderable force. The portion of the tendril
between its base and the point of attachment
is obviously unable to twine round the support
like the free apex ; and therefore the irritation
caused by the contact extending to the portion
that is not in contact produces a different form
of curvature, consisting in a rolling up of this
portion into the form of a corkscrew, as shown
in Fig. 487 «, w, w'. This coiling is similar to
that already mentioned as taking place of its
own accord in many tendrils which do not take
hold of a support, especially in the circum-
stance that the under or dorsal side of the
tendril is always the concave one ; but it differs
from a spontaneous coiling in being always the
result of irritation, occurring invariably when
tendrils take hold of a support, and also in
taking place some time (half a day to a day)
after the attachment, at a time when the tendril
is still perfectly sensitive and growing rapidly
in length; while the spontaneous coiling occurs
only with the cessation of growth and of irri-
tability. The coiling which is the result of the
irritation caused by contact also takes place
much more rapidly than that which is sponta-
neous ; both can be readily observed by noticing
older tendrils which are still straight and have
not attached themselves, and younger ones on the same shoot that are attached and
already coiled up. The coiling of tendrils attached to supports is therefore a result
of stimulation in the same sense as the twining of the free portion round a support ;
and it is only the physical impossibility of also twining round the support that forces
the portion of the tendril between its base and the support to coil up like a cork-
screw. The coiling of this intermediate portion, like the curvature of a longer piece
of a tendril in consequence of the contact of a single point, is a proof that the local

3 K 2

Fig. 487.— Coiling of a tendril of Bryom'ii dioica. B a
portion of the branch from which the tendril springs by
the side of the petiole b and the axillary bud k; the
lower part of the tendril u is straight ; the upper part x
has coiled round a twig A; the long intermediate
part between the rigid basal portion « and the point of
attachment x has coiled spirally, and thus raised the
branch ^; w iv' the two spots where the direction of the
coil is reversed.

868 MECHANICS OF GROWTH.

irritation is communicated along the tendril. The whole consequence of irritation
does not however end with these phenomena ; for tendrils that are fixed to a sup-
port also increase subsequently in thickness, sometimes very considerably, like the
petioles oi Solanum jasminoides ; they become woody, and have a longer term of life
than those which have coiled spontaneously, or generally than those that have not
attached themselves.

There is still another point in which attached tendrils differ from those that
have coiled spontaneously. In the latter all the coils of the spiral run in one direc-
tion ; those of a tendril attached to a support have, on the contrary, points (Fig. 487,
w, w') at which the direction changes ; between any two of these points is a number
of coils in the same direction, those beyond them being in the opposite direction ; in
long tendrils with close coils there are often as many as five or six of these points.
Darwin has already shown that this is no special property of tendrils, and still less
a specific result of irritation, but is rather a physical necessity; for if a body which
coils up is fixed at both ends so that no twisting can take place at either end, the
coils must necessarily be produced in opposite directions in order that the torsions
which are unavoidably produced may counterbalance one another. This behaviour
of fixed tendrils can be imitated by cementing a narrow stretched strip of india-
rubber firmly along another strip which is not stretched, and then releasing the
former; it contracts and forms the inside of a spiral, the outer side of which is
formed by the strip that is not stretched. If the double strip is held at each end
and first stretched out straight and then relaxed, coils will be produced, some to the
right, others to the left, as in a tendril. If one end is now let go, the strip will twist
itself anew into a spiral.

Since all the movements of tendrils that have been described are the result of
growth, they take place only when the external conditions of growth are favourable,
and the more energetically the more favourable they are ; this is the case when food
is abundant, temperature high, and the plant contains abundance of sap, the result
of a copious supply of water combined with small loss by transpiration. Under
these conditions tendrils can, as I have shown, carry on their nutation and sensitive
movements even in the dark, and can twine and coil round supports. An instance
is afforded by plants of Cucurbita Pepo, the upper parts of which are grown in a
dark vessel, and which are nourished by green leaves exposed to light.

As regards the mechanism of the curvatures caused by contact, as well as the
coiling of free tendrils, it cannot be doubted that we have here to do with processes
of growth and of its alteration by transverse pressure on the side which is growing
less rapidly. The tendrils are only sensitive to contact or pressure so long as they
are in a growing state. A curvature due to irritation may be effaced during growth,
in the same manner as the curvature of growing shoots caused by concussion ; but
if the irritation from the support lasts for a longer time and a coiling takes place,
the difference in length between the convex and concave surfaces becomes per-
manent. The cells of the convex are longer than those of the concave surface (as
in roots which have curved downwards or nodes of Grasses which have curved
upwards); in thick tendrils which coil round slender supports the difference in
length is so great that it strikes the eye at once without measuring. De Vries's
recent experiments on tendrils that have not yet coiled, which he marked with trans-

Vth

" tlT

TWINING OF TENDRILS. 869

rse Streaks and measured after they had coiled, show that the growth of the con-
X surface is more considerable, that of the concave surface less so than in the

ortions of the same tendril that have remained straight above and below the curved

art. A tendril of Cucurhita Pepo twined round a support 1-2 mm. thick; after the
rvature was complete, the increment of the curved part for each millimetre of

riginal length was 1*4 mm. on the convex surface, while on the concave surface
t was only o*imm. ; the mean increment on both surfaces in the portion that
emained straight amounted to 0*2 mm. If the growth which takes place in the
lentire tendril at the time of contact with a support is small, a considerable accelera-
tion occurs on the convex surface, but in general there is no elongation on the
concave surface, or there may even be a contraction ; in the case of a tendril of
Cucurbita this contraction amounted to nearly one-third of the original length.

Similar alterations in the length of the convex and concave surfaces are observ-
able in the spontaneous coiling of free, as well as in the coiled portion of attached

ndrils between the base and the point of attachment ; and since in these cases
the amount of growth which takes place in the entire tendril is usually small a short
time previously, the contraction of the concave surface is, according to de Vries, a
very common phenomenon.

The conclusion to be derived from these phenomena and from others not
described here is that the growth of the surface not in contact is first of all increased
by the pressure of the support; the support presses the surface that is in contact,
and the pressure which the concave surface undergoes arrests its growth, or even
causes a contraction in it. It seems probable that a relaxation of the parenchyma
of the surface in contact (by giving off water to the parenchyma of the upper sur-
face) and a consequent elastic contraction of its cell-walls contribute to this result ;
at least this seems the only explanation of the contraction of the surface in contact
in the case of tendrils the growth of which has already become slow. We have
however as yet no knowledge of the mode in which the slight pressure of a light
thread or that of the revolving tendril on a support causes this alteration of growth
not only at the point of contact, but along the entire tendril.

The only cause of the spontaneous coiling of tendrils when not fixed to a sup-
port is that the upper surface continues to lengthen for a considerable time after
the growth of the under surface has ceased. The cells of the growing upper sur-
face probably withdraw from those of the under surface a portion of their water (as
the inner layers of the pith from the outer layers, see p. 805), which causes the
latter to become shorter, and the former to become longer.

Without entering further into the numerous questions of a purely mechanical
character connected with the curving of tendrils, it may at least be explained why
thick tendrils are unable to twine round very slender supports. If two tendrils are
compared one of which twines round a slender, the other round a thicker support, it
will be seen that in the former the proportional difference in length of the outer and
inner sides must be greater than in the latter. If a thick and a slender tendril twining
round supports of equal thickness are compared, the proportionate difference in
length of the outer and inner surfaces will be greater in the former than the latter case;
and if the support is supposed to decrease constantly in thickness, the difference will
increase more rapidly in the case of the thick than in that of the slender tendril, and

870 MECHANICS OF GROWTH.

the question arises whether the difference in growth of the two surfaces of the tendril
can reach to any given amount or not. The difference in length between the two
surfaces caused by unequal growth has, in fact, a limit, as is shown by experiment.
The slender tendrils of Passiflora gracilis twine firmly round threads of silk ; the
thick tendrils of the Vine on the other hand twine only round supports which are at
least from 2 to 3 mm. thick. The most strongly curved tendril of a Vine which
I could find had twined firmly round a support 3*5 mm. thick, and in a nearly circular
coil ; the mean thickness of the tendril at this spot was 3 mm. The concave surface
of a coil was nearly 11 mm., the convex outer surface nearly 29 mm. long, the
proportionate length of the two surfaces therefore nearly as i : 2*6. If this tendril
3 mm. thick were forced to twine round a support only 0*5 mm. in thickness, an
almost circular coil would have on the concave surface a length of i-6mm., on the
convex surface a length of 20*4 mm.; the relative length of the two surfaces would
therefore be as i : 13; and it does not seem possible for growth to cause so great
a difference in length between the two surfaces of a tendril. If, on the other hand,
the problem were to cause a tendril 0-5 mm. thick to twine firmly round a support
of the same thickness in nearly circular coils, it would only be necessary that
the inside of a coil should be i*6mm., the outside 47 mm, long, or that the pro-
portion between the two surfaces should be as i : 3.

In order for a tendril to attach itself firmly to a support, it is not sufficient that
its coils should merely be in contact with it; they must be firmly appressed to it.
That this is actually the case is seen when a tendril is made to twine round a smooth
support, and the support is then withdrawn ; when, as de Vries has shown, the coils
become at once narrower and increase in number. This fact shows also that a
tendril which is irritated by contact with a support endeavours to form coils the
radius of whose curvature is less than that of the support, provided the support
is not too slender nor the tendril too thick.

The cases are very instructive, in reference to the pressure which the coils of
tendrils exercise on their supports, where leaves are embraced by strong tendrils, and
are folded and compressed by them.

What has now been said is merely intended to draw attention to the more important
mechanical principles which must be taken into account in the twining of tendrils.
The biology of climbing plants and of those furnished with tendrils, so fertile in extra-
ordinary adaptations, cannot be gone into in detail. On this subject the reader will
find in Darwin's treatise quoted above a mass of beautiful observations most admirably
described.

Since the physiological function of tendrils is to take hold of supports (generally
other plants) in order to allow the slender-stemmed plant which is furnished with them
to climb up, the point of greatest importance is for the tendril to be brought into con-
tact with a support. This is usually effected with extraordinary perfection by the
revolving nutation not only of the tendril itself but also of the apex of the shoot that
bears it at the time when it is sensitive, thus causing every object anywhere within reach
of the tendril which could be used as a support to be brought almost inevitably into
contact with it. The apex of the shoot which bears the tendril usually describes an
ascending elliptic helix, the revolution being completed in from one to five hours. As in
the case of twining stems, a strong positive heliotropism would be injurious, as it would
often carry the tendril away from the supports. Some tendrils appear in fact to be
not heliotropic (those of Pimm according to Darwin), in others a weak positive helio-

MOVEMENTS OF GROWING LEAVES AND FLORAL ORGANS. 87 1

tropism is shown by the fact that the revolving nutation takes place more quickly
towards the light than away from it. Some tendrils, strikingly those of the Virginian
Creeper and Bignonia capreolata^ have the remarkable power of developing broad discs
at the end of their branches when they remain in contact for some time with hard
bodies, which attach themselves like cupping glasses to rough surfaces, and enable the
plant to climb up vertical walls when it finds no slender support round which it can coil.
In this case it is obviously necessary that the tendril should turn towards the wall which
serves as its support in order to become attached to it, and this is effected by negative
heliotropism. which causes the tendril to approach the wall shaded by foliage, where it
now performs its revolving movements of nutation — one might almost say its groping
movements — creeps along the surface, finds out the crevices and depressions, and
developes its adhesive discs.

Sect. 26. — Movements of growing Leaves and Floral Organs produced
by variations of Light and of Temperature ^ In the foregoing paragraphs we
have become acquainted with the movements of curvature of growing organs which
take place when the external conditions are constant, movements which are pro-
duced by the more rapid growth of the one or of the other side of the organ under
the action of purely internal causes. These movements were termed * spontaneous
nutations.' Amongst the organs endowed with spontaneous nutation we found that
tendrils are peculiar in being sensitive to contact on one side and in that the slight
pressure of the support induces a more rapid growth of the free surface, and a much
less rapid growth of the surface with which it is in contact. Many growing foliage-
leaves and floral organs, possessing, Hke tendrils, a bilateral organisation, are stimu-
lated to curve by variations of temperature or of the intensity of light, the growth of
one side or the other being either accelerated or retarded.

It is not all growing leaves and flowers that are sensitive to these meteorological
influences ; among plants which are very closely allied some do and some do not
possess this property. Pfefifer mentions, as examples of sensitive growing leaves, in
addition to the very sensitive leaves of Lnpatiens nolitangere, those of Chenopodieae,
Atripliceae, Solanese, Mimulus tigrinus, Mirabilis Jalapa^ of species of Silene and
Alsine, and of many Compositae; to this list Batalin adds Malva rotundifolia^ (Eno-
thera sp., Portulacca oleracea, Linum grandiflorum, Stellaria media, Gnaphalium uligi-
nosum, various species of Polygonum, Senecio vulgaris, Sida Napma, Rumeax Hydrola-
pathum, IpomcBa purpurea, and Brassica oleracea, and doubtless further investigation
will increase the number. The movements of these leaves are not efl'ected by means
of special organs, but it is the petiole or the lower part of the lamina of the leaf
which curves, under the meteoric influences, upwards or downwards accordingly as
the growth in length of the upper or of the lower surface has been accelerated by
them. The mode of antagonism of the two sides of the bilateral organ is difi^erent
in diff"erent species : in some the leaves are raised at night, as in Chenopodiumy
Brassica, Polygonum aviculare, Stellaria, Linum, in others the leaves fall, as in

^ Pfefler, Physiologische Untersuchungen, Leipzig 1873, and Sitzungsber. der Ges. zur Beford.
der ges. Naturwiss. zu Marburg, 1873. — Batalin, Flora, 1873.

[Pfeffer, Die Periodischen Bewegungen der Blattorgane, 1875. — Darwin, Movements of Plants.
— Wiesner, Die heliotropische Erscheinungen, II.]

In these works full references are given to the older literature of the subject.

872 MECHANICS OF GROWTH,

various species of Impatiens, in Polygonum Convolvulus, and in Sida NapcBa. The
amplitude of these curvatures which follow the alternation of day and night appears to
increase and diminish with the increase and the diminution of the rapidity of growth.
*■ In young leaves,' says Batalin, ' the curvatures are the greatest, and in Chenopodium
and Stellaria^ for example, they go so far that at night the young leaves close
to form very large buds : the older leaves curve but litde, the oldest not at all/

Among floral organs it is more especially the movements of the petals and of
the tubes of gamopetalous flowers which have attracted attention; no movements
which could be included in this category are known to occur in stamens or styles.
The movement consists in the curvature of the petal or of the segment of a corolla
outwards at certain times of the day and inwards at others, in such a way that in the
ordinary course the flower opens and shuts once in a day. The opening occurs
usually in the morning or during the day when the intensity of light and the tem-
perature are increasing, the closing towards evening when light and heat are diminish-
ing, although the contrary is sometimes the case. In the Compositae the movements
of the individual flowers efl'ect not so much their own opening and closing as that of
the whole capitulum.

Among the very numerous instances of motile flowers the following have been
more particularly investigated: Crocus, Tulipa, Colchicum, Onii'ihogalum, Anemone^
Ranunculus, NymphcEfl, Malope, many Compositae, especially Taraxacum, Leontodon,
Scorzonera, Hieracium, Calendula, Venidium, Bellis, &c.

As in the case of leaves so here there are no special motile organs, but certain
parts of the corolla continue to grow for a considerable time and are stimulated by
meteoric influences to a more rapid growth either of their upper or their under
surface (or internal and external), which efi^ects the opening or closing of the
flower. The region of curvature usually lies in the basal half of petals, but in Oxalis
rosea in the upper half. Among the Compositae with ligulate corollas there are
some in which the motile zone lies immediately above the tube at the base of the
ligulate corolla {Venidium, Bellis, Calendula), whilst in others, such as Taraxacum,
Leoniodon, &c., the tube itself undergoes curvature ; in both cases the centre of the
capitulum is the centre of the movements which take place inwards or outwards
along radii proceeding from it.

All the movements which are now under consideration agree in this respect,
that the plane of curvature of the organ (leaf, petal, tube) coincides with its median
plane, which is also the plane of symmetry, since it is the two sides of the organ
which are diff'erent, that is the anterior and the posterior, which produce the curva-
tures in consequence of the diff'erent way in which they react to the influence
exercised upon them by variations of light and of temperature.

The following paragraphs give an account of the mechanism of the movements
produced by variations of light and of temperature, taken from the exhaustive
researches of Pfeff"er with some additions from Batalin.

1 . The movements which are now under consideration differ from the periodical
movements and the movements due to stimulation which are manifested by leaves
possessing motile organs, in that they only take place so long as growth continues,
and that they cease with it.

2. They are effected in consequence of a more vigorous growth of the internal

MOVEMENTS OF GROWING LEAVES AND FLORAL ORGANS. 873

(upper) surface of the organ due to an increase, within certain limits, of the tempera-
ture or of the intensity of light ; when the temperature and the intensity of light are
diminishing the growth of the external surface is greater than that of the internal.
In the former case the curvature is convex inwards (opening), in the latter it is
convex outwards (closing ^). This is of course only the case when the diurnal condi-
tion of the organ is the open one ; when the contrary is the case the meteoric
influences affect the internal and external surfaces in just the opposite way.

3. The curvature of the growing organs which are sensitive to meteoric in-
fluences is not efl"ected like that of fully-developed motile organs by an alternating
expansion and contraction of the tissue, but by an alternating more vigorous growth
of one side and then of the other, so that the organ, whilst making these move-
ments, continues to increase in length. This by no means excludes the possibility
of a slight shortening of the concave side occurring temporarily just as in the case of
growing tendrils and of geotropically curved stems and nodes of Grasses.

4. Many of the organs now under consideration are especially sensitive to
changes of temperature, others to variations in the intensity of light. Many
are affected by very slight variations, others are less sensitive, and thus form a
connection with those leaves and flowers which exhibit no such movements. In
many cases each variation of temperature or of light has an immediate effect, in
other cases the effect is produced only after the lapse of a considerable time since
the last movement.

5. From the various differences mentioned in the preceding paragraph it
becomes evident why certain flowers (and leaves) open very early in the morning,
and others only later in the day; and why it is that some are affected by every
change of weather, whereas others complete their daily period with great exactitude.

If we ask, finally, what the biological meaning of these phenomena may be
in the economy of the plant, it is not easy at present to give any satisfactory answer
in so far as leaves are concerned ^. The opening and closing of flowers, however,
has an obvious connection with the process of pollination ^ ; the flowers which are
open by day are visited by the winged insects which effect pollination, and their
closure in the evening, or during cold damp weather during the day, serves as
a protection to the pollen in the anthers. Like many similar useful adaptations,
these can be readily explained on the Darwinian theory, since they depend upon
the further development of properties which belong also to allied plants but are less
developed in them, and are not accompanied in them by corresponding collateral
arrangements.

^ In order to be able to apply the expressions ' opening ' and ' shutting ' to foliage-leaves these
organs may be regarded as standing on a short axis or even as being in the bud,

^ [Darwin includes these movements of leaves, as w^ell as those described in the next chapter,
under the head of nyctitropic or sleep-movements. The object of the closing up of the leaves at
night is, he believes, to diminish the radiation from them and thus to prevent injury due to an
excessive fall of their temperature. In addition to the nightly sleep there is a diurnal sleep {Para-
heliotropism of Darwin), in which the leaves present their margins to the incident light ; the object
of this is to protect their chlorophyll from the action of too intense light. (,See Darwin, Move-
ments of Plants, p. 445. — Wiesner, Die natUrlichen Einrichtungen zum Schutze des Chlorophylls,
Wien 1876. — Stahl, Ueb. sogennante Compasspflanzen, Jena 188 1),]

^ [On the protection of the pollen from the influence of the weather, see Kerner, Die Schutz-
mittel des Pollens gegen die Nachtheile vorzeitiger Dislocation und Befeuchtung, Innsbriick 1873.]

874 MECHANICS OF GROWTH,

(a) The most important result of PfefFer's researches is doubtless the establishment
of the fact that these movements depend not upon alternate expansion and contraction
of the tissue, as was formerly thought, but upon modifications of growth. We must
therefore distinguish between these movements and those of special organs which are
no longer growing, and we may classify the former along with heliotropic and geotropic
curvatures and with the movements of tendrils. It must not, however, be forgotten that
all those external and internal conditions which increase or diminish the turgescence of
the tissues must also accelerate or retard growth ; hence it follows that the same causes
which modify the state of tension of a fully-developed organ may also modify the growth
of organs which are still growing. If this takes place to a differing extent in the two
sides of a bilateral organ, movements produced by growth will be exhibited. It might
be suggested that all the movements of curvature treated of in this and in the fol-
lowing chapter should be considered together. I quite agree with this suggestion as
far as it goes, but an account contained in a text-book must possess clearness and
precision, above all things, in the arrangement of the matter in hand. In the present
incomplete state of our knowledge of the movements of curvature, these objects will be
best attained if those movements which are results of growth be sharply separated from
those which are independent of growth.

(&) As regards Lewves the following may be appended to the account previously given.
In order to demonstrate that each upward and downward movement of the leaves of
Chenopodium album is accompanied by an increase in length, Batalin fixed a straw seven
or eight centimetres long as an indicator to the base of the lamina of a leaf attached to
a stem which had ceased to grow ; the indicator projected laterally from the leaf and its
movements were recorded by a tracing made by its free end upon a surface of sooted
paper. It became apparent that the curves described by the point of the indicator cor-
responding to each upward and downward movement did not coincide, but formed a
zigzag line tending away from the stem.

According to Pfeffer, the leaves of Impatiens, Chenopodium, Nicotiana, and Wigandia
exhibit, when in continuous darkness, a movement resembling that of the ordinary daily
period, but this periodic movement does not continue for any length of time. The
circumstance that the movement takes place, under these conditions, with the same
intervals of time as in the ordinary period when the plant is exposed to the alterna-
tion of day and night, opposes the assumption that the movement is due entirely to
internal causes, that is, that there is any 'independent periodicity.' On the contrary,
Pfeffer is inclined to assume that we have in this an instance of persistent effect whereby
those movements are produced in total darkness vi^hich had been previously brought
about by the daily alternation of light and darkness. These movements, produced by
persistence of effect, are accompanied by the growth of a particular side at a particular
hour. It is not certain if, in addition to this, spontaneous nutations take place at shorter
intervals of time.

From Pfeffer's manuscript I take the following. When motile leaves are placed in
the dark an acceleration of the growth of both sides is the result, just as greater
turgescence and tension of the tissues is produced in the motile organs of leaves which
have ceased to grow {Mimosa, Papilionacese).

A slight shortening of the side which is becoming concave may occur, as in the case
of the geotropic curvature of the nodes of Grasses and of the curvature of stimulated
tendrils.

The property in virtue of which the leaves respond by an acceleration of growth
to exposure to darkness is gained by previous exposure to light, but quite indepen-
dently of assimilation. Leaves of Impatiens nolitangere make a distinct movement when
replaced in darkness after an exposure of five minutes to light, and after an exposure of
ten minutes the movement is well-marked. Growth is accelerated when the plant is
replaced in darkness and takes place with greater rapidity than if the plant had r,emained
continuously in the dark. The leaves of other plants {Sieges bee kia, Chenopodium) require to

MOVEMENTS OF GROWING LEAVES AND FLORAL ORGANS. 875

be exposed to light for a longer time in order that a movement may take place when
they are again in darkness. Each movement produced by darkness is followed by a kind
of rebound ; thus the downward curvature of the leaf of Impatiens is followed by a rise.
The rise takes place more quickly in the light.

The rigidity of the leaves of Impatiens is not materially affected by their assuming the
nocturnal position.

A rise of temperature at 3 a.m. distinctly accelerated the assumption of the diurnal
position by the leaves of Impatiens, but it seemed to have little or no effect upon other
meteorically sensitive leaves {Chenopodium),

(c) Flowers (a). Pfeffer made measurements to ascertain the amount of growth
during curvature upon flowers which perform movements at all times in consequence
of slight changes of temperature (see infra). He found the Crocus (jvernus and luteus)
to be especially adapted for this purpose. In this flower the zone of curvature of the
six segments of the perianth lies above their separation from the tube, and occupies
from one-fourth to one-sixth of each segment of the perianth. A portion of this zone
about 3 mm. in length is especially capable of curvature, and upwards and downwards
from this the capacity gradually diminishes. Black marks were made upon the most
motile portions to serve as a means of measurement. The measurements were made
with a magnifying power of eighty diameters. One division of the micrometer eye-piece
was equivalent to 0*0076 mm. The following table may serve as an illustration : —

Crocus 'vernus.
Lengths of the marked portion.
On the external surface.
Closed. Open.

214 divisions. 214 divisions.

217 „ 218 „

187 „ 188 ,»

209-5 » 210 „

196 „ 196 „

On the internal surface.

214*5 „ 220 „

181 „ 185 „

208 „ 210-5 „

205 „ 212 „

191*5 „ 196-5 »

In the closed position, the portion measured was nearly straight; in the open position,
it was concave outwards, the curvature having a radius of from 15 to 30 mm.

These and other measurements made upon Tulipa, Oxalis, 'Taraxacum, Leontodon, and
Fenidium show that the side towards which the movement takes place undergoes no
perceptible elongation, whilst the other side increases considerably in length.

It is possible, in the case of Crocus, to remove the epidermis of one side and to
determine that the same movements are effected as before in consequence of varia-
tions of temperature. This took place even when the epidermis of both sides was
removed. The negative tension and the elasticity of the epidermis therefore play
but an unimportant part in producing the movement.

The growth of the motile zone is by no means at a standstill when the flowers
are not performing any movements ; on the contrary, it is continuing slowly and
uniformly in the two antagonistic halves, and may be retarded or accelerated in either
half at any time by a variation of the temperature or of the intensity of light: a
movement (curvature) will of course be the result.

(3) The effect of Fariations of Temperature may become apparent at any time in
many flowers, such as those oi Crocus and of Tulipa. If they be placed first in a cold

876 MECHANICS OF GROWTH,

and then in a warm atmosphere, it is easy to observe that an increase of temperature
causes a movement of opening, and a decrease a movement of closing. Pfeffer succeeded
in making a Crocus flower open and close eight times in one day. The opening is more
complete if the flower has remained closed for a considerable time, and -vice 'versa.
Particularly sensitive Crocus flowers will open or close in as few as eight minutes in
consequence of a variation of temperature amounting to 5° C. : a variation of from
12° to 22° C. will cause it in three minutes. Immersion in warm or cold water has
the same effect. It is possible to ascertain, by appropriate arrangements, that Crocus
flowers are sensitive to a variation of 0*5° C. Flowers of Tulips are not so sensitive,
but they will react to a variation of 2° G, A reversed variation of temperature is not
immediately followed by a reversal of the movement ; this continues for some time before
it exhibits the effect of the more recent stimulus. The lower limit at which variations of
temperature will still induce movements lies, for the Crocus, above 8° C, according to
Pfeffer. The flowers of Leontodon hastilis, Hier actum 'vulgatum, Scor%onera hispanica^ and
of Oxalis rosea, open between 8° and 10° C, whilst between 1° and 3° C. they remain
closed in the light.

When a certain maximum of temperature is exceeded, a partial closure of the flowers
of Crocus and of Tulipa begins, although previously the opening had become more com-
plete with every rise of temperature. These phenomena are rendered intelligible by
Sect. 19.

Pfeff'er mentions, after Crocus and lulipa, the flowers of Adonis vernalis, Ornithogalurn
umbellatum, and Colchicum autumnale, as being very sensitive to variations of temperature ;
and in a less degree those of Ficaria ranunculoides, Anemone nemorosa, and Malope trifida,
all of which perform movements at any time of the day in consequence of variations
of temperature, the more energetically the longer the period since the last movement.
This is very evident in Nymphoea alba, Oxalis rosea and valdi'viana, Mesembryanthemum
tricolorum and echinatum, and in all motile flowers of Compositae. When these have
closed in the evening a rise of temperature from 10° to 28° C. produces scarcely
any opening; in the morning, on the contrary, a rise of temperature causes them to
open even in the dark.

(y) The action of Light. Sudden obscurity suffices to cause the closure of open
flowers. Evident closing was observed in Calendula officinalis, Leontodon hastilis, and
in Venidium calendulaceum, when the flowers, which had fully opened in diff'use daylight,
were placed in darkness from eleven to twelve o'clock in the morning ; the temperature
varied between 19° and 20° C. In the afternoon, after longer exposure to light, the
closing consequent upon sudden obscurity is more marked. It appears also in the
case of Compositae and of Oxalis, that a sudden increase of the intensity of light
causes a more vigorous opening if the flowers have been previously kept in darkness
for a considerable time. The motile organ reacts the more vigorously to variations
in the intensity of light, as also to variations of temperature, the longer the time since
the occurrence of the last movement due to a stimulus acting in the opposite direction.

According to Pfeffer, it is only darkness which accelerates growth ; but a consi-
derable time must elapse before the acceleration is perceptible in one (the inner) of
the two antagonistic masses of tissue. At the same time this side becomes compressed
by the other.

1 1 appears from Pfeff'er's manuscript that as in the case of leaves so in flowers which
are motile and which are sensitive to variations in the intensity of light, a persistent effect
may be observed of such a kind that the daily periodicity of movement brought about by
the alternation of day and night will continue to be manifested for some time in
continued darkness. An instance of this is afforded by lolpis barbata. The ligulate
peripheral flowers of Bellis perennis curve outwards between seven and eight o'clock in
the morning in August (at a window facing south), and close between five and six o'clock
in the evening. In obscurity, the opening begins one or two hours later, and the closure
at night is incomplete. The flowers of Taraxacum officinale, Leontodon hastilis, and of

MOVEMENTS OF GROWING LEAVES AND FLORAL ORGANS. Syy

Barkhausia rubra, placed in the dark towards evening and allowed to remain there,
only half-opened on the following day, and remained partially open during the night.
On the second day the movements were much feebler. These movements could not be
attributed to variations of temperature.

(S) The differing sensibility of leaves to variations of temperature on the one hand, and
to variations in the intensity of light on the other, is evident from Pfeffer's statements.
The flowers of Crocus and of Tulipa which are so very sensitive to variations of tempera-
ture, close in consequence of sudden obscurity, and open when exposed to light, with
an energy which is sufficient to overcome the counteracting effect of a temperature-
stimulus. Considerable variations of temperature may however reverse the opening or
closing effected by light or darkness. In Oxalis, Nymphaa alba, Taraxacum, and
Leontodon hastilis, however, the closing in the evening cannot be prevented by a rise of
temperature, and a fall of temperature does not arrest the opening in the morning.
But if these flowers be kept in darkness during the day-time, they may be made to open
in the evening by a rise of temperature. It is necessary, in order that this opening
may take place, that a considerable time shall have elapsed since the occurrence of
the stimulating effect of darkness.

The conditions which are here stated explain why it is that many flowers growing
in the open manifest an exact daily periodicity, whereas others open and close at all
periods of the day in consequence of sudden changes of the weather.

It must be mentioned, in conclusion, that many flowers close in consequence of an
excessive intensity of light, as they do when the heat is too great. Thus Oxalis
'valdi'viana, Calendula, Venidium, and others close when direct sunlight falls upon them.
However, it is still uncertain whether the light or the heat is the active cause.

(e) It is scarcely possible at present to formulate a brief general explanation of the
phenomena exhibited by leaves and flowers which are sensitive to the action of meteoric
influences, for we do not fully understand the internal changes effected by these stimuli
nor the mutual relations of the two antagonistic halves of the organ. It is important to
discover whether a variation of temperature or of light acts in each case upon both the
antagonistic halves in contrary ways, or if one side only is sensitive undergoing changes
which affect the other side indirectly, as occurs in the case of the sensitive motile organs
of Mimoseae and probably also in the case of twining tendrils.

As a general result, it is to be remembered that each variation of temperature and
of light acts the more energetically as a stimulus the longer (within certain limits) the
time which has elapsed since the action of the last stimulus in the contrary direction.
The matter may be put thus : a difference, which disappears at a later period, is pro-
duced between the upper and the lower side of a growing organ by each stimulus due to
temperature or to light ; the more nearly this difference has disappeared the more easy
it is for a fresh stimulus to produce an effect, that is to produce again a difference
between the upper and the lower surface, or, in other words, to cause a movement or
a curvature.

CHAPTER V.

PERIODIC MOVEMENTS OF THE MATURE PARTS
OF PLANTS AND MOVEMENTS DEPENDENT
ON IRRITATION.

Sect. 27. — Introduction \ The greater number of the movements which are
brought into play during growth — as the curvatures caused by heliotropism or geo-
tropism or by the pressure of supports on tendrils and climbing plants — produce
new permanent conditions, since it is growth that is modified. It is only when the
action has been a very transitory one that heliotropic or geotropic curvature or that
of tendrils due to irritability can again be effaced by further growth. During these
processes the organ is advancing towards maturity; the changes which have not
been effaced are therefore, as it were, stereotyped.

The case is quite different with the changes now to be described. They take
place in organs whose growth is completed, but whose structure allows the tissues
to assume different conditions which alternate under the influence of external or
internal causes.

In those movements which occur during growth the tension of the tissue is
concerned only so far as any change in it reacts on growth and modifies it. Periodic
movements and those due to irritation, on the contrary, depend entirely on changes
in the tension of the tissues, which, in this case, are fully developed only when the
organ has attained maturity. These alterations of the tension of the tissues do not
however induce new permanent conditions, but can be effaced; every change is
again reversed by internal forces, and the previous condition restored so long as
there has been no structural injury.

Various objections might be raised to the distinction drawn between the move-
ments exhibited by growing organs and those performed by organs which have
ceased to grow. It might be argued that motile organs begin to be irritable and to
perform movements whilst they are still growing. In reply to this it may be urged
that the motility persists after growth has ceased, and that it is only then that it is
fully developed, whereas the motility of the organs considered in the previous chapter
ceases with growth. A possible objection is that the motile organs under consider-
ation have not ceased to grow, as a matter of fact, at the time when they are
especially irritable and in periodic movement, for they are capable, when in this

^ [Pfefifer, Physiologische Untersuchungen, 1873; id., Die periodischen Bewegungen der Blatt-
organe, 1875. — Darwin, Movements of Plants, 1880.]

INTRODUCTION. 879

condition, of curving geotropically and heliotropically. I observed both these sets
of phenomena, for example, in the motile organs of Phaseolus, and in the irritable
filaments of the Cynarese. These heliotropic and geotropic curvatures are necessarily
accompanied by growth. To meet this it may be pointed out, (i) that the periodic
movements and the movements due to irritation which we are now considering are
not dependent upon growth but upon an alternate contraction and dilatation of the
cells; (2) that there are other organs which are also capable of renewed growth
under abnormal conditions after their growth under normal conditions has ceased.
We found this to occur in the nodes of Grasses. When these have ceased to grow
in the erect position, they exhibit sharp geotropic curvatures due to a vigorous
growth of the lower surface when they are placed horizontally. The periodically
motile and irritable organs evidently possess the same property. They are not only
irritable and periodically motile after their growth under normal conditions has
ceased, but they are capable of renewed growth under abnormal conditions. The
conditions are abnormal when light falls upon these organs from one side only, or
when they are placed in an unusual position which is more or less nearly horizontal ;
then heliotropism and geotropism are brought into play.

Now that sufficient stress has been laid upon the distinction between organs
which are motile while growing and organs which are motile after they have ceased
to grow, the points of resemblance of the two may be considered. The first of
these is the fact that for each kind of movement which is exhibited by the special
motile organs, a corresponding kind may be observed in growing organs. Thus,
the spontaneous periodic movements of the former correspond to the spontaneous
nutations of growing stems, leaves, tendrils, and flowers. The variations of tempe-
rature and of the intensity of light which act as stimuli upon the former, by increas-
ing or diminishing the turgescence of the tissues, also affect many growing leaves
and cause flowers to open or shut, by accelerating the growth of one side or the
other. The motile organs of Mimoseae, Oxalidese, and Cynareae, which are sensitive
to contact, correspond to the growing tendrils and roots which are sensitive to
pressure.

This comparison tends to show that the causes which induce contraction and
dilatation of the cells of motile organs by modifying their turgidity, may also retard
or accelerate the growth of growing organs. The deeply-seated connection be-
tween these phenomena will become evident if what was said in Sect. 14 as to the
causes of growth be compared with the following account of the causes of the
periodic movements and of those due to stimulation. According to my theory, the
hydrostatic pressure which the cell-sap exercises, in consequence of its increase in
quantity by endosmosis, upon the extensible cell-wall, is an essential condition of the
growth of the cell ; fresh solid matter is deposited between the micellae of the
stretched cell-wall, it therefore grows, and thus growth is a perpetual over-stepping
of the limit of elasticity of the stretched cell-wall. Everything which increases the
turgescence of the cell promotes its growth, everything which diminishes the tur-
gescence is prejudicial to its growth. If these effects are produced in different
degrees upon the two sides of a growing organ, corresponding curvatures will be
produced. If these effects are produced in a mass of tissue the cell-walls of which
have ceased to grow but are very extensible and very perfectly elastic, an increase of

88o PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION.

turgidity will cause an expansion, a diminution of turgidity will cause a contraction
of the mass of cells, and these changes in volume will be accompanied by corre-
sponding curvatures of the organ. The conditions which modify the turgescence of
growing cells may be identical with those which modify the turgidity of cells which
have completed their growth ; in the former case every variation of turgescence
involves an alteration of the volume of the cell which is made permanent by growth,
in the latter case the alteration of volume is only temporary, and can be effaced by a
variation of the turgidity in the opposite direction. From these considerations it
becomes apparent that the study of the phenomena of movement will contribute to
the development of a mechanical theory of growth, and vice versa.

Sect. 28. Review of the phenomena connected with periodically
motile and irritable parts of plants. It is remarkable that all organs at pre-
sent known as coming under this category are, in a morphological sense, foliar
structures, as green foliage-leaves, petals, stamens, or occasionally parts of the carpels
(styles or stigmas). It is the more striking that no axial structures or parts of stems
are contractile in this sense, because the contractile parts of leaves are usually cylin-
drical, or at least are not expanded flat, and therefore possess the ordinary form of an
axis. There is this further agreement in the anatomical structure of all parts which
exhibit these phenomena; — that a very succulent mass of parenchyma envelopes
an axial fibro-vascular bundle or a few bundles running parallel to one another;
the elements composing these bundles being only slightly or not at all lignified, and
therefore remaining extensible and flexible, a fact of importance in reference to
the possibility of the movement, which consists of flexions upwards and downwards,
generally in the median plane of the organ, the fibro-vascular bundle thus forming
the neutral axis of the curvature \ The mass of parenchyma which envelopes the
fibro-vascular bundle often has the form of a pulvinus, and does not contain in its
outer layers any air-conducting intercellular spaces, or only very small ones, while in
the inner layers they are larger, especially in the immediate vicinity of the bundle ;
these being, according to Morren, Unger, and Pfeffer, wanting only in the irritable
stamens of Berberis and Mahonia. The tension of these layers of tissue which is
generally very considerable, is caused by the stronger turgidity of the parenchy-
matous cells on the one hand and the elasticity of the axial bundle and epidermis on
the other hand. As far as observations go at present, especially those made on the
larger contractile organs, the tendency to extension is greatest in the middle layers
of the parenchyma between the epidermis and the axial bundle, but the elastic
resistance of the epidermis is less than that of the bundle.

If we now consider the nature of the movements in reference to the causes which
directly operate to produce them, we may, in the present state of our knowledge,
distinguish between three diff'erent kinds, viz.

(i) Those periodic movements which are produced entirely by internal causes,
without the cooperation of any considerable external impulse of any kind. Such
movements may be termed automatic or spontaneous.

(2) Spontaneously motile foliage-leaves are also sensitive to the influence of lights
in such a way that within certain limits any increase in the intensity of the light

^ This is also true for Dion<za if the motile parts and not the whole leaf be considered.

•■'. SI

MOTILE AND IRRITABLE PARTS OF PLANTS. 88 1

causes such a curvature of the contractile organs as to place the leaves in an ex-
panded and completely unfolded position ; while any decrease in the intensity of the
light produces the opposite curvature, causing the leaves to fold up. The expanded
position is called that of waking or //le diurnal position, the opposite one that of sleep,
or the nocturnal position. In consequence of this sensitiveness to fluctuations in the
light, these organs make periodic movements depending on the alternation of day and
night, which, being induced by external causes, must be clearly distinguished from
the automatic or those brought about by internal causes ; and the more so because
both kinds usually occur in the same organ, and are combined in various ways\
In their sensitiveness to variations in the intensity of light these fully-developed
organs resemble the growing organs referred to in Sect. 26. It has not yet
een determined if any such parallelism exists with reference to variations of
emperature.

(3) In a smaller number of instances periodically motile foliage-leaves, as well
as some reproductive organs which do not exhibit periodical movements, are irritable
to touch or concussion. If a particular spot of the organ is onfy lightly touched or
subjected to a slight rubbing from a solid body, the side which is touched becomes
oncave or contracts. The same effect is produced if a stronger impulse acts on
any other part of the irritable organ, which then excites the irritable part. If the
motile part has curved in consequence of the mechanical irritation, it afterwards re-
sumes its previous position, and is then again irritable.

The biological significance of these various forms of movement in the economy
of the plant is known only in a few instances, as in the case of irritable stamens,
where the insects that visit the flowers cause the irritation and consequent alteration
in the position of the stamens, these movements being serviceable for the conveyance
of the pollen either to the stigma of the same flower (as in Berberis^) or to those of
other flowers (as in Cynaracese). We have no knowledge, on the other hand, of any
purpose in the economy of the plant served by the periodic and irritable move-
ments of foliage-leaves ^

(i) The spontaneous periodic mo'vement is seen most conspicuously in the few cases in
which the period extends only over a few minutes, and the oscillation of the organ takes
place by day and night under a sufficiently high temperature, as in the small lateral
leaflets of the trifoliolate leaf oi Desmodium gyrans (the Indian ' Telegraph-Plant'), and
the labellum of the flowers of Megaclinium falcatum (an African Orchid). The lateral
leaflets of Desmodium gyrans^ are attached to the common petiole by slender petiolules
4 to 5 mm. in length, the petiolules being the organs by the movements of which the
leaflets are carried round, their apices describing nearly a circle. One revolution takes,
when the temperature is above 22° C.,from 2 to 5 minutes; the motion is often irregular,

' This distinction, partly founded on facts that have long been known, is very necessary for
a clear insight into the phenomena, and was first brought forward by me in the treatise on the
various immobile conditions of the periodically motile and irritable parts of plants ('Flora,' 1863).

2 [H. M tiller (Befruchtung der Blumen durch Insekten, Leipzig 1873) has shown that the
irritability of the stamens of Berberis is a contrivance for cross-fertilisation rather than' for self-
fertilisation.]

^ [See note 2 on page 873.]

* For further illustrations see Meyen, Neues System der Pflanzen-Physiologie, 1839, "^ol. III.
P- 553- [The first account of Desmodium gyrans, based on Lady Morisou's observations, is by
Broussoriet, Mem. Acad, de Paris, 1784, p. 616.]

3L

882 PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION.

sometimes interrupted, and then recommencing suddenly in jerks. The labellum of
Megarl'mium falcatum ^ narrows below into a claw traversed by three slender fibro-
vascular bundles, the curving of this portion imparting to the labellum a swinging motion
up and down. In a much larger number of other foliage-leaves endowed with periodic
motion the spontaneous periodicity is almost entirely concealed by the contractile parts
being also very sensitive to light, so that a cursory observation detects only the daily
period, or the different positions by day and night. If however these plants, or even
cut branches placed in water, remain for some days in the dark or in artificial light
of unvarying intensity, it is seen that the periodic movements do not cease, but continue
when the temperature also is constant, i.e. independently of any irritation resulting
from change of temperature. Under these circumstances the leaves are in a constant
slow motion, indicated by the varying positions at short intervals (as e.g. in Mimosay
jicacia lophantha, Trifolium incarnatum and pratense, Phaseolus, various species of Oxalisy
as O. Acetosella, &c.^). After a certain time these movements cease. The behaviour of
the lateral leaflets of Desmodium gyrans and of the labellum of Megaclinium falcatum
on the one hand, and that of leaves which assume different positions by day and by night
on the other hand, offer a contrast in the following respect ; in the former the internal
periodic causes of the movement are stronger than the irritation of the light to which
they may happen to be exposed, while in the latter these internal causes are outweighed
by the irritation caused by the varying amount of light under ordinary conditions, so that
only the daily periodicity induced by the alternation of day and night is apparent. To this
last category belong the movements of the compound leaves of Leguminosae, of many
species of Oxalis, and of Marsilia. In the Leguminosae the common petiole is often
attached to the stem by a larger contractile organ or ^ pul'v'mus \' and in all the cases
just named the petiolule of each leaflet possesses a similar organ. If, as in the bipinnate
leaves of Mimosa^ there are secondary common petioles, these are also attached to the
primary petiole by contractile organs. These organs always consist of an axial fibro-
vascular bundle surrounded by a thick layer of turgid parenchyma. The other parts
of the leaves, the petiole as well as the lamina, are not spontaneously contractile, but
the alterations in their position are caused by the curvatures of the organs at their base.
The movement is either a curving upwards and downwards, as in Phaseolus, Trifolium,
Oxalis, and the common petioles of Mimosa, or is directed from behind and below in a
forward and upward direction, as in the leaflets of Mimosa.

(2) Sensitiveness to variations in the intensity of light is exhibited with peculiar
distinctness in the leaves of Leguminosae and Oxalideae and of Marsilia, and the conse-
quent movements are effected by the organs which also produce the spontaneous periodic
movements^. These organs occur also in the leaves of many other plants, as Cannaceae
and Marattiaceae, but their irritability has not as yet been investigated.

In the diurnal position produced by increasing intensity of light the leaves generally
have their surfaces completely unfolded and expanded flat ; in the nocturnal position
they are on the contrary folded up in different ways, being turned upwards, downwards,
or sideways. The leaflets of Lotus, Trifolium, Vicia, and Lathyrus are, for example,
folded upwards at night, those of Lupinus, Robinia, Glycyrrhiza, Glycine, Phaseolus, and
Oxalis downwards ; the common petiole of Mimosa turns downwards at night, that
of Phaseolus becomes erect ; the leaflets of Mimosa and Tamarindus indica^ turn laterally
forwards and upwards in the dark, those of Tephrosia caribcea backwards. When the
petiole and other parts of the same leaf are contractile, the curvatures of the various
motile parts may differ ; thus, for example, the petiole of Phaseolus turns upwards in the
evening, while the leaflets turn downwards ; the petiole of Mimosa on the other hand

1 C. Morren, Ann. des sci, nat. 1843, 2nd series, vol. XIX. p. 91.

2 For further proof see Sachs, Flora, 1863, p. 468, where the literature of the subject is quoted.

3 [See Somnus Plantarum, P. Bremer, Linn. Amoen. Acad. iv. p. 333.]
* See Meyen, Neues System der Pflanzen-Physiologie, vol. III. p. 476.

MOTILE AND IRRITABLE PARTS OF PLANTS. 883

turns downwards while the leaflets turn forwards and upwards, till they partially cover
one another in an imbricate manner. The folding-up of a leaf in the diurnal position
when suddenly placed in darkness is the more energetic the longer the period of illu-
mination has been, and inversely. If a plant which has been exposed to alternations of
day and night be kept in darkness for a considerable time, the dailj' periodicity may
continue for a time, according to PfefFer, as a persistent effect, and then the more rapid
spontaneous movements make their appearance. In cases in which they take place
with considerable force {Oxalis, Trifolium) this persistent effect is not exhibited. In
general the sensitiveness to the action of light is very different in difl:erent species.

(3) Many leaves endowed with periodic motility or sensitiveness to light are also
irritable to contact and concussion ^, as those of Oxalis Acetosella, stricta, corniculata, purpurea,
carnosa, and Deppei ^, Robinia pseud- Acacia ^, various species of Mimosa, as sensiti'va, pros-
trata, casta, •vi'va, asperata, quadri'ual'vis, dormiens, pernambucina, pigra, humilis, and pellita^
jEschinomene sensiti'va, indica, and pumila, Smithia sensiti'va, Desmanthus stolonifer, trique-
trus, and lacustris. In the greater number of these plants a rather violent or often
repeated concussion is requisite to set the parts in motion, which then always assume
the position of sleep ; in other words, mechanical stimulation acts in the same way as a
diminution of light. The sensitiveness is greater in Oxalis {Biophytum) sensiti'va and
Mimosa pudica, where a very slight concussion or simple contact on the contractile
organ suffices to cause immediate and considerable motion, which is due, when
the plant is highly irritable, to conduction of the stimulus to the parts not touched.
Although the position assumed in consequence of contact or concussion resembles the
nocturnal position, yet it differs from it in its conditions ; the former is accompanied by
flaccidity of the contractile organ, the latter by increased turgidity.

Among irritable stamens may be enumerated the various species of Berberis^ i^-g-
'vulgaris, emarginata, cretica, and cristata), and of the sub-genus Mahonia. In contact
with the corolla when at rest, they curve concavely inwards when the base of the inner
side of the filament is lightly touched, so that the anther comes into contact with the
stigma.

There is a greater diversity in the phenomena produced by a slight blow or friction
on any part of the filaments of various Gynaraceae (as Centaurea, Onopordon, Cnicusy
Carduus, and Cynara) and Cichoriacese (as Cichorium and Hieracium), The filaments
which spring from the tube of the corolla bear the five firmly attached (not coherent)
anthers, which together form a tube through which the style grows up while the pollen
is escaping. At this time the filaments are irritable ; when at rest they are curved con-
cavely outwards as far as the width of the corolla-tube will permit ; on contact or
concussion they contract, become straight, and hence come into close contact along
their whole length with the style which they enclose, lengthening again after some
minutes and resuming their curved form. Since each separate filament is independently
irritable, touching a single filament or a blow on one side only of the capitulum will irri-
tate, according to circumstances, only one, two, or three of the filaments, and by the
contraction of one side the whole of the reproductive organs will be bent to one side.
By the displacement connected with this or the pressure of the other filaments on the
corolla they are also irritated, and thus arises an irregular oscillating or twisting motion
of the reproductive organs of the flower. If the whole capitulum is shaken, or if the
hand is passed over the surface of the flower, or the flower is blown into, a ' creeping *
motion ensues of all the flowers in the capitulum. This phenomenon occurs only while
the style is growing through the anther-tube and the pollen is being emptied into the
tube ; the motion of the filaments eflfected by insects causes the anther-tube to be drawn

* [Movements may also be caused by chemical stimuli; see Darwin, Insectivorous Plants, 1875.]
2 From Unger, Anatomic und Physiologic der Pflanzen, 1853, p. 417.

^ Mohl, Flora, 1832, vol. II. No. 32, and his Vermischte Schriften.

* Goeppert, Linnsea, 1828, vol. HI, p. 234 et seq.

3 L 2

884 PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION.

downwards and a portion of the pollen thus to escape above it, which is then carried away
by insects to other flowers and capitula where the stigmas are already unfolded^.

Among irritable female reproductive organs are the lobes of the stigmas of MlmuIuSf
Martynia, Goldfussia anisophylla, &c., which close when their inner side is touched,
evidently in order to retain the pollen brought to them by insects. More striking are
the movements which follow a light touch on the gynostemium of Stylidium, a genus
■ almost peculiar to Australia (e.g. S. adnatum and graminifoUum). The cylindrical
gynostemium which bears the stigma and close beside it two anthers is, when at rest,
turned sharply downwards ; irritation causes a sudden elevation and even reversal to
the other side of the flower.

A more detailed description of these and other contractile organs will be foun d in
Morren's treatise named below '^.

(4) Mobile and immobile condition of the motile parts of plants^. The parts of plants
endowed with periodic motion and irritability may present alternately two different
conditions according to the external influences to which the plants are subjected.
Their properties may be suspended for a shorter or longer time, and may give place
to a condition of immobility which again disappears if the external influences are
favourable, provided the organ is not in the meantime killed. This immobile condition
differs from that caused by death in the fact that it is transitory, and that the internal
changes which cause it are reparable.

The following particulars are taken from the detailed illustrations in my work already
quoted.

(i) Transitory rigidity from cold occurs in the leaves of Mimosa pudica when the
influences are otherwise favourable if the temperature of the surrounding air remains
. for some hours below 15° C; the lower the temperature falls below this point, the
more quickly does the rigidity set in ; the irritability to touch and concussion disappears
first, then that to the action of light, and finally also the spontaneous periodic movement.
The lateral leaflets of Desmodium gyrans are, according to Kabsch, immotile when the
temperature of the air is below 22° G.

(2) Transitory rigidity from heat occurs in Mimosa within an hour in damp air at
40° C, within half an hour in air at 45° C, in a few minutes in air at 49° to 50° C. ; the
sensitiveness returns after exposure for some hours to air at a favourable temperature.
In water the rigidity from cold of Mimosa sets in at a higher temperature, 'vi%. in a
quarter of an hour between 16° and 17° C, and the rigidity from heat at a lower
temperature than in air, nji^. in a quarter of an hour between id" and 40° C* During
the rigidity from heat, whether in air or water, the leaflets are closed, as after irritation,
but the petiole is erect, while when irritated it is directed downwards.

* These phenomena were discovered as long ago as 1 764 by Count Battista dal Covolo, and
are well described by Kolreuter in his preliminary Nachrichten von einigen das Geschlecht der
Pflanzen betreffenden Versuchen; 3rd Appendix, 1766, pp. 125, 126.

^ C. Monen, On Stylidium, Mem. de I'Acad, roy. des sci. de Bruxelles, 1836; on Goldfussia,
ditto, 1839; on Spar ma7inia africana, ditto, 1 841 ; on Megaclinium, ditto, 1862. Also on Oxalis,
Bull, de I'Acad. roy. des sci. de Bruxelles, vol. II. No. 7 ; on Cereus, ditto, vols. V and VI. [On the
irritability of the stamens o( Ruta, see Carlet, Comp. rend., August 25, 1873, and May 18, 1874;
Heckel in Comp. rend., July 6, 1874. On Sparmannia, Cistus, and Helianthevium, see Heckel, in
Comp. rend., March 23 and April 6 and 20, 1874.]

^ Sachs, Die voriibergehende Starrezustande periodisch bev/eglicher und reizbarer Pflanzen-
Organe, Flora, 1863, No. 29 et seq. — Dutrochet, Mem. pour servir, vol. I. p. 562. — Kabsch, Bot.
Zeit. 1862, p. 342 et seq.

* A plant oi Mimosa immersed in water of from 19 to 2i-5°C. remains sensitive to impact
and light for eighteen hours or more. Bert's statement (Recherches sur le mouvement de la
sensitive, Paris 1867, p. 20) that Mimosa remains irritable up to 56 or even 60° C. is not suffi-
ciently confirmed, and is opposed to all that we know about the superior limits of temperature
for vegetation.

MOTILE AND IRRITABLE PARTS OF PLANTS. 885

(3) Transitory rigidity from darkness. If plants whose leaves are periodically motile
and irritable to light and concussion, as Mimosa^ Acacia, Trifolium^ Phaseolus, and Oxalisj
are placed in the dark, the spontaneous periodic movements take place without the
changes in position caused by the action of light, and therefore all the more clearly,
and the irritability to touch is also not at first injured. But this motile condition
disappears completely when the darkness lasts for one day or more. If a plant ren-
dered rigid by exposure to darkness is again placed in the light, the motile condition is
not restored for some hours or even for some days.

Perfect darkness is however by no means necessary in order to produce rigidity. It
may be brought about by placing a plant that is very dependent on light, like Mimosa^
for some days in a deficient light, as in an ordinary dwelling-room, at some distance from
the window.

In contrast to the rigidity caused by darkness, I have applied the term Phototoniis to the
normal motile condition resulting from the alternation of day and night. A plant in this
condition, if placed in the dark, will, as we have seen, remain for some time (hours or even
days) in a state of phototonus, which then disappears gradually; the plant is therefore,
under normal conditions, in a state of phototonus even during the night. In the same
manner a plant which has become rigid in continued darkness retains its rigidity for some
time (hours or even days) after being exposed to light. The two conditions therefore
pass over into one another only slowly.

In the case also of rigidity caused by darkness, the irritability of Mimosa to con-
cussion disappears first, and then the spontaneous periodic motion. In the same manner
a plant which has thus become rigid reassumes first of all its periodic movement, then its
irritability.

The position of the various parts of the leaves of Mimosa when in a state of rigidity
caused by darkness is different from that caused by darkness in phototonic plants, and
also different from that produced by rigidity due to heat. In the first case the leaves
remain quite expanded, the petiolules directed downwards, the common petiole almost
horizontal.

Changes in the intensity of the light produce the same effect as irritants, but only on
healthy phototonic plants ; leaves which have become rigid from exposure to darkness
show no irritability to variations in its intensity until they have again become photo-
tonic from long-continued exposure to light. A plant of Acacia lophantha^ left for five
days in the dark, was found to have lost during the last forty-eight hours every trace
of its spontaneous movements. It was then placed in a window, where within two
hours it directed its leaflets strongly downwards, the sky being cloudy, and other small
changes of position took place in the petiolules. In this condition the plant was still
rigid ; when it was then placed about noon in the dark with another phototonic plant
of the same species, the position of its leaves did not change, the leaflets remained
expanded, while the other plant within an hour closed its leaflets and assumed the most
complete nocturnal position. Both plants were then once more placed in the window,
when the first again retained the position of its leaves unchanged, while the normal
phototonic plant expanded its closed leaflets in an hour, the sky being still cloudy. By
the evening the lowest six leaves still remained rigid and expanded, but the upper eight
or nine leaves closed ; the next morning all the leaves again expanded into their normal
diurnal position \

Irifoliiim incarnatum exhibited similar phenomena, with only immaterial differences.

* [Bert (Bull, de la Soc. hot. de France, vol. XVII, 1871, p. 107) found that the irritability of
the leaves of Mimosa was destroyed by placing them under bell-glasses of green glass almost to the
same extent as if placed in the dark ; the plants being entirely killed in twelve days under blackened,
in sixteen days under green glass ; plants placed in the same manner beneath white, red, yellow,
violet, and blue glasses being still perfectly healthy and sensitive, though varying in the rapidity
of their growth.]

886 PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION.

It is worth noting that in the plants observed by me the positions of the leaves in-
duced by the rigidity caused by darkness resemble the diurnal more than the nocturnal
position of phototonic plants. Rigidity produced by darkness is apparently only ex-
hibited by organs containing chlorophyll, for, according to a communication from
Pfeffer, the stamens of Cynaraceae, though developed in the dark, are irritable. This,
as well as the fact that individual organs of a plant may be made rigid by darkness^,
shows that this condition is not due to an excessive accumulation of carbonic acid
in the tissues.

(4) Transitory rigidity from drought I have observed only in Mimosa pudica. If the
earth in the pot in which a plant is growing is left unwatered for a considerable time,
the irritability of leaves perceptibly diminishes with the increasing dryness, and an almost
complete rigidity ensues, causing the common petiole to assume a horizontal position,
and the leaflets to expand. Leaves which have lost their irritability are not withered
nor flaccid ; but the watering of the soil causes a return of the irritability within two or
three hours.

(5) Transitory rigidity resulting from chemical influences. In this category I include
especially the condition termed by Dutrochet^ Asphyxia, which occurs in Mimosa when
placed in the receiver of an air-pump. While the air is being pumped out, the leaves
fold up, no doubt in consequence of the concussion ; but the leaflets then expand, the
petiole becomes erect, and while the leaves assume the same position as after pro-
longed withdrawal of light, they become rigid and possess neither periodic motility
nor irritability to concussion. When brought into the air the plant again becomes
motile. It can scarcely be doubted that the effect of the vacuum is essentially a result
of the removal of the atmospheric oxygen, and therefore causes rigidity by suspending
the respiration.

Kabsch^ confirmed these statements, and showed that the stamens of Berberisy Ma-
honia, and HeUanthemum also lose their irritability in 'vacuo, regaining it in the air.

The cessation of the irritability of the stamens of these plants which Kabsch states to
take place when they are placed in nitrogen or hydrogen gas may also be ascribed to a
simple suspension of respiration, the irritability returning on access of air. The destruc-
tion of the irritability which takes place, on the authority of the same observer, in the
stamens of Berberis in pure carbon dioxide or in air containing more than 40 p. c. of this
gas must, on the contrary, be considered a positively injurious chemical action of the
nature of poisoning. If they remain from three to four hours in carbon dioxide, the
irritability returns only after some hours on replacing them in air. Carbonic oxide
mixed with air in the proportion of from 20 to 25 p. c. destroys irritabihty, while nitrous
oxide produces no effect. The stamens, on the other hand, bend towards the pistil in
nitric oxide, and lose their irritability in i^ or 2 minutes. Ammoniacal gas appears
to cause transitory rigidity after a few minutes^.

Kabsch states that rigidity ensues after from i^ to 2 hours in pure oxygen, the
stamens again recovering in the air.

The vapours of chloroform or of ether destroy the irritability (also for variations
of light ?) of motile organs, without however causing death unless the action be too
prolonged. If entire plants of Mimoseae or branches which have been cut off be

* Pfeffer, Physiol. Unters., Leipzig 1873, p. 66.
^ Dutrochet, Mem. pour servir, vol. I. p. 562.

^ Kabsch, Eot. Zeit. 1862, p. 342.

* [J. B. Schnetzler (Bull, de la Societe vaudoise des Sciences naturelles, 1869) points out that
the substances which destroy the contractility of animal ' sarcode ' also destroy the irritability of the
stamens of Berberis and the leaves of Mimosa. Curare has no prejudicial effect in either case ;
while nicotine, alcohol, and mineral acids destroy both. In the Comptes rendus for April 23rd,
1870, is a record of a series of experiments on the efi'ect of chloroform on the irritability of the
stamens of Mahonia.']

MECHANISM OF THE MOVEMENTS, 887

introduced into an atmosphere nearly saturated with these vapours, the irritability
will disappear in a few minutes. If the organs have been previously stimulated, they
now expand (without being irritable) in consequence of their becoming more rigid.
The action of the vapour of ether or of chloroform is a purely local one : only those
organs which are exposed to it lose their irritability^.

(6) Frequent stimulation at short intervals (vibration) causes the pulvinus of
Mimoseae to become insensible to stimuli, although they expand during the continuance
of the stimulation and take up a position of rest such as would have been produced
by the first shock alone. It is not for from five to fifteen minutes after the cessation
of the shocks that irritability returns^.

(7) Transitory rigidity caused by electrical agency^ was observed by Kabsch in the
gynostemium of Stylidium. A weak current produced the same result as concussion;
a stronger current destroyed the irritability, which however returned after half an hour.
In Desmodium gyrans^ on the other hand, the leaflets which had been rendered rigid by
cold (22° G.) were again made motile by the action of induction-currents.

Sect. 29.— Mechanism of Movements caused by Contact or Concus-
sion*. This has been studied with success by many observers in the articulations
at the bases of the petioles of Mimosa pudica^ in the small motile organs of the
leaflets of Oxalis Aceiosella, and in the filaments of the Cynarese. There is but little
doubt that other motile organs are essentially similar to these types.

Generally the motile organs are nearly cylindrical in form, but somewhat
flattened horizontally. A tough, but not brittle, flexible lignified fibro-vascular bundle
lies in the axis of the organ, and is surrounded by several layers of succulent
parenchyma which is invested by a feebly-developed epidermis. The turgid paren-
chyma tends to stretch the axial bundle and the epidermis, whereas these, more
especially the bundle, offer a resistance to the expansion of the parenchyma.

It is the parenchyma which is irritable. It may be the parenchyma of one side
only of the organ which is irritable, as in Mimosa (the lower surface only of the large
articulations), or of both sides (filaments of Cynareae).

The conditions of the Irritability are the following : i. that the parenchymatous
cells tend to absorb water continually and thus to stretch their cell-walls, that is, to
be strongly turgid ; and 2. that a slight concussion of the irritable cells causes an
escape through their walls of a portion of the water which they contain. This
sudden change produced by a stimulus probably aff'ects, according to Pfeff"er's
reasoning, the protoplasm of the parenchymatous cells: their cell-walls are not
irritable and contribute to the movement merely in virtue of their elasticity. When
the movement has ceased, the absorption of water recommences and their turgidity
and the irritable condition are restored.

The Movement itself is produced by the action of the elasticity of the stretched
cell-walls which comes into play at the moment when the turgid cells give off water.
The cell-walls contract elastically in proportion to the quantity of water which
escapes from the cells. The water passes into the intercellular spaces of the

* Pfeffer, 1. c. pp. 64-66.

2 Pfeffer, 1. c. p. 58.

3 Kabsch, Bot. Zeit. 1861, p. 358.

* The very extensive literature on this subject has been collated by Pfeffer in his 'Physio-
logische Untersuchungen,' Leipzig 1873.

888 PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION.

irritable tissue and finds its way, for the most part, out of the organ into other
tissues. As a consequence the volume of the stimulated tissue diminishes. The
tissue of the other side of the organ, which has not been stimulated, is turgid and
expands at the time when the tense epidermis of the stimulated side is contracting
elastically : the latter side becomes concave, the former convex, and the stimulated
organ curves so that the parts which are connected with it are passively raised or
lowered accordingly as the concavity of the organ, which is always on the stimu-
lated side, faces upwards or downwards.

The organ is not irritable immediately after this has taken place, for the con-
tracted cells are not sufficiendy turgid to admit of a further escape of water. After
some time the contracted cells absorb water, their turgidity increases, their walls,
become tense, the volume of the individual cells and therefore that also of the whole
irritable tissue becomes greater, the epidermis is stretched, and a curvature is effected
in an opposite direction to that which was effected by stimulation, the stimulated
side of the organ becomes convex, and then the organ is again irritable.

From what has been said it is evident that the volume and the turgidity (rigidity)
of the whole organ diminishes during the movement produced by stimulation, that
the restoration of irritability is associated with increased turgidity and volume, and
finally, that the irritability and the amplitude of the movement must be greater,
ceteris paribus, the more turgid the organ is.

This account of the nature of the irritability and of the phenomena of the
movement is derived from Pfeffer's acute observations. He completed the investiga-
tions of his predecessors, Lindsay, Briicke, Hofmeister, Sachs, Cohn, Unger and
others, by demonstrating the escape of the water from the stimulated cells, a fact
which had not been clearly ascertained before.

(a) The Sensithe Plant^ {Mimosa pudicd). The leaf when fully developed is bipinnate,
and consists of a petiole from 4 to 6 cm. long with two pairs of petiolules 4 to 5 cm. in
length, and on each of these from fifteen to twenty-five pairs of leaflets 5 to 10 mm. long
and I '5 to 2 mm. broad. All these parts are connected by contractile organs; every
leaflet is immediately attached to the rachis by such an organ from 0*4 to 0*6 mm. long,
and this again to the primary petiole by another similar organ from 2 to 3 mm. long and
about I mm. thick. The base of the petiole itself is transformed into a nearly cylin-
drical contractile organ 4 to 5 mm. long and 2 to 2*5 mm. thick, which is furnished, like
those of the petiolules, with a number of long stiff hairs on the under side ; the upper
side being only slightly hairy or not at all.

Each of the contractile organs consists of a comparatively very thick layer of
parenchyma with a feebly developed epidermis without stomata, and penetrated by
an axial flexible but only very slightly extensible fibro-vascular bundle, which separates
into several bundles where it emerges into the channelled petiole. The parenchyma
consists of roundish cells enclosing, in the eight layers which surround the axial
bundle, large air-conducting intercellular spaces which become much smaller in the
eighteen or twenty outer layers of cells, and are entirely wanting in those immediately
beneath the epidermis. These intercellular spaces are in communication with one

^ Dutrochet, Mem. pour servir, vol, I. p. 545. — Meyen, Neues System der Pflanz.-Phys,, vol. III.
p. 516 et seq. — Briicke, in Miiller's Archiv fur Anat. und Phys., 1848, p. 434; ditto, in Sitzungs-
berichte der kais. Akad. der Wiss. Wien, vol. L, July 14, 1864. — Hofmeister, Flora, 1852, No, 32 et
seq. — Sachs, Handb, der Exp,-rhys , 1866, p. 479 «/ s^y.— P. Bert, Recherches sur les mouvements
de la seusitive, Paiis 1867.

MECHANISM OF THE MOVEMENTS, 889

another from the fibro-vascular bundle to the middle layers of tissue ; the very small ones
of the outer layers have the appearance of isolated triangular cavities, and when
cut (therefore in the irritated state) seem to be full of water. The cells of the
under side of the 'pulvinus' are thin-walled, those of the upper side have much thicker
walls (about three times as thick) of pure cellulose. Together with a moderate quantity
of protoplasm (including a nucleus) and small chlorophyll-granules and starch, each of
the cells contains in its cavity a large globular drop, consisting, according to Pfeffer,
of a concentrated solution of tannin surrounded by a delicate pellicle \ The young organs
however manifest sensitiveness when the cell-walls of the upper side are not thicker than
those of the other side and the globular drop has not yet made its appearance.

A slight concussion of the whole plant causes the contractile organs of all the
primary petioles to curve downwards, those of the petiolules forwards, those of the
leaflets forwards and upwards. The primary petioles which were previously turned
obliquely upwards then become horizontal or turn obliquely downwards, while the petio-
lules and leaflets close. This condition is identical externally with the nocturnal position
of the leaves, but differs internally, inasmuch as a concussion still acts as a stimulus
in the latter case and causes especially a stronger depression of the primary petiole.
The irritated contractile organ is also flaccid, as Briicke has shown, and is more flexible
than before the irritation, the weight being the same ; in the nocturnal position, on
the contrary, the organ is more rigid and less flexible than in the diurnal. In the
contractile organs of the primary petioles and petiolules a light touch on the hairs
on the under side is sufficient to produce the movement, and in those of the leaflets
the lightest touch on the glabrous upper side. When the temperature is high and
the air very damp, the irritability is much greater, and any local irritation incites
movements in the neighbouring organs, often in all the leaves of a plant, a phenomenon
which has been termed Conduction of the irritation. If, for example, one of the
anterior leaflets is cut off" by a pair of scissors, or its contractile organ is touched, or
if it is placed in the focus of a burning-glass, it takes the position which is normally
the result of irritation, the next lower pair of leaflets then follows, and in succession
those at a greater distance; after a short time the leaflets of an adjoining petiolule
begin to fold together from above upwards, and the same with the other petiolules.
Finally, often after a considerable time, the primary petiole bends downwards; the
phenomenon is then transferred to the primary petiole of the next leaf below, and
also probably to that of the next one above; their petiolules and leaflets taking also
similar positions. Thus in the course of a few minutes all the leaves are set in motion ;
sometimes particular parts are passed by which only begin to move subsequently. The
conduction of the irritation appears to take place more easily from above downwards
than from below upwards, both in the leaves and the stem. If the plant is left to
itself the leaflets and petiolules again expand after a few minutes, the primary petioles
become erect, and the leaves are again irritable.

If the parenchyma of the upper side is cut away as far as the central fibro-vascular
bundle from the large contractile organ of a primary petiole, the petiole afterwards
again becomes erect, and more so than would otherwise be the case; and the organ thus
treated retains a smaller degree of irritability. If, on the other hand, the parenchyma
is removed from the under side, the petiole turns sharply downwards, and its irritability
is destroyed. It follows from this that it is only the under side that is irritable; the
parenchyma of the upper side takes only a subordinate part in the movement, as will be
shown more clearly presently.

If one of the large contractile organs is cut away close to the stem, it curves down-
wards, and a drop of water escapes from it. If it is now split lengthwise through the
central fibro-vascular bundle into an upper and lower half, the former bends still more

^ Similar globular drops are stated by Unger to occur also in Glycyrrhiza and Desmodium
gyrans.

890 PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION,

strongly downwards, while the lower bends only slightly or becomes nearly straight.
These curvatures are still more clearly seen if the two halves are again divided length-
wise by a cut at right angles to the previous one ; the four pieces then also manifest a
small lateral curvature inwards. Again, if the upper and lower parenchyma are separated
from the central fibro-vascular bundle by two cuts lengthwise, the former bends strongly
downwards, the latter slightly upwards; they also increase so much in length as con-
siderably to exceed the central bundle. These and other experiments show that a con-
siderable tension of the parenchyma exists with reference to the central bundle even
when the contractile organ has been stimulated and has lost its water, and that in this
condition the tension is greater between the parenchyma of the upper side of the fibro-
vascular bundle than between the parenchyma of the under side and the bundle.

If one of the contractile organs of the petiole which has been treated in this way is
placed in water, in order to replace the loss of water which has taken place during the
operation and thus to produce a condition similar to the normal one, the downward
curvature of the upper half becomes still stronger, but the under side also curves strongly
upwards, and its tissue, previously flaccid, becomes very tense and almost cartilaginous,
as in the other half. This shows that the turgidity of the parenchyma of the under side
had decreased more than that of the upper side from the loss of water resulting from the
operation, and that it increases also more rapidly when re-absorbing water. In other
words, the irritable under side both gives oft' and re-absorbs water more readily than the
upper side. The upper parenchyma always has a tendency to press the central bundle
downwards ; but the lower parenchyma tends to press it upwards only when it contains
much water ; when therefore the organ contains but little water, it must be bent down-
wards, and can only be bent upwards when the quantity of water in it is larger. It
has already been mentioned that Briicke first discovered the important fact that the
organ becomes flaccid on stimulation ; he pointed out that the angle which a petiole
makes with the stem varies much more, when the position of the stem with reference
to the horizon is altered, if its contractile organ has been stimulated than if the organ
is still irritable and at rest. I drew attention to the fact which had been previously
established by Dutrochet and by Meyen, that the leaves will move and that their
contractile organs will become flaccid if an incision is made so deeply into the stem
that the wood is penetrated, provided that an escape of water is produced by the
wound. This and other facts seemed to indicate that the movement produced by
stimulation is associated with an escape of water from the irritable organ. Pfeffer,
however, first brought forward a complete proof that this was the case, and obtained
a clearer insight into the processes attendant upon stimulation.

He first of all determined, by careful measurements of the same organ in the two
conditions, that the volume of the lower parenchyma which contracts in consequence of
stimulation decreases, while that of the upper parenchyma which lengthens increases ;
but the increase of the upper half is much less than the decrease of the lower. It
follows that the whole organ decreases in volume when irritation causes it to bend
downwards. This decrease of the lower parenchyma is the result of loss of water, as is
shown by the following experiment. After cutting through the contractile organ at
the base of the petiole where the central fibro-vascular bundle is still undivided, the
organ is at first not sensitive (and bent downwards) ; but if the plant is placed in air
saturated with moisture, it again becomes sensitive after a shorter or longer time ; when
irritated, water escapes each time very rapidly from the incision in considerable quantity
when the tissues of the plant are abundantly supphed with water. This water (Pfeffer
has shown that this can be clearly demonstrated by experiment) comes from the paren-
chyma, and almost entirely from that portion which surrounds the central bundle and
contains large intercellular spaces. It is apparent sometimes only beneath and at the
sides of the fibro-vascular bundle, sometimes also above it. Sometimes Pfeffer observed
also the section of the fibro-vascular bundle to exude moisture. When a powerful
stimulation is applied to the under side of an organ from the upper side of which the

MECHANISM OF THE MOVEMENTS. 89 1

parenchyma has been removed, water may sometimes be seen to escape also from the
horizontal cut surface of the parenchyma. It is therefore certain that during the
movement produced by stimulation water escapes from the lower parenchyma ; it gives
off a small portion of it to the upper parenchyma (as is shown by the measurements
that have been quoted), a larger portion flows off at the sides through the intercellular
spaces, and a smaller portion apparently enters the central fibro-vascular bundle. The
whole amount of water that escapes from the lower parenchyma is so small that it
is no doubt at once absorbed by these parts at the moment of irritation.

Since water escapes from the parenchymatous cells of the under side when stimulated,
and passes into the intercellular spaces, the air must be at least partially expelled from
the latter ; and this is evidently the cause of the darker colour of the irritated parts
already observed by Lindsay. PfefFer fixed a petiole in the normal condition so that the
contractile organ could not bend when irritated ; when he touched a point of the irrit-
able side he saw the darker colour spread instantaneously from the point of contact. No
other explanation of this phenomenon is possible than that the air is expelled from the
intercellular spaces and replaced by water, which would cause a smaller amount of light
to be reflected from the interior. The expelled air will collect, in consequence of the
laws of capillarity, in the larger intercellular spaces round the central fibro-vascular
bundle, from which it will easily reach the petiole.

In the diurnal position of the organ slight transverse folds are seen to run along
both sides which after stimulation become more shallow on the upper but deeper on the
under side, showing that the consequent curvature causes a slight passive compression of
the under side. This side first of all contracts in consequence of its loss of water and of
the elasticity of its cell-walls, and then becomes still further compressed by the down-
ward curvature of the upper side.

How it comes about that a slight touch or concussion should cause an escape of
water from the strongly turgid cells of the lower side, followed by an energetic
reabsorption, cannot for the present be explained^. Pfeffer's observations on the
stamens of Cynareae seem to warrant the assumption that the protoplasm of the irritable
cells undergoes a change, in consequence of a touch or of concussion, of such a nature
that it becomes more permeable to water, and that the water which has passed through
the protoplasmic layer simply filters through the cell-wall, which then contracts in
virtue of its elasticity.

The propagation of the stimulus in Mimosa, to which frequent reference has been
made above, has been shown by Pfeffer (Jahrb. f. wiss. Bot. IX. p. 308), in confirmation
of Dutrochet's results and of my own, to be efl^ected by the fibro-vascular bundles.
Since each movement of a leaf produced by stimulation is accompanied by an escape
of water from its parenchyma, the water of the axial bundle and of the bundles
connected with it is set in motion. If, when an incision is made into the wood of a
stem, a drop of water exudes, a movement of the water in the fibro-vascular system
is set up, which aff'ects also that of the axial bundle of the contractile organ and of
the irritable parenchyma.

In the contractile organs of the leaflets of Oxalis Acetosella'- , where the anatomical and
mechanical contrivances are similar to those of Mimosa, this compression is much
stronger, and the under side contracts when the organ is irritated. Pfeffer states
that a decrease in volume also takes place, and since a very considerable elongation of
the upper parenchyma is required for the movements, there must be a more con-
siderable transference of water from the under side. The organs of Oxalis differ
from those of Mimosa in remaining irritable when the intercellular spaces are injected
with water; but when in this state they become flaccid on irritation; it is probable

* [For a discussion of this subject see Vines, The Influence of Light on the Growth of Uni-
cellular Organs, Arb. d. bot. Inst, in Wurzburg, II. i, 1878.]
2 See Sachs, Bot. Zeit. 1857, pi. XIII.

89^ PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION,

therefore that a portion of the water passes from the contractile organ into the tissue
of the petiole and lamina. The depression of the leaves of O. Acetosella and stricta
when sunlight falls suddenly upon them is, like the irritable movements, attended with
flaccidity, and according to Pfeffer, is of the same nature.

(b) The external features of the phenomena of irritability exhibited by the stamens
of Gynaraceae^ in the normal condition have already been described. For a close
examination of them it is necessary to remove single flowers from the capitulum, and to
cut away the corolla from below as far as the point of insertion of the filaments, or to
cut across the corolla-tube, stamens, and style above the insertion of the filaments, and
to fix the reproductive organs which are thus isolated by means of a pin in damp air.
When the filaments have recovered from the irritation caused by this operation, they are
convex outwards. The filaments are flat and strap-shaped ; they consist of three or four
layers of long cylindrical parenchymatous cells, separated by thin straight walls, and sur-
rounded by a layer of epidermal ceils of similar form, strongly cuticularised and growing
out in many places to hairs, each of which is cut off by a longitudinal wall. Intercellular
spaces of considerable size lie between the parenchymatous cells ; through the middle of
the parenchyma passes a delicate fibro-vascular bundle, which, like the epidermis, is
strongly stretched by the turgid parenchyma.

If the flower has been dissected according to the plan first described, and one of the
filaments, curved convexly outwards and fixed below to the corolla, above to the anther-
tube, is touched, it becomes straight and therefore shorter and in contact along its
whole length with the style. If all the filaments are touched, it is seen that they have
considerably decreased in length so as to draw down the anther-tube. After a few
minutes they resume their original length and curvature, and are then again irritable.
If the corolla has been dissected according to the second mode, where the filaments are
cut away and can move freely below, it is easy to see that every time they are touched
a curvature immediately ensues ; if the outer side is touched, it becomes at first concave,
then convex; if the inner side is touched, it becomes concave, and sometimes after-
wards convex. The contraction of the stimulated filament begins at the moment of
contact, after some time reaches its maximum, and the organ then at once begins again
to lengthen, at first quickly, then more slowly.

With regard to the mechanism of these movements, we are in possession of Pfeffer's
most acute observations made for the most part upon the filaments of Cynara Scolymus
and Centaurea jacea. The following is a summary of his most important results.

The filaments are from 4 to 6 mm. long in these species: the tangential diameter
of those oi Cynara is 0*42 mm., the radial 0*2 mm.; in Centaurea the measurements are
0*24 and 0*14 mm. The axial fibro-vascular bundle is thin and delicate. The irritable
parenchymatous cells are in Cynara two or three times, in Centaurea from four to six
times as long as they are broad, and their transverse walls are at right angles to the long
axis. All the walls of the cells, even of those forming the bundle, are thin : only the
external walls of the cells of the epidermis are somewhat thickened. The abundant
cell-sap of the parenchymatous cells is surrounded by a parietal layer of protoplasm
of moderate thickness, in which lies a nucleus. The protoplasm exhibits rotation. Some
tannin and a considerable quantity of glucose is dissolved in the cell-sap.

The filaments are irritable throughout their whole length, that is, they will contract
at any point if touched. Pfeffer succeeded by especial contrivances to magnify the
contractions one or two hundred times. The contraction may amount to from eight
to twenty-two per cent, of the length of the filament when at rest. It is accompanied
by a thickening of the filament, which is, however, too slight to suggest that the
contraction produces merely a change of form; it rather indicates a considerable

^ Cohn, Contractile Gewebe im Pflanzenreich, Breslau 1861 ; ditto, Zeitschrift fiir wiss. Zoologie,
vol. XII. Heft 3.— Kabsch, Bot. Zeit. 1861, No. 4.— Unger, Bot. Zeit. 1862, No. 15, and 1S63,
No. 46. — Pfeffer, Physiologische Untersuchungen, p. 80.

MECHANISM OF THE MOVEMENTS. 893

decrease in bulk. This decrease is due to an escape of water from the cells into the
intercellular spaces, from which it flows, as Pfeffer directly observed, when a filament
is cut across, just as is the case with the organs of Mimoseae. If the intercellular
spaces be injected with water, the filaments remain irritable and on stimulation the
escape of fluid from the cut surface is more evident.

The filaments are very extensible and at the same time very perfectly elastic.
They may be stretched to twice their usual length, and, on being released, they return
to their original dimensions.

When the filament is irritable, the axial bundle and the epidermis are stretched by
the turgid parenchyma, and even after stimulation a tension of the same kind but less
considerable still exists.

The possible assumption that the movement is due to an increase of the elasticity of
the cell-walls under the influence of a stimulus, a contraction of all the cells and an
escape of water from them being the result, is shown by Pfeffer to be incorrect, for
the elasticity of the stimulated and of the unstimulated filament is the same.

The assumption that the permeability of the cell-walls is suddenly increased by
a stimulus and that thus the escape of water is rendered possible, is also shown to be
very improbable. PfeflTer points out that the water filters through under high pressure, and
proves that the ordinary permeability of organic membranes is quite sufficiently great to
permit of the passage of the small quantity of water which escapes from stimulated cells.
It is therefore unnecessary to assume a sudden change in the properties of the cell-wall
which increases its permeability.

Pfeffer overthrows Hofmeister's theory that the escaping water comes not from the
vacuole of the cell but from the cell-wall itself, by the fact that the lateral walls of the
parenchymatous cells become thicker on contraction, and he might have added that on
this theory a contraction of empty cells but not of full tense cells was possible.

After having shown — though without absolute proof — how improbable it is that the
cell-wall undergoes a sudden change in consequence of stimulation, Pfeff'er goes on to
point out how probable it is that some change is produced in the protoplasm which lines
the cell-wall as a closed sac. For a complete discussion I refer the reader to his
exhaustive treatment of the subject; I will only append the following account for
the sake of clearness. It is evident that if permeability of the tense cell-wall remain
unaltered, the escape of water from the cell may depend upon the permeability of the
layer of protoplasm which lines the cell-wall. If it is not permeable, it becomes more
closely pressed to the cell-wall by the increased hydrostatic pressure effected by
endosmosis ; if now any force affects the protoplasm in such a way that the protoplasm
becomes permeable to water, an escape of fluid will take place not only through the
protoplasmic layer, but also through the cell-wall which has already been shown to
be sufficiently permeable. It has now to be shown that the occurrence of such a change
in the protoplasm is possible, and to be explained why it is that this suddenly increased
permeability of the protoplasm ceases after the movement, a fact which is essential to
the restoration of the irritability. On these points I would refer the reader to the
explanations given in PfefFer's work; I would only add that such changes in the
permeability of protoplasm as are here assumed are already known to occur. When the
protoplasm of a cell of Spirogyra contracts before conjugation, it must necessarily become
more permeable, for most of the water escapes from it ; this escape does not take place
when the cell is turgid and actively growing. If the cell-wall of the conjugating cell
were very tense and if it were at the same time very elastic, it would contract simul-
taneously with the protoplasm, and would permit of the escape of the water through it.
As a matter of fact the cell-wall of Spirogyra is not very tense, and it is rigid, so that
it does not materially alter its form when the protoplasm contracts ; the water which
escapes through the protoplasm therefore occupies the space between it and the cell-wall.
It may be objected that this contraction preparatory to the conjugation of the cell of
Spirogyra is not the result of the action of an external stimulus ; this is quite true, but

894 PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION.

it serves to prove that an alteration of the permeability of protoplasm is possible.
Moreover it is known that the protoplasm of many cells contracts when they are
subjected to pressure from without, and this is only possible if an escape of water takes
place. If, under these circumstances, the cell-wall remains fixed, it is because it was but
slightly stretched and comparatively inextensible before. If the wall of a cell which has
been so treated were to contract as considerably as the protoplasm, we should have
a result similar to that which was assumed above with Veference to a stimulus. These
considerations suggest that the irritability, in its narrowest sense, is a property of the
protoplasm only, and that it is essential to the existence of irritable organs that the
irritable protoplasm should be surrounded by cell-walls which are tense in consequence
of the turgidity of the cells and which can follow the contractions of the protoplasm in
virtue of their perfect elasticity.

(c) The irritable stamens oi Berberis^ differ considerably in the mechanism of their
movements from those of Cynareae, especially in that they are irritable not upon the outer
side but upon the inner side only ; further, the irritable parenchyma includes no inter-
cellular spaces, but between the thin-walled cells there is a quantity of * intercellular
substance' which is capable of swelling. If the inner side of the filament be touched,
it curves throughout its whole length. Pftflfer was able to observe in this case also that
when the filament is cut across, stimulation causes the escape of a drop of water from the
cut surface.

(d) Too little is at present known of the mechanism of the irritable gynostemium of
Stylidium, of the leaves of Dionata muscipula, of the glands upon the leaves of Drosera, of
the segments of the stigma of Mimulus, &c., to admit of brief yet satisfactory
treatment ^.

Sect. 30.— Mechanism of the Movements produced by Variations of
Temperature and of Light ^. If plants with motile leaves, like Papilionacese and
Oxalideae, after having remained in the light, are suddenly placed in the dark, the
leaves after some time take up their nocturnal position, closing upwards or down-
wards according to the species (Sect. 28). If light is now let in upon the plant
in the state of sleep, the leaves again open and assume their diurnal position.
Placing them in the shade has the same effect as complete darkness, but not so
strongly.

These facts show that fluctuations in the intensity of the light cause curvatures
of the motile parts of plants. If these parts are also irritable to concussion, as in
Mimosa and Oxalis acetosella, darkness causes a similar position of the leaves to
concussion. But the internal conditions are, as has been mentioned, very different
in the two cases ; for the folding up caused by darkness is associated with an increase

* linger, Anat. und Physiol, der Pflanze, 1855, p. 419. — Kabsch, Bot. Zeit., i86i,p. 26, — Pfeffer,
Physiol. Unters., p. 157.

^ See also Unger, Anat. u. Phys., 1855, p. 419 Suringar (on Z)rosfrfl), Vereenigung voor de

Flora van Nederland eng. den 15 Juli, 1853. — Nitschke (on Drosera), Bot. Zeit. i860. — Schnetzler (on
Berberis), in Bulletin de la Societe vaudoise des Sci. Nat. X, 1869. — Kabsch (on Berheris, Mitnulus,
Sec), Bot. Zeit. 1861.— Kabsch (on Stylidium), Bot. Zeit. 1861.— A. W. Bennett, The Movements
of the Glands oi Drosera, Quart. Journ. Micr. Sci. 1873.

[See also Kurz, Anatomic des Blattes der Dioncea muscipula ; Du Bois Reymond's Archiv, 1876 :
also M unk, Die elektrischen und Bewegungs-Erscheinungen am Blatte der Dioncea Muscipula, ibid. ;
further, Darwin, Insectivorous Plants, 1875.]

^ Dutrochet, Mem. pour servir, vol. I. p. 509. — Meyen, Neues Syst. der Pflanz.-Phys. vol. III.
p. 487. — Sachs, Bot. Zeit. 1857, Nos. 46, 47. — Bert, Recherches sur les mouvements de la sensitive,
Paris 1867. — Millardet, Nouvelles recherches sur la periodicite de la sensitive, Marburg 1869. —
Pfeffer, Physiol. Unters., Leipzig 1873.

MECHANISM OF SPONTANEOUS PERIODIC MOVEMENTS. 895

in the rigidity of the part, and therefore with an increase in its turgidity; while
in that caused by irritation there is a decrease in all these, as Briicke was the first to
show in the case of Mimosa. In the leaves of Phaseolus which are not irritable
to concussion Pfeffer also found an increase of rigidity in the nocturnal position.
Conversely the diurnal position caused by the action of light or an increase in its
intensity is the result of a diminution of the rigidity or turgidity. The effect pro-
duced upon the turgidity of motile organs by variations in the intensity of light
causes upward and downward curvatures, since the variation of the turgidity of the
one side of the organ is more considerable and occurs more rapidly than that of
the other. A rise of temperature, on the contrary, which affects the motile part
directly, is, according to Pfeffer, associated, in Oxalis, and in a less degree in
Phaseolus^ with increase of rigidity, and therefore also of turgidity, and causes a
movement towards the nocturnal position, and hence a stronger turgidity of the
upper side. When, on the other hand, an increase in the intensity of the light and
a rise of temperature act on a contractile organ at the same time, its curvature is
a resultant of the two changes ; according as the one or the other preponderates,
the leaf approaches more nearly the diurnal or the nocturnal position. Beyond this
we have less certain knowledge as to the action of variations of temperature, than
we have with reference to light.

Pfeffer, who has been especially engaged for a considerable time in the study of the
mechanism of the movements produced by stimulation, has supplied me with the follow-
ing : — ' The tendency to expand is increased by darkness equally in both the antagonistic
halves, and in the tissues of the organ generally : light has the contrary effect, and the one
half always reacts more powerfully than the other. The expansive force increases more
rapidly in the half which is becoming convex, but it may become more considerable in
the other half: it is for this reason that every movement produced by removal into
darkness is followed after a time by a movement in the opposite direction, tending to the
resumption of the position of equilibrium. It is quite certain that darkness not merely
produces a closure but that it has a persistent effect, just as the movement imparted to
a pendulum persists for a time whilst the amplitude of the oscillations is rapidly
diminishing.'

Bert ^ showed that if a Mimosa be continuously exposed to light for five days, the
amplitude of its periodic movements diminished considerably, whereas the irritability
increased. Pfeffer also found that continuous illumination for a period of one or
more days arrested the daily periodic movements of Acacia lophantha. If a plant so
treated be placed in the dark, closure takes place and then opening, and in continuous
darkness for one or more days opening and closing repeatedly alternate. When the
plant had been exposed to a strong light before being placed in the dark, the interval
between the movements of opening and closing was about 24 hours. This is also the
case if the plant be placed in the dark in the morning, so that the first closure takes place
during the day. Pfeffer regards these phenomena (like those of motile flowers and
of growing leaves) to be due to the persistent effect of the previous alternation of day
and night; they are not to be confounded with the spontaneous periodic movements
of these plants, for these latter continue when the former have disappeared, and the
interval at which they occur is shorter.

Sect. 31. — Mechanism of spontaneous periodic Movements. The ex-
istence of spontaneous periodic movements which are not directly produced by the

1 Bert, Mem. de I'Acad. d. Sci. Phys. et Nat. de Bordeaux, 1866, Bd. VIII.

Sg6 PHENOMENA OF SEXUAL REPRODUCTION.

action of external influences has been already mentioned in Sect. 28, the move-
ment of the lateral leaflets of Hedysarum gyrans being cited as a striking example.
The rapid upward and downward movement of these leaflets take place, if the
temperature be sufficiently high, both in continuous illumination and in continued
darkness. The spontaneous movements of the leaflets of Oxalis acetosella and of
Trifolium praiense are also very evident ; the leaflets move in continued darkness and
at a constant temperature through an angle of from 30° to 90° in from i to 4 hours.

It was pointed out in the foregoing paragraph that these spontaneous move-
ments are independent of the persistent effect of the stimulating influence of light.
Pfeff"er clearly made this out in the case of the large terminal leaflet of Hedysarum
{Desmodiuni) gyrans, which makes upward and downward movements of small
amplitude in short periods of time.. Whilst the daily periodicity (Sect. 30) is
manifested only for a short time in prolonged darkness, the spontaneous movements
continue. In Trifolium and Oxalis the persistence of the daily periodicity is very
slight, whereas the spontaneous movements are very evident in prolonged darkness.

It can scarcely be doubted, after what has been already said with reference
to the various movements of growing and of mature organs, that the spontaneous
periodic movements are eff"ected by variations of turgidity, that is by the absorption
and the escape of water. Since they are not accompanied, as Pfefl"er has observed,
by any alteration of the rigidity of the organ, it is probable that one half of the
tissue gains in expansive force what the other loses and at the same moment ; this
almost amounts to saying that first one and then the other half of the tissue absorbs
water from the other half.

Batalin (Flora, 1873) asserts that each periodic movement in Mimosa and Phaseolus
is accompanied by a slight increase in length. Simple calculation suffices, however, to
show that the periodic movements of these organs cannot be due to periodic growth of
the upper and under surfaces.

The question why it is that the *two sides of the organ are alternately more or less
strongly turgid cannot be answered at present any more than the question why it is that
first one side and then the other grows more rapidly in growing leaves, stems, tendrils,
&c. which exhibit nutation.

CHAPTER VI.

THE PHENOMENA OF SEXUAL REPRODUCTION.

Sect. 32^. Sexuality consists essentially in the formation, in the course of
development of the plant, of reproductive cells of two diff'erent kinds, which have no
independent power of further development, but which, by their coalescence, give
rise to a product which possesses that power.

^ The facts upon which the considerations contained in this section are based are fully detailed
in Book 11, with references to the literature.

THE NATURE OF SEX UAL I TV. 897

It is only in a comparatively small number of cases, and in plants of very
simple structure, like the Desmidiese, Mesocarpeae, and Pandorinese, that the two
cells which coalesce are alike in their mode of production, size, form, and behaviour
when coalescing ; and even in these cases they probably differ internally, since it is
difficult to explain on any other hypothesis the necessity for their union into a pro^
duct capable of development (the Zygospore). In some other Conjugatse, as Spiro-
gyra, this internal differentiation is exhibited at least to the extent that the contents
of one of the conjugating cells pass over into the other, the contents of which remain
stationary. But usually, as in most Algae ( Vaucherta, CEdogonium, ColeochcBte, Fucus^
&c.), and in all Characese, Muscinese, and Vascular Cryptogams, a great variety
of differences are manifested between the sexual cells as to size, form, motility,
mode of production, and the share they take in the formation of the product of
the union. This differentiation presents, especially in the Algge, a most complete
series of gradations between the conjugation of similar cells and the fertilisation
of oospheres by antherozoids, any boundary line between these two processes
being unnatural and artificial. The difference also between the sexual cells is de-
veloped only gradually and step by step, like the external and internal differentiation
of plants ; and it is this that renders it probable that in the lowest forms of the
vegetable kingdom, as in the Nostocaceae, no process at all of this kind exists, or
that at all events there are plants of extremely simple structure in which no such
process occurs* ;

Wherever there is an evident external difference between the two sexual cells,
one behaves actively in the union, and loses in the process its individual existence,
the other behaves passively, absorbing into itself the substance of the active one, and
furnishing by far the larger proportion of the first materials for the formation of tha
immediate product of the union. The former is termed the male cell or antherozoidi
the latter ihQ/emale cell or oosphere.

These most essential features of the sexual process may also be recognised in
the fertilisation of the Ascomycetes and Florideae, although the external appearance
of the female organ, the carpogonium, on the one hand and of the male organ
(in certain Ascomycetes at least) on the other hand, are strikingly different from
those which occur in any other class of plants. '

The usual condition of the female cell during the sexual process (except in the
Ascomycetes and Florideae) is that of a naked primordial cell (oosphere), formed
either by simple contraction of the protoplasm of a cell previously enclosed within a
cell- wall (as in the oogonium of Vaucheria, (Edogomum, and ColeochcBle, and in the
archegonium of Muscineae, Vascular Cryptogams, and Gymnosperms) or by the
division of the protoplasm of a mother-cell combined with contraction and rounding
off of the daughter-cells (as in Saprolegnia and Fucaceae), or by free cell-formatiori
(as in the embryo-sac of Angiosperms). In all these cases the oosphere is spherical
or ellipsoidal, except that in the Angiosperms it is sometimes elongated ; in general
its form is the simplest that the vegetable cell can assume. The rounding off is not
connected with any internal differentiation ; at least where any internal differentiation
is exhibited (as in the formation of chlorophyll and the granular contents in CEdo-
gonium and other Algae), the phenomenon is a secondary one in the process of
fertilisation. The oosphere is never actively motile, even when, as in the Fucaceae,

3 M

898 PHENOMENA OF SEXUAL REPRODUCTION.

after it is extruded it is set in rotation by the attached antherozoids ; it usually
remains enclosed in the mother-cell that produces it (the oogonium of Algae and
Fungi, the archegonium of Muscineae, Vascular Cryptogams, and Gymnosperms,
and the embryo-sac of Angiosperms), where it awaits fertilisation. While the male
cell loses during the union its character as an individual cell, the oosphere is
rendered capable of a more complete individual existence, which is first indicated
by the invariable formation of a wall of cellulose, even when the oosphere results
simply from the contraction of the protoplasm of an oogonium and still remains
enclosed in its cell-wall, as in (Edogonium and Vaucheria, In this respect the
zygospore of Conjugatae and Mucorini behaves also like a fertilised oosphere
or oospore.

The male cell is more variable in its form and in its behaviour in the process
of fertilisation. It always moves to the oosphere which remains at rest ; in the
Florideae it is carried passively by the water ; in the Fucaceae, in Vaucheria, (Edogo-
nium, and other Algae, in all Characeae, Muscineae, and Vascular Cryptogams, it
swims actively. In other cases the male organ becomes attached in its growth to
the female organ, as in the antheridial branches (pollinodia) of some Saprolegnieae
and of some Ascomycetes, and the pollen-tube of Phanerogams. The great variety of
fortn of the male cell becomes especially conspicuous if we compare the roundish
swarm-spore-like antherozoids of (Edogonium and Coleochcete with the filiform anther-
ozoids of Characeae, Muscineae, and Vascular Cryptogams, and with the rounded
non-motile antherozoids of the Florideae which (like the spermatia of Lichens) possess
a cell-wall. The form is in each case evidently adapted to produce the right
kind of motion in order to convey the fertilising substance to the female organ in
a manner in harmony with its structure ; while in the fertilisation of the latter the
quality of the substance only is concerned.

According to the present state of our knowledge it may be assumed that
fertilisation essentially consists in a union of protoplasm and nuclear substance
derived from the male organ with protoplasm and nuclear substance of the female
organ. In conjugation this union is brought about by the coalescence of the two
conjugating cells. In the fertilisation of (Edogonium and Vaucheria, the entrance
of the antherozoid into the protoplasm of the oosphere and its absorption in it
has been observed by Pringsheim. The antherozoids of Muscineae and Ferns
were observed by Hofmeister, and those of Marsilia by Hanstein, to enter the
archegonium, those of Ferns by Strasburger to penetrate to the oosphere itself.
It must therefore be inferred from analogy that in Phanerogams a union ^ takes
place of some substance contained in the pollen-tube with the oosphere ; and in
certain Ascomycetes of the contents of the poUinodium - with those of the asco-
gonium. It would be impossible otherwise to explain how in these cases the
mere contact of the often thick-walled pollen-tube with the embryo-sac, or of
the pollinodium with the ascogonium, can effect fertilisation, while in the former
cases such a complete coalescence of the male and female cells is necessary for
this purpose.

* [See pp. 524 and 584. J

THE NATURE OF SEXUALITY. 899

The product resulting from the sexual process is usually a new individual which
has no longer any organic connexion with the mother-plant and is not united with
it in growth. This is the case in the Muscineae, where the sporogonium, and in
Phanerogams, where the embryo, is nourished by the mother-plant, but there is no
actual continuity of tissue between it and the latter. The case is quite different in
the Ascomycetes {e. g. Lichens, Eurotium, and Erysiphe) and Floridese, in which the
female organ itself or certain cells connected with it are stimulated by fertilisation
to produce new shoots from which results a fructification containing spores ; and it is
only after the completion of this complicated vegetative process brought about by the
sexual union that the spores are set free, and produce new individuals independent
of the mother-plant.

The reproductive cells of the same plant do not differ merely externally ; the
inability of either to originate by itself a new course of development, while the two
together produce an organism capable of germinating, shows that the properties of
the two are complementary to one another. The sexual differentiation, or difference
between the male and female cells, which is neutralised by the act of fertilisation,
has been preparing for a longer or shorter time ; the product which is the result of
fertilisation owes its formation to the neutralising of the sexual difference. In the
Conjugatae and other families where the sexual difference is extremely small or even
imperceptible, the preceding processes of development are also alike; the mother-
cells of the two kinds of reproductive cells even to the earliest stage of development
do not differ externally. But where the sexual difference is greater, it is fore-
shadowed in the preceding processes of development. Thus the mother-cell of the
antherozoids of (Edogontum differs in form from that of the oosphere ; and this is
especially seen in the development of the (Edogoniese with ' dwarf males.' In
Vaucheria the branches which subsequently become antheridia differ at an early
stage from those which form the oogonia. The sexual differentiation of the Cha-
raceae is inaugurated long beforehand in the great difference in the development of
the antheridia and carpogonia, the position of the two organs on the leaf being also
different. In the Muscineae and Vascular Cryptogams again preparation is made
for the production of the antherozoids and oospheres in different ways by the
formation of antheridia and archegonia. But this preparation is not confined to
the difference between the organs which immediately produce the reproductive
cells ; in many classes of plants it even goes back so far that the entire plant
developes as a male or as a female plant, producing only male or only female
reproductive organs. This occurs in some Algae, Characeae, Muscineae, and in
the prothallia of some Vascular Cryptogams.

The fact is very remarkable that this preparation may be carried back in the
development of the individual even beyond the limit marked by the alternation of
generations. In the Algae, Characeae, Muscineae, Ferns, and Equisetaceae, the nature
of the alternation of generations is such that the sexual differentiation is developed
in one of the generations, while it is neutralised in the succeeding generation. In
these cases therefore we have a sexual and an asexual generation in the course of
the development of the same individual ; the asexual generation is the product
of the neutralising of the sexual differentiation of the sexual generation. The two
generations, especially in Muscineae and Vascular Cryptogams, differ essentially from

3M2

90p PHENOMENA OF SEXUAL REPRODUCTION.

ii morphological point of view ; they follow altogether different laws of development ;
one of their limits always occurs in the fertilised oosphere. The prothallium de-
veloped from the asexual spore of Ferns and Equisetaceae is, for example, morpho-
logically a thallus without leaves or roots, while its physiological significance is
determined by the production of antheridia and archegonia. From the fertilised
oosphere on the other hand is produced the Fern or Horsetail, characterised
morphologically by the differentiation of stem, root, and leaf; but sexually this
differentiated plant is neuter, producing neither male nor female cells, but only
^sexual spores. If the process of development of Rhizocarpeae and Selaginelleae is
compared with these phenomena, it will be seen that in these classes the two genera-
tions, the prothallium and the spore-forming leafy plant, stand essentially in the same
;"elation to one another as in Ferns and Equisetaceae, only that the sexual differentia-
tion goes back to the spore itself; the spores are of two kinds, large female spores
•which produce the small female prothallium, and small male spores which produce
a still smaller prothallium and antherozoids. The preparation for this sexual difference
is manifested even in the asexual generation, by the sporangia producing only female
or only male spores according to their position. In Salvinia the preparation goes
back still further, each entire capsule producing only female or only male sporangia.
It has already been pointed out how in Phanerogams the embryo-sac corresponds to
the large, the pollen-grain to the small spore of heterosporous Vascular Cryptogams,
and the endosperm to the prothallium. The endosperm of Phanerogams no longer
appears as an independent structure, but only as a constituent part of the preceding
generation; in Angiosperms it is often from the first rudimentary and sometimes
entirely absent, and the female sexual cell, the oosphere, is then the immediate pro-
duct of the embryo-sac which corresponds to the large spore. The true sexual
generation therefore becomes less and less important ; as such it becomes devoid of
significance, while the sexual differentiation is carried back to the spore-forming
generation, in which it determines the formation of the two kinds of reproductive
organs, i. e. the pollen-sacs and ovules ; the flower may be exclusively male or
female (monoecious diclinous), and, where the plant is dioecious, the sexual differ-
entiation affects the entire individual, which is either male or female. In all Crypto-
gams, on the other hand, dioecism is only displayed in one (the sexual) generation
jn the course of development of the individual.

The process of development brought about by fertilisation or the union of the
reproductive cells is usually not confined to the resulting embryo, but shows itself
also in a variety of changes in the mother-plant itself. In Coleochcete the oospore
becomes invested with a cortical layer; in Characeae the enveloping tubes of the
jcarpogonium grow after fertilisation, their coils increase in number, and their mem-
-branes become lignified on the inside ; in the Hepaticae a variety of envelopes arise
from the mother-plant ; in the Mosses the vaginule and in all Muscineae the calyptra
becomes developed ; the tissue of the prothallium which surrounds the growing
embryo of Ferns grows at first rapidly along with it ; in Phanerogams the entire
ndeyeiopment of the seed and fruit depends on the changes caused in the mother-
plant by the fertilisation of the oosphere. The two most remarkable cases occur in
Florideae and Ascomycetes on the one hand, and in Orchideae on the other hand.
In the former fertilisation does not in general directly cause the formation of an

THE NATURE OF SEXUALITY. 901

embryo, but brings about processes of growth in the mother-plant, in consequence of
which the cystocarp is produced in Floridese and the spore-fruit in Ascomycetes. In
the Orchideae the action of the pollen-tube is visible on the mother-plant even before
fertilisation; Hildebrand has shown (Bot. Zeit. 1863, p. 341) that in all Orchids
which he examined the ovules were not in a condition to be fertilised at the time of
pollination ; and in some (as Dendrobium nobile) they have not even begun to be
formed ; it is only during the growth of the pollen-tubes through the tissue of the
stigma and style that the ovules become so far developed that fertilisation can at
length be effected. In the Orchideae the formation of the female cell is therefore a
result of pollination; it is determined by the action of the male pollen- tube on the
tissue of the mother-plant^

When the embryo is being developed within the mother-plant, as in the Muscineae
and Vascular Cryptogams, it obtains its food-material from the plant; and this is
connected in the Vascular Cryptogams with complete exhaustion and the dying off
of the prothallium. In Phanerogams not only does the embryo usually acquire a
considerable development, even within the fruit, but a great quantity of the pro-
ducts of assimilation is also withdrawn from the plant by the accumulation of
reserve-material in the seed and by the development of the fruit; in many cases
the plant itself is also completely exhausted, all its disposable formative sub-
stances are given up to the seed and the fruit, and it dies off (monocarpous
plants). It is clear that all these changes and the various movements of materials
in the mother-plant connected with them are results of fertilisation, results of
immense importance caused by the union of microscopic cells, imponderable by
the best balance.

(a) A careful consideration of the phenomena occurring among the Thallophytes
would probably lead to the formation of a tolerably clear idea of the mode of the
Development of Sexuality in the Vegetable Kingdom. The space at our disposal will
only suffice for a few general remarks.

The labours of Pringsheim^, which open up the way for a complete theory of
sexuality in the future, seem to indicate that the conjugation of the motile cells of
Pandorina, Ulothrix, &c. is one of the primitive phases of a sexual act. If this be so,
then it follows that a sexual coalescence of cells first made its appearance when the
Thallophytes had already attained a considerable degree of morphological and of phy-
siological development. The same result is reached by a consideration of the fact
that sexuality is apparently absent in the Hydrodictyese^ and in the immediate allies
of Ulothrix, and that comparatively highly-developed forms, such as the Rivularieae,
exist among the Protophyta in which no trace of sexuality can be discovered.

If it be also remembered that the simplest forms of sexuality, conjugation and the
formation of zygospores, occur in very different groups of the Thallophytes, and that
the mode of conjugation varies with the form and habit of the plants and that it may
be very different in different groups, the thought is at once suggested that the sexual
coalescence of cells may have commenced at different times and quite independently in

' [For a summary of the instances in which pollen appears to have influenced the fruit of the-
mother-plant, see C. J. Maximowicz, Journ. Roy. Hort. Soc, new series, vol. III. p. 161 ; ^nd Darwin,^
Animals and Plants under Domestication, vol. I. p. 397.]

^ Monatsber. d. k. Akad. der Wiss. in Berlin, 1869.

5> [Conjugation has since been observed in Hydrodiciyon (see page 251).]

90a PHENOMENA OF SEXUAL REPRODUCTION.

different groups of Thallophytes. The variety of the modes in which the auxospores are
formed among the Diatomeae indicates that conjugation has become developed here from
the first quite independently of the connection of this group with the Conjugatae; and
in the latter group so many modes of the formation of zygospores occur that possibly
the development of sexuality began in several different species included within it. The
mode in which conjugation takes place among the Zygomycetes seems to indicate that
the sexual act originated independently, and it is still difficult to trace a historical
connection between the fertilisation of the Ascomycetes and of the Florideae and that
of any other group of Thallophytes. It is more feasible, as Pringsheim has already
suggested, to regard the fertilisation of the CEdogonieae and of the Vaucheriaceae as
being a further development of the conjugation of two motile cells. If, as appears
to be the case, a sexual act originated at different times in different Thallophytes, a
series of further developments should be found to correspond to each distinct origin,
but at present it is impossible to say from which of these origins the formation of the
archegonia of Mosses and of Vascular Cryptogams and that of the reproductive organs of
Phanerogams has been evolved.

The question arises with reference to the simplest phases of sexuality, as to whether
conjugation was effected primarily in consequence of a sexual differentiation of the cells,
or whether conjugation preceded any sexual differentiation and that this only made its
appearance as a secondary phenomenon when plant-forms had become more highly deve-
loped ^ The former of these two alternatives is supported by those phenomena which
indicate a mutual action at a distance of the conjugating cells and which have been fre-
quently mentioned by observers as a sort of mutual search. The latter is borne out by the
consideration that the conjugating processes are developed from those points only of the
mycelium, in Zygomycetes and in many Ascomycetes, at which they are in contact. It
is known that similar, but quite infertile, connections occur between mycelial filaments,
and there are good grounds for believing (Sect. i6) that these phenomena of growth are
induced by pressure ; and therefore it may be reasonably inferred that the growth of the
conjugating filaments and the formation of zygospores is a further development of the
sterile coalescence of mycelial filaments (in the form of the letter H) which is simply the
result of pressure. If it be admitted that the sexual coalescence of cells originated at
diflferent times and in different plant-forms, it may also be admitted that in one case
a sexual diflferentiation first took place which rendered a coalescence necessary, while in
other cases the processes of growth initiated by pressure resulted in the developing of
conjugating organs which were sexually differentiated.

(b) Parthenogenesis '^ is the term used to express the fact that plants which possess
normal male organs of fertilisation and in which embryos are developed by the fer-
tilisation of the oospheres may occasionally develope embryos from female cells which
have not been fertilised, but which are nevertheless capable of complete development.
This phenomenon, which is of frequent occurrence in the Animal Kingdom, especially
among Insects, has been satisfactorily observed in only a few cases among plants. The
doubts as to the parthenogenesis of the Calebogyne ilicifolia which is cultivated in Europe
still exists. It appears, however, that Chara crinita is represented in certain places
by the female form only, and that nevertheless it bears an enormous number of spores
which are capable of germination. The most satisfactory cases of parthenogenesis
are those of Saprolegnia ferax and Achlya polyandra ^. Pringsheim has shown that
their oospheres are usually fertilised, but that frequently they germinate and develope

^ [From the fact that, as in Ulothrix for instance, the microzoogonidia come to rest and
germinate as well withont as with previous conjugation, it appears probable that the latter is the
correct view.]

^ Braun, Die Parthenogenesis bei Pflanzen, in den Abhandl. der Berl. Akad. 1856.— Pringsheim,
Jahrb. f. Wiss. Bot. IX. [See page 593.]

^ [From de Bary's researches it appears that parthenogenesis is the rule in the Saprolegnia;.]

THE NATURE OF SEXUALITV. 903

new plants without fertilisation and without any difference in the mode of their germination
from that of true oospores, with this single exception, that the quiescent period of the
parthenogenetic cells is shorter.

Pringsheim's account of the development of the parthenogenetic forms of these
plants which is appended here is of great interest : — ' The successive generations both
of Saprolegnia ferax and of Achlya polyandra produced by cultivation become smaller,
and the number of male filaments diminishes in each succeeding generation until they
finally cease to be formed, and thus the monoecious forms become replaced by purely
female ones.' These observations show that as the result of continued cultivation combined
with the action of certain unfavourable conditions which accompany every attempt at
cultivation, the formation of male sexual organs at length ceases. Possibly it is in
consequence of the action of similar adverse conditions upon its internal constitution
that Chara cr'mita ceases to form antheridia after having grown for a considerable time
in certain waters. Possibly also these internal disturbances may affect the nature of the
female cells, though they are developed in the usual external form, so that they are not
sexually differentiated or only imperfectly so. In this case the effect would be one of
the obliteration of the existing sexual differentiation, or in other words, a case of
retrograde metamorphosis, and this is quite as conceivable as the first origin and the subse-
quent development of sexuality. In future investigations of the subject attention ought
to be paid to the question whether the oospheres developed upon plants of Saprolegnia
ferax and of Achlya polyandra which bear antheridia also are capable of parthenogenetic
development, or whether this property belongs only to oospheres developed by plants
destitute of antheridia ^. However difficult it may be to answer this question by experi-
ment, it must be done before it can be possible to decide whether or not the development
of the male organs deprives the oospheres of their power of independent development, so
that in proportion as the development of the male element diminishes the partheno-
genetic property increases. Since we may assume that the essential object of fertilisation
is to give to the oosphere something which it lacks but which is necessary for its further
development, a parthenogenetic oosphere must possess, independently of fertilisation, that
which it requires for its further development, that is, it is not sexually differentiated, and
this probably because the differentiation of the male element has been suppressed.

(c) The Effects of Sexual Coalescence. Since nearly all plants, and more especially
the majority of Thallophytes, are capable of reproducing themselves asexually, and since
this' is the usual mode of reproduction in many species, it may well be asked what the
significance of sexual reproduction really is. If sexuality is merely concerned in the
development of new individuals, it is difficult to understand why the asexual reproduction
should not suffice. This question is of especial interest with reference to those lower
forms of Thallophytes which reproduce themselves through many generations by asexual
cells which may be either motile or non-motile : with reference to many Phanerogams,
for instance, the Conifers, it seems as if without sexuality, which induces the formation
of the seed, no reproduction would be possible.

The significance of sexuality is seen in quite another light when those plants are
considered which exhibit a distinct alternation of generations, such as the Ferns,
Equisetaceae, the Mosses, and others. In Sect. 29 of Book I, I endeavoured to show
that the alternation of generations, wherever it occurs in the Vegetable Kingdom,
is produced by sexuality, and that without it no such alternation is possible. In all
cases of well-marked alternation of generations an organism which finally bears sexual
organs is developed from a spore which has been produced asexually; fertilisation
initiates a new process of development which closes with the development of the spore.
Before fertilisation, there is merely the organism developed from the spore, the first
or sexual generation : after fertilisation the second or asexual generation is developed

^ [According to de Bary's recent investigations, the former of these two answers to the question
is the correct one.]

904 PHENOMENA OF SEXUAL REPRODUCTION.

M'hich bears spores. If we compare the histological and the morphological development
: of the two generations, it becomes strikingly evident, among the Vascular Cryptogams
at least, that the generation which has been developed as the result of fertilisation
is much more highly organised than the generation (prothallium) which has been
developed from the spore. In the Mosses it might appear that the contrary is the
^ case, for in these plants the sexual generation is the one which grows independently and
which is differentiated into leaf and stem ; however, the histological differentiation of the
sporogonium is far more perfect than that of the moss-plant, so that it is true for Mosses
also that the product of fertilisation is the more highly organised of the two generations.
All these cases of evident alternation of generations lead to the conclusion that
a process of development of a more complex kind is initiated by the sexual act.
To a certain extent this is true also of the Zygomycetes, the zygospore being a more
highly organised cell than any of those of the mycelium, and it can scarcely be doubted
with reference to the Coleochaeteae, the Gharaceae, and the Florideae that the sexually-
formed spore-fruit is histologically the most complex product of these plants. Although
the same cannot be said with reference to the Oosporese and to the Conjugatae, still this
by no means affects the significance of the sexual act for other plants. A complete
discussion of the facts would, on the contrary, probably show that the higher develop-
ment of the Pandorineae, of the Conjugatae, and of the Diatomaceae when compared with
the Protophyta has been probably promoted by the evolution of sexuality, even though
this is not expressed by a well-marked alternation of generations. The Phanerogams
afford a similar, though exactly opposite case : in them the alternation of generations is
exhibited in only a rudimentary form, for, in the course of the development of these
plants from some primitive type allied to the Vascular Cryptogams the sexual generation
(prothallium) has been reduced to its simplest expression. Whereas in the Oosporeae
the sexual generation is the predominating one and the product of fertilisation is but
imperfectly developed, in Phanerogams it is the generation produced in consequence
of a sexual act which comes to be completely developed, and it is the sexual generation
(Prothallium, Endosperm) which is rudimentary. In the latter case we have the end, in
the former the beginning of phytogenetic series ; in the latter the alternation of genera-
tions is disappearing, in the former it is in the first stage of its evolution. If therefore
we desire to understand the significance of sexuality in the history of the development
of a single plant or in that of the whole Vegetable Kingdom, we must fix our attention
upon those groups in which an alternation of generation is evident: in such 'cases
(Vascular Cryptogams, Muscineae) the effect of sexuality is obvious. We may then con-
clude that the coalescence of the male with the female cell causes the development of an
organism which is more highly differentiated both histologically and morphologically.

Sect. 33. Influence of the origin of the reproductive cells on the
product of fertilisation. The male and female cells or the organs that produce
them are formed at a greater or lesser distance from one another on the same
plant, or on different individuals of the same species. The male and female cells
of the same species may thus be more or less nearly related to one another as
having been immediately or more remotely derived from the same parent-cell. The
question arises what influence this genetic relationship of the male and female cells
exercises on the product of fertilisation. At present we are unable to lay down any
general law in this respect ; but the overwhelming weight of evidence points to the
law that the sexual union of nearly related cells is detrimental to the preservation
of the plant, and in general the more so the further the morphological and sexual
differentiation of the species has advanced. Only in a few plants of low organ-
isation does a fertile union take place between sister-cells, as in Rhynchone7na
among Conjugatse. But in most Algae and Fungi (as Spirogyra, (EdogotiiuTn,

INFLUENCE OF THE ORIGIN OF THE CELLS ON FERTILISATION, 905

''ucus platycarpus, &c.) the reproductive cells of the same plant are not so closely
related, and especially where fertilisation is caused by actively or passively motile
Jantherozoids, there being at least a possibility of their meeting with oospheres of
pnore remote origin. Even in Vaucherta, where the antheridium is the sister-cell of
|the oogonium, the curving of the former, and the direction in which the antherozoids
jscape, indicates that fertilisation does not usually take place between the contiguous
organs, but between those more remote or even between those produced by different
individuals. The tendency for fertilisation to occur only between reproductive cells
of as remote relationship as possible within the same species is manifested in a great
variety of contrivances, the simplest being that on each individual of the sexual
generation only male or only female organs are produced. Thus between the two
uniting reproductive cells there lies the entire course of development of the two
plants when the plants are derived from the same mother-plant, and a still longer
course of development when they are derived from different mother-plants. This
distribution of the sexes, which is generally termed Dicecism, occurs in all classes
and orders of the vegetable kingdom, showing that it is a useful contrivance for the
maintenance of different species. Thus we find this phenomenon in many Algae, as
in most Fucaceae, in some Saprolegniese and Characeae {Nitella syncarpa^ &c.), in
many Muscineae, in the prothallium of many Ferns {Osmunda regalis) and of most
Equisetacese, and in many Gymnosperms and Angiosperms.

If the plant which produces both kinds of sexual organs is large or at least
highly differentiated, distance in the relationship of the two kinds of reproductive
cells is still attained by the male and female organs being produced on different
branches ; and this phenomenon, which is in general termed Monoecism, is also
common in the vegetable kingdom, as in some Algae, many Muscineae, and a very
large number of Gymnosperms and Angiosperms'.

But another condition which, according to the law just stated, should appa-
rently be very unfavourable, is also of very common occurrence in the vegetable
kingdom, namely, that the reproductive organs are in close contiguity, and the sexual
cells are therefore of near even if not always of the closest affinity. Thus, for
example, the same cellular filament of CEdogonium produces both male and female
cells, the same Vaucheria-&i2imQn\. antheridia and oogonia in close proximity, the same
conceptacle of Fucus platycarpus produces both oogonia and antheridia ; the
carpogonia of most Characeae are produced close beside the antheridium on the same
leaf; the archegonia and antheridia of some Mosses (species oiBryum) are collected
together in hermaphrodite receptacles, the prothallia of many Ferns produce both
kinds of reproductive organs side by side ; in the flowers of Angiosperms herma-
phroditism is the typical and most common arrangement. But in all these cases
where the aim is apparently to favour the union of sexual cells nearly related to one
another, there are at the same time contrivances which hinder the male cells from
reaching the contiguous female cells ; or at least to render it possible that this
should not always happen. This fact was first recognised by Kolreuter (1761) and
Karl Conrad Sprengel (1793), and has been further illustrated recently by Darwin,

^ The arrangement of the reproductive organs termed Polygamy is also a contrivance intended
to hinder perpetual self-fertilis-ation of a flower or of an individual.

906 PHENOMENA OF SEXUAL REPRODUCTION.

Hildebrand, and others ^ In spite of the hermaphrodite flowers of Phanerogams
and the similar sexual arrangements of Cryptogams, it appears very certain that the
union of nearly related sexual cells must be unfavourable to the perpetuation of most
plants, since such various and often astonishing means are provided in order to
prevent self- fertilisation when the sexual organs are contiguous.

One of the simplest and commonest means for ensuring cross-fertilisation is
Dichogamy^ i, e. the arrangement by which the two kinds of reproductive organs,
when they are contiguous, are mature at different times, so that the sexual cells
which are in close contiguity and are therefore nearly related are not capable of
performing their respective functions simultaneously. The male cell must in these
cases unite with a female cell in a different group of sexual organs. This is in fact
usually the case with the hermaphrodite flowers of Angiosperms, as also with most
prothallia of Ferns'and the monoecious Characese, in which the carpogonium is situated
close to the antheridium but becomes mature only at a later period (this is very
strikingly the case in Nitella flexilis). Insects are the main agents in the conveyance
of the pollen to the stigma of other flowers of dichogamous Phanerogams, for which
purpose the parts of the flower possess special adaptations which will be described
presently. In the dichogamous species of Nitella and prothallia of Ferns the motility
of the antherozoids is sufficient to enable them to reach the archegonia of neigh-
bouring prothallia, or the carpogonia on other leaves of the same plant, or even
on other plants of the same species. Whether the Algae named above and some
Muscineae are dichogamous is doubtful ; but the motility of the antherozoids renders
it possible for them to reach the oospheres of other plants or those on other
branches of the same plant.

Among Angiosperms, in addition to the common occurrence of dichogamy,
there are also other contrivances of a very different nature which have the sole
purpose of transferring the pollen of hermaphrodite flowers, by the help of insects,
to the stigma of another flower of the same or of a different plant. In most
Orchidese, Asclepiadese, Viola, &c., the reproductive organs of each individual flower
are developed at the same time, but at the time of maturity mechanical contrivances
exist which prevent the pollen falling on the stigma of the same flower ; it must be
carried by insects to other flowers.

In other cases, as Hildebrand has shown in the case of Corydalis cava"^, the
pollen does actually fall on the stigma of the same flower, but is there impotent,
having the power of fertilising only when it falls on the stigma of a different flower,
and only perfectly when carried to the flower of a different individual of the same
species. Such a plant is therefore only morphologically hermaphrodite; it is

^ K. C. Sprengel (Das neu entdeckte Geheimniss der Natur im Bau und in der Befruchtung der
Blumen, Berlin, 1 793, p 43) first gave expression to the pregnant idea, ' Since a large number of
flowers are diclinous and probably at least as many hermaphrodite flowers are dichogamous,
Nature appears to have designed that no flower shall be fertilised by its own pollen.' Darwin (On
the Various Contrivances by which Orchids are Fertilised, London 1862, p. 359) says, 'Nature
tells us in the most emphatic manner that she abhors perpetual self-fertilisation ; ' and again, • No
hermaphrodite fertilises itself for a perpetuity of generations.' [This last observation was first
made by Andrew Knight in 1799 (Phil. Trans, p. 202). — See Darwin, The Effects of Cross- and
Self-Fertilisation in the Vegetable Kingdom, 1876.]

^ [Ueber die Befruchtung von Corydalis cava, Jahrb. fiir wiss. Bot. 1866.]

INFLUENCE OF THE ORIGIN OF THE CELLS ON FERTILISATION, goj

physiologically dioecious. J. Scott states that Oncidium microchilum exhibits the
same phenomena, the pollen not being potent on the stigma of the same flower
while cross-pollination ensures fertilisation^; the pollen and stigma are therefore
without function except to the stigma and pollen of a different flower. Similar phe-
nomena have been described by Gartner in the case of Lobelia fulgens and Verbascum
nigrum^ and in species of Bego?iia by Fritz Miiller^.

No less remarkable is another contrivance for the mutual fertilisation of diff"erent
individuals of plants with hermaphrodite flowers, — Dimorphism^ (or Heterostylism),
consisting in a diff'erence between diff"erent individuals of the same species with
reference to their reproductive organs. In one individual the flowers all have a long
style and short filaments, while in another individual aU the flowers have a short style
and long filaments, as in Linum perenne, Primula sinensis, and other species of
Primula. It sometimes happens also, as in Lythrum Salicaria and many species of
Oxalis^, that the reproductive organs in the flowers of diff'erent specimens of the
same species exhibit three diff'erent relative lengths {Trimorphism), there being an
intermediate length of style between the long-styled and the short-styled forms. In
these cases of dimorphism and trimorphism Darwin and Hildebrand have shown that
fertilisation is possible only (in the case of Linum perenne") or at least has the best
result when the pollen of the long-styled flower is carried to the short-styled stigma
of another plant, and vice versa ^. Where there are three different lengths of style,
fertilisation succeeds best when the pollen is carried to the stigma which stands at the
same height in another flower as the anthers from which the pollen came. It will be
seen that this is but an expansion of the same rule.

While in the very numerous diclinous, dichogamous, dimorphic, and trimorphic
flowers, insects carry pollen from one flower to another, it is comparatively rare for
cross-pollination to take place without the help of insects. This occurs in some
Urticacese, as Pilea and Broussonetia, where the anthers emerge suddenly from the
bud and scatter their light pollen in the air like a fine cloud of dust, which is then
blown to the female organs of other flowers. In the Rye the arrangement is still
simpler ; the flowers open separately, usually in the morning ; the filaments elongate
rapidly and push the ripe anthers out of the pales; the anthers then hang down at
the end of the long filaments, open, and allow the heavy pollen to fall down, thus
reaching the stigmas of other flowers lower down in the same spike or in neigh-
bouring spikes, being assisted in this by the oscillations of the haulm under the
influence of the wind**.

^ According to Fritz MiiUer (Bot. Zeit. 1868, p. 114), in some species of Oncidium the pollen-
masses and stigmas of the same individual have a positively poisonous effect on one another.

2 Fritz Miiller, Bot. Zeit. 1864, p. 629.

3 [Darwin, On the Tvt^o Forms, or Dimorphic Condition, in the Species of Primula, Journ.
Proc. Linn. Soc. Bot. 1862, p. 77 ; ditto, On the Existence of Tv^o Forms, &c. of the Genus Linum,
ibid., 1863, p. 69; ditto, On Trimorphism in Lythrum Salicaria, ibid., 1864, p. 169; ditto, On the
Character and Hybrid-like Nature of the Offspring from the Illegitimate Unions of Dimorphic and
Trimorphic Plants, Joum. Linn. Soc. 1868, p. 393; ditto, The Different Forms of Flowers on Plants
of the same Species, 1877.]

* Hildebrand, Bot. Zeit. 1871, Nos. 25, 26.

^ [DarMnn has given the name oi legitimate to the union of two distinct forms, illegitimate to the
fertilisation of long- or short-styled plants by pollen from flowers of their own form.]

^ [For a detailed account of the very remarkable phenomena connected with the pollination of

908 PHENOMENA OF SEXUAL REPRODUCTION.

In connection with the tendency so clearly evidenced even among Cryptogams,
and still more among Phanerogams, to prevent self-fertilisation within the same
hermaphrodite group of sexual organs, it is a very remarkable fact that there are a
number of plants among Angiosperms which form two kinds of hermaphrodite
flowers, VIZ. large flowers which can generally be fertilised by the pollen of other
flowers, and small, more or less depauperated flowers, sometimes underground, which
never open \C lei's togamous Flowers], the pollen emitting its tubes immediately from
the anthers and thus fertilising the ovules. There occur therefore in these cases
difl"erent kinds of flowers on the same individual, one kind being adapted for cross-,
the other kind exclusively for self-fertilisation ^ This occurs, for example, in Oxalis
Aceiosella, where the small flowers are formed close to the ground when the larger
flowers have already ripened their fruit ; in Impatiens Noli-me-iangere, Lamium
amplexicaule, Specularia per/olmta, many species of Viola, as V. odoraia, elaiior,
canina, mirabilis, &c., Ruellia clandesltna, many Papilionaceae, as AmphicarpcEa, and
Voandzeia, Commelyna bengalensis, &c. When in these cases the large typically de-
veloped flowers are fertile, cross-fertilisation with other flowers of the same species
must happen occasionally in the course of generations, and the small depauperated
self-fertilised flowers then seem to be a subsidiary contrivance whose purpose is
altogether unknown. It is however remarkable, and apparently in contradiction to
the general rule, that the large normal flowers sometimes exhibit a tendency to infer-
tility (as in species of Viola) or are altogether unfruitful (as in Voandzeid), so that
reproduction depends in such cases mainly or entirely on the cleistogamous self-
fertilised flowers. But since there are many questions in connection with this subject
which find their solution in the foregoing facts, these rare exceptions cannot over-
throw the general law^.

In other cases, as in most Fumariaceae, Canna indica, Salvia hirla, Linum usita-
lissimum, Draba verna, Brassica Rapa, Oxalis micrantha and sensiliva, the pollen
must also, according to Hildebrand, owing to the position of the sexual organs, fall
on the stigma in the same flower, and is potent ; but in such cases, since the flowers
are visited by insects, an occasional crossing with other flowers is not impossible.
Even among Orchidese, where we find the most wonderful contrivances to prevent
self-fertilisation, Darwin found an instance in Cephalanthera grandiflora in which the
pollen-tubes are emitted from the pollen-grains on to the stigma while the former

Rye and other cereals, see Hildebrand in Gardener's Chronicle, March 15 and 22, and May 24, 1873 ;
also A, S. Wilson, Trans. Bot. Soc. Edin. XI. 506 and XII. 84. Flowers the pollination of which
is effected by the wind are termed anetnophilous, in contradistinction to the entomophilous, or those
pollinated by the agency of insects.]

^ H. V. Mohl, Einige Beobachtungen iiber dimorphe Bliithen, Bot. Zeit. 1863, Nos. 42, 43.
[See also A. W. Bennett on the closed self-fertilised flowers oi Impatiens in Journ. Linn. Soc. 1872,
p. 147; ditto, Pop. Sci. Rev. 1873, p. 337. In Junais hifoniiis the pollen-tubes are emitted while
the pollen-grains are still enclosed in the anther, perforating the wall of the latter. Henslow, On
the Self-fertilisation of Plants, Trans. Linn. Soc, Series II, vol. I, 1879.]

^ [Herrmann Miiller (Nature, vol. VIII. p. 433 et scq.) has pointed out the existence of another
kind of dimorphism, in which a species presents two different forms of flowers, one adapted to self-
fertilisation, smaller and less brightly-coloured, growing in situations where there are but few
insects, the other adapted to cross-fertilisation, larger and more brightly-coloured, growing where
insects abound. These two forms have occasionally been described as distinct varieties or even
species.]

INFLUENCE OF THE ORIGIN OF THE CELLS ON FERTILISATION. 909

are still in the anthers ; but according to Darwin's experiments the number of good
seeds produced is smaller when the plant is allowed to fertilise itself than when
pollination is effected by foreign pollen with the help of insects.

A clear comprehension of the phenomena of dichogamy, dimorphism, and the other
contrivances for ensuring cross-fertilisation, can only be obtained by a careful study of
numerous individual cases ^.

It is more clearly seen in the fertilisation of flowers than almost anywhere else how
exactly the development of the organs is adapted to the fulfilment of a perfectly definite
purpose. Each plant has its own peculiar contrivance for the conveyance of the pollen
to the stigma of another flower. It is not possible to make many general remarks on
this subject ; the following may suffice here.

It must be noted in the first place that insects^ carry pollen undesignedly while seek-
ing the nectar of flowers which has been produced exclusively for their attraction. Flowers
which are not visited by insects, and Cryptogams which do not require them, do not
secrete any nectar. The position of the nectaries, usually concealed deep at the bottom
of the flower, as well as the size, form, arrangement, and often also the movement of the
parts of the flower during the time of pollination, are always of such a nature that the
insect — sometimes of one particular species— must take up particular positions and make
particular movements in obtaining the nectar, and thus cause the masses of pollen to
become attached to its hairs, feet, or proboscis, and afterwards, when assuming similar
positions, to be applied to the stigmas of other flowers. In dichogamous plants the
movements of the stamens, styles, or branches of the stigmas assist this end, taking place
frequently in such a way that at one time the open anthers occupy the same position
in the flower that the receptive stigmas do at another time, so that the insect, when
taking up the same position, touches the open anthers in one flower and the receptive
stigmas in another flower with the same part of its body. The same result is also ob-
tained in dimorphic flowers, the pollination being in these ca^es efficacious when anthers
and stigmas which occupy the same position in different flowers are made mutually to
act on one another. But there are besides many other contrivances, most variable in
their nature and often perfectly astonishing, for effecting the conveyance of pollen by
insects. ^ few examples may suffice.

^ See especially K. C. Sprengel, Das neu entdeckte Gelieimniss der Natur, &c., Berlin 1793. —
Darwin, On the Fertilisation of Orchids, London 1862. — Hildebrand, Die Geschlechtervertheilung
bei den Pflanzen, u. das Gesetz der vermiedenen u. unvortheilhaften stetigen Selbstbefruchtung,
Leipzig 1867. — Strasburger in Jenaische Zeitschrift, vol. VI, 1870, and Jahrb. fiir wiss. Bot.
vol. VII, where the mode of fertilisation of Gymnosperms, Marchantieae, and Ferns is described.
[The most complete account of the phenomena of the reciprocal adaptation of flowers and insects
to cross-fertilisation is contained in Herrmann MUller's Befruchtung derBlumen durch Insecten u. die
gegenseitigen Anpassungen beider, Leipzig, 1873, where also is a resume of the literature of the sub-
ject. See also Kolreuter, Vorlaufige Nachricht von einigen das Geschlecht betreffenden Versuchen,
Leipzig 1761. — Delpino, Ulteriori osservazioni suUa dicogamia, Milan 1868-1870. — Axell, Om
Anordningarna for fanerogama vaxternas befruktning, 1869. — Darwin, On the Agency of Bees in
the Fertilisation of Papilionaceous Flowers, Ann. and Mag. Nat. Hist. 3rd series, vol. II. p. 461.
—Ogle in Pop. Sci. Rev. 1869, p. 261, and 1870, p. 45 (on Salvia). — Hildebrand in Leopoldina,
1869 (Compositae) ; ditto, in Monatsber. der Berlin. Akad. 1872 (Grasses). — Farrer in Ann. and
Mag. Nat. Hist. 1868; Nature, vol. VI, 1872, p. 478 et seq. (Papilionacere). — A.W. Bennett, in Pop.
Sci. Rev. 1873, p.337.— H. Muller, in Nature, vols. VIII, IX, and X.— Sir J. Lubbock, On British
Wild Flowers considered in relation to Insects, London 1875.]

2 J Q Kolreuter first recognised the necessity of insect help, and described special contrivances
for pollination, in his Vorlaufige Nachricht von einigen das Geschlecht der Pflanzen betreffenden
Versuchen, 1761.

9TO

PHENOMENA OF SEXUAL REPRODUCTION.

(i) Dichogamous Flowers^ are either protnnclrous or protogynous"^. In the former the
stamens are developed first, their anthers opening at a time when the stigmas are still
undeveloped and not yet receptive ; the stigmatic surface is only developed later, and
usually not till the pollen has been carried away from the anthers by insects ; they can
then only be fertilised by the pollen of younger flowers. To this category belong the
various species of Geranium, Pelargonium, Epilobium, Mal'va, Umbelliferae, Compositae,
Gampanulaceae, Labiatae, Digitalis, &c. The phenomena referred to, especially the
movements of the stamens and stigmas, are so readily observed in these cases, e.g. in
Geranium and jilthooa, that no further description is necessary. In protogynous flowers

Fig. 4,W.—Aristolochia Clematitis: a piece of a stem st with
petiole b ; in the axil of this are flowers of different ages ; i, i
young flowers not yet fertilised ; 2, 2 fertilised flowers, the pedicels
bent downwards ; k swollen part of the tube of the perianth r ; y the
inferior ovary (natural size).

Fig. 489. — Aristolochia Clematitis: the perianth cut
through longhudinally. A before, B after pollination
(magnified).

the stigma is receptive before the anthers in the same flower are mature ; when these
subsequently open and allow the pollen to escape, the stigma has already been pollinated
by foreign pollen or has even withered up and fallen off (as in Parietaria diffusa) ; and
the pollen of these flowers can therefore only be applied to the fertilisation of younger

^ F. Delpino, Ulteriori osservazioni sulla dicogamia nel regno vegetabile, Atti della soc. Ital.
di sci. nat. vol. XIII, 1869, and Bot. Zeit. 1871, No. 26 et seq. ; ditto, in Bot. Zeit. 1869, p. 792.

^ [For a list of British protandrous, protogynous, and 'synacmic' plants (or those in which the
male and female organs are mature at nearly the same time), see A. W. Bennett in Journal of
Botany, 1870, p. 315, and 1873, p. 329.]

t

INFLUENCE OF THE ORIGIN OF THE CELLS ON FERTILISATION. 91 1

flowers. To this class belong Scrophularia nodosa, Mandragora 'vernalis, Scopolia atropoides,
Plantago media, Lunula pilosa, Anthoxanthum odoratum, &c. Among protogynous flowers
Aristolochia Clematitis is characterised by striking and peculiar contrivances.

In Fig. 489 A is shown a young flower cut through lengthwise; the stigmatic sur-
face n is already in a receptive condition, but the anthers are still closed; a small
fly /, which has brought on its back a mass of pollen from an older flower, makes its
way in through the narrow throat of the perianth, and runs about in the globular
swelling k ; as many as from six to ten, flies are not unfrequently found in one flower.
They are shut up and cannot escape, because the throat of the perianth r is furnished
with long hairs moving as on a hinge, which present no impediment to the entrance of
the insect, but prevent its escape like a trap. While the insect is moving about in
the cavity, its back laden with pollen comes into contact with the stigmatic surface
and pollinates it, in consequence of which the lobes of the stigma curve upwards, as
is shown in Fig. 489 B, n. As soon as this has taken place, the anthers, previously
closed, open; they are laid bare by the change in the position of the stigmas, and
are rendered accessible by the withering up of the hairs at the bottom of the cavity
of the flower, which has now become wider. The flies which have now carried their
pollen on to the stigmatic surface can therefore creep down to the open anthers,
where the pollen again becomes attached to them. By this time the throat of the
perianth r has again become passable, the net-work of hairs in it having died and
withered away after the pollination of the stigma. The insect, laden with the pollen
of this flower, can now escape, and again performs the same work in another flower.
But while the changes which have been described are taking place inside the flower,
its position has also altered. As long as the stigma is still receptive, the pedicel is
erect and the perianth open outwards (Fig. 488 / /), so that the visiting flies find a door
hospitably open. But as soon as the pollination of the stigma has been effected, the
pedicel bends sharply downwards just beneath the ovary, and when the flies, again
laden with pollen, have flown out of the flower, the standard-like lobe of the perianth
above the mouth of the tube (Fig. 489 B) closes, preventing the entrance of the flies,
whose visits would now be useless.

(2) Flowers in 'which the anthers and stigmas are mature at the same time, but self-
fertilisation is hindered or prevented by the position of the organs and by mechanical contri-
vances. The pollen is in these cases also usually carried to the stigma by insects, but
generally in such a manner that the stigma can only be pollinated by the pollen from
another flower, though sometimes, as in Asclepiadeae, pollination from the same flower
is not impossible in addition to cross-fertilisation. The contrivances for this purpose
are astonishingly numerous, and sometimes so complicated that their purpose can only
be detected by very careful investigation. To this category belong, for example, the
various species of Iris, Crocus, and Pedicularis, many Labiatae, Melastomaceae, Passiflor-
aceae, and Papilionaceae. Among the most interesting examples are the Asclepiadeae,
in which however the contrivances could be explained only by lengthy descriptions and
a large number of illustrations \ In Salvia pratensis and some other species of this genus
the mechanical contrivance for preventing self-fertilisation and for ensuring crossing^ is
extremely beautiful and easy to understand. Fig. 490 represents a flower of S. pratensis
seen from the side; at n is the two-lipped stigma in a receptive condition; and indicated
by a dotted line inside the upper lip of the corolla is the position of one of the two sta-
mens. If a pin is inserted into the tube of the corolla in the direction of the arrow, the
two stamens spring out, as indicated at « ; if a humble-bee inserts its proboscis in order
to obtain the honey, the open anthers strike the back of the insect, and some of the
pollen adheres to a particular part ; when the bee places itself in the same position in

* For a fuller description, see R. Brown, Observations on the Organs and Mode of Fecundation
in Orchidese and Asclepiadeae ; Trans. Linn. Soc. 1833, and Hildebrand in Bot. Zeit. 1867, No. 33.
^ For further details, see Hildebrand, Jahrb. fiir wiss. Bot. vol. IV, 1865, p. i.

9T2

PHENOMENA OF SEXUAL REPRODUCTION.

another flower, the pollen is rubbed off its back on to the stigma. The cause of the
stamen springing out in this way is made sufficiently clear in Fig. 490 B. This shows the
short true filamentsyy which adhere by their bases to the sides of the corolla-tube, and
bear at their upper end the long connective c x, which oscillates readily about its point
of attachment. Only the upper longer and slender arm of each connective c bears an
anther-lobe a, the lower shorter arm x is without an anther, and is applied to that of the
other stamen in such a manner that the two form together a kind of arm-chair. When
the proboscis of the bee in search of honey penetrates the flower in the direction of

Fig. 4go.— Salvia pratensis : a corolla with stigmas n and fertile anther-lobes a ; B stamens removed from corolla.

the arrow, the lower arm of the connective is pressed down, and the upper arm"c is
made to move forward, and thus to strike the back of the insect.

In the Pansy {Viola tricolor) we have quite a different contrivance for preventing the
possibility of self-fertilisation. In Fig. 491, A and B, is shown the position and arrange-
ment of the parts of the flower. The cavity of the flower enclosed by the petals is
completely filled up by the anthers and ovary, with the exception of the tubular spur of
the inferior petal in which the nectar collects, secreted by the appendages of the two
inferior stamens. The only entrance to this nectary, which therefore lies behind the

Fig. 491.— Fzo/a tricolor: A longitudinal section through the flower (natural size); 5 the ovary fertilised and swollen ;
the filaments have been ruptured and the anthers drawn up by the growth of the ovary ; C the stigma with its orifice o and
lip Ip, on the style gr (magnified) ; / sepals, Is prolonged base of the sepals, c petals, cs spur of the inferior petals or
nectary; fs appendages of the two inferior stamens projecting into the spur, which secrete the nectar, a the anthers,
n stigma, v bracts ; D horizontal section through the ovary with the three placentae sp and ovules sK\ E horizontal
section through an unripe anther.

reproductive organs, is through a deep channel in the inferior petal, lined with hairs.
The upper and lateral petals incline towards one another in front of the ovary which
is surrounded by the anthers, and above the channel in such a manner that the entrance
to it is entirely filled up by the capitate stigma B, n. The stigma is seated on a flexible
style (C, gr)^ is hollow and opens by an orifice which faces the hairy channel of the
lower petal ; the lower and posterior margin of this orifice has a lip-like appendage.
The anthers open of their own accord, and the pollen in the form of a yellow powder

INFLUENCE OF THE ORIGIN OF THE CELLS ON FERTILISATION.

913

collects below and behind the stigma among the hairs of the channel. An insect
which has already brought pollen on its proboscis from another flower inserts its pro-
boscis beneath the stigma through the channel into the nectary. The foreign pollen,
which is attached to the proboscis, is thus rubbed off on to the lip of the stigma, it is de-
tained by the viscid secretion which fills up the hollow of the latter, and subsequently
emits its pollen-tubes through the canal of the style. While the insect is sucking
the nectar in the spur, the pollen of this flower, which lies in the channel behind the
stigma, becomes attached to the proboscis; when the proboscis is again drawn out,
this pollen does not come into contact with the viscid stigma, the lip being drawn
forward by the motion of the proboscis, and the orifice of the stigma protected. The
pollen that is removed from this flower is now carried, in the manner described, to
the stigma of another flower. If the insect were to insert its proboscis again into the
nectary of the same flower, the pollen would be detached into the cavity of its own

Fig. 492. — Epipactis latifolia: A longitudinal section through a flower-bud; B open flower after removal
of the perianth with the exception of the labellum/; C the reproductive organs after removal of the perianth
seen from below and in front ; D as &', the point of a lead-pencil b inserted after the manner of the proboscis of
an insect; /J' and F the lead-pencil with the pollinia attached; yA' ovary, / labellum, its bag-like depression
serving as a nectary, n the broad stigma, en the connective of the single fertile anther, / pollinia, h the rostellum,
XX the two lateral gland-like staminodes, i place where the labellum has been cut oflf, s the gynostemium.

stigma ; but, as Hildebrand has remarked, insects do not usually do this, but suck up
the nectar only once, and then visit another flower. The proceedings of the insect
may be imitated by inserting a fine sharp pin beneath the stigma into the channel
and again withdrawing it, and filling with the pollen thus removed the stigmatic
cavity of another flower.

The contrivances for cross-pollination in Orchids, as numerous as they are compli-
cated and ingenious, have been described in d&tail by Darwin in the work already
named ^ One of the simpler cases, and the most frequent in its main features, may be
briefly described in the case of Epipactis latijolia. At the time when the reproductive
organs are mature, the flower stands, in consequence of a torsion of its pedicel, so that
the true posterior leaf of the six that form the perianth (the labellum) hangs in front and

* See also Wolff, Beitriige zur Entwickelungsgeschichte der Orchideen-bllithe, in Jahib. ftir wiss.
Bot. vol. IV, 1865.

3 N

914 PHENOMENA OF SEXUAL REPRODUCTION.

downward ; it is hollowed out in its lower part, and is thus transformed into a receptacle
for the nectar which it secretes (Fig. 492, 5, 73, /). The sexual organs, borne on the
gynostemium 5 (in C), project obliquely above this nectary; the stigma forms a disc
with several lips hollowed out and viscid in the centre, the surface of which is in-
clined obliquely above the nectary. The two gland-like staminodes x x stand right
and left beside the stigma; above the stigma and covering it like a roof lies the
single fertile anther, of considerable size, which is again on its part protected above
by its cushion-like connective en; the lateral walls of the two anther-lobes burst
lengthwise right and left, so that their pollen-masses (pollinia) became partially exposed,
the pollen-grains remaining attached to one another by a viscid substance. In front of
the middle of the anther and above the stigmatic surface is the rostellum ^, a peculiarly
metamorphosed part of the stigma (see ^) ; the tissue of the rostellum is transformed
into a viscid substance covered only by a thin membrane. The flower of Epipactis is
not fertilised if left to itself; the pollinia do not fall of their own accord out of the
anther, and would even then not reach the stigmatic surface ; they must be carried away
by insects to the stigma of other flowers. The mode in which this is eff'ected is ex-
plained by inserting the point of a black-lead pencil into the flower in a direction towards
the bottom of the labellum and beneath the stigmatic surface; if it is then pressed
slightly against the rostellum, and again withdrawn slowly in this position (D), the viscid
mass of the rostellum or adhesive disc of the pollinia to which the pollen-masses are
attached remains sticking to the pencil. The pollinia are now completely removed from
the two anther-lobes by the withdrawing of the pencil, as is shown in E and F. If the
pencil with its pollinia attached is now again inserted into another flower in the direction
of the bottom of the labellum, the pollinia necessarily come into contact with the viscid
stigmatic surface and adhere firmly to it ; when the pencil is again withdrawn they are
left behind, being partially or entirely torn from the pencil. In consequence of the form
and position of the parts of the flower, an insect which settles on the anterior part of the
labellum would in the same manner be able to creep into the bottom of the nectary with-
out disturbing the rostellum ; but when it again crept out after obtaining the nectar, it
would strike against it and carry off" the pollinia ; and on crawling into a second flower,
these would come into contact with the viscid stigma, and would remain attached to it.
In some other Orchidese the contrivances are much more complicated.

(3) The ripe pollen has often to remain for a considerable time in the open anthers,
in the case of flowers in which pollination is eff'ected by insects, before it is carried
away. During this time it might be blown away by the wind or wetted by rain or dew.
In order to prevent this, numerous and very different contrivances exist which protect
the pollen. For details see Kerner, Die Schutzmittel des Pollens (Innsbruck, 1873).

Sect, 34. — Hybridisation \ In the preceding paragraphs we have spoken
only of the union of the reproductive cells of the same plant, or of two individuals of
the same species. We learn however from experience that a fertile sexual union can
take place between plants which are specifically distinct. A union of this kind is
called Hybridisation^ and its product a Hybrid. According as the union takes place
between different varieties of one species, diff'erent species of one genus, or between

* J. G, Kolreuter, Vorlaufige Nachricht von einigen das Geschlecht der Pflanzen betreffenden
Versuchen u. Beobachtungen, Leipzig 1761; Appendices in 1763, 1764, and 1766. — W. Herbert,
On Amaryllidacese, with a treatise on cross-bred vegetables; London, 1837. — Gartner, Versuche vi.
Beobachtungen liber die Bastarderzeugung im Pflanzenreich ; Stuttgart, 1 849. [See notice by
Berkeley, Journ. Roy. Hort. Soc. vol. V, 1850, p. 156,] — Wichura, Die Bastardbefruchtung im
Pflanzenreich, erlautert an den Bastarden der Weiden (with two nature-printed plates) ; Breslau,
1865. [See abstract by Berkeley, Journ. Roy. Hort. Soc. new series, vol. I, 1850, p. 57. — Focke,
Pflanzen Mischlinge, 1881.]

HVBRIDISA TION. 9 1 5

two species belonging to different genera, the resulting hybrid may be termed a
variety-hybrid, species-hybrid, or genus-hybrid.

Among Cryptogams only a few instances of hybridisation are known with
certainty. Thuret (Ann. des sci. nat., 1855) obtained hybrid plants by bringing
antherozoids of Fucus serratus into contact with oospheres of F. vesiculosus. In
some other families of Cryptogams forms have been found which have been sup-
posed, from their characters, to have a hybrid origin. Thus A. Braun (Verjiingung,
p. 329) adduces instances of hybrids between Mosses ^ PhyscGmitrium pyriforme and
Funaria hygrometrica, and between Physcomitrium fascicular e and Funaria hygro-
metrica, and between the following species of Ferns — Gymnogramme chrysophylla
and G. calomelana, G. chrysophylla and G. distans, and Aspidium Filix-mas and
A . spinulosum ^.

The most important observations from a scientific point of view, which have
given us the clearest insight into the nature of the difference of sex, are however
those made on hybrids between flowering plants, resulting from the artificial convey-
ance of pollen from one species to another. Nageli has collected the results of
many thousand experiments on hybridisation made by Kolreuter in the last century,
and more recently by Knight, Gartner, Herbert, Wichura, and other observers. The
following facts are taken chiefly from Nageli's resume^.

I. Only those forms which are closely related genetically can produce hybrids.
They are formed most easily between different varieties of the same species ; with
greater difficulty — but are still possible in a great number of cases — between two
species of the same genus ; of hybrids between species which belong to different
genera only a very few instances are known, and it is probable that in these cases
the species ought to be included in the same genus. The facility with which
hybrids can be produced varies extremely in different orders, families, and genera of
Angtosperms. The phenomenon is frequent among Liliacese, Irideae, Nyctaginese,
Lobeliacese, Solanaceae, Scrophulariacese, Gesneraceae, Primulaceae, Ericaceae, Ranun-
culaceae, Passifloraceae, Cactaceae, Caryophyllaceae, Malvaceae, Geraniaceae, CEnothe-
reae, Rosaceae, and Salicineae. It does not occur at all, or only very ejcQeplionally,
in Gramineae, Urticaceae, Labiatae^ Convolvulaceae, Polemoniaceae., Qroi^iitariaceae,
Papaveraceae, Cruciferae, Hypericineag, and Papilionaceae. Even genera of the same
order or family differ in this respect. Among Caryophyllaceae, the species of
Dianthus hybridise easily, those of Silene only with difficulty; among Solanaceae,
the species of Nicotiana and Datura have a tendency to produce hybrids, while
those of Solatium, Physalis, and Nycandra have not; among Scrophulariaceae,
Verbascimi^ and Digitalis, but not Pentstemon, Linaria, or Antirrhirfum ; among
Rosaceae, Geum, but not Potentilla.

' [See also H. Philibert, L'Hybridation dans les Mousses (Grimmia), Ann. des sci. nat. 1873,
vol. XVII. p. 225.]

^ [See also T. Moore on Adiantum farleyense, Journ. Roy. Hort. Soc. new series, I. p. 83 ;
Berkeley on Asplenium ebenoides, Scott, ibid. p. 137.]

=* Nageli, Sitzungsber. der k. bayer. Akad. der Wiss. in Miinchen, Dec. 23, 1865, and Jan. 13,
1866, Also Kemer, in Oesterreich. BoV Zeitsch. Wien. XXI.

* [Siachys ambigua Sm. is considered to be a hybrid betweea S. sylvatica and S. palustris.']

' [On hybridity in the genus Verbascum, see Darwin, Journ Linn. Soc. 1868; p. 437']

3 N 2

K

yi6 PHENOMENA OF SEXUAL REPRODUCTION.

Hybridisation between species belonging to different genera has been observed
between Lychnis and Silene, Rhododendron and Azalea^ Rhododendron and Rhodora,
Azalea and Rhodora, Rhododendron and Kalmm, Rhododendron and Menziesia^,
jEgtIops and Triti'cum, and between Echinocaclus, Cercus, and Phyllocactus, to which
must be added a few wild forms which appear to be genus-hybrids.

2. Besides the near genetic relationship, the possibility of the production of
hybrids depends also on a certain relationship between the parent-plants, which is
manifested only in the result of hybridisation, and which Nageli calls 'Sexual Affinity.'
This kind of affinity is not always concurrent with the external resemblance of the
plants. Thus, for example, hybrids have never been obtained between the Apple
and Pear^, Anagallis arvcnis and ccBrulea, Primula officinalis and elalior, or Nigella
damascena and saliva^ nor between many other pairs of species belonging to the same
genus which are very nearly allied to one another; while in other cases very dis-
similar forms unite, as ^gilops ovata with Triticum vulgare, Lychnis diurna with
Z. Flos-cuculi, Cereus speciosissimus with Phyllocactus Phyllanihus, the Peach with the
Almond. A still more striking proof of the difference between 'sexual and genetic
affinity is afforded by the fact that varieties of the same species will sometimes be
partially or altogether infertile with one another, as e.g. Silene inflata var. alpina with
var. angustifolia, var. latifolia with var. liiloralis, &c.

3. When a sexual union is possible between two species A and B, A can usually
produce hybrids when fertihsed by the pollen of B, and B when fertilised by the
pollen of A (reciprocal hybridisation). But there are cases in which A can only be
the male and B only the female parent plant, the pollination of A by B yielding no
result. Thus Thuret found, as has already been mentioned, that Fucus vesiculosus
produces hybrids with the antherozoids of F. serratus, while the oospheres of the
latter species could not be fertilised by the antherozoids of the former. Gartner
states that Nicotiana paniculata produces hybrid seeds when acted on by the pollen
of N. Langsdorfii, while the latter does not under the influence of the pollen of the
former. Kolreuter easily obtained seeds of Mirahilis Jalapa with the pollen of
M. longiflora, while more than two hundred experiments on pollinating the latter by
the former species extending over eight years produced no result.

4. Sexual affinity presents a great variety of gradations. At one extreme we
have complete infertility under the influence of the pollen of another variety or
species, the pollen-tubes not even entering the stigma, and the pollinated flower
behaving precisely as if no pollen had reached it ; the other extreme is shown in
the production of numerous hybrids, which not only grow vigorously, but are them-
selves fertile. The lowest degree of the action of pollen of a different kind consists
ih various changes taking place in the parts of the flower of the mother-plant, the
ovary or even the ovules also growing, without any embryo being produced.
A higher degree is manifested in the production of ripe normal fruits and seeds

' [The history of the plant which is here intended is given in the Botanical Gazette, vol. III.
p. 82. It was raised from seed oi Bryanthvs {Menziesia) empetriformis, supposed to be fertilised by
the pollen of Rhodothammis (Rhododendron) Chamcecisttis. It is figured under the name of Brynnthus
erechis in Paxton's Flower Garden, vol. I. t. 19; but it agrees well with specimens of its female
parent from the Rocky Moimtains, and is probably therefore not a hybrid at all.]

^ [An instance to the contrary is recorded in the Proc. Acad. Philadelphia, 1871, vol. I. p. 10.]

HYBRIDISATION. 917

containing embryos, but these embryos having no power of germination. Further
steps are indicated by the number of embryos which have the power of germination
that are produced in the ovary ^

5. When pollen from different species is applied simultaneously to the same
stigma, only one kind is potent, viz. that from the species which has the greatest
sexual affinity to the one that is pollinated. And since, as a general law, pollen is
most efficacious on a different flower of the same species — in other words, the highest
degree of sexual affinity occurs between different individuals of the same species —
when a stigma is pollinated at the same time with pollen of the same and of another
species, the first only is potent. But since, on the other hand, hybrids are sometimes
more easily produced between varieties than between individuals of the same variety,
in this case the foreign pollen may be prepotent over that of the same kind. When
the pollen of different species reaches the stigma at the same time, and if that which
reaches it later has a greater sexual affinity, it can only be potent when the first is
not potent or acts injuriously. In Nicotiana the production of hybrids can no longer
be prevented by its own pollen after two hours, in Malva and Hibiscus after three
hours, in Dianthus after five or six hours.

6. The hybrid is possessed of external characters intermediate between those of
its parent-forms, usually nearly half way between ; less often it resembles one of the
parent-forms more nearly than the other, and this is more often the case with variety-
hybrids than with species-hybrids. It follows that in reciprocal hybrids from the
species A and B, the hybrid A B is generally similar externally to the hybrid B A,
though the two forms may differ somewhat internally. Thus, according to Gartner,
the hybrid Nicotiana paniculato-rustica is more fertile than the reciprocal hybrid
Nicotiana rustico-paniculata'^. An internal difference between reciprocal hybrids is
also shown by the fact that one is more variable than the other ; thus, according to
Gartner, the progeny of Digitalis purpureo-lutea is more variable than that of D. luteo-
purpurea, the progeny of Dianthus pulchello-arenarius more variable than that of
D. arenario-pulchellus.

When two species A and B hybridise, and the one species A exercises a
greater influence on the form and properties of the hybrid than the other species
B, the hybrid or its descendants, if fertilised by A, will revert more quickly to the
parent-form A than it will to the parent-form B if fertilised by it. Thus Gartner
states that the hybrid of Dianthus chinensis and D. Caryophyllus reverts to the
latter form after three or four generations if repeatedly fertilised by it, while it
requires fertilisation for five or six generations by D. chinensis in order to revert to
that form.

7. The characteristics of the parent-forms are as a rule so transmitted to the
hybrid that the influence of both is manifested in all its characters, producing a
fusion of the different peculiarities. This is more evident in the species- than in
the variety-hybrids ; in the latter some of the non-essential characters of the parents
sometimes present themselves in the offspring uncombined side by side ; e.g. various

^ See Hildebrand, Bastardirungsversuche an Orchideen, Bot. Zeit. 1865, No. 31.

^ In this mode of designating hybrids, the name of the male parent-plant stands first; thus
Nicotiana rustico-paiiiciilala is the product of the fertilisation of N. paniculata by the pollen of
N. riisiica.

91 8 PHENOMENA OF SEXUAL REPRODUCTION.

kinds of streaks and blotches instead of a mixing of the colours of the flowers. Thus
a hybrid which Sageret obtained from Cucumis Chafe (female) with C. Melo Cania-
lupus (which had a reticulated rind) had a yellow flesh, a reticulate marking of the
rind and moderately prominent ribs like the male parent, but white seeds and an
acid flavour like the female parent. Another hybrid from the same species had, on
the contrary, the sweet flavour and yellow flesh of the male, with the white seeds and
smooth rind of the female parent. To this category belongs also the hybrid of
Cytisus Laburnum and purpureus [known as Cytisus Adami\ some of the branches
of which partially or entirely resembled one and some of them the other parent-form.
I have found what seemed to be a hybrid Antirrhinum majus^ in which the inflor-
escence bore on one side of the axis only dark-red, on the other side only yellow
flowers, while between the two halves stood a single flower which was half red and
half yellow.

8. In addition to its inherited properties, the hybrid usually possesses characters
of its own by which it is distinguished from both its parent-forms. One of these
new characters, which occurs especially with variety-hybrids, is the tendency to vary
more strongly than its parent-forms. Species-hybrids are usually weak in their
sexual properties ; those derived from nearly related parent-species are, on the other
hand, more vigorous in their growth than their parent-forms, while hybrids resulting
from the union of species less nearly related are generally feebler in their develop-
ment. The luxuriant growth of the hybrids from nearly allied species is displayed
in their more numerous and larger leaves, in their taller and stouter stems, more
copious root' system, and larger number of shoots (stolons, scions, &c.). Hybrids
have also a tendency to a longer duration of life ; those of annual or biennial parent-
forms often live a number of years, probably in consequence of their producing
a smaller number of seeds. Hybrids are also characterised by commencing to
flower earlier, and continuing to do so longer and more abundantly, than the parent-
forms ; sometimes they produce an extraordinary number of flowers, which are also
larger, more enduring, and of brighter colour and stronger odour. They have also
a tendency to become double, their staminal and carpellary leaves to increase in
number and develope into petals. Along with this luxuriant vegetative growth, the
sexual organs are usually weak, and this in every possible degree. ' The stamens,'
says Nageli, ' are, it is true, in some cases perfect externally, but partially or altogether
infertile, the pollen-grains not attaining their proper development; while in others
the stamens are altogether abortive and reduced to rudiments. The pistils (gynae-
ceum) of hybrids are in most cases not distinguishable externally from those of the
parent species, but their ovules have no power, or only to a slight degree, of
becoming fertilised; either no oospheres are formed, or the embryos which begin
to be developed from the oospheres perish sooner or later. Under favourable
circumstances, when fertile seeds are produced, their number is smaller, and they
manifest a certain degree of feebleness in their slow germination and the short
duration of this capacity.' The feebleness of the sexual function is in some
variety-hybrids scarcely perceptible, in others but small ; in general it is the more
marked the more distant the genetic and sexual affinity of the parent-forms. When
species-hybrids have the power of producing seeds by self-pollination, and this is
repeated in the progeny, their fertility generally diminishes from generation to

HYBRIDISATION. 9T9

generation ; though this phenomenon probably depends less on the sexual feebleness
of hybrids than on the circumstance that their flowers have probably been generally
fertilised with their own pollen, instead of being pollinated from other flowers or
other individuals of the same hybrid. Nageli's rule holds true in the general way,
that the male organs of species-hybrids are functionally weak to a higher degree
than the female organs, although the rule is not without exceptions.

9. * Hybrids usually vary less in the first generation, the less the degree of
affinity between their parent-forms ; species-hybrids therefore less than variety-
hybrids ; the former are often characterised by a great uniformity, the latter by a
great variability. When hybrids are self-fertilised, the variability increases in the
second and succeeding generations the more completely it was absent from the first;
and three diff"erent varieties arise more certainly the less the affinity between the
parent-forms; viz. one corresponding to the original (hybrid) type, the two others
bearing a greater resemblance to the two parent-forms. But these varieties show
but little constancy, passing easily into one another, at least in the earlier genera-
tions. An actual reversion to one of the two parent-forms (with pure breeding-in)
takes place especially when the parent-forms are very nearly related, as in variety-
hybrids and those from species that approximate to varieties. When this reversion
occurs in other species-hybrids, it appears to be limited to those cases where one
of the parent-species exercised a preponderating influence in the hybridisation.'
(Nageli, /. c.)

10. When a hybrid is made to unite with one of its parent-forms, or with
another parent-form, or with a hybrid of diff'erent origin, the product is termed a
' derivative hybrid ;' and this may again on its part unite with one of the parent-
forms or with a hybrid of diff'erent origin. When a union is eff"ected between a
hybrid and one of its own parent-forms, and the hybrid thus obtained unites again
with the same parent-form, and so on through several generations, the derived
progeny approach more and more nearly in their characters to those of this parent-
form, until they come to resemble it in all respects. According as one or the other
of the parent-forms is taken, a larger or smaller number of generations are required
to eff"ect the perfect reversion ; and this behaviour has been reduced by Nageli to a
numerical expression (formula of heredity), which indicates in numbers the amount
of influence exercised by a species in reference to the hereditary transmission of
its qualities in hybridisation. In proportion as the derivative hybrid approaches
one or the other of its parent-forms, its hybrid nature gradually decreases, and its
fertility at the same time increases.

When a hybrid unites with a new parent-form or with a hybrid of another
species, a derivative hybrid results in which three, four, or more species (or varieties)
are combined ; Wichura has united as many as six diff'erent species of Willow in one
such derivative hybrid. Hybrids of this kind, which may conveniently be termed
* combined hybrids,' usually follow the same rules with reference to their form and
other characters as hold good in the case of simple hybrids. Combined hybrids
become less fertile the larger the number of diff'erent parent-forms that are united in
them ; and they are usually very variable. Wichura showed, from his own observa-
tions and those of Gartner, that hybrid pollen produces a greater variety of forms
in its progeny than does the pollen of true species.

920 ORIGIN OF SPECIES.

The results of hybridisation are important \yith respect to the theory of sexuality,
because there is no boundary-line or essential distinction between the self-fertilisation
of pure species or varieties and their fertilisation by other species or varieties; and
because in the latter case — in other words in hybridisation — certain peculiarities of sexual
differentiation and union are rendered more evident. The two extremes of the con-
ditions under which a fertile union of sexual cells is possible lie at a great distance from
one another, but are connected by very numerous transitions. One extreme is presented
in the genus Rhynchonema, where a fertile sexual union of sister-cells takes place
regularly; the other extreme is furnished in genus-hybrids, where the uniting cells
belong to very different forms of plants whose descent from a common ancestor dates
back to a remote antiquity. But the great majority of phenomena in the vegetable
kingdom show that sexual union is usually most productive when the cells stand neither
in too close nor in too remote an affinity to one another ; self-fertilisation is in the vast
majority of cases as carefully avoided as the hybridisation of different species or genera.
The phenomena may be comprised in the statement that the original form of sexual
differentiation was probably the simultaneous formation of male and female organs in
close juxtaposition on the same plant, but that sexual union is more potent and more
favourable for the maintenance of the race when the closely contiguous cells do not
unite, but those of different descent, a certain mean amount of difference of descent
being established as the most favourable. This mean of the difference of descent
associated with a maximum of sexual potency is obtained when the sexual cells belong
to different individuals of the same species \ The arrangements described in the pre-
ceding paragraphs which are manifested in polygamy, diclinism, dichogamy, dimorphism,
the impotence of pollen on the stigma of the same flower (as in Corjdalis and
Oncidium), the mechanical contrivances for rendering self-fertilisation impossible (as
in Aristolochia Clematitis, many Orchidese, &c.), are different means for promoting the
cross-fertilisation of individuals belonging to the same species or for rendering it alone
possible.

CHAPTER VII.

THE ORIGIN OF SPECIES,

Sect. 35. — Origin ofVarieties. The characters of plants are transmitted to
their descendants, or, in other words, are hereditary. But, in addition to the inherited
properties, new characters may arise in a smaller or larger number of the descendants
of a plant which were not possessed by the parent-plants. Thus, for example,
Descemet obtained in 1803^, among the seedlings from Rohinia Pseud-acacia, an
individual without spines; Duchesne, in 1761^, among seedHngs of the Strawberry,
one with simple instead of trifoliolate leaves ; and Godron*, among seedHngs of
Datura Taiula, one with smooth instead of spiny capsules.

^ [See Darwin, Variation of Animals and Plants under Domestication, vol. II. chap, xvii, where
several illustrations of the law are given.]]

"^ See Chevreul, Ann. des sci. nat. 1846, vol. VI. p. 157, [Journ. Roy. Hort. Soc. vol. VI,-
1851, p. 61.]

2 For further details, See Usteri, Annalen der Botanik, vol. V. p. 40.

* See Naudin, Compt. rend. 1867, vol. LXIV, p. 929.

ORIGIN OF VARIETIES. ~ 92 1

The characters which arise in single descendants are often only individual,
t. e. they are not again transmitted to their descendants. Thus the seeds of the un-
armed Robinia produce again spiny plants resembling, not their immediate ancestor,
but more remote ones ; while in other cases the new character is hereditary, though
at first perhaps only partially so, the new form making its appearance only in a
certain proportion of the descendants, while the others revert to the original form, as
in Duchesne's unifoliolate Strawberry.

When a new character is transmitted by inheritance to new generations, the
number of individuals that revert to the primitive form often decreases from gener-
ation to generation, or the hereditary permanence of the new character increases ;
they become more and more constant, and sometimes even as much so as those of
the primitive form. Such new constant forms are termed Varieties^,

The same parent-form may produce a smaller or larger number either simul-
taneously or in succession, sometimes even hundreds, of new forms ; and this is
especially the case with cultivated plants. The enormous number of varieties of the
Dahlia, differing in the colour, size, and form of the flowers and in their mode of
growth, now cultivated in our gardens, have been derived since 1802 from the simple
yellow-blossomed primitive form of Dahlia variabilis. The great variety of Pansies,
distinguished chiefly by the colour of their flowers, have resulted since 1687 from the
cultivation of the Viola tricolor of our fields with small flowers almost uniform in
colour^. Still more numerous are the varieties of Cucurbiia Pepo, differing not only
in the form of their fruit but also in all other characters ; and the same is the case
with the Cabbage {Brassica oleracea) and a vast number of other cultivated plants.

Some plants have a special tendency to variation ; among native species, for
example, the fruticose Rubi, and those of Rosa and Hieracium ; others, on the con-
trary, are distinguished by great constancy in their characters, as for example Rye,
which has as yet produced no hereditary varieties, notwithstanding long cultivation ;
while the nearly related species of Wheat (especially Triticum vulgare, amyleum and
Speltd) are distinguished by a number of old varieties and an ever-increasing number
of new ones.

By far the greater number of hereditary varieties are the product of sexual repro-
duction ; thus among Phanerogams the new characters appear suddenly in individual
seedlings which differ at once from the parent-plant in these respects. Sometimes
however it happens that particular buds develope differently from the other shoots
of the same stock ; and of this Bud-variation^ two different cases must be carefully
distinguished, since their significance is altogether different. In the one case the ab-
normal shoot of a stock which itself belongs to a variety resembles or reverts to
the primitive form ; and this therefore is an instance not of the production but of
the cessation of a new form. In the Botanic Garden at Munich there is, for example,
a Beech-tree with divided leaves, itself a variety, a single branch of which bears the

^ For examples, see Hofmeister, Allgemeine Morphologie, p. 565.

2 Darwin, The Variation of Animals and Plants under Domestication, vol. I. p. 368 et seq.

3 [T. Meehan adduces a number of remarkable instances of bud-variation in which hybrid-
isation could not have taken any part;— in Rtibtts which rarely produces seeds in the wild state,
Cofivolviihis Batatas, which seldom flowers in America, &c. See Proceedings of the Philadelphia
Acad, of Nat. Sci. Nov. 29, 1870.]

g22 ORIGIN OF SPECIES.

ordinary undivided entire leaves, or has reverted to the primitive form. In the second
case new characters not previously displayed arise on particular shoots of a stock.
Thus, for instance, single shoots of the Myrtle are sometimes found with leaves in
alternating whorls of threes, instead of in pairs ; but these shoots again produce from
the axils of their leaves the ordinary branches with decussate leaves. Knight (see
Darwin, /. c. vol. I. p. 375) observed a Cherry (the May Duke) with one branch bear-
ing fruit of a longer shape which always ripened later. The common 'Moss-Rose'
is considered by Darwin (/. c. p. 379) to have probably arisen by * bud-variation'
from R. centifolia ; the white and striped Moss-Roses made their appearance in 1788
from a bud of the common red Moss-Rose ; Rivers states that the seeds of the simple
red Moss-Rose almost always again produce Moss-Roses \

Those changes which are produced in a plant by the nature of its food and
other external conditions must not be confounded with variation. Specimens of the
same plant often differ conspicuously in the size and number of their leaves, shoots,
flowers, and fruits, according as their supply of food has been abundant or deficient ;
deep shade frequently occasions the most striking changes in the habit of plants that
usually grow in sunshine ; but these changes are not hereditary ; the descendants of
such individuals revert, under normal conditions of light and nutrition, to the original
characters of the species.

Those characters, on the contrary, which may become hereditary or form the
groundwork of varieties, arise independently of the direct influence of soil, locality,
climate, or other external influences ; they appear seemingly without any cause. We
must therefore assume either that external impulses which are altogether imper-
ceptible first cause an imperceptible deviation in the process of development, which
is always extremely complicated, and that this variation gradually increases until it
becomes perceptible, or that the processes in the interior of the plant itself react
upon one another in such a manner as to cause sooner or later an external
change.

The fact that wild plants, when cultivated, usually begin to produce hereditary
varieties, shows that the change in the external conditions of life disturbs to a
certain extent the ordinary process of development ; but it does not show that par-
ticular external influences produce particular hereditary varieties corresponding to
them ; for under the same conditions of cultivation the most diff"erent varieties arise
simultaneously or successively from the same parent-form. The same is the case
also in nature with wild plants ; in the same locality under precisely the same vital
conditions a number of varieties are often found by the side of their parent-form, and
the same variety is often found in the most diverse localities ^ It is for this very
reason — because varieties are to so great an extent independent of external influences
— that they are hereditary. A change produced in a plant by moisture, shade, or any
similar cause, is not hereditary, because its descendants, when placed under other
vital conditions, acquire again other non-permanent characters. That hereditary
characters, or those which may become so, are not produced by external influences.

^ [See also M. J, Masters, On a pink sport of the Gloire de Dijon Rose, Journ. Roy. Hort. Soc.
new series, vol. IV. p. 153. — Braun, Abhand. d. Berl. Akad. 1859, p. 219.

^ Further details on this important subject are given by Nageli in the Sitzungsberichte der kon.
^aver. Akad. der Wiss. Dec. 15, 1865.

ORIGIN OF VARIETIES. 9^5

is proved most conclusively by the fact that seeds from the same fruit produce
diflferent varieties, either exclusively or together v^^ith the hereditary parent-form.

Although the production of varieties and the form they assume are not the
direct results of external influences, yet the continuance of the existence of a variety
may be determined by these influences. When a variety is produced, the question
arises whether it will thrive best in damp or in dry ground, in sunny or shady places,
and so forth ; whether it can reproduce itself under these circumstances, or whether
it will die out. The conclusion follows that hereditary varieties arise independently
of direct external influences, but that the continuance of their existence depends on
external causes. A variety which occurs only in a particular locality is not produced
by the conditions of this particular locality ; but it alone furnishes the peculiar con-
ditions of life which this particular variety requires, while other varieties which have
arisen at the same place disappear.

It has already been shown in Sect. 34 that hybrids show in general a tendency
to the production of varieties. Two diff"erent sets of hereditary characters are com-
bined in a hybrid, and there is hence a strong tendency towards the formation of new
characters which may be more or less hereditary. Hybridisation is therefore one of
the most important means at the command of the horticulturist for disturbing the
constancy of inherited characters and producing a number of varieties from two dis-
tinct ancestral forms ^. But even the ordinary sexual union of two individuals of a
species, as in dioecious, dichogamous, or dimorphic plants, may be considered as a
kind of hybridisation ; in these cases also the individuals which unite must cer-
tainly be different, since otherwise their cross-fertilisation would be no more pro-
ductive than self-fertilisation. In these cases therefore two sets of characters which
diff"er, though it may be but slightly, also unite in the descendants ; and if a hybrid
from two different species exhibits a strong tendency to variation, the cross-fertil-
isation of two diff'erent individuals of one and the same species may at least give
rise to a slight tendency in the same direction. It is therefore probable that in the
cross-fertilisation of different individuals — towards which there is always a tendency
in nature even in hermaphrodite flowers — we have a perpetual cause of variation in
plants. But this is by no means the only cause of variation, as is shown by the
existence of bud-variation, and by the reflection that the difference between indi-
viduals which produce a variable progeny is itself due to slight variation.

A great number of facts point to the conclusion that almost every plant has a tendency
to vary continually and in different directions, while every new character which is not
produced directly by external agencies tends at the same time to become hereditary.
If notwithstanding this many wild plants and some cultivated ones are very constant and
produce no varieties which can be distinguished externally, this is mainly the result of the
fact that the newly produced varieties are unable to exist in the conditions by which they
are surrounded, or at least soon disappear, a point to which I shall recur more in detail.
The hereditary transmissibility of acquired characters exhibits itself in a most peculiar
way when it does not affect the whole of the parent-plant, but only a particular branch.
A still more remarkable case was observed by Bridgman. He noticed that the spores
from the lower inner part of the lamina of the leaves of the varieties Scolopendrium

^ See also Naudin, Compt. rend. 1864, vol. LIX. p. 837. [Journ. Roy. Hort. Soc. new series,
vol, I, p. 1.]

924 ORIGIN Of" SPECIES,

'vulgare laceratum and S, 'vulgare Crista-gaUi, which was of the normal form, uniformly
produced plants of the normal parent-form, while those produced on the outer abnormal
part of the leaf reproduced the special varieties*.

Sect. 36. — Accumulation of new characters in the reproduction of
varieties. The difference between a variety and its parent-form, or between the
varieties of a common parent-form, is usually at first small and affects only a few
characters. But the descendants of the variety may again vary, the new characters
may thus become intensified, and other new characters of a different kind may be
added to them. The amount of difference between parent-form and variety and
between the various varieties of the same parent-form thus becomes greater ; and if
the tendency of the characters to become hereditary increases with the increase of
their difference, the variety comes at length to differ so greatly from the parent-form
that their genetic connection can only be proved historically or by the existence of
transitional forms. This is the case with many of our cultivated plants, as e. g. the
Pear, which varies much even in the wild state, but under cultivation has altered its
mode of growth, form of leaf, flower, and especially its fruit, to such an extent that
it would be impossible to suppose the finest sorts of Pears to be descendants of the
wild Pyrus cotfimunis, if Decaisne had not proved their genetic connection by the
study of the transitional forms (Darwin, /. c. vol. I. p. 350). In the same manner it
scarcely admits of a doubt that all the cultivated kinds of Gooseberry are descended
from the wild Ribes Grossularia of Central and Northern Europe ; and Darwin
brings forward historical evidence to show that the size of the fruit has been con-
tinually increased by cultivation since 1786, so that in 1852 it had attained the
weight of 895 grs. Darwin found that a small apple 6^ inches in circumference
weighed as much (/. c. p. 356). The different varieties of Cabbage are all descended
from one parent-species, or, according to Alph. de Candolle, from two or three
closely related ones still growing in the neighbourhood of the Mediterranean. In
this case hybridisation has also cooperated j the varieties are for the most part
hereditary but without any great constancy. The extent of the variation which has
taken place under cultivation is shown by the existence on the one hand of shrubby
forms with branching woody stems, 10 to 12 or even 16 feet high, on the other
hand of the round Cabbage with a short stem and a spherical, pointed, or broad
head consisting of leaves closely packed one over another ; and again of the Savoy
with its curled blistered leaves, the Kohl-Rabi with its stem swollen below, the Cauli-
flower with its crowded monstrous flowers, &c.^

In the case of many cultivated plants the original wild form is unknown. It is
possible that in a few cases it may have disappeared; but it is more probable
that the varieties which have arisen under cultivation have gradually acquired such a
number of new characters that their resemblance to the wild parent-form can no
longer be traced. This is probably the case with the cultivated Cucurbitaceae,
Gourds, Bottle-Gourds, Melons, Water-Melons, &c., the hundreds of varieties of which

^ Ann. and Mag. Nat. Hist, third series, vol. VIII, 1861, p. 490 ; Darwin, /, c. vol. II. p. 379.
Also Nageli, Sitzungsberichte d. k. bayer. Akad. d, Wiss, 1866, p. 274.

^ See Melzger, Landwirthschafdiche Pflanzenkunde, Frankfurt a. M. 1851, p. 1000; and
Darwin, /. c. vol. I. p. 323.

ACCUMULATION OF NEW CHARACTERS. 925

have been traced back by Naudin to three primitive forms, Cucurhila Pepo, maxima,
and moschaia, neither of which however is known in the wild state. These original
forms have been as it were evolved from the resemblances and differences of the
numberless varieties, and have only an ideal existence ; it is doubtful whether either
of them ever actually existed, or whether these ideal parent-forms do not merely
correspond to three principal varieties which arose from a single primitive form
which possibly still exists, or from the hybridisation of several. The characters of
many of these varieties are perfectly hereditary, and all the organs show the greatest
degree of variation ; how great and various these differences are is seen from the
fact that Naudin has divided the group of forms which he includes under the name
Cticurbita Pepo into seven sections, each of which again includes a number of
subordinate varieties ^ The fruit of one variety exceeds that of another variety
more than two thousand fold in size; the original form of the fruit is probably
ovoid, but in some varieties it is elongated into a cylinder, in others abbreviated into
a fiat plate ; the colour of the rind varies almost infinitely in the different varieties ;
in some it is hard, in others soft ; some have a sweet, others a bitter flesh ; the
seeds vary in length from 5 or 7 to 25 mm.; in some the tendrils are of enormous
size, in others they are altogether wanting ; in one variety they are transformed into
branches which bear leaves, flowers, and fruits. Even characters which are normally
constant throughout entire natural orders become extremely variable in the Gourds ;
thus Naudin (Compt. rend. 1867, vol. LXIV. p. 929) describes a Chinese variety of
Cucurhita maxima which has a perfectly free or superior ovary, whereas it is inferior
elsewhere in the Cucurbitaceae and in the nearly allied orders^. The varieties of
Melon {Cucumis Melo) Naudin divides into ten sections, which differ also not only
in their fruit, but also in their leaves and their entire habit or mode of growth.
Some Melons are no larger than small plums, others weigh as much as dd lbs. ;
one variety has a scarlet fruit; another is only i inch in diameter but 3 feet long,
and is coiled in a serpentine manner in all directions, the other organs being
also greatly elongated. The fruits of one variety can scarcely be distinguished
externally or internally from Cucumbers; one Algerian variety suddenly splits up
into sections when ripe (Darwin, I.e. vol. I. p. 357).

The behaviour of the genus Zea is similar to that of Cucurhita. The cultivated
varieties of Maize are probably descended from a single primitive wild form which
has been cultivated in America for a very long period ; but it seems doubtful whether
the native Brazilian species, the only one known in the wild state, with long glumes
enveloping the grains, is the primitive form ; if it is not, then no plant is now known
which can be considered as the ancestral form of our numerous and extremely diverse
varieties of Maize. In this case also continued cultivation has increased the amount
of difference between the different varieties, as well as to a prodigious extent that
between them and the primitive form ; and the separate varieties are distinguished
from one another by a number of different characters. Some are only \\ feet high,
others as much as 15 to 18 feet; the grains stand on the rachis in rows varying from

^ See Metzger, Landwirthschaftliche Pflanzenkunde, p. 692, and Darwin, /. c. vol. I. p. 358.

"^ Hooker states that a specimen of Begonia fri%ida at Kew produced, in addition to male and
female flowers with inferior ovary, also hermaphrodite flowers with superior ovary. This'variation
was the product of seeds from a normal flower. (Darwin, /. c. p. 365.)

92^ ORIGIN OF SPECIES.

six to twenty in number ; they may be white, yellow, red, orange, violet, streaked
with black, blue, or copper- red; their weight varies sevenfold; their form also varies
extremely; there are varieties with three kinds of fruit of different form and colour
on one rachis ; and a great number of other differences also occur \ These
instances may suffice to show to what an extent the amount of deviation of the
varieties of a primitive form may increase under cultivation ^.

It is much more difficult, and to a great extent impossible, to prove directly to
what extent the variation of wild forms can increase without cultivation, because
historical evidence is in this case generally impossible, or can only be obtained indi-
rectly or conjecturally. But since the laws of variation are unquestionably the same
in the case of wild as of cultivated plants-^al though they operate in the two cases
under different conditions — we may for the present at least assume as probable that
plants vary as greatly in the wild as in the cultivated state. We shall however in the
sequel have to examine a number of weighty considerations which lead to the con-
clusion that variation has produced infinitely greater effects in originating the various
wild forms of plants than those which we perceive in cultivated varieties ^.

The variation of cultivated plants shows that there is only one cause for the
internal and for the external hereditary resemblance between different plants, and
that this cause is the common origin of similar forms from one and the same
ancestral form. When we meet with corresponding phenomena in wild forms, and
when we find that with them dissimilar forms are connected by a series of inter-
mediate forms, just as we find to be the case between the primitive forms of culti-
vated plants and their most abnormal varieties, we are forced to the conclusion that
in wild plants also a similar affinity is the only cause of resemblance. The extra-
ordinarily numerous forms, for example, of the widely distributed genus Hieraciuvt
present phenomena similar in many respects to those of cultivated Gourds, Cab-
bages, &c. In addition to a number of forms which are considered to be species,
there are a still greater number of intermediate forms, some of which only are
hybrids, the greater part perfectly fertile varieties. Nageli ^, who has made this genus
the subject of close study, says : — * If an attempt is made to unite into a single
species all the types which are connected by perfectly fertile transitional forms, we
should find only three species of native Hieracia, which have been erected by some
authors into distinct genera : — Pilosella (Piloselloidese), Hieracium {Archieracium),
and Chlorocrepis [H. staiicifolium). Between these three groups we have, at least
in Europe, no transitional forms ; hybrids between Piloselloideae and Archieraciujn

^ See Darwin, /. c. vol. I. p. 365, and Metzger, /. c. p. 207. No great value with reference to
variation and the constancy of varieties must be set on the result of experiments on cultivated plants,
since the possibility of hybridisation was not excluded. Some varieties of Maize appear to hybridise
with difficulty.

^ Further material will be found collected in Darwin's and Metzger's works already quoted, and
in De Candolle, Geographic botanique, Paris 1855.

^ [H. Hoffmann gives in the Bot. Zeit. for April 27 and May i, 1874, an account of an inter-
esting series of experiments on the extent^ to which the characters which distinguish the allied
species Papaver Rhceas and dubium and Phafeoh/s vulgaris and multijlorus can be made to vary by
cultivation, and on the tendency of the cultivated varieties to revert to the parent-form. See also
his 'Riickblick auf meine Variations- Versuche von 1855-1880,' Bot. Zeitg. 1881.]

* Sitzungsberichte der kon, bayer, Akad. der Wiss. March 10, 1866.

ACCUMULATION OF NEW CHARACTERS. 927

have been erroneously stated to occur, but the alleged hybrids are either true Pilo-
seiloideae or true Archieracia. . . . According to the present state of our knowledge,
no other hypothesis is possible but that all the various species of Hieracium have
sprung from the transmutation (descent with variation) of forms which have either
disappeared or are still in existence ; and a large number of the intermediate forms
•still occur which must have had their share in producing several new species by the
splitting up of ©ne original species, or which would have occurred in the transform-
ation of a still living species into one derived from it. In the case of Hieracium the
species have not become so completely separated by the suppression of the inter-
mediate forms as is the case in most other genera.'

By the term Species is meant the aggregate of all the individual plants which have the
same constant characters, these characters being different from those of other somewhat
similar forms. It is clear from what has already been said that the only distinction
between varieties of a known primitive form which have become constant, and the wild
species comprised within a genus, is that in one case their descent is known, in the other
it is not. The various cultivated varieties of a primitive form which have become con-
stant are linked together by intermediate forms in which the progressive accumulation
of new varietal characters may be perceived ; but these intermediate forms may dis-
appear, and then there is a more or less wide chasm between the various varieties them-
selves, as well as between them and the primitive form. Both of these cases occur also
in wild plants. In some genera, like Hieracium, species the extreme forms of which
differ greatly are connected together by a number of intermediate forms which occur
along with them. The analogy of cultivated plants justifies us in considering these
intermediate forms (so far as they are not hybrids) as varieties in a progressive state
of development, some particular descendants of which have advanced furthest in the
accumulation of new properties. But usually the intermediate forms, the connecting
links so to speak between the ancestral and the derived forms, have disappeared ; and
the species of the same genus are then completely separated from one another, and their
characters are at once distinguishable. The different species of the same genus agree
among one another in a number of inherited characters, and are distinguished by
only a few constant characters ; the amount of resemblance is much greater than the
amount of difference. The same relationship therefore exists, but in a greater degree,
between the various species of one genus as between different varieties of the same
primitive form. Since no other explanation is known of this relationship than common
descent with variation and the heredity of the new characters, we are entitled to
consider the species of a genus as varieties of a common ancestral form which have
developed further and become constant,— the original form having possibly actually
disappeared or being no longer recognisable as such. There is therefore no natural
boundary-line between variety and species ; they differ only in the amount of divergence
of the characters and in the degree of their constancy. Just as a number of varieties are
included in the idea of a species — the varietal characters being neglected in the diagnosis
of the species — so several species are united into a genus by including in the diagnosis of
the genus a maximum of their common characters. But since it is impossible either to
determine by measure or by weight the most important characters of a plant, it is
ditficult and to a certain extent impossible to define, i.e. to determine by convention
what amount of differentiation is necessary in order to classify two different but similar
forms as species rather than varieties. In the same manner it is left to a great degree to
personal judgment to decide whether two different but icimilar groups of forms should
be regarded as two species each including several varieties, or as two distinct genera
each including several species. The only object actually presented to the eye is the
individual (and even this not always as a whole) ; the ideas Variety, Species, Genus are
abstract ideas, and indicate a progressive scale of the differences between individuals

928 OR TO IN OF SPECIES.

which is small in the variety, larger in the species, and still larger in the genus. But in
all three cases the points of difference are accompanied by a much greater amount of
resemblance ; and since in the phenomena of variation vi^e learn that from forms which
are similar others are derived which are constantly becoming more different by the
continual accumulation of differences, we assume that the higher degree of variation
of similar forms which we express by the terms Species and Genus has resulted from
the accumulation of new characters in the variation from one ancestral form.

Sect. 37. — Causes of the progressive development of varieties. The
characters of the cultivated varieties of one parent-form show, as Darwin was the
first to point out, a constant striking and remarkable relation to the purpose for
which the plant was cultivated by man. The varieties of Wheat differ from one
another only slightly in the form of the haulm or leaves, which are of but small im-
portance to mankind ; but they show a great variety and extent of difference in the
form and size of the grains, and the quantity of starch and proteid contained in
them, z. e. in the characters of that part of the plant for the sake of which Wheat is
cultivated, and in those properties of this part which under various circumstances are
especially useful to mankind. The varieties of the Cabbage, on the other hand, scarcely
differ at all in their seeds or even in their seed-vessels or flowers, the external pro-
perties of which are useless to man, and the internal properties only of value because
the seed has to reproduce the variety; the varieties of Cabbage differ exclusively in
the development of those parts which are used as vegetables, and to which therefore
cultivation is directed. The object of cultivation is therefore, retaining the taste and
value as food for man, sometimes to increase the succulence of the tissues, sometimes
to attain as large a size as possible, sometimes to alter the time of the year at which
the vegetable can be used. These and a number of other properties are furnished
by the different varieties. The varieties of Beet differ only slightly in their flowers,
more in their leaves, according as they are grown in the garden as ornamental
foliage-plants or as agricultural crops ; the varieties in the latter case differ from one
another in the size and shape of the roots and the amount of sugar they contain,
properties which make the plant valuable on the one hand as food for cattle, on the
other hand for the manufacture of sugar. Fruit-trees of the same kind differ but
little in general in their roots, leaves, flowers, or stems, but to an extraordinary
extent in the size, shape, colour, smell, taste, period of maturity, and keeping-pro-
perties of the fruit, according to the special purpose or prevalent mode in which it
is employed. In garden-flowers it is generally the flowers and especially the corolla
and inflorescence that differ in the varieties of a species, because the greater number
are cultivated only for the shape, size, colour, or odour of the flowers.

This relation of cultivated varieties to the requirements of man is explained if
we suppose that only those varieties were cultivated, at first undesignedly afterwards
designedly, in which some character useful to man was more strongly manifested
than in the others ; those individuals were selected which best answered to a definite
requirement ; they alone were further cultivated ; the particular character was again
strongly displayed in some of their descendants, and only these individuals were
again selected for reproduction ; and the desired character was thus continually in-
creased in strength. Other characters of the plant also varied at the same time, but
they were disregarded, and the individuals in which they occurred were not preserved

CAUSES OF THE PROGRESSIVE DEVELOPMENT OF VARIETIES, gig

for reproduction, and no increase of these characters consequently took place from
generation to generation.

The greatest service which Darwin has rendered to science is to have shown
that wild plants are also subject to conditions of life the effect of which consists in
this, that only some of the varieties of one primitive form maintain themselves and
increase their peculiarities, while others perish. The relationship of the varying wild
plant to its environment in the broadest sense of the word is however different from
that of the cultivated plant to man ; man protects his charges in order to preserve
them ; he places them under favourable conditions in order that those properties
which are useful to him may become freely developed. Wild plants, on the contrary,
have to protect themselves against all injury from without ; their existence is con-
tinually threatened by other plants or animals or by the hostility of the elements ;
and in this Struggle for Existence, as Darwin has appropriately termed it, only those
individuals are able to maintain themselves which are best able to resist the prejudicial
influences to which they are exposed ; and only those varieties which happen to be
the best endowed in these respects will reproduce themselves and further develope
their special properties. Hence the characters of wild plants, as far as they are not
of a purely morphological nature, always show a perfectly definite relationship to the
conditions in which they are placed; the form and other characters of the organs
have essentially for their object to secure the existence of the plant under the local
conditions of its habitat ; varieties and species which are not endowed with qualities
to endure the struggle for existence perish. The struggle for existence acts there-
fore in a certain sense similarly to the selection of the breeder ; as the breeder de-
velopes only that which is suited to his own purposes, so in the struggle for existence
only those varieties survive and reproduce their kind which are better adapted,
through some property which they possess, to endure the struggle. Thus, finally,
through imperceptible variation, through the destruction of those characters which
are not beneficial, and through the further development of the useful ones — in one
word, through what may be termed metaphorically Natural Selection by means of
the struggle for existence, — forms are produced which are as well or even better
adapted for the purpose of self-preservation than cultivated plants are for the pur-
poses of man. By the undesigned reciprocal influences of plants and of their living
and physical environments, specialities of organisation finally arise which could
scarcely be better adapted for the preservation of the plant under its special local
conditions, and which give the impression of being the result of the greatest
ingenuity and foresight.

On the other hand, certain specialities of organisation which are essential in the

struggle for existence may disappear in consequence of continued cultivation.

[ildebrand points out^ that Peas, Beans, Lentils, Cereals, Buckwheat, all develope
under cultivation large heavy seeds which cannot be self-sown, so that these plants

?hen left to themselves do not become wild, but disappear in consequence of having
^lost the specialities of organisation which effect the dispersion of their seeds and
which protect them from animals. The same is the case with cultivated plants the

fruits of which have been modified for the use of man, and have become useless in

Hildebrand, Die Verbreitungsmittel der Pflanzen, Leipzig 1873.
30

930 ORIGIN OF SPECIES.

the struggle for existence in the wild condition ; such are cucurbitaceous plants,
fruit-trees, &c. Cultivation may so far affect edible fruits that they cease to develope
seeds, as in the better kinds of Pears, in Grapes, Figs, Oranges, Dates, Bread-fruit,
and Bananas ; thus the first and most important element in maintaining the struggle
for existence, namely, reproduction by means of seeds, is destroyed.

In order to understand clearly how the struggle for existence has caused the
existing wild forms of plants to be so admirably adapted to their specific conditions
of life, it must be borne in mind that all plants are continually varying to a very
slight extent, and that the variation affects all their organs and all their characters,
although usually to an imperceptible amount. On the other hand, the struggle for
existence in plants (as well as in animals) is a perpetual and never-ceasing one, in
which the smallest advantage that the plant has obtained through variation in any
one direction may be of the utmost importance for its perpetuation.

The struggle which the plant carries on by means of its capacity for variation
has two different aspects. On the one hand its tendency is to adapt its organisation
completely to the conditions of food and growth afforded by the climate and the
soil. It is evident that the organisation of a submerged water-plant must be different
from that of a land-plant ; that the assimilating organs of a plant that grows in the
deep shade of a wood must be differently constructed from those of a plant exposed
daily to bright sunshine, and so forth. The conditions of life of all plants growing at
a great elevation and in Arctic countries must be different from those growing in
the lowlands of the Tropic and Temperate zones. If we had to do only with these
general conditions of plant-life, the struggle for existence would be a comparatively
simple process. It would be easy to imagine how, among the varieties of a primitive
form which grew in water, there would be some which would be occasionally sub-
jected to a subsidence of the water, and how these would give birth to descendants
which would gradually assume the character of marsh- and finally of land-plants,
as is well illustrated in the case of Nasturtium amphibium^ Polygonum amphibium^
&c.^ It may also be supposed that some of the descendants of a plant exhibit a
somewhat greater power of resisting frost, that this property increases in the course
of generations, and that thus a form which can at first only bear a temperate climate
gradually produces varieties which can endure a more and more severe climate ; and
so forth. Even these comparatively simple conditions would necessarily lead to a
great diversity in the varieties descended from one ancestral form; for each
adaptation to new conditions of climate or locality would act in different ways ;
i.e, varieties of different descriptions would take up and carry out in different
ways the struggle against the influences of the elements.

But the struggle for existence and the changes occasioned by it in the organ-
isation of plants are greatly complicated by the fact that every plant, while struggling
to adapt itself to its special conditions of life, has also to protect itself at the same time
against a number of other plants and against the attacks of animals j or, what is more
to the point, its capacity for variation enables it to make use of particular favourable
t»nditions which are offered to it by other plants and animals in order to take ad-

^ A special interest attaches in this connection to Hildebrand's observation on Marsilia in Bot.
Zeit. 1870, No. I, and Askenasy's on Ranunculus aquatilis and divaricatus in Bot. Zeit. 1870, p. 193
et seq.

CAUSES OF THE PROGRESSIVE DEVELOPMENT OF VARIETIES. 93 1

vantage of them ; as parasites of their hosts, dichogamous and other flowering plants
of the visits of insects, &c. These relationships are endless in their diversity, and
can only be illustrated by examples.

We must here call special attention to a remark of Darwin's ; that the indi-
viduals of the same species or variety are competitors for position, food, light, &c.
The fact that plants of the same species have the same requirements itself gives rise
to a struggle for existence among them ; and the same is the case, though to a some-
what smaller but still to a great extent between the different varieties of the same
primitive form, to a less extent between different species and genera. The result of
these relationships is seen on the one hand in the fact that, in the case of plants which
live socially, only the most vigorous seedlings arrive at full maturity, while the weaker
ones are smothered, as may be seen in any young plantation ; on the other hand, that
species and genera which differ greatly from one another can thrive side by side,
because their requirements are different and the competition between them is less.

From the fact that plants whose organisation differs can thrive better side by
side on the same §oil in consequence of the diminished competition between them,
Darwin drew the important and pregnant conclusion that in the propagation of the
varieties of one primitive form those new forms must be the best able to maintain
themselves in the wild state which differ most from the primitive form and from one
another, whereas the intermediate forms must be gradually dispossessed. This is the
reason why the connecting forms between the different species of a genus are so
often wanting, although the conclusion cannot be avoided that the species arose
by variation from a single ancestral form, and by the propagation of varieties.

In its broader features (and on that account more conspicuously) the struggle for
existence between the various forms of plants, the competition for space, food, and light,
is manifested in the luxuriant growth of what we term weeds in our gardens and fields.
Our cultivated plants are able to bear our climate, and the soil supplies what they
require for their vigorous growth. But a number of wild plants are still better adapted
to the climate ; and they grow still more vigorously, rapidly, and luxuriantly on cultivated
soil, and their seeds or rhizomes are everywhere present in enormous quantities. If
the cultivated plants are not carefully protected from the weeds, the latter soon dis-
possess them of the ground which was set apart for them. Every country and every soil
has its own peculiar weeds ; i.e. under any particular external conditions there are always
certain forms of plants which thrive best and drive out the cultivated plants. To a
certain extent we have a measure of the amount of advantage which weeds have over
cultivated plants in the amount of labour bestowed by man on their destruction in
order to preserve and maintain his nurselings. The primitive forms of our cultivated
plants are mostly natives of other countries, where they are not only sufficiently adapted
for the climate, but are able to sustain competition with their neighbours.

The number of species or of individuals of any species which we find in a meadow,
a marsh, &c. is not a matter of chance ; it does not depend merely on the number of seeds
of one or another species produced or brought to the locality ; every one of these species
would, if it alone existed there or were protected by cultivation, of itself cover the space of
ground in a short time ; but there is a definite relationship between the numbers of indi-
viduals of the different species when left to themselves, a relationship which depends on the
specific power of each particular species to maintain itself in the struggle with the rest^.

^ [How the relationship subsisting between the species in permanent pastures may be disturbed
by the application of different manures, may be seen in Lawes and Gilbert's paper on this subject
in Journ. Roy. Agric. Soc. vol. XXIV, 1863.]

302

g^2 ORIGIN OF SPECIES.

How complicated may be this relationship in the cases of only two nearly related
forms of plants in their struggle for existence in particular localities has been described
as exhaustively as clearly by Nageli in the case of various Alpine plants. *Thc interne-
cine war,* he says \ * is obviously most severe between the species and races that are
most nearly related, because they require the same conditions of existence. Achillea
moschata drives out A, atrata, or is driven out by it ; they are seldom found side by side ;
while each grows along with A. Millefolium. It is clear that Achillea moschata and atrata^
being extremely similar to one another externally, make similar demands on their en-
vironment, while A. Millefolium, which is less nearly allied to both, does not properly
compete with them, because it requires other conditions of existence. Still less do

plants of different genera or orders compete with one another In the Bernina-

Heuthal (Upper Engadin) Achillea moschata^ atrata, and Millefolium occur in profusion,
A. moschata and Millefolium on slate, A. atrata and Millefolium on limestone ; where
the slate ends and limestone begins, A. moschata always ceases and A, atrata takes its
place. Both species are therefore here strictly circumscribed as to soil, and this I
have found to be the case also at various spots in Biindten, where both species occur
together. But where one species is absent the other is widely distributed. A. atrata is
then found indiscriminately on slate or limestone : and although A, moschata does not
apparently grow so readily on limestone as A. atrata does on slate, yet in the neighbour-
hood of the primary rocks it is found on a distinctly calcareous formation along with the
vegetation characteristic of it. In the Bernina-Heuthal I found in the midst of the slate
which was thickly covered with A. moschata a large erratic block of limestone covered
with a crust of soil scarcely an inch thick, upon which a patch of A. moschata had
established itself, because it did not here meet with any competition from A. atrata. . . .
A similar relationship was observed in certain districts between Rhododendron hirsutum
and ferrugineum, Saussurea alpina and discolor, and between species of the genera Genti-
ana, Veronica, Erigeron, Hieracium, &c.' The obvious objection, that there cannot
possibly be any struggle between two forms of plants as long as there is space for
both in the area in question, rests on an incorrect basis, and is disposed of by Nageli
as follows : — * Upon a slate slope are a million plants of A. moschata ; they obviously do
not occupy the whole space, for a hundred millions or more could find room there ; but
the rest of the space is occupied by other plants. There is here a condition of equi-
librium, which has been produced in reference to the nature of the soil and the preceding
climatic influences. The number one million gives us also the proportion which A.
moschata is able to maintain in relation to the rest of the vegetation ; and the objection
that there would still be plenty of room for A. atrata is an untenable one. If the space
were accessible to species of Achillea generally, it would be occupied by the species which
is already present, and which in any case has the advantage, A. moschata. If we now
imagine that the two species happened for once to be intermixed on the slate slope,
perhaps in consequence of artificial transplanting, in equal quantities, say 500,000 plants
of each, A, moschata would thrive the better of the two, as the soil contains but little
lime; A. atrata would become weaker and its tissue less matured, and would in conse-
quence have less power to withstand external prejudicial influences, as summer frosts,
long-continued rainy weather, or persistent drought, &c. If we suppose, for example,
that every twentieth or fiftieth year a severe frost occurs at the time of flowering
which destroys half the plants of A. atrata, while the more vigorous A. moschata re-
sists it, the voids are again filled up by the dispersion of the seeds ; but more plants of
A. moschata spring up than of A. atrata, because the number of individuals of the latter
was reduced by the frost to 250,000, while that of the former remains at 500,000. The
million plants of Achillea on the slope will in consequence be composed of say 670,000

^ Sitzungsber, der kon. bayer. Akad, der Wiss. Dec. 15, 1865. The manner in which the arrange-
ments for the protection of the pollen with reference to certain insects on the one hand, and to
climatic influences on the other, determine the distribution of certain plants, is clearly indicated
by Kerner (Schutzmittel des Pollens, Innsbruck 1873).

L

RELATION OF MORPHOLOGICAL NATURE OF ORGANS TO ADAPTATION. 933

A. moschata and 330,000 A, atrata. After a second frost, which again destroys one half
of the individuals of A. atrata, we should have about 800,000 of A, moschata to 200,000
of A. atrata. In this manner the number of the latter would decrease with every
unusual summer frost, until at length it entirely disappeared, a nearly-allied hardier species
becoming distributed over the locaUty in its place.' In conclusion, the following remark
by the same author may be added : — ' From such a course of reasoning the conclusion
might perhaps be drawn that this result would always take place, and that one of two
plants would always be crowded out, because the two could hardly be precisely equally
hardy. But this conclusion would be unsound, because it would hold good only for
plants whose conditions of existence were as nearly as possible alike. We can imagine
another case in which the two species suffer injury from altogether dissimilar external
influences (one, e.g., from spring frost, the other from dry heat), so that sometimes the
number of individuals of one species, sometimes that of the other species diminishes,
and where moreover the production and the germination of the seeds are affected by
altogether dissimilar external influences, so that sometimes the one sometimes the
other species increases more rapidly and occupies the vacant spots. The numerical
proportion of the two species must in this case be variable, but neither is able to
expel the other.'

Just as the struggle between two species is the result of their thriving more or less
vigorously on a soil of a particular chemical nature, so also the need for more or less
water, light, heat, &c. can determine also the nature of the struggle for existence.
Nageli gives some examples of the first case. When Primula officinalis and elatior occur
together in a district, they are sometimes sharply separated from one another, P. offici-
nalis preferring the dry, P. ^elatior the damp spots. Each is most vigorous in its own
locality, and may expel the other. But when only one species occurs, it is not so
particular ; P. officinalis will choose damper, P. elatior drier situations, than if they were
in company. Prunella 'vulgaris and grandiflora behave in the same manner in reference
to poorer and more fertile soils ; as also do Rhinanthus Alectorolophus and minor, Hieracium
Pilosella and hoppeanum.

These examples may suffice to show what is meant by the Struggle for Existence. It
must however be borne in mind that such a struggle must arise in reference to every
vital phenomenon of a plant, and to each of its relationships to the external world,
especially to the animal kingdom; and that its course must vary for the same plant
in diflferent localities. An understanding of the Theory of Descent, and especially
an insight into the causes of the perfect adaptations of the organisation of the plant
to its vital conditions which are often extremely local, depend essentially on a clear
comprehension of the struggle for existence.

Sect. 38. — Relationship of the morphological nature of the organ to
its adaptation to the conditions of plant-life. Every plant is very accurately
adapted (though not absolutely so) to the conditions and circumstances under which
it grows and is reproduced; its organs have the shape, size, mode of develop-
ment, power of movement, chemical properties, &c. needful for this purpose. If
this were not the case, the plant would inevitably perish in the struggle for existence.
But the vital conditions are extremely various, and undergo, in the course of time,
endless changes. The diversity in the characters of plants corresponds to this infinite
variety in the conditions of Ufe ; and yet even in the more highly differentiated classes
there are only three or four morphologically distinct forms of structure, axis (cau-
lomes), leaves (phyllomes), roots, and trichomes, which suffice for these conditions,
while maintaining a constant morphological character through numberless variations
in their physiological properties. This relationship has already been described in
chap, iii of Book I as the metamorphosis of the morphological members of a plant.

934 ORIGIN OF SPECIES,

understanding by metamorphosis the adaptation to various physiological purposes of
morphologically equivalent members. The diversity in the physiological development
is related to the conditions of life of the plant ; and to this extent Metamorphosis is
synonymous with what we have here termed Adaptation, and which has also been
described as Accommodation. When we speak of Purpose in the structure of a
plant, we mean in fact nothing more than that the form and the other characters of
the organ are adapted to the conditions of its life, which may be at once inferred from
the very fact of the survival of the plant in the struggle for existence. The terms
Purpose, Adaptation, and Metamorphosis express therefore the same thing, and may
be used as synonymous, as we have already repeatedly done^.

For the discussion of the questions to be treated of in the following paragraphs
it is important to have as clear a conception as possible of the relationship of adapt-
ation to the morphological nature of the organs, and of the great constancy of
morphological characters and the infinite diversity of metamorphosis; for this re-
lationship can be explained by no other theory than that of descent.

In its most general features the relationship of adaptation to the morphological
nature of organs is manifested in the fact that all the various morphological members
perform the most different functions and in an infinite variety of ways; in other words,
that the morphological nature of the parts of a plant is not directly determined by
their function, nor is the function of an organ determined directly by its morpho-
logical nature. Thus, for example, trichomes sometimes take the form of a pro-
tective envelope (mostly in buds), sometimes of glands, sometimes of absorptive
organs (as root-hairs), sometimes of asexual organs of reproduction (as the sporangia
of Ferns), &c. The leaves again are usually organs of assimilation containing chlo-
rophyll ; but they may also be employed as protective envelopes to winter-buds (in
most of our native woody plants), as reservoirs for reserve food-materials (in the
seedlings of flowering plants and in bulbs). In the majority of plants the leaves
bear the sporangia: in flowering plants the flowers are composed of peculiarly
metamorphosed leaves. In many slender-stemmed Angiosperms the leaves are
transformed into tendrils, in order to raise up the slender stem and fix it to neigh-
bouring supports ; the leaves of Nepenthes produce at their apex an appendage
which forms a pitcher provided with a moveable lid and filled with the fluid
which it itself secretes ; some of the leaves contained in flowers are developed into
nectaries and then perform the function of glands ; not unfrequently they are trans-
formed into hard woody spines ; in other cases they are sensitive to irritation,
contractile, and so forth. The parts of the axis are scarcely less varied in their
development ; sometimes they cling round upright supports ; sometimes they are
woody and able to retain themselves in an erect position ; sometimes they are slender
swaying branches, or thick fleshy succulent masses {Cactus), or round tubers filled

^ Many recent writers seem to be almost too anxious to avoid the use of the word ' purpose,'
because it seems to suggest antiquated teleological views. The word 'useful' which they would
substitute for ' purposive ' has also a teleological significance in human affairs. If every word
which has once been used to express an incorrect theory is to be discarded, the resulting diminu-
tion of the vocabulary would soon produce an evident impoverishment of the language. The
mission of science is not to explain and alter words, but the ideas to which they correspond. Ought
we to give up the use of the word ' root ' in Botany, because it formerly bore a very different
meaning from that which is now attached to it ?

RELATION OF MORPHOLOGICAL NATURE OF ORGANS TO ADAPTATION. 935

with food-materials {Arum, Potato), or they become tendrils (the Vine), or spines
{Gleditschid) ', sometimes they assume the form of foliage-leaves {Ruscus, Xylo-
phyllum, &c.). The adaptations of roots are less numerous ; usually filiform, slender,
cylindrical, and provided with root-hairs for absorbing water and dissolved mineral
substances, they become tuberous reservoirs for reserve food-materials in the Dahlia ;
their tissue is loose and contains air and they resemble swimming bladders in
Jussieua ; in the Ivy, Ficus repens^ &c., they are simple organs of attachment for the
stem ; in Vanilla aromatica they play the part of tendrils ; but they never produce
sporangia or sexual organs.

According to the definition already given of Purpose in the vegetable organ-
isation, its relationship to the morphological nature of the organ can also be illus-
trated by keeping in view the end to be served, i. e. the condition of the plant which
is most favourable in the struggle for existence, and then observing the means em-
ployed for attaining this end, i, e. what members of the plant become adapted, and
what metamorphosis they undergo. A few examples will explain this^

It is obviously useful for the greater number of flowering plants — in other
words advantageous in the struggle for existence — that their stem should grow
rapidly to a certain height, because the conditions of assimilation (light and warmth)
are thus most perfectly fulfilled, and because — which is perhaps of greater importance
— the flowers are more easily detected by insects on the wing, and the pollen trans-
ferred by them from one flower to another. Even where (as in many Coniferae, &c.)
the light pollen is carried by the wind to the female flowers, this is accomplished
better when the flowers are at a considerable height from the ground ; and finally by
this means the dissemination of the seeds by the wind or by frugivorous birds is pro-
moted, or their scattering by the bursting of the fruits. That these arrangements for
propagation are especially promoted by the upright growth of the stem is evident from
the fact that in the large number of plants which develope their leaves in a rosette close
to the ground or on a stem that creeps along it, a rapidly ascending flower-stem is
formed only just before the unfolding of the flower-buds. Still more strikingly is
this the case in parasites and saprophytes {Orobanche, Neottia^ &c.), which vegetate
below and blossom above ground. If we recognise these and other special purposes
of upright growth, it is of interest to see in what various ways this one purpose is
attained in difi'erent species of plants. In many shrubs the growing stem is endowed
with sufficient firmness and elasticity to support in an upright position the weight of
the leaves, flowers, and fruits ; if it happen to be broken down, or if it must raise
itself from a previously creeping position, advantage is taken of the property of
geotropism. But the slender haulms of Grasses are not themselves endowed with
this power; and in their case the basal portion of each leaf-sheath forms a thick ring
the tissue of which retains for a long time its power of growth ; and when the haulm
is bent by the wind, or is in its early stage prostrate on the ground, the elevation into
an erect position is brought about by the surface of the node which faces the ground

^ In these examples I am compelled to confine myself to the most important points. Most of
the adaptations are so complicated that a detailed description of them in even a single plant would
require a great deal of space. What was said in the fourth chapter of this Book on climbing plants
and in the sixth on the adaptation of the foliar organs of a flower to the purpose of cross-
fertilisation may be consulted.

93 6 ORIGIN OF SPECIES.

growing rapidly and strongly; a knee-shaped bend is thus formed by which the
upper part of the haulm is raised up. If, on the contrary, the stem is perennial, and
has to bear a great weight of branches, leaves, and fruits, contrivances of this kind
are not sufficient, and then the tissue becomes woody ; if the weight of the crown
increases year by year, the stem also becomes thicker each year, as in dicotyledonous
trees and Conifers ; if the weight of the foliage does not increase, as in Palms, the
stem only retains the same thickness. In such cases a considerable quantity of as-
similated food-material is necessary in order to produce the massive solid stem, while
in many other cases the elevation is attained at the expense of a very small amount
of organic substance, as in climbing and twining plants, such as are found in the
most widely separated families of Angiosperms. Plants with a twining stem like the
Hop presuppose in general the existence and proximity of other plants which are able
themselves to grow upright and round which they twine ; and in order that such a
neighbouring support may be more easily and certainly taken hold of, the slender
stem of climbing plants is endowed with a power of revolution by which the apex
is carried round in a circle and is enabled to come into contact with the stem of an
upright plant, up which it then climbs.

The greater number of plants provided with tendrils are also dependent on the
proximity of. erect plants round which they can climb ; they are characterised by an
extreme parsimony in the employment of organic substances for the purpose of an
erect growth. Sometimes (as in the Grape- Vine) the tendrils are axial structures
furnished with minute leaves and branching from the axils of these ; but much
more commonly (as in Clematis or Tropceolum) the petioles, or (as in Fumarid) the
branched narrowly-divided lamina, or most often the metamorphosed apical parts of
the foliage-leaves (Cobcea scandens, the Pea and other Papilionaceae) are developed in
a filiform manner and perform the function of tendrils. The morphological signi-
ficance of the tendrils of Cucurbitacese is not yet perfectly determined; but they
are probably metamorphosed branches. Tendrils occur only in those plants whose
stem is not able to bear in an erect position the weight of the foliage, flowers, and
fruits ; in the genus Vicia, for example, all the slender-stemmed species have leaf-
tendrils ; but in the thick-stemmed erect V. Faba they are rudimentary. The office
of tendrils is to twine round the slender branches and the leaves of other neigh-
bouring plants, and thus to fix the apex of the stem as with cords on various sides
while it is growing upwards. The adaptation of tendrils, i. e. their endowment with
useful properties corresponding to their purpose, is, as Darwin has shown, not only
extremely diverse, but exhibits also very different grades of perfection, like that of
climbing stems. Some tendrils are only of slight use ; sometimes (as in some
species of Bignonid) they are merely helps to an imperfectly climbing stem ; but
where they are perfectly adapted to their function, a variety of properties concur in
a remarkable way to increase to a maximum this kind of adaptation to the use of the
plant. The tendrils radiate in different directions from the growing apex of the
shoot, which makes movements of revolving nutation by which the tendrils are
brought into the greatest variety of positions, they themselves also revolving at the
same time, so that within a certain area, often not a very small one, they assume an
infinite number of positions, by which they must almost inevitably be brought into
contact with some support, such as a branch or leaf, lying within this area. The

RELATION OF MORPHOLOGICAL NATURE OF ORGANS TO ADAPTATION. 937

supports are, so to speak, sought out in the most industrious manner ; when one is
touched by a tendril, the tendril bends in consequence of the stimulation and
twines firmly round it; and when several tendrils do the same in different direc-
tion from the stem, it hangs suspended between the points of support. If this
were all, the attachment would be a very weak one, and the elevation of the stem
would only take place slowly ; but the whole contrivance is perfected in the most
ingenious way. When the tendrils have fixed themselves by their extremities, they
draw the stem towards the support by twisting themselves spirally. When several
tendrils do this in different directions, the stem which is suspended between them
is tightly stretched, and the tenacity of the tendrils is at the same time con-
siderably increased by the twisting. Many tendrils, while very tender at the time
when they are sensitive, become afterwards hard and woody, and some become
much thicker ; this is strikingly the case in Clematis glandulosa and Solanum jasmi"
noides. But the most perfect adaptation is shown in the tendrils of the Virginian
Creeper, Bignonia capreolata, and some other plants : it is most perfect in Ampelopsis
hederacea. As in the Grape -Vine, the tendrils are here branched axial structures,
and are to a much greater extent negatively heliotropic ; their power of twining
round slender supports is but slightly developed, but when, in consequence of their
negative heliotropism, they come into contact with a wall, or in the wild state with a
rock, trunk of a tree, &c., there is formed in the course of a few days on each branch
of the tendril which touches the support with its curved and hooked apex, a cushion-
like swelling which afterwards expands into a red flat disc, and becomes firmly
attached by its surface to the support. The adhesion of this organ of attachment is
probably at first occasioned by an exudation of viscid sap ; but the attachment to
the support is caused mainly by this organ forcing itself into all the depressions in
the surface of the support and growing over the slight elevations. After this has
taken place the whole tendril becomes thicker; it contracts spirally, the stem to
which it belongs being thus drawn towards the wall, rock, &c. ; then it becomes
woody, and the firmness of its tissue and the power of retention of the disc are so
considerable that, according to Darwin ^ a tendril ten years old and furnished with
five of these discs can support a weight of 10 lbs. without giving way and without
the disc becoming detached from the wall. Since a shoot which is growing upwards
forms a number of tendrils, this attachment to the flat support is a very effectual one,
and enables the plant to endure the annually increasing weight of the stem which is
gradually becoming thicker and more woody; and in this way it climbs over the
walls and roofs of buildings more than 100 feet high. The fact is very interesting
that those tendrils of the Virginian Creeper which do not come into contact with the
wall or rock die after some time, and wither up into slender threads which then fall
off, no adhesive disc having been formed on them. But in order that these peculiar
tendrils may more readily come into contact with the support, the upright shoot
is scarcely at all positively heliotropic, since this property would cause it and its
tendrils to move further away from the supports ; while the young shoots which ex-
hibit very slight heliotropism become erect under the influence of gravitation; other-
wise the whole of the contrivances connected with the tendrils would be purposeless.

^ [Movements and Habits of Climbing Plants, 1875.]

938 ORIGIN OF SPECIES,

If looked at merely superficially, the mode in which the Virginian Creeper
climbs up rocks, walls, and thick trees, presents a certain resemblance to the climb-
ing of the Ivy ; but in fact the adaptations of the two are altogether different. It
has already been shown how negative heliotropism causes the leafy branches of the
Ivy to become closely pressed to the support, and how the summit of the branch at
first exhibits slight positive heliotropism, so that the slight convexity is in contact
with the support. At this point of pressure rows of aerial roots afterwards arise (not
in consequence of pressure, for they make their appearance also on branches which
hang free) which apply themselves to the inequalities of the bark of the tree or of
the rock which serves as a support, and thus fix the Ivy-stem to it. Other weak-
stemmed plants attain the same object (that of elevating their assimilating and
flowering shoots) by apparently much simpler means, as the Bramble, Rose, and
some cHmbing Palms hke Calamus, &c., whose long shoots spread over neighbour-
ing plants and are supported by them, their hooked prickles and other similar con-
trivances assisting in this.

It is of service to many plants in the struggle for existence that they should
keep firm possession of the piece of ground they have once occupied, without
forming for this purpose large woody masses, like trees and shrubs. The under-
ground parts of such plants are perennial, and they send up separate shoots in each
vegetative period to be exposed to the light and air where they will be able to
assimilate, to produce flowers, and to scatter their seeds. This persistence of the
underground parts has the advantage that the plant, although it assimilates and
grows only at particular times of the year, is not compelled to seek each year, like
annual plants, a new locality in which its seeds may germinate. The collection of
reserve food-materials underground gives strength to the plant ; it developes its buds
beneath the soil to such an extent that at the right time they can grow up quickly
at the expense of the rich supply of material. Every year very strong shoots are put
forth, while in annual plants a number of feeble seedlings perish annually before
some of them attain sufficient strength to protect themselves from the shade and
humidity to which their neighbours subject them. Plants whose underground parts
are perennial have in particular the power of resisting long and severe frost and the
greatest variations of temperature, because these only penetrate slowly beneath the
soil. It is for this reason that so large a number of Alpine and Arctic plants belong
to this class. They are also able to grow in localities which are much too dry for
the germination of the seeds of annual plants, because moisture is retained at a great
depth for a longer period than near the surface. Numerous other advantages might
also be mentioned which are compensated for in annual plants by other adaptations \

This permanence of the underground parts is attained in the greatest variety of
ways. Sometimes the plant possesses slender creeping underground shoots in which
the reserve food-materials are collected and which themselves rise above the surface
at a particular time, as in many Grasses ; or sometimes the leafy stems are developed
from lateral buds, as in Equisetum; or there are thick stout stems from which shoots

* [This subject — and especially the relation of peculiar habits of life to the power of resisting
great cold — is very fully discussed in Kerner's treatise Die Abhangigkeit der Pflanzengestalt von
Klima und Boden, Innsbruck 1869.]

RELATION OF MORPHOLOGICAL NATURE OF ORGANS TO ADAPTATION, 939

ippear each year at the same place. In some cases the whole plant is annually
renewed; all the parts which existed the previous year die off, and a complete
rejuvenescence of the individual is accomplished underground. In the Potato and
Artichoke only the apical parts of the underground lateral shoots swollen into tubers
remain over till the next year, the whole of the rest of the plant having perished. In
many of our native Orchids the rejuvenescence takes place in a similar way (see
p. 219 and fig. 158); and one of the most interesting cases of annual rejuvenes-
cence occurs in Colchicum auiumnale (see fig. 422). In these cases, with the excep-
tion of the Orchids, the reserve food materials accumulate in underground parts of
the axis; in other cases this takes place in the swollen roots, which remain in
connection with the underground part of the stem that bears the new buds, as in the
Hop, Dahlia, and Bryony. In bulbs again the reserve accumulates in the leaves
(bulb-scales) which surround the bud that developes into the new plant. The re-
serve often collects in cataphyllary leaves of peculiar development; in Allium Cepa
in the lower part of the leaf-sheaths, which persist through the winter, while the
upper parts of the leaves die off.

We have already in the last chapter spoken of the immense variety of the
contrivances which have for their object the partial or entire prevention of the
self-fertilisation of plants, in order to produce a stronger and more numerous off-
spring by the sexual union of different individuals ; and only a few examples need
now be mentioned. Just as the form, size, colour, position and movements of the
parts of the flower are almost invariably adapted to facilitate the conveyance of
pollen from one flower to another, generally by insects, and often also to render
self-fertilisation impossible ; and as a great diversity even of those forms of flowers
which are constructed on the same morphological type results from this, so the
properties of ripe seeds and fruits are no less adapted^ to bring about the dis-
semination of the seeds. Fruits which are very similar from a morphological point of
view may nevertheless assume physiological properties which are altogether different,
and fruits which are very different morphologically may become extremely similar in
consequence of their adaptation to the purposes of dissemination. The service
rendered by insects in the fertilisation of diclinous, dichogamous, dimorphic, and
many other flowers, is performed by birds in the dissemination of a number of seeds
which are concealed beneath fleshy edible pericarps ; in some cases, as the Mistletoe,
it is scarcely possible to imagine any other mode of dissemination than the eating of
the berries by birds. Dry fruits or the seeds which are shed by dry fruits are often
provided with an apparatus adapted for transport by the wind, the morphological
value of which is as various as possible. The wings on the seeds of the species
of Abies are outgrowths of a superficial layer of the tissue of the scale, those on the
seed of Bignonia muricata originate from the integument of the ovule ; the wings of
the indehiscent fruits (samaras) oi Acer, Ulmus, &c. are outgrowths of the pericarp;
the crown of hairs on the seed of Asclepias syriaca evidently performs the same
service as the pappus of many Compositse which is a metamorphosed calyx. In
these cases it is obvious that the wind carries the seeds or fruits; in other cases

* It is scarcely needful to mention again that this mode of expression has only a metaphorical
meaning from the stand-point here assumed, and is only used for the sake of convenience,

940 ORIGIN OF SPECIES.

animals of considerable size perform this ofBce involuntarily, the hooked or rough
fruits becoming attached to them and afterwards falling off ^

In most of these adaptations, both their purpose and the mechanical con-
trivances for its attainment are easily recognised; but not unfrequently the latter
require a closer examination and some reflection in order to understand them.
Among many other cases of this kind one only may be mentioned here which
any one can easily observe for himself. The fruit of Erodium gruinum and other
Geraniaceae'^ splits up into five mericarps each of which has the form of a cone with
the apex pointing downwards, containing the seed and bearing above a long awn.
When moist this awn is stretched out straight, but if it becomes dry while lying on
the ground the outer side of the awn contracts strongly, causing the upper end to
describe a sickle-shaped curve, which brings its point against the ground, the cone
being thus placed with its apex downwards. The lower part of the awn now begins
to contract into narrow spiral coils, causing the cone to turn on its axis and to
penetrate the ground, and the erect hairs on it which point upwards retain it there
like grappling-hooks. After the cone has penetrated the ground, the twisted part
of the awn does the same, driving the part which contains the seed further and
further into the soil. If the mericarp now becomes moistened, the coiled part
attempts to straighten itself, but its coils are held by the hairs which stand on
the convex surface ; and thus this movement also contributes to drive the cone
deeper into the soil. Whether therefore the moisture is greater or less, the me-
chanical contrivance produces the same effect, namely, to drive the part of the
mericarp which contains the seed into the soil.

Some of the contrivances found in plants are extremely striking, from the concur-
rence of the most different properties for the attainment of a perfectly definite purpose
corresponding only to certain specific conditions of life, as the adaptation of the Virginian
Creeper to climbing up vertical walls, the contrivance to prevent self-fertilisation in the
flowers of Aristolochia Clematitis, the bursting of the fruit of Momordica Elaterium^
and a thousand similar cases. The most remarkable instances are generally connected
with the ordinary arrangements, or even with other extreme cases, by a number of the
most diverse intermediate or transitional forms. These transitional forms have been
described in detail by Darwin in the case of climbing and twining plants, and the
fertilisation of Orchids, in his works already mentioned, and by Hildebrand in the case of
the fertilisation of Sal-via^, and of the dissemination of seeds.

Sect. 39. — The Theory of Descent. The facts and conclusions which have
been indicated rather than described are the foundation of the Theory of Descent.
This theory consists in the hypothesis that the most unlike forms of plants have a
relationship to one another of the same kind as that which the varieties gradually
developed from one ancestral form bear to it and to one another. It supposes
that the different species of a genus are varieties derived from one progenitor
which have undergone further development ; and that in the same manner the various
genera of an order owe their common characters to their descent from one and the

* [A remarkable instance of this is recorded by Dr. Shaw (Journ. Linn. Soc. vol. XIV, 1874,
p. 202), in the introduction into South Africa and enormously rapid distribution of a European
plant, Xanthium spinosum, by the spiny achenes clinging to the wool of the Merino sheep.] See also
Hildebrand, Die Verbreitungsmittel der Pflanzen, Leipzig 1873.

^ See Hanstein, Sitzungsber. der niederrheinischen Ges. in Bonn, 1868.

3 Jahrbuch fiir wiss. Bot. vol. IV, 1865. Also Verbreitungsmittel der Pflanzen.

THEORY OF DESCENT.

941

same older ancestral form, and their differences to variation and to the accumulation
)f new characters in successive individuals through a long series of generations.
'he theory of descent goes still further, and assumes the same mutual relationship
itween the various orders of a class, between the various classes of a group, and
inally between the various groups. It considers variation in the course of repro-
'duction to be the cause of all the differences among plants, and the heredity of
the varietal characters to be the cause of the agreement which subsists even be-
tween the most diverse forms of plants. What we call the common law of growth
of a class, or in other words its Type, is the result of all the plants of this class being
iescended from one ancestral form or Archetype, as Darwin terms it. That which
ras long since termed in a merely metaphorical sense the affinity between different
forms of plants is, according to the theory of descent, an actual affinity or blood-
Telationship in various degrees. The differences have arisen in the course of a long
series of generations, by the descendants of the same archetype continuing to vary
by the variation of the different individuals in different ways, and by the continual
and necessary increase of the differences between them under diverse conditions of
climate, especially under the conditions imposed by the struggle for existence, in
order that they may still be capable of maintaining themselves. At the same time
numberless varieties, species, and genera have gradually disappeared, because they
were not sufficiently adapted for the struggle for existence under the new conditions
caused by geological changes, and in consequence of the appearance of other forms
which were better adapted to resist them.

The scientific basis for the theory of descent rests in the fact that it alone is able
to explain in a simple manner all the mutual relationships of plants to one another,
to the animal kingdom, and to the facts of geology and palaeontology, their distribu-
tion at different times over the surface of the earth, &c. ; for this no other hypothesis
is necessary than descent with variation and the continued struggle for existence which
permits those forms only to persist that are sufficiently endowed with useful pro-
perties, the others perishing sooner or later. Moreover both these hypotheses are
supported by an infinite number of facts. The theory of descent involves only one
hypothesis that is not directly demonstrated by facts, namely that the amount of
variation may increase to any given extent in a sufficiently long time. But since the
theory which involves this hypothesis is sufficient to explain the facts of morphology
and adaptation, and since these are explained by no other scientific theory, we are
justified in making this assumption.

The theory of descent explains intelligibly how plants have obtained their
extraordinarily perfect adaptations for supporting the struggle for existence ; this
struggle has itself been the means of their obtaining them by the * Survival of the
Fittest,' that is, by permitting the existence and propagation of those newly-formed
varieties alone which are endowed with the various characters that render them
best fitted to the climate and to resist the rivalry of competitors, the attacks of
animals, &c. In this manner adaptations are gradually developed from a slight and
imperceptible beginning by the accumulation of useful characters which have the
appearance of being the result of the most careful and far-sighted calculation and
deliberation, or sometimes even of the most cruel caprice (as in the fertilisation of
Apocynum androscemi/olium by flies which are tortured to death in the process).

942 ORIGIN OF SPECIES,

The fact that members which are morphologically similar are adapted for the
most various functions is explicable when we consider that the morphological
features in the structure of plants are those which are most certainly transmitted
unchanged to posterity, either because they have no direct relation to the struggle
for existence, or because they have proved useful in the various relations of life ;
as for example the differentiation into stem, root, leaves, &c., and into the different
tissue-systems, by which the division of physiological labour and the acquisition of
the most various properties useful for the struggle for existence are facilitated. The
structure of the Thallophytes and of the Hepaticae shows that these morphological
differentiations do not exist in the first or lowest forms of plants, but that they come
gradually into existence ; but when once fully developed they are preserved in the
course of further variations, because they are never prejudicial, but often on the
contrary advantageous for the purposes of adaptation.

The perfect heredity of morphological characters gives rise to a very remarkable
phenomenon, the production of functionless members. It is obvious that hereditary
peculiarities may have lost their use under the new conditions of life of the de-
scendants, because the physiological requirements of the plant are supplied by other
means, by fresh adaptations. Of this nature are, for example, the minute leaves
on the root-like shoots of Psilotum, the formation of endosperm in the embryo-sac
of many Dicotyledons whose embryo afterwards grows so vigorously as to supplant
the endosperm, while it becomes itself filled with reserve food-materials which in
other cases are stored up in the endosperm for the seedling. The most striking
illustration however is the behaviour of parasites and saprophytes destitute of
chlorophyll, which are found in various orders of plants, and the near allies of
which form large green leaves containing chlorophyll, while these produce leaves
similar in a morphological sense, but which are neither large nor green, and
sometimes degenerated so as to have become obsolete. The explanation of this
phenomenon is at once afforded by the theory of descent, viz. that the parasites
and saprophytes which contain no chlorophyll are the transformed descendants of
leafy ancestors which did form chlorophyll, but which have gradually become ac-
customed to take up the assimilated food-materials of other plants or the available
products of their decomposition ; and the more they did this the less needful did
it become for the plants themselves to assimilate. The green leaves therefore
became meaningless and ceased to form chlorophyll; but without chlorophyll the
leaves were of Httle or no service to the new form, and therefore as little substance
as possible was employed in their development, and they gradually degenerated.

Looked at from the point of view of the theory of descent, the natural system
of the classification of plants represents their blood-relationship to one another.
A species consists of all the varieties which are descended from a common ancestral
form; a genus of all the species produced from an older progenitor, and which
have become in the course of time further differentiated ; an order includes all the
genera which have been derived by variation from a still older ancestral form ; and
the first primitive form of all the orders comprised in a group belongs to a still
older past ; finally there must have been a time when a primordial plant originated
the whole series of development ; and this must have produced in its varying de-
scendants the primitive types of all the later forms. The relationships of the various

THEORY OF DESCENT. 943

i classes and groups described at length in Book II might be represented by lines,
(which should express their actual affinity to one another; and the system of
diverging lines which would thus be obtained might be compared to an irregular
[system of branching.

It has frequently been attempted to draw up genealogical trees for the whole
Vegetable Kingdom and for the various groups of it, but the attempts made
hitherto are by no means satisfactory, for the incompleteness of our knowledge
as to the real relationships leaves too much room for the play of the imagination
and of subjective impressions. I will merely suggest here, as I did repeatedly in
Book II, that in endeavouring to form such a genealogical tree, the simplest forms
of the various types or classes must be especially considered, for it is in them
that the evidences of descent from common ancestral forms will most easily be
detected. With each of these simple forms, which do not differ widely from each
other, is connected a branching developmental series in which variation is taking
place independently of other series ; in consequence of this variation the difference
between the members of one series and those of another is increasing, so that the
most perfect forms of different types are those which are the most widely separated.

The theory of descent requires that the various forms of plants must have arisen
at different times, that the primitive forms of the separate classes and groups existed
at an earlier period than the derived ones ; and palaeontological research, although at
present it has but a very small amount of material at its disposal, supports this view.

In the same manner it is a necessary consequence of the theory that each plant-
form must have originated at a definite spot, that it must have spread gradually more
widely from that spot, that its change of locality in the course of generations must
have depended on climatic conditions, the competition of rivals, &c., and that its
distribution must have been impeded by hindrances or assisted by means of
transport ^ The geographical distribution of plants has already determined in
the case of many forms the spots on the surface of the earth or centres of distri-
bution from which they have gradually spread ; it has shown how the distribution
has been hindered sometimes by climate, sometimes by chains of mountains, some-
times by seas ; how more recently formed islands have been peopled by the plants
from the neighbouring continents which have become the ancestors of new species ^ ;
how some species when transported to a new soil (as European plants in America
and vice versa) have sometimes carried on a successful struggle for existence with the
native plants and have increased enormously. In the distribution of plants at present
existing, as for instance Alpine plants, it is possible to recognise the influences of
the last great geological changes, of the entrance and disappearance of the glacial
epoch and of earlier periods.

When we reflect what a number of generations our cultivated plants must have
passed through before any considerable amount of new properties were manifested in

^ Keraer has given an illustration of what can be accomplished in this direction in the rela-
tionships, geographical distribution, and history of the species of Cytisus from the primitive form
Tubocytisus, in his pamphlet Die Abhangigkeit der Pflanzengestalt von Klima und Boden, Inns-
bruck 1869,

^ See Dr. Hooker, On Insular Floras, Gardener's Chronicle, Jan. 1867; Ann. des sci. nat.
5th series, vol. IV. p. 266. [Wallace, Island Life, 1880.]

944 ORIGIN OF SPECIES.

their varieties, and how long it takes for these new properties to become hereditary,
and further how enormous is the diversity of hereditary properties, we are forced to
the conclusion that an inconceivably long period must have elapsed since the
appearance of the first plants on the earth. But geology and the physical nature of
the globe require as great a space of time for the explanation of other facts ; and a
few millions of years more or less is a matter of but little consequence in the expla-
nation of facts which require lapse of time in order to reach a given magnitude.

The first rudiments of the Theory of Descent, which holds good for the animal as for
the vegetable kingdom, may be traced to Lamarck, at the commencement of the century,
in his Zoologie Philosophique (Paris, 1801); it was afterwards advocated by Geoffroy
St. Hilaire ; but it is only since the publication of Darwin's work * On the Origin of
Species by means of Natural Selection' (London, 1859) that it has become an integral
part of science. Darwin's great service to science is to have established as a fact the
struggle for existence which all living beings have to fight, and to have proved its action
in the maintenance or destruction of new forms. It is only by means of the struggle for
existence that the motive principle is recognised, and that the theory of descent is enabled
to solve the great problem why parts which are morphologically similar are adapted for
such different functions ; and conversely also to show how purpose in organisation, and at
the same time the relations of affinity among plants, can be explained. Darwin considers
the Natural Selection which the struggle for existence brings about as the sole cause
of the increasing differentiation of plants which are undergoing variation ; he starts with
the hypothesis that every plant varies in all directions without any definite tendency to
become further developed in any one particular direction. He attributes to the struggle
for existence alone the power of securing the perpetuation of one or more varieties
among the countless numbers which are produced, and is convinced that in this way not
only is a perfect adaptation of the new forms effected, but morphological differentiation
is also carried further. Nageli ^ assumes, on the contrary, that each plant has in itself
a tendency to vary in a definite direction, to increase the morphological differentiation,
or, as it is commonly expressed, to perfect itself. The great differences of a purely
morphological nature between the classes and smaller divisions of the vegetable kingdom
may then owe their existence to this internal tendency towards a higher and more varied
differentiation ; while the struggle for existence brings about the adaptation of the
separate forms. The great services of the theory of descent remain in either case.

In all future research it will be of primary importance to distinguish clearly between
those peculiarities of plants which have no reference to the external world, which are,
that is, purely morphological, and those without which it would be impossible for the
plant to continue to exist under certain external conditions. It appears to be certain
that the latter are only produced by adaptation in the struggle for existence^.

The first and simplest plants had no ancestors ; they arose by spontaneous generation.
Whether this took place only once ; whether only one or a number of primitive plants
were produced simultaneously, giving origin in the latter case to diflferent developmental
series, or whether, as Nageli supposes, spontaneous generation has taken place at all
times, and is now taking place, giving rise to new developmental series, are questions
which still await solution, and which we cannot follow out further here.

* Nageli, Entstehung und BegrifFder naturhistorischen Art, Munich 1865.

^ Pringsheim has recently drawn attention to certain phenomena in the Sphacelarise which
show that a continued development with increasing morphological differentiation may take place
independently of the struggle for existence (Abh. Berl. Acad. 1873). Cultivated plants show, on
the other hand, that a considerable accumulation of new physiological properties may take place
unaccompanied by important morphological changes.

APPENDIX,

BOOK I.

Page 8. Cell-formation by Rejwvenescence. In most cases the cell produced con-
tains only one nucleus, but several may be present, as in the zoogonidium of Vaucheria
and in the oosphere of those Saprolegnieae in which there is only one oosphere in the
oogonium.

The ivhole of the protoplasm of the cell is not necessarily involved in this process, as
is shown by the development of the antherozoid in many cases.

Page 9. In the process of conjugation in Spirogyra the nuclei of the conjugating cells
were observed by Schmitz (Sitzber. d. niederrhein. Ges. in Bonn, 1879) to coalesce.

Page 10. From the researches of Schmitz on the Myxomycetes (Sitzber. d. nieder-
rhein. Ges. in Bonn, 1879), it appears that the nuclei of the cells which coalesce to form
the Plasmodium do not fuse but remain distinct : this case of coalescence of cells cannot,
therefore, be any longer regarded as an instance of cell-formation by conjugation.

Free cell-formation. From the account of this process given in the text, it is evident
that the expression 'free cell-formation' is now used in a sense different from that which
it originally possessed. In its original sense it implied the development de novo of
fresh nuclei around which the protoplasm became aggregated so as to form cells: it is
now applied to those cases in which many nuclear divisions take place before any cor-
responding cell-divisions occur. Taken in this sense, free cell-formation differs only in
degree from cell-division, and it is not possible to distinguish sharply between them :
for example, the development of the pollen-grains of Dicotyledons is usually regarded as
coming under the head of cell-division, but it may equally well be considered to be a case
of very limited free-cell formation in which only four nuclei are produced by division
before any cell-division takes place.

It must not be assumed that there is no such thing as a formation of nuclei de no'vo.
Strasburger (Bau u. Wachsth. d. Zellhaute) has pointed out that the appearance of the male
pronucleus in the oosphere during fertilisation (see p. 584) is an instance of it, inasmuch as
the nuclei of the pollen-grain do not pass directly into the oosphere, but break up and
become diffused either in the pollen-grain or in the pollen-tube. Johow has observed the
breaking-up and the reconstitution of nuclei in Chara (Bot. Zeitg. 1881).

Page 12. For minute details as to the development of spores and of pollen-grains,
see Strasburger, Bau und Wachsthum der Zellhaute, 1882.

Page 15. Cell-formation by Budding and Abstriction. Further instances of this are
afforded in the formation of the spores of Pellia epiphylla and some other Liverworts.
The mother-cell of the spores grows out into four protuberances, each of which becomes
shut off by a septum in the narrow pedicle and forms a spore (Strasburger, Zelltheilung und
Zellbildung, 1880).

Page 16. Vegetati-ve Cell-formation. In the case of naked (primordial) cells division
takes place by the gradual constriction of the protoplasm : this has been observed by
Schmitz (Mittheil. aus der zool. Stat, zu Neapel, I, 1878) in the formation of zoogonidia of
Halosphcera viridis ; by de Bary in the amoeboid zoogonidia of Myxomycetes ; by Pring-
sheim in the oospore of (Edogonium ; and by Kirchner in the oospore of Folvox minor.

Page 17. The Behaviour of the Nucleus during Division. The accounts of the structure
of the nucleus and of its behaviour during division given by Flemming and by Strasburger
do not agree in all points. The following are the principal differences between them :
(i) Flemming holds that the chromatin only exists in the form of fibrillae ; (2) he does
not agree with Strasburger that, in the splitting of the equatorial plate, any division of

3 P

94<5 APPENDIX. BOOK I.

the chromatin fibrillae takes place, but he considers that two groups of fibrillae are formed
which travel to the poles of the spindle; (3) he is of opinion that the equatorial plate
consists of chromatin and the rest of the spindle of achromatin, whereas Strasburger
states that the equatorial plate consists of the whole of the nuclear substance (both
chromatin and achromatin) and that the spindle consists of cell-protoplasm ; (4) Flem-
ming does not admit that the fibrillae, derived from the equatorial plate, which travel to
each pole, undergo fusion to form a new nucleus. (See Quart. Journ. Micros. Sci. 1882.)

The illustrative cases of cell-division which have been given in the text refer only to
cells which contain a single nucleus. In cells which are or are about to become multi-
nuclear, — in cases, that is, in which nuclear division is not followed by cell division,—
the process of nuclear division is usually simpler. In the older internodal cells of the
Characeae, in older parenchymatous cells of Lycopodium and of some Phanerogams
{Taraxacum, Glyceria, Sempervl'vum, Cereus, Solarium^ etc.), and occasionally in Falonia (in
all of which cases the cells become multinuclear), the nucleus simply divides by con-
striction, the chromatin granules being shared equally between the two new nuclei
without any indications of karyokinesis. Division may begin again in the two new
nuclei even before they are separated from each other. This process of nuclear division
has been termed fragmentation. In Falonia and in Codium a rudimentary form of karyo-
kinesis has been observed by Schmitz and by Berthold (Mittheil. der zool. Stat, zu
Neapel, II, 1880), which appears to occur commonly among Thallophytes. In such a case
the nucleus becomes elongated ; its ends enlarge, whereas the middle part remains narrow ;
the ground-substance of the nucleus now presents a faint longitudinal striation, and the
chromatin-granules either simply travel to the two ends of the nucleus, or become rod-
shaped and aggregate in the equatorial plane to form a rudimentary nuclear disc, which
splits in the ordinary way, each half travelling to one end of the nucleus. The narrow
middle portion now undergoes absorption, and the two ends round themselves off to con-
stitute two new nuclei. Treub (Sur des cellules v^g^tales ^ plusieurs noyaux. Arch.
N^erland., XV, 1880) has observed the division of the nucleus in the multinuclear bast-
fibres and laticiferous cells of various Phanerogams, and finds that it takes place in the
manner described in the text for uninuclear cells, except that no cell-plate is formed.
See also Johow, Die Zellkerne von Cbara, Bot. Zeitg. 1881.

An illustration of the independence of cell-division with regard to nuclear division
is afforded by Cladophora. The cells of this plant are multinuclear, and Strasburger has
found that the division of the nuclei takes place in the manner observed by Treub in
bast-fibres and laticiferous cells : the division of the cells bears no relation, either in
time or space, to that of the nuclei ; the new cell-wall is formed in much the same
manner as in Spirogyra.

Page 19. Schmitz has come to the conclusion {loc. cit. 1880) that the cell-wall is
formed by the actual conversion of a peripheral layer of the protoplasm into cellulose.
He is also of opinion that stratified cell-walls are formed by the deposition, one within
the other, of successive layers, and not by intussusceptive growth with subsequent differ-
entiation. These views are also held by Strasburger ; Bau und Wachsthum der Zell-
haute, 1882 (see p. 960).

Both Schmitz and Strasburger hold that the surface-growth of cell-walls is not due
to the intercalation of new solid particles (intussusception), but is simply the expression
of the stretching of the cell-wall by the cell-contents.

Frommann (Protoplasma, 1880) believes that he has been able to trace a connexion
of the protoplasm through the walls of adjacent cells. That protoplasm can pass through
closed cell-walls is beyond doubt. See Strasburger, loc. cit. p. 247.

Page 23. The formation of bordered pits. A very different account from that in
the text is given by Mikosch (Unters. lib. Entstehung und Bau der Hoftiipfel, Sitzber. d.
k. k. Akad. in Wien, LXXXIV, 1881). Sachs has found (Ueb. die Porositat des Holzes,
Arb. d. bot. Inst, in Wiirzburg, II, 1879) that in the spring-wood at least the bordered pits
are closed by a membrane. See also Strasburger, Bau und Wachsth. d. Zellhaute, 1882.

APPENDIX. BOOK I. g^y

Page 29. With regard to the growth of the cell- wall by intussusception, see note
above referring to page 19.

Page 30. With regard to the growth in thickness of the cell-wall, see the note
referring to page 19.

Page 36, 1 6th line from the top: for * molecules' read 'micellae.*

Page 37. Protoplasm. According to Schmitz (Unters. ueb. die Struktur des Pro-
toplasmas und der Zellkerne, Sitzber. d. niederrhein. Ges. in Bonn, 1880), a net-work
of fine fibres exists in protoplasm, the meshes of which are occupied by a homogeneous
fluid : the net-work readily stains with haematoxylin, but the fluid remains colourless. In
the endoplasm, more particularly, a greater or smaller number of fine granules, termed by
Hanstein microsomata, which stain deeply with haematoxylin, are present. (See also
Frommann, Beob. iib. Structur und Bewegungserscheinungen des Protoplasma, Jena, 1880.
—Hanstein, Das Protoplasma, Heidelberg, 1880: id., Einige Zuge aus der Biologic des
Protoplasmas, Botanische Abhandlungen, IV. 2, 1880. — Strasburger, Zellhaute, 1882.— On
the Chemistry of Protoplasm, see Reinke, in Unters. aus dem Bot. Lab. der Univers.
Gottingen, II, 1881.)

Page 44. The Nucleus. Schmitz has detected one or more nuclei in a number of
Thallophytes {Batrachospermum moniliforme, Codium, Faucheria, Caulerpa, Conferva, Gon-
grosira, Schizogonium, Monostroma, Chlamydomonas, Chroolepus, Saprolegnia, Mucor, Sac-
charomyces, Mycoderma, Oidium, Exoascus, Peziza, Morchella, Ascobolus, Myxomycetes, etc. •
(Sitzber. d. niederrhein. Ges. in Bonn, 1879 and 1880): he believes that the Phycochro-
maceae and the Schizomycetes are the only plants in which no nucleus is present.

With regard to the function of the nucleus, it does not appear that it regulates the
process of cell-division. Strasburger is of opinion (Bau und Wachsthum der Zellhaute,
1882, p. 241) that it is of importance in the 'nutrition of the cell, more especially as regards
the formation of proteids.

With regard to the structure of the nucleus, see page 18.

Chemically the nucleus consists of a substance termed nucle'in, to which Miescher has
given the formula C29 H49 Ng P3 O22. (See Hoppe-Seyler, Physiologische Chemie ; also
Zacharias, Die chemische Beschaffenheit des Zellkerns, Bot. Zeitg. 188 1.)

Page 48. On the development of chlorophyll-granules, see Mikosch, Unters. lib.
die Entstehung der Chlorophyllkorner, Sitzber. d. k. k. Akad. in Wien, LXXVIII, 1878.

Page 49. Structure of chlorophyll-granules. Pringsheim has observed (Ueb. Licht-
wirkung und Chlorophyllfunction in der Pflanze, Jahrb. f. wiss. Bot., XII, 1881) that
when green parts of plants are treated with dilute acids or with warm water the colouring-
matter exudes in viscid drops, the colourless basis remains behind and presents a trabecular
structure in the interstices of which the colouring-matter was previously contained.

Page 49-51. Crystalloids and Aleurone-grains. On Rhodospermin, see note on
page 289. — Schimper, Unters. iib. die Proteinkrystalloide der Pflanzen, Strassburg, 1879 :
id., Ueb. die Krystallisation der eiweissartigen Substanzen, Zeitschr. fiir Krystallographie,
1880. — Vines, On the Proteid Substances contained in the Seeds of Plants, Journal of
Physiology, III, 1881. — Julius Klein, Die Krystalloide der Meeresalgen, and Die Zellkern-
Krystalloide von Pinguicula und Utricularia, Jahrb. f. wiss. Bot. XIII, 1881.

An aleurone-grain consists of a mixture of proteids, some of which are soluble in
water, and others in either saturated or dilute solution of common salt; the former
belong to the chemical group of peptones, the latter to the globulins. In some cases the
grain does not dissolve in salt solution ; this is probably due to a chemical alteration of
the globulins into albuminates. The crystalloids consist chemically of globulins or of
altered globulins (albuminates).

Page 57 et seq. In the account of the growth of starch-grains the word * micella' is
to be substituted for 'molecule' (see Bk. Ill): it is also preferable to use the word
' hilum ' instead of ' nucleus.'

With reference to the mode of growth of starch-grains, it appears from Schimper's
observations (Unters. iib. das Wachsthum der Starkekorner, Bot. Zeitg. 1881) that they

3 p 2

948 APPENDIX. BOOK I.

grow by apposition and not by intussusception. He is led to the conclusion that a starch-
grain is a sphero-crystal, built up of prismatic crystalloids. (By a * crystalloid ' is meant here
and elsew here a crystal which is capable of swelling-up.) This conclusion is supported by
Meyer {ibid.), and by Strasburger (Bau und Wachsthum der Zellhiiute, 1882), but is severely
criticised by Niigeli (Unters. lib. das Wachsthum der Starkekorner, Bot. Zeitg. 188 1).

It is known that when starch-grains are formed in parts of plants exposed to light,
they arise in connexion with the chlorophyll-granules. Schimper has made the interesting
observation (Bot. Zeitg. 1880; also Researches upon the Development of Starch-grains,
Quart. Journ. Micr. Sci., 1881) that when they are formed in parts of plants not exposed
to light they arise in connexion with small masses of protoplasm which he terms ' starch-
forming corpuscles' {Starkebildner). That they are closely related to the chlorophyll-
granules is shown by the fact that if cells containing starch-forming corpuscles are exposed
to light, the corpuscles turn green and become in fact chlorophyll-granules. Errera
suggests the name ' amidoplasts ' for these bodies.

Schimper observed that the point of attachment of the starch-grain to the chlorophyll-
granule or starch-forming corpuscle lies in the line of the long axis of the grain, — that is,
in the line of most rapid growth, — and at its broader end ; the hilum is near the free
narrow end of the grain. These facts afford a strong argument in favour of the growth of
the grain by apposition.

Page 64, line 9 from the top. For * erecta ' read ' cvecta.^

Page 65. On the distribution of calcium carbonate, see Molisch, Ueber die Abla-
gerung von kohlensaurem Kalk im Stamme dicotyler Holzgewachse, Sitzber. d. k. k. Akad.
in Wien, LXXXIV, 1881.

Page 68. Cystoliths. See also Richter, Beitr. zur genauern Kenntniss der Cysto-
lithen, etc., Sitzber. d. k. k. Akad. in Wien, LXXVI, 1877, and Melnikoff, Unters. iib. das
Vorkommen des Kchlensauren Kalkes in Pflanzen, Diss. Inaug., Bonn, 1877.

Page 70. The statement made here with reference to the formation of the endo-
sperm in the embryo-sac of Phanerogams is not quite accurate : compare p. 585.

For a more complete account of the Morphology of the Tissues, see de Bary,
Vergleichende Anatomic der Vegetationsorgane der Phanerogamen und der Fame, 1877.

Page 77. Leitgeb has found (Die Athemoffnungen der Marchantiaceen (Sitzber.
d. k. k. Akad. in Wien, LXXXI, 1880) that the hypodermal chambers of these plants is not
formed, as described in the text, by the separation of the epidermal cells from the sub-
jacent tissue : these chambers make their first appearance as pits which become overgrown
by the epidermal cells which form their limits, and the communication between the cavity
of the pit and the external air may be continuous from the beginning, or the pits may
become completely closed in by the overgrowth of the surrounding epidermal cells, the
communication being restored on the development of the stoma. In both cases the cells
forming the stoma are not derived from a single mother cell; in the former case the
stomatal cells are formed by the cutting off of contiguous segments from the cells bounding
the opening ; in the latter, by the cutting off of segments in a similar manner from the
cells lying over the centre of the chamber, and by the subsequent separation of these
segments so as to form an opening between them. The stomatal cells may then divide so
as to form a series of superposed cells, and thus the complex stoma of Mar chant ia, for
example, is produced.

Page 86. On the development of laticiferous vessels, see Schmalhausen, Beitr. z.
Kenntniss der Milchsaftbehalter der Pflanzen, M6m. de TAcad, imp. de St. Petersbourg,
XXIV, 1877; also Scott, The Development of Articulated Laticiferous Vessels, Quart.
Journ. Micr. Sci., 1882, and Schmidt. Bot. Zeitg., 1882.

Page 88, line 2 from the bottom. The statement in the text that sieve-tubes occur
only in the fibro-vascular bundles is not correct. Scattered bundles of them occur in the
stems of many Dicotyledons and Monocotyledons ; in the periphery of the pith in Solanum
tuberosum, Dulcamara, species of Nicotiana, Datura, and Cestrum, in many Campanulaceae,
and among Composites in Gundelia Tournefortii, and in the genera Lactuca, Scor%onera^

APPENDIX. BOOK I. 949

Sonchus, ^ragopogoriy and Hieracium ; in the cortex of thick-stemmed Cucurbitaceae, such as
Cucurbita, Lagenaria, Cucumis, Ecbalium^ and in many species of Potamogeton {P. natansy
lucens, pectinatus).

In Strychnos no sieve-tubes are formed in the secondary phloem, but they occur in the
xylem (de Bary. Vergleichende Anatomic, p. 594).

Page 90. For a full account of the structure of sieve-tubes, and of their transverse
connexion, see Wilhelm, Beitr. z. Kenntniss des Siebrohrenapparates Dicotyler Pflanzen,
Leipzig, 1880: also, Janczewski, Etudes comparees sur les tubes cribreux, Cherbourg,
1881.

Page 93. On the development of resin-ducts, see Kreuz, Beitr. ziir Entwicke-
lungsgeschichte der Harzgange einiger Coniferen, Sitzber. der k. k. Akad. in Wien,
LXXVI, 1877.

Page 98. Stohr (Ueb. Vorkommen von Chlorophyll in der Epidermis der Phanero-
gamen-Laubblatter, Sitzber. d. k. k. Akad. in Wien, LXXIX, 1879) has found chlorophyll-
granules in the epidermal cells of the leaves of a large number of plants.

Page 102. Mention should be made, under the head of Stomata, of the water-
stomata or water-pores which occur on the leaves of those plants (such as Alchemilla, Zea,
many Aroids, Saxifragacese, and Crassulaceae) which excrete drops of water. In some of
these an ordinary stoma serves as a water-stoma ; in others the water-stoma is larger than
the ordinary stoma, and its guard-cells are incapable of opening and closing the aperture.
In the Saxifragacese and Crassulaceae the mesophyll-cells beneath the water-stoma are
differentiated so as to form a more or less well-defined mass of tissue, the water-gland,
which appears to effect the excretion of saline substances (principally calcic' carbonate) in
solution in the excreted water. In each such gland a fibro-vascular bundle terminates.
See Gardiner, Quart. Journ. Micr. Sci., 1881 ; also de Bary, Vergleichende Anatomic, pp.
55, 113, 389.

Page 105. On the development of the stomata of Marchantia, see the note above
which refers to p. 77.

Page 106. On Cork, see von Hohnel, Ueber den Kork und verkorkte Gewebe
iiberhaupt, Sitzber. d. k. k. Akad. in Wien, LXXVI, 1877. When cork is developed in roots
it is formed by the division of the cells of the pericambium, the primary cortex being
gradually thrown off.

Page 108. On Lenticels, see Haberlandt, Beitrage zur Kenntniss der Lenticellen,
Sitzber. d. k. k. Akad. in Wien, LXXII, 1875, and Kreuz, ibid.y Entwickelung der Lenti-
cellen an beschatteten Zweigen von Ampelopsis hederacea.

Page 112. In addition to collateral and concentric bundles the following may also
be distinguished : —

1. BicoUateral bundles, in which (as mentioned on page 1 1 1) there is a layer of phloem

on the inner as well as on the outer side of the xylem; Cucurbitaceae,
Melastomaceae, Cichoriaceae, Solanacese, Asclepiadaceae, Apocyneae, Strychnosy
Daphne, Eucalyptus Globulus, and probably also Metrosideros, Callistemon, Mela-
leuca, Myrtus, and the other species of Eucalyptus,

2. Radial Bundles, in which the xylem and the phloem strands lie on different radii.

This arrangement obtains almost universally in roots, exceptions being found

in those of Dioscorea Batatas, of Ophrydeae, and perhaps in those of Sedum

Telephium and its allies, in which the bundles are collateral. The bundles

in the stems of Lycopodiaceae are rather to be regarded as radial than as

concentric as is done in the text. (De Bary, Vergl. Anat.)

Page 114. The Fibro-vascular System of Roots. On the transition from the fibro-

vascular system of the stem to that of the root, see Sophie Goldsmith, Beitr. z. Ent-

wickelungsgeschichte der Fibrovasalmassen im Stengel und in der Hauptwurzel der

Dicotyledonen, Diss. Inaug. Zurich, 1876; Gerard, Passage de la Racine a la Tige, Ann.

Sci. Nat. sen 6, t. XI, 188 1.

Page 116. The 'vessels' (tracheae) in most plants are really tracheides : this is the

950

APPENDIX. BOOK I.

case with the peripheral ends of fibro-vascular bundles in all plants ; with the vascular
elements of the secondary xylem of Conifers and Cycads, and with most of those of the
secondary xylem of woody Dicotyledons and of many Monocotyledons ; with the vascular
elements of Ferns and their allies (true nessels are known only in Pierh aquilina and in the
root oi Athyrium Filix fcemind). See de Bary, Vergleichende Anatomic, p. 172.

The definition given in the text is not satisfactory : a tracheide is a closed vascular
cell ; true 'vessels are formed by the fusion — that is, by the absorption of the intervening
septa — of trache'ides.

Page 118. De Bary (Vergleichende Anatomic) distinguishes the following kinds of
prosenchymatous xylem-elements in Dicotyledons (see also p. 651): —

(i) True Woody Fibres : differing from tracheides in the absence of the internal
spirally-fibrous layer of their wall, and in that the pits are narrow, elongated
transversely, and oblique ; they contain a small residue of protoplasm,
(2) Fibres (secondary wood) : septate or unseptate, of two kinds,

a. resembling the true woody fibres, but differing from them in that

they almost always contain starch, and sometimes chlorophyll or
tannin ;

b. shorter fibres, tending to parenchymatous form (hence termed ersatZ'

fasern or ' intermediate cells ').
Page 130, line 13 from the top. Dele the word * cortex.'

„ line 16 from the top. For ' phloem {K) ' read ' cortex (jR).'

Page 131, line 20 from the top. For ' secondary cortex' read 'secondary phloem.*
Page 132, line 25 from the top. Dele the word 'phloem.'

Page 135. The cambium which is formed outside the primary xylem-bundles arises
in the pericambium.

Page 136, section (c). The following is to be substituted for the last two para-
graphs of this section ; —

1. The successive rings of bundles originate in the primary cortex: Menispermeae,

Cycadese, Avicennieae.

2. They originate in the primary phloem : Phytolacca.

3. They originate in the secondary phloem: Wistaria^ Bauhmia, Rhynchosia (Legu-

minosae) ; Securidaca (Polygalaceae) ; Gnetum; Doliocarpus (Dilleniaceae) ;
Phytocrene (Olacineae). (See de Bary, Vergleichende Anatomic, 1877.)

Section (d). The account of the mode of growth of the stems of Bignoniaceae given
by de Bary {loc. cit.) differs somewhat from that in the text. According to him four
symmetrically placed fibro-vascular bundles of the ring of primary bundles in the stem are
from the first larger than the others, all the bundles being connected by a normal cam-
bium-ring. When growth in thickness begins it proceeds normally at all points except in
the four above-mentioned bundles ; in these the formation of xylem elements is very much
smaller than in the adjoining bundles, whereas the formation of bast is much more con-
siderable. The cambium layer consequently loses its originally circular outline and
becomes deeply infolded where it extends inwards to each of these four bundles. Where
it lies radially with respect to the primary bundles, that is, where it is parallel to the
circumference, the cambium-layer gives rise to xylem and phloem elements in the normal
manner: where the infoldings take place, that is, where it lies upon the sides of the bundles
in which the wood has been normally developed, it forms only parenchyma, so that con-
siderable masses of cortical tissue are formed between the normally developed bundles^
corresponding in position to the four abnormally developed bundles.

Page 137, line 2 1 from the bottom. None of the scattered bundles in the pith of
Piperaceae, Amarantaceae {Amarantus, Euxolus), or Nyctagineae are cauline : they are leaf-
traces (de Bary, Vergl. Anat.). The structure of these abnormal forms will be rendered
more intelligible by the following considerations. The primary bundles in the dicotyle-
donous stem are in some cases all leaf-traces (e. g. Ricinus, Cucurbita), but more commonly
other primary bundles which are cauline are formed between the primary leaf-traces.

APPENDIX. BOOK I. 95 1

When all the bundles lie in a ring, the characteristic structure of a young dicotyledonous
stem is seen in a transverse section ; when they do not lie in a ring, the appearance pre-
sented by a section is that of an abnormal stem. In the instances mentioned above the
latter is the case ; but a more marked instance of this is afforded by Podophyllum, for
example, in which, owing to the irregular distribution of the primary bundles (which are
here all leaf-traces), a transverse section of the stem somewhat resembles that of a mono-
cotyledonous stem.

The account given in the text of the formation of the cambium-ring in Cbavica
(Piperaceae) will be correct if for * cauline bundles ' ' inner leaf-trace bundles * be sub-
stituted : that given of the Begoniaceae, in which the internal bundles are cauline, is
correct.

Section 19. For an account of the laws according to which cell-divisions take place
in growing organs, see Sachs' important paper Ueb. die Anordnung der Zellen in jiingsten
Pflanzentheilen, Arb. d. bot. Inst, in Wiirzburg, II. i, 1878. The following are the more
important points to which he draws attention : —

1. The walls are formed at right angles to those which they intersect.

2. The planes of the walls in a growing-point are classified thus :

a. Periclinal, those which are curved in the same direction as the surface

(seen in longitudinal section).

b. Anticlinal, those which intersect the surface and the periclinal walls at

right angles; they thus constitute a system of orthogonal trajec-
tories for the pericHnal walls.

c. Radial, those which pass through the axis of growth and intersect the

surface at right angles.

d. Transverse, those which intersect both the axis of growth and the

surface at right angles.
The relation of the periclinal and anticlinal planes are illustrated by the following
cases : —

a. If the outline (in longitudinal section) of the growing-point is a para-

bola, the periclinals will constitute a system of confocal parabolas of
different parameter, the focus of the system being at the point of
intersection of two lines of which one is the direction of the axis
and the other of the parameter. In this case the anticlinals being
the orthogonal trajectories of the periclinals, constitute a system of
confocal parabolas the axis and focus of which coincide with those
of the periclinals.

b. If the outline of the growing-point is a hyperbola, the periclinals will be

confocal hyperbolas with the same axis but different parameter;
the anticlinals will be confocal ellipses, with the same focus and axis
as the periclinals.

c. If the outline of the growing-point is an ellipse, the periclinals will be

confocal ellipses ; the anticlinals will be confocal hyperbolas.

It must not be supposed that the outline of a growing-point is necessarily one or
other of these well-known geometrical forms ; they are selected merely because they serve
to illustrate clearly the rectangular intersection of the cell-walls as they are formed, and
because, when the nature of the periclinal and anticlinal curves is unknown, their relations
may be inferred by analogy.

An interesting deviation is found in those roots (such as those of Papilionaceae) in
which the apex of the plerome are open, /. e. are directly continuous with the tissue of the
root-cap ; this is to be ascribed to the fact that the periclinals at the end of the root
become parallel or even tend to diverge upwards.

3. The arrangement of the cell- walls in these planes is most perfectly seen in the

growing-points of plants which do not possess a single apical cell but in
which there is a small-celled meristem. A large apical cell interrupts the

95^ APPENDIX, BOOK I,

continuity of the planes which are evident in the divisions which take place
in the segments cut off from the apical cell.
4. It is probable, contrary to the generally received view, that, in confocal grow-
ing-points, the apical cell represents the most slowly growing portion,
whereas in non-confocal meristem-protuberances the most active growth
may take place at the apex. Westermaier however has come to the con-
clusion (Ueb. die Wachsthumsintensitat der Scheitelzelle, Jahrb. f. wiss.
Bot., XII, 1 881) that the maximum of increase in volume within the apical
region is exhibited either by the apical cell itself or by the youngest
segments.
Page 140. On the apical growth of Metzgeria furcata, see Goebel, Ueb. das Wachs-
thum von Metzgeria furcata und Aneura^ in Arb. d. bot. des Bot. Inst, in Wiirzburg,
11. 2, 1879.

When the segments are cut off from the apical cell by oblique walls these are parts of
anticlinals which are completed by the secondary divisions in the adjacent segments (see
fig. 150 A),

Page 146. On growing-points of stems without an apical cell, see Schmitz, Beob.
ueb. die Entwickelung der Sprossspitze der Phanerogamen, Halle, 1874.

Page 147, line 3 from the bottom. It is only in certain cases that the root-cap of
Phanerogams is derived from the dermatogen.

On the structure of the growing-points of roots, see Janczewski, Recherches sur le
developpement des racines dans les Phan^rogames, Ann. d. Sci. Nat., ser. 5. t. XX, 1874 :
— Treub, Le Meristeme primitif de la Racine dans les Monocotyledones, Leyden, 1876: —
Eriksson, Ueb. das Urmeristem der Dikotylen-Wurzeln, Jahrb. f. wiss. Bot. XI, 1878: —
Flahault, Recherches sur I'Accroissement terminal de la Racine chez les Phanerogames,
Ann. d. Sci. Nat., ser. VI. t. 6, 1878: — Olivier, Rech. sur I'appareil tegumentaire des
Racines, Ann. d. Sci. Nat., s6r. 6, t. XI, 188 1. A good account is also given in de Bary's
Vergleichende Anatomic.

From Janczewski's researches, of which the following is a brief abstract, it appears
that in many cases there is a distinct meristematic layer, which he terms the calyptrogen,
from which the root-cap is derived : —

Type I. The meristem consists of four distinct layers, Plerome, Periblem, Der-
matogen, and Calyptrogen : Hydrocharis Morsus Ranee, Pistia Stratiotes.
Type 2. A distinct Plerome and Calyptrogen ; the Periblem and the Dermatogen
have common initial-cells : many Monocotyledons (Juncaceae, Haemo-
doracesE, Cannaceae, Zingiberacese, Typha, Cyperaceae, Gramineae, Com-
melineae, Potameae, Juncagineae, Sagittaria, Limnocharis, Stratiotes).
Type 3. A distinct Plerome ; the Calyptrogen, Periblem, and the Dermatogen
have common initial cells (Treub) : many Monocotyledons (Liliaceae,
Astelieap, Xerotideae, Aspidistreae, Ophiopogoneae, Amaryllideae, Hypo-
xideae, Dioscoreae, Taccaceae, Bromeliaceae, Musaceae, Orchideae, Palmae,
Pandaneae, Cyclantheae, Aroideae (except Pistia), Irideae, Pontederieae,
Sparganium, Butomus, Alisma (?).
Type 4. A distinct Plerome and Periblem ; the Dermatogen and the Calyptrogen
have common initial cells : most Dicotyledons {Helianthus annuus,
Fagopyrum, Raphanus sati'vus, Myriophyllum, Salix, Casuarina stricta,
Linum usitatissimum, Primulacese).
Type 5. A group of initial cells common to all four layers : some Dicotyledons

{Cucurbita, Pisum, Phaseolus, Cicer).
Type 6. A distinct Plerome and Periblem only ; hence there is no true epidermis
or root-cap, these being formed simply by the outer layers of the peri-
blem (cortex) : Gymnosperms.
Cryptogams. According to the observations of Strasburger (Coniferen und Gne-
taceen) and of Bruchmann (Ueb. Wurzeln von Lycopodium und Isoetes,

APPENDIX. BOOK I, 9^3

Jena, 1874) the structure of the growing-point of the root oi Lycopodium
is the same as that described above under Type i.
According to Bruchmann the structure of the growing-point of the root of Loetes

agrees with that described under Type 4.
At the growing-point of the root in the Marattiaceae there is a group of several
large polygonal cells. From these segments are cut off parallel to the base to
form the root-cap, and internally segments are cut off to increase the Plerome.
The plerome-segments are further divided by longitudinal walls, and the more
external of the cells thus formed constitute the cortex, which is differentiated
at a lower level into Dermatogen and Periblem. These roots are thus inter-
mediate in their structure between those which have a single apical cell and
those which have a small-celled meristem. In the case of stems, those of
certain Selaginellx {arborescens, Per'villei, JVallichii, Lyalli) occupy an analogous
position.
Page 149. For a discussion of the physiological causes of the morphological differ-
entiation of Plants, see Sachs, Stoff und Form der Pflanzenorgane, Arb. d. bot. Inst, in
Wiirzburg, II. 3, 1880, and 4, 1882: see also Vochting, Ueb. Organbildung im Pflanzen-
reich, 1878, and F. Darwin, The Theory of the Growth of Cuttings, Journ. Linn. Soc
XVIII, 1881. ''

Page 153. On the Anatomy and Morphology of the Leaf, see J. Ghatin, De la
Feuille, Paris, 1874; and Goebel, Beitr. zur Morphologic und Physiologic des Blattes,
Bot. Zeitg. 1880 and 1882.

Page 163, fig. 122. The root represented here affords a good illustration of the
structure of the apex described above under Type 2 ; it is obvious that the original
interpretation was the right one, namely, that the root-cap is not developed from the
dermatogen.

Page 166. On the development of the lateral roots, see Janczewski, Recherches sur
le Developpement des Radicelles dans les Phanerogames, Ann. d. Sci. Nat, ser. 5, XX,
1874.

In the Equisetacese the plerome- (bundle-) sheath consists of two layers (Van
Tieghem), and it is from cells of the inner layer that the lateral roots are developed.
Line 5 from the bottom. For ' vascular ' read ' xylem.'

Page 168, paragraph (e). For another case of the conversion of a root into a stem,
see Goebel, Ueb. Wurzelsprosse von Anthurium longifolium, Bot. Zeitg. 1878.

Page 170. Wel'ujitschia produces two leaves in addition to the cotyledons (see Bower,
Q^J. M. S. 1881).

Page 171. It has been found (see p. 400) that the shoots of Equisetaceae are not
endogenous.

Page 172, paragraph (a). See Heinricher, Ueb. Adventivknospen an der Wedel-
spreite einiger Fame, and. Die jiingsten Stadien der Adventivknospen an der Wedel-
spreite von Asplenium bulbiferum, Sitzber. d. k. k. Wien. Akad. LXXVIII, 1878, and
LXXXIV, i88r. He finds that in Asplenium the adventitious bud is developed from a
single superficial cell.

Page 173, paragraphs (b) and (c). On the adventitious development of organs, see
Hansen, Vergl. Unters. ueb. Adventivbildungen bei den Pflanzen, Frankfurt, 1881.

Paragraph (e). It has been shown by Janczewski and by Famintzin that the lateral
buds of Equisetaceae are not of endogenous origin.

Page 184, paragraph (d). It is pointed out in Book II that the branching of the
stem of the Lycopodiaceae is not dichotomous in all cases.

Page 187. Phyllotaxis. On this subject see Schwendener, Mechanische Theorie
der Blattstellungen, Leipzig, 1878. According to him the relative positions of lateral
members depends upon (i) the relative size of the lateral members (when they are deve-
loped close together), and (2) the increase in length and in thickness of the axis bearing
them.

954 APPENDIX. BOOK II.

Page 204, paragraph (4). Sachs points out (Ueb. orthotrope und plagiotrope Pflan-
zentheile, Arb. d. bot. Inst, in Wiirzburg, II. 2, 1879) that most monosymmetrical or
bilaterally symmetrical organs present not only two symmetrical (right and left) halves,
but also dorsal and ventral halves which are of different internal structure; such organs he
describes by the term dorsi'ventral. When this is the case the two halves react differently
to external forces (light, gravity, etc.), and the organ is, according to his terminology,
plagiotropic. Some bilateral organs are therefore plagiotropic, but this peculiarity is not
confined to them, for some polysymmetrical organs are plagiotropic also (see Book III.
p. 854 ; also Goebel, Ueb. die Verzweigung dorsiventraler Sprosse, Arb. d. bot. Inst, in
Wiizburg, II. 3, 1880). The term * actinomorphic ' is often used as synonymous with
* polysymmetrical.'

Page 209. On the morphology of Begonia, see Eichler, Ueb. Wuchsverhaltnisse der
Begonien, Sitzber. d. Ges. naturfor. Freunde, Berlin, 1880.

Page 224, line 1 6 from bottom : dele ' germinal vesicle.'

For an account of sexual reproduction more in accordance with our present know-
ledge, see Book III. p. 896, et seq.

Page 226. Alternation of Generations in Thallophytes. It is extremely doubtful if
any real alternation of generations can be detected in the life-history of Thallophytes,
more particularly in the one {Penicillium) given as an example in the text. The sexual
and the asexual modes of reproduction both occur, but their relation in time and space is
not so definite as to warrant the comparison of the life-history of one of these plants with
that of a Moss or a Fern. In some, such as the Coleochseteae, sexual individuals occur
periodically with a number of intervening asexual generations ; the same might be said of
some of the jJEcidiomycetes (such as yEc. Berberidis) if only the existence of a sexual act
were proved. These remarks are also applicable to the accounts of the life-histories of
various plants given in the Introduction to the Thallophytes in Book II. (See Pringsheim,
Jahrb. f. wiss. Bot. XI, 1878 ; and Vines, Journal of Botany, 1879.)

BOOK II.

Page 246. Cyanophyceae (or Phycochromaceae, or Schizophyceae). Goebel (Bot.
Zeitg. 1880, p. 490) has observed the formation of zoogonidia in Merismopedia.

Janczewski, Observ. sur la Reprod. de quelques Nostocacees, Ann. Sci. Nat. sir. V, 19.

The segments into which the filamentous forms of this group break up have been
termed by Thuret Hormogonia.

Page 248. Rostafinski, Quelques mots sur VHsematococcus lacustris, MIm. de la
Soc. Nat. des Sc. Nat. de Cherbourg, 1875 ; Dyer, Sexual Reproduction of Thallophytes,
CLJ. M.S. 1875.

For ' Clorophyll ' read ' Chlorophyll.'

Schizomycetes. The formation of gonidia in these plants was first observed by Cohn
in Bacillus (Beitr. z. Biol. d. Pflnzn. I). The filaments elongate very rapidly and become
diffluent, and then rows of highly refractive gonidia make their appearance in them.

The following additional references will suffice as an introduction to the very ex-
tensive recent literature. Dallinger and Drysdale, On the Existence of Flagella (cilia) in
Bacterium Termo, Monthly Microscop. Journ. XIX, 1875: — Warming, On new forms of
Bacteria, Vidensk. Meddel., Copenhagen, 1875 :— Suringar, On Sarcina, Amsterdam, 1866:
— Papers by Cohn, Koch, and others, in Cohn's Beitr. z. Biol. d. Pflnzn. vol. 2, and vol. 3. I,
1877-79:— Geddes and Cossar Ewart, Proc. Roy. Soc. 1878 {Spirillum): — Reports in
Q1.J. M. S., vols. XVIII and XX, 1878-80) :—Kuehn, Ein Beitrag zur Biologic der Bac-
terien, Dorpat, 1879 :— Nencki, Beitrage zur Biologic der Spaltpilze, Leipzig, 1880 :— Lister,

APPENDIX. BOOK II.

955

On the Relation of Micro-organisms to Disease, Q^ J. M. S. 1881 : — Prazmowski, Unters.
lib. Entwick. und Fermentwirkung einiger Bacterien-Asten, 1880:— Papers in Nageli's
Unters. ueb. niedere Pilze, Munich, 1882 ; also Die niederen Pilze, 1877 (sanitary).

Page 249. Saccharomycetes. Nageli, Ueb. die chemische Zusammensetzung der
Hefe, in Sitzungsber. d. Akad. d. Wiss. zu Miinchen, 1878; also, Theorie der Gahrung,
Munich, 1879 :— Hansen, Recherches sur la Physiologic et la Morphologic des Ferments
alcooliques, Copenhagen, 1881: — Reess, Ueb. den Soorpilz {O'idmm albicans, Robin),
Erlangen, 1877.

Page 250, note 3 : for * Lebrbuch' read ' Lehrbuch.'

Page 258. Note 2 : see also Book I. p. i6.

Page 260. Diatomacese. For a full account of this group, see Pfitzer, Die Bacil-
larlaceen (Diatomaceen), in vol. II of Schenk's Handbuch der Botanik, part of the Scientific
Encyclopedia published by Trewendt, Breslau, 1882.

Page 261. Myxomycetes. See de Bary, Morph. und Physiol, der Pilze, Flechten
und Myxomyceten : — Cooke, Myxomycetes of Great Britain, 1877.

It has already been pointed out that the nuclei of the myxoamoebse which coalesce to
form the Plasmodium remain distinct, p. 945 ; hence the Plasmodium can no longer be
regarded as the equivalent of a zygospore, and the position of the Myxomycetes among the
Zygomycetes is untenable.

Page 264. Zygomycetes. Van Tieghem, Nouvelles Recherches sur les Mucori-
nees, Ann. d. Sci. Nat., ser. 6, t. I :— Brefeld, Ueb. copulirende Pilze, Sitzber. d. Ges.
naturforsch. Freunde, Berlin, 1875, and Weitere Unters. {Mortierella), ibid. 1876. In
Mortierella the zygospore becomes enclosed in a capsule of pseudoparenchyma formed by
a felt of hyphae. Brefeld classifies the Zygomycetes as follows : —

1. Mucorini (incl. Chsetocladiacese) with simple zygospores ; conidia formed by free

cell-formation or by abstriction.

2. Mortierellese ; zygospore enclosed in a capsule; conidia formed by free cell-

formation.

3. Piptocephalideae ; zygospore possessing a temporary growing point and under-

going division to form three cells, one of which is the functional zygospore ;
conidia formed by division with subsequent rounding-off.

There is some ground for believing that the Entomophthoreae belong to the group of
Zygomycetes (see note on p. 277).

Page 271. Siphoneae. Since conjugation of zoogonidia has been observed to take
place in Botrydium and in Acetabularia these plants ought to be included among the
Zygosporeae in accordance with the classification followed in this work. See infra, note
on the Fucoideae.

Page 275. Parthenogenesis of Saprolegnieae. De Bary concludes from ffis observa-
tions (Beitr. z. Morphol. u. Physiol, d. Pilze, IV, 1881) that, in Saprolegnia, Achlya, and
Aphanomyces, even when antheridia are formed and come into contact with oogonia, no act
of fertilisation takes place, that is, no part of the contents of the antheridium enters the
oogonium : hence the oospore is in all cases parthenogenetically produced.

The following is a brief resume of the results of his observations on the oosporous
Fungi: —

1. Pythium ; most of the protoplasm of the antheridium passes into the oosphere.

2. Phytophthora ; a small quantity of the antheridial protoplasm enters the oosphere.

3. Peronospora ; probably the same process as in the preceding genus.

4. Saprolegnia, Achlya, Aphanomyces ; the antheridial tube does not open into the

oosphere, and no passage of substance can be observed.

5. Saprolegnia (5. torulosa, asterophora) ; the antheridia are closely applied to the

oogonia, but no antheridial tubes, or only rudimentary ones, are developed.

6. No antheridia developed.

Page 278, line 9 from bottom; for ^ BulbocrcBte' read ^ BulbochcBte.'

Page 281. Fucoideae. This group, like the Siphoneae (see supra), includes forms

95^ APPENDIX. BOOK II.

in which conjugation and others in which fertilisation takes place. To be consistent with
the classification followed in this work, it ought to be divided into two sub-groups, one
belonging to the Zygosporeae, the other to the Oosporeae. Such a subdivision of the group,
and this holds equally with regard to the Siphoneae, would be obviously unnatural : the
most satisfactory mode of meeting the difficulty would be to combine the Zygosporeae and
the Oosporeae into one group. The following are the principal Orders of Fucoideae or
Melanophyceae (Falkenberg, Die Algen, Schenk's Handbuch, vol. II) : —
Order I. Fucaceae: reproduction by fertilisation ; no zoogonidia.
„ 2. Cutleriaceae : sexual cells both motile, the female being the larger ; asexual

reproduction by zoogonidia.
„ 3. Phaeosporeae : sexual cells both motile at first, but the female cell comes
to rest before fertilisation ; asexual reproduction by zoogonidia. (Fer-
tilisation observed by Berthoid in Scytosiphon lomentarius and Ectocarpus
siliculosus.)

a. Sphacelarieae.

b. Ectocarpeae ; Mesoglceaceae ; Desmarestieae.

c. Phyllitis ; Scytosiphon; Colpomenia ; Asperococcus ] Punctaria,

d. Laminarieae.
„ 4. Tilopterideae.

„ 5 (?) Dictyotaceae.
It is doubtful if the Dictyotaceae ought to be regarded as an Order of Fucoideae, for
they differ from the other members of this group in that they produce tetraspores, and in that
their antherozoids are not motile ; in these respects they approach the Florideae. Probably
the Dictyotaceae constitute a group of Algae intermediate between the Fucoideae and the
Florideae.

In an interesting paper on Hy drums (Akad. d. Wiss. Krakau, 1881) Rostafinski
groups together the brown Algae as follows: —
Phaeoideae.

1. Diatomaceae.

2. Syngeneticae {Chromophyton, Hy drums) ; no sexual reproduction (agamic).

3. Phaeosporeae;

{a) agamae,
{b) isogamae,
{c) oogamae.

4. Cutleriese.

5. Fucaceae.

6. Dictyoteae.

Page 330. Lichens. Further evidence in support of the composite nature of
Lichens is afforded by the discovery that the fungal-element of the Lichen-genus Corat
Fries, is a basidiomycetous Fungus. (Contribuzioni alio studio del genere Cora, Fries, del
Dottore Oreste Mattirolo, Nuov. giorn. bot. ital. 1881 ; also Bot. Zeitg. 1 881, p. 865.)

^cidiomycetes. On Hemikia 'vastatrix, a fungus which most probably is to be
referred to this group and which attacks Coffee plantations, see Marshall Ward, Q^.J. M.S.,
1882.

Page 335. Ustilagineae. Woronin, Beitrag zur Kenntniss der Ustilagineen ; being
No. 5 of de Bary and Woronin's Beitrage zur Morph. und Physiol, d. Pilze, Frankfurt,
1882.

Page 341. Relationships of the groups of Fungi. A short account may be given
here of de Bary's views respecting the affinities of the groups of the higher Fungi as
expressed in No. 4 of his Beitrage (1881). He considers that the Ascomycetes are con-
nected with the Peronosporeae through the Erysipheae : the Uredineae form one of the
more highly developed groups of the Ascomycetous series. Among Basidiomycetes the
Tremellineae are closely connected with those Uredineae which have no ^cidium-forra
{e. g. Chrysomyxa Abietis), their basidia being regarded by de Bary as homologous with the

APPENDIX. BOOK II. 957

teleutospores of the Uredineae ; it appears probable that all the other Basidiomycetes have
sprung from the Tremellineae.

The Ustilagineae form a group of which it is difficult to trace the affinities. De Bary
concludes, from a consideration of the simpler forms, such as Entyloma and Protomyces, that
they are connected with the Chytridiaceae through Nowakowski's Gladochytrieae. (Gohn,
Beitrage, II.)

Page 342. For a full account of the Muscinese, see Goebel in Schenk's Handbuch,
vol.11, T882.

Page 351. According to Goebel the embryo does not in all cases undergo the
successive divisions which produce the octants: in some cases {Sphxrocarpus, Targionia
Michelii) it is spindle-shaped, and undergoes at first only transverse divisions, four
octant-cells being subsequently formed at the upper end by longitudinal divisions.

The relative differentiation of the sporogonium in the Hepaticae is tabulated by
Leitgeb as follows : —

1. The sporogonium consists of a parietal layer enclosing a mass of sporogenous

cells : Riccia, Oxymitra.

2. Of the internal cells some are sporogenous, whereas others are sterile and act as

deposits of nutriment : Cors'm'ia, Riella, Notothylas.

3. The sterile cells develope into elaters : most Hepaticae.

4. The axis of the capsule is occupied by a mass of sterile cells, the columella, which

is covered above by the sporogenous layer: Anthocerotex (some species of
"Notothylas ?).

Page 355, note i. On the development of the stomata of Marchantia^ see Ap-
pendix, p. 948.

Page 386, note i. From the researches of Schmitz (Sitzber. d. niederrhein. Ges.
zu Bonn, 1880) and of Zacharias (Bot. Zeitg. 1881, Ueb. die Spermatozoiden) it appears
that the old view held by Hofmeister and Schacht is the correct one, that, namely, the
nucleus of the mother-cell does not disappear, but becomes actually converted into the
antherozoid, forming the greater portion of it, the remainder being derived from the pro-
toplasm of the mother- cell.

Line 24 and 27 from top: for 'neck-cell' read 'canal-cell of the neck.'

Page 438. On the development of the spores, see p. 13.

Page 444. Rhizocarpeae. The Female Prothallium. From the researches of
Berggren (Om Azollas prothallium och embryo, Lunds Univ. Arsskrift, t. XVI ; also Bot.
Zeitg. 188 1, and Nature, vol. 25. p. 327), it appears that the prothallium oi A%olla caroli-
niana resembles that of Sal'vinia. On germination the endospore of the macrospore
ruptures along its three edges, and the prothallium projects as a convex disc which is only
one cell thick at its margin. Shortly after this a single archegonium is developed near the
centre, consisting of four cells forming the ventral and of four forming the neck portion.
When mature the prothallium is nearly hemispherical, and its cells contain chlorophyll.
No mention is made of any canal-cell in the archegonium.

After fertilisation the oospore is divided by a transverse (basal) wall, and then by a
median and a transverse wall, so that it consists of octants. Each octant is then divided by
a wall parallel to the first (basal) and thus the embryo comes to consist of sixteen cells.
The four uppermost cells (nearest to the neck of the archegonium) give rise to the foot ; of
the four lowermost cells, one becomes the apical cell of the stem, the second developes
into a leaf-like organ, the third and fourth produce the scutiform leaf. It appears that
these two organs can scarcely be regarded as true leaves; they seem to have the same
morphological value as the stem.

Page 461-485. The alternate pages should be headed * Dichotomeae ' instead of
* Filicineae.'

Page 464. Apical growth of Psilotum. It is only the subterranean shoots of
Psilotum which have a single apical cell ; the subaerial shoots have a group of dividing cells
at their apices (Strasburger, Bot. Zeitg. 1873).

95^ APPENDIX. BOOK IT.

Page 478. An interesting abnormality has been observed by Goebcl (Bot. Zeitg.
1879) in Isoetes lacustris and echinospora. In a number of specimens no sporangia had been
developed on the leaves, but in the place of each sporangium a young plant had been
produced by budding.

Page 481, seventh line from top. For * delevopment ' read ' development.*

Page 495, line 18; for ^ Cocus' read ^ Cocos.*

Page 504. Cycadeae. On the development of the pollen sacs and pollen of Zamia
murkatay see Treub, Ann. d. Sci. Nat, ser. 6, t. XV, 1882.

Page 506. Treub {loc. cit.) makes the following statements with regard to the
development of the ovule of Ceratozamia longifoHa.

Each scale of the female cone bears two sporangiferous lobes, in each of which a
macrosporangium is developed.

The macrosporangium is visible in the lobe before any external differentiation can
be detected.

The macrosporangium subsequently consists of a group of sporogenous cells (arche-
sporium ?), surrounded by an external parietal layer and by an internal parietal layer
consisting of several rows of cells.

Only one of the sporogenous cells gives rise to a macrospore. This cell undergoes
no further division, but constitutes the single macrospore in the manner in which the
embryo-sac is generally formed.

Shortly after the differentiation of the macrosporangium in the interior of the
sporangiferous lobe, two new bodies are formed on the lobe superficially to the macro-
sporangium ; these are the integument, and a mass of tissue immediately over the macro-
sporangium which Treub terms the nucellus.

The macrosporangium is, according to Treub, perfectly homologous with a
sporangium of Ophioglossum ; the nucellus and the integument are therefore new forma-
tions which have no equivalents in Cryptogams.

Page 519, line 6 from top. For ' are ' read ' is.'

On the Morphology of the female flower in the Coniferae, see Eichler, Ueb. die
weiblichen Bliithen der Goniferen, in Monatsber. d. k. Akad. d. Wiss., Berlin, 1881. As
the result of comparative investigation of the female flowers in the various families of
Coniferae, he concludes that in the Araucarieae, Abietineae, and Taxodineae each cone is a
single flower, and that the scales of the cone are simple leaves (carpels) bearing ventral
outgrowths (seminiferous scales) on which the ovules are borne ; in the Cupressineae the
ovules are borne in the axils of the carpeilary leaves ; in the Taxineae (incl. Podocarpese)
the ovules are borne on the carpeilary leaves [Microcachys, Dacrydium, Podocarpus)y in
Phyllocladus they are axillary, in Taxus and Torreya they are terminal on lateral shoots, no
carpels being present. In these two genera each ovule represents a single female flower,
whereas in all the other genera and families the female flower consists of an aggregate of
carpels bearing ovules either directly or in their axils. Eichler considers that Taxus and
Torreya lead from the Coniferae to the Gnetaceae.

Eichler finds that it is no longer possible to hold Celakovsky's view that the ovule is
either a modified leaf or a bud. (See note, p. 574, on the morphological significance of the
placenta and of the ovule.)

Page 526, top line; for ^ Juniperus^ read ^Thuja.^

Page 530. Gnetaceae. On the Embryology of the Gnetaceae, see the paper by
Bower, Q^J. M. S., 1882, of which the following is a brief abstract: —

It appears to be the rule in Gnetum Gnemon (though not in all species of the genus)
that the embryo is not developed until the seed begins to germinate : long tubular
suspensors are however found in the endosperm of the ripe seed. On germination, em-
bryos are formed at the apices of these suspensors, the mode of their development being
similar to that which obtains in other Gymnosperms. It has been observed that the
suspensors branch and that an embryo is developed at the extremity of each branch,
a curious form of polyembryony : only one of the numerous embryos persists.

APPENDIX. BOOK IT. 959

After the two cotyledons of the developing embryo have made their appearance, an
outgrowth, similar to that in Wel<wttschia, is produced at the base of the hypocotyledonary
stem ; but in Gnetum its position relatively to the planes of the cotyledons is not fixed, as
in IVelivitschia, the point at which it is formed being determined by the action of gravity,
so that it always developes on the under side of the hypocotyledonary stem. Further, the
organ is larger in Gnetum, and the pith and vascular tissue take part in its formation,
whereas in JVel^itschia it is derived only from the cortex and epidermis of the hypocoty-
ledonary stem.

In the three genera of Gnetaceae this organ is developed for the transfer of nutritive
material from the seed to the seedling, the size of the organ being proportional to that of
the seed and to the quantity of reserve material.

Page 534, line 2 from bottom (note). For 'Eilcher' read ' Eichler.'

Page 574. The morphological significance of the ovule. See above, note on the
female flowers of Goniferae.

Page 576. On the development of the ovule of the Loranthaceae, see Treub,
Observations sur les Loranthacees, Ann. d. Jardin botanique de Buitenzorg, vols. 2 and 3,
Leyden, 1882.

In Loranthus sphcerocarpus there is, according to Treub, a central placenta which
bears three or four free lateral segments; these Treub regards as rudimentary ovules. In
each of these several embryo-sac-mother-cells (archesporial cells) are formed, but only
one embryo-sac becomes fully developed : it is developed from the uppermost cell of the
row formed by the division of the mother-cell.

In the Viscum album and articulatum there is no placenta and no ovule, but the
embryo-sacs are developed in the tissue of the carpels ; in V. album there is a relation
between the number and position of the embryo-sacs and the carpels, but in V. articulatum
this is not the case. In spite of the degradation of these plants, the mother-cells of the
embryo-sacs are nevertheless of hypodermal origin as in the other Angiosperms.

Page 578, line 8 from top. For ' maculatam ' read ' maculatum*

In addition to the instances here given of deviation from what may be regarded as
the typical mode of development of the embryo-sac, it may be added that in some cases it
is the uppermost of the cells of the row formed by the division of the archesporial cell
which developes into the embryo-sac, e. g. Loranthus sphserocarpus (Treub), Pyrethrum
hahaminatum (Marshall Ward), Agr aphis patula (Treub and Mellink). For the most
recent researches on the embryo-sac of Angiosperms, see Guignard, Ann. d. Sci. Nat.,
s^r. 6, t. XIII, 1882.

Page 580. The Synergidse. Strasburger (Bau und Wachsthum der Zellhaute,
1882) considers that the longitudinal striation (Filiform Apparatus of Schacht) mentioned
in the text is due to the presence of delicate canals which are filled with protoplasm ; the
body of the Filiform Apparatus is probably not protoplasmic.

Page 588. Development of the Embryo. See also Rech. embryol. sur V Orchis
maculata, Monteverde in Melanges Biologiques de I'Acad. Imp. de St. Petersbourg, 1880;
Guignard, Rech. d'embryogenie veg6tale comparee, Ann. d. Sci. Nat., ser. 6, t. XII ; and
Treub, Notes sur I'Embryon, le Sac embryonnaire, et I'Ovule, in Ann. d. Jardin bot. de
Buitenzorg, III, 1882.

Treub's researches refer to Peristylus grandis and to A'uicennia officinalis. With
regard to the former he finds that the embryo remains at first rudimentary, whereas the
suspensor grows rapidly and until it projects through the micropyle ; it then branches,
and the branches become closely applied to the placenta. At this time the embryo begins
to develope, and there is no doubt that it does so in consequence of the supply of nutritive
material which is absorbed from the placenta and transmitted by the suspensor. These
observations confirm Treub's previous conclusions as to the function of the suspensor in
Orchids.

With regard to Avicennia, Treub's observations complete our knowledge of this
curious 'viviparous' plant long ago described by Griffith (On the development of the

960 APPENDIX. BOOK HI.

ovulum in Avicennia, Trans. Linn. Soc. XX, 1846). In the first place Treub points out
that the ovule is not naked, as Griffith states, but that it has an integument developed from
the dermatogen in the same way as that described for Thesium ebracteatum by Warming
(Ann. Sci. Nat., s6r. 6, t. V, 1878). One of the hypodermal cells enlarges and becomes
the archesporial cell : this is divided transversely into two, the lower becoming the em-
bryo-sac, the upper dividing transversely into two superposed tapetal cells. These tapetal
cells are peculiar in that they persist for a considerable time, whereas in most plants they
are absorbed before fertilisation. The embryo-sac enlarges, pushes aside the tapetal cells,
and absorbs the epidermis at the micropylar end of the ovule. After fertilisation the
embryo is soon to be seen surrounded by endosperm cells, and at one side of this group of
cells there is a large cell, termed by Treub the ' cotyloid ' cell, which elongates towards
the apex of the embryo-sac. The endosperm now grows and projects through the micro-
pyle, until finally it is quite external to the ovule. It still encloses the embryo, but as the
embryo grows it ruptures the endosperm and the cotyledons project, the radicular end
remaining inserted in the endosperm. During this time the cotyloid cell has enlarged,
branching posteriorly in the ovule and penetrating anteriorly into the placenta. It acts as
an absorptive organ, taking up nutritive substances from the ovule and the placenta, and
transmitting them to the endosperm and thus also to the embryo. The radicular end of
the embryo is peculiar in that it is destitute of a root-cap ; but previously to the dehiscence
of the fruit adventitious roots, generally four in number, provided with root-caps, spring
from it close to its attachment to the suspensor.

Page 603. On the flowers of Orchids, see Gerard, Sur I'Homologie et le Diagramme
des Orchid^es, Ann. d. Sci. Nat., s^r. 6, t. VII.

Page 611. On the symmetry of the flower, see also Eichler, Ueb. einige zygo-
morphe Bliithen, in Sitzber. d. Ges. naturf. Freunde, Berlin, i88o.

Page 629. On abnormal fibro-vascular bundles in Monocotyledons, see Kny, Ueb.
einige Abweichungen im Bau des Leitbiindels der Monokotyledonen, Sitz. d. Bot. Ver. d,
Prov. Brandenburg, 1881.

Page 650, lines 5 and 9 from bottom ; for ' secondary cortex ' read ' secondary phloem.*

Page 653. On abnormal modes of thickening of the stem in Dicotyledons, see
Appendix, p. 950.

BOOK III.

Page 663. The Condition of Aggregation of organised structures. In his work on
the structure and growth of the cell-wall (Ueb. Bau und Wachsthum der Zellhaute, 1882),
Strasburger dissents entirely from Nageli's theory of the structure of organised bodies
which is given in the text. A short account of his views may be given here.

Strasburger comes to the conclusion that the forces which hold together the solid
particles of organised bodies are of a chemical, as opposed to a physical, nature. The
chemical molecules are not grouped together into micellae by cohesion, and the micellae
are not connected into organised substance by attraction, as Nageli would have it, but the
molecules are linked together by chemical affinity, probably by means of multivalent atoms,
into networks. Further, the water present is retained, according to Strasburger, in the
intermolecular meshes by capillarity. The phenomenon of ' swelling-up ' is therefore one
of intermolecular capillarity, and depends upon the mobility of the molecules about their
position of equilibrium.

Nageli's micellar theory received considerable support from his observations upon
the appearances presented by organised bodies (starch-grains, cell-walls) when examined in
polarised light. He found that they were doubly refractive, and further that their double
refraction was not aff'ected by tension, strain, etc. It was from these facts that he inferred

APPENDIX. BOOK III. 96 1

the crystalline form of the micellae. Strasburger argues that cell-walls and starch-grains
consist of numerous lamellae which are in different states of tension and are at the same
time very firmly adherent ; it is to these tensions that the optical phenomena in question
are due, and it is not probable that any mechanical force applied could so far modify
these tensions as to produce an alteration in the optical phenomena.
Strasburger's views may be summarised as follows: —

1. Organised bodies consist of molecules of solid substance united by chemical
affinity; the water which they contain is retained by intermolecular capillarity.

2. The doubly refractive properties of starch-grains and cell-walls depend upon
tensions.

3. The directions of swelling-up are determined by a certain anatomical structure.

4. The increase in surface of organised bodies depends upon stretching and swelling-
up (see also p. 946).

5. Increase in thickness or in bulk depends upon apposition.

Page 671. On the growth of artificial cells, see Traube, Experimente zur physikal-
ischen Erklarung der Bildung der Zellhaut, ihres "Wachsthum durch Intussusception, und
des Aufwartswachsens der Pflanzen, Bot. Zeitg. 1875, p. 56 ; also Reinke, ibid.^ p. 425,
Bemerkungen iiber das Wachsthum anorganischer Zellen ; further, Sachs and Traube,
Bot. Zeitg. 1878.

Page 678. On Transpiration, see also Wiesner, Untersuch. ueb. den Einfluss des
Lichtes und der strahlenden Warme auf die Transspiration der Pflanze, Sitzber. d. k.
Akad. d. Wiss., Bd. LXXIV, Wien, 1876 (also Ann. d. Sci. Nat., ser. 6, t. IV, 1876).

In note 4, for * 1836 ' read ' 1856.'

Page 688. On the absorption of water by roots, see Vesque, De I'influence des
matieres salines sur I'absorption de I'eau par les racines, Ann. d. Sci. nat., ser. 6, t. IX,
1880.

Page 701. Absorption of substances by roots. See Phillips, On the Absorption of
metallic oxides by plants, American Journal of Science, 1882.

Page 703. On the chemistry of Assimilation, see Reinke, Theoretisches zur As-
similationsproblem, Bot. Zeitg. 1882 : also Strasburger, Bau und Wachsthum der Zellhaute,

p. 237.

Page 707. It has been recently ascertained that glycogen occurs as a reserve-
material in many plants : see Errera, L'Epiplasme des Ascomycetes et le Glycogene des
V^g6taux, Brussels, 1882.

Page 712. From observations made on Euphorbia trigona, Treub comes to the
conclusion that the laticiferous cells serve as channels for the transmission of amylaceous
substances, and that the starch-grains which they contain are transitory (Ann. du Jardin
bot. de Buitenzorg, III, 1882),

Page 734, paragraph 3. See also Holzner, Beob. ueb. die Schiitte der Kiefer oder
Fohre und die Winterfarbung immergriiner Gewachse, Freising, 1877.

Page 737. On the action of light, see also Pauchon, Rech. sur le r61e de la lumiere
dans la germination, Ann. d. Sci. Nat., ser. 6, t. X, 188 1.

Siemens, On the Influence of Electric Light upon Vegetation, Proc. Roy. Soc,
1880; also. On some Applications of Electric Energy to Horticulture and Agriculture,
London, 1881.

Page 739 (2). See also Famintzin, De I'influence de I'intensit^ de la lumiere sur
la decomposition de Tacide carbonique par les plantes, Ann. d. Sci. Nat., s6r. 6, t. X, 1880.

Page 744 {b). Famintzin, La decomposition de I'acide carbonique par les plantes
expos6es a la lumiere artificielle, Ann. d. Sci. Nat., ser. 6, t. X ; Deherain et Maquenne,
Sur la decomposition de I'acide carbonique par les feuilles ^clairees par des lumieres artifi-
cielles, Ann. d. Sci. Nat., ser. 6, t. IX (1880).

Page 761. On the function of chlorophyll, see further. Bonnier, Du role physiol-
ogique de la chlorophylle, Ann. d. Sci. Nat., ser. 6, t. X, 1881 ; and Hansen, Geschichte der
Assimilation und Ghlorophyllfunction, Arb. d. bot. Inst, in Wurzburg, II, 1882.

3 Q

962 APPENDIX. BOOK III.

Page 768. On the action of electrical currents on growing roots, see Elfving, Ueb.
eine Wirkung des galvanischen Stromes auf wachsende Wurzeln, Bot. Zeitg., 1882.

He finds that when a root is placed vertically between two electrodes, it curves
towards the positive electrode ; the curvature is evidently connected with the growth of
the root, the current effecting a retardation. Continued exposure to the action of the
current causes death.

When the current is parallel to the long axis of the root it appears to retard growth
when it runs in opposition to the direction of growth.

Experiments with negatively heliotropic roots {Brassica oleracea^ Lepidium sativuniy
Sinapis alba), the direction of the current being transverse, showed that they curved
towards the negative electrode.

Page 769. For further researches on DioncBa, see Burdon-Sanderson, On the Elec-
tromotive Properties of the leaf oi Dionsea, Phil. Trans. 1882.

Page 787. Turgidity does not necessarily cause an elongation of cells ; it may also
cause them to become shorter and thicker. See de Vries, Ueb. Verkiirzung pflanzlicher
Zellen durch Aufnahme von Wasser, Bot. Zeitg., 1879.

Page 812. Light has some influence on the development of root-hairs on the
gemmae of Marchantia ; see Zimmermann, Ueb. die Einwirkung des Lichtes auf den
Marchantienthallus, Arb. d. bot. Inst, in Wiirzburg, II, 1882.

Page 815. On the growth in length of stems, see further, Wiesner, Die undulirende
Nutation der Internodien, Sitzber. d. k. k. Akad. d. Wiss. in Wien, LXXVII, 1878.

Page 839. Geotropism. See F. Darwin, On the Connexion betvi^een Geotropism
and Growth, Journ. Linn. Soc, vol. XIX, 1882.

Page 854, note 2, Further, F. Darwin, On the power possessed by leaves of
placing themselves at right angles to the direction of incident light, Journ. Linn. Soc,
XVIII, 1881.

Page 862. On Climbing Plants, see Schwendener, Ueb. das Winden der Schling-
pflanzen, Monatsber. d. Bed. Akad., 1881 ; Sachs, Notiz iiber Schlingpflanzen, Arb. d. bot.
Inst, in Wiirzburg, II, 1882.

Page 865. See de Vries, Over de Bewegingen der Ranken van Sicyos, Amsterdam,
1880.

Treub has recently drawn attention to a new group of climbing plants, those namely
which climb by means of irritable hooks, the effect of irritation being an increase in the
thickness of the hook. Such are Uncaria (Rubiaceae), Ancistrocladus (Dipterocarpeae),
Artabotrys (Anonaceae), Lwvunga (Aurantiaceae), Olax (Olacineae), Hugonia (Linaceae),
Strychnos (Loganiaceae). This group is not to be confounded with Darwin's * hook-
climbers,' the hooks of which are not irritable. (Treub, Sur une nouvelle categoric de
plantes grimpantes, Ann. du Jardin botanique de Buitenzorg, III, 1882.)

Page 871, Sect. 26. Vochting, Die Bewegungen der Bliithen und Friichte, Bonn,
1882.

Page 920, Sect. 35. For some interesting observations as to the relation between
plants and external conditions, made on the Flora of Scandinavia, see Bonnier et Flahault,
Observations sur les Modifications des V6getaux suivant les conditions physiques du milieu,
Ann. Sci. Nat. ser. 6, t. VII, 1879, and Flahault, ibid. t. IX, 1880.

INDEX.

Abies, pp. 514, 518, 939

(Figs. 350, 353).
Abietineae, 518, 527, 958.
Abortion, 219, 602.
Absorption, 700, 961.
Absorption of assimilated

substances, 720.
Acacieae, 557.
Acalypheae, 661.
Acanthaceae, 65, 68, 658.
Acanthus, 648.
Acarospora, 325.
Accumulation of characters,

924.
Acer, 535,615,617, 651,940.
Acerineae, 610.
Acetabularia, 65, 272, 955.
Achenium, 615.
Achillaea, 932.
Achlya, 12, 273, 902, 955

(Figs. 8, 9, 178, 179).
Achromatin, 18, 946.
Aconitum, 495, 538,607,608.
Acorn, 614.
Acorus, 115 (Fig. 96).
Acrocarpous Mosses, 370.
Acropetal order of develop-
ment, 170.
Acrosticheae, 441.
Acrostichum, 435, 436, 441.
Actinomorphic, 954.
Actinostrobese, 527.
Acyclic, 600, 641.
Adaptation, 934.
Adhesion, 219, 546.
Adiantum,i82, 424, 425,427,

442, 915 (Figs. 293, 295-

297).
Adventitious formations, 1 7 1,

173, 433, 639, 953.
-^cidiomycetes, 244, 330,

956.
iEcidium, 330.
jiEsculineae, 660.
iEsculus, 644 (Fig. 455).
iEthalium, 262, 841.
Agapanthus, 22.
Agaricus, 336 (Figs. 2 25, 226,

227).

Agave, 556, 594, 621.
Aggregatae, 658.
Agrimonia, 578.
Agrostemma, 579.
Ailanthus, 652 (Fig. 97).
Akebia, 534 (Fig. 357).
Albumen, 492.
Albuminoids, 706.
Albuminous, 587.
Alchemilla, 568, 677.
Aldrovanda, 649.
Alepyrum, 542.
Aletris, 127.
Aleurone, 51, 947.
Algae, 231, 241.
Alisma, 589, 626, 952.
Alismaceae, 626, 631 (Fig.

431)-
Alliaria, 574.
Allionia, 581.
Allium, 17, 88, 581, 623, 624,

629,939(Figs. 15,73, 424,

425).
Almond, 637.
Alnus, 555, 652.
Aloe, 127, 193, 629 (Fig. 152).
Aloineae, 136, 496.
Alopecurus, 596.
Alpinia, 625 (Fig. 429).
Alsineae, 661.
Alternate arrangement, 189,

601.
Alternation of generations,

222, 225, 233, 239, 342,

385, 486, 488, 899, 903,

954-

Althaea, 546, 552, 553 (Figs.
42, 83, 360, 377, 378,
381).

Aluminium, 695.

Alyssum, 544.

Amaranthaceae, 137,645,661.

Amaranthus, 653, 950.

Amentiferae, 656.

Amidoplasts, 948.

Ammonia, 698.

Amoeboid-movement of pro-
toplasm, 39, 263.

Amorpha, 599.

3 Q 2

Amorphophallus, 183, 624

(Fig. 141).
Ampelideae, 644, 660, 866.
Ampelopsis, 217, 865, 866,
Amphigastria, 356.
Amphithecium, 375.
Amygdaleae, 651, 662.
Amyrideae, 660.
Anacardiaceae, 644, 660 (Fig.

452).
Anagallis, 565, 6i6 (Fig.392).
Ananasineae, 632.
Anaptychia, 323, 325 (Figs.

218, 219).
Anatropous, 492, 570.
Anchusa, 598.
Ancistrocladus, 962.
Andreaea, 373, 374 (Fig. 255).
Andreaeaceae, 361, 380.
Androecium, 490, 541.
Androgonidium, 279.
Aneimia, 105, 421, 423, 440

(Fig. 106).
Anemone, 640.
Anemophilous, 494, 908.
Aneura, 346, 356, 358, 952.
Angiocarpous, 306.
Angiocarpous Lichens, 323.
Angiopteris, 93, 416 (Figs.

291, 292).
Angiosperms, 497, 534.
Angle of divergence, 1 8 8, 201.
Anisocarpae, 655, 658.
Anisostemonous, 641.
Annual ring, 132,652, 813.
Annular vessels, 22, 23.
Annulus, 338, 381, 435.
Anona, 557.
Anonaceae, 657, 962.
Anterior, 600.
Anthela, 596.
Anthemis, 599.
Anther, 491, 541 (Figs. 360,

382).
Anthericum, 577.
Antheridium, 234, 268, 290,

299, 343, 349, 387, 394,
411, 423, 442, 469, 487,
515, 899 (Figs. 177, 179,

964

INDEX,

181, 183, 185, 187, 189,

190, 198, 200, 201, 235,

237, 239, 241, 253, 254,

262, 275, 287, 293, 294,

309, 329, 330-
Antherozoid, 224, 234, 268,
271, 278, 279, 282, 287,
290, 298, 371, 386, 394,

424, 443, 446, 469, 524,
897 (Figs. 2,177,183,185,
187, 188, 198, 241, 254,
262, 275, 293, 309, 310,

314, 329)-

Anthoceros, 346, 352 (Figs.
237, 238).

Anthoceroteae, 352.

Anthophore, 539 (Fig. 361).

Anthurium, 586, 953.

Antipetalous, 601.

Antipodal cells, 486, 580.

Antirrhinum, 616.

Antisepalous, 601.

Apetalous, 641.

Apex, 158, 175, 202.

Aphanocyclae, 655, 657.

Aphanomyces, 273,955.

Apheliotropic, 756.

Apical cell, 137, 173, 289,
293, 347, 397, 401, 413,
429, 449, 450, 463, 464,
476 (Figs. 108-112, 115,
116, 120, 121, 138, 142,
192, 244, 279, 283, 284,
3i5«, 336).

Apical growth, 158.

Apocynaceae, 86, 658, 949.

Apogamy, 425.

Apopetalous, 539.

Apophyllous, 539.

Apophysis, 385 (Fig. 273).

Aposepalous, 539.

Apostasiaceae, 633.

Apostrophe, 750.

Apothecium, 308, 323 (Figs.
7, 205, 218, 219).

Apple, 614, 662.

Aqueous Tissue, 98, 123.

Aquifoliaceae, 660.

Aquilegia, 570, 643 (Figs.
396, 451).

Aralia, 654.

Araliaceae, 661.

Araucaria, 507, 509, 5 10, 5 1 3,
518,527.

Araucarieae, 507, 527, 958.

Arbutus, 543 (Fig. 527).

Arc-indicator, 826 (Fig. 480).

Arceuthos, 517.

Archegonium, 343, 349, 353,
355, 357, 359, 372, 386,
394, 411, 423, 425, 445,
446, 462, 471, 487, 498,
506, 521, 529, 582, 899
(Figs. 237, 240, 243, 256,

263, 276, 287, 294, 295,
312, 330, 331, 354, 355).

Archesporium, 375, 388, 437,
491, 577 (Figs. 305, 337).
Archetype, 941.
Archidium, 374, 380 (Figs.

264, 265).
Archieracium, 926.
Arcyria, 262.
Areca-Palm, 587.

Aril, 495, 513, 570, 618 (Fig.

348).
Aristolochia, 862, 910, 940

(Figs. 488, 489).
Aristolochiaceae, 656.
Aroideae, 65,84, 93, 120,540,

631, 952.
Artabotrys, 962.
Arthrotaxis, 518, 527.
Artocarpeae, 656.
Arum, 583,585, 933.
Asarineae, 656.
Asarum, 535 (Fig. 358).
Ascent of water, 685.
Asclepiadeae, 85, 658, 906,

911, 949.
Ascobolus, 238, 309 (Fig.

204).
Ascogenous hyphae, 308, 310,

315, 324 (Fig. 204).
Ascogonium, 309, 329, 898

(Figs. 204, 206, 207, 208).
Ascomycetes, 244, 308, 899.
Ascospore, 238, 308 (Figs.

7, 205, 207, 208, 209, 219).
Ascus, 226, 238, 308, 310

(Figs.7, 205,207, 208, 209,

219).
Asexual generation, 225,227,

239,343,351,387,395,412,

416,425,447,462,472,488,

899.
Asexual propagation, 348.
Asexual reproduction, 223.
Ashes, 36, 695.
Asparagin, 718.
Aspergillus, 312 (Fig. 208).
Asperifolieae, 599.
Asphodelus, 552.
Aspicilia, 323.
Aspidieae, 442.
Aspidium, 426, 427, 429, 432,

433,435,436,437,438,439,

442, 915 (Figs. 302, 303,

304, 306).
Asplenieae, 422.
Asplenium,i44, 172,437,442,

915, 953 (Figs. 112, 127,

305)-
Assimilation, 703, 729, 737,

961.
Asterophyllites, 408.
Astragalus, 561, 615.
Astrapaea, 555.

Astrocarpus, 609.

Atelanthera, 605.

Atherurus, 622.

Atom, 664.

Atrichum, 367, 368.

Atriplex, 653.

Aurantiaceae, 646, 660, 962.

Automatic periodic move-
ments, 880, 895.

Autumnal wood, 813.

Auxanometer, 827(Fig.48i).

Auxospore, 261.

Avicennia, 652, 959.

Avicennieae, 950.

Axial cylinder, 115, 127, 133,
468.

Axial longitudinal section,
203.

Axial placentation, 563, 564.

Axil, 175,489.

Axillary placentation, 574.

Axillary shoots, 175, 489.

Axis of growth, 159, 203,
206.

Azolla, 448, 450, 452, 454,
460, 957.

Bacillarieae, 260.
Bacillus, 246, 249, 954.
Bacteria, 249, 954 (Fig. 166).
Bactrospora, 325.
Baeomyces, 324.
Balanophora, 634.
Balanophoreae, 241, 576, 648,

649, 662.
Balsamineae, 660.
Bambusa, 603 (Fig. 409).
Bangiaceae, 289.
Barbula, 364, 368, 385 (Fig.

250).
Bark, 106, 108.
Bartramia, 362, 365.
Basal growth, 159.
Basal wall, 351.
Base, 177, 202.
Basidiomycetes, 240, 244,

305, 335, 956.
Basidiospore, 15, 336 (Fig.

227).
Basidium, 15, 330, 333, 336,

338 (Fig. 227).
Bassorin, 705.
Bast, III.

Batrachospermum, 289.
Bauhinia, 653, 950.
Beech, 587.
Beggiatoa, 249.
Begonia, 204, 209, 639, 907,

925, 954-
Begoniaceae, 137,662.
Benthamia, 614.
Berberideae, 646, 657.
Berberis, 332, 595, 647, 880,

S83, 894.

INDEX.

i)6i

Berry, 526, 616.

Betula, 652.

Betulaceae, 656.

BicoUateral bundles, 949.

Bicornes, 659.

Bifurcation, 177.

Bignonia, 137,617, 652, 866,

936, 939.
Bignoniaceae, 136, 658, 950.
Bilateral structure, 204, 854,

954.
Bilirubin, 758.
Biota, 514, 527.
Biscutella, 570 (Fig. 396).
Bisexual, 490.
Bixaceae, 659.
Blackberry, 614.
Blasia, 347, 348, 352, 357.
Blastocolla, 10 1.
Bleeding of wood, 677.
Bloom on plants, 99.
Boletus, 96, 336 (Fig. 79).
Boragineae, 597, 658.
Borago, 598.
Bordered pits, 23, 25, 531,

533,946.
Bostrychoid cyme, 180 (Fig.

136).
Bostrychoid dichotomy, 178

(Fig. 134).
Bostryx, 180, 597.
Botrychium, 410 (Figs. 287,

289).
Botrydium, 271, 955.
Botrytis, 311.
Bract, 214, 540, 595.
Bracteole, 297, 304, 49 1, 595'
Branch system, 176.
Branching, 169.
Branching of leaves, 182.
Branching of roots, 181.
Branching of stem, 184.
Brasenia, 575.
Brassica, 599, 605, 921.
Brizula, 542.
Bromeliaceae, 632.
Bromine, 695.
Broussonetia, 907.
Bryaceae, 377, 381.
Bryineae, 362.

Bryonia, 867, 939 (Fig. 487).
Bryophyllum, 173, 640.
Bryopsis, 272.
Bryum, 96, 365, 375, 377

(Figs. 80, 249).
Bud, 156.

Bud-rudiment, 297.
Bud- variation, 921.
Bulb, 216, 620, 623, 704, 707,

715 (Figs. 421, 424, 470).
Bulbil, 172, 223, 228, 295, 368,

622 (Fig. 250).
Bulbocapnos, 634.
Bulbochaete, 278,

Bundle-sheath, 112, 113, 115,
123, 124, 167, 407, 434,
468, 483.

Burmanniaceae, 633.

Burseraceae, 660.

Butomaceae, 601.

Butomus, 558, 568, 606, 626
(Fig. 382).

Buxbaumia, 366, 369.

Buxineae, 661.

Byttneriaceae, 661.

Cabombeae, 657.
Cactaceae, 638, 662.
Cactus, 216, 935.
Caeoma, 331.
Caesalpineae, 633, 662.
Caesalpinia, 133.
Caladium, 87.
Calamites, 404, 405, 407.
Calamodendron, 407, 408.
Calamostachys, 408.
Calamus, 629, 938.
Calanthe, 567 (Fig. 394).
Calcium, 699.

Calcium carbonate, 65, 948.
Calcium oxalate, 52, 65, 88,

699.
Calcium sulphate, 699.
Calendula, 876.
Callistemon, 651, 949.
Callithamnion, 51.
Callitrichaceae, 662.
Callitriche, 570.
Callitris, 517, 527 (Fig. 352).
Callus, 810.
Calodracon, 114, 129.
Calothamnus, 544 (Fig. 365).
Calycanthaceae, 662.
Calycanthus, 651.
Calycereae, 659.
Calyciflorae, 655, 662.
Calycium, 324.
Calyculus, 540.
Calypogeia, 359.
Calyptra, 344, 350, 35 1, 355,

374 (Figs. 236, 240, 246,

257, 263-266).
Calyptrogen, 489, 500, 952.
Calyx, 538.

Cambiform tissue, 119.
Cambium, 82, no, 112, 129,

810, 813.
Cambium-ring, 127, 130, 133,

531,650.
Camellia, 20, 651 (Fig. 16).
Campanula, 642, 812 (Figs.

442, 479).
Campanulaceae, 86, 642, 658.
Campylotropous, 492, 570.
Canal of the style, 568 (Fig.

395).
Canal-cell, 350, 374, 386,

395, 412, 424, 446, 472,

499, 521, 522, 582, 957

(Figs. 236, 256, 294, 312,

331).
CandoUea, 646 (Fig. 461).
Canna,6i,625,586(Fig.625).
Cannabineae, 535, 656.
Cannabis, 10 1.
Cannaceae, 99, 628, 632, 952

(Fig. 430).
Caoutchouc, 712.
Cap-cell of root, 145 (Figs.

112, 120, 121, 138).
Capillary attraction, 684.
Capillitium, 262 (Fig. 172).
Capitulum, 596 (Fig. 126).
Capparidese, 604, 657 (Fig.

412).
Caprifoliaceae, 642, 658 (Fig.

440).
Capsella, 590, 591 (Fig. 403).
Capsule, 616.
Capsule, of Muscineae, 344,

374, 375 (Figs. 236-240,

246, 263-268).
Caragana, 652.
Carbo-hydrates, 715.
Carbon, 695, 696.
Carbon dioxide, 696, 703,

721, 737, 744-
Carcerulus, 614.
Carex, 577, 600.
Carica, 87, 708.
Carpel, 214, 490, 516, 518,

526, 536, 559, 567, 574.
Carpellary leaf, 493, 517.
Carpinus, 555, 651.
Carpogonium, 236, 285, 286,

287, 290, 297, 301, 307,

309, 312, 313, 314, 329,

897 (Figs. 164, 188, 189,

190, 197, 199, 202, 204,

207, 208).
Carpophore, 615.
Carpospore, 236, 305.
Carposporeae, 236, 244, 284.
Caruncle, 618.
Caryolopha, 598.
Caryophylleae, 563, 570, 661,

915.
Caryophyllineae, 645, 661.
Caryopsis, 615, 617.
Cassia, 561, 615.
Castanea, 651.

Casuarina, 116, 491, 541, 651.
Casuarineee, 662.
Catalpa, 588, 648.
Cataphyllary leaves, 186,213,

509, 622, 623, 624, 640.
Catharinea, 199, 217.
Caulerpa, 158, 161, 272, 947.
Cauline bundles, 137, 156,

440, 468, 482, 483, 030,

649, 950.
Caulome, 150, 151, 157,

g66

INDEX.

Caulotretus, 136.
Cedreleae, 660.
Cedrus, 509, 527.
Celastrineae, 645, 660.
Celastrus, 602,651 (Fig. 407).
Cell, Primordial, 5.
Cell, Structure of, i.
Cell-division, 8, 12, 16, 275,

762.
Cell-families, 81, 246.
Cell-multiplication, 8.
Cell-nucleus, 2, 17, 37, 44,

945, 947-
Cell-plate, 13, 18.
Cell-sap, 2, 62.
Cell-wall, I, 19, 946.
Cells, Bundle of, 81.
Cells, Filament of, 80.
Cells, Formation of, 7, 945.
Cells, Formation of the com-
mon wall of, 72.
Cells, Forms of, 5, 809.
Cells, Group of, 81.
Cells, Layers of, 80.
Cellular tissue, i.
Cellulose, 2, 19, 708.
Celosia, 566, 64^ (Fig. 460).
Celtideae, 656,
Centaurea, 883, 892.
Centradenia, 543, 548, 651,

(Fig. 364).
Central cell, 350, 353, 374,

386, 395, 411, 424, 446,

472, 506, 521 (Figs. 294,

312, 331).
Centranthus, 643 (Fig. 443).
Centrifugal force, action of,

772.
Centrifugal infloresence,596.
Centripetal infloresence,595.
Centrolepidaceae, 542.
Centrolepis, 542.
Centrospermae, 630, 655, 66 1 .
Cephalanthera, 908.
Cephalotaxus, 66, 518, 527.
Cephalotus, 640.
Ceramieae, 72, 290.
Ceramium, 51.
Cerastium, 563, 564, 616,
Ceratonia, 20, 35, 36, 73,

652 (Fig. 39).
Ceratophyllaceae, 662.
Ceratophyllum, 49, 98, 586,

648, 649.
Ceratopteris, 164, 422, 430,

433, 436.
Ceratozamia, 501, 503, 506,

958 (Fig. 345).
Cercis, 185, 208 (Fig. 155).
Cereus, 99, 203, 216.
Cerinthe, 598.
Ceropegia, 89.
Cerorchideae, 557.
Ceroxylon, 99.

Chaetocladium, 267, 955.
Chaetomorpha, 281.
Chaetophora, 281.
Chalaza, 492, 571.
Chamaecyparis, 527.
Chantransia, 290.
Chara, 152, 293, 902 (Figs.

115, 191-197, 203).
Characeae, 18, 39, 174, 237,

238, 241, 244, 292, 897,

898, 899, 900, 905.
Characteristic forms of

leaves and shoots, 211.
Chavica, 137, 951.
Cheiranthus, 578, 652.
Chelidonium, 87, 618, 647

(Fig. 464).
Chemical processes, 695.
Chenopodiaceae, 574, 661.
Chenopodium, 536, 577, 578,

871 (Fig. 359).
Chimonanthus,634(Fig.434).
Chlaenaceae, 660.
Chlamydococcus, 253.
Chlamydomonas, 246, 253,

947-
Chlorantheae, 656.
Chlorine, 695, 699.
Chlorofucine, 765.
Chlorophyll, 45, 58, 241,

244, 260, 282, 289, 691,

697, 703, 729, 737, 743j

757, 961.
Chlorophyll band, 10, 16.
Chlorophyll-bodies, 6, 45, 48,

257 (Figs. 3, 5, 43, 170,

171).
Chlorophyll-granules, 6, 9,

10, 17, 46, 47, 747, 750,

947 (Figs. 5, 44, 45).
Chlorophyll spectrum, 759

(Fig. 476).
Chlorophyllan, 758.
Chlorophytum,756, 833, 838.
Ghordarieae, 283.
Chorisis, 605.
Chromatin, 18, 945.
Chroococcaceae, 81, 246, 318,

328.
Ghroolepus, 329, 947.
Chrysobalaneae, 662.
Ghrysodium, 433.
Chrysomyxa, 331, 956.
Ghrysosplenium, 601.
Chrysotannin, 767.
Chytridinese, 251, 264, 957.
Cibotium, 428, 435, 441.
Cichoriaceae, 86, 93, 883,

949.
Cichorium, 23, 883 (Fig. 20).
Gicinal dichotomy, 178.
Cicinus, 180, 597.
Cilia, 4, 39, 234.
Cilia in Mosses, 381, 383.

Cinnamomum,643 (Fig. 450).

Circaea, 646.

Circulation of protoplasm, 3 9.

Circumnutation, 855.

Cirsium, 93, 640.

Cissus, 653, 866.

Cistineae, 659.

Citrus, 91, 593,616, 646 (Fig.

462).
Cladonia, 319, 327 (Fig. 222).
Cladophora, 15, 281, 946.
Clathrate cells, 89.
Claviceps, 316, 317.
Claw, 539.

Cleistogamous flowers, 908.
Clematis, 174, 540, 554, 936

(Fig. 129).
Cleome, 605.
Climbing plants, 862, 936,

962.
Climbing stems, 217, 862.
Glinostat, 773.

Closed bundles,

439,

629.

Closing-cells, 446.
Glosterium, 260.
Clusiaceae, 660.
Coalescence of cells, 75.
Cobaea, 569, 936.
Cobalt, 695.
Cocconema, 261.
Gocculus, 135, 653.
Cocoa-nut, 585.
Cocos, 495.

Coefficients of Heat Expan-
sion, 727.
Coelastrum, 70.
Coelebogyne, 593, 902.
Cceloblastae, 244, 269.
Coenobium, 252, 278 (Figs,

167-169).
Coenogonium, 323, 329.
Goffea, loi, 586.
Coffee-berry, 587.
Cohesion, 222, 538, 601.
Colchicum, 584, 622, 627,

939 (Fig. 422).
Coleochaete, 236, 237, 240,

288, 897, 900 (Figs. 186,

187).
Coleochaeteae, 237, 241, 244,

286, 290, 306, 329, 904.
Coleorhiza, 165, 588, 619

(Figs. 123, 124).
Coleosporium, 331.
Coleus, 652.
Collateral bundles, 112.
Collateral chorisis, 605.
Collema, 319, 328, 329 (Fig.

212).
CoUemaceae, 318.
Collenchyma, 24, 95, 98, 123,

654 (Fig. 21).
Colleter, 10 1.

INDEX,

9^7

Colloids, 670.
Colouring-matters, 764.
Colours of leaves in autumn,

734.
Columella, 345, 353, 354,

375, 379, 380, 381, 441

(Figs. 238, 266, 273).
Columnea, 222, 548,61 r (Fig.

416).
Columniferae, 661.
Colymbea, 507.
Combined hybrids, 919.
Combretaceae, 662.
Comesperma, 653,
Commelynaceae, 88, 632 (Fig.

86).
Common bundles, 155, 156,

407, 496, 530J 629, 649.
Compositae, 93, 176, 194, 53S,

540, 555, 571, 572, 575,

587, 596, 609, 613, 615,

617, 639, 643, 659, 871,

872, 910 (Fig. 445).
Compound glands, 8^, 91

(Figs. 76, 77).
Concentric bundles, 112.
Conceptacle, 281 (Fig. 184).
Concussion, Irritability to,

88r.
Condition of aggregation of

organised structures, 663,

960.
Conducting tissue for the

assimilated food-materials,

711.
Conducting tissue of style,

568.
Conduction of heat, 725.
Cone of growth, 138.
Confervacesp, 281, 329.
Conidia, 223, 239.
Coniferae, 93, 104, iii, 113,

130, 158, 174, 181, 185,

204, 213, 486, 490, 497,

507, 531, 743,958.
Conjugatse, 9, 244, 252, 257,

897.
Conjugation, 8, 9, 224, 234,

253, 257, 261, 264, 897.
Connective, 491, 541 (Figs.

364, 490).
Contortae, 658.
Contractile organs, 756, 888.
Convallaria, 579, 624.
Convolvulaceae, 88, 555, 658,

915.
Convolvulus, 217, 862.
Copper, 695.
Corallina, 65.
Corallineae, 289.
Corallorhiza, 164, 214, 241,

620, 697.
Cordyline, 129.
Coriaria, 187 (Fig. 144).

Cork, 20, 33, 95, 106, 415,

949 (Fig. 90).
Cork-cambium, 82, 107 (Fig.

90).
Corm, 622 (Figs. 422, 423).
Cormophytes, 151.
Cornus, 540.
Cornaceae, 66 r.
Corolla, 538.

CoroUiflorae, 632, 655, 662.
Corona, 213, 539 (Fig. 361).
Corpusculum, 486, 498, 521.
Corrosion by roots, 702.
Cortex, 96, 108, 120, 289,

293.
Cortical sheath, 650.
Coryanthes, 677.
Corydalis, 535, 604,618, 906.
Cosmarium, 259 (Fig. 171).
Costus, 623, 625.
Cotyledon, 214, 426, 447,

448, 473, 499, 501, 507,

587, 618, 634, 704, 715.
Crambe, 544.
Crassulaceae, 104, 194, 645,

652, 661, 949.
Crataegus, 108, 651.
Craterospermum, 257.
Cremocarp, 615.
Crest, 618.

Crinum, 98, 586, 618, 628.
Critenchyma, 123.
Crocus, 568, 580, 581, 583,

872, 875 (Fig. 423).
Crown, 299.

Crozophora, 644 (Fig. 453).
Crucibulum, 71, 339 (Figs.

55, 228, 229, 230).
Cruciferae, 104, 494, 573,

574, 578, 595, 599, 604,

607, 616, 633, 634, 637,

641, 647, 657 (Fig. 413).
Cruciflorae, 646, 655, 657.
Crustaceous Lichens, 319.
Cryptomeria, 527.
Crystalloids, 49, 55, 289,

672, 947, 948 (Fig. 48).
Crystals, 53, 64, 84 (Fig. 52).
Cucumis, 569, 925.
Cucurbita, 22, 29, 32, 36,

43, 58, 89, 495, 545, 554,

634, 635, 637, 868, 925,

952 (Figs. 34, 35, 74, 98,

99,368, 379, 444)-
Cucurbitaceae, iii, 217, 555,

570, 587, 643, 654, 658,

865, 924, 936, 949.
Cunninghamia, 518,527,533.
Gunninghamieae, 527.
Cunonia, loi, 102.
Cunoniaceae, 661.
Cuphea, 579.
Cupressineae, 209, 493, 507,

526.

Cupressus,509,5i6, 517, 527,

532.
Cupule, 348, 369, 540 (Figs.

233, 251)-

Cupuliferae, '535, 587, 633,
656.

Curvature of concussion, 7 8 7.

Cuscuta, 217, 241, 634, 637,
649, 721, 865.

Cuscuteae, 658.

Cuticle, 34, 98.

Cuticularisation of the cell-
wall, 20,33 (Fig. 37).

Cutlerieae, 956.

Cyanophyceae, 244, 246,954.

Cyatheaceae, 391, 435, 438,
441.

Cyathium, 647.

Cycadeae, 93, 486, 488, 489,
490, 491, 496, 497, 498,
500, 501, 515, 519, 531,

573, 950, 958.
Cycas, 493, 499, 503, 531

(Fig. 343).
Cyclantheae, 631, 952.
Cyclanthera, 541.
Cyclic, 600, 608, 641, 657.
Cyclomyces, 336.
Cydonia, 578.:
Cylindrocystis, 258.
Cyme, 179, 597.
Cymose infloresence, 596.
Cymose umbel, 179, 596.
Gynanchum, 598.
Cynara, 733, 883, 892 (Fig.

473).
Cynaraceae, 93, 881, 883,

886.
Cynareae, 887, 891, 894.
Cynoglossum, 54, 598.
Cyperaceae, 538, 627, 631,

952.
Cyperus, 596 (Fig. 372).
Cypripedium, 548, 557, 603.
Cystocarp, 292.
Cystococcus, 249, 329.
Cystocoleus, 329.
Cystolith, 36, 65, 68, 948

(Fig. 53).
Cystopteris, 430.
Cystopus, 235, 275, 277 (Figs.

180, 181).
Cystoseira, 282.
Cytineae, 656.
Cytisus, 599, 943.

Dacrydium, 527.
Dahlia, 26, 63, 119, 166, 713,
921, 939 (Figs. 25, 26, 29,

51,74).
Daily periodicity of growth,

823.
Dammara, 511, 513, 5 19, 527*
Danaea, 416.

968

INDEX.

Datura, i6i, 6i6.
Daucus, 493.
Davallieae, 442.
Decussate, 189, 198.
Dedoublement, 549,605, 645.
Definite inflorescence, 596.
Degradation of chlorophyll,

47.

Degradation-products, 705.

Dehiscent fruits, 615.

Delesseria, 289.

Delphinium, 578, 579, 608.

Dentaria, 640.

Deposits in the cell- wall, 36.

Derivative hybrid, 919.

Dermatogen, 147, 500, 590,
59i> 592, 952 (Figs. 114,
122, 125).

Descent, Theory of, 940.

Desmanthus, 883.

Desmidieae, 252, 258, 260.

Desmodium, 757, 881, 884.

Deutzia, 36.

Development of Sexuality,
901.

Development of the mem-
bers of a branch-system,
176.

Diageotropism, 854.

Diagonal plane, 600.

Diagram of Shoot, 188, 189,
191, 192, 193, 194, 196
(Figs. 146, 148, 149, 150,

151, 152,153).
Diagrams, Floral, 601 (Figs.

147, 406-414).
Development of Varieties,

928.
Diaheliotropism, 854.
Dialypetalae, 659.
Diandrae, 658.
Dianthus, 540, 917.
Diastase, 708.
Diatomaceae, 47, 244, 252,

260, 956.
Diatomine, 260.
Diatoms, 36, 260.
Dicentra, 604.
Dichasium, 179, 597 (Fig.

135)-
Dichogamy, 906.
Dichotomeae, 391, 460.
Dichotomy, 169, 177, 178,

460 (Figs. 133, 134, 137,

138).
Diclinous, 490, 500, 907.
Dicotyledons, 13, 14, 129

133, 486, 498, 555, 590,

633.
Dictamnus, 92, 175, 563, 601,

605,856 (Figs. 76, 77, 131,

388, 389, 414, 485).
Dictyostelium, 262.
Dictyota, 177 (Fig. 133).

Dictyoteae, 283, 956.
Dicyclic, 601.
Didymium, 262 (Fig. 172).
Differentiation of cell-wall,

32.
Differentiation of tissues, 1 3 8 .
Digitalis, 599, 917.
Dilleniaceae, 136, 646, 653,

657, 950.

Dimorphism, 907.

Dioecious, 490, 500.

Dicecism, 905.

Dion, 502.

Dionaea, 769, 894, 962.

Dioscoreae, 632, 952.

Diosmeae, 660.

Diospyrineae, 659.

Diplostemonous, 600.

Dipsacaceae, 658.

Dipterocarpeae, 660.

Directions of growth, 202,
206.

Discomycetes, 308.

Discophorae, 661.

Displacement, 219.

Diurnal and nocturnal posi-
tions of organs, 881.

Divergence, Angle of, 188.

Dorsiventral, 854, 954.

Dorstenia, 221, 656.

Doubling of the flower, 542.

Draba, 605.

Dracaena, 127, 128, 621, 629
(Fig. 104).

Draparnaldia, 281.

Dried substance of plants,

695.
Drimys, 651,
Drosera, 161, 597, 894.
Drupe, 616.
Dryadeae, 221, 662.
Dudresnaya, 292.
Dwarf males, 279, 280, 899

(Figs. 163, 183).

Ebenaceae, 659.
Echeveria, 197,597.
Echium, 598, 614.
Ectocarpeae, 283.
Ectoplasm, 40.
Egg-apparatus, 580.
Elaeagnaceae, 662.
Elaeagnus, 559 (Fig. 384).
Elaeis, 54.
Elaphomyces, 315.
Elasticity, 779, 784.
Elater, 23, 344, 345, 350,

352, 403, 404 (Figs. 18,

236, 238, 240 bis, 246,

286).
Elatineae, 662.
Electricity, 768, 962.
Elementary constituents of

the food of plants, 695.

Eleutheropetalae, 655, 659.
Eleutheropetalous, 539.
Eleutherophyllous, 539.
Eleutherosepalous, 539.
Elodea, 750.
Embryo of —

Dicotyledons, 634.

Monocotyledons, 618.
Embryo-sac, 486, 492, 498,
506, 520, 529, 576, 900,
959 (Figs. 397-401).
Embryology of —

Angiosperms, 587, 959.

Coniferae, 525 (Figs. 354,

355).
Cycadeae, 506.
Dicotyledons, 590 (Figs.

403-405).
Equisetum, 395 (Fig.

277).
Ferns, 425 (Figs. 296-

298).
Gnetaceae, 958.
Hepaticae, 351,353,355,

360, 957 (Figs. 237,

238, 240, 246).
Isoetes, 472.
Monocotyledons, 589

(Figs. 402, 404).
Mosses, 375, 957 (Figs.

257, 264).
Ophioglosseae, 412.
Rhizocarpeae, 447 (Figs.

311, 313-315).
Selaginella, 471 (Figs.

331, 332).

Embryonal tubes, 529.

Emergences, 161.

Empetraceae, 662.

Empetrum, 570.

Empirical diagram, 602.

Empusa, 277.

Emulsin, 708.

Enantioblastae, 625, 632.

Encephalartos, 501.

Endocarp, 594, 615,

Endocarpon, 329.

Endogenous formations, 162,
170, 359, 400.

Endophyllum, 330, 331.

Endoplasm, 40.

Endosmotic force, 672.

Endosperm, 472, 486, 492,
495) 497, 498, 506, 521,
529, 585, 618, 633, 715,
900 (Figs. 124, 331, 346,
348, 354, 355, 40O) 40I5
4^9, 434, 435).

Endospore, 32, 344, 404,421,

437.
Endostome, 570.
Endothecium, 375, 556.
Energy of growth, 821.
Entomophilous, 494, 908.

Entomophthoreae, 277
Epacrideae, 644, 659 (Fig.

454).
Epen, 121.
Epenchyma, 121.
Ephebe, 321 (Fig. 216),
Ephedra, 528, 530, 651.
Epibasal cell, 351, 395, 426,

447,473.
Epicalyx, 540.
Epicarp, 594, 615.
Epidermal tissue, 79, 94.
Epidermis, 97 (Figs. 37, 79-

89).
Epigynae, 658.
Epigynous, 559.
Epilobium, 601 (Fig. 380).
Epimedium, 570, 646 (Fig.

396).
Epinasty, 857.
Epipactis, 913 (Figs. 396,

492).
Epiphragm, 341, 383 (Fig.

273)-
Epiphyllum, 538.
Epipogium, 164, 620, 633,

697, 721.
Epispore, 31, 444, 446, 458,

556 (Figs. 33, 310, 311,

314, 315, 324).
Epistrophe, 750.
Equisetaceae, 142, 225, 385,

390, 392.
Equisetineae, 390.
Equisetum, 13, 36, 143, 173,

393 (Figs. 10, III, 128,

274-286).
Eranthis, 540.
Ergot, 316 (Fig. 209).
Ericaceae, 601, 644, 659 (Fig.

454)-
Eriocauloneae, 632.
Erodium, 644, 940.
Eryngium, 559 (Fig. 383).
Erysiphe, 311 (Fig. 207).
Erythrophyll, 767.
Erythroxylaceae, 660.
Escallonia, 570.
Escallonieae, 661.
Etiolation, 754.
Etiolin, 743.
Eucalyptus, 640, 949.
Eucyclae, 655, 659.
Eucyclic, 601.
Eudorina, 278.
Eugenia, 652.
Euonymus, 570, 651.
Euphorbia, 85, 86, 179, 189,

597, 618, 647 (Figs. 74,

148).
Euphorbiaceae, 86, 644,661.
Euphorbieae, 661.
Eupodium, 419.
Eurotiaae, 312.

INDEX,

Eurotium, 312 (Fig. 208).
Eusporangiata, 388.
Evernia, 329.
Exalbuminous, 587.
Excipulum, 323, 324 (Fig.

218).
Exobasidium, 336.
Exogenous formations, 154,

170.
Exospore, 32, 344, 404, 421,

438.
Exostome, 570.
Extensibility, 779, 784.
External sheath, 420.
Extine, 32, 505, 514, 553,

555(Figs. 35, 36, 350, 351,

381).
Extra - axillary branching,

175, 639.
Extrorse, 557.

Fagus, 584.

False dichotomy, 179 (Fig.

135)-
False tissue, 71.
Fascicular cambium, 130.
Fascicular tissue, 79.
Fascicular xylem, 130.
Female prothallium, 444,

470.
Female reproductive cell,

224, 897.
Ferments, organised, 248,

249.
Ferments, unorganised, 60,

708.
Ferns, 421.
Fertilisation, 224, 233, 267,

495, 523, 582, 897.
Festuca, 542.
Fibrovascular bundle of —
Dicotyledons, 649 (Figs.

93, 95)-
Equisetum, 407.
Ferns, 439 (Figs. 94,

308).
Gymnosperms, 530.
Ligulatae, 482.
Lycopodiaceae, 468.
Marattiacese, 420.
Monocotyledons, 629

(Fig. 92).

Ophioglosseae, 415.

Phanerogams, 496,

Rhizocarpeae, 459.

Fibrovascular bundles, 79,

108, 387, 949.
Ficaria, 577.
Ficus, 36, 68, 85, 98, 220

(Figs. 53, 159).
Fig, 220,594, 614.
Filament, 491, 541.
Filices, 229, 391, 421.
Filicineae, 390, 409.

969

Filiform apparatus, 580, 582,

959-
Filobacteria, 249.
Fissidens, 198, 364, 366.
Flexibility, 779.
Flexibility of internodes,

785.
Float, 395.
Floral diagram, 60 1.
Floral formulae, 606, 625,

642.
Florideae, 51, 238, 244, 288,

898.
Flower, 490, 538, 599, 625,

641.
Flowers of tan, 262.
Fluorescence of chlorophyll,

760.
Fluorine, 695.
Foliaceous Lichens, 319 (Fig.

211).
Foliage-leaves, 214.
Foliose Hepaticae, 347.
Follicle, 615.
Fontinalis, 153, 195, 199,

372 (Figs. 116, 267).
Food-materials, 696.
Foot, 344, 351,387, 395,426,

448, 472, 473 (Figs. 238,

264, 265, 296, 313, 314).
Foramen, 492.
Formative materials, 705.
Fossil Equisetaceae, 407.
Fossil Lycopodiaceae, 484.
Fourcroya, 557, 568.
Four-fold pollen-grains, 557.
Fovea, 475 (Fig. 334).
Foveola, 475.
Fovilla, 556.
Fragaria, 568, 578.
Fragmentation, 946.
Francoaceae, 661.
Frangulineae, 660.
Frankeniaceae, 659.
Fraxinus, 652.
Free cell-formation, 10, 945.
Freezing, effects of, 731.
Frenela, 517, 527.
Fritillaria, 192, 621, 677

(Figs. 151, 421).
Fruit, 495, 593, 614.
FruUania, 347, 359, 362.
Fruticose lichens, 319.
Fucaceae, 3, 281, 956.
Fuchsia, 572.
Fucoideae, 244, 281, 955.
Fucoxanthine, 766.
Fucus, 3, 235,281,897,905,

915 (Figs. 2, 184, 185).
Fumaria, 613.
Fumariaceae, 604, 646, 657

(Fig. 411).
Funaria, 47, 97, 362, 367,

368,370,371,373,381,915,

970

INDEX,

936(Figs. 45,'82,247, 353,

254, 256, 257, 266, 268-

272 .
Fundamental tissue, 79, 120.
Fungi, 227,241, 697, 956.
Funiculus, 492, 570.
Funkia, 15,23,551,572,581,

593 (Figs. 12,19,375,376,

398, 399)-

•Gagea, 495, 598.

Gamopetalae, 655, 658.

Gamopetalous, 221, 539.

Gamophyllous, 539.

Ganiosepalous, 221, 539.

Garidella, 608.

Gases, movements of, 691.

Gasteromycetes,7i,3o7,338.

Gelatinous lichens, 320.

Gelidium, 289.

Gemmae, 172, 223, 342, 348,

369, 423 (Figs. 234, 251).
General conditions of Plant

Life, 725.
Generating tissue, 82.
Generations, Alternation of,

222, 233, 342, 385, 486,

488, 899.
Genetic spiral, 190.
Gentianaceae, 658.
Genus, 927.
Genus-iiybrid, 915.
Geoglossum, 311.
Geographical distribution of

plants, 943.
Geotropism, negative, 839.
positive, 839 (Figs. 482,

484).
transverse, 854.
Geraniaceae, 660.
Germination, 715, 728, 731.
Germination of seeds, 501,

507, 619, 635 (Figs. 346,

4i9» 435, 436).
Germination of spore?, 258,

259, 262, 268, 293, 333,

346, 361, 393, 416, 421,

442, 461, 469, 515, 521,

580, 582.
Gesneraceae, 658.
Geum, 221, 579 (Fig. 161),
Ginkgo, 520, 578.
Glabrous, 99.
Gladiolus, 555.
Glands, compound, 91 (Fig,
76).
simple, 84.
Glandular hairs, 10 1, 161

(Figs. 77, 359).
Glans, 615.
Glaucium, 588.
Gieba, 307, 341 (Fig. 231).
Gleditschia, 218, 639, 652,

935.

Gleichenia, 431.
Gleicheniaceae, 391, 441.
Globoids, 52 (Fig. 48).
Globularia, 648.
Globulariaceae, 658.
Globule, 298 (Figs. 197, 198,

200, 201).
Gloeocapsa, 246 (Fig. 165).
Glceocystis, 248.
Gloeothece, 247.
Gloxinia, 584.
Glucose, 712.
Glume, 631.
Glumiflorae, 631.
Glyptostrobus, 527.
Gnetaceae, 497, 498, 500,527,

95.8.
Gnetum, 527, 578, 959.
Gomphrena, 580.
Gonidium, 223, 239, 318.
Gonium., 253.
Goodeniaceae, 659.
Gramineae, 631 (Fig. 409).
Grand period of growth,

817.
Granulose, 56, 60.
Graphideae, 329.
Graphis, 319.
Grass, flower of, 603.
Grasses, embryo of, 618.
Grateloupia, 292.
Gravitation, action of, 770,

839 (Fig. 482).
Grossulariaceae, 661.
Growing point, 138 (Figs.

108-114).
Growth —

action of electricity on,

962.
action of gravitation on,
770, 839 (Figs. 477,
482).
action of light on, 752,

832, 962.
action of temperature

on, 730, 828.
causes of, 775.
directions of, 202.
in length, 815.
in length of the root and

stem, 137.
in thickness of the cell-
wall, 19, 32.
in thickness of the root,

127, 133.
in thickness of the stem

and root, 125.
in thickness of the stem

of monocotyledons,

127.
mechanics of, 773.
of starch-grains, 57.
periodicity of, 823.
Gruinales, 660.

Guard-cells of stomata, 98,
103 (Figs. 61-65, 84-89).

Gum, 36, 93, 705.

Gum -passages, 93 (Fig. 66).

Guttiferae, 660.

Gymnadenia, 578.

Gymnoascus, 307, 308.

Gymnocarpous, 306.

Gymnocarpous lichens, 323.

Gymnogramme, 421.

Gymnosperms,497, 498, 534,
897.

Gymnosporangia, 331, 335.

Gymnostachys, 626 (Fig.

433)-
Gymnostomous, 381.
Gymnostomum, 383.
Gynaeceum, 490, 557, 627,

641 (Figs. 386, 387).
Gynandrae, 633.
Gynobasic style, 563, 568

(Figs. 389, 390).
Gynophore, 547, 563 (Figs.

371, 388).
Gynostemium, 547, 627, 633

(Figs. 372, 492).

Haematococcus, 954.

Haemodoraceae, 632.

Hairs, 99, 150, 160 (Figs.
42, 83).

Hakea, 124, 652.

Halidrys, 282.

Haloragideae, 662.

Halymenia, 292.

Haustoria, 275, 812 (Figs.
175, 276).

Heat-
action of, 731, 884.
conduction of, 725,
production of, 723.
radiation of, 725.

Heat-expansion, Coefficients
of, 727.

Heating apparatus for the
microscope, 736 (Fig.474).

Hebenstreitia, 648.

Hedera, 651.

Hedychium, 625 (Fig. 429).

Helianthemum, 577, 597.

Helianthus, 567 (Fig. 393).

Helicoid cyme, 1 80, 597 (Fig.
136).

Helicoid dichotomy, 178
(Fig. 134).

Heliotropism, 832.
negative, 756, 833.
positive, 756, 833.
transverse, 854.

Heliotropium, 598.

Helleborus, 495.

Helobiae, 630.

Helvella, 311.

INDEX.

971

Hemerocallis, 597, 598.
Hemicyclic, 600, 641.
Hemileia, 956.
Hepaticae, 345, 346.
Heppia, 328.

Heracleum, 610 (Fig. 415).
Hermaphrodite, 490.
Herminium, 218 (Fig. 158).
Herpothamnion, 291 (Fig.

189).
Hesperideae, 660.
Hesperidiam, 6i6.
Heterocyst, 247.
Heteroecism, 332.
Heteromerous, 601.
Heteromerous lichens, 320.
Heterosporeae, 389.
Heterostylism, 907.
Hibiscus, 616.
Hieracium, 926.
Hildenbrandtia, 289.
Hilum, 492, 570, 618.
Himanthalia, 282.
Hippocastaneae, 644, 660

(Fig. 455).
Hippocrateaceae, 660.
Hippurideae, 662.
Hippuris, 155,537, 649 (Figs.

119,360).
Histology of —

Dicotyledons, 649.

Filices, 438.

Gymnosperms, 530.

Ligulatae, 481.

Lycopodiaceae, 467.

Marattiaceae, 420.

Monocotyledons, 629.

Ophioglossaceae, 415.

Phanerogams, 496.

Rhizocarpeae, 459.
Holargidium, 605.
Holly, 34, 35 (Fig. 37).
Hollyhock, 43, loi (Figs.

42, 83).
Homoiomeroiis lichens, 320.
Hook-climbers, 962.
Hop, 862, 939.
Hormogonia, 954.
Hoy a, 29, 85, 668 (Figs. 30,

466).
Humiriaceae, 660.
Hyacinthus, 77 (Figs. 61-64).
Hybrid, 914.
Hybridisation, 914.
Hydnora, 576.
Hydnoreae, 656.
Hydnum, 336.
Hydrangea, 652.
Hydrangeae, 661.
Hydrilla, 630.
Hydrilleae, 631.
Hydrocharideae, 625, 631.
Hydrocharis, 624, 626.
Hydrodictyeae, 244, 255.

Hydrodictyon, 256.
Hydrogen, 695, 697.
Hydropeltidineae, 655, 657.
Hydrophyllaceae, 658.
Hydropleon, 664.
Hydrophylleae, 539 (Fig.

483).
Hydrotropism, 845.
Hymenium, 306 (Figs. 205,

218, 219, 227).
Hymenomycetes, 336.
Hymenophyllaceas, 421, 441.
Hymenophyllum, 422.
Hyoscyamus, 598.
Hypecoum, 604.
Hypericineae, 544, 660.
Hypericum, 545, 602 (Figs.

367, 408).
Hyphae, 84, 307 (Figs. 55,

205, 214-217, 225, 229).
Hyphal tissue, 84.
Hypobasal cell, 351, 395,

426, 447, 472.
Hypochlorin, 710.
Hypocotyledonary axis, 507,

635.
Hypoderma, 95, 98, 123 (Fig.

102).
Hypodermiae, 335.
Hypoglossum, 289.
Hypogynae, 658.
Hypogynous, 559.
Hyponasty, 857.
Hypophysis, 589 (Figs. 402--

405).
Hypothallus, 323.
Hypothecium, 324.
Hypsophyllary leaves, 214,

595-

Iberis, 649.

Ice, Formation of, 731 (Fig.

473).
Idioblast, 84, 91.
lUicium, 615.
Imbibition, 790.
Irapatiens, 555, 637.
Incombustible deposits in

cell-wall, 36.
Indefinite inflorescence, 595.
Indehiscent fruits, 615, 616.
Indusium, 419, 435, 455,

575 (Figs. 303, 304).
Inferior ovary, 566 (Figs.

393, 394).

Inflorescence, 490, 495, 594.

Innovation, 185, 342.

Insect-agency in pollination,
494, 906 (Figs. 488-492).

Insertion of leaves, 155.

Integument, 492, 498, 570^
575 (Figs. 397, 398).

Intercalary growth of cell-
wall, 22.

Intercalary vegetative zone,

536, 816.
Intercellular spaces, 75, 76,

93, 98 (Figs. 58-66, 78).
Intercellular substance, 72,

76, 119.
Interfascicular cambium, 130.
Interfascicular phloem, 130.
Interfascicular xylem, 130.
Intermediate cells, 119, 651.
Intermediate tissue, 123.
Internodes, 156.
Interposed members, 60 r.
Intine, 32, 505, 514, 553,

555 (Figs. 345, 351, 380,

381).
Intrapetiolar buds, 639.
Introrse, 557.

Intussusception, 29, 58, 666,
Inulin, 62, 707 (Fig. 51).
Involucel, 596.
Involucre, 540, 596.
Iodine, 695.
Ipomaea, 599.

Irideae, 625, 632 (Fig. 427).
Iron, 696, 699.
Irritability, 865, 878.
Isatis, 175 (Fig. 132).
Isoeteae, 389, 392, 460.
Isoetes, 173, 182, 215, 388,

468, 958 (Figs. 138, 329,

330, 333, 334).
Isocarpeae, 655, 659.
Isomerous, 601.
Isosporeae, 389.
Isostemonous, 600.
Ivy, 78, 837, 935, 938 (Fig.

66).

Jasminiaceae, 658^

jasminum, 651.

Jerusalem artichoke, 42,

63.
Juglandeae, 662.
Juglans, 633.
Juliflorae, 655.
Juncaceae, 632.
Juncagineae, 626, 631 (Fig.

432).
Jungermannia,347,356(Figs.

244, 246).
Jungermannieae, 343, 349,

351, 356.
Juniperineae, 527.
Juniperus, 509, 511, 513,

523 (Figs. 349, 355).
Jussieua, 935.
Justicia, 555.

Kalmia, 616.
Karyokinesis, 17.
Kaulfussia, 416.
Kenenchyma, 83,
Kerria, 651.

972

Kitaibelia, 540.

Klugia, 598.

Knight's experiments on the

influence of gravitation,

853.

Labellum, 567, ^13 (Figs.

394, 492).
Labiatae, 548, 579, 585, 613,

633, 648, 658.
Labiatiflorae, 658.
Labium, 475 (Fig. 334).
Laburnum, 613.
Lamina of Leaf, 212, 640.
Laminarieae, 283.
Lamium, 549, 578, 640 (Fig.

373)-
Larix, 509, 511.
Lateral budding, 169 (Figs.

128-132).
Lateral plane, 600.
Lateral roots, 166 (Fig. 125).
Lateral shoots, 173.
Lathraea, 50, 648.
Laticiferous cells, 85, 961

(Figs. 69, 71).
Laticiferous vessels, 86, 705,

712, 94.8 (Fig. 72).
Latticed cells, 89,
Lauraceae, 643.
Laurineae, 657.
Leaf, 150, 153, 157, 187.
Leaf, development ot, in

Ferns, 430.
Leaf-bearing axes, 153.
Leaf-blade, 212, 640.
Leaf-branching, 182.
Leaf-forms, 211.
Leaf-sheath, 396.
Leaf-spines, 215.
Leaf-stalk, 212, 640.
Leaf-tendrils, 214, 865.
Leaf-trace, 155, 496.
Leaf- veins, 213.
Leaflet, 212.
Legume, 615.
.Legumin, 719.
Leguminosae, 633, 718.
Lejolisia, 181, 291 (Fig. 190).
Lemaneaceae, 289.
Lemna, 164, 621, 626, 628.
Lemnaceae, 630.
Lempholemma, 328.
Lentibulariaceae, 659.
Lenticels, 108.
Lepidium, 604.
Lepidodendron, 484.
Lepidostrobus, 485.
Leptogium, 328 (Fig. 215).
Leptosporangiata, 388.
Lessonia, 283.
Leucin, 718.
Leucojum, 628.
Levisticum, 183 (Fig. 140).

INDEX.

Leycesteria, 642 (Fig. 440).

Libocedrus, 509.

Libriform Fibres, 35, 118
.(Fig. 97)^

Lichens, 231, 241, 318.

Lichina, 328.

Lichnoerythrine, 766.

Lichnoxanthine, 766.

Light-
action of, 737.
chemical action of, 743.
mechanical action of,
749, 832.

Lignification of the cell-wall,

20,34,705-
Ligulatae, 392, 461, 468.
Ligule, 212, 392, 475, 539,

624 (Figs. 361, 425).
Liliaceae, 584, 602, 606, 628,

632 (Fig. 406).
Liliiflorae, 625, 632.
Lilium, 223, 623.
Limnanthaceae, 660.
Linaceae, 660.
Linaria, 640.
Lingula, 475 (Fig. 334).
Linnea, 579.
Linum, 563.
Liriodendron, 652.
Lithium, 695.
Lithocysts, 84.
Lithospermum, 598.
Loasaceae, 659.
Lobelia, 642 (Fig. 442).
Lobeliaceae, 86, 613, 658.
Loculicidal dehiscence, 616.
Lodicule, 538, 627.
Loganiaceae, 65.8.
Lomentum, 615.
Lonicera, 640, 642 (Fig.

440).
Loranthaceae, 576, 585, 599,

662, 959.
Loranthus, 588.
Lunularia, 348, 355.
Lupinus, 52, 718 (Fig. 47).
Luzula, 577.

Lychnis, 536, 539 (Fig. 361).
Lycogala, 262.
Lycopodiaceae, 184, 385, 392,

461, 484, 953.
Lycopodieae, 184, 389, 392,

460.
Lycopodineae, 460.
Lycopodium, 72, 38S, 389,

461, 466, 515 (Figs. 326-

328).
Lygodmm, 217, 432, 435,

862.
Lysimachia, 579, 638,
Lythrarieae, 662.

Macrocystis, 283.
Macrosporangium, 453, 459,

476, 480, 491 (Figs. 319-

321, 325, 335)-
Macrospore, 31, 386, 389,

442, 455, 457, 458, 470,

491, 497 (Figs. 33, 310,

311, 314, 330, 331, 338).
Macrozamia, 503.
Macrozoogonidia, 281.
Magnesium, 52, 696, 699.
Magnolia, 549, 612, 652.
Magnoliaceae, 600, 608, 641,

657.
Mahernia, 579.
Mahonia,543, 880 (Fig. 362).
Maianthemum, 606.
Maize (Figs, i^ bis, 41, 50,

58, 92, 117, 122-124).
Malachium, 563.
Malaxis, 622.
Male Prothallium, 442.
Male reproductive cells, 224,

897.
Malope, 540.
Malpighiaceae, 660.
Malvaceae, 661.
Manglesia, 547 (Fig. 370).
Manubrium, 29S (Fig. 198).
Marattia, 64, 93, 416.
Marattiaceae, 391, 416, 882.
Marchantia, 23, 78, 348, 350,

356, 962 (Figs. 18, 65, 89,

233-236,241-243).
Marchantia, stomata of, 75,

106,948 (Figs.65, 89, 233).
Marchantieae, 343, 352, 355.
Marcgraviaceae, 660.
Marsilia, 162, 184, 443, 446,

447, 451, 456, 459, 882

(Figs. 120, 121, 142, 310,

314, 315, 317, 321, 325).
Marsiliaceae, 391, 460.
Mazus, 648.

Mechanics of growth, 773.
Mechanism of —

heliotropism, 837.

geotropism, S53 (Fig.
482.

movements, 887.
Median plane, 188, 600.
Median wall, 426.
Medullary rays, 120, 129,

131, 531,649 (Fig. 105).
Medullary sheath, 130, 531,

650 (Fig. 105).
Megacarpaea, 605.
Megaclinium, 88 r.
Megalospora, 325.
Melaleuca, 651.
Melampsora, 331.
Melampyrum, 648.
Melastomaceae, 662.
Meliaceae, 660.
Melilotus, 493.
Melobesia, 65.

INDEX,

9n

Melobesiaceae, igo.
Members, 149.
Menispermaceae, 643, 653,

657 (Fig. 449).
Mentha, 638.
Menyanthes, 161, 595.
Mericarp, 614, 615.
Merismopedia, 247, 954.
Meristem, 80, 137.
Mertensia, 431.
Mesembryanthemeae, 653,

662.
Mesocarp, 594, 615.
Mesocarpeae, 257.
Mesocarpus, 257.
Mesophyll, 213.
Mesotaenium, 258.
Metamorphosis —
chemical, 708.
of organs, 149,151,934.
Metaplasm, 37, 40.
Metastasis, 703.
Metzgeria, 140, 181,347, 356,

952 (Figs. 109, no, 137,

232).
Micella, 19, 664.
Micellar aggregate, 664.
Michauxia, 642, 646.
Micranthae, 631.
Microcachrys, 527.
Micropyle, 492, 570 (Figs.

352, 397,400).
Microsomata, 18, 947.
Microsporangium, 453, 455,

459, 475, 476, 479 (Figs.

309, 319-321, 325, 334,

335).
Microspore, 386, 389, 442,

469, 491, 497 (Figs. 309,

310,329, 331).
Microzoogonidia, 256, 281.
Mid-rib, 213.
Middle (central) lamella, 25,

34, 72 (Figs. 24, 32, 38,

57).
Mignonette, 187 (Fig. 145).
Mimosa, 882, 888.
Mimoseae, 550, 557, 662, 879.
Mirabilis, 538, 653.
Mistletoe, 634, 837.
Mnium, 342, 363.
Molecular forces, 663.
Momordica, 940.
Monocarpellary, 560.
Monocarpic, 594.
Monocarpous, 560.
Monochlamydeae, 655, 656.
Monocleae, 352, 354.
Monocotyledons, 498, 618.
Monocyclic, 601.
Monoecious, 490, 900.
Moncecism, 905.
Monopodial inflorescence,

595.

Monopodium,i77 (Figs. 119,

123, T32).
Monosymmetrical, 204, 611.
Monotropa, 214, 241, 577,
578, 584, 634, 649, 697,
721.
Monotropeae, 659.
Monsonia, 644.
Monsterineae, 84 (Fig. 70).
Moreae, 656.
Morchella, 311.
Morel, 311.
Mosses, 342.
Mosses (true), 381.
Mougeotia, 258.
Movement —

action of light on, 756,

885.
ciliary, 4, 232, 240,751.
dependent on oxygen,

722, 886.
influence of temperature

on, 729, 884.
of chlorophyll granules,

750.
of protoplasm, 38, 40,

261, 730, 749, 769.
of water, 674.
Movements —
induced, 878.
of gases, 691.
of nutation, 855, 857,

862, 866.
of nutation, influence of
light and temperature
on, 871.
periodic, 878.
periodic, influence of
light on, 882.
Mucilage, conversion of the
cell-wall into,2o, 33,35,705.
Mucor, 265, 266.
Mucorini, 265, 267.
Miihlenbeckia, 651.
Mulberry, 594, 614.
Multilateral structure, 205.
Musa, 555, 621.
MusaceaE,62 5,632 (Fig. 428).
Muscari, 174 (Fig. 130).
Musci, 345, 361.
Muscineae, 342.
Mushroom, 307, 336 (Figs.

225-227).
Mycelium, 226, 305, 307
(Figs. 174, 175, 180, 181,
204, 208, 225).
Myosotis, 598.
Myosurus, 561, 578.
Myricaceae, 657.
Myriophyllum, 49, 171.
Myristica, 492, 570, 587.
Myristicaceae, 657.
Myrosin, 708.
Myrsinaceae, 570, 659.

Myrtaceae, 662.
Myrtiflorae, 662.
Myxoamoebae, 1 0,39,252,261,

955 (Fig. 173).
Myxomycetes, 10, 41, 244,

261, 945.

Naiadeae, 573, 6r8, 630.
Naias, 491, 541, 557,565,566,

573,575, 627.
Nardus, 603 (Fig. 409).
Nasturtium, 930.
Natural Selection, 929.
Natural System, 942.
Nectar, 494, 569.
Nectary, 494, 569.
Negative geotropism, 839.
Negative heliotropism, 756,

833.
Nelumbiaceae, 641, 657.
Nemalieae, 289, 290.
Nemalion, 238 (Figs. 164,

188).
Neottia, 168, 214, 697, 721,

935.
Nepentheae, 656.
Nepenthes, 640, 677, 688,

934.
Nephrolepis, 428, 431, 433.
Nerium, 539, 652.
Nickel, 695.
Nicotiana, 494.
Nidularieae, 339.
Nigella, 542.
Niphobolus, 429, 430.
Nitella, 17, 49, 298, 300, 302,

905 (Figs. 14, 198-202).
Nitophyllum, 290.
Nitric acid, 698.
Nitrogen, 691, 696, 698.
Node, 156.
Nostoc, 247.
Nostocaceae, 247, 328.
Nothoscordum, 593.
Nucellus, 487, 497 (Figs. 348,

349, 352, 354, 355, 383,

391, 397, 398).
Nuclear disc, 18, (Fig. 10).
Nuclein, 947.
Nucleoli, 38, 44.
Nucleus, 2, 38, 44, 947.

division of, 17.
Nucule, 299.
Nuphar, 575, 586, 617, 648

(Fig. 68).
Nut, 615.

Nutation, 855 (Fig. 485).
Nutmeg, 587.
Nutrition, 775.
Nyctagineae, 137, 653, 661,

950.
Nyctitropic, 873.
Nymphaea, 538, 549, 5?6,

612, 648.

974

INDEX,

Nymphxaceae, 536, 586, 600,
633, 641, 654, 657.

Oak, 614.

Obdiplostemonous, 601,

Ochnaceae, 660.

Ochrolechia, 326.

Octamerous, 642.

Octants, 351.

CEdogonieae, 244, 278.

CEdogonium, 9, 22, 235, 279,
897, 945 (Figs. 17, 163,
182, 183).

(Enothereae, 662.

Oidium, 311.

Oil, 52, 91,706,715.

Oleaceae, 643, 658 (Fig. 448).

Olfersia, 436.

Omphalaria, 328.

Omphalodes, 598.

Onagrariese, 555.

Onopordon, 640.

Oocardium, 248.

Oogonium, 3, 234, 267, 271,
273, 279, 281 (Figs, 163,
177, 179, 181, 183, 185).

Oophore, 225, 342,385,421,
488.

Oosphere, 224, 234, 267, 350,
355, 373, 425, 446, 486,
523, 580, 897 (Figs. 163,
177, 179, 181, 183, 185,
236, 240, 256, 295, 314,

331, 354,355, 399).
Oospore, 224, 234, 343, 471,

585.
Oosporese, 244, 267.
Opegrapha, 329.
Open bundles, 650.
Opening and closing of

flowers, 872.
Operculum, 345, 381 (Fig.

266).
Ophioglosseae, 385, 391, 410.
Ophioglossum, 410, 412, 414,

958 (Figs. 288, 290).
Ophrydeae, 557, 603, 628.
Opuntia, 538, 566.
Crchideae, 557, 576, 579,

583, 586, 593, 603, 618,

627, 633, 908, 960 (Fig.

410).
Orchis, 571, 578 (Figs. 397,

418).
Order of succession of the

parts of the flower, 608.
Organic centre, 203.
Organs of plants, 149.
Origin of species, 920.
Ornithogalum, 580, 588.
Orobanche, 214, 241, 634,

649, 935-
Orobancheae, 585, 658.
Orthostichy, 188 (Fig. 154).

Orthotropic, 854.
Orthotropous, 492, 570.
Oscillatorieae, 246, 247.
Osmunda, 423, 435.
Osmundaceae, 391, 410, 437,

440.
Ouvirandra, 625.
Ovary, 493, 497, 558 (Figs.

382-395).
Ovule, 487, 491, 497, 498,
570 (Figs. 343, 347-
349, 397, 398, 400).
development of, in An-

giosperms, 576.
development of, in Go-

niferae, 520.
development of, in Gne-
taceae, 529.
Oxalic acid, 699.
Oxalideae, 660, 879.
Oxalis, 882, 891.
Oxygen, 691, 696, 698.
Oxy-salts, 698.
Ozothallia, 282.

Paeonia, 614.

Paleae, 150, 435, 602, 631.

Paliurus, 599.

Pallisade - parenchyma, 84,

533, 735-
Palmaceae, 624, 631.
Palmella, 248.
Palmellaceae, 46, 244, 248,

329-
Palms, 620, 623, 629, 938.
Pandanaceae, b^L.
Pandorina, 234, 254 (Figs.

162, 167).
Pandorineae, 244, 253.
Panicle, 596.

Panicled inflorescences, 596.
Pannaria, 328.
Papain, 708.
Papaver, 87, 616, 641, 647,

654 (Fig. 464).
Papaveraceae, 87, 647, 657.
Papayaceae, 87, 659.
Papilionaceae, 607, 609, 613.
Pappus, 538, 617, 940.
Paraphyses, 309, 310, 325,

336, 338, 343, 370, 435,

441 (Figs. 205, 219, 227,

253)-
Parasites, 214, 242, 649, 720,

942.
Parastichy, 194.
Paratonic action of light,

757-
Parenchyma, 83 (Fig. i).
Parietales, 659.
Paris, 188, 606 (Fig. 147).
Parmelia, 327.

Parnassia, 642,856 (Fig. 44 1 ).
Paronychieae, 645, 661.

Parthenogenesis, 275, 593,

902.
Passiflora, 536, 865, 870.
Passifloraceae, 659.
Pastinaca, 183 (Fig. 140).
Paullinia, 653.
Pectinaceoussubstances,7o6.
Pediastrum, 70, 256 (Figs.

54, 169).
Pedicularis, 588, 648.
Peduncle, 491.
Peganum, 610, 644.
Peissomelia, 289.
Pellia, 352, 360, 395, 422,

945-
Peltigera, 319.
Pelvetia, 282.
Penicillium, 226, 306, 314.
Pentamerous, 641.
Peperomia, 579.
Perianth, 490, 528, 538.
Periblem, 147, 163, 500, 952

(Fig. 114).
Pericambium, 115, 134, 167

(Figs. 96, 106, 114, 125).
Pericarp, 594, 615.
Perichaetium, 343, 371.
Periderm, 95, 106.
Peridium, 306, 333, 340 (Fig.

231).
Perigynae, 661.
Perigynium, 34.3, 350 (Fig.

236).
Perigynous, 559.
Periodic movements of or-
gans, 878.
Periodicity of growth in

length, 817, 823.
Periodicity of tension, 808.
Perisperm, 492, 586.
Peristome, 381 (Figs. 266,

267, 273).
Perithecium, 308, 312, 313,

317, 323 (Figs. 207, 208,

209).
Permanent tissue, 82.
Peronosporeae, 244, 275, 955.
Persea, 555.
Pertusaria, 319, 326 (Fig.

220).
Petal, 538.
Petiole, 212, 640.
Peziza, 11, 310 (Figs. 205,

206).
Phaeosporeae, 283, 956.
Phalloideae, 341.
Phallus, 340 (Fig. 231).
Phascaceae, 377, 380.
Phascum, 375.
Phaseoius, 24, 134, 135, 147,

561, 636 (Figs. 21, 106,

107, 113, 385, 437).
Phegopteris, 430, 441.
Phelloderm, 107 (Fig. 90).

INDEX.

975

Phellogen, 82, 107 (Fig. 90).-
Philadelpheae, 661.
Philodendron, 93.
Phloem, III, 119.
Phloem-ray, iii, 131.
Phloem-sheath, 440, 468.
Phlomis, 564 (Fig. 390).
Phoenix, 618 (Fig. 419).
Phosphorescence, 723.
Phosphorus, 696, 699.
Phototactic, 752.
Phototonus, 757, 885.
Phragmidium, 332.
Phycocyanine, 246, 766.
Phycoerythrine, 289, 766.
Phycomyces, 265, 267 (Fig.

174).
Phycophaeine, 282.
Phycoxanthine, 260, 766.
Phyllactidium, 329.
Phyllanthacese, 661.
Phyllantheae, 661.
Phylliscium, 328.
Phylloclade, 217.
Phyllocladus, 158, 217, 509,

510, 527.
Phyllade, 476.
Phyllode, 222.
Phylloglossum, 462, 463.
Phyllome, 150, 157.
Phyllophoraceae, 289.
Phyllophyte, 151.
Phyllopode, 484.
Phyllotaxis, 187, 194, 953

(Figs. 146-1.54).
Phyllotaxis of Ligulatae, 476.
Physalis, 638.
•Physarum (Fig. 173).
Physcia, 327 (Fig. 222).
Physma, 327 (Fig. 222).
Phytelephas, 587.
Phytocrene, 653, 950.
Phytolacca, 645, 653 (Fig.

459)-
Phytolaccaceae, 136, 645,

661.
Phytophthora, 275.
Picea, 517.
Pileus, 336, 337.
Pilobolus, 267.
Pilularia, 31, 451, 455, 457,

458, 460 (Figs. 33, 318,

320, 322-324).
Pine-apple, 614.
Pinus, 25, 30, 72,74, 94, 105,

124, 508, 527, 533 (Figs.

23, 24, 32, 60, 78,88, 102,

103, 346, 356).
Piperaceae, 137, 492, 496,

564, 573, 575, 633, 654,

656, 951.
Piperineae, 655.
Piptocephaiidae, 267, 955

(Fig. 175).

Pistia, 585, 628.

Pisum, 52 (Fig. 46).

Pitcher-like organs, 677.

Pith, 120.

Pitted vessels, 24, 116 (Figs.

25-27, 97).
Pittosporeae, 93, 660.
Placenta, 452, 492, 574.
Placentation, 561, 563.
Plagiotropic, 854,954.
Plane of insertion, 188.
Plane of symmetry, 204.
Plantagineae, 643, 658 (Fig.

447).
Plasmodium, 10, 39, 252, 262,

841, 955-
Platanacese, 656.
Platanus, 639.
Platycerium, 170, 432.
Pleomorphy, 232.
Pleon, 664.
Pleospora, 316.
Plerome, 115, 147, 163, 500,

952 (Fig. 114).
Plerome sheath, 115, 124,

415 (Fig. 96).
Pleurocarpous Mosses, 370.
Pleurococcus, 246, 253, 329.
Plocamium, 289.
Plumbagineae, 659.
Plumule, 499, 593.
Podocarpeae, 527.
Podocarpus, 500, 510, 527.
Podophyllum, 647.
Podospliaera, 238, 312.
Podostemoneae, 662.
Point of insertion, 188.
Polanisia, 605 (Fig. 412).
Polar nuclei, 580.
Polarised light, 665.
Polemoniaceae, 658.
Pollen, Development of, 12,

14, 15, 32, 505, 551, 552
(Figs. 12, 34, 374, 375,

378, 379)-
Pollen-grain, 23, 34, 224,
487, 491, 498, 506, 514,

553, 555, 900 (Figs. 36,
345, 350, 351, 378, 380,
381).

Pollen-sac, 491, 505, 512,
542, 551, 552, 556 (Figs.

374, 377).
Pollen-tube, 33, 515, 522,

554, 568 (Figs. 35, 345,
354, 355, 376, 395).

Po lination, 494, 583.
PoUinium, 557, 913.
PoUinodium, 236, 307, 309,

312, 313, 898 (Figs. 204,

207).
Polyblastia, 329.
Polycarpae, 631, 657.
Polycarpcllary, 561,

Polycarpic, 594.
Polycarpous, 560.
Polychidium, 328.
Polycyclic, 601.
Polyembryony, 526, 593.
Polygala, 136, 612 (Fig. 417).
Polygalaceae, 136, 653, 660.
Polygamous, 535, 905.
Polygamy, 905.
Polygonaceae, 493, 565, 662.
Polygonatum, 186, 619 (Figs,

143, 420).
Polygonum, 580, 638, 930.
Polypodiaceae, 391, 421, 441,
Polypodiese, 441.
Polysymmetrical, 204, 611.
Pblytrichum, 365, 366, 368,

384 (Fig. 273).
Pomeae, 662.
Pomegranate, 616.
Pontederiaceae, 632.
Populus, 637, 640.
Pore-capsule, 616.
Porlieria, 651.
Porocyphus, 328.
Portulacaceae, 661.
Posterior, 600.
Potamogeton, 625, 630.
Potamogetoneae, 586, 631.
Potassium, 696, 699.
Potato, 50, 59.
Potentilla, 609, 651.
Pottia, 361.
Pressure, Effect of, on

growth, 809.
Prickle, 100.
Primary bast, 650.
Primary cortex,i27,i 30, 532,

649.
Primary endosperm, 582.
Primary fibro - vascular

bundle, 129, 649.
Primary meristem, 80, 82,

137.
Primary root, 164, 166.
Primary tissue, 80.
Primary wood, 531, 650.
Primine,57i (Figs. 397, 398).
Primordia, 609.
Primordial cell, 5.
Primordial epidermis, 147.
Primordial utricle, 42.
Primulaceae, 493, 565, 574,

644, 659 (Fig. 456).
Primulineae, 659.
Procambium, no.
Products —

of degradation, 705.
of degradation of chloro-
phyll, 47.
Pro-embryo, 293 (Fig. 191).
Pro-embryonic branch, 296.
Prolification, 490, 503.
Promycelium, 3 30(^Fig. 224).

976

INDEX.

Pro-nucleus, 495.
Prosenchyma, 83, 950.
Protandrous, 910.
Proteaceae, 662.
Proteids, 51.
Proten, 121.
Protenchyma, 121.
Prothallium, 225, 385, 393,

394j 427, 4445 447, 47i,

486, 498, 521, 522, 582.
Protococcus, 248, 329.
Protogynous, 910.
Protomyces, 335, 957.
Protonema, 226, 342, 346,

361, 362, 363, 376 (Figs.

247, 284, 250, 259).
Protophyta, 244, 245.
Protoplasm, 2, 37, 947.
Prototaxites, 272.
Pseudaxis, 178, 180.
Pseudocarp, 221, 594, 614.
Pseudo-parenchyma, 84,307.
Pseudopodium —
of Mosses, 369.
of Sphagnum, 379 (Fig.
263).
Pseudotsuga, 533.
Psiloteae, 392.
Psilotum, 164, 460, 464, 942,

957.

Psoralea, 81 (Fig. 69).

Pteris, 24, 27,28,30,35,109,
113, 123, 144, 216, 424,
426, 428, 429, 431, 439
(Figs. 22, 27, 28, 31, 38,
84» 91, 95, loi, 112, 156,
294, 296, 299-301, 307,
308).

Puccmia, 331,333, 334 (Figs.
223, 224).

Pulvinus, 880, 882, 889.

Punctum vegetationis, 138.

Punica, 651.

Purpose, 934.

Puschkinia, 495.

Putamen, 616.

Pycnidium, 308, 316, 326.

Pycnophycus, 282.

Pyrenomycetes, 316.

Pyrola, 562, 634 (Fig. 387).

Pyrolaceae, 659.

Pyrus, 559.

Pythium, 273.

Pyxidium, 616.

Quercus, 593, 636 (Figs. 438,
439)-

Raceme, 595.

Racemose branching, 179.

Racemose inflorescence, 595.

Racoblenna, 328.

Radial bundles, 949.

Radiation of heat, 725.

Radicle, 593, 635.

Radula, 343, 346, 356, 362

(Fig. 245).
Rafflesiacea^, 634, 656.
Ramalina, 327.
Ramenta, 428, 435.
Ramondieae, 658.
Ranunculaceae, 634, 657.
Raphe, 492, 570.
Raphides, 65, 84, 88.
Reaumuriaceae, 660.
Receptacle —

of Flower, 221,490, 559,
614 (Figs. 159-161).

of Mosses, 370 (Figs.
253, 256).
Receptacles for secretions,

93.

Receptive spot, 271,280, 425.

Reciprocal hybrids, 916.

Regular flowers, 611.

Rejuvenescence of the cell,
8, 945-

Reproductive cells, 223, 896,

Reseda, 608 (Fig. 145).

Resedaceae, 608, 659.

Reserve-materials, 704.

Reservoir of reserve-mate-
rials, 704.

Resin, 705.

Resin-passages, 93, 94, 532,

949 (I^^ig. 78).

Respiration, 721.

Restiaceae, 632.

Resupination, 604.

Retardation of growth by-
light, 755, 832.

Revolving nutation, 855.

Rhamnaceae, 660.

Rheum, 565 (Fig. 391).

Rhinanthus, 579, 648.

Rhizantheae, 656.

Rhizine, 319 (Fig. 214).

Rhizocarpeae, 385, 391, 442,

957.
Rhizocarpon, 323.
Rhizoclonium, 281.
Rhizoid, 231, 293, 296, 367

(Figs. 80, 191, 248).
Rhizome, 216.
Rhizophore, 168, 477.
Rhododendron, 557.
Rhodoraceae, 601, 659.
Rhodospermine, 50, 51, 298.
Rhus, 644 (Fig. 452).
Rhynchonema, 904, 920.
Ribes, 107 (Fig. 90).
Riccia, 344, 351, 354 (Figs.

239, 240).
Riccieae, 352, 354.
Ricinus, 53, 112, 114, 182,

544, 635, 705, 716 (Figs.

48, 93, 94, 139, 366, 435,

471).

Rivularia, 248.
Rivularieae, 247, 328.
Robinia, 584, 652.
Roccella, 329.
Roestella, 331, 335-
Root, 150, 162.
Root-cap, 144, 162, 952
(Figs. 112, 114, 120-122).
Root-hairs, 162.
Root-pressure, 676, 685,

688 (Figs. 467, 469).
Root-sheath, 165 (Figs. 123,

124).
Root-system, 164.
Roots —

branching of, 164, 181

(Figs. 125, 138).
fibrovascular system of,
114, 949 (Figs. 96,
106).
growth in thickness of,
133 (Figs. 106, 107).
Rosa, 220 (Fig. 160).
Rosaceae, 662.
Rose-hip, 221, 614.
Rosette, 521.
Rosiflorae, 662.
Rostellum, 913 (Fig. 492).
Rotation of protoplasm, 39,

43.
Rubiaceae, 643, 658 (Fjg.

446).
Rubus, 578.
Rudbeckia, 599.
Ruscus, 217, 935,
Rutaceae, 660.
Ruteae, 660.

Sabal, 191 (Fig. 149).
Sabina, 516, 527.
SabuHna, 578.
Saccharomyces, 244, 249.
Saccharomycetes, 244, 249,

955.
Sagittaria, 74, 624 (Fig. 59).
Salicineae, 660.
Salisbura, 512, 527 (Fig.

347).
Salix, 599.

Salvia, 912, 940 (Fig. 498).
Salvinia, 191, 391, 443, 444,

445, 449, 450, 453 (Figs.

150, 309, 311-313, 3»6,

319).
Salviniaceae, 391, 459.
Samara, 615.

Sambucus, 650 (Fig. 465).
Samolus, 566, 599.
Samydaceae, 659.
Sanguisorba, 578.
Sanguisorbeae, 662.
Santalaceae, 576, 662.
Santalum, 579, 580, 581.
Sap- vesicles, 42.

INDEX.

977

Sapindaceae, 653, 660.
SapindesE, 660.
Saponaria, 535.
Sapotaceae, 659.
Saprolegnia, 897, 955 (Fig.

163).
Saprolegnieae, 235, 244, 272,

955.
Saprophytes, 214, 242, 620,

649j 697, 721, 942.
Sarcina, 249, 954 (Fig. 166).
Sarcocarp, 615.
Sarcogyne, 325.
Sarracenia, 640.
Sauromatum, 183, 624.
Saurureae, 656.
Saxifraga, 562 (Fig. 386).
Saxifragaceae, 642, 661, 949.
Saxifragineae, 661.
Scabiosa, 593.
Scalariform vessels, 25, 27,

I Hj 439 (Figs. 27, 94, 308).
Scale-leaves, 186, 214.
Scattered arrangement, 188.
Schizaea, 432.
Schizaeaceae, 391, 440.
Schizandreae, 657.
Schizocarp, 614, 615.
Schizomycetes, 244, 248,

954 (Fig. 166).
Schultz's solution, 69.
Sciadopitys, 519, 527.
Scilla, 597.

Scirpus, 625 (Fig. 426).
Scitamineae, 632.
Sclerantheae, 66 1.
Scleranthus, 645 (Fig. 458).
Sclerenchyma, 35, 84, 122,

125 (Fig. 28).
Scleroblasts, 83, 84.
Sclerotium, 306, 315, 317.
Scolecite, 310.
Scorpioid cyme, 180, 597

(Fig. 136).
Scorpioid dichotomy, 178

(Fig. 134).
Scorzonera, 87 (F?g. 72).
Scrophularia, 90 (Fig. 75).
Scrophulariaceae, 658.
Scutellum, 166, 618 (Figs.

123, 124).
Scutiform leaf, 444, 447

(Fig. 311).
Scytonema, 248.
Scytonemeae, 248, 328.
Scytosiphon, 956.
Secondary —

bundles, 136, 653, 950.
endosperm, 582.
meristem, 82.
phloem (bast), 127, 131,

650.
products of metastasis,
705.

Secondary —
roots, 164.

wood (xylem), 127, 131,
531, 650, 651.
Secretion-canals, 93.
Secundine, 571.
Securidaca, 136, 653, 950.
Sedum, 597 (Fig. 85).
Seed, 486, 593, 618, 633.
Segmentation of the apical

cell, 139, 951 (Figs. 108-

112).
Selagineae, 658.
Selaginella, 47, 80, 122, 471,

473, 476, 477, 479, 480,

481, 482, 483, 486 (Figs.

44,67, 100, 331, 335-341).
Selaginelleae, 392, 481.
Senebiera, 604,
Senecio, 578.
Sepal, 214, 538.
Septate Fibres, 118, 651

(Fig. 97).
Septicidal dehiscence, 616.
Septifragal dehiscence, 616.
Sequoia, 509, 527.
Serjania, 653.
Serpentarieae, 656.
Seta—

of Cyperaceae, 538.
of Muscineae, 344, 351,
374 (Figs. 266, 273).
Sexual —

affinity, 916.
generation, 225.
reproduction, 223, 896.
reproductive cells, 224,
897.
Sexuality, development of,

901.
Sheath-teeth, 400 (Figs. 278,

282).
Shells, formation of, in the

cell-wall, 32 (Figs. 35-38).
Shield, 298 (Figs. 198, 200).
Shoot, 158, 215.
Sieve-cells, 22.
Sieve-plates, 22, 89, 118

(Figs. 98, 99).
Sieve-pores, 89.
Sieve-tubes, 88, 118, 948

(Figs. 74, 98, 99)'
Sigillaria, 485.
Sileneae, 661.
Silicon, 696, 700.
Siliqua, 616.
Silphium, 598, 640.
Silver grain, 130, 651.
Simarubeae, 660.
Simple glands, 84.
Simultaneous whorls, 187.
Sinningia, 580.
Siphoneae, 271, 955.
Sirogonium, 258.

3»

feirosiphon, 248, 328.
Sisymbrium, 599.
Sisyrinchium, 578.
Skeleton of cell- wall, 36.
Skeleton of starch-grain, 60.
Sleep of plants, 873.
Sodium, 696, 699,
Soft bast, 119.
Solanaceae, iii, 219, 597,

658.
Solanum, 556.
Solorina, 326.
Solubility of starch-grains,

61.
Sorby's researches on chlo-
rophyll, 764.
Sordaria, 316.
Soredial branch, 321 (Fig.

217).
Soredium, 326 (Fig. 221).
Sorus, 435 (Figs. 292, 304,

321).
Spadiciflorae, 631, 884.
Spadix, 595.
Sparmannia, 651, 884.
Spathe, 540, 624.
Spathularia, 311.
Special mother-cells, 15, 32,

554 (Figs. 34, 378, 379)-
Species, 927.
Species, Origin of, 920.
Species-hybrid, 915.
Spectrum of chlorophyll,7 59.
Spermatia, 308, 326, 329,

333,898.
Spermogonium,3o8, 3 16,326,

330, 333 (Fig. 223).
Spermothamnieae, 290.
Sphacelarieae, 283, 944, 965.
Sphaceha, 317.
Sphaeria, 316.
Sphaerobacteria, 249.
Sphaerococcus, 289.
Sphaeromphale, 329.
Sphaeroplea, 235, 269.
Sphaerotheca, 311.
Sphagnaceae, 377.
Sphagnum, 376, 377, 378,

379 (Figs. 81, 258-263).
Sphenophyllum, 408.
Sphere-crystals, 63, 64, 420

(Fig. 51).
Spicate inflorescences, 595.
Spicular cells, 66, 84 (Fig.

52).
Spike, 595.
Spilonema, 328,
Spine, 218.
Spiraea, 651.
Spiraeeae, 662.
Spiral arrangement, 190.
Spiral flowers, 600.
Spiral theory of phyllotaxis,

201.

978

INDE X.

Spiral vessels, 22, 90, 114?
116 (Figs. 75, 95).

Spirillum, 249, 954 (Fig.
166).

Spirobacteria, 249.

Spirochaete, 249.

Spirogyra, io„ 16, 46, 234,
258, 946 (Figs. 5, 6, 13,
170).

Spirulina, 249.

Splitting of the cell-wall, 73
(Figs. 58, 59).

Spongy parenchyma, 84.

Spontaneous periodic move-
ments, 880, 881, 895.

Sporangium, 388, 402, 414,

436, 453, 455, 456, 459,
466, 475, 476, 479, 480
(Figs. 285, 290, 292, 304,
305, 319-322, -325, 327,
334, 335, 337, 338).

Sporastatia, 323.

Spore, 223, 228, 382, 404,
438.

Spores, Mode of formation
of, 12 (Figs. 7, 10, 188,
205, 223, 227, 268-270,
286, 305, 306, 322, 323,

337).
Sporidia,33o, 334(Fig. 224).
Sporocarp —

of Garposporeae, 236,

284 (Figs. 164, 187-

190, 199, 204, 207,

208, 218).

of Rhizocarpeae, 451,

452,453,455,456,459
(Figs. 317-321, 325).

Sporogonium, 226, 342, 350,
354, 355, 360, 374, 379,
380, 381 (Figs. 238, 240,
246, 257, 263-266).

Sporophore, 225, 343, 387.

Spur, 570 (Figs. 396, 491).

Squamarieae, 292.

Stamen, 490, 505, 514, 529,
541, 543, 544, 545, 54^
(Figs. 344, 347-350, 362-
366, 490).

Staminal leaves, 541.

Staminode, 548.

Staphisagria, 608.

Staphylea, 651.

Staph yleaceae, 660.

Starch, 6, 56, 707, 747.

Starch-forming corpuscles,
948.

Starch grains, 46, 56, 61
(Figs. 45, 49).

Staurospermum, 257.

Stem, 150, 157.

Stem-tendrils, 217, 865.

Stephanosphaera, 255 (Fig.
168).

Sterculia, 547 (Fig. 371).
Sterculiaceae, 661.
Stereocaulon, 319.
Sterigma, 326.
Stichococcus, 329.
Sticta, 320 (Fig. 214).
Stigeoclonium, 4, 8 (Fig. 3).
Stigma, 493, 558, 568 (Figs.

382, 385-387, 390-392,

395, 491, 492).
Stigma, of Muscineae, 343,

373-

Stigmaria, 485.

Stigmatic cells, 351.

Stinging hairs, 100.

Stipulatae, 390, 410.

Stipule, 212, 215, 640.

Stipules of Chara, 295.

Stolon, 217.

Stomata, 77, 102, 103, 104,
105, 106, 678, 949 (Figs.
84-89).

Stone, 122, J 25.

Stone-cells, 84.

Stone-fruit, 616.

Stratification of the cell-
wall, 19, 27, 946.

Stratioteae, 631.

Stratiotes, 952.

Strawberry, 221, 594, 614.

Strelitzia, 555.

Striation of the cell-wall,
19, 27 (Figs. 28, 30, 32).

Stroma, 316 (Fig. 209).

Strontium, 695.

Strophiole, 618.

Struggle for existence, 929.

Struthiopteris, 428, 433,

435.
Strychnaceae, 658.
Strychnos, 587, 949.
Style, 558, 567 (Figs. 383,

386, 388-390, 392, 393,

395, 491)-

Stylidieae, 659.

Stylogonidium, 239.

Stypocaulon, 139 (Fig. 108).

Styracaceae, 659.

Suberous change of cell- wall,
20.

Subhymenial layer, 310, 323,
325, 333, 338 (Figs. 7, 205,
218, 219, 227).

Successive whorls, 187.

Succulent tissue, 83.

Sugar, 708.

Sulphur, 696, 698.

Sulphuric acid, 698.

Sunflower,7i,i54, 171 (Figs.
118, 126).

Superior gynaeceum, 560.

Superposed, 189, 601, 645.

Surface-growth of the cell-
wall, 21, 946.

Survival of the Fittest, 941.
Suspensor, 472, 499, 523,

588 (Figs. 332, 354, 355,

399, 400, 402-405).
Swarm-spore, 4, 12, 240.
Swartzieae, 662.
Swelling -up, 35, 6j, 668,

778.
Swimming of swarm-spores,

38.
Symmetry, 204, 537, 610,

960.
Sympetalae, 658.
Sympetalous, 221, 539.
Symphoricarpus, 642 (Fig.

440).
Symphyllous, 539.
Symphytum, 577, 598.
Sympodial inflorescence, 597.
Sympodium, 178, 180.
Synacmic, 910.
Synalissia, 327.
Synandrae, 658.
Syncarp, 614.
Synchitrium, 252.
Synergidse, 580, 959 (Fig.

399)-
Synsepalous, 221, 539.
Syntagma, 664.
Syringa, 652.
System, Natural, 942.

Taccaceae, 632.

Tagma, 664.

Tamariscineae, 660.

Tannin, 706.

Tap-root, 637.

Tapetum, 388, 437, 49 1 (Figs.

305, 322, 337, 374, 377).
Targioneae, 356.
Tasmannia, 651.
Taxineae, 527, 958.
Taxodineae, 527, 958.
Taxodium, 510, 517, 527.
Taxus, 513, 522, 527 (Figs.

348, 354).
Tecoma, 137, 654.
Teeth, 381, 383.
Teleutospore, 3 30 (Figs. 223,

224).
Temperature, Influence of,

725.
Tendril, 214, 217, 865, 934,

936 (Fig. 486).
Tension —

in plants, 787.
of tissues, 794.
Terebinthaceae, 93, 660.
Terebinthineae, 660.
Terminal branching, 176.
Ternstromiaceae, 660.
Testa, 486, 492, 593 (Figs.

346, 435, 436, 438).
Tetracyclae, 655, 657.

INDEX,

979

Tetragonidia, 289, 956 (Fig.

190).
Tetragonieae, 662.
Tetramerous, 641.
Tetraphis, 369 (Figs. 251,

252).
Tetrapoma, 605,
Tetraspora, 248.
Thalloid Hepaticae, 346.
Thallome, 151, 158.
Thallophytes, 151, 231, 244.
Thallus, 151.
Thamnidium, 267.
Theca, 344, 374 (Figs. 266,

267, 271-273).
Theobroma, 635.
Theoretical diagram, 602.
Theory of descent, 940.
Thesium, 599, 648.
Thickening-ring, 129, 147,

653.
Thread-indicator, 827.
Thuja, 514, 527 (Fig. 351).
Thujopsidae, 527.
Thunbergia, 34 (Fig. 36).
Thymelaeaceae, 662.
Thymelaeineae, 662.
Thyrsopteris, 419.
Tilia, 652 (Fig. 463).
Tiliaceae, 646, 661.
Tissues, Forms and Systems

of, 79-
Tissues, Morphology of, 70,

948.
Tmesipteris, 462.
Torenia, 581.
Torreya, 519, 527,958.
Torsion, 859.
Torus, 490, 559.
Trabeculsp, 479 (Fig. 334).
Tracheides, 75, 116, 949.
Traction, Action of, on

growth, 809.
Tradescantia, 554, 577.
Trama, 338 (Fig. 227).
Transfusion-tissue, 533.
Transpiration, 678, 961.
Transpiration current, 682.
Transport of assimilated sub-
stances, 711.
Transverse wall, 426.
Trapa, 586, 592, 633 (Fig.

125).
Traube's artificial cells, 671,

961.
Tree-ferns, 215, 433.
Tremellineae, 336,956.
Trichoblast, 85 (Figs. 70,

466).
Trichogyne, 238, 288, 290,

329 (Figs. 164, 188-190).
Trichomanes, 421, 441.
Trichome, 150, 160.
Trichomes, of Ferns, 435.

Trichophore, 238, 288, 292.
Tricoccae, 661.
Tricyclic, 601.
Trifolium, 599.
Triglochin, 577, 626.
Trimorphism, 907.
Triphragmium, 332.
Tritonia, 577.
Trochodendron, 651.
Tropaeolaceae, 660.
Tropaeolum, 14, 586, 640,

865, 936 (Fig. 486).
Truffle, 314.
Tsuga, 527,533-
Tuber, 215, 216, 226, 704,

935.
Tuber (truffle), 308, 315.
Tuberaceae, 314.
Tubiflorae, 658.
Tuhpa, 715 (Fig. 470).
Tiillen, 24, 810.
Turgidity, 781, 788, 962.
Turneraceae, 659.
Twining of climbing plants,

862.
Twining of tendrils, 865.
Twining stems, 217, 862.
Tyloses, 24, 810.
Type, 941.

Typha, 491, 541, 542, 564.
Typhaceae, 573, 631.
Tyrosine, 718.

Udotea, 272.
Ulex, 652.
Ulmaceae, 656.
Ulmus, 584, 940.
Ulothricaceae, 257.
Ulothrix, 252, 253, 257, 901,

902.
Ulvaceae, 281.
Umbel, 596.

Umbel, Cymose, 185, 596.
Umbellicaria, 325.
Umbelliferae, 661.
Umbelliflorae, 661.
Uncaria, 962.
Unequal growth, 854.
Unguis, 539.
Unicellular plants, 79, 232,

692.
Unilateral cicinal cyme, 597.
Unilateral helicoid cyme,

597.
Uredineae, 330, 956.
Uredo, 330.
Uredospore, 330, 333 (Fig.

223).
Urn, 344, 374.
Uromyces, 332.
Uropedium, 603.
Urticaceae, 100, 656.
Urticeae, 656.
Urticineae, 656.

Usnea, 320, 321, 327 (Figs.

213, 217, 221).
Ustilagineae, 335, 956.
Utricular vessels, 88 (Fig. 7 3).
Utricularia, 639.

Vaccinieae, 659.
Vaccinium, 648.
Vacuole, 3, 38, 40.
Vaginula, 344, 374.
Valeriana, 643 (Fig. 443).
Valerianaceae, 643, 658.
Vallisneria, 43, 769.
Vallisnerieae, 631.
Valonia, 946.
Vanilla, 935.
Variation, 777.
Variation of hybrids, 918.
Varieties, Origin of, 920.
Variety, 921.
Variety-hybrid, 915.
Vascular Cryptogams, 385.
Vaucheria, 41, 244, 269, 270,
271, 897 (Figs. 40, 176,

177).
Vegetable ivory, 587.
Vegetative cone, 138.
Veins, 122.
Velamen, 98.

Velum, 337, 475 (Fig. 225).
Venation, 213, 624, 641.
Ventral canal-cell, 350, 386,

395, 425, 446 (Figs. 256,

295).
Veratrum, 596.
Verbascum, 577.
Verbenaceae, 658.
Veronica, 599.
Verrucarieae, 329.
Versatile anther, 543.
Verticillate flowers, 600.
Vessels, 80, 116, 949.
Vibratile cilia, 4, 240.
Vicia, 635, 882, 936 (Figs.

436,477)-
Victoria regia, 640.
Vine, 837, 935, 936.
Viola, 568, 586, 912 (Figs.

395, 400, 401, 491)-
Violaceae, 659.
Virginian creeper, 837, 937.
Viscum, 578.
Vitis, 639, 865 (Fig. 457).
Volkmannia, 408.
Volvocineae, 244, 278.
Volvox, 278, 945.

Water, Ascent of, from the

root, 685.
Water, Currents of, in the

wood, 679.
Water, Exudation of, 676.
Water, Movements of, 674.
Water of crystallisation, 3 1 .

980

INDEX.

Water oforganisution, 3 1,62.
Water-pores, 949.
Watsonia, 584,
Wax, 99.
Wellingtonia, 509.
Welwitschia, 66, 170, 528,

53ij 953.
Wendungszellen, 299, 302

(Fig. 202).
Whorl, 170, 187.
Whorl, Spurious, 170, 187.
Widdringtonia, 527.
Wistaria, 653, 950.
Wood, HI.
Woodwardia, 433.
Wrangelieae, 290.

Xanthium, 215, 940.
Xanthophyll, 765.

Xanthoxylaceae, 660.
Xylaria, 316.
Xylem, iii.
Xylem-portion of fibrovas-

cular bundle, 116.
Xylem ray, in, 131.
Xylophylla, 217, 935.
XyrideiE, 632.

Yeast-fungi, 249.
Yew, 512.
Yucca, 127, 629.

Zamia, 501, 505, 958 (Figs.

342, 344)-
Zanichellia, 618.
Zea, 580, 620, 925.
Zinc, 695.
Zingiberaceae, 625, 632 (Fig.

429).

Zoogonidia, 12, 240 (Figs. 3,
4, 9, 162, 167, 168, 169,
173, 176, 178, 180, 182).

Zoospore, 234, 254 (Figs.
167, 168, 181, 183).

Zoosporeae, 253.

Zostera, 557.

Zygnema, 46, 258.

Zygnemeae, 257.

Zygogonium, 258.

Zygomorphic, 204,610, 611,
612, 613.

Zygomycetes, 9, 244, 955.

Zygophyllaceae, 660.

Zygospore, 9, 224, 234, 254,
258, 265, 897 (Figs. 162,
167, 168, 170, 171, 174,

175).
Zygosporeae, 244, 250.

1^

THE END.

O

BINDING ClICT. JAN 131972.

QK

S23
1882

Biolo^

Sachs, Julius von

Text-book of botany,
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