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Marine Biological Laboratory

BrrffiwrH Sept. 20, 1948

Accession No.

31727

Given By
Place,

The Macmillan Co.

New York City

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A

BOTANY OF THE LIVING PLANT

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-4?»i

-''.^s^r'-.

GIANT TREES OF CALIFORNIA

Showing proportionately an approximate limit of size of the plant body.

See chapters X. and XXXVI.

BOTANY OF THE
LIVING PLANT

BY

F. O. BOWER, Sc.D., LL.D., F.R.S.

EMERITUS PROFESSOR OF BOTANY IN THE UNIVERSITY OF GLASGOW

IN THE THIRD EDITION HE WAS ASSISTED BY

J. M. F. DRUMMOND, M.A., F.R.S.E.

PROFESSOR OF BOTANY IN THE UNIVERSITY OF MANCHESTER

AND

GEORGE BOND, Ph.D., D.Sc.

LECTURER IN PLANT PHYSIOLOGY, UNIVERSITY OF GLASGOW

IN THIS FOURTH EDITION BY

C. W. WARDLAW, Ph.D., D.Sc, F.R.S.E.

PROFESSOR OF CRYPTOGAMIC BOTANY IN THE UNIVERSITY OF MANCHESTER

AS GENERAL EDITOR AND IN PARTICULAR IN THE REVISION

OF THE SECTION ON THE CRYPTOGAMS

MACMILLAN AND CO., LIMITED
ST. MARTIN'S STREET, LONDON

1947

COPYRIGHT

First Edition 1919.
Second Edition 1923.
Third Edition 1939.
Fourth Edition 1947

PRINTED IN GREAT BRITAIN

PREFACE TO FOURTH EDITION

A textbook of any progressive Science needs to be revised from time
to time, if it is to serve the student as it should do. This book, fii
issued a quarter of a century ago, has been twice revised. In the pro-
duction of the Third Edition the author gratefully acknowledged
the help of Professor Drummond in respect of his new Chapter XXXV.,
on Heredity and Variation, and of Dr. Bond in the general revision of
the parts dealing with Physiology. The exhaustion of that Edition
has given the opportunity for another general revision of the Text,
together with some additions of fresh matter. In producing this Fourth
Edition the author wishes to acknowledge the general help of Professor
Wardlaw, and in particular his revision of Chapters XXL to XX\ III.
Both Professor Wardlaw and myself have also made certain other
additions, as required. The result of these changes has been to

modernise the Text.

F. O. BOWER.

PREFACE TO THIRD EDITION

The present Volume was framed in 1919 upon the Courses of Elemen-
tary Lectures on Botany given in Glasgow University during a h
succession of years. Those Courses were progressively re-modelled
and developed as time went on, while the clastic limits of a book
have allowed the introduction of sundry additions. And so in
successive issues the text has expanded. But the main oh
throughout has been in the first place that of presenting the individual
plant as a living, growing, self-nourishing, self-adapting creature, 1
I always endeavoured to sketch it in the Lecture Room, and to dt
monstrate it in the Laboratory.

The subject-matter is parcelled out into a series of E:
one self-contained. They arc designed so as to fii together and ton

vi 1

viii PREFACE TO THIRD EDITION

a continuous treatise. The omissions are palpable enougn, tor no
attempt has been made after encyclopedic writing. The material itself
is such as will be reckoned elementary. Elementary and fundamental
should be held as equivalent terms when applied to those facts and
principles upon which the Science itself is built : and it is to these
that the available space has been devoted.

The book has not been written in conformity with the schedule of
work prescribed by any University or School : nor is it designed to
meet the requirements of any specified examination. Its object has
been to present a true picture in as simple terms as possible. Pro-
ceeding from the known to the unknown, it opens with a description,
structural and functional, of familiar Flowering Plants. The conse-
quent inversion of the evolutionary aspect of the Vegetable Kingdom
as a whole will probably be criticised. But a definite break has been
made at the end of Chapter XIX. Here follows a Chapter on " Evolu-
tion, Homoplasy, Homology, and Analogy." It is designed as an
introduction to the scientific comparison of Plants. The illustrative
material for this is then supplied by the description of a progressive
series of forms, starting from some of the simplest and proceeding to
those that are more complex. These have been selected in harmony
with general opinion as to the trend of evolutionary history. It
hardly needs to be said that those selected suggest only the barest and
most general outline by means of which the progress of Descent can
be sketched. Nevertheless they give a rational foundation, however
slender, for the generalisations advanced in Chapters XXXIII. to

XXXVI.

In preparing the text of this Third Edition, twenty years after the
first and thirteen years after retirement, I have had the advantage
of help from younger men actually engaged in teaching. The Chapters
relating to Physiology, viz. III., VII.-IX and parts of Chapter XI.
and XII., have been amended and largely re-written, and new figures
added to them, bv Dr. G. Bond, Lecturer on Plant Physiology in
the University of Glasgow. Professor M. Drummond of Manchester
University has revised the text of the Thallophyta at various
points, and has re-written Chapter XXXV., under the title of
Sex and Variation. He has also added paragraphs on Vitamins
to Appendix II., which deals with Vegetable Food-stuffs. In these
amendments we acknowledge valuable suggestions from Dr. F. W.
Sansome and Prof. I. M. Heilbron. To all of these I am deeply
indebted for their help. I have myself added short statements on
the Psilophytales and Equisetales, so as to aid comparison. And

PREFACE TO THIRD EDITION ix

finally, I have added in Chapter XXXVI. a condensed statement
" The Relation of Size and Form in Plants ". Some may be surpri
to find that subject introduced into a current Text-book ; holding
this to be premature until opinion is more settled than it is al pn
Whether or not adherence be given to the views expressed in the Chap-
ter, it seems desirable to raise the topic, (i) as a means of impressing
the necessity for uniformity of scale in thought, in measurement, and
in illustration, where such questions are discussed ; (ii) as pointing
the contrast between primary and secondary development ; and
(hi) as localising the study of primary mouldings in relation to the
growing point itself.

More than 200 of the illustrations have been specially prepared for
this book. My own original drawings are initialed. Almost an equal
number were drawn for the first edition by Professor J. McLean
Thompson of Liverpool University, and these are signed by him.
Many beautiful French blocks have been borrowed from Figuier's
Vegetable World, and a considerable number have been taken with
permission of the publishers from Strasburger's Textbook. By arrange-
ment with the S.P.C.K., many of the excellent drawings of Fungi by
the late Professor Marshall Ward are embodied. They are taken from
his Diseases of Plants, now long out of print. The sources of these
illustrations, and of some others not mentioned here by name, are
specified in the rubrics, and their use is gratefully acknowledged.

Finally, I wish to thank critics and friends who have, in print or

verbally, suggested amendments to the Text : and in particular

Dame Helen Gwynne-Vaughan, G.B.E., Professor W. H. Lang, and

Dr. S. Williams. But I hold myself fully responsible for the text

as it stands.

F. O. BOWER.

Ripon, 1939.

CONTENTS

PACE

Introduction ------- - i

DIVISION I
ANGIOSPERMS OR HIGHER FLOWERING PLANTS

CHAPTER

I. Seed. Germination. Form of the Established Plant

II. The Cellular Construction c? the Plant 16

III. The Living Cell - ' - - - 29

IV. The Tissues of the Stem ------- 40

V. The Leaf ------- - 69

VI. The Root -------- -82

VII. The Water-Relation ------- 95

VIII. Synthesis, Storage, and Breakdown - - - - 114

IX. Growth, Irritability and Movement
X. The Mechanical Construction of the Plant-Body
XI. Modifications of Form in the Vegetative System of the
Higher Plants --------

XII. Irregular Nutrition - - - - - - -22

XIII. Vegetative Propagation ------- J44

XIV' The Inflorescence, and the Flower - - - - - 1

XV. The Stamen and Pollen-Sac ------

XVI. The Carpel and Ovule - - - -

XVII. Pollination and Fertilisation ----- $cc

XVIII. The Embryo and the Seed ------

XIX. The Fruit and Seed-Dispers:
XX. Evolution, Homoplasy, Analogy, Homology -

DIVISION II
THALLOPHYTA

XXI, Introductory to Thallophvta

XXII. Green Algae (Chlorophyceae)

xi

^\72 7

xii CONTENTS

CHAPTFR PAGE

XXIII. Brown Algae (Phaeophyceae) - 37$

XXIV. Fungi. Introductory ------- 391

XXV. Phycomycetes --------- 402

XXVI. ASCOMYCETES --------- 418

XXVII. Basidiomycetes -------- 431

XXVIII. The Bacteria - - - - 448

XXIX. Introduction to Land-Vegetation - 454

DIVISION III

BRYOPHYTA

XXX. Musci and Hepaticae : Mosses and Liverworts - - 460
Appendix on Psilophytales ------ 478

DIVISION IV

PTERIDOPHYTA

XXXI. Filicales. Ferns - - - - - - - -481

XXXII. Lycopodiales. Club Mosses - - - - - -511

Appendix on Equisetales. Horsetails - - -522

DIVISION V

GYMNOSPERMS
XXXIII. Coniferae. The Scots Pine -

525

GENERAL CONCLUSION

XXXIV. Alternation of Generations, and the Land-Habit - 543

XXXV. Heredity and Variation - - - - - - 557

XXXVI. The Relation of Size and Form in Plants - - 589

Appendix A. Types of Floral Construction in Angio-

sperms - - - -- - - - - -605

Appendix B. Vegetable Food-Stuffs and Vitamins 653

Index and Glossary ------- 665

BOTANY OF THE LIVING PLANT

INTRODUCTION.

The vegetation of any ordinary country-side consists of a vast number
of distinct kinds of Plants, large and small, simple and complex.
They are mixed up without any apparent system or order. The
object of the scientific study of this mixed vegetation is to know as
much as possible about the various Plants that compose it. The form
of each kind of Plant when fully grown will have to be noted, as well
as the way it grows so as to attain that form. The way the Plants
nourish themselves is also an important question. And finally we
shall enquire how they increase in numbers : for some die off from
time to time, and their places are constantly being taken by new
Plants.

This study of Plants and of their vital activities cannot be carried
out with success by merely examining the mass of Plants all together.
They must be taken singly, and examined individually. One can
then be compared with another. On the basis of such comparisons
we may form opinions as to their probable relationships, and even
approach a view regarding their origin. To make such a study
methodical and coherent, the Plants recognised must be arranged
according to their characters. They must in fact be classified, and
the classification should then indicate their natural affinities. In such
a Natural Classification those which are relatively simple in then-
structure and mode of life should be placed first, and the most
elaborate last.

We may take as an example of a very simple Plant that green
powdery growth which is often found on the bark of trees, on wooden
rails, and other places, in damp situations. This growth is composed
of individual grains which are very numerous, but so small that tl
are only visible to the naked eye when present in large number

B.B. A

BOTANY OF THE LIVING PLANT

D

Fig. i.
Cells of Pleurococcus Naegelii. Chod.

Highly magnified, the powder is seen to consist of single spherical
cells, or groups of cells (Fig. i). Each of these cells is an individual
Plant, and it multiplies by division. The results of such repeated
divisions may remain for a time coherent, thus forming groups of
varying number. But finally they separate, and each single cell can
continue its life as a distinct unicellular organism. It is called

Pleurococcus Naegelii. It may be
held as taking a low place in the
scale of vegetation, and would be
classified near to the beginning of our
Series.

On the other hand, ordinary herbs,
shrubs and trees are examples of more
elaborate organisation. Each one of
these is composed of various large and
complex parts, which are united to
form the complete Plant. The sev-
eral distinct functions which they
perform benefit the whole. Such
n=nucieu°sWS *£££^$S*$*± Plants may attain large size, and very
e shows packed SS&ng from divi- complicated structure, as in the case
sioa. (After chodat.) Q| forest trees (Frontispiece). Each

of these Plants is an independent individual. Their increase in number
is by Seeds, produced through the process of flowering. The pro-
duction of seed is an involved and elaborate process, as will be seen
when it is described in later Chapters. Partly on the ground of
their complex structure, and partly because of the elaborate method
of their propagation, such Flowering Plants, or Seed-Plants, are ranked
as higher in the scale. Between such extremes as Pleurococcus and a
Flowering Plant other intermediate types may be ranked according
to their structure and their method of increase. And so a Series
may be found leading from those which are comparatively simple,
by gradual steps, to those which are more elaborate. Such a Series
is believed to illustrate roughly, and with some degree of truth, the
course which the Evolution of the Vegetable Kingdom has actually
followed. The simpler examples are held to represent such types as
appeared earlier in the History of Descent, and thus to be more
primitive. The types which are more complex in structure, and in
their method of propagation, are believed to have appeared later in
the History of Descent, and are regarded as derivative. This agrees
essentially with the sequence of fossils embedded in successive

INTRODUTION

Geological Strata : so that the positive evidence of the Rocks sup-
ports, so far as it goes, the grouping of Plants by comparison.

On such grounds as these, Plants may be sorted into five main
Divisions, each comprising several Classes. They may be tabula'
as follows, while examples are given of familiar Plants, illustrating
the sort of living organisms which belong to each Class :

Examples.
fSeaweeds and Freshwater Weeds
IMushrooms, Mildews, Moulds
/Mosses ....

I Liverworts - - - -
I Ferns - - - - -

Club-Mosses

Horse-Tails -

Pines, Firs, Yew -
fOak, Sunflower, Potato, Bean
IGrasses, Lilies, Palms -

}

i

Class.
Algae
Fungi
Musci

Hepaticae

Filicales

Lycopodiales

Equisetales

Coniferales -

Dicotyledons

Monocotyledons

I M vision.
- Thallophytes.

I - Bryophytes.

- Pteridophytes.

Gymnosperms.
I Angiosperms.

A natural way of studying these would doubtless be to start from
the simplest and most primitive, and to proceed to those which are
more advanced : — that is, to follow the course which we believe
that Evolution has taken. It is, however, easier to begin the study
of the Living Plant from those of larger size, which are already
familiar objects to everyone, than from minute and unfamiliar
organisms, which can only be examined microscopically. It will
thus be best for us to take the Higher Flowering Plants first, and
to hold over the lower organisms to the end.

One further general statement may be made regarding the Series
as thus laid out. It relates to the mode of life of the Plants con-
cerned. Many of the Thallophytes are water-growing Plants, such
as Seaweeds, and the Algae of freshwater streams and pools ; most
of them grow only where abundant moisture is present. The Mosses
and Ferns, though they appear as land-living Plants, require external
liquid water for completing one essential stage in their life-history :
without it they fail. On this ground they may be -ailed the
"Amphibians" of the Vegetable Kingdom. But the Seed-Plants
are not thus dependent upon external liquid water. The gen<
conclusion follows that Vegetation began in the water, that it spread
later to the land, and that it found its climax in the Seed- Bearing
Plants of the Present Day. This is the fundamental idea xv h» h should
underlie all Ecology, that branch of the Science which connotes

4 BOTANY OF THE LIVING PLANT

study of Plants in relation to their surroundings. The broadest of all
ecological conceptions is that Plants were in their ultimate origin
aquatic : that they have gradually emancipated themselves from
dependence upon an aquatic habitat : and that the highest terms
of the series are characteristically Dwellers on Dry Land.

Conclusions like these have been drawn by Zoologists with regard
to the Animal Kingdom. Both branches of living things, viz.
Animals and Plants, probably originated in the water. Certain of
the very simplest Animals and Plants are so alike that it is difficult
to draw any line separating at such early stages the one Kingdom
from the other. It is therefore concluded that they probably had
a common origin, but that in the course of Evolution they diverged.
The most distinctive feature which separates them is that of Nutrition.
Plants advanced along the direct line of Self -Nutrition. They form
their own organic food from inorganic materials. These are ultimately
Carbon Dioxide and Water, together with Mineral Salts. Their green
colouring matter plays an essential part in the process of their
nutrition from such simple sources. Animals, on the other hand,
advanced along Predatory Lines. They take their food in more
elaborate form as material already organic : that is, either from bodies
which are living, or such as have been produced by living organisms.
This is seen both in Herbivorous and Carnivorous Animals. Pursuing
these divergent lines, both Animals and Plants established them-
selves upon exposed land-surfaces, and both show in their higher
terms abundant evidence of their fitness for living in the surroundings
which they have adopted.

Since Animals take their food as organic material — that is, in
a sense at second hand, and do not construct it for themselves — it is
obvious that at some stage or other they are dependent for it upon
the Vegetable Kingdom. This gives Plants a special claim on the
attention of Biologists : for the Green Plant is, in point of fact, the
essential source of supply of organised material to all other forms of Life
upon the Earth' 's Surface.

DIVISION I.
ANGIOSPERMS OR HIGHER FLOWERING PLANTS

CHAPTER I.

SEED. GERMINATION. FORM OF THE ESTABLISHED

PLANT.

The Higher Plants are called Seed-Plants, because they bear Seeds.
The Seed is a detachable part of the parent Plant, which contains the
germ of a new individual. When mature, it is usually hard and dry.
It can stand drying up without losing its vitality. In this state it
may remain dormant for a considerable period, often for years, and
may withstand conditions which would be unfavourable for active
life, such as extremes of heat and cold. But when the conditions are
favourable, the active life of the germ, which has been in a state of
suspense in the dry seed, may be resumed. The test of vitality of the
seed is whether or not it will germinate when exposed to suitable
conditions.

If a dry seed of a Bean, such as may be bought in a seedsman's shop,
be dissected, its parts may be easily recognised. But the dissection
will be more readily carried out if it be soaked in water for twenty-four
hours. The effect of the soaking will be that it will increase in bulk
and in weight. The swelling is due to the imbibition of water, which
is a property of dry vegetable tissues. A distinction must be drawn
between such swelling and growth. Swelling by imbibition is
reversible process, and is not a manifestation of life. A dead b<
will swell equally with a living one. If either be dried again, it will
shrink back to its original bulk. Growth, on the other hand, is a result
of vital activity. It involves, as we shall see in Chapter IX., p. 139,
a redistribution of organic material. This is an irreversible proc.

5

6 BOTANY OF THE LIVING PLANT

The seed which has once germinated cannot be restored to its original
state again. It is the same with all other changes in life : the prior
condition can never be exactly resumed after vital changes have occurred.

Fig 2.

Common bean (Vicia Faba). i. ii., seed covered by seed-coat, iii., germ, with
seed-coat and the nearer cotyledon removed, iv. v., successive stages of germina-
tion slightly reduced.

The Bean-Seed, as shed by splitting of the Bean pod, appears as
a roughly discoid body (Fig. 2). It consists of an embryo or germ,
protected by an external Seed-Coat. On its edge, close to a slight
involution of the margin, is an elongated scar, the hilum, marking
the point of attachment to the parent plant. If the tough brown
Seed-Coat be dissected off, the Germ or Embryo will be disclosed,
filling the whole space within. It consists of two large flattened
Seed-Leaves, or Cotyledons, which are attached at the base, right

SEED. GERMINATION

i

and left, to a curved body which lies between them. t ol tin-

compressed between the Seed-leaves; it is the leafy bud or plumule,
which is to grow into the shoot of the seedling. Pointing in t he-
opposite direction to this is the first root or radicle, the conical tip
of which is close to one end of the hilum. The parts thus recognised
are present in all normal seeds of Dicotyledons, which take their name
from the paired Seed-Leaves. But the form and proportions of i
seed and of the germ may vary in different plants, and certain
additional tissues may in some cases be present. The Seed-L
of the Bean are fleshy in texture, and are stored with materi
which serve on germination for the nutrition of the other parts of
the germ.

The conditions necessary for the germination of a living seed
constructed are : (i) the presence of moisture ; (ii) free access to atmo-
spheric air ; and (hi) a suitable temperature. The ordinary conditions
of spring-time would meet these requirements, if the seed were buried
in an open porous soil. For the soil would be moist, and the air would
be free to penetrate its pores ; while the rising temperature of the
season wTould meet the third requirement in the case of seeds of
temperate climates.

Supposing these conditions to have been fulfilled in the case of
a living Bean-Seed, germination ensues. The first external change
is the rupture of the seed-coat at a point close to the end of
the scar. Through this the pale young root protrudes, and as it
elongates its conical tip at once penetrates vertically downwards
into the soil. When the root has attained a length of several inches,
a curved shoot emerges through the slit between the bases of the
two seed-leaves. Clearly this is the result of growth from the leafy
bud seen between the cotyledons in the seed. This shoot turns
upwards, and soon projects above the level of the soil. Growth is
thus seen in both parts, the material required for the enlarging
tissues being supplied from the fleshy seed-leaves. This is used
in building up the enlarged root and shoot. No drying-up will
make the root or shoot shrink back to their original size or form,
nor can the material drawn from the seed-leaves be by any means
replaced. Such growth is an irreversible process, as is all growth
in Plants.

That the root points downwards and the shoot upwards is not a
haphazard result. Experiments with seeds placed in various positi
in the soil show that the behaviour of these two parts of the seedling
is constant, and suggest that it is a response to some external infll

8 BOTANY OF THE LIVING PLANT

that applies to them all. In this case the influence is Gravity. The

subject of external influences will be discussed in detail later (see

Chapter IX.). Meanwhile it must suffice to say that an influence

such as Gravity, which acts on a living organism so as to produce

a change in it, is called a stimulus. The effect which the living

organism shows is called the response. The effect of Gravity upon

the growing shoot or root, so as to make the one turn upwards and

the other downwards, is an example of response to stimulus, and such

a response is one of the essential indications of Life.

It is familiar to every gardener that, up to a certain point, the

higher the temperature the quicker his seedlings appear above

ground. But plants vary in their relation to temperature, and that

necessary for germination is not the same for them all. Thus most

cereals can germinate at a temperature very near to the freezing

point, whereas Maize and the Kidney Bean require a minimum

temperature of about 9° C. All the functional activities of the Living

Plant have such a relation to temperature. The case of germination is

merely one example of a general condition of Life. This subject will

also be taken up again in Chapter IX.
The root and shoot established on germination are capable of

continued growth, which is followed in both cases by the formation

of lateral appendages. Thus a Root-System and a Shoot-System

are established, the former being buried in the soil, the latter rising

above the level of the soil, and constituting the part of the plant

ordinarily seen (Fig. 2, v.). If the soil be carefully washed away from

the root-system of a Bean-seedling after the main root has attained

about eight inches in length, it will be seen to consist of a primary, or

tap-root, which grows directly downwards and bears horizontal lateral

roots. The smallest and youngest of these are nearest to the tip of the

main root, and the largest and oldest are most remote from it. These

may in like manner bear lateral roots of still higher order, radiating

in all directions. Thus a complex root-system is built up. The

extreme tip of each root comes naked out of the soil, and is pellucid

and slimy to the touch, so that it readily slides past obstacles as it

penetrates the soil. But about three-quarters of an inch back from

the tip the particles of soil adhere to the root, showing that from that

point backwards a close relation is established between the root and

the soil. It will be seen later that this is due to the presence of

numerous minute root-hairs.

When the shoot of the Bean has grown to the length of about

six inches, it will be seen to consist of a central Stem terminated by

SEED. GERMINATION 9

a bud. Leaves, of which the lowest are pale-coloured scales, a
borne laterally on the stem. Passing upwards from the base suco
sively larger leaves are met, each with broad green lobes and a sheath-
ing base. Passing on from these mature leaves, we come to the bud.
Dissection of the bud shows that it is composed of a series of suc«
sively more minute leaves, very delicate in texture, closely overlapping
one another, and all seated on the immature axis. A bud is thus a
compact young shoot, consisting of a short stem overlapped by crowded,
immature leaves.

As the shoot develops, a new bud appears in the axil, or angle
of insertion of each leaf upon the stem. Even the cotyledons of the
Bean may bear such axillary buds. They are constructed like tin
apical bud, and on development each may repeat the characters of
the main shoot. Provision is thus made for multiplication of shoots,
so as to form a branched shoot-system.

The Bean-Seed itself has throughout germination remained below
ground. Its fleshy seed-leaves do not emerge from the seed-coat,
but are there gradually emptied of their nutritive store, and finally
rot away. Their sole function is storage of food-material for the
germ. But in other plants the behaviour is different. A good example
is seen in the Charlock. Its seed is nearly spherical, with slightly
flattened sides. A lateral scar of attachment (hilum) is seen, as in
the Bean, and the seed is covered by a tough leathery seed-coat.
Within this is the embryo, with two cotyledons, a bud or plumule, and
a first root or radicle. But here the cotyledons are folded sharply in
a median plane, giving a compactness to the embryo, which fills the
spherical seed (Fig. 3, i. ii.). On germination the seed-coat is burst by
the enlarging germ, and the root emerges as before, curving at once
downwards (iii.). But here the part of the stem or axis below the
seed-leaves grows quickly in length. At first it shows a strong
arching curve (iv.). But later this is straightened out, with the
consequence that the cotyledons and the bud are carried ab<
ground, still covered by the protecting coat. This soon falls away,
and the cotyledons expand, diverging as green leaves, with the bud
between them (vi. vii.). The latter elongates as it grows older,
forming the leafy shoot. Thus the parts of the seedling of the
Charlock are numerically and relatively as in the Bean, and in both
cases the food-supply is in the fleshy cotyledons. The difference
lies in the fact that in the Bean the active growth is in the region oi
the axis above the insertion of the seed-leaves; in the Charlock
is in the region below them. They are thus raised above the

10

BOTANY OF THE LIVING PLANT

and exposed to the light. They expand as green leaves, and help the
nutrition. Thus in the Charlock the seed-leaves serve first for storage,
and afterwards for carrying out nutrition. It is not an uncommon
thing in plants for a part to serve more than one purpose, sometimes
simultaneously, sometimes successively.

Fig. 3.

Charlock (Brassica Sinapis). i. ii., seed with and without seed-coat, iii.-vii.,
successive stages of germination. Natural size.

A third type of germination may be seen in the Castor-Oil plant
(Fig- 4)- The seeds are large, and covered by a mottled, brittle
seed-coat, which is easily removed, disclosing the semi-transparent
contents. These consist of a thin film covering the massive oily
endosperm, a nutritive store which is not represented in the previous
examples. It can easily be split down the middle in the plane in
which the seed is flattened (ii. vi.). The germ is then disclosed,
having the same number and relation of parts as in the other examples.
But here the cotyledons are thin and papery, and the whole germ is

SEED. GERMINA'J ION

i)

immersed in the nutritive endosperm. The chiei store ol food is thi

not in the germ itself, but in the surrounding tissue.

The germination of the Castor-Oil seed corresponds in its external
features to that of the Charlock. But here the germ, lying in d

Fig. 4.

Castor oil [Ricinus communis). L, seed seen from ^S^bAhSSSkS

tudinal and in transverse section, jv. v.^seedbng^s^d^buretbut^t^

still enveloped in endosperm, vi., .the ,samet S^^"gl\^ stable, -d SeSng,
straightening, endosperm still adhering to cotyledons n^ -Jta^
with expanded cotyledons and first plumular leaves. (| natural sue .)

contact with the endosperm, extracts the food from it, and absorbs it
into itself, while the endosperm gradually shrivels. As the seed-coat
is thrown off, the cotyledons turn green and expand. Thi dry
mams of the endosperm may then be seen still for a tune adhering
to their lower surfaces; but ultimately it falls away. b tn<

cotyledons act first as suctorial organs, and later expand into nourisl
ing green leaves. The root and shoot thus established may de>
further into a root-system and a shoot-system, as in previous 1

12

BOTANY OF THE LIVING PLANT

The leafy shoot of the Sunflower is produced in a manner very-
like that of the Castor Oil. But it differs from it and also from its
own smooth lower parts in having a harsh roughened surface. This
is due to hairs of various size. The coarser types of them are seated
on conical outgrowths, called emergences, often of considerable size.
Such surface-growths, or dermal appendages as they are called, are
inconstant in occurrence, and irregular in distribution in plants, as
compared with the foliar appendages. They vary greatly in character

in different plants, and being so incon-
stant, they are held as less important
than the axis and leaf.

Such examples as those now given illus-
trate some of the differences of propor-
tion, and of function which may occur
in seedlings, while the general plan of
construction is the same. In each case
the result of germination is the establish-
ment of a seedling with its root-system
in the soil, and its shoot-system exposed
to the air. These regions are directly
continuous one with another at the level
of the soil. Together they form the
living and growing organism. They serve
distinct functions, but they co-operate in
promoting the general life of the plant.

Each of these tivo regions of the plant-
body once established is capable of indefinite
extension (Fig. 5). The radicle continues
its apical growth, and can form an un-
limited number of lateral roots ; these
may again repeat the process. In all of
them also the root-tip may continue to
grow indefinitely. Thus a constantly
increasing provision is made for the
growing plant as regards mechanical support, and physiologically
for the supply of water and salts from the soil. On the other
hand, the stem is also gifted with continued apical growth, and
it has the power of forming an unlimited succession of leaves, of
which the oldest are nearest to the base and the youngest distal,
while those at the extreme tip are closely grouped so as to form a
terminal bud. Further, in the axil, or angle between the base of

i.o.h.

Fig. 5.

Diagram suggesting plan of un-
limited growth of a Flowering Plant,
with multiplication of roots and
branches.

CONTINUED EMBRYOLOGY

13

each leaf and the stem, a fresh bud may appear, which repeats the
chief characters of the terminal bud. Each bud is capable of
developing into a lateral branch similar to the main shoot, and
so on. The increase in number of shoots or of roots is in fact on
a very prolific scale. In herbs, such
as the Sunflower, Bean and Castor Oil,
this mode of development is not carried
far ; but still the unlimited possibility
exists in the plan of their construction.

It is precisely the same scheme carried
out further which gives rise to shrubs
and trees. In some of them the develop-
ment upon this plan may be continued
for centuries, and the organism may
attain very great size and a high com-
plexity of branching. The result of such
continued growth may be very well
studied on the twigs and branches of
trees in winter, when the leaves have
fallen, or in the spring when the winter
buds are bursting. For instance, on the
Horse Chestnut (Fig. 6), each shoot is
terminated by a bud, composed of exter-
nal bud-scales, which enclose the closely
folded foliage leaves awaiting expansion
in the succeeding season. The woody
stem below is marked by opposite pairs
of semicircular scars, where the leaves
of the preceding season fell away in
autumn. Immediately above each scar
an axillary bud may be seen, which is
capable of developing into a new branch ;
but frequently these remain dormant
until the distal apex is arrested or
destroyed. Some distance down the
stem a zone will be found marked transversely by many narrow scars
close together. This is the lower limit of the preceding year's growth,
and the scars are those left when the bud-scales fell away. Similar
zones may be found successively lower down, marking the limits
of the increment of growth of earlier years. Each year's growth
leaves its record on the outer surface of the branch. Thus, passing

Fig. 6.

Twig of Horse Chestnut in winter,
indicating the end of the increment of
growth of 1916, the limits of increment
of 1917, and the bud to be expanded in
1918, with scars of bud-scales and of
foliage leaves, and axillary buds and
lenticels. Natural size.

i4 BOTANY OF THE LIVING PLANT

from below upwards along the twig, its annual history can be read,
till we arrive finally at the terminal bud, which is already providing
for the development of the next year's shoot.

Any Land Plant built upon such a progressive scheme as this
requires as it grows additional provision for mechanical support, and
for the conduction of water and other supplies from the soil. This is
achieved in various ways, as will be seen later. But the most prevalent
is the method seen in forest trees, in which the trunk and branches
thicken according to the demands of the enlarging body. It is common
knowledge how the wood of their trunks is marked internally by
annual rings, so called because normally one ring of new tissue is
added each year. A similar growth is seen in their roots, both regions
showing an automatic increase to meet the growing demands, which
are both mechanical and physiological.

The most important factor in determining the conformation of the
plant-body in all the Higher Plants is the continued growth at the
apex of stem and root. The life of the Higher Plants may be described
as an indefinitely continued embryology, the increase in the number of
parts being in a geometric ratio. In this it differs essentially from
that of the Higher Animals, in which the parts of the body are laid
down once for all in the initial steps of development, and the body
is of a circumscribed and limited type.

But though the body of the Plant is thus theoretically unlimited
in its plan, in actual practice limits are imposed. It would be a
physical impossibility to develop all the potential parts of so complex
a system. Many buds remain dormant. In others seasonal con-
ditions may check or stop apical growth. Various mechanical or
physiological injuries may intervene, caused it may be by wind or
frost. Animal or fungal attack may destroy many embryonic buds.
Consideration must also be given to the physiological drain of
flowering. This appears to be effective in the case of herbs, and
especially of annuals, as in the Sunflower or Bean. By such influences
the theoretically unlimited plan of development is restricted within

bounds.

It is upon the scheme laid down in the preceding pages that the
body of all the Higher Plants has been constructed. The number
and exact position of the leaves may vary, and consequently the
number and position of the branches, since these arise from axillary
buds. The form and proportion also of the axes and leaves is open
to great difference of detail ; they are frequently adapted biologically
to the conditions under which they live. But these are only minor

CONTINUED EMBRYOLOGY 15

modifications, which may make the plan more obvious in some
cases than in others. Examination of ordinary herbs, shrubs and
trees from the point of view suggested here should be practised upon
the varied vegetation seen on any country walk. Such observations
will show the constancy of the scheme of organisation of the Higher
Plants, even in complicated cases. They will also illustrate in what
various ways the number, form and proportion of the parts may differ.
Thus there comes about that great diversity in appearance shown by
the plants that make up ordinary vegetation, though underlying the
construction of them all there is still a consistent plan. The salient
feature of this plan of construction of the Higher Plants is the capacity
for an indefinite vegetative increase in size and complexity of the
individual, which is based upon their " Continued Embryology" This
is centred in the Growing Point.

CHAPTER II.

THE CELLULAR CONSTRUCTION OF THE PLANT.

The apical points of Stem and Root, described in the previous chapter,
cannot fail to have attracted attention, by reason of their continued
powers of growth and of forming new parts. The perpetual youth
of the extreme tips is their leading character. Passing back from
these we see parts in successive stages of development up to full
maturity. This shows from external observation that new parts
originate there. To understand how this takes place, a study of the
internal structure will also be necessary. Such study is called Anatomy ;
in other words, large and solid bodies must be cut into in order
that their construction may be made out. Two courses are open
for such study. A start may be made from the mature parts, such as
the fully formed stem, leaf, or root, in which the structure is very
complicated. Or the young embryonic tip itself may be examined first.
Since the construction is much simpler at the tip where the tissues
are still young, it will be found best to take this first. Moreover,
upon the result of this examination it is possible to base a general
idea of the construction of the whole Plant-Body and of all its mature
parts.

If an apical bud of a water-plant, such as Hippuris or Elodea, be
dissected under a magnifying power of ten to fifteen diameters, a
succession of overlapping leaves will be found, those lying within
being constantly smaller than those outside them. The series may
be followed inwards till the last are too minute for recognition with
the simple lens. In the centre is a projecting cone of soft colourless
tissue, with a dome-like ending. This is the apex of the stem, or
growing point (Fig. 7).

If a median longitudinal section be cut through a bud of Hippuris
so as to traverse this cone to its extreme tip, it would on microscopic

16

THE CELLULAR CONSTRUCTION OF THE PLANT 17

examination show that it is not of uniform texture, like paraffin wax.
It is built up of a number of structural units, or cells, of more or less

Fig. 7

External view of the growing point of Hippuris, showing the smooth apical cone
bearing alternating whorls of lateral leaves, the youngest nearest to the tip.
Magnified.

cubical form, which are all essentially similar, and are arranged with
some degree of regularity (Fig. 8). More highly magnified, they are
seen to be all separated from one another by definite, but very thin

Fig. 8.

Median longitudinal section of the apex of a quite young bud of Hippuris.
c=epidermis. pe = perib\em. />/ = plerome. After De Bary. ( x 300.)

cell-walls (Fig. g). Each unit comprises a granular mass of material,
colourless and semi-liquid in the living state, which is the cytoplasm.
b.b. b

18 BOTANY OF THE LIVING PLANT

In a central position in each of them is a more highly refractive,
spherical body : this is the nucleus. Embedded in the cytoplasm,
and often difficult to observe, are other minute roundish bodies, which
are colourless : they are the plastids. The collective term proto-
plasmic body, or protoplast, is applied to all the contents enclosed

Fig. 9.

Young thin-walled cells from the growing point of Tradescantia, each with a relatively
large nucleus, containing a highly refractive nucleolus. Many plastids are present in the
cytoplasm. After Schimper. ( x 800.) The minute size of the cells may be realised by
measurement of their diameter as seen in Fig. 9, and division of the results by the magni-
fication as stated : the result is a diam. of about -02 mm.

within the cell-wall. In older tissues the cell-walls are often so
conspicuous that the units of construction were called " cells " by
the earlier observers, from their comparison with the partitioned
honeycomb. That name is still retained for them. But it is now
fully recognised that it is the protoplast and not the cell-wall that is the
essential part, for it is in it that the active vitality is centred.

Increase of Cells by Division.

As the tissues increase with the general growth of the apical region,
the number of cells composing it increases by cell- division. An
examination of the tissues themselves will show how this is carried
out. Very frequently cells may be found in the apical cone grouped
in pairs, and separated by a very thin wall. These plainly indicate
that a division of a pre-existent mother-cell has recently taken place,
so as to form two usually equal daughter-cells from one parent cell.
The new cell-wall thus formed is inserted at right angles upon the
older walls. If the cells always divide into nearly equal halves, and
if the new walls are fixed at right angles upon the older walls, the
result must necessarily show some degree of regularity in the arrange-
ment of the cells that are formed. In some cases that regularity is
very striking. The scheme of construction in the case of the apex
of Hippuris would be like that shown in Fig. 10, and it is found that
in plants at large the young tissues are arranged according to similar

THE CELLULAR CONSTRUCTION OF THE PLANT

19

schemes. Thus in the young state the axis, and, it may be said more
generally, the plant-body throughout, is partitioned up into cells in
somewhat the same way as a house is partitioned into rooms. And
their arrangement is not at haphazard, but according to laws. //
may be stated generally, as a fact of experience, that the whole of the
plant-body, whether young or mature, is made up of such cells, or their
derivatives. This generalisation used to be spoken of as the " cellular

Fig. 10.

Diagram illustrating the plan of arrangement of cell-walls in the apex of the stem
of an Angiosperm. XX = axis of construction. EE = external surface. PP=peri-
clinal curves. AA =anticlinal curves. (After Sachs.)

theory." But it is now so fully demonstrated that the fact may be
enunciated as a positive conclusion. It will be seen later what are
the modifications which such cells undergo so as to produce the
mature tissues of the plant, which often differ widely in form and
structure from the young cells that give rise to them.

From a comparison of cells in various states of division it is possible
to construct a connected history of the process (Fig. 1 1). The nucleus
takes the initiative (i.-iv.). By complex changes, which will be
described in detail later, it divides into two exactly equivalent parts,
which at first lie in the longer axis of the cell, embedded in the still
undivided cytoplasm (v.-vii.). Then a delicate film of cell-wall is
formed between them, inserted at right angles to the pre-existent walls,
cutting the cell into two nearly equal parts, each containing a nucleus
(viii.-ix.). Such simple divisions are called somatic, belonging to the
soma or plant-body, to distinguish them from certain divisions which
involve further complications connected with the reproductive process.
The number of somatic divisions is indefinite, and the numerous cells
to which they give rise are while young thin-walled, and all alike.

20

BOTANY OF THE LIVING PLANT
i. ii. in.

IV.

VII.

V.

vm.

Is? •:»-'. ---v.';. •■••-.• -j?' -v-

i&te-M

& -•" •- • ■' £ ™ti • j i ill 11 •?■? #;'M

sSS^/^Ka

Mil-M^Li^iiM

IX.

Fig. ii.

I. -IX. Successive stages, drawn from different individual cells of the same root
of Allium cepa by Dr. J. M. Thompson ( x 730). They illustrate the steps in the pro-
cess of division of a vegetative cell. Such division accounts for the normal increase
in number of cells in the " soma," or plant-body : it is therefore called somatic
division. Details of behaviour of the nucleus in division will be given later
(Chapter XXXV).

The division of the cell being constantly as described, it follows that
every cell arises from a pre-existent cell. Every nucleus is derived
from a pre-existent nucleus by division, and is never produced de novo.

THE CELLULAR CONSTRUCTION OF THE PLANT 21

It was formerly believed that plastids also arise only by division, but
there is now some doubt whether this is always the case, although
their multiplication by division is very common. It is different with
the cell-wall. In cell-division it appears as a new film deposited from
the protoplasm : while, as we shall see, it may be absent altogether
from reproductive cells. It is thus a body of secondary importance,
as compared with the more constant constituents of the cell.

Differentiation of Tissues.

Passing in the examination of the longitudinal section of any
bud from the growing point downwards, successively older tissues

Fig. 12.

Parenchyma-cells from the cortex of the root of Fritillaria : longitudinal section
(X550). A, very young cells, not yet vacuolated. B, older cells with numeious
vacuoles containing cell sap. s. each surrounded by the protoplasm, p. C, older cells,
with larger vacuoles filled with sap, s. The protoplasm (p) lines the cell-walls
internally, and embeds the nuclei (k), which may be suspended centrally, or placed
laterally. The large cell to the left has a single large central vacuole, or cell-cavity.
(After Sachs.)

22 BOTANY OF THE LIVING PLANT

are seen, till the mature parts are reached. Various changes appear
in the cells. They alter their form and the character of their walls
and contents. As a rule the cells enlarge greatly. An important
change in the cytoplasm, which is usual in plant-cells, accompanies
this growth. It is known as vacuolization, and it may be well illus-
trated in the cells developing into the pith or cortex of an ordinary
stem. Starting from the embryonic state, where the wall is very
thin, and the cytoplasm and nucleus fill the whole space enclosed by
it (Fig. 12, A), the volume begins to increase with age and the wall
thickens. But the volume of the cytoplasm does not keep pace with
that of the whole cell, and vesicles or drops of clear liquid appear
within it.1 These are called vacuoles, and they are filled with vacuole
fluid, or cell-sap, which is water with certain substances dissolved in
it (Fig. 12, B). The vacuoles are always completely enclosed in the
cytoplasm, which controls them and the substances dissolved in them.
The vacuoles may vary in number, size and position, and the position
of the nucleus is also inconstant ; sometimes it lies laterally in the
peripheral cytoplasm ; usually it is central. As the vacuoles enlarge
they may run together, and finally form a continuous cavity, in the
middle of which the nucleus is frequently suspended by radiating
threads of cytoplasm (Fig. 12, C). A condition is thus arrived at
which is characteristic of many cells in the mature state.

Other cells may undergo changes of a much more marked character
than this, as they pass from the young to the mature condition.
Such changes fit them for performing their several functions in that
commonwealth of units of which the mature plant consists. Division
of labour is characteristic of the mature tissues of all the Higher
Plants, and it is the structural differentiation of the constituent cells
that makes this possible. It will be well here to explain briefly the
chief changes which may be traced in the different tissues, as they
are developed from the uniform embryonic cells that compose the
apical cone.2

(i) Changes in Size and Shape of the Cells.

Practically all cells grow as they mature, and a simple case in
which the change of form is only slight has been seen in Fig. 12 (A-C).

1 It is now known that minute vacuoles are present in embryonic cells.

2 This analysis of the changes during differentiation finds its proper place
here, and forms the natural foundation for any rational study of mature
tissues. But it is open to the student to read it either before or after those
tissues have been described.

THE CELLULAR CONSTRUCTION OF THE PLANT 23

But in the course of their growth cells may also assume various
shapes. Usually there is elongation, and the ends become more or
less oblique. This naturally follows from the fact that the part of
which they are constituent units, such as stem, leaf, or root, itself
grows in length. Such changes often involve a readjustment of the
cells among themselves by a sliding process, which is specially obvious

Fig. 13.

Various forms of cells, (i.) cell of the parenchyma from the cortex of the. root of
a Buttercup, almost spherical ; (ii.) oblong cell of the medullary ray of Lime ; (hi.)
stellate parenchyma of pith of the Rush ; (iv.) wood-fibre of the Lime ; (v.) fibrous
tracheid of Lime.

x = intercellular spaces ; n = nucleus; cyt = cytoplasm ; vac = vacuole; st = starch-
grains, (i. ii. iii. x 200 ; iv. v. x 75.)

where the cells become elongated or very wide when mature. They
seem then to push the surrounding cells aside, so that the appearance
of the mature tissue composed of them may differ strongly from that
of the embryonic tissue from which they sprang. Special names used
to be applied to all the different characteristic forms which cells
might assume. But this is not necessary except in extreme cases ;
thus the old name parenchyma is kept for a tissue of roughly spherical
or oblong cells with square ends, while long thick-walled cells with
pointed ends are called fibres (Fig. 13, i.-v.).

(2) Changes in the Thickness of the Wall : Pits.
During the growth of the cell the wall is stretched, and like a
rubber sheet it would become thinner as it yields, were it not for
the deposit of new cell-wall substance by the cytoplasm. In ordinary
cell-walls this is effected by apposition of successive layers upon the
surface of the wall, and so quickly is this carried out that the stretch-
ing wall of an enlarging cell actually grows thicker instead of thinner

24

BOTANY OF THE LIVING PLANT

Fig. 14.

Section of a cell of Hoya camosa,
with greatly thickened, stratified,

as it stretches. As the limit of size of the mature cell is approached
and the stretching ceases, the thickening of the wall may be more

rapid, and it is probably this which
causes the cessation of growth. The
thickening may be continued till in
extreme cases a large proportion of the
space within the original film of cell-wall
is filled up. Often this original wall may
be recognised in the mature state as a
" middle lamella," where two or more
thickened walls adjoin (see Fig. 16, B,
p. 27). In the mature cells with thick
walls the layers of stratification can
often be clearly seen (Fig. 14). But the
thickening of the walls is seldom uniform.

and pitted walls. The pits are very p„rfa:n Qrpc,c arP Ipff- thin onr| a note-
narrow, and often branched. (After Lertain areas are leit tnin, ana a note

vonMohi.) worthy feature is that the thin areas in

adjoining cells usually correspond. Such thin areas are called Pits,
and the partitioning wall is called the pit-membrane. Pits are of use
in facilitating the physiological communication between cells, and
practically all mature cells show pits of some sort on their walls
(Figs. 15 £, 16 B).

(3) Changes of Substance of the Cell-walls.
In the young state the cell-walls are composed of a carbohydrate
substance, namely cellulose, together with more or less of those
pectin-substances which form the basis of fruit-jellies. Such cell-walls
are at first yielding and plastic like putty, but they become more
resistant and elastic as they grow older. As they become mature
the chemical and physical nature of the walls may change. Some
walls become lignified or woody, and are then mechanically more
resistant and harder. Such walls give its character to the wood of
tree-trunks. Others become suberised, or corky, and are then im-
pervious to the passage of water. Bottle-cork consists of masses of
dead cells with corky walls. Others may become gummy, or mucila-
ginous, and are liable to swell greatly on access of water, which they
thus retain. Gum arabic as sold in shops consists of hardened
amorphous masses of gum exuded from the stem of certain Acacias.
By such changes the cells may become fitted to perform different
specific functions in the mature parts.

THE CELLULAR CONSTRUCTION OF THE PLANT 25

(4) Absorption of Cell-wall.
Though in the young cells the wall completely encloses the proto-
plasm, it may be partially broken down and absorbed before maturity.
This most commonly occurs in those longitudinal rows of cells which
are destined to form vessels ; and usually it affects the transverse,

A B

Fig. 15.

A , a longitudinal row of cells from the root of Maize, still with complete septa,
nuclei, and cytoplasm, from which a vessel would be formed by absorption of the
septa, and disappearance of the protoplasts. B, a mature vessel of Sunflower, with
thickened and pitted walls and no protoplasmic contents, cut in slightly oblique
longitudinal section. The arrows indicate free passage through holes formed by
absorption of the septa. The longitudinal lines on the pitted walls show the limits
of the adjoining cells. {A x ioo ; B x 165.)

but sometimes also the longitudinal walls. The septa between the
cells being thus removed, two or more cells may be thrown together
so as to form a continuous tube. Such a tube is called a Vessel
(Fig. 15). Other cases of absorption of walls may also occur, but
that leading to the formation of wood-vessels is the most important,
and the most frequent.

(5) Changes in the Protoplasmic Body of the Cell.
The common change of vacuolisation has already been described.
Other changes result in the deposit and removal of contained bodies

26 BOTANY OF THE LIVING PLANT

such as oil-globules, starch-grains, or crystals in the cytoplasm.
But the most marked change is the disappearance of the protoplasmic
body itself, so that it is not represented in the mature structure.
This is found to be the case in the vessels of the wood, in the cells
of cork, and in some other tissues (Fig. 15, B). Since the vital
activities reside in the protoplasm, those tissues where it is absent,
consisting only of cell-wall, are no longer actively living, though
passively they may still perform functions important in the life of
the plant.

(6) Changes in the Plastids.

These bodies are minute, and difficult to see in the young cells
(Fig. 9). But as the cells mature they may become more numerous
by division, and more prominent by their size and colour. In many
cells of vegetative parts they turn vivid green, and are called chloro-
plasts, or chlorophyll-corpuscles (Fig. 51, p. 76). They are present in
myriads in any green leaf, and collectively give the green colour to
the parts in which they occur. Other plastids may take red, or yellow
colours, as in petals, or in fruits, and they are called chromoplasts.
Others remain colourless, and are called leucoplasts, or starch- forming
corpuscles, because they are actively functional in the deposit of in-
soluble grains of starch in tubers, and elsewhere (Fig. 81, p. 124).

By such changes as those described under the headings (1) to (6),
the young embryonic cells may be transformed into the various
tissues that make up the mature parts. Originally the cells were all
alike ; as they become mature they are liable to be differentiated
and specialised for different functions. It may be held as probable
that what is seen in the individual development is a reasonable guide
to what actually took place in the evolution of the race. It is prob-
able that plants with little or no differentiation of tissues, that is
Cellular Plants such as the Algae, preceded in the history of Evolution
the more complex Vascular Plants. These with their higher state
of tissue-differentiation constitute the leading feature of the Flora
of exposed Land-Surfaces.

Continuity of Protoplasm.

The cells of living tissues all share in a common physiological life,
and are in intimate relation to one another. But cell-walls separating
adjoining cells do not form complete barriers between their protoplasts.
In most mature tissues a Continuity of Protoplasm may be demon-
strated, from cell to cell. It is established by means of fine connecting

THE CELLULAR CONSTRUCTION OK THE PLANT 27

threads. These pass for the most part through those thin areas of
pit-membrane, where the distance to be traversed is the shortest.
Occasionally they may also extend through the thicker regions of the
cell-wall. Examples are shown in Fig. 16, A, B. The prevalence of
Protoplasmic Continuity, now generally demonstrated for the tissues
of Plants, forms a structural foundation for their physiological study.

B

Fig. 16.

Continuity of protoplasm through the walls of plant-cells. A. Cells of the
pulvinus of Robinia, after treatment with sulphuric acid to swell the walls, and
staining of the protoplasm with methyl violet. ( x 55°-)

B Cell-wall of a single cell of the endosperm of Lodoicea, showing the pits and
the protoplasmic threads, traversing both the thin pit-membranes and the thickened
regions of the wall. ( x 400.) (After Gardiner.)

There is reason to believe that the protoplasm is the seat of physio-
logical activity, and since the protoplasm of adjoining cells is con-
nected by threads traversing the cell-walls, whole tracts of tissue will
be able to share a common life. This leads us to expect that organs
will react as a whole under external stimulus, and that though the
cell may appear to be an individual structural unit, still each cell
takes its place as a constituent of that physiological commonwealth
which we call the Plant-Organism.

While we thus recognise the physiological importance of the con-
tinuity of protoplasm through the cell-walls, it should be remembered
what circumstance it is that has made it necessary. It is the presence

28 BOTANY OF THE LIVING PLANT

of the cell-wall itself. The encysted state of the cell is a feature of all
advanced types of Plant-Organisation. It was probably secondary
in origin, and its existence is amply justified by the strength and
protection which it affords to the defenceless protoplast. Moreover,
it has made possible the building up of large mechanically stable
plant-bodies, whether buoyed up in water or supporting themselves
in air. The presence of cell-wall may appear to have complicated
the problem of physiological interchange by interposing barriers
between the protoplasts. But this difficulty has been surmounted
by those threads of protoplasm, which traverse the walls, and link
up the protoplasts into a continuous living system.

CHAPTER III.

THE LIVING CELL.

In order to understand that " continuous living system " of which
the plant body consists, it will be necessary to consider in more detail
the structure and physiological properties of the cell-units which
compose it. As stated in the previous chapter, the mature plant does
not consist entirely of living cells, but it is in them that the physiolo-
gical activities of the plant are concentrated.

Fig. 17.

Optical longitudinal section of a single cell of a hair of the Cucumber. Externally
is the cell-wall (C.W.). Its inner surface is lined by a layer of cytoplasm (cyt.),
surrounding a large vacuole (V.). In this the nucleus (N.) is suspended centrally
by numerous cytoplasmic threads. Movements may be seen in these during vitality,
which convey ' chloroplasts (chl.), and even crystals (cry.), thus showing active
circulation in the living cell. (After Sachs.)

The visible structure of typical living cells has already been described,
but the main features will be revised at this point, with the help of
Fig. 17, which shows a single cell from one of the coarse hairs that
roughen the surface of the shoot of the Cucumber. Externally the
cell is limited by a definite cell-wall of almost uniform thickness. Its

29

30 BOTANY OF THE LIVING PLANT

ends are partitioned off from the adjoining cells by transverse septa
marked by shallow pits, a condition which is usual for the septa, or
party-walls between cells. Protoplasmic connections probably pass
through these pits. Each protoplast (p. 1 8) is thus enclosed by an
elastic envelope of cell-wall, composed chiefly of cellulose and normally
saturated to a greater or lesser extent with imbibed water. A film
of cytoplasm lines the whole internal surface of the cell-wall, and thus
completely invests the large internal cavity, or vacuole, which is
filled with watery cell-sap. A highly refringent nucleus (as a rule
more sharply delimited than in the Figure), surrounded by a sheath of
granular cytoplasm, is suspended in a central position by threads of
cytoplasm, which traverse the vacuole. Their arrangement is irre-
gular, with frequent branchings ; but most of them converge towards
the nucleus, which is thus indicated as a functional centre of the
whole protoplast. Granules and inclusions of various sorts are seen
immersed in the cytoplasm. The largest of these are the plastids,
which in this cell are green and are termed chloroplasts, but many
other smaller bodies are present, including occasional crystals. The
nucleus, cytoplasm and plastids are the actual living components of
the cell, and collectively form the protoplast.

In many living cells the protoplasm exhibits streaming movements.
It is probable that the movement is initiated in the cytoplasm, the
plastids and other cell-inclusions being carried along passively. The
plastids may be carried back and forth along the threads that suspend
the nucleus, as in the cucumber-hair cell, or along the cytoplasmic
lining of the cell.

Properties of the Living Cell.

Of the various properties of the typical living plant-cell, that of
effecting chemical transformation is one of the most prominent and
important. In connection with the nutrition of the plant there
proceed within its constituent cells very diverse chemical reactions,
complex substances being elaborated from simple raw materials
derived from the environment of the plant. Some of these activities
are restricted to particular cells in the Higher Plants : thus the
process of photosynthesis (see Chapter VIII.) occurs chiefly in the
cells of the leaf. Other types of chemical activity are exhibited by
all living cells. These chemical activities, involving also transforma-
tions of energy, are grouped together under the general term Meta-
bolism, and are considered in more detail in Chapter VIII. Growth

THE LIVING CELL 31

in size and complexity is another feature of the cell, the materials
for it being prepared by the metabolic activities mentioned. Finally
the living cell is sensitive or irritable to certain outside influences
which act as stimuli. Irritability is most conveniently discussed in
relation to whole organs of plants, and is considered in Chapter IX.

In order that the cell may display its full activity, certain conditions
must be provided. Suitable raze materials must be present before
constructive processes and growth can occur, while a source of energy
must be to hand. For most purposes this energy is derived from the
oxidation of sugars previously built up in photosynthesis, and so the
presence of oxygen is for most cells a necessary condition to normal
activity. In the particular constructive process known as photo-
synthesis, light energy is utilised, and thus light must be available
for this process to be carried out. The cell structures normally
contain a high proportion (over 80 per cent.) of water and adequate
supplies of this must be available. Its absence leads to death or to a
suspension of cell activities, as in dormant seeds where the cells are
in a state of desiccation. A suitable temperature must be provided,
as mentioned in Chapter I. In most plants vital activity is only
manifest between 0° C. and about 45 ° C, with greatest activity pre-
vailing between 250 C. and 350 C. ; exposure of cells to temperatures
above or below the former range is liable to result in death (see also
Chapter IX.). Vital activity is suspended by exposure of cells to
substances such as chloroform or ether, which act as narcotics. From
the effects of these the cells may subsequently recover, while other
substances such as alcohol, or compounds of copper, act as poisons
and kill the cell.

The operation of some of these factors can conveniently be studied
in observations on the protoplasmic streaming, which, as already
mentioned, is shown by some cells. The rate of streaming serves as
an index of the general activity of the cell. Low temperatures slow
down the streaming, higher temperatures accelerate it until a certain
temperature (the optimum) is reached, above which the cell is injured
and the movements soon cease. Deprival of oxygen or exposure to
narcotics or poisons causes the movements to stop.

Protoplasm.

AH these attributes of the cell — metabolism, growth, irritability —
have their genesis in the protoplasm. We must therefore enquire
why such remarkable properties should reside in the protoplasm.

32 BOTANY OF THE LIVING PLANT

There is no doubt that chemically and physically protoplasm is very
complex indeed, and the study of its nature is attended by great
difficulties. Under the microscope the cytoplasmic part of the
protoplasm has the appearance of a clear medium with many sus-
pended granules. Experimental evidence shows that protoplasm is
usually liquid in nature, though considerably thicker or more viscous
than water. In some cases the consistency is more that of a jelly.
The nucleus and plastids are somewhat denser parts of it.

Chemical analysis has indicated that protoplasm is a heterogeneous
mixture of a great variety of chemical substances, the Proteins being
present in greatest quantity, both in the nucleus and the cytoplasm
(see Chapter VIII.). Associated with them are fats and their deriva-
tives, and mineral salts. All these substances are dissolved or sus-
pended in the water which constitutes over 80 per cent, by weight
of protoplasm. The constituents of protoplasm are to a considerable
extent in a colloidal condition, and some knowledge of the special
properties of colloidal substances and solutions is absolutely essential
to a consideration, however elementary, of the living cell ; for there
is no doubt that it is largely the colloidal nature of its constituents
that endows protoplasm with its remarkable powers. The cell-wall
and some of the metabolic substances occurring in cells are also
colloidal in nature. Graham, in his researches of the middle of last
century, divided soluble substances into two classes, as the result of
his studies on dialysis, — the term applied to the passage of substances
in solution through membranes such as parchment. He found that
certain substances passed freely through such membranes, and since
generally speaking they could also be readily obtained in a crystalline
condition he called them Crystalloids. As examples we have sugars
and soluble salts. Other substances were found not to dialyse, or
only very slowly, while a further character was their amorphous
nature when dehydrated. These he termed Colloids. Many plant
and animal products are of this type. As examples gelatin (a protein),
agar-agar, gums and starch may be quoted. These substances give
a special type of solution in water, known as a colloidal solution. In
such a solution the substance is present in the form of relatively
large particles, either because the molecules are aggregated into
groups, or because the individual molecules are very large ; or again
because they take into association a number of molecules of water.
Such solutions are intermediate between the true solutions given by
crystalloids, and suspensions or emulsions. The particles are per-
manently suspended in the water, they are invisible under the highest

THE LIVING CELL 33

powers of the microscope, and will pass through a filter paper, though
they cannot pass through the minute pores of parchment, as already

mentioned.

In the dry state colloids are often characterised by a high affini
for water, leading to imbibition when placed in water. In some cases
the colloid dilutes itself so far as to go into solution, as does gelatin.
In others the colloid, though imbibing water to a considerable extent,
remains in a semi-solid, jelly state. This is the case with the cellulose
of the wall. The swelling of dry seeds in water is due to imbibition
both by cell-walls and by protoplasmic colloids.

Many colloidal solutions, while liquid at higher temperatures or
in greater dilution, set to form a jelly on cooling or on concentration.
Gelatin and starch paste are examples. The fact that protoplasm
exists sometimes as a liquid, sometimes as a jelly, points to its
colloidal nature. Protoplasm appears to consist of an extremely complex
system, in which proteins are present in colloidal solution in an aqueous
medium containing also mineral salts and other soluble substances.
Associated with the proteins are fats and other constituents, such as the
protoplasmic catalysts or enzymes. They will be considered in Chapter
VIIJ. The surface of colloidal particles, presented to the aqueous
medium, is the seat of important phenomena. Crystalloidal substances
which may be present in the water tend to accumulate on the surface
of the particles by a process known as adsorption. This is likely to
accelerate any chemical reactions that may occur between different
substances adsorbed side by side on colloidal particles ; and there
is little doubt that the great chemical activity of protoplasm is in
part due to this adsorption, especially on to the surface of the proto-
plasmic catalysts or enzymes.

It is a matter for surprise that many different chemical reactions
should be able to proceed at once within the minute confines of a
cell. As will be seen later, some of these reactions are known to be
restricted to the plastids, and it is often assumed that there is an
invisible separation of the protoplasm into further separate areas,
each devoted to a particular type of reaction. The nucleus is the
bearer of the hereditary characters of a plant, as is described later.
It must also play an important part in the other activities of the cell,
directing them in such a way that the plant as a whole, in a suitable
environment, acquires those morphological and physiological attri-
butes which characterise its race.

B.B.

34 BOTANY OF THE LIVING PLANT

Osmotic Phenomena in Plant Cells.

Most free cell-wall surfaces tend to bulge outwards. This is seen
even in embryonic cells (Fig. 9), and is a general feature of mature
cells of the epidermis (Figs. 22, 47). A tendency to convexity also
appears in the free cell-walls of the Cucumber hair (Fig. 17). Such
observations are evidence of a feature common in the living cells
of plants, namely, the state of Turgor (or Turgescence), arising
from the presence of internal pressure (Turgor Pressure) acting
equally upon the whole inner surface, and tending to round off the
contours of the cell. This state of turgor is due to the osmotic pro-
perties of the cell.

At this point we may consider an experiment in which the arrange-
ment shown in Fig. 17A is employed. A parchment thimble is initially
filled with a solution of sugar and securely attached to a rubber
stopper fitted with a long glass tube. The thimble is then immersed
in water. Very soon liquid commences to rise in the tube, and if the
sugar solution is sufficiently concentrated a column several yards
high may be obtained in a day or two. Evidently water is passing
from the outer vessel into the sugar solution, and this is leading to
the development of a considerable hydrostatic pressure within the
thimble. This movement of water through a membrane is known as
Osmosis and would be produced with a solution of most crystalloidal
substances. The motive agent in the movement of the water is what
is known as the Osmotic Pressure of the sugar solution : the precise
nature of osmotic pressure is uncertain, but its existence is connected
with the fact that while the molecules of water pass through the
membrane rapidly, those of sugar penetrate only slowly. A measure
of the osmotic pressure of a given solution is obtained by noting the
maximum hydrostatic pressure that is developed when the solution
is enclosed in an apparatus of the type shown in Fig. 17A, using a
semi-permeable membrane (see below). Such measurements reveal
that by osmosis, hydrostatic pressures of considerable magnitude
may be set up. Thus the osmotic pressure of a 10 per cent, solution
of cane sugar is equal to approximately 8 atmospheres, or 120 pounds
per square inch : that is to say, the solution would set up a hydro-
static pressure of this magnitude when enclosed in a suitable apparatus.
Osmotic pressure varies with the concentration of a solution, and
within limits there is a direct relation between the two values.
Osmosis does not occur only from pure water into a solution : if
one solution is separated by a membrane from a second solution of

THE LIVING CKLL

35

greater osmotic pressure, water will pass by osmosis from the first
into the second solution, the effective osmotic pressure in this case
being the difference between the values for the two solutions.

The parchment membrane used in the experiment described above
allows the sugar molecules to diffuse slowly into the outer water ;
the membrane is to a certain extent permeable to the sugar. The
concentration and therefore the osmotic pres-
sure of the contents of the thimble thus fall,
and some of the water initially taken into the
thimble leaks out again. A permanent osmotic
retention of water is only possible with a
membrane which, while giving free passage
to water molecules, denies passage to the mole-
cules of the osmotically active substance.
Such a membrane is said to be semi-permeable
to the solution, and it is only with such mem-
branes that the osmotic pressure of a solution
can be determined directly. Copper ferro-
cyanide forms a well-known artificial mem-
brane which is semi-permeable to solutions
of sugar and of various other substances.
For purposes of experiment it is usually de-
posited within the minute pores of a porous
pot, which thus gives it rigid support.

The plant cell forms an osmotic system.
The cell-sap of the vacuole consists of a solu-
tion of osmotically active substances, such as
sugars, organic acids and salts, and normally
exerts a total osmotic pressure of 5 to 20
atmospheres. This solution is enclosed in the
membrane formed by the thin lining of cyto-
plasm. The cell-wall gives support to the
cytoplasmic lining : its presence makes possible the development
of a high degree of turgor within the cell. Under normal conditions
the plant cell is in a state of inflation or of turgor (p. 34), set up by
the osmotic absorption of water. This condition can be removed
without harm to the cell by immersion in a solution of potassium
nitrate or of cane sugar which is sufficiently concentrated to be of
higher osmotic pressure than the cell-sap. Such a solution is said
to be hypertonic to the sap ; one of lower osmotic pressure than the
sap would be termed hypotonic . For many plant cells a 5 per cent.

Fig. I7a.

Apparatus for the demon-
stration of Osmosis. The
parchment thimble contains a
sugar solution, while the outer
vessel is initiaDy filled with
water.

36

BOTANY OF THE LIVING PLANT

solution of potassium nitrate is hypertonic and will withdraw water
from the cell-sap. The effect of immersion in such a solution can be
most readily observed microscopically in cells which contain a soluble
pigment in the cell-sap, giving greater definition to the outlines of
the vacuole and of the protoplast. Sections of the Red Beet may be
used, or strips of epidermal cells of leaves such as Cyclamen, where
a red pigment is again present. The initial effect of immersion is a
shrinkage of the whole cell as the pressure of the contents on the
wall is relaxed (Fig. 18, i. and ii.). In a very short time the protoplast
breaks contact with the cell-wall and shrinks to a further extent.
The cell is now said to be plasmolysed (Fig. 1 8, iii.). The space between

Fig. 18.

Young parenchymatous cell (i.) in the turgid state, (ii. and iii.) successive ap-
pearances of cell after immersion in a hypertonic solution of potassium nitrate.
it'=wall; c = cytoplasm; n = nucleus ; s = cell-sap; e= nitrate solution which has
passed through the cell-wall. (After De Vries.)

the cell-wall and the protoplast is filled with the nitrate solution,
illustrating the fact that the cellulose cell-wall is readily permeable
to water and to dissolved, crystalloidal substances. Turning to the
cytoplasm, it will be noticed that there is no outward leakage of the
red pigment, the colour of which is intensified by the concentration
of the sap. The cytoplasmic lining constitutes a membrane that is in
life to some degree semi- permeable to the sap.

If plasmolysis be carried out slowly by means of dilute solutions, fine
threads of cytoplasm may often be seen to stretch from the cell-wall to the
contracted mass (Fig. 19). This indicates under normal conditions of the
cell an intimate relation between the two bodies, which is in accordance
with the deposit of the cell-wall from the protoplasm. But it is uncertain
what relation, if any, these cytoplasmic threads have to the threads that
establish protoplasmic continuity through the cell-wall (Chapter II.).

THE LIVING CELL

17

The shrinkage of the protoplast of the plasmolysed cell continues
until the sap is concentrated to such a degree that its osmotic pressure
becomes equal to that of the external solution : equilibrium is then
established, provided that the dissolved substance of the external
solution is unable to enter the cell. If it docs enter, the osmotic activity
of the cell-sap is gradually increased and eventually the protoplast is
able to take up water from the outer solution and so to recover its
turgor. This recovery or de-plasmolysis can in any case be rapidly
induced by transferring the cells from the plasmolysing solution into

Fig. 19.
Cells from the prothallus of Nephrodium villosum after treatment with 3 per cent,
solution of common salt ( x 550). Drawn as observed about 15 minutes after plasmo-
lysis ; the threads are very fine, but appear proportionally thicker in the figure
than they actually are.

water. The osmotic properties of the cell-sap now result in a rapid
intake of water, just as water passed into the parchment thimble
in the experiment shown in Fig. i;a. The protoplast swells and soon
regains contact with the wall : through continued entrance of water
the elastic wall becomes distended. As the wall now exerts a pressure
on the expanding protoplast, the capacity of the cell for further
absorption of water is thereby curtailed. The effective osmotic
pressure conducive to further absorption is clearly the difference
between the osmotic pressure of the cell-sap and the inward
pressure of the wall. This effective osmotic pressure is distinguished
as Suction Pressure. As the absorption of water continues a st.

38 BOTANY OF THE LIVING PLANT

is eventually reached at which the available osmotic forces are unable
to stretch the wall any further. The suction pressure is now nil, and
the cell has no capacity for further absorption of water. On the other
hand the pressure of the cell-contents on the wall — known as the
Turgor Pressure — is now at its maximum, the whole of the osmotic
pressure of the cell-sap being exerted on the wall. The cytoplasmic
membrane and the wall of such a turgid cell are in a state of inflation,
and may be compared with a blown-up football, the membrane
corresponding to the bladder, the cell-wall to the case.

The possible development of turgor pressures of values ranging
from 5 to 20 atmospheres within relatively thin-walled cells will
naturally occasion some surprise. Actually under normal conditions
plant cells are not fully turgid, as is indicated by the swelling usually
shown on placing a living tissue in water. Also it should be remem-
bered that most cells are exceedingly small, and what would be
impossible in a larger structure may be quite possible in a smaller
(Chapter X.).

By command of such high osmotic forces as those above quoted,
upon which the growth of the cells depends, the organs of plants are
able to overcome considerable resistances (Chapter IX.). Thus the
root forces its way into the soil, while the shoots of plants occasionally
lift asphalt or flagged pavements. The turgor of the individual
cells also gives such degree of firmness and rigidity as is seen in
sappy and herbaceous plants, which they lose on wilting or withering
or on plasmolysis (Chapter X). Like the football or the pneumatic
tyre they are rigid when distended, but flaccid and limp when the
pressure falls. In pneumatic tyres it is a pressure of compressible
gas that gives the mechanical effect : in the plant cell it is a turgor
caused by an accumulation of incompressible liquid. But in both
cases the mechanical effectiveness depends upon the resistance of
the elastic outer cover.

The Living Cell and Dissolved Substances.

It is frequently stated that the cytoplasmic lining of the cell is
semi-permeable to the cell-sap solution. It is, however, obvious that
the mineral salts of the sap must have entered the cell from without, and
unless the cell contains chloroplasts any sugars that are present must
also have passed from another cell into the one in question. In fact
the physiology of the plant requires that there shall be free diffusion
of dissolved substances from one living cell to another. It seems that

THE LIVING CELL 39

the cytoplasm must be appreciably permeable to salts, sugars,
amino acids and such crystalloidal substances, although under ex-
perimental conditions the degree of this permeability appears to be-
lower than one would expect. It is possible that movement from
cell to cell of substances in solution also occurs through the cyto-
plasmic connections already mentioned.

Experiments show that some classes of substances penetrate the
cell very quickly, as is the case for example with alcohol (poisonous
in higher concentrations), urea and other organic compounds. There
has been much speculation as to why some substances are able to
pass the cell-membrane more easily than others. There is some
evidence that membranes, both natural and artificial, act as molecular
sieves, allowing smaller molecules to pass through, but holding up
larger ones ; though this theory only accounts for some of the facts.

If a cell is killed the cytoplasmic membrane immediately becomes
very much more permeable to dissolved substances, its organisation
having been destroyed. Thus, while Beet cells are in life semi-
permeable to the red pigment of the sap, if the cells are killed, by
heat or by exposure to chloroform, the red pigment quickly begins to
diffuse out of the cells into the external liquid in which they may
be lying.

In some cases the entrance of dissolved substances into a cell
proceeds according to the laws of ordinary diffusion, continuing until
the concentrations inside and outside the cell are equal in respect of a
particular substance. This holds for various organic compounds. If
the substance is being constantly used up in one way or another inside
the cell, or is passing into neighbouring cells, equality of concentration
may never be reached, and entrance will continue indefinitely. A
similar consideration applies to the entrance of inorganic salts, and of
the ions produced by their dissociation : but with these entrance may
continue even after the state of equal concentration has been reached.
This is a fact of great importance in connection with the absorption
of mineral salts by the root-hairs from the very dilute soil-solution
(Chapter VII.). It is clear that here forces over and above those
productive of ordinary diffusion are operative, though their nature
is still problematical.

CHAPTER IV.

THE TISSUES OF THE STEM.

The mature tissues of a Plant are not homogeneous as they are in
the apical bud. At first they are all soft. But as they pass over
to the mature condition, while certain tissues retain their relatively
thin-walls, others become indurated, forming strands which are
mechanically resistant. This is illustrated in a familiar way in the
shoots of garden vegetables. If these are allowed to mature too far
they become stringy, owing to the development of toughened strands.
Where the succulent tissues preponderate the harder strands form
isolated threads embedded in the softer tissues. In other cases they
may be fused into larger tracts, and this is especially so as they grow
older. They thus form in tree trunks and twigs of woody plants a
cylindrical core, from which smaller strands extend outwards into the
leaves and branches. The mature Shoot, with its constituent axis
and leaves, is thus composed of a relatively firm skeleton, consisting
of the Vascular and Fibrous System : this is embedded in the softer
Ground-Tissue ; and the whole is covered on the outside by a con-
tinuous skin of the Epidermis. The first of these serves for conduction,
and gives mechanical strength : the second carries on the functions
of nutrition and storage : while the epidermal system may give external
protection.

In order to obtain a more exact idea of the general construction of
the shoot of a Flowering Plant either the firmer strands may be
dissected out by hand, or their position may be studied by means
of sections. It is only in large and herbaceous plants that the former
method is effective ; but it is well to cam7 it out in some such plant
as the Sunflower, for this gives a better understanding of the results
obtained by sections. By either method it is possible to trace the
course and connections of the strands through the softer tissues, and

40

THE TISSUES OF THE STEM

4i

so to construct the vascular skeleton of the shoot. Such a skeleton
is shown for Clematis in Fig. 20. Here the leaves are arranged in
pairs, and from each leaf three vascular strands pass into the stem.

Each strand curves downwards before
reaching the centre of the stem, and
after taking a straight downward
course to the level of the next pair
of leaves it forks. The two shanks
are then inserted right and left upon
the strands that enter there. If a
transverse section be cut at any
point between the pairs of leaves,
the section shows six main strands
arranged in a ring, with smaller
strands between them (Fig. 21). A
very simple connected system of

Fig. 20.

Clematis viticella. End of a branch
which has been made transparent by
the removal of the superficial tissues
and treatment with caustic potash. The
emerging strands have been slightly dis-
placed by gentle pressure. Ihe two
uppermost pairs of young leaves (W1, bl2)
are still without leaf -traces; v, apical
cone. (S. after Naegeli.)

F03.

Fig. 21.

Transverse section of an internode of a stem of
Clematis, showing a ring of six larger and six smaller
vascular strands, surrounding the central pith, and
covered externally bv the thick cortex. In the
adjoining skeleton it is only the six larger strands
that are represented. The collenchyma massed at
the projecting angles is dotted. ( x 15.)

vascular strands is thus formed, and it illustrates the arrangement
usual for them in the shoots of Dicotyledons. There are differences
in the number of the strands entering from each leaf in various
examples. In some there is only one strand, in others more. The

42 BOTANY OF THE LIVING PLANT

arrangement of the leaves on the stem may also vary, as well as the
distance through which the strands keep a separate course down the
stem. Such differences will naturally alter the number and relations
of the strands. But as they all regularly take the curved course in
the way shown in Clematis, the vascular strands will appear in all
ordinary Dicotyledons to be arranged in a circle in the transverse
section. Within that circle lies the central column of pith. Out-
side it is a more or less broad band of cortex, and externally is the
epidermis. The pith and cortex thus embed the vascular system,
and are sometimes called collectively the ground-tissue. It is in
herbaceous Dicotyledons that these tissues are most easily studied,
such as the Bean, Sunflower, or Potato.

Stems of Herbaceous Dicotyledons.

The superficial layers of the stem in herbaceous Dicotyledons
show well-marked characters, and are relatively simple in construction
(Fig. 22). Starting from the outside, the epidermis appears as a

col) S

Chd

Fig. 22.

Superficial tissues of the stem of Potato as seen in transverse section. ( x 100.) ^^epi-
dermis ; d;/=outermost laver of the cortex containing chlorophyll corpuscles. co//=collen-
chyma ; <W = endodermis 'with starch-grains. The protoplasmic contents of the cells are
omitted on the right, so as to make the cell-walls appear more prominent.

single superficial layer of living cells, each with its lining of cytoplasm.
The outer wall of each cell is thickened, and covered externally by
a thin film of cuticle, which, being highly impervious to water, con-
trols the loss by evaporation. The epidermis thus forms a skin
projective both mechanically and physiologically. It is, however,

THE TISSUES OF THE STEM 43

interrupted here and there by breathing pores, or stomata ; but none
of these happen to have been traversed in the section drawn. They
serve for ventilation of the tissues, and will be described later
(compare Figs. 47, 48, 51).

Beneath the epidermis is a band of cortex, limited internally by the
endo dermis, which is the innermost layer of its cells. They are easily
recognised while young by the starch-grains which they contain (dark
blue with Iodine solution). The outermost layer of the cortex consists
of thin-walled cells containing green chloroplasts, while triangular
spaces occur where their walls meet. It is chiefly this layer that
gives the green colour to the stem. Beneath it lies a broad band of
cells with their cellulose walls thickened at their angles, which gives
them added strength. It is called collenchynta, and it passes over
gradually to an inner, thin-walled cortex, with cells of larger size, and
triangular intercellular spaces. These spaces are formed by splitting
of the cell-walls as they pass from the young to the mature stage, at
the angles where they meet. Since in life they are filled with gases
they form a connected ventilating system, which communicates with
the outer air through the stomata. The cells of the cortex are all
living cells, each with its protoplasmic lining. Gaseous interchange
is thus provided for them by the connected system of air-channels.
But this communication is cut off inwards, or at least restricted, by
the layer of the endodennis, in which the cells fit closely together, and
form a barrier between the outer tissues and those which lie within.
Thev are thin-walled, and in addition to the starch which thev contain
while young, they are characterised by a band of corky substance on
each radial wall, which being thrown into folds as the wall shrinks
on death of the cell, gives the appearance of a dark dot. The endo-
dermis is actually the innermost layer of the cortex, and it delimits
the tissues that lie within. The whole aggregate of these may be
styled the central cylinder, or more briefly the stele.

The cortex is a variable tissue. The Potato gives an example of the bulk and
character that is usual for herbaceous stems, in which the stele is relatively
large and the cortex narrow. In young woody stems the stele is smaller
in proportion at first, and the cortex usually broader, as it is in Clematis
(Fig. 21). Aquatic plants have a contracted stele, and the cortex is more
bulky still, with large intercellular spaces ; as in Hippuris, or the Pond Weeds.
On the other hand, in some Monocotyledons, and especially in the Grasses,
it may be extremely narrow, so that the stele takes up nearly the whole trans-
verse area (Fig. 29). It may also vary in constitution. Sometimes it consists
wholly of soft green parenchyma, as in Hippuris : or this tissue may be
associated in various ways with strengthening collenchyma : or sometimes

44

BOTANY OF THE LIVING PLANT

it may be hard and woody, as it is in the haulms of Grasses. It may also
contain occasional tissues, such as resin-passages (Sunflower, Ivy, Pine),
or milk-vessels and cells (Dandelion and Spurge, see p. 54). But however
variable, it is constantly present between the epidermis and the stele.

The stele comprises the ring of vascular strands, together with the
tissues that envelop them, and the central column of pith. The
pith is a bulky mass of thin-walled parenchyma, its living

Fig. 23.

Transverse section of a vascular strand of Scrophularia nodosa. e = endodermis.
/> = pericycle. />/r/ = phloem. s= sieve-tubes. c = cambium. ,ry = xylem. p.v.—
pitted vessel, px = protoxylem. ( x 150.)

cells resembling those of the inner cortex. Similar tissue may
extend between the vascular strands outwards to the endodermis.
A single strand of the stem of the Figwort thus isolated is shown in
transverse section in Fig. 23. It is typical of an herbaceous Dicoty-
ledon. The endodermis (e), indicated by its starch-grains, defines
the inner limit of the cortex. The vascular strand is roughly oval in
section, but it is not strictly circumscribed. It consists of two main
regions which differ in structure and in function. A softer region,

THE TISSUES OF THE STEM 45

which lies peripherally, is called the phloem, or bast (phi) ; a firmer
region, which lies next the pith, is called the xylem or wood (xy).
Between them are very thin-walled cells showing active division ;
this is the cambium (c). The transition between this and the wood
appears sudden in the drawing ; but the radial rows in which the
cambial cells are arranged may be traced inwards into the wood,
and also outwards into the bast, showing that all the tissues are
structurally related.

The most marked constituents of the xylem or ivood are radial rows
of tracheae, or vessels which are elements with thick woody walls. The
smallest are nearest to the pith ; these are the protoxylem (px), or
first-formed tracheae of the wood. The larger tracheae are pitted
vessels (p.v.), and the transition from one to the other is gradual.
There is no protoplasm in any of these, but they are embedded in
tissue of which at least some cells retain it, and are thus alive.
Near to the protoxylem the tissue is thin-walled, but towards the
cambium it is thick-walled and woody, so that here the xylem forms
a firm and coherent mass. The phloem or bast has no woody walls,
but is soft, with cellulose walls and protoplasmic contents. The most
marked constituents are the rather wide sieve-tubes (s), which are em-
bedded in thin-walled parenchyma. A layer of more or less definite
cells, called the pericycle (p, p) adjoins the endodermis internally.

A very incomplete idea is obtained by seeing the tissues only in
transverse section. It is like seeing the ground plan of a building
without the " elevation." The tissues must also be examined in
longitudinal section, so that each unit is followed throughout its
length, and the character of its lateral walls disclosed. If a longitu-
dinal section be taken in a radial plane through the stem of the Figwort,
so as to cut through the middle of a vascular strand like that in
Fig. 23, its appearance would be as in Fig. 24. To the right are the
external tissues, and the pith to the left. Starting from the epidermis
(epid) its cells are oblong, with their outer walls thickened and cuticu-
larised. A stoma has been cut through longitudinally (arrow). Below
is a thin-walled cortex, with intercellular spaces. There is no collen-
chyma here, and the cortex is a narrow band. It is limited internally
by the closely-fitting oblong cells of the endodermis (endod), with
numerous starch-grains, which marks off the cortex from the stele.
The vascular tissue shows up as a compact strand of more elongated
elements, the phloem being thin-walled, and the xylem thick-walled
and woody. The sieve-tubes are now seen to be long cylinders, tra-
versed here and there by oblique septa of beaded appearance. 1 hese

46

BOTANY OF THE LIVING PLANT

are the sieve-plates which are perforated, while the cytoplasm that

lines the tubes is collected in a mass above each plate. The tubes

are embedded in long prismatic cells of the phloem-parenchyma,

and in this stem no bast-fibres are present.

The cambium consists of long, very narrow cells, with thin walls

and dense protoplasm. In each radial row of them the cells are as

a rule of the same length, showing that they are the result of division

of a single parent cell.

capita endod epid

pith

xylera

phloem

-H

M

^

Fig. 24.

Median longitudinal section through a vascular strand of Scrophularia nodosa,
similar to that shown in Fig. 23. The arrow indicates the pore of a stoma, and points
towards the centre of the stem. ( x 150 )

The xylem is more varied in structure, and if the section happened
to have followed one of those radial series of vessels seen in the trans-
verse section, its appearance would be as shown in Fig. 24. Starting
inwards from the cambium, a series of fibrous tracheides would be
met. They are elongated and pointed, with thick lignified walls
bearing small pits. They surround and embed a larger pitted vessel,
which appears as a wide tube without any protoplasmic contents,
and is limited by a thick, pitted, woody wall. About half-way down
the section it is marked by a ring ; this indicates where an oblique
septum divided two of those cells from the fusion of which the vessel

THE TISSUES OF THE STEM 4;

was derived. That septum was in fact occupied by .1 large round

pit, of which the margin remains as the ring, while the thin pit-
membrane has been absorbed. Further inwards the vessels are
successively smaller, and show very characteristic markings. In this
particular case there is first another pitted vessel, but narrower,
with the pits arranged with some spiral indication. In the inner
vessels complete spiral bands of thickening are coiled within the
thin wall. In the successive vessels nearer the pith the spirals are
more loosely coiled, till finally there is no continuous spiral, but a
series of more or less regular rings (annular vessels). Such vessels
are characteristic of protoxylem, and they abut directly on the paren-
chymatous pith.

The structure of the protoxylem is a consequence of the earliest vessels
being developed while the stem is still growing in length. While the thickening
is being deposited upon their walls they are being constantly stretched, and
those earliest formed will be stretched most. The annular thickening appears
in these. The closer spirals of those formed later show that they have been
stretched less. Finally, where the stretching has been slight, the spirals run
together laterally, leaving only irregular pits between them. The thickening
rings and spirals are effective in keeping the cavity of the vessels open against
the pressure of the surrounding cells. On the other hand, a sufficient area
of thin pit-membrane is necessary to allow of the exit of water from the
vessels to them.

Stems of Aquatic and Climbing Plants.

The plan of construction of the vascular bundle of Scroplmlaria is
that general for Angiosperms. The bundles, however, vary greatly
in size and composition in different types, and this is closely related
to the needs of the plants in question. A marked variant is seen in
aquatic plants, where the need of water-supply is not pressing. There
the woody tissue is reduced sometimes to the vanishing point (Elodea) ;
in less extreme cases the stele is contracted, and the individuality
of the vascular strands is not maintained (Hippnris). In plants with
a climbing habit, on the other hand, the vascular strands are isolated,
with bands of soft parenchyma between them, while the wood-
vessels are large, and the phloem plentiful. A case in point is seen
in the Cucumber (Fig. 25). Here, though the vessels are few, their
radial rows can be traced, with the protoxylem directed centrally.
A few vessels of enormous size associated with pitted tracheides
replace the more numerous vessels of herbaceous stems. The most
marked modification is in the phloem, which is much more plentiful.
A large mass of it with numerous large sieve-tubes is seen in the

48

BOTANY OF THE LIVING PLANT

normal position ; but additional sieve-tubes are present also on the
side adjoining the pith. Cambial activity is evident between the
xylem and the outer phloem, and a few divisions are also seen between
the xylem and the medullary phloem.

The stems of climbing plants, and especially of the Cucumber and
Vine, have been habitually used to demonstrate the structure of
sieve-tubes, because there they are specially large. In Figs. 25, 26 the

Fig. 25.

Transverse section of a vascular strand of the Cucumber, showing plentiful
phloem both on peripheral and central sides of the xylem (bicollateral). The
vessels of the xylem are few, but very large. s/=sieve-tubes. £.w. = pitted vessels.
i># = protoxylem. c = fascicular cambium. ic = interfascicular cambium. ( x 75.)

sieve-tubes appear nearly circular in transverse section ; they are
limited by a cellulose wall, and a cytoplasmic lining invests the wall
during life. This readily contracts from the wall when the tissues are
cut across, and the internal pressure relieved. That is the state in
which they are usually observed. Associated with each is a small
companion-cell, often triangular in transverse section, and with dense
nucleated cytoplasm. Where a sieve-plate is included in the section
it will present a surface perforated by dot-like pores. The contents
are densely aggregated round the plate. Under a high power, when

THE TISSUES OF THE STKM

49

treated with solvents that remove the contents, and stained, the
structure of the perforated cellulose wall is well seen (Fig. 26). Longi-
tudinal sections show that the
tubes are partitioned at intervals
by transverse or oblique septa,
each of which bears a sieve-
plate that occupies its whole
area (Fig. 27) The cytoplasm
contracted in the preparation
appears as a thick cord which
widens out so as to cover the

sieve-area, wi

th which it is closely spim§^-£&g{
re are no nuclei in /ApaOD^

related. Thei

mature sieve-tubes. In fine sec-
tions suitably stained the con-
tinuity of the cytoplasm by
threads traversing the pores can
be easily seen. But another way
of demonstrating the continuity between the sieve-plates.

1 • •« „..i_i»,._- face view. 6 = phloem parenchvma

is by treatment with sulphuric y v

acid which destroys the cell-wall, while the more resistant cytoplasm

retains its outline (Fig. 27, D).

B

Fig. 26.

Phloem and cambium (cb) of Cucumber ( x 200).
c.c. = companion cells. s/=sieve-tubes cut through

s/>=sieve-plate in sur-
F.O.B.

IK'l/fili

_ 0 OQOOOB.

if ;

1 \ n

Fig. 27.

Sieve-tubes of Cucurbita Pepo. /l=surface view of a sieve-plate. B, C = longi-
tudinal sections showing segments of sieve-tubes. D = contents of a sieve-tube
after treatment with sulphuric acid, showing continuity through a sieve-plate.
s = companion cells. u = mucilaginous contents, pt =peripheral cytoplasm. c =
callusplate. f*=small lateral sieve-pit, with callus plate. ( x 540.) After Str.isburger.

B.B.

50 BOTANY OF THE LIVING PLANT

As a tube grows older a mass of callus substance is formed around
the cellulose framework (Fig. 27, A), embedding it and extending into
its pores, so that finally they may be quite closed. In most plants a tube
which has been thus closed does not resume its function {e.g. Cucumber)
(Fig. 27, C). But in some cases (e.g. Vine) the autumn-formed
callus, which is a readily soluble carbohydrate, may be re-absorbed
in the spring, and the sieve-tube resumes its activity. There is con-
siderable evidence that the transfer of metabolic materials, such as
sugars and amino acids, from one part of the plant body to another
takes place through the sieve-tubes (see Chapter VIII.). The function
of the nucleated companion-cells, which show great constancy of
occurrence, is still unknown.

The sieve-tubes are sometimes called bast-vessels because of the analogy
in development and structure between them and the vessels of the wood. In
both cases a number of cells fuse to form the vessel. In the wood-vessel the
walls separating these cells are occupied by one or more large pits. As the
walls thicken with woody deposits these thin pit-membranes break down,
while the original protoplasm is absorbed. The cavities of the cells thus
coalesce into a continuous tube, which is filled in life by sap, with or without
bubbles of gas. They serve as open channels of transit for water with sub-
stances in solution. But the distance through which they are continuous as
open tubes is usually limited to a few centimeters, though sometimes con-
siderably greater.

Similarly the sieve-tubes originate from a number of cells usually attached
end to end. The terminal walls bear the sieve-plates, each plate is thickened
in a reticulate manner, and the meshes are styled sieve-fields, which are
actually individual pits. Each of these is stopped when young by a pit-
membrane which is perforated by fine threads. These perforated membranes
are then completely absorbed, so that a thick rope of protoplasm replaces the
fine threads. Thus technically speaking the sieve-tube also is a cell-fusion.
But at maturity its walls still consist of cellulose ; the protoplasts lose the
nuclei they originally contained, and the tube is filled with a vacuolated
column of non-nucleated cytoplasm, which is continuous through the open
pores of the sieve. The analogy of their development with that of the wood-
vessels is close, but the contents and the function are different.

Stems of Monocotyledons.

In the Monocotyledons both the arrangement and the structure of
the vascular strands may differ from that in the Dicotyledons, though
the general plan is essentially the same. The cortex in Monocotyledons
is reduced, and the stele is distended, containing isolated vascular
bundles, but no cambium. The vascular strands are sometimes dis-
posed in a simple ring round a central pith, as in Tamus or Schoenus
(Fig. 28). In other cases their regularity is disturbed, the largest

THE TISSUES OF THE STEM

5i

of them lying isolated towards the centre, as in Molinia (Fig. 119,
p. 185). But in sections of Palms, Maize, and Sugar-Cane such

Fig. 28.

Young shaft of Schoenus nigricans cut transversely : centrally is the pith,
surrounded by an irregular ring of vascular bundles. The dotted patches near the
periphery are mechanical tissue. ( x 35.)

strands appear more numerous, and they are seen to be scattered
throughout the pithless stele (Fig. 29). This is a consequence of the

4tw ,4

.1/72

jtn

Fig. 29.
Transverse section of an internode of Zea.
Mais. pr = primary cortex. />c = peric\rle.
o> = vascular bundles. gc = conjunctive paren-
chyma ( x 2.) (After Strasburger.)

Fig. 30.
Diagram showing the course of the vascular
bundles of the "Palm type" of Monoco-
tyledons. The numbers indicate tlie suc-
cession of the alt. mating leaves. m= median
bundle. (After De Bary.) (From Strasburger.)

52

BOTANY OF THE LIVING PLANT

fact that each of the largest strands on entering the stem from the
leaf slants sharply inwards, but short of the centre it curves again
outwards, and gradually approaches the periphery. There it fuses
with other strands. As the strand is thickest in the middle of this
course, the consequence is that the strands appear fewest and largest
at the centre, and smaller but more numerous near to the periphery

Fig. 31.

Transverse section of a vascular strand from the internode of Zea Mais, a = annular
tracheid. sp = spiral tracheid. m, m' = vessels with bordered pits. v= sieve-tubes.
s= companion cells. cpr= compressed first tissues of phloem. /= intercellular space.
vg = sclerotic sheath. /= conjunctive parenchyma. ( x 180.) (After Strasburger.)

(Fig. 29). Thus the difference between the arrangement of the
strands in Dicotyledons and Monocotyledons is not fundamental.
As a matter of fact the one type graduates by intermediate steps into
the other.

Similarly the structure of the strand itself is on the same plan in
both, the most conspicuous difference being the absence of cambium
in the Monocotyledons. One of the larger strands towards the centre
of the stem of Maize shows features usual in them (Fig. 31). It is

THE TISSUES OF THE STEM 53

embedded in thin-walled conjunctive parenchyma of the stele, and is
surrounded by an indefinite sheath of sclerotic fibres. It consists of
xylem directed as in the Dicotyledons to the centre of the stem, and
phloem towards the periphery. The xylem is represented by two or
three annular or spiral vessels of the protoxylem (a, sp.)} adjoining
a large air-space (/), and two large pitted vessels (m, m'), with a bridge
of fibrous tracheides coupling them together. The number of these
vessels may vary, especially near to the nodes. Together they form
a V-shaped group, and the phloem is fitted between the limbs of
the V. It consists entirely of sieve-tubes (v), and associated com-
panion-cells (s).

Vascular bundles in which the xylem and phloem run alongside
one another, as in the stems of Dicotyledons and Monocotyledons,
are called collateral. The xylem is usually directed centrally, and
the phloem peripherally in the stem. The case of the Cucumber,
where extra phloem adjoins the protoxylem, is described as hi-
collateral (Fig. 25). When a cambium is present the bundle is
described as open (Figs. 23, 25), when cambium is absent it is closed

(Fig- 31).

The uniformity of the cylindrical structure of the stems of Flowering

Plants is very striking. The reason for it is to be found in the fact
that it satisfactorily meets the requirements. The stem has at once
to serve for the physiological transfer of material, and for the mechani-
cal support of the leaves and branches. The cylindrical form, or
even the hollow cylinder serves these purposes well. A parallel may
be drawn with bones. The marrow-cavity corresponds mechanically
to the pith of the stem, while in either case the harder tissue forms
an external cylinder. In the case of Birds, however, the bones may
be hollow, as in a Grass-haulm. The result in either case is high
mechanical strength combined with lightness (see Chapter X.).

In addition to the tissues thus described, which are generally distributed
in the stems of land-living plants, there are others of occasional occurrence.
These are often characteristic of certain families of plants. The most im-
portant of them are the laticiferous tissues, and various glandular, secretory,
and excretory cells or groups of cells, containing crystals, essential oils, and
other bodies. The laticiferous tissues consist of continuous tubes widely
spread through the parts of the plants that contain them, and filled with a
milky white, or sometimes red or yellow juice, called latex, which exudes
whenever the tissues are cut, or otherwise damaged. This latex coagulates
on exposure to the air, or on addition of certain chemicals. Rubber is
prepared from such coagulated latex, the foundation of the coagulum and of
Rubber being the complex hydrocarbon, Caoutchouc. Laticiferous tissue
exists in two different types. In the first the tubes result from the fusion of

54

BOTANY OF THE LIVING PLANT

many distinct nucleated cells, by the absorption of their septa : while the
contents which have previously assumed the character of the secretion flow
together, though their nuclei may still persist (Fig. 32, B). New branches
may also be formed which anastomose freely, so that a dense network of tubes
results. This type of latex-vessel is found in the Chicoriaceae, Campanulaceae
and Papaveraceae, and they are found also in Manihot and Hevea (the chief
rubber-yielding tree), among the Euphorbiaceae. The other type, occurring
in most of the Euphorbiaceae, the Urticaceae and Apocynaceae, consists of
much-branched cells which do not juse. They may be recognised in the young

A

Fig. 32.

A. — Diagrammatic section of the hypocotyl of Tragopogon, showing the position
of the latex-vessels relatively to the other tissues.

Z?.— Latex-vessels from a longitudinal section of a cotyledon of Tragopogon,
showing the absorption of the cell-walls in progress. (After Scott.)

embryo : they grow with the growth of the various tissues, branching fre-
quently, and thrusting their ends between the cells of other tissues. They
contain the milky latex and numerous nuclei, which divide actively near to
the growing tips of the tubes.

The function of the latex-tubes is uncertain. They have been regarded
as storage elements or as a channel for transport of organic materials through
the plant. Their contents include proteins and carbohydrates, caoutchouc
and resin. It is uncertain to what extent these materials can be again used
by the plant. The structure of the latex-tubes, and their close relation to
the mesophyll of the leaf and to the phloem of the vascular strands (Fig. 32, A),
lend some support to the suggestion that the tubes are connected with
transport.

Calcium oxalate is found as solitary crystals laid aside in cells scattered
through the tissues of many plants (Figs. 34, 47). But it may also take the
form of numerous long acicular crystals, or raphides, which lie parallel to one

THE TISSUES OF THE STEM

55

another in special cells (Fig. 33). The whole bundle is often enveloped in
mucilage which, swelling on any perforation of the cell from without, ejects
the crystals. Being sharply pointed they are found to serve as a protection
to the plants in which they occur against gnawing by
snails, and other animals. They are chiefly seen in the
Monocotyledons.

Secretory cells or cell-fusions containing essential oils
are characteristic of certain families. For instance,
those borne on the glandular hairs of the Labiatae,
which are the source of their aromatic scents : or the
sunken cysts seen in the Rutaceae : those in the rind
of the Orange are good examples of these bodies.

Woody Stems of Dicotyledons.

The apical bud is potentially capable of un-
limited growth and production of new leaves ; the
bud in the axil of each leaf may also grow into a
branch similarly endowed. Correlated with these
developments the mechanical and conducting
tissues of the shoot are augmented by radial
growth. This is particularly evident in plants
which continue growing for a number of years, and
thus attain large size. In them the increase in
girth takes place by secondary growth, through the
activity of the tissue called the Cambium. Cam-
bium is also present in herbaceous Dicotyledons,

1*

J

Fig. 33.
Cell of the

cortex
of Dratana rubra, rilled

but it is soecially active in enlarging the trunks and with mucilaginous matter

1 and containing a bundle

branches of shrubs and trees, rrom the bulky of raphides, r. (xi6o.)

, r ... . . . r ■ 1 (After Strasburger.)

column formed by its activity the superficial

tissues may be peeled off, separating with special ease in the spring.

The line of easy rupture is the cambium itself, and the reason why

it splits so easily is that in the spring it is actively growing, and its

cells are then thin-walled and weak. It will be necessary first to

examine this tissue in detail, since it produces such important

changes.

In herbaceous stems of Dicotyledons, such as the Sunflower, Fig-
wort, or Cucumber (Figs. 23, 25), the cambium is seen between the
wood and bast of the vascular strands. In wood' stems it occupies
a similar position, as in the Elm (Fig. 34). The difference is only
one of the proportion of the tissues, and of the activity of the
cambium. Where the strands are separate, as in herbaceous stems,
the cambial activity may be seen to bridge over the spaces between

56

BOTANY OF THE LIVING PLANT

the strands, thus completing the cambial ring. The cambium then
forms a complete cylinder. The parts within the strands are called
fascicular, and those between them inter -fascicular cambium [i.e.
Fig. 25). In woody stems where the strands are closely grouped,
this distinction is not so obvious.

Fig. 34.

Transverse section of a single vascular strand from the young stem of the Elm.
«=endodermis. sc=sclerotic pericycle. st = sieve-tubes. cc = cambium. p.v.=
pitted vessels. £*=protoxylem. mr= medullary rays. P=pith. ( x 150.)

The cambium is recognised by the repeated division of its thin-
walled cells by tangential walls, so that radial rows of cells are pro-
duced. As there is no limit to the repeated divisions, a great increase
of the tissues may be the result. The clearest evidence of division
is in the centre of each radial row, and it has been concluded from
careful comparison of many such rows that in each of them there is
an ultimate initial-cell, which retains its identity, giving off on the
one side cells formative of wood, and on the other cells formative
of bast. This has been styled SaniVs law of cambial division

(Fig- 35).

THE TISSUES OF THE STEM

?7

In transverse section each cambium cell appears oblong, with the
broader sides facing inwards and outwards, while the narrower run
radially (Figs. 25, 26, 35). If a section be taken longitudinally
through the cambium in a tangential plane, the cells appear with
pointed ends interlocked one with another (Fig. 36). If a radial
section be taken, so as to follow one of the radial rows, the individual
cells will appear long and narrow, with square ends (Fig. 24).

Cross section through a radial row of
cambial zone in Pinus sylvestris, after
Sanio. ( x 400.) H=side next the wood.
i = the conjectural initial cell. (From De
Bary.)

Fig. 36.

Tangential section through the cambium of the
Elm, showing the elongated form of the prismatic
cambium cells. w = thc groups of cambial cells
forming medullary rays. ( x 100.)

Putting together the results of these three sections, the form of the
cell as a solid body would be flattened prismatic ; it is placed with
its pointed ends directed up and down, and its broader faces inwards
and outwards. The cells have very thin walls, and plentiful cyto-
plasm, with a large nucleus. In fact they show the characters of
embryonic tissue. The cells given off from the initial cell, after
further division pass over gradually to the mature state, forming
additions to the tissues already present. Those which lie internally
are added to the wood, those externallv are added to the bast

58

BOTANY OF THE LIVING PLANT

outside the ring. If this process be continued, the structure of the
stem will become like that shown in Fig. 2>7, C-

2

Fig. 37.

Diagrams of secondary thickening in stem of Dicotyledon, based on transverse
sections of the hypocotyl of Ricinus. A represents the stem before origin of inter-
fascicular cambium. B, same after it has been formed. C, after it has produced
internally a broad ring of secondary wood, and externally a narrower ring of secon-
dary phloem. 2? = primary cortex. M=pith. £=phloem. #=primary xylem.
6 = bast fibres at periphery of phloem, fc = fascicular cambium. ic = interfascicular
cambium, fh = wood of primary bundle. i/fe = wood developed from inter-
fascicular cambium. ifp = phloem developed from interfascicular cambium. By
the intercalation of the secondary tissues the primary bast, b, b, b, is removed some
distance from the primary wood x, x. In C the principal medullary rays extend the
whole distance through the ring, the secondary rays only part of that distance.
(After Sachs.)

Three different results accrue from the growth of the vascular
column, viz. increased power of conduction, of storage, and of mechanical
resistance. Three different types of tissue are involved, and though
differing in detail, all of these are to be found as a rule both in the
internal woody column and in the external band of bast. The
tissues in question are, the vessel for conduction, the parenchymatous

THE TISSUES OF THE STEM

59

cell for storage and other functions, and the fibrous cell for mechanical
resistance. The forms of these various products of the cambial
cells, as seen in the stem of the Lime, are shown all to the same

scale in Fig. 38, i.-x.

When a large vessel of the wood is being formed, cambial cells in
a longitudinal row widen greatly, pushing aside the adjoining cells.
The lateral walls become thickened, and usually pitted, but the end
walls are absorbed, and the protoplasm disappears. Thus they

v/.

/v

I

F.o.S

Mm.

Fig. 38.

Cambium cell of the Lime, and its various products in the secondary wood and
secondary bast, all drawn to the same scale, and seen in tangential section. 1.-
nucleated cambium cell. ii. = fibrous tracheid. iii. = group of cells of wood-
parenchyma, iv. v.=single lengths of vessels, the oblique terminal walls having
been absorbed, vi. vii. = wood fibres, bent to save space in the figure, via. = group
of bast-parenchyma. ix. = single length of sieve-tube, with oblique terminal walls
perforated. x.=bast fibre bent to save space in the figure. ( x 75)

become tubes for transit of the sap that fills them. As they have lost
their protoplasm they are dead elements. Smaller vessels develop
similarly, but with less disturbance of the neighbouring cells (iv. v.).
When wood-fibres are being formed the cambial cells elongate, and their
pointed ends bore their way upwards and downwards, with a sliding
readjustment among themselves. As considerable tracts of cells may
develop thus alike, and as the cells themselves take a more or less
sinuous course, they become interlocked, almost like the strands of
a rope. At the same time their walls become greatly thickened, and
woody, and their protoplasm disappears. Their function is thus not
vital but mechanical (vi. vii.). When wood-parenchyma is being formed

60 BOTANY OF THE LIVING PLANT

the cambial cells widen and undergo divisions, transverse and some-
times longitudinal, into a number of square or oblong cells. Their
walls become thick, woody, and pitted, but the cells retain their
cytoplasm and nucleus. They are living cells, and are often stored
with starch (iii.). Analogous changes occur in the maturing of tissues
of the phloem. When sieve-tubes are being formed one or more sieve-
plates appear on the oblique terminal walls of the cambial cells ; the
cytoplasm is continuous through these, and they act as bast-vessels
for transit of plastic materials (ix.). The formation and function
of bast-fibres, if present, and of bast-parenchyma, corresponds in
essentials to that of the fibres and parenchyma of the secondary
wood (viii. x.).

One other component of the vascular column remains to be de-
scribed, viz. the medullary ray. The name is derived from the fact
that in transverse section radial lines of tissue, which look structurally
like the medulla or pith, run part or the whole way from the cambium
inwards through the wood, and are also continued outwards through
the bast. Such rays are narrow plates of tissue, and though they
extend far in a radial direction, they are continued only a short distance
up or down (Fig. 39). They are composed of brick-shaped cells with
their longer axis horizontal ; these cells often have thick pitted walls,
and many of them retain their protoplasm and nucleus. They link up
with the parenchyma of wood and bast, forming a connected system
of living tissue extending inwards and outwards from the cambium
(Figs. 40 and 41). The rays appear as bright streaks in the mature
wood, and are the silver grain of carpenters. Two types of rays may
be distinguished : primary rays, which intervene between the original
vascular strands, and extend the whole way from cortex to pith ;
and secondary rays, which are initiated subsequently in the cambium,
as the stem increases in bulk. These extend only part way through
the vascular ring. The tissue of the rays is derived from special
cambial cells, which are readily recognised in transverse sections by
the fact that they are not so narrow radially as the cells of the ordinary
cambium (Fig. 41). The form of the rays, and of the cells com-
posing them, should also be observed in tangential sections (Fig. 36),
in which it may be seen that minute triangular intercellular spaces
occur where three cell-walls meet. Thus a ventilation-system extends
through the rays inwards into the vascular trunk. Examined in
radial longitudinal sections it is apparent that the rays are, as their
name implies, narrow plates of tissue extending far radially, but
only a short distance vertically (Fig. 39). They serve as means of

THE TISSUES OF THE STEM

61

ventilation, and of interchange of materials in a radial direction
through the vascular column.

The complete ring of cambium is capable of indefinite activity of
growth and cell-division. Cells produced on its inner margin develop
as wood-vessels, wood-fibres, or wood-parenchyma by changes such
as those above described, and they form additions to the internal
column of wood. Cells produced on the outer margin of the cambial
zone develop as sieve-tubes, bast-fibres, or bast-parenchyma, by

1

sm

Fig. 39.

Medullary rays of the Pine (em, tm, sm), seen in a radial section through the
cambial region (c). Phloem to the left, xylem to the right. ( x 240.) Strasburger.

changes such as those above described, and they form additions to
the external zone of phloem or bast. The internal additions are the
more active, so that the woody column grows more rapidly than the
bast. This is suggested by Fig. 37, C, and the proportions are actually
shown for the Lime in Fig. 40. The relations of the wood and
bast to the cambium, and that irregularity of arrangement of the
several constituents which is usual for Dicotyledons, are suggested
in detail in Fig. 41. The chief bulk of the wood and bast consists
of tissue-elements elongated in a vertical direction. This structure
would make radial interchange a difficulty were it not for the fact
that the medullary rays penetrate radially inwards and outwards

62

BOTANY OF THE L1YIXG PLANT

from the cambium, and thus facilitate radial communication. All
the arrangements in the woody trunk are such as to admit of develop-
ment being indefinitely continued. The woody column increases by
successive additions from without : the bast by additions from within.
The former presents no mechanical difficulties : but the continued
addition of tissue from within will tend to stretch the outer-lying

M.R*$-r-^

Fig. 40.

Transverse section of stem of Lime, cut in the spring of its fifth year. Drawn
by Dr. Thompson. P = pith. XyJ-Xy.v xylem of first to fifth season. M.R.=
medullary ray. Sp.W. =spring wood. A u.W. = autumn wood. P/t/.= phloem.
Cfc. = cambium. Cor. = cortex. C/c. = cork. (xn.) The small outlined square of
tissue is represented on higher scale in Fig. 41.

layers of the bast, cortex, and epidermis. The effect of this is seen
in the Lime, where the medullary rays widen in the phloem into
broad parenchymatous masses, which intervene between the wedges
of bast. Thus the stretching of the outer layers is indirectly provided
for (Figs. 40, 41. M.R.). But in other trees the difficulty may be
overcome in other ways (see p. 65).

In temperate climates the activity of the cambium is interrupted
each winter. This leaves its mark on the woody column in the form

THE TISSUES OF Till. STEM

63

of annual rings, each of which represents the increment of growth
of one year. Consequently it is possible by counting them to estimate
the age of the trunk (Fig. 40). The reason why these rings are recog-
nisable is that the wood formed in the spring has larger and more
numerous vessels and on the average thinner walls than that formed
as the season advances. The physiological advantage of this may be

m.r.

auWs

Fig. 41.
The tissues in the small outlined square in Fig. 40, magnified 200. au.W = autumn
wood of fourth season. sp.W = spring wood of fifth season. c6 = cambmm. phl.=
phloem. *u.r. = medullary ray. V, V = vessels of wood. S = sieve-tubes.

that the arrangement ensures an increase in the water-conducting
tissues in spring, called for by the new leaves; but in the later
part of the season, when nutrition is high, more material is spent in
thickening the walls, thus adding to the mechanical strength. The
result of continued development of a woody trunk in the manner
described is to give a column constructed after the plan shown in
Fig. 42, where the relation of the annual rings and medullary rays
is clearly seen.

64

BOTANY OF THE LIVING PLANT

As trunks grow old the colour and quality of the central wood changes in
many trees, but not in all. It becomes darker in colour, and harder, and it
is distinguished as heart-wood. It is prized by joiners for its strength and
durability, as distinct from the more superficial sap-wood, which is paler in
colour, softer in texture, and more liable to the attacks of vermin, or fungi,
when used in joinery. The change from sap-wood to heart- wood follows on
the death of the wood-parenchyma and medullary rays. The sap-wood, as
its name implies, is functional for conveyance of sap and for storage. The
heart-wood being dead serves only the purpose of mechanical support. It is,

I* ~h

Fig. 42.

Wedge cut out of a four-year-old stem of Pine, in winter. Though the Pine is a Gymno-
sperm, the construction of its trunk is on the same plan as that seen in Angiosperms.
q= transverse view; /= radial view; t = tangential view. /=spring,s = autumn wood. m=
medulla. £ = protoxylem. h, h = resin-passages. 1, 2, 3, 4 = successive annual rings. ms=
medullary ray in transverse, ms', ms", in radial, ms", in tangential view. c= cambium.
fe=bast. 6r=bark. ( x 6.) Strasburger.

however, liable to be attacked by certain fungi in the living tree, which bring
about its decay. Trees hollowed by such means, though mechanically
weakened, retain their external sap-wood and cambium, and so possess in
the more superficial tissues all that is otherwise necessary for normal life.

The occurrence and proportion, as well as the mutual arrangement of the
component tissues are variable in different stems. It is this that gives the
characteristic qualities to their wood and bast. Thin walls, and relatively
few fibres result in soft wood, as in the Lime. Thicker walls, and numerous
fibres grouped in solid masses give a hard wood, like that of the Oak or
Laburnum. Fibres may be absent from the bast, as in the Currant ; or they
may be present in large numbers, forming irregular masses, as in the Lime,
which gives the " bast " for tying up garden plants. The grouping among
themselves of the several tissues composing the wood and bast appears

THE TISSUES OF THE STEM 65

at iirst sight confused. To secure conduction, mechanical strength, and
storage it is necessary that each of the tissues that meets these needs must
be continuous in order to be effective. There is no general or regular rule of
their arrangement for all stems. The complex problem is solved by different
trunks in different ways. A careful microscopic analysis of the masses of
wood and bast is necessary for the understanding of any individual case.
But such analysis shows, for instance in the wood, that the three tissue-types
form each their own connected system, though all are fused together into the
woody column. The clearest example of this is found in the medullary
rays, which are intimately related to the parenchyma of wood and bast, and
thread together these apparently isolated tracts radially, so as to form a
connected living system of storage-cells, which finds its inner limit at the
barrier of the dead heart- wood.

The growing vascular column is covered externally as it expands
by the cortex and epidermis. These must necessarily yield in some
way to the increase within. Sometimes they simply stretch, and
this is usual in most stems for a time. But as a rule the epidermis
and some of the cortex dries, and peels off, owing to the formation
of cork. The nature and function of a corky tissue is that it forms
an impervious barrier to the passage of water. As bottle-cork it is
used for this purpose. In the plant a layer of cork, wherever formed,
will cut off any tissues outside it from physiological interchange with
the tissues within. The first layer may be succeeded by other layers
formed more deeply, and cutting off successive bands of deeper
tissue. The layers may encroach into the phloem, from which
successively the outermost, that is the effete layers, would thus
be cut off by an impervious barrier from the active tissues within.
All tissues lying outside the innermost cork-layer are called collectively
Bark, which is consequently a dead tissue, including it may be
epidermis, cortex and phloem. As the stem continues to grow, the
bark being dead does not keep pace with it, but splits into fissures,
or peels off in scales. Thus the characteristic appearance of the
fissured trunks of the Oak or Elm may arise, or the scaly surface of
the boles of the Scots Pine or Sycamore. The thick masses of bark
in old stems give also mechanical protection, while they form a non-
conducting barrier against excessive heat, as witnessed by the survival
of Australian Eucalypts after forest fires.

Cork may originate from the epidermis (Apple, Sorbus), but more
commonly from the outer cortex, often from the layer immedi-
ately below the epidermis (Elm, Birch, Figs. 43, 44). Divisions appear
by walls parallel to the outer surface, and are repeated so that
from each parent-cell a row of cells is produced. A certain cell in
each row remains narrow and thin-walled, and it continues to grow

B.B. E

66

BOTANY OF THE LIVING PLANT

and divide, acting as a cambium-cell. The cells thrown off from it
on the inner side take the characters of additional cortex ; those on
the outer side enlarge, their cell-contents are absorbed, and their

Fig. 43.

Diagrams illustrating successive steps in the formation of cork in the cells (2)
directly below the epidermis ( 1) . A shows the first division. B shows the state after
repeated divisions, resulting in a radial row of cells. Of these the outer (a) are
cork ; the innermost (c) are phelloderm, which adds to the cortex, (b) represents
the cork-cambium.

thin walls are changed to impervious cork (Fig. 44 : parts right and
left of the drawing). As the cells fit closely without intercellular
spaces, they form a complete protective covering. Partly owing to
the growth in bulk within, but chiefly because the cork cuts it off

Fig. 44.

Transverse section of the stem of Elder, traversing a lenticel. £=epidermis.
£/t=phellogen, or cork cambium. Z=spongy cells filling the lenticel. ^=phellogen
of the lenticel. ^>d=cork, or phelloderm. ( x go.) Strasburger.

from the living tissues, the epidermis soon dries up, splits, and peels
away.

The covering of cork having its cells closely fitting together is not
only impervious to water, but also to gases. Thus the living tissues

THE TISSUES OF THE STEM 67

would be cut off entirely from the outer air, were it not for interrup-
tions of the continuity here and there. These are called lenticels,
and they may be seen with the naked eye on most stems, as brownish,
slightly swollen spots (Fig. 6, p. 13). A lens shows that here the
epidermis is split, and that powdery tissue lies within. Microscopi-
cally it is seen that in place of the closely fitting cork-cells those of the
lenticel are rounded, with intercellular spaces, so that the tissue is
spongy, and allows ventilation into the cortex (Fig. 44, /.). The
lenticels remain for years, and may grow to a large size, as may be
seen on the surface of many woody trunks, where they often
determine the position of the fissures of the bark. The brown
crumbling spaces in bad bottle-corks are the lenticels, which traverse
the otherwise impervious cork of the Cork-Oak.

The stem of a Dicotyledon showing secondary thickening as thus
described is mechanically a stable structure. Its form is that of a
cone with its base at the level of the ground. There it often widens
out into a broad " stool," which helps to give it stability. Many
large trees of the tropics form radiating buttresses at their base,
which are still more effective. The stems of Monocotyledons are
constructed differently. Most of them do not increase in bulk at the
extreme base ; but developing stronger above than below, assume the
form of an inverted cone, with its apex at the level of the soil.
This unstable structure is propped up by roots, which act like oblique
struts ; this may be very clearly seen in large plants of the Maize, or
of the Screw Pine. In many large Palms, however, the base of the stem
distends with age. This is due to a general expansion of the con-
junctive parenchyma, which may be accompanied by the formation
of additional deeply-seated vascular strands. In some few Mono-
cotyledons, however, as in Dracaena, there is a cambial increase. It
arises in the pericycle, outside the primary bundles, and it forms
new vascular strands which are closed, and embedded in a sclerotic
parenchyma. Physiologically the result is the same as in Dicoty-
ledons, but it arises in a different manner (see Chap. XXXVI, p. 590).
Those who have followed the foregoing description of the woody
Dicotyledons will see how admirably the trunk meets the requirements.
Their shoot-system is constructed on a scheme of indefinite expansion,
consequent on continued apical growth and branching. The demands
that will be made upon the trunk and branches are well illustrated
by any Sycamore or Beech tree exposed to a wind in early summer,
after the leaves are expanded and the tree is in full flower. The strain
of the wind-pressure is transferred from the leaves to the twigs, and

68 BOTANY OF THE LIVING PLANT

from them downwards to the branches, and finally to the cylindrical
trunk. It culminates towards the base, where there is the greatest
leverage. The butt must stand firm from whatever quarter the wind
may blow. It cannot yield as the branches can without loosening its
hold on the ground. Actually it is here that the trunk is thickest.
As it tapers upwards it becomes more pliant, and yields to the wind :
but a condition of successful resistance is the perfect recovery of
trunk, branches, and twigs when the wind falls. The mechanical
effectiveness of the tissues within the several parts is demonstrated by
any tree that stands erect and unbroken after a wind.

It is a familiar fact that a detached twig soon wilts by evaporation,
and more quickly in a wind than in still air. It requires water in
order to recover. In a tree exposed to the wind the young shoots
normally retain their firmness. During the wind they will have been
exposed to more than normal loss by evaporation, but still they are
firm, showing that they have been supplied with sufficient water to
make good their loss. The trunk, and the distributing agency of its
branches and twigs have carried out their water -conducting function.
Clearly the water-conducting system meets the requirements.

The rapid production of leaves and flowers in early summer is in
itself evidence of the storage capacity of the trunk and branches.
The material required for their formation is gained previously. The
rapid development in the spring depends upon its transfer from the
storage tissues in the trunk and branches to the buds that wTere
dormant during the winter. Thus the tree demonstrates by results
obvious to any observer the efficiency of the tissues of the trunk and
branches for mechanical resistance, water-conveyance, and the
storage and transfer of materials. Not only this, but also the method
of thickening of the stem is such that it meets the growing demands
of the enlarging plant. Finally, the adjustment of the surface tissues
to the increasing bulk is peculiarly effective. Not only does the de-
velopment of cork, and of that heterogeneous covering of bark, give
protection to the surface of the increasing stem, but it provides for
the removal of effete tissue. The old phloem, with its cells charged
with tannin, crystals of calcium oxalate, and many other substances
no longer required, together with old and collapsed sieve-tubes,
would be a useless burden to the stem. It is cut off by cork, and shed
with the decay of the outer layers of bark. The trunk that shows
such features as these is highly organised indeed. It is characteristic
of those plants which are recognised as the most advanced, viz. the
Seed-Bearing Plants, and particularly the Dicotyledons.

CHAPTER V.

THE LEAF.

Everyone knows roughly what is meant by the term Leaf. It is
commonly a flat, stalked structure, which exposes a large area of
green tissue to the light and air, usually in a more or less horizontal
plane ; and in a great many of our native plants it falls off in the
autumn by detachment at the base of the leaf-stalk. But this general
conception of the leaf is not of universal application, and it does not
define the leaf in a scientific sense. The essential features of the leaf
are not found in its form or direction, or in its detachment in autumn,
for all of these are liable to vary. The features that are constant for
leaves, and so define them morphologically are, that they arise as
lateral outgrowths from the apical cone, that they spring from the super-
ficial and underlying tissues, and appear in acropetal succession ; also
that they do not repeat the characters of the shoot itself. It follows
from their origin that their tissues are continuous at the base with
the tissues of the axis. The conceptions of axis and leaf are cor-
relative : the axis is a spindle bearing leaves, and the leaf is a
lateral appendage upon the axis. Axis and leaves together constitute
the Shoot. This conclusion holds notwithstanding that both stem
and leaf may vary greatly in form and proportion. Particularly the
leaf is variable in outline : it may have a stalk or petiole, which bears
the blade or lamina at its end : or there may be no stalk, and the blade
is then seated directly upon the axis. Further, the blade itself shows
the greatest variety in outline ; it may be a simple flat expansion
(entire), or it may be cut up in various ways into parts (pinnae), and
these may be again subdivided. We need not follow this further than
to recognise the fact that the outline of the leaf varies greatly. It
may even show differences in the individual, as is seen on comparing
the bud-scales and foliage leaves of any ordinary plant with those

69

70

BOTANY OF THE LIVING PLANT

leaves of the floral region which, however diverse in appearance, are
all none the less leaves (see Figs. 260-264, pp. 349-351)-

The leaf differs habitually from the stem in its symmetry, though
this difference does not apply to all cases, and cannot be held as in
itself distinctive of the one from the other. The stem is usually radial
in symmetry, being developed equally all round. But the leaf is as
a rule a flattened organ, showing what is described as dorsiventral
symmetry, and having more or less clearly defined surfaces. In most
leaves one face is turned upwards to the sky, and the other down-
wards ; but this is by no means constant. It is important for
clearness of description to distinguish these two faces by some more
constant character than that of direction up or down. The most
constant is their relation to the axis which bears the leaf. One of the

faces is directed in the young bud towards
the axis which bears it ; it is therefore
defined as the ad-axial, though as it is
usually directed upwards it is often called
the upper surface. The other faces away
from the axis in the bud, and is defined as
the ab-axial, and as it is usually directed
downwards it is styled the lower. But the
terms " upper " and " lower " are merely
descriptive of the positions that are usual :
they are not scientific definitions as the
others are. The Common Garlic shows how
necessary this precision is, for in most of its leaves, by a twist of
the leaf-stalk, the adaxial face is turned downwards.

Since the tissues of the leaf are continuous at the base with
those of the stem, the constitution of the petiole may be expected
to resemble that of the stem. But its form is more or less
flattened or channelled on the upper (ad-axial) surface. This is in
accordance with the mechanical requirements ; for it has to support
the weight of the blade, and hold it with some degree of firmness
in its horizontal position. The necessary strength is secured by
a form which in transverse section would appear semicircular, or
Semilunar, as it is in such a leaf as the Sunflower (Fig. 45). In
this case it is traversed by three large vascular strands, which
with some smaller strands are arranged in a semicircle. The
surrounding tissues are essentially like those of the young stem, but
there is no stelar tract defined by a general endodermis ; here each
strand is surrounded by its own sheath. The xylem of each strand

Fig. 45.

Transverse section of the petiole
of the Sunflower. ( x 6.)

THE LEAF

71

as it curves out from the stem is directed upwards (ad-axial), and
the phloem downwards (ab-axial), and these relative positions are
regularly maintained throughout the leaf (Figs. 20, 47). The petiole
appears to be structurally little more than a means of junction between
the axis and the blade. Its presence brings two advantages : that
the blade is carried some distance outwards from the stem, and
thus the probability of one leaf overshadowing another is avoided ;
and secondly, the narrow petiole allows the blade to yield to the
pressure of wind, instead of rigidly resisting it.

Fig. 46.

Skeleton of the lamina of Ivy. Natural size.

It is the blade, or lamina, which is the distinctive feature of the
foliage leaf. As shown in such common types as the Sunflower,
Dahlia, Cabbage, or Sycamore, the blade consists of a skeleton or
framework of thickened and mechanically firm ribs, which support
an expanse of relatively thin and delicate tissues. A prominent
mid-rib runs up continuously from the leaf-stalk to the tip of the
blade, and branch-ribs of successively smaller size pass off from it
towards the margin. On removing the softer tissues by reagents,
the vascular system can be demonstrated as a leaf-skeleton (Fig. 46).
It is then seen that vascular strands of the midrib and of
the stronger lateral ribs, give off thinner lateral branches ; that
smaller branch-veins pass off from these, and with further ramifica-
tions and many fusions form a fine network traversing the thinner
areas of the blade. Such a reticulate venation is characteristic of

72

BOTANY OF THE LIVING PLANT

the leaves of Dicotyledons. In Monocotyledons the main veins
run parallel to one another, but still they are connected laterally by
transverse branches. Thus the vascular system, of which the leading
function is conduction, is very effectively placed for carrying out its
purpose : for it is connected below with the conducting system of
the stem, and it spreads throughout the blade, and reaches to its
extreme tip and margin.

A superficial examination gives only a very imperfect idea of the
structure of this important part of the plant. Transverse sections

CLeC ax.

Transverse section of the midrib of the leaf of Aspen (Populus tremula), extending
to the thinner expanse of the blade right and left. *<£=upper epidermis. Ze = lower
epidermis. />£ = palisade parenchyma. s£=spongy parenchyma. coZZ = collen-
chyma. #y = xylem. £/iZ = phloem. sZ=stoma. Note numerous crystals of
calcium oxalate, ada* = adaxial or upper, a6ax=abaxial or lower leaf-surface.
( x 75-)

through the thin lamina so as to traverse one of the ribs trans-
versely, reveal the characteristic structure upon which the functional
activity of the foliage leaf depends (Fig. 47). The upper (ad-axial) face
is easily distinguished from the lower (ab-axial) by the fact that the
larger veins project strongly from the latter surface. Where the vein
is traversed it shows on a reduced scale the same construction as the
petiole, and like it the ribs are often strengthened by collenchymatous
tissue, as is seen in the Aspen. Here again the xylem of the vascular
bundle is directed towards the upper (ad-axial) face, and the phloem
towards the lower (ab-axial). The tissues which compose the thinner
expanse of the lamina bear definite relations to these opposed surfaces,
and to the incidence of light upon that which faces upwards. They
will be described, starting from the upper (ad-axial) face, as they might

THE LEAF 73

be seen in the leaf of the Aspen, or as shown more in detail in the
Privet (Fig. 48).

The upper surface is covered by a continuous layer of epidermis,
composed of oblong cells with their outer wall thickened and cuticu-
larised, better seen in Fig. 52, p. JJ. The lower surface is similarly
covered by epidermis, but the continuity of its cells is here and there
interrupted by pores (stomata), which allow communication between
the outer air and the intercellular spaces within. Between these two
epidermal layers lies the tissue of the mesophyll. Towards the upper
surface its cells are arranged with some degree of regularity, frequently

Fig. 48.
Transverse section of lamina of Privet [Ligustrum vulgar e). ab.ax. = the abaxial
or lower surface. The protoplasts are omitted on the left-hand side. u.e. = upper
epidermis. I.e. = lower epidermis. i.sp.= intercellular space. v= vascular strand.
sJ=stoma. ££ = palisade parenchyma, s.p. =spongy parenchyma. cry = crystal.
( x 75.) The palisade parenchyma is poorly developed in the section of this shade-
loving leaf. Compare Fig. 47.

in two layers. The cells of these layers are of oblong form, and stand
parallel with one another, the ends of the outermost layer abutting
on the epidermis. From their form and arrangement they are called
the palisade-parenchyma. Towards the lower surface the cells are
less regular in form and arrangement, and as the intercellular spaces
are very large and numerous this tissue is described as the spongy
parenchyma. The whole mesophyll is composed of thin-walled cells,
with living cytoplasmic lining, a nucleus, and very numerous discoid,
green Chloroplasts. To these is due the full green colour of the
leaf. The intercellular spaces, so conspicuous in the spongy paren-
chyma, are continuous though of smaller size between the palisade-
cells, and they connect with the stomata. A specially large space is
usually present opposite the pore of each stoma. The whole mesophyll

74 BOTANY OF THE LIVING PLANT

is thus permeated by a ventilating system of air-channels, which may
communicate through the pores of the stomata with the air outside.

Most foliage leaves have such a structure as that described. But
the leaf-blade of different plants fluctuates almost as much in the
details of its internal construction as it does in its outline : this may
even be seen in some degree in those of the same individual plant.
Leaves may vary in thickness from the delicate, almost filmy leaf of
shade-loving plants, to the leathery texture of those exposed to the
sun in dry climates. The leaves may be smooth in surface, as is usual
in water-plants, or covered with rough hairs like the Sunflower, or
with a dense woolly protection like the Alpine Edelweiss. In-
ternally they may have only a single layer of palisade parenchyma,
as in most shaded leaves ; or two, which is common in leaves exposed
to the full sun ; or more. They may be strengthened by mechanically
effective tissue, often placed just below the upper epidermis (hypo-
derma), as in the leathery Cherry-Laurel: or distended by water-
storage cells, usually occupying the middle of the leaf, as in the
succulent Stonecrop, or Aloe. Notwithstanding such differences
as these, and many others, their construction is as a rule based upon
the same essential scheme as that described.

The leaf is the chief organ of nutrition in green plants. An essential
point of structure to this end is the perforation of the epidermis by
the stomatal pores, for this gives the opportunity of gaseous inter-
change between the intercellular spaces and the outer air. Stomata
occur here and there in the epidermis of the stem and petiole, and
even upon the various parts of the flower. But it is on the surfaces
of the leaf -blade that they are found in the greatest numbers. Some-
times, as in the Sunflower and many herbaceous plants, they are
present on both sides of the blade ; but frequently they are fewer on
the upper epidermis, or even absent, as in many woody plants. They
are often very minute, especially in the Dicotyledons, so that large
numbers, usually 1 00 to 300, may be counted on each square millimetre
of surface. The following table gives the result of countings per sq.
mm. on the leaves named :

Lilac ....
Oak ....

Apple -
Water Lily-
Wheat -

Examined microscopically in surface view the epidermis of a
Dicotyledon appears as a film of tabular cells, often with sinuous

Upper.

Lower.

IOO

I50

0

346 .

0

246

460

0 (submerged)

47

52

THE LEAF

7?

outline (Fig. 49). The stomata form part of the epidermal layer,
but their cells differ in form from the rest. Each stoma is composed
of two guard-cells attached by their ends, so that between them there
is a pore that may be either open or closed. In a microscopic pre-
paration of a living epidermis taken on a sunny day, and mounted
in water, the pores, governed by the still living cells, will be seen to

Fig. 49.

Part of the lamina of Tropaeolum, seen as a transparency, in surface view from
above ; showing the sinuous outline of the epidermal cells, with stomata. Below
the epidermis the palisade-cells are seen end-on, with large intercellular spaces,
especially below the stomata. The vascular veins are more deeply shaded. ( x 175.)

be widely open, as they are represented to be in the drawing of
the leaf of Tropaeolum. Access is thus readily given to the inter-
cellular spaces within. Fig. 49 further shows the mesophyll visible
through the transparent epidermis. The palisade-cells are here
seen end-on ; and it is more apparent than in transverse section
how well ventilated this tissue actually is. Almost all its cells touch
another cell laterally ; but in all of them a very considerable propor-
tion of the wall-surface is freely exposed to the gases contained in
the intercellular spaces. These spaces are specially large in the

76

BOTANY OF THE LIVING PLANT

neighbourhood of the stomata. Lastly, part of the vascular network
is shown dimly in a still lower plane. The strands lie between the

palisade and the spongy paren-

A.

B

chyma, and the conducting
system is thus in near relation
to the bases of the oblong pali-
sade cells.

In the Dicotyledons the
stomata are relatively small,
and irregular in their orienta-
tion. They are shown in section
in Figs. 47, 48, and in surface
view in Fig. 49. In Monocotyle-
dons they are usually larger,
and their orientation regular.
The large stoma of Narcissus
serves as an example (Fig. 50, A).
Its guard-cells have a charac-
teristic outline, each with a
projecting ridge on its oblique outer and inner walls. The stoma
is here slightly sunk below the outer surface of the leaf (Fig. 51). The

Fig. 50.
Drawings from the same Stoma of Narcissus, in
surface view. A , in the open ; B, in the closed state.
F. O. B. ( x 250.)

Fig. 51.
Stoma of Narcissus in transverse section, showing its relation to the adjoining
epidermal cells and to the mesophyll below. Two of the cells of this tissue are
drawn in detail, that to the right as seen in surface view from without ; that to the
left in optical section. The chloroplasts are black. ( x 300.) b.O.ti.

level of the stoma relatively to the leaf-surface varies in different
types according to their habitat. In plants of temperate climates

THE LEAF

77

it sometimes projects, or lies about level with the general surface.
But in plants of dry climates it is apt to be sunk inwards. This

Fig. 52.

Stoma of Aloe depressed below the well-developed epidermis. The thick cuticle

is shown black. ( x 300.) F. O, B.

is seen in slight degree in Narcissus (Fig. 51), but more distinctly
in Aloe (Fig. 52), a succulent plant with strongly cuticularised

Fig. 53.

Part of a transverse section of the xerophytic leaf of Hakea, showing a stoma
greatly depressed below the well-developed, and cuticularised epidermis, which is
propped out by thick-walled sclerotic cells. ( x 150.) F. O. B.

epidermis. The stomata themselves are of the same type as Narcissus,
but seated at the bottom of deep pits. A more extreme case is seen

78

BOTANY OF THE LIVING PLANT

in Hakea (Fig. 53). Additional control over evaporation of water

is gained by this means (Chapter XL).

Seeing that the epidermis serves for protection, and for regulating

the ventilation of the leaf, both functions of a secondary character,

it seems clear that the mesophyll is the tissue of prime importance.

The cells of the palisade-parenchyma
are oblong in form, and each is bounded
by a thin cellulose wall, which is
rounded off at the angles so as to pro-
rp vide the intercellular spaces (Fig. 51).
The cell-wall is lined internally by a
film of cytoplasm, within which is a
large vacuole filled with sap. The
nucleus may be suspended in the middle
of the cell, but more frequently it
is embedded laterally in the cyto-
plasm. The most marked features of
a these cells are the chlorophyll-corpuscles,
or chloroplasts, which are discoid in
form and of a full green colour. They
are always embedded in the cytoplasm,
and are as a rule so placed that one
flattened side faces to the cell-wall,
the other to the central vacuole. The
cells of the spongy-parenchyma re-
semble the palisade cells in all essential
points except in their form, which is
very irregular, and in the fact that
their chloroplasts are fewer (Fig. 48).

Fig. 54-
Varying positions taken by chlorophyll

As the cytoplasm has been seen to be

grains in "the cells of Lemna trisnlca under capable of movement, it can alter the
illumination of different intensity. T, in r .

diffuse daylight. S, in direct sunlight, position of the chloroplasts embedded in

N, at night. The arrows indicate the ■. Ar>art from anv other disturbing cause
direction of the light. (After Stahl.) ir- APart Irom anY OUier aibxurumg tdUbe,

they collect at the cell-surfaces adjoining
the intercellular spaces. Their movements are regulated by external con-
ditions, of which one is the aeration of the cell. Another, and apparently a
stronger influence is light. In diffused light they place themselves so as
to present their flattened surfaces to the incident rays (T, Fig. 54) ; in
intense sunlight they present their edges, and so protect themselves from
its harmful effects (S, Fig. 54).

The origin of the chloroplasts is primarily from the plastids of the embryonic
cells, which enlarge and assume a green colour. These plastids multiply by
fission, and it is easy to see that the chloroplasts do the same. Comparison

THE LEAF

79

of a number of them shows some with an elongated form, others with a median
constriction, and others again grouped in pairs as though resulting from a
recent fission (see Fig. 76, B, p. 115) : but it is doubtful whether it can be
said that plastids in general arise only by fission of pre-existing plastids.

The structure of the palisade-parenchyma, with its oblong cells,
is admirably suited to accommodate large numbers of the chloro-
plasts, which thus present for the most part their edges to the light
incident upon the upper surface of the leaf. The cells of the spongy
parenchyma, with their irregular form, are less specially fitted for
this, and usually they contain fewer chloroplasts than those of the
palisade. They are also less important in that they get most of
their light at second hand, that is, after it has passed through the
palisade tissue. About midway between the upper and lower epidermis
the smaller vascular strands of the reticulum may be found traversing
the mesophyll, and in intimate relation with it (Figs. 47-49). They
vary in complexity from considerable bundles downwards, to a
minimum size where the xylem may be represented by a single tracheid.
The sieve-tubes stop short before this point is reached ; but certain
richly protoplasmic cells, like large companion cells, extend further
than they towards the vein-endings. Each strand is surrounded by
a parenchymatous sheath, the cells of which are in contact with the
cells of the mesophyll. Thus the conducting system penetrates and
thoroughly permeates the green tissues of the lamina.

The characteristic function of nutrition carried on by the leaf is
discussed more fully in Chapter VIII. Here it may be stated simply
that in the presence of light the cells containing the green pigment
(chlorophyll) build up new organic compounds from carbon dioxide
obtained from the atmosphere, and water. The process is known
as Photosynthesis ; it is of fundamental importance to the
plant.

In the leaves of many plants the period of this activity is limited by
the season, and autumn with its lowering temperatures and shortening
days leads to the fall of the leaf. At the base of the petiole a band of
corky tissue is formed in a transverse plane. Immediately above it
the cells become rounded off by increase of the intercellular spaces.
This is called the absciss layer , because the line of fracture is determined
by the weakness of its cells, and it is here that the leaf falls away.
The scar is covered by the layer of protective cork, while the vessels
and sieve-tubes running up to its surface are constricted by the pressure
of the adjoining cells (Figs. 6, 55). Thus the fall of the leaf causes
no open exposure of the living tissues liable to attack by intrusive

8o

BOTANY OF THE LIVING PLANT

organisms. Prior to the fall of the leaf its tissues are depleted of all
useful materials, which are transferred down the petiole to the axis.
This is accompanied by changes in the cell-contents which give the
varied autumnal tints. What falls away is then little more than an
empty shell. Its removal leads to a great reduction of the exposed
surface of the plant, with the result that there is less loss from evapora-
tion, and less resistance to the winter winds. As a whole the plant

enters a dormant condition in autumn,
partly determined by the climatic
conditions, partly by the absence of
those organs which play so active a
part in its vegetation. But the fall
of the leaf is not an inherent feature
in any group of plants, nor does the
absciss layer form any constant limit
between leaf and stem. For example,
the British Oak (Quercus robur) is
deciduous, that is, it drops its leaves in
autumn ; but the Holm Oak (Quercus
Ilex), which is a native of the Mediter-
ranean region, remains evergreen. The
common Cherry (Prunus cerasus) drops
its leaves in autumn ; but the Cherry
Laurel (Prunus law o- cerasus) is ever-
green. Thus though the leaf-fall is a
very striking feature of many trees
and shrubs in temperate climates, it
is really nothing more than a seasonal,
and often a specific, adaptation. In
many woody plants it does not occur
at any regular intervals, while in most
herbaceous plants, and especially in
annuals such as the Sunflower or Bean, the whole shoot simultaneously
ceases its vegetative activity, leaf and axis remaining connected
till they rot.

From the description which has been given of the structure of the
leaf it will be seen how well that organ is fitted for carrying out the
duty of nutrition, while exposed to the ordinary climatic conditions.
In the first place a broad expanse of green tissue suitably orientated
makes for a maximum interception of light. In terms of photosynthetic
activity the larger this area the better. Since the active cells

Fig. 55-

Vertical section through the base of a
petiole (pet) of a Horse-chestnut at its
junction with the axis, showing the absciss
layer (ab) with cork (ck) beneath it. (l) = a
lenticel. When the leaf falls the scar is
protected, and the axillary bud (ax. b) is
left attached. F. O. B. ( x 10.)

THE LEAF 81

are those of tne mesophyll, it is essential that their thin cellulose
walls, saturated with water, shall be exposed for aeration. If simply
and directly exposed to the atmospheric air they would quickly dry up
and shrivel, but they are protected on either side by the epidermis,
which is a continuous layer covered by the impervious cuticle. Access
to the atmosphere is still maintained, though under control, through
the numerous stomata : by means of the intercellular spaces that
permeate the mesophyll it is extended onwards to the individual cells.
The epidermis also gives mechanical strength. The mesophyll with its
thin walls and spongy texture would by itself be too weak to maintain
its form, and resist the impact of winds. It is bound together by the
firmer epidermis. The thin expanse of the blade is further stiffened by
the framework of the midrib and veins. (Fig. 46.) These illustrate in
modified form the same methods of mechanical strengthening as the
stem itself. (Figs. 45, 47.) Often the blade is strengthened also by a
marginal band of hardened tissue, which acts like a hem. Lastly, the
whole lamina is attached to the leaf-stalk, which, though sufficiently
rigid to support it, will yet yield to the impact of wind, and so avoid
mechanical damage.

On the other hand the conducting system is continuous from the
axis outwards through the leaf-stalk, and on through the midrib and
veins to the ultimate branchlets which ramify throughout the meso-
phyll. Thus there is efficient provision for the transit of water
from the axis to the remotest points of the lamina, and conversely
materials may also be transmitted backwards from these to the leaf-
base and into the axis. Such transit does actually take place, the
first through the woody tract of the vascular strand, the other through
the bast. In short, a foliage leaf is fundamentally a structure with
adequate provision for mechanical strength, and for the transit of
materials backwards or forwards between the axis and its distal
points, which is secured by its conducting system : at the same time
it exposes a large area cf green tissue to the light, and affords to each
cell ready access to the atmospheric air.

b.b. h

CHAPTER VI.

STRUCTURE OF THE ROOT.

The origin of the primary, or tap-root from the radicle of the embryo,
the development of the root-system from it, and the relation of
that system to the shoot in the normal Flowering Plant have been

Fig. 56.

Diagram illustrating the arrangement of tissues in the transverse section of a root.
rh= root-hairs, exod =exodermis. pilif= piliferous layer. ^nrf = endodermis. per —
pericycle. #y = xylem. phi — phloem. £=pith.

described in Chapter I. Its fixation in the soil by means of its root-
hairs has also been noted. The details of its structure will now be
examined, so that the facts mav serve as an introduction to the
study of the functions which the root has to perform.

The root is typically cylindrical, and accordingly its transverse

82

STRUCTURE OF THE ROOT

83

section will be circular. The general arrangement of the tissues,
exposed in a section cut about two or three inches behind the apex of
any ordinary root, is more regular than that usual in stems. It is
shown diagrammatically for a thin root, such as that of a Pea seedling,
in Fig. 56. There is a superficial covering which may be held as
corresponding to the epidermis. Within it lies the cortex, and
centrally the stele. But here the cortex is relatively bulky, while
the stelar column is much more contracted than is usual in stems.
This disposition of the tissues is
typical for roots at large.

The superficial layer consists of
an unbroken series of thin-walled
cells, without any cuticle upon
their outer walls. It is called
the piliferous layer because many
of its cells are extended out-
wards as root-hairs (Figs. 57, 66).
Below the piliferous layer comes
the bulky tissue of the cortex, of
which the outermost and the
innermost layers are sharply de-
fined, while the massive band of
tissue between them consists in
the young root of a featureless,
thin-walled parenchyma with in-
tercellular spaces. The outer-
most layer, lying directly below
the piliferous layer, and with its
cells alternating with these, is called the exodermis. Its cells fit closely
together, and they show a sharply defined corky band upon their radial
walls, which often extends with age to the other walls (Fig. 57). This
throws the control of the passage of water upon the protoplasts
of the living cells so long as the root is young. The exodermis is
thus a living physiological barrier, as is also the jnnermost layer,
which is called the endodermis. Its cells also are in close lateral
relation one to another, while the radial walls have a corky
band which serves a similar purpose (Figs. 58, 59). The cortex
thus composed forms the larger part of the area of transverse section,
and it may be regarded as a water-reservoir round the stele, con-
trolled both on its outer and its inner surfaces by the protoplasm
of living cells.

Fig. 57.

External tissues of the root of Ruscus, showing
each cell of the piliferous layer grown out into a
root-hair. ex= exodermis. The section shown was
probably taken above the absorbing region.
( x 100.)

84 BOTANY OF THE LIVING PLANT

The most prominent tissues of the stelar column are certain well-
defined strands of xylem and phloem. In small roots like those of the
Cress or Onion there may be only two of each of these ; and the strands
may meet at the centre, there being no pith at all. In larger roots of
Dicotyledons there are usually more of them, four or five being common
numbers (Fig. 59). But the number is not constant in roots of the
same species, or even of the same individual. In Monocotyledons

Fig. 58.

Transverse section of the stele of the root of Acorns— a Monocotyledon. cort=
cortex. emi=endodermis. per =-pericyc\e. phl=ph\oem. />nry=protoxylem.
p.v.= pitted vessel. ( x 150.)

the number is usually larger still, and it may run to a very high figure
in roots of Palms, or Screw Pines. In such roots a pith is present,
and it may be of considerable bulk. A typical structure of the stele
for a simple Monocotyledon is seen in the root of the Sweet Rush,
Acorus (Fig. 58). The cortex and endodermis surround the stele
itself, of which the superficial layer is the thin-walled pericycle, here
a very regular row of cells. There are seven groups of xylem, and
seven of phloem alternating with them. Each group of xylem is
composed of smaller vessels of the protoxylem (pr.xy), which are
directed to the periphery, while successively larger, pitted vessels
(p.v.) constitute the later-formed metaxylem. The alternating groups

STRUCTURE OF THE ROOT

85

of phloem are not strongly developed, and the whole is compacted
by cells of the conjunctive parenchyma, which fill up the interspaces
and extend to the centre. Here its cells instead of forming a soft
pith become sclerotic with age, so that the lignified tissues are all
welded together into a central, mechanically resistant strand.

Internally to the phloem cell-divisions may be seen at several points in the
Fig. 58. These are in the position where in other roots a cambial activity
arises. Here, however, as in Monocotyledons generally, the divisions pro-
ceed nc further. It will also be noticed that the intercellular spaces in
the cortex are large. Acorus is a swamp-growing plant, and the tissues of
water-plants are characterised by large intercellular spaces.

The arrangement of the vascular tissues thus seen in roots, with
the xylem and phloem alternating on different radii, is described as

Fig. 59.

Transverse section of the stele ot a young root of Ranunculus, showing the central
metaxylem not yet developed. Lettering as before. ( x 200.)

radial. It is in sharp contrast to the collateral arrangement charac-
teristic of stems, where the xylem and phloem are upon the same
radius, the phloem being outermost. Moreover, while in stems the
protoxylem is directed centrally, in roots it is peripheral in position.
Evidence of the centripetal succession of development can easily be
seen in sections of young roots. Fig. 59 shows such a section from

86

BOTANY OF THE LIVING PLANT

the Buttercup, which has five protoxylem groups, a number not
uncommon for Dicotyledons. But of these only the protoxylem
vessels are as yet developed ; the vessels of the metaxylem are still
thin-walled, but they extend to the centre of the pithless root, and
they form a solid star of xylem when mature.

Since the arrangement of the vascular tissues is radial in the root, but
collateral in the stem, it is obvious that a readjustment must take place where
the one passes into the other. The change is effected in seedlings in various
ways, at or near to the level of the soil. The xylem-masses rotate upon their
axes, and this is combined with splittings and fusions of the strands in some
cases, so that the peripheral protoxylem of the root becomes central in the
stem and the xylem-masses range themselves internally to the phloem-
masses. Thus without break of the continuity of the conducting tracts,
the characteristic structure of the root passes upwards into that of the
stem.

In order that the root may absorb water from the soil, a close relation
with the soil must be established. This is effected by the root-hair.

Fig. 60.

Representation of root-hairs in the soil. £ = piliferous layer of the root. h, h'=
root-hairs grown out from its cells, and adjusting their growth to the solid fragments
of the soil. Each of these is covered by a film of water, which is shaded ; while the
clear spaces indicate the air-cavities in the porous soil. (After Sachs.)

The parent cell is usually oblong in form, and from a point about the
middle or upper end of its outer face the hair arises as a cylindrical
process, which penetrates between the particles of the soil. It adjusts
its form to the spaces between them, while the nucleus passes out into

STRUCTURE OF THE ROOT

8;

the growing hair, and the delicate cell-wall is lined by a thin film of
cytoplasm surrounding a central vacuole. A gummy softening of the
wall near the distal end leads to a most intimate connection with any
solid particle. The state of the root-hair in the soil is suggested by
Fig. 60. As the water in the soil is held in the form of surface films

Fig. 6r.

Seedling of Carpinus Betulus. h =
hypocotyl. c = cotyledons. hw=imiii-
root. s;c = lateral roots. r=root-hairs.
£=epicotyl. Z = foliage leaf. Natural
size. Strasburger.

Fig. i>2.
Localisation of growth near to tlic root-tip
of Vicia Faba. In I. the root-tip has been
marked with 10 zones by lines 1 mm. apart.
In II. the same root after 22 hours. The lines
nearest the tip are now most separated owing
to the growth having been there most active.
(After Sachs, from Strasburger.)

covering the several particles, the root can by its hairs tap those films,
however thin they may be in a dry soil. It is important further to
realise how numerous these root-hairs are. It has been estimated
that over two hundred of them may be borne on one square millimetre
of surface of the root of a Pea, while in other plants, for instance the
Maize, the number may be still higher. The effect of this is greatly
to increase the possible absorptive surface of the root.

88

BOTANY OF THE LIVING PLANT

The hairs arise in acropetal order. Individually they are functional
only for a short time, as is seen from the fact that though active at
a short distance backwards from the growing root-tip, at a further
distance from the apex they may have already shrivelled away,
so that the older part of the root no longer preserves its intimate
relation to the soil. Thus as the root-system extends, it taps an
ever enlarging area of the soil, while it is actively absorbent only
at the outer limit of the area invaded (Fig. 61).

As soon as any part of the root is anchored in the soil by its out-
growing hairs, it could not possibly increase in length without tearing

them away from their hold. But
of this there is no sign. It fol-
lows, therefore, that the growth
in length of the root must be
restricted to the region beyond
the youngest root-hairs. It may
be demonstrated that the growth
is thus restricted, and that the
most rapid growth is close to
the tip, by making marks with
Indian ink at equal distances
upon the outside of a growing
root. The root should then be
kept in condition as near as may
be to the normal. After a period
of twenty-four hours, if the dis-
tance between the marks be
compared with the original scale,
it will be seen that the two do not tally. It will easily be seen where
the greatest elongation has been (Fig. 62). The most rapid growth
in an average root is about 5 mm. from the tip. It diminishes
gradually from that point in both directions, and ceases at about
10 mms. from the tip. This restriction of the growth in length to a
short region behind the tip is characteristic of roots, and is in
sharp contrast to what is seen in ordinary stems. In them the
growth in length may be spread sometimes over a length of several
decimetres.

A consequence of the growth of the region beyond the anchoring
root-hairs is that the root-tip itself is forced forward. As in the stem,
so in the root the apical tissue is embryonic, and of delicate texture.
Nevertheless this delicate root-tip is driven through the soil, forcing

*«.

AZ.

Tit*.

Fig. 63.

Root-tip of Barley, cut in median longitudinal
section, and placed apex upwards. R.C=root-
cap. caJ = calyptrogen layer, which renews the
tissue of the cap. CI = common initials for pili-
ferous layer (Pil) and the periblem (Peri). Pler=
central cylinder of plerome, giving rise to the stele.
{ x no.) (After Janczewski.)

STRUCTURE OF THE ROOT 89

aside the solid particles in its course. A study of its structure explains
how injury to the apex is avoided. External observation shows that
the conical apex itself is semi-transparent and slimy to the touch.
By this sliminess it readily slides past obstacles, losing occasionally
some superficial cells in the process. But the structure of the root-tip
as seen in section explains the protection better ; it is well shpwn for
the common Barley in Fig. 63. The actual growing point is protected
by a calyptra, or root-cap. The superficial pilifcrous layer as it ap-
proaches the apex, curves inwards, though still preserving its identity.
Outside it lies a cap of tissue (R.C.)} which though thick at the actual
tip, gradually thins off as it spreads backwards. Its superficial cells are

Fig. 64.
,4=Root-tip of Buckwheat ( x 120). B=root-tip of Pea ( x 60). /?.C = root-
cap. Pi/ = piliferous layer. Pert' = periblem. Pter = plerome. GM = the general
meristem in the Pea from which the different tissues are gradually derived.
(After Janczewski.)

only loosely attached. Their walls, being gummy and swollen with
the water in the soil, are easily rubbed away. But the loss is made
good by growth and cell-division arising from the innermost cells
just outside the incurved piliferous layer. Internally the central
stele continues up to the apex, ending in a rounded dome. At that
point a single layer of cells intervenes between it and the root-cap.
This layer gives rise by successive divisions to the piliferous layer and
the cortex. There are thus in this type, which is common for Mono-
cotyledons, three definite strata at the apex of the root, and each
gives rise to a tissue which takes special characters as it matures.

The stratification of the growing point of roots is not always on the plan
described, though that is the usual type in Monocotyledons. In Dicotyledons
various conditions are seen, of which two examples may be given. The most
common is that seen in the Sunflower and Buckwheat (Fig. 04, A), where the
stele is as before a distinct column ending in a definite dome. The cortex
is also a distinct tissue covering it, but reduced to a single layer at the extreme
tip. The piliferous layer has a joint origin with the root-cap, by periclinnl

90

BOTANY OF THE LIVING PLANT

divisions of cells just outside the tip of the cortex. The chief difference is
that the piliferous layer is distinct in origin from the cortex, while in the
Barley they were seen to have a common origin. A second type is illustrated
by the Pea and other Leguminosae. (Fig. 64, B.) Here the stele, cortex,
piliferous layer, and root-cap all originate from a common mass of meristem,
which occupies the apex, and segregates gradually into the several tissues
as the cells mature. Such facts show that no theory of " germinal layers "
can have any general application in the development of the plant-body.

The normal increase in number of roots is by the formation of lateral
rootlets, which originate from deeply-seated tissues, and force their way

b

a

bericu

'enl '°*

Fig. 65.

Origin of a lateral root from the pericycle, as seen in longitudinal section of
Reseda. In (a) the pericycle has divided by periclinal walls to form four layers to
which the tissues named are referable ; the endodermis has yielded. In (b) the
formative tissues are clearly recognised. The endodermis (end) has developed as a
digestive sac. ( x 100.) (After Van Tieghem.)

out of the parent root. Such an origin is described as endogenous, and
is in contrast to the exogenous origin of leaves, where the surface-tissue
remains continuous over the new growth. The lateral root springs
from the cells of the pericycle, usually at a point opposite to one of the
protoxylem-groups. If the parent root be cut longitudinally through
the point where a lateral rootlet is being formed, the cells of the
pericycle opposite the protoxylem will be found in active division (Fig.
65, a). Later the tissues thus formed give rise to the central stele (pier.),
the cortex (cort.), piliferous layer (pilif.), and root-cap (calyp.), of the

STRUCTURE OF THE ROOT 91

lateral root (Fig. 65, b). By reason of its deep-seated origin the tissues
of the lateral root are intimately connected with the tissues of the main
root. At its tip it pierces the cortex of the parent root by a process of
digestion, through the activity of cells of the endodermis (end.), though
in some cases cells of the cortex also act in the same way. They form a
glandular digestive sac which softens the cells outside, so that they yield
to the growing rootlet within. The substances of the digested cells
are absorbed by the rootlet as it forces its way out from the parent.

The details of division of cells in the formation of a rootlet vary in different
cases. It is here illustrated for the type usual in Dicotyledons. The cells
of the pericycle divide by periclinal walls, so as to form several layers, and by
anticlinal walls, so that each layer consists of numerous cells. The innermost
layer forms the plerome of the young root, giving rise to its central stele (pier.).
The next outer layer is the periblem, giving rise to the cortex (peribl.) ; while
the third outermost layer gives rise on the one hand to the root-cap (calyp.),
and on the other to the piliferous layer {pilif.) ', the two having a joint origin,
as already explained in the case of the Buckwheat. The consequences of this
mode of origin are that the vascular tissues of the rootlet connect directly
with the vascular tissues of the main root. This is essential to the effective
transfer of materials. But the cortex, piliferous layer, and root-cap of the
lateral root are all distinct in origin from those of the parent. Physiologically
this discontinuity is not a matter of importance.

The individual root thus growing indefinitely at its apex, and bearing
an increasing number of lateral roots, is subject to increasing demands
upon it, as a means of transit from the distal absorbing region to its
attachment at the base of the plant. In Monocotyledons (Fig. 58),
and in some Dicotyledons (Fig. 59) there is no special development
of tissues to meet this, but there is usually an increase in the number
of adventitious roots, as in Palms, Maize, Onion, etc. In most
Dicotyledons, and especially in those that are woody, there is a
process of secondary thickening by means of a cambium, analogous to
that seen in their stems. Cell-divisions appear in the conjunctive paren-
chyma lying internally to the several groups of phloem (Fig. 66, a).
Arcs of cambial activity are thus formed, which soon spread to the
tissue of the pericycle lying peripherally to the protoxylem (Fig. 66, b).
The several arcs are thus linked together to a continuous band, in the
form of a corrugated cylinder. As in the stem, this cambium produces
secondary wood internally, and secondary bast externally, to an
indefinite degree. But at first the cells lying peripherally to the
protoxylem form only parenchyma, so that a broad medullary ray
appears externally to each group of the primary wood. In the
secondary wood additional medullary rays and annual rings may

92

BOTANY OF THE LIVING PLANT

a

Fig. 66.

a, b, Successively older sections of the root of the Bean [Vicia Faba), showing the
beginning of cambial activity. In (a) divisions appear (cb) in the conjunctive
parenchyma internally to the phloem, but the pericycle (per) lsquiescent. In (b) the
cambium has already formed xylem internally (2nd xy), and phloem externally
(2nd phi), and cell division has spread to the pericycle, and is continuous outside the
protoxylem (pr.xy). ( x 100.)

appear, as in the stem. Consequently an old root of a Dicotyledon
acquires a structure very like that of an old stem (Fig. 67). The
difference may be often observed even in old roots by looking for the

STRUCTURE OF THE ROOT

93

primary wood. If the protoxylem is central the section has come
from a stem ; it it is peripheral relatively to the rest of the primary
wood, then the section has come from the primary region of the root.
As the distal part of a young root grows older the root-hairs upon
it shrivel. In Monocotyledons, and in some Dicotyledons the cortex
is retained ; but as it grows old its cells lose their turgor, and the
cortex shrinks. In bulbous Monocotyledons, and in some Dicoty-
ledons the whole root then shortens, anchoring the plant firmly in

prjcg^

m

pr.phl-

m r

/

/ m

phl"-.-\

V pr.pld
--^pp.phl

pr.xy

p.mr

pr.xg

Fig. 67.

Diagram A shows arrangement of tissues in a young root of a Dicotyledon before
cambial activity begins. B. the same when cambium can be clearly recognised.
C. after secondary thickening has progressed. c=cortex, present in A and B, but
in C it has been thrown off. pr.phl= primary phloem. />/;/" =secondary phloem.
pr.xy= primary xylem. .vy"=secondary xylem. £.wr=primary medullary ray.
mr" =secondary medullary ray. ;/i=pith. F.O.B. Compare Fig. 37, p. 58.

the ground, while evidence of the shrinkage is seen in transverse
wrinkles on its surface. In most woody Dicotyledons the bulky cortex
itself collapses and finally peels off. This is due to the formation of
a band of cork, which originates from the pericycle, and cuts off the
outer-lying tissues from physiological connection with those within, so
that they perish (Fig. 67, C). Since the cortex makes up a very large
proportion of the whole bulk of the root, the consequence in such
cases is that at first the root appears to become thinner as it grows
older. But as a matter of fact the central stele has meanwhile been
increasing in bulk, and it is protected externally by the band of cork
which originates from the pericycle. Since this cork may be further

94 BOTANY OF THE LIVING PLANT

developed as in stems, the old root is covered by a band of bark not
unlike that of a woody stem.

The stem, leaf, and root, as seen in ordinary Flowering Plants, have
now been described. In subsequent chapters the Plant-Body can
therefore be considered as a connected whole, and some idea gained
of its physiological position as a concrete living organism. Normally,
with its root in the soil and its shoot in the air, it acts as a sort of
intermediary between the two regions in which it lives. One phase
of the Botanical interest would be to see how the Plant plays off the
one medium against the other, drawing meanwhile its nourishment
from both. Another phase is to follow the material abstracted and
to see how it is used in Life, and how on the disorganisation of the
Plant after Death that material is finally restored to its original
source. Vegetation may thus be looked upon as an active factor in
that interchange and circulation of material which is constantly
taking place at the surface of the Earth's crust.

CHAPTER VII.

THE WATER-RELATION.

If you neglect to water plants grown in pot culture in the greenhouse
the plants will wither : if the withering has not gone too far the plant
may recover after watering ; but if the neglect has been too prolonged
the plant will die. These facts are familiar to everyone who has
experience of indoor cultivation of plants ; and though the problem of
water-supply may perhaps be less obvious in plants growing naturally
in the open, it is no less a grave one for them also.

In discussing the relation of plants to water it is necessary in the first
place to realise how high is the proportion of water contained in ordi-
nary plants. If a block of wood be cut out from the trunk of a living
tree, and weighed, and after drying out thoroughly at 100° C, it be
weighed again, it would be found to have lost about half its weight.
Thus water forms about half of the weight of so solid a tissue as
the wood in the normal living state. In succulent tissues of the
leaves or young stems, or in the tissues of herbaceous plants, the pro-
portion is much larger. In the case of the fresh Cabbage it amounts
to about 92 per cent., and in the Lettuce, as cut fresh for a salad, to
about 95 per cent. Thus only about 5 per cent, of a crisp Lettuce
consists of the substance of protoplasm, and cell-walls. The living
plant may then be regarded as a structural framework retaining within
it a very high water-content. The water exists there in various forms.
A large part of it appears as liquid water, filling the vacuoles of cells, or
in the cavities of the vessels, and it can be seen as such microscopically.
But a considerable proportion of it is absorbed into the substance of
the protoplasm or the cell-walls and starch-grains, as water of im-
bibition, upon which their swollen condition depends. Some may be
present as " water of constitution," entering more intimately into
relation with the substances of which the plant body is composed.

95

96 BOTANY OF THE LIVING PLANT

The significance of water to the plant is manifold. It is the medium
in which the protoplasm conducts its chemical reactions and it is
actually used up in a number of chemical operations, e.g. in photo-
synthesis ; it is the solvent through which all materials enter the
plant cell from its environment, and thus they pass from point to
point in the plant : finally, its accumulation at an early stage under
pressure within the cells leads to their enlargement, and subsequently
gives firmness to them and to the organs that they compose.

It is clear that an active plant will require constant supplies of
water for its chemical operations, and to meet the needs of new tissues.
But over and above the small amount of water required for these
purposes, a very much greater quantity is needed to make good the
loss incurred by the evaporation of water (or Transpiration) from the
aerial organs.

Absorption of Water by the Plant.

Land-plants rely on the soil for their water supply. But the
absorption of water is mainly restricted to the younger parts of the
root system, and especially to the regions bearing root-hairs. The
structure and distribution of these hairs have been described in the
previous chapter, where it was pointed out that their presence increases
the absorptive area of the root very considerably, while the hairs
make intimate contact with particles of the soil (Fig. 60), facilitating
absorption of water present as films round those particles. Each
root-hair-cell forms an osmotic system, as described for plant-cells
in general in Chapter III. ; the cell-sap normally has an osmotic
pressure of 5 atmospheres or more, so that the hairs are in a position
to carry out osmotic absorption of water under suitable conditions.

The soil in which the root system develops is a complex mixture
of materials, organic and inorganic, holding within it water to a greater
or lesser degree. No soil in the open is ever actually dry, and normally
the amount of water contained is considerable. Soil-water is of
different types. Thus we may recognise as gravitational water that
which is only present in badly-drained soils, or after rain. Of the
water retained by a well-drained soil, some is held by capillarity in
the minute channels between the particles, or in the form of films
round the particles ; while some is held by imbibition in the colloidal
constituents of the soil, such as clay and humus. Humus, using the
term in a broad sense, is the decaying organic matter of the soil,
derived from previous generations of plants. Leaf-mould is a type

THE WATER-RELATION

of humus. It is important to realise that the soil-water is mobile,
and tends to distribute itself uniformly through a mass of soil
that if water is drawn off at any point in the soil there will be a certain
compensatory flow of water towards that point. The consequence
of this will be that a root can to some extent draw on the whole
reservoir of water which the soil in its neighbourhood contains.
Dissolved in the soil-water are small quantities of salts such as
nitrates, carbonates, phosphates and sulphates of sodium, potassium,
calcium, etc., the total concentration being usually less than o-i per
cent.

The study of soils is a science in itself. Only a few general features can
be mentioned here. The constituents of a soil fall into several groups.
The first group includes those which together form the general framework
of the soil. This is composed firstly of the coarser particles of sand and
gravel, consisting for the most part of pure silica : secondly, of finer
particles forming clay, which consists of silica combined with oxides of iron,
aluminium, and other metals. There is no sharp line to be drawn between
the former and the latter, and the name " silt " is sometimes applied
with varying meaning to material that is intermediate between them, while
the name " loam " is used for soils containing more fine clay than sand.
These components of the soil all owe their origin to the weathering of rocks
of inorganic origin.

A second group of soil-constituents includes those derived ultimately from
organic life. The most important is calcium carbonate, produced by the
weathering of chalk and limestone ; this is a regular constituent of loams.
It has important effects on the condition of soils, preventing sourness, and
making a heavy clay soil more- workable. Lime (calcium oxide) has a similar
effect. Another substance of this type is calcium phosphate, which is partly
of organic origin, but partly derived from the decay of rock.

A third group consists of those organic substances collectively called humus
(see above), which represent the intermediate products of the decay of plants
which have previously grown on the soil. As these die their leaves and other
parts are carried down by earth-worms into the soil, and there the materials
which compose them are gradually broken down into simpler compounds.
This is largely the result of the activities of the micro-organisms of the soil
(see below). These simple compounds are then available for absorption by
existing plants. There is thus a circulation of material between the soil and
the vegetation which it carries : food is extracted from it, but restoration
of the substances is made on the decomposition of the plant-body.

Any ordinary soil contains water in more or less quantity, and being porous,
it may be permeated by atmospheric gases. The supply of water comes from
rain, or from the sub-soil. It is diminished by evaporation and by drainage,
and the water-content of the soil at any moment represents the balance
between gain and loss, being liable to constant change. The constitution of
the soil, and the nature of its surface are factors which affect it. For instance,
sand retains water less than clay : a fine surface broken by rake or hoe checks

B.B. G

98 BOTANY OF THE LIVING PLANT

evaporation. About 60 to 70 per cent, of the volume of an arable soil is
made up of soil-constituents as above detailed, and about 30 to 40 per cent,
is accounted for by spaces between the soil particles, occupied by air (which
may be somewhat different in composition from atmospheric air) and by water.
In badly drained soils the spaces are largely occupied by water, and aeration
is correspondingly reduced. The soil-water carries in solution small quantities
of any constituent of the soil that is soluble. Examples of materials commonly
found in solution in the soil are mentioned above.

Besides the plants rooted in it, the soil houses a vast population of other
organisms. Earthworms are constantly at work burrowing through the soil,
passing large volumes of it through their alimentary canal, and voiding it
at the surface as worm-castings. They bring up material from the lower
layers to the surface, and conversely they draw down into their burrows
leaves and other decaying parts, thus serving as tillers of the soil. Many
other animals are also present, such as wire- worms, eel- worms, etc. There
are also multitudes of microscopic forms of life, such as the Protozoa, Fungi
and Bacteria. The latter play a very important role in the decay of organic
materials in the soil, and in other ways by the varied chemical changes which
they bring about (see Chapters VIII. and XXVIII.).

The root-hair in exercising its function of absorbing water has to
face a certain resistance from the soil. This arises from the physical
forces of capillarity and imbibition, and from the slight osmotic
activity which the soil-water, with its salts in solution, possesses.
The magnitude of this last factor is unusually high in salt-marshes
and other sea-side situations, where the salt-content of the soil may
be considerable. But the fact that plants usually manage to keep
that condition of turgor, on which their firmness and mechanical
rigidity largely depend, indicates that the absorptive forces of root-
hairs are adequate to overcome these resistances to a requisite extent
(see Chapter X.). It is probable that plants utilise chiefly the capil-
lary water, since the colloidal particles of soil cling very tenaciously
to their imbibed water, and the absorptive forces of the root-hairs
may be inadequate to overcome these forces of imbibition. Reference
has been made to the osmotic pressure of the root-hairs. The actual
proportion of this osmotic force that is available for absorption at
any moment depends on the degree to which the root-hairs are
removed from complete turgor. It was pointed out in Chapter III.
how a fully turgid cell has no absorptive capacity (or Suction Pressure),
while this capacity is at its maximum in a plasmolysed cell. Only
if there is a steady movement of water out of the root-hairs into
the interior of the root will absorption by the root-hairs continue, for
otherwise complete turgor would soon be reached and absorption
cease.

THE WATER-RELATION

99

This brings us to the first section of the route which most of the
water absorbed by the root-hairs pursues through the plant, in the
so-called Transpiration Stream. The water passes laterally from
the root-hairs across the cortex of
the root, through the endodermis
and pericycle and into the xylem
vessels (Fig. 68). Until the xylem
is reached the water passes through
living cells, each cell forming the
usual osmotic unit. As in the case
of the root-hairs, so with these
inner cells, if there were not a
constant movement of water from
the innermost living cells into the
xylem vessels, all the living cells
would soon become fully turgid,
and a static condition would be
reached. The fact that a constant
movement of the type mentioned
does exist results in the mainten-
ance of an absorptive capacity in
those inner living cells. These
in consequence take up water
osmotically from the cells lying
next outside them, a process which
extends outwards until the root-
hairs and soil are reached. A
lateral flow of water from the soil
through the living cells of the
root is thus maintained till it frrows,:ll Yhl root"hairs. we.re proportionaii:

longer than they appear in the drawing. The

reaches the Xvlem. The move- cell-contents are shown in some cases, dia-

j ' U,V/ grammatically. r.h. = root-hair ; c= cortex ;

ment of water from the inner- «= endodermis ; *y= xylem . (*i33.) (With

acknowledgments to Pmestley.)

most living cells into the xylem

will be reserved for later consideration. At this point we leave it
in its journey through the plant, and go on to study the course of
events above soil-level.

Fig. 68.

Part of a transverse section of a Pea root to
show the direction of movement of water (see

Transpiration.

We have seen in previous chapters how the leaves and young
stems of the plant are covered by a protective cuticle, which is highly

ioo BOTANY OF THE LIVING PLANT

impervious to water. The cuticle is, however, interrupted by the
very numerous stomata. Within the organs, and connected with the
stomata, we find a ramifying system of air-spaces, which ventilate the
internal tissues. They are thus brought indirectly into communication
with the atmosphere, making possible that exchange of carbon dioxide
and oxygen which the functions of photosynthesis and respiration
require. It is clearly possible that there will be loss of water by
evaporation from the tissues so ventilated, just as clothes hung out
in the open-air dry by evaporation.

Experiment confirms that from the aerial organs of plants, especially
the leaves, there is an extensive evaporation of water, to which the
term Transpiration is applied. If leafy shoots are placed under a
bell-jar, a deposit of condensed water-vapour soon appears on its
inner surface. Further, if pieces of dry cobalt chloride paper be fixed
on the surface of a suitable leaf and protected from the atmosphere,
a rapid change of colour from blue to pink will indicate the liberation
of water-vapour from the leaf-tissues. The actual evaporation is
generally held to occur from the cell-walls of the mesophyll, which
contain imbibed water, into the atmosphere of the air-spaces : thence
the water-vapour diffuses through the stomata into the outer air.
The path can be traced in Figs. 48 and 49. There is also some trans-
piration through the cuticle of leaf and stem ; but that the greater
part is through the stomata is indicated by an experiment in which
pieces of cobalt chloride paper are placed on both surfaces of a
leaf showing stomata on the abaxial surface only. The change in
colour is found to occur considerably more rapidly in the lower
piece of test-paper than in the upper. Alternatively two similar
leaves may be taken and a thin film of vaseline smeared on the
adaxial surface of one, and on the abaxial surface of the other ; here
again leaves with stomata on the lower surface only are to be used.
It will be found by weighing that the first leaf loses water by trans-
piration much more quickly than the second, in which the stomata
are covered up.

We thus realise that transpiration is for the most part an unavoid-
able result of aeration of the tissues. In times of water-shortage
transpiration may be a definite disadvantage to the plant, and it
has been remarked that more plants perish or are retarded in growth
through lack of water than from any other cause. Many plants,
especially those inhabiting dry situations (the so-called Xerophytes,
see Chapter XI.), display structural modifications tending to reduce
transpiration in times of drought. To a plant, however, that is

THE WATER -RELATION

IOI

well supplied with water, it is probable that transpiration has i ertain
advantages. In the first place, evaporation has a cooling effect which
may prevent harmful overheating of the leaf especially when exposed
to strong sunshine. Secondly, the flow of water up the plant that
is initiated by transpiration, probably accelerates the movement
of mineral salts from the roots to the upper part of the plant (see also
p. no). The fact that there is a subsidiary mechanism which tends
to provide for a current of water through the plant in the absence
of transpiration suggests that the maintenance of such a current
is of importance to the plant (see Root Pressure, p. 108).

Fie. 69.

Potometer of the Ganong pattern. When necessary the air-bubble (shown in
black) can be driven back to the right-hand end of the capillary tube by admitting
water from the funnel.

The actual amount of water lost in transpiration may be consider-
able. For example it has been estimated that a Birch tree may lose
as much as 600 lbs. of water during a hot dry day. Measurements
of transpiration can conveniently be made on plants growing in pots.
The pot and soil must be carefully sealed up in metal shells roofed
over with sheet rubber so that evaporation can occur only from the
plant. The whole arrangement is weighed at intervals and the loss
in weight during a period represents the transpiration. An instrument
called the potometer is frequently used in experiments on transpiration,
though what is thus measured is actually the rate of absorption of
water by the cut shoot fixed into the apparatus (see Fig. 69). The
whole apparatus being initially rilled with water, absorption by the

BOTANY OF THB LIVING PLANT

, • wiU , a movement oi water from the small

the right into the bent an !'lary tube-

\n luced into this tube by raising its end out of

the beak moments. The number of graduations

luently t r.i I by the bubbl( tin time gives an in-

rption, and since normally absorption is

approodmati [ual to transpiration, we have here an indirect

metho transpiration.

With the potometer it can be shown that exposure of the shoot

current oi air from an electric fan increases transpiration.

1 xposure of the shoot to specially

B

dried air also increases transpira-
tion, while exposure to a very
moist atmosphere decreases it.
These results we should expect
from experience of ordinary
evaporation; but the further
observation that transpiration
at night is very much less than
in the daytime would not have
been entirely expected. This
ilt may be in part due to
differences of temperature : but
it is also referable to the influ-
=a ence of the stomata. Some in-
70. formation concerning these struc-

Stocna v A- in the tiirp«; was aiven in Chanter V

oper 150.) 1 on rures was given in ^napter v.,

where it was pointed out that

the stomata] guard-cells are adjustable, so that the pore can

her open or closed. Generally speaking the stomata open

in light and close in darkness, the difference being due to a

or prevailing in the guard-cells in the presence of light

a in When turgor is high the guard-cells tend to

nd the pore opens : but when turgor falls the cells

tend 1 traight position, and the pore closes (Fig. 70).

re due to a'terations in the amount of osmo-

in the sap of the guard-cells. These, unlike

other epidermal cells, regularly contain chloroplasts, and in the

light irs will presumably be produced by photo-

: in saying this we anticipate information to be given in

tpter YIII. It 1- now believed that a production of sugars

THE WATER-RELATION [03

more important to the action of the stoma, is as follows. Observation
shows that in darkness abundant starch grains are present in the
chloroplasts of the guard-cells. When light falls on the leaf,
these starch grains disappear owing to their conversion to sugar,
probably through enzymic action. The increase in sugar content
of the guard-cells thus produced leads to an intake of water from
the neighbouring epidermal cells : for these have as a rule no chloro-
plasts and no starch reserves, and experience no increase in sugar
content when exposed to light. This intake of water and consequent
increased turgor in the guard-cells produces the opening, as described.
Observation indicates that when the light fails in the evening the
sugars in the cell-sap of the guard-cells are largely converted back
into starch, with a resultant fall in turgor and closure of the pore.
So that the effect of light intensity on stomatal aperture is due to
its exertion of a close control on the starch-sugar balance of the
guard-cells, though at present it is somewhat uncertain how this
control arises. That the opening of the stoma depends on turgor
is readily proved by treating a living preparation from a leaf, showing
open stomata, with a 5 per cent, solution of common salt. The
osmotic withdrawal of water and loss of turgor quickly results in
closure of the stomata, the guard-cells ultimately becoming plas-
molysed.

The mechanics of stomatal movement are complex, and are bound up
with the structure of the guard-cells. While the latter varies a good deal,
the chief features are these. The two guard-cells, attached at their ends,
are usually curved, the wall facing the pore being shorter than that in contact
with the adjoining cells. The inner and outer slopes of the face next the
pore bear each a projecting ridge (Fig. 71). The open stoma, with its tense
cells, requires more room than the closed stoma. That room has to be gained
by forcing the adjoining cells aside. Where the cell-walls are thick, special
thin areas of cell-wall are found which are effective as joints or hinges, allowing
the cells to adjust themselves mutually when the pore opens. It is the in-
crease in turgor, produced as explained above, acting on cells of the form and
structure of the guard-cells, that makes the pore open in the presence of
light. The internal pressure being equal over the whole internal surface,
since the convex wall presents a larger area, it will stretch more than the
concave wall when turgor increases ; this would in itself produce a greater
curvature and opening of the pore, even if the walls were all of the same
thickness. But they are not. The ridges of thickened cell-wall on the outer
and inner faces of each guard-cell make the average thickness of the wall
greater on the side next the pore. This will accentuate the curvature of the
cells when the turgor rises. A further effect of the increased turgor will
be to make the section of the cell-cavity approach as nearly as possible to
the circular. This too will result in withdrawing the projecting surfaces

BOTANY OF THE LIVING PLANT

„h.n All tl„ I together in

:»lI,K the Sl . . _.n:j

V „ thai the stoma* provide !^> raPld

tWeen the leaf 1 -n.i the ■t"°£**;

„f th-.r huge Dumbei . but also for the less

ob> .n that far mon diffusion ? > *nU wru tokea place through a

Wch toma, than through a port « dimensions.

1 hl's prim iplr I .1.1 DC illustrated t.v .- fen ■:, I > to I M" Omenta on the diffusion
oJ ,, ^ metal discs securely hxed to the tops

Fie. ft.
oa in median section, and also in surface-view. The
■w the position of its nuard cells in the open state ; the dotted
iw tbeir position in I 1 state.

<.f l;I ten- jars containing water. The evaporation through the

■i be followed by noting the loss in weight of the jars. In

irticular i ment the evaporation from a jar covered by a disc with

entimeter in diameter was compared with that from a

second jar in which the di* bore four smaller holes equal in combined area

to that of the ringli in the first jar. The evaporation from the second

rably r than that from the first. Thus subdivision

of j f&ciency as channels of diffusion. The efficiency

of mi like those of stomata is therefore very high.

Th it night, togetlicr with the fact that the

t of the epidermi rooted-over by the cuticle, through which

to a limited extent only, results in the avoidance

THE WATER-RELATION 105

of unnecessary loss of water by transpiration during a period when
there is little need for gaseous exchange with the atmosphere. It
was formerly believed that the stomata exercised a close control
over the transpiration from the leaf during the whole day and adjusted
it in accordance with the available supply of water. It was supposed
that if transpiration tended to exceed supply, the turgor of all the
leaf cells including the guard-cells would tend to fall, leading to
reduced stomatal opening and to a check to transpiration. Experi-
ment has indicated, however, that frequently a very considerable
reduction in stomatal aperture must occur before transpiration is
itself reduced, the reason being that when the stomata are wide
open their diffusive capacity may not be fully utilised. Transpiration
at such times is under the control of other factors, such as atmospheric
conditions or the water supply at the seat of the process (see below).
When, however, transpiration is so excessive as to produce visible
loss of leaf turgor, as shown by flagging or wilting, the stomata
usually tend to close completely and leaf moisture is thus conserved.
Independently of any stomatal regulation, transpiration does tend
to keep step with the supply of water to some extent : for if the latter
becomes inadequate the cell-walls in the leaf become somewhat
drier, and evaporation is correspondingly reduced.

Ascent of Water through the Plant.

Since in our climate leaves usually present a turgid appearance,
except perhaps on a hot summer's day, it is obvious that the water
lost by transpiration from them is continuously replaced. The
replacement is effected by an upward transport of water absorbed
by the root from the soil. This current of water, flowing through
the plant from the root hairs to the leaves, is known as the Trans-
piration Stream. Experiment proves that the stream of water ascends
the plant through the vessels and tracheides of the xylem, a tissue
which forms a continuous system through root, stem and leaf ; more-
over, its elaborate system of pitting gives the impression of its being
adapted for aiding the conduction of liquid. If plants or cut shoots
are stood in dye solutions until signs of the dye are seen in the upper
parts of the shoots, sections then taken from the stem will show that
the xylem tissues alone are stained. The water of the transpiration
stream thus indicated contains small quantities of mineral salts,
and of organic substances, and the contents of the xylem elements
are for this reason sometimes referred to as sap.

io<S

BOTANY OF THE LIVING PLANT

Leaf. Stem
and Root

r

Root

J

phylL

V n mentioned that 600 lb ter may he evaporated

a the in t! ne day. Clearly, very

ill be needed to rai 1 quickly from the

jnsl ,; ravity and to overcome internal

leration applies in leaser degree to smaller

pk, ting to this end in the plant

much discussion, and although in the Cohesion
, The a fairly satisfactory explanation is
» now forthcoming, there is admittedly much

ill to be elucidated.
N In the region where transpiration occurs
tin- cell-walls bordering on the air-spaces
lost ter by transpiration. It may be

nmed that the walls imbibe a corres-
ponding amount of water from the proto-
plasts which they enclose, creating in each
protoplast an increased osmotic suction
which leads it to absorb water from a
neighbouring cell. This cell in turn absorbs
water from a third cell, the process being
repeated until we arrive at a cell which
is in contact with a tracheide or vessel
of one of the fine leaf-veins. This cell
takes water from the xylem element (Fig.

72).

At this point reference must be made

to the Cohesion of water. Because of its

h« path of the liquid nature, the water under experiment

Fie. 72

i>

throuch

. trum Mr.i>- must be supported in a rigid envelope be-
fore this property can be demonstrated.
a suitable arrangement. Before being sealed, the
« urved glass tube (actually about one metre in length) is partly filled
1 water, which is then boiled so as to wet the sides of the tube
thoroughly and to expel the air. The tube is then sealed-off. If
it 1. a properly set up, the tube can be brought by careful tilting

into the position shown, with the water hanging in the long arm.
The film of water in contact with the glass clings to it, while the
• of the water retains its continuity as the result of its cohesive
power The cohesion is due to the mutual attraction between the
mole- ules of water, and the weight of the column of water is insuffi-
cient to pull the moleculi : .rt. Other experiments have proved

THE WATER-RELATION

107

that very high tensions are necessary to break down the cohesion
of such a column of water.

In consequence of its cohesion, a column of water could be pulled
up the tube supporting it if suction were to be applied to the top
of the column ; just as a vertical steel rod can be lifted by its upper
end. In the plant, the vessels of the xylem provide a series of rigid
tubes, through which continuous columns or threads of water may
stretch from the leaves down to the roots. While it is true that
individual vessels are of limited length only, yet the walls separating
one vessel from its neighbours are thoroughly per-
meated with water, and this provides for continuity
of the water columns. When, as described above,
the leaf-cell abstracts water from the xylem of the
vein, the abstracted molecules draw up more behind
them by cohesion. It is suggested that the wholesale
abstraction of water from the leaf -veins during trans-
piration pulls the water of the transpiration stream
up the trunk or stem. That at least is how the
Cohesion Theory explains the ascent of the water.
Summarising, we may say that the ascent of the water
is regarded as being mainly initiated by an osmotic
suction arising in the leaf- cells as the result of trans-
piration, and this pulls the water up by virtue of the Fkj
cohesive properties of water enclosed in rigid tubes. Experiment to show

. , c cohesion of water.

There is considerable evidence in favour 01 (After Dixon.)
this theory. Calculation shows that the leaf-cells,
with an osmotic pressure of 10 to 20 atmospheres, are in a position
to exert an osmotic suction sufficient to account for the ascent of
the transpiration stream to the top of the tallest tree, while the
cohesive power of water is ample to withstand the resulting tension.
The theory implies that the water in the vessels is in a state of
tension, not of pressure as would be the case if the water were being
forced up from below. A simple experiment confirms this. If a
seedling is placed with its roots and lower part of the stem in eosin
solution, and a cut be made across the stem below the level of the
dye, it will be observed that the eosin ascends the plant very quickly,
and in less than a minute may appear in the veins of the leaf.
Evidently the opening of the conducting strands by cutting relieves
a state of tension and leads to a rapid absorption of the dye.

The Cohesion Theory requires the existence of continuous columns
of water stretching right through the plant. These must be free of

io8

r\N\ « i| THE LIVING PLANT

bubbl< «'t bubbles would d< stroj i ohesion

in the < olumns oi water, di» ontinuity tl veloping by the expan-

n ,,, the bubble columns are subject to tension.

It has »>• emarked that, in point ct, the water-columns

lin the • 'i<ii include air-bubbles: but the upholders of

the Cohesion Theory maintain that the contents are free from bubbles

m .1 sufficient proportion of the vessels
to provide the necessary degree of con-
tinuity : moreover, that the effect of
the air-bubbles is only local.

Any tension in the xylem vessels will
spread downwards into the roots, and
will there promote the flow of water into
the xylem from the adjacent living cells.
Tins will activate the osmotic chain which
across the cortex, and continued
absorption of water from the soil will
ensue (see Fig. 72, also p. 99).

It appears, however, that there is an-
other mechanism capable of inducing
movement of water into the vessels of
the root and up through the plant.
This is suggested by the phenomena of
Exudation and of Root Pressure. If a
plant is severed a few inches above the
soil on a warm day, and the cut sur-
Fic. 74. face immediately examined with a lens,

ggSj it will appear dry. A drop of water
placed upon it will at first be absorbed,
but in a short time it will be found that

:it>> tin- tube,
the prcsvirr can !*• measured fan

I the h' .1 mei

water emerges from the cut surface,

1 -suing from the region of the vessels.

The -tump i- .nd to 'bleed," and may do so very extensively.

Thus a vine ha- been observed to exude an average of 500 c.c.

of liquid per day over .1 period of several week-. As might

\ii< late is not pure water, but contains small quan-
titi' mineral and of organic matter. Occasionally the latter is

more abundant, a- in the Sugar Maple, where the exudate from
incisions made into the trunk in spring, before the leaves are fully
formed, may contain .; per cent, of sugars. The use of apparatus
similar to that ill -1 in Fig. 74 -hows that a considerable pressure

THE WATER-RELATION

109

(Root Pressure) may be developed in exudation from a root system,
amounting in some cases to 1 atmosphere or more : it is possible
that in the intact, uninjured plant Root Pressure may reach consider
ably higher magnitudes.

A phenomenon that is probably due to Root Pressure is to be seen
in plants that are provided with water glands. Drops of water issue
from the leaves of such plants when conditions are unfavourable to
transpiration, though still favouring absorption, as during a warm
night (Fig. 74 a). The water exuded in this way from the leaves of
Grasses at night is often mis-
taken for dew. It frequently
contains salts in solution, and
these may form an obvious
incrustation round the water
gland, as for example in var-
ious Saxifrages. The deposit
here is largely calcium car-
bonate. Exudation from leaves
is very common in tropical
rain-forests, where the humid
atmosphere depresses trans-
piration.

It appears then that a mechanism is present in the roots of
plants which forces water up through the plant when transpiration
is not operating, as is the case at night or in a decapitated plant.
One suggestion is that special osmotic arrangements cause the inner-
most living cells to pump water into the xylem (see Figs. 68 and 72).
The endodermis, with its radial walls made impermeable to water
by the strip of corky material previously mentioned, may play an
important part in preventing the water which is accumulated under
pressure in the xylem vessels from leaking out of the stele along the
cell-walls, and perhaps eventually finding its way back to the soil.
The Root Pressure mechanism may co-operate with that contemplated
by the Cohesion Theory in raising the transpiration stream when trans-
piration is in progress. On the other hand, in many experiments only
feeble Root Pressure has been detected, while it is known that during
active transpiration the contents of the xylem are in a state of tension
rather than of pressure. Hence the opinion has for some time been
prevalent that Root Pressure plays only a minor part in promoting
the upward stream. Probably the significance of Root Pressure in
the plant has not yet been properly evaluated. Lastly, there has all

Fig. 74 a.

Exudation of water from the margin of the leaf of
Tropaeolum, through water stomata which are perma-
nently open. (After Strasburger.)

no BOTANY OF THE LIVING PI ANT

hool ni botanists who have held that the- living cells
thai lie along tl oi the transpiration stream, both in the

A and in the item, play e more important part in raising water
than the uphokta l >h< on Theory would ascribe to them.

Absorption oi Salts prom thi Soil.

Water is not the only substance derived from the soil. The root-
hairs also absorb a variety of inorganic salts that are present in

solution in the soil-water (\ , Analysis of plants reveals that

in addition to carbon (derived from atmospheric carbon dioxide)
hydrogen and oxygen (derived from water), a great variety of elements
may be detected within the tissues. Those most commonly present
are nitrogen, potassium, sodium, calcium, magnesium, iron, phos-
phoric, sulphur, silicon, chlorine, manganese, aluminium, zinc and

on. All these elements arc derived from the soil, and are absorbed

the plant in the form of salts.

Information concerning the absorption of dissolved substances by plant
cells m general has been given on pp. 38-39. The extent of absorption
rtuular salt or ion by a plant is affected by its concentration in the
soil and by the permeability of the root-hairs towards it: also by the rate
at which it p from the root-hairs into the other parts of the plant, and

1>\ the opcr.it ion of factors not properly understood and referred to on p. 39.
The entrance of salts into the root-hairs is a process essentially independent
of the entry of water. It could continue in the absence of the latter process,
while conversely the mere presence of a particular salt in the soil-water
does not 1 inly mean that the salt will pass into the root-hairs along

with any water that is being absorbed. It seems probable, however, that
the movement of water will hasten the absorption of salts to which the root-
hair is permeable, as well as the subsequent transport of the salts to the
DO of the root. There ta fairly general agreement that the salts are carried
,11) tM ' rial organs of the plant in the transpiration stream, as already

mentioned. The structure of the endodermis necessitates that all materials
diffusing through this layer should pass through the protoplasts, wall-diffusion
ng prevented by the corky strips on the radial walls (p. 83). Thus only
materials to which the protoplasts are permeable can penetrate into the
inner part of the root and undergo transport to the other parts of the plant.

elements listed above only seven are usually regarded as

plant development, namely, nitrogen, phosphorus,

sulphur, p mi, calcium, magnesium and iron. This view is

based on riments in which advantage is taken of the fact that

plants will grow with their roots in aqueous solutions instead

ot in soil Any d( ombination of salts ran be supplied in solution

THE WATER-RELATION

in

to the roots, and the effect on growth studied. The usual arrangement
is shown in Fig. 75. A solution containing the above seven elements
in suitable form usually results in satisfactory growth. Knop's
culture solution is often employed in these experiments, and is made
up as follows : calcium nitrate, 1-0 gm. ; potassium nitrate, potassium
di-hydrogen phosphate and magnesium sulphate, 02 gm. each ;
ferric chloride, a trace ; distilled water, I litre. By suitable adjust-
ments of this formula, modified culture solutions can be prepared

Fig. 75-
Water Cultures of Barley. The plants in the left-hand jar had been grown in a
solution containing all the essential elements. Certain elements were omitted from
the other jars, as labelled. The glass tubes allow of aeration of the solutions.
Photo. G. B. ( x i.)

from which particular elements have been omitted. Thus instead
of magnesium sulphate, potassium sulphate could be added, giving
a magnesium-free solution. The growth of a plant in such a solution
could then be studied, and by this procedure it is found that omission
of any of the seven elements mentioned prevents proper growth (see
Fig. 75). In addition to these elements it now appears that certain
others, for example boron and manganese, must be accessible, to
some plants at least, in order to secure proper growth. These elements
are only needed in minute quantity, and it is probable that they are
present in sufficient amount as impurities in ordinary water cultures.

,n BOTANY 01 THE LIVING PLANT

musl be supplied in particular forms

i iably utilised by the plant. Thus sulphur

and phosphorus must be in the form of sulphate and phosphate

trvely, while nitrogen is mosl suitable in the form of nitrate,

<>r of unmonium Baits.

Though the reasons why the elements mentioned should be essen-
tial to plant lopment scarcely fall to be considered at this point,
it may be mentioned thai nitrogen, sulphur and phosphorus are
requir protein syntl while magnesium is a constituent of
olorophyU molecule. The absence of iron also interferes with
chlorophyll formation and leads to a condition of chlorosis, in which
the l< have a sickly yellow colour. The reason of this may
that iron acts as a catalyst in the building up of the complex
molecule of chlorophyll. Iron and its compounds may also serve as
in the cell in other connections, while the same applies
to potassium. Calcium is essential since it is a constituent of the
pectic bodies which, as mentioned on p. 24, enter into the composition
of the cell -wall, particularly of the middle lamella. The application
within the plant of elements such as boron is at present uncertain.
In addition to these uses of elements derived from the soil, it is certain
that inorganic salts and ions figure in the actual make-up of proto-
plasm (Chapter III), and in regulating its activity.

The Soil and Plant Distribution.

The essential materials required from the soil are much the same
for all plants. It is, however, a matter of common observation in the
1 and in the garden that a soil which favours certain plant species
1- unfavourable to others. Some plants require more of a particular
rial than do oth. This applies especially to water, and

the water-content of the soil, which depends on the climate, on
make-up of the soil and on topographical conditions, is a very
important factor in plant distribution. Requirements also vary with
I to mineral nutrients. Thus some plants, such as the Nettle
^ rtlca til"1 to have a high nitrogen requirement and

Irish best in the vicinity of dwellings or in other positions where
n and nitrates are relatively abundant. Among culti-
thep high potassium, and swedes and turnips a

hphosphoru uirement. The distribution of plants is markedly
thePr< »n tlu-M.il of lime or other compounds of cal-

cium. Some plan* - heep's Sorrel (Rumex acelosella), Heather

THE WATER-RELATION 113

(Calluna vulgaris) and Bog Moss (Sphagnum), thrive best under a
deficiency of lime, and regularly inhabit such soils. Other plants,
such as Dog's Mercury (Mercurialis perennis), are lime-loving. It is
not so much that the calcium requirements of plants varies, but
rather that lime is the chief basic substance of the soil, and its presence
affects the properties of the soil very considerably, especially the
soil-reaction. Some plants prefer an acid soil, others a neutral or
slightly alkaline one. Plants are also liable to be affected by the
presence of non-essential salts such as sodium chloride. Most plants
are unable to make satisfactory growth in a soil, such as that of a
salt-marsh or other maritime area, in which sodium chloride is rela-
tively abundant, because of the harmful effect of the salt. The group
of plants known as Halophytes (Chapter XI.) are, however, able to
thrive in such places, and their development is actually improved by
the presence of the salt. Enough has been said to indicate the com-
plexity of the relationship between the plant and the soil, the number
of factors that come into play, and the degree to which soil factors
affect the distribution of plants.

B.B.

( 1IAPTER VIII.

SYNTH! STORAGE AND BREAKDOWN.

Livi ells and the plants which they compose are characterised

narkable chemical activity. They promote a great variety

hemic. d transformations which collectively constitute the Meta-

boli the plant. We shall consider first the constructive phases

of plant metabolism. One of the most important properties of the

green plant is its ability, when supplied with a few simple raw materials,

elaborate or synthesise within its cells a great variety of organic

sub- vhich are utilised in the further growth of the plant, and

in other activity The raw materials, comprising carbon dioxide,

and mineral salts, are said to undergo assimilation into the

substance of the plant. In a study of the synthetic activities of the

plant the process of Photosynthesis, in which sugars are elaborated

from carbon dioxide and water, occupies a very prominent position

and will now be considered.

Photosynthesis.

:m implies, photosynthesis is a constructive process in
which tli -nee of light is a necessary condition. It is of twofold

importance to the plant, for not only is it an essential step in the
manufacture of carbohydrates for incorporation into new cell-walls
new proteins, but it is also a process whereby light-energy
derived from the sun is fixed and stored in the plant for future use.
This • n of solar energy occurs because the green plant is

I with a mechanism for conducting the synthesis of sugars
from carbon <;. and water, an energy-consuming reaction, at the

CX] inlight. The amount of energy utilised,

114

SYNTHESIS, STORAGE AND BREAKDOWN 115

expressed in terms of heat units, is indicated in the following equa-
tion which sums up the process of photosynthesis.

6C02 + 6H20 + 674,000 calories = C6H1206 + 602

That is, the synthesis of a gramme-molecule (180 gm.) of sugar

requires the equivalent of 674,000 calories of heat-energy. It will

be seen from this equation that

photosynthesis consists of the

formation of sugar from carbon

dioxide and water, energy being

utilised and oxygen liberated.

The green pigment of plants,
Chlorophyll, is a key substance in
photosynthesis. The pigment is
located in special protoplasmic
bodies, the Chloroplasts, which are
restricted to those organs of the
plant that are exposed to light :
they are especially abundant in
the mesophyll of the leaf, which is
the chief organ of photosynthesis.
Experiment proves that only those
parts of the plant that contain
chlorophyll can carry on photo-
synthesis. To confirm this, varie-
gated leaves may be used, and it
can be shown that only the green
parts of the leaf can conduct
photosynthesis. In order to de-
tect the occurrence of photosyn-
thesis one method is to test for
the presence of the end-products.
Sugars themselves are not so easily
detected as the higher carbo-

„ Jlt.LJLaj / shows the inclui

plastid has swollen in water. (After Sachs.)

plants is very quickly formed

from the initially-produced sugars. A plant with variegated
leaves is exposed to sunlight for several hours, at the end of
which time a leaf is to be detached, and the outlines of the yellow
and green parts noted. The leaf is dipped in boiling water and
then immersed in methylated spirit, which is heated over a water-

Fig. 76.

A. Chloroplasts in cells of the leaf of Funaria,
showing small starch-grains included in them.
B shows stages of division of the chloroplasts.

hydrate Starch, which in many ^J?,,*^^ starch"grains -after

|l6 r.,,| \\\ OF Mil- LIVING PLANT

h until all the green pigment ha n extracted. The leaf is then

immersion in hoi water and placed in iodine solution. It

rill ,und that the ar< the leal which were originally green

nnw take blue-black colour, due to the presence of starch within

tlu. tl while the non-green parts show no such coloration. A

mii. lamination o\ the leal would show that the starch is

Mllv deposited within the- chloroplasts (Fig. 76), while other

ments indicate thai the oxygen of photosynthesis is liberated

n the chloroplasl ee later). We may therefore conclude that

tstricUd to green parts of the plant and that it proceeds

in the chloroplasts.

,<• chemical and physical properties of chlorophyll have been

clog 1. Chlorophyll is a complex substance built up

RED

ORANGE.

YELLOW GREEN BLUE

VIOLET

7000A.

6000 &

5000 A

4000A.

Fig. 77-

rptfcn Spectrum at an ether-solution erf Chlorophyll (actually the a form).

lack and thaded regions are respectively those in which Dthe light
r., I.t.lv or partially absorbed. The wave-lengths in Angstrom

unr iwn below. (Re-drawn from Willstatter and Stoll, 1918.)

a the clement- carbon, hydrogen, oxygen, nitrogen and magnesium :
and it is interesting to notice that it is related chemically to Haemo-
ibin, the red blood-pigment of mammals. The necessity of iron
for 1 hlorophyll formation, as already mentioned, may be due to that
substance acting as a catalyst in the building-up of the complex
molecule of the pigment. Chlorophyll can be extracted from the
by alchol or other organic solvents, and if such an extract is
I through a spectroscope it will be found that of the various
colours which make up sunlight, certain red rays are strongly ab-
sorbed by the pigment, and the blue-violet rays slightly less strongly,
while the intervening green and vellow rays mostly pass through
tbsorbed see Fig. ;;). The green leaf itself has similar absorptive
propertii Various experiments, some of which are mentioned
below, ii. that if plants nrc exposed to light of different colours,

ynthe I rapid in the red ana (at least according to

some m\ 0 in the blue-violet rays: i.e. the same rays

thai absorbed by chlorophyll. From this it may be concluded

that chlorophyll al the particular light-rays the energy of which

. used in photosynth*

SYNTHESIS, STORAGE AND BkKAKIX )\V\ n;

It should be mentioned that two slightly different forms of chlorophyll

(a and b) are always present in the chloroplasts, the significance of this being
unknown. Accompanying the green pigments are two yellow pigment"
Carotin and Xanthophyll, whose function in the chloroplasts is also uncertain!
It may be noted that the carotin of plants is of great importance to animal
nutrition, since it is from that substance that Vitamin A is produced in the
animal body. These yellow pigments also occur separately from chlorophyll
in yellow and red flowers and fruits and other organs, for example the carrot.

Fig. 78.

Results of experiment to show that Photosynthesis only proceeds in parts of the
plant that are exposed to light. See Text.

Since light-energy is utilised in photosynthesis, it is not to be
expected that the process should continue in plants or parts of plants
that are deprived of light. If a potted plant is placed in darkness,
application of the iodine test will show that the amount of starch
present in the leaves gradually decreases until a negative result is
finally obtained. This usually requires from twenty-four to forty-
eight hours. The explanation is that the starch originally present
has been used up for respiration (see p. 133), or transported in soluble
form to other parts of the plant : and owing to the absence of light
no new starch has been photosynthesised. If the plant is returned
to light, starch formation is resumed. If, in a particular leaf, light

BOTANY "i THE LIVING PLANT

,K. 0„ ■• ,,„ pai the leaf, starch formation

wiI! to those parts. To a demonstration of

thi il may be prepai rom a piece ol opaque material, such

,1 t0 the upper surface of a starch-free leaf. After

• J hoi I the leaf to light, application of the iodine

. thai only the areas that were exposed to light will

blue-black colour, indicating that photosynthesis has

limit, parts (Fi( 78). Clearly the effect of light is

light r. 1 be used in photosynthesis only by those

that tually illuminated.

md plant depends on the atmosphere for the carbon dioxide

in ph, This is indicated by an experiment in which

r. previously freed of carbon dioxide, is passed through

a bell-jar which 1- d to a glass plate and contains a number

with their stalks in water. The leaves should be initially

;.. 117). Although the bell-jar is placed in a good

light, a subsequent test will show that under these conditions no

photosynthesis occurs. Alternatively, normal air may be passed

illuminated leaves and the issuing air shown to contain less

1 dioxide than it did originally.

rbon dioxide is present in the atmosphere to the extent of 3 volumes

r 10.000 volumes of air. This is an average figure and may be departed

from at certain times and places. Thus in the neighbourhood of extensive

n the carbon dioxide-content of the atmosphere may during the

middle of the day be appreciably below the figure mentioned, owing to photo-

' tin. there is a constant evolution of carbon dioxide from the

.1. due to the respiration of soil organisms, and the proportion of the gas

present in the atmosphere Dear the soil is liable to be considerably above

the aver are. The average figure is kept constant because the various

proce ting it, photosynthesis and plant respiration being the chief,

equilibrium. The amount of carbon dioxioc available to the plant

it tirst sight to be very small, though it must be remembered that

if mo I the ^ absorbed at any point their places are taken by

others diffusing from neighbouring regions of the atmosphere. It is, how-

:i that tin- photosynthetic process is often hindered by the low

proportion of carbon dioxide that is present in the atmosphere (see p. 119).

In order to reach the actual site of photosynthesis, the molecules

of carbon dioxide musl first diffuse through the stomatal pores

Fig. 79)i these being usually open in daytime, to which period

naturally restricted by considerations of lighting.

or vapours can be absorbed or evolved by the leaf tissues

through the stomata with surprising rapidity (see p. 104), and it is

SYNTHESIS, STORAGE AND BREAKDOWN

119

quite certain that the fact that the carbon dioxide has to diffuse
through the stomata in no way restricts photosynthesis, provided
the stomata are appreciably open. The gas then diffuses through the
continuous system of air-channels that permeates the leaf. According
to the general view, the carbon dioxide next becomes dissolved in
water present in the cell-walls bounding the air-spaces, and subse-
quently diffuses in solution into the cytoplasm of the cells, where the
chloroplasts are situated (Fig. 79). So long as the gas continues

Fig. 79.

Part of leaf of Narcissus in transverse section. Two of the cells of the mesophyll
are drawn in in detail : that to the right as seen in surface view from without ; that
to the left in optical section. The chloroplasts are shown black. ( x 300.) F. O. B.

to be used up in photosynthesis by the chloroplasts, there will be a
constant inward flow of further carbon dioxide from the external
atmosphere, because of the tendency of a gas to diffuse from a region
of higher concentration to one of lower. In the case of submerged
water-plants the carbon dioxide present in the water round about
the plant is that utilised in photosynthesis. The carbon dioxide
enters these plants in solution rather than in the gaseous form :
it should be remembered, however, that in land-plants also entrance
into the actual cells can only be effected in solution.

There is no doubt that the rate of photosynthesis is under natural conditions
frequently restricted by the low carbon dioxide-content of the atmosphere,
which can be termed the limiting factor at such times. Experiment shows
that the rate of photosynthesis can be accelerated by increasing the supply
of carbon dioxide. The growth of the plant is consequently benefited, and
experiments in greenhouses have proved that by enrichment of the air with
additional carbon dioxide the yield of tomato and other crops can be very

120

BOTANY OF THE LIVING PLANT

materially increased, though so far there has been no extensive application
of this knowledge to practical operations. Ultimately a point is reached at
which the gas begins to exert a harmful effect on the plants, though previous
to this it may be observed that continued addition of carbon dioxide produces
little further benefit ; probably some other factor such as light intensity is
inadequate to support higher rates of photosynthesis, and it now becomes
the limiting factor.

It is by virtue of the evolution of oxygen during photosynthesis that
plants can be said to " improve the air " in the presence of light, a

Fig. 80.
Arrangement for showing that oxygen is given off in Photosynthesis.
A, before exposure to light, the tube is filled with water. B, after
exposure for some time. A large volume of discharged gas has
collected in the tube. (After Noll.)

property which was noticed many years ago. The oxygen that is pro-
duced in photosynthesis diffuses from the chloroplasts through the
cytoplasm and cell-walls and thence in the gaseous form through the
air-spaces of the leaf, and finally emerges into the atmosphere, apart
from that utilised in respiration (see p. 133). The production of the
gas is most easily demonstrated in water-plants. If a cut shoot of
Canadian Pond Weed (Elodea) lying in water is exposed to bright
sunlight, a continuous stream of bubbles may be seen to escape from
the cut end of the shoot, especially if the water has previously been
enriched with extra carbon dioxide. The temperature also must be
favourable. The gas can be collected by the arrangement shown in
Fig. 8o, and tests will show that the gas contains a high proportion of

SYNTHESIS, STORAGE AND BREAKDOWN 121

oxygen, produced by photosynthesis. Some of the oxygen so produced
diffuses away from the plant in solution, but owing to the low solubility
of the gas in water it accumulates under pressure in the gaseous form
inside the intercellular spaces and escapes from any cut surface. The
rate of production of bubbles gives a rough index of the rate of photo-
synthesis, and experiment would show that the bubbling slows down
if the light-intensity be considerably reduced, and would stop if
water free of carbon dioxide were to be supplied to the plant. Or
again the method could be employed to compare the rates of photo-
synthesis in light of different colours, and to demonstrate the fact
that up to a point increase in temperature hastens photosynthesis,
if other factors are favourable. Another method, chiefly perhaps
of historical interest, for demonstrating the liberation of oxygen
involves the use of certain bacteria which show movements only if
supplied with oxygen. If a filament of Spirogyra (a simple water-
plant, see pp. 372-3) is mounted in water containing the bacteria, and
air excluded, the movement of the bacteria continues in the vicinity
of the plant, provided it be exposed to light. The production of oxygen
is thus indicated. A special tendency has been detected for the bac-
teria to accumulate near the chloroplasts. Other experiments
involving the use of methods of gas analysis show that the volume of
oxygen produced in a given time by photosynthesis is equal to that of
the carbon dioxide absorbed.

It is impossible to attempt here any detailed consideration of the
chemical changes that may be involved in the building-up of sugars
from carbon dioxide and water. There is no doubt that the synthesis
involves a number of intermediate reactions, and though actually we
have little information about these, it has often been supposed that
first the substance formaldehyde is produced according to the
equation : QQ^ + Ha0 = h • CHO + 02

and that six molecules of the aldehyde then become united together
to give sugar : 6H • CHO =C6H1206-

This latter reaction is easily effected in the test tube by subjecting
formaldehyde to the action of alkalies. Each of these steps may
itself involve a series of intermediate reactions. The evidence in
favour of the formaldehyde theory is rather slender, and indirect rather
than direct in nature ; but it should be realised that the investigation
of the chemistry of photosynthesis is beset with great difficulties.
There are two obvious lines of attack, namely, to test for the presence

BOTANY OF THE LIVING PLANT

of formaldehyde ii lis during photosynthesis, and to try to

:n the plant Btarting from artificially supplied
maldehyde rather than from carbon dioxide: neither has yielded

Other in ators have attempted to obtain a

phot ' carbohydrates from carbon dioxide and

in artificial ms where the chemistry of the process could

tudied. Exti cted chlorophyll has been introduced

into Bome of tl While some investigators claim to have

■ vntlu inder such conditions, all such claims

:i sub i strorjg criticism, and so far this line of investi-

tionhasn isted very mtich in the elucidation of photosynthesis

in the plant.

I: a :i\ believed thai the r61e of chlorophyll is not limited to the

I ii m of light-energy, and to the application of that energy to the

vnthft. stem ; but that the pigment also participates, possibly

in the ity of a catalyst, in the chemical changes of photosynthesis.

One well-known theory assigns a function of this type to the pigment in

:u<n with the formation of the formaldehyde, which is believed to

an intermediate product in photosynthesis.

Appl n of analytical methods to leaves shows that sugars are

the firs! products of photosynthesis, as already stated. It is probable

that the simpler suj Glucose and Fructose, are those actually

med first (see next section). In many plants the initially-formed

to a considerable extent converted quickly into Starch,

which acts as a temporary7 reserve substance. But in a number of

plants, as for example many Monocotyledons, no starch is formed and

cumulates as such.

Estimates of the rates of carbohydrate-synthesis under natural
ditions have yielded very variable results: but in order to give
of the extent of photosynthesis, it may be mentioned
that Sat I Deluded from his measurements that as much as 25

irbohydrate may be formed within a square metre of
S t" during ummer's day. The carbohydrate of the

I would be mostly in the form of starch, and if this quantity of
out on scales it will be seen to be of very consider-
able bulk ; in forming that amount of starch the leaf would deprive
50 cubic metres of normal atmospheric air of its carbon dioxide.

In this consideration of photosynthesis we have looked at the
Pr( from tl indpoint of the plant itself: the process is also

of profound significance to the animal kingdom, a matter which is
Considered al I 1 of this chapter.

SYNTHESIS, STORAGE AND BREAKDOWN 123

Chemosynthesis — Certain bacteria are able to synthesise organic carbon-
compounds from carbon dioxide and water without the assistance either of
light or of chlorophyll by a process known as chemosynthesis. An example
is provided by the nitrifying bacteria (p. 452). The necessary energy for the
synthesis is obtained chemically by the oxidation of environmental materials.
Certain other bacteria contain a green pigment related to chlorophyll and
carry on a photosynthetic process resembling to some extent that of higher
plants.

Carbohydrates.

In the preceding section we have chiefly considered the synthesis of one
type of carbohydrate — the sugars. From these there arises within the plant
a whole range of more complex carbohydrates, by processes independent
of light or chlorophyll : these may occur in any part of the plant. Carbo-
hydrates figure very prominently in plant metabolism, and this is a convenient
point at which to review them. They will be considered in order of increasing
complexity. It will be noticed that carbohydrates are all compounds of
carbon, hydrogen and oxygen, in which there are twice as many hydrogen
atoms as there are 3.: oxygen. In treating of them frequent reference will
be made to Enzymes : a later section of the present Chapter will be devoted
to a special consideration of these highly important protoplasmic catalysts
(p. 128). The carbohydrates may be grouped as follows :

(1) Monosaccharides . — Most of these have the molecular formula C6H1206 ;
and the most important examples in the plant are Glucose (Grape Sugar)
and Fructose. It has been mentioned that the initial product of photo-
synthesis consists probably of one or both of these sugars, and they are very-
common in plant-cells, both of the leaf and of other parts. The plant-cell
is able to convert one into the other, though no enzyme promoting the
conversion is known.

(2) Disaccharides. — These have the molecular formula C12H22011 and like
the first class are sugars. The molecule of a disaccharide is built up by the
combination of two monosaccharide molecules with the elimination of a mole-
cule of water. The most important example, Sucrose (Cane Sugar), prepared
commercially from plant sources (Sugar Cane or Sugar Beet), is built up by
combination of glucose and fructose in the manner mentioned, a com-
bination which is probably accelerated in the plant by the enzyme Invertase.
It can easily be demonstrated that this enzyme at other times accelerates
the decomposition of sucrose into glucose and fructose. Sucrose and these
two monosaccharides are together the commonest sugars of the plant. Another
disaccharide, Maltose (Malt Sugar), has also been detected in plants in small
quantity. It is produced by the action of the enzyme Diastase on starch,
but in the plant the maltose is quickly converted by the enzyme Maltase
into glucose, from which maltose is built up.

(3) Polysaccharides.

These are complex substances built up by the combination of many mole-
cules of monosaccharide sugars, again with elimination of water ; the empirical
formula for most of them is (C6H10O5)n. The molecular weights of the poly-
saccharides are not definitely known. Unlike the sugars, the polysaccharides
are tasteless, colloidal substances.

BOTANV OF THE LIVING PLANT

. i it a common polysaccharide in plants, and is built up from many

molecules. LI forma ubstance, either of a

in fee chloroplasta oi the leal where its | e in

„ Ulth pi a noted: or of a more permanent

IMt. many kinds. It ia almosl always deposited

in the form ol grains within plastids, either the « nlortplasts of green organs,

,,r . tponding but colourless Uucoplasts ol uon-green organs (Figs.

j1( g, | lh,- presence within plant tissues oi an enzyme, Diastase,

TQB.

Fig. 8i.
inn Potato. A, with minute leucoplasts surrounding the nucleus.
B already fanning starch, stained darkly with iodine. C shows

furt irmation, and one cubical protein crystal. ( ■ 220.)

which promotes the decomposition of starch into maltose, can easily be
demonstrated, and it ia generally assumed that the same enzyme at other
times builds up starch from maltose. This enzyme appears to be located
in the pi. 1st ids. Starch grains show a characteristic stratification, which is
related to their growth by apposition (Fig. 82), while the blue-black coloration
with iodine is also distinctive. They may be simple or compound, and are
larger in storage organs than in the leaf.

Inulni i> another reserve polysaccharide, built up from fructose, and differ-
farther from starch in its restricted occurrence in plants, and by its exist-
in .1 state of colloidal solution in the cell-sap. It is present in large
quantity in the storage organs of members of the Compositae, e.g. Dahlia
roots and Artichoke tubers.

Uulose is the substance from which the walls of plant-cells, especially
of young cells, are largely built up, and is thus of structural rather than of
nutrition. d importance. The cellulose is deposited by the protoplast. Its
molecule ia larger than that of starch, but it again is built up from a large
number of glucose mole ules, presumably under the influence of enzymes.
An enzymi m 1 apable of converting cellulose to glucose has been detected

in a few of the lower plants. Cellulose ia a colloidal substance with a strong
attr.u tion for water ; hence the cell-wall usually contains a high proportion
of imbibed water. 1 the original cellulose may become impregnated with

other substances Mich as Lignin (in xylem, and particularly in fibrous ele-
ments), giving increased mechanical strength : or Cutin and Suberin, in
lermal and cork cells, to which they give impermeability to water (see

SYNTHESIS, STORAGE AND BREAKDOWN

125

A somewhat different substance known as Reserve Cellulose occurs in certain
seeds (Lupin, Date) : and the cell-walls are in consequence much thickened
(Fig. 83). As its name implies, this substance is of nutritional rather than
structural significance. During germination it is converted into sugars under
the influence of an enzyme Cytase.

Fats, although not carbohydrates, may be considered here. Fatty sub-
stances may figure in the make-up of protoplasm, as already mentioned,
and so be present in any living cell. It is in seeds and fruits that fats are best

Fig. 82.

Starch grains from the Potato. A. simple ; B, half
compound ; C, D, compound grains. c, organic
centre, or nucleus of formation. ( x 540.) (After
Strasburger.)

Fig. 83.

Cell-wall of a single cell of the endo-
sperm of Lodoicea, consisting of reserve
cellulose which forms the thickened
regions of the wall. ( x 400.) (After
Gardiner.)

known and most abundant, forming in many cases the chief non-nitrogenous
reserve substance. This for example is the case in the seeds of the Castor-oil
plant {Ricinus) and in various nuts, — Brazil, Walnut and Hazel (see Appendix
B). The fats exist in the form of globules in the cytoplasm : they contain
the elements carbon, hydrogen and oxygen, and are organic salts produced
by reaction between fatty acids and glycerine, a basic substance. There is
no doubt that fats arise from sugars within the plant cell, though the steps
in the transformation are obscure. It is believed that fatty acids and glycerine
are separately formed from sugars, and the two then react together. During
the germination of fatty seeds a reverse change occurs, the fats being re-
converted into sugars. The well-known enzyme Lipase effects the formation
of fats from fattv acids and glycerine, and under other conditions it promotes
the reverse reaction.

[26

. B01 VNY OF THE LIVING PLANT

P ITEIN S HESIS.

Proteins and carbohydral re the most important constituents
of the plant body, though there is In addition a great range of sub-
Btan >ther typ< •- we saw in Chapter III., the especial im-

portan proteins lies in the fad that the living substance, proto-

plasm, i- largely made up from them. Chemical analysis shows that
proteins and their derivatives may account for as much as 60 per

•it. of the dry weight <<t protoplasm.

The protein molecule contains the elements carbon, hydrogen,
oxygen, nitrogen and sulphur (the latter in very small proportion).
It ia an dingly complex structure with a molecular weight

running into many thousands.

Ji

Fig. 84.

A great many different pro-
teins have been detected in
plants, and each plant con-
tains a variety of them. They
are all built up in the main
from Amino -acids. These are
organic acids containing the
amino group, - NH2. Glycine,

A, cell from the endosperm of Ricinus in water, Acnarrir Arirl Trwr^rnT^ricm^

which causes the outer coat of the aleurone grains to ^P^rL1(- AUU, irypiopnane

swell. B. is dated .ilcurone grains in oil. A: = albumen „nrl fVQfinp mo\r hp nampH

tab. g globoid ( > 540.) (After Strasburger.) dnu ^yatme mav De namea

as examples. Some of these
amino-acids are themselves quite complex, but it is probable that
several hundreds of amino-acid molecules are required for the forma-
tion of a single protein molecule. Certain proteins, distinguished
as iVwc/Voproteins, contain phosphorus in addition to the above
elements. The nuclear structures of the cell are believed to con-
sist largely of these. As mentioned in Chapter III., proteins give
colloidal solutions, and it is doubtless in this state that proteins are
t-nt in the protopla-m.
In addition to the protoplasmic proteins there are also reserve
ems, regularly presenl in seeds and other storage organs either
in a crystalline form (Fig. 81), or in the form of grains, known
Aleurone grains. These may be quite complex in structure, as
t'»r example in the Castor Oil seed (Fig. 84). Here the aleurone
in contains within an outer coat of protein a protein crystal, and
a so-called globoid consisting of protein associated with inorganic
groups. Enzymes that are capable of resolving proteins into their
constituent amino-acids are presenl in plant tissues: this process

SYNTHESIS, STORAGE AND BREAKDOWN 127

occurs extensively during germination. Information regarding
these protein-splitting enzymes is as yet rather incomplete : for
the present purpose it will suffice to refer to them collectively as the
Proteases.

For the synthesis of proteins in the plant a supply of nitrogen
is required. We must therefore consider the sources from which
nitrogen is secured for this purpose. The typical plant is dependent
for its nitrogen supplies on combined forms of the element, absorbed
from the soil, and is incapable of utilising the free nitrogen of the
atmosphere. Water culture experiments of the type described on
p. hi lead to the conclusion that plants are incapable of making
appreciable growth if no compound of nitrogen is supplied to them.
Further, experiments show that for the majority of plants nitrates,
ammonium salts, and to a lesser extent nitrites, are the most suit-
able sources of nitrogen ; and there is no doubt that it is in these
forms, especially the first two, that the plant obtains its nitrogen
from the soil. Soluble organic forms of nitrogen also occur in soils,
and may be used to some extent. The supply in the soil of soluble
nitrogen-compounds is maintained as the result of a constant cir-
culation of nitrogen between the plant world, the soil, and the atmo-
sphere. There is also a somewhat similar circulation of carbon.
After the death of a plant (or animal) the materials of which it is
composed become eventually added to the soil, and under the influence
of bacteria and fungi they pass through processes of decay. The
nitrogen present in the proteins is restored to the form of nitrate as
the result of a series of transformations described in detail on p. 452,
though some absorption by plants may occur before the nitrate
stage is reached. An important part is played by those soil bacteria
(Azotobacter, Clostridium and Bacillus radicicola) that possess the
special faculty, denied to the great majority of organisms, of utilising
or "fixing" atmospheric nitrogen. The second of these is discussed
on the page quoted, while the third, which lives symbiotically with
certain higher plants, is considered in Chapter XII. The activities
of these organisms result in an enrichment of the soil as regards
nitrogen, though this is counterbalanced by other factors which lead
to a loss of combined nitrogen.

The nitrates or other nitrogen-compounds are absorbed by the
root hairs, and are probably transported through the plant by the
transpiration stream along with other salts, of which sulphates and
phosphates are also needed for protein synthesis. While the synthesis
of sugars is dependent on light and on the presence of chlorophyll,

(Js BOTANY 01 rm LIVING PLANT

and therefore confined to aerial and green tissues, protein synthesis

■anally independent of light, and <An probably take place in any

living cell in the presence of suitable raw materials. It is likely, how-

that the leaf cells are the most important sites of protein synthesis,

it i- here thai sugars are most abundant. We have little definite

information with regard to the intermediate stages in the synthesis

proteins within the plant, though it can be assumed that the process
consi sentially in the interaction of nitrogenous compound- with

the sugars of photosynthesis or their derivatives. A common view
has been that by interaction of ammonia, derived from the inorganic
nitrogen absorbed from the soil, with organic acids arising from
sugars a whole ran^e of amino-acids is produced in the plant. These

ids may then become linked up to form protein under the in-
fluence of the enzymes mentioned above, — the proteases, — which
.it other times catalyse the splitting of proteins. This view is, how-
ever, rendered rather uncertain by experimental evidence which
indicate- that substances known as Amides (especially Asparagine)
may figure prominently in protein synthesis. Energy is required
for the synthesis of proteins. In this case the energy is not derived
directly from the sun's rays, but only indirectly through the process
of respiration as discussed below (p. 133).

Enzymes.

A number of enzymes have already been mentioned. These agents
play a most important part in plant metabolism, since they enable
the protoplasm to carry out at high speed chemical transformations
which otherwise would not occur at all, or only very slowly. Enzymes
are in many respects comparable to the Catalysts of chemistry, and
they are in effect protoplasmic catalysts. As an example of ordinary
catalysis we have the very rapid combination of hydrogen and
oxygen to form water in the presence of finely divided platinum, the
latter acting as a catalyst. The combination of the gases is extremely
slow in it- absence. An important feature of catalysis, including
enzymic catalysis, is that the catalyst remains unchanged at the end
of the reaction and can be used over and over again.

The chemical nature of enzymes is uncertain, though it is agreed
that they are complex substances, probably present in a colloidal
form in the cell. It is to this colloidal nature, and the resulting high
adsorptive capacity, that the catalytic properties of enzymes are in
part attributed [Chapter III.). It was formerly believed that they

SYNTHESIS, STORAGE AND BREAKDOWN 129

were proteins, though examination of the relatively pure preparations
of enzymes that have been obtained in later years has not in every
case confirmed this belief. It is, however, very difficult to prepare
enzymes free from contamination with other cell-contents. Enzymes
may be extracted from plant tissues with water or other solvents
after the cells have been disrupted by pounding, or the cell-
membranes rendered more permeable by pre-treatment with alcohol,
or by freezing. For example, a crude extract containing the enzyme
diastase can be prepared from germinating cereal grains by mashing
the grains in water and standing the mixture aside for a time. Experi-
ment shows that the extract is able to effect the conversion of starch
into sugar, so that evidently an enzyme may retain its activity after separ-
ation from the plant. Extracts so prepared contain a mixture of en-
zymes in additions to other cell-contents, and special methods have
to be adopted in order to isolate a particular enzyme.

The following is a list of the better known enzymes of the plant,
together with the reactions which they characteristically catalyse
outside the plant : —

Invertase catalyses Sucrose -> Glucose + Fructose.

Diastase ,, Starch -> Maltose.

Maltase ,, Maltose -> Glucose.

Lipase ,, Fats -> Fatty acids + Glycerine.

Proteases ,, Proteins -> Amino-acids.

Zymase ,, Sugars -> carbon dioxide + Alcohol.

Every plant cell almost certainly contains all these enzymes and
others not mentioned here.

Many of the reactions proceeding within the plant are reversible,
that is they may proceed in either direction. For example during
the daytime starch is formed in the chloroplasts of the leaf from
sugars, while at night the starch is converted back into sugars.
Under the conditions usually obtaining in experiments with enzymes
outside the plant, it is the breaking-down actions (presented in the
above table) that are most manifest : but in certain cases it has been
shown that the opposite, that is a building-up or synthetic property,
is also possessed by enzymes. It is assumed that this finding applies
to all enzymes concerned with reversible reactions. Circumstances
prevailing within the cell will decide the predominating activity of
such an enzyme at any moment, the relative concentration of sub-
stances concerned in the reaction being especially important. Thus
in a reaction A +± B, if substance A is considerably in excess of B,

B.B. I

I30 BOTANY OF THE LIVING PLANT

lh, tic will proceed chiefly from left to right. As the concentra-
tion of B increases the right to left action commences and finally
a itate oi equilibrium la reached. If, however, B is removed as fast
M it is formed, or if if is converted into an insoluble and chemically
inactive Bubstance, the left to right action will continue to predominate.
fhe period oi germination of seeds or of renewal of growth of other
resting organs is one characterised by intense enzyme activity. The
reserve substances (starch, protein, fats, reserve-cellulose, etc.) of

B

Jill

Fig. 85.

A-F, Leucoplasts from tuber of Phajus, showing various stages of development
of ttarcb grains. 1 -4. Various stages of the corrosion of starch-grains in germinating
Barley. (After Strasburger.)

the resting organ are mostly insoluble and indiffusible. Before being
transferred from the region of storage to the growing points, they
are converted into soluble diffusible forms. This mobilisation of
reserves is promoted by appropriate enzymes, each acting in a down-
grade fashion as indicated in the above list. Fig. 85, 1-4, illustrates
the process of solution of starch grains under the influence of diastase
during germination of barley.

Translocation and Storage.

The division of labour between the different organs of the higher
plant requires that there shall be a constant movement of metabolic
materials from point to point within the plant, to which the term
Translocation is applied. The chief directions of translocation are :
(a) from the leaves to actively growing parts, (b) from the leaves to
Murage organs, and (c) from regions of storage to actively growing

SYNTHESIS, STORAGE AND BREAKDOWN 131

parts. In addition we have the upward movement of salts from the
roots to the aerial parts of the plant, to which the term translocation is
sometimes extended. This has already been discussed in Chapter YII.

The leaf is the organ in which the synthesis of carbohydrates and
probably of proteins is chiefly located, and provision must be made
for the conveyance of these manufactured materials from the leaves
to the apices of root and shoot, or to regions where there is cambial
activity. Further, the rate of synthesis of materials in the leaf is
usually in excess of that demanded by current growth. The balance
of material is deposited in various parts of the plant in the form of
carbohydrate (starch, sugar, inulin, reserve cellulose), fat, or protein
and may serve for future use. Any parenchymatous tissue may serve
for storage. For instance, in trees which lose their leaves in autumn
and form a new suit of them in the spring, the material for these is
prepared in the previous season, and stored in the medullary rays,
wood parenchyma, and cortex of the trunk and branches. But in
many cases, especially in herbaceous plants, storage is effected in parts
which may become greatly distended, as in the turnip, carrot, and
potato (see also Chapter XL). Seeds habitually contain stored
material, the product of the activity of the parent plant. They meet
the needs of the seedling in the early stages of growth. Storage, in
one form or another, is then a common phenomenon in plants. It
involves the translocation of the materials from the point of produc-
tion, usually the leaves, to the point of storage.

The translocation of manufactured materials out of the leaf to
regions of growth and storage proceeds constantly, but is most
easily detected when the leaf is in darkness. Synthetic processes
are then stopped or slowed down. Suitable tests show that both
the carbohydrates and protein of the leaf decrease in amount over-
night, chiefly as the result of translocation. Thus, application of
the iodine test will show that the starch content of the leaves of a
given plant is much less in the morning than in the previous evening,
while if the plant is kept in darkness for a further period all the
starch will disappear.

At a later date the reserve substances stored in the various tissues
mentioned above will be required for growth and undergo trans-
location, as already noted. At all times materials to be translocated
must be brought into a soluble form, i.e. they must be mobilised.
Materials undergoing translocation are mostly crystalloidal in
nature, though there may also be some movement of colloidal
materials. There is evidence that sucrose is the usual form in which

BOTANY OF THE LIVING PLANT

carbohydrates are branslocated, while nitrogenous compounds may
mn\ imino-acids, or as amides.

I, [s now held by most authorities that longitudinal translocation
metabolic materials in the plant is chiefly through the sieve-tubes
the phloem, excepl for the upward passage of inorganic salts,

which in all probability occurs with the
T< £1 transpiration stream in the xylem (see

J I Chapter VII.). The structure of the

V f 0 sieve-tubes (see p. 48) appears to facili-

tate a lengthwise flow of materials
through them, inasmuch as they are
elongated and the cross-walls are per-
forated to form the sieve-plates, per-
mitting of free movement of material
from one tube to the next. Further,
the contents of sieve-tubes are rich in
carbohydrates and nitrogenous sub-
stances. More direct evidence is fur-
nished by ringing experiments. It can
be shown that the removal over a short
zone of a leafy stem of the tissues
FlG. 86. external to the cambium interferes with

Lower parts of willow cuttings which th downward translocation of materials

had been induced to form adventitious

mots by standing in water. The right- r ^ Jeaves . as indicated by re-

hand cutting was initially ringed ' '

■hotly above the base. Note the effect duced growth of the parts of the plant
.•11 the position of root-production. ( x §.) » r r

below the ring, and by accumulation of
< arbohydrates and nitrogenous compounds above the ring. A tree is
Usually killed if its trunk is ringed, because the ring interrupts the
flow of food substances from the leaves to the roots, which are in
consequence gradually starved. The result of ringing cuttings is
illustrated in Fig. 86. The production of the roots above the ring
it least partly due to the arrest of the flow of nutritive materials
which diffuse from the upper part of the cutting to the region of
root-initiation. Corresponding results have been obtained in connec-
tion with the upward movement of organic materials through the stem,
such as occurs in a deciduous tree in spring, wrhen reserves stored in
the trunk are transported up to the opening buds. These ringing
experiments indicate that the conveyance of organic materials is
effected not in the xylem but in some more external tissue :
and the considerations already mentioned point to the conclusion
that the phloem is the tissue in question. This view has received

SYNTHESIS, STORAGE AND BREAKDOWN 133

confirmation in more critical experiments, which have revealed a
close correspondence between the daily fluctuations in the concen-
tration of carbohydrates and nitrogenous substances in the leaf-cells,
and those in the sieve-tubes of the stem below the leafy zone.

The movement of soluble materials through the sieve-tubes is
not a matter of simple diffusion, although it has certain resemblances
to a process of that type : such as the fact that often, though not
always, translocation occurs from regions of higher concentration
of a particular substance to those of lower. But the rate of trans-
location is greatly in excess of that which could be accounted for by
diffusion alone. One suggestion is that passage of material through
the sieve-tubes is hastened by a circulation of the protoplasm. But
this phenomenon, though fairly common in plants, is not regularly
observed in sieve-tubes. Another suggestion calls in the aid of
osmotic forces. At present, however, we have no definite information
of the forces productive of translocation.

Though the sieve-tubes provide the main channels for translocation,
it is clear that since relatively few cells are in direct contact with those
elements, translocation from one organ to another must involve
in both organs a cell-to-cell movement of materials in tissues other
than the phloem. It has been suggested that materials in solution
may pass rapidly from cell to cell through the finer protoplasmic
connections (see p. 26), as they are known to do through the larger
protoplasmic connections traversing the sieve-plates.

Respiration.

A living organism requires a constant supply of energy. The
most obvious way in which energy may be used in a plant is in the
various chemical syntheses ; in the particular instance of photo-
synthesis the energy of light from the sun's rays can be utilised
directly, but for the rest, and for other vital activities, an internal
source of energy is necessary. The readiest source of energy
available for the ordinary purposes of men is by the combustion
of fuel, such as coal or wood : that is, its oxidation, the ultimate
products being carbon dioxide and water. The latent energy of the
fuel is converted on combustion into kinetic energy. This supplies
the motor impulse for engines of various kinds. In a somewhat
similar way there proceeds, within the cells of the living plant or
animal, what may be described as a slow physiological combustion

134 BOTANY OF THE LIVING PLANT

of materials, yielding chemical energy for the cell-processes. The
term Respiration is applied to any such energy-releasing processes
in plants, some type of which is carried on in every living cell.

The type of respiration normal for the great majority of plants
consists in the oxidation of sugars to carbon dioxide and water ;
it may be summarised by the equation :

C6H1206 + 602 = 6C02 + 6H20 + 674,000 calories.

A considerable amount of energy is evidently produced by the com-
bustion of sugar : it is equal to that required for the building-up of
sugars (see p. 114). The process represented by the above equation
is in effect that of photosynthesis reversed, though the intermediate
stages are very different, as we shall see later. The sugars oxidised
in respiration have been previously manufactured in the plant by
photosynthesis. Since the sugar molecules are broken down in the
process, we may see in respiration a phase of metabolism which is
destructive in nature, as compared with the constructive phases
which we have so far considered. In photosynthesis the plant
stores up potential chemical energy within the sugar molecules,
while it is able by means of the respiratory process to release that
energy as required and at any point in the plant body.

The equation indicates that respiration in the plant, as in the
animal, is attended by the absorption of oxygen, which is obtained
from the atmosphere (or from photosynthesis) and by the evolution
of carbon dioxide. While in the animal there may be mechanical
inhalation and exhalation of the gases involved the respiratory
interchange in the higher land-plants depends on simple gaseous
diffusion between the atmosphere and the air spaces of the tissues,
in aerial organs via the stomata and lenticels. In roots oxygen
enters dissolved in the water which the root absorbs, while the
carbon dioxide produced in respiration escapes from the roots in
solution. Similar considerations apply to submerged aquatic plants.

The evolution of carbon dioxide by plants is most readily demon-
strated in the case of germinating seeds or of flowers, in which respira-
tion is specially active ; while photosynthesis, which tends to mask
respiration, is absent. If a stream of air, from which the carbon
dioxide initially present has been removed by passage through
caustic soda, is drawn through a flask containing such respiring
material, and then through a further flask containing lime-water, a
copious precipitate will soon be formed in the latter. The evolution
of carbon dioxide is thus demonstrated.

SYNTHESIS, STORAGE AND BREAKDOWN

135

By a suitable elaboration of this arrangement the amount of carbon dioxide
evolved could be determined and an index of the rate of respiration so ob-
tained. It could be shown that respiration quickens with increasing tem-
perature until levels are reached at which the cell-structure is adversely
affected ; also that actively-growing material such as germinating seeds
respire more quickly than mature or dormant parts of plants. This observation
is indicative of a close connection between respiration and growth. If by
any treatment respiration is checked, growth is correspondingly depressed.
Practical advantage of this has been taken in the storage of fruit and vegetables.
By storing these in an atmosphere which is initially rich in carbon dioxide,
respiration is reduced, the development and over-ripening of the material
is slowed down, while the growth of micro-organisms is likewise discouraged.

Fig. 87.

Arrangement for demonstrating gaseous interchange in Respiration, a, shows an
earlier stage of the experiment ; b, a later stage. See Text.

Another instructive experiment dealing with the respiratory
interchange of plants is shown in Fig. 87. A quantity of flower-buds
or of germinating seeds is placed in a long-necked flask, which is then
fixed with the neck projecting downwards into a dish of mercury.
A strong solution of caustic potash is then floated above the mercury
in the neck. The potash serves to absorb the carbon dioxide which
we have seen to be produced by the respiring material. At the same
time the mercury is observed to rise up the neck of the flask (see
Fig. 87, b), indicating that some part of the original atmosphere of
the flask has been absorbed by the respiring material. Actually it
is the oxygen that is so absorbed, as is indicated by the observation
that the mercury continues to rise until the volume of the gas within
the flask has been reduced by about one-fifth.

I36 BOTANY OF THE LIVING PLANT

In photosynthetic cellfl such as those of the leaf, the breakdown
(,t bu| on side by Bide with their synthesis, though only a

small proportion of the sugars manufactured in photosynthesis is

msumed in the respiration of the leaf. One result of this predomin-
ant oi photosynthesis is thai in green organs such as leaves, when

*>sed to light, the gaseous interchange of photosynthesis, which
we have seen consists in t he absorption of carbon dioxide and

ilution of oxygen, masks that of respiration and is alone in evidence.
In order to demonstrate the latter process in such organs light must
be excluded, photosynthesis being thereby prevented while respiration
continues, since it is independent of light.

If we refer back to the equation for respiration we see that the
volume of oxygen absorbed should be equal to that of carbon dioxide
produced; so that the so-called Respiratory Quotient, that is the
r.itio of carbon dioxide given off to oxygen absorbed, should equal
unitv. Experiments show that this is usually approximately true.
There are exceptions, one being provided during the germination of
seeds with fatty food reserves, the fats providing the initial fuel for

miration. Here a greater volume of oxygen is absorbed than of
carbon dioxide produced, giving a quotient of less than unity (fre-
quently in the region of 0-6). The probable explanation is that a
preliminary to respiration proper is here the conversion of the fats
to sugars. Oxygen is needed for this, over and above that subse-
quently required for the oxidation of the sugars. Conversely the
volume of carbon dioxide evolved may exceed that of oxygen ab-
sorbed ; carbon dioxide may even be produced without any absorp-
tion of oxygen. This type of respiration is displayed by plants when
they are deprived of oxygen. It is known as Anaerobic Respiration,
as distinct from the normal or Aerobic Respiration carried on in air,
such as we have so far been considering.

Anaerobic respiration can be demonstrated by passing a few
germinating peas up into an inverted test-tube completely filled
with mercury and supported in a dish also containing mercury. After
some hours gas will be found to have accumulated over the mercury,
and will continue to increase in amount for several days. On testing
the gas it will be recognised as carbon dioxide. In this type of respira-
tion sugar is again consumed, though here the products are carbon
dioxide and ethyl alcohol. There is in fact evidence that in this
anaerobic respiration of higher plants we have a process very similar
to the well-known alcoholic fermentation promoted by the fungus Yeast.
Higher plants do not, however, live very long if they are deprived

SYNTHESIS, STORAGE AND BREAKDOWN 13;

of oxygen, probably because the alcohol produced is poisonous to
their tissues, or because the energy liberated by anaerobic respiration
is insufficient to maintain life.

There is no doubt that the respiratory processes are very complex
and involve a chain of intermediate reactions, many of which are
those of catalysis activated by enzymes or other protoplasmic agents
present in the cell. This conclusion is forced upon us by consideration
of the fact that in the laboratory carbohydrates are relatively difficult
of oxidation. It is widely believed that in normal aerobic respiration
there are two chief phases, only the second of which actually requires
atmospheric oxygen. In the first phase the sugars are thought to
be broken down to simpler compounds, possibly through the agency
of the enzyme zymase which is present in higher plants, though
known best in yeast. This first phase is considered to be common
to both normal aerobic respiration and to that of the anaerobic type.
If oxygen is present the products of the first phase are believed to
be oxidised with the final production of carbon dioxide and water.
In this oxidative phase enzymes known as oxidases and dehydrases
are thought to play a part. If on the other hand oxygen is absent,
the degradation of the sugar is less complete, and the products of
the first phase are converted to alcohol and carbon dioxide. By
this scheme normal and anaerobic respiration are linked up and
regarded as alternative developments of a common initial phase.
Yeast differs from higher plants in that it continues to exhibit the
anaerobic form of respiration even when oxygen is present.

Some of the respiratory energy of the plant is evolved as heat,
as may be shown if steps are taken to prevent loss of the heat by
radiation. Thus germinating seeds if enclosed in a thermos flask,
with precautions to prevent bacterial growths, will show an appre-
ciable rise in temperature. The amount of heat so produced in
plant tissues is small, and under normal conditions is quickly lost
by radiation, so that body heat is not a characteristic of plants.
The temperature of plant-organs, at least in darkness, is much the
same as that of the surrounding atmosphere.

Concluding Remarks.

In this study of metabolism we have seen how the green plant
utilises simple materials from its environment and from them ela-
borates carbohydrates, proteins, and other complex substances.
This ability to make use of entirely inorganic materials for purposes

i38 BOTANY OF THE LIVING PLANT

imitation is practically limited to the green plant. As men-
tioned in the Introduction the activities of the plant in this way are
of fundamental importance to animals, as well as to non-green plants ;
such organisms being dependent for their food on the organic materials
lynthesised by the green plant. Animals can utilise organic food
only, and the food of an animal is always derived, directly or in-
directly, from plants. An herbivorous animal feeds directly on plant
materia] While a carnivorous animal feeds on other animals,
the latter will probably be herbivorous in diet. And thus the so-called
"food-chains" are established which always terminate in the green
plant. This holds not only on land but also in the sea, where minute
plants such as the Diatoms are the ultimate source of the food of
all marine animal organisms.

In animals food is used partly for purposes of body-building, in
which connection the Vitamins, essentially of plant origin, are of
recently-discovered importance ; also as a source of energy for vital
activities. The energy which an animal obtains by respiratory oxida-
tion of food materials is derived from the sun, and was originally
fixed by some plant, in the process of photosynthesis. Thus the whole
organic world depends on the sun for the energy which its vital
processes require ; all this energy is originally trapped by green
plants in photosynthesis. The future may," however, see some reduc-
tion in the dependence of the animal kingdom on plants ; for means
may be devised for synthesising food materials without the co-opera-
tion of the plant.

In addition to the energy which we consume physiologically, the
vast supply of energy which modern civilisation requires is largely
vegetable in origin. Materials such as wood, coal, peat and prob-
ably petroleum have been ultimately derived from plants, and
the energy liberated from their combustion was originally fixed
in photosynthesis. Thus we return into present currency balances
of the sun's energy stored in the Earth's crust from an earlier
age.

These general remarks are, however, a digression from the study
of the plant. Returning to the green plant itself, its constructive
metabolism provides new material. This is required in the first
instance for the nourishment and growth of its several parts : that
is for the maintenance of the individual. But secondly, it is required
for the increase in number of individuals. The propagation of the
race can only be carried out when sufficient material is at hand
from which to form new germs.

CHAPTER IX.

GROWTH, IRRITABILITY AND MOVEMENT.

In the preceding chapter we considered the processes of synthesis in
which raw materials derived from the environment are built up within
the plant into substances such as sugars and various organic nitro-
genous compounds. Except for the proportion that is consumed in
respiration, these products of synthesis are ultimately utilised in the
formation of new tissues, leading to the growth of the plant. We
have now to consider various aspects of growth processes in the plant
and of associated phenomena. This includes a study of the sensitive-
ness or irritability which growing organs of the plant exhibit towards
certain directive influences to which they are exposed during their
development. And it will lead to a general consideration of phenomena
of movement in plants.

I. Growth.

Growth is one of the most conspicuous features of Life. Its most
obvious sign is increase in Size. But as applied to a living organism
the term means something more than mere enlargement. It involves
change and transfer of materials which cannot by any means be
restored to their original state. Nor is it merely that the existing
plants or parts of them enlarge as seen from without : for additional
organs are successively developed, so that growth of the plant
commonly leads to an increasing complexity of form, and also of
internal structure.

I. Germination.

In entering on the studv of Growth it will be convenient to con-
sider it as it is seen in Flowering Plants, and to trace it from the

i39

I40 BOTANY OF THE LIVING PLANT

.,„,„.„;„„ ()t lll(. I(li | hapter [.), The external conditions

nducing to germination of the dormant seed are :

Xhc pr« of moisture, which causes swelling, and the passage

fr<jm that state of desiccation which its tissues show in the

dormant seed. The mobilisation of the food reserves soon

follows (sec Chapter Y1II.).

(b) Access to air : actually it is the oxygen component that is

needed for respiration.

(c) Temperature within the range to which vital activity is re-

stricted.

While in a majority of cases light is without effect, in some seeds germination
is retarded or even prevented by the incidence of light, while in others light
has a benefit ial effect. The retarding effect obtains in the seeds of some
Phloxes, the benefii ial one in seeds of Mistletoe {Viscum album), Tobacco and
Purpl< .strife [Lythrum salicaria). The full explanation of these effects

1-, not j < t available

It does not follow that germination will always occur when the external
conditions are favourable. The seed may continue to show dormancy as the
result of the operation of some internal factor. Seeds of many species are
incapable of germinating immediately after liberation from the parent plant.
This may be because the embryo is immature at the time of separation from
the parent, as it is in the seeds of the Lesser Celandine (Ranunculus ficaria),
Marsh Mangold (Caltha palustris), the Ash (Fraxinus excelsior), and perhaps
the Sycamore (A cer psendoplatanus) . When the seeds of such plants are placed
under suitable conditions for germination, the development of the embryo
is completed, but germination is arrested until that stage is reached. In
the Sycamore and Ash germination may be held up in this way for months,
and so tide over the winter. In a number of other cases the testa has pro-
perties which for a certain time prevent germination. It may show a high
degree of impermeability towards water, so that no absorption proceeds
even if the seeds are submerged. This is the case in seeds of many members
of the Leguminosae, as for example Gorse (Ulex europaeus), Clover (Trifolium
spp.) and Sweet Pea (Lathyrus odoratus). The testa gradually becomes
permeable to water, but germination may be delayed for months or even
years ; there is, however, great variation in this respect among the seeds
from a single plant. The advantage of this may be that the germination
of the seeds produced by a parent plant is spread over several months or
years : the chance of some of the seedlings finding favourable conditions for
their development is thereby increased. By chipping the impermeable testas
of these seeds prompt swelling and germination can be secured. In other
cases the seed coat is at first relatively impermeable to oxygen and carbon
dioxide, respiration and hence germination being impeded : in other cases the
seed coat restrains the sprouting of the embryo for a time mechanically.

The period over which seeds retain their capacity for germination,
though denied conditions suitable for it, — that is their viability, —

GROWTH, IRRITABILITY AND MOVEMENT 141

varies very considerably. Seeds of Willow (Salixs pp.) lose it a few
days after liberation from the parent plant ; those of a number of
other native trees lose their viability a few months after shedding.
But records are available of the long survival by seeds of periods of
enforced dormancy. Thus those of a leguminous species, taken from
a dried herbarium specimen, were still capable of germination though
known to be over 150 years old. Examples are also known of seeds
having lain dormant buried deep in the soil for very long periods.
When grassland, which has at some previous period been arable, is
again ploughed up, a crop of characteristic weed-plants of arable
land may be obtained in the first season. There is evidence that
the seeds producing these plants have lain dormant at low levels in
the soil for periods up to fifty years. It is possible that the dormancy
here is due to the high concentration of carbon dioxide present at
low levels in the soil. This will arrest their respiration, and hence
their growth (see p. 334).

2. The Vegetative Phase of Growth.

(a) General Features.

Germination having been effected, the young plant enters upon
a period of growth in which the plant body characteristic of the
species is built up. The particular form assumed by a plant is the
result of the interaction of external conditions of growth with the
internal hereditary factors (Chapter XXXV.).

Growth is initiated in the meristematic regions of the plant, where
new cells are formed. Meristems are found at the apices of shoots
and roots, and also in the form of cambium, often present in older
parts of the plant. In a meristematic cell new protoplasm is elaborated,
and this is usually accompanied by an increase in the size of the cell.
In due course division of the nucleus and protoplasm occurs, and with
the laying down of a new wall two daughter cells are formed. The
meristematic regions are characterised by the constant repetition of
these events. The influences that induce the continued activity of
the meristems are but little understood : there is, however, evidence
that cambial activity in the stem is induced by an activating
substance diffusing from apical regions (p. 1 50). The formation of
new protoplasm and of cell-walls in the meristematic regions neces-
sitates a supply of organic materials such as amino-acids, amides
and sugars. These are transferred to the growing points from the
leaves, where they are originally manufactured ; or from storage

l42 BOTANY OF THE LIVING PLANT

organs (p, I JO). The total mass of the meristematic cells at the
apex Of a root or shoot remains relatively constant because the
cella on the side of the meristcm remote from the apex are suc-

rivcly ceasing to divide, and pass into the phase of enlargement.
Much the same applies to cambial activity. In an organ of limited

>Wth, such as a leaf, all the cells soon pass into the later phases
of development.

On entering the phase of enlargement the newly-formed cells under-
go a very considerable increase in size, due not so much to further
elaboration of protoplasm within them, as to the intake of water.
It is to this that most of the increase in size of the growing plant
is due. This enlargement of new cells, with the attendant formation
of vacuoles, has already been described on pp. 21-22, though the
mechanism was not there considered. It was formerly believed that
cell-enlargement was due to an increase in osmotic pressure of the
cell-sap. This might be produced by the conversion of insoluble
into soluble, crystalloidal substances, e.g. starch might be converted
to sugar. Experiment has, however, failed to reveal evidence of such
increase. It is now believed that the intake of water is due rather
to an increase in the plastic extensibility of the walls of the newly-
formed cells, although presumably there is a production of additional
osmotic substances while the stretching is in progress, so as to counter-
balance the tendency to dilution caused by the absorption of water.
During the stretching of the cells additional layers of cellulose are
deposited on the walls, resulting in an increase in thickness rather
than the decrease which would otherwise follow. The final arrest
of the stretching may be due to the continued strengthening of the
walls in this manner. The stretching of the cell-wall during growth
is permanent : plasmolysis of the mature cell does not result in the
shrinkage of the cell-walls to the dimensions which they possessed in
the newly-formed stage.

A considerable interval may elapse between the emergence of
cells from the meristematic phase and their enlargement. Instances
of this are seen in the resting buds of trees. By meristematic activity
the various parts of the shoot for the forthcoming year are laid down
within the buds during the summer and autumn : only in the sub-
sequent spring do the cells undergo enlargement, causing the unfolding
of the buds.

In roots and stems the enlargement of cells is chiefly a matter
of elongation. This follows with some regularity on the emergence
of the cells from the meristematic phase, so that behind the growing

GROWTH, IRRITABILITY AND MOVEMENT

143

points of root and shoot is a region of cell-elongation, which is also
the region in which the extension of the organ itself chiefly occurs.
These growth zones can be located by marking on a young shoot or a
root a series of initially equidistant dots of Indian ink (Figs. 88, 89).
After a day or so the distances between the dots are re-measured,
and the amount of elonga-
tive growth in this way is
ascertained for different
parts Of the organ. In the
shoot the greatest elonga-
tion wrill be shown in a region
considerably below the tip,
wThile it gradually diminishes,
on the one hand towards the
tip and on the other down-
wards, until a point is
reached where growth has
ceased (Fig. 88). This de-
monstration at once accounts
for the general contour shown
by growing shoots. In the
apical bud the leaves are
closely grouped because the
axis has not yet extended
to its full dimensions. The
length of the internodes (or
intervals between the leaves)
increases downwards till their
growth is complete, and
their full length has been
attained. Below that point
the stem has become rigid,
owing to general thickening
of the cell-walls as they ma-
ture. In roots the cells enter
on the phase of elongation with less delay and complete it more
rapidly than in stems : with the result that the region of most active
growth is more compact. It is localised within a few millimetres of the
tip of the root (see Fig. 89, also p. 88). This proximity of the zone of
growth to the apex may facilitate the penetration of the soil by the
root : if it were further back, bending of the apical part might follow,

Fig. 88.

The left-hand figure shows a growing shoot at the
beginning, the right hand-figure at the end of the period
of observation. See Text. (After Errera.)

,u BOTANY OF THE LIVING PLANT

a long nail tends to bend when it is being driven into hard
wood. It will be seen later thai the elongation of the cells of shoot

.,n«l root i- subject to control by the apical tissues.
The elongation of cells in the zone of most active growth of a
m or a i liable to be proceeding, at any moment, at varying

rates on different sides of the organ.
In this event the apical part of the
organ will tend to curve away from
the side of greatest growth. If the
tendency towards most rapid elonga-
tion moves regularly round the stem,
the tip will exhibit a revolving move-
ment, and will pursue a spiral course
through space. The shoot-apices of
many plants do exhibit such a revolving
movement or circumnutation, as was
first discovered by Darwin. But the
movements are not readily discernible
owing to their slowness and small
amplitude. Darwin devised a method
of magnifying the movements and of
projecting a record of them on to a
plane surface (Fig. 90). It will be seen
that the movements are slow and some-
what irregular, although there is a rough
revolution round a central point. These

Fig. 89. r

Localisation of growth sear to the nutatory movements are of much greater

root-tii) of I'icia Faba. In I. the root •■• i 11 • 1 • t.

baa been marked with 10 zones 1 mm. amplitude and have special significance

■part In II. the same root after • . 1 1 r . ■ ■ 1 1 • -1

rhe lines nearer to the tip are in the shoots ot twining plants, and in the
ESfojn!eei?moBt **£? there.6 ^ftS tendrils of climbers (pp. 215, 2 1 6). Nut-

^trasburger.) . i r j •

atory movements are also lound in roots.
During the enlargement of the cells, or subsequent to it, structural
• litfcrcnti.it ion sets in: this leads to the production of vascular
elements from some cells, photosynthetic tissue from others; and so
on, according to their position within the growing organ (Chapter II.).
Tin- mark- the last stage in the formation of the cell : when completed
the cell is mature and normally it does not exhibit further growth.

{b) Measurement of Growth.

There are 1 attributes of the growing plant, or plant-organ,

that can be mad.- the basis of observation when an estimate of growth

.

GROWTH, IRRITABILITY AND MOVEMENT

145

9 - '

&V

Cito^-**

/O. 30

/o ro

is required. Thus we can measure increase in linear dimensions, in
volume, or in weight : the impression that is gained of the growth of
a plant or organ is, however, liable to vary according to the particular
index of growth that is adopted. The growth of a root or a stem
is usually measured by noting the increase in length over a given
time. For this purpose the horizontal microscope, of which different
patterns are available, may be employed, or one may use the anxano-
meter, in which the growth is magnified by means of levers or pulleys
for the purpose of measurement.
If the growth of the plant as a
whole is required it is usual to
observe the increase in its weight.
A special value as a growth-
index has been placed on the
dry weight of the plant, i.e. the
weight of matter remaining after
the water originally present has
been driven off by drying the
plant tissues in an oven. What
this method measures is the net
amount of environmental mater-
ial that has been incorporated
into the actual substance of the

plant, irrespective Of whether Record of circumnutation in a seedling made by
• . , . , , . Malins Smith. Note that the direction was reversed

this has been USed tor growth Or before 8.o a.m. in this record.

deposited as a storage material.

The determination of dry weight involves killing a plant, so that
consecutive observations cannot be made on a single plant : a series
of similar plants must be available when growth is to be measured
in this way.

(c) Factors affecting Growth.

Since in its growth the plant utilises the products of its assimilatory
processes, such as the synthesis of carbohydrate and of protein, it
is obvious that factors affecting those processes may also affect
growth. Thus growth will be hindered if adequate supplies of raw
materials such as carbon dioxide, water and nutrient salts are not
to hand : experiments indicating the importance of nutrient salts
have been described on p. III. Light must also be present if auto-
trophic growth is to occur, while oxygen is required for respiration.

Some of these factors have specific effect on growth, and these have
b.b. k

/*.*8/«*.

Fig. 90.

I46 BOTANY OF THE LIVING PLANT

„(lW to be considered, while in addition other factors, not so far
mentioned, also affed growth.

Light

light is necessary for photosynthesis, in which essential

materials for growth are prepared, it is obvious that there can be no

growth of the autotrophic plant in the absence of light unless supplies
rganic materials are available. Given such supplies

growth will occur. Thus the root system is normally able to develop

in darkness because it draws supplies of organic materials from the

aerial organs, which are exposed to
light : while growth of both root
and shoot may be obtained in dark-
ness from such structures as seeds,
tubers, or rhizomes, etc., where
reserve substances are present,
provided other conditions are suit-
able. The growth so obtained may
however be of a very different type
from that in light, making it clear
that light has a direct effect on
growth. This statement applies
chiefly to the shoot, the growth
of roots being usually but little
affected by the presence of light.
The shoot grown in darkness is
distinctly abnormal in appearance,
and is said to be etiolated. The
result of etiolation varies a good
deal, but in many species stem-
elongation is much more rapid than
in illuminated shoots, so that the
etiolated shoot becomes very tall,
an effect which is due to a greater
elongation of the cells rather than
to more rapid cell-division. Differ-
entiation of the tissues is incom-

plete in the etiolated stem, while the growth of the leaves, or more
ly of the leaf blades, is arrested (Fig. 91). Chlorophyll formation

1^ inhibited, the shoot having a white or yellow colour.
The appearance of an etiolated shoot suggests that in addition

to its nutritive effect, light has a direct effect on the growth of the

«V .r'A^S-

1 IG. 01.

N'.nn.il fl-fti and I plants of Broad

!'•• ID 1 .. :.i I .iba). ( • }.)

GROWTH, IRRITABILITY AND MOVEMENT 147

shoot : it controls the growth of the stem, induces leaf development,
and promotes internal differentiation. To some extent the light
may operate through its effect on the growth-regulating substances
of the shoot (see p. 148). In addition the reduced elongation of the
stem may be associated with a general decrease in cell-turgor
which results from the occurrence of more rapid transpiration in
the presence of light. Experiment shows that the blue and violet
constituents of white light are the most effective in this connection :
for instance, a plant grown in red light is almost as markedly etiolated
as one grown in darkness.

Although growth in the usual sense of the term is obtained in dark-
ness in the examples quoted above, yet the plant as a whole is steadily
decreasing in dry weight, since respiration is not counter-balanced
by assimilation of fresh material. The dry weight of those parts
of the etiolated plant that are actually developing — the root and
shoot — would, however, show some increase, since food will be
drafted into them from the storage tissue.

It is the want of sufficient light for normal development that makes
plants in crowded greenhouses and dwelling rooms grow " leggy,"
with unduly lengthened stems and leaf-stalks. Crowding of field
crops has a like effect, and often leads to the " laying " of corn under
heavy wind and rain before harvest, the plants being top heavy
and their stems weak. The apparent stimulating effect of darkness
on stem elongation has the advantage to the plant that the young
shoots of seeds or other organs sprouting underground grow rapidly
up through the soil, and are able to enter on photosynthesis earlier
than would otherwise be the case. In consequence of the controlling
effect of light, the rate of elongation of shoots is more rapid during
the night than in the daytime, provided the temperature is not too
low : while the short stature and rosette form of alpine plants is
partly due to the high light-intensity to which such plants are exposed.

Temperature.

It has previously been remarked that vital activity in general,
which includes growth, is only possible within a certain range of
temperature, which for many plants is approximately 0° to 450 C. ;
though there is considerable variation in this respect. Species from
temperate regions have lower temperature requirements for their
growth than those from the tropics. Alpine and arctic plants have
still lower requirements. It is a common fact of experience that
in temperate climates a rise of temperature from the normal accelerates

I r BOTANY OF THE LIVING PLANT

.hi ,, othcr conditions are favourable. In order to force plants

rows them in hot-houses, or in frames slightly
NVil'rn. nentative changes in rotting manure upon which

rram( i. The rise of temperature in spring and early summer

„• in the stimulation of growth at that season, while

i lv the tall of temperature in autumn is at least a contributary

the period of dormancy in vegetation at large which then sets

The high temperatures prevailing in tropical regions, combined

with the generally favourable conditions, result in very rapid growth :

thus sho : the Giant Hamboo may grow in length over a foot a

day, a rapidity which can be followed by careful observation with a

hand lens.

Exposure to temperatures in the region of o° C. results in growth and other

\ ital acth ities being arrested, but it does not necessarily kill plants ; neverthe-

in many plants, such as Tomato, Potato or Dahlia, fatal results may fol-

I n mi, li i • hough there may be a direct effect of the low temperature

on the protoplasm, it is the formation of ice in the tissues that contributes

tii- death of the plant. The ice forms in the intercellular spaces and

to the withdrawal of water from the vacuoles and protoplasm of the

lis. The protoplasm is killed by such dehydration, probably because its

proteins become coagulated. In addition, the development of ice within

th .-s may cause mechanical injury. By suitable treatment many plants

hardened," and are then able to withstand exposure to moderately

low temperatures without injury. Some plants, especially those inhabiting

polar regions, are so constituted that they can withstand exposure to very

low temperatures, in some cases as low as -50°C, without being killed.

The full explanation of this ability to withstand low temperatures is unknown.

1 xposure of plant organs to low temperatures results in the conversion of

any starch that may be present into sugars : the sweetening of frosted potatoes

cample of this. Experiment shows that in the presence of sugars,

proteins are less easily coagulated by freezing than otherwise. Also, hardy

plants or organs tend to be relatively low in water content : less ice can

therefore be produced, while of the water that is present an unusual proportion

may be bound up as water of imbibition in colloidal substances, and not

idily subject to freezing.

Water.

ill-enlargement is dependent on an adequate supply of water.
For normal growth the plant must have access to such a supply:
the plant will otherwise be stunted owing to the failure of the cells
to attain their normal size.

Internal Regulation of Growth. Plant Hormones.

An obviou are in the growth of a plant is that there is co-

ordination of the growth of its various parts, which is adjusted

GROWTH, IRRITABILITY AND MOVEMENT

149

in the interests of the organism as a whole. As a result the plant
body is well-balanced and forms an efficient physiological system.
This co-ordination, or correlation of growth arises from the exercise
of influence of different parts mutually upon one another. Its exist-
ence is frequently more obvious when a part of a plant is removed,
and the effect of the removal on
the rest of the plant is studied.
Thus, though many of the buds
on trees and shrubs normally
remain dormant, if the upper
growth is severely pruned or defoli-
ated through the action of some
pest, the dormant buds may resume
growth. For this reason regular
trimming of a hedge results in
denser growth below. In herbac-
eous plants, such as the Broad
Bean or Chrysanthemum, if the
terminal bud is removed, one or
more of the lateral buds, which
previously had remained dormant,
will become active, and continue
the growth of the shoot. In these
examples it would appear that the
growth of the dormant buds is
arrested through the exertion of
some influence by existing parts.
Not all correlative influences are
inhibitive. Thus the buds on a
cutting (p. 248) have been shown
to exert a promoting effect on the
formation of roots at the base :
while in the spring developing buds
on trees stimulate cambial activity in the branches bearing them.
It might be suggested that the dormancy of lateral buds was
simply due to inadequacy of food materials for their development.
Experiments have made it clear that this nutritive factor is not the
only one, and the present tendency is towards the view that corre-
lation within the plant is to a large extent due to the operation of
specific chemical growth regulators, an idea which Sachs advanced
many years ago. These substances are produced in one part of a

Fig. 92.

Lemon cuttings, the lower ones initially
treated at the base with a very dilute solution
of a substance promoting root-formation, the
upper ones untreated. Photographed 2% weeks
after the cuttings were taken. (From Cooper,
after Thimann and Went, 1937.)

BOTANY OF THE LIVING PLANT

plant and undergo transference to some other part, the growth of
^sequence modified. Such substances are now termed
Plant Harm they correspond to those products of the animal

>m to which the term Hormone was originally applied For
imple, there is evidence that the dormancy of lateral buds is due
the production within the terminal bud and young leaves of a
hormone which p ,l.»wn t he stem and inhibits lateral bud develop-

ment. A similar explanation is now forthcoming for the promoting
• ol buds on cambial activity and root formation. Chemical
have been discovered which simulate the effect of the
natural hormone By supplying such substances to the plant a
particular effect may be obtained more rapidly than would normally
be the case. Thus the formation of roots on stem-cuttings can
frequently be hastened by preliminary treatment with a variety of
chemicals (see Fig. 92).

The particular instance of hormone-action in plants that has been most
in-, i still remains to be considered. Experiments suggest that the

etching of cells in the zone of cell-elongation in shoots (pp. 142-3) is markedly
bed by a hormone which is chiefly produced by the tissues at the tip
of the shoot, and then diffuses back to the region of cell-elongation. A
Itate of affairs may exist in roots. These effects are of special import-
ance in connection with the tropic curvatures of shoots and roots (see p. 153).

1 far most of the work on this particular hormone-effect has been carried
out on the young shoot or sprout of seedlings of Oat or other Grass, since
it provides very convenient material for experiment. Actually it is the
outer cylindrical sheath, or coleoptile, of the shoot that is used in these

cperiments. This sheath encloses the developing foliage leaves. The actual
formation of the cells of the coleoptile is completed at an early stage, and the
Bobaeqnent growth of the organ, which occurs chiefly in the basal and central
zones, is due simply to cell-elongation. The coleoptile is an organ of limited
growth and is soon split open by the development of the leaves.

cperiment shows that if the extreme tip of the organ is amputated,
the growth of the coleoptile rapidly diminishes to a very low level. That
tl. t is not merely due to injury is suggested by the observation that

if the tip is immediately replaced in position after amputation the reducing
rowtfa is much less than in the absence of the tip. It appears
then thai the tip in some way promotes the elongation of cells in the lower
part of the organ, and there is convincing evidence that a growth-promoting
horn: 1 by the tip. For instance, a number of coleoptile tips may

be plan "1 on a thin shed of agar-jelly for a time. If small blocks of the agar
are then prepared and placed on decapitated coleoptiles, the growth of these
will Ik- inn. h stimulated : evidently as the result of the diffusion of some

nbstance out of the agar into the organ. If the blocks are placed asymmetri-
cally on the coleoptile a curvature away from the side below the agar block
will develop -• I , \ considerable amount of information about the

GROWTH, IRRITABILITY AND MOVEMENT 151

chemical nature of this growth-promoting hormone, which is known as
Auxin, has been obtained, and a number of other substances, natural
and synthetic, have been found to stimulate the growth of the coleoptile
much like Auxin itself. The man-
ner in which cell-elongation is
affected is not yet clear, though
there is evidence that the hormone
increases the plasticity of the cell-
walls (seep. 142). Similar experiments
to those described for the coleoptile Fig. 93.

have been carried out with shoots of Curvatures in decapitated Oat coleoptiles

resulting from the application of agar blocks
some dicotyledonous plants with like containing a substance of similar action to Auxin.

results • thus the stem anex annears The first leaf was partly pulled, °,ut of th* cole/
results . tnus tne stem apex appears optile at the commencement of the experiment,

to produce a hormone which promotes and in each plant it is seen projecting beyond the
„ , . top of the amputated coleoptile. (From Thimann

cell-elongation in lower regions of the and Went, 1937.)

stem.

The position with regard to cell-elongation in the root is rather obscure.

Parallel experiments to those described above suggest that root elongation

is somewhat accelerated by amputation of the tip, and that auxin and other

substances which promote the growth of shoots retard that of roots. The

explanation of these observations must await further investigation.

3. Physiological Aspects of Reproduction.

So far we have considered chiefly the vegetative phase of the
growth of plants, in which the plant body, consisting of root and
shoot systems, is built up. At the same time, many of the remarks
in the preceding section apply to growth processes as a whole. Sooner
or later in the growth and development of the higher plant repro-
ductive structures are produced, first the flowers and from these the
fruits and seeds. The manner of development and the structure of
these organs are considered in later Chapters: at present we are
primarily concerned with the study of the factors that induce the
formation of reproductive organs. In the case of annual and biennial
plants the origination of reproductive structures happens but once in
the life-cycle, and after their formation the plant dies. There is
some connection between the events, for if flowering and fruiting
are prevented the life of an annual plant can often be prolonged
considerably. In perennial plants, however, there is normally an
annual production of flowers, though the first flowering may not
occur until the plant has attained a considerable age. Thus the
first flowering in the oak may be delayed until the tree is forty years
old.

Much attention has been paid to the study of the conditions that
affect flowering and fruiting. Illumination is again important.

IM BOTANY OF THE LIVING PLANT

Howerin [uently tends to be arreted in plants growing in

inipl, in honeysuckle and other plants growing

r . though some plants flower freely in such

Th, , Kperiments of Klebs in this field of m-

him to the belief that {lowering is favoured by a

rbohydrates within the tissues, especially in

mparison with nitrogenous substances. Defective light will tend,

t on photosynthesis, to prevent this condition. An

t nitrogenous manure will also have the same result, and it

h | uently been observed to lead to vigorous vegetative growth,

Dut , production. In the presence of abundant reserve food

; may, however, occur in darkness, as in the case of some

bulbou- plants.

The length of daily illumination is also an important factor in
termining the time of flower formation under natural conditions:
artificially adjusting this the time of flowering can in some cases
►ntrolled, though a number of plants are relatively insensitive
to day-length. In <ome plants, of the so-called short-day type, flower-
ing , hastened by exposure to artificially shortened days.
samples are Dahlias, Chrysanthemums, and the Runner Bean.
- ich plants tend in these latitudes to produce their flowers in the
shorter days of autumn: but by exposing Dahlias to shortened days
from the time they were planted, flowering has been secured in July
instead of September. In long-day plants flowering can be hastened
by exposure to lengthened days, using artificial light for the purpose-
• lis .ire examples of this type, and they normally produce their
flowers in the summer months. Exposure of these plants to shortened
da ' nds to suppress flowering. These effects of the length of daily
illumination are not yet understood, but they have considerable
practical importance : thus in introducing a new crop into a particular
m, the influence of length of day upon flowering and fruiting
must be considered, and those varieties selected which are best fitted
local conditions in this respect.

•arc may also affect reproduction. It has been found that exposure
of a plant to low temperature during germination may hasten the onset of
flowering. This has Ken applied practically, and is described as vernalisation.
A Miml seen m biennials, such as the turnip, which tends to flower

in the tirbt year if exposed to frost.

4. Periodicity in Growth.

The plant exhib'ts frequent changes in its growth activities. Thus during
the course of the yearly growth -season the phase of vegetative activity is

GROWTH, IRRITABILITY AND MOVEMENT 153

succeeded by one in which reproductive structures are formed. In the autumn,
or earlier in some plants, growth tends to cease : the leaves and other aerial
organs may be lost, and a phase of dormancy opens. In annuals the death
of the plant occurs at this stage, the race being carried on by the seeds,
which themselves exhibit dormancy. It is these and many other types of
rhythmic change in its activities that justify the statement that periodicity
is a characteristic feature of the growth of the plant.

It would appear at first sight that many of these periodic changes could
be ascribed without much hesitation to the direct effect of obvious climatic
factors. Closer examination reveals a more complex condition : more obscure
environmental or internal factors are then discovered to be involved. It
has been pointed out that the dormancy of some seeds during the winter is
due, not solely to the unfavourable external conditions, but also to internal
factors (p. 140). Again the onset of dormancy in shrubs and trees in autumn,
often accompanied by leaf -fall, might be ascribed to the direct effect of
unfavourable climatic conditions of autumn and winter. But protection
of the plant from the unfavourable weather will not prevent leaf-fall, and
will not induce the dormant buds to grow out. Several months must elapse
before the buds can be induced to open by merely exposing them to a favour-
able temperature, though more drastic treatment may " force " the buds
into activity. Thus buds of lilac and of other shrubs can be induced to open
by immersion of the plant in a warm bath of water in the region of 300 C,
or by exposure to ether vapour. Continuous illumination from an artificial
source has been seen to break down the dormancy of buds of the beech. The
dormancy of potato tubers can be broken by treatment with certain chemicals,
a method which has been used in countries where the climate permits of suc-
cessive crops of potatoes in one season.

II. Irritability and Movement in Plants.

The organs of plants normally show definite orientation. Main
stems grow vertically upwards, main roots vertically downwards :
leaves occupy a horizontal or oblique position, while the lateral
branches of shoot and root usually grow out obliquely. These posi-
tions are those in which the respective functions of the organs are
most efficiently performed, and the assumption of them is due to
the sensitiveness or irritability of plant organs towards certain
directional influences. Gravity is the chief of them, though the
direction of incident illumination is also very important for the shoot.
These influences act as stimuli to the organs of the plant, which
during their growth assume a definite orientation with regard to the
direction of their impact. When the organs of the plant become
displaced from their customary orientation, or when the direction
of one of these stimuli is changed, then as the result of curvatures
and torsions which soon appear, the organs or the younger parts
of them tend to resume their normal relation to the stimulus. In

, BOTANY OF THE LIVING PLANT

wch circumstance* a type oi movement of plant organs is exhibited

In adjusting the <.nmt.it.un of its organs with regard to directional
stimuli the plant exhibits Tropic responses, or Tropisms. The re-
spouse to the force of gravity is termed Geotropism, that to the direc-
0j light, Phototropism. These phenomena will now be considered
in more detail.

i. Geotropism.
That the upward growth of the main stem and the downward
growth of the mam root are due to the influence of gravity might
dedui ed from the consideration that gravity is the only directional
influence of general operation that could be involved. Confirmation
that inference is provided by experiments with Knight's wheel
(sec p. 155). The root, growing towards the source of the gravity
tmulus, is said to be positively geotropic, while the main shoot,
rowing upwards and away from the centre of the earth, is negatively
otropic : organs such as lateral shoots and roots and many leaves
which place themselves in oblique or transverse positions are plagio-
tropic. The assumption of a strictly transverse position with
gard to the direction of gravity is sometimes distinguished by the
1 term diageotropism. Stolons, runners and some rhizomes
and leaves provide examples of this. An organ may exhibit different
types of geotropism in the course of its development. Thus in seed-
lings the upper part of the plumule, or of the hypocotyl in epigean
types, is frequently bent over and points downwards, becoming
upright after emerging from the soil (Chapter I.). This is, at least in
part, due to changed geotropic response. An advantage to the seed-
ling following from this is that the terminal bud is less liable to be
damaged by being dragged through the soil than by being pushed.
The upper part of the peduncle of the Poppy is also positively geotropic
before flowering and points downwards, but during flowering and fruit-
production the upright posture is gradually assumed. Some flowering
become positively geotropic as the fruits ripen. This is the case
in the Monkey-nut {Arachis), where the fruits are as a result pushed
d <>•.>. awards into the soil.

If the organs of a plant are displaced from their normal orientation,
in which a state of equilibrium exists with regard to gravity, then
•ropic curvatures may be exhibited. Thus if a seedling is fixed
horizontally or obliquely in a moist atmosphere, in the course of a
few hours curvature- will develop and will gradually bring the apical
parts of the root and stem back into the normal orientation (Fig. 94).

GROWTH, IRRITABILITY AND MOVEMENT 155

These geotropic curvatures appear in the zones of cell-elongation
(p. 143). They are due to the setting up of different rates of growth
on the upper and lower sides of the horizontal or oblique organ.
In a horizontally-placed stem growth becomes greater on the under
side of the organ than on the upper, while the opposite obtains in
the root. But it is only the apical part of the root or shoot that is
restored to its normal orientation : generally speaking the older
and basal part is incapable of longitudinal growth, and shows no
change of orientation. Geotropic
curvatures may also occur in plagio-
geotropic organs when they are dis-
placed.

The length of time required for the
development of a visible geotropic
curvature in a horizontally-placed
root or shoot (known as the reaction Seedling of Be^\ 94after having been

timp\ varies PTPaflv according to the germinated, and the radicle had grown

n?ne) varies greauy accurumg lu uic downwards and piumuie upwards, its posi-

material and the conditions. In some ^Tn^SL"0^ SZZSfSi

cases a reaction time as low as fifteen X^^^^ST^i^X

minutes is obtained, though values ft8 latter negatively geotropic. (Dr. j. m.

' ° Thompson.)

of one hour or longer are more usual.

The organ need not, however, be in the position of stimulation for
the whole of this period, but only for a part of it ; the necessary
time for effective stimulation being known as the presentation time.
It is possible to prevent the usual geotropic curvatures of roots
or shoots fixed in a horizontal position, by arranging for them to
be slowly but continuously turned about their long axes. For this
purpose the rotating Klinostat is used, and finds frequent employ-
ment in experiments on geotropism. In a root or shoot that is thus
revolved round a horizontal axis, that region of the organ which at
one moment is lowermost will in a short time be uppermost. Any
tendency to react to gravity which developed in the first position
would be cancelled by an equal but opposite tendency in the second,
and so on all round the organ, which therefore continues to grow
horizontally unless some other factor intervenes. A forerunner of
the Klinostat was the device known as Knight's wheel, used in experi-
ments carried out at the beginning of last century. Knight fastened
seedlings to the rim of a wheel which was rapidly rotated in a vertical
plane, a considerable centrifugal force being in this way set up.
Gravity was eliminated for the same reason as in the Klinostat :
but it was found that the direction of growth of the organs was now

BOTANY OF THE LIVING PLANT

trolled by the centrifugal force. The shoots grew towards the

Keel, the roots away from it. The fact that centrifugal

ible to control the orientation of plant-organs in these

eriments confirms the view that under normal conditions gravity

the determining influence, since centrifugal force resembles gravi-

tational fofce in that both will cause internal pressures within the cells.

Fig. 95.

hi longitudinal section of the root cap of Roripa amphibia, showing
Btan h paint settled to that side of the cells which was lowermost. 73=dermato-
calyptn

ue of root-cap of Hrlianthus annuus after 24 hours in a horizontal position
Arrow (O) shows axis of root. (S) shows direction of gravity.

I m igrammatii representation of the statolith cells of the root-cap of Pisum
• t rest ; b, 30 minutes in horizontal position ; c, J, cells fixed in inter-
mediate positions

(After Nemec.)

( toe well-known theory suggests that the sensitiveness to gravity is confined
to certain special cells which contain relatively heavy cell-inclusions, free
move in the cell : these will therefore tend to lie against the physically
a all. These inclusions are termed statoliths and it is suggested that
they normally consist oi Btaxcfa grains. It is certainly the case that many
plant-organs include cells containing starch grains which are free to move
in ] toplasm, though generally speaking their position is fixed. In

root-, tt» b cells arc usually present in the root-cap (Fig. 95), while in stems

GROWTH, IRRITABILITY AND MOVEMENT 157

they are often present in the starch-sheath in the younger part of the stem.
When a root or a "stem is displaced from its normal position and placed, say
horizontally, the statoliths under the influence of gravity move until they
lie against the wall which is now lowermost (Fig. 95, B, C). They will then
be pressing on a part of the cytoplasm that is not customary : the theory
suggests that this unusual pressure activates some mechanism which results
in growth being altered in such a way as to produce a curvature. This curva-
ture ultimately results in the statoliths returning to their normal position.

Though the statolith theory of gravity-perception is not universally
accepted, there is considerable evidence in its favour. Experiment shows
that in roots the sensitiveness to gravity is largely confined to the tip, i.e. to
the region where the statoliths are present. The remaining part of the root,
even the zone of cell-elongation where the geotropic curvatures are effected,
is relatively insensitive. That this is so is suggested by the observation
that a root which has been decapitated can be placed horizontally without
any resulting curvature, although the zone in which curvature would be
expected has not been removed. It might be suggested that this was merely
due to the injury involved in decapitation, but the fact that the ability to
execute curvatures is restored if the tip is replaced in position by means
of a drop of water or a thin layer of gelatin is against this view. In the
coleoptile of cereals the position is similar to that in the root : the statoliths
occur in the apex and sensitiveness to gravity is chiefly resident there. In
stems the apical part is in general again the most sensitive, though there is
not the same degree of localisation of sensitiveness as in the root. This is
in agreement with the occurrence of the statoliths, which are distributed
through the younger part of the stem.

Much attention has recently been paid to the importance of hormones
in the production of geotropic curvatures. Evidence has been obtained
that the development of a curvature follows on a re-distribution of the
growth -controlling hormone, which, as we have seen (p. 150), diffuses back
from the apices of coleoptiles, of shoots and probably of roots, to the zones
of cell-elongation. Under normal conditions the hormone arrives at the
zone of elongation equally distributed all round the organ : when the
organ is placed in an unusual position with respect to gravity the movement
of hormone appears to be in some way so affected that more arrives on the
lower than on the upper side of the growing zone. Since the hormone is
growth-promoting in the stem and coleoptile, and growth-retarding in the
root, an upward curvature is the result in the former, a downward in the
latter. According to this explanation the importance of the apex in geo-
tropic curvatures is that it is the source of the growth-controlling hormone.
The significance of the statoliths in these events is uncertain.

It has been remarked that geotropic curvatures in general are
only shown by young still-growing parts of plant organs, the mature
parts being relatively incapable of curving. An exception to this
statement is provided by the stem (haulm) of cereal Grasses, in
the lower part of which curvatures may be developed. The nodes
of these shoots, which are really the swollen bases of leaf-sheaths,

BOTANY OF fHE LIVING PLANT

a

retain the power oi growth. When the shoot is placed in a horizontal
or oblique position the nodes resume growth on the under sides,

with tin- result tli.it the- upper part of the shoot
again becomes erect (Fig. 96)- In this way
real plants which have been " layed " by
wind and rain stand up again if they are not too
old. Similar responses are to be seen in the

Carnation.

So far as leaves are concerned, the curvature

,„ of the upper part of the stem brings the

&rtrt younger leaves back into their normal position

*2HS£S3& £2 with regard to gravity. Leaves on older parts

| «<£*£ «t- have an independent capacity for adjusting

their position by means of curvatures and

torsions of the leaf stalk. In pulvinoid leaves,

which will be considered later, these occur especially at the pulvini.

2. Phototropism.
By Phototropism is meant the capacity of plant-organs to orientate
themselves in definite relation to the direction from which light
falls on them. Stems tend to orientate themselves so that they
point towards the source of light, and are said to be positively photo-
tropic. With overhead illumination the stem grows vertically :
lateral illumination frequently results in curvature (Fig. 97). The
curvature that develops in such a
laterally illuminated stem is again
due to differential growth, the cells
in the zone of elongation that are
away from the light tending to
elongate more rapidly than those
on the more brightly illuminated
side. The result is that the apical
part of the -tern is moved over until
it 1- dire* ted towards the source of
light. Before such curvature can

■-. .^^:^^- •--.. v •••■•-..■.

Fig. 97.
Phototropic curvatures in the hypocotyls of

develop, the tendency of the shoot J|£s££ {f^s) seedlines "^nated from
to remain upright as a result of its

geotropism has to be overcome : the position which is finally
umed is the resultant of the operation of the two tropisms. The
same applies to other phototropic responses.

GROWTH, IRRITABILITY AND MOVEMENT 159

Although earth-roots in general show no phototropic response,
those of some plants, such as Mustard (Sinapis), show negative photo-
tropism, i.e. growth away from the source of light. As in geotropism,
we see that here again the phototropic response of these roots is the
converse of that characterising stems, a fact for which there is as
yet no agreed explanation. The aerial roots of Ivy and the tendrils
of Virginian Creeper [Ampelopsis) are also negatively phototropic,
a property of obvious significance in relation to their function (see

also p. 217).

Leaves are on the whole diaphototropic, placing themselves so that
their upper surface is presented at right angles to the incident light,
a position which secures a maximum incidence of light on the leaf,
and is obviously a very suitable one for photosynthesis. Under
natural conditions of growth, light usually falls on the leaves of a
plant from a variety of angles : it is the direction from which the
most light comes that determines the position which the individual
leaves take up during their development. Phototropic responses
of leaves are usually prominent in plants grown in dwelling rooms or
against a wall. The adjustments of leaf-position in response to the
direction of light are mostly due to growth-curvatures and torsions
of the petiole. In the case of plants growing in tropical or other
regions where they are exposed to very intense sunlight, leaves may
take up a quite different position, one in which the incidence of light,
especially of the strongest midday rays, is minimised. Some examples
of this are given later in the section on Xerophytes. A further
well-known instance is provided by the so-called Compass plants,
of which the European Lactuca scariola is an example. When this
plant is growing in a position subject to strong sunlight, the leaves,
though actually produced spirally on the stem, twist until they all
come to lie in a vertical plane running North and South, i.e. in the
position in which they will be least exposed to the hot midday sun-
light. In this way overheating of the leaves and excessive transpira-
tion may be avoided.

As in geotropic responses, the type of phototropic response exhibited by
a particular organ may vary during its development. Thus the flower stalks
of Ivy-leaved Toadflax (Linaria) are positively phototropic during flowering,
but subsequently turn away from the light, with the result that the fruits
tend to be deposited in the crannies of some wall on which the plant usually
grows. Experiment has also revealed that the type of phototropic curvature
exhibited by an organ may vary with the strength of the light-stimulus. Thus
although positive phototropism is usually obtained with the coleoptile of

l6o BOTANY OF THE LIVING PLANT

nditions of light intensity and period of stimulation

urvatures have been obtained.

tain extenl the law of " quantity of stimulus " applies to photo-

ponses. The result of briei exposure to a strong unilateral light

ma) be the same ... thai of a longer exposure to a weaker light. The length

potme th.it ifl required to produce a subsequent curvature {i.e. the

presentation turn-) thus depends on the intensity of light that is used. Similar

considerationa actually apply to geotropism.

eotropic curvatures, so in those of phototropism there is
evidence that hormone re-distribution is involved. For the most
part this i- provided by experiments on the coleoptile, where there
is a similar localisation of sensitiveness and curvature as is encountered
in that organ with regard to gravity. If the tip is covered by an
opaque hood, and thus screened from light, the organ becomes much
less sensitive to lateral illumination, although the zone where curva-
ture would normally occur is exposed to the light. It thus appears
that the one-sided light acts more on the apex than directly on the
actual growing cells. Experiments suggest that there is in the later-
ally-illuminated tip a re-distribution of auxin, more accumulating
on the shaded side. This inequality is transmitted down to the
growth zone and is manifested there in a positive curvature, since the
hormone is growth-promoting (in this connection see Fig. 93). There
may, however, be in addition a direct, growth-retarding effect of
the light on the growing cells. Similar considerations apply to the
phototropic responses of stem structures in general.

3. Other Tropisms.

In addition to gravity and light, certain other factors exert directive
influences on the growth of plant organs. Roots are frequently sensitive to the
distribution of moisture in the rooting medium, and curve in their growth
towards the region of greater moisture-content, exhibiting the property known
rotropism. This particular tropism has not been very intensively in-
ted, and there is some doubt as to the degree to which it is developed
by rot ' eneral. The blocking of field drains by the invading roots of

neighbouring trees is probably an example of hydrotropism. Aerotropism, as
a result of which an organ is sensitive to the distribution of gases, especially
. has been detected in roots, which tend to grow towards a higher
1 of oxygen, as is perhaps seen in " pot-bound " root-systems.
Pollen tiilK-s, which grow away from a source of oxygen, behave in the
Opposite fashion (] ;a). Chemotropism, or sensitiveness to the dis-

tribution of chemical substances in the vicinity, is especially important
111 fun ter XXI Y.), since as a result their hyphae grow towards

d material : in the higher plants it is exhibited by roots. Hapto-

GROWTH. IRRITABILITY AND MOVEMENT

161

tropism, or curvature in response to contact, is best known in tendrils (p.
216). Contact of the tendril with a solid object causes growth on the side

Fig. 97 a.

Pollen-grains germinated in a nutritive medium under a cover glass, of which the
margin is shown. The tubes curve away from the margin, that is, away from
the supply of oxygen. (After Molisch.)

away from the point of contact to be considerably accelerated, with the
result that the object, provided it is of suitable dimensions, tends to become
entwined by the tendril.

4. Nastic Movements.

So far we have studied the effect on plant organs of directional
stimuli such as gravity, line of incident light, unequal distribution
of water and so on. Many plant organs in addition display a sensi-
tiveness towards non-directional and uniformly distributed stimuli
such as variations in light intensity and temperature, or mechanical
shock, and show change of orientation or movement under the
influence of such stimuli. In these cases the new orientation is
determined not by the stimulus, as in tropic responses, but by the
structure of the plant. Such responses are said to be of the Nastic
type. Whereas tropic movements are due to growth changes and
are therefore confined to growing organs, nastic movements are
mostly, though not always, due to changes of turgor. Such changes
can occur in mature parts, provided that mechanically the organ is
suited for bending. They are frequently more rapid and striking
than tropic movements, though of less importance biologically.

The opening and closing movements shown by many flowers and by
some foliage leaves are examples of nastic responses. They usually
occur in the morning and evening, the evening movements being
commonly termed " sleep movements." It so happens that the
opening and closing of flowers are due to growth changes in the petals

-B.B .

l6a BOTANY OF THE LIVING PLANT

M rianth ambers, rather than to turgor changes. For example,
the opening and < losing of Crocus and Tulip flowers are due to differen-
ti J growth on the inner and outer surfaces of the perianth members,
temperature being the controlling factor. A fall in temperature
induces closur, of the flower, while a rise results in opening. In
other plants, such as Dandelion, changes in light intensity are more
important than those of temperature. The closure of flowers at
night or in wet weather serves to protect the inner floral organs
from damage by low temperatures or by rain. In some plants,
such as the Night Scented Stock (Matthiola), the flowers open

II.
I.

Fig. 98.
Shoot of Mimosa pudica. I. with leaves in the normal day-position. II. in the
night-position assumed at dusk, or after stimulation. B = inflorescences. (After
Strasburger.)

at night and close in the daytime, which suggests an adaptation
to pollination by moths.

Sleep movements of foliage leaves are to be seen, for example, in
(lover, Wood-sorrel and Mimosa, all of which have compound leaves.
In the daytime the leaves are open and the leaflets occupy a horizontal
position, but with the approach of night the leaves close up. In
(lover this is the result of the leaflets rising, while in Wood-
sorrel they droop. In Mimosa the pairs of leaflets fold their upper
surfaces together, and the pinnae converge, while the petiole falls
into a pendant position (Fig. 98). These movements are made
possible by the presence of structures (pulvini) which act like hinges

GROWTH, IRRITABILITY AND MOVEMENT 163

or joints at the base of each leaflet in Clover and Wood-sorrel, while
in Mimosa they are also present at the base of the pinnae and of
the leaf stalk. Externally the pulvini can be recognised as swollen,
dark green regions, while internally the vascular tissues are found to
be contracted to a compact central strand, surrounded by a broad
cortical band of parenchyma which forms the motor tissue. The
movements of the leaves are brought about by differential changes
of turgor in the motor tissues on the upper and lower sides of the
pulvini, induced chiefly by changes in light intensity. An increase
in turgor on one side will lead to an expansion of that half of the
pulvinus, a decrease to a contraction. A corresponding movement
of the leaflet or other part governed by the pulvinus will follow.

The point which leads to Mimosa being commonly called the
Sensitive Plant is that in response to mechanical shock the leaves
rapidly assume the same appearance as they take up more gradually
in the evening (Fig. 98). The disturbance of walking roughly through
a patch of Mimosa pudica results in a broad track of completely
transformed vegetation : it must, however, be admitted that the
value of this sensitiveness to the plant is far from obvious. If the
stimulus be applied gently, steps in the leaf movement can be ob-
served. A soft touch at the sensitive lower surface of the hinge
at the base of the petiole makes the whole leaf fall, and the response
may then be extended outwards to the pinnae and successive pinnules.
Or if the distal pair of pinnules be pinched, or stimulated with the
hot head of an extinguished match, the stimulus received distally
will extend downwards. The leaflets will fold in successive pairs,
and finally the leaf will fall. If the stimulus be sufficiently strong
its effect may extend along the stem to other leaves. It thus
appears that the state of excitation set up at the point of stimula-
tion may be conducted through the plant.

The method of conveyance of excitation through the plant is still the sub-
ject of investigation. It has been found that if a stem of Mimosa is cut right
across and the two parts then joined together by a glass tube filled with
water, application of a stimulus below the interruption will result in closure
of leaves above it. The inference from such experiments is that a chemical
substance of hormone-type is produced at the point of stimulation, and
that it is conducted through the plant, affecting the pulvini in turn as they
are reached. The path of conduction of the stimulus is still in doubt. An
earlier suggestion was that the conduction of excitation is effected in a
mechanical fashion as a wave of pressure running through peculiar elongated
elements that are present in the phloem of Mimosa, in addition to the sieve
tubes. It was suggested that the pressure waves are set up as the result of
the movement in the pulvinus where the stimulus was first received, and

l64 BOTANY OF THE LIVING PLANT

on through the elements mentioned: much as a wave of pressure can

,t £££, a rubber tnbe filled with water. If one observer ^mches

OU(. (.I1(1 oi lt a aecond observer can feel the wave at the other end of the tube.

\. in the Bleep movements of Mimosa, so in the shock movement, changes

targof in the pulvini are responsible. On simulation the cells of one

ride of the motor tissue suddenly lose the power to retain the water of their

rouoles with the result that the water filters out into the intercellular

.paces and that side oi the pulvinus shrinks. Thus in the mam pulvmus

iU, ents oo u. at the under side on stimulation, with the result that the

petiole falls (Fig, 98, II.)- B

e

Fig. 99.

Leaves of Droscra rotundifolia ; enlarged. A, in the receptive state before
stimulation. B, after stimulation, viewed from above, with tentacles partly
in- urved. (After Darwin.)

Other striking nastic movements are those of Carnivorous Plants
(see also Chapter XII.). In Sundew (Drosera) the spathulate leaves
bear numerous radiating tentacles, each terminating in a spherical

md, which secretes a viscid juice containing protein-splitting
enzymes (Fig. 99, A). This acts like bird-lime, detaining any small
insect that touches it. The tentacles are sensitive to contact and to
the presence of chemical substances, especially those containing
nitrogen, such as ammonium salts or proteins. When such stimuli
are applied the tentacles, as the result of unequal growth, begin to
close inwards, sometimes within a few minutes after application of
the stimulus (Fig. 99, B). An insect that is responsible for stimulat-
ing the tentacles thus becomes enveloped by them, and undergoes
digestion by the enzymic liquid which they secrete. In another
native carnivorous plant, the Butterwort (Pinguicula), the presence
ts leads to an inrolling of the leaf margins, again tending to
envelop the insects, and to hasten their digestion by a secretion

GROWTH, IRRITABILITY AND MOVEMENT

165

Pig. 100.

Leaf of Dionaea in the receptive state.
(After Darwin.) ( x 4.)

from glandular hairs on the upper surface. The movements of the
Venus Fly Trap [Dionaea), an American plant, are very strik-
ing. Each of the rosette of
leaves of the plant bears at
its distal end a two-flapped
mechanism, like the covers of
a book, mobile along the
median line as a hinge (Fig.
100). The flaps are furnished
with marginal spines, while
three sensitive bristles rise
erect from the upper surface of
each. Under favourable condi-
tions, a touch on any of these
six bristles results in the immediate closure of the flaps, with interlock-
ing of the marginal spines. Any insect touching them would be cap-
tured within the trap ; and as the inner
leaf-surfaces are furnished with secret-
ing glands, digestion follows, with the
usual absorption of the soluble products
of digestion. The movement is due to
combined turgor and growth changes.

5. Hygroscopic Movements.

All the instances of movement in plants
that we have so far considered, whether
tropic or nastic, depend en changes of growth
or turgor, and therefore involve living cells.
There is, however, another class of movements
in plants which stand in no direct relation to
living cells, and which are physical rather
than physiological in nature. These move-
ments can take place in dead plant organs
and are due to changes in water content of
the tissues, in combination with special struc-
tural features. Such hygroscopic movements
are seen in the dehiscence of many fruits, and

Fig. ioi.

Fruits of Cardamine hirsuta. The upper- are 0ften due to tensions set up through un-
most are unripe. Those below them are . . ' ___ . 4-u^

ripe, and the carpellary walls, splitting equal contraction of different layers 01 tne
away from below, curve so quickly as to ,. f ., f .. ^ drjes The tensions

throw the seeds forcibly outwards. wau OI tne Iruit ab 1U U11CS-

ultimately lead to the rupture of the fruit,

and in the sudden relaxation the seeds may be thrown out and scattered. An

instance of this is provided by the native Hairy Bitter-Cress (Fig. 10 1), but a

more notable one is the Sand Box Tree (Hura crepitans), a native of tropical

,ft BOTANY OP THE LIVING PLANT

I ,. ^ ,„ this plant explodes with a report Like a P^;**. *£
throws the h isoul to a distance oi many yards (Fig. 102). These and

other explosive frnita will be considered again on p. 324. Mention may also be
milll. ()f th, hygroscopic awns of the grass Stipa. through the twisting move-
meats of whii h the seed tends to be pushed into the soil ; and also of the move-
ments of the peristome teeth of Mosses. An example of a rather different type
of hygroscopic movement is seen in the dehiscence of the Fern sporangium,
which depends on changes in volume of the water present in the cavities

Fig. 102.

Whole fruit of Hura crepitans, before rupture of its woody carpels. (After Le Maout.)

a, b, single carpels after the explosion, showing each coccus with gaping halves. The

rupture happens suddenly, each coccus taking a wider shape ; the cocci and seeds

are thus thrown asunder. c = a single large seed. The tree is native in Tropical America.

of specialised cells of the annulus (Fig. 103). When exposed to a dry atmo-
sphere these cells gradually lose the water with which the cell-cavities are filled,
diminishing mass of water within each cell retains its continuity because of
the cohesive properties of water, and also continues to adhere to the cell walls :
the latter therefore suffer a deformation and the annulus as a whole curves back-
Fig. 103, A , C) . A point is ultimately reached at which the continuity of
the water within the cells is broken. The cells immediately resume their normal
shape and the annulus recovers with a sudden jerk (Fig. 103, B, D), throwing out
the spores. The rolling and unrolling of certain Grass leaves (p. 188) is also
due to changes in volume of the water present in special cells.

It is evident from what has been said in the latter part of this Chapter
that the several organs of the plant exhibit a wide variety of move-
ments and curvatures. The plant as a whole is. however, fixed in
ition, a feature of difference from the animal which is to be

GROWTH, IRRITABILITY AND MOVEMENT

167

associated with their divergent methods of securing nutrition. The
animal goes in search of its food : while the raw materials which the
plant requires for its nutrition reach it by processes of diffusion. Such
movements as are exhibited by the individual organs of the plant
involve a quite different mechanism from that in animals. With the
exception of the hygroscopic movements, they are all dependent on
variations of the osmotic turgor of living cells or of the resistance
of cell-walls ; it is structurally impossible that the mechanical
effects of the movements should exceed, as they mostly fall
short of, the limit of the pressure of the protoplast upon the con-

A -3

Fig. 103.

Fern sporangia : A, with the cells of the annulus darkly shaded, and curved
strongly backwards by drying of its cells. B, the annulus after its sudden recovery,
while the previous position is shown in dotted lines. C shows in detail cells of the
annulus in A, and D shows similar cells in the state seen in B. See Chapter XXXI.

taining walls. Such movements of the plant-body are brought about
in an essentially different way from those positive contractions of
muscle-fibres, which are the source of movement in the animal body.
In this, as in so many other features, the two kingdoms show evidence
of their initial divergence. However parallel their behaviour may
appear to be, when fully analysed it becomes apparent that it is
analogy, rather than any closer correspondence, that holds between
them. For we do not find either that contractile muscular fibres
exist, or any specialised nerve-system in the body of the plant.
Such movements as plants show, and even the conveyance of stimuli,
are brought about without them.

Two other types of movement are encountered within the plant king-
dom. Locomotion is displayed by some Algae (Chapters XXI.-XXII.)
and by ciliated reproductive cells, such as the sperm of the Fern
(Chapter XXXI.). Movement of cell contents also is common in plants.
It usually takes the form of protoplasmic streaming (Chapter III.).

CHAPTER X.

1 Hi: MECHANICAL CONSTRUCTION OF THE PLANT-BODY.

The texture of any normally growing Plant is relatively firm and
clastic, so that it keeps its form, and after yielding to pressure tends
recover. Even young shoots show this; but it is a much more
prominent feature in older plants. Woody trunks, large leaf -stalks
of Palms, and old roots consist of hard masses of resistant tissue.
They also may yield to the pressure of the wind, and they recover
very perfectly after it is over. But if the limit of elastic recovery is
passed, any part, young or old, may be so damaged that it is of no
further use to the plant. A trunk may be shattered, a limb or a leaf
may be severed, and so lost ; or soft cells may be crushed between
harder tissues, as may be seen in any leaf -blade roughly folded at
too sharp an angle. To minimise such risks it is necessary that
plants shall be mechanically constructed so as to resist those stresses
and strains which are likely to befall them in their ordinary course
of life.

Individual plants often attain large size. The Brown Tangles of
the colder oceans, such as Macrocystis, may be several hundred feet in
length, and are among the largest of living organisms. Their leathery
body i^ anchored to rocks, and buoyed up by sea-water, though exposed
to its currents and waves. To maintain the form and attachment of
so large a plant offers a quite considerable mechanical problem, which
i- -hired by other water-plants in proportion to their size. But the
quirements in the case of aerial plants are much more exacting, for
th re not buoyed up by a medium of high specific gravity. They
must be stiff and firm of texture. Forest trees grow upwards to a
height sometimes of 300 feet or more, and there hold aloft the dead
weight of branches, leave-, and fruits. Not only must this be done
in still air, but they must also be ready to resist successfully the impact

168

MECHANICAL CONSTRUCTION OF PLANT-BODY 169

of winds. In large plants this presents a serious engineering problem,
and especially so in view of the fact that perfect elastic recovery
after the wind-pressure ceases is a condition of its successful solution.
A similar mechanical problem varying with the size, presents itself
in relation to every living plant that grows in the air. The solution
of such problems is based upon the fact that the cells forming the plant-
body are encysted. The necessary firmness which they show depends,
in one way or another, upon
the fact that a cell-wall sur-
rounds each soft and slimy
protoplast.

There is reason to believe that
the evolution of multicellular plants
started from simple unicellular be-
ginnings. Those primitive creatures,
the Flagellates, of which Euglena is
an example, may be regarded as
illustrating at the present day the
sort of organisms from which the
evolution of the higher forms may
have started. Euglena shows two
phases in its life : an active phase
of motion, in which the plant exists
as a primordial cell, that is, as
a protoplast without a cell-wall.
(Fig. 104, A, B, C.) But there is
also a second quiescent phase, in
which the protoplast is encysted, fi
that is, surrounded by a cell-wall.
(Fig. 104, D, E.) It is probably
from some such source as this
that the evolution of the plant-
body of all the higher forms origin-
ated. For the encysted state of
Euglena corresponds structurally
and mechanically to that of the cells composing the ordinary tissues of
plants ; and it has been seen that the whole plant-body, however com-
plicated, is built up of such encysted cells or their derivatives (Chapter II.).
But the primordial phase is still represented in sexual propagation. The
cells directly involved in that process, even in Flowering Plants, are
primordial cells, and in many of the lower forms they are still capable
of movement in water, like the active Euglena. The primordial proto-
plast has the advantage of free movement, but it is mechanically weak.
Its soft slimy consistency may suffice for small unicellular organisms ; but it
would be quite impossible to construct large plants from such cells without
some means of mechanical strengthening, especially if they are to live in air.

Fig. 104.

Euglena gracilis. A Flagellate, showing in A,
B, C, the motile condition, without cell-wall.
D, is a resting cyst, with cell-wall. E, shows
a cyst germinating. (Highly magnified.) (After
Strasburger.)

I70 BOTANY OF THE LIVING PLANT

It is oo the basis of the encysted state, strengthened and protected by cell-wall
tlM, ,,,,, plant-bodies have been made possible. But the mechanical
advantage conferred by the cell-wall has been gained at the sacrifice of
mobility Moreover, the mechanical framework offers an obstacle to physio-
iogical activity, and this may sometimes be a serious difficulty. The method

the plant-body is thus a compromise between the need for
, ical strength, and for the carrying out of the vital processes. That
the result is in favour of the plant is shown by the success which vegetation
has achieved.

Whether economy of material is actually as important in plants as it is
in the construction of bridges or ships by the engineer, such a principle
is undoubtedly manifested in the construction of many plants. In
bot h i • he less material that is used the lighter the structure will be.

In the plant the material used has to be gained through photosynthesis.
Where the formation of a large vegetative system is involved it must
be mechanically strong enough to maintain its form. Our chief interest
will lie in seeing how plants use their materials so as to be mechanically
effective. It will be found that the methods of its use run parallel
to those adopted by man to gain similar results. In plants there are
two distinct methods of securing mechanical resistance together with
economy. One is through turgor of the cells, the other is by the forma-
tion of specific mechanical tissues. The former plays the chief part
while the tissues are young, the latter is effective in the mature parts
of the organism. But in their action they are not distinct from one
another. Both may be effective in the same part, and at the same
time. For the dependence on turgor gradually passes over to depen-
dence on the specific mechanical tissues, as the shoot develops and
its requirements become greater.

Rigidity as based on Turgor.

_The fact that living cells are normally turgescent has already been
discussed in Chapter III. The firmness and rigidity of the tense cell
was there compared with the condition of an inflated football, or of a
pneumatic tyre. The elastic cellulose wall corresponds to the outer
cover, and the protoplast to the bladder, or to the inner tube. The
withered or plasmolysed cell loses its power of mechanical resistance
like a punctured tyre, or a deflated football. This condition holds for
every normally living encysted cell while young, whether isolated as
in some Algae, or forming a unit of some larger structure.

In the evolution of the higher forms from simpler organisms it
might appear that the simplest way of extending the plant-body

MECHANICAL CONSTRUCTION OF PLANT-BODY 171

would be to enlarge the single cell as a non-septate sac. The best of
this simple but ineffective method has been made by the Algal family
of the Siphonales. But it is only under favourable conditions that it
can succeed, for it is a method of construction with obvious limitations.
L'nless buoyed up by water it is unsuitable except in the case of small
organisms. Some relatively small members of the Siphonales, such
as Protosiphon or Vaucheria, live in damp situations exposed to the
air. But all the larger forms are submerged, and live usually in still

r

T

Fig. 105.

Caulerpa prolifera. a, growing apex, b, young thallus-lobes. r = znizoids.
Notwithstanding its elaborate form this plant is a single non-septate sac. (t Nat.
size.) (After Strasburger.)

lagoons or pools. In Valonia ventricosa, which is a sea-weed, the
form of the sac is simply spherical, or pear-shaped, and may be an
inch or more in diameter. But in other genera it is more elaborate
in form, and may extend to a foot or so in length. Some mimic
curiously the creeping shoots of aerial plants ; for instance Bryopsis
and Caulerpa (Fig. 105). Many other of the larger forms, however,
grow with numerous branches matted together, giving mutual support
(Codium), or even cemented together into a solid mass by deposits
of lime (Halimeda). Such structural modifications as these show that
the non-septate sac is too weak a method of construction for practical

1 " I
1 / -

BOTANY OF THE LIVING PLANT

I, has only been adopted by a few organisms, the chief of which
ertain Algae living in still water, a medium of nearly the same
vity as themselves. Thus buoyed up the action of gravity
upon them is minimised.

If • spherical robber balloon be filled with water so as to be turgescent, it

long as ,t is submerged. But if it be lifted out into the

an it changes its form according to the support, and the larger it is in proportion

to the linnn its skin the greater will be the deformation. A very large

one with ■ weak skin will burst. These simple facts are in accordance with

■ general principle which rales for similar structures of various size. Their

strength increases as the square of their dimensions, their weight as the cube

their dimensions. So long as the structure rests in a still medium, of its

.u approximate specific gravity, no mechanical difficulty need arise. But

Tig. io6.

Part of a transverse section of Canlerpa, showing the thick outer wall, and the
r- tii ulate rods of cellulose, which act as ties, and give added rigidity. F. O. B.
( • 50.)

in a medium of less specific gravity the demand for rigidity rises in a higher
ratio than the dimensions of the structure. The result of this applied to
plants is that a method of construction which suffices for small organisms,
consisting largely of water, and exposed to the air, will not suffice for those
of larger dimensions. The mechanical demand on the turgor and strength
of the cell-wall in order to maintain form rises in a more rapid ratio than the
it may be noted that all large I and- growing plants are septate ; also that
land-growing Siphonales are small, and the larger ones are all aquatic.
In the of the non-septate cell of large dimensions exposed in the air,

the wall would have to be of such thickness in order to maintain the form of
the organism under the influence of gravity, as to be on the one hand wasteful
of material, while on the other it would present a formidable obstacle to
physiological transit. Such thickening is seen in the large species of Valonia
and ( auUrpa, even though they grow submerged. In both of these genera
the cell-wall i iderably thickened ; but in the latter additional firmness

-•( ured for the otherwise feeble structure by numerous cellulose rods,
which stretch across the internal cavity and act as ties (Fig. 106). In other

MECHANICAL CONSTRUCTION OF PLANT-BODY 173

large types some accessory means of strengthening has to be adopted, such
as matting the branches together as in Codium and Penicillus : or cementing
them together with lime as in Halimeda. These are to be regarded as con-
cessions to the mechanical imperfection of the non-septate construction.

Cases of non-septate tissues exist in the body of some of the higher plants,
but as they are embedded in other tissues they are not exposed to mechanical
demands. Examples are seen in the latex-cells and vessels, as in the
Euphorbiaceae and Cichoriaceae (Fig. 32, p. 54) ; and in the young
embryo-sacs of the Flowering Plants. (See p. 295.)

All ordinary plants of large size are septate. This is specially
necessary where they live exposed in the air, and are thus subjected
to greater strains than if floating in water. In the embryonic region

Fig. 107.

Diagram illustrating the plan of arrangement of cell-walls in the apex of the stem
of an Angiosperm. AA'=axis of construction. EE = external surface. PP=pri-
clinal curves. A A = anticlinal curves. (After Sachs.)

the cells are seen to divide into equal parts, each newer cell-wall being
inserted at right angles on the older walls. This leads to a cell-net
the exact detail of which depends upon the external form of the
part (Fig. 107). The disposition of the walls is from the first such
as to give added mechanical strength, together with other advantages.
But in the young cell the walls are extremely thin, and are composed
of pliant material. In the young shoot the mechanical strength is
almost entirely dependent on the turgor of the individual cells.
When they are tense their walls do not act as mere props or stays, as
do the floors or partitions of a house ; but the turgor gives individual
rigidity to each cell, and through them collectively to the whole part
which they construct. A high mechanical effect is thus gained in a
succulent structure with extreme economy of material. This may be
illustrated by the case of the Lettuce. We have seen that a crisp

l7A BOTANY OF THE LIVING PLANT

citable for a salad gives by weight 95 per cent, of water,
and only 5 per cent, of organic material for cell-walls and proto-
plasts Such a structure is in fact a very slight organic framework
containing water. I he mechanical effectiveness of the internal turgor
the cells, and the insufficiency of the mere partitioning of a young
or succulent part is shown by comparing a crisp fresh leaf with one
which has withered, or has been plasmolysed.

There is, however, another factor which increases the mechanical

effectiveness of succulent parts in the young state, viz. the mutual

ions of tissues. If a fresh young stem of Sunflower or Elder, or

any extending part of an herbaceous plant, be slit longitudinally into

quarters, these take strong curves. The outer surface of each quarter

becomes concave, the inner faces of each
quarter convex. The curves become more
marked after the cut stem has been
steeped in water. These curves show that
the relations of the tissues in the living
stem are not passive. (Fig. 108, 2a, 2b.)
That the phenomenon is one of turgor of
the cells may be shown by allowing the
slit stem to wither, or by plasmolysing it
with a salt solution, when the curves dis-
appear, and the parts become limp. On
the other hand, if the several tissues be
completely separated from a measured
length of a fresh stem, and be themselves
measured after separation, the column of
pith will be found to have elongated, and
the outer tissues to have contracted. To
bring them back to their original state
the pith would have to be compressed and the outer tissues stretched.
(Fig. 108, I.) This is in fact their condition in the growing stem.
The pith tends to elongate, but is held back by the outer and firmer
-, which are thus kept tense. The relation of the inner and
outer tissues is then analogous to that of the protoplast and wall
in the turgescenl i ell, and the mechanical effect is the same. // is thus
I that the firmness of succulent stems is due in large degree not only to
the turgor of the individual cells, but also to the mutual tensions of their
tes. Similar relations also hold for the tissues of leaves and roots,
but these need not be described in detail.

Fie. io8.

I, Shoot of Sunflower with pith
separated by a cork-borer from the
r tissues. 2a, split stem of Dan-
delion. 2b, after immersion in water.
• r Strasburger.)

MECHANICAL CONSTRUCTION OF PLANT-BODY 175

Rigidity as based on Specific Mechanical Tissues.

Such methods serve to give the necessary mechanical strength to
young parts. But as the tissues grow older, their walls become thick-
ened. They are then less susceptible to turgor, as they are also more
resistant to growth. Moreover, in the older parts the mechanical
demand for support increases with the increasing burden of leaves
and branches. These demands are met by specific mechanical tissues,
fitted by their thickened walls to offer greater resistance. Though the
effect of turgor is characteristic of young plants, and that of the
specific mechanical tissues of the mature, there is no definite limit

Fig. 109.

Collenchyma from the stem of the Potato, seen in transverse section.
The walls only ara shown. ( x 31,0.)

between the action of each. Dependence on the one merges gradually
with age into dependence on the other, and in the growing part both
sources of support may be effective at the same time.

There are two types of specific mechanical tissue, (a) Collenchyma,
which is found in growing herbaceous stems and leaves ; and (b) Scleren-
chyma, which is characteristic of more mature parts. Collenchyma
consists of cells which retain their living protoplasts, and thus remain
physiologically active, while chloroplasts are frequent in its cells.
These cells usually have the form of 4-6-sided prisms, with transverse
or oblique ends, and are sometimes transversely partitioned. The
cell-walls are composed of cellulose, which is swollen in life with
water. They are thicker at the angles of the prisms than at their
flattened sides, where the thinner membrane allows of ready physio-
logical interchange. This gives the tissue, when seen in transverse
section, the appearance shown in Fig. 109. It is thus a tissue which,

»rfi

BOTANY OF THE LIVING PLANT

^. O 3.

Fig. i io.

though mechanically effective, is not fully specialised for giving

th. The cells do not offer rigid resistance to elongation, but

they themselves grow with the growth of the part they support,

offering .1 persistent though plastic resistance. The position of the

Uenchyma is usually peripheral, closely below the epidermis. This

gives it its full mechanical
effect. In fluted stems it is
usually massed at the project-
ing ridges, and between them
it is often interrupted by thin-
walled green tissue of the
cortex. (Fig. no.) Function-
ally it takes an intermediate
place in the individual develop-
ment between the state depen-
dent upon turgor and the full
rigidity of the mature state.
The resistance which it offers
to elongation is one of the
chief factors in producing the
mutual tensions of tissues
above described in the succu-
lent elongating stem.
The Sclerenchyma is, however, the more important specific mechanical
tissue. Its effectiveness depends partly upon its own characteristics,
partly upon its distribution. It consists of cells with thickened
walls, which are usually but not always lignined. When mature its
protoplasts are no longer functionally active, and may be represented
only by vestigial remains. It is then practically a dead tissue. The
form of the cells varies. Sometimes they are cubical or oblong with
Is, and may be isolated, or disposed in groups. Such stone-
cells, or sclereids, give a hard gritty texture to the parts where they
ur, as in the bark or pith of various woody plants. The gritty
urc of the fruit of the Pear is due to nests of such stone-cells. But
the sclerotic cells are elongated, and variously branched,
i the sclereids of the leaf of Tea, or unbranched with flattened
a in the leaf of Hakea (Fig. in). Such cells are commonly
They stiffen the parts where they occur by the resistance
which their thickened walls offer to compression. The form, structure,
and function of the sclereids of Hakea, which prop out the firm
epidermis and thus give the leaf its remarkable stiffness, may be

Flowering stem of Aslrantia in transverse section
( x io.) The collenchyma is dotted.

MECHANICAL CONSTRUCTION OF PLANT-BODY 177

compared with that of the hollow steel columns used for supporting
shop-fronts.

A more frequent, and mechanically a more effective type of strength-
ening cell is seen in the elongated sclerenchyma- fibre. Such cells are
commonly associated in masses, often forming strands which run
continuously for long distances. These strands of the Flax supply
the material for linen, of the Jute for sacking, of Hemp, New Zealand

Fig. hi.

Part of a transverse section of the xerophytic leaf of Hakea, showing a stoma
greatly depressed below the well-developed epidermis, which is propped out by
• thick-walled sclerotic cells. ( x 150.) F. O. B.

Flax, etc., materials for cordage, while similar strands of the Coco-
Nut, and other Palms are worked up into mats, brushes, etc. In the
plant they frequently accompany the vascular strands, and are often
associated with the phloem as bast-fibres, or with the xylem as wood-
fibres (compare Fig. 38). But there is no necessary association with
vascular tissues, and the sclerenchyma is often quite independent of
them. The mechanical cells themselves are elongated, with ends
pointed and sides flattened, so that they fit closely together. (Fig.
112, A, B.) The cells of Hemp are about 100 times as long as broad, in
linen the proportion is about 200 to I ; in the extreme case of the Rhea
fibre (Boehmerid) the length has been estimated at 1500 times the
breadth. The lignified walls may be so thickened that the cell-cavity
is obliterated. They are thus practically rods of resistant material.

B.B.

M

i78

BOTANY OF THE LIVING PLANT

Each develops from a single embryonic, or cambial cell. As it elon-
gates, its pointed ends slide between those of other fibrous cells, taking
a sinuous course. The result is that the cells of the strand interlock,
and when a longitudinal stress 1 is applied, the resistant rods press
laterally upon one another, so that the greater the stress the more
closely are they united. Mechanically such strands act like solid
metal wires.

Fig. 112

A , Transverse section of sclerenchvma of stem of Sunflower. The larger sections
show cells cut through the middle of the fibre, the smaller near to the pointed end.
B, the same in longitudinal section, showing the pointed ends of the cells. Small
pits are present, in surface view, and in section. F. O. B. ( x 300.)

Characteristic features of the resistance of plant-fibres, as contrasted
with those of certain metal wires of similar transverse section, show
how different is the behaviour of the two under stress.

Name.

Limit of Elasticity
in Kg. per sq. mm.

Breaking stress in
Kg. per sq. mm.

Elongation at limit

of Elasticity per
1000 units of length.

Dasylirion -
New Zealand Flax -
Hyacinth -
Garlic -
Nolina -

17.8

20

12.3

14.7

25

21.6

25

16.3

17.6

• •

13-3

13

50

38

14-5

Silver Wire ...
Wrought Iron - - -
Steel -----

II

13-13
24.6

29

4O.9

82

0.67
1.20

1 Stress and strain. When a force is applied to a piece of material, the
material is said to be under stress. In mechanical construction those portions
intended to be under pull are called ties, and those under push, struts. Ties are

MECHANICAL CONSTRUCTION OF PLANT-BODY 179

The first column shows the maximum burden per unit of transverse
section of the fibrous strand, or of the metal wire respectively, under
which the limit of elastic recovery is not overstepped. The fibre of
Nolina (25) is actually superior to steel (24.6). Other fibres compare
favourably with silver and wrought iron. The second column shows
the burden per unit of transverse section which causes rupture ; that
is, it states the limit of teyiacity. It is seen that in this metals are
distinctly superior. But the table brings out a very important feature
of plant-fibres, that their limits of elastic recovery and of tenacity
are very nearly coincident, while those of metals are widely apart.
Metals are ductile, fibres are not. The importance of this in the plant-
body lies in the provision thus made for perfect recovery after strain.
Great breaking strength would be of no value if the plant subjected
to the strain were permanently deformed.1 The third column shows
the elongation which the strand or wire suffers at the limit of elastic
recovery, stated in terms of units of length per 1000 of the strand as
a whole. Here the difference between the fibres and the wires is
strongly marked, the fibres yielding in much higher degree than the
metal wires. This again meets the requirements of a plant exposed
to forces, such as wind. For the tissues while resisting the force
very efficiently, yield to it, but recover very perfectly when the force is
removed.

In late years metal straps have been used largely in concrete con-
struction, reinforcing the concrete in which they are embedded.
Ordinary herbaceous plants are constructed on the same principle.
The sclerotic strands correspond to the metal straps, the surrounding
parenchyma with its turgescent cells corresponds mechanically to the
concrete. The office of the latter in either case is to keep the resistant
straps in place, while the straps resist the tensions which would
produce loss of form. In the reinforced concrete a high degree of

said to be under tensile stress when pulled, and struts under compressive
stress when pushed. The term strain is used to signify the change of length
or other dimensions or the change of form which occurs in a material under
stress.

1 Collenchyma is, however, exceptional. While its absolute strength is
little inferior to that of bast fibres, its limit of elasticity is much lower : and
it is thus liable to permanent elongation. But this is an advantage in growing
stems in which it usually occurs ; for it offers no rigid barrier to growth, while
it is sufficiently resistant to meet sudden stresses. It is like the Second
Chamber in the Constitution of a State, which resists a new initiative ; but if
the initiative be continuously pressed, it yields. Meantime stability is main-
tained.

,Ro BOTANY OF THE LIVING PLANT

rigidity is necessary, or the concrete would crack. But in the plant-
body, with its elastic cells and tenacious fibres, a considerable change
of form is allowable in yielding to the stress without permanent
injury following. The herbaceous plant thus has a distinct superiority
over any building of reinforced concrete, for the embedding medium
is itself elastic. The conditions are most nearly matched by the
covers of certain motor tyres, where resistance must be coupled with
elasticity, and fibre is embedded in the rubber.

In the 'economical use of material the disposition of the specific
mechanical tissues is important, both on grounds of lightness of the
structure, and the physiological expense of the substance used. The
problem of obtaining the best mechanical effect with the least
expenditure varies with the requirements to be met. The girder
principle, which has been adopted by engineers as a means of
securing a high degree of mechanical efficiency with economy of
material, is frequently illustrated in the construction of plants. It
is even seen in plants of the Coal Period, such as Cordaites, which
lived ages before the origin of man. The common type is the double-
strap girder, which gives in transverse section the figure (i). If a
girder of such construction be fixed in the position indicated by the
figure, and loaded in the middle while it is supported at the ends,
there will be a tendency to curvature which will compress the upper
strap or flange, while the lower strap will suffer tension. The resistance
to these stresses will depend upon the two straps being held rigidly
in their relative positions by the connecting plate. The material will
be most economically used if it be concentrated in the form of the
upper and lower straps at the regions of greatest strain. The con-
necting plate may even be replaced in " latticed girders " by a system
of connecting ties, which follow the lines of greatest stress. The
wider the upper and lower straps are apart, consistent with their being
held rigidly in place, the better the result will be. The principle is
illustrated by the use of girders, or simple combinations of them, in
the construction of bridges, floors, and shop-fronts. More complicated
arrangements giving columnar construction are seen in lattice-signal-
posts, and large gasometer-frames. The latter offer close analogies
with certain types of stem-construction in plants.

In plants girder-construction depends upon differences of mechanical
resistance of tissues. An illustration of a simple case is given in Fig. 1 13
from the leaf of Cyperus. On either side of the vascular strand there
is a band of thick-walled resistant woody sclerenchyma. Each is close
to the upper and lower surfaces respectively, indeed three cells of the

MECHANICAL CONSTRUCTION OF PLANT-BODY l8l

lower epidermis are themselves sclerotn . These bands represent the
upper and lower straps of the girder, while the less resistant vascular
strand represents the con-
necting plate. The girder is
kept in place by the softer
tissues, but especially by the
firm layers of epidermis.

The requirements that are
most commonly illustrated
in plants fall under three
heads, which are typified by
parts of normally growing
plants, though they are sub-
ject to great variety with
the varying form of the
plant-body :

(a) Columnar require-
ment, as in an up-
right stem.

(b) Stiffening of flattened surfaces, as in the leaf-blade, and pro-

tection of the margins against tearing.

(c) Rope-requirement, as in roots of upright plants exposed to

wind.

Fig. 113.
Transverse section through a leaf of Cyperus, showing a
vascular strand with a strand of resistant sclerenchyma
above and below, constituting a girder. ( x 300.) F. O. B.

(a) The Columnar Requirement.

The columnar requirement, for support of the growing dead-weight
of branches, leaves, and fruits, is met in large Dicotyledons chiefly
by the woody column, which grows in proportion to the growing need
for support. It is cylindrical, so as to meet all winds equally. It is
composed of mixed xylem-tissues, with continuous woody walls.
The most important of these mechanically are the wood-fibres, and
the resistant quality of the wood is roughly in proportion to their
preponderance. But all the tissues,— vessels, wood-parenchyma, and
medullary rays,— contribute in varying degree to the mechanical ettect.
This is enhanced as the central tissues die, and are converted into
heart-wood. The method of construction is that of the solid column,
now acknowledged by engineers to be not the most economical of
material. But in Dicotyledons the retention of this rather primitive
method is to be explained as a compromise, the success of which is
evidence of its fitness to meet the circumstances. For the living part

[82

BOTANY OF THE LIVING PLANT

of the growing trunk affords in addition to mechanical strength, room
also for storage, and a means of transmission of materials.

[n the stems of Palms, where a large terminal tuft of leaves has to
be supported al a great height against winds, the mechanically effec-
tive tissues are massed towards the periphery, while the central regions
arc Bofter. This may be compared with the hollow metal column
filled with concrete, which has the effect of preventing the metal skin

A B

Fig. 114.

Stem of Bamboo including one node at its upper end. In A it is seen from outside,
with transverse scar of the leaf-insertion, and the circular scar of its axillary branch.
In li it is cut so as to show the hollow, and the septum which gives added strength.
(Reduced to £.)

from " buckling." On a smaller scale this construction is found also
in the stems of pithed Rushes and Sedges, and in many herbaceous
I dicotyledons. It is only a step from such arrangements to that seen
in the Bamboo, or on a smaller scale in the haulm of Grasses ; and in
some Dicotyledons, such as the Umbelliferae. Here the thin-walled
pith present in the young state breaks away, leaving a central cavity
surrounded by a cylinder of firmer tissue. In this case the comparison
is with the hollow column so largely used in metal construction by
man. But it is liable to " buckle," as has been found in the masts and

MECHANICAL CONSTRUCTION OF PLANT-BODY 183

spars of racing yachts, in which internal ties of metal have been used
to meet that risk. In the haulms of Grasses, and conspicuously in
the large Bamboos, hard woody septa at the level of the leaf-insertions
serve the same purpose (Fig. 114). With-
out these it would not be possible for
hollow stems to uphold the huge head of
leaves, sometimes one hundred feet above
ground, against all winds, as the Giant
Bamboos are able to do.

The hollow cylindrical stem of a Bamboo,
or of a straw of wheat, may be imagined as
corresponding to a series of crossed girders
(Fig. 115), with the straps or flanges all fused
laterally, so as to form a firm peripheral
band. The effective material is placed as
far as possible from the centre. The straps
*being fused laterally, the connecting plate or web of the individual girders
can be dispensed with, and the stem is accordingly hollow. But in many
cases, and especially in young stems of Dicotyledons, the relation of the

Fig. 115.

Diagram of crossed girders.
Text.

See

Fig. 116.
Flowering stem of Astrantia in transverse Transverse section of an internode of a

section. ( ■■■ 10.) The collenchyma is dotted. stem of Clematis, showing a ring of six

larger and six smaller vascular strands,
surrounding the central pith, and covered
externally by the thick cortex, with six
projecting bands of collenchyma. ( * 15.)

structure to girder-construction is more plainly seen, since in them the straps
of mechanical tissue are not fused laterally. This gives reality to the con-
ception. Thus, in the stem of Astrantia, or more clearly in Clematis or Lamium,
the bands of mechanical tissue are isolated, and alternate with softer tissues
which keep them in their position (Figs. 1 16, 1 17). In Astrantia, the resolution
of the whole arrangement into a girder-construction is less obvious because

[&J

BOTANY OF THE LIVING PLANT

0f tlu- 1 irge Dumber of the strands, and their slight irregularities ; and this is

lUa I ^cotyledons. But in the case of Clematis only three
, enter into the construction, so that the method appears clearer :

and still more so in Lamium, where there are
only two. There are other points of wide
application illustrated in these cases. The
stems are fluted, with projecting angles, and
one strap of mechanical tissue is seated in
each. This gives added strength on the
principal of the fluted column, the depth of
each girder being thereby increased. The
second point is that one of the stronger
vascular strands is opposite each of the
mechanically strengthened ridges, so that
the construction of the stem, as it grows
older and the vascular strands become
mechanically more effective, resembles
that of a number of peripheral girders

l.oB.

Fig. 117.

Transverse section of stem of Lam ium ,
showing projecting angles of collen-
rhyma (dotted), opposite four larger

ular strands: an arrangement equi- disposed in a ring. This method is seen*
v.d.,n to two crossed girders. (xi<.) in the • frames supporting large gasometers.

The central tissue may even be replaced by a cavity filled with air,
which gives added point to the comparison. The simpler construction of
the stem in Lamium may be compared mechanically with that of
a lattice signal post. In it four bands of metal occupy the four
angles, and are kept in place by a lattice
work of thin straps, while the centre is
hollow. So in Lamium (Fig. 117), the
projecting angles contain the chief
mechanical tissue. The softer tissues
hold them in place, while there is a
central pith-cavity. It is immaterial
exactly how the mechanical arrange-
ments are analysed in such stems as
these quoted. The point is that the
girder principle can be recognised in
them all, with strengthening strands
isolated and peripheral. But in Dico-
tyledons these arrangements are apt to
be lost sight of as the stems grow older,
owing to secondary thickening, and the
irelopment of the vascular ring into

ttral column of wood (p. 58, etc.). Transverse section of 'the shaft of SC1>/,MS

I he Stem thus assumes at last the type of {Eleocharis) caespilosv.s, showing four large

iv. i;j 1 T , . . girders, with smaller and less perfect girders

the solid column. In large tropical trees between them. Centrally is a large cavity.

a further mechanical device is often seen Thc dotted areas indicate thin-walled

, . water-storage tissue. ( x 62.)

at the base. By unequal thickening broad

flanges are produced, which radiate outwards, and act in support like

thr 1,ult: ,f a Gothic tower. The final result of such development might

be compared with the outline of the greatly widened base of the Eiffel Tower.

MECHANICAL CONSTRUCTION OF PLANT-BODY 185

Similar principles hold also for the stems of Monocotyledons ; but the
circumstance that their closed vascular strands are usually scattered over the
transverse section tends to complicate their scheme of construction (p. 51).
On the other hand the absence of cambial thickening makes its recognition in the
mature stem easier. Commonly the sclerotic tissue accompanies each vascular
strand, as bands following its course, or even as a sheath encircling it (Fig. 113).
Such an arrangement is in itself comparable with a girder, and occasionally,
where there is a ring of vascular strands thus invested, the construction is
simply a circle of sub-epidermal girders, as in Scirpus (Eleocharis) caespitosus
(Fig. 118). Sometimes the sclerotic bands may be quite separate from the

Fig. 119.

Transverse section of the flowering shaft of Molinia coerulea. Centrally is a large
cavity. Thin-walled tissue is left clear : sclerotic tissue is dotted, vascular strands
cross-hatched. The peripheral vascular strands are embedded in a continuous ring
of mechanical tissue. ( x 40.)

vascular strands, though usually opposite to the strongest of them, as in
Schoenns nigricans (compare Fig. 28, p. 51). These simple arrangements are
very like those seen in the Dicotyledons. But in most Monocotyledons the
vascular strands are scattered over the transverse section, and this introduces a
good deal of variety of detail. Where the vascular strands are thus scattered,
and the mechanical construction more complicated, the sclerotic bands may be
fused together laterally in various ways. A continuous sclerotic ring may thus
be formed, and the vascular strands be embedded in it, with or without
flanges projecting from it (Fig. 119). Such structure developed on a larger
scale, and with more general fusion of the sclerotic tissues, leads to the con-
dition seen in the large Bamboos and Palms.

The case of Moli)iia coerulea proves how effective this mechanical strengthen-
ing actually is. The structure of the haulm is shown in transverse section

a

1 86 BOTANY OF THE LIVING PLANT

|„ p^ , ,., \t its base it is about ,V of an inch in diameter, but it is there

ie degree by its sheathing leaves. It may grow in favourable

}o inches before bearing its inflorescence. Thus the

mis to its diameter as about 500 to 1, and still it can uphold

the infl nee at its distal end, together with the weight of the fruit when

ripe. Tins extreme proportion of length to diameter suffices for a small

Gran , but it cannot be maintained indefinitely for larger structures. For

the weight of a structure varies as the cube of its dimensions, while the strength

vanes only as the square. There must be then a limit of size beyond which

it becomes impossible for a certain type of structure to be maintained. For

instance, according to the proportion of the Molinia stem, a ten-foot fishing

should only be J inch in diameter at the butt ; but it is thicker than

\ striking case is seen in the Giant Bamboos. Ewart quotes one

60 metres high, and 40 cms. in diameter at the base. That is a proportion

nlv about 150 to 1. For a plant of that size these dimensions have been
calculated as about the limit possible, though the proportions are less striking
than those of the smaller Molinia. There is in point of fact a size-limit for
any plan of plant-construction based upon a certain quality and use of
material : beyond this limit the stem will either bend or break. In the
Bamboo, which approaches the limit, the extreme top does bend in a graceful
curve ; as it does also in the haulms of most Grasses, which like Molinia
approach the limit of mechanical resistance from their actual dimensions,
structure and materials.

(b) Stiffening and Protection of Flattened Surfaces.

The mechanical problems affecting the dorsiventral leaf differ from
those of the radially constructed stem. The end to be gained is the
largest possible expanse of blade, with the least possible risk from
winds, and the employment of the least possible material. The elastic
petiole allows the blade as a whole to yield before the wind. In the
extreme case of the Aspen, where the least breath causes a shiver of
the leaves, the petiole is laterally compressed so as to be very flexible.
But in most leaves it is stiffer, and semilunar in section, with the convex

!e downwards, an arrangement which secures efficient support for the
blade, while still permitting some freedom of movement (Fig. 45, p. 70).
Another feature of mechanical importance is the cutting of the larger
leaves into segments, so that while the aggregate area may still be con-

lerable, no unduly large surface is exposed to the wind. The risks of a
large leaf-area are two : first, that of folding into so sharp a curve as
to crush the soft mesophyll between the firmer layers of epidermis;
and second, that of tearing from the margin inwards. The former
is met by the vascular venation, which is often accompanied by
sclerotic strands, and by enlargement of the surrounding tissues so

MECHANICAL CONSTRUCTION OF PLANT-BODY 187

as to form strong ribs. The latter danger is met by arched venation,
which is often aided by marginal deposits of sclerenchyma. lhe
whole blade is held together by the upper and lower layers of epidermis,

'■■■A--- ~fJM • •*** is? K» Lb-i VHf H

.-•■N

■ \..-

^V

Fig. 120.

Fig. 121.

Fig. 122.

T7 , .«, Transverse section of the leaf of Pkormium tenax, New Zealand Flax.
Fig. 120. lransverse secuuu w ^ Deschampsia caespitosa.

in positions opposite to the aqueous areas in Fig. 120. (t.v.u.)

which, having a thickened outer wall, form a firm skin over the softer
mesophyll within. Sometimes the mesophyll may be itself sclerotic
in places, as it is in many Monocotyledons.

The structural stiffening of the flattened blade against folding is
best illustrated in the leaves of Monocotyledons, for there the parallel

BOTANY OF THE LIVING PLANT

nation makes the transverse section appear simpler. All the main
ins arc then cu1 at right-angles, and in simple cases each may present
an appearance as in Fig. 113, P- 181, °f Cyperus. Sclerotic strands
follow the veins on either side, forming with each a girder, as above
explained. Since these girders run parallel, and are held in place by
the firm upper and lower epidermis, the construction of the whole is
on the same principle as that of a lattice-girder railway-bridge, in
winch also a high degree of rigidity is required, together with economy
of material. In some, as in the old Charing Cross railway-bridge, the

Fig. 123.

Photograph of the skeleton of a Dicotyledon leaf, showing reticulation witn
successive intra-marginal arches. (Natural size.)

tracks run between the girders, that space being left vacant in the
nstruction. In many leaves of Monocotyledons the corresponding
space between the girders is occupied by mechanically ineffective
mesophyll, while in some there are large thin-walled cells for water-
ge (Fig. 120). It thus appears that the requisite stiffness of
Monocotyledon-leaves is gained by means very similar to those
employed by engineers to obtain like results in bridges. But irregulari-
ties are frequent, especially in thick leaves. The girders may be in-
mplete, or the sclerotic bands may be fused laterally. But still the
girder-principle may be recognised as underlying such deviations.

I he most interesting variants are those seen in xerophilous grass-leaves,

which curl automatically so as to check transpiration. Native examples are

Sheep's Fescue, the Marram, Lyme, and Tussock Grasses (Figs.

120-122). Their mechanism shows a reduction of the mesophyll with a

MECHANICAL CONSTRUCTION OF PLANT-BODY 189

corresponding involution of the surface between the girders, while the latter are
specially deep. The upper leaf-surface is thus marked by parallel grooves,
the aqueous cells at the bottom of each furrow being large and thin-walled.
The lower side of the leaf is sclerotic, so as to maintain its outline, while the
upper is liable to shrink with drought. Such shrinkage draws the margins
together. The sloping faces of the grooves, which bear the stomata, meet, and
transpiration is checked. Access of water, on the other hand, swells the
aqueous cells, and the leaf flattens out again.

The Monocotyledon leaf, with its parallel venation, is usually secure
from marginal tearing. A prominent exception is seen in the Banana,
where the huge leaf seldom appears
perfect in plants grown in the open.
It has a midrib from which the veins
run out parallel towards the margin, on
the plan of a feather. These leaves
are readily slit to ribbons by the wind,
from the margin inwards, since there
is no sufficient marginal protection.
In most Palms a similar subdivision
of the leaf is carried out during de-
velopment, by the plants themselves.
Certain tissue-tracts dry up, cutting
the blade into segments, and giving
the appearance when mature of a
true pinnation. For smaller leaves
tearing from the margin is still a risk,
and there are structural arrangements Part of leaf £ l~splenium horridlim,

which nrevent it The commonest is showing " gussets " dotted, at the base of
wiiicn prevent it. 1 lie commoner lb indentations> (Slightly enlarged.) F.O.B.

the curving of the veins into intra-

marginal arches (Fig. 123). Several series of these of successively
smaller size towards the margin, effectually check tearing. The
most dangerous spots are naturally the indentations in toothed or
deeply cut leaves. These are often protected by small " gussets '
of indurated tissue at the base of each sinus. Good examples are seen
in Ferns, as in the genus Asplenium (Fig. 124) ; or a special strongly
arched vein may run across the point of deepest indentation ; while
not uncommonly a patch of sclerotic cells may be fused with it at
the point of danger (Fig. 125). This is often continued along the
margin as a sclerotic band, which serves, like the hem of a hand-
kerchief, to prevent a marginal tearing. Similar hems are found in
many ordinary leaves, but conspicuously in xerophilous plants.
Good examples are seen in the Date Palm, and in the Gum Trees. The

190

BOTANY OF THE LIVING PLANT

marginal stiffening becomes actually aggressive in spinous leaves, such

the Holly, Barberry, and Gorse. A particular instance of a like

sclerotic development that serves a peaceful end is seen in the Sand

A

Fig. 125.

" Gussets" at margins of the indentations of leaves (A) of the Elm, {B) of the
Sycamore, showing their relation to the vascular network, and to the mechanically
strengthened margin. ( x 14.) F. O. B.

Sedge. It burrows with its creeping rhizome through the sand. The
apical bud has its successive scale-leaves developed to a point, tipped
with hard sclerenchyma, by means of which it passes through soft
objects like a brad-awl. These are a few examples of the mechanical
adaptability of the leaves of Flowering Plants.

(c) The Rope-Requirement.

Those parts of the plant, such as roots or rhizomes, which hold it
upright in the soil against the impact of winds are subject to longitu-
dinal tension, as on a rope or string. In
cordage, in order to resist such tension,
the fibres are twisted together so as to be
grouped in as small a transverse area as
possible. This method secures the even
distribution of the stress over them all.
A similar condensation of the mechanical
tissues, but without the twisting, is usual
in roots. Their stele is small compared
with the whole transverse section, and it is
frequently pithless. But often the xylem
stops short of the centre, which is then
occupied by sclerotic tissue. This links
up the xylem, so as to form with it a
resistant central cord (compare Fig. 58, of Acorus). In larger roots
a pith may be present, surrounded by a dense ring of mechanically
effective tissue composed in the same way (Fig. 126). But still the

Fig. 126.

Transverse section of root of Rus-
ius showing large proportion of cortex
to the contracted and pithed stele.
:z.) F.O.B.

MECHANICAL CONSTRUCTION OF PLANT-BODY

stele is compact as compared with that of the axis. Underground
rhizomes show a similar construction. Their stele is contracted,
and their cortex widened, as is seen in the Marram Grass, and still

B

Fig. 127.

Sections of stems of two Sedges. A, Rhizome of Carex arenaria with mechanical
tissue condensed centrally, as resistant to the rope-requirement. ( x 14.) B, Carex
vulgaris, aerial stem constructed to meet the columnar requirement. ( x 25.)

more clearly in the Sand Sedge, where the cortex is very weak and
lacunar, while the stele is compactly cemented together with sclerotic
tissue, so as to form a solid core (Fig. 127, A). This is in sharp con-
trast to the aerial stems of most Sedges (Fig. 127, B). Stems support-
ing heavy, pendent fruits show a like structure ; also submerged and
some climbing stems, all of which are liable to longitudinal tension. The
similar modification of plants of such various habit, when subjected
to the same mechanical demand, indicates
that the rope-like concentration is adaptive.
The problem for strut-roots, such as are
seen at the base of the stem of the Maize, is a
mixed one ; for the roots on the windward
side of the plant suffer longitudinal tension,
while those on the lee side act as oblique
struts, and are subject to columnar pressure.
The structural requirements are thus opposed.
The mechanical tissues in such roots are found
to form two systems (Fig. 128). The stele is
compact, and cemented together in a hollow
ring, which is, however, wider than in the underground roots. It is
suited to resist longitudinal tension, but is not highly specialised to
meet it. The cortex is, however, sclerotic, the thickening being
greatest at the periphery. This is suited to meet the columnar

Fig. 128.

Transverse section of strut-
root of Zea Mais. The mechani-
cal tissue of the cortex is dotted.
(xi2.) F.O.B.

192 BOTANY OF THE LIVING PLANT

lircment. Thus the mixed problem is solved by a structure that

serves both purposes.

Such cases as those quoted in the last paragraphs confirm the view
which follows inevitably from the study of the distribution of the
mechanical tissues generally in plants, viz. that those methods are
adaptive, and have been acquired in the course of Descent in accord-
ance with the requirements. The structure is as a rule hereditary,
though the conditions to which the developing parts are exposed may
have an influence in determining the quantity of sclerotic tissue formed
in the individual part. There is reason to believe that the forces
acting on the developing part serve as stimuli, increasing or even
causing the formation of the mechanical tissue. This is in accordance
with the well known fact that those tendrils of climbers which grasp
a support develop much more strongly, and are able to bear a greater
load than those which do not. In the large woody climbers of the
tropics the difference is often very marked, the fixed tendril developing
as a large woody structure.

While such observations are in themselves interesting in their
bearing on the quantity of the mechanical tissue present, they do not
explain the origin of the methods of its distribution. The degree
of parallelism which those methods show to the methods of modern
engineers in using their stone, steel, and concrete, is remarkable.
While we admire the efficiency of the result in either case, and especi-
ally the economy of material made possible by such methods, it is
to be borne in mind that the priority of initiative undoubtedly lies
with the Plant. For many of the methods represented by ancient
types of vegetation have been adopted by man only within the last
few decades. Nor is there evidence that engineers ever took, as well
they might have done, any suggestion from the study of the engineering
methods of Plants. What we see in the two cases is accordingly a
result of parallel, or homoplastic development. Similar results have
been acquired independently along two quite distinct lines of evolu-
In the one case the results have followed from human calculation
and experiment, in the other they are described as " adaptive." But
still an open question how the mechanically effective structures
n in plants, and produced during development, are causally
related to the requirements of the adult state.

CHAPTER XI.

MODIFICATIONS OF FORM IN THE VEGETATIVE
SYSTEM OF THE HIGHER PLANTS.

It has been seen that the primary construction of the Higher Plants is
according to a general and uniform plan (p. 14). But the scheme may
be worked out in detail in very different ways. Either the root or the
shoot, or both, may vary in form and proportion in different plants.
Such differences are clearly related to the conditions under which the
plants grow. Since it is possible in very many cases to see that the
particular form taken is suitable for successful life under corresponding
particular conditions, such forms are described as being specialised, or
adapted to them. Those special modifications of form which lead to
success are described as adaptations, and will be so grouped here :
though with the same reservation as to causality as that expressed at
the close of the preceding chapter with reference to internal structure.

Plants may be recognised as being thus adapted to the medium in
which they grow ; as, for instance, in water or in air : to other physical
conditions, such as the action of gravity, or the direction of light : to
the climate to which they are exposed, hot or cold, dry or moist,
equable or with marked seasons, quiescent or stormy : or to the
soil which provides them with requisite supplies : or they may
show special features which enable them to take advantage of other
surrounding circumstances. Prominent examples of this are seen in
plants which straggle or climb over others ; or in those which, by taking
to some irregular form of nutrition, derive advantage from their
neighbours as parasites, or in other ways. Such modifications of
form according to circumstance often lead to difficulties in recognition
of the category of parts to which the modified organs rightly belong.
A stem or even a root may carry out photosynthesis. A leaf may
be effective only for climbing, or for protection. It becomes therefore
b.b. 193 N

h,} BOTANY OF THE LIVING PLANT

necessary to observe some order and method in the study of parts so
variable. The classification of parts of the Plant will be taken up
in Chapter XX. : meanwhile a few examples illustrating the adapt-
ability of the plant will be described in the present chapter. But it
would be impossible in such a book as this to treat so wide a subject
exhaustively. It must suffice to refer to the Natural History of Plants,
by Kerner and Oliver (Blackie & Son, 1895, 4 vols.), where many
adaptive features of plants are described and illustrated ; or for the
consequences of adaptation as shown in the distribution of British
Plants to Types of British Vegetation, by Tansley (Camb. Univ.
Press).

Biology of Season and of Duration.

If we attempt to sketch a general, that is a non-specialised type of
Flowering Plant, it would have a cylindrical upright stem, bearing
leaves with petiole and lamina radiating out on all sides of it, and with
axillary branches repeating the characters of the main shoot. Its
root-system would consist of a tap-root and lateral roots of successive
orders, all fibrous. A young Sycamore or Apple-tree would answer
this general description. Further, it seems probable that the perennial
state was prevalent, or even constant among early Vascular Plants.
For it is seen almost exclusively in living Pteridophytes and Gymno-
sperms, and it is characteristic of the early fossils. So that in this
respect, as also in their general form, an Apple or a Sycamore may be
held as representing a type of vegetative construction usual for early
Flowering Plants.

In one marked feature, however, the Sycamore and the Apple are cer-
tainly adaptive. Both are deciduous, that is they drop their leaves in
autumn, as do most of our British trees and shrubs. Leaf-fall is clearly
related to season. It brings the biological advantage of reducing the
transpiring area at the time of low temperature, when the activity of
the roots in the cold soil declines. It is in fact a provision against
what may be called physiological drought, for the roots in the cold soil in
winter are unable to make up for any great loss of water by transpira-
tion. But many familiar plants retain their leaves during the winter,
as " evergreens." They are mostly shrubs with leathery leaves, and
many of them have been introduced from southern lands, such as the
Rhododendron and Cherry-Laurel, from the Levant ; but Holly, and
Yew, and Ivy are native evergreens.

The evergreen state is more common in plants of lands where the seasons
are equable, and it is probably a primitive state, while the deciduous habit

MODIFICATIONS OF FORM 195

has been acquired in species that have spread to regions with marked seasons.
On the other hand, in hotter climates than our own, a dry and hot season may
also be tided over by many plants by a fall of their leaves, and a new suit
of leaves is usually formed after the commencement of the rains. The
physiological advantage is similar to that of our autumn leaf-fall at home.
Protection against drought is gained. But the drought thus met in the tropics
is a real lack of water in hot weather. In very hot dry seasons our own trees
sometimes drop their leaves in the same way. Thus by a simple modification
plants can limit their transpiring area temporarily. However prominent
the fall of the leaf in autumn may appear to be, it is not a fundamental
feature, but only a special adaptation to season.

The annual habit may similarly be regarded as an accommodation
to seasonal change. The seed is more resistant to extremes of
temperature and drought than the growing plant. If then the
vegetative development, from germination to flowering and fruiting,
can all be completed within one growing season, an adverse period
can be safely passed as seed, and the species will survive. Practically
this has proved more effective in temperate than in tropical climates,
as is shown by the prevalence of annuals in the temperate Flora. On
the other hand, annuals are few in forest areas, which are less favour-
able to their growth than open ground. It is worthy of note that the
Arctic and Alpine Floras consist almost entirely of perennials. This
is easily understood, since the vegetative season is there too short for
the completion both of vegetation and propagation. The fact is
illustrated by the Alpine Flora of the Scottish Hills, which is distinc-
tively Arctic in its character.

Perennation and Storage.

Perennation, that is the maintenance of the individual from year
to year, presents no difficulties where the seasons are equable, as in
many tropical areas: here perennials, growing steadily on from
year to year, form a leading element in the Flora. But in temperate
regions, with their strongly marked seasons, various adaptations of
the vegetative shoot besides that of leaf-fall may be seen, especially
in herbaceous plants, for tiding over the winter. They are associated
with the storage of material, which is thus carried over from one season
to the next (Chapter VIII.). The simplest case is that of biennial
plants, such as the Evening Primrose and Foxglove ; or, among culti-
vated plants, the Turnip, Carrot, Beet-root, or Onion. These in their
first year store their surplus nutriment in the vegetative organs at the
base of the plant, and use it up in flowering and fruiting in the following

BOTANY OF fHE LIVING PLANT

,,-, after which the plant dies. Extreme cases of this method,
wnCrC the itive period may extend over several years, and is

terminated by flowering, fruiting, and death of the individual, are
en in certain Bamboos, in Agave, and conspicuously in the large
pka. Tins plant, after years of vegetative growth, flowers
with an inflorescence thirty or forty feet in height, fruits profusely,
and <1.

Fig. 129.
Perennial stock of Iris, a, b, c, successive yearly growths. (After Figuier.)

On the other hand, if the flowering be not profuse the perennation
may go on indefinitely, as in ordinary bulbs and herbaceous plants.
Each year a surplus of food-material is laid aside in underground
parts. In autumn the aerial parts may die away, but the stock remains
dormant and usually buried underground. Its store is thus protected
1 the rigours of winter, till in spring fresh shoots develop similar
to those of the previous year. Most of these plants form their foliage
I es first, and they have the advantage of developing more rapidly
than in germination, as they can draw on the store already in hand in
the stock. But some flower at once, even before their vegetative

MODIFICATIONS OF FORM

197

leaves .ire fully formed, as in the Christmas Rose [Helleborus), the
Crocus, and Snowdrop.

For the disposal of their store a slight distension of the tissues is
pften sufficient in these herbaceous perennials. This is seen in the
Iris (Fig. 129), where the short stock grows onwards from year to year,
bearing fresh leaves each season and axillary buds, and storing each
year's surplus in the massive stem. In other cases the various parts

Fig. 130.
Tuberous roots of the Dahlia, (After Figuier.)

Fig. 131.

Corm, or storage stem of Crocus.
(After Figuier.)

may be considerably changed in their proportions. Thus the roots
of the Dahlia are swollen to hold inulin (Fig. 1 30), and root-storage is
also seen in the native Orchids, in Ranunculus Fkaria, and in Spiraea
Filipendula. But it is more frequently the stem, or rhizome as it is
called when underground, that is distended for storage. The familiar
corm of the Crocus is the base of an upright stem of the previous
season's growth, which is swollen for storage. The store is depleted
in the spring by the development of leaves and a terminal flower upon
a new axillary bud. The foliage leaves on that axillary bud serve to
nourish it as the season progresses, and its distended base then remains

198

BOTANY OF THE LIVING PLANT

as the swollen corm for the succeeding season. The membranous
bases of its withered leaves cover the corm externally, while their
axillary buds may provide additional corms. The perennation is
thUs carried out by a sympodial series of storage-corms (Fig. 131).
Similar distended axes are found in many other perennials, e.g. the
Tuberous Buttercup (Ranunculus bulbosus), and the Pig-Nut (Cono-
podium denudatum). In other cases the storage is in lateral branches
borne in large numbers, as in the Potato and Jerusalem Artichoke.

Fig. 132.
Bulb of the Hyacinth. A , seen externally ; B, in median section. (After Figuier.)

These will be considered later in relation to vegetative propagation,
to which such developments readily lead. The bulb, as in the Hya-
cinth, Snowdrop, or Lily, is similarly an upright, abbreviated, perennial
shoot, with its growth interrupted by dormant periods. Its biology
corresponds to that of the corm ; but here the chief storage region is
not the axis, which remains small and broadly conical, but the bases
of the leaves. The wrhole bulb is in fact a perennating bud, the apex
of which terminates in a flower, or inflorescence, while the growth is
then continued by one or more leafy buds formed in the axils of the
storage leaves (Fig. 132). The plants quoted are sufficiently distinct

MODIFICATIONS OF FORM 199

from one another to show that they are all cases of independent
adaptation, though the method of their perennation is essentially
the same in each.

Such arrangements are biologically suited to life under strongly
marked seasons. The plant starts the active season of each year with a
sufficient store of nutrition already in hand to support rapid flowering.
In the remainder of the active season the store for the next year is
acquired by the expanded foliage leaves, and laid aside in the ripening
bulb. The bulb-dealer is understood to sell fully ripened bulbs : flower-
buds are already present in them, and only wait to expand. The pur-
chaser simply offers the conditionsfor active growth, and for the transfer
of the store to the flowering region. But to ensure a repetition of the
flowering in the next year he must fully ripen the bulb again as before.
This is often difficult, or impossible in the case of room-culture, or in
towns. Hence the dealer has a safe and continued market, based on the
ignorance, or the lack of opportunity of the public. The professional
bulb-grower secures normal perennation, with seasonal flowering ; the
purchaseris apt to forget that its continued success depends on nutrition
being maintained till the green leaves shrivel, and functional activity
ceases for the year. This dormant state, in which the bulb or corm is
bought, is itself an accommodation to seasonal drought. The bulb-
habit is widely spread, but it is specially characteristic of countries like
Southern Europe and the Cape, with a moist spring, but a dry and
hot summer.

Symmetry, and its Modifications.

The Root-System, developing in the soil, finds a medium in which
the conditions of temperature and moisture are relatively constant ;
but its form is liable to be strongly influenced by the texture of the
soil. Growing roots yield readily to the mechanical resistance thus
offered by any large obstacle. But if the roots develop in water, or
if the texture of the soil be fine and uniform, as it is in prepared garden
soil, the root-system develops with a regular symmetry. When, as
in Dicotyledons, there is a definite tap-root, this grows vertically
downwards, and the lateral roots radiate from it equally in all direc-
tions. Except for the effects of mechanical resistance, the root-
system of ordinary plants shows little departure from this regular
symmetry, while the individual root is typically cylindrical. It is
different where, as in epiphytes, the roots are aerial. Thus those
Orchids, which normally grow perched on the branches of trees, but are
cultivated in hanging baskets or on cork, often have roots of a flattened

BOTANY OF THE LIVING PLANT

m which follow closely the surface to which they become attached.

nally they may even become green, and act as effective organs

Photosynthesis. But these are exceptional cases. Speaking

ncrally the root of Flowering Plants retains its uniform cylindrical

outline and the whole root-system is built up as regularly or radially

.metrical. This fact may rightly be related to the uniformity of its

usual surroundings.

"I lie Shoot, on the other hand, is exposed to much more varied con-
ditions than the root. It may be developed in water, still or moving ;
or if developed in air, it may be subjected to various degrees of lighting
and moisture, and to winds from any quarter, as well as to the various
incidence of gravity. It is possible to trace, in the different forms of
the shoot which we see,, a relation to and fitness for its surroundings.
It would be strange if the shoot, which is so adaptable individually
as we have seen it to be (Chapter IX.), should not show variety
of conformation in the race, seeing that its surroundings are so
diverse. It may not be possible to correlate all its forms directly, or
even indirectly with circumstance. The difficult question of the
actual method, by which such adaptive features as we recognise may
have been produced in Descent, must be also left aside. But we may
agree to accept as results of adaptation those features which harmonise
^^ y with the surroundings : and from

W si /f^K \ >w 1 tn*s Pomt °f v*ew tne snoot anc* its
^f I /[ x^x/^X Parts maY be studied comparatively.
// / sf VxVv An ordinary upright shoot develops

as a rule with radial symmetry,
that is equally all round the central
axis. The axis being cylindrical
meets equally the impact of all
winds, and its leaves radiate out
from it as a centre, occupying a
circular area whose radius is the
length of the mature leaf. This type
is probably a primitive one, and is very general. But it may be worked
out variously in detail as regards the arrangement of the leaves, as
well as in their form, so as to secure an approximately equal exposure
of all the leaves to the incidence of light. It is obviously undesirable
that one leaf shall overshadow another, and it is interesting to observe
the various ways in which this may be avoided.

Following on the paired seed-leaves, the plumular leaves of Dicoty-
ledons are often paired also, and at right angles to the prior pair

Fig. 133.

Transverse section through the apical bud
of Eptlobium angustifolium, L., showing a
symmetrical^ ■• 2, or decussate system. (After
Church.)

MODIFICATIONS OF FORM

201

{decussate arrangement). This arrangement may be maintained
through life, as it is in the Dead-Nettie, Willow-herb (Fig. 133), Lilac,
Horse-Chestnut, or Sycamore. The upright shoot of the Sycamore
is a good example of the way in which the circular area round the axis
is put to the best use by leaves arranged on a decussate plan. Each
successive pair fits into the gap between those of the preceding pair.
But if the internodes were short, as they are in the young state, the
higher would overshadow the next pair but one of lower leaves.
This difficulty is met by the lower pair having longer petioles, so that

Fig. 134.

Young leafy shoot of Sycamore seen from above : showing how with very little
overlapping the leaf-blades form a mosaic. The spaces unoccupied centrally will
be filled as the younger leaves expand.

their blades are carried out beyond those of the leaves immediately
above them, forming a compact " leaf-mosaic " (Fig. 134).

The decussate is the simplest of the cyclic or whorled arrangements,
where two or more leaves are seated at the same level. But in
other cases the number of the leaves at the same level may be
not two only, but three, four, or more. As in the decussate plan the
leaves of each succeeding whorl alternate as a rule with those of the
preceding, so that they occupy the spaces between them, an arrange-
ment that is very convenient in the packing of the crowded parts into
small compass in the bud. A transition to higher numbers in the cycle
may be seen in the individual plant. Thus in Fuchsia, which has
usually decussate leaves, a very strong shoot may bear alternating
whorls of three. In Lysimachia vulgaris, and in the Privet, a like

BOTANY OP THE LIVING PLANT

tbility is common. It is styled meristic variation, and probably
on in producing it is the size of the apical cone, which, when

large proportionally to the leaf-primordia, can accommodate a larger
Dumber of young leaves at the same level. Such variations are

.union in the floral region, where cyclic arrangements prevail.
[Compare Floral Diagrams in Appendix A.)

But in most Dicotyledons, and very generally in Monocotyledons,
the arrangement of the leaves is alternate] that is, they are seated
singly, each at a different level upon the axis. The arrangement is

Fig. 135.

A F. Ground-plans of buds of Sunflower of different ages : but these drawings are not
uniform in scale. See Text. (After Church.)

often such that an ascending spiral line may be drawn round the
mature stem so as to thread together the bases of them all. Such
arrangements are therefore described as spiral. That the cyclic and
spiral modes of arrangement are not essentially distinct from one
another is shown by the fact that both may appear successively in
the same plant. For instance, in the Sunflower, the seedling starts
with paired cotyledons, followed by decussate leaves of the plumule
(Fig. 135, a), which arrangement may be maintained for a time (c) ;
but sooner or later irregularities appear (b), leading to an alternate
arrangement (dj, which becomes more complex in the upper vegetative
on (e), and culminates in the very complex structure of the
flowering head (Fig. 135, f). It will be unnecessarv for us to trace

MODIFICATIONS OF FORM

203

these successive stages out into detail, though they are found to
follow certain definite methods. The point is that from a cyclic
beginning a spiral disposition is arrived at. As the individual plant
develops and its apex expands, the complexity of the arrangement
of its appendages increases. The individual life of the Sunflower
illustrates a relation that is usual, viz. that complex spiral arrangements

Fig. 136.

Transverse sections through the apical buds of branches of Araucaria excelsa of different sizes.
(After Church.) The uppermost is of a branch of the first degree (7+11): the lower, left, a
branch of the second degree (5+8) : the lower, right, one of the third degree (3+5).

are found where a widened axis develops with short internodes, and
where the crowded primordia of leaves are of relatively small size.
Such spirals occur either in the vegetative or the floral region. Very
beautiful examples are seen in the vegetative shoots of Araucaria
excelsa of various size (Fig. 136). A biological consequence is that
with verv numerous leaves each obtains a maximum exposure to
light incident from above. Moreover, since the branching is axillary
in the flowering plants, the position of the branches themselves will

:<l[ BOTANY OF THE LIVING PLANT

folio* the arrangement of the leaves. This still further defines the
ial form of the mature organism.

Very striking phenomena of spiral leaf-arrangement are afforded by many
p] tnt-shoota This- is the after-effect of rhythmic sequence in leaf-formation
and ia Been most clearly in transverse sections at the level of the bud-apex, as
in Fifl 136. Such patterns present systems of intersecting curves, which may
be expressed numerically ; the spiral effect being due to the fact that the
numbers as counted in different directions, left or right, are unequal, as, for
instance, 7 : ii, 5 : 8, 3 : 5. When the numbers are equal the system is whorled.

Fig. 13;.

Accurate drawing of a shoot of Rhododendron, seen from above. The successive
leaves are numbered, and it is seen that the ninth is covered by the first, while an
imaginary spiral including all their bases successively, will have encircled the stem
thriii-. That is, the angle of divergence between any two successive leaves is f.
(Reduced to J.)

The fact that the numbers commonly observed at the shoot-apex, or residual
in older constructions, are in the great majority of cases successive numerals
of the Fibonacci ratio-series, 1 : 2 : 3 : 5 : 8 : 13, etc. (other ratios being
exceptional), has been taken to imply that the arrangement, as also working
out in terms of angular divergence of successive members as the Fibonacci
angle of approximately 137^, gives the optimum effect in distributing the
laminae to incident light (Wiesner) ; but though the mechanism may
be effective in such a direction it does not necessarily follow that this is the
1 sal f.ictor of the phenomenon (Church).

The different types of spiral seen in the mature shoot may be designated
according to the angle between the median planes of the successive leaves.
This is called the angle of divergence, and it may be expressed as a fraction of

MODIFICATIONS OF FORM 205

the complete circle. It is found that in many plants, and even in whole
families, certain angles are constant in the mature shoot. This gives a com-
parative, or even a systematic value to their observation. For example, in the
Grasses, and in Iris, the angle of divergence is £, that is, the leaves are alter-
nately on opposite sides of the stem, the third being above the first : they thus
constitute two longitudinal rows. In the Sedges, Veratrum, and other Mono-
cotyledons the angle is \, the fourth leaf being above the first, and their
arrangement being in three longitudinal rows. In the Rosaceae and many
other Dicotyledons a common angle of divergence is jths, and the leaves are
arranged in five rows. Consequently the sixth leaf will be directly above the
first, while the imaginary spiral threading their bases together will have passed
twice round the axis before reaching it. Other more complex arrangements
are expressed by divergences of f (Rhododendron) (Fig. 137), and A [Dracaena],
etc. These higher divergences go along with shorter internodes, and compact
grouping of the leaves ; while their overlapping is avoided by the spiral
arrangement.

Various theories have been propounded to account for these facts. The
old spiral theory assumed an inherent tendency to spiral organisation in plants,
and, deductively, attempts were made to read spiral construction into all
shoots. Subsequently a theory of contact-pressures was suggested, according
to which the spirals resulted from mechanical arrangement of the leaf-primordia
upon the axis, comparable to those of marbles in a flat frame. But though
such pressures may in certain cases have an effect upon the arrangement of
the parts as they mature, they do not explain the initial steps. For when the
primordia first appear they are not in contact with one another. In point of
fact the exact position of the primordia of leaves upon the axis, and their initial
arrangement relative to one another, can at present only be referred to inner
causes as yet unknown ; such as localisation of hormone within the growing point
(see Chapter XXXVI.).

Some plants develop with bilateral symmetry, having anterior and posterior,
or right and left sides, which are alike. The flattened shoots of the Prickly
Pear, or of Phyllocactus, are examples, also certain Mosses (Fissidens). But
the headquarters of this type of symmetry, which is uncommon in Flowering
Plants, is in the Marine Algae, and a good example is seen in the common
Bladder Wrack (Fucus, Fig. 280, p. 379).

DORSIVENTRALITY.

While radial symmetry is the rule in upright shoots, those in which
the axis is oblique or horizontal usually diverge from the radial in
more or less degree. They show an obvious relation to the direction
of gravity, light, etc., developing differently on the sides directed
upwards and downwards. Such developments are styled dorsiventral.
Sometimes the effect appears only in the later development of the
axes or appendages ; but in more pronounced cases the initial arrange-
ment of the leaves is itself dorsiventral.

Examples of the former are seen in the lateral branches of many
trees, and conspicuously in many Conifers, such as the Spruce.

BOTANY OF THE LIVING PLANT

While the main axis of the "Christmas Tree" is upright and
,,1 the lat.-r,! branches appear flattened. This is due partly

to the fact that the lateral
branches of higher order have
been developed from buds
seated right and left on the
main branches ; partly to the
curvature of the leaves upwards.
Nevertheless the construction
of these dorsiventral branches
is essentially radial. If the
terminal bud of the main stem
be removed, one or more of
the lateral buds, which would
normally grow into flattened
lateral branches, will grow up-
wards and take its place. With
the vertica-1 position, it regains
its radial symmetry. Very
much the same may be seen
in the Sycamore, or the Horse
Chestnut, both of which retain
the radial structure of the
leading shoot. Their lateral
branches are slightly dorsiven-

Fig. 138.
Strongly dorsiventral branch of Lime, seen from
above. Its leaves form a compact leaf-mosaic.
(Reduced to J.)

tral, but they do not modify the symmetry of the lateral branches
so much as the Spruce.

On the other hand, the upright radial shoot, or leader, in many trees
may itself become dorsiventral as growth proceeds, while the lateral
\ "ranches are conspicuously so. Consequently the whole tree when
full grown will be made up of flattened shoots, the lateral branches
spreading out like fans. (Fig. 138.) In the Elm, Beech, and Lime
this change of the leading shoot comes early. Their seedlings are
radially constructed. In the Elm and Beech the dorsiventral structure,
with alternate distichous leaves (in place of the decussate arrangement)
app n the second year of the seedling, and continues from then

onwards. Bui though the whole tree is thus built up of dorsiventral
shoots, these may form collectively a radial crown, as in the Elm.
Such examples of the transition from radial to dorsiventral symmetry
are carried out in each individual plant. They indicate that the
radial is the more primitive state, and the dorsiventral derivative.

MODIFICATIONS OF FORM

20/

In many herbaceous plants the dorsiventral habit is more pronounced
than in trees. The whole shoot may take a creeping horizontal
position, or a climbing habit by lateral attachment, as in the Ivy.
The lopsidedness then becomes marked, and different degrees of it

Fig. 139.

Successive sympodial shoots of Carex. Each shoot is cf radial construction, but the
whole system is dorsiventral. (After Figuier.)

may be noted. Thus in the creeping rhizome of Carex, while the
leaves show the l-3rd divergence, the axillary bud of some downward-
directed leaf, frequently the fourth, alone is developed. The apex of
the main shoot then turns upwards, while the axillary bud grows on,
continuing the false-axis (sympodium). Thus the individual shoot

BOTANY OK TIIK LIVING PLANT

lr

is radially constructed, but the sympodial system is dorsiventral.
ig. 139.) Inothen the mature shoot may be itself dorsiventral,
though that condition is only acquired in the course of the individual
development, and is not shown by the apical bud. This is seen in
the rhizomes of Acorns, where the apical bud shows a bilateral sym-
metry, with alternate leaves. But in the mature rhizome the two
mw shifted to the upper surface, giving a dorsiventral condition,

with roots arising from the lower surface. In other cases again, and

conspicuously in Ferns, the dorsi-
ventrality is fixed already at the
growing point, and is not the
mere result of any subsequent
displacement of parts. This is
seen in the Common Polypody,
which has from the first distichous
leaves placed obliquely on the
upper surface of the rhizome,
while roots only arise on the lower
side. This arrangement is already
apparent at the growing point
(Fig. 140). Such examples show
various steps in the impress of
dorsiventrality on the vegetative
shoot.

Lopsidedness may also appear in the inflorescence of many flowering plants,
e.g. in many grasses, such as Dactylis. But, as we shall see later, it is in the
flower that dorsiventrality becomes most marked, and of great biological
importance. In all cases among the Higher Plants the dorsiventral is probably
a condition which has been derivative from a primitive radial state.

Fie. 140.

PolypoJium vul^are. ( x 6.) Median section
through prothallus and embryo, showing one
series only of the distichous leaves I|, lu /s, 1-.
R — Tools, ap — apex of axis. The youngshoot
becomes inverted, growing backwards over the
prothallus. (F.O.B.)

Plant Communities.

It is thus seen that Climate and other conditions of life are frequently
related to special modifications in the plant, which meet special
As the conditions will be substantially similar for all
plants which grow in a district or specific area, they may collectively
take characters which they share in common. Thus they form
characteristic Communities. The water-relation more than anything
else determines such adaptations. Where the climate or conditions
are dry, so that water must be carefully conserved, the plants so
adapted are termed Xerophytes. Where water is abundant, or the

MODIFICATIONS OF FOKM

209

vegetation is actually submerged, the plants specially developed under
such conditions arc termed Hydrophytes. Where the water is
brackish or salt, plants assume the characters of Halophytes. But
where the conditions as regards temperature and water-supply are
not extreme, though there may be marked seasons, the vegetation
would be described as consisting of Mesophytes. Such groupings
cannot be drawn with definiteness. They must be held as generally
descriptive rather than mutually exclusive. In point of fact the

Fig. 141.
Succulent stem and flowers of a Cactus. (After Figuier.)

communities overlap and graduate one into another. Naturally
the distinctive characters impressed upon each community are best
illustrated by extreme types.

Xerophyte Vegetation is characterised by various features which
nave the effect, individually or collectively, of controlling transpira-
tion, and thus making the best of a limited water-supply. The leaf-
area is reduced, and its texture is often fleshy, so as to serve for
water-storage, as in the Stonecrop, Aloe, or Onion. In other cases
leaf-reduction may go along with a corresponding distension of the
stem, which becomes green, and takes over the function of photo-
synthesis. This is seen in the tropical Euphorbias, and Cactaceae :

B.B. O

BOTANY OF THE LIVING PLANT

the former especially on the dry areas of the ( >ld, the latter of the New
Worl • Insomi is the stem swells to an almost spherical

term, by which humus the greatest possible proportion of bulk to
: t. uned. By virtue of the water stored up within its tissues
a , pl.mt can live for several months without any external supply

of water. A spiny or thorny character is common in Xerophytes (Fig.
141 1. and is a marked feature in dry districts such as the veldt of
ith Africa. A consequence of this is protection against the attack
of herbivorous animals. Many Xerophytes possess extensive and
deep root-systems (e.g. Wehuitschia) ; moreover, their root-hairs are
characterised by high osmotic pressures and this allows them to
dehydrate the soil more thoroughly than is usual in Mesophytes.

Along with these features go various other structural modifications.
Thickened epidermal walls and cuticle are common (Fig. 142), and this

Fig. 142.
Part of a transverse section of the xerophytic leaf of Hakea, showing a stoma
lv depressed below the well-developed epidermis, which has greatly thickened
, covered by a thick, continuous cuticle. ( > 150.) F. O. B.

together with a free development of mechanical tissue frequently gives the
es a leathery texture. Hairiness is common. The stomata are frequently
sunk in deep pits (Figs. 52, 142) in which a pocket of moist air collects and is
likely to impede diffusion through the pore. Hairs have a like effect on eva-
poration from the cuticle. Protection of stomata is also achieved in other
• s by the lengthwise rolling of the leaf in such a way that the stomata lie

MODIFICATION OF FORM 211

on the inner surface. The rolling may be permanent as in Empetrum and
members of the Heath family, or may only appear during dry spells, as in
various Grasses (p. 188). In some Xerophytes the leaves occupy a vertical
rather than a horizontal plane, due to a bending of the petiole, as in certain
species of Eucalyptus. The incidence of the sun's rays on the leaf is thereby
much reduced. A similar feature is commonly found in those plants in which
the leaves are replaced by flattened stems or petioles. Butcher's Broom
(Ruscus) and certain Acacias are examples of these (Fig. 259, p. 344).

Recent experimental work has revealed that when water is available some
Xerophytes show a surprisingly extensive loss of water by transpiration,
perhaps related to the need for rapid photosynthesis and growth during
the infrequent wet spells. They cannot as a class be said to show low
transpiration at all times. During dry periods, however, the possession
of heavy cuticle, of hairs and of protected stomata which may remain
closed, doubtless tends to reduce transpiration. A further property of
Xerophytes is the ability of their protoplasm to withstand desiccation to an
unusual extent.

Modifications like those shown by xerophytes are seen also in other
plants where water-supply is for other reasons difficult, as it is in those
which live attached to the branches or trunks of other plants (Epi-
phytes). Since they have no direct access to the soil, they must receive
and store the water from rainfall, or condense it from a moist atmo-
sphere. This is the condition of many tropical Orchids and Bromeliads.
In the latter a special surface-protection is afforded by scurfy peltate
hairs, while others serve for absorption of water.

Again, in Arctic and Alpine plants many xerophytic characters are
presented, such as deep rooting, leaf-reduction, succulence, waxy
surface, or hairy coverings. These are probably related to the
condition of physiological drought caused by the prevailing low
temperature of the soil, which checks the activity of root-absorption :
while the shoot, in clear weather and in a wind, may be exposed
to conditions which would stimulate transpiration to a dangerous
degree. The same applies to the temporary reduction of leaf-area
of deciduous trees in winter (see p. 194)-

Halophytes living on the sea-shore, or in salt-marshes, also show
characters similar to xerophytes, such as reduced leaves and succulence
(Salicomia), and development of spines (Salsola). To explain this it
has been assumed that halophytes experience physiological drought
because of the high osmotic pressure of the saline soil solution. Experi-
mental investigation has not supported this theory, for halophytes
transpire freely and appear to have no difficulty in securing water,
while despite their succulence they cannot withstand drought. Their
resemblance to xerophytes may be an apparent one only.

212

BOTANY OF THE LIVING PLANT

On the other hand, Hydrophytes, which grow in wet situations or

|]y Subn I, are independent of the risks of water-supply.

|cav< • often finely divided, giving a large proportion of

sur! to bulk, as in the water Buttercups. They are mostly

rennials. The Water-Lily (Nymphaea) will serve as a good example.
Its thick stock is rooted in the mud and bears floating leaves, with broad
but thin lamina and smooth surface. Stomata are absent from the
submerged parts, but are present on the exposed upper surface
The shoot has little mechanical or woody tissue, but
contains large air-spaces which give buoyancy. The air-spaces
also allow of the storage of carbon dioxide or oxygen, and of gaseous
diffusion from point to point within the plant. The texture of such
plants is limp. Their parts dry up quickly in the air, owing to
deficient cuticular protection. These characters, which are com-
mon in the Hydrophytes, are in sharp contrast with those of xerophyte
types.

The Mesophyte vegetation remains to be considered. Excepting in the
higher temperature, and the greater intensity of lighting of the latter,
the Temperate Zones and the Tropics are alike in presenting con-
ditions very favourable to growth, so long as extremes of season and
of water-supply are excluded. In the low lands of the temperate
zones and of the tropics, many areas exist where vegetation is easy.
Here, when supplied with seed produced by prolific methods, the soil
becomes covered with a dense investment of herbage or of woody
plants, in which the potential individuals are more numerous than the
ground can carry. Over-population is the character of the sward of
any field, as it is habitually of natural woods and forests. Two points
emerge from the contemplation of such native or natural growth.
One is that plants of very diverse outline and construction may thrive,
mixed indiscriminately together. The normal types of Monocoty-
ledons and of Dicotyledons seem to succeed equally well side by side.
This indicates that under such conditions there is little need for
specialised development. The second is that the overpopulation leads
to competition for space and light. Evidence of this is found in the

mmonness of stunted plants, crowded out by the stronger. Any
area of densely overgrown ground in a lowland field or wood shows
in a convincing way how important access to sunlight really is. The
I),an: ■ in fact, in a race for the light, and the tallest plants win.

It is upon this fact that the most striking adaptive feature of the
Mesophytic and Tropic vegetation is based, viz. the Climbing Habit.

MODIFICATIONS OF FORM 213

The Climbing Habit.

The biological advantage gained by the climbing habit is that the
plant which adopts it reaches the light with a minimum expenditure
upon its stem. A plant standing alone has to form a strong supporting
column. To do this requires a considerable expenditure of material
on tissues which are of little physiological use beyond giving mechanical
support. If then such support can be attained in some other way,
so much material will be gained. That the expenditure is really
saved by climbing plants is seen from their anatomy ; for their stems
show vessels relatively few and large, few other tissues of the wood,
and in herbaceous types, though cambium may be present, there is
an absence of tissue-masses formed by cambial thickening. There is,
however, a well-developed phloem, which in some cases is duplicated
on the side next the pith. The vascular strands thus constructed
contain little fibrous tissue, and are usually isolated one from another
by intervening tracts of soft parenchyma (compare Fig. 25, p. 48).
The result is that climbing stems are relatively weak and flexible,
while their leaves, flowers, and fruits may be large. These facts
demonstrate their dependence upon attachment to some stronger
support.

The methods of climbing are various, and they are assumed by
representatives of many distinct families ; not uncommonly by isolated
species in a genus that does not climb as a rule. But in some families
of plants many genera and species are climbers, as in the Leguminosae,
Sapindaceae, and Bignoniaceae. The habit is much more frequent
in Dicotyledons than in Monocotyledons. Several Ferns have also
adopted a very successful climbing habit. This widespread and often
isolated occurrence of climbing, as well as the variety of the methods
involved, suggests that the habit has been acquired along many
distinct lines of Descent. Instances of marked homoplasy are numerous.
While climbing is common in our native Flora, it is most frequent here
in herbaceous plants, such as Vetches, Convolvulus, or Hop. They may
be annuals, like the Black Bindweed ; or perennials with an under-
ground root-stock, like the Hop, or Black and White Bryony. Some
few are woody, as the Honeysuckle, and Clematis and Ivy. While this
is less common in temperate Floras, it becomes a very marked feature
of Tropical Forests. There the huge woody " lianes " develop their
leafy shoots high up amid the branches of the lofty canopy of trees,
while their flexible but woody stems hang down like ropes, connecting
the shoot above with the root-system in the soil. But such climbers

214

BOTANY OF THE LIVING PLANT

of large size do not differ essentially in their methods from the smaller
climbers of Temperate zon<

The methods of climbing may be ranged under three heads : (i)
Straggling, (2) prehensile, and (3) adhesive climbing. The first of these
is the least specialised. It is successfully practised at home by
Cleavers {Galium aparine), and in the south by the Wild Madder
(Rubia peregrina), herbaceous plants which thread their way through
undergrowth or hedge, supporting themselves partly by stiff whorls
of leaves expanding at right angles to the axis, partly by hooked
pnekles borne chiefly on the projecting angles of stem and leaves.

In the Tropics the straggli)ig method gives very successful support to larger
woody plants. (Fig. 143, i.-vii.) In many cases widely spreading branches

V//

Fig. 143.

Various woody stragglers collected in Cevlon, showing various parts reflexed for

support. (1.) axillary shoots of Sageretia ; (ii.) stipules of Zixypkus ; (hi.) prickles

ma ; (iv.) prickles of Calamus ; (v. vi.) axillary branched shoots of Carissa ■

(vii.) retlexed pinnae of Desmonchus. '

in the axils of decussate leaves are an important aid, as in a species of
Lantana, which was introduced into Ceylon as a decorative plant. It has
taken possession of large tracts of abandoned coffee-land, favoured partly by

MODIFICATIONS OF FORM

215

B

its straggling habit, partly by the spread of its pulpy fruits by birds. The
widely spreading branches bear hooked prickles on their projecting angles,
which are effective in aiding support (iii). In other cases hooks that help the
straggling are produced from other parts. The climbing Rattan Palms of the
genus Calamus bear them on the concave side of the whip-like leaf-apices,
or of the axillary buds ; for it is sometimes
the one, sometimes the other, which serves
in this genus as the climbing organ (iv.).
In the Jujube (Zizyphus) there are woody-
stipules to the leaves which are borne by
the curved, whip-like branches. Of these
stipules the one that is downwardly-
directed of each pair is sharply reflexed,
while the other points forwards (ii.). A
parallel is seen in Sageretia, but in this case
it is the axillary buds that are effective,
for the lower of each pair forms a recurved
hook, while the upper develops upwards
as a leafy shoot, (i.) The mechanical effect
is exactly the same as in the Jujube, but
the parts used are different. In the
Palm Desmonchus, it is the distal pinnae
that are reflexed, and act almost like the
flukes of a patent anchor (vii.). A very
similar mechanical effect is shown by the
reflexed axillary branches of Carissa (v. vi.).
Such examples illustrate in what varied
ways straggling may be made an effective
method of support. They involve such
diverse parts as emergences, stipules, pinnae,
and axillary branches. In fact any part
of the shoot-system may be used. The
instances come from most diverse families
of Dicotyledons and Monocotyledons.

: ii:. >

Fig. 144-

Climbing by prehensile methods has

• ,1 r Twining stems. A , Sinistrorse shoot of

gained more attention than straggling, PharbitiS b Dextrorse shoot of Myrsi-
because it is so well represented in the #**«. (After strasburger.)
Native Flora, and because the advantages which it brings are so
obvious. The attachment to the support may be by a twining stem,
the Hop, Scarlet Runner, or Convolvulus; or by tendrils of

as in

various sorts, and by prehensile leaves. The twining of a stem is
partly due to the execution of circumnutatory movements by the
apical part of the stem, the sweep of the nutatory spiral being greatly
increased by the horizontal or oblique orientation which the upper
part of the stem assumes. If the stem comes in contact with an up-
right support of suitable thickness it laps round it with a continuous

216

BOTANY OF THE LIVING PLANT

upwards. There is evidence that the twining is to some
tent a geotropic response of a special type. There is here little
morphological change beyond an elongation of the internodes, and
frequently a delay in the development of the leaves till their support is
mred. Such climbers may be dextrorse, following the hands of a watch
(Hop), or sinistrorse, showing the reverse, which is more common, as in
( onvolvulus or Phaseohis (Fig. 144). There is here little or no contact
stimulus, the twining being a nutatory and geotropic phenomenon.

1 hit tendrils grasp their support as a consequence of contact-stimulus,
which reacts by disturbing the growth while young. The tendril

Fig. 145.

Portion of stern of Sicyos, a Cucurbit, with tendril attached to support, x— point
of reversal of the coiling of the tendril. See Text. (After Strasburger.)

is a cylindrical whip-like organ, usually with a hooked tip. Its
sensitiveness is sometimes localised along a definite line. During
growth it shows movements of circumnutation : if it then comes in
contact with a support, inequality of growth causes the tendril to
lap round it. (Fig. 145.) Its morphological origin may be various.
In the Garden Pea, Vetch, and Cobaea it obviously represents the
distal region of the leaf, including several pinnae ; or it may be
the excurrent tip of the lamina, as in Gloriosa ; or extended parts
of the lower region of the leaf may be prehensile, while the lamina
or pinnae develop normally after the lower region has grasped the
support, as in Corydalis, Clematis, and Solanum jasminoides ; or
lateral ' stipular " structures may be represented by tendrils, as in
Smilax ; or again, the tendril may be referable to a whole shoot, as

MODIFICATIONS OF FORM

21/

in the Grape-Vine ; and probably a like interpretation may be
applied to those of the Passion Flower and the Cucurbits. Thus
various parts of the shoot, or the whole of one, may in different cases
develop as structures called tendrils, and act as prehensile organs.

Once they are attached, tendrils strengthen their tissues. As
growth ceases, the part between the distal attachment and the base
is usually thrown into spiral curves ; and as both ends are fixed, these
are necessarily equal in number in reverse directions. The elastic
tissues of the spirally coiled tendril act like a spring in resisting wind,
and recover when the pressure is relieved (Fig. 145).

Adhesive Cliynbers attach themselves by application of some part
of their surface very closely to the surface of the support, following
its minute irregularities. The result is that
they are affixed so closely that they will
often break before quitting hold. Roots
require little adaptation to this function.
The Ivy is a native type of a number of
plants of other lands, often large and
woody, which attach themselves in this
way to tree-trunks, rocks, etc. Such roots
of attachment are " adventitious," that is,
they are formed not from the root-system,
but at points on the shoot, which are usually
determined by the external conditions.
The roots sometimes lap round the support,
with a prehensile action, as in many of
the large Aroids.

The familiar case of the Virginia Creeper
(Ampelopsis) is morphologically identical
with its relative the Grape-Vine, but the
tendrils are attached by adhesive discs.
The tips of the branched tendril move away
from the light, and this leads to contact

,, Climbing shoot of Ampelopsis

with the support : a rock, wall, or tree- Veitchn. The tendrils (R) have

..... attached their adhesive discs to the

trunk. After Contact each tip Widens into wall-surface behind them. (After

a disc, which at first secretes an adhesive tra6 urger'
cement. This together with its very close application to the inequali-
ties of surface gives a firm attachment. Subsequently the tissues
harden, and the tendril may assume spiral curves, which give a
spring-like resistance (Fig. 146).

k lis

■

Fig. 146.

21 i

UUTANY OF THE LIVING PLANT

Correlation of Growth.

The examples of external adaptation thus selected show that the
Vegetative System of the Higher Plants is liable to various modifica-
tions of form and appearance, and that these often have definite
relation to the surroundings under which the plant grows. But such
modifications are subject to the limiting principle of Correlation.
Correlation of Growth involves the fact that where one part is developed

Fig. 147.

Lower parts of a Potato plant, Solatium tuberosum. The swollen tuberous stems
bear correlatively small scale-leaves. (After Baillon, from Strasburger.)

larger than usual another part is liable to be correspondingly reduced.
This applies especially to the shoot, and it may be illustrated by many
familiar examples. The succulent stem of a Cactus (Fig. 141, p. 209),
distended for water storage, bears correlatively small leaves. The
same is seen in the swollen tuber of the Potato, with its correlatively
reduced scale-leaves (Fig. 147). An extreme and peculiar case is that
of Wehvitschia, where two enormous plumular leaves increase in size
through a long term of years, but the main axis which produced them
is an enlarging stump, bearing no further leaves upon it.

Correlation applies not only between leaf and axis, but also between
tla- various parts of the leaf. Thus in the young Broad Bean, and still
more clearly in the genus Bauhinia, the two basal pinnae develop
to a large size, while the distal part of the leaf is represented only by

MODIFICATIONS OF FORM

219

a minute apical spur between them. In Lathyrus aphaca it is the
stipules which become large foliar expansions, while the lamina itself
is linear (Fig. 148). In such cases, which might be multiplied in-
definitely, extra development of one part is accom-
panied by the correlative reduction of another, as
compared with normal examples. But there is
no exact numerical ratio that can be put upon
the proportions. They suggest in general
terms, rather than with any exactness, that
the excessive expenditure from the total
amount of available material on one part
leaves a deficiency for others. There is no
doubt that this principle of correlation has
a very wide application in determining the
adult proportions of parts in plants.

Correlation is neither a cause nor an
explanation of adaptabilitv, which remains a Lathyrus Aphaca. s, stem;

r . . «, stipules ; b, leaf-tendril.

quite independent problem. It is important (i size.) (After strasbm-ger.)
to see clearly what is meant when the word "adaptation" is used.
It has been used by some biologists to indicate those special modifica-
tions of the plant which arise in relation to the environment. In another
view all plant structures are to be regarded as the expression of the
specific hereditary substance during growth and development under
a certain set of conditions. The structures so developed may, or may
not, be adaptations. Other things being equal, those structures which
are advantageous to the plant will tend to be perpetuated by the
process of natural selection. Thus although many modifications in
plant structure appear to be closely adapted to the environment it
is not necessary to say that they actually arose in relation to that
environment. The advantage which certain features confer upon the
plants that show them often appears obvious enough. But it should
be realised that their recognition as adaptations is no more than an
assumption : it is rarely well founded. This applies to many of the
peculiarities of form discussed in this chapter. They have been
grouped under the heading of adaptations for convenience. The study
of " adaptation " is an attractive phase of biology. But it has led to
much facile or even sentimental writing, which has in it little of the
scientific spirit, and still less of true scientific method.

CHAPTER XII.

IRREGULAR NUTRITION.

So far the plant has been regarded as a self-supporting organism.
Starting from the seed with its small supply of food, it has been seen
to have the power of acquiring, from the soil and through photo-
synthesis, the material necessary for its development. The great
majority of plants have this ability. They build up their substance
by assimilation of simple inorganic materials derived from the environ-
ment. (See Chap. VIII., p. 147.) To such a type of nutrition, depen-
dent on inorganic raw materials, the term Autotrophic is applied.

While the typical plant thus depends on an autotrophic nutrition,
a number of plants have not the ability to assimilate purely
inorganic materials : hence they must have at their disposal a supply of
organic substances before growth is possible. The nutrition of these
plants is said to be irregular or Heterotrophic. Some plants are wholly
dependent on irregular nutrition, others are only partially dependent
on it. Organic food can only be derived from some other organism,
living or dead. If this food is taken from a living organism, plant or
animal, this organism is called the Host, while the dependent organism
is called a Parasite upon it. Sometimes the dependent organism
feeds not upon the living host, but upon the dead body, or upon the
products of its decay. Such a dependent is called a Saprophyte.
Parasitic and saprophytic plants frequently show marked modifica-
tions of form, usually regarded as results of reduction. There is no
sharp line that can be drawn between these two conditions of para-
sitism and saprophytism, for sometimes the parasite causes death
but continues to feed upon the corpse, and so is first a parasite and
afterwards a saprophyte. A peculiar place is taken in this respect
by the Carnivorous plants which digest small animals. They capture

220

IRREGULAR NUTRITION 221

the living animal, but feed upon its dead body. Lastly, there is a
condition known as Symbiosis, where two organisms exist together in
special association : this may be regarded as a kind of parasitism
in which the parasite is held in check, and a state of balance arises.

There is one fact which is common to all these irregular methods of
nutrition. Contact with the source of supply is necessary for the
establishment of any of these dependent conditions. The circum-
stances of a crowded vegetation naturally favour this. The matted
roots of any sod give the opportunity for root-parasitism, such as
is seen in the Yellow Rattle ; the close contact of climbing plants
with their support offers facilities for stem- parasitism, as in the Dodder.
The various types of Algae attached to the submerged surfaces of
water-plants, or it may be actually growing in the intercellular spaces
of aerial parts, are common examples of close contact. This state
has probably led on from mere association to that physiological
dependence which is seen in certain Fungi. The decaying remains of
a crowded vegetation persist for a long time as humus or leaf-mould,
which itself supplies the most common source for saprophytic nourish-
ment. Whether the frequency of these phenomena is to be explained
by the advantage which the dependent organism gains by securing its
nourishment " ready-made," or whether there is another explanation,
cannot be decided on the evidence so far available.

Irregular nutrition is not restricted to any one Family or Group of
Plants, but it has become the leading character of some of them. The
Fungi are the chief examples of it. But as they are very highly
specialised in this relation, while the fossil history shows that irregular
nutrition was established very early in them, they will be held over for
special study in later chapters (Chaps. XXIV. to XXVIIL). For the
present, the illustrations will be taken from the Flowering Plants. Some
families of them appear to be specially prone to physiological depen-
dence. For instance, the Loranthaceae, and Orobancheae. In other
cases isolated genera have adopted the habit ; for instance, the
Dodder among the Convolvulaceae, or Cassytha among the Laurels.
Such facts lead to the conclusion that irregular nutrition among
Flowering Plants is a relatively late and sporadic departure from the
state of nutrition characteristic of the Green Plant.

Partial Parasites.
Certain plants which have adopted a parasitic habit still retain
their chlorophyll, though their colour is apt to be yellowish rather
than the full green. They are thus able to carry on photosynthesis,

> •. »

BOTANY OF THE LIVING PLANT

and to produce at least part of their own nourishment. They are only
partial parasites. This state is seen in the Loranthaceae, with the
familiar example of the Mistletoe (Viscum), a plant which grows fixed
on the branches of various trees. It occurs occasionally on the Oak,
on which it was in early days recognised as the mysterious " Golden
Bough." Other native green parasites are the Eyebright {Euphrasia
officinalis), and the Yellow Rattle (Rhinanthus Crista-gaM), while
Cassytha, a very omnivorous parasite of the tropics, has also a green

colour.

These plants are all fixed by means of haustoria or suckers upon the
host-plant in such a way that the tissues of the one come into close

Fig. 149.

Root of Louse-wort (Pedicular is), which like Eyebright and Yellow-Rattle, is fixed
by suckers upon the roots of the host, here represented black. (After Maybrook.)

relation with the tissues of the other. In the case of Eyebright and
Yellow Rattle there are suckers upon the roots, and they penetrate
the roots of the grasses with which the plant grows. The close juxta-
position of the roots in the sod offers a ready opportunity for the
parasite (Fig. 149). The effect of the parasitism upon the Grasses in
a meadow is such that patches infested by Yellow Rattle can often be
recognised from a distance by the poverty of their growth. In
Cassytha t he suckers arise from the shoot, and the close vegetation of
the tropical undergrowth gives the necessary contact at many points.
In the case of Mistletoe, and of its near relative Loranthus, the
opportunity for parasitic attachment arises from the fact that their
fruits are viscid ; in fact Bird-Lime is derived from them. The berries
are eaten by birds which reject the sticky seeds, leaving them attached
to the twigs on which they perched. Here the seeds germinate, and

IRREGULAR NUTRITION 223

a sucker penetrates the living tissues of the host. In Viscum a
suctorial system spreads from the original centre within the tissues
of the host, penetrating along the region of the cambium. But in
Loranthus the shoot of the parasite creeps along the outside of the
host, and puts in suckers at intervals (Fig. 150).

All these green parasites establish a relation with the conducting sys-
tem of the host, especially with its xylem. Water with its dissolved

Fig. 150.

Loranthus parasitic externally upon a branch of an Alligator Pear, by means of
haustoria penetrating its tissues at intervals. Ceylon. (\ natural size.)

salts is then drawn off from the transpiration stream. In the root-
parasites this supply is additional to what they can themselves absorb.
But in those attached to the shoot their whole supply is thus obtained.
It is uncertain whether or in what degree organic supplies may also be
abstracted. In any case the presence of chlorophyll shows that these
green parasites are not wholly dependent upon their host, but can
themselves carry on photosynthesis. Some at least of the root-para-
sites can grow quite well without the parasitic connection, though this
is naturally impossible in shoot-parasites such as Mistletoe, where
normal roots are absent.

Complete Parasites.

In these, though the plant may show various colours, the green of
chlorophyll is almost absent, and it is evident that the parasite leads a
heterotrophic existence at the expense of the host. A familiar example
is the Dodder (Cascuta), a genus represented in the British Flora by
three species. It belongs to the Convolvulaceae, and shares with
Convolvulus the twining habit (Fig. 151). Clover fields are sometimes
attacked by one of these Dodders (Cuscuta trifolii), and the infected
patches can be seen from a distance by the reddisli colour of the
parasite and the stunted growth of the clover upon which it preys.
Examination shows the Dodder to have cylindrical stems, which twine

22 }

BOTANY OF THE LIVING PLANT

closely round the host plant, and are attached by numerous suckers
llong the surface of contact. Though the seedling germinates in the
soil the parasite after making its attachment to the host, loses its hold
on the soil. It thus becomes entirely dependent on the host for its

Fig. 151.

Cuscuta europaea, on the right germinating seedlings. In the middle a plant of
Cuscuta parasitic on Willow : b, reduced leaves of the floral region ; Bl, flowers.
On the left cross-section of the host, showing haustoria, H, in intimate contact
with the vascular strands. (After Strasburger.)

supply of water and soluble salts. Since it has little or no chlorophyll,
it is also dependent on the host for its organic supply. It is in fact a
complete parasite.

A marked feature is the absence of foliage leaves in the vegetative
region of the Dodder. There are not even any cotyledons on the
embryo. This is to be connected with the parasitic nutrition ; there

IRREGULAR NUTRITION

225

is no self-nutritive function in the parasite, and the leaves which
would normally carry it on are not developed. Such redaction of the
vegetative system is usual in complete parasites. But this reduction
does not apply to the floral region. The flowers of the Dodder, which
are produced in dense heads, arise each in the axil of a bract, and struc-
turally they show the characteristic features of the Convolvulaceae.
From these facts the conclusion seems justified that Dodder is a type
related to Convolvulus, and that the twining habit has led to its
parasitism. Whereas Convolvulus is dependent by twining only for
mechanical support, Dodder has gone a step further and has become

Fig. 152.

Cuscuta europaea

Dodder, shown above in the drawing, into the stem of the host. See Text

Section vertically through a sucker, which projects from the stem of the

(X35-) *

dependent upon its host for its complete physiological support also.
Further, as it is an isolated parasitic genus, and its flowers are like those
of Convolvulus, though on a smaller scale, it appears probable that
its parasitic habit has been acquired relatively late in its evolution.

The attachment of the Dodder to its host is by means of suckers,
which probably represent highly modified roots. The details of the
connection appear to be variable in different species, and perhaps on
different hosts. In specimens of Cuscuta europaea the facts appear
strongly to support their root-character (Fig. 152). First an adhesive
disc projects from the stem of the parasite, and becomes closely
appressed to the surface of the host, attaching itself by rhizoid-like
hairs. Endogenous tissues then burst through like a root, and pene-
trate the tissues of the host. The superficial cells of the penetrating
sucker then grow out into tubes of varying length. Some of these
apply themselves to the wood, others to the bast, others to the pith

B.B.

226

BOTANY OF THE LIVING PLANT

Fig. 153.

Median section of a young plant of
Orobanche seated upon the root of its
host. (After Hovelacque.) ( x 20.)

and cortex, thus tapping both storage and conducting tissues. Where
the sucker impinges upon a vascular strand a continuous xylem-

connection may be established ; in the
phloem also a close relation of the sieve-
tubes of the parasite with those of the
host has been shown.

The Broomrape (Orobanche), and the
Toothwort (Lathraea) are further examples
of parasites with complete physiologi-
cal dependence. Both of these are root-
parasites, with attachment to the host by
haustoria, which penetrate the tissues. The
Toothwort which infests the roots of Hazel
is classified in the Scrophulariaceae, close to
the Eyebright and Yellow Rattle, which are
themselves partial root-parasites. But it
differs from them in having become entirely dependent physiologically upon
its host. The leaves are still represented on the underground shoot, and

thfir curiously reduced and altered form gives rise to

the name of Tooth-Wort. But the flowering shoot

rises above ground, displaying flowers with structure

characteristic of the Family.

The Broom-rapes [Orobanche), which attack various

plants, woody or herbaceous, are closely related to

Lathraea. They show a greater modification of the

shoot, which attaches itself on germination to the

root of the host, developing a brown tuberous body,

without leaves, and shut off from the light. By means

of a sucker it burrows with a broad surface into the root

of the host plant, and establishes a close relation with

its conducting tissues (Fig. 153). The flowering shoot

with its brownish leaves rises above ground, bearing

numerous flowers. Their structure shows that it is a

form related to the Toothwort, but its vegetative

system is still more reduced, leaves being absent from

the base of the tuber. This reduction runs parallel

to but distinct from that seen in Convolvulus and

Dodder. The two sequences provide a good example

of homoplasy, or parallel development, and show

that parasitism may originate separately in distinct

families, though the steps of the consequent modifi-
cation may be alike.

An example of a still further reduction of the vege-
tative system of a complete parasite is seen in

Rafflesia, which grows enclosed within the tissues of

its host. It infests the stems and roots of Cissus,

traversing the tissues with branched filaments of cells, which provide no sem-
blance of stem, leaf, or root. The vegetative system is, in fact, reduced to the

Fig. 154.
Flower-buds of Rafflesia
bursting their way out
from the root of Cissus.
(After Robert Brown.)
Much reduced.

IRREGULAR NUTRITION 227

level seen in filamentous Fungi. But when flowering follows, large buds are
formed deep down in the tissue of the host. These burst through, and develop
as flowers (Fig. 154). In Rafflesia Arnoldi each flower is thirty inches across
when full blown, and has a very peculiar and complex structure (Fig. 155).

Fig. 155.
Flower of Rafflesia Arnoldi. Much reduced. (After Robert Brown.)

It thus appears that while in parasites the vegetative system shows reduction,
which may at times reach an extreme as in Rafflesia, the flower may never-
theless be disproportionately large and elaborate, and produce very numerous
seeds. Biologically their number may be held as an offset to the risk of not
finding the proper host on germination.

Mycorrhiza.

The roots or other underground organs of many flowering plants
regularly grow in close association with the filaments (or hyphae) of
a fungus : the term Mycorrhiza is applied to this association. There
is evidence that at least in some cases of mycorrhiza the higher
plant gains organic food and so displays an irregular nutrition. The
mycorrhizal association has usually been regarded as an example of
Symbiosis (or " living together ") from which both higher plant and
fungus derive benefit, but at present we are not in a position to assess
with any certainty the extent of such benefit. Probably any advan-
tage accruing to the higher plant is connected with the special sapro-
phytic faculty which the fungal partner has of absorbing organic
materials from the decaying vegetable matter of the soil. Some of
these materials, or derivatives of them, may become transferred from
the fungus to the higher plant with which it is associated in the
mycorrhiza. Indeed in those higher plants described as Complete

2JlS BOTANY OF THE LIVING PLANT

brophytes (sec later) there appears to be complete dependence on
organic food derived from the fungus of the mycorrhiza.

Two types of mycorrhiza are recognised. In the first the fungus
lives outside the root tissues of the plant with which it is associated ;
this is described as ectotrophic, and it regularly occurs in many trees,
e.g. Beech, Oak and Pine ; also in some herbaceous plants. In the
second type the fungus penetrates into the cortical cells of the root
of the higher plant, and this is styled endotrophic mycorrhiza. This
type is found in the Heaths and Orchids. But the two types of
mycorrhiza are linked by intermediate conditions, and in many

imples of so-called ectotrophic mycorrhiza there is some degree of
penetration into the cells.

Fig. 156.

Part of the superficial tissue of a root of Sarcodes, covered by a felt of fungal
hyphae (h), in which the dark lines (r, c) are cast-off layers of the root-cap. The
outermost layer of cells of the cortex (c) is covered by a piliferous layer (e), but
the root-hairs are replaced by conical cells between which the fungal hyphae have
forced their way. (After Oliver.) Greatly magnified.

(a) Ectotrophic Mycorrhiza.
Externally roots showing typical ectotrophic mycorrhiza appear
wrapped round by a covering of fungal origin and are short and
thick, and repeatedly branched. Sections show that the roots are
covered by a thick felt of matted fungal threads which usually envelop
the apical part of the root completely. The development of the
dense mantle of fungus may be due to the excretion from the root
surface of a substance favourable to fungal growth. Fig. 156 shows
part of a section through the root of Sarcodes, a native of
N. America, with ectotrophic mycorrhiza. The fungal mantle is seen

IRREGULAR NUTRITION'

229

closely investing the superficial cells of the root, and forcing them
apart ; but still they appear healthy and active. The fungal fila-
ments do not as a rule pene-
trate the cells themselves, so
that the investment is morpho-
logically external. It is, how-
ever, so complete when fully
developed that the surface of
the root has no direct contact
with the soil, and must take its
supplies through the medium
of the fungus. The hyphae at
the outer surface grow out into
absorptive filaments that take
the place of the root-hairs. In
this particular type the root-
cap is shed off in layers, which
are held in the fungal mantle.
Like the native Monotropa
(Bird's Nest) growing in Fir
and Beech woods, and develop-
ing ectotrophic mycorrhiza,
Sarcodes is a complete sapro-
phyte and shows, as with the
complete parasites, a reduced
vegetative system including an
almost complete lack of chloro-
phyll (Fig. 157). The only
visible source of nutrition of
these plants is from the abund-
ant decaying vegetable matter
(humus) of the woodland soils
in which they live, presumably
through the intermediary of the
fungus. These higher plants
would thus appear to be sapro-
phytes at second-hand.

The toadstools of various
types which are commonly

found in WOOds are the Spore- shoot,' with' broad sheathing scales below, and a ter-

minal inflorescence with prominent bracts. Reduced

bearing parts of fungi which (After Oliver.)

Fig. 157.
Whole plant of Sarcodes, showing the mycorrhizic
root-system, from which arises a bulky flesh-coloured

BOTANY OF THE LIVING PLANT

otherwise grow below soil level (p. 459), and it has been shown that
it is tl ame fungi that participate in the mycorrhiza of the roots oi

tree. A good deal of work has been carried out with a view to
determining the importance of ectotrophic mycorrhiza to trees,
chiefly in connection with afforestation schemes. There is evidence
thai in some types of soil the tree does not flourish unless the mycor-
rhizal association is properly developed, though the explanation of this
is not yet clear. Benefit to the tree might arise as already mentioned
by absorption of organic food derived by the fungus from the humus
of the soil, and subsequently transferred to the tree. A further sug-
gestion is that the fungus may supply water and mineral salts more
rapidly than the tree could absorb these for itself.

Whether the fungus gains much from the association with the
higher plant is uncertain, especially where the latter is of the com-
pletely saprophyte class, such as Monotropa or Sarcodes ; but there is
evidence that in some cases the fungi of tree-mycorrhiza develop
unsatisfactorily apart from their normal associate.

(b) Endotrophic Mycorrhiza.

In this type of mycorrhiza the fungal filaments penetrate and
inhabit the cells of the higher plant and come into intimate relation
with their protoplasts, though normally these do not appear to be
adversely affected. Besides the Orchids and Heaths endotrophic
mycorrhiza is present in many other flowering plants and in some
lower plants, for example in Lycopods, and certain Liverworts. As
with the ectotrophic type, most of the plants with endotrophic
mycorrhiza are normal in appearance ; but some, such as the Bird's
Nest Orchid (Neottia), are almost devoid of chlorophyll and are
classed as complete saprophytes, being obviously of irregular nutrition.

Special attention has been given to the endotrophic mycorrhiza
of the Heath family and of the Orchids. The fungus concerned in the
mycorrhiza of the common Heather [Calluna) has been identified as
a species of the fungal genus Phoma. The fungus enters the fine
young roots of the Heather in spring, from the soil, and is to be
found within the cortical cells, these internal hyphae being connected
with others growing on and near the surface of the root (Fig. 158).
The existence of the fungal filaments within the cells of the root
is of limited duration, for during the summer and autumn the filaments
undergo what appears to be a process of digestion under the influence
of enzymes of the Heather plant. While this digestive process,

IRREGULAR NUTRITION

231

which has been compared with phagocytosis in animals, is perhaps
to be regarded as a method of keeping the fungus in check, its products
may be of use to the plant as nutrient substances. A proportion
of these food substances may have been originally derived from the
Heather itself but some part may have been obtained by the fungus
from the humus of the soil, and by digestion this is placed at the disposal
of the plant. In addition this particular fungus may, like the root-
nodule organism, have the faculty of fixing atmospheric nitrogen
(see p. 235). Any nitrogen so fixed would be transferred to the host
by digestion. There is no evidence that food materials obtained
from the fungus are essential to the existence of the adult Heather

Fig. 158.
A single superficial cell of the young root of Common Heather {Calluna vulgaris)
showing the endotrophic fungus, and its penetration of the cell-walls. (After
Rayner.) ( x 1500.)

plant, though it is possible that the plant is thereby enabled to thrive
better in the rather unfavourable habitats which it frequents. Some
experiments have indicated that the association with the fungus is
beneficial or even essential to the proper development of seedling
Heathers, but this point is not finally settled. A special feature of
the mycorrhiza of Heather is that the fungus is not confined to the
roots, but is to be found also in the aerial organs.

In the Orchids, we find that while the majority are green and
normal in appearance, a number are complete saprophytes and show
the usual reduced vegetative system. The mycorrhizal fungus,
known as Rhizoctonia, inhabits the cortical cells of Orchid roots,
and frequently of the tubers that characterise many Orchids. These
internal hyphae are connected up to a certain extent with other
hyphae growing in the soil. In some of the infected cells the fungus
soon undergoes digestion, as in Heather ; while in others, termed
Host cells, the fungus persists unharmed, at least for a considerable
period (Fig. 159). These two types of cell (digestive and host) may
be very regularly arranged, a well-known example being provided

23a BOTANY- OF THL LIVING PLANT

in the root of Neottia. Here again the digestive process may result
in the transference of food materials from the fungus to the higher
plant There is no evidence that in the green Orchids the adult plant
is actually dependent on any such additional food : but the position
different with those Orchids classed as complete saprophytes,
of which Corallorhiza (Coral-root) and Neottia (Bird's Nest Orchid)

Fig. 159.

Section through the mycorrhizal region of the tuber of the Orchid, Phalaenopsis.
At the top of the figure are normal cells of the host ; at its lower limit are cells
crowded with fungal filaments, but still retaining their nuclei. Between these
zones are cells in which the fungal filaments have undergone digestion. An amor-
phous mass, or " clump," of undigested material remains, while the nuclei are
lobed. (After Bernard.) ( x 98.)

are native examples. The former grows in Pine woods with its freely
branched rhizome embedded in rich humous soil. The rhizome
produces scale-leaves but no roots, while the aerial part of the plant
is a simple scape bearing only colourless scales and at the top a
raceme of small pale flowers. The tissues of the rhizome are freely
infected with mycorrhizal fungus. Neottia also grows in woods, and
if the plant is dug up the underground portion, which has some
imagined resemblance to a bird's nest, is found to consist of fleshy
roots crowded upon a short central rhizome which also bears scale
leaves. The plant throws up an aerial stem bearing only scale-leaves
and flowers, all being of a pale brown colour (Fig. 160). Rhizome
and root are freely infected with fungus. Both these plants, being

IRREGULAR NUTRITION

233

almost devoid of chlorophyll, are, like Monotropa and Sarcodes,

obviously incapable of normal plant nutrition. The general view,

which is not entirely satisfactory, is that

during the course of evolution the plants

have come to rely chiefly on organic food

secured from the mycorrhizal fungus ; and

this in turn derives its supply from the

abundant humus of the soil which these

plants inhabit.

It is, however, probable that in the

seedling stage all Orchids, green or sapro-
phytic, are to some extent dependent on

assistance derived from the mycorrhizal

fungus. Observation shows that in the

presence of the fungus, infection of the

Orchid seedling frequently occurs at a very

early stage (Fig. 161). The difficulty of

securing germination of Orchid seeds in

horticultural practice is well known. Ex-
periment has shown that usually Orchid

seeds will not develop under sterile condi-
tions. Inoculation with the fungus is fol-
lowed by normal germination (Fig. 162).

The explanation may be that the Orchid

seed requires a stimulus before germination

will take place ; the stimulating substance,

and additional food, may be provided

by the fungus. The nutritional aspect

is supported by experiments which show that if Orchid seeds

are provided with organic
food such as sugars, normal
development will proceed in
the absence of the fungus.
Both of these observations
have been put to practical
application in horticulture,
and Orchid plants can be
raised (a) by sowing seeds
on special soil infected
with the mycorrhizal fun-
gus, or (b) by feeding the seedlings with organic materials. The

Fig. 160.

Lower part of a plant of Neottia
nidus-avis, showing the dense mass
of roots springing from a central
rhizome, which is concealed by the
roots. s=scale-leaf. /=flower.
r =root. ( x §.)

Fig. 161.
Seed of Neottia infected by fungus at a very early
stage in germination. The outer line represents the
testa. (After Bernard.) ( x ioo.)

234

BOTANY OF THE LIVING PLANT

second method has been termed Asymbiotic germination, and is the
more successful.

It seems probable that in the association called endotrophic mycor-
rhiza the initiative comes from the fungus, by an essentially parasitic

Fig. 162.
Germination of the orchid Odontoglossum. All sections are median longitudinal.
(1) a seed ; (2) shows the extent of development after three months under ger-
minating conditions in the absence of the fungus ; (3) shows the development
obtained in a similar period in the presence of the fungus. From this tuberous
structure the root and shoot later arise. Note the heavy infection and signs of
digestion. p= absorbing hairs (through which infection occurs). s = stoma. ( x 100.)
(From Bernard, Ann. Sci. Nat.)

attack from the soil into the tissues of the host plant. By digestion
the intrusive fungus may be kept under control, while incidentally
the higher plant gains materials which may be of value for its own
nutrition. The same view can be taken of ectotrophic mycorrhiza,

IRREGULAR NUTRITION

235

where, however, the parasitism is of a milder type. The two organisms,
the fungus and the higher plant, essentially antagonistic to each other,
may be looked upon as being in a state of balance ; and that this
leads to a prolonged symbiotic existence together, from which both
may gain advantage. In the above treatment the possible benefit
to the higher plant has received most attention, but the fungus
probably gains at least a favourable en-
vironment, and some food materials from
the higher plant.

The symbiotic condition presented in
mycorrhiza occupies a middle position be-
tween two extremes. The one is that of
mortal disease, where one organism of an
association causes the ultimate death of
the other. The other extreme is that of
immunity, where though two organisms may
be in relation, the one has no power over
the other. There is evidence that under
conditions unfavourable to the development
of the higher plant the state of balance
in the mycorrhiza may be upset, and that
the fungus develops more pronounced
parasitic tendencies.

Root-Nodule Plants.

Plants of the family Leguminosae (which
include Peas, Beans, Clovers and Lupins)
are regularly characterised by the presence
of nodules on their roots. The nodules yield
another example of Symbiosis, in this in- Root of ^fL, ^ nu.
stance between a flowering plant and a bac- ™e/°usc/°°K nod^es- Reduced-

0 r (After Strasburger.)

terium. The lower organism is again perhaps

best regarded as being essentially parasitic in nature. It inhabits the
cells not of the root itself, as in endotrophic mycorrhiza, but of those
special structures, the nodules, which arise on the roots and which
may be compared with other swellings produced on plants by the
presence of a foreign organism, e.g. galls.

The nodules may be either spherical or cylindrical, simple or
branched, according to the particular plant concerned, and they are
present on main and lateral roots (Fig. 163). The nodule consists of

2l6

BOTANY OF THE LIVING PLANT

a centra] mass of enlarged cells, crowded with bacteria which do
not, however, destroy the nucleus or other parts of the host cell
(Fig. 104, 1, 2). Vascular strands traverse the outer tissues of the
nodule and connect up with the stele of the root, while the presence

K

0 Jd

* * a?

0

S

Fig. 164.

1. Young Nodules (K) on a root (W) of Vicia Faba. B=large-celled tissue filled
with masses of Bacteria. M=meristem. T = tracheids. (x 60.) 2, a cell infected
with Bacteria, and smaller non-infected cells. ( x 320.) 3, an infected root-hair.
( ■' 320.) 4, Bacteroids. 5, unaltered Bacilli. (X1200.) (After Strasburger.)

of a meristem at the apex of the nodule provides for its continued
growth. The bacterium (known as Bacillus radicicola) exists in the
soil in a motile condition and penetrates the root-hairs of suitable
leguminous plants. Inside the root-hair the bacterium multiplies
very rapidly, and in the form of an " infection thread " advances
along the hair (Fig. 164, 3), penetrating a group of cortical cells,
which begin to divide, and the nodule is thus originated. The bacteria

IRREGULAR NUTRITION

237

leave the infection thread and by continued multiplication eventually
fill the cells of the developing nodule, though they are restricted to
its tissues and do not infect any other part of the plant. The bacteria
within the nodule-cells have at first a spherical or rod form, but
eventually most of them pass into an enlarged, frequently forked
condition, known as the Bacteroid form (Fig. 164, 4, 5). The formation

Fig. 165.
Plants of Soya Bean (Glycine) growing in nitrogen-free sand. Those on the left
bore root-nodules, those on the right were free of nodules. Photo. G. B. ( x {.)

of nodules usually begins during the first few weeks of development
of a young leguminous plant, and the nodules persist in an annual
until the close of the life-cycle of the plant approaches, when their
tissues decay.

Unlike the great majority of plants, nodulated leguminous
plants are able to make good growth in rooting media free of nitrate
or other nitrogenous compounds, although other necessary salts are
provided : a result which indicates their ability to utilise atmospheric
nitrogen in protein synthesis. On the other hand, if the medium is

BOTANY OF THE LIVING PLANT

initially sterilised and if other precautions are taken to prevent
infection and nodulation, the leguminous plant develops signs of
starvation and makes poor growth (Fig. 165). Evidently it is the
presence of the bacterium which enables nitrogen fixation to proceed
in nodulated plants, and there is general agreement that the bacterium
itself effects the fixation within the nodules, though the chemical
stages in the process are at present uncertain. In some way the
nitrogenous products of fixation are transferred from the bacterium
to the surrounding cytoplasm of the nodule cells, perhaps by a diges-
tion of bacteria under the influence of plant enzymes, or through
excretion of nitrogenous compounds by the bacteria. There is a
steady passage of these products of fixation from the nodules into
the rest of the plant, to its great advantage, especially when the soil
is deficient in nitrogen. No doubt the bacteria appropriate to their
own use certain carbohydrate materials from the plant. The environ-
ment provided by the nodule cells of the Leguminosae seems to be
particularly favourable to the process of nitrogen-fixation, for al-
though the bacterium can readily be grown on a variety of prepared
nutrient media, there is as yet no satisfactory evidence that appre-
ciable fixation of nitrogen proceeds under such conditions.

The significance to the organic world in general of this fixation
of nitrogen by B. radicicola in association with a leguminous plant
has already been stressed. After the completion of the life-cycle
of the plant the nitrogen fixed during its development is added to
the soil, and in due course it becomes available for uptake by plants
in general. We have here a process whereby soil fertility is main-
tained. In agriculture the manurial effect of leguminous crops
such as clover and lupins has been known for centuries. It is,
however, possible that leguminous plants confer benefit not only on
future generations of other plants, in the manner explained, but
also on contemporary plants of other families that may be growing
in the vicinity. For there is evidence that at least under some con-
ditions there is a leakage from the nodules into the soil of nitrogenous
compounds, available for absorption by other neighbouring plants.

It is frequently advantageous in agricultural practice to inoculate the
seeds of a leguminous crop with the correct bacterium, in order to
ensure plentiful nodule-formation. This is especially the case when
a new crop is being introduced into a district, since the correct bacter-
ium may not be present in the soil. It should be mentioned that a
number of different races of the nodule bacterium exist, each capable
of infecting only a limited number of different leguminous species.

IRREGULAR NUTRITION

239

It has been established that the red colour which the central tissue of the
nodule usually shows is due to haemoglobin.

Root nodules are not entirely restricted to the family Leguminosae, but are
also found for example on the Alder tree, Bog Myrtle, Sea Buckthorn, and a
few other plants. The infecting organisms here are different from that in
leguminous types, but they also possess the faculty of nitrogen fixation.

Carnivorous Plants.
The predatory methods of Carnivorous Plants have been considered
from the point of view of the reception of stimulus, and the consequent
movements, in Chapter IX. Another aspect of them is in respect
of nutrition. The plants which show this peculiar habit grow under
conditions where the supply of combined nitrogen is difficult, such as
in peaty or humous soil, where the native Pinguicula and Drosera are

B

Fig. 166.

Leaves of Drosera rotundifolia : enlarged. A , in the receptive state before
stimulation. B, after stimulation, viewed from above, with tentacles partly
incurved. (After Darwin.)

habitually found. Venus' Fly Trap and Sarracenia grow on boggy
moors in America, while Nepenthes is an epiphyte. In such positions
nitrates and other salts would naturally be deficient. On the other
hand, all of these plants contain chlorophyll, and can thus construct
carbohydrate for themselves. In point of fact the carnivorous habit
is not essential to their existence, though experimentally it is found
that a moderate supply of animal food is beneficial.

Apart from the mere mechanism of capture, which is very various,
the physiological treatment of the prize is fairly uniform. Its success
depends upon secretion of digestive juices from certain localised,

240

BOTANY OF THE LIVING PLANT

pl.Hi.lul.il- cells. This has been studied very thoroughly in the Sundew
:, in which the mechanism of capture has already been
described (p. 164). The secretion which plays so important a part
is produced by the glandular head of the tentacle, which receives its
vascular supply through a strand traversing its stalk (Fig. 166}. The
digestive fluid is exuded by the epithelium that covers the surface
of the gland. On contact with an insect, or with a small piece of

nitrogenous matter, such as a cube of
white-of-egg, the gland is stimulated to
greater secretion. A proteolytic enzyme,
which breaks down the complex protein to
simpler soluble substances, is given out, and
the secretion takes an acid reaction. The
emission of the enzyme and of the acid is, as
in the gastric secretion of the animal stomach,
dependent upon the absorption of nitro-
genous matter from the stimulating body :
a piece of indigestible matter produces less
secretion and without proteolytic powers.
The body of the insect, or the cube of white-
of-egg, enveloped in the secretion, is slowly
digested, and the dissolved material, together
with the secretion itself, absorbed into the
cells of the leaf. In the white-of-egg the
rounding of the edges of the cube can easily
be followed. All that remains of an insect
when digestion is complete are the insoluble
.>. L 7!?' l6y' chitinous parts. The process of digestion in

Pitcher of Nepenthes, with part °

of the wall removed to show the the Butterwort and Venus' Flv Trap is

fluid (F) secreted by the glands .

bome on the inner surface, essentially the same as in Sundew. The dif-

(i natural size.) (After Stras-

burger.) ference lies in the varying perfection of the

mechanism.
In Nepenthes the pitcher-shaped leaves are effective traps for
luring small animals into the fluid that partly fills each pendent urn
(Fig. 167). In Botanic Gardens these are often choked by the partially
digested remains of ants, cockroaches, and other victims. There is
no motile mechanism that catches them, but only a static trap. The
pitcher's lip slopes inwards, and is cartilaginous and smooth, with
secreting glands at its inner rim. Insects attracted by the secretion
into a dangerous position on the smooth sloping surface lose their
footing and fall into the pitcher, from which, owing to the absence of

IRREGULAR NUTRITION

241

foothold on the converging walls, there is no escape. Death and
digestion follow.

The secretion within the pitcher is exuded by numerous large, button-
shaped glands upon its inner surface. Each is covered on its upper
side by a downward drooping hood, which effectually prevents its use
as a step in climbing up out of the fluid. The presence of an insect
in the base of the pitcher stimulates the glands to secrete digestive
enzymes which convert the proteins of the insect body into soluble
compounds suitable for absorption by the cells of the pitcher. Some-

Fig. 167 a.

Bladder of Utricularia vulgaris in front view. The semi-circular
entrance is seen, closed by the lid, which is attached to the rim
along the top and part-way down the sides. Four bristles project
from the base of the lid. Attached to the rim of the entrance are
the antennae and other hairs. ( x 33.) (From Skutch, New Phyt.)

what similar pitchers are present in Sarracenia and Cephalotus, though
no secretion of enzymes has been demonstrated in them.

The trapping arrangements in the Bladderwort (Utricularia), a
native aquatic insectivorous plant, are quite different (Fig. 167 a). The
numerous minute bladders, replacing leaf-segments, are normally filled
with water, and are "set" as the result of an osmotic withdrawal
of water from the interior of the bladder by glands on the inner side
of the wall. The bladder-walls are in this way drawn in and brought
into a state of tension. If now an aquatic animal touches the bristles
which project from the lid of the bladder and so opens the lid, the

B.B.

242 BOTANY OF THE LIVING PLANT

tension is released and the walls spring back to their original shape,
producing an inrush of water which sweeps the animal into the
bladder. There is no escape since the lid cannot open outwards.
Death and decay eventually occur and the products are absorbed

by the plant.

The extraordinary forms and mechanisms thus seen in carnivorous
plants seem to accentuate the importance for them of the gain which
follows on this accessory nutrition. Yet all of these plants can live
without it, while a surfeit of animal food may be experimentally
shown to be harmful to them. These plants stand out as some of
the strangest results of special adaptation, and strike the observer
as showing a grotesque disproportion between the end gained and
the means adopted to secure it.

Thus many plants, and often those in which we should least expect
it, have other methods of nutrition than the autotrophic process.
The degree of dependence upon irregular methods varies greatly.
The habit is not restricted to any one family or group of plants.
It has been seen that sometimes single species or genera, sometimes
whole families, are affected. These phenomena are chiefly found
in advanced families such as Leguminosae, Orchids, Heaths, or
Orobancheae, rather than in those held to be primitive. All these
facts taken together lead to the conclusion that irregular nutrition
among Flowering Plants is secondary. Its methods have been adopted
individually, and comparatively late in Descent, by organisms of
which the ancestors were autophytes. Moreover it has not started
along any single line of Descent, but along many. In this, as in
so many special adaptations, homoplasy, or parallel development,
is frequently illustrated. The advance has been along lines of
opportunism. Close crowding has encouraged it. Use has been made
of such circumstances as offered in order to achieve the end of the
plant's existence. That end is not merely the maintenance of the
individual, but the propagation of the race by new germs. The case of
Rafflesia illustrates this in a striking though extreme manner (Figs.
!54, 155)- The vegetative system is reduced, in accordance with
its parasitism, to the level of fungal hyphae. But its flower is of
enormous size, complex in structure, and results in a great output
of seeds, each containing a new germ (ovula numerosissima, as
Robert Brown called them). The nutritive system, though reduced,
is still effective for nourishing this flower. Thus the propagation of
the race takes precedence over the vegetative development. This

IRREGULAR NUTRITION 243

is the ultimate lesson taught by the study of irregular nutrition,
and by the morphological degradation of the vegetative system,
which so often follows on its successful practice. Propagation is
the real end. Vegetative development is only a means to that end. The
whole vegetative system may be regarded as a physiological scaffold,
while the mechanism of propagation is the substantive building
which is erected by means of it.

CHAPTER XIII.

VEGETATIVE PROPAGATION.

a*

In the life of any organism there are two chief phases, which are not
always distinct from one another, and may overlap. The one secures

the maintenance of the individual, the
other increases the number of indivi-
duals. Thus far the former only has
been followed. It has been seen how
the Plant is established on germination,
and developed as an organism which
can maintain itself. It is able, more-
over, to acquire material in excess of
its immediate needs. This is in itself
a necessary condition of increase in
number, for there must be at least a
sufficiency of material for forming the
new germ or germs. But it is not
possible to put any measure on the
amount of material required to be at
hand before increase takes place. An
unusually early propagation in a Seed-
Plant is seen in the Potato, where
the seedling may form a tuber from
the axillary bud of each cotyledon
(Fig. 168).

There are two methods of increase.
One is by Vegetative Propagation, which consists simply in separation
of a part of the plant-body as a being physiologically independent of
the parent. During its early development that part is nourished by
the parent. The separation may finally be completed by the death

244

Fig. 168.

Seedling of the Potato, showing how
the buds (a*) in axils of the coty-
ledons (r) develop as tubers. (After
Percival.)

VEGETATIVE PROPAGATION 245

of the parent, as in the Potato ; or it may be the result of rupture
or death of the tissue connecting it with the parent, as in the bulbils of
the Orange Lily. It is the separation that defines the new individual.
In origin it was a part of the parent plant, the characters of which it
retains and repeats. The process may be simply described as budding ;
or more specifically as somatic budding, as it involves the detachment
of some part of the soma, or plant-body.

The other method is by sexual reproduction, which involves the
fusion of two sexual cells, or gametes, to form a new cell, the Zygote.
This is also the starting point of a new individual. The two gametes
are more or less distinct from one another in origin and character.
The offspring shows features derived from both of the parent gametes.
But it differs in some degree from either of them. The process is not
then a mere act of repetition, as the budding is. On the contrary,
Sexual Reproduction may be a source of something different from either
parent, though it shares the qualities of both (see Chapter XXXV.).

Vegetative propagation is a very wide-spread means of increase
both of wild plants and of those in cultivation, and there is considerable
variety of detail in the way in which it is carried out. In Flowering
Plants it consists in the independent establishment of buds. Such
buds may be produced in the normal sequence, as axillary buds ; or
they may be produced out of the normal sequence, as adventitious
buds. Examples of each will first be taken from plants growing
naturally, and later it will be seen how the cultivator in the exercise of
his art makes use of these, or actually induces their production
artificially.

The propagation by buds formed in the normal sequence sometimes
involves no modification of the shoot, and is so simple a process that it
can hardly be distinguished from ordinary normal growth. An example
is seen in the Canadian Water-Weed (Elodea). The shoot produces
axillary buds which grow into long branches. Either mechanical
rupture, or progressive decay from below, may sever the physiological
connection, and the branch becomes a new individual. Elodea shows
also the indefinite degree to which this vegetative propagation may
extend ; for since the plant was introduced into Britain about the
middle of the nineteenth century, it has spread throughout the water-
ways, notwithstanding that only the female plant was introduced ;
and, being dioecious, it does not propagate here by seed. This
simplest of all methods of vegetative increase in numbers is very
common. Ordinary perennials, such as Grasses and Sedges, give
abundant examples of it.

BOTANY OF THE LIVING PLANT

In other plants some slight modification of the axillary buds may
be seen giving biological advantages. For instance, there may be an
elongation of the basal internodes, leading not only to vegetative
increase, but also to a wide extension of the area occupied. A " runner '
is thus formed, and the bud is carried out to a distance from the parent
plant. It there roots in the soil without competing with the parent.
Tins is seen in the Strawberry (Fig. 169), the Silver-Weed, the Bugle,
and many other creeping herbs. In other cases storage, either in
the axis or the leaves of the bud, gives it an additional advantage at
the start, especially in plants subjected to seasonal change. Such
buds may be produced above ground, as in the bulbils of the Lily or

Fig. 169.

Strawberry Plant, bearing axillary buds developed as runners, with long internodes
which may branch and root at their distal ends. (Reduced to |.)

Onion ; but more frequently they are buried, as in the Potato (see
Fig. 147, p. 218), or Artichoke. Perennation as well as increase in
numbers is secured by such measures as these. Sometimes the
parent survives, as in Saxifraga granulata, or Scrophularia nodosa;
in other cases it dies, as in the Potato and Jerusalem Artichoke. In
the latter cases, since each tuber is borne on an elongated stalk and
can grow into a new plant, both spread and increase are secured, as
well as perennation. It will be unnecessary to illustrate further the
manifold varieties of detail shown in vegetative propagation by means
of buds produced in the normal sequence. In nature a very large
proportion of individual plants may be traced as having been pro-
duced in this way. But it is more common in herbaceous than in
woody plants.

Adventitious buds, that is, buds formed in positions where buds are

VEGETATIVE PROPAGATION

247

Fig. 170.

Leaf of Bryophyllum after culture on moist soil, with
adventitious buds borne at its margins.

not normally present, serve the same end, but they are less frequent.
The case most commonly quoted is that of Bryophyllum, one of the
House-Leek family, whose fleshy leaves may be induced to form buds
in the notches of the leaf-
margin, by pegging them
down on moist soil (Fig.
170). These buds root
themselves in the soil,
and as the leaf decays
they remain as substan-
tive plants. A similar
case occurs in the
familiar Cuckoo Flower
{Cardamine pratensis). In
old plants the radical leaves lie along the surface of the soil, and
buds are formed on the upper surface, usually at the forking
of the veins. In Malaxis paludosa, a small native swamp Orchid,
minute buds appear at the tips of the leaves. Frequently
the adventitious bud-formation appears under some stress of
circumstances. But this is most evident in those cases where they
appear upon roots. For instance, if a Poplar or an Elm be cut down,
the root-system is left still alive in the soil. It contains a large supply
of plastic material, which it uses in the formation of buds. They

originate without order from the region
of the cambium, and rise above ground
as " suckers " (Fig. 171). Fruit trees,
such as plums, if severely pruned, also
produce similar suckers. These are
familiar objects in the vegetable garden,
where plums are trained against a
wall and pruned. Such adventitious
developments are clearly related to a
check of the aerial shoot, and may be
held to be a method of recovery from it.
A special interest attaches to the fact

Root of Populus alba, bearing adven- . .

titious buds, which come above ground that vegetative propagation is common

as suckers. (Reduced.) , . . T . ,

among alpine plants. In them vege-
tative buds easily detached replace flower buds. This is seen in
Polygonum viviparum, where the lower part of the inflorescence bears
as a rule bulbils only, while a variable number of flowers occupy its
distal end. The rare Saxifraga cernua seldom flowers in Britain, but

Fig. 171.

248 BOTANY. OF THE LIVING PLANT

usually bears bulbils instead. Certain Grasses frequently show this
state, e.g. Poa alpina, Deschampsia caespitosa, and particularly the
mountain variety of the Sheep's Fescue (Festuca ovina, var. vivipara).
Such developments may be held as a response to stress of circum-
stances. The short season and alpine conditions being unfavourable
for flowering and fruiting so as to set seed, the formation of vegeta-
tive buds gives a greater certainty of survival. Any advantage
that follows from sexual propagation is sacrificed to attain that

certainty.

Such examples suggest how various are the ways in which Flowering
Plants propagate by budding. It is, however, rare among Gymno-
sperms. In lower forms, such as Ferns, Horse-tails, and Club-mosses,
it is common. Among the most prolific of all plants in this way are
the Mosses, some of which are practically unknown in fruit. Finally
in the Algae, and especially in the Fungi vegetative propagation is
conspicuous as a source of increase. This raises the question whether
this method might not suffice for all practical purposes. Cases are
known among cultivated Flowering Plants where it is continued
indefinitely. The cultivated Banana and the Pineapple are seedless.
The Sugar Cane rarely flowers. The Jerusalem Artichoke has been
grown regularly in British gardens for two centuries from tubers.
There appears in fact to be no definite limit to repetition of increase
by budding. It has always been favoured by horticulturalists for
the good reason that the qualities of the strain or variety are as a
rule retained, while in propagation by seed those qualities are liable
to be modified or lost. It is the weakness, as it is also the strength,
of somatic budding that there is as a rule repetition rather than evolu-
tionary change.

This repetition is exactly what the horticulturalist requires when
dealing with special strains of cultivated plants. His methods depend
on the maintenance of the desired strain through buds produced in
normal sequence, or adventitiously induced. Cuttings and slips are
merely parts of the shoot of the parent plant bearing one or more
normal buds. They are kept under circumstances to promote root-
formation, which takes place best if the cut be made just below a
node. The shoots must be selected of the right age and condition
to secure success (see also pp. 149-150). Greater certainty follows
on layering, in which the shoot is not separated from the parent,
but pegged down in the soil, with or without a notch or ring, cut so as to
check the downward flow of plastic material (p. 132). This promotes
root-formation, after which the shoot may be detached (Fig. 172).

VEGETATIVE PROPAGATION

249

This method is used for rapid production of established plants in the
case of currants, vines, and various fruiting stocks. But some plants

Fig. 172.
Propagation by layering.

(After Figuier.)

are refractory and difficult to root. In such cases the stem below the
shoot it is desired to establish may be nicked with a knife, and packed
with wet moss or soil. Roots may then be formed, after which the
shoot may be severed.

Budding and grafting are
methods commonly used for
woody plants, but latterly they
have been employed also with
success in succulent plants.
These processes consist in the
insertion of a single bud, or of a
shoot bearing a number of buds,
not in the soil, but upon the
corresponding tissues of some
related plant. In the case of
shield budding, which is largely
practised in the propagation
of varieties of roses, a bud is
removed from the plant which
it is desired to propagate, together with an area of superficial tissues
separated at the cambium layer. A surface for its reception is

1

Fig. 173.

Method of shield-budding or cushion-grafting.
(After Figuier.)

25o BOTANY OF THE LIVING PLANT

prepared by a T-shaped cut into the tissues of the stock that is to
receive it, and the tissues down to the cambium are separated from
the woody column (Fig. 173)- The cambium-layer of the shield is
placed in contact with the wood, and the whole is bound up with bast
and wax to exclude air and intrusive fungi. The two living tissues
form each a callus : the two unite, and their junction is such that the

Fig. 174.

Cleft-grafting.
(After Figuier.)

Fig. 175.

Approach-grafting or inarching. (After
Figuier.)

woody column of the stock provides the transpiration stream to the
alien bud. Grafting is essentially the same process ; but a woody
shoot with a number of buds is removed, and inserted upon a corre-
spondingly cut surface of the stock, so that the active cambial tissues
of both shall be in contact (Fig. 174). Inarching or approach-grafting
is similar, but has the advantage of both plants remaining rooted till
the union is complete (Fig. 175). After the bud or graft has fully
united with the stock its own root is cut away. Meanwhile the head
of the stock having been removed, the graft or bud takes its place.

VEGETATIVE PROPAGATION 251

The bud or graft retains its original qualities. But according to the
vigour of the stock it may mature earlier in the season, and fruit more
profusely than upon its own root. Besides such advantages, time is
also saved. For it is much quicker to insert a graft or bud upon an
established stock than to raise an equally strong plant from a cutting.

The graft, or bud, or scion, as it is often called, need not necessarily be of
the same species as the stock upon which it is placed. For instance, the
Peach may be grafted on a Plum stock, the Apple on the Pear, the Pear on the
Quince, or the Medlar on the Hawthorn. But the affinity must be close, such
as within the Natural Order. The stock often influences the scion, though
the latter retains its essential characters. The size, age of coming into fruit,
or the period of maturing of the fruit may be affected ; but such changes are
ascribed rather to the nutritional capacity of the stock than to any more
profound cause. On the other hand, a more intimate association of the
characters of the stock and of the scion is occasionally set up. Reputed
Graft-Hybrids exist, which appear to share the characters of stock and scion.
The most notable of these is Cytisus Adami, of which the parental forms are
stated to have been the common yellow Laburnum and Cytisus purpureas,
the latter having been inserted on the former. The plant which resulted has
been widely propagated. It shows usually purple flowers, but certain branches
" throw-back " to the common yellow form.

Others have also been raised artificially ; the most notable of these were
from grafts between Solarium nigrum, the black Nightshade, and S. Lyco-
persicum, the Tomato. A wedge or saddle-graft is made, and after the tissues
have united the graft is cut through transversely : this causes a callus
to be formed with numerous buds. Most of these show only the characters
of the stock, or of the scion : but some show those characters intermingled.
For instance, the shoot might be almost equally divided so that one side of it
is Nightshade the other Tomato. Such monstrous forms are called " chim-
aeras," and the above instance would be distinguished as a sectorial chimaera,
owing its origin to a lateral coalescence of the tissues of scion and stock,
without any actual fusion of the cells. Others are called periclinal chimaeras,
where the superficial cells of the apex arise from one source, the inner cells from
the other, but again without cell-fusion. It appears from detailed examination
of its tissues that Cytisus adami is a chimaera of the latter type. Chimaeras are
not hybrids in the true sense of the word. There is no nuclear fusion, but the
buds arise from a mechanical coalescence of tissues from the two parents at
the junction of stock and graft. Each retains its own individual qualities,
however closely the two may appear to be physiologically related together.

But occasionally a true graft-hybrid may occur, produced by fusion of cells
and of their nuclei. An example is seen in Solatium darwinianum raised by
Winkler, which is found to be intermediate between Nightshade and Tomato
even in the number of its chromosomes. The nuclei of the former show on
division 72 chromosomes, but those of the latter have only 24. The germ-

cells of 5. darwinianum are found to have 48, that is — — — . It may there-
fore be concluded that a fusion of a nucleus of Nightshade (72) has actually

252

BOTANY OF THE LIVING PLANT

occurred with one of Tomato (24), giving 96, which on reduction to form the
m-cells appears as 48. If this be so, then 5. darwinianum is a true graft-
hybrid. (For an explanation of the behaviour of chromosomes in Reduction
see Chapter XXXV.)

The horticulturalist also induces the formation of adventitious buds,
and some plants respond freely. If a lamina of Begonia or of Gloxinia
be cut transversely across the main ribs, and be cultivated in heat on
damp soil, buds may be formed in relation to any cut vein. These buds

root themselves in the soil as
new plants. It is stated that
each bud arises from a single
cell of the parent leaf. (Fig.
176.) Certain Fern-rhizomes,
and even the bases of their
leaves behave in a similar
way ; but it is in the Mosses
that there is the most re-
markable profusion of this ad-
ventitious development from
single cells of the injured part.
If moss plants be chopped up
into small pieces, any piece
in which an uninjured cell
remains may start a new

vegetative

growth

a

, and lead
new moss

Fig. 176.

Part of leaf of Begonia, bearing adventitious buds
after cultivation, in heat, on moist soil.

ultimately to

plant.

There are certain weeds of

farm land which depend upon
a somewhat similar vegetative multiplication for their survival, when
the land is worked by plough and harrow. The Couch Grass [Triticum
repens), and the Common Horsetail (Equisetum arvense) are cases in
point. Any node serves to provide new buds ; and as the long under-
ground rhizomes are broken up in preparing the soil, this does not
eliminate, but tends to spread the weed.

It thus appears that vegetative extension and propagation of the
individual is a very wide-spread feature, both in Flowering Plants
and in those lower in the scale. It is effective in wild life, as well
as under the hand of the gardener. A very considerable proportion
of the perennial plants which we see have been so produced. This
applies especially to the Grasses and Sedges, whose perennial

VEGETATIVE PROPAGATION 253

rhizomes are constantly growing forward, and as constantly rotting
progressively from the base. But probably the most prominent, and
at the same time familiar example of all is the Bracken Fern, which
covers immense areas, and is widely spread all over the world. Its
underground rhizomes branch freely ; if a single specimen be dug up,
and followed backwards, the brown region of decay is soon reached.
Young seedling Brackens are rarely met with in the open. Here then
is a case where the apical growth and branching of the individual are
practically unlimited, and where its vegetative increase in number of
physiologically independent units appears to be unlimited too. It
may be held as a type of that vegetative spread and multiplication
which, though it involves no special development for the purpose,
is frequent among perennial plants.

CHAPTER XIV.

THE INFLORESCENCE, AND THE FLOWER.1

In Flowering Plants Sexual Reproduction is carried out in the Flower.
It results in the production of Seed. Contained in each ripe seed is
the Germ of a new individual. The Flower which serves this uniform
purpose of producing new germs may take an infinite variety of
forms in plants. But however various the appearance of the Flower
may be in outline, or in the number or complexity of its parts,
comparison shows that the organs which are directly connected with
the sexual process, and the details of that process, are in all cases
essentially the same. This suggests that the differences are accessory,
and that the propagative process itself is the real end.

The Flower.

The Flower is found to consist of parts which fall into certain definite
groups, or kinds of organs. But they may vary greatly in number,
while all the kinds of organs need not be represented in the same
flower. Some like the Water-Lily, or the Rose or Quince, consist of
numerous parts representing all the kinds of organs (Fig. 177). In
other cases the flower in the strict sense may comprise only a few parts,
or in extreme cases only a single one of them, as in the Spurge (Fig. 178).
There is usually a prolongation of the stalk to bear the floral parts ;
but it terminates abruptly and is apt to be more or less widened out
laterally, so that the appendages can be closely crowded upon it.
This widened tip of the stem is called the floral receptacle and since its
apical growth stops, the result is that the flower is always distal,

L This chapter will be best understood after a number of the types of Floral
Construction described in Appendix A have been dissected and examined.

254

THE INFLORESCENCE, AND THE FLOWER

255

that is, it is borne at the end of its stalk. The parts which the recep-
tacle bears may be grouped as : —

(1) Sepals, which are the lowest and outermost parts. They are
usually leaf-like, being firm and green in texture. They constitute
the Calyx, the office of which is protective to the inner parts of the
young bud (Fig. 177, Sep.).

(2) Petals, which lie internally to them, and are usually delicate in
texture and in tint. They constitute the Corolla, and serve chiefly
for attracting attention by colour and scent (Fig. 177, Pet.).

Fig. 177.

Vertical section through a flower of the Quince, Cydonia (Rosaceae). up— sepals.
£rt=petals. si=stamens. c = apices of the carpels, elongated into styles. ou = ovules.
n= nectaries. The receptacle is here hollowed out, so that the carpels appear sunk
down into a cavity. (After Church.)

(3) Stamens, which are inserted internally to the petals, and are
usually club-shaped, and yellow in colour. Each bears Pollen-Sacs,
commonly four in number. The stamens are styled collectively the
Androecium,1 and their function is to produce Pollen (Fig. 177, St.).

(4) Carpels, one or more of which occupy the centre, and are
usually of pod-like form, and either green or colourless (Fig. 177, c).
They are styled collectively the Pistil, or Gynoecium,1 and their

1 The correct spellings of these words, as based on derivation, are
androecium and gynaeceum. But as it is inconvenient to maintain this
difference in spelling in view of the cognate meanings of the terms, it is
best to sacrifice strict accuracy, and to assimilate the spellings. The words
will therefore stand in the text as androecium and gynoecium.

256

BOTANY OF THE LIVING PLANT

functions are to receive the pollen, and to enclose and protect
the Ovules. (Fig. 177, ov.) Under favourable conditions each ovule
is able to produce a single new germ, and to develop into a mature

Seed.

The relations of these floral parts to the receptacle are similar to
those of the foliage leaves to the stem ; for they arise laterally upon
it, and their succession is such that the oldest are the lowest, or outer-
most, and the youngest the inner-
most, or nearest to the tip. No
buds are produced in their axils.
As in the foliage shoot the appen-
dages may be arranged spirally,
or in whorls, but in the flower the
latter is the more common. The
members of the successive whorls
usually alternate with one another.
This is convenient for their close
packing in the bud. The, parts of
the flower are as a rule closely
aggregated together, while those of
the vegetative shoot may be separ-
ated by long internodes. But all

Fig. 178. J is

(i) single male flower of Spurge (Enphor- normal leafy shoots terminate in a

bia), consisting of one stamen, with abortive 111 1 , 1

perianth, (ii) Single female flower, consisting bud, and SO, at least in the young
of three carpels, and an abortive perianth. ,1 1-1 a.w~ „1„~

(iii) Single male flower of Anthostema State, the tWO are alike in thlS alSO.

up or laceae). There are thus marked analogies

between the foliage shoot and the flower. Both are constructed on the
same plan. There is, however, one absolutely distinctive character which
separates them. It is the presence in the Flower of the Organs of
Propagation, called Sporangia. These have no correlative in the
vegetative region. They are organs of a separate category altogether.
Accordingly the Flower may be defined as a simple Shoot which bears
Sporangia.

In the Flowering Plants the Sporangia are of two sorts, viz. Pollen-
Sacs and Ovules. In very many cases these are both present in the
same flower, which is then called Hermaphrodite. But in others only
one or the other is present. When the flower contains stamens
bearing pollen-sacs, but no carpels, it is described as Staminate ; when
it has carpels bearing ovules, but no stamens, it is called Pistillate.
The biological importance of these differences of distribution is great,
as they are closely related to the mechanism of intercrossing.

in

THE INFLORESCENCE, AND THE FLOWER * 257

The two tvpes of sporangia, viz. pollen-sacs and ovules, and the parts that
bear them, viz. the stamens and carpels, are often described as organs of sex, and
flowers in which only one or the other occur are called " male " or " female."
It is better to call them staminate and pistillate. All the old terminology of the
flower has been based upon a misconception. It should be clearly understood
that there has been an error of description. It is almost impossible to eradicate
that error without discarding much of the current terminology, which it is
hardly necessary to do provided that the difficulty is clearly realised.
The early writers suffered from an imperfect knowledge of fact, and a want
of proper comparisons. For our present purpose those comparisons cannot
be made intelligible till certain plants lower in the scale have been examined.
At the moment the conclusion must be stated without the detailed grounds
for it. It must suffice to say here that the sporangia of the Higher Plants
produce SPORES {pollen-grains, and embryo-sacs), and it is from these that the
actual sexual organs originate. But neither stamens nor carpels, nor pollen-sacs
nor ovules, are themselves organs of sex. They are all parts of the neutral plant,
specialised in relation to the sexual organs which it is their idtimate function to
produce. (See Chapters XV. and XVI., also XXXIV., XXXV.)

The Inflorescence.

The Flower denned as above is usually marked off from the vegeta-
tive system that bears it as a definite unit. When borne singly, as
in the Tulip or Buttercup, no one has any doubt what is meant by
the term. But such units are often borne in large numbers together
upon a common branch-system, or Inflorescence, as in the trusses of a
Horse-Chestnut, or a Lilac ; and sometimes the flowers of an inflores-
cence are so closely packed together that the whole may be mistaken
for a single flower, as in the Daisy. It is thus necessary to analyse
the branch-systems that bear the individual flowers.

The Inflorescence often presents marked features, and as these recur
in related forms they have their value in classification. The methods
of branching in an inflorescence, which are often very complicated,
are the same as those found in the vegetative region. Here as there
axillary branching prevails. The leaf, in the axil of which a flower-
bud arises, is usually small and simple by reduction ; sometimes it
is abortive. These reduced leaves are termed Bracts. Where they
are borne upon axes of relatively higher order they are commonly
smaller, and may be styled Bracteoles. But there is no real difference,
except in their relation in the branch-system. The bracts serve
to protect the buds while young. The production of many flowers
together, so as to form a conspicuous group, even though the flowers
may be individually small, brings advantages in mutual protection ;
but still more in relation to the transfer of the pollen. In particular,
b.b. R

258 BOTANY OF THE LIVING PLANT

a complicated branching not only gives the opportunity for a larger
output of seeds, but it may also provide a succession of flowers
which bloom during a prolonged period, instead of simultaneously;
thus the physiological drain of flowering is distributed in time.
These are among the biological advantages gained by complicated
inrtorescences.

The characteristic features which inflorescences show depend upon four
main factors : (i) the arrangement of the leaves (bracts) in the axils of which
the branches arise ; (ii) the proportion of intercalary growth of the several

4Mb,

Man

Ml

A

2>

Fig. 179.

Diagrams of common types of Inflorescence. A, B, definite ; C, D, E, indefinite.
The numbers indicate the succession of the flowers. See text.

axes ; (iii) the number of flower-buds produced ; and (iv) the succession
in which the buds mature. Of these the most important is the last, and
inflorescences may be classed according to its consequences as Definite or
Indefinite. If a distal flower blooms first, that will stop the apical growth
of the main axis, and all further flowrers must be borne on lateral axes. Such
inflorescences are termed Definite, or Cymose (Fig. 179, A, B). They com-
monly develop sympodially, that is, the lateral axes grow so as to overtop
the main axis. But if the buds on lateral branches bloom first, the apex
may still continue to grow, and to form additional bracts and flowering buds.
Such inflorescences are called Indefinite, or Racemose (Fig. 179, C, D, E).
They usually work out along monopodial lines : that is, the main axis remains
dominant, and the lateral axes are accessory.

The simplest Cymose or Definite inflorescence is illustrated by the Tulip,
with its solitary flower terminal on the peduncle, or main flower-stalk. But

THE INFLORESCENCE, AND THE FLOWER 259

in the bulb below, an axillary bud matures during the season, which will
repeat the flowering axis in the next year, and so on. The tulip is then a
definite inflorescence, with an interval of a year between its flowers. Its
branch-system is sympodial, each succeeding axis overtopping its predecessor.
The Buttercup shows a similar condition, but its sympodial character appears
in the flowering shoots of the single season. (Fig. 179, A.) In these cases

ft

>»

Fig. 180.

Inflorescence of Centaury : a dichasium.
(After Figuier.)

Fig. 181.

Inflorescence of Verbena : a

spike. (After Figuier.)

the leaves are alternate, so that each lateral branch is solitary. But if the
leaves which serve as bracts in a cymose inflorescence are opposite, as they
commonly are in the Pink Family, and in many Gentians, the main axis will
bear two lateral branches at the same level. (Fig. 179. B.) The result is
what is called a Dichasium (Fig. 180). The difference depends here upon the
leaf-arrangement, but the method is the same as before ; and a number
of sympodia are the result, instead of only one. Various other Cymose in-
florescences are built up on fundamentally the same principle, but differing in
the orientation and succession of the bracts, and consequently of their

26o BOTANY OF THE LIVING PLANT

individual flowering branches, or pedicels. Examples are seen in the common
Rock Rose, and in Echeveria.

The simplest Racemose, or Indefinite inflorescence is the Spike, where
flowers in an acropetal sequence are seated directly in the axils of the bracts
borne by the main axis or peduncle, as in Verbena (Fig. 181). Or the lateral
flower-stalks (pedicels) may be elongated, giving the condition of the typical
Raceme as in the Currant (Fig. 182). Or again the pedicels may be themselves
branched, as in the Vine, giving the panicle (Fig. 183). Such differences
depend partly upon differences of intercalary growth, partly upon branching
of a higher order. In all of them the distal buds develop latest. It happens,
however, not uncommonly that the characters may be mixed. For instance,

Fig. 182.
Inflorescence of Currant : a raceme. (After Figuier.)

a cymose tendency may appear in the higher branchings of a panicle ; as is well
seen in the inflorescences of Figwort, where the terminal flower of the lateral
branches blooms before those seated below.

The racemose type is particularly subject to extreme differences of growth in
length. The result is seen in certain inflorescences which characterise large
families. The most important are the Umbel and the Capitulum, Both result
from suppression of growth in length. If the axis be abbreviated in the part,
that bears the pedicels these will all appear to originate from the same level,
giving a candelabrum-like branching, called a simple Umbel (Fig. 179, D).
The subtending bracts are also grouped into a close investment just below
the group of branches It is called collectively an involucre and serves for
protection in the young state (Fig. 184). The branching may be repeated
in each of the pedicels, each being provided with a partial involucre of bracts.
The result is the Compound Umbel (Fig. 185). But as in other complicated
inflorescences, the bracts of the partial, and even the general involucre, are

THE INFLORESCENCE, AND Till. FLOWER

261

liable to be reduced, or entirely absent. The closely grouped buds protect
one another while young, so that the bracts become superfluous, and arc
liable to be suppressed. Such inflorescences are characteristic of the Umbelli-
ferae ; but various degrees of abbreviation of the axes are found in other
families, giving rise to modifications of the raceme or panicle sometimes
described as corymbose.

Fig. 183.
Inflorescence of the Vine : a panicle.

(After Figuier.)

If, however, intercalary growth be reduced both in the peduncle and the
pedicels, all the flowers will appear aggregated in a dense head. The axis
of the whole inflorescence is then usually enlarged into a general receptacle,
upon which numerous flowers are seated. Such an inflorescence is called a
Capitulum (Fig. 179, £)• It is characteristic of the Compositae. Here again
the bracts form a general involucre protecting the whole head, while a bracteole
normally subtends each flower borne on the receptacle (Fig. 1 86) . But as these
are closely packed, they must mutually protect one another. The bracteoles
are then superfluous, and are often absent, as they are in the Oxeye Daisy
and the Dandelion (Fig. 480, App. A) . Similar capitula are found in the Sheep s
Bit (Jasione) among the Campanulaceae, and in the Teasel and Scabious

262

BOTANY OF THE LIVING PLANT

among the Dipsaceae. It is in fact a character recurrent in several distinct
families, though it finds its headquarters in the Compositae. Its biological
effect is that an inflorescence acts functionally in the same way as a single
flower.

Fig. 184.

Inflorescence of Astrantia : a simple
umbel. (After Figuier.)

Fig. 185.

Inflorescence of Chervil : a compound
umbel. (After Figuier.)

Inflorescences usually develop on a radial plan, especially those of indefinite
type. But many definite inflorescences appear distinctly dorsiventral. These

~-v

Fig. 186.
Inflorescence of Daisy : a capitulum. (After Figuier.)

are so arranged that each flower as it blooms is directed upwards, thus secur-
ing prominence at the time of pollination. This is seen in the Forget-me-nots

THE INFLORESCENCE, AND THE FLOWER

263

and Rock-Roses. In the Grasses, which arc racemose, even upright
inflorescences may be dorsiventral. This is seen in the Cock's-foot {Dactyhs),
and the Mat-weed (Xardus).

Methods of Comparison of Flowers.

No attempt will be made here to describe fully the wide range of
difference in construction of Flowers, nor to treat those differences
systematically, as a basis for a natural grouping into Families. Such
details will be left over to Appendix A. It must suffice to illustrate
the methods by which such comparisons are most easily presented, and
to state the leading factors upon which these differences depend.
Certain essential facts of floral construction may be obtained from a
median vertical section (Fig. 177, p. 255). This will give the form
and proportion of the receptacle, and the relative levels of the
successive organs which it bears. But it
cannot indicate their number, or fully disclose
their position relative one to another. Such
facts may be obtained by observation from
above, and be plotted into a floral diagram
(Fig. 187). This allows of the representation
of each constituent part. It also gives the
orientation of the flower relative to the axis
and subtending bract. The side next the
axis is described as posterior, that towards
the bract anterior. The plane including the
axis and the midrib of the bract is called Floral di^arf 7of Liliaceous
the median plane ; that at right angles to it ^XdrdiAiboveE^!Snts^
the transverse plane. It is thus possible to jj*j^ the bract is shown below,

plot the constituent parts in ground plan, and

to describe them in their relation to these planes. But the floral
diagram gives no record of the elevation. Accordingly it must be
used in conjunction with vertical sections in order to complete the
study. A compact mode of registering both is found in the floral
formula. If S represent the Sepals, P the Petals, A the Androecium,
and G the Gynoecium, the number of parts of each may be added
as a numeral (00 indicates an indefinite number). Where the Calyx
and Corolla are not differentiated, P may stand for Perianth. Where
the parts of one category form more than one whorl, this may be
shown by giving a separate figure for each. The mutual relations
of the parts can be indicated by brackets, thus showing where parts
are united. The position of the outer parts relative to the gynoecium

264

BOTANY OF THE LIVING PLANT

is suggested by a line above or below the latter. Other details
arc sometimes introduced into floral formulae, but it is best not
to overload them. As examples the following formulae for common
flowers may be given :

Lily, P3+3, A3+3, G(3). (Compare Fig. 187.)
Buttercup, S5, P5, A 00JG 00 . (Fig. 452, App. A.)
Myrrhis, S5, P5, A5, G (2). (Fig. 468, App. A.)
Primrose, (S5), (P5), A0+5, G (5). (Fig. 470, App. A.)

[For details which will explain these formulae see Appendix A.]

Factors leading to Differences of Floral Construction.

The leading factors upon which the differences of floral construction
chiefly depend will now be stated and discussed. They are these :
(i) Differences in the arrangement of parts on the receptacle,
(ii) Meristic differences,
(iii) Fusion of parts.
(iv) Pleiomery.
(v) Meiomery.

(vi) Various development of the floral receptacle,
(vii) Differences of symmetry.

Each of these will be discussed and illustrated,
(i) The arrangement of the parts upon the receptacle may be either
spiral, as in Adonis (Fig. 188) ; or cyclic, as in Ornithogalum (Fig. 187) ;

Fig. 188.

Floral diagram of Adonis.
(From Strasburger.)

Fig. 189.

Floral diagram of Helleborus.
Church.)

(After

or an intermediate condition [hemicyclic) may be found between them,
as in some of the Buttercup Family (Fig. 189). The cyclic type is

THE INFLORESCENCE, AND THE FLOWER 265

however prevalent, especially in highly organised flowers. Since the
spiral is characteristic of many primitive flowers and these graduate
into cyclic types, the facts suggest that the cyclic state may often
have been derived in Descent from condensation of a spiral scheme.
Whether arranged in whorls or in spirals, the parts of successive series
alternate as a rule. This allows of their being closely packed in the
bud. They are formed in acropetal succession. Occasionally it is
otherwise, but the most prominent exceptions occur where an organ
has been reduced, as in the calyx of the Compositae.

(ii) Meristic differences. The number of parts in each successive
category may differ in different flowers, and these are called meristic
differences. In spiral types the numbers are relatively large and in-
definite (Figs. 188, 189) ; in cyclic types they are smaller, and usually
definite (Fig. 187). Where the whorls are well defined the actual
numbers of parts in each may be compared, and are found to vary
in different flowers. Some fundamental number ', commonly three, four,
or five, then rules in the construction of each flower. Such flowers
may be described as trimerous, tetramerous, or pentamerous respec-
tively. But the fundamental number rarely holds through all the
whorls, though the Flax is an example of this (S5, P5, A5, G5). Usually
the androecium shows larger, and the gynoecium smaller numbers
than the sepals or petals.

Not only are meristic differences common between different families,
genera, or species, but even between different flowers of the same inflorescence.
As regards families, the Crassulaceae show meristic variation in high degree,
the fundamental figure rising in the House-leek to as many as twenty, whereas
in Sedum it is commonly five. Within the family of the Liliaceae Maianthemum
has 2-merous flowers, most Liliaceae have 3-merous, but Paris has 4-merous,
or even 5-merous flowers. Within the Primulaceae Glaux sometimes has
4-merous, Primula 5-merous, and Lysimachia 6-merous flowers, and others
have still higher numbers. Within the genus, Gcntiana campestris has
4-merous, and G. amarella 5-merous flowers, while species of Saxifraga may
show flowers 5-, 6-, or 7-merous. In the same inflorescence Adoxa and Pitta
both show meristic variation. In Rxita the terminal flower is 5-merous, and
the lateral flowers 4-merous. In Adoxa, as a rule, the terminal flower is
4-merous, and the lateral flowers 5-merous. Such facts are a warning against
any undue faith in numbers of parts as themselves indicative of affinity.

(iii) Fusion of parts. In some simple flowers like the Buttercup
all the floral parts are separate, or free from one another. This state
is probably primitive, and corresponds to the condition seen in most
vegetative buds. But in many flowers certain parts are found to
be fused together in the mature state. There is a real continuity of
tissue between them. A familiar instance is the Primrose, where the

266

BOTANY OF THE LIVING PLANT

corolla can be pulled away in one piece, though its margin clearly
shows five petaline lobes. Further, if the corolla of the Primrose be
opened out, five stamens will be seen attached to the inner surface of
its tube. So not only is there a cohesion of the five petals to form a
tubular corolla, but also an adhesion of the stamens to it. COHESION
of parts of the same category [such as petals with petals), and ADHESION
of parts of different category [such as stamens to petals), are common in
flowers, and may be held as secondary modifications of their free condition,
as seen in the primitive state.

This view is borne out by the study of development. For where the parts
are fused in the mature state, they still originate as separate papillae of tissue
from the growing point of the flower, just as the foliage leaves usually do.
It is later that the growth extends from the individual bases of these papillae
into the region between them. Consequently when mature they appear as
though borne up on a common base. This is well shown in the flower of the
Compositae, in which there is cohesion of the petals to form the tube of the
corolla, and adhesion of the stamens to the inner surface of that tube.
Fig. 198, p. 273, (v) and (vi) show how that adhesion arises. In (v) the stamens
and petals are independently inserted on the hollowed receptacle ; but the line
where basal growth will take place is indicated. In (vi) the result is seen ;
for they are there borne up on a common base, which has been the result of
that growth.

(iv) Pleiomery. By this is meant that the number of parts of one
category is greater than the fundamental number for the whole flower.

It is most frequently seen in the androecium, so
that the stamens are in excess of the other parts.
It may be a question exactly how this comes
about in each individual example. Branching
or fission of originally single parts may account
for some cases ; interpolation of additional parts,
where there is room for them on the receptacle,
may explain others. The distinction between
these is not always clear ; it turns upon compari-
son, and the observation of details of development.
The essential feature is, however, that more parts
FlG' I9°' of one category are produced than the other whorls

Group of three stamens .

of vdiozia, taking the of the flower would give reason to expect.

place of one stamen in the
normal Liliaceous flower.
(After Eichler.) Fission is most easily recognised where two or more

stamens stand side by side in the place normally occu-
pied by one ; it then sometimes happens that they arise from a common
stalk. The case of Vellozia (Fig. 190) gives a good example. Interpolation
of extra parts may give very similar results. Sometimes it is individual

THE INFLORESCENCE, AND THE FLOWER

267

parts, sometimes additional whorls that are added. All these methods
appear exemplified in the Rosaceae. The following diagrams may be
quoted as illustrating the pleiomeric variations within that family, though
without suggesting any actual line of Descent (Fig. 191). In Sibbaldia,
the pentamerous flower has five stamens. Quillaija has two whorls of five
(diplostemonous), and this probably represents the fundamental type for the
Rosaceae, as it corresponds to that of related families with the formula (S P ,
A5+5- G5)- But in the Rosaceae the matter does not stop there. Further steps
are taken till an indefinite number of stamens is arrived at. For instance,

Z J£.

Fig. 191.

Floral diagrams of various Rosaceae (carpels omitted). I. Sibbaldia cuneata,
and some species of Agrimonia. II. Agrimonia odorata : the first whorl of five
stamens is followed by one of ten. III. Potentilla : the pentamerous corolla is
succeeded by a whorl of ten stamens, alternating with ten stamens of the second
whorl. IV. Rubus idaeus (special case). The pentamerous corolla is followed by a
whorl of ten stamens, and from one to four stamens according to the growth of
the zone of the floral axis are interpolated in the intervals between each pair of the
first stamens, not only one as in III. There are three at a : one at b : three at c :
two at d : two at e : two at /: four at g : two at h : three at j : two at k.
(After Goebel.)

in Agrimonia odorata there is an outer whorl of five and an inner of ten (II.).
In Potentilla there may be two whorls of ten stamens each (III.) ; in Mespilus
there may be four whorls of ten each; while in Rubus (IV.), taking account only
of the two outermost whorls, that next to the pentamerous corolla consists of
ten stamens, but it is followed by numerous stamens disposed in irregular
groups varying from one to four ; those groups alternate with the stamens of
the outer whorl. This points to an irregular interpolation of extra stamens.
Such comparisons suggest that in the Rosaceae three sources of pleiomery of
stamens have occurred, (i) fission, (ii) interpolation of individual stamens, and
(iii) interpolation of extra whorls of stamens.

The diplostemonous state, where the stamens are twice as many as the
petals, is common in those Dicotyledons and Monocotyledons which have
cyclic flowers. Many polypetalous Dicotyledons show it, but with slight

BOTANY OF THE LIVING PLANT

modification of position of the stamens, which suggests that it may have
originated in different ways. It is represented also in the Gamopetalous
1 )icotyledons : these are divided into Tetracyclicae which have only one
whorl of stamens, and Pentacyclicae which have two ; making in all five
cycles of floral parts. The Pentacyclic type is prevalent also in the
Monocotyledons, as shown by the Liliaceous type of flower, with its many
derivatives. The biological meaning of such facts is to be found in
the larger supply of pollen thus provided to meet the risks of failure in
pollination.

5T .

Fig. 192.

Flower of Scrophularia nodosa, which is pentamerous. The five stamens are
represented, but the posterior stamen (sf) has no anther : it is merely a vestigial
staminode which marks the place of a normal stamen.

In " doubled " flowers similar features of pleiomery are seen. " Doubling "
often consists merely in a petaloid development of stamens ; and it most
commonly occurs in flowers with an indefinite number of stamens. But it
often involves also an actual increase in number of the petaloid parts, effected
by methods of fission, as well as of interpolation of extra whorls, or of extra
individual parts.

(v) By Meiomery is meant that the number of parts of one category
stands below the fundamental number for the whole flower. It is
most often seen in the gynoecium ; but it appears also in the androe-
cium, especially in flowers where the mechanism is highly specialised.
It occurs less frequently in the outer floral parts. In the gynoecium

THE INFLORESCENCE. AND THE FLOWER

269

it may be referred to a fading out of the activity of the floral shoot,
or even a deficiency of room for the full number of parts. The result
is that the carpels are frequently fewer than the other parts. For
instance, in the Compositae, the Umbelliferae, and most Gamopetals
there are only two carpels in the pentamerous flower. But in many
cases it is clear that the smaller number is the result of abortion of
parts which comparison with allied plants would show as actually
present. Sometimes those parts are represented by vestigial remains,

Fig. 193.

Flower of Veronica Chamaedrys, typically pentamerous, but the posterior sepal
is abortive : the two obliquely posterior petals are fully fused to form apparently
one : only the two obliquely posterior stamens are developed.

marking the position which those parts should hold, though they do
not come to functional maturity. A good case of a vestigial stamen
(st.) is seen in Scrophularia (Fig. 192).

Meiomery may appear in any of the floral parts ; often it is seen in several of
them in the same flower. A complete whorl may be absent : for instance the
corolla in the Pearl-Wort (Sagina apetala) in the Pink Family, and Glaux among
the Primroses : or one of the whorls of stamens may be absent, as in the Prim-
rose. The most marked examples in the androecium are related to increasing
precision of the floral mechanism. For instance in the Orchidaceae, derived
from an Amaryllidaceous type with six stamens, Anastasia has three,
Cypripedium two, and Orchis only one— the anterior stamen. Ginger has also
only one, but it is the posterior. All of these are highly specialised types :
their meiomery by abortion has followed parallel, but quite distinct lines. The
Valerianaceae show various degrees of abortion of the stamens ; but they also

270 BOTANY OF THE LIVING PLANT

have a reduced gynoecium. Here three loculi are present in the ovary, but
only one bears a fertile ovule. The same is the case in the Oak ; also in the
Coco-Nut. Here the three depressed scars on the shell indicate the three
carpels, but only the one that can be pierced by a pin matures its seed, and

forms a germ.

A beautiful case of meiomery, involving several steps, is seen in the
Scrophulariaceae. The flower is typically pentamerous, but it becomes

M.

TL.

E

Fig. 194.
Dissections of flowers of Lychnis dioica. I., II., VIII., the pistillate flower in
which the stamens are represented only by staminodes (st). III., IV., IX., the
staminate flowers in which the gynoecium is represented only by a vestigium (gyn).

reduced to apparent tetramery. In the Mullein (Verbascum) the formula
is S5, P5, A5, G2. But in Scrophularia the posterior stamen is represented
by a non-functional staminode (Fig. 192), while in others of the family it
may be absent. In Veronica the two anterior stamens are also abortive.
The two posterior petals fuse so that the corolla appears to be four-lobed ;
the posterior sepal, which is present in the Mullein, is represented in some
species of Veronica by a sepal smaller than the rest ; in other species it is
absent, as in Veronica Chamaedrys and the most of the Rhinantheae. Thus
the flower, though typically pentamerous, has by stages of meiomery become
apparently tetramerous. (Fig. 193.) Similar changes occur in the Plantains
and Teasels.

THE INFLORESCENCE, AND THE FLOWER

271

Such examples serve to show that meiomery by abortion may affect anv
of the series of parts, and not [infrequently more than one of them in the same
flower. It is probably the cause of greater divergence of detail in flowers
than any other factor.

The most important cases of meiomery are those where one or other
of the essential organs may be wholly abortive. It frequently occurs
that flowers typically hermaphrodite may be staminate or pistillate, by
abortion of one or other of the essential parts. A good example is
seen in Lychnis dioica (Fig. 194). The Pink family to which it belongs
have usually hermaphrodite flowers ; but here the species is dioecious,
which means that some plants have only staminate others only
pistillate flowers. An examination of each of them shows that in the
staminate flowers an abortive gynoecium occupies the centre (iv.) ; in
the pistillate flowers ten staminoides, or abortive stamens, surround
the base of the ovary (ii.). Since these parts correspond in position
to the parts normally present in allied plants, they indicate that
L. dioica is dioecious by abortion. There is evidence that this form
of reduction has been of frequent occurrence in the evolution of
Flowering Plants.

Such results are sometimes seen in extreme form, and nowhere better than in
the Spurge. Comparison indicates that the Euphorbiaceae are related to the
Geranium Family, members of which are
typically hermaphrodite. Some of the Spurge
Family [e.g. Andrachne, Phyllanthns) have
calyx and corolla represented, but only
stamens or carpels, never both. Others
show steps of further reduction of floral
structure, till in Euphorbia itself the staminate
and pistillate flowers reach a very simple
condition. The former appear as a single
stamen, with a ring half way up its stalk.
This represents the abortive perianth. The
pistillate flower consists of three coherent
carpels, with a rim below, which represents
again the abortive perianth (Fig. 178, p. 256).
The facts justify the conclusion that there
is here a very advanced state of meiomery.
Such extreme reduction is usually connected
with a close crowding of numerous flowers FlG

into an aggregated inflorescence. Vertkal section o{ flower o{ Mymmm

as an example of an hypogynous flower.

(vi) Various development of the floral (After Figuier.)
axis or receptacle accounts for very con-
siderable differences of floral construction. As a rule the parts of
the flower are closely packed upon the shortened and distended

272

axis. In
(Fig.

Fig. 196.

Vertical section of flower of the Peach, as
an example of a perigynous flower. (After
Figuier.)

BOTANY OF THE LIVING PLANT

primitive types, such as the Buttercup, or Mousetail
the receptacle is conical, and the sepals, petals, stamens

and carpels succeed one another
upon it without any interval. Where
the stamens are thus seated below
the carpels the condition is de-
scribed as hypogynous, and the
ovary superior. Occasionally in such
types the axis may be elongated,
so that there is an interval between
the series of parts. In the Passion
Flower the stamens and carpels
are thus carried up a considerable
distance above the sepals and petals. In the Caper Family the
carpels alone are raised thus on an elongated axis. More frequently
there may be a local widening out of the receptacle, in the form of
a ring or cup, by growth of tissue beneath the
insertion of the lower parts. The sepals,
petals, and stamens may together be carried
outwards upon its margin, while the gynoecium
occupies the centre of the cup. This occurs
frequently in certain families, and is well seen
in the Rosaceae (Fig. 196, Peach). Occasion-
ally an isolated genus shows it, as in Subularia,
among the hypogynous Cruciferae. It may
be regarded as a local modification of the
hypogynous state, and is described as peri-
gynous.

A more important modification is that
which leads to the sinking of the gynoecium
downwards into the tissue of the abbreviated
axis. This gives the epigynous condition,
with so-called inferior ovary (Fig. 197, Fuchsia).
The way in which this comes about is best
illustrated by observing the development of a
flower of an epigynous type, as may be easily
done in the Sunflower or others of the Com-
positae (Fig. 198).

If median sections be cut of a young head of Sunflower, the general receptacle
will be seen to bear flowers of different ages, the oldest at the outside and the
youngest nearest the centre. Each arises in the axil of a bract, and the youngest

Fig. 197.

Vertical section of flower
of th; Fuchsia, as an ex-
ample of an epigynous flower.
(After Figuier.)

THE INFLORESCENCE, AND THE FLOWER

2— -*
/ 3

may have the form of a simple convex papilla (i.). But as the growth at the
centre is slower than at the periphery, the flower becomes first flattened and

then hollowed. Five rounded bosses appear on the margin of cup, whi< h arc
the five petals (ii.). The hollow surrounded by them deepens, and five other
bosses appear internally to, and alternating with them.

These ar-e the stamens

S*,^

i -

ovj-

Fig. 198.
i -viii. Successive stages in the development of the individual flower of the
Sunflower; i.-vi. in vertical, vii., viii., in transverse section. £ = tiower. 0-
bracteole. /> = petal. sr=stamen. c = carpel. s=sepal (i.-v., vii. and vin. -bo
vi. x 16.) The shaded zone in vi. is the result of intercalary growth originating at
the dotted line in v.

(hi.). By their formation the hollow is narrowed, and presently from its
shoulders two other upgrowths are formed. These are the carpels (iv.).
Meanwhile two small outgrowths may be seen at the outside, which previously
was smooth. These are two teeth representing the reduced calyx, which thus
appears delayed out of the normal succession. The carpels are in contact
above and enclose a cavity, which is the ovarian cavity, and the organic apex
of the 'flower lies at the bottom of it (v.). It is here that the single ovule arises
(vi.), and thus the ovary containing it lies apparently below the other parts.
ItVdescribed as inferior, and the flower as epigynons. But it is clear that the
b.b. s

274

BOTANY OF THE LIVING PLANT

succession of parts, excepting the reduced calyx, is acropetal, and the carpels
are actually nearest to the apex. Thus the epigynous state results from the
relatively slow growth of the apex, which is overtopped by the stronger growth
around it.

(vii) Differences of Symmetry. In many flowers, such as the Butter-
cup, Rose, or Tulip, the parts may develop equally on all sides of the
axis, giving a radial or actinomorphic symmetry. This is believed to
be a primitive condition, and it is found in flowers which are not
highly specialised. It prevails in spiral or hemicyclic types. In
others the development may be unequal on different radii, so as to
give the flower a lop-sided form ; but almost always so that it can
be divided by at least one plane of symmetry into two equivalent

ABC

Fig. 199.

Diagrams of zygomorphic flowers. A, of Aconite (Ranunculaceae). B, of the
Laburnum (Leguminosae). C, of Toadflax (Scrophulariaceae). (After Eichler.)

halves. Such flowers are called dorsiventral, or zygomorphic, and the
Aconite, Laburnum, and Toadflax are examples (Fig. 199). The plane
of symmetry is usually the median plane ; but sometimes it is oblique,
as in the Horse Chestnut ; or transverse, as in Corydalis. Zygomorphy
is very common, it occurs in most families, and it is characteristic of
some throughout, as in the Orchidaceae, or Labiatae. Its importance
lies in relation to the transfer of pollen by animal agency. The most
perfect mechanisms for this end are found in zygomorphic flowers.
It is a derivative or specialised state, and comparison shows that it
has been acquired by modification from the radial state, along many
distinct lines of descent.

The Biological Specialisation of the Flower.

Diversity of floral structure is very fully illustrated in the Angio-
sperms. But however varied, or sometimes even extravagant the
structure of flowers may appear to be, each flower should, in point

THE INFLORESCENCE, AND THE FLOWER 275

of final analysis, be regarded as a simple shoot bearing sporangia,
modified in the course of its evolution along lines of specialisation
such as those specified above. Structurally its shoot-nature is evident
in the less specialised examples, though in the more highly specialised
it may be so hidden by peculiarities of form as to be recognisable
only after careful analysis and comparison. Moreover, those several
factors of specialisation, which have been discussed and illustrated
in the preceding pages, may take effect in the most varied combination,
so that the analysis which is required to reduce a complicated flower
to terms of construction comparable to that of an ordinary leafy
shoot may demand both skill and insight. But this is greatly aided
if the student can realise, in the first place, the probable evolutionary
steps by which the peculiar structure was produced : and secondly,
the biological advantage which follows from it. The most obvious
end served by the mechanism of the flower is the transfer of pollen
from the stamen to the carpel. This is a necessary step, though by
no means the only important one, in normal reproduction. For
carrying this out Seed-Plants are dependent upon external agencies,
such as wind, water, or animal activity. Since Plants are fixed in
the soil and immobile, such agencies must be resorted to if the advan-
tage of intercrossing is to be secured. It is such considerations as
these which make the variety and intricacy of the forms which flowers
show biologically intelligible, and give a special interest to the study
of their evolutionary origin.

It is possible by comparison to follow the general trend of evolu-
tionary advance from floral types which may be held to be relatively
primitive to those which are advanced and specialised. The former
are more like the normal vegetative shoots in the arrangement and
mutual relations of their parts : the latter diverge in more or less
marked degree from that simple state. It is generally accepted
that a primitive type of Angiospermic flower was hermaphrodite, and
that its numerous parts were inserted separately, and in acropetal
order upon an elongated receptacle ; and that they were arranged
after a spiral plan. This type of construction is seen in the Mag-
noliaceae, and other related types, such as the Buttercups and Water-
lilies. On the other hand it seems probable that very simple unisexual
flowers also existed in early times, such as are seen in the Willows.
It is significant that both the Magnoliaceae and the Salicaceae occur
early as fossils, and are in fact recorded from low horizons of the
Cretaceous Period. It is, however, in the hermaphrodite types that
the lines of floral specialisation can be most readily traced.

BOTANY OF THE LIVING PLANT

An early line of specialisation led towards a cyclic arrangement
of the parts, their number becoming at the same time more definite.
The outer whorls were differentiated as the protective calyx and
the attractive corolla. These steps are illustrated among the Ranun-
culaceae, which include both hemicyclic and cyclic flowers (Figs. 1 88,
189). The best examples of the cyclic state are, however, found among
the more advanced types, where that feature is constant (Figs.
187, 192). Once the cyclic flower was established, other biologically
effective modifications followed. The sepals became united into
the mechanically stronger gamosepalous calyx. The petals also
became coherent, thus acquiring mutual strength, and at the same
time fencing round the honey-secretion at its base. Moreover the
gamopetalous corolla has often assumed forms that suit the con-
venience of the insect-visitors : colours and odours that attract ;
and it even shows distinctive lines that guide the eye to where the
honey lies. The stamens become adherent to the corolla-tube.
Such features go habitually with that lop-sided or zygomorphic
development which secures a convenient alighting platform, and
ensures a definite position for the visitor. The result of this precise
mechanism is that there is economy in the amount of pollen necessary
to make pollination a reasonable certainty.

So long as the flower is not highly specialised a large production of

pollen, and a large number of carpels and ovules will be advantageous

or even necessary, for this will multiply the chances of successful

propagation. It is then natural to find that pleiomery of stamens

and carpels is common in non-specialised types. For instance, the

Ranunculaceae (Figs. 188, 189), and Rosaceae (Fig. 191), the Lime,

and the Mallow are all polypetalous flowers of radial construction,

and they have as a rule numerous stamens and carpels. But their

haphazard methods of pollination and of seed-distribution entail

waste. Economy comes with specialisation, such as is seen in the

Gamopetals, and in the more highly adapted Monocotyledons. In

them meiomery appears both of stamens and carpels. The former

may be reduced from the fundamental number of the flower to two

Veronica (Fig. 193), Salvia, Cypripedhwi], or to only one (Centranthus,

hrhis, Ginger). The carpels of highly adapted flowers often number

only two, and they may be reduced to one, as in the Papilionaceae.

I he ovules themselves are frequently solitary where the seed is large,

and its chances of distribution and germination are good, as they

in the Coconut and Cherry, and in the Grasses. Thus there is

a biological rationale underlying pleiomery and meiomery, both of

THE INFLORESCENCE, AND THE FLOWER 277

which may be held as advances from a relatively primitive state,
where the fundamental number ruled through all the floral
whorls.

The behaviour of the floral receptacle is biologically the most
important of all. Primitive flowers are hypogynous, exposing their
carpels on an elongated receptacle (Fig. 195). By shortening its
growth the epigynous state is produced (Figs. 177, 197), and the
carpels are immersed in the tissue of the axis. Thus they secure at
once the nursing advantages of additional protection, and of a near
relation of the ovules to the sources of supply. All the factors upon
which diversity of floral structure depends are biologically intelli-
gible. All lead to a higher probability of successful propagation.
It is, however, worthy of remark that the flower does not really
differ in respect of them from the foliage shoot : for all of the modifica-
tions can be matched by special instances of development in the
vegetative region. But features that are exceptional in the vegetative
shoot are common in the floral region, and it is that which makes
the structure of the flower seem peculiar. The real distinction
between the vegetative and the propagative shoot lies not in form,
or texture, or colour, or number or relation of the parts, but in the
fact that sporangia are present in the flower, while they are absent
from the foliage shoot.

The fact that the flower is constructed fundamentally on the same plan as
the foliage shoot did not escape the attention of the early botanists. More-
over, they noted that it is universally preceded by some form of vegetative
shoot in the individual life of the higher plants. In annual plants this is
obvious enough : it is only after the establishment of the leafy plant that the
flower-buds make their appearance. But in the case of many of the plants
that expand their flowers in the early spring the matter is not so simple, and
one is apt to forget the swollen underground parts from whose stores the
flowers draw their material. It is needless to elaborate by examples the
simple fact that in all cases nutrition must precede propagation. It was this
fact that formed the foundation of Goethe's Theory of Metamorphosis. He
recognised under this name the process by which one and the same organ, for
instance the leaf, presents itself to us in various modifications, such as the
foliage leaf, sepal, petal, or stamen. He distinguished as " progressive meta-
morphosis " those changes of type of the appendages which proceed from the
cotyledons or seed-leaves, through the foliage region and bracts to the flower,
and finally to the perfected fruit. On the other hand he designated as " retro-
gressive metamorphosis " that process by which the succession appears
reversed ; as for instance in abnormal or doubled flowers, when a stamen or
a carpel develops as a petal, or even as a foliage leaf. These general ideas
of the relation of the vegetative and floral regions were amplified and made
more definite by later writers, and were for a long time widely held. Thus it

BOTANY OF THE LIVING PLANT

, ,1 belief that the flower had resulted from changes wrought in some

p, tat shoot.

we direct our attention solely to the Flowering Plants this opinion
might stand. But as the nineteenth century drew on, knowledge of the lower
forms greatly increased, especially in the case of such plants as the Ferns and
Club-Mosses. This has supplied the material necessary for a revised theory of
the origin of the flower. Checked by these comparisons we may now figure, on
a basis of fact, rather than of semi-poetical surmise, how the flower as distinct
Erom the vegetative region originated in the higher plants. The main point
to bear in mind is that the propagative function must have recurred in each
fully completed life-cycle throughout descent. Hence the production of
sporangia was never an innovation, and they cannot at any time in the course
of descent have been imposed upon a pre-existent vegetative system, as
Goethe's theory would assume. The facts suggest that the whole shoot of
relatively primitive Vascular Plants was non-specialised. It served general
purposes, both for vegetation and propagation. But in the course of evolution of
higher types differentiation took place, so that a certain region became exclusively
vegetative, while another produced sporangia. Thus a theory of Segregation
takes the place of a theory of Metamorphosis. The vegetative phase naturally
comes first in the individual life, so as to supply the necessary materials for
propagation. Once these two pristine functions were allocated to distinct
regions of the plant, each was open to its own distinct specialisation. And so
it comes about that while in simple cases there may be some similarity
between the flower and the foliage shoot, the two may diverge widely in more
advanced types. But, however greatly they may differ, the flower and the
foliage shoot are to be held as the results of segregation of parts of an
originally general-purposes shoot. This gives a natural meaning to those
structural resemblances, which are sometimes so striking, between the foliage
shoot and the flower which it ultimately bears.

In such discussions as this the antithesis between the flower and the foliage
shoot is apt to be drawn more strongly than the facts warrant. The flower
is not always clearly defined from the vegetative region. Comparison of the
flowers of Cactaceae and Magnoliaceae, and of some Grasses such as
Streptochaete, show that bracts may merge gradually into floral organs. A
further comparison with Conifers, Cycads, and Club-Mosses will confirm the
view that the two regions have not always been as distinct as they now appear
to be in the Higher Flowering Plants.

In the earlier part of this Chapter the more important lines have
been sketched along which the analysis of floral construction may
be undertaken. Such analysis is a necessary basis for comparison,
and ultimately for classification. But this will not be pursued further
al present. It is also necessary as a step towards realising how the
mechanism of the flower works. An engineer cannot properly under-

ind his engine till he has taken it down. But the engine will not
work till the parts are again assembled. Similarly the student must
not be content with the mere analysis of the flower. After analysis

THE INFLORESCENCE, AND THE FLOWER 279

he must assemble the mechanism again, either mentally, or by
reference to another specimen, and study it as a whole. He will
then be in a position to understand how the various features which
the flower shows may each contribute to the biological end which
the flower has to serve. This aspect of floral construction has been
lightly sketched in the latter part of the Chapter. It is important
to realise that each flower functionates as a whole, just as much as
any machine. Each part has its own share leading towards the
common end. That end is the production of the germ contained in
the seed. The transfer of the pollen is only one step towards that end,
and a comparatively early one. It is carried out in relation to the
showy parts unfolded at the time of blooming, and thus gains an
undue prominence. But the interest does not stop when the flower
fades. Fertilisation, which follows after blooming, is actually the
central feature, for it initiates the germ. It is a necessary prelude
to the nursing of the germ within the ovule. The protection of
the ovule meanwhile within the carpel, itself either free or sunk
into the tissue of the axis, is also important as contributing to the
final result. Not only then should the flower itself be studied as a
whole, but the propagative process also ; and Pollination, with its
accessories of form, colour and scent, should be put into its proper
place as one incident only, though an essential one, in the complete
propagative story.

N0te, — Special attention is drawn to the detailed description of various types
of floral structure given in Appendix A. The facts there described will
illustrate the preceding chapter. They are placed in the Appendix not because
they are unimportant, but so that the description of the reproductive process
should not be broken by a mass of detailed facts, however apposite those
facts may be to the process itself.

CHAPTER XV.

THE STAMEN AND POLLEN-SAC.

All the facts brought forward in the preceding chapter may be
observed under a hand lens, or under low powers of the microscope.
But observation under higher powers is necessary for obtaining an
intelligible grasp of the details of propagation in Seed-Plants. The
minute structure of the outer envelopes can be dismissed briefly.
The sepals, which are usually green and relatively firm in texture,
repeat in their structure, though on a reduced scale, the features of
foliage leaves. Their epidermis bears stomata, and the mesophyll

fe^sv

C

Fig. 200.

Single cell of a petal of Senecio, showing numerous chromoplasts of semi-crystalline

form. ( x 800.) (After Schimper.)

below, though small in quantity, contains chlorophyll, and is traversed
by vascular strands after the manner of other leaves. The petals also,
though wider in expanse and more delicate in structure, are con-
structed on a similar plan. But the green of chlorophyll is replaced in
them by other colours. The blues, reds and intermediate shades
of flowers are due to pigments (Anthocyanins) dissolved in the

1-sap. Yellow colours are given by pigments confined to bodies
called Chromoplasts, which are frequently of irregular crystalline
form (Fig. 200). Usually these yellow pigments are the same as

cur in the leaf, namely Carotin and Xanthophyll (see p. 117).
Sometimes both sources of colour may be present in the same petal,
and even in the same cell. The streaky colouring of Parrot Tulips
i from the irregular distribution of the chromoplasts in addition
to the soluble colouring. The outer floral envelopes take no direct
part m propagation. Indirectly they serve that purpose : the sepals

280

THE STAMEN AND POLLEN-SAC

281

by the protection which they give to the inner parts of the young bud ;
the petals by giving scent, and a wide expanse of coloured surface, by
which means the flower is made attractive. Both parts are subject in
special cases to modification of form and character in relation to
pollination ; but these details must be left aside for the
present. After the flower is full blown, the petals wither
and fall away. The sepals frequently fall away also, as
in Crucifers and in the Buttercup. Sometimes they
drop even before the flower is full blown, as in the
Poppy. In other cases they persist, as in the Rose,
Violet, or Pea, though their further use is not obvious.
The stamens which lie within are essential for pro-
pagation. Their function is to produce pollen-grains
which, after further development, give rise each of
them to two male fertilising cells, or gametes. The
form and structure of the stamens may vary in detail
in special cases. But in the great majority of flowers
they conform to one simple type, consisting of a
cylindrical stalk or filament, which is continued up-
wards into the distal anther (Fig. 201). This is two-
lobed, the lobes being attached laterally along the filament. While
young the outer surface of the anther is turgid, and unbroken. When
ripe it opens, sometimes by pores, as in Solanum, but usually by
longitudinal slits, right and left, which gape widely, as in Iris or Caltha.
Thus the pollen is shed as grains, usually separate and of yellow colour.
The ripening of the stamens coincides with the blooming of the flower ;

u

Fig. 201.

Stamens of Iris :
that on the right
shows dehiscence.
(After Figuier.)

B

Fig. 202.

Sections of the anther of Caltha. A, before dehiscence, showing the four pollen
sacs still closed. B, after dehiscence, showing the sacs open, and the slits gaping
widely. Centrally lies the vascular strand, shaded. (Enlarged.) F. O. B.

but in cases where the stamens are numerous they may open in
succession, so that the shedding of the pollen may be spread over a
lengthened period. After the pollen is shed the stamens usually fall
away, or sometimes they persist in a withered state ; but they serve
no further function.

If the anther of any ordinary stamen be cut transversely it is found

BOTANY OF THE LIVING PLANT

to contain four sporangia, or pollen- sacs, two being placed on either
side of a central connective, which is simply a continuation upwards
of the filament. A single vascular strand which traverses the filament
is continued upwards into the anther, where it fades out usually
without any branching (Fig. 202, A). Stamens arise as exogenous
growths from the axis, and in acropetal succession. In " doubled "
(lowers they not infrequently appear as transformed into petals, or
even into green leaves. This suggests a foliar nature. But for a full
understanding of their relation to foliage leaves comparison will be

Fig. 203.

Lobe of anther of Caltha cut transversely, showing two pollen-sacs at maturity,
with the fibrous layer immediately below the epidermis. For details see text.
( x 100.) F. O. B.

needed with corresponding parts in early fossils. For close comparison
the material is gradually becoming available : but at the moment that re-
lation appears as an evolutionary problem rather than a demonstration.
It may, however, be accepted that stamens are parts specialised for
bearing pollen-sacs or micro -sporangia. Each pollen-sac is enclosed
till it is ripe by a wall consisting of several layers of cells. In most
stamens the slit of dehiscence runs longitudinally, following the line
where the walls of the two sacs of one anther-lobe join with the
septum that separates them (Fig. 201). There the cell-walls are thin
and the cells themselves are rounded off by intercellular spaces, so
that they easily come apart. The slit thus formed gapes widely, owing
to the action of the walls of the pollen-sacs. Below their superficial
epidermis lies a layer of fibrous cells, the inner cell- walls of which are
thicker than the outer, while fibrous bars running outwards along
their lateral walls prevent radial collapse. The effect on these cells of

THE STAMEN AND POLLEN-SAC

283

drying as the anther ripens is that their outer walls shrink, while
the thick inner wall retains its form. The result of the shrinkage
of the outer side of the fibrous layer will naturally be to reduce the
curvature of the convex sporangial wall, and the slit gapes widely
(Fig. 202, B). The dusty pollen-grains are then readily removed.
There are various differences in detail of the dehiscence of anthers,
particularly in highly specialised flowers. But in all ordinary flowers
where the pollen is dry and powdery, the way in which it is set free is
essentially like that described.

Fig. 204.

Pollen-tetrads and pollen-grains of Caltha. (i) a tetrad, with each cell uninucleate,
(ii) an older tetrad, with the grains almost separated, each containing two nuclei,
(hi, iv) mature pollen-grains, showing the larger vegetative cell, and the smaller
antheridial mother-cell. ( x 550.) F. O. B.

The pollen itself varies greatly in different plants, in the form and
size of the grain, in the sculpturing of its walls, in colour, and in the
dryness or stickiness of its surface. But these differences are only
external. There is a remarkable uniformity of the internal structure of
the pollen-grains of Flowering Plants (Fig. 204, iii, iv). They are two-
celled. One cell is usually the larger, and it is called the vegetative cell,
because it is from it that the pollen-tube is formed, and it does not take
any direct part in the reproductive process. It consists of cytoplasm
and a nucleus. The smaller cell is the antheridial mother-cell ; it also
has cytoplasm and a nucleus, and is marked off from the other by a
plasmic film, not by a cell-wall. The two-celled state of the pollen-
grain may be attained while it is still in the unopened pollen-sac.

The early stages of segmentation of the stamen may be followed by study
of the flowers of Chrysanthemum (Fig. 205, i.-iv.). The stamen first appears as
a rounded papilla of tissue. It is covered by a layer of cells which divide only

BOTANY OF THE LIVING PLANT

/.

by walla perpendicular to the surface, so that the layer maintains its identity.
It develops into the epidermis (i.)- The young stamen soon shows four
angular projections, which represent the four pollen-sacs. They project
owing to the active growth and division of the hypodermal cells (ii. hi.).
In the earliest state the hypoderma also appears as a single layer all round
(n.) ; but as it grows older certain of its cells enlarge at the angles of the
ion, and divide by walls parallel to the surface. Outer parietal cells are

thus cut off from inner cells which are sporo-
genous, and give rise ultimately to the pollen
(ii. hi.). The outer cells undergo further
division by walls parallel to the first, forming
usually three cells each (iv.). Of these the
outermost cells provide, after further growth
and division, the fibrous layer; while the inner-
most, which adjoin the sporogenous cells,
form part of the nutritive tissue known as
the tapetum. The sporogenous cells them-
selves, which are shaded in Fig. 205, ii.-iv.,
are easily distinguished not only by their
origin, but also by their dense protoplasmic
contents. The steps thus described are found
to be very constant in the anthers of Flower-
ing Plants. They provide material for com-
parison with other sporangia. There is some
latitude, however, in the number of the
divisions, and the figures show this even in
the single case of Chrysanthemum. Where
the stamen is large and its walls thick, as in
the Lily, more numerous divisions of the
hypodermal cells may provide a sporangial
wall thickened by extra layers.

As the pollen-sac develops, the layer next
below the epidermis first acquires a store of
starch (Fig. 207) which is converted later into
the thickening of the walls characteristic of
the fibrous layer (Fig. 203). Within this
lie the featureless cells of the intermediate
layer. The group of sporogenous cells in-
creases in size, and often also in number of
cells by division. Each of these cells is called a pollen-mother-cell. The group
of them occupies the centre of the projecting sporangium, and is invested by
the continuous sheath of the tapetum (shaded in Figs. 206, 207). This appears
Jingle layer of large cells with very thin walls and granular contents.
is the result of development of the cells on all sides adjoining the
sporogenous group. Fragmentation of the nucleus frequently happens as
ow old. Its function is to nourish the developing pollen. As the
re the cells of the tapetum gradually collapse, their substance
sorbed by the pollen. In the mature pollen-sac only vestiges of
them remain lining the sac internally (Fig. 203).

Fig. 205.

Successive stages in the early develop-
ment <<t the anther in Chrysanthemum.
(After Warming.) ( x 300.) Thesporo-
11s cells are shaded. For details,
xt.

THE STAMEN AND POLLEN-SAC

285

The pollen-mother cells are all derived by division from the inner product
of the hypodermal cells. They vary in number in different anthers. Caltha
gives an average example, in which from 14 to 16 appear in each transverse
section of a pollen-sac (Fig. 206). They are characterised by very thin walls,
a dense, non-vacuolated cytoplasm, and a proportionately very large nucleus.
At first they are closely fitted together, without intercellular spaces (a). But
presently the pollen-sac distends, and the pollen-mother cells round themselves
off, and become suspended individually in a liquid medium which fills the spaces
between them (b). Meanwhile the tapetal cells, from which the liquid probably
arises, retain their form, though their nuclei often multiply by fragmentation.

Fig. 206.

a, b, successive stages of development of the contents of the pollen-sac of Caltha.

See text. ( * 100.) F. O. B.

As the pollen-mother-cells separate they enter on the tetrad-division. Each
nucleus divides first into two, and then into four. These rapidly repeated
divisions are characteristic of all spore-formations, and have an important rela-
tion to the constitution of the nuclei themselves (see p. 563). The four resulting
nuclei are first enclosed in the single protoplast. But soon each is separated
by a partition-wall, and is surrounded by a quarter share of the cytoplasm.
The pollen-tetrad is thus constituted. The four cells are still enclosed by the
common wall ; but later each cell deposits a special wall round itself. The
common wall, which was of a mucilaginous character, is then dissolved, with
the result that the four cells become dissociated as independent pollen-grains
(Fig. 204, i, ii). Meanwhile the single nucleus of each has divided, giving the
two nuclei present in the mature grain (Fig. 204, iii, iv).

This description of the development seen in Caltha applies for the develop-
ment of the pollen in all ordinary Dicotyledons. In Monocotyledons the

BOTANY OF THE LIVING PLANT

ssentially the same. But the pollen-mother-cell, after the first
division, is itself partitioned by a wall into two cells, in each of which
the second nuclear division follows. The final result is thus the same except
that the arrangement of the tetrad is not tetrahedral, as it usually is in Dicoty-
ledons But the grains lie in pairs, the longer axis of one pair being at right

Fig. 207.

A later stage of development of a pollen-sac, showing the young fibrous layer,
containing starch. The tapetum (shaded) surrounds the sac in which the tetrads
float freely. ( x 100.) F. O. B.

angles to that of the other. These are minor points ; in all essentials the
tetrad-division which produces the pollen is the same throughout Flowering
Plants. Figs. 204, 206, 207.

For the present this brief description must suffice. But later
(Chapter XXXV.) the details of behaviour of the nuclei in this impor-
tant process of tetrad-division, and chromosome-reduction will be
described and discussed at greater length (p. 563). The pollen-grains,
as their development shows, are produced from internal tissues of the
plant, and are set free by rupture of the superficial tissues. Such
bodies are called spores, and the pollen-grains being of relatively small
size are called micro-spores. A step in their production is the tetrad-
division. The tetrad breaks up later into its four constituent spores.
Tetrad-division is a constant feature in the production of spores in all
spore-bearing Plants, such as Mosses, Ferns> and Seed-Plants. When a
marked feature such as this recurs with constancy in a large group of

THE STAMEN AND POLLEN-SAC 287

organisms, even though they may differ in many respects, it may be
assumed that it has seme special significance. In this case the im-
portance of the tetrad-division lies partly in the provision of numerous
easily transferred bodies, which carry out an essential function in the
propagative process. But a more important point is that in the
course of the tetrad-division the nuclei undergo the change called
REDUCTION. Certain formed bodies, chromosomes, present in con-
stant number in each nucleus on division, are reduced, in the course
of the process, to half their original number in each nucleus. All
the products of further division of nuclei so reduced have the same
smaller number. The ordinary nuclei of the plant have a number of
chromosomes which may be represented as " 2x," and they are called
diploid. The nuclei that result from reduction are found to have only
"*" chromosomes, and are described as haploid. The constitution of
the nucleus and its behaviour in reduction will be discussed again in
detail in Chapter XXXV. Meanwhile we note that an essential change
has occurred during the tetrad-division, and that the nuclei produced within
the pollen-grain are themselves haploid. It is then not simply a separa-
tion of vegetative cells that occurs in the production of pollen. The
grain as it leaves the anther conveys with it nuclei that differ in an
essential feature from those of the ordinary vegetative cells of the
Plant. The process of reduction in the tetrad initiates a sexual phase,
or Gametophyte. The result of its further development will be certain
cells, which are capable of taking a direct part in sexual reproduction :
they are in fact the male gametes, or male sexual cells.

CHAPTER XVI.

THE CARPEL AND OVULE.

The Gynoecium, or Pistil, occupies the centre of the Flower. Its office
is to produce ovules, and after their fertilisation to nourish and protect
them, together with the new germ that each may contain. This
nursing function is continued till the time of ripeness, when the Seeds
are shed. The Gynoecium is thus the most persistent part of the
Flower. While the sepals, petals, and stamens are liable to fall away
after the period of blooming, the gynoecium remains attached until
the seed is ripe, continuing to draw from the receptacle the nourish-
ment required for the germ. The term Fruit is applied to the whole
Pistil when fully matured.

The Gynoecium is composed of Carpels, which may vary in number
in different cases from many downwards to one. Associated with
them are the Ovules, or Mega-Sporangia, which also vary from large
numbers in some cases down to one in others. Two parts of the
gynoecium may be distinguished by their structure and function. A
distal region, which offers at the time of blooming a receptive surface
for the pollen-grains : this is called the stigma ; and a basal part,
distended as the ovary, which encloses the ovule or ovules. Frequently,
and especially in syncarpous pistils, an elongated region intervenes
which is called the style (Fig. 209).

The Carpels are leaves. Often that can easily be recognised, as

in the Pea, where the pod represents a single carpel ; or in Caltha,

where there are many (Fig. 208). In this case they are all separate

from one another (apocarpous), and their foliar nature is undisguised.

Each leaf is folded so as to envelop the ovules borne on its margins,

the midrib is turned outwards from the centre of the flower

(Fig. 210). The foliar nature of the carpels is less easily recognised

where they are united (syncarpous), as they are in the Lily. But even

288

THE CARPEL AND OVULE

289

there a transverse section shows by its outline, by the arrangement of
the vascular strands, and by the position of the ovules that the com-

Fig. 208.

Whole gynoecium of Caltha,
consisting of many carpels, all
separate.

Fig. 209.

Pistil, or gynoecium of Lily, show-
ing the relative positions of ovary,
style, and stigma. F. O. B.

pound structure is referable in origin to three fused leaves (Fig. 211).
Moreover cases of partial fusion are found, for instance in Colchicum,
where the three carpels are fused below, but extend upwards as separate

Fig. 210.
Transverse section through the separate
carpels, composing the gynoecium of
Caltha. F. O. B.

Fig. til.
Transverse section of the syncarpous
ovary of Lily, showing the three folded
and fused carpellary leaves, bearing
ovules on their margins. F. O. B.

styles. Their relative positions are, however, the same as of those
in the completely syncarpous Lily. Biologically the advantage of a

B.B.

BOTANY OF THE LIVING PLANT

coherent gynoecium over that with separate carpels lies in the more
tive protection and nutrition of the ovules. But it presents

sh difficulties in the liberation of the seeds when ripe.

The structure of the gynoecium is further complicated by the fact
that the carpels are frequently sunk into the tissues of the enlarged
receptacle, a condition which serves still more completely the purposes
of protection and nutrition of the ovules ; for it brings them closer
to the nutritive supply that comes from below, and it makes a thicker
protective wall possible (Fig. 212). Naturally this goes along with
fusion of the carpels. The result is a massive body formed partly

Fig. 212.

To the left, median section of the flower of Saxifrage, showing the carpels half
sunk in the receptacle, and coherent for the greater part of their length. (After
Figuier.) A, similar section of the Quince, showing an apparently inferior ovary,
but the styles and stigmas are separate. B, same for Apple, showing styles united,
but stigmas still separate. (After Warming.)

from the floral receptacle, partly from the carpels. It is syncarpous,
and being apparently below the other floral parts it is styled inferior.
The development of such a gynoecium has been traced in the structur-
ally simple, though highly specialised case of Chrysanthemum, where
there is only one cavity of the ovary, and one ovule (Fig. 198, p. 273).
But in relatively primitive cases there are several loculi, corresponding
to the number of the carpels, and there may be many ovules in each,
as in the Quince (Fig. 212, A), or Iris (Fig. 213).

Various intermediate states serve to explain how the inferior syncarpous
ovary may have come into existence. For instance, in Saxifraga the two
carpels are united through about half their length, and are partially sunk
below the other parts in the tissue of the axis (Fig. 212). Other intermediate
states are seen in the Pomeae. For instance, in Cotoneaster the flower is little
removed from the perigynous state. In Cydonia the ovary is more dis-
tinctly inferior, but the carpels are not fully united, each having a separate
style, while the apex of the abbreviated axis lies in a deep depression between
them (Fig. 212, A). In the closely related Apple their fusion is more advanced,
so that there is a common style, but a distinct stigma for each carpel (Fig. 212,
B). The last step of fusion would be their coalescence to a single stigma, as is

THE CARPEL AND OVULE 291

seen in the Lily (Fig. 209) . Such fusion of carpels, with or without their sinking
into the receptacle, has developed progressively, and has appeared repeatedly
in distinct evolutionary lines. The result is a solid and massive gynoecium,
whether with superior or inferior ovary. A transverse section of the inferior
ovary may still show evidence of its carpellary origin almost as clearly as in
the superior ovary. This is seen in Iris, where notwithstanding that the
carpels are sunk in the receptacle, their struc-
ture, and even the arrangement of the chief
vascular strands resembles in some degree that
seen in the superior ovary of the Lily (Fig.
211). The conclusion follows that in such cases
the gynoecium is still to be referred in origin
to foliar structures, more or less completely
fused with or sunk into the tissue of the
receptacle.

The structure of the carpel, where it
is distinctly leaf-like, as it is in the pod FlG- 2I3-

. . Transverse section of the inferior

Of the Pea Corresponds in essentials tO ovary of Iris. Compare the superior
,\. r 1 • i-r j a ovary of Lily, Fig. 211. F. O. B.

that of a foliage leaf, but simplified. A

vascular strand usually traverses each margin, as in Caltha (Fig. 210).
This is related to the fact that the ovules are seated there ; or, as
it is described, the placentation is marginal It is probable that
this was the regular primitive position for ovules. But sometimes
they appear scattered over the inner surface of the carpellary wall,
as in the Flowering Rush (Butomus), the Poppy, or the Water-Lily.
This is described as superficial placentation, and it probably originated
by the spread of the ovules to the surface. Sometimes they appear as
though seated on a prolongation of the axis into the ovarian cavity,
as in the Pinks and Primroses (Appendix A, Fig. 47 1, of Lychnis). This
is called free-central placentation, and it also is probably derived from
the marginal type, by breaking away the partitions, or septa, of the
syncarpous ovary, leaving the margins with their ovules at the centre.
Thus the marginal was probably the primitive, as it is certainly the
prevalent position for the ovules on the carpellary leaf ; and this holds
equally for syncarpous ovaries (Figs. 211, 213).

The Stigma, or receptive surface for the pollen-grains, is at the distal
end of the carpel. Where the carpels are separate each has its own
stigma, a condition which is probably the primitive state. It is seen
in Caltha and the Buttercup, the stigmatic area being recognised by
its roughened surface. Even where the carpels are united below into
a syncarpous ovary, each may separate upwards to form a distinct
stigma, as in the Apple or Quince (Fig. 212, A, B). But in cases which
are regarded as more advanced the fusion of the carpels may extend

BOTANY OF THE LIVING PLANT

t0 their tips, and a single stigma is the result, as in the Lily (Fig. 209)
Datura (Fig. 214). Even here the receptive surface is lobed, and
the number and position of the constituent carpels is indicated by
that of the stigmatic lobes. From external observation this is often
the readiest guide to the composition of the gynoecium. For instance,
the two-lobed stigma of the Compositae accords with the facts of

*n9

Fig. 214.
Stigma of Datura with pollen-grains adhering to its surface. (After Figuier.)

development of their flowers, in which two carpels make their appear-
ance (Fig. 198). In Datura the two lobes clearly indicate the two
carpels of the Solanaceae.

The roughness of the stigmatic surface is due to the outgrowth of
the superficial cells as papillae (Fig. 214), the size of which is found to
bear a relation to the size of the pollen-grains. The cells are thin-
walled, with active protoplasts ; frequently they are moist, or secrete
a sticky juice, which helps to detain the pollen-grains in contact with
the surface. The grains themselves have sometimes a sticky exterior,
which serves the same end. A still more important feature is that the
style, by its elongation carries the stigma upwards to a level suitable for

THE CARPEL AND OVULE 293

the deposit of (lie pollen (Fig. 209). In some cases the style is absent,
as in the Buttercup or Nettle. It is a feature of variable occurrence,
resulting from intercalary growth which is adjustable according to the
proportions of the other parts. Extreme cases are seen in the Crocus
or in Colchicum, where the ovary is underground, while the style,
which is six inches or more in length, carries the stigma several inches
above ground, to a level a little in advance of the stamens. In the
Gamopetalous Dicotyledons, and in many Monocotyledons the cylin-
drical style is proportional in length to the tube of the corolla, as is
seen in Tobacco and Gloxinia, or in Lilium auratum and Narcissus.

Fig. 215.

Transverse section of the style of Salvia, showing the cells of the conducting
tissue (c) with swollen mucilaginous walls (m). (After Capus.)

The style is sometimes traversed by an open channel, so that
direct access can be gained to the ovarian cavity from the stigma ;
this is the case in the Violet and Mignonette. But where the channel
is narrow it is commonly filled with a mass of mucilage derived from
epithelial ce*lls which clothe its surface. This is seen in the Lily and
Rhododendron (Fig. 225, p. 305), in both of which a separate groove
from each of the stigmatic lobes leads downwards to the common
conducting canal filled with mucilage. In other cases there is no
actual canal, but a column of lax tissue with mucilaginous walls tra-
verses the style, and serves as a conducting tissue. This is found in
Salvia (Fig. 215) ; also in the Corn Cockle (Agrostemma), and in the
Mallow. In such cases the conducting tract is connected upwards
with the separate stigmas, while downwards the channel branches so
as to lead to the several loculi of the ovary.

BOTANY OF THE LIVING PLANT

The ovule at the period when it is ready for fertilisation is more or
less oval in form, and it is seated upon a stalk, the funiculus, which is
usually short (Fig. 2 16). It consists of a central body of conical form,
which is called the nucellus. This is the actual mega- sporangium. It is
invested by one, and frequently by two integuments, which are attached
to its base, and cover it closely, leaving only a very narrow channel
open at the apex, which is called the micropyle. The opposite end,
where it is attached to the funiculus, is called the chalaza. A vascular
strand, springing from the vascular system of the carpel, traverses
the funiculus, but stops at the chalazal end of the nucellus. This
leads up the supplies to the base of the sporangium. The form of
the ovule varies. Sometimes it is straight, as in the Rhubarb, or Dock
(Fig. 22 1, .p. 303) ; sometimes the body of the ovule is itself curved, as in
the Kidney bean or Shepherd's Purse. In the great majority of cases
the body of the ovule is straight, but it is inverted or anatropous, so
that the micropyle lies close to the attachment of the funiculus on the
carpel. This is seen in Fig. 216, which shows an ovule of Caltha cut
in median section, at the time wThen it is ready for fertilisation. The
nucellus is the essential part of the ovule, the integuments and the
funiculus being accessory. They provide respectively for external
protection, and for the conduction of supplies. Moreover, the nucellus
is the part first formed. In a young state k may be found already
well advanced, though the integuments are incomplete, and the
funiculus is only beginning to assume that curvature which results
in the inversion of the mature ovule (Fig. 217, p. 296).

At the period of blooming the nucellus consists of a peripheral
covering of thin-walled cells, of varying bulk in different groups of
plants : it encloses one large cavity, wThich, though its contents are
complex, is developed from a single cell. This is the Embryo-Sac, or
Megaspore. It attains its large size by encroaching on the adjoining
cells as it develops, by a process of digestion ; this leads to their collapse,
and the final absorption of their substance. The sac is limited by a very
thin cell-wall, and is lined by dense granular protoplasm. Within it seven
nuclei are seen, of which one of large size is near to the centre (Fig.
-io, /. n). As these contents of the embryo-sac are almost constant
in Flowering Plants, and are all accessory to the production of the new
germ, they demand special attention. There are two groups of three
cells each, one fixed at the micropylar end, the other at the chalazal
end of the embryo-sac. The latter are often large, with well-marked
nuclei, each of which is surrounded by an area of granular cytoplasm
marked off by a plasmic film, not by a cell-wall. It is called the

THE CARPEL AND OVULE

295

antipodal group [ant.)} and it occupies the base of the embryo-sac, just
above the chalazal ending of the vascular strand. At the micropylar
end is another group of three cells, called the egg-apparatus (e.a.). One
of the cells projects further into the cavity than the other two : it is the
Ovum, or egg-cell which after fertilisation initiates the new germ. The
other two, called the Synergidae, are of equal size, but smaller than the

Fig. 216.

Median longitudinal section of an ovule of Caltha, at the period of fertilisation.
/=funiculus. cA=chalaza. o.t'n<=outer integument, i. in*=inner integument.
«wc = nucellus. m=micropyle. e. a = egg-apparatus, ant = antipodals. /.n. = fusion
nucleus. ( x no.)

ovum. The egg-apparatus is attached just below the micropyle.
In Caltha two layers of cells are seen to intervene (Fig. 216, nuc), but in
other plants the number may be larger ; on the other hand in many
ovules the embryo-sac is found to abut directly on the micropyle.
The large cavity of the embryo-sac is filled by vacuolated cytoplasm,
while in the centre the large fusion-nucleus (/.».) with a prominent
nucleolus is suspended by cytoplasmic threads. Though ovules of
Flowering Plants may vary in form, in the complexity of their

296

BOTANY OF THE LIVING PLANT

mstraction, in the number of the integuments, and even in the
number of their embryo-sacs, there is a marked constancy in the
number and position of the bodies contained in the embryo-sac at
the time of fertilisation.

The following description of the developments the ovule relates primarily
to the type seen in the relatively primitive family of the Ranunculaceae. It
appears at first as a rounded papilla of tissue, which develops directly into the
nucellus or megasporangium. By active growth and cell-division it is carried

Fig. 217.

Median section of a young ovule of Caltha, anatropous curvature still incomplete,
and the uucellus only partially covered by the integuments. The spore-mother-cell
has divided once, and the second division to form the tetrad is already indicated
by the nuclear spindles. ( x 200.) F. O. B.

up upon the elongating funiculus. Meanwhile by outgrowth of a ring of tissue
at the base of the nucellus the inner integument first appears. The outer
integument follows as a growth on the side wThich will be turned outwards as
the ovule becomes inverted ; later it coalesces with the stalk so as to invest
the nucellus on all sides except that of the stalk (Fig. 217). As the ovule
grows older the curvature increases till it is completely inverted. Meanwhile
the integuments extend over the nucellus, covering it in, except for the
narrow channel of the micropyle (Fig. 216).

1 he chief interest lies in the origin and development of the embryo-sac. It

has been stated that the nucellus is a megasporangium, and the embryo-sac a

tspore. It is because of the manner of their development that these parts

so recognised. The young nucellus first appears as a hemispherical

THE CARPEL AND OVULE

297

upgrowth : it consists internally of a number of radial rows of cells, covered
by a superficial layer. The latter divides in Caltha into two or more layers at
the tip of the nucellus, forming a cap of tissue covering the radial rows within.
It is from the central row of the internal cells that the embryo-sac arises. The
condition in Caltha is relatively simple. The terminal cell of the central row
undergoes division into two, and then into four (Fig. 217). This is in fact a
tetrad-division, and the mother-cell which divides is of hypodermal origin.
It has been shown that this division is accompanied by reduction of chromosomes
of the nuclei to the half number, as in the pollen-tetrad. The resulting four cells
are arranged in a row ; pollen-tetrads are sometimes found to have the same
arrangement. The conclusion follows that the tetrad thus produced in the ovale is
the correlative of a single pollen-tetrad, and each of the four cells might become a
spore. This actually happens in the pollen-sac ; but in the ovule as a rule only
one of the four potential spores develops further. The
lowest cell of the four enlarges at the expense of the
others, which collapse, and are crushed out of shape. The
embryo-sac encroaches also on the surrounding cells of
the nucellus, which give way to allow of its increase in
size. Thus, as shown by its development, the embryo-
sac is the single spore of a tetrad : as it develops to a
large size it is styled a mega-spore.

In other cases the structure may be more complex than
in Caltha. A considerable number of Flowering Plants
show rapid growth and division of the superficial cells
at the tip of the nucellus, so as to form a considerable
pad of tissue covering the hypoderma. This is seen in
Rosa livida (Fig. 218), which also shows numerous hypo-
dermal cells with dense contents, each divided into a
cell-row. The basal cell of each row is an embryo-sac
mother-cell ; the distal cells are parietal cells, com-
parable with the parietal cells of the pollen-sac (compare Fig. 205). There
is thus in Rosa a plurality of mother-cells, as is usual in pollen-sacs ; while
the parietal cells, which are absent in Caltha, are here present, and strengthen
the comparison with the pollen-sac. Such conditions, which are not infrequent
among the more primitive Dicotyledons, indicate that the pollen-sac and the
ovule, though differing in form, have essential features in common. Both
are sporangia, though they have diverged in details of development.

Other ovules again are simpler than Caltha. For instance, in Monotropa
the nucellus is represented only by a single layer of cells, surrounding one
central row, which consists of the embryo-sac itself and two sister cells (Fig.
219, i.). Here there are no parietal cells, and the tetrad itself is represented
only by three cells, the uppermost cell of the tetrad not having undergone the
second division. The two sister cells are later disorganised, and make way for
the embryo-sac ; and finally the single layer of the nucellus is also absorbed
(Fig. 219, viii.) ; so that at fertilisation the embryo-sac is all that represents the
nucellus. A still simpler condition as regards the origin of the embryo-sac
though not of the whole structure, is seen in Li Hum. Here, as in most Mono-
cotyledons, the parietal cells are absent. Further, the hypodermal cell of the
central row itself becomes directly the megaspore-mother-cell, and reduction

Fig. 218.

Young nucellus of ovule
of Rosa livida. See text.
(Alter Strasburger.)

j B01 ANY OF THE LIVING PLANT

takes place not in the preparation of the mother-cell, but during the first
lln ithiD it. The whole development is here condensed, and stages

seen elsewhere are entirely omitted.

There is thus considerable latitude in the details of the nucellus, associated
with marked differences in its bulk. Speaking generally, the nucellus is more
bulky in relatively primitive types, such as the Rosaceae and Ranunculaceae,
tnd especially in the Fagales. It is less bulky in advanced types, such as the
( ,,„„ .pctalous Dicotyledons, or the Orchids. But the former often make up
tins by the elaborate structure of their single integument.

In sharp contrast to this variability of the nucellus, or megasporan-
gium in Flowering Plants, is the dead level of uniformity shown by
the embryo-sac or megaspore, and its contents. So great is this that,

v/

m

Fig. 219.

Stages in the development of the embryo-sac of Monotropa, after Strasburger.
See Text below, (i.-vii. x 400. viii. is less highly magnified.)

putting aside the few exceptions that exist, one description will serve
for all Angiospermic Plants. It is based upon the observations of
Strasburger on the transparent ovules of Monotropa (Fig. 219). The
lowest of the three cells resulting from division of the megaspore-
mother-cell is the future embryo-sac (i). It enlarges at the expense
of the other two, which collapse, and their disorganised remains
appear as a cap covering the micropylar end of the sac (ii-vi). The
nucleus divides into two parts, which pass to opposite ends of the sac,
while a vacuole appears between them (ii). These again divide (iii),
with the result that two nuclei are formed at either end (iv). These
four divide once more (v), and the resulting nuclei arrange themselves
in the characteristic way common to embryo-sacs (vi). Three are
grouped at either end, each surrounded by its own area of cytoplasm,
limited by a plasma-film. Those at the chalazal end form the antipodal

THE CARPEL AND OVULE 299

group ; those at the micropylar end form the egg-apparatus. The
latter consists of the two synergidae which occupy the extreme apex,
and the ovum attached rather lower. The odd nuclei from each end
approach one another, and finally coalesce (vi, vii) to form the central
fusion-nucleus. The embryo-sac is then ready for fertilisation.

All the nuclei resulting from this development of the contents of
the embryo-sac are haploid. Reduction has already taken place in
the divisions of the mother-cell. The embryo-sac, or megaspore,
being one of the products of that operation, its nucleus is already
reduced. The whole group of nuclei, together with the cytoplasm
that surrounds them, may be recognised as the sexual phase or gameto-
phyte. It is characterised by differing in the constitution of its nuclei
from the ordinary vegetative cells of the plant : all its cells are
primordial cells, that is, they are not surrounded by cell-walls, but
are delimited by plasmatic films. The Ovum is that cell of the
gametophyte which will be fertilised. It is the female gamete, or
sexual cell, which is to take part in sexual reproduction.

CHAPTER XVII.

POLLINATION AND FERTILISATION.

" Pollination " and " Fertilisation " are often used as synonymous
terms. This may have been natural when in 1793 Sprengel published
his novel observations under the title " The Secret of Nature discovered
in the Structure and Fertilisation of Flowers." But at the present
day there is little excuse for such laxity. It is well to be clear in the
correct meaning of these words when applied to the Higher Flowering
Plants. By " pollination " is meant merely the transfer of the pollen
from the anther to the receptive surface of the stigma, and it was this
that was discussed by Sprengel, and later by Darwin and others. By
11 fertilisation " is meant the actual coalescence of two cells : the one
is the male gamete derived from the development of the pollen-grain ;
the other is the ovum contained within the ovule. Obviously some
interval must elapse between the events of pollination and fertilisa-
tion ; it is usually short, but may in extreme cases be as long as
a year, or more. Pollination precedes fertilisation in the Flowering
Plant, and is a means to that end ; but fertilisation is the end itself.

The mechanical problem of propagation in Flowering Plants is
complicated by the fact that the Plants themselves are immobile.
1 teing rooted in the ground they cannot like the Higher Animals move
to seek their mate. The cells that are to carry out fertilisation, viz. the
male and female gametes, are produced more or less widely apart from
They are themselves incapable of movement, while one
of them (the ovum) is deeply embedded in the tissues of the ovule,
and covered in by the carpel also. This brings the advantage of
protection and nutrition of the germ, but it also greatly complicates
the problem of sexual propagation. The steps that are necessary
to carry it out are first the transfer of the movable, though non-
motile pollen-grain from the anther to the surface of the stigma of the

300

POLLINATION AND FERTILISATION 301

same, or a corresponding flower. That transfer is called Pollination.
The second step is the germination of the pollen-grain, with the forma-
tion of a pollen-tube, which makes its way from the stigma to the
micropyle of the ovule, and conveys the contents of the pollen-grain or
microspore to the embryo-sac, or megaspore. The final step is the
fusion of the male gamete, which the tube conveys, with the female
gamete or ovum. This fusion is called Fertilisation. These several
events must be considered separately and in their natural succession.

The distance through which the pollen-grain must travel from the
pollen-sac to the stigma varies greatly. In flowers such as the Butter-
cup, containing both stamens and carpels {hermaphrodite), the distance
may be small. But in many cases only stamens or carpels are produced
in the individual flower, and the grains must then be transferred from
one to the other. If the staminate and pistillate flowers are borne on
the same plant, the condition is described as diclinous, as in the Hazel,
Beech, or Oak. They may, however, be borne on distinct plants,
which are then styled dioecious, as in the Rose-Campion, or Willow.
These are examples of the separation of the pollen-sacs and ovules in
space. But there may also be separation of them in point of time. For
even where they are seated side by side the pollen-sacs may not shed
their pollen at the time when the stigma is ready to receive it. Two
possible cases exist. The stamens may shed their pollen before the
stigma of the same flower is fully matured, as in the Willow-herb, or in
the Compositae. This is the more common state, and it is described
as protandrous. Or the stigma may mature first, and be no longer
receptive when the stamens of the same flower shed their pollen.
This is the less common state, and it is seen in the Figwort and
Plantain. It is described as protogynous. Obviously the practical
effect is the same as the separation in space, for in either event the
pollen must be brought from elsewhere, if fertilisation is to succeed.
In such cases the distance to be traversed may be considerable, and
the plant has no means of its own for making the transfer.

Use is then made of outside agencies, such as the movements of wind
or water : or advantage is taken of the mobility of animals. The
mechanism of flowers has been specialised in the most remarkable
manner in accordance with these methods of transfer. Where use
is made of wind the flowers produce abundance of dry dusty pollen,
easily shaken out in clouds from anthers often balanced on very
flexible filaments. The stigmas meanwhile are much branched and
feathery, so as to expose a large surface for catching the grains. These
features go with close grouping of the flowers, which are individually

•TANY OF THE LIVING PLANT

small and inconspicuous. Where animal agency is used, the flowers
arc attractive and conspicuous by their scent, by honey-secretion,
and by widely expanded floral envelopes of bright colour. The latter
attract the eye, the former the other senses of the animal, for instance
a bee, and lead her to visit the flower for her own purposes of gathering
honey or pollen. Incidentally the floral mechanism is so arranged,
in size and form of the parts, that pollen, often of a sticky nature, is
deposited on her body as she visits the flower. The flower may be so
formed as to lead her, for sake of convenience, to take a definite
position : consequently the pollen is deposited on a definite part of
her body (Fig. 220). The result of repeated visits to a succession of

Pollination of Salvia pratensis. i, Flower visited by Humble Bee, showing the
projection of the curved connective from the helmet-shaped upper lip, and the deposit
of the pollen on the back of the Bee. 2, Older flower, with»connective withdrawn
and elongated style. 4, the staminal apparatus at rest, with connective enclosed
within the upper lip. 3, the same when disturbed by the entrance of the proboscis
of the Bee in the direction of the arrow. /=filament. c = connective. s = the
obstructing half of the anther, which produces no pollen. (After Strasburger.)

flowers of like construction will then be that, if the stigmas correspond
in position to the spots on which she bears the pollen, they may
probably receive some part of it. Thus unwittingly she will have been
the agent of transfer of the pollen from the pollen-sac to the receptive
stigma.

Such mechanisms have been elaborated in the course of Descent in
an infinite variety of detail. This is the biological meaning of the

tractive features which flowers have assumed. It may even be seen
how certain floral types have been adjusted in relation to the visits
of certain animals, and show development parallel with them. A
good instance is that of the Aconite and the Humble Bee, to whose
visits the Aconite flower offers convenient access. A study of their
distribution across Europe and Asia shows that the northern limit
of occurrence of the two organisms almost exactly coincides. This
suggests the importance of the Humble Bee in the transfer of the

POLLINATION AND FERTILISATION

303

pollen of the Aconite, while the food which the flower offers may in
some measure react in determining the distribution of the bee. The
methods of transfer of the pollen may thus be varied. But the essential
feature of them all is the same, viz. the conveyance of an immobile
body essential for propagation from the pollen-sac, where it is pro-
duced, to the surface of the stigma
where it can germinate. (For numerous
instances of the methods of pollination,
as illustrated by various examples de-
scribed in detail, see Appendix A.)

The germination of the pollen-grain
takes place normally on the stigma.
(Figs. 221, 222.) But it can be induced
in a nutritive medium, apart from the
stigma, such as a solution of cane sugar
of suitable strength. This makes it
possible to observe the origin and
behaviour of the pollen-tube. The
germination may be very rapid. From
fresh pollen of the Wild Hyacinth,
placed in a 7-10 p.c. solution, pollen-
tubes will be produced at a normal
summer temperature in about 15 min-
utes, and in an hour will have grown
to a length several times the diameter
of the grain. In some cases the struc-
ture of the wall of the grain does not FlG- •"• , ,

Ovary of Polygonum Convolvulus at

indicate where the tube will be formed, time of fertilisation. /s=baseof ovary.

... . /«>== wall of ovary. /i = funiculus. cha =

But in Others its Origin IS determined chalaza. «u = nucellus. tfiz" = micropyle.

,, T , „r11 11 1 ii = inner integument, ie = outer mtegu-

StruCturally. In the WlllOW-nerbS and ment. * = embryo-sac. ek = central fusion

~ . . . r • • nucleus. ei = egg-apparatus. an = anti-

Geraniums three points 01 origin are podai ceils. g=styie. n=stigma. p=

. . t-, ,, pollen-grains. ps= pollen tubes. ( x 48.)

present on each grain. frequently (After strasburger.)

their number is greater, as in the Corn

Cockle (Fig. 222, A) ; but of the 40 or 50 points of exit there seen, only

one gives rise to a tube. A curious exception is seen in the Mallow,

where numerous tubes emerge, which firmly anchor the grain. (Fig.

222, B).

The effect of external influences upon the growth of the tube can be
studied in culture-experiments. If grains be germinated in a suitable
solution under a cover-glass, the tubes, as they first issue, point indis-
criminately in all directions. But soon those near the margin turn

^-J

BOTANY OF THE LIVING PLANT

inwards from the sunn.- of free Oxygen: they are negatively aero-
/,,, | ig. 223). If a similar culture be prepared, and a piece of

J^\±.

A

Fig. 222.

A = Pollen-grains of Corn Cockle (Agrostemma) showing numerous possible points
of origin for pollen-tubes, but only one tube, which penetrates at once a papillar cell
of the stigma. B = a similar condition in Mallow [Malva), but here numerous
small tubes are formed for attachment. (After Strasburger.) ( x 120.)

the style and stigma of the same species be introduced with the
pollen-grains, the tubes curve towards it, and especially towards the

Fig. 223.

Pollen grains germinated in a nutritive medium

under a cover glass, of which the margin is shown

lne tubes curve away from the margin, that is

away from the supply ot oxygen. (After Molisch.) '

Fig. 224.

Result of culture of pollen-tubes of
Xarcissus Tazetta in neighbourhood of
the style and stigma, in 7 per cent,
sugar after 16 hours. Diagrammatic.
(After Molisch.) ( x 10.)

surface (Fig. 224). They are positively chemotropic. Their be-
haviour on the stigma, where they take a course in close contact
with the moist cell-walls, shows that they are also positively hydro-

POLLINATION AND FERTILISATION

>05

tropic (p. 1 60). These three factors are effective in deciding the
course of the tube when it germinates normally upon the stigma.
They lead it to apply itself closely to the surface cells.

On germination the contents of the pollen-grain pass over into
the growing tube. The nucleus of the vegetative cell with its cyto-
plasm usually passes out first, while the
antheridial-mother-cell is embedded in the rear-
ward part of the vegetative cytoplasm. It soon
divides to form two gametes, the nuclei of which
follow the vegetative nucleus (Fig. 226). As
the tube lengthens, the grain as well as the
older part of the tube is thus emptied of its con-
tents. Successive lengths are then shut off from
the distal part of the tube that is still full, bv

\, J Fig. 225.
plugs of Cellulose, SO that as the tube advances Transverse section of the
•4--„„i.-it 'ii -i. T-i style of Rhododendron, show-
it IS Still possible tO preserve ltS turgor. Thus ing the five-rayed channel

provided, the tube can advance through long S whh SBST ut
distances to reach the ovule. (Compare S^S^S

Fip" 22 I ^ ^ots 'n the sect^on- (F. O- B.)

From the surface of the stigma, the pollen-tube, under the com-
bined influence of its negative aerotropism, positive hydrotropism, and
positive chemotropism, is brought into close relation with the moist

stigmatic tissues. Where there
is an open channel the pollen-
tube does not need to penetrate
the tissue. Even where, as in
Lily or Rhododendron, the
channel is filled with mucilage
the tubes penetrate the secre-
tion, but not the cells which pro-
duce it (Fig. 225). There is little
apparent difference in those
cases where, as in Salvia, there is
conducting tissue with mucilagi-
nous walls (Fig. 215) ; for there
the pollen-tubes penetrate the
mucilaginous middle lamella,
passing between the cells them-
selves. This is in fact the
commonest wray for the tube
to enter the tissue of the stigma,

Fig. 226.

A, Pollen-tube of Orchis latifolia teased out from
the ovary. t/ = vegetative nucleus, g, g = gametes.
( x 500.) B, pollen-tube of the same penetrating
the micropyle : its gametes still in the tube. The
two synergids and the ovum (shaded) are clearly
shown. ( x 300.) (After Strasburger.)

B.B. U

I'.OTANY OF THE LIVING PLANT

and it is well illustrated in the Grasses. Here the tubes force their
between the stigmatic cells, penetrating the middle lamella of
their walls. But occasionally the cells of the stigma are themselves
perforated. This is seen in the Corn Cockle (Agrostemma), where the
pollen-tube traverses the delicate cell-wall of the stigmatic papilla
(1-il;. 222, A). The protoplast of the perforated cell is not killed, and
it may even continue its movements for a time, and retain its tur-
ence. The tube passes out between the walls of the subjacent
conductine cells, and continues its course in that way. The Mallow
behaves similarly, with the further feature that a number of tubes are
formed from each of the large grains, serving to fix the grain on the

surface of the stigma. One tube,
however, grows larger than the rest,
and conveys the essential contents
of the grain (Fig. 222, B). It thus
appears that pollen-tubes behave
upon the surface of the stigma like
the filaments of parasitic Fungi,
which similarly either follow the
surface of the invaded tissue or
grow between its cells ; but some-
times they penetrate the cells them-
selves. There is no doubt that in
its course the pollen-tube also
draws nourishment from the tissue
it traverses.

Passing thus down the style and
into the cavity of the ovary, the
tube is often conducted mechani-
cally by directing hairs towards
the ovule, which in the common
inverted type has its micropyle
close to the wall of the ovary.
The last part of the course is
believed to be influenced by the
sSSbESS8 °f the embry°-sac- (From synergids ; in some cases a drop of

fluid, derived perhaps from them, is
exuded from the micropyle. Whatever the influences may be, the tube
enters the micropyle and impinges closely on the apex of the nucellus ;
where that tissue has already been absorbed, it may advance directly
upon the embryo-sac, close to the egg-apparatus (Fig. 226, B).

Fig. 227.

A. embryo-sac of Helianthus annuus (after
•ischin). B, the male nuclei more highly
magnified. ps = pollen-tube. s,s2=svnergidae.
*fa,*Pi male nuclei. ov = egg-cell. <?& = central
fusion-nucleus of embryo-sac. a = antipodal
cells. spt fertilises the egg ; sp.2 fuses with the

POLLINATION AND FERTILISATION

307

The passage of the pollen-tube direct to the micropyle is the usual, and
probably the primitive course. Fertilisation in that way is called porogamy.
But in a considerable number of plants it takes a course through the superficial
tissues of the ovule. Sometimes it passes through the funiculus to the chalazal
end of the embryo-sac, as in the Walnut and Casuarina : this is called
chalazogamy. Sometimes an irregular course may be pursued, by traversing
the integuments, as in the Elm. But here the course appears to be very
inconstant. It is doubtful whether these irregularities have any special
significance, but it is worthy of remark that they occur in relatively primitive
Families of Flowering Plants.

The pollen-tube on entering the micropyle conveys with it the two
male gametes enclosed in the cytoplasm of the tube (Fig. 226, A).
Probably the turgor of the contents has its effect in rupturing the
soft tip of the tube, and extruding its contents. The nuclei of the two
gametes can shortly afterwards
be recognised in the embryo-sac.
The one passes into the ovum and
fuses with its nucleus. The result
of fusion of the male and female
gametes is the zygote. The other
passes on to the central fusion-
nucleus and coalesces with it
(Fig. 228). The mechanism of the
movements within the embryo-sac
is uncertain. It has been suggested
that protoplasmic streaming may
assist it. On the other hand, the
peculiar form wThich the male nuclei
sometimes take suggests indepen-
dent movements, like those of the
sperms of lower plants to which
they correspond functionally (Fig. 227, B). Meantime the synergids
shrivel, and begin to disorganise. Clearly their function is com-
pleted on fertilisation.

In the case of Lilium the more or less spiral form of the male nucleus, when
it penetrates the ovum, has been seen to be retained till it is applied to the
nucleus of the ovum. But the nuclei gradually become alike in shape, size,
and structure. Both are in the resting condition, and have a nucleolus
(Fig. 228). The nuclear membrane then disappears at the place of contact,
their cavities become one, the chromatin-reticulum of the one unites with that
of the other, and the resulting fusion-nucleus can scarcely be distinguished
from the nucleus of an unfertilised egg. Finally the nucleoli fuse also. The
details of the fusion of the second male nucleus with the central fusion-nucleus

Fig. 228.

Behaviour of the male and female nuclei of
Lily in fertilisation. (Mottier.) A, vermiform
male nucleus applied to the egg-nucleus (Lilium
Martagon). B, egg-cell of Lilium candidum
showing sexual nuclei in act of fusing. The
nuclear membranes have disappeared at the
place of contact.

BOTANY OF THE LIVING PLANT

of the embryo-sac in Lilium resemble those in the egg, but the process is
complicated by the fact that it may synchronise with the fusion of the two
po] ir miclei, so that a triple fusion may be seen actually in progress (Fig. 229).
But in most plants the polar nuclei fuse before the access of the second male
gamete.

The act of Fertilisation in the Higher Flowering Plants is thus a
double one, involving the ovum and the central fusion-nucleus on
the one hand, and the two male gametes on the other. The nuclei
o{ the male gamete and ovum are both haploid, being cells of the

metophyte generation, derived from spore-mother-cells which have
undergone reduction. The fusion of the gametes restores the original
number of chromosomes. The nucleus of the zygote is diploid, and that
diploid cell originates the new germ. The central fusion-nucleus had

Fig. 229.
Fusion of the second male nucleus with the polar nuclei in Lilium Martagon.
A . an S-shaped male nucleus applied to the upper polar nucleus. B, the second
male nucleus (shown only in part) and the two polar nuclei close together. C, all
three nuclei fusing. (After Mottier.)

already resulted from the fusion of the two polar nuclei. On its
fertilisation by the second male gamete a third nucleus coalesces with
it. This triple fusion is unique, so far as present observation extends.
It may have its physiological importance in relation to the develop-
ments that follow, for the triple fusion initiates the endosperm.

An important feature characterising the intricate changes in the
embryo-sac of Flowering plants is the extraordinary constancy in
the number and behaviour of the cells involved. Plants which differ
widely in form, internal structure and biological character, as well as
in the number and relation of their floral organs, show a remarkable
uniformity in these details. Exceptions do exist, but they are few
relatively to the majority which conform. This indicates that
probably each step is significant in the success of the sexual propaga-
tion, though it is not possible to assign with certainty its exact function
to each. One general conclusion follows from comparison with forms
lower in the scale, though the foundations for it can only be given on
a later page ; it is that the parts directly involved in the sexual

POLLINATION AND FERTILISATION 309

propagation of Flowering Plants represent, in a vestigial form, parts
which are more fully represented in more primitive organisms. . They
all belong to the Gametophyte, or Haploid Generation. Here they
appear as only a few cells contained on the one hand in the micro-
spore, or pollen-grain, and its tube ; on the other in the megaspore,
or embryo-sac. But these represent in their position in the life-
cycle that haploid generation which will be seen later in larger pro-
portions and greater independence in such plants as the Ferns and
Mosses. In either case, however, it is Syngamy, that is the fusion of
primordial gametes to form a zygote, which closes the haploid phase.
This event forms the starting point for the new diploid individual,
which is the Germ or Embryo.

CHAPTER XVIII.

THE EMBRYO AND THE SEED.

In normal cases if no pollen-tube arrives at the micropyle of the ovule,
and the ovum is consequently not fertilised, the ovule develops
no further. But if fertilisation by a pollen-tube has been carried
out, changes follow not only in the ovum itself and the other contents
of the embryo-sac, but also in other parts of the ovule. The term
Seed is applied to the body which results from this further development
of the ovule, while the germ which originates from the fertilised ovum
is called the Embryo.

After fertilisation the earliest changes appear in the embryo-sac
itself. The synergidae shrivel, and a cell-wall is deposited round
the fertilised ovum, or Zygote, which remains attached at the micro-
pylar end of the embryo-sac. It soon elongates, its free end extending
into the cavity of the sac. Meanwhile its nucleus, which has resulted
from the fusion of the male and female nuclei, divides, showing the
double number of chromosomes characteristic of the diploid genera-
tion. The zygote then divides into two cells by a transverse wall,
and this may be followed by further divisions in planes parallel
to the first; so that a simple filament, or Pro-embryo, is formed
(Fig. 230, i). The very first division of the zygote stamps the polarity
of this pro-embryo, and from that point on defines its apex and base.
The basal end of the pro-embryo remains attached to the micropylar
end of the sac, the apical end projects into the cavity, and produces
the embryo. It is a significant fact that in the great majority of
Flowering Plants, though not in all, this filamentous stage, showing
polarity and consisting of a varying number of cells, appears first.
It has the practical result of carrying the embryo deep down into
sac, but it may also have a phyletic meaning, as indicating an
ancestral filamentous construction. The pro-embryo then differ-
mto two regions : a free apical part which develops into the

310

THE EMBRYO AND THE SEED

3ii

massive Embryo, and an attached basal part which remains filament-
ous. This is called the Suspensor, since it holds the embryo in a
definite position during its early development, surrounded and
nourished by the semi-fluid contents of the enlarging sac.

The Embryo.

Among Dicotyledons the embryo of Capsella, the Shepherd's
Purse, in which the development was first followed out in detail,

Fig. 230.
Embryos of Capsdla in various stages of development, (i-v after Famintzin ;
vi-viii after Hanstein.) The hypophysis and its products are shaded. All these
embryos have the apex upwards, and the root downwards. But it is to be remem-
bered that the root always points to the micropyle of the ovule, as seen in Fig. 231.

serves as a very general type. The filamentous pro-embryo (Fig. 230, i.)
has the cell at its basal or micropylar end greatly enlarged. The cell
at the apical end is relatively small at first, but it gives rise to the
greater part of the embryo, a smaller part originating from the next
lower cell. The apical cell enlarges into a spherical form, and divides
into octants, by walls at right angles to one another and to the outer

3"

BOTANY OF THE LIVING PLANT

surface (anticlinal). Their position is uniform, and the first is usually
longitudinal (ii.), but their succession may vary. This suggests that
no grc.it morphological value can be set on their order of appearance
(iii. iv.). Later each octant divides into an outer and inner cell, by a wall
parallel to the surface (periclinal) ; the superficial cells thereafter divide
only by anticlinal walls, and the layer thus produced is called dermato-

gen, because it forms the epidermis
(v.-viii.). The inner cells divide
again periclinally to form an inner
and an outer series ; this is more
regular in the lower tier of octants,
which will form the hypocotyl and
root. The inner series constitutes
the plerome, which forms the
stele ; the outer is the periblem,
which forms the cortex (vi.-viii.).
Meanwhile the cell of the pro-
embryo adjoining the lower tier
of octants (the hypophysis, here
shaded) has enlarged, and divided
(vi. vii.), so as to form a group of
cells which encroach into the
spherical embryo. It provides
the apex of the root, which is thus
attached to the suspensor, and it
is always directed towards the
micropyle. The upper tier of
octants soon gives rise to two pro-
jecting lobes (cotyledons), which
bear no constant orientation
relative to the first segmenta-
tions. Between them is a smooth
groove, where the plumule will arise later. It is now possible to
recognise the position of all the parts of the germ, viz. the radicle, the
two cotyledons, and the plumule between them. In the Shepherd's
Purse the seed is exalbuminous (p. 317), and the embryo develops fast
in bulk, and in length (Fig. 231). But the ovule is of the type with a
:urved embryo-sac. The embryo, as it grows, adapts itself by curving
Iso, and soon fills the greater part of the sac. Meanwhile the plumule
at last appears at the base of the groove between the cotyledons,
position coincides with the intersection of the octant walls.

Fig. 231.

Shepherd's Purse. Photomicrograph of young
seed, showing embryo, endosperm centrally, and
developing testa on the outside. ( x 125.) The
micropyle is directed upwards and to the left,
and the root-tip is directed towards it. (After
Coulter and Chamberlain.)

THE EMBRYO AND THE SEED 313

Accordingly its position was denned by the first segmentations of
the zygote. Thus the embryo of a typical Dicotyledon springs from
the two distal cells of the filamentous pro-embryo. The larger part,
including the cotyledons, plumule, hypocotyl, and most of the root,
arises from the distal cell ; the tip of the root originates from the
cell next below it ; the rest of the pro-embryo acts as an organ of
attachment.

While the Capsella-type shows the embryogeny usual in Dicotyledons,
aberrant forms are not uncommon. But as they are mostly sporadic in their
distribution they do not suggest any consistent basis for morphological argu-
ment. It is rarely that a family includes many aberrant types : an exception
is seen in the Leguminosae, where the peculiarities are most marked in the
suspensor. Of the rest, the most interesting variants are the pseudo-mono-
cotyledonous embryos. In certain plants that are clearly Dicotyledons in
their general characters, only one cotyledon appears. This is seen in Carum
bulbocastanum, Eranthis hyemalis and Cyclamen persicum ; and it is probably
due to abortion of one of the cotyledons. But in some cases it is believed to
result from a lateral fusion of the two cotyledons to form one, as in Ranun-
culus Ficaria. Much other evidence suggests that the Monocotyledonous
state is derivative from that of the Dicotyledons. This conclusion is further
countenanced by the fact that occasionally Monocotyledons are found with
two cotyledons (Agapanthus), or even four growing points may appear on a
peripheral zone (Cyrtanthus) . In such cases the apex of the axis would be
central and terminal, as in the Capsella-type. But in most Monocotyledons
it is lateral, as it is in Alisma.

The development from the zygote in Alisma or Sagittaria is held as
typical for a large number of Monocotyledons. The first divisions give
rise to a three-celled pro-embryo, of which the basal cell (q) is enlarged,
and does not divide further (Fig. 232, i.). The middle cell divides
into several cells (m, n, 0, p, Fig. 232, ii.). The distal cell divides
into quarters, and subsequently by anticlinal and periclinal walls so as
to form the single terminal cotyledon (Fig. 232, ii.-v., /). Meanwhile
in the tier of cells below this (Fig. 232, ii. to v., m) a lateral depression
begins to appear, which develops into the apex of the axis. This is
then lateral in origin, while the cotyledon is terminal. The hypocotyl
and root-tip spring from the next two tiers (n-o, Fig. 232, ii.-v.). It
thus appears that the reference of the several parts to cells of the
pro-embryo differs in the Alisma-type from that in the Capsella-type.
The most remarkable difference lies in the lateral origin of the apex
of the axis in Alisma, while in the Capsella-type the cotyledons are
lateral, and the apex distal.

In a number of Monocotyledons the embryo differs from the Alisma-type.
The most interesting are those in which the apex of the axis originates from

3M

BOTANY OF THE LIVING PLANT

the terminal cell of the pro-embryo. This occurs in the Dioscoreaceae and
Commelinaceae (Fig. 233), also in Zanichellia and others. There is reason

Fig. 232.

Successive stages of development of the embryo of Alistna, after Famintzin.
/, m, n, o, p, q represent successive cells of the pro-embryo, and the tissues derived
from them by division.

to believe that the stem-tip in the embryo of the Monocotyledon was originally
terminal, as in the Dicotyledons, but that in most types it has been forced
to one side by the strong growth of the single cotyledon. If this were so the
A lisma-type would be a derivative, and secondary condition, and an apparent

anomaly would thus be explained. For
among Vascular Plants the embryos of
the Alisma-ty-pe are the only exceptions
to an otherwise general rule ; which is,
that the apex of the shoot bears a con-
stant and close relation to the centre of
the distal tier of cells composing the
embryo.

The very peculiar structure of the em-
bryo in Grasses has caused much discus-
sion (App. A, Fig. 503, p. 658). The
question has arisen whether the " scutel-
lum," which faces the endosperm, and
acts as a sucker from it, is or is not
the cotyledon. The view now held as
probable is that the cotyledon is highly
specialised. A basal part of it appears
as the scutellum, the distal part of it as the " cotylar sheath." The origin
of the plumule appears here also to be from the apex of the embryo.

B

Fig. 233.

Embryos of Tamus. (After Solms-Laubach.)
A, younger; B, older. Si$»= suspensor.
H = hypophysis. E, body of "the embryo.
Here the embryology is more nearly of the
type of Capsella.

THE EMBRYO AND THE SEED

315

The three primary antipodal cells (ant. Fig. 216, p. 295) have no
wall up to the time of fertilisation. Their subsequent behaviour is
variable. Sometimes they are at once disorganised ; but in most
other cases they remain functional. They may grow to large size, as
in many Ranunculaceae ; or they may undergo fragmentation of
nuclei, and even cell-division, so as to form a considerable tissue, as
in the Compositae and other Gamopetals. Their use appears to be to
act as intermediaries between the vascular supply and the enlarging
embryo-sac, before the endosperm is organised as a tissue. To that
end they sometimes develop as suckers penetrating the chalaza. But
in any case they only help towards the final end, which is the full
development of the germ.

The Endosperm.

The triple fusion, of the two polar nuclei with the second male
gamete, has already been noted (Figs. 227, 229). The first division of

/v «

cJ"'

Fig. 234.
Successive stages of development of the endosperm in Myosvrus. (After Stras-
burger.) (i.-ii. and iv.-vii. x 400 ; iii. x 170.) i. shows state at fertilisation.
ii., embryo-sac much enlarged, and first division of the fusion-nucleus, iii. shows
embryo-sac still more enlarged ; it is on a lower scale of magnification, iv.-vii.,
stages of cell-formation round the numerous nuclei, derived by division from the
fusion-nucleus.

the resulting triple-fusion-nucleus usually precedes that of the zygote
(Fig. 234, i. ii.) : it is repeated synchronously, in rapid succession, so
that the numerous nuclei formed are found to be in corresponding
stages of division, and their number at any moment is some power of

3i<5

BOTANY OF THE LIVING PLANT

two. Their chromosome-number is at first diploid, but the number is
not strictly maintained. Meanwhile the embryo-sac grows rapidly,
and the large central vacuole is surrounded by a thin peripheral film of
cytoplasm, in which the free nuclei are embedded (Fig. 234, ii.-vi.).
Partition-walls are formed later, isolating each nucleus in its own cyto-
plasmic area. Sometimes several may be present in a single cell, but
when this is so they commonly fuse together. The embryo-sac is thus
lined internally by a flattened layer of uni-nucleate cells, surrounding a
large central vacuole (vii.). These cells then grow inwards, and divide,
encroaching on the central cavity : this they ultimately fill with the

compact tissue of the endo-
sperm, which embeds the
embryo. Sometimes, how-
ever, a central cavity may
still remain. This is the case
in the Coconut, where the
cavity is filled with the
" milk," which is actually
vacuole-fluid, while the white
flesh is the tissue of the endo-
sperm, which has not filled
the very large embryo-sac.

The above description applies
to ordinary types of ovule. But
there is a considerable variety
of detail in the behaviour of the
embryo-sac and its contents after
fertilisation. Frequently the first
division of the fusion-nucleus is
followed by a cell-wall dividing
the sac into two chambers ; this
is seen in some Monocotyledons, and in many Dicotyledons, especially among
the Gamopetals. Sometimes the development proceeds no further ; but usually
divisions may be continued in one or the other, or in both of the chambers.

More marked modifications are connected with the nourishment of the
embryo-sac. In relatively primitive types like Myosurus the sac merely
increases greatly in size, encroaching upon the surrounding tissues, which
make way for it. Their cells collapse and their substances are absorbed into
the growing sac, which acts thus parasitically upon them (Fig. 234, ii.).
But in more specialised types, such as the Gamopetals, the nursing of the
embryo-sac is more exact. Their ovules have only a single integument,
while the embryo-sac soon crushes the small nucellus out of existence. The
innermost layer of the integument then abuts on the growing sac, and
forms an epithelial jacket of prismatic cells (Fig. 235, n.j.). This serves

Fig. 235.

Median section of an ovule of Rhinanthus minor,
showing haustoria. (After Balicka-Iwanowska.) s = sus-
pensor. em=embryo. e.s.=embryo-sac./wn.=funiculus.
n.j. = nutritive jacket, nt. — nutritive tissue. c.A. = cha-
lazal haustorium. m.ft. = micropylar haustorium.

THE EMBRYO AND THE SEED 317

as a permanent nourishing tissue, which acts till the embryo-sac is well
advanced.

In addition to this the embryo-sac itself may frequently put out local
haustoria, which penetrate to favourable sources of nourishment. A good
example of this is seen in Rhinanthus (Fig. 235), where, in addition to the
epithelial jacket (n.j.), haustoria are formed at both ends of the sac. The
chalazal end (c.h.) extends so as to reach a mass of nutritive tissue (nt.) close to
the end of the vascular strand. From the micropylar end a similar haustorium
(m.h.) passes through the micropyle, and traverses the funicle towards the
same source of supply. The haustorial connections may be still more
elaborate in other plants (Plantago). Such arrangements indicate the impor-
tance of the nourishment of the sac, especially in its earlier stages. They
also provide interesting analogies with the behaviour of parasites, whether
among the Fungi or in Flowering Plants.

The function of the endosperm is to provide temporary nourishment
for the embryo which it surrounds. But the amount of the supply,
and the time when it is yielded to the embryo
may vary. Two main types of seed arise
accordingly. In the first the embryo grows
slowly, and keeps in. close touch with the
endosperm, which remains relatively large
till the seed is ripe ; it embeds the embryo
and is stored with food. The result of this
is the " albuminous " seed (Fig. 239). It is
probably a relatively primitive state, and it is FlG- 236-

. . f ... 1 t-> 1 Vertical section through a

found in such families as the Kanunculaceae Peppercorn. «m= embryo.

,,, .. , . .,» ,1 « = endosperm. * = perisperm.

andMagnohaceae, and in mostMonocotyledons. The testa is shaded. ^r=Peri-

nr 11 /— i_ j r j-l • carp. (After Baillon.)

Moreover, all Gymnosperms have seeds 01 this

type. In the second the embryo develops more quickly. It absorbs
the available nourishment early, so that at ripeness little or nothing
remains of the endosperm. Its function has been temporary. Such
seeds are called " ex-albuminous " (Fig. 239). Intermediate states
are found, as in the Leguminosae, which, though usually held to be
exalbuminous, have in many cases a band of mucilaginous endosperm
covering the embryo (Fig. 237). While the substances stored in the
endosperm provide for the further growth of the germ, they also supply
the staple food of man in the various cereal grains.

In some cases the store of food for the embryo may in part be out-
side the embryo-sac, in the chalazal region of the nucellus. Such tissue
is called perispenn, and it is found in the Peppercorn (Fig. 236),
or the seed of the Water-Lily. The difference from an ordinary
albuminous seed is morphological rather than physiological. But
in the great majority of cases the nucellus is obliterated early, owing

I'.UTANY OF THE LIVING PLANT

to the precocious growth of the megaspore which it envelops while
young. Accordingly, it is not represented as a rule in the ripe
seed, except by the remains of its tissue, which are crushed between

the firm seed-coat and the endosperm
or embryo within.

On the other hand, the integument
or integuments persist, developing into
the testa, or seed-coat. Their tissues
become indurated, of stony or leathery
texture ; but there is a good deal of
variety in the detail. Usually the
outermost layer, but not infrequently
some layer more deeply seated, develops
its cells in prismatic form and thick-
walled. Others may also harden : but
the inner, softer layers are often com-
pressed. The tissues lose their cell-
contents, serving only the purpose of
protection to the germ and the stores
within. (Fig. 237.) When this condi-
tion is reached in many-seeded ovaries

Fig. 237.
Section through testa, and mucHage-
endosperm of seed of Gymnocladus cana-
densis. />« = palisade layer. s=sup-
porting layer. /> = thick- walled tissue.
These form thr testa. sch= mucilaginous
endosperm of this Leguminous seed.
(After Nadelmann.)

Fig. 238.
Young carpel, and fruit of Copaifera.
a indicates the arillus which spreads from
the micropylar end of the pendent seed.
(After Baillon.)

the tissue of the funicle dries, and being brittle, the connection
the seed and the parent plant is severed. It is now indepen-
dent, and the new individual has to fend for itself.

< ier developments, having special biological value, are sometimes formed
mg the ripening of the seed. Superficial cells may grow into long hairs,

THE EMBRYO AND THE SEED

319

as in the Cotton, Willow, or Poplar. These are effective in the transfer of the
seed by the wind. Succulent bodies may sometimes be formed by local
hypertrophy ; such as the massive enlargement of the micropylar region to
form the " caruncle " of the Spurges. A growth may proceed from the base of
the ovule, appearing as an extra integu-
ment, or arillus (Fig. 238). The Mace
sold by grocers is an example of this.
It appears after fertilisation as a partial
covering, highly coloured and strongly
flavoured, round the true seed, which
is sold as the Nutmeg. The bright
orange sheath round the ripe seeds of
the Spindle Tree is of the same
nature. In both cases the aril is
exposed as the fruit ripens, and its
presence is believed to promote distri-
bution of the seeds by Birds. But
such developments are infrequent.

c.

Each seed contains normally a
single germ, together with a store
of nutriment either in the germ
itself or in the accessory tissues.
(Fig. 239.) As it ripens it dries
out. In this state, after separation
from the parent, it may undergo
a period of rest. But sooner or
later its function is to establish a
new individual. In order that this
may be most effectively done it is
important that each seed should
have the chance of independent
germination, a condition which is
secured by dispersal of the seeds.

Each seed thus represents a single matured megasporangium, or
ovule. It is a composite body comprising parts derived from three
generations. The seed-coat, and the perisperm when present, consist of
tissues of the parent diploid plant. The endosperm is usually held
as representing, in a specialised and altered state, the haploid female
prothallus, or gametophyte generation. The germ, which results
directly from fertilisation, represents the new diploid generation.
That these are all closely related in the seed of Flowering Plants
is a late and derivative state, which cannot be properly understood
till certain lower types have been described, and compared with it.

Fig. 239.

Seeds in median section : the upper (Capsella)
exalbuminous, the lower {Datura) albuminous.
/= funiculus. m=micropyle. <= testa. <r=» endo-
sperm- c = cotyledons. £/=plumule. r = radicle.
(Enlarged.) (Dr. J. M. Thompson.)

CHAPTER XIX.

THE FRUIT AND SEED-DISPERSAL.

The effects of Fertilisation extend in Flowering Plants beyond the
ovule or megasporangium to the carpels, and often also to the floral
receptacle. The mature Pistil or Gynoecium is called the Fruit.
Sometimes this term is applied in a strict sense to the ripened pistil
only. But it may be used in a more extended sense to include the
receptacle, or even other parts when they also undergo changes con-
sequent on fertilisation. This wider use of the term accords with
the definition of the Flower as a simple shoot bearing sporangia.
The Fruit would then be the whole of that simple shoot developed in
the interest of the ovules which it bears.

According to the structure of the flower that produces it, the fruit
may comprise one carpel or more. The carpels may be separate or
united. They may in more specialised types be sunk down in the
tissues of the receptacle. Such differences of the gynoecium existed
already in the flower, and they remain as the fruit ripens, giving
variety to the construction of its different types. It would be possible
to analyse and describe the several kinds of fruits on the basis of their
morphological structure, and it is necessary to do this when the chief
purpose is systematic classification (see Appendix A). But another
method is to examine them from the biological point of view. The
function of the fruit is then taken as the basis of their study, and the
attempt is made to see how each type is fitted for its performance.
That is the aspect which will now be developed.

The functions of the maturing fruit are the protection and nourish-
ment of the ripening ovules, and the dispersal of the seeds. It will
not be necessary to dwell upon the first, for it is so obvious. In
superior fruits the ovule, or ovules, are covered in by carpels, and
attached to them by their funicles. In this way effective protection

320

THE FRUIT AND SEED-DISPERSAL 321

and nourishment are afforded during development. But they are
still better secured in inferior ovaries ; for in these the ovules are
surrounded also by the massive tissue of the receptacle, while they
are brought nearer to the source of vascular supply by their deeply
sunken position. The protection becomes more effective as the
maturing of the gynoecium proceeds, for its tissues frequently become
more bulky and succulent. But still more is this so in others which
harden with age. Extreme cases are seen in nuts or stone-fruits,
where the woody tissues of the carpel reinforce, or sometimes mechani-
cally replace the seed-coats. Since the seed with its nutritive store
offers attractions to animals as food, the biological importance of
such strong mechanical protection should be duly recognised.

But the biological fact which has dominated the evolution of the
fruit, as regards its development after fertilisation, more than any
other is the need for the dispersal of the seeds. The greater their
number the greater is the need for it. Sooner or later each seed
should have the opportunity of germination. This is carried out
with the best prospect of success where each is isolated from its
neighbours. Moreover, a wide dispersal leads to the spread of the
species, and thus helps it in that competition for room in an over-,
crowded world which has been called the Struggle for Existence.
The size of the individual seed is an important factor in the problem
of dispersal. Clearly, the larger the seed the better is the chance of
successful establishment of the young plant on germination ; for the
larger the store it carries with it the larger the vegetative system it
will be able to form before it has to depend on its own resources.
But the larger the seed the less easily will it be transferred from point
to point. There are thus two conflicting factors of success. In the
course of evolution plants have severally struck their own balance
between these opposing factors, and the variety of fruit-construction
shows in what different ways the problem may be solved. The success
of each may be measured by the survival, numbers, and spread of
those plants which have adopted them. These are the underlying
conditions which should be kept in mind in studying the structure
of the fruit, and its relation to seed-dispersal.

It is probable that a primitive type of gynoecium was apocarpous, containing
a number of ovules. Such a carpel is seen in the pod of a Pea, or in the follicle
of Caltha or Aquilegia. Along several lines of Descent comparison suggests
that there has been a reduction in the number of ovules to a single one. This
is illustrated in the Ranunculaceae. The Helleboreae have follicles and are
probably a central type {e.g. Helleborns, Aconitum, Caltha, Fig. 240). In

B.B. X

BOTANY OF THE LIVING PLANT

the ( irpd of 4 nemone only a single ovule matures, though several are initiated
but remain vestigial (Fig. 241. a). In Rue and Buttercup only a single ovule
is present (Fig. 241, b). On the other hand, the number of carpels in the
HeUeboreae is about five: but a much larger number is seen in Anemone
or Buttercup ; the more numerous carpels make up for the reduction of the
ovules to one in each. Similarly in the Leguminosae a primitive type is seen,
with many ovules in each carpel in the Pea or Bean. But Copaifera contains
but two ovules, of which one only ripens into an unusually large seed (Fig. 238,
p. 318). Again, in the Rosaceae, in Spiraea or Cydonia each carpel is many-
ovuled ; but in the Potentilleae, Rubeae and Roseae each has only one.
The loss of number is, however, made up by the greater number of carpels.
In the Pruneae, on the other hand, the single carpel contains two ovules, of
which usually only one matures ; but it grows into a large kernel. The

a

Fig. 240.

Fig. 241.

Fig. 242.

Fig. 240. Follicles of Aconite. (After Figuier.)

Fig. 241. a, Carpel of Anemone, with abortive ovules, b, Carpel of Thalictrum.

(After Prantl.)

Fig. 242. Achene or nut of Buttercup. (After Figuier.)

gynoecium of the Oak is trilocular, with two ovules in each ; but only one of
the six ovules matures into the seed in the large acorn. The Coconut again
shows three loculi, but only one matures its single very large seed. A similar
condition on a smaller scale is seen in Valerianaceae, while the gynoecium, with
a single loculus and a single ovule in each flower, becomes the rule in the Teasels
and Composites : these may be held to be examples of an extreme state.

Such evidence clearly indicates a progressive reduction in number of ovules
produced and matured in the cases quoted. It often goes with increase in
size of the individual seed, as in the Plum, Oak, and Coconut, which gives
greater certainty of successful germination. But some families show the
converse, viz. that there has been a progressive increase in number of the
seeds, though the individual size of the seeds is diminished. The most marked
examples are seen where some irregular form of nutrition makes germina-
tion hazardous : for instance, in the mycorhizic Orchidaceae, where over
a million seeds have been estimated as the product of a single capsule. In
many of the mycorhizic Ericaceae the seeds are numerous ; and the same is

THE FRUIT AND SEED-DISPERSAL 323

seen in the parasitic Rafflesiaceae or Balanophoreae. In such cases the large
number of seeds covers the risk of germination.

Such differences in number and size of the seeds must profoundly affect the
mechanical problem of seed-distribution. The minute seeds of an Orchid
can be scattered by a breath, but this would have no appreciable effect upon
a Coconut in its natural husk.

Dissemination of Seeds

Seeds being immobile require some agent outside themselves
for their dispersal. Various means are effective. Sometimes the
dispersal may be dependent upon the structure of the pistil, as in
explosive fruits. But usually it is carried out by means of transfer
outside the plant, such as currents of air, or water, or the movements
of animals. There is an obvious analogy in this between seed-dispersal
and the transfer of pollen. In both cases the immobile plant depends
upon external agencies for transfer of essential parts. But the two
present quite distinct problems. In pollination the end is to deposit
the pollen-grains upon the stigma, and the more accurately this is
done the better. In seed-dispersal the end is simply the wide separa-
tion of the individual seeds. Accuracy is of less importance.

In primitive types of fruit, as the carpellary wall matures, its tissues
commonly become hardened. Where the seeds are numerous they
require to be set free for germination. The natural course is by
splitting or dehiscence of the carpel ; and this is carried out in various
ways. A split most naturally follows along the line of the coalescent
margins of the folded leaf. This is the case in the follicle of Aconite
(Fig. 240). A similar split may also sever the carpel at its midrib,
as in the pods of Peas and Laburnums. The case of syncarpous fruits
is not so simple. Their capsules open sometimes by longitudinal
slits, as in Cardamine (Fig. 101) or Hura (Fig. 102), sometimes
by transverse slits, sometimes by valves or pores : and this may occur
either in superior or in inferior fruits. The seeds can thus escape,
leaving the carpellary structure behind as an empty husk. On the
other hand, in cases where the carpels are separate, and the number-of
the seeds in each is reduced to one, dehiscence is unnecessary, while
the firm tissue of the carpel may form an additional protective wall.
The resulting achene, or nut, is then shed bodily, as if it were a single
I seed, though it is strictly speaking a fruit. This is seen in the Butter-
cup (Fig. 242), or Potentilla. It may happen also in syncarpous
pistils where only one seed matures, as in the Hazel, Acorn, or Coco-
nut. A peculiar case is that of the Umbelliferac, where the inferior

, BOTANY OF THE LIVING PLANT

splits into two halves, each corresponding to one of its two
carpels, and each containing a seed. Each half is practically equiva-
lent to an achene (Fig. 490, vi-> Appendix A, p. 638).

To release the seeds as separate bodies is one thing, to disseminate
them is a different proposition. By various modifications of the
ripening pistil this second end may also be secured, sometimes by the
activity of the pistil itself, but oftener through the medium of external
agencies. The plant itself may disperse its seeds by means of various
mechanical arrangements, which often result from the sudden release
of strains set up in the carpellary walls as they dry in the process of
ripening. An instance is seen in the common Hairy Cress (Cardamine

kirsuta). Its fruit is the usual siliqua of the
Crucifers ; but the lateral valves, on splitting
from their base, curl so sharply upwards that
the seeds are forcibly thrown out (Fig. 101).
The method of the Geranium is similar,
though differing in detail. Here only one
seed is ripened in each of the five carpels.
By splitting from the base, and sudden curling
of the carpel upwards, the relatively large
seed is slung out at a distance (Fig. 243).
One of the largest and most effective of these
explosive fruits is that of the Sand-box Tree
(Hum, Fig. 102). It is composed of 12 to
18 woody carpels, each containing one seed
about I inch across. As the fruit hangs ripe
upon the tree, the carpels suddenly split apart,
and their woody shells take a twisted form, thus relieving the previous
strains. By the sudden change the seeds may be thrown to a distance
of some thirty yards from the tree. Another method is seen in the
tricarpellary capsule of the Violet. When ripe the three carpels
separate, and each boat-like carpel then presses its margins laterally
together as it dries, and pinches the smooth seeds, which are thus shot
out to a distance. Such examples illustrate the mechanical methods
of dispersal seen in dry fruits.

In some pulpy fruits the principle of the squirt is used. The fruit
of the squirting Cucumber (Ecb allium) hangs pendent as it ripens,
and its outer pericarp is kept tense by the semifluid contents. When
ripe it breaks away at the stem, opening a basal pore. As it drops
the contents are sprayed out of it, seeds and all, to a considerable
distance, scattering as they fall.

Fig. 243.

Sling-Fruit of Geranium.
(After Figuier.)

THE FRUIT AND SEED-DISPERSAL

325

Dissemination by Wind and Water.

More frequently the motor impulse is from without, the chief
agents being wind, water or some moving animal. Wind acts directly
upon small seeds, such as those of Orchids,
scattering them as it would so much dust.
It may act less directly where the seeds are
larger, and the dehiscent fruit is borne on
a stiff stalk, as in the gaping follicles of
the Aconite, which, shaken by the wind,
scatter their seeds all round the parent
(Fig. 240) : or in the Poppy (Fig. 244) or
Canterbury Bell, which do the same ; but
here the dehiscence is by pores, the prin-
ciple being that of the Pepper-box.

The wind would no doubt influence the
fall of any seeds ; but the development
upon them of tufts of hair, or of broad
thin wing-like surfaces, enhances its effect
upon their transfer, even where the seeds
are relatively large. Such developments
are sometimes upon the seed itself, as in the
case of dehiscent fruits : or they may be
formed by the carpellary walls where the
fruit is one-seeded, or where it breaks into
one-seeded parts. The development of hairs on the seeds themselves
is seen in the Willow and Poplar, in Cotton (Fig. 245), and in the

Fig. 244.

Capsule of Poppy opening by
pores below the star-shaped
stigma. (After Figuier.)

Fig. 245.

Seed of Cotton with superficial hairs.
(After Figuier.)

Fig. 246.

Fruit of Valerian. (After
Figuier.)

Willow-herb. When in any of these the fruit splits, the seeds are set
free, each with its hairy parachute, which supports it in the breeze,

326 BOTANY OF THE LIVING PLANT

so that it may be conveyed to a distance before reaching the ground.
In many Valerianaceae (Fig. 246) and Compositae (Fig. 247), where

<

the fruit is one-seeded and does not
split, the calyx is persistent, and
developing as the feathery pappus
buoys up the inferior achene when
set free, so that it may be conveyed
to a distance before reaching the
ground. The development of flat-
tened wings upon seeds or fruits is
closely analogous. Examples in
the case of winged seeds are seen
in the Bignoniaceae (Fig. 248), and
of winged fruits in the Elm, Ash
or Sycamore (Fig. 249). It is note-
worthy that winged seeds and fruits
are most common where the plants
that bear them are of some stature,
or are climbers ; so that they have
to fall a considerable distance. The
wind has thus a chance of scatter-
ing them far and wide.

For large seeds a more effective
means of transit is by water, which
in commerce is a very efficient
method for goods generally. Given

a movement of water and a floating seed or fruit, dispersal is easy.

The Water Lily is an example. The large berry ripens under water.

It there splits, and the coherent mass of seeds, each with bubbles

held in its aril, floats to the surface.

There the seeds separate, drifting

about till the bubbles are liberated

by the decay of the aril. The seeds

thus dispersed then sink. More

striking examples are seen in lit-
toral or estuarine plants, of which

the seeds or fruits are often very

large : for provided they float the

size is immaterial. In Barringtonia

(Myrtaceae), Scaevola (Goodeniaceae), and Heritiera (Sterculiaceae),

which are all estuarine plants, the relatively large fruits have a fibrous

Fig. 247.

Fruit of Dandelion, with pappus as
parachute. Note the absence of bracteoles
on the general receptacle.

Fig. 248.

Winged seed of one of the Bignoniaceae.
(Reduced.)

THE FRUIT AND SEED-DISPERSAL

327

fruit-coat (pericarp), with air-spaces. Their large fruits can thus
be easily floated away as they drop, by a stream or by tides. But

Fig. 249.
Samara, or winged fruit of Sycamore, dividing into two.

(After Figuier.)

extreme cases are seen in Nipa, Cocos, and Lodoicea, all of them littoral
and estuarine Palms. Their fruits have fibrous husks with air-
chambers, and this serves to float them. Each contains a single

Fig. 250.

Fruiting annual plants of Salsola, caught at a wire fence, as they were rolled by
wind over level sand, on the coast near Adelaide, Australia.

seed. Those of Lodoicea, the Double Coconut of the Seychelles, are
the largest known. They may be carried long distances by ocean
currents.

BOTANY OF THE LIVING PLANT

Some plants show curvatures of the fruit-stalk, or of the shoot
aerally, which aid in the dissemination, or even in the exact deposit
of the seeds. The fruit-stalk of the Ivy-leaved Toad-flax may be
seen to direct its fruit to the crannies of the rock or wall in which it
grows, so that its seeds are shed directly on the spot suitable for their
germination. Some denizens of arid soil curl up their branches as
they dry into a sphere, which when detached may be rolled great
distances by the wind over flat ground, scattering their seeds as
they go. This is seen in the " Rose of Jericho " and in the Grass,
Spinifex. It is shown graphically in a species of Salsola which
grows on the coast near Adelaide (Fig. 250).

Dissemination by Animals.

In various ways Plants may make use of the free movements of
animals, which while serving their own ends become the involuntary
agents of dispersal. The living seeds may be conveyed by them

Fig. 251.
Fruits with hooked outgrowths, effective in transfer by animals. A = A grimonia
(Le Maout.) B = Galium (Le Maout). C = Cynoglossum. D = Geum. E = Bidens.

externally, attached to their coats or other parts of their bodies :
or internally, as ingested food : or they may be actually carried by
them intentionally. For the first no special development is actually
necessary in the seed or fruit. They may stick to the feet of animals
clogged with mud, especially birds. Darwin removed the soil from
the foot of an injured partridge, and obtained from it no less than
82 seedlings. But many fruits and seeds are provided with means
of attachment ; where the seeds are small, a sticky glandular secretion
may serve, as in some Salvias. But many fruits develop as " burrs,"
being provided with hooks of various origin, which attach them to
fur or feathers. In the Burdock the tips of the bracts are hooked :
in Bidens spines representing the calyx bear reflexed teeth (Fig.
251, E) : in Cleavers (B), or the Carrot there are hooked emergences
on the wall of the inferior ovary : in the " Echinella " section of the

THE FRUIT AND SEED-DISPERSAL

329

Buttercups, or in the Hounds-tongue (Cynoglossum) (C) they are on
tho superior achene : in the Water Avens (Genm rivale) (D) a single
hook is formed half way up the style of each carpel. Such instances
taken from different families, and involving quite
different parts, show that these effective develop-
ments have originated repeatedly, and indepen-
dently of one another.

A second and more prevalent means of dispersal
is internally, as ingested food. It is secured by
development of succulent tissues in close relation
to the seeds. Here it is found that most diverse
parts are involved, even in nearly related plants.
Pulpy fruits occur in almost every family ; even
among the Grasses, which have characteristic dry
grains, certain Bamboos bear a succulent fruit.
The pulpy tissue often involves the whole carpel-
lary wall, and the seeds are embedded in it : this
is the case in the berries of the Grape, or Currant,
the one being a superior the other an inferior
ovary (Fig. 252). Or it may be only the outer
part of the wall that is pulpy, while the inner is
stony, as in the drupe of Cherries or Plums (Fig. 253). Sometimes
parts other than the pistil itself may be involved, for instance the
receptacle may be convex, and succulent, and bear the dry achenes
(which are the true fruits in the restricted sense) upon its surface, as in

Fig. 252.

Berries of the Currant.
(After Figuier.)

Fig. 253.

Drupe of Cherry.
Figuier.)

(After

Fig. 254.

Succulent receptacle of Strawberry.
(After Figuier.)

the Strawberry (Fig. 254). Or it may be concave, and the achenes
be borne within its hollow cavity, as in the " hip " of the Rose. In
the Fig it is the massive axis of the condensed inflorescence that

330

BOTANY OF THE LIVING PLANT

becomes concave and pulpy, while the achenes produced by the
numerous flowers are contained within it (Fig. 255). Lastly, the

perianth may be persistent, as it is in
each of the aggregated flowers of the
Mulberry, and becoming pulpy they
embed the true fruits, which are
achenes (Fig. 256). It is needless to
multiply instances. Those quoted
suffice to show how various are the
parts that offer attractions by their
succulent development to fruit-eating
animals. Colour, scent, flavour and
organic content, all attract them to
these fruits as food. In the haste of
feeding they do not exclude, but bolt
the seeds, which may thus be carried
by them internally to a distance before
being voided with their excreta.

In these cases the animal is an un-
witting agent. Another method, less
effective but still to be reckoned with,
is by means of intentional transfer of
fruits or seeds for the animal's own purposes. Squirrels, some birds,
and ants hoard stores of food in the form of nuts, etc. ; sometimes
the store is not fully exhausted, so that the live seeds germinate apart
from the parent plant. But more exact results

come where masses of esculent tissue, often contain-
ing oil, occur on the surface of the seed, in the form
of caruncles, as in the Violaceae, Euphorbiaceae
(Fig. 479, vii.-viii., Appendix A), and Leguminosae.
Observations have been made on the actual transfer
of the seeds of Gorse by ants, and the spread of that
plant on certain moor-lands can be definitely ascribed
to this agency.

Lastly, man himself is the most potent agent in
the distribution of plants, though his influence is a " Mulberry" com-
as often destructive as constructive. He consciously K^su^uient"^-
introduces plants of economic value to new areas, ^nths ePclose eac£ a

1 » dry achene. (Alter

and clears off the native flora to make way for their F»g™er.)
cultivation. But he also unconsciously carries with him the seeds
of certain plants, which appear as " weeds " wherever he goes. The

Fig. 255.

Succulent hollowed axis of the inflores-
cence of the Fig, bearing achenes within.
(After Figuier.)

THE FRUIT AND SEED-DISPERSAL 331

Nettle, the Shepherd's Purse, and the greater Plantain are the
commonest of these.

All such means for the dissemination of seeds as those described
are to be held as offsets to the limitation imposed upon plants by
their fixity of position. They are themselves immobile, as are also
their seeds ; but a spread of their seeds is essential for the survival
of a species, or its spread to new stations. The measure of its success
may be illustrated by a few examples. Darwin remarks that no
cultivated plant has run wild on so enormous a scale as the Cardoon
Thistle [Cynara cardunculus), introduced from Spain to La Plata.
It has spread so as to cover large areas to the exclusion of other
plants. Its " pappus " carries its fruits down the wind, after the
manner of other Composites, and its spread shows the effectiveness
of the method. A more recent example is that of Lantana aculeata,
a native of Mexico, which was introduced into Ceylon as a garden
plant in 1828, and has since spread all over the island, taking up waste
land to the exclusion of other plants. It is spread by birds, which
eat its pulpy fruits. Where forest fires occur in Canada the " Fire-
weed " (Epilobium angustifolium) at once occupies the cleared
ground. It reaches the sterilised surface by its light seeds being
supported in the wind by superficial hairs. A census was made
some years ago of plants found growing in humus borne on the stumps
of pollard Willows near Cambridge. The seeds or fruits from which
they sprang would have had to be raised about eight feet above
the ground. Of the total of nearly 4000 records, 44.62 p.c. were
plants with fleshy fruits, 25.18 p.c. had winged or feathered fruits
or seeds, 16.47 P-c- naol burred fruits, IO.75 p.c. had seeds so
light or small as to be easily wind-borne. Thus the presence of
all the plants observed upon the stumps, excepting about 3 p.c, is
accounted for by recognised methods of seed-dispersal. Such
examples show the practical results of transport of seeds by the
methods described, and give some idea also of the rapidity of their
effects.

An experiment on the grand scale was made in the formation of a completely
new Flora of the Island of Krakatau, and fortunately its results were followed
by competent observers, who kept careful records. These form the best
authenticated story of the natural formation of a plant-population in an area
where none was living before. Up to 1883 the islands forming the small group
in the Sunda Strait between Java and Sumatra, of which Krakatau is the
largest, were covered by dense vegetation. From May to August of that
year successive volcanic eruptions resulted in the complete sterilisation of
the surface, which was covered with hot stones and ashes. Thus, on cooling

BOTANY OF THE LIVING PLANT

an uninhabited desert was exposed, lying at a distance of fully twelve miles from
the nearest vegetation. Since then a new Flora has sprung up upon the islands.
This has been studied at intervals. The late Dr. Treub, who visited Krakatau
in 1886, concluded that the first colonists were blue-green Algae associated
with Diatoms and Bacteria. These formed a suitable nidus for the spores of
Mosses and Ferns, and for the seeds of Flowering Plants adapted for dispersal
by winds. On the beach were found the fruits and seeds of Flowering Plants
carried by water, some of which had germinated ; many of them belonged to
the characteristic strand-flora of the Malay region. But the plants introduced
by animals or by man were not found by him on this visit, which took place
only three years after the eruption. In 1897 Penzig visited the island, and
estimated that of the Flowering Plants noted 60.39 p.c. had reached it by
ocean currents, 32.07 p.c. by wind agency, and only 7.54 p.c. had been trans-
ported by fruit-eating animals and man. On a subsequent visit by a party of
botanists in 1906, the results as stated by Ernst show that though these pro-
portions for Flowering Plants were not exactly maintained, still the largest
number were borne by water-transit, and the smallest by animal agency. Thus
for an oceanic island the most effective agency of transit is water ; wind-
carriage takes a middle place, and transit by animal agency is the least
effective of the three. These are the results which would naturally have been
anticipated.

Plant-Population.

A very important factor in the maintenance and spread of a species
is the actual number of germs produced. The third Chapter of
Darwin's Origin of Species deals with the geometrical ratio of increase
of living things. Following Malthus he there points out that there
is no exception to the rule that every organic being increases at so
high a rate that, if not destroyed, the earth would soon be covered
by the progeny of a single pair. Linnaeus had already calculated
that if an annual plant produced only two seeds, and their seedlings
next year produced two, and so on, then in twenty years there would
be a million plants. This is, however, a very slow rate of breeding.
The following table gives the results of careful computation by Kerner
of the number of seeds produced in a single season by an average
specimen of each :

Henbane (Hyoscyamus niger) - 10,000

Radish (Raphanus Raphani strum) - - 12,000

Plantain (Plantago major) - 14,000

Shepherd's Purse (Capsella Bursa-pastoris) - 64,000

Fleabane (Erigeron canadense) ... 120,000

Tobacco [Nicotiana Tabacum) - - - 360,000

Flixweed (Sisymbrium Sophia) - ~ - 730,000

THE FRUIT AND SEED-DISPERSAL 333

The Orchidaceae are extreme cases of productivity : the estimates
of seed-production in them are as follows :

Per Capsule.

Per Plant.

Cephalanthera

6020

24,080

Orchis maculala

6200

186,300

A cropera

- 37!.250

74,000,000

Maxillaria

- 1.756,44°

Such figures convey little more than a general impression of vast-
ness : but evidently the number of germs produced is far in excess of
the actual requirement to make up directly for losses by death. There
is in fact an immense margin, which may be regarded as a very
efficient reserve to meet all the contingencies involved in the establish-
ment of the germ till it reaches propagative maturity. Such a
reserve is necessary, for the risks of youth are great. Many seeds
fall victims to the predatory attacks of animals, which naturally
divert to their own uses the food-stores laid by for the germ. Many
never reach a situation fit for their germination. Many young
plants are killed off almost at once by unfavourable conditions,
such as unsuitable temperature, or drought, or unseasonable changes
while in the defenceless condition of the seedling. Competition with
the same or other races of plants destroys others. Fungal attack
also takes its toll, and especially in the seedling state. But notwith-
standing the number, and insistence of these risks, an overplus
remains in any surviving species. This not only keeps the race in
being, but in most cases provides for its spread into fresh areas,
where, however, it is liable to be checked by various limiting factors.
Moreover, the large numbers, and the competition which necessarily
follows, provide material for Natural Selection to work upon : and it
is the fittest that will be the most likely to survive.

On the other hand, carefully recorded cases under the favoured
conditions of cultivation illustrate how prolific plants may be, even
where the productivity in number of seeds is not specially high. A
good example is seen in the " Marquis " Wheat, which was derived
from a single head in 1903, as a result of hybridisation. It has since
been spread through Canada and the United States. In 1918 it was
sown on 20,000,000 acres of land, and yielded some 300,000,000 bushels
of grain. So wonderful a result in a cultivated plant under control,
in a limited number of years, illustrates the effect of a geometrical
ratio of increase, such as might often be possible in Nature, if the
circumstances were equally favourable.

, BOTANY OF THE LIVING PLANT

Latent Period.

After seeds are shed and distributed they usually undergo a period
of rest. During the autumn and winter of temperate climates they
ome buried in the soil. They naturally fall into chinks and
crannies, and are often covered by rotting leaves ; they are also washed
into the soil by rain, or drawn below by the restless activity of earth-
worms, or covered by their castings. But some work their own way
into the soil by hygroscopic movements, as in Avena, or Stipa, or
Erodium. A few even bury their fruits as they mature by geotropic
curvature of the fruiting stalk, as in the Earth-nut (Arachis), or the
subterranean Clover. In one way or another they become covered,
and in the dormant season there is plenty of time for the process.

A latent or resting period is commonly determined by climatic

conditions of drought in the tropics, or of cold in temperate climates.

But a latent period may be self-induced, in which case the seeds will

not germinate till after a period of rest. In this respect seeds vary.

Some will germinate immediately they are matured if the conditions

are favourable. It is this that makes the difficulty with cereal crops

in a warm, wet autumn : for those conditions stimulate immediate

germination, and the grain is liable to sprout in the stook before it

can be harvested. But as a rule a period of rest follows on ripening.

The seed dries out, and in that state it remains stationary, but retains

its vitality, being specially resistant to extremes of drought and

heat. Examples of an obligatory resting period have been quoted

in Chapter IX., p. 140. If such seeds are collected in autumn, and

exposed to conditions favourable for germination, they remain passive

till the spring, when they will germinate almost simultaneously.

Other plants again have, in addition to seeds that germinate in the

first year, other seeds which require a longer rest : Laburnum, wild

Mignonette, and field Clover are stated to be among these, while

the Cockleburr (Xanthium), with its two fused fruits, is also said to

germinate one in the first, the other in a later season.

4

Akin to such questions is that of the length of time during which seeds can
retain their vitality. This varies in the individuals of any sample of seeds
saved under apparently uniform circumstances. As the period is lengthened
the proportion of seeds that germinate diminishes. Oily seeds retain their
vitality a shorter time than starchy seeds ; and those kept dry retain it
longer than those kept damp. But to extend this possibility of survival
to the so-called "mummy-wheat" is too long a step to seem probable. A.
de Candolle, after examining the evidence up to 1882, concluded that no

THE FRUIT AND SEED-DISPERSAL

i -> "

grain taken from an ancient Egyptian sarcophagus and sown by horticul-
turists has ever been known to germinate : nor is there any trustworthy
evidence up to the present date. The condition of grains known to have
been taken from mummies is as though they were charred, and the germ
perished. On the other hand, seeds of Nelumbium from Sir Hans Sloane's
collection were germinated by Robert Brown after being dry for at least
1 20 years.

Low temperature arrests physiological activity : but Brown and Escombe
showed that the vitality of seeds is not destroyed by exposure even to extremes
of cold. They submitted seeds of twelve plants of different affinities to a
temperature varying from 183 to 193 degrees below zero centigrade, for over
100 hours. As a result their powers of germination showed no appreciable
difference and they produced healthy plants.

Cycle of Life.

The germination of the seed, and the re-establishment of the
sporophyte as its result, completes the normal cycle of life of the
Flowering Plant. The leading incidents of that cycle may be repre-
sented by a diagram, which will serve later as a means of ready

VeCPsop',

Sporophyte,
(Diploid)

Microspore. Mecaspore.

CoNTOrrsOr

Embryo -Sac

(Haploid)

CONTENTS Of

Pollen-grain -tube

Fig. 257.

comparison with types of vegetation lower in the scale (Fig. 257). It
will be found in them all that the leading events succeed one another in
a sequence that is uniform, however different the details may appear.
Two critical points in the cycle are marked by the fact that the
individual life is there presented in each case by a single cell. They
are the Spore and the Zygote : the former follows on reduction, and

336 BOTANY OF THE LIVING PLANT

haploid ; the latter results from fertilisation, and is diploid.
Between these, and derived respectively from them, are two phases
of cellular amplification, each forming a soma, or plant-body. The
one is the Sporophyte, or rooted plant, which springs from the Zygote,
and is diploid ; the other is rudimentary in the Flowering Plants,
though it is more fully represented in lower forms. It is initiated
by the haploid spore, and is itself haploid. It consists here of only
a few cells contained on the one hand in the embryo-sac, on the
other in the pollen-grain and tube. It is called the Gametophyte.
These two phases together constitute the alternating Cycle of Life
of the Flowering Plant, which thus shows vestigial relics of an
Alternation of Generations.

This succession of events may often be obscured by vegetative
propagation, by means of buds originated in various ways. But, as
shown in Chapter XIII., this is a mere process of extension, or repeti-
tion of the individual sporophyte. It appears as though it extended
the cycle, but it does not really introduce any new feature. It may

£ Sporophyte.

*5P0R0PHYT£,

Iegaspomnciun. * Embayo

MlCMSPORANGIUM f ^im

Microspore. Megaspore.

Fig. 258.

be fitly represented as a supplementary cycle outside the main
diagram (Fig. 257).

The doubling of part of the main cycle indicates sex-differentiation,
the male and female developments running parallel. This is a feature
which is very prominent in Flowering Plants, and for them the cycle
is thus doubled for about three-quarters of its extent It will be found
on applying a similar method successively to plants lower in the scale
that in them the sexual differentiation becomes progressively a less
marked feature. But meanwhile it is important to note that the
extent of this differentiation is not itself constant in Flowering Plants.
In certain plants the flowers are hermaphrodite, as in the Buttercup.
The diagram as in Fig. 257 applies to such cases. The sex- differentia-
tion appears in them only on formation of the pollen-sacs and ovules,
which are in the same flower. But in others the plant is itself either
male or female, and the species dioecious, as in the Willow. This

THE FRUIT AND SEED-DISPERSAL 337

condition may be represented by an amended diagram (Fig. 258).
For us here the point of importance is this. That the stage at which
difference of sex is first recognisable in the individual life is not fixed
for all Flowering Plants. In hermaphrodite plants it appears only
in the several organs of the individual flower. In dioecious plants the
difference extends to the whole sporophyte plant. These observations
will be of value for comparison with forms lower in the scale of
vegetation. But such questions cannot be fully treated till these
organisms have been described. Nor can the origin and real nature
of that complex body, the seed, be properly understood till it can be
examined comparatively. A knowledge of the propagative methods
of more primitive plants must first be acquired ; and the comparisons
which follow from it will be taken later (Chapter XXXIV., p. 543).

B.B.

CHAPTER XX.

EVOLUTION, HOMOPLASY, ANALOGY, AND HOMOLOGY.

In the preceding pages the Higher Plants have been described with
only occasional allusion to organisms which, being simpler in their
structure and mode of life, are placed lower in the scale of the Vege-
table Kingdom. In taking the Highest Plants first, a similar sequence
has been followed to that which has ruled in the development of the
Science itself, as its history plainly shows. It was natural that the
earlier observers should direct their attention to those plants which
were most obvious, and seemed to them to be the most useful and
common. Consequently the structure of the science was founded at
first upon observation of Flowering Plants. Later, when botanists
began to examine smaller and simpler organisms, the tendency was
to interpret them in terms of the Higher Plants. But this procedure
is a practical inversion of the course which the real history of the
Vegetable Kingdom is now believed to have taken. Comparison of
the simpler organisms with the more complex has led with ever
increasing certainty to the conclusion that the latter were derived by
Descent from the former. The simpler organisms are now held to
represent types such as appeared earlier upon the Earth. The more
complex types, on the other hand, are held to have been derived
from ancestors simpler than themselves, and to have been later in
their origin. This being the view generally entertained, it will be
well for us to break off for the moment from the discussion of the
higher forms, and to set back to those lowest in the scale. By doing
so, and then by proceeding gradually to those which are more com-
plex, it will be possible to trace in general those lines along which
Evolution appears to have progressed in producing the Higher Forms
of Vegetation.

While we thus consciously adopt the general conclusion that the

338

EVOLUTION, HOMOPLASY, ANALOGY, HOMOLOGY 339

origin of the various forms of Plant-Life was by gradual Evolution
through long ages of Descent, it must be clearly understood that this
is nothing more than a theory. Evolutionary History as a whole is
not, and cannot be, a result of actual demonstration. It has not
been possible to produce detailed proof of the evolutionary history of
any wild species, or genus, whatever the probability may seem to be
as to the source from which it sprang. But so many facts can be
explained by the theory of Evolution that it is now generally accepted
as giving the most intelligible account of the probable origin of the
Organic World as we see it.

The evidence upon which its probability rests, is derived (i.) from
Comparisons based on external form and internal structure : (ii.) from
a study of the Ontogeny, which means the history of development of
the individual from the egg to the adult : (iii.) from Palaeontology,
which means the study of fossils in their stratigraphical sequence : this
demonstrates their Distribution in Time : and (iv.) from the evidence
given by the Geographical Distribution of organisms upon the Earth's
surface at the present day. If the theory be true, the results derived
from these several methods of enquiry should coincide, and that
coincidence should apply not only for Plants, but for the Animal
Kingdom as well. It is the high degree of coincidence seen in the
evidence derived from all of these sources, for the two Kingdoms
of Living Beings, which has led to the general acceptance of a Theory
of Evolution.

The facts of Variation and of Heredity constitute the basis upon
which Evolution is believed to have worked. In Plants, as in
Animals, the offspring shows a general resemblance to the parents.
But that resemblance, however close, is not exact. The individual
variations from type may be greater or less : often they are very
slight indeed. They differ also in respect of their hereditary trans-
mission to a succeeding generation. Some are not transmitted, so
far as present evidence goes, and consequently they are not available
in Evolution. These are called Fluctuating Variations or Modifications.
Others are evidently heritable and are transmitted to the offspring ;
these are called Mutations. The problem of variation is dealt
with more fully in Chapter XXXV. In the meantime it may be
noted that any process in Nature that would bring about a
summing together or accumulation of the heritable differences which
exist would tend to establish in the race such differences as characterise
species, genera, and successively larger groups. Such a process is
found to be actually at work in Natural Selection. It produces its effect

340 BOTANY OF THE LIVING PLANT

by rejection of the less fit, and survival of the fittest. Its recognition as
a feature affecting all wild life was a novel factor introduced into
biological Science by Darwin.

The efficiency of Natural Selection as a means of accumulating
new characters depends upon the fact that the production of germs
is carried on at a higher rate than would suffice simply to restore
losses by death. This has already been shown for Flowering Plants
in Chapter XIX., p. 332, where extreme cases of prolific propagation
have been quoted. The lower organisms as a rule reproduce pro-
lifically also, and often with extreme rapidity. An overplus of young
individuals is indeed a general feature of plant-life. Since each of the
offspring is normally capable of reproducing as prolifically as the
parent, if all came to maturity the numerical increase would be in
a geometrical ratio. But all of the individuals produced do not and
cannot come to maturity, so as again to propagate their species.
There could never be room or opportunity for nutrition of them all
upon the earth's surface. Their number leads to a competitive
struggle for existence. In ordinary vegetation the number of estab-
lished plants remains approximately constant in any given area as
the result of that struggle. If this be so, the average mortality of
the offspring of an annual would involve all of the germs of each
season but one : and in the case of perennials only a fraction of that
number of survivors would come to functional maturity. It is upon
the enormous overplus of individuals produced that Natural Selection
works. The most likely to survive are the strongest, or those which
present some favourable variation from type. Those that are weaker
or less suited to their environment are the more likely to succumb.
The result of Natural Selection will therefore be the Survival of the
Fittest together with the perpetuation of favourable variations from type.

A special Mechanism of Heredity is shown to exist by the facts of
sexual reproduction, as seen in the higher types both of Plants and
of Animals. Such reproduction is referable to the sexual cells alone.
In Flowering Plants it has already been seen that the new individual,
with all its characteristics of structure and function, is derived from
the ovum fertilised by the male gamete. These cells must then be
the bearers of all those characteristics which are transmitted from
the parent to the offspring. For the maturing of the sexual cells,
and the part which their nuclei are believed to take in the segregation,
transmission, and distribution of hereditary features, reference must
be made to Chapter XXXV., where the details of the chromosome-
cycle are given for the Higher Plants, together with a brief account

EVOLUTION, HOMOPLASY, ANALOGY, HOMOLOGY 341

of Mendelian segregation and its implications. Though the corre-
sponding facts have not yet been recorded for the lower organisms
with the same detail as for the higher, there is reason to believe that
they are of general application wherever sexuality exists : for the
details so far observed in the lower resemble closely those for the
higher forms. The conclusion may then be applied generally that
the sexual cells are the bearers and the distributors of hereditary
characters.

But there are many simple organisms, often unicellular, in which
no specialised phenomena of sex are known. They may show a
multiplicity of forms ranked as genera and species, notwithstanding
that they are sexless. This suggests that the absence of a specialised
mechanism of heredity does not entail an absence of variation, nor
even of the hereditary transmission of characters. Probably such
simple organisms as these are really primitive. Sexual repro-
duction developed as a later condition and brought with it various
advantages : among the most important is the orderly distribu-
tion of the hereditary characters (Chapter XXXV.). It thus appears
that a Theory of Evolution is of general application to organisms
higher or lower in the scale, and that it finds its true basis in the
facts of variation. An essential feature is that certain structural
variations are heritable. By Natural Selection these are sifted out,
perpetuated, and even accumulated. Other factors may also be at
work : but these are commonly held to be the most important. The
central problem of Evolution will therefore lie in the origin of those
variations that are heritable : while the mechanism of their transmission
to, or distribution among, the offspring naturally takes its place as
accessory.

Using some such evolutionary theory as a working basis, a
succession of organisms may be passed in review, starting with those
that are relatively simple and proceeding to those that are more
complex. But in doing this it is necessary to remember that Evolu-
tion has not been a simple matter. Organic life has progressed along
a great number of distinct Lines of Descent, which may have been
divergent, parallel, or convergent as regards their characters. More-
over, while the progressions which may be traced will in the main be
from simpler to more complicated states of organisation, the converse
may sometimes be the case. It has been seen in Chapter XII. how
parasitism in Flowering Plants, which in part or in whole relieves
them of the function of self-nutrition, often leads to a reduction
of the vegetative system. It has been noted that Cuscuta has

342 BOTANY OF THE LIVING PLANT

dispensed with its foliage leaves as a consequence of its parasitism.
An extreme case is seen in Rafflesia with its huge flower, while its
absorbent vegetative system has been reduced almost to the level
of that of parasitic Fungi (p. 227). Thus a simpler structure may
sometimes appear as a consequence of evolutionary changes, though
in the great majority of cases evolution leads to greater complexity.
Such considerations as these must be borne in mind while examining
and comparing organisms from an evolutionary point of view. Many
other precautions are also necessary before Lines of Descent can be
traced with any degree of probability. If we figure to ourselves the
whole plan of Descent of Plants as a highly ramified tree, or group
of trees, the great majority of living species would correspond only to
the distal twigs, while their connections downwards to earlier branches,
which represent their ancestry, are mostly wanting. At best they are
suggested perhaps by a few isolated, archaic forms, or vaguely sketched
in by occasional fossil remains of earlier time. It may even be a ques-
tion whether there is really any connection downwards to a single
trunk : for it is quite reasonable to suppose that the Evolution of the
Vegetable Kingdom may have been not monophyletic, that is, along
a single main line or phylum, but polyphyletic, that is, along a plurality
of lines. Notwithstanding the uncertainty on points even so impor-
tant as this, a belief in Evolution appears to be fully justified by the
facts. Origin by Descent should therefore form the constant back-
ground to any intelligent comparison of living Plants when taken in
progressive sequence, leading as it habitually would from those which
are relatively simple to those which are more advanced.

Homoplasy.

In Chapter XI. (p. 193) special modifications of form which lead
to successful life under special conditions have been designated
adaptations. There is reason to believe that they have arisen in the
course of Descent in relation to those conditions : in fact, that the
plants which show them have been adapted to their environment. If
two or more races of plants quite distinct from one another in Descent
have developed independently under similar conditions, and been
adapted thus to those conditions, a similarity of contour or of structure
may be expected to result. A parallelism of development, or as it
is called Homoplasy, would then be seen. Such similarities in distinct
races would be described as homoplastic. Having been distinct in origin

EVOLUTION, HOMOPLASY, ANALOGY, HOMOLOGY 343

throughout, such similarities are clearly no real evidence of relation-
ship of the plants which show them. A good example of this is seen
in Cactus and Euphorbia, two genera widely apart in the System of
Flowering Plants. Both of these genera show turgid succulence of
the stem and reduction of their leaf-area as an adaptation to con-
ditions of drought (Fig. 141). However similar to one another in
vegetative structure such plants may appear to be, the difference of
their floral characters shows that they are not really akin. More-
over, the succulent Cacti are characteristic of the American Con-
tinent and the Euphorbias of Africa. Thus both their floral structure
and their distribution suggest that their vegetative resemblance
indicates homoplasy. Similar arguments will apply to the climbing
habit by means of twining stems or tendril-like leaves : to the spinous
development of stems and leaves : to parasitism : to zygomorphy in
vegetative shoots and in flowers : to gamopetaly and epigyny : and to
many other familiar features that have been already noted in Flower-
ing Plants. Such characteristics cannot safely be held as evidence in
themselves of affinity of the plants which show them, since they may
appear independently in plants otherwise quite distinct. They are
often merely examples of Homoplasy. Such homoplasy, or parallel
development, is thus a famil;ar fact in Flowering Plants. In the study
of the lower organisms the observer should be prepared to meet with
similar consequences of adaptation, resulting from a like accommoda-
tion to the same conditions of life in organisms not closely related by
Descent. He should be willing to agree that in them also apparent
similarity of characters may not be a sign of affinity, but only of
homoplastic adaptation. Especially will this be so for the vegetative
system, which is not so reliable a guide as the more stable organs of
propagation. All parts are, in point of fact, subject to adaptation
under the varied circumstances of life. Consequently it is not upon
single characters, which may be only homoplastic adaptations, but
upon the sum of all the characters, external and internal, that the affinity
of plants, whether high or low in the scale, is to be judged. This is
the broad base upon which a Natural System of Classification should
rest.

Homology and Analogy.

The fact of widespread homoplastic adaptation makes it necessary
to have some more reliable basis for the classification of the parts
of the Plant than mere comparison of their external form or even
of their internal structure. Such classification of parts must be based

344

BOTANY OF THE LIVING PLANT

upon their origin, and upon the place which they take relatively to
other parts at the time when they first appear. Following this
method, those parts of the individual, or of different individuals, species

Fig. 259.

Shoot of Butcher's Broom (Ruscus aculeatus) after Figuier. Note the small
scale-leaves in the axils of which the flattened axes (phylloclades) arise ; also the
flowers arising from the surface of the phylloclades in the axils of other scale-leaves.

or genera, are distinguished as homologous which have the same relation
to the whole plant-body, whatever their function or external conditions
may be. On the other hand, parts may resemble one another in form
or in function, though they may differ in their relation to the whole
plant-body. Such parts are described as analogous one with another.

EVOLUTION, HOMOPLASY, ANALOGY, HOMOLOGY 345

Examples of analogy of parts have already been noted : for instance,
in those woody climbers which support themselves by reflexed hooks
(Fig. 143). These hooks may actually be axillary buds (Sageretia,
Carissa), or reflexed pinnae (Desmoncus), or recurved stipules
(Zizyphus), or merely superficial prickles (Lantana, Calamus). Func-
tional similarity in parts of diverse origin is seen also in tendrils,
which may actually be axes (Vitis, Bauhinia), or leaf-blades (Lathyrus
aphaca, Fig. 148), or distal parts of the leaf (Cobaea, Vicia cracca),
or lateral appendages at the base of the leaf (Smilax). Again, spinous
development may appear in stems (Crataegus), or leaves (Berberis),
or stipules (Acacia), or in roots (Ac author hizd) . On the other hand,
axillary buds may develop flattened and leaf-like, as in the " phyllo-
clades " of Ruscus (Fig. 259). But their position, and the fact that
they bear flowers proclaim their shoot-character, notwithstanding
their leaf-like appearance. Such examples might be indefinitely
multiplied, showing how common is homoplasy in parts that are
only analogous, and not really comparable one with another in point
of origin and position.

There may be various degrees of that closer correspondence of
parts which would justify their being held as homologous. The
strictest conception of homology is that designated Homogeny. Lan-
kester defined as homogenous those structures which are genetically
related in so far as they have a single representative in a common
ancestor. This definition implies repetition of an individual part
bearing a definite relation to the whole organism, just as the hand of
a child repeats in position and in its qualities the hand of the mother.
Clearly, in Plants with their continued embryology, the recognition
of such individual correspondence is rarely possible. The indefinite-
ness in number of the appendages produced by any growing shoot
precludes it. And yet the leaves of a shoot are comparable in other
respects. Thus it is only in a less stringent sense than homogeny that
the homology of the foliar appendages can be admitted in organisms
which produce an acropetal succession of them as flowering plants do.

It may be suggested that the successive production of appendages
on the growing axis of a plant finds its correlative in the " serial
homology," or, as it has been called, the " homodynamy " seen in
animals. But the " segments " of the animal body have not their
counterpart in the plant-body. Attempts have been made, it is
true, to reduce the shoot to an articulate series of constituent parts,
each consisting of a portion of the axis bearing one or more appen-
dages. But though it is possible to analyse some few shoots in this

346 BOTANY OF THE LIVING PLANT

way, the method is not generally applicable, and the analysis itself
bears evidence of its artificiality. It should be realised that the
evolution of the Higher Animals and of the Higher Plants has pursued
distinct lines. The two Kingdoms cannot be assumed to have
adopted like methods of advance.

In Plants lower in the scale the categories of parts are less clearly
defined than in the Higher Flowering Plants, and in the simplest
types the several parts cannot be distinguished at all. This suggests
that such categories of parts do not represent any essential plan of
construction applicable to plants generally, but that they are con-
sequences of progressive evolution in organisms that meet in a similar
way the requirements of a life common to them all. It raises also
the question whether on the one hand all the parts classed together,
such as leaves, had really a common origin by Descent. There is
reason to think that leaf-like bodies have originated separately in
numerous distinct phyletic lines : for instance, among the Higher
Algae, in the Bryophytes, and in Vascular Plants. In fact, while
we group those parts that are held as homologous into certain morpho-
logical categories, such as leaf or axis, these are not to be held as
definite or of universal application. They are rather to be regarded
as being based upon such uniformity of result as has been achieved
in Descent by various types of organisms. This view gives a true
or evolutionary basis to the distinction between homology and
analogy. Those parts are held to be " homologous " which had
fallen relatively early in Descent into such relationships as char-
acterise that category of parts to which they are referred : those
are held to be only " analogous " which have undergone their specific
modification relatively late in Descent, after the characters defining
their morphological category had already been acquired.

Morphological Categories of Parts.

Classifying the parts of the Higher Plants by the relations which
they bear to the whole plant-body, and not merely according to
function, they are found to fall into certain categories, and the parts
so grouped together are held to be " homologous " one with another,
whether they be borne on the same or on different individuals.
The seedling plant consists of Root and Shoot (Figs. 2, 4). The
primary shoot is the whole product of the plumule on germination
and the primary root is the product of the radicle of the seedling.
Each of these parts is distinct from the other in origin and character,

EVOLUTION, HOMOPLASY, ANALOGY, HOMOLOGY 347

and each is capable of indefinite amplification and of the production
of parts similar to itself. Since these regions of the plant are distinct
in origin, and in their relation to the whole plant-body, no homology
can be recognised between them or tfreir parts. The distinction
between these regions seen in the Phanerogamic seedling is of wide
application. Root and Shoot constitute the fundamental categories of
parts seen in Vascular Plants, and to one or other of them all the parts
of the vegetative system may be referred.

The Root presents few morphological difficulties, its cylindrical
shape being remarkably uniform. It has been shown in Chapter VI.
how lateral roots originate endogenously and in acropetal succession
on the main root. They repeat its characters, and these may be
repeated again in roots successively of higher order. However
complex the root-system may become, it is easily analysed, and all
its branches are held as equally of root-nature and homologous.

The Shoot being both complex and variable in the relation of its
parts, presents many morphological problems. The fundamental
relations of its constituent parts, the axis and leaf, have been defined
in Chapter V., p. 69. The relations of the vegetative and floral
regions have been discussed in Chapter XIV., p. 277. Both are
referred in origin to the " general-purposes-shoot," from which they
are held to have been derived by a process of segregation. It may also
be held that scale and foliage leaves, bracts and bracteoles, and the
successive series of sepals, petals, stamens, and carpels are all types
of foliar appendage borne upon the axis. They are all so far homo-
logous that their relations to the axis which bears them are those
already defined : for they arise as lateral outgrowths from the
apical cone, springing from superficial and underlying tissues. They
appear in acropetal succession, and do not as a rule repeat the char-
acters of the axis which bears them. Their apparent differences
depend upon the fact that they are specialised in their development
to serve particular purposes in the plant.

The shoot, thus composed of a simple axis and leaves, is the unit
from which even the most complex plant-bodies are built up. In
carrying this out, the shoot-unit may be multiplied in three ways :
(i) by axillary branching, (ii) by distal branching, and (iii) by adven-
titious buds.

The first (i), that is axillary branching, is the common type for
Flowering Plants, a new apex being constituted in the angle between
leaf and stem, and the new shoot repeating the characters of the
original one (Figs. 5, 6).

348 BOTANY OF THE LIVING PLANT

The second (ii), viz. distal branching, arises from the apical cone
above the youngest leaves and independently of them. It is not typical
of the Higher Flowering Plants, but it is a marked feature in the
Pteridophyta, and it is well seen in Lycopods and Ferns, and also
in the Algae (Fig. 289). It commonly results in the formation of
two equal shoots in place of one, by dichotomy. But various stages
of inequality of the forking are found which lead from equal dichotomy
to definite dominance of one shank over the other ; so that a mono-
podial branching may result, where one shank takes a definite lead
over the other, and actually precedes it in time. It is regarded as
probable that an association of the weaker shank of a distal branching
with the base of the next lower leaf may have been the actual source
of axillary branching. In this case the two types of branching are
not essentially different from one another, and both may be held as
repetitions of the original shoot.

The third (hi), viz. adventitious budding, has already been described
in Chapter XIII., p. 246. Whether produced naturally or artificially
induced, the buds appear in abnormal position and number, and are not
to be held as parts of the shoot-unit but as accessory, resulting from
a new growth-stimulus. Adventitious roots may also be formed at
various points on the shoot-system, but neither are they to be held
as parts of it. Adventitious buds and roots may both be regarded
merely as repetitions of parts by establishment of a new growth-
stimulus at points which do not follow the usual sequence. The
parts produced may be held as generally homologous with those
which appear in the regular succession.

The leaf of Dicotyledons varies greatly in size and form. It is
commonly described as consisting of Lamina and Petiole, with or
without stipules (Chapter V.). While this treatment may be con-
venient in relation to its functional activity, it does not give any
satisfactory morphological insight into the construction of the typical
leaf of a Dicotyledon. It is in reality a simple Rachis, or a modified
branch-system based on it. That this is so may often be clearly seen
from the cutting of the margins of the blade, or in its venation
(Figs. 260-3). The fact is, however, disguised by (1) the dorsiventral
structure, which leads to the branches, or pinnae, being ranged in two
lateral rows : (2) by the frequent condensation of the branches borne
by the rachis into a coherent blade : and (3) by the intercalation in
the course of its development of a petiole of greater or less length. In
order to place the morphological treatment of a Dicotyledon leaf upon
a rational basis the rachis should be distinguished from the pinnae and

EVOLUTION, HOMOPLASY, ANALOGY, HOMOLOGY 349

stipules, and these from the pinnules, or branches of higher order. It
will then be seen that the axis bears to the rachis a like relation to that

Fig. 260.
Pinnate leaf ot Robima, after Figuier.

Fig. 261.
Doubly pinnate leaf of Gleditschia, after Figuier.

which the rachis bears to the pinnae, and, in doubly pinnate leaves, the
pinnae to the pinnules (Figs. 260, 261). The construction of the whole

35o BOTANY OF THE LIVING PLANT

shoot thus becomes intelligible, and comparison is facilitated with the
stem and fronds of the Pteridosperms, Cycads, and Ferns, in which

Fig. 262.
Leaf of Sweet Chestnut (Castanea vesca), after Figuier. Here the fundamental
pinnate branching is shown by the veins, but the whole is condensed into a coherent
blade.

the leaf is obviously a branched system (Chapter XXXI.). Rachis,
pinnae, and pinnules are in fact categories of parts of the shoot which
rank naturally with the category of the axis itself. They all represent

Fig. 263.

Stages of transition between scale and foliage leaf, in Ribes, after Figuier.

correlative grades in the branching of the shoot as a whole. But the
distal branch-system of a leaf thus composed is so often greatly

EVOLUTION, HOMOPLASY, ANALOGY, HOMOLOGY 351

condensed, by marginal fusion of lobes and veins, that it appears
collectively as a unit, and so it seems to justify its designation col-
lectively as the " lamina " (Figs. 262-3).

Explanatory terms for the three regions, into which the leaf appears to be
divided in many Dicotyledons, help in upholding this view of it as a branch-
system. The rachis springs laterally from the stem : and three parts of it may
be commonly distinguished by their form and function, as :

(1) The hypo-rachis or leaf -base, to which the stipules are attached, if present ;
though frequently there are none.

(2) The meso-rachis or petiole, which results from an extension of a middle
region intercalated between the leaf-base and the blade. If the intercalation
of the petiole begins above the extreme base the position of the stipules
suggests that a first pair of pinnae had been left behind in the process.

(3) The epi-rachis, which embodies the whole of the distal branch-system.
It is composed of lateral rows of potential pinnae, or of these either separate
or fused in various degrees laterally, to form the flanges of a coherent blade or
lamina (Figs. 260-3).

The value of such terms is that they fix the attention on the branch-system
as a whole, though its branching may be obscured, or even absent in many
flowering plants, especially where the leaves are small,
and in the Monocotyledons (Fig. 264). A diverse
development of the individual leaves of many Di-
cotyledons may accentuate the character of these three
regions, even in the leaves of the same individual
plant. For instance those of Ribes (Fig. 263) : for
they are differentiated in form according to their
function. To the left in the figure is the normal foliage
leaf, with its sheathing base, its petiole, and its
lamina, all present. In the leaf to the right the
protective hypo-rachis is largely developed, and the
other parts are vestigial : while the two middle figures
show the petiole, or meso-rachis, in various degrees
of interpolation between the other parts. Thus each
region of the leaf may be specialised to perform its
several functions : (1) bud protection, (2) adjustment
of leaf-mosaic to avoid overshadowing, and (3) ex-
posure of receptive surface for sunlight. But all
three involve parts of one rachis.

The leaf of a typical Monocotyledon, such as Funkia (Fig. 264), presents a
distinction of sheathing base and blade similar to that in many Dicotyledons.
But the broad expanse of the latter originates not by fusion of pinnae, but by
lateral expansion of mesophyll between the parallel veins of the distal part of the
rachis, which is here unbranched. Thus we see that the blade of Funkia is not
the developmental homologue of that of an ordinary Dicotyledon, but rather
its analogue.

Doubts may thus arise as to the strict correspondence between leaves of
Dicotyledons and Monocotyledons. But when we come to the leaves of Mosses
and Liverworts any doubts of strict homology with those of Flowering Plants

Fig. 264.

Funkia grandiflora, upper
part of petiole and ' ' iamnia ".
(Reduced after Mrs. Arber.)

352 BOTANY OF THE LIVING PLANT

approach to a certainty. For they are parts of the gametophyte or haploid
phase, not of the diploid sporophyte, and there is reason to believe that those
two somatic phases have been distinct throughout Descent (Chapter XXX.)-
Further, any strict homology between the leaves of Vascular Plants and the
leaf-like' parts seen in highly organised Algae must be held as still more doubt-
ful (Chapters XXI. -XXIII.). Before any such comparisons can be accepted
as having strict morphological value, the argument must be based not on
general preconceptions or surmises, but on the demonstration of similar
evolutionary sequences, among- organisms nearly related to one another.
Hitherto this has not been done.

The truth seems to be (i) that in plants at large, whether in the sporophyte
phase or in the gametophyte, advantage has been taken of the evolutionary
development of lateral flattened surfaces, and their expansion as organs of
photosynthesis; (2) that such development may have arisen either in the
diploid or in the haploid soma : and (3) that it may have arisen independently
along a plurality of evolutionary lines. In other words, that the development of
the category of leaves has been widely polyphyletic.

Minor appendages, in the form of Hairs and Emergences are pro-
duced irregularly, and often in large numbers, scattered over the sur-
faces of the shoot. They may appear at any point on axis or leaf, and
also on the surface of roots, as root-hairs (Figs. 56, 57, 60, 61, 68).
The Hair is usually defined as a product of the epidermis only : but
the Emergence as involving also the subjacent tissue. This is, how-
ever, an arbitrary distinction, though it separates roughly the more
delicate from the coarser. As a morphological category hairs and
emergences take a quite subsidiary place, and do not rank equally in
position or constancy with the leading categories of axis, rachis,
pinna, or pinnule.

The sporangia have been described for the Higher Flowering Plants
in Chapters XV. and XVI. : those of Pteridophyta and Bryophyta in
Chapters XXX. to XXXII. Our question will be, what relation do
sporangia bear to the categories of the vegetative parts on which they
may be seated. As regards the Higher Plants their origin has been from
time to time referred to Metamorphosis of various parts of their
vegetative system. But Von Goebel, in 1881, laid it down, on the
basis of wide comparison with those plants lower in the Scale of
Descent, that Sporangia are organs sui generis, as much as are
shoots and roots : and that they are not referable, through metamorphosis,
to any other category of parts, whatever their position relatively to these.
That view may be generally adopted, and recent palaeontological
discoveries have tended to confirm it. For instance the Bryophyta
with their leafless sporogonia, have been traced back to the Carboni-
ferous Period, while the Psilophytales, including the leafless Hornea

EVOLUTION, HOMOPLASY, ANALOGY, HOMOLOGY 353

(Fig. 372a), belong to the Devonian Age. Both of these suggest that
there is no need to call in " metamorphosis " to account for the early
establishment of spore-bearing parts. These fossils show that in them
tetrad-division has been an earlier and more stable feature of the Hof-
meisterian cycle than the evolution of leaves themselves. (See Chapter

XXXIV., p. 545-)

The sporangia of the Higher Plants vary in size, disposition and
number. But however inconstant in these points, there is essential
uniformity in the development of their contents, which when mature
are represented by the pollen-grains (microspores) and the embryo-
sac (or megaspore) (Chapters XV., XVI.). In either case a fundamental
feature in their production is the tetrad-division of the spore-mother-
cell. (See pp. 282-3, and 296-7.) This event involves Meiosis, and is
inherent in spore-formation, not only in the Higher Flowering Plant
but also in such of those lower in the scale as possess sexuality. In
fact syngamy and meiosis may be regarded as correlated phenomena,
which recur constantly, not only in the Higher Plants but also in those
lower in the scale — as will be seen in Chapters XXI. to XXXIII.
This is in itself further evidence of the homology of sporangia at large.

In the lower forms of vegetation, more clearly than in the specialised
Flowering Plants, another category of parts is seen, viz. the gaynetan-
gia, or sexual organs, which produce respectively the male and female
sexual cells or gametes. In all but the most primitive plants the game-
tangia may be readily distinguished as male and female. The former
are called antheridia and they produce spermatozoids, which are usually
numerous from each ; the latter are called oogonia or archegonia, and
they produce one or more eggs respectively. The gametangia are
described for the leading types of Pteridophyta and Thallophyta in
Chapters XXI. to XXXIII. In the more primitive types, and par-
ticularly among the Algae, the gametangia are all alike : but in the
more advanced they differ according as they are male or female. Hence
it may be concluded that they are all homologous, though differen-
tiated in relation to sex, and to the act of syngamy. The events of
syngamy or fertilisation, and of meiosis or reduction appear as com-
plementary features, alternating in the Hofmeisterian Cycle (Fig. 427).
This is seen most obviously in the Archegoniatae (Chaptejs XXX. to
XXXII.) . In the Algae and Fungi it is less obvious, owing to their
smaller size, and their more rudimentary sexual differentiation. On the
other hand, in the Phanerogams the process of syngamy is obscured,
partly by simplification of the parts directly involved in their sub-
aerial fertilisation. Notwithstanding these differences, as seen in the

BB Z

353A

BOTANY OF THE LIVING PLANT

whole evolutionary sequence, there can be little doubt of the Homology
of the sexual process in them all, and in the gametangia which produce the
gametes themselves.

The determination of all questions of homogeny, homology, and
analogy must ultimately be based upon a knowledge of Descent.
Until the phyletic lines for any series of plants are demonstrated the
recognition of such relations of their parts cannot rest on more than
carefully balanced opinion. A comparative examination of organisms
lower in the scale, and ultimately a grouping of them into phyletic
sequences, will therefore be necessary before a final basis can be found
for the classification of their parts. The point of view from which
their study is at present to be approached is that they will supply a
basis, however imperfect, for such a classification. This will not only
help to explain their own mutual relations, but also to illuminate the
morphology and classification of plants higher in the scale of Vegeta-
tion. It is only by such comparative study that the details of the

structure and propagation seen in
the Higher Flowering Plants can be
reasonably interpreted. In par-
ticular, the Seed, which is the most
distinctive feature of the Higher
Plants, cannot be properly under-
stood unless it be shown by com-
parison with more primitive plants
how that very complex body came
into existence.

Sporangium

Phylloid

~ Telome

Sporangienstand PhylLoidstand

Telomstdnde
Fig. 264A.

Diagrams of "Telome und Telomstande,"
after Zimmermann ; fertile to the left, sterile
to the right.

Telome.

The term " Telome " has been re-
cently introduced by Zimmermann to
connote a category additional to the
parts of the sporophyte already recog-
nised in this chapter as taking part in
the organisation of cormophytic plants
(see Zimmermann " Phylogenie der
Pflanzen." Jena, 1930, pp. 58-70). The
telome is associated specially with spore-
production, and is in fact a primitive
spore-producing organ, whether the in-
dividual be sterile or fertile. Its recog-
nition tends to clarify the morphological
analysis of plants of complex construc-
tion. But it is most readily recognised

EVOLUTION, HOMOPLASY, ANALOGY, HOMOLOGY 353B

in certain ancient fossils of relatively simple form, such as the Psilophytales
(Fig. 3 72 a) : while each sporogonium of a Bryophyte might also be held as an
isolated telome (Fig. 352).

The fertile telome consists of a distal tract containing spores, seated on a
stalk usually traversed by a vascular strand that conveys nutrition from below.
Here as elsewhere the function of nutrition precedes spore-formation, hence the
distal position of the fertile cyst, or sporangium. This biological succession of
events underlies the scheme of development of all self-nourishing vegetation.
It has been further suggested that the whole wealth of form that characterises
living cormophytes may have been built up from such telome-units, by branch-
ing and general elaboration that has resulted in the vegetative system that bears
the fertile region distally.

The detailed realisation of cumulative growth in producing from individual
telomes complex bodies, such as are seen in the Higher Plants, has been imagined
as yet, rather than demonstrated by actual developmental observation, or by
comparison of successive stages of branching. And a vast amount of compara-
tive study, of early fossils as well as of living plants, will be required before
such an origin could be accepted as other than frankly speculative, rather than
demonstrational.

DIVISION II.

THALLOPHYTA.

CHAPTER XXL

THALLOPHYTA.

Introductory.

The Angiosperms have been described in some detail in the preceding
Chapters. They form the chief constituent of the Flora of the Land,
and are recognised as the highest types of Plant-Life. But the
Gymnosperms, Pteridophyta, and Bryophyta are also prominent
features in Land-Vegetation. For instance large areas are covered
by Pine Forests : Ferns, Club-Mosses, and Horsetails are world-wide
in their spread : and Mosses and Liverworts are present in quantity
wherever there is sufficient moisture for them to nourish. These
large Divisions of the Vegetable Kingdom are held to occupy a middle
position between the highest and the lowest Plants. The consideration
of them will, however, be held over for the present, until that residuum
has been considered which remains after all of these have been excluded.
The residuum comprises the lowest and simplest of plant-organisms.
These are very numerous both in individuals, and in species and
genera. Individually they are often small and inconspicuous, and
many of them are dwellers in water. Nevertheless, on the basis
of their nutrition and of the encystment of their cells they are properly
ranked as Plants. They are collectively designated Thallophyta,
or thalloid Plants, since a general feature in them is the absence of
that differentiation of the shoot into axis and leaf which is charac-
teristic of the higher forms. They include the Algae, Fungi, and
Lichens.

354

THALLOPHYTA 355

It must not be assumed that all the organisms thus grouped under
the common head of Thallophyta are necessarily akin to one another.
They are found to be naturally separable into distinct groups or
phyla. The plants belonging to the several phyla may be so arranged
as to show progress from simpler to more complex forms. Such
sequences probably represent with some degree of accuracy Lines
of Descent. Commonly the simpler terms of these distinct phyla are
more alike than those which are more advanced. Thus the Lines of
Descent are divergent, and the Thallophytes would therefore appear
to represent a brush of phyletic lines radiating outwards from some
simpler source. Though there is at present no trustworthy evidence
that any of the Thallophyta now seen living have themselves achieved
that highest development seen in the Land-Vegetation, many have
advanced far in their evolution. In mere size the Brown Seaweeds
include the gigantic Tangles, which are among the largest of living
organisms. In complexity of propagative method no group of
Plants shows more intricacy than the Red Seaweeds. In physio-
logical resource the Fungi are the most diverse. But each of these
includes simple types, which link up more easily with other classes of
organisms than do the extremes. Each phylum appears to have
worked out its own divergent line of advance independently of the
rest. Some degree of parallelism in the progressions may then be
anticipated, and is actually found to exist.

The readiest basis of distinction of these natural groups of Thallo-
phyta is by colour. The most important cleavage is according
to the presence or absence of chlorophyll, or of some of its deriva-
tives. This separates the Fungi, which have no chlorophyll or
kindred colouring matters, from the Algae, which have. The Algae
again fall into distinct groups on the basis of colour-difference. Those
which have full green chlorophyll, such as is seen in Land-Vegetation,
are designated the Chlorophyceae. Others are characterised by
their olive green or brown colour, which is due to a mixture of pigments
of which chlorophyll is one. It is characteristic of the Brown Tangles,
or Phaeophyceae. A third series have a prevalent red colour also
due to a mixture of pigments, which again includes chlorophyll, and
they are called the Rhodophyceae, or Red Seaweeds. These colour-
distinctions are not absolutely constant but, together with other
characteristics, such as the nature of the flagella, or organs of propul-
sion of the motile stages, and the forms in which storage materials
occur in the cells, they afford a true indication of the several distinct
groups.

J56 BOTANY OF THE LIVING PLANT

It may be objected that, while the Thallophytes are classified by colour,
in Flowering Plants such differences were not taken into account in their
classification. But in dealing with Organic Nature, which has progressed
along individual lines, consistency of method in classification is not possible,
if the grouping is to follow the course which evolution has apparently taken.
The reason why the method adopted for Flowering Plants will not apply
for the Thallophytes is that in the former the change to irregular nutrition
happened late. The seed-bearing parasites are plainly Flowering Plants
that have changed their mode of nutrition. But in the case of the Fungi
we are dealing with a very ancient change. Fungi existed in the Palae-
ozoic Period. Thus their irregular nutrition will have influenced their
development from very early times.

The colourings have a physiological meaning. The absence of chlorophyll
indicates dependent nutrition, as in the Fungi. The colours distinctive of
the three groups of Algae are related to photosynthesis. A brown or red
tint makes self-nutrition possible deep down in sea-water. Speaking
generally, the Red Seaweeds are prevalent at the lower levels while the Browns
extend from the highest levels downwards, but stop short of the greater
depths. The Greens are more widely diffused, but they occur mostly at the
higher levels, and they are the prevalent Algae of fresh water.

If we accept this general view of the Thallophytes, it becomes
a question whether there is any living group of organisms which
represents approximately a source from which they may have
originated. It is a very general opinion that such a source is to
be found among the Flagellatae, a family which it is difficult to
refer definitely either to the Kingdom of Animals or of Plants. It
includes many of those organisms which cause certain diseases in
man and other animals, and these are more definitely animal in
their characters. But others, such as Euglena, possess features
characteristic rather of Plants. Euglena is found commonly in
summer, colouring the foul water that drains from manure heaps
a bright green. The organism is then seen in the motile state, as a
free-swimming, naked protoplast of elongated form, propelled by
a single flagellum (Fig. 265). There is a central nucleus (n), several
green-coloured chromatophores which vary according to the con-
ditions (ch), a contractile vacuole (v), communicating by a canal
or funnel with the exterior, and a red eye-spot or stigma lying at
the junction of canal and vacuole. The flagellum passes down-
wards through the canal, and is attached by a branched base to
the inner surface of the vacuole. In this state Euglena can feed
itself by photosynthesis, but it probably obtains simultaneously
some degree of saprophytic supply from the foul water in which it
lives. When well nourished it may contain large paramylon bodies,
but not starch. It multiplies by fission, the nucleus dividing first

THALLOPHYTA

357

(Fig. 266). When starved for a lengthened period an encysted
form is assumed. The chromatophores diminish in size and colour,

Fig. 265.
Euglena gracilis. A. Form with green chromatophores {ch) ; n = nucleus; v==
vacuole and red eye-spot; g = flagellum. £ = hemi-saprophytic form with small
green chromatophores. C = colourless saprophytic form occurring in nutrient
solutions, in absence of light. D = resting cyst of the form C : r=red eye-spot.
E= germination of the resting cvst of the form A . by division into four daughter cells,
which later escape. (After Zumstein.) (A, C x 630 ; 5x650; D, E ■< 1000.)
(From Strasburger.)

and storage materials appear in the contracted protoplasm, which is
then surrounded by a thick wall (Fig. 265, D). In this state it can

IS

Tic. 2

-C5.

Successive stages of fission of Euglena : semi-diagrammatic.

resist conditions that are adverse. But when these are favourable
again the cyst germinates ; its wall becomes mucilaginous ; the

358 BOTANY OF THE LIVING PLANT

contents usually divide into two or four (E. gracilis), or even more parts
(E. viridis), which, after showing movement within the wall, are finally
set free as naked protoplasts (E.). Some Flagellates show sexuality ;
this has been noted also for Euglena, but needs confirmation.

The common Euglena viridis does not grow well in spring water,
but it flourishes in water containing organic impunities. Probably
photosynthesis is its chief mode of nutrition, but it can also act as a
partial saprophyte. This is more clearly seen in E. gracilis, which
has been shown to be either autotrophic or purely saprophytic accord-
ing to circumstances. Fig. 265, C, shows the colourless form grown in
the dark in a nutritive medium. The chromatophores are reduced
to small pale plastids, but still the organism appears well nourished.
This saprophytic type can then be restored to the autotrophic condition
by exposure to light. It thus appears that certain Flagellates may
temporarily or permanently make use of a saprophytic mode of
nutrition.

Organisms which show characters so versatile suggest several distinct
lines along which evolution is possible ; and those lines if realised
would give rise to features characteristic of the largest groups of
living beings. The motile green form with the capacity for
photosynthesis, if it becomes encysted, loses its motility while it
achieves protection. The encysted form of Euglena after division of
its protoplast is so like certain Algae allied to Palmella that it has
been called the " Palmella- state" The resulting cells remain grouped
for a time. If that state became permanent, and the divisions
numerous, a cell-colony would be formed of a type characteristic for
certain simple types of Plants. But the protoplasts of Euglena may
after division escape and become motile again as primordial, that is
naked cells, a condition which is seen repeated commonly in the
propagative cells of Plants up to the Gymnosperms. Thus the
encysted state of Euglena suggests a possible mode of initiation of the
encysted construction characteristic of Plant-Forms (compare Chapter
X.). But most of them still retain the primitive primordial cell in
reproduction.

The saprophytic mode of life in Euglena — or parasitic, as it is in
many other Flagellates, which are then independent of light and
chlorophyll — suggests a distinctively animal existence. Here motility
is retained, and encystment appears only as an occasional incident.
This behaviour of Euglena, an organism in which sexuality has
not hitherto been verified fully, indicates that the segregation of
Animals and Plants may have antedated sexuality. But such ideas

THALLOPHYTA 359

must not be taken for more than they are worth, since they raise
questions which cannot be definitely answered. Nevertheless they
are worthy of consideration as giving a point of view which will
have its value in directing the study of the lower organisms, whether
of the Animal or of the Vegetable Kingdoms.

The instability of nutritional method in Euglena — and especially
its mixed nutrition, partly photosynthetic and partly saprophytic, as
it grows strongly in foul water — finds its parallel in the mixed nutrition
of many land-living Plants. It seems probable that irregular nutrition
has been widespread, from very low forms such as Euglena to the
highest Flowering Plants (Chapter XII.). At various points in the
series the dependence upon physiological supply other than by photo-
synthesis may have been accentuated. The parasitic Seed-Plants,
such as Viscum or Cuscuta, and the saprophytes, such as Neottia or
Monotropa, are cases where it was adopted relatively late, in forms
with their character already stamped as Seed-Plants. The various
groups of Fungi are cases where the physiological dependence was
established early, but after the encysted state had been definitely
adopted. A similar segregation, but earlier still, with absence of
encystment, would account for the establishment of the Animal
Kingdom.

Such suggestions as these are based upon the actual facts observed
in simple organisms referable to the Flagellatae. It may be uncertain
whether or no these forms were or were not like the original sources
from which Vegetable and Animal Life sprang. They serve, however,
to give some idea of the possible origin and early relations of the
larger groups of living organisms, and of their differentiation on
the basis of nutrition, and of certain fundamental features of their
structure. In point of habitat the significance of these comparisons
cannot be mistaken ; for all these organisms are either aquatic, or at
least they live where water is readily available. It has been concluded
from this general fact that Life, whether of Animals or of Plants,
originated in the water, and probably in the first instance in the water
of the ocean itself. This is the position here adopted as a Working
Hypothesis, to be accepted until it is disproved. In studying those
few selected examples of the Thallophyta which it is possible to
describe in this book, they will be held as illustrations of primitive
Plant-Life. But the members of any definite natural affinity may
be seriated, so as to illustrate progress from simpler to more complex
conditions, and so it may be found possible, upon a basis of comparison
— but always open to correction as new knowledge is acquired —

3oo BOTANY OF THE LIVING PLANT

to trace the probable origin of many of those features which charac-
terise organisms higher in the scale. For instance, by comparison
of members of more than one natural sequence of living organisms,
the suggestion is clearly given how differentiation of sex arose. This
is only one instance of those illuminating consequences which follow
from the comparative study of organisms lower in the scale with
those which are more advanced. It is in fact upon the cumulative
effect of such comparisons that it has been found possible to base a
rational theory of Descent, applicable to Organic Life as a whole.

Towards the end of the last century Klebs called attention to the
affinity between the Flagellata and the Algae : the view now generally
held is that the old distinction between the two cannot be maintained.
Indeed, from the botanical standpoint, it has been suggested that the
term Flagellata should be discarded. The most primitive group of
plants, the Protophyta, would then include both the flagellate and
algal types of organisation, such characteristics as motility, the
possession of a cell wall and sexual reproduction, having appeared
at different stages in the evolution of the different Classes of this large
and varied group.

CHAPTER XXII.

GREEN ALGAE (CHLOROPHYCEAE).

Green Algae are a heterogeneous assemblage of forms. Some are
marine : others live in fresh water. Some are unicellular, some
colonial (Chlorococcales, Volvocales) : others, which are multicellular,
consist of a simple filament, with various degrees of its branching
(Ulotrichales) ; or they may form widened flat expansions (Ulvaceae) :
others again are coenocytic, not being partitioned into cells or only
partially septate as in the Siphonales. A frequent feature is the
enlargement of the chloroplast into a body often of complicated shape.
One or more of these chromatophores may be present in the cell, and
each may contain one or more highly refractive pyrenoids, spherical
bodies which act as centres for the formation of starch. The variety
of the form and structure of the plants is matched by the diversity
of their propagation. Some multiply by simple fission [Pleurococcus
Naegelii) : others undergo conjugation of equal, non-motile cells
(Conjugatae) ; but most of them produce motile zoospores produced
in cells which may be called sporangia, and gametes produced in cells
which may be called gametangia. The latter show in more than one
natural series evidence of a progressive sexual differentiation (Ulotri-
cales, Siphonales, Fig. 275). The effect of a general study of these
Algae is to suggest that they may all represent steps in advance from
the Protophyta ; and that they contribute many distinct lines in
which an increasing complexity of development of their encysted
phase, and a differentiation of sex have been independently achieved.
A few examples will be described which illustrate the great variety of
structure and propagation that these plants show.

Volvocales and Chlorococcales.

The Volvocales include unicellular and colonial organisms which
are typically motile throughout the vegetative phase or readily resort

361

362 BOTANY OF THE LIVING PLANT

to the motile condition. In the simpler forms the individual consists
of a single ciliated cell : in the more complex forms many such cells
are aggregated together. The fact that the Volvocales are sometimes
claimed by zoologists shows how closely the two Kingdoms are related
by these primitive creatures. A simple example of them is seen in
Sphaerella (Haemato coccus) pluvialis, a unicellular organism frequent
in water-butts and puddles (Fig. 267). Its motile stage is an oval
cell (A) with a protoplast containing a large chromatophore, and
several pyrenoids. It is surrounded by a mucilaginous cell-wall,
through which two cilia project. It readily becomes encysted forming
" Red-snow " in high latitudes, which is its " Palmella " stage.
From this, after division (B), the contents escape as motile zoospores,
which may grow and again divide. Gametes are formed by division
of a mother-cell into 8, 16, or 32, which escape as motile isogametes
all alike (D), and conjugate in pairs (E) to form encysted zygotes

(F,G).

Chlamydomonas has a similar life-history. In some species sexual
reproduction takes place by the fusion of isogametes whereas in others
the gametes are of different size (heterogametes). In the latter a
fertilisation of the larger female gamete (egg) by the smaller male
gamete (spermatozoid) occurs. The zygote undergoes a reduction
division on germination.

The sex-difference thus indicated in Chlamydomonas is much more
marked in Volvox globator. This organism appears in ponds or pools
of fresh water, in the form of hollow free-swimming spherical colonies
(Fig. 268, A). Each is composed of a film of cells embedded in
mucilaginous cell-walls, through which pairs of cilia protrude on the
outer surface. The whole colony shows a slow rolling movement
due to their activity. It propagates vegetatively by subdivision
of certain larger cells, each forming a daughter-colony within the
parent, which are set free by its disorganisation. A greater interest
attaches to the sexual propagation. Large non-motile female cells
(eggs) project into the cavity (D) and are there fertilised by minute
motile spermatazoids produced by subdivision of antheridial cells (B).
A thick-walled resting zygote is the result, which may germinate to
form a new colony.

This series of members of the Volvocales illustrates steps in the
origin of sex. In Sphaerella the gametes are all alike. In Chlamy-
domonas they are alike in some species, but unequal in others in point
of size. In Volvox that inequality is more marked, and the behaviour
of the large inactive egg is contrasted with that of the minute and

GREEN ALGAE

363

/5\*

Fig. 267.

A , B, Sphaerella (Haematoccus) pluvialis ( x 360); A , swarming cell ; B, formation
of swarm-spores C-G, Haematococcus Butschlii ; C, formation of gametes ( x «.oo) ■ D,
gamete ; E, conjugation of two gametes ; F, G, zygotes ( x 800). (C-G, after Bloch-
mann, from Strasburger.)

t!i"2->*^»»*«^sS;

Fig. 268.

Volvox globator. A, colony showing various stages of development of ova and
spermatozoids ( x 165) ; B, bundle of spermatozoids formed by division of a single cell
( x 530). C, spermatozoids ( x 530). D, egg-cell surrounded bv spermatozoids in the
mucilaginous membrane ( x 265). (After Cohn, from Strasburger.)

364

BOTANY OF THE LIVING PLANT

numerous motile spermatozoids. The series will be found to run
parallel in these respects to those in other groups of Algae.

In organisms such as Chlamydomonas and Sphaerella there may be a
brief, non-motile phase at the time of vegetative reproduction. In the
Chlorococcales this non-motile phase tends to be prolonged, the zoo
sporic phase being of comparatively brief duration. The common soil
alga, Chlorococcum humicolum, Fig. 269, consists of small spherical cells

Fig. 269.

Left : Chlorococcum humicolum. A-F, various stages in the life history. A, B, C,
non-motile cells; D, E, F, motile stages ( x 800). (From Chapman's, "An Intro-
duction to the Study of Algae." Cambr. Univ. Press.)

Right : Pleurococcus Naegelii Chod. H, cell structure and division ; n, nucleus :
c, chloroplast. (After Fritsch and Salisbury.) G, packet of cells resulting from
division. (After Chodat.)

each with a cell wall, a parietal chloroplast and internal structure not
unlike that of Chlamydomonas, except that the stigma and contractile
vacuole are absent. This type of cell is typical of many of the Chloro-
coccales. On attaining a certain size, the protoplast undergoes a suc-
cession of divisions into 2, 4, 8, etc. parts; each part becomes
ovoid in shape and develops two flagella. The retaining membrane
is then ruptured and these naked zoospores are set free. After
a period of movement, the flagella are withdrawn, a cell wall is
formed, and a new non-motile vegetative phase is begun. The close
assemblage of many zoospores may lead to the production of a green
stratum consisting of closely addressed cells of irregular shape and
unequal size. In some instances the swarmers behave as gametes,
sometimes of unequal size, and these fuse together in pairs to form
a spherical zygote which becomes a new individual. No cases of
oogamy are known in this Order. Like the Volvocales, which are pro-
bably closely related, the Chlorococcales include both unicellular and

GREEN ALGAE

365

colonial forms, the remarkable organism, the Water-net, Hydrodictyon

reticulatuyn, representing the highest state of organisation in the Order.

The very common unicellular green alga, Pleurococcus Naegelii, Chod.
(Protococcus viridis), Fig. 269, found as <. green incrustation on the windward
side of tree-trunks, walls, etc., was formerly placed in this group. It is now
considered to be a very reduced member of a considerably more advanced
group, the Chaetophoraceae. Multiplication is by cell-division, Fig. 269, no
motile stage and no resting stage being known to occur.

Ulotrichales and Oedogoniales.
These Algae show some advance in development of their plant-body,
owing to repeated cell-division, the products of which remain associated
together to form simple fila-
ments, as in Ulothrix or Oedo-
gonium ; or the filaments
may be branched, as in
Bulbochaete ; or flattened
expansions may be formed,
as in Ulva or Enter omorpha.
The plants inhabit salt or
fresh water, or may even
grow in moist air, as Hormi-
dium does. Ulothrix, which
may be taken as a first ex-
ample, is commonly found
attached to stones washed
by a quickly running stream:
but some of its species are
marine. The unbranched
filament consists of a series
of discoid cells, each with a
zonal chloroplast, and it is
attached by a basal rhizoid
(Fig. 270, A). Its propaga-
tion though varied is rudi-
mentary like its vegetative
structure. Motile zoospores
may be produced either
singly from a cell, or by
division of its contents ;
they escape through an
opening of the cell-wall into

Fig. 2?o.

Ulothrix zonata. A, young filament with rhizoid r
( x 300). B, portion of filament with zoospores escaping
from a zoosporangium. C, single zoospores. D, forma-
tion and escape of gametes from a gametangium.
£= gametes. F = conjugation. G= zygote. H= zygote.
/ = zygote after period of rest. K = zygote after division
into zoospores. (After Dodel Port.) (B-K x 482.) (From
Strasburger.)

366 BOTANY OF THE LIVING PLANT

the water (B). The cell which produces them may be called a zoo-
sporangium. According to the number of the divisions the zoospores
may differ in size. The large macrozoospores have each four cilia
attached to the narrower end of its pear-shaped body (C) ; the smaller
microzoospoies have four or two cilia. After a period of movement,
they settle, form a cell-wall, and affix themselves to some solid sub-
stratum : growing out transversely to their former axis and dividing,
each may form a new filament. The gametes are also produced in a
similar way, from a cell which may be called a gametangium : but their
divisions are more numerous, their size smaller, and they bear only
two cilia (E). The gametes, which are all alike in size and form,
escape from the cells : if those from different filaments meet they
coalesce in pairs, the result being a four-ciliate zygote, which soon loses
its cilia, settles and forms a cell-wall. After a period of rest it
germinates, the contents dividing, and escaping as zoospores, which
grow into new filaments (/, K) ; the first division is a reduction-
division.

Ulothrix takes a low place both as regards structure and propagative
method. The differentiation of its sexual cells is imperfect. Not
only is there no distinction of sex in the form of gametes, but
occasionally the gametes may themselves germinate without fusion.
They are strikingly similar in form and origin to the zoospores. The
facts are in accord with the theory that fusion of gametes (syngamy)
is a means of strengthening otherwise weak cells, which were originally
organs of vegetative propagation.

Oedogonium and Bulbochaete are also filamentous Algae, but with more
elaborate structure of their cells. Various species are very commonly
found attached to stones or submerged parts of plants in quiet fresh
water. Their cells are uninucleate, and contain a single reticulate
chromatophore. They may be propagated by motile cells, or zoo-
spores, which are formed from the whole content of a cell. Each
escapes through a transverse slit in the wall into water, having an
oval form, with a fringe of cilia round the colourless anterior end
(Fig. 271, A, B). After a period of movement the zoospore settles, forms
a cell-wall, and grows directly into a new individual. The plants are
readily distributed by this means. The sexual organs of Oedogonium
are antheridia and oogonia, which differ in size. The oogonium is a
large barrel-shaped cell, containing a single egg. It opens at maturity
by a transverse slit, as in the liberation of the zoospores ; but the
ovum remains in situ, and is motionless (Fig. 271, C, D). At the same
time cells, of the same of of a separate filament, undergo repeated

GREEN ALGAE

367

divisions to form short discoid cells, which are the antheridia. Each,
on opening in the same way, sets free two spermatozoids, the result

Fig. 271.

Oedogonium. A = escaping zoospores. B=free zoospore. C -sexual organs
before fertilisation. D =in process of fertilisation. 0 =oogonia ; a = dwarf-males.
S =spermatozoid. (After Pringsheim, from Strasburger.) ( x 350.)

of division of its protoplast. They resemble the zoospores in form,
but are smaller.

In some species special small plants (dwarf males) are produced from a
special type of swarmer known as an androspore, these being produced singly
within flat, discoid cells {androsporangia) which result from repeated trans-
verse division of the filament. The androspores attach themselves in the
neighbourhood of the oogonium, and dividing into a few cells, liberate their
spermatozoids close to the opening (Fig. 271, C, D).

Fertilisation follows by fusion of the spermatozoid at the receptive
spot of the ovum, and the coalescence of the nuclei has been observed.
The zygote forms a firm protective wall : it is
stored with nutriment, takes a brown or red
colour, and may enter a period of rest. Its
germination presents a point of special interest.
The outer wall bursts and the contents escape,
contained within a delicate membrane. The
protoplast then divides usually into four cells,
which ultimately escape as motile zoospores
(Fig. 272). It has been ascertained in Oedo-
gonium capillare that the zygote nucleus under-
goes a reduction division : hence the Oedogonium
plant is typically haploid. Unfertilised oogonia
may occasionally germinate directly. In some
instances the zygote gives rise to a single large

B.B. A 2

Fig. 272.

Bulbochaete intermedia.

A = oospore. Z? = formation
of four zoospores in the
germinating oospore. (After
Pringsheim, from Strasbur-
ger.) ( x 250.)

368

BOTANY OF THE LIVING PLANT

and presumably diploid swarmer. Hence, in addition to the regular
features of the life-cycle, certain irregularities may occur, as in other
classes of plants. All species with dwarf males are dioecious, i.e. the
male and female gametes are produced on different plants.

A point of interest in these, as it is also in other Confervoid Algae,
is that they illustrate degrees of sexual differentiation. Ulothrix has
isogametes : Oedogonium heterogametes. It is not suggested, however,
that these particular genera are closely related to one another by
descent. Another feature is the similarity of the gametes to the
zoospores, of which they appear as smaller examples. This suggests
that gametes may actually be in origin of the nature of zoospores
specialised in relation to sexual fusion.

Siphonales.

The Siphonales have already been discussed in Chapter X., in re-
spect of their peculiar structure (p. 171, Fig. 105). They are coenocytes
that is, they are not partitioned into separate cells, but the plant-body
consists of a large non-septate sac, limited by a cell-wall, and kept
firm enough by internal turgor to preserve its form in the quiet water
in which as a rule these plants live. This condition is shown in a less
complete form in some members of the Order, where septa occur at in-
tervals, but the protoplasts lying between these are multi-nucleate, and
hardly warrant their designation as cells. This is also seen in Clado-
phora, a very common genus of fresh- and salt-water Algae. Clearly the
non-septate state is mechanically ineffective. It has been pointed
out (p. 172) how, as a set off against it, the more complicated forms
of the Siphonales have acquired additional mechanical strength,
either by internal cellulose ties, as in Caulerpa (Fig. 273), or by matting

Fig. 273.

«>?"? ?f a transverse section of Caulerpa, showing the thick outer wall, and the
reticulate rods of cellulose, which act as ties, and give added rigidity. F. O. B.
I x 5°-)

GREEN ALGAE

369

of their branches together, as in C odium, or by cementing those
branches together, as in Halimeda. But these are concessions to
an essentially weak construction. It is only possible to carry it to
any considerable size when living in water, and all the larger forms
are marine. Vaucheria is an exception : for though many of its

Fig. 27t.

Vaucheria sessilis. A = young sporangium. B= zoospore, with the spor-
angium from which it has escaped. C =a portion of the peripheral zone of a zoo-
spore. D = a young plant, with rhizoids, developed from a zoospore. (A , B, after
Gotz ; D after Sachs ; C after Strasburger.) (From Strasburger.)

species float in water, some live on moist soil, exposed to the air.
But the members of the genus consist only of simple or branched
filaments, and when living aerially they lie procumbent.

The cytoplasm of these plants contains many small chloroplasts,
and numerous nuclei usually lying internally to them. Centrally
is a large vacuole. In Vaucheria the product of photosynthesis
appears as oil, but others of the Siphonales may contain starch, which

37o BOTANY OF THE LIVING PLANT

is commonly present in Green Algae. The general physiology of a
coenocytic, or, as it has been called, a non-cellular plant, is probably
like that of any ordinary green plant. The difference lies in the
mechanical construction.

Vegetative propagation is carried out in various Siphonaceous
Algae by non-motile, or by motile cells, produced in large numbers
in "special branches, and liberated into the water. Vaucheria is an
exception, in that the whole contents of such a branch-ending, which
are previously shut off by a septum, are discharged as a single ciliated
zoospore, large enough to be seen with the naked eye. The escape

Ftg. 275.
Gametes of various Siphonales, illustrating differentiation of male and female.
i=Acetabularia, isogametes ; ii = Bryopsis ; iii=Codium; iv =Vaucheria. In ii
and iii the gametes are unequal, but still motile ; iv in the large egg is stationary,
while the smaller spermatozoid is motile. (Taken from Oltmann's Algae.)

is effected in the early morning ; after a period of movement the
zoospore comes to rest, and germinates directly into a new plant
(Fig. 274). In structure the zoospore shows the cilia in pairs, each
pair related to a nucleus which is superficial, while the chloroplasts
lie within. The origin and structure of the zoospore suggests that
it represents the undivided contents of a whole zoosporangium, such
as may be seen in other Siphonales.

The Siphonales reproduce sexually : but degrees of difference in
size of the gametes are seen. The differentiation thus indicated must
be held as distinct from, though parallel with that already described
in the Volvocales and in the Ulotrichales and Oedogoniales. In
Acetabularia the gametes are of equal size, and those produced from
different gametangia, or from different plants fuse in pairs (Fig. 275, i.).

GREEN ALGAE

371

In Bryopsis the size is unequal (ii.) ; while Codhim shows still greater
inequality (iii.).

In Vaucheria, Fig. 2 75 (»v.), which is the most advanced of all in sexual
differentiation, the large non-motile egg, retained in the oogonium,
is fertilised by a small spermatozoid.
The facts suggest again a progression
from isogametes to a distinction of
egg and spermatozoid, but in a series
of Algae quite distinct from those
previously described.

The sex-organs of VaucheHa arise
close together as short lateral branches
(V. sessilis), or borne together on the
same branch (V. terrestris, Fig. 276).
The male, or antheridia, are horn-like
curved bodies ; the female, or oogonia,
are oval. In the antheridium a sep-
tum cuts off the multi -nucleate pro-
toplast from the parent tube : each
nucleus becomes the centre of a
spindle-shaped spermatozoid ; and
these escape, with their paired cilia
pointing fore and aft, through an
opening at the distal end. The
oogonium also at first contains
numerous nuclei embedded in protoplasm stored with many globules
of oil. But, as the ovum matures, all the nuclei but one wander
back into the parent filament, which is then shut off from the oogonium
by a wall. Meanwhile a beak has formed at the distal end of the oogon-
ium, which then opens by swelling of the wall, a portion of the colour-
less contents being emitted. The uninucleate egg then lies open for
fertilisation by the small motile spermatozoids (Fig. 275, iv.). It
appears that self-fertilisation from an adjoining antheridium is the
rule. The fusion of the two nuclei has been observed to form the
nucleus of the zygote. Then follows storage of further oil, a change of
colour of the contents, and the formation of a thick wall. In this
state, as an oospore, a period of rest follows. Germination takes place
by rupture of the thick wall, and the direct formation of a new filament
from the contents.

In such organisms as these it has been found that there is no obligatory
succession of events in the life-history. It lies in the hand of the experimenter

Fig. 276.

Sexual branch of Vaucheria terrestris,
bearing distally a curved male gametan-
gium or antheridium as it is calLd : and
right and left oval oogonia.

372 BOTANY OF THE LIVING PLANT .

to determine by altering the conditions what form of propagative organ shall
be produced. This has been shown with particular clearness in the case
of Vaucheria. If the plant be kept first in a flowing stream of water, as it
may be in a glass tube, it simply grows vegetatively. If it be then transferred
to still water, zoospores are produced. This also follows on flooding, in the
case of terrestrial forms. If it be desired to produce sexual organs, the plant
should be well nourished, for instance by exposure to good light, or cultivation
in a weak sugar solution. A rise of temperature also encourages their pro-
duction. Speaking generally, light produces sexual organs, shade zoospores.
A third type of propagative organ is formed in some species (V. geminata).
Under dry conditions the filament, dividing up into short lengths, forms
aplanospores with thick walls. These can stand drying up. All of these
propagative organs are biologically suitable to the spread and survival of
the plant in its native habitat. When flooded in cool weather it forms
zoospores. When there is risk of drying up in summer after exposure to
light and heat it forms zygotes or aplanospores, which can tide over the
period of drought ; but on germination, whether of the zoospores, zygotes,
or aplanospores, either zoospores or sexual organs may be formed first on
the young plant, according to the conditions.

In Vaucheria it is probable that reduction takes place at the first nuclear
division in the germinating oospore ; and hence the vegetative plant is
typically haploid. But in Acetabularia and related genera the plant is diploid,
with reduction occurring at the formation of the gametes.

CONJUGALES.

A considerable series of common Green Algae belong to the Con-
jugates, a group which stands aloof from the Chlorophyceae in the
more restricted sense. One large family of these is the Zygnemaceae,
filamentous Algae native in still water : the most familiar example
being Spirogyra. Another family, the Desmidiaceae, are mostly
unicellular, and very beautiful. They are found in quantities in peaty
pools. The two families are grouped together because of the structure
of their uninucleate cells, which contain complicated chromato-
phores ; and because both show conjugation of non-motile gametes.

The well-known genus Spirogyra includes numerous species, of
which the filaments float commonly unattached in still fresh water,
and with no distinction of apex and base. They are slimy to the
touch, owing to their outer wall being mucilaginous. Each filament
is partitioned by transverse septa into cells, each of which may be
detached from its neighbours by shock, when its convex ends demon-
strate its internal turgor. Growing on and dividing, each may form
a new filament. Each cell is practically an individual (Fig. 277, cell h).
It is cylindrical, the proportion of length to breadth varying in
different species. Within the external wall is a layer of colourless
cytoplasm surrounding a central vacuole, in the middle of which the

GREEN ALGAE

373

single nucleus is suspended by colourless threads. The most marked
feature is the green spiral chromatophore, which gives the genus its
name. One or more of these, according to the species, may lie embedded
in the peripheral cytoplasm, coiled corkscrew-fashion. As in the
Conjugales generally, pyrenoids occur in the chromatophores : they

Fig. 277.

Two filaments of Spirogyra, which illustrate various stages of conjugation, a, b,
have formed outgrowths which have met at c ; the protoplast d is contracted to a
dense sphere. The next lower pair of cells show conjugation, the protoplasts
fusing at /. In g, conjugation is complete, a zygote having been formed by the
fusion of two protoplasts, h shows a cell not in conjugation. (After Kny.)

are highly refractive, and form centres for the formation of starch,
while the threads that suspend the nucleus usually run out to them.

Vegetative propagation is simply by division of the cells, which
occurs during the night, and may be continued indefinitely. As the
season progresses the filaments conjugate. Adjacent filaments put
out processes from cells opposite one another, which meet, flatten,
and fuse at their tips, the intervening wall being absorbed. Their

374

BOTANY OF THE LIVING PLANT

protoplasms contract : one of them contracts earlier than the other ;
it then passes bodily through the now open tube, and its cytoplasm
coalesces with that of the other cell, while its chloroplast becomes dis-
organised (Fig. 277). The nuclei remain for a time distinct. The zygote
changes to a reddish colour, fats are stored in it, and a thickened wall
is formed. Freed from the parent filaments it remains dormant.
On germinating the outer wall ruptures, while the inner covers the
enlarging protoplast. The nuclei fuse, and later undergo tetrad-
division, with reduction. But only one of the four survives as the
nucleus of the cell from which, by division, the new haploid filament
arises.

In Mesocarpus the conjugation is similar, except that the zygote
is formed actually in the tube connecting the conjugating cells.

Conjugation in the Desmids is essentially similar. The behaviour of the
zygote on germination has been followed (Fig. 278), and there also, after

1.

2.

4- 5- 6.

Fig. 278.

Syngamy and germination of a Desmid, Closterium, after Klebahn. 1, zygote before
nuclear fusion ; 2, first nuclear division . 3, binuclear stage ; 4, second nuclear division ;
5, two cells, each with a small and a large nucleus ; 6, formation of two new Closterium
cells each with one large nucleus ; the other is disorganised. (From Oltmanns.)

delayed fusion, the fusion-nucleus divides first into two and then into four ;
but two are atrophied, while the others remain as the nuclei of the two new
cells formed on germination. Thus in the Conjugales there is a tetrad-
division, and a presumable reduction which follows on conjugation, just as
tetrad-division follows on sexual fusion elsewhere.

It is different in the Diatoms, a distinct class of unicellular Algae with very
many forms, marine and fresh-water, great numbers of which are found in the

GREEN ALGAE 375

floating " Plankton." Their chloroplasts are brown, and the uninucleate
protoplast is enclosed between two silicified shells with delicate sculpturing,
which fit over one another like the two parts of a pill-box. Vegetative
division results in regular decrease in size of the cells, till a limit is reached,
from which recovery is usually by conjugation, resulting in Auxospores. The
nuclei of the conjugating cells of certain types, such as Rhopalodia, hrsc divide
into four : each cell then divides into two gametes with two nuclei in each
of them, one large and one small : the gametes then fuse in pairs, the larger
nuclei also fusing, while the smaller disintegrate. Here also there is a tetrad-
division, but it precedes conjugation, while in the Conjugales it follows. It
may then be concluded that the vegetative phase of the Conjugales is haploid,
while that of certain Diatoms is diploid. These facts have their importance
in questions to be discussed in Chapter XXXIII.

Examples such as these from the Green Algae show how diverse
those plants are in structure and in propagative method. It may
be held that they sprang from some common source, including motile
and non-motile forms, such as the Protophyta : and this would seem
probable from their characters, both vegetative and propagative.
But the differences which they show suggest a plurality of lines of
parallel development. More especially does this emerge from their
comparison in respect of sexual differentiation. The steps of distinc-
tion of male and female gametes correspond in several large Orders,
e.g. Volvocales and Ulotrichales, though these series are sharply
distinct in vegetative structure. The only possible conclusion from
such facts is that the distinction of the sexes has been achieved not
only once, but in a number of distinct evolutionary series. There has,
in fact, been parallel development, or as it is styled, homoplasy. The
spermatozoids and ova of Volvox, Oedogonium, and Vaucheria are not
then to be held as homogenous, that is, produced from a common
ancestry that bore spermatozoids and eggs ; but homoplastic, that is,
each has arrived as a result of independent sexual evolution from
some ancestry which had not male spermatozoids or female eggs, but
undifferentiated gametes as the propagative organs.

While a majority of Green Algae are typically haploid organisms,
and undergo reduction at the first nuclear division in the zygote,
there are others, e.g. among the Siphonales, which are typically
diploid organisms. Thus the Green Algae give some indication of the
very considerable diversity of life-cycle which is characteristic of the
Algae as a whole. This aspect is discussed in greater detail at the end
of Chapter XXIII.

376

BOTANY OF THE LIVING PLANT

Myxophyceae (Cyanophyceae).
The Blue-green Algae are unicellular, or filamentous. They are
found living either in water, or on surfaces which are habitually moist.

A common type is seen in Gloeocapsa,
where the oval or spherical cells have a
swollen cell-wall. This holds the cells
together after fission, in rounded colonies
which break up by disorganisation of
the wall. It is commonly found on the
inner surfaces of the glass of damp
greenhouses (Fig. 279). Oscillatoria is a
filamentous type, which is common on
damp walls and rocks. Its pale green
filaments show slow swinging move-
ments, hence the name (Fig. 279A).
They consist of disc-shaped cells, which
multiply by division. In some of the larger forms granules of
irregular form are found in the so-called central body, which react
to stains like chromatin and divide before cell-division takes place.

Fig. 279.

Gloeocapsa polydermatica. A, in pro-
cess of division ; B, to the left, shortly
after division ; C, a later stage.
( x 540.) (From Strasburger.)

Fig. 279A.

A, Oscillatoria princeps : a terminal portion of a filament ; b, portions from the
middle of a filament, properly fixed and stained ; t, cells in division. ( x 1080.)
a, UsctUatona Froeiichii. ( x 540.) (From Strasburger.)

The filaments, which are unattached, may break up at any point
into several shorter lengths ; but in other cases special cells (hetero-
cysts) occur at intervals in the filaments, which appear to determine
their brewing up into shorter lengths, as in Nostoc or Rivularia.
Some of these fission-Algae take part in the formation of Lichens :
thus Collema has Nostoc as its Algal constituent. Others, such as

BLUE-GREEN ALGAE 377

Anabaena, lead an endophytic life, contributing probably to an
irregular nutrition, as in the roots of Cycads. In the establishment
of the new Flora of the volcanic Island of Krakatoa (p. 331), Blue-
green Algae were among the first colonists, taking their part in the
preparation of an organic soil for larger developments to follow.
Certain of these Algae allied to Anabaena often appear suddenly in
large quantities on the surface of fresh water, causing the pheno-
menon known as 4< water bloom," or the ' breaking of the meres."
One of these, with a deep red colour {Trichodesmium erythraeum),
floats in ocean-waters, and becomes prominent when massed together
by wind and tidal streams. It has thus attracted attention in various
oceans, and has given its name to the " Red Sea."

CHAPTER XXIII.

BROWN ALGAE (PHAEOPHYCEAE).

The Brown Algae, or Phaeophyceae, include a large proportion of the
Seaweeds commonly found between the tide-marks, and extending
downwards to greater depths. Some of them are delicate filamentous
growths, branched or unbranched (Ectocarpales). Others are larger
and more complicated in structure, with ribbon-shaped thallus
(Dictyota). Some, of leathery texture, attain gigantic dimensions,
the Tangles of the colder oceans being among the largest of living
beings (Laminaria, Nereocystis, Macrocystis). The most familiar
are the species of Fucus found on all British coasts, of which F. vesi-
culosus is the Common Bladder Wrack. The smaller forms show
gradual steps of increasing complexity, from the simple septate
unbranched filament, through various modes of branching and
cortication, to the massive tissue-formation seen in the larger Tangles.
Even the largest of them may thus be referred ultimately in origin
to the simple filament

The thallus of the larger forms is usually flattened, and bilaterally
symmetrical (p. 205). It shows forked branching, often very perfectly
dichotomous, and in a single plane. The result is that the whole
frond is fan-shaped, as is seen particularly well in the native species
Fucus serratus (Fig. 280). The thallus is attached by a holdfast to
some firm body such as a rock ; but the gulfweed (Sargassum bacciferum)
is exceptional in floating freely, in which state, however, it probably
propagates vegetatively : organs of sexual reproduction only occur on
attached plants. The holdfast of Tangles applies itself so closely
to the irregularities of the surface that the stalk will often break before
it would come away. In Fucus it is discoid ; in Laminaria and others
it may be branched and root-like. But its function is only mechanical,
not absorbent. The thickness of the stalk which arises from it is
proportional to the size of the thallus it has to moor. A plant so

378

BROWN ALGAE

379

constructed is well fitted to resist the swirl of the waves, keeping its
hold, and though pliant, retaining the form of its leathery frond.
Its form gives a large proportion of surface to bulk for exposure to its
liquid environment. Plants are normally buoyed up by the water,

Fig. 280.

Drawing of a plant of Fucus serratus, showing the fertile
distal ends of the longer branches, (i Nat. size.)

but when exposed between tide-marks they subside to form a dense
stratum whereby the plants mutually protect one another from drying.
There is in many of these seaweeds a localised apical growth with
distal branching. In Fucus the growing point lies in a depression at
the extreme tip. In others (Dictyota, Sphacelaria) the apex projects.
The segmentation of the apical cell is often very regular, the form and
succession of the segments varying in accordance with the form of
the tip itself. In other cases intercalary growth is dominant. This
is seen in the simple filaments of the Ectocarpales, and in the more
complex thallus of Cutleria. But it Is shown on the large scale in

38o

BOTANY OF THE LIVING PLANT

Laminaria, where a new frond is formed each season between the
old one, which is thrown off, and the persistent stalk (Fig. 281).

In simple forms the cells are all alike. Soft cell-walls surround a
uni-nucleate protoplast, which includes simple brown chromatophores.
The plastid pigments are chlorophyll, xanthophyll and carotin, to-
gether with fucoxanthin which masks
the others and is peculiar to the group.
The products of photosynthesis are
soluble carbohydrates like laminarin
or mannitol. In larger forms the
tissues are differentiated. For in-
stance, in Fucus the cells of the super-
ficial layer are thin-walled, and divide
actively ; they are covered externally
by a layer corresponding to cuticle.
They are the chief seat of photosyn-
thesis, and of tissue-formation. Pass-
ing inwards from this layer the
mucilaginous cell-walls become more
and more swollen, so that the deeper-
seated tissues of an old thallus con-
sist of a bulky mucous matrix, in
which the cells themselves appear as
a complicated network (Fig. 282).
Centrally there is a firmer conducting
cord, which is well defined in old
stalks of the larger Tangles. It
contains manv tubes with sieve-
structure and callus, closely com-
parable to the sieve-tubes of Vascular
Plants, and serving like them for
transport. In large stalks an ill-defined cambial activity provides
for thickening and increased mechanical strength. This is still
further secured by " intrusive hyphae," which burrow through the
softer tissues, and brace them together. In this way they acquire
their tough and resistant but yet pliant character. We thus see
that both in external form and internal structure the Brown Sea-
weeds cover a wide range, from the simple to the complex.

In their sexual propagation they also show an advance, which runs
in some degree parallel with their structural progress. The propagative
cells are produced in sporangia and gametangia. The former which

Fig. 281.

Laminaria, thallus showing a new frond,
intercalated between the stalk and the old
frond, which is being thrown off. (Re-
duced. ) (After Strasburger.)

BROWN ALGAE

38i

produce zoospores are borne both by the simpler and more advanced
types but they are absent in Fucvs. Successive steps in differentiation
of the sexes may be found within the Brown Algae. The simplest
forms produce isogametes which are motile. More complex forms show
differentiation of sexes, the small spermatozoids being motile, but the
larger ova are non-motile primordial cells.

Fig. 282.

Fig. 283.

Fig. 282. — Mature male conceptacle of Fucus serratus, filled with branched
antheridia hairs. (After Thuret.) Fig. 283. — Mature female conceptacle of Fucus
serratus, ontaining unbranched hairs, and oogonia. (After Thuret.) Incidentally
these drawings show the structure of the mature thallus (p. 358).

The Phaeophyceae, which comprise eleven Orders, collectively
illustrate : (i) an advance in vegetative organisation from simple
filamentous plants to plants of large size and elaborate organisation ;
(ii) alternation of generations from simpler types in which the gameto-
phytic and sporophytic generations are alike in their vegetative
development to those in which the sporophyte shows a marked
preponderance over the gametophyte ; and (iii) an advance from
isogamy to oogamy.

The propagative cell of the Brown Seaweeds is very constant
in its primitive form. It is actively motile in water when set free.

382

BOTANY OF THE LIVING PLANT

It is a pear-shaped, nucleated protoplast, with two cilia attached
laterally so that one is directed forwards the other backwards. They
are inserted close to a red eye-spot, which as a rule is closely related
to a yellow chromatophore (Figs. 284, B ; 287, 3). The constancy
of this type shows the probable phyletic unity of the Brown Algae.

Fig. 284.

A, Pleurocladia lacustris. Uni-
locular sporangium with its contents
divided up into zoospores, a = eye-spot.
chr = chromatophore. (After Klebahn.)
B= Chorda filum, zoospores. (After
Reinke.) (From Oltmanns' Algae.)

Fig. 285.

Ectocarpus siliculosus. i, female gamete sur-
rounded by a number of male gametes. 2-5, stages
in the fusion of gametes. 6, zygote after 24
hours. 7-9, fusion of the nuclei as seen in fixed
and stained material. (1-5 after Berthold ; 6-9
after Oltmanns.) (From Strasburger.)

In the simplest of the Phaeophyceae, the Ectocarpales, the complete life-
cycle, as in Ectocarpus or Pylaiella, involves an alternation between (i) a diploid
plant bearing asexual, unilocular sporangia, Fig. 284, in which reduction
division takes place during the formation of the zoospores, and (ii) a haploid
plant, bearing plurilocular sporangia ( =gametangia) from which haploid
isogametes are liberated and fuse in pairs, Fig. 285. Thus, a zoospore from a
unilocular sporangium gives rise, on settling and germinating, to a haploid
gametophytic plant ; a zygote on germination gives rise to a diploid sporo-
phytic plant. The haploid and diploid plants are so similar in size and struc-
ture as to be described as identical ; hence they may be considered to exemplify
homologous or isomorphic alternation. Both phases of the life-cycle have
accessory means of vegetative propagation : diploid plants may be propagated
over several generations by diploid zoospores released from plurilocular
sporangia ; haploid plants may be similarly propagated by swarmers (apo-
gamous gametes) from the plurilocular sporangia or gametangia. The sporo-
phytic plant is always recognisable as that which bears unilocular sporangia

BROWN ALGAE

383

as well as plurilocular sporangia. Environmental conditions influence the
preponderance of one or other of the two alternating phases : thus plants of
Ectocarpus in Northern Europe are mainly sporophytic whereas those on the
Mediterranean coast are mainly gametophytic

The Ectocarpale- show various stages in sexual differentiation. Although in
Ectocarpus the male and female gametes are morphologically identical, physio-
logical differences may be observed Those gametes which are functionally
female tend to move more slowly and for a shorter time and to become centres
of attraction for the still motile males. In some species the gametes are of
unequal size (heterogamous) and in related genera within the Order there may
even be an advance to incipient oogamy. In Cutleria (Cutleriales) the two
gametes differ obviously in size (Fig. 286).

Fig. 286.

On left, Cutleria multifida, showing the smaller, male gametangia (or plurilocular
sporangia) and the larger female. Top-left are spermatozoids, top-right are ova.
Below are three stages of fertilisation. ( x 500.) (After Reinke.)

On right, Laminaria digitata. A, Male gametophyte ; a, empty anthendia.
B, C, D, Female gametophytes (B is large, C small, while D is reduced to a single
oogonium) ; og, oogonium ; 0, ovum. E, young sporophyte, still seated on the
empty oogonium. (A x Coo ; B x 292 ; C x 322 ; D x 625 ; E x 322. After
H. Kylin.)

In Dictyota dichotoma (Dictyotales) there is also isomorphic alternation of
generations, the sporophytic plant bearing tetraspores. Separate plants in
Dictyota dichotoma bear respectively tetraspores, oogonia, and antheridia.
The nuclei in the first of these plants are diploid, with 32 chromosomes. The
nuclei of the male and female plants are haploid, with only 16 chromosomes.
Reduction takes place when the tetraspores are formed, and these on ger-
mination give the sexual plants. The analogy of the tetraspores with the spore-

B.B. 2B

BOTANY OF THE LIVING PLANT

tetrad in Land Plants is obvious. But here the haploid gametophyte and the
diploid sporophyte are alike in form.

In contrast to the isomorphic alternation seen in the Ectocarpales, Dictyotales,

the Laminariales afford remarkable examples of fc alternation.

In this Order the large plant itself— for example I rim as already

:ribed. Fig. 2S1— is the diploid sporophyte : the gametophyte, by contrast,

minute, filamentous plant. In club-shaped, unilocular sporangia, borne on

roush patches on the frond of Laminaria, reduction takes place and haploid

zoospores are liberated. These give rise to minute, filamentous male and

female gametophytes Fif - --6) ; the former are branched and bear antheridia

while the latter consist of a few cells, and in extreme cases may be reduced

to a single cell, and bear oogonia. The naked egg-cell which emerges from the

oogonium and remains attached thereto, grows into the diploid sporophyte on

fertilisation.

The Fucales are conspicuously oogamous. The sexual organs are in
conceptacles, cavities hollowed out of the thallus (Fig. 280). Fucus
serratus (Common Wrack: bears the male and female organs on distinct
plants, but in many species they may appear in the same conceptacles.
A median section through a male conceptacle shows how the flask-
shaped cavity opens to the outside by a narrow pore, and is at
maturity filled by richly branched hairs, which arise from the tissues
bounding the conceptacle, and bear the numerous minute antheridia,
or nude ganietangia (Fig. 282). The female conceptacle is of like
structure, but bears unbranched hairs, and associated with them are
the large oogonia, or female gametayigia, which are large enough when
mature to be seen with the naked eye (Fig. 283).

The antheridium itself is an oval unicellulcr body, surrounded by
a cell-wall. When young it contains a single nucleus, which divides
to form 64 nuclei, each of which becomes the centre of a spermatozoid.
The cytoplasm divides into as many portions, and each is found to
contain a red eye-spot beside the nucleus (Fig. : 9 ~. : . The contents
slip out when ripe from the ruptured outer wall, as a mass still
surrounded by the inner wall : this soon deliquesces, and sets them
free as 64 motile sperr .::;:: .;: each with the characters usual for
the Brown Seaweeds (3, 4). The oogonia though larger are of the same
pattern. Each has at first one nucleus ; but here it divides to form
only eight, and the cytoplasm undergoes cleavage into eight large

• 2. They are also shed in the same way, and round themselves
off as 8 non-motile eggs (5). [See also Fig. 288, D.E.)

A comparison of the antheridium with the oogonium in the Fucaceae shows
that they are probably results of differentiation from a common source. When
an oogonium is to be formed, a cell of the wall of the conceptacle projects with-
out branching into the cavity*, and divides to form a stalk-cell and an

BROWN* ALGAE

J»5

oogonium

(Fig. 288, A, B. st.). In Sarcophycus, however, the stalk is
branched, and a succession of oogonia may be produced, as is seen in the
antheridia of Fucus. On the other hand, the antheridial hair of Fucus rr
start precisely in the same way as the oogonium, the terminal cell forming the
first antheridium. But the growth does not stop there : the stalk-cell shoots
out laterally and produces another antheridium, and the process may be con-
tinued with irregular sympodial repetition (Fig 287, I.). This finds its biologi-

fi k

Fucus 1, group of antheridia. 2. part of an anthendium showing developed
spennatozoidsT 3, spermatozoid ; - spot; * -and isolated antheridia

liberating spermatozoids. 5, ovum surrounded by spermatozoids. 6. secuon tnrougn
a fertihsed egg ; ek = nucleus of egg ; spk = nucleus of sperm ; sp = s^"°**f:
(1, 4, 5 after Thuret ; 2, 3 after Guignard ; 6 after Farmer.) (From Stra^burger.)

cal explanation in the need for a continued supply of numerous spermatozoids,
so as to secure fertilisation over a prolonged period. The large number is
further ensured bv the divisions m each cell being continued to 64.

The sohtarv ooeonia. with their few ova, find their biological elucidation
in the facts that the ova are large and have a strong chemotactic influence
on the motile spermatozoids. Their s e? a high decree of certa

of successful germination if once fertilised. Their attractive influence secures
a hieh probability of fertilisation, notwithstanding their immobility, which
has followed on increase in size. But Fucus is not the last term of the series
of reduction of the oogonium. In Ascophyllum only four ova are matured in

336

BOTANY OF THE LIVING PLANT

each, in Pelvetia two, and in Himanthalia and others only one. Vestigia
of the atrophied eggs are found, which clearly indicate that their number
has been reduced. Thus the Brown Algae form a coherent series of sexual
differentiation. Their sexual cells probably originated from motile cells
all alike. The first functional though not formal distinction of sex is seen in
Ectocarpus siliculosus (Fig 285). Steps in loss of motility are seen in other
species, and in Cutleria (Fig. 286). while in Fucus the large ovum is entirely non-
motile (Fig. 287). Finally, in Himanthalia a single ovum occupies the whole
oogonium : its large size giving such security of germination as to justify the
reduction in number to one. The whole series thus illustrates steps in sexual
differentiation, which are biologically intelligible.

Fig. 288.

Fucus A, oogonium, the contents of which have divided into eight eggs. £ =
oogonium, from which tbe contents (C) have been extruded. D, E, liberation of
the eight eggs ; st = stalk ; mes = middle — end = inner layers of the oogonial wall.
(After Thuret.) (From Strasburger.j

In Fucoids which live submerged the gametes may be set free at
any time. But when exposed between tide-marks it will be chiefly
on the rising tide that they are liberated. Exposed to the air the
mucous thallus shrinks on drying, and the pressure of the contracting
tissue may be seen to squeeze out the ripe antheridia and oogonia,
which may be recognised by their yellow or olive colour, through the
open pores of the conceptacles. If these be collected fresh in separate
watch-glasses in sea water, the final liberation of the minute motile
spermatozoids (Fig. 287, 4) and of the much larger non-motile eggs
(Fig. 288, E) can be easily followed. If then a drop of water containing
the former be added to a drop containing ova on a slide, many sperma-
tozoids will be seen to collect round each ovum, which thus shows

DROWN ALGAE

387

its attractive influence on their movements (Fig. 287, 5). But only
one penetrates each egg, and its nucleus has been followed on its
course through the cytoplasm till it fuses with the nucleus of the
ovum (Fig. 287, 6). The rest at once move away, as though a repellent
influence from the egg had replaced the previous attraction.

The immediate consequence of fertilisation is the deposit of a
cell-wall covering the zygote. It settles on some solid substratum,

Fig. 289.

I.-IV. Drawings direct from successively older plants of Fucus serratus, showing

the regular dichotomy.

and germinates directly into a new Fucus plant. Stones on a rocky
shore where Fucoids grow may be found in summer covered by a
dense growth of myriads of these young plants (Fig. 289).

In the life-history of Fucus the increase in numbers is exclusively through
the sexual process. There is no production of zoospores, nor any non-sexual
propagation, as there is in the simpler Brown Seaweeds. The Fucus plant is
diploid and reduction takes place in the first divisions respectively of the
antheridial and oogonial cells. The haploid phase is extremely brief and there
is, strictly speaking, no alternation of generations as in Laminaria. By com-
parison with the very reduced gametophyte of Laminaria, it has been suggested

388 BOTANY OF THE LIVING PLANT

that in the Brown Algae there has been a progressive retrogression of the
haploid phase culminating in the condition found in Fucus. In this view, the
first two nuclear divisions in the antheridium and oogonium are regarded as a
tetrad division within a sporangium ; the subsequent developments are held
to represent all that remains of the greatly reduced gametophyte generation.
Such conceptions, should, however, be viewed with caution. An alternative
hypothesis, for which there is evidence among several of the less specialised
groups, is that the reproductive mechanism in the Fucales had its origin in a
tendency for the asexual zoospores (produced in unilocular sporangia) to
behave as gametes. As the Fucales are undoubtedly advanced forms with a
long history behind them, the matter under discussion is clearly one which
calls for a suspension of judgment until further data are available.

It seems clear from the peculiarity and the constancy of form of
their motile propagative cells that the Brown Seaweeds are a natur-
ally related group of organisms. They illustrate steps of advance in
respect of form, structure, and propagative method ; and these
follow for the most part along parallel lines. Accordingly they
may be held to represent a progressive series. That they are ranked
as Thallophytes is no sufficient reason for holding them all as primitive.
Their higher terms show high structural adaptation to their require-
ments, and they are in their own habitat eminently successful plants.
These may therefore be held as the ultimate exponents of an evolution
limited by its surroundings, and distinct from other lines of Descent.

RED ALGAE {RHODOPHYCEAE).

The Red Algae (Rhodophyceae) are a separate group, distinguished by
their method of propagation from all others. They are mostly marine,
spreading from the zone between tide-marks to deeper levels, and finding
their limit at about 150 feet below low-water mark. Their colour varies
from pink to purple, or reddish brown. This is due to chromatophores
containing red pigment which masks the chlorophyll. The colouring has its
relation to light. The greatest activity of photosynthesis is in light comple-
mentary to the colour of the plant. Ordinary green plants make special use of
the rays at the red end of the spectrum ; but for Red Algae rays further
along the spectrum are effective., and it is the rays towards the blue end of
the spectrum which penetrate into the depths of sea water. But all of the
Rhodophyceae are not red. It is significant that Lemanea, which is exposed
to ordinary sunlight in shallow fresh-water streams, is olive-green.

In form the Red Algae are various, but never large. They include plants
which in form and colour are among the most beautiful, and therefore are
prized by collectors. They may consist merely of branched septate filaments :
or fronds, variously thickened and flattened, may be formed by matting
and webbing of many filaments together. Often they are fan-shaped and

BROWN ALGAE

389

sometimes lime-encrusted. In the warmer waters some of these contribute
to the formation of Coral reefs, and have been active in this way from early
geological times. The chief feature they have in common is their method
of sexuality, which may be illustrated in the simple case of Nemalion. The
male organs are unicellular, the whole content of each cell escaping as a naked,
non-motile spermatium (Fig. 290, 1). The female organ is a carpogonium,
consisting of a cell with an enlarged base, and elongated upwards into a fila-
mentous trichogyne. This receives the non-motile spermatium, and then
shrivels. Carpospores then arise directly or indirectly from the enlarged

O

Fig. 290.

Nemalton multifxdum. 1, Branch bearing antheridia to the left and a carpogonium
to the right, with spermatia, some of which adhere to the trichogyne. 2-5 are
successive stages of development ot the very simple fruit bearing the carpogonial
buds. (After Kny.)

base. Fertilisation is indirect, in the sense that the nucleus of the spermatium
received by the trichogyne is passed down to the base of the carpogonium,
where fusion and bud-formation follow (Fig. 290, 4, 5). This is the lead-
ing character of the Red Algae, and it is worked out in some of them with
extreme complication of detail in the method of transfer of the nucleus. An
example of a not infrequent type of fruit-body of Red Seaweeds is shown in
Harveyella (Fig. 291), where the carpogonium produces branches (black in
the figure), from the ends of which the carpospores are given off.

In some Red Algae such as Nemalion, which is one of the simplest of them
all, no other propagative organs are known. But in most of them tetraspores
are found, in the production of which reduction of chromosomes has been
demonstrated. The nuclear cycle has been fully worked out in the genus

39o BOTANY OF THE LIVING PLANT

Polysiphonia, where tetraspores and sexual organs are borne on separate
plants. The carpospore is diploid and on germination gives rise to a tetra-
sporic plant (sporophyte) which bears tetraspores, reduction taking place
in their formation. The haploid tetraspore grows into the sexual plant.
The fusion of a spermatium and a carpogonium produces the diploid carpo-
spore : and so on. There is thus a regular alternation of tetrasporic and
sexual plants But in form these are closely alike. The case is comparable
with that of Dictyota ; and questions of the origin of alternation in the Red
Algae are raised similar to those for the Brown.

Alternation of Generations in the Algae.

The very considerable diversity of life-cycle shown by the Green, Brown and
Red Algae has been made evident in the preceding pages. At least four main
types can be distinguished. Thus most of the Chlorophyceae and Red Algae
such as Nemalion are haploid organisms, with reduction taking place at the
first division in the zygote. In these organisms the diploid condition is confined
to the zygote itself. In a second type of life-cycle, typical of some of the
Siphonales probably of all the Bacillariophyceae (Diatoms), and of the Fucales
among the Brown Algae, the plant is diploid ; reduction division takes place
at the formation of the gametes, and the haploid condition is limited to the
brief phase prior to the fusion of the gametes. A third type consists of the
alternation of two morphologically similar individuals, one haploid and gameto-
phytic, the other diploid and sporophytic. Such isomorphic or homologous
alternation is found in the Cladophorales and Ulvaceae among Green Algae, in
Ectocarpales and Dictyotale< among Brown Algae, and is typical of all the more
advanced Orders of Red Algae ; in the Brown and Red Algae reduction division
takes place in the unilocular sporangia and tetrasporangia respectively.
Lastly, heteromorphic alternation is met with in such Brown Algae as the
Cutleriales and Laminariales , and it may be, if we accept one interpretation
given above, also in the Fucales. In these instances the very large and con-
spicuous sporophytic plant stands in marked contrast to the minute gameto-
phyte, but both are equally required to complete the cycle. Such Algae not
only remind us strongly of the life-cycle in the Pteridophyta, but might be
regarded as another instance of the rise and predominance of the diploid
generation. But from a knowledge of the other types of life-cycle given
above it will be seen that while the Hofmeisterian doctrine of alternation —
founded originally for higher plants — applies in detail to some Algae, it cannot
be extended to the group as a whole. (See Chapter XXXIV.)

Comparison with the Bryophyta, Pteridophyta, and Seed-Plants suggests
that it is in the tetraspores of the Algae that the nearest correlatives are found
to the spores produced in them by tetrad-division. In so far as their place
in the chromosome-cycle is concerned these spores may be held as comparable
with those of Archegoniate Plants. In Chapter XXXIV. the general relation
of the somatic to the chromosome-cycle is considered at some length. The
view is there advanced that the latter is the more stable in plants at large, and
is therefore to be held as the more important in comparison, with its normally
alternating events of syngamy and reduction.

CHAPTER XXIV.

FUNGI. INTRODUCTORY.

The character which all Fungi have in common is a negative one :
the absence of chlorophyll, or of any kindred colouring matter by means
of which they can carry on photosynthesis. They are therefore
sometimes called colourless ; but many of them, and especially the
large Toadstools, are brightly coloured, though with pigments quite
distinct from chlorophyll. The absence of chlorophyll is associated
with the fact that they are able to acquire their supplies indirectly :
that is, either by a parasitic or a saprophytic habit. These methods
of nutrition have already been discussed in Chapter XII., as they
are seen in Seed-bearing Plants. A mere negative character, such
as the absence of chlorophyll, is not in itself a sound basis for classifica-
tion. At once the doubt is aroused whether all the Fungi are really
" blood-relations." Such Phanerogamic parasites and saprophytes
as were described in Chapter *XII. are without chlorophyll. But
they are for the most part referable to well-known Natural Families,
a fact which signifies that their ancestors were green, and that the
parasitic or saprophytic habit was adopted relatively late in the
Evolutionary History, after the seed-habit was already established.
No one would suggest that these plants should be ranked as Fungi,
nor assume that they are all " blood-relations."

It is probable that a like story accounts for the origin of the Fungi,
and that they also sprang from self-supporting ancestors. But there
is reason to believe that at least the majority of them adopted irregular
nutrition early. The opportunity for parasitism and saprophytism
was open from the earliest times, wherever organisms grew closely
crowded, or their decaying bodies were massed together. The study
of fossil plants has shown that organisms characteristically fungal
existed from the earliest geological horizons from which there is any

39i

392

BOTANY OF THE LIVING PLANT

reliable record. In the Devonian Period, long before there were any
Flowering Plants, there is abundant evidence of Fungal structure
existing under conditions favourable to that habit. It does not,
however, follow that all organisms which adopted a " fungal " habit
early were allied to one another, nor that all Fungi originated at
the same time. The existence of parasitic and saprophytic Seed-
Plants is a warning against such an assumption. We should rather

Fig. 291.

Harveyella mirabilis, growing as a colourless parasite on the thallus of Rhodomela,
one of the Red Algae. Longitudinal section of Rhodomela bearing the parasite, with
a mature cystocarp, the fertile filaments of which are black. The cells of the
host with food-material are dotted ; those which are exhausted are left blank. (After
Sturch.)

be prepared to recognise that " Fungi " have originated along more
than one line of Descent, and probably at different times, from very
early periods omcards. It is natural to seek for some Algal origin
for them, for in many features they resemble Algae. At least two
general sources can be suggested, though the actual points of connec-
tion by descent may have been numerous. One is from non-septate
Algae, such as the Siphonales. This might have given rise to those
non-septate Fungi which are called Phycomycetes, from their
Alga-like features. The other is from septate Algae ; and the view

FUNGI. INTRODUCTORY 393

has often been put forward that the septate filamentous Red Algae
gave rise to some of the septate Fungi ; but these may have arisen
independently from a more primitive source, the Protista, i.e., primitive
organisms not definitely distinguished as plants or animals.

The lower organisms, and especially those of aquatic life, live habitually
in close juxtaposition. As a rule any large Seaweed or submerged fresh-water
plant bears innumerable smaller organisms attached to its surface. Sometimes
they penetrate into its tissues. Some by preference frequent certain hosts.
Thus Polysiphonia fastigiata is regularly borne on the Brown Tangle Asco-
phyllum nodosum, and its filaments extend deeply into its tissues. The proof
of actual parasitism is here incomplete, though the regularity of occurrence
arouses suspicion. But there can be no doubt of the physiological dependence
in Harveyella mirabilis, which grows as a colourless parasite penetrating
the tissue of Rhodomela, one of the Red Algae (Fig. 291). By structure and
propagative organs the parasite is clearly another Red Alga, which acts like
a true Fungus. Among the Green Algae, Coleochaete, one of the Chaetophor-
ales, grows habitually on the surface of submerged plants. The allied
Cephaleuros virescens, though still green, penetrates the tissues of the leaves
of Camellia. Other similar cases might be quoted from allied septate Algae.
Again, the green Siphonaceous Phyllcsiphon lives habitually in the intercellular
spaces of the leaf of Arisarum, causing discoloured patches. Such examples,
which might be greatly extended, show how juxtaposition may give opportunity
for parasitic encroachment. They are seen in modern living forms referable
to recognised groups of Algae. They suggest how in the past fungal parasitism
may have arisen. They also prove that the " Fungal " condition may be
arising now, as in the past, and along a plurality of evolutionary lines.

The Fungi are very various in habit, and in form. The most
familiar types are the large Mushrooms and Toadstools, — or " seats
of death," so called in allusion to the poisonous character of some
species. Many of these grow on decaying humus, and like the Common
Mushroom are saprophytes. Others are parasites, like the large shelf-
fungi (Polyporus), which grow out from the trunks of trees, and
are the cause of the perishing of the heart-wood in hollow timber ;
or like the Honey- Agaric (Armillaria mellea), which kills forest trees
by attacking their soft and nutritious cambium (Fig. 292). But
apart from these there are multitudinous smaller Fungi, such as
the parasitic Mildews and saprophytic Moulds, while the unicellular
Yeasts show the simplest structure of them all. However complicated
and various their structure may be, it is based upon the simple or
branched filament, or hypha. The whole system of such hyphae is
called a mycelium. Such filaments may grow singly, as in the Moulds
and Mildews, or they may be massed together so as to form the
complex bodies of the larger Fungi. When closely appressed the
septate filaments may seem to form a definite tissue ; but it is in

394

BOTANY OF THE LIVING PLANT

origin always a false tissue, or pseudo- parenchyma, made up from
independent filaments, not a true parenchyma produced by segmenta-
tion of cells with a common origin. Many Fungi form large solid
masses of such pseudo-parenchyma, which are called sclerotia, and
serve for storage during a resting period (Fig. 293). The hyphae are
limited by a cell-wall, composed of substance differing in its reactions
from ordinary cellulose : they may be septate or non-septate, and in
the former case there may be considerable variety in the number of

Fig. 292.

Base of a young tree (s) killed by Armillaria mellea, which has attacked the roots,
and developed rhizomorphs (r) and fructifications. To the right the fructifications
have been traced by dissection to the rhizomorphs that produced them. (After
Marshall Ward.)

the minute nuclei in their colourless protoplasts. Chromatophores
are absent, and there is no starch, its place being taken by glycogen,
or by globules of oil. Thus structurally the cells of Fungi resemble
those of Algae, but without the chloroplasts or chlorophyll.

The success of the Fungal nutrition, whether parasitic or sapro-
phytic, depends greatly upon their power of penetration of the nutritive
medium. It has been shown in various cases that this is due to a
digestive secretion, and the same probably applies generally. A
highly refractive drop may sometimes be seen on the end of the
hypha, which is believed to contain a digestive ferment (Fig. 294, i. ii.).

FUNGI. INTRODUCTORY

395

I

'//;]

P

El:

A ferment has been extracted from large cultures of a certain Botrytis,
and found to act upon cell-walls, causing them to swell. Such swelling
is a feature of the perforation by the
invading hypha, which first softens
the cell-wall, and then penetrates the
softened mass, finally emerging on the
other side (Fig. 294, iii.-viii.). This
power of perforation has been found
in certain cases to depend upon the
nutrition of the Fungus : for instance,
Sclerotinia sclerotioriun can only pene-
trate living tissue after a period of
saprophytic nutrition. There is, how-
ever, another side to such questions in
the case of parasitic attack : viz. the
power of resistance of the victim,
which depends partly upon the thick-
ness of the protective walls, but pro-
bably also on the presence or absence
of inhibiting substances. Thus fungal
attack may be regarded as a balance
of physiological powers between the
invader and the host. In fact, it
stands on a footing similar to that of
mycorhiza in Phanerogamic Plants, or of conditions of symbiosis
generally (see Chapter XII.).

KV

Fig. 293.

Sclerotium of Ergot of Rye (Claviceps),
a mass of pseudo-parenchyma formed in
the ovary of Rye : above is the style still
covered by remains of " Honey-Dew "
(see p. 425, Fig. 325, a).

Fig. 294.

Successive stages of the penetration of cell-wall of Lily by the hypha of the
Fungus causing the Lily disease. (After Marshall Ward.)

396

BOTANY OF THE LIVING PLANT

It is along such lines that the explanation must be sought for the
condition known as " epidemic," where by a sudden outburst a disease
becomes prevalent. Examples have been seen in the Irish Potato
Famine, the Coffee Disease of Ceylon, or the Lily Disease which in
1888 made the cultivation of Lilies in the Thames Valley a failure.
In such cases the disease is not necessarily a new one. The novelty
lies in the success of the invader. It appears to be due to a change
of balance between attack and resistance. That balance may be
affected either by physiological strengthening of the parasite, or by
weakening of the host. Sometimes the same circumstances may
affect both. In the Lily Disease and the Potato Disease a cold wet

Fig. 295.

Portion of the root of a Crucifer malformed owing to the presence of Plasmodiophora.
(After Woronin ; from Marshall Ward.)

season, while it favours the fungus, produces a thin-walled, watery
host, readily susceptible to attack. A similar epidemic of " damping
off " by Pythium may at any time be induced by cultivation of Cress
overcrowded, in moisture and heat (see p. 400).

The effect of the parasitic invasion may be the death of the host,
where vital parts are destroyed, as in attacks by Pythium, or Armillaria
mellea (Fig. 292). But in many cases the attack is tolerated by the
host, with only partial injury. It is often the leaf, or only certain
tissues of the leaf, which are attacked, the result being a loss of
efficiency by the host while the parasite gains access to the sources
of supply. The host may even be stimulated to greater action, with
the effect of swelling and extra divisions of its cells. The result
may be various malformations, such as are seen in the familiar leaf-curl
of Peaches or the swollen patches of Cluster-Cups (Fig. 337, p. 437)-

FUNGI. INTRODUCTORY 397

The attack may, however, be upon the stem or root, or even the
ovary. The effect is to produce swellings and malformations such
as those of the roots of Crucifers, called " Club-root " (Fig. 295),
or of the grain of " Ergot of Rye " (Fig. 293).

Some Fungi lead a constantly parasitic life, as the Rusts (Uredineae)
do. Others are as constantly saprophytic, like the Saprolegniae.
Others again may be sometimes the one, sometimes the other : and
this may be so in the individual life. Thus Pythium, the " Damping
off Fungus," attacks the seedling-host and kills it, but continues to
live on the corpse (p. 400). It is first parasitic, then saprophytic.
But the converse has been shown in Sclerotinia sclerotiorum, where a
period of saprophytic nourishment is a necessary condition for its
success in perforating the living host. It has been regarded as a fungus
which is in course of " education " for passage from the saprophytic
to the parasitic life. It is thus impossible to lay down any general
rule of priority for parasitism or saprophytism : and it is only in
certain cases that the one habit or the other can be assigned to any
definite systematic group.

The life of Fungi is very varied. No organisms show greater resource
in the acquisition of food. But their propagative methods are no
less effective. Originally sprung from aquatic organisms, some
show this clearly in their reproductive organs, which often involve
motility of zoospores in external water, as in many Phycomycetes.
But the more advanced types are commonly propagated by means
which are clearly related to life in the air. In most Eumycetes
minute bud-like bodies called conidia are borne in prodigious numbers,
and they are small enough to be carried as dry dusty bodies through
the air. The conidia of common Moulds are present everywhere
about the dwellings of man : so that any suitable medium is apt to
be invaded by them, provided the conditions of temperature and
moisture are favourable. This explains the apparently spontaneous
appearance of moulds on bread, leather, jam, etc., when kept in a
confined space. The spread of fungal diseases is usually by similar
means. One of the most surprising facts in this relation is the very
constant recurrence of certain Fungi on isolated and restricted media.
The horns of sheep cast away on a Scottish hill-side are commonly
invaded by a horn-destroying Fungus, Onygena. But any one such
horn may be isolated far from any visible source of infection.
This shows the ubiquity of fungal germs. It suggests also the other
side of the question, the vast number of germs that never find the
suitable nidus.

398

BOTANY OF THE LIVING PLANT

Like other Thallophyta, Fungi may bear organs of sex, which lead
up to the production of accessory spores with attendant reduction
division. They thus show a life-cycle with successive phases com-
parable with that of autophytic plants. But in many of these which
are regarded as the most advanced, and especially in the larger
Agarics, sexual propagation is not normally carried out and the organs
of sex may be actually absent. Various stages of functional per-
version, and of atrophy of the sexual organs, are illustrated by less
advanced Fungi. Most Fungi have thus two forms of propagative
organs in their life-history : conidia, which are minute bud-like bodies
easily detached from the parent, which they produce vegetatively ;

Fig. 296.

Development and fertilisation of the ovum of Pythium. The granular protoplasm
of the oogonium (c) collects into a ball, and the antheridium sends in a fertilising
tube. In (b) and (c) transmission of contents into the ovum is shown, (d) The
ovum has formed a cell-wall, and lies loosely in the oogonium. Highly magnified.
(After Marshall Ward.)

and spores (p. 401), which are formed as a consequence of a sexual
process, or take the same place in the life-cycle in those cases where
sexuality is absent. Some Fungi have more than two modes of propa-
gation. The propagative bodies may differ in appearance, though
they are really mere stages in one life-history : for instance, conidio-
phores and spore-fruits. Sometimes in parasitic Fungi these stages
may appear on different hosts, as in the Rust of Wheat, where the one
stage is on the Wheat, another on the Barberry (p. 432). Natur-
ally, before the facts were fully known, such stages were liable to
be regarded as different Fungi, and described by distinct names.
For instance, the Rust on Wheat was called Puccinia, while the
stage on the Barberry was described as Aecidium. Later these
were proved to be merely parts of the same life-history, and they
were given a single designation. But in many Fungi the life-h?story

FUNGI. INTRODUCTORY

399

is not yet fully known, often because one stage is commoner, or more
obvious than another. Those Fungi in which the knowledge of the
life-cycle is incomplete are called "Fungi Imperfecti,\ and they
constitute a large proportion of the described species.

It will be gathered from the preceding pages that the Fungi provide
characters, vegetative, propagative, and also functional, which will
serve for their classification, though
the data may often be insufficient
for a final decision. Those Fungi
which have non- septate hyphae
are called Phycomycetes. Their
structure is relatively coarse, and
corresponds in this, as well as in
the absence of septa, to what is
seen in the Siphonales. They are
divided into two sub-classes,
according to the sexual organs.
Where these can be distinguished
as male (antheridia) and female
(oogonia) they are called Oomy-
cetes (Fig. 296). To these belong
such parasites as the Perono-
sporeae, for instance, the Potato
Fungus (Phytophthora) ; also the
saprophytic family of the Sapro-
legniae, which includes the
" Damping-off Fungus " of seed-
lings (Pythium). The latter live
on vegetable or animal matter
decaying in water. In the second
sub-class there is conjugation of
similar bodies to form a zygo-

Fig. 297.
Mucor Mucedo. Different stages in the forma-
tion and germination of the zygospore, i = two
conjugating branches in contact. 2 = septa tion
of the two conjugating cells. 3 =more advanced
stage • the conjugating cells are still distinct
from one another. 4 =ripe zygospore (b) between
5 = germinating zygospore

the suspensors (a).

with germ- tube bearing a sporangium. (After
Spore, as in the Common MOUld, Brefeld.) (1-4x225, 5 x circa 60.) (From
,„. N ^p, 1 Strasburger.)

Mucor (Fig. 297). These are de-
signated the Zygomycetes and have been referred in origin to Algae
of the type of the living Siphonales. The Phycomycetes are dealt
with in Chapter XXV.

The great bulk of the Fungi are probably distinct in Descent from

these. The structural distinction lies in their septate hyphae. The

constituent 4: cells " between the septa vary in length and in nuclear

condition. In some cases the nuclei are small and numerous : in

b.b. 2 c

4oo BOTANY OF THE LIVING PLANT

others there is a pair of nuclei, or only one. These septate Fungi are
sometimes called Eumycetes. The sexual organs of many of them
show a resemblance to those of Red Algae, especially when there is
a female oogonium with trichogyne. But in many of them such organs
of sex have not been found, and there is reason to believe that they
are no longer sexually produced. They are divided into two sub-
classes, according to the method of production of their spores. In

Fig. 298.

Portion of the hymen ium of the Morel
{Morchella esculenta). a =asci, each con-
taining eight ascospores. p = paraphyses.
sh = subhymenial tissue. ( x 240.) (After
Strasburger.)

Fig. 299.

Honey Agaric (Ar miliaria mellea). A,
young basidium with two primary nuclei.
P^ after fusion of the two nuclei. C =*a
basidium of Hypholoma appendiculatum
before the four nuclei derived from the
secondary nucleus of the basidium have
passed into the four basidiospores
D = passage of a nucleus into the basidio-
spore. (After Ruhland.) (From Stras-
burger.)

the first the spores are commonly eight in number, and are produced
internally within a closed body, the ascus. These are called ascospores,
and the sub-class the Ascomycetes (Fig. 298). To them belong
many Moulds, Ergot of Rye, the edible Truffle, etc. Examples will
be described in Chapter XXVI.

In the second sub-class the spores are produced externally, com-
monly to the number of four, upon a body called a basidium. They
are called basidiospores, and the sub-class the Basidiomycetes
(Fig. 299). To them belong the Mushrooms and Puff-balls ; also the
large series of parasitic Rusts, which being more primitive in their
characters than the rest, give probable clues to the origin of the
Basidiomycetes. They will be described in Chapter XXVII. Both
the ascospores and the basidiospores are to be held as equivalent to

FUNGI. INTRODUCTORY

401

tetraspores, being related either actually to a process of nuclear reduc-
tion, or at least holding a position in the cycle following closely upon
the period when reduction would occur in a normal cycle where sexual
organs are present.
The Fungi may then be grouped thus :

Class. Sub-Class.

Phycomycetes - \Oomycetes

(non-septate) [Zygomycetes

ASCOMYCETES

(septate ; with asci)
Basidiomycetes ....

(septate ; with basidia)

Examples.

Pythuim, Phytophthora.
Mucor, Empusa.
Sphaerotheca, Euro-

tium, Peziza.
Puccinia, Ustilago

A gar tens, Boletus,

Scleroderma.

A Note on Terminology.

Propagative cells of Fungi which are produced asexually and externally are
usually described as conidia : those produced within a spore-case or sporangium
are usually described as spores. The stalks or filaments bearing sporangia or
conidia are described as sporangiophores and conidiophores respectively.
Some spores may be no more than vegetative organs of reproduction, but
others, such as ascospores and basidiospores, follow on a process, or substitute
process, of sexual reproduction and occur at a definite point in the nuclear
cycle.

CHAPTER XXV.

PHYCOMYCETES.— (a) OOMYCETES.

Two of the commonest and most destructive of fungal parasites will
serve to illustrate the Oomycetes. They both belong to those non-
septate Fungi which habitually produce distinct male and female
organs, comparable to those seen in the higher Siphonaceous Algae,
such as Vaucheria. Like them also they include in their life-history
a stage where zoospores are motile in water. This, together with
their close dependence upon moisture during vegetation, justifies for
them the name Phy corny cetesy or Alga-like Fungi.

The " Damping-off Fungus " (Pythium debaryanum).

When Mustard and Cress are sown thickly, and kept too warm
and damp, the seedlings are liable to the disease of "damping-off",

the plants quickly rotting with an unpleasant
smell. Many other seedlings, and especially
Cucumbers and Melons, are subject to it ; in
fact, the disease is one of the commonest diffi-
culties of the gardener, and ruins the efforts of
many amateurs. It makes its appearance at
definite spots in the seed-beds, and if not checked
it spreads thence in ever-increasing circles. The
first sign is the collapse of a seedling, owing to
the shrinking of its cortex, usually at some
point above the soil-level, the stem being no
longer able to support the weight above (Fig.
Fic 300. 300). If the diseased plant be examined micro-

A young Cress-seedling .... . .,, , - 1,1 -jji j

attacked by Pythium at d, scopically its tissues will be found to be riddled

just above the ground-line c. , , 111 11 1 11

6=root. a = cotyledons and through and through by .rather coarse colourless

plumule. (Nat. size.) (After ,, , 111 n 11 j cii j -x.u

Marshall Ward.) threads, enclosed by a cell-wall and tilled with

402

PHYCOMYCETES— (a) OOMYCETES 403

granular protoplasm which contains numerous nucl The threads
traverse the cell-walls of the host with the greatest case (Fig. 301),
while collapse of the cells and loss of mechanical firmness lead to the
falling over of the diseased plant. Left to itself in moist air the disease
may spread from plant to plant, the hyphae passing out from the
tissues and forming cottony growths through the damp air : they

Fig. 301.

Small portion of cellular tissue of a Potato, showing the passage of the hyphae of
Pythium through the cell-walls at b. At a, hyphae are seen in an inter-cellular
space, one of which has then entered the large cell. Highly magnified. (After
Marshall Ward.)

are coarse enough to be seen with the naked eye. The affected seed-
lings soon become a putrid mass of decay. The fungus that causes
the trouble is Pythium debaryamim, which belongs to the large family
of the Peronosporales. Most of these plants live actually in water,
and cause decay in submerged plant- and animal-matter. One of
them, Achlya, appears with a high degree of certainty on dead flies,
if left floating in foul water.

Pythium propagates both vegetatively and sexually. The vegeta-
tive propagation is by sporangia (Fig. 302, c), formed usually from
the ends of the hyphae, by their swelling to an oval form. They con-

404

BOTANY OF THE LIVING PLANT

tain fine-grained protoplasm and are shut off by a septum. The
sporangium is readily detached and germinates directly if the circum-

Fig. 302.

Portion of tissue near d in Fig. 300, highly magnified. The hyphae are seen
running in all directions ; at a, one passes through a stoma ; at c, a sporangium
L> about to form. (After Marshall Ward.)

stances are favourable. It then grows out into a fresh hypha which
may directly infect a new victim. In other circumstances the ger-
minating filament may expand into a spherical body, and the contents

Fig. 303.

Germination of a sporangium of Pythium debaryanum in water. The tube put
forth at a begins to swell at the end {b, c), and dilates (d), receiving all the protoplasm,
which rapidly breaks up into zoospores (e). The whole process occupies about a
quarter of an hour. Highly magnified. (After Marshall Ward.)

passing into it undergo division into a number of zoospores capable of
movement (Fig. 303). These escape by rupture of the vesicle as
minute colourless, kidney-shaped bodies with two active cilia.
Provided water is present, they can swim to and even climb up the
stem of other seedlings, and, coming to rest, perforate the superficial

PHYCOM YCETES— {a) ( )( )M Y( KTES

405

cell-walls, causing a new infection. It is in this way that the attack
commonly appears some way up the stem. This method of propaga-
tion may be continued throughout the season.

It is found that the disease is liable to reappear in the following
year in seedlings grown on soil that has been badly affected before.
This has been explained by the discovery of an alternate mode of
sexual propagation, which produces oospores : these retain their
vitality through the winter. If an infected plant be kept moist,
or even immersed in water for a few days, the hyphae begin to form

Fig. 304.

Development and fertilization of the ovum of Pythium. The granular protoplasm
of the oogonium (c) collects into a ball and the antheridium sends in a fertilising
tube. In (b) and (c) transmission of contents into the ovum is shown, (d) The
ovum has formed a cell-wall, and lies loosely in the oogonium. Highly magnified.
(After Marshall Ward.)

swellings at their ends, like the sporangia but larger (oogonia). Pre-
sently in these the protoplasm begins to draw away from the wall,
and rounds off as a central sphere (the ovum). Meanwhile another
branch, or the end of another hypha, grows up with a smaller swelling,
which is also cut off by a septum (the antheridium). It comes into
contact with the first, and puts out a slender tube which penetrates
the cell-wall and extends to the ovum (Fig. 304, b). This is the
fertilising tube, which transmits its contents into the ovum. After
fertilisation the zygote surrounds itself with a cell-wall, which soon
thickens, but retains a smooth surface. This is the oospore, which
can retain its vitality through the winter (Fig. 304, d). The oospores
will not germinate at once, but require a period of rest. In the spring
under favourable conditions the thick wall bursts and a hypha is
produced, which soon develops sporangia and zoospores as usual.
The initial infection of cultures of seedlings in each year is from this
source, the resting spores being present in the soil. But besides this
the fundus can continue its life as a saprophyte . for its activity does

406

BOTANY OF THE LIVING PLANT

not end with the death of its host. There is in fact no absolute line
between the parasitic and the saprophytic habit.

The risk of attack on the seedlings may be reduced by avoiding
too close sowing and too moist culture ; also by avoiding the use
of soil in which infected seedlings have been raised. The risk may be
further reduced by raising the soil before sowing to a temperature
that will kill all the germs that it contains.

The Potato Fungus (Phytophthora infestans).

The Potato Fungus is the cause of what is commonly known as
the " Potato Disease," which is always a risk to the crop in damp,
warm seasons. In the years 1 845 -1 850 the disease already known

Fig. 305.

A Potato-leaf, showing the spots and patches of " Potato-disease ", due to Phyto-
phthora infestans. In the darker patches the tissues are quite dead. The margins
of the spots would show the hvphae of the fungus projecting from the surface. (After
Sorauer ; from Marshall Ward.)

in America assumed epidemic virulence in Ireland, causing the great
famine. Since that time the potato crop has never been entirely
free from it. The disease makes its appearance upon the leaves and
stems as spots at first small and pale-coloured, but as they enlarge
the centre of each becomes brown, and extends, though still with a
pale margin, till the spots run together, and the whole leaf or even the
whole shoot may be affected (Fig. 305). If leaves with young infected
patches be examined on a damp still day, or better, if they be kept in
moist still air under a bell-glass, white glistening filaments will be
found on the lower surface. They are large enough to be seen with
the naked eye, and are the sporangiophores of the fungus. They bear

PHYCOMY* ETES— (a) OOMYCETES

40/

*%zm

numerous small white powdery bodies, which are really comparable
with the sporangia of Pytkium ; these are very readily detached.
These sporangiophores spring from the mycelium, which permeates
the mesophyll of the leaf in the
diseased patch (Fig. 307). The
mycelium consists of coarse non-
septate, branched hyphae, which
traverse the intercellular spaces,
coming into close contact with the
moist walls of the cells. They are
also able to penetrate the softer
middle-lamella of the cell-walls,
where two cells adjoin, and this
brings them into still closer relation
to the cells, and the nourishment
which these can supply (Fig. 306).
Though the cells of the potato are a
not as a rule perforated, they lose
their vitality and collapse, probably
owing to a toxic influence. The
brown discolouration at the centre
of the infected spots is due to their
decay.

The rapidity of the spread of the
disease is one of its most surprising
features. The fact that it habitu-

Fig. 306.

Piece of the tissue of the stem of a Potato-
plant, showing the hyphae of Phytophthora
penetrating the middle lamella of the cell-walls,
ally Spreads down the prevailing a=nucleusof a cell. Highly magnified. (After

wind indicates

Marshall Ward.)

that it is due to
wind-borne sporangia. The sporangiophores project through the
stomata on the lower surface of the leaf and branch repeatedly
(Fig. 307). The end of each branch may swell into an inverted pear-
shaped sporangium, which is constricted off from its very thin stalk.
If growing in still air the first sporangium may be turned aside, the
stalk growing on sympodially, and proceeding to form a second
sporangium, and so on (compare Fig. 179, p. 258). Thus a suc-
cession of sporangia may be produced for a considerable time, each
readily detachable and easily borne by the wind.

Germination takes place only in presence of moisture. The proto-
plasm of the sporangium divides into about ten parts, which by rupture
of its apex escape into the water in the form of zoospores, very like
those of Pythium ; they are motile for a time by means of two cilia

408

BOTANY OF THE LIVING PLANT

(Fig. 308). Coming then to rest, the cilia are dropped: each zoo-
spore rounds itself off, and, investing itself with a wall, puts out a
hyphal tube. If this takes place on the surface of a potato leaf, as it
well might do under conditions of rain or heavy dew, all is ready
for the infection. This may be either by entry through the pore of a

Fig. 307.

Section of Potato-leaf, in the tissues of which is the mycelum of Phytophthora.
The hyphae run between the cells and send out through the stomata, a, c, d, the
aerial branches which bear the sporangia, b. The dark parts of the tissue of the leaf
show where the cells are dying from the effects of the parasite. Highly magnified.
The normally upper surface of the leaf is here turned downwards. / is a glandular
hair of the Potato. (After Marshall Ward.)

stoma or by direct perforation through the epidermal wall (Fig. 309).
By either route the germ-tube of the parasite may reach the inter-
cellular spaces and establish a new infection.

It is not only the leaves but also the stems and tubers of the
Potato-plant that may be traversed. The mycelium spreads through
the tissues down the haulms to the tubers. Young tubers may be
infected by way of the " eyes ", the lenticels or through wounds.
Tubers thus infected may decay at once under moist conditions
and when other saprophytic organisms are present, otherwise a
dry-rot results. Tubers in the latter condition may be among

PHYCOMYCETES— (a) OOMYCETES

409

those harvested. But such tubers often decay during the winter.
If however, they are not heavily infected and are used as " seed '

d

9

J

Fig. 308.

Stages of germination of one of the sporangia of Phytophthora. a, the ripe
sporangium in water, b, protoplasmic contents breaking up into blocks, which
separate and escape (c, d) as minute kidney-shaped zoospores (e), each with two
cilia. /, g, the zoospores coming to rest, and losing its cilia, h, i, j, k, stages of
germination of the zoospore. Highly magnified. (After Marshall Ward.)

potatoes for a new crop, the mycelium perennating in the tubers provides
a source of infection in that sporangiophores develop on short aerial

Fig. 309.

Germination of zoospores of Phytophthora on the epidermis of Potato. At (a)
the germ- tube is entering a stoma. At (c) it bores directly through the cell-wall.
Highly magnified. (After Marshall Ward.)

4io

BOTANY OF THE LIVING PLANT

shoots which grow out from the tubers. There may be also other
sources of reinfection. The control measures to be taken are to destroy
by lire all infected haulms and leaves, to avoid carefully the use of
tainted " seed " tubers ; and, as a preventive, to spray the young
growing crop with suitable disinfectants, especially if the season is
wet in the middle summer. But a more hopeful line of prevention is
by the use of " immune varieties," which are able to resist the attack
of the parasite.

Fig. 310.

Fertilisation of the Peronosporales. 1. Peronospora parasitica, young multi-
nucleate oogonium (og ), and antheridium (an). 2. Cystopus Candida. Oogonium with
the central, uninucleate egg {os), and the fertilising tube (a) of the antheridium which
introduces the male nucleus. 3. The same. The fertilised egg (0) surrounded by
periplasm (p). (After Wager, x 666.) (From Strasburger.)

No mention has been made of sexual reproduction in the Potato
Fungus. As a matter of fact sexual organs have not been proved to
exist in Phytophthora infestans under normal conditions of life. Like
some other plants it seems to be able to propagate itself indefinitely
without the recurrence of sexual reproduction. But under special
conditions of saprophytic culture the sexual organs have been
produced, though it is still doubtful whether they are ever formed
when the Potato Fungus is growing on the living host. In other
members of the Peronosporales the details which have been frequently
observed show a striking parallelism with those of Vaucheria. The
sexual organs have been found in Peronospora and Cystopus to be
formed within the host-plant (Fig. 310). The oogonium appears
as a spherical swelling on the end of a hypha, while a thinner branch,

PHYCOMYCETES— (a) OOMYCETES 411

arising as a rule below it on the same hypha, forms the anther idium.
Each is shut off by a septum, and contains dense protoplasm with
numerous nuclei. A single egg, or ovum, is differentiated in the
middle of the oogonium, by passage of all the nuclei but one to a
peripheral position. The uninucleate ovum is then delimited from
the multinucleate periplasm. The anthcridium penetrates the
oogonium by means of a fertilising tube, the apex of which opens
into the ovum, and transmits a single male nucleus (Fig. 310, a).
The resulting zygote soon surrounds itself with a membrane, while
the periplasm contributes to the thickening of its wall. A period
of rest may ensue. On germination the contents of the zygote
divide, giving rise to a number of zoospores, which may cause a fresh
infection in the same way as those produced from the sporangia.

The parallel between this structure and that of Vaucheria is close
as regards the origin of the uninucleate ovum. The method of
fertilisation by a fertilising tube in place of the liberated spermatozoids
moving freely in water, offers an interesting parallel with the pollen-
tube in Seed-Plants. In both cases the male gamete is conveyed
to the female by a method suitable for land-living plants. Comparison
shows that both are modifications of organs originally developed
to secure fertilisation through the medium of external water. The
question will naturally arise whether any fungal type still shows a
motile male gamete. This is found in Monoblepharis, a fungus that
lives saprophytically in water (Fig. 311). Here a terminal oogonium
contains a single ovum, which is fertilised by spermatozoids, each
motile by a single cilium. ' This case is unique among Fungi. It
holds a place comparable with the motile spermatozoids of the Cycads
and Ginkgo, in that it gives evidence of the transition from an aquatic
to a terrestrial type of fertilisation. (Compare Chapter XXXIV.)

The origin of such Phycomycetous Fungi as those described may
have been from some Siphonaceous source. In the case of parasites
the first step would be the adoption of an endophytic life, as in
Phyllo siphon. This would be naturally followed by parasitic nutrition
and loss of chlorophyll and chloroplasts. As regards the propagative
organs, the sporangium of Pythium or of Phytophthora are such as to
make for easy detachment and transport through the air. But it
shows its real nature on germination by producing zoospores which
are liberated in water. In this connection it may be noted that in the
Peronosporales as a whole there is a marked tendency for the sporan-
gium to become, or to behave as, an air-borne conidium, as in Peron-
ospora in which zoospores are unknown. Sexual reproduction in the

412

BOTANY OF THE LIVING PLANT

Peronosporales is not unlike that seen in Vaucheria : the former do not
liberate free spermatozoids but transfer a single gamete by a tube, as
in the higher land-living Plants. The oogonium retains its character,
as in Vaucheria. Such considerations may be held to justify the
recognition of Fungi included among the Oomycetes as possibly of

1 Z

Fig. 3tt.

Monoblepharis sphaerica. End of a filament with terminal oogonium (o) and an
antheridium (a), i, before formation of the gametes. 2, spermatozoids (s) escaping
and approaching the opening of the oogonium. 3, osp, ripe zygote and an empty
antheridium. (After Cornu. x 800.) (From Strasburger.)

Siphonaceous origin, from types already advanced in their sexuality.
On the other hand it has been suggested that the origin of the Phy-
comycetes is to be sought among the colourless Protophyta and that
they have developed along lines parallel to, but independent of, those
leading to the Green Algae.

PHYCOMYCETES.— (b) ZYGOMYCETES.

The Mucors.

The Zygomycetes include many common Moulds. They are mostly
saprophytes, though some of them are parasites not only on other
plants and animals, but even on one another. They are characterised
first by their coarse non-septate hyphae, but more particularly by
the manner of their sexual reproduction, which results from the
fusion of two similar, multinucleate branch-endings, to form a single
large resting-spore, or zygospore. The chief representatives are
the Mucors (Mucorales) found on decaying organic matter in moist
circumstances. If moist bread, horse-duns:, brewer's grains or
other organic substances be kept warm for a day or two under a
bell-glass, Mucorineous Moulds will almost certainly appear. They

PHYCOMYCETES— (b) ZYGOMYCETES

413

are often of considerable stature. Phycomyces nitens, which comes
commonly on brewer's grains, may be several inches in height, the
coarse sporangiophores ending in sporangia easily seen with the naked
eye.

Fig. 312.

A plant of Mucor, showing the mycelium of branched hyphae (m) and sporangio-
phores (g). A is a single sporangium more highly magnified, containing spores.
(After Brefeld.)

Another common type is Empiisa muscae (Entomophthorales),
which lives parasitically on the House Fly. In autumn the infected
flies become sluggish, and finally resting on a window pane, or else-
where, they appear as though surrounded by a white halo. This is
formed by the conidia thrown off to a distance by the stalks that
bear them, which radiate outwards from the body of the fly killed by
the fungus in its vegetative stage. Thus, while most of the fungi
of this group are saprophytes, some may be parasitic, even on
animals.

414

BOTANY OF THE LIVING PLANT

If the stalks, or sporangiophores, of Mucor, or Phycomyces be followed
to their base, they are found to arise from finer filaments forming a
profusely branched mycelium, which traverses the substratum. A
good idea of its nature is obtained by culture from the spore on a
glass surface (Fig. 312). It is then seen how, starting from the spore
as a centre, the mycelium may radiate outwards, with successively
finer branches of its non-septate tubes. Each of the thick upright
sporangiophores bears one sporangium on its end, which when ripe
consists of a brittle external wall surrounding many spores embedded

Fig. 313.

I, Mucor Mucedo, a sporangium in optical longitudinal section, c = columella.
»i=wall of sporangium. sp=spores. 2. Mucor mucilagineus, a sporangium
shedding its spores ; the wall (w) is ruptured, and the mucilaginous matrix (z)
is greatly swollen. (After Brefeld, 1 x 225 ; 2 x 300, after v. Tavel.) (From
Strasburger.)

in a mucilaginous matrix, while centrally is a large columella (Fig.
313, 1). It is difficult to see this structure satisfactorily in ripe
sporangia mounted in water, owing to the swelling of the mucilaginous
matrix, which bursts the wall and scatters the spores (Fig. 313, 2).
Their dissemination may thus normally take place by the agency of
water, though Mucor can also disperse its spores in dry air.

Though this is the case for the typical Mucors, there are many Mucorineous
Fungi in which the dissemination of spores is through the air. They are
found among those smaller forms which live parasitically on the larger Mucors,
and frequently appear upon old cultures of these as flocculent growths attached
by suckers to their sporangiophores {Chaetocladium, Piptocephalis). In
these parasites the sporangiophores are profusely branched, and they bear
many unicellular, conidium-like bodies, easily detached, and carried by the
wind. That they are really sporangia of very reduced form is indicated by
intermediate types, with minute sporangia, which contain a few spores.
Even Mucor itself, when starved, may produce such small sporangia, which
show how the still simpler condition may arise. The family thus illustrates a
transition from water-dispersal, by spores produced internally in few large
sporangia, to dispersal through the air of sporangia reduced to a single cell,

PHYCOMYCETES— (b) ZYGOMYCETES 415

and produced in larger numbers. The latter may be held as a derivative
condition.

A very peculiar mode of dispersal is by forcible projection of the whole
sporangial head. This is seen in Pilobolus, whence its name. This fungus
appears with a high degree of certainty on dung kept under a bell-glass. The
sporangium is constructed like that of Mucor ; but at ripeness it breaks away
from the stalk, which has become turgid with sap under osmotic pressure.
By the principle of the squirt this Huid is thrown out to a distance of some
inches, carrying with it the sporangial head. A somewhat similar projection
happens in Empusa, but here it is a sporangium reduced to the state of a single
" conidium " that is discharged ; these conidia by adhering to any solid body,
cause the halo previously mentioned. There is thus a considerable variety
in the methods of dissemination in the Zygomycetes.

The spores of many of the Mucorales contain more than one
nucleus. There are said to be two in Pilobolus, and many in Sporodinia.
In the latter, which grows parasitically on large saprophytic species of
Boletus, the sporangial head is first shut off by a septum : this becomes
convex, and forms the central columella. The large polynucleate
mass of protoplasm filling the head then undergoes cleavage into a
number of parts, each containing several nuclei. These round them-
selves off and form the spores. Such cleavage of the contents of the
sporangium is the typical method of formation of spores in the
Mucorales. The final result is the same in all : viz. germination under
favourable circumstances to form a new non-septate, and poly-
nucleate mycelium.

The mycelium of Mucor may vary in its mode of growth according
to the medium. If it be submerged in fluid it shows the Oidium-
condition, where, dividing into short lengths, each of these may
increase by a process of budding not unlike a yeast. Or again, if
the conditions are unfavourable, the starved mycelium may divide
transversely, and the portions become thick-walled, as so-called
Chlamydospores, a state reminiscent of the behaviour of Vaucheria
geminata under like conditions (p. 372). When the circumstances
are again favourable, either of these states may pass over into the
normal mycelium again.

The chief alternative mode of propagation is, however, by the
production of Zygospores. In many Mucorales this is normally
a rare event, in others common. One dominating circumstance, the
fact of a difference of the nature of sex, has been observed, which
may explain the rarity of its occurrence. The essential feature
is the coalescence of the ends of two equal club-shaped hyphae to
form a single fusion-body, which is the zygospore. Two hyphal

B.B.

2 D

4i6

BOTANY OF THE LIVING PLANT

branches {pro gametangia), either of the same mycelium (Sporodinia),
or of distinct myceiia [Mucor stolonifer, and other Mucors), growing
towards one another, meet at their apices (Fig. 314). From the end of
each a conjugating cell or gaynetangium containing protoplasm with

many nuclei is cut off by a trans-
verse septum from the basal sus-
pensor. Their apices flatten, and
the gametangia fuse, the wall
separating them being absorbed.
Many nuclei are involved ; these
fuse in pairs and the result on
maturation is a zygospore (Fig.
314, 4). The large protoplast is
stored with nutritive material,
while the outer wall thickens,
often forming characteristic bosses
externally (Fig. 314, 3, 4)- In
this condition the zygospore may
undergo a period of rest, and is
resistant to unfavourable con-
ditions. But suitable conditions
induce germination, and they de-
termine what follows. Sometimes
there is a formation of a vegetative
mycelium, sometimes, as in Fig.
314, 5, an immediate formation of

Mucor Mucedo. Different stages in the forma- a sporangium I the Spores prO-
tion and germination of the zygospore, i =two . , « i r

progametangia in contact. 2 =septation of the duced in the latter are the tirSt
two gametangia. 3 = more advanced stage ; the

conjugating cells are still distinct from one stage of the new gametophyte.
another. 4 =ripe zygospore (b) between the .

suspensors (a). 5 =germinating zygospore with A remarkable fact IS the OCCUT-

germ-tube bearing a sporangium. (After Brefeld.) '

(1-4x225; 5 x circa 60.) (From v. Tavei.) rence in some Mucors 01 Azygo-

(From Strasburger.) ,

spores," that is, bodies that re-
semble zygospores in their structure and covering, but are produced
without any fusion.

According to their power and readiness for conjugation the Mucorales
have been divided into two groups. The Homothallic, which form zygospores
on two branches of the same mycelium, so that by sowing a single spore
zygospores may be obtained. This is seen in Sporodinia, in which the zygo-
spores may often be obtained in the open in autumn. Others are called
Heterothallic, where the presence of two individuals of two different types
is necessary for the production of zygospores (Rhizopus, Mucor, Phycomyces)
These types are distinguished as + and - , rather than as male and female,

Fig. 314.

PHYCOMYCETES— (b) ZYGOMYCETES 417

because in many species there is no recognisable difference of form or structure,
but only of function. If either be cultivated pure, and apart, the mycelium
bears no zygospores. But if cultures of the + and - types be started apart
and meet, a profuse formation of zygospores appears along the line of junction
(Fig. 315). The facts thus disclosed give a ready explanation of the rarity of
zygospores in certain cases, and their frequency in others.

The facts of the life-history in the Zygomycetes show a less direct
dependence of these plants on external liquid water than in the
Oomycetes, for there are no zoospores motile by cilia. Still the dis-
semination of the spores in the Mucors is through swelling of mucilage

Fig. 315.
Result of a plate-culture of the heterothallic Mucor hiemalis, made by Prof.
Drummond. + and — strains were started on opposite sides of the plate. The
dark line transversely between these shows where the cultures meet, and the zygo-
spores were formed. (£ natural size.)

in water, or ejection where liquid pressure gives the propulsive power.
The series with branched sporangiophores, and wind-borne conidium-
like bodies is a step still further away from dependence upon the
water-medium. Comparison suggests for the more primitive sporangia
such as Mucor an origin from a sporangium like that of a Siphonaceous
Alga. The loss of motility of the spores which is involved is readily
understood in organisms living in moist air in place of water.

The formation of zygospores presents the unusual condition of the
fusion not of single cells as in the Conjugatae, but of coenogametes
to form a coenozygote. There is reason to believe that numerous
nuclear fusions take place : in fact that the formation of the zygospore
is a fusion of gametangia, rather than of single gametes. If two
gametangia, like those of Codium or Bryopsis, were to fuse as a whole,
in place of opening and shedding their gametes to fuse singly, the
result would be very like what is seen in the Mucorales.

If we consider the sexual reproduction of the Phycomycetes as a
whole, the Zygomycetes and the Oomycetes would appear to repre-
sent two distinct lines of evolution, the Oomycetes showing the
more advanced condition, in which sexual differentiation extends
to the gametangia themselves.

CHAPTER XXVI.

ASCOMYCETES.

The Fungi belonging to the Ascomycetes, the first sub-class of the
septate section, are very various in habit. Many are parasites,
often on leaves and stems of Flowering Plants : for instance the

Mildews, such as Sphaerotheca. Others
are saprophytes, such as the small and
prevalent Moulds, Aspergillus and Pent-
cillium. Others again form large fruiting
bodies, such as those of Peziza, or the
edible Truffle (Tuber), or the Morel (Mor-
chella). Some are parasitic on animals, as
in the case of Cordyceps, which invades
caterpillars and the larvae of Cock-
roaches. The Ascomycetes are thus not
only a large but very varied group of
Fungi.

Their characteristic feature is a club-
shaped or oval body, the Ascus, within
which Ascospores usually to the number
s'1 of eight are contained (Fig. 316). Such
asci may occasionally be produced singly
FlG- 316. jn very simple forms, such as Sphaero-

MoreMA/orcL/S^s^SaPalascL theca ; but they are commonly associated
SsluTT^o.) wSiSKESS together in large numbers, in fruit-bodies

of various form. In many cases the
development of the asci has been found to follow on the formation
of sexual organs, of which the female is an oogonium, sometimes with
a receptive trichogyne, as in the Red Seaweeds. The ascospores may
therefore be held to be of the nature of post-sexual tetraspores formed

418

ASCOMYCKTKS

419

after reduction carried out in the nuclear divisions within the ascus.

In other cases the sexual organs have not been found, and there is
reason to believe that normal sexuality has passed into abeyance in
many of these parasitic and saprophytic plants, though the asci still
remain as morphologically representing the products of the oogonium.
In most of these Fungi there is also propagation by means of minute
conidia, which are buds not sexually produced. They are borne on
conidiophores, which are various and characteristic in form.

The young unicellular ascus has originally two nuclei, which later fuse.
The resulting nucleus then typically undergoes three successive divisions
involving a process of reduction. Each of the eight nuclei then becomes
a centre of free cell-formation ; an area of cytoplasm around each is
delimited by a cell-wall, leaving a residuum of cytoplasm which embeds
the spores till ripeness. Ultimately, the spores are liberated, in the
majority of cases by forcible ejection, more rarely by disintegration
of the ascus-wall. Sometimes the spores are shot out for a distance of
a foot.

The Mildews (Erysiphales).

The Mildews are Ascomycetous Fungi, parasitic on the leaves of
various plants. They take their name from the fact that the patches

Fig. 317.

a -a germinating eonidium of an Erysiphe, showing how the young gam-tube
at once attaches itself by a haustorium to the epidermis, b -a haustorium in
section. (After De Bary, highly magnified.) (From Marshall Ward.)

of the disease appear white and floury, owing to the formation of
their conidia, produced from a cobweb-like mycelium, which grows
on the outside of the infected leaf. Its hyphae are attached to the

420 BOTANY OF THE LIVING PLANT

leaf by haustoria, which penetrate the outer walls of the epidermal
cells, the connection being established immediately on the germination
of the infecting conidium (Fig. 317). As the season progresses
small dark specks make their appearance, large enough to be seen
with the naked eye. These are the fruits, or perithecia, each con-
taining one or more asci. In damp weather these parasites develop
quickly, a fact that has drawn almost superstitious attention to them.
A very common example is seen in Podosphaera clandestina, which
infests the leaves of the Hawthorn in any warm and wet autumn.

Fig. 318.

Piece of epidermis of Hop, showing mycelium (b) and perithecia (a) of the Hop-
mildew on its surface, h is a large hair. At (b) the first beginnings of a perithecium.
Magnified. (After Marshall Ward.)

A still simpler one, Sphaerotheca Hamuli, causes a disease on the
cultivated Hop, though it may occur also on many other plants.
The effect of Mildews on the infected plant is that the diseased areas
take a pale colour, nourishment being withdrawn from them. But
there is no malformation.

After the Hop Mildew has established itself, and formed a branched,
septate mycelium, certain hyphae grow vertically upwards from
the leaf-surface without branching : they segment transversely into
short lengths, which become detached in basipetal series as conidia,
easily removed when ripe by a breath of air. They will germinate
on a moist leaf, and cause new infections during the summer (Fig. 3 17)-
But later the fruit-bodies (perithecia) appear, which provide for
the winter's rest (Fig. 318). They arise where the branched hyphae

ASCOMYCETES

421

cross or touch one another. Two short branches grow erect, and a
uni-nucleate cell is shut off from the end of each. One which is
larger is recognised as the oogonium, the other which is smaller is the
antheridium. They fuse, and the nucleus of the latter passes into
the former, which then becomes surrounded by an investment of
up-growing filaments, forming an outer shell to the fruit. Meanwhile
the oogonium divides into a row of cells, and the penultimate cell forms
the single ascus, with eight ascospores (Fig. 319, C. s.). Protected by

a_ A

Fig. 319.
Various stages in the development of the perithecium of the Hop-mildew,showing
the contact of the two short branches (^4), one of which (p) gradually becomes
invested by enveloping branches (B). The envelope forms the wall of the perithecium,
and the single ascus is formed from the enclosed branch. (C. D.) Highly magnified.
(After De Bary.) (From Marshall Ward.)

the outer shell, and attached by long filaments which grow out from
their surface, the fruits remain fixed to the leaves, which fall and
rot. In the following spring they rupture, and the ascospores are
shed. In favourable conditions germination ensues.

This is one of the simplest of Ascomycetous fruits. In Erysiphe
and others hyphae are produced from the female organ, and these
bear numerous asci. The fruit-bodies of the larger Ascomycetes are
in general more complex.

Moulds (Plectascales).

If bread, or any organic body such as leather or jam, be kept in
a closed damp space, for instance under a bell-glass, or in a damp
cupboard, it will become mouldy. This is due to the germination
upon it of air-borne germs of Moulds that are always present about
the dwellings of man. These Moulds at first form isolated patches
on the bread, which then run together, covering the whole surface,
and also penetrating inwards. There are many different Moulds that
may thus appear, and they grow mixed together, or they may be
segregated into distinct, purer patches. Where this occurs the two

422 BOTANY OF THE LIVING PLANT

commonest are readily distinguished by their stature and colour.
Low-growing, velvety, blue-green patches are Penicillium crustaceum ;
coarser, olive-green patches, with mop-like heads, of size visible
tc the naked eye, are Eurotium (Aspergillus) herbariorum. As the
patches of the latter grow older, minute yellow specks may appear
upon them : these are the Eurotium-irmts, a stage originally described
as a distinct fungus. A breath will carry away the numerous conidia
from such a culture in a dense cloud. They form part of the ordinary
dust of dwellings, and this accounts for the constant appearance of the
Moulds on organic substrata where the conditions are favourable to
heir growth, as in the moist air in a close cupboard, or under a bell-jar.

Fig. 320.

Conidiophores of Eurotium herbariorum (to the left) and of Penicillium crustaceum
(to the right). (From Strasburger.) Highly magnified.

If a sample of Eurotium be taken, the branched and septate my-
celium is seen to ramify over and penetrate into the organic sub-
stratum, deriving nourishment till able to propagate. Stout branches
then rise upright as conidiophores, which swell upwards into a spherical
head (Fig. 320). On this numerous conical sterigmata bud forth, each
giving rise to a chain of conidia, formed in basipetal succession. The
oldest is distal, and successively others are abstricted off : an arrange-
ment which provides for the due nourishment of each, and the ready
removal of those that are mature by any breath of air ; for these
minute polynucleate conidia are very lightly attached. They
germinate readily in water or damp air, and the mycelium permeates
any nutritive medium ; thus they serve for the quick spread of the

ASCOMYCETES

423

Mould. The corresponding conidiophores of Penicillium are con-
structed on a similar plan, but arc much smaller. Instead of bearing
a mop-like head, they are repeatedly branched, giving them a brush-
like appearance, while from the end of each branch a basipctal chain
of conidia is abstricted, as before (Fig. 320).

The alternative method of propagation follows in Eurotium on
a rise of temperature, and results in the production of small yellow
fruits. When ripe each contains numerous asci, and spores. Similar

Fig. 321.

i, Section through part of a fruit of Penicillium ; a, b, pseudo-parenchyma toils
covering ; d =ascogenous hyphae. 2, 3, ascogenous hyphae with asci, more highly
magnified. 4, ascospores. (After Brefeld.)

fruits are formed also in Penicillium, but more rarely. The develop-
ment originates in either case from a twisted oogonial branch, which
is associated with an anther idium. As in Sphaerotheca these sexual
organs become enveloped in a pseudo-parenchymatous covering,
derived from the mycelium that bears them. The oogonium divides
into a number of cells, from which strong hyphae arise. These are
nourished by the surrounding tissue, and produce the numerous oval
asci, each with eight ascospores. In Penicillium the structure of the
fruit-body is more complicated than in Eurotium (Fig. 321). In
either case the result at ripeness is that the fruit appears as a dry
spherical sac, filled with ascospores set free by the disappearance of

424 BOTANY OF THE LIVING PLANT

the ascus-walls, and of the nutritive tissues that embedded them
(Fig. 321, 2, 3, 4). On germination the ascospores form a new
mycelium like the original one.

In Eurotium both the antheridial and oogonial cells are multinucleate,
and it appears that sometimes a normal sexual fusion takes place. But in
others the antheridium degenerates, and sexual fusion is replaced by a fusion
of oogonial nuclei in pairs. Thus Eurotium illustrates that degradation
of sexuality, evidence of which is common among the Fungi.

The type of life-history seen in these simple Mildews and Moulds is common
for other Ascomycetes, though it may be worked out with greater complication.
But in many of them the sexual organs are so degraded that they, or their
equivalents, if present, have hitherto escaped observation. There is normally
an alternation of generations, the critical points of which are the sexual
fusion in the oogonium and the reduction which precedes the formation
of the ascospores. The stage of the ascogenous hyphae, which intervenes
between these events may be regarded as a diploid sporophyte. The rest is
held to be the haploid gametophyte, which is liable to indefinite repetition by
means of conidia. The fruit-body is then a composite structure, consisting
essentially of the ascogenous hyphae, constituting the sporophyte, which is
enveloped in a covering derived from the mycelial gametophyte. The nearest
analogies are with the fruiting bodies of the Red Seaweeds.

On the fertilisation of the oogonium in some Ascomycetes the male and
female nuclei fuse in pairs and diploid nuclei result. These pass out into the
ascogenous hyphae which grow out from the oogonium. The young ascus
typically develops from the penultimate cell of an ascogenous hypha. This
cell always contains two nuclei. These fuse together and therefore produce a
nucleus which is tetraploid. Subsequently, as the ascus develops, three
successive nuclear divisions take place, thereby producing the eight nuclei
for the eight ascospores. The first division is a meiotic division, the second is
a mitotic division, and the third a second meiotic division. This remarkable
phenomenon of a second reduction division, on which a considerable amount
of investigation has been carried out and which is still a matter for controversy,
is known as br achy meio sis. Where no genuine sexual, or substitute sexual,
fusion has taken place in the oogonium, only a single reduction division takes
place in the young ascus.

OTHER TYPES OF ASCOMYCETOUS FUNGI.

Both the mycelial and the fruiting stages of Ascomycetous Fungi
are subject to modifications according to habit and circumstance,
and either may attain large size in some of the representatives of the
family. The mycelium may, by repeated branching and knotting
together of its hyphae, form dense masses stored with nutritive
material, hard and dark coloured, called sclerotia. When the rest of
the mycelium is killed off by dry or cold weather these remain un-

ASCOMYCETES

425

injured, and may germinate after a period of rest, forming at first
fresh superficial mycelium and conidia ; but, later on, outgrowths
may spring directly from them, as in some species of Peziza, which
bear broad disc-like fruits. Those Ascomycetes which have such
flat open fruits as are seen in Peziza are ranked as Discomycetes
(Fig. 322). The most notorious sclerotia are those of Claviceps

Fig. 322.

A, a sclerotium of Peziza, which has germinated and
given rise to numerous trumpet-shaped discs. B, section
through such a sclerotium (sc), and the Peziza disc (d) to
which it has given rise. b= stalk. a=asci. Magnified
and slightly diagrammatic. (From Marshall Ward.)

purpurea, the Ergot of Rye, a fungus which causes a disease on
Rye-crops. The fungus attacks the ovaries of the Rye and other
Grasses at the flowering period, spreading over them and causing
the condition known as " Honey-Dew." This is the conidial stage,
and it is spread from plant to plant by insects, which are attracted
by a sugary secretion in which the conidia float (Fig. 323, a, b ;
also Fig. 293, p. 395). But the effect becomes more apparent as the
Rye-crop ripens, for in place of the normal grains long curved bodies
project from the ear (Fig. 323, c). These are the sclerotia of the fungus,
which fail off at the time of ripening of the grain. They are the
commercial source of supply of a useful drug. In this resting stage
the winter is passed. In spring the sclerotia germinate, forming
numerous pinhead-like growths, which bear the flask-shaped peri-
thecia characteristic of the large group of the Pyrenomycetes (Fig. 323,
d, e). Finally in these the asci and thread-like ascospores are
matured at about the time when the Grasses flower. It has been
proved experimentally that hyphae from the germinating ascospores

426

BOTANY OF THE LIVING PLANT

invade the Grass-flowers, causing the development of the conidial
stage again. The life-history is here essentially the same as before,
but with the addition of the resting sclerotium.

& o

°%\

o~ ^

Fig. 323.

a, b = conidial stage of Claviceps, developed in the flower of Rye. c = sclerotia
replacing the grains of the ear of Rye. d, e = germination of the sclerotia in spring.
See Text. (After Tulasne.) (From Marshall Ward.)

The fruit-bodies are very complex in some of the larger saprophytic
Ascomycetes. An extreme case is seen in the edible Morel (Morchella
esculenta), in which the external hymenial surface is convoluted and
thereby accommodates a vast number of asci (Fig. 324). It is possible to
refer this to an elaboration of the Discomycetous type, as it is seen
in Peziza (Fig. 322). But in the Truffle [Tuber) the equally numerous

ASCOMYCETES

427

asci are borne internally, in the large underground tuberous fruit

(Fig. 325).

ASCOLICHENES.

There is a series of Ascomycetous Fungi which live in symbiotic
relation with Algae, and thus constitute compound bodies which are
called Lichens. The physiological relation of the two distinct organ-
isms is not unlike that of the Fungus and Host-plant in mycorrhiza,

Fig. 324.

Morchella esculenta, the fruiting
body of the Morel. The convo-
luted folds of surface are covered
by the hymenial layer, bearing
asci. (* nat. size.) (After Stras-
burger.

Fig. 325.

a Truffle. The fructification in vertical
a = cortex. c=dark veins of compact
hvphae. d = air-containing tissue, h = ascogenous hyphae,
with numerous asci. (After Tulasne ; from Strasburger.)

Tuber rufutn
section ( x 5).

but there is here no intra-cellular digestion (Chapter XII.). The
Lichens are very various in form. In simple cases they may be
filamentous, as in Ephebe, which is like a filamentous Alga with a
fungus growing in its mucilaginous walls. Some appear as flat
gelatinous thalli, readily swelling with water, as in Collema, which is
based upon the gelatinous Alga, Nostoc. Others are more firm in
texture, and form variously flattened thalli, more or less closely
attached to the substratum of rocks, roofs, or tree-trunks, etc. Others
again are erect or pendulous, and often branched, rising freely from
their base of attachment. In texture they are brittle when dry, but
more or less leathery when moist, and they vary greatly in colour
from g«rey to more vivid yellow, or even red. They are curiously

43S

BOTANY OF THE LIVING PLANT

susceptible to impurities in the air, and are therefore absent from
urban areas (Fig. 326).

Their structure shows two distinct constituents. Certain cells
have algal characters, and often closely resemble Algae known in
the free state ; they contain chlorophyll or some related colouring
matter, and are photosynthetic. They are distributed variously

B

Fig. 326.

A =Xanthoria (Parmelia) parietina, the common foliaceous yellow Lichen.
B =Cladonia rangiferina, a fruticose Lichen. Both bear ascus-fruits, and are shown
natural size. (After Strasburger.)

in the thallus, often in a definite gonidial layer. These cells are
closely invested by the fungal constituent, which is composed of
septate and branched hyphae, twigs of which enwrap the algal cells,
establishing intimate physiological relations (Fig. 327). Not only
does this dual organism flourish, but it may also propagate as such.
In wet weather many Lichens are covered by a mealy powder, extruded
from within. Examination shows that it is composed of soredia,
which are bodies containing both constituents of the thallus ; one or
more algal cells are enveloped in a weft of fungal hyphae. Each
soredium is thus able to grow directly into a new Lichen.

The fruiting bodies of the Lichens are, however, produced from
the fungal constituent only, and most of them closely resemble those
of Discomycetous and Pyrenomycetous Fungi in form and con-
struction (Fig. 326, A). In the Iceland Moss (Cetraria Islandica),
which is officinal, the fruits appear as marginal discs. When cut
vertically they show a superficial hymenium, with numerous asci
arranged as in Peziza (compare Fig. 322, B, p. 42 5) . Male sexual organs

ASCOMYCETES

429

are sometimes borne, as minute, non-motile bodies (spermatid), which
are produced in flask-shaped spermogonia, contained in the marginal
teeth of the thallus. Female organs have been seen in some of the
Lichens (Collema, etc.) to have the form of a coiled oogonium, with
a receptive trichogyne that projects to receive the non-motile sper-
matium, as in the Red Seaweeds. But it seems probable that in many
of the Lichens, as in many advanced Fungi, the sexual organs even
if morphologically present are not functional.

A B

Fig. 327.
Cladonia furcata. A = vertical section of the thallus showing the inverted gonidial
layer below the cortical sheath ( x 330). B =part of the same highly magnified, to
show the mode of attachment of the hyphae to the gonidia ( x 950). (After Bornet.)

The establishment of a new Lichen from the germination of the
ascospore depends upon the presence of the algal partner. This
is not left to chance, but is provided for, in some cases at least, by
11 hymenial gonidia " : these are small algal cells which develop in
close relation to the asci. When the fungal spores are ejected some
of these adhere to their sticky walls, and thus the two partners
germinate together from the first.

The close similarity in structure, nutritional behaviour, and pro-
pagative method with the Ascomycetes makes it probable that the
Asco-Lichens have arisen from Ascomycetous Fungi which have
adopted a symbiotic relation with Algae, and become dependent

430 BOTAXY OF THE LIVING PLANT

upon them. Now, some of the Lichen-Fungi have been cultivated in
nutrient media when they produce a small thallus without gonidia.
And again, the algal cells have been isolated and have been found to
continue their normal life as free- growing Algae. Indeed they have
been found to agree in detail with certain species of the lower Algae.
Finally, synthetic experiments have been successfully carried out in
building up a Lichen from its two constituents when grown together.
The proof that the Lichen consists of a coalition of two organisms
living in symbiotic relation seems thus complete. That the symbiosis
is a mutual advantage is clear from the healthy growth. It may be
held that the alga contributes fresh organic substance by photo-
synthesis, while the fungus supplies water and soluble salts, which it is
specially able to extract and convey.

CHAPTER XXVII,

BASIDIOMYCETES.

The Basidiomycetes form the second sub-class of the septate Eu-
mycetes. They include most of the large Fungi, such as the Mush-
rooms, Toad-stools, Shelf-Fungi, and Puff-Bails. These are almost all
saprophytes. But the Basidiomycetes also
include the Rusts and Smuts, which are
parasitic forms causing disease. Some of
these are the most injurious pests to cereal
crops, such as the Rust of Wheat or the
Smut of Oats. Some Basidiomycetes also
take part in the formation of certain types
of Lichens. They are thus very varied
in their habit, and include many familiar
objects. The characteristic feature is the
Basidium, which takes a place in the life-
cycle corresponding to the ascus in the
Ascomycetes ; for in both of them there
is nuclear reduction, and both produce
post-sexual spores equivalent to Tetra-
spores. But while in the ascus they are
formed internally (Fig. 316, p. 417), in the the basidium have pass

J \ t> ° ' r tx//j ""* four basidiospores. Z)=passage of a

basidium they are borne externally, as RuhS^
Basidiospores (Fig. 328).

In the Basidiomycetes normal sexuality has not been shown to exiit, while
it is only in the Uredinales or Rusts that organs are found which, though not
always functional as such, are held to be of the nature of sexual organs. This
would indicate that the Uredinales are relatively primitive types of the
Basidiomycetes. It may be held as probable that all these Fum;i were derived
from a sexually reproducing ancestry ; but that the sexuality is in abeyance
in the more advanced parasites and saprophytes, while evidence of it remains
in the more primitive Rusts.

b.b. 431 2E

Fig. 328.

Honey Agaric (Armillaria mellea).
A, young basidium with two primary
nuclei. B, after fusion of the two
nuclei. C =a basidium of Hypholoma
appcndiculatum before the four nuclei
derived from the secondary nucleus

432

BOTANY OF THE LIVING PLANT

The basidium resembles the ascus in containing two nuclei when young.
As in the ascus these first fuse with one another, and then follows division,
first into two then into four, which is accompanied by nuclear reduction.
In the more primitive types (Uredinales, Tremellales), the basidium divides
after the fusion : in the Rusts it appears as a short mycelium (promycelium),
from which four cells are partitioned off ; each of these puts out a short
beak, or sterigma, and bears a basidiospore (Fig. 335, p. 436). In the more
advanced types, such as the Toad-stools and Puff-balls, the basidium does
not divide into distinct cells ; but sterigmata are formed, and each produces
a basidiospore, which behaves in the same way as in the Uredinales. The
difference is that the basidium remains undivided (Fig. 328).

Uredinales. — Rust-Fungi.

The Rust-Fungi are very prevalent parasites on the shoots of many
plants, and as their mycelium lives in the intercellular spaces of
the infected tissues, and draws
nourishment from the cells, the
host-plant suffers, though it
is not immediately killed. If
the host be perennial, the fall
of the leaf or dying down of the
aerial shoot rids it temporarily
of its enemy, and the parasite
has to make a new infection in
the succeeding season. It is only
occasionally, as in Gymnospor-
angium on the Juniper, that the
fungus perennates in the host.
A very large number of Rust-
Fungi are known, and they show
a high degree of specialisation
in their parasitism, being mostly
restricted to certain genera, or
even species of host ; while in
not a few cases their elaborate
life-cycle is completed by stages
of growth upon two distinct and
successive hosts. This is de-
scribed as Heteroecism. The name
Rust is derived from the fact that FlG- 32g-

.1 tt j Upper portion of a stalk of Wheat, with groups of

tne LVeaOSpOreS produced by the uredospores (u) on the leaves, and dense masses

of teleutospores (p) on the fast-ripening leaf-sheath
SOme- and straw. (After Marshall Ward.)

fungus in summer — and

BASIDIOMYCETES 433

times called summer-buds — are of the colour of rust of iron. They
are found in such quantity as to attract public attention, and this
provided the name.

The most familiar example, as it is also economically the most
important, is the Rust of Wheat, Puccinia graminis. In June and
July the green leaves of the Wheat are often seen to lose their colour
(Fig. 329). Yellow patches appear between the veins, and run
together into lines that follow the softer mesophyll. The epidermis
bursts, and innumerable orange bodies are set free, which are easily
carried as dust by the wind. These are the Uredospores of the Rust.
A part of a field thus diseased is a centre of infection, and the fungus
may often be seen to spread from it down the prevailing wind. This
production of spores, which represents so much material robbed from
the developing crop, may continue till autumn ; then gradually the
patches of disease change in colour to a dark purple-brown. This is
due to the formation in them of a new kind of propagative body, the
Winter-bud or Teleutospore, which is firmly attached to the straw,
and dies down with it, or is removed with the crop. These spores
retain their vitality till the next spring, when they germinate (Fig.

335, P- 436).

The result of their germination has been shown experimentally
to be the infection of the leaves of the Barberry plant, and the pro-
duction of a second stage, which appears as red or yellow blotches,
thicker than the healthy parts of the leaves that bear them : small
dark spots open on the upper surface (spermogonia), and numerous
widely gaping cups (aecidia) are clustered together on the lower
surface. This stage was first described as 4' Cluster-Cups," and
regarded as a distinct fungal disease under the name of Aecidium
berberidis (Fig. 330). But it is now known that the spores produced
by the cups are able on germination to cause a new infection of the
leaves of the WTheat plant, which results again in the growth of a
mycelium bearing the uredospores. There are thus two stages
of the disease, the one on the Wheat or other Grasses, the other on
the Barberry. Long before it was proved that these two different -
looking diseases were only stages in one life-history, a connection
between the two had been suspected. It was thought that the
Barberry was in some way injurious to Wheat. But it was not till
late in the Nineteenth Century that the cycle was completely demon-
strated. A similar hetcroecismal life is now known for numerous
species of Rusts. One of the commonest is Puccinia car ids, of which
the uredospores and teleutospores are on leaves of species of Carex,

434

BOTANY OF THE LIVING PLANT

and the aecidium-stage on the common Nettle, causing contorted
swellings upon its stem and leaves. Thus the Rust of Wheat is an
example of a life-history that is not uncommon.

Fig. 330.

Part of shoot of Barberry with leaves attacked by Puccinia graminis which forms yellow

cushions, or cluster -cups, on the leaf -blades and stalks. (After Marshall Ward.)

Sections through a diseased leaf of wheat in summer reveal the
branched and septate hyphae closely packed in the intercellular
spaces, and investing the green cells. They accumulate below the
epidermis, forming a dense mass or sorus, the end of individual hyphae
swelling into the uredospores, which, increasing in bulk and number,
burst the epidermis, and are shed (Fig. 331). Each spore is covered

^i^^giii^^^OT^r

Fig. 331.

Longitudinal section of a leaf of Wheat, showing a tuft of uredospore bursting
through the epidermis. Highly magnified. (After Marshall Ward.)

(It will be realised from this illustration that uredospores are really conidia :
for historic reasons, however, it is convenient to retain the term uredospore.)

by a thick wall, containing dense protoplasm with oily globules, and
two nuclei. Their walls are marked by thin spots round the equator.
It is from these spots that the germ- tubes emerge when grown in wrater
(Fig. 332)- K this germination takes place on a wet leaf of wheat, the
tube growing over the surface finds entry by a stoma (Fig. S33), and

BASIDIOMYCETES

435

at once gains access to the nutritive cells. In about a fortnight the
infected spot will be producing fresh uredospores.

The teleutospores may arise from the same spot as the uredospores,

Fig. 332.

Germinating uredospores, showing various stages of development of the germ-
tubes, a, b, c. Very highly magnified. (After Marshall Ward.)

but later in the season. They differ from them in being closely packed
and firmly attached, as well as in structure. Each spore is spindle-
shaped, and is partitioned into two cells, each with a dark brown coat
(Fig. 334)- Like other cells of the mycelium and the uredospores

Fig. 333-
Longitudinal section of a leaf of Wheat, showing a germ-tube from a uredospore
passing through a stoma (a) into the intercellular space (b). Very highly magnified.
(After Marshall Ward.)

themselves, each cell contains two nuclei. They do not germinate till
the following spring. In March or April, after a few hours in water, each
cell puts out a delicate tube (Fig. 335). This, after segmenting to form
four distal cells, constitutes what has been called the promyceliutn,

436

BOTANY OF THE LIVING PLANT

which is really a septate basidium. The first step is the fusion of the
two nuclei to form one. Then follows its division into two and into

Fig. 334.

Longitudinal section through sorus of the teleutospores of Puccinia, on a stalk

of Wheat-straw. Highly magnified. (After Marshall Ward.)

four — in fact a tetrad- division, followed by partitioning of the four cells.
Each cell then forms a process, or sterigma, on the end of which a

Fig. 335.

m Two teleutospores of Puccinia germinating. In the one to the left each cell has
given ofi a promycelium, or basidium (a, a) ; to the right only the lower cell has
done so, and the promycelium has given rise to four sterigmata, bearing sporidia
(s, s) or baeidiospores as they are better called. Very highly magnified. (After
Marshall Ward.)

BASIDIOMYCETES

437

swelling appears, and into it the protoplasm and nucleus pass. These
are sometimes called sporidia : each is the product of a reduction-
process and is recognised as a basidiospore (Fig. 335, s, s). Their

Fig. 336.

Sporidia, or basidiospores, of Puccinia, germinating on the epidermis of a Barberry
leaf, and putting out germ-tubes, which penetrate the cell-walls. Very highly
magnified. (After Marshall Ward.)

peculiarity is that they do not cause infection of the Wheat, but can
penetrate the epidermis of the Barberry. Acting on suspicions
already aroused, De Bary succeeded in carrying out an experimental
infection in 1864. He found that the basidiospores, easily shed from

Fig. 337.
Vertical section through a patch of aecidia (<?) and spermogonia (s) on the
Barberry leaf, showing the swelling of the diseased part. The small aecidium
to the right has not yet burst. Highly magnified. (After Marshall Ward.)

the sterigmata, germinate to form a germ-tube, which can directly
penetrate the epidermal wall of the Barberry (Fig. 336). This initiates
the second phase : and as the basidiospore itself is haploid, so is the
stage produced from it on the Barberry. Each cell has only a single
nucleus, which may be held to indicate a haploid condition, as in the
gametophyte.

438

BOTANY OF THE LIVING PLANT

A section through an infected spot on the Barberry leaf shows the
effect of the attack in the greatly increased thickness as compared
with the normal leaf (Fig. 337)- The enlarged cells of the mesophyll
appear enveloped by fungal hyphae which choke the intercellular
spaces. They are massed chiefly at points towards the upper and
lower surfaces, to form bodies of considerable size. The first are
the flask-shaped spermogonia com-
posed of hyphae pointing radially
inwards, while from the end of each
a minute non-motile spermatium is
abstricted. These have not them-
selves been found capable of causing
infection ; but they may stimulate
development and are possibly male
organs {sp). The spermogonia secrete
a fluid, sometimes described as " nec-
tar " in which large numbers of
spermatia may be present. Projecting
beyond the spermogonial opening or
ostiole are long flexiwus hyphae. The
bodies on the lower surface are larger
and develop into the cup-like aecid-
ium-fruits. Each is composed of an
outer sheath or peridium, while the
cup is filled with filaments rising from
the base, from each of which a chain
of aecidiospores is produced. The
oldest are distal, and they are shed
succession from the downward-

m

Fig. 338.

Phragmidium violaceum. A, portion of a
young aecidium ; st, sterile cell ; a, fertile
cells : at a2 the passage of a nucleus from
a neighbouring cell is seen. B, formation
of the first mother-cell, sm, from the basal

turned cups (Fig. 337,*). When ripe ^ $ & \£S££>^ *jSinT
they are bi-nucleate. De Bary in 1865 cala!7 celln(z) have arisen ;jm2 the second

J j ~> mother-cell. D, ripe aecidiospore ; note

Showed that if SOWn On young grass- the paired nuclei. (After Blackman.) (From

J ° ° Strasburger.)

leaves they infect them, and produce

the Rust again. Thus there are two stages in the life-cycle, which
differ in host and in propagative organs : the one has paired nuclei,
and may be held as a diploid sporophyte : it grows on the Grass. The
other has a single nucleus in each cell, and may be held to be a haploid
gametophyte : it grows on the Barberry.

The problem of sexuality in the Rusts is still not entirely solved. In some
species, cluster-cups develop after infection by a single sporidium ; in others
mycelia from two sporidia must meet, or a mycelium must be stimulated by

BASIDIOMYCETES 439

spermatia of a different strain. In other words it is now recognised that where-
as some Rusts are homothallic, others like Puccinia graminis are hetcrothailic
(see p. 416) ; the latter require the association of hyphae from two comple-
mentary strains before cluster-cups and aecidiospores are developed. This
association may be brought about by the overlapping of pustules. Insects
visiting the spermogonia for " nectar " bring about a transference of spermatia
and these may become intimately associated with flexuous hyphae of the
opposite sex. Spermatial nuclei then enter the hyphae and the subsequent
nuclear divisions and migrations bring about the cytological condition necessary
for the development of aecidiospores. In all cases, nuclear pairing is believed
to occur at the base of young aecidia. It has been traced in Phragmidium
violaceum, as consisting in the passage of the nucleus of one cell into a neigh-
bouring cell (Fig. 338, A), very much as has been seen in certain apogamous
Ferns (Fig. 403, p. 508). But here the nuclei do not fuse at once ; the receptive
cell remains bi-nucleate, and divides as such into a chain of bi-nucleate spores
(a) and sterile intercalary cells (2) (Fig. 338). Each chain of spores appears to
be initiated in this way. A similar process has been seen in P. speciosum, and
other Uredinales. This discovery made it possible to relate the life-history
of the Uredinales to that of other plants. The state with paired nuclei is
held as the correlative of the diploid sporophyte, and that with a single
nucleus in each cell as the haploid gametophyte. The events may then
be summarised as follows : —

A. Sporophyte on Grass B. Gametophyte on Barberry

(paired nuclei). (uni-nucleate).

i. Mvcelhim. viii. Mycelium.

ii. Uredospore (repeating the ix. Spermatium.

mycelium). • x. Nuclear association.

iii. Teleutospore (winter's rest). xi. Aecidiospore (bi-nucleate)
iv. Nuclear fusion in germinating (Fig. 338, D).

teleutospore. xii. Infection of Grass-leaf.

v. Basidium ( =promycelium).

equivalent of a spore-tetrad,

with reduction.
vi. Basidiospore ( =sporidium).
vii. Infection of Barberry plant.

There are many of the Uredinales that do not show so elaborate a series
of stages in the life-cycle as the Rust of Wheat. The uredospores may be
omitted, or the aecidia ; or some may have neither aecidia nor uredospores.
The spermogonia may also be omitted : so that in these last the life consists
of a repetition of teleutospores with subsequent germination.

The Rust disease is difficult to cheek, and its distribution is world-
wide. One obvious measure would appear to be to remove the
alternative host, the wild Barberry. This has been done in wheat-
growing districts in England, but definite consequences from it are

440

BOTANY OF THE LIVING PLANT

uncertain. It is significant that Wheat-Rusts abound in South
Africa, Australia, and parts of India, where no species of Berberis
are indigenous. The most effective remedy is to plant only the seeds
of varieties of wheat that are known to be immune to the disease,
by resisting the infection. Progress has already been made in the
production and circulation of such immune varieties. But owing
to the minute specialisation characteristic of many Rusts a variety
may be immune in one country and susceptible to the same Rust
in a different climate : so delicate is the balance which exists between
the attacking and resistant powers of two organisms. (Compare
Chapter XII., on Irregular Nutrition.)

USTILAGINALES (SMUTS).

The Smut-Fungi (Ustilaginales) are also parasites on Grasses,
certain of them causing diseases on Oats, Barley, Wheat, and Maize,
which culminate in the fruiting Ear. The diseased grain is replaced
by a mass of dusky spores, corresponding in their behaviour to the
teleutospores of the Uredinales (Fig. 339). For like them they ger-

Fig. 339.

Teleutospores or brand spores of Ustilago germinating, and giving off basidiospores
or sporidia. (a) germinated in water only, b, c, d, e in nutritive solutions, where they
continue to sprout. Very highly magnified. (After Brefeld, from Marshall Ward.)

minate after the winter's rest, forming a basidium (promycelium) with
effective basidiospores (sporidia). The germs may have remained
in the soil of the field from the previous season ; or the crop may
have been harvested and the straw used for bedding, passed to
the manure-heap, and then carted out on to the land again. Either
way the soil in which the grain germinates will have been infected.

BASIDIOMYCETES 441

A further point of importance is that in a nutritive liquid, like the
foul water of the manure-heap, the spores formed on germination
continue to multiply by budding, thus increasing the chances of
infection (Fig. 339, d, e). The detached spores then conjugate and
begin a bi-nucleate stage, which is able to penetrate the tissue of the
seedling corn but not of the adult. The plant, once infected, grows on
as though quite healthy till the flowering period. Then the parasite,
the mycelium of which has followed its growth internally, fastens on
the ovary where nutritive material is concentrated, and diverts the
food from the formation of the grain to the nutrition of a mass of its
own spores. For prevention of the disease " dressing " of the seed-
grain with disinfecting mixtures is practised. But equally important
is to prevent the manure being contaminated by the spores from
the smutted crop of a previous year.

Hymenomycetales.

The life-history of the Rust of Wheat has been described in some
detail as giving an example of a Basidiomycete which still shows

Fig. 340.

Fomes igniarius. Section through an old fructification, showing annual zones
of growth, a = point of attachment upon the tree which is its host. The porous
hymenium is directed downwards, (h nat. size.) (From Strasburger.)

evidence of sexuality, both morphologically and physiologically ;
though it is altered from what was probably its normal and original
course. In the rest of the Basidiomycetes such evidence is wanting.
They may provisionally be held to be saprophytes and parasites which
were descended from an ancestry with normal sexuality, but have
advanced further in the elimination of their sexual process. In some
species variation is ensured by the association of myeelia of different
origin. The basidia (Fig. 328) are borne on fruit-bodies, which are
often large and brightly coloured. They arise from a mycelium
which acquires the necessary nourishment sometimes parasitically
but more often from saprophytic sources. The basidia are borne in
various ways, and this gives distinctive characters to the main groups

442

BOTANY OF THE LIVING PLANT

of these Fungi. Thus in the Gasteromycetes the fructification is closed,
the basidia being produced internally, and the spores set free by rup-
ture, as in the Puff-Bails. In the
Hymenomycetes the basidia are borne
collectively in a definite layer called
a hymenium, exposed to the air, from
which the spores are shed, as in the
Mushrooms, Toadstools, and Shelf-
Fungi (Figs. 340, 341).

The mycelium may obtain nourish-
ment in various ways. It is some-
times parasitic as in the Honey Agaric
(Armillaria mellea), which penetrates
the trunks of forest trees, ravaging the
cambium, and killing them (see Fig.

292). Many OI the bhelf-rungl [Foly- Qf the radiating, downward-directed gills

x ,, ,1 To the right a young fructification or

pOrUS) grOW paraSltlCally at the ex- -button" Mushroom. (Reduced.) (From

pense of the heart-wood of trees, Strasbur§er)

making them hollow. The infection comes through injury by wind,
which exposes the internal tissues to the invading spores. The
mycelium may live for years, digesting the lignined walls, till it is

Fig. 341.

Psalliota (Agaricus) campestris. Mush-
room. The hymenium covers the surface

Fig. 342.

Radial longitudinal section through wood infested with Forties igniarius.

Highly magnified. (After Hartig.)

sufficiently nourished to form a fruit-body (Fig. 342). On the other
hand the Dry Rot Fungus (Merulius lacrymans) lives saprophytically,
its mycelium digesting the substance from dead wood-work in houses

BASIDIOMYCETES

443

and ships, where confined in a elose damp space. Later it forms the
cake-like fruit bodies. The Common Mushroom is an example of a
very common habitat of saprophytic mycelium, viz. in the sod of
grass-land. It is found especially where horses have been grazing
(Fig. 343)- But the mycelium can be bought in bricks of mush-

W

Fig. 343- , T v

JisSEM SSJgSSS «»*«&fi» Sack!

room-spawn," made up of a compost of dung, clay and loam in
which it can be seen as fine white threads, ether spr admg as s ng 1c
hyphae, or as numerous hyphae runmng parallel so as to form thicker
strands If the brick be broken into pieces and spread through a
mLr compost, and kept warm and moist in the dark the myce ,um
grows ; in a few weeks it forms mushrooms of vano sue Th • hu
stages of development of mushrooms appear to be rap.d. This i due
o he fact that'the tightly packed threads that compose , , I button
mushroom undergo rapid extension, w.th absorption of water. (Figs.

444

BOTANY OF THE LIVING PLANT

341,343-) The apparently fresh formation of mushrooms from day
to day in the fields is thus accounted for. The button-mushrooms
are hidden in the grass till the extension takes place.

t sh , h

Fig. 344.

Structure of the hymeniumof the Mushroom Psalliota (Agaricus) campestris. A is
a vertical section through the pileus traversing several gills (/). B shows the
structure of one gill more highly magnified : hy = the hymenium. C shows a small
area of the hymenium in section ( x 350). The basidia project, each bearing two
basidiospores, that number being exceptional but regularly present in the Mushroom.
(After Sachs.)

The Mushroom as commonly known is the fruiting-body, borne
on the nutritive mycelium. It has the usual toad-stool form, with a
stalk or stipe bearing the hemispherical pileus (Fig. 341). As in all
large fungal bodies, it consists of false tissue. The hyphae composing
it take first a parallel course so as to form the stipe, they then diverge
upwards so as to form the wide-spread pileus. In the button stage
the margin of the pileus is connected with the stipe by a thin covering
of the velum ; but this is ruptured as the mushroom expands, leaving
a ring round the stipe (Fig. 341). The radiating gills, which hang verti-

BASIDIOMYCETES

445

cally from the lower surface, are thus freely exposed. It is upon the
gills that the hymenial layer, bearing the basidia, is borne. The colour
of the gills is at first pink, but it gradually grows dark brown with age.
This is due to the colour of the spores (basidiospores) produced in large
numbers all over its surface. If a young expanded pileus be laid

Fig. 345.
Coprinus sterquilinus. Section through a gill, showing hymenium (hym), subhymenial
layer (sub), and trama (tr) ; the basidia {b), each bearing four basidiospores, or only the
sterigmata (st), from which the spores have been already thrown off. (After Buller.)

face downwards upon a sheet of paper, after a few hours a print of
the gills will have been traced by the deposit of their spores.

A vertical section through the gills shows that they consist of a
rather lax central region, which supports the more compact hymenium
that completely covers their surface (Figs. 344, 345). It is composed
of narrow paraphyses which surround the more bulky basidia, the
ends of which project, and in the Mushroom bear each two sterigmata,
with a basidiospore on the end of each. The number two is, however,
exceptional among Basidiomycetes. The basidia of the Honey-
Agaric, or of Coprinus, are typical (see Fig. 345)- There the fusion-
nucleus divides into four, four sterigmata are formed, and one of the
nuclei squeezes through the narrow channel into the basidiospore
which each sterigma bears.

446 BOTANY OF THE LIVING PLANT

In the Mushroom the germination of the spores presents a difficult
problem. It seems probable that one incident, and perhaps a neces-
sary one, is that they should pass through the alimentary tract
of some herbivorous animal : for the spores would naturally be
taken in with the grass they eat, while the Mushroom grows on old
pastures manured by their dung. Related fungi do not always
present such difficulties, and are readily raised from spores. In the
case of the Mushroom no other propagative bodies exist, nor have
any sexual organs been recognised. The fruit-bodies arise from
a mycelium with paired nuclei. The young basidium contains
two nuclei which later fuse. The fusion-nucleus undergoes two
divisions — involving reduction — and each of the four nuclei formed
becomes the nucleus of a basidiospore. The fact that in the Hy-
menomycetes there is a fusion of paired nuclei followed by reduction
shows that the basidiospores are of the nature of tetraspores.

On the germination of the basidiospore a considerable mycelium of uni-
nucleate cells may be formed. Between different hyphae fusions or anastom-
oses are common, the attendant nuclear migrations restoring the binucleate
condition. Such binucleate hyphae, on further growth, may give rise to the
fructifications described above. Some of the Hymenomycetales are self-
compatible so that fusions may take place between hyphae of the same
mycelium and thus the fructifications may eventually be obtained from the
germination of a single spore. But, as pure culture experiments have shown,
many others are self-incompatible and hyphal fusions and nuclear transferences
between compatible strains are necessary for the development of fructifications.

Basidio-Lichens.

Certain Basidiomycetous Fungi take part in the composition of
Lichens, but this is much less frequent than in the Ascomycetes.
The most familiar example is the genus Cora, found not uncommonly
in the tropics, growing on the ground or on trees. Its form is not
unlike a Stereum, or Thelephora ; and, like them, its hymenium is
on the lower surface. The fact that the fungal constituent of some
Lichens can be referred to Basidiomycetous Fungi may be held as
a final proof of their being compound organisms (see pp. 427-428).

Fungi as Subaerial Plants.
The Fungi, sprung perhaps from various Algal sources, show some
degree of adjustment to subaerial life parallel to that already seen in
Green Plants. But as the hypha is the basis of their construction,
their vegetative system gives less opportunity for adaptive change
than their propagative organs.

BASIDIOMYCETES 447

The most striking modifications are seen in the Phycomycetes,
which bear similarity to non-septate Algae in having motile zoo-
spores and gametes. Among the Oomycetes the aquatic origin
is clearly retlected in the germination of the sporangia, where each
bursts and gives rise to zoospores motile in water (Figs. 303, 308,
pp. 404, 409). The sporangium in fact appears to be like the sporan-
gium seen in many Algae, but reduced in size and detachable, thus
permitting of its distribution by air currents, though its germina-
tion is still carried out in water. Similarly in the Zygomycetes the
large sporangium of Mucor is also effective for water-distribution of
its spores, though they are not themselves motile (Fig. 313, p. 4J-0-
But within the family of the Mucorales the sporangium' is liable to be
reduced in size, with increase in its numbers, till in Chaetocladmm
and Piptocephaiis it matures as a single detachable cell : in fact it is
an air-borne conidium, the large numbers of these conidia compen-
sating for the reduction from the sporangial condition. The Oomycetes
and Zygomycetes thus suggest a parallel progression from sporangia
producing numerous spores to small wind-borne bodies, ranking as
conidia. The profuse propagation of the Eumycetes by various
types of detachable unicellular buds, also called conidia, acts biologi-
cally in the same way, as a means of subaerial propagation and dis-
tribution. But the phyletic origin of such conidia was probably
from a source distinct from that of the Phycomycetes.

On the other hand, the sexual organs of Pythium and Cystopus
correspond in form and general characters to those of the Siphonales,
such as Vaucheria. But in place of a dehiscent antheridium, shedding
spermatozoids motile in water, a fertilising tube is found, which,
like a pollen-tube, transfers its contents directly to the ovum (Fig.
310, p. 410). It is in fact an antheridium, which being subaerial in its
development does not dehisce to set free motile gametes. In the
Mucorales the zygospores may very probably be regarded in the same
way, but referable to a more primitive state of sexuality. Two
distal gametangia, instead of dehiscing and their separate gametes
fusing, conjugate as a whole, producing the coenocytic zygospore
(Fig. 314, p. 416). Such examples accord with the general reference
of Land-living Plants in their origin to an aquatic ancestry ; and
they illustrate how the modifications in Thallophytes may run
parallel with those of organisms higher in the scale. They go far to
support the general thesis that Plant-Life originated in the water
and spread later to land-surfaces.

B.B. 2F

CHAPTER XXVIII.

THE BACTERIA.

The Bacteria include a vast number of minute saprophytic and
parasitic organisms which have multiplication by fission and the
absence of the photosynthetic pigments as common features. They
are among the simplest of organised plants and include the most
minute of living beings. They stand rather isolated from the other
Thallophytes. Most of them have more or less conspicuous gelatinous
walls, surrounding a protoplast in which, though granules of chromatin
may be detected, there is no fully organised nucleus. Sexuality is
absent. In contrast to this simplicity of structure and life history
they show the greatest possible variety of physiological activity :
in fact, therein lies their special interest and importance. Indeed
some which appear identical in form may have different physiological
powers, and are accordingly distinguished as " physiological species."

Their cells may be spherical (Coccus), or rod-shaped (Bacillus), or
slightly spiral (Vibrio), or strongly spiral (Spirillum), or straight and
slender (Cladothrix), or grouped in cubical packets (Sarcina). They
have a superficial membrane, and protoplasmic body, sometimes with
chromatin-granules, but no definitely formed nucleus. Many of them
are motile, and bear cilia varying in number and position in dif-
ferent types. Their multiplication is by fission. Their mode of life is
best illustrated by an example.

The Hay-Bacillus (B. subtilis) can be obtained in any decoction of
hay, in hot or even in boiling water. If the fluid is filtered and set
aside for 48 hours it will be found to be swarming with ciliated Bacilli,
while at the surface a scum is formed, which is the " zoogloea "
condition of the same plant. In old hay the Bacillus is in the resting
condition, as spores, the protoplasm having contracted away from
the wall, and being surrounded by a thick membrane (Fig. 346, c).

448

HIE BACTERIA

449

The spores can resist even the temperature of boiling water, and pass
still living into the decoction. There they germinate into active

Bacilli, motile in the liquid
by cilia (Fig. 346, a, d).
But those which rise to the
surface lose their motility
(b), though continuing to
divide ; they form thick
gelatinous walls, and so
they remain associated to-
gether as the scum of the
zoogloea (e). If the supply
of organic material is ex-
hausted they pass again
into the resistant spore-
stage. It thus appears that
a single boiling of the
medium containing spores
of B. subtilis is not enough
to sterilise it, for the spores
can resist 100° C, at least
for a time. For complete
sterilisation it is necessary after boiling to incubate the culture at a
favourable temperature of 370 C. for 48 hours, during which time the
spores will all pass into the active but vulnerable state. Then a
second boiling will completely sterilise the liquid. This method is
commonly used in the preparation of media for the culture of Bacteria.
It is important to realise the great rapidity of multiplication of
Bacteria. Under favourable conditions B. subtilis is found to divide
once in about 20 minutes. If this pace be continued by all the
progeny for 8 hours the result from a single Bacillus would be over
16 millions. It is not, however, the rapid multiplication and easy
transfer of these minute bodies alone which gives the Bacteria their
importance. A still more interesting feature is the variety of their
physiological powers. Being as a rule parasitic or saprophytic plants,
they depend for their supply of food and energy upon breaking down
more complex organic compounds into simpler ones : and they do this
in the most various ways. The end products of such changes are
chiefly carbon-dioxide and water. In fact, Bacteria are the great
scavengers of the world, restoring organic material to the sources from
which it came. But in the course of the process many steps may

Fig. 346.

Bacillus subtilis. a, d, motile cells and chains of cells ;
b, non-motile cells and chains of cells ; c, spores from the
zoogloea ; e, the zoogloea. (After A. Fischer, a-d x
1500 ; e x 500.) (After Strasburger.)

45o BOTANY OF THE LIVING PLANT

intervene : and by-products may be produced which are sometimes
useful, though often harmful to other organisms, and to Man.

Where suitable food-material is available Bacteria may multiply
indefinitely. But there are important external checks which control
them. Many Bacteria are susceptible to injury by light. This has
been shown for the Anthrax-Bacillus, by growing it on culture-
plates, partly exposed to light and partly shaded ; and the results
have been verified in various others. The destructive effect lies
in the blue-violet end of the spectrum. After incubation in the dark
for three or four days, the area of a plate exposed for a time to such
rays at the beginning of the experiment will remain clear, while the
shaded portion will be covered by bacterial colonies : this shows that the
bacteria exposed to light had been killed. Such facts are of prime im-
portance in relation to general health : for sunlight thus offers a natural
and wide-reaching check upon the spread of many harmful germs.

The relation of Bacteria to the free oxygen of the air is also a matter
of importance. Like other organisms they may be distinguished
as aerobic and anaerobic, according to their dependence upon the
presence of free oxygen or their independence of it (p. 136). But
no sharp line can be drawn between these two modes of life. Many
Bacteria carry on their life with absorption of oxygen, like other
plants. If they have a power of movement they are attracted by
oxygen, crowding around air-bubbles. But some of the most dele-
terious, such as the Bacillus of Tetanus, flourish only in the absence
of free oxygen, obtaining their supply of energy at the expense of the
organic material which they destroy. Some Bacteria that cause
butyric fermentation behave in this way. Such activity may be
compared with that of the bottom-yeast of beer-vats ; in both cases
the activity is anaerobic. It is this mode of life, together with the
toxines which result from it, that makes the Tetanus-Bacillus specially
dangerous in wounds.

Such questions as these are, however, the material for more special
treatises than this. It must suffice here to have pointed out that the
partial decompositions due to bacterial action are of most varied
importance, economically and socially. Physiologically they may be
referred for the most part to that degradation of organic material
which supports parasitic and saprophytic Life.

On the basis of nutrition Bacteria have been classified into three
groups : (i) Prototrophic or autotrophic, those which require no organic
compounds at all for their nutrition. These are represented by the
nitrifying Bacteria which live in open nature, in the soil, and are never

SCHIZOPHYTA 45 1

parasitic, (ii) Metatrophic, those which cannot live unless they have
organic substances at their disposal, both nitrogenous and carbonaceous.
They occur in the open, and are saprogenic and sometimes parasitic
(facultative parasites), (iii) Par atrophic, those which develop normally
only within the living tissues of other organisms, and are true and
obligatory parasites, such as the germs of Tubercle or Diphtheria.

This classification may be extended, however, to all other organisms.
All green autophytes are prototrophic in the same sense as the first
group of Bacteria. All fungi and animals are metatrophic, except
the parasitic forms, which are paratrophic. Thus in point of fact the
Bacteria exemplify types of nutrition which run parallel with those
seen in larger organisms.

Bacterial germs are widely diffused in air, water, and soil, as well as on or
within living organisms, whether animals or plants. Their activities are so
various that it will be best to illustrate them by a few examples rather than
to mention many.

Among the Bacteria that live in water Crenothrix polyspora is notorious for
choking pipes of water-supply, and making the water undrinkable, though
apparently not poisonous. Lille, Rotterdam, Berlin, and Cheltenham have
suffered from it. It is probably world-wide in distribution : but being an
Iron-Bacterium, it finds a special opportunity for development in the water-
conduits of towns. It shares with other Iron-Bacteria, such as Leptothrix
ochracea, the power of oxidising oxide of iron to the hydrated oxide, which is
deposited in the walls of its cells, and when these are massed together, it
appears as those ochre-coloured deposits of bog iron ore not uncommon in
the beds of ferrugineous streams. These are filamentous Bacteria, and they
grow attached at their base to solid objects (Fig. 347). Somewhat similar are

o0o

oo<?3>p 5

Q O

Fig. 347-
Crenothrix polyspora. (After Ellis.) (Mature specimen, x 250.)

the Sulphur-Bacteria, such as Beggiatoa, which grow in sulphurous springs.
They separate sulphur from sulphuretted hydrogen, which is then deposited
in their cells. The green and purple sulphur bacteria are remarkable for the
fact that they depend for their carbon supply on carbon dioxide and sunlight.
They are, in fact, photosynthetic, though the process differs in certain respects

from that in green plants.

Both the Iron- and the Sulphur-Bacteria are prototrophic, that is independent
of organic compounds for their nutrition. It is otherwise with Bacillus radicicola
already described in relation to the nodules of the Leguminosae (pp. 127,
235). These bacteria exist freely in the soil, and when the opportunity offers
they penetrate the root-hairs. The Bacillus within the nodule is paratrophic

452 BOTANY OF THE LIVING PLANT

but with a special power of fixing free nitrogen. This it shares with another
soil-bacterium, Clostridium pasteurianum, which however is metatrophic :
when grown in a nutritive solution with sugar it is found to take up nitrogen
from the air. In either case free nitrogen of the air is brought into combination.
Other Bacteria are engaged in bringing about changes in nitrogenous bodies
already present. Various putrefactive organisms, chiefly bacteria, break
down more complex organic bodies into simpler. In the case of nitrogenous
compounds the organic nitrogen is ultimately liberated as ammonia. This
ammonia is probably " mineralised " before it can be used again by green
plants. It is oxidised in the soil, and combined with a base to form nitrate.
This is known as " nitrification." It is carried out by certain prototrophic
bacteria everywhere present in the soil. The change is effected in two steps :
the nitrite bacterium (Nitrosomonas) oxidises the ammonia into nitrous acid,
and the nitrate bacterium (Nitrobacter) converts it into nitric acid (Fig. 348).

\

€t>

e.

Fig. 348.

Nitrifying bacteria. (After Winogradsky.) a, Nitrosomonas europaea, from
Zurich : b, Nitrosomonas javanensis, from Java : c, Nitrobacter, from Queto. (From
Fischer, Vorl. ii. Bacterien. x 1000.)

Both organisms are aerobic, and they are always present together in Nature,
so that the compounds formed by one are immediately taken up by the other,
and the end-product is nitric acid in the form of nitrate, which is then available
as plant-food.

These examples of organisms in water and in soil will serve to suggest the
activity of Bacteria in Nature. They have also wide-reaching effect in
manufactures. For instance, acetic-acid bacteria convert alcohol into vinegar :
butyric bacteria cause the " retting " of Flax and Hemp : bacteria take part
in the preparation of Indigo ; while the flavours of cheese, butter, and
tobacco depend for their market-value upon the exact type and conduct of
the "partial decomposition of their constituents by bacterial action.

We are most directly interested in those bacteria that affect Man, and other
animals. Many metatrophic forms flourish on the mucous membranes of
the mouth, nose, and alimentary canal, etc., and accompany the individual
through life without doing harm. But many other paratrophic bacteria,
entering the tissues, are the active causes of disease. Thus, suppuration is
caused by various Cocci : acute lobar pneumonia by a Diplococcus : anthrax,
or malignant pustule, by Bacillus anthracis : lock-jaw by Bacillus tetani :
tubercle by Bacillus tuberculosis : cholera by the " comma-bacillus," Vibrio
cholerae, etc. (Fig. 349.) The actual intrusion of the organism is the first
essential of disease, but the serious conseauences are due to the action of
poisons, or toxines, produced by the micro-organisms, and liberated into the
system of the host. The host defends itself by the action of the white blood-
corpuscles, and other cells, which take up and digest the invading bacteria :
these cells have accordingly been called " phagocytes." The process is physio-

SCHIZOPHYTA

453

logically similar to that carried out by the cells of the digestive tract in the
mycorrhizic Orchidaceae, described in Chapter XII. p. 231. But there is also
another line of defence which forms the foundation of the serum-treatment
now so widely applied. It is based on the fact that in certain instances, when

Us

a.

00&5

i'J

%

t^s

^>

\

r

Fig. 349.

b, erysipelas cocci ; c, gonorrhoea cocci ; d, splenic

Pathogenic Bacteria, a, Pus cocci ,
fever bacilli ; f, diphtheria bacilli ; g, tubercle bacilli ; h, typhoid bacilli ; i, colon bacilli ;
k, cholera bacilli. (From A. Fischer, Vorl. u Bacterien. x about 1500.)

a bacterial toxine is introduced into the circulation of an animal in suitable
amounts, there is developed in the blood-serum of the animal a substance
which has the property of neutralising the toxine, and is called an antitoxine.
Various specific antitoxic sera are thus prepared, and are used either as
preventive or as curative agents. This is not the place to discuss the pheno-
mena of immunity, but it may be stated that immunity is of two main
types : — it is directed either against the growth of the bacteria, on the one
hand ; or against the action of their toxines, on the other.

During recent years a number of important anti-bacterial drugs have been
synthesised and given wide clinical application. The discovery of the non-
toxic, anti-bacterial substance Penicillin, produced during the growth of the
green mould Penicillium notation and related species on suitable media, marks the
beginning of a new era in the treatment of wounds and diseases, Staphylococci,
Streptococci and Pneumococci being among the organisms most readily affected.

The invasion of plant-tissues by Bacteria does not appear to be so common

as of animal tissues. The reason probably is that the cell-walls prove an

obstacle to ready infection : but a good example is seen in the Crown Call,

which occurs on various cultivated plants, and especially on fruit trees. It

is prominently seen on the Paris Daisy (Chrysanthemum frutescens), on which

it is very destructive to nursery stock, being highly contagious. The galls

are formed just under the ground on the collar or root, and grow rapidly to

a large size, decaying at the end of the season, but forming new galls in the

following season round the edge of the old wound, and so on. The causal

organism is Bacterium tumefaciens, which has been isolated, and gives like

characters both in America and in Britain. All infected stock should be

burned, and quicklime worked into the tainted soil.

It will be gathered from the examples mentioned above that the various
modes of life of Bacteria, however peculiar, are such as to rank with the
activities of the Fungi, and that they may be compared with those seen in
other representatives of the Vegetable Kingdom.

CHAPTER XXIX.

INTRODUCTORY TO LAND- VEGETATION.

The Thallophyta are essentially aquatic in their characters. Most
of the Algae are actually water-living organisms : and though some
of them exist on land-surfaces, they are restricted as a rule to moist
situations, and none of those now living have achieved a dominant
position in subaerial vegetation. In the last paragraphs of Chapter
XXVII. (p. 444) it has been shown how Fungi, though referable in
origin to Algal sources, and showing evidence of it particularly in
the Phycomycetes, have in many of their characters become specialised
so as to accommodate them to subaerial life. Even extreme examples
of this mode of life are still traceable by comparison to an Algal
origin. The conclusion may then be held as justified for the Thallo-
phyta generally that they originated in water.

We pass now to Plants which have acquired a firmer hold on
exposed Land-Surfaces, though still retaining features which show
that they were ultimately of aquatic origin. They may be held as
the Amphibians of the Vegetable Kingdom, being specialised in their
vegetative structure to subaerial life, but still retaining certain
features which indicate their dependence on external liquid water.
The most striking of these is that their fertilisation is carried out by
spermatozoids motile in water (zoidiogamic) . The life-cycle cannot
be completed without this stage, which is clearly reminiscent of their
flagellate origin. The Plants in question are sometimes styled the
Archegoniatae, a term which comprises the relatively simpler
Mosses (Bryophyta) and the more complex Ferns (Pteridophyta).
The feature which they have in common justifying that inclusive
name is the female organ, or Archegonium (Fig. 350). This is a
more or less flask-shaped body enclosing and protecting a single
ovum or egg, which is thus enveloped in the tissue of the parent.

454

INTRODUCTORY TO LAND-VEGETATION

455

The ovum of Land Plants cannot, as in Fucus and other aquatic
plants, be safely extruded from the parent to fend for itself. An
unprotected primordial cell would have a poor chance of surviv.il
if exposed to the drying influence of the air. The ova of the

Fig. 350.
Archegonia of Polvpodium vulgare. A , still closed. o = ovum. K' = canal-cell, tf "^ventral
canal-cell. B = an archegonium ruptured. ( x 240.) (After Strasburger.)

Archegoniatae are accordingly retained within the tissue of the parent,
and are produced singly. The archegonium is in fact a nursing organ,
and the constancy of its occurrence, with only minor differences of
detail in the Bryophyta and Pteridophyta, may be taken as evidence
how essential to survival of subaerial plants is the protection and
nursing of the germ. It may be said generally for subaerial Animals

REDUCTION.

5YNGAMY.

and Plants that an internal embryology in one form or another is an
essential factor of life upon exposed land-surfaces. It is this which
follows on fertilisation of the ovum of the Archegoniatae so safely
hidden within the archegonium.

456

BOTANY OF THE LIVING PLANT

The origin of a somatic development, or formation of a plant-body,
may be at either of two points in the life cycle : often at both
points. This has already been foreshadowed in such Thallophytes

Fig. 35-^.
Catharinea (Atrichitm) undulata. The leafy gametophyte, or Moss Plant, bearing
capsules, or sporogonia, which are the dependent sporophyte generation. (After
Schimper.)

as Dictyota (p. 384), and Polysiphonia (p. 387), both of which show
two alternating " generations." The limits between these are on the
one hand the zygote, in which by syngamy the chromosomes
are doubled in number : on the other the tetrad-division, in which

INTRODUCTORY TO LAND-VEGETATION

457

by reduction the original number of the chromosomes is restored.
The cycle may be represented diagrammatically by Fig. 351, and
the two phases, the haploid (" X ") and the diploid (" 2X ") constitute
the alternating " generations.'' They appear as the gametophyte and
the sporophyte respectively. These " generations " may bear varying
proportions one to the other, that which is the more prominent and
obvious being referred to in popular phraseology as " the plant." But
it is, in fact, only one phase of the complete life history. For instance,
in the Bryophytes it is the X -generation or gametophyte which is the more
conspicuous, and in a Moss the leafy body which is called the " Moss-
Plant " turns out to be the gametophyte and bears the sexual organs,
while the sporophyte is the capsule dependent upon it (see Fig. 352).
This is so for all of the Bryophytes. In
the Pteridophytes, however, it is the 2X-
generation or sporophyte which is pre-
dominant, and the leafy structure known
as the " Fern-Plant " is the sporophyte
generation, while the gametophyte is the
relatively small prothallus which pro-
duces it (Figs. 353, 354). This is so for
all of the Pteridophytes. When it is
remembered that in Dictyota and Poly-
siphonia the two generations are very
much alike, this difference of balance
will seem less strange than it might
otherwise appear to be. For the pro-
gression to a leafy shoot may in fact
have been carried out in either of them.
Most of the Archegoniatae possess
a shoot composed of axis and leaves, a
type of development of the plant-body which is continued in Seed-
Plants, and is evidently suited, by its relatively large proportion of
surface to bulk, to the conditions of subaerial life. Exposure to the
air of organs charged as they are with water leads to transpiration.
Their ventilation-system of intercellular spaces opening at the
numerous stomata facilitates this. The loss is made good from an
absorptive system in the soil, which is itself nourished by the activity
of photosynthesis in the shoot. This implies both water-conduction
and transit of plastic materials by means of a vascular system, such as is
found in all of the larger forms of land-living plants, in which the
•• plant " is in all cases the sporophyte generation (Fig. 257, p. 335).

Fig. 353.

Adiantum Capillus Veneris. The
prothallus, pp, seen from below has a
young Fern-plant attached to it. b =
first leaf. w, w' = first and second
roots, h = root hairs of the prothallus.
( x about 30.) (After Sachs.)

458

BOTANY OF THE LIVING PLANT

The climax of the sporophyte-generation is the formation of
spores. It is better to call those spores which follow immediately
on the tetrad-division specifically tetraspores. The Archegoniatae
probably sprang from some Algal source which had its chromosome-
cycle already defined, and produced spores which were all alike
(homosporous). The earliest land-plants were similarly homosporous.
In the first instance a high output of spores was the end to be gained,

Fig. 354.

Adult plant of Dryopteris (N ephrodium) Filix-mas, grown in the open. Much reduced.
An example of the Sporophyte.or diploid generation of a Fern, established independently
in the soil.

and the whole development of the sporophyte in the primitive Arche-
goniatae serves this object, though with different degrees of efficiency.
In the dependent capsule of the Bryophytes life is short. Spore-
production happens once for all in each individual. All the spores
of one capsule mature at once, and the capsule itself dies. It is the
gametophyte of a Moss that perennates. But in the Pteridophytes
the gametophyte is short-lived. It is the neutral generation which is
predominant, rooted in the soil, and usually perennial. The plant may
in that case produce not one crop of spores only, but many in successive
years. Moreover, in the more advanced types a succession of separate

INTRODUCTORY TO LAND-VEGETATION 459

sporangia may be borne in each season, and the spore-production
may thus be spread over a long period. This with many accessory
features marks the Pteridophytes as higher in the scale of Land-
Vegetation than the Bryophytes, foreshadowing that climax of
Land-Vegetation which is seen in the Flowering Plants. Nevertheless,
apart from the differences of balance and of special development of
the two generations, the life-cycle of the Bryophytes and Pterido-
phytes is essentially alike. The Archegoniatae may then be held as
the primitive vegetation of the Land, sharing with the Seed-Plants
the advantages of subaerial life, but still tied down by their primitive
method of fertilisation to habitats where external liquid water is at
least occasionally accessible.

DIVISION III.
BRYOPHYTA.

CHAPTER XXX.

MUSCI AND HEPATICAE : MOSSES AND LIVERWORTS.

The Bryophytes include two Classes, represented by very numerous
species, widespread in all lands except in those of persistent drought.
They are the Musci or Mosses, and the Hepaticae or Liverworts.
These form a very natural alliance, and indeed are distinguished from
one another only by minor characters. Everyone knows the general
appearance of Mosses, as low-growing leafy plants, chiefly found in
moist surroundings. The Liverworts, with a similar habitat, have
commonly a flattened form described as a thallus ; but some of
them bear small leaves. Thus the Bryophytes may be either leafy or
thalloid. All the Bryophytes show a cycle of life of the same general
plan, having two alternating generations. That green and often
leafy structure which is recognised as the " Moss or Liverwort- Plant "
turns out on examination to be the gametophyte. It bears the sexual
organs, while the sporophyte, which is produced from them and
bears the spores, is the well-known Capsule, or Sporogonium (Fig. 355).
In all the Bryophytes the spore-bearing generation is dependent upon
the gametophyte throughout its existence. It never fixes itself
directly in the soil. Thus the leading morphological feature is the
relatively high vegetative development of the sexual generation, which
is able to carry on active nutrition and propagation, and commonly
persists as a perennial. There is no elaborated root-system. It
is true Mosses and Liverworts have rhizoids ; but both depend for
their water-supply not only upon localised absorption by these, but
also upon general absorption by their whole surface, as opportunity

460

MUSCI AND HEPATICAE

461

offers for it. In cither case they are dependent for their normal
development on sufficient water-supply : and this directly determines

Fig. 355-

Catharinea {Atrichum) unduJata. The leafy gametophyte, or Moss Plant, bearing
capsules, or sporogonia, which are the dependent sporophytc generation. (After
Schimper.)

their distribution. A rough estimate of the dampness of climate in
a locality may even be founded on the proportion of the Bryophyta
in its Flora.

462

BOTANY OF THE LIVING PLANT

Musci, or Mosses.

Mosses are usually gregarious. The leafy plants are often massed
together in tussocks or cushions with their small stems upright, and
occasionally branched. Sometimes they may be isolated and
straggling, with more frequent branchings. They are fixed in the
soil or some other substratum by numerous rhizoids springing from
their base (Fig. 356), or from a creeping rhizomatous shoot from which
the upright stems arise (Fig. 355). Their stature is never great,
and often they are very minute. Though they are commonest where
moisture is plentiful, and sometimes grow actually in water [Fontinalis),

Fig. 356.

Lower part of stem of a Moss (Barbula muralis) with protonema. a-b shows the
soil-level. B is a young gemma. kn = a bud that would grow into a new plant.
(After H. Muller.)

or along its edge (Porotrichum alopecurum), they often flourish in
stations apparently the most unpromising, such as exposed rocks or
roofs, tree-trunks, and wall-tops. Here they may be dried to crispness
in summer. But they recover at once after a shower- of rain. This
capacity of resisting drought, and of instant recovery by surface-
absorption of water, is one of the causes of their biological success ;
for by entering thus a state of physiological inhibition, they can tide
over extreme conditions.

The best way of presenting the life-history of a Moss is by starting
from the spore (tetraspore) 9hed from the ripe capsule. The spores are so
minute that they are readily carried as dust by the breeze. A striking
instance of their ubiquity is seen where ashes are left after a fire in
woods, or even on cinder paths. A certain Moss, Funaria hygro-
metrica, commonly makes its appearance there, though none of the

MUSCI AND HEPATICAE

463

species may be seen in the near neighbourhood. But occasionally tlic
method of spread is more precise. Thus the spores of some Mosses
are sticky, and readily carried by insects. This is so with the dung-
infecting Splachnum, the agent of its spread being the dung-fly.
Scattered in one way or another, the spore germinates in presence
of moisture, giving rise to filaments, which as they grow are partitioned
into cells, and soon branch. Some of the branches are exposed at the
surface of the soil, and develop chlorophyll. Others penetrating
the soil are colourless, or have brownish walls ; they serve as rhizoids

Fig. 357.
a, b, c, germination of Moss-spores to form protonema <* = formation ^ol j bud
laterally upon the protonema. e = diagrammatic plan of the segmentations of d, as
seen from above. (After H. Muller.)

(Figs. 356, 357). The filamentous system thus produced is called pro-
tonema, and the formation of Moss-Plants is regularly preluded by this
filamentous stage. If grown in dim light the protonema may increase
indefinitely, but with full exposure it sooner or later forms Moss-plants.
These arise as buds, each taking the place of a branch of the protonema,
and may be held to be a condensed form of it. The segmentation
of the bud is from an initial cell, by walls with rather more than
120 degrees of divergence, as shown in ground plan (Fig. 357, 4 e)-
Each segment gives rise to a leaf of the Moss-Plant, borne on the
upward-growing stem. Bud-formation does not check the growth of
the protonema, which may still extend indefinitely. Branches from
the rhizoids may anywhere rise above ground, sooner or later

B.B.

2G

4^4

BOTANY OF THE LIVING PLANT

forming new buds (Fig. 356). In this way the usual gregarious habit
of Mosses is established.

In the smaller Mosses the structure of the leafy Plant is very simple.
The leaves may consist only of a single layer of green cells (Fig. 76,
p. 115), with a strand of elongated cells forming a central vein, which
stops at their bases : the stem is here traversed by an independent
conducting cord (Mnvwm). But in Polytrichum, and other large
Mosses, there is a conducting system consisting of a central column
of water-conducting tissue, upon which strands from the leaves are

YTP

,/ /•yd-_ BMy/..

— ^**j» — ■

Upl.

Fig. 358.

Transverse section of the central tissues of an aerial leafy stem of Polytrichum
commune, showing entry of leaf-traces into the mantles of the central cylinder. The
leaf-traces are numbered from without inwards. amyl = starchy parenchyma.
kydr=hydvom. lept=leptom. hyd. sA.=hydrom-sheath. rud. per. = rudimentary
pericycle. x 200. (After Tansley and Chick.)

applied. Each of these consists of hydrom (xylem) and leptom
(phloem) (Fig. 358). Thus in the gametophyte of the larger Mosses
a structure is seen which offers an analogy with that of the sporophyte
of Vascular Plants.

A curious structure is seen in the leaves of Polytrichum, and some other large
Mosses, which is probably effective in collecting and retaining water during
rain. The flat blade bears on its upper face numerous longitudinal plates of
chlorophyll-parenchyma, sometimes overlapped by the membranous margins
of the leaf. In P. commune (Fig. 359) the distal cells of each plate are enlarged,
so that its chlorophyll-cells abut upon an almost closed space. As the leaf
flattens when moist and curls its margins upwards when dry, the access of
atmospheric air to the parenchyma is controlled as it is by the automatic

MUSCI AND HEPATIC A J-. 465

stomata in Vascular Plants. But this is only an analogy, for the surfaces
of the lamellae are actually the outer surface of the leaf thrown into deep
folds, and the leaf itself is part of the gametophyte, not of the sporophyte.

An example in the Moss-Plant of extreme simplicity is seen in Buxbaumia,
which habitually grows on humus soil, or rotting tree-stems. Its male plant
consists of only a single hollowed leaf, surrounding an antheridium. The
female consists of a few leaves, and neither are green. There is an extensive
green protonema, but the rhizoids show a hypha-like habit, and establish
very close relations with the humous substratum. The sporogonium itself is

Fig. 359-

Half of a transverse section of a leaf of Polytrichum commune, showing the longi-
tudinal plates cut in section.

relatively large in Buxbaumia. The evidence of saprophytism is strong, and
it seems probable that many Mosses share that mode of irregular nutrition
in varying degree.

Among the many special adaptations seen in the gametophyte of Mosses
one of the most peculiar is that of the Bog-Mosses (Sphagnum) : it is shared
in some degree by the quite distinct genus, Leucobryum. The tissues of stem
and leaf include not only living cells with active protoplasts, but also dead
cells of larger size, with their walls propped out by annular or spiral fibrous
thickenings, and opening by round pores to the outside. They form a capillary
system by which water is retained as in a sponge. It is this structure which
gives Sphagnum its value for surgical dressings. These Mosses occupy large
areas under cold wet climates, and their dead bodies are the chief constituent
of peat.

The permanent establishment of new Moss-Colonies is largely due
to the profusion of their methods of vegetative propagation. Pro-
tonema is a regular preliminary to the formation of Moss-Plants. A
filament may arise from any undamaged cell, either of the plant itself
or of the protonema. If a sod on which Mosses are growing is inverted
and kept moist, protonema and ultimately a new crop of Moss-Plants

466

BOTANY OF THE LIVING PLANT

will arise from the rhizoids already there. If leaves or stems be
chopped up, any undamaged cell may grow out under favourable
circumstances into protonema, giving rise to a new crop. But
besides this, in many Mosses certain parts are so developed during
normal life that they are readily detached as gemmae, which may start
new colonies in fresh stations. The protonema itself may break into
short lengths (Funaria), or bulbils may be formed upon it (Barbula)
(Fig. 356, B), or gemmae may be formed on the surface of the leaves

Fig. 360.

Leaf -gemmae of Aidacomnion palustre.
The drawing shows the gemmae, and scars
where some have been shed. (F. O. B.)

Fig. 361.
Meesia uliginosa, Hedw. (After
Hedwig, 1787-) Showing antheridia
(an), and archegonia (ar), with para-
physes (/>), on same axis.

(Grimmia), or in terminal cups (Tetraphis). Whole leaves, slightly
modified for the purpose, may sometimes be detached, as in Aula-
comnion palustre (Fig. 360). In any case protonema is formed first,
and subsequently Moss-Plants as buds upon it.

Frequently it is by suctumeans that Mosses are spread. But a more
certain transfer to longer distances is by the minute spores produced
in the capsule, or sporogonium, which thus reveals itself as the sporo-
phyte generation (Fig. 355). This like other sporophytes results from
propagation by sexual organs borne by the gametophyte. The
antheridia and archegonia of Mosses are sometimes borne on the ends

MUSCI AND HEPATICAE

467

of the main stem (acrocarpic), sometimes on short lateral 1 tranches
(pleurocarpic), and this character is useful in the classification of
Mosses. They are often protected by specially developed ' peri-
chaetial " leaves, which give an almost flower-like appearance (Poly-
trichum). In some Mosses the antheridia and archegonia are grouped
together, as in Meesia (Fig. 361) ; but commonly they are separate,
either on distant branches of the same plant (Funaria hygrometrica) ,
or on different plants (Polytrichum, Buxbaumia). Their presence
upon the Moss-Plant makes it evident that it is the sexual generation,
or Gametophyte.

Fig. 362.
i.-vi. Stages in development of the antheridium of Funaria hygrometrica, ziter
Campbell ( x 400). vii. Spermatozoids of Funaria, after Campbell, and Sachs.
( x 800.) viii. Empty antheridium of Andreaea, with paraphysis, after Kuhn.
(* I35-)

The analogy between the arrangement and distribution of the stamens and
carpels in the flowers of Angiosperms and the sexual organs in the perichaetia
of Mosses is obvious. But it must always be remembered that the two sides
of this comparison are essentially different. The antheridia and archegonia
of Mosses are the real sexual organs borne by the gametophyte, and they
contain the gametes ; the ovules and pollen-sacs of Angiosperms are parts
of the sporophyte, specialised so as to produce the highly modified gameto-
phytes, which in their turn produce the gametes.

Both types of sexual organs project freely from the surface of the
plant. Each originates from a single cell, by a segmentation which
shows a continued apical sequence, and is quite distinct from that

468

BOTANY OF THE LIVING PLANT

seen in the sexual organs of Pteridophytes. The antheridium
(Fig. 362, i.-viii.) is a club-shaped body, seated on a short massive
stalk, and it is frequently large enough to be seen by the naked eye.
It consists of a peripheral wall of tabular cells, covering a mass of
cubical spermatocytes (vi.). It bursts when ripe at the distal end
(viii.). There is often a special cap of mucilaginous cells, which produce
and control the pore of exit. The spermatozoids can then escape in a

Fig. 363.
i.-v. Stages in development of the archegonium of Funaria, after Campbell ( x 4°°)-
vi. Mature archegonium of Andreaea, after Kiihn (X250). i. shows cover-cell
separated from central-cell (shaded), ii. iii. cover-cell (x) undergoing segmentation
as an initial cell, giving rise to three rows of lateral and one of basal segments : the
former constitute the " neck," the latter are the canal-cells, iv. shows the ovum (ov),
ventral canal-cell (v.c.c), and canal-cells {ex.). V. shows the apex of the neck before
rupture, with canal-cells {ex.) within.

thin stream, embedded in mucilage from which they soon escape. In
cases where the perichaetial leaves face upwards, a shower of rain
would bring the rupture about, and the mucilaginous contents may
be seen and collected on a slide in a drop of water. The biciliate
spermatozoids may then be observed in active movement (Fig. 362, vii.).
The archegonium is a flask-shaped body with a long neck (Fig. 363).
It is seated on a massive stalk, and it also arises from a single superficial
cell. When mature it consists of a peripheral wall, which in the lower
ventral portion is double, but the neck consists of a single layer built
up of six rows of cells, as against four in the Pteridophyta. The wall

MUSCI AND HEPATICAE

469

encloses a central series, consisting of canal-cells (c.c.) which may some-
times be very numerous, a ventral-canal-cell (v. c.c), and the ovum (ov.).
At maturity the end of the neck opens in presence of water, owing to
pressure of mucilaginous swelling within ; a funnel-like channel then
leads down to the ovum (Fig. 363, vi.). Spermatozoids, motile in the
water, may be seen to enter it, and there is reason to believe that their

movements are directed by dif-
fusion from it of some soluble
substance, such as cane sugar.
It will be noted how perfect is
the protection of the ovum
within the archegonium, and
that the protection continues
after fertilisation. This reten-
tion of the ovum within the
parent plant is a general
feature of subaerial vegeta-
tion, and may be held to have
been a leading feature in its
success.

The result of fertilisation is
the Sporogonium (Fig. 355, p.
461). It usually appears as
a radially constructed body,
seated in the tissue of the
Moss Plant, and bearing at the
end of a long stalk (seta) a more
or less oval head (the capsule),
which at ripeness- contains very
numerous spores. It is covered
at first by a hood or cap
(calyptra), which falls off at
maturity, disclosing a lid, or
operculum. This finally separates by a transverse split, and falling
away opens the capsule, just as the lid might be taken off a covered
jar. In most Mosses a fringe of ragged filaments, the peristome, is
thus disclosed, which by their hygroscopic movements serve to
distribute the dry and dusty spores (Figs. 364, 367). The sporogonium^
is usually green while young, but yellowish or brown when rM.e.
This is due to photo-synthetic tissue, which is specially developed
at the enlarged base of the capsule (apophysis), where also stomata

Fig. 364.
Median section of an immature sporogonium
of Funaria. s=seta. ap.= apophysis. u' = water-
storage-tissue. s*. = stomata. sp.s. =spore-sac.
arch. = archesporium. col = columella. p = peristome.
op = operculum. Based on a drawing by Haber-
landt. ( x 20.)

4/0

BOTANY OF THE LIVING PLANT

may be found, providing for ventilation
(Fig. 364, St.). But such tissues dry up at
maturity, so that the capsule is then full
of the yellowish dusty spores.

The sporophyte thus constructed is
dependent throughout its existence upon
the gametophyte : it bears no appendages,
and normally it never branches. The
spores all originate from one continuous
spore-sac contained in the capsule. These
features are common for the Bryophyta,
and mark a simpler grade of evolution of
the diploid generation than is seen in the
Pteridophyta and Seed-Plants. Whether
this is a primitive simplicity, or a con-
sequence of reduction, is a question which
can only be discussed on grounds of broad
comparison.

The Moss Sporogonium is without any of
those lateral appendages which are so con-
spicuous a feature in Vascular Plants. Its
external form contrasts with theirs in being
simple. It is essentially a spindle, with polarity
defined as apex and base. Transverse expan-
sion accounts for the origin of the oval capsule
on the end of the seta. But this capsule is a
complicated body in the higher Mosses. Its
complexity arises from an advance in inter-
nal structure, which thus contrasts with its
simplicity of external form. To understand it
the best approach is through development.

The zygote first divides by a basal wall,
which is transverse or slightly oblique to the
axis of the archegonium. This at once defines
the polarity. It is succeeded by a brief apical
growth in the epibasal half, with a two-sided
apical cell. The hypobasal half also segments,
but less regularly, boring its way downwards
into the tissue of the parent (Fig. 366). A
spindle-shaped body is thus produced (Fig.
365, A, B). Subdivision of the segments gives
a central tract (endothecium), and a peripheral
tissue (amphithecium) (Fig. 365, C). The
former is the exclusive source of spore-forma-
tion ; the latter produces the external tissues

B

9 •■ 9

Fig. 365.

Ceratodon purpureus (after Kienitz-
Gerloff) . A , B, young embryo seen
from points of view at right angles to
one another. C = an older embryo.
gg = outer limit of the endothecium.
sp = outer spore-sac.

Ml 'SCI AND HEPATICAE

47 1

of the wall, parts of which are photosynthetic while young. Thus far the
sporogonium is enclosed in the growing venter of the archegonium (Fig. y>6).

As it develops further the lower part remains thin,
forming the seta, which may be traversed by a
conducting strand (Fig. 364). But the distal part
enlarges to form the capsule. A layer of cells is
there cut off from the periphery of the endothecium,
and acquires dense contents. This is the arche-
sporium, which is shaped like a barrel without
ends {arch. Fig. 364). Within it is the large-celled
water-storage tissue of the columella. The amphi-
thecium, limited now by a superficial epidermis with
stomata, forms a lacunar photo-synthetic tissue,
with a large and continuous air-space outside the
archesporium. This tissue is specially active in that
region called the apophysis, where the stomata are
most frequent (st., Fig. 364). In some Mosses it
is enlarged as a very effective organ of nutrition.

As the development proceeds the cells of the
archesporium divide repeatedly, forming a thick
cylinder of sporogenous cells surrounding the
columella, and limited externally by a double
layer of cells of the amphithecium. This con-
stitutes the spore-sac. Its cells then separate,
and rounding off in a liquid that fills the sac, each
undergoes tetrad-division, and finally produces four
spores. Reduction takes place as usual, common
numbers of chromosomes for Mosses being 12-6.
The numbers are low for Bryophytes generally.
The mature spore is very minute, and almost
spherical, and it contains globules of oil.

Meanwhile above the fertile region certain inner
cells of the amphithecium undergo changes of induration of the cell-walls to
form the peristome, which is closely related to the liberation of the spores
{p., Fig. 364). As its structure differs in
detail in various Mosses, it provides facts
valuable in their classification. The case of
Fontinalis serves as a good example of a com-
plicated peristome, as it is seen after the oper-
culum falls away. It is double (Fig. 367). The
inner peristome forms a sort of connected lattice-
work which will allow the spores to pass singly
through its pores, but prevents them all falling out
at once. The outer consists of 16 teeth, which arc
really strips of thickened cell-wall, separated from
one another by the breaking down of the thinner
lateral connections. They show movements with

Fig. 366.

Young sporogonium of Physco-
mitrella patens, shown in outline,
shortly before the rupture of the
archegonial wall. (After Hy.)

Fig. 367.
Fontimdis, apex of capsule (K)

changes of moisture in the air, and catching one after shedding the operculum
. . . - ., /i- 1 _.•__ a/> = outer peristome. «j£> =

on another by their rough edges, they give flicking appcri™™l
jerks on release, which throw the spores to a ( x 50.)

(After Schimper.)

4;2 BOTANY OF THE LIVING PLANT

distance. The spores thus shed germinate to form protonema, as seen
above (Fig. 357, p. 463).

The Mosses show by their ubiquitous spread that they are a success-
ful type of Land Vegetation, though restricted by dependence on
water not only for their fertilisation, but also for their physiological
activity. Their capacity for retaining their vitality under drought,
and their subsequent recovery often saves them. But the feature which
leads most directly to their success is their profuse vegetative propaga-
tion by protonema and by gemmae. Not only do the latter secure
their spread, but the former provide also for their persistence in the
soil whenever conditions at the surface are unfavourable. The
prevalence of Mosses in all relatively humid climates is thus easily
explained.

Hepaticae or Liverworts.

The Life-Cycle of Liverworts is on the same plan as that of the
Mosses, the gametophyte being the predominant generation. In the
simpler types it is thalloid, and may be forked. Pellia, which is
common on moist clay banks, is constructed like a large flat thallus,
of form similar to that of a Fern-prothallus, with rhizoids on the
lower surface, but no other appendages. Most Liverworts, how-
ever, bear appendages. Thus the thalloid Riccia has scales upon

Fig. 368.

Vertical section through part of the thallus of Targionia, showing the cavities opening
by pores on the upper surface, and containing filaments of chlorophyll-cells. ( x 75.)

the ventral (lower) surface of its fleshy thallus. Moreover, its
upper surface is deeply penetrated by narrow air-canals, each bounded
by four rows of chlorophyll-containing cells, of which the outermost
may be enlarged. The result is a ventilated photosynthetic structure.
In the series of the Marchantiales this ventilated construction is
further developed, so as to render the thallus very efficient for photo-
synthesis on land. For instance, in Targionia (Fig. 368), the flattened
thallus, bearing ventral scales below, and fixed by rhizoids in the
soil, is differentiated into a massive lower region chiefly for storage

MUSCI AND HEPATICAE

473

(though it is also penetrated by a mycorrhizic fungus through the
root-hairs), and an upper photosynthetic region marked by large,
overarched air-chambers. Each of these communicates with the
outer air by a large pore, which is more or less under control. From
the floor of the chamber arise active green cells, grouped in simple,
or branching filaments. Such developments, with varying detail,
are characteristic of the Marchantiales. The analogy with Angio-
spermic leaves is obvious, but the origin of the structure is here quite
different, being chiefly due to surface-involution.

Fig. 369.
A, Scapania nemorosa, dorsal view of the leafy shoot, which bears a sporogonium
at its tip. D, Frullania tamarisci, view of leafy shoot from below, to show the
ventral row of leaves, and the two lateral rows, of which the lower lobes form
pitchers. A has the " succubous," B the " incubous disposition of the
leaves. (After Cavers.)

A distinct line of vegetative advance is shown by the Junger-
manniales, in which successive steps may be found from the thalloid
state, through various forms of marginal lobes, to a full leafy develop-
ment. In the truly leafy Liverworts there is a ventral row of leaves,
and a row of lateral leaves on either side. These leaves are more or less
clearly two-lobed, and the lobes are often unequal (Fig. 369, -l ■
Sometimes a lobe may become highly specialised, as in Frullania
(Fig. 369, B), where that which is downward-directed develops as a
water-sac, or pitcher, effective in collecting and holding water in this
epiphytic or rock-dwelling genus. On the other hand, in Trichocolea

474

BOTANY OF THE LIVING PLANT

the leaves may be divided into narrow laciniae, which collectively hold
water as in a sponge. Thus it appears there is a wide scale of
adaptation of the gametophyte in Liverworts. Its results offer
analogies with the special adaptations seen in the sporophyte of
Flowering Plants.

The sexual organs are essentially similar to those of the Mosses ;
but there are differences in their segmentation. This suggests that

Fig. 370.

A — archegonium of Riccia trichocarpa, showing ventral canal-cell (v) and ovum. ( x 525-)
B = ripe archegonium of Riccia glauca. ( x 260.) (After Campbell.)

their origin may have been distinct. In the thalloid Liverworts they
are always borne on the morphologically upper surface ; but by
various means they are carefully protected, being sometimes sunk
deeply in the thallus (Fig. 370). . In the Jungermanniae they are
covered in by envelopes, the number and variety of which give useful
features in classification. A particular interest attaches to those
which develop a " marsupium," that is a nursing-sac surrounding the
archegonia, and penetrating deeply into the soil. Such arrangements,
both in the vegetative structure of the gametophyte and in the

MUSCI AND HEPATIC A K

47?

disposal and protection of the sexual organs, suggest that the Liver-
worts are making the best of subaerial life, to which their simple
structure is not in itself well suited. Their fertilisation is by means
of spermatozoids motile through water.

The sporogonium itself is on a simpler scale than that of the Mosses.
Excepting the peculiar group of the Anthoceroteae, it is not structur-

Fig. 371.

A, Ripe capsule of Aneura pinguis in longitudinal section. From the summit an
elaterophcre hangs into the spore-cavity, in which are many spores and detached
elaters. Magnified. (After Goebel.)

B, Capsule of Pellia calycina, burst, and emptied, showing the valves of the wall
recurved, and an elaterophore rising from the base, bearing many threads. (After
Goebel.)

ally specialised for carrying on photosynthesis, nor is there any
complete columella. Moreover the ripe sporogonium is longer enclosed
in the archegonial wall ; but it bursts it at maturity, when the seta
elongates, bearing outwards the spherical head. There is no oper-
culum, but the relatively thin wall bursts, usually into four vah
and the spores, interspersed among fibrous elaters that help to distri-
bute them, are exposed as a flocculent mass to the breeze, and are
scattered in the dry state. Though the details are different from those
of the Mosses, the end is the same (Fig. 371).

476

BOTANY OF THE LIVING PLANT

On the other hand, certain Liverworts have very simple sporogonia.
This is conspicuously the case in the Ricciaceae (Fig. 372), where it is
spherical, with no distinction of apex and base, and no elaters. The
sporogonial wall is one layer thick, and is disorganised at ripeness.

Fig. 372.

Ricciocarpits natans. Young sporogonia still surrounded by the archegonial
wall. The younger ( x 666) shows the wall of the sporogonium shaded, surrounding
the sporogenous cells. In the older ( x 560), these are separated as the free
spore-mother-cells. (After Garber.)

The spores are scattered on decay of the thallus. This is the simplest
condition of the sporophyte known in Archegoniate Plants. It is a
familiar subject of comparative discussion whether the simplicity
that Riccia shows is really primitive or the result of reduction from
some more complex type.

Comparing the facts from the Mosses and Liverworts it is apparent
that both are " amphibians " in the sense that they live on exposed
land-surfaces, but cannot complete their life-cycle without the
presence of external liquid water. This tends to restrict them to
moist situations. In any organism with a life-cycle punctuated by
the two stages of the spore and the zygote, there are two possibilities
of somatic expansion, viz. in the diploid sporophyte and in the haploid

MUSCI AND HEPATICAF 477

gametophyte. In the Bryophytes the second alternative has b<
fully exploited. Their characters depend upon the development
of the gametophyte to the highest condition in which it is seen in
Land Vegetation. The details of this development run parallel
with those of the sporophyte in Vascular Plants, so that the two
present a series of analogies. The most striking are seen in the
organs of photosynthesis, in the conducting tracts, and in the grouping
of the organs of sex. In the sporophyte of Vascular Plants the
typical photosynthetic organ is the leaf-blade, with its ventilated
mesophyll and stomatal control. In the gametophyte of Mosses
and Liverworts a similarly ventilated structure is seen in the leaves
of some of the larger Mosses (Fig. 359), and in the thallus-structure
of the Marchantiales (Fig. 368). These are, however, parts of the
gametophyte, and the ventilated structure is here produced mainly
by involution of the outer surface, while in Vascular Plants it
arises from intercellular splitting of the cell-walls. The physio-
logical end is the same in both cases, but the place and the means
are different. Plainly these are the results of parallel evolution, or
homoplasy.

So also the conducting tissues seen in the stem of large Mosses, such
as Polytrichum, show in their connections with the leaves, as well as
in their construction of hydrom and leptom, similarities with the
conducting system of Vascular Plants (Fig. 358). But again the
comparison is between the gametophyte on the one hand and the
sporophyte on the other ; while the isolation of such phenomena in
the larger Mosses indicates that the conducting tissues are an
adaptive feature specially developed in them, and not general for
all Mosses. Again the similarity of structure to that seen in Vascular
Plants must be held as homoplastic.

There is also a very peculiar analogy between the flowers of Angio-
sperms and the so-called " flowers " of Mosses, where the perichaetial
leaves surround the sexual organs, as the perianth surrounds the
androecium and gynoecium in Flowering Plants. There is even
parallelism in the distribution of the sexes, for such ' flowers ' in
Mosses may be hermaphrodite or unisexual. Notwithstanding this
likeness it is necessary to keep clearly in mind that such comparisons
deal with essentially different things, though both involve the sex-
distinction (p. 256). The interest of them lies in the fact that the
similarity exists at all.

Such comparisons show how nearly the evolution of the gameto-
phyte in the Bryophyta may follow along the same lines of adapta-

478 BOTANY OF THE LIVING PLANT

Hon as the sporophyte of Vascular Plants. The end is the same
for both, viz. to develop on land as large a vegetable system as
possible, 'so as to provide material for the largest possible number
of germs. The one phylum has solved it by enlargement of the
sporophyte, which thus becomes the substantive " Plant " of
vascular types. The other has solved it by elaboration of the
gametophyte, which has similarly become the substantive " Plant "

of the Bryophytes.

For a less condensed treatment of the Bryophyta see Primitive

Land Plants, Chapters I. to VI.

The Psilophytales.

Till recently it had been thought that among Plants of the Land the widest
gap of organisation lay between the Bryophyta and the Pteridophyta, not-
withstanding that the life-cycle of both is essentially the same. Even now
the distinction seems wide when only living examples of them are compared.
But in late years there have been discovered from rocks of Devonian Age, and
widely spread geographically, fossils grouped as a new Class of the Psilo-
phytales. Their sporophytes only are known, but these are clearly vascular
land-plants of rudimentary organisation. They are rootless, and the simplest
of them leafless. The general character of two of the best known types is
shown in Fig. 372 a, which represents reconstructions of them after Kidston and
Lang. Hornea, the smaller, has a tuberous base attached within the soil by
rhizoids. From this a bifurcating cylindrical shaft rises erect and leafless.
Some of its finer branches bear terminal sporangia. The larger plant, Asteroxy-
lon, has dichotomously branched leafless rhizomes without absorbent hairs :
their finer branchlets ramify in the peat in which the plants grew. From these
sprang large branching and erect trunks. They had localised growing points,
and bore simple microphyllous leaves ; but these were absent from the smaller
distal branchlets which dichotomised freely, and bore terminal sporangia.
The structure of these plants is very completely known. They are quoted
here as examples of early vegetation of the land, which offer illuminating
comparison on the one hand with the sporogonia of the Bryophytes, and on
the other with the sporophytes of the simpler types of Pteridophytes previously
known.

There are obvious restrictive defects in the organisation of the Mosses and
Liverworts, which account for their dwarfed habit. In the sporophyte we note
its physiological dependence on the gametophyte, and the absence of continued
apical growth and branching. ' But the diploid phase of vascular plants is
habitually fixed in the soil and diffuse in form : these features appear in rudi-
mentary outline in the very ancient Psilophytales. Hence we may rightly see
in them types which suggest that varied advance which characterises the
Pteridophytes, and vascular plants generally. Without suggesting close
affinities by descent with any of these, the early existence of the Psilophytales

MUSCI AND HEPATIC A K

479

suggests an intermediate state of that elaboration <>f the sporophyte which
has resulted in the characteristic Flora of the Land. By the discovery of these
and other archaic forms the gap in organisation between the Bryophyta and

Aster vxylon MackieL

Hornea Lignieri

Fig. 372 a.
Reconstruction by Kidston and Lang of the Rhynie fossils named above.

Reduced in scale.

the Pteridophyta has been narrowed, not only in theory but by observed fact.
Nevertheless in the absence of the gametophyte of these fossils, the method ot
establishing the independence of the sporophyte remains still an open problem.

B.B.

2 H

48o BOTANY OF THE LIVING PLANT

But light is shed upon it by the Psilotales, which of living plants appear to be
the nearest allies of the Psilophytales. Their prothallus and embryology have
now been observed. In them we see that the freedom of the sporophyte follows
simply and directly on decay of the prothallus. With such facts in view, the
relation of the Bryophyta to the Pteridophyta becomes more intelligible now
than it was before the discovery of the Psilophytales, or the completion of the
life-history of the Psilotales. For a more complete treatment of the Psilophy-
tales, see Primitive Land Plants, Chapters VII., VIII.

DIVISION IV.

PTERIDOPHYTA.

CHAPTER XXXI.

FILICALES.

The Pteridophyta include the Club-Mosses (Lycopodiales), the
Horse-tails (Equisetales), and the Ferns (Filicales), together with
certain other less familiar types of Plants, some of which are only
known as Fossils. All of these Classes were represented in the
Palaeozoic Period : there is thus no doubt of their extreme antiquity,
which is shown by their characters as well as by their history.
They differ from the Bryophyta in that the leafy " Plant " is the
diploid generation or Sporophyte, and it bears spores : whereas
in Mosses and Liverworts the " Plant " is the haploid or sexual
generation, and it has been seen that it bears antheridia and
archegonia.

A difference in proportion of the leaves of the sporophyte plant Is
very marked among the Pteridophyta. In the Lycopodiales and Equi-
setales the leaves are small (microphyllous), and the axis rela-
tively large (Figs. 404A-406, 413A). But in the Filicales the leaves
are relatively large (megaphyllous) , and often highly branched, giving
the well-known character of the foliage of Ferns (Figs. 354, 373). In
both the shoot is traversed by a conducting system of vascular tissue,
often highly elaborated, and accordingly the whole Division has
sometimes been styled the Vascular Cryptogams. In all of them the
gametophyte is relatively inconspicuous, and is described as the
Prothallus. The life-cycle is essentially uniform for them all. and
their natural relationship may be accepted, notwithstanding the
differences which distinguish the several Classes of them. For the

4S1

482

BOTANY OF THE LIVING PLANT

present purpose it will suffice to select a few examples, of which the
most important will be some common Fern from the large series of
the Filicales, and Selaginella from among the Lycopodiales.

Filicales.

The living Ferns include more than 7000 species, widely spread over
the earth's surface from the Equator to the Arctic Regions. Some

Fig. 373.
Dryopteris Filix-mas, Rich. Fertile leaf about one-sixth natural size, the lower
part with the under surface exposed. To the left a single fertile segment, bearing
kidney-shaped son, enlarged about seven times. (After Luerssen.)

39 of these are British. Though a few live actually in water, others
are distinctly xerophytic, and able to resist extremes of drought.

FTLICALE? A'

But the vast majority live in moist and often in shaded positions,
and, as will be seen later, external liquid water is necessary for the
completion of the normal cycle of their life. The Fern Plant is with
very few exceptions perennial. It consists of a shoot which may or
may not be branched, and is attached to the soil by numerous fibrous
roots. The shoot consists of axis and leaves, as in the Flowering
Plants, but usually without axillary buds. The leaves are large in
proportion to the stem that bears them. Often they are highly
branched, with two rows of lateral pinnae that are branched again
repeatedly. This, together with their delicate texture, gives the
feathery appearance to the leaves of most Ferns (Fig. 373). It is
specially conspicuous in the large Tree-Ferns, where each leaf may
be many feet in length. This habit (megaphyllous) contrasts strongly
with that of the Lycopods with their small unbranched leaves
(microphyllous).

The Coal Period has been sometimes described as ' The Age of Ferns." It
is true that large-leaved Fern-like Plants were frequent then. But many of
these have been lately shown to have been Seed-bearing Plants (Pteridosperms),
whereas Ferns have no seeds. The question has then been raised whether
true Ferns existed at all at that early time. There are at least three types
which can only have been Ferns that did live then, and Botryopteris, of which
the stele is shown in Fig. 375, is an example. Some Ferns resembling these
early fossils survive to the present day. But many of the Ferns we know are
relatively modern. It is doubtful whether at any time a more varied Fern-
Flora has existed on the earth than at the present day. If that be so, the
present is as much the age of Ferns as any that has gone before.

The Ferns show a comparatively primitive cycle of life. It consists
of two alternating and physiologically independent phases, or genera-
tions, the one diploid (non-sexual), which is the sporophyte, the
other haploid (sexual), which is the gametophyte. The former is what
is known commonly as the Fern-Plant ; the latter is a small green
scale-like body, which is called the Prothallus. As the former is the
better-known phase, it will be described first.

Male Shield Fern and Bracken.

The large-leaved shoot of the Fern Plant may grow upright, and
usually unbranched, as in Tree-Ferns, and the Royal Fern (Osmunda) ;
or obliquely, as in the common Shield Fern (Dryopteris) (Fig. 374. A) ;
or horizontally with a creeping habit, as in the Bracken [Pteridium)
or the Common Polypody. When upright, the internodes are short,
and the numerous leaves take that basket-like grouping so well seen

484

BOTANY OF THE LIVING PLANT

in the Shield Fern. When creeping the internodes are longer, so that
the leaves are isolated, as on the underground rhizome of the Bracken,
the leaves being here the only part above ground. The stems of
Ferns have unlimited apical growth, and sometimes fork at their

Fig. 374.

Dryopteris Filix-mas, Rich. A, stock in longitudinal section. v= the apex. st=
the stem, b — the leaf -stalks, b' = one of the still folded leaves. g= vascular strands.
B = a leaf -stalk, bearing at k a bud with root at w, and several leaves. C is a similar
leaf -stalk cut longitudinally. D = a stock, from which the leaves have been cut away
to their bases, leaving only those of the terminal bud. The spaces between the leaves
are filled with numerous roots, rr, wf . E, stock from which the rind has been removed
to show the vascular network, g. F=a mesh of the network enlarged, showing the
strands which pass out into the leaves. (After Sachs.)

ends. But buds may also appear at the leaf-bases, a condition seen in
the old leaves of the Shield Fern (Fig. 374, B). A general peculiarity
of Ferns is the crozier-like curvature of their young leaves, the adaxial
face growing at first more slowly. But later it catches up the abaxial
face, so that the leaf flattens out as it matures. This habit is effective

FILICALES

485

in protecting the curled tip of the leaf, since in Ferns apical growth
is long continued, and the apical tissues arc delicate. The stem and
leaves, especially while young, are often densely covered either with
hairs (Osmunda) or chaffy scales (Dryopteris), which protect the
young parts against drought, but are liable to fall away later.

In their general construction Ferns resemble Flowering Plants.
They have a superficial epidermis, and a conducting system of vascular
tissue, embedded in ground-tissue which is parenchymatous : but often

Fig. 375-
Transverse section of a fossil-Fern, Botryopteris cylindrica, showing a protostele
with solid central core of xylem, and peripheral phloem. This is a fossil from
the Palaeozoic Period, and it illustrates how perfect the preservation of structure
may sometimes be in fossils of very early times.

it also encloses strands or islands of hard brown sderenchvma, while
hard stony or horny sheaths frequently form the surface of stem and
leaf-stalk. The epidermis and ground tissue call for no detailed
description. The chief interest lies in the vascular system. In
ancient fossil Ferns, such as Botryopteris, in many primitive living
Ferns, and generally in young sporelings there is a simple stele of a
type called a " protostele," having a solid xylem-cure. and phloem
surrounding it. This is believed to have been the primitive strueture
for them all. It is well shown in Botryopteris (Fig. 375). Occasionally
this state may be retained through life [Hymenophyllum, Lygodium).

486

BOTANY OF THE LIVING PLANT

But in the vast majority of Ferns the stele expands as the plant
grows stronger, and the leaves larger. In various ways it becomes
segregated into a number of vascular strands (meristeles), arranged in
a cylindrical network (Fig. 374, E. F). Each mesh in Dryopteris
corresponds to the insertion of a leaf -base, and is called a foliar mesh,
or gap. The vascular strands that run out into the leaf, called col-
lectively the leaf-trace, arise from the margin of it, and the cortex and

Fig. 376.

Transverse section of rhizome of Bracken, showing the outer and inner series of
meristeles, and the irregular bands of sclerenchyma between them. These are
embedded in soft ground-parenchyma, with a hard sclerotic rind. ( x 10.)

pith are in direct communication through the foliar gaps. This is
readily understood if the vascular skeleton be dissected out, as it is
seen in Fig. 376 a. In transverse sections the meristeles would appear
as a ring of isolated tracts, and this gave rise to the misleading term
" polystelic " as applied to such stems. But the dissection shows
that they are all parts of one cylindrical network, which arises through
dilatation of the protostele of the sporeling (compare Figs. 374, 375)-
A similar disintegration of the leaf-trace may be followed in Dryop-
teris, and many of the larger Ferns. A simple strand appears to have
been the primitive type of trace, and it is seen generally in the leaves
of their sporelings. But a plurality of strands appears in the adult

I- 1 LIC ALES

leaf-stalk of most modern Ferns (Fig. 374, D). The several strand
Dryopteris can be traced into the pinnae and pinnules where they fork
freely and end blindly (Fig. 373). But in broad-leaved Ferns such
as the Adder's Tongue, and particularly in those of more advan

Fig. 376 a.

Dictyostele of the Male Shield Fern dissected out, showing the overlapping
leaf-gaps which allow communication between cortex and pith. (After Reinke.)

type, such as Onoclea and Woodwardia, a network of veins may arise
by their lateral fusion, after the manner of Dicotyledons (Fig. 46).
Both types serve the same purpose, that of supply within the
flattened blade ; but the netted venation is functionally the more
efficient. Moreover, comparison shows that it is a derivative state,
based on an originally forked venation.

It thus appears that the vascular system in the leaves of Ferns resembles
that of Flowering Plants more nearly than does that of their adult stems.
The diversity in vascular structure between the stems of Ferns and those of
Seed Plants arises from an essential difference in their way of solving a
fundamental problem of support and of supply (see Chapter XXXVI). In the
Bracken and Shield Fern the conducting tracts are all of primary origin. They
may be traced continuously to their source immediately below the growing
point itself, and their outline there corresponds to that which they show when
mature. As will be seen in Figs. 376-378, there is no cambium in their make-up.
Ferns by their stelar elaborations make the best of this primitive scheme of
vascular construction. On the other hand, cambial activity, as described fur
Dicotyledons and Gymnosperms in Chapter IV, is an automatic means of
meeting the growing demands of increasing size. But it is a morphological
afterthought : the tissues it produces being of secondary origin.

4»8

BOTANY OF THE LIVING PLANT

For the study of the tissues composing a vascular strand a rhizome
with long internodes, such as the Bracken, gives the best results.
In a transverse section taken between the leaf-insertions an outer and
inner series of vascular strands is found, separated by an incomplete
ring of sclerenchyma. The outer series corresponds to the mesh-work
of Dryopteris, the inner are accessory or medullary meristeles (Fig.
376). Each one is circumscribed by a complete endodermis. This

Fig. 377.
Part of a transverse section of a meristele of Bracken. g=ground parenchyma.
e= endodermis. ph = phloem with sieve-tubes. *y = xylem, with large scalariform
tracheides. Some smaller tracheides lying centrally are the proto-xylem. Note
that no intercellular spaces are seen within the endodermis. ( x 75.)

is usual in Ferns. Each consists of a central core of xylem, surrounded
by phloem ; in fact they repeat the main structure of the protostele
itself. A transverse section of one of them, examined under a high
power, gives the following succession of tissues (Fig. 377). Passing in-
wards from the starchy ground-tissue, with intercellular spaces (g), the
layer of brownish cells of the endodermis (e) forms a continuous barrier,
delimiting the strand sharply. Within it follows the pericyde, with
its cells not very regularly disposed, but corresponding roughly to the
cells of the endodermis, both having been derived by division from a
single layer. Within this comes the phloem (ph), with large sieve-tubes

FILICALES

489

as the characteristic elements. They are thin-walled, with watery

contents. The lateral walls where two adjoin bear the sieve-plat
which are recognised by glistening globules that adhere to them. They
are embedded in parenchyma, which extends inwards into the xylem,
and may be called collectively conjunctive parenchyma. The chief
features of the xylem (xy) are the tracheides, which are relatively large,
with a very characteristic polygonal outline. They have woody
walls, and no protoplasmic contents. Where two adjoin the walls are

Fig. 378.
Longitudinal section of meristele of Bracken. Lettering and magnification as in Fig. 377.

flattened, and of double thickness, showing that each has its own share
of the thickening, which overarches the pit-membrane as in the pits of
Conifers. The structure is in fact essentially the same, only in Ferns
the pits are liable to be extended transversely. But where the
tracheide abuts on parenchyma-cells the pits are narrower. Internally,
and usually about the foci of the elliptical meristele, smaller tracheides
are found. These are the first-formed tracheides, or protoxyUm.
The meristele of a Fern is thus concentric in construction ; it is
strictly delimited, and has no provision for increase in size.

A transverse section gives only one aspect in which such complicated
tissues should be studied. Its interpretation is aided by longitudinal

490

BOTANY OF THE LIVING PLANT

tions (Fig. 3/8). It is then seen that the sieve-tubes, which are
elongated and pointed, bear their numerous sieve-areas upon the
lateral walls : and that the spindle-shaped tracheides bear also upon
their lateral walls those transversely elongated pits which give them
the so-called scalariform appearance.

The tracheide of the Fern resembles that of the Pine in being of spindle-
form, with its thickened lignified walls marked by bordered pits. (See p. 530.)

Fig. 379.

Tracheides of Pteridinm. A =the end and
about one third of the length of a tracheide with
part of the lateral wall in surface view, showing
scalariform marking ( x ioo). Z?=part of A
magnified 200. C«=thin longitudinal section
through a lateral wall where two tracheides
adjoined ( v 375). D= similar section through
oblique wall at / ( x 200) . there the pit mem-
branes are not visible. (After De Bary.)

Fig. 380.

Sieve-tubes of Pteridium. .4= end
of a tube separated by maceration
( x 100). B = longitudinal section
through phloem showing one sieve-
tube with the sieve-plates (sj) in surface
view, c, c are walls shown in section,
bearing sieve-pits ( x 200).

But whereas the pits in the Pine are circular, those in the Fern are liable to be
transversely elongated, as is natural in tracheids so wide as these are.
Their features are well seen in longitudinal sections, but better if they are
isolated by maceration (Fig. 379, A). The elongated pits he parallel to one
another, and this is specially well seen where two wide tracheids have faced
one another. From the ladder-like appearance that results they have been
called scalariform tracheids. Examined under a high power the double out-
line of the pits is seen, and when the pits are small and circular the similarity
to those of the Pine is plain (Fig. 379, B). In most Ferns the pit-mem-
branes persist, but in Pteridium they appear to be liable to be broken down,
and the cavities thrown together, technically as in vessels. The tracheides

FTLICALES 491

of the protoxylem are seen in longitudinal section to be spiral or reticulate,
as in other Vascular Plants (Fig. 378, p.xv).

The sieve-tubes are also spindle-shaped, and are without companion-cells.
Their cellulose walls are swollen. Where two sieve-tubes adjoin, numerous

thinner sieve-areas of irregular outline are borne. They arc found to be
perforated by very fine protoplasmic threads extending between highly
refractive globules that adhere to the walls (Fig. 380). Such tracheides and
sieve-tubes are characteristic of Ferns, and with differences of detail, they
are present in other Pteridophytes as well.

The anatomy of the leaf in Ferns resembles that of Seed-Plants down even
to the collateral structure of the vascular strands. Being chiefly shade-
loving plants chlorophyll is usually present in the cells of their epidermis,
and the differentiation of the mesophyll into palisade and spongy parenchyma

Fig. 381.
Transverse section of part of pinnule of Dryopteris ( x 150), showing epidermis,
and the spongy mesophyll, with an internal glandular cell.

is not marked (Fig. 381). In these respects they resemble the leaves of Angio-
sperms of similar habit. In the roots of Ferns, as in those of Seed-Plants, there
is a superficial piliferous layer, a broad cortex, and a contracted stele. But
usually the inner cortex is very strongly lignified, up to the endodermis, which
is thin-walled (Fig. 382). The pericycle which follows is variable, sometimes
being greatly enlarged as a water-storage- tissue. The protoxylems are
peripheral, and two or sometimes more in number, the phloem-groups alter-
nating with them. In fact the root of a Fern is constructed essentially on the
plan of that in Seed-Plants. As there is no secondary thickening the roots of
Ferns are all fibrous. The lateral roots arise opposite to the protoxylems,
and there they originate from definite cells of the endodermis, which may often
be recognised beforehand by their size and content-.

While we recognise the substantial similarity of Ferns and Seed-Plants in
respect of form and structure of stem, leaf, and root, these plants dilter in tin-
construction of their apical meristetns. In Seed-Plants these are small-celled
tissues, and more or less definitely stratified (pp. 17, 89). In Ferns such
as Osmunda, Dryopteris or Polypodium, a single large cell, the apical or
initial cell, occupies the tip of each growing part. It has a definite

492

BOTANY OF THE LIVING PLANT

shape ; it shows continued though slow growth, and segments are cut off
from its sides in definite succession. As the whole tissue of the stem,
leaf, or root is derived from such segments, the whole of each part is referable
in origin to its apical cell, which maintains its identity throughout. The form
of the cell in roots, in most stems, and in some leaves (Osmunda) is that of a
three-sided pyramid ; but where the organ is flattened, as in some stems
(Pteridium), and almost all leaves, it has two convex sides, and is shaped like

Fig. 382.

Transverse section of a root of a Fern (Pellcea) ( x 150). Outside lies the sclerotic
cortex, limited internally by a definite endodermis. There are two groups of pro-
toxylem ; a very broad pericycle, of 3 or 4 layers, surrounds the vascular tissues.

half of a biconvex lens. In the former case the segments are cut off in regular
succession from the three sides (Figs. 383, 385), in the latter alternately from
the two sides (Fig. 384). The further subdivision of the segments to form
the tissues is represented in surface view for the case of Osmunda in Fig. 383 :
and Fig. 384 shows, in the surface view of a young leaf of Ceratopteris how
the whole member may be built up from such segments. In roots the seg-
mentation is complicated by the origin of the root-cap. This is provided by
a segment cut off from the frontal face of the pyramid, after each cycle of
three has been cut off from its sides (Fig. 385). Thus every fourth segment
goes to form the protective cap, and renews it from within. Not only does
the leaf also show continued growth and apical segmentation from its two-
sided apical cell, but the lateral wings or flaps originate by the activity of
rows of marginal cells. There is also a definite segmentation seen in the
origin of the sporangia. Thus Ferns have not stratified meristems like

FILICALES

Seed-Plants. The tissues of all their parts originate from segmentation
of superficial cells. This is a general character of the Pteridophyta, though

Fig. 383.
Apex of stem of Osmunda regalis, seen from above,
showing the three-sided apical cells of stem, and of leaf ;
and the bases of the older leaves shaded. The succes-
sive segments of the apical cell form the whole of the
apical cone. ( x 83).

Fig. 384.

Young leaf of Ceratopteris, in surface view, after
Kny ; showing two-sided apical cell ; and the
marginal series, continuous round the young
pinnae. The latter do not correspond in number
to the segments from the apical cell.

the details of their segmentation and the number of the initial cells are open
to variation.

Fig. 385.

( x 250.) A = longitudinal section through apex of th-> n rans-

verse section through the apical cell of the root and neighbouring segments of
Athyrium. (After Naegeli and Leitgeb.) r = apical cell, k, I. m, n =-successivt- laj
of root-cap. o = dermatogen. c = limit of stele. (From Sachs.)

Thus constituted the Fern-Plant carries out its Life on Land in
essentially the same way as Seed-Plants. The structural differences
are those of detail, the most important being the absence of secondary
thickening in the stem. These plants have no automatic provision for
increasing mechanical strength with size. In Tree-Ferns this deficiency
is made up for partly by masses of hard brown sclerenchyma. which

494

BOTANY OF THE LIVING PLANT

accompany and enclose the flattened meristeles ; and their margins
are usually curved outwards, thus securing increased mechanical
resistance on the same principle as in corrugated columns. Their
strength is further increased according to size and age by the develop-
ment of masses of sclerotic, adventitious roots, matted together to

form a thick investment to the original
trunk, and adding to its stability by
a method comparable mechanically to
a cambial thickening, though quite
different in origin. But such mechani-
cal provision for increase in size is only
partially effective. There is no evi-
dence that Ferns ever ranked among
the largest of living Plants.

Many Ferns increase in number by
vegetative propagation. This may follow
simply on continued growth and
branching, as in Pteridium, where the
rhizome forks frequently. Whenever
progressive rotting extends from the
base beyond a branching, the two
apices grow on as independent plants.
In this way the Bracken multiplies
habitually. In Dryopteris buds are
formed near the bases of the leaves in
old plants. Again, as rotting proceeds
from the base, these buds become
isolated, and root themselves as new
individuals (Fig. 374, B} C). In other

Fig. 386.

A pinna of a Fern (Woodwardia) show-
ing many sporophytic buds on the upper
surface. They correspond in position to
son on the lower surface, which are
abortive, and they may be held to be sub-
stitutionary growths.

Ferns, as in the various species of

Aspleniuvi so commonly grown in
dwelling rooms, buds or bulbils arise
on the lamina. Being very lightly
attached to the leaf they are readily
shed, and root themselves independently in the soil. In some cases
vegetative buds may replace the sori (Fig. 386). Such vegetative
propagation of the Fern-Plant is a mere repetition of the sporophyte
generation. But sooner or later the Fern-Plant bears the spores,
which start the alternate generation.

The spores are produced on certain leaves of the mature plant
which are therefore called sporophylls, to distinguish them from those

FILICALES

which are only nutritive. In Dryopteris nutritive ! ind

sporophylls are alike in outline. The young plant only produces the
former. But the leaves of older plants bear on their lower surfa
and chiefly in the apical region, numerous groups of organs which
green or brown according to age. These are called sorit and consist
of sporangia with certain protective structure The sori vary greatly

in size and form in different Ferns, which are classified according

Fig. 387.

Vertical section through the sorus of Dryopteris Filix-mas. (After Kny.)

The adaxial surface is uppermost.

to their characters. In Dryopteris, as the name implies, they are
kidney-shaped, as is seen in Fig. 373. Each sorus is seated on a
vein, which provides its necessary nourishment. It is protected by a
covering called the indusium, of kidney-like outline, beneath which are
numerous sporangia. If a leaf bearing mature sori -be laid on a sheet
of paper to dry, with its lower surface downwards, the indusia shrivel,
and the bursting sporangia shed the spores in such numbers that t hex-
give a clear print of the outline of the sporophyll upon the pa]
The spores are dark-coloured, very minute, and are produced in
millions.

B.B. 2 1

496

BOTANY OF THE LIVING PLANT

A vertical section through the sorus of Dryopteris shows an
enlarged receptacle, traversed by the vascular strand. The indusium
rising from it overarches the numerous sporangia which are attached
basally by long thin stalks (Fig. 387). The head of each sporangium is

Fig. 388.
Successive young stages in the segmentation of the sporangium of Dryopteris

Filix-mas. (After Kny.)

shaped like a biconvex lens ; its margin is almost completely sur-
rounded by a series of indurated cells, which form the mechanically
effective annulus. This stops short on one side, where several thin-
walled cells define the stomium, or point where dehiscence will
take place (Figs. 387, 389, 4a). Within are the dark-coloured
spores, which on opening a ripe sporangium carefully in a drop of
glycerine may be counted to the number of 48. Normally the
sporangia open in dry air, and the dry and dusty spores are forcibly
thrown out.

FILICALES

497

The origin of a sporangium is by outgrowth of a single supcrliu.il < ell of the
receptacle, which undergoes successive segmentations as illustrated in Fig. 388.
1-3. A tetrahedral internal cell is thus completely segmented off from a single
layer of superficial cells constituting the wall. The Conner undergoes fun
segmentation to form a second layer of transitory nutritive cells called the
tapetum (Fig. 388, 6-12), subsequently doubled by tangential fission (Fig. 389,1).

Fig. 389.

Later stages of development of the sporangium of Dryopteris b'ilix-mas.

(After Kny.)

The tetrahedral cell which still remains in the centre, having grown mean-
while, undergoes successive divisions till twelve sporc-mothcr-cclh are formed
(Fig. 389, 2-7). These become spherical, and are suspended in a liquid which,
together with the now disorganised tapetum, fills the enlarged cavity of the
sporangium. Each' spore-mother-cell then divides twice to form a Spore-
tetrad: in this process, just as in the formation of pollen-grains and other
spores, the number of chromosomes is reduced to a half. Finally the resulting
cells separate on ripening as individual spores, each covered by a protecting
wall, rugged and dark brown at maturity. Owing to the absorption of the
liquid contents of the sporangium the separate spores are dry and dusty, and

49§

BOTANY OF THE LIVING PLANT

are readily scattered. Since each of the 12 spore-mother-cells forms four
spores, their number is 48 in each sporangium. Each mature spore consists
of a nucleated protoplast, bounded by a colourless inner wall, and a brown
epispore bearing irregular projecting folds.

Meanwhile the wall of the sporangium has differentiated into the thinner
lateral walls of the lens-shaped head, and the annulus, which is a chain of about
16 indurated cells surrounding its margin (Fig. 389, 4a, 46). These form a
mechanical spring, which on rupture of the thin-walled stomium becomes
slowly everted as its cells dry in the air, and then recovering with a sudden
jerk throws out the spores to a considerable distance (Fig. 390). Dry con-
ditions are necessary for this last phase of spore-production, viz. the dis-
semination of the numerous living germs. Each spore is a living cell, and
may serve as the starting point for a new individual.

Fig. 390.

A = sporangium with annulus everted. B, a similar sporangium after recovery by
a sudden jerk. C, condition of cells of the everted annulus. Z) = cells of annulus
before eversion (see p. 167).

The dry conditions which are necessary for the dissemination of
the spores do not suffice for their further development. Moisture
and a suitable temperature are required for their germination. The
outer coat then bursts, and the inner protrudes, cell-division appearing
as the growth proceeds (Fig. 391). The body that is thus produced
is called the prothallus, and it may vary in its form according to the
circumstances. It usually grows first into a short filament attached
by one or more rhizoids to the soil (4). It then widens out at the
tip to a spatula-like and finally to a cordate form (Fig. 392).
But when closely crowded the filamentous form may be retained
longer (Fig. 393, 1). The body of the prothallus, exclusive of the
downward-growing rhizoids, consists of cells which are essentially
alike, arranged at first in a single-layered sheet. The peripheral parts
retain this, but in the central region, below the emarginate apex, the

FILK'AI.KS

490

cells divide by walls parallel to the flattened 3urfa md thu

massive central cushion is formed. The mature cells are thin-walled,
with a peripheral film of protoplasm surrounding a central vacuole,
and embedding the nucleus and numerous chloroplasts : intercellular
spaces are absent (Fig. 19, p. u). The whole body is thus capablt

Fig. 391.

Successive stages in germination of the spores of Dryopteris Filix-mas, to forni

the prothallus. (After Kuy.)

an independent physiological existence, nourishing itselt by absorption
from the soil, and by photo-synthesis (Fig. 392). But there is a large
proportion of surface to bulk, and no serious resistance is ottered to
the evaporation of water from it in dry air. Comparing t he prothallus
with the Fern-Plant as regards the water-relation, it is plainly
adapted for life on land, and more immediately dependent on
moisture.

The prothallus thus constituted is capable in some of

500

BOTANY OF THE LIVING PLANT

vegetative propagation by " gemmae." But this gametophytic
budding is less common here than in the Bryophytes.

The dependence on moisture is still more obvious in the behaviour
of the sexual organs which the prothallus bears. These are male and
female, and they may be found on the same prothallus (Fig. 392), or
on different prothalli (Fig. 393, 1). In the former case the antheridia,

Mature prothallus of Dryopteris Filix-mas, as seen from below, bearing antheridia
among its rhizoids, and archegonia near to the apical indentation. (After Kny.)

or male organs, commonly appear first, and the archegonia, or female
organs, later. There may thus be a separation of the sexes either in
time or in space. The flattened prothallus of the ordinary cordate
type usually bears both sex-organs. When growing under normal
circumstances on a horizontal substratum it produces them on its
lower surface, the antheridia in the basal or lateral regions, the
archegonia upon the massive cushion. The latter develop in acropetal
order, the youngest being nearest to the incurved apex of the pro-
thallus. The position of the sexual organs is evidently favourable

FIUCALES

to their continued exposure to moist air, or to liquid water which is
necessary for carrying out their function.

The antheridium, which arises by outgrowth and segmentation of
a single superficial cell (Fig. 393, 2, 3), consists when mature of a
peripheral wall of tabular cells, surrounding a central group of sperma-
tocytes (Fig. 393, 4, 5). The antheridium readily matures in moist air,

Fig. 393.
i, an attenuated male prothallus of Dryopteris Filix-MOS. 2 -of develop-

ment of the antheridium. 6, 7, ruptured antheridia. 8, a spennutozoid highly
magnified. (After Kny.)

but it does not open except in presence of extrrn.il water. '1 his
causes swelling of. the mucilaginous walls of the spermatocytes, and
increased turgor of the cells of the wall. The tension is relieved by
rupture of the wall at the distal end, and the spermatocytes are ex-
truded into the water; in this the remaining cells of the wall assist by
their swelling inwards, and consequent shortening (Fig. 393. '' • The
spermatocytes thus extruded into the water which caused the rupture,

502

BOTANY OF THE LIVING PLANT

soon show active movement, and the spermatozoid which had already
been formed within each of them escapes from its mucilaginous sheath,
and moves freely in the water by means of active cilia attached
near one end of its spirally coiled body (Fig. 393, 8).

The archegonium also originates from a single superficial cell, and
grows out so as to project from the downward surface of the thallus.
It consists when mature of a peripheral wall of cells constituting the
projecting neck, and a central group arranged serially. The deepest-
seated of these is the large ovum, which is sunk in the tissue of the
cushion ; above this is a small ventral-canal-cell, and a longer canal-cell
(Fig. 394, A). If prothalli be grown in moist air, and only watered
by absorption from below, the archegonia having no direct access

Fig. 394.

Archegonia of Polypodium vulgare. A, still closed. o=ovum. K' — canal-cell. AT"=ventral
canal-cell. B — an archegonium ruptured, (x.240.) (After Strasburger.)

to liquid water will remain closed. Fertilisation is then impossible.
But if they are watered from above, as they would be by rain in the
ordinary course of nature, the external water will bathe them,
and rupture will result. This may be observed in living archegonia
which have been kept relatively dry, and then mounted in water.
The neck bursts at the distal end, owing to internal mucilaginous
swelling, and its cells diverge widely. The canal-cell and ventral-
canal-cell are extruded, and the ovum remains as a deeply seated
spherical protoplast, while access to it is gained through the open
channel of the neck (Fig. 394, B). Thus the same condition leads to
the rupture both of the male and female organs. In nature a shower
of rain would supply the necessary water, which would serve also
as the medium of transit of the spermatozoids to the ovum. But the
movements of the spermatozoids are not subject to blind chance. It
has been shown that diffusion of a very dilute soluble substance, such

MI KALES

503

as malic acid, into water serves as a guide, the spermatozoida moving
towards the centre of diffusion. Probably it is in this way that ti

A

Fig. 3Q5.

Fertilisation in Onoclea sensibilis : the arrows indicate direction to the growing
point. A =a vertical section through an archegonium probably within ten minutes
after entrance of the first spermatozoid. ( x 500.) B = vertical section of the venter
of an archegonium, containing spermatozoids, and the collapsed egg with a sperma-
tozoid within the nucleus. Thirty minutes. ( x 1200.) (After Shaw.)

are attracted to the neck of the archegonium, which thev mav be
seen to enter, and finally one spermatozoid coalesces with the ovum

Fig. 396.

Horizontal section of an egg, showing coiled male tmcleos within the female. Twelve

hours. ( . 1200.) (After Shaw.)

(Fig, 395). Fertilisation is effected by entry of the male nucleus into
the female nucleus, and their complete fusion (Fig. 396). Thus the

504

BOTANY OF THE LIVING PLANT

presence of external water is essential for fertilisation in Ferns.
Their normal life-cycle cannot be completed without it.

The immediate consequence of fertilisation is growth and segmenta-
tion of the zygote, which first secretes a cell-wall. It divides into
octants, four of which constitute an epibasal hemisphere, directed
towards the apex of the parent thallus, giving rise to axis and leaf of
the sporeling ; four form a hypobasal tier, which gives rise to the
first root and a suctorial organ called the foot. These parts are soon
distinguishable by their form and structure, and are seen in their

Fig. 397.
Embryo of Adiantum continuum in the enlarged venter of the archegonium, so far
advanced as to show the parts of the embryo. The epibasal hemisphere is to the
left, the hypobasal to the right. L=leaf or cotyledon. K=root. 5 = stem.
F =foot. (After Atkinson.)

relative positions, but still enclosed in the enlarged venter of the
archegonium, in Fig. 397. Soon the cotyledon and first root burst
their way out : the former expands as the first nutritive leaf, the latter
buries itself in the soil (Fig. 399). At first the young Fern-Plant is
dependent upon the prothallus that encloses it, but by means of
its cotyledon and its root it soon becomes self-dependent, and the
prothallus rots away. It is then only a matter of time and opportunity
for it to attain characters similar to those of the parent Fern-Plant.

These are the salient features in the life-cycie of a Fern as it is seen
in its simplest form. They may be represented graphically to the
eye in a diagram (Fig. 400, p. 506). The two most notable points are
those where the individual is represented only by a single cell, viz. the

FILKALKS

505

spore, and the zygote. These are two landmarks between which
intervene two more extensive developments, on the one hand the
sexual generation or prothallus, on the other the spore-bearing
generation, or Fern-Plant. If the events above detailed recur in
regular succession the two phases of life will alternate. Of these the
one bears sexual organs, containing sexual cells or gametes, and
it may accordingly be called the gametophyte ; the other is non-
sexual, but bears sporangia containing the spores, and is accordingly
called the sporophyte. The study of Ferns, and of Pteridophytes at
large, leads to the conclusion that this regular alternation is typical

Fig. 398.
Adiantum Capillus Veneris.

The

prothallus, pp, seen from below has
a young Fern-plant attached to it.
6 = first leaf, w, ifl'=first and
second roots, h = root hairs of the
prothallus. ( x about 30.) (After
Sachs.)

Fig. 399.
Adiantum Capillus Veneris. Longitudinal section
through the prothallus, pp, and young Fern-plant £.
fc = root hairs of prothallus. a = archegonia. 6 = the
first leaf. u> = the first root of the embryo. ( x 10.)
(After Sachs.)

for them all. These two alternating generations differ not only
in form but also in their relation to external circumstances, and
especially in the water-relation. The sporophyte is structurally a
land growing plant, with nutritive, mechanical, and conducting
tissues, and a ventilating system. Not only is it capable of under-
going free exposure to the ordinary atmospheric conditions, but
dryness of the air is essential for the final end of its existence,
viz. the distribution of its spores. On the other hand, the gameto-
phyte is structurally a plant ill-fitted for exposure, with undiffer-
entiated and ill-protected tissues and no ventilating system, while
the object of its existence, viz. fertilisation, can only be secured
in the presence of external liquid water. As regards the water-relation

506 BOTANY OF THE LIVING PLANT

the whole life-cycle of a Fern, or of Pteridophytes generally, might
not inaptly be designated as amphibious, since the one phase is
dependent on external fluid water for achieving its object of pro-
pagation, while the other is independent of it.

fporvpfit/tic
buaefina

POROPHYTE

ZYGOTE
/t

rfnf/iecu/ium

/IreheaoMum

Oametophyte

■budlavuj

Fig 400.
Diagram illustrating the cycle of life of a Fern.

The normal cycle thus presented to the eye involves differences of nuclear
condition of the alternating phases, those differences being established re-
spectively by fertilisation and by the tetrad-division. The sporophyte or
Fern-Plant is diploid, and the number of chromosomes is usually very large
(about 90 for Athyrium, 144 for Dryopteris pseudo-mas, but 32 for Marsilia).
This number is reduced to one-half in the tetrad-division of the spore-mother-
cells, and the spores on germination produce the gametophyte which is haploid.
But in fertilisation, when the gametes fuse, the diploid number is restored.
This normal cycle corresponds to that seen in higher forms, the substantive
Plant being in all cases the diploid sporophyte.

The cycle as thus defined is liable to certain modifications. Some involve
the introduction of new incidents, others the excision of certain phases.
For instance, buds may be produced either on the Fern-Plant or on the pro-
thallus, which repeat respectively the one or the other (Fig. 386, p. 494). These

MI I- AU-.S

50/

are merely amplifications of the soma, without any change of constitution of
the tissues, or of the nuclei. But others are oi the nature of shortn ata

instance, a prothallus may arise from the Fern Plant without th<- intervention
of spores (apospory), as in certain forms of Athyrium or Polystichum (Fig. 401,

1

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A, B). Or a Fern-Plant may spring directly from a prothallus without the
sexual process {apogamy), as in Ptcris erotica (Fig. 402, C). Such example- show
that the events of the life-cycle are not immutable. But they raise difficulties
of interpretation in terms of chromosomes.

In the relatively simple case of Dryophris pseudo-mas, var. polydactylum,
a young sporophyte is produced as a direct outgrowth from the prothallus. By
a careful examination of the bud-forming tissue it has been found that the bud

Fig. 402.

Pteris cretica ; prothallus seen from below, bearing an apogamous bud derived not by
fertilisation but by direct growth from the cushion. (After De Bary.)

Fig. 403.
Dryopteris pseudo-mas, var. polydactylum. Tissue of prothallus where an apo-
gamous growth is to be found, showing on the left a cell with two nuclei, while an
adjoining cell has none. At the centre a nucleus is seen passing through a perfora-
tion of the wall, and fusing immediately with that of the cell it enters. (After
Farmer, Moore, and Miss Digby.)

FILICALES

509

* &

./-

frcn

is preceded by a sort of irregular fertilisation. The nu< letlfl |

cell through a pore in the cell-wall into the next cell. I here it hues with the

nucleus of the invaded cell (Fig. 403). Doubtless there is here a donblin|

the chromosomes, as in normal fertilisation ; and such a cell, like a f.
zygote, may serve to initiate the sporophytic bud. The proo
styled pseudomixis to suggest a comparison with sexuality, while marking it^
distinctness from it.

In other cases careful investigation has shown that a gametophyte may
be diploid. Transition from one generation to the other may then be re]
while uniformity of chromosome-number is maintained throughout. Thi
seen in Athyrium filix-foemina, var. clarissima,
where the number is 90, approximately that
for the normal sporophyte of that species.
The same is the case for certain plants of
Marsilia Drummondii, which are diploid
throughout, with 32 as the number. It is
probable that the converse is the case for
Dryopteris pseudo-mas, var. cristata (Fig. 404),
for the chromosome-number throughout was
found to be variable, from 60 to 78, while in
that species the normal number for the sporo-
phyte is 144. Not only do such cases show
that the usual chromosome-cycle may be
departed from, but also that the external
characters are not directly dependent upon
the chromosome-number.

The cycle of life of a Fern shows more
clearly than that of any of the Vascular
Plants hitherto described the antithesis of
the two generations which constitute it.
Each can grow, nourish itself, and even
multiply independently of the other. It
is true that the young sporophyte is nursed
temporarily in the parent prothallus. But

this is only a transient event and is soon over. A similar nursing
period, with much greater adaptive detail that lends added efficiency,
is seen in the Seed-Plants. The main difference between the Seed-
Plant and the Fern lies in their spores. The former are lit (cros porous,
the latter are, with few exceptions, homosporous. The advantage of the
large female spore is that it contains already a supply of nourishment
for the young germ after fertilisation, so that a vegel rive prothallus
is not necessary. Especially is this so in Seed-Plants where the spore
is retained in the tissue of the parent, and can draw nourishment
continuously from it. On the other hand, in the primitive homo-
sporous state, as it is seen in the Bryophytes and Pteridophytes, the

Fig. 404.
Dryopteris pseudo-mas, v. cristata
(Cropper). Drawing by Dr. Lang
showing apogamous transition from
prothallus to sporophyte, and sub-
sequent aposporous transiting from
sporophyte to prothallus at the apex
and margins of the leaf.

5io BOTANY OF THE LIVING PLANT

individual spores are small and they are cast out in large numbers to
fend for themselves. It is then incumbent on each spore, when it
germinates, at once to increase its slender store, otherwise it cannot
produce gametes, or nourish the resulting germ. Hence the inde-
pendent vegetative existence of the gametophyte, and its immediate
formation of photosynthetic tissue when it germinates.

Further, the difference in biological relation of the two generations
to water is very marked. The prothallus, which is semi-aquatic, is the
less prominent, and its growth is normally limited in size and duration.
The Fern-plant, which is definitely terrestrial in structure and function,
is in the ascendent, and its growth is unlimited in size and in duration.
A Fern is like a man with one foot in the water and one on land. But
the foot that is on land is more firmly set than the other. In the
Bryophytes, however, the balance is the other way : it is the gameto-
phyte-foot that is more securely placed, and the sporophyte is
dependent on it, not temporarily, but up to the time of maturity of
its spores. There is no doubt that the Pteridophytes are a real
advance on the Bryophytes, as regards success in growth on land.
The essential features of superiority of the Pteridophytes over the
Bryophytes consist, first, in the establishment of the sporophyte as
an independent Plant, rooted in the soil : secondly, in the pro-
duction of spores, not matured simultaneously and once for all in
a single capsule, but in numerous capsules matured independently,
and spread over a long period of time, even over many seasons where
the plant is perennial. This feature is universal for all the higher
types of the Vegetable Kingdom. Its effect is to increase the possible
output of spores : and it tends to make fertilisation a more rare
event, instead of a recurrent necessity for the survival of the race.
This is an obvious advantage for land-living plants which retain
their primitive method of fertilisation, as the Ferns do. In point of
fact, it is in the larger Pteridophytes, such as the Tree-Ferns, that
the climax of numerical production of homosporous spores has been
attained.

For a more explicit description of the Ferns see Primitive Land
Plants, Chapters XVI. to XXIII.

CHAPTER XXXII.

LYCOPODIALES.

The Lycopodiales, or Club Mosses, to which Lycopodium and
Selaginella belong, are Vascular Plants of varied land habit. They
have relatively small leaves [micro phyllous), borne upon a prepon-
derating axis which is usually branched, and is rooted in the soil.
The branching of both root and stem is typically dichotomous, but
frequently transitions may be seen to moyiopodial branching : that
is, where a new branch arises laterally below the apex of the originat-
ing part. Many early fossils belonging to this Class, for instance Lepi-
dodendron and Sigillaria, were tree-like : others were relatively small,
as are all the living Lycopods. More or less definite fertile cones or
strobili are borne on the ends of their branches. Somewhat compressed
in the axil of each leaf is a single sporangium that opens when ripe
like an oyster, by a marginal slit. These characters are common for
the Lycopodiales, modern and ancient.

This Class is represented in the British Flora by some five species
of Lycopodium (Fig. 404 a) : they are native on heaths and moors,
chiefly in hilly districts. Nearly 100 other species are widely spread
through the temperate zones and the tropics. They are mostly
low-growing plants, but some are epiphytes. There is in Britain
only one native species of Selaginella (S. spinulosa), a minute plant
of mossy hill-sides (Fig. 404 b). But over 300 species of the genus arc
spread through the tropics. Many species are in cultivation. They
are mostly low-growing and shade-loving plants of straggling habit.
Another British representative of the Lycopodiales is that curious
inhabitant of freshwater lochs, hoetes lacustris, with its long let
crowded upon a short succulent stock, which is fixed in the mud
at the bottom, by dichotomising roots. These relatively inconspicuous
plants are the meagre present-day representatives of the Lyco-
podiales, a type of which grew to tree-like size in the Coal Teriod, and
b.b. 511 2K

512

BOTANY OF THE LIVING PLANT

contributed largely to the organic remains preserved as Coal.
Isoetes appears as the nearest living ally of these fossil plants.

The Lycopodiales are divided into two series, the Eligulatae,
in which a single sporangium is borne in or near to the axil of each

leaf of the fertile cone, but
without any ligule. Here
the sporangia and spores
are all alike, the plants
being homosporous. The
Eligulatae include the gen-
era Lycopodium and Phyl-
loglossum. But Selaginella
and Isoetes are included in
the Ligulatae, in which
the sporangium is accom-
panied by a minute scale
or ligule borne on the upper
surface of each leaf : it
is inserted on the distal
side of the sporangium
(Figs. 407-409). Here the
sporangia are of two types,
producing respectively male
microspores and female
megaspores (heterosporous).
Most of the fossil Lycopods
belong to the Ligulatae.

Having chosen Dryop-
teris and Pteridium as the
chief examples of homo-
sporous Ferns, — a condi-
tion which is shared by
Lycopodium among the
Club Mosses, — Selaginella
will be taken for special
treatment as a type of these, while it also illustrates the hetero-
sporous state.

Selaginella.

A primitive type of Selaginella had an upright radial axis, with
leaves of equal size all round it ; and this is the case in 5. spinulosa.

LYCOPOD/UM,
S E LA CO. L.

J

Fig. 404 a.

Whole plant of Lycopodium, Selago, showing ascending
and upright habit, as developed when growing protected
by Heather. The equal dichotomy and the alternating
sterile and fertile zones are seen, and the forking of the
roots. Distally bulbils are borne. Reduced.

LYCOPODIAU-.S

513

But most living species have a much-branched, dorsiventral shoot of

an " espalier " type, sometimes simulating highly compound lea-
(Fig. 405). On these shoots the actual leaves are disposed in four
longitudinal rows, those on the lower Hanks being larger, those on the
upper smaller. Such shoots are commonly propped up by root-like
organs (rhizophores) borne at the forkings of the shoot, and themselves
showing very regular dichotomy. They are not actually roots, but

Fig. 404 b.

Plant of Selaginella spinulosa, with root-system springing from swollen knot at base of
the upright hypocotyl. Here there are no rhizophores. Natural size.

on reaching the ground they give rise to roots endogenously : hence
their name. Structurally Selaginella is relatively simple. The
vascular system is. essentially of the same type as in simple Ferns.
It consists of sharply circumscribed stelar tracts, with tracheides but
no vessels, and peripheral phloem. Each is surrounded by an endo-
dermis, which in many species shows the cell- laterally separated as
radiating " trabeculac." In the smaller species the stele remains
simple, but in some of the larger it may be disintegrated, somewhat

5U

BOTANY OF THE LIVING PLANT

after the manner of Ferns. In the larger fossils, however, cambial
increase was introduced, providing a massive tract of secondary
wood, to serve the upright and tree-like trunks of Lepidodendron and
Sigillaria. A vestige of such structure still persists in the abbreviated
and succulent stock of Isoetes.

The greatest interest lies not in the structure of the Ligulatae, but
in their sporangia, and the germination of the spores : for these give
lines of comparison with the Seed-Plants on the one hand, and with
homosporous types on the other. The strobilus or cone that bears
them is distal on a vegetative branch, and even in the flattened

Fig. 405.

Part of the shoot of Selaginella Martensii, showing its " espalier " form and minute
unequal leaves. It is seen from above, and the forking rhizophores are directed
downwards. (Nat. size.) (After Goebel • from Strasburger.)

species of Selaginella it has the radial form, all the sporophylls being of
equal size (Fig. 406, A). A longitudinal section shows that a short-
stalked sporangium is borne in the axil of each. These sporangia
are of two sorts, which are associated in the same strobilus but dis-
posed in various ways in different species. In S. inaequalifolia those
on the right side of the section shown in Fig. 406, B, are all mega-
sporangia, with four large spores in each ; those on the left-hand side

LYCOPODIAI.I-.S

515

are microsporangia, containing main* sin. ill mi< rospores. In form I

sporangia arc alike; they differ in the number and size of their
contents. A mature microsporangium, with its subtending sporophyU

and ligule, is shown in median section in Fig. 407. The line of deh
cence is distal, where the cells of the wall are smallest, and the

Fig. 406.

Selaginella inaequaii folia. A, fertile branch, half natural size. B, its tip in
longitudinal section, and enlarged, with microsporangia to the left, and D
sporangia to the right. (After Sachs.)

structure of the cells of the wall is such as to lead to its valves being
everted as they dry on ripening, so that the spores are shed. The
mature megasporangium behaves in a similar way, but the spores arc
ejected forcibly by pressure of the everted valves upon them (Fig.
408). The spores fall upon the soil and germinate together. Thus
both the megaspores and microspores are shed from the parent plant.

516

BOTANY OF THE LIVING PLANT

Fig. 407.

Microsporangium of Selaginella apus, in
median vertical section, containing numerous
microspores. The ligule is seen in Figs. 407,
408, as a small tongue-like bodv, rising from
the base of the leaf . (X55.) (After Miss Lyon.)

Fig. 408.

Megasporangium of Selaginella apus, in median
vertical section, showing three of the four mega-
spores. (x2i.) (After Miss Lyon.)

Fig. 409.

A, B = radial sections through young sporangia. C = transverse section of one
more advanced. D — tangential section. E =radial section of an older sporangium,
with ligule : the tapetum is shaded, and the sporogenotrs cells lie within. (A , B, C,
D = 35o; £ = 200.) F. O. B.

LYCOPODIALES

517

The sporangium arises in the axil of the Bporophyll, just within the lignle.

After the first segmentations arc past it is found to consi I I a short tl
stalk, bearing the slightly flattened sporangial head. This contain up

of sporogenous cells, from which later a surrounding tapetal lay< I is ( ut off.
Outside this is a wall composed of two layers of colls (Fig. .)<".. / . ,. Later the
spore-mother-cells round themselves off, becoming isolated in .1 liquid that
fills the enlarging cavity. Up to this point it
is impossible to tell which type of spore the
sporangium will produce. This fact indicates
that the megasporangium and microsporangium
are differentiated from one original type. In
the case of a microsporangium all the spore-
mother-cells undergo the tetrad division, and
a large number of microspores is the result.
But if it is to be a megasporangium, only one
(in some species two) of the spore-mother-cells
develops further (Fig. 410), the rest becoming
disorganised. The four megaspores, with
rugged walls, occupy the whole sporangium
at maturity (Fig. 408).

Fig. 410.

Selaginella spinulosa. Section of
megasporangium, showinc: the single
fertile tetrad still very small, and the
rest ot the sporogenous cells arrested.
( x 100.) F. O. B.

The germination of both types of
spores may begin before they are shed,
but it is continued on the moist soil.
The microspore first partitions off a lenti-
cular cell, which, as it appears to perform no function, and does not
develop further, may be held as vestigial, representing the vegetative
region of a male prothallus. The rest of the contents segment to

A B

Fig. 411.

A , microspore of S. apus after germination. B the same M below extiwioo

of spermatozoids. (After Mi- Lyon.)

form a wall of eight sterile cells surrounding a numerous group of
spermatocytes (Fig. 411, 4 This is in fact the correlative of an
antheridium: the whole male prothallus thus consists of a

5i8

BOTANY OF THE LIVING PLANT

vestigial vegetative region, and a single antheridium. In certain
cases starved Fern prothalli may be found of almost equal sim-
plicity. A mucilaginous change appears in the walls of the
central mass of cells. Meanwhile their protoplasts form each a single
curved spermatozoid, motile in water by two cilia. Swelling of the
mucilage by water bursts the wall of the spore, and the spermatozoids
escape (Fig. 411, B).

*US I .^^ ar ..71 ■*-

Fig. 412.

Embryology of Selagxnella denticulata, after Bruchmann. I. -III. ( x iai) show
germination of megaspore. IV. vertical section of megaspore showing prothallus,
archegonia, and embryo {em) with suspensor {sus) ( x 50). V . = a mature archegonium.
VI.-IX.=stages of developing embryo. X.=spermatozoids of S. cuspidata, after
Belajeff. ( x 250.)

The germination of the megaspores produces an internal tissue of
greater extent, which may be styled the female prothallus. Its
development begins below the meeting of the three converging ridges
of the tetrahedral spore, and it extends into the spore-cavity, which
is stored with nutritive material (Fig. 408). The increase in bulk of
the contents ruptures the wall of the spore along the three converging
ridges, so that the surface of the tissue is exposed (Fig. 412,1.). Near
the central point the first archegonium appears, while laterally others
may be formed later, but not in regular succession. A vertical
section through a megaspore of 5. denticulata shows it completely filled
with tissue of the prothallus, while its exposed surface bears rhizoids
(rh) and archegonia in various stages of development (ar) (IV.). The

LYCOPODIAU-.S

archegonium consists of a neck (;/) composed of two tiers of four cells
each : a canal-cell (c.c), ventral-canal- Cell {v.c.c), and the ovum [ov.) ;
all of which were derived by segmentation from a sin uperficial
cell of the prothallus (V.). When mature the neck is open, the
canal-cell and vcntral-canal-cell have disappeared, and the ovum,
which is now a rounded primordial cell, is open to access of the
spermatozoids. As both types of spore germinate together, fertilisa-
tion is readily carried out. If the germination of the two types of spore
is compared with that seen in Ferns, it appears that while in both the

B

A

Fig. 413.

Embryos of Selaginella denliculata, after Bruchmann. el — suspensor. a/ = root.
/=foot. W = basal wall. A=hypocotyl. s = ape.\. k, ft1 = rotyledons. / = ligulc.
(After Bruchmann.)

vegetative region of the prothallus is reduced, and does not take part
in active nutrition, the antheridium and the archegonium are in
essential correspondence with the like parts in Ferns.

After fertilisation the zygote secretes a protective wall, and elongat-
ing in the axis of the archegonium, segments repeatedly (Fig. 412,
VI.-VIII.)toform a filamentous suspensor (s) which thrusts the embryo
down deep into the prothallus (IV.). The distal cell soon enlarge
and divides. It gives rise centrally to the apex of the stem [st)t with
cotyledons right and left (c, c). Meanwhile unequal growth turns it
to one side, and the convex side enlarges into the suctorial ot '
(IX. /). Lastly, the first root (wtt Fig. 4' 5, At B) is initiated by
periclinal divisions close to the attachment of the suspensor, and on
the same side of the embryo. All the parts of the young embryo have
thus been produced, while the apex of the axis occupies the distal
position from the very first in the curved embryo. As the axis and
root grow they protrude from the ruptured spore, the root turning
downwards, and the elongating hypocotyl turning upwards (Fig. 41 J,

520 BOTANY OF THE LIVING PLANT

II. III.). The cotyledons already bear ligules (/, Fig. 413). The
further development is merely a matter of continued growth and
branching, which result in the establishment of a young Selaginella-
plant similar to the parent.

If the embryology of Selaginella is compared with that of a Fern,
the most marked difference is seen to lie in the presence of the fila-
mentous suspensor. There are, however, certain primitive Ferns in
which a suspensor is present, as it generally is in Lycopods and Seed-
Plants. It seems probable that it is really a primitive organ, but that
it has been eliminated in the more modern Ferns, and some other
Pteridophytes, though retained by Seed-bearing Plants.

The leading events in the life-cycle of Selaginella correspond clearly
to those of a homosporous Fern (see Diagram, Fig. 400). The micro-
phyllous sporophyte is the correlative of the megaphyllous Fern.
Sporangia are borne by both, and chromosome-reduction in the
formation of the spores establishes in both their haploid state. The
germinating spores produce the prothalli, and ultimately antheridia
and archegonia. After fertilisation a new sporophyte arises in both
cases from the zygote. All these events follow in the same succession
in Selaginella and in Dryopteris, and the several stages may be held
as homologous. The chief difference lies in the two distinct types of
spore in Selaginella. The distinction of these appears late in the
individual development. At first all the sporangia are alike, with
numerous spore-mother-cells in each. The microsporangia behave
like those of homosporous types, such as Lycopodium or Dryopteris,
all the spore-mother-cells undergoing tetrad-division and forming
microspores. But in the megasporangium of Selaginella only one of
them as a rule shows tetrad-division, the rest giving up their substance
to that one (Fig. 410). The result is that each individual spore of
the megasporangium is hypertrophied as a megaspore, and stored
with nutritive material. The conclusion to be drawn from these
facts is that the difference between these spores, which are then
described as heterosporous, has been secondarily acquired from the
primitive homosporous state, and that it is founded on nutrition.
Comparison supports this inference : for there is evidence of like
progressions from the homosporous to the heterosporous state in the
Equisetales and in the Filicales. In the latter, the Hydropterideae
also show identity of early development of the sporangia, whether
they are to be megasporangia or microsporangia. It is concluded,
therefore, that heterospory has been homoplastic in distinct phyla :
that is, that it has been initiated more than once in Descent.

LYCOPODIAI.KS 521

The biological advantage which follows 011 the adoption of hetero*
spory lies in the fact that a large and well-nourished megaspore forms

a better starting point for the new embryo than an independent
prothallns which has to elaborate its own supply. The megaspore
draws upon the ample resources of the parent sporophyte, and when
shed it already contains sufficient food to start the embryo well on
its way. This is seen in the germinating megaspores of Selaginella,
where the sporeling may attain considerable size before the store is
exhausted (Fig. 412, III.). Its root and shoot are then able to take
up nourishment independently of the prothallus. This heterosporous
condition had already been adopted by Palaeozoic Lycopods, so that
it is not a modern device.

Adoption of the Seed-Habit.

Certain of the Palaeozoic Lycopods had, however, gone a step
further than the state seen in Selaginella. They retained the megaspore
within the tissue of the parent, so that its nutrition could be continued
without the interruption caused by shedding of the spore. In Lepido-
carpon and Miadesmia a seed-like structure actually existed, though
not exactly of the same type as that which has become a constant and
permanent feature in Seed-Plants. A similar condition, but carried
out in greater perfection, is seen in certain Fern-like Plants of the
Palaeozoic Period. They are called Pteridosperms from the fact that
they produced seeds containing a megaspore. This retention of the
megaspore finds its biological justification in the fact that thereby
the nutrition of the germ is still more effectively secured than it is
by simple heterospory. // thus appears that certain Pteridophxta
illustrate steps essential in the institution of the Seed-Habit. These
are, first, the adoption of heterospory, and, secondly, the retention of the
megaspore upon the parent plant.

The establishment of the Seed-Habit was undoubtedly the greatest
evolutionary advance towards a specific Flora of the Land. Its effect
is shown by the prevalence of Seed-Plants upon exposed Land-
Surfaces. In any ordinary landscape it is the Seed-Plants, whether
Angiosperms or Gymnosperms, which strike the eye, and appear to
make up the majority of the Vegetation. Though the Angiosperms
date back only to the Cretaceous period, the more primitive Gymno-
sperms are of much earlier origin, while the archaic Pteridosperms,
Fern-like Plants which bore seeds sometimes of considerable size,
carry the Seed-Habit far back into the Palaeozoic Age. Thus,

522 BOTANY OF THE LIVING PLANT

though the Angiospermic development is relatively recent, the Seed-
Habit was initiated in the very remote past.

It has been pointed out that the Pteridophyta retain their primitive
zoidiosamic fertilisation, a fact that has tended to restrict their
spread on exposed land-surfaces. Also it has been noted that certain
of them have taken the forward steps to heterospory, and to the
retention of the megaspore upon the parent plant. Both of these
essential steps towards the Seed-Habit can be adopted while still
retaining zoidiogamic fertilisation. It will be seen later that some
Seed-Plants still carry out their fertilisation by means of freely moving
spermatozoids, thus giving strong evidence of their Descent from a
Pteridophytic source. But the vast majority of Seed-Plants have
adopted the siphonogamic mode of fertilisation, by means of a pollen
tube, as already described for the Angiosperms in Chapter XVII.
This finally emancipated them from the embarrassing tie which
zoidiogamy imposed. Combining the Seed-Habit with siphonogamic
fertilisation the Seed-Plants became in actual fact Plants of the Land,
independent of water except such as can be extracted from the soil
by their roots. This was probably the chief factor leading to their
present dominant position.

The Equisetales.

The Equisetales, or Horse-tails, can only be briefly described here, though
they should not be omitted : for they figured largely as the Calamarians in
the primary rocks from the Devonian Period onwards, while the Class survives
in the cosmopolitan genus Equisetum. Whereas the Calamarian fossils often
attained tree-like proportions, the living types are relatively small. Never-
theless the general organisation of the Class is very uniform, the stem being
dominant and the relatively small leaves disposed upon it in successive
whorls with intervening internodes. This type of construction is sometimes
described as " articulate." It is shared by another Class of fossils, the Spheno-
phyllales : but these are all extinct.

As in the Ferns, the sporophyte of the Equisetales is the substantive plant,
while the gametophyte is relatively small. The habit of an Equisetum is seen
in Fig. 413 a, with its webbed leaf-sheaths present alike on the rhizomes and
on the aerial shoots : both of these may be branched. Buds and related roots
arise at the nodes. The result is a certain uniformity of the microphyllous
habit, its differences depending mainly upon the degree of development of
the branching. A solitary distal fertile cone or strobilus may be seen on
each fertile branch. It consists of a central axis bearing lateral sporangiophores,
disposed less regularly than the leaves. Each sporangiophore supports a
number of large sporangia pendent from its peltate end.

Anatomically the internode presents in transverse section a well-marked
epidermis with stomata leading to a broad, well-ventilated and photosynthetic

EQUISETALES

523

Fig. 413 a.

Equisetum pratmse, Khrh. Rhizome with unbraih hod fertile ihootl (•), I fertile
shoot which has begun to form branches (b), and a young sterile shoot (r). Natural
(After Duval-Jouve; from Rabenhorst's Krypt. Flora.)

524 BOTANY OF THE LIVING PLANT

cortex. Centrally there is a large pith-cavity. Round this vascular strands
are disposed in a regular cylinder, their number corresponding to that of the
leaves in each whorl, and with details related to the moist habitat of these
plants. There is a near analogy with the structure of certain seed-plants,
but there is no persistent cambium in the living species of Equisetum : though
in the Calamarian fossils, while the primary scheme is the same, an active
ring of cambium led to the development of a massive woody trunk.

The sporangia are large, and each contains numerous homosporous spores.
These germinate and produce autotrophic, dorsiventral prothalli, bearing
numerous erect lobes. The gametangia are essentially of the Fern type : but
the embryo appears as a spindle-shaped body, the initial cell at its apex being
defined by the first segmentations. It is difficult to bring these features
into any near relation either with those of Ferns or of Lycopods. Perhaps
the nearest comparison among living plants would be with Tmesipteris.
But the facts of segmentation of the zygote bear a special value, since
Equisetum is the only genus of the ancient Articulate Plants in which the
embryology is known. The Horse-tails appear as an isolated type in modern
vegetation, and present a phyletic problem of their own.

A more full description of the Equisetales will be found in Primitive Land
Plants, Chapter X.

DIVISION V.

SEED-PLANTS.— G YMNOSPERMS.

CHAPTER XXXIII.

CONIFERAE: THE SCOTS PINE.

The description already given in Chapters I. to XIX. has presented
the structure and development of the Higher Flowering Plants, or
Angiosperms, in which the ovules are protected by a carpellary wall,
and the pollen-grain is received upon a stigmatic surface. These Plants
appeared relatively late in Geological Time. Their records date back
only to the Cretaceous Period. Comparative evidence supports the
conclusion that they are the culminating types of Vegetation, and
rightly occupy the highest position. Their ready adaptability to
their surroundings has contributed to their success in the struggle for
existence, as shown by the profusion of their forms now living. In
fact, they are the dominant types of the Present Day.

But Seed-Plants long ante-dated the Angiosperms. Seeds existed
in the Devonian Period. They belonged to forms corresponding more
nearly to those living Plants which are collectively named GytntUh
sperms, than to the Angiosperms. This gives a special interest to the
study of the living representatives of the Gymnosperms, a class which
have as their leading characteristic the free exposure of their seeds, a
carpellary protection being absent. Being simpler in their propagative
methods, as they are also in certain structural features, and having
existed in earlier geological periods than the Angiosperms, they are
naturally held as the more primitive Seed-Plants, and as such they
may be expected to offer features valuable for comparison with the
Pteridophyta. This is found to be the case, and it is this which makes
the study of them of special value.

5^5

526

BOTANY OF THE LIVING PLANT

The living Gymnosperms may be regarded as the survivors of a
large class of Plants of earlier periods, and accordingly their representa-
tives appear rather isolated and distinct from one another. They
comprise the Cycadales, Ginkgoales, Gnetales, and Coniferales. The

last is the leading Class of living Gymnosperms, and it includes the
greatest number of species. They arc called the Coniferales from the

A B

Fig. 414.

,<4«= mature cone of the Scots Pine, natural size. />', a single scale of th^ cone,
seen from above, showing the two ovules, together with the outline of the film of
tissue which separates with the ripe seed. Enlarged. (After Le Maout.)

fruiting body usual for them, which is composed of closely packed
hard woody scales, as js well exemplified by the ordinary Fir Cone
(Fig. 414, A.) In some, however, a definite woody cone is not formed.

Cycadales.

The Cycadales arc represented at the present day by nine genera, and
abont 100 species of Fern-like Plants, widely distributed in Tropical and
Sub-Tropical regions. These are the relics of a Flora which appears to have
reached a climax in numbers and importance in the Oolitic and Cretaceous
Periods. In habit they are megaphyllous, often with upright stems, and
pinnate or sometimes doubly pinnate leaves. The texture of their leaves is
stiff, leathery, and even spinous, and they are constructed on a plan like that
of the Marattiaccous Ferns, but specialised for withstanding drought. Their
Stems are thick and fleshy, and comparison of their internal structure points
again to the Marat tiaceous Ferns, to which some of the related fossils corre-
spond in marked degree.

The Cycads are reproduced by seeds. The ovules (megasporangia) from
which they are matured arc borne freely exposed upon the margins of the
carpels, or mcgasporophylls : these are often associated in closely packed

CONIFKkAK: TI1K S< < )T< I'INK

52;

cones. On other plants staminal cones arc borix The tamens, or im< ro«
sporophylls, bear numerous pollen-sacs (microspore oil their lower nil

The pollen-grains (microspores) from these gain
direct access to the micropyle of the exposed
ovule, and form short pollen-tubes, \vln< b
have been found in Cycas and Zamia to dis-
charge motile spcrmatozoids as the fertilising
bodies into a liquid secreted by the nucellus
(Fig. 415). In many respects the living
Cycadales show points for comparison on the
one hand with the primitive Ferns, on the
other with the remaining Gymnospcrms.
But the feature which possesses the greatest
comparative interest is that the motile sper-
matozoid is retained by them as the means
of fertilisation. Ginkgo biloba, the only living
representative of the Ginkgoales, a family
well represented in the Jurassic Period, is
also fertilised by motile spcrmatozoids. The
existence of zoidiogamic fertilisation in two
families of ancient Seed-Plants still living, and so distinct as these, is held
as strong evidence of the origin of Seed-Plants from a Pteridophytic ancestry.

Fig. 415.
End of pollen-tube of Zamia, a
Cycad. showing the proth.illi.il cell
(v), the sterile sister-cell ($),
the two spermatozoids. a, before
movement of the spermatozoids has
commenced, b, after beginning of
ciliary motion. ( x about 75.) (After
Webber, from Strasburger.)

CONIFERALES.

The Scots Pine.

The leading class of living Gymnosperms is the Coniferales, so named
from the fruiting body with its hard woody scales, as seen in t lie-
ordinary Fir-cone. The vast forests of such Conifers, existing in
temperate and sub-arctic zones, are the sources of the supply of soft-
wood, wood-pulp, turpentine and pitch. In the native British Flora
the Gymnosperms are represented only by Conifers, such as Scots
Pine (Pinus sylvestris), the Yew (Taxus baccata), and the Juniper
(Juniperus communis) ; but many more arc familiar in cultivation
in shrubberies and woods. Over the world at large they include
a number of other forms, somewhat loosely related, but with
common features that indicate their primitive character. Among
them are some of the largest and oldest of living organisms, such
as the Big Trees of California (Sequoia, see Frontispiece). Another
well-known and peculiar form is the Monkey Puzzle (Araucaria

imbricata) .

The seed of the Scots Pine, and of other Conifers, produces on ger-
mination a seedling with a dominating main axis, winch grows upright,
and keeps as a rule its radial construction. Radiating groups of

B.B. 2t

52.S

BOTANY OF THE LIVING PLANT

branches are borne at intervals upon it, which take a more or less
flattened form ; and as they do not grow so strongly as the main
stem the result is the pyramidal habit so well seen in the Christmas

Fig. 416.

Branch-end of Pinus nigra (Arnold) bearing laterally two shoots of un-
limited growth, and a cone replacing a third one. Each is covered by numerous
"foliage-spurs" bearing two "needles." At the base of the figure these are fully
developed; above they are half-grown, the shoot having been cut in spring. Two
young female cones ($), at the distal end, are at the period of pollination. (After
Groom.)

Tree (Picea), and in the young Scots Pine. Sometimes this habit is
maintained throughout life ; but often, as in the Scots Pine, the form
becomes irregular as the tree grows older. The Coniferae are as a
rule closely gregarious, and they then form very exclusive forests.
The lower branches die off in the crowded woods, giving the clean

CONIFERAE: THE SCOTS PINE 5:

trunks without knots that arc specially valued as timber, and supply
naturally-formed masts, spars, and telegraph-pol<

The Scots Pine, like most of the family, is chara< d by lea

of relatively small size, simple form and stiff texture. These are
xerophytic features, and are well illustrated by the " needles " of the
Pine. Their structure, with sunken stomata, a well-developed
cuticle, and a large proportion of bulk to surface bears this out.
Hairs are absent from their smooth surfaces.

In some of the Coniferae the vegetative leaves are all of essentially the same
type, as in the Juniper. But in the Scots Pine some of them are developed
as protective scales, others as green foliage leaves, and the mutual arrangement
of these two types is very characteristic. It is closely connected with the
fact that all the axillary buds do not develop alike. Those at the end of the
annual increment of growth are unlimited, and form the radiating group of
branches of each successive year already noted. Those lower down develop
only as limited foliage spurs, which remain short, bearing only a few mem-
branous scales, and distally a few long green " needles " (Fig. 416). In the
seedling plant green foliage leaves may follow the cotyledons on the main
axis. But in the later stages the main axis and the woody branches bear only
scale-leaves, while the green needles are always borne on the foliage-spurs. In
the different species of Pinns the number of needles on each spur varies : in
P. monophylla it may be only one. The Coniferae are mostly evergreens like
the Scots Pine, Yew, and Juniper. But some, like the Larch, shed their
leaves in autumn, or even their short leafy shoots, as in Taxodium.

The root-system starts in all cases with a tap-root ; but it seldom
maintains its lead. Lateral roots arise from it, and they form the chief
attachment of the mature tree, which is often shallow-rooted. Some
of them, as in the Scots Pine, are mycorrhizic, the roots being invested
by a fungal felt, which acts as an intermediary between the root and
the soil (p. 228). But as seedlings can be raised in pure cultures with-
out the fungus, its presence, however advantageous, is not necessary.

The external characters of the Coniferae thus briefly sketched
stamp the appearance of most of them. The general plan of their
Plant-body or sporophyte is the same as that seen in Angiosperms.
It is the working out of the details that gives the special characl
of the Coniferae. Their habit is easily distinguished from that
of the broad-leaved Dicotyledons, and still more easily from the
Palms and other large Monocotyledons. A feature which has its
bearing upon the habit and spread of the family is the rarity of
vegetative propagation. In Nature it hardly ever occurs, and the
forester finds it impracticabl<\ Virtually all individuals arc raised
from seed. This is in marked contrast to the An-iospcrms.

530

BOTANY OF THE LIVING PLANT

There is no need to describe the minute structure of the vegetative
organs, since it corresponds in essentials to what has been seen in

Angiosperms. It must suffice
to note certain features of
comparative importance. The
stem is constructed on the
same plan as that of the woody
Dicotyledons, with indefinite
secondary thickening of the
vascular ring originating from
a cambium (compare Fig. 35,
p. 57). It results in the Scots
Pine in a woody trunk marked
by annual rings and medullary
rays, while externally are
phloem and a scaly fissured
bark, (see Fig. 42, p. 64). Resin-
passages permeate all the
tissues irregularly. They are
specialised intercellular spaces,
lined by an epithelium, which
deposits the sticky resin in the
passages. It exudes from them
under pressure of the surround-
ing tissues whenever the plant
is broken or cut. The most
notable structural feature is
that the wood is composed
entirely of tracheides, each
developed after tangential
1"%™! division from a single cambial

= cam-

flG. 417.

Tracheides of Pine, seen in radial section
successive tracheides of one radial row. cb
bium. t = young pit of cambium, f, T=older pits. 11 /rp- . , ,-A Thpv arP lini-
5/ = pits of larger area facing the oblong cells of the cen \rifc>- 41/,/* 1 "cy die uin

medullary ray. ( x 550.) (After Sachs.) form in shape, as the cambium

is, and are not deformed as the wood of the Dicotyledons is, by
sliding growth, or by unequal development of the individual cells.
The wood is consequently of that even texture seen in " deal."
The tracheides are lignified, and their radial walls are marked by
circular bordered pits.

The bordered pit which is found widespread in the tracheae of vascular
plants, is seen in perfection in the wood of Conifers. The pit originates as a
circular area of wall which remains thin, while the rest of the wall thickens.

CONIFERAE: THE SCOTS PINE

: *

But the thickening encroaches upon the area of th< pi< and overan 1 •
418, C, D, E). As seen in surface \nw a double outline then appears. "1 1 1 « -
outer circle corresponds to the area ol tin- pit membrane, tin mri' 1 to th< hunt
of the overarching ; and the greater the thickening the further these outlii

will be apart. Meanwhile the centre of the pit-membrane itseli tin. k<
forming the " torus " (A, C), which serves mechanically to meet th<- n>k ol
rupture following on any unequal pressure on the t\v<. Bides. For the torus
would, as the membrane yields, press against the overarching lip (li). '1 he
prevalence of bordered pits indicates that they are functionally important.
They may be recognised as a compromise between the conflicting requirements
of ready transit of liquid between thick-walled cells and the maintenance of
mechanical strength. For the former a large pit-membrane is an advanta

B.

Fig. 418.

Bordered pits of tracheides of Pine. A = a whole tracheide in transverse section
with pit sin its radial walls. B shows the torus pressed to one side. C, I), E illustrate
the development, and the relation of the structure as seen in section to the double
outline as seen in surface view. £ shows this in the young state. D rather more
advanced. C, mature.

but it would weaken the wall. This difficulty is met by the overarching
as seen in the bordered pit, by which the strength of the woody wall is
maintained.

The phloem consists chiefly of sieve-tubes (v, Fig. 419), which are also arranged
in regular radial rows, but without companion-cells. They have cellulose walls
and sparing contents. Cells of phloem-parenchyma are also present. The
sieve-pits (vt) are mostly on the radial walls, and thus correspond in position
to the bordered pits in the tracheides. The secondary tissues are traversed In-
medullary rays, as in Dicotyledons (em, sm, tm). They include cells that
retain their protoplasm, while minute intercellular spaces pass radially inwards
between their cells. They serve accordingly for radial ventilation, as well
as for storage within easy reach of the conducting phloem. Though the plan
of construction of the vascular tissues of Gymnosperms is the same as in
Dicotyledons, the details of their development are not so elaborate

The chief comparative interest of such a plant as the Scots Pine
lies not so much in the form and structure of the sporophyte-plant,
as in the details of its propagation. This is carried out, as in t he Ailgio-
sperms, by organs grouped as Flowers, which are " male " or "female"
In the Scots Pine these may be borne on the same tree, though often
en distinct branches. The female flower, pink and succulent at

532

BOTANY OF THE LIVING PLANT

pollination, matures into the hard woody cone, from which the name
Coniferae is derived (see Figs. 414, 416). When ripe it consists of a
central axis bearing in a complex spiral numerous woody ovidiferous
scales. As the cone ripens the scales turn back, and two seeds may
be seen freely exposed on the upper surface of each. When fully
ripe each seed separates from the scale, together with a thin film of
superficial tissue, which on detachment helps to float it away on

SM

Fig. 419.

Radial section of Pine stem, at the junction of wood and bast. Phloem to
the left, xylem to the right. s = autumn tracheides. *=bordered pits. c=
cambium. r=sieve tubes. itf=sieve pits, tm = tracheidal medullary ray cells.
sm= medullary ray cells in the wood containing starch. sm' = the same in the
bast. £wi=medullary ray cells with protoplasmic content. ( x 240.) (After
Strasburger.)

the breeze (Fig. 4 14, B). The seed is protected by a seed-coat, covering
a bulky endosperm, with a large embryo enclosed in it, which has
many cotyledons, a plumule and radicle. The seed is thus " albu-
minous," and in essential points it corresponds to that of Angiosperms.
But in the Scots Pine it takes two years to produce, and the details
of its production give important features for comparison.

Both the types of flower are axillary in their origin. The male
flowers are produced in large numbers, replacing the weak foliage
spurs (Fig. 420). The female take the place of the stronger branches
of unlimited growth, and are produced in smaller numbers (Fig. 416).

coNii-Kk.u. : Tin-. .-< i rrs pine

533

As these project at the time of pollination above the end of the •
tending shoot, they arc in the best possible position for receiving I
wind-borne pollen. The male flower is enveloped below by niembr
scales, and bears distally numerous sporophylU or stamen: . h with

Fig. 420.

Pinus nigra, Arnold. Shoot bearing mak (lowers in place of 1

spurs. (After Groom.)

two pollen-sacs on its under side (Fi-. 4- f, BtC). Th< pollen-grains are
peculiar in bearing right and left of the grain itself air-containing sacs
(wings) which give a low specific gravity to the whole grain, and so
aid its transfer by the breeze (Fig. 421, D). At the tunc when r
shed, the grain of Pinus contain.-, in addition to the vestigial remains
of two obliterated cells of the male prothallus, one nucleated cell

534

BOTANY OF THE LIVING PLANT

attached laterally (the generative cell), and a free nucleus (the tube
nucleus) enclosed in cytoplasm which fills the rest of the grain.

Fig. 421.
Pinus montana. A longitudinal section of a ripe male flower (xio). B, longitu-
dinal section of a single stamen ( x 20). C, Transverse section of a stamen ( x 27).
D, a ripe pollen grain of Pinus sylvestris. The obliterated prothallial cells are not
shown. ( x 400.) (After Strasburger.)

The male flower is thus a simple shoot bearing sporangia. The
female flower may also be regarded as a simple shoot. It consists of
an axis bearing numerous scales that are at first succulent, but finally

Fig. 422.
Median section of ovule and scales in Pine at time of pollination (after Coulter).
b. sc= bract scale, ov. sc = ovulif erous scale. « = nucellus. t' = integument. tn =
micropyle. e.s. = embryo-sac. As the two ovules lie side by side, only one of them
is seen in the radial section. p = pollen-grain on nucellus.

woody. They are arranged on a complex spiral plan. One of these
scales removed from the young pink cone at the stage of pollination
shows a double structure. A smaller lobe, sometimes called the

CONIFERAE: THE SCOTS PINK

bract-scale, bears on its upper surface a larger and thickened bl

sometimes called the ovuliferous scale (Fig. 422). It seems probable
that this is a local upgrowth of tissue from the surface of the forn.
though as the cone grows older it becomes woody, and is by far the
more prominent feature of the two. Other interpretations of the
cone have been given ; but if this view be accepted, then the whole
cone is a simple flower bearing many complex sporophylls. Attached

nc

Fig. 423.
Median longitudinal section of an ovule of Picea excelsa at time of fertilisation.
* = embryo-sac filled with tissue of the female prothallus. a = archegoiuum.
showing venter (a) and neck (c). n = nucleus of ovum, nr = nucellus. />--pulkn-
grains. / = pollen-tube. » = integument. ( x 9.) (After Strasburger.)

to the upper face of the ovuliferous scale are two ovules, which are
not enveloped by a carpel as in the Angiosperms. but are fully
exposed, with their wide micropyles directed downwards. Each ovule
consists of a nucellus corresponding to that of the Angiosper:
surrounded by a single integument, and with a wide micropyle
(m, Fig. 422). The pollen-grains being produced in enormous
numbers, and floating away on the dry air of a June day, are
scattered over the female cones, of which the scales then stand apart
to receive them. A drop of fluid extruded from the micropyle

536

BOTANY OF THE LIVING PLANT

catches them and is then absorbed. Thus the pollen- grains are landed
directly on the apex of the nucellus, where they are constantly to be
found in sections cut through the ovule {p, Fig. 423). Excepting
that there is no receptive stigma the process is not unlike that in
wind-pollinated Angiosperms.

Differences of great comparative interest lie in the details
within the ovule itself. The ovule originates as in Angiosperms,
and as in them the embryo-sac is one cell of a tetrad produced

- 0/

Fig. 424.
Picea vulgaris. A = longitudinal section through apex of female prothallus, and
one archegonium. n=neck. v.cc = ventral canal cell. <w = ovum. B = neck seen
from above. C = entry of pollen-tube with gametes into the canal of the arche-
gonium. (A x 100 ; B, C x 250.) (After Strasburger.)

in the young nucellus ; its nucleus is therefore haploid. (See
p. 297.) But an essential difference is that in Pinus, and all other
Gymnosperms, nuclear division proceeds to a high number in
the young embryo-sac. Cell formation follows, and the sac is thus
filled before fertilisation by a bulky tissue of the endosperm, or
female prothallus (Fig. 423, e). At the time of fertilisation, which
in Pinus and Picea happens about the middle of June of the
year after pollination, the female prothallus bears at its micro-
pylar end three to six large archegonia, of which two commonly
appear in a median longitudinal section of it (Fig. 423, a). Each of
these originates from a single superficial cell of the prothallus, and

CONIFERAE: THE SCOTS PINE

consists of a large nucleated ovum, with a small lenticular ventral-
canal-cell lying above it, which is cut off from the ovum shortly before
fertilisation. Covering this is a group of cells two or more tiers in
depth, forming the channel of the neck (Fig. 424, A). The ovum
of Pinus, as is not uncommonly the case in Gymnosperms, is large
enough to be seen with the naked eye. The archegonia lie in a slight
depression of the surface of the prothallus. The last step before
fertilisation is the collapse of the ventral-canal-cell, which takes no
direct part in propagation.

em

A

A' •.••-...:•'•.• • •»:•,... -A J.''.vl->*.--.v:Vv-::-V: ■-•**:••..,..„_.

Fig. 425-
Pi>i!<s Laricio, stages of embryology. .4 shows two tiers of four cells at base of
the archegonium. B shows three tiers, and the last division proceeding in the owest
tier. C shows the tier of suspensors (s) elongating, and carrying forward the lowest
embryonic tier. (After Coulter and Chamberlain.)

In the Scots Pine a whole year elapses between the pollination of
the young pink cone and the act of fertilisation. But in Conifers
generally the times are different. With or without a lengthened
interval each pollen-grain, germinating on the apex of the nucellus,
forms a pollen-tube, which penetrates the nucellus, passing towards
the apex of the prothallus. Meanwhile its generative cell lias divided
to form a stalk-cell and a body-cell. The former breaks away from its
attachment, and the contents of the grain enter the tube. The body-cell
divides during transit to form the two male gametes. Thus provided,
the tube enters the neck of an archegonium, and the gametes are
transferred into the ovum (Fig. 4-M, O- The nucleus of one of the

538

BOTANY OF THE LIVING PLANT

gametes fuses with the nucleus of the ovum. The result of the fusion
is the zygote. Both the ovum and the male gamete were haploid,
and the consequence of their fusion is to initiate the new diploid phase,
which forms the embryo. The detail of embryology is variable in
different genera of Conifers, and rather complicated.

In Pinus and its allies the nucleus of the zygote divides at once, first into
two and then into four. The resulting nuclei sink to the base of the egg,
lying in a single plane. Divisions follow to form four tiers consisting of four
cells each (Fig. 425). Of these the lowest but one elongate as the suspensors (s);
the lowest form the embryonic tier (e). In the Pines these may either form
together a single embryo, or they may separate, each borne on its own suspensor,
and so four embryos may result from one fertilisation. As there are several
archegonia, and each may be fertilised, a high degree of polyembryony is
possible. As a rule one embryo in each ovule secures the ascendency over the
others, and the rest are absorbed. It has been calculated that in the Scots
Pine only about one per cent, of the potential embryos are matured.

The maturing embryo, borne on the end of its elongating suspensor,
is thrust downwards into the substance of the endosperm. With

less regular segmentation than that
in Angiosperms it matures into the
germ, which in the ripe Pine-Seed is
cylindrical in form. It is terminated
by an apical cone, round which coty-
ledons, varying to the number of
fifteen, are arranged in a ring. The
radicle is massive, with the large root-
cap characteristic of Gymnosperms.
The female prothallus persists as the
nutritive endosperm ; the nucellus is
crushed between the enlarging endo-
sperm and the hardening seed-coat.
The seed is thus albuminous, which is
the case for all Gymnosperms, a fact
which suggests that this is the
primitive state for the seed generally.
Its parts when mature correspond in
nature and position to those of the
seed in Angiosperms (Fig. 426, I.).
A period of rest is followed by
germination of the seed. The
embryo, drawing upon the nutritive
endosperm, enlarges, and the radicle

I. Median

Fig. 426.

Pine seed and germination,
section of seed ; y=micropolar end.
11. germination. III. Ditto later; s =
seed-coat; e=endosperm; c = cotyledons.
a> = primary root; Ac=hypocotyl. (After
Sachs.)

CONIFERAE: THE S< OTS PINE

projects and grows down (Fig. 420, II. The hypocotyl elon
and the seed is carried above ground. The tips of the cotyl

remain within it till the store is exhausted, when it is c tnd

the cotyledons expand round the central plumule (Fig. 420, III.).
Thus the seedling is established.

Comparison between Pteridophytes and Seed-Plants.

The Gymnosperms offer natural lines of comparison on the one
hand with the heterosporous Pteridophyta, and on the other with the
Angiosperms. The general features of the life-cycle are the same in
them all. Hence it may be concluded that the comparisons are
legitimate, as between cognate organisms. In all of them the sporo-
phyte is the dominant " plant," while the gametophyte shows pro-
gressive degrees of elimination as an independent vegetative structure.
Comparing Pinus as an average Gymnosperm with Selaginella, which
may be taken as a fair example of a heterosporous Pteridophyte, the
" plant " bears in either case two kinds of sporangia. The micro-
sporangium of Selaginella corresponds to the pollen-sac of the Pine,
since both produce spore-mother-cells, and these after tetrad-division
form microspores, which are shed on rupture of the sporangial wall.
The megasporangium of Selaginella corresponds to the nucellus of the
ovule of Pinus, which is, however, protected by the extra covering
of the integument. In the ovule, as in the megasporangium of Sela-
ginella, a tetrad-division leads up to the formation of the megaspore.
In Pinus only one megaspore or embryo-sac is matured. In
Selaginella commonly four, and occasionally more : but sometimes
two, or even one only. Thus in point of origin, in the manner of
production, and sometimes also in number, the megaspore of Sela-
ginella corresponds to the embryo-sac of Pinus. An essential
difference may appear to lie in the fact that in Selaginella the mega-
spore with its thick protective wall is shed on rupture of the
megasporangium : while in Pinus the megaspore or embryo-sac is
retained within the nucellus, and is thin-walled. But this retention
of the megaspore within the sporangium gives the biological advantage
of continued nutrition, to which a thick and rugged wall would be an
unnecessary obstacle. The facts thus point to the conclusion that
the thin-walled embryo-sac of Pinus is a retained megaspore, and that
we see in it a derivative state which has been universally adopted by
Seed-Plants. (Compare Fig. 412, IV., with Fig. 423). ]t has already
been noted that certain fossil Lycopods had done the like.

540 BOTANY OF THE LIVING PLANT

Turning next to the products of germination of the spores, that is,
to the gametophyte generation, a comparison must be made of the
reduced and modified prothalli resulting respectively from the micro-
spores and megaspores.

The microspore of Selaginella produces a single vegetative cell, and
an antheridium with a wall and many spermatocytes, each of which
gives rise to a freely motile spermatozoid. In the pollen of Pinus
two vegetative cells are formed, and obliterated in the developing
grain, and three further divisions follow. But there is no cellulose
antheridial wall, and only two spermatocytes appear, which remain
non-motile. Clearly this is a state further reduced and specialised as
compared with Selaginella. But it has been noted in Cycas and
Ginkgo that the gametes are motile spermatozoids, and in Microcycas
they are numerous. Such instances show that the products of ger-
mination of the microspores of Gymnosperms are sometimes motile
like those of Selaginella and other Pteridophytes : but they suggest
that in other cases the motility of the gametes has been lost in the
course of evolution. This change is also seen in Seed-Plants generally.
(Compare Figs. 227, 229.)

The megaspore of Selaginella begins to produce a prothallus while
still within the sporangium, and continues its development after it is
shed. It bears archegonia which are exposed by rupture of the rugged
wall, and are fertilised by spermatozoids moving freely from the
germinating microspores. One or several embryos are produced.
Similarly in Pinus the female prothallus (endosperm) is formed within
the megaspore (embryo-sac), and produces archegonia. But it
remains embedded in the sporangium (nucellus), where fertilisation
takes place by non-motile gametes conveyed by the pollen-tube to
the neck of the archegonium. Several of these may be fertilised, and
a plurality of embryos be initiated. It follows that the contents of
the megaspore of Selaginella and of the embryo -sac of Pinus are
homologous, both being female prothalli, produced by germination of
the megaspore. (Compare Figs. 412, IV., and 423.) The only new
structure is the integument of the ovule of Pinus : the rest are modifi-
cations in accordance with Life on Land, of parts already present.
The biological probability of the several steps disclosed by this
comparison is such as to justify their acceptance as evolutionary
history. Those steps are, the retention of the megaspore upon the
parent plant, and the loss of its protective wall : the development of
the pollen-tube (perhaps as an extension of a single antheridium) :
and the loss of motility of the gametes.

CONILKRAE: THE SCOTS PINE 541

Comparison on the other hand with the Angiosperms shows tfa I
while there is a general correspondence in the propagative method,

the balance between the alternating generations is still more lines
in them than in such Gymnosperms as Pinus. The germination of
the microspore of the Angiosperm shows only a single division prior
to the formation of the two gametes. But a still broader difference
is seen between the contents of the embryo-sac in Gymnosperms and
in Angiosperms : this is especially apparent at the time of fertilisation.
In the former the embryo-sac then contains a massive tissue of the
endosperm (female prothallus), with fully formed archegonia hardly
differing from those of some Pteridophytes (Fig. 423). But in the
Angiosperms it contains only the egg-apparatus, the antipodals, and
the central-fusion nucleus with its cytoplasm (Fig. 216). The differ-
ence is so wide, and the reduction of the female gametophyte so far-
reaching, that it is still a problem for the comparative morphologist
whether the several contents of the Angiospermic embryo-sac are
really comparable to prothallial structures. All this points to the con-
clusion that in the Angiosperms we see a final state of reduction of
the female gametophyte, the initial steps of which were taken when
heterospory and retention of the embryo-sac on the parent were first
adopted.

The general result which follows from such comparisons is that the
Gymnosperms are confirmed in their position as primitive Seed-
Plants. This harmonises with the fact that they are represented far
back in Geological History. The application to them of the name
Archisperms appears justified. If their origin by Descent is to be
traced, it is to plants of the nature of the Pteridophytes that we
should look : and by preference to some Fern-like source. The
modifications which they show are all explicable as adaptations to
the Land-Habit. More especially is this the case in their substitution
of siphonogamy for zoidiogamy. With the loss of motility of the
spermatozoid the last direct index of their aquatic ancestry was
relinquished, and these primitive Seed-Plants became in the full
sense Plants of the Land.

Those more recent, and still more advanced Plants of the Land,
the Angiosperms, owe their dominant position to their greater
adaptability. Its results have been illustrated for their vegetative
system in Chapters X., XL: and for their propagative system in
Chapter XIV. Ample evidence of it is also provided by the plants
described specifically in Appendix A. The chief features of their
further advance in the propagative region are seen, first, in the

542 BOTANY OF THE LIVING PLANT

elaboration of the floral envelopes, by which are secured the attraction
and mechanical direction of the animal agents of pollination.
Secondly, in the gynoecium. Here the carpels envelop the ovules,
giving a much more efficient protection than the integument alone
can do. In more advanced epigynous types the carpels and the ovules
within them are sunk down into the massive tissue of the axis, thus
giving additional security, as well as ready nutritive supply. Lastly,
the evolution of a receptive stigma, and of the conducting tissue of
the style, combined with the wonderful mechanisms of pollination
seen in Angiosperms, stand in strong antithesis to the primitive and
haphazard methods seen in Gymnosperms. Such features, together
with those more specialised details of the contents of the pollen-tube
and embryo-sac already mentioned, leave no doubt in assigning the
highest place in the Vegetation of the Land to the Angiosperms.

If any further testimony were required to the accuracy of this
comparative conclusion, it would be found in the fact that it is in
general harmony with the chronological succession of plant-remains
demonstrated by Geologists.

CHAPTER XXXIV.

ALTERNATION OF GENERATIONS, AND THE LAND-HABIT.

The expression " Alternation of Generations " was brought into
prominence by Steenstrup, who applied it to the succession of phases
in the life-history of Medusae, Trematodes, and other Animals. He
defined it as " the remarkable natural phenomenon of an Animal
producing an offspring which at no time resembles its parent ; but
itself brings forth a progeny which returns in its form and nature
to the parent Animal." The publication of Steenstrup's essay pre-
ceded the demonstration by Hofmeister of the life-history of Mosses,
Ferns, and Conifers. These researches disclosed phenomena of
Alternation in Plants superficially so like those in Animals that it
was natural to use the same terms in describing them. But later
it has become clear that the resemblance is not an exact one, and that
the " generations " in Plants differ more essentially from one another
than those in Animals, to which the terms were originally applied.

In many Plants the distinctness of the sporophyte and gameto-
phyte is marked by form and structure; for instance, thai between
the prothallus and the Fern-plant. Nevertheless some contemplated
the possibility of the sporophyte having originated as a modification
of a gametophyte, and described the alternation as one of ' homo-
logous " generations. Others, impressed with their distinctness not
only in form but also in their probable origin as indicated by com-
parison, held the two generations to be " antithetic." thai is, distinct
in their origin and history from one another. The discussion of this
question seemed likely to pass into an inconclusive dialectic, when
a fresh point was given to it by the discovery that in Plants there
is a prevalent nuclear difference between the two generations. In
such Plants as may be held to be normal, the sporophyte was found to
have diploid nuclei, and the gametophyte nuclei that arc haploid

b.b. 543

544 BOTANY OF THE LIVING PLANT

This distinction is not matched by any corresponding known difference
in Animals, in which the whole body appears to be consistently
diploid. Thus while Botanists have assumed the term " Alternation
of Generations " first used in relation to Animals, they now apply
it to a phenomenon in Plants which proves to be peculiar to them.
The descriptions already given of the life-histories of Plants have
provided many facts which may now be drawn together into a
comprehensive statement on Alternation, and on the changes and
modifications which it shows in relation to habit.

In normal Plant-Organisms which possess sexuality the fusion of two
nuclei in syngamy has been found to result in a doubling of the number

REDUCTION. SYNGAMY.

X

Fig. 427.

of chromosomes in the zygote. This has been demonstrated in so
many well-authenticated cases that it may be held as a general con-
sequence. At some other point in their life-cycle, normally before
another sexual fusion occurs, there is a tetrad-division, which results
in the reduction of the chromosomes again to the original number.
The second process may be held to be complementary to the first,
and it appears to be necessary if the number of chromosomes is to
be kept within limits after repeated syngamy. The life-cycle in
sexually propagated plants is thus made up of two phases : the one
intervenes between syngamy and reduction and is diploid, i.e. with
2x chromosomes. It is commonly styled the sporophyte, or non-
sexual generation, because it usually terminates in the production
of non-sexual spores. These spores are consequent on a tetrad-
division, and may be styled specifically tetraspores. The other is
haploid, i.e. with x chromosomes. It is commonly styled the gameto-
phyte, or sexual generation, because it normally results in the formation

ALTERNATION OF GENERATIONS 545

of gametes. The cycle thus constructed may be represented as in
the diagram, Fig. 427. Since the two phases follow one another in
regular succession, this phenomenon is that which is now understood
as the normal Alternation of Generations in Plants. It is also referred
to as the Hofmeisterian Cycle.

The deviations from the normal cycle known as apogamy and apospory
have been illustrated by examples from Ferns, pp. 506-509. They connote,
respectively, a direct vegetative transition from the gametophyte to the
sporophyte without the act of syngamy, and a converse transition from the
sporophyte to the gametophyte without the intervention of spores. Though
recorded instances of such deviations from the normal are numerous, they are
not standardised as the normal cycle is in archegoniate plants. This suggests
that they are ex post facto events, illustrating the potentialities of plants
at the present day rather than evolutionary features forming part of the
history of the past.

It seems probable in many, though not perhaps in all phyla, that
nuclear fusion and reduction remained constant features in each com-
pleted life-cycle throughout Descent. In that case two opportunities
for somatic amplification were possible in Evolution from simpler
forms. The one between syngamy and reduction would give rise to the
body of tissue called the sporophyte : the other between reduction and
syngamy would give rise to the gametophyte. If the fusion and reduc-
tion retained their identity throughout Descent, these two somata can
never have been homologous : that is, homogenetic by descent. They
must have been " antithetic " throughout, however nearly they may
resemble one another in their characters. If they both develop in the
same medium they, being in fact merely phases of the same organism,
might be expected to resemble one another very closely.

That is found to be actually the case in certain Algae and Fungi. For in-
stance, in Dictyota among the Brown Algae, and Polysiphonia among the Red,
the two generations appear identical (pp. 383-390) ; they seem only to differ in
their chromosome-number, and in the propagative organs which they bear.
There may be a difference of potentiality between the haploid and diploid
phases ; but it need not be realised where the circumstances are uniform, as
when both grow in water or in moist conditions. In that case both may appear
alike. But even in an aquatic environment the two generations may show
marked differences as in Laminaria.

The Hofmeisterian Cycle.
The inconclusiveness which marked the early discussions on alternation
arose partly from an assumption that what is seen normally in Ferns or Ifom
and described as the Hofmeisterian Cycle, is a standardised life-pattern for
plants at large. There was also insufficient knowledge of the facts of nuclear
structure and behaviour, particularly in the Thallophytes. But the most
notable defect was the failure to take into account the diverse conditions to

546 BOTANY OF THE LIVING PLANT

which archegoniate organisms are exposed. The problem before the nascent
plant possessing sexuality has been to adjust its somatic development to the
nuclear cycle. But these are two essentially different things. The somatic
phases must undergo development in relation to external conditions, whereas
the normal succession of syngamy and meiosis in the nuclear cycle is not
affected by them. There is thus no ground for assuming that the two should
be interlocked in any rigid scheme, such as that presented by the archegoniate
cycle. All life-cycles do not, as a fact, lend themselves to interpretation in
terms of the Hofmeisterian Cycle. While " antithetic alternation " is exem-
plified in a high degree by the vegetation of the land, it should not be forgotten
that it is also characteristic of some of the Brown Algae, e.g. the Laminariaceae.
On the other hand, " homologous alternation " is seen only among the Thallo-
phyta, and particularly the Algae. Such facts uphold the conclusion of Olt-
manns, that just as sexuality may be held to have arisen repeatedly and
independently in various groups of the lower organisms, so may various higher
families have carried out independently the establishment of two generations.
Continuing this line of thought Von Goebel has affirmed that the doctrine of
alternation founded originally for the Higher Plants (from the Bryophyta
upwards) cannot be extended to all plants. From such views it follows that
the relatively stable alternation, as described by Hofmeister for the Arche-
goniatae, should be discussed separately from others, and on its own merits.
To secure a clear verbal contrast it will be well to drop the old terminology,
which never was explicit ; and to substitute " interpolation theory " in place
of antithetic, and " transformation theory " for homologous. These words
are explicit in conveying the opposed theories of origin of the alternation,
which differ in the fixity or otherwise of somatic development in relation to
syngamy and meiosis.

Among the Algae somatic alternation may be absent. In those Algae
which have been distinguished as haplobiontic, the life-cycle, as in the Red
Alga Scinaia and a majority of Green Algae, includes only a single somatic
phase ; and this is usually haploid. On the other hand the life-histories of
Dictyota and Polysiphonia are diplobiontic, with regularly alternating haploid
and diploid somata, though these are uniform in outline and evenly balanced.

With the ground thus freed from the trammels of the old assumption of
uniformity in alternation, we may next enquire into the subaerial conditions
under which the diplobiontic type of alternation seen in the Archegoniatae
originated. It appears that the effect of Life on Land has been to stabilise the
somatic and cytolcgical relations of the Hofmeisterian Cycle. We may picture
to ourselves the working out of the conditions affecting their early evolution as
follows. Those simple Archegoniatae which formed the primitive Flora
of the Land probably sprang from green haplobiontic Algae, inhabiting
shallow fresh water or the higher levels between tide-marks. Here sexual
reproduction would be effected through the medium of external liquid water.
If other conditions were favourable this could be carried out by them at any
time, provided water be present and the sexual organs mature. But certain
types escaped competition or availed themselves of a varying water-level,
by establishing themselves on land where access to liquid water was only
an occasional occurrence. In these the sexual process would only be effected
at times of rain or copious dews. Less dependence could then be placed upon

ALTERNATION OF GENERATION 547

sexuality for propagation, and some alternative method oi ina aid

be an advantage. This was provided by interpolation 0/ a new ft phase,

the act of reduction being delayed by progressive Sterilisation and developn

of the new sporophytc. The natural result would be a multiplicity of
mother-cells, and a higher spore-output: moreover, sue h accord with

the known facts of life among the simplest archegoniate plants. spore

dispersed under dry conditions starts a new individual, yet all those produ<
by one sporophyte would be consequent on a single act oi fertilisation. The
organism thus leads an amphibious existence, tied down by occasional fer-
tilisation through water, but with the resulting spores numerous, and d
seminated in dry air.

The idea of this interpolation of the sporophyte as a factor in the evohr
of archegoniate plants is not new. It was first brought into prominence
by Naegeli as illustrating a principle of evolution in plants in his Abstam-
tnungslehre of 18S5, though the fact of sterilisation of fertile cells to form
additions to the sterile tissues in a plant possessing sexuality was well known
at an earlier date (see Chapter XXXIV. : also Bower, Primitive Land Plants,
Chapter XXIV., pp. 489-491). Such sterilisation was formulated by Naegeli
thus : he states that " the phenomenon of reproduction of one stage becomes
at a higher stage that of vegetation. The cells which in a simple plant are set
free as germs and constitute the initials of new individuals, become in the next
higher plant part of the individual organism, and lengthen the ontogeny to a
corresponding extent" {Abstammiingslehre, 1885, p. 352).

Such an interpolation of a sporophyte, hypothetically sketched here, is
historically probable. Not only would an increased fertility be secured
under variable circumstances but also, as Svedelius has pointed out, the
multiplication of delayed acts of reduction will increase the number of nuclear
combinations, and the plant thereby multiplies its chances of forming new
heritable characters. Given conservatism in the mode of fertilisation, which
is assumed to be inherited from an algal ancestry, an interpolated alternation
would be biologically stabilised as an outcome of migration to land. There are
no close links connecting any living green Algae with any archegoniate
plants. The often-quoted genera, Ulothrix and Coleochaete, as also the isolate >1
class of the Characeae, all stand on the haplobiontic plane, reduction being
involved in the first division of the zygote. None of them has hit off the in-
novation of postponing reduction by the interpolation of a diploid soma fitted
for subaerial life. The focal point of an interpolation theory is that certain
littoral organisms did this. Once the step is taken the wide morphological gap
between aquatic and subaerial vegetation explains itself : for the advancing
archegoniate type would by originating a diploid soma have achieved three
biological ends of supreme importance to any land-living organism : (1) a
multiplication of possible combinations of heritable characters, giving the n
material for variation .; (ii) an opportunity for wide dissemination by air-borne
spores ; and (iii) relief from dependence on repeated syngamy for numerical
increase. The last was probably the most important for land-living plants.
The superiority thus gained by the early plant-amphibians will have favoured
the advance of the interpolated sporophyte, and the haplobiontic algal ancestors
would be left hopelessly behind. Hence the wide gap between any green Alga
and any living archegoniate plant.

548 BOTANY OF THE LIVING PLANT

The Land Habit.
No circumstance of life has more profoundly affected Organic
Evolution than the progression from water to land. The further the
comparative study of the simpler living beings is carried, the more
the conclusion is confirmed that the birth-place of Animals and Plants
was in the water. The comparative study of the Higher Animals and
Plants demonstrates fully that it is in subaerial conditions that both
have reached their highest development. Accordingly it becomes a
question of supreme interest what are the effects impressed upon the
organism by a Land Habit in place of its original aquatic surroundings.
Certain factors stand out clearly. First, the mechanical requirement
for support and maintenance of form. The body, whether of Animal
or Plant, is nearly of the specific gravity of water. When immersed it
is buoyed up, and provided the water be not itself in violent motion
there is little demand on the organism for mechanical resistance. It
is different, however, with subaerial organisms, which require not only
to support their own weight in the lighter medium of air, but must also
maintain their form under the impact of winds and other stresses in-
cident to life on land. The larger the organisms the more insistent will
be their demand. Such questions of mechanical strength have
been taken up for Plants in Chapter X. and need not be discussed
again here. A second factor is the need for protection of the proto-
plast against loss of water by evaporation. The evidence of its
importance is seen in the very general presence of a cuticularised
wall covering all the exposed surfaces, as well as in the simple fact
that no Land-living Plant sheds its ova from the parent, as so many
Algae do. A third factor is the need for internal aeration of the
tissues wherever they assume large bulk. Another which has contri-
buted to the moulding of Plant Organisms living on land is the require-
ment of a large surface of exposure to light and air for photosynthesis.
Such factors as these must be considered in their effect on the Evolu-
tion of Organisms showing sexuality and alternation, as they adapted
themselves to a Land Habit.

Among the various plants that show the Hofmeisterian alternation
the search has naturally been for types of sporophyte which may
be held as primitive. The smallest were those first selected as possible
links for comparison : and in particular, Riccia. But in view of new
detailed knowledge and of the fossil evidence, the trend of opinion
is now towards Bryophytes in which both generations bore photo-
synthetic equipment fitted for subaerial life, as seen in the Antho-
cerotales (p. 475), the Sphagnales, and many Bryales (p. 47^). The

ALTERNATION OF GENERATIONS 5;

sporogonia of these possess stomata and internal ventilation, thou
frequently their stomata are imperfect and functionless. Such str
ture suggests for them a descending rather than an ascending scalt

vegetative development. If this view be correct it will not be among

the smallest and structurally simplest BryophyU-s tl | e should
seek for the key to the early history of the sporophyte in Archegoniate
Plants.

A most important feature of the Land Vegetation is the retention
of the ovum within the parent plant. It is enveloped in the archc-
gonium. The archegonium itself is so constant in the earlier Land
Vegetation that on it is based the name " Archegoniatae," so often
applied collectively to the Mosses and Ferns. The explanation of
the constancy of form and structure of the archegonium is to be
found in the imperative need for the protection which it offers to the
ovum, but without excluding access of the spermatozoid at the recep-
tive period. The immediate consequences of this retention of the
ovum are seen in the fact that Archegoniate Plants, or their Gymno-
spermic derivatives, form the bulk of the early Fossil Flora, and are
an integral part of the Land Flora of the present day. But such
organisms have not cut themselves wholly adrift from their original
mode of life. They are still dependent upon external liquid water
for the fertilising act itself, since it is through water that the male
gamete moves to the egg. Moreover, the gametophyte with its
relatively delicate structure is essentially dependent upon moist
conditions for its normal growth.

Rise and Decadence of the Gametophyte.
The gametophyte has never made a real success of Life on Land,
as measured by its size and structure. But this in itself makes the
study of its partial success the more interesting. In Ferns and the
thalloid Liverworts it is commonly a flattened thin or fleshy body
of undifferentiated tissue, capable of self-nourishment and absorption
from the soil. In extreme cases, growing in very moist and shaded
conditions, it may even be filamentous, while the Alga-like habit n.
be accentuated in vegetative propagation by gemmae. The thalloid
Liverworts also show this ; but in their larger forms the upper surface
of the thallus may be alveolated, and the cavities occupied by photo-
synthetic tissue, so as to make them efficient for self-nutrition in
dry air, as in the leaves of Flowering Plants (p. 472, Fig. 368). In the
Mosses and leafy Liverworts, after a preliminary filamentous staj
a leafy plant is formed after the fashion of the leafy sporophyte. In

550 BOTANY OF THE LIVING PLANT

the larger forms it may develop a conducting system, while some-
times, by involution of their surface, its leaves may acquire a structure
efficient for photosynthesis combined with water-control (pp. 464,
465). But the size of these gametophytes is never great, and often
very minute. Even in its most successful forms the sexual generation
suffers from the disability of an imperfect internal ventilation. It is
essentially semi aquatic, and often saves itself, as the Mosses do, by its
power of dormant vitality under drought, and its readiness in surface-
absorption whenever water is available. Thus constituted the gameto-
phyte is a constant menace to the success of the Archegoniatae, as
Land-living Plants. Finally, its dependence on external water for
fertilisation has tended to tie the lower Archegoniatae down to limited
habitats, from which they have never been fully emancipated.

Some of the most archaic plants that have survived, such as the Psilotaceae,
Lycopods, and Ophioglossaceae, have underground prothalli with endotrophic
mycorrhiza, and saprophytic nutrition (Chapter XII.) Incidentally it may
be noted that internal ventilation is absent from these prothalli, even when
their size is great. This is quite exceptional. It must suffice here to state
these facts without detailed description. In view of the disabilities of the
gametophyte for life on land, the underground habit and the form of sapro-
phytic nutrition which these plants possess may well have been conditions
which have determined their survival.

The difficulty presented by this dependence of the gametophyte
upon external water has been met in the Higher Flowering Plants by
a repetition of the method already so successful in the first conquest
of the Land, viz. the retention of the vulnerable part upon the parent.
First the ovum was retained, as in the Archegoniatae, then the whole
prothallus which bears it, as in the Seed-bearing Plants. For this
the way was prepared by the sexual differentiation of the spores.
Within each of the phyla of Ferns, Lycopods, and Equiseta, this
differentiation has taken place. In each case the original state was,
as in all Bryophytes, homosporous, with all the spores alike, and
commonly yielding on germination a bi-sexual prothallus (p. 500, Fig.
392). The first step is a separation of the sexes on distinct prothalli. A
purely male prothallus has no permanent duty, but only the temporary
function of producing spermatozoids. It may therefore, and it does,
remain small. But the female prothallus has both to produce ova
and to nourish the embryo after fertilisation. This can best be
carried out by a large prothallus, which will develop better from a
well-nourished spore. This is the physiological rationale of the origin
of the megaspore as distinct from the microspore, as seen in Selaginella,

ALTERNATIONS OF GENERATIONS 551

which is heterosporous (p. 514). Tl me condition is also in

certain Ferns (Marsilia, .-holla), in many fossil Lycopods, and

occasionally in Equiseta also. But still the megaspore in th< 1 plants
is shed from the parent before fertilisation, and is then ndent on

its own resources. The longer the period of connection with the
parent the better. A further advantage was then gained by retention
of the megaspore itself upon the parent plant until the embryo is far
advanced. The sporangium which thus retains its megaspore is called
an ovule, and this matures into the seed, which is characteristic of all
the Higher Plants of the Land (p. 521). The prevalence of the Seed-
habit is the token of its success.

While a certainty of protection of the prothallus and of continued
nutrition of the embryo is thus secured by retention of the mega-
spore, or embryo-sac, upon the parent, the steps of progress involved
have reacted adversely upon the gametophyte generation. The
separation of the sexes tended to relieve it of the necessity for self-
nutrition. Provided the microspores are numerous, and each has a
sufficiency of material to form an antheridium and spermatozoids,
that would meet the requirements, and little or no vegetative tissue
is needed. This condition is characteristic of heterosporous plants.
It is seen in Selaginella (p. 517, Fig. 41 1), and in the pollen of Gymno-
sperms and Angiosperms with their vestigial male prothalli (Figs. 222,
421). On the other hand, the megaspore requires to be well nourished,
in the interest of the archegonia, and of the embryo which each will
bear. But it receives its supply from the parent plant, rather than by
its own self-nutrition. Thus in the case of a megaspore of Selaginella
the female prothallus is little more than a storage tissue, and a basis for
archegonia (Fig. 412, iv.). Self-nutrition is reduced, or entirely absent
as in the Gymnosperms ; and, finally, in the Angiosperms the female
gametophyte is so transformed that it may be difficult to homologise
the contents of the embryo-sac at all with a female prothallus (Fig. 216).

The spermatozoid motile in water remained, however, as the me.
of fertilisation even after the adoption of the Seed-habit. It is still
seen in the Ginkgoaceae and the Cycads, though its unpractical nature
is evident (p. 527). The last step of emancipation from the original
aquatic method of propagation was the substitution of the motile
spermatozoid by the non-motile gamete delivered by the pollen-tul
(Figs. 226, 424). Thus the most critical point of each life-cyi le, viz.
fertilisation, was finally adapted to Life on Land. And so by a
series of steps the gametophyte is reduced, altered, and in some ca»
almost obliterated. It has paid the penalty of its inability to adapt

552 BOTANY OF THE LIVING PLANT

itself thoroughly to sub-aerial conditions. The climax of the game-
tophyte on land is attained in the hornosporous Mosses and the leafy
Liverworts. It appears of independent, though limited growth in
the hornosporous Ferns. But with heterospory it fades into insignifi-
cance, and in the Higher Seed-Plants it survives as a mere relic.

Rise of the Sporophyte.
Although Brown Algae such as the Laminariales afford evidence of
the rise of the sporophyte generation it is in the progressive series of Land-
living plants that this phenomenon is most strikingly disclosed. By
adaptation of form and structure it has met, in its highest terms
successfully, the requirements for mechanical strength, for protection
under drought, for exposure of a large surface for photosynthesis, and
for ventilation of extensive nutritional tissues. In all these respects it
is the superior of the gametophyte, and perhaps the structural feature
that has contributed most to its supremacy is the ventilation-system
of intercellular spaces, controlled by stomata at their exits to the open
air. This differentiates the sporophyte from the gametophyte more
clearly than any other structural character, and stamps it as adapted
to sub-aerial life. The end of its vegetative existence is the formation of
spores (tetraspores). The more numerous these are (other things being
equal) the better the chance of survival of the species, and of its spread.
The vegetative development may be regarded as a means to that end,
and in hornosporous forms its nutritive capacity imposes a natural
limit on spore-numbers. The dispersal of the spores is dependent in
primitive land forms upon a dry atmosphere. This is in strong
antithesis to the necessity for external liquid water for fertilisation,
which is the end of their gametophyte.

The sporogonium of the Bryophyta is usually held to represent
the most primitive type of sporophyte among Land-living Plants,
and recent discoveries of very early fossils tend to support that
opinion. Its limited plan of construction, its ephemeral character,
the absence of appendages, and its dependence throughout life upon
the gametophyte, are all indications pointing in the same direction.
Moreover, the fact that the spore-production in each sporogonium
arises from one undivided sporogenous tissue, not from a number of
distinct sporangia, also points to the same conclusion. Within the
Bryophyta the various forms may be seriated so as to give probable
indications of progressive advance in various important characters.
These suggest that in very primitive types apex and base were
defined early. A sterile stalk and central columella were acquired,

ALTERNATION OF GENERATION 553

and later, specialised methods of spore-distribution. These rose to
high efficiency in the higher Mosses (Figs. 355, 367). A well-formed
epidermis with stomata is found in some of them, while beneath
this lie assimilating tissues as well ventilated as in the leaves of
Vascular Plants (Fig. 364). Nevertheless, the simple form, the
limited apical growth, the absence of appendages, and above all the
want of any direct connection with the soil, stamp even the most
elaborate sporogonium as an only partially efficient structure. To
achieve higher development it would be necessary to break away
from so restricted a plan, which in itself is only practically possible
where the gametophyte shows high elaboration, so as to supply the
nourishment the sporophyte cannot wholly acquire for itself.

The Homosporous Pteridophytes are free from such restrictions.
The features in wThich they show superiority to the Bryophyta as
spore-producing plants are : (i) an unlimited capacity for apical growth ;
(ii) the possession of lateral appendages ; (iii) a direct access to the
soil by a root-system ; (iv) an improved conducting system ; (v) a
well-ventilated photosynthetic system, with elaborate external form,
and more complete differentiation of vegetative from propagative
regions of the plant ; (vi) the formation of numerous distinct spor-
angia ; and (vii) the production of the sporangia not simultaneously
but in succession, or even delayed, so as to spread the physiological
drain over a long period. Possessed of these features the sporopfr
develops as an independent, self-nourishing organism, unlimited in
plan, in period of life, and in power of spore-production. It is the
sphere of Special Morphology to trace the lines along which these
various features may have been acquired. But the result of them is
seen in varying proportion and efficiency in any ordinary Fern, or
Lycopod, or Horse-tail. These are characteristic examples of the
primitive Vascular Plants of the Land. They depend upon the vege
tative development of a freely-rooted sporophyte for their legitimate
success, while still retaining their homosporous state. In point of
size the acme of achievement of the Homosporous Pteridophytes
now living is to be found in the Filicales ; though they still show :
the most part a leafy shoot which serves general purposes, and is not
strongly differentiated into vegetative and propagative regions.

It was the adoption of the heterosporous state and the retention of
the megaspore and its prothallus within the megasporangium, or
ovule, that paved the way for the full possession of the Land by Seed-
bearing Plants. Plants thus finally broke away from dependen
on external water for their fertilisation. Seed-production is carried

554 BOTANY OF THE LIVING PLANT

on in a compact Strobilns, or Flower. Evidence of the steps of
segregation of the '' general purposes shoot " into distinct nutritive
and propagative regions may still be traced in favourable cases (see
p. 278). These regions once established took each its own independent
line of specialisation in the evolution of Seed-bearing Plants. The
vegetative region which appears first in the individual life commonly
develops normal foliage ; but under special conditions it may become
xerophytic, scandent, parasitic, or saprophytic, the adaptive nature
of the change being usually evident (Chapters XL, XII.). The propa-
gative region or Flower also became specialised in relation to its
functions. It appears later, and it is distal, since nutrition is necessary
before propagation can be carried out. This distal position, while it
removes the flower further from the water-supply, offers the best
opportunity for transfer of pollen, whether by wind or by animal
agency. The functions of the Flower are : to produce sporangia ;
by its structure to offer facilities for pollination, with fertilisation as its
consequence ; to protect and nurse the new germs up to the period of
ripeness of the seed ; and, finally, to secure seed-dispersal. The means
by which these ends are attained are almost infinitely various.
Examples are described at length in Appendix A. It is the high degree
of adaptability of the Seed-bearing Plants to subaerial conditions, so
as to secure these ends, that has given them their supremacy. The
pollen-grains, usually dry and dusty, retain essentially the character
of the microspores of the Pteridophyte. They thus allow of either
self-pollination or of inter-crossing in various degrees, in organisms
themselves non-motile. Commonly they are exposed at the time of
flowering to dry air and full sunlight. The antheridial mother-cell
within each grain is protected by the cuticularised and often coloured
coat of the grain from injury by drought or intense light : its contents
are not set free into water as in the Amphibious Pteridophytes, but
flow into the security of the pollen-tube : there the male gametes are
formed and are passed on to their destination as the tube grows
(Chapter XVII.). Similarly, the ovum is never exposed. Its pro-
tection against all risks is secured by a succession of tissue-envelopes.
The carpel, one or two integuments, and finally the nucellus all take
their part in this duty. The ovum itself, a primordial cell not differ-
ing essentially from the exposed egg of Fucus, thus deeply sunk in
living tissue, is immune to the risks of subaerial life. It is in a position,
when fertilised, to draw its supplies during the nursing period from
the embryo-sac, and the surrounding envelopes. Such conditions,
combined with the effective and often elaborate means of distribution

ALTERNATION OF GENERATIONS

of the ripe seeds already described (Chapter XIX ), account for the
Seed-Plants becoming the chief constituent of Land-Vegetation.
The disabilities of the gametophyte for land-life have been led.

In the more adaptive sporophyte the most vulnerable points in I
cycle of life, viz. the period of fertilisation, and the first stages of
development of the embryo are effectively protected. Thus the sporo-
phyte has become virtually the Plant of the Land, and the gametophyte a
mere vestige.

A strong antithesis has been drawn between the relative failure of
the gametophyte and the ultimate triumph of the sporophyte in sub-
aerial life. The two generations differ normally in chromosome-
number. This seems to suggest that a higher potentiality and ini-
tiative in variation lies with the diploid state. It may not be possible
to lay it down as a general proposition that a double number of
chromosomes is an index of greater power of adaptability ; but it
is a significant fact that the highest somatic evolution both in Animals
and Plants has been attained by diploid, not by haploid tissues.
The gametophyte is the sexual generation, and the sexual organs
are borne by it. In this it stands in strong antithesis to the sporophyte,
which is a non-sexual or neutral generation, by nature and by origin.
But the steps in the obliteration of the gametophyte, and in the evolu-
tion of the seed have been accompanied by a tendency for sexual
differentiation to encroach more and more on the morphology of the
sporophyte generation. This will be apparent on comparison of the
life-cycle of a homosporous Fern (Fig. 400, p. 506) with that of a
Flowering Plant (Fig. 257, p. 335). The final culmination of this
found in those Seed-Plants which are dioecious, such as the Willow,
or the Yew (Fig. 258, p. 336). In these some individuals bear only
staminate, others only pistillate flowers, and the plants are thus ranked
as " male " or " female," though in point of fact they represent the
neutral generation. The end result is thus seemingly a paralhl
between the Higher Plants and the Higher Animals . 1 xuality.

In both the individual appears to be either " male " or " female." But
this similarity is superficial rather than real, for it has been attained
along quite distinct evolutionary trends in the two Kingdoms. In the
Higher Animals there is a true sex-difference between individual.-,
the one producing male, the other female gametes. In the Flowering
Plants the individual is the neutral sporophyte, which does not itself
produce gametes. But in the course of Descent certain distinctr
features relating to sex have become increasingly evident in the
sporophyte or neutral generation. Accordingly the Flowering Plant

556 BOTANY OF THE LIVING PLANT

has secured such advantages as follow from sex, through its retention
of the sexual generation within it.

It thus appears that there are three forms in which somatic de-
velopment may be related to the nuclear cycle characteristic of
organisms possessing sexuality : (i) that in which a haploid soma
intervenes between the events of reduction and syngamy : this is
characteristic of haplobiontic Algae ; (ii) that in which there is
present not only the haploid soma as before, but also a diploid
soma which intervenes between the events of syngamy and reduction :
this is characteristic of the Archegoniatae, and the derivative Seed
Plants ; it is also seen in some Thallophytes, and is typical of the
diplobiontic Algae ; and (iii) that in which a diploid soma intervenes
between the events of syngamy and reduction, there being no haploid
soma : this is characteristic of Animals, and of certain Algae, e.g.,
Fucus, and members of the Siphonales (Green Algae). The fact that
a haploid soma may exist without the diploid, while a diploid may
exist without the haploid, appears to strengthen the view that the
two phases seen in the Archegoniatae differ in the history of their origin :
and to support for them the theory of interpolation, of which that
historical difference is an essential feature. It is a recognised principle
in Morphology to fix the attention upon those features that are most
constant in their occurrence, and to accord to them the higher impor-
tance. Syngamy and its complementary Reduction thus take a prior
place, while somatic developments appear as inconstant incidents in a
more stable nuclear cycle.

Summary.

The sexual act probably originated polyphyletically among primitive Algae,
in some of which an early differentiation of the gametes can be traced by com-
parison with allied forms (Fig. 275). The two leading events of the sexual
cycle are Syngamy and Reduction. In haplobiontic plants such as the
Characeae they follow in direct succession. Here reduction takes place in the
first division of the zygote : in Chara the zygote develops directly into a new
haploid individual like the parent. There is thus no alternation in the full
Hofmeisterian sense.

What gave rise to the Hofmeisterian Cycle appears to have been a post-
poning of the event of reduction and, by the interpolation of a diploid soma,
the fitting of a new phase into the cycle of life. This innovation once esta-
blished was seized upon in the Evolution of Plant Life on Land ; for it brought
important advantages, which are detailed on p. 547. The final result has been
the establishment of a Land Flora such as we see living on any land surface
today ; and it is dominated by the diploid soma.

CHAPTER XXXV.

HEREDITY AND VARIATION

Sexual Reproduction.

In the great majority of Plant Organisms certain sexual cells called
Gametes are produced, which fuse in pairs. The process is called
Syngamy, or Sexual Fusion. The result of it is the production of a
single cell, the Zygote, which forms the starting point for a new indi-
vidual. Though such syngamy is a very wide-spread fact among
living things, whether Animals or Plants, it is not universal. Some
primitive organisms are without it. The whole series of the Schizo-
phyta are examples of this, while sexuality is rare or doubtful in
Euglena and Pleurococcus. In certain Plants also of advanced organisa-
tion syngamy may be absent, as in some Flowering Plants and
Ferns.

A comparison of related organisms low in the scale, which show
syngamy, suggests that in the first instance the fusing gametes wi
alike in size and behaviour, though more or less distinct in their origin.
Such cells are called isogametes, and the process of their fusion is de-
scribed as conjugation. It is seen in both Animals and Plants of low-
organisation. This condition, where the gametes show no char
distinction of sex, is believed to represent a primitive state from which
distinction of sex was later derived. The isogametes themselves m
be motile or non-motile. The former is seen typically in various
green and brown Algae : Ulothrix (Fig. 270, p. 365), Acetabular ia
(Fig. 275, p. 370), and Ectocarpus siliculosus (Fig. 284, p. 380) are
cases in point. Conjugation of non-motile isogametes occurs in the
Conjugatae, such as Spirogyra (Fig. 277, p. 373).

If two organisms, each consisting of only a single cell, fuse to
form one, the immediate result is a diminution in number to one

557

558 BOTANY OF THE LIVING PLANT

•

half. This occurs in various instances, and Spirogyra is a case in
point, for each cell of its filament is properly recognised as an indivi-
dual. The same occurs in various other unicellular Animals and Plants.
As these are probably primitive, they suggest that in the first instance
syngamy was not a means of increase in number of individuals, though
in all the Higher Plants and Animals this appears to be its natural
consequence. Some believe that the chief advantage following on
sexual fusion in such simple organisms lay in nutrition. If during
repeated fissions cell-division wTas more rapid than nutritive recupera-
tion, then fusion of two cells would be a possible form of recovery.
But such fusio#n appears to bring with it also a stimulus to fresh
activity of growth and division, which may break out at once, though
in primitive organisms it follows usually after a period of rest. With
this fusion there follows also the pooling of such qualities as the fusing
cells themselves possess. So far as these qualities can be transmitted
to the offspring, the mechanism of fusion offers the opportunity for
it (see later) ; and it is significant that the fusing gametes are as
a rule distinct in origin. For instance, the pairing gametes of Ulothrix
(p. 365), or of Ectocarpus (p. 380), originate from different gamet-
angia ; and the distinctness of origin is still more marked in many
plants higher in the scale. It seems probable that such advantages
as these, viz. nutritive recovery, stimulus to further development,
and hereditary transmission, have favoured a constant recurrence
of syngamy. In the long run hereditary transmission has been the
most important.

Differentiation of Gametes.

Fusion of isogametes once established led to sexual differentiation
in many distinct phyletic lines, both of Animals and Plants. Com-
parison of closely related forms is the basis of this conclusion ; and
a particularly convincing example is seen in the Brown Algae (pp.
380-382). The distinction is there found to be first a difference in
behaviour rather than of form [Ectocarpus siliculosus). Next, a differ-
ence in size as well as in behaviour marks the female as distinct from
the male, as in E. secundus. In Cutleria that difference is still more
accentuated, and the larger female gamete soon loses its motility. In
Fucus the difference in size is very great indeed, and the large female
egg is never motile at all. Various other phyletic lines could be

HEREDITY AND VARIATION

quoted showing similar examples of sexual differentiation (Fig. 275).
The question naturally arises why should such progressions exist in a
plurality of distinct phyletic lines? That the differentiation of
gametes has occurred more than once indicates a probability that
some real advantage had attended it.

The advantage appears to lie in the fact that the larger the amount
of nutriment embodied in the egg, the better nourished the offspring
will be in its first stages, and the better accordingly will be the chance
of its passing successfully through the dangerous risks of youth.
But the larger the egg the less mobile it will be. Even in the liquid
medium into which the eggs of Algae are often shed, a large body is
less easily moved than a small one. We naturally associate with this
the fact that the larger eggs have lost their motility. This is, how-
ever, immaterial so long as the spermatozoids remain small and
actively motile, provided that the egg can influence their movements,
and so act as a centre of attraction to them. It has been seen that
the eggs are able to do this (p. 382). Such advantages as follow from
the stimulus of fusion, and the pooling of the hereditary factors of
the two sexual cells can still be secured by these means. Thus the
nett advantage lies with the plant which can, without sacrificing
the benefits that follow from syngamy, secure also for its offspring an
increased probability of successful germination. Conjugating organ-
isms, with their small equivalent gametes, may be regarded as a plant-
proletariat that produces numerous offspring with little physiological
capital ; so that each individual must depend chiefly upon its own
efforts. The organism that shows differentiation of its garnet
with an enlarged well-nourished egg is like a capitalist, whose progeny
starts life well furnished with a capital of reserve food. Other things
being equal, ultimate success will lie with such organisms. Both
Kingdoms of Living Beings show how successful the results of
differentiation have actually been : for all their higher terms have
differentiated gametes.

A large naked egg, such as that of Fucus, may be a successful
enough means of propagation in water. But it could not develop
into an embryo exposed to the drying influence of the atmosphere.
A necessary condition of Life on the Land is thus the protection in
one way or another of the egg and the embryo. In the Evolution
of Land-living Plants and Animals this necessity has played a leading
part. The result is seen in the various forms of internal embryology :
that is, the envelopment of the egg and of the embryo within the
tissues of the parent. This brings also the collateral advantage of

B.B. 2N

56o BOTANY OF THE LIVING PLANT

its continued nutrition. How this has worked out in detail in Mosses
and Ferns, and ultimately in the Seed-Plants, has been discussed in
Chapter XXXIV. Here it must suffice to remark that the sub-
sequent changes seen in such Plants have to do with the details
of transmission and of nursing of the gametes. They do not involve
any further distinction of the male and female gametes, in respect of
size or structure, than that already established among the Higher
Thallophyta.

The Nucleus and Heredity.

To form an opinion on the function of the gametes in sexual pro-
pagation, a knowledge of their structure is necessary ; also of the re-
lation of their structure to that of other cells of the plant-body. The
nucleus, which takes a prominent part in the process of fertilisation,
and itself constitutes almost the whole of many spermatozoids, will
specially claim attention. Moreover, the facts already recognised in
Chapter II, p. 20 — that every nucleus is derived from a pre-existing
nucleus, and that in ordinary cell-division the parent nucleus is
partitioned into two exactly corresponding halves — suggest that the
nucleus has a special relation to the facts of heredity. The detail of
the process of nuclear division will then be the natural starting point
for the further discussion of Heredity and Variation.

NUCLEAR DIVISION

The Resting Nucleus.

The " resting nucleus " of a vegetative cell — that is the nucleus when
not in a state of division — has the appearance of a meshwork (nuclear
reticulum) of fine threads embedded in a clear ground-substance
(nuclear sap) and delimited from the cytoplasm by an exceedingly
delicate nuclear membrane (Fig. 428, a). The stains commonly used
in cytological studies do not affect the whole of the meshwork to the
same extent, certain parts becoming more heavily stained than the
rest. It has therefore long been customary to describe the nuclear
reticulum as composed of faintly staining linin with localised masses
of deeply-staining chromatin. But there may be doubt as to the
morphological distinctness of these two components. Enclosed in the
nucleus are one or more denser, rounded, heavily staining bodies, the
nucleoli.

HEREDITY AND VARIATION

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562 BOTANY OF THE LIVING PLANT

Mitosis and Meiosis.

There are two distinct normal methods of nuclear division : (a) mitosis
or somatic division and (b) meiosis or reduction division. Mitosis
takes place in the cells of the vegetative plant body (somatic cells) by
which it increases the number of its cells during growth. Meiosis is the
process, comprising two nuclear divisions of special type, which
initiates the gametophyte (haploid) stage in the life-cycle of any plant
that reproduces sexually.

Mitosis.

Mitosis is a complicated process and for convenience of description may be
regarded as taking place in a series of stages to which distinctive names have
been given.

During prophase (Fig. 428, b and c) the chromosome reticulum becomes
resolved into a definite number of chromonemata ; these are long slender
threads, more or less twisted, and double. The threads gradually become
shorter and thicker and finally take the form of V- or L-shaped deeply staining
rods, the chromosomes. Each chromosome, like the chromonema from which it
is derived, is a double structure, being divided lengthwise into two equal
halves, the chromatids.

In the second stage or metaphase (Fig. 428, /), the chromosomes arrange
themselves in the equatorial plane — i.e. the plane of division of the cell — with
the point of the V or L facing inwards. At the same time delicate protoplasmic
threads {spindle fibres) appear in the dividing cell and group themselves into a
spindle-shaped figure (Fig. 428, g) placed at right angles to the equatorial
plane. The chromatids are of uniform thickness, except for certain con-
strictions, the number and position of which are specific for each chromosome ;
there is always at least one constriction, the attachment-constriction or
centromere, which is believed to control the movements of the chromatid.

At the third stage or anaphase (Fig. 428, g-i), the halves of each chromo-
some (chromatids) separate and pass to the opposite poles of the spindle. It
is thought that this separation is initiated by a mutual repulsion between the
centromeres.

The final stage or telophase (Fig. 428,7'-/) is essentially a reversal of prophase.
The " daughter chromosomes," as the separated chromatids may now be
called, lengthen into slender threads which become entangled together and
reconstitute a " daughter nucleus " at each pole of the cell. In the resting
stage the chromosomes lengthen and become threadlike, and again split
lengthwise in preparation for the next division.

The nucleoli and the nuclear membrane disappear during the early stages
of mitosis and reappear at telophase. The new cell-wall most commonly
arises as a thin lamella (" middle lamella " or " primary cell-wall ") within
the cell-plate, a specialised layer of the cytoplasm which becomes gradually
differentiated during telophase in the equatorial plane.

The outstanding features of mitosis are three .

(1) For the cells of each kind of plant (or animal) there is a definite number
of chromosomes which normallv remains constant for all the somatic divisions.

HKRKDITV AND VARIATION

(z) Each chromosome separates into exactly equal halves (chromatids)
which arc distributed to opposite poles <>t the dividing cell.

Hence, (3) each daughter nucleus receives eld exactly equal share "f tin-
chromosome material of the parent QU< leus (Fig. 420).

Proph

ase

B

Y

AtetepAase

1

(division cft/ip

nuc/eus \

Anaph

ase

V

Te/op/iase

(ftro nuc/ei)

a, atfacnmenf c<7/7sfr/cf/0/z .

Fie. 429.
Diagram showing the changes undergone by two chron. 111 the

course of mitosis. (From Crane and Lawrence alter Darlington.)

If, as is generally assumed, the heritable qualities of the organism are local-
ised in the chromosomes— as so-called genes— mitosis dearly provides a
mechanism by means of which these qualities are transmitted unchanged
each somatic division. The fact that new individuals produced by veg« tative
propagation (Chapter XIII) exactly resemble the parent plant bears out this

conclusion.

Meiosis.

The formation of tetraspores (such as pollen-grains, spon a t Pteridoph
and Bryophytes, tetraspores of Rhodophyceae, ascospores and basidiospon
involves two divisions of the nucleus accompanied by one division of the
chromosomes; as a result, the chromosome number is halved. Mine four
tetraspores are produced from each spore mother cell, the whole proa
is often described as a tetrad-division.

564

BOTANY OF THE LIVING PLANT

firs/ Division

Prophase

potnt of cross/fig orer

\

chiasms

f ,

/<^\ attachment

Metaph

ase

Anaphase

Seconcf D/y/sion

Mefaphase

Encf of
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f

•p.. fig. 430.

Diagram of Meiosis. For explanation, see text. (From Crane and
Lawrence after Darlington.)

HEREDITY AND VARIATION

The essential features of meiosis arc ilh. I in 1

the case of mitosis it is convenient to distinguish a nu:
very complicated proo

Heterotype I m : Proj > a

In the first stage {leptotene) (Fig. 430, a, 431, 1) the chroma
as long slender threads with a general resemblance t<> the chn
in the first stage of mitosis, except that the threads are single instead
double.

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■*

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• • • 1

• / i

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y ..Oi.i; ' ' ' yf

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Fig. 431.

Meiosis— early stages in Trillium erection. 1, leptotene ; 2, EygOteoe ; r ra
tene. (From Sansome, after Huskins and Smith, by courtesy of Scientific Horti-
culture.)

At the second stage {zygotene) (Fig. 430, b, 431, 2) the chromosomes become
associated in pairs. This association is not a random one ; the chromosomes
that pair are the corresponding, similarly constituted or homologous ones, one
of each pair being derived from the male gamete, and the other from the female
gamete, which initiate this sporophyte generation.

In the third stage {pachytene, Fig. 431, 3) the paired chromosomes api
shorter and thicker as a result of twisting round one another.

At the fourth stage {diplotene, Fig. 430, c, d) the chromosomes split length-
wise into halves so that each pair now consists of four intertwined threads
(chromatids), held together in pairs by the centromeres, which do not split.
At this stage, or immediately prior thereto, the members of a homologous
pair interchange some of their parts, a process known as crossing-over, only two
out of the four chromatids breaking and rejoining at any one point. The
homologous chromosomes then begin to separate, but not completely, being
still held together at certain points— chiasmata— as a result of crossing-over
(Fig. 433). These four stages collectively constitute pt ; \

Metal
During the fifth and sixth stage (diakinesis and metaphass) the four chroma
tids continue to shorten and thicken ; the nucleoli disapp ■ Duel

spindle is formed ; and the chromosome-pairs arrange themselves In the

equatorial plane.

At the seventh stage (first anaphase, Fig. 432, 6) the paired chromosomes
part company, two chromatids— held together by the unsplit centromere-
passing to one pole of the cell and two to the other pole (Fig. 430, /).

566

BOTANY OF THE LIVING PLANT

Telophase.
At the eighth and final stage of the first division {first telophase, Fig. 432, 7)
two daughter nuclei are formed and pass into a short resting stage {interphase).

Homotype Division.

A second division follows quickly in which the centromeres now split ; as a
consequence the process closely resembles an ordinary mitosis, the chromo-
some-halves passing to opposite poles and thus into different tetraspores
(Fig. 432, 8-10).

The outstanding points of meiosis are :

(1) It involves two nuclear divisions with only one division of the chromo-
somes. Hence,

(2) It leads to a halving of the chromosome number (reduction).

(3) Interchange of parts (crossing-over) occurs between the chromatids
of each pair of homologous chromosomes.

(4) The first division of meiosis distributes the four chromatids of each
pair of homologous chromosomes, so that each daughter-cell receives two.

(5) The second division distributes these two chromatids so that each
granddaughter-cell (tetraspores) receives one.

(6) The four chromatids of each chromosome pair are thus distributed at
random among the four members of a tetrad of tetraspores.

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10

Fig. 432.
Meiosis — later stages in Pinus Banksiana. 1st Division : 6, anaphase ; 7, telo-
phase. 2nd Division : 8, metaphase ; 9, telophase ; 10, four pollen nuclei. (From
Sansome, after J. M. Beal, by courtesy of Scientific Horticulture.)

Provided that each type of tetraspore is viable, the gametophytes and
ultimately the gametes formed by them will exhibit the same distribution of
genes as the spores from which they are derived. The whole gametophyte
generation and the gametes themselves will have the reduced or haploid
number of chromosomes.

HEREDITY AND VARIATION

Syngamy is the fusion of gametes to produce i tli-

standing the fact that in many Algae the garnet of the natan

primordial cells with a considerable amount of cytoplasm, and in
higher types, such as Mosses, Ferns and Seed Plants, the cytoplasm
of the male gamete is reduced to negligible proportions, fertilisation is

Fig. 433.

Camera lucida drawings of chromosomes in the prophase of meiosis. A, Ltltum
pardalinum ; B, Fritillana lanceolata. Note the correspondence between the sue
and sequence of the particles of the pairing chromosomes. In 13 the four strands
(chromatids) are clearly seen. There is one chiasma in each case. (From Crane and
Lawrence after Belling.)

essentially a process of nuclear fusion. The first division of the zvl
is an ordinary mitosis (apart from cases like that of Spirogyra, where
the zygote itself undergoes meiosis on germination), with equal
distribution of chromosome-halves (chromatids) ; and all subsequent
vegetative divisions follow the same course. As botli the nude and the
female gametes are haploid, syngamy restores the full number of
chromosomes. The diploid sporophyte i- thus initiated, and the
normal nuclear cycle, diploid sporophyte- haploid gametophyte
diploid sporophyte, may be repeated indefinitely.

Chromosome Complkmini (Karyotype).

It has already been noted that the chromosome number i< constant
for all the somatic divisions of any normal plant. Thus the Garden

568

BOTANY OF THE LIVING PLANT

Pea has 14 chromosomes, the Gooseberry 16, Maize 20 and the Tomato
24 ; these, of course, are the diploid numbers, each chromosome being
present in the sporophyte cells in duplicate. The actual chromosome
number, though characteristic of a particular species, or race, is
apparently not in itself of special significance ; that is to say, there is
no correlation between chromosome number (or chromosome size) on

M.long.

kin

I w A B q Cz c3 d) d2 d3 0I4

M.ten,

till

M.monsir.j

IllilfMf

A B C| C2C3 c^c^c^oU

Fig. 434.

Somatic chromosome complements (karyotypes) of three species of Muscari—
M. longipes, M. tenuiflorum and M. monstrosum. (From Sharp's Introduction to
Cytology, by courtesy of the McGraw-Hill Book Co., Inc. After Delaunay, 1926.)

the one hand, and taxonomic position or grade of evolution on the'
other, at any rate in so far as the major taxonomic groups are con-
cerned. The constancy of the chromosome number, however, is im-
portant as evidence that the chromosomes, though not ordinarily
distinguishable as such except at nuclear division, do nevertheless
retain their individuality during the resting stage. Further support
for this view is provided by the many cases in which the several pairs
of chromosomes are visibly — i.e. morphologically — distinct ; an
example is shown in Fig. 434. The type of chromosome complement
characteristic of a race or individual has been termed its karyotype.
Comparison of karyotypes within limited circles of affinity has thrown
light on problems of taxonomy and evolution (see below under " poly-
ploidy," pp. 581-584).

Hybridisation.

If the gametes involved in producing a succession of generations were
uniform throughout in their origin and hereditary constitution, the
organisms produced might be expected to remain constant. But the

HEREDITY AND VARIATION

parents that produce the gametes are not themselves as a rule exactly
alike, and the gametes produced by them will therefore differ. The
dissimilar can breed together within certain limits. Meml races

or varieties commonly interbreed freely ; sometimes -pecies and even
genera are interfertile. The artificially produced generic hybrids of
certain Orchids are a well-known feature of horticultural shows;
and other cases are known of fertile hybrids arising from crosses
between species of distinct genera, such as the " Raphanobrassica "
derived from a cross between Radish and Cabba^ The breeding
together of members of distinct races, species, etc., is called hybridisa-
tion, and the offspring are known as hybrids. Since the experimental
study of hybrids has thrown much light on the mechanism of inherit-
ance, it will be treated in some detail ; but before this can be done,
the nature of variation must be briefly considered.

The characteristics of organisms have been classed under two heads :
those which are heritable and those which are not. The latter category
includes such features as can be related directly to the impress of
external circumstances upon the parent ; the former comprise those
features which cannot be so related. Mutilations can be quoted as
examples of characters not transmitted ; likewise the immediate
accommodations of the growing parts to the impact of gravity, light,
etc., such as have been described in Chapter IX. However effective
these may appear to be in determining the mature form of the parent,
there is no reliable evidence that such modifications are transmitted
to the offspring. But there are other larger or smaller deviations from
type, appearing suddenly and individually, which have not been
referred directly to known causes and are found to be [heritable.
Individuals showing variation of this type are called mutants and the
process of their formation mutation. It may be emphasised again
that modification leaves no permanent impress upon the organism
so as to affect its gametes. On the other hand, it has now been esta-
blished with certainty that mutation is the chief source of the varia-
tions upon which Natural Selection can work, and that it has therefore
played an important part in Evolution.

Mendelian Segregation.

It has long been known that offspring produced by the crossing of
closely related forms, whether of animals or plants, do not alw.
come true to type. But it remained for Mendel to discover, in the
latter half of the nineteenth century, the laws, since verified by many

570 BOTANY OF THE LIVING PLANT

observers, which operate in the distribution of the characters of the
parent forms among the progeny. The following description of one
sample of Mendel's experiments is based, with Professor Punnett's
permission, upon passages in his book on Mendelism.

In the selection of a plant for experiment Mendel recognised that
two main conditions must be fulfilled. In the first place the plant
must possess evident differentiating characters ; and, secondly, the
experimental plants must be protected from the influence of foreign
pollen during the flowering period. In Pisum sativum Mendel found
an almost ideal plant to work with. The flowers are self-fertilising,
whilst complications from insect-interference are practically non-
existent. There are numerous varieties of the Garden Pea exhibiting
characters to which they breed true. Mendel selected a certain number
of such differentiating characters and investigated their inheritance
separately for each character. Thus in one series of experiments he
concentrated his attention on the stature of the plants. Crosses were
made between tall and dwarf races, which previous experience had
shown to come true to type with regard to these characters. It mat-
tered not which was the pollen-producing and which was the seed-
bearing plant ; in every case the result was the same. Tall plants
only resulted from the cross. For this reason Mendel applied the
terms dominant and recessive to the tall and dwarf habits respectively.
Seeds collected from the hybrid plants (F1 generation) and sown the
following year, gave both tall and dwarf plants among the progeny
(F2). Every individual was either tall or dwarf, and no intermediates
appeared. In one series of experiments Mendel obtained 1064 F2
plants, of which 787 were tall and 277 dwarf ; that is the dominant
and recessive characters occurred in the second generation of hybrids
(F2) in the approximate proportion of 3 : 1.

In the following year seeds from the F2 generation were sown as
before and produced the F3 generation. From the seeds of the dwarfs
came only dwarfs, i.e. the recessive character bred true. The tall plants
however, of the F2 generation now revealed themselves as being of
two kinds. Some of them produced seed giving rise to tall plants
only ; others formed seed from which sprang both tails and dwarfs in
the ratio of 3 : 1. The former, which evidently carried only the tall
character, are " pure " dominants ; the latter, which carried both tall
and dwarf characters, are " impure " dominants. By observation of
the F3 and subsequent generations, Mendel showed that the pure
dominants and the recessives always bred true, resembling in this
way the original parents. The impure dominants, on the other hand,

HEREDITY AND VARIATION

571

always showed segregation, giving dominants and recessives in the
constant ratio of 3 : I. Since the pure dominants are only half as
numerous as the impure dominants, it follows that the impure domin-
ant on being self-fertilised, produces as offspring pure dominants,
impure dominants and recessives in the proportion of 1 : 2 : :. The
case of only one pair of characters has been considered here ; but
Mendel showed that the rule holds good for all the various pairs of
differentiating characters (seven in all) studied by him ; and since
his time his conclusions have been verified in numerous instances
both in plants and in animals.

A general scheme may be constructed to show the result of crossing
individuals which each bear one of a pair of differentiating character-
If D represent the pure dominant ; if the impure dominant, which
cannot be distinguished from it by appearance, be represented by
(D) ; and if R represent the recessive, then the following will be the
scheme of inheritance (Fig. 435).

D DIDJ [D] R D [D] ID] R R

Fig. 435.

Scheme of inheritance of Dominant. D, and Recessive. R. characters reM.ltmc
from thT casing of individuals which each bear pne of , ,,, fetttet

characters, through three generations /„ /., fy (After Punni tt )

In any sexually reproduced organism the gametes form the link
between successive adult generations. The characters peculiar to
the adult must therefore be represented in their heritable constitution.
In a tall Pea some at least of the gametes, whether male or female,

572 BOTANY OF THE LIVING PLANT

must " carry " the tall character ; for from an impure tall three-
quarters of the offspring are tall. If the race of tall Peas proves
experimentally to be pure for that character, all the gametes must
" carry " that character, and that alone. The union of two such
gametes will give a zygote carrying the gene for tall character only.
Such a zygote is known as a homozygote. But a zygote formed by the
union of two dissimilar gametes — e.g. in the case of Peas where one
•'carries " the tall and the other the dwarf character — is termed a hetero-
zygote. The plant produced from a heterozygote frequently shows the
form of the pure dominant and can be distinguished from it only by the
test of breeding. That, nevertheless, the recessive character is present
in it is shown when such heterozygotes are bred together, one-quarter
of the progeny proving recessive. It is the elements in the gametes cor-
responding to the differentiating characters of the zygotes that are now
known as genes (Johannsen). Pairs of genes corresponding to pairs of
contrasting characters, such as tallness and dwarfness in the Pea, are
called allelomorphs or alleles. The heterozygote is formed by the union
of two dissimilar gametes, and consequently the cells of the individual
into which it grows must contain both allelomorphs. In order to recon-
cile these statements it must be supposed that at some cell-division
previous to the formation of gametes a primitive germ-cell divides into
two dissimilar portions. Instead of the two allelomorphs passing in
association into both daughter-cells, the gene corresponding to the
" dominant " characterpasses into one, and the " recessive " geneinto the
other. From this it follows that every gamete contains one of a pair of
allelomorphs only, i.e. it is pure for that gene. In other words, a simple
heterozygote produces gametes of two kinds, and produces them in equal
numbers. Each gamete is pure for one of each pair of allelomorphs. The
genes are said to segregate in the formation of the gametes.

If we now return to the details of the tetrad-division described
above (pp. 563-567), it is seen that the segregation postulated as the
result of Mendel's experiments is actually effected. Nuclei of two
types are there segregated, each tetrad having two of each type.
From the cells containing these nuclei (tetraspores) the gametes
which share their genes are ultimately derived. Two types of gametes
are thus produced, as the Mendelian experiments require for their
explanation. The results arrived at first by the actual experiments
in crossing, and thereafter explained on the basis of the cytological
details, will be made clear by the diagram (Fig. 436).

The zygotes are represented by squares, the gametes by circles.
Every zygote, being formed by fusion of two gametes, is double and

HEREDITY AND VARIATION

573

contains a pair of allelomorphs corresponding to a pair of contrasting
characters. These genes arc represented by rectangles, the ' es-
sive " gene being shown in black. As applied to the Peas of Mend-
experiment, the original parents (Px) are pure tall and pure dwarf,
the latter being the pollen-parent. In Fx the heterozygote contains
genes for both tallness and dwarf ness, but the plants are all tall like
the tall parent. On producing gametes, these plants yield equal

f?

Gametes

of F?

rY(heterozygbte)

Lr

/ s

\ P

&«.&

©-'

1 — 1

.*«

*^

1 1
1 1

a-

~^

^

(Z=3

9,

i

^

o

t5

^1

Ch-

B^ BL**

R zygotes

Fig. 436.

Scheme illustrating the segregation of characters of a heterozygote in tetrad-
division. See Text. (After Punnett.)

numbers of two kinds, containing "tall" and "dwarf" ger
respectively. Every ovule which contains the -cue for tallness may
be assumed to have an equal chance of being fertilised by a " tall "
or by a " dwarf " pollen-grain, so that " tall " ovules will giv to

equal numbers of homozygous and of heterozygous tails. Similarly
with "dwarf" ovules. Hence of every four zy. in 1.,. on I

average one will be homozygous for talkn nother homozygous

for dwarfness, and the remaining two heterozy These pro-

portions, which correspond to those actually observed in breed-
ing experiments, are represented in the middle column of the F,
zygotes.

574 BOTANY OF THE LIVING PLANT

The fundamental assumption made by Mendel was that of the
" purity of the gametes " ; this still holds good, except in the case of
polyploids (see below, p. 581), where both allelomorphs may occur in
the same gamete. The two subsidiary assumptions, which are also
borne out by observation, are (1) that the different kinds of gametes
are produced in equal numbers ; and (2) that these gametes mate at
random.

Genetics.

For various reasons the far-reaching importance of the simple
principles laid down by Mendel on the basis of his own experiments
was not recognised for more than thirty years. The re-discovery of
his work in 1900 gave a fresh stimulus to the experimental study of
inheritance, and since that date this field of investigation has devel-
oped to a remarkable extent so as to constitute a separate province of
research known as Genetics. Much of this expansion has been due to
the discovery of the chromosomes (unknown in Mendel's day) and to
the increasing precision in our knowledge of the mechanism of nuclear
division and in particular of the details of meiosis. At the present
time genetics and cytology are indissolubly linked, and the mcst
striking advances in the study of inheritance have been achieved by
" cytogenetical " methods. Only a few of the more generally impor-
tant developments in genetical knowledge will be mentioned here.

Examples of Segregation in Hybrids.

(a) Involving One Pair of Characters.

Mendel's classical experiment with tall and dwarf Peas which has been
discussed in detail above, illustrates what happens in a " monohybrid "
cross, i.e. one in which the parents differ in only a single Mendelian character.
In the case described, the tall habit is completely dominant to the dwarf
habit, so that the " hybrid tall " (heterozygous) individuals of the F2 genera-
tion are not visibly distinct from the " pure tall " (homozygous) plants.
Such complete dominance is by no means universal ; in many other cases
the Fx individuals are more or less intermediate, in respect of the differ-
entiating character, between the two parents. Thus, if a true-breeding
crimson-flowered Snapdragon {Antirrhinum majus) is crossed with a true-
breeding ' ivory-flowered " (cream-coloured) individual (Fig. 437), the
Fx plants all have pink flowers. The F2 generation segregates into 25 per
cent, crimson, 25 per cent, ivory, and 50 per cent, pink-flowered progeny.
The crimson and the ivory individuals are homozygous ; the pink-flowered
plants are heterozygous and on being " selfed " segregate just like the Fx
plants. Because of this incomplete dominance of crimson over ivory in the

MKkKIHTY AND VARIATK

Snapdragon, the F2 generation t h u- comprises three visibly distini t forms or
pheuotvpcs. In the Pea, on the contrary, where taHne

over dwarfness, the " pure tails " and the " hybrid tails " arc pfa illy

identical; their different behaviour after being fed," bo*

that they have different genetic constitutions, i.e. that they re]
genotypes.

Fig. 437.

"Mono-hybrid" Segregation. Cross between ia crimapq faa^m

black) and an ivory Snapdragon (shown white). I be offe]
grey) The F2 generation produced by self-fertilising an F, indivulu
crimson-, pink- and ivory- flowered plants, in the proportion indicated m 0*
(From Shull's Heredity by courtesy of the McGraw-Hill Book Co., Incj

(6) Involving Two Pairs of Charade) ,
A " dihybrid " cross is one between two individuals differing in two in-
dependently segregating Mendelian characters; ral men
examined and explained by Mendel in t! . As our first example we BhaU
consider the cross between an " ivory " Snapdragon with normal flowers and
a crimson Snapdragon with " peloric " flowers (Fig. 43») i " *Ottld
plained that, whereas the normal form of tin- flower in Snapdra(
morphic, races exist in which all the flowers are actinomorphie (" |
and thus quite unlike the usual type in appearance. From what has K-. n

B.B. 2 0

576

BOTANY OF THE LIVING PLANT

above (p. 574), we should expect the Fx plant from such a cross to be pink-
flowered. This is actually the case, and in addition the flowers are all

Fig. 438. l»J

" Dihybrid " Segregation. Cross between an Ivory, Normal (1) and a
Crimson, Peloric (2) Snapdragon. (3) =FX (Pink, Normal). 4, 5, 6, 7 :
four out of the six F2 phenotypes (in the diagram no distinction is made
between pink and crimson flowers — see text). (After E. Baur.)

zygomorphic, the normal flower-form being fully dominant to the peloric form.
In F2 six phenotypes appear, namely : Iq the ratios of

Crimson, Normal -

Pink, Normal ...

Crimson, Peloric -

Pink, Peloric -

Ivory, Normal -

Ivory, Peloric =.

i\ths

rVths

rVth

Aths

i^ths

}

3

J-th

1^

HEREDITY AND VARIATION

allelomorphs.

If the genes involved be represented as follows :

A =gene for Crimson flower colour |

a= „ Ivory ,, ,, J

B = ,, Normal flower form ^ „ .

b= „ Peloric „ „ } ^^'""^Phs.

then the genotype of the Ivory Normal parent is aaBB, that of the ( rm
Peloric parent AAhb. If these genes segregate Independently their i
gametes will be aB and Ab, and the genotype of the F, AaBb. A plant of this
constitution will produce four different kinds of gametes, viz. AB, Ab. aB, and
ab, in equal numbers. Random mating between these can take place m
teen different ways. The simplest method of showing the results is by m<
of a " chequerboard " (Fig. 439).

Pollen

AB

Ab

aB

ab

O

eLab

n

(1
AABB

(2

AABb

(3

AaBB

(4
AaBb

(0

Crimson,
Normal

Crimson,
Normal

Pink,
Normal

Pink,
Normal

Ab

(5
AABb

(6
AAbb

(7
AaBb

(8
Aabb

Crimson,
Normal

Crimsoq,
Peloric

Pink,
Normal

Pink,
Peloric

aB

(9
AaBB

(10
AaBb

(11
aaBB

(12
aaBb

Pink,
Normal

Pink,
Normal

Ivory
Normal

Ivory,
Normal

(13

(14

(15

(16

ab

AaBb

Aabb

aaBb

aabb

Pink,
Normal

Pink,
Peloric

Ivory,
Normal

Ivory,
Peloric

Fig. 439. r. 1 •

"Dihvbrid" Segregation. Cross between an Ivory, Normal and a Cnmson. l«
Snapdragon. Chequerboard showing the segregation in the P, generation in detail.

If it is borne in mind that :

Every individual with a double "dose " of A will be Crimson.

„ „ single dose of A will be Pink.
La< kino a will t> 1 \ »ry.

with a double or a single dose ol B will rmaL

lacking B will be Peloric.

>»
it
ft
>>

578 BOTANY OF THE LIVING PLANT

then it will be seen at once that the plants resulting from

Combinations 1, 2 and 5 must be Crimson, Normal (Tsffths)

those from „ 3, 4, 7, 9, 10 and 13 „ Pink, Normal (T%ths)

6 „ Crimson, Peloric (tV^3)

8 and 14 „ Pink, Peloric (yVths)

11, 12 and 15 „ Ivory, Normal (TVths)

16 ,, Ivory, Peloric dVth)

If all red-flowered plants, whether crimson or pink, are grouped together the

ratio is :

9 Red, Normal : 3 Red, Peloric : 3 Ivory, Normal : 1 Ivory, Peloric.

This 9:3:3:1 ratio is the typical F2 ratio for a " dihybrid " cross. It is the neces-
sary consequence of random distribution of two independent pairs of allelo-
morphs by meiosis followed by random mating between the four resultant
types of nuclei. Naturally, a close approach to the theoretical ratio, in this
and in all other cases of segregation, will be realised in practice only when a
sufficiently large F2 generation is grown. In the present instance there are
six F2 phenotypes, because of the incomplete dominance of the crimson char-
acter. If both crimson flower-colour and normal flower-form had been incom-
pletely dominant, there would have been nine phenotypes ; and if both had
been fully dominant, there would have been only four. The number of distinct
genotypes is in any case nine. Four of these, Nos. 1, 6, 11 and 16, are homo-
zygous and when selfed will perpetuate their respective phenotypes ; the
remainder are heterozygous in one or in both characters and, on being selfed,
will show further segregation. The combinations of particular interest are
No. 16, the homozygous Ivory Peloric and No. 1, the homozygous Crimson
Normal. In these it will be seen that the characters of the two parents have been
recombined. This must necessarily arise in F2 whenever the original parents
differ in two or more independent Mendelian characters.

The inheritance of fruit-colour in the Tomato furnishes another example of
the genetical behaviour of plants differing in two pairs of genes, when crossed
with one another. Tomatoes are most commonly bright red with a shiny
surface ; but races are also known with matt red, golden yellow or pale yellow
fruits. The fruit-colour depends upon two pairs of genes. " R " gives rise to
red flesh (mesocarp) ; " r " to yellow flesh. " Y " produces a deep yellow
opaque skin (epicarp) ; " y " a faintly coloured translucent skin. The geno-
type of the ordinary bright red tomato is RRYY (giving red flesh plus deep
yellow skin). An RRyy tomato (red flesh plus pale translucent skin) is matt
red ; an rrYY tomato is golden yellow (yellow flesh plus deep yellow skin) ;
and an rryy tomato is pale yellow (yellow flesh plus pale translucent skin).
This case is of special interest because it illustrates a very wide-spread pheno-
menon, namely the production of a particular phenotype through the inter-
action of the effects of different genes.

In the Tomato, the interaction is of an unusually simple and obvious nature,
the outward appearance of the fruit being determined by the various combina-
tions of two different skin-types with two different flesh-colours. In most
cases, the mechanism of the interaction is far more complex and recondite.
In the Snapdragon a very considerable number of genes are involved in the
production of the wide range of flower-colours found among the cultivated

HEREDITY AND VARIATION 9

races of this plant ; almost every one ol the known shad< olonx arises

through the interaction of two or more of these geni

Two further instances of interaction require brief notice. In the

Chinese Primula (Primula sinensis) the normal (palmatifid Of " palm "
shape depends upon the presence of at least seven principal ii ting

" dominant " genes. The " recessive " genes which have been identified by
corresponding characteristic leaf-shapes are ' fern," ' tongue," ' oak,"
" maple," " claw " and two different " crimping " genes. All th<
discovered in the first instance through their effects upon leaf -form, arc known
to influence the characters of other parts of the plant as well, in parti nlar
the shape of the corolla. A parallel case is that of the Japanese Morning
Glory (Pharbitis Nil). Here the normal leaf-form depends upon eleven
(ten " dominant " and one " recessive "). The gene termed " willow " not
only produces the leaf-shape indicated by the name, but also causes the
cotyledons and corolla-segments to be narrow and makes the flower function-
ally unisexual (male) ; the " maple " gene, besides affecting the form of the
leaf, produces a polypetalous condition in the flower. These two cases ha
been mentioned for two reasons. First, the phenotypic characters involved
are more clearly of taxonomic and of biological significance than the minor
differences in flower-colour, which are not obviously important biologically,
while they are regarded as trivial by taxonomists. Secondly, these ca
illustrate the relation of certain genes to several distinct phenotypical char-
acters which appear to have no direct morphological or physiological connec-
tion with one another ; it is probable that most if not all genes are of this
pleiotropic nature.

Linkage.
So far, Mendelian inheritance has been considered purely in relation to
genes which segregate independently of one another. In terms of the chromo-
some interpretation of inheritance, independently segregating genes are tl.
which are located in different chromosomes. As a corollary, it would seem at
first sight to follow that genes located in the same chromosome should always
be transmitted in association with one another. If this association v.
absolute, then clearly all the known genes of a particular plant should
capable of classification into a number of groups corresponding to the nun
of chromosomes, the members of each group being always transmitted to-
gether, but segregating independently of the members of any other group.
' The actual state of affairs is not so simple as this. When any plant Of animal
is subjected to intensive gene-analysis, the genes are found to fall into a number
of linkage-groups, corresponding to, or at any rate aot exceeding, the char-
acteristic number of chromosomes. All the km m o genes - 4 Make, 6 ff example
(over 300), can be arranged in ten linkage-groups—corresponding to the
(haploid) chromosome number. The members of a links up show

varying degrees of association in inheritance— linka< i1 is called with

each other. In Mendelian experiments linkage reveals itself by the regular
occurrence of marked departures from the theoretical numerical ratio
segregation, certain characters appearing in association with one another
either more or less frequently than accords with normal expectation.
first case of such an aberrant ratio to be detected occurred in the early work ol

58o BOTANY OF THE LIVING PLANT

Bateson and Punnctt on the Sweet Pea. In this plant, purple flower colour is
dominant to red, and erect standard to " hooded " (lax) standard. The experi-
ment which led to the discovery of linkage was a so-called " back-cross "
between a Purple, Erect plant heterozygous for both characters with a Red,
Hooded plant homozygous for both recessive characters. The genotype of
the Purple, Erect parent may be represented as AaBb and that of the Red,
Hooded parent as aabb. The gametes of the former will be of four types, viz.
AB, Ab, aB and ab ; those of the latter can be of one type only, viz. ab. The

O U e D i N Rl » G P x C t < A V

O U «.' 4 1 N « L' SGP X«TF

Fig. 440.
Diagram of two homologous chromosomes, maternal and paternal, indicating that
similar genes are placed at the same level in the length of the chromosome. Compare with
Fig- 433- (From Shull's Heredity, by courtesy of the McGraw-Hill Book Co., Inc.)

next generation would therefore be expected to consist of the four categories
Purple, Erect (AaBb), Purple, Hooded (Aabb), Red, Erect (aaBb) and Red,
Hooded (aabb) in equal numbers. Actually, the parental types (Purple,
Erect and Red, Hooded) were found to be greatly in excess, each forming
49*5 per cent, of the whole, whereas the two re-combinations (Purple, Hooded
and Red, Erect) appeared to the extent of only «5 per cent, of each. This
result was difficult to understand at the time. The view was therefore enter-
tained by geneticists that the phenomena of linkage and crossing-over were
present at gametogenesis. Meanwhile the cytologists were able to show that,
in chiasma-formation (see above, p. 565), a related chromosome mechanism
was, in fact, present. The explanation depends upon two additional assump-
tions, viz. (1) that every gene occupies a definite position (locus) on a chromo-
some and (2) that the genes are arranged along the chromosomes in a linear
series like beads on a string, allelomorphic genes occupying corresponding loci
on the chromosomes of a homologous pair, the " dominant " and " recessive "
genes of each allelomorphic pair thus being " opposite numbers " (see Fig. 440).
These assumptions are supported by adequate cytogenetical evidence which
cannot, however, be discussed here.

The cytological basis of linkage is illustrated diagrammatically in Fig. 441.
Here three pairs of genes are involved A-a, B-b, C-c, the ' dominants "
A, B and C having come from one parent and the " recessives " a, b and c from
the other. A and B are situated close together, whereas the locus of C is far
removed from both. If crossing over occurs more or less at random along
the chromosomes, the chances of it occurring between A-a and B-b are
small, but there is a much greater chance of its happening somewhere between
B-b and C-c. A and B, and similarly a and b, will therefore tend to pass to
the gametes in association with one another. Crossing over may not take
between B-b and C-c either ; in that case the gametes produced will be
ABC and abc. When, however, crossing over does happen in this region, the
gametes will be ABC, ABc, abC and abc, the proportion of ABc and abC
gametes — the " cross-over types " — depending on the frequency of crossing
over between B-b and C-c. The further apart two genes are on a chromosome,
the greater will be the frequency of crossing over ; conversely, the closer the genes
lie together on the chromosome, the more often will they be transmitted to the same

HEREDITY AND VARIATION

gametes, i.e. the closer the lit ige be.

loci of two genes are sufficiently far apart foi ... -. it. or i

over to take place between them, they will i .th, u

A
B

A a

Bb

Co
a

B

rr\

A

B

«- 1

a-
b

3

rb

Ctjc

C*k

>

c

f S

Fig. 441.

The cytological basis of linkage (diagrammatic) a, two somatic chromosomes
carrying the genes A, B, C and a, b, c, respectively ; each chromosome
two chromatids, b, During the close pairing of the chromosomes in mekxts, bn
occur in the chromatids, which join up with different partners, i.e. segments oi the
chromatids become interchanged, c, At anaphase of the first division the '
chromosomes with their interchanged segments are distributed to different nuclei,
and at anaphase of the second division each of the four daughter cbrooa
(d, e, f, g) is distributed to a different tetraspore. d and g are non-crc> md

e and / cross -over types. The zygotes from gametes of those four I
independent inheritance for gene C and linkage between A and B. From Crane .mil
Lawrence.)

of the fact that they lie on the same chromosome. By means of intensive
gene-analysis leading to the establishment of linkage groups, it is poasibl
construct " chromosome-maps " indicating the relative positions of the known
genes in the several chromosomes.

Polyploidy.

While gene-mutation — a change in the constitution of a gene — is the most
usual source of heritable variation, there an ral Other ways in which the

hereditary material may undergo alteration. A portion of a chromosome D
break loose and (a) remain separate, or {b) rejoin in the inverted positj
(c) become attached to another chromosome or i combine with other
detached chromosome-segments, sometimes to form .1 ring. Such " chi
some-rearrangements " are shown diagrammatically in Fig. 442. The peculiar
behaviour of many Evening Primroses [Oenothera spp.), which, although
complex hybrids, breed practically true, is connected \\ith the formation of
chromosome-rings. Chromosome-rearrangements, though <>f frequent occur-
rence, will not be considered further here. An important ..
polyploidy.

582

BOTANY OF THE LIVING PLANT

A polyploid organism is one which possesses more than two " sets " of
chromosomes in its somatic cells. Polyploids are very common among culti-
vated plants. The chromosome complements of some of the cultivated forms
of Rubus are shown in Fig. 443. The Raspberry (the variety figured is " Super-
lative ") is a normal diploid with seven chromosome pairs (two " sets ") .

A
B

C
D

a

B

D

a

B

H

A B

A
D

B
C

D C

b c d e f g

Fig. 442.

Diagram illustrating types of chromosome rearrangement. The letters indicate
successive segments of the chromosomes, a, normal chromosome ; b, fragmentation ;
c, inversion ; d, deletion ; e, a pair of normal chromosomes ; /, reciprocal trans-
location of segments C and A, resulting in g, " ring-formation " at meiosis. (From
Crane and Lawrence.)

the " Mahdi " is triploid, with three sets ; the Veitchberry tetr aphid, with
four sets ; the Loganberry hexaploid, with six sets ; and the Laxtonberry
heptaploid, with seven sets.

/

Or -U

Superlative2n=l4 Mahdi 2n=2l

Vfe!tchby2a=28

1

*A

%

Loganberry 2n =42 Laxtonberry 2n- 49

Fig. 443.

Somatic chromosome complements of some diploid and polyploid
Rubi. For explanation, see text. (From Crane and Lawrence. After
Crane and Darlington.)

Polyploids can arise in several different ways. For example (1) the chromo-
some number of a somatic cell may become doubled (Fig. 444, a, d). This type
of polyploidy can be induced artificially in certain plants, e.g. in the Tomato
by encouraging the development of adventitious buds from a wound-callus.

HEREDITY AND VARIATION

5«3

(2) The chromosome number may be doubled in aba

behaviour at meiosis. Thus at meiosis in the I , g< m th.

Rapkanus x Brassica, the Radish and Cabbage chromosomes cannot j

owing to the wide difference in their genie constitution. quenti

first metaphase follows an abnormal course, and sometimes all the chromo-
somes come to lie in one daughter-cell ; .it th< rod division, two ' t< •
spores " result, each containing a double complement oi 1 hromosorm

So much for the cytolo-K ,il origin of polyploids. Polyploids may, Km
be classified from another point of view, Auto-p><lvploids arc derived from a
single parent species or race ; in their cells, therefore, tin- 1 hromosome com-
plements are all similar (Fig. 444, d), e.g. the tetraploid Tomato In allo-
polyploids the chromosome sets are not all alike ; allopolyploidy is usually

UUb

1

Fig. 444.
Diagram showing the origin and constitution of the chromosome
complements of auto- and allo-tetraploids. a and b, diploid spa ies ;
c, diploid hybrid from ax 6; d, auto-tetraploid derived from doub-
ling of the chromosome sets in a ; e, allo-tetraploid derived from
doubling of the chromosome sets in c. (From Crane and Lawrence.)

a result of artificial or natural hybridisation (Fig. 444, e). One of the I
known of allo-polyploids is Primula Kewensis, which arose from an artificial
cross between P. floribunda and P. vcrticillata. Both of th<
9 pairs of chromosomes, and the original (diploid) hybrid also had 9 p
and was almost completely sterile. Later this gave ri usly to a

tetraploid race having two sets oi floribunda and two ticillata chromo-

somes, sterile with both the original parents but fertile with its own t
and breeding nearly true. An allo-polyploid which has arisen in nature is the
Rice-grass, Spartina Townsendii, which was first noted on Southampton Water
in 1870, since when it has spread rapidly along the South « I and has even
crossed the Channel to France. It is believed to have originated h©m B natural
cross between two older species, viz. S. strict* -known as a native of Britain
for at least 300 years— and S. alterni flora, a m. eni introduction from

North America. The (somatic) chromosome numb.
70 in 5. alter niflora; and I26(= 2 ■ | jS I 35)) in >'■ *"o Hi. The i

breeds true and is sterile with both its presumptive parents. It is of intc

584 BOTANY OF THE LIVING PLANT

that, where it has come into competition with either 5. stricta or S. alterniflora,
the new species has gained the upper hand.

Another example of polyploidy in Nature is provided by Biscutella laevigata,
a Crucifer native to Central Europe and Italy, which has some races with 18
chromosomes (diploid) and others with 36 (tetraploid) . The tetraploid races
occur over a wide and continuous geographical area, ranging from the Alps
to the Balkans. The diploid races, on the other hand, occupy several com-
paratively small and discontinuous areas in the valley-systems of the Rhine
and of some other large rivers. The explanation suggested is that the diploid
races are representatives of an ancient (inter-glacial or pre-glacial) type, while
the tetraploid races are post-glacial immigrants. A similar state of affairs has
been noted in the case of several North American species of Tradescantia.

That polyploidy las played a considerable part in producing new species is
indicated by a more general line of evidence. The species of Angiosperms for
which the chromosome numbers are known (several thousands) belong to
about 500 genera representing a diversity of Families ; roughly 16 per cent,
of these genera are made up of species forming polyploid series. Thus in
various species of Chrysanthemum the somatic numbers are 18, 36, 54, 72 and
90, i.e. the basic number is 9, and the condition of the several species ranges
from diploid to decaploid. In Solarium the basic number is 12, and the series
ranges from 24 to 144. In Papaver, finally, two series can be distinguished
with 7 and 11 as their respective basic numbers.

The general importance of polyploidy has been summarised as follows by
Crane and Lawrence : it increases the effective range of hybridisation ; it
combines the products of specific differentiation within a single (new) species ;
and it increases the potential range of species variation. There may also be an
increase in vegetative vigour, frost-resistance, and resistance to pathogens.
In this connection, it is significant that so many valuable cultivated plants,
for example Wheat and Oat, are polyploids.

Genetics and Evolution.

In the foregoing condensed and therefore necessarily somewhat
dogmatic account only a few of the more important aspects of genetics
have been considered. Among the interesting topics which have been
passed over as unsuitable for inclusion in an elementary discussion
are : the nature and inheritance of sex ; hybrid vigour ; the definition
of a " species " ; the origin and biological significance of dominance ;
the causes of mutation ; the nature of genes ; and the mechanism
of the action of genes upon developmental processes. The very
simple cases of Mendelian inheritance and of polyploidy which have
been considered in some detail may, nevertheless, provide a basis for
some general reflections upon the relation of genetical data to the
wider problem of evolution.

The outstanding features of Mendelian phenomena are two :
(1) Inheritance is seen to he particulate, the characters of the parents
being transmitted individually to the offspring by genes situated at

HEREDITY AND VARIATION

definite loci in the chromosomes. There is no evidence of that " blend-
ing inheritance" which pre-Mendelian biolo 1 to be the
rule. Although • cytoplasmic inheritam anno! be ruled oul
non-existent — indeed, there is evidence thai in some instan< ea it m
important— it is probable that the part that it plays in heredity ia sub-
ordinate to the action of the nuclear genes. It should be mentioned tl
while, for technical reasons, Mendelian inheritance has been studi
chiefly in higher organisms, it has been observed also anion- M
Fungi and Algae (including unicellular types such as Chlamydomonas).

(2) Hybridisation followed by segregation has been an important
source of new forms (re-combinations). Hybridisation experiments,
again for technical reasons, are commonly carried out with ra
differing in comparatively few genes ; but what is known about
" wide crosses " (between widely differing species or between distinct
genera) suggests that the observed divergences from straightforward
Mendelian segregation are due partly to the large number of ge;
involved, and partly to secondary complications such as polyploidy,
chromosome re-arrangement and indirect effects of cytoplasmic
differences. ,

Mutation is the basis of heritable variation and thus of evolution.
" Heredity is essentially a conservative process. Evolution is possible
only because heredity is counteracted by another force opposite in
effect, namely mutation " (Dobzhansky). Strictly, the term mutation
should be applied only to gene-mutation, i.e. to a change in the con-
stitution of a gene ; but it is often more convenient to use the term
in a wider sense so as to include such changes as chromosome-rearran
ment and polyploidy, which may likewise give rise to heritable varia-
tions.

Nothing definite can be predicated as yet regarding the rate of origin
of new species by gene-mutation alone. It is on the other hand clear
that new species can arise by a sudden or " cataclysmic " method,
viz., through polyploidy following upon hybridisation. So far, poly-
ploidy has been observed only within certain groups of organisms,
mainly among plants ; but within these limits there can be little doubt
as to its importance as a subsidiary method of specie- production.

The persistence of a new form, however produced, depends 0-

tially on the action of some mechanism of isolation, which prevents
the new type from disappearing through free inter-crossing with pre-
existing types. Geographical separation and any structural featui
or physiological conditions (cross-incompatibility or hybrid-sterility)
that prevent fertile union between different species, provide such

556 BOTANY OF THE LIVING PLANT

mechanisms. The subject is a difficult one and has hitherto received
insufficient attention.

The hypothesis of Natural Selection, which was one of the pillars
of the original Darwinian theory, is quite consistent with modern
genetical views, provided that it is recognised, first, that selection
operates only within genetically mixed populations and has no effect
on those which are genetically uniform (" pure lines ") ; and, secondly,
that the hereditary variations that provide the material upon which Natural
Selection can operate arise by mutation, i.e. that it is mutants and not
modifications [see above, p. 569) which undergo selection.

There remains the vexed question of adaptation. It can hardly be
disputed that a high degree of adaptation to a particular environment
is one of the most striking properties of living things. On the other
hand, it is impossible to believe that every individual mutation is in
itself adaptive ; indeed there is abundant evidence to the contrary.
In addition it is probable that some heritable characters are " neutral,"
in that they do not affect the chances of survival of the individual
one way or the other, though caution is indicated in this connec-
tion in view of the pleiotropic action of many genes. Adaptation
may be thought of as arising by an " integration," into genotypes
possessing a definite survival value, of a succession of mutational steps
which are individually non-adaptive. It should be realised that the
number of possible gene-combinations within a species greatly exceeds
the number of individuals of that species, so that there is ample scope
for the origin of adaptations by a process of " trial and error," especi-
ally as the same mutation may occur on many separate occasions.
The " opportunism " of adaptation in certain evolutionary trends
(e.g. in the evolution of the pollen-tube from a rhizoid to a gamete-
carrier) is suggestive. It must however be admitted that it is not
possible at present to offer a satisfactory explanation of the origin of
adaptation in general.

In conclusion, it may be worth while to quote a summary of Sewell
Wright's views on Evolution as envisaged by a geneticist : " The
most general conclusion is that evolution depends upon a certain
balance among its factors. There must be gene mutation, but an
excessive rate gives an array of freaks, not evolution ; there must be
selection, but too severe a process destroys the field of variability,
and thus the basis for further advance ; prevalence of local inbreeding
within a species has extremely important evolutionary consequences,
but too close inbreeding leads merely to extinction. A certain amount
of cross-breeding is favourable, but not too much. In this dependence

HEREDITY AND VARIATION 587

on the balance the species is like .1 living organism. At .ill I

organization life depends on the maintenance ol
among its factors." Further it may he noted that : " If the population
size is small, favourable mutations may actually be lost, and evolution-
ary changes may proceed against the pressure and diro tion oi -«-|.

tion. In large populations, however, selection will act to an amount
approximately proportional to its intensity."

Irregular Propagation.

Some plants may eliminate normal sexual propagation, substituting for it
in various ways other means of increase in numbers. Thus they forego the
advantages which follow from sexuality, but not infrequently they secure
greater certainty of propagation. The commonest cases are where vegetative
propagation replaces partially or completely the reproduction by seed : a
condition common in Nature, and seen in special degree in cultivated plants
such as the Potato, Jerusalem Artichoke, Sugar-Cane, Banana, and Pine-
Apple (Chapter XIII.). In the viviparous habit of Alpine Plants the substi-
tution of vegetative buds for flowers is probably a biological accommodation
to the shortness of the Alpine summer. In other cases there may be an
apparent maturing of good seeds, though the embryos within them arc
not sexually produced. Thus in Funkia, Coelebogyne, and others, numerous
embryos arise by adventitious budding from the tissue of the nucellus, and they
project like normally produced embryos into the embryo-sac. The nucellar
tissue in such cases was already diploid : so that here there is neither reduc-
tion nor sexual fusion. They are peculiar examples of sporophytic budding-
But as they involve a loss of sexuality, they may be described under the
general term of Apogamy : or better, of Apowixis, by which is meant quite
generally the absence of syngamy where it would normally occur.

Somewhat similar states, which however involve the contents of the embi
sac, are found in Alchemilla, Thalictrum, Taraxacum, and Hieracium. In them
embryos may be formed from an ovum without fertilisation. But here the i
itself has been found to be diploid, for reduction had been omitted in the
development of the embryo-sac. Technically this has been described M
*' somatic parthenogenesis," which implies that the embryo springs from the
ovum but the ovum was itself diploid. A like condition h
Marsilia Drummondi, and in Athyrium filix-foemina, var. dan In ^ich

cases again no fertilisation is necessary to arrive at the diploid state. A n
rare condition is that where an egg that is really haploid develops as though rt
had been fertilised. This is rare in Seed-Plants, but it has been
It has also been found to occur in Chara crintia.

Contrasted with these cases of apomixis. where no sexual fanon
are various conditions which may be rank- **•** * > ' ■

nuclear fusion is seen, with consequences like those following on normal
syngamy But the nuclei involved are not produced in the normal m
Examples have already been described for N*pkrodHum ■ ■ '•

dactvlum (p. 508. Fig."4o3) I and in thai unusual x^
which occurs in the formation of the fruit In the Uregineae (p. 43*. ' '- *W-

588 BOTANY OF THE LIVING PLANT

In the latter the association is ultimately followed by nuclear fusion,
which thus, though deferred, takes its place in the cycle. The fact that
similar nuclear fusions precede the formation of ascospores (p. 419) and of
basidiospores (p. 432) suggests that such methods are probably wide-spread
among Fungi. The nuclei involved in such cases spring not from any widely
distinct sources, but from cells closely related in position and in origin. It is
remarkable that these and other irregularities in the sexual cycle are found
commonly among Plants represented by very numerous closely related forms.
If their numerous species and varieties have resulted from mutation, then it
would appear that excessive mutation may have had some influence in
producing those irregularities. In organisms which mutate freely, the
Mendelian sifting out, and preservation of each heritable mutation, would be
a less vital matter than it is in more stable forms. This may in some degree
account for such deviations from the normal sexual reproduction as have just
been described.

CHAPTER XXXVI.

THE RELATION OF SIZE AND FORM IN PLANTS.

The Obconical Form.

Living Plants have now been considered in their various aspects of
Form, Structure, Function, and Propagation. But there is still
another point of view which emerges as the result of comparative
measurement. A relation has thus been found to exist between Form
and actual Size ; but hitherto its
study has been almost wholly omitted
from the general discussion of the
plant-body. Moreover, there has
been a very frequent neglect of uni-
formity in scale of the illustrations
used in comparison, which has tended
to obscure the issue. But once the
relation between Size and Form is
brought into view on a uniform scale
of measurement for each example,
many observed data acquire new
meanings, both functional and evol-
utionary.

The discussion of this relation
starts from the fact that green
plants are accumulators of material
gained by photosynthesis. They are
in no equivalent degree expenders.
Hence they naturally work to a
favourable balance of material. As the surplus of supply is u
up in the growth of a plant of primary development, which has
polarity in relation to some fixed -tame, its outline tends to expand
from the base upwards, and the plant takes a more or less obconical
form, with the vertex or cusp directed downwards. A simple example
of this type is seen in the young plant of Fucus (Fig. 445)- I* :

589

Fin. 4
Young spon .-linn "f 1 ' ncus MttCttJOMH in

longitudinal and •

Rostafinsld.) The form is obconical, with
circular transverse outline : it is only lata
that it 1 ■• laterally oompreawd up-

ward-. (Sei

590

BOTANY OF THE LIVING PLANT

frequent feature in sporelings and embryos, and it also appears in
many adult plants. But the result is unpractical, both mechanically
in regard to stability, and physiologically in regard to the absorption
and transmission of materials. Plants so constructed present an ever
more insistent problem of well-being as their size increases.

Conical and Obconical Form Contrasted.

The prevalence of this type in plants of primary construction is

apt to be lost sight of by the student, owing to the customary practice

of presenting first to him such relatively advanced examples

as Dicotyledons and Gymnosperms. His mind is thus familiarised

with the idea of conical form tapering upwards, as seen in any forest

tree : and especially in the Coniferae, such as Sequoia. It is also

conveyed by descriptions of cambial thickening (Chap. IV.), or by

A BCD

r

Fig. 446.

Diagrams not drawn to uniform scale, showing various methods of development
of plants of primary obconical construction as their size increases. A = Plant of
Maize or of Screw Pine, with prop-roots. B =Stem of Cocos or Oreodoxa showing the
widely obconical base with attached roots, followed by the cylindrical trunk.
C = Dracaena, with obconical primary development, supported by secondary thicken-
ing. D =Tree Fern, with its obconical stem supported by a massive sheath of roots.

comparison of sections of woody stems taken successively from
below: or again by diagrams like those of Fig. 37. But timber trees
are products of relatively advanced evolution, involving secondary,
that is, cambial activity. The student should therefore distinguish
clearly between such conical stems, and the obconical contours that
are apt to follow on primary development. Examples of these, which
have retained that primary plan unaltered to the adult state, are
shown diagrammatically for well-known plants in Fig. 446. The
rubrics suggest how each has made the best of this seemingly dis-
advantageous scheme as its size increases. There are, in point of fact,
two outstanding types of organisation of dendroid plants : the one
having an obconical primary stem, which bears an enlarging distal

THE RELATION IX SIZE AND FORM y\ PLANTS <

bud .md branches only occasionally, as in Ferns and Palms : th<
takes a conical form as a consequence oi cambial thickening Fig. 5).
Here there is profuse branching with numerous relatively small

distal buds, as in forest trees. When young the two types do not
seem to differ essentially from one another : but as greater dimensii
are reached they tend to diverge, not only in external form but also
internally.

Principle of Similarity.

Galileo's Principle of Similarity applies to all structures, gri
and small, living or not living. In accordance with it, if the form of
an enlarging solid body remains unaltered, its bulk increases as the
cube, but its surface only as the square of the linear dimension 3.
In living organisms it is through limiting surfaces, whether external
or internal, that physiological interchange is effected. It may be
assumed that, other things being equal, the amount of substance
transferred will be proportional to the area involved. Hence the
importance of the principle here applied to obconical plants of
primary development : for provided any surface of transit be con-
tinuous, it would increase at a lower ratio than the bulk that it
encloses ; and if its character remains unchanged during growth
there would be a constant approach to a point of functional in-
efficiency. A remedy may, however, be found in change of form as
the growth proceeds : and this is actually to be seen in most spore-
lings. Any elaboration of so simple a contour as the inverted cone would
tend to uphold the surface-volume ratio.

Examples and Illustration

The mouldings that occur in individual development of the primary plant-
body, as it advances onwards from the spore or the zygote, may appear
either externally or internally. A few examples will show how, by-elaboration
of form, certain difficulties that arise from increasing si/e have actually been
met in Nature. Obconical sporelings present relatively simple methi
For instance in the green Alga, FritschieUa (Fig. .\\:) the germinal filament
first enlarges upwards, and then branches profusely, forming .1 <list.il brush.
The bulk of each thread equals approximately th.it .>t the first, though
their collective bulk is great. By this elaboration <>t form the propor-
tion of surface-exposure of the whole tends to be maintained. A fern
prothallus (Fig. 391), or a young Fucus (Figs. 2 )), widens into a thin

expanse with flattened sides : this tends by a different method to uphold
the surface-volume ratio. In the tissue of a young growing BporeKng of
Riccia, clefts appear in the enlarging upward growth, and these open
B.B. 2 1'

592

BOTANY OF THE LIVING PLANT

the surface : thus the enlarging thallus is ventilated, and the cell-surface
facing the atmosphere tends to be maintained internally (Fig. 448). In
Mosses the obconically enlarging protonemal buds become leafy upwards
(Figs. 355. 356) : here also the thin leaves as external outgrowths tend to
uphold the surface-volume ratio, but by a method the converse of internal
ventilation. These simple examples suggest four distinct methods for gaining

.sec

■pr

JV . « m m

Fig. 447.

Fritschiella tuberosa. A small mature
plant with a single rhizoid. c/=cluster; pr=
primary; sec = secondary branch. ( x 350.)
(After Iyengar.)

Fig. 448.

Riccia trichocarpa. Young sporeling showing
spore and germ-tube, k ; rhizoid, r ; and the
thallus enlarging upwards with sunken apex,
and intercellular spaees beginning to develop.
( x 85.) (After Campbell.)

that end in obconical sporelings, as their size increases. They are all taken
from gametophytes. But both of the alternating phases of the life-cycle
are subject to the same demand. For instance, in the sporeling of Poly podium
(Fig, 140) the stem is obconical : it increases to about four times its original
diameter before its seventh leaf is reached. But here again the surface-
vonime ratio tends to be upheld by foliar development, combined with in-
ternal ventilation. In point of fact these four methods may be variously
combined among themselves in the organisation of vegetation generally.
For instance, flattening and internal ventilation take part in the construction
of any ordinary leaf-blade; and branching also in the widest sense may be

THE RELATION OF SIZE AND FORM IN PLANTS 593

involved, giving the condition seen in compound leaves and in leafy shoots
at large. When they are associated also with apical growth and lobation
of various orders, they lead to that due balance of surface and bulk which
has made the realisation of diffuse land-vegetation possible, up to the limit
of mechanical resistance (see Chapter X.).

Statement by Measurement.

To demonstrate by comparison to uniform scale that there is a
real though not an exact relation between Size and Form, it will be
convenient to turn to definite internal tracts, and particularly to the

&

Fig. 449.

Outlines of the xylem of Coenopterid steles, all drawn to the same scale, to show their
relative sizes ( x 5). 1 = Botryopteris cylindrica. 2=Ankyropteris Grayi. 3=ditto,
larger. 4=Asterochlaena laxa. The elaborateness of outline increases with the size.

conducting tissues of plants of primary construction. In the fossil
Fern Botryopteris the small protostele is cylindrical (Fig. 375), and
circular in transverse section. But in those of larger size its
section is stellate, with projecting flanges, the surfaces between them

594

BOTANY OF THE LIVING PLANT

being hollowed. The larger the stele the deeper the involutions,
and even the number of flanges increases with the size of the whole.
This is shown in Fig. 449, where all are drawn to the same scale.
With differences of detail similar results may be obtained from the
xylem in leafy shoots of Lycopodium, in the leafless rhizomes of
Psilotum, and again in roots, particularly in those of Monocotyledons
where cambial thickening is absent (Fig. 58). The larger the root the
more numerous are the flanges of protoxylem (p. 84). Even in the
unicellular Alga, Closterium, where there are in each cell two flanged
chloroplasts, the number of flanges varies according to the size of
the cell in different species. The measurements taken from such
varied objects gain in cogency by tabulation, as they are presented
below for the leafless rhizomes of Psilotum, the leafy stems of Lyco-
podium, the roots of Colocasia, and for optical sections of the cells
of Closterium.

TABLE SHOWING RELATION OF SIZE TO STRUCTURE IN LEAFLESS RHIZOMES. LEAFY
SHOOTS. ROOTS. AND IN CELLS OF DESMIDS.

NAME

DIAMETER OF
STELE IN MM.

NUMBER
OF RAYS

APPROXIMATE
RATIO

Psilotum : Sections of rhizomes

from Bertrand's figures,
Arch. Bot. du Nord, Lille. 1881

130.
151.
175.
161.
162.

20
32
40
60
70

2
3

4

5

7

10.0
103
10.0
12.0
10.0

Lycopodium : sections of stems, as
in " Size and Form," Fig. 7, after ■
Ward law

' 3.
4.
5.
6.
7.

19
26
38
66
88

6

7
11
19
22

3.2
3.7
3.5
3.5
4.0

Colocasia : sections of roots, as in

" Size and Form," Fig. 53, after

Ward law

1.

2.

3.

.4.

1 5.

5

8

19

27

83

5

9

14

21

38

1.0
.9
1.4
1.3
2.2

SECTIONS OF CELLS OF SPECIES OF
CLOSTERIUM

DIAM. OF CELL
IN MM. (x810)

NUMBER OF

RAYS OF

CHLOROPLAST

APPROXIMATE
RATIO

C. D/anae (Fig. 37)
C. juncidium (Fig. 48)
C. angustatum (Fig. 36) -
C. strio/atum (Fig. 42) -
C. Lunula (Fig. 4)

10
12

20
23
53

6

7

10

13

15

1.6
1.7
2.0
1.8
3.5

These figures for Closterium are based on drawings by Miss Carter.
Ann. of Botany, XXXIII, pi. XIV, XV.

THE RELATION OF SIZE AND FORM IN PLANTS 595

These varied examples show how uniformly increase in Size is
accompanied by elaboration of Form. As they stand, the uniformity
of these results points to some definite relation between Size and
Form. The degree of constancy shown in the ratios is not a matter
of chance, whatever may be its physiological significance. On the
other hand the results have not been found referable to any external
agency : they are certainly not dependent on the influence of appen-
dages, for in three of the instances quoted there are none. The
degree of constancy in the ratios in the last column is not exact :
nevertheless it suggests some inner influence that controls elaboration
of form in accordance with increase of size in each of the several cases.

Problem of Supply to Distal Buds of Vascular Plants.

In plants of advanced but primary organisation, obconical growth leads
towards enlargement of the distal bud. This is often associated with a
marked restriction, diminution, or even absence of branching, as in many
Ferns and Palms (Fig. 446). In these, as development proceeds, the
axis may expand to great bulk. Thus, in Amphicosmia, a well-grown

Fig. 450.

Apex of stem of a large plant of Amphicosmia Walkerae, shown natural size :
with the arrangement of the leaves in trimerous whorls upon the flattened apical
plateau.

Fern, but not of the largest size, the flattened apex, which may bear
four cycles of leaf-primordia upon it, was found to be fully four centi-
metres in diameter (Fig. 450). Its bulk is enormous as compared with
the buds of forest trees, such as the Beech (Fig. 451), where the growing

596

BOTANY OF THE LIVING PLANT

point of a twig lies buried as a minute speck within the bud itself,
which is less than three millimetres in diameter. But the Beech has

the advantage of cambial thickening, which
Ferns and Palms have not. Hence the pro-
blem of provision of channels of supply to
stem and leaves differs in the two types : the
one shows " exogenous " growth, after the
old terminology : the other is " endogenous ".
While the former depends upon secondary
cambial increase and accretion of successive
layers of conducting tissue in the woody
trunk, the latter shows distal expansion of the
primary stele. The first of these modes of
increase has been described at length on pp.
55 to 68 : the latter is less clearly under-
stood, and it will be reconsidered here, as
it is seen in Ferns, and in Palms.

The general structure of an adult Palm stem,

as seen in transverse and longitudinal sections,

Winter-buds of the Beech (Fagus with its distended and ill-defined stele, and

S^jfSSl^g^SS numerous vascular strands scattered through the

proportion of exposed surface in pith, leads out to the successive leaf-bases, as

the defoliated state. Natural size. , ., , . ™, ., , « -i—

(After Strasburger.) described m most Textbooks (see p. 50, rigs. 29,

30). But few botanists who are familiar with the

structure of an adult Palm or Maize plant could give a detailed account of

the development of their vascular systems from the seedling upwards : few

also will have themselves dissected the adult buds, or have realised that

Fig. 452.

Loxsoma Cunninghami. Diagram showing the form of the medullated stele at
a node of the rhizome, ss = solenostele ; U= departing leaf- trace ; lg = leaf -gap.
The arrow points towards the apex of the rhizome. (After Gwynne-Vaughan.)

greater or less expansion of the primary stele in Monocotyledons stands in
a direct relationship to formative activities in the enlarging distal bud, and
its developing leaves. The stele, moreover, is responsible for distributing

THE RELATION OF SIZE AND FORM IN PLANTS 597

the supply of nutrients required in the growing region. All this is, however,
achieved in Palms in the absence of secondary accretion. As such primary
development is more readily apparent in Ferns than in Monocotyledons, the
former are selected here to illustrate steps of stelar expansion in relation
to the enlargement of the terminal bud.

The Expanding Stele in Ferns.

The obconical enlargement habitual in the stems of sporeling Ferns is clearly
seen in Polypodium vulgare (p. 208, Fig. 140). Not only does the axis enlarge
upwards, but the stele likewise expands. Comparison shows the steps usual in
them to be through medullation to solenostely (Fig. 452), and by overlapping
of closely disposed leaf-gaps to dictyostely (Fig. 376A). This has been described

Fig. 453.

Series of transverse sections of the stem of Pteris (Litobrochia) podophylla, all drawn
to the same scale, so as to show the great increase in size, and the progressive complexity
of the conducting tracts (shaded) as the stem expands conically upwards : also the
successively enlarging leaf-bases attached laterally in each. ( x 4.)

briefly on pp. 485-7. It is in fact general in Leptosporangiate Ferns. For full
details it must suffice to refer to Ferns, Camb. Press, Vol. I., Chapters VII.,
VIII. : Primitive Land Plants, Chapter XVII. and Size and Form, Chapter
VIII. : where references are given to the extensive literature on the subject.
This short precis of facts, together with the illustrations which follow, will
serve as a basis for discussion of the relation of obconical growth to the
expanding primary stele of the adult Fern.

In the Ferns these elaborations all start from the simple protostele, which
expands upwards with the enlarging shoot. A central pith is then formed, at

598 BOTANY OF THE LIVING PLANT

first small but often attaining considerable size in the adult (Figs. 140, 374). In
all advanced Ferns the stelar tissue is shut off from the pith by an inner
endodermis, giving the cylindrical structure known as solenostelic (Fig. 452).
Further the tube is apt to be interrupted by foliar and other gaps, so that
it appears as a network of meristeles surrounding the column of pith. This
allows of ventilation inwards, a matter of some importance where the column
is large, as it is in the Shield Fern (Fig. 374). In many Tree Ferns, such as
Dicksonia, the pith may measure several inches in diameter. At first it consists
of relatively inert storage parenchyma, without any conducting tissue to aid
transit within its bulk. But this physiological difficulty has been met in many
large Ferns by the formation of accessory conducting tracts that are present
in the pith. An example of this has already been seen in the inner meristeles
of the Bracken (Fig. 376). These medullary strands often take the form
of concentric rings, as in Pteris podophylla (Fig. 453) • Their number increases
with the size of the stem. In the largest (6) three complete rings are seen
and the inception of a fourth : and in extreme cases as many as a dozen
may be found, all fitting concentrically within one another. This has been
seen in the fossil genus Psaronius. In others, however, as in Platycerium,
isolated strands appear dotted over the transverse section, as in the stems of
Monocotyledons (Figs. 29, 30).

In the very diverse vascular types of Ferns and Monocotyledons the con-
ducting system of each vegetative shoot forms a connected whole, and each is
of primary origin only. The stelar expansion of each is correlative with the
obconical enlargement towards the apical bud. The stolons of Nephrolepis
give a clear demonstration of the relation of stelar complexity to size. Since
both stolons and tubers are leafless, the modifications of form of the vascular
strands are not affected by leaf-insertions, but are determined in the growing

bud itself.

Where the size of the stolons in Nephrolepis is small the form is cylindrical,
and it is traversed by a solid stelar core. But where the stolon itself swells
distally into a pear-shaped tuber the stele may become variously fluted, or even
disintegrated, as it is in Nephrolepis cordifolia (Fig. 454, A and B). At its base
the solid stelar core expands first into a solenostele, and later disintegrates
into a ring of meristeles (C). But as it passes upwards into the conical tip it
gradually re-constitutes itself ; and the protostelic state is resumed (Fig.
454, A). The conclusion from such facts is that these converse changes,
basal and apical, are related locally to the dimensions of the tuber. Such
facts accord with what is seen in the leafy shoots of Ferns at large (Figs. 374,
375, 376, 376.^, 455). Both of these phenomena are presented by primary
tissues defined at the growing point itself.

In a vertical section of the sporeling of Polypodium vulgare (p. 208, Fig. 140)
the apical region appears broad and flattened, and the leaves arising from it
are solitary. But it is a relatively small plant, its apical region being in dia-
meter about ^„ th that of the adult Tree Fern shown in surface view in Fig. 450.
Here the young leaves are crowded round the growing point, each requiring its
own quota of nourishment. The problem of supplying all the leaf-primordia
may be held as particularly acute, at the time when a new succession of them
is developing. In various degrees they will call for supplies from the expand-
ing shoot through each leaf-base. Actually very little is known of the mode of

THE RELATION OF SIZE AND FORM IN PLANTS 599

distribution of nutrients to the growing region from below. But structurally
we see in many Ferns how distribution of nutrients may be helped by accessory
strands which traverse the column of the pith, either in the form of a single
protostele, a solenostele, or a concentric group of them {Psaronins) : or again
there may be a number of separate medullary strands. In point of fact, there
is considerable variety in these medullary systems. But they are all primary,
differing fundamentally from the secondary growths of cambial origin, which are
characteristic of " exogenous " plants.

Fig. 454.

Nephrolepis cordifolia. A = stolon bearing a tuber, in which the protostele breaks
up into a cylindrical network, contracting again at the apex. R =root (after Sahni).
B = transverse section of a protostelic stolon ( x 5). C = transverse section of tuber,
showing ring of meristeles each limited by endodermis. Diameter of stolon, i-6 mm.
Diameter of tuber, n mm. (From Size and Form, p. 132.)

The functional analogy between such medullary systems as those of Ferns
and the vascular strands scattered through the pith of the expanded stele of
Palms, Maize, or Sugar Cane, appears obvious, though their morphological
origin is not the same : nor is their physiological effectiveness identical.
Nevertheless, both follow on stelar expansion. Both types might be included
under the old term "' endogenous " : but since the plants in which they appear
have no near affinity, and arise along quite distinct evolutionary trends,
their comparison cannot be held as more than one of analogy.

Whether small or great the apical bud acts as a physiological unit, and the
stelar system serves it as a common unit of supply from below to the apical
region. In the more advanced Ferns the conducting tissues are sheathed
by endodermis, which by the nature of its cell-walls acts when mature as a
physiological barrier (Figs. 377, 378). But its control is not complete, being
subject to progressive phases of development. In its adult state it may limit
gaseous interchange, and serve as an efficient boundary in guiding supplies
of nutrients in solution upwards. But as the younger tissues of the growing

600 BOTANY OF THE LIVING PLANT

point are approached from below the control is relaxed : in passing finally to
the procambial region a point is reached where outward diffusion from the
stele is no longer so restricted : the distal growing point may thus be supplied
with water and solutes by upward diffusion over the whole cross-sectional area.
This may roughly be compared with the spout and perforated " rose " of a
watering pot. In either case the separate streams merge at last into a common
supply to the region immediately below the growing point.

The apical structure of Palms and other Monocotyledons corresponds func-
tionally with that in Ferns, though the strands differ in the former by the absence
of endodermis surrounding each strand (Fig. 31). In both the existence of a
medullary vascular system may find its explanation in the need for bringing
adequate supplies towards the centre of a progressively expanding distal bud,
in plants of primary construction, where the apical bud and leaves are large, and
are served by stems without secondary increase.

Large and Small Buds Compared.

We may now return to the opening of this discussion on p. 589. In
the words of Herbert Spencer, green plants are accumulators of
material gained by photosynthesis. They are in no equivalent degree
expenders. Hence they naturally work towards a favourable balance
of material. One consequence of this is a prevalence of primary
expansion upwards, and the development of large distal buds : as seen
in Ferns and Palms. Here the medullary vascular tracts may serve for
the transit of nutrients through the inert pith towards the centre of
the apical region, with its acropetal succession of young leaves. In
fact the conducting systems of Ferns and Palms, etc., on the one hand,
and of woody forest trees on the other, have solved their respective
problems of advancing size and internal structure in quite different
wavs. In the former, there has been adherence to their original scheme
of primary obconical expansion of the shoot with dilation of the stele in
relation to the single, enlarging terminal bud : in the latter, what is
probably a more efficient method both for mechanical support and
for supply has been evolved ; that is, for the development of a multi-
tude of small buds that initiate a profuse branching. In these the
vestigial relics of the old primary wood translocate a sufficient supply
to meet the earliest demands of each small bud ; but this is supple-
mented without delay by continued secondary increase through
cambial activity which meets both nutritional and mechanical needs.
The final result of this difference in solving the fundamental pro-
blems of Size may be seen in any mixed forest, where conical woody
plants with small buds are dominant, and form the canopy beneath which
the obconical and big-budded types shelter.

THE RELATION OF SIZE AND FORM IN PLANTS 601

The Size Problem in the Bryophytes.
Those surfaces of a plant which are in contact with a surrounding
medium, such as air or water, are styled presentation-surfaces, whether
these be external or internal. For instance, in order to uphold gaseous
interchange in subaerial plants it is essential to maintain a due pro-
portion of surface to bulk ; and the like is needed also in partially or
completely submerged plants. To this end Mosses and Liverworts
have adjusted their form and structure up to a point, though without
realising their opportunities to the full. Accordingly they afford
interesting comparisons with other plants. What is commonly lacking
in their organographic make-up is the combination of sufficiently
elaborate external contour, with or without internal ventilation. For
instance the gametophyte of the Jungermanniales is commonly leafy,
though without internal ventilation (Fig. 369) : on the other hand, the
Marchantiales have an internal type of ventilation of the branched
and fleshy thallus (Fig. 368) : meanwhile the unbranched sporo-
gonia of both are without ventilation. In the Mosses the plant is
leafy, and often profusely branched : but even the largest of them
lack the ventilated structure of Vascular Plants. Their sporogonia,
however, have internal ventilation, with localised stomata (Fig. 364) :
but they are unbranched and leafless. Neitlier phase of the life-cycle
in these plants has secured full efficiency, by combining well-developed
ventilation with branching of a leafy shoot, as Vascular Plants have
done. In this we may see a reason for their limited stature. As a Class
they are structurally doomed to be dwarfs.

Combination of Organographic Factors.
That combination which is wanting in the Bryophytes is, however,
present, together with other helpful features, in most Vascular Plants.
The success of their vegetative system in developing to large size
is chiefly based on such features as, (i) continued apical growth ;
(ii) branching of various orders ; (iii) internal ventilation ; (iv) a
plastic primary conducting system ; and (v) secondary cambial
increase in many, but not in all. Collectively, and in various com-
bination, such features as these lead to the diffuse form and elaborate
structure seen in Land Vegetation. On the other hand, by comparing
the striking results that have followed in Vascular Plants with the
limited success of the Bryophytes, we may measure the importance
of those combinations which the Mosses lack. But, as a consequence
of the superiority of the Higher Plants based on such advantages as

602 BOTANY OF THE LIVING PLANT

these, the Bryophytes do not offer the best field for study of the Size
and Form Relation. There are in fact three grades of vegetation,
differing in the degree of their adjustment to the progressive demands
of increasing Size, though they are not sharply marked off from one
another :

I. Rudimentary organisms, of submerged or semi-aquatic habit,
which are for the most part built up on the cylindrical filament,
such as Algae and Fungi. In these the maintenance of the surface-
volume ratio does not present serious difficulty.

II. Ordinary Land Plants, which have met in full the demands of
increasing size by the aid of structural changes recognised as second-
ary, such as internal ventilation and cambial increase.

III. Primitive Land Plants known as the Archegoniatae, which
occupy a middle position, their organisation in many instances being
based on primary structure only.

The study of Morphology in Plants seems to have progressed as
though there were no Size Problem. One reason for this has probably
been that the simplest plants of aquatic habit do not present the
question in any acute form : while the higher plants of land habit
have solved their problem by adaptive adjustment. But the earliest
denizens of the Land, that is the Archegoniatae, have either carried
out such adjustments imperfectly, or not at all. It is through them
that the Problem of Size and of its consequences may be most readily
grasped : and in none of them is the evidence so clear as it is in the
Mosses and Ferns.

Mechanical Limitations.
The foregoing pages have touched upon those various devices by
which plants meet more or less fully the physiological demands
that follow inevitably on increase in Size. But the direct demand
for mechanical stability stands at the back of all progressive organisation.
As the size increases the strength of a structure increases as the
square of the linear dimensions, but the weight or mass as the
cube, provided the form and material remain the same. We have
seen in Chapter X. how the simplest steps to secure stability are based
on the turgidity of the encysted cell, which suffices for small organisms,
chiefly aquatic. In land plants of larger size the necessary resistance
may be secured by the help of cell-partitioning and of specialised
sclerotic tissues, still of primary origin. A climax of such primary
development has been reached with amazing success by Tree Ferns,
Bamboos, Screw Pines, and Palms. But the most successful develop-

THE RELATION OF SIZE AND FORM IN PLANTS 603

ment of all is based on secondary cambial activity, as in the Gymno-
sperms and Dicotyledons. This provides an automatic increase, not
only in the channels of supply but also in tissues of mechanical resis-
tance. Theoretically there is no limit to growth in photosynthetic
plants, based on continued embryology, combined with a geometrical
ratio of branching, internal ventilation, and cambial increase. But
actually there are spatial and mechanical checks on this ambitious
scheme. The final check of all for subaerial plants is mechanical
inability to support an indefinitely increasing load of stems, branches
and leaves, and to resist the impact of winds. Even automatic
cambial increase does not suffice. Overstepping the limit of resistance
results in fallen trees, and stripped branches, twigs, and leaves. These
mark various degrees of failure under the final test of mechanical
stress.

Adaptation of Form in Submerged Parts.

Chapters V. and VIII. have shown how the internal tissues of the larger
Plants of the Land are ventilated by a system of intercellular spaces, which
open through the pores of the stomata and through porous lenticels. Thus
they provide for that gaseous interchange with the open air that is essential
for the subaerial life of Land Plants. This has been fully discussed by Haber-
landt as illustrating the Principle of Maximum Exposure laid down in his
Physiological Plant Anatomy (Engl. Ed., p. 276, etc.). The effect will be
proportionate to the area of the internal surfaces thus exposed.

A contrast to the condition thus seen in the vast majority of subaerial plants
is presented by those in which the green leaves are submerged wholly or in part.
Since stomata are absent from such surfaces as face the water the ventilating
system is sealed up within the investing epidermis. In them gaseous inter-
change can be conducted only by the slower process of diffusion through the
unbroken outer surface. The sum of its activity will then depend upon the
surface-volume ratio of the plant as a whole. This physiological problem faces
all submerged green plants : it is in fact one of size and form : and the success
of each individual will depend upon the measure in which that ratio is main-
tained. As we have seen, any elaboration of a simpler to a more complex form
tends to increase it. A few familiar examples may serve to illustrate how by
modification of form of the submerged parts the surface- volume ratio tends to
be upheld. It is usually by disintegration of the leaf-blade, such as is seen in
isolated genera and species belonging to diverse distinct families. These
plants suggest that they are specially adapted to meet the physiological
demands consequent on a submerged habit, and the absence of stomata.

Such examples are presented in varying degree by the many different forms of
Ranunculus aquatilis Linn, (Ranunculacae), by Ceratophyllum (Callitrichineae),
Cabomba (Cabombaceae), Potamogeton (Naiadaceae), Hottonia (Primulaceae)
and Myriophyllum (Halorageae). In each of these examples belonging to six
different families, the fully submerged leaves are disintegrated in diverse
degrees, forming narrow lacineae : while those leaves of them that float on the
surface of the water, together with those that are fully subaerial, have

604 BOTANY OF THE LIVING PLANT

coherent or even entire blades, sometimes with stomata on the upper surface,
as in Nuphar. Further, in Cabomba, or in the variety of Ranunculus aquatilis
described as " heterophy litis, Fries," two distinct types of leaf are borne on the
same plant : — those submerged are dissected, those floating or subaerial being
lobed, or even entire. Lastly, in Aponogeton fenestralis (Juncaginaceae), a
denizen of still waters, the broad submerged blade is perforated ; which gives
as regards the surface-volume ratio, practically the same result as dissection.
In all such cases, provided the thickness of the leaf be not increased, the
elaboration of form will necessarily tend towards maintaining the surface-
volume ratio, notwithstanding the absence of stomata from the submerged
surfaces. Thus the consequent difficulty of gaseous interchange that this
entails in parts that are submerged tends to be overcome.

Conclusion

In the preceding pages limiting surfaces of the plant-body, whether
external or internal, have been discussed in terms of the Principle of
Similarity. That progressive elaborations of form and structure take
place as the size of the individual increases has been demonstrated on
a wide basis. Indeed, the existence of a Size and Form correlation
may now be accepted as a fact. The basic problem that awaits solu-
tion, and concerning which we have very little precise knowledge, is
that of the mechanism by which the relationship is brought about. It
has been seen that the adult form and structure of plants admit of
interpretation in terms of function. But however plausible the
biological advantage of each adjustment in form or structure, follow-
ing on change in size of plants, may seem to be, the many inter-related
factors, which are responsible for bringing about these adjustments,
are at work primarily in the apical growing region, and must be
studied there.

APPENDIX A.

TYPES OF FLORAL CONSTRUCTION IN ANGIOSPERMS.

A description of a few types of Flower, together with notes on the
Natural Families to which the Plants that bear them belong, are
here added, so as to illustrate more fully the methods of floral con-
struction described in Chapter XIV. They have been selected partly
because they are common Flowering Plants easily accessible to all ;
partly because they represent characteristic features of the Natural
Families whose products are of importance to Man ; partly also because
of their biological interest in relation to the production and dispersal
of seeds. A study of such examples will give some idea of the various
forms which the flowering shoot may assume. A few added notes
will help to explain in the several examples the biological advantages
which follow from the form adopted.

CONSPECTUS OF THE PLANTS DESCRIBED.

ORDER.
LlLIALES -

Orchidales
Glumales -

MONOCOTYLEDONEAE.

FAMILY.

Liliaceae
Amaryllidaceae

Iridaceae
Orchidaceae
uncaceae -
. Cyperaceae -
IGramineae -

a

EXAMPLES.

(i) Tulip, (2) Squill.

(3) Snowdrop, (4) Pheasant's

Eye.
(5) Iris, (6) Crocus.

(7) Orchis.

(8) Wood-Rush.

(9) Cotton-Grass, (10) Sedge.
(11) Rye-Grass.

DICOTYLEDOXEAE.
(Dicotyledoneae — Choripetalae).

Salicales -
curvembryeae

polycarpicae

Salicaceae -
Caryophyllaceae

Ranunculaceae

605

(12) Goat Willow.

(13) Ragged Robin, (14)

Campion.
(15) Marsh Marigold, (16)
Buttercup, (17) Aconite.

6o6

BOTANY OF THE LIVING PLANT

ORDER.

Rhoeadales
Geraniales

Tricoccae
Saxifragales

ROSALES -

Leguminales
Umbellales

FAMILY.

(Papaveraceae

(Cruciferae -

Geraniaceae

Euphorbiaceae

Saxifragaceae

Rosaceae

Leguminosae
Umbelliferae

EXAMPLES.

- (18) Poppy.

- (19) Mustard, (20) Wallflower.

- (21) Geranium, (22) Pelar-

gonium.

- (23) Spurge.

- (24) Saxifrage, (25) Currant.

- (26) Apple, (27) Strawberry,

(28) Rose, (29) Cherry.

- (30) Trefoil, (31) Pea.

- (32) Cow-Parsnip.

Bicornes -
Primulales

Personatae

Verbenales
Synandrae

(Dicotyledoneae — Sympetalae).
(a) Pentacyclicae.

■ Ericaceae - - (33) Heath, (34) Bilberry.
Primulaceae - (35) Primrose.

(b) Tetracyclicae.

fSolanaceae -
\Scrophulariaceae

Labiatae
Compositae -

(36) Nightshade, (37) Potato.
(38) Figwort, (39) Foxglove,

(40) Speedwell.
(41) Dead-Nettie, (42) Sage.
(43) Groundsel, (44) Ox-eye,

(45) Cornflower, " (46)

Dandelion.

MONOCOTYLEDONEAE.

These Plants are characterised by the embryo bearing only one
cotyledon. The leaves are as a rule alternate, with simple form, parallel
venation, and a broad sheathing base. The stem and root show no
secondary thickening of the type usual in Dicotyledons, their vascular
strands having no cambium. The flower is constructed usually of
five alternating whorls of parts, and each whorl is commonly trimerous.
Most of the Monocotyledons are perennials. They include Grasses,
Sedges, Orchids, and Palms. Many are rhizomatous and bulbous
plants that are grown for the beauty of their flowers.

ORDER: LTLIALES.

. This very large Order includes plants which are naturally related
together by their floral structure, though diverse in vegetative habit.
The type dates back to the Cretaceous Period, and it may be accepted
as underlying the floral construction of most of the Monocotyledons.
Most of them are perennials, with creeping or bulbous stock (Lily,
Tulip, Solomon's Seal) : but some are tree-like (Draccena), or shrubby

APPENDIX A

607

(Ruse us), while some are climbers (Smilax, Diosccrea). They have

for the most part entire leaves, with sheathing base, and parallel

venation : but the Dioscoreaceae are exceptional in having broad

reticulate leaves.

The flowers are constructed on a type which may be accepted as a general
underlying plan for Monocotyledons, being com-
posed of five alternating whorls of parts. The
number of parts in each whorl is commonly
three ; but other numbers may be found, such as
two (Maianihemum) , or four, or even five (Paris,
Aspidistra). The floral formula is P. n + n, And.
n + n, Gyn. (n), and the floral diagram as in
Fig. 455. The more primitive Liliiflorae have
hypogynous flowers, but they are epigynous in
the Amaryllidaceae and Iridaceae, a condition
regarded as later and derivative. The ovary has
one loculus for each carpel, and the anatropous
ovules are seated on their incurved margins, which
are fused to form an axile placenta (Fig. 457) . The
flowers are usually of large size, and are often
conspicuous by colour and scent (Lily, Tulip) . They show steps of progressive
fitness for the nursing of the ovules, by various degrees of fusion of the
carpels, and of sinking of the ovary from the superior position of the

Liliaceae to the inferior of the Amaryllidaceae and
•>. Iridaceae. Progressive steps may also be traced in the

' perfection of the pollination-mechanism.

Fig. 455.

General floral diagram for trimer-

ous Flower of the Liliales.

-St

3

— -Ov

Fig. 456.

Superior gynoecium
of Lilium, showing
relative position of
ovary (or), style (sty),
and stigma (stig).
F. O. B.
B.B.

Family : Liliaceae. Example : The Tulip.

The Liliaceae may be held to represent a primitive type
of the Liliiflorae. They include a large number of familiar
bulbous plants, such as Garlic, Hyacinth, Tulip. The
latter may be taken as an example.

(1) The Tulip plant (Tidipa gesneriana) at the flowering
period consists of the underground bulb, bearing roots
downwards from the margin of the disc-like stem, upon
which the storage-scales of the bulb are seated. From its
apex rises an elongated stem bearing a few foliage leaves,
and a single terminal flower which is radially symmetrical.
Provision for the next season is made by one or more
buds in the axils of the bulb-scales, which grow into new
bulbs, and each may produce a flower. Compare bulb of
Hyacinth (Fig. 132, p. 198). The analysis of the flower is
as follows :

Perianth segments 3 + 3, polyphyllous, inferior.

Androecium stamens 3 + 3, free, hypogynous.

Gynoecium carpels 3, syncarpous, superior. Stigma
three-lobed, sessile. Ovary trilocular. Placentation
axile. Ovules numerous, anatropous (Figs. 456, 457).

2Q

6o8

BOTANY OF THE LIVING PLANT

The floral diagram (Fig. 455) shows the regular alternation of the successive
whorls of three parts. As those of each whorl are all of equal size, and except-
ing the carpels all separate from one another, the Tulip may be held as a
relatively primitive type of Liliaceous flower. But the syncarpous state here
seen is probably not the most primitive. In Colchicum, with its Crocus-like
habit, the carpels are incompletely fused, and each has its separate style and
stigma : an indication of a primitive apocarpous state. Other members of the
Order show various steps in cohesion and adhesion of the outer parts. For
instance, in (2) the Wild Hyacinth (Scilla nutans, Sm.) the stamens are adherent
to the perianth-segments (epiphyllous). In the Grape-Hyacinth (Muscari),
and the Lily of the Valley {Convallaria)
the segments of the perianth are
coherent into a bell. In some Lilies
the perianth may form a long tube,
while the style is proportionally
elongated. But still the ovary is
superior ; even in Colchicum, where it
is below ground, it stands above the
insertion of the long tube-like perianth.
In others, as in Hemerocallis, the gamo-
phyllous flower is zygomorphic. Thus
the primitive state seen in the Tulip
may be modified in relation to pollina-
tion by insects.

Pollination. The flower of the Tulip
is conspicuous by its size and colour ;
but there is no honey, though in the
nearly allied Fritillaria a large honey-
gland lies at the base of each perianth-
segment. The Tulip is visited by insects for its pollen, and so crossing may
be effected ; but it is not a specialised mechanism.

The fruit of the Liliaceae is either a capsule, splitting by longitudinal slits,
and so shedding the seeds, which are flattened and readily carried by the
wind ; or it may be a berry as in Lily of the Valley, or Asparagus, and thus
be distributed by birds.

The Liliaceae are world-wide in distribution. Many are cultivated for the
beauty of their flowers. Some yield fibres for cordage (Phormium), others
valuable drugs (Aloe. Colchicum, Sarsaparilla (Smilax) ) : others are grown as
vegetables, e.g. Onion, Asparagus.

Fig. 457.

Transverse section of the superior ovary of
Lily, showing the three syncarpous carpels,
bearing the anatropous ovules on their infolded
margins. F. O. B-

Family : Amaryllidaceae. Examples : Snowdrop, Narcissus.

Those Liliales which have the same floral plan as the Liliaceae, but with
an ovary inferior, are grouped as Amaryllidaceae. But there is no sharp
line of demarcation between the hypogynous and the epigynous types. Some
genera show an intermediate state, their half-inferior ovary suggesting
how the carpels may have sunk into the tissue of the receptacle, thus giving
the ovules better protection, and nearer proximity to the sources of supply.
The floral diagram is the same for the Amaryllidaceae as for the Liliaceae.
(3) The Snowdrop (Galanthus nivalis, L.), or the Snow-flake (Leucojum, L.)

APPENDIX A 609

illustrate a primitive state of this epigynous type where all the parts of
perianth and androccium are separate. A more advanced type, showing
not only epigyny but also cohesion of the perianth, and adhesion of the
stamens to it, is seen in Narcissus.

(4) In the Daffodil (A*. Pseudo-Narcissus, L.), the same floral diagram
(Fig. 455) and floral formula apply. But here the coherent perianth springs
from the summit of the inferior ovary as a tube which separates upwards into
six widely spreading segments. At the level where they diverge the tube
appears to be continued into a wide trumpet-shaped corona. This is an
accessory formation, and only appears late, after the other parts have been
formed. From the inner surface of the perianth-tube, near to its base, arise
the six epiphyllous stamens. They are closely grouped round the central style,
whose stigma projects beyond them. Honey-secretion is provided by three
deep glands in the septa of the ovary, and it flows into the base of the tube.
The size of the flower allows entry to humble-bees, which, passing from flower
to flower, make cross-pollination probable.

Narcissus poeticus, L., the Pheasant's Eye, is a species of similar construction,
but with white perianth, and a corona fringed with red. Its tube is, however,
narrow, and the anthers and stigma almost fill its opening. Thus it is inacces-
sible to bees ; but its white colour, heavy scent, and the length of the narrow
tube fit it for pollination by long-tongued, night-flying moths. These three
types of Amaryllids show how the same floral structure may be modified as a
mechanism for pollination by different types of insects.

The Amaryllidaceae are less numerous in genera and species than the
Liliaceae, but are widely spread, especially in the Mediterranean region, and
at the Cape. They include many bulbous plants, and some that yield fibres
(Agave).

Family : Iridaceae. Example : Yellow Flag.

(5) Those Liliales which have inferior ovary and only three stamens, are
grouped as the Iridaceae, of which the native Yellow Flag (Iris Pseudacorus, L.)
serves as sdi example. It has a branched and strongly rooted perennial
stock (see Fig. 129, p. 196) and each branch ends in an annual foliage-shoot,
with sword-shaped leaves, sheathing at the base. The apex of certain shoots
extends upwards into the cylindrical flowering stems, which bear their first
flower distally ; a second flower arises subsequently in the axil of a bract
below it, and others may follow, in sympodial arrangement.

The large yellow flower is composed of the following parts (Fig. 458) :

Perianth, segments 3+3, gamophyllous at the base, superior ; the outer

series broad and recurved, the inner narrower, and erect.
Androecium, stamens 3+0, free, epigynous, anthers opening outwards. It

is the inner series that is absent.
Gynoecium, carpels 3, syncarpous, styles three petaloid ; ovary inferior,

trilocular with axile placentation ; ovules numerous, anatropous

(Fig. 213, p. 291).

The fruit is a dry loculicidal capsule, and the flattened albuminous seeds are
scattered by the wind.

Pollination. The flower of Iris is more specialised in relation to insect-

6io

BOTANY OF THE LIVING PLANT

agency than most of the Iridaceae. Many of them, such as Ixia, or Sisy-
rinchium, have flowers not unlike the Snowdrop, but with only three stamens.
Gladiolus has the same, but it is slightly zygomorphic. (6) Crocus has a
tubular perianth greatly elongated, so that while the ovary is seated just above
the underground corm, the perianth, stamens, and stigmas are above ground.
But Iris is the most specialised of them all. Its peculiar features are that the
three stamens, which open outwards, are enclosed each between one of the
broad outer perianth-segments and one of the three broadly petaloid styles
(Fig. 458, in, iv.). The tip of each style is two-lobed, and bears a project-
ing lip on its lower and outer surface. This is the stigma. The fact that
the styles are opposite the stamens, and these opposite the outer perianth-
segments, shows that it is the inner stamens that are wanting (v.). Honey is

Fig. 458.
Iris Pseudacorus, L. I. Complete flower. II. Same cut in median section.
III. Flower with perianth removed. IV. Single lobe of petaloid style, with stigmatic
lip. V. Floral diagram.

secreted by the inner surface of the perianth-tube, and collects round the base
of the styles. Each third of the flower may be pollinated independently by
humble-bees, which force their way between the perianth-segment and its
opposing style. On entering, if they bring pollen, it is swept off on the project-
ing stigmatic lip ; the stamen then deposits a fresh supply of pollen upon his
body, which he carries away. Self-pollination is mechanically impossible ; but
cross-pollination results with high probability from a succession of visits, either
to other thirds of the same flower or to different flowers. In many forms of
Iris the parts fit so exactly as to exclude small and weak insects that would
not effect pollination, but this exclusion is less perfect in others. Iris may
be held to show a culmination of pollination-mechanism as seen in the Liliales.
The Iridaceae are widely spread; but are specially frequent in the Cape
Flora. They include plants grown for their showy flowers (Crocus, Iris,
Gladiolus, Ixia). The dried rhizomes of species of Iris give the "Orris-root,"
and the stigmas of Crocus sativus are the source of Saffron.

APPENDIX A

611

ORDER : ORCHIDALES.

Family : Orchidaceae. Example : Spotted Orchis.

(7) The spotted Orchis {Orchis maculata, L.), which flowers early, in damp
grassy ground, will serve as an example of a still higher specialisation of

Fig. 459.
Whole plant of Orchis maculata, including the swollen myeorhizic roots. (After Figuier.)

the flower for cross-pollination by insect-agency, which characterises the
Orchidaceae. The plant is perennial, and in summer is seen to consist of an

6l2

BOTANY OF THE LIVING PLANT

upright axis bearing sheathing leaves, and a distal spike of flowers. This
springs from the apex of a palmate storage-tuber of the previous year, now
shrivelled ; while a second similar tuber, young and plump, is developing as
storage for the next season (Fig. 459). The new tuber bursts through
from the axil of one of the lowest leaves, and bears a terminal bud seated on
swollen mycorhizic roots directed obliquely downwards. Normal roots also
arise from the base of the shaft. By a succession of such tubers the plants
perennate from year to year.

Each flower of the spike is sessile in the axil of a large leafy bract, and
is of an epigynousLiliifloraltype, but specialised so as to be an accurate

Fig. 460.
Flower of Orchis maculata, L. I. Whole flower in frontal view. II. Lateral
aspect, perianth partly removed. III. Young fruit. IV. Floral diagram. V. VI.
Pollinia in erect and curved positions. 1-6 = perianth-segments ; (1) is really the
anterior, but by resupination the posterior segment ; 6 is the labellum, actually
posterior, but by resupination the anterior segment. £? = bract ; s/>=spur ; ov=
ovary; s=stigma ; r = rostellum.

mechanism for pollination (Fig. 460, 1.). The inferior ovary itself constitutes
the stalk of attachment of the sessile flower. It shows ridges, the spiral
turns of which indicate that the flower has been inverted by a half-circle-
twist of the ovary (resupinate) (11. in.). The posterior side is thus turned to
the front facing the bract. This is essential for the success of the strongly
zygomorphic flower as a pollinating mechanism.

The ground-plan of the flower is shown in Fig. 460, iv. ; it consists of :
Perianth, segments 3 + 3, polyphyllous, superior. The three outer segments
are small, and of about equal size, the odd one being anterior (1). The three
inner are very unequal. The posterior segment, turned by resupination to
the fronf, forms a large platform, or labellum (6), and it is dilated downwards
into a long spur (sp). The two smaller, together with the outer anterior
segment, form a hood-like group over the column, which rises just behind

APPENDIX A 613

the open entrance to the spur. It is the result of fusion of the single stamen
with the short style.

The Androecium is represented only by a single anterior , fertile stamen, which,
owing to the resupination, faces the observer : each of its two purple
anther-lobes shows when ripe a longitudinal slit of dehiscence. The down-
ward-directed, but really apical end of the anther is covered by the small
globular rosiellum, which obstructs the entrance to the spur (Fig. 460, 1. 11.).

The Gynoecium consists of three carpels, syncarpous and inferior. Of the
three stigmas one which is not receptive is represented by the rostellum (r),
the other two are merged into a hollow oval stigmatic surface (5) situated
below the rostellum. A transverse section of the ovary, preferably of a
flower already fertilised, shows a single cavity, with three parietal placentas,
and very numerous minute ovules.

The Fruit matures as a dry capsule, splitting by six longitudinal slits into
three larger and three smaller strips, which remain united at their ends.
The minute seeds are scattered by the wind.

Pollination. The flower being resupinate, the large posterior labellum pro-
jects forward as a convenient platform for the visiting insect, while the entry
to the spur is presented directly to his proboscis just below the column.
But the passage is obstructed by the rostellum. On inserting his proboscis
this is pushed aside. As it breaks away it lays bare two sticky discs, which
adhere to his proboscis, their cement setting firmly in the few seconds during
which he is engaged in probing the honey-containing tissue at the base of the
spur. On his withdrawing it, the coherent contents of the two anther-lobes
are themselves withdrawn, and appear as club-shaped pollinia, fixed by their
sticky discs in an erect position. But in a few seconds this position changes,
and they curve strongly forwards (Fig., 460, v. vi.). Meanwhile, if he has flown
to another flower, they are in such a position that as he inserts his proboscis,
they will impinge directly on its stigma, which is below the rostellum. The
pollen, which is in coherent masses, is then held by the sticky stigma. Thus
cross -pollination is effected with a high degree of certainty, while self-pollina-
tion is mechanically impossible. The efficiency of the mechanism is shown
by the constancy with which the Spotted Orchis sets its fruit as it grows in
the open. This is only one of the very various methods of pollination seen
in this wonderful family. For further details reference may be made to
Darwin's Fertilisation of Orchids.

The Orchidaceae are a very large family, chiefly tropical, and often epiphytic,
though represented in the British Flora by many ground-growing species.
Their interest is chiefly biological and spectacular. One of the few useful
products is Vanilla, the dried fruit of an American climber.

ORDER : GLUM ALES.

The Cyperaceae and Gramineae, which are grouped under this heading,
have in common hypogynous flowers, more or less specialised in relation to
pollination by the wind, while their flowers are often grouped in dense in-
florescences. The Juncaceae may be associated with them as a Liliifloral type
only slightly modified. The construction of their flowers may be referred in
origin to the Liliaceous type, but the perianth is inconspicuous, and the

614

BOTANY OF THE LIVING PLANT

pentacyclic, usually trimerous structure is more or less reduced in the number
of the parts. The name Glumiflorae refers to the fact that the bracts are
usually stiff and dry, and are called Glumes or Paleae, which constitute the
" chaff " of Grasses. The gynoecium is superior, and though it may be tri-
locular with numerous ovules in the less specialised forms, in the more
advanced Sedges and Grasses it is unilocular, and contains only a single ovule.
The fruit then matures as a Grain, or Nut. The plants are mostly annual or
perennial herbs, frequently with long internodes. The alternate leaves are
sheathing below, with a simple grass-like blade, and often a ligule projecting
upwards at the junction of sheath and blade.

Family : Juncaceae. Example : Field Wood-Rush.

(8) The Field Wood-rush (Luzula campestris, Willd.) is a perennial very
common on poor grass land. It has a Grass-like habit, but its flowers are

Fig. 461.
J uncus lamprocarpus. a, Part of an
inflorescence ; single flower {b) and gynoe-
cium (c) magnified. (Strasburger.)

Fig. 462.

Eriophorum augusiifolium. 1, inflorescence. 2, a
single spikelet. 3, single flower. 4, flower with bract
removed. 5, fruit. (After Hoffmann ; 1, about nat.
size; the others x 3-5.) (Strasburger.)

constructed on the Lily-type. The root-stock produces leaves with sheath
and blade, but no ligule. The axis elongates upwards into an inflorescence

APPENDIX A

615

bearing numerous flowers in a compact cymose group, with chaffy bracts.
The flower is constructed of :

Perianth, segments 3+3, polyphyllous, inferior, dry and chaffy.

Androecium, stamens 3 + 3, free hypogynous.

Gynoecium, carpels 3, syncarpous, superior, with three long feathery
stigmas, united below into a short style. Ovary trilocular, with one ovule
in each loculus (Fig. 461, b, c).

Fruit, a capsule dehiscent loculicidally. Seed with starchy endosperm.

Pollination. The flower is strongly protogynous ; the feathery stigmas
project, while the perianth is still closed over the unopened stamens. Later
the stigmas wither, the perianth expands, and the anthers burst, setting
free the dry dusty pollen, which is readily shaken out, and carried away by
the breeze. There is no honey-secretion, or other attraction for insects,
but cross pollination is almost certain by the agency of wind. Self-pollina-
tion is prevented by the marked protogyny. Nevertheless fruit is almost
uniformly produced.

The chief genus is Juncus, to which the Rushes belong. They are mostly
plants of moist habit, and of little feeding value for stock. Their presence
in grass-land is an indication of the need for draining.

atr. O

Family : Cyperaceae. Examples : Cotton-Grass, Sedge.

(9) The Cotton-Grass (Eriophorutn vaginatum, L.) is a tufted perennial
herb of swampy moorlands, marked by its single cottony heads when in fruit.

The flowering head rises about a foot from
the root-stock, and is composed of a single
spikelet of flowers in the axils of glume-
like bracts. Each hermaphrodite flower
is constructed as follows :

Perianth, inferior, represented by
numerous bristles, which are developed
relatively late, and mature into the
" cotton " of the fruit.

Androecium, stamens three, hypogynous,
representing those of the inner whorl.

Gynoecium, carpels three, syncarpous,
and superior, with three stigmas. The
ovary is unilocular, with a solitary ovule
(Fig. 462).

Pollination is effected by the wind, which
also carries out the transfer of the fruit.

Fruit, a trigonous nut, with the cottony
tuft of the persistent perianth attached
at its base. The floral structure suggests
a modification of the Liliaceous type, by
cottony development of the perianth, loss of the three outer stamens, and
reduction of the ovary to a single loculus with one ovule.

(10) For the structure of a Sedge, any of the following species of Car ex will
serve (C. glauca, Murr ; pallescens, L. ; pendula, Huds. ; hirta, L. ; flava, L. ;
or binervis, Sm.). The Sedges are perennial herbs of swampy ground, which

A, floral diagram of a male flower of
Carex ; B, of a female flower with three
stigmas ; C, of a female flower with two
stigmas. D, diagram of female flower of
Carex. E, diagram of the hermaphrodite
spikelet of Elvna : a, secondary axis ; utr.
utricle or bract of secondary axis. (After
Eichler.) (Strasburger.)

6i6

BOTANY OF THE LIVING PLANT

put up long flowering shafts, each bearing several spikes of unisexual flowers
(Fig. 463). In the species named, one or more of the distal spikes bears only
male flowers : the lower lateral spikes bear female flowers. In the axil of each
glume-like bract of the male spike is a male flower consisting only of three
stamens, with no perianth or gynoecium. In a similar position on the female
spikes there are found flask-shaped bodies (perigynia), through the open throat
of which at flowering a three-branched stigma projects. The perigynium is
a bract enveloping the female flower, which has no perianth, and no androecium,
but consists of three, or sometimes two carpels, syncarpous and superior. The
ovary is unilocular, the ovule solitary, and the fruit a nut. Here the flower is
still more simple than in the Cotton-Grass, for there is no perianth, and as
the flowers are unisexual, all that remains are the three stamens in the male,
and the three carpels in the female. Pollination is by the wind. A large
number of flowers of simple structure are aggregated in the inflorescence :
cross-pollination is therefore probable.

The Sedges are of little value for fodder. Drainage oi the wet ground in
which they grow promotes the more valuable Grasses, from which they are
distinguished by their solid stems and leaf-divergence of J, while in Grasses
it is \ : also by the median position of the embryo in the seed, while in Grasses
it is placed laterally.

Family : Gramineae. Example 1 Common Rye-Grass.

(11) The Rye-Grass (Lolium perenne, L.) is a common grass of meadows
and road-sides ; its variety italicum is often cultivated for fodder. It has

Fig. 464.

Inflorescence and flower of Lolium perenne. I. shows part of the main rachis, with
one lateral spikelet in the axil of the single glume (gl). II. a single flower with its
two paleae (p), pendent stamens, and feathery stigmas {st). III. flower dissected
out, showing lodicules (I). IV. the gynoecium. V. floral diagram: g/ = glume;
gl' = glume which is wanting in Lolium ; p = paleae ; / = lodicules.

leafy stolons and ascending flowering shoots, and it is easily recognised
amongst common Grasses by their long flattened form. Its flowers open in

APPENDIX A

617

succession, so that it can be obtained in the flowering state throughout the
summer. The flowers can be made to open at any time by keeping them warm,
and in water.

The inflorescence is borne on a long stalk, above the uppermost foliage
leaf, which has a split sheath, ligule, and lamina. It is a compact, com-
pound spike, composed of spikelets placed edgewise, alternately on its two
sides, with one terminal. Each lateral spikelet, which consists of an axis
bearing flowers alternately in two rows, arises in the axil of a bract, or outer
glume (Fig. 464, 1.). In most Grasses there is a second, or inner glume, on
the side opposite to the outer ; but this, being unnecessary for protection in
the dense inflorescence of the Rye-Grass, is not present, except in the
terminal spikelet. Each flower is ensheathed in two further bracts or

paleae : the lower and outer is
anterior, the upper and inner is
posterior, and the flower itself
lies between them. At flower-
ing they gape widely apart, so
as to expose the parts of the
flower (I). If a flower be found
in this condition, or if the lower
palea be forced back, the flower,
as seen from the anterior side,
will show the following parts :
(i) Two lodicules, which are
minute, colourless, hypogynous
scales, right and left of the
median plane (/). It is by their
swelling that the paleae are
forced apart at the time of
flowering.

(ii) Three stamens (st), hypo-

Fig. 465.

gynous, free, with long flexible

Part of a median longitudinal section of a grain of
Wheat, showing embryo and scutellum (sc). i>s = vase,
bundle of scutellum; ce — its columnar epithelium;
i' = ligule ; c— sheathing part of cotyledon ; pv = \ege-
tative cone of stem; /i£ = hypocotyl ; / = epiblast;
r = radicle; cl = root sheath ; m — micropyle ; /> = funi-
culus; u/> = vascular bundle of funiculus; /=lateral
wall of groove ; cp=pericarp. ( x 14.) (After Stras-
burger.)

filaments, and versatile anthers,
bearing powdery pollen. One
stamen is median and anterior,
the two others obliquely pos-
terior.

(iii) The gynoecium, lying
centrally, consists of a pear-
shaped superior ovary, grooved on the posterior side, and bears distally, right
and left, two feathery stigmas. Dissection shows a single ovule in the ovary.
The number of such flowers in each spikelet varies : 8 to 10 are common
numbers in the Rye-Grass, and they open at intervals in acropetal succession.
The flower may be held to be of Liliifloral type, reduced in relation to wind-
pollination. The perianth is represented by the two lodicules, corresponding
to the oblique anterior segments of the inner series, which being of use in
separating the paleae at flowering have survived. The stamens correspond in
position to the outer whorl of the Liliiflorae, while the gynoecium is held to
consist of a single carpel, corresponding to the anterior carpel of the Liliiflorae.

6i8

BOTANY OF THE LIVING PLANT

This floral structure is very constant in the Grasses, but the flowers are variously
disposed in their inflorescences. The Rye-Grass may be taken as a good
example for the Family, and it is easily recognised.

The inconspicuous flowers, versatile anthers, dry dusty pollen, and expanded
feathery stigmas clearly indicate wind-pollination, with promiscuous inter-
crossing. Most Grass-flowers are homogamous, that is, stamens and stigmas
mature simultaneously, but some are protogynous (Alopecurus) .

The fruit is a dry nut, containing one albuminous seed and a lateral embryo.
Its structure is well illustrated by the grain of wheat or maize (Fig. 465).

The Gramineae are the most important Family in supplying the wants of
man. Their fruits are the various cereal grains (Appendix B) : their foliage
gives fodder for animals : sugar is yielded by the Sugar Cane ; and the Bamboo
serves most various uses to the dwellers in the Tropics.

DICOTYLEDONEAE.

These Plants are characterised by the embryo bearing two coty-
ledons. The leaves are net-veined, usually with a narrow base, and
a definite petiole. The stem and root show secondary thickening by
means of a cambium. The flowers are usually pentamerous, or
tetramerous, with distinct calyx and corolla. The plants are perennial
or annual, many of the former developing as shrubs or trees.

The Dicotyledons are divided into two large series, according to the
separateness or coherence of their petals. This distinction does not
serve a like purpose in the classification of the Monocotyledons ; it
has already been seen that the very natural Family of the Liliaceae
is variable in this respect. But in the Dicotyledons the same varia-
bility within natural families is exceptional : therefore this dis-
tinction serves to give a natural separation of them into Polypetals,
or Choripetalae, with the petals all separate from one another ;
and the Gamopetals, or Sympetalae, where there is a coherence of
the petals to form a united, usually tubular corolla.

The former is undoubtedly the more primitive state. It repeats the
condition usual in the vegetative region, and it is characteristic of
those less specialised flowers which on many other grounds are held
as less advanced. The gamopetalous state is characteristic of flowers
which are more specialised as pollinating machines, and they may
therefore be held as more advanced. But there is no reason to hold
all plants showing gamopetaly as necessarily related to one another :
this would involve the assumption that the advance had happened
only once in the course of Evolution. It seems probable that in a
plurality of evolutionary lines the advance was made to gamopetaly,
and the student should be prepared to recognise this in any sequence

APPENDIX A

619

in which comparison makes it appear probable. In accordance with
the views thus briefly sketched the Polypetals, or Dicotyledoneae —
Choripetalae, will be taken first.

DICOTYLEDONEAE— CHORIPETALAE.

ORDER : SALICALES.
Family : Salicaceae. Example : The Goat Willow.

(12) The numerous native species of Willow are trees, or shrubs, or dwarf
undershrubs, which live in damp situations ; almost any of them would serve to
illustrate their very simple floral structure. In the large shrubby Goat- Willow
(Salix caprea, L.) the flowers appear grouped to form the well-known Catkins
or " Palms." These are of two sorts, distributed on different plants {dioecious)

a

Fig. 466.
Catkins of the Willow, a, male ; b, female. (After Figuier.)

(Fig. 466). The male catkins appear bright yellow when in bloom, from
their projecting stamens ; the female catkins are more slender, and of olive-
green colour. In each case the catkin is a spike. Its main axis bears
darkly-coloured bracts, and in the axil of each of these is a single very simply-
constructed flower.

The male flower (Fig. 467, a) consists of two stamens, each with a long
filament, which bears the anther, with sticky, not dusty pollen. There is no
perianth, nor gynoecium ; but on the side next the stem is a nectary, which
secretes honey freely at flowering (Fig. 468, A, d). Other species may have
three, four, or more stamens, but no other floral parts (Fig. 468, C). The
female flower (Fig. 468, b) is also axillary. It consists only of a gynoecium of
two carpels joined by their margins to form a unilocular, superior ovary with

620

BOTANY OF THE LIVING PLANT

two-lobed stigma. The ovules are numerous, and the placentation parietal.
A honey-gland is present here also between the flower and the main axis
(Fig. 469, A,d).

Fig. 467.

Flowers of Willow {Salix alba), a, male ; b, female ; in each case the subtending
bract is also shown. (After Figuier.)

Pollination. The flowers of both catkins are visited freely by insects, both
bees and moths, for honey or for pollen. Self-pollination is obviously im-
possible, for the plants are dioecious ; but crossing follows as a natural
consequence of the conveyance of the sticky pollen to the protogynous female
catkins by insect-visitors.

<0>~d>

gffluv/7/

Fig. 468.

Floral diagrams of male flowers of Willow.
A=S. caprea. B = S. purpurea. C = S. pen-
tandra. (After Eichler.)

Fig. 469.

Floral diagrams of female flowers of Salix.
A =S. caprea. B=S. alba. (After Eichler.)

The fruit is a tough capsule, which splits longitudinally, exposing the seeds,
each with a tuft of silky hairs attached to its base, by which it is transferred
by the wind.

ORDER : CURVEMBRYEAE.

Family : Caryophyllaceae. Examples : Ragged Robin, Red

Campion.

(13) The Ragged Robin (Lychnis Flos-cuculi, L.) is a herb of damp grassy
ground, with perennial root-stock from which arise upright stems with simple
leaves in alternate pairs. The inflorescence is a definite, regular dichasial
cyme : that is, the main axis ends in the first flower ; branches arising in the
axils of the last leaves again terminate each in a flower, and so on (compare
Fig. 179 B, p. 258). The flower is of a radial type, with peculiar tattered pink
petals, which gives the name (Fig. 470). Its constitution is as follows :

Calyx, sepals 5, gamosepalous, inferior, dilated below, and serving as a
mechanical support to the weaker parts within.

APPENDIX A

621

Corolla, petals 5, polypetalous, inferior, deeply notched, and again divided,
bearing paired ligules at the sharp angle of the claw of each.

Androecium, stamens 10, free, hypogynous, of varying length during flower-
ing. The 5 outer opposite the sepals, the 5 inner opposite the petals.

Fig. 470.
The Ragged Robin (Lychnis Flos-cuculi, L.) I. whole flower. II. same in section.
III. petal with ligule, abaxial view. IV. petal with ligule and petaline (inner)
stamen, adaxial view. V. gynoecium. VI. floral diagram.

Gynoecium, carpels 5, syncarpous, superior ; ovary unilocular, ovules
numerous, on free central placenta. Styles and stigmas 5, rising separately
from the apex of the ovary.

Pollination. Nectar is secreted at the base of the stamens, and the flowers
are visited by many insects, especially butterflies and moths. The flowers
are pvotandrous ; in the first stage the 5 outer stamens shed their pollen at
the entrance of the tube ; later the 5 inner stamens do the same ; finally the
5 stigmas grow up and fill the entrance to the flower. An insect visiting the
flower in either of the first stages will remove pollen on its proboscis, which
it may deposit on another flower in the third stage. Intercrossing is thus
probable, though self-pollination is possible.

(14) In the Red Campion {Lychnis diurna, L.), two types of plant are found :
some with thinner stems and smaller leaves bear only staminate flowers, others
with more robust habit bear pistillate flowers. These should be examined
and compared (Fig. 471). In the staminate flowers the Calyx and Corolla are
essentially as above described. Also the 10 stamens, but they are of unequal
lengths, and they surround a minute green central process, which represents
the abortive gynoecium. The pistillate flowers are of like structure ; but here
the androecium is represented by ten small conical staminodes, while the
gynoecium is fully developed, with five carpels, a large ovary with five styles
and stigmas, and numerous ovules on a free central placenta.

Pollination. Comparing the two species : in both, nectar is secreted at the
base of the tube, and protected by hairs, while the weak petals and stamens
are supported by the firm gamosepalous calyx. In the Ragged Robin the
sexes are separated in time, the flowers being markedly protandrous, which
gives a probability of intercrossing as a consequence of repeated insect- visits.

622

BOTANY OF THE LIVING PLANT

In the Red Campion the sexes are separated in space, being borne on distinct
plants. This renders self-pollination impossible, and cross-pollination obliga-
tory. That the latter is the derivative state is clearly shown by the presence
of the abortive stamens, and pistils.

M.

JL.

ZF

Fig. 471.

Dissections of flowers of Lychnis diurna. I., II., VIII., the pistillate flower in
which the stamens are represented only by staminodes (st). III., IV., IX., the
pistillate flowers in which the gynoecium is represented only by a vestigium (gyn).

The fruit is a dry capsule, which opens by teeth at the distal end (Fig.
471. VII.), and the numerous curved, albuminous seeds are scattered as it is
shaken in the wind.

The products of the Order are unimportant . It is related to the Goose-
foot Family (Chenopodiaceae).

ORDER: POLYCARPICAE.

Family : Ranunculaceae. Examples : Marsh-Marigold, Buttercup,

Monkshood.

The Buttercup Family is relatively primitive, as indicated by the variability
of its floral construction, by the number of its parts, and the character and

APPENDIX A

623

Fig. 472.

composition of the perianth : also by the fact that all the parts are inserted
separately upon a conical receptacle. It includes herbs or shrubs of
temperate and cold climates, with alternate
ex-stipulate leaves, having palmate venation.
They are mostly acrid, and poisonous.

(15) A simple type of their floral construction
is seen in the Marsh-Marigold {Caltha palustris,
L.), common in wet places. It is a coarse herb,
with creeping rootstocks and cordate radical
leaves. The flowering stems are sub-erect, with
a few leafy bracts and cymose branching. The
large yellow flowers consist of :

Perianth, a single series of petaloid sepals,
5 or more, imbricate in bud : the outermost
obliquely anterior ; polyphyllous, and inferior.

Androecium, stamens indefinite (80 to 150),
free, hypogynous, dehiscent by lateral slits.

Gynoecium, carpels 5 to 10, apocarpous,
superior. They are follicles, with margins
turned centrally, to which the numerous

ovules are attached in two rows (Fig. 472). Pistil of Caltha, with numerous apo-
Each has a terminal stigma, while near the carpous carpels. Enlarged.

base of each, on either side, is a group of honey-secreting hairs. Ovule

anatropous.

The pollination is not highly specialised ; the symmetry is radial ; the

attractions are colour, and honey on the carpels ; there is slight protandry

and the stamens mature in succession,
so that the supply of pollen is prolonged.
There is a probability of intercrossing,
but self-pollination is possible.

The fruit is a group of follicles, which
open by their ventral sutures, and gap-
ing widely upwards allow their seeds to
escape (Fig. 473, A).

(16) The Buttercup {Ranunculus
acris, L., or other species) is more
specialised, having both calyx and
corolla; but the flower is constructed on
a similar plan ; as follows : (Fig. 474.)
Calyx, sepals 5, polysepalous, inferior,
imbricate in bud, the outermost being
obliquely anterior.

Fig. 473.
A, Follicles of Aconite. (After Figuier.)

B, Achene or nut of Buttercup. (After Figuier.)

Corolla, petals 5, polypetalous, alternating with the sepals, inferior yellow,
with a honey-pouch on the upper face of each, near the base (Fig. 474, 2).

Androecium, stamens indefinite, free, hypogynous ; the outermost maturing
earliest.

Gynoecium, carpels indefinite, apocarpous, superior : each contains only a
single anatropous ovule ; otherwise similar in form to the fewer and larger
carpels of Caltha.

B.B. 2 R

624

BOTANY OF THE LIVING PLANT

The general conditions of pollination are the same as in Caltha, but with
honey at the base of the coloured petals.

Fig. 474-

Buttercup [Ranunculus acris, L.). i, Flower in median section. 2, a single petal.
3, the gynoecium. 4, the same in section. 5, floral diagram.

mm

^m,

Each carpel matures as a dry indehiscent nut, falling away with its single
seed within (Fig. 473, B). Comparing with Caltha the flower is more
elaborate and probably derived from the type of the Helleboreae, by con-
version of the outermost stamens first into honey-
leaves, as in Helleborus or the Globe-Flower, then
into large honey-bearing petals, as in the Buttercup
(compare Fig. 189, p. 264). The carpels meanwhile
had their ovules reduced to one each, while the pro-
pagative power was made up by increase in the
number of carpels.

(17) The Monkshood (Aconitum Napellus, L.) is a
perennial with swollen storage roots, commonly grown
in gardens : it is an important drug. Its inflores-
cence is a raceme, developing as a cymose panicle
below. The flower shows median zygomorphy
(Fig. 475), and consists of :

Calyx, sepals 5, polysepalous, inferior, corresponding
in number and position to the Buttercup, but petaloid ; the posterior sepal
enlarged as a hood.

Corolla, petals usually 8, of which the two obliquely posterior are elongated
into stalked glandular spurs (nectaries), covered by the hooded sepal ; the
rest are small ; polypetalous, inferior.

>Ǥgp>

Fig. 475.

Floral diagram of Aconi-
tum. (After Eichler.)

APPENDIX A

625

Androecium, stamens indefinite, spirally arranged, free, hypogynous.

Gynoecium, carpels usually 3, apocarpous, superior. Follicles and their
dehiscence as in Caltha.

Comparing this flower with Ranunculus or Helleborus, it is clearly a zygo-
morphic development of the same type. The flower is protandrous. The
sepals give colour-attraction, and the honey is conveniently placed for humble-
bees in the posterior, spurred petals, while the whole is sheltered by the hood.
The protandry makes cross-pollination highly probable from successive visits.
After shedding their pollen the filaments curve back so as to expose the
receptive stigmas. Self-pollination is thus improbable.

ORDER : RHOEADALES.

Family : Papaveraceae. Example : The Red Poppy.

(18) The Common Corn-Field Poppy (Papaver Rhoeas, L.) is an annual
which ripens its seeds before the corn is cut, and it is thus ready to spring

Fig. 476.

Red Poppv (Papaver Rhoeas, L.). I. flower and buds, with sepals falling away.
II. flower in median section. III. gynoecium. IV. ripe fruit, with dehiscent pores
below the star-shaped stigma. V. floral diagram.

again in the next season. The plant, which is bristly and contains a milky
juice, consists of a leafy stem branched below. The solitary flowers are
terminal on their long hispid stalks, the buds hanging down, but the stalks
are straight when flowering, and in fruit (Fig. 476). The flower consists of :

Calyx, sepals 2, polysepalous, inferior, falling off at the opening of
the bud.

Corolla, petals 4, polypetalous, inferior, wrinkled in bud, two lateral, two
antero-posterior.

Androecium , stamens indefinite, free, hypogynous.

626

BOTANY OF THE LIVING PLANT

Gynoecium, carpels 8-12, syncarpous, superior ; the stigma sessile, star-
shaped ; the rays of the star indicate the number of the carpels. The ovary
is unilocular : beneath each ray of the stigma (that is, at the junction of
the carpels that compose it) a flat partition extends radially towards the centre,
but without reaching it. The small and numerous ovules are borne super-
ficially on these plates. In others of the Poppy family the carpels may be
fewer, and in Chelidonium only two, as in the Cruciferae. Fruit, a dry capsule
opening by pores below the stigma. Seeds with oily endosperm : they are
scattered out by wind shaking the pore-capsule (see Fig. 244, p. 325).

Pollination. The showy flower attracts insects which come to collect
pollen. There is no honey, and the flower is radial : it is not a highly special-
ised type. Promiscuous cross-pollination may follow from insect visits, but
self-pollination is also possible. Papaver somniferum, L., the Opium Poppy,
belongs to the family.

Family : Cruciferae. Example : The Charlock.

(19) The Charlock, or Field Mustard (Brassica Sinapis, Visiani), is the
common weed that colours cornfields yellow in early summer. It is an
annual, with stem and leaves bristly with stiff hairs. Its germination is

Fig. 477.

Charlock [Brassica Sinapis, Visiani) . I. flower, with parts slightly displaced.

II. ripe and dehiscent fruit. III. floral diagram.

shown in Fig. 3, p. 10. Its inflorescence is a raceme, but with the bracts
abortive (Fig. 179 C). The flower, which is of radial symmetry, consists of :

Calyx, sepals 4, polysepalous, inferior ; in two pairs, the outermost being
antero-posterior, the inner lateral.

Corolla, petals 4, polypetalous, inferior, with a long basal claw. They are
cruciform, and alternate with the sepals.

Androecium, stamens 6, free, hypogynous: two are short and lateral, opposite
the lateral sepals : four are longer in two pairs, opposite the antero-posterior
sepals.

Gynoecium, carpels 2, syncarpous, superior ; ovary bilocular, with many
ovules ; an ill-defined style, and a stigma with antero-posterior lobes. The

APPENDIX A 627

ovules are curved, and seeds ex-albuminous (Fig. 477, I. II.). Fruit, a "siliqua"
which is a dry capsule, the lateral carpellary walls of which split from the
base upwards, leaving the two placentas as a frame with the transparent
septum stretched between them (Fig. 101). The septum is called "false'
because it is formed late, by ingrowths from the two opposite placentas, the
ovary being originally unilocular, as it is in the Poppies and in the Capers.

Pollination. The flowers being grouped are conspicuous, and are visited
for their pollen, and for honey. The honey-secretion is by glands at the
insertion of the short lateral stamens. Insects passing from flower to flower
and inserting their proboscis, will probably effect intercrossing ; but self-
pollination is possible, and it is even provided for by the longer stamens
coming in contact with the stigma as the style elongates. It is not a highly
specialised type of flower. It is very constant in the Cruciferae, and may be
equally well studied in the Wallflower (20). The structure is probably di-
merous throughout, but with a fission of the median petals to form divergent
pairs, and of the median inner stamens to form the four longer. This is
expressed in the floral formula : — S. 2 + 2. P. 2., x , A. 2 + 22, G. £2).

The construction of the flower would then be theoretically as follows :

Two antero-posterior sepals.

Two lateral sepals.

Two antero-posterior petals (by fission resulting in the four oblique petals).

Two lateral stamens, short.

Two antero-posterior stamens (by fission resulting in the four long stamens) .

Two lateral carpels.

Comparison with the Poppies and the Caper Family shows this to be the
probable interpretation of the Cruciferous Flower, and that it is thus referable
to a dimerous origin, with regularly alternating whorls.

The Cruciferae provide a large proportion of garden vegetables, such as
Cabbage, Cress, Turnip, Horse-radish, etc.

ORDER : GERANIALES.
Family : Geraniaceae. Example : The Field Geranium.

(21) The Field Geranium (Geranium pratense, L.) is a strong- growing herb
with opposite, palmate, and stipulate leaves. The inflorescences are lax
cymose panicles (Fig. 478). Each flower consists of :

Calyx, sepals 5, polysepalous, inferior.

Corolla, petals 5, polypetalous, inferior, alternating with the sepals.

Androecium, stamens 10, free, perigynous, with filaments widened at the base.
The five petaline stamens are external, the five sepaline are internal, a con-
dition described as "obdiplostemonous," and notable as an apparent departure
from the rule of alternation of successive whorls.

Gynoecium, carpels 5, syncarpous, superior, with single style bearing five
distinct stigmas. Ovary with five loculi, each containing two ovules, of which
only one usually matures.

The dry fruit is characteristic. The style remains as a firm woody beak.
At ripeness each carpel suddenly splits away longitudinally from the beak
and curving sharply, hurls out its seed to a distance (Fig. 243, p. 324).

The floral formula is S. 5, P. 5, And. 5 + 5, G. 5.

628

BOTANY OF THE LIVING PLANT

Pollination. The flower is showy, and attracts also by honey secretion
outside the bases of the stamens. It is markedly protandrous. The five
outer stamens open first, followed by the five inner, but the stigmas remain
closely appressed together (in.), and expand only after the pollen is shed, when
the anthers curve away from them (iv.). This separation of the sexes both in
time and space makes self-pollination highly improbable, and the plant de-
pends upon cross-pollination resulting from insect visits. In other species
of Geranium, especially the smaller-flowered, self-pollination occurs, the separa-
tion of the sexes being less marked. On the other hand in the Scarlet

Fig. 478.
Geranium pratense,L. I. whole flower. II. the same in section. III. the stamens
at time of dehiscence, the stigmas (stg) still closely appressed. IV. later stage with
stigmas {stg) recurved. V. gynoecium at same stage as IV. VI. ripe fruit dehisced.
VII. floral diagram.

Geranium of gardens (Pelargonium) (22), the flowers are slightly zygomorphic,
and there is a deep honey-gland sunk in the pedicel opposite to the posterior
sepal, a specialisation still more perfected in the Nasturtium (Tropaeolum).

ORDER: TRICOCCAE.

Family : Euphorbiaceae.

Example : The Caper- Spurge, or other
Species.

The Euphorbiaceae, or Spurges, are a very large Family, of which the genus
Euphorbia is an extreme type. They have reduced, unisexual flowers, which
are sometimes isolated, with their floral envelopes developed, as in Phyllanthus ;
but in Euphorbia and others the flowers are closely grouped together, so that
a whole inflorescence may appear, and even functionate as a single flower.
The less reduced types indicate that their relation is with the Geraniales, from
which they may be regarded as an interesting reduction-series.

(23) Euphorbia is represented in the British Flora by many species. They
are herbs or small shrubs with smooth surface, and milky juice. Their leaves

APPENDIX A

629

are exstipulate. but that is not general for the Family. The inflorescence is
very complicated, the apparent unit being the flower-like cyathium, which is
itself a very compact, compound spike (Fig. 479, 1. 11.). These units are
borne like flowers on an inflorescence, which is usually a cymose umbel. The
cyathium itself consists of an external cup, which looks like a calyx, but is
really formed of five coalescent bracts, forming an involucre. On its margin four,
or occasionally five, yellowish glands are borne, a blank space being left on
one side ; there two teeth of the bracts are found, where the missing gland

Fig. 479'

Euphorbia Lathy 7 -us, L. I. flowering shoot. II. a single cvathium. III. Cyathium
th involucre removed. IV. same in section. V. the involucre. VI. a single male

wi

flower. VII. ripe seed with caruncle.

cyathium.

\ III. same in section. IX. diagram of a

might be. Within the cyathium a single stalked female flower occupies the
centre : it projects from the cup, and hangs over the side between the two
bracts which are not separated by a gland. It consists of a gynoecium of three
syncarpous carpels, having three styles with bifid stigmas. It is trilocular, and
one pendulous, anatropous ovule lies in each : the upward-directed micropyle
is covered by a fleshy outgrowth known as the caruncle, which is character-
istic. At the base of the ovary is a distended ring, held to represent the
abortive perianth. The gynoecium is thus superior.

Around the female flower are a number of structures which look like stamens :
they are associated with minute hairy bracts. Each of these is a male flower

630 BOTANY OF THE LIVING PLANT

(vi.), consisting of a single stamen with a bilobed anther. The stalk which looks
like a filament bears about half way down a constricted joint, which is believed
to mark the place of an abortive perianth. The part below it would then be a
pedicel, above it the filament. In Anthostema the perianth is better developed
(Fig. 178, hi. p. 256). If these conclusions be correct, then the Cyathium
is properly regarded as a condensed inflorescence.

The fruit is a capsule ; when it is ripe the carpels separate elastically from a
central column. This type of carpel, though in larger number, is seen in Hura
(Fig. 102, p. 166), another member of the Family. This type of carpel is known
as a coccus, hence the name Trococcae, for the number is usually three. The
similarity to the fruit of the Geraniaceae is striking.

Pollination. The stigmas in any Cyathium have as a rule ceased to be
receptive before the pollen of the same cyathium is shed. Thus the in-
florescences are protogynous.

Important Rubber-yielding plants belong to this Family (e.g. Manihot), and
some drugs, e.g. Ricinus.

ORDER: SAXIFRAGALES.

The Saxifragales probably represent a type from which a number of deriva-
tive groups have sprung. A general floral formula for them is S. n, P. n,
And. n +n,G. n, with the ovary superior, and in the simplest examples, such as
A stilbe, the carpels are separate and many-seeded pods. This type may be varied
by increase in number of the stamens, or of the carpels ; by the sinking of the
carpels more or less completely into the receptacle, so as to give a half-inferior
or an inferior ovary ; and in some cases by reduction of the number of the
carpels to two or only one. The number of ovules may also be reduced. But
still the main framework of the flower remains the same. Most of the related
plants bear stipulate leaves.

Family : Saxifragaceae. Example : Meadow Saxifrage.

(24) The White Meadow Saxifrage (Saxifraga granulata, L.) is a frequent
herb of grass-land, and banks. It bears at its base pink bulbils, by which it
multiplies vegetatively, associated with radical leaves. The flowering stem
bears leaves below, and a definite cymose inflorescence above, with few large
flowers. The whole plant has glandular hairs. The flower-stalk widens out
into a green hemispherical region : this encloses the base of the ovary, which
is thus half-inferior ; while the other floral parts are inserted on its margin
(Fig. 480). The flower consists of :

Calyx, sepals 5, polysepalous, seated on the margin of the receptacle ; the
odd sepal is posterior.

Corolla, petals 5, polypetalous, alternating with the sepals.

Androecium, stamens 5 + 5, the petaline outermost (obdiplostemonous)
inserted round the half-sunk carpels, i.e. half-epigynous.

Gynoecium, carpels 2, half -inferior, oblique ; united in their lower part,
but separate above, with distinct styles and capitate stigmas (half-syn-
carpous). Ovary bilocular, ovules numerous, placentation axile.

Fruit, a dry capsule, with longitudinal dehiscence.

APPENDIX A

631

Pollination. The flowers arc protandrous, the inner series of stamens
ripening first ; then the outer. Nectar is secreted at the upper surface of the

ovary. The flowers are visited by flies
and small bees, and repeated visits will
give a high probability of cross-pollination.
But the flower is not highly specialised,
and self-pollination is possible.

The London Pride (Saxifraga umbrosa,
L.) will serve as an alternative example,
though the flowers are smaller and more
numerous, and the carpels are not so
deeply sunk. It is pollinated by small
flies.

(25) The Red-Flowering Currant (Ribes
sanguineum, Pursh) is native in North
America, and is commonly grown in
gardens. It serves as an example of the
Ribesiaceae, which are usually grouped with the Saxifragaceae, notwithstand-
ing their inferior ovary, and five stamens. The Gooseberry or Currant of
gardens would serve equally well.

Fig. 480.

Median section of the flower of Saxifrage,
showing the carpels half sunk in the recep
tacle, and coherent for the greater part of
their length. (After Figuier.)

Fig. 481.
Inflorescence of Currant : a raceme.

(After Figuier.)

Fig. 482.

Berries of the Currant.
(After Figuier.)

The Inflorescence is a pendulous raceme (Fig. 481), arising in the axil of a
foliage leaf of the previous season. The flowers are hermaphrodite and
actinomorphic, composed as follows :

Calyx, sepals 5, polysepalous, superior, crimson, ; odd sepal posterior.

Corolla, petals 5, polypetalous, alternating with sepals, paler coloured.

Androecium, stamens 5, alternating with petals ; seated on rim of recep-
tacular tube.

632

BOTANY OF THE LIVING PLANT

Gynoecium, carpels 2, syncarpous, ovary inferior, unilocular, with numerous
ovules seated on lateral placentas, ovules anatropous.

Fruit, an inferior berry (Fig. 482).

Pollination. The flowers are attractive by colour, and by grouping in
racemes. Honey is secreted at the base of the receptacular tube. The flowers
are very slightly protogynous, and are pollinated chiefly by bees ; but self-
pollination is also possible.

ORDER : ROSALES.

The Rosales are herbs, shrubs, and trees, with alternate stipulate leaves.
They are widely distributed, especially in temperate zones, and are largely
represented among cultivated flowers and fruits. The flowers are actino-
morphic, and usually pentamerous ; but the stamens are often numerous.
The Order is specially instructive from the variability in development of the
receptacle, so that it includes perigynous and epigynous types. There is also
great variability in the number of the carpels. But still it is a very natural
group, the flowers being referable to the same fundamental construction as
the Saxifragaceae, to which they are closely allied.

Family : Rosaceae. Examples : Apple, Strawberry, Rose, Cherry.
(26) The Apple (Pyrus Malus, L.) is a small tree with long vegetative
shoots and short spurs, upon which the flowers are borne. The leaves are

Fie. 483.

Vertical section through a flower of the Quince (Cydonia). sep=sepa\s. pet =
petals. si = stamens. c = apices of the carpels, elongated into styles. cw=ovules.
n = nectaries. The receptacle is here hollowed out, so that the carpels appear sunk
down into a cavity. (After Church.)

stipulate. The flowers appear in groups, one terminal on the spur, the rest
in the axils of the bracts below it. Each flower, together with two bracteoles, is

APPENDIX A

633

borne on an elongated stalk, which swells immediately below the calyx into
the enlarged inferior ovary. It is thus epigynons (Fig. 212, B, p. 290), and
consists of :

Calyx, sepals 5, polysepalous, superior ; the odd sepal is posterior.

Corolla, petals 5, polypetalous, superior, alternating with the sepals.
- Androecium, stamens indefinite, free, epigynous.

Gynoecium, carpels 5, syncarpous, inferior ; five distinct stigmas are borne
on styles separate above, but more or less distinct below. Ovary with five
loculi, and several ovules in each (Fig. 484, A).

Fruit, consists of the inferior ovary crowned by the persistent calyx. The
five carpels are sunk in the succulent tissue of the receptacle, from which they
are not distinctly marked off. Their inner cartilaginous wall forms the
" core," and one or more " pips," or seeds, are contained in each.

A

B

Fig. 484-
A, diagram of Apple. (After Eichler.) B, diagram of Potentilla Comarum.
C, diagram of Cherry. (After Eichler.)

Pollination. The flowers are attractive by colour, and by easily accessible
honey secreted on the concave surface within the stamens. They are slightly
protogynous, but are not highly specialised. Insects collecting honey and
pollen will carry out cross-pollination, but self-pollination is also possible.

(27) The Wild Strawberry (Fragaria vesca, L.) is a perennial herb with
ternate, stipulate leaves, borne upon a sympodial rhizome. The apex of the
leafy shoot of the preceding year grows up into the inflorescence of the current
year, while it is upon a lateral bud from it that the foliage leaves are borne.
The inflorescence is cymose. Potentilla Comarum will serve as an alternative
example (Fig. 484, B). The flower consists of :

Calyx, sepals 5, polysepalous, seated at the margin of the widened receptacle.
Between the sepals are five additional green lobes, forming what is called an
epicalyx, believed to represent the fused pairs of stipules of the sepals, the
vegetative leaves being stipulate. (The number of sepals in cultivated straw-
berries may be more than five.)

Corolla, petals 5, polypetalous, alternating with the sepals.

Androecium, stamens indefinite, free, perigynous. They are arranged
with some regularity in whorls ; the outermost is of 10, representing five
stamens which have undergone fission.

634

BOTANY OF THE LIVING PLANT

Gynoecium, carpels indefinite, apocarpous superior, seated on the spherical
receptacle. Style springing from the side of each ovary, which contains
only one ovule.

Fruit. A number of dry nuts seated on the receptacle, which has become
distended and succulent, while the calyx is persistent as the " hull " (Fig. 485).

Pollination. The flowers are conspicuous by their white petals, and honey
is secreted on the receptacular cup between stamens and carpels. They are
slightly protogynous. The result of repeated insect visits will thus be a
probability of cross-pollination, though self-pollination is also possible.

Fig. 485.

Succulent receptacle of Strawberry.
(After Figuier.)

Fig. 486.

Vertical section of flower of the Peach, as an example
of a perigynous flower. (After Figuier.)

(28) The flower of the Dog Rose (Rosa canina, L.) is constructed on a
plan similar to that of the Strawberry, but without the epicalyx. The chief
difference is in the receptacle, which instead of being convex with the carpels
carried up on the hemispherical axis, is hollowed into a sunken cavity. This
encloses the numerous bristly carpels, while their stigmas project above.
When mature the receptacle becomes succulent as in the Strawberry, forming
the " hip," with the nutlets or true fruits within.

(29) The Cherry (Primus Cerasus, L.) has a construction of the flower
like that of the Rose, but with only one carpel borne in the hollow, cup-like
receptacle (Fig. 484, C) . It consists of :

Calyx, sepals 5, inserted upon the margin of the cup-like receptacle. The
odd sepal is posterior.

Corolla, petals 5, polypetalous, alternating with the sepals.

Androecium, stamens indefinite, free, perigynous, i.e. inserted on the margin
of the receptacular cup.

Gynoecium, carpel 1, superior. The swollen ovary contains two ovules.

Fruit, a drupe. The receptacular cup here dries up, and falls away. The
wall of the ovary differentiates into a superficial skin, a middle region of succu-
lent pulp, and an inner stony layer. The stone of the mature cherry contains
as a rule only one kernel, which is the exalbuminous seed, developed from one
of the ovules. Sometimes, however, both are matured. The Drupe thus
constructed is the type of fruit of Plums, Apricots, Peaches, etc. (Fig. 487).

Pollination. The Cherry flower is not highly specialised. Anthers and
stigmas ripen simultaneously. Honey is secreted on the hollow surface of the
cup. It is visited by various short-lipped insects.

APPENDIX A

635

The variability of Rosaceous flowers is illustrated by such examples. A
striking feature is the diversity of origin of the succulent pulp, which gives
value to their fruits. In the Strawberry it arises from the convex receptacle ;

Fig. 487.

Drupe of Cherry. (After
Figuier.)

Fig. 488.

Grouped drupels of the Raspberry.
(After Figuier.)

in the Rose from the concave receptacle ; in the Apple and Pear partly from
the receptacle, partly from the carpels ; in the Raspberry it is from the middle
layer of the tissue of the very numerous carpels (Fig. 488) ; in the Cherry
and Plum from the corresponding tissue of the single carpel. The Almond of
shops is the seed taken from the drupe of Amygdalns.

ORDER : LEGUMINALES.

This is one of the largest and most important Orders of Plants, and it is
cosmopolitan. It is characterised by its gynoecium, which consists of a single
carpel, ripening as a Legume, or Pod, as in the Pea. It is divided into three
Families : the Mimoseae, which are the most primitive, having flowers of
radial symmetry, as in Mimosa and Acacia ; the Caesalpineae, which have
zygomorphic flowers, as in the Tamarind, or Cassia ; and the Papilionaceae,
or Pea-flowers. The last of these only are represented in the British Flora.
The Order yields most varied products of importance : food-stuffs, timbers,
drugs, and dyes, etc.

Family : Papilionaceae. Examples : Trefoil, Pea.

(30) The Bird's Foot Trefoil (Lotus comiculatus, L.) forms many straggling
branches from a central root-stock. The leaves have green " stipules " which
are really basal pinnae, and three distal lobes, hence its name. The flowering
branches are leafless, but bear distally a leafy bract and a radiating group of
flowers. Each flower is strongly zygomorphic, its plane of symmetry being
vertical and median, as it is in all of the Papilionaceae (Fig. 489). It is com-
posed thus :

Calyx, sepals 5, gamosepalous, inferior, the odd sepal being anterior. The
calyx-tube gives mechanical support to the internal parts, and is slightly
widened in a perigynous manner.

Corolla, petals 5, polypetalous, inserted separately on the slightly widened
receptacle, i.e. perigynous. The petals alternate with the sepals. The

636

BOTANY OF THE LIVING PLANT

posterior petal is the large vexillum or standard ; the two lateral are the
alae or wings, which invest the anterior carina, or keel. The latter is formed
from two obliquely anterior petals, inserted by separate stalks, but fused
distally, so as to enclose the stamens and carpel.

Androecium, stamens 10, perigynous : nine are united by their stalks into
a tube ; the tenth, which is posterior, is separate to its base. The anthers are
completely enclosed in the carina.

Gynoecium, carpel i, apocarpous, superior. It is a pod, containing numerous
ovules, with placentation on the posterior margins. The style is longer than

Fig. 489.

Flower of Lotus corniculatus, L. I. flower compleie. II. with vexillum removed.
III. with alae and part of carina removed. IV. carina slit longitudinally. V. one of
the alae. VI. stamens of different length. VII. carpel in section. VIII. floral
diagram. s= sepals; v = vexillum ; A = alae; e = carina; s = stamens; st= stigma.

the stamens, and bears a capitate stigma. The pod is almost surrounded by
the united filaments, but access to the honey-secretion, which is on the enlarged
receptacle round its base, is gained through the slits right and left of the
separate stamen. The fruit when ripe is a dry pod, splitting longitudinally
into halves.

Pollination. The mechanism of the flower is elaborate, and secures cross-
pollination notwithstanding the close relation of anthers and stigma, which
are both enclosed in the funnel-like carina. For the stigma is not receptive
until it has been rubbed, and remains infertile till the insect-visitors, which
are bees, arrive. Searching for nectar, and guided by the converging red
lines on the standard, the bee alights on the projecting wings ; its weight is
transmitted by their interlocking surfaces to the keel, which is thus depressed,
and yields. The stiff stamens and carpel do not yield, and so first the stigma,
and then the anthers with their pollen project through a pore which is left
open at the tip of the keel. When the weight of the bee is removed the keel
rises, and the stamens and stigma are again enveloped, and are ready for a

APPENDIX A 637

fresh insect visit. The effect of such visits will be, first, to rub the stigma, and
make it receptive, while as it emerges first from the keel it receives any pollen
brought from other flowers by the bee ; second, to deposit a fresh supply of
pollen on the insect. A cross-pollination is thus a virtual certainty.

(31) The flower of the Garden Pea (Pisum sativum), may be taken as an
alternative type, the construction being essentially the same as in Lotus,
though differing slightly in details of its mechanism. The weight of the visit-
ing-insect depresses the interlocking wings and keel as before ; but the latter
is closed only along the anterior margin, so that when it is depressed the
stirrer stamens and carpel rise out of the boat- like keel, and come in contact
with the lower surface of the insect. The style bears a brush of hairs, which,
as it rises, sweeps out the pollen on to the insect's body ; but the stigma reaches
its body first, and receives thus such pollen as it may have brought. The
flower is elastic and recovers, making successive visits possible. The mechan-
ism is less precise than in Lotus, but still very effective. It requires a strong
insect, and in absence of cross-pollination self-pollination is possible.

These examples serve to illustrate the exact mechanism of the Papilionace-
ous flower, and the way in which slight differences may affect the process of
their pollination.

ORDER; UMBELLALES.

Family : Umbelliferae. Example : Cow Parsnip.

(32) The Cow Parsnip (Heracleum Sphondylium, L.) is a coarse perennial
herb, with massive storage stock, which sends up the annual leafy and flowering
shoot. The stem is hollow and fluted, and bears alternate leaves with broad
sheathing base, and irregularly cut lamina. The main inflorescence is terminal,
but others may arise in the axils of the upper leaves. It is a compound
umbel (p. 262, Fig. 185). The flowers are individually small, but many being
grouped together, and all at the same level, the aggregate inflorescence becomes
a conspicuous feature. Each flower is borne upon a slender hairy stalk, which
widens out just below the flower itself into a flattened green body. This
is the inferior ovary, and the flower is epigynous (Fig. 490). Care should be
taken to select perfect flowers for observation, as the parts fall away early.
The flower consists of :

Calyx, sepals 5, superior, present as minute teeth visible between the petals.
The odd sepal is posterior.

Corolla, petals 5, polypetalous, free, superior. Each is notched at the free
edge. In the marginal flowers the petals are unequal, the outermost being the
largest.

Androecium, stamens 5, free, epigynous, alternating with the petals ; bent
inward in bud, but straightening when mature.

Gynoecium, carpels 2, syncarpous ; stigmas 2, styles widening downwards
into two yellowish green nectaries. Ovary inferior, bilocular, with one pendu-
lous ovule in each loculus.

Fruit, a flattened oval body, which matures dry. When ripe it splits into
two halves (mericarps), attached at first by a slender middle column/from which
they later break away, and are readily carried by the wind. Each mericarp
contains a single albuminous seed, and is marked by elongated oil-glands, four
on the outer and two on the inner flattened sides* .

6*8

BOTANY OF THE LIVING PLANT

Pollination. The aggregate inflorescence attracts the attention of insects
from a distance : the slight zygomorphy increases its effect. It is visited by
various insects, the exposed honey being accessible alike to short-tongued
and long-tongued. The individual flowers are protandrous, the stamens often
falling before the stigmas are mature. Cross-pollination is probably by insects
crawling from flower to flower, but self-pollination is still possible.

Fig. 490.

Heracleum Sphondylinm. I. whole flower seen from above. II. the same seen
from the side. III. gynoecium. IV. fruit. V. ditto in section. VI. mature fruit
with mericarps separate. VII. fruit in transverse section. VIII. floral diagram.

There is in the Cow Parsnip, but still more in other Umbelliferae, a partial
separation of the sexes in space as well as in time ; since the pistil is
degenerate in the later-formed flowers, they are practically male. That
is seen markedly in Astrantia and in Myrrkis. The condition is described as
andro- diclinous, and clearly it will promote cross-pollination, while it also
secures the pollination of those hermaphrodite flowers which are formed
late, and are protandrous.

The Umbelliferae are a numerous and wide-spread Family of herbs, often
strong-smelling, and poisonous. But it includes cultivated plants, such as
Carrot, Parsley, Celery, Caraway. Some yield drugs (Asafostida), and oils.

DICOTYLEDONEAE—SYMPETALAE.

Those Dicotyledons which have their petals united, that is sympetalous or
gamopetalous, are held to show in this respect a position in advance of the
Polypetalous types. A further character which they have in marked degree
is that their flowers are strongly cyclic, and the number of parts more definite

AITKXIHX A

639

than in flowers of more primitive construction. They are divided into two
series on the broad feature of the number of the whorls in which their parts
are arranged. Those with five whorls have the general formula, S. v, P. n,
A. n + n, G. n - , and they are styled the Pentacyclicae. Those with only four
whorls have the general formula, S. 11, P. n, A. n, G. n - , and they are styled
the Tetracyclicae. The number of the carpels is usually below the typical
number («). The number of the stamens is also frequently less than (>i)
or n + n, especially in those flowers where the pollination-mechanism is
specialised.

(a) PENTACYCLICAE.

ORDER : BICORXES.

Family : Ericaceae. Example : Cross-leaved Heath.

(33) The Cross-leaved Heath (Erica Tetralix, L.) is a shrubby moorland
plant, mycorhizic like the rest of the family, bearing minute stiff leaves, studded
with red, stalked glands. The margins of the leaves are reflexed, so that
the lower stomatal area is concave, and more or less closed. All these features
are xerophytic.

Erica Tetralix.

Fig. 491.
I. whole flower from outside. II. flower in section.

III. floral diagram.

The flowers are borne in dense racemes, and are pendulous on pedicels
bearing two bracteoles (Fig. 491). They consist of :

Calyx, sepals 4, polysepalous, inferior, glandular.

Corolla, petals 4, gamopetalous, inferior, globose, with a narrow opening,
through which the capitate stigma projects.

Androecium, stamens 4+4, free, hypogynous, with curved filaments;
anthers dehiscing by two distal pores, which face downwards. From the base
of each anther two divergent spurs project outwards to the inner surface of
the corolla.

b.b. 2 s

640 BOTANY OF THE LIVING PLANT

Gynoecium, carpels 4, syncarpous, superior, style elongated, with capitate
stigma. Ovary with 4 loculi, ovules minute, numerous, on an enlarged axile
placenta. Honey-disc round the base of the hairy ovary.

Fruit, a loculicidal capsule, from which the minute seeds are shaken by wind.

Pollination is by bees, which hang on to the pendent flowers. The bee first
touches the sticky stigma, depositing pollen it may have brought from another
flower ; then inserting the proboscis, it collides with the spurred stamens,
shaking out a shower of dry pollen. Thus there is a high probability of cross-
pollination, though self-pollination is possible by some falling upon the stigma
of the same flower. The gamopetalous corolla with narrow mouth, and the
spurred stamens exclude small thieving insects.

(34) Compare the Bilberry (V actinium Myrtillus, L.), in which the floral
structure is essentially the same, but the ovary is here inferior. In this genus,
and in the Ericaceae generally, there is frequent meristic variation, the flowers
being either tetra-merous or penta-merous.

ORDER : PRIMULALES.
Family : Primulaceae. Example : Common Primrose.

(35) The Primrose {Primula vulgaris, Huds.) is a perennial with its stock
covered with old leaf-bases, and ending in a rosette of leaves of the current
year. The flowers are borne singly in the axils of bracts, which succeed the
foliage leaves. There are two types of flower which are borne on distinct
plants : " pin-eyed," with the stigma occupying the centre of the flower, and
" thrum-eyed," where its place is taken by five anthers. It will be seen,
however, that in number and arrangement of the parts both are alike : the
difference is one of proportion of development of the parts (Fig. 492). For
convenience a " pin-eyed " flower may be taken ; it consists of :

Calyx, sepals 5, gamosepalous, inferior, forming a tube supporting the corolla.
The odd sepal is posterior.

Corolla, petals 5, gamopetalous, inferior, alternating with the sepals, and
forming a long narrow tube below, with five lobes diverging at right angles

from it.

Androecium, stamens 5, epipetalous, inserted with very short filaments
half-way up the tube of the corolla, and opening inwards (Fig. 492, 11.).
Note that the stamens are opposite the petals (anti-petalous) .

Gynoecium, carpels 5, syncarpous, superior ; style elongated so as to carry
the pin-headed stigma to the throat of the corolla. Ovary turgid, unilocular ;
ovules numerous, placentation axile.

Fruit, a capsule opening distally by ten teeth, which become reflexed.
The number five of the carpels is inferred from comparison with other flowers,
and from the parts of the Primrose itself. The ten teeth of the fruit support this
view. The anti-petalous position of the stamens, and the number (n), instead
of (n + n), suggests that five sepaline stamens have disappeared : this conclusion
is supported by the fact that in Samolus, Lysimachia, etc., five small
staminodes are present in the place where the missing stamens should be.
The family shows meristic variation, the whorls varying in number of parts
from four to nine. Trientalis and Lysimachia are specially variable.

APPENDIX A

641

Pollination. Compare first the " thrum-eyed " type of flower. The parts
are numerically the same as in the " pin-eyed " ; but the style carries the
stigma only half-way up the corolla-tube, corresponding in level to the stamens
of the " pin-eyed." The stamens are inserted at the throat of the corolla,
corresponding in level to the stigma of the " pin-eyed." The effect of this
" dimorphism " is to increase the probability of intercrossing as a consequence

Fig. 492.

Primula vulgaris. I. short-styled type of flower (thrum-eyed) in section. II. long-
styled type (pin-eyed). III. pollen of short-styled. IV. of long-styled types.
V. stigmatic papillae of short-styled. VI. of long-styled type. VII. floral diagram.

of repeated visits from bees to the two types of flower, which are borne on
different plants. The sticky pollen deposited on the proboscis of the bee
from the " pin-eyed " type will correspond in level to the stigma of the " thrum-
eyed," and the pollen of the latter to the stigma of the former. These are
what have been called " legitimate " crosses, and they have been shown to be
more prolific than the " illegitimate ' crosses, between parts of unequal
length. But self-pollination is not precluded. The gamopetalous corolla is
effective in excluding smaller insects, while bees are attracted by honey,
colour, and scent.

(d) TETRACYCLICAE.
ORDER : PERSON ATAE.

This Order includes a large number of showy plants of temperate and
tropical climates, with tetracyclic, gamopetalous flowers, having the general
formula S. 5, P. (5), A. 5, G. (2). The ovary is superior, and bilocular, and the
number of ovules borne on the axile placenta is usually large.

042

BOTANY OF THE LIVING PLANT

Family : Solanaceae. Examples s Nightshade, Potato.

(36) The Deadly Nightshade (Atropa Belladonna, L.) is a perennial herb
of shrubby habit, with entire leaves having a clammy glandular surface. It
bears its flowers solitary in the axils of leafy bracts. The whole inflorescence,
which is cymose, starts with a single terminal flower : below this strong
branches develop, the ultimate branchings of which are complicated by
adhesions. The flower consists of :

Calyx, sepals 5, gamosepalous, inferior ; the odd sepal is posterior.

Corolla, petals 5, gamopetalous, inferior, alternating with the sepals ; very
slightly zygomorphic.

I. • 7.

Fig. 493.

Solatium tuberosum. I. flower. II. pistil, and persistent calyx. III. stamen with
porous dehiscence. IV. seed in median section. V. floral diagram.

Androecium, stamens 5, hypogynous, epipetalous ; filaments curved.

Gynoecium, carpels 1, syncarpous, superior, placed obliquely to the median
plane ; style elongated, stigma capitate, ovary bilocular, ovules numerous,
placentation axile. A honey-disc surrounds the base of the ovary.

Fruit, a large black berry, surrounded by the persistent green calyx. Seeds
albuminous, embryo curved.

Pollination. The colour and honey-secretion offer attractions to bees,
especially humble-bees, while the gamopetalous corolla and the stiff hairs
at the base of the filaments tend to exclude small crawling insects. The
stigma and anthers mature almost simultaneously. The stigma projects
beyond the curved stamens, thus there is a probability of cross-pollination
from visits from humble-bees, but the flower is not highly specialised.

(37) The Potato (Solatium tuberosum, L.) is an herbaceous plant that
reproduces itself by tubers (Fig. 147, p. 218). But it commonly flowers also
in cymose inflorescences, which are without bracts. The flowers of the
cultivated varieties are apt to show abnormalities. The normal structure is
like Atropa in number and arrangement of parts. But the corolla is wheel-
shaped, and expanded in a vertical plane, while the five projecting stamens
open by terminal pores (Fig. 493, in.) . The stigma projects beyond them.
There is no honey-secretion. The native habitat is South America. The
arrangement of the flower might lead to crossing if the suitable insects
were present, but here insects rarely visit the flowers. Self-pollination is
possible, and fruit is often set. The fruit is a berry.

APPENDIX A

643

ORDER: PERSONATAE.

Family : Scrophulariaceae. Examples : Figwort, Speedwell.

(38) The Figwort (Scrophularia nodosa, L.) is a common plant of moist
soil, with upright four-angled stems bearing decussate leaves, and terminating
in lax cymose panicles of tawny purplish flowers. They are zygomorphic,
and strongly protogynous (Fig. 494). Each flower consists of:

Calyx, sepals 5, slightly gamosepalous, inferior. The odd sepal is posterior.

Corolla, petals 5, gamopetalous, inferior ; two-lipped.

Fig. 494.

Scrophularia nodosa. I. flower in anterior view with stigma recurved, and the
stamens dehiscent (late stage). II. same in section. III. flower seen laterally with
projecting stigma (early stage). IV. floral diagram.

Androecium, stamens 4, epipetalous ; the fifth posterior stamen represented
by a prominent staminode below the upper lip.

Gynoecium, carpels 2, syncarpous, superior, antero-posterior ; style elongated
stigma capitate. Ovary bilocular, with numerous ovules on an enlarged axile
placenta. A yellow honey-disc surrounds the base of the ovary. Fruit a
dry capsule, which splits septicidally, and liberates the numerous albuminous
seeds, with straight embryos.

Pollination. The tawny colour of the flower attracts wasps, which are
the pollinating agents. The flowers are strongly protogynous. While the
stamens are still tightly packed in the globose corolla, the stigma protrudes
so as to meet any visiting insect, and receives any pollen she may bring (111.).

644

BOTANY OF THE LIVING PLANT

Later it is strongly recurved, and the stamens then straighten their filaments,
carrying their anthers outwards, partly blocking the lower side of the corolla-
tube (i. II.). They will thus deposit their pollen on the ventral surface of the
visiting insect, conveniently for transfer to the stigma of flowers in the earlier
female stage. The posterior stamen from its position could not do this : it
is superfluous, and is reduced to a staminode. Cross-pollination is thus highly
probable, but self-pollination is possible by pollen falling from the anthers
upon the still receptive stigma.

The Foxglove (Digitalis purpurea, L.) has a similar structure, but its wide
bell is suitable for humble bees, the pollen being deposited on their backs.

Flower of Veronica Chamaedrys, typically pentamerous : but the posterior sepal
is abortive : the two obliquely posterior petals are fully fused to form apparently
one ; only the two obliquely posterior stamens are developed.

The flowers are here protandrous. Cross pollination is probable, but self-
pollination is still possible. Comparison of the Foxglove with the Figwort
shows how, with the same plan of floral construction, there may be differences
in the detail and in the agent of cross-pollination.

(39) The Germander Speedwell (Veronica Chamaedrys, L.) is a common
perennial of road-sides and banks. It has long ascending shoots bearing
decussate leaves. The racemose inflorescences arise in the axils of the upper
leaves. Each flower has a slender stalk, and consists of the following
parts

Calyx, sepals 4, slightly gamosepalous, inferior. Though the number
appears to be four, comparison with other species of Veronica, and with other
related plants, such as the Foxglove, shows that a fifth sepal, which should be
median and posterior, is here wanting (Fig. 495, 11.).

Corolla, petals apparently four, gamopetalous, inferior, alternating with
the sepals and forming a wheel-shaped (rotate) corolla, which readily falls

APPENDIX A 645

away in one piece. Comparison with related plants shows that the large
posterior petal is really the equivalent of two obliquely posterior petals fused
together. In front view the petals are marked by lines, or honey-guides, con-
verging to the centre of the flower (Fig. 495, 1.).

Androecium, stamens 2, epipetalous, diverging widely right and left. Com-
parison shows that these correspond to the two obliquely posterior stamens,
while those obliquely anterior are abortive, as well as the median posterior
stamen (Fig. 495, 1., in., vn.).

Gynoecium, carpels 2, antero-posterior, syncarpous, superior. The single
style projects between the diverging stamens, and bears a capitate stigma.
Ovary bilocular, ovules numerous, placentation axile.

Fruit, a dry capsule.

Pollination. The flower is a specialised and reduced example of the Scro-
phulariaceae. The change from a pentamerous to an apparently tetramerous
type follows from the abortion of the posterior stamen, which leads to abortion
of the posterior sepal and fusion of the two obliquely posterior petals. The
obliquely anterior stamens are also abortive, not being necessary for effective
pollination by flies. The rotate corolla is expanded in a vertical plane, with
the style and two stamens projecting horizontally. The insect alighting
on the flower gains a foothold by grasping the stamens, drawing them together
so that they deposit pollen on the under surface of his body. On going to
another flower the stigma receives this, before an additional supply from that
flower can be deposited. The result is a high certainty of cross-pollination,
with high improbability of self-pollination ; and it is effectively carried out
without the three stamens that are abortive.

ORDER : VERBENALES.

_

Family : Labiatae. Examples : Dead-Nettle, Sage.

(40) The Labiatae are a very large Family including herbs and shrubs
spread through warm and temperate regions, and characterised by their
four-angled stamens and decussate leaves. They have often an aromatic
smell : Mint, Sage, and Lavender are examples. Their floral structure is
very constant. The flowers are either solitary or in axillary cymes. The
White Dead Nettle (Lamium album, L.) illustrates the leading features. Its
flowers, which are in crowded " verticillasters," show their cymose arrange-
ment by the fact that the flower directly in the axil of the leafy bract opens
first, and those right and left successively later. Each flower is strongly
zygomorphic, with its median plane vertical (Fig. 496). It consists of :

Calyx, sepals 5, gamosepalous, inferior, odd sepal posterior.

Corolla, petals 5, gamopetalous, inferior ; strongly two-lipped. One large
petal forms the anterior lower lip, two smaller petals guard the entrance to
the tube laterally, the upper lip forms a hood, composed of two obliquely
posterior petals. The corolla is easily removed in one piece, its tube is narrow
below, but widens upwards.

Androecium, stamens 4, epipetalous. The fifth posterior stamen is absent,
its place being inconveniently behind the style. The anthers, of unequal
length and opening downwards, lie below the hood of the corolla. In this
Family sometimes the outer, sometimes the inner pair are the longer.

646

BOTANY OF THE LIVING PLANT

Gynoeclum, carpels 2, syncarpous, superior, antero-posterior ; style elon-
gated ; stigma two-lipped, lying between the pairs of anthers, with lobes
widely divergent, the anterior lobe directed downwards. The ovary 4-partite,
with one anatropous ovule in each. It is really bilocular, with two ovules
in each loculus ; but it becomes " falsely " quadrilocular by intrusion of a
septum between each pair of ovules. Nectaries are found at the anterior
base of the ovary. The nectar accumulates in the narrow lower part of the
corolla-tube, protected by a fringe of hairs which grow inwards from the tube
of the corolla just above the ovary.

Fig. 496.

Lamium album. I. flower seen laterally. II. same in frontal view. III. dissection
showing ovary and style, and the base of the corolla-tube, with insertion of stamens
and fringe of hairs. IV. ovary as seen from above. V. floral diagram.

Fruit, four dry nutlets, two being derived from each carpel. They remain
till shed, enclosed by the persistent calyx.

Pollination is by bees, which alight on the lower lip of the corolla and insert
Jie proboscis into the tube. The bee's body fills the space between the upper
and lower lips, so that its back presses against the stamens and stigma. The
anterior lobe of the stigma projects further downwards than the stamens, so
that it first touches the back of the bee, receiving pollen if she has brought any
from another flower. She then receives pollen from the anthers which open
downwards. The flower is homogamous, that is, the stigma is receptive at the
time when the pollen is shed. Self-pollination is therefore possible, but there
is a high probability of cross -pollination.

(41) The Dead Nettle is highly specialised, and deposits the pollen on a
limited area of the insect's body ; but a still higher degree of specialisation is
seen in Salvia pratensis, or other species. The plan of the flower is the same as

APPENDIX A

647

in Lamium, but, as the mechanism is more precise, sufficient probability of
pollination can be secured with greater economy of pollen than in other Labi-
atae (Fig. 497). Only the two obliquely anterior stamens are matured, the
posterior are represented by minute staminodes, or are quite abortive. The
anthers of the two well-developed stamens have the " connectives " between
the anther-lobes elongated, so that they are separated by about half an inch.
Each anther is affixed midway on the short stout filament of the stamen by
a flexible joint, so as to be able to move like the lever of an Egyptian well.
One of the lobes is directed forwards, and this develops normal pollen ;
the other is directed backwards and develops as a sterile knob. This is so
placed as to block the entrance to the corolla-tube, while the fertile lobe rests

Fig. 497.

Pollination of Salvia pratensis. i, Flower visited by Humble Bee, showing the
projection of the curved connective from the helmet-shaped upper lip, and the deposit
of the pollen on the back of the Bee. 2, older flower, with connective withdrawn
and elongated style. 4, the staminal apparatus at rest, with connective enclosed
within the upper lip. 3, the same when disturbed by the entrance of the proboscis
of the Bee in the direction of the arrow. /= filament. c = connective. s = the
obstructing half of the anther, which produces no pollen. (After Strasburger.)

under the hood of the corolla. The flower is strongly protandrous, the
style being hidden, and the stigma-lobes appressed at the time of shedding
of the pollen.

If a large insect, perching on the lower lip of the flower of Salvia, inserts its
proboscis, the sterile lobes will be pressed upwards, and this will cause the
fertile lobes to descend, depositing the pollen over a definite area of the insect's
back. In older flowers that have already shed their pollen, the style elongates :
its lobes diverge and take such a position that the stigma touches that region
of the insect's body which received the pollen from the younger flowers.
Cross-pollination thus follows on repeated visits to flowers of various ages ;
and it is effected with a high degree of certainty, though in each flower only
two half-anthers are fertile. Economy of pollen follows on perfection of the
mechanism.

ORDER : SYNANDRAE.
Family : Compositae. Examples : Ox-Eye, Dandelion, Cornflower.

The Family of the Compositae consists mostly of herbs. It is of world-
wide distribution, and is the largest Family of Flowering Plants. It is char-
acterised by having the gamopetalous flowers collected into capitula, or heads.

648

BOTANY OF THE LIVING PLANT

Each head is surrounded by a common involucre of protective bracts. The
whole head is equivalent biologically to a single flower, and behaves as such,

..">

Fig. 499.

Floral diagram for Compositae.
(After Eichler.)

Fig. 498.
Inflorescence of Daisy : a capitulum. (After Figuier.)

though morphologically it is a closely packed, spicate inflorescence. The
Dandelion and Daisy are familiar examples (Fig. 498). The structure of the

individual flower is already known in the case of
0 the Sunflower, by the study of its development

..-*.-.^ (Chapter XIV., p. 273, Fig. 198). Each flower is

there seen in the normal position, i.e. in the axil
of a bract ; it consists of 5 petals, 5 stamens, and
2 carpels. The transverse section of the flower
approaching maturity shows these parts arranged
as in a floral diagram. The odd petal is anterior ;
the stamens alternate with the petals, and the
carpels are antero-posterior (Fig. 198, viii.). The
ovary is inferior, unilocular, and contains one ovule.
This structure is fundamental for all Compositae
(Fig. 499).
As in similarly crowded inflorescences (for in-
stance the cyathium of the Spurges, p. 629), the crowding brings with it
reduction of the individual flowers, but it does not go so far in the Compositae
as in the Spurges. The most usual modification is the reduction of the calyx,
its protective function having devolved upon the involucre. Sometimes it
is absent, as in the Daisy ; or it may be represented by two or three teeth, as
in Bidens (Fig. 251, E). But most frequently it is replaced by a number of
bristles composing what is called the " pappus," which serves as a means of
fruit-dispersal, taking the form of a " pappus " composed of bristles, which
spread like a parachute (Fig. 247, p. 326). The bract subtending each flower
is often abortive, as in the Dandelion and Daisy. The flowers themselves
though typically hermaphrodite are liable to become unisexual by abortion.
These are all features of reduction, following on the aggregation of the flowers
in the compact inflorescence.

The flowers may develop in three different ways, though all are fundamen-
tally of the same construction, having the general formula, S, (5, or less, or o,

or x), P. 5, A. 5, G. (2). The first type is radially symmetrical, with five equal

» '

petals. This is probably the original type, and is characteristic of the florets
of the disc (Fig. 500, in., iv.). A second type is seen in the ray-florets (Fig.
500, v.), in which the corolla is tubular below, but the three anterior petals are
elongated into a long strap-shaped ray, as shown by the three distal teeth ;
the two obliquely posterior are reduced or absent. These ray-florets are

APPENDIX A 649

frequently female, or neuter. A third type is the ligtdaie floret, in which the
corolla is split on one side, and all the five petals, as shown by the five distal
teeth, are elongated into a strap-shaped ray ; but here all five join in its
formation (Fig. 502). According to the type of flower the Family is divided
into two Sub-families : (i) The Tubuliflorae, in which the flowers are all tubular,
or the outer may be developed as ray-florets (Fig. 500). They have watery
juice. Examples are the Groundsel, Daisy, Sunflower, and Cornflower,
(ii) The Lignliflorae, in which all the flowers are ligulate (Fig. 502). They
have milky juice. Examples are Dandelion or Hawk-weed.

(i) Tubuliflorae.

(42) The Common Groundsel (Senecio vulgaris, L.) is one of the commonest
weeds of cultivated ground. It is annual and herbaceous, with branched
leafy stem, bearing a few heads drooping when young, erect when old.
The single head, examined in full flower, shows a green involucre of bracts, the
outer short, the inner long, with black tips. These surround numerous
tubular disc-florets. Ray-florets are usually absent. The single disc-floret
consists of :

Calyx, replaced by numerous bristles (pappus), rising from the top of the
inferior ovary.

Corolla, petals 5, gamopetalous, superior, rather longer than the pappus.
The five equal teeth are borne at the end of the corolla-tube, which is narrower
below and widens upwards into a bell.

Androecium, stamens 5, epipetalous, alternating with the petals, inserted
by five distinct filaments on the throat of the corolla-tube at the point where
it dilates. Anthers united into a tube (syngenesious).

Gynoecium, carpels 2, syncarpous, ovary inferior, unilocular, with one
ascending, anatropous ovule. Style elongated, bearing in fully matured
flowers two antero-posterior lobes, which diverge beyond the tube of the
anthers.

Fruit, a brown striated nut, bearing the wide-spread pappus at the tip,
by means of which it is distributed by the wind.

Pollination. If several flowering heads be examined from above it will be
seen that the flowers mature in acropetal succession, the oldest being outside.
The corolla bursts, the syngenesious and introrse anthers protrude, and the
pollen is driven out of them by the elongating style, the stigmatic lobes being
still appressed ; later these expand, exposing their inner receptive surfaces.
The flowers are thus protandrous. There is honey-secretion in the corolla-
tube, but the flowers are rarely visited by insects and self-pollination is
certainly common.

(43) The Ox-Eye Daisy {Chrysanthemum Leucanthemum, L.), belonging
also to the Tubuliflorae, is a common perennial of dry ground (Fig. 500). The
capitulum is solitary on the end of a stem, which widens out to form the general
receptacle. From its margin arises the involucre of bracts, with membranous
margins. Within are numerous florets inserted on the receptacle, but without
any bracts subtending them. Centrally are the yellow florets of the disc,
peripherally the white ray-florets. Each disc-floret consists of :

Calyx, represented only by a rim round the upper limit of the inferior
ovary. There is no pappus.

650

BOTANY OF THE LIVING PLANT

Corolla, petals 5, gamopetalous, superior.

Androecium, stamens 5, alternating with the petals, epipetalous, inserted
by separate filaments upon the throat of the corolla, but with the anthers
united laterally into a tube (syngenesious).

Gynoecium, carpels 2, antero-posterior, syncarpous. Ovary inferior, uni-
locular, containing a single anatropous ovule. Style elongated, and bearing
up through the tube of the anthers the two lobes of the stigma, which diverge
beyond it in the later stages of flowering (Fig. 500, vii.).

'■■■■ W

Fig. 500.

I. Whole capitulum of Chrysanthemum. II. same in median section. III. disc-
floret in earlier (male) condition. IV. same in later (female) condition. V. ray-
floret. VI. style and stigma. VII. disc-floret in section. VIII. floral diagram.

The white ray- floret consists of :

Calyx, as before.

Corolla, petals 5, gamopetalous ; tubular below, elongated above into a
narrow strap-shaped ray representing the three anterior petals, the two
posterior being here obsolete.

Androecium, absent.

Gynoecium, as before (Fig. 500, v., VI.).

Fruit. Each flower produces a dry achene, which at maturity is shaken
out from the protecting involucre. There is no pappus.

Pollination. The mechanism is here essentially the same as in Groundsel,
but with addition of the attractive ray-florets. In the first flowering stage the
disc-florets offer pollen, in the second stage the expanded stigmas to the insects
that are attracted by the colour and honey (iii. iv.). Any crawling insect will
effect crossing. But if this fails self-pollination is also possible.

(44) A third more elaborate type is seen in Cornflower (Centaurea Cyanus,
or C. montana will serve). The general structure is the same, but the ovoid
head is tightly enclosed by the appressed bracts with brown margins. The
receptacle is flat and bristly. The flowers are all tubular, but the outermost
are neuter, and coloured, with long tubular two-lipped and 5-lobed corolla,
and abortive stamens and ovary. The inner florets are hermaphrodite, and
of the usual type, with pappus of short unequal bristles. The lower part of
the corolla-tube is tubular and narrow, the upper is globose, bearing five

APPENDIX A

651

A

distal lobes. The syngenesious anthers form a dark purple tube, with a
terminal beak. The style bears below the stigma-lobes a ring of bristles,

which acts like a sweep's brush upon the
pollen. The flowers are protandrous as
before. The filaments are curved and sensi-
tive, contracting on the stimulus of touch.
This is received by hairs radiating out
from them ; honey is secreted at the base
of the corolla (Fig. 501).

The insect visitors are most commonly
bees. Inserting the proboscis into the tube
of a floret with stigma not yet receptive, the
filaments are stimulated ; they straighten
and contract, drawing the anther-tube
downwards. The bristles of the style thus
brush out the pollen at the moment the
insect is there, and it is deposited on his
body. If he then passes to a floret with
stigmas expanded cross-pollination is ensured.
But self-pollination is also possible by curva-
ture of the stigmas to touch the pollen
carried on the stylar brush. These examples
show how differences of detail in the florets
of the Tubuliflorae may be effective in
pollination : the fundamental facts being
a protandrous condition, and an aggregated
inflorescence.

- 6

B

Fig. 501.
Stamens and style of Centaurca-
A, in the unstimulated, B, in the
stimulated state. The style in the
latter projects beyond the anthers,
and the pollen has been brushed out.
(After Strasburger.)

(ii) Liguliflorae.

(45) The Common Dandelion, or any Hawk-weed, will serve as an example.
The Dandelion (Taraxacum officinale, Web.) is a perennial herb, with massive
storage root, a rosette of radical leaves, and solitary, long-stalked heads.
The tissues are traversed by branched latex-tubes containing milky-juice.
The head consists of an involucre of bracts (Fig. 5°2» *), seated at the margin
of a naked, pitted general receptacle (gr.). Within are numerous ligulatc
florets, which are all alike, and have the same number and relation of parts
as in the Tubuliflorae. But the split ligulate corolla shows by its five teeth
at the distal end that it is composed of five petals.

The pollination-mechanism is founded on protandry. The elongating style
sweeps out the pollen during the first stage of flowering ; the stigma then
expands and is receptive during the second stage. The heads expand in
sunshine, and intercrossing is possible by many different insects. Self-pollina-
tion is also possible by the recurved stigmas coming in contact with pollen
adhering to the style. It has, however, been found that in certain cases the
fruit of the Dandelion can be matured without any pollination at all, even
in buds from which the anthers and stigmas have been all cut away before
flowering.

The fruiting head is the well-known Dandelion " clock," a type which is

652

BOTANY OF THE LIVING PLANT

characteristic for many of the Compositae. The individual fruit is a dry
inferior achene or nut, attached by a long beak to the parachute-like pappus.

Fig. 502.

Head of Dandelion in vertical section, i — involucre. gr = general receptacle.

See Text.

These fruits are easily detached by wind, being exposed on the convex growth
of the receptacle, owing to the curving back of the involucre (Fig. 247, p. 326).
They are thus scattered long distances by the wind. The success of the
Compositae as a Family depends largely upon the certainty of each floret
producing a good fruit, and on the effective dispersal of the fruit by the
wind.

APPENDIX B.

VEGETABLE FOOD-STUFFS.

The Plant-body, containing as it does digestible proteins, carbohydrates,
and fats, together with certain mineral salts, is a natural food for Man.
Primitive Man used what Nature supplied. But with civilisation came
cultivation of the selected plants which best met his needs. Continued
cultivation led to improvement in quantity and quality of the crop ;
and though certain supplies are still drawn from natural sources, it is
the cultivated plants that yield by far the greater proportion of the
vegetable foods. They are so varied in origin, and in the parts used,
. chat an exact scientific classification is difficult. They may be roughly
grouped for practical study under four heads : as (i) Roots and Shoots,
(ii) Legumes, (hi) Fruits, and (iv) Cereal grains. Naturally the parts
where the material is stored compactly for the use of the Plant itself
are those which are of most value to Man. It is to the roots and tubers,
and to the fruits and seeds that he looks for his best supplies of food.
On the other hand, in the kitchen garden profuse vegetation is en-
couraged so as to obtain in the shortest possible time a large quantity
of succulent tissue, with the least proportion of woody fibre. Halo-
phytes have provided many of the original stocks from which garden
vegetables have sprung, the original sources of Cabbage, Sea-Kale,
Beet, Asparagus, and Spinach were all coastal plants, while the Potato
and Carrot are at home on marine sands.

The analysis of average samples affords some knowledge of the
feeding value of each. The results of such analyses for a few of the
vegetable foods in common use are given in the following tables, which
have been extracted from Konig's Die menschlichen Niihrungs- unci
Genuss-Mittel, and other sources. But there may be considerable
variation from sample, and the figures should be held as a guide to an
estimate of feeding-value rather than as any exact statement applicable
to all cases.

The Potato (Solanum tuberosum, L. Solanaceae) grows abundantly
on the sand of the sea-shore in the archipelago of S. Chili (Fig. 147,
p. 218). At the time of the discovery of America its cultivation was
practised with every appearance of ancient usage from Chili to New
Grenada. It was introduced probably in the latter half of the sixteenth
century into Virginia and North Carolina, and imported into Europe at
the time of Raleigh's Virginian voyages, between 1580 and 1585, first by
the Spaniards and afterwards by the English. The tubers, which are

653

654 BOTANY OF THE LIVING PLANT

axillary buds specialised for storage and propagation, are deficient in
fats, and contain little proteins. More than four-fifths of the organic
substance is in the form of starch. Hence potatoes are used with
meat and fats to make a well-balanced meal. Some of the protein is
in the form of cubical crystalloids, located near to the corky rind, and
thus liable to be removed by peeling the potato (Fig. 82, p. 124).

The Beet {Beta vulgaris and B. maritima, L. Chenopodiaceae) grows
wild on sandy shores in the Mediterranean region, extending northwards
to our own coasts. Its originally slender root became fleshy from the
effects of soil and cultivation, and Vilmorin has shown that it is one of
the plants most easily improved by selection. It has been cultivated
since before the Christian era. The fleshy root, characterised by
repeated cycles of separate vascular strands, as seen in transverse
section, contains cane-sugar in its sappy parenchyma. The analysis
shows, for garden Beet, over 14 per cent. ; but in specially selected
and cultivated sugar-beets the percentage is higher. It has long been
grown as a garden vegetable for winter use ; but latterly it has become
the chief European source of Sugar.

The Parsnip (Pastinaca sativa, L. Umbelliferae) ranks high as a
nutritious vegetable. The original type is native in Britain ; it has
been cultivated since Roman times. The root, distended by cultivation,
is apt to be fibrous on poor soils ; but when well grown it contains a
high percentage of digestible carbohydrates.

The Onion {Allium cepa, L. Liliaceae) was used as a condiment by
the Egyptians, Greeks, and Romans. It appears to have originated
from a wild species of the Middle East. The distended leaf -bases
form a bulb containing a large deposit of sugar. But it is as a
condiment that it is specially valued. Other species of Allium give
Garlic, Shallots, Chives, etc.

The Carrot {Daucus Carota, L. Umbelliferae) is the enlarged root of
the species native in Britain.

The Cabbage, Kale, Cauliflower, and Turnip (Cruciferae) are repre-
sented by many varieties. They may all be attributed to one or another
of four Linnaean species, viz. Brassica oleracea, napus, rapa, and
campestris. Some varieties are cultivated for their leaves, as Cabbages ;
or for their crowded inflorescences, as Cauliflower ; or for the oil in their
seeds, as Colza and Rape ; others again for the fleshy swellings of the
root, or lower part of the stem. In Turnips and Swedes the hypocotyl
is swollen ; in Kohl-rabi the epicotyl. All of these were ultimately of
European or Siberian origin, and some of their ancestral forms grow wild
on our coasts. Their cultivation was diffused in Europe before the Aryan
invasion. The analyses show that their value as foods is not high, though
they contain a fair proportion of digestible proteins and carbohydrates.
Celery {Apium graveolens, L. Umbelliferae) is derived from the wild
species widely spread from Sweden through Europe and the Near
East. It was known to the Greeks. In cultivation it is blanched by
earthing up, so as to diminish its bitterness. The feeding value is
about equivalent to Winter Kale.

Spinach (Spinacia oleracea, L. Chenopodiaceae) was not known to

APPENDIX B

655

the ancients. It was new to Europe in the sixteenth century, being
introduced from the Near East. It is not known in the wild state. The
feeding value of its leaves is below that of Kale.

The Garden Lettuce (Lactuca Scariola : var. saliva, Compositae) is
derived from the wild species native in temperate and southern Europe.
It was used by the Greeks and Romans as a salad, and several varieties
were already known to them. It is notable for its high water-content.

TABLE OF ANALYSES OF ROOTS AND SHOOTS.

N.B.— Vegetables used in the fresh state have a very high water-
content. This must be taken fully into account in considering their
value as foods.

Name.

Water.

Nitro-
genous
substances.

Fats.

Digestible

carbo-
hydrates.

Cellulose
and lignin.

Ash

Potato

74-98

2-08

0-15

2I-OI

0-69

I 09

Beetroot

82-25

1-27

0-12

14-40

I-I4

0-82

Parsnip

79-31

1-32

l6-36

i-73

1-28

Onion -

85-99

1-68

o-io

IO-82

071

070

Carrot

86-79

1-23

0-30

9-17

1-49

1-02

Turnip

87-80

i-54

0-21

8-22

1-32

09I

Cauliflower -

90-89

2-48

o-34

4'55

0-91

0-83

Winter Kale

80-03

3-99

0-90

11-63

i-88

i-57

Celery -

84-09

1-48

o-39

n-8o

1-40

084

Spinach

88-47

3-49

0-58

4-44

o-93

2 09

Lettuce

94*33

1-41

0-31

2-19

o-73

1 03

TABLE OF ANALYSES OF LEGUMES.

The Legumes are notable for the high protein-content of their seeds.
The water-content of the parched seeds averages about 13 per cent.
Consequently the percentage of the other constituents appears to stand
high as compared with the previous table, and with the analyses of
Green Peas and French Beans.

Name.

Water.

Nitro-
genous
substances.

Fats.

Digestible

carbo-
hydrates.

Cellulose
and lignin.

Ash.

Bean - - -
Parched Peas
Lentils
Soya Beans -
Arachis

13-49
13-92

12-33
12-71

7-71

25-3 1

23-J5

25-94
38-18

31-12

1-68
1-89

i-93

I4°3
46-56

48-33
52-68

52-84

3J-97
9-39

8-o6
5-68

3-92
4-40
2-16

3J3
2-68

3-°4

4-71
306

Green Peas -
French Beans

78-44

88-75

6-35

2-72

o-53
0-14

1200
6-6o

1-87
i-i8

o-8i
o-6i

B.B.

2 X

656 BOTANY OF THE LIVING PLANT

The Broad Bean (Vicia Faba, L.) was cultivated in Europe in pre-
historic times. It was probably introduced during the earliest Aryan
migrations, its wild habitat having been south of the Caspian, while a
related species {V. narbonensis) is still wild in the Mediterranean region.
The large percentage of protein in the Bean is represented by numerous
small aleurone grains, and the protoplasmic matrix in the cotyledonary
cells, while large starch-grains account for most of the digestible carbo-
hydrates. The thick cell-walls make up 8 per cent., while the ash is
unusually high. Beans are difficult of digestion, but very nutritious.

The Garden Pea (Pisum sativum, L.) was introduced into Europe by
the Aryans from the Near East, but it no longer exists in a wild state.
It has been found among the relics of the Bronze Age, and even of the
Stone Age. In point of analysis it corresponds nearly to the Bean.
It is, however, commonly used in the immature state, as " green peas " ;
but the analysis of these, putting aside the high water-content, corre-
sponds in essential feeding-value to that of dried peas, while they are
more readily digestible.

The Lentil (Ervum lens, L.) was cultivated from prehistoric times
in western Asia, and in Egypt, and it has been found in the remains of
the Swiss lake-dwellings, but it is no longer known in the wild state.
Its analysis corresponds to that of Beans, but notably with a smaller
proportion of cellulose and lignin.

The most important of all the Legumes for the future may be the Soya
Bean (Glycine soja, Sieb. et Luce, and other species), which is of very early
cultivation in the Far East. The analysis shows that while the protein
content is extremely high, oil replaces a considerable proportion of the
digestible carbohydrate. The chief supply was formerly from Manchuria,
but its cultivation has now spread to many other countries, some effort
having been made to introduce the crop to Britain ; as, with the excep-
tion of Rape and Linseed, no oleaginous seeds are grown in this country.

The Pea-nut, or Monkey-nut (Arachis hypogaea, L.) is believed to
have originated in Brazil. Seeds have been found in Peruvian tombs.
Thence it was conveyed to Africa and Asia, and it is now cultivated
in all hot countries, either for the seed or for the oil which they contain
in so large a proportion, replacing most of the digestible carbohydrates.

The French Bean, or Haricot (Phaseolus vulgaris, Savi) was probably
of American origin, its seeds having been found in Peruvian tombs.
There is no evidence that it has been long cultivated in Europe or Asia.
In its qualities and uses it resembles the Pea and Bean. Its immature
pods, used as a vegetable, are inferior in food-value to green Peas.

FRUITS.

The chief interest in Fresh Fruits, apart from their high water-content,
lies in the proportion of sugar and of free acids (but see p. 662). Upon
the former depends their value for the production of wines, in which the
Grape takes precedence. The Vine (Vitis vinifera, L. Vitaceae) grows
wild in W. Asia, S. Europe, and N. Africa. Both Semitic and Aryan
nations knew the use of wine, and Egyptian records carry back the

APPENDIX B

657

cultivation of the grape to 4000 b.c. The grape-sugar, of which it con-
tains over 14 per cent., is the starting-point for alcoholic fermentation.
But it is also important in the dried state, giving their value to raisins,
and to dried currants, which are small dried grapes. The Apple (sugar
7-22 per cent.) and Pear (8-26 per cent.) give respectively Cider and
Perry, while the Currant (6-38 per cent.) and the Gooseberry (7-03 per
cent.) are also used in the preparation of British wines. But the rela-
tively large proportion of free acids in these detracts from their value.

TABLE OF ANALYSES OF FRESH FRUITS.

Name.

Water.

Nitro-
genous

Free

Sugar.

Other
digestible

Cellulose
and

Ash.

substances
0-36

hydrates.

lignin.

Apples

84-79

0-82

7*22

5-8i

IS*

0-49

Pears -

83-03

0-36

0-20

8-26

3*54

4-30

0-31

Plums -

84-86

0-40

I-50

3-56

468

4-43

o-66

•

Peaches

80-03

0-65

092

4-48

7-17

6-o6

069

Apricots

8l-22

0-49

1-16

4-69

6-35

5-27

0-82

Cherries

79-82

0-67

0-91

10-24

1-76

6-07

0'73

Grapes

78-17

o-59

0-79

I4-36

1 96

3-60

o-53

Strawberries

87-66

o-54

o-93

6-28

1 46

2-32

o-8i

Raspberries

8574

0-40

1-42

3-86

o-66

7*44

0-48

Blackberries

86-41

051

1-19

4'44

1-76

5-21

0-48

Gooseberries

8574

0-47

1-42

7-°3

1 40

3-52

042

Currants

84-77

0-51

2-15

6-38

090

4-57

0*72

The actual nutritive value of fresh fruits is usually small. But in
the dried state those which contain sugar, and the kernels of oily nuts,
are of high value, as shown by the following table :

ANALYSES OF DRIED FRUITS.

Name.

Water.

Nitro-
genous
substances.

Fats.

Digestible

carbo-
hydrates.

Cellulose
and lignin.

Ash.

Almond
Hazel Nut -
Walnut

6-02
711
7-18

23-49
17-41

J5-77

53-°2
62-60

57-43

7-84

7-22

I3-03

6-51
3-17
4-59

3- 1 2

2-49

2- OO

Raisins
Dried Figs -

32-02
31-20

2-42
4-01

o-59

6204
40-79

1-72

I-2I

2-86

A comparison of the constituents of kernels such as the Almond, and
of dried fruits such as the Raisin or Fig, shows that together they supply-
in suitable proportions the proteins, fats, and digestible carbohydrates
required for food.

658

BOTANY OF THE LIVING PLANT

CEREAL GRAINS.

By far the most important vegetable foods are the Cereal Grains,
which are the fruits of various Grasses (Gramineae). The general con-
struction of all of these grains is the same, and the structure of the grain
of Wheat will serve to illustrate it for them all.

The Wheat-Grain is oval, and hairy at the apical end, but smooth
at the base where is the scar of attachment. A lateral groove running
longitudinally marks the pos-
terior side ; the anterior side
is convex, and shows near its
base an area which is de-
pressed and wrinkled when
dry. This marks the position
of the germ, which is thus
basal and faces the anterior
side of the grain (Fig. 503).
The greater part of the grain
is made up of a mass of endo-
sperm : this together with the
germ is covered by the fruit-
coat (pericarp) and seed-coat
(testa), which jointly form a
hard brittle shell, separated in
milling from the inner parts
as Bran.

A microscopic examination
shows that the germ consists
of thin-walled tissue densely
stored with oily protoplasm,
but with no starch. The endo-
sperm is also thin-walled, but
contains much starch closely
packed (Fig. 482, am) ; but a
superficial layer of its cells is
distinguished by the absence of starch, while as it contains numerous
aleurone grains it is recognised as the aleur one-layer (al). The Bran
consists of compressed and thickened cell-walls of woody texture,
containing a deposit of silica. The outer band (p) represents the
fruit-coat or pericarp : the inner (/) represents the seed-coat or
testa.

The constitution of the Wheat-Grain as shown by analysis may vary
considerably according to sample. This is shown even in so important
a feature as the proportion of nitrogenous substances. " Soft Wheat '
may contain only io-8o per cent, of protein, while " Hard Wheat " has
been found to contain 13-83 per cent. In some Russian Wheats it
may even rise above 17 per cent. These facts are mentioned to show
that the results of analysis must be taken as a general guide rather than
as a statement of constant fact.

Fig. 503.

Part of a median longitudinal section of a grain of
Wheat, showing embryo andscutellum (sc). us = vasc.
bundle of scutellum"; ce = its columnar epithelium;
r=ligule; c=sheathing part of cotyledon; £y=vege-
tative cone of stem ; &/> = hypocotyl ; / = epiblast ;
r = radicle; cl = root-sheath ; m = micropyle ; /^funi-
culus; vp = vascular bundle of funiculus; /= lateral
wall of groove ; c/> = pericarp. ( x 14.) (After Stras-
burger.)

APPENDIX B

659

It is important as a basis for judgment of the results of milling to
know by analysis the distribution of the constituent substances in the

Fig. 504.

Part of a section of a grain of Wheat. £=pericarp; t = seed-coat, internal to
which is the endosperm; a/ = aleurone grains; «;w = starch grains; n = nucleus.
( x 240.) (After Strasburger.)

Wheat-Grain. This is shown by the subjoined table, taken from Dr.
Hutcheson's book on Food :

Wheat.

Water.

Nitro-
genous
substances.

Fats.

Digestible

carbo-
hydrates.

Cellulose
and lignin.

Ash.

Whole Grain, 100% -

14-5

II-O

1-2

690

2-6

i-7

Bran, 13-5%

12-5

16-4

3-5

43-6

1 80

6-o

Endosperm, 85%

130

10-5

o-8

74-3

07

0-7

Germ, 1-5%

12-5

35"7

i3-i

3I-2

i-8

57

A large portion of the grain (85 per cent.) is endosperm ; the bran
amounts to 13-5 per cent., and the germ only to 1-5 per cent. The
latter is, however, important, for it contains a high proportion of
proteins, of fats, and of ash. Since the germ flattens in roller-milling,
it can be sifted out. The highly nitrogenous and fatty body thus
extracted may then be added to ordinary flour in varying proportions,
giving different kinds of germ-bread. Bran is characterised by its
large proportion of cellulose and lignin (18 per cent.), which is indi-
gestible by man, but more available for herbivorous animals. There
is in it, however, a large quantity of nitrogenous substance, owing
to the adherence of the aleurone-layer of the endosperm to the flaky
scales of the fruit-coat ; the silica in the latter accounts for the large
percentage of ash in bran (6 per cent.). Thus bran contains an undue
proportion of proteins in which the grain as a whole is deficient. Its
value in bran-mash for horses is therefore easilv understood. The

66o

BOTANY OF THE LIVING PLANT

endosperm, which forms 85 per cent, of the grain, is broken down in
the process of milling into fine flour, semolina, and other products. Its
analysis shows that, while about three-quarters of it consists of starch,
there still remains in it about 10 per cent, of protein, which, as " gluten,"
forms the basis of the dough of bread when moistened with water.

The purpose of milling of grain was in the first instance simply to
grind it into small parts. The bread of primitive Man was doubtless
" wholemeal " bread. But even in the old stone-grinding the products
were usually graded roughly as bran, pollard, sharps, middlings, and
fine flour. Sometimes the coarser products were reground, and the
fine flour again extracted from them ; but mostly they were regarded
as " offals," and were fed to stock in various forms. More recently
in the process of roller-milling the grain is comminuted more accurately
oy successive stages, being passed through rollers with successively
finer ridges. The products of these successive ' breaks " are sifted
partly by screens, partly by air-blasts, so arranged that their various
grading can be very perfectly carried out. The end-product of greatest
importance is the flour. The finer this flour is graded the less percentage
of it will be yielded from the milled grain, but the whiter will be the flour
and the bread made from it. In practice the highest percentage of finest
white flour that can be obtained is about 70 per cent, of the weight of
the whole grain. Higher extraction results in a flour showing some degree
of brownness owing to the inclusion of a proportion of bran. In deciding
the type of flour that is to be produced, various aspects have to be con-
sidered, such as nutritional value and acceptability, and there has been
some difference of opinion on the matter. In war-time the need for
economy gives added importance to a relatively high extraction. During
the 1939-45 War, British national flour has been based on an extraction
of 80-85 per cent, of the wheat grain.

A comparison of the analyses of average samples of the grains in
common use by Man gives a basis for estimating their relative values
as foods. The average of a large number of different samples of each
is given in the subjoined table :

TABLE OF ANALYSES OF CEREAL GRAINS.

Name.

Water.

Nitro-
genous
substances.

Fats.

Digestible

carbo-
hydrates.

Cellulose
and lignin.

Ash.

Wheat

I3'37

12-04

1-91

69-07

1-90

I-7I

Rye -

13-37

io-8i

1-77

70-21

1-78

206

Barley

If05

9-66

1-93

66-99

4-95

2-42

Oats, average from

all lands -

12-11

1066

4-99

58-37

IO-58

3-29

Oats, England and

Scotland -

12-11

13-05

-6-15

53-16

11-89

3-64

Maize -

13-35

10-17

4-78

68-63

1-67

1-40

Rice (not cleaned)

n-99

6-48

1-65

70-07

6-48

3-33

APPENDIX B 661

Wheat [Triticwn vulgare, Yillars) has. been cultivated from pre-
historic times in Europe and Egypt, and in China records of it go back
to 2700 b.c. It is represented by numerous varieties. Those in which
the ripened grain detaches itself naturally from the husk are referred to
one species {Triticum vulgare, Villars) ; those in which the ripe grain
is closely contained in the husk are distinguished as Spelts (7\ Spelta,
L.). The separation of these was probably prehistoric. There is no
certain evidence of the place of origin of wheat, for it is not found wild,
but its probable home was in the Near East. It is the chief staple food
of the white races.

Rye (Secale cereale, L.) probably had its origin in the countries north
of the Danube, and its cultivation was hardly earlier than the Christian
era. It does not greatly differ in its nutritive qualities from wheat,
and is largely grown in central and northern Europe.

Barley (Hordeum distichon, L., vulgare, L., and hexastichon, L.) is
among the most ancient of cultivated plants. It has been found wild
in the Caspian region. It is chiefly used for malting and brewing, for
which its low content of proteins is suitable.

The Oat (Avena sativa, L.) is not now found wild, but it was probably
derived from a form native in Eastern Temperate Europe. It was
cultivated anciently in Italy and Greece, and its grains have been found
in Swiss lake-dwellings, and in early German tombs. Though a coarse
grain with much cellulose and lignin in its outer coats (10 to 12 per cent.),
its high percentage of fats (5 to 6 per cent.), proteins (12-13 per cent.),
and ash (3 to 4 per cent.), mark it as superior to any other Cereal as a
staple food. Moreover, a comparison of the average of analyses of
Oats from all sources with those from England and Scotland shows
that these stand the highest of all ; a fact which justifies, and should
encourage, the prevalent use of porridge and oat-cake.

Maize [Zea Mays, L.) is of American origin. At the time of the
discovery of the New World it was found to be one of the staples of
widespread agriculture from Peru northwards, but it has not been
found in the wild state. Its large grain is very hard, by reason of
the close packing of the starch-grains in the endosperm. Starch is
also present in the relatively large germ. Its analysis shows a rather
high percentage of fats ; but the hardness of the endosperm gives a
gritty texture to its products, and makes thorough cooking necessary.

Rice (Oryza sativa, L.) is indigenous in India, and perhaps also in
China. It is a more widespread staple food than any other, supporting
about one-third of the human race. It has a rough husk, which
represents 6 per cent, of the grain, and is cleaned off before exportation.
This tends to remove the aleurone layer also, and to carry off a pro-
portion of the proteins from a grain that is already deficient in them
(6.48 per cent.). The analysis of cleaned rice, i.e. after removal of the
husk, shows a very high percentage of digestible carbohydrates, with a
marked deficiency of proteins and fats. This justifies its use in curries
and puddings, in the preparation of which fats and proteins are added.
But as a staple food without additions it leaves much to be desired.

662 BOTANY OF THE LIVING PLANT

FOOD VALUES. VITAMINS.

The relative values of the various vegetable food-stuffs of which
analyses have been given above, may be assessed in the first instance
by considering their " fuel values ", i.e. the number of " Calories "
which each produces when completely oxidised (one [large] Calorie is
the amount of heat required to raise the temperature of one kilogram
of water by one degree C. — or that of one lb. of water by 4 degrees F.).
It has been calculated by physiologists that an average healthy man,
weighing 70 kg. (11 stone) and doing a moderate amount of physical
work, requires each day food with a total fuel value of 3300 Calories.
Assuming each gram of protein or carbohydrate to provide 4 Calories
and each gram of fat 9 Calories, the normal metabolic requirements of
the average man will be met by a diet comprising 100 grams of protein,
100 grams of fat and 500 grams of carbohydrate per day.

The fuel value of any substance is however only one aspect of its
suitability for human food. As regards proteins, in particular, quality
is quite as important as quantity. Digestibility and palatability of the
food must likewise be taken into account. Among the drawbacks of a
purely vegetarian diet is the monotonous and unpalatable character of
many vegetable staple foods. Rice, for example, quite apart from its
very low fat and protein content, makes an insipid dish and hence an
unsatisfactory staple diet unless seasoned by the addition of highly
flavoured condiments such as curry or soy. The widespread instinctive
preference for a mixed diet is moreover biologically justified, not only
by the need for a proper balance between protein, fat and carbohydrate,
but also because adherence to such a diet is the natural way of ensuring
a sufficient and regular supply of the essential mineral salts and of the
no less indispensable vitamins.

Vitamins are " accessory food factors ". Though they are not them-
selves food substances in the ordinary sense, a certain amount of each
vitamin must be supplied to the human body if full health is to be main-
tained. That amount is very small ; perhaps not more than one
millionth part of the daily food ration ; but, on the other hand, a
minimum supply of every vitamin is absolutely necessary, since lack of
any one results in a characteristic " deficiency disease ". The best
known of these ailments are scurvy and rickets.

Scurvy has been known for centuries, especially among seafaring
peoples. Its connection with a diet lacking fresh vegetables and fruit
was first clearly demonstrated by Lind in 1757. Captain Cook was able
to keep his crews free of the disease by making them eat fresh food as
often as possible. It is now known that the active principle in fresh
foods which prevents the development of scurvy is the " anti-scorbutic
vitamin " or " Vitamin C ".

Rickets is ordinarily caused by lack of sunlight aggravated by a diet
poor in natural fats. The specific " anti-rachitic " agent is " Vitamin
D ", one of the richest sources of which is cod-liver oil (and similar fish
oils). Most other natural foods contain comparatively little of this

APPENDIX B 663

vitamin, hence the importance of sufficient sunlight, the ultra-violet rays
in which transform any inactive " provitamin " present in the body into
the active vitamin.

Certain eye-troubles (such as night-blindness) and an unhealthy con-
dition of the mucous membranes result from a deficiency of " Vitamin A ".
This vitamin is readily obtained from fresh green vegetables (e.g. spinach
or cabbage) and from yellow root vegetables (e.g. carrot), in fact from all
foods rich in the chromatophore pigment carotene, which is the provita-
min. The vitamin is also present in fish-liver oils and in ox-liver. In
this case the provitamin is synthesised solely by plants ; the transfor-
mation of carotene into the active vitamin, on the other hand, takes
place so far as is known only within the animal body.

Other serious diseases due to vitamin deficiency, but less familiar in
Europe, are ' beri-beri " ("Vitamin B± " deficiency) and "pellagra"
(deficiency of the " P.P. Vitamin ").

At the time when the vitamin theory of deficiency diseases was first
put forward (19 12) and for many years afterwards, little or nothing was
known about the chemical nature of vitamins, which were indeed long
regarded as mysterious if not as miraculous agents. More recently,
however, most of them have been isolated in the pure state, analysed
and their constitution determined ; several have even been synthesised.
It has already been noted that " Vitamin A" is a derivative of the com-
plex hydrocarbon carotene. " Vitamin C " is ascorbic acid, C6H806 and
' Vitamin D " is a modification, under the influence' of ultra-violet
radiation, of ergosterol, C28H440.

We are, in fact, now in a position to administer all the principal
vitamins in the pure form as curative or preventive agents, if need be.
The first line of defence against deficiency diseases, however, should be
a good mixed diet, including daily supplies of fresh vegetables and
fruit ; in addition, ample sunlight is essential, or, where sunlight is
deficient, regular doses of cod-liver or halibut oil.

INDEX AND GLOSSARY

Abaxial-surface, of the leaf, that
facing away from the stem in
development, 70, 72 (Figs. 47, 48).

Abortion, where a part normally pre-
sent is not fully developed, 269
(Fig. 193) ; of a complete whorl of
floral-parts, 269 ; of ovules, 322
(Fig. 241) ; of loculi of Oak,
Coconut, etc., 322 ; of floral-parts
in Lychnis, 621 (Fig. 470).

Absciss-layer, the layer of cells along
which the leaf breaks away from
the stem in autumn, 80 (Fig. 55).

Absorption of cell-wall, 25 (Fig. 15).

Acacia, phyllodes of, 211.

Acanthorhiza, 345.

Accessory food-factors (vitamins),
662.

Acetabularia, 370 (Fig. 275) ; iso-
gametes of, 557.

Acetic acid bacteria, 452.

Achene, a one-seeded nut, 323 ; of
Buttercup, 322 (Fig. 242).

Achlya, on dead flies, 403.

Aconitum, zygomorphy of, 274 (Fig.
199) ; pollination by Humble-Bee,
302 ; follicles of, 322 (Fig. 240),
623 (Figs. 473, 475).

Acorus (sweet-rush), root of, 84 (Fig.
58) ; dorsiventrality of, 208.

Acquired characters, doctrine of non-
inheritance of, 569.

Acrocarpic, applied to Mosses which
fruit at ends of stems, 467.

Acropetal, applied to a succession of
appendages in which the latest
formed are nearest the apex of the
part that bears them : of leaves,
69 ; of parts of flower, 265.

Actinomorphic, or radial symmetry
of a shoot, where development is
equal on all sides, 200 (Figs. 133,

134). 274 (Fig- l87)-

Adaptation, special modification

which arises in relation to the

environment, 193, 219, 342 ; origin

of, 586.
Adaxial surface, of the leaf, that

facing the stem in development,

68, 72 (Figs. 47, 48).
Adhesion, the fusion of parts of

distinct categories in the flower,

266.
Adhesive-climbing, 217 (Fig. 146).
Adiantum, embryo of, 504-505 (Figs.

397-399).

Adonis, floral diagram of spiral
flower, 264 (Fig. 188).

Adsorption, 33.

Adventitious, applied to buds formed
not in the normal sequence, pro-
pagation by, 252 (Fig. 176), 348 ;
do. roots, 348.

Aecidium on Barberry, 398, 433 (Fig.

33o), 437 (Fig- 337)-
Aecidium-buds, or spores, 433, 438 ;

of Phragmidium, 439 (Fig. 338).
Aerobic organisms, those which live

normally exposed to atmospheric

air, 136-137.
Aerotropism, response to the stimulus

of unequal aeration, 160 ; positive

in roots of pot-bound plants, 160 ;

negative in pollen-tubes, 161 (Fig.

97» A).

Agaricus campestris (Mushroom),
401, 402 (Fig. 344).

Agrimonia, floral construction of, 267
(Fig, 191) ; hooked-fruits of, 328
(Fig. 251).

Agrostemma (corn-cockle), conduct-
ing tissue of style, 293 ; pollen-
tubes of, 304 (Fig. 222).

Air bubbles in vessels, 108.

Alae, lateral petals of Pea-flowers, 636
(Fig. 489).

665

666

BOTANY OF THE LIVING PLANT

Albuminous-seed, in which endo-
sperm persists till ripeness, 319
(Fig. 239).

Alchemilla, somatic parthenogenesis
in, 587.

Alcoholic fermentation, 136.

Alder, root-nodules of, 239.

Aleurone-grains, 126 (Fig. 84).

Aleurone-layer, in wheat, 659 (Fig.

504)-
Algae, sea-" weeds " and fresh-water

" weeds," 3, 355 ; brown (Chap.

xxiii.) ; green (Chap, xxii.) ; red,

388 ; blue-green, 376 ; homoplasy

in, 375-

Alisma, embryology of, 313 (Fig.
232).

Allelomorphs, pairs of genes corre-
sponding to pairs of contrasting
characters, such as tallness or
dwarf ness in the Pea, 572.

Allium, somatic division of cells in,
20 (Fig. 11).

Allo-polyploid, a polyploid the chro-
mosome sets of which are not alike,
583 (Fig. 444).

Almond, analysis of, 657.

Aloe, succulent leaf of, 74 ; stoma of,
77 (Fig. 52).

Alopecurus, 618.

Alpine flora, chiefly perennial, 195.

Alpine plants, xerophytic features of,
211.

Alternation of Generations in seed
plants, 336, 456 ; in Algae, 390 ; in
general, 543 (Chap, xxxiv.).

Amaryllidaceae, 608.

Amides, 128 ; translocation of, 132.

Amino-acids, 126, 128 ; translocation
of, 132.

Ampelopsis, adhesive climbing of,
217 (Fig. 146).

Amphibians, organisms, whether
plants or animals, which are de-
pendent on external liquid water
for completion of their life-cycle,
3, 454, 476, 547.

Amphicosmia, bud of, 595 (Fig. 450).

Amphithecium, external tract of cells
in the young moss sporogonium,
470 (Fig. 365).

Anabaena, 377.

Anaerobic organisms, those which live
normally or can exist without
access to atmospheric air : anaero-
bic respiration, 136-137.

Analogy, 343, 346 ; defined, 344.

Analysis, of roots and shoots, 655 ;
of legumes, 655 ; of fruits, 657 ;
of cereal grains, 660.

Anatomy, 16.

Anatropous, the inverted form of

ovule, 294 (Fig. 216).
Andreaea, antheridium of, 467 (Fig.

362) ; archegonium of , 468 (Fig. 363).
Androecium, applied collectively to

all the stamens of a single flower,

255-
Anemone, abortive ovules of, 322

(Fig. 241).

Aneura, sporogonium of, 475 (Fig.

37i).
Angiosperms, seed-plants with their

seeds protected by carpels, 3, 5,

Chapters i-xix.
Animal agency, in pollination, 302

(Fig. 220) ; in seed-dispersal, 328.
Animal kingdom, establishment of,

359 ; nutrition of, 4 ; dependence

of, 138.
Ankyropteris, stele of, 593 (Fig. 449).
Annual habit, applied to plants which

germinate and develop, flower, and

fruit in one season, 195.
Annual rings, 62 (Fig. 40).
Anterior-side, that side of a flower

which faces the bract, 263.
Anther, the part of stamen bearing

pollen-sacs, 281 (Figs. 201, 202).
Antheridia, 353 ; of Fungi, 399 ; of

ferns, 501 (Fig. 393) ; of mosses,

467 (Fig. 362) ; of Oedogonium,

366 (Fig. 271) ; of Pythium, 405

Fig. 304) ; of Fucus, 384 (Fig. 287);

of Vaucheria, 371 (Fig. 276).
Antheridial mother-cell, a cell within

the pollen-grain of flowering plants

which divides to form the male

gametes, 283 (Fig. 204).
Anthocerotales, 475, 548.
Anthostema, 256 (Fig. 178).
Anthrax bacillus, effect of light on,

450-

Anticlinal, applied to cell-walls, or
lines of cell-walls which run ap-
proximately at right angles to the
outer surface of the part, 19 (Fig.
10) ; these cut the periclinal walls
at right angles.

Antipodal cells, a group usually of
three primordial cells attached at
the chalazal end of the embryo-sac,
295 (Fig. 216), 298 (Fig. 219) ;
after fertilisation, 315.

Antirachitic vitamin D, 662.

Antirrhinum majus (Snapdragon),
inheritance of flower colour, 574
(Fig. 437) ; inheritance of flower
shape, 575-578 (Figs. 43^, 439).

Anti-scorbutic acid (vitamin C), 663.

INDEX AND GLOSSARY

667

Antithetic alternation, 543, 546.

Antitoxines, 453.

Apical meristems, of Ferns, 493
(Figs. 383, 385).

Apium graveolens (Celery), analysis
of, 655.

Aplanospores, non-motile propaga-
tive spores of Algae, 372.

Apocarpous, applied to the gyno-
ecium when composed of separate
carpels, 289 (Fig. 210), 321.

Apogamous nuclear-pairing in Phrag-
midium, 438 (Fig. 338).

Apogamy, where a sporophyte springs
directly from a gametophyte with-
out syngamy, 508 (Figs. 402, 403) ;

545. 587-
Apomixis (or Apogamy). loss of

sexuality, 545, 587.

Apophysis, basal part of capsule in
Mosses, 469 (Fig. 364).

Apospory, transition directly from
sporophyte to gametophyte, with-
out intervention of spores ; in
Ferns, 507 (Fig. 401), 545.

Apple, 622 (Figs. 483, 484, A) ; analy-
sis of, 657 ; cork of, 65 ; deciduous
leaves of, 194 ; flower of, 290
(Fig. 212) ; stigma of, 291.

Apposition, of layers in growth of
cell-wall, 23.

Apricot, analysis of, 598.

Aqueous cells, in folding leaves of
Grasses, 187 (Figs. 120-122).

Arachis hypogaea (Peanut, Monkey-
nut), geotropic curvature of fruit,
334 ; analysis of, 655.

Araucaria, leaf arrangement of, 203.

Archegoniatae, 454 (Chap, xxix.) ;
origin of, 546 ; size-relation in, 597.

Archegonium, 353 ; of . Pine, 535
(Figs. 423, 424) ; of Pteridophytes,
455 ; of Selaginella, 518 (Fig. 412) ;
of Ferns, 502 (Fig. 394) ; of Mosses,
468 (Fig. 363).

Archesporium, the cell or group of
cells from which the spores of a
sporangium or sporogonium origi-
nate ; of Moss, 469 (Fig. 364), 471.

Arctic flora, chiefly perennial, 195.

Arctic plants, xerophytic features of,
211.

Arillus, an extra integument formed
after fertilisation, 318 (Figs. 238).

Arisarum, 393.

Armillaria mellea, parasitism of, 393
(Fig. 292), 443 ; basidium of, 431
(Fig. 328) ; rhizomorphs of, 394.

Aroids, prehensile roots of, 217.

Artichoke, 124.

Articulate plants, 524.

Ascent of water, 105.

Ascogenous hyphae, in fruit of Asco-
mycetes, 400, 401, 424 (Fig. 321).

Asco-lichenes, 426.

Ascomycetes, fungi which produce
asci, 418 (Chap, xxvi.) ; alterna-
tion in, 424 ; cytology of, 424.

Ascophyllum, 385, 393.

Ascospores, spores produced in asci,
418 (Fig. 316) ; of Penicillium, 423
(Fig. 321) ; as tetraspores, 401, 418.

Ascus, the characteristic spore-bear-
ing body of the Ascomycetes, 418
(Fig. 316) ; development of, 419,
424.

Ash (Fraxinus) delayed germination,
140.

Asparagine, 128.

Asparagus, 608.

Aspen, lamina of, 72 (Fig. 47) ;
petiole of, 186.

Aspergillus, 418, 422 (Fig. 320).

Asplenium, sporophytic budding in,

494.

Asterochloena, stele of, 593 (Fig. 449).

Asteroxylon, 478 (Fig. 372 a).

Astilbe, 630.

Astrantia, mechanical construction
of stem, 183 (Fig. 116) ; simple
umbel of, 262 (Fig. 184), 638.

Asymbiotic germination, 234.

Athyrium Filix-foemina, v. claris-
sima, chromosomes of, 509 ; apo-
spory in, 507 (Fig. 401) ; par-
thenogenesis in, 587.

Atmospheric nitrogen, fixation of,
127.

Atropa Belladonna (Nightshade), 642.

Aulacomnion, gemmae of, 466 (Fig.
360).

Autophyte, a plant which is com-
pletely self-nourished, 220.

Auto-polyploid, a polyploid the chro-
mosome sets of which are all alike,
583 (Fig. 444).

Autotrophic nutrition, complete self-
nutrition, 220 ; in Bacteria, 450.

Auxanometer, an instrument for
amplifying and measuring growth,

145.
Auxin, action of, 151 (Fig. 73).

Auxospores of Diatoms, 375.

Avena sativa (Oat), analysis of, 660 ;
origin of, 661.

Axillary buds, buds arising in the
angle between stem and leaf, 9, 80
(Fig. 55) ; branching, 347.

Axis, morphological category of, 347.

Azotobactcr, 127.

668

BOTANY OF THE LIVING PLANT

Azygospore, a body resembling a
zygospore, but produced without
syngamy : in Mucor, 416.

Bacillus, rod-shape of Bacteria, 448 ;
anthracis, 452 ; B. subtilis (Hay

Bacillus), 448 (Fig. 346) ; B. radi-
cicola, in root-tubercles, 127, 136

(Fig. 164).
Bacteria in general, Chap, xxviii, 448 ;

in soil, 98, 451 ; aerobic and an-
aerobic, 450 ; nutrition of, 450 ;

cilia of, 448 ; rapid multiplication

of, 449 ; effect of light on, 450.
Bacterium tumefaciens, causing

Crown Gall, 453.
Bacterioids, turgid forms of Bacillus

radicicola found in root-tubercles,

237 (Fig. 164).
Bamboo, mechanical construction of

stem of, 182 (Fig. 114).
Banana, marginal tearing of leaf,

189.
Barberry, 345 ; as host for Aecidium

(Puccinia), 433 (Fig. 330) ; diseased

patches on, 437 (Fig. 337).
Barbula muralis, protonema of, 462

(Fig. 356) ; bulbils of, 466.
Bark, 65.
Barley, root-tip of, 88 (Fig. 63) ;

analysis of, 660 ; origin of, 661.
Basidio-Lichenes, those Lichens in

which the fungal constituent may

be classed under the Basidio-

mycetes, 446.
Basidiomycetes, fungi which produce

basidia, 400, 431 (Chap, xxvii.) ;

absence of sex in, 431.
Basidiospores, spores produced on

basidia, 400, 431 (Fig. 328), 445

(Figs. 344, 345, ; like tetraspore,

43i. . .

Basidium, the characteristic spore-
bearing body of the Basidio-
mycetes, 400, 431 (Fig. 328), 445
(Figs. 344, 345) ; comparison with
ascus, 431 ; reduction in, 432 ;
septate of Puccinia, 436 (Fig. 335) ;
of Ustilagineae, 440 (Fig. 339).

Bast-fibres, 59 (Fig. 38), 177.

Bast-parenchyma, 59 (Fig. 38).

Bauhinia, correlation in leaf, 218,

345-
Bean (Vicia Faba), 6 (Fig. 2), 92 (Fig.

66) ; etiolation of, 146 (Fig. 91) ;
correlation in leaf of, 218 ; root-
nodules of, 235 ; analysis of, 655 ;
origin of, 656.
Bean seedling, geotropism of, 155
(Fig. 94).

Beech, ectotrophic mycorrhiza of,
228 ; bud of, 596 (Fig. 451).

Beetroot (Beta vulgaris), origin and
analysis of, 655, 656.

Beggiatoa, 451.

Begonia, propagation of, by adventi-
tious buds, 252 (Fig. 176).

Beri-beri, 663.

Berry, a fruit with the whole pericarp
succulent, 329 (Fig. 252).

Bicornes, 639.

Bidens, 648 ; hooked-fruits of, 328
(Fig. 251).

Bignoniaceae, climbing habit of, 213.

Bilateral symmetry, where the sides
of an organ or shoot are alike, as in
many sea-weeds, 205, 378 (Fig.
280).

Bilberry, 640.

Bird's-nest Orchis (Neottia), mycor-
rhiza in, 230-233 ; saprophytism
of, 232 (Figs. 160, 161).

Biscutella laevigata, polyploidy in,

584.
Bitter Cress, explosive fruit, 165 (Fig.

101).
Blackberry, analysis of, 657.
Bladderwort (Utricularia), 241 (Fig.

167, A).
Bladder-wrack, 378.
" Bleeding," 108.
Blue-green Algae, 376.
Boehmeria, fibrous cells of, 177.
Bog iron ore, 451.
Bog Moss (Sphagnum), 113.
Bog Myrtle, nodules, 239.
Boletus, 401 ; Sporodinia on, 415.
Bordered pits, of Conifers, 530 (Figs.

417, 418).
Botryopteris cylindnca, stele of, 485

(Fig. 375). 593 (Fig- 449).

Botrytis, 395.

Bottom yeasts, 450.

Brachymeiosis, 424.

Bracken, 483 ; vegetative propaga-
tion of, 253 ; rhizome in section,
486 (Fig. 379) ; meristele of, 488
(Figs. 377, 378).

Bract, a reduced leaf, subtending a
flower, 257.

Bracteole, a reduced type of bract,
borne on a relatively higher branch
of an inflorescence, 257.

Bract-scale of Conifers, 534 (Fig. 422).

Bran, analysis of, 659.

Branching, axillary, 347 ; distal,
348 ; adventitious, 348.

Brassica, 10 (Fig. 3) ; Cabbage,
Kale, Cauliflower, Turnip, 654, 655.

Break-down (Chap, viii.), 114.

INDEX AND GLOSSARY

669

Breaking stress, the smallest burden
per unit of transverse section of a
strand which will cause rupture,
178, 179.

Breeding, rate of, 333.

Bromeliads, epiphytic, 211.

Broomrape (Orobanche), parasitism
of, 226 (Fig. 153).

Brown Algae, 378 (Chap, xxiii.).

Bryales, 476, 549.

Bryophyllum, adventitious buds of,
247 (Fig. 170).

Bryophyta, the lower archegoniate
plants, including Mosses and Liver-
worts, 3 ; saprophytism in, 465 ;
description of (Chap, xxx.), 461 ;
size-problem of, 591, 601 ; failure
to combine ventilation with branch-
ing, 601.

Bryopsis, thallus of, 171 ; 370 (Fig.

275) ; 4*7-
Buckwheat, root-tip of, 89 (Fig.

64. A).

Bud, a compact young shoot, 9 ;
dormant, 14, 80 (Fig. 55), 149, 150 ;
axillary, formed in normal se-
quence, 245 ; adventitious, 247
(Figs. 170, .171) ; carpogonial buds
of Red Algae, 389 ; summer and
winter buds of Uredineae, 433,
437 ; big and small buds compared,
600.

Bulb, a storage bud : of Hyacinth,
198 (Fig. 132) ; ripening of, 199.

Bulbochaete, 365, 367 (Fig. 272).

Buttercup (Ranunculus), root of, 85
(Fig. 59) ; Water-Buttercups, 212 ;
flower of, 623, 624 (Figs. 473 B,

474).
Butterwort (Pinguicula), 164.

Buxbaumia, saprophytism of, 465.

Cactus, succulent stem of, 209 (Fig.
141 ; correlation in, 218.

Calamarian fossils, 522.

Calamus, Rattan Palm, straggling
habit, 214 (Fig. 143, iv.), 345.

Calcium carbonate in soil, 97.

California, giant trees of (Frontis-
piece), 14, 527.

Calluna (Heather), endotrophic my-
corrhiza, 230 (Fig. 158).

Callus, a carbohydrate substance de-
posited round the sieve-plate, 49
(Fig. 27), 50.

Calories, 662.

Caltha (Marsh Marigold) structure of
anther, 281 (Figs. 202, 203) ;
pollen-tetrads of, 283 (Fig. 204),
286 (Fig. 207) ; carpels of, 289

(Figs. 208, 210) ; anatropous ovule
of, 294-296 (Figs. 216, 217) ; floral
construction of, 622, 623.

Calyptra, the cap covering the capsule
in most Mosses, developed from the
archegonial wall, 469 (Fig. 364).
The same term is also applied
to the Root Cap of Vascular
Plants.

Calyptrogen, the layer of cells which
gives rise to the root-cap, 88
(Fig. 63).

Calyx, the outermost series of floral
parts, composed of sepals, 255.

Cambium, an actively dividing forma-
tive tissue (secondary meristem),
55-58 (Figs. 34-36, 41) ; fascicular
and interfascicular, 56 ; of root, 91
(Fig. 66) ; of Conifers, 502 ; pro-
ducts of, 50 (Fig. 38) ; form of
cells of, 59 (Figs. 35, 36).

Camellia, 393.

Canadian weed (Elodea), 120 ; vege-
tative propagation of, 245.

Canal-cells, of Fern, 478 (Fig. 394) ;
of Moss, 468 (Fig. 363).

Cane sugar (sucrose), 123.

Caoutchouc, 53.

Caper Family, 568.

Capillarity in soils, 96.

Capitulum, a head, as in Compositae,
where numerous small flowers are
grouped on the widened axis, or
general receptacle, 262 (Fig. 186),
647, 652 (Figs. 498, 502).

Capsella, ovule and embryo of, 311
(Figs, 230, 231, 239).

Capsule of Bryophytes, 461 (Fig. 355),
469 (Fig. 364), 475 (Fig. 371).

Carbohydrates, 123 ; used to form
protein, 126 ; synthesis, rates of,
122 ; storage of, 124, 130 (Figs.
81, 85) ; in foodstuffs, 653-661.

Carbon dioxide, in air, 118 ; in
water, no; utilised in photo-
synthesis, 118; given off in res-
piration, 134.

Cardamine hirsuta, explosive fruit of,
165 (Fig. 101), 324.

Cardamine pratensis, adventitious
buds of, 247.

Cardoon, spread of, in La Plata, 331.

Carex, rhizome of, 207 (Fig. 139) ;
host for Puccinia, 433 ; flowers of,
615 (Fig. 463).

Carina, or keel of Pea-flowers, com-
posed of the two obliquely anterior
coherent petals, 636 (Fig. 489).

Carissa, straggling by axillary
branches, 215 (Fig. 143, v. vi.).

670

BOTANY OF THE LIVING PLANT

Carnivorous habit, as source of com-
bined nitrogen, 164, 220, 239.

Carnivorous plants, those which cap-
ture animals, and digest nourish-
ment from them, 239.

Carotin, 117, 663.

Carpels, the floral parts bearing
ovules or megasporangia : they
constitute the gynoecium, 255,
288 (Chap. xvi.).

Carpinus, seedling of, 87 (Fig. 61).

Carpogonium, the female organ of
some Algae and Fungi : of Red
Algae, 389 (Fig. 290) ; 400.

Carpospores, of Red Seaweeds, 389.

Carrot, analysis of, 655.

Caruncle, a swelling of the micropylar
region, characteristic of Euphor-
biaceae, 319, 629 (Fig. 479, vii.
viii.).

Cassytha, parasitism of, 221, 222.

Castanea, leaf of, 350 (Fig. 262).

Castor oil (Ricinus) 10 (Fig. 4), stor-
age of oil, and of protein, 126 (Fig.
84).

Casuarina, chalazogamy in, 307.

Catalysts, 128.

Categories of parts, 346.

Catharinea, 461 (Fig. 355).

Catkins of Willow, 619 (Fig. 466).

Caulerpa, non-septate thallus of, 171
(Fig. 105) ; cellulose rods of, 172
(Fig. 106), 368 (Fig. 273).

Cauliflower (Brassica), analysis of,
655 ; origin of, 654.

Celery (Apium graveolens), analysis
of, 655 ; origin of, 654.

Cell, the structural unit, 17, 18 ; size
of, 18 (Fig. 9) ; shape of, 23 (Sig.
13) ; growth of, 141 ; properties of
living cell, 30, 31 ; entrance of
dissolved substances, 39 ; move-
ment of contents, 167.

Cell-division, 18, 20 (Fig. 11), 561.

Cell-plate, a specialised layer of the
cytoplasm, within which the pri-
mary cell-wall is laid down at cell-
division, 562 (Fig. 428).
Cell-sap, fluid filling a vacuole, 35

(Figs, 12, 17).
Cell-theory, 19.
Cellular construction, 16, 17.
Cellulose, a carbohydrate which forms
the greater part of young cell-
walls, 24, 124 ; in foodstuffs, 655,

657-
Cell-wall, 17, 18, 24 ; permeable, 35.

Centaurea (Corn Flower), 650.

Centaury, dichasium of, 259 (Fig.

180).

Central body, in Blue-green Algae,

376.
Centrifugal force, effect of, 156.
Cephaleuros, 393.
Cephalotus, urns of, 241.
Ceratodon, embryo of, 470 (Fig. 365).
Ceratopteris, young leaf of, 493 (Fig.

384).
Cereal grains, 658.

Cereals, geotropic recovery of, 157
(Fig. 96).

Cetraria Islandica, 428.

Chaetocladium, parasitism of, 414 ;
air distribution of, 447.

Chaetophoraceae, 365.

Chalaza, the point at base of the
nucellus where the vascular strand
of the funicle stops, 294 (Fig. 216).

Chalazogamy, where in seed-plants
fertilisation is by a pollen-tube
traversing the chalaza, 307.

Chara, 547 ; Chara crinita, genera-
tive parthenogenesis in, 587.

Charlock (Brassica sinapis), 9, 10
(Fig. 3), 626 (Fig. 477).

Chelidonium, 626.

Chemosynthesis, 123.

Chemotropism, positive of pollen-
tubes, 160, 304 (Fig. 224).

Cherry, 634, 635 (Figs. 484, C : 487) ;
analysis of, 657.

Cherry-laurel, hypoderma of, 74.

Chervil, compound umbel of, 262
(Fig. 185).

Chiasma, region of a chromosome in
which interchange of material takes
place between homologous chromo-
somes, 565 (Figs. 430, 433).

Chimaeras, 251, 362.

Chlamydomonas, 364.

Chalmydospores of Mucor, 415.

Chlorococcales, 361.

Chlorococcum, 364 (Fig. 269).

Chlorophyceae, 355, 361 (Chap. xxii.).

Chlorophyll, the green colouring
matter of plants, 115, 117; spec-
trum of, 116 (Fig. 77).

Chloroplasts (or chlorophyll cor-
puscles), 43, 73, 76 (Fig. 51) ; 115,
117 (Figs. 75, 76).

Chlorotic state, pale yellow in absence
of iron, 112.

Chorda, 381 (Fig. 284).

Choripetalae, Dicotyledons with
separate petals (polypetalous), 605,
619.

Christmas Rose (Helleborus), 197-

Christmas Tree (Picea), 528.
Chromatid, a half-chromosome, 562
(Fig. 429), 565 (Figs. 430, 433).

INDEX AND GLOSSARY

6/i

Chromatophorc, 361 ; of Spirogyra,

373 (Fig- ^77)-
Chromonema, the slender thread of

karyotin which later becomes a

chromosome, 502 (Fig. 420).

Chromoplasts, colouring plastids in
petals or fruits, 280 (Fig. 200).

Chromosomes, bodies composed of
karyotin, which segregate in de-
finite number in the dividing
nucleus, 562 (Figs. 428, 433) ; num-
bers of, 567 ; re-arrangement of,
581 (Fig. 442) ; reduction of, 565
(Figs. 430, 432.

Chrysanthemum, development of
anther in, 284 (Fig. 205) ; C.
leucanthemum (Ox-eye Daisy), 649
(Fig. 500) ; C. frutescens, Crown
Gall of, 453.

Cilia, of Euglena, 356 (Fig. 265) ; of
Volvox, 363 ; of Fucus, 382 (Fig,
287) ; of Fern, 501 (Fig. 393) ; of
Moss, 467 (Fig. 362) ; of Zamia,
527 (Fig. 415).

Circulation of nitrogen in nature, 127.

Circumnutation, spontaneous move-
ment of stem and root in normally
growing seedling, 144 (Fig. 90).

Cissus, host of Rafflesia, 226 (Fig.

155)-

Cladonia, 428 (Fig. 326) ; 429 (Fig.

327)-
Cladophora, 368.

Cladophorales, isomorphic alterna-
tion in, 390.

Cladothrix, straight and slender forms
of Bacteria, 448.

Classification, natural system of,

343-

Claviceps (Ergot of Rye), 395 (Fig.
293) ; sclerotia of, 425 (Fig. 323).

Clay, 96, 97.

Clematis, 41 (Figs. 20, 21) ; mechani-
cal construction of stem, 183
(Fig. 116) ; prehensile leaf of, 216.

Climbing, by straggling, 214 (Fig.
143) ; prehensile, 215 (Figs. 144,
145) ; adhesive, 217 (Fig. 146).

Climbing habit, 213.

Climbing plants, stem-structure of,
213 ; methods of, 213.

Closed-bundle, having no cambium,
53 (Fig. 31).

Closterium, conjugation of, 374 (Fig.
278) ; flanged chloroplast of, 594.

Clostridium, 127, 452.

Clover, day and night movements of,
162 ; seeds of, 140.

Club-mosses (Chap, xxxii.), 510.

Club-root, 396 (Fig. 295).

Cluster-cups of Rust Fungi, 396, 437

(Fig- 337)-
Coal, origin of, 138.

Cobalt chloride, 100.

Coboea, tendril of, 216.

Coccus, spherical form of Bacteria,

448 ; causing suppuration, 452.

Cockle-burr (Xanthium), latent period

of, 334-
Coconut, milk of, 316 ; floating fruit

of, 327-
Codium, matted filaments of, 171,

369 ; gametes of, 370 (Fig. 275,

hi.).
Coelebogyne, sporophytic budding,

587.

Coenocyte, a multinucleate proto-
plast not divided into cells : of
Siphonales, 361, 368.

Coenogamete, a gamete in which
many nuclei are involved, 417.

Coenopterid steles, flanging of, 593
(Fig. 449).

Coenozygote, a zygote formed by the
union of coenogametes each con-
taining many nuclei, 417.

Coffee disease, 396.

Cohesion, the fusion of parts of
the same category in the flower,
266.

Cohesion theory of ascent of water,
106 (Fig. 73).

Colchicum, carpels of, 289 ; style of,
293, 608.

Coleochaete, 393, 547.

Coleoptile of grasses, 150, 160.

Collateral bundle, where wood and
bast run longitudinally parallel on
the same radius, 53 (Figs. 23, 34).

Collema, 376, 427, 429.

Collenchyma, 42 (Fig. 22) ; structure
of, 175 (Fig. 109) ; physical quali-
ties of, 179.

Colloids, 32.

Colocasia, stele of root, 594.

Column, or Orchidaceae, 612 (Fig.
460).

Columnar requirement, for mechani-
cal resistance in stems, 181.

Combination of organographic fac-
tors, 602.

Companion-cells, cells adjoining
sieve-tubes, from which they are
derived by late longitudinal divi-
sion, 48 (Figs. 25, 26, 27).

Compass plants, 159.

Complete parasites, those which are
wholly dependent on parasitism
for nutrition. They are without
chlorophyll, 223.

B.B.

2 u

672

BOTANY OF THE LIVING PLANT

Compositae, wind-borne fruit of, 326
(Fig. 247) ; structure of flower of,
647 (Figs. 498-502).

Conceptacle of Fucus, 381 ; mature
structure, 381 (Figs. 2S2, 283) ;

384-
Concrete, reinforced, 180.

Cone of Coniferae, 526 (Fig. 414).

Confervoideae, 368.

Conical and obconical form con-
trasted, 590 (Fig. 446).

Conidiophore, a hyphal branch which
bears conidia, 401 ; of Ascomy-
cetes, 422 (Fig. 320).

Conidium, the air-borne, not sexually
produced, propagative cells of
Fungi, 397, 401, 422 (Fig. 320).

Coniferales, 3, 527 ; female cones of,
526 (Fig. 414).

Conjugatae, 372.

Conjugation, syngamy of equal ga-
metes, 361, 372, 399 (Fig. 297), 557.

Conjunctive parenchyma, those par-
enchyma cells which fill up the
spaces between vascular elements
of the stele, 85 (Figs. 58, 59), 92
(Fig. 66), 489 (Figs. 377, 378)-

Connective, the region between two
anther-lobes,' greatly extended in
Sage, 302 (Fig. 220), 647.

Constitution, water of, 94.

Constructive metabolism, those chem-
ical changes which relate to for-
mation of new organic material,

134-
Continued embryology, 14, 15.

Continuity of protoplasm, the con-
nection of one protoplasmic body
with another by threads traversing
the cell- wall, 26 (Fig. 16).

Contractile vacuole, 356.

Control by stomata, 102.

Convallaria, 608.

Convolvulus, twining stem of, 215-
217 ; flowers of, 225.

Copaifera, arillus of, 319 (Fig. 238).

Copper ferrocyanide, 35.

Coprinus, hymenium of, 445 (Fig.

345)-
Cora, 446.
Coral-root (Corallorhiza), mycorrhiza

in, 232.
Cordaites, 180.

Cordyceps (Ergot of Rye), 418.
Cork, 65 (Figs. 43, 44), 80 (Fig. 55).
Corn, " laying of," 157 ; recovery,

158 (Fig". 96).
Corn-flower, 650.
Corolla, the inner floral envelope

composed of petals, 255.

Corona, a late additional appendage
of the corolla, 609.

Correlation of growth, where one part
is developed larger than usual and
another part is reduced, 218.

Cortex, the tissue lying between epi-
dermis and stele, 42 (Fig. 22) ; of
root, 82 (Fig. 56) ; of old root,

93-
Corydalis, transverse zygomorphy of,

274.
Corypha, its single flowering, 196.
Cotton, hairs on seeds, 325 (Fig. 245).
Cotton-grass, 614 (Fig. 462).
Cotyledons, the first seed-leaves

borne on the embryo, 6, 312 (Fig.

230).
Couch grass, vegetative propagation

of, 252.
Cow-parsnip, 637 (Fig. 490).
Crassulaceae, meristic differences in,

265.
Crataegus, 345.
Crenothrix, 451 (Fig. 347).
Cress-seedling, damping-off of, 402

(Fig. 300).
Crocus, 197, 610 ; style of, 293 ;

storage corm of, 197 (Fig. 131).
Crossing-over, the interchange of

parts between the chromatids of a

pair of homologous chromosomes

at meiosis, 565, 580 (Figs. 430, 433,

441).
Crown Gall, 453.
Cruciferae, 626 (Fig. 477).
Crystalloids of Graham, 32.
Crystalloids, proteid storage bodies of

crvstalline form, of potato, 124

(Figs. 81, 84).
Cucumber, 29, 48 (Figs. 25, 26, 27) ;

bi-collateral bundle of, 48 ; hair of,

29 (Fig. 17).
Cultivated plants, vegetative pro-
pagation of, 248.
Currant (Ribes), raceme of, 260

(Fig. 182), 631 (Figs. 481), 482) ;

analysis of, 657.
Curvembryeae, 620.
Cuscuta (Dodder), parasitism of, 223

(Figs. 151, 152).
Cuticle, a thin layer of corky nature

covering exposed surfaces, 42, 75

(Figs. 51-53) ; of xerophytes, 210

Fig. 142).
Cutin, 124.
Cutleria, 379 ; gametes of, 383 (Fig.

286) ; 386.
Cutleriales, heteromorphic alterna-
tion in, 390.
Cuttings, 149 (Fig. 92).

INDEX AND GLOSSARY

6/3

Cyanophyceac, 376.

Cyathium, a condensed spicate in-
florescence contained in a cup-like
involucre : of spurge, 629 (Fig.

479).
Cycadales, 526.
Cycas, motile male gametes of, 527

(Fig. 415).
Cycle of Life, in seed-plants, 335

(Fig. 257).
Cyclic arrangement of leaves where

two or more are seated at the same

level, 201 ; of parts of flower, 263

(Fig. 187).
Cydonia (Quince), flower of, 255

(Fig. 177).
Cynara, spread of, in La Plata,

33i-

Cynoglossum, hooked-fruits of, 328

(Fig. 251, C).
Cyperaceae, 615.
Cyperus, girder construction in, 181

(Fig. 113).
Cystopus, fertilisation in, 410 (Fig.

3io) ; 447-
Cytase, the digestive ferment for

cellulose, 125.
Cytisus adami, reputed graft-hybrid,

251-

Cytoplasm, the protoplasmic body of
the cell exclusive of the nucleus,
17, 29 ; movements of, 30 ; per-
meability of lining, 36, 38, 39.

Daffodil, 609.

Dahlia, 197 ; storage roots of (Fig.

130) ; storage in, 124.
Daisy, capitulum of, 262 (Fig. 186).
Damping-off disease, 396, 397, 399 ;

general account of, 402.
Dandelion, capitulum of, 651 (Fig.

502) ; clock, 326 (Fig. 247).
Dasylirion, qualities of fibres of, 178.
Date, storage in, 125.
Datura, stigma of, 292 (Fig. 214.)
Daucus Carota (Carrot), 654, 655.
Dead-nettle (Lamium), 645 (Fig.

496).
Dead-weight, mechanical support of,

181.
Deciduous, applied to plants which

drop their leaves at certain seasons,

194.
Decussate arrangement, of leaves in

successive alternating pairs, 201 ;

of Sycamore (Fig. 134) , of Epilo-

bium (Fig. 133).
Deficiency disease, due to want of

necessary vitamin, 662.

Dehiscence, splitting, especially of
sporangial or carpellary walls : in
anthers, 282 ; in fruits, 323.

Dehydrases, 137.

De-plasmolysis, 37.

Dermatogen, a layer of cells, usually
superficial, giving rise to the epi-
dermis, 312 (Fig. 230, v.-viii.).

Descent, lines of, 355.

Deschampsia caespitosa, viviparous
habit, 248.

Desmidiaceae, 374 (Fig. 278).

Desmoncus, straggling by reflexed
pinnae, 215 (Fig. 143, vii.), 345.

Destructive metabolism, 134.

Dextrorse twining, following hands
of watch, 2 1 6.

Dialysis, 32.

Diaphototropism, 159.

Diastase, ferment converting starch
to sugar, 123 ; action of, 129, 130.

Diatoms, 374, 375 ; life-cycle of, 390.

Dichasium, a definite or cymose
inflorescence, where two lateral
branches arise at about the same
level, 250 (Fig. 180).

Dichotomy, a forking into two equal
branches, 348.

Dicksonia, pith of, 598.

Diclinous, where staminate or pistil-
late flowers are borne on the same
plant, 301.

Dicotyledoneae, 605, 619.

Dicotyledons, seed-plants (Angio-
sperms), having an embryo with
two seed-leaves, 3, 7 ; herbaceous
stems of, 42 ; woody, 55 ; root of,
85 (Fig. 59) ; old root, 92 (Figs.
66, 67) ; mechanical construction
of stem, 1 81 ; embryology of, 311
(Fig. 230).

Dictyota, 378, 384 ; apex of, 379 ;
tetraspores of, 383, 390 ; alterna-
tion in, 383, 390, 545.

Differentiation : of tissues, the gradu-
ally acquired distinction of charac-
ter as the cells mature from an un-
differentiated embryonic tissue,
21 ; of sex, 557 ; of gametes, 558 ;
in Brown Algae, 382 ; in Green
Algae, 370 ; under enzymes in
Heather, 230.

Digestion, intra-cellular, 232 ; by
Carnivorous Plants, 240.

Digestive ferment, of Fungi, 394
(Fig. 294).

Digestive sac, the layer of cells which
softens the outer-lying tissues for
the passage outwards of the lateral
root, 90 (Fig. 65).

674

BOTANY OF THE LIVING PLANT

Digestive glands of Drosera, 240 (Fig.

166).
Digestive tract, in mycorrhiza, 231

(Figs. 159, 160).
Digitalis purpurea, Foxglove, 644.
Dimorphism, of Primrose, 640 (Fig.

492). .

Dioecious, where staminate and pistil-
late flowers are borne on different

plants, 301.
Dioecism, in Willow, 336, 619 (Figs.

467-469) ; by abortion in Lychnis

dioica, 271 (Fig. 194).
Dionaea, motile leaf- traps of, 165

(Fig. 100).
Dioscorea, 607.
Dioscoreaceae, embryology of, 314

(Fig. 233).
Diplobiontic Algae, 546, 556.
Diplococcus, 452.
Diploid, having double the typical

number of chromosomes (2x), as

shown by each nucleus on division ;

this is characteristic of the sporo-

phyte, 308 ; in Green Algae, 375 ;

in Ferns, 457. See Chap, xxxiv.
Diplostemonous, where the stamens

are twice as many as the petals,

267.
Disaccharides, 123.
Disc-florets, of Compositae, 648 (Fig.

500).
Discomycetes, 425 (Fig. 322), 428.
Disease, mortal, 235 ; epidemic, 396.
Dispersal of seeds, 323.
Dissolved substances, entrance of,

Distal branching of shoot, 348.

Divergence, angle of, in leaf arrange-
ment, the angle between the median
planes of successive leaves, 204
(Fig. 137).

Division of nucleus, somatic (mitosis),
562 (Figs. 428, 429) ; tetrad-division
(meiosis), 563 (Figs. 430, 431).

Dock, straight ovule of, 303 (Fig. 221).

Dodder (Cuscuta), parasitism of, 223
(Fig. 151) ; suckers of, 225 (Fig.

152).

Dog Rose, 634.

Dog's Mercury (Mercurialis) , 113.

Dominant, that one of a pair of unit
characters which remains apparent
in all the offspring of the first cross,
e.g. tallness in Peas, 570 (Fig.

435)-

Dormancy of seed, 140.

Dorsiventral symmetry, where an
organ or shoot develops unequally
on two sides, in relation to gravity,

light, etc., 205 ; of lateral branches,
186 ; of rhizomes, 206, 207.

Double fertilisation, in Helianthus,
306-308 (Fig. 227).

Doubling of flowers, 268.

Dracaena, secondary thickening of,
67 ; raphides of, 54 (Fig. 33) ; leaf
arrangement {irsch) in, 205.

Dressing of seed-grain, 441.

Dried fruits, analyses of, 657.

Drosera, motile tentacles of, 164
(Fig. 99) ; carnivorous habit of,
239 (Fig. 166) ; digestion in, 240.

Drought, physiological, 211.

Drupe, a fruit with succulent middle
layer of the pericarp, and stony
inner layers, 329 (Fig. 253) ; 635

(Fig. 487).

Dryopteris filix-mas, Chapter xxxi.,
p. 481 ; leaf of, 482 (Fig. 373) ;
stock of, 484 (Fig. 374) ; vascular
system of, 484 (Fig. 374), 486 (Fig.
376 a) ; meristele of, 488 (Figs.
377, 378) ; leaf-structure of, 491
(Fig. 381) ; sorus of, 482 (Fig. 373),
495 (Fig. 387) ; sporangium of,
496-498 (Figs. 388-390) ; prothal-
lus of, 499-501 (Figs. 391-393)-

Dryopteris pseudo-mas, v. cristata,
apospory and apogamy in, 509
(Fig. 404).

Dry Rot fungus (Merulius), 442.

Dry weight as growth index, 145.

Duration, biology of, 194.

Dwarfs, 570, 601.

Dwarf-males, of Oedogonium, 367
(Fig. 271).

Earthworms, 98.

Eating Pea (Pisum sativum), used in
Mendel's experiments, 570.

Ecballium, squirting fruit of, 324.

Ecology, the study of plants in rela-
tion to their surroundings, 3.

Economy of material, 180.

•Ectocarpales, 378, 383.

Ectocarpus, gametes of, 381, 383 ;
siliculosus, gametes of, 381 (Fig.
285) ; sex distinction in, 386.

Ectotrophic mycorrhiza, where the
fungus lives outside the tissues of
the host, 228 (Figs. 156, 157).

Edelweiss, hairy covering of, 74.

Egg-apparatus, a group of primordial
cells at the micropylar end of
the embryo-sac, consisting of two
synergidae and the ovum, 295
(Fig. 216), 299 (Fig. 219).

Egg, or ovum, 332, 353 ; protection
of, in Land Plants, 455.

INDEX AND GLOSSARY

6/5

Elasticity, limit of, that is the degree
of elongation which a strand or
wire will sutler and recover its exact
length when the stress is removed,
178-179.

Elements, of plant-food, no.

Eligulatae, those Lycopods which
have no ligule, 512.

Elm, vascular bundle of, 56 (Eig. 34) ;
cambium of, 57 (Fig. 36) ; bark of,

65.
Elodea, Canadian weed, 16, 120 (Fig.

80) ; vegetative propagation of,

245-
Elymus, Lyme Grass, leaf-structure,

187 (Fig. 121).

Embryo, a new individual resulting
from syngamy, 6 ; immature, 140 ;
initiated by syngamy, 311 (Figs.
230, 232 ) ; of Pine, 537 (Fig. 425) ;
of Selaginella, 518 (Figs. 412, 413) ;
of Fern, 504 (Fig. 397).

Embryo-sac, the large cell or mega-
spore enclosed in the nucellus in
Seed Plants, which contains the
ovum and other cells, 295 (Fig.
216) ; development of, 296 (Fig.
217), 298 (Fig. 219).

Embryology, internal, of Land Plants,

455. 559-

Emergences, appendages of the epi-
dermis, together with subjacent
tissue, 12, 352.

Empetrum, rolled leaf, 211.

Empusa Muscae, 401, 413 ; explosive
dispersal of conidia of, 415.

Encysted state, where a proto-
plast is surrounded by a cell-wall,
169 ; of Euglena (Fig. 104, D,
E).

Endodermis, the layer of cells de-
limiting the stele (= phloeoterma),
42 (Fig. 22), 44 (Fig. 23), 46 (Fig.
24) ; in root, 84 (Figs. 58, 59) ;
function of, 109 ; in Fern, 484
(Figs. 377, 378) ; at apex of Ferns,
598-600.

Endogenous origin, development of a
new part from deeply-seated tissue,
e.g. roots, 90 (Fig. 65).

Endosperm, a nutritive tissue pro-
duced within the embryo-sac ; it
surrounds the embryo, and often
persists till the ripeness of the
seed, which is then described as
albuminous, 319 (Fig. 239) ; in
albuminous seeds, n (Fig. 4) ;
function of, 319 ; in wheat, 658 ;
analysis of, 660 ; of Coniferae, 535
(Fig. 423), 538 (Fig. 426).

Endothecium, the central tract of
cells of the young Moss sporo-
gonium, 470 (Fig. 365).

Endotrophic mycorrhiza, where the
fungus occupies the living cells of
the host, 230 (Figs. 159, 160).

Energy and photosynthesis, 138.

Energy of light-rays, 116 ; needed by
cell, 133 ; liberated by, 134 ; fix-
ation of, 138.

Enlargement of cell, 141.

Enteromorpha, 365.

Entomophthorales, 413.

Enzymes, 33, 128.

Ephebe, 427.

Epibasal hemisphere, the part of an
embryo lying above the basal wall :
in Ferns, 504.

Epicalyx, 633 (Fig. 434, B)„

Epidemic, 396.

Epidermis, 42 (Fig. 22), 46 (Fig. 24) ;
as seen in surface view of leaf, 75
(Fig. 49).

Epigynous, of flowers, where the
gynoecium is sunk in the abbre-
viated receptacle, so that the ovary
appears to be below the other
floral parts, 272 (Figs. 197, 198),
277.

Epilobium, symmetry of shoot, 200
(Fig. 133).

Epiphytes, plants which live attached
to the branches or trunks of other
plants ; water-supply of, 211 ;
xerophytic features of, 209.

Epi-rachis, 351.

Epithelium, in style of Rhododen-
dron, 305 (Fig. 225).

Equisetales, the Horsetails, 3, 522
(Fig. 413, A).

Equisetum arvense, vegetative pro-
pagation of, 252 ; sporangiophores
of, 522.

Ergosterol, 663.

Ergot or Rye (Claviceps), 395 (Fig.
293). 397 i general account of, 425
(Fig. 323).

Ericaceae, Heaths, mycorrhiza in,
230 ; flower of, 639 (Fig. 491).

Eriophorum, 615 (Fig. 462).

Ervum lens (Lentil), analysis of, 655 ;
origin of, 056.

Erysiphe, description of, 419 ; haus-
toria of, 420 (Fig. 317).

Erysiphales, 419.

Essential elements, 112.

Ethyl alcohol, 136.

Etiolation, result of growth of a plant
in the absence of light, 146 (Fig. 91).

Eucalyptus, vertical leaves of, 211.

676

BOTANY OF THE LIVING PLANT

Euglena, 169 (Fig. 104), 357 (Figs.
265, 266) ; instability of nutri-
tional method, 356.

Eumycetes, 400, 431.

Euphorbia (Spurge), xerophytes of
Old World, 210 ; simple flowers
of, 256 (Fig. 178), 343, 629 (Fig.

479).
Euphorbiaceae, reduction of flowers

in, 271 ; transfer of seeds of, 330 ;

floral construction in, 629 (Fig.

479).
Eurotium, see Aspergillus, 401, 422

(Fig. 320).

Evergreens, plants which retain their
leaves through the year, 80, 194.

Evolution theory, a working hy-
pothesis, 339 (Chap. xx.).

Exalbuminous seed, in which the
endosperm is absorbed before ripe-
ness, 319 (Fig. 239).

Exodermis, in many roots a special-
ised layer below the piliferous
layer, 83 (Fig. 57).

Exogenous origin, development of
new parts from superficial tissues,
e.g. leaves, 17 (Fig. 7), 90.

Extension, the elongation of a part
already formed, 143 (Fig. 88).

Exudation of water, 108, 109 (Fig.

74. A).

Eyebright (Euphrasia), a green root-
parasite, 222.

Eye-spot, 357 (Fig. 265).

Fall of leaf, 80 (Fig. 55), 194.-

False tissue of Fungi, 394.

Families, Natural, 605 (Appendix A).

Fats, storage of, 125.

Female gamete, or ovum, or egg, 294
(Fig. 216), 305 (Fig. 226) ; of Coni-
ferae, 536 (Fig. 424) ; of Fern, 502
(Figs. 394, 396) ; of Fucus, 386
(Figs. 287, 288).

Ferments, or enzymes, 128 ; proteo-
lytic, which break down complex
protein into simpler substances,
240 ; digestive, of Fungi, 393 (Fig.

294).
Ferns, 481 (Chap, xxxi.) ; distal

branching, 348 ; life-cycle of, 506
(Fig. 400) ; mechanism of sporan-
gium, 166 (Fig. 103) ; vascular
supply to big buds, 595, 600.
Fertilisation, the coalescence of male
and female gametes to form a
zygote, 301 ; in Flowering Plants,
306-307 (Figs. 227, 228) ; in Ferns,
503 (Figs. 395, 396) ; in Fucus, 386

(Fig. 287) : indirect in Red Sea-
weeds, 389.

Fertilising tube, of Pythium, 398
(Fig. 296), 405 ; in Peronosporeae,
410 (Fig. 310).

Festuca ovina, viviparous habit of,
248.

Fibonacci series, 204.

Fibre, a cell much longer than broad,
with pointed ends, 23 (Fig. 13), 59

(Fig. 38).

Fibrous cells, mechanically effective
in opening the anther, to shed the
pollen, 282 (Fig. 203) ; develop-
ment of, 285 (Fig. 206).

Fig, hollow succulent inflorescence of,
330 (Fig. 255) ; analysis of, 657.

Figwort (Scrophularia), vascular
strand of, 44 (Fig. 23), 46 (Fig. 24) ;
flower of, 643 (Fig. 494) ; abortive
stamen of, 268 (Fig. 192).

Filament, the stalk of a stamen, 281
(Fig. 201).

Filicales, the Ferns, 3 ; general de-
scription of, Chap, xxxi., p. 481.

Fine flour, 660.

Fissidens, a Moss, bilateral sym-
metry of, 205.

Fission, or branching of parts, in the
flower, 266 ; of stamens in Vellozia,
266 (Fig. 190) ; of cells of Euglena,
357 (Fig. 266).

Fission Fungi (Bacteria), 448.

Fixation of atmospheric nitrogen
(Heather), 231.

Flagellatae, 356, 359, 360.

Flagellum, of Euglena, 357.

Flattened surfaces, stiffening of, 186 ;
protection of margins of, 189 (Figs.
124, 125).

Flexuous hyphae of Rust Fungi, 438.

Flies, killed by Empusa., 413, 415.

Flora of Land, origin of, 546.

Floral construction, 605 (Appendix
A).

Floral diagram, explained, 263 (Fig.

l87)' a t

Floral formula, a compact mode of

registering the component parts of

a flower, 264.

Flower, a simple shoot which bears
sporangia, 256 (Chap, xiv.) ; de-
velopment of, 273 (Fig. 198) ; parts
of, 255 (Fig. 177) ; definition of,
256 ; comparison of, 263 ; bio-
logical specialisation of, 274 ; of
Conifers, 531 (Figs. 416, 420, 421).

Flowering, its relation to storage, 195.

Flowering Plant, 2, 5.

" Flowers " of Mosses, 467.

INDEX AND GLOSSARY

677

Fluting of stele, 593 (Fig. 440).
Foliage-spurs, of Pine, 529 (Fig. 416).
Foliar appendages, borne on axis, 347.
Foliar-gaps of Ferns, 486 (Figs. 374,

376. A).
Follicle, a separate carpel splitting

along its margins, and containing

several seeds, 321 (Fig. 240).
Fomes, 441 (Figs. 340, 342.)
Fontinalis, aquatic habit of, 462 ;

peristome of, 471 (Fig. 367).
Food chains, 138.
Food of plants, 114 ; supplied in

solution, 131.
Food-stuffs (Appendix B), 653.
Food- values, 662.
Foot, the suctorial organ in the

embryo : in Ferns, 504 (Fig. 397).
Forcing of plants, 153.
Form, modifications of, Chap. xi. ;

relation to size, 589 (Chap, xxxvi.).
Formaldehyde, 121.
Foxglove (Digitalis), 644.
Fragaria vesca (Strawberry), 633

(Fig- 485).
Fragmentation, the direct division

of nuclei, without formation of a

spindle, 284.

Free-central placentation, of ovules,
when seated on an apparent pro-
longation of the floral axis into the
ovary, 291.

French-bean (Phaseolus vulgaris),
analysis of, 655 ; origin of, 656.

Fritillaria, 21 (Fig. 12), 608.

Fritschiella, sporeling, 592 (Fig. 447).

Frost, effect on plants, 148.

Fructose, 123.

Fruit, the whole pistil or gynoecium
when matured, 320.

Frullania, structure of leaves of, 473
(Fig. 369).

Fuchsia, epigynous flower of, 272
(Fig. 197).

Fucoxanthin, 380.

Fucus, bilateral symmetry in, 205 ;
external characters of, 378 (Fig.
280) ; structure of, 378 (Figs. 282,
283) ; sexual organs of, 384 (Figs.
287, 288) ; fertilisation of, 386 ;
young plants of, 387 (Fig. 289) ;
gametes of, 385 ; absence of alter-
nation in, 387 ; life cycle, 556 ;
obconical sporeling of, 589 (Fig.

445)-
Funaria, habitat of, 462 ; sexual

organs of, 467 (Figs. 362, 363).
Fundamental number, that number

of parts in the flower which rules

in the construction, so that flowers

appear tri-merous, tetra-merous,
etc., 265.

Fungal attack, 394 (Fig. 294).

Fungi, those Thallophytes which are
without green chlorophyll, 3, 355,
391 ; origin of, 392 ; polyphyletic
in origin, 393 ; sex organs in, 398 ;
irregular nutrition of, 221 ; intro-
ductory, 391 ; early occurrence of,
391 ; subaerial adaptations of,
446 ; non-septate, 399, 400 ; sep-
tate, 399, 420.

Fungi Imperfecti, those of which the
knowledge of the life-cycle is
incomplete, 399.

Fungivorous habit, where a plant is
able by digestion to absorb the
substances of a fungus into itself ;
parallel with the carnivorous habit,
232 (Fig. 159).

Funiculus, the stalk of an ovule, 294
(Fig. 216).

Funkia, leaf of, 351 (Fig. 264) ; sporo-
phytic budding in ovule, 587.

Fusion of parts, of flower, 265.

Fusion-nucleus, the central nucleus
of the embryo-sac, which results
from fusion of the two polar
nuclei, 298 (Fig. 219).

Galanthus (Snowdrop), 608.

Galileo, Principle of Similarity, 591.

Galium, straggling habit of, 214 ;
hooked-fruits of, 328 (Fig. 251).

Gametangia, 351, 361 ; in Algae, 380.

Gamete, a sexual cell, 281, 300, 308,
352, 544 ; fertilisation by fusion of
gametes, 300 ; of Brown Seaweeds,
380 (Figs. 284-288) ; of Ulothrix,
365 (Fig. 270) ; of Siphonales, 370
(Fig. 275) ; non-motile of Conju-
gatae, 373 (Fig. 277) ; motile male
of Monoblepharis, 411 (Fig. 311) ;
protected in Land Plants, 549, 557 ;
differentiation of, 558.

Gametophyte, the sexual phase in the
life-history of plants showing alter-
nation, 287 ; in Flowering Plants,
299 ; haploid, 544 ; its water-
relation, 549 ; its rise and de-
cadence, 549-552-

Gametophytic budding, where the
gametophyte is reproduced by buds
of gemmae from a parent gameto-
phyte ; in Ferns, 500 ; in Mosses,
466 (Fig. 360).

Garlic, twisted leaves of, 70 ; quali-
ties of fibres of, 178.

Gasteromycetes, 442.

6yS

BOTANY OF THE LIVING PLANT

Gemmae, of Mosses, 466 (Fig. 360).

General purposes shoot, 347 ; segre-
gation of, 554.

General receptacle of Compositae, 648,
652 (Fig. 502).

Generative parthenogenesis, where an
embryo is formed without fertilisa-
tion, from a haploid egg, 587.

Genes, the elements in the gametes
corresponding to the differentiating
characters of the zygote, 563, 572 ;
interaction of, 578 ; pleiotropic,
579. 586 ; loci of, 580 ; gene-muta-
tion, 585.

Genetics, the experimental study of
inheritance and variation, 574-584 ;
and Evolution, 584-587.

Genotype, an individual with a given
genetic constitution : also that
constitution itself, 575.

Geotropism, response to stimulus of
gravity : positive and negative,
154 (Fig. 94) ; geotropic recovery,
158 (Fig. 96) ; lateral, 215 (Fig.
144).

Geraniaceae, 628 (Fig. 478).

Geraniales, 627.

Geranium, sling fruit of, 324 (Fig.
243) ; flower of, 628 (Fig. 478).

Germ, a young plant, or embryonic
individual, 254 ; of Wheat, 658
(Fig. 503) ; analysis of, 659.

Germination, renewal of activity of
the dormant seed or spore, 5,
7, 139 ; conditions governing, 140.

Geum, hooked-fruit of, 328 (Fig.

251, D).
Gills, of Mushroom, 444.
Ginkgo, motile male gametes of, 527,

551-
Girder-principle, of disposition of

tissue, 181 (Fig. 113), 183 (Fig.

115), 187 (Figs. 120-122).
Gleditschia, leaf of, 349 (Fig. 261).
Gloeocapsa, 376 (Fig. 279).
Gloriosa, climbing leaf -tip, 216.
Gloxinia, propagation by adventitious

buds, 252.
Glucose (Grape Sugar), 123.
Glumales, 613.
Glumes, 614, 616 (Fig. 464).
Gluten, 660.
Glycine soja (Soya Bean), analysis of,

655 ; origin of, 656.
Glycogen, reserve carbohydrate of

Fungi, 394.
Goat Willow (Salix), 619.
Gonidial layer, the stratum in a

Lichen thallus where the green cells

lie, 428 (Fig. 327).

Gooseberry, 631 ; analysis of, 657.

Gorse, seeds of, 140 ; transferred by
ants, 330.

Graft-hybrid, the reputed result of
complete fusion of tissues of graft
and stock, so that the characters of
both are mingled, 251.

Grafting, 249 (Figs. 173, 174).

Gramineae, 616 (Fig. 464).

Grape (Vitis vinifera), origin of, 656 ;
analysis of, 657.

Grape-hyacinth (Muscari), 608.

Grape sugar (Glucose), 123.

Grasses, coleoptile of, 150 (Fig. 93) ;
haulm of, 185 (Fig. 119) ; leaf-
arrangement, 205 ; embryo of,
314 ; flower of, 616 (Fig. 464).

Grass-leaves, curling under drought,
187, 211 (Figs. 120-122) ; water
exudation from, 109.

Gravity, effect of, 8 ; stimulus of,
154 ; perception, theory of, 156

(Fig. 95)-

Green Algae, 361, Chap. xxii.

Green peas, analysis of, 655.

Grimmia, gemmae of, 466.

Groundsel, 649.

Ground-tissue, 40.

Growing-point, the tip of the stem
or leaf or root, where now tissues
are being produced, 16 (Figs. 7, 8,
10) ; powers of, 604.

Growth, increase in bulk with re-
distribution of organic material,
5, 7 : (Chap, ix.) 139-153 ; annual in-
crement of, 13 (Fig. 6) ; conditions
of, 31 ; effect of temperature upon,
147 (Fig. 90) ; effect of light upon,
146 ; localisation of, 85 (Fig. 62) ;
measurement of, 144 ; correlation
of, 149 ; zones of, 143 ; periodicity
of, 152 ; vegetative phase of, 141 ;
hormone promoting, 150 ; growth
and respiration, 135.

Guard-cells, the cells that control the
pore of a stoma, 75 (Figs. 49, 50),
76 (Figs. 51, 52), 102 (Figs. 70,
71) ; water-stomata, 109 (Fig. 74,
A).

Gummy walls, of mucilaginous

character, 24.
Gussets, of mechanical tissue in

leaves, 189 (Figs. 124, 125).
Gymnocladus, testa and endosperm

of, 318 (Fig. 237).
Gymnosperms, seed-plants with their
ovules exposed, such as the Pines
and Firs, 3, 525 (Chap, xxxiii.).
Gymnosporangium, on Juniper,

432.

INDEX AND GLOSSARY

6/9

Gynoecium, applied collectively to
all the carpels of a single flower,

255-

Haematococcus (see Sphaerella),

3°3-

Haemoglobin, in root-nodules, 239.

Hairs, appendages of the epidermis,
12 ; effective in seed dispersal,
325 (Fig. 245), 351.

Hairiness, 210.

Hakea, stoma of, 77 (Fig. 53) ;
sclereids in leaf of, 177 (Fig. in) ;
xerophytic leaf of, 210.

Halimeda, matted and cemented
filaments of, 173, 369.

Halophytes, 113, 211.

Haplobiontic Algae, 546, 556.

Haploid, having tihe simple number
(x) of chromosomes in each nucleus,
as in the gametophyte generation,
299 ; in Algae, 375 ; in Ferns, 506 ;
in Ascomycetes, 424 ; in Puccinia,

437-
Haptotropism, 160.

Hardening of plants, 148.

Harveyeila, parasitism of, 389 (Fig.

291), 392.
Haustoria in ovules of Rhinanthus,

316 (Fig. 235) : of mildews, 419

(Fig. 317).
Hay-bacillus, 448 (Fig. 346).
Hazel nut, analysis of, 657.
Heart- wood, 64.
Heat, evolution of, 137.
Heath (Erica), curled leaves of, 106,

211 ; endotrophic mycorrhiza of,

230, 231 (Fig. 158) ; flowers of,

639 (Fig. 49i)-
Heather (Calluna), 113 ; endotrophic

mycorrhiza of, 231 (Fig. 158).
Helianthus (Sunflower), root-cap of,

156 (Fig. 95) ; development of

flower of, 273 (Fig. 198) ; double

fertilisation of, 306 (Fig. 227).
Heliotropism, response to stimulus of

light, 158 (Fig. 96) ; diaphoto-

tropic, 159.
Helleboreae, follicles of, 322 (Fig. 240).
Helleborus, floral diagram of, 264

(Fig. 189), 624.
Hemerocallis, 608.
Hemicyclic, arrangement of parts of

flower, 264 (Fig. 189).
Hemp, fibres of, 177.
Hems, of sclerotic tissue at leaf

margins, 190 (Fig. 125).
Hepaticae (Liverworts), 3, 460, 472 ;

archegonia of, 474.

Heracleum (Cow Parsnip), 637 (Fig.

490).
Herbaceous Dicotyledons, stem of,

42.
Heredity (Chap, xxxv.) ; mechanism

of, 341.
Hermaphrodite, where male and

female organs are on the same

individual : applied to flowers

when they contain both stamens

and carpels, 256, 301.
Heterocysts, of certain Cyano-

phyceae, 376.
Heteroecism, where the life-cycle of a

parasite is completed by stages on

distinct hosts, 432.
Heteromorphic alternation, 390.
Heterosporous, applied to vascular

plants in which there are distinct

magaspores and microspores, 352 ;

a derivative state, 517 ; adoption

by Pteridophytes and seed-plants,

539, 553-
Heterothallic, in Mucorales, where

zygospores are only produced on
meeting of branches of two differ-
ent mycelia, 416 (Fig. 315) ; in
Rusts, 439.

Heterotrophic, applied to nutrition
by some accessory or irregular
method, in addition to or even
superseding self-nutrition, 220
(Chap. xii.).

Heterotype division, another name
for meiosis : conveying the fact
that the resulting nuclei are of a
type different from their pre-
decessors, 565 (Fig. 430).

Heterozygote, formed by union of
two genetically dissimilar gametes,

572.
Hieracium, somatic parthenogenesis

in, 587.
Higher animals and plants compared,

555-
Hilum, scar of attachment of a seed

to the parent plant, 6.
Himanthalia, 386.
Hip, of Rose, a succulent hollow

receptacle, 329.
Hippuris, Mare's tail, 16, 17 (Figs.

7, 8), 43, 47-

Hofmeistenan Cycle, 353, 390, 545.

Holdfast, 378.

Holly, indurated leaf -margin of, 190.

Homodynamy, 345.

Homogeny, in parts genetically re-
lated by having a single repre-
sentation in a common ancestor,

345-

68o

BOTANY OF THE LIVING PLANT

Homologous alternation, 381, 384,

390, 543, 546-
Homology, 343 (Chap, xx.) ; denned,

344-
Homoplastic development, 342 (Chap.

xx.).

Homoplasy, where similar morpho-
logical results are produced by
adaptation in two or more distinct
evolutionary lines, 192, 342 ; in
Algae, 375.

Homosporous, applied to archegoni-
ate plants in which there is only
one type of spore, 352 ; a primitive
state, 520 ; fully exploited in
Pteridophytes, 553.

Homothallic condition, in Mucorales,
where zygospores are produced on
meeting of two branches of the
same mycelium, 416 ; in Rusts,

439.

Homotype-division, the second divi-
sion in the spore-tetrad, carried
out like any somatic division,
565 (Fig. 430).

Homozygote, formed by union of
two gametes similar in respect of a
given character, or characters, 572.

Honey Agaric, parasitism of, 393
(Fig. 292) ; 445.

" Honey Dew," 395 (Fig. 293) ; 425
(Fig. 323).

Hooks, in seed dispersal, 328 (FigC

251)-
Hop, twining stem of, 215.

Hop-mildew (Sphaerotheca), 420

(Figs. 318, 319) ; 421.
Hordeum (Barley), analysis of, 660 ;

origin of, 661.
Horizontal microscope, 145.
Hormidium, 365.
Hormones, 148 ; in geotropism, 157 ;

in phototropism, 160.
Hornea, 352, 478 (Fig. 372 a).
Horse-chestnut (Aesculus), 13 (Fig.

6), 80 (Fig. 55).
Horsetails, vegetative propagation

of, 252 ; characters of, 522 (Fig.

413 a).
Host, a plant or animal that supplies

food to a parasite, 220.
Host-cells in mycorrhiza, 231 (Fig.

159).

Humble-bee, agent for pollination of
Aconite, 302.

Humus, leaf-mould, decaying vege-
table matter of the soil, 96, 221.

Hura (Sand-box Tree), explosive fruit
of, 165 (Fig. 102) ; dehiscence of
fruit, 324, 630.

Hyacinth, qualities of fibres of, 178 ;

perennation of, 198 (Fig. 132).
Hybrid, the offspring of a cross be-
tween parents belonging to distinct

races or species, 568-581.
Hybridisation, the breeding together

of members of distinct races or

species, 568-581.
Hydrodictyon, the Water-Net, 365.
Hydrom, water-conducting tissue of

Mosses, 464 (Fig. 358).
Hydrophytes, plants adapted to life

in presence of plentiful water,

212.
Hydrotropism, response to stimulus

of unequal water-supply, 160.
Hygroscopic movements, due to

changes in degree of moisture,

165 (Figs. 100-103) ; of seeds and

fruits, 324.
Hymenial gonidia, algal cells in the

hymenium of a Lichen, 429.
Hymenium, the layer bearing asci

or basidia, in Fungi or Lichens,

428 ; of Hymenomycetes, 442

(Fig. 387) ; of Mushroom, 442,

445.
Hymenomycetes, 441.

Hypertonic, and hypotonic solutions,

35-
Hypha, the fungal filament, 393 ;

non-septate, 399 ; septate, 399 ;
traversing tissue of host, 395 (Fig.
294. 404 (Fig' 302), 407 (Fig.
306).

Hypholoma, 400 (Fig. 299), 431.

Hypobasal tier, the part of an embryo
lying below the basal wall : in
Ferns, 504.

Hypocotyl, region of stem below the
cotyledons, n (Fig. 4), 58; struc-
ture of (Fig. 37).

Hypoderma, tissue below the epider-
mis, often mechanically strength-
ened, 74.

Hypodermal cells, those lying below
the epidermis, 284.

Hypogynous, of flowers, with stamens
and other outer parts seated
below the gynoecium, 271 (Fig.

195)-
Hypophysis, cell giving rise to the

root-tip in the embryo of Dicoty-
ledons, 311 (Fig. 230).
Hypo-rachis, 351.

Ice in tissues, 148.

Iceland Moss (Cetraria), 428.

Imbibition, water of, 5, 33, 96.

INDEX AND GLOSSARY

68 1

Immune varieties, of Potato, 410; of
Wheat, 440.

Immunity, where two organisms may
be in relation, but neither has
power against the other, 235 ; to
bacteria, 453.

Inarching, or approach-grafting, 250
(Fig. 175).

Indigo, preparation of, 452.

Indusium, of Ferns, 495 (Fig. 387).

Infection-threads of root-nodule,
236.

Inferior, applied to the ovary wrhen
sunk below the level of the other
floral parts, 272 (Figs. 197, 198).

Inflorescence, a common branch-
system bearing a number of flowers,
257 (Chap, xiv.) ; definite or cy-
mose inflorescence, 258 (Fig. 179) ;
indefinite or racemose, 258 (Fig.
179) ; radial and dorsiventral, 262.

Inheritance, Chap. xxxv. : parti-
culate, 584.

Integuments, the coverings investing
the nucellus of an ovule, 294 (Figs.
216, 217).

Intercalary growth, in Algae, 379.

Intercalation in leaves, 351.

Intercellular spaces, 43, 73 (Fig. 48),
75 (Fig. 49), 76 (Figs. 51-53).

Interchange of gases, in photo-
synthesis and respiration, Chap.

vin.

Interpolation of parts, where extra
primordia appear in spaces norm-
ally unoccupied, 266 (Fig. 191).

Interpolation-theory of alternation,

546, 547-

Intrusive hyphae, in algal construc-
tion, 380.

Inulin, storage of, in Dahlia, 124.

Invertase, an enzyme which converts
cane sugar into invert sugar, 123,
129.

Involucre, a group of protective
bracts, 260 (Fig. 186), 648 (Fig.
498), 652 (Fig. 502).

Iridaceae, 609 (Fig. 458).

Iris, perennial stock of, 196 (Fig.
129) ; leaf arrangement of, 205 ;
dehiscence of anthers, 281 (Figs.
201, 202) ; flower of, 609 (Fig. 458).

Irish Potato Famine, 396.

Iron, as catalyst, 112 ; in chlorophyll-
formation, 116.

Iron Bacteria, 451.

Irregular nutrition, obtaining organic
substance by some other process
than by photosynthesis (Chap,
xii.) ; secondary in Flowering

Plants, 242 ; in Thallophytes,

Chap. xxiv.
Irregular propagation, 587.
Irritability, power of response to

stimulus, 31, 153-167.
Isoetes lacustris, 511 ; stock of, 514.
Isogametes, sexual cells of equal size,

362, 370 (Fig. 275), 557 ; in

Phaeophyceae, 381.
Isolation mechanisms, in Fvolution,

585.
Isomorphic alternation, 381, 384.
Ivy, skeleton of lamina, 71 (Fig.

46) ; climbing shoots negatively

phototropic, 159 ; adhesive roots,

217.

Juncaceae, O14.

Juncus lamprocarpus, 614 (Fig. 461).
Jungermanniales, 473 (Fig. 369) ;
size-structure correlation in, 601.
Juniper, 527.

Karyotype, the type of chromosome
complement characteristic of a race
or individual, 567 (Fig. 434).

Kidney Bean, 8 ; analysis of, 655.

Klinostat, a clock-work arrangement
for slowly rotating a plant under
experiment, 155.

Knight's Wheel, 155.

Knop's culture solution, 111.

Krakatau, new flora of, 331, 377.

Labellum, of orchids, 612 (Fig. 460).

Labiatae, 645 (Fig. 496).

Laburnum, hard wood of, 64 ; zygo-
morphy of, 274 (Fig. 199).

Lactuca, as a compass-plant, 159.

Lactuca scariola v. sativa (Lettuce),
percentage of water in, 95 ; analy-
sis of, 655.

Lamina, the leaf-blade, 69 ; struc-
ture of, 72 (Figs. 47, 48), 75 (Fig.
49) ; venation of, 188 (Fig. 123) ;
nature of, 348, 351.

Laminaria, 378 (Fig. 281) ; 380 ;
gametophyte of, 383 (Fig. 286) ;
alternation in, 384 ; life cycle of,

387. 545. 552.
Lamium, mechanical construction of

stem of, 184 (Fig. 117) ; L. album
(Dead-Nettie), 645 (Fig. 496).

Land Flora, origin of, 546.

Land-habit, Chap, xxxiv : 548.

Land plants, internal embryology of,
Chap, xxxiv.

682

BOTANY OF THE LIVING PLANT

Land vegetation, introductory to,

454 (Chap. xxix.).
Lantana, straggling habit of, 214

(Fig. 143, hi.) ; spread by birds in

Ceylon, 331, 345.
Lastraea, see Dryopteris.
Latent period, 334.
Lateral geotropism, of twining stems,

215.

Lateral roots, origin of, 90 (Fig.

65)-
Latex cells, of Euphorbiaceae, 54.

Latex vessels, of Cichoriaceae, 54.

Lathraea, parasitism of, 226.

Lathyrus aphaca, correlation in leaf,
219 (Fig. 148).

Laticiferous tissues, 53 (Fig. 32).

Latticed girders, 180.

Layering, 158 (Fig. 93).

Leaf, definition of, 69 ; structure of,
(Chap, v.) ; structure suited to
function, 80 ; phototropic response
159 ; arrangement, whorled or
cyclic, 201 ; alternate, 202 ; fall,
80 (Fig. 55), 153 ; symmetry of,
200 ; rolling of, 210 ; branch-
system of, 348 ; origin of, 352.

Leaf-mosaic, the fitting of the leaves
together, so as fully to occupy
space exposed to light without
overlapping, 201 (Figs. 134, 137,

138).
Leaf-mould, source of saprophytic

nourishment, 221.
Leaf-scar, surface of separation of

leaf from axis, 13 (Fig. 6), 78 (Fig.

55).
Leaf-trace, in Ferns, 486.

Legume or pod, a separate carpel,
splitting along both margins and
midrib, and containing several
seeds, 321, 323, 635.

" Legumes," analysis of, 655.

Leguminales, 635.

Leguminosae, climbing habit of, 213 ;
root-nodule of, 235 ; flowers of,
636 (Fig. 489).

Lemanea, 387.

Lemna, movement of chloroplasts in,
78 (Fig. 54).

Lenticels, breathing pores through
corky covering of a stem or root,
13 (Fig. 6), 66 (Fig. 44), 80 (Fig.

55)-
Lentil (Ervum lens), analysis of, 655 ;

origin of, 656.

Lepidocarpon, seed-like organ of, 521.

Lepidodendron, 511.

Leptom, of Mosses, 464 (Fig. 358).

Leptothrix, 451.

Lesser Celandine, delayed germina-
tion, 140.

Lettuce (Lactuca scariola), high
water-content of, 95 ; analysis of,

655-
Leucobryum, 465.

Leucojum, 608.

Leucoplasts, or starch-forming cor-
puscles, 124 (Figs. 81, 85).

Lianes, woody climbers of large size,
213.

Liberation of oxygen in photosyn-
thesis, 121.

Lichens, symbiosis of, 427 (Figs. 326,
327), 446 ; Myxophyceae a con-
stituent of, 376.

Life, indications of, 30.

Life-cycle of Algae, 390 ; of Flower-
ing Plants, 335 (Fig. 257) ; of
Ferns, 506 (Fig. 400) ; of Bryo-
phytes, 456, 544.

Light, necessary for photo-synthesis,
108 ; local effect of, 117 (Fig. 78) ;
source of energy, 114; retarding
influence on growth, 146 ; effect
on growing organs, 158 ; on ger-
mination, 140.

Light-rays absorbed by chlorophyll,
116 (Fig. 77).

Lignified walls, of woody character,
24.

Lignin, 124.

Ligulatae, those Lycopods which
have a ligule, 512.

Ligulate florets, of Compositae, 649,
651 (Fig. 480).

Ligule of Selaginella, 515 (Figs. 407-
409) ; of Lychnis, 621 (Fig.
470).

Liguliflorae, 651.

Liliaceae, meristic differences in, 265,
607.

Liliales, 606 (Fig. 455).

Lilium, origin of embryo-sac in, 297 ;
fertilisation in, 307 (Fig. 228), 607
(Fig. 455).

Lily, bulbils of, 245 ; syncarpous
pistil of, 289 (Figs. 209, 211) ;
stigma of, 292 ; of the Valley, 608.

Lily Disease, 395.

Lime Tree, products of cambium of,
59 (Fig. 38) ; secondary thickening
of, 62 (Figs. 40, 41) ; leaf-mosaic
of, 206 (Fig. 138).

Lime (calcium oxide), 97 ; in plant
distribution, 112 ; in soil reaction,

113.

Limit of elasticity, 178 ; measured

by the greatest burden per unit
of transverse section which can be

INDEX AND GLOSSARY

683

supported without losing the power
of perfect recovery.

Limit of mechanical resistance, 603.

Limiting factor, 119.

Linen, fibres of, 177.

Lines of Descent, 355.

Linin, anastomosing threads in the
nucleus which bear the chromatin,
560.

Linkage, the tendency of certain
genes to be associated in inherit-
ance, due to their being located in
the same chromosome, 579-581
(Figs. 440, 441).

Lipase, the enzyme which decom-
poses fats, 125, 129.

Loam, 97.

Locomotion, 167.

Locus, in genetics, the definite posi-
tion on a chromosome occupied by
a given gene, 580 (Fig. 440).

Lodicules, two tumid scales in the
flower of Grasses, held to represent
two obliquely anterior segments of
the perianth : 617 (Fig. 464).

Lodoicea, protoplasmic continuity in,
27 ; reserve-cellulose in, 125 (Fig.
83) ; floating fruit of, 327.

Lolium (Rye-grass), 616 (Fig. 464).

London Pride, 631.

Long-day type, 152.

Loosestrife (Lythrum), 140.

Loranthus, parasitism of, 222, 223
(Fig. 150).

Lotus corniculatus, 635 (Fig. 489).

Lousewort (Pedicularis), root para-
sitism of, 222 (Fig. 149).

Loxsoma, solenostele of, 596-597
(Fig. 452).

Lupin, reserve cellulose in, 125.

Luzula, 614.

Lychnis flos-cuculi, 620 (Fig. 470) ;
diurna, 621 (Fig. 471).

Lycopodiales, the Club Mosses, 3,
511 (Chap, xxxii.) ; seed-habit in,
521.

Lycopodium, mycorrhiza in, 230 ;
distal branching of, 348 ; stele of,
594-

Mace, the arillus of the nutmeg, 319.

Macrocystis, 378.

Maianthemum, 607.

Maize, 8 ; stem of, 51 (Figs. 29, 30,

31) ; strut-roots of, 191 (Fig.

128) ; analysis of, 660 ; origin of,

661.
Malaxis paludosa, adventitious buds

of, 247.

Male Fern (Dryopteris), 483-502 ;
stock of, 484 (Fig. 374) ; leaf and
sorus of, 482 (Fig. 373), 495 (Fig.
387) ; anatomy of, 484-493 (Figs.

374-385)-

Male flower of Coniferae, 533 (Figs.
420, 421).

Male gametes, male sexual cells, 287,
305 (Fig. 226) ; behaviour of, in
fertilisation of Seed Plants, 306-
309 (Figs. 227-229) ; of Pine, 536
Fig. 424) ; of Zamia, 527 (Fig.

415). 540.

Malformations, caused by Fungi, 396
(Fig. 295).

Malic acid, in fertilisation of Ferns,
502.

Maltase, 123, 129.

Maltose, 123.

Malva, pollen-tubes of, 304 (Fig.
222, B).

Mannitol, 380.

Maple sugar, in sap, 108.

Marchantiales, structure of, 472.

Marginal placentation, of ovules in-
serted on the margins of the car-
pels, 291.

Marsh Marigold (Caltha), delayed
germination, 140 ; anther of, 281
(Fig. 202) ; pollen-sac and pollen
of, 282-286 (Figs. 203-207) ; carpels
of, 289 (Figs. 208, 210) ; ovules of,
294 (Figs. 216, 217), 623 (Fig. 472).

Marsilia Drummondii, chromosomes
of, 506 ; parthenogenesis in, 587.

Marsupium, a nursing sac containing
the young sporogonium of certain
Liverworts, 474.

Materials, translocation of, 130.

Mechanical construction, Chap, x.,
p. 168.

Mechanical function of woody stem, 67.

Mechanical limitations of size, 168,
186, 603.

Mechanical stimulus, 163.

Mechanical tissues, 175 ; physical
qualities of, 178, 179 ; disposition
of, 180 ; in stems, 181 ; in leaves,
186 ; in roots, 190.

Media for bacterial culture, sterilisa-
tion of, 449.

Median plane, in a floral diagram,
the plane including the axis, and
the midrib of the bract, 263.

Medullary rays, plates of paren-
chymatous tissue running radially
from the cambium, inwards into
the wood, and outwards into the
bast : 57 (Figs. 36, 37, 39), 62
(Figs. 40, 41), 64 (Fig. 42).

684

BOTANY OF THE LIVING PLANT

Meesia, sexual organs of, 466 (Fig.

361).

Megasporangium, the female spor-
angium containing one or more
megaspores. In Flowering Plants
it is the ovule : 288, 295 (Figs.
216, 217).

Megaspore, where spores are sexually
differentiated the female spore.
In Seed Plants it is the embryo-
sac : 294 (Fig. 216) ; development
of, 296 (Fig. 217), 298 (Fig. 219),
352, 535 (Fig- 423).- 539 ; retention
of, 551-

Meiomery, where the number of parts
of one category stands below the
fundamental number for the flower,
268.

Meiosis, the process whereby the
chromosome number is halved
(" reduced "), 353, 562, 563 (Figs.
430-433) ; meiosis and Mendelian
segregation, 572 (Fig. 436) ; and
linkage, 580 (Figs. 440, 441).

Membrane, permeable or semi-per-
meable, 35.

Mendelian segregation, see Segrega-
tion.

Mendel's laws, 569-572.

Meristele, of Ferns, 486, 487 (Fig.
376) ; structure of, 488 (Figs. 377,

378).

Meristic differences, differences in the
fundamental number of parts in
different flowers, 265.

Meristic variation, divergence in cer-
tain cases in the number of parts,
where a definite number is usual ;
as in successive whorls of leaves :
202, 265, 604.

Merulius, Dry Rot Fungus, 442.

Mesocarpus, 374.

Mesophyll, the parenchyma between
upper and lower epidermis of the
lamina, 72 (Figs. 47, 48) ; of
Narcissus, 119 (Fig. 79).

Mesophytes, plants living under con-
ditions that are not extreme, 212.

Meso-rachis, 351.

Metabolism, chemical change with-
in the organism, 30: 114, 134,

137-
Metamorphosis, Goethe's theory of,

277 ; sporangia, due to, 352.
Metatrophic bacteria, 451.
Metaxylem, the later-formed part of

the primary xylem, 85 (Fig. 59).
Miadesmia, seed-like organ of, 521.
Micropyle, a narrow channel leading
• to the apex of the nucellus of an

ovule : the channel for the pollen-
tube, 295 (Fig. 216).

Microsporangia, 352.

Microspore, where spores are sexually
differentiated, the male spore,
characterised by its smaller size.
In Seed Plants the pollen-grains
are microspores : 286, 352 ; of
Selaginella, 515, 517 (Figs. 407,

4"). 540-

Middle lamella, 112.

Migration to land, 547.

Mildews, 393, 418 ; general account
of, 419; (Figs. 317-319).

Mimosa pudica, sensitive plant, 162
(Fig. 98) ; movement under me-
chanical shock, 163.

Mistletoe (Viscum), parasitism of, 140,
222.

Mitosis, division of the nucleus in
connection with the division of
somatic cells, 562 (Figs. 428, 429).

Mnium, conducting tissue of, 464.

Mobilisation of reserves, 130.

Modification, a (non-inheritable) fea-
ture of an individual, directly re-
lated to environmental influences,
569. 586.

Molinia, mechanical construction of
stem, 185 (Fig. 119).

Monkey-nut, geotropism in, 154.

Monkey-puzzle (Araucaria), 527 ; leaf
arrangemeut of, 203 (Fig. 136).

Monkshood (Aconitum), pollination
of, 302 ; flower of, 624 (Figs. 475,

473. A).
Monoblepharis, 411 (Fig. 311).
Monocotyledons : Seed Plants (Angio-

sperms) having an embryo with

one seed-leaf : stem of, 50 (Figs.

28-31), 67 ; stomata, 76 (Figs.

50, 51) ; root of, 84 (Fig. 58) ;

mechanical construction of stem,

182 ; of leaf, 187 (Figs. 120-122) ;

embryology of , 314 (Figs. 232, 233) ;

flowers of, 606-619 (Appendix A) ;

stelar structure of, 596.
Monopodial branching, where a new

branch arises laterally below the

apex of the original part, 511.
Monosaccharides, 123.
Monotropa, ectotrophic mycorrhiza

of, 229 ; nucellus and embryo-sac

of, 298 (Fig. 219).
Morchella (Morel), 400 (Fig. 298) ;

418 ; 426 (Fig. 324) ; asci of, 417

(Fig. 316).
Mortal disease, 235.
Moss-plant, origin on protonema,

463 (Fig- 357)-

INDEX AND GLOSSARY

685

Mosses, 460 (Chap. xxx.).

Motility, of gametes, loss of, 540.

Motor influences on movement of
water, 107.

Moulds, Chap, xxiv., 393, 421.

Movement, 153-167 (Chap, ix.) ; in
growing parts, 153 ; stimuli of,
153 ; of protoplasm within the cell,

30.

Mucilage, in channel of style, 293,
305 (Fig. 225).

Mucor, 399, 401, 414 ; sporangia of,
414 (Figs. 313, 314 ; heterothallic
culture of, 417 (Fig. 315) ; zygo-
spores of, 416 (Fig. 314).

Mucorales, reduction of sporangium
to unicellular conidium, 414.

Mulberry, aggregate fruit of, with
succulent persistent perianth of
each flower, 330 (Fig. 256).

Multiplication, rapid, of Bacteria,
449.

Mummy- wheat, 335.

Muscari (Grape-Hyacinth), 608.

Musci, 3, 462, 472 ; saprophytism of,

465- *

Mushroom, 440, 442 (Figs. 341, 343) ;
germination of, 446.

Mushroom " spawn," 443.

Mustard, phototropism in, 158 (Fig.
97), 626 (Fig. 477).

Mutant, a new species, race, or indi-
vidual produced by mutation, 569,
586.

" Mutations," heritable deviations
from type, which have not been
referred directly to known causes,
339. 569 ; beneficial, preserved by
Mendelian segregation, 569.

Mutualism, a living together of two
organisms with joint physiological
action, 227-238.

Mycelium, an aggregate of fungal
hyphae, 393 ; septate, and non-
septate, 399.

Mycorrhiza, the coalition of fungal
filaments with the living tissues of
other organisms, 227 ; in orchids,
231 ; of roots, 230.

Myosurus, hypogynous flower of,
271 (Fig. 195) ; embryo-sac and
endosperm of, 315 (Fig. 234).

Myrrhis, 638.

Mvrsiphyllum, dextrorse twining
stem, 215 (Fig. 144).

Myxophyceae (Cyanophyceae), 376.

Narcissus, stoma of, 76 (Figs. 50, 51),
102 (Figs. 70, 71) ; leaf-section of,

it-) (Fig. 79) ; chemotropic pollen-
tubes, 304 (Fig. 224) ; flower of,
609.

Narcotics, 31.

Nastic movements, 161 ; in flowers,
162.

Natural families, 605 (Appendix A).

Natural selection, 339.

Natural system of classification, 343.

Needles of Pine, 529.

Nelumbium, germination of seeds of,

335-
Nemalion, 389 (Fig. 290).

Neottia (Bird's Nest Orchis), mycor-
rhiza in, 232 (Figs. 159, 160, 161).

Nepenthes, pitcher of, 240 (Fig. 167)

Nephrodium, Chap. xxxi. ; see Dry-
opteris, pp. 482-510.

Nephrolepis, stolons and tubers, 598
(Fig. 454).

Nereocystis, 378.

Nettle (Urtica dioica), 112 ; host for
Puccinia caricis, 434.

Neutral generation, dominant on
land, 552.

New Zealand flax, qualities of fibres
of, 178 ; leaf structure, 187 (Fig.
120).

Night-flowering, 152.

Nightshade (Atropa), 642.

Nitrifving Bacteria, 123, 450, 452

(Fig. 348).

Nitrobacter, 452 (Fig. 348).

Nitrogen, sources of, 127 ; fixation
of, in plants with root-nodules, 237.

Nitrosomonas, 452 (Fig. 348).

Nodule-formation, significance of,
238.

Nolina, qualities of fibres, 178.

Non-septate sac, as mode of con-
struction, 170 (Figs. 105, 106).

Nostoc, 376 ; part of Lichen, 427.

Nucellus, the body of tissue forming
the centre of an ovule, and enclos-
ing the embryo-sac ; the mega-
sporangium : 294 (Figs. 216, 217).

Nuclear division, 560-567 (Figs. 428-
433) ; nuclear reticulum, 560.

Nuclear membrane, 560 (Fig. 428).

Nuclear pairing in Phragmidium,
436 (Fig. 338).

Nuclear sap, 560.

Nuclear spindle, 562 (Fig. 428).

Nucleolus, a highly refractive body,
probably of reserve substance, in
the nucleus, 560 (Fig. 428), 562.

Nucleo-proteins, 126.

Nucleus, a definite spherical or oval
body, reproduced by division,
which acts as centre of the activity

686

BOTANY OF THE LIVING PLANT

of the cell : 18 (Fig. 9), 29 ; its im-
portance in syngamy or fertilisa-
tion, 557 ; nucleus and heredity,
560 ; structure of resting nucleus,
560 ; somatic division of (mitosis),
562 (Figs. 428, 429) ; tetrad divi-
sion of (meiosis), 563 (Figs. 430-

433).

Nutrition, autotrophic or hetero-
trophic, 22 (Chap, viii.) ; of Bac-
teria, its classification, 450.

Nutritive lacket, a layer of nutritive
tissue surrounding the embryo-sac
in Gamopetals : 316 (Fig. 235).

Nymphaea, floating leaves of, 212.

Oat, analysis of, 660 ; origin of, 661.

Obconical expansion in Ferns, and
Palms, 596.

Obconical form, problem of, 589 ;
in sporelings, 591 ; in stems of den-
droid plants, 590 (Fig. 446) ; in

Ferns, 593. 597 (FiSs- 140. 453) I

supply to buds big and small, 595,

600.
Odontoglossum, mycorrhizic infection

of, 234 (Fig. 162).
Oedogoniales, 365.
Oedogonium, 365, 366 (Fig. 271).
Oidium-condition, of Mucor, 415.
Onion, bulbils of, 246 ; analysis of,

655-
Onoclea, fertilisation of, 503 (Figs.

395> 396).

Onygena, sporadic occurrence of, 395.

Oogonium, the organ in Algae and
Fungi, which contains one or more
female gametes, or ova ; 352 ; of
Fucus, 384 (Fig. 288) ; of Oedo-
gonium, 366 (Fig. 271) ; of Vau-
cheria, 370 (Figs. 275, 276) ; of
Oomycetes, 399, 410 (Figs. 296,

Oomycetes, 399 (Chap, xxv.), 402,

447-
Oospore, of Pythium, 405.

Oosporeae, aquatic origin of, 411.

Open bundle, one possessing active-
cambium, 53.

Operculum, the lid which falls away
from the ripe capsule in Mosses,
469 (Fig. 364).

Ophrydeae, mycorrhizic germination
of, 233 (Fig. 161).

Opium Poppy, 626.

Orchidaceae, 611.

Orchids, epiphytic, 191 ; endotrophic
mycorrhiza of, 231 (Figs. 159, 160,
161, 162).

Orchis, meiomery in, 269 ; flower of,

612 (Figs. 459, 460).
Organic material, formation of new,

4, Chap. viii.
Organographic factors, combination

of, 602.
Orobanche, parasitism of, 226 (Fig.

153).
Orobancheae, 221.

Oryza (Rice), analysis of, 660 ;
origin of, 661.

Oscillatoria, 376 (Fig. 279 a).

Osmotic control, upset by shock,
164.

Osmotic phenomena of cells, 34.

Osmotic pressure, 34.

Osmunda (Royal Fern), 483 ; apex
of, 493 (Fig. 383).

Ovary, the part of the pistil contain-
ing an ovule or ovules, Chap, xvi.,
p. 288 : superior, where the ovary
is borne by the elongated recep-
tacle above the other parts, 271
(Fig. 195) ; inferior, where it
appears sunk in the shortened re-
ceptacle, and below the outer parts,
272 (Figs. 197, 198).

Ovule, the megasporangium of flower-
ing plants, which ripens into the
seed : 294 (Figs. 216, 217) ; of
Pine, 534 (Figs. 422, 423) ; mega-
spore retained within, 539.

Ovuliferous scale, of Coniferae, 534
(Fig. 422).

Ovum, or egg, the female gamete,
294 (Fig. 216), 298 (Fig. 219), 305
(Fig. 226), 306 (Fig. 227), 353 ; of
Pine, 536 (Fig. 424) ; of Selagin-
ella, 518 (Fig. 412) ; of Fern, 502
(Fig. 394) ; of Moss, 468 (Fig. 363) ;
of Riccia, 474 (Fig. 370) ; of Fucus,
384, 385 (Figs. 287, 288) ; of Oedo-
gonium, 366 ; of Pythium, 398
(Fig. 296) ; retention of, in arche-
gonium, 549, 550.

Oxidases, 137.

Oxygen, given off in photosynthesis,
120 (Fig. 80) ; absorbed in respira-
tion, 135 (Fig. 87).

Paleae, inner chaffy bracts of

the Grasses, 616, 617 (Fig. 464),

658.
Palaeozoic Lycopods, 521.
Palisade parenchyma, 72 (Fig. 47), 75

(Fig. 49), 76 (Fig. 51).
Palm, stem of, 51 ; mechanical

construction in stem of, 182 ;

subdivision of leaf of, 189.

INDEX AXD GLOSSARY

687

Palm-type of structure of stem, 49
(Figs. 29, 30) ; obconical base of,
590 (Fig. 440, B).

Palmella-state, of Euglena, 358, 362.

Panicle, an indefinite inflorescence in
which each pedicel branches, bear-
ing several flowers, 261 (Fig. 183).

Papaver (Poppy), 625 (Fig. 476),
P. somniferum, 626.

Papaveraceae, 625.

Papilionaceae, 635.

Pappus, feathery bristles representing
the calyx in the Compositae, 326
(Fig. 247), 648-652.

Parallel-development, 192 ; in Algae,

375-
Parallel venation of Monocotyledons,

72.
Paraphyses of Mushroom, 445.
Parasite, and host, 220 ; an organism

that derives organic supply from

some other living organism, 220 ;

partial, 222 ; complete, 223.
Parasitic habit of Fungi, 391 (Chap.

xxiv.).
Paratrophic bacteria, 451.
Parenchyma, cells roughly oblong in

form, and not much longer than

broad, 23 (Figs. 12, 13, 16), 73 (Fig.

48).
Parietal cells, which form inner wall

of pollen-sac, 284 (Fig. 205).
Parmelia, 428 (Fig. 326).
Parsnip, origin and analysis of, 655.
Parthenogenesis, somatic and gener-
ative, 587.
Partial parasites, those which are only

partly dependent on parasitism for

nutrition ; they are usually green,

221.
Pastinaca sativa (Parsnip), 655.
Pea (Pisum sativum), analysis of,

655 ; origin of, 656 ; root-tip of, 89

(Fig. 64, B) ; tendril, 216.
Pea Xut or Monkey Nut (Arachis

hypogaea), analysis of, 655 ; origin

of, 656.
Peach, analysis of, 657 ; perigynous

flowers of, 272 (Fig. 196).
Peach Leaf Curl, 396.
Pear, analysis of, 657.
Peat, origin of, 465.
Pedicel, a flower-stalk of a higher

order of branching, 260.
Peduncle, a flower stalk, 258.
Pelargonium, 628.
Pellaea, root of, 492 (Fig. 382).
Pellagra, 663.

Pellia, capsule of, 475 (Fig. 371).
Pelvetia, 386.

b.b. 2X

i '' nicillin, 453.

Penicillium, 418, 422, 453 (Fig. 320).

Pentacyclic, of flowers, with five
cycles of parts, 268.

Pentacyclicae, gamopetals with five
cycles of floral parts, 639.

Pentamerous, or 5-merous, flowers,
with parts in whorls of five, 265.

Peppercorn, perisperm of, 317 (Fig.
236).

Perennation, persistence from season
to season, 195.

Pericarp, product of carpellary wall ;
in Wheat, 658.

Perichaetium, of Mosses, leaves sur-
rounding the sexual organs, 467.

Pericycle, tissue immediately within
the endodermis, forming a peri-
pheral band of the stele, 45 (Fig.
23), 56 (Fig. 34), 84 (Fig. 58) ; in
Ferns, 488 (Figs. 377, 378).

Perigynium, a bract enveloping the
female flower in Carex, 616 (Fig.
463).

Perigynous, of flowers, where the
receptacle is laterally enlarged,
forming a cup on the margin of
which the sepals, petals and
stamens are seated : 272 (Fig. 196).

Periplasm, protoplasm surrounding
the ovum ; in the oogonium of the
Peronosporeae, 411 (Fig. 310).

Perisperm, tissue of the nucellus
persistent in the ripe seed, 317
(Fig. 236).

Peristome, in Mosses, a mechanical
structure surrounding the lip of
the dehiscent capsule, which is
effective in scattering the spores,
471 (Fig. 367).

Perithecia, flask-shaped cavities filled
with asci, in the fruits of some
Ascomycetes : 420 (Fig. 318), 421
(Fig. 319).

Permeable membrane, 35.

Peronospora, sexual organs in, 410
(Fig. 310).

Peronosporeae, 399 : sexual organs
of, 410 (Fig. 310).

Personatae, 643.

Petals, the parts constituting the
inner floral envelope, or corolla,
255 ; structure of, 280 (Fig. 200).

Petiole, the leaf-stalk, 70 (Fig. 45),

348.
Peziza, 401, 418, 425 (Fig. 322).

Phaeophyceae, 355, and Chap, xxiii.

Phagocytes, 452.

Phajus, leucoplasts of, 130 (Fig.

85).

688

BOTANY OF THE LIVING PLANT

232

Phalaenopsis, mycorrhiza in,
(Fig. 159).

Pharbitis, sinistrorse twining stem of,
215 (Fig. 144).

Phaseolus vulgaris (French or Haricot
Bean), analysis of, 655 ; origin of,
656.

Pheasant's Eye, 609.

Phelloderm, the inner product of
cork-cambium, 66 (Figs. 43, 44).

Phellogen, the cambium that pro-
duces cork, 66 (Figs. 43, 44).

Phenotype, of an individual, the
aggregate of its visible or demon-
strable personal characters : also
an individual with a given aggre-
gate of such characters, 575.

Phloem, the bast-region of the vas-
cular strand, 45, 48 (Figs. 23, 24,
25) ; of Fern, 488 (Figs. 377, 378).

Phloem parenchyma, 49 (Fig. 26), 59
(Fig. 38).

Phlox, 140.

Phoma, in mycorrhiza, 230.

Phormium, leaf structure of, 187 (Fig.
120).

Phosphorus, necessary to form pro-
teins, 126 ; supply of, no.

Photosynthesis, construction of new
organic material under the influ-
ence of light in green parts : 114 ;
localised, 117 (Fig. 78) ; evolution
of oxygen, 120 (Fig. 80) ; chemistry
of, 121 ; rate of, 122 ; products of,
122 ; and energy, 138 ; activity of,
on a summer's day, 122 ; by Sul-
phur Bacteria, 451.

Phototropism, 158 (Fig. 97) ; positive
and negative, 158-159.

Phragmidium, 438 (Fig. 338).

Phycomyces, 413, 414, 416.

Phycomycetes, 392, Chap. xxv. ; sub-
aerial adaptation of, 447.

Phyllanthus, 628.

Phylloclades of Ruscus, 345 (Fig.

259).

Phylloglossum, 512.

Phyllosiphon, 393, 411.

Physiological drought, a deficiency of
water due to inability of the plant
to absorb enough to replace loss,
not to a want of water outside it,
211.

Phytophthora infestans (Potato Fun-
gus), 399. 401. 4°6 (Figs- 3°5-3°9) ;
sexual reproduction in, 410.

Pileus, domed head of Mushroom,
444.

Piliferous layer, the superficial layer
of the root, which bears the root-

hairs, 83 (Fig. 57) ; origin of, 88,
89 (Figs. 63, 64).
Pilobolus, explosive dispersal of,

415-

Pinguicula (Butterwort), carnivorous
habit of, 164, 239.

Pinnule, 349.

Pinus, Chap, xxxiii., p. 527 ; cam-
bium of, 57 (Fig. 35).

Pinus montana, male flower of, 534
(Fig. 421).

Pinus nigra, male flowers, 528 (Fig.
420) ; female flowers, 539 (Fig.
416).

Piptocephalis, parasitism of, 414 ;
air dissemination of, 447.

Pistil, an old term for the gynoecium,
or carpellary region of the flower,
255, 288.

Pistillate, applied to flowers or plants
which bear carpels, but not
stamens, 256 ; by abortion in
Lychnis, 270 (Fig. 194).

Pisum sativum (Garden Pea), recep-
tive cells in root, 156 (Fig. 95) ;
used in Mendel's experiments, 570 ;
analysis of, 655.

Pit, an area of cell-wall that remains
thin, 24 (Figs. 14, 15, 16).

Pitcher Plant (Nepenthes). 240 (Fig.
167).

Pith, 42 (Fig. 21), 44 (Fig. 24).

Placenta, the surface of insertion of
the ovule or ovules, 291 ; free-
central, 621, 640 (Figs. 470, 472).

Placentation, the mode of insertion
of the ovules, 291.

Plagiotropism, 154.

Plankton, floating organic life, 375.

Plantago, haustoria of embryo-sac,

317-
Plant cell, an osmotic system, 35.

Plant communities, plants of a local-
ity subjected to and adapted for
life under common conditions, 188.

Plant population, 312.

Plants and animals, 138.

Plasmatic membrane, 34-35.

Plasmodiophora, 396 (Fig. 295).

Plasmolysis, separation of the proto-
plast from the cell-wall, by its
contraction, due to loss of water,
36 (Figs. 18, 19).

Plastids, minute bodies in the cyto-
plasm, which multiply by fusion,
and give rise to chloroplasts,
chromoplasts, or leucoplasts : 18

(Fig. 9)-
Platycerium, 598.
Plectascales, 421.

INDEX AND GLOSSARY

689

Pleiomery, in the flower, where the
number of parts of one category is
greater than the fundamental num-
ber for that flower, 266.

Pleurocarpic, of Mosses which fruit
laterally, 467.

Pleurocladia, zoospores of, 381 (Fig.
284).

Pleurococcus, 2 (Fig. 1) ; 361, 364
(Fig. 269) ; 365 (see Protococcus).

Plum, analysis of, 657.

Plumule, the apical leafy bud of the
embfyo, 7.

Plurilocular sporangia, 382 (Fig. 286).

Pneumacoccus, 453.

Poa alpina, viviparous habit of, 248.

Podosphaera clandestina (Hawthorn
Mildew), 420.

Poisons, 31.

Polar nuclei, fusion of in Monotropa,
298 (Fig. 219).

Pollen-grains, of Flowering Plants,
283 (Fig. 204) ; development of,
285 (Figs. 206, 207) ; germination
of, 3°3, 3°4 ; of pine. 534 (Fig.
421), 535 (Fig. 423)-

Pollen-mother-cells, those cells which
after tetrad-division, give rise to
pollen, 285 (Figs. 206, 207).

Pollen-sac, the microsporangium of
Flowering Plants, containing the
microspores, or pollen-grains, 222,
282 (Fig. 203), 352 ; development
of, 284 (Figs. 205, 206).

Pollen-tetrad, a group of four cells,
resulting from the tetrad-division
of a pollen-mother-cell, 283 (Fig.
204), 564 (Fig. 430).

Pollen-tube, formation of, 283 ; cul-
ture solutions for, 303 ; negative
aerotropism of, 160, 161 (Fig, 97,
A) ; influences upon growth of, 161,
304 (Figs. 222, 223) ; of Pine, 535
(Fig. 423, 424) ; of Zamia, 527
(Fig. 415).

Pollination, the transfer of pollen-
grains from the pollen-sac to the
stigma, Chap. xvii.

Pollinia of Orchis, 612 (Fig. 460).

Polycarpicae, 622.

Polygonum viviparum, bulbils in
place of flowers, 247.

Polyploid, an individual possessing in
its somatic cells more than the
normal (diploid) number of chromo-
some sets, 581-584 (Figs. 443, 444) ;
polyploid series, 584.

Polypodium, dorsiventrality of, 208
(Fig. 140) ; archegonia of, 502
(Fig. 394) ; sporeling of, 592, 597.

Polyporus, 393. -H2-

Polysaccharides, 123.

Polysiphonia, alternation in, 390, 393,

546.
Polystichum, apospory in, 507 (Hg.

401).
Polytrichum, conducting tissue of,

464 (Fig. 358) ; leaf structure of,

465 (Fig. 359) ; perichaetia of, 467.
Poplar, adventitious buds on root,

247 (Fig. 171).

Poppy, geotropic changes in, 154.

Poppy (Papaver), pore capsule of,
325 (Fig. 244), 625 (Fig. 476).

Populus alba, adventitious buds on
root, 247 (Fig. 171) ; tremula,
lamina of, 72 (Fig. 47).

Porogamy, where in Seed-Plants
fertilisation is through the micro-
pyle, 3°5 (Fig. 226).

Porotrichum, aquatic habit of, 462.

Posterior, side of a flower next the
axis, 263.

Potassium, absorption from soil, no.

Potato, cortex of, 42 (Fig. 22) ; cor-
rection in, 218 (Fig. 147) ; leuco-
plasts of, 124 (Fig, 81) ; starch-
grains, 125 (Fig. 82) ; early forma-
tion of tubers in, 244 (Fig. 168) ;
vegetative propagation of, 218
(Figs. 147, 168) ; tissue attacked
by Pythium, 401 (Fig. 301) ; flower
of, 642 (Fig. 493) ; origin of, 653 ;
analysis of, 655.

Potato Disease (Phytophthora in-
festans) 396, 406 (Figs. 305"309).

Potentilla, floral construction of, 267
(Fig. 191), 633 (Fig. 484).

Potometer, an instrument for measur-
ing the absorption of water, 101
(Fig. 69).

Presentation time, 155, 160 ; sur-
faces, 601.

Primary phloem, bast formed without
cambial activity, 58 (Fig. 37).

Primary xylem, wood formed with-
out cambial activity, 58 (Fig. 37).

Primordial cell, a naked protoplast,
169 (Fig. 104).

Primrose (Primula vulgaris), 640
(Fig. 492).

Primula Kewensis, an allo-polyploid,

583.
Primulaceae. meristic differences in,

265, 640 (Fig. 492).
Primulales, 640.
Principle of Maximum Exposure,

603.
Principle of Similarity, 591.
Productivity, by seeds, 332.

690

BOTANY OF THE LIVING PLANT

Pro-embryo, the first filamentous
development from the zygote in
Seed-Plants, 311 (Fig. 230, i.) ; of
Monocotyledons, 314.

Progametangia of Mucor, 416.

Promycelium, of Puccinia, 435 (Fig.

335).

Propagation, irregular, 587 ; vegeta-
tive, Chap. xiii.

Propagative cells of Fungi : ter-
minology, 401.

Protandrous, term applied where in
the flower the stamens mature
before the stigmas, 301.

Proteases, 127-129.

Proteins, 32, synthesis of, 126 ;
storage of, 126 (Figs. 84, 85).

Proteolytic ferment, which breaks
down complex protein into simpler
substances, 240.

Prothallus, (female) of Pine, 535 (Fig.
423), 513 ; male of Selaginella,
517 (Fig. 411) ; female, 518 (Fig.
412) ; of Fern, 499 (Figs. 391, 392) ;
retention on parent plant, 509, 549.

Protista, 393.

Protococcus, 2 (Fig. 1), 364, 365 (see
Pleurococcus).

Protogynous, applied where stigmas
are receptive before the pollen of
the same flower is shed, 301.

Protonema, preliminary filamentous
stage of Mosses, 462 (Figs. 356,

357)-
Protophyta, 360, 361, 375.

Protoplasm or protoplast, the living
body of the cell, 18 (Figs, 9, 12) ;
continuity of , 26 (Fig. 16) ; stream-
ing of, 30, 31, 167 ; the ultimate
receiver of stimuli, 139, 153.

Protostele, a stele with a solid xylem-
core, 485 (Fig. 375), 593.

Prototrophic bacteria, 450.

Protoxylem, the first formed elements
of the wood, 47 (Fig. 24) ; in root,
84 (Figs. 58, 59) ; in root after
secondary thickening, 92 (Figs. 66,
67) ; in Fern, 488 (Figs. 377,

378).
Protozoa, in soil, 98.

Prunus cerasus (Cherry), 633 (Figs.
484, C, 486).

Psalliota (Agaricus), campestris, 401,
442 (Figs. 341, 343, 344) ; sapro-
phytic habit of, 443.

Psaronius, concentric stelar rings of,
598.

Pseudomixis, a fusion of nuclei which
initiates a sporophyte, but not by
regular syngamy, 509 (Fig. 403).

Pseudo-monocotyledonous embryos,
where in Dicotyledons by abortion
or fusion only one cotyledon ap-
pears, 313.

Pseudo-parenchyma, of Fungi, 394.

Psilophytales, 352, 353 B, 478.

Psilotales, 480.

Psilotum, mycorrhiza in, 210 ; stele

of, 594-

Pteridium (Bracken), meristeles of,
486 (Fig. 376), 488 (Figs. 377, 378) ;
tracheites of, 490.

Pteridophyta, the higher Arche-
goniate plants, including Ferns,
Club Mosses, Horse tails, etc., 3,
481 (Chaps, xxxi., xxxii.) ; com-
pared with Seed Plants, 539.

Pteridosperms, 521.

Pteris, root apex of, 493 (Fig. 385) ;
P. cretica, apospory in, 507 (Fig.
401) ; P. podophylla, concentric
vascular rings of, 597 (Fig. 453).

Puccinia caricis, 433.

Puccinia graminis (Rust of Wheat),
398, 401, 433 (Figs. 329, etc.).

Puff-balls, 429.

Pulpy fruits, 329.

Pulvini, 162.

Pure line, a genetically uniform popu-
lation, the offspring of a single self-
fertilised homozygous individual,
586.

Pylaiella, 381.

Pyrenoid, 361.

Pyrenomycetes, 425.

Pyrus malus (Apple), 632 (Fig. 484).

Pythium, attack on cress-seedling,
396, 399, 4OI> 4°2 (Fig. 300) ;
hyphae of, traversing host-plant,
403 (Figs. 301, 302) ; sporangia
of, 404 (Fig. 303) ; oospores of, 405
(Fig. 304) ; fertilising tube of,

405-

Quillaija, floral construction of,

267.
Quince, flower of, 255 (Fig. 177), 290
" (Fig. 212), 632.
Quotient, respiratory, 136.

Raceme, an indefinite inflorescence in
which each pedicel bears one flower,
260 (Fig. 182).

Rachis, 348.

Radial construction of stele, as seen
in roots with protoxylem external,
and alternating wood and bast, 84
(Figs. 58, 59).

INDEX AND GLOSSARY

691

Radial symmetry, where an organ or
shoot develops equally all round,
200.

Radicle, first shoot of the embryo, 7.

Raffiesia, parasitism of, 226 (Figs.
154, 155) ; flower of, 227 ; numer-
ous seeds of, 242.

Ragged Robin, 620 (Fig. 470).

Raisin, analysis of, 657.

Ranunculaceae, 622 (Figs. 472-475).

Ranunculus, 623 (Fig. 474).

Raphides, 54 (Fig. 33).

Raspberry, 635 (Fig. 488) ; analysis
of, 657.

Ray-florets, of Compositae, 648 (Fig.
500).

Receptacle, the dilated floral axis on
which the parts of the flower are
inserted, 254 ; various develop-
ment of, 271 (Figs. I95-IQ7) I
general receptacle of capitulum,
262 (Fig. 186) ; arrangement of
parts upon, 264 ; succulent, of
strawberry, 329 (Fig. 254) ; of
sorus in Ferns, 495 (Fig. 387).

Recessive, as in dwarf-habit of Peas,
570 (Fig. 435).

Re combinations, Mendelian, geno-
types appearing in the second
generation of hybrids, in which
some of the characters of the two
original parents have been inter-
changed, 578, 585.

Red Algae, 387.

" Red-Sea," algal origin of name,

377-

Red Snow, 362.

Reduction, a nuclear change by which
the number of chromosomes is
halved, 267, 533 (Fig. 429).

Reduction division, the first division
in the spore-mother-cell by which
the number of chromosomes is
reduced to one half, 533, 536 ; its
relation to Mendelian segregation,

572.
Reproductive organs, production of,

151-

Reseda, lateral roots of, 88 (Fig.

63)-
Reserve cellulose, 125 (Fig. 83) ;

proteins, 126 (Fig. 84).

Reserve materials, storage of, 131 ;
mobilisation of, 130.

Resin passages, of Coniferae, 502.

Respiration, 133 ; demonstration of,
135 (Fig. 87) ; rise of temperature
in, 137 ; aerobic and anaerobic,
136; a means of liberating energy,

i~37.

Respiration and growth, 135.

Respiratory quotient, 136.

Response to stimulus, 8.

Rest, period of, 334.

Resupinate, of flower, rotation
through half a circle so that pos-
terior side appears anterior ; in
Orchis, 612.

Reticulate venation, of leaf of Dicoty-
ledon, 71 (Fig. 46).

Retting of Flax, 452.

Rhea, fibres of, 177.

Rhinanthus, haustoria in ovule, 316
(Fig. 235).

Rhizoctonia, a mycorrhizic Fungus,
231 (Figs. 158, 159).

Rhizoids, of Mosses, 462 (Fig. 356).

Rhizomorphs, root-like strands of
Armillaria, parasitic, 394.

Rhizophores, root-bearing organs of
Selaginella, 514 (Fig. 405).

Rhizopus, 416.

Rhododendron, evergreen, 194 ; leaf
arrangement, 204 (Fig. 137) ;
pollen-tubes in style of, 305 (Fig.

225).
Rhodomela, 393 (Fig. 291).
Rhodophyceae, 355, 387 ; alternation

in, Chap, xxxiv.
Rhoeadales, 625.
Rhopalodia, 375.
Rhynie fossils, 479 (Fig. 372 a).
Ribes, leaf of, 350 (Fig. 263), 631

(Figs. 459, 46°)-

Riccia, archegonia of, 474 (Fig.
370) ; sporogonium of, 548 ; ob-
conical sporeling of, 592 (Fig.
448).

Ricciocarpus, sporogonium of, 476
(Fig. 372).

Rice, analysis of, 660 ; origin of,
661.

Ricinus, (Castor Oil), 10 (Fig. 4) ;
hvpocotyl after thickening, 58
(Fig. 37) ; oil, 125 (Fig. 84).

Rickets, a deficiency disease, 662.

Ringing experiments, 132 (Fig. 86).

Rivularia, 376.

Robinia, leaf of, 349 (Fig. 260).

Roller-milling, 600.

Root, 7 ; structure of, Chap. vi. ;
activity of, q6 ; secondary thicken-
ing of, 92 (Fig. 66) ; construction
of, 82 (Fig. 56) ; lateral movement
of water, 99 (Fig. 68) ; zones of
growth in, 143 (Fig. 89) ; negative
phototropism, 159, 170 (Fig. 126) ;
storage in, 190 (Fig. 130) ; sym-
metry of, 199 ; modified as suckers
in Dodder, 225 (Fig. 152) ; cate-

692

BOTANY OF THE LIVING PLANT

gory of, 346, 347 ; of Fern, 491-492

(Fig. 382).
Root-cap, the pad of tissue constantly

renewed which covers the growing

point, 88 (Figs. 63, 64) ; starch-
grains in, 156 (Fig. 95).
Root-hairs, 8, 83 (Fig. 57), 86 (Figs.

58, 60, 61) ; absorption of water by,

98 ; infection of Bean root through,

236 (Fig. 164).
Root-nodules, 235 ; details of, 236

(Fig. 164).
Root-parasitism, as in Yellow-rattle,

221 (Fig. 149).
Root-pressure, 108 (Fig. 74).
Root-system, 8 ; symmetry of, 199.
Root-tip, structure of, 88 (Figs. 63,

64) ; receptive of stimulus of

gravity, 156.
Roots and shoots, size-structure cor-
relation in, 593, 594.
Rope-requirement, of roots, 190 (Fig.

126).
Roripa, root-cap of, 156 (Fig. 95).
Rosa canina, 634.
Rosa livida, nucellus of, 297 (Fig.

218).
Rosaceae, leaf arrangement of, 205 ;

floral construction of, 267 (Fig.

191), 632 (Figs. 483-488).
Rosales, 632.
Rose of Jericho, seed-dispersal in,

328.
Rostellum, of Orchis, abortive third

lobe of stigma, 613 (Fig. 460).
Rubia peregrina (Wild Madder),

straggling habit of, 214.
Rubus idaeus, floral construction of,

267 (Fig. 191).
Runner of Strawberry, an elongated

axillary bud, 246 (Fig. 169).
Ruscus (Butcher's Broom), root of,

190 (Fig. 126) ; phylloclades of,

344 (Fig- 259).
Rust Fungi, 397, 400, 401, 431, 439

(Figs. 329-338).
Rust of Wheat, 432 (Figs. 329, 338) ;

immunity of wheats against, 440.
Rye, ovary attacked by Claviceps,

395 (Fig- 293). 425, (Fig. 323) \
analysis of, 660 ; origin of, 661.
Rye-grass, 616 (Fig. 464).

Sage, 647 (Fig. 497).
Sageretia, straggling by help of
axillary shoots, 214 (Fig. 143, i.),

345-
Salicales, 619.

Salicornia, succulence of, 211.

Salix (Willow), 619 (Figs. 466, 467).

Salsola, a spiny halophyte, 211 ;
wind-dissemination of seeds, 327
(Fig. 250).

Salts, in soil water, 97 ; enter plant
by root, no.

Salvia, conducting tissue of style, 293
(Fig. 215) ; mechanism of pollina-
tion, 302 (Fig. 220) ; flower of, 647
(Fig. 497).

Sand-box Tree (Hura), explosive
fruit of, 165, 166 (Fig. 102).

Sand-sedge, burrowing tip of, 191 ;
rhizome of, 191 (Fig. 127).

Sanio's law of cambial division, 56

Fig. 35)-
Sap, 105.

Sapindaceae, climbing habit of, 213.
Saprogenic bacteria, 451.
Saprolegniae, 397, 399.
Saprophyte, an organism that derives

organic supply from the substance

of some dead organism, or from the

products of its decay, 220.
Saprophytic fungus, in mycorrhiza,

227 ; initiative of, 230 ; habit of

fungi, 391 ; life in Euglena, 356.
Sap-wood, 64.
Sarcina, cubical packet-form of

Bacteria, 448.
Sarcodes, ectotrophic mycorrhiza of,

228, 229 (Figs. 156, 157).
Sarcophycus, 385.
Sarracenia, 239, 241.
Saxifraga, exudation from, 109 ;

vegetative propagation of, 247 ;

flower of, 290 (Fig. 212) ; 630

(Fig. 480).
Saxifragales, 630.
Scalariform tracheides, of Fern, 488

(Figs. 377-379).
Scapania, structure of, 473 (Fig. 369).
Scarlet-runner, twining stem of, 215.
Schizophyta, 448 : {see Bacteria,

Chap, xxviii.).
Schoenus, construction of stem, 51

(Fig. 28), 185.
Scinaia, 546.
Scion, a bud or graft inserted on a

stock, 251.
Scirpus, mechanical structure of stem,

184 (Fig. 118).
Sclereids, stone-cells, 176 (Fig. in).
Sclerenchyma, hard mechanical tissue

175, 176 ; of Sunflower, 178

(Fig. 112) ; of Ferns, 485 (Fig.

376).
Sclerotia, hard storage-masses of
fungal tissue, 394 (Fig. 293) ; 424
(Fig. 322).

INDEX AND GLOSSARY

693

Sclerotic cells, with thick woody

walls, of Hakea, 77 (Fig. 53).
Sclcrotinia, varying virulence of, 395,

397-
Scots Pine (or Scotch Fir), ecto-

trophic mycorrhiza of, 228 ; char-
acters of, Chap, xxxiii.

Screw Pine, stem of, 67, 603.

Scrophularia, vascular strand of, 44
(Fig. 23), 46 (Fig. 24) ; abortion of
fifth stamen, 268 (Fig. 192) ;
flower of, 643 (Fig. 494).

Scrophulariaceae, meiomery in, 268
(Figs. 192, 193) ; flower of, 643
(Figs. 494. 495)-

Scurvy, a deficiency disease, 662.

Scutellum, suctorial organ of grass-
embryo, 658 (Fig. 503).

Sea Buckthorn, root-nodules, 239.

Season, biology of, 194.

Seaweeds, Brown, 378 ; Red, 388.

Secale (Rye), analysis of, 660 ; origin
of, 661.

Secondary growth, by activity of
cambium, 55 (Figs. 34-37) ; of
root, 91 (Figs. 66, 67).

Sedge (Carex), stems of, 191 (Fig.
127) ; leaf arrangement of, 207 ;
flowers of, 615 (Fig. 463).

Seed, the result of maturing of a
megasporangium or ovule, de-
tached at ripeness, having an
embryo within : 5, 256, 311, 521;
viability of, 140 ; structure of, 310 ;
dispersal of, Chap. xix. ; reduc-
tion in number of, 322 ; increase
in number, 322 ; of Neottia, 233
(Fig. 161) ; wTinged of Bignoniaceae,
326 (Fig. 248) ; 353 a.

Seed-coat, the covering of the seed
developed from the integuments,
6, 318 (Fig. 237) ; of Coniferae, 538
(Fig. 426).

Seed-habit, origin of, 521, 550.

Seed-leaves, the first leaves or coty-
ledons formed on an embryo, 7.

Seed Plants, 2, 5.

Segregation, theory of, 278.

Segregation of chromosomes in tetrad-
division, 566 (Fig. 430), 563 (Fig.
430, B) ; Mendelian, 571 ; of genes,
572 (Fig. 436) ; in hybrids, 574.

Selaginella, Chap, xxxii. ; ligule of,
512 ; rhizophores of, 513 (Fig. 405);
sporangia of, 516 (Figs. 406-410) ;
heterospory in, 512 ; embryology
of, 518 (Figs. 412, 413).

Semi-permeable membrane, 35.

Semolina, granules of endosperm
of wheat, sifted out in milling, 660.

Senecio, chromoplasts in petal of,
0 (Fig. 200) ; flower of, 649.

Sensitive filaments of Centaurea, 651
(Fig. 501).

Sensitive plant (Mimosa), movements
of, 163 (Fig. 98).

Sepals, the outermost series of floral
parts, constituting the calyx, 255 ;
structure of, 280.

Septate structure, of plants, 173,
(Fig. 107).

Sequoia, Big Tree of California
(Frontispiece), 527 ; conical stem
of, 590.

Sera, 453.

Seta, stalk of the capsule in Mosses,
461 (Figs. 355, 364), 469.

Sex, Chap. xxxv. ; functions of, 531.

Sex-difference, reflected back to
sporophyte, 555.

Sexual act, polyphyletic, 556.

Sexual-differentiation, in Algae, 370
(Fig. 275), 381.

Sexual fusion, 307, 308, 382, 386.

Sexual organs, 353 ; of Ferns, 501
(Figs. 393, 394) ; of Mosses, 467
(Figs. 362, 363) ; of Fucus, 384
(Figs. 287, 288) ; of Fungi, 398
(Fig. 296), 410 (Fig. 310) ; of
Ascomycetes, 421 (Fig. 319).

Sexual reproduction, significance of,
558 ; its difference from vegetative
propagation, 245.

Sheep Sorrel (Rumex acetosella), 112.

Shelf Fungi, parasitism of, 393, 431,
442 (Fig. 340).

Shepherd's Purse (Capsella), curved
ovule of, 319 (Fig. 239).

Shoot, a morphological unit, made
up of a stem bearing leaves later-
ally, and in acropetal succession, 7 ;
shoot-system, 8 ; zones of growth
of, 143 (Fig. 88) ; symmetry of,
199 ; multiplication of, 347 ; gen-
eral purposes, 347 ; axillary and
distal branching of, 347, 348.

Short-day type, 152.

Sibbaldia, floral diagram of, 267 (Fig.
191).

Sicyos, tendril of, 216 (Fig. 145).

Sieve-plate, the perforated area of
wall of a sieve-tube, 48, 49 (Figs.

25-27)-
Sieve-tubes, conducting tubes of the

phloem : 44 (Figs. 23, 24) ; 48
(Figs. 25-27) ; 56 (Fig. 34) ; trans-
location by, 132 ; of Conifers, 532
(Fig. 419) ; of Ferns, 488 (Figs.

377- 378)-
Sigillaria, 511.

694

BOTANY OF THE LIVING PLANT

Siliqua, the fruit of Cruciferae, 626
(Fig. 477).

Silt, 97.

Silver grain, tangential aspect of
medullary rays, 60.

Silver wire, qualities of, 178.

Similarity, Principle of, 591.

Sinistrorse twining, reverse of hands
of watch, 215 (Fig. 144, A).

Siphonales, non-septate Algae, 171
(Fig. 105), 361, 367 ; vegetative
propagation of, 370 ; gametes of,
370 (Fig. 275) ; life cycle of, 390,
556 ; comparison with Fungi, 447.

Size and form, relation of, 589
(Chap, xxxvi.) ; relation shown by
measurement, 593 (Table).

Sleep movements, 161 ; in Clover,
Mimosa, etc., 162 (Fig. 98)

Smut Fungi, 431, 440.

Snowdrop and Snowflake, 608.

Soil, constituents of, 96 ; influence
on plant-distribution, 112.

Solanaceae, 642 (Fig. 493).

Solanum : jasminoides, prehensile
leaf of, 216 ; porous dehiscence of
anthers of, 281 ; tuberosum
(Potato), seedling of, 244 (Fig.
168) ; correlation in, 218 (Fig.
147) ; graft-hybrids of, 251 ; origin
of, 653 ; analysis of tubers, 655 ;
flower of, 642 (Fig. 493).

Solar energy, fixation of, 114.

Solenostele, 596 (Fig. 452), 597.

Sols, 32, 35.

Somatic budding, separation of buds
from the soma or plant-body, 245.

Somatic phases, 546 ; relation to
nuclear cycle, 556.

Somatic division of nucleus, 561 (Fig.
428), 562 (Fig. 429, A); of cells,
19, 20 (Fig. n).

Somatic parthenogenesis, where an
embryo is formed without fertilisa-
tion from a diploid egg, 587.

Soredia, organs of vegetative propa-
gation of Lichens, 427.

Sori of Ferns, 482 (Fig. 373), 495

(Fig. 387).

Soya Bean (Glycine soja), nitrogen-
free germination of, 237 (Fig. 165) ;
analysis of, 655 ; origin of, 656.

Spartina Townsendii, a natural allo-
polyploid, 583.

Specific mechanical tissues, 175 ;
physical qualities of, 178, 179 ;
disposition of, 180.

Spermatium, a naked non-motile
male gamete ; of Red Seaweed,
389 (Fig. 290).

Spermatocytes, cells giving rise to
spermatozoids ; of Selaginella, 517
(Fig. 411) ; of Ferns, 501 (Fig. 393).

Spermatozoids, motile male fertilising
bodies ; of Zamia, 527 (Fig. 415) ;
of Selaginella, 517 (Figs. 411, 412) ;
of Ferns, 501 (Fig. 393), 503 (Fig.
395) ; of Mosses, 467 (Fig. 362) ;
of Brown Seaweeds, 381, 383 ; of
Fucus, 384 (Fig. 287) ; of Oedo-
gonium, 366 ; of Monoblepharis,
412 (Fig. 311).

Spermogonia, flask-shaped bodies
containing spermatia : in Lichens,
429 ; in Puccinia, 438 (Fig. 337).

Sperms, or male gametes, 353.

Sphacelaria, apical cell of, 379.

Sphaerella, 363 (Fig. 267).

Sphaerotheca, 401, 418 (Figs. 318,
319). 420.

Sphagnales, 548.

Sphagnum (Bog Moss), 465.

Sphenophyllales, 522.

Spike, an indefinite inflorescence with
sessile flowers, 259 (Fig. 181).

Spikelets, of Glumales, 616 (Fig. 464).

Spinach (Spinacia oleracea), analysis
of, 655 ; origin of, 654.

Spindle-fibres, in dividing nucleus,
562 (Fig. 428).

Spindle Tree (Euonymus), arillus of,
318 (Fig. 238).

Spiral arrangement, of leaves, where
an ascending spiral line may be
drawn round the stem, threading
together the bases of them all, 202 ;
of parts of flower, 264 (Fig. 188).

Spirillum, strongly spiral shape of
Bacteria, 448.

Spirogyra, 121 (Fig. 277) ; conju-
gation of, 372, 373.

Splachnum, habitat of, 463.

Spongy parenchyma, 72 (Figs. 47, 48).

Sporangiophores, hyphae bearing
sporangia, of Mucor, 401, 413 (Fig.
312).

Sporangium, an organ of propagation
producing spores internally, 256,
352 ; of Fern, mechanism of, 167
(Fig. 103), 496 (Figs. 388-390) ; of
Selaginella, 516 (Figs. 407, 408) ;
of Brown Algae, unilocular, 382
(Fig. 284) ; plurilocular, 382 ; of
Fungi, 401 ; of Mucor, 413 (Figs.
312, 313.)

Spore-mother-cell, of Ferns, 497 (Fig.
389) ; tetrad-division of, 564 (Fig.
43o).

Spores, of Selaginella, 515 (Figs. 407,
408) ; of Ferns, 494 (Figs. 387-

INDEX AND GLOSSARY

695

391) ; in Mosses, germination of,
463 (Fig. 357) ; of Bacteria, 448
(Fig. 347) ; of Fungi, 398, 401.

Spore-sac, in Mosses, the part of the
capsule which originates, nourishes,
and directly invests the spores, 469
(Fig. 364).

Sporidia, the tetraspores of Puccima,

437-

Sporodinia, 415, 416.

Sporogenous cells, which give rise to
pollen-grains or other spores : in
anther, 284.

Sporogonium, 461 (Fig. 355) ; struc-
ture of, 469 (Fig. 364) ; develop-
ment of, in Moss, 470 (Fig. 365).

Sporophylls, leaves bearing sporangia
of Ferns, 494.

Sporophyte, of Bryophyta, 460, 469 ;
rise of, 552.

Sporophytic budding, formation of
vegetative buds on the sporophyte,
which form new sporophytes, 506
(Fig. 400).

Spotted Orchis, 611 (Figs. 459, 460).

Spruce, dorsiventral symmetry of
lateral branches, 206.

Spur, 529 (Fig. 178).

Spurge (Euphorbia), simple flowers
of, 256 (Fig. 178) ; 629 (Fig. 479).

Squill (or Wild Hyacinth), 608.

Squirting Cucumber, fruit of, 324.

Stabilisation of the Hofmeisterian
Cycle, 547.

Stamens, the floral part bearing
pollen-sacs or microsporangia :
collectively the stamens constitute
the androecium, 255 ; structure of,
281 (Figs. 201-207).

Staminate, applied to flowers or
plants which bear stamens but not
carpels, 256 ; by abortion in
Lychnis, 270 (Fig. 194).

Staminode, an aborted stamen : in
Lychnis, 271, 622 (Fig. 471).

Standard flour, 660.

Staphylococcus, 453.

Starch-grains, formed in photosyn-
thesis, 115 (Fig. 76), 124 (Figs. 81,
82) ; storage of, 130 ; conversion
to sugar by diastase, 130 (Fig. 85) ;
as receivers of the stimulus of
gravity, 156 (Fig. 95).

Statoliths, 156 (Fig. 95).

Steel, qualities of, 178.

Stele, the aggregate of vascular
tissues in a stem or root, with or
without a pith, and limited ex-
ternally by an endodermis, 41
(Fig. 21) ; of Fern, 485 (Fig. 375) ;

of root, 84 (Fig. 58) ; fluting of,
593 ; size-structure correlation in,
Chap, xxxvi., 589.

Stem, apex of, 16, 18 (Figs. 7, 8) ;
tissues of, 40 (Chap, iv.) ; her-
baceous, 42 ; aquatic and climbing,
47 ; of Monocotyledons, 50;
woody, 55.

Stem-parasitism, as in Dodder, 221,
224 (Fig. 151), 225 (Fig. 152).

Stereum, 446.

Sterigmata, conical processes upon
which conidia or spores are borne,
422 (Fig. 320) ; of Puccinia, 432,
436 (Fig. 335) ; of Coprinus, 445

(Fig. 345)-
Sterilisation, of an organic medium,

449 ; theory of, 547.
Stigma, the receptive surface of the

carpel, 292 (Figs. 214, 221, 222).
Stimulus, a cause of reaction, 8 ;

conveyance to a distance, 153.
Stipe of Mushroom, 444.
Stipules, lateral appendages at base

of leaf-stalk, prehensile of Smilax,

216 ; of Lathyrus aphaca, 219

(Fig. 148), 348.
Stomata, breathing pores through the

epidermis, 73 (Fig. 48), 75 (Fig. 49),

76 (Figs. 50-53) ; their number, 74 ;

their position, 76, 77 (Figs. 50, 53) ;

effect of light on, 102 ; control by

turgor of, 102-104 (Figs. 70, 71) ;

closure at night, 104, 105 ; water-

stomata, 109.
Stomium, the point of rupture of a

Fern sporangium, 498.
Stone-cells, with hard woody walls,

176.
Stonecrop, succulent leaves of, 74,

209.
Storage, 114, 130 (Chap, viii.) ; by

woody stem, 68 ; in parenchyma,

130 ; its importance in perenna-

tion, 195.
Storage materials, 130.
Straggling, methods of, 214 (Fig. 143).
Strawberry, runner of, 246 (Fig. 169) ;

flower of, 633 (Fig. 463) ; fruit of,

634 (Fig. 463) ; analysis of, 657.
Streptococcus, 453.
Strobilus, or flower, 554 ; of Arche-

goniatae, 514 (Figs. 405-406).
Struggle for existence, 340.
Strut-roots, of Maize, mechanical

structure of, 191 (Fig. 128).
Style, an elongated region often

intervening between the ovary

and the stigma, 288 (Fig. 209) ;

proportionate length of, 292.

6p6

BOTANY OF THE LIVING PLANT

Suberin, 124.

Suberised walls, of corky character,
24.

Substituted sexuality, 587.

Subularia, perigynous flower of, 272.

Succulence, in xerophytes, 209 (Fig.
141).

Suckers, organs by which parasites
extract nourishment from the host,
225 (Figs. 151, 152), 226 (Fig. 153) ;
shoots which appear above ground,
formed as adventitious buds on
roots, 247.

Sucrose (cane sugar), 123, 131.

Suction, of water from above, 107.

Suction pressure, 37.

Sugar, 123 ; formed in photosyn-
thesis, 115 ; storage of, 124 ; de-
rived from starch, 129.

Sugar-beet, 654.

Sugar Maple, 108.

Sulphur, necessary to form proteins,
127.

Sulphur Bacteria, 451.

Summer buds or spores (uredo-
spores), of Puccinia, 433 (Figs. 331-

333)-
Sundew (Drosera), motile tentacles of,

164 (Fig. 99) ; leaves of, 239 (Fig.

166) ; digestion by, 240.

Sunflower, germination of, 12 ; petiole
of, 70 (Fig. 45) ; transition of leaf
arrangement, 202 (Fig. 135) ; de-
velopment of epigynous flower of,
272 (Fig. 198).

Sunken stomata, 210 (Fig. 142).

Superficial placentation of ovules
inserted on the carpellary surface,
291.

Surface-volume ratio, 591.

Suspended animation, 334.

Suspensor, the" part by which the em-
bryo is attached, 311 (Fig. 230), 313
(Fig. 232) ; of Selaginella, 518 (Figs.

412, 413)-
Suspensor of Mucor, 416.
Sweet Chestnut (Castanea vesca),

leaf of, 350 (Fig. 262).
Sweet Pea, seeds of, 140.
Swelling, increase of bulk by taking

up water into organised substance,

5-
Sycamore, deciduous leaves of, 194 ;

leaf -mosaic, 201 (Fig. 134) ; de-
layed germination of, 140.

Symbiosis, a mutual existence of
two organisms with joint physio-
logical action, 221 (Chap. xii.).

Symmetry, the proportions of a shoot
or root resulting from equal or un-

equal growth round the axis of con-
struction : of root, 199 ; of shoot,
200 ; of flower, 274 ; radial or
actinomorphic, where development
is equal all round, 200, 274 ; dorsi-
ventral or zygomorphic, where
there are distinct upper and lower
faces, 70, 205, 274 (Fig. 199) ; bi-
lateral, 205.

Sympetalae, Dicotyledons with co-
herent petals (gametopetalous, 638.

Sympodium, a false axis built up of
parts which are branches of suc-
cessively higher order, 207 (Fig.

139).
Synandreae, 647.

Syncarpous, applied to carpels when
united, 289 (Figs. 209, 211).

Synergidae, two cells which, with the

ovum, form the egg-apparatus, and

co-operate with it in fertilisation,

295 (Fig. 216), 305 (Fig. 226).

yngamy, the fusion of gametes, 309,

353. 544. 557. 567-
Synthesis, 114 (Chap, viii.) ; of pro-
teins, 126.

Tamus, 50 ; embryo of, 314 (Fig. 233).
Tangles, large size and mechanical

demands of, 168, 378, 380.
Tapetum, a layer of cells surrounding

the spore-mother-cells in sporangia;

of pollen-sac, 284 (Figs. 205, 206) ;

of Fern sporangium, 497 (Fig.

389, i-)-
Taraxacum (Dandelion), flower of,

652 (Fig. 502) ; somatic partheno-
genesis in, 587.

Targionia, structure of, 472 (Fig. 368).

Taxodium, 529.

Taxus (Yew), 527.

Teleutospores (or winter spores or
buds), of Puccinia, 433 (Fig. 334) ;
germination of, 435 (Fig. 335).

Telome, morphology of, 353 a.

Temperature ; effect on germination,
8 ; suitable for growth, 31 ; effect
on growth, 147 ; on flowering, 152 ;
on movements in cell, 31.

Tenacity, limit of, 179.

Tendril, haptotropic, 161, 216 (Fig.
145), 219 (Fig. 148).

Tensions of tissues, mutual, 1 74 (Fig.
108).

Testa, the seed-coat developed from
the integument, 318 (Fig. 237) ; of
Wheat, 658 ; impermeability of,
140.

Tetanus Bacillus, anaerobic, 450.

INDEX AND GLOSSARY

697

Tetracyclic, of gamopetalous flower
with four cycles of parts, 268.

Tetracycliceae, 641.

Tetrad, a group of four spores re-
sulting from division of one spore-
mother-cell, 563 (Fig. 430) ; its
relation to Mendelian segregation,

572.

Tetrad-division (meiosis) of a spore-
mother-cell, first into 2, then into
4, forming the tetrad ; reduction
of chromosomes accompanies it,
285 ; in formation of pollen, 286
(Fig. 207) ; in formation of em-
bryo-sac, 296 (Fig. 217) ; in
sporangia of Higher Plants, 353,
563 (Fig. 430, 564).

Tetramerous, with parts in whorls of
four, 265.

Tetraploid, see Polyploid.

Tetraspores, of Red Seaweeds, 389 ;
of Dictyota, 383, 545 ; of Fungi,
401.

Tetraphis, gemmae of, 466.

Thalictrum, somatic parthenogenesis

in, 587-

Thallophyta, plants with no clear
distinction of stem and leaf, 3, 354
(Chap, xxi.) ; alternation in, 546.

Thelephora, part of Lichen, 446.

Tissues, of stem, 40 (Chap, iv.), of
leaf, 69 (Chap, v.) ; of root, 82
(Chap, vi.) ; mutual tensions of,

174 (Fig- Io8)
Tmesipteris, 524.

Toad-flax, 159 ; zygomorphy of, 274
(Fig. 199) ; deposit of seeds, 328.

Toadstools, 391, 393, 431.

Tomato, inheritance of fruit-colour,
578.

Tooth-wort, parasitism of, 226.

Torus of bordered pits, 531 (Fig. 418).

Toxines, 452.

Tracheae, a general term including
tracheides and vessels. They have
woody walls and no cell-contents
when mature, 45.

Tracheide, a cell with complete woody
walls and no cell-contents : fibrous,
46 (Fig. 24), 59 (Fig. 38) ; of Coni-
ferous wood, 530 (Figs. 417-419) ;
of Fern, 488 (Figs. 377"379).

Tragopogon, latex vessels of, 54.

Transformation theory, 546.

Translocation of materials, 130.

Transpiration, exhalation of water-
vapour, 99-105 ; transpiration
stream, 99, 105 ; path of, 106 (Fig.
72) ; stomatal control of, 105 ;
significance of, 100 ; measurement

of, 101 (Fig. 69) ; conveys salts,

100.
Transverse plane, in a floral-diagram

a plane perpendicular to the median

plane, 263.
Trefoil (Lotus), 635 (Fig. 489).
Tremellales, 432.
Trichocolea, structure of, 473.
Trichodesmium, of ' Red Sea,"

377-
Trichogyne, the receptive filament of

Red Seaweeds, 389 (Fig. 290) ; of

Fungi, 400, 418 ; of Collema, 429.

Tricoccae, 628.

Trimerous, with parts in whorls of
three, 263 (Fig. 187), 265.

Triticum repens, vegetative propa-
gation of, 252.

Triticum vulgare, analysis of, 660 ;
origin of, 661.

Tropaeolum, structure of lamina, 75
(Fig. 49) ; exudation of water from
leaf, 109 (Fig. 74, A).

Tropic responses, 154.

Truffle (Tuber), 401, 418, 426 (Fig.

325)-
Tubercle Bacillus, 451.

Tubercles (Nodules), on Leguminous
roots, 235 ; fixation of nitrogen in,
236-237.

Tuberous stems, 177.

Tubers of Nephrolepis, vascular sys-
tem of, 598 (Fig. 454).

Tubuliflorae, 649.

Tulip, chromoplasts in petals, 280 ;
flower of, 607.

Turgor (turgescence), the tense con-
dition of living cells owing to pres-
sure of the protoplast on the wall,
34, 38 ; of stoma, 100, 103 (Figs.
70, 71) ; rigidity based on, 170.

Turnip (Brassica), analysis of, 655 ;
origin of, 654.

Twining stem, 215 (Fig. 144)

Ulotrichales, 361, 365, 375.
Ulothrix, 365 (Fig. 270), 368, 547.
Ulva, 365.

Ulvaceae, 361 ; isomorphic alterna-
tion in, 390.
Umbellales, 637 (Fig. 490)-
Umbelliferae, fruit of, 323 ; flower of,

637-
Unilocular sporangia, 381.

Uredinales, 397, 401, 431 (Figs. 329-
338) ; life-history of, 432.

Uredospores (or summer-buds, or
spores of Puccinia), 432 (Fig. 331) ;
germination of, 434 (Figs. 332, 333).

698

BOTANY OF THE LIVING PLANT

Ustilaginales (Smuts), 440.

Ustilago, classification of, 401 ; ger-
mination of, 440 (Fig. 339).

Utricularia, bladders of, 241 (Fig.
167, A).

Vaccinium, 640.

Vacuole, a cavity in the cytoplasm,
filled with cell sap, 22 (Figs. 12, 13),
29 (Fig. 17).

Valerian, wind-borne fruit of, 325
(Fig. 246).

Valonia, non-septate sac of, 171.

Variation, 339, 569, 585.

Vascular plants, success of, 602.

Vascular strand, of Dicotyledons, 41
(Figs. 20, 21), 56 (Fig. 34) ; of
Cucumber, 48 (Fig. 25) ; of Mono-
cotyledon, 52 (Fig. 31).

Vascular system, 40 (Figs. 20. 21) ;
of Palm-type, 51 (Figs. 29, 30) ;
of Male Fern, 484 (Figs. 374,
376).

Vascular tissue, 44 (Figs. 23, 24).

Vaucheria, non-septate tubes of, 171 ;
aplanospores of, 372 ; life-history
of, 369 (Figs. 274-276) ; compari-
son with Fungi, 402, 410, 411, 412,

447-
Vegetable foodstuffs, 653 (Appendix

B).
Vegetation, three grades of, 602.
Vegetative-cell, in pollen-grain, the

cell from which the pollen-tube

is formed, but does not itself

take part in syngamy, 283 (Fig.

204).
Vegetative propagation, increase in

numbers by detachment of a part

of a vegetative plant-body, Chap.

xiii., p. 244 ; rare in Conifers, 529 ;

in Ferns, 494.
Veins of leaf, 71 (Fig. 46).
Vellozia, fission of stamens of, 266

(Fig. 190).
Velum, of Mushroom, 444 v., (Fig.

34L 343)-
Venation, parallel and reticulate,

71-72.
Ventilating-system, of intercellular

spaces of sporophyte, 553 ; absence

of in large underground prothalli,

550-
Ventral-canal-cell, of Pine, 536-537

(Fig. 424) ; of Selaginella, 518 (Fig.

412) ; of Fern, 502 (Fig. 394) ; of

Moss, 468 (Fig. 363).
Venus' Fly-trap (Dionoea) , carnivorous

habit of, 165 (Fig. 100), 239.

Verbena, spike of, 259 (Fig. 181).

Verbenales, 645.

Vernalisation, 152.

Veronica, meiomery in, 269 (Fig.
193) ; flower of, 644 (Fig. 495).

Vessel, result of fusion of two or more
cells, by absorption of septa and
protoplasmic contents, to form an
open channel, 25 (Fig. 15), 43, 46
(Fig. 24), 59 (Fig. 38) ; laticiferous,
54 (Fig. 32).

Vestigial remains, parts imperfectly
developed which marks the place
where normally a fully developed
part would be, 268 (Fig. 192).

Vexillum, the posterior petal of Pea-
flowers, 636 (Fig. 467).

Viability of seeds, 140.

Vibrio, slightly spiral-shape of Bac-
teria, 448, 452.

Vicia, 235 ; root-nodules of (Figs.
163, 165).

Vicia Faba (Broad Bean), 6 (Fig. 2) ;
growth of root, 144 (Fig. 89) ;
analysis of, 655 ; origin of, 656.

Vine (Vitis vinifera), panicle of, 261
(Fig. 183) ; origin of, 656.

Violaceae, transfer of seeds, 330.

Virginia Creeper (Ampelopsis Vetchii),
climbing shoots negatively photo-
tropic, 159 ; adhesive climbing of,
197 (Fig. 146).

Vitamins, 138, 662.

Viviparous habit, of Alpines, 587.

Volvocales, 361, 375.

Volvox globator, 362, 363 (Fig. 268).

Walnut, analysis of, 657.

Water, proportion of, in plants, 95 ;
in the soil, 96 (Fig. 60) ; of con-
stitution, 95 ; of imbibition, 95 ;
gravitational, 96 ; significance of,
96 ; absorption of, by root-hairs,
98 ; lateral movement of, 99 (Fig.
68) ; ascent of, 105 ; cohesion by,
106 (Fig. 73) ; conduction of
woody stem, 68 ; storage in xero-
phytes, 209 (Fig. 141).

Water-bloom, 377.

Water-culture, recipes for, no (Fig.

75)-

Water-lily, floating leaves, 212 ; peri-
sperm of, 317.

Water-Net (Hydrodictyon), 365.

Water-relation, Chap, vii., p. 95 ; its
effect on adaptive organisms,
208.

Water-transport, of seeds and fruits,
326.

INDEX AND GLOSSARY

699

Weeds, unconscious transfer by man,

330.

^Velwitschia, deep-rooted, 210 ; cor-
relation in, 218.

Wheat, mummy, 334, as host of
Puccinia, 432 (Fig. 329) ; infection
by aecidium spores, 433 ; by
uredospores, 434 (Fig. 333) ; struc-
ture of grain, 658 (Figs. 503, 504) ;
analysis of, 660 ; origin of, 661.

Whole-meal, 660.

Wild-hyacinth (Scilla), 608.

Willow (Salix), cuttings, 132 (Fig.
86) ; viability of seeds, 141 ;
flowers of, 619 (Figs. 444, 447).

Willows (pollarded), flora of, 331.

Wilting, 105.

Wind, as pollinating agent, 301 ; its
use in dissemination of seeds,

325-
Winter-kale (Brassica), analysis of,

655 ; origin of, 654.

Winter-spores, or buds (Teleuto-

spores) of Puccinia, 433 (Fig.

334) ; germination of, 435 (Fig.

335)-
Wood-fibres, 59 (Fig. 38).

Wood-parenchyma, 59 (Fig. 38).

Wood-rush, 614 (Fig. 461).

Wood-sorrel, movement in, 162.

Woodwardia, sporophytic budding

in, 494 (Fig- 386).
Working hypothesis, of evolution,

339-
Wrought-iron, qualities of, 178.

Xanthophyll, T17.

Xanthoria, 428 (Fig. 326).

Xerophytes, plants adapted for life
under conditions of drought, 208 ;
their characters, 209 ; succulence
of, 209 (Fig. 141) ; structural
modifications of, 210 (Fig. 142) ;
sunken stomata of, 210 (Fig. 142).

Xylem, the woody region of the vascu-
lar strand, 45 (Fig. 23), 46 (Fig. 24).

Yeast in fermentation, 136, 137, 393.
Yellow-flag (Iris), 609 (Fig. 458) ;

stock of, 196 (Fig. 129).
Yellow-rattle (Rhinanthus), green

root-parasite, 222 ; compare Fig.

149.
Yew (Taxus), 527.

Zea (Maize), vascular bundles of, 51
(Figs. 29, 31) ; analysis of grain,
660 ; origin of, 661.

Zamia, 527 (Fig. 415).

Zizyphus, straggling by stipules, 214
(Fig. 143, ii.), 345.

Zoogloea, of Bacillus subtilis, 448
(Fig. 346).

Zones of growth, 143 (Fig. 88).

Zoospore, of Ulothrix, 365 (Fig.
270, of Oedogonium, 367 (Fig. 271).

Zoosporangium, of Pythium, 404
(Fig. 303).

Zoospore, a spore motile in water :
of Ulothrix, 365 (Fig. 270) ; of
Oedogonium, 367 (Fig. 271) ; of
Bulbochaete, 367 (Fig. 272) ; of
Vaucheria, 369 (Fig. 274) ; of
Potato Fungus, 408 (Fig. 308) ; in-
fection of Potato by, 408 (Fig. 309).

Zygnemaceae, 372.

Zygomorphic, lop-sided symmetry
within the flower, 274 (Fig. 199).

Zygomycetes, 399, 401, 412.

Zygospores, of Mucor, 399, 415 (Fig.

3H)-
Zygote, the product of fusion of two

gametes (syngamy) : of seed-
plants, 310, 315 (Fig. 234) ; of
Ferns, 503 (Fig. 396) ; of Spiro-
gyra, 373 (Fig. 277), 557.
Zymase, 129, 137.

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