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THE PLANT CELL:

ITS MODIFICATIONS AND VITAL PROCESSES.

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Plate II

1. Portion of a transverse section of stem of one year’s growth. Note the
primary bundles, and betw'een them the interfascicular cambium and evidences of
secondary xylem and phloem. Resin -canals ate scattered throughout the cortex. Note
also patches of sclerenchyma at tlie outer mat gin of cortex.

2. Portion of a transverse section of a dicotyledonous stem, showing arrangement of
the xylem, cambium, phloem, and bast-fihres.

3. Portion of a transverse section of the young stem of LupnlvK, showing the cambial
region and young xylem and phloem elements.

4. Part of a transverse section of the stem of Finns, showing the characteristic
dicotyledonous arrangement ; large resin-canals are seen in the cortex.

(See Chaps, iv. and v.)

THE PLANT CELL

ITS MODIFICATIONS AND VITAL PROCESSES.

A MANUAL FOB STUDENTS

HAEOLD A. HAIG, M.B., B.S. Bond.

(LATE BUCKNILL SCHOLAR, UNIVERSITY COLLEGE, LONDON).

5llu6tratct) 1Wumcrou6 Drawings anD
Ipbotomicrograpbs.

COLL. REG.

LONDON:

CHARLES GRIFFIN AND COMPANY, LIMITED;
EXETER STREET, STRAND.

1910.

PREFACE.

In the following pages the Author’s aim has been to
deal with the Study of Structural and Physiological
Botany from a biological standpoint, in which the work-
ing substance of a cell, — viz., the protoplasm — is given
the first place in importance ; the subsequent changes
which are produced in form, function, &c., being looked
upon as being due to the sole agency of the protoplasm,
influenced by the various physical and chemical stimuli
which may be brought to bear upon it. This method
of dealing with the cell, whether animal or vegetable,
has been found, in the writer’s experience, to be a
rational and useful one when such a wide subject as
Biology is first approached by the student. The section
on Cell-division has been presented in rather full detail,
on account of the great importance attached nowadays
to cytological phenomena in which the nucleus is in-
volved. With regard to the illustrations, a few photo-
micrographs have been inserted, and these, it is hoped,
will give a rather more realistic aspect to one or two of
the more difficult sections, such as those on Embryology
and Nuclear Division.

The Author’s thanks are due to Professor Oliver
(University College, London) for several valuable sug-
gestions, and for the help afforded whilst the Author
was a student at University College.

H. A. H.

Hendon, 1909.

CONTENTS

INTRODUCTION.

Definition of a Cell— Tissues — Function — Outline of Classification
of Plants,

CHAPTER I.

The Nature and Reactions of Protoplasm.

Composition and Constitution of Protoplasm— Water of Constitu-
tion— Irritability — Importance of Water and Oxygen—
Turgidity — Conditions for Continued Activity of Proto-
plasm, ...........

CHAPTER II.

The Study of a Living Assimilating Cell.

A. The Fully Differentiated Assimilating Cell — Structure of the

Cell — Vital Processes and Action upon the Cell of Re-
agents— Significance of Starch-formation — Definition of
Assimilation in its Wider Sense,

B. TM Young Undifferentiated Cell — Plastids — Vacuole Forma-

tion— Growth of the Cell, .......

CHAPTER III.

Cells of the External Tissues and Certain Supporting
AND Protective Tissues in Plants.

The Epidermis and Structures in Connection with it — Der-
matogen — Stomata, their Structure and Function — Hairs,
their Structure and Function — Root Hairs — Cortex —
Mesophyll — Cork and Cork Cambium — Collenchyma —
Sclerenchyma,

PAGES

1-6

7-12

13-18

18-22

23-41

Vlll

CONTENTS.

PAGES

CHAPTER IV.

Meristem.

Primary and Secondary Meristems — Cambiums — Dividing Cells
of Rudimentary Tissues and their Mode of Growth —

Growth of Cell-wall — Cambium, its Origin, Structure, and

the Tissues arising from it, 42-56

CHAPTER V.

The Vascular Tissues.

The Phloem — The Xylem — The Medullary Rays — Endodermis —
Pericycle — Medulla or Pith — Appendix to Chapter V.

— Origin of Primary Wood Elements — Vascular Elements
in Bryophyta,

CHAPTER VI.

Isolated Tissues or Cells having a Specific Function.
Secretory Cells of Oil Glands — Resin Canals' — Mineral and
Organic Matters separating out in Cells — Idioblasts —
Laticiferous Vessels and Cells, ...... 78-87

CHAPTER VII.

Cells occurring amongst the Lower Plants.

Cells of Fungi — Cells of Algje, viz. , Spirogyra, Vaucheria^

Sphcerella, Melosira, 88-99

CHAPTER VIII.

Cell-division.

Amitotic Cell-division — Mitotic Nuclear Division, with Division
of the Cell — The Structure of the Quiescent Nucleus —

Mitosis, its Details and Probable Mechanism, . . . 100-116

CHAPTER IX.

Cells having the Function of Reproducing the Species.

General Considerations — Reproduction in Angiosperms — The
Microspore, its Origin and Maturation — The Macrospore,
its Origin and Maturation — Fertilisation and Embryo-
formation — Origin of the Endosperm — Reproduction in
Gymnosperms — Reproduction in Pteridophyta (Homo-
sporous and Heterosporous) — Reproduction in Bryophyta,

Fungi, and Algse — Homology and Table of Homologies, . 117-155

CONTENTS.

PAGES

CHAPTER X.

Chemical and Physiological Studies in Connection with
THE Cell.

General Coihsiderations — Metabolism — Essential Food-Materials
— Enzymes — Constructive Processes — Details of Vital
Processes — Starch — Chlorophyll — Elaboration of Nitrogen
— The Cell-sap and Sap-conduction — Transpiration and
Root-pressure — Gaseous Interchange during Assimilation
and Respiration — Assimilation of C02and H2O — Variations
of Protoplasmic Activity under the Influence of Different
Physical Agencies — Production of Heat, Light, and Elec-
trical Phenomena in the Cell, ...... 156-188

APPENDIX TO CHAPTER X.

The Physics of the Absorption of Water, Salts, and Gases

by the Cell, 189-191

Index, 193-207

EREATA.

Frontispiece, line 1, for Samhucns read Liqoidiis.

„ line 7, for Lupulus read Sambvcus.

Page 12, lines 13 to 15 should read thus : —

“ is meant a condition of tension inside a cell resulting from the intake
of water by osmosis (see Chap, x.) until an equilibrium is set up
between the sap inside the cell and the fluid outside.”

Page 127, line 37, after “oospore” insert “elongates into a structure
known as the proemhryo which has a cell cut off from its lower end,
and this,” &c. The remainder of the proembryo forms the snspensor.
Page 128, line 4, omit the words “and, in some cases, the foot (an absor-
bent organ).”

LIST OF PLATES AND PHOTOMICROGRAPHS.

Transverse Sections of Various Stems, . . Frontispiece

PLATE PAGE

I. and II. Phases in Mitosis, to face 103

III. Stages in the Maturation of the Embryo-sac

in Angiosperms, ..... ,, 123

IV. Stages in Endosperm Formation in Angiosperms, ,, 128

FIG.

6a. Young Cells of the Endosperm of Caliha palustris^ . 21

зба. The Cambial Region of Stem of Samhiicus, . to face 54

збб. Transverse Section of a Primary Bundle in a

Dicotyledonous Stem, .... ,, 55

51. Transverse Section of the Root of P^Vm6•, . ,, 68

65a. Resin-canals in the Xylem of Finns Stem, . ,, 81

06 and 97. The Completely-matured Embryo-sac of if e//e6on<s, ,, 126

104. Archegonia in the Prothallium of Finns, . ,, 135

THE PLANT CELL.

INTRODUCTION.

In order to study the life-phenomena of any organism, and to
arrive at a definite explanation of them, it is often found necessary
to enquire into its minute structure ; and, in the case of the
plant, the study of the cell, including its form, growth, component
parts, and the varied conditions under which it can exist, is an
essential part of the science of Botany, as a branch of Biology.
The facts and features brought to light by the microscope, coupled
with those pertaining to Chemistry and Physics, have long afforded
proofs of statements and observations which were formerly only
regarded in the light of speculation.

In the following pages the object will be to give a con-
cise and correct idea of the principal structural elements of
plant-tissues ; stress has been laid upon practical microscopical
observations and reactions with various reagents, for these,
although generally simple to perform, sometimes afford a very
clear demonstration of important life-factors. A brief account
has also been given of the most important chemical and physical
phenomena occurring in a cell.

Cells; Types met with in the Plant. — From a purely biological
standpoint a cell or protoplast is defined as “a mass of protoplasm,
sometimes with and sometimes without a definite limiting mem-
brane, having situated in its substance (except in a few cases) a
nucleus and often accessory portions, such as plastids, chloroplasts,
vacuoles, and food-granules of various kinds.” The cases between
which this definition distinguishes are : —

i. The Amoeboid Cell (plasmodia). — Here the protoplasm is not,
during at least the greater part of its existence, limited by any

THE PLANT CELL.

firm cell-boundary, but is motile and creeps about, and also in-
gests food by means of protrusions (pseudopodia) pushed out from
its clear outer portion, or ectoplasm (see Chap. i.).

ii. Cells possessing Definite Cell-walls during their whole
Existence. — Here a firm limiting membrane is present, and the
protoplasm although capable of moving within these limits,
cannot move freely from one position to another. Further on, it
will nevertheless be seen that “ pits ’’ or perforations may exist in
the cell-wall, by means of which the protoplasmic contents of
adjacent cells are put into communication with one another ; and
at times the protoplasm may pass slowly through these “ pits ”
so as to vacate one cell-cavity for another, leaving its former
casing quite empty.

iii. Motile cells, often possessing no differentiated cell-wall, but
the outermost portion of the protoplasm is much firmer than the
inner portion, thus forming a more or less resistant boundary.
These cells have one or more protrusions of the firm outer
protoplasm (ectoplasm), known as cilia, which are active in
producing movements of translation or rotation (swarmspores).

We shall notice more especially the second of these sub-
divisions.

Tissues and their Arrangement : Function : Classification of
Plants according to Evolution. — Before proceeding to the detailed
study of the various cells of which a plant is made up, it is
necessary to examine, briefly, the manner in which cells are
grouped into tissues, and the nomenclature, general arrange-
ment, and function of these as they occur in such organs of a
plant as stem, root, and leaf ; and it will also be convenient to
have an outline of the main groups and subdivisions into which
the vegetable kingdom is divided, from the point of view of
evolution.

In lower plants, such as the Algae, there are often found cells
living as single organisms during their whole existence, and yet
others are joined together so as to form a colony, such as a
filament, or a flat, or round mass of cells, which live together
forming what is known as a cell - community. Occasionally
plants of a low order are observed, which are to all external
appearances somewhat highly differentiated (Fucus, Laminaria),
but which, nevertheless, when their internal structure is examined,
are found to be of comparatively simple organisation.

INTRODUCTION.

On passing upwards through the Fungi to the Liverworts and
Mosses, and so on to the Higher Plants, it is noticed first, that
the external configuration of a plant becomes, as a rule, more
complex, there being a subdivision of the whole into various
organs ; and secondly, that this subdivision coincides with a
correspondingly complex internal organisation ; it is found, in
fact, that where in the lower plants all the vital functions take
place in the one or perhaps a very few cells, in the higher plants
a division of labour obtains whereby separate functions are
relegated to as many separate tissues.

In the higher plants in which well-marked organs, such as
stem, leaf, and root, are found, it will be seen that the cells
composing these organs may be grouped into tissues, which have
the following nomenclature and general arrangement from with-
out inwards ; —

>^tem or Root.

External Tissues : (a) Epidermis.

{h) Cortex;* at times continuous with (r).

Internal Tissues : (c) A general fundamental tissue in which
lie the vascular bundles, or,

{d) A well-marked central cylinder (bounded by a tissue
known as endodermis), which comprises the vascular system,
and the pith or medulla.

Besides these main tissues, there occur in various positions in
the cortex, fundamental tissue, or central cylinder, certain other
tissues, which, as a rule, have a supporting or protective function ;
such are cork, collenchyma, and sclerenchyma, the occurrence
and features of which will be examined in due course (Chap. iii.).

In leaf -structures, the tissue arrangement may be of two
kinds — viz., {a) the bifacial, or (h) the centric. In the former is
found externally the epidermis on both upper and under surfaces
of the leaf, and internally a tissue known as mesophyll, in
which lie vascular elements and at times other tissues of a sub-
sidiary nature. In the latter or centric type there is present a
general arrangement not unlike that found in the stem of the same
plant — viz., externally, epidermis and mesophyll; internally,
a central cylinder in which are to be seen the vascular bundles.

*The cortex is only external with regard to the central cylinder. It is,
however, convenient to deal with it among the external tissues.

THE PLANT CELL.

(The leaves of Pinus and Hakea belong to the centric type.) In
some leaf-structures, such as those of mosses, a much simpler
arrangement obtains, the leaf being, perhaps, only two or three
cells thick, and the vascular system quite rudimentary.

The subdivision of the tissues of stem or root-structures in
plants into epidermis, cortex, and central cylinder, occurs typically
in the Dicotyledons and Coniferae, and these tissues are set apart
early in the young stem or root; in the Monocotyledons and the
Higher Pteridophyta (ferns) is found the arrangement noted
in (c) (see supra) — viz., an external epidermis, and internally a
ground-tissue in which lie several separate vascular bundles, no
well-marked central cylinder existing, although in the young
shoot a central cylinder may be detected.

In plants below the Pteridophyta the main grouping of tissues,
into external and internal may often hold good, but the differentia-
tion is not so marked as it is in the higher types, and, finally,,
when the Fungi and Algae are considered, the vascular tissue&
cease to exist per se, and the plant becomes a structure known a&
a Thallus (Thallophyta) the component tissues of w’hich conform
to one or at most a few simple types, and are not always to
be differentiated into internal and external groups.

With regard to the general nomenclature of tissues, those
in which the component cells have equal, or nearly equal, dimen-
sions whichever way they are measured, are termed paren-
chyma; whilst those where the cells have an elongated shape,
one dimension being possibly ten or twenty times the other,,
are known as prosenchyma. Amongst the latter are scleren-
chyma, bast fibres, cambial elements, and elements of the-
xylem and phloem, all of which will be examined in detail
(Chaps, iv. and v.).

The functions of the cells in the various tissues will be to a
certain extent studied together with their structural' details, but
broadly speaking it may be here stated that the following
tissues — viz., epidermis, cortex, and the mesophyll of leaves —
function in assimilation, transpiration, and elaboration of food
materials, the epidermis being also often protective in nature;
the cork, collenchyma and sclerenchyma are mainly protective,
and confer elasticity and rigidity upon an organ in which they
are present ; whilst the wood and bast confer rigidity, and are
essentially concerned in the conduction of sap (the phloem

INTRODUCTION.

possessing the special functions of conducting and storing
the constituents of elaborated sap).

There are, moreover, certain isolated tissues* in plants, such
as glands and resin-canals, which have special functions, and these
will be examined in due course (Chap. vi.).

In lower plants, such as the A\gx, all functions, — viz., assimila-
tion, respiration, nutrition as a whole, and reproduction — may
be carried on in the one or perhaps the few cells of which the plant
is made up ; and thus, the division of labour which obtains in a
plant composed of many tissues, is absent in the lower forms.

In higher plants the function of reproducing the species is
relegated to Avell-marked special organs, and the processes occur-
ring in these will be examined in detail in Chapter ix. ; certain
well-defined types being selected for this purpose.

In Chapter vii. ; the phenomena involved in the production of
fresh cells from pre-existing ones (cell-division) will be gone
into, and, finally, in Chapter x. the physiology and chemistry of
the cell will be considered.

An outline of the main groups and subdivisions into which
the vegetable kingdom is divided will be found of use for
purposes of reference, although it is not here intended to deal
with botany from the point of view of classification.

Such an outline as the following will indicate the main genea-
logical relationships of members of the plant kingdom : —

Not isolated in the strict sense of the term (see Chap. vi.).

THE PLANT CELL.

CRYPTOGAMIA (non-flowering plants)

Non-Vaseular Cryptogams

Vascular Cryptogams

Thallophyta Bryophyta

Algee Fungi Hepaticse Musci

Pteridophyta

Heterosporous

types

= Hy drop ter idete
Selaginellece
Isoete?e

Honiosporous
types
= Filices
Equisetineae
Lycopodiaceoe

PHANEROGAMIA (flowering plants)

Angiospermse Gymnospermse

Monocotyledons Dicotyledons Conifer® Cycade®

Gnetace®

The arrows show the order in which the table should be read;
it indicates that the line of evolution of the Higher Plants has
been by way of the Heterosporous Pteridophyta and the Cycadese,
certain fossil types probably intervening.

The dotted line shows tlie homological connection between the
Heterosporous Pteridophyta and the Gymnosperms (the subject of
homology will be referred to in detail at the end of Chap. ix.).
The chief variations in the structure, etc., of the cell, will be
found amongst the Higher Plants — viz., higher ferns, Monocoty-
ledons, Dicotyledons, and Gymnosperms (Coniferje), and these will
be the main groups used in dealing with jdant histology.

CHAPTER I.

THE NATURE AND REACTIONS OF PROTOPLASM.

The vital or working substance in every living cell is the
protoplasm, a material which has a very complex chemical
and physical composition and constitution. Resolved into its
elementary components, dead or “ fixed ” protoplasm may be
said to be made up of Carbon, Hydrogen, Oxygen, Nitrogen,
and Sulphur, and, in the case of the nucleus, in addition,
Phosphorus ; these elements are united in certain definite
proportions and aggregated into complex molecules or groups of
molecules. Certain mineral substances are also always found
in close connection with the protoplasm, but not, however, in
chemical combination (metaplasm). On the other hand, living
protoplasm has probably a very different constitution as compared
with the dead substance, and since it has been found impossible
to correctly analyse the living material, its true composition still
remains hypothetical; but chemists have from time to time
constructed formulae which have been assumed to represent the
composition of dead protoplasm, and which have shown it to
be made up mainly of a combination of proteid, amine, and
carbohydrate molecules.

On examining a young living cell microscopically, the proto-
plasm appears as a nearly transparent substance, with here and
there highly refractive granules ; in the middle of the cell is the
nucleus, a specialised portion of the protoplasm, and sometimes
there are one or more vacuoles, or fluid-filled spaces, which
resemble oil-drops in appearance (see Fig. 1).

It is probable, as will be seen later, that in many plant-cells the
protoplasm is made up of two main portions — viz., a firmer,
clearer external part, known as ectoplasm, and a more granular
fluid inner part, known as endoplasm ; in cells “ fixed ” and

* Termed by Huxley “ the physical basis of life” {Methodj and ResvUs).

THE PLANT CELL.

stained in a special manner this distinction can sometimes be
made out, but in the living cell it is not easy to do so, except in
such cases as Aethalium ^ or Amoeba (an animal organism not
unlike Aethalium), where the demarcation is very distinct (see
Fig. 2). In such a cell, which is a naked mass of protoplasm,
the ectoplasm is capable of responding to stimuli, protrusions
known as pseudopodia being pushed out in all directions; it is

G F

Fig. 1. — A Single Cell from a Root-Tip, fixed, and stained to
SHOW THE VARIOUS PARTS. — A, Cell Wall ; B, protoplasm, here
granular, owing to coagulation, and partly to the presence of
microsomata ; C, vacuoles filled with cell sap ; D, the nucleus : the
clear part just outside the nuclear membrane may be taken to
represent the kinoplasm ; E, chromatin particles arranged upon a
network of linin, the latter being faintly represented ; F, nucleoli
fplasmosomes) ; G, the centrosomes (probably absent in higher plants).

probable, however, that it is the endoplasm which receives the
stimulus, which, after it has passed into the cell by way of

* One of the Myxomycetes.

THE NATURE AND REACTIONS OF PROTOPLASM.

the ectoplasm, causes the more fluid internal part to push out
the ectoplasm, as it were. Ciliary action may possibly be
explained on this hypothesis ; and in the case of the absorptive
cells (root-hairs) of roots the ectoplasm is able to exercise a

1 2

Fig. 2. — A3iOiBA PRINCEPS. — The figures show demarcation into clear
ectoplasm and granular endoplasm, and change in shape of an
organism at intervals of one minute. The nucleus and the “con
tractile vacuole” (an excretory structure) are seen in the endoplasm.

marked selective capacity over the absorption of food materials
(salts in solution in the soil).

THE PLANT CELL.

Apart from these two main portions, the protoplasm has been
supposed to have a somewhat complex physical constitution';
some cytologists produce evidence to show that it has a spongy
basis, or spongioplasm, which is firm in consistency, and forms
a sort of network, in the meshes of which a more fluid portion,
or hyaloplasm, exists. On the other hand, Blitschli supposes
that it possesses a foam-like structure not unlike that seen in
emulsions of clove-oil, bicarbonate of soda, and water ; latterly,
however, the idea has been gaining ground that living protoplasm
has a quite homogeneous constitution, as careful investigators have
failed to detect any special structural basis in it,* whatever may
have been observed in preparations of fixed” and stained
protoplasm.

The chemical composition of living protoplasm is also, as was
stated above, somewhat hypothetical; but one fact is well-
established — viz., that the living substance always contains a
certain amount of water of constitution. Protoplasm, even of
the driest seeds capable of germination, contains this combined
water, and once it is removed, either by desiccation or treatment
with dehydrating agents, death occurs owing to its extraction.

There is one property of living protoplasm which completely
characterises this substance — viz., its capacity of responding to
stimuli, whether these be mechanical, chemical, or produced by
light, heat, gravity, or electricity ; a comprehensive term for this
property is “ irritability,” and, as instances of its possession by
the living substance, may be cited the following : — Aethalium,
a mass of naked motile protoplasm, when subjected to power-
ful illumination withdraws to a position where the light is less
intense; and Amceha, a similar, although, correctly speaking,
animal organism, draws in its pseudopodia at once if a harmful
stimulus, such as that produced by a crystal of sodium chloride
in its vicinity, is brought to bear upon it.

Fundamentally, there is no essential difference between proto-
plasm which is “naked,” as in Aethalium, and that which is
enclosed within a cell- wall ; in the latter case the living substance
may be, and often is, endowed with the power of movement
round the enclosing membrane, and light, heat, electricity, and
other physical and chemical agencies are found to produce
measurable effects when brought to bear upon it.

* Wilson, The Cell in Inheritance and Development.

THE NATURE AND REACTIONS OF PROTOPLASM.

With regard to the influence of heat, it has been determined
that a certain temperature, which varies for different cells, is
required, in order that the protoplasm may carry on to the best
of its ability the complex processes involved in the manufacture
of nitrogenous and carbohydrate food; and it is a well-established
fact that chloroplasts in the cells of green parts of plants are
markedly affected b}' light (see Chap, x.), these chloroplasts being
in the main protoplasmic in nature {i.e., specialised portions of
the protoplasm).

Other physical agencies, such as gravity and moisture, have
a powerful “directive action’^ upon the protoplasm of cells of
the growing-point of roots and stems; while certain chemical
substances (enzymes and malic acid) have a marked influence in
causing the attraction of swarmspores and the growth of "pollen-
tubes. This attraction is known as “positive chemotaxis.”
AVith regard to the movement of the protoplasm round a cell
(so-called “streaming” or “rotation,” see Chap, ii.), Hofmeister
regarded this as depending upon variations in the absorptive
capacity for water shown by the living substance at different
points of a cell ; in this case also, it is necessary to take into
account the influence of temperature, and possibly" differences in
electrical potential at various points in a cell. The phenomena
of surface tension may, however, account for some of these
protoplasmic movements.

Experimenting upon the vitality of seeds, one investigator
discovered that those capable of germination were, when stimu-
lated by an electric current, also capable of producing a so-called
“ blaze-reaction ’’ — viz., an electric response-current in a definite
direction when included in circuit with a sensitive galvanometer
— and he showed that this current was evidence of the vitality
of the protoplasm of the seeds experimented upon. The reaction
was in all probability due to chemical changes set up by stimula-
tion in the living substance, of the nature of slow oxidations,
giving rise to changes in electrical potential. In the above
experiments it was found, moreover, that if the “ water of consti-
tution ” in the protoplasm were first of all removed by drying at
high temperatures, or alcohol, no blaze-reaction resulted, pointing
definitely to the fact that this loosely combined water was essen-
tial to the maintenance of vitalit}'. Another factor which is also

* Prof. Waller.

THE PLANT CELL.

essential to the continued activity of protoplasm in plant-cells
is the presence of oxygen, either as a gas or in a compound,
whereby, just as in the animal cell, the protoplasm is oxidised,
giving rise to the evolution of heat ; many of the bye-products
formed in cells are the result of oxidation processes, whereby,
finally, complex compounds are broken down into carbon dioxide
and water (see also Chap x.). In addition to the water of
constitution mentioned above, protoplasm requires an extra
supply of water (in which certain essential salts are dissolved) for
vital processes, and this it derives from the soil, air, or water
surrounding the cells of a plant; and here a very important
point arises — viz., the" question of “turgidity,” — by which term
is meant an equilibrium between the sap inside a cell and the
fluids outside, this balance being known physically as osmotic
equilibrium. Turgidity has been shown to favour growth, and
it is a common experience that slack or withering parts of a
plant soon cease to live (see Plasmolysis, Chap. ii.).

To recapitulate then, it may be said that the following con-
ditions are necessary to the continued activity of protoplasm ; —

(a) A certain temperature, which, in most plants, is something
above zero Centigrade.

{h) Access to moisture.

{d) A requisite degree of turgidity in the case of an enclosed
protoplast, and, in addition,

(e) Protoplasmic continuity in the case of a cell-community
between the living cells of the same plant. This factor
is important, and will be considered more fully later ; and
if) The presence of certain assimilable food-materials and
mineral salts (see Chap. x.).

yote. — Protoplasm is soluble in dilute eaustic potash and also in
solutions of sodium or potassium hypochlorite : the nucleus also being
dissolved. The living substance (cytoplasm) is also dissolved by solutions
of pepsin or trypsin ; the nucleus (chromatin) resists pepsin, but dissolves
in trypsin solution. At a certain temperature (between 70° and 80° C.)
protoplasm passes into a condition known as “heat-rigor,” when all
functions cease, the living substance being killed (coagulation).

CHAPTER II.

THE STUDY OF A LIVING ASSIMILATING CELL.

A. The fully Differentiated Assimilating Cell.

Before passing on to the consideration of the various modifica-
tions which are met with in plant cells, it is advisable to examine
a typical living cell in which some of the more well-defined vital
processes may be easily demonstrated. Such cells are to be
found in the green assimilating tissues of plants, such as the
mesophyll of leaves, and the outer part of the cortex of herb-
aceous stems.

Vallisneria spiralis, a water-plant, affords very good material to
work with in this respect, as the cells of the leaf of this plant are
typical assimilating cells, the term assimilation being understood
in its true botanical sense, as, for example, in the taking in of
carbon dioxide and water, and the elaboration of these into
carbon-compounds in the chlorophyll bodies, oxygen being evolved
during the process.

If a leaf of Vallisneria be taken, and a small portion of it
mounted in water and examined under the half-inch power of
the microscope, the following details may be made out by focuss-
ing into various planes : —

i. The outermost layer of the leaf, composed of elongated cells rect-
angular in shape, and forming the epidermis.

ii. Internally as regards these, somewhat elongated cells rounded off
at the angles : it is with these cells for the most part that the leaf carries
on the process of assimilation.

iii. Smaller cubical cells, which occur near the edges of the leaf.

Using a higher power of the microscope (^" objective) it is
possible to distinguish in any of these cells (i. or ii.) the follow-
ing features (see Fig. 3) : —

(a) The cell-wall, a delicate membrane enclosing the other parts of
the cell or cell-contents.

THE PLANT CELL.

Fig. 3. — a, Two cells from the leaf of Vallisneria : the left-hand one shows
cell-walls and the peripheral layer of protoplasm in which are seen the
nucleus and chloroplasts. The arrows show the direction of rotation
of the protoplasm ; the central clear space is the “central vacuole.”
The right-hand cell shows the younger cell and chloroplasts containing
granules of starch in their interior in process of formation. &, Forma-
tion of starch-granules in chloroplasts in a cell of Begonia leaf,
c, Formation of starch-granules in chloroplasts of a cell of Vallisneria
leaf.

THE ASSIMILATING CELL.

(6) A layer of protoplasm, lining the inner surface of the cell-wall.*

(d) Numerous oval green ehloroplasts, also lying embedded in the
protoplasm.

(e) The central vacuole, filled with cell-sap, enclosed by the
protoplasmic sac ; in the smaller cells several vacuoles may be present.

These several parts should now be examined in detail; and
for this purpose it is as well to use a small “ stop ” on the sub-
stage diaphragm of the microscope (or on the iris-diaphragm
often fitted to the condenser) in order to cut off the peripheral
illuminating rays, and thus obtain a very much sharper definition
of each object examined.

With these precautions it is possible to make out that the
cell-wall is a delicate membrane of a transparent homogeneous
material : in this case it is not always possible to make out that
the boundary-wall of adjacent cells is in reality double, unless
very careful focussing is made, but that this is so will be readily
seen in many other tissues which will be examined further on.

The inner edge of the layer of protoplasm is now more
distinct, and the protoplasm itself is seen to be composed of a
clear substance in which are suspended the ehloroplasts, and some
small granules, these latter being either of a protoplasmic
nature t or food-granules (starch, etc.).

Lying in the protoplasm, and, as a rule, close to the cell-wall,
is the nucleus, an ellipsoidal body with a centrally situated
round spot, the nucleolus ; the main substance of the nucleus in
the living cell appears to be nearly homogeneous, but certain
reagents, such as acetic acid, show up a distinct reticulum, and
some stains, notably safranin and haematoxylin bring out other
features, which will be examined in Chap. viii.

By the time these observations have been completed, there
will probably have occurred a phenomenon which first appears in
the more internal rectangular cells. If closely watched the
protoplasm of some of these cells will be seen to be moving
slowly round the cell, carrying with it granules, nucleus and
ehloroplasts. This movement is known as “ rotation ” or
“ streaming,” and up to a certain point its rate increases with
the temperature ; it is the endoplasm which really moves, the

* This layer is the “primordial utricle {primordialschlauch) of von

Mohl.

t So-called “ microsomata.”

THE PLANT CELL.

ectoplasm forming a very delicate firmer layer next the cell-wall,
which certainly moves slowly, but not so fast as the more fluid
endoplasm. It is, however, hardly possible to distinguish opti-
cally between ectoplasm and endoplasm in the living cells of
Vallisneria leaf ; but in root-hairs these two portions may be made
out as distinct from one another, when the protoplasm is observed
under a high power.

The chloroplasts are most conveniently examined in the
smaller cubical cells near the edge of the leaf. Each chloroplast
is ellipsoidal in shape, small when compared with the nucleus,
and of a light greenish-yellow colour, the latter being due to the
presence of chlorophyll, a pigment which permeates the substance
of the chloroplast; the ground-substance of each chloroplast is,
however, protoplasmic in nature.

The effect of certain simple reagents upon the living cell must
next be studied ; and for this purpose it is usual to employ : —

(а) A solution of acetiC acid in water, 20 per cent, strength.

(б) A solution of iodine in a dilute solution of potassium iodide, until
the whole is of a dark sherry-red colour.

{d) Iodine solution, followed by a drop of concentrated SUlphUPiC
acid.

The reaction of the cell and its parts to these reagents will
now be described.

{a) Acetic acid, 20 per cent, solution in water, will, if added
(one drop under the cover-slip of the preparation) to the water
in which the portion of Vallisneria leaf is mounted, produce the
following effects : —

i. The protoplasm shrinks away from the walls of the cell observed,
and retracts towards the middle of the cell-cavity, strands or “ bridles ”
of protoplasm being observed which at first connect the main mass with
the cell-wall.

ii. The whole cell will shrink somewhat, the w'alls becoming convex
inwards.

iii. The nucleus takes on after a short time a punctate appearance,
probably due to coagulation of certain substances in its interior.

The retraction of the protoplasm is known as “ plasmolysis,”
and is dependent upon the disturbance of the osmotic equilibrium

* The exact quantities are as follows : — 0‘2 grm. iodine added to a
solution of 70 c.c. cone, zinc chloride and 10 grms. of potassium iodide.

THE ASSIMILATING CELL.

of the cell, whereby water is extracted from the cell-sap contained
in the central vacuole ; the substance causing this disturbance,
here acetic acid, is known as the plasmolyte. The reaction
shows that the protoplasm lines the cell-wall in the form of a
sac, which encloses the central vacuole ; concentrated solutions
of any salt (for instance, sodium chloride) act as plasmolytes,
the osmotic balance being so delicate that any but the most
dilute solutions will upset this balance causing plasmolysis.
Certain solutions of a definite strength and known as isotonic
solutions do not cause plasmolysis (see section on Osmosis,
Chap. X.).

(b) Iodine solution added to a fresh preparation causes at first
a partial plasmolysis, which, however, does not obscure the
following important effects ; —

i. A darkening of some of the granules in the protoplasm ; these are
starch-granules fully formed.

ii. A darkening of portions of the chloroplasts, this being due to the
effect of the iodine upon granules of reserve Starch undergoing formation
in the substance of these bodies.

iii. The nucleus and nucleolus are coloured brown (reaction for proteid).

(c‘) Schulze's solution, added to a fresh preparation, acts first upon
the cell-wall, which turns blue ; the other effects noticed are
similar to those of {li), except that the starch-granules turn a
somewhat brilliant blue colour in contradistinction to the rather
deeper blue caused by iodine solution alone.

(d) Iodine solution followed by a drop of pure sulphuric acid turns
the cell-'wall blue. This reaction shows that the cell-wall, more
especially that of the young cell, is composed of cellulose ; the
first action of Schulze’s solution shows the same thing. Pure
sulphuric acid alone will cause the protoplasm to assume at first
a pink colour (when sugar is present) owing to its action
upon the sugar, furfuraldehyde being produced. Cellulose is
dissolved by strong sulphuric acid with the formation of
derirose.

The presence of granules of starch in the interior of the
chloroplasts, a point brought out by reaction {b), indicates that
these bodies are active starch manufacturers and storers. In
Valllsneria and Begonia leaves all stages in the production of starch-
granules may be traced in the chloroplasts, from the minutest
particle shown up by the iodine solution, to the fully-formed

THE PLANT CELL.

granule, where only the thinnest film of the substance of the
chloroplast remains (see Fig. 3).

In Begonia leaf (cells of the mesophyll) starch-granules are
formed at first in the interior of chloroplasts, but subsequent
growth proceeds at the side of these structures ; in Vallisneria, on
the other hand, the granules are seen to be centrally situated
from beginning to end. Moreover, even in the apparently fully-
formed grains, a delicate film of chloroplast substance is always
to be detected, stretched over the grain.

In this formation of starch in the chloroplasts of cells from
the green parts of plants is to be found a partial demonstration
of assimilation ; for a chloroplast is able, by means of its chloro-
phyll, to utilise during the daytime certain of the rays of white
light falling upon the leaves, these rays being turned to account
in the decomposition of the carbon dioxide which enters the cells
after having gained admission through certain pores (stomata)
existing in the epidermis (photosynthesis). In the substance of
the chloroplast certain somewhat complex chemical reactions take
place which result in the formation of carbon compounds, such as
starch, sugar, or cellulose from the carbon dioxide and water
supplied ; and in this process oxygen is evolved and passes out
again through the stomata.

The whole process above described is, correctly speaking, only
part of the assimilatory reactions taking place in the cell ; for, as
will be pointed out more fully later on, nitrogenous substances
are also elaborated and assimilated in the leaf-cells, and the
materials resulting from this elaboration (amido-acids) are made
use of by the protoplasm in the complex processes involved in
formation of fresh protoplasm and nutrition of the cell as a whole.
Nevertheless, this preliminary study of the assimilation of carbon
dioxide and water in the chloroplasts, with the optical demonstra-
tion of the final result — viz., formation of starch granules — is a
useful introduction to the investigation of other and possibly
more complex vital processes taking place in the cell (see Chap. x.).

B. The Young Undifferentiated Cell.

A cell, such as the assimilating cell of Vallisneria leaf, does not
present the same features throughout its whole existence — viz.,
peripheral protoplasm, chloroplasts, and the phenomenon of

THE ASSIMILATING CELL.

“ rotation.” In fact, it is only in the adult cell that these are to
be seen. The cells of a very young leaf or a rudimentary organ
of any kind present very different features ; in the first place, the
protoplasm almost entirely fills the cell-cavity, and the nucleus is
situated in the geometrical centre of the cell. Moreover, chloro-
plasts do not, as a rule, appear as such in the cells of organs
which will ultimately become green until those organs have been
exposed to light (there are a few exceptions to this statement —
e.(j., the seed leaves of Finns and the green layer in the cortex of
stems just internal to the cork), but are replaced by structures
known as plastids or leucoplasts, which are, so to speak, chloro-
plasts in which as yet no chlorophyll has been formed (see Fig. 4).

Fig. 4. — Youn'c Cells from a Root-tif. — The cytoplasm fills the cell-
cavity, and the nucleus is a relatively large structure. The small
oval l)odies are plastids.

The protoplasm of such a young cell does not show the
streaming ” movement seen in some older cells, and the cell-sap
is small in amount, does not at first form vacuoles in the proto-
})lasm, but exists in it somewhat as water does in the meshes of
a sponge. The cell-wall is very thin, and gives the characteristic
^‘blue” reaction for cellulose when treated with iodine and
sulphuric acid ; acetic acid will cause a shrinking away of the
protoplasm from the wall, but not to the same extent as in older
cells, and the nucleus will, under these conditions, show the
punctate appearance before mentioned. The plastids are turned

THE PLANT CELL

Fig. 5. — Young Cells from the developing Endosperm of Caltha
palustris. — The cells have been recently dividing, and the nuclei
show numerous nucleoli. The cytoplasm has shrunk away from the
cell-wall somewhat.

Fig. 6. — A SLIGHTLY OLDER CeLL THAN THOSE OF FiG. 5 FROM THK
Endosperm of Caltha. — Vacuoles have formed and the protoplasm
has been thrown into “bridles” passing from a central mass in which
lies the nucleus to the peripheral layer.

THE ASSIMILATING CELL.

brown by iodine solution, since no starch has as yet been formed
in them; at times, however, starch may be formed in them by
a different process to that which takes place in chloroplasts, the
available energy for this being derived not from light rays, but
some other source.

The general shape of the young cell is in section often oval, or,
if there is much lateral pressure due to other cells, polyhedral ;
thus, if the pressures are equal in every direction, and the cells
of equal size, the geometrical shape of a cell is that of the regular

Fig. Ga. — A Photomicrograph showing Young Cells ob’ the Endosperm
OF CaJ.tha paluatris. — Bridles of protoplasm are to be seen passing
between adjacent cells.

dodecahedron ; but, as a rule, the pressures are not always equal,
and since the cells are not always of the same size, the shapes met
with are often irregular (see Fig. 5).

As growth proceeds, the cell-sap which exists in the meshes of
the protoplasm gradually collects into vacuoles, this being due to
the relatively unequal growth in volume of the cell-cavity and in

THE PLANT CELL.

mass of the protoplasm, the former preponderating ; at this stage
the nucleus is usually embedded in a central mass of protoplasm,
whilst “bridles” of varying breadth pass from this mass to
a layer of protoplasm lining the cell-wall internally. In still
older cells the protoplasm forms a layer lining the wall, and
encloses a central vacuole, the nucleus lying somewhere in this
peripheral layer.

Starting from the young undifferentiated cell as the simplest
type many subsequent modifications are to be found, and in the
following pages the main object will be to study in detail the
changes in structure, size, and function which occur in cells of
different parts of plants, according to the position they occupy
and the conditions brought to bear upon them.

Note. — The permanent microscopical preparation of the young cell is
readily carried out by first “fixing” a root-tip or other embryonic tissue
in. Flemming’s solution (see note at end of Chap, viii.), washing, after
fixing, in distilled water for some hours, and then hardening in alcohol,
and transferring to methylated spirit ; sections, either longitudinal or
transverse, should then be made from this, and these stained with
hematoxylin (Delafield’s) and fuchsin, using the stains in dilute solution,
and staining with each separately. The section is then dehydrated with
alcohol and spirit, cleared with clove-oil, and mounted in xylol balsam
(Canada balsam thinned with a little xylol). Very beautiful preparations
may be made by this method.

CHAPTER III.

CELLS OF THE EXTERNAL TISSUES AND CERTAIN
SUPPORTING AND PROTECTIVE TISSUES IN PLANTS.

A. CELLS ARISING FROM THE DERMATOGEN.

1. The Epidermis and Structures in connection with it.

The epidermis forms the outermost layer of cells occurring in
such of the higher, and also lower, plants as possess differentiated
organs ; the layer forms, as a rule, a protective covering to the
more delicate tissues beneath, and, moreover, is intimately con-
cerned in the function of transpiration and the admission and
means of exit of the gases of respiration and assimilation,
matters which will be examined more fully when the stomata
are studied.

Fig. 7.— The Dekmatooen from the Apex of a Bud.— The outer layer
of young cells represents the derniatogen, the deeper cells belonging
to the “ peribleni.”

Every epidermal cell is at first, like all young cells, a thin-
walled undifferentiated structure; the developing epidermis is
l>est examined in thin longitudinal sections taken through the
apex of a young shoot of Abies or Pinus. In such a section the
following features may be noted (see Figs. 7 and 34) : —

(a) An outer layer of small cubical cells, filled w'ith protoplasm, and
possessing relatively large nuclei.

THE PLANT CELL.

(6) A tissue composed of more oval or polyhedral cells with large nuclei
lying internal to {a) ; this is the periblem, from which arise laterally the
cortex and mesophyll of leaves.

Fig. 8.— Portion of a Transverse Section of the Leaf of Ficus
elaslica. — c, Cuticle ; I, lamellae of the outer walls ; e, epidermal cells;
jO, “Palisade” parenchyma. (In this case the epidermis is three-
layered, a somewhat uncommon occurrence.)

The outermost layer of cells {a) is the dermatogen, and from it
the epidermis is derived. Every cell of this layer is capable of
dividing, and fresh cell-walls are formed during these divisions
at right angles to the surface of the bud. The layer thus, as
a rule, remains only one cell thick, since no walls parallel to the
surface (tangential walls) are formed. An exception to this is
seen in Ficus elastica (leaf), where the epidermis is three-layered.
The dermatogen cells are, however, soon modified, so as to form

Fig. 9.— Epidermal Cells from the Leaf of Hippuris, showing the per-
sistence in them of cytoplasm and nucleus and the presence of
chloroplasts.

permanent epidermal cells, which may or may not possess proto-
plasmic contents; in the majority of instances these latter are

OUTER CELLS AND TISSUES.

absent, but in a few cases, such as Hippiiris and Vallisneria, they
possess not only protoplasm and nucleus, but also chloroplasts or
plastids (see Fig. 9).

The walls of the cells forming the dermatogen are composed
of unaltered cellulose.* When, however, the permanent stage is
reached, the outer walls no longer consist of pure cellulose, but
are considerably modified with regard to their chemical com-
position. In fact, the external wall becomes often greatly
thickened, and, in addition, the outermost layers of the external
wall become converted into a substance known as cutin (see
Fig. 10), which, when certain reagents are added, may be made

Fig. 10. — Portion of a Transverse Section of the Leaf of Piniis
sylvestris. — c, Cuticle ; e, epidermal cells, tlie m alls being made up of
two distinct layers; hy, hypodermis ; m, cells of the mesophyll
containing chloroplasts.

to swell and separate from the wall. It will be noticed that it
is, in general, only the outer wall which becomes modified in this
manner ; the side and internal walls are, as a matter of fact,
often thickened, but not to such an extent as the external one.
But just internal to the epidermis there occurs in some stems
and leaves (Pinus) a layer of cells known as the hypodermis, the
component elements of which possess very thick walls which

* In which a form known as pectose occurs in large amount.

THE PLANT CELL.

make up for any deficiency in strength of the epidermis (see
Figs. 8 and 10).

In surface mev: epidermal cells present a variety of shapes;
thus they may be rectangular, polyhedral, or sinuous in contour
(see Fig. 11). In all cases, however, a regular pattern is
preserved, the component cells fitting close so as to leave no
intercellular spaces, except where stomata occur.

In section, some epidermal cells may show minute perforations
or “ pits ” in their inner walls. These pits have been functional
in permitting of the passage of the protoplasm from the epidermis
into the deeper cells just internal to it when the work of the
living substance has been completed ; they may be seen in the
epidermal cells of Smilax.

Fig. 11. — Epidermal Cells of Sedum, seen in surface view.

Note. — Epidermis maybe studied in any of the higher plants. Great
thickening of the outer wall may be seen in the epidermal cells of
the Holly leaf, and of Viacum album, leaf of Pinus sylvestris, and Ficus
elastica. The cuticle may be caused to separate by the use of caustic
potash ; and by the use of Schulze’s solution, the part of the wall which
still remains unaltered cellulose may be distinguished from the rest.

2. Structures to be observed in connection with the Epidermis.

These are : —

(a) Stomata (occurring in leaves, petioles, petals, and some stems).
\h) Hairs, of varied shape, size, and function.

{a) Stomata are apertures or intercellular spaces occurring at
certain points in the epidermis, which permit of the passage of
the gases of the atmosphere into spaces surrounded by the

OUTER CELLS AND TISSUES.

mesophyll cells of a leaf, or cortical cells of a stem ; they also
allow of the exit of aqueous vapour during transpiration, a most
important function, and also of oxygen during assimilation.

A single stoma arises by the division of a young epidermal cell
into two, and these separate slightl}- along the line of junction
known as the middle lamella, leaving an opening which leads
into the afore-mentioned space (see Fig. 15, a). The walls of
these cells become greatly thickened, but the cell-contents persist ;
and a certain amount of apparent subsidence may take place, as
in Pinus, so that ultimately the cells, which are known guard-
cells, come to lie somewhat below the general level of the
epidermis (see Fig. 15, h).

Fig. 12. — A yTOMA FROM THE Leaf OF Smilax, seen in surface view. The
two crescentic guard-cells possess cytoplasmic contents and chloroplasts.

Fig. 13. — A Stoma fro:u the Leaf of Iris, in surface view (from a
photomicrograph).

The primary cells may divide more than once, the last division
of all resulting in the formation of guard-cells ; the first-formed
cells are then termed ‘’subsidiary.” Subsidiary cells are well
seen in the leaf of Secliim (see Figs. 12 to 14). In surface view
guard-cells are usually crescentic in shape.

A section across a stoma will show the following features : —

i. An outer passage, the “vestibule,” bounded, as a rule, by epidermal
cells, or at times by subsidiary cells. Tlie guard-cells lie at the inner end
of the vestibule, and are very close together, leaving only a very narrow
entrance into

THE PLANT CELL.

ii. The respiratory cavity, which lies deeper than the guard-cells,
and is surrounded by the thin-walled cells of the mesophyll, or, in the case
of herbaceous stems, by the outermost cortical cells (see Fig. 15, h).

From this preliminary examination of the structure of a
stoma, it is possible to deduce its function. If the mesophyll
cells* of a leaf are studied, it will be found that they conform in
structural characters to the type of thin-walled assimilating-cell
which was examined in Chapter i. In each cell there is seen a
layer of peripheral protoplasm, in which are suspended chloro-
plasts and nucleus ; moreover, a large amount of watery cell-sap
is present in the central vacuole, and during the daytime aqueous
vapour is being constantly given off through the thin walls into

Fig. 14. — A Stoma from the Leaf of Sedum, showing the subsidiary cells
(1,2, and 3 show the order of formation of the walls of these latter).

the respiratory cavity of the stoma. This process is known as
transpiration, and a current, the transpiration current, is kept up
by the evaporation of moisture through the stomata, so that
water is drawn up from the stem and root to replace that
evaporated from the mesophyll cells. Transpiration is readily
demonstrated by placing a leafy plant under a bell-jar, in the
sunlight, when the moisture evaporated through the stomata will
condense upon the inner surface of the bell-jar. (For further
details of transpiration see Chap. x.).

In some plants there are contrivances (hairs) in connection

* Sometimes called the spongy parenchyma.

OUTER CELLS AND TISSUES.

with stomata whereby transpiration may be regulated, in order
to cope with such conditions as draught ; otherwise certain plants
would wither in a few hours. Stomata are usually more
numerous on the under surface of a leaf than on the upper
aspect, and in some leaves may be greatly reduced in number, in
order to prevent excessive loss of water (leaves of plants in the
Canary Islands, belonging to the genus Cactus).''^

Fig. 15.— a, The formation of a Stoma (two stages) in the Leaf of
Pt'umis laurocerasus. b, A Stoma from the Leaf of Pinus, seen in
section. — v, Vestibule ; c, epidermal-cells ; .7, guard-cells ; Ay, hypo-
dermis ; rpc, respiratory cavity ; c, mesopliyll cells.

The stomata have, however, another and most important
function — viz., that of admitting the gases of the atmosphere

* The stomata also undergo certain changes whereby the aperture, or
stoma proper, is closed at times, by variations in the turgidity of the
guard-cells ; this occurs at night time.

THE PLANT CELL.

to the mesophyll cells, generally in a state of solution in aqueous
vapour, these gases (COg and O.^) being required for purposes of
respiration and assimilation ; and besides admitting these gases,
the stomata also permit of exit to the gases produced and
evolved during respiration and assimilation — viz,, COg and Og
respectively. During the daytime, whilst light is impinging on

Fig. 15a. — Thansverse Section of a Bifacial Leaf (semi-diagram-
matic).— e, Epidermis of upper and under surfaces; c, cuticle;
St, stoma; b, vestibule of stoma; </, guard-cells; r.c, respiratory
cavity ; p.p, palisade parenchemya ; s.p, spongy parenchyma ;
i, intercellular spaces. A section of the leaf-trace (bundle) is seen
in the spongy parenchyma.

a plant, transpiration and assimilation proceed to the greatest
extent in the green parts of the plant, respiration being over-

OUTER CELLS AND TISSUES.

shadowed by the former. At night, however, respiration is more
apparent, whereas transpiration and assimilation of CO.2 are at a
minimum, although growth as a whole is probably going on at
an increased rate.

Some of the experiments demonstrating these vital processes
will be described in detail in Chapter x. ; but it was necessary
to make brief mention of them here, since it seems more rational
to study the function of a given structure, or cell, in connection
with its histological details.

Note. — Stomata are best studied by examining thin strips or sections
of the epidermis of such leaves as Holly, Piniis, Hakea, and Iris. Guard-
cells may be stained with methyl-green, which picks out these cells to the
exclusion of others. Subsidiary cells are seen in the epidermis of Sedum
leaf, and developing stomata in the young leaf of Priinns iaurocerasus
(see Fig. 1.5, a).

Fig. 16. — Simple Hairs from the Petiole of Rhododendron (Leaf).

{h) Hairs are structures which arise from epidermal cells, and
are either simple or compound ; thus they may be : —

i. Simple un branched or branched unicellular hairs.

ii. Multicellular hairs.

iii. Secretory hairs, which are often multicellular, but at times
unicellular.

Simple unbranched hairs may occur on leaves, petioles, or
bud-scales. They originate from epidermal cells, the outer walls
of which are pushed out at an early stage, the protoplasm flowing
into the protrusion. Soon, however, in most cases, the proto-
plasm leaves the hair and passes into the deeper cells, after its
work has been done in connection with the growth of the hair
(see Fig. 16). Some simple hairs retain their protoplasmic
contents throughout their whole existence, as, for instance,

THE PLANT CELL.

root-hairs (trichomes), where the ectoplasm forms a distinct layer
which exercises a marked selective capacity over the absorption
of salts in the soil (see Fig. 21) ; but the function of most simple
hairs is in the main one of protection either from excessive cold,
heat, or mechanical injury. Occasionally a large number of hairs
are aggregated together to form one variety of emergence ;* and
in the case of the long felt-like hairs which cover buds (Hazel
and Alder), these are mainly useful in protecting the latter from
the effects of frost. Simple hairs may at times be branched
(stellate hairs).

Fig. 17. — A Compound Hair from Rhododendron — at the base are six
small cells. The small projections on the wall of the large upper cell
are composed of carbonate of lime.

Multicellular hairs are those which, having retained their
protoplasm, divide and form several cells lying in one or more
planes (see Figs. 1 7 and 1 9) ; such hairs may be stellate, sickle-
shaped, or shield-shaped. Stellate hairs may, however, be a
variety of branched unicellular hair; and in the case of the
long simple hairs, a wall may arise which cuts off the elongated
portion from the original epidermal cell (see Fig. 16).

* These emergences may also have cells from the deeper layers in their
structure ; some of them are glandular and possess an internal secretory
layer.

OUTER CELLS AND TISSUES.

Secretory hairs are occasionally composed of one or, at most,
a few cells, the apical one of vvhich forms the glandular portion
(Pelargonium).

In the leaf of Pelargonium (see Fig. 18) the hair consists of
three distinct cells — viz., a basal cell and two upper ones — the
apical one being spheroidal in shape and possessing protoplasmic
contents which manufacture a sticky secretion. Such a hair is
known as a capitate glandular hair, and occasionally these hairs
serve as organs of absorption for ammonia and nitric acid
existing in the atmosphere."'

Another type of secretory hair is seen in the stinging nettle
(Urtica urens) ; each hair is here an elongated cell which arises
from an epidermal cell of the stem or leaf, having a broad base
.surrounded by a cup-shaped receptacle formed by a large number

Fig. 18. — A Capitate Glandular Hair from the Leaf of
Pelargonium.

of small cells which have been produced by the divisions, in an
early stage, of adjacent epidermal cells. The whole hair tapers
towards the apex, which is extremely delicate and surmounted
by a small knob; internally are seen protoplasm and nucleus.
Formic acid (strictly speaking, an excretion, and not a secretion)
is formed in the hair, and it is this substance which produces the
stinging sensation and rash when the fine broken apex of the
hair penetrates the skin (see Fig. 20). The hair of the nettle is
thus seen to be mainly protective in function.

In Rhododendron secretory hairs arise which are composed of
many cells, each of these possessing protoplasm, and secreting
a sticky substance (see Fig. 19).

* Kerner and Oliver, Natural History of Plants, vol. i.

34 THE PLANT CELL.

Fig. 19.— A Compound Glandular Hair from Rhododendron.
small projections on the surface are globules of oily secretion.

Fig. 20. — A Glandular Hair of the Nettle {Urtica urens).-
broad base of the hair is embedded in a cushion of small cells.

The

-The

OUTER CEIvLS AND TISSUPJS.

THE PLANT CELL.

B. CELLS ARISING FROM THE PERIBLEM.

1. The Cortex.

The cortex will be described here under the external tissues,
but, strictly speaking, it is only an external tissue when con-
sidered relatively to the central cylinder (where the latter is
present) ; nevertheless, it is convenient to class it with the
external tissues. Cortical cells are formed by the subsequent
growth and modification of cells of the periblem — viz., that
layer which is just internal to the epidermis in the young shoot.
Each adult cell of the cortex is, in most cases, a typical assimilat-
ing cell, with the exception that cortical cells in roots do not
contain chloroplasts, but plastids. As a rule, the shape of a
cortical cell is oval, or more often rectangular or polyhedral in
section, and such a cell would be termed parenchymatous, since
no diameter is much in excess of the others.

In stems which possess a well-marked central cylinder the
cortex extends radially from the epidermis (or hypodermis) to
the endoderrnis or starch-sheath. In herbaceous stems all the
cells may possess chloroplasts, but where the layer is of any
extent only the outermost cells possess chlorophyll. At times
a well-defined layer of cells possessing chlorophyll is met with
at the outer margin of the cortex, this being known as the
phelloderm; but it is formed from a tissue known as cork-
cambium, and, as such, will be examined later.

2. Cells of the Mesophyll of Leaves.

The mesophyll in leaves is that tissue which exists between
the epidermis of the upper and under surfaces, in the case of the
bifacial leaf, and in the centric type is the mass of cells which
intervenes between the epidermis and the central cylinder.
A layer of columnar cells known as palisade parenchyma is often
present between the true mesophyll or spongy parenchyma and
the epidermis of the upper surface of a bifacial leaf, and these
palisade cells are characterised by the presence of large numbers
of chloroplasts (see Fig. 1 5a). Each cell of the spongy paren-
chyma is thin-walled and possesses protoplasm and chloroplasts,
and is chiefly concerned in the processes of transpiration, and

OUTER CELLS AND TISSUES.

the assimilation of carbon dioxide (during the daytime) ; but
the palisade cells are far more powerful than those of the spongy
parenchyma as assimilators of carbon dioxide, and this by reason
of the large amount of chlorophyll they possess.

In the centric leaf of Pinus each cell of the spongy parenchyma
has curious infoldings of the cell-wall, which, in the adult cell,
are known as trabeculae. These increase the available transpir-
ing surface of the cell (see Fig. 10, m).

In bifacial leaves the palisade cells are arranged in groups
which converge by their bases on to single cells of the spongy
parenchyma, knovui as collecting cells ; this arrangement facili-
tates the diffusion of sugar formed in the palisade parenchyma
into the other cells of the mesophyll (see also Chap. x.).

C. CELLS OF CERTAIN SUPPORTING AND PROTECTIVE TISSUES
OCCURRING IN PLANTS IN VARIOUS POSITIONS.

Under this heading will be described : —

(a) Cork.

(fe) Collenchyma.

These tissues are found in varied positions in the stem, root,
or leaf of higher plants, chiefly the Dicotyledons, Monocotyledons,
Coniferae, and higher Ferns ; and in all cases their function is to
confer elasticity and rigidity,* and act as a means of protection
to more delicate tissues.

{a) Cork is found in the form of layers of varying thickness in
the stem or root ; in the latter it is produced from a zone of
actively dividing cells known as the pericycle, which occur just
internal to the endodermis in those roots which possess a well-
marked central cylinder.

Cork-cells may arise from epidermal-cells, in stems, and in this
case are cut off in the first instance from the inner portions of
these cells ; but. as a rule, the first division going to produce the
cork-forming layer occurs in the first layer of cortical cells just
l>elow the epidermis.

If a longitudinal section be taken of a young stem of Sambucus

* Rigidity in .succulent plants is greatly aided by turgidity of the
living cells.

THE PLANT CELL.

the cork is distinguished as a layer of cells some five or six deep,
lying just internal to the epidermis ; one or two lines of these
cells may be seen to possess protoplasmic contents, the outer cells
being empty and often pressed together (see Figs. 22 and 23).

Fig. 22. — A Portion of a Longitudinal Section through the Young
vStem of Sa77ih2iais to show the Cork.— Cork-cells; x, cork-
cambium ; ph, phelloderm.

Fig. 23. — Older Cork-Cells from the Potato Tuiser.— /■, Compound
cork-cells being cast off.

The cells containing protoplasm are known collectively as the
cork-cambium* or phellogen, and, strictly speaking, should come

* A similar layer occurs in leaf-petioles at the time of separation of the
leaf in the autumn ; it is known as the absciss-layer, and separation takes
place along the middle lamellae.

OUTER CELLS AND TISSUES.

under the heading of meristem, to be considered later. The
cork-cambium produces on its outer aspect fresh cork-cells, and
on its inner aspect, at times, a layer of cells possessing chloro-
plasts, known as the phelloderm (see supra). In some plants —
e.g., Quercus sessiliflora — several separate zones of cork may be
found at different depths in the cortex. Cork forms, at times,
a la}"er of considerable thickness, which affords no mean protec-
tion to the cortical tissues of the stem^; the walls of the freshly-
formed cork-cells are composed of pure cellulose and pectose, but
they soon become toughened and rendered more elastic by the
deposit in them of a substance known as suberin. Older cells of

Fig. ‘24. — Foktion of a Transvkkse Section of the Young Stem of
Finns to snow the formation of Cork. — e, Epidermis ; a, cuticle ;
hy, liypcKlermis ; sc, cork-camViium ; c, cortical cells. ,

the cork contain air only, so that cork-tissue is of low specific
gravity and very elastic in nature. In some stems openings are
formed in the “ bark,” through which the more superficial of the
cork-cells are continually shed ; these apertures are known as
lenticels, and are caused by the thinning of the epidermis at
certain points and subsequent rupture, leading to an aperture
which is of value in admitting air into the intercellular spaces of
the cortex.

THE PLANT CELL.

(6) Collenchyma. — This tissue usually occurs just internal to
the epidermis, in such stems as that of Cucurhita ; the cells com-
posing it are in reality the outermost cortical cells, at the angles

of junction of which the inter-
cellular substance becomes con-
verted into a material, highly
refractile in appearance, which
when dry is not unlike dried
mucilage. On the addition of
water or caustic potash solution
it swells up to many times its
original bulk, and it may be
stained with methylene blue
(see Fig. 26). Collenchyma
confers elasticity upon the

Fig. k-CoKTicAL Cells FROM THE «"ter layers of the cortex,
Young Stem OF Pi/iws.—w;, Cell- and, as a protective layer
wall ; k, chloroplasts lying in the against mechanical shock, must
^ of great service to the plant.

(c) Sclerenchyma. — In vari-
ous parts of a plant there occur elongated fibres, massed
together into bundles or zones of greater or lesser extent;
they are to be found in the outer parts of the rhizome

Fig. 26. — Collenchyma from the Stem of Cucurhita (transverse
section). — On treatment with dilute caustic potash the intercellular
substance at the angles of the cells swells up to twice or three
times its original volume.

or petioles of Ferns, and also surrounding the vascular
bundles in these plants. Each fibre arises at an early stage from

OUTKR (JELLS AND TISSUES.

elongated cells (prosenchyma), the walls of which become greatly
thickened (sclerised), and at times deeply pigmented, usually
a brown colour; as soon as the thickening is completed the
protoplasm leaves the cell-cavity and passes through “ pits ” in
the walls into other cells. A transverse section across a patch
of sclerenchyma shows rather irregular rounded or polyhedral
elements with very thick walls, the latter being perforated here
and there by narrow “pits” joining the cavities of adjacent
fibres. The walls also show concentric striations, pointing to
the fact that the various thickening layers have been laid down
at different times {cf. growth of the cell- wall by accretion,
Chap. iv.). On treatment with iodine solution the fibres stain
a yellowish-brown. In longitudinal sections of stems or roots the
fibres are seen to be elongated fusiform elements, not composed,
as would appear, of single cells, but of several which have united
end to end, the intermediate end-walls becoming absorbed.
The fibres join one another obliquely, and have tapering ends.

Note. — Sclerenchyma may be studied in transverse and longitudinal
sections of the stems of Zea mais, Finns, and the rhizome or petiole of
PteiHs aquilina. Tilia and Euphorbia stems are also good. In Zea mais
the fibres are arranged round the fibro-vascular bundles; the “pits” in
the walls are well seen in transverse sections of the leaf of Sansevieria and
of the stem of Eupho'rbia. In the stem of Finns the sclerised elements are
the bast-fibres ; they are of an oval, flattened shape in transverse section.

Fig. 27. — A Small Patch of Sclerenchyma from the Leaf of Sanse-
riena, seen in Transverse Section.— Note the laminated structure
of the walls of the fibres and the “pits” connecting their cavities.
The large cells surrounding the patch are cells of the mesophyll.

C H A P T E R I y .

MERISTEM.

The various organs and adult tissues of which a plant is made up
arise from young undifferentiated tissue which occurs in certain
positions, notably at the apices of young shoots and roots, and
in the form of zones of dividing cells in stems and roots, and
at times in other positions ; this rudimentary tissue is known a§
meristem, and the first few cells of this tissue as promeristem,
A meristematic tissue from which the primary tissues arise
(viz., that producing the first wood and bast, and the epidermis,
pith, and cortex) is known as a primary meristem, whereas that
arising from previously differentiated cells (viz., that producing
cork, secondary wood and bast) is known as secondary meristem.
Amongst the latter would be classed the various cambiums met
with (cambium proper, cork-cambium, pericycle) and certain
layers known as intercalary meristem, which arise in such organs
as young leaves towards the base, and which, in this case,
function in the transverse growth of the leaf.

Meristem may thus be defined as “ a tissue which, during
some part of the existence of a plant, is, as regards its component
cells, either in a condition of active cell-formation or else remains
capable of renewed activity after periods of quiescence.'’ The
cambiums may be described as zonal meristem.

A. DIVIDING CELLS OF ANY RUDIMENTARY TISSUE AND
THEIR MODE OF GROWTH.

Cells of embryonic tissues are in structural details similar to
the type of young undifferentiated cell which was examined in
Chapter ii., B., where it was seen that protoplasm almost filling
the cell-cavity, large nucleus, cell-sap, and plastids were the main
cell-contents.

In a tissue where rapid cell-formation is in progress (meristem),

MKKISTEM.

It is possible to detect here and there cells in which typical divisioii'-
tigures (mitosis) can be made out, especially where a thin section is
cut and stained as directed in the note at the end of Chapter ii.
(see Fig. 28). In these young cells the cell-walls are very thin,
and on account of tiirgidit}' are a good deal on the stretch ; the
polyhedral shape so often observed in the cells of young tissues
is due partly to mutual cohesion and j^cessure ; and, moreover, a
certain amount of intercellular matrix (which forms the middle-
lamella) is soon secreted which tends to make the cells cohere.
If the intercellular substance is dissolved by certain reagents,*
the cells may be made to separate from one another, and

Fig. *2S. — VorN(; Dividing; Cclls from a Rudimkntary Tissck. — In
one cell the nucleus is undergoing division (mitosis).

they then tend to resume the si)heroidal shape. The direc-
tions in which fresh cell-walls are formed is determined to a
certain extent by the directive action of the protoplasm, and
by tlie relative position of the cells in the young tissue. In
Imds or root-tips it is })ossible to make out two main modes
of wall-formation with regard to their direction in s[>ace, and
these are known by the terms synclinal and anticlinal. The
svncliual walls are formed more or less parallel to the external
contour of the bud, or the contour of the central c^dinder, whilst
anticlinal walls are those formed at riglit angles to these. The

* Scludze’s Macerating Mixture (see infra).

THE PLANT CELL.

general shape of the synclinal and anticlinal surfaces, when cut
by a plane passing through the longitudinal axis of the bud,
would thus be parabolic, the two sets of parabolae cutting one
another at right angles, and the foci of both sets of curves would
be within the area of the growing point of the shoot.

The contents of the young cells consist, as has been mentioned,
of protoplasm, nucleus, sometimes plastids, and cell-sap, in which
latter certain salts are held in solution. The plastids (when
present) manufacture starch, not quite in the same manner as the
chloroplasts, but from certain elaborated materials (sugar) brought
to the cells from the leaves, and from the starch thus built up the
protoplasm is able to manufacture cellulose, for the purpose of wall
formation. The production of cellulose is, however, not a simple
matter, since it has been shown that in the production of the cell-
plate, or partition wall dividing a cell into two during the later
phases of cell-division (see Chap, viii.), the protoplasm undergoes an
almost direct transformation into cellulose by the splitting off of its
carbohydrate molecule, the remaining proteid and amine portions
being then free to combine with carbohydrate derived from other
sources in the cell. Moreover, it has been found that manj^ stages
ordinarily exist between protoplasm and cellulose, and that starch
before it can be utilised must first be converted into dextrins and
sugar by the agency of enzymes, and it is probably this sugar
which is made use of by the protoplasm. In the latter process
oxidation possibly has a large share. The unlignified cell-wall
has a large amount of pectose in its composition, pectose having
the same generic formula as cellulose; the middle-lamella, in fact,
consists of calcium pectate.

Physically speaking, growth of the cell-wall takes place in two
Avays, viz. ; —

(a) Growth by intUSSUSCeption, fresh particles of cellulose being
intercalated between those already existing.

(h) Growth by accretion— fresh layers of cellulose are laid down
one after the other, .somewhat after the manner in which crystals increase
in size.

Both these processes are going on together in the cell ; growth
of the wall in surface-area being effected by intussusception,
whilst growth in thickness of the cell-wall proceeds by accretion.

In this connection it is necessary to give a feAV instances of
the secondary thickening of the cell-Avall by accretion. The

MEKISTEM.

walls of endosperm cells in some plants (Date, Sagm taedigem^
Phytelephas) become after a time enormously thick, the cell-
cavities being still connected by means of “ pits ” which traverse
the walls of adjacent cells. The thickening takes place mainly
by the deposition of layer after layer of cellulose, but, as a rule,
other substances are also deposited which confer upon the walls
great toughness {Phytelephas).

The cells of the pith of some plants {Hoya carnosa) have
extremely thick walls, through which pass “ pits,” usually simple
in nature. As a rule, however, adult pith-cells are thin-walled
(Sambucus) and contain nothing but air.

Epidermal cells often possess, as has been seen (see supra) very
thick outer walls {Viscum album, Holly), and at times layer after
layer can be distinguished ; in such cases treatment of the walls
with caustic potash usually results in a separation and swelling
of the cuticle, followed by a swelling of the layers of the outer
wall. The thickening of the walls of sclerenchymatous fibres
and wood elements also takes place mainly by accretion.

The wall of the young cell is not, however, devoid of inter-
stices ; indeed the fact that salt molecules of different sizes can
penetrate into the cell through the wall, points definitely to the
existence of such interstices. Naegeli looked upon the cell-wall
as being constituted somewhat as follows : —

i. The ultimate molecules (micellie) of cellulose have spaces between
them. Each micella is supposed to be surrounded by a watery envelope.

ii. These molecules are again grouped into larger particles (tagmata)
between which larger spaces exist. Thus a sort of complex meshwork is
produced, which permits of the passage of certain substances.

It is highly probable that some such structure is present in
the cell-wall of a young cell, and that molecules of salts can pass
through. In this connection, how^ever, the study of root-hairs
offers an exi>lanation of the absorption of salts into the interior
of the cell, which cannot be arrived at by simply considering the
structure of the cell-wall. It is, in fact, highly probable that the
ectoplasm lining the inner aspect of the wall of the root-hair
exercises a selective capacity upon the absorption of salts in
solution from the soil, some salts being admitted to the exclusion
of others ; and as in the root-hairs, so in the young thin-walled
cells of a rudimentary tissue, although in this case the materials
supplied to the cell are, as a rule, not raw, but elaborated, the mole-

THE PLANT CELL.

cules being larger than those of salts. Later on, during the life
of the cell, the walls are generally too thick to allow of the above-
mentioned process of absorption, and then the presence of “ pits,”
or perforations in the walls, becomes a factor of great importance
in the transference of food materials and water from cell to cell ;
and it has already been seen that the cytoplasm also passes
slowly from cell to cell by means of the same “ pits.”

yote. — Cells of young developing tissues (meristeni) may be studied in the
young endosperm of Caltha palnstris (see Fig, 5), or in sections of root-tips
or apices of stems. The same method of fixing, hardening, and staining may
be used as in the preparation of the young undifferentiated cell. (Note
at end of Chapter ii.). Caltha is the marsh marigold, and the endosperm
starts developing from the beginning to the middle of June, after the
petals have fallen, and the carpels have just started to ripen. Transverse
sections of the carpels will cut the ovules longitudinally, and a large
number of sections ma}^ be rapidly examined, the thinnest and best being
selected for mounting.

B. ZONAL MERISTEMATIC TISSUES.

Under this heading are included : —

i. The Cambium (stem and root).

ii. Cork-eambium (stems),

iii. Peri cycle (roots).

The second of these has been already examined under cork-
tissues. The pericycle will be examined in Chap. v.

In cambium wall-formation during cell-division takes place in
only two directions, generally speaking — viz., the radial and the
tangential directions in a stem or root; thus the walls produced
in such a tissue have always a fixed orientation, being either
situated along a radius, or perpendicular to radii of the organ in
which they occur. In rudimentary tissues other than cambium
it was pointed out above that the main directions of wall-forma-
tion were either synclinal or anticlinal with regard to certain
fixed planes in the bud, and that walls might be formed at times
in almost any direction in space. In the tissue now to be
studied, however, a marked regularity in the directions of wall-
formation is preserved. The planes in which walls are formed
are always parallel to either a fixed perpendicular, or transverse
plane in the organ.

- The statements made wdth regard to thickening and growth

MKRISTExM.

of the cell-wall and the absorption of food materials apply equally
to cambium as to other rapidly-dividing young tissues ; and it
will be seen that the change from the typical thin-walled cambial
cell to the ^modified elements met with in the wood and bast is
often a very rapid one.

Fig. 29. —Portion of a Transverse Section near the Apex of a Youni^
Shoot of Pinus. — e, Epidermis ; Ic^ periblem (rudimentary cortex) ;
.r, rudimentary cambium ; pxy, protoxylem ; pph, protophloem ;
7/t, medulla ; r, resin-canals.

Fig. 30. — Diagram illustrating the Arrangement of Primary and
Secondary Vascular Tissues in a Dicotyledonous (or Coniferous)
Stem. (A transverse section near the apex of a young stem.) —
e, Epidermis ; /, fundamental or ground-tissue ; pc, procambial
strands, inner parts protoxylem, outer parts protophloem ; x, meri-
stera zone (cambium) of a procambial strand ; ifc, dotted circle
indicating the position where the interfascicular cambium will arise.

TffE PLANT CELL.

I. The Cambium (found as a distinct layer of meristem in the
stem and root of Dicotyledons and Coniferse). — Before passing on
to the detailed description of the cambium, it is necessary to
examine briefly the arrangement of the tissues in the vascular
region of a dicotyledonous stem, together with the early origin
of the cambial layer, and its subsequent history.

The vascular region proper is that part which lies internal to
the endodermis or starch-sheath, a ring of cells which is found
immediately internal to the cortex in dicotyledonous or coniferous

Fig. 31. — A Transverse Suction across an older Stem. — e, Epidermis;
s, cork-layer ; c, cortex ; end, endodermis ; x, cambium ring (the line
points to an interfascicular portion, ifc) ; ph, phloem ring ; p.ph, rem-
nants of the original protophloem; 1, P, 1, 2, etc., the ring of xylem,
made up of xylem elements derived from the fascicular and inter-
fascicular cambium respectively; p.xy. protoxylem ; m, medulla or
pitli.

stems ; all the tissues internal to the endodermis are included in
the term “ central cylinder,”* and comprise, from without inwards,
the bast and phloem, the cambium, the wood, and, in the centre,
the pith or medulla (see Figs. 31, 32, 33, and 34). All the tissues,
however, contained in this central cylinder are not, in the true

* A central cylinder or “plerome” is also to be found in the young
monocotyledonous stem, but the primary cortex appears to merge into it,
there being no endodermis proper.

MERISTEM.

sense of the term, vascular — the vascular tissues proper being the
wood and soft-bast, and possibly the cambium — these tissues being
functional in the conduction of sap, raw and elaborated, to and
from the leaves respectively. The other tissues of the stem, such as
cortex, young pith, &c., derive their supply of elaborated sap
more by osmosis through the phloem than by direct conduction.

The term cambium is applied, in stems and roots of Dicoty-
ledons and Conifers, to a narrow zone of meristem situated
between the woody portions of the fibro-vascular bundles and

Fig. 32 (serai-diagrammatic). — A Tra.nsverse Section through a First
Year’s Stem of Pinus. — e, Epidermis; hy, hypodermis ; s, cork-
layer ; c, cortex ; r, resin-canals ; end, endodermis ; ph, phloem ;
X, cambium layers; ifc, interfascicular cambium; xy, xylem ; p.xy,
protoxylem.

that portion known as the phloem or bast ; it is functional in
producing on its inner aspect fresh elements of the wood or
xylem, and on the outer aspect fresh phloem elements. The
origin of the cambial layer can be traced back to an early period in
the growth of stem or root ; a transverse section, for example, just

THE PLANT CELL.

below the apex of a young shoot of a Dicotyledon (or Conifer)
will show, when examined under a low power of the microscope,
the following details : —

(а) A general fundamental or gPOUnd-tiSSUe.

(б) A few patches, circularly arranged, towards the centre of the
section, which are, in reality, sections across the rudimentary primary
vascular bundles, or, as they are sometimes called, the pPOCambial
strands (see Figs. 29, 31, and 32).

Fig. 33 (diagrammatic). — A Transverse Section through an Older
Stem op Pinua, showing the complete Ring of Wood and Bast. —
ky Cortex ; md, medullary rays (secondary, see infra) ; wi, pith.
Other letters the same as in Fig. 32.

Each primary vascular bundle is composed of three portions,
viz.: —

(i.) An inner part made up of a few embryonic wood-elements having
spiral or annular thickenings on their walls, and known as pPOtOXylem.

MERISTEM.

(ii. ) An outer part made up of thin-walled elements, the rudimentary
phloem, or ppotophloem.

(iii.) An intermediate part composed of thin- walled meristematic cells,
the rudimentary cambium of the primary vascular bundles (fascicular
cambium).

Further down the stem the primary bundles are differentiated
into typical xylem, cambium, and phloem, and between the
primary bundles it is found that certain cells of the ground-tissue
have remained or have subsequently become meristematic ; these

Fig. 34. — A Longitudinal Section through the Apex of a Young
Shoot (Dicotyledon or Pinus). — d, Dermatogen ; p, periblem ; pc,
procambial strands ; m, medulla ; b, bracts ; k, leaf-buds ; I, lateral
offshoots from procambial strands. The portion included between the
procambial strands (pc) is the “central cylinder.”

■cells, in fact, will give rise to intermediate patches of cambium,
the so-called interfascicular cambium. The interfascicular cam-
bium produces, in like manner to the cambium of the primary

THE PLANT CELL.

bundles, xylem upon its inner aspect and phloem upon its outer
aspect, but, as will be readily understood, there is no protoxylem
or protophloem to be seen in these portions, as they are
secondary fc«*mations (see Figs. 30, 31, and 32, ifc).

The fascicular and interfascicular cambium unite during the
first year’s growth, and thus is produced a complete ring of
meristem in stem (or root) which gives rise to fresh annual rings
of xylem and phloem (see Figs. 31, 32, 33). The whole process
is known as secondary thickening.

In roots, although the ultimate disposition of xylem, cambium,
and phloem is similar to that just described, the protoxylem and
protophloern alternate with one another, and are not situated
upon the same radial lines in the young root.

Fig. 35. — Diagram of a Transverse Section through a Young Mono^
COTYLEDONOUS Stem — 6, Epidermis ; /, fundamental tissue ; 5, fibro*
vascular bundles (black = xylem, dotted = phloem).

In the Monocotyledons and higher Ferns no persistent ring of
meristem analogous to the cambium of Dicotyledons exists, and
the fibro-vascular bundles are made up of xylem and phloem
formed early from certain rudimentary elements ; generally
speaking, in Monocotyledons, the phloem is found between the
arms of a V-shaped mass of xylem (see Fig. 35); whilst in the
higher Ferns the phloem surrounds a centrally situated mass of
xylem in each separate bundle. Thus the bundles of Monocoty-
ledons and Ferns are termed “closed” bundles, in contradistinc'
tion to those of Dicotyledons and Conifers, which are known as-

MERISTEM.

“open” bundles — viz., bundles capable of receiving fresh annual
rings of xylem and phloem by the activity of the cambial layer.
In a few instances, however {Dracaena), the stems of certain
Monocotyledons possess zones of meristem from which fresh
annual rings of fibro-vascular bundles are produced, these bundles
being, however, always of the closed variety ; and it may be here
mentioned that petioles of bifacial leaves in Dicotyledons possess
only scattered closed bundles, there being no cambial layer in

Fig. 36. — Portion of a Transverse Section through the Stem of
Ricinua communis. — x, Cambium ; ph, phloem ; xy, xylem (one large
vessel is seen amongst the tracheides) ; md, medullary ray.

each bundle, and thus no possibility of secondary thickening.
In centric leaves {Fhius), on the other hand, where a central
cylinder is present, there may be, for a short time, a narrow
zone of cambium between the xylem and phloem portions of
the fibro - vascular bundles. In some dicotyledonous stems
(Podophyllum peltatum) scattered vascular bundles occur instead
of a well-defined ring of wood and bast; this is known as
anomalous stem-structure. Other instances also occur.

widL

THE PLANT CELL.

For the purpose of studying the structural details of the
cambium both transverse and longitudinal sections should be
taken of the stems and roots selected. In transverse section the
layer has much the same appearance as the cork-cambium, each
cambial element having a somewhat flattened rectangular shape,

Fig. 37. —A Longitudinal Section through
THE Cambial Region of Vinca major
(stem). — cc a; ar, Cambial cells; <ph, young
sieve-tubes of the phloem ; j&r, phloem-
parenchyma ; xy-^, young wood elements
(pitted tracheides); xy^, older wood-
elements.

Fig. 38 (semi-diagram-
matic). — A Longi-
tudinal Section
through the Cam-
bial Region of
Pinus. — Cj, Cg, Cg,
Cambial elements ;
Cj remains active, Cg
and Cg going to form
xylem and phloem
elements respec-
tively.

and in careful preparations the protoplasm is seen to fill the cell-
cavity almost entirely (see Figs. 36 and 36a). In longitudinal
sections (see Figs. 37, 38, and 39) each cambial cell is observed

[To face p. 54.

3(ia. — A PnoTOMTCRooRAPir .sirowixo ttfe Camrial Region Xylem
and I’liEOE.M IN Transverse Section, from the Young Stem of
ScunhucHS. — X, Cambial cells; .s, sieve-tubes in the phloem; v, large
vessels (annular) in the xylem.

[To face p. 54.

Fig. — Photomickookaph showing a Transverse Section of a

Primary Bundle ( DicoU'leclon). Note the central cambium, the
phloem at the top, and the xylem at the bottom.

MERISTEM.

to be an elongated (prosenchymatous) element possessing granu-
lar protoplasm in which lies a very elongated fusiform nucleus.
The wall of the element is very thin at first, but a cell
which has just been formed on either side by the division of
a cambial element soon undergoes modification into a xylem or
a phloem element, the wall being then thickened and otherwise
altered (see Chap. v.). In dividing, there are usually only one

Fig. 39.— A Longitudinal Section in the Region of the Inter-
fascicular Cambium of the Stem of Pinus. — x, Cambial cells ;
y, young sieve-tubes ; h, bast-fibres ; ^2> yo^^ig wood-elements

(tracheides with “bordered pits”); m, medulla; mdi, ground-tissue
rays.

or two lines of cells forming active cambial elements. Supposing
(see Fig. 38) that there is one such line of cells, Cp and that this
line of cells has already produced the elements and Cg, then
it is found that the original line q remains active, Cg and Cg

THE PLANT CELL.

going to form permanent elements of the xylem and phloem.
Occasionally, however, several lines of cells may be active.

In the division of a cambial cell the nucleus probably divides
en masse (amitosis), and does not undergo mitotic division, a
process which would take too long a time for its completion.
The elongated fusiform shape of the nucleus is also further
evidence of its mass-division.

Note. — The cambium may be studied by taking transverse and longi-
tudinal sections of any quickly-growing dicotyledonous or coniferous stem
or root ; in some roots — e.gr., Horse-radish — the cambium may appear to
form a rather wide zone on account of the absence of any great amount
of thickening in the elements just cut off on either side. For staining
cambium, fuchsin and hsematoxylin are good stains to use, the protoplasm
being stained by the fuchsin and the nuclei by the logwood. The tissue
used for studying cambium should, if good preparations are required for
keeping and demonstration, be first fixed with Flemming’s solution or
2 per cent, solution of chromic acid. Suitable plants for studying this
layer are Ricinus (stem). Horse-radish (root), Piims (stem or root), and
Cucurhita (stem).

CHAPTER V.

THE VASCULAR TISSUES.

In describing the elements composing the conducting tissues of
plants it must be remembered that similar elements may occur
in all of the four great groups, Dicotyledons, Conifers, Mono-
cotyledons, and Pteridophyta ; but it is convenient, when
considering the vascular tissues, to take those groups in which
the greatest variety of conducting elements occur, and in this
respect, the Dicotyledons and Conifers afford much the widest
scope for investigation. Moreover, by doing this a more rational
sequence will be preserved, seeing that it has just been shown
in the preceding Chapter how the xylem and phloem arise in
Dicotyledons and Coniferae from the cambial layer ; and, in
addition, certain other important tissues occur in Dicotyledons and
Conifers, such as the medullary rays, endodermis, and pericycle,
which, although not strictly speaking vascular tissues, are neverthe-
less included in the central cylinder, and have important functions.'^
In some instances the other groups — viz,. Monocotyledons and
Ferns — possess conducting elements which are important to study,
and these will be incidentally described ; but, in the majority of
cases, it will be found that Dicotyledons and Coniferse possess in
their vascular system a sufficient variety of conducting-element
to enable the student to gain a very fair idea of the more
important of these.

Therefore, in the following description of the component
elements of the vascular tissues, the order of examination set
forth below will be found convenient : —

(A) The Phloem [produced by the cambium (Dicotyledons and
Coniferse) upon its outer aspect].

(B) The Xylem [produced by the cambium (Dicotyledons and
Conifene) upon its inner aspect].

* The endodermis and pericycle occur also in Monocotyledons and
Pteridophyta, both being present in the roots of either group ; and an
endodermis is to be found round each of the separate bundles in the
rhizomes of Pteridophyta.

THE PLANT CELL.

In addition, the endodermis, pericycle, and medulla or pith will
be briefly described as component tissues of the central cylinder.
It should, however, be remembered that in those plants which
do not possess a strictly limited central cylinder, or a well-defined
zone of cambium, phloem and xylem elements may be met with in
the so-called closed vascular bundles, similar in many respects to
those occurring in the xylem and the phloem of the more highly
differentiated groups — viz., Dicotyledons and Coniferse.

(A) The Phloem or Bast.

The cells produced by the cambium on its outer or cortical
aspect go to form a tissue consisting almost entirely of elements
known as sieve-tubes. The undifferentiated cells originating
from the cambium are at first quite thin-walled, but soon changes
take place which result in : —

(i. ) Thickening of the lateral and end- walls,

(ii.) The formation of special areas known as sieve-plates upon the
end -walls.

These sieve-plates are formed as follows : — thin areas are left in the
end-walls during the development of the sieve-tube, and after a time these
thin areas, which coincide with one another in adjacent end-walls, unite,
the intermediate middle-lamella becoming absorbed. The other portions
of the end -walls become much thickened ; and in some cases several such
sieve-areas may be present in the end-walls of tubes ( Tilia), the number
of actual perforations, or pits, which may be present in each sieve-area
being perhaps twenty, thirty, or more.

Sieve-tubes may be readily examined by taking transverse and
longitudinal sections of such a stem as that of Cucurhita. In
transverse section each sieve-tube is seen to be of a somewhat
irregular shape; lying just outside the cambial layer, and close
to the tube — being, in fact, cut off from the main cell — is
to be seen a smaller cell, known as the companion-cell, which
appears full of granular contents. In such a section the tubes
are usually recognised by their sieve-areas, which may be made
more evident by staining the section with eosin (see Fig. 40).

In longitudinal sections each sieve-tube is seen to be an
elongated element, with its narrow companion-cell lying next to
it along its whole extent. Towards the middle line of the tube
are to be seen the contracted protoplasmic contents — that is to
say, if ordinary spirit-preserved material is being used for the

THE VASCULAR TISSUES.

40.— a, Two sieve-tubes in transverse section, showing sieve-plates, s,
and companion-cells, c {Cucurbita). h, Portions of two adjacent sieve-
tubes seen in longitudinal section. Note the sieve-plate, its perfora-
tions being plugged by callose. The granular mass on the upper
surface of the sieve-plate is the callus. The cytoplasm is contracted
towards the centre of the tubes. c. Portions of two sieve-tubes
showing a pervious sieve-plate.

THE PLANT CELL.

examination. But in careful preparations made from material
fixed in a special manner, it will be found that the protoplasm
of each sieve-tube really lines the inner surface of the wall as a
thin peripheral layer, in which lie the nucleus and drops of
mucilage and food-granules, the central
space being occupied by a large vacuole
filled with cell-sap.

The companion - cell is filled with
granular protoplasm, and small “pits”
in the adjacent walls of tube and com-
panion-cell put the protoplasts of the two
elements into communication with one
another.

If iodine solution be added to a fresh
longitudinal section, certain granules in
the cytoplasm near the sieve-plates turn
brown, a reaction which points to the
presence of proteid. Globules of mucilage
are also to be seen in the mass of con-
tracted protoplasm near the sieve-plate.

Towards autumn, a mass of a substance
known as callose is formed on either
side of each sieve-area, the whole com-
pleted mass being the callus. It stains
yellow if treated with solution of aniline
sulphate, and bright red with eosin. The
callus is deposited by the agency of the
cytoplasm, and functions as an effectual
plug, which stops up the perforations in
the sieve-plate. In the spring of the
following year the callus becomes ab-
sorbed, and the sieve-tube becomes once
Fig. 41.— A COMPLETE more functional, but after two or three
Sieve-tube from Cu- years a given sieve-tube becomes obli-
plasm (contracted) ; terated, others having been formed in
sieve-plate ; c, callus ; the meantime.

X, companion-cell. Closely connected with the phloem is a

tissue which occurs typically in the leaves
of some plants, notably the centric leaf of Finns. This tissue
is known as transfusion - tissue, and its component cells are

THE VASCULAR TISSUES.

characterised by the presence in their walls of small bordered-
pits (for the structure of bordered-pits see pp. 67 and 68 on

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the xylem), and are known as transfusion-cells (see Figs. 4 1 a, 416),
The tissue lies outside the bundles, internal to the endodermis,.
and its communication with the phloem of each bundle may be

THE PLANT CELL.

readily traced (see Fig. 416). The function of this tissue is to
aid in the downward translocation of elaborated food-material,
which passes in from the mesophyll through the endodermis into
the transfusion-cell, and so into the phloem.*

The sieve-areas in the sieve-tube of Pinus are situated, not on
the end-wall, but laterally on the radial walls, so that this would
seem to facilitate the inward diffusion of elaborated sap from the
mesophyll into the phloem. In bifacial leaves elaborated sap
passes directly from the cells of the palisade layer and spongy
parenchyma into thin-walled phloem cells, situated on the under
side of the endings of the leaf-bundles.

In ultimate function sieve-tubes and their companion-cells act
in the translocation and storing of elaborated food-materials, and
in addition each tube is possibly concerned in the manufacture or
further elaboration of certain of these food-materials. The elabo-
rated sap from the mesophyll cells of the leaves finds its way into
the phloem of the leaf-traces, and so downwards by means of the
perforations in the sieve-plates. These perforations are large,
and through them large quantities of sap, food-granules, and cyto-
plasm can pass at a time. All the way down the stem and root
elaborated sap can, after being perhaps further changed in the sieve-
tubes and companion-cells, find its way by osmosis into the cortex
externally, and the cambium internally; and in the spring the
stored nitrogenous and carbohydrate food in the tubes is converted,
by means of enzymes, into soluble proteids and carbohydrates,
which pass out laterally by means of osmosis into the tissues
requiring fresh elaborated food for the purposes of growth and
general nutrition (see also Chap. x.).

Subsidiary Elements of the Phloem. — These are : —

{a) Phloem-parenchyma.

(6) Bast-fibres.

(a) The phloem-parenchyma is composed of small thin-walled
cells lying between the sieve-tubes, and possessing protoplasm and
reserve starch. Functionally these cells form a sort of supple-
mentary tissue to the sieve-tubes, and are useful in the storage
of carbohydrates.

* Transfusion-cells exist also in the endodermis of the roots of Iris ; and
in Pinus leaf the transfusion-cells on the xylem side of the bundles permit
•of the passage of water from the wood into the mesophyll.

THE VASCULAR TISSUES.

(b) The bast-fibres are situated outside the phloem proper,
and are individually elongated sclerenchymatous elements, which
form a layer of varying thickness; in the stem of Finns they
are oval and compressed when examined in transverse section,
and possess minute “ pits ” in their thick walls. Bast-fibres are
not formed as such, annually by the cambium, but result from
the modification of elements formed in previous years. Function-
ally they serve as a protective and supporting layer to the more
delicate phloem lying internal to them (see Fig.

B. The Xylem.

The elements formed by the cambium in Dicotyledons and
Conifers upon its inner aspect — viz., the rudimentary xylem —
are at first elongated thin-walled cells (prosenchyma), which,
liowever, soon undergo the following modifications : —

1. A general thickening and chemical change in the cell-wall,
known as lignification (deposit of lignin).

2. The production of localised apeas of thickening, the inter-
mediate portions remaining thin (thin wall-areas).

3. The thin wall-areas later on often became absorbed in adjacent
portions of cell-wall, leading to the formation of actual pepforations
or “pits.” At times, however, a thin partition remains unabsorbed,
this being usually formed by the middle lamella, which is, in reality,
an intercellular substance.

During these changes the cell undergoes an elongation, but, as
a rule, not much increase in its other dimensions. At times the
adjacent end-walls of elements may become absorbed, leading to
the formation of vessels of relatively great length; or the
elements may remain single, the end-walls persisting, when they
are known as tracheides.

The best method of classifying wood-elements is by means
of the various thickenings and “ pits ” occurring on their walls,
and in this manner it is possible to distinguish the following
varieties : —

(a) Tracheides or vessels with simple “ pits ” in their walls. —
The pits are at first only thin wall-areas, but subsequently the
middle lamella may become absorbed, leading to definite
perforations (see Fig. 42, a).

THE PLANT CELL.

(b) Tracheides or vessels with reticulate* thickenings on their
walls, the reticulate markings consisting of localised thickened
areas, the intermediate parts remaining thin (see Fig. 44, c).

(c) Elements possessing both the modifications (a) and (b) —
viz., reticulo-pitted vessels or tracheides (see Fig. 42, b) ; in this
case the pits occur in the areas enclosed by the reticulations, and
are often of the “ bordered ” variety (see next heading, d).

{d) Tracheides or vessels possessing “ bordered-pits.” — These
“ pits ” occur in two main varieties — viz., the simple bordered-pit,

a h

Fig. 42.— a, A Pitted Tr.^cheide
FROM Xylem of Quercus. h,
A Pitted and Reticulate
Tracheide {Quercus).

Fig. 43. — Portions of Annular
and Spiral Vessels from the
Protoxylkm of Dicotyledon-
ous Stems.

where an upraised thickened margin occurs round a simple
circular or oval pit, and the “ bordered-pit,” which is met with in
the tracheides of such a plant as Finns — almost to the exclusion
of other elements. These pits occur only on the radial walls of
tracheides, and have the following structure : —

i. In surface view each “pit” is circular in contour, the diameter
occupying almost the entire breadth of a tracheide. The central part, or

* The scalariform vessel met with in the xylem of Ferns is one form of
reticulate element (see Fig. 44, c).

THE VASCULAR TISSUES.

lumen of the pit, is surrounded by a raised thickened bOPdeP (see Figs.
47 and 48).

ii. In tangential sections of the stem of Pinus bordered-pits may
be studied in section. It is found that the raised borders coincide in
position on corresponding parts of the cell-wall in two adjacent tracheides

(see Fig. 48), and that these thickened
parts enclose between them a space,
the pit - chamber, into which the
lumina of the two halves lead. Stretch-
ing across this chamber is the middle
lamella, in the centre of which occurs
a thickening, fusiform in section, and
known as the tOPUS. By this arrange-
ment the lumina of the “pit” may

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Fig. 44.— a. Portion of a Reticulo-
piTTED Vessel (Bog-oak).

6, Portion of a Vessel with
Small Bordered Pits, different
in structure to those of Pinus
(Bog-oak).

c, Portion of a Scalariform
Vessel (Pteris).

Fig. 45. — A Pitted Vessel (re-
cently-formed).—e, End-walls
which become absorbed ; p,
pits in surface view ; p^, pits
in section.

THE PLANT CELL.

be closed at times, owing to the forcing up of the torus against either
of them.

The development of bordered-pits may be observed in longitudinal
sections of the young stem. The tracheides which have been just formed
by the cambium show thin circular wall-areas, round which the first
indication of the border soon appears (see Fig. 50).

(e) Tracheides or vessels with annular or
are to be seen in the protoxylem of Dico-
tyledons and Coniferae, and also in the xylem
of fibro-vascular bundles of Monocotyledons
and Ferns (see Fig. 43). The endings of the
leaf-bundles also show these elements.

(/) Wood-fibres are also seen at times,

spiral thickenings

Fig. 46. —A Wood Fibre. Fig. 47. — Details of Xylem Elements (Pwms).

— p, Small pits ; these — Tracheides with bordered-pits (p) on the

are slit-like, with a radial walls, tt, tracheides; /, lumen of

thickened border. pit ; c, tangential wall ; r, reticulations.

THE VASCULAR TISSUES.

although more rarely than the other above-mentioned elements.
They are formed by the junction end to end of several tracheides,
and they have thick walls in which occur slit -like pits surrounded
by a narrow thickened border (see Fig. 46).

(g) Lying amongst the other elements a few cells occur in
some stems, which are termed wood-parenchyma. Each cell is
thick-walled, with simple pits in the walls, and internally are to

Fig. 48. — Borde:red - pits
( p) IN Tangential Sec-
tion.—Tlie middle lamella
is seen in section, and the
torus appears as a central
thickening on this ; m,
medullary ray.

ocy^

Fig. 49. — Tracheides seen in Transverse
Section. — xyi, Young tracheides; xy2-, older
elements.

Fig. 50. — Portion of a Tracheide showing
Bordered-pits in course of Formation
( Pinus).

THE PLANT CELL.

be seen protoplasmic contents and starch. Wood-parenchyma
is, in fact, a tissue set apart for the supplementary storing of
carbohydrates, and is analogous to the phloem-parenchyma.

All these elements of the xylem, with the exception of the
wood-parenchyma, are functional in the upward conduction of
raw unelaborated sap from the root to the leaves, the moving
forces being the transpiration current and root-pressure (see
Chap. X.). The tracheides are especially useful in this process,
it being probable that the sap passes partly by means of the cell-
cavities and partly through the cell-walls. The various forms of
“ pits ” occurring in the walls may possibly be of use in sap-
conduction, but, as a matter of fact, these pits function more as
a means of exit for the protoplasm after it has finished its work
in the xylem-elements. Those elements of the central cylinder
of Dicotyledons and Conifers which now remain to be studied —
viz., the medullary rays, the endodermis, the pericycle, and the
medulla — are not conducting tissues, but are, nevertheless, of
importance from several points of view.

Note. — To facilitate the practical examination of the xylem those plants
in which the various elements ma}'^ be studied will now be mentioned. In
all cases both longitudinal and transverse sections should be made of the
stems or roots : —

Pitted and reticulate vessels and tracheides. Bog-oak, Hazel, RicinuSy
and Lime (stems of all these).

Tracheides with bordered-pits : Pimis (stem or root).

Scalariform (reticulate) vessels : Pteris and other Filicineae.

Spiral and annular elements : protoxylem of Dicotyledons and Coniferae,
and xylem of Monocotyledons and Ferns. Also the bundles in many leaves.

Wood-parenchyma : Hazel (stem).

In showing up wood-elements either solution of aniline sulphate or
iodine solution may be used. The latter reagent also shows up the endo-
dermis (starch-sheath) wood-parenchj^ma and medullary rays, since the
starch in the cells of these tissues turns dark blue.

In the examination of individual separate elements wood may be
macerated in Schulze^ s macerating mixture,* which dissolves the middle
lamellae, and the resulting mass teased out, washed in distilled water, and
the separate elements stained with methyl-green solution.

C. The Medullary Rays (see Figs. 52, 53, and 54).

Medullary rays are of two kinds, viz. : —

(a) Primary or gPOUnd-tiSSUe rayS.

{h) Secondary or tpue medullary rays.

* Dilute nitric acid (20 per cent.), to which 2 to 3 per cent, of a
saturated solution of chlorate of potash has been added.

[To face 'p. 68.

Fig. 51. — A PnoTOMICROCKAl’H OF PoRTIOX OF A TrANSVER.SK SECTION
ACROSS A Root of rhnn* to show tlie ti’aclieides of tlie xylem in
section. In the centre is seen the tri-nuliate protox}'lf‘in, and enclosed
l)etween the Y-shaped arms of this three large resin-canals.

THE VASCULAR TISSUES.

(a) Ground-tissue rays are those portions of the fundamental
or ground-tissue in the stem or root of Dicotyledons and Coniferse
which exist between the primary vascular bundles before the
interfascicular cambium has arisen. When the secondary bundles
are formed by the activity of this cambium the cells of these
columns of tissue become, at first, compressed, and, finally, almost

obliterated. The component cells of the rays are similar in
structure to those of the cortex or ground-tissue, and the
centrally-situated mass of fundamental tissue, which is at first

THE PLANT CELL.

connected to the cortex by means of the rays, becomes, later on,
the pith or medulla.

(h) True or ‘secondary medullary rays are, on the othei hand,
formed from special cells of the cambium, and stretch out from
this both' ways into the xylem and phloem. At times a ray may
pass right through both, so as to connect pith and cortex ; but,
as a rule, the rays end in the xylem and phloem (see Fig. 52).

the course of a medullary ray through
the xylem and phloem and into the
cortex. — X, Cambium ; xi/, xylem ;
p/ij, phloem ; ph.2, bast-fibres.

Fig. 54. — Medullary Ray
SEEx IN A Tangential
Longitudinal Section
OF A Stem of Pinus. —
t, Tracheides ; md, me-
dullary ray cells.

THE VASCULAR TISSUES.

In transverse sections of such a stem as Pinus (first or second
year’s growth) these rays may be seen as single lines of oval
elements which show up well by treating the section with iodine
or Schulze’s solutions, the reaction being due to the darkening
of the starch-granules in the component cells of the ray by the
reagent (see Fig. 53).

To examine the origin and relations of the true medullary
rays it is necessary to make longitudinal sections in directions
parallel to, and at right angles to, radii of the stem — viz., radial
longitudinal and tangential longitudinal sections.

In radial sections, which include the cambium, each ray is
seen to originate from certain cells of the cambial layer.
Usually more than one cambial cell is active, and often as many
as five or six may be the forerunners of the same number of
radial lines of ray-cells. As observed in such a section, the shape
of each component cell of the ray is rectangular, and numerous
simple pits may be detected in the rather thick cell-walls. The
contents of each cell consist of protoplasm and starch-granules
(see Fig. 52).

In tangential sections of the xylem, each ray appears as a
spindle-shaped perpendicular line of cells, ranged one above
the other (see Fig. 54). The number of cells in the tier
depends, of course, upon the relative position of the plane of
section.

In function the true medullary rays act as reservoirs of carbo-
hydrate food-material, being supplemented to a certain extent by
the w^ood-parenchyma. In the early spring, when the sap is
beginning its upward movement in the xylem, the starch in the
ray-cells is converted by the agency of the enzyme diastase into
dextrins and sugar, of great value to the cambium and young
xylem and phloem elements, before the sap has started to be
elaborated in the leaves in quantity sufficient for the needs of
the plant.

Note. — The true medullary rays are best studied in the stems of Pinus,
or the Lime. In the latter, the rays are, in transverse section, seen to be
very broad towards the cortical ends and narrow towards the pith, the
increased breadth at the outer extremity being due to the occurrence of
radial, as well as tangential, divisions in the component-cells of the ray.
Sections of stems should be treated with Schulze’s solution or iodine to
show up the rays.

THE PLANT CELL.

(6) Portion of the Endodermts of the Root of Iris.—e, Endodermis cell; c, cortical cells;
p, pericyclic fibres (sclerenchyma).

THE VASCULAR TISSUES.

Subsidiary tissues occurring in the Central Cylinder of Dicoty-
ledonous and Coniferous Stems and Roots.

These are : —

i. The endodermiS or Stareh-Sheath (occurring in both stem

and root).

ii. The pericycle (roots only). This tissue is meristematic.

iii. The medulla or pith, with its medullary sheath.

i. The endodermis, bundle-sheath, or starch-sheath is a ring
of cells, only one cell thick, which marks the limits of the central
cylinder in a stem or root of more than one year’s growth
(Dicotyledons, Coniferje).’^ Its component cells at times contain

Fig. 56. — Endodermis Cells in a Longitudinal Section of the young
Stem of Corylus avdlana (Hazel).— e, Endodermal cells with starch-
granules (stained with iodine) ; h, bast-fibres ; c, cortical cells.

starch-granules, and in many cases a distinctive feature is the
presence of peculiar fusiform thickenings on their radial Avails.
Functionally this layer forms a starch reservoir, but in many

* The separate bundles or schizosteles in tlie rhizomes of certain Pterido-
]»hyta (Pteris) are also surrounded by an endodermis.

• THE PLANT CELL.

stems and roots may be protective in nature, since the outer
walls are often very thick (see Figs. 55, 56). The endodermal
cells do not always contain starch, as the latter is being constantly
used up by the cambium, &c.

ii. The pericycle is a meristematic zone of cells occurring in
the root, just internal to the endodermis ; it may be several cells
in thickness, and from it are produced, (a) the secondary or
lateral roots, (b) a ring of cork, and (c) the pericyclic fibres;
the latter being elongated thick-walled sclerenchymatous fibres,
not unlike bast-fibres, and often taking the place of these. The
pericycle may consist of one layer of cells only, and may even be
absent altogether in some roots, and at times it may arise from
the outermost cells of the central cylinder.

iii. The medulla or pith is formed in stems and roots of more
than one year’s growth by the remains of the central groxmd-

Fig. 57.— Young Cells or the Rudimentary Pith of Lupulus humulus. —
a, Cell-wall ; p, cytoplasm ; nucleus.

tissue of the younger stem. Near the apex of the shoot the
cells of the centrally situated portion of the ground-tissue are
young undifferentiated elements possessing protoplasmic contents
(see Fig. 57). Older cells of the medulla are, except in some
succulent herbaceous stems, devoid of protoplasm, and contain
only air; and their walls are at times very thick, and per-
forated by numerous simple pits (see Figs. 58, 59). A
medullary sheath formed of somewhat rectangular thick-walled
elements, in the walls of which either simple or small bordered-
])its are sometimes seen, may occasionally be present. This so-
called sheath is only a slightly modified layer of the outermost
cells of the medulla.

TH^: VASCULAR TISSUES.

The function of the pith is to form a highly elastic tissue
which reduces the weight of the stem, and acts as a sort of
“ cushion ” to the central cylinder. At times, however, the pith-

Fig. 58. — Older Cells of the Pith of Corylus avellana. Note the
simple “pit” both in surface view and in section.

Fig. 59. — Pith-elements from the Stem of Vinca major. — a, Cell-
wall ; p, simple pits.

cells break down, leaving the centre of the stem hollow (Bambusa),
except at the “ nodes.” The pith is in no sense of the term a
conducting tissue.

THE PLANT CELL.

APPENDIX TO CHAPTER V.

Origin of the first Wood-Elements of the Proeambial Strands.

The origin of the first WOOd-elementS may be studied in longitudinal
sections of the apex of a young shoot {Pinus). Here they take the form
of fusiform cells, characterised by the presence of large oval thin wall-

Fig. 60. — Portion of a Longitudinal Section near the Apex of a
Young Shoot of Finns, to show the origin of the first wood-
elements. — c, Meristematic cells which soon undergo modification to
produce {x) the young wood cells (note the oval thin wall-area,
so-called “pits”); ph, rudimentary phloem elements (protophloem);
m, ground-tissue ; k, cortex (periblem).

THE VASCULAR TISSUES.

areas separated by bars of thickened wall ; these thickenings become,
further down, the annular and spiral bands of the elements of the
protoxylem (see Fig. 60). External to these fusiform cells are to be seen
somewhat elongated cells filled with protoplasm and with long spindle-
shaped nuclei. These cells form the rudimentary meristem and phloem
elements, which, further down the shoot, are differentiated into young
cambium and protophloem.

In this connection it is interesting to note that spiral vessels occur in
such plants as the Mosses and Liverworts. In the simple leaf of Funarior

Fig. 61. — Portion of a Moss-leaf, showing (in optical section) indica-
tions of a simple vascular system. — xy. Spiral vessel ; p, protective
cells just outside the vessels ; c, cells of the green assimilating tissue
of the leaf (mesophyll).

a few elements having the characteristics of the spiral vessels, with rather
broad thickening spirals, are to be seen towards the central axis of the
leaf (see Fig. 61). These are surrounded by a few elements of a thick-
wall^ nature, and outside these latter comes the green assimilating tissue
of the leaf. Nevertheless the Bryophyta are not included amongst the
vascular Cryptogams.

CHAPTEE VI.

ISOLATED TISSUES OR CELLS HAVING A SPECIFIC
FUNCTION.

The tissues and cells which will now be described are, so to
speak, only isolated in so far as they have special functions to
perform. It should, however, be clearly understood that their
protoplasmic contents communicate with those of the cells of
surrounding tissues, and that no living cell in a plant can be
looked upon as being completely isolated from the other cells of
the community.

Under the above heading will be studied : —

(а) SeCPetOPy cells of oil-glands.

(б) Resin-eanals.

(d) Idioblasts.

(e) LatieifePOUS cells and vessels.

A. Secretory Cells of Oil-Glands.

Oil-glands are of wide occurrence in the higher plants, and
may be found in almost any position in the stems, leaves, or in
connection with the parts of the flower. The essential cells of
any gland are the secretory cells, which, as a rule, line a central
cavity as a layer one or two cells thick, into which cavity a
special oily secretion is poured, or freed by the breaking down of
the secretory cells.

One type of such a gland occurs in the outer layers of the
cortex of fruits belonging to the genus Citrus {Citrus aurantii).
If a thin section be taken of the cortex in a direction perpen-
dicular to the surface, the following structure may be made out,
using a low power of the microscope : —

i. Externally, the epidePHliS of the fruit.

ii. Internally, cells of the COPtex (pericarp).

iii. The oil-glands lying quite near the surface.

CELLS WITH SPECIFIC FUNCTIONS.

Each gland is made up of the following parts (see Fig. 62) : —

(a) An external layer of rather thick -walled cells, arranged con-
centrically.

(/3) An internal layer of thin-walled cells full of granular contents.
This is the so-called endothelial layer.

(y) A central cavity, in which may be seen globules of oil and a
small quantity of cell-debris.

The cells of the endothelial secretory layer break down and
disorganise, thus setting free the oily secretion. Mixed with
this oil is a certain amount of cell-sap, which confers considerable

Fig. 62. — A Section across an Oil-gland in the Outer Cortical
Tissue (pericarp) of Citrus Aurantii.—e, Epidermis; c, cortex;
ep, endothelial secreting layer ; g, oil-globules lying in the central
cavity ; p, thick- walled cells just outside the gland.

turgidity upon the gland. At times the gland may burst through
the cortex and epidermis, setting free the secretion on the
external surface.

These structures occur typically in the cortex and xylem of
stem and root of Pinus, and also in the leaves, where they are
surrounded by the cells of the spongy parenchyma.

In a transverse section of the stem or leaf each resin-canal
is seen to possess the following parts (see Figs. 63, 64, 65, 65a) ; —

i. An outer layer of thick-walled elements one or two deep. This layer
forms the guaPd-Ping“ of the canal.

B. Resin-canals.

THE PLANT CELL.

ii. An internal layer of very thin-walled cells (endothelial layer)
which are full of a granular protoplasm.

iii. A central cavity, the section across the “duct,” in which may
be seen globules of liquid resinOUS material. This is set free into the
duct by the breaking down of the endothelial cells.

Fig. 63. — A. A Fully-formed Resin-
canal IN Transverse Section. —
en, Endothelial layer ; g, protec-
tive fibres ; r, resin-globules.

B. A Young Resin-canal, show-
ing an internal mass of granular
secretory cells (en), with as yet no
central cavity or duct. — g, Protec-
tive-cells.

Fig. 64. — A Resin-canal (r) in
THE Xylem of Pinus. — m.
Medullary ray.

In longitudinal section (see Fig. 66) the elements composing
the guard-ring are seen to be a variety of sclerenchymatous
fibre, and are very thick-walled, with small cell-cavities. The
endothelial layer is made out internal to the guard-ring, forming
on either side of the central duct a line of parenchymatous cells
with granular contents. The development of resin-canals may

To face p. 81.]

Fig. 65a. — A Photomicrograi'h showing two Resin-canals in the
Wood of Finns. Note also the shape of the tracheides in transverse
section, and the medullary rays lying close to both canals.

CELLS WITH SPECIFIC FUNCTIONS.

Fig. 65.— A Resin-Canal from
Pinus Stem, in longitudinal
section.— en, Endothelial layer;
g, thick • walled protective
cells (fibres) ; r, globules of
resinous material lying in the
duct.

Fig. 66. — A Drawing (from a photomicrograph) of a Transverse
Section of the Centric Leaf of Pimis to show distribution of
resin-canals. — e, Epidermis; Ay, hypodermis ; st, stoma; m, meso-
phyll ; r, resin-canals lying in the mesophyll ; end, endoderrais ;
6, fibro-vascular bundles.

THE PLANT CELL.

Fig. 67. — a, Raphides in a cell of the stem of Draccana ; these crystals are
large spindle-shaped ones, h, A bundle of small needle-shaped
raphides in a cell of Dracmna. c, Clustered crystals from the leaf of
Begonia. c?, A quadratic crystal from Begonia leaf. All these
crystals are composed of calcium oxalate, Ca(C02)2«

CELLS WITH SPECIFIC FUNCTIONS.

he observed in transverse and longitudinal sections of the young
shoot or leaf of Finns. The first stage seen is one where a small
column of young cells possessing granular contents is set off from
the surrounding cortical or mesophyll cells, and invested by a
ring of rather thick-walled elements, which ultimately form the
guard-ring. The latter soon become differentiated, whilst the
central cells of the internal mass break down to form the duct,
the remaining granular cells persisting as the endothelial layer
(see Fig. 63, B). This mode of origin of a resin-duct is known
as lysigenous origin, in contradistinction to the schizogenous
method, in which the central cells of a future canal become
merely separated from one another
along the middle lamellae. It is
probable that the resin is formed
as a product of disintegration of
the cell-walls of the endothelium,
and is thus not a direct secretion
of the cytoplasm.

C. Cells in which Mineral or
Organic Matters may sepa-
rate out under certain con-
ditions.

The materials which separate
out in these cells are not always,
strictly speaking, secretions, but
more often of the nature of excre-
tions, to be got rid of later on
by oxidative processes, removal to
other parts, or other reactions in
the cell ; and the substances thrown
out of solution, such as crystals,
etc., would often remain in solution
in the cell-sap were it not for the fact that in order to examine
them sections of the tissue have to be made, the mere process
of cutting and exposure to the air causing, in many instances,
spontaneous crystallisation. Sometimes, however, mineral matters
separate out in the living cell. In this connection it is convenient
to examine here the following structures : —

Fig. 68. — Clustered Crystals

IN SOME OF IHE CeLLS OF

A Bud-scale of Prunus
laurocerasus.

THE PLANT CELL.

1. Crystals occurring in certain cells.

2. Crystalloids.

3. Cystoliths.

1. Crystals of oxalate of lime, Ca(C02)2, may occur in the
following forms : —

«• Raphides, or elongated acicular crystals, found singly or in sheaves
in the cells of the eortex in the stem of Draccma (see Fig. 67). They also
occur in the root of Hyacinthus and many other plants.

Quadratic crystals occurring singly in cells of the leaf of Begonia.

y. Clustered crystals, also occurring in leaf-cells of Begonia, and in
other tissues (see Fig. 68).

These crystals are distinguished from those of other salts by
the fact that, on addition of dilute hydrochloric acid, they dissolve
without effervescence, whilst they are insoluble in aceiic acid.

Oxalic acid is a bye-product of metabolism in the cell, and it
combines with calcium to form calcium oxalate, which separates
out, in this case, in the living cell.'"

Fig. 69.— A Cystolith of Carbonate of Lime (CaCOg) formed in an
Epidermal Cell of the Leaf of Ficus elastica. Note the “core”
of cellulose upon which numerous layers of carbonate of lime are
deposited.

2. Crystalloids, or, as they are often called, spheroids, of a
substance known as inulin (a carbohydrate), separate out in the
cells of the tubers or petiole of Dahlia when these are treated with
alcohol. Inulin takes the place of starch or sugar in these cells.
The spheroids have a peculiar concentric and radiating structure
(see Fig. 71, a) which is very characteristic. Large spheroids of

* Occasionally crystals of oxalate of lime are found in the walls of cells
(mesophyll cells of Wellingtonia).

CELLS WITH SPECIFIC FUNCTIONS.

inulin occur in the Artichoke, which extend through many cells.
Crystalloids of a proteid nature are also found at times in the
cell (c/. “Aleurone grains,” Chap. x.).

3. Cystoliths are structures in certain of the deeper epidermal
cells of the leaf of Ficus elastica (see Fig. 69). The main mass

Fig. 70. — a, Latici FERGUS vessels in transverse section of Euphorbia
stem, h, Laticiferous vessels in a longitudinal section {Euphorbia).
Note the branching and union of vessels by short side branches.

of the cystolith is composed of amorphous carbonate of lime
[CaCOJ, which is deposited in the form of concentric layers or
small clusters upon an axial core of cellulose which projects into

THE PLANT CELL.

the cell-cavity from the outer cell-wall. On the addition of
acetic acid the carbonate of lime dissolves with effervescence,
leaving the core of cellulose intact. It is worthy of note in
this connection that the epidermis of the leaf of Ficus is three-
layered, a somewhat unusual occurrence.

D. Idioblasts.

Certain isolated cells occur at times in various parts of a
plant which have the specific function of secreting or excreting
substances detec tible by the employment of special tests ; such
cells are known as idioblasts. One of the commonest forms is
the tannin-cell, which is found in the cortex of such plants as
Pinus and Quercus. In the former tannin-cells are recognised by
treating a fresh transverse section of the stem with a dilute
solution of percliloride of iron (FeClg), when these cells turn black,
owing to formation of tannate of iron.

Another form of idioblast occurs in the petiole of the leaf of
Nymphcea. In this case large stellate cells are found at the
points of junction of the numerous strands of cells composing the
ground-tissue of the petiole, these stellate cells having walls
which are characterised by the presence on them of small pro-
jections formed of oxalate of lime (see Fig. 71, c). Their
function is not obvious.

E. Laticiferous Cells and Vessels.

These elements are characterised by the presence in them of
a secretion known as latex, a thick or thin milky fluid composed
of a mixture of gums, proteids, and resins, which at times
coagulates spontaneously, or on heating (india-rubber).

The vessels in which this latex occurs may be seen in
transverse and longitudinal sections of Euphorbia stem, or in the
stem of Ficus elastica ; in longitudinal sections the vessels are
seen to be branched, and communicate here and there by means
of short lateral passages. They are formed by the early develop-
ment of elongated passages which arise by the lengthening and
branching of prosenchymatous cells, and these, by their further
differentiation, give rise to a system of branched canals in the
cortex of the stem. The latex is formed from the protoplasm

CELLS AVITH SPECIFIC FUNCTIONS.

lining the vessel ; numerous dumb-bell shaped starch-grains are
also often present lying in the latex.

Laticiferous cells are, strictly speaking, a form of idioblast.
They are large oval cells, the protoplasm of which manufactures
the milky secretion, the process being probably of the nature of
an oxidation or breaking-down of the cytoplasm.

Fig. 71.— a, Spheroids of Inulin in a cell of Dahlia tuber, b, A
TANNIN-CELL lying amongst the cortical cells of Pinus stem, c, A
STELLATE IDIOBLAST formed at the junction of several strands of
“tubular” cells in the flower-stalk of Nymphcea. The nodules on
the wall are composed of oxalate of lime.

CHAPTER VII.

CELLS OCCURRING AMONGST THE LOWER PLANTS.

Having now examined some of the most important elements
going to build up the tissues of Higher Plants, it becomes
necessary for the student to inquire into the structural details of
cells as they occur amongst the Lower Plants; and in this
respect it will be convenient to start by examining briefly the
chief form of cell met with in the Fungi, and then to take a few
well-marked types occurring amongst the Algae, and examine
these in detail. Many of the Algae are unicellular organisms,
and, as such, are easy to study ; they are, moreover, very
interesting, since in them vital processes occur which are often
diflicult to demonstrate in the cells of higher plants, but which,
in these types of lower organism, escape the confusion often
consequent on the examination of complex tissues.

A. Cells occurring amongst the Fungi.

In the more highly differentiated members of the Fungi,
although certain variations occur, the tissues are composed of
cells ^ which conform to a simple type — viz., a tubular or
parenchymatous thin-walled element. The cells are joined
together to form long filaments, which are known as h3rphsB, and
sections of fungal tissues generally show a dense interwoven
mass of these hyphse, cut across in many directions. In a few
members — viz., the Lichens — algal cells are found living
together with hyphse, forming what is known as a symbiotic
community; and these plants are often propagated by small
masses known as soredia, composed of a certain number of hyphae,
amongst which are embedded a few algoid cells.

* Each of these so-called cells is in reality a “coenocyte” — viz., it
possesses many small nuclei — and is thus composed of many potential
protoplasts.

CELLS IN LOWER PLANTS.

In the lowest members of the Fungi — i.e.^ Schizomycetes, or
fission Fungi, as they are sometimes called — unicellular organisms
either rod-shaped (Bacilli), or in the form of small spheroidal
cells (cocci), are found, which may be joined together into
chains, or occur in the form of masses of varied shapes {strep-
tococci, staphylococci). Many of these forms are motile cells, such
as the Bacillus typhosus, Bacillus subtilis, Proteus vulgaris, etc.
Occasionally, as in the Streptothrix group, long branching filaments
are formed, composed of large numbers of rod-like cells {Actin-
omyces, Bacillus mycoides) ; but the life-histories and vital processes
of the Schizomycetes are, nowadays, considered to belong to the
domain of bacteriology rather than that of botany pure and
simple, and it is not here intended to give more than a brief
survey of the general characters of the group.

In the higher Fungi, although, as was above stated, the chief
type of tissue met with is that composed of hyphal filaments,
there occur, nevertheless, variations in this tissue, more especially
in connection with the processes of reproduction ; thus, in the
propagation by spores, the ends of hyphal filaments are modified
so as to become divided up into large numbers of spores or
gonidia, (exogenous spore-formation), and in yet other instances
spores may be formed inside special organs, the asci (ascospores)
in the process of endogenous spore-formation. No tissues corre-
sponding to the vascular tissues of Higher Plants occur in Fungi,
and even the highest members are only to be distinguished by
the variety and form of their fructifications, the vegetative part
of the plant being nearly always small and insignificant, and
known as a mycelium.

The cells composing the hyphal filaments possess protoplasm,
many nuclei, and cell-sap, but no chlorophyll ever appears in
them; and in place of starch, oil-globules are found in the
cell.* Moreover, the fungi are able to absorb, by means of their
mycelia, organic materials direct from the substratum on which
they grow, so that the processes of elaboration of nitrogenous
and carbohydrate material from salts and other raw material
supplied are not necessary in the members of this great group.
In those plants next above the Fungi — viz., the Bryophyta
{Hepaticce and Musci) — vascular tissues of a rudimentary type

* Glycogen in Fungi .seems also at times to take the place ot sugar or
starch in the higlier plants.

THE PLANT CELL.

may be met with, and the cells composing the other tissues of
these plants, although, as a rule, simple in type, have some
resemblance to those met with in Higher Plants. It will, how-
ever, be seen that in plants below the Fungi — viz., the Algae —
cells are often met with which in all respects agree with the
typical assimilating cell, which was studied in Chapter ii.

It is not intended here to pursue the study of the cells of
Fungi any further, but to proceed to the examination of a few
of the more well-defined types of lower plant organism met
with amongst that group of the Thallophyta known as the Algae.

B. Cells occurring amongst the Algae.

The types here selected for study will be : —

CL Spirogyra (belonging to the Conjugatce).

h. Vaucheria (belonging to the Siphonece).

c. Sphaerella (belonging to the Volvocinece).

d. Melosira (belonging to the Diatomacece).

a. Spirogyra. — This plant is one of the filamentous Algae, in
which a large number of cylindrical cells are united end to end
to form a colony. It is found at the bottom of ponds in
the form of large interwoven masses of a light green colour.
Each of the cells composing a filament is relatively a large
one, and, when examined microscopically, may be seen to be
composed of the following parts (see Fig. 72, 1) : —

i. Externally, a delicate cell-wall.

ii. An internal layer of eytoplasm, lining the inner surface of the
cell-wall and enclosing a central vaCUOlC ; from this layer bridles pass
to a central mass in which lies

iii. The nucleus. This body possesses a well-defined central spot, the

nucleolus.

iv. A spirally wound ribbon-shaped Chlorophyll band, which lies
next the cell-wall embedded in the peripheral cjdoplasm. The edges of
the band have a sinuous appearance, and the axial portion seems to be
rather thicker and more retractile than the lateral parts. Arranged at
regular intervals along the axial portion of the band are to be seen rounded
refringent structures, which are known as the pyrcnoids ; these, as will
soon be seen, are active Starch-formcrs and Storcrs, and require
special examination. For this purpose a fresh preparation of a filament
may be treated with a drop of weak iodine solution^ when the following
effects may be noted : —

CELLS IN LOWER PLANTS.

a. Each cell will undergo a partial plasmolysls (see Fig. 72, 3).

/3. The nucleus, and especially the nuCleoluS, will turn bPOWn
(reaction for proteid).

•y. The pypenoids are acted upon as follows : — The central portions
may stain a yelloivish-broivn, whilst the outer parts turn blue. This
reaction shows that Starch is present at the periphery of the pyrenoid.

Under a high power of the microscope it is possible to study
the pyrenoids more closely. It will then be found that some of
these bodies are completely surrounded by a ring of starch,
whilst others have only a few separate granules arranged round
them in the form of a circle (see Fig. 72, 2).

Fig. /2, — 1. A .Single Cell from a Filament of Spirogyra. Note the
delicate layer of peripheral cytoplasm, the nucleus held in the middle
of the cell by strands or bridles of protoplasm. The spirally wound
chlorophyll band has numerous pyrenoids arranged in line along its
axial portion.

2. Portion of the Chlorophyll Band after the cell has been
treated with iodine solution. The uppermost pyrenoid has a ring of
small separate starch-granules round it ; the lower one is completely
surrounded by a ring of starch.

3. Plasmolysis in a Cell of Spirogyra. Bridles of protoplasm
pass at first from the plasmolysed portion to the cell-wall.

THE PLANT CELL.

In a living Spirogyra cell it is often possible to detect small
granules in the cytoplasm in the vicinity of the pyrenoids, and,
by carefully focussing these, and cutting off the peripheral rays
of illumination to render them sharper in definition, it will be
seen that they are vibrating rapidly to and fro. It is probable
that this is not quite the same sort of movement as the well-
known Brownian vibration of small particles in the protoplasm,
which is a physical phenomenon, but is evidence of protoplasmic
activity, since the vibrating particles are situated chiefly over the
pyrenoids, and the bulk of these latter bodies is protoplasmic
in nature, each pyrenoid in fact being looked upon as a
plastid.

If Schulze’s solution be used as a reagent the cell-wall will stain
blue, showing that it is composed of pure cellulose. The other
reactions are the same as those noticed in a, /3, and 7.

For the complete study of Spirogyra very instructive prepara-
tions may be made by first fixing filaments in very dilute
Flemming’s solution or chromic acid per cent.). The filaments
are then washed in distilled water, treated with alcohol for a
few minutes, and stained with the Ehrlich- Bioncli triple stain
(composed of methyl-green, fuchsin, and orange G). By this
method the cytoplasm is stained pink, the nucleus green, and the
chlorophyll band and pyrenoids reds of different shades.

The cells of which a filament of Spirogyra is made up form
very good examples of typical assimilating cells, in which the
production of starch (or sugar) forms a large part of the pro-
cesses of assimilation. Moreover, this starch is only formed in
the presence of light, as may be easily demonstrated by growing
filaments in the- dark for some days, when the cells will, if
treated with iodine solution, show the pyrenoids devoid of any
peripheral starch-rings. The nitrogenous substances requisite for
formation of the proteid and amine parts of the cytoplasm mole-
cule are derived from the dilute solution of nitrites and nitrates
in the surrounding water, these being, together with water itself
and other salts, assimilated mostly during the absence of light.
The oxygen necessary for respiration is also derived from the
surrounding water, in which traces of oxygen are dissolved ; and,
possibly, some of the oxygen evolved from the cell during the
assimilation of carbon dioxide is dissolved in the water and used
again for purposes of respiration. Carbon dioxide exists in the

CELLS IN LOWER PLANTS.

water of ponds, dissolved to a slight extent,* but quite sufficient
for the needs of Spirogyra and other algal plants.

A filament grows in length only by the division of its terminal
cells, and this division involves, as a rule, the mitotic division of
the nucleus (see Chap, viii.) ; but by suitably altering the com-
position of the medium in which the filaments are growing, one
observer has succeeded in changing the type of nuclear division
in Spirogyra from the “mitotic” into the “amitotic” form, in
the latter of which the nucleus divides en masse. This experi-
ment is one of great interest, as it shows that adaptation to
altered conditions may take place even in these low forms of
vegetable life.

Conjugation in Spirogyra Avill be considered later under
“ Reproduction ” (Chap, ix.).

h. Vaucheria. — In this plant, which is also a filamentous Alga
growing under conditions similar to those which obtain in the
case of Spirogyra, the filament is composed of one long tubular
cell, and not of a colony of separate cells joined end to end.

Preparations of the fresh living filament may be first examined.
On mounting a filament, including its free-growing end, in water,
the following features will be noticed under a medium power of
the microscope : —

i. A delicate cell- wall forming the outer boundary of the filament.

ii. A narrow layer of cytoplasm lining the inner surface of the cell-
wall ; in this layer are to be seen (see Fig. 73, 1) —

iii. Large numbers of small oval chlOPOplastS, and

iv. Numerous small nuclei. These are usually only to be detected by
first fixing a filament in weak Flemming’s solution, washing, and staining
with carmine or h(cmatoxylin, with or without preliminary treatment of
the cell with alcohol to extract the chlorophyll. The nuclei lie close to-
the cell- wall, and arise by repeated divisions of pre-existing nuclei.

The above structure — viz., peripheral protoplasm — in which lie
numerous nuclei, determines Vaucheria to be a coenocyte, a term
which denotes that a large number of “ potential ” cell-units are
present, enclosed by a common cell-wall (c/. Cells of fungal
hyphse).

V. Oil-globules are to be seen in the central space enclosed by the
protoplasm (central vacuole). Oil is manufactured by the chloroplasts
of Vaucheria in the place of starch (or sugar). On the addition of iodine
solution the nuclei turn a brownish colour, but no starch-granules show up.

* The dissolved CO2 is present in the form of COg . H^O, or carbonic
acid.

THE PLANT CELL.

Fig. 73. — 1. The end of
A Filament of Fau-
cheria, stained so as to
show the nuclei and
represented as seen in
optical section. Note
the peripheral cyto-
plasm, in which lie
numerous chloroplasts
and the nuclei (shown
black in the figure).
The larger round bo-
dies are oil globules
which float in the cell-
sap in the central
cavity.

2. Plasmolysis in
Vtnicheria.

3. Formation of a
SWA RMSPORE (see text).

4. Freeing of the

SVVARMSPORE by split-
ting of the apex of the
terminal cell- wall.

iiiijiii,

CELLS IN LOWER PLANTS.

The cytoplasm in a fresh Faucheria filament will, if watched,
and especially if the slide be warmed, be found to exhibit the
phenomenon of ‘‘ rotation ” or streaming, similar to that which
was observed in the cells of Fallisneria (Chapter ii.).

Plasmolysis is very striking in Faucheria. On the addition of
a drop of 20 per cent, acetic acid or strong salt solution, the
cytoplasm retracts from the wall, bridles at first connecting the
retracted portion with the cell-wall ; ultimately the protoplasm
forms a retracted axial cord, the oil-globules being often forced
out of it and lying between it and the wall of the filament
(see Fig. 73, 2).

Faucheria is often reproduced by an asexual method — viz.,
by means of swarmspores — which may be described at this
point. A portion of the cytoplasm with several nuclei and
chloroplasts is cut off from the free end of the filament by the
formation of a thin partition wall. This mass of cytoplasm soon
acquires a delicate external layer of ectoplasm, and from this
latter numerous short vibratile cilia arise.* The cellulose
wall at the free end of the filament then ruptures and sets
free this swarmspore, which at once begins to move rapidly
through the water by means of its cilia. At a certain period,
however, it becomes fixed by one extremity to an object, the
cilia vanish, and the cytoplasm develops a wall of cellulose.
After a period of quiescence this “ encysted ” spore becomes
active and produces a fresh Faucheria filament, the thick wall
bursting and the cytoplasm growing out into an elongated mass
which is soon coated by a thin wall of cellulose. The nuclei and
chloroplasts also undergo division, and soon a typical filament
is reproduced.

The sexual method of reproduction will be described in
Chapter ix.

c. Sphasrella. — This organism occurs in several forms, and
the one which will be described here is known as Sphcerella
pluvialis, occurring in pools of rain-water which have lain a
few hours.

Sphcerella, in its free-swimming stage, is a motile cell, the
motility being consequent on the possession of two vibratile

* Pairs of these cilia arise just opposite each of the numerous nuclei,
which latter are arranged in line all round, immediately internal to the
outer firm boundary.

THE PLANT CELL.

Fig. 74. — 1. A SINGLE Free-swimming Individual of Sphcv.rella (see text).

2. The same treated with Alcohol and Iodine Solution.

3. The Palmella Stage, where two individuals have been formed
by division, the whole being surrounded by a gelatinous capsule.

4. The First Stage in the Reproduction of Sphardla by con-
jugation.

5. Fusion of two of the resulting Swarmspores by their anterior
ends.

CELLS IN LOWER PLANTS.

cilia. The movements are of two kinds — viz., rotatory and
translatory, and these are so rapid as to make examination of
the living organism somewhat difficult. If a single organism be
examined microscopically, the following structure will become
apparent (see Fig. 74): —

i. A verv delicate outer membrane of cellulose, giving the “blue ”
reaction with Schulze’s solution.

ii. Internal to this, and separated from it by a considerable space, a
pear-shaped mass of cytoplasm, which may be teen to be composed of
a very thin layer of eCtoplasm, and an internal endoplasm, in which
many chlorophyll bodies are present.

iii. Somewhere in the endoplasm the nucleus, a crescentic structure,
may possibly be detected ; but in the living cell this is not easy.

iv. At the anterior end of the cell may be seen the tWO Vibratile
cilia, which spring from the ectoplasm and pass through the outer mem-
brane of cellulose.

v. Near the point of origin of the cilia, the red cye-spot can be
distinguished; the function of this body is not obvious, but that it can be
affected by light is not improbable.

Delicate “bridles” of cjdoplasm stretch from the inner mass to the
internal surface of the outer membrane.

If such a cell be treated with alcohol, and then iodine, solution
be added, tiie pyrenoids will show up, a starch-ring being
present round each of these; only three or four pyrenoids are,
as a rule, present; the chlorophyll exists in the small chloro-
plasts in the endoplasm, and is extracted by the alcohol.

The cilia are in reality hollow protrusions of the ectoplasm,
into which the endoplasm suddenly flows, only to withdraw
again with equal rapidity. During this process each cilium is
(juickly bent in one direction, and straightened again, the sup-
position being that a cilium is thinner on one aspect than on the
oi>t)osite side. These sudden movements of the cilia have the
effect of moving the whole organism through the water, or of
producing rotatory movements.

With the exception of jEthalium (or the protozoan Amoeba)
Sphcerella is the lowest form of plant cell which has been studied ;
in the case of such organisms, moreover, the animal kingdom is
closely approached, Sphcerella being very similar in structure to Noc-
tiluca (Protozoa). With regard to the distinction between a lower
plant and a lower animal organism the possession of chlorophyll
does not afford much help, as both plant and animal may possess
this pigment (c/. Hydra). The assimilation of carbon dioxide is,
however, a distinctive feature, starch being formed by plants,

THE PLANT CELL.

while no analagous process takes place in animals, with the
exception, perhaps, of the Tunica ta, which are able to manu-
facture cellulose for their outer casings.

Eeproduction in Sphcerella takes place in two ways, viz,,
an asexual method and a sexual one, in the latter of which the
conjugation of two similar individuals takes place. In the
former, or asexual process, a single cell divides into two,
which become encapsuled by a gelatinous cyst, secreted by the
cell, and common to both individuals : further divisions arise,
resulting in the production of a number of cells, pairs of these
being enclosed in cysts, and the whole enclosed by a gelatinous
mass similar in str ucture to that enclosing the daughter cysts. In
this stage the whole mass is said to be in the palmella phase (see

Fig. 75. — Two Cells of a Chain of Diatoms {Melosira). Note the
peripheral protoplasm, the central nucleus held in position by
“bridles,” and the two somewhat irregularly shaped chlorophyll
bodies. The frustule of the Diatom is seen to be striated in a longi-
tudinal direction : the double nature of each frustule is not shown in
the figure (see text).

Fig. 74, 3), and, later on, the separate cells may be freed from
their cysts and become free-swimming organisms once more. At
certain times, however, reproduction takes place by the con-
jugation of two similar small ciliated motile cells which have
arisen by the division of the original cell into a large number of
equal ciliated individuals (see Fig. 74, 4 and 5). These
become freed by the bursting of the original cell-membrane of
parent-cell, and, whilst swimming freely, two of these bodies
approach one another, and meet by their anterior ciliated ex-

CELLS IN LOWER PLANTS.

tremities. After a short period they fuse, and the resulting
mass develops a delicate membrane like the original parent-cell,
and two vibratile cilia.

d. Melosira (Diatomacece). — Diatoms are members of the Algae,
characterised by the possession of silicified cell-walls, which are
often beautifully marked. The various markings met with serve
in many cases to distinguish the different genera, as also does the
enormous variety of shapes which these organisms can assume.

The cell-wall of a diatom is known as the frustule, and
contains enough silica in its composition to enable it to retain
its form and markings, even after it has been heated to a white
heat. Melosira is here chosen for examination, as this genus
shows the structure of the cell very clearly.

The living cell has the following structure (see Fig. 7 5) : —

(a) An external case or eell-Wall, composed in reality of two parts,
one of which fits into the other, pill-box fashion. The wall is marked
longitudinally by closely-set parallel lines, which are only apparent under
a high power.

{b) An inner peripheral layer of cytoplasm lining the inner surfaces
of the two halves of the frustule; from this layer “bridlCS” of proto-
plasm pass to a central mass, in which is suspended the nUClCUS.

(c) Two laterally situated masses of a brownish-green colour are to be
seen in the cell. These are the chlorophyll bodies or chromato-
phores. They are semi-fluid in consistency, and internally have sinuous
borders.

Melosira occurs in chains of varying length, there being often
one hundred or more individuals in a chain. The isolated cell
is capable of protruding a portion of its cytoplasm between the
two halves of the frustule, and uses this as a pseudopodium for
purposes of locomotion. Occasionally, however, diatoms are
able to move by causing currents of water to pass through their
interior and out again.

Cell-division in Diatoms takes place lengthwise between the
two halves, and the cell- wall of the new individual is enclosed
within the ruin of that of the mother-cell, so that repeated
divisions lead to a progressive decrease in the size of individuals.
At times, however, large forms known as auxospores arise, and
these by their divisions go to produce a smaller series. Auxo-
spores arise by the conjugation of two smaller individuals, the
resulting cells subsequently decreasing in size on division.

100

CHAPTER VIII.

CELL-DIVISION.

Having now examined some of the chief modifications of the
plant-cell, and gained an outline of the more important vital
processes to be demonstrated in it, attention may now be
directed to the manner in which fresh cells may arise from pre-
existing ones; and in this respect it is found that, in the
majority of instances, the nucleus is the structure in a cell which
undergoes the most marked changes. In the higher plants, in
fact, cell-division is always preceded by division of the nucleus.

Of the types of cell-division met with there are two main
varieties — viz., the amitotic and the mitotic. In the former the-
nucleus divides en masse, the cytoplasm becoming aggregated
round the resulting nuclei, after a process of redistribution ;
whilst in the latter, or mitotic type, certain changes take place
in the structure of the nucleus which lead to the development
of a well-marked karyokinetic or division-figure, followed by the-
formation of a partition-wall dividing the original cell into two.

A. Amitotic Cell-division.

This type is comparatively rare in the Higher Plants; it
occurs, however, in cambial cells, and it is also seen in old
internodal cells of Tradescantia virginica. In the lower plants it
may occur at times, as in the case of the nuclei of Vaiicheria,.
and in internodal cells of Cliara fragilis.

In amitosis, the nucleus becomes constricted in the middle,
and this constricted part becomes narrower, until finally the
original nucleus has split into two daughter-nuclei. This type
of nuclear division is looked upon by many as an evidence of
degeneration (more especially in animal cells); but in a few cases
it is a sign of the need of rapid division, where time and space-
will not allow of the more highly differentiated mitotic type.

CELL-DIVISION.

101

In some cases of free cell-formation the nuclei may divide
amitotically. the cytoplasm of the original cell becoming distri-
buted round the several nuclei resulting from the division.
Cell-walls may be subsequently formed cutting off separate cells
from one another. The formation of endosperm in Phanerogams
takes place in a somewiiat similar manner, although mitosis is
here the usual type of division of the nuclei.*

B. Mitotic Nuclear Division, followed by division of the cell.

1. This process almost always precedes division of the cell in
Higher Plants and most of the lower plants, although in the
latter case differences may be seen during some of the phases.

In order to properly understand mitosis, it is necessary first of
all to examine more fully than has been done hitherto the
structure of the quiescent nucleus. To do this, powers of the
microscope, ranging from the ^ inch to the yV inch oil immersion,
should be employed, and preparations of the cell for the purpose
of examining the nucleus should preferably be made in the
manner described in the Note at the end of Chapter ii., young
growing tissues, such as a root-tip of Allium or Hyacinthus
serving very well for material to work with. The preparation
having been made, a cell should be selected for examination in
which the nucleus is still intact, and as yet shows no signs
of karyokinesis. Such a nucleus will, under the yV inch objective
and a suitable eyepiece of the microscope, be magnified about
800 or 900 diameters, and will shoAv the following structure
(.see Fig. 76, 1, Fig. 81a, and Fig. 1) : —

(tt) An external boundary, the nUCleaP membPane, which is pro-
bably the innermost firmer portion of the kinoplasm, or layer of the
cytoplasm just outside the nucleus. The nucleus may, in fact, be looked
upon as a space filled with fluid and bounded by the kinoplasm, in which
space certain other structures are suspended

(6) Internal to tlie nuclear membrane, a clear portion, of a fluid nature,
the .so-called nUCleaP plasm, in which are suspended :

{(1) (Granules of a substance known as ehPOmatin, arranged at some-
what irregular intervals upon the linin network ; here and there rather

* Such a formation of cells is sometimes known as “multicellular
formation.” Free cell-formation results in the production of distinct
i.solated cells, as in the case of the production of ascospores in the Fungi.

102

THE PLANT CELL.

larger masses of chromatin occur, termed net-knotS or kapyosomes
(see Fig. 1, Chap. i.). The chromatin is so-called on account of its capacity
to take up stains like hcematoxylin and safranin.

(e) Other structures which, like the chromatin, are able to take up
certain stains, are the nucleoli or plasmosomes. These lie in the
spaces between the linin network. There may be only one large nucleolus
present situated centrally.

Fig. 76.— 1. A Quiescent Cell from a Growing Root-tjp. The nucleus
is situated centrally in the granular cytoplasm, and shows externally
the nuclear membrane, and internally the clear nuclear plasm, the
linin network upon which are seen at intervals the chromatin granules
and a few karyosomes ; two nucleoli are present.

2. The Initial Phase or Mitosis (early prophase). The chromatin
granules have increased in size, and are becoming arranged in the
form of a definite chain upon the linin thread.

To face p. 103. ]

PLATE I.

Plates I. ami II. (PhotomicrogTaphs showing Vaiiuus Phases in Mitosis).

1, 2, 3, and 4 sliow the spireme stage ;

5, 6, and 7, the monaster stage seen from the side ;

8 and 9, tlie secondary chromosomes separating ;

10, 11, and 12, later stages of the metaphase ;

13 (right-hand cell), 14, 15, and 16, end-stages (telophase).

Mostly from longitudinal sections of root-tips of Alhum and HyacniUiux. 16. o
the endosperm of CaHha. 15. Prom a cortical cell of Larix cone. 14. Irom endosperm
of Calf ha.

To face p. 103.]

. CELL-DIVISION.

103

Tile chromatin is the essential substance in the nucleus, and
in chemical composition is identical with nuclein, a material
which contains phosphorus in its molecule. The nucleolus is
composed of a substance known as paranuclein, or parachromatin,
and, during mitrosis, may possibly be partly converted into
chromatin, or a body from which chromatin may subsequently
be formed.

In the cells of most plants below the mosses, and also in
certain cells during the reproductive cycle, in some of the
higher plants (Cycads) two peculiar structures are to be seen
lying close to the nucleus in the kinoplasm. These are the
centrosomes, and in lower plants and most animal cells, even
during vegetative divisions, they appear to possess an important
rdle. In the following description of mitosis the centrosomes
will be omitted, as in Higher Plants they are in all probability
absent, at least during ordinary vegetative divisions.

2. The Details and Mechanism of Mitotic Nuclear Division,
or Karyokinesis* (see Plate I., Diagrams 76 to 80, and
Figs. 81 A to 90 inclusive).

The mitotic process is most conveniently divided into five
stages, the first four being termed phases, while the last
involves the formation of the cell-plate, or rudimentary par-
tition wall which divides the parent-cell into two. Thus it is
possible to distinguish between (a) Prophase, (|8) Metaphase,
(y) Anaphase, and (§) Telophase, in each of which certain
changes take place in the nuclear structures. It will be best to
study each of these phases sejmrately and in order.

(a) The Prophase, in which the nucleus prepares for division.
— At the beginning of mitosis certain conditions must be present
in a cell in order that the nucleus may be provided with
adequate powers to complete the process. These conditions are :

(a) The presence of an adequate supply of soluble nitrogenous
food and carbohydrates (elaborated food- materials from the leaves).

(h) The maintenance of an optimum temperature.

(d) Protoplasmic continuity between adjacent cells of a dividing

tissue.

* See an article by the author in Knowledge and Scientijic News, Feb.,
1909, on “The Mechanism of Nuclear Division.” Also one in same
magazine, Aug., Sep., 1909, on “ Mitosis in Higher Plants.”

104

THE PLANT CELL.

There are possibly other factors, especially in connection with
the increase in mass of the chromatin, which must require a supply
of phosphorus-containing food material, but these cannot be gone
into fully, the chemistry of the process being somewhat obscure.

Microscopically, the first change to be noticed in the nucleus
is the increased capacity which this structure shows in the
taking up of such stains as hsBmatoxylin or safranin. In this
respect it is the chromatin-granules which show this increased
staining capacity, the nucleoli not showing much difference at
first (early prophase). Next, the chromatin -granules become
more regularly arranged upon the linin network, and soon the
appearance is presented of a definite chain of granules set at
equal or nearly equal intervals apart upon a continuous coiled
thread of linin (see Fig. 76,2). At a slightly later stage, careful
observation has shown that each chromatin-granule becomes
divided into two, so that there are then two parallel rows
of granules arranged regularly upon two threads of linin, the
latter structure also having undergone a similar fission to the
granules.'^ During this process, the chromatin-granules have
increased in size, and approached one another, so that, finally,
there seem to be two I'arallel threads coiled with the limits
of the nuclear membrane (see Fig. 76, 2, and Fig. 77, 3). A
good resolving power of the microscope is necessary to make out
the dual nature of the chromatin band. In the endosperm of
Fritillaria, and root-tip of Hyacinthus, during mitoses, it is, how-
ever, fairly obvious.

These changes complete the early prophase, and the coiled
chromatin-band is now known as the spireme or skein (see
Fig. 77, 4). Traces of the nueleoli may still be seen at this
stage, but the nuclear membrane has already become indistinct.

The phenomenon now occurs of the breaking up of the
spireme into a number of equal lengths of chromatin, known as
the primary chromosomes : this is effected by either mechanical
rupture or chemical absorption occurring in the linin-thread at
several equidistant points (see Fig. 78, 5). The number of the
primary chromosomes varies in different plants, and may be as
many as twenty-four {Lilium)', and it is obvious that each
primary chromosome is a double structure.

* This was definitely shown to occur in Hellehorus foetuhis, by Mottier,
and it can be observed in most cases.

CELL-DIVISION.

105

Fig. 77. — 3. A Later Prophase.
The chroniatin-gramiles have
each undergone fission into
two.

4. A Complete Spireme.
The chromatin-granules have
increased in size, and lie so
close together as to produce
the appearance of a double
thread.

Fig. 78. — 5. A Later Prophase.
The double band of chromatin has
been split up into four primary
chromosomes, each of these pre-
serving its double nature. The
achromatic spindle is now begin-
ning to appear, the fibrils of the
spindle converging to two poles
which are at opposite ends of
the cell.

6. A Final Prophase (monaster
stage) seen in surface view. The
primary chromosomes have been
guided into the median equatorial
plane of the spindle, so as to
form a star- shaped figure. Each
chromosome lies with its bend
towards the centre of the median
])lane. The prophase is now
completed.

106

THE PLANT CELL.

Careful focussing will now often bring to light fine refractile
lines radiating from points near either end of the long axis of
the nucleus, and passing towards the median equatorial plane of
the nucleus and cell; these lines are the achromatic fibrils.*
The fibrils soon become more obvious, and radiate from points
(poles) on either side of the nucleus, in the form of cone-shaped
bundles, which (partly) become attached by their central ends to
the primary chromosomes; some of them, however (the central
ones), pass from pole to pole without being attached to chromo-
somes.

The next thing noticed is a change in position of the primary
chromosomes. Each of these becomes bent into the shape of a
U or a V, and appears to be dragged (or guided) by certain of
the achromatic fibrils into the median equatorial plane, where it
takes up a position in which the bend of the V looks towards
the centre of that plane, whilst the free ends of the V look away
from the centre towards the circumference. The achromatic
fibrils have together, at this stage, the shape of a spindle, and
form what is known as the nuclear spindle or amphiaster ; and
the primary chromosomes form in their equatorial position what
is termed the monaster or single rosette, since this is the form of
the figure when seen in surface view (see Fig. 78, 6, and Fig. 85).

Those fibri’s of the spindle which have been influential in
pulling or guiding the chromosomes into their median position
are known as the mantle-fibres; they have been assumed to be
contractile, and they lie on the outer surface of the spindle in
the form of cone-shaped bundles radiating from the poles at
each end of the cell. This assumption is, however, partly
hypothetical, since some observers will not allow of any of the
fibrils being contractile, but bring forward evidence to show
that chemotaxis plays a role in the movements of the chromo-
somes, especially during the metapbase. It is, however, quite
possible in some cases to make out the cone-shaped masses of
fibrils at the beginning of the metaphase (see Plate I., 6 and 8).

The prophase is completed by the time the primary chromo-
somes have assumed the equatorial position : no trace of nucleoli
can be detected at the completion of this phase, and the nuclear
membrane has vanished.

* So-called because they do not stain with those dyes which the chro
matin takes up.

CELL-DIVISION.

107

Fig. 79. — 7. An Early Metaphase.
The two halves of each chromo-
some are separating from one
another in the form of V-shaped
secondary chromosomes ; the
achromatic spindle is now quite
a marked feature.

8. A Latkr Mktaphase,
showing the two systems of
secondary chromosomes travel-
ling away from one another
towards the poles of the
spindle.

Fig. 80. — 9. A stage where the two
systems of daughter-chromosomes
here almost reached the poles of
the spindle (late metaphase). The
line of dots along the middle of
the spindle indicates the line
along which the cell-plate will
form.

108

THE PLANT CELL.

Fig. 81. — An End-stage
(Anaphase) from the
Developing Endo-
sperm OF Caltha palus-
tris. The two daughter-
nuclei are formed, but
no cell -plate has yet
appeared in the spindle.

Fig. 81a. — A Quiescent
Nucleus lying in the
Cytoplasm at the Apex
OF THE Embryo-sac of
Lilium (four - nuclei
stage).

CELL-DIVISION

109

Fig. 82. — Early and Late Spireme Stages (from a photomicrography
Hyacinthus root-tip).

Fig. 83. — A Complete Spireme (from a photomicrograph, AUmm root-tip),

110

THE PLANT CELL.

.Fig. 84. — The Embryo-sac of Lilium martagon showing the nucleus of the
sac at the phase where the primary chromosomes (split and twisted)
have just been formed (late prophase). Three large nucleoli are
present (from a photomicrograph).

CELL-DIVISION,

111

Fig. 85. — The Monaster Stage in a Cell of the Root-tip of Hyacinthus.
The chromosomes are in reality double, but the duplication cannot be
seen when the loops are seen in surface view (from a photomicrograph).

Fig. 86. — The Monaster Stage seen from the side (root-tip of
Allium).

112

THE PLANT CELL.

(/3) The Metaphase. — This phase commences by the separation
of the two halves of each longitudinally split chromosome, the
bends being the first portions to come apart; the free-ends
remain in contact for some time, but at last, by the agency of the
mantle-fibres, these are drawn asunder. The appearance now
presented is that of two systems of so-called secondary or
daughter-chromosomes travelling away from one another, there
being an equal number of loops in each system (see Figs. 87, 88).

Fig. 87. — Two Cells from Allium Root-tip, showing the early meta-
phase and late metaphase respectively. Note the cone-shaped
bundles of mantle-fibres in the left-hand cell.

In surface view, or in slightly oblique sections of tlie cell, it is
now possible to make out two “ rosettes ” of chromosomes ;
between these are to be seen certain fibres of the achromatic
spindle which persist until the cell-plate has been formed, and
even after this. These fibrils are composed of the so-called
interzonal fibres, and are not contractile in nature.

The later metaphase shows the two systems of secondary

CKLL-DIVISION.

113

chromosomes close to the poles of the spindle ; this stage is also
known by the terms — double-rosette or diaster-stage (see Fig.
79, 8, Fig. 80, 9, and Fig. 88). On tlie lines of the chemotactic
theory, the passage of the two systems of chromosomes is effected,
not by the agency of the mantle-fibres of the achromatic spindle,
but by the attractive influence of certain substances in the
vicinity of the poles of the spindle (enzymes) upon the chromo-
somes ; nevertheless, distinct cone-shaped bundles of the spindle

Fig. 88.— A Metaphase Stage from Allium Root-tip, showing two
systems of loops widely separated (from a photomicrograph).

fibres can be seen attached in many cases to separate chromo-
somes along their whole length, so that the theory of the pulling
action of the mantle-fibres cannot be lightly dismissed. It is
possible that a compromise must be made, both chemotaxis and
the action of the mantle-fibres being taken into account.

(y) and (S) The Anaphase and Telophase ; formation of the
cell-plate. — The final or end-stages in mitosis comprise (a) In-
volution of the secondary chromosomes, and (b) The formation
of the cell-plate.

114

THE PLANT CELL.

(a) After the daughter-chromosomes have reached opposite
poles of the achromatic spindle, a short period of quiescence
supervenes ; then the chromosomes become joined end to end so
as to form a typical spireme at each pole (dispireme stage), the
band of chromatin being of a single and not a double nature.
Each spireme is then broken up into chromatin-granules
arranged upon a linin thread, and nucleoli once more make their
appearance (see Figs. 81, 89).

Fig. 89. — An End-stage from a
Cell of the Young Female
Cone of Larix Europcea. The
cell-plate is just beginning to
form.

Fig. 90. — Formation of the Cell-
plate (complete partition wall
between two daughter - cells).
The daughter- nuclei are complete
(from Hyacinthus root-tip).

{h) Whilst these changes have been going on in the daughter-
nuclei at each pole of the spindle, small thickenings appear on
the fibrils of this structure in the median equatorial plane :
these are the first indications of the cell-plate. Gradually these
thickenings enlarge, and at last join one another all round, so

CKLL-DIVISION.

115

tliat a delicate film or plate is produced, separating the two
halves of the spindle from one another ; concomitantly with this
change, the whole spindle contracts somewhat towards the
median plane, and this contraction has the result of bulging out
the circumference of the spindle, so that ultimately it touches the
side walls of the cell. In this manner the cell-plate comes to
extend right across the cell and constitutes the rudimentary
partition-wall separating the two resulting cells from one another
(see Fig. 90). The cell-plate is in its later stages composed of
ehemically pure Cellulose, and it has been shown that during its
formation the protoplasm (or kinoplasm) becomes directly con-
verted into cellulose by the splitting off of its proteid and
nmine portions.

The two halves of the spindle lying on either side of the cell-
j)late persist, and the achromatic fibrils become ultimately con-
verted into bridles of cytoplasm, which communicate with one
iinother through minute “pits^’ in the partition-wall.

The origin of the achromatic spindle is somewhat hypothetical;
thus it has been supposed to arise from the kinoplasm, just out-
side the nuclear membrane, but at times it seems that it takes its
origin from the nuclear plasm. Some observers state that the
spindle-fibres arise early and lie as a sort of feltwork just outside
the nucleus, which, as mitosis proceeds, pushes its way into the
interior of the nucleus towards the chromosomes. In Stypocaulon
and Erysiphe^ according to Harper, the spindle is an intra-nuclear
formation, so that here it would appear to arise from the
Jiuclear-plasm.

In those cases where, as in lower plants, centrosomes are
formed in the cell, the achromatic spindle is an early formation,
and arises between the centrosomes, close to the nucleus, during
the early prophase. Moreover, fibrils, known as the “astral
rays,” stretch out in all directions from the poles or centrosomes,
and not only into the interpolar region.

With regard to variation in form of the chromosomes, it
may be mentioned that at times the chromosomes may take on
the form of rings instead of loops, the free ends of the loops
remaining united for some time. This is known as heterotypic
mitosis, the process above described being normal or homotypic
mitosis. Moreover, it is an interesting fact, that during the
reproductive divisions in the microspore and embryo-sac of

116

THE PLANT CELL.

Higher Plants, the number of the primary chromosomes is
“ reduced ” to half of what it is during the vegetative division of
the same plant. This is known as cell-division with reduction.
It occurs, for example, in Osmunda regalis in the mitoses occurring
in cells of the prothallus (gametophyte).

Note. — Some of the various “fixing ” reagents used for the preparation
of material for the study of mitosis, and also a list of plants and organs
suitable for this study, will be mentioned here.

(a) Flemming^s Sohition. — This is a very useful reagent for the rapid
fixation of quickly-growing tissues, such as root-tips, one modification of
it being as follows : —

This solution may be used more dilute if required. Root-tips fix in it
in about 12 hours, and, after fixing, should be well washed in distilled
water and transferred progressively to the following strengths of alcohol :
— 50 per cent., 70 per cent., 90 per cent., and, finally, absolute alcohoL
By this method the tissue is hardened. For preserving after hardening,
the root-tips or other tissue should be kept in pure methylated spirit.

(b) Chromic Acid. — This may be used in 5 per cent., or 2 per cent.,
or \ per cent, solutions in distilled water. It is not so good as Flemming’s
solution as a fixing reagent. It may be made more useful oy the addition
of acetic acid.

(d) A Mixture of Acetic Acid and Alcohol (about 55 per cent, strength)
is sometimes used as a fixing agent.

(e) A Solution of Corrosive Sublimate in Alcohol (2 to 5 per cent.) is at
times a useful fixing reagent. It is used mostly for animal tissues, more
especially for the fixing and hardening of larval tissues.

The most serviceable reagents for the fixation of plant-tissues, parti-
cularly for those in which it is desired to study mitosis, are Flemming’s
solution and Chromic acid solution^ since these, if used dilute, will not
cause much preliminary shrinking of the cytoplasm.

Plants suitable for the study of cell-division are the following : —
Hyacinthus (root-tips of water-cultures). Allium (root-tips of water-
cultures), Fritillaria (endosperm, root-tips of water-cultures), Lilium
maturation stages in the embryo-sac, pollen mother-cells), Larix (cortex
and medulla of young ? cone).

Longitudinal and transverse sections should be made by cutting with a
flat razor in split pith, or tissues may be hardened and embedded in
celloidin or parafin, and microtome sections made, a somewhat more
lengthy process.

5 per cent, chromic acid,
2 per cent, osmic acid.
Glacial acetic acid, -
Distilled water up to

10 c.c.

5 c.e
1 c.c.
50 c.c.

117

CHAPTER IX.

CELLS HAVING THE FUNCTION OF REPRODUCING
THE SPECIES.

In this chapter a very important subject will be dealt with —
viz., reproduction — and it will be found convenient, to start
with, to examine the reproductive cycles as they occur in the
three great groups of the Higher Plants — i.e., Angiosperms,
Gynmosperms, and Pteridophyta. In all these, certain primary
essential cells arise in special organs, the reproductive organs.
Thus, in Angiosperms and Gymnosperms, the male cells, or, as
they are usually termed, the microspores or pollen-grains, arise
in the anthers of the stamens of a flower; and the female cell —
viz., the macrospore or embryo-sac — is formed in the nucellus of
the ovule, or sporangial portion of the flower. These two
elements, the microspore and macrospore, undergo, first of all, a
process known as maturation, in which certain cells are formed
in each structure ; one of these cells, in each of the fully matured
sexual elements (microspores and macrospores) being the
effective cell, which, by fusion with its counterpart, results in
the production of a fully-fertilised cell, from which the embryo-
plant is reproduced.

Thus, to put the process into tabular form, the following
stages are noted : —

Microspore Macrospore <-

I i

^ Effective cell $ Effective cell

Fertilised cell

Embryo-plant

= spore-forming or asexual generation.

118

THE PLANT CELL.

Ill the Higher Pteridophyta (Ferns), the reproductive cycle is
somewhat more complicated, inasmuch as two separate genera-
tions are produced. One of these — the sexual form — is known
as the gametophyte, and is the product of the germination of
the spore, the latter arising in a special organ, the sporangium,
which occurs on the fern-plant proper. The other, or asexual
generation, is the sporophyte, and is the result of the fusion of
two effective cells produced in special organs, which arise on the
under surface of the gametophyte, or prothallus, as it is some-
times called. Thus, tabulating as before, the following sequence
is noted : —

>Spore

Gametophyte = prothallus = the sexual generation

$ Effective cell $ Effective cell

Sporophyte = Fern-plant iiroper

= the asexual generation.

Tlie arrows indicate in both tables the completion of the cycle.

This reproductive cycle, then, includes two distinct genera-
tions, and, for this reason, tlie Higher Pteridophyta are said to
exhibit the phenomenon of an alternation of generations.

With these few introductory remarks, it is possible to proceed
to the study of the formation and maturation of the essential
cells in Angiosperms, Gymnosperms, and Pteridophyta, and, in
all, the process of fertilisation and formation of the embryo-
plant will be shortly described. With regard to tlie Pterido-
phyta, it may be mentioned that two main types can be
recognised, viz., one in which only one kind of spore is produced
by tlie s[)orophyte, or spore-bearing jilant, and the other in
which two kinds of spore are produced. In the latter, two
separate gametophytes or prothallia are formed, and the effective
cells thus arise on two separate sexual generations, there being
in this case a sort of double alternation of generations. The
two types are known respectively as Homosporous (one spore

REPRODUCTIVE CEIXS.

119

only) and Heterosporous (two kinds of spore), and both will be
studied on account of the valuable comparisons which can be
made between their reproductive cycles and those which occur
in the Angiosperms and Gymnosperms.

After reproduction in the higher types of plant has been
studied, a brief description will be given of the process as it
occurs in the Bryophyta, Fungi, and Algae. It is not intended
to examine very fully reproduction in the Fungi, so that only
an outline of this will be given. The important point is to gain
a clear idea of the reproductive cycles met with in the three
great groups mentioned above.

A. Reproduction in Angiosperms.

In the Angiosperms the essential primary sexual elements are
the following : —

a. The microspore or pollen-grain.

h. The macrospore or embryo-sac.

Each of these undergoes a process of maturation, and at the
completion of this fertilisation takes place.

a. The Microspore : its Origin and Maturation. — The pollen-
grains are produced in certain parts of the anthers of a flower ;
usually four rudimentary masses of cells are set off in the young
anther, and these are the archesporial cells. These cells arise by
the division of a primary archesporial cell, and of the resulting
mass the outermost cells give rise by further divisions to a
sheathing layer known as the tap e turn ; the remaining inner
mass form the pollen mother-cells, from which the microspores
are ultimately produced (see Fig. 91, a).

At a somewhat later period each mother - cell undergoes
division into two cells, the resulting cells dividing again (see
Fig. 91, h, c, cl), so that ultimately there are four nucleated
masses of cytoplasm enclosed within the original wall of the
parent-cell. Each of the four masses is a potential microspore,
and soon assumes a thin wall of cellulose which, later on,
l)ecomes modified in a manner which will be described. A large
number of mother-cells are often present, and it will be seen that
the number of microspores ultimately formed is four times as
great as that of the mother-cells.

120

THE PLANT CELL.

Fig. 91. — a, A portion of a longitudinal
section through a young anther of
Polygonatum, .showing part of the
pollen-sac, filled with pollen mother-
cells in all stages of division, h, A
single mother-cell which has divided
into tw'o, the cell-plate having just
been formed, c, The same after the
second division, walls being formed
at right angles to the first 'vall.
d, Four complete daughter - cells
(young microspores) still enclosed
by the wall of the original mother-
cell. c, A mature microspore. Three
cells are present, the uppermost one
being the prothallial cell, the middle
one the antheridial or generative
cell, and the lowest the vegetative
cell, from which ultimately the
pollen-tube is formed.

REPRODUCTIVE CELLS.

121

After a time the wall of the parent-cell is ruptured or becomes
absorbed, and the young microspores come to lie free in the
pollen-chambers or sacs (microsporangia), which are lined by the
remains of the tapetum. The immature microspore presents the
following features : —

i. A thin cell-wall externally.

ii. Internally, gramilar cytoplasm, in which are a large nucleus and
food-granules (starch, &c.).

The wall soon becomes modified so as to consist of two
distinct layers (see Fig. 91, e) — viz., an outer one, the extine,
which is thick, and often beautifully marked by reticulations,
projections of various shapes, or thin-wall areas ; and an inner
one, the intine, which encloses the cytoplasm. From the mark-
ings on the extine it is often possible to distinguish tlie genus or
species from which the pollen was derived.

• Maturation of the microspore consists in the formation of
certain cells by the division of the cytoplasm and nucleus of the
main cell. At first a small cell is occasionally cut off, which is
known as the prothallial cell, the significance of which will be
pointed out when the Heterosporous Ferns are considered. Next,
a cell is cut off from the remaining larger cell ; this is the
so-called “ generative ” cell, and is the effective maU cell in fer-
tilisation. Thus, in the mature microspore there are present
three cells enclosed within the intine (see Fig. 91, r), viz,: —

a. The prothallial cell ; this cell is generally absent. It may, however,

seen in the niicrospore of Sparganium.

/3. The generative cell.

y. The vegetative cell, this being the large cell left after the forma-
ation of a and /3. It is this cell which forms the pollen-tube during
fertilisation.

All these maturation changes in the microspore may take place
whilst it is resting on tlie stigma of the ovary. The further
changes which take place — viz , formation of the pollen-tube and
division of the generative nucleus — are best considered under
fertilisation.

h. The Macrospore : its Origin and Maturation. — The macro-
spore or embryo-sac is contained at the apex of the nucellus of
the ovule (macrosporangium) in Angiosperms, and has the
following origin : — The terminal hypodermal cell of the axial row
of cells in the young nucellus is the so-called archesporial cell or

122

THK PLANT CELL.

Fig. 92. — 1. The Youngj
Nucellus of an Angio-
SPEEM {LUium). The
dark cell is the hypo-
dermal terminal cell of
the axial row of cells,
and is the mother-cell of
the embryo-sac (arche-
sporial cell).

2. The archesporial
cell has divided into
two — viz., an upper or
tapetal cell, and a lower
larger cell.

8. The tapetal cell
has divided into three
secondary tapetal cells,
and the lower one has
had a small cell — the
cap-cell — split off from
its upper end. The
lowest cell, e, is the
young embryo-sac.

4. The cap -cell has
divided into two, and
the embryo sac (macro-
spore) is enlarging.

To face p. 123.]

1. A photomicrograph of tlie eml)ryo-sac ami its nucleus at the stage where the latter
is about to undergo its first mitotic division (Lilniw).

2. A photomicrograph showing the completed first division (Liliiiw).

3. A slightly later stage than 2, showing the completed daughter-nuclei.

4. End of the second division, four nuclei being now present.

The next stage would he that where each of the nuclei in 4 have divided again, leading
to the presence of eight nuclei, four at each end of the emhryo-sac.

Plate III. (Stages in the Maturation of the Emhryo-sac).

REPRODUCTIVE CELLS.

123

mother-cell of the embryo-sac (see Fig. 92, 1). This cell divides
into an upper or primary tapetal cell, and a lower larger
cell; the primary tapetal cell gives rise to three or more
secondary tapetal cells, which, later on, become obliterated by
pressure.

The lower larger cell has cut off from its upper end a cell
which soon divides into two (so-called cap-cells), one of the
resulting cells dividing again ; so that, finally, there are pre-
sent in a typical case seven cells (see Fig. 92, 2, 3, and 4),
viz. : —

i. The three tapetal cells.

ii. An intermediate tier of three cells, so-called cap-CellS ; and

iii. A large lower cell, which is the rudimentary embryo-sac ; the
cap-cells (ii. ) become obliterated as well as the tapetal cells (i. ) by the
pressure caused In' further growth of the embrvo-sac.*

The young embryo-sac (macrospore) is a large cell possessing
cytoplasm and a relatively large nucleus. In the latter are, as a
rule, several nucleoli and an open chromatin reticulum. The
embryo-sac increases in size enormously, and soon comes to be
one of the largest cells present in the plant.

* Maturation of the macrospore consists in the occurrence of
certain changes in the cytoplasm and nucleus of this cell which
result in the production of the essential female cell or egg-cell,
and certain other accessory cells or nuclei, which will now be
described (see Plate iii.). The first change noticed is the division
of the nucleus of the embryo -sac into two by the mitotic method
of nuclear division ; another division then takes place in each of
these nuclei, so that there are now four nuclei present in the
cytoplasm, situated usually at the angles of regular figure (see
Plate iii., 4, and Fig. 93). A further division of each of these
four nuclei results in the production of eight nuclei, four of
which become massed together at the upper pole of the embryo-
sac and four at the lower pole. Of these eight nuclei, one from
each end [)asses to the middle of the embryo-sac, and these remain
for a time close together ; they are the so-called polar nuclei (see
Fig. 94), and in a short time they fuse to produce the definitive
nucleus (see Fig.s. 95, 96, and 97). At this stage there are then
present in the embryo-sac seven nuclei, three at each end and

See CToebel, Ontlinefi of Classification and Morphology.

124

THE PLANT CELL.

Fig. 93. — The Young Ovule of Caltha palnstris, showing the embryo-sac
at the stage where two mitotic figures are present, end of the second
division (from a photomicrograph).

REPRODUCTIVE CELLS.

125

one larger one in or near the centre. The cytoplasm becomes dis-
tributed round them in such a way as to lead to the presence of
three cells at each end, and the central definitive nucleus is usually
held in a central mass by “ bridles ” of protoplasm. Of the six
cellular structures present the three at the upper end constitute
the two synergidae (lying uppermost) and the egg-cell, the latter
of which is the effective female cell (also termed the oosphere).

a photomicrograph).

whilst the three at the lower end are the antipodal cells. At
this stage the macrospore is completely matured and ready for
fertilisation.

[Most of the preceding stages described may be seen in
Plate iii. and Figs. 93 to 97 inclusive.]

126

THE PLANT CELL.

Fig. 95. — The Completely Matured Embryo-sac of Lilium. — It shows
at the upper end the synergidae, one of the generative nuclei, and the
egg-cell nucleus, and, at the lower end, the large spheroidal definitive
nucleus and three antipodal cells.

[To face p. 126.

Fig. 9(5. — The Completely Matured P]mbryo-sac of Helleborus niger.
Note the relatively large nucleoli. (From a photomicrograph.)

rvEPRODUCTIVE CELLS.

127

c. ' Fertilisation and Subsequent Changes. — The matured
microspore lying on the stigma of the pistil of an Angiospermous
plant now undergoes the following further changes : — First, the
large vegetative cell of the microspore (see p. 121) grows
down into the conducting tissue (the central loose tissue) of the
style in the form of an elongated cell, the pollen-tube. This
process is brought about by the action of enzymes in the tube,
which dissolve the cellulose walls of the cells of the conducting
tissue ; a further action of these enzymes being the conversion
of the starch in the cells into dextrins and sugar, which furnish
nutriment to the tube during its progress. Having reached the
cavity of the ovary, the pollen-tube is attracted towards the
micropyle of an ovule, tlie tip penetrates the micropyle, and
grows through the micellar tissue to the upper pole of the
cmbryo-sac (the attraction of the tube being probably of the
nature of positive chemotaxis). The next change which occurs
is the passage of the nucleus of the generative cell (see p. 121)
to the apex of the pollen-tube, where it divides into two. One
of the resulting nuclei then passes between the synergidse, which
are situated at the upper pole of the embryo-sac, and, having
reached the egg-cell or oosphere, penetrates into this cell,
and, after a short time, fuses with the egg-cell nucleus, the
cytoplasm also fusing with that of the egg-cell. This pro-
cess completes the fertilisation of the oosphere, which now
becomes the oospore. The other nucleus resulting from the
division of the generative nucleus also passes between the
synergidae, and fuses with the definitive nucleus, which now
becomes the endosperm nucleus. Round this latter the remain-
ing cytoplasm of the embryo-sac soon collects, and divisions
•occur, resulting in the formation of the early endosperm nuclei,
which lie free in the cytoplasm in the middle of the embryo-sac.
This cytoplasm, with its nuclei, later on, lines the wall of the sac.

The fusion of the second generative nucleus with the definitive
nucleus completes the process known as double fertilisation, a
phenomenon which has recently been shown to occur in the
majority, if not all, of the Angiosperms.

The fertilised egg-cell, or oospore, divides, after a short period
•of quiesence, into two cells, viz. — an upper or epibasal cell, and
a lower or hypobasal cell, which bear an important relation to
the position of the rudimentary tissues to be shortly formed from

128

THE PLANT CELL.

them. Thus, from the epibasal cell are subsequently produced
the young stem, first leaf, and the cotyledons (or cotyledon, in
the case of monocotyledons) ; whilst from the hypobasal cell, the
root, and, in some cases, the foot (an absorbent organ), and the
so-called hypocotyledonary portion of the stem arise. The
manner in which these tissues arise is, briefly, by the formation
of octants of cells, from which, by subsequent synclinal and
anticlinal divisions, the rudimentary tissues are developed.

Before the embryo-plant (spore-forming or asexual generation)
thus formed is completed, the endosperm (or secondary prothal-
lium, so-called ; see section on ‘‘ Homology ”) has increased to
a great extent, and cells Avith definite cell-walls have arisen.
At first the endosperm nuclei lie free in the shell of cyto-
plasm lining the embryo-sac, and no Avails are formed until

Fig. 97a. — A Single Cell from the Endosperm of Ccdtha palustris,
showing: — iv, cell-Avall; n, nucleus; p, protoplasmic “bridles” passing
through the cell- wall ; v, vacuoles.

a considerable number of nuclei have been produced. After
a time, hoAvever, the cytoplasm grows in thickness, and
fresh nuclei are produced centripetally. Ultimately cell-walls
are formed simultaneously between a large number of nuclei,
there being a peculiar formation of radiating inter - nuclear
achromatic spindles, across which Avails are formed. (In Caltha
'palustris, very beautiful preparations of the developing endo-
sperm may be made, Avhich shoAv this internuclear wall-

Pl.ATK IV. (Stages ill Eiulosperni-fonnation).

1. Nucleus resulting from third division of the eiidosiierm nucleus {Calthn).

■2. Three stages in the mitoses’of nuclei lying free in the cytoplasm of the embryo-sac.

:i. End-stages in the mitoses of two nuclei of the early endosperm of Caltha. Note the
oval daughter-nuclei.

4. Portion of the sheet of endosperm of Calthn, showing a large number of free nuclei
just previous to multicellular formation.

.s. ^■oung endosperm cells just subse»iuent to wall-formation. Note the large number
<»f nucleoli in each nucleus

f>. Somewhat older cells of Calthn endosperm, showing in each cell two daughter-nuclei
ami a well-marked achromatic spindle. Partition walls not yet formed.

REPRODUCTIVE CELLS.

129

formation [see Plate iv.]). Endosperm is thus produced
after the method known as multicellular formation (see p. 101).
In the later endosperm cells of Caltha the intercommuni-
cating cytoplasmic fibrils may be readily made out (see
Fig. 97a).

Fig. 98 (Diagrammatic). — The Microspore of Pinus and the stages
TAKING place DURING ITS MATURATION.— The young unmatured
microspore, h, A prothallial cell has been cut off. c, The larger cell in
It has had another cell cut off from it : this is the antheridial cell ;
V, the vegetative cell, d, The antheridial cell has divided into a
“stalk” cell, 8, and the true generative cell, x, p, is the pollen-tube
formed by the elongation of the vegetative cell, e. The nucleus of the
generative cell has divided into two, which travel towards the apex of
the pollen-tube and lie there in a mass of cytoplasm. (Drawings
made from figures at the British Museum of Natural History.)

130

THE PLANT CELL.

Note. — The study of the development of the microspore may be readily
carried out in sections of the young flower of Polygonatum. Longi-
tudinal sections of the fixed and hardened flower buds will cut across the
young anthers. Sections should be stained with safranin and haematoxylin
to show up the mitotic figures in the divisions of the pollen mother-cells.
The study of the maturation of the microspore is difficult to carry out, and
rather beyond the scope of the practical work noted in these pages.

The development of the embryo-sac and its maturation stages can,
however, be readily studied in Lilium martagon, using young flower-buds.
Hellehorus niger is also a useful plant for the later stages, as also is Caltha
palustris. Transverse sections of the young ovary of Lilium will cut
across the ovules and embryo-sac longitudinally, and often in the same
section three different stages may be recognised.

The development of the embryo is best observed in Capsella bursa-
pastoris. In this plant the early 'growth of the embryo may be made out
by selecting ovaries of various sizes, placing them in glycerine and water,
and gently squeezing them, in order to flatten the ovules and force out
the embryos. Development of endosperm can be studied by taking trans-
verse sections of the ripening carpels of Caltha (fixed and hardened), and
staining to show up the nuclei, lying free in the cytoplasm of the embryo-
sac. Hellehorus niger may also be used for this purpose.

Fig. 99. — Structure of the Immature Microspore. — w. Extine;
intine ; w, nucleus lying in the cytoplasm ; Ih^ lateral lobes formed
from the extine.

B. Reproduction in Gymnosperms.

The Gymnosperms are interesting from the fact that in them
the reproductive cycle forms a sort of link between the process as
it occurs in Angiosperms and that taking place in the hetero-
sporous Pteridophyta. The Cycadeae show perhaps the most
resemblance in this respect, but the type here selected will be
PinuSf in which genus all the more important details may be
readily made out.

As in the Angiosperms, the microspore (pollen-grain) and the
macrospore (embryo-sac) form the primary sexual cells, and in
each of these certain processes of maturation take place, which
lead to the formation of the effective cells in reproduction.

REPRODUCTIVE CELLS.

131

a. The Microspore : its Origin and Maturation {Finns). — The
origin of the microspore takes place, as in the Angiosperms, by
the setting apart of an archesporium in the anther, from which
are produced an outer sheathing layer, the tapetum, and an inner
mass of pollen mother-cells ; each of the latter gives rise to four
rudimentary microspores, which are set free later on into the
cavity of the pollen-sac or micro sporangium (see Fig. 100)

Fig. 100.— A Section (Longitudinal) of two Pollen-sacs (Micro-
sporangia) OF Pinus. — sk^ Microsporophyll j ps^ pollen-sac; eps, wall
of the pollen-sac : several pollen-grains are seen inside ; /, A, fibro-
vascular bundles.

The tapetal cells in this case may give rise to a secondary
tapetum. The young unmatured microspore (pollen-grain) has
the following structure : —

132

THE PLANT CELL.

i. An outer wall, which soon becomes thickened, the extine, from
which are produced two lateral lobeS ; these possess reticulations, and
are useful in buoying up the microspores during dispersion by the wind
(see Fig. 99).

ii. An inner and thinner wall, the intine.

iii. Internally, cytoplasm and a large nucleus.

Maturation of the microspore takes place either in the pollen-
sac or whilst it is lying upon the apex of the nucellus in the ovule,
and consists in the cutting off of certain cells from the main mass ;
thus, the first cell to arise is the prothallial cell, which may divide
again. The second division, which cuts off a cell from the larger
remaining cell, gives rise to the antheridial or generative cell ;
whilst the large cell now left is the vegetative cell, from which
the pollen-tube is produced. Later on the generative cell divides
into a stalk-cell and the generative cell proper (see Fig. 98), this
usually occurring after the pollen-tube has been formed. The
later changes are best described under fertilisation. It is note-
worthy that in Ginkgo and the Cycadece the generative cells
are further differentiated into antherozooids (ciliated motile cells)
of a peculiar type."^ This process thus links these groups with
the Pteridophyta (Heterosporous type).

b. The Macrospore : its Origin and Maturation. — The embryo-
sac (macrospore) has an origin similar to that of the Angio-
sperms. An archesporial cell arises just beneath the epidermis
(or oftener rather deeper) of the nucellus of the ovule
(macrosporangium), this latter being situated upon the upper
surface of a carpellary leaf (macrosporophyll) of the female
cone (see Fig. 101). A primary tapetal cell is cut off from
the apical end of the archesporial cell, and also cap-cells from
the lower larger cell. The remaining large cell is the embryo-
sac (macrospore), which soon enlarges to many times its
original size. The next change which occurs is the division
of the cytoplasm and nucleus of this macrospore into a number
of free cells which soon develop cell-walls and undergo further
division, and the tissue which is ultimately formed by this
process fills the embryo-sac, and is known as a prothallium
(incorrectly termed endosperm).

At the upper (micropylar) end of this prothallium now arise

• * For a very good account of the formation of the antherozooids of
Ginkgo, see The Journal of Applied Microscopy and Laboratory Methods
for May, 1902.

REPRODUCTIVE CELLS.

133

several peculiar flask-shaped structures, the archegonia,* and the
early formation of an archegonium (see Fig. 102) is as follows: —
One of the apical cells of the prothallium divides into two,
and the lower of these divides again. The lowest or larger
cell is the archegonium proper, and contains an oosphere com-
posed of cytoplasm and large nucleus. The upper cells, by
further divisions at right angles to the former ones, give rise
to cells which separate in the centre and leave a space, the
canal of the archegonium (see Figs. 102 and 103). A few cells
are soon cut off from the upper part of the oosphere, the lowest

Fig. 101.— A Longitudinal Section of the Fruit-scale (Macrosporo-
phyll) and Bract of Pinus, to show relations of the embryo-sac
and nucellus. — si-, Fruit-scale; hr, bract; nh, nucellus ; mp, micro-
pyle ; es, embryo-sac (prothallium or “endosperm” already formed) ;
sm, the “samara” or wing of the ovule ; ph^^ ph^, the xylem
and phloem of the scale and bract respectively : note that the relative
positions of the.se are reversed in the fruit-scale, the phloem being
uppermost. ,

* The “ corpuscula ” of earlier writers. Each archegonium was formerly
erroneously looked upon as a separate embr3^o-sac, but, strictly speaking,
the corpuscula = oospheres.

134

THE PLANT CELL.

being the so-called ventral canal-cell, the other two being the
neck canal cells.

Fig. 102. — Diagrams showing the formation of the Archegonia in
THE Embryo-sac (Macrospore) of Finus. — 1. The young embryo-sac
with its nucleus. 2. The cytoplasm and nucleus have divided to form
a tissue the prothallium (“endosperm”) in the sac; x is the cell from
which an archegonium will arise. 3. The cell x has had a cell, Xi,
cut oflF from it. 4. The cell x^ has divided into cells rcg ^3 > ^3
the rudimentary “ body ” of the archegonium. 5. The cells a? and a;.2
are later divided by walls at right angles to the previous ones, and a
space arises, the canal, which is lined by the cells so formed. The
cytoplasm of x^ usually has a few cells cut off from its upper portion,
these being the so-called ventral and neck canal-cells ; the remainder
forms the oosphere.

To face p. 135.]

Fig. 104. — a, Photomicrograph showing three archegonia, h. Photomicro-
graph showing to the left tiie canal of an archegonium leading into
the archegonium proper. A pollen-tube is seen just penetrating
the upper end of the embryo-sac ; the generative nucleus is to be seen
close to the upper end of the right-hand archegonium.

REPRODUCTIVE CELLS.

135

[This process of maturation resembles that occurring in
the macrospore of the heterosporous Pteridophyta (MarsUea,
Salvinia)t qua vide where oogonia (= archegonia) arise upon
a special female prothallium produced in that spore: see,
however, “ Homology,” at end of chapter.]

Fig. 103. — Three Archegonia at the Apex of the Embryo-sac of
Pimis. — ar, Archegonium ; in the left-hand one an oosphere with its
nucleus is present : the middle one shows at its lower end signs of
division ; end, endosperm (prothallium) ; c, canal of an archegonium ;
pt, pollen-tube.

c. Fertilisation and Subsequent Changes (Pinus). — The process
of fertilisation consists essentially in the fusion of a generative

■ 136

THE PLANT CELL.

Fig. 105. — 1. An archegonium at the apex of the embryo-sac of Pinus,
containing an oosphere which has just received a generative nucleus
from the microspore (small nucleus just above the larger one of the
oosphere).

2. The first two divisions at the lower pole of the oospore, subsequent
to fertilisation.

3. Shows the appearance at the end of the third division. The
lowest cells are the embryonal cells ; the lowest but one (6) are the
cells from which the suspensors are produced.

REPRODUCTIVE CELLS.

137

nucleus derived from the microspore (see p. 132) with one of the
oospheres contained in the archegonium at the upper pole of the
prothallium in the macrospore (embryo-sac). The pollen-tube^
derived from the further growth of the vegetative cell of the
microspore, grows through the tissue at the apex of the nucellus
of the ovule until its tip rests upon the upper cells of the
prothallium at the top of the embryo-sac in the vicinity of the
canal of an archegonium. The nucleus of the generative cell
now travels to the tip of the pollen-tube, and, lying in a mass of
cytoplasm existing there, divides into two (see Fig. 98, e). One
of these nuclei (the so-called male pro-nucleus) penetrates the
canal of an archegonium, being probably attracted by a substance
(enzymic in nature) secreted by the neck canal cells, and enters
the oosphere, Avhere it lies for a short time close to the nucleus of
the oosphere (the so-called female pro-nucleus). Fusion of these
two nuclei now occurs, and the resulting nucleus travels to the
lower end of the fertilised oosphere, or oospore, as it is now called.

This nucleus, and the cytoplasm at the lower end, now divide,
giving rise to two cells devoid of cell-walls. In each of the
resulting cells a division at right angles to the direction of the
former one arises, so that four cells lying in the same plane are
produced. Each of these four cells divides again twice, so that,
finally, there are present at the lower pole of the oospore four
rows of cells, there being three cells in each vertical row. The
lowest cell of each of these tiers is a potential embryonal cell
(pro-embryo), the middle cell in each row is the suspensor-cell,
and the upper cell later on disappears, or forms, with the remains
of the oospore above, pabulum for the lower cells (see Fig. 105).

The suspensor - cells soon elongate greatly and push the
embryonal cells before them deep into the prothallium. Each of
the embryonal cells then divides into two cells by a somewhat
oblique wall, the uppermost being the epibasal cell and the
lower one the hypobasal cell. These two cells are divided again
by a wall at right angles to the first, and the next division
results in the formation of an octant, from the segments of
which the rudimentary organs are produced somewhat after the
same manner as in Angiosperms, subsequent growth proceeding
from a primary apical cell, which forms the apical tissues.

In the above process, then, four embryonal cells are formed,
but in reality only one becomes a fully developed embryo.

138

THE PLANT CELL.

This is an instance of polyembryony, a phenomenon which
occurs in some Angiosperms, notably Funkia cwdata. The
other embryonal cells, as a rule, form embryos which are later

Fig. 106. — Embryo - sac of Pinus, showing PIlongation of the
SusPENSORS. — 1, Basal cells, which probably form pabulum for the
others, or else abort ; 2, the suspensor cells, greatly elongated ;
3, the pro-embryonal cells, one of which will become an embr}'©-
plant by subsequent division ; osp, remains of the oospore in the
archegonium.

REPRODUCTIVE CELLS.

139

on absorbed, whilst the final developing embryo grows at the
expense of the prothallium contained in the macrospore ; more
than one oosphere is usually fertilised, but the embryos formed
from these do not get beyond a certain stage.

Note. — The practical examination of the reproductive cycle in the
Gymnosperms is readily carried out taking longitudinal sections of
male and female cones of Pinus at various stages of their growth. The
development of the microspore can be made out by this method, and the
maturation is often to be observed when the pollen-grain is lying at
the apex of the nucellus of an ovule in the female cone.

The formation of the embryo-sac and its prothalliura may be studied by
taking sections of very early female cones of Pinus from the time when
they are about 4 mm. in length onwards. Archegonia are soon formed
after the prothallium is complete, but some care is necessary in selecting
cones for this examination.

Fertilisation and subsequent changes are best seen in sections of ovules
of cones gathered from June 1st to the end of that month, although, in
some Conifers, fertilisation takes a long time. The later changes,
just previous to and after the elongation of the suspensors, are more
readily obtained than the earlier ones. In Pimis fertilisation takes two
years to accomplish.

C. Reproduction in the Pteridophyta.

In this great group of plants, as was pointed out above, an
alternation of generations is met with — that is, a sexual
generation formed from the spore alternates with an asexual
generation whicli arises as the result of the fusion of the two
effective cells produced in special organs upon the sexual plant
or gametophyte. Moreover, two main types of reproduction are
met with in Pteridophyta. In the one, or Homosporous type,
only one kind of spore is produced, and this, by its germination,
produces the gametophyte or prothallus ; whilst in the other, or
Heterosporous type, two kinds of spore are found — viz., micro-
spore and macrospore — in each of which a separate prothallium
is formed. Tims, the microspore in the latter case produces a
male prothallium upon which organs comparable to antheridia
arise in which the male effective cells are formed, and the
macrospore produces a female prothallium upon which an organ
arises in which the female efiective cell originates.

The Homosporous type is exemplified in Pteris or Aspidium,
which belong to the order Filicineae; and the Heterosporous
type is seen in Marsilea or Salvinia, members of the order
Hydropteridea?. Each of these types will be examined in order.

140

THE PLANT CELL.

I. Reproduction in the Homosporous Pteridophyta {Aspidium).
— The stages and structures to be examined here are : —

a. The spOPe and sexual generation (gametophyte or pro-
thallus).

h. The sexual organs arising on the sexual generation, and the
essential cells formed in these organs.

c. Fertilisation and the origin of the sporophytc or asexual
generation.

These will be considered in detail.

CL The Spore and Sexual Generation (Gametophyte). — The
spores are formed in certain well-defined structures known as
the sporangia. These arise either from one epidermal cell
or a group of cells upon small cushions of tissue, the sori,
which are formed upon the under surfaces of the sporophylls
or spore-bearing fronds of the fern. Usually these sori are
found at the endings of the lateral branches of the leaf-traces of
the frond, and their position varies according to the genus.
A sheath of cells known as the indusium often covers over each
sorus, but in some cases the sori are naked. Each sporangium
is composed of three parts (see Fig. 107), viz.: —

i. The stalk, which grows from the sorus.*

ii. The spore-chamber or sporangium proper, which is thin-
walled, and composed of small translucent cells. Inside the chamber are
seen the spores, which are produced at an early stage by the divisions of
an archesporial cell which forms the spore mother-cellS, each of these
latter going to produce four spores.

iii. The annulus, a curved portion at the back of the sporangium.
This is composed of peculiar cells, each having thick walls perpendicular
to the surface, and thinner very elastic outer walls. The annulus acts as
an elastic layer which helps to stretch open the spore-chamber when the
thin front wall ruptures t and sets free the spores.

A single mature spore is a simple spheroidal cell which
})ossesses two walls, an outer thick wall and an inner thin one.
Internally are cytoplasm, large nucleus, and a few food-granules
(starch). (See Fig. 107, 2.) At a certain period, determined
by the relative humidity of the atmosphere, rupture of the thin
anterior wall of the sporangium occurs, and the spores are freed,

* A stalked gland arises in some cases from the stalk close to the
sporangium proper, and is characteristic of the species Aspidium Jilix -mas.
It is not represented in Fig. 107.

+ In connection with the rupturing of the anterior wall two peculiarly-
shaped cells are found, between which the rupture occurs along the middle
lamella. The two cells constitute the so-called “ stomium.”

REPRODUCTIVE CELLS.

141

being, in fact, shot out by the effect of the elastic recoil of the
annulus, and, after falling upon a suitable substratum, each spore
germinates. The outer wall of a mature spore is much wrinkled
in the genus Aspidium (in the figure this is not represented).

Fig. 107. — Details (Diagrammatic) of the Reproductive Cycle in
Aspidium (Homosporous Ferns). — 1. A mature sporangium, showing
the spore-chamber with spores inside, the stalk, and the curved
annulus. 2. A single spore. 3. Germination of the spore by the
splitting of the thick outer wall and protrusion of the cytoplasm
contained in the thin inner wall. 4. An antheridium from the under
surface of a mature prothallus ; a few mother-cells of the anthero-
zooids are seen in the central cavity. 5. A single antherozooid,
showing the rounded head with vesicle attached, and the tail, at
the end of which are the cilia. 6. An oogonium sunk in the
under surface of the prothallus. Note the oogonium proper con-
taining the oosphere, and the canal, in which are the canal-cells.
7. The first two divisions of the oospore.

142

THE PLANT CELL.

Germination of a spore consists in the swelling and ultimate
rupture of its outer wall, and protrusion of the inner thin-walled
cell in the form of a tubular growth. This protrusion is soon
divided into two cells by the formation of a wall, and subse-
quently a large number of cells are formed, the whole mass
being the rudimentary gametophyte or prothallus (sexual genera-
tion). Chloroplasts soon appear in the cells of this structure,
which ultimately takes on the form of a small cordate mass,
notched or bilobed at the broader end. It is flat, and only a
few cells thick. The upper surface is smooth, and on the under
surface are found towards the apex — (i) A number of rhizoids,
which serve both as organs of attachment and absorption.
Each rhizoid is somewhat like a long root-hair, only thicker,
(ii) The sexual organs. These are the antheridia and oogonia ;
their origin and subsequent changes will be described separately.

b. The Sexual Organs, with their Origin, and the Essential
Cells. — As mentioned above, the male organ (antheridium) and
the female organ (oogonium) arise in the under surface of the
gametophyte, or prothallus. Each antheridium is, when mature,
^ rounded structure, which is formed from a single cell of the
under surface of the prothallus, this cell undergoing certain
divisions which result in the formation of an external layer of
cells, enclosing a mass of cells known as the mother-cells of the
antherozooids. These latter form the essential male, or fer-
tilising elements (see Fig. 107, 4, 5). The maturation of an
antherozooid consists in the occurrence of changes in the nucleus
and cytoplasm of the mother-cell. During this process the
nucleus (chiefly the chromatin portion) becomes elongated and
specially curved, one end being thicker than the other, and over
the whole a thin film of cytoplasm is present. At the thin end,
or tail, are two or three long vibratile cilia, formed, in all pro-
bability of ectoplasm, or kinoplasm; whilst at the thicker end,
or head, is a vesicle, which is cytoplasmic in nature, and con-
tains a few vacuoles and granules of food-material (probably
starch).

Each antherozooid is, by virtue of the possession of vibratile
cilia, a motile cell, and swims about in the droplets of moisture
on the under surface of the prothallus. Its further history is
perhaps better postponed until after the study of the oogonia.

The oogonia are also formed on the under surface of the

REPRODUCTIVE CELLS.

143

prothallus, and, as a rule, are sunk in the tissues of that structure.
Each mature oogonium is a flask-shaped organ, composed of two
portions — viz., the oogonium proper, or venter, a spherical recep-
tacle sunk in the prothallus, and a canal leading from this to
the surface. These two parts arise from a single cell of the
under .surface of the prothallus, this undergoing division into two,
the lower cell being again divided into two. The lowest, or
rather the deepest of these, becomes the oogonium proper,
whilst the two upper ones undergo divisions at right angles
to the former plane, lateral cells being formed, and a passage
— the canal — arises between them, along the point of union of
the planes of division which contains the axial cells. The
protoplasmic contents of three small central cells cut off early
from the cytoplasm in the venter of the oogonium
form the ventral and neck canal-cells, whilst the remaining
large mass in the cavity of the oogonium is the oosphere (see
Fig. 107, 6), this being the essential female cell. The neck
canal-cells are later on converted into a gelatinous plug, which
contains a substance (malic acid) capable of attracting the
antherozooids (positive chemotaxis).

c. Fertilisation and the Formation of the Embryo -sporophyte
(Asexual Generation).— Fertilisation is accomplished by the
passage of one antherozooid into the oosphere by way of the
canal of the oogonium. At a certain period the neck canal-cells
secrete a substance (malic acid or an enzyme) which has a
powerful attraction for the antherozooids, and one of these bodies
finds its way down the canal, passing through the mucilaginous
plug which now fills that space.

After penetrating the oosphere, the antherozooid fuses with
the nucleus of the oosphere, and this body, being thus fertilised,
becomes the oospore.

The oospore is soon divided by an oblique wall into an
epibasal and a hypobasal cell. A second wall at right angles
to the first is then formed, and of the four cells now present, the
two upper ones go to produce the stem (rhizome) and first leaf
of the sporophyte, whilst the lower two give rise to the root,
and an absorbing organ, the foot. The foot remains sunk in
the prothallus whilst the first leaf grows upwards, usually
through the notch of the prothallus, the rhizome and root
growing horizontally and downwards respectively. In con-

144

THE PLANT CELL.

nection with the formation of the embryo-sporophyte, it should
be mentioned that the young tissues are produced by the
divisions arising in what is known as an apical cell. This cell
is pyramidal in shape, with the base outwards, and walls are
produced in it, parallel to the three sides of the pyramid. Fresh
tissues, even lateral buds, all have this form of cell at their
apices, and growth is thus entirely apical at first, the subsequent
walls arising in other planes. The apical cell is found not only
in the Pteridophyta, but also in the Bryophyta, and is typical
of both these groups of plants.

Note. — The study of the reproductiv^e cycle in the Homosporous
Pteridophyta may be carried out in Pteris or Aspidium (Filicinese). The
spores are readily examined by brushing off a number of sporangia into a
drop of water on a slide, and observing rapidly under the microscope,
when the annuli will stretch open, and the spore-chambers rupture,
freeing the spores.

The prothalli are best examined by growing spores on moist humus,
so prepared as to exclude moulds, and watching the stages of growth, in
order to pick out prothalli showing the various phases in the formation
of the antheridia and oogonia, and, later on, of the embryo-sporophyte.
Fresh gametophytes may be examined in glycerine and water, or the
structures may be fixed and hardened and sections taken in split pith, or,
after embedding in celloidin, by means of a special microtome.

II. Reproduction in the Heterosporous Pteridophyta (types
Marsilea, Salvinia). — A brief description of this type of repro-
duction in Pteridophyta is necessary on account of the important
comparisons to be made between it and that occurring in Angio-
sperms and Gymnosperms.

In Marsilea, one of the Hydropterideae, two kinds of sporangia
are found — viz., micro sporangia and macrosporangia, in special
organs, the sporocarps. In the former, a number of small spores,
or microspores, are produced; and in the latter, a few large
spores, or macrospores, arise.

A microspore, on being freed by the rupture of its sporan-
gium, germinates, and produces a small male prothallium, which
is enclosed within the limits of the thick outer coat or exospore
of the microspore. Upon this prothallium, antheridial cells are
formed, in which antherozooids arise, somewhat after the same
manner as those of Aspidium. A macrospore, when freed from
its sporangium, also germinates, and gives rise to a somewhat
larger female prothallium, upon, or in, which an oogonium is
formed, containing, after the cutting off of certain canal-
cells, the oosphere. There are thus two separate gametophytes

REPRODUCTIVE CELLS.

145

(sexual generation) or prothallia, and the process occurring in
the Homosporous type — viz., formation of the prothallus from the
one spore — might be looked upon as the fusion of two prothallia,
produced by the germination of a potentially double (male and
female) spore.

The further history of the cycle in Marsilea consists in the
freeing of the motile antherozooids, and the fusion of one of
these with the oosphere nucleus, the oosphere being then known
as the oospore. From the oospore the embryo-sporophyte
(asexual generation) is again produced. Other Heterosporous
Pteridophyta are the Selaginelleae and Isoetese. Equisetum gives
rise to spores all of the same size, but the sexual organs arise on
separate prothallia (dioecism). The determination of the sex of
the prothallium in this case is largely a question of nutrition.

Reproduction in the Bryophyta, Fungi, and AlgaB.

In order to complete the survey of the reproductive
processes occurring in plants it is necessary to examine briefly
the main variations occurring in the reproduction of Mosses,
Liverworts, Fungi, and Algae. It is not intended here to give
an exhaustive account of these, as this would involve the con-
sideration of many subsidiary groups, and would, moreover, lead
to inevitable confusion. For a full account of many of these
the student may be referred to Goebel’s Outlines of Classification
and Special Mmpliology, or any of the larger text-books.

1. Reproduction in the Musci (Bryophyta). — In these plants
an alternation of generations exists, the plant arising from the
oospore — viz., the sporophyte — nevertheless, remaining in a special
organ found in connection with the fructification of the moss-
plant. Thus, antheridia and oogonia arise on the moss-plant,
which is here the gametophyte, or sexual generation, in special
fertile shoots, and the result of fusion of an antherozooid with
the oosphere (the product being the oospore) is the forma-
tion of a mass of cells known as sporogenous cells, inside a
special organ, the sporogonium. The sporogonium and sporo-
genous cells are thus comparable to the sporophyte, or asexual
generation, and the ripe spore, on germination, gives rise to a
rudimentary cellular structure, the protonema, from which the
moss-plant or gametophyte, is produced. The moss-plant proper

146

THE PLANT CELL.

is thus homologous with the prothallus of Homosporous Pteri-
dophyta; but, as has just been seen, both gametophyte and
sporophyte are united in the same plant. The musci are also
propagated by a vegetative method — viz., by means of gemmae,
which are small cellular offshoots of the gametojohyte.

2. Eeproduction in Hepaticae (Bryophyta). — Here there is
also an alternation of generations, and the reproductive organs,
antheridia and oogonia, are found on the gametophyte, or sexual
generation (which has often the form of a simple flattened
structure), upon the under surfaces of special fructifications. At
times a type of vegetative propagation occurs, in that gemmae,
or buds, composed of a few cells, are formed, usually at the
bottom of small cup-shaped receptacles {Marchantia). The result
of the fusion of an antherozooid with the oosphere in an
oogonium is the oospore, which divides and forms a sporogonium,
which is the sporophyte, or asexual generation. In the sporo-
gonium the spores are produced, and germination of a spore
results in the production of the thallus, gametophyte, or sexual
generation once more. In Riccia, the sporogonium is quite a
simple structure, whilst in Anthoceros it forms a more complicated
growth.'^

3. Reproduction in the Fungi (Thallophyta). — Two main
methods of reproduction occur, viz., an asexual, by means of
spores, and a sexual type (Phycomycetes), often of the nature
of conjugation. In the former, or asexual method, spores are
often formed at the ends of special hyphal branches or
gonidiophores, the spores being here known as gonidia (Mucor).
At times, on the other hand, the ends of certain hyphae develop
into special strutures known as asci, which are enclosed in an
ascocarp, and spores (ascospores), to the number of eight, are
formed in these in rows, being, later on, freed by rupture of the
asci. Spores may also be formed in sporangia (endospores).

In the more highly differentiated Fungi (Agaricus, &c.), large
fructifications are formed, and on the under surfaces of the ter-
minal parts of these — viz., the hymenium — delicate lamellae are
produced, from the two surfaces of which spores arise upon
hyphal structures known as basidia, there being four spores to

* The sporogonium at times develops a “foot,” which attaches it to
the gametophyte. In Anthoceros {Hepaticce) and Funaria [Musci) the
foot is well marked.

REPRODUCTIVE CELLS.

147

each basidium ; the fructification may be a closed structure, as
in the Truffle, or Puff-ball, and the spores here originate in
special hyphal filaments or asci {Gasteromycetes) .

In the lower fungi {Zygomycetes and Oomycetes), at times, a
sexual mode of reproduction occurs, in that two similar hyphae
approach one another and meet by their somewhat club-shaped
extremities. Fusion of the adjacent cells then occurs, and the
resulting body puts on a thick pigmented wall, and is known as
a zygospore. In a few cases (Eurotium) two somewhat dis-
similar organs may be produced from adjacent filaments. Thus,
a large globular organ containing cytoplasm may arise, corre-
sponding to an oogonium, and this is fertilised by the content
of a smaller organ ( = antheridium) which has arisen close by or
from the same hyphal filament just below. The resulting mass
then becomes an oospore, and can reproduce the fungus, the
fertilised mass often forming swarmspores, which are freed later
on by the rupture of the oogonium, each being capable of
forming a hyphal filament, the promycelium.

In the Schizomycetes, or fission-fungi (Bacteria), the only
methods of reproduction are the vegetative, by simple fission,
and the reproduction by spores. The spores may arise either
by the development of large forms (arthrospores) on a main
chain of organisms, or by endogenous formation, the spore being
formed by the aggregation of the cytoplasm in certain of the
members of a colony, and the production of a thick wall round
the resulting mass. Kupture of the original wall of the parent-
cell then frees the spore thus formed {Bacillus mycoicles, Tetanus
hacillus, B. anthracis). These few instances will serve to show
the great variety of methods of reproduction in the Fungi.

4. Reproduction in the Algae (Thallophyta). — It is necessary
here to take a few well-defined types for study : thus Spirogyra,
Fucus, and Vaucheria afford three distinct varieties of sexual
reproduction in this group of plants.

{a) Conjugation in Spirogyra (see Fig. 108, 1, 2, and 3). —
This alga has already been seen to possess the ordinary vegeta-
tive mode of reproduction, but, in addition, a method known as
conjugation sometimes occurs, especially when the surrounding
conditions do not favour vegetative reproduction (towards
autumn or in colder weather). In conjugation, two similar fila-
ments, adjacent and parallel to one another, undergo with regard

148

THE PLANT CELL.

to certain cells of these filaments, a change, which results first of
all in the pushing out of small protrusions from the cell- walls of
adjacent cells (see Fig. 108). These protrusions grow out
laterally from the main cells until they meet, and then the
partitions between them become dissolved, thus leading to the
formation of a tubular passage joining the two cells ; the cyto-
plasm, chlorophyll band and nucleus of one cell then passes along
this passage into the other cell and fuses (with the exception of
the chloroplasts) with the cytoplasm and other structures of the
latter. The resulting mass is known as a zygote (each of the
original masses being the gamates), and soon takes on a thick wall
of cellulose ; the chlorophyll band of the receiving cell persists,
whilst that of the other aborts. It is usual to look upon the

Fig. 108.— Conjugation in Spirogyra. — 1. The protoplasm in two adja-
cent cells has contracted, and a protrusion from each cell has already
been formed. 2. The protrusions have met, the intermediate wall
has been dissolved, and the protoplasmic contents of one cell are
passing into the cavity of the other. 3. Fusion (conjugation) of the
two masses has occurred, and the resulting “zygote” has assumed a
wall of cellulose.

“ fertilising ” gamete as the male cell, and the other or receiving
gamete as the female element. At a later date the encapsuled
mass is set free from the cavity of the original cell, by bursting
of the wall of the latter, and soon germinates; germination
results in the formation of an elongated cylindrical cell which
divides into two and so on, so as to produce ultimately a filamen-

REPRODUCTIVE CELLS.

•149

tons colony. In one species {Spirogym quinina) five adjacent cells
conjugate with five others of a parallel filament at the same time.

(b) Reproduction in Fucus vesiculosus. — Fucus is an alga
which to all external appearances seems highly differentiated,
there being a system of branching organs which give a false
aspect of stem and leaf structures; internally, however, the
histology is seen to be of a siniple type, the main tissue being
composed of elongated tubular cells joined end to end so as to
form an open network. Externally there is, however, a simple
type of epidermis, immediately underneath which is a zone of
small “cortical” elements. The whole plant conforms, however,
to the type known as a thallus.

The organs of reproduction are situated in special parts of the
thallus, o nd consist of antheridia and oogonia which arise in spaces
known as conceptacles (male and female) found sunk in the tissue
of the thallus at the ends of somewhat club-shaped branches.
An oogonium arises first of all from a single cell at the bottom of
a female conceptacle, and this cell divides into two. The lowest
of these is the basal cell, the upper one being the oogonium
proper. The contents of the oogonium form the oosphere, and
this is a simple mass of nucleated cytoplasm (see Fig. 109, 1, 2,
3). The male organ or antheridium arises in the form of a
special branching system of tubular cells from the bottom or
sides of a male conceptacle. The terminal and a few of the
lateral cells of this branch contain c3doplasm and nuclei which
divide to form the mother-cells of the antherozooids. The
antherozooids when mature are set free by the bursting of the
wall of the parent-cell as free-swimming motile cells. Each
antherozooid is a small nucleated pear-shaped body, possessing an
eye-spot, and two laterally situated vibratile cilia (see Fig. 109, 5).

The oosphere now undergoes a process of maturation. In
this process the original cell divides into eight equal-sized
egg- cells, each of which is a potential sexual cell. At a certain
period the egg-cells are set free by the rupture of the wall of the
oogonium, and lie in the conceptacle or in the sea-water in the
vicinity of the main plant. Some hair-like structures, the
paraphyses, which arise from cells at the bottom and sides of the
conceptacle, possibly serve to retain the egg-cells in the chamber,
so that occasionally fertilisation may take place in the con-
ceptacle itself.

150

. THE PLANT CELL.

2 .3

Fig. 109.

REPRODUCTIVE CELLS.

151

Fertilisation consists in the fusion of an antherozooid with an
egg-cell, the nuclei of both participating in this fusion ; as a rule,
a large number of antherozooids may be seen swimming round
one of the spheroidal egg-cells, but only one motile cell is needed
for the purpose of fertilisation (see Fig. 109, 6).

After fusion, the fertilised egg-cell (oospore proper) is divided
by tw'o walls into four cells, and subsequent divisions result in
the formation of a pear-shaped structure which, after a time,
becomes fixed by a branching “ foot ” at one extremity to a
suitable support. The foot does not function as a root, or
absorbing organ, but only as a means of attachment.

(c) Reproduction in Vaucheria (see Fig. 110, 1, 2, 3, and 4).
The reproduction of Vaucheria by means of swarmspores has
already been studied. The other method is a sexual one in

Fig. 109.— Rei’Roduction in Fucus vesiculosiis (Diagrammatic).— 1. Sec-
tion across a female conceptacle showing oogonia springing from the
bottom of it and [numerous paraphyses. 2. A single oogonium, com-
posed of a basal cell and an oogonium proper, which contains the
oosphere. 3. Division of the oosphere into eight egg-cells previous to
fertilisation. 4. An antheridial “ branch ” from a male conceptacle ;
the antheridia spring from the end and sides near the top of the
“branch.” The antheridia contain the mother-cells of the anthero-
zooids. 5. A single antherozooid, with lateral eye-spot and two
vibratile cilia situated laterally. 6. An egg-cell, freed from the
oogonium and surrounded hy antherozooids ; one of these will
ultimateh’ fuse with the cytoplasm of the egg-cell, the nuclei also
fusing.

which the antherozooid and the oosphere form the effective cells.
An antheridium and an oogonium arise on the same filament
close to one another, by the formation of protrusions of the cell-
wall into which a certain amount of cytoplasm flows with nuclei
and chloroplasts. The antheridium is a small curved structure,
and the apical part becomes cut off from the lower portion by a
thin partition-wall. In this apical part the cytoplasm and
nuclei are soon difierentiated into a number of ciliated anthero-
zooids, these being somewhat similar in structure to the anthero-
zooids of Fucus. The oogonium arises close to an antheridium,
in a similar manner to the latter, by the cutting off of the pro-
trusion from the main filament by a thin partition-wall ; the
cytoplasmic contents of this protrusion form the oosphere.

152

THE PLANT CELL.

Fig. 110. — Reproduction in Vaiicheria (Diagrammatic). — 1. Portion of a
filament of Vaucheria, with two protrusions arising near one another.
2. One of these protrusions has been converted into an antheridium
(the smaller one), the other into an oogonium, both being shut off
from the main filament by their walls. 3. The oosphere in the
oogonium has secreted a plug of mucilage, and antherozooids are
being attracted by this. 4. The result of fusion of one of the anthe-
rozooids with the oosphere ; the oospore has assumed a wall of
cellulose, and later will be freed from the oogonium by rupture
of the wall of the latter.

KEPRODUCTIVE CELLS.

153

Fertilisation takes place by the passage of an antlierozooid
into the oosphere, a plug of mucilage containing a chemical
substance secreted by the latter acting as a means of attraction
for the antherozooids (positive chemotaxis) ; fusion then occurs,
and the result of this is an oospore, which soon assumes a thick
wall of cellulose. After a period of quiescence the wall of the
oogonium ruptures and frees the spore, which germinates, form-
ing a typical Vauclieria filament.

Note. — The study of the reproductive processes in the Algse is often best
carried out by first growing filaments, &c., in an aquarium, ^o that j)lants
may be gathered and examiii^d at frequent short intervals. Spirogyra
“conjugates” tow'ards autumn as the water is getting colder, and so is
unfavourable for vegetative reproduction. The same applies to Vauclieria.

In Fucus, the conceptacles are found in the club-shaped swollen ends of
certain fertile branches of the thallus, and externally look like small “pits”
or dimples in the surface of these. Sections may be taken in the transverse
direction, and these will often cut the conceptacles at the sides of the branch
in a direction perpendicular to the surface.

Antheridia and oogonia in Fucus arise in separate conceptacles.

The Homology of the Various Types of Reproduction.

By the term Homology, used in connection with reproduction,
is meant a comparison of the various stages in the reproductive
cycles of different groups of plants, and is interesting from the
fact tliat there may often be traced in the higher types studied,
remnants of phases whicli are more or less marked and of
importance in the cycles occurring in lower types. The study of
Homology is thus the only reliable method of placing a plant in
its correct position in the scale of evolution, and as such, should
be given due consideration in the study of Botany.

The comparison of tlie various reproductive cycles which have
been examined is best made by drawing up a table, showing, in
each group, the successive stages met with during maturation of
the primary sexual elements, up to the time when fertilisation is
completed by the union of the effective cells produced during
the maturation process in each element. Such a table would be
somewhat as is seen in the table of Homologies, facing p. 154,
the male and female elements being distinguished by the
symbols ^ and $ respectively.

From this table it may be seen that in Angiosperms, the
antipodal cells, formed during the maturation of the embryo-sac

154

THE PLANT CELL.

(macrospore) are to be regarded as a remnant of a former
female prothallium, which is represented in the Gymnosperms
by the prothallium formed early in the embryo-sac, and which
is existent in the macrospore of the Heterosporous Pteridophyta.
The synergidae, in like manner, have been looked upon as
remnants of archegonia, or oogonia, occurring in Gymnosperms,
or Pteridophyta, as special organs growing upon the female
prothallium or gametophyte (sexual generation). The prothallus
of Homosporous Pteridophyta is in reality a double structure,
homologically, although the spore from which it is produced
shows no signs of a mixed or hermaphrodite nature. It is not
homologous to the prothallium of Gymnosperms alone, but to
the combined prothallial cell of the microspore, and the pro-
thallium of the embiyo-sac or macrospore. The antherozooids
of the Pteridophyta and lower types are homologous, not with
the whole microspore of Angiosperms and Gymnosperms, but
with the generative cells only in that structure.

In the Heterosporous Pteridophyta, the male prothallium
formed in the microspore is homologous with the prothallial cell
formed early during the maturation of the microspore in some
Angiosperms {SjMrganium) and Gymnosperms, and the female
prothallium formed in the macrospore is homologous with the
prothallium formed early in the embryo-sac of Gymnosperms,
and, as was above stated, probably with the antipodal ells
produced during the maturation of the macrospore in the
Angiosperms.

In the Cycadese (Gymnosperms) antherozooids or spermato-
zooids are met with which are produced in the microspore, and
the prothallial cells cut off early in this microspore somewhat
recall the male prothallium formed in the microspore of Hetero-
sporous Pteridophyta. The Cycads probably form the nearest
existing link between the Angiosperms and the Pteridophyta,
more especially with the Heterosporous members of that group.

The phenomenon of double fertilisation in the Angiosperms
has no parallel in the lower groups. By this method a secondary
prothallium is produced in the macrospore (embryo sac), by
means of which the embryo is nourished during the period
which precedes germination, and for a short time after it. In
the lower groups, however, the embryo-sporophyte depends
partly for its first nutriment upon the cells of the prothallus

TABLE OE HOMOLOCHEB.

\To/axxp. 154.

ANGIOSPERMS.

Miepospope

ProthaUial cell
(usually absent)
Genepative cell

Synergidiu
Antipodal cells
-Definitive nucleus
Oosphepe

Endospepm

Spopophyte

= spore-forming generation.

Antipodal cells = Rudiments of a ppimapy
ppothallium or gametophyte (female pro-
thaUium of Macrospore, Heterosporous
Fteridophyta).

Synepgidse= Rudiments of fopinep Arche-
gonia, or Oogonla of Fteridophyta.

Endospepm is the result of double fep-
tilisation and = a secondapy ppothal-
lium.

Ppothallial cell in Microspore (when
present) = Rudiment of Male pPOthal-
lium homologous with Male ppothallium
in Micpospope of MaPSilea (Hetero-
sporous Fteridophyta).

Genepative cell= Anthepozooids of lower
types.

GYMNOSPERMS.

MicpOspope

Ppothallium

Apchegonia

Oospope

Embpyonal cells
Spopophyte

Rimarks :

Ppothallium in Macrospore=Female ppo-
thallium produced in MaCPOspOPe of
Marsilea (Heterosporous Fteridophyta).

Apchegonia = Oogonla of either Homo-
sporous or Heterosporous Fteridophyta.

Genepative cell = Anthepozooids of lower

types.

In triut-po'and Cycadece {Zamia) Antherozooids
are produced in the Microspore in the xjlace
of non-motile generative cells.

FTERIDOPHYTA.

(A) Homospopous type.

Ppothallus = Gametophyte

I — sexual generatioii

S Antliepidia
Anthepozooids

Oogonia 9
Oosphepes

Oospope

Spopophyte

= asexual generation.

(B) Hetepospopous type.

MacPOspope 9

Male ppothallium
I -- sexual
I generatiot

Anthepidla

Anther

Spopophyte

= asexual generation

Altepnation of Generations occurs in
these Groups.

(A) Musci and Hepatiese.

Moss-plant = Gametophyte

I = sexual generation

S Antherldia
Anthepozooids

Oogonia 9

Oosphere

Sporogenous cells
= Spopophyte

= asexual generation

Alternation of Generations occurs here, the
Spopophyte and Gametophyte being
united in the same plant, the Sporo-
gonium producing a “foot” which attaches
it to the Gametophyte.

Vegetative propagation hy Gemmae in both

(A) Asexual reproduction by
means of spopes, viz.

Gonidia (exogeiioits).
Ascosporea (endogenous).
Uredospores,

Teleutospores (exogenous).
&c., &c.

(B) Reproduction by conju-
gation of two similar
hyphal cells : the product

= a Zygospore.

i. A structure resembling
an Oogonium, together
with

ii. A structure resembling
an Antheridium,

the product being an Oo-
spore.

Type = Eurotiiim.

(H) Vegetative propagation by
means of —

i. Sopedia (Lichens).

(A) Asexual reproduction by
means of swapmspopes.

Types = Vauchei’ia.

(B) Reproduction by conju-
gation of two similar pro-
toplasts (Gametes) : the

product of union of two

Gametes = a Zygote.

Type = Spirogyra.

the essential cells in—

i. Antheridia, together
with

ii. Oogonia,

the product being an Oo-
spore.

Types = Vaucheria.

Fucus.

(Edogonium.

Ckara.

[lEPRODUCTIVE CELLS.

155

c in Hoinosporous Pteridophyta, and those of the female pro-
thallium in the macrospore of Heterosporous Pteridophyta.

In the Bryophyta an apparent anomaly is encountered, for it
is not, as would at first appear, the moss-plant (Musci) which is
the sporophyte, but a rudimentary mass of cells, the sporogonium
and sporogenous cells. The gametophyte,. or product of ger-
mination of the spore is here a much more highly differentiated
plant than the sporophyte, and in Musci forms the moss-plant
proper. In the Hepaticae, the sporophyte and gametophyte
are, as in the Musci, fused in the one plant, a sporogonium
being produced, which corresponds to the sporophyte, or asexual
generation. Germination of a spore produced in the sporo-
gonium results in the formation once more of the protonema, from
which arises the moss-plant or Liverwort proper (gametophyte,
or sexual generation).

In the Thallophyta, the homologies become somewhat limited.
The antheridia and oogonia are, of course, homologous structures
to those found in the Bryophyta and Pteridophyta, but beyond
this it becomes very difficult to trace their reproductive relations,
although attempts have been made to do so.

Certain divergencies from the main type of maturation of the
embryo-sac in Angiosperms are sometimes met with. Thus, in
Peperomia, the primary nucleus of the embryo-sac divides into
sixteen, instead of eight nuclei, and these are uniformly distri-
buted through the cytoplasm, instead of forming an egg-apparatus
(synergidae and egg-cell) and antipodal cells. No polar nuclei are
met with in this case.

In Sparganium simplex, the antipodal cells divide many times,
and give rise to a mass of one hundred and fifty, or even more,
cells ; in this plant also a prothallial cell is met with in the
microspore. These instances are interesting, as they point to a
sort of reversion to ancestral processes.

Parthenogenesis, or the development of an unfertilised egg-
cell, is known only in the case of Ohara crinita (Algae). The
development of parthenogenetic eggs is more common in the
animal kingdom, notably in the case of Daphnia (water-fleas).

156

CHAPTER X.

CHEMICAL AND PHYSIOLOGICAL STUDIES IN
CONNECTION WITH THE CELL.

It is now necessary to direct attention to some of the more
important chemical and physiological processes to be observed in
the living plant, processes which are, moreover, to be looked
upon as the reflection on a large scale of what is going on in
each living cell.

Some of these processes may be demonstrated in the cell itself
by the use of suitable reagents, and yet others are only to be
detected by the employment of experimental methods involving
the use of the whole, or, at any rate, a large part of a plant.
It was, moreover, seen in Chapter i. that the vitality of the
protoplasm depends upon the maintenance of certain conditions,
such as an adequate supply of water and oxygen and the co-
existence of a suitable temperature, and also that protoplasmic
continuity between the living cells in a cell-community was
a necessary factor. Therefore, in the performance of laboratory
experiments upon plants, or parts of plants, it is often essential to
ensure the presence of those conditions under which the plant
investigated exists in nature ; otherwise the results of experiment
will be inaccurate and hardly expressions of natural processes.

A. The General Chemistry and Physiology of the Cell.

Before proceeding to the detailed study of some of the more
readily demonstrable chemical processes taking place in the cell,
it is advisable to have an outline of the chemistry and physiology
of that structure, looked at from a general point of view. The
protoplasmic contents of a typical assimilating cell may be looked
upon as a very efficient energy-transformer and utiliser, in which
the principle of the conservation of energy holds good, just as it
does whatever the working substance may be.

CHEMICAL AND PHYSIOLOGICAL STUDIES.

157

The vital processes involv^ed in the building up during deoxi-
dation and subsequent breaking down of the protoplasm during
oxidation are included under the comprehensive term meta-
bolism, the building up process being known as anabolism,
and the breaking down katabolism. Thus, it is usual to speak
of the nitrogenous and carbohydrate metabolism of a cell, these
two being, in fact, the main vital phenomena in assimilation.
Moreover, wherever metabolic activity is proceeding, water must
always be present, since every chemical reaction in the cell
involves this substance. In this respect, then, it would be quite
as correct to use the term assimilation in connection with water
as it is in the case of the formation of carbon-compounds from
the CO2 derived from the medium surrounding a plant, or the
manufacture of proteids from the nitrogenous raw materials
supplied ; for in both these latter cases water is assimilated quite
as much as carbon and nitrogen.

For growth to proceed satisfactorily in a green plant there are
certain elements, in addition to those already mentioned (see
p. 7), which have been found to be absolutely indispensable.
These are Potassium, Phosphorus, Calcium, Magnesium, and Iron.
Sodium does not appear to have the same importance as Potassium
in metabolism, and most plants can do without it ; in Fungi, on the
other hand. Calcium may be dispensed with, but Iron is necessary
to the normal growth of these plants. Certain elements — viz.,
Ca, Mg, K, and Fe — are always found in the ash produced by the
combustion of protoplasm, but it is probable that salts of these
metals exist in the living substance not in any chemical combina-
tion, but rather as substances which it is extremely difficult to
get rid of during analysis. They are thus termed metaplasm.
The ash of plants contains in its composition many more
elements than the four mentioned above, but, as in the case of
Iodine and Bromine in sea-plants. Silica in cereals, and, at times.
Aluminium, such elements are not absolutely essential to growth.
Nearly every element, including some of the rarer ones (rubidium,
thallium, &c.), has been found in the ash of various j^lants, but it
appears that only Potassium, Magnesium, Calcium, Iron, Sulphur,
Nitrogen, Phosphorus, Oxygen, Carbon, and Hydrogen (with,
perhaps, sodium and silicon) have any real metabolic value.
As will be shortly seen. Iron is essential for the formation of
chlorophyll.

158

THE PLANT CELL.

There is another very important consideration to be taken
into account in many of the vital processes which go on in a cell,
and that is the formation and action of those peculiar bodies
known as the enzymes (unorganised ferments). The chief
feature about these substances is the fact that very small
quantities of them will produce very marked and extensive
ehemical changes in other substances. Their action may be
expressed by the term catalytic, somewhat after the mode of
operation seen in the reaction between oxide of manganese and
chlorate of potash in the manufacture of oxygen.

Of these enzymes the best known (in plants) are diastase,
which converts starch into dextrine and sugar,* and certain
peptic ferments which are present in the leaf-cells of such plants
as Drosera and Carica papaya. The process which takes place
when an enzyme acts is known as hydrolysis.

In the plant-cell, just as in certain animal cells (cells of glands),
the enzymes are probably formed by protoplasmic activit}^, a
precursor known as a zymogen, being first of all produced, and,
subsequently, by the action of water or an acid upon the
zymogen the enzyme is formed (see p. 159). The importance of
enzymes in a cell is undoubted, as upon their action depend most
■of the chemical changes involved in the conversion of reserve
carbohydrate and proteid into forms more suited for direct use
by the cytoplasm.! The anabolic processes taking place in a
-cell are in many instances very complex, and it is only in a few
cases that distinct intermediate stages can be recognised, when
such substances as starch and proteid are converted into proto-
plasm. Recently the views concerning nitrogenous metabolism
have undergone a certain amount of revision, particularly when
it was shown that some plants {Bacteria) were able to utilise the
free nitrogen of the air, and convert it into substances which
were of further value to plants as sources of nitrogenous food-
material (see infra). A similar instance is that where filaments
of Beggiatoa are able to utilise sulphuretted hydrogen existing
in solution in natural springs, converting it into sulphur and
sulphuric acid by a process of oxidation. The kataholic side of

* Some forms of diastase (cybase) can dissolve cellulose.

t The action of enzymes increases up to an optimum temperature
ranging from .30° to 45° C. Enzymes are destroyed at a temperature of
from 60° to 70° C. Darkness or subdued light appears to favour their
action.

CHEMICAL AND PHYSIOLOGICAL STUDIES.

159

metabolism is often quite as complex as the anabolic. The
oxygen required by the cell for the purposes of oxidation is
obtained either from the air or water surrounding it or from
easily reducible substances in the cell itself. Most of the ox}^gen
produced during the assimilation of CO2 and H^O is, as will be
shortly proved, evolved from the cell as free oxygen, and is not
utilised for the purposes of oxidation, although, in the case of
water-plants, some of it may be dissolved in the water and
re-utilised. As will be pointed out a little further on, the inter-
mediate reactions involved in nitrogenous katabolism occasionally
result in the formation of such bodies as alkaloids or glucosides ;
and many of the bye-products of both carbohydrate and nitro-
genous metabolism consist of organic acids, such as oxalic, tannic,
meconic, ulmic, &c., which combine with bases present in the
cell-sap to form definite salts which at times separate out in
the sap {vide raphides). These bodies — viz., the alkaloids,
glucosides, and organic acids — are, as a rule, removed to those
cells of a plant where they will have no further action upon
metabolism.

Constructive processes in the cell are partly anabolic, and
partly katabolic ; thus, the building up of fresh protoplasm from
proteids, carbohydrates and amido-acids (see infra) is an anabolic
process, whilst the formation of cellulose, wood, and cork are
instances of katabolic construction, cytoplasm being broken down
again in these latter.

The Enzymes (ferments), which have been mentioned above,
are formed by the protoplasm by a sort of double process — viz,,
anabolic to start with, and the substances so produced (zymo-
gens) are broken down again (katabolism) to form the ferment.

Oils and fats arise in the cell during metabolism by a break-
ing down of the cytoplasm during oxidation; and many of the
non-nitrogenous vegetable acids met with are products of kata-
bolism, but in a few instances they may be formed as bye-
products during anabolic processes (oxalic acid).

The formation of the cell-wall by the cytoplasm has been
shown to be connected with the deposition of microsomafa upon
the wall, and the conversion of these into cellulose (or pectose)
by a process of self-decomposition (secretion). The cell-plate
{vide Chap, viii.) is formed in much the same manner.

In a few cases the formation of oils and fats has been shown

■160

THE PLANT CELL.

to take place in the substance of small structures comparable to
plastids, known as elaioplasts ; in these bodies glycerine and a
fatty acid are combined to form the oil or fat.

The deposition of starch in the plastids and chloroplasts is in
the main a process of secretion ; the sugar, which is first formed
during photosynthesis (see infra), being utilised for the purpose
of starch-formation (storage); this process is thus katabolic in
nature.

The cell obtains energy for the purposes of elaboration of
food from several sources, viz.: —

a. Light.

h. External heat.

c. Internal heat liberated during oxidation. *

The influence of heat upon vital activity in a cell increases
up to a certain point, the so-called optimum temperature, after
which it again decreases.

With regard to the relation between heat and chemical
action, the following reservations must be made: — Some
reactions require for their completion heat from outside or from
the cell itself, and these are known as exothermic reactions,
whilst others evolve heat during their progress, and are called
endothermic reactions. In the former case the cell loses a
certain amount of energy, whilst in the latter energy is gained.
Occasionally reactions occur which may be exothermic or endo-
thermic according to circumstances, and these are known as
reversible reactions. In the case of the energy of light rays
(radiant energy) it will be seen further on that the chloroplasts
are able to transform the radiant energy into energy of chemical
action (actinic), and in this manner the chloroplast is enabled to
form starch (or sugar) from the raw materials COg and HgO
supplied to it.

Occasionally the energy of chemical action (oxidation) is
intense enough in plant-cells to cause luminosity (certain
Bacteria). This phenomenon is, however, not so frequent in
plants as in animals (see infra).

The absorption of water by germinating seeds is often
attended with a considerable evolution of heat, due partly to

* A large part of the energy of a plant is derived from the oxidation of
carbohydrates during respiration.

CHEMICAL AND PHYSIOLOGICAL STUDIES.

161

actual combination of the water with protoplasm (comparable to
tlie formation of HgSO^ : a’H^O when water is added to sulphuric
acid). This is an endothermic reaction, and is a source of energy
to the cell. In this case, also, the heat evolved during respiration
must be taken into account. On the other hand, the action of
enzymes mentioned above is in the main one of breaking down of
complex compounds into simpler bodies (hydrolysis), and, as such,
often requires heat from 'outside — viz., it is an exothermic re-
action, and involves a loss of energy to the cell. The cellulose and
woody framework of a plant represent a store of 'potential energ'y,
whilst the oxidative processes in the cell liberate an amount of
kinetic energy, which appears in the form of heat.

B. Details of Vital Processes.

Having now obtained an outline of the main vital processes to
be considered in the cell, it is necessary to examine in detail a
few of the more important of these, and, where possible, try to
elucidate some of the intermediate stages in the formation of the
essential food-substances elaborated by a cell from the raw-
material supplied.

In this respect the following will be described : —

i. Starch and starch-formation.

ii. The relation existing between chlorophyll, lig’ht, and the
assimilation of CO2 and H2O.

iii. The formation of elaborated nitrogenous food.

iv. The cell-sap and the mechanics of sap-conduction.

V. The evolution of oxygen during assimilation, and of carbon
dioxide and water during respiration.

vi. The assimilation of carbon dioxide and water from the
surrounding medium.

vii. Variations of protoplasmic activity under different con-
ditions, especially those concerned with growth in light Of varying
refrangibility, and the effect of gravity and other physical
agencies upon growth.

viii. The production of heat, light, and changes in electrical
potential in cells of plants ; action of eleCtriC Currents upon
cytoplasm.

Each of these must be considered in detail ; v. and vi.
include experiments which demonstrate the processes mentioned.

162

THE PLANT CELL.

i. Starch and Starch-formation.

Starch, which has a composition represented by the general
formula CgHj^Og,* is a carbohydrate belonging to the group of
polysaccharides. It occurs in plant-cells, either alone, or in chloro-
plasts or plastids, in the form of granules and grains of various
sizes and shapes. In order to examine starch, a small portion of a
thin slice of a potato-tuber should be placed in a small drop of
water on a slide, and gently squeezed. The slice is then
removed, and the now somewhat opalescent drop covered with
a cover-slip and examined under a low power. Numerous
starch-granules are then seen, which, when examined by trans-
mitted light, have a semi-translucent retractile appearance, but
by reflected light are white and opaque. The size of the indi-
vidual granules varies from a small circular particle to the large
oval grains many times the size of the former; and, by using
the -g-in. objective, cutting off the peripheral illuminating rays,
and using somewhat oblique illumination, one of the larger
granules may be seen to possess the following structure
(see Fig. Ill) ; —

а. A dark spot situated somewhat eccentrically : this is the hilum
of the grain.

б. Outside this alternating layers of light and dark lamellse, ar-
ranged round the hilum, but, as a rule, thicker on one side of the hilum
than the other (see 4, Fig. 111).

c. If the plane mirror of the microscope be used for illuminating, the
rays will be partially polarised ; and if, after these rays have passed
through the starch-granule, they be again passed through a Nicol’s
prism (analyser) in the eye-piece of the microscope, they can be
analysed by rotating the prism (contained in the eye-piece) so that its
axis assumes different inclinations. The result of this analysis shows that
a granule of starch is made up of alternating zones of two substances
which rotate the plane of polarisation in different directions, and
that one of these substances contains more water in its composition than
the other. The starch-grain is thus anisOtropiC.

In form, the larger granules in cells of potato-tubers are
oval, whilst the smallest are circular, no hilum being present in
these latter. In the cells of maize endosperm, the granules are
polyhedral, and in the rhizome of Iris, dumb-bell shaped.

In potato, much of the reserve starch in the tuber-cells is

* Usually found together with a certain amount of water of constitu-
tion. Cellulose is represented by the formula ^(CeHioOs).

CHEMICAL AND PHYSIOLOGICAL STUDIES.

163

formed at first in plastids,'" and by the time the tuber is full
grown, all the plastids have been converted into starch. During
the examination of the chloroplasts in the cells of Vallisneria

Fig. 111. — Various Starch-grains from Cells of the Potato-tuber.
1. Small granules in which the first signs of a hilum and concentric
laminae can be detected. 2, 3, 4. Grains of various shapes and sizes
in which the hilum and lamination are well marked. 5, 6. Grains in
which the starch is deposited regularly at first, and subsequently
somewhat irregularly. 7. A plastid from the rhizome of Iris
germanica, showing the formation of starch at both ends. 8. The
resulting dumb-bell shaped granules formed from 7. 9. Effect of

boiling water upon a starch-grain : the outer envelope is the farinose,
the inner granular portion (stained with iodine) is the granulose.

* Some of the starch is, however, formed in the tuber by the trans-
location of carbohydrate from the cells of remote parts, and starch is
then reformed by the plastids from sugar, &c.

164

THE PLANT CELL.

leaf (Chap, ii.), it was seen that the starch-granules formed in
them were produced (or rather stored) during the daytime in
the presence of light, but in the formation of starch in the
plastids in the absence of light a somewhat different process goe&
on, although the ultimate product is the same. Thus, in
plastids it is highly probable that elaborated food (sugar)
from the leaves is used, and gradually worked up by the proto-
plasm, each plastid being, as has been pointed out, a specialised
portion of the cytoplasm of a cell, and as such, capable of acting
as a “plastic” body. After the plastid or chloroplast has been
completely converted into starch, the further growth of each
granule goes on by a process of accretion, the main cytoplasm of
the cell forming successive layers. The hilum in the larger
granules may at times indicate the position of a former plastid,
but is more often produced by splitting of the centre of the granule,
producing a tri-radiate figure. Compound and semi-compound
starch-grains are also found in the cells of the potato-tuber.

The blue reaction of starch with iodine indicates the forma-
tion of a definite but rather unstable chemical compound, which
is readily destroyed by heating or treatment with alcohol.

Boiling in water causes the granules to swell, and finally a
sort of sac or shell is produced, formed by an external substance
known as farinose, enclosing a granular substance, or granulose
which takes up iodine. Caustic potash also causes a swelling of
the granules, and a dilute acid or a solution of diastase will
dissolve the granules, especially on gently warming, with the
formation of dextrin.

The chemistry of starch-formation is rather complex. It is
not intended here to give more than a brief outline of the
process, which is, as yet, somewhat undetermined. It may,
however, be mentioned, that a good deal of the starch in plants
is the result of anabolic processes, and not of katabolic.
The main feature in these processes seems to be the elimination
of oxygen. Experimental evidence points to the fact that there
are many stages between COg, HgO, and starch in the anabolism
of these substances by the chloroplasts, and between protoplasm
and starch during the katabolisni of the living substance.

It has been thought that formaldehyde is an intermediate
product during the formation of starch (or sugar) in chloroplasts
or plastids, and that this, by elimination of water, is converted

CHEMICAL AND PHYSIOLOGICAL STUDIES.

165

into starch, thus ; — CO2 + HgO = CHgO + Og (the O2 being
evolved during assimilation), then GCH.^O — HgO = OgH^^O^
(starch).'^ At times it appears that cane-sugar may be formed
by the polymerisation of formaldehyde.

The above equations, however, are by no means a complete
representation of starch-formation, it being probable that other
intermediate stages occur. Moreover, it appears that sugar is
the carbon-compound formed as the final product, the starch
being produced subsequently by a process of secretion and
stored in the chloroplast, and the sugar first formed may be
oither cane-sugar or a hexose {i.e., dextrose). Some of the
sugar is used up at once for formative purposes, and it is the
remainder that is stored. In some cases, it seems that the proto-
plasm may be converted into starch by oxidation and splitting off
of the proteid and amine parts of the molecule (katabolic). In the
formation of the cell-plate (cellulose), some such process as this
appears to take place, starch or sugar being here used for
reconstructive purposes. The synthesis of cellulose is, however,
a rather more complex process than would appear from the
molecular composition of that substance, and the ectoplasm
next the cell-wall is probably here the Avorking substance, a
process analogous to secretion taking place.

The initial process, in Avhich the chlorophyll synthesises COg
and HgO to form sugar, is termed photosynthesis. The later
reactions involved in the production of reserve starch in the
chloroplasts are more the result of chemosynthesis.

ii. {a) The Relation existing between Chlorophyll, Light, and
the Assimilation of Carbon Dioxide. (6) Pigments other than
chlorophyll, (c) The conditions governing chlorophyll for-
mation.

{a) Chlorophyll is the green colouring matter Avhich, as has
been seen, exists in the chloroplasts, probably dissolved in an
oily substance, Avhich permeates the substance of these struc-
tures. In reality, in alcoholic solution chlorophyll is made up
of a mixture of two pigments — viz., a greenish one known as
phyllocyanin, and a yellow one, phylloxanthin.f If a leaf or other

* See Vines, Physiology of Plants.

t Recent researches seem to point to the fact that chlorophyll is a
single pigment which is readily decomposed (by alcohol or boiling Avater)
into the above-mentioned tAA'o pigments.

166

THE PLANT CELL.

green part of a plant be placed in alcohol for some hours, the
chlorophyll is extracted, and, on adding benzine in equal volume
to this alcoholic extract, and shaking up the mixture, the benzene
separates the phyllocyanin and floats on the top of the alcohol,
the latter liquid retaining the phylloxanthin.

By making an alcoholic extract of chlorophyll alkaline with
caustic potash, and examining the extract by means of the
spectroscope (the tube or special vessel containing the chlorophyll
solution being placed in the path of rays of white light before
they reach the prism), some characteristic absorption bands may
be seen in the spectrum of white light v/hich has passed through
the tube containing the chlorophyll. In all, seven such bands
occur in various parts of the spectrum, and they indicate that
chlorophyll absorbs certain of the rays of sunlight and allows
others to pass. The absorption bands are situated as follows
in the spectrum (see Fig. 112) : —

ROY G B I V

If 1 Z C 3 4 5 6 7

Fig. 112, — The Absorption Spectrum of Chlorophyll (see text). —
6, c, Fraunhofer lines in the red. d. The sodium band in the yellow
portion of the spectrum. 1, 2, 3, 4, 5, 6, and 7 are the bands in the
spectrum formed by the absorption of certain rays by the chlorophyll
(alcoholic slightly alkaline solution).

One band (the I band) occurs in the red, between the Fraun-
hofer lines B and C, another in the orange whilst a third and
fourth are in the green portion of the spectrum. There are
also three broad bands in the blue and violet at the other
end (chemical rays).

The significance of these bands is as follows; — Of the white
light which reaches a chloroplast, only those rays which are
indicated by the position of the absorption bands in the spec-
trum are made use of for the purposes of the assimilation of
COg and water, the other rays passing through. The chloroplast
is a specialised portion of the cytoplasm of a cell, and as such is
able to transform the energy derived liy the sifting out by

CHEMICAL AND PHYSIOLOGICAL STUDIES.

167

chlorophyll of certain light-rays into energy of chemical action
(luring the processes of starch-formation ; and in this process, as
has been seen, COg and water are assimilated, and certain inter-
mediate carbon compounds are formed, to be quickly broken
down or synthesised again (photosynthesis). It is probable that
the red rays (the least refrangible) are the ones most utilised in
these reactions, and since the more refrangible violet and blue
rays are also absorbed to a certain extent, it seems likely that
they are also used to further the process. Possibly these rays
are converted into others of a lower refrangibility, or, as is more
rational to suppose, the red ra}^s may be converted into those
which are known to further certain chemical actions (cf. infra,
Timiriazeff’s experiment).

If green parts of plants are kept in the dark for some time
they become etiolated — that is, the chlorophyll disappears (or
etiolin is formed), and the chloroplasts are unable to assimilate
carbon dioxide and water and produce sugar to the same extent
as before. In the case of plastids, in parts of plants which are
not greeji, formation of starch probably takes place by the con-
version by these structures of elaborated material (sugar) from
the leaves into other carbon-compounds, and finally into starch
(chemosynthesis). In some cases, also, it is probable that the
cytoplasm undergoes a gradual conversion into starch by the
s})litting off of the proteid and amine portions of its molecule.

Chlorophyll has been found in those parts of plants which
have never been exposed to the light, such as the seed-leaves of
Piniis, and in the phelloderm formed from the cork-cambium
in certain stems (see Chap. iii.). In such cases it is probable
that chlorophyll is formed in a somewhat different manner to
that in which it arises in parts exposed to the light, or else that
just enough light penetrates to these tissues for the pigment to
be formed.*

(b) Pigments other than Chlorophyll occurring in Plant-cells.
— Red, blue, and yellow colouring matters may exist in cells,
singly or combined, either in the form of chromoplasts contain-
ing the pigment, or dissolved in the cell-sap. In the cells of
the petals of Tropceolum, angular chromoplasts are to l)e

* It can he demonstrated that the light needed for clilorophyll-forma-
tiou need not necessarily he so intense as that needed for CO2 assimilation
(.see Darw in and Acton, Practical Plant Physiology).

168

THE PLANT CELL.

found in the cytoplasm, each of these containing an orange-red
pigment. The basis of each chromoplast is protoplasmic in nature.

In the carrot, the cells of the cortex possess reddish
crystalloid bodies of a proteid nature, which contain carotin.
This pigment has some chlorophyll in its composition. Such
pigments are usually formed in the presence of light, oxygen and
Fe-salts, much as chlorophyll is in the chloroplasts (see infra).
It is probable that the chemical composition of many of them is
not far removed from that of chlorophyll, particularly in the case of
j^ellow or greenish-yellow pigments, some of which bear a definite
relationship to phylloxanthin. In many instances the colouring
substances exist in a cell dissolved ^in the cell-sap, as in the
Beet-root and pericarp of many fruits. The red pigments
belonging to this group are changed to green or blue on the
addition of an alkali, and when acid is added to such an
alkaline solution, the red colour returns when neutralisation is
complete (c/. action of acids and alkalies upon litmus, a
vegetable pigment). The blue colouring matter in many cells is
known as anthocyanin.

The whiteness of many petals is due to the presence in tlie
cells of chromoplasts (leucoplasts), which reflect the rays of
white light falling upon them almost entirely. Intercellular
spaces and the convexity of the outer walls of the epidermal
cells may also contribute towards this result.

The function of many of these pigments is often, as in the
case of the petals of flowers, of the nature of an adjuvant to
fertilisation, insects being attracted by brilliantly-coloured petals;
but where a greenish-yellow pigment is present in definite
chromoplasts, ,the assimilation of carbon-dioxide may at times
occur. The majority of the pigments existing in chromoplasts
may be extracted from them by alcohol; where the colouring
matter is in solution in the cell-sap, as in the Beet-root,
boiling kills the cytoplasm, and upsets the osmotic balance
of the sap in the cells, leading to an outward diffusion of the
pigment. The colouring matter of such permanent elements
as those of wood and sclerenchyma exists in the cell-walls, and
is the product of the decomposition of substances deposited in
the walls at various times. Such pigments have, of course, no
vital significance once they have been deposited.

CHEMICAL AND THYSIOLOGICAL STUDIES.

169

in the Chloroplasts have been determined to be the fol-
lowing : —

i. The action of light. In darkness, green plants become etiolated —
that is, the chloroplasts lose their green tint, a j^ellowish one being sub-
stituted, which is due to the formation of etiolin.

ii. The presence of oxygen.

iii. The presence of traces of an iron-salt in the soil. Without this iron,
the chlorophyll is not formed. The influence of iron upon the colouring of
petals of Hydrangea is well known, and seems to point to the necessity of
the same conditions for the formation of other pigments than chlorophyll.

Etiolated plants will, when again exposed to light, develop
chlorophyll, provided the ^ther conditions of its formation be
present; and it has been shown that photosjmthesis can proceed
to a limited extent in chloroplasts in which only etiolin is present.

iii. The Formation of Elaborated Nitrogenous Food {Proteids).

The assimilation of nitrogen wdiich takes place chiefly in the
leaf-cells and other green parts of a plant is a subject which is
difficult to deal with from an elementary point of view, seeing
that it involves complicated synthetic and analytic reactions
between organic and inorganic compounds in a cell. The
nitrogen is obtained from nitrites (or nitric acid) and nitrates,
as well as ammonia at times {cf. absorption of ammonia b}--
capitate hairs). In many cases reserve proteids are present in
the cell-sap, which, when acted upon by enzymes, are converted
into albumoses and peptones, and these are then gradually built
up into protoplasm by the further agency of the living substance.
The conversion of proteids into albumoses and peptones is mainly
a process of hydrolysis, the elements of water entering into the
reactions, a fresh compound being then formed bj" the splitting up
or reconstitution of the previous one. The formation of proteids
in a cell involves synthesis of a high order. During this process
various waste-products are formed, notably oxalic acid, and in
some cases this acts upon calcium nitrate in the cell-sap, forming
calcium oxalate, the released nitric acid being again assimilated.
S})eaking generally, in the synthesis of protein, the following
substances are involved in the reactions ; —

Nitrites, nitrates, phosphates, sulphates, and clilorides of K, Ca, Mg, Fe,
and at times other metals ; water, carbon-compounds (starch, &c.), ammonia,
asparagin, and nitric acid. The actual chemistry of the process is a subject

170

THE PLANT CELL.

which is as yet somewhat undecided, and beyond the scope of an elemen-
tary text-book. It may, however, be mentioned, that the synthetic
processes involve the interaction of substances known as amido-acids
(asparagin), and a carbohydrate, together with a sulphur-contpJning
compound. The assimilation of nitrogen is thus a process of cheiTlO-
synthesis as opposed to photosynthesis, and can proceed in the
absence of light.

With regard to the ultimate fate of carbohydrate and nitro-
genous materials in the cell, it is important to remember that of
these essential food-substances, there are two main parts — viz.,
that which is at once utilised by the cell-protoplasm, and known
as circulating proteid, or carbohydrate, and that which forms
stored or reserve food. As has been seen, the various enzymes
are constantly at work converting reserve starch and proteid into
soluble substances, which can pass from cell to cell, from the parts
where they are manufactured or stored to remoter cells of a plant.
If a leaf in which starch has been actively formed during the
daytime be examined early, before the next day’s assimilation
has started, it will be found that during the night-time all the
starch has been used up, in fact, has been converted into sugar,
which has been transported to other parts as circulating food-
material. The same may be said of the asparagin (an amido-
acid), formed in the leaves ; this substance quickly passes away
from the leaf-cells, and is, together with carbohydrates and
sulphur- compounds, constructed into proteid and protoplasm in
remoter parts. It has been shown that hydrocyanic acid can at
times be used for proteid construction in the place of an
amido-acid.

iv. — The Cell-sap and the Mechanics of Sap-conduction.

The cell-sap is a fluid which varies somewhat in composition
according to the part of a plant from which it is taken. Thus,
the watery sap which is present in the root-hairs of a root, and
is conducted upwards by the woody portions of root or stem,
contains far less solid matters in solution than the sap of the
elaborating cells of the leaf, or of the downward-conducting
elements of the phloem. Nevertheless, from a general point of
view, it may be said that the cell-sap is made up of the follow-
ing substances : —

a. Water.— In some cases 08 per cent, of the sap is composed of water.

h. Mineral matters in solution — viz., salts of sodium, potassium.

CHEMICAL AND PHYSIOLOGICAL STUDIES.

171

lithium, magnesium, calcium, and at times silicon. Of these, the
nitrates and nitrites of sodium and potassium, the phOSphatCS and
chlorides of the same metals, and the salts of calcium and silicon,
occur in the sap of most plants, silica being present in the sap of cereals
to a considerable extent. In sea-weeds and shore-plants, iodides and
bromides of sodium and potassium are also found. Sulphates of the
above metals also occur.

c. Dissolved Carbohydrates (sugars— viz., glucose, cane-sugar,
and mannite, inuliii), and amides (asparagin). Dextrins also occur
as intermediate products between starch and sugar ; gumS, such as
arabinose, tragacanth, &c., may also be present. Proteids, albumoses,
and peptones are also present at time.s. Some of the proteids— e.g'.,
gluten — are insoluble.

d. Soluble Alkaloids and Glueosides. — These are bodies which
are intermediate products in the katabolism of nitrogenous substances
and protoplasm in plant-cells. The alkaloids are amides — viz., are
NII3, in which one or more H atoms are replaced by a radicle. The
glueosides are nitrogenous bodies composed of amines combined with
glucose. Both of these bodies separate out sooner or later in the cell,
and are rendered harmless .

Examples of the alkaloids are : quinine {Cinchona), nicotine (a liquid
alkaloid contained in the leaves of Nicotiana tahaca), atropine (in the
berries of Atropa belladonna), strychnine (in the seeds of Strychnos mix
vomica) ; and, as instances of glueosides, may be given, digitoxin (Z)^g'^7a^^•s
purpurea), picrotoxin, and many others. In the case of the leaves of
Prunus Laurocerasus, a glucoside amygdalin is present, which, when
acted upon by emulsin (an enzyme), breaks up into hydrocyanic
acid, glucose, and benzoic aldehyde, this being one way in which the
glueosides are split up in plants (hydrolysis).* The glueosides may
l)e confounded at times with tannin, with which substance glucose often
exists in loose conibination.

e. Ferments. — These are the unorganised ferments or enzymes, such
as diastase, papain, and the peptic ferment in the leaf-cells of
Drosera. Tliey are very important bodies. Diastase converts Starch
into dextrins (achroodextrin, erythrodextrin) and sugar (glucose),
and the peptic ferments convert the proteids into albumoses and
peptones, bodies more suitable for assimilation than the proteids. [The
organised ferments belong to the Fungi, and one of the best known
is Saccharomyces, the yeast-fungus.] Other ferments are invertase,
which inverts cane-sugar, cytase, lipase (a fat-splitting ferment), and
synaptase.

/. Organic Acids.t — These may be present as acids, or in combination
with mineral or organic bases in the cell. The acids found may be
oxalic, malic, citric, racemic, and tartaric. Tannic and gallic
acids arc often present, and salicylic acid is found in the cells of
Gaidtheria proenmhens as iueth3d-salicylate. The acids are in many
cases l)3’e- products of cell -metabolism —viz. , oxalic acid. Calcium

* Another such instance is where salicin is split up by means of
S3maptase into saligenol and glucose.

t Inorganic acids (viz., HNO3) ma3' also be present at times, and NH3
(ammonia) ma3’ occasionall3' be found. In the latter case, however, amido-
acids are soon formed.

172

THE PLANT CELL.

oxalate has been shown to be formed by the decomposition of caleium
nitrate in leaves, the calcium combining with oxalic acid, and the nitPiC
acid being subsequently assimilated.

g. Fats and waxes oceur at times in the cell, and should be men-
tioned here, although they can hardly be looked upon as being soluble in
the cell-sap. They are formed by the decomposition of protoplasm,
and, possibly, at times, by other methods. The waxes are excreted
by the epidermal cells of some leaves, and form short rods set at right
angles to the surface on the outer aspect of the external walls. In this
manner a layer of wax is formed which prevents water from collecting
on the leaf.

The cell-sap, then, contains many substances of the nature of
raw food-materials, some elaborated food- substances (such as
sugar, inulin, amido-acids, and proteids), ferments, and a good
many excretory products, or bye-products of the breaking down of
the cytoplasm. Other materials are also present, such as resins
and gums, oils, &c., which are not soluble in the cell-sap, but
which are products removed as soon as they are formed. In
many cases the resins, &c., may be looked upon as products
of the degradation of the cell- wall, and form striking instances
of substances thrown out of a cell which may nevertheless
be of great value to the plant. The manner in which
raw food-materials, such as salts of potash, sodium, silica, &c.,
enter the plant, has been partly considered already during
the examination of root-hairs and the young cell (Chap. iv.).
It was there pointed out that the ectoplasm of the root-hair
exercised a selective capacity over the absorption of salts in
dilute solution in the soil; in the one case, perhaps, salts of
sodium and potassium being taken in to the exclusion of others ;
and in the other case, possibly salts of silica being admitted
as well.

A closer consideration of the phenomena of osmosis is not
inappropriate at this point ; the absorption of the dilute solution
of earthy salts by the root-hairs is dependent upon the imesence
in the central vacuole of the hair of substances which are osmoti-
cally active — that is, which exert a distinct attraction upon the
molecules of water and salts outside the cell At a certain stage
the so-called osmotic pressure inside the cell reaches a limit at
which internal diffusion or endosmosis ceases and equilibrium
exists. But the water in the vacuole of the root-hair is being
constantly withdrawn by reason of the suction action of the
transpiration current (see p. 174), and also by further osmosis all

CHEMICAL AND PHYSIOLOGICAL STUDIES.

173

the way up the root, and, consequently, the osmotic balance of the
hair is being as constantly disturbed, so that fresh water and salts
are drawn in again from the soil. The turgid condition of a cell
when the upper limit of osmosis has been reached determines a
certain stretching of the cell- wall (turgidity), and this stretching
is a great aid to the growth of the wall in area.

Solutions may be prepared which are said to be isotonic —
that is, when a cell is placed in them neither influx nor exit of
water takes place. But a cell, such as a root-hair, when placed
in a solution which is ever so slightly greater in concentration
than the sap in the hair, will suffer a certain amount of
exosmosis — that is, water will pass out, and the hair will
shrink (see Plasmolysis, Chap. ii.).

In speaking of selective absorption by the root-hairs, it was
shown that certain salts may be taken in to the exclusion of'
others in the soil; this is quite true, but, nevertheless, the
factors determining this selective absorption, depend not only
upon the regulating influence of the ectoplasm, but also upon the
physical nature of the salt in solution in the sap inside the hair —
that is, the osmotic activity of the substances in the cell must be
considered in addition to the selective influence of the ectoplasm.

In addition to these few statements with regard to osmosis
and turgidity, it must be mentioned that during the distribution
of the assimilable food in a plant, the question of the osmotic
nature of the substances in the cells to which this food passes, is
a highly important one, and is, moreover, one which chiefly
determines whether or no such a substance as sugar, for instance,
shall be taken into any given cell. The fact that the surplus of
the circulating food is converted into reserve food, leads to a
constant movement of the diffusible materials towards the
storing cells (see also Appendix at end of Chap x.).

AVith regard to the absorption of nitrites and nitrates by
the root-hairs, it is an interesting fact that certain Bacteria
exist in the soil which are capable of converting ammonia and
free nitrogen into nitrites and nitrates. Several species of
Bacteria probably exist, each one taking on a single stage in
this process. In the Leguminosse there are certain tubercles to
be found on the rootlets, and these tubercles have been shown to
be composed of dense masses of Bacteria {B. radicicola) belonging
to a species which is able to convert the free nitrogen of the

174

THE PLANT CELL.

groimd-air into ammonia, nitrites, and nitrates. The latter
salts are then absorbed by the roots.* The bacteria are known
as “nitrifying bacteria,” one of the forms being Closterium
Pasteuriannm.

In Chap. iii. it was mentioned that certain glandular capitate
hairs were able to absorb ammonia from the atmosphere. It
should be understood, however, that it is most probably ammo-
nium nitrite which is absorbed, since this substance exists at times
in the air (cf. Thorpe’s Inorganic Chemistry^ vol. i.). The capacity
of working up nitrogen possessed by the different species of soil
bacteria has been put to practical test of recent years in con-
nection with the raising of cereals.

The manner in which the dilute solution of salts, or raw sap,
is drawn up through the vascular tissues of the root and stem
until it finally reaches the leaves of a plant must next be
examined, and in this connection it is necessary to consider two
phenomena. The first of these is the transpiration current,
and the second root-pressure.

The transpiration current is an upward flow of sap through
the wood of root and stem, caused primarily by the suction
action produced by the evaporation of water from the leaves.
It has been found that about 98 per cent, of the radiant energy
absorbed by a plant is utilised in evaporating the water of
transpiration. This evaporation takes place through the
stomata, the mesophyll cells surrounding the respiratory cavity
of each stoma, constantly giving off, during the daytime,
moisture, which collects in and is subsequently evaporated
from the respiratory chamber. The effect of this loss of water
from some of the mesophyll cells is to draw in water by osmosis
from cells of the spongy parenchyma, which are more remote,
and ultimately from the annular and spiral elements of the
leaf' traces, which, as has been seen, lie in the mesophyll of a
bifacial leaf ; and since these elements are the terminations of
the fibrovascular bundles of the petioles and ultimately of the
stem, water is being constantly sucked up from elements
•containing sap of progressively decreasing concentration (see
Fig. 113).

In this manner a current — the so-called transpiration current

* See Muir and Ritchie, Manual of Bacteriology, 1907 ; also Detmer
.and Moore, Practical Plant Physiology.

CHEMICAL AND PHYSIOLOGICAL STUDIES.

175

— is put into action, and during the daytime, for the most part,
water is evaporated from the leaves, and as constantly replaced
l)y raw sap drawn up from the roots, and, ultimately, the soil.

In direct sunlight and in hot dry weather, the transpiration
current is much more rapid than in diffuse daylight, or colder
weather; also, the relative humidity of the atmosphere influences
the rate of evaporation of water from the leaves, so that, in
moist weather, the current may be greatly diminished.

With regard to the elements of the xylem which function
most in this upward conduction of sap, it has been found that
the tracheides exercise by far the greatest influence. At times
air-huhhles in the tracheides may act, either l)y reason of
capillarity, or a sort of pumping action, as distinct aids to the
how. It was formerly thought that the cell-walls formed the

alburnum or young wood; the heart-wood or duramen is always dry.

In tracheides with border ed-pits, the flow of sap may at
times be prevented from passing in the transverse direction by
reason of the forcing of the torus on to the lumen of either side
of the pit.

That the transpiration current is not the only factor at work
in producing the upward flow of sap in a plant is shown by
cutting across a stem below all the leafy parts, when it will be
found that sap constantly exudes through the cut xylem; by
connecting up the cut surface with a manometer (a mercurial
pressure-gauge) the force producing this exudation may be
measured, and is sometimes found to be considerable. This

176

THE PLANT CELL.

force is known as root-pressure, and is the result both of turgidity
and of the damming up of the sap at certain levels in the root
and stem until a considerable rise of pressure has been produced.
As a result, when transpiration is at a minimum root-pressure
is at a maximum, and vice versa'"

By these two methods, then, the dilute solution of salts (raw
sap) is either sucked up or forced up through the xylem to the
leaves, in the cells of which active assimilation of CO2, H2O, and
nitrogen is proceeding. After the raw sap has been elaborated
and added to during photo- and chemo- synthesis it becomes
the elaborated sap, and, as such, passes into the thin-walled
phloem elements lying on the under surfaces of the leaf-
traces in bifacial leaves. It then passes down by means of
the large perforations in the sieve-plates of the sieve-tubes, and
also laterally by osmosis, and is finally distributed by osmosis
into the cortex, young shoots, cambium, medullary rays, and
other tissues needing elaborated food-material. Some of this
food-material is used at once, but towards the end of the
‘‘growing” months a good deal of it is converted into reserve-
material for use during the early months of the succeeding
spring. Such reserve-material exists in large quantities in
bulbs, tubers, corms, fruits and the phloem, medullary rays,
wood-parenchyma, and starch-sheath. The conversion of this
stored starch, proteid, &c., during the early spring is due in
most cases to the action of enzymes (diastase, synaptase, &c.).
Occasionally carbohydrates (i.e.^ starch - grains) “ wander ” from
cell to cell, being first dissolved by enzymes, and then
reconstructed in more remote parts.

Other forms of reserve food occur, the chief amongst them
being

a. Oils and fatS in many seeds.

h. Cellulose (in endosperm). This is dissolved by the enzyme
eytase.

c. Inulin (a carbohydrate) occurring as the spheroids in Dahlia.

d. Aleurone grains. These are found typically in the endosperm-
cells of Ricinus, and are composed of two parts — viz., a crystalloid of
a proteid nature, and a globoid — the latter being a double phosphate
of magnesium and calcium. Smaller aleurone grains are also found in
Zea mais in the starch-containing cells just outside the endosperm.

* Both root-pressure and transpiration exhibit periodic diurnal fluctua-
tions, which are dependent upon a property of the protoplasm known as
rhythm or periodicity.

CHEMICAL AND PHYSIOLOGICAL STUDIES.

177

e. Glucosides. The glucose formed by the action upon these bodies
of various enzymes is useful as circulating food-material.

/. Protein reserve, such as gluten, zein, and, at times, the
araido-acid asparagin, although the latter is usually more a form of
circulating than reserve food.

The various processes, both physical and chemical, which take
place in a green plant, from the absorption of raw food-materials
to the manufacture, utilisation, and storing of elaborated food in
the leaves and other parts, may, for greater convenience of
reference, be put into tabular form as follows : —

A. Absorption of Raw Materials by the Roots.

Substances absorbed = a. Water.

h. Salts in solution (chiefly salts of Ca,
Mg, K, and Fe).

B. Upward Conduction of Raw Sap by the Wood.

Moving forces = c. Transpiration current.

d. Root-pressure.

C. Elaboration of Raw Sap in Leaves and other Green Parts.

Processes involved = e. Intake of COg, HoO, and Og ; outgo of Og.

/. Photosynthesis : COg and HoO being

synthesised in the chloroplasts to
form (1) Formaldehyde; (ii.) Sugar,

g. Starch stored in the chloroplasts, and

gradually transformed into sugar by
enzymes, and used for cellulose
formation and circulating food.

h. Formation of amido-compounds, some

being used at once to form protoplasm.

D. Translocation, Utilisation, and Storage of Elaborated
Compounds.

Processes involved = i. The sugar and amido-compounds con-
ducted by means of phloem and osmosis
to tissues requiringelaborated food (cir-
culating carbohydrates and proteids).

k. Manufacture of proteidS from sugar,
amido-compounds, and a sulphur-
compound, some being used at once,
and some stored. Manufacture of
wood and cellulose.

L Storage of surplus proteid and carbo-
hydrate in various tissues ; subse-
quent conversion of these by enzymes
into more assimilable food = digestion.

Of the processes above noted, /, I, and k, are mainly anabolic, whilst
g and I are mainly katabolic ; wood and cellulose [k) are, however,
katabolic formations.

178

THE PLANT CELL.

Certain Experiments Demonstrating Life-processes in the
Cells of Plants.

Three experiments for the demonstration of important vital
processes may be considered at this point (v. and vi.), viz.: —

а. The evolution of oxygen during assimilation.

б. The evolution of CO2 during respiration.

c. The retention in the plant of carbon dlOXide during assimila-
tion (this being used during the formation of sugar preparatory to
starch-formation).

a. The evolution of oxygen during assimilation is readily
demonstrated in the case of some water-plants (Vallisneria, Elodea,
Fotamogeton). If such plants are grown in an aquarium, bubbles
of gas will often be seen to rise from the leaves to the surface
during the action of sunlight. These bubbles, if collected in
a test-tube (filled with water and inverted over the water in the
aquarium) and tested, will be found to consist of pure oxygen.

In land-plants more care is required to demonstrate the same
process. A plant is taken and enclosed in a large vessel contain-
ing atmospheric air to which a known extra volume of COg has
been added, the whole being placed in sunlight. After some
hours the gas in the vessel is tested, and is found to contain
less CO2 and more Og in proportion than was originally the case.
Here the COg evolved during respiration may be neglected, as it
is relatively small in the daytime during assimilation.

b. The Evolution of CO2 during Respiration (see Fig. 114). —
A plant is taken and placed in a vessel (bell-jar) of 10 litres
capacity (A, Fig. 114). Connected with this vessel are (i.)
two bulbs, C, containing sticks of caustic potash, and (ii.) an
aspirator, B. By means of the latter a slow current of air
(freed from COg by the absorbing action of C) is drawn through
the apparatus. In the vessel A is placed, previous to the start-
ing of the experiment, a small dish, D, which contains a saturated
solution of caustic potash. The aspirator is then stopped, the
bulbs shut off, and the whole apparatus placed in a dark place
for some hours. Under these conditions no assimilation of COg
can take place, for as fast as it is given off during respiration
it is absorbed by the potash in the dish D.

The experiment is stopped after about twelve hours, and the
COg absorbed is estimated by precipitating with baryta water,

CHEMICAL AND PHYSIOLOGICAL STUDIES.

179

and the resulting barium carbonate collected, washed, dried, and
weighed. From the weight thus obtained the volume of COg
evolved may be readily calculated. If, in the above experiment,
the gas remaining in the vessel A be tested, it will be found to
contain less 0., in proportion to N than atmospheric air, thus
proving that oxygen has been used for the purposes of internal
oxidation of carbon compounds or protoplasm, and an equivalent
volume (or nearly so) of COg evolved. During respiration in
the plant -cell water is formed as well as carbon dioxide.^

Fig. 114. — Apparatus for Demonstrating the Evolution op COg
FROM A Plant during Respiration. — A, Large bell-jar; P, plant;
D, small dish containing a saturated solution of caustic potash ;
C, tubes containing sticks of caustic potash to absorb the COg of the
atmosphere before it is drawn into the jar A ; B, aspirator.

In connection with respiration it is necessary to mention that
many plants possess amongst their internal tissues air-spaces
and intercellular spaces which are filled with oxygen or atmo-
spheric air. These spaces form at times channels of aeration of
considerable extent, more especially in such plants as Nymphcea,
NuphaVy and Equiseium. The value of such aeration is manifest,
in that the oxygen needed for respiration surrounds masses of

* Some plants (anaerobic bacteria) are able to exist in the absence of
free oxygen, and in them a process known as intramolecular respiration
goes on.

180

THE PLANT CELL.

internal tissue, and is thus directly available. The system of
spaces lying amongst the spongy-parenchyma of a bifacial leaf is
an instance of a similar provision for adequate aeration.

c. The assimilation of COg or rather the intake of COg . HoO
( = carbonic acid) into plant-cells for the purposes of sugar-^
formation is demonstrated by placing a quickly-growing plant in
a vessel, which is then filled with a mixture of air moisture
and COg in known proportions, the COg being in excess of what
it is in atmospheric air. This vessel is then placed in sunlight
for about six hours, and at the end of that time the experiment
stopped and the gases in the vessel tested.

It will be found that as a result of the intake of CO2 into the
mesophyll cells of the leaf by way of the stomata, there is less
CO2 in the vessel in jDroportion, and more oxygen. The respira-
tory constants of the plant in daylight should be known, but the
error caused by the gases interchanged during respiration is a
small one.

These three experiments show that a continuous gaseous
interchange is taking place between the leaf-cells of a living
plant and the surrounding atmosphere (or water), and that
the cell requires oxygen for purposes of respiration or tissue-
oxidation. Plants surrounded by an inert gas, such as nitrogen
or hydrogen, die in a short time ; and moisture must also be
present in the air, for the CO2 and O2 gain entrance into a cell
dissolved in water, the former as a definite compound, CO2 . H2O,
or carbonic acid.

vii. Variations of Protoplasmic Activity under Different
Conditions.

Under this heading will be considered : —

a. The influence of light upon the direction Of growth of
organs.

h. The influence upon metabolism of light rays of varying
refrangibility.

c. The action of gravity or a centripetal force in any direction
upon the direction of growth.

d. The influence of mechanical and chemical stimuli upon
protoplasm.

a. The influence of light-rays (direct sunlight especially) upon
growing organs is usually referred to under the comprehensive

CHEMICAL AND PHYSIOLOGICAL STUDIES.

181

term heliotropism. Organs which grow towards a source of light
are said to be positively heliotropic, whilst those which grow
away from it are negatively heliotropic. In this sense stems and
flowers are positive, whilst roots are negative, and most leaves
are transversely heliotropic, setting at right angles to the rays of
light. In most instances there is a certain position with regard
to the direction of the incident rays of light which an organ
takes up in preference to others, and this is known as the
optimum position ; thus, to refer to the lower plants, .ffithalinm,
a mass of naked protoplasm, if subjected to powerful illumina-
tion, will withdraw to a position where the light is not so
powerful. Many leaves assume a position which permits not
of the maximum amount of light falling upon their upper
surfaces, but of just that amount which is found to coincide
with the requisite intensity of assimilation in these organs. Light
has thus a kind of tonic effect (phototonus) upon the growth
and position of organs. The explanation of the movements of
an organ caused by light is to be found in the fact that those
cells of the organ which are nearest the source of illumination
transpire most freely, and are not so turgid as the opposite
parts ; and since turgidity favours growth, it follows that the
remoter parts will grow more strongly than those nearest the
source of light, and, by so doing, will cause a curve in the organ
concave towards the light incidence. In this manner arise the
curvatures produced in some stems and flower stalks. It is well
known that roots grow much faster in the dark than in the
light; this can readily be shown by growing water-cultures of
Hyacintlius in transparent white vases and in opaque ones
respectively, when it will be found that the roots springing
from the bulb in the dark jar are, after some days' growth, much
longer than those from the bulb in the transparent one. In this
connection it may be mentioned that the violet and ultra-violet
rays of the spectrum have most influence upon the formative
action of protoplasm.

It is an interesting fact that the chloroplasts in the palisade-
cells of a leaf take up positions which vary according to the
intensity of illumination of the upper surface of the leaf ; thus,
in very intense illumination they become arranged along the
side-walls, presenting only their edges to the incident rays
(apostroplie), whilst in medium illumination they are situated

182

THE PLANT CELL.

along the upper and under walls, presenting their broader
surfaces to the light (epistrophe). This is an instance of adapta-
tion so as to ensure an “optimum” intensity of assimilation.*
In Mougeotia (one of the Conjugatse) the chloroplast is in the
form of a band in each cell of the filament, and this band rotates
into a position which enables it to receive the optimum intensity
of illumination; in strong illumination it presents its edge to
the incident rays.

b. The Influence upon Metabolism of Light Eays of Vary-
ing Refrangibility. — An experiment devised by Engelmann
(described in the Botanische Zeitung for 1881, p. 447) illustrates
in a striking manner the influence of the red, yellow, and violet
portions of the spectrum upon the intensity of assimilation of
COg. A filament of Spirogyra is mounted in water along the
middle of an opaque microscope slide, so as to traverse three
transparent circular portions of the slide (see a, Fig. 115). These
three spaces are illuminated by red, yellow, and violet light,
R, Y, and V respectively (see Fig. 115). A small culture of
Proteus vulgaris (a motile organism which is markedly attracted
by oxygen) is now introduced under the cover-slip of the
preparation, and, the filament being carefully focussed under
the microscope, the following observations may be made : —

i. In the vicinity of those cells of the filament illuminated by the red
rays, a vast swarm of motile bacteria become aggregated.

ii. Fev/er bacteria exist in the region of the cells illuminated by the

yellow rays.

iii. Very few organisms are to be seen near the cells lighted by
the violet portion of the spectrum. Engelmann employed a substage
prism in order to produce a spectrum, the red, yellow, and violet portion
of which thus illuminated the spaces in the slide from beneath.

This shows that more oxygen is being evolved from those
cells of the filament illuminated by the red rays than from those
under the influence of the yellow or violet rays, and that, therefore,
assimilation is more intense in red than in other illumination.

Another experiment, due to Timiriazeff*, and depending upon
the formation of starch in the leaf-cells, is as follows (see Fig.
115)

A given leaf of a plant is selected on a certain day, and, before
any light has fallen upon it, is covered, with the exception of a

* The small leaves of Lemna trisulca are excellent organs for the
observation of this phenomenon.

CHEMICAL AND PHYSIOLOGICAL STUDIES.

183

small strip, with an opaque sheath of tinfoil. Upon the uncovered
strip a small spectrum of sunlight is thrown by means of a suitably
arranged prism, and the whole left for some hours. The leaf is
then cut olf and immersed in alcohol, to dissolve out the chloro-
phyll, and subsequently washed in distilled water and transferred
for a time to a vessel containing a solution of iodine in potas-
sium iodide. It will then be found upon examining the leaf
after again w^ashing, that in the part which was illuminated by
the spectrum a band of “stained” starch is present, the staining

Fig. 115. — (a) Engelmann’s Bacteria Experiment. — S, Glass slide
(opa^iue) ; R, Y, V, three spaces left clear, which are illuminated by
red, yellow, and violet rays respectively ; /, filament of Spirogyra.

(6) Timiriazeff’s Experiment. — L, Beam of white light ; P, prism ;
R, V, spectrum thrown upon a leaf.

being deepest in that part which was previously illuminated by
the red rays, and weakest at the opposite or violet end.
Photosynthesis (followed by storing of starch) is thus more
intense under the influence of rays of low refrangibility.

These two experiments show conclusively that assimilation of

184

THE PLANT CELL.

CO2 proceeds most actively under the influence of the rays of the
spectrum which are of low refrangihility.

c and d. The Action of Physical, Tactile, and Chemical Stimuli
upon Protoplasm. — The effect of gravity (geotropism), or a cen-
tripetal force, upon protoplasm, is very marked at times. Thus,
there arises a directive action which causes root-tips to curve
downwards into the soil, even when seedlings are placed horizon-
tally ; and stem- structures will, on the contrary, curve upwards.
Eoots are thus said to be positively geotropic, whilst stems
are negatively geotropic, and the curves shown by these struc-
tures under such conditions are known as geotropic curvatures.
If seedlings are grown upon the edge of a rotating wheel,
the plane of rotation being at right angles to the line of
action of gravity, the roots will grow out along the radii
of this wheel, or at any rate along the line of action of the
resultant of the centripetal force and the force of gravity
(Knight’s experiment). This shows that a force acting in any
direction, which is strong enough to overcome gravity, will
determine the directive action of the protoplasm in root-cells.
An instrument known as the clinostat is used nowadays for
the demonstration of geotropic curvatures ; it consists of a flat
wheel which can be kept rotating by means of a clockwork
device at any constant speed which is desired.^

With regard to rhizomes and stolons, the effect of gravity is
to produce growth in a horizontal direction. This is known as
diageotropism, and the growing apex of a rhizome will, if placed
vertically, soon return to the horizontal position.

The influence of gravity is manifested in all these cases by an
increased growth of one side of the organ, and a diminished
growth of the opposite side {vide action of light), so that a
geotropic curvature results.

If the cells upon one side of the apex of a quickly-growing
stem be more turgid than those on the other, increased growth
will take place on that side, and a corresponding curvature of the
apex away from that side will be produced; this curvature is
known as a nutation. At times a periodic alteration in the
turgidity of the cells all round the apex takes place in succession,
and the curve thus produced is a circle, or since growth in

See Detmer and Moore, Practical Plant Physiology.

CHEMICAL AND PHYSIOLOGICAIi STUDIES.

185

length is also proceeding an ellipse or spiral ; this is then
known as a circnmnutation.

In flat (dorsiventral) organs an upward or downward or side
to side movement will be produced by the same variations in
growth, and the phenomenon is here known as epinasty or
hyponasty as the case may be. These movements are known as
movements of variation.

So-called nyctitropic movements are due to curvatures in organs
(leaves) owing to the periodic variations in turgidity of certain
cells, which are determined by the altered conditions of growth
as night approaches. Th^cells which cause these movements
are usually situated at the oases of the petioles (in leaves), so
that a drooping of the organ occurs, or if the organ is a flower,
a closing of this.

The twining or revolving movements of stem-climbers are
due to the action of gravity, which causes an increased growth on
either the right or left side of the growing internodes of the
apex of a shoot. In this manner, either right-handed (dex-
trorse) or left-handed (sinistrorse) curves may be produced,
which serve to twine the stem round a support.

In the case of the nodes of grass-haulms, the resting tissue of
these parts may be stimulated by the action of gravity, so that
the lower aspect of the node exhibits increased growth ; this
causes an upward bending of the haulm.

Tactile stimuli produce measurable effects upon certain grow-
ing organs. At times, especially in certain climbing plants (tendril-
climbers), the contact of a resisting body, such as a wall, or stick,
causes increased growth in those cells of the plant-organ which
are remote from the stimulus, and this has the result of producing
a curvature which enables the organ (tendril, &c.) to entwine
the ^pport. In Drosera, contact stimuli will cause the glandular
hairs of the leaf to curve over and enclose a small particle which
has fallen upon them; and if this particle happens to be of
an organic nature (albumen, small fly, &c.), a further action
results — viz., secretion of a peptic ferment in certain cells of
the leaf.

In Mimosa pudica, tactile stimuli applied to one of the small
pinnae of the bi-compound leaf, causes first of all a drooping of
the pinna, then of the lateral leaflet, and finally of the whole
leaf This result depends upon the disturbance of the osmotic

186

THE PLANT CELL.

balance (turgidity) in the cells of the pulvini (small cellular
cushions), situated at the bases of the leaves and leaflets, whereby
turgidity is lessened. The stimulus in this case passes along
certain cells surrounding the vascular bundles, and travels by
means of cytoplasmic connecting “ bridles ” between adjacent cells.

Chemical substances exert at times a powerful attractive
influence upon protoplasm; and, on the other hand, they may
repel the living substance. The growth of pollen-tubes is
brought about by such stimuli, as also is the attraction of anther-
ozooids towards the oospheres of Pteridophyta (see supra). In
the latter instance, enzymes often seem to be the chemical
substance producing the attraction, the process being known as
positive chemotaxis. Negative chemotaxis is seen in the repul-
sion of jEthalium, produced by strong salt solution or acetio
acid. The selective action exercised b}^ the ectoplasm of root-
hairs is also of a kindred nature.

The influence of moisture (hydrotropism) is sufficient at times
to determine the direction of growth of an organ (root). The
stimulus is in the main of a chemical nature.

These few remarks on the chemistry and physiology of the
cell may serve to emphasise the fact that the cytoplasm is, as
v.^as incidentally mentioned in Chapter i., capable of responding
to a variety of stimuli ; or, as is often said, possesses the pro-
perty of irritability. It has also been seen that protoplasm is
capable of transmitting stimuli from one cell to the cells of
remote parts ; and, finally, evidence has been put forward
showing that growth, as a whole, is the result of the action
upon the cytoplasm of the various physical and chemical agencies
v/hich are from time to time brought to bear upon it.*

viii. — The Production of Heat, Light, and Changes of Electrical
Potential in a Cell, and the Action of Electric Currents
upon the Cytoplasm.

The subject of heat-production in the cells of plants is one
which, from the experimental point of view, is often beset with
difficulties, owing to the inability to measure such small changes

* Huxley long ago maintained that the vital properties of the proto-
plasm were the result of the disposition of the molecules of which it is
composed, and that no such term as “ inherent vitality ” was necessary in
escribing the attributes of the living substance. [Method and Results, 1893.)

CHEMICAL AND PHYSIOLOGICAL STUDIES.

187

in temperature as may at times arise ; but occasionally a con-
siderable rise may be observed, more especially in the case of
germinating seeds. If the bulb of a delicate thermometer be
surrounded in a vessel by some seeds of Pimm sativum, and water
added, after a time a rise of some 4° C. will be observed, this
being due partly to the absorption of water by the cells of the
cotyledons, but mostly to respiration.

The internal tissues of quickly-growing stems will sometimes
indicate a slightly higher temperature than the outer tissues.*
In this case, a thermo-electric needle in circuit with a sensitive
galvanometer must be used, the needle being so arranged as to
be non-polarisable (that is, coated with a substance which is not
acted upon by the acid sap).

In the growing spadix of Aroidese, also, a considerable eleva-
tion of temperature has been noted (10° to 12° C.); and certain
bacteria {B. suhtilis) also cause a great rise of temperature
during growth (c/. firing of hayricks).

The evolution of radiant energy takes place in some of the
lower plants, as in the case of the Bacterium phosphm'eum, one of
the Schizom}"cetes.t So intense is the radiation in this instance,
that pea-seedlings will grow towards a vessel in which a culture
of these organisms is growing. Other bacteria, such as those
producing the “ phosphorescence ” of decaying fish, are also
capable of evincing light-rays.

The luminosity in such cases as these is dependent upon the
oxidation going on in the cells.

Differences of electrical potential have been observed to be
present between the upper and under surfaces of certain leaves
(leaves of Victoria regia, &c.), and it has been shown that if the
internal and external tissues of some stems are connected through
a circuit in which a sensitive galvanometer is included, a current
will flow from the internal to the external parts (Becquerel).
Here, also, non-polarisable electrodes are essential.

In Chapter i., the blaze-reaction mentioned in connection
with experiments upon the capacity of germination possessed by
seeds, was adduced as evidence of changes in electrical potential
produced by oxidative changes of low intensity proceeding in
the dormant cytoplasm of the cells composing the seeds ; in fact,

* See Becquerel, Phyaiologie Vegetate.
t See Knowledge and Scientijic News, Feb., 1909.

188

THE PLANT CELL.

wherever chemical actions are proceeding, however small these
may be, there will variations in electrical potential be produced,
a current flowing from the regions of greater activity to those
of less.

Strong electric currents have the effect of causing an imme-
diate contraction of the cytoplasm of a cell {Sjnrogyra, or Elodea
Canadensis), from which recovery is impossible. Currents of a
less intense nature cause either a partial retraction from the
cell-wall, with, later on, after the current has been stopped,
resumption of function. Upon swarmspores and antherozooids
swimming in water, a strong current has the peculiar efi'ect of
polarising these bodies, so that they set with either one or the
other extremity facing in a definite direction — viz., either with or
against the direction of flow of the current.

189

APPENDIX.

THE PHYSICS OF THE ABSORPTION OF WATER, SALTS,
AND GASES BY THE CELL.

The question of the absorption of raw food-materials by the
living cell is a most important one, and involves, as has been seen
on p. 172, the consideration of osmosis. This process, which
must be studied a little more in detail, is partly vital and partly
physico-chemical, and concerns not only the absorption into the
cell of water and salts, but also of the gases COg and O2, which
are used during assimilation and respiration respectively. If a
bladder composed of moistened vegetable parchment be filled
with a solution of sodium chloride in water and immersed in
distilled water, water will pass into the bladder, and a certain
small amount of salt will also escape in the opposite direction.
The inward diffusion of water is known by the term endosmosis,
whilst the exit of salt is known as exosmosis. After a certain
time a condition of equilibrium is reached, when neither endos-
mose nor exosmose occur, and this state is known as osmotic
equilibrium, the bladder being turgid.

If the concentration of two solutions of a salt separated by a
permeable membrane is the same, no permanent interchange of
water and salt occurs, but if the dilution of one of the solutions
be altered ever so little by the addition of more solvent, then
osmosis occurs. Nevertheless, even when the state of balance
has been reached, it is assumed that a constant interchange of
equal quantities of salt is taking place, so that a condition of
rest never obtains.

In the case of the living plant-cell (root-hair, &c.), the state
of things is somewhat modified by the fact that the membrane
separating the two solutions (which are here on one side, the

190

THE PLANT CELL.

cell-sap, and on the other side, either a dilute solution of earthy
salts, or of a gas) is by no means an inert one like parchment,
but is composed of several parts — viz., a layer of cellulose, then
a layer of ectoplasm, then a layer of endoplasm, and, finally,
lining the central vacuole, another very delicate pellicle of
ectoplasm (hyaloplasm); consequently, the osmotic phenomena
observed in the case of the cell are not quite equivalent to the
purely physical processes observed to take place when experi-
menting with the parchment membrane.

The endosmosis of earthy salts into a root hair is, as has been
seen, governed to a certain extent by the osmotic properties of
substances in the cell-sap; during the metabolism of the cell,
bye-products are formed, which it is found exert an attraction
upon the salts and water outside in the soil, and consequently,
these are drawn in by endosmosis, whilst a small amount of the
above bye-products (chiefly organic acids) escapes by exosmosis.
The condition of turgidity thus set up is always present in such
a cell as a root-hair, and, in fact, in any cell which is growing
to any extent, this condition being, indeed, essential to the
growth in extent of the cell-wall.

A state of absolute equilibrium is, of course, rarely reached,
since the salts absorbed are as constantly removed by diffusion
into adjacent cells, and by the osmotic effect of the transpiration
of water from the leaves which leads to a progressively decreasing
concentration of the sap in the cells below, and in the tracheides
of the wood in stem and root (c/. Transpiraton). The passage of
the soluble elaborated foods (sugar, amido-acids), from the leaf-
cells to other parts of a plant is effected mainly by osmosis ; the
elaborated food is being constantly used up either for formative
processes or storage, and this removal of soluble foods from the
cell-sap leads to a corresponding intake of these substances from
adjacent and remote cells. The bye-products of metabolism are
also, as was noted above, useful aids in promoting osmosis in
this respect.

The gases COg and Og must, before they are taken into a cell,
be dissolved in water; the cell-walls of the mesophyll cells in a
leaf are saturated with moisture, and the gases, entering by
means of the stomata are led to intercellular spaces surrounded
by the cells of the mesophyll (spongy parenchyma). If, for
instance, the percentage of CO2 in a cell is smaller than that

APPENDIX.

191

in the intercellular space, this gas will pass into the cell by
endosmosis, and the same may be said of oxygen ; during the
assimilation of COg and water during the day-time in the cells of
the mesophyll, oxygen is one of the bye-products, and this, since
its osmotic pressure and percentage in the cell-sap are greater
than those of the same gas in the intercellular spaces, will escape
to a certain extent by exosmosis, leading to the evolution of
oxygen from the surface of the leaf through the stomata.

A given solution of either salts or a gas is said to be isotonic
with another solution, when no permanent interchange takes
place between their saline or gaseous constituents when they are
separated from one another by a permeable membrane. In the
plant cell, although for a short space of time, the condition of
osmotic balance may be present, this condition rarely lasts any
length of time, since the absorbed substances are being constantly
used up. The main conditions for adequate osmosis in the
plant-cell are : —

a. The stability of the cell-wall and ectoplasmic membrane.

b. The presence of dilute solutions of salts, &c., in the soil or
cell-sap ; adequate dilution is necessary, since, before molecules of
salt can pass through the membrane by osmosis, they must be
ionised, that is to say, the atoms or atomic groups must be
separated from one another by the solvent.

In the plant-cell, however, the ectoplasm exerts a regulating
action (selective capacity), which, to a certain extent, modifies
osmosis ; and the membrane of separation becomes, in a measure,
comparable to the semi-permeable membrane — viz., that which
permits of the entrance or exit of certain molecules (salts, acids),
but not of others (colloids). Moreover, in many instances, salts
may be absorbed which do not appear to have any influence
upon metabolism, but gain an entrance on account of the
smallness of their molecules, or atomic groups into which they
are split up.

193

INDEX.

Abies, Structure of bud, 23, 24.

Absorption, of water and salts, 9, 45, 172, 173, 189 ; of gases, 29, 30, 190 ;

spectrum of chlorophyll, 166.

Absciss layer, in faUing leaf, 38.

Accretion, Growth of cell- wall by, 44.

Acetic acid, used as a plasmolyte, 16 ; in Flemming’s solution, 116 ; as
a solvent of carbonate of lime in cystoliths, 86.

Acids, Presence of, in cells, 84, 159, 171 ; Formation of, 158, 171.
Achromatic spindle, 106, 115.

Actinomyces, 89.

Aeration of plants, 39, 179.

Aethalium, 8, 97, 181.

Alburnum, Function of, 175.

Alcohol, used as a fixing agent, 116 ; as a solvent of chlorophyll, 165.
Aleurone grains, 176.

Algae, Position of, 6 ; Types of cell amongst, 90 ; Reproduction in, 93,
95, 98, 99, 147.

Alkahs, used in demonstrating cuticle, 45 ; Action of, upon vegetable
colouring matters, 168 ; Action of, upon coUenchyma, 40.

Alkaloids, Formation and fate of, 159, 171 ; Varieties of, 171.

AUium, for material in study of mitosis, 116.

Alternation of generations, 118, 154.

Aluminium, Presence of, in plants, 157.

Amitosis, 100 ; in Tradescantia hairs, 100 ; in Chara, iOO ; in Cambial
cells, 58, 100 ; in Spirogyra, 93.

Amido-acids, in proteid formation, 159, 170, 177 ; as a circulating food,
170, 177 ; as a reserve food, 177 ; Asparagin, one of the, 170, 177.
Amines, 7, 171.

Ammonia, Absorption of, by plants, 33, 169, 174 ; Conversion of, by bacteria,
158, 173.

Amoeba, 8, 97.

Amygdalin, Decomposition of, by emulsin, 171.

Anaerobic respiration, 179.

Anabolism, 157, 159, 177.

Analyser, Use of, in study of starch, 162.

Anaphase ; see Mitosis.

Angiosperms, Position of, 6; Reproductive cycle in, 117, 119; see also
Homology.

194

INDEX.

Aniline, Sulphate of, as a reagent for wood-elements, 71.

Annular vessels, 69, 71.

Anomalies, in stem-structure, 53 ; in reproductive cycles, 155.

Anther, Microspore formation in, 119, 131.

Antherozooids, of ferns, 142 ; of Fucus, 149 ; of Vaucheria, 151 ; of Cycas
and Ginkgo, 132.

Antheridium, see Antherozooids.

Anthoceros, Sporogonium of, 146.

Anticlinal walls, 43.

Antipodal cells, 125, 153, 154.

Apical cell, 144.

Apostrophe, 181.

Archegonium, Formation of, 133.

Arrangement of tissues, 3, 47 ; in vascular region of dicotyledons, 48, 49.
Arthrospore, 147.

Arum, Rise of temperature in spadix of, 187.

Ascospore, of fungi, 89, 146 ; Mode of formation of, 101, 146.

Asexual generation, 118, 140, 145 ; see also Table of Homologies.

Asparagin, see Amido-acids.

Assimilating cells, 13, 36, 92.

Assimilation, of CO2 and H2O, 18, 30, 180 ; of N, 18 ; of HCN, 170 ; see
also Photosynthesis, Proteids, Constructive processes.

Aster, see Mitosis.

Auxospore, 99.

Bacteria, 89, 147 ; Nitrifying, 173, 174 ; Production of heat by, 187 ; Pro-
duction of light by, 187.

Bambusa, Pith of, 78.

Bast-fibres, 41, 65.

Beet-root, Colouring matter in sap of, 168.

Begonia, Starch-formation in, 17.

Benzene, as solvent for phyllocyanin, 166.

Bifacial leaf, 3, 30, 36 ; Ending of bundles in, 174.

Bridles, Cytoplasmic, 21, 97, 99.

Bromine, occurrence in plants, 157.

Bryophyta, Position of, 6 ; Reproduction in, 145, 154 ; Vascular elements
in, 77.

Bud, Structure of, in Abies, 23 ; Storage of food in, 176.

Bundles, Fibrovascular, 50 ; Closed, 52 ; Open, 53.

Bye-products, of metabohsm, 84, 159, 171 ; as aids to osmosis, 173, 190.

Calcium, as essential food- material, 157 ; Crystals of oxalate of, 84, 159 ;
Carbonate of, 85 ; Nitrate of, in assimilation of N, 172 ; Presence of,
in aleurone-grains, 176 ; essential to growth of fungi, 157.

Calcium pectate. Presence of, in middle-lamella, 44.

Callus, 62.

INDEX.

195

Caltha, Formation of endosperm in, 128 ; as material for preparation of
young cell, 46 ; for stages in reproductive cycle, 130.

Cambium, Origin and position of, 48 ; Details of, 50, et seq. ; Amitosis in
cells of, 58, 100 ; Tissues arising from, 49, 60, 65.

Canal, of archegonium, 133 ; Resin, 79, et seq.

Canal-cells, 134, 143.

Cane-sugar, Formation of, during photosynthesis, 160, 165, 177.

Cap-cells, 123, 132.

CapseUa, as material for study of embryo -formation, 130.

Carbohydrates, forms met with in plants, 162, 165, 171 ; Manufacture of,
17, 18, 164 ; Storage of, 17, 64, 71, 76, 164, 170, 176.

Carbon, Source of, 20, 93, 157 ; Presence of, in protoplasm, 7 ; Assimilation
of, 18, 30, 167, 180.

Carbonic acid, as form in which CO2 is absorbed, 93, 180.

Carotin, in crystalloids of carrot, 168.

Caustic potash, as solvent of protoplasm, 12 ; see also Alkalis.

Cell, Types of, 1, 2 ; Definition of, 1 ; Division of, 100, et seq. ; Growth
of, 21, 44.

Cell-division, 100 ; see Mitosis.

Cell-plate, Formation of, 44, 114, 165.

Cell-sap, 15 ; Composition of, 170 ; Conduction of, 64, 71, 174, 176 ; Elabor-
ation of, 164, 167, 169, 176, 177.

Cellulose, in cell- wall, 17, 19, 25, 44 ; Reactions for, 17 ; Formation of,
159, 165, 177 ; Composition of, 44, 162.

Cell-waU, Pits in, 2, 26, 41, 46, 62, et seq. ; Structure of, 17, 25, 39, 44 ;
Growth and secondary thickening of, 44, 159, 165 ; Reactions of, 17,
41.

Central cyhnder, 3, 24 ; Arrangement of tissues in, 3, 48, 49.

Centric leaf, 3 ; of Pinus, 37 ; of Hakea, 4.

Centrifugal force. Influence of, upon disective action of protoplasm, 184.

Centrosome, 101, 115.

Chemistry, General, of ceU, 156, et seq. ; of starch-formation, 18, 164 ;
of proteid construction, 169, 170.

Chemotaxis, 11, 106, 143; Positive, 11, 143, 186; Negative, 186.

Chlorophyll, Occurrence of, in plants, 16, 165 ; Presence of, in chloroplasts,
16, 165 ; Absorption spectrum of, 166 ; Relations of, to fight and
assimilation of CO.^, 18, 165, et seq. ; Occurrence of, in animals, 97 ;
Conditions governing formation of, 169 ; in Diatoms, 99.

Cliloroplasts, occurrence in ceU, 15 ; Starch-formation in, 17, 18, 167 ;
Origin of, 19 ; in SphtereUa, 97 ; in Vaucheria, 93.

Chromatin, as a constituent of the nucleus, 101, 104 ; Changes in, during
mitosis, 103, 104 ; Relations of, to paranuclein, 103 ; Phosphorus in,
103.

Chromic acid, used as a fixing agent, 58 ; in Flemming’s solution, 116.

Chromoplasts, 167, 168.

CTiromosomes, Formation of primary, 104; Formation of secondary, 112.

Cilia, Movements of, 9, 97 ; of Sphoerella, 97 ; of swarmspore of Vaucheria,
95.

Circulating foods, 170, 176, 177.

Circumnutation, 184.

Gtrus, Oil-glands of, 78.

Classification, Outline of, 6.

196

INDEX.

Coenocyte, of Vaucheria, 93 ; of fungal hyphse, 88.

CoUenchyma, 40.

Colony, Definition of, 2.

Colouring matters, in cell-sap, 168 ; in chromoplasts, 167, 168
Companion-cell, 60.

Coniferae, Position of, 6.

Conjugatae, see Spirogyra.

Conjugation, in Spirogyra, 148 ; in fungi, 146, 147 ; in Sphaerella, 98.
Constructive processes. Anabolic, 159, 160, 177 ; Katabolic, 159, 160,
177.

Cork, Origin of, 37, 38 ; Functions of, 38, 39.

Cork-cambium, 38.

Cortex, Origin of, 36.

Cotyledons, Formation of, in Angiosperms, 128.

Crystals, raphides, 84 ; clustered, 84 ; Quadratic, 84.

Crystalloids, in carrot, 168 ; in Dahlia cells, 84 ; in Aleurone -grains,
176.

Curvatures, Geotropic, 184 ; Helio tropic, 181 ; due to variations in tur-
gidity, 181, 184.

Cuticle, 25.

Cutin, see Cuticle.

Cycadeae, Position of, 6 ; Antherozooids of, 132.

Cystoliths, of Ficus, 85, 86.

Cytoplasm, see Protoplasm.

Definitive nucleus, 123.

Degeneration, evidenced in amitosis, 100.

Deposits, in cell-waU, 168 ; from cell-sap, see Crystals.

Dermatogen, 24 ; Modification of, 25 ; Cells formed from, 23, 24.

Development, of microspore, 119, 131 ; of macrospore, 121, 132; of resin-
canals, 83 ; of stomata, 27 ; of antherozooids, 142, 149, 151 ; of
archegonia, 133 ; of heat in cells, 160, 161 ; of hairs, 31, et seq. ; of
first wood-elements, 79.

Dextrin, 74 ; as intermediate product during action of diastase upon
starch, 74, 171.

Dextrose, presence in plants, 165, 171, 177.

Diageotropism, 184.

Diastase, occurrence in plants, 74, 158, 171 ; Action of, upon starch, 74,
158, 171.

Diaster, see Mitosis.

Diatoms, Structure and reproduction in, 99.

Dicotyledons, Position of, 6 ; Arrangement of tissues in stem of, 4, 48,
49, et seq.y and Chap. v.

Dispireme stage, see Mitosis.

Double fertihsation, 127, 154.

Dracaena, Raphides in cells of cortex in, 84 ; Cambial ring in, 53.

Duramen, Function of, 175.

INDEX.

197

Ectoplasm, in Amcsba, 7, 8 ; in Aethalium, 8 ; in enclosed protoplasts, 9,
32 ; Functions of, 32, 172, 173, 190, 191 ; Structure of, 7 ; of Sphserella,
97.

Egg-cell, see Oosphere.

Elaboration of sap, 164, 165, 167, 169, 176, 177.

Elaioplasts, 160.

Electric currents. Action of, upon protoplasm, 188 ; Production of, in cells,
187.

Embryo, Formation of, in Angiosperms, 128 ; in ferns, 143, 144 ; in Gymno-
sperms, 137 ; Octant-formation in, 128, 137.

Embryo-sac, see Macrospore.

Emergence, 32 ; Glandular, 32.

Emulsin, Action of, upon amygdabn, 171.

Endodermis, 3, 48, 76.

Endogenous, in connection with spore-formation, 89, 147.

Endoplasm, Structure of, 7 ; Motility of, 8, 9, 15.

Endosperm, Formation of, 128.

Endospores, 146.

Endothelial layer, of resin-canals, 80 ; of glands, 79.

End-stages, see Mitosis and Telophase.

Engelmann, Experiments of, on value of red rays in metabolism, 182.
Energy, Sources of, to ceU, 160 ; Conservation of, 156 ; Conversion of,
into other forms, 160, 166.

Enzymes, Occurrence of, in plants, 64, 74, 113, 127, 158, 159, 171, 177 ;

Varieties of, 158, 171.

Eosin, Use of, as stain for caUose, 60, 62.

Epibasal cell, 128, 137, 143.

Epidermis, Origin of, 23, 24 ; Details of, 24, 25, 26 ; Structures observed
in connection with, 26, et seq. ; Functions of, 4, 26, et seq.

Epinasty, 185.

Epistrophe, 182.

Equatorial stage, see Mitosis.

Equisetum, ProthaUia of, 145.

Etiolin, 167, 169.

Euphorbia, Laticiferous vessels of, 86.

Eurotium, Reproduction in, 147.

Excretions, as bye-products of metabohsm, 83, 84, 159, 172 ; of formic
acid, 33.

Exogenous, in connection with spore-formation, 89.

Exothermic reactions, 160, 161.

Extine, 121, 132.

Farinose, 164.

Fascicular cambium, 57.

Fats, Formation of, in cell, 159, 160 ; Storage of, 176.

Ferments, see Enzymes.

Ferns, Scalariform vessels in, 66, 67 ; Reproduction in, 139, et seq.

198

INDEX.

Fertilisation, in Angiosperms, 127 ; in Gymnosperms, 135, et seq. ; in ferns,
143, 145.

Ficus, Cystoliths in, 85 ; Latex in, 86 ; Epidermis of leaf of, 24
Fission-fungi, see Bacteria.

Flemming’s solution, 116.

Food-materials, 157, 171, 172, 177.

Foods, see Chap. X.

Formaldehyde, as an intermediate product in sugar-formation, 164 ;

Polymerisation of, 165, 177.

Formic acid, in Urtica, 33.

Free ceU-formation, 101.

Fucus, Reproduction in, 149.

Fungi, Cells of, 88 ; Reproduction in, 89, 146, 147 ; Glycogen in, 89.
Funkia, Polyembryony in, 138.

Gamete, 148.

Garnet ophyte, of Angiosperms, 154; of Gymnosperms, 132, 154; of ferns,
140, 142, 144, 154.

Gases, Absorption of, by cell, 18, 26, 30, 92, 93, 190.

Gemmae, of Mosses, 146 ; of Liverworts, 146.

Generative cell, of Angiosperms, 121 ; of Gymnosperms, 132.

Geotropism, Positive, 184 ; Negative, 184 ; Dia-, 184.

Germination, Rise of temperature during, 160, 183 ; Blaze-reaction during,
11, 187.

Ginkzo, Antherozooids of, 132.

Glands, Oil-, in Citrus, 78, 79.

Glandular hairs, of Pelargonium, 33 ; of Drosera, 185.

Glucose, see Dextrose.

Glucosides, Occurrence of, in plants, 159, 171 ; Decomposition of, by
enzymes, 171 ; Association of, with tannin, 171 ; Composition of,
171 ; as a reserve food, 177.

Glycogen, in fungi, 89.

Gonidia, 89, 146.

Granulose, 164.

Ground-tissue, 3, 4, 50, 77.

Ground-tissue rays, see Primary medullary rays.

Growing-points, see Meristem.

Growth, of cell- wall, 44 ; of cell as a whole, 21, 22 ; Influence of physical
agencies upon, 181, 184, 185 ; of wood-elements, 65.

Guard-cells, 27, 29.

Gymnosperms, Position of, 6 ; Arrangement of tissues in stems of, 4 ;
Reproduction in, 130, et seq.

Hairs, Simple, 31, 32 ; Compound, 32, 33, 34 ; Glandular, 33, 34.
Hakea, Centric leaf of, 4.

Hardening and fixing reagents, 116.

Haematoxylin, as a nuclear stain, 102, 104.

INDEX.

199

Heat, Production of, in ceU, 161, 187 ; produced in spadix of Arum, 187 ;
produced during germination, 160, 161, 187 ; Loss of, by evaporation
of water from leaves, 174.

Heliotropism, 181.

Heterosporous ferns. Reproduction in, 118, 144.

Heterotypic mitosis, 115.

Hilum, see Starch.

Hippuris, epidermis of leaf, 24, 25.

Hofmeister, theory of rotation of protoplasm, 11.

HoUy, epidermis of leaf, 26, 45.

Homology, 6, 153 ; see also Table opp. p. 154.

Homosporous ferns. Reproduction in, 118, 140.

Homotypic mitosis, 115.

Huxley, views on constitution of cytoplasm, 186 ; definition of protoplasm,

Hyacinthus, Root- tip of, as material for study of mitosis, 116.

Hyaloplasm, 10.

Hydrogen, Assimilation of, 157 ; as constituent of protoplasm, 7.

Hydro pteridiae, see Heterosporous ferns.

Hydrotropism, 186.

Hymenium, of fungi, 146.

Hyphse, 88, 146, 147.

Hypobasal cell, of Angiosperms, 128 ; of ferns, 143.

Idioblasts, 86.

Intercellular spaces, 26, 39, 179.

Interfascicular cambium, 51.

Intussusception, 44.

Inulin, in cells of Dahlia, 84 ; in cell-sap, 171 ; as stored food, 176.

Iodine, as reagent for starch, 16, 17, 164 ; as reagent for proteid, 17 ; a
component of sea- weeds, 157, 171 ; as reagent for sclerenchyma
and hgnin, 71.

Iron, essential to growth of plant, 157 ; essential for formation of chloro-
phyll, 168, 169 ; Perchloride of, as a test for tannin, 86.

Irritabihty, 10, 186.

Isoetes, see Heterosporous ferns.

Isolated tissues, 78, et seq.

Isotonic solutions, 17, 173, 191.

Karyokinesis, see Mitosis.

Katabolism, 157, 159, 177.

Kinoplasm, 101 ; Origin of nuclear spindle from, 115.

Klinostat, 184.

Knight, Experiment of, upon influence of centripetal force upon plants.

184.

200

INDEX

Lamella, in walls of sclerenchymatous fibres, 41 ; in starch-grains, 162.
Larix, as material for study of mitosis, 116.

Latex, 86.

Laticiferous vessels, 86, 87 ; -cells, 87.

Leaf, Bifacial, 3, 30, 36, 37 ; Centric, 3, 37 ; Elaboration of food in, 18,
167, 169, 177 ; Transpiration from, 28, 174, 175 ; Movements of, 181,
185.

Leguminosse, Root tubercles in, 173.

Lenticles, Formation and frmction of, 39.

Leucoplasts, see Plastids.

Lichens, 88.

Light, Influence of, upon transpiration, 181 ; essential to formation of
chlorophyll, 169 ; Action of chlorophyll upon, during photosynthesis,
18, 167 ; influence of fight of varying refrangibifity upon metabofism.
182 ; Influence of, upon growth, 181.

Lignin, 65.

Lime tree, for examination of medullary rays, 74.

Linin, see Nucleus.

Lower plants. Cells of, 88, et seq.

Lysigenous origin, in glands or canals, 83.

(■

Macrosporangium, 121, 132, 144.

Macrospore, Origin and structure of, in Angiosperms, 121 ; in Gymno-
sperms, 132 ; in Heterosporous ferns, 144 ; Maturation stages in, 123,
133, 144.

Magnesium, occurrence in plants, 157, 170 ; in aleurone-grains, 176.

Mafic acid, in ferns, as a cause of chemotaxis, 143.

Mantle- fibres, see Mitosis.

Marsilea, see Heterosporous ferns.

Mass-division of nucleus, see Amitosis.

Maturation, of microspore, 121, 132, 144 ; of macrosnore, 123, 133, 144.
Medulla, 45, 48, 77.

Medullary rays. Primary, 72 ; Secondary, 73.

Medullary sheath, 77.

Meristem, Apical, 43 ; Cambiums, 42, 48, et seq. ; Intercalary, 42.
Metabofism, 157.

Metaphase, see Mitosis.

Melosira, see Diatoms.

MiceUse, 45.

Microscope, Special observations with, 7, 13, 23, 90, 93, 97, 99, 101.
Microsomata, 15, 159.

Microsporangium, 121, 131, 144.

Microspore, Origin and structure of, in Angiosperms, 119; in Gymno-
sperms, 131 ; in Heterosporous ferns, 144 ; Maturation of, 121, 132,
144.

Middle-lamella, 38, 43 ; in bordered-pits, 68.

INDEX.

201

Mitosis (Chap, viii.) anaphase, 113, 114 ; prophase, 103, et seq. ; metaphase,
112, 113; telophase, 114; formation of cell-plate, 114, 115; origin
of achromatic spindle, 106, 115 ; origin of primary chromosomes, 104 ;
spireme stage, 104 ; dispireme stage, 114 ; monaster stage, 106 ;
diaster stage, 113 ; Centrosomes in, 103, 115 ; mantle-fibres, 106,
112; Chemotaxis in, 106, 113.

Mimosa, Sensative leaf of, 185.

Mo hi, von. Primordial utricle of, 15.

Monaster stage, see Mitosis.

Monocotyledons, Arrangement of tissues in stems of, 4, 52 ; Disposition
of bundles in stems of, 52 ; Presence of cambial zones in stems of, 53.

Mosses, Reproduction in, 145 ; Vascular elements of, 80.

MotUe cells, 2, 89, 96, 97, 132, 142, 144, 149, 151 ; Action of electric currents
upon, 188. %

Mougeottia, Rotation of Chlorophyll band in, 182.

Movements, of protoplasts (naked), 2, 8, 10 ; of protoplasm in a cell, 11,
15 ; Geo tropic, 184 ; Heho tropic, 181 ; of variation, 185 ; of swarm-
spores, 11, 95 ; of small particles in Spirogyra, 92 ; of cilia, 97.

Mucdage, in sieve-tubes, 62.

Multicellular formation, 101, 129.

Mycehum, 89, 147.

Myxomycetes, 8.

Nageli, theory of constitution of cell-wall, 45.

Nettle, Stinging hairs of, 33.

Nitrogen, Assimilation of, 18, 158, 169 ; Conversion of, by bacteria, 158,
173 ; forms in which N enters plant, 33, 171, 173, 174.

Nitrates, Absorption of, 173, 174 ; formed by bacteria, 173.

Nitrifying bacteria, 158, 173.

Nitrites, Absorption of, 33, 173 ; formed by bacteria, 158, 173.

Nucellus, 121, 132.

Nuclear membrane, see Nucleus.

Nuclear plasm, see Nucleus.

Nuclear spindle, see Mitosis.

Nuclein, see Chromatin.

Nucleolus, see Nucleus.

Nucleus, 1, 7, 15, 101, 102, 103 ; Structure of quiescent, 15, 101, 102, 103 ;

Changes in, during mitosis, 103, et seq. ; Linin-network of, 101.
Nutation, 184.

Nyctitropism, 185.

Nymphaea, Idioblasts in, 86, 87.

Octants, see Embryo.

Oil, in Vaucheria, 93 ; in higher plants, 78, 79, 159, 160, 176 ; in fungi,
89 ; -glands, 78, 79.

Oogonium, of Vaucheria, 151 ; of ferns, 142 ; of Fucus, 149 ; of fungi, 147.

202

INDEX.

Oosphere, of Angiosperms, 125 ; of Gymnosperms, 133 ; of ferns, 143 ;
of Vaucheria, 151 ; of Fucus, 149 ; of Bryophyta, 145, 146-

Oospore, see Oosphere.

Optimum temperature, 160 ; illumination, 181.

Organic material, Absorption of, by fungi, 89.

Osmosis, 49, 64, 172, 189.

Osmotic equilibrium, 12, 172, 189.

Oxalic acid. Occurrence of, in plant-cells, 84, 171 ; as a bye-product during
metabolism, 84, 159, 171.

Oxidation, 12, 44, 157, 159, 161, 165, 187.

Oxygen, Evolution of, during assimilation, 13, 18, 27, 165, 178 ; essential

I to respiration, 30, 160, 178 ; essential to formation of chlorophyll,
169 ; essential to kataboHc constructive processes, 159 ; essential to
mitosis, 103.

Palisade cells, 36, 37.

Palmella stage, 98.

Parenchyma, Definition of, 4.

Paranuclein, 103.

Parthenogenesis, in Chara crinita, 155.

Pectose, in young cell- wall, 25, 44 ; in middle-lamella, 44.

Pelargonium, Glandular hair of, 33.

Peperomia, Anomalies in maturation stages of, 155.

Peptic ferments, see Enzymes.

Peptones, occurrence in plants, 171.

Periblem, 24.

Pericycle, 37, 46, 77.

Phanerogams, Position of, 6.

Phelloderm, 39.

Phellogen, see Cork-cambium.

Phloem, Origin of, 49, 51, 60 ; Functions of, 64.

Phosphorus, in nucleus, 7, 103 ; as phosphates in cell-sap, 171 ; in aleurone-
grains, 176 ; as an essential element in plants, 157.

Photosynthesis, 18, 165, 167, 177, 180 ; Sugars formed during, 160, 165,
180.

Phototonus, 181.

Phyllocyanin, 165.

Phylloxanthin, 165.

Pinus, Bordered- pits in stem of, 66, 68 ; Medullary rays of, 74 ; Tracheides
of, 66, 68, 71 ; Reproduction in, 130, et seq. ; Resin-canals of, 79, 80,
et seq.

Pith, see Medulla.

Pits, Simple, 26, 41, 45, 65 ; Bordered, 66, 68 ; Origin of bordered, 68.
Plant characteristics, 97, 98.

Plasmodium, of Amoeba, 1, 8 ; of Aethahum, 8*

Plasmolysis, in VaUisneria, 16 ; in Spirogyra, 91 ; in Vaucheria, 95.
Plasmosome, see Nucleolus.

Plastic materials, see Starch, Proteids, Protoplasm, Cellulose B ood.

INDEX.

203

Plastids, in young cell, 19 ; in rhizome of Iris, 163 ; Starch-formation in,
21, 163 ; Fat-formation in, 160.

Polarised Hght, Use of, in studying starch-structure, 162.

PoUen-tube, Growth of, 127, 186 ; Formation of, 121, 132, 137.
Polyembryony, 138.

Potassium, as essential element in plants, 157.

Potato, Starch-grains in, 162 ; Cork of, 38.

Procambial strands, 50.

Promycelium, 147.

Prophase, see Mitosis.

Prosenchyma, Definition of, 4.

Proteids, Manufacture of, 169, 177 ; Storage of, 170, 176, 177.

Proteus vulgaris. Attraction of, by oxygen, 182.

Prothallial cell, in microspore of Angiosperms, 121 ; in microspore of
Gymnosperms, 132.

Prothallium, of Angiosperms, 154 (see Table opp. p. 154) ; of Gymnosperms,
132 ; of ferns, 142, 144.

ProthaUus, 118, 142, 144.

Protonema, 145.

Protophloem, 51.

Protoplasm, Definition of, by Huxley, 7 ; Composition of, 7, 10 ; Move-
ments of, 8, 9, 11, 15 ; Rotation of, 15 ; Staining of, 17 ; Growth of,
21 ; Conditions essential to continued activity of, 12 ; Water of con-
stitution of, 10 ; Solvents of, 12 ; Effects of nhvsical and chemical
stimuli upon, 184.

Protoplast, Definition of, 1.

Protoxylem, 50, 69.

Prussic acid. Assimilation of, 170.

Pseudopodia, of Amoeba, 2, 8 ; of Diatoms, 99

Pteridophyta, Reproduction in, 118, 119, 140, 144 ; Position of, 6.

Pulvinus, 185.

Pyrenoids, of Spirogyra, 90 ; of Sphyerella, 97.

Quercus, for study of wood-elements, 7 1 ; cork-layers in Q. sessiliflora, 39.
Quiescent nucleus, 101.

Quinine, see Alkaloids.

Radlajl, sections, 74 ; wall-formation in cambium, 46.

Raphides, in Dracaena, 82, 84 ; Composition of, 84.

Reproduction, general considerations, 117, 118, 119.

Resin, Formation of, 83, 172.

Resin-canals, in Pinus, 79, 80, et seq. ; Development of, 81, 83.

Respiration, Products of, 12, 30 ; Oxygen essential to, 12, 30, 159, 179 ;
Anaerobic, 179.

Rcsting-stage, in spore of Vaucheria, 95, 153.

Reticulate vessels, 66, 71.

Rhododendron, Glandular hairs of, 33.

204

INDEX.

Riccia, Sporogonium of, 146.

Ricinus, Cambium of, 53, 58.

Root-hairs, 9, 16, 32, 35 ; Osmotic phenomena in, 190.
Root-pressure, 71, 175, 176.

Rosette-stages, see Mitosis.

Rotation, of cytoplasm, 15, 95 ; movements caused by cilia, 97.

Safranin, as a nuclear stain, 15, 102, 104.

Salts, Absorption of, 9, 32, 172, 173, 190 ; essential to green plants, 157.

Salvinia, see Heterosporous ferns.

Sambucus, Cork in stem of, 38.

Sansevieria, Sclerenchyma in leaf of, 41.

Sap, Raw, 170, 172, 176, 177 ; Upward conduction of, 71, 174, 175, 176,
177 ; Elaboration of, 172, 176, 177.

Salicylic acid. Occurrence of, in plants, 171.

Schizogenous, origin of ducts, 83.

Schulze’s solution, as reagent for cellulose and starch, 17, 92 ; as reagent
for stem-sections, 74.

gclerenchyma, 40, 41, 65.

Secondary thickening, of stems, 52 ; of cell- wall, 44, 45.

Secretion, 159, 160, 165.

SelagineUa, see Heterosporous ferns.

Selective capacity, 9, 32, 172, 191.

Sieve-plate, 60.

Sieve-tube, Origin and structure of, 60, 62.

Silica, in walls of Diatoms, 99 ; as occasional constituent of sap, 157, 170,
171.

Siphonete, see Vaucheria.

Sodium, occurrence in plants, 157, 170.

Soredia, 88.

Sparganium, prothallial cell of microspore, 121, 155 ; divisions of antipodal
cells, 155.

Spectrum, absorption of chlorophyll, 166.

Sphserella, 95, et seq. ; Structure of, 97 ; Pyrenoids in, 97 ; Reproduction
in, 98.

Spheroids, 84, 87, 176.

Spireme, see Mitosis.

Spirogyra, Structure of, 90 ; Pyrenoids in, 90, 91, 92 ; Reproduction in,
93, 147 ; Amitosis in, 93 ; Chlorophyll band of, 90 ; Plasmolysis in,
91.

Spongioplasm, 10.

Spongy parenchyma, 28, 36.

Spores, of Homosporous ferns, 118, 119, 140; of Heterosporous ferns, 118,
119, 144 ; of fungi, 89, 146, 147.

Sporophyte, of Angiosperms, 117, 128 ; of Gymnosperms, 137, 139 ; of
ferns, 143, 145.

Stains, Nuclear, 102, 104 ; Plasmatic, 17, 22, 92 ; for guard-cells, 31 ;
for wood, 71 ; for sieve-plates, 62 ; for sclerenchyma, 41.

INDEX.

205

Starch, Formation of, in chloroplasts, 17, 18, 160, 162 ; in plastids, 21,
160, 163 ; Storage of, 17, 18, 160, 165, 177 ; Conversion of, into sugar,
44, 171, 176, 177 ; Structure of grains of, 162 ; Reactions for, 17,
162 ; Hilum of grain of, 162.

Starch-sheath, 48, 76 ; see also Endodermis.

Stems, Structure of, in Dicotyledons and Gymnosperms, 4, 48, et seq. ; in
Monocotyledons, 52 ; Anomalous structure in, 53 ; Climbing of, 185 .

Stimuli, Mechanical effect of, upon protoplasm, 185 ; light, 181 ; chemical,
186 ; gravity, 184 ; water, 186 ; Conduction of, in Drosera hairs, 186.

Stomata, of Iris, 27 ; of Sedum, 27 ; of Pinus, 27, 29 ; Origin of. in Prunus,
29, 31 ; Function of, 28, et seq., 174.

Suberin, in cork-cells, 39.

Sugar, Formation of, in plant-ceUs, 37, 160, 165 ; as a circulating food,
170, 177 ; Forms of, met with, 165, 171.

Sulphm’, as constituent of protoplasm, 7 ; assimilated by Beggiatoa, 158 ;
essential to growth of plants, 157.

Sulphuric acid, as reagent for protoplasm, 17 ; Presence of, in sap as sul-
phates, 171.

Suspensor-cells, 137.

Swarmspore, of Vaucheria, 95 ; of SphaereUa, 97.

Symbiotic community, 88.

Synclinal walls, 43.

Synergidge, 125 ; see also Homology, 154, and Table.

Synthesis, of carbohydrates, 164, 165, 177 ; of amido-acids, 170 ; of amido-
acids, S-compound, and a carbohydrate to form proteid, 159, 170, 177.

Tagmata, 45.

Tangential walls, 46 ; secretions, 74.

Tannin-cells, 86.

Tapetum, 119, 123, 131, 132.

Telophase, see Mitosis.

Temperature, Range of, for vitality of cytoplasm, 12 ; Rise of, during
germination, 160, 161, 187.

Tendrils, Climbing of, 185.

ThaUophyta, Position of, 6 ; Cells of, 4, 88, et seq. ; Reproduction in, 146,
et seq.

Timiriazeff, Experiment of, in connection with influence of red rays upon
starch-formation, 182.

Torus, in bordered- pits, 68 ; Function of, 68.

Tracheides, 65, 66, 69 ; Function of, 71, 175 ; of Pinus, 66, 68.
Transfusion-cells, 62, 63.

Translocation, of food in plants, 62, 176, 177 ; by phloem, 62, 176, 177.
Transpiration, current, 28, 71, 174, 177 ; from leaf, 28, 174 ; Influence of
light upon, 175 ; Energy required for, 174.

Tropaeolum, Chromoplasts of, 167.

Tunicata, Cellulose in, 98.

Turgidity, Factors determining, 12 (see Erratum, p. viii), 17, 172, 173,
90 ; Influence of, upon growth, 12, 173, 190.

206

INDEX.

Ulmic acid, as a bye-product of metabolism, 159.
Unicellular plants, 2, 96.

Urtica, Stinging hairs of, 33, 34.

Vacuoles, in cytoplasm, 7, 15 ; Origin of, 21 ; Contents of, 15.

VaUisneria, Assimilating cells of, 13 ; Starch-formation in chloroplasts of,
17, 18 ; Plasmolysis in, 16, 17 ; Rotation of protoplasm in, 15.
Variation, Movements of, 185.

Vascular bundles. Origin of, 50, 51, 52 ; Arrangement of, in Dicotyledons,
50 ; in Monocotyledons, 52.

Vascular cryptogams. Position of, 6.

Vaucheria, Structure of, 93 ; Swarmspores of, 95 ; Sexual reproduction in,
95, 151 ; Plasmolysis in, 95 ; Rotation of protoplasm in, 95 ; Encysted
spore of, 95, 153.

Vegetative cell, in microspore, 121, 132.

Vegetative propagation, see Gemmce^ Soredia.

Ventral canal- cell, see Canal-cells.

Vessels, Formation of, 65; Reticulate, 66; Pitted, 65, 66; Annular, 69;

Scalariform, 66 ; Function of, 71.

Vitahty, Conditions favouring, in cytoplasm, 12, 156.

Volvocinese. see Sphcerella. ■

Waller, Prof., Experiments of, in connection with germination of seeds, 11.
Water, Absorption of, 172, 173, 189 ; Importance of, to the cell, 10, 12 ;
of constitution, 10 ; Evaporation of, from leaves, 28, 174 ; Conduction
of, 71, 174, 175, 176, 177 ; Assimilation of, 18, 157, 159, 167, 180 ;
in cell-sap, 17, 171.

Wax, Occurrence and formation of, in plants, 172.

Wood, see Xylem.

Wood-parenchyma, 70.

Xanthophyll, see Phylloxanthin.

Xylem, Origin of first elements of, in bud, 50, 79 ; Origin of, from cambium,
49, 52, 65 ; Details of, 65, et seq. ; Functions of, 71, 175 ; of open
bundles, 53 ; of closed bundles, 52.

INDEX.

207

Yeast, an organised ferment, 171.

Zea mais, Sclerenchyma in, 41.

Zygnemaceae, see Spirogyra.

Zygospore, see Reproduction in fungiy 147.
Zygote, of Spirogyra, 148.

Zymogens, as precursors of enzymes, 158, 159.

BELL AND BAIN, LIMITED, MITCHELL STREET, GLASGOW

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