The plant cell : its modifications and vital processes, a manual for students

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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 working 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 involved. With regard to the illustrations, a few photomicrographs 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 suggestions, and for the help afforded whilst the Author was a student at University College.

H. A. H.

Hendon, 1909.

b

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 Reagents— Significance of Starch-formation — Definition of Assimilation in its Wider Sense,

B. The 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— Dermatogen — 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

23-41

Vlll

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 (Homosporous and Heterosporous) — Reproduction in Bryophyta,

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

CONTENTS.

IX

PACKS

CHAPTER X. Chemical and Physiological Studies in Connection with

the Cell.

General Considerations — 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 COo and HA) — \ ariations of Protoplasmic Activit}' under the Influence of Different Physical Agencies — Production of Heat. Light, and Electrical 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

ERRATA.

Frontispiece, line 1 , for Sambucus read Lupulus.

„ line 7, for Lupulus read Sambucus.

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 proembryo which has a cell cut off from its lower end, and this,” &c. The remainder of the proembryo forms the suspensor. Page 128, line 4, omit the words “and, in some cases, the foot (an absorbent 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.

6ct. Young Cells of the Endosperm of Caltha palustris, . 21

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

збб. Transverse Section of a Primary Bundle in a

Dicotyledonous Stem, .... ,, 55

51. Transverse Section of the Root of Pinus , . ,, 68

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

96 and 97. The Completely-matured Embryo-sac of Helleborus, ,, 126

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

THE PLANT CELL.

INTRODUCTION.

In order to study the life-phenomena of any organism, and to ai'rive 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 concise 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 membrane, 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 ingests 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 subdivisions.

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 arrangement, 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 othei’s 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 appeai’ances 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 without inwards : —

Stem or Root.

External Tissues : (a) Epidermis.

(b) Cortex;* at times continuous with (e).

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 ( b ) 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 subsidiary 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 Conifers®, 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 differentiation is not so marked as it is in the higher types, and, finally r when the Fungi and Algte arc considered, the vascular tissues cease to exist per se, and the plant becomes a structure known as a Thallus (Thallophyta) the component tissues of which 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, dimensions whichever way they are measured, are termed parenchyma; 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 sclerenchyma, 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 aro 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 Algae, all functions, — viz., assimilation, 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 well-marked special organs, and the processes occurring 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 genealogical 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- Vascular Cryptogams Vascular Cryptogams

Pteridophyta

Thallophyta Bryophyta

Heterosporous

Selaginellete Isoetete

PHANEROGAMIA (flowering plants)

Angiospermoe <- Gymnospermse

Monocotyledons Dicotyledons Conifer® Cycadete

Gnetaceai

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 Cycadete, certain fossil types probably intervening.

The dotted line shows the 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, Monocotyledons, Dicotyledons, and Gymnosperms (Coniferse), and these will be the main groups used in dealing with plant 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 protoplasm 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” ( Method and Results).

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 JF

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 (plasmosomes) ; 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

Fig. 2. — Am (Eli a Pkinceps. — 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 “contractile 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, Biitschli 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 powerful illumination withdraws to a position where the light is less intense ; and Amoeba, a similar, although, correct^ 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 protoplasm which is “ naked,” as in Aetlialium, 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.’’ With 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 stimulated 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 stimulation 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 constitution ” 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 essential to the maintenance of vitality. 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 conditions are necessary to the continued activity of protoplasm : —

(a) A certain temperature, which, ill most plants, is something

above zero Centigrade.

(b) Access to moisture.

(c) The presence of oxygen.

(cl) 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

(/) The presence of certain assimilable food-materials and mineral salts (see Chap. x.).

Note. — Protoplasm is soluble in dilute caustic 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 modifications 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 ma)^ 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 herbaceous 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 tyjncal 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 focussing into various planes : —

i. The outermost layer of the leaf, composed of elongated cells rectangular 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 (i" objective) it is possible to distinguish in any of these cells (i. or ii.) the following features (see Fig. 3) : —

(a) The eell-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, b, Formation 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.

1 5

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

(c) The nucleus, lying somewhere in this layer.

(<f) 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 substage 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 f 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 luematoxylin 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