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Full text of "An introduction to the structure and reproduction of plants"



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QK 45.F85 



An Introduction to the structure and rep 





3 1924 001 698 905 





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AN INTRODUCTION TO THE STRUCTURE 
AND REPRODUCTION OF PLANTS 



BY THE SAJ/E ATTHORS 

AN INTRODUCTION TO THE 
STUDY OF PLANTS 

Uy F. E. Fritcii, D.Sc, I'h.I)., F.L.S., an.l 
!•:. J- Salisbury, D.Sc, F. L.S. With over 250 
Illustrations. Fourtli Edition. Demy Svo. 71. 6ii. 
net. This volume completely covers the Limdon 
University Matriculation Syllabus. 

" I'nis book is one of a very small cl.iss, the kind tliat 
makes teachers wonder whatever their pupils did in the past 
without them." — Kdiicationat Times. 

" Certainly one of the very best of the eletnentary textbooks 
.if btptany we have seen." — Joii7-7iaI ef Edii^ation. 

ELEMENTARY STUDIES IN 
PLANT LIFE 

By V. E. Fritcm, D.Sc, Ph.D., F-L.S,, and 
E. J. Salisbury, D.Sc, F.L.S. With over 150 
IIlustratLons. Crown Svo. Third Ethliun, 
3,f, Gd. net. 

" It would not be t;asy to find a bouk licttcr :^uited to the 
needs of junior students." — Nature. 

"There is plenty of practical work, the style \% easy and 
natural, the illustrations are good and numerous. Alti.i^icthLT 
this is one of the soundest and most att.iacti\'e books fur 
beginners we have seen." — School ]]'o>'ld. 



LONDON: G. UELL AND SONS LTD. 
York House, Portugal St., W.C.2 



PLATE I 




Fig. 2IO. — Photomicrograph of transverse section through the young 
capituUim of the Dandelion (Taraxacum). [Photo: IC. J.S.l 



[i't'OiUi-<l>it'ci:, 



AN INTRODUCTION TO THE 

STRUCTURE AND 
REPRODUCTION OF 

PLANTS 

1!V 

F. E. FRITCH, D.Sc, Ph.D., F.L.S. 

rROFES~>OK OF BOTANY, EAST LONDON COLLEGE, 

(."NIVERjilTY OF LONDON 

AND 

E. J. SALISBURY, D.Sc, F.L.S. 

LECTl'KER IN riOTANV AND FELLOW OF TNIVER'^ITV 
COLLEGE, UNIVERSITY i>F LONDON 




' ''/./ 



LONDON 

G. BELL AND SONS LTD. 

1920 



(55 
hi 



t^l 



First Published, 1920 



PREFACE 

This volume has been prepared in response to the demand for a 
sequel to our I ntroduction to the- Study of Plants, from which 
the minute structure and details of life-history, that require the 
use of a microscope for their proper comprehension, were 
purposely omitted. 

Whilst the present volume is complete in itself, frequent 
references to the earlier book are included, and the two together 
form a comprehensive introduction to the science of Botany. 
Not only is the syllabus for all Higher School Examinations 
amply covered, but the two volumes will also be found to meet 
the needs of the first-year student at the Universities. Like its 
predecessor, however, the present work was compiled for those 
who really desire to learn something of the role of plants in nature, 
of their structure and of their mode of life, rather than merely to 
furnish another examination textbook. 

With the aim of giving a better survey of the \'egetable Kingdom 
and presenting a more balanced concept, we have abandoned the 
study of isolated types in favour of a more general account, 
indicating the range of form and reproductive methods within 
each group. This has not resulted in an undue increase in the 
subject-matter, since we have omitted much developmental detail 
which has little educational value or significance, except for the 
specialist. With the object of widening the outlook of the student 
on the two aspects of botanical science treated in this volume, 
special chapters have been included dealing with ecological 
anatomy, and variation and heredity. iVIoreover, we have intro- 
duced such physiology as is more appropriately considered in 
relation to microscopic structure. 

Features, whether of plant-anatomy or plant-chemistry, which 
are of commercial importance are emphasised throughout, and 
it is hoped that this may ser\'e to combat the frequent ignorance 



vi PREFACE 

of botanical students with respect to tlie economic aspects of their 
subject. We have again been at pains to illustrate the subject- 
matter, as far as possible, by reference to British or commonly 
cultivated plants, especially in that part of the book dealing with 
the different groups. 

The index has been compiled with the greatest care, and includes 
numerous subject-headings with a Adew to rendering the large 
number of facts dealt with of the widest possible use. Some two- 
thirds of the illustrations are original, and have been especially 
prepared for this book. For a number of these we are indebted 
to Mrs. F. E. Fritch, who is responsible for those initialled H. F. ; 
the remainder are the work of one or other of the authors. The 
source of all illustrations that have been copied is acknowledged 
in the figure descriptions. For the use of the photograph 
illustrating Fig. 222 we are greatly indebted to Prof. Bateson, 
F.R.S., and Miss Pellehew. We also have to thank Mr. W. B. 
Johnson for the photograph illustrating Fig. 200, Prof. F. W. 
Oliver, F.R.S., for Figs 188-191, Mr. E. M. Cutting, M.A., for 
Figs. 136- 13Q, the proprietors of the Indiarnhber Journal (through 
the instrumentality of Mr. H. Wright, A.R.C.S.) for Fig. 79, 
and Dr. H. C. I. Gwynne-Vaughan for permission to reproduce 
Figs. 10-12. 

We are glad to have this opportunity of expressing our gratitude 
to the following specialists, who have kindly read through various 
sections of the manuscript and ofiered criticisms and suggestions : 
Prof. A. E. Boycott, F.R.S. (the section on Bacteria), Mr. E. M. 
Cutting, M.A. (the chapter on Fungi), Prof. F. W. Ohver, F.R.S. 
(the Cycads and Pteridosperms), and Dr. F. G. Pope (chemical 
sections). We are also indebted to the works of numerous 
authorities on special branches of the subject, and, among these, 
would especially mention that of Haas and Hill on Tlic Chemistry 
of Plant Products, which we have repeatedly consulted in connec- 
tion with the chemical matter. 

F. E. F. 
E. J. S. 

London, 

May 1920. 



CONTEXTS 



PART I.— THE ANATOMY OF PLANTS 

CH-VPIER I'AGE 

I. The Structure and Physiology of the Plant- 
cell ........ I 



II. Growing Points and Cell-division . . . i6 

III. The Differentiation of Plant-cells . . 27 

I\'. The Non-lia'ing Contents of Cells. . . 40 

V. Cell-contents (By-products, etc.) ... 58 

VI. The Structure of Roots .... 65 

VII. The Structure of the Young Stem . . 76 

\'III. The Epiderjiis. ...... 91 

IX. The Structure of the Leaf .... 107 

X. Secondary Thickening ..... 117 

XI. Cork-formation, etc. ..... 135 

XII. Secretory Organs 144 



XIII. ANATOMY IN Relation to the Habitat . . 15S 

vii 



VIU 



CONTENTS 



PART IT— THE LIFE-HISTORIES AND REPRODUCTION 
OF PLANTS 

CHAPTER PAGE 

XIV. Simple Vegetable Organisms . . . i8o 

XV. Pond-scums, Seaweeds, etc. (Alg.e) . . 195 

XVI. Reproduction among the Alg^ . . . 212 

XVII. The Structure and Reproduction of the 

Fungi ....... 230 

XVIII. Physiology of Fungi, Lichens, Bacteria . 252 

XIX. Liverworts and Mosses .... 269 

XX. Ferns ........ 2S9 

XXI. Horsetails and Clubmosses .... 310 

XXII. The Cycads and Certain Extinct Seed-bearing 

Plants ....... 322 



XXIII. The Conifers 

XXIV. The Angiosperms . 
XXV. Heredity and Evolution 

Appendix 

Index .... 



334 
354 
374 
398 
411 



THE 

STRUCTURE AND RE- 
PRODUCTION OF PLANTS 

PART I 

THE ANATOMY OF PLANTS 

CHAPTER I 

The Structure and Physiology of the Plant-cell 

Every living organism, whether animal or plant, is composed 
of one or more minute units called cells, and this applies even 
to those forms of life which are so small that they can only be 
seen with the help of a microscope. The green powdery cover- 
ing so often present on tree-trunks and palings consists of multi- 
tudes of single-celled plants [PUurococcus, Fig. 102, p. 192), whilst 
millions of cells together form the body of a garden weed or 
tree. Organisms of the latter type are described as inulticellidar, 
whilst those consisting of a single cell are termed unicellular . 

The structure of a cell will best be realised if a typical example 
is studied, such as that obtained by stripping off the skin (or 
epidermis) from the inner surface of an Onion-scale. If a small 
portion of this be mounted in water, and examined under the 
low power of a microscope (see Appendix I), it will be seen to 
consist of a large number of oblong cells connected together 
without any intervening spaces to form a tissue (Fig. I, A). 
The network of delicate lines separating the individual units 
is constituted by the cell-n-alls, which are all joined to one another. 
In each cell a colourless, somewhat dense granular substance 
(the cytoplasm, Fig. i, B, Cy.) is visible, particularly around the 
I 



2 STRUCTURE OE THE PLANT-CELL 

edge, whilst near the centre or against one side is situated a 
rounded shining body, the nucleus (Fig. i, B, N). 

These cells consist, then, of three parts, the cell-wall (Fig. i, 
B, C.W.), which is not living and is merely a protective case, 
the cytoplasm, and the nucleus; the last two constitute the 




Fig. I. — Epidermis of Onion bulb-scale. A, small portion in surface view. 
B, a single cell mucfi enlarged. C, a scale in vertical section. In 
A and C the nuclei are shown black. Cy., cytoplasm ; C.ll'., cell- 
wall; ep., epidermis ; iV, nucleus ; k, nucleolus ; Va., vacuole. 

living part or protoplast, composed of protoplasm. The proto- 
plasm is probably a very complex mixture of proteins, fatty 
bodies, etc., composed mainly of the elements _carbon, hydrogen, 
oxygen, nitrogen, and sulphur (often together with phosphorus, 
especially in the nucleus). It has somewhat the consistency 



STRUCTURE OF THE PLANT-CETL 3 

of the white of an unboiled egg, and usualh' contains large 
numbers of small granules which are partly of the nature of 
food-bodies and partly waste-products. The fact that not all, 
though some, of the properties of living protoplasm are exhibited 
by cells which are ground to pulp suggests that the particular 
characteristics of the cytoplasm are not entirely an outcome of 
its chemical constitution, but are to some extent a consequence 
of its ultimate structure. The nucleus is the most important 
part of the protoplast, a fact which will become more apparent 
in later chapters (Part II), when the reproductive processes of 
plants are studied. It appears to be essential for many 
of the vital activities of the cell. A demonstration of this is 
afforded by the fact that, if the unicellular animal Stenlor is 
broken into pieces, the fragments which contain portions of the 
nucleus develop into new individuals, whilst the others, after 
a short time, perish. Moreover, the nucleus is generall}' found 
in that part of the cell which is in process of active growth, 
e.g. at the tip of a growing root-hair. 

The structure of the Onion-cell can be more clearly distin- 
guished if the strip of epidermis be stained with a drop of iodine 
(see Appendix II), and a single cell examined under the high- 
power objective. The protoplasm will have assumed a yellowish 
tint, whilst the nucleus appears pale brown. This colour 
reaction of the protoplast is one characteristic of proteins gener- 
ally. The nucleus (Tig. i, B, X) is bounded by a thin nuclear 
membrane, whilst within it one or two small round bodies, the 
nucleoli [n), are now plainly visible, since they are stained more 
deeply than the rest. The cytoplasm does not completelj' lill 
the cell, but there is a large space or vacuole {Va.) occupj-ing the 
greater part of the central region ; this vacuole, apparently 
empty, is in reahtj" filled with a watery fluid, the cell-sap. Close 
observation shows that the cytoplasm is not evenly granular 
throughout, but that there is a very thin clear layer immediately 
within the cell-wall. This layer is a result of physical forces 
operating at the surface of the cytoplasm, and a similar clear 
layer can be detected at the surface abutting on the vacuole. 
These layers are spoken of as the plasmatic membranes. 

If another strip of Onion epidermis be mounted in concen- 
trated sulphuric acid, the cell-walls wiU swell and disappear. 



4 STRUCTURE OF THE PTANT-CELL 

Subsequent addition of iodine gives a blue colouration to the 
dissolving walls, a reaction characteristic of cellulose, of which 
most thin ccll-nicmbranes largely consist. 

It must be realised that the cells just examined, like all 
plant-cells, are developed in three dimensions, a fact which can 
be verified b}' cutting a section transversely across the Onion- 
scale, when the epidermal cells will appear as flat tabular struc- 
tures (Fig. I, C). 

As a second example, one of the cells forming the purple 
hairs (Fig. 2, A) on the stamens of the Common Spiderwort 
[Tradescantia) may be examined.^ These show the same struc- 
ture as those of the Onion, but the cell-sap is here of a purple 
colour. The nucleus, surrounded by a small mass of cj'toplasm, 
is not uncommonly suspended in the middle of the vacuole, 
the enveloping cytoplasm being connected with that lining the 
cell-wall by a number of protoplasmic strands (Fig. 2, B). If the 
granules in these strands are closely observed, they will be seen 
to exhibit a continuous streaming movement which can be 
accelerated by slight warming, but ceases on the addition of a 
poison [e.g. alcohol), whereby the cell is killed. A temporary 
cessation of the movement can be brought about by mounting 
the cells in water to which a trace of some an.-esthctic [e.g. chloro- 
form) has been added. On returning the hairs to ordinary water, 
however, streaming of the granules is soon resumed. A similar 
effect is ol^tained if the cells are mounted in water which has 
been boiled and subsequently cooled, the result showing the 
necessity of oxygen for the performance of such movements. 
The movement is really due to a flowing of the cytoplasm, and 
this phenomenon betrays the fluid consistency of the latter. 
No movement can be recognised, however, in the plasmatic 
membranes, which are therefore probably? of a denser character. 

All living plant-cells display the features above described, 
but in many cases other structures are present, and of these the 
commonest are bodies known as plastids. In those cells of the 
plant which are exposed to the light the plastids become the 
depositories of the green pigment, or cldorophyll , and are then 

' If material of the Spiderwort is not availal)lc, a good substitute is 
furnistied by the unicellular hairs lining the inner surface of the corolla- 
tube of the White Dead-nettle, in which, howevcr,the cell-sap is uncoloured. 



STRUCTURE OF THE PLANT-CELL 5 

known as chloroplasts. On examining a k-af of the Canadian 
Pondweed [Elodea canadensis) under the high power, each of 
the more or less rectangular cells will be found to contain a 
number of chloroplasts, which will be seen from one of two 
aspects (Fig. 3). Some, situated along the sides of the cell, 
are ^•iewed in profile and appear as flattened ellipses {OS.). 




Fig. 2. — Staminal hairs of the Spiderwort (Tradescanlia). A, a single 
hair ; B, a single cell showing the nucleus and strands of cytoplasm 
passing across the vacuole ; C, a hair plasmolysed with sea-water. 

whilst others hing against the upper or lower walls, and conse- 
quently seen from the surface, present a round or oval form {Sit.). 
Each chloroplast is thus a solid body which has more or less 
the shape of a biconvex lens. 

If attention be centred on a sin,gle cell, the chloroplasts lying 
against the upper face can be brought clearly into view by 
turning the fine adjustment ; on focussing to a lower level. 



6 STRUCTURE OF THE PLANT-CETL 

however, another layer of ehloroplasts appears Ijelonging to the 
same ceU. We can thus hken each cell to an oblong box lined 
along the whole of the inner surface with a thick viscid fluid 
(the cytoplasm) in which are embedded the ehloroplasts, the 
cavity of the box representing the vacuole (Fig. 3, Va.) with its 
cell-sap. 

By watching the ehloroplasts, particularly in the elongated 
cells of the midrib, they will be observed to exhibit movement 
similar to that of the granules in the cells of the Spiderwort, and 
here as there this is actually due to a flow of the cytoplasm. 




Fig. 3. — Two culls of the Canadian Pondwccd, one in surface view (Sji.), 
and the other in optical section {O.S.). The ehloroplasts are shown 
black. At the left a single chloroplast showing starch grains {St.). 
Va., vacuole. 

Such a streaming movement probably takes place in the cyto- 
plasm of all living cells, but in many cases it is so slow that 
it cannot be demonstrated. By its means food-substances, etc., 
are more tj^uickly distributed from one part of the cell to the 
other than b)? mere diffusion. 

When leaves of the Canadian Pondweed, or those of other 
plants, are kept in spirit for some little time, all the chlorophyll 
is removed in solution. The ehloroplasts, though now colourless, 
ha\'e undergone no change of shape, and are therefore merely 
specialised parts of the cytoplasm \vhich held the chlorophyll. 

The green cells of all the higher plants contain numerous 



STRUCTURE OF THE PLAXT-CELL 7 

chloroplasts, essentially similar in form to those just studied. 
In the simple tj'pes of plant-life [e.g. among the Alga:-), however, 
the cells are often provided with but a single chloroplast or a 
Umited number of them, and these are frequently of a much 
more elaborate pattern. Spirogyra, which is exceedingly common 
in stagnant water, furnishes an extreme instance. The plant 




Fig. 4. — Single cell of a Spirogyra fdameat showing the spiral chloroplast 
(ch.), the pyrenoids {p.) surrounded by starch, and the nucleus (».) 
supported by cytoplasmic strands [si.). S, separating wall between 
adjacent cells. 

here consists of a single thread or filament composed of cylin- 
drical cells joined end to end. According to the species, each 
cell contains, apart from the cytoplasm and nucleus, one or 
more chloroplasts in the shape of green spiral bands, usually 
with a jagged edge (Fig. 4, ch.). These spiral chloroplasts are 
here, again, situated in the lining la3-er of cj'toplasm. At inter^-als 
bright bodies, consisting of protein ancl known as pyrenoids, are 



8 PLASMOLYSIS 

embedded within tlieir substance (Fig. 4, p.) ; but these arc more 
evident after treatment with iodine, whereupon tliey become 
blue, owing to the fact that each is surrounded by a laj'er of 
starch. Pyrenoids are not found in the cliloroplasts of the 
higher plants, but are quite frequent amongst the Alga; (see 
p. 207). 

Returning to the cells in tlie liairs of the Spiderwort, we 
will mount them in 2 per cent, natural or artificial sea-water 
(see Appendix III) ; the resulting phenomena could be equally 
well observed by using any cells with coloured sap, e.g. those 
forming the lower epidermis of the Mother-of-Thousands [Saxi- 
fraga sarmentosa) , or those in the petal of a Pieony. In the 
cells thus treated the lining layer of cytoplasm has contracted 
away from the wall, so that a clear space is visible between it 
and the latter (Fig. 2, C). In this condition the cell is said to 
be plasmolysed , and the phenomenon is spoken of as plasmolysis . 
If the sea-water be replaced by ordinary water, the cell-contents 
increase in volume, so that the plasmatic membrane regains its 
original position in contact with the wall and the cell resumes 
its normal appearance. By alternately substituting sea-water 
and tap-water, this sequence of events may be repeatedly ob- 
served. 

In the normal cell the pressure of the sap within the vacuole 
keeps the lining layer of cytoplasm distended and forced against 
the wall, in much the same way as the bladder of a football, 
when inflated with air, is pressed against the leather cover. If 
the air be allowed to escape, the bladder shrinks and a space 
is left between it and the cover. Similarlj/, the contraction of 
the protoplast of the cell, when surrounded hy sea-water, must 
be attributed to a decrease in volume of the ccU-sap owing to 
leakage into the outside liquid. The recovery, when placed in 
water, obviously implies an increase in volvTmc of the sap, and 
this can only be due to some of the water around having passed 
into it. 

It is a well-known physical phenomenon (osmosis) that, when 
two solutions of different concentrations are separated by certain 
kinds of membranes termed " semi-permeable membranes " (which 
may be of a fluid consistency), there is a passage of water from 
the weaker solution to the stronger until both attain the same 



PLASMOLYSIS 9 

concentration. This is due to the fact that such membranes, 
whilst readily permeable to water, are for all practical purposes 
impermeable to the dissolved substances. The hning layer of 
protoplasm, or probably more correctly the plasmatic membranes, 
exhibit these features. Consequently, when cells are surrounded 
by solutions which are more concentrated than the cell-sap, 
water passes out and the protoplast contracts. On the other 
hand, when replaced in water, the cell-sap is the more concen- 
trated solution, and the protoplast becomes distended until its 
further expansion, and further increase in the volume of the 
sap, is prevented by the cell-wall. In a healthy plant, supphed 
with sufficient water, all the living cells are thus distended to 
their utmost, that is to saj' they are turgid , a condition which 
plays an important part in maintaining the stability of herba- 
ceous organs (cf. F. & S\, p. 102). 

These phenomena plainly show that the cytoplasmic hning 
is readily permeable to water, but not appreciably to the sub- 
stances dissolved in the sap, nor to those in the solution around. 
If it were permeable, the concentration inside and outside the 
cell would rapidly become equal as a result of diffusion and 
plasmolj'sis could not occur, nor would turgescence of the cells 
be possible. 

The plasmatic membranes exhibit their semi-permeable char- 
acter, however, onl}' so long as the protoplast remains alive ; 
hence after death both the substances dissolved in the cell-sap 
can pass outwards, and external solutions can diffuse inwards. 
If cells of the Spiderwort, etc., mounted in water, are killed by 
gently heating the slide over a spirit-lamp, the coloured sap 
wiU be found to escape into the surrounding liquid ; moreover, 
it wiU be found impossible to bring about plasmolysis or to 
render such killed cells turgid. These results, apart from showing 
that the semi-permeable character of the cytoplasm is altered 
by death, demonstrate that the cell-waU is permeable to sub- 
stances in solution. The permeabihty of the cytoplasm, when 
dead, to the cell-sap may be exhibited on a large scale by placing 
slices of Beetroot in hot water. 

A moment's thought, however, will show that the plasmatic 

1 I.e. Fritch and Salisbury, An Introduction to the Study of Plants, 
which in the subsequent pages will always be briefly referred to in this way, 



10 OSMOTIC PRESSURE 

membranes cannot be completely impermeable, since most of 
the substances found dissolved in the soil-water can be detected 
withm the plant. Moreover, plants will thrive for months or years 
in a water-culture solution, ^ from which analysis shows that 
mineral salts have been absorbed (cf. also below, p. 15). 

Were it not for the restraining influence of the wall, which 
is only slightly elastic, but possesses considerable strength, a 
plant-cell placed in pure water would increase in size until it 
became ruptured. This actually occurs when the root-hairs of 
salt-marsh plants are suddenly transferred to water, for these, 
like other marine plants, have a highly concentrated sap. Owing 
to the restriction on the dimensions of the cell, under normal 
circumstances, the intake of water is arrested when the wall 
has reached the limit of its stretching capacity. The more con- 
centrated the sap the greater its attraction for water, and hence, 
when a cell is surrounded by water, = the cytoplasm will be forced 
against the cell-wall with a pressure proportional to the strength 
of the sap. This pressure is spoken of as osmotic pressure, and 
in many plant-cells is very considerable, as is well illustrated by 
the following figures, which give the osmotic pressures in atmo- 
spheres in a few selected cases ; 

Leaves of Acer pseudoplatanus . . I4'52 

,, Hedera helix . . . lyGS-iS'y 

,, Pteris aquilina . . . 7^44 

Rhizome of Iris germanica . . . 9^97 

Owing to the small size of the cell, it is not possible to estimate 
the osmotic pressure of the sap directly, but an indirect measure 
is afforded by the strength of the solution necessary to bring 
about plasmolysis. By using artificially prepared semi-permeable 
membranes (see Appendix IV), the osmotic pressure of any given 
solution can be calculated in terms of atmospheric pressure. In 

1 Cf., F. it S., pp. 125, 126, sec footnote on p. 9. 

2 Even when a cell forms one of a tissue, it will receive water from all 
sides as long as the water-supply is normal, since all parts of a healthy 
plant imbibe water (cf. p. 26). Until a cell has reached the limit of its 
stretching capacity, water will be withdrawn into its vacuole from any 
of the surrounjiag cells which are more turgid. 



OSMOTIC PRESSURE ii 

general the osmotic strength of a solution depends upon the num- 
ber of molecules it contains (though this does not apply to dilute 
solutions of many salts), so that when these are present in equal 
proportions, two solutions [e.g. of cane-sugar and grape-sugar) 
have the same osmotic pressure. Such eqitimolecular solutions 
are obtained by dissolving substances, in a litre of distilled 
water, in proportions equivalent to their molecular weights. 
If the number of grammes of the compound dissolved in a htre 
of water is equivalent to the molecular weight, we have a so-called 
molecular solution, briefly indicated by M. Solutions of other 
strengths are indicated as 0-5 M, 0-2 M, etc. Since the weight 
of a substance depends on the number and mass of the atoms 
composing its molecule, a 10 per cent, solution of a complex com- 
pound, such as inuhn or dextrin, will contain fewer molecules 
than a 10 per cent, solution of a simpler compound, such as 
grape-sugar or cane-sugar ; moreover, cane-sugar, which has a 
higher molecular weight than grape-sugar, will exhibit a lower 
osmotic pressure than the latter when in solutions of the same 
percentage strength. These important facts are illustrated in 
the following table : 



Grape-sugar 
Cane-sugar 
Dextrin . 
Albumen 



In order to determine the osmotic pressure of anj^ cell, a 
solution of sea-water (see Appendix III), strong enough to bring 
about plasmolysis, is first obtained. We next proceed to find 
a sUghtly weaker solution such as will fail to plasmolyse the 
cell. By experimenting with a series of solutions of intermediate 
strengths, one can eventually be found which causes very Uttle 
contraction of the protoplast, the plasmatic membrane only 
receding very slightly from the angles of the cell. The osmotic 
strength of the solution which produces this effect is somewhat 
stronger than that of the cell-sap, since, at first, the contraction 
of the protoplast is accompanied by shrinkage of the stretched 
cell-wall. The osmotic pressure of the sap may, however, be 
taken as approximately that of the solution which just fails to 



Molecular 


Molar concentratioa 


Osmotic pressure 


weight. 


of 10 


per ceut. solution. 


iu atmospheres. 


180 




o'555 


12-43 


342 




0'292 


6-54 


— 







ca. 2'2 


ca. 13,000 




o'ooS 


0-17 



12 PERMEABILITY 

produce any visible plasmolysis (for equivalent pressures, see 
Appendix V).^ 

Another method, in which the average osmotic pressure of a 
whole tissue is determined, depends upon the curvatures con- 
sequent upon the different stretching capacities of the walls of 
the component cells. '^ For this purpose one can make use of 
short lengths of the stalk of a Dandehon-inflorescence which 
are split lengthwise into four portions. When placed in water 
liquid is absorbed by the cells, but, since those towards the 
outside of the stem have thicker walls than the inner ones, the 
latter have a greater stretching capacity ; as a consequence the 
strips curl up and form rings. A strength of sea-water that 
causes neither increase nor decrease in the curvature will have 
approximately the same osmotic pressure as the cell -sap. Stems 
of many herbaceous plants can be utilised in this way. 

Attention has already been drawn to the fact that the proto- 
plasm must be permeable to the compounds dissolved in the 
soil-water or in a water-culture solution. The permeability of 
the protoplasm can be demonstrated by placing young shoots 
of the Canadian Pondweed in a solution of methyl blue, so weak 
that it has but a very faint tint. If the cells be examined after 
some days the sap is found to be of a deep blue colour, indicating 
that the dissolved dye has passed through the cytoplasm in 
considerable amount. If the methyl blue remained unaltered 
on reaching the vacuole, only sufficient could have entered to 
bring about a concentration equivalent to that of the solution 
outside the cells. But the deep blue colour shows that the dye 
has accumulated within the sap, and this is due to the com- 
bination of the methyl blue with the tannin in the latter to form 
a substance to which the plasmatic membrane is impermeable. 
In this way the concentration of the dye vv'hich enters the cell- 
sap is continually being reduced to a strength below that outside. 
As a consequence more and more methyl blue diffuses in, and 
thus the deep blue colour is gradually produced. 

A further demonstration of permeability of the cytoplasm is 

1 Very accurate determinations of osmotic pressure are made by indirect 
means depending on the relation between osmotic pressure and tire tem- 
perature at which a liquid (e.g. the expressed sap of a plant-organ) freezes. 

2 Cf. F, & S., pp. 103, 10-1, see footnote on p. g. 



PERMEABILITY 13 

obtained when filaments of Spirogyra (and many other Algas) 
are placed in a dilute (i per cent.) solution of caffein. If the 
effect be observed under the microscope, a very fine greyish 
precipitate is seen to appear in the vacuole, which, as it accu- 
mulates, renders the cell more and more opaque. This precipitate 
is again a consequence of combination between the tannin in 
the ceU-sap and the caffein. 

Both examples illustrate a very important phenomenon, 
viz. that plant-cells can absorb and accumulate considerable 
quantities of various substances from very dilute solutions, pro- 
vided that within the cell they are changed into some other 
form which does not readily pass through the plasmatic mem- 
brane. This fact is significant when it is remembered that many 
compounds are only present in the soil-water in very small 
amounts. The phenomena just discussed also explain the 
accumulation of food-reserves (e.g. insoluble starch) in large 
quantities in the cells of storage-organs. When these reserves 
are utihsed, it is clear that they must be changed into a form 
which can diffuse to the growing organs, starch, for example, 
being transformed into sugar. This process can be simulated 
by placing leaves of the Canadian Pondweed, whose cell-sap 
has acquired a deep blue colour in the way above described, 
in a very dilute solution of citric acid (i per cent.). The latter 
passes into the cells and changes the blue pigment into a form 
capable of diffusing through the plasmatic membrane. Since 
its concentration inside the cell is greatly in excess of that outside, 
diffusion takes place and the sap gradually loses its deep blue 
colouration. 

Although, in the numerous processes of diffusion that occur 
between the living cells of the plant, complex organic substances, 
analogous to methyl blue and caffein, are often involved, the 
mode of absorption of the simple inorganic compounds in the 
soil-water is of primary importance. It is a well-known fact 
that mineral salts bring about contraction of the protoplast of 
plant-cells (including root-hairs), when used singly and in a 
sufficiently concentrated solution. In recent j'ears, however, the 
American botanist Osterhout has shown that many, perhaps all, 
of these cases of so-called plasmolysis are due, not to imperme- 
abilit}' of the plasmatic membrane to the compound in question, 



14 ANTAGONISM 

but to a poisonous or toxic effect of the latter upon the proto- 
plasm. Two compounds, however, which separately exert such 
a harmful effect can often, if mingled in certain proportions, 
provide a solution which produces no plasmotysis and is not 
poisonous. 

For example, if germinating zoospores of Vaiichcria (cf. p. 217) 
are placed in weak solutions of sodium chloride (ranging from 
0'0937 to o"ooi M), plasmolysis ensues more or less rapidly (in 
the case of the weaker solutions only after an interval of a day 
or more). If, however, calcium chloride is added to the salt 
solution in the proportion of one molecule of the former to one 
hundred of the latter, the young plants of Vaucheria can be 
subjected to strengths up to as much as o'l M without any 
contraction of the cell-contents. Indirectly the same effects 
can be observed by studying the growth and duration of life 
of young plants of the same Alga in the different solutions : 
in sodium chloride alone or calcium chloride alone there is prac- 
tically no growth and death soon takes place ; whilst in a mixture 
of the two in the proportions mentioned above, growth is as 
vigorous, and the plants remain alive as long, as in glass-distilled ^ 
water or in much diluted sea-water. 

Similar observations can be made on the development of 
the root-system of Wheat -seedlings, although these are not so 
sensitive to the poisonous solutions. In the same way potassium 
chloride and magnesium chloride are toxic when used separately, 
though when mixed in suitable proportions the poisonous effect 
largely disappears. Analogous results have been obtained with 
animals and animal-cells. 

It is not only the chlorides that give such results, for similar 
effects have been obtained with nitrates, sulphates, etc. All 
these compounds are employed in such dilute solutions that 
dissociation of the molecules into ions will have taken place. 
Since in the different experiments much the same amount of 
chloride is present, the striking results obtained must be due 
to some effect of the metallic ions, probably upon the proteins 
of the plasmatic membrane. The prevention of the poisonous 
action of one ion by one or more other ions is spoken of as 

1 Ordinary distilled water often contains traces of copper, etc., which 
are exceedingly harmful to most vegetable organisms. 



AXTAGOXISM 15 

antagonism, and is probably greatest between ions of different 
valencies. A mixture of ions in solution in such proportions 
that they exert no toxic effect is called a balanced solution, and 
when a solution of this nature produces contraction of the proto- 
plast, the effect is a true plasmoh^sis. In this category may be 
placed sea-water, which is a mixture consisting mainlj' of sodium 
chloride, magnesium chloride, magnesium sulphate, potassium 
chloride, and calcium chloride, and for this reason sea-water 
suitably diluted forms the best medium for determining osmotic 
pressures in the plant-cell. 

The fact that many mineral salts produce false plasmolysis 
proves that they are capable of penetrating the protoplast ; and 
even when in so dilute a solution as to have no deleterious effects, 
there is no reason to suppose that their power of penetration is 
diminished. The compounds in the soil-water are similarly 
capable of passing into the plant, although so many are present 
that the influence of the one upon the other cannot be over- 
looked. As yet, however, our knowledge on this point is quite 
inadequate. The complexity of the phenomena is illustrated 
by the fact that plants do not absorb compounds in the same 
proportions as they occur naturallj' in the soil-soluticn. One 
of the most striking instances of such selective absorption is 
furnished bj- Seaweeds, which contain as much as 0'5 per cent, 
of iodine in their ash, whilst iodides are present in sea-water in 
almost imperceptible amount. 



CHAPTER 11 

Growing Points and Cell-division 

In some of the simpler forms of plants [e.g. Spirogyra) almost 
every cell may exhibit growth and division, but, in the vast 
majority, the cells originate in the first instance by division from 




Fig. 5. — End of a branch of Sphacelaria. The successive segments cut 
off from the apical cell (a) are lettered in order from the youngest to 
the oldest (h — e], and show increasing subdivision. 

so-called growing points. These, in the case of a Flowering 
Plant, are situated at the tips of the stem and root and of their 

«6 



OROWIXG POINTS 



17 



respective branches. A similar apical position of the growing 
point usually obtains amongst the lower plants. 

A good example is afforded by the little Seaweed Sphacdaria, 
which is not uncommon in rock-pools along the sea-shore. Here 
each branch ends in a single large cell (Fig. 5, a), which consti- 
tutes the growing point. During the active season of the year 
this cell is constantly increasing in size, and, when it has attained 
a certain length, it becomes halved by the formation of a cross- 




FiG. 6. — Growing point (a) and adjacent part of the thallus of Dictyota. 
Lettering as in Fig. 5. 

wall (Fig. 5, &). The upper portion again enlarges until a new 
septum arises, whilst the lower half (termed a segment) under- 
goes further division (Fig. 5, c-e) to form the cells of the mature 
plant. The Seaweed Dictyota furnishes another good instance 
of a unicellular growing point, whose mode of division will be 
understood by reference to Fig. 6. The daughter-cells or seg- 
ments in these two cases are cut off in a single series parallel to 
one face of the cell. 

In most plants having a unicellular growing point the apical 



i8 GROWING POINTS 

cell has the form of a three-sided pyramid with a rounded base 
directed outwards (Fig. 7, a.c.) ; such a cell is described as 
tetrahcdral. Segments arc here cut off in succession from each 
of the three flat inner faces, these segments undergoing further 
subdivision to form the mature tissues. This type is found m 
Mosses and Ferns, and can be readily examined in a longitudinal 
section through the tip of the stem of a Horsetail Fern [Equisetmn) 
(Fig. 7). In such a section the apical cell [a.c.) of course appears 



cue. 




Fig. 7. — Growing apex of the Giant Horsetail [Eqicisclitni niaxinnini) in 
longitudinal section, showing the tctrahedral apical cells of the stem 
and leaf [a.c.) ; /, leaf. 



triangular, and two only of tire three series of segments are visible. 
In the roots of Ferns and other Cryptogams which possess such 
a unicellular growing point its mode of division is similar, but 
segments are also cut off parallel to the rounded base, the cells 
produced by their subdivision forming the root-cap. 

The growing point of the higher plants consists of a tissue 
of small actively growing and dividing elements, which takes 
the place of the single cell of the lower forms. Such a tissue 
is called a meristcm, and is best examined by cutting longitudinal 
sections through the apex of stem or root. In both organs the 



GROWiXG POINTS I9 

cells arc arranged in several layers (Fig. S) which can be traced 
back into the chfferent regions of the mature plant. As the cells 
of the growing point multiplj', those that remain near the apex 
retain their power of division, whilst those that come to lie 
further back gradually pass over into permanent tissue. 

In the stem the outermost layer of the meristem usually 
divides only by walls at right angles to the surface to produce 
a single layer of cells which in the more mature re.gion can be 
recognised as the epidermis (Fig. 8, cp). The innermost cells 




E.J.S. 

Fig. 8. — Growing point of the stem of the Glare's Tail (Hippuris) in 
longitudinal section, showing the regions of vascular cylinder (T'.C), 
cortex (C), and epidermis (ep.), and the single meristcmatic cell (i.) 
from which the central cjdinder arises. 

derived from the gro\\ing point divide in all directions, and can 
be traced back into the central region of the stem, which latter 
consists of the vascular strands and pith (Fig. 8, V.C.). The 
middle laj-ers of the meristem also segment in like manner, and 
develop into the tissue situated between the epidermis and the 
vascular strands, a part of the mature stem known as the cortex 
(Fig. 8, C). There are thus, at the growing point of the stem, 
three meristematic regions, known as the dermatogcn, perihlem, 
and plerome, which respectively give rise to the epidermis, 
cortex, and vascular cylinder of the adult. 



20 



GROWING POINTS 



At the growing apex of the root four meristematic layers are 
often distinguishable. Three, the dermatogen, periblem, and 
plerome, serve the same purpose as in the stem ; but external to 
the dermatogen is a fourth layer, the calyptrogen, which cuts off 
segments towards the outside only, and thus gives rise to the 




Fig. 9. — Koot-tip of the Hyacinlli [SciUa] in longiludinal section showing 
the growing point and the root-cap. Sonic of the nuclei (blacl-;) 
cxliibit piliases in nuclear division (cf. p. 12). 

root-cap (Fig. q). The latter is constantly replenished liy the 
growing point, as the older cells in front become disorganised. 
In manj/ roots, however, the epidermis and root-cap arise from 
a common rncristematic layer, and there are other modifications 
of the growing point of the root \\hich it is nnnecessar}' to describe 
in detail. 



NUCLEAR DIVISION 



21 



The intercalary meristems found at the base of the internode 
in Grasses, and at the top of the internode in Labiatc'e, consist 
of a transverse plate of small meristematic cells in which no 
definite regions can be distinguished. 

In the process of cell-division the nucleus ahvaj'S takes the 
lead, dividing into two parts, or daughter-nuclei, which become 
separated from one another by the development of an intervening 







Fig. io. — Early phases of mitosis (after Fraser and Snell). A, resting 
nucleus ; B, spireme ; C, formation of chromosomes ; D, establish- 
ment of nuclear spindle, ch., chromosomes ; ti , nucleolus. 



wall. Thus two new cells are established, each with its own 
nucleus similar in all respects to that of the cell from which they 
originated. Amongst some of the lower plants, and as a rare 
phenomenon in the higher, the nucleus merely divides into two 
portions by a median constriction. This is spoken of as direct 
nuclear division [amitosis). 

More usually, however, there is a sequence of complex changes 
in the nucleus preparatory to and during division, and this whole 



22 NUCLEAR DIVISION 

process is designated mitosis or indirect nuclear division.'' All 
stages can be seen in a thin longitudinal section through the 
growing point of a higher plant (cf. Fig. 9) ; but in order to 
observe them readily, it is necessary to employ material which 
has been carefully preserved and suitably stained (see Appen- 
dix VI). 

The nucleus is usually oval in form (Figs. I, N ; 10, A), and, 
when not actually dividing, is spoken of as a resiing nucleus, a 
term, however, which is apt to be misleading, since in this state 
it is probably just as active. Immediately within the nuclear 
membrane (p. 3) there is a more or less irregular network of 
deeply-stained substance, the chromatin (Fig. 10, A). The 
nucleoli («). which are likewise deeply stained and consist of 
similar material, are situated just within this reticulum. The 
whole of the central region is occupied by the unstained nuclear 
sap, a denser peripheral portion of which contains the chromatin 
network. 

The first change, indicating that division is about to occur, 
is a gradual simplification of the chromatin network, owing to 
the disappearance of some of the cross-connections and the 
closing up of some of the meshes. Later the chromatin network 
appears to consist of a number of irregular, ill-defined segments 
still exhibiting attachment to one another, especially near the 
ends. In the next stage, as a result of further concentration 
and disappearance of all except the terminal chromatin-conncc- 
tions, there remains a single thread exhibiting a split along its 
whole length, and irregularly coiled around the nucleolus or 
nucleoli (Fig. 10, B). This is known as the spireme stage. 

Soon after this the nucleoli disappear, their chromatin prob- 
ably having been absorbed into the thread. The latter now 
contracts somewhat, thus becoming thicker, and breaks up into 
segments or chromosomes (Fig. lo.CandD, cii.). The number 
of these has been found to be practically constant for the vege- 
tative cells of any particular species. 

Meanwhile the cytoplasm surrounding the nucleus becomes 

denser, and numerous streaks become apparent, radiating from 

each end of the cell towards the nucleus (Fig. 10, C, D), whose 

membrane has by this time practically disappeared. The radia- 

1 The antiquated term /iaryokinesis is now rarely employed. 



NUCLEAR DIVISION 



23 



ting lines gradually extend into the central part of the nucleus 
and meet in its middle. They thus form a spindle-shaped group, 
termed the nuclear spindle (Figs. 10, D, and 11), the two points 
from which the indi\'idual streaks or fibres originate being called 
the poles. It is debatable whether the spindle fibres represent 
specialised strands of c^'toplasm, or are merely the expression 
of physical forces operating within the cell. 

Since the chromosomes result from the breaking up of a 
double thread, each is split longitudinally, but this feature is 
often unrecognisable at this stage. The chromosomes now become 
grouped in the equatorial region of the nuclear spindle, and 














Fig. II. — Mitosis (after Fraser and Snell). A, splitting of chromosomes. 
B, equatorial grouping of same. Chromosomes are shaded. 

seem to become attached to some of its fibres (Fig. 11, A). In 
the next stage these apparently shorten, and the two halves of 
each chromosome become separated (Fig. 11, B), and begin to 
move towards the opposite poles, which they ultimately reach 
(Fig. 12, A). The diverging pairs of chromosomes often form 
loops or V's, according to the mode of separation of the two . 
halves, and during their passage to the poles each frequently 
exhibits a longitudinal spHt comparable to that present in the 
parent-chromosome. This spht probably forms the plane of 
separation of the two halves at the next nuclear division. 

On reaching the poles the chromosomes give rise to two 
daughter -nuclei (Fig. 12, B), which pass into the so-caUed resting- 



24 NUCLEAR DIVISION 

stage by a scries of changes broadly the reverse of those taking 
place at the commencement of nuclear division. That is, the 
individuality of the chromosomes becomes obscured, owing to 
the formation of numerous cross-connections between them, and 
through the separation of the two halves of each chromosome, 
which only remain joined at the ends and by fine processes in 
between. With the appearance of nucleoli and a new nuclear 
membrane, the resting nucleus with its chromatin-reticulum is 
again established. 

From the original nucleus two daughter-nuclei are hence 
formed, cither of which contains a half of each of the chromo- 
somes of the parent-nucleus. As the process is repeated at every 
division, all the vegetative cells ^ of the plant come to possess 
nuclei with the same number of chromatin-m asses. 

In addition to the contractile fibres which were attached 
to the chromosomes, other fibres extend through the cytoplasm 
from pole to pole. These persist for some httle time after the 
establishment of the daughter-nuclei, and develop thickenings 
in the equatorial plane, an appearance probably due to a modi- 
fication of the cytoplasm in this region. These thickenings be- 
come more conspicuous as they extend horizontally across the 
parent-cell, till finally they join to form a complete diaphragm 
of modified cytoplasm, the cdl-platc (Fig. 12, B). Within the 
latter there is formed a thin membrane of cell-wall substance 
which separates the two daughter-cells, and is traversed by the 
fine cytoplasmic connections representing the fibres. The latter 
persist, even in the adult condition, so that the protoplasts of 
adjacent cells of most plants are connected by living matter. 
Subsequently further layers, consisting largely of cellulose, are 
deposited on either side of the original membrane, which is the 
only part of this separating wall common to the two daughter- 
cells. These additional layers frequently become chemically 
changed in various ways, but the original separating wall, 
termed the middle lamella (Figs. 14, 15, m.l.), does not usually 
undergo such profound modification, so that it is readily dis- 
tinguished by its different densit}^ and often stains more deeply .2 

1 For details of nuclear division in reproductive cells, see p.p. ^0=5, 3ori, 

= The separating wall in many Algrc arises as an ingrowing diaptiragm 

(Fig. 10 |, E, p. 196), and is not always directly related to nuclear division. 



CELL-DIVISION 25 

The further modification of the cell-v,all will be dealt with in 
the next chapter. 

From the foregoing it is e\ident that, when cell-di\-ision takes 
place, the nuclei of the daughter-ceUs are derived from that 
of the parent-cell. This fact holds generally throughout the 
Vegetable and Animal Kingdoms, the nucleus of any given cell 
always being the product of a pre-existing one. The origin of 
a new nucleus from the cytoplasm is unknown. In hke manner 
it is probable that all chloroplasts (and other kinds of plastids) 
are'derived bj- division from pre-existing speciahsed cytoplasmic 








B 



V 



Fig. 12. — Mitosis (after Fraser and Snell). A, chromosomes have sepa- 
rated and reached the poles. B, establishment of daughter-nuclei and 
development of cell-plate. 

bodies. During the enlargement usuall}' foUomng upon ceU- 
division considerable increase of the cytoplasm may take place ; 
but it should be emphasised that new protoplasm is only formed 
in connection with a pre-existing protoplast, the independent 
origin of li\-ing matter being unknown. In other words, both in 
the Animal and \'egetable Kingdoms, one generation is merely 
a continuation of the previous one, the reproductive cells of 
any one di\'iding and enlarging to form the body of the next. 
We can now consider how the cells produced at the gro\T,ing 
point gradually pass over into the mature condition. Just below 
the apex they have dense cytoplasmic contents, are more or less 



26 CELL-DIVISION 

rectangular or polygonal, and fit closely together, without any 
interspaces. A little further back, representing an older phase, 
they increase appreciably in size and often become more or less 
rounded off, as a result of which small spaces, the intercellular 
spaces (Fig. 27, i.p), appear between them, particularly at the 
angles. 

Lhe increase in volume of the cells is almost entirely due to 
the intake of water which results from the formation of substances 
that bring about an increase in osmotic pressure. These sub- 
stances are produced during the living processes carried on in 
the cells, and, as a consequence of their solution, a number of 
small vacuoles containing cell-sap appear in the cj'toplasm. 
These vacuoles gradually increase in size, and ultimately coalesce 
to form one [e.g. Onion, Fig. i) or few large vacuoles. Apart 
from its presence in the vacuoles, however, water also permeates 
the protoplasm and cell-wall, forming the so-called imbibition- 
water. 

With the gradual assumption of the mature condition the cells 
generally lose their power of division, though this faculty may 
be again stimulated into activity, as when a plant is wounded. 
The growth of any organ of the plant is thus mainly the outcome 
of multiplication of cells at its growing point, their subsequent 
increase in volume leading both to a lengthening and gradual 
widening of the tissues. As they reach maturity the new units 
mostly become specialised in various directions to serve diverse 
needs. Those, however, which have not undergone profound 
alteration, iDut have retained their juvenile character, often 
retain also their power of division and capacity to develop in 
different ways as the demands of the organism may dictate. 
It is the visible enlargement of plants that is popularly spoken 
of as growth, l:>ut in rcahty this involves the three phases of 
cell-division, enlargement, and the final drfferentiation which 
will form the subject-matter of the next chapter. 



CHAPTER III 

The Differentiation of Plant-cells 

Four principal types of element ^ are found in vegetable tissues. 
A large part of the plant-body is composed of cells which usually 
appear rounded or polj-gonal in transverse sections, and in most 
cases contain a lining protoplast. In longitudinal sections thej' 
are often rectangular in form, seldom more than two or three 
times as long as broad, and pro\'ided ^\ith square or rounded 
ends (cf. Fig. 9). Such cells, described as parenchymatous, 
frequently have thin walls, and often form an extensive tissue 
known as parenchyma. 

In contrast to this t}-pe are other cells, termed prosenchyma- 
tous, in which the length greatly exceeds the breadth, such 
elements being found more particularly in the mechanical and 
conducting tissues. These ceUs usually have pointed ends and 
thickened walls (Fig. 16, C, D), and the ^ndth, as seen in trans- 
verse section, is commonly small as compared ^^ith that of a 
parenchimatous element (Fig. 16, B). OccasionaUv forms of 
ceUs are encountered which are transitional between these two 
types. 

The elements of the third type are of quite a different char- 
acter, and are termed vessels (Fig. 17). They are the principal 
water-conducting structures in the wood of Flowering Plants, 
possess no h\'ing contents when mature, and their longitudinal 
walls are thickened in the ^-arious ways to be described below 

I These, and the various modifications described in the present chapter, 
are most easily studied by teasing out [i.e. tearing to pieces with a pair 
of needles) small poriiions of a Celer\--stalk or :Marro-\v-stem that have 
been previously boiled for some time in water ; the middle lamellae are 
therebj' dissolved, so that the indi\ldual cells readily separate from one 
another. 

27 



28 FORMS OF PLANT-CELLS 

(p. 36). In the mature condition they appear as long wide 
tubes or cyhnders, but they are actually derived from vertical 
rows of cells by the more or less complete breaking down of the 
cross-walls. Perforated septa thus occur at intervals in the 
course of the vessel, and in woody plants are often obliquely 
radial with reference to the organ as a whole. The cross-walls 
may almost entirely disappear (as in the Maple and Oak, 
Fig. 17, F), but most commonly a distinct rim persists, only the 
central part being absorbed {e.g. the Lime or the Poplar, 
Fig. 17, E). In still other cases the walls break down in such 
a way that a number of cross-bars remain {e.g. the Alder, 
Fig. 17, D). 

The elements of the fourth type likewise serve for conduction, 
in this case of elaborated food-materials. They are known as 
sieve-tubes,'^ and are located in the bast or phloem, a tissue that 
in most stems lies just outside the wood. The sieve-tubes, hke the 
vessels, are derived from vertical rows of cells whose cross-walls 
are perforated by a large number of fine pores through which the 
protoplasts of the adjacent units become connected. They retain 
their living contents in the mature condition, and the membranes 
remain relatively thin (Fig. 13). When the cross-walls are seen 
from the surface they present the appearance of a sieve, the 
meshes of which constitute the pores ; these walls arc spoken of 
as sieve-plates (Fig. 13, s.p.). 

Before the sieve-plates are fully developed each cell undergoes 
longitudinal division into two unequal portions. Of these the 
larger constitutes a segment of the sieve-tube whose nucleus 
degenerates, whilst the smaller, which retains its nucleus, forms 
a so-called companion cell (Fig. 13, c.c). In some cases two, or 
even three, companion cells may be produced before the nucleus 
of the sieve-tube segment dies away. The narrow companion 
cells have dense granular contents in contrast to the wider 
sieve-tubes, which possess but a thin lining layer of cytoplasm 
and a large central vacuole, features by which they are often 
readily recognised in transverse sections. 

Behind the growing point the cells have been seen to exhibit 
a steady increase in size until they attain their full dimensions. 

' See also p. 82. Sieve-tubes of a different type occur in Ferns and 
Gymnosperms (pp. 293, 339). 



THICKEXIXG OF THE CELL-WALL 29 

During the phase of enlargement the walls undergo no appreciable 
thickening, but surface growth takes place. This is probably 
accompUshed by stretching of the elastic membrane and inter- 
calation of new particles of ceU-wall substance between those 
already present. When fully grown more or less marked thicken- 
ing of the K-all occurs, successive layers formed by the agency 
of the li\'ing protoplast being deposited on the inner surface. 
Subsequently the wall often undergoes considerable chemical 
changes, as a result of which it may become more or less im- 




FiG. 13. — Phloem-Structure of the Vegetable Marrow (Ciiciirbita) in longi- 
tudinal (left) and transverse (right) sections, c, cambium ; ex., com- 
panion cells ; p, phloem parenchyma ; s.p., sieve-plate. 

permeable to water, so that the contained protoplast dies. \\'hen 
the thickening is at all conspicuous, the successive laj'ers gener- 
all}' appear more or less distinct (Fig. 14, D; 16, B). Jhisstratifica- 
tioii is due to the fact that the layers adjacent to one another 
are of unequal density, so that one is more transparent and 
consequenth' brighter than another. 

As a general rule the thickening is not uniform over the 
whole inner surface of the wall, small areas commonly remaining 
thin (Fig. 14, B). The intimate relation between adjacent cells 
is e^■idenced b}' the fact that such thin areas, interrupting the 



30 SIMPLE PITS 

layers formed by the protoplast of one cell, coincide with the 
thin areas on the outer sides of its walls, where the thickening 
has been deposited by the protoplasts of the surrounding cells. 
At these points, known as simple pits, the original middle lamella 
(the pit-mcnihrane, Fig. 14, A, pm.) alone separates the adjacent 
cells. In these cases the bulk of the protoplasmic connections 
(cf. p. 24) pass through the pit-membrane. 

Good examples of simple pits are seen in sections of Elder 
pith (Fig. 14, B). In surface view they appear as irregularly 
scattered oval or elliptical areas which are more transparent 




Fig. 14. — Thickened cell-walls. A, Portion of the endosperm of the Date 
in section. B, Pith-cell of the Elder. C, A group of stone-cells of 
the Pear, together with a small part of the adjacent flesh. D, Two 
isolated stone-cells, m.l., middle lamella; p., pits seen in section ; 
pm., pit-membrane ; Ps., pits in surface view ; s.c, stone-cells. 



than the rest [Ps.). In section (/>.) the walls appear broken at 
sundry points ; but careful focussing shows that the apparent 
gaps are really bridged by a thin line, the pit-membrane. In 
certain cases [e.g. the endosperm of the Date (Fig. 14, A), the 
cotyledon-walls of the Lupine and many other Leguminosffi, and 
cells of the cortex of the Mistletoe] the thickening is so extreme 
that the cavity of the cell becomes much reduced, and the pits 
then appear as deep depressions within the wall. It is in such 
tissues that the protoplasmic connections between cell and cell 
can most readily be demonstrated. 



COLLEXCHYMA 



■',1 



The thick-walled elements of the plant are of considerable 
mechanical importance, since they form skeletal tissues which 
are mostly devoid of intercellular spaces. When such mechanical 
elements occur in young growing organs, where too great a 
rigidity' is disadvantageous, they take the form of coUenchyma, 
which differs from other strengthening tissues in the fact that its 
cells are Uving. Collench3-ma is frequent immediately beneath 
the epidermis in young stems (Fig. 34, s) , in the midribs of young 
leaves (Fig. 51, M), and in the adult stems of some herbs (e.g. 
Hogvveed). In its most tvpical form it is characterised by 
extreme thickening of the corners of the cells, as seen in trans- 
verse section (Fig. 15), the thickened angles appearing bright 




Fig. 15. — Collenchymatous tissue from the stems of the Burdock (Arctium) 
(left) and Dead Xettle (Ln)Hnri)!) (right), ep., epidermis ; );!./., middle 
lamella. Both in transverse section. The cell-contents are shaded. 



and shining owing to their highly refractive character. Some- 
times, however, coUenchj'ma exhibits uniformly thickened waUs, 
interrupted only by pits. The thickening, unlike that of most 
mechanical elements, is practically unaltered cellulose, which 
possesses considerable stretching power, so that coUench\Tna is 
well suited to act as a supporting tissue in A'oung growing organs. 
AU cellulose-waUs ^ exhibit tliis character, and, moreover, are 
readily permeable to water. Cellulose is one of the more 
complex carboh3-drates, belonging to the series of the poh'sac- 
charides, and composed of a large number of CgHioOs groups. 
It is scarcely coloured by iodine solution, but is stained blue 

' Jlingled with the cellulose in the walls of many plant-cells, especially 
those of succulent fruits (e.g. Apples, Gooseberries) and storage roots 
(e.g. Beetroot), are so-called pectic substances which are soluble in water. 



33 SCLERENCHYMA 

if the application of iodine is followed by that of strong sul- 
phuric acid. The acid causes swclhng of the wall, and this 
is followed by complete solution, the blue colour with iodine 
being due to one of the products. Cellulose-walls are likewise 
dissolved by ammoniated copper oxide (cuprammonia, see 
Appendix II). A blue colouration and sweUing also results from 
the application of chlor-zinc-iodide (Schultze's solution. Appen- 
dix II), but it is necessary to use the freshly made compound, 
as it soon decomposes. For general purposes useful stains for 
cellulose walls are methyl blue and haematoxyhn (Appendix VI), 
the latter giving a deep purple colour. 

The remaining mechanical elements form tissues spoken of 
as sclcrcncliyma. They are distinguished from collenchyma by 
a chemical change of the thickened walls known as lignification, 
in consequence of which the cell-contents die. Such lignifred 
walls are much harder and stronger than those consisting of 
cellulose, features which are the cause of the toughness of wood. 
They possess relatively Uttle elasticity, and when wet are not 
readily permeable to water and practically impermeable to air. 
The exact nature of the chemical change is not known, but it 
is sufficiently profound to lead to marked differences in reaction. 
Thus such walls are insoluble in cuprammonia, whilst the lignin 
is dissolved by Eau dc Javclle (Appendix II). Iodine solution 
stains lignificd walls brown, and a similar colouration is olDtained 
with chlor-zinc-iodide. Anihne chloride, or sulphate, which does 
not affect cellulose-walls, yields a brilliant yellow colouration 
and, as it does not stain starch, is particularly useful when 
that substance is present in quantity. An alcoholic solution of 
phloroglucin, followed by strong hydrochloric acid, produces a 
scarlet colouration. Lignified walls stain yellow or brown with 
hematoxylin, and are deeply stained by gentian violet. The 
latter is most effective in conjunction with Bismarck brown, the 
wood and other sclercnchyma becoming violet and the ordinary 
cell-waUs brown. 

There arc two principal tj'pcs of sclerenchymatous elements, 
viz. stone-cells and fibres ; of these the former are more or less 
parenchymatous, the latter prosenchymatous in shape. The 
gritty character of the flesh of the Pear is due to stonc-cdls 
which, in a thin section, will be found to occur as little clusters 



FIBRES 33 

(Fig. 14, C) irregularly scattered through the thin-walled par- 
ench3-ma forming the flesh. The cavity of each stone-cell is 
very much reduced and inconspicuous owing to the strong 
thickening of the wah (Fig. 14, C and D,s.c.). This shows distinct 
stratification, and is traversed by a number of dark hues {p.) 
radiating from the centre and branching as they approach the 
surface. These are the pits (spoken of as pit-ca)ials when the\- 
exhibit this narrow elongated form) which serve for the trans- 
ference of nourishment to the protoplast during the process of 
thickening, on the completion of which the hving matter cUes. 
T)i& fibre is a narrow, very much elongated, cell with tapering 
pointed ends, and in the adult condition its protoplast is fre- 
quently dead. The walls are in general strongly thickened and 
hgnified (Fig. 16, B-D), and usually bear a number of oblique 
slit-shaped pits. In the mature plant fibres are generally the 
most important form of mechanical element, and compose a 
large proportion of the wood of thicker branches (cf. p. 121) - 
these wood-fibres sometimes bear a modified t^-pe of bordered pit 
(see below). The corte.x in the young stems of manv woody 
plants exhibits a continuous mechanical ring composed of alter- 
nating groups of fibres and stone-cells. 

Plant-fibres can undergo considerable elongation and can 
bear very heavy strains without losing the power of again con- 
tracting to their original length ; as a matter of fact, loads 
varying from 14 to 25 kilogrammes are required to produce 
permanent lengthening. The powers of fibres in this respect 
may be compared with those of metal rods {e.g. of wrought iron 
and steel) which, whilst they become permanently stretched 
under similar strains, exhibit far less extension before the limit 
of their elasticitj- is reached. Fibres wiU thus permit bending 
on the part of a plant-organ (under the influence of the wind, 
for example), and will not give way, even under considerable 
strain ; moreover, their elasticitj' wiU bring about a return to 
the normal position as soon as the strain is relieved. WTiilst 
the breaking strain (10-12 kilogrammes) for collenchyma is not 
much less than that for many fibres, it exhibits permanent 
elongation under quite low tension, so that it is especially suited 
to the mechanical needs of a growing organ where constant 
extension is taking place. 
3 



34 



FIBRES 



Owing to their great strength, fibres are of considerable 
economic importance, being used for the manufacture of 
textiles, rope, etc. They are usually prepared from herbaceous 
plants by removing the softer tissues or allowing them to rot 



H 



u 



R 




D 



EJ.S. 



Fig. i6. — CoUenchyma of Dead Nettle stem in longitudinal section (A), 
and fibres from the stem of the Hop in transverse (B) and longi- 
tudinal sections (C, D). c.c, cell cavity ; c.to., cell-wall ; /, fibres ; 
p, parenchyma. 



away. Important fibre-yielding plants are the Flax {Limim 

jisitatissimiim), New Zealand Flax (PJwrmhini ienax), Hemp 

[Cannabis saliva), Jute (species of CorcJwrus, mainly cultivated 

in Bengal), whilst Raffia-tape or bast is obtained from the Palm 



VESSELS 



35 



Raphia} Moreover, fibres treated in various ways are the 
source of wood-pulp and other paper-making materials. 

The main function of vessels (Fig. 17) is water-conduction, 
but they also are elements of mechanical importance on account 
of their thick walls. These are necessitated by the frequent 
existence of negative pressures in the water-conducting tracts. 




Fig. 17. — Vessels showing various types of thickening and perforation. 
A, spiral ; B, annular ; C, portion of A in longitudinal section showing 
attachment of thickening to wall ; D, end of vessel of Alder (Alniis) 
showing ladder-like perforation ; E, vessel of Poplar (Populus) ; 
F, vessel of Oak (Quercics) ; G, reticulate vessel of Marrow [Ciiciir- 
bita). b., bordered pit ; p., perforations between segments of vessel. 



such as obtain, for instance, when transpiration is active. At 
these times collapse of the vessels, under the positive pressure 
exerted 'by the surrounding elements, is prevented by the rigidity 
of their walls. The type of thickening varies greatly, depending 
largety on whether the vessel is formed in a part of the plant 
that is still gro\ring or in one that has reached maturity. In 
the former case the thickening must be of such a nature as to 

' Regarding cotton see p. 103. 



36 BORDERED PITS 

admit of stretching of tlie wall, which would otherwise be 
ruptured. 

The first-formed vessels of the wood, which differentiate a 
short distance behind the growing point, exhibit continuous 
spirals or disconnected rings of thickening {spiral and annular 
vessels, Fig. 17, A, B). These arc deposited on the inner side 
of the original vessel wall, to which, however, they are only 
attached by a narrow connection, as can be seen in optical 
section (Fig. 17, C). As the organ elongates the spirals are drawn 
out like a spring, and the rings become more widely separated, 
by the gradual stretching of the unthickened part of the wall. 
If, however, growth in length is very considerable (as, for instance, 
in many Monocotyledons), complete rupture may ultimately take 
place, so that in the mature plant only an irregular canal re- 
mains to indicate where the firstTormed vessels were situated 
(Fig. 40, ^.f.). 

The vessels formed after completion of growth often exhibit 
reticulate thickening (Fig. 17, G), which appears as a more or 
less irregular network of ridges deposited on the inner side of 
the original wall. In many cases careful examination shows 
that the meshes of this reticulum possess the characters of the 
bordered pits about to be described. 

Such bordered pits are very common on the vessels in the 
older wood. An examination of the surface of the vessel wall 
under the high power of the microscope shows that each pit is 
provided with a broad harder (Fig. 17, E, h. ; also Fig. 18, B, h). 
This is either circular in outline, or more commonly polj'gonal 
as a result of dense crowding, the actual aperture of the pit 
appearing round (Fig. 18, B, />) or more or less slit-shaped. In 
section the border is recognised as a dome-shaped cover formed 
by the thickening of the \\all, which around the pit arches awaj' 
from the midcUe lamella (i.e. pit-membrane) ; the top of this 
low dome is perforated Ijy the aperture leading into the pit 
(cf. also Fig. 197, D, p. 341). Where two vessels adjoin one 
another, such a perforated dome occurs on either side of the 
common wall, so that these bordered pits coincide. The pit- 
membrane possesses a central thickened area, the torus, which, 
owing to its modified character , is impervious alike to air and water. 
When the pit-membrane, which is easily chsplaced, is forced 



BORDERED PITS 



37 



to one side or the other, the torus closes up the aperture of the 
dome, and thereby any passage of water or of air from one 
vessel to the other is prevented. Such displacement will occur 
when the pressure in one vessel differs from that in the adjacent 
one, so that the bordered pits operate as safety-valves whereby 
differences of pressure in the wood become locaHsed. 

The conduction of water in the higher plants is mainly effected 
by means of vessels, but another kind of conducting element. 




Fig. 1 8. — Tracheids. A, Spiral type, from the leaf of Hog's Mercury 
Mercurialis). B, Bordered-pitted, from the stem of the Scotch Inr 
(Pinits). b, border of bordered pit ; ;»./., middle lamella ; p, aperture 
of bordered pit ; tr., tracheids. 

the tracheid, which is likewise dead, is also found in the wood 
of thicker branches and, more commonly, in leaves. Each 
tracheid is formed from a single cell which is more or less 
elongated in form [i.e. prosenchymatous), and usually bears 
bordered pits (Fig, i8, B) or spiral thickening (Fig. i8. A). The 
wood of the Fir and other Conifers is entirely made up of long 
tracheids, similar in appearance to fibres, but bearing pronounced 
bordered pits (Fig. i8, B). Very narrow bordered pits appearing 
as oblique slits are not uncommon in fibres proper. 



38 



MUCILAGE AND GUM 



The walls of mature cells often undergo chemical changes 
other than hgnification, some of which [e.g. the change affecting 
the cells of the cork, p. 136) will be described later, but a few 
special cases may conclude the present chapter. In a number 
of plants the thickening of the walls of certain cells takes the 
form of layers of mucilage, whose exact chemical constitution is 
not known. These, in the dry condition, are hard and horny, 
but in the presence of moisture they soften and swell up con- 
siderably. Indeed, such mucilaginous walls possess a remarkable 




Fig. ig. — Transverse section of the leaf of a Nettle (Urlica) showing a 
cystolith (C ). Ch., Cuticle ; £/>., epidermis ; A, hair; Pa., palisade 
tissue : Sp., spongy parenchyma. 

power of absorbing and holding water, and arc consequently 
particularly prevalent in plants characteristic of dry situations. 
The slimy character of many Seaweeds [e.g. the common Bladder- 
wrack) is due to the mucilaginous nature of nearly all the cell- 
walls, and here the change in bulk, as between the dry and wet 
condition, is especially evident. Mucilaginous walls stain deeply 
and rapidly with aniline blue. 

The gums formed by certain plants arc probably very similar 
chemically to the mucilages, and, Uke thenr, appear, in many 
cases at least, to arise as a result of chemical alteration of the 



SILICIFIED AND CALCIFIED WALLS 39 

cell-wall. Diverse species of Acacia yield gums of economic im- 
portance, e.g. gum arable (from A. Senegal), catechu used in 
tanning (from A. catechu), etc. ; gum tragacanth, which is far 
less soluble, is obtained from species of Astragalus. 

Another frequent modification consists in the deposition of 
mineral substances within the cell-wall. For example, in the 
Horsetails and most Grasses the membranes of the outer cells 
are impregnated with silica, to which many Grass-leaves owe 
their sharp cutting edges. This silicification increases the sta- 
bility of the plant and also serves as a protection against snails, 
etc. A siliceous envelope is very characteristic of certain minute 
unicellular Algae known as Diatoms (cf. p. 206), which often occur 
in countless numbers in fresh and salt water. 

In a few cases the surface of the plant is encrusted with 
carbonate of lime, as in some Stoneworts (Chara) and certain 
other lowly members of the Vegetable Kingdom (cf. p. 204). 
Local deposits of lime on special ingrowths of the walls are 
not uncommon in the epidermal cells in certain groups of 
Flowering Plants. The resulting structures are known as cysto- 
liths, and are well seen in the leaves of the Stinging Nettle 
(Fig. ig, C. ) and of various kinds of Fig [e.g. in Ficus elastica). 
If sections of the leaves are placed in weak acetic acid, the 
carbonate of lime dissolves with effervescence and the framework 
of the cystohth becomes visible. 



CHAPTER IV 

The Non-livixg Contents of Cells 

Apart from the living constituents, cells usually contain numerous 
substances which are either dissolved in the sap or, when in- 
soluble, occur as solid bodies or suspended drops. These sub- 
stances can for the most part be grouped as food-bodies or as 
by-products, according as they are known to be employed in 
the nutrition of the plant or arc believed to be of no further 
nutritive value. 

Among the commoner food-substances are ^'arious carbo- 
hydrates {e.g. sugars, starch, etc.), oils, and proteins, all of which 
are built up by the plant from simple inorganic compounds by a 
scries of complex changes. Durmg the earlier part of the season 
such food-substances are used directly to supply the necessary ma- 
terials for growth, but subscquentl}'-, with decreasing demands, 
a large proportion arc stored up for future use. In perennial 
plants they accumulate in those organs which persist from year 
to year {e.g. bulbs, tubers, etc., and, in the case of woody per- 
ennials, the stem- and root-systems).'- Similar food-substances 
are, moreover, laid up within the cotyledons or endosperm of 
all seeds. 

One of the most important food-substances is stairli, which 
is insoluble in the cell-sap. It is often the first easily recognisable 
product of photosynthesis in a green leaf which has been exposed 
to light. On microscopic examination the starch appears as very 
small shining grains, mainly within the chloroplasts (Fig. ], St.). 
These grains gradually increase in amount during the day, but 
generally disappear over-night, and are consequently spoken of 
as transitory starcli. Their gradual accumulation on a bright day 

^ Cf. 1''. cv S., chapter xii. 
40 



STARCH-GRAINS 41 

is due to the conversion of the sohible carbohydrates {e.g. sugars) , 
which are produced more rapidly than they can be removed, 
into insohible starch. In darkness, wlien carbon dioxide as- 
similation ceases, the accumulated starch is changed back into 
sugar and transferred to other parts of the plant. 

In contradistinction to this transitory starch, that which 
accumulates in storage-organs usually takes the form of rather 
large grains which originate within colourless plastids, known as 
leucoplasts. The latter occur in those cells which are not exposed 
to light , and differ from chloroplasts only in the absence of 
chlorophyll, which is generally not produced in darkness. Leu- 
coplasts, however, readily change to chloroplasts. When a 
Potato-tuber, for instance, is exposed to light, it turns green 
through the formation of chlorophjdl within the leucoplasts. 
In a few cases large starch-grains are actually produced within 
green plastids [e.g. in a small greenhouse plant known as Pelli- 
onia), and these provide particularly appropriate material for 
studpng the mode of formation of the grains. 

Thus, in a transverse section of the stem of Pdlionia the 
outermost cells (Fig. 20, a) are seen to contain chloroplasts [ch), 
in some of which there is a bright shining dot, the transitory 
starch-grain (s). The larger starch-grains, seen nearer the centre 
in various stages of development, may be supposed to have 
originated Hkewise, as small bodies within the chloroplasts there 
situated (Fig. 20, b). But in the mature condition these grains 
have enlarged to such an extent that the enveloping chloroplasts 
appear merely as green caps to one side of them (Fig. 20, c). 
On staining with dilute iodine both small and large grains take 
on the blue colouration typical of starch. 

The large starch-grains that can be scraped out of a Potato 
are more suitable for the studj' of details of structure. Examined 
in a drop of water, each shows a number of asjanmetrical layers 
arranged concentrically around the darker hilum, or point of 
origin (Fig. 20, d). This stratification indicates the manner of 
growth of the starch-grain, the successive layers being distin- 
guished, presumabh', by differences either in chemical or physical 
structure. In the grains of the Potato (Fig. 20, d) and PeJlionia 
(Fig. 20, c) the point of origin is towards one end [e.xccntric type). 
On the other hand, the hilum, in those from the cotyledons of 



42 



STARCH-GRAINS 



the Pea or Bean, lies in the centre {centric type, Fig. 20, /and 
g), and the laj'ers are arranged symmetrically aroimd it. The 
dry grains of Pea, Bean, etc., exhibit a number of radiating 




Fig. 20. — Starch-grains, a-c, stages in tlic development of grains within 
the chloroplastsof Pf//(o;n'ii; chloroplasts shown blaclc. c]i., cliloroplasts ; 
5, starch-grains ; d and e, Potato starcli ; / and g. Pea starch ; 
h, compound starch-grain ot Rice. 



cracks, which appear as dark irregular lines (Fig. 20, ff) ; these 
seem to be due to the drying up of some of the imbibed water 
which permeates the substance of all starch-grains. 



STARCH-GRAINS 43 

Whilst most of tho grains of Potato-starch show but a single 
hilum, an occasional one will be found to possess two or three, 
each with its own system of laj'ers (Fig. 20, c). This results from 
the development of several grains wthin the same leucoplast, 
growth of each occurring independent^ until they meet ; in 
some cases deposition of starch continues ^^•ith the formation of 
laj'ers common to the whole group (Fig. 20, t'). Such structures 
are called compound grains. In some plants {e.g. in man}' cereals) 
numerous grains arise in each leucoplast, so that the compound 
structure may consist of a thousand or more units. Thus a 
starch-grain of the Rice (Fig. 20, h) or Oat is marked out into 
a number of small areas, each representing a constituent unit. 

Starch is a carbohydrate belonging to the group known as 
polysaccharides — that is to say, its highh' complex molecule is 
built up of CoHioOj groups present in larger numbers than 
in the case of cellulose (cf. p. 31), but here as there the actual 
number is unknown. The grains are regarded b\' some as con- 
sisting of a number of alternating amorphous lavers, while others 
believe them to possess a crj'stalline structure. Under polarised 
light the}' exhibit a black cross with the hilum as its centre. 
WTien warmed in a drop of water starch-grains swell, lose their 
pronounced stratification, and ultimatelv pass into a " solution " 
similar to that obtained when a thin starch-paste is treated with 
an excess of boihng water. This "solution" is, however, col- 
loidal in character, and therefore wiU not diffuse through an 
organic membrane. A similar result is obtained when starch 
is treated with caustic potash. Comparatively few dj'es colour 
starch, although gentian violet and eosin are notable exceptions. 

The accumulation of starch-reserves by plants is a feature 
of the greatest economic importance, constituting as they do a 
very important article of human diet. The cereals, the pulses, 
and Potatoes, all of which contain a high percentage of starch 
(cf. p. 53), furnish a sufficient illustration. Rice, which perhaps 
constitutes the most widely consumed food in the world, is the grain 
of Oryza sativa, a Grass cultivated in most parts of the Tropics 
where the necessarj' water for submerging the rice-fields is 
available. Tapioca is obtained from the root-tubers of the 
Cassava or Mandioc [Manihot utilissima, \\'idely grown in the 
Tropics), whilst sago is the starch found in the pith of various 



44 INULIN 

Palms (mainl)' species of Mctroxylon , cviltivated in tlic Malaj? 
region), from which it is extracted after the appearance of the 
inflorescence. 

The wide chstribution of starch as a form of storage of car- 
bohydrate material can probably be related to its insoluble 
character, in consequence of which it exerts no osmotic pressure. 
The small amount of moisture in seeds renders them unsuited 
to the storage of soluble carbohydrates, and doubtless explains 
the frequent occurrence of starch in the endosperm and cotjde- 
dons. In succulent storage organs, on the other hand, soluble 
carbohydrates often occur. One of the most important is inulin, 
another of the complex polysaccharides, though with a molecular 
weight smaller than that of starch. As a result, in spite of its 
solubility and frequent high concentration, inulin has but little 
effect on the osmotic pressure of the cells containing it (cf. p. n). 

Inulin is found especially in the Compositae and the aUied 
family of the Campanulacea;, but also in the bulb of the Wild 
H^'acinth [Scilla nutans) and in other JVIonocotyledons. Soluble 
carbohydrates of a similar chemical constitution are, moreover, 
encountered in many members of the latter group [e.g. the gra- 
minin of Grasses, the irisin of the Iris, etc.). 

In sections of a fresh Dahlia-tuber, Salsify-root, or Jerusalem 
Artichoke, the intact parenchymatous cells will be seen to contain 
a faintly yellow oil-like liquid, the inulin. On placing the sections 
in spirit, the inulin is deposited as a finely granular precipitate. 
In sections of material kept for some weeks in spirit, so that 
the latter has only penetrated slowly into the tissues, the inulin 
will be found as big spherical or lobed masses deposited on the 
cell-walls. These sphere-crystals (Fig. 21, In.) usually show con- 
centric la5?ers, whilst radial lines traversing them indicate the 
numerous needle-like units of which they are Inuilt up. On 
warming in water, the precipitates and sphere-crystals of inulin 
readily dissolve. Iodine gives a brown colouration with inulin 
which is but little different from that of the reagent itself. 

Sugars are among the most important of the soluble carbo- 
hydrates present in plants. They possess much simpler molecules 
than the polysaccharides just considered, benig either mono- 
saccharides with the general formula CeHi-iO,,, or disaccharides 
with the formula C12H22O11. Disaccharides and polysaccharides 



SUGARS 



45 



are so called since thev respectively split up, under certain cir- 
cumstances, into two or man)- molecules of monosaccharides. 
Among the monosaccharides found in plants, the commonest 
are dextrose or glucose (popularly' known as grape-sugar) and 
fructose or levulose (fruit-sugar), whilst of the disaccharides 
sucrose or saccharose (cane-sugar) and maltose (malt-sugar) 
deserve mention. Owing to their simple molecules they produce 
a relativety high osmotic pressure, although for solutions of 
equal strength this is greater in the case of the monosaccharides 




Fig. 



21. — Sphere-crystals of inulin [In.] in the cells of the tuberous root 
of a Dahlia. 



than in that of the disacchaiides. For this reason no doubt 
grape-sugar and cane-sugar, the two which function as food- 
reserves, are only found in very minute quantities in seeds 
(except for the cane-sugar in the Sweet Corn, a variety of Maize). 
On the other hand, grape-sugar is one of the principal carbo- 
hydrate-reserves in the bulb of the Onion, whilst cane-sugar 
occurs in the Sugar Beet {Beta), in the pith of the stem of the 
Sugar Cane {Sacchariim officiiianim), and in the Sugar Maple 
{Acer sacchannum) (see p. 124). The sugar is extracted from 
the sliced Beet with the aid of warm water, whilst in the case 



46 SUGARS 

of the Sugar Cane the juice is crushed out of the canes with 
the help of rollers. In all cases the crude sugar is subjected to 
subsequent processes of refinement. 

Fructose is most abundant in succulent fruits, and is an 
important constituent of honey ; in both, however, it is mixed 
with, usually smaller amounts of, grape and cane sugars. In 
such cases the sugars are, of course, not of the nature of food- 
reserves, but serve a biological purpose in connection with seed- 
dispersal and cross-pollination. 

All the four sugars above mentioned are found in fohage- 
leaves, though in proportions that vary greatly both during the 
day and night and at different seasons of the year. It is still 
an open question whether glucose or sucrose is the first sugar 
to be formed in photosynthesis ; but there is no doubt that the 
other two, Hkc the transitorj? starch, are secondary products. 
Maltose appears to be produced invariably during the solution 
of starch within the plant, and is consequently found not only 
in foliage-leaves, but in germinating Barley (malt) and other 
starch-containing seeds. The fructose found in leaves, on the 
other hand, is formed by the breakdown of cane-sugar. 

The latter is readily split (in the presence of water, so-called 
hydrolysis) into two molecules of monosaccharide hy boiling the 
solution with a few drops of some mineral acid {e.g. hydrochloric 
acid) ; one molecule of glucose and one of fructose are obtained, 
the mixture being known as invert sugar. Cane-sugar is similarly 
converted into invert sugar by the agency of an enzyme invertase 
found in most plants. In the same way the polysaccharides 
above discussed can be split up with the formation of disaccha- 
rides or monosaccharides, as the case may be. For instance, a 
starch " solution " boiled with a few drops of a mineral acid 
becomes clearer, and the ordinary reaction to iodine gradually 
disappears ; the colour assumed with this reagent is now reddish, 
owing to the presence of simpler polysaccharides known as dcx- 
trins. If the boiling be continued, the whole of the starch " solu- 
tion " ultimately breaks down into simple glucose. Similarly 
inulin gives rise to fructose. In the plant starch and inulin are 
acted upon by ferments, diastase and inidase respectivelj', which 
effect like changes, except that diastase breaks down starch into 
the disaccharide maltose, which in its turn is acted upon by 



SUGARS 



47 



an enz3'me maltase with the production of two molecules of 
glucose . 

The sugars are readily distinguished from one another by 
certain characteristic reactions. Thus glucose, fructose, and 
maltose all reduce Fehling's solution (which contains cupric 
oxide, Appendix II) with the formation of a red precipitate 
of cuprous oxide, and are consequentlv known as reducing sugars. 
Sucrose, on the other hand, is a non-reducing sugar, giving no 
precipitate with Fehling's solution, until it has been inverted 
by boiling or enz\Tne-action. It mav be added that neither 
inulin nor the dextrins effect reduction of this reagent. 

For microchemical purposes, especially when but small quan- 
tities of sugars are present, the foUomng procedure is more 
advisable. The sections are mounted in a drop of a solution 





Fig. 22. — Osazones. A, of glucose; B, of maltose (after Plimmer) 



of phenylhydrazine h^'drochloride in gh'cerine ^^-ith which a 
drop of a solution of sodium acetate in gtycerine (Appendix II) 
is thoroughly mixed. The preparation is heated for about half 
an hour (although a longer period is often necessary) and 
aUowed to cool. The phenylhydrazine reacts ^\•ith many of the 
sugars to form insoluble yellow cr^'staUine compounds, known 
as osazones. Glucose and fructose produce the same osazone, 
whose crystals appear as long needles arranged in sheaves 
(Fig. 22, A) ; that of maltose forms rosettes or plates of broad 
needles (Fig. 22, B), whilst in the case of cane-sugar no osazone 
is produced. 

Another method of storage of carbohydrate-material takes 
the form of strongly thickened ceU-waUs (cf. p. 30 and Fig. 14, A), 
as in many seeds (Date, Lupine, Coffee, etc.). Such walls consist 



48 GLUCOSIDES 

of so-called reserve-celluloses, polysaccharides which differ some- 
what from ordinary cellulose and break down more readily into 
simple sugars. 

Of very common occurrence in plants are complex compounds 
known as gliicosides, which, in the majority of cases, consist of 
glucose combined with one or more aromatic substances or other 
organic residue. On boihng with dilute mineral acids they split 
up into their constituents (hydrolysis). The decomposition of 
the glucosidc within the plant is eflected by special ferments 
which generally occur in distinct cells, so that the chemical 
process is not initiated until, for some reason (e.g. injury), ferment 
and glucoside come into contact. Thus, a glucoside amygdalin 
occurs in the seeds of the Bitter Almond (but not in the culti- 
vated form), whilst the appropriate ferment emulsin is situated 
in the skin ; on crushing the seeds decomposition of the amygda- 
lin into glucose, benzaldehyde, and prussic acid takes place, this 
last being responsible for the poisonous properties. The hot taste 
of many members of the Cruciferje [e.g. Horse Radish, Cress) is 
due to the formation of mustard oil (together with glucose and 
potassium hydrogen sulphate) by the action of a ferment [myrosiii) 
on another glucoside (sinigrin or myronate of potash). 

In the two instances just given the glucosidcs undoubtcdlj' 
render the plant distasteful to animals, but in many cases they 
seem to serve as a means of storing glucose in a form which does 
not diffuse readily. Thus the leaves of many Willows contain 
a glucoside salicin which, during the night, is split up by the 
enzyme salicase into glucose and saligenin ; the former is re- 
moved, whilst the latter combines with the new sugar formed 
the next day. To the glucosides also belong : — the saponins found 
in the Soapwort [Saponaria), Quillaia-hs.i-'k, and many other 
plants, and easily recognised by the formation of a froth ^^•hen 
shaken up with water ; the active principles of the Foxglove 
(Digitalis), the most important being digitalin, which has a pro- 
found effect on the action of the heart ; and the inchcan of the 
Woad (Isatis tinctoria) and of the Indigo-plant (I iidigofcyii) , the 
latter being the source of natural indigo. ' 

1 Vanillin, the cause of the aroma of Vanilla (obtained from the pod 
of Vanilla planifolia, a tropical Orchidaceous climber), is an aldehvde, 
similar to those often combined with glucose to form glucosides. 



OILS AND FATS 49 

Oils or fats form another important group of reserve-sub- 
stances found especially in those seeds in which carboh^'drates 
are either scanty or absent {e.g. Castor Oil, Sunflower, etc.). 
Drops of oil are, however, not uncommon in the ordinary vege- 
tative cells, and are particularly abundant in some lower plants 
[e.g.inthe Alga Vaitcheria (cf. p. 20S), and intheErgot (Claviceps)]. 
The vegetable fats are compounds (esters) of glvcerine with 
various fatty acids (palmitic acid, oleic add, etc.), and are de- 
composed by ferments known as lipases into these two con- 
stituents. In most cases thej' occur as fluids \\ithin the plant, 
although the fat of the Cocoa (Theobroina cacao), the so-called 
Cocoa-butter, fonns an exception to this. 

The oil appears in the cell-sap or protoplasm as shining 
globular drops of varying size which are readilv soluble in ether, 
benzene, etc. ; some are even soluble in alcohol [e.g. those in the 
seeds of the Castor Oil). When a considerable quantity of fat 
is present (e.g. in the Brazil-nut), it can be squeezed out by 
pressure on to a piece of filter-paper, producing a greasy mark. 
The oil-globules turn pink or red on treatment with Scharlach 
Red (Appendix II), and assume a blackish colouration with 
osmic acid, which, however, also stains proteins and tannins in 
the same wa}'. If sections of oil-containing material be placed 
in a solution of concentrated potash and ammonia in equal pro- 
portions, the globules after some time lose their sharply defined 
outUne, and often become replaced by needle-shaped crystals. 
The change, spoken of as saponification, is due to the breaking 
up of the oil into glycerine and the fatty acid, the latter uniting 
with the alkali to form the corresponding salt. 

Many plant-fats are of considerable economic importance ; 
thus, olive oil is obtained from the fleshy fruit-waU of the Ohve 
[Olea europcBa, mainlv cultivated in the Mediterranean region) ; 
coconut oil, used in the preparation of margarine, from the 
ripe seeds of the Coconut Palm {Cocos iiiicifera) ^ ; and Castor 
Oil from the seeds of the Castor Oil plant (Ricimcs communis). 
Other important commercial sources are "Linseed (Liniim- iisita- 
tissimum), Cotton-seed [Gossypium spp.), Pea-nuts (Arachis 
hypogcea, \nth. 38-50 per cent, of oil), and Soja-beans (Glycine spp.). 
In most cases the oil is extracted after crushing, the ultimate 

1 Copra is the commercial name for the dried liernel of the Coconut. 



50 PROTEINS 

residue forming so-called " oil-cake," whicli is extensively used 
for the feeding of cattle. Various vegetable oils are, moreover, 
employed in the manufacture of soap. 

In the plant the fats are formed from carbohydrates and, since 
they contain a much smaher amount of oxj'gen than the latter, 
the volume of carbon dioxide evolved during their formation is 
considerably greater than that of the oxygen taken in. Con- 
versely, when fats undergo change into carbohydrates during 
the germination of a seed, a large quantity of oxygen is absorbed 
in proportion to the carbon dioxide evolved. In correspondence 
with their low specific gravity, fats are a frequent form of non- 
nitrogenous food-reserve in seeds depending on wind-dispersal. 

The carbohydrates or fats found in the various storage-organs 
are always accompanied by nitrogenous food-reserves, the 
most important and widespread of which are the proteins. We 
have already seen that very complex combinations of proteins 
are organised to form the living protoplasm, but simpler proteins 
often occur as non-hving constituents of the ordinary vegetative 
cells, and are especially abundant m the diverse storage-organs. 
In the former case they may either be dissolved in the cell-sap 
or appear as crystal-like bodies, termed crystalloids, which may 
even be lodged in the plastids or nuclei. Succulent storage- 
organs, such as tubers, often likewise contain dissolved proteins, 
or these may take the form of crystalloids, as in the outer layers 
of a Potato ; but not uncommonly a considerable part of the 
nitrogenous matter in these cases is a mixture of simpler com- 
pounds known as amides (e.g. asparagin in the Potato and 
glutamin in the Beetroot). 

In seeds proteins generally occur as small grains which are 
well seen in the cotyledons of a Pea or the endosperm of the 
Castor Oil. If a section of the former be treated with iodine, 
the minute protein granules take on a brown colouration, in 
sharp contrast to the blue or blackish starch-grains with which 
they arc intermingled. On warming a section in a few drops of 
Millon's reagent (yVppendix II), the whok' assumes a brick-red 
colour which microscopic examination shows to be due to the 
proteins. Heating with concentrated nitric acid gives a yellow 
colouration which, on addition of ammonia, changes to orange 
xanthoprotcin reaction). 



PROTEINS 



51 



The proteins of oil-containing seeds {e.g. Castor Oil, Brazil- 
nut) occur in the form of especially large granules, known as 
aleitrone grains. These appear to arise, as the seed dries during 
ripening, from the entire contents of vacuoles rich in protein- 
substance. Although the structure of aleurone grains is relatively 
complicated, they exhibit the characteristic protein-reactions 
mentioned above. If a thin section of the Castor Oil or Brazil- 
nut (from which the fat has been removed by soaking in alcohol 
and benzene respectiveh") be stained with iodine, the brown- 




FiG. 23. — Aleurone grains {AL] from the endosperm of the Castor Oil 
plant [Ricinus) showing the globoid (GI.) and cr^-stalloid [Cr.). 



coloured grains are readilv seen under the microscope. Each 
shows a bounding membrane, the original membrane of the 
vacuole, enclosing amorphous protein in which two or more 
bodies are embedded. One of these, the crystalloid (Fig. 23, Cr.), 
is large and more or less angular, whilst the other, the globoid, 
is smaller and rounded (Fig. 23, Gl.) : both consist of protein, 
but in the globoid, of which more than one may be present, 
this is combined with a double phosphate of calcium and mag- 
nesium. Bv mounting sections in water the amorphous ground- 
mass of the aleurone grain mav be dissolved, and the bounding 
membrane rendered clearlv visible : on the other hand, treatment 



52 PROTEINS 

with dilute potash causes both bounding membrane and crys- 
talloid to swell and disappear. 

The protein-granules of the Pea, Bean, etc., are often regarded 
as small aleurone grains devoid of crystalloid and globoid. 
Similar structures occur in abundance in the so-called aleurone 
layer found at the periphery of the endosperm of Grasses ("V\Tieat, 
Maize, etc.), just within the coat of the grain (Fig. 24). When 
the latter is detached, the protein-containing layer generally 
comes away with it, hence the greater nourishing properties of 
wholemeal bread as compared with that manufactured from 
white flour. For the same reason peeled potatoes are not so 
nutritious as those boiled in their sliins, since the outer layers 
of the tuber contain protein -crj/stalloids (cf. above). 

Proteins, owing to their complex molecules, are not readily 




Fig. 24. — Aleurone layer [A.L.)oi Wheat as seen in a transverse section. 
The starcli-containing cells (S.C.) lie immediately beneath. 

diffusible, and consequent!}' become converted into simpler, 
freely-diffusing compounds before they are transferred to parts 
where growth is occurring {e.g. in a germinating seed). This 
conversion is brought about by so-called proteolytic enzymes {e.g. 
pepsin, erepsin), which are in general similar to those occurring 
in the digestive tracts of animals, although some uncertainty 
exists as to the exact nature of those present in plants. The 
chief compounds produced, by the action of these enzymes on 
proteins, are firstly peptones and subsequently amino-acids. 
The former still exhibit the general characteristics of proteins, 
although they diffuse more easily, while the latter no longer 
possess protein -properties. Amino-acids are exceedingly cont- 
mon in growing and other parts of the plant {e.g. leucin in the 
buds of the Horse Chestnut, tyrosin in seedhngs of the Lupine, 
etc.). 



EXTRACTION OF ENZYMES 



53 



Approximate Food-coxteni of ^'ARIous Plant-products 
(In percentages of the fresh weight) 





starch. 


Fats. 


Proteins. I 


Cocoa 


4'2 


50-4 


I3'3 


Maize 


68-42 


4'5 


9-8 


Oat 


580 = 


5'2 


io'4 


Rice 


75'o 


O'.T 


90 


Wheat . 


67-92 


1-8 


12-3 


Potato-tubers . 


20-0 


o'i5 


I '9 


Lettuce , 


2-2 


o'3 


I '4 


Broad Bean 


48-0 2 


16 


23'o 


Pea-nuts . 


5'^ 


440 


30 '0 



In \de\v of the great importance of enzymes in the acti\'ities 
of the plant, the present chapter may be concluded \\'ith a brief 
consideration of their mode of action. It should be realised 
that such substances are probably of universal occurrence in 
li-vdng ceUs. A very large number of enzymes are now known, 
and others are continualh' being discovered, but only a few 
can be mentioned here. The following table gives an epitome 
of some of the principal enzymes found in plants : 



Enzyme. 
Diastase. 
JIaltase. 
Invertase. 
Inulase. 
Cytase. 

Emulsin, ^Nlyrosin, etc. 
Lipases. 

Proteases (proteolytic en- 
zymes) . 
Zymase (in Yeast, p. 256). 



Substance acted upon. 

Starch. 

Maltose. 

Cane-sugar. 

Inulin. 

Reserve-cellulose. 

Glucosides. 

Oils. 

Proteins. 
Monosaccharides. 



Products. 
Dextrin and rvlaltose. 
Glucose. 

Glucose and Fructose. 
Fructose. 

Mannose and Galactose. 
Glucose, etc. 
Glycerine and Fatty Acids. 

Peptones and Amino-acids. 
Alcohol and Carbon Dioxide. 



The method of extraction of diastase described on p. 169 of 
our hitroduction to the Study of Plants is appUcable to many 



1 In most cases the percentages given are calculated for the entire 
nitrogenous organic matter. 

2 Total carbohydrates. 



54 EXTRACTION OF ENZYMES 

enzymes. The material should always be ground to a fine 
powder, from which an extract is made, usually with cold water ; 
if the filtered solution is at all bulky, it will be convenient to 
reduce its volume bv evaporation at a temperature not exceeding 
50° C. The solution is treated with an excess of alcohol and 
the resulting precipitate allowed to settle. It is then filtered 
off, redissolved in distilled water, and can be purified by a further 
precipitation with alcohol. 

A solution of invertase which will invert cane-sugar is readily 
obtained, according to Plimmer.i in the following manner. A 
quantity (100 grams) of ordhiary Yeast is ground up with about 
6 grams of calcium carbonate. The resulting paste is treated 
with 5 c.c. of chloroform or ether (to kill the Yeast cells), and 
allowed to stand exposed to the air for three or four days, after 
which the enzyme is precipitated from the filtrate with an equal 
volume of alcohol. An impure solution of invertase, which will 
demonstrate the inversion of cane-sugar, can, howc^'er, be 
obtained by simply mashing Yeast in water with the addition of 
a little ether, and filtering oif the sohd matter (preferably through 
an asbestos filter). 

Lipase can be prepared from the seeds of the Castor Oil by 
cutting up the endosperm into small pieces, and soaking these 
for a short time in a small quantity of ether in order to remove 
the oil. The material is then ground into a pulp with a very 
dilute (o'5 per cent.) solution of acetic acid, which sets free the 
enzyme. The insoluble matter is filtered oft\ washed till the 
filtrate gives no acid reaction, and the residue is shaken up with 
a small amount of water. If some of this suspension be added 
to a little ohA'e oil, an acid reaction will soon be obtained due 
to the formation of fatty acids consequent upon the decom- 
position of the oil. 

The preparation of proteolj-fic enzymes from plant-material 
is a matter of some difficulty and beyond the scope of this book. 
The action of such ferments is, ho\ve\'er, ^\•ell illustrated by the 
use of peptonising powders (containing the enzMiie pepsin) in 
rendering milk more digestible for nn-alids. 

The action of enzymes is \-ery often a h\'drol)'tic one, that 

1 Praclical Oi'i;aiiic and Bioclicniislry, p. 31)9. 



ENZYMES— MODE OF ACTION 55 

is to say, the compound is broken down with the addition of 
water, thus : 

Ci.H,,On + H,0 = CeHi.Oe + C,}1,,0, 1 
cane-sugar water glucose fructose 

(CeHioOa)^ + "H.O = nCoH,,Oe 
inuHn water fructose. 

In other cases, however, the action appears to be different, as, 
for instance, that of the fermenting enzymes (cf. p. 256) and of 
the oxidising enzymes or oxidases which are in part responsible 
for the change in colour of the cut surface of an Apple, and of 
many Fungi, when exposed to air. Evidence is accumulating in 
support of the view that all the chemical processes depending on 
enzyme-action are reversible, taking place in one direction or the 
other according to the prevailing conditions. Thus, for example, 
it is beheved that the building up of starch from sugar and the 
reverse process are both dependent on the same set of enz}anes. 
Tittle is known as to the chemical nature of enzymes, but all 
of them are colloids, and as a consequence the rate of diffusion 
through parchment and similar membranes is either very slow 
or practically nil. 

Our knowledge of this class of substances is almost entirely 
confined to their mode of action. The reactions influenced by 
enzjmies are all such as require an appreciable interval of time 
for their completion, so that it is possible to measure their rate 
under anv given set of conditions. One of the most important 
aspects of enzMne-activit}' is the small quantity of the enzj'me 
necessary to bring about a pronounced chemical change ; thus 
invertase is stated to invert 200, coo times its weight of cane- 
sugar. Moreover, at the end of the reaction the enzjTne appears 
to be unaltered both as regards amount and characteristics. 
In both these respects enzymes resemble the so-called catalytic 
agents employed in certain chemical processes. As examples 
we mS-Y mention the use of small quantities of manganese dioxide 
to accelerate the liberation of oxygen from chlorate of potash, 

1 Although the formulae for glucose and fructose are the same, these 
two compounds differ in the arrangement of the atoms -within their 
molecules. 



56 ENZYMES— MODE OF ACTION 

and the effect of traces of colloidal (finely divided) platinum, 
known as platinum black, in causing the explosive combination 
of oxygen and hydrogen at ordinary temperatures. In either 
case the reaction is one that would also take place in the absence 
of the catalytic agent, but with this difference, that the rate 
would be very much slower. Similarly, there is reason to believe 
that the changes brought about by enzymes would also occur in 
their absence, but at so slow a rate as not generally to be capable 
of recognition. In this connection it is well to recall that much 
the same effect as is produced by the enzyme can often be 
attained by the use of other catalytic agents (e.g. boihng with 
small quantities of mineral acids). 

Although the enzyme appears unaltered at the end of the 
reaction, it is almost certain that combination of some kind 
with the substances undergoing change takes place while the 
process is proceeding. In view of the large surface which is 
presented by colloids, it is very probable that this combination 
is a ph5'sical one (adsorption), and it is thought by many that, 
in the hydrolytic enzymes, for instance, the water and the com- 
pound undergoing hydrolysis are brought into intimate contact 
at the surface of the ultramicroscopic ferment particles. 

Enzymes differ from most other catalysers in that each is 
usually only effective in accelerating one or few particular re- 
actions (cf . p. 53) , and they are, moreover , very sensitive to heat and 
light. The rate of the reaction is doubled or trebled with every rise 
of 10° C. , but soon a temperature is reached (usually about 60' C.) 
at which most enzymes are destroyed. It may be added that 
heating which suffices to kill the protoplasm lea\'es ferments 
unharmed. Strong light destroys them very quickly, an effect 
for which the violet end of the spectrum appears to be mainly 
responsible. Many chemical compounds [e.g. sulphuretted hy- 
drogen, prussic acid, chloroform, etc.) arrest enzyme-action to 
a more or less marked degree according to their concentration. 
If the products of enzj'me activity are not removed, a retardation 
of the process is at once manifest, continued action of diastase, 
for extunple, being dependent upon the reiuowd of the maltose. 
In some cases the accumulation of the products of the reaction 
actually exerts a poisonous effect, as, for instance, that of the 
alcohol produced by Yeast. Usually, however, the substances 



ENZYMES— MODE OF ACTION 57 

resulting from enzyme-action are removed by the plant as soon 
as formed. 

Enzymes frequently act only in the presence of another 
substance, an activator, which is commonly a salt, acid, or alkali, 
although sometimes more complex. These activators differ from 
the enzjanes in being able to diffuse through a parchmerrt mem- 
brane, and can consequently be separated from the latter by 
dialysis. 1 Examples are afforded by the small quantitj- of acid 
requisite for the action of pepsin, and the necessity of the pres- 
ence of certain complex phosphatic compounds for the action of 
the zymase of Yeast. 

Just as these substances have the effect of accelerating enzyme 
action, so, too, there are others which exert a retarding 
influence, and there is every reason to believe that either the one 
or the other can be produced as may be required by the living 
cell. Moreover, the rate of the reaction depends on the amount 
of the enzyme present, and this latter is regulated bj^ the cell's 
activity. It is clear, then, that the character and vigour of 
catalytic activity is subject to considerable modification, and 
is indeed intimately related to the momentary requirements of 
the organism. 

1 That is to sa)', placing the mixture of enzyme and activator in a 
parchment tray floating on a large bulk of water. 



CHAPTER V 

Cell-contents (By-products, etc.) 

The by-products comprise chemical compounds formed during 
the metaboUsm of the plant which do not, as far as our present 
knowledge goes, appear to play any further part in the elabora- 
tion of food-substances. This does not, however, mean that 
they fulfil no functions in the living organism, for they maj? be 
of importance in warding off the attacks of preying animals, in 
the creation of attractive mechanisms, in the reduction of trans- 
piration, etc. 

One of the most widespread of the by-products is oxalate 
of lime, which may occur in practically every organ and tissue 
of the plant. It is produced by the neutralisation of the oxalic 
acid formed during metabolism and, being insoluble, appears in 
the shape of crystals which assume diverse forms. Large 
solitary crystals (Fig. 25, C), each occupying the greater part of 
the cell-contents, are very common ; but most frequent are 
clusters, or rosettes, of crystals radiating from a dark centre 
(hlg. 25, A). Another widespread type takes the form of bundles 
of needle-shaped crystals {raphides), generally situated in enlarged 
cells containing mucilage (Fig. 25, B). This last type is particu- 
larly characteristic of the Monocotyledons, though by no means 
lackmg in Dicotyledons {e.g. Enchanter's Nightshade). In some 
few cases, especially in the Solanaceae, the oxalate of lime is 
deposited in the form of a powder-like mass of numerous very 
minute crystals (so-called eryslal-saiid). 

Crystals of calcimn oxalate arc always found in quantity 
where active metabohsm is going on. They are often very 
abundant in the tissues adjoining activel}' secreting organs 

58 



CRYSTALS 



59 



(cf. p. 144) ; also in the leaves of deciduous plants, just prior 
to leaf-fall, features which respectively emphasise that this 
substance is a by-product, and that it is not generall}' useful to 
the organism. 




Fig, 2j. — Crystals. A, Cluster crystals from leaf of Dog's ilercury (Mer- 
curialis). B, Raphides from leaf of Enchanter's Nightshade (Circua), 
C, Solitary crystals from phloem of Horse Chestnut (Mscidits). 



The crystals of calcium oxalate are not soluble in acetic acid, 
but readity dissolve in sulphuric acid, with the production of 
calcium sulphate. The latter, being itself insoluble, becomes 
deposited promiscuousl y in the form of needle-like crystals . These 
tests serve to verify tfie presence of oxalate of lime, but in most 



6o ORGANIC ACIDS, TANNINS 

cases their application is unnecessary, since crystals of other 
inorganic compounds are very rare. 

Many by-products occur in solution in the cell-sap, and of 
these the commonest are diverse organic acids and bodies known 
as tannins. The former are responsible for the frequently acid 
reaction of the sap, and are especially abundant in unripe fruits. 
As examples mention may be made of malic acid (m Apples), 
citric acid (in Lemons), tartaric acid (in Grapes), etc. The name 
of tannins is given to diverse organic substances, whose chemical 
constitution is not fully established, but all of which possess 
an astringent taste and arc characterised by the following 
reactions ; They reduce Fehling's solution, are precipitated by 
the salts of many metals {e.g. basic lead acetate), and take on a 
blue-black or greenish colour with ferric chloride. Dilute iodine 
solution, together with a little lo per cent, ammonia, gives a 
■ briUiant red colour even with small quantities of tannins, whilst 
they are readily precipitated by dilute solutions of caffein 
(cf. p. 13). 

Tannins are particularly common in the bark of trees (e.g. 
Oak, Alder), in unripe fruits [e.g. Pear, Acorn), in leaves [e.g. 
Bracken), and occur abundantly in certain abnormal growths 
{e.g. Oak-galls). They are often accompanied by a yellow or 
reddish pigment which facilitates recognition of the cells in 
which they occur. The bark of certain trees (e.g. Oak), owing 
to the large quantity of tannin present, has long been emploj'ed 
in the conversion of hide into leather, which process depends on 
the coagulation, by the tannin, of the albuminous substances 
contained in the animal skin. The reactions of tannins with 
ferric salts have been extensively used in the preparation 
of ink. 

Another group of by-products, encountered particularly 
in certain families of Flowering Plants (e.g. Ranunculace;e, 
Scrophulariacea;, Solanacea;, UmbelUferae), are the alkaloids. 
These are complex basic organic compounds containing nitrogen, 
which are either dissolved in the cell-sap or present in the solid 
state ; in the plant they are often combined with organic acids. 
The alkaloids are of such importance, owing to their poisonous 
and medicinal properties, that the following list of some of the 
more familiar is given : 



ALKALOIDS 



6i 



Alkaloid. 
Aconitine 

Atropine 



Cephacline 

Cocaine . 
Coniine . 
Hyoscyamine 

^lorphine 

Nicotine . 

Quinine 
Strychnine 



Action. 
Poisonous (used as 

an antip3'retic) . 
Poisonous (various 

medicinal uses) . 



Emetic (active 
principle of ipe- 
cacuanha). 

Local an;Esthetic . 

Poisonous (para- 
lytic effect) 

Poisonous (seda- 
tive) 

Narcotic (active 

principle of opium 
Poisonous 

Febrifuge . 
Poisonous, heart 

and respiratory 

stimulant. 



Source, etc. 

Leaves, root, etc., of Jlonkshood 
(Aconitum napelliis). 

All organs of Deadly Nightshade or 
Belladonna (Airopa belladonna), 
seeds especially of Thornapple (Da- 
tura stramonium, Fig. 26) (Solan- 
aceae) . 

Root of P.';_y(;/!0^nfl ipecacuanha (Fam. 
Rubiaceae, Brazil). 

Leaves of Erylhroxylon coca (Fam. 

Linacese, Bolivia and Peru). 
Seeds of Hemlock (Conium macula- 

tum, Umbelliferaj). 
Henbane (Hyoscyamus niger), Airopa 

belladonna, Datura stramonium 

(Fig. 26). 
Young fruits of Opium Poppy (Pap- 
) aver somniferum). 
Leaves of Tobacco [Xicoliana taba- 

cutn, Solanacea;). 
Bark of Cinchona spp. (Rubiace^). 
Seeds of Strychnos nux-vomica (Fam. 

Loganiaceae). 



The alkaloids as a whole are not characterised bj' anj^ very 
specific reactions, but thej' are precipitated from solution by 
many different reagents (e.g. iodine in potassium iodide, tannic 
acid). They give very marked colour -reactions with various 
substances ; thus a section of the rhizome of the Monkshood 
treated with a little 50 per cent, sulphuric acid shows a bright 
red colouration in the parenchj'ma adjoining the vascular strands, 
as a result of the presence of aconitine. 

The ptomaines, which are basic in character, are compounds 
produced during the decomposition of flesh, etc., by the 
agency of Moulds and Bacteria, but it is not altogether certain 
that the effects of so-called "ptomaine-poisoning" are solely 
due to these substances. Such stimulants as the caffeine of 
tea-leaves, coffee-beans, cocoa-beans, and Kola {Cola acuminata), 
and the theobromine present in the Cocoa, are derivatives of 
purine and very similar to the alkaloids. 

The characteristic and often pleasing odour of many Labiatfe 
[e.g. Lavender, Mint, etc.) and L'mbellifera; [e.g. Fennel), as well 



02 



ETHEREAL OILS, TERPENES 



as of flowers, is due to the presence of so-called volatile or ethereal 
oils, which are composed of mixtures of hydrocarbons {i.e. 
compounds containing carbon and hydrogen only), known as 
terpenes, and of their oxygen derivatives. Examples are : 
lavender oil from the flowers of the Lavender [Lavendula) ; 
peppermint oil, which contains the antiseptic menthol, from the 
Peppermint {Mentlia piperita) ; bergamot oil, used in the manu- 




FiG. 25. — Photograph of part of the shoot, including two fruits, of the 
Thoniapple [Datura strnnionnim), which contains the alkaloids atro- 
pine and hyoscyamine. [Plioto E. J. S.] 



facture of Eau de Cologne and other perfnnres, from the 
Bergamot Orange (Citrus aiirantium var. bcri^ainia), a variety of 
the ordinary Orange ; oil of aniseed from the Aniseed [Pimpindla 
anis'um) ; and the numerous oils from the many species of Euca- 
lyptus (Myrtacete). Many of these are used commercially in the 
preparation of perfumes. Similar oils are the essential principles 
of such spices as Cloves (the flower-buds of luit^oiia carvophvHata) 



ETHEREAL OILS, TERPENES 63 

Cinnamon (the bark of young twigs of Cinnamommn zeylanicum) , 
Ginger (the rhizome of Zingiber officinale), Pepper (the berries of 
Piper nigrum), etc. Moreover, the active principle of the Hop 
[Hiumilus liipiilus), which is contained in special hairs (cf. p. 105) 
borne on the bracts of the female catkins, and that causing the 
odour of Tea, likewise belong to the ethereal oils. 

Camphor is a solid terpene-derivative obtained from the wood 
of the Camphor-tree [Cinnamomum campJiora), whilst turpentine 
is a mixture of terpenes which flows from the resin-passages 
(cf. p. 339) in the trunks of various species of Pines (especially 
Finns pinaster) and of the Spruce Fir (Picea excelsa), when cuts 
are made in the surface. After the oil of turpentine has been 
chstilled off, the solid residue left is rosin. 

Most of the terpenes are colourless, highly refractive liquids, 
which evaporate completelj' if sections containing them are 
heated on a shde for about ten minutes. They are readily 
soluble in alcohol, chloral h^'drate, glacial acetic acid, etc. Com- 
pounds of this nature, often together with other substances, 
commonly occur in speciallj' differentiated ceUs or intercellular 
spaces within the tissues of the plant (cf. pp. 151, 152), or in 
glandular hairs (p. 105). The ethereal oils are sometimes 
combined with glucose, etc., in the form of glucosides {e.g. the 
mustard oil of Cruciferie, cf. p. 48), and become hberated only 
after coming in contact with the appropriate enz\Tne. 

The colours of flowers are due in manj' cases to pigments, 
which are classed as anthocyanins, dissolved in the cell-sap of the 
petals, the colour being red, blue, or violet according to the 
acid or alkaline reaction of the sap. Such pigments are also 
frequently present in the vegetative organs, as, for instance, in 
the Beetroot and in the lea\'es of the Mother-of-Thousands [Sa.xi- 
fraga sarmentosa). Their development appears to be stimulated 
by excessive transpiration and intense illumination, conditions 
which are realised in alpine and arctic regions where high coloura- 
tion is a conspicuous feature. 

The yellow and red colouration of many flowers [e.g. Garden 
Nasturtium) and fruits [e.g. Tomato) is, however, not due to 
substances in the cell-sap, but to the presence of pigments 
(carotin, etc.) in special plastids, termed chromoplasts. In many- 
cases the pigment occurs in the latter in a crystalline form. 



64 PIGMENTS 

Carotin is present in considerable quantity in the root of the 
Carrot, to which it imparts the orange colour. 

A considerable number of plants are still used as the source 
of dyes ; thus the dried stigmata of Crocus sat km s yield saffron, 
and the rhizome of Citrctima longa (tropical Asia) the yellow dye 
turmeric. 

It is doubtful whether any of the substances dealt with in 
this chapter are produced for their intrinsic utility to the plant. 
But it is quite possible that the poisonous alkaloids, the astringent 
tannins, or the pungent resins may render the plants containing 
them distasteful to herbivorous animals, and so prove indirectly 
Ijeneficial. Similarly the antiseptic character of resin may well 
be of service in protecting a wound over which it has congealed. 
Tyndall showed that the vapour of ethereal oils tends to diminish 
the passage of heat through air, so that their common occurrence 
in plants of dry situations may be a means of checking evapora- 
tion by retarding the heating effect of sunshine. It may be, 
however, that some of the substances here referred to constitute 
a means of storage of food-material, and this applies especially 
to some of the tannins which seem to have the structure of 
glucosides. 



CHAPTER VI 

The Structure of Roots 

Ix the mature plant the different types of elements are grouped 
together to form tissues subserving diverse functions. Thus, for 
instance, one tissue, which is always superficial and has the general 
function of protecting the underh-ing parts, constitutes the 
epidermis. The vascular tissues, comprising the wood or xylem 
and the bast or phloem, form the respective conducting sj'stems 
for water and mineral salts, and for elaborated food-substances, 
whilst others — largeh' parenchymatous in character — go to form 
the general matrix or ground tissue. It must not be supposed, 
however, that, because there is one general function, the com- 
ponent elements of these and other tissues are necessarily uniform 
in character, a fact that will be best reahsed by the study of 
concrete examples. 

In every j'oung root we can distinguish a number of regions, 
beginning \\ith the root-cap, covering the growing point (Figs. 9 
and 30, r.c), and followed successively by the zone of elongation, 
the zone bearing root-hairs, and the older part where the root- 
hairs have withered and the lateral roots (Fig. 30, l.r.) are seen 
emerging through the surface-layers, commonly in four or five 
vertical rows. In a longitudinal section through the tip of the 
root the cap is found to consist of a number of concentric layers 
of thin-waUed parenchj-matous cells, whose arrangement becomes 
less regular towards the outside (Fig. 9). The outermost cells 
gradually become mucilaginous and break down, whilst the 
substance of the cap is constantly renewed from the underlying 
meristem. A Uttle way behind the gro',\ing point (p. 20) the 
middle of the root is occupied by a continuous, rather denser 
strand of elongated elements, which develop into the vascular 
tissue, and which offer a marked contrast to the thin-waUed 
parench\-matous cortex around. • 

5 ' 65 



66 



ROOT-HAIRS 



The general structure of the root can best be studied in a 
cross-section through the mature region, the Creeping Buttercup 
[Ranunculus repens) furnishing suitable material for a first ex- 
ample. Under the low power of the microscope the broad paren- 
chymatous cortex (Fig. 27, C), whose cells contain numerous 
starch-grains [s.), and the central conducting strand, are again 




Fig. 27. — Transverse .section of the central part of the root of the Creeping 
Buttercup [Raninwiilus repens). The walls of the xylem elements are 
shown black. C, inner part of cortex ; ea., cambium ; c.c, companion 
cell ; en., endodermis ; i.p., intercellular spaces ; p., pericycle ; 
pt.xy., protoxylem ; s., starch ; s.t., sieve-tube. 

sharply contrasted. At the edge of the section is a layer of 
shrunken cells (Fig. 28, r.), some of which are prolonged into 
shrivelled root-hairs. This epidermis withers above the zone 
of root-hairs and, since its chief function is their production, 
it is more usually termed the piliferoits layer. It is only in 
sections cut nearer the growing tip that its cells are as yet un- 
contracted, and can be seen to form a single layer. 

The root-hairs, each arising from a separate cell (Fig. 28, ;-.), 



PILIFEROUS LAYER AND EXODER.^IIS 



67 



appear as tubular unbranched outgrowths with bluntly rounded 
tips. The greater part of any root-hair is occupied bj' a large 
vacuole continuous with that of the epidermal cell and filled 
with sap. There is consequently only a thin lining layer of 
cytoplasm, which is best seen near the tip where the single 
nucleus usually lies embedded. 

Beneath the withered pihferous layer, in the older part of 
the root, lies the exodermis (Fig. 28, Ex.), a layer of protective cells 
which are on the whole rather smaller than the adjacent cells 
of the cortex. Their bro^^'nish, sUghtly thickened walls are 
chemically changed {i.e. suberised, cf. p. 136) in such a way that 




Fig. 2S. — Piliferous layer (in part withered) and exodermis of the root of 
the Creeping Buttercup (Ranunculus repens). Co., cortex ; Ex., e.xo- 
dermis ; )'., root-hairs arising from cells of piliferous layer ; 5/., starch. 

they are almost impermeable to water ; but this alteration does 
not usually take place tiU the root-hairs begin to wither, so that 
the water they absorb can readily pass inwards to the vascular 
cyhnder. In some cases, however {e.g. in most Monocotyledons), 
where the exodermis differentiates at an early stage, thin-walled 
passage-cells (Fig. 33 P.), through which the water travels, occur 
at regular intervals. 

The cell-waUs of the cortex become thinner towards the centre, 
and intercellular spaces (Fig. 27, i.p.) are abundant. The latter 
form a continuous system which permits of gaseous exchange 
with the aerial parts of the plant. The vascular strand is de- 



68 VASCULAR STRAND 

limited from the cortex by two well-defined layers of cells, an 
outer, the endodermis (ni.), and an inner, the pericyclc (p.) (cf. 
below) . 

In sections stained with aniline chloride a fom'-raj'ed group of 
yellow elements, composing the lignified wood or xylem (Fig. 27), 
is seen to occup}' the greater part of the conducting strand. In 
the bays between the four arms of the xylem are oval groups of 
small-celled unstained tissue, the phloctn, in which the wide and 
empty-looking sieve-tubes (s.t.) are plainly distinguished from 
the narrower companion-cells (c.c.) with their dense contents 
(cf. p. 28). Each phloem-group is separated from the adjacent 
xylem by one or two layers of parenchyma (fa.). 

The xylem consists chiefly of dead, empty-looking elements, 
the vessels, of which those at the centre of the conducting strand 
are the largest, whilst the remainder become progressive!}' smaller 
in passing outwards along any one of the rays. The end of each 
xylem-ami, immediately beneath the pericycle, is thus occupied, 
by a strand of the narrowest vessels (Fig. 27, pt.xy.). In cross- 
sections, through younger parts of the root, a larger or smaller 
number of the central vessels will appear thin-walled and un- 
lignified, showing that differentiation of the xylem takes place 
from without inwards. The small peripheral elements are con- 
sequently spoken of as protoxylem, and the larger, later-formed 
ones, as metaxylcm. 

Longitudinal sections passing through one of the xylem-arms 
will show that the vessels of the protoxylem are spirally thickened, 
whilst those of the metaxylem bear bordered pits. The walls 
between the larger vessels, in the transverse section, exhibit a 
thin dark line down the middle (the middle lamella), with the 
thickening layers on either side. These latter are not homo- 
geneous, however, but appear to consist of short dark lengths 
alternating with lighter and narrower portions, where the pits 
are seen in optical section. 

The ceUs of the endodermis (Fig. 27, en.) are distinctly smaller 
than the adjacent cortical cells and somewhat flattened. Their 
radial walls are thicker and look darker than the others, although 
when sharply focussed in optical section the)' appear bright owing 
to their highly refractive character. On treatment of a section 
with strong sulphuric acid, the membrane swells and dissolves, 



VARIATIONS IN ROOT-STRUCTURE 69 

except for the radial walls, which persist unaltered, implying 
that they are chemically different from the other walls of the 
endodermal cells. These features of the radial walls are com- 
monly exhibited by the endodermis of roots. 

The purpose of the endodermis is still obscure, but such a 
layer, showing the characteristic thickening of the radial walls, 
is found in the roots of many Flowering Plants, as well as in the 
stems of aquatics (cf. p. 171). The firm lateral connection of the 
cells due to the thickening renders this layer an efficient sheath 
to the vascular system. The considerable thickening of the 
entire endodermal wall which obtains in some cases (cf. below), 
suggests a probable mechanical value. In the un thickened state 
the chief function of this layer may perhaps be to cut off the 
water-conducting strand from the air-spaces of the cortex, a 
delimitation rendered necessary by the frequent negative pressure ^ 
within the vascular system. Not uncommonly the layer of 
cortical cells next to the endodermis develops characteristic 
thickenings on the transverse and radial walls, a feature well 
seen in the roots of many Cruciferas (e.g. White Mustard), and 
no doubt of mechanical value. 

The pericyde which lies immediately within the endodermis 
is another continuous layer not characterised by any structural 
peculiarities (Fig. 27, p.). 

Many of the features just described are typical of roots 
generally, viz. the aggregation of the vascular tissue near the 
centre ; the alternation of phloem and xylem resulting in a radial 
structure ; the peripheral location of the protoxylem ; the wide 
cortex ; the differentiation of the endodermis ; and the occurrence 
of exodermis and piliferous layer. Roots differ among one 
another in two principal respects — namely, as to the number of 
phloem- and xylem-strands, and in the presence or absence of 
parenchyma (pith) in the centre of the conducting tissue. As 
regards the former feature, there may be two or more of such 
strands, roots being described as diarch, triarch, tetrarch (Fig. 27), 
pentarch (Fig. 30), etc., according as the number of alternating 
xylem- and phloem-groups is two, three, four, or five, etc., whilst 
when they are numerous the structure is said to be polyarch 
(Fig. 29). Thus the root just examined is tetrarch, that of the 
1 Cf. F, & S., p. 121. 



70 



VARIATIONS IN ROOT-STRUCTURE 



Wallflower diarch, and that of most Monocotyledons polyarch. 
A pith composed of parenchyma, which is sometimes thick- 
walled, is frequent in the roots of herbaceous Dicotyledons 
(Fig. 30) and in Monocotyledons (Fig. 29), but in woody 
Dicotyledons and Conifers the xylem-groups often meet at the 
centre. Roots also vary in the manner of thickening of the 
mature cells of the endodermis. 




I'"iG, 2y. — Transverse section of the central part of the root of the Ins. 
C, cortex ; en., endodermis ; m.xy., metaxylcm ; pa., passage cells ; 
pc, pericycle ; ph., phloem ; p.xy., protoxylem. 

A transverse section of the root of the Iris illustrates these 
features. It exhibits the typical Monocotyledonous structure, 
viz. a central pith and numerous alternating groups of xylem 
and phloem (Fig. 29). The endodermis [en) is conspicuous owing 
to the marked thickening of all but the outer walls of most of 
its cells. Opposite the protoxylem groups [p.xy.), however, the 
cndodermal cells are often thin-walled, and such. passagc-cdls [pa.) 



LATERAL ROOTS 



71 



serve for the transference of water through the endodermis, 
which is otherwise impermeable. A thickened endodermis inter- 
rupted by thin-walled passage-cells is found particularly amongst 
Monocotyledons (see also Fig. 33, Pa.). 

The mode of origin of the laterals can be readily studied in 
longitudinal sections through a Bean root. The lateral roots 
arise by di\'ision of pericyclic cells, either opposite the protoxylem 




Fig. 30. — The root of a Bean seedling in surface-%'iew and in transverse 
section showing the origin of the lateral roots [i.r.). C, cortex; 
ph., phloem ; p.c, pericycle ; r.c, root-cap; ;rj'., xylem. 

strands (Fig. 2'^, I.r.), or between these and the phloem, so that 
in Dicotyledons they form vertical series equal to or double the 
number of xylem groups. In Monocotyledons, owing to the 
numerous protoxylem-strands, and the early decay of the tap- 
root, this arrangement is often obscured. In the course of their 
further development, the side-roots push their way through the 
cortex, so that, by the time they emerge at the surface, the 
protective root-cap is fully formed (Fig. 30, r.c). This so-called 



72 RELATION OF STRUCTURE AND FUNCTION 

endogenous origin contrasts with tire superficial origin of the 
branches of the stem, and can be related to a need for protection, 
until the young root has developed sufficiently to withstand the 
resistance of the soil. 

It is now possible to inquire how the structure of the root 
is related to its two principal functions, viz. absorption of water 
from the soil and the anchorage of the plant. Absorption of 
water takes place through those regions in which the piliferous 
layer is still intact and living. The outgrowth of its cells into 
root-hairs, which are the chief organs of absorption, affords an 
enormously increased surface over which the latter can take 
place. The size and number of the root-hairs tend to decrease 
with increasing wetness of the soil, and in some marsh-plants 
[e.g. Marsh Marigold) and a considerable number of aquatics 
they may be altogether absent. Here, owing to the high water- 
content of the soil, an enlargement of the absorbing surface is 
unnecessary. The root-hairs not only function in absorbing 
the water with its dissolved mineral salts, but themselves play 
a part in rendering substances in the soil available. The older 
part of the root which does not absorb is amply protected by 
the exodermis. 

Many plants characteristic of soils rich in humus exhibit an 
intimate relation of fungal threads with their roots or other 
underground organs. In some cases these threads form a dense 
weft over the whole surface {ectotrophic mycorrhiza , e.g. Beech, 
Birch, etc.), and appear to replace the absent root-hairs. In 
others the Fungus can be seen occupying a definite zone within 
the cortex [endotrophic mycorrhiza, e.g. Bird's Nest Orchid, 
(Fig. 31, i1/. ), Heather, etc.]. The advantage of association with 
the Fungus would appear to depend mainh' on the po\ver of the 
latter to break down and absorb the organic material which is 
then in part utilised by the Flowering Plant. 

The root is suited to its anchoring function bv its more or 
less extensive branching, and the central location of the me- 
chanical elements which enable it to withstand the pulling strain 
to which it is subjected when the shoot sways to and fro in the 
wind. In most roots the mechanical tissue is constituted merelv 
by the xylem, as well as by the pith ^^■hen the latter is thick- 
wahed, but in the Pea, Bean, and other Lcguminos;e groups of 



RELATION OF STRUCTURE AXD FUNXTION 73 

fibres are developed in relation to the phloem. When roots serve 
not only for anchoring, but also for the support of the plant, a 
modified structure may obtain. Thus, a cross-section of the 
prop-root of the Maize (Fig. 32) contrasts ^\^th one of an ordinary 
root of this plant in the presence of a special cortical ring of 
mechanical elements and the larger size of the central cylinder, 




Fig. 31. — Transverse section of the outer part of one of the underground 
branches of the Bird's Nest Orchid (Neottia nidus-avis), showing the 
vascular strand (T'.S.), the cortex (C.) and the mycorrhizal zone (.1/.). 



so that the xylem also is more peripheral. In both these 
respects the mechanical construction of such a prop-root ap- 
proaches that of a stem (cf. p. 87). 

The aerial roots of many tropical epiphytes {e.g. Orchids) 
also exhibit a modification of structure in correspondence with 
their special method of absorption and their function as assimi- 
latory organs. The cortical cells frequently contain chlorophj-il, 



74 AERIAL ROOTS 

a feature doubtless merely related to their growth in the light, 
since ordinary terrestrial roots [e.g. those of the Pea) will often 
become green when exposed to illumination. In extreme cases 
[e.g. Tmniophyllum) the entire carbon dioxide assimilation is 
carried out by means of the aerial roots, which assume a leaf -like 
appearance, whilst the true leaves are reduced to mere scales. 
The absorption of water in such roots is accomplished with the 
aid of a tissue known as the velamen (Fig. 33, F.), formed by 
a remarkable development of the epidermis. The dermatogen 
usually divides to form several, or many, layers of cells, which 
ultimately lose their living contents and often exhibit a peculiar 
spiral or reticulate thickening. Moreover, large holes frequently 




Fig. 32. — Diagrams of a normal absorbing root (B) and of a prop-root (A) 
of the lUaize (Zea mais). Tlie sclerenchymatous tissues are indicated 
by shading. V ., vessels. 

develop in the walls of many of the cells, as a result of which 
any moisture faUing or condensing on the surface is rapidly 
absorbed. The cxodermis (Fig. t,^,, Ex.), situated at the inner 
edge of the vciamen, is interrupted by thin-wallcd passage-cells 
(P.) for the inward transference of the water absorbed. In 
dry weather air fills the cells of the vciamen, so that they 
appear white and opaque ; but when occupied by moisture they 
become translucent and the green colour of the cells beneath 
is visible. 

The ccUs in the central region of the root-cap in many roots 
contain numerous large starch-grains, which readily move from 
one part of the cell to another, when the direction of the root 
is altered. Thus, in the normal erect position they lie against 
the cytoplasm hnmg the lower walls, but if the root be placed 



AERIAL ROOTS 



75 



horizontally they rest on the side walls. Similar readily movable 
bodies (not always consisting of starch, however) have been 
found in herbaceous stems which are sensitive to gravity. Since 
detailed experiments have established that the stimiUus of 
gravity is most readily perceived in the root-tip,^ some Botanists 




Fig. 33. — Transverse section through part of a root of an epiphytic Orchid 
(Dendrobiuin), showing the velamen {!'.). Co., cortex ; En., endo- 
dermis; Ex., exodermis ; P. and Pa., passage cells ; Xy., xylem. 

regard the impact of these faUing starch-grains as the mechanism 
fof perceiving orientation. Xo adequate proof of this hypothesis 
has, however, yet been given, the mechanism of gra^'it3'-percep- 
tion being still unsolved. 

1 Cf. F. &. S, p. 209. 



CHAPTER VII 

The Structure of the Young Stem 

The comparatively slight variation in the anatomical construc- 
tion of roots can be related to their very uniform environment 
(the soil) ; in fact, the only marked departures from the normal 
are associated with special needs, as in aerial roots, prop-roots, 
etc. Stems, on the other hand, develop under very diverse 
conditions as regards mechanical strains, illumination, supply 
of moisture, etc., and consequently display much more varied 
structure. Moreover, the general plan of construction of the 
stem differs from that of the root in several important respects. 
The organisation of a Dicotyledonous stem can be studied 
in a cross-section through the uppermost internode of a young 
Sunflower (Fig. 34). The most obvious feature is that the 
vascular tissue is a ring broken up into distinct vascular bundles. 
In each the thick-walled xylcm {xy.} is towards the inside and 
the thin-walled phloem [ph.) towards the outside, the two thus 
lying on the same radius (constituting a collateral bundle), in 
contrast to the alternate arrangement of these tissues obtaining 
in the root. By the disposition of the vascular bundles in a 
ring the ground tissue is marked out into three regions, namely 
the cor/cx (Fig. 34, c.) on the outside, the pitli (p.) in the centre, 
and the rays (often called medullary rays, r.) connecting the 
two and separating adjacent bundles. The terms pith, cortex, 
and rays are, however, purely topographical, and do not neces- 
sarily imply any differentiation between the component ceUs, 
which in fact, in most cases, are largely thin-walled and paren- 
chjmiatous in all three regions (cf. Fig. ;6, Co. and p.). The 
edge of the section is bounded by an epidermis 1 which differs 

1 l'"(jr a detailed coiisideralion ni epidermis and stomata, see the next 
chapter. 

76 



DICOTYLEDONOUS STEM 



77 



from that of the root in having strongly thickened outer walls. 
It forms a continuous layer except where interrupted by the 
presence of stomata ^ or lenticels (p. 139) . Its function is to 
protect the more dehcate internal tissues, and in correspondence 
with this it does not mther at an early stage like the piliferous 
layer. Occasional cells grow out into hairs (Fig. 34, h.), but 
these are quite different in character from the root-hairs. 

Immediately within the epidermis are several layers of 
collenchymatous cells (Fig. 34, s.), thickened mainly on the 




Fig. 34. — Photomicrograph of a transverse section of the young stem of 
the Sunflower (Helianihiis). c, cortex; /;., hair; p., pith; ph., 
phloem; r, raj'S ; s, collenchyma ; xy., xylem. 



tangential walls, whilst between the cells of the inner layers are 
relatively large intercellular spaces. Such spaces are also abun- 
dant in the remainder of the ground tissue, and form small tri- 
angular cavities between the polygonal cells of the pith. 

At the inner limit of the cortex there is a single layer of cells 
distinguished b}' the frequent presence of large starch-grains 
(Fig. 36, sh.), which can be rendered more prominent by staining 
vnt\i iodine. The layer in question bends outwards around each 

1 For a detailed consideration of epidermis and stomata, see the next 
chapter. 



78 



DICOTYLEDONOUS STEM 



vascular bundle, and so presents a sinuous outline. Such a 
starch-sheath is not uncommon in herbaceous Dicotyledonous 
stems (cf. also Fig. 57, S.s.), and its readily movable starch-grains 
have been regarded as fulfilling a similar function in perceiving 
geotropic stimuU as that attributed to the starch-grains in the 
cells of the root-tip (cf. p. 75). The central cyhnder, which is 
separated off from the cortex by the starch-sheath, is termed 




Fig. 35. — A single vascular bundle from the stem of the Sunflower [Heli- 
anihus) in transverse section, ex., companion cells (shaded) ; /., fibres 
of the pericycle ; M.xy., metaxylem ; p., parenchyma of the rays; 
ph.p., phloem-parenchyma ; Pt.xy., protoxylem ; s.p., sieve-plate. 

the stele, and comprises the vascular bundles and the accom- 
panying ground tissue. 

The zone of tissue between the starch-sheath and the phloem 
constitutes the pericycle, which, in the Sunflower, is composed of 
groups of fibres (as yet not fully thickened) opposite the bundles 
(Fig- 35. /• ; Fig. 57, F), and elsewhere of parenchj'ma (Fig. 
34). In other plants the pericycle is often parenchymatous 
throughout, consisting of one or more layers of cells. The term, 



VASCULAR BUNDLE 79 

as in the case of the root, is a purely topographical one, being 
applied to the region situated between the conducting strands 
and the starch-sheath or a similar continuous layer of cells. 

In the individual bundles the phloem can be distinguished 
by the shining appearance of the cell-walls, which resemble those 
of the young fibres, as well as by the unequal size of its com- 
ponent elements (Fig. 35). The large, empty-looking sieve-tubes, 
exhibiting an occasional sieve-plate {s.p.)'^ and associated with 
small companion cells {ex.), which have dense contents, are inter- 
mingled \\ith ordinary'- parenchyma-ceUs [phloem-parenchyma , 
ph.p.) . Two or three layers of flattened thin-walled cells, situated 
between the phloem and the xjdem, constitute the cambium 
(of. Fig. 57, C, p. 118), a meristematic region which becomes 
active in the older stem and forms additional vascular tissue 
(cf. Chapter X). 

The wood consists of radial files of vessels separated by rows 
of smaller parenchymatous cells (the wood-parenchyma, Fig. 35). 
The smallest vessels (Pt.xy.), which represent the protoxylem 
(cf. p. 68), are situated nearest the pith, while the larger 
metaxylem-elements (M.xy.) are towards the outside, a further 
point of contrast to the root. The wood-parenchyma cells have 
more or less thickened walls, which are Hgnified like those of the 
vessels, but they nevertheless retain their living contents. Small 
cells resembling the wood-parenchyma, but not hgnified, form 
a sheath (the medullary sheath, Fig. 34) at the inner edge of each 
bundle. 

A longitudinal section of a Sunflower-stem passing radially 
{i.e. parallel to a radius) through one of the bundles ^^^ll show 
the same succession of tissues and enable us to complete our 
picture of the various elements (Fig. 36). The short and some- 
what bulging epidermal cells are succeeded by those of the 
collenchyma, which are manv times longer than broad, and 
have strongly thickened longitudinal walls (cf. Fig. 16, A, of 
Dead-nettle). The thin-waUed rectangular cells of the cortex 
(Fig. 36, Co.) are much shorter. Next come the starch-sheath 
{sh.) , recognisable by its large starch-grains, and the typical fibres 
of the pericycle (/.). The detailed structure of phloem and cam- 
bium (c.) will be studied in another stem, but in the former the 
^ The sieve-plates are not readily recognised in the Sunflower. 



8o 



STEM OF CUCURBITA 



long, apparently empty, sieve-tubes (s.) and the narrow, densely 
granular, companion cells (c.r.) are readily distinguished. The 
vessels of the metaxylem (p.v.) bear very dense spirals or occa- 
sionally bordered pits, whilst in the protoxylem the thickening 
takes the form of rings or a loose spiral {px.). The narrow 
elongated elements sometimes seen between are the wood- 
parenchyma cells. 

Further insight into the structure of the Dicotyledonous stem 
will be afforded by a study of that of the Vegetable Marrow 





\. 



w ^ ' 



^t\ 




Fig. 36. — Longitudinal section througli a vascular bundle of the Sun- 
Rower {Helianihus). c, cambium; c.c. , companion cells ; Co., cortex ; 
/..fibres of pericycle ; p., pith-parenchyma ; p.v., pitted vessel of meta- 
xylem ; px., spiral vessel of protoxylem ; s., sieve-tube ; sh., starch- 
sheath. 



(Cucurbita) (Fig. 37). The transverse section differs from that 
of the Sunflower in the hollow pith, in the presence of a broad 
ring of sclerenchyma {Sd.) some little way beneath the epidermis, 
and in the arrangement and construction of the vascular bundles. 
A pith-cavity is a frequent feature in the stems of herbs (e.g. 
Labiatai, Umbellifene, Graminere, etc.), and can be related to 
cessation in the growth of the pith-cells, so that rupture occurs 
as a result of the continued enlargement of the stem. 

In the stem of the Marrow there are two rings of vascular 
bundles, of which the larger and inner alternate with the smaller 



BICOLLATERAL BUNDLES 8i 

and outer (Fig. 37). Two groups of phloem occur in each bundle, 
one external (Ph.'), separated from the xylem b\- the cambium 
(Ca), the other internal (Ph."), separated from the protoxylem 
(P.xy.) by ordinary parenchyma. Such bundles, termed bicol- 
lateral, are not uncommon in certain families of Flowering Plants, 




Fig. 37. — Diagram of a transverse section of the stem of tlie Vegetable 
Marrow {Cucurbila), Ca,, cambium ; Col., coUencliyma ; Ep., epi- 
dermis ; jl/..r_y., metaxj'lera ; PA.', outer phloem ; PA.", inner phloem ; 
P.xy., protoxylem ; Scl., sclerenchyma (black). 

such as the Cucumber -family or Cucurbitaceae (to which the 
Marrow belongs), and the Potato-family or Solanacefe, and offer 
a marked contrast to the normal collateral type seen in the Sun- 
flower. The elements of both xylem and phloem are remarkably 
large, and are therefore especially suited for a study of their 
detailed structure, which can be rendered more distinct by 
staining with eosin (see Appendix VI). 
6 



82 PHLOEM 

In the phloem many of the large somewhat thick -walled 
sieve-tubes appear empty ; but here and there, where the plane 
of section coincides with a separating waU, there is a deeply 
stained sieve-plate (Fig. 13, s.p.), recognisable by its dotted 
structure. Under the high power the sieve-plate exhibits an 
irregular network of thickening, completely covered by a thin 
layer of cytoplasm, the greater part of each mesh being occupied 
by an open pore through which communication with the next 
segment of the sieve-tube is established. In contact with each 
sieve-tube are one or more small, often more or less triangular 
companion cells (Fig. 13, c.c), with dense contents. The 
numerous phloem-parenchyma cells (Fig. 13, p.) are more or 
less intermediate in size, but without either the thickened wall 
of the sieve-tube or the dense contents of the companion cell. 
Both inner and outer phloem show the same structure. 

In longitudinal sections (Figs. 13, left, and 38) the horizontal 
sieve-plates [s.p), stained red with the eosin, occur at frequent 
intervals in the course of the sieve-tubes. When viewed in 
optical section, they present an interrupted appearance, due to 
the alternation between the pores and the bars of thickening 
with their covering of cytoplasm. Thick, highly refractive masses 
(stained red by the eosin), composed of a carbohydrate known as 
callose, are often found on one or both sides of the sieve-plates. 
Such deposits of callose, bringing about a closure of the latter, 
arise sooner or later in the sieve-tubes of most plants, and are 
often permanent, marking the end of the activity of the element 
in question ; in some woody plants and in many Monocotyle- 
donous rhizomes this is, however, a periodic phenomenon, the 
callose being formed in the autumn and becoming redissolved 
at the advent of spring. Callose is distinguished by being 
insoluble in ammoniated copper oxide, but soluble in a i per cent, 
solution of caustic potash, and by its reddish-brown colouration 
with chlor-zinc-iodide ; it becomes deeply stained bj' a dilute 
aqueous solution of aniline blue, which should be ahowed to act 
for half an hour. 

Here and there the plane of the section passes through a 
companion cell (Fig. 13, c.c.) in contact with its sieve-tube. The 
former tapers off towards the sieve-plates above and below, so 
that its length coincides with that of the sieve-tube segment 



XYLEM 



83 



from which it was cut off. The large nucleus can generally' 
be seen embedded in the dense cj'toplasm which completely fills 
the cell. The phloem-parenchj-ma cells (Fig. 13, p.) are readity 
distinguished h\ their shape from the other elements. The cells 
of the cambium (Fig. 38, Ca.) appear much elongated in the 
longitudinal section and have abruptl}^ pointed ends. 

In the transverse section of the u-ood one can recognise, 
as in the Sunflower, small protoxylem -elements towards the 
interior (Fig. 37, P.xy), large metaxylem-vessels beyond [M.xy.), 




Fig. 38. — Longitudinal section through tlie stem of the Vegetable Marrow 
(Cucurbita). Ca., cambium; Coll., collenchj-ma ; Ep., epidermis; 
M.xy., metaxylem ; Par., parenchymatous cortex; Ph.' and Ph.", 
outer and inner phloem ; P..\y., protoxjrlem ; Scl., sclerenchyma. 



and numerous intermingled wood-parench\TQa cells. Where two 
vessels are in contact, the middle lamella between the pitted 
thickening lavers of the common wall can be clearly made out. 
In longitudinal sections the broad metaxylem-vessels (Fig. 38, 
M.xy.) show reticulate thickenings which are in marked contrast 
to the spiral thickenings of the protoxylem-vessels [P.xy.) ; the 
shght constrictions occurring at short intervals mark the limits 
of the cells from which the vessel was built up, and, in thick 
sections, the ring-like remnants of the original septa can often 



84 



MONOCOTYLEDONOUS STEM 



be distinguished. The exceptionaUy kirge vessels are a marked 
feature of many other climbers besides the Vegetable Marrow. 

Stems of Monocotyledons (of which the Maize furnishes a 
typical instance) usually exhibit a large number of bundles which 
frequently appear scattered throughout the whole of the ground 
tissue (Fig. 39), so that a definite cortex and pith cannot be 
distinguished, i.e. there is often no sharply circumscribed stele 
as in the orchnary Dicotyledon. The bundles, though differing 
in certain details, show the same general structure, being col- 






\94 * 










^X£h>'-^tiil^ir-^H':. . . JlOsi 



Fig, 39. — Photomicrograph of part of a transverse section of tlie stem of 
tlie Maize (Zea iiuiis). p., phloem ; p.x., proto.xj-leni. 

lateral with xylem and phloem on the same radius and with the 
protoxylems {p.x.) directed inwards. 

The phloem (Fig. 39, P-). which often has similar sliining cell- 
walls to that of the Sunflower, is an oval or roimded group of 
tissue composed of sieve-tubes (Fig. 40, A, 5./.) and small com- 
panion cells (C ). The latter are more rounded than is usual in 
Dicotyledons, and this, combined \\'ith the absence of paren- 
chyma,' leads to the remarkably uniform appearance of the 
phloem. There is no cambium between the xylcni and the 
phloem, an important respect in which the Monocotyledonous 
bundle usually differs from that of the Dicotyledon. The shape 
1 Phloem-parenchyma is present in some monocotyledons. 




85 



86 VARIATIONS IN STEM-STRUCTURE 

of the xylem as a whole is often roughlj' that of a Y, the stem 
of which is occupied by a radial row of protoxylern-vessels (Fig. 
40, A, Px.), whilst the arms are formed by a pair of large 
metaxylem-vessels (Mx.). The phloem tends to be sunk between 
the latter (cf. Fig. 40, A), but the degree of sinking varies con- 
siderably in different stems, and in extreme cases the phloem 
may even be completely surrounded by the xylem (as in the 
Sweet Flag, Acorns calanms, Fig. 40, B). As usual, a certain 
amount of wood-parench)'ma occurs between the vessels. In 
the stems of the Maize and many other Monocotyledons which 
exhibit rapid elongation, the protoxylem-vessels undergo early 
rupture and give rise to an irregular cavity (cf. p. 36) in which 
remnants of the spiral thickenings are recognisable (Fig. 40, A, 
P.c). The bundles are commonly enveloped in a sheath of 
small, thick-walled cells (often fibres, t'.g'. in the Maize, Fig. 40, A), 
a feature also encountered in some Dicotyledons [e.g. Buttercup). 

The typical Monocotyledonous stem thus differs from that 
characteristic of Dicotyledons in the large number, scattered 
arrangement, and general form of the bundles, in the usual ab- 
sence of a cambium, and in the detailed structure of the phloem. 
Certain Dicotyledons (e.g. Buttercup and its allies), however, 
show resemblance to Monocotyledons in the shape of the bundle, 
in the absence of phloem-parenchjmia, and the very feeble de- 
velopment of the cambium. Moreover, the stem of the Dicoty- 
ledon may exceptionally possess a relatively large number 
of scattered bundles [e.g. Meadow Rue, Thaliclriim) , whilst that 
of Monocotyledons may in its turn exhibit a ring-like arrange- 
ment of the bundles [e.g. Black Bryony, Tamils, Fig. 41) and a 
feeble development of cambium (cf. p. 117). 

In certain Dicotyledons which possess a normal ring of 
vascular bundles additional ones occur in the pith (e.g. 
Spinach, Water Dropwort, Oenantlie eroeata) or cortex (e.g. Box, 
Btixus sempcrvircns). Such medullary and cortical bundles are 
probably in most cases merely strands entering from the leaves 
which have not yet taken their normal place in the vascular 
cylinder (cf. p. 114). These bundles either show the normal 
orientation (i.e. with the xylem towards the inside), or 
they exhibit the reverse arrangement ; in many plants, moreover, 
they consist of phloem only. 



MECHANICS 



87 



The functions of the stem arc manifold, but among the most 
important are those of bearing leaves and flowers, and of con- 
stituting a connecting-Unk between the root-s^-stem and the 
fohage. The shoot of the ordinary erect plant is most hable to 
bending, under the influence of the wind, etc., and thus contrasts 
with the root, which is exposed chiefly to pxiUing strains. Re- 
lated to this the mechanical elements of the stem are more or 
less symmetrically arranged near the periphery. In the young 
stem the upright position is maintained by the frequent sub- 
epidermal collenchjmia (Figs. 15, 34, and 37), by the xylem of 




r'iG. 41. — Stem of the Black Bryony (Tamils communis). A, Diagram of 
transverse section. B, Photograph of a small portion on a larger 
scale, p., phloem ; Scl., sclerenchyma-ring ; ,r. , metaxylem vessel. 



the bundles, and by the turgidity of the living cells combined 
Mith tissue-tensions. 1 As the stem matures, additional mechani- 
cal tissue is often furnished by the development of pericycUc 
fibres ((?.o'. Sunflower, Fig. 35, /".), of a thick-walled sheath to the 
bundles (Monocotjdedons, Fig. 40, A, Buttercup), or of a zone 
of sclerenchjTna in the cortex [e.g. Vegetable Marrow, Fig. ^j, 
Scl.). Even in the ordinary Monocotvledonous stem the bundles 
are far more numerous near the edge (cf. Fig. 39); moreover, Mono- 
cotyledons frequently exhibit a copious development of peri- 
pheral sclerenchjTna, to which, for instance, the hardness of a 

1 Cf. F. & S., p. 103. 



88 ASSIMILATION AND CONDUCTION 

Bamboo is largely due. In slender underground stems aiding in 
the attachment of the plant, and therefore chiefly exposed to 
tension, a much more centralised disposition of the mechanical 
elements is observed than in the upright shoot. This is well seen 
in many Sedges and Grasses. An extreme condition is reached 
in water-plants, where, owing to the prevalent pulling strain, such 
mechanical tissue as is present becomes concentrated at the 
centre of the stem (cf. p. 170). 

Most young stems exhibit chloroplasts in the cortical cells 
(even when these are collenchymatous, as in the Campion), the 
carbon dioxide requisite for photosynthesis being obtained from 
the intercellular spaces between them, which communicate with the 
atmosphere by means of occasional stomata (cf. p. 96) in the 
epidermis. In some cases [e.g. Umbellifera;) the outer cortical 
zone consists of alternating bands of mechanical and assimilatory 
tissues, the former occupying the ridges, the latter situated 
beneath the furrows on the surface of the stem. Here the stomata 
are restricted to the strips of epidermis overlying the assimilator)- 
tissue. The cortex of the stem may even become the chief seat 
of the assimilatory function ; but this is exceptional, occurring 
mainly in plants inhabiting dry situations {e.g. Broom). 

Various experiments ^ show that the vessels are the channels 
by which water and mineral salts are passed from the root into 
the leaves. The phloem, on the other hand, serves to conduct 
elaborated food-substances, the proteins appearing to travel 
mainly by way of the sieve-tubes. If, for instance, the stem 
of the Vegetable Marrow be dipped into boiling water so as to 
coagulate the contents of the sieve-tubes, the abundant proteins 
can be demonstrated by heating longitudinal sections with 
Millon's reagent. Opinions chffer as to whether the carbo- 
hydrates are transferred by the phloem, or, as some Botanists 
believe, mainly by way of the cortical- and phloem-parenchyma. 
The storage of food-substances in perennial organs (e.g. rhizomes, 
tubers, etc.) is effected chiefly by the ground tissue, which in these 
cases is thin-walled. 

Whilst it has been seen that the arrangement of the tissues 
differs materially in root and stem, the ground tissues in the 
two organs pass over imperceptibly into one another. The 
1 Cf. F, & S., pp. no. III. 



TRANSITION FROM STE:M TO ROOT 



protoxj'lem-strands are directly continuous, wiiilst the metaxylem 
is de\'eloped towards the exterior in the stem and towards the 
interior in the root, and occupies a more or less intermediate 
position in the region {i.e. the hj'pocotji) where the transition 
from the one to the other type of structure takes place. The 
phloem is similarly continuous, and the transition from root to 
stem mainly involves a lateral and outward displacement accom- 
panying the enlargement of the vascular cylinder (Fig. 42). 
The transition between the two organs is, however, often 
very complex, being accom- 
panied b}' sphtting or union 
of conducting strands, as a 
result of which the number of 
protoxylem-groups in stem and 
root do not necessarily corre- 
spond. 

The dcAxTopment of the 
different regions of the axis 
from the gro^ving points has 
already been described in 
Chapter II, but some additional 
details as to the manner of 
origin of the vascular tissues 
^\ill serve to ampUfy the 
picture. These first appear in 
the plerome a short distance 
behind the tip as so-called 
procambial strands, composed 
of very narrow, elongated, thin-walled cells with pointed ends, 
dense protoplasmic contents, and prominent nuclei. They 
develop by repeated longitudinal di\dsions in the ceUs of the 
plerome, in which transverse diidsion almost ceases at an early 
stage, the narrow segments thus formed subsequently elongating. 

In the stem each procambial strand gives rise to a vascular 
bundle, whilst in the root there is usually a single strand from 
which the whole vascular cylinder is differentiated. The inner- 
most elements, in each of the procambial strands of the stem, 
become the spirally thickened protoxj'lem-vessels, whilst simul- 
taneously the ouferiTjost elements develop into the first-formed 




Fig. 42. — Diagrams showing one 
type of transition from the 
vascular structure of the root 
to that of the stem. The dotted 
areas represent phloem and the 
shaded xylem ; protoxylem 
sho^vn black, i?., root; S., stem; 
the remaining cross-section, 
hypocotj'l. 



90 DEVELOPMENT FROM THE GROWING POINT 

phloem {protophloem). As the strands are traced further and 
further from the apex {i.e. from younger to older stages) 
the differentiation of xylem and phloem proceeds towards the 
middle of each. Ultimately all that remains is a narrow strip 
of thin-walled cells between xylem and phloem, which in the 
case of Dicotyledons gives rise to the cambium. In the root 
alternating groups of protoxvlem and protophloem arise at the 
periphery of the procambial strand, whilst the later-formed 
elements, and the pith if present, develop from the central regi.on. 
Both leaves and branches originate close behind the growing 
point of the stem as superficial outgrowths {i.e. are exogenous, 
Figs. 7, 8), a mode of development contrasting markedly with 
the internal (endogenous) origin of lateral roots. Leaf-rudi- 
ments, at first, consist merely of a mass of periblem enveloped 
by a protrusion of the epidermis (Fig. 8), but as they enlarge 
procambial strands are formed within them by division of certain 
cells. The differentiation of these strands gradually extends 
inwards till they ultimately join those of the stem. As a 
result the vascular bundles of leaf and stem are continuous in 
the mature condition (p. 114, Fig. 55). When vascular strands 
occur in the stipules they are branches of those in the leaf -base. 
The strands arising in the j-'ovmg branches also become connected 
with those in the stem. 



CHAPTER VIII 

The Epidermis 

The surface of the shoot is protected by a skin, or epidermis, 
composed of one layer of hving cells which possess certain marked 
characteristics. In transverse sections the cells usually appear 
somewhat flattened with slightly convex outer walls, whilst the 
lateral and inner ones are generally straight ; moreover, they fit 
closely together without intercellular spaces (Eigs. 19 and43,£'^.). 
The epidermal cells have living contents, usually contain plastids, 
and possess large vacuoles filled with watery, generally colour- 
less, sap (Fig. 43, £/>.). Chloroplasts are not developed in the 
cells in well-illuminated situations, but are often present in the 
epidermis of submerged aquatics or of land-plants when growing 
in the shade (cf. p. 169). 

The outer epidermal waUs in most cases are more strongly 
thickened than the others, and so changed (cutiailarised) as to 
render them more or less impermeable to water vapour and 
gases. The exact nature of the modification is not known, but 
it appears to consist essentially in impregnation with substances 
of a fatty or waxy character ; these are most abundant in the 
outermost region of the external walls, which forms a continuous, 
relatively impermeable, layer over the whole epidermis (except for 
the stomata) known as the cuticle (Eigs. 43 and 46, C«.) . Where the 
walls are strongly thickened an intervening zone (the cuticular- 
ised layers) , in which cuticularisation has not progressed to the 
same extent, can sometimes be distinguished between the cuticle 
and the unaltered cellulose on the inside. The cuticle is often 
yellowish in colour, whilst the cuticularised layers, when present, 
appear faintly yeUow and less transparent than the colourless 
cellulose. The cuticle may be quite smooth, but in some cases 
[e.g. Hellehorus fcetidns) it is provided with numerous minute 

91 



92 



CUTICLE AND CUTICULARISATION 



ridges which appear as a faint striation in surface sections. 
Cuticularisation may sometimes extend to the lateral walls, 
which in these cases are generally thickened, so that in section 
they appear as pegs projecting inwards. 

The cuticle is insoluble in cuprammonia and concentrated 
sulphuric acid, being the only part of the epidermis that does 
not dissolve in the latter reagent. With iodine and sulphuric 
acid it yields a brown colouration, and it is easily stained with 




Fig. 43. — Transverse sections through the leaves of the Holly (A) and 
Ranunculus auricomus (B). In the former only half of the vertical 
extent of the leaf is shown. Cu., cuticle ; li['-, epidermis ; Hy., 
hypodermis ; Nn., nucleus ; Pal., palisade layers ; Sp., spongy 
parenchyma. The chloroplasts are shown black. 



Scharlach red, probably as a result of the presence of fatty 
substances. 

Owing to its toughness, the cuticle, especially when strongly 
developed, renders the epidermis more efficient as a protection 
against mechanical injury; but its main function is certainly 
the restriction of transpiration to the stomata. In correspond- 
ence with this its thickness depends mainly on the nature of 
the habitat ; thus the cuticle is most strongly de^'cloped in 
plants of dry situatioits [cp,. Gorse, Fig. <Si, B, Psamma, Fig. 85, A, 
etc.), whilst it is extremely thin in submerged aquatics (Fig. 89), 



EPIDERMIS 93 

where absorption of water takes place over the whole surface. 
Most young organs, before they have attained their full size, 
possess but a very thin cuticle and exhibit considerable cuticular 
transpiration ; hence the necessity for other devices for the 
reduction of transpiration {e.g. hairs, folding of leaves, etc.). 
Even a very thick cuticle is, however, not completel\- imperme- 
able, and its efficiency in reducing transpiration is sometimes 
augmented by a covering of wax secreted bv the epidermal cells ; 
when present in any considerable quantity (e.g. lea\'es of the 
Sea Holly, fruit of the Plum, etc.), this gives the surface a bluish 
tinge. Such coverings of wax also prevent the collection of 
moisture on the surface of the plant. Thev are readilv rubbed 
off, but may be subsequenth' renewed. 

Not uncommonlj- the epidermis can be easih' stripped off 
and so examined in siii-face rieii\ when it will again be seen to 
form a continuous la^'er (Fig. 44, B-D) without intercellular 
spaces, the only gaps being constituted bj' the stomata to be 
described below. The shape of the cells in such surface sections 
is very varied. In stems (Fig. 44, D) and in the leaves of most 
Monocotyledons (Fig. 44, C) they are usually considerably 
elongated parallel to the longitudinal axis, whilst in Dicotvle- 
donous leaves they are in most cases roughlv isodiametric (Fig. 
44, B). Where the lamina is thin the lateral walls of the epi- 
dermal cells are often undulated (Fig. 44, B). Through this 
wavy outline the cells interlock, so that the surface of contact 
between them is increased, and the risk of tearing bv the wind 
is proportionally diminished. It is significant that the lower 
epidermis, which has numerous points of weakness constituted 
by the stomatal perforations, generally exhibits this undulation 
to a more marked degree than the upper. This feature is also 
more pronounced in the leaves of certain aquatics than in those 
of the corresponding land forms. 

The epidermis keeps pace with the increasing size of the 
underlying tissues bj' the division of its cells, but usuall}' all the 
septa are at right-angles to the surface, so that it remains a single 
la^'er. This subdivision is plainly recognisable in the stem of 
the Runner Bean, where the spindle-shaped epidermal cells, 
after reaching a certain size, become divided by transverse septa 
(Fig. 44, D). 



94 



EPIDERMIS 



In plants of dry habitats the epidermal cells may attain a 
considerable size {e.g. in the Sea Purslane, Arenaria peploides, 

and in the Prickly 
Saltwort, Salsola 
kali), and serve for 
the storage of water, 
which is possibly 
alwaj/s a function of 
this layer, though 
here to a much 
greater extent than 
is normally the rule. 
In extreme cases 
water may be stored 
in localised enlarge- 
ments, which often 
project as water- 
containing hairs or 
bladders {e.g. the Ice- 
plant, Mesembryan- 
themum crystallinum, 
and the Silver Goose- 
foot, Obione portnla- 
coidcs, Fig. 44, A, //..). 
In times of drought 
the adjacent assimi- 
latory tissues with- 
draw moisture from 
these cells, which 
consequently con- 
tract, the side walls 
becoming undulated ; 
when water again be- 
comes plentiful, the 
cells fill and the walls 
graduallj' straighten 




Fig. 44. — Structure of the epidermis. A, 
Transverse section of leaf of Silver Goose- 
foot (Obione portulacoides) showing the 
bladder-like hairs [h.), the aqueous tissue 
(a.), and the palisade layer (p.) B, Sur- 
face section of leaf of Ground Ivy (Nepeia 
glechoma). C, Surface section of leaf of 
Onion. D, Surface section of stem of 
Runner Bean [Phaseolus innltifioriis). 
St., stomata. 

out. The epidermis 

is not uncommonly aided in the storage of water by the 

development of one or more additional layers, which may be 



EPIDERMIS 95 

formed by the division of the dermatogen {Ficus elastica), or by 
differentiation from the underlying periblem (as in the Holly, 
Fig. 43, A, Hy.). The cells of such a hypodcrm (cf. also p. iii) 
resemble the epidermal cells in the possession of large vacuoles 
and the absence of chloroplasts, but usually have thinner 
walls. It will be realised that in all cases leaf and stem are 
surrounded by a water-jacket tending to dmimish the heating 
effect of the sun. 

Another device serving for water-storage consists in the 
development of thick mucilaginous internal waUs (cf. p. 38) 
by the epidermal cells [e.g. Hollyhock, Sj-camore, etc.). These 
are inconspicuous in sections mounted dry, but swell up con- 
siderably, often projecting deeply into the assimilatory tissues 
[e.g. HoUyhock), when placed in water. This device tends to 
retard the evaporation of water during periods of drought. 

The pronounced thickening and cuticularisation of the outer 
walls of the epidermal cells, and the absence of intercellular 
spaces between them, endues the epidermis as a whole with 
considerable strength. It hence serves as a mechanical protec- 
tion, but also (in combination with the hj'poderm, when present) 
as a light-screen for the underlying chlorophyU. Moreover, the 
polished cuticle, especiaUj' characteristic of tropical plants, 
serves to reflect a large part of the light and heat rays faUing 
upon it. Most important of all, it checks evaporation from the 
general surface of the plant, and indeed, where the cuticle is 
thick, transpiration is almost entirely restricted to the special 
apertures or stomata which will be considered below. 

Certain modifications of the epidermis are associated wdth 
special physiological functions. Thus in many shade-loving 
plants the outer waUs of the epidermal ceUs are markedly convex 
[e.g. Wood Sorrel; Moschatel, Adoxa, Fig. 45). Where this is 
the case, each cell functions as a plano-convex lens focussing a 
localised patch of bright light at its base. It has been suggested 
that this acts as a mechanism for the orientation of the leaf, 
since, when the direction of illumination is altered, the position 
of the patch of light likewise changes. The protoplasm upon 
which this light falls is assumed to be sensitive, so that an 
adjustment of the leaf follows until the patch of light is again 
located in the normal position. But quite apart from any value 



96 



STOMATA 




that this may possibly have in enabhng the plant to place its 
leaves at an appropriate angle to the light/ the concentration 
of the latter may well be of value in connection with carbon 
dioxide assimilation. 

The general characteristics of stomata are readily studied by 
stripping off a piece of the epidermis from a fresh leaf of the Iris. 
Scattered among the colourless elongated cells are oval stomata, 
each consisting of two bean-shapecl guani-cells (Fig. 46, C, g.c. ; 
see also Fig. 44, C) surrounding the narrow elliptical pore by 
means of which the intercellular spaces of the leaf communicate 
with the atmosphere. The guard-cells contrast with the ordinary 
epidermal cells in containing numerous chloroplasts and starch- 
grains, and are especialty distin- 
guished by the uneven thickening 
'^^'^r^ ~~'°'\=^ '^^ their walls, which are thin on 

_ /(^ ^^ ^ ^ -~n!'— ^'^'^ ^^'^'^ away from the pore, but 

markedly thickened around the latter 
(Fig. 46, C). Each guard-cell has 
a well-developed protoplast and a 
prominent nucleus. The ordinary 
epidermal cells almost meet above 
the guard-cells, so that the latter 
are only plainly visible on focussing 
to a lower level (Fig. 46, C) ; in 
this way the pore comes to be 
situated at the bottom of a miiriature 
hollow (veslibtile) constituted by the surrounding cells, and the 
stoma consequently occupies a sheltered position. This feature 
is especially marked in plants of dry situations, but in those 
growing in damp, shady habitats the guard-cells are often level 
with or raised above the adjacent epidermis (Fig. 81, C, D). 

The overarching of the guard-cells by the adjoining epidermal 
cells is equally obvious in a transverse section (Fig. 46, D) of 
the Iris-leai. Here each guard-cell appears appro.\imately oval 
in form and provided with thick walls, the thickening being 
especially marked adjacent to the pore, and giving rise to a 
slight upwardly projecting ridge at the outer edge of the aperture. 
In many plants, however, the walls of the guard-cells are not so 

' a. ¥. & s., p. 212. 



Fig. z|5. — Transverse section 
through part of the upper 
epidermis and paUsade 
layer ot tlic leaf of the 
IMoschatel [Adoxa moscha- 
teU'nia). The chloroplasts 
are shown black. 



stomata 



97 



Uniformly thickened, ttiose remote from the pore being relatively 
thin, whilst those towards the aperture show a marked decrease 
in thickness opposite the middle of the pore {e.g. Onion, Fig. 
46, E) ; in such cases the outer ridges are well marked, and 
there is frequently a second pair of ridges on the inner side. 
As a result of this unequal distribution of the thickening, the 
cavities of the guard-cells taper abruptly in the direction of the 




Fig. 46. — Structure of stomata. A, C, F, and G, in surface view ; B. D, 
and E, in transverse section. A, B, Jlillet-Grass (MilUum effiisum). 
C, D, Iris gennanica. E, Onion (Allium cepaj. F, JIadder (Riibia 
peregrina). G, Sediitn spectabile. cii., cuticle ; ep., epidermal cell ; 
g.c, guard-cells ; s.c, subsidiary cells. 

pore, so that a ma.ximum thickness is obtained towards the 
upper and lower sides (Fig. 46, E) ; in some plants this is so 
marked that the whole ca^dty merely appears as a slit {e.g. Grass, 
Fig. 46, B). Beneath the stoma lies an air-space, the respiratory 
cavity (Fig. 51, R.), serving for the immediate interchange of gases 
and water-vapour between the intercellular system of the plant 
and the air around. 

The vertical leaf of the Iris, in which both surfaces arc alike, 
bears almost the same number of stomata on each. ■ Moreover, 

7 



98 STOMATA 

the latter are all placed parallel to the axis, as is usual for the 
elongated leaves of Monocotyledons (Fig. 44, C) and the epidermis 
of young stems ; in the latter case, however, the stomata are often 
few in number (Fig. 44, D). Those of horizontal dorsiventral 
leaves {e.g. of most Dicotyledons) are chiefly situated on the lower 
surface, being sometmies altogether absent from the upper [e.g. 
most trees), and, moreover, exhibit an irregular arrangement 
(Fig. 44, B). In some families the guard-cells are accompanied 
by so-called subsidiary cells (two in the Madder, Fig. 46, F, s.c, 
and Bedstraws, three or more in the House-leek and Stone-crop, 
Fig. 46, G, s.c.) which, differ in size and shape from the other 
epidermal ceUs, and probably form part of the mechanism of 
the stoma. 

The stomatal apparatus originates from a mother-cell which 
is cut off by means of a curved septum from one of the ordinary 
epidermal cells. Soon a vertical wall is formed parallel to the 
long axis of the mother-cell, separating the future guard-cells, 
which gradually acquire their distinctive thickening. Mean- 
while the middle lamella between them breaks down, except at 
the two ends, to form the pore. Subsidiary cells, when present, 
are usually cut off from the mother-cell before the guard-cells 
are produced, but in some cases they are formed by division of 
the surrounding epidermis. 

On mounting a strip of the fresh epidermis of some leaf in 
water, the open pores of the stomata are readily visible, being 
usually occupied by air. On transference to glycerine or a 5 
per cent, sugar solution, which will reduce the turgor of the 
guard-cells, the width of the pores decreases appreciably {i.e. 
they " close "), but when returned to water the turgor is restored 
and they again open. Measurement shows that, though there 
is usually no marked change in length, there is an appreciable 
increase in width, when the stoma opens (cf. Fig. 47). The 
alteration in form of the guard-cells, to which the variation in 
the size of the stomatal aperture is due, is thus primarily deter- 
mined by changes in turgescence. 

The mechanism is, however, directly dependent on the un- 
equal distribution of thickening which, in transverse section, 
has been seen to be mainly localised around the upper and lower 
edges of the pore ; on the other hand, the middle of the convex 



MECHANISM OF STOMATA 



99 



wall next to the pore, as well as the curved wall remote from 
the pore, remain comparatively thin (Fig. 46, E). When the 
guard-cells are turgid and the thin walls stretch, the pull exerted 
separates the thick walls, thus opening the pore (Fig. 47, op.). 
As seen in section, opening of the stoma is accompanied by a 
flattening of the con\-ex walls on each side of the pore and an 
increased convexity of the unthickened part. As a result there 
is movement of the guard-cells at the lines of junction with the 
adjacent epidermis both above and below, due to the bulging 
of the walls away from the pore. 
Where both the outer and the inner 
walls of the epidermal cells are 
thickened, there is a sudden thinning 
at the points of junction with the 
guard-cells to admit of their move- 
ment [e.g. Onion, Fig. 46, E) ; but 
when the external wall alone is 
thickened, such a hinge occurs only 
on the outside [e.g. Iris, Fig. 46, D). 
The flattening of the convex waUs 
adjacent to the pore is facilitated 
by the thinner middle portion, which 
constitutes a similar hmge. A good 
illustration of the action of a stoma, 
depending likewise on unequal dis- 
tribution of thickening, is afforded 
by the change in form of the two 
halves of a split Daffodil-scape, tied 
together at their ends with the outer sides in contact, and 
placed alternately in salt solution and water. ^ 

The influence of external conditions on the size of the sto- 
matal pore can be indirectlv studied by means of the porometer,^ 
or by the foUomng method adopted b}' Lloj'd : Leaves, gro\nng 
in the particular conditions to be studied, are detached from 
the plant and immediately placed in strong alcohol, which fixes 
the guard-cells in their momentarv condition. A strip of epi- 
dermis can now be removed without alteration, and the average 

1 Cf. F. & S., p. 117. 
' F. & S., p. 369. 




Fig. 47. — Half of a stoma, 
seen in perspective, show- 
ing the positions of the 
guard-cells in the open 
(op., firm lines) and closed 
(cl., dotted lines) con- 
ditions, ep., epidermis ; 
g., guard-cell; p., pore. 



100 MECHANISM OF STOMATA 

size of the stomatal apertures directly measured under the 
microscope (cf. Appendix VII). 

Either of these methods will serve to demonstrate the ten- 
dency of the stomata (except in many shade and marsh plants, 
e.g. the Water Plantain, Alisma plantago) to " close " when the 
shoot becomes flaccid owing to deficiency of water, although, 
as shown especially by the porometer, they often open more 
widely at the first commencement of wilting. " Closure " is 
also brought about b\' a change from light to darkness, which 
emphasises the fact that alteration in turgidity of the plant 
as a whole is not necessarily' involved. It may be noted that 
Fungi, which have no stomata, also exhibit diminished trans- 
piration in darkness. The sensitiveness of the stomatal mecha- 
nism is so great that even the effect of temporary shading, 
as by a big cloud, can be observed with the help of the porometer. 
Moreover, shaking of a leaf may cause a more or less marked 
temporary " closure," and for this reason it is best to allow a 
short interval to elapse, after fixing the porometer, before read- 
ings are taken. 

We thus realise that the stomatal mechanism furnishes an 
automatic control on the escape of water-vapour from the leaf. 
The rate of transpiration has been shown to decrease as the 
humidity increases, though it becomes zero only when the air 
is slightly supersaturated. 

The stomata are therefore highly irritable, and the ultimate 
cause for their response to different conditions must, at all events 
in part, reside in the living protoplasm of the guard-cells. It 
would seem that conditions causing " closure " of the stoma 
must lead to a reduction in the permeability of the protoplasm, 
and vice versa. In the case of the stomata present on the cap- 
sule of Mosses (cf. p. 284), it has been found that the guard-cells, 
when open, have nearly five times the osmotic pressure of the 
surrounding epidermal cells, but when " closed " have the same 
osmotic pressure. 

A much modified type of stoma is found in the Gramineae 
and Cyperacefe (Sedge-family), though the mechanism is essen- 
tially similar. The much elongated guard-cells surround a large 
pore having the form of a flattened hexagon (Fig. 46, A). In 
the middle portion of each guard-cell the outer and inner walls 



SPECIAL TYPES OF STOMATA— HAIRS loi 

arc so strongly thickened as to leave only a small slit-shaped 
cavity (Fig. 46, B), but the end-portions are thin-walled and 
somewhat enlarged (Fig. 46, A, g.c). On either side of the 
stoma is a thin-walled subsidiary cell {s.c). These latter, to- 
gether with the terminal portions of the guard-eeUs, function 
in much the same way as the thin-x^-alled part of an ordinary 
guard-cell, whilst the thick median portions correspond to the 
thickened walls of the latter. 

Submerged leaves of water-plants usually bear no stomata, 
whilst in floating leaves they are restricted to the upper surface, 
and exhibit marked differences from the ordinary type. The 
guard-ceUs, in transverse section, are here roughh" triangular 
through the inner walls being bevelled off towards the outer 
edge of the pore, where the thickening is most pronounced.' 
As the upper surface of such floating leaves is usually covered 
by a fine waxy bloom {e.g. Water Lily, Nyniphcea), this side of 
the leaf is not readily wetted, and hence the formation of water- 
films across the stomatal apertures is prevented. 

Not uncommi)nly a few, or even man^', of the epidermal cells 
(cf. Fig. 37) grow out into more or less elongated, often branched, 
processes called hairs, which are especially common on leaves 
and, when numerous, are very obvious to the naked eye. They 
may remain unicellular (Fig. 48, B, C), or become multicellular 
(Fig. 48, D) by the formation of septa whose development is 
probably related to mechanical requirements. Nearly every type 
of hair, whether branched or not, may be uni- or multi-cellular 
without any appreciable difference in outward form ; but the 
presence or absence of septa, as well as the structure of the hair, 
are often characteristic of whole groups of plants. The walls of 
the hairs are of varying thickness, and either consist of cellulose 
or have undergone chemical alteration. 

One of the most important junctions of hairs, when numerous, 
is to bring about a decrease in the rate of transpiration. The 
moist air entangled between these hairs is sheltered from the 
wind, so that it is not readily removed by air-currents, nor does 
it diffuse rapidly into the dry atmosphere around. As a result 
of the presence of this moisture-laden air in the immediate 
neighbourhood of the leaf-surface, transpiration of water-vapour 
from the interior through the stomata is retarded. These cover- 



102 



HAIRS 



ing hairs are usually dead and, at maturity, occupied only by 
air ; such dead hairs often appear white, showing that a large 
proportion of the hght falling upon them is totally reflected, 
hence they also afford protection against the heating effect of 
the sun and against excessive illumination. Hairs thus serve 
to reduce transpiration and act as a screen to the underlying 

chloroplasts. An 
analogy could be 
drawn with the 
interior of a wood- 
land, where be- 
neath the trees 
the air is cool, 
shady, and damp, 
just as it is be- 
neath the hairs on 
a leaf. Not un- 
commonly hairs 
are only present 
on the young leaf, 
falling off as the 
latter matures [e.g. 
1^1 an e. Horse 
Chestnut, etc.). 

The simplest 
type of covering 
hair is unbranched 
and usually tapers 
towards the tip 
(Fig. 48, B-D). 
Short stiff hairs 
of this kind are 
found in some Boraginacere [e.g. I"orget-me-not, etc.), but 
they are often much longer, and cither interwoven to form 
a woolly tangle [e.g. Coltsfoot, Thistles, young leaves of the 
Horse Chestnut, etc.) or all disposed in the same direction, giving 
a silky appearance to the surface of the leaf [e.g. Silverwced). 
Long unbranched hairs from the seed-coat of the Cotton-plant 
[Gossypium spp.), a member of the Mallow-family (Malvacea:-), 




Fig. 40. — Hairs. A, of Deittzia; B, of Cynoglos- 
sum ; C, of Shepherd's Purse (Capsella) 
(unbranched type) ; D, of Vegetable Marrow 
[Cucurbiia) ; E, of Hop [Humulus). 



HAIRS 103 

are the source of cotton. 1 They are unicellular and may reach 
a length of 2-5 centimetres and, in the plant's economy, serve 
for the dispersal of the seeds by the wind. Similar hairs, with 
the same function, occur on the seeds of the Willow, Willow- 
herb, and many other plants. - 

The epidermal ceUs of many petals are drawn out into very 
short processes, or papillce, which are the cause of the velvety 
surface and prevent wetting. A similar production of papillae 
is not uncommon on the stomatal surface of the leaves of tropical 
plants growing in damp situations [e.g. tropical rain-forest). 

Branched hairs assume very diverse forms, only a few of 
which can be mentioned. In the MuUein and Plane they are 
tree-hke, and consist of an erect multicellular axis from which 
numerous tapering branches radiate at 
intervals. More frequent are so-called 
stellate hairs, in which there is but one 
set of radiating branches terminating a 
short stalk which, however, is some- 
times practically absent [e.g. Deiitzia). 
Multicellular hairs of this kind are 
typical of the Lime-family (Tiliacece) 
and Mallow-family (Malvacere), whilst Fig. 49.— Peltate hair of 

. ,, , 1-/: J • Sea Buckthorn (Hit- 

uniceUular ones are exemplihed m , , , , -^ x 

^ pophce rhamnoides). 

Deidzia (Fig. 48, A). Stellate hairs are 

also found in the Cruciferfe, but here they are accompanied by 
simpler forms with onh' two or three branches and by un- 
branched hairs. 

A very efficient transpiration-check is afforded by the peltate 
hairs, which are well seen in the Sea Buckthorn [HippcphcB, 
Fig. 49), and in Elaagnits, where, owing to the large number of 
these scale-like structures, the under-surface of the leaf has a 
characteristic silvery appearance. The short stalk of these hairs 
is surmounted by a horizontal expansion, consisting of a large 

1 The walls of these hairs are practicall}' pure cellulose, ilany hairs, 
which from their length might be used for the manufacture of textiles, 
are slightly lignified, and consequently too brittle to spin. Kapok of 
commerce is obtained from the slightly cuticularised hairs lining the fruit 
of the Silk-Cotton tree {Eriodendroii aiifractiiosiiiii}, wh-.ch is common in 
the Tropics. 

2 SeeF, & S., p. 286, 




104 HAIRS 

number of unicfllular rays which arc joined together at their 
base but free at their tips. 

Certain climbers are materially aided in clinging to their 
support by the possession of stii^ hairs. Thus the ridges on the 
stem of the Goosegrass {Galium aparinc) bear numerous reflexcd 
unicellular hairs, shaped like a hook and having a stout base 
and a very strongly thickened tip. In the Hop the ridges are 
similarly beset by stiff two-armed hairs seated on a small elevation 
of the epidermis ; the two arms arc situated in the vertical plane, 
and the downwardly directed one is considerably longer than 
the other. Extreme types of multicellular climbing hairs arc 
furnished by the prickles of the Rose and Bramble. 

Whilst the walls of the ordinary covering hairs are generally 
not appreciably thickened, both branched and unbranched hairs 
may possess thick walls which are frequently silicified or calcified ; 
when thus stiffened, they constitute a chcvaitx-dc-jrisc against 
small animals {e.g. slugs). Good examples are afforded bj' the 
unbranched bristle-hairs of many Boraginacese {e.g. Borage, 
Comfrcy, etc.) and the branched types found on the Stocks and 
other Crucifera;. Their effect is often accentuated by the pres- 
ence of numerous minute teeth on their surface (Fig. 48, A). 

One of the most striking examples of hairs acting as a de- 
terrent to animal attacks is, however, furnished by the uni- 
cellular stinging hairs of the Nettle {Urtica) (Fig. 50, D). Each 
is borne on a multicellular stalk in which is embedded the thin- 
walled swollen base of the actual stinging hair. The upiper part 
of the latter is comparatively thick-walled and tapers gradually 
to near the apex, where it suddenly enlarges to form a tiny 
bead-like tip (Fig. 50, E). The lower part of the wall is calcified, 
the upper part silicified. The living protoplasmic contents often 
show distinct streaming movements, and include a large vacuole 
fihed with acrid sap. When an animal brushes against one of 
these hairs, the little tip breaks off, leaving exposed a fine needle- 
like point formed by the upper tapering part of the hair. As a 
result of the pressure of contact, this fine tube penetrates the 
skin, and the compression of the bladder-hke base injects the 
contained fluid into the wound. 

The hairs of man3.' plants produce secretions ' which are often 
' Walt'"- and sugar-secreting hairs are considered in Cliaptcr Xll. 



GLANDULAR HAIRS 



105 



of the nature of ethereal oils (cf. p. 621. Such glandular hairs 
are multicellular and generaUy consist of a basal cell, which is 
usually sunk in the epidermis, a projecting stalk, and a glandular 
head (cf. Fig. 71, g.h.), but are othem-ise of very diverse form. 
In the case of the Chinese Primrose {Primula sinensis, Fig. 50, 
A-C) and the Garden Geranium (Pelargonium], the head is 
formed by a single ceU and the stalk by a varying number of 
cells. In the Labiatfe (e.g. White Deadnettle) the head is com- 




FiG. 50. — Glandular hairs of Chinese Primrose (Primula sinensis, A-C) 
and Stinging Hair of Xettle (Vytica, D, E). In A-C the secretion 
is shaded, and in C and E only the greatly magnified tip of the hair 
is shown. 



posed of four or more ceUs disposed in a plane parallel to the 
surface of the leaf. Extended division of the head leads to the 
peltate type of glandular hair, such as is seen in the Hop (Fig. 
48, E) and the Black Currant. 

All the cells of such glandular hairs are living, those of the 
head which are concerned in active secretion being specially 
characterised by dense protoplasmic contents and large nuclei. 
Small droplets of secretion can often be recognised ^^ithin the 
3'oung glandular cells, but in the mature condition the ethereal 
oil is found deposited between the cuticle and the ceUuIose-layer 



io6 GLANDULAR HAIRS 

of the outer wall (Fig. 50, B), so that after solution of the oil 
b}' means of alcohol a space is evident beneath the cuticle. The 
volatile oils produced by these glands are the cause of the fra- 
grant perfume of many herbs [e.g. Lavender), and in some cases 
play apart in the reduction of transpiration (cf. p. 64) or render 
the plants distasteful to animals. 



CHAPTER IX 

The Structure of the Leaf 

The blade of a dorsiventral foliage leaf exhibits the following 
general structure in transverse section. Beneath the colourless 
epidermis (Fig. 51, Ep) of the upper side are one or more laj'ers 
of vertically elongated cells constituting the palisade tissue [Pa), 
which is especially concerned with carbon dio.xide assimilation ; 
its cells are deep green owing to the numerous chloroplasts. 
Between the palisade layer and the lower epidermis lies the 
loose " spongy " tissue (Sp.), which is composed of irregular cells 
separated by many and often conspicuous intercellular spaces 
[In.). This tissue contains fewer chloroplasts and communicates 
with the external atmosphere by way of the stomata, its chief 
function being to facihtate gaseous exchange. Pahsade and 
spongy tissues together constitute the thin-walled ground-tissue, 
or mesophyll, of the leaf. Here and there the section will pass 
through veins, some cut transversely, others obliquely or longi- 
tudinally ; the veins include the vascular tissue with xjdem 
towards the upper and phloem towards the lower side, and each 
is surrounded b3' a well-defined layer of cells, the hmdle-sheath 
[Sh). 

Good material for a detailed study of the structure of an 
ordinary leaf is furnished by the Fuchsia. The features of the 
epidermis have been fuUy described in the previous chapter and 
require no further mention . The palisade cells are four to six times 
as long as broad and form a single layer (Fig. 51, Pa). They are 
attached on the one hand to the upper epidermis, and on the other 
to the rounded ceUs constituting the uppermost layer of the 
spongy parenchyma. Narrow intercellular spaces, extending the 
whole depth of the palisade laj'er, occur at intervals between 
the cells, but these spaces are only apparent here and there 

107 




■O OS 



&. 



u 








1' 












rrl 










ir.i 






o 








iTl 




X 




rt 


^ 




d 


Oi) 




cfi 


r'r 


Ti 


- 


rf 
O^ 


u^ 


I/l 


,1^ 


^ 


rt 










■n 


d 








k. 


d 


r 1 



s «> 














u. 


rri 


rrt 












H 


rfi 


U 




? 


> 


i-i 








CJ 


O 


[^ 












t^ 







jo8 



PALISADE TISSUE AND CHLOROPLASTS 109 

in the transverse section (cf. Fig. 51, Pa.). In sections 
parallel to the surface of the leaf the pahsade cells appear 
rounded (being cut transversely, Fig. 52, A), so that they have 
the form of a number of closely packed cylinders placed side by 
side and interspersed with regularly disposed vertical spaces (i.p.), 
where the curved surfaces are not in contact. 

The numerous lenticular cliloroplasts ' form an almost con- 
tinuous layer in the cytoplasm lining the vertical walls, a feature 
well seen in 
both trans- 
verse and sur- 
face sections 
(Fig. SI, Pa.; 
Fig- 52, A ; 
see also Fig. 
43). This 
peripheral 
position is 
clearly favour- 
able to the 
rapid absorp- 
tion of carbon 
dioxide from 
the adjacent 
intercellular 
spaces. More- 
over the 
chloroplasts, 
as a result, present their edges to the light so that the chlorophyll 

1 For the general properties of chlorophyll, see F. & S., p. 13c. It is 
now known that the chloroplasts of all plants contain two green (chloro- 
phyll a and chlorophyll 6) and two yellow pigments (carotin and xantho- 
phyll), the former being present in considerably greater quantity than 
the latter. The chlorophjdls, which differ but slightly from each other, 
are complex compounds of Carbon, Hj'drogen, Oxj-gen, Nitrogen, and 
Magnesium, whilst the j-ellow pigments are of simpler composition ; 
carotin has the formula C40H56, and xanthophyll the formula CjoHjoOo. 
A rough separation of the green and yellow pigments may be effected by 
shaking up an alcoholic extract with benzene and allowing the liquids to 
settle ; the alcohol then contains the jrellow, the benzol floating above it 
the green, pigments. 




Fig. 



52. — Sections parallel to the surfaces of a Fuchsia 
leaf, cut respectively through the palisade laj^er 
(A) and through the spongy parenchyma (B). In 
each case the lower figure shows a small portion 
on an enlarged scale, i.p., intercellular spaces. 



liO SPONGY TISSUE 

is protected from the injurious effects of intense illumination. 
Owing to the considerable length of the palisade cells, there is 
accommodation for a large number of chloroplasts in each. In 
some plants, however, the chloroplast-bearing surface is increased 
by the development of special infoldings of the wall [arm-palisade , 
e.g. in the leaves of the Elder (Fig. 53, a.p.) and White Lily]. 

In the transverse section of the leaf of the Fuchsia two or 
three palisade cells frequently join by their lower ends on to one 
and the same cell of the spongy parenchyma (Fig. 51 ; see also 
Fig. 53, c.c). The cells in question are usually broadened at 
their upper ends, so that they are more or less funnel-shaped. 
Such cells are usually observed in leaves having the kind of 
structure here described ; in fact more than three palisade cells 
may be found thus connected with one spongy cell {e.g. India- 
rubber plant, i^j'cMS elastica ; Oleander, Nerimn oleander, Fig. 82, 
Col). It is probable that the assimilation-products (carbo- 
hydrates, etc.) formed in the palisade cells pass into these collecting 
cells and from them diffuse, via other spongy elements, to the 
veins. 

The spongy parenchyma, in its most typical form {e.g. in 
Euphorbia amygdaloides), consists of irregularly lobed cells 
attached to one another by their projecting arms (cf. Fig. 82), 
so that wide intercellular spaces occur between them. In the 
leaf of the Fuchsia and in many other cases, however, the cells 
are more rounded and the interspaces consequently smaller (Fig. 
51, In. ; Fig. 52, B). The rather few chloroplasts in the spongy, 
as compared with the palisade, parenchyma may be related to 
the fact that the former tissue receives relatively little light. 
The layer in contact with the lower epidermis is not uncommonly 
continuous (except for the gaps constituted by the often large 
respiratory cavities beneath the stomata. Fig. 51, R.), and its 
cells may even show a palisade-like form ; in this case they 
generally contain rather numerous chloroplasts {e.g. Fuchsia, 
Corn Cockle), and serve to utilise the light reaching the 
under-side of the leaf. 

The chief function of the spongy tissue is to constitute an 
extensive intercellular system communicating on the one hand 
with the atmosphere by way of the stomata, and on the other 
hand with the entire aerating system of the rest of the plant. 



DORSIVENTRAL LEAVES 



III 



The spongy cells also serve to conduct elaborated food-materials 
in various directions to the adjacent veins, a function to which 
the}' are ^^■ell suited bv virtue of their irregular shape. 

The structure of the mesophvl! just described is characteristic 
of dorsiveniral leaves generallv, the foUowing being the chief 
modifications. The palisade tissue not uncommonl\' consists of 
several layers (e.g. Wallflower, Holh', Fig. 43, A), a feature 
especiall}' encountered in leaves exposed to strong illumination, 
whUst leaves developed in dull light may have httle or no pahsade 
tissue. In 
some cases 
[e.g. the 
House -leek) 
the assimila- 
torv* cells have 
quite a differ- 
ent form, being 
elongated 
parallel to the 
midrib, whilst 
in transverse 
section they 
appear more or 
less rounded. 
Plants grow- 
ing in dn." 
sunny situa- 
tions often ex- 
hibit a second layer of colourless cells beneath the upper 
epidermis. The cells of this hypodenn (p. 95, Fig. 43, Hy.) are 
usuallv thick-walled, and consequently form with the epidermis 
a strong surface skin, besides further screening the underMng 
chloroplasts from excessive illumination. 

Variegated leaves usually exhibit a similar structure to that 
of the normal foliage of the same species, except that locahsed 
areas show no development of chlorophyll ; these areas in con- 
sequence appear white or yellow. It need hardly be said that 
the structure of reduced fohage-leaves [e.g. scale-leaves of rhi- 
zomes, bud-scales, etc.) is of a much simpler character. Such are 




Fig, 



53. — Transverse section through part of the leaf 
of the Elder (Sambucus) showing the upper 
epidermis (e.) ; ■ the arm-palisade cells [a. p.) ; the 
collecting cells {c.c.) ; and the spongy parench3-ma 
[s.p.). i., intercellular space. 



112 VASCULAR SYSTEM OF LEAF 

usually colourless with a homogeneous mesophyll, and often 
possess no veins. 

Movement of chloroplasts in conformity with the intensity of 
the illumination is well seen in a few plants {e.g. Duckweed, 
Lenina ; Moss-leaves, etc.). In these cases the chloroplasts 
occupy a profile position on the vertical walls when the light is 
intense, whilst when weak the chloroplasts pass to the horizontal 
walls, so that their full surface is presented towards the source of 
illumination. Similarly the leaves of some plants [e.g. Bracken ; 
Yorkshire Fog, Holeits mollis) take up fixed light-positions, at 
an oblique angle to strong sunlight, so that much of this is 
reflected from the leaf-surface and the chloroplasts are con- 
sequently protected. When growing in shady situations, 
however, the leaf -blades are placed at right-angles to the in- 
cident rays. 

The vascular tissue of the leaf is very extensive, forming 
an irregular network (reticulate) in Dicotyledons and a very 
regular (parallel) system in most Monocotyledons.'- The repeated 
branching facilitates not only the delivery of water and mineral 
salts to all parts of the leaf, but also the rapid removal of elabor- 
ated food-substances. The vascular system, however, also con- 
stitutes a supporting skeleton for the lamina, in which it is often 
aided by accompanying mechanical tissues ; the latter arc found 
especially in the larger veins, and consist of strands of collenchvma 
(Fig. 51, M .) or sclerenchyma, which run both above and below 
the vascular bundles or sometimes on the lower side onl}'. 

A transverse section through one of the prmcipal veins of 
the Fuchsia-lcdl shows a single collateral bundle (cf. p. 76) 
enveloped in a sheath of one or more layers of large transparent 
thin-walled parenchyma-cells (Fig. 51, Sh. ; cf. also Fig. 82, V .) ; 
the accompanying collcnchyma (A/.) is developed especially on 
the lower side. The xylem, which is adjacent to the palisade 
tissue, consists of rows of vessels alternating with wood-paren- 
chyma, the protoxylem being directed towards the upper epi- 
dermis ; the phloem is of the normal type and lies towards the 
lower side. Between xylem and phloem a cambium can often 
be recognised (Fig. 51) ; this is especially the case in evergreen 

1 The Cuckoo-pint (Ai'iim niciiiilalini/) and Rlack Bryony (Tcdhus com- 
munis), f<jr instance, have a venation similar to tliat of Dicotyledons. 



VASCULAR SYSTEM OF LEAF 



11^, 



leaves [e.g. Holly) which remain on the plant for more than one 
year. 

In passing to the finer and finer ramifications of the vascular 
system a gradual simplification in structure is apparent. The 
differentiation of the phloem becomes less and less distinct, its 
place being taken by a more or less uniform tissue of thin-walled 
elongated cells, whilst at the ultimate terminations of the bundles 
it often disappears 
completely. Simi- 
larly the xylem- 
vessels gradually 
give place to re- 
latively short spiral 
or reticulate tra- 
cheids (Fig. i8. A, 
p. 37), the amount 
of wood-paren- 
chjnna diminishing 
till it dies out. 
Thus the bundle- 
ends usually consist 
only of tracheids 
surrounded by the 
sin gl e -1 a y ered 
parenchyma- 
sheath. 

In larger leaves 
[e.g. Sunflower) the 
midrib often con- 
tains several 
bundles, and ex- 
treme cases are seen in the Docks (Rumex) and the Rhubarb 
{Rheum rhaponticiim) , where quite a large number of strands 
occur. On the whole the vascular supply is proportional to 
the size of the leaf, and this is true also of that of the petiole. 
In small leaves [e.g. Mouse-ear Chickweed, Cerastiimi, Fig. 55) 
the latter frequently contains but a single vascular strand 
orientated as in the lamina, whilst in larger ones there are 
several bundles usually grouped in the form of a C with the 
8 




Fig. 54. — Diagrams showing the petiolar struc- 
ture of A, Sea Holly [Eryngiiwi maritinium) ; 

B, Dog's Mercury {Mercurialis perennis) ; 

C, Black Bindweed (Polygonum convolvulus) ; 

D, Bishop's Weed {/Egopodium podagraria). 
The shaded part of the bundles represents 
xylem, the unshaded part phloem. Sc, 
sclerenchj'ma. 



114 CONNECTION BETWEEN TEAF AND STEM 

opening towards the concave or flattened upper surface and with 
the protoxylems directed uiwards (Fig. 54, A-D). The part of 
the ground-tissue inchided within tire group of strands is spoken 
of as pitli, and that outside as cortex, the peripheral laj/ers of 
the latter not uncommonly consisting of 
mechanical tissue (Fig. 54, Sc). 

The bundles of the petiole can be traced 
backwards some little distance into the 
cortex of the stem, following a slightly 
oblique course, so that, in sections cut 
transversel}' just below the nodes, the one 
or more bundles {leaf-trace bundles) passing 
from the leaf into the stem appear cut 
''*W^ \ ^ W^ obliquely in the cortex. After penetrating 
\ '1 . r;'I #7 some little way into the latter the bundles 

turn abruptly downwards and run vertically 
through one or more intcrnodes, ultimately, 
with (Fig. 55) or without previous branch- 
ing, fusing laterally with strands derived 
from other leaves. The bundles traversing 
the stem are therefore merely downward 
continuations of those found in the leaves, 
i.e. the bundles of stem and leaf are common. 
The vascular strands passing to the axillary 
buds (cf. Fig. 69, p. 142) are branches of 
those serving the leaves, whilst the vascular 
supply of stipules originates in a similar 
manner. 

In Monocotyledons, where the sheathing 
leaf-bases completely encircle the axis, 
numerous strands pass from each leaf into 
the stem. The median \-ascular bundles 
pass almost to the centre of the stem 
before thej' bend downwards ; subsequently they pursue a 
longitudinal direction, at the same time returning obliquely 
towards the periphery. The more laterally placed strands 
follow a similar course, but do not penetrate to the same 
depth. As a result the vascular bundles from the successive 
leaves usually appear irregularly scattered through the cross- 




FiG. 55. — Diagram- 
matic representa- 
tion of the vas- 
cular system in a 
small portion of 
the stem of the 
Mouse-ear Chick- 
weed (Cerastium) 
(modified from 
Prantl). Only the 
bases of the 
leaves, with a 
single vascular 
strand, are shown. 



MECHANICS— LEAF-MOVEMENTS 1I5 

section (cf. p. 84), but tend to be more densely crowded near the 
outside (cf. Fig. 39, p. 84). The bundles are thus again common 
to stem and leaf. 

The customary disposition of the mechanical elements in 
petiole and midrib in the form of an arc admits of considerable 
flexibility under the influence of wind, etc., but at the same time 
is well suited to withstand the vertical strain caused by the 
weight of the leaf-blade. In peltate leaves {e.g. Marsh Penny- 
wort, Hydrocotyle ; Garden Nasturtium, TropcBolum, etc.), where 
the strain is equally distributed, the petiole, however, exhibits 
a corresponding radial construction, as in stems. The broadly 
expanded lamina of the ordinary leaf is exposed to risk of injury 
under the tearing action of wind and hail, but this is to a large 
extent obviated by the strengthening network formed by the 
veins, and especially by the occurrence of marginal mechanical 
elements. The latter either form arched connections running 
parallel to the leaf margin, between the finer vascular strands 
[e.g. Red Currant), or bundles of fibres {e.g. Iris) occupying the 
same position. Moreover, the epidermal cells at the edge of the 
leaf are usually especially thickened and have a pronounced 
cuticle . 

In leaves which normally exhibit sleep-movements ^ {e.g. False 
Acacia, Robinia pseudacacia ; Runner Bean, Phaseolus miilti- 
floriis ; Sensitive Plant, Mimosa piidica), and in which the 
petiole takes up different positions in darkness and in light, the 
swollen leaf-base, or pulvinus (Fig. 56,/).), is the region of move- 
ment. This is facilitated by a flattening of the vascular 
tissue in a plane at right angles to the direction of curvature 
(Fig. 56, Pu.), the necessary rigidity being attained by a greater 
development of the cortex, which consists of large, turgid, thin- 
walled parenchyma-cells. The fall of the leaflet at dusk is 
accompanied by a decrease in the turgidity of the cortical cells 
on the lower side of the pulvinus, whilst at the same time some 
of their watery sap escapes into the intercellular spaces. This 
is due to a change in the permeability of the protoplasm and, as 
a result of the infiltration of the intercellular spaces, the whole 
pulvinus acquires a more transparent, deeper green appearance. 
At dawn the cells of the pulvinus once more become turgid and 
1 Cf. F. & s. p. 221. 



ii6 



LEAF-MOVEMENTS 



the leaf assumes the day -position. In the case of the Sensitive 
Plant (Mimosa pudica, Fig. 56, B) a similar but very sudden 
fall in turgiditv results when the leaf is touched or injured. 

The leaves so far considered exhibit a marked difference of 
structure and appearance between the upper and lower surfaces 
(dorsiventral type), but this is not always the case. In the Iris 




Fig. 56. — A, Pulvinus of J<unner Bean [Phascolus) and diagrams of trans- 
verse sections of tlic pulvinus {F'lt.) and the petiole {Pe.). B, Pulvinus 
of Sensitive Plant IMiniusci pudica). p., pulvinus ; s., stipules. 



and other Monocotvledons, where the leaf -blades stand vertically, 
the structure is identical on the two sides (isobilateral type). In 
still other cases, particularly in succulents among British plants 
(e.g. Wliite Stonecrop, Scdum album), the leaves arc more or less 
cylindrical, and, apart from the dor5i\"entral arrangement of the 
vascular l)undles, exhibit radial organisation (Fig. 86, p. 165), 
with a palisade la\X'r extending uniformly round the periphery. 



CHAPTER X 

Secondary Thickening 

It was pointed out in the preceding chapter that the vascular 
supply of the leaf is roughlj' proportional to its size (p. 113), 
and in the same way the vascular system of the stem is correlated 
with the area of leaf-surface which it bears. With the annual 
increase of foliage exhibited by all woody perennials, a need for 
additional conducting elements arises, and this want is supplied 
through the activity of a mcristem (the cambium, p. 79) situated 
between the xjdem and phloem of the bundles. Cambium is 
found in this position in all Dicotyledons and Conifers (see p. 340), 
but in Monocotyledons is present only in certain cases and as a 
rudimentary vestige (e.g. in the leaf-sheaths of the Maize and in the 
leaves of many other Grasses). The division of the cells of the 
cambium leads to the formation of additional conducting ele- 
ments, accompanied by a gradual increase in the size of the 
stem, spoken of as secondary thickening. Enlargement does not, 
however, always imply cell-division, since in Palms, where no 
active cambium occurs, there is increase in girth mainly due to 
growth of the cells alread}' present. 

The cambium may be regarded as arising from an unaltered 
remnant of the procambial strand (cf. p. 90), which has retained 
its powers of division, but until it becomes active it is difficult 
to recognise. The actual cambium is established by the appear- 
ance of two parallel tangential walls in the persisting procambial 
elements. There is thus cut out a single laj'er of radially flat- 
tened cells (Fig. 57, C.) which, as seen in longitudinal section 
(cf. Fig. 38, Ca., p. S3), have an elongated form and pointed ends. 
Subsequent division of these cells takes place parallel to the two 
tangential faces whereby files of segments are produced, both 
on the outer and inner side, those adjacent to the phloem be- 

117 



ii8 



CAMBIUM 



coming differentiated as additional (secondary) phloem, those 
adjacent to the xylem as additional (secondary) xylem. 

Subsequent to the development of the cambium within the 
bundles a similar division by two parallel tangential walls takes 
place in certain of the cells of the medullary rays, and, where a 

starch-sheath is 
present, such 
divisions will be 
recognised as 
occurring in the 
pericj'clic layer 
(e.g. hj'pocotyl 
of the Castor 
Oil plant. Fig. 
57, I.e.). In 
this way a cam- 
bium between 
the bundles 
[ititei'faseieular, 

Fig- 57. I-C-) 
links up with 
that within the 
bundles [intra- 
faseieular, Fig. 
57, C.) to form 
a complete cam- 
bial ring. In 
some plants, 
however, the 
formation of 
inter- and intra- 
fascicular cam- 
bia takes place 
almost simultaneously. The subsequent di\-ision of tlie inter- 
fascicular cambium is like that of the intrafascicular, so that 
the whole cambial cylinder cuts off segments on both sides. 
In woody perennials the cambium resumes its function each 
year, although division is arrested during the winter months 
(cf. below). 




Fig. 



57. — Transverse section of a small portion of 
the hypocotyl of the Castor Oil plant (Ricimis 
communis). C. , intrafascicular cambium ; Co., 
cortex; Coll., coUenchyma ; F., pcricyclic fibres ; 
i.e., interfascicular cambium ; P., parench>'ma ; 
Ph., phloem ; Px., protoxylcra ; S.s., starch 
sheath. 



GENERAL FEATURES OF SECONDARY THICKENING 119 

The secondary wood thus added on the inside forms a larger 
and larger core each year (Fig, 58). Since it is composed of 
hard persistent tissue, there is practically no compression of the 
wood, which progressively accumulates, so that the increase in 
girth of the stem serves as a rough measure of the amount of 
tissue added. To this enlargement the secondary phloem con- 




FiG. 58. — Diagram showing the arrangement of tissues in a transverse 
section of a woody trunk about tAventy-four years old. The large 
vessels of the spring wood of each annual ring are shown as black 
dots. b., bark ; c, re.gion of cambium ; p., pith ; Ph., secondary 
phloem ; pr., primary ray ; Sy., secondary ray. 



tributes but little, since this tissue is mainly thin-walled, and 
the outer earlier-formed elements become compressed more and 
more, as a consequence of the increasing pressure resulting from 
the growth of the wood and the annual formation of intercalated 
phloem. The cambium keeps pace with the enlargement of the 
circumference of the secondary wood by tangential stretching and 
occasional radial divisions in its cells. 

The pressure on the outer tissues, due to the interpolation 



120 GENERAL FEATURES OF SECONDARY THICKENING 

of secondary vascular elements between the primary xylem 
and phloem, becomes more and more marked as the years go 
b}' and its effects are most pronounced in the cortex. Moreover, 
the steady increase in size of tire woody core results in a gradual 
enlargement of its circumference, so that the softer tissues beyond 
become tangentially stretched. This tension can be readily 
demonstrated by making an extended vertical incision through 
the cortex of a three-j'ear-old twig of the Ash. The edges of the 
cut are seen to separate immediately owing to transverse con- 
traction of the thin-wahed tissues. In nature the tangential 
tension is exemplified by the irregular longitudinal fissures which 
arc so marked a feature of the older bark of many trees. 

At certain points the segments cut off from the cambium on 
either side develop into parenchymatous cells which differ from 
the other elements of the wood and phloem in being radially and 
not longitudinally elongated (Fig. 60, A, m.r.). In transverse 
sections of a secondarily thickened stem these medullary rays 
appear as a number of radiating streaks, one or more cells in 
width (Fig. 58, pr. and Sr). Some of these rays extend from 
cortex to pith [pr.) and, since they correspond in position to the 
original rays between the vascular bundles, are called priumry 
medullary rays; others (the secondary medullary rays, Sr), 
however, though traversing the greater part of the secondary 
phloem, penetrate only to a varying depth into the secondary 
wood. In some plants [e.g. Aristolochia) the primary rays are 
very wide, and practically the whole of the interfascicular cambial 
strips participate in their formation. 

The secondary wood is composed of four main types of ele- 
ments, viz. vessels, tracheids, wood-fibres, and wood-parenchyma, 
but transitions between the different types are not infrequent. 
The component elements often exhibit a distinct radial arrange- 
ment [e.g. Horse Chestnut), though this may be somewhat 
obscured when the vessels are large and numerous [e.g. Lime, 
Elm). The vessels of the secondary wood usually bear denselj' 
crowded bordered pits (Fig. 60, V; Fig. 17, E, b, p. 35), often 
arranged in distinct vertical series, and not uncommonly show 
additional reticulate or spiral thickenings deposited on the inner 
surface of the wall. The septa are frequently placed obliquely 
(Fig. 60, C, V) with reference to the radial plane, a feature which 



SECONDARY WOOD 



121 



can be related to the peculiar form of the cambial segments from 
\vhich the vessels are derived. The perforations are varied 
(cf. p. 35 and Fig. 17, D-F), but the type in which a number 
of cross-bars remain is commoner than in the primarj' wood. 
Tracheids (cf. p. 37) differ from vessels in being derived from 
single segments of the 
cambium, which show no 
open perforations in the 
end-walls ; they are usually 
much shorter than the 
vessels, but of about the 
same \\idth. The vessels 
and tracheids are the 
water-conducting elements 
of the secondary wood, and 
the former are generally 
much more numerous than 
the latter ; in fact in some 
cases there are practically 
no tracheids (e.g. ^^'illow). 
The typical wood-fibres 
closel}' resemble those de- 
scribed on p. 33, having 
pointed ends and more or 
less thickened walls, which 
are provided with oblique 
slit-shaped simple pits (Fig. 
60, /). Such fibres are 
connected by transitions 
with others which bear 
bordered pits with oblique 
slits (e.g. Beech), and differ 

but little from the fibrous tracheids of the Conifers (see p. 340). 
In the typical wood-fibres the walls are lignified and the contents 
dead; but , fibre-like cells with living protoplasts occur in the 
secondary wood of the Sycamore and of many herbs. Such fibres 
are occasionally septate (e.g. Vine) and constitute transitions 
to wood-parenchyma. Where vessels are in contact with the 
ordinary mechanical fibres, pits are not developed on the walls. 




Fig. 



59. — Transverse section of the outer 
part of the stem of the Horse Chest- 
nut IJEsculus) showing the secondary 
phloem (Sec. ph.) and the cambium 
(Cam.), c.c, companion cells ; /, 
fibres of pericycle ; ^I.r., medullary 
ray ; P. ph., primarj- phloem ; s.t., 
sieve-tube. 



122 MEDULLARY RAYS 

The wood-parenchyma cells of the secondary xylem resemble 
those of the primary xj'lem, in form and in the possession of a 
living protoplast ; the walls are, however, commonly thicker 
and often lignified, the horizontal ones showing prominent pitting 
(Fig. 60, p). Wood-parenchyma cells may be generally dis- 
tributed throughout the wood [e.g. Birch, Beech), or more especi- 
all}' confined to the autumn-formed wood and the immediate 
vicinity of the vessels, sometimes completely ensheathing them 
[e.g. Ash). The parenchyma in contact with the vessels bears 
simple pits corresponding in position to pits of the vessel wall, 
which latter are here either simple [e.g. Oak) or bordered [one- 
sided bordered pits), like the remaining pits of the vessel wall. 

The structure of the niedidlarv rays can only be fully appre- 
ciated by a study of their appearance in transverse, radial, and 
tangential, longitudinal sections. In the transverse section the 
cells, except for the fact that their long axis is placed radially 
and not vertically, are very similar to those of the wood-paren- 
chyma, though not uncommonly having somewhat thinner walls. 
The latter bear simple pits which are often particularlj' numerous 
on the tangential walls (Fig. 60, B; Fig. 62, m.). The simple 
pits of the radial walls and the bordered pits of the vessels com- 
bine to form one-sided bordered pits like those described above. 

Attention has already been drawn to the variable width of 
the rays. In general the secondary medullary rays are narrow 
and often only one cell wide ; the primary ones in most trees 
do not greatly exceed the secondary in width, but in many herbs 
the contrast is extremely marked. Each ray is a plate of cells 
of which the full vertical extent is seen in tangential longitudinal 
sections (Fig. 60, C, in.r.). The secondary medullary rays are 
very limited in this direction, rarely exceeding ten or twelve 
cells in height, whilst the primary rays usually extend through 
an entire internode. Each ray, as seen in tangential section, is 
somewhat spindle-shaped as a result of the tapering of the cells 
at the upper and lower margins (Fig. 60, C, m.r). 

In radial longitudinal sections the secondary rays are cut 
parallel to their flat faces, and appear as so many narrow paren- 
chymatous strips passing at right-angles across the longitudinal 
grain of the wood (Fig. 60, B). The detailed structure of the 
ray here somewhat resembles that of a brick wall, the alternation 



M .i 










V2 



123 



124 LIVING ELEMENTS OF SECONDARY WOOD 

of the cells simulating the "bond" of the bricks. The com- 
ponent cells show plainly the radial elongation referred to above, 
but sometimes those at the upper and lower margins are short 
and not markedly lengthened (e.g. Willows). The radial, like 
the vertical, extent of the secondary rays is seen to be limited 
(cf. p. i2o), the rays penetrating inwards from the cambium for 
varying distances. Should the radial section pass through one 
of the primary rays, the latter will be seen to show a far greater 
development, both in the vertical and radial directions. 

Apart from the occasional presence of fibres with protoplasmic 
contents, the wood-parenchyma and the meduUarj' rays are the 
only living constituents of the secondary wood. The living 
cylinder constituted by the phloem and cortex is thus connected 
with numerous inwardly directed plates of living tissue, the 
medullary rays. Of these, however, only the primary ones 
extend to the pith, where they are linked up b)/ a second cylinder 
of living cells, the medullary sheath (p. 79), which invests the 
inner margins of the primary xylem strands. The radiating 
plates formed by the secondary medullary rays are, however, 
not isolated, even where they pass between the dead elements 
of the secondary wood, since they are connected both vertically 
and horizontally bj? bands of wood-parenchyma cells. 

The mass of vessels and fibres is thus permeated by a con- 
tinuous system of living elements connected with the food- 
conducting tissues. The functions of this system are twofold : 
firstly, to conduct elaborated food-substances to the cambial 
region, the living cells of the wood, and the medullary sheath; 
secondly, at certain times of the year, the cells serve for the 
storage of food-substances, e.g. starch, as can be shown by the 
application of iodine to a section of a twig in autumn. When 
this starch is utilised during the sprouting of the buds in spring, 
it is changed into sugar, and is then transferred in the water 
ascending the vessels to the growing regions, hence the sweet 
character of the sap which exudes in bleeding. ^ The aeration 
of the secondary tissues is effected by narrow intercellular spaces 
which are more particularly associated with the living elements. 

1 Cf. F. & S., pp. 109, 169. Maple-sugar is derived from the evaporated 
sap of Acer sacchariniim (United States), obtained by tapping the trees in 
spring. 



ANNUAL RINGS 125 

The major part of each annual addition to the secondary' wood 
usually consists of vessels and fibres, but the proportion of these 
latter varies with the kind of plant and during each season's 
growth. The wood formed in the spring {spring-wood , Fig. 60 
A, S.) mostly contains a much larger percentage of vessels 
than that produced later [autumn-wood , A.) ; its vessels, 
moreover, are often larger and have thinner walls, and 
the same may be true of the fibres. This difference can be 
related to the sudden demand on the water-supply in the spring 
on the part of the newly-expanding leaves, whilst later in the 
season provision can be made for the growing mechanical re- 
quirements 1 of the plant by an increased proportion of fibres. 
As a consequence there is a sharp boundary between the dense 
small-celled autumn-wood of one season and the wide-celled 
spring-wood of the next (Fig. 60), and this leads to the marking 
out of the secondary wood into a succession of annual rings 
(Fig. 58), by means of which the approximate age of a trunk 
can be estimated. Occasionally, however — as, for example, when 
a new set of leaves is produced to replace a first crop killed by 
frost or devoured by caterpillars — their expansion is accompanied 
by the formation of a second zone of spring-wood, so that two 
" annual rings " arc formed in a single season. The width of 
the annual ring is mainly determined by nutrition, though the 
variations in thickness in one and the same ring are probably 
the result of mechanical strain. Such asymmetrical annual rings, 
with a maximum development on the upper or lower side, are 
commonly found in horizontal branches. An extreme condition 
is seen in the "buttress-roots" observed in many tropical trees 
{e.g. species of Ficus). 

The constant addition of new wood is probably mainly 
necessitated by changes in the central earlier-formed xylem, as 
a result of which it becomes useless for purposes of conduction, 
although such changes often increase its value as a mechanical 
support ; it is then spoken of as heart-wood in contrast to the active 
sap-wood beyond. The elements of the heart-wood often become 
impregnated with tannins, resins, etc., which are frequently 
accompanied by dark-coloured pigments ; in some cases the 

1 A feature that is probably also connected with the growing mechanical 
strain is the increased length of the fibres in the later-formed annual rings. 



126 HEART-WOOD 

latter are extracted and afford useful dyes, e.g. the logwood 
(hiematoxylin) obtained from Hcematoxylon campechianum (Tropi- 
cal America). The employment of mahogany', walnut, etc., in 
cabinet-work is largely due to the rich colouration of the heart- 
wood and the high polish which its hard character enables it to 
take. In the Ebony-tree (Diospyros) the living sap-wood is white 
and even soft, the ebony of commerce being the mature, very 
hard, and jet-black heart-wood. In some cases {e.g. Beech) little 
heart-wood is formed, most of the xylem remaining functional. 

The impregnating substances are often antiseptic, and prevent 
decay by inhibiting the development of Fungi and Bacteria, thus 
increasing the durability of the wood. Teak ( I'edona grandis) 
owes its great value as a tropical timber to the presence of an 
oil which renders it immune from the depredations of wood-boring 
insects ; it is also the cause of its peculiar scent. The liability 
of many Willows to develop hollow trunks at an early stage may 
be attributed to the absence of antiseptic substances from the 
heart-wood. 

The cavities of the water-conducting elements in the heart- 
wood are frequently blocked in various ways, most commonly 
by the ingrowth of structures known as tyloses (Fig. 6i). These 
are bladder-like intrusions through the pits, from the wood- 
parenchyma cells, into the vessels, and are sometimes so numerous 
as to fill the latter completely with a false tissue resembling 
parenchyma. They are bounded by the extended thin-walled 
pit-membrane, which undergoes a certain amount of surface 
growth, and occasionally becomes thickened and lignified [e.g. 
False Acacia, Robinia pseudacacia). Each of the young tyloses 
is living, containing cytoplasm, cell-sap, and sometimes also a 
nucleus ; but when they have reached their full size both the 
tyloses and the wood-parenchyma cells ^ of the heart-wood die, 
so that the whole of the latter consists of dead elements. Tyloses 
are also produced in herbaceous stems {e.g. Vegetable Marrow, 
Fig. 6i), but here their function is obscure. The plugging of 
the vessels of the heart-wood is, however, not always effected 
in this way, since in other cases mineral deposits (lime in the 
Elm) may take their place. 

' The latter are invariably dead in the mature heart-wood, even when 
no tyloses arc formed. 



PROPERTIES OF TIMBER 127 

The mechanical properties of timbers depend largely on the 
proportion of fibres, on the number and size of the vessels, and 
on the thickness of the iibre walls. Thus, so-called close-grained 
woods {e.g. Box, Holly, etc.) are characterised by having abun- 
dant fibres and small vessels, whilst the coarse or open grain 
of wood (seen in the Lime, Horse Chestnut, etc.) is due to the 




Fig. 61. — Tyloses in a vessel of the Vegetable Marrow [Cucurhita] as seen 
in transverse section. On the right the connection with the wood- 
parenchyma is shown. 

large number of wide vessels and often to the thin character of 
the fibres as well. The " soft woods " of commerce are mostly 
obtained from Conifers whose wood is solely composed of long 
narrow tracheids (cf. pp. 340, 341), having thinner walls than the 
fibres of hard-wood trees. 

The employment of timber for commercial purposes is deter- 
mined by such qualities as closeness of grain, ease of working, 
elasticity, toughness, durability, colour, figure, etc. Thus, the 



128 



PROPERTIES OF TIMBER 



resilience of the wood of certain types of Willow is responsible 
for their use in the manufacture of cricket-bats, whilst the ancient 
esteem of Yew {Taxits haccata) for bows, and the modern use of 
Ash for aeroplane-construction, are the outcome of the elasticity 
and toughness of these woods. The clastic limit of Ash is only 
reached under a strain of about 5,000 lb. per square inch as com- 
pared with 3,500 lb. for Douglas Fir. Such qualities are probably 
not only the result of length of fibre, but also one of physical or 







Fig. G2. — Transverse sections of the secondary wood of the Sycamore [Acer 
pseiidoplataiius) (right) and the Oak (Quercus) (left). Note the fibres 
(/), which are mucli thicl^er-walled in the latter than in the former.' 
fu, medullary ray ; T', vessels. 

chemical constitution of the xylcm walls. Liability to splitting 
is often due to a large size of the constituent elements, and it is 
the reverse condition vdiich renders the timlier of the Hornbeam 
[Carpinus) so suitable for cog-wheels, etc. Tlie " figure " of 
timber, to which many woods owe their \-alue in cal)inet-work, 
is dependent on the direction of the fibres and the prominence 
of the medullary rays when cut in certain planes. 

To render timber suitable for use, the cut log needs to undergo 
a process of seasoning, which involves the drying out of the 



Timber trees ng 

sap and wafer', a process that in the open occupies from six 
months to a year or even longer in the case of hard woods, and 
about half the time for Coniferous timber. Artiticial means 
{e.g. heating in special sheds or by electricit}') are now frequently 
eraployed in order to shorten this period. The greatest care is 
necessary in seasoning to avoid undue stresses and strains, \\'hich 
bring about warping and splitting of the trunk, defects that 
commonly develop if the process be unduly hastened. This is 
mainly due to the fact that the tangential shrinkage is consider- 
ably greater than the radial. ^ 

Another common defect is the " knots," which are constituted 
by the vascular tissues of lateral branches that have become 
embedded in the wood of the trunk ; in forestry their develop- 
ment is checked by dense planting. 

In the following list are enumerated some of the more im- 
portant timbers and the trees from which they are derived : 

A. Coniferous. 
Douglas Fir or 

Oregon Pine . Pseudotsiiga doiiglasii (Rocky Mts.) (woodwork in build- 
ings ).= 
Larch . . Larix europeea (Europe) (sleepers, pit-props, etc.). 

Pitch Pine . Pinus palustris (United States) (constructional work). 

Red Deal . . Pinus sylvestris (Britain and Scandinavia) (building 

work) . 
Wliite Deal . Picea excelsa (Norway) (paper-pulp, floor-boards, etc.). 
"White Pine . Abies pectinaia (S. Europe) (joinery). 

White or Yellow 

Pine . . Pinus strobus (N. America) (joinery). 

B. Dicotyledonous. 

Ash . . . Fraxinus excelsior (Europe) (wheelwright's work, tool- 

handles, aeroplanes, etc.). 

Beech . . Fagus sylvatica (Europe) (furniture and tools). 

Black Walnut . Juglans nigra (N. America) (furniture). 

Blue Gum . Eucalyptus globulus (Australia) (constructional work, 

piers). 

Birch . . Betula spp. (N. Europe) (furniture, cabinet-work, bob- 

bins, etc.). 

Cedar Wood . Cedrela spp. (America) (cigar-boxes). 

1 The tangential shrinkage is often nearly double, and for this reason 
radial cracks, that open wider and wider as the trunk dries, are not un- 
common. 

2 A few only of the more important uses are mentioned in each case. 

9 



1 10 



SliCONDARY PHLOEM 



Chestnut . . Castanea saliva (Europe) (constructional work). 

Elm . . Ulmiis spp. (Europe) (coffin-boards). 

Hickory . . Carya spp. (N. America) (shafts, spokes, etc.). 

Jarrah . . Eucalyptus marginala (Australia) (constructional work, 

wood-paving blocks, etc.). 

Lignum vita; . Guaiacum officinale (America) (shaft-bearings, etc.). 

Lime . . Tilia spp. (Europe and N. America) (cabinet-work). 

Mahogany. . Sunetenia niahagoni (West Indies, Peru) (cabinet-work). 

Maple . . Acer sp-p. (Europe and N. America) (furniture). 

Oak . ■ . . Quercus spp. (Europe and America) (numerous uses, 

especially building). 

Poplar . . Populus spp. (Europe) (packing-cases, etc.). 

Sandalwood . Satitalnm album (E. Indies) (cabinet-work). 

Satinwood . Chloroxylon swieienia (E. Indies) (cabinet-work). 

Teak . . Tectona grandis (India, Java) (cabinet-work). 

The secondary phloem, like the primary, is composed of sieve- 
tubes, companion cells, and phloem-parenchyma, but these are 
often accompanied by fibres. The septa in the sieve-tubes are 
not uncommonly oblique and provided with several perforated 
areas separated by bar-like thickenings [compound sieve-plates). 
In many herbaceous plants, however {e.g. Vegetable Marrow, 
CiicitrMta) , the septum is horizontal and bears but one perforated 
area. Functionless sieve-like areas are sometimes present as 
isolated patches on the longitudinal walls of the sieve-tubes (cf. 
Fig. 13, left). In some cases the phloem-parenchyma and sieve- 
tubes, with their companion cells, are produced in such regular 
sequence as to form alternating tangential bands. The fibres, 
which show no distinctive features, are likewise often disposed 
in layers separating the zones of thin-walled elements [e.g. Lime, 
Mallow, etc.). Vertical series of phloem-parenchyma cells, occu- 
pied by crystals of oxalate of lime, and elongated secretory 
elements [e.g. tannin-sacs, cf. p. 151) are not uncommon. The 
older secondary phloem which has passed out of use is, in the 
case of woody plants, not infrequently shed with the bark (p. 140). 

The parenchyma rays of the secondary phloem are continuous 
with those of the xylem and exhibit the same general structure, 
except that the component cells remain thin-walled. In certain 
Flowering Plants [e.g. Lime, etc.) the outer ends of the primary 
medullary rays exhibit a marked V-shaped enlargement, whereby 
the secondary phloem becomes divided up into a number of 
wedge-like groups. 



SECONDARY THICKENING OF ROOTS 



131 



A transverse section of any unthickened Dicotyledonous root 
will show a narrow band of two or three laj'ers of parench^-matous 
cells between each phloem-group and the adjacent xylem. Prior 
to the commencement of secondar}- growth tangential division- 
walls arise in these cells, leading to the production of a cambium 
(Fig. 27, ca.,p. 66) 
like that of the 
stem. In this way 
there originate as 
many cambial 
strips (Fig. 63, A, 
B, C.) as there are 
groups of phloem, 
and, by the active 
division of the 
former, secondary' 
xylem is produced 
on the inside and 
secondary phloem 
on the outside. 
Differentiation of 
the cambium is at 
first confined to 
the inner surface 
of each phloem- 
group, but by slow 
degrees it extends 
along the sides of 
the xjdem-arms 
till finally, by the 
development of 
tangential divi- 
sion-waUs in the 
cells of the pericycle opposite the protoxjdems, a complete 
lobed cylinder of cambium is established. 0\\ing to the late 
development of the cambium opposite the protoxylem-groups, 
secondary thickening is at first more extensive in the bays, so 
that the outline of the cambium, at first lobed, gradually 
becomes circular. 




Fig, 



63. — Diagrams showing secondary thickening 
in the root. A, a tetrarch, and B, a diarch 
root, before thickening, showing the position 
of the cambium (C). C, a tetrarch root after 
secondary thickening has been going on for 
some time. Co., cork ; Cor., cortex ; En., 
endodermis; PA., phloem ; P.;., primary ray ; 
P.xy., protoxylem ; Xy., primary xylem ; 
2Xy., secondary xjdem, 



132 SECONDARY THICKENING OF ROOTS 

The root soon comes to possess a broad ring of secondary 
wood and phloem (Fig. 63, C), similar to that of the stem, and, as 
in the latter, traversed by primar^^ {P-f-) a^nd secondary rays. 
The former, which, especially in herbs, often attain a considerable 
width, are situated along the same radii as, and are equal in 
number to, the primary xylem-groups {P.xy.). When these pri- 
mary rays are broad and consist only of parenchyma, separate 
wedges of secondary vascular tissues result. The radial extension 
of the parenchymatous rays is not so marked as that of the 
wood, so that certain diarch roots, after secondary thickening, 
sometimes exhibit a band-like structure {e.g. Nettle). The 
secondary rays, like those of the stem, are generally narrow. 

The vessels and wood-parenchyma of the secondary wood of 
the root are relatival}' more numerous and more evenly dis- 
tributed than in the stem, so that the annual rings are usually 
less conspicuous. In its detailed structure the secondary phloem 
is similar to that of the stem, and, except for the points already 
mentioned, the same is true of the secondary wood. In the 
root, as in the aerial axis, the production of secondary tissues 
forces the primary phloem progressively farther from the centre. 
Old,- secondarily thickened roots resemble stems very closely, 
but, when the central tissues are preserved, the original root- 
structure can be traced by following down the primary rays 
and locating the protoxylem-groups at the periphery of the 
metaxj'lem. 

The storage of food in fleshy roots is effected by thin-walled 
parenchyma, which is often copiously developed both in the 
secondary phloem and in the secondary xylem. The vessels of 
the latter then form isolated groups, which are either scattered 
or arranged in radial files {e.g. Salsify, Tragopogon, Fig. 78, B) 
in the storage tissues, a distribution facilitating rapid transference 
from the storage cells when growth is resumed. In such roots 
the limits of secondare' xylem and phloem are often difficult to 
distinguish, unless the cambium be first located. 



The process of secondary' thickening so far described is that 
normally found in the vast majority of Dicotyledons and Conifers, 
but a few so-called anomalous types merit a brief consideration. 



ANOMALOUS TYPES OF SECONDARY THICKENING 133 



In many members of the Spinach-family (Chenopodiaccit) the 
normal cambium functions only for a short time and, after forming 
a narrow strip of secondary xylem and phloem, ceases to divide. 
Thereupon another cambium arises in the inner part of the 
cortex, and a new strip of 
xylem and phloem is produced 
till this cambium in its turn 
ceases to be active. This 
process is repeated again and 
again, each successive cam- 
bium forming xjdem on the 
inner and phloem on the outer 
side. In the root of the Beet 
[Beta) the successive cambia 
form continuous rings of xylem 
and phloem, its fleshy char- 
acter being mainly due to the 
extensive development of the 
latter tissue. As a general 
rule, however, xylem and 
phloem are produced only at 
certain points, and appear as 
secondary bundles separated 
by the thick-walled ground- 
tissue, developed from the re- 
maining parts of each cambium 
[e.g. Silver Goosefoot, Obione 
portulacoides) . 

Another uncommon 
method, shown by certain 
Monocotyledons which ex- 
hibit marked secondary in- 
crease in girth, is well illus- 
trated by the Dragon-tree of 
Teneriffe [Dracana) , the stems 

of which may attain a diameter of ten feet or more. The bundles 
of the young stem here exhibit the usual scattered arrangement ; 
but in the cortex, immediately beyond the vascular region, there 
arises a cambium (Fig. 64, Ca.) which cuts off segments mainly 




Fig. 64. — Transverse section of a 
portion of an old stem of 
Dvaccena showing secondary 
thickening. C, cortex ; Co., 
cambium; P., phloem; P.b., 
primary bundle ; S. 6. .secondary 
bundle ; Xy., xylem. 



134 ANOMALOUS TYPES OF SECONDARY THICKENING 

towards the inside, the small number cut off on the outside 
simply forming an addition to the cortex (C). Some of the 
inner segments divide and undergo gradual differentiation to 
form secondary bundles {S.b.), whilst the remainder, retaining a 
somewhat radial arrangement, become thickened and lignified. 
The secondary bundles embedded in this thick-walled tissue are 
concentric with central phloem (P. ) , but the latter is often very 
scanty in amount ; the xylem contains no vessels consisting 
of fibrous tracheids only. 

Very complicated types of anomalous thickening are exhibited 
by the woody climbers [lianes) of tropical forests. In many of 
these the old trunks develop cambial rings about several centres, 
each such cambium producing a separate xylem-core, so that a 
rope-like structure composed of several intertwined woody strands 
results. 



CHAPTER XI 

Cork-formation, etc. 

One result of secondary thickening is a marked enlargement of 
the periphery of stem or root, in consequence of which the outer 
tissues are subjected to increasing tension (cf. p. 120). These, 
the epidermis and cortex, ordinarily have but a limited power 
of stretching, and, as soon as this limit is reached, they rupture 
and no longer form an effective covering for the underlying tissues. 
This function is henceforth fulfilled by a secondary protective 
tissue, the cork, formed by the active division of another cambium, 
the phellogen or cork-cambium, which arises somewhere in the 
cortex. In a few cases [e.g. in some cultivated Maples, viz. Acer 
striatum) the epidermal and cortical cells are capable of limited 
growth and division, and here the formation of a cork-cambium 
is correspondingh' delayed. 

Cork-formation takes place in essentially the same way in both 
stem and root. The phellogen invariably arises hy the develop- 
ment of two successive tangential walls, in the case of the stem, 
most commonly in the cells of the subepidermal layer (Fig. 65). 
The cells are thus each divided into three segments, of which the 
central constitute the actual cork-cambium [c.c), whilst the outer 
form the first layer of cork and the inner the first, and often the 
onl}', layer of a tissue known as plicUodcrm [Ph.] . WTiilst the outer 
and inner segments undergo no further division, the cells of the 
phellogen divide again and again, one of the two products of 
each division becoming differentiated as cork or phelloderm, 
while the other remains as the cell of the cork-cambium. In 
the majoritv of cases, however, these divisions of the phellogen 
lead to the cutting-off of cells on the outside only, so that no 
further formation of phelloderm takes place. 

The activity of the cambium results in the development of 

i.3p 



136 



CORK 



a continuous tissue consisting of numerous radial files of cells, 
each file (Fig, 65, 1-6) representing the product of one cork- 
cambium cell. This tissue is the cork and, apart from the 
absence of intercellular spaces between its cells, it is especially 
characterised by a chemical modification of the cell-walls spoken 
of as suherisation. This latter renders them practically imper- 
vious alike to gases and to liquids, features to which cork owes 
its utilisation in closing bottles. 




Fig. 65. — Transverse section through the cork layer in the stem of the 
Elder (Sambuciis). ex., cork-cambium ; cii., cuticle ; ep., epidermis ; 
Ph., phelloderm ; 1-6, successive segments of the cork in order of 
production. 



Suherisation is known to be due to the presence of various 
fatty compounds, and hence the walls of the cork are coloured 
by the same reagents (Scharlach red, etc.) as are employed for 
staining fats. A yellow -brown colouration is assumed \"\'ith chlor- 
zinc-iodide, and a yellow one with strong potash. A marked 
green colouration is obtained by treating cork-cells with a freshly 
prepared alcoholic solution of chlorophyll, which is allowed to 
act for about a quarter of an hour in the dark. On boiling with 
concentrated potash, large yellow globules often escape from the 



CORK 137 

walls. Subcrised membranes, moreover, are highly resistant, being 
insoluble both in cuprammonia and concentrated sulphuric acid. 
Suberisation of the walls ensues soon after the cork-cells arc cut 
off from the cambium, and it is almost unnecessary to add that, 
as a result, the cells die, their contents ultimately consisting of 
air or more rarely of pigmented bodies (often tannins and their 
derivatives) . The characteristic white appearance of the surface 
of the Birch (Bdula) is due to the presence in the cells of the 
cork of solid granules of a substance known as betulin. 

The cells of the cork vary considerablj' in shape, although 
very commonly flattened. The walls are often relatively thin. 
The radial walls are frequently thrown into folds, whilst the 
tangential ones are often almost straight (cf. Fig. 65), facts that 
can be related to the tangential tension and radial compression 
set up by the increasing girth of the enclosed axis. 

In a few cases (e.g. Currant, Fig. 66, Laburnum, etc.) cells 
are likewise cut off by the phellogen towards the inside, so that 
a more or less extensive phelloderm (Ph.) is formed. The cells 
of this tissue also are arranged in radial files, but the walls remain 
unsuberised. The phelloderm consists of living cells, and thus 
merely serves to augment the primary cortex, although where 
the walls are thickened it has an additional mechanical value. 

It has already been mentioned above that the cork-cambium 
most commonly develops in the subepidermal layer of cells, but 
not infrequently it arises in deeper-seated cortical layers (Fig. 66) 
or even in the pericycle. A good instance is furnished by the 
Barberry (Fig. 67), where the phellogen (C.C.) develops just inside 
the ring of mechanical tissue [Sc.) occupying the inner part 
of the cortex. In roots, too, it almost invariably arises in 
cortical cells in the immediate neighbourhood of the pericycle 
(Fig. 63, C, Co.). The origin of the cork-cambium from the 
epidermis is seen in Willows, as well as in the Rose, Apple, Haw- 
thorn, and other members of the Rosacese. 

The cork not only prevents the excessive transpiration which 
would ensue from rupture of the epidermis consequent upon • 
secondary thickening, but also takes over other protective func- 
tions of that tissue. Suberised walls possess considerable strength, 
though their elasticity is slight, and the cork consequently forms 
a mechanical envelope whose efficiency is heightened by the 



^8 



FUNCTIONS OF CORK 



close connection between its cells. The frequent presence of 
air in the latter retards excessive heating by day or excessive 
cooling by night. Moreover, the waste substances commonly 
encountered in the walls or cavities of the cork-cells are anti- 
septic, excluding access of various parasites to the living tissues 




Fig. 66. — Transverse section through the peripheral part of the stem of 
the Currant (Ribes), sliowing the cork (Co.), the cork-cambium (C.C), 
and the extensive phelloderm [Ph.), of four to six layers of cells- 
Beyond the cork are seen cortex and epidermis. 



within. As soon as cork-development commences, therefore, the 
parts concerned become ensheathed in an almost impermeable 
protective layer which would practicaUy sever all direct com- 
munication between the internal tissues and the atmosphere, 
but for the formation of localised patches of loose tissue, the 



LENTICELS 



139 



lenticels, whose development often commences slightly before 
that of the cork. 

The first lenticels usually arise beneath the stomata of the 
young stem, where strips of cambium are formed in the sub- 
epidermal layer by the customary tangential divisions. These 
cambial strips divide 
very actively, cutting 
off segments on both 
sides. Those on the 
inner give rise to radial 
rows of phelloderm, 
whilst those on the 
outer remain thin- 
walled and unsuberised, 
but sooner or later round 
off and lose all connec- 
tion with one another 
(Fig. 68, I.). This loose 
tissue 1 is formed in con- 
siderable bulk, so that 
it leads to a gradual 
elevation and ultimate 
rupture of the overlying 
epidermis. The tissue 
of the lenticel is thus 
exposed, and air from 
the surrounding atmo- 
sphere can freely circu- 
late between its cells 
and, by way of the 
narrow air-spaces be- 
tween the cells of the 
phellogen and phello- 
derm, into the inter- 
cellular spaces of the cortex. The rounding-off of the constituent 
cells and consequent development of the intercellular spaces 
vary considerably, so that the lenticel may be spongy («.,§'. Elm, 
Birch) or relatively compact {e.g. Willow, Spindle-tree). The 
t Forming the so-called " complementary tissue." 




Fig. 67. — Transverse section through the 
deep-seated cork in the stem of the 
Barberry (Berberis) showing two layers 
of cork-cells (Co.) situated between 
the cork-cambium (C.C.) and the zone 
of sclerenchyma (Sc). 



140 



BARK 



strips of cambium giving rise to the lenticels subsequently become 
continuous with the corl^-caml^ium (Fig. 68). 

\Miere the corlc is deep-seated, the tissues external to it are 
practically cut off from all sources of food and all connection 
with the interior, and therefore die away. These dead tissues, 
on the outside of the cork, are shed sooner or later, leaving the 
latter exposed as hark. In some plants [e.g. Beech, Oak) the 
same cork-cambium continues to divide year after year, although 
inactive during the winter, so that a thick mass of cork is formed. 
This is also the case in the Cork Oak [Qnercus sttber), which is 
the main source of the commercial article. The first cork, which 




c,c- 



Fig. 68. — Transverse section through a lenticel (/.) of the Elder {Sambucus). 
c.c, cork-cambium ; ni, mechanical tissue. 

here arises subepidermally, is, howe\'(T, of no value, being re- 
moved \\-lien the tree is ten to fifteen years old, the cortex thus 
exposed forming a new phellogen which gives rise to the 
thin-walled cork of commerce. This is peeled off every eight to 
twelve years. 

In most woody plants the first-formed phellogen ceases to 
di^•ide, and indeed itself becomes changed into a la\'er of cork, 
at a comparatively early stage. A new cambiimi their arises at 
a deeper level in the cortex, produces a fresh zone of cork, and 
then in its turn passes out of action, to be succeeded b^' another 
situated still deeper. The bark formed in this \\'ay consists of 
alternating layers of cork and dead cortex, and comprises all 
the tissues beyond the most recently established phellogen. When 



BARK 141 

the original cork-cambium is deep-seated, the subsequent cambia 
are continuous cyhnders; whilst when the first is superficial, the 
later ones are often merely vertical strips whose margins are in 
contact with one another. As the bark gets thicker and thicker 
the outer portions are subjected to a growing tension, so that, 
being dead tissue, fissures appear at the surface as irregular 
longitudinal furrows well seen in the Oak and Elm. In many 
cases, however, the oldest bark is shed, and this takes place 
either as rings {ring-hark, e.g. Birch, Cherry) or as scales (scale- 
bark, e.g. Plane, Scotch Fir), according as the successive cork- 
cambia are continuous cylinders or separate strips.'- 

After the shedding of the first-formed cork, lenticels may 
develop from any part of an active phellogen. As a result of 
subsequent stretching they assume various forms, which are often 
characteristic of individual species. Thus on the bark of the 
Birch and Cherry they appear as slightly raised horizontal lines, 
whilst in the Poplar they are usually lozenge-shaped. The 
cork-wings developed in many varieties of woody plants {e.g. Cork 
Elm, Cork Maple, etc.) are due to the copious formation of cork 
which, consisting as it does of dead cells, necessarily splits at 
the surface ; the regularity of the fissures may, however, depend 
in part on excessive local activity of the cork-cambium. 

It will have become apparent that cork serves to protect 
surfaces which would otherwise be exposed, and indeed it is 
even found covering wounds and the scars left b}^ the shedding 
of leaves, flowers, branches, etc. The fall of leaves is preceded 
by the differentiation of a definite separating layer (abscission 
layer) which is produced, with or without division, from the cells 
at the base of the leaf -stalk (Fig. 69, S.). The layer in question 
is generally recognisable by the smaller size of its ceUs, and its 
position is often indicated externally by a sUght constriction of 
the petiole. Subsequently the middle lamellse between the cells 
of this layer become mucilaginous and break down, so that for 
a short time the leaf is connected with the stem solely by the 
epidermis and vascular bundles. Beneath the abscission layer 
a protective laj'er of cork, which subsequently becomes con- 
tinuous with that of the stem, is developed, either before or 
after the detachment of the leaf. The smooth layer of cork 

1 Regarding commercial uses of bark, see pp. 48, 60, 62. 



142 



CALLUS 



covering the leaf-scar is interrupted only where the vascular 
bundles are broken across, and here the vessels are usually 
plugged by tjdoses. The detachment of flowers and the pheno- 
menon of self-pruning, by which small branches of certain trees 
[e.g. Poplar) are regularly shed, is due to a similar development 

of a definite abscission-layer. 

When a branch is artificially re- 
moved, the living cells, and especially 
those of the exposed cambium, give 
rise to a large-celled, thin-walled tissue 
which at first forms a ring, but often 
gradually becomes broader till the 
whole wounded surface is completely 
covered, the outermost cells usually 
becoming suberised. The tissue thus 
formed is called a callus, and may 
subsequently produce adventitious 
shoots {e.g. pollarded trees) or adven- 
titious roots {e.g. cuttings). Regions 
of injury in the parenchymatous tissues 
{e.g. of cortex or leaf) are commonly 
isolated from the healthy surrounding 
tissue by the formation of cork from 
a cambium produced by tangential 
divisions in the uninjured cells nearest 
the wound. In this way the attack 
of a parasitic Fungus maj' often be 
localised. 

The cut surfaces of scion and 
stock, brought into contact in the 
process of grafting, become intimately 
associated owing to fusion of the 
intact cells, which is usually accompanied by some cell-division. 
Renewed activity in the tissues of a plant may likewise result 
from wounding by various organisms, either vegetable or animal. 
The local enlargements produced are termed galls (Fig. 70), 
and in their formation the, probably chemically, stimulated cells 
may either enlarge or undergo division. 

Little or no tissue differentiation exists in galls caused by 




Fig. 69. — Diagrammatic 
longitudinal section 
through part of a node 
of the Sycamore (Acer 
pseitdoplatanus), show- 
ing the abscission layer 
(S.). a.x., axillary bud ; 
p., petiole; I'., vascular 
bundles. 



GALLS 



143 



plants (e.g. those of the Finger and Toe disease, Fig. 135, or those 
produced by many Rust Fungi), and externally they commonly 
present an irregular and very variable form. Many animal 
galls offer a striking contrast in these respects, not only ex- 
hibiting a definite external structure, but, as in many of those 
produced by gall-flies, a complex internal differentiation. In 




Fig. 70. — Pliotograph of galls (g.) on leaves of Dogwood 
(Coruiis sanguinea). [Photo E. J. S.] 

some of these cases the central region, where the larva of the 
gall -insect resides, consists of nutritive tissue composed of cells 
with dense cytoplasm, rich in protein material, and serving for 
the animal's nourishment. This region is in some cases sur- 
rounded by a la3'er of sclerenchj'ma, whilst the outermost zone 
consists of parenchymatous tissue. The latter is usually well 
supplied with intercellular spaces, contains little or no chloro- 
hyll, and is oiten traversed by vascular tissue. 



CHAPTER XII 

Secretory Organs 

Referenxe has already been made in Chapter V to the various 
by-products which are formed during the constructive and de- 
structive processes in plants and which often collect as so-called 
secretions. This term is also applied to the sugary liquid pro- 
duced by nectaries (both floral and extrafioral) , the digestive 
juices formed by insectivorous plants, and even the watery 
exudations appearing at different points on leaves under certain 
conditions. These diverse secretions are very commonly the 
result of the activity of specialised cells or groups of cells, which 
may be classed under the general heading of secretory organs or 
glands. Their products may pass direct to the exterior, or may 
be retained in special cavities or canals within the body of the 
plant. Many of the secretory organs are superficial, and some 
of these are developed as hairs, for instance the glandular hairs, 
already described (p. 105), which secrete various by-products. 

Another widespread type, known as a hydathode, serves to 
remove excess water, either as liquid or vapour. In some cases 
these hydathodes take the form of glandular hairs, their 
cells possessing dense protoplasmic contents and large nuclei 
(Fig. 71, A). Good examples are found on the leaves of the 
Runner Bean, where they are bent, club-shaped structures situated 
near the veins and consisting of a row of thin-walled cells, the 
terminal cell often being divided into two by a vertical wall 
(Fig. 71, A). Many semi-parasites (e.g. Yellow Rattle, Bartsia, 
etc.) bear numerous water-secreting hairs situated chiefly along 
the veins of the leaves. These hydathodes (Fig. 71, B) have 
the form of small domes consisting of living cells, viz. a large 
basal cell [b.c.) embedded in the epidermis, a short stalk-cell {s.c), 
and a head composed of two to four cells lying side b}' side 

144 



HYDATHODES 



145 



(Fig. 71, B, left-hand figure) ; in the middle of the head there 
is a small space between the cells, and above this the overlying 
cuticle is pierced by a minute hole (/>.) through which the water 
is secreted. Both here and within the leaf cavities of the Tooth- 
wort (LathrcBa), where similar hydathodes occur (Fig. 71, C, h.), 
glandular hairs (g.h.) of another type are present, which may 
take part in the secretion of water. 




Fig. 71. — Water-secreting hairs. A, Runner Bean (Phaseolus multiflorus) . 
B, Yellow Rattle (Rhinanthiis) . C, Toothwort (Lathraa squamaria). 
The left-hand figure of B from the surface, the others in vertical 
section, i.e., basal cell ; ^. A. , glandular hair ; /;., hydathode ; ^.,pore; 
s.c, stalk cell. 



Another kind of hydathode, exhibiting quite a different 
mechanism, is much commoner among British plants. These 
hydathodes are not hair-like, but occur generally above the 
bundle-endings, being especially located at the leaf-apex (Grasses) 
or on the tips of the leaf-teeth (Lesser Celandine). In all these 
cases the water escapes from so-called water-pores, which are 
situated in the epidermis of the hydathode (Fig. y^- St.) and 
resemble ordinary stomata, except that they are often larger 
and that their pore remains permanently open in correspondence 
10 



146 



HYDATHODES 



with the absence of the characteristic thickenings in the guard- 
cehs ; in surface sections they chiefly differ in their more rounded 
form (Fig. 72). In some plants but a single water-pore is asso- 
ciated with each hydathode {e.g. Enchanter's Nightshade, 
Fuchsia), but in others they arc numerous {e.g. Wild Strawberry), 
and occasionally grouped in shallow depressions that can be 
recognised with the unaided eye {e.g. Marsh Marigold). 

In a longitudinal section through an entire hydathode of this 




Fig. 72. — Water-pores {IV. p.) of the Lesser Celandine (Ficaria verna) seen 
from the surface. St., a normal stoma on the same scale for com- 
parison. 



type (Fig. j^) the end of the vascular bundle, which is here seen 
to consist of tracheids only, usually enlarges somewhat, often in 
a cup-shaped manner. Between the tracheids and the overlying 
water-pores there is commonl)' a small-celled tissue, the cpithem 
{ep), composed of cells with prominent nuclei and dense c^'to- 
plasmic contents ; there is often, however, a space immediately 
beneath the epidermis. The epithem is tra\-ersed bj' a system 
of fine intercellular spaces through which the water passes from 
the tracheids to the water-pores. In many Monocotylcdonous 
water-plants {e.g. Water Plantain, Alisina plantago) the tissue 



HYDATHODES 147 

above the bundle-end breaks down completely, so that the 
escaping water passes directly to the exterior. 

Hydathodes can often be recognised at a very early stage, 
and are probably most active in the young leaf, which develops 
in an almost saturated atmosphere within the expanding bud. 
Active secretion of water takes place when the hydrostatic 
pressure within the plant becomes excessive, that is when trans- 
piration is slow and absorption considerable, a condition that is 
most frequently attained at night, but which can be artificially 
produced by placing a well -watered plant under a beU-jar in a 
warm room.^ The hydathodes can therefore be regarded as 
safety-valves which avert blocking of the intercellular spaces 




Fig. 73. — Hydathode of Lesser Celandine (Ficaria verna) in vertical section 
(after Salisbury), ep., epithcm tissue ; St., water-pores. 

with water under the conditions just mentioned. In the case 
of the semi-parasites they probably serve to get rid of the water 
which is absorbed in excess from the host. The liquid exuded 
from hydathodes is not pure water, but usually contains a very 
small percentage of dissolved salts. In some cases, however, the 
amount of the latter may be so considerable that they remain 
behind as an incrustation when the water evaporates. Thus 
in the Saxifrages {Saxifraga) a little white scale consisting of 
carbonate of lime forms on the leaf-teeth over each of the 
hydathodes. 

The exudation of water from the hair-like hydathodes of 
semi-parasites and the Runner Bean is due to active secretion 
on the part of the protoplasts of the constituent cells. As a 
1 Cf. F. & S., p. 121. 



148 NECTARIES 

consequence secretion of water ceases, if tiie cells be killed by 
painting the surface of the leaf with a solution of corrosive 
sublimate or other poison. Hydathodes possessing water-pores 
are, however, mainly passive in their action, the water being- 
forced out by hydrostatic pressure through the intercellular 
spaces of the epithem, and not by active secretion on the part 
of the living protoplasm. 

Insects are most commonly attracted to flowers by honey 
formed in special structures termed nectaries. The honey is 
produced by the active secretion of cells belonging either to the 
modified epidermis (usually palisade- or papilla-like, the latter 
in the ^^'ood Anemone, Anemone nemorosa) or to the underh'ing 
tissue. The nectary of the Hogweed or other common member 
of Umbelliferffi, forming the disc on the top of the ovary, is seen 
in a cross-section to consist of a mass of small glandular cells 
having the customary thm walls, abundant protoplasm, and 
large nuclei, and covered bj- an epidermis containing numerous 
stomata. The prominent cuticular ridges often exhibited by the 
epidermis of these nectaries may serve to retain the secreted 
fluid in situ. In the Buttercup the secreting tissue is similarly 
composed of small cells, but there are no stomata, so that the 
nectar only escapes by filtration through the outer membrane. 

Extrafloral nectaries on the vegetative organs occur in quite 
a number of plants, c.^'. on the under-surfaces of the stipules of 
the Broad Bean [Vicia faba) , on the leaf-bases of the Black Bind- 
weed [Polygonum convolvulus), at the forks of the fronds of the 
Bracken {Pteris aquiUiia. Fig. 74, A), and on the upper part of 
the petioles of the Guelder Rose {Vihitrmtm opulus). In the first- 
named plant the nectary appears as a dark depression, which 
when cut across is found to be composed of a palisade-like layer 
of secreting hairs, each consisting of an oblong head of several 
cells, borne on a short stalk. In the Guelder Rose the nectaries 
take the form of short cup-like projections, each supplied with 
a vascular bundle, the actual secreting surface being situated in 
the depression at the tip, and being similar in structure to that 
of the floral nectary of the Hogweed. The structure of the 
nectaries of the Bracken will be apparent from a reference to 
Fig. 74, C. 

In all nectaries it is primaril}' the secretion of osmotic sub- 



DIGESTIVE GLANDS 



149 



fC3 



stances on the surface (largely sugars) which initiates the exuda- 
tion of liquid, and the mechanism can be well imitated by placing 
sugar in a hollow scooped out of a Potato. ^ The osmotic sub- 
stances secreted by extrafioral nectaries tend to withdraw water 
from the plant when it is turgid, and these organs are indeed 
most active in a moist warm atmosphere. They may well serve 
primarily, therefore, to fulfil the same purpose as hydathodes. 
Floral nectaries, however, have another more important function, 
namely the attraction of 
insects. 

Glands whose liquid 
secretion is a digestive 
fluid containing proteo- 
lytic and other enzymes 
are present on the leaves 
of most insectivorous 
plants. By their agency 
the nitrogenous 
materials of the cap- 
tured insects are gradu- 
ally dissolved and 
rendered available for 
absorption. In the 
Butterwort (Pingtiicula) 
the digestive glands are 
found on the upper 
surface of the leaf and 
take the form of small 

hairs (Fig. 75, d.) composed of a basal cell, a short stalk-cell, 
and a usually eight-celled head. Similar glands occur also on 
the under-surface, but these are probably mainly concerned in 
the removal of water. The digestive glands are accompanied 
by other long-stalked glands ()».), whose umbrella-like heads 
secrete the sticky mucilaginous matter to which the insects 
adhere. In the Pitcher-plant (Nepenthes) the glands occur 
on the inner surface of the pitcher ; they are short-stalked 
structures, with a very robust oval head of palisade-like secreting 
cells. 

1 Cf. F. & S., p. 257. 




Fig. 74. — Extrafloralnectary of the Bracken 
(Pteris aquilina) (after Lloyd). A, Tlie 
fork of a frond showing the nectary (n.). 
B, A stoma from the nectary in surface 
view. C, The nectary in section. 
el., glandular cells ; St., stoma. 



150 



DIGESTIVE GLANDS 



The tentacles of the Sundew (Drosera) are much more elabo- 
rate, being supplied with a vascular bundle which extends into 
the slightly enlarged tip. In the latter the bundle is enveloped 
by three distinct layers of cells, of which the innermost is thick- 
ened after the manner of an endodermis, while the two outer 
layers which contain the crimson pigment are those actually 
concerned in secretion ; the superficial one has a palisade-like 
structure. The tentacles of the Sundew, like many other digestive 
glands, serve not only for the secretion of the digestive fluid, 

but also to 
^lYV. absorb the pro- 

ducts of its 
action ; in addi- 
tion they pro- 
duce the ad- 
hesive mucilage 
which forms a 
thick glistening 
laj'er over the 
head of the 
tentacle. 

The Bladder- 
wort {Utricii- 
laria) is stated 
to secrete no di- 




FiG. 75. — Transverse section through a small part of 
the leaf of the Butterwort (Piiiguicula), showing 
the short digestive glands (d.) and a mucilage- 
secreting hair (»(.). 



gestive fluid, the 



four-armed hairs 
which occur on 
the inner surface 

of the bladders serving solely for the absorption of the products 
of decay of the minute bodies of the entrapped animals. 

Various digestive ferments are likewise secreted hy the sur- 
face of the cotyledons of albuminous seeds whereb\' the food- 
substances in the endosperm are changed into a diffusible form. 
In general there is no differentiation of a special secreting laj-er, 
although in Grasses the palisade-like epidermal cells of the 
scutellum, in contact with the endosperm, are glandular in char- 
acter. These same cells are also concerned in the absorption 
of the digested food-substances. 



SECRETORY CELLS AND CAVITIES 151 

Secretions of the nature of b\'-products (ethereal oils, resins, 
etc.),^ although often deposited in glandular hairs, are frequently 
lodged within the body of the plant. In some plants {e.g. Bay 
Laurel, Laiirus nobilis) such secretions are found in isolated cells 
{secretory cells), often differing, apart from their contents, in shape 
and in their larger size from the cells of the surrounding paren- 
chymatous tissues in which thev usually occur. Typical instances 
are furnished by the so-called tannin-sacs, which are generally 
characterised by a slightly elongated form ; extreme examples 
are found in the cortex and pith of the Elder {Samhiicits). 

More striking are the secretory cavities, i.e. large intercellular 
spaces, approximately isodiametric in foiTn, and again usually 
lodged in the parenchymatous tissues of the plant. In many 
cases the} appear as transparent dots \\'hen leayes containing 
them are held up to the light, a phenomenon well seen in the 
St. John's Wort {Hypericum perforatum) and in the Rue (Ruta 
graveoleiis). The cayities are filled with an oily secretion, to 
which these plants owe their peculiar odour. In cross-sections 
of the leaves of the St. John's Wort the cayities appear more or 
less circular, each being lined with a layer of thin-walled, some- 
what flattened cells (the epitlieUum, cf. Fig. 76, B, S.), which 
discharge the secretion into the central space. The cayities of 
this plant originate by a gradual separation of the cells, a type 
of deyelopment spoken of 3.s scliizogeiioiis, and recognisable even 
at maturity b}' the presence of a well-defined epithelium ; similar 
schizogenous cavities are encountered in the leaves of the Myrtle 
{Myrtus communis). 

In those of the Rue, on the other hand, the cavities arise by a 
disorganisation of the secretnig cells whose remains (cf. Fig. 76, A) 
persist at the periphery, this mode of origin being described as 
lysigenous ; thus at maturity no epithelial la3'er is present. 
Similar lysigenous cavities are encountered in the heads of the 
Clove {Eugenia caryophyllata , Fig. 76, A) and in the skin of the 
Orange. The secretory cavities of the Rue immediately adjoin 
the upper epidermis of the leaf, and the secretion in this instance 
gradually escapes to the exterior through a special pore, some- 
what resembling a stoma, except that the slit is zigzagged and 
surrounded h\ four cells in place of two ; bending of the leaves 

' Regardins; the nature and function of these secretions see Chapter V. 



15-2 



SECRETORY CANALS 



{e.g. by the wind) leads to momentary distortion of the secretory 
space and consequent emission of part of the secretion. In the 
Mallows {Malva spp.) many parts of the plant contain irregular 
lysigenous cavities due to the confluence of cells with muci- 
laginous walls. 

In many plants, and especially in the stems and roots, the 
secretions are present in elongated structures, the secretory canals, 
which are generally schizogenous in origin, the bounding epithe- 
lium being developed throughout their entire length. In cross- 




FiG. 76. — Secretory organs. A, Lysigenous secretory cavity of the Clove 
(Eugenia caryophyllala). B, Schizogenous secretory^canal of the Ivy 
(Hedera helix) in transverse section. S,, secretory epithelium ; Sc., 
sclerenchyma sheath. 



sections these canals appear as rounded (Fig. 76, B) or oval 
cavities, whilst in longitudinal sections they are seen to be 
extensive sinuous tubes which frequently branch and fuse, thus 
forming a system often pervading all the parenchymatous tissues 
of the plant. The secretory canals are commonly [e.g. leaf of 
the Scotch Fir, Pinits syhestris ; petiole of the Ivy) enveloped 
by a sheath of thick-walled ceUs (Fig. 76 B, Sc), which prevents 
compression or collapse through turgor of the surroundmg tissue. 
They often occur in the immediate neighbourhood of the phloem 
of the vascular bundles, as in the Ivy [Hedera helix) and the 



LATICIFEROUS CELLS 153 

Umbelliferje. In the St. John's Wort and certain other cases 
the secretory cavities of the leaf are replaced by canals in the 
stem, but it may be pointed out that the difference is .one of 
shape, not of kind. 

It is a familiar fact that in some plants a milky, though 
sometimes coloured, juice (brilliant orange in the Greater Celan- 
dine, Chdidonium majits) issues from every cut or broken surface. 
This latex is especially found amongst British plants in members 
of the Poppy-family (Papaveraceffi) ,Spurge-f amily (Euphorbiacese) , 
Harebell-family (Campanulacese) , the tribe Liguliflorffi of Com- 
positae , and in the White Convolvulus (Convolvulus septum) ; but 
it is still more characteristic of certain tropical genera. The 
latex is contained in much elongated tubes which constitute a 
branched sj'stem throughout the thin-walled tissues of the plant 
and which conform to one of two types. 

In the Spurges {e.g. Wood Spurge, Euphorbia amygdaloides) 
the laticiferous tubes can be recognised already in the embryo 
as several isolated cells, situated just outside the rudimentary 
vascular system of the cotyledonary node. Each of these lati- 
ciferous cells elongates considerably as the seedling develops, 
insinuating itself between the surrounding parenchymatous cells, 
and this process of growth continues throughout the life of the 
plant. Thus, even in the adult condition, the number of lati- 
ciferous cells remains the same as in the embryo. In the course 
of their elongation the laticiferous cells develop frequent branches 
which fohow a more or less longitudinal course into all the organs 
of the plant, including the different parts of the flower, but the 
branches do not fuse with one another. In spite of this extensive 



grow 



b 



th, which leads to the penetration of the latex-tubes even 
into the ultimate branches — in tropical Spurges as much as 
50 feet above the ground — no cross-walls arise in these elements. 
On the other hand, as elongation and branching occur, repeated 
nuclear division takes place, the numerous minute nuclei in the 
adult laticiferous cell being embedded in the lining layer of 
cytoplasm which envelops a continuous vacuole occupied by 
the latex. 

In transverse sections through the mature stem of a Spurge, 
the branches of the laticiferous cells (Fig. 77, 1.) will be seen at the 
Duterhmit of the phloem (ph.) as a riumber of large circular elements 



154 



LATICIFEROUS VESSELS 



with thick white walls. Longitudinal sections, cut tangentially 
to the phloem, show the characteristic form of the tubes, and 
branching can often be recognised (Fig. 77, B). The granular 
latex, which has been coagulated bv the preservative (spirit), 
contains curious starch-grains somewhat resembling minute 
knuckle-bones. These, and the thick walls of the tubes, are 
peculiar to the Spurges, but in other respects the features just 
described are applicable to all laticiferous cells. 

By contrast laticiferous vessels, which are characteristic of 
PapaveracCt'e , Canipanulace;E, and Compositc'e, are formed from 
rows of cells (which may run m any direction, though preva- 
lently longitudinal) 
b\' the partial, or 
usually complete, 
breaking down of the 
cross-walls. Latici- 
ferous ^'essels are 
nsualh' not recognis- 
able in the embryo, 
but arise at a later 
stage in development. 
They too form an 
extensi\-e system in 
all parts of the plant, 
most commonly near 
or within the phloem. 
They are readily distinguished from the laticiferous cells, how- 
ever, by the occurrence of freciuent fusions between their 
branches, as a result of which they form a highly irregular 
network (Fig. 78, C). The mode of origin of these elements can 
seldom be recognised in the adult condition, but in the Greater 
Celandine {Clielidonium niojiis) longitudinal sections show remains 
of the partially absorbi'd trans^'erse septa quite clearl\-. 

Laticiferous vessels are abundant m the secondary phloem 
of the fleshy roots of the Dandelion [Taraxacum) or Salsify 
(Tragopogoii , Fig. 78, .\] ; m transwrse sections of preser\-ed 
material they are plainly recognisable by their lirown contents. 
They have comparatively thin walls and present a very irregular 
shape (Fig. 78, B, /.), which is due to the plane of section often 




Fig. 77. — Laticiferous cell'; (/.) in the stem of 
a Spurge (Euphorbia) in transverse (A) and 
longitudinal (B) sections, pa., parenchyma 
of cortex ; ph., secondary phloem : .r_v,, 
secondary xylcm. 



LATEX 



155 



more or less coinciding with that of a cross-connection between 
the vertical components of the sj'stem ; moreover, owing to the 
thin walls of the laticiferous vessels, the pressure of the surround- 
ing elements leads to distortion. The dense irregular network 
resulting from the numerous cross-connections is a very prominent 
feature in a radial longitudinal section (Fig. 78, C, /.). 

Latex, like milk, is an emulsion, the fluid basis of which is 
a solution of diverse 
substances (mineral salts, 
sugars, proteins, tannins, 
etc.). In some cases it 
includes an important 
active principle of the 
plant ; for example, in 
the Opium Poppy [Pa- 
paver somniferuni) the 
alkaloid morphine. The 
suspended particles in- 
clude on the one hand 
oil-drops, on the other 
granules of resin, gum, 
protein, and caoutchouc, 
whilst, as already noted, 
starch-grains occur in 
the latex of the Spurges. 
On exposure to air latex 
as a general rule congeals 
rapidly, a change often 
accompanied by dis- 
colouration 1 ; the ' ' set- 
ting " is partly due to evaporation of water, but mainly to 
a confluence of the oil-globules and suspended particles. The 
coagulation of the latex, hke that of blood, is of advantage in 
protecting and rapidly covering a wounded surface ; moreover, 
the " dressing" in this case is even antiseptic. The laticiferous 
elements further serve as food-reser\'oirs, and in this connection 

1 This feature is especially marked in the case of the latex of the 
Lacquer-tree (Rhus vermicifera), where the action is due to an oxidising 
enzyme (cf . p. 55) which converts the white juice into a dark shining varnish. 




Fig. 78. — Laticiferous vessels in the root 
of the Salsify (Tragopogon) . A, Dia- 
grammatic representation of a small 
part of a transverse section, showing 
the distribution of the laticiferous 
vessels (I.), in relation to the cam- 
bium (c), and the vessels of the 
xylem (!'.). B, a small part of the 
secondary phloem enlarged. C, Longi- 
tudinal section, p., parenchyma. 



156 



LATEX 



it may be noted that the latex of starved plants becomes thin 
and water}'. Moreover, the frequent association of these 




Fig. 79. — Row of Para-rubbcr trees (Hevca brasilicusis) on Gallev Beach 
Estates, Ceylon. (Reproduced by permission of tlie proprietors of 
the Indidrnbhcy Journal.) 



elements with the phloem, and the often intimate contact 
between them and the assimilatory tissues, seem to indicate a 
role in the transport of elaborated food-material. The by- 



RUBBER AND GUTTA-PERCHA 157 

products, which are not uncommonl}' present in considerable 
quantity in the latex, probably render these plants distasteful 
to animals. 

Those plants, whose latex contains a considerable percentage 
of caoutchouc-particles, arc of great economic importance, since 
they are the source of the rubber and gutta-percha of commerce. 
Para-rubber is obtained from a member of the Spurge-family 
(Hevea brasiliensis) in which, however, the latex is contained in 
laticiferous vessels. Other kinds are Ccara-rubber (from RIanihot 
glaziovii, a member of Euphorbiacea;) , African rubber (from 
species of Landolphia, which belong to the family Apoc^'nacece) , 
and those obtained from the Indiarubber plant (Ficiis elastica) 
and from CastiUoa elastica (Central America). Gutta-percha is 
derived from diverse members of a tropical family, the Sapotace;e, 
but here the latex is contained in vertical rows of cells. 

The latex is obtained by making V-shaped or herring-bone 
cuts in the bark of the tree and collecting the exuding juice in 
a small cup. The flow is maintained by paring off thin slices 
from the lower edges of the sloping cuts, so that the latex-tubes 
are kept open. After the latex has been artificially coagulated 
it is washed, and thereupon the raw product is vulcanised. The 
process of vulcanisation or curing involves a combination with 
sulphur in varying proportions, according to whether soft rubber 
or vulcanite is required. 



CHAPTER XIII 

Anatojiy in Relation to the Habitat 

The fundamental organisation of the plant is essentially the 
same for that of the desert as for that of the lake or mountain- 
top, but the detailed structure is nevertheless subject to con- 
siderable modifications in harmony with the differing conditions 
of the environment. The most striking of these modifications 
are related to the conditions of water-supply in the varied habitats 
in which vegetation occurs, but light and other factors may also 
play a part in moulding the structure of the plant. The effect 
of diverse conditions is most patent when the self-same species 
occupies two different habitats, as in the case of the sun- and 
shade-forms of many common plants, and the land- and water- 
forms of aquatics. 

The anatomical features exhibited by plants that have to 
economise their water-supply ^ may be taken first. Such economy 
may be necessitated by diverse factors, the most important of 
which are deficiency of water in the soil [e.g. sand-dunes) , extreme 
transpiration (as on a heath), or conditions which retard absorp- 
tion by the roots (e.g. moorlands) ; these factors may either act 
separately or several may operate simultaneously. Many of the 
structural peculiarities, associated with environments in which 
such factors prevail, are of the nature of transpiration-checks , 
whilst others are connected with the storage of water during 
times of plenty to be gradually utilised during periods of drought. 
Among the former the most important are : development of a 
thick cuticle, depression of the stomata below the general surface 
(cf. p. 96) and other modifications of the stomata, restriction of 
the latter to grooves or pits, copious production of hairs (p. loi), 
and reduction of the leaf -surf ace. - 

' Such plants arc frequently spoken of as xerophytes. 
^ See also F. & S., chapter xiii. 
158 



CLADODE OF RUSCUS 



159 



Several of these features can be observed in a cross-section of 
the cladode of the Butcher's Broom {Riiscits aculeatus, Fig. 80). 
The epidermis {cp.) has thick outer walls furnished with a pro- 
nounced cuticle, but there is no appreciable depression of the 
stomata. The latter [St.] are protected by the development, on 
the external portions of the contiguous faces of the guard-cells, 
of two pairs of ridges which extend upwards towards the pore ; 
the outer pair are the more prominent, whilst the smaller inner 




Fig. 80. — Transverse section through part of a cladode of the Butcher's 
Broom (Riiscus aculeatus), showing a single stoma (St.), the thick- 
walled epidermis (ep.), and the aqueous tissue (Aq.) below the assimi- 
latory zone. The chloroplasts are shown black. 



ones, which correspond to those present in most stomata, almost 
meet in the middle line. In consequence two antechambers are 
interposed between the actual pore of the stoma and the outside 
air, the larger and outer being known as the vestibule. The upper 
and lower epidermis are alike as regards thickening of the walls 
and distribution of stomata, and this, as well as the absence 
of a marked pahsade layer, can be related to the " edge-on " 
position which this modilied branch assumes. The assimilatory 
tissue, which is almost uniformly developed, exhibits rounded 



i6o TRANSPIRATION CHECKS 

cells having slightly thickened walls with simple pits and 
separated by narrow intercellular spaces (Fig. 80). The 
central region of the cladode is occupied by large water-storing 
cells (aqueous tissue, Aq., cf. below, p. 166) Which are enveloped 
by the assimilatory tissue. The arrangement of the vascular 
system is like that of a leaf. 

The development of a vestibule impedes the escape of 
the water-vapour exhaled from the stoma, and consequently 




Fig. Si. — Stomata. A, Holly {Ilex aqiiifolium). B, Gorse (Ule.x europcBiis). 
C, Brooklime (Veronica beccabunga). D, Yellow Pimpernel (Lysi- 
machia veniorum). 



effects reduced transpiration. Protection is more commonly 
attained by the location of the stoma at the base of a depression, 
a condition already noted in the leaf of the Iris (Fig. 46, D, p. 97). 
More pronounced examples are afforded by many plants with 
leathery leaves. In the Holly (Fig. 81, A) each stoma is situated at 
the base of a canal which is formed mainlj' as a result of the great 
thickening of the cuticle. Similarly protected stomata are well 
seen in sections through the stem-spines of the Gorse (Fig. 81, B), 
where the cuticle also reaches an extreme development. Plants 
of damp situations offer a marked contrast in these respects, 



STOMATAL GROOVES AND HOLLOWS 



i6i 



the cuticle being relatively thin and the stomata often slightly 
raised above the general surface (Fig. 8i, C, D).i 

Retarded transpiration is also very effectively attained by 
restriction of the stomata to grooves or hollows whose commu- 
nication with the exterior is often partially occluded by an 
outgrowth of hairs. A good example is furnished by the common 
Oleander [Nervum oleander). If the lower surface of a leaf of 






_ -^jCDC 




Fig. 82. — Transverse section of part of the leaf of the Oleander {Nerium 
oleander) showing a stomatal chamber. Col., collecting cells ; 
e , epidermis ; p., palisade layers ; St., stoma ; V , vascular bundle. 



this plant be examined with a lens, a large number of light- 
coloured patches appear dotted between the principal veins, 
each patch being due to a tuft of hairs arising from one of the 
numerous hollows. In transverse sections (Fig. 82) the latter 
are seen to extend inwards for slightly more than one-third the 

1 Plugging of the stomatal apertures with particles of wax is observed 
in some plants (e.g. certain Conifers), which thereby' impede the escape of 
water-vapour and consequently check transpiration, which is almost 
entirely cuticular. 



l62 



STOMATAL GROOVES AND HOLLOWS 



thickness of the leaf. The stomata (St.) are confined to the 
portions of the epidermis within these depressions, and inter- 
spersed among them are numerous thick-walled unicellular hairs. 
Each stoma is raised, on a papilla-like ring of cells, above the 
level of the epidermis lining the hollows, a fact which is not 
surprising when it is realised that, since the depressions contain 
a damp atmosphere, the stomata within them develop under 
the same conditions as those of ordinary leaves growing in moist 
situations. Other strildng features of the Oleander-leaf, apart 
from the thick cuticle on the exposed surface, are the extremely 

lacunar spongy 
tissue, and the 
presence of 
two layers of 
water - storing 
hypoderm be- 
tween the 
epidermis and 
the palisade 
tissue. 

In the She- 
Oaks (Cas'iia- 
rina) of Aus- 
tralia, which 
supply valu- 
able timbers, 
the leaves are 
scale-like and carbon dioxide assimilation is carried on by the 
green twigs, which possess longitudinal grooves (Fig. 83) to 
which the stomata (S.), protected by hairs, are restricted. The 
stomata are located at the sides of each furrow, and are 
situated close to the assimilatory tissue, which is developed in 
the same position as in other green stems (cf. below, p. 167). 

In the leaves of the Heather [Cullinia, Fig. 84, B) the stomata 
are confined to a single groo\-e [stomalal diamhcr, St.) situated 
on the under-surfacc of the leaf. The upper or outer surface 
is protected by a thick cuticle, and the aiicrture of the groove 
is closed by interlacing hairs (Fig. 84, B). The form of the 
ti ansvcrse section is roughly that of an in\-erted triangle, with 




Fig. S3. — Transverse section of a young assimilating 
stem of the Slie-Oak (Casuarina) witli the 
stomatal grooves ; one of the latter is shown 
enlarged on the right, ch., assimilatory tissue ; 
S., stoma ; Scl., sclcrenchyma. 



ROLLED LEAVES 



163 



the groove occupying a small area in the lower angle and sur- 
rounded by the very lacunar mesophyll. The rolled leaves of the 
Crowberry (Enipetrum) have a similar organisation, except that 
the stomatal chamber is much larger (Fig. 84, A). 

More extreme types of this kind are found in Grasses, where, 
moreover, the leaf is often capable of rolling and unrolling in 
response to changes in humidity. An excellent example is 
afforded by the leaves of the Marram Grass (Psamma arenaria), 
which clothes many young dunes in great profusion. A section 
across the roUed-up leaf is more or less circular in outline 
(Fig. 85, A), and is bounded on the outer 
[i.e. under) surface by an epidermis pro- 
vided with a thick cuticle [Cii) and devoid 
of stomata ; within are several layers of 
rounded thick-walled cells (Fig. 85, B, m.). 
The inner {i.e. upper) surface has a corru- 
gated appearance, being produced into a 
number of longitudinal ridges, each of 
which is traversed by a vascular bundle 
iy.h.). The intervening grooves are 
flanked by assimilatory tissue consisting 
of more or less rounded cells [A.t.). The 
scattered stomata (S.) correspond in posi- 
tion with this tissue and are consequently 
confined to the furrows, where additional 
protection is furnished by numerous stiff 
interlocking hairs (Fig. 85, A). 

The epidermal cells at the base of each groove are exceptionally 
large {hinge-cells, Fig. 85, h. and h.c.) and, being relatively thin- 
walled, are the first to lose water and shrink when transpiration 
is excessive. As a result the inner surface of the leaf contracts 
in width {i.e. transversely'), so that the fiat edges formed by 
the marginal ridges are brought together and the leaf as a whole 
becomes tubular. When the water-supply is plentiful, the 
reverse action takes place. 

The preceding paragraphs will have indicated the manifold 
devices that are employed to restrict transpiration from the 
stomata, and such are particularly characteristic of plants of 
dry situations. The efficacy of the sunken stoma and of pro- 




FiG. 84. — Diagrams of 
transverse sections 
of the leaves of the 
Crowberry [Eni- 
petrum nigrum) (A) 
and Heather (Calluna 
vulgaris) (B). St., 
stomatal chamber. 




a 



7: -a 



« 3 ^ 



e O 



6 ^ 

c/1 H 



o -^ '^ 



o " 



S5 



< 2 ^ 



o 



164 



CENTRIC LEAVES 



165 



tective hairs can be gauged by comparing the rates of evaporation 
of water from three similar wide-moutlied bottles (about 8 oz. 
capacity) , the neck of one being protected by a cardboard collar 
about I J inches high, and that of a second by a similar collar 
filled with thistle-down or cotton-wool. Each bottle should be 
filled originally with the same volume of water, and this should 
again be measured at the end of the experiment. After a few 
days' exposure in the open air, preferably when it is windy, it 
will be found 
that the bottle 
with thistle- 
down has lost 
least water, 
whilst that 
unprotected 
by a collar 
has lost most. 
There are, 
however, 
many other 
peculiarities 
that are asso- 
ciated with 
plants exhibit- 
ing a struc- 
ture that tends 
to reduce 
transpiration. 
A feature 

which often accompanies a reduction of leaf-surface is the 
absence of markedly dorsiventral structure. In its extreme 
form this results in the leaf acquiring radial organisation, 
i.e. it becomes centric, and it may then closely resemble a 
stem. Such is the leaf of the Jointed Rush [J uncus articnlatus) , 
whose sheathing base passes over into an almost cylindrical 
lamina with a slight concavity on the upper side which 
faces the stem. The transverse section (Fig. 86) presents 
an epidermis with thick outer walls and a pronounced cuticle. 
The sunken stomata [St.) are distributed at intervals 




Fig. 



E-J.S. 



86. — Transverse section ol the leaf of the Jointed 
Rush (Jiincus articnlatus). The upper figure shows 
a diagram of about half the section, the lower a 
small segment on a larger scale. C, assimilatory 
tissue ; p., parenchyma ; p.c, central cavity ; St., 
stoma ; V .b., vascular bundles. 



i66 CENTRIC LEAVES 

around the whole periphery, but in surface sections are seen 
to be arranged in approximately longitudinal series. Beneath 
the epidermis is a typical palisade tissue (C) of two to four layers 
of cells, interrupted by large gaps (the respiratory cavities) below 
the stomata. Within the assimilatory tissue follows an irregular 
ring of typical Monocotylcdonous bundles {V.b.) accompanied by 
sclerenchyma. The large central cavity {p-c), which is bounded 
by parenchyma IJ).), is interrupted at intervals by transverse 
septa containing vascular elements. The Jointed Rush affords 
an excellent example of those plants which, though growing in 
wet places, exhibit a structure that suggests a need for economy 
of water (cf. p. 177). The reason is not yet fully understood. 

A less extreme type of centric structure is found in Hakea, 
a native of Australia, in which the leaves are pinnately branched, 
the cylindrical segments being slightly flattened on the upper 
surface. A transverse section of a pinna shows the customary 
thick cuticle and deeply sunk stomata. Beneath the epidermis 
are two layers of palisade cells which completely encircle the 
central parenchyma containing three prominent vascular bundles. 
Stretched between the epidermis and this central region 
are occasional mechanical elements having thick whitish walls 
and slightly dilated ends, and probably serving to prevent collapse 
of the palisade tissue during periods of drought. The vascular 
bundles are accompanied, both on their upper and lower sides, 
by strands of sclerenchyma. The central region in which the 
bundles are embedded consists of colourless water -storing 
(aqueous) tissue in which are scattered occasional large tannin- 
cells having deep brown contents. A similar type of centric 
leaf is seen in the Scotch Fir [Pinus, cf. p. 343). 

Aqueous tissue, already observed in the Butcher's Broom 
(Fig. 80, Aq.) and Hakea, is a most prominent feature of succu- 
lents. In the leaves of the Stonecrop [Scdinji), the Prickly 
Saltwort [Salsola kali), and the Sea-blite [Sucvda], the large 
colourless and thin-walled cells in which water is stored up form 
the bulk of the leaf-tissue. The aqueous tissue occupies the 
centre of the leaf, with the assimilatory tissue towards the 
periphery ; in the Prickly Saltwort these tissues are sharply nrarked 
off from one another (cf . also Fig. 44, A , p. 94) , but in the other two 
cases there is quite a gradual transition between them. Similar 



ASSIMILATING STEMS 167 

water-storing tissue is encountered in stem-succulents (e.g. Cactus) . 
Through the loss of water, by transpiration and absorption by 
the adjacent assimilatory cells during periods of drought, the 
cells of the aqueous tissue shrink, and this results in the walls 
becoming tlirown into small folds which disappear, during the 
wet season, as the plant regains turgidity. Loss of water from 
such aqueous tissue is often retarded by the presence of thin 
mucilage in the cell-contents which exudes from a broken surface 
as a slimy fluid [e.g. in the Ice-plant, Mesembryanthemuni). 

Attention has already been drawn to the presence of chloro- 
plasts in the outer cortical cells of j'oung stems (cf. p. 88) ; 
but, where the leaf-surface is small, the stem may retain its 
assimilatory powers for some years after the inception of 
secondary thickening, and may even become enlarged by wing- 
like outgrowths whereby its efficiency is increased. The Broom 
[Cytisus scoparius), the Whortleberry [Vaccinium myrtiUits, 
Fig. 87), and the Gorse [Ulcx) furnish examples of such assimi- 
latory sterns.^ In the first-named downward prolongations from 
the margins of the leaf-bases give the stem a ridged appearance. 
A cross-section shows the usual thick-wallcd epidermis beneath 
which, in each ridge, there is a strand of fibres; but, except for 
these, the whole periphery of the cortex consists of a layer of 
palisade tissue sircceeded b)' four or five laA'ers of closely packed 
cells, also containing chloroplasts. The stomata occur at in- 
tervals throughout the parts of the epidermis overlj'ing the 
assimilatory tissue. In other respects the stem exhibits the 
normal structure of a secondarily thickened axis. 

The leaves of the Whortleberry, though not greatly reduced, 
are usually deciduous, so that during the winter the plant only 
assimilates by means of its 'winged stem. Beneath the epidermis 
is a continuous zone of assimilatory tissue (Fig. 87, as.), con- 
sisting of rounded or polygonal cells, uninterrupted by fibres. 
The inner cortex is formed by a network of chlorophylLcontaining 
cells in which the meshes are occupied by large aqueous ele- 
ments [aq.). 

It has been possible, in the preceding pages, to recognise a 
more or less intimate relation between the construction of the 
plant and the circumstances under which it lives. The cjfed 
1 See also F. & S., p. 175. 



i68 



SUN- AND SHADE-LEAVES 



of environment upon structvirc is, however, best illustrated by 
the changes exhibited in one and the same organ when growing 
under diverse conditions, and leaves are particularly plastic in 
this respect. A comparison of leaves of the same species growing 
under different conditions as regards light-intensity and degree 
of humidit}?, factors which often go hand in hand, helps to 
emphasise the significance of some of the features above con- 
sidered. Thus, the structure of the leaves of the Ground Ivy 
[Nepeta gleclwma), growing in an exposed sunny situation, is 
quite chstinct from that of the leaves of the same species, de- 




FiG. 87. — Diagram of transverse section (A), and detail of small portion (B), 
of the assimilating stem of the Whortleberry (Vacciniuin myrtiUus). 
aq., aqueous tissue ; as., assimilatory tissue ; /., fibres ; p., pith ; 
ph., phhx'm ; xy., xylem. 



veloped in the shade of a wood, and a similar contrast is found 
between the sun- and shade-forms of many other plants ^ {e.g. 
Enchanter's Nightshade, Circcra lutetiana ; Dog's Mercury, Mer- 
curialis percnnis ; Yellow Deadnettle, Lamiitm Galeobdolon, 
Fig. 88). Comparable anatomical differences are encountered, 
in trees, between the leaves on the outside of the canopy and 
those growing in the shade of the interior. 

In general shade-leaves are larger, thinner (cf. Fig. 88), and 
very commonly, if the leaf be lobed or compound, not so deeply 
cut as the corresponding sun-forms. The colour is usually a 
1 Cf. F. & S., p. 197. 



SUN- AND SHADE-LEAVES 



169 



fresher green, owing to little decomposition of the chlorophyll 
(through the absence of strong light) and the greater translu- 
cency of the leaf ; moreover, chloroplasts are not infrequent in 
the epidermal cells (e.g. Bracken). In the case of hairy leaves 
(e.g. Yellow Deadnettlc, Dog's Mercury) the production of hairs 
is usually much reduced in the shade-form. A similar difference, 
between the exposed and sheltered parts of the same plant, is 
exemplified by the more numerous hairs on the stem-leaves as 
compared with the radical leaves of species having rosettes. In 
the Meadowsweet (Spircea iilmaria) the lower leaves, protected 




Fig. 88, — Transverse sections of the sun- (A) and shade- (B) leaves of 
the Yellow Deadnettle (Lamiitm Galeobdolon), on the same scale, i.s., 
intercellular spaces ; p., palisade tissue ; s., stoma (in A, cut longitu- 
dinally) . 



among the herbage, are smooth and green on the underside, 
whilst the corresponding surface in the exposed upper leaves is 
of a greyish tinge owing to the downy felt of hairs. 

The cuticle and outer epidermal walls of the shade-forms 
are thinner than in the sun-forms, a feature which can be related 
to the greater humidity of the air. A comparison of strips of 
epidermis from the two kinds of leaves shows that the vertical 
walls of the epidermal cells tend to be straighter in the sun-form 
where the leaves are thicker (cf. p. 93). The palisade cells 
(Fig. 88, p.) are shorter and the number of palisade layers, 
as compared with lea^•es growing in bright light, exhibits rcduc- 



170 ALPINE PLANTS— AQUATICS 

tion (e.g. Beech), accompanying which there is a relative increase 
of spongy tissue, whose intercellular spaces (I'.s.) become much 
more conspicuous. The last-named feature may probably 
facilitate transpiration in a humid atmosphere. As a general 
rule the stomata tend to be level with, or even raised above, 
the epidermis in the shade-form, whilst very commonly more 
or less depressed in the sun-form. On the other hand, they are 
generally more numerous in an equivalent area of the latter, 
the fewer stomata in the shade-form being possibly related to 
the greater humidity of the surroundings. 

Analogous differences to those exhibited in sun- and shade- 
leaves are observed between the leaves of plants growing near 
the tops of high mountains and those found in lowland districts ; 
this is especially exemplified when the same species occurs in 
the two kinds of habitats {e.g. Dandelion). In this country, 
owing to the relatively low height of the mountains and the 
humid atmosphere at their summits, such differences are not 
well marked. But elsewhere {e.g. in the Alps) the leaves are 
commonly thicker and smaller, and have a better-developed 
palisade tissue, than those of the corresponding lowland form, 
though they possess a looser texture owing to the large inter- 
cellular spaces. A rosette- or dwarf-habit is very common (cf. 
Fig. 221) ; and to this may probably be related the frequent 
presence of more numerous stomata on the upper, as compared 
with the under, surfaces of the leaves, since the latter are 
closely adpressed to the humid soil. 

The relation between anatomical structure and habitat is 
nowhere more plainly shown than in the aquatics ^ among Flower- 
ing Plants, which, however, in many cases betray distinct evidence 
of their origin from terrestrial ancestors. All the submerged 
organs of such plants arc modified to suit the exceptional con- 
ditions of the environment. This is well exemplified by a study 
of the transverse section of the stent of an aquatic like the 
Hornwort {Ceratophyllum demerT.um. Fig. 89), \\iiich grows com- 
pletely under water. The vascular system forms a single central 
strand, as in a root, in correspondence with the fact that here, 
as there, the main strain is a pulling one. In fact, such a com- 
pletely submerged plant is everywhere supported by the sur- 

1 Cf. F. & S,, pp. 331 el seq. 



AQUATICS, STRUCTURE 



171 



rounding water, so that protection is alone required against the 
longitudinal tension due to the force of the current. A small 
intercellular space (c), surrounded by three or four layers of 
thin-walled parenchyma-cells, occupies the centre of the strand. 
This space is actually a longitudinal canal (the xylem-canal) 
formed by the breaking down of elements of the procambial 
strand which, in less extreme aquatics [e.g. Water Milfoil, Myrio- 
phyllum) , give rise to xylem. Beyond the enveloping parenchyma 
lies the phloem, which can be recognised by its large, empty- 
looking sieve-tubes (Fig. 89, s.t.). The absence of xylem, whilst 




Fig. 89. — Habit, and transverse section of the stem, of the Hornwort {Cera- 
tophyllum demersum). c, xylem-canal ; en., endodermis ; 5., inter- 
cellular space of cortex ; s.t., sieve-tube. 

the phloem is well represented, can be related to the fact that 
the aquatic absorbs water over its whole surface, whilst con- 
duction of elaborated food-materials remains as necessary as in 
a terrestrial plant. 

The vascular strand is sharply bounded towards the cortex 
by a well-defined endodermis (Figs. 8g and 90, en.), showing 
suberised thickenings on the radial walls, and immediatelj? wthin 
is a pericycle, just as in many young roots. The wide cortex 
beyond consists of thin-walled parenchyma in which a ring of 
large intercellular spaces (s.) surrounds the central strand. The 



172 VASCULAR SYSTEM OF AQUATICS 

epidermal cells have thin outer walls with a very thin cuticle, 
and there are no stomata. 

The structure of the leaf , as seen in cross-section, is essentially 
similar to that of the stem. Its centric character may perhaps 
be related to the absence of illumination from any particular 
direction, owing to the diffuse character of the light and the 
constant movement of the leaves by the water. ^ 

Those aquatics, whose lower parts only are submerged, 
approach more nearly to land-plants in their internal structure. 
In theMare's-tail {Hippiiris, Fig. 90) , for instance, the xj'lem (Xy.), 
which is necessary to supply water to the aerial parts, though 
not extensive, forms an obvious zone situated immediately 
within the phloem (p.) . Moreover, there is a large central pith {j)i.) 
whose presence canbe related to the growth of the shoots above the 
water and the consequent bending strains to which these are 
exposed. Hence a more peripheral disposition of the mechanical 
elements than occurs in submerged plants is rendered necessary. 

Even the more extreme aquatics, however, often preserve 
indications of their terrestrial ancestry in their vascular system. 
Thus spiral vessels commonly persist at the nodes [e.g. Pota- 
mogeton), and transient spiral vessels occur in the J'oung 
internodes. In the different species of Pondweed can be 
found various stages of the concentration and reduction of 
the vascular system. These features are least pronounced in 
the Floating Pondweed [Potamogcton natans), which develops a 
relatively large inflorescence rising some two or three inches out of 
the water, and further possesses floating leaves. The large central 
cylinder (Fig. 91) encloses eight to ten fairly well-defined vascular 
bundles, each of which is separated from its neighbours by two 
to three layers of thin-walled parenchj'ma, and consists of a 
large x^'lem-canal (C.) with accompanj'ing phloem (P.). The 
likewise broad-leaved, but completely submerged, P. lucens pre- 
sents a more marked concentration of the bundles, whose indi- 
viduality is consequently less pronounced. Lastly, the narrow- 
leaved P. pedinatus, which is also totally submerged, exhibits 
but a single xylem-canal surrounded by phloem, as in Cerato- 

1 A similar reduced structure is encountered in tlie Fennel-lea^-ed 
Pondweed (Potamogeton pccliiinli(s) and in completelj- submerged marine 
aquatics [e.g. the Grasswrack, Zostcra). 



VASCULAR SYSTEM OF AQUATICS 



173 



phyllum. It may be noted that the leaf-traces exhibit a similar 
reduction, one bundle passing to each leaf in the last-named 
species, whilst in the two former the trace consists of three 




Fig. 90. — Portion of transverse section of the stem of the Mare's-tail 
(Hippuris). Co., lacunar cortex ; En., endodermis ; p. phloem ; 
pi., pith ; Xy., xylem. 

bundles. In view of the reduction which water-plants show in 
respect to the vascular tissue, it is scarcely surprising that they 
rarely exhibit cambial activity. A trace of cambium can, how- 
ev^' be detected in the stem of the Mare"s-tail (Hippiwis). 



174 



AfiRENCHYMA 



Other examples of adaptation to the habitat-conditions in 
aquatics are : — restriction of the stomata to the upper surface 
of floating leaves, development of a covering of wax on the 




Fig. 91. — Transverse section of the vascular strand of the Floating Pond- 
weed [Potamogeton natans). C, xylem-canals ; E., endodermis ; 
I., air-canal; P., sieve-tube. 

latter, and the aerenchyma encountered in certain marsh-plants 
(e.g. Purple Loosestrife, Lyiliritm salicaria ; Gipsj'wort, Lycopus 
ciirofcetts ; Hairy Willow-herb, Epilobiiim Iiirsiilitin). Aeren- 



AERENCHYMA 



175 



chyma is a very lacunar secondary tissue, formed in place of 
cork by the phellogen, which cuts off ceUs only on the outside. 
These remain thin-walled and living and, as they enlarge, par- 
tiahy separate, and so produce a system of wide air-spaces (often 
concentric in their arrangement) to which the spongy character 
is due. The function of this tissue is to supply air to the sub- 
merged parts, and it is particularly well developed in plants 
growing in water-logged 
soil. 

The numerous large 
air-canals of aquatics are 
often segmented by plate- 
like septa or diaphragms 
composed of many small 
cells separated by minute 
intercellular perforations. 
These latter are too small 
to admit of the passage 
of water, and thus pre- 
vent the injection of the 
air-canals, when fragments 
of water-plants become 
detached, as normally 
occurs in vegetative re- 
production ; owing to the 
perforations the flow of 
air is not obstructed. 

Aquatics display a re- 
markable plasticity of 
structure, particularly in 
relation to their anatomy, as a result of which many can grow 
either completely submerged or on the mud near the edge 
of the water. A comparison of such land- and water-forms , 
belonging to the same species, clearly shows the adaptational 
significance of most of the characters of aquatics. Thus, in a 
cross-section of the stem of the land-form of the Water Starwort 
[CallitricJie, Fig. 93, a), the cortex consists of closely packed 
rounded cells with small intercellular spaces between them, 
whilst that of the water-form (Fig. 93, d) is mainly occupied by 




Fig. y2. — Photomicrograph of a small 
portion of the aerenchyma (a) of the 
Marsh Samphire (Salicornia.) Co., 
cortex; A'3'.,xylem. (Photo E. J. S.) 



176 



LAND- AND WATER-FORMS 



two large air-canals separated only by narrow strips of tissue. 
Thicker outer walls and a distinct cuticle characterise the epi- 
dermal cells of the land-form (compare Fig. 93, b and e). The 
vascular strand of the latter has an almost continuous ring of 



lOQOQaOC 




Fig. 93. — Transverse sections of the stems of the land- («-f) and water- 
forms {d-f) of the Starwort (Callilriclie siagnalis). a and d, entire 
sections ; h and e, epidermis ; c and/, vascular strands, la., lacuna ; 
Xy-, xylcm-vessels. 

xylcm (Fig. 93, c) in contrast to the one or two xylem elements 
bordering the central canal [la.) in the water-form, whose vascular 
strand is slightly smaller (Fig. 93, /). Similar differences are 
encountered between the structure of the stems of the two forms 
in other aquatics. 



LAND- AND WATER-FORMS 177 

The leaves of the Starwort do not differ appreciably in the 
two forms, since the blades are thin and broad in both, but 
there is a distinct cuticle and a better developed vascular system 
in that of the land-plant. Since stomata are in this case present 
even on (the upper sides of) the submerged leaves, the only 
difference with respect to them is that they remain closed 
in the water-form. A marked contrast is, however, pre- 
sented by the leaves of the two forms of the common Water 
Buttercup (Ranunculus aqtiatilis, Fig. 94). Those of the land- 
form (L) have a definite palisade layer, which occupies the bulk 
of the mesophyll, and the epidermis is devoid of chloroplasts 
and provided with stomata through which gaseous exchange 




Fig. 94- — Transverse sections of the leaves of the land (L) and water (W) 
forms of the Water Buttercup (Ranunculus ajuatilis). 

takes place. In the leaves of the water-form (W), on the other 
hand, palisade tissue is absent and there are large and con- 
spicuous intercellular spaces. There are no stomata in the 
epidermis whose cells contain chloroplasts. Similar differences 
can be observed between the floating and submerged leaves of 
this plant, but the contrast is more striking in the Mare's-tail 
(Hippiiris), where the leaves borne above and below water are 
of the same form. 

A combination of aquatic characteristics with others, usually 
encountered in the vegetation of dry habitats, is not uncommonly 
exhibited by plants rooted in boggj' ground, but whose shoots 
are exposed to conditions tending to encourage excessive trans- 
piration. These conditions may in part explain the phenomenon, 



17.8 



BOG-PLANTS 



which is, however, prolDably an outcome of complex causes. 
Excellent examples are furnished by the Cotton Grass [Erio- 
phormn), the Rush (cf. p. 165), and the Bulrush [Scirpus), all 
typical of such loco.lities. The principal feature reminiscent of 
aquatics is the vast system of intercellular air-canals which 
serves to supply the vuiderground organs with oxygen. 

The numerous examples cited in this chapter have shown 
that specialised structure and habitat often go hand in hand. 
Thus the plants of a salt-marsh are mostly succulents, those 
of a desert often have small leaves or assimilating stems, many 
heath-plants have rolled or hairy leaves, whilst those of a wood 




Fig. 95. — Transverse section of Clover-stem (CI.) showing penetrating 
haustoria (/(.) of Dodder {Cuscuta) [Cii.], whose stem is cut longi- 
tudinally, v., vascular tissue of Dodder. 



mostly possess large thin leaf-blades without marked checks to 
transpiration. Certain types of structure are, moreover, asso- 
ciated with certain families, as evidenced by the succulence of 
most members of the .Stonccrop-familj? (Crassulacea;) and Cactus- 
family (Cactacefe), the small-leaved habit of most plants of the 
Heather-family (Ericace;e), and the at]uatic character of all 
members of the Pondweed-fan-iily (Potamogetonacere). 

Modifications of structure in relation to the special mode of 
life are also exhil^ited by the pcirasilL's and saprophytes amongst 
Flowering Plants. ^ A striking feature of both is the feeble 
development of the xylem, no doubt in relation to the reduc- 
tion of the leaves {e.g. in the parasitic Dodder and in the 

' Cf. F, & S., pp. T39 et seq. 



PARASITES AND SAPROPHYTES 179 

saprophytic Bird's-nest Orchid). The haustoria of parasites 
arise from the stem (Dodder, Cuscuta, Fig. 95) or root (Cow- 
wheat, Melampyrum) as outgrowths (/».), which flatten out in a 
sucker-hke manner in contact with the surface of the host. The 
centre of the sucker grows out as a peg-hke process, which pene- 
trates the cortex and ultimately reaches the vascular tissue of 
the host, where it often widens out in a fan-shaped manner 
(Fig. 95, h.). At first this process consists of undifferentiated 
cells, but later those adjacent to the xylem become tracheids, 
whilst those in contact with the phloem develop as phloem-like 
elements. 

By the connection thus established between the conducting 
elements of host and parasite, the assimilatory products and 
absorptive system of the former become available for the parasite, 
which regulates any excess of water thus obtained by means of 
the hydathodes already described (cf. p. 144). The parasite, 
being relieved of those functions which necessitate elaboration 
of the vegetative structure, is enabled to utilise almost its entire 
energies for the purpose of reproduction. Simplification of 
structure, in every other direction but that which tends towards 
an increase in the output of seeds, is a marked characteristic of 
these organisms. Even the ovules (cf. p. 365) and embryos are 
simpler in construction than those of most green plants, the 
material economised in this way presumably making possible a 
quantitative increase. 



PART II 

THE LIFE-HISTORIES AND REPRODUCTION 
OF PLANTS 

CHAPTER XIV 

Simple Vegetable Organisms 

Plants exhibit a great variety of external forms, but nevertheless 
can be collected into groups having many features, both of 
structure and life-history, in common. In the preceding 
portion of this book the highly elaborated structure of the 
Flowering Plant has been almost exclusively considered, but 
very many vegetable organisms are, of course, far simpler in every 
way. For example, whole groups of lower plants lack true 
roots and possess no vascular system. A specialised conducting 
tissue consisting of xylem and phloem is only encountered 
in the Flowering Plants (Angiosperms), in the Gymnosperms 
[e.g. Fir-tree and other Conifers), and in the Ferns and their 
allies (Horsetails, Clubmosses, etc.), whilst it is lacking in 
Mosses, Liverworts, Fungi, and Seaweeds. 

The Vegetable Kingdom can therefore be conveniently 
divided into vascular and non-vascular plants.^ This difference 
may perhaps be related to the fact that a considerable per- 
centage of the latter flourish in wet or damp habitats, for it is 
only amongst some of the larger and definitely terrestrial 
Mosses that anything simulating the vascular tissues of higher 
plants is developed. The majority of non-vascular plants 
possess a type of body called a tliaUiis which exhibits no definite 
stem and leaves, in the sense in which these terms are used 
among higher plants, and is in fact in many cases a mere flattened 

' Since many so-called vascular plants possess no true vessels, these 
terms are apt to be misleading, but long use justifies their retention. 

iSo 



STRUCTURE OF CHLAMYDOMONAS i8i 

cellular expansion (Figs, io8, 109). The further classification of the 
Vegetable Kingdom (cf. Appendix, IX) is based mainly on the 
methods of reproduction and the structure of the reproductive 
organs, as will become apparent in the course of the subsequent 
chapters. 

A considerable number of the simplest types of plants 
consist of a single cell, and such minute organisms, by their 
abundance, often cause the green colouration of small stagnant 
pools. Some of the forms commonly responsible for this 
phenomenon belong to the genus Chlaniydomonas , which will 
serve to illustrate the structure of these plants, one of whose 
most striking characteristics is a power of movement from 
place to place, usually associated with animals. 

If a drop of water containing species of this genus be examined 
under the microscope, the individuals will be seen to move 
rapidly across the field of view. Each consists of a spherical 
or oval cell, about one-fiftieth of a millimetre in diameter. The 
cell is bounded by a thin wall which is often produced into a slight 
colourless papilla at the front end (Fig. 96, d), i.e. that which is 
foremost during movement. There is a single large chloroplast, 
having the shape of a deep cup with a very thick base and with 
the opening directed forwards (Fig. 96, d, ch.). Within the thick- 
ened part is embedded a pyrenoid (cf. p. 7), surrounded by a 
sheath of small starch-grains (Fig. 96. p.), but after active 
assimilation starch may also be found stored in other parts 
of the chloroplast. Adjacent to the rim of the latter lies a small 
red speck or streak of modified cytoplasm known as the eye-spot 
(stigma) (Fig. 96, s.), which is believed to be concerned with the 
perception of light intensity (cf. below). A single nucleus («.) 
lies in front of the pyrenoid suspended by cytoplasmic strands 
within the cavity of the chloroplast. 

If a stationary individual be observed under the high power 
of the microscope, a pair of small circular highly refractive 
vacuoles will be recognised in the clear cytoplasm at the front 
end (Fig. 96, C. V.) ; when these are watched closely they will be 
seen, alternately, to enlarge gradually and then suddenly to 
collapse. Similar contractile vacuoles occur in many unicellular 
plants, as well as among the lower animals, and they probably 
serve for the excretion of waste products to the exterior. 



l82 



STRUCTURE OF CHLAMYDOMONAS 



The movement of the Clilajnydomonas-individusds is accom- 
pHshed by means of two delicate thread-hke outgrowths, the 
cilia (Fig. 96, a and d), whicli can be detected arising close 
together at the front end, and are usually as long, or longer 
than, the body of the cell. They are recognisable under the 
high power in stationary individuals, but are more readily 
seen after adding a drop of iodine, which has the effect of killing 




Fig. 96. — Individuals and vegetative reproduction of Chlamydomonas. 
a, single individual of C. reinhardii ; d. ditto of C. angulosa ; 6 and 
c, Palniella-stage oi C. monadina ; e, iront end oi3.ce\loiC. reinhardii, 
greatly magnified ; /, g, h, and i, stages in the vegetative propagation 
of C. media. <:/)., chloroplast ; C.W, contractile vacuole ; n., nucleus; 
p., pyrcnoid ; s., eye-spot. (a-c and e after Goroschankin, d after 

Dili, f-i after Klebs.) 

the organisms without much change, and not only brings out the 
cilia, but also makes the nucleus more distinct. The cilia, as 
a matter of fact, are whip-like prolongations of the cytoplasm 
which, by their rapid backward strokes, pull the plant through 
the water, the movement being accompanied by a rapid rotation 
of the organism upon its axis. In returning to the front position, 
the cilia are not stretched out, and thus do not counteract the 
effect of the back stroke, the movements being thus comparable 



MOVEMENT OF CHLAMYDOMONAS 183 

to those of the arms in swimming. The action is of essentially 
the same type wherever ciha are the organs of traction. The 
rate of travel of these organisms is very slow as measured by 
inches, but is rapid relative to their size. Thus, a CJiIamydo- 
monas occupies a mere fraction of a second in traversing a distance 
equal to its own length, whilst in the case of a liner this evolution 
requires several seconds. 

The direction of movement is influenced by various external 
stimuli, such as light, distribution of chemical substances, etc. 
If some water containing Chlamydomonas be placed in a glass 
bottle covered, except for a small aperture on one side, with 
brown paper, after exposure to illumination for some hours, 
the organisms will be found to concentrate in a dense cluster 
at the spot where the beam of light penetrates. If, however, 
the latter is very intense, the plants swim away from the illumi- 
nated region, which consequently becomes colourless. The 
influence of the direction and intensity of the light on such 
movements is spoken of as pJiototaxis. Just as in the case 
of the movements of the protoplasts within the cells of higher 
plants (cf. p. 4), the movement of these unicellular organisms 
can be temporarily arrested by slight traces of anaesthetics. 

The Chlamydomonas-individnol, with the help of its chloro- 
plast, is able to manufacture food from simple inorganic sub- 
stances like any other green plant, and as a result the cell 
grows. After attaining a certain size it comes to rest, draws 
in its cilia, and begins to form daugldcr-indivuhtah (Fig. 96,/-)'). 
The protoplast contracts slightly awaj' from the wall and, after 
nuclear division has taken place, gradually constricts into 
two equal portions (/), each containing half the nucleus, chloro- 
plast, etc. The resulting segments may di^"ide again («•-/«), and 
these even for a third time (J), the successive divisions taking 
place in planes at right-angles to one another. Each segment 
develops a ceh-wall and two ciha, and thus 2, 4, or 8 new 
individuals are constituted which, apart from size, resemble the 
parent in every respect. 

This process of vegetative reproduction is completed by the 
rupture or dissolution of the membrane of the parent-cell, 
with consequent liberation of the daughter-individuals. Since, 
under favourable circumstances, the succession of events just 



i84 CHLAMYDOMONAS, VEGETATIVE REPRODUCTION 

described recurs about every twenty-four hours, one individual 
would in the course of a week give rise to 2,097,152 ! Hence the 
often rather sudden appearance of such organisms in huge 
numbers in small pieces of water. It is to be noticed that the 
protoplasm of the parent is entirely incorporated in the bodies 
of its offspring, the dead cell-membrane alone remaining 
behind ; but for the destruction of a large proportion, such 
organisms might be regarded as potentially immortal, in the 
sense that death from senile decay is unknown. 

Under certain undetermined conditions the daughter-indi- 
viduals fail to produce cilia and remain at rest within the 
parent cell-membrane, which gradually becomes mucilaginous 
(Fig, 96, c). The daughter-individuals assimilate and grow 
and sooner or later divide again, their membranes in their turn 
becoming mucilaginous. This may be repeated indefinitely 
until large gelatinous masses, enclosing numerous cells and known 
as Palmella-stages (Fig. 96, b), are produced. In the temporary 
adoption of this sedentary mode of life, Chlamydomonas and 
similar organisms exhibit a more marked resemblance to the 
majority of plants. On the return of favourable conditions, 
the individual cells acquire cilia and, escaping from the en- 
veloping mucilage, resume the motile condition. 

For a long time multiplication may be purely vegetative, 
but sooner or later — usually when growth is checked by a 
deficiency in nutritive salts — another method of reproduction 
sets in. This too is accompanied by division, but the resulting 
segments are more numerous, 16 or even 32 being formed ; 
these are liberated as sexual cells or gametes (Fig. 97, a), which 
only differ from ordinary individuals in being considerably 
smaller and, in most species of Chlamydomonas, naked {i.e. 
devoid of a cell-wall). They move for a short time, but soon 
meet in pairs, whose cilia become entangled, and thereafter a 
gradual fusion (Fig. 97, b and c) of the two protoplasts and of 
their nuclei (Fig. 97, _;', A') takes place. There results a single 
cell (termed a zygote) which moves for a brief period with the 
aid of its four cilia (Fig, 97, d) and then comes to rest. The 
cilia are withdrawn, the protoplast assumes a spherical shape 
and secretes a thick stratified membrane, and large quantities 
of a reddish-yellow oil appear in the cytoplasm. The refulting 



CHLAMYDOMONAS, sexual reproduction 185 

body (Fig. 97, e, /), known as a zygospore, sinks to the bottom 
and enters upon a resting condition. 

The process of reproduction just described is known as a 
sexual one since, as in all such processes, the fusing together 
of two distinct individuals is involved. Despite the fact that 
the gametes are outwardly all alike, there is evidently some 
internal (probably chemical) difference between them, for it 




Fig. 97. — Sexual reproduction of Chlainydomonas. a, two gametes o£ 
C. pertyi ; b and c, stages in the fusion of the same ; d, zygote, and e, 
zygospore of C. pertyi ; /, zygospore of C. reinhardii ; g, microgamete, 
and h, macrogamete of C. monadina ; i-k, stages in fusion of gametes 
of the same, ma., nucleus of macrogamete ; mi., ditto of microgamete ; 
«., fusion-nucleus (in k) ; p., pyrenoid. (All after Goroschankin.) 



has occasionally been observed that only gametes derived from 
distinct parent-individuals are attracted towards one another 
and fuse together. In one species of Chlamydomonas [C. mona- 
dina), however, sexual union always takes place between gametes 
which differ both in size and behaviour. Some, produced by 
few divisions of an individual, are large (macrogametes. Fig. 97, h), 
relatively sluggish in their movements, and soon come to rest 
(although the ciha persist), whilst others, formed by numerous 



i86 CHLAMYDOMONAS, SEXUAL REPRODUCTION 

divisions in the parent-cells, are small and quick-moving micro- 
gametes (Fig. 97, g). The zA'gote is produced by one of the 
latter approaching a passive macrogamete and fusing with it 
(Fig. 97, i^k). 

This kind of sexual union in which the two gametes are unlike 
is described as anisogamoits, in contrast to the isogamoits process, 
with fusion of similar gametes, found in the majority of species 
of Chlamydomonas. Since in higher forms of plants, where the 
differentiation between the gametes is more extreme, the female 
are motionless and the male alone motile, the state of affairs 
obtaining in C. monadina can be regarded as a simple phase of 
differentiation of sex. 

C. monadina is also peculiar in having gametes provided 
with a cell-membrane (Fig. 97, g and h), a feature also seen 
in a few other species of this genus and of its immediate allies, 
although in all other members of the Vegetable Kingdom the 
sexual cells are naked. 

Prolonged desiccation and extremes of temperature leave 
the thick-walled zj'gospores unharmed, even when the pool 
dries up. As the caked mud flakes and becomes powdery, 
both it and the zygospores are whirled away in windy weather 
as dust. If the zygospores are blown into water, their colour 
sooner or later changes to green, owing to the absorption of the 
yellowish oil, and the contents divide successively into a small 
number of parts which are liberated as new organisms by the 
bursting of the thick membrane. It is in this way that Chlamy- 
domonas and similar forms reach, and develop in, almost any 
suitable piece of water. 

The genus Carlcria, individuals of which are not infrequent 
in fresh-water, resembles Chlamydomonas in all essential re- 
spects, except for the possession of four cilia (Fig. 98, F). 
Another motile unicellular form, commonly found in small 
pools of water, is variously known by the generic names Hama- 
tococciis, Sphaerella, and sometimes Protococcits ; the last name 
is, however, altogether antiquated. The individuals (Fig. 98, 
A and C) , though moving with the aid of two ciha and of much 
the same shape and size as those of Chlamydomonas, differ 
somewhat in structure. The protoplast is separated from the 
firm bounding membrane by a wide transparent layer of mucilage 



HiEMATOCOCCUS (SPHAERELLA) 



187 



which forms the inner part of the thick wall. This mucilage is 
traversed by a number of, usually branched, thread-like pro- 
longations of the protoplast extending up to the surface layer 
(Fig. 98, A, C), and comparable to the cytoplasmic strands in the 
pit-canals of higher plants. Practically the whole of the 




Fig gS. — Hcsmatococcus (Sphaerella). A and C, Hcemalococctts phivialis, 
motile individuals (after Schmidlc). B, D, and E, H. droehakensis 
(after Wollenwcbcr). B, Resting cell ; D, single individual in optical 
section, showing the chloroplast (f.) ; It, the same, in surface view, 
showing the numerous contractile vacuoles ; F, an individual of 
Carteria (after Takeda). «., nucleus ; p., pyrenoid ; S., ej'e-spot. 



peripheral region of the cytoplasm is occupied by an ill-defined 
reticulate chloroplast (Fig. 98, D, c), containing a number (2-8) 
of scatteredjpyrenoids (p.), and bearing an eye-spot (S.) near the 
front end. The contractile vacuoles (Fig. 98, E) are numerous 
and irregularly distributed. 



H.^MATOCOCCUS (SPHAERELLA) 



The normal green colour of the individuals of Hcsmatococcus 
is often obscured by the accumulation of a bright red pigment 
(hjematochrome) which appears particularly at certain times, 
when, with the withdrawal of the cilia and thickening of the 
cell-membrane, the individuals round off to form characteristic 
resting-stages (Fig. 98, B). If these occur in quantity they may 
lend a deep red colour to the water or mud. This simple method 
of entering upon a resting-stage is also encountered in a few 
species of CJdaniydomonas {e.g. in red snow), but is much rarer 
there than in Hcsmatococcus. Reproduction of the latter is 
effected in almost the same manner as in Chlaniydomonas , the 

sexual process 
always being 
isogamous. 

Chlamyd 0- 
monas, Carteria, 
and Hcematococ- 
ciis are but re- 
presentatives of 
a whole class 
of simple green 
plants, all of 
which swim 
about like ani- 
mals during the 
greater part of 
their life.i and 

ni^st of which are inhabitants of fresh-water. In some 
genera the unicellular individuals are combined in different 
ways to form definite groups or colonies. Such is Eudorina 
(Fig. 99), which is commonly to be found in small pieces of water 
among the slimy growth co\'cring submerged parts of higher 
aquatics. The colony here consists of 16, or more commonly 32, 
globular cells embedded at regular intervals in the peripheral 
portion of a spherical or oval mass of mucilage. Each indi- 
vidual cell is almost identical in structure with a Chlamydo7nonas- 
plant, the two cilia projecting well beyond the mucilage-invest- 

> In their mode of nutrition, however, tliey arc altogether plant-hke, 
and there is nu justification for pUxcing them in the Animal Kingdom. 




Fig. 99. — Eudorina elegans. A, \'egctative colony ; 
the cilia are seen on three of the individuals. 
B, A colony showing vegetative reproduction, 
the daughter-colonies being in various stages of 
development. 



COLONIES (EUDORINA, VOLVOX) 1S9 

ment, and often being easily recognisable where they traverse the 
latter (Fig. 99, A). By the united action of the cilia of all 
the individuals, the whole colony moves rapidly through 
the water, exhibiting a simultaneous rotation upon its axis. 
Reproduction is usually effected by the subdivision of each 
of the constituent cells to form as many small daughter-colonies 
(Fig. 99, B), which are subsequently liberated by the breaking 
down of the mucilage-investment of the parent. Occasional 




Fig. 100. — Volvox. A, Colony of V. aureus (after Klein) enclosing 
five daughter-colonies, the latter already exhibiting the large cells 
(d.) from which another generation will be developed. B, Section, and 
C, surface view of single cells of V . globalor (after Meyer) . g., mucilage- 
layer of cell-wall. 



specimens of Gonium (Fig. loi, E), in which 16 Chlamydomonas- 
Uke cells are combined to form a flat plate, are not uncommonly 
associated with Eudorina. 

The spherical colonies of Volvox (the globe animalcule), 
sometimes found in large quantities in fresh-water, are far more 
elaborate. Since they are composed of several thousand cells, 
they are of quite appreciable dimensions (usually about the size 
of a pin's head) and readily visible to the naked eye (Fig. 100, A). 
The constituent cells (Fig. 100, B, C) show more resemblance to 



190 VOLVOX, PLEUROCOCCUS 

H amatococcus than to Chlamydomonas, since they have thick 
mucilaginous walls (g.) traversed by protoplasmic processes, 
which correspond in adjacent individuals. Movement is effected, 
as in Eiidorina, by the combined lashing of the pairs of cilia 
borne by all except a few large cells (Fig. i : o, A, d.) located in 
that part of the colony which is always directed backward 
during movement. It is these large cells alone that divide to 
form daughter-colonies, the latter being at first liberated into 
the hollow interior of the parent (Fig. loo. A), where they grow 
until set free by its rupture. 

Volvox thus affords a simple example of the setting apart 
of certain cells for special purposes ; most are purely vegetative 
and perish with the death of the parent, whilst a few are destined 
to undergo division and persist as daughter-colonies. This is 
in marked contrast with Eudorina, where every cell is capable 
of performing all the hfe-functions (nutrition, growth, repro- 
duction, etc.). The specialisation exhibited by Volvox brings 
with it the death of the greater part of the colony, a feature which 
is generally associated with division of labour. Eudorina and 
Volvox also exhibit a rather advanced type of sexual reproduction 
which is, however, so rarely observed that its description may be 
dispensed with. 

A large proportion of the simple unicellular and colonial green 
plants are, however, non-motile throughout the vegetative phase 
of their life-history, and only exhibit movement, if at all, in 
connection with reproduction. They thus recall the Palmella- 
stages of Chlamydomonas, to which some, indeed, show consider- 
able resemblance, e.g. the bright green gelatinous masses of 
Tetraspora, commonly found in spring in small ponds, and con- 
taining numerous cells arranged in groups of four (Fig. loi, D). 

As an example of a motionless unicellular plant, the very 
widely distributed Pleurococciis, which commonly forms the green 
powdery covering on tree-trunks, palings, etc., may be studied. 
Under the microscope the green powder is seen to consist of 
small groups of 2, 4, or rarely more, cells, intermingled with 
which are more or less numerous rounded individuals (Fig. 102, A), 
all representing different stages of Pleurococcus. Each cell 
has a moderately thick wall, and contains a single nucleus and 
a lobed chloroplast (Fig. 102, B). The isolated cells constitute 



PLEUROCOCCUS 



191 



the adult form, and sooner or later divide, commonly by two 
successive walls at right-angles to one another, to form four- 
celled packets. Each daughter-ceU is thus the quadrant of 
a sphere, but with subsequent growth it rounds off and separates 
from its neighbours, giving rise again to the adult form. This 
process of vegetative propagation is, when conditions are favour- 
able, repeated at frequent intervals, and in this way Pleitrococcus 
rapidly covers large areas.' 




Fig. ioi. — Various colonial Green Alg-^. A, Scenedesmus quadricauda. 
B, S. oblijuus, var. dimorphus. C, Pediasirum. D, Small part of a 
colony of Tetraspora (the cells are shown black). E, Goniiim. 



The ordinary cells of Pleitrococcus possess a remarkable 
power of withstanding drought, almost comparable to that 
of the zygospores of Chlatnydomonas and other simple forms 

1 In wet weather, when the cells are covered with a surface film of 
moisture, the protoplasmic contents sometimes divide to form a number 
of masses which are liberated by the breaking open of the cell-wall as 
naked biciliate motile elements, known as zoospores (cf. p. 214) ; the latter 
sooner or later come to rest and, withdrawing their cilia, secrete a membrane 
and form new Pleurococcus-individila.ls. Gametes exhibiting an isogamous 
sexual process are also stated to be produced at times. 



192 



MOTIONLESS COLONIES— PLANKTON 



in which the vegetative individuals are extremely susceptible 
to adverse conditions. It is this property which enables 
Pleurococais to exist, and even flourish, in the exposed dry 
habitats that it usually frequents. Although retaining its normal 
green appearance throughout the year, its activities are more 
or less completely arrested during prolonged periods of drought, 
when its only source of moisture is dew. 

Many of the motionless colonial green forms are common 
in freshwater pools where they occur entangled among the 
filaments of the pond-scums or in the shmy growth on the sur- 
face of larger water-plants. 
As examples we may mention 
the four- or eight-celled 
colonies of Scenedesmus (Fig. 
loi, A, B), and the character- 
istic disc-like plates of Pedi- 
aslritm, in which particularly 
the marginal cells are often 
of very distinctive form (Fig. 
loi, C). In both cases the 
ordinary course of reproduc- 
tion consists in the division 
of the contents of each cell to 
form a new colony. 

Forms like Scenedesmus, 
Pediastrum, Endorina, etc., 
together with many of the uni- 
cellular Diatoms and Desmids 
to be mentioned in the next chapter (pp. 206, 2og), very commonly 
occur in considerable numbers floating freely in the surface-layers 
of lakes, rivers, etc. (Fig. 103). These floating microscopic 
plants or vegetable Plankton form the food for many aquatic 
animals, and may at times occur in such prodigious quantity 
as to lend a visible colouration even to large pieces of water, 
a phenomenon known popularly as " water-bloom," and for 
which the Blue-green Alga; (p. 234) are most frcqucnth' responsible. 
It may be added that the character of the Plankton usually 
differs markedly in different seasons of the vear. 

The Vegetable Kingdom taken as a whole comprises sedentary 




Fig. 102. — Pleurococcus. A, Group 
of cells, under the low power. 
B, Single cell, and C, pair of 
cells, under the high power. 
ch., chloroplast ; n., nucleus ; 
w., cell-wall. 



PLANTS AND ANIMALS COMPARED 



193 



organisms, and consequently the capacity for movement has 
come to be regarded as an essentially animal characteristic. 
But the motile habit has been seen to occur in quite a consider- 
able number of unicellular and colonial aquatic plants, and 
is by no means confined to them, being observable also among 
higher forms (e.g. Mosses, Liverworts, Ferns, etc.), although 
here restricted to certain reproductive cells. The forms studied in 
this chapter force us to recognise that the power of movement 
cannot serve as an invariable distinction between the two 
Kingdoms. 

Nevertheless, viewing \ 
them as a whole, the Vege- 
table Kingdom may be 
described as essentially 
sedentary, and the Animal 
Kingdom as essentially 
motile. This distinction 
may be related to the 
necessity for animals to 
move from place to place 
in search of food, whilst 
plants, depending as they 
do almost solely on simple 
chemical compounds, can 
best obtain these by being 
stationary. It will, for 
instance, be clear that, 
for the terrestrial plant, 
the intimate contact which is necessary between root and 
soil is totally inconsistent with a motile habit. Another dis- 
tinction between higher plants and animals is the possession 
by the latter of highly developed sense-organs. This too can 
probably be related to the motile habit, with the concomitant 
necessity for rapid response to the everchanging conditions of the 
environment. It is significant that in animals like the Hydrazoon 
Obelia, which have a motile and sedentary phase in their life- 
history, the more specialised organs of sense occur in the former. 
Contractile vacuoles and eye-spots are found alike in many 
lowly plants and animals, and cannot be said to be characteristic 




Fig 



103. — Photograph of freshwater 
Diatom-Plankton ; the two princi- 
pal forms present are the filament- 
ous Melosira, and the star-shaped 
colonies of Asterionella (from 
Wesenberg-Lund) . 



194 PLANTS AND ANIMALS COMPARED 

of either. In fact, the only satisfactory criterion as to the 
inclusion of an organism in the one Kingdom or the other 
is the method of nutrition. Plants absorb their food in liquid 
or gaseous form, and, with the exception of those lacking 
chlorophyll, build up their bodies from simple inorganic com- 
pounds ; animals feed on complex organic substances, and take 
up a large part of their food in the solid form. 

The most important distinction between the two Kingdoms 
thus depends on the presence or absence of chlorophyll ; but 
even this fails in some cases, as, for example, in certain uni- 
cellular motile forms which may or may not possess a green 
pigment according to the conditions under which they live. 
Just as it is amongst the simple unicellular plants and animals 
that we find the closest resemblances, so too it is the complex 
organisms of the highest groups, as exemplified by Flowering 
Plants and Mammals, that exhibit the greatest distinction 
and portray the salient plant and animal characters in the most 
marked degree. 



CHAPTER XV 

Pond-scums, Seaweeds, etc. (Alg^) 

The organisms considered in the last chapter are simple 
representatives of a lowly group of the Vegetable Kingdom 
termed the Alga, to which also belong the Pond-scums and the 
Seaweeds. The Algs, with very few exceptions, possess 
chlorophyll and feed in the same way as other green plants. 
They are distinguished mainly by the relatively simple con- 
struction of their body, which varies from a single cell to a 
multicellular thallus of some complexity, and by the simple 
character of their reproductive organs. 

The simplest type of multicellular thallus is a filament or 
row of cylindrical cells, all nearly identical, both in structure 
and function. Examples are furnished by Ulothrix (Fig. 115, c), 
Spirogyra, and Qidogoniuni (Fig. 117, h), genera whose species 
commonly occur as floating tangled masses in ponds and stagnant 
ditches, although some are found in flowing water. In most 
such forms every cell is capable of division, but in (Edogonium 
this is restricted to occasional cells, recognisable by the presence, 
at one end of the cell, of a succession of fine rings (known as 
caps), formed singly at each division (Fig. 117, h, c). 

The habit of the plant becomes slightly more complex when 
the filaments are branched, as in Cladophora (Fig. 104, A) 
and Edocarpus (Fig. 119, /). Species of the latter commonly 
occur as brown tufts or tresses attached to diverse substrata 
in the rock-pools on the seashore, whilst those of the former, 
though also found in similar situations, are commoner in well 
aerated fresh-water. Each branch of a Cladophora terminates 
in a pointed apical cell, with specially dense green contents. 
These cells are the growing points, by whose enlargement and 
division the branches gradually lengthen, the segments cut off 

195 



196 



THALLUS OF GREEN ALG.^ 



from them not uncommonly undergoing no further division. 
Branching originates by the outgrowth from the upper ends 
of the cells, just beneath the septa, of small protrusions which 
are cut off to form the apical cells of the new branches 



--af. 








Fig, 104. — Cladophora. A, Small part of a plant of C. glomerata (after 
Migula). B, Attaching cell (after Brand). C, Apex of a branch with 
three zoosporangia, the top one having liberated its contents through 
the aperture seen on one side (after Oltmanns). D, Single cell stained 
to show nuclei («.), pyrenoids (p-), and the net-like chloroplast (c. 
(after Wettstein), E, Small part of a cell showing the manner of in- 
growth of the septum (cf. p. 24) separating two daughter-cells (after 
Strasburger). 



(Fig. 104, A) ; the degree of branching varies greatly in the 
different species and even in different individuals. 

The lowest cell of the main C/rtrfo/)/;ora-filament is modified 
as an organ of attachment, has scanty contents, and is often 
considerably branched (Fig. 104, B). Basal attaching cells 



THALLUS OF GREEN ALG^ 



197 



are found in young stages of most filamentous Algae (Figs. 115, a ; 
117, g), but in forms like Spirogyra, (Edogonitim, and Ulothrix, 
the filaments usually break off and become free-floating as they 
grow older, except in the case of species inhabiting flowing water. 
The lobes of the attaching cells grow into all the irregularities 
of the substratum 
(Fig. 104, B), to which 
they cling after the 
manner of a sucker, 
adhesion being often 
increased by the secre- 
tion of a cementing 
substance. The great 
efficiency of the at- 
taching cells is very 
evident in those 
species of Cladophora 
which grow on wave- 
beaten rocks or in 
rapidly flowing 
streams. 

The restriction of 
the power of division 
to the terminal cells 
of Cladophora marks 
a considerable step in 
the direction of divi- 
sion of labour as com- 
pared with Ulothrix, 
and this is also seen 
in the formation of 
reproductive units, 
which is almost con- 
fined to the cells of the lateral branches (cf. p. 215). A more 
extreme condition is seen in Draparnaldia (Fig. 105), which 
is not uncommon in slowly flowing water. Here the main 
axes, which serve almost solely for support, consist of large 
cells with smaU chloroplasts, whUst assimilation and reproduction 
are relegated to the densely branched laterals. 




Fig. 105. — Photograph of part of a plant o 
the Green Alga Draparnaldia, seen under 
the low power of the microscope. 
[Photo : E. J. S.] 



igS THALLUS OF GREEN ALG^ 

Division of labour is also well marked in Ectocarpus, where 
the thallus usually exhibits a differentiation into upright and 
prostrate portions, both of which are branched and filamentous, 
the latter acting as the organ of attachment. The growth 
of the erect portion takes place by the division of certain cells 
near the ends of the branches, beyond which the latter usually 
consist of almost colourless tapering cells (Fig. 119, b). 

An unusual type of thallus is that of V aucheria , the species 
of which form rather coarse dark green wefts on damp soil {e.g. 
by streams or in greenhouses) or in pools of fresh or salt water. 
The branched filaments, though of considerable width and 
length, arc uninterrupted by septa (Fig. 118, A), and might as a 
consequence be regarded as consisting of single cells. Since, 
however, each contains numerous nuclei (Fig. 118, D, n.), it is 
better compared with a multicellular organism where too the 
cytoplasm exhibits continuity (cf. p. 24), but where mechanical 
support is afforded by the walls separating the uninucleate por- 
tions. As a matter of fact the filaments of Vaucheria depend 
for their rigidity entirely upon turgor, and very readily collapse. 
Vaucheria displays a division of labour similar to that of Clado- 
phora, inasmuch as it often possesses an attaching organ and 
growth is localised at the tips of the branches. 

At low tide on rocky shores one often sees thin crinkled slimy 
sheets of a vivid green colour ; these belong to the Alga Ulva 
(the Sea Tcttuce). The thallus, which may reach a diameter of 
a foot or more, consists of two superposed layers of cells of a 
uniform character throughout (Fig. 116, E, F) except where 
they constitute the attaching base. Very young plants of Ulva, 
like those of most higher Algas, begin as a simple unbranched 
filament, whose cells, however, soon undergo division in several 
directions to produce the flat thallus. 

Most of the genera hitherto studied are so-called Green Alg;c 
{ChlorophycecB),^ the majority of which inhabit fresh-water. The 
bulk of the Seaweeds, however, are brown or red owing to the 
presence of special pigments in the chloroplasts side by side with 
the chlorophyll. The colouring matters arc readily extracted with 

1 To this group also belong the Stoneworts (Chnia), which, however, 
exhibit much greater elaboration in their vegetative and reproductive 
structures than other Green Alg.c. 



BROWN ALG^ (PH^OPHYCE^) 199 

water from dead specimens, which then assume a green colour. 
It is possible that these pigments serve to screen the chlorophyll 
against the strong light to which Seaweeds are at times exposed, 
although other functions have been attributed to them. 

Many Brown Seaweeds [PhcBophycecB), of which Eciocarpus 
(Fig. 119, /) has already furnished a relatively simple example, 
attain to very considerable 
dimensions, are far bulkier 
than any Algse yet noticed, 
and evince marked division 
of labour. Thus in Lamin- 
aria, which occurs in the 
zone just below low- tide 
level, the thallus consists of 
three distinct regions (Fig. 
106) : viz. a richly branched 
holdfast, by which the Alga 
is anchored to rocks, a stout 
cylindrical stalk which may, 
in some species, be several 
feet in length and as much 
as an inch in diameter, and 
a correspondingly large 
leathery blade. The latter 
either takes the form of a 
broad ribbon (L. saccliarina) 
or of a deeply divided frond 
like the palm of a hand (L. 
digitata, Fig. 106). Whilst 
the stalk and attaching organ 

are perennial, the blade is renewed, usually in the spring of each 
year, by means of a small-celled meristem situated at the top of 
the stalk ; the new frond therefore appears at the base of the old 
one, which eventually becomes detached by the action of the waves. 
The holdfast is composed of numerous cells with strongly thickened 
walls, and develops additional branches as the plant grows older, 
whilst simultaneously the stalk slowly increases in thickness. ^ 

' This Alga not infrequently develops on rock-fragments that arc too 
small to anchor the adult, in consequence of which the thalli and attached 




Fig. 106. — Young plant of Laminaria 
digitata, about one-fifth natural 
size. 



200 



FUCUS (THALLUS) 



The Seaweed Macrocystis, which is a close ally of Laminaria 
and particularly common in the Southern Hemisphere, attains 
to enormous dimensions, often measuring as much as 400 feet 
from end to end. 

Another large Brown Alga is the common Bladder Wrack 
{Fucus vesictdosus, Fig. 108), which, however, is found on rocks 
between low and high tide-levels, so that it is uncovered for several 
hours at a time (Fig. 107). An entire plant often attains a length 







Fig. 107. — Photograph of Fucus vesiculosus on rocks between 
tide-levels. [Photo : E. J. S.] 

of from one and a half to three feet, and exhibits three regions 
similar to those of Laminaria, but the stalk is short and the frond 
repeatedly forked (Fig. 108). Each portion possesses an obvious 
midrib (;«.), which is thicker and more pronounced in the older 
parts where it gradually passes over into the stalk. The latter 
is indeed nothing else than the persistent midrib of the first- 
formed part of the thallus. The holdfast is similar to that of 
Laminaria. 

rocks get carried out to se^. This may sometimes take place on a cori- 
giderable scale. 



FUCUS (THALLUS) 



201 



Tlie slight notch at the tip of each branch of the frond harbours 
an apical cell by whose divisions growth is ei^ected. The paired 
air-bladders (Fig. io8, a.b.), which appear as occasional large 
swellings on either side of the midrib, and to which the specific 




Fig. io8. — Fiicus. On the left, part of a thallus of the Bladder Wrack 
(F. vesictilosiis) ; on the right, of the Serrated Wrack {F. serratus). 
a.b., air-bladder ; /., fertile conceptacles ; m., midrib of thallus ; 
5., sterile conceptacles. 

name is due, increase the buoyancy of the plant when submerged, 
and may also, like the air-canals of aquatic Phanerogams, serve 
for purposes of respiration. 

Scattered irregularly over the whole surface of the frond are 
slightly protruding dots (Fig. io8, s.) marking the positions of 



202 FUCUS (THALLUS) 

small cavities {conceptacles) in the thallus. These are lined with 
hairs which often project as a minute tuft through the tiny aper- 
ture leading to the exterior. Usually some of the branches of the 
frond have swollen ends (Fig. io8) provided with much more 
conspicuous (fertile) conceptacles enclosing the sexual repro- 
ductive organs (cf. p. 224). 

The Serrated Wrack (Fiicus serratus), which is equally common 
on rocky shores, is distinguished by the toothed margin of the 
frond, the absence of air-bladders, and the less distinct swelling 
of the ends bearing the fertile conceptacles (Fig. 108, right). 

A transverse section through the frond of any Fiicus (Fig. 
121, B, p. 225) presents three regions. At the outside is a small- 
celled assimilating zone {a.), of which the most obvious part is the 
palisade-like surface-layer whose cells show occasional tangential 
division-walls. The cells of the central medulla {m.) are con- 
spicuous for their thick mucilaginous walls, by which the small 
protoplasts are widely separated. The elements in question, 
whose function is partly mechanical and partly conducting, 
are of considerable length, but roughly follow the direction of 
the thallus, and hence appear more or less oval in transverse 
section. Between the medulla and the surface region is a zone 
of relatively large storage-cells (S.) with highly refractive contents 
that are presumably products of assimilation. The outermost 
layer of the thallus is meristematic and adds to the assimi- 
latory region, whose innermost cells gradually enlarge to form 
elements of the storage tissue. Similarly the medulla slowly 
increases at the expense of the adjacent storage-cells, so that, 
although there is an obvious differentiation into three regions, 
the same cells may perform different functions in successive 
periods of the life of the thallus. 

A cross-section of the stalk or frond of Laiiiinaria shows 
essentially the same construction, and here the high specialisation 
of the Brown Algfe is evidenced by the presence of sieve-like 
areas, analogous to those of sieve-plates, on the cross-walls of 
many of the elongated elements of the medulla. Similar struc- 
tures occur in Fncus, but are not so easily recognised. 

Clothing the rocks, which for the greater part of the season 
are only reached by spray, one finds the Seaweed Pclvdia, which, 
though closely allied to Fncus, is much smaller, and in which the 



PELVETIA 



20^ 



swollen fertile ends are a particularly conspicuous feature (Fig. log). 
The structure of the thallus shows no important differences, but 
the branches of the frond are much narrower and channelled on 
their upper surface, features which aid in the retention of mois- 
ture ; moreover, the thallus is thus mainly illuminated by oblique 
light, so that the heating effect of the sun, and therefore transpira- 
tion, is presumably diminished during the hot hours of the day. 
The principal differences exhibited by Pelvetia can therefore be 
related to the special conditions of its habitat. 




Fig. 109. — Plant of Pelvetia eanaliculata, about natural size, showing the 
basal attaching disc and the prominent fertile conceptacles. 



On rocky shores the Brown Algae Pelvetia, Fticus, and Lami- 
naria often form three zones at successively lower levels, occurring 
in the order named. Other members of the group are free-floating, 
as, for example, Sargassitm hacciferum (Fig. no), huge stretches 
of which characterise the Sargasso Sea in the ^Mid-Atlantic. Many 
of the larger Brown Algse are edible, and indeed extensively 
cultivated in Japan, whilst in the West of Scotland Fucus and 
similar forms are abundantly used as manure. 

Most of the Red Algae {Rhodophycea) are much smaller plants 
preferring weakly illuminated habitats, so that they either grow at 
considerable depths below low-tide level or in shady rock-pools. 



204 



RED ALG/E (RHODOPHYCE^) 



In many the thallus is branched and thread-Hke (e.g. Calli- 
thamnion, Ceramium, Fig. iii, C), wliilst in others it is flattened 
{Plocammm, Chondriis, Fig. in, A). In the former the larger 
branches usually consist of several rows of cells, whilst in the latter 
a number of distinct tissue-regions can often be distinguished. 
An interesting form, often very abundant in the rock-pools, is 
Corallina (Fig. in, B), whose branched thallus is composed of 
numerous pinkish-white segments loosely jointed to one another, 

and densely encrusted 
with carbonate of lime. 
Similar calcareous 
Algcc, in part of larger 
dimensions, are com- 
mon in tropical seas, 
where they often play 
an important part in 
the production of 
coral-reefs, and com- 
parable forms are 
known to have contri- 
buted largely to the 
formation of certain 
limestone rocks. Di- 
verse tropical Red 
Algae are the source 
of agar-agar, a sub- 
stitute for gelatine 
extensively used for 
bacterial cultures, 
whilst Carrageen 
[Chondrus crispus, Fig. in, A) furnishes a ^■aluable invalid diet. 
There is still another large group of Algae [Cyanopliycccr] 
named after the prevailing colour, which in this case is blue-green. 
The forms in question, though relatively rare in the sea, are com- 
mon in fresh-water, and often particularly characteristic of damp 
terrestrial habitats, such as rocks exposed to a constant trickle 
and the muddy sides of watercourses. Their great adaptability 
to varied conditions is also shown by their playing the leading 
role alike in the vegetation of hot springs and in the frigid lakes 




Fig. 1 10. — Part of a plant of Saygassum 
baccifermn, about natural size, showing 
the numerous air-bladders to which the 
specific name is due. 



BLUE-GREEN ALG/E (CYAN0PHYCE.5;) 



205 



of the Antarctic continent. The Blue-green Algse are either uni- 
ceUular {Chroococcus, Fig. 112, A), colonial {GUmcapsa, Fig. 112, C), 
or filamentous [Oscillatoria ; Lyngbya, Fig. 112, F; Nosioc, 
Fig. 112, B). The cells possess either no nucleus in the ordinary 
sense, or one of very simple construction, and are in nearly every 
case devoid of a definite chloroplast. The cells or filaments, as 
the case may be, are often contained in large numbers in mucila- 
ginous envelopes (e.g. Glceocapsa, Nostoc), to which may be partly 




Fig. III. — Red Alga;. A, Carrageen (Choiidrus crispus), the dark 
patches being due to groups of carpospores (p. 228) . B, Corallina. 
C, Apex of a branch of Ceramium (after Kiitzing), with groups of 
tetraspores (shown black). 

attributed the capacity of many species to survive considerable 
periods of drought. 

Common members of the group are : Glceocapsa forming exten- 
sive gelatinous coverings on damp substrata and, under the 
microscope, resembling a blue-green Palmella-stAge (Fig. 112, C) ; 
Oscillatoria and Lyngbya, whose undifferentiated and unbranched 
filaments of discoid cells (Fig. 112, F) occur as sheets or bundles 
in freshwater pools, on moist stonework, etc. ; Tolypothnx, a 
branched filamentous form, usually aquatic (Fig. 112, E) ; and 
the mainly terrestrial Nostoc, the species of which take the form^ 



206 



DIATOMS 



of variously shaped gelatinous clumps (Fig. 112, B) in which 
are embedded numerous tortuous chains of rounded cells, inter- 
rupted here and there by slightly larger colourless ones [lieterocysts, 
seen also in Tolypothrix, Fig. 112, E, h.) of uncertain function. 
The Cyanophyceas often play a very important part in nature as 
the first plants to colonise bare ground. 

In every habitat where other Algfe occur there can usually 
be found unicellular, or more rarely colonial, forms known as 
Diatoms (Fig. 113) which possess quite special characteristics of 




Fig. 112. — Diverse Blue-green Algx-. A, Chroococcus. B, Single thread 
from colony of Nostoc (shown natural size in b). C, Glceocapsa. 
D, Anabrsna (withs-pore, Sp.). E, Tolypothrix. F,Lyngbya. G, Rivti- 
laria. h., heterocysts ; sh , sheath. 

their own, and whose exact relationship to the other groups is not 
clear. The individuals are either unattached, and in that case often 
endowed with a power of fairly rapid movement, or else fixed to 
the surface of larger Alga:: and other aquatics. The Diatoms 
are brown or greenish in colour, though a few species are colour- 
less and saprophytic, and each plant is provided with a cell-wall 
richly impregnated with silica, and usually bearing a symmetrical 
and often highly elaborate pattern of sculptured markings (Fig. 
113). With the death of the organisms the practically unaltered 
siliceous shells sink, so that, where Diatoms are plentiful, deposits 



DIATOMS 



207 



of almost pure silica slowly accumulate at the bottom of the 
water. Instances are afforded by the extensive beds of " dia- 
tomaceous earth " found at DolgeUy in Wales, at Bihn in Bohemia, 
and elsewhere, some being of marine, others of freshwater origin. 
Such deposits are utilised commercially in the preparation of 
dentifrices and, owing to the very small size of the individual 
particles, for mixing with nitroglycerine in the manufacture of 
dynamite. The extensive 
vegetable Plankton of the 
sea at times consists almost 
entirely of Diatoms. 

Although several groups 
of Algae are designated ac- 
cording to the prevailing 
colour of their chloroplasts, 
they are characterised by 
many more important 
features, and especially by 
the nature of their repro- 
ductive processes (see next 
chapter). The chloroplasts, 
in general, assume the most 
complex forms among the 
Green Alga, where there is 
often but a single one in 
each cell, a condition al- 
ready noticed in Chlamy- 
domonas and its aUies (cf. 
p. 181). Moreover, the 
chloroplasts of the Green 

Algse commonly possess one or more pyrenoids and, during active 
assimilation, starch is formed as a reserve-product, first around 
the pyrenoids, and then in the general substance of the plastid. 
Whilst most of the colonial [e.g. Scenedesvius) and less differ- 
entiated filamentous forms [e.g. Ulothrix) have a relatively 
simple undivided chloroplast, greater complexity is met with 
in many of the more highly organised Green Algae. 

The chloroplast of Ulothrix is a curved band having the form 
of an incomplete cylinder (Fig. 115, a, c), which occupies the 




Fig. 113. — Various Diatoms (only the 
siliceous shells are shown), a and 
c, Navicula ; b, Nitzschia ; d, Cym- 
hella ; e, end-view, and /, side-view 
of cells of the colonial Melosira. 



2o8 CHLOROPLASTS OF ALG^ 

lining layer of cytoplasm within the thin cell-wall ; it contains 
one or more pyrenoids.^ In Cladopliora and (Edogoniimi , on the 
other hand, the chloroplast is perforated and becomes a network 
with numerous scattered pyrenoids. In CEdogoniimi (Fig. 117, «) 
the meshes are elongated and more or less parallel to one another, 
whilst in Cladophora (Fig. 104, D) the network is irregular and 
often consists of numerous separate disc-shaped portions, 
some of which possess pyrenoids. Other peculiarities of the 
cell-structure of Cladophora are the numerous small nuclei 
(Fig. 104, D, H.), that are recognisable after careful staining, on 
the inner side of the chloroplasts, and the thick stratified wall 
to which this Alga owes its coarse texture ; the former feature 
is in marked contrast to the single nucleus found in the cells 
of Ulothrix and especially obvious in Qidogonium (Fig. 117, a, «.). 

The non-septate threads of Vaucheria possess numerous 
discoid chloroplasts which are lodged in the lining layer of cyto- 
plasm and lack pyrenoids (Fig. 118, D, c) ; this xA.lga also contrasts 
with other Chlorophyceje in producing no starch, the excess 
food being stored as oil. In the Red and Brown Algge (Fig. iig, 
d, cli.), as well as in Diatoms, the cells contain several chloro- 
plasts which usually have a more or less lobed outline, and not 
uncommonly enclose structures resembling pyrenoids ; in these 
groups the cells generally possess one or two nuclei. 

The most elaborate types of chloroplasts are, however, found 
in Spirogyra and its allies, which belong to a group of the Green 
Algffi known as the ConjugatcB, practically confined to fresh-water. 
Some of these are filamentous like Spirogyra (Fig. 4), whose 
spiral chloroplasts, one or more to each cell, have already been 
described (p. 7), and Zvgnema (Fig. 122, F), where the cells 
contain two star-shaped chloroplasts with a conspicuous pyrenoid 
[p.) at the centre of each. A large number of the Conjugata; 
are, however, unicellular forms, named Desmids (Fig. 114), 
which resemble Spirogyra and Zygnema in their methods of 
reproduction, but in many cases have even more elaborate 
chloroplasts. 

The Desmid-cell usually exhibits two symmetrical halves, 

' In the Alga Honnidiiiiii, which occurs very commonly on damp soil, 
and in which much the same type of chloroplast is encountered (I'ig. no, D), 
the character of the latter is often very readily recognised. 



DESMIDS 



209 



each containing one or two chloroplasts, and not uncommonly 
separated by a median constriction, where the single nucleus 
is situated {e.g. Cosmaritim, Fig. 114, D, E). The wall is often 
richly sculptured or provided with spinous outgrowths (Fig. 
114, A, D). In Closterium (Fig. 114, G), species of which are 




Fig. 114. — Diverse Desmids. A, Micrasterias. B and C, Eiiastrum. 
D and E, Cosmariiim {in E the top left-hand figure shows the cell 
in side-view ; the lower left-hand figure the cell in end-view). 
F, Staurastrum (the right-hand figure shows the end-view) (after 
Ostenfeld). G, Closterium. H, Cylindrocystis. I, zygospore of 
Cosmarium meneghini (after West). J, Desniidhim (filamentous). 
K, Pleurot(snium. The cell-contents are shown only in E-H. 
»., nucleus ; p., pyrenoids. 



very frequent, each half of the, usually semilunar, cell is occupied 
by a chloroplast consisting of a central rod which contains a 
row of pyrenoids {p.) and bears a number of I'adiating longi- 
tudinal plates ; the latter appear as dark green streaks when the 
cell is viewed from the surface. The small vacuoles, commonly 
seen at either end of the cell, enclose minute crystals of gypsum 
14 



210 DIVISION OF LABOUR 

exhibiting Brownian movement, and are a peculiar feature 
of tliis genus. Similar elaborate chloroplasts are found in 
Cosmarium (Fig. 114, E), where the two halves of the cell are 
usually rounded or oblong, and Micrasterias (Fig. 114, A), 
where the outline of the cell as a whole is often very complex. 
This last feature reaches a climax in those Desmids which often 
abound in the surface-water of lakes, and where, as in other 
unicellular floating organisms, the increased surface due to 
their complicated outline considerably augments their buoyancy. 

The extreme variety of the chloroplasts amongst the Algre 
is in striking contrast with their comparative uniformity amongst 
Flowering Plants. It may be remarked, however, that the 
chloroplast of the simple Alga is as much the assimilating organ 
as is the leaf in the higher plant, where, too, a great diversity 
of structure in an apparently uniform habitat is found. 

The Algse afford an excellent illustration of the fact that 
division of labour is associated with increased complexity' of 
structure. In many filamentous Green Algte all the cells, except 
that serving for attachment, may be alike in form and play an 
equal part in growth and division {e.g. Ulothrix, Spirogym). At 
the other extreme the larger Brown Algte not only exhibit a 
relegation of attachment, growth, and reproduction to definite 
parts of the thallus, but the units of which the latter is built up 
also show a certain specialisation into conducting, assimilating, 
and meristematic elements. So too, within the cell itself, definite 
bodies are organised for the carrying on of particular functions. 
Whilst in the simply constructed Blue-green Alga; it is difficult, 
or even impossible, to recognise either chloroplast or nucleus, 
such division of labour is well marked within the cell in the 
majority of plants. The complexity which the chloroplast 
sometimes attains merely affords an extreme illustration of this 
principle. 

It is probable that the complex tj'pe of cell arose from a much 
simpler one, and that similarly the multicellular organism 
had its origin in the unicellular, as is usually the case in the course 
of the individual development (cf. next chapter). High efficiency 
for particular conditions of life demand complexity of structure 
which, however, like all specialisation, tends to diminish the 
adaptability of the organism, to reduce its cajiacity for meeting 



ALG,^ AT DIFFERENT SEASONS 211 

changed conditions. It is in harmony with this that only a small 
part (viz. the relatively unspecialised gametes, see next chapter) 
of highly specialised organisms persists from one generation to 
the next, whereas in the case of a simple organism, the whole 
may survive in the bodies of its offspring. We may see in this, 
in the case of the more highly differentiated forms, a provision 
whereby the next generation is temporarily relieved of the 
trammels of the specialisation of its parents, and thus probably 
becomes better fitted to meet the extremely varied conditions 
to which the different individuals are subjected during their 
development. 

The diverse freshwater Algse are not found in equal quantity 
aU the year round, many disappearing more or less completely 
during the warmer months or during the winter. A maximum 
abundance is usually attained during the spring (the time at 
which sexual reproduction occurs in many forms), whilst 
renewed development may take place in the autumn. The 
different genera and species frequently succeed one another 
to a more or less marked extent, so that a number of phases 
can be distinguished in the algal vegetation of small ponds, 
etc. This periodicity is probably conditioned by a multi- 
plicity of factors, among which varying temperature and gas- 
content of the water play an important role. Some forms 
{e.g. Cladophora, several Diatoms), however, are encountered 
all the year round, although in varying abundance. 

Many marine Algae appear to persist throughout the year, 
although their development is often retarded during the summer 
or winter, as the case may be. The late winter is often the period 
at which sexual reproduction occurs. The power of with- 
standing low temperatures, which is so manifest in the case 
of the Seaweeds, also appertains to many freshwater Algae, 
even such delicate forms as Desmids, for instance, often occurring 
as healthy individuals beneath the ice-sheet in the depth of winter. 



CHAPTER XVI 

Reproduction among the Alg^ 

Just as the vegetative structure of the Algx shows progressive 
stages in complexity, so also do the processes of reproduction, 
although specialisation in the one respect docs not always go 
hand in hand with specialisation in the other. Thus Cladophora, 
with its markedly differentiated vegetative system, shows the 
same simple reproductive processes as does Ulothrix, and 
Edocarpus is scarcely more advanced. Our consideration of 
the life-history of Chlaniydomonas and its allies (Chapter XIV) 
has already shown that new individuals may be formed in various 
ways, and, if we survey the Alga; as a whole, it is especially in 
relation to the sexual method of reproduction that progressive 
differentiation becomes most apparent. It will be convenient 
at the outset to study an Alga like Ulothrix, which, while exempli- 
fying the various types of reproductive processes customary 
in the group, shows them in their simplest form. 

At times of active growth this Alga reproduces vegetatively 
by the mere splitting or fragmentation of its filaments into short 
lengths which develop into new threads. Not infrequently, 
however, a more specialised mode of vegetative multiplication 
obtains, spoken of as asexual reproduction owing to its general 
resemblance to the sexual method except for the absence of 
fusion. Both the sexual and asexual reproductive cells are 
motile, although the ordinary Ulothrix-iils^aiewi is without 
any power of movement. 

Asexual reproduction may take place in some or all of the 
cells of a filament. At its commencement, the protoplasts round 
off slightly and thereupon usually divide, along successive 
planes at right-angles to one another, into 2, 4, or even 8 separate 
parts, the number depending upon the size of the cell (Fig. 115, &). 

212 



REPRODUCTION OF ULOTHRIX 213 

The products of division {sp.), each of which has a chloroplast 
and pyrenoid of its own, are hberated through a small round 
hole formed in the side-wall, but remain enveloped for a few 
seconds in a thin bladder of mucilage (Fig. 115, b). Meanwhile 




Fig. 115. — Ulothrix zonaia. a, two young plants recently developed 
from zoospores ; b, thread showing stages in the development of 
zoospores (sp.) ; c, short length of vegetative filament showing 
chloroplasts (c) vT.th pyrenoids (p.) ; d, zoospore ; e and /, young 
plants developed from zoospores ; g and h, two forms of zygospores ; 
i, germination of same ; j, gamete ; k and /, stages in fusion of 
gametes ; m, the resulting zygote ; n and 0, stages in development 
and liberation of gametes (g.). a.c, attaching cell, (b and c, j-ni, 
after Klebs, all the rest after Dodel-Port.) 



each portion has developed four ciha, two contractile vacuoles, 
and an eye-spot, so that in all essential respects it resembles a 
naked Chlamydomonas. 

The pear-shaped motile elements thus formed are the 



214 REPRODUCTION OF ULOTHRIX 

asexual reproductive cells or zoospores (Fig. 115, d), which can 
swim after the manner of a Chlamydomonas for several hours, 
and thus travel some distance away from the parent-filament. 
During most of this time they seek out regions of the water which 
are well, though not excessively, illuminated, but ultimately 
their sensitiveness to light alters and they move towards darker 
spots, where they come to rest on the surface of stones, submerged 
parts of Flowering Plants, or other Alga. The zoospore flattens 
out against the substratum and the cilia are withdrawn ; a cell- 
wall is secreted (Fig. 115, e) and then, by gradual elongation 
and division, there is produced a new filament (Fig. 115, / and a), 
which soon breaks away from its attachment and becomes 
free-floating. Such asexual reproduction is obviously very prolific. 

The gametes are formed and liberated in exactly the same way 
as the zoospores, except that in the cells producing them division 
into 16 or even 32 parts is not uncommon (Fig. 115, n, 0). The 
isogamous sexual cells {j), which differ from the zoospores onl}' 
in their small size and in having but two ciha, behave just as 
in Chlamydomonas, those from different filaments ordinarily 
fusing together in pairs (k, I) to form a quadriciliate zygote (in) 
which, soon after, comes to rest and secretes a thick wall [h). 
The resulting zygospore remains in a dormant condition during 
the hot season, and may be dispersed in the same way as in 
Chlamydomonas (cf. p. 186) ; occasionally it forms a short 
outgrowth which becomes attached to some substratum (g). 
On germination, the contents divide into a small number of 
parts each of which gives rise to a new thread (Fig. 115, i). It 
appears that under certain conditions the gametes lose the ten- 
dency to fuse with one another, and round off singly to produce 
spores which in all respects resemble the z^'gospores, an indication 
that their sexual character is not very pronounced. 

At certain times the cells of the r/o//!rj,v-filament develop 
mucilaginous walls and commence to divide along successive 
planes at right-angles to one another. Since the products 
become rounded off and themselves secrete nuicilaginous walls, 
a condition is assumed closely resembling the Palmella-stagcs 
of Chlamydomonas (Fig. 116, A). These may persist for some 
considerable time, but sooner or later the cells are liberated as 
^oospores which produce new filaments. At times of extreme 



REPRODUCTION OF ULOTHRIX 



215 



drought, the ceUs of the ordinary [^/o//;;'J«-threads, after thicken- 
ing their walls and becoming laden with food-reserves, often 
fall apart and form as many separate resting spores (Fig. 116, B). 
During the greater part of its life-history Ulothrix is thus 
sedentary like the majority of plants, but motility is definitely 
associated with the customary methods of reproduction. At 
such times a form is assumed resembling that of the unicellular 




Fig. 116. — A and B, Ulothrix. A, Thread passing into Paliiiella-stage 
(after Cienkowski). B, Thread showing formation of resting spores 
(after Fritch). C and D, Hormidium (after Klebs). C, Thread under- 
going fragmentation. D, Short length of filament. E and F, JJlva 
lactuca (original). E, A small part of the thallus, seen from the 
surface under the low power. F, A few cells, more highly magnified. 
ch., chloroplast. 

organisms which are motile throughout their existence. The 
reproductive cells in question are differentiated into asexual 
zoospores and sexual gametes, but it will be gathered that 
this distinction is here no very sharp one. 

The reproduction of CladopJiora is essentially similar to that of 
Ulothrix, but the zoospores and gametes are produced in large 
numbers (Fig. 104, C), and their formation is usually restricted 
to the cells of the finer branches. During periods that are 



2i6 VEGETATIVE PROPAGATION 

unfavourable to vegetative growth {e.g. the cold months of the 
winter), the cells of this Alga often become laden with food- 
reserves and develop exceptionally thick walls. 

All the higher Algse resemble Uloihrix and Cladophora 
in the restriction of motility to the reproductive phase, which, 
however, exhibits a varying degree of specialisation in the different 
forms. Vegetative propagation by fragmentation {i.e. cell- 
separation without preparatory division), as among aquatic 
Flowering Plants, is very common. In Spirogyra, for instance, 
the threads not uncommonly break up into their constituent 
cells, each of which can divide to form a new filament. In 
the common soO. Alga Hormidium this is the customary method 
of propagation during a great part of the year (Fig. ii6, C). 
The thick-walled threads of Ctadophora above mentioned often 
fragment in a similar manner, when renewed growth takes 
place. Formation of new individuals by cell-division is the 
commonest form of reproduction in Desmids, Diatoms, and the 
unicellular Blue-green Algje, whilst the filamentous members 
of the last-named group propagate abundantly by mere frag- 
mentation. In such massive forms as Fucus, the same end is 
attained by the detachment of small adventitious branches 
of the thallus, which are often formed in bunches at points of 
injury, and are especially characteristic of the unattached species 
of this genus which occur on salt-marshes. 

The majority of the Green Alga, with the exception of the 
Conjugatfe, reproduce asexually by means of zoospores, but 
these often possess a more elaborate structure than those of 
Ulothrix. For example, in CEdogotiium, where they are pro- 
duced singly from the ordinary cells, they are much larger and 
bear a ring of ciHa a little way behind the colourless front end 
(Fig. 117, c). If filaments of this Alga are brought indoors, 
zoospores are usually formed within a few hours, and, with a 
little patience, their development and liberation can be observed. 

In the first place the protoplast contracts slightlj' away from 
the wall, and soon after this a colourless area, marking the future 
front end of the zoospore, arises on one side of the cell (Fig. 117, U). 
Around the edge of this area the numerous short cilia sprout 
out, appearing as so many fine lines. Thereupon the wall 
breaks across, near one end of the cell, and the shorter piece 



ZOOSPORES 



217 



hinges back to form an aperture through which the contents 
slowly ghde (c). At the moment of hberation the almost spherical 
zoospore is surrounded by a thin bladder of extruded mucilage (d), 
but it almost immediately commences to move away with the 
help of its cilia, though rather more slowly than in the case of 
Ulothrix or Cladophora. Sooner or later the zoospore becomes 
attached to some submerged object by its colourless front end 




Fig. 117. — (Edogonium. a, single cell, highly magnified (after Schmitz), 
showing the net-like chloroplast with pyrenoids {p.), and 
the single nucleus {".) : b-g, (Edogonium concaienatum (after Hirn) ; 
b-d, stages in formation of zoospores ; e, liberation of ditto ; / and g, 
germination of zoospores (in g the characteristic attaching cell is 
seen) ; h, CEdogonimn pachyandrium (after Hirn), showing cells with 
caps (c), and an oogonium [oog.). 



(Fig. 117, /), and the cilia are withdrawn ; then a cell-wall 
is secreted and division takes place to form a new filament, 
whilst the end in contact with the substratum grows out into 
the branched holdfast (Fig. 117, g). 

In the case of Vaiicheria, zoospore-production commences 
with the swelling of the tips of the branches, which become 
filled with abundant cytoplasm, chloroplasts, etc., and are finally 



2l8 



ZOOSPORES 



cut off by separating walls to form so-called zoosporangia 
(Fig. ii8, B). Within each the contents round off and the 
numerous nuclei take up a position outside the chloroplasts. 
A pair of cilia arise opposite each nucleus, so that a large multi- 

ciliate oval zoo- 
spore (Fig. ii8, 
C) is formed, 
which escapes 
into the water 
by the breaking 
down of the tip 
of the sporan- 
gium. Its move- 
ments are slow 
and usually cease 
after a short 
time with the 
withdrawal of 
the cilia ; a thin 
wall is formed 
and the two ends 
lengthen into 
tubes, of which 
one frequently 
penetrates the 
soil or mud and 
becomes a colour- 
less attaching 
organ (Fig. Ii8, 
E, r.). 

Zoospores are 
also encountered 
in some of the 
Brown Algre, e.g. 
in Eciocarpiis 
and Laminaria. In the former they develop in considerable 
numbers in oval sporangia, each borne laterally on a branch 
of the filament (Fig. iiq, f), and liberating its contents by 
rupture of the apex. The pear-shaped zoospores have two 




Fig. ii8. Vaucheria. A, Portion of a plant show- 
ing the branched non-septate thallus and tlic 
colourless rhizoid-like attaching organ ()-.). 
B, Zoosporangium. C, Zoospore. T>, Small 
part of thallus, showing the numerous chloro- 
plasts (c), and nuclei («■). E, Germinated 
zoospore. (C after Oltmanns ; rest original.) 



ZOOSPORES 



219 



cilia, one directed forwards and the other backwards during 
movement, and these are attached to one side adjacent to the 
chloroplast and the prominent eye-spot (Fig, 119, g). 

The production of zoospores serves as a rapid means of 




Fig. 119. — Ectocarpus, a, small part of a thread showing a gametangium 
(g.), liberating gametes ; b, tip of a branch showing hair-like termi- 
nations ; c, small part of a thread bearing a zoosporangium (s.) ; 

d, part of thread showing cell-contents and chloroplasts (ch.) ; 

e, branched filament bearing several gametangia (g.) ; /, plant of 
E. littoralis, natural size ; g, zoospore ; h and i, stages in fusion of 
gametes ; k, zygospore, still showing two chloroplasts and two eye- 
spots, (a, after Thuret ; h and d-/ after Migula ; c and g after Reinke ; 
h~h after Berthold.) 

multiplication and dispersal at times when vegetative activity 
is at its height, but this method lacks the stimulus which sexual 
fusion appears to provide. As a matter of fact continued 
asexUqil j,eproduction has, in certain cases {e.g. some Diatoms), 



220 SEXUAL REPRODUCTION (ECTOCARPUS) 

proved to be detrimental to the organism. On the other hand, 
there are quite a number of Algre in which a sexual process is 
unknown, as in the whole group of the Cyanophycete and in 
many Desmids. Apart from these, however, most Algte re- 
produce sexually at some time or other in the course of the year, 
most commonly during the spring. In many cases sexual 
fusion results, as in Ulothrix, in the production of resistant 
spores and, in contrast to the asexual method, is frequently 
associated with the onset of conditions adverse to the plant's 
growth and nutrition. 

The fusing gametes are outwardly alike in Ulothrix and 
Cladophora, as well as in all species of Chlamydomonas, excepting 
C. monadina. The dissimilarity in size and behaviour of the 
sexual cells seen in the last-named species (cf. p. 185) is paralleled, 
or even more emphasised, in the higher Algje. A relatively 
simple instance is afforded by Ectocarpiis, where the gametes 
are produced in special gametangia , occupying the same lateral 
position as the sporangia, but differing in their more elongated 
shape and in being divided into numerous compartments in 
each of which a single gamete is formed (Fig. 119, a,g.). The 
sexual cells are smaller, but otherwise resemble the zoospores. 
Despite their structural uniformity, some gametes are relatively 
sluggish, and, after a brief period of movement, become attached 
to any suitable substratum by a disc-like expansion at the end 
of their forward cilium, whilst others move actively' and for a 
much longer time. The latter ultimately collect in groups around 
the others and, sooner or later, an active gamete fuses with a 
resting one (Fig. 119, h and /). There is thus a marked difference 
in behaviour between the two fusing cells, but the differentiation 
into active males and passive females is here purely physiological, 
although in a few species of Edocarpits the two gametes differ in 
size and other respects. 

The distinction between the two sexes is much more marked in 
forms like Qidogo7tiiim, Vaitcheria, and Fucus. where one sexual 
cell (the female or egg) is large, motionless, and provided with 
plentiful food-material, whilst the other (the male or sperma- 
tozoid) is small, actively motile, and possessed of very scanty 
cytoplasm. The two kinds of gametes arc usually formed in 
special sexual organs differentiated from ordinary vegetative 




Fig. I20. — Ooganious sexual reproduction among the Algae. A-D, 
Qidogonium. A, Part of tliread of 0. pachyandrium showing antheridia 
(a.) with spermatozoids (sp.) ; B, oogonium of 0. concatenatutn, with 
two dwarf-males (d.m.) ; C, oogonium of 0. diplandrum showing the 
spermatozoid penetrating into the egg at the receptive spot (r.s.) ; 
D, antheridia of 0. landsboroughi, with escaping spermatozoids. 
E-G, Vaucheria sessilis. E, Oogonium (?) and antheridium ((J) ; 
F, oogonium with oospore and dehisced antheridium ; G, spermato- 
zoids. H-L, Funis. H, Oogonium with surrounding hairs ; I, 
branched hair bearing numerous antheridia (a.) ; J, young plant ; 
K, liberation of spermatozoids ; L, spermatozoid. (A, B, and E> after 
Hirn ; C after Juranyi ; E-G after Sachs ; H-K after Thuret ; L after 
Guignard.) 



222 SEXUAL REPRODUCTION (CEDOGONIUM) 

cells; that producing the egg {ovum) is termed the oogonium, 
whilst that forming the spermatozoids is known as the 
antheridium. 

In (Edogoni'um the oogonia are more or less o\'al cells which 
may arise in any part of the filament and occur either singly or 
in short chains (Fig. 117, /;, oog.). At one point the wall of the 
oogonium develops a small papilla by the breaking down of 
whose tip an aperture for the entry of the male cell is created ; 
in some species of (Edogonium, however, opening is effected by a 
complete transverse split in the wall. The single egg, formed by 
the contracted protoplast of the oogonium, develops a small 
colourless area, adjacent to the aperture, known as the receptive 
spot (Fig. 120, C, r.s.), and at this point a quantity of mucilage 
is extruded shortly before fertilisation. 

The antheridia are small and tabular, being formed by re- 
peated transverse division of cells of the filament (Fig. 120, A, a.). 
Each produces one, or more commonly two, spermatozoids {sp.) 
which are diminutive, though almost colourless, replicas of the 
zoospores, and which are liberated in the same manner (Fig. 120, 
D). Should a spermatozoid in the course of its movement come 
into the neighbourhood of an oogonium, it appears to be attracted, 
probably by some chemical substance ^ in the extruded mucilage, 
and, passing through the aperture, penetrates into the egg 
(Fig. 120, C), the cytoplasm and nucleus of the one thereupon 
fusing with those of the other. Neither sexual cell can develop 
independently, an indication of specialisation as compared 
with Ulothrix. 

It is customary to speak of such sexual union as fertilisalion, 
the egg being said to be fertilised by the spermatozoid. The 
effect of the fusion of a spermatozoid with an egg is probably 
of the nature of a chemical and physical stimulus, without which 
further development of the ovum is impossible. This view is 
supported by the fact that the eggs of Sea-urchins have been 
induced to develop into embryos by mere immersion in suitable 
solutions, whilst those of the Frog have been caused to undergo 

'■ The influence exerted by chemical substances on the direction of move- 
ment of motile elements is spoken of as cheniotaxis, and the positive chemo- 
taxis evident in sexual union is only one uf many examples of such 
chemical stimulation (cf. p. 232). 



SEXUAL REPRODUCTION ((EDOGONIUM) 223 

the first stages of development by mere pricking. Moreover, 
in plants, fertilisation sometimes stimulates other cells near the 
egg to develop into embryos (cf. p. 372). 

The fertilised eggs, or oospores, develop thick protective waUs 
and fatty pigmented contents and, as the filaments containing 
them die away, sink to the bottom of the water. Here they pass 
through a prolonged resting period, and, should the pond dry 
up, may be dispersed by the wind. In the few cases in which 
their germination has been observed, they gave rise to three or 
four ordinary zoospores, which were set free by the bursting of 
the thick membrane. 

The sexual organs are arranged in various ways in the different 
species of CEdogonium, male and female sometimes occurring in 
the same filament (moncecious forms), sometimes in different 
filaments (dioecious forms) ; in the latter case the male plants 
often consist of only a few cells [dwarf males, Fig. 120, B,^.wi.), 
and arise from special smaller zoospores which become attached 
to the female plant, on or near an oogonium. 

It will be evident that CEdogonium exhibits considerable 
specialisation in its methods of multiplication, and not the least 
conspicuous feature is the division of labour manifest in the 
oogamoii-s sexual reproduction. Owing to its stationary character, 
the egg can possess the greater bulk which a more adequate 
provision of food-material for the benefit of the next generation 
necessarily entails (cf. p. 220). Since the spermatozoids con- 
tribute nothing to this food-supply, they can be correspondingly 
smaller, and therefore, without additional strain on the organism, 
produced in larger numbers, whereby the chance of fertilisation 
occurring is greatly increased. The probability of fusion between 
the two gametes is, moreover, doubled by one of them remaining 
stationary. The greater certainty of sexual union admits of a 
corresponding decrease in the production of eggs, which will 
afford as many offspring as would a larger number of motile 
female gametes. These remarks apply with equal force to all 
plants in which oogamy occurs. 

The oogonia and antheridia of Vaucheria are produced near 
one another as outgrowths of the main filament or of short lateral 
branches, from which in either case they become separated by 
a septum (Fig. 120, E). Their relative positions are very 



224 SEXUAL REPRODUCTION (VAUCHERIA) 

diverse, but the adjacent sexual organs usually mature almost 
simultaneously, so that self-fertilisation is probably the rule. 
The more or less oval oogonium ($) develops a protrusion on 
one side, whose tip becomes mucilaginous, and breaks down to 
form the aperture through which the male cell enters. The 
contents are rich in chloroplasts and at first multinucleate, but 
in the mature egg, which possesses a pronounced receptive spot 
opposite the opening, only one large nucleus persists. 

The antheridium is a coiled tube tapering slightly near the 
apex, and giving rise, by division of its contents, to numerous 
minute spermatozoids (Fig. 120, E, ^) ; the latter are pear- 
shaped bodies with two laterally attached cilia, a very small 
yellowish chloroplast, and a prominent nucleus (Fig. 120, G), 
and are liberated by a breaking open of the tip of the antheridium 
(Fig. 120, F). The attraction of the spermatozoid towards the 
egg is probably again connected with the extrusion of mucilaginous 
matter by the latter. After fusion, the oospore becomes en- 
veloped by a thick wall and accumulates large stores of reserve 
oil (Fig. 120, F) ; it then enters on the usual resting period, 
which is ultimately terminated by the direct development of a 
new plant. 

Apart from the vegetative propagation above described, 
Fucus exhibits only sexual reproduction, the antheridia and 
oogonia being developed in the large fertile conceptacles occupying 
the swollen tips of the thallus (Fig. 108, /). In some species 
(e.g. F. platycarpus) the two kinds of sexual organs occur in the 
same conceptacle, but in F. vesiculosus and F. serratns there are 
distinct male and female plants. The globular cavities of the 
conceptacles (Fig. 121, A) are separated from the rest of the 
thallus by a wall [w.) composed of several la^'ers of flattened 
cells, from whose inner surface arise numerous unbranched 
multicellular hairs which bend towards, and indeed often protrude 
from, the small aperture ; in the fertile conceptacles the sexual 
organs are interspersed among these hairs (Fig. 121, A). 

The oval oogonia possess a thick transparent several-layered 
membrane, and are seated on a short stalk which arises directly 
from the wall of the conceptacle (Fig. 120, H ; 121, A). At 
maturity the contents are divided into eight uninucleate eggs, 
containing abundant chloroplasts and separated by delicate 



SEXUAL REPRODUCTION (FUCUS) 225 

septa. The antheridia are oval cells, which likewise possess 
relatively thick walls and occupy the ends of most of the short 
lower segments of richly branched hairs (Fig. 120, I).i In each 
are formed numerous minute bicihate spermatozoids (Fig. 120, L), 
containing a well-marked nucleus, but only traces of a chloroplast. 
The mature antheridia have a yellowish colour, which they impart 
to the entire conceptacle, and by this means, in the dicecious 
species, the male plants can be distinguished. 







Fig. 121. — The Bladder Wrack [Fiiciis vesictilosus). A, Transverse section 
through a fertile conceptacle containing oogonia, in different stages 
of development. B, Small part of a transverse section through the 
thallus, more highly magnified, a., assimilating laj'er ; m., medulla ; 
s., storage cells ; w., wall of conceptacle. 



WTien the se.xual cells are ripe, the outermost layer of the 
antheridium or oogonium, as the case may be, breaks open and 
sets free the contents which remain enclosed in the inner part 
of the wall. The gradual extrusion of the packets of ova, or 
spermatozoids, from the opening of the conceptacle often occurs 
between the tides, and is probably largely brought about by 
expansion of the mucilage, secreted by the hairs, combined with 

' These antheridial hairs are best examined by teasing out the contents 
of a male conceptacle in a drop of water. 

15 



226 SEXUAL REPRODUCTION (FUCUS) 

desiccation and contraction. Tlie sea-water dissolves the mem- 
branes still enveloping the sexual cells (Fig. 120, K), and the ova, 
which have now assumed a spherical form, become fertilised by the 
actively moving spermatozoids. The oospore secretes a thin mem- 
brane and immediately, without a resting period, develops into 
a new FMCws-thallus. The young plant is at first spherical, but at 
an early stage produces the basal holdfast (Fig. 120, J) and acquires 
a strap-shaped form, and this is soon followed by branching. 

In Pelvetia, where both sexual organs occur in the same 
conceptacle, the oogonium has an exceptionally thick wall and 
produces only two eggs. Extrusion of the sexual cells takes 
place in the same way as in Fiicus, but the ova retain their 
thick mucilaginous investment which the spermatozoids have 
to penetrate, and which envelops the young plant during the 
early stages of development. 

Fiicus and Pelvetia differ from most other oogamous plants 
in the number of eggs and in the fact that the latter are fertilised 
outside the plant, in both of which respects these Algje appear 
relatively unspecialised. Normally the female organ contains 
but a single ovum (cf. Qldogoniiim, VancJieria, and the higher 
plants), and in this connection it is interesting to note the re- 
duction of the eggs to two in Pelvetia, though here also the 
nucleus divides into eight parts, six of which abort. 

The Conjugata; (cf. p. 208) owe their name to a very special 
type of sexual reproduction [conjugation), in which neither gamete 
is free-swimming. In the filamentous forms, such as Spirogyra 
and Zygnema, two threads become ranged parallel to one another, 
and their opposing ceUs develop finger-like protrusions which 
grow towards each other till they meet and fuse (Fig. 122, B) ; 
after this the separating wall breaks down, so that an open tube 
(the conjugation canal, Fig. 122, c.c.) is established. In 
Spirogyra and many species of Zygnema the development of 
processes always commences a little sooner on the one filament 
than on the other, and a similar difference is observed with 
respect to the contraction of the protoplasts which now ensues. 
The cells of the filament that first put out processes, in these 
cases, act as males, since their contents commence to glide over, 
through the conjugation canals, into the opposite cells (Fig. 
122, B, li), with whose passi\'e (female) protoplasts they fuse. 



SEXUAL REPRODUCTION (CONJUGAT/E) 227 

In some species of Zygnema (e.g. Z. pectinatnm), however, the 
events leading up to conjugation take place simultaneously in 
the two filaments, and the fusing protoplasts meet in the middle 
of the conjugation canal ; in such forms there is no outward 
differentiation of sex, the gametes being isogamous as in Chlaniy- 
domofias and Ulothrix (Fig. 122, D). But even in the case first 




Fig. 122. — Sexual reproduction in various Conjugata;. A, Spirogyra 
weberi, showing lateral conjugation (after Petit). B and C, Spirogyra 
bellis. B, successive stages (a and b) in conjugation ; C, completed 
conjugation. D, ladder-like, and E, lateral conjugation in Zygnema 
peciinatum. F, small part of filament of same, c.c, conjugation 
canal ; n., nucleus ; p., pyrcnoid ; z., zygospore ; ^ = male cells. 

described the gametes are merely distinguished physiologically, 
although a difference of size occurs in one of the related genera. 
The sexual process of the Conjugata; therefore, whilst peculiar 
in itself, shows the same analogous series of stages to that 
observed in other Algse. 

In some species, both of Spirogyra a.nd Zygnema, sexual union 
may take place between adjacent cells of the same filament 



228 SEXUAL REPRODUCTION (CONJUGATE) 

(Fig. 122, A and E), the conjugation canals (cc.) forming loop- 
like connections between their contiguous ends. Here the threads 
must be regarded as including cells of both sexes, the zygospores 
as before being formed either in the conjugation canal or in one 
of the two cells. The fact that both methods of conjugation may 
occur simultaneously in the same mass of Spirogyra or Zygnema 
indicates that sexual differentiation between the filaments is not 
very profound. 

In all cases the zygospores (Fig. 122, z.) secrete a thick several- 
layered wall and pass through a prolonged resting period, during 
which they may be distributed in the customary manner by the 
wind. On germination the membrane usually bursts at two 
places, the contents growing out in the one direction to produce 
the new filament, and in the other direction to form the colourless 
attaching cell. 

Among the Desmids where, except in a few forms, sexual 
reproduction is rarely observed, fusion takes place between the 
liberated protoplasts of two individuals, which usually become 
enveloped in mucilage. The empty halves of their cell-walls 
are often recognisable near the resulting zygospores (Fig. 114, I), 
which frequentl}' have elaborately sculptured membranes. In 
some of the Diatoms a sexual process of an analogous type is 
encountered. 

The Red Algs possess only motionless reproductive cells, and 
exhibit a very complex se.xual process whose description is 
beyond the scope of this book. ]\Iention may, howe^'e^, be made 
of the copious production of filamentous outgrowths from the 
female organ after fertilisation, the ends of these threads giving 
rise to special asexual reproductive cells known as carposporcs. 
The dense clusters thus produced are often conspicuous as minute 
oval patches of a darker colour (Fig. iii. A). The ordinary 
asexual cells, so-called tctraspores, arc produced in fours in small 
usually spherical sporangia, and arc readily recognised on the 
thalli under the microscope (Fig. ill, C). 

The examples of reproductive processes among the Algas 
might be multiplied considerably, but sufficient have been 
described to show the diversity of methods by which the same 
end, namely the multiplication and perpetuation of the species, 
is attained. It is the result rather than the means which must 



REPRODUCTION OF ALG7E 229 

be regarded as the more important biological phenomenon. The 
methods of reproduction have been seen to vary, not only 
amongst closely related forms, but even in one and the same 
species. But, with all this variety, the outcome is a cell, or 
cells, each capable of giving rise to a new plant. 

Why reproductive bodies, capable of resisting adverse con- 
ditions and of remaining dormant for considerable periods, should 
be more particularly produced by sexual fusion, is a question as 
yet unanswered. It is, however, easy to recognise the importance 
of a second type of multiplication, by cells which, being un- 
provided with either food-reserves or resistant walls, can be 
formed rapidly and in large numbers. The association of this 
type with asexual reproduction may perhaps be related to the 
greater facility of responding to favourable conditions. 



CHAPTER XVII 

The Structure and Reproduction of the Fungi 

The Alga; are not the only plants in which the body is a thallus 
of relatively simple construction, but the remaining representa- 
tives of the Thallophyta, the lowest class of the Vegetable King- 
dom, are characterised by the absence of chlorophyll. They are, 
consequently, like the colourless saprophytes and parasites among 
higher plants, dependent upon organic material elaborated by 
other organisms. The plants in question are grouped as Fungi and 
show many peculiarities, both in vegetative structure and the 
nature of their reproductive processes. 

A considerable number derive all their nourishment from 
other living plants or animals, such parasites, exemplified by the 
Smut of Wheat, the Gooseberry Mildew, the Potato Blight, the 
Salmon and Silkworm diseases, often doing serious harm to their 
host. Numerous Fungi, however, live upon decaying organic 
matter {e.g. many Moulds and Toadstools), and these saprophytes 
play an important part in nature in connection with processes 
of decay. 

The plant-body is of a peculiar type, consisting generally of 
a loose weft, the mycelium (Fig. 125, a), composed of very delicate 
branched threads or liyphcr, which are usually colourless, and 
which may or may not be septate (Fig. 129, a). The narrow 
diameter of the hyph^e facilitates their penetration either into 
the interior of a host (parasites), or between the particles 
of decaying organic material (saprophj'tes). The hyph;e, more- 
over, secrete at their tips various cnz3'mes (cf. p. 53), which 
bring about solution of the obstructing cell-walls and also convert 
the organic material into a readily assimilated form, a single 
species of Fungus producing a number of different enzymes, 

230 



MYCELIUM AND HYPH^— PHYCOMYCETES 231 

according to the substratum upon which it occurs. The extreme 
simplicity of the vegetative structure may well be compared 
with that of parasitic Flowering Plants (cf. p. 179), some of 
which — encountered only in the Tropics — have as a matter of 
fact a plant-body so reduced that it resembles a mycelium. 

In some Fungi the wall of the hypha; consists of cellulose, 
but much more commonly of a complex nitrogenous compound 
similar to the chitin found in animals, together with other sub- 
stances such as pectose, callose, etc. Embedded in the lining 
layer of cytoplasm in the lower forms are numerous minute 
nuclei, but in the septate hyphse of the higher types there are 
usually only one or two in each cell ; neither plastids nor starch- 
grains are ever present, but there are often small oil-drops and 
sometimes crystalline albuminous bodies. The central vacuole 
is prominently developed. Where abundant food-storage occurs, 
as, for instance, in the reproductive cells, it is customary to find 
the polysaccharide glycogen, which can be recognised by the deep 
brown colouration assumed with iodine. In coloured hyphse, 
such as occur in species of Peziza, etc., the pigment is generally 
confined to the cell-wall. 

The Fungi are classified in three groups — Phycomycetes, 
Ascomycetes, and Basidiomycetes — each of which has so many 
characteristic features that it will be convenient to consider them 
separately. The Phycomycetes, which are not modified to so 
marked an extent as the other two groups, include forms which 
usually show a well-marked sexual process, and which, in this 
and other respects, resemble Algge such as Vaucheria. The 
hyphas, for example, contain numerous nuclei, and often only 
exhibit transverse walls in relation to the formation of repro- 
ductive bodies. The group includes many common parasites, 
such as Cystopus (the White Rust of Crucifera;, Fig. 123, A), 
Pythinm debaryanum (the cause of the " damping off " of seed- 
lings, Fig. 124, B), Phytophthora infestans (the Potato Blight), 
Empnsa (responsible for a disease of house-flies), as well as the 
saprophytes Mucor (the black Mould appearing on jam, bread, 
etc.), Saprolegnia, and Achlya (the last two frequent on decaying 
water-plants). 

Cystopus, a species of which often attacks the Shepherd's 
Purse, furnishes a typical example, whose life-history can easily 



232 CYSTOPUS 

be studied. The parts affected by the Fungus, most commonly 
situated in tlie region of the inflorescence, are swollen and con- 
torted (Fig. 123, A), and exhibit a white surface which looks as 
though it had been whitewashed. Such enlargement, or hyper- 
trophy, is a frequent symptom of fungal attack, and is an outcome 
of the abnormal development of the diseased tissue, whose cells 
undergo increase in size with, or without, division. A longitudinal 
section through such a blister (best stained with eosin) shows the 
hyphffi of the parasite ramifying in all directions within the 
intercellular spaces and middle lameUs of the host (Fig. 123, B, /;.). 
Here and there, however, small club-shaped branchlets (the 
haustoria, S.) will be observed penetrating into the actual cell- 
cavities, and by this means the Fungus absorbs food-material 
elaborated by the host. 

Near the surface of the stem the hyphse are more densely 
packed, and their almost parallel branches form a pile-like felt 
(the hymenumi, Fig. 123, C) which ruptures the overlying epi- 
dermis and causes the white appearance above mentioned. The 
slightly swollen ends of the hyphs: of the hymenium exhibit 
various stages of constriction, resulting in the gradual formation 
of chains of spherical structures called gonidia (Sp.), the oldest 
of which is farthest away from the point of origin. As the short 
fragile stalks connecting the gonidia with one another get broken 
across, the latter are removed by the wind, and sometimes travel 
many yards before reaching the ground. 

When rain or heavy dew causes a sufficient accumulation of 
moisture, the contents of the gonidia divide into se'S'eral parts, 
which are liberated as minute colourless zoospores (Fig. 123, G), 
swimming by means of a pair of cilia. Many doubtless perish 
before reaching a suitable host, but should they encounter 
seedlings of a Cruciferous plant, they come to rest on the surface, 
secrete a membrane, and elongate into a short hypha which 
penetrates into the interior by way of a stoma. The stimulus 
directing the movement of the zoospore towards the host-plant 
is probably a chemotactic one (p. 222), whilst the growth of the 
hypha into the interior affords an example of positi^'e chemo- 
tropism. For some weeks further development of the Fungus 
consists in the ramification and gradual spread of the hj'pha; 
through the tissues of the host, until a sufficiently large haustorial 




Fig. 123. — The White Rust of Crucifcrae [Cystopiis candidus). A, Diseased 
inflorescence of Shepherd's Purse, showing the white patches where 
the gonidia of the Fungus are being formed. B, Hypha (A.) with haus- 
toria (S.), as seen in a longitudinal section between the cells (Ce.) of 
the host. C, Transverse section near surface of host, showing hyphae (A.) 
and gonidia (Sp.). D, Antheridium (a.) and oogonium (0.) (after De 
Bary). E, The same in section at the time of fertilisation (after 
Stevens). F, Germination of oospore (after De Bary). G, Zoospores 
(after De Bary). (Figs. A-C, original.) 



233 



234 CYSTOPUS 

system has been created to supply the material necessary for 
the production of gonidia. 

Sexual reproductive organs are usually produced towards the 
end of the host's flowering period {i.e. when the supply of nutri- 
ment probably becomes deficient), and arise in the interior of 
the infected regions. They consist of spherical oogonia (Fig. 
123, D, 0.), generally situated at the ends of the same hyphffi as 
bear the club-shaped anthcridia {a.) at a slightly lower level, 
although in some cases the two kinds of sexual organs are formed 
on neighbouring hyphae. Both are multinucleate and, during 
development, undergo differentiation of their protoplasmic con- 
tents into a denser central and a less dense peripheral region 
(Fig. 123, E) ; the former constitutes the egg in the case of the 
oogonium and the male gamete in the case of the antheridium, 
while the outer region plays no part in the sexual fusion. 

The antheridium becomes applied to the female organ and 
puts out a slender tube which, piercing the oogonial wall, pene- 
trates through the peripheral cytoplasm up to the egg (Fig. 
123, D, E). The tip of the tube thereupon opens and the male 
gamete passes through it to fertilise the ovum, the process in- 
volving nuclear and cytoplasmic fusion in the usual way. The 
product becomes invested by a thick dark-coloured wall (Fig. 
123, F). After the decay of the host the oospores, which con- 
stitute the resting-stage in the life-history, may remain dormant 
in the soil for a considerable period. When conditions suitable 
for germination occur, the contents divide to form numerous 
zoospores which, after rupture of the thick wall, infect seedlings 
in the way already described. 

The Potato Blight {Phytophthora injestans) and the damping- 
off Fungus [Pythium debaryamim) have life-histories very similar 
to that of Cystopns, except that their gonidia can, under certain 
circumstances, germinate direct into a new plant {i.e. without 
forming zoospores). In both cases the mycelium is intercellular, 
and the asexual reproductive organs alone appear on the surface 
of the host. In the Potato Blight the oval or elliptical gonidia 
are formed singly at the ends of branched hypha;, which emerge 
through the stomata of the diseased leaves (Fig. 124, A). If 
blown on to the lea\'es of another Potato-jilant, the gonidia 
grow out direct into an infecting hypha ; whilst, if they fall on 



PHYTOPHTHORA 



233 



the ground, they can, in the presence of moisture, produce zoo- 
spores, as in Cysiopns. The first signs of disease are discoloured 
spots exhibiting a dark central region surrounded by successive 
zones of greyish and pale green tissue, which rapidly become 
brown or even blackish ; closer inspection discloses the white 




Fig. 124. — Asexual reproduction in various Oomycetes. A, Small part 
of epidermis of Potato-leaf, infected with Blight (Phytophthora infes- 
ians), showing branched hypha; bearing gonidia [g.) emerging from 
the stomata. B, Seedling of Cress which is " damping off," due to 
an attack of Pythium debaryarium ; the point at which the hypocotyl 
is giving way is indicated by an arrow. C, Hypha with sporangia 
of the same. D, Young, and E older, sporangia of Saprolegnia, 
showing numerous zoospores [Sp.). (A after Strasburger ; B after 
Miyake ; C after Hesse ; D and E after Thuret.) 



tufts of hyphse bearing the gonidia, especially on the lower surface 
of the infected leaf. The hyphee of the parasite gradually spread 
to the underground parts, thus infecting the tubers, so that 
early removal of diseased shoots is advisable. 

An attack of Pythium results in a rapid softening of the 



236 PYTHIUM AND SAPROLEGNIA 

hypocotyls of the diseased seedlings (Fig. 124, B), which soon give 
way at tliis point and collapse. The rounded gonidia, which are 
borne on simple or forked hyphge (Fig. 124, C), usually give rise to 
zoospores without becoming detached, so that the disease rapidly 
spreads from one seedling to another. The sexual reproduction 
of Pythiiim and PhytopJifhora is practically identical with that 
of Cy slop us. 

Owing to the ease with which the Fungi just considered 
produce zoospores, which of course require a film of moisture 
in which to swim, spells of damp, warm weather are particularly 
favourable to their spread and development. Indeed, the 
damping-off of seedlings through attacks of Pythimn only occurs 
in conditions of excessive humidity due to overwatcring or 
overcrowding. 

In the aquatic Phycomycetes reproduction by zoospores is 
naturally the rule. The well-known Salmon disease is caused 
by one of these Fungi {Saprolegnia ferox), which grows on the 
gills of the fish, where its wefts cause asphyxiation. The biciliate 
zoospores are produced in large numbers in tubular sporangia 
(Fig. 124, D, E), whilst the only essential difference in the sexual 
reproduction of this genus lies in the development of several, or 
even many, eggs in each oogonium. In many of the species of this 
genus, moreover, the eggs develop into oospores without fertili- 
sation (so-called apogamy), although functionless antheridia may 
be formed. 

In contrasting the Fungi hitherto described with the Algje, 
one of the most striking peculiarities, apart from the absence 
of chlorophyll, is the non-motile character of the male gamete. 
Spcrmatozoids, as a matter of fact, are known to occur only in 
one small group of the Phycomycetes. This feature may be 
related to the fact that the Fungi as a whole are a terrestrial 
group, living under conditions {e.g. in the interior of a host-plant) 
in which the necessary moisture for the movement of spcrmato- 
zoids is not available. 

The saprophytes among Phycomycetes arc well exemplified 
by Miicor, which thrives on all kinds of decaying substrata 
(especially horse manure), upon which its mycelium forms a 
white weft (Fig. 125, a). Numerous absorptive branches pene- 
trate downwards into the source of nourishment, and sooner or 



MUCOR 



237 



later conspicuous, dark brown or black, spherical sponmgia 
{sp.) appear at the ends of relatively thick upright hyphre, 
which in some species are branched. An ally of Miicor 
[Rhizopus stolonifer), that occurs very commonly on stale bread 
and horse dung, spreads very rapidly by hypha resembling 




Fig. 125. — Mucor. a, mycelium, slightly magnified, showing two of the 
long-stalked sporangia (sp.) ; 6, sporangium, much enlarged, in 
optical section, showing the numerous spores and the central column 
(Co.) ; c, dehisced sporangium in which only the column and a 
small part of the wall remains ; d and e, conjugation of gametes (g.) ; 
/, mature zygospore, (n, 6, and/after Brefeld ; cafter Sachs; dande 
after De Bary.) 



minute Strawberr}' runners, at the end of each of which a tuft 
of absorptive threads and sporangia is produced. 

The wall of each sporangium (Fig. 125, h) is beset with 
numerous minute needles of oxalate of lime, whilst the swollen 
end of the hypha below projects into the cavity as a central 
column [Co.) ; between this and the wall are many small thick- 
waUed spores embedded ina mucilaginous substance. The latter 
swells in the presence of moisture, and thus contributes to the 



238 MUCOR 

bursting of the sporangium. It is also responsible for the adhesive 
nature of the spores, which are so widely disseminated by the 
wind that they are almost ubiquitous. They are extremely 
resistant, and are capable of remaining dormant for long periods. 
On germination they grow direct into a new plant without the 
production of zoospores. 

Under certain circumstances {e.g. growth in a liquid) the 
hyphas of Mucor become septate, the protoplasts of the individual 
compartments contract, and secrete a thick dark-coloured wall. 
The special resting spores thus formed are known as chlamydo- 
sporex, which likewise produce new plants direct. 

Sexual reproduction is rare, since in most species it only 
takes place between two mycelia belonging to distinct strains. 
The difference is, however, purely physiological, since the two 
are not distinguishable outwardly. The gametes are produced 
within swollen club-shaped branches whose end-portions become 
separated off by cross-walls (Fig. 125, d,g.). Two perfectly similar 
branches meet by their tips, one being derived from each of the 
two plants (Fig. 125, d), and the intervening membrane breaks 
down, whereupon fusion of the protoplasts and of their nuclei 
ensues (tj. The product, deriving nutriment from the respective 
mycelia, subsequently undergoes slow enlargement to form a 
large spherical zygospore with a thick black wall (Fig. 125, /). 
Germination is direct, as in the case of the spores. 

The sexual process of Mucor is thus isogamous and analogous 
to that of the Conjugatffi, where likewise an entire plant is often 
of one sex or the other (cf. p. 227). In the parasitic Fungus 
which attacks house-flies [Empitsa musca) the fusing cells are, 
however, of unequal size, and thus show a structural distinction. 

Mucor and Empusa belong to a subdivision of the Phyco- 
mycctes, known as the Zygomycetes, all characterised by this 
type of sexual process. The other forms previously considered 
are oogamous and classed as Oomycetes , but they are also dis- 
tinguished by the ease with which they produce zoospores. The 
Zygomycetes arc much more markedly adapted to terrestrial 
conditions from this point of view, since all of them reproduce 
by means of motionless spores. 

The second great group of Fungi, the Ascomyceles, are char- 
acterised by their method of spore-formation and by the absence 



ASCOMYCETES— PE2IZA 



239 



of a true sexual process in the vast majority of cases. Common 
parasites belonging to this group are the Mildews [Erysiphacea;), 
the Ergot of Rye {Claviceps purpurea, Fig. 128, A), and the 
Vegetable Caterpillar [Cordyceps) ; but there are numerous sapro- 
phytes, such as the ubiquitous Blue Mould [Penicillium), the 
C u p-fungi 
(speciesoi Peziza, 
Fig. 126, C), the 
Stag's Horn 
Fungus {Xylaria, 
Fig. 126, A), the 
Morel {M or- 
cliella, Fig. 126, 
D), and Nedria, 
which is the 
cause of the 
bright red pus- 
tules so common 
on decaying 
branches and 
sticks. 

A general 
idea of the As- 
comycetes can be 
obtained from an 
examination of a 
species of Peziza. 
The septate my- 
celium of this 
Fungus is peren- 
nial and ramifies 
in the decaying 
substratum {e.g. dead trunks and branches, soil rich in humus), 
its presence only becoming apparent in autumn, when conspicuous, 
and often brightly coloured, cup-shaped fruit-bodies (apothecia, 
Fig. 126, C) are produced at the surface. In a vertical section 
through one of these (Fig. 127, B) the hyphffi are seen to be so 
densely compacted as to produce a false tissue, the elements of 
which are quite irregularly arranged, except for those hning the 




Fig. 126. — Fruit-bodies of various Ascomycetes. 

A, Xylaria hypoxylon (Stag's Horn Fungus). 

B, Geoglossum. C, Peziza (Cup-fungus). D, 
MorclieUa (Morel). E, Sclerotinia, showing 
apothecia arising from sclerotium. 



240 



PEZIZA 



inner surface of the cup. These form a pahsade-Uke layer (the 
hymenium, h.) composed of numerous elongated sporangia or 
asci (Fig. 127, A, a-j), interspersed with the slender hair-hke 
ends of barren hyphffi [p.). Each ascus contains eight eUipsoidal 
ascospores {e, f), which are liberated when mature through a 
terminal aperture on contact with moist air. Mere breathing 
on a ripe fruit-body may often cause the liberation of a cloud 
of spores. 




Fig. 127. — Peziza vesiculosa. B, Section of half an'apothecium (diagram- 
matic), and A, Small part of the hymenium enlarged, showing asci 
in progressive stages of development [ci-J). h., hymenium ; p., barren 
hyphae of same ; s., small-celled subhymcnium. (After Goebeh) 

The ascus is typical of the Ascomycetes as a whole, and 
constitutes one of the chief characteristics. For, by contrast 
with the Basidiomycetes (cf. p. 245), the spores are produced 
within the mother-cell, whilst in contradistinction to the Phyco- 
mycetes they are nearly always only eight in number, though some- 
times fewer {e.g. two or four, as in the Truffle), or more numerous 
(sixteen or thirty-two). In many cases the asci, as in Peziza, 
are grouped together in compact and often large fruit-bodies, 
the hymenium cither covering a great part of the exposed surface 
(as in the Morel, Morchella, Fig. 126, D, and Gcoglossum, Fig. 
126, B) or being completely enveloped within sterile hypha; ; 



ERGOT (CLAVICEPS) 



241 



the last is the case in the Truffles [Tulier), whose fruit-bodies 
are, moreover, subterranean. 

Ergot {Claviceps) infests the ovaries of Rye, Oats, and other 
Grasses, becoming very conspicuous at the time of harvest, as 
a result of the gradual replacement of the grains by a black 
banana-shaped mass (about half an incli long) of closely inter- 



FlG, 




12a. — Ergot of Rye (Claviceps purpurea). A, Head of Rye, with a 
number of black sclerotia (s.). B, Longitudinal section of gyni-cium 
of Rye-flower, showing the dense hyphaj forming the sclerotium in 
the lower part, and the looser mass of hypha; producing gonidia in 
the upper. C, A small part of the latter in section, highly magnified, 
showing the budding off of gonidia. D, Germinating sclerotium. 
E, Vertical section through one of the swellings arising from the 
latter, showing numerous perithecia (p.). F, Part of same, highly 
magnified, to show perithecia with asci (a.). G, Three asci and (on 
the right) four of the thread-like ascospores. (A and D after Wett- 
stein ; the remainder after Tulasne.) 



woven hyphje (Fig. 128, A, s.). This constitutes a resting-stage 
of the Fungus, and is so hard that the term sclerotitmi ^ is applied 
to it. In transverse section all the hyphse, and especially those 
at the periphery, are seen to have very thick walls, whilst the 

1 Similar sclerotia occur, as resting-stages, in the life-cycle of several 
other Ascomycetes, e.g. Scleroiinia (Fig. 126, E), a close ally of Peziza, 
whose cup-shaped apothecia arise from the sclerotia. 
16 



242 ERGOT (CLAVICEPS) 

more central ones, forming the lighter-coloured region, are laden 
with food-reserves. The sclerotia drop off in the autumn and 
remain dormant in the soil until the following spring. Then 
they send up one or more stalked swelhngs (Fig. 128, D), in 
which are embedded numerous flask-shaped cavities {peritJiecia, 
Fig. 128, E, p.) communicating with the exterior by small pores 
(Fig. 128, F). Each pcrithecium is lined with a hymenium 
similar to that of Pcziza, but the ascospores developed within 
the asci are in this case thread-hke (Fig. 128, G), so that they 
are readily distributed by the wind. If caught by the stigma 
of a Grass-flower the spores germinate and the hypha grows 
down through the style into the ovary, thus bringing about a 
fresh infection. 

By slow degrees the contents of the ovarj' are replaced bj? a 
dense hyphal mass with deep surface furrows (Fig. 128, B, upper 
part). From the ends of the superficial hypha;, which are more or 
less parallel to one another, large numbers of minute oval gonidia 
are budded off (Fig. 128, C), and at the same time the surface 
secretes a sugary liquid. This attracts insects, to whose bodies the 
gonidia adhere, and so a rapid spread of the disease from flower 
to flower is brought about. Later in the summer the production 
of gonidia ceases and the outer hj'phte blacken, whereby' the 
resting sclerotium is formed. 

The bright red pustules of Ncdria and the branched sclerotia 
of the Stag's Horn Fungus (Fig. 126, A) harbour flask-shaped 
perithecia similar to those of the Ergot. 

In some Ascomycetes reproduction by gonidia is far more 
frequent than the formation of asci, as, for instance, in the two 
common Moulds Penicilliitm (Fig. 129, a, h) and Eurotium 
(Aspergillus, Fig. 129, /). Here the gonidia are budded off in 
chains from the terminal branchlets of erect h3'phae which, in 
the case of Eurotium (Fig. 129, /), are strongly swohen at their 
apices. Both Fungi also occasionally produce spherical ascus- 
fruits (Fig. 129, d), which arise from special sexual organs 
(Fig. 129, c), although it is doubtful whether any actual fusion 
of cell-contents occurs. 

In the White Mildews [Erysiplmcccr) ^ formation of gonidia 

' The Mildews arc the cause of many famiUar diseases of cultivated 
plants, as instances of wliich may be mentioned the Cooscberry Mildew 



Mildews (erysiphace^) 



243 



is again relatively common. The mycelium in these parasites 
develops externally, on the surface of the leaf (Fig. 130, b), the 
haustoria alone penetrating into the epidermal cells. The 
mildewed appearance is usually due to the extensive production 
of chains of gonidia from the ends of unbranched upright hyphae 
(Fig. 130, c), such chains being very striking in the Mildew 




Fig. 129. — a—b, Penicillium. a, small part of mycelium with gonidio- 
phores ; b, one of the latter enlarged, c-g, Eurotium (Aspergillus). 
c, very early stage of fruit-formation, showing the coiled hypha (female 
organ) from which the asci arise ; d, mature fruit ; e, an ascus from 
the interior of the same ; /, gonidiophore ; g, small part of apex of 
same, showing tlie way in which the gonidia are budded off. (a and b 
after Brefeld ; the remainder after De Bary.) 



commonly found on Forget-me-not leaves (due to a species of 
Oidiiim). Later in the season many of these forms develop 

(Sphaerolhcca mors-ui'ts), the Rose Mildew |S. paniwsa), Eyysiphe polygoni 
(on Field Peas and Cucumber), and E. graminis (on Wheat). Many 
so-called Mildews do not, however, belong to the Ascomycetes, but are 
Phycomycetes, whose richly branched gonidial-bearing hyphje give a 
whitish appearance to the leaves ; such are the Cabbage Mildew [Perono- 
spora parasitica) and Grape Mildew (Plismopora viiicola). 



244 



MILDEWS (ERYSIPHACE^) 



numerous small dark specks (Fig. 130, a), the ascus-fruits, on 
the greyish-white mycelium. Under the microscope they are 
seen to be almost spherical structures, provided with very diverse 
hair-like appendages and without an aperture (Fig. 130, b.). 
The hard black wall ruptures irregularly, exposing one or more 
small asci (Fig. 130, e). 

In the Hop Mildew (Spliaeroiheca castagnei) and one or two 




Fig. 130. — The Hop Mildew (Sphaerolhcca cas/ngiiei). a, part of the leaf 
of the Hop, with the ascus-fruits ; b, small part of the surface, greatly 
magnified, showing the superficial hyphjc {Ii.) and two ascus-fruits, 
each with numerous long appendages (ap.) ; c, production of a chain 
of gonidia (g.) ; d, sex organs in apposition ; e, ascus ; /, young fruit. 
[a after Wcttstein ; b, c, and e after Tulasnc ; d and /after Harper.) 



Other cases it has been established that the fruit arises from 
club-shaped sexual organs of unequal size, the larger functioning 
as the female (Fig. 130, d). Their tii)s become closclj' adprcsscd, 
but, owing to the great difficulty of establishing such facts, there 
exists a marked difference of opinion as to whether or not there 
is a fusion analogous to that above described for the Phycomy- 
cetcs. The same doubt attaches to all cases of sexual fusion 



BASIDIOMYCETES 245 

that have been investigated in the Ascomycetes. Since, how- 
ever, a production of sexual organs has been observed, in one 
case or another, prior to the formation of all the different kinds 
of ascus-fruits encountered in this group, the view is generally 
held that its members are to be regarded as being descended 
from Fungi which exhibited a sexual process, now functionless, 
at least in the vast majority of cases. In fact, in most Asco- 
mycetes no traces of sexual organs are to be found. 

The young asci in all cases arise from binucleate cells, the 
first step in their development consisting in the fusion of the 
two nuclei, and this is regarded by many as giving the same 
stimulus as a sexual fusion. The single nucleus thus produced 
undergoes successive division into eight, whereupon membranes 
are formed independently around each nucleus and the adjacent 
cytoplasm, so that eight ascospores are cut out ; a small portion 
of the cytoplasm remains, however, which is not incorporated 
in the latter. 

The Basidiomycetes, which are likewise characterised by a 
special mode of spore-formation in which a definite number (4) 
of spores is usually constricted off from the mother-cell, are 
altogether devoid of sexual organs. The group includes many 
diverse Fungi, mostly saprophytes, familiar examples being the 
Mushroom {Agaricits, Fig. 132), various Toadstools {e.g. Coprinns, 
Boletus), Puff-balls [Lycoperdon, Fig. 134, C), etc. The Smuts 
[U stilaginecB) and Rusts (Uredinece) are usually also included in 
the Basidiomycetes, but exhibit many peculiar features of their 
own. 

The Rusts are of special importance as being the cause of 
many serious diseases of crops, and of these the Rust of Wheat 
{Pitccinia graminis) is, unfortunately, all too common. Like 
many other members of the group, it possesses a very complicated 
life-history, whose phases, in this species, occur on two different 
hosts. In summer the parasite attacks the leaves and stems of 
various Grasses, and betraj's its presence by the development of 
orange-coloured streaks upon them. These are due to clusters 
of unicellular thick-waUed spores (summer- or nredo-spores) of an 
orange colour and beset with numerous minute spines ; each spore 
arises at the end of a projecting hypha (Fig. 131, b, «.). After 
detachment they may be blown by the wind on to another 



246 



RUSTS (UREDINE./E) 



appropriate Grass, whereupon hyphse grow out through special 
thin areas of the wall (Fig. 131, /), to start a fresh generation 
of the Rust. In this way the disease rapidly spreads during 
the summer months. 

In rare cases the Fungus may persist through the winter by 




Fig. 131. — Rust of Wheat (Puccinia graminis). a, small part of leaf of 
Barberry, showing a group of iccidia ; b, group of uredospores (».) 
and one teleutospore [t.) ; c, germinating teleutospore (only the hypha 
growing from one of the two cells is shown at its full length), with 
developing basidiospores {ba.) ; d, group of teleutospores, seen in 
section of loaf of Wheat ; e, section of Barberry-leaf, showing cluster- 
cups or Eccidia {ae.) in two stages of development, and spermogonia 
(s.) ; /, germinating uredosporc. (b, d, and/ after De Bary ; c after 
Tulasne ; e after Sachs.) 



means of these spores, and possibly also as a parasite on wild 
Grasses. As a general rule, however, special winter- or idcido- 
spores arc produced towards autumn, and these remain dormant 
in the soil until the following spring. The formation of teleuto- 
spores is evidenced by a darkening of the spots on the Grass- 
leaves. Microscopic examination shows that they are now caused 



RUSTS (UREDINE.E) 247 

by clusters of biccllular spores (Fig. 131, d), again borne singly 
at the ends of projecting hyphffi, and provided with a thick dark 
brown membrane (Fig. 131, h, t.) which has a thin germinal 
pore in each cell. 

With the advent of spring both cells of the telcutospore put 
out a short hypha composed of four cells (Fig. 131, c), each of 
which gives rise to a short process bearing a small spherical 
gonidium (basidiospore, ha.). The latter is only capable of further 
development if carried by the wind to a plant of the Wild 
Barberry {Berber is vulgaris). In that case a mycelium is pro- 
duced within the new host, and the presence of the disease is 
soon manifested by the appearance, usually on the under-surface 
of the leaf, of groups of small orange-coloured cups (the cluster- 
cups or cecidia, Fig. 131, a). The minute specks, recognisable 
on the upper surface of the leaf, are caused by small flask-shaped 
cavities {spermogonia, Fig. 131, e, s.) containing reproductive 
cells of unknown function. 

In a vertical section (Fig. 131, e) the hyphae of the ?ecidium 
lae.) are seen to form a compact bounding wall and a dense 
interwoven mass at the base, whilst occupying the floor of the 
cup is a palisade-like hymenial layer whose vertical hyphse bud 
off rows of orange-coloured cecidiospores. The latter, if carried 
by the wind to Wheat or other Grasses, give rise to a new 
uredo-form. 

Whilst the cells of the ordinary mycelium in the Barberry-leaf 
are uninucleate, they become binucleate in the young aecidium, 
and the resulting spores are similarly provided with two nuclei. 
The binucleate condition persists throughout all the uredo-forms, 
and the cells of the young teleutospores show the same feature, 
but fusion of the nuclei occurs as the latter mature. 

The existence of the Fungus on different host-plants, at 
different stages of its life-cycle, is paralleled among animal para- 
sites {e.g. Malarial Parasite, Tapeworm), and affords one means for 
the extermination of the disease, viz. by the eradication of one 
host. This is, however, only partially successful, since Wheat 
Rust occasionally appears in successive years, even where the 
Barberry does not grow {e.g. Australia), which is probably due 
to the persistence of uredospores through the winter. In some 
Rusts there is no uredospore phase {e.g. Pitccinici anemones), 



24S SMUTS (USTILAGINE.E)— MUSHROOM (AGARICUS) 

whilst in others [e.g. Puccinia malvaceariim) only teleutospores 
are known. ' 

Some of the Rusts exhibit very extreme specialisation in 
relation to definite host-plants, possessing biologic strains which 
can only develop on one particular species. Others, however, 
can attack a variety of related hosts, and, in the continuance 
of a disease of cultivated crops, wild plants may often play an 
important part in bridging the interval of a rotation. 

The Smuts [Ustilagincce) are characterised by the sooty black 
mass of spores which are formed by the breaking up of the hyphs 
into unicellular chlamydospores (cf. p. 23S) with a thick pig- 
mented wall. They occur extensively as diseases of various 
Cereals [e.g. Oats, Wheat, Maize), infesting either the leaves 
(Fig. 134, G) or the ovaries. 

The common Mushroom (Agaricns campestris) affords a typical 
example of the Basidiomycetes proper. The mycelium, which 
inhabits soil rich in humus, and is present in considerable amount 
in so-called Mushroom spawn, is composed of binucleate cells. 
The hyphffi, as in many other Basidiomycetes, tend to be inter- 
woven in bundles, so that the mycelium appears thicker and 
coarser than in other Fungi. The overground edible portion is 
the reproductive body which first appears on the mycelium as 
a knob-like swelling (Fig. 132, b) composed of densely interwoven 
hyphffi, but later, as it gradually enlarges, broadens out at the 
top (Fig. 132, c, a). In the mature condition it consists of a 
stalk and an umbrella-shaped cap (cf. also Figs. 137, 138), with 
a large number of radiating plates or gills, which bear the 
hymeniiim, protruding from the under surface. A little way below 
the cap the stalk is surrounded by a membranous ring of broken 
tissue (the anniiliis, Fig. 138) which, before the expansion of the 
cap, extended continuously from the edge of the latter to the stalk, 
thus constituting a protection for the developing gills (Fig. 132, a). 
In a vertical section through the cap (Fig. 132, a, c) the middle 
of each gill [d, /) is seen to consist of longitudinally arranged 
hyphfe. These are composed of rather large cells (/.), and diverge 

'■ Other well-known and widespread Rusts are those occurring on the 
Hollyhock {Puccinia malvaceariim), on the Wood Anemone (P. anemones), 
and that causing purple spots on the leaves of the Blackberry [Pliragmidium 
hulbosum). 



MUSHROOM (AGARICUS) 



249 



at their ends to form the superficial paUsade-like hymenium (li.) 
and a round-celled subhymenium (s.). The former comprises 
two kinds of club-shaped hyphal terminations : some, the 
basidia {ha.), bear at their apex two, or four, short processes, 
from the end of each of which a hasidiospore is formed, whilst 
the others are purely sterile, and probably play a part in the 




Fig. 132. — The common Mushroom (Agaricus campestris). h, c, and a, 
successive stages in the development of the fructification (in a the 
annulus is distinct, but yet unruptured) ; e, transverse section through 
small part of cap, showing gills ; d, one of the latter enlarged ; /, sur- 
face of a gill, in section, highly magnified, ha., basidium ; /i., hyme- 
nium ; s., subhymenium ; t., large cells of middle of gill. (After Sachs.) 

detachment of the spores. This method of spore-formation is 
that characteristic of Basidiomycetes generally, four being the 
usual number produced. The colour of the spores varies con- 
siderably in different forms. 

The enormous production of spores in this and other similar 
Fungi can be gauged by placing the mature caps, with the gills 
downwards, on a sheet of white paper, when, after a short time, 
the outline of each gill will be marked by the spores which have 



250 



OTHER BASIDIOMYCETES 



been shed. It has been estimated that a moderate-sized specimen 
will produce some 1,800,000,000 spores, and other allied species 
form spores in even greater profusion ! 

In the genus Boletus, whose fruit-body has the same general 
form as that of the Mushroom, the underside of the cap presents 
the structure of a honeycomb, consisting of a multitude of vertical 
tubes, the inner surfaces of which are lined with hymenium 
(Fig. 134, E). A similar construction is seen in the Bracket 
Fungus [Polyporus sqitamosus), whose thick tough fruit-bodies 
are commonly found on decaying tree-trunks, to which they are 
attached along one side of the cap (Fig. 133). Some of the 
related Fungi [e.g. Dadalea qucrcina) have woody fructifications 

which may persist for several 
years. In Hydnuni (Fig. 
134, F) the hymenium covers 
the numerous pointed pro- 
jections arising from the 
underside of the cap. 

As further instances of 
the diversity of form pre- 
sented by the fruit-bodies 
of the Basidiomycetes, men- 
tion may be made of the 
purple encrusting fruits of 
the Fungus responsible for 
the Silver-leaf disease of the 
Plum [Stereum purpiireum), of the Puff-balls [Lycoperdon, Fig. 
134, C), whose spores form a powdery mass within the pear- 
shaped fructification, and of the Coral Fungus {Clavaria, Fig. 
134, B), where the fruit-body is richly branched and bears the 
hymenium over its entire surface. 

In conclusion, a brief reference must be made to the Slime 
Fungi (Myxontycetes), whose relation to other Thallophyta is 
exceedingly obscure ; in some respects they show decided re- 
semblances to Protozoa, although the methods of multiplication 
recall those habitual among lower plants. The Slime Fungi are 
most evident in damp weather, when the large naked protoplasmic 
masses (Plasmodia) , constituting the vegetative phase, creep out 
from the crevices of the decaying tree stumps, humus, or other 




Fig. 133. — The Bracket Fungus {Poly- 
porus sqiiamosus). [Photo: E. J. S.] 



SLIME FUNGI (MYXOMYCETES) 



251 



substratum. Small, often rounded sporangia, containing 
numerous spores, are formed, especially in the autumn, and are 
sometimes very conspicuous owing to their brilliant colouration 
{e.g. the yellow-coloured Flowers of Tan, common on tanner's 




Fig. 134. — Various Basidiomycetes. A, Cantharellus. B, Clavaria cinerea. 
C, Lycoperdon. D, Scleroderma vulgare (on the left entire fructifica- 
tion, on the right the latter in vertical section, showing the wall and 
the dark mass of contained spores). E, Surface-view of small part 
of hymenium of Boletus. F, Hydniim repandum. G, Usiilago longis- 
sima (on leaf of Glyceria aqiiatica). 

bark). The group also includes some parasites, one of the most 
noteworthy [Plasmodiopliora hrassicce) being that responsible for 
the disease known as " Finger and Toe " in Cabbages, etc. 
(Fig. 135)- 



CHAPTER XVHI 

Physiology of Fungi, Lichens, Bacteria 

Many of the Fungi play a very important role in the economy 
of nature. The saprophytes, in association with Bacteria, are 
largely responsible for the decomposition of vegetable remains, 
and without them the whole surface of the earth would become 
buried under the bodies of plants and animals. Through their 
agency the material locked up in the raw humus of the soil is 
transformed into simpler chemical compounds, and rendered 
available for the use of higher plants. A striking instance of 
this function is afforded by those cases in which the Fungus, 
responsible for the decomposition of humus, is intimately asso- 
ciated as a mycorrhiza with the underground organs of Flowering 
Plants (cf. p. 72 and Fig. 31). The processes of decay initiated 
by saprophytes are, however, often detrimental to the interests 
of man, as in the case of the Basidiomycete [Meriiliiis lacrymans) 
responsible for so-called " dry rot " of timber, and the diverse 
and almost ubiquitous Moulds which all too readily develop upon 
articles of food. 

The parasitic species frequently do great damage to cultivated 
plants and to animals. In some cases the parasite sooner or 
later brings about the death of its host {e.g. Silver-leaf Disease 
of Plum), but more commonly [e.g. Mildew, Rust) the diseased 
plant, though injured, continues to live as an unhealthy indi- 
vidual and to maintain the parasite which grows at its expense. 
A considerable number of parasites can also exist as saprophj'tes, 
so that they remain alive after the host has died [e.g. PytJiiiim 
and many Smuts). Conversely, certain saprophytes {e.g. Mncor) 
can occasionally act as parasites, generally after access has been 
obtained at a point of injury. The so-called wound-parasites 

25^ 



SUSCEPTIBILITY AND IMMUNITY 253 

[e.g. Ncctria, a species of which causes Coral-spot Disease of 
various trees) belong to this category, though their saprophytic 
phase is of short duration. The majority of Fungi, however, 
are either strictly saprophytic or strictly parasitic. 

In many cases special conditions, such as excess of moisture 
{e.g. Pythium), the general state of health of the host, or acci- 
dental injuries to the latter, may be instrumental in bringing 
about the attacks of parasitic Fungi. Epidemics of such wide- 
spread diseases as the Potato Blight and the Gooseberry Mildew 
have, for instance, often been associated with particularly damp 
warm seasons. Fungi proper are rarely the cause of disease in 
man, but it may be mentioned that various skin diseases {e.g. 
Ringworm, Favtis) are due to Fungi. 

Owing to the small size of the spores. Fungi often become 
widely disseminated by the wind, but so far as can be gauged 
by the careful study of the spread of plant diseases due to these 
parasites, infection by wind-borne spores -seldom occurs beyond 
a few miles. Long-distance carriage is generally to be attributed 
to transport in infected plants or plant-fragments, through human 
or other agency, hence the value of careful inspection and control 
of imported horticultural produce. It is owing to such intro- 
duction that the parasitic Fungus-flora of Botanic Gardens is 
so extraordinarily rich. 

In dealing with the Rust Fungi, mention was made of the 
fact that a particular species or strain of these parasites may 
be so specialised as to be able to attack only one particular kind 
of host, and the same is true of the Mildews. There is thus 
often a difference, with regard to susceptibility to a certain disease, 
between the various races of a cultivated plant ; for instance, 
some varieties of Potato and Wheat are immune to Blight and 
Rust respectively, and would tend to be grown in regions in 
which these Fungi were known to be prevalent. Much has also 
been done by the production of immune hybrids (cf. p. 384) 
between immune and non-immune races. The ravages of a 
disease may decrease in intensity after it has been rampant for 
some years, the host presumably becoming adapted to the 
presence of the parasite ; thus the Hollyhock Rust {Puccinia 
nialvaceariim) , when first introduced into Europe about 1870, 
played great havoc with its host, but now, though Hollyhocks 



2^4 



REMEDIES 



arc still commonly attacked, they do not appear to suffer 
appreciably. 

Artificial co)iirol of fmigal diseases is accomplished by diverse 
means, but in each case success depends upon a knowledge of 
the life-history of the parasite. The method of treatment varies 
according as the parasite attacks the overground or underground 
organs of the host. Several of the Fungi considered in the last 

chapter {e.g. Potato Blight) 
afford instances of the 
former mode of attack. 
Diseases of the subter- 
ranean parts are exempli- 
fied by the pernicious Wart 
Disease of the Potato, due 
to a lowly Fungus of a 
peculiar kind, and that 
known as Finger and Toe 
(Fig. 135 and p. 251), which 
attacks Turnips, Cabbages, 
etc., causing irregular 
swellings upon the root- 
system. 

One of the remedies 
most commonly employed 
against Fungi infesting the 
overground parts is spray- 
ing with a fungicide which, 
while deleterious to the 
parasite, leaves the host 
practically unharmed. For 
this purpose Bordeaux 
mixture, consisting of a solution of copper sulphate and slaked 
lime, is one of the most popular. Where the disease is subter- 
ranean, application of unslaked lime is often successful, as, for 
instance, in the case of Finger and Toe. Such diseases are, 
however, far more difficult to eradicate than those which 
develop overground. They are often best counteracted by 
growing only such crops, in the infected soil, for several seasons 
in succession, as are not attacked b\' the parasite in question. 




Fig. 135. — Brussels Sprouts attacked 
by " Finger and Toe " [Plasmodio- 
phora brassicie.) [Photo : E. J. S.] 



PLATE II 




Fig. 136. — Pleurotus ostreitus. 
Fig. 138. — Armillaria mellea. 

[All after photos by Mr. 



Fig. I},-; .- -Amaniiopsis vagiiiata. 
Fig. 139. — Polystictis versicolor. 
E. M. Cuttins;, M.A.l 



=341 



EDIBLE FUNGI— YEAST 255 

In many cases, unfortunately, no adequate remedy has yet been 
discovered, and the only advisable procedure, in the event of 
an outbreak of disease, is to burn all infected plants, so as to 
prevent the spread of the parasite. 

Fungi are, however, not only of importance in causing decay 
and disease, but also afford several greatly prized articles of 
diet {e.g. Mushrooms, Truffles, Morels, etc.), although their actual 
food-value is probably small. The great majority of the British 
Basidiomycetes are innocuous, but there are a certain number 
of species, some very widely distributed, which harbour deadly 
poisons (alkaloids, etc.), and attention may be drawn to the fact 
that such Fungi are by no means alwaj's highly coloured (Fig. 137) . 
As examples of poisonous forms we may mention the Fly Toad- 
stool {Amanita muscaria) and the Death Cap {Amanita 
phalloides). The sclerotia of an Australian species of Polyporits, 
which may attain the size of a football, are eaten by the natives. 
An edible Fungus of another kind is seen in the so-called Vegetable 
Caterpillar {Cordyceps), where a dense mass of hyphs or sclero- 
tium completely replaces the internal organs of the animal ; this 
parasite is extensivelj' cultivated in parts of the East and used 
as a condiment. Fungi are not often employed in medicine, 
except for the powdered sclerotium of the Ergot {Claviccps) 
which contains a nitrogen base having the property of causing 
muscular contraction. 

The production of most alcoholic beverages is due to the 
activity of Yeasts {Saccharomyces), whose exact relationship to 
other Fungi is not quite clear. The Yeast-plant (Fig. 140) con- 
sists of oval cells, which are either isolated {a) or adhere together 
in short chains (c), each the product of a peculiar method of 
division of a single individual. The thin-walled cells contain a 
large central vacuole (Fig. 140, e, va.), and, in contact with the 
latter at one point, a nucleolus with surrounding chromatin (w.), 
which become apparent on staining the living cells with a dilute 
aqueous solution of methylene blue ; vacuole, nucleolus, etc., 
together probably represent the nucleus. The Yeast-cells often 
contain large glycogen- vacuoles {g.), as well as small bodies {v.), 
stained deeply by methylene blue, and known as volutin-granules, 
which appear to constitute another kind of reserve. 

When a cell has reached a certain size, it gives rise to a small 



256 



YEAST (SACCHAROMYCES) 



outgrowth (Fig. 140, b) which slowly enlarges and assumes the 
form of the parent, from which it becomes separated by gradual 
constriction ; if this process of budding takes place rapidly, the 
cells do not immediately separate, and thus the chains (Fig. 140, c) 
above mentioned are formed. A stage, capable of a prolonged 
resting period, can also be obtained, for instance by growing 
Yeast on the surface of a raw Potato ; under these circumstances 
the cell-contents undergo division into four, and each part 
becomes surrounded by a thick wall (Fig. 140, d). 

There are a number of different species of Yeast which 




Fig. 140. — Yeast {Saccharomyces). a-c, various individuals, showing 
general form and multiplication by budding ; d, individual containing 
restingspores ; e., cell-structure, ch., chromatin threads ; g., glycogen 
vacuole ; «., nucleolus with surrounding chromatin ; v., volutin 
granules; wa., nuclear vacuole, (a-rf after Wcttstcin ; e after Wager.) 



ferment various sugars and split them up into alcohols (mainly 
ethyl alcohol) and carbon dioxide. The chemical change is 
brought about by an enzyme zymase (cf. pp. 53, 55) which can, 
with some difficulty, be extracted from the cells ; in addition 
the latter contain invertase and other ferments. The mode of 
action of zymase is complicated and not yet fully understood, 
but it is known that fcnncntalion depends on the contemporary 
presence of phosphates (cf. p. 57). The alcohol present in beer, 
wine, etc., is formed by the action of Yeasts, while the carbon 
dioxide simultaneously evolved is compressed into cylinders and 
sold as a by-product. In the manufacture of beer, malt {i.e. germi- 



ALCOHOLIC FERMENTATION— LICHENS 257 

nated Barley) is treated with hot water, and supplies part of the 
sugar, various other sugars being used according to local practice. 
Hops are added to the liquid to provide the peculiar flavour, 
and the whole of this wort is then fermented in vats.' The 
success of brewing depends upon the employment of pure races 
of Yeast and the realisation of the right temperatures at the 
different stages of the process ; a small supply of oxygen has 
also been shown to be favourable to active fermentation. 

In the fermentation of sugars a considerable quantity of 
energy is liberated, and herein possibly lies the advantage to 
the Yeast-plant. It appears likely that some of this energy 
is utilised in synthesis. The process of alcoholic fermentation 
shows much resemblance to anaerobic respiration, but there is 
this difference, that the compounds broken down are outside 
and not within the organism, so that no loss of weight is in- 
volved. It may be added that Yeasts are not alone among 
Fungi in fermenting sugars, since spores of Mucor, for example, 
if placed in a sugary liquid, will reproduce by budding and cause 
alcoholic fermentation, forming so-called Mucor-ytd&i. 



We have already noted that a parasitic Fungus may some- 
times do very little harm to the host, and such cases are but a 
step removed from those in which the balance, between the 
Fungus and the organism with which it is associated, is so perfect 
that both are mutually benefited. Such a condition is found in 
the Lichens, whose body is composed of algal cells embedded 
in a weft of fungal hyphse. Lichens, with but few exceptions, 
are subaerial, being commonly found on peaty soils, rocks, tree- 
trunks, old walls, etc. In conjunction with various Alg£€ (com- 
monly Cyanophyccce) and Mosses, Lichens play an exceedingly 
important part in the primary colonisation of rock surfaces and 
of other ground laid bare of vegetation, being conspicuous, for 
instance, among the early vegetation of burnt heaths. The black- 
coloured blotches, which are such a familiar sight on the pebbles 

' If all the starch in the malt is allowed to undergo conversion into 
sugar, and the fermented product is distilled, whisky results ; brandy is 
similarly obtained by distilling the alcohol from fermented grape- juice. 
The Japanese saki is obtained by the fermentation of rice. 

17 



258 



LICHENS 



of a shingic-bcach, are due to various Lichens {e.g. Rhizocarpon 
confervoides, Fig. 141, C), which are here the iirst colonisers. 

The shape of the thallus is very diverse, and, as a general 
rule, almost entirely determined by the Fungus. It most com- 
monly takes the form of fiat, lobed expansions which are often 




Fig. 141. — Various common Lichens. A, Usnea barbata. B, Partnelia 
physodes, on twig. C, Rhizocarpon confervoides, on pebble from shingle- 
beach at Pevensey. D, Xantlioria parietina (showing numerous 
apothccia). E, Cladonia sp., showing the upgrowths on whiclr the 
apfjthccia arc borne. 

almost circular [e.g. Parmdia, Xanthoria, Fig. 141, D). Other fre- 
quent types are those exhibiting repeated branching (Fig. 141, B) ; 
in such the segments may be upright {e.g. Iceland Moss, Cctraria 
islandica, Fig. 143) or hanging {e.g. Old Man's Beard, Usnea, 
Fig. 141, A). In some cases the thaUus is highly gelatinous, 
so that it is much more conspicuous in wet than in dry weather 
{eg Collema). 



LICHENS 259 

The Algffi found within the tha]li of Liehens are forms which 
can also grow independently in the terrestrial habitats in which 
the latter occur ; examples are afforded by the unicellular green 
CystococcHS (found in the Lichen Cladonia) and the filamentous 
blue-green Nostoc (found in Peltigera). The Fungi concerned 
are, on the other hand, unable to lead an independent existence 
in nature, and are only capable of growth when associated with 
their appropriate Alga. A single species of Alga may be coupled 
with several different Fungi to form as many distinct Lichens, 
but each Fungus has only one algal associate. It has even 
proved possible to produce a Lichen artificially by sowing the 
spores of its fungal constituent among the appropriate algal cells. 
The vast majority of Lichen Fungi belong to the Ascomycetes, 
as shown by their fructifications, which closely resemble 
small rather flattened Peziza-cuYis (Fig. 141, D). In Cladonia 
(Fig. 141, E), where the thallus itself is often inconspicuous, 
the apothecia are borne on the edges of variously shaped 
upgrowths. 

Since the hyphse at the two surfaces of the thallus are usually 
densely compacted and thick-walled (Fig. 142, C), the Alga {al.) 
embedded in the interior is well protected during times of drought. 
As a general rule, moreover, tufts of hyphae project from the 
underside of the thallus into the substratum, absorbing moisture 
which is held there by capillarity, and thus the algal cells are 
kept supplied during dry periods. In the case of Lichens growing 
on rocks, the penetration of these hyphal tufts is facilitated by 
the secretion of solvents, and it is to this property that such 
Lichens owe their important role in the disintegration and 
colonisation of bare rock-surfaces. 

Whilst the Alga thus obtains protection and a supply of 
moisture, the Fungus no doubt profits by absorbing part of the 
photosynthetic products of the former, with whose cells some 
of its hypha; come into intimate contact (Fig. 142, B). In some 
cases, in fact, the hyphas are stated actually to penetrate the 
algal cells, so that the partnership borders closely on parasitism. 
Despite this intimate relation, the enveloping hyphje are usually 
loosely arranged, so that a well-marked system of intercellular 
spaces admits of the inward diffusion of carbon dioxide (Fig. 142, 
C, Jiy.). In transverse sections the algal cells are seen as patches 



26o 



LICHENS 



^-g^^^^^^^^^dmsj 



scattered throughout the thallus [e.g. Collema) or occupying 
definite zones (e.g. XantJwria parietina, Fig. 142, C). 

The association between Alga and Fungus is paralleled by 
the so-called " green cells " found in several aquatic animals 
[e.g. freshwater Sponges, Hydra viridis) ; these are due to uni- 
cellular Algse 
(species of Chlor- 
ella, etc.), and in 
all such cases the 
partnership be- 
tween plant and 
animal appears to 
be of mutual 
benefit. In this 
connection it may 
be noted that 
various Algae are 
found commonly 
inhabiting the 
intercellular 
spaces of certain 
higher plants [e.g. 
Duckweed, the 
Liverwort AntJw- 
ceros), but in these 
cases the Algte 
appear to be no- 
thing more than 




Fig. 142. — A, soredium, and C, section through 
the thallus, of the Lichen Xatithoria parietina. 
al., algal cells ; hy., fungal hyph.T;. B, an 
algal cell from the Lichen Cladonia, illustrating 
the intimate relation between it and the 
fungal hypha2. (B after Bornet.) 



space-parasites 
seeking protec- 
tion. 

The ordinary 
cup-like fructifi- 
cations of Lichens originate from the Fungus, and the resulting 
ascospores reproduce the latter only, so that germination of 
the spores must take place in the presence of the appropriate 
Alga, if a fresh Lichen is to be formed. But multiplication in- 
volving the dual organism also occurs ; in this case algal cells 
surrounded by interlacing hypha: become detached, and when, 



BACTERIA 



261 



as usually, this takes place on a large scale, the surface of the 
thallus acquires a powdery appearance. The individual granules 
(so-called soredia, Fig. 142, A), being distributed by the wind, 
afford a rapid means of propagation of the Lichen. 

The bright colours of many Lichens are due to the presence 
of pigments which are remarkable for their " fast " character, 
and have been employed in dyeing. The well-known chemical 
indicator litmus is obtained from species of Roccella. Iceland 
Moss {Cetraria islandica. Fig. 143), which forms a slimy fluid 
when boiled with 

water, is used as an jr-a 

invalid food. ..tM*k\^ 



The Bacteria con- 
stitute an extremely 
important group of 
Thallophyta whose re- 
lationships are very 
obscure. The majority 
of species are colour- 
less unicellular plants 
of extremely small 
size. The larger speci- 
mens are not more 
than i/iooth mm. in 
length and about one- 
tenth as wide, whilst 

the globular forms average i/ioooth mm. in diameter. Some 
are so small as to be almost, if not quite, invisible under the 
highest powers of the microscope. They occur in almost every 
possible situation, and live under the most varied conditions. 
They are not killed by cold, and some can survive for many 
months at the temperature of liquid air. Whilst most die if 
heated to about 50° C, a few (thermophilic Bacteria) live in 
fermenting hay and manure heaps which, owing to their activity, 
may attain a temperature of 70° C.^ Many Bacteria also in- 
habit the waters of hot springs. 

1 Hence the employment of manure for hot -beds, et?, 




Fig. 143. — Iceland Moss (Cetraria islandica), 
part of a large specimen, about natural 
size. 



262 GENERAL FEATURES OF BACTERIA 

Bacteria are rod-shaped (Bacillus, Fig. 144, d, f, Bacterium), 
spherical {Micrococcus), or curved (Spirillum, Fig. 144, k). 
The rod-shaped individuals may be joined end to end to form 
long filaments (Fig. 144, d), whilst the Coccf/s-f orms may occur 
in chains (Streptococcus, Fig. 144, b), in cubical packets (Sarcina), 
or in irregular masses (Staphylococcus, Fig. 144, a). The individuals 
may be capable of movement owing to the possession of cilia, 
which arc only visible after careful staining, or they may be 
devoid of these, and merely exhibit Brownian movement in corre- 
spondence with their minute dimensions. In the elongated forms 
the cilia are either situated in tufts at the two ends (Spirillum, 
Fig. 144, k), or else arise all over the body (Bacillus, Fig. 144, e, f), 
but there are some genera possessing only one or two cilia re- 
stricted to one end of the cell (Pseudomonas, Fig. 144, /). 

Very little is known regarding the internal structure of the 
cells, and it is a matter of doubt whether or not thej' possess 
a true nucleus ; plastids are of course absent. The cell-wall 
appears to consist of a protein, and may probably be regarded 
merely as a differentiated outer layer of the cytoplasm. The 
latter contains various substances, of which the commonest are 
glycogen (cf. p. 231), granules of volutin (p. 255), and fat-globules 
which appear as brighter specks in the cytoplasm. 

Multiplication of the cells by division, under normal circum- 
stances, ensues with great rapidity, often taking place once 
every hour, or even more frequently. In some of the elongated 
forms the daughter-individuals are separated off by a mere 
constriction of the cell (e.g. Bacterium spp.), whilst in other 
cases the latter is subdivided by a transverse septum which sub- 
sequently splits down the middle. In certain Bacteria inhabi- 
ting water or other fluids, the daughter-individuals develop thick 
mucilaginous walls, and in consequence adhere together in \-ast 
numbers to form an often iridescent pellicle on the surface, or 
thick pale-coloured Palmel la-like ifoating masses (zooglcea-stages). 

Bacteria survive unfavourable conditions by the formation 
of resting spores, produced within the cells by a localised con- 
centration of the greater part of the protoplasm, which then 
stains differently to the peripheral portion. This central region 
eventually becomes surrounded by a thick wall, and at maturity 
the remainder of the cell is often empty (Fig. 145, b, c). The 



REPRODUCTION OF BACTERIA 



26? 



spores are frequently located in special parts of the cell, giving 
the spore-forming individuals a characteristic appearance, as 
in Bacillus tetani, where they occupy a dilation of one end 
(Fig. 145, a). In certain forms (e.g. Bacillus amylohacter) more 
than one spore may be developed by each individual. Bacterial 
spores are often very resistant to extremes of temperature, and 
can indeed in some cases (e.g. Bacillus siiblilis) withstand pro- 
longed boiling. 

Most sorts of Bacteria live a free life in water, earth, etc. 
Many exert a very beneficial action in promoting decay and main- 




O^ 




Fig. 144. — Various Bacteria (magnified about 1,000 times), a, Slaphylo- 
cocciis aureus ; b, Streptococcus pyogenes ; c, Pneuinococcus \ d, Bacillus 
anthracis ; e, B. typhosus ; /, B. tetani ; g, Microspora comma ; h, Spi- 
rillum of relapsing fever (possibly Protozoal) ; A, Spirillum rubrum ; 
I, Pseudomonas spp. j represents a blood corpuscle on the same scale, 
and the line below, the width of a fine human hair, or about half the 
thickness of a sheet of newspaper. (a-h from Muir and Ritchie ; 
k-l after Engler and Prantl.) 



taining the circulation of nitrogen and carbon dioxide in nature, 
whilst a small number are extremely harmful, living as parasites 
on higher animals and plants, and causing such diseases as 
typhoid, cholera, plague, etc. WTiilst some, such as Bacillus 
subtilis, which is found in infusions of hay, etc., require a free 
supply of ox3^gen (aerobic species), others, such as the organism 
responsible for lock-jaw (Bacillus tetani), will grow only in the 
almost complete absence of oxygen (anaerobic species). Hence 
the value of oxidising agents (e.g. peroxide of hydrogen) for the 
cleansing of wounds, 



264 PURE CULTURES 

It will be realised that the identification of the pathogenic 
forms {i.e. those causing disease), in particular, is of great im- 
portance. Owing, however, to the small dimensions of the 
individuals, it is as a rule impossible to identify the species by 
microscopic examination alone. Resource is therefore had to 
the various reactions of Bacteria under conditions of culture. 

Bacteria can be grown in the laboratory on various artificial 
media, such as extract of meat, decoctions of fruit, etc. Under 
such conditions, and at temperatures suitable for each species, 
growth is very rapid, and a single organism soon multiplies to 
such an extent that the mass of its offspring is visible to the 
naked eye. In making cultures all the vessels and instruments 
employed are scrupulously cleaned and freed from live Bacteria 
{sterilised), either by heating at high temperatures or by washing 
with special disinfectants, since even small quantities of dust 
are replete with bacterial spores. 

The different kinds of Bacteria present in any material to 
be examined can be separated from one another and isolated 
by using the various culture media mentioned above, with the 
addition of gelatine or agar-agar (p. 204). A little of the material 
containing the organisms is well mixed with a considerable 
quantity of such a medium, liquefied by warming, so that the 
individual bacilli are widely separated from one another. On 
allowing the mixture to cool the Bacteria are immobilised ; they 
soon grow, and each of the widely separated individuals gives 
rise to a small group (colony), visible to the naked eye, which can 
be transferred to another lot of culture medium. In this way 
cultures can be obtained which have arisen from single organisms 
and consist of one species only {pure cultures) ; in them the form 
of the Bacteria, their mode of growth, the formation of pigment 
(usually outside the cehs) and other chemical substances, can 
be studied and the species identified. 

Some Bacteria are entirely parasitic {e.g. the Micrococcus 
which causes spotted fever) and cannot live apart from their 
host, whilst some are semi-parasitic {e.g. Bacillus tetani) and can 
live either in earth, etc., or in the body of a Mammal. Each 
parasitic species can generally use only one or two species of 
Mammal as hosts ; children, for example, do not have distemper, 
and dogs and cats do not have measles and chicken-pox. Simi- 



BACTERIA AND FERMENTATION 265 

larly Bacillus carotovoriis causes " soft rot " of Carrot, but does 
not attack Parsnips. 

The evil effects of parasitic Bacteria are due to poisons {toxins) 
wliich they produce ; these are proteins in nature, and those 
which liave been isolated are the most poisonous chemical sub- 
stances known. The infected animal resists the invading or- 
ganism, partly by the amceboid cells of the blood (leucocytes) 
which devour them, and partly by producing chemical substances 
which neutralise the toxins. The power of the body to make 
a successful resistance is much improved by practice. Hence 
one attack of an infectious disease often enables a person to 
destroy that particular bacillus at once if it gets into the body 
a second time, and so another attack of the disease is avoided. 

A large number of Bacteria obtain energy ' by bringing about 
processes oi fermentation. Thus the Vinegar Bacteria [Bacterium 
aceticiim, etc.) convert alcohol into acetic acid," whilst the lactic 
acid Bacteria cause the souring of milk, changing the milk sugar 
(lactose) into lactic acid. Another product of bacterial activity 
is butyric acid, which is the chief cause of the rancid character 
of bad butter. The putrefaction of meat is likewise due to the 
agency of Bacteria, which in this case decompose protein sub- 
stances. Each type of cheese is the product of a definite bacterial 
and Fungus flora. In many cases fermentation is due to a group 
of organisms ; for instance, the so-called ginger-beer plant, used 
in the manufacture of the beverage of that name, consists of a 
Yeast (p. 255) associated with certain Bacteria. 

It is to Bacteria, moreover, that we owe the decomposition 
of the cellulose in dead leaves, etc., whereby undue accumulation 
is prevented, and the carbon is again brought into circulation 
as carbon dioxide. The processes of decay involved in the " ret- 
ting " of fibres (e.g. Flax, Hemp, Jute), the " curing " of tobacco, 
and the conversion of sewage are likewise brought about by 
members of this group. 

The nitrates of the soil, upon which plants are dependent 

1 In some cases part of this energy is dissipated as heat (cf. p. 261 ) 
or light (e.g. the phosphorescence of bad meat). 

^ In the commercial production of vinegar either wine or spirit is 
used as the raw material, and to these vinegar is added in order to in- 
troduce the necessary Bacteria. 



•266 NITRITE- AND NITRATE-BACTERIA 

for their supply of nitrogen, would, owing to their ready solu- 
bility, soon become depleted, were it not that they are continually 
being reinforced by the action of Bacteria. The decaj' of plant 
and animal bodies, which is likewise due to bacterial agency, 
leads to the production of a large number of waste products, 
of which one of the most important is ammonia. The latter 
combines \\'ith the calcium carbonate to form ammonium car- 
bonate, and this is oxidised to a nitrite by the so-called Nitrite- 
Bacteria which belong to the genus Nitrosomonas. The nitrites 
in their turn are converted into nitrates by the Nitrate-Bacteria 
(Nitrohactcr) , and in this wa}' the ammonia, liberated by the 
decay of dead organisms, again becomes available to living plants. 

By means of these oxidative processes the Bacteria in ques- 
tion gain the energy necessary for their vital activities. It has 
been found possible to cultivate them only in the absence of 
organic matter, but in nature the presence of the latter in the 
soil seems even to be beneficial. Nevertheless these organisms 
appear to be capable of building up organic substance from 
simple compounds, utilising for this purpose the energy obtained 
in the oxidative processes which thev carry on. These nitri- 
fying Bacteria cannot flourish in acid soils, or in such as contain 
an appreciable amount of free ammonia. This probably explains 
their paucity in many soils which are rich in humus [e.g. moor- 
lands) or poor in lime. Where the decay of organic matter takes 
place on a large scale, as in the guano-fields of Chile, so large a 
quantity of nitrates may be formed that the}? accumulate as an 
efflorescence on the surface of the soil. This is the mode of 
origin of Chile saltpetre (potassium nitrate). 

The beneficial action of these organisms is to some extent 
countered bj' the breaking down of nitrates with the evolution 
of nitrogen by such Bacteria as Bacterium denitrificaiis [denitrifi- 
cation). There are, however, others fthe nitrogeii-Jixing Bacteria), 
which actuall)' have the power of fixing the free nitrogen of the 
air, with the formation of organic nitrogen compounds, although 
the chemical processes involved are obscure. The most impor- 
tant of the organisms concerned is Azotohacter (hlg. 145, d), an 
aerobic form which obtains its energy by the breakdown of 
carbohydrates, a process setting free a considerable amount of 
carbon dioxide, The amount of nitrogen fixed is proportional 



NITROGEN-FIXING BACTERIA 



267 




to the amount of organic material decomposed. Another of the 
nitrogen-fixing Bacteria, Clostridium pasteurianitm (Fig, 145, e), is 
anaerobic, the principal product of its activity, apart from nitro- 
genous compounds, being butyric acid. It is the organisms 
mentioned that are largely responsible for the gradual increase 
in the nitrogen-content of unmanured grassland. 

Even in the time of the Romans the inclusion of Leguminous 
plants in a rotation of crops was recognised as beneficial. E.x- 
perience has shown that cultivation of Clovers, Sainfoin, Lucerne, 
etc., materially increases the nitrogen- 
content of the soil, especially if the crop 
is subsequently ploughed into the field (so- 
called green manuring). This phenomenon 
remained unexplained until it was dis- 
covered that the swellings upon the roots 
of Leguminous plants are inhabited by 
Bacteria (Bacillus radicicola, Fig. 145,/), 
capable of fixing free nitrogen, and present 
in every healthy soil. Infection of the 
root takes place through the root-hairs, 
probably when the organism is in the 
motile phase ; having penetrated the root- 
hairs, the Bacteria pass into the adjoining 
cells, which are thereby caused to divide, 
so that a gall-like structure arises. Within 
the cells of this nodule rapid multiplication 
of the Bacteria ensues, probably at the 
expense of carbohydrates furnished by the 
Leguminous plant. At the same time, 
however, the latter profits by the nitro- 
genous material formed by the Bacteria, the removal of which is 
indeed necessary for their continued activity. In the mature 
condition large numbers of the Bacteria, within the cells of the 
nodules, assume an irregular form, and become digested by the 
action of the host. A limited number persist unaltered, and 
return to the soil as the roots decay away. 

The relation between the nodules and these nitrogen-fixing 
Bacteria is shown by the fact that Leguminous plants, grown 
from seed in soil which has been thoroughly sterilised by heating. 



Fig. 145. — a~c, spore- 
formation in Bac- 
teria, a, Bacillus 
tetani ; b, Bacillus 
of malignant oede- 
ma ; c, Bacillus 
cedamitis. d-f, ni- 
trogen-fixing Bac- 
teria. d,Azotobacter; 
e, Clostridium pas- 
teurianum sp. ; /, 
Bacillus radicicola. 
(After Engler and 
Prantl, and Ellis.) 



268 THREAD-BACTERIA 

fail to develop any nodules, and are just as dependent on a 
supply of soil-nitrates as other green plants. It appears that 
different strains of these Bacteria infect different Leguminous 
plants, and that normally those of a particular strain only attack 
other individuals of the same species. Similar nodules of a 
larger size occur on the roots of the Alder and the Bog Myrtle 
{Myrica gale). 

Brief reference maj' be made to ihe so-caWeAThread-Baderia , 
whose exact relation to the other forms is, however, dubious. 
In many respects they show a closer approximation to the Blue- 
green Algje than to the Bacteria proper, A common form, 
Beggiaioa, which is like a colourless OsciUatoria, is frequent in 
waters rich in sulphuretted hydrogen ; the organism in question 
obtains the energy necessary for the building up of organic 
substances by the oxidation of the sulphuretted hydrogen to 
sulphur. The Iron-Bacteria {Lepiothrix, Crenothrix, etc.), whose 
branched threads are abundant in ferruginous \'\aters, and are 
often the cause of the brown deposit of ferric hydroxide, are 
further examples of these forms, 



CHAPTER XIX 

Liverworts and Mosses 

The plants so far considered all agree in possessing relatively- 
simple sexual organs, consisting of more or less modified cells, 
whose protoplasmic contents, with or without division, give rise 
to the gametes. The sexual organs, moreover, are usualty, except 
in such forms as Fiiciis and Pelvetia, distributed over the entire 
thallus. On the other hand, in Liverworts (Hepatic^) and Mosses 
(Musci), which are grouped as Bryophyta, the second class of the 
Vegetable Kingdom, the sexual organs are not only much more 
elaborate, but are commonly restricted to definite portions of 
the plant-body. The latter usually also exhibits greater com- 
plexity of structure, which may be related to the fact that the 
Bryophyta are on the whole terrestrial plants in contrast, for 
instance, to the essentially aquatic Algje, Other distinctive 
features of the Bryophyta will become apparent in the subsequent 
pages. 

The majority of Liverworts are damp-loving plants, many 
of them growing in situations that are covered with moisture 
in the wetter seasons of the year [e.g. along the sides of water- 
courses), whilst some are even truly aquatic. Many Mosses, on 
the other hand, can flourish in habitats that are comparatively 
dry for a great part of the year, extreme examples being furnished 
by the Hair Moss (Polytrichtim jimiperimtm) , found on dry 
heaths, and the Wall Moss [Tortida muralis, Fig. 150, D), common 
on old walls, rocks, etc. In relation to this difference of 
habitat, Mosses generally display a structure which is more 
elaborate, and better suited to resist drought, than that of 
Liverworts. 

The body of the Liverwort in its simplest form is a small flat 
green ribbon-like structure, often repeatedly forked, and growing 

269 



270 



STRUCTURE OF LIVERWORTS 



in close contact with the soil {e.g. the common Liverworts Pellia, 
Fig. 146, B, and Marcliantia, Fig. 146, E). At the base of the 
notch situated at the tip of each lobe of the thallus lies the 
growing point (Fig. 146, D, g.p.), which commonly consists of a 
marginal row of cells. The middle part of each segment is 




Fig. 146. — Various thalloid Liverworts. A, Small part of the thallus of 
Fegatella, seen from the under-surface, showing the dense weft of 
rhizoids. B, Group of thalli of Pellia (ordinary form), one with a 
young sporogonium (s.). C, Pellia, form characteristic of xcry wet 
habitats. D, Thallus of Fegatella from the upper surface, showing 
the antheridial discs (a.). E-G, Marchantia. showing respectively 
gemma-cups (g.), antheridial head, I', and archegonial head, G. g.p., 
growing point ; m., midrib. (All figures approximately natural size.) 



generally somewhat thickened like, a midrib (Fig. 146, in.), 
and projects to a more or less marked extent on the lower side 
of the thallus. Water and nutrient salts are absorbed by 
numerous fine thin-walled unicellular hairs (rhizoids, Fig. 146, A ; 
Fig. 152, r.), which grow out into the soil from the lower surface 



STRUCTURE OF LIVERWORTS 



271 



often mainly from the region of the midrib ; these rhizoids also 
function as organs of attachment. 

The underside of the thallus, in many Liverworts, also bears 
one or more rows of fiat overlapping scales (Fig. 149, s.), which 
are one cell thick, and not uncommonly purphsh in colour. 
Some forms {e.g. Marchantia) exhibit, m the axils of these scales, 




XI5 



Fig. 147. — A leafy Liverwort (Cephalozia bicuspidata). A, Portion of a 
plant. B, Sporogonium showing capsule (c.) before dehiscence. C, The 
same with dehiscent capsule, showing elaters. D, Elaters (el.) and 
spores {sp.). 

a second type of rhizoid, characterised bj' possessing a wall 
with numerous peg-like internal thickenings (Fig. 149, A, B) 
The capillary channels, between the dense weft of rhizoids and 
the overlapping scales, are no doubt of importance in the reten- 
tion of water for use during periods of drought. 

The Liverworts, however, also include leafy types which 
somewhat resemble Mosses in habit, and, in general, grow in 



272 



LEAFY LIVERWORTS 



rather drier situations than do the simple thalloid forms ; the 
leaves, unlike those of Mosses, are usually lobed (Fig, 147, A), and 
sometimes even deeply divided. Some of the commonest of 
the foliose Liverworts belong to the genera Lophocolea and 
Cephalozia, whose structure is typical of these forms. Here 
there is a prostrate stem, bearing on either flank a row of over- 
lapping, sessile, 
two-lobed leaves 
(Fig. 147, A) and, 
on the underside, 
a few small scales 
with tufts of 
rhizoids arising 
from their base. 
The two lobes of 
the leaves are 
often folded to- 
gether. In some 
of the foliose 
forms the leaves 
are complicated 
by the modifica- 
tion of the lower 
lobe into a water- 
receptacle. Frul- 
lania, a common 
epiphyte on tree- 
trunks, affords 
an extreme ex- 
ample, in which 
this lobe is de- 
veloped as a 
smah, helmet-shaped pitcher (Fig. 148). In the leafy Liverworts 
the growing point of the stem is invariably a single cell. 

The thallus of most Liverworts exhibits little anatomical 
differentiation. In such an one as P cilia, for example, all the 
cells, apart from their elongated form in the region of the midrib, 
are similar in shape, and most of them contain the numerous 
smell discoid chloroplasts which are typical of Bryophyta 




Fig. 148. — Photomicrograph of a small portion of the 
epiphytic Liverwort FruUania tamarisciiii, show- 
ing the pitcher-like lower lobes of the leaves. 
[Photo, E. J. S.] 



Anatomy of liverworts 



273 



(Fig. 152, A). An equally simple structure is displayed by 
most of the foliose forms, the leaves being invariably only one 
cell thick, and usually devoid of a midrib. In Marchantia and 
some of its allies, however, the upper part of the thallus, which 
is always the principal assimilating region, shows considerable 
complexity (Fig. 149, A). It is subdivided into a large number 
of shallow polygonal chambers, each of which is roofed over by 
an epidermis, and communicates with the exterior through a 




Fig. 149. — Structure of Marchantia. A, Small part of a transverse sec- 
tion through the thallus. B, Part of a peg-rhizoid, much enlarged, 
showing the thickenings {pe.). C, Surface-view, showing a single 
pore. D, Gemma, seen from the surface, a., assimilatory filaments ; 
m., mucilage-cell ; p., pores ; r., rhizoid ; s., scales. 



central barrel-shaped pore [p.). From the floor of each chamber 
arise numerous short filaments of green cells {a.), which form 
the assimilatory system of the thallus. The whole of the lower 
portion of the latter consists of large colourless cells, serving 
in the main for the storage of food-reserves, and in part showing 
reticulate thickenings. On the surface of the thallus the assimi- 
latory chambers appear to the naked eye as a number of small 
diamond-shaped areas, each with a minute central dot corre- 
sponding to the pore. 
18 



274 



STRUCTURE OF MOSSES 



In the Mosses there is a definite stem, bearing three or more 
rows of alternate sessile leaves. The plants either have a pros- 
trate habit {e.g. Hypnnm, Fig. 150, C), like that of most leafy 
Liverworts, or grow erect, as in the Hair Mosses (PolytricJmm, 
Fig. 150, A, Fiinaria, etc.). The lower part of the stem, which 
is buried in the soil, forms a kind of rhizome bearing nmiierous 
rhizoids, and sometimes small scales as well. The rhizoids, 

which may also 
develop from the 
pai't of the stem 
just above the 
soil, are out- 
growths of the 
superficial cells, 
but differ from 
those of Liver- 
worts in being 
multicellular and 
branched ; they 
usually have 
brown mem- 
branes, and are 
divided by 
oblique septa, 
i ust behind which 
the branches 
originate. 

The leaves of 
Mosses are never 
lobed, are at- 
tached to the 
stem by a broad cushion-like base, and usually, except in the 
region of the midrib, consist of a single layer of cells (Fig. 151. B). 
The midrib (;«.) is generally well differentiated, and its presence 
forms a point of contrast with the leafy Liverworts. The branches 
of the stem arise from below the leaves, and, in some of the 
erect forms, are produced in such large numbers from the base 
that the plants exhibit a densely tufted habit (e.g. Tortilla, 
Fig. 150, D). Many of the Bog-mosses {Sphagnum) are likewise 




Fig 



150. — Various Mosses. A, Plant of Polytrichum, 
bearing a sporogonium whose capsule is covered 
by the calyptra (ca.). B, Male plant of the 
same, showing the antheridial head (a.). C, 
Hypnnm, with sporogonium. D, Torlida 
muralis. E, Male plant of Miiiitm, with an- 
theridial head (a.). 



STRUCTURE OF MOUSES 



275 



richly branched. The pecuhar habit of these Mosses is partly 
due to the fact that some of the branches elongate considerably, 
and hang down alongside the main stem (Fig. 151, F). The 




Fig. 151. — Structure of Mosses. A, Portion of a leaf of the Bog-moss 
{Sphagnum), showing cells with chloroplasts (ch.) and the large empty 
cells with their thickenings and pores {p.). B, Leaf-apex of Mniiini, 
with the midrib {in.). C, Leaf-cells of Hypnum, from the surface, 
showing thickened walls and pits. D, Transverse section of the stem 
of JMnium hornum (after Bastit), showing conducting strand {Cs.) and 
storage-cells (C). E, Leaf-cells of Hylocomium, from the surface. 
F, Habit of Sphagiuim (after Schimper), showing three sporogonia. 



branching of many prostrate Mosses takes place in a pinnate 
manner {e.g. Hypmtni, Fig. 150), whilst that of the erect forms 
is usually forked. Growth of the stem and its branches is 



276 ANATOMY OF MOSSES 

effected bj' means of three-sided apical cells, similar to those 
of Equisetum (cf, p. 18). 

Mosses show a greater degree of anatomical complexity than 
Liverworts, as is well illustrated by the invariable presence in 
the stem of a distinct conducting strand, consisting of small, thin- 
walled, much elongated cells (Fig, 151, D, Cs.) ; in some of the 
larger Mosses {e.g. Polytrichuni) these cells may be of two kinds, 
serving respectively for the conduction of water and elaborated 
food-materials. The outer tissues of the stem, as seen in trans- 
verse section, consist, except in Mosses occupying clamp habitats, 
of cells with strongly? thickened walls which are often reddish- 
brown in colour (Fig. 151, D). Between this peripheral 
mechanical cylinder and the central conducting strand are larger 
cells (C) with thinner walls, ^'^■hich seem to function mainly for 
food-storage. 

The cells composing the leaves are either narrow and rhom- 
boidal (Fig. 151, E), or spindle-shaped (as in many species of 
Hypnnni, Fig. 151, C), or almost isodiametric {e.g. Funaria and 
Mniiim, Fig. 151, B) , whilst those of the midrib («i.) , when present, 
are elongated ; the cells at the margin are often produced into 
teeth, and not uncommonly strongly thickened. The remaining 
cells may also be somewhat thick-walled, and in that case fre- 
quently bear well-marked pits {e.g. Hvpnum, Fig. 151, C). In 
a few Mosses {e.g. Tliiiidiiim) the assimilating surface is increased 
by outgrowths from the stem or by longitudinal lamella:, running 
parallel to the midrib {e.g. Catharinea nndulata, an abundant 
Moss in many woods). 

The leaves of Bog-mosses {Sphagnnm) are peculiar in being 
composed of two kinds of cells (Fig. 151, A). The green assimi- 
lating cells {ell.) are of narrow form, and are arranged as a 
reticulum whose meshes are occupied hy large spirallv thickened 
transparent cells which are dead and empty. The latter com- 
municate with the exterior bv means of one or more holes {p.) 
in their walls. These large colourless cells readilv fill with 
water, and it is owing to this that so much liquid can be 
squeezed out of a handful of Bog-moss. Siniilarl)', dry Sphagnum 
can suck up a great deal of moisture, hence its emplo\nient as 
an absorbent in surgery, or for molasses m the manufacture of 
Molassine Meal. Tn the drv condition, the air in the dead cells 



ANTHERIDIA OF PELLIA 277 

obscures the green colour of the Hving ones, and causes the whole 
plant to appear whitish. The Moss Leucobryuni, common on 
wet heaths, possesses leaves with a somewhat similar structure. 

Both Liverworts and Mosses propagate abundantly by vege- 
tative means, most frequently by fragmentation of the thallus. 
In many Liverworts, and some few Mosses, the thallus forms 
special bodies called gemma:, which consist of a varying number 
of cells and are often of characteristic shape (Fig. 149, D). In 
Marchantia they are formed in special cup-like outgrowths on 
the upper surface of the thallus (Fig. 146, E, g.), but in the 
leafy Liverworts they are usually budded off from the tips of 
the shoots. An abundant production of gemma: is likewise seen 
in the Moss Aiilacomnion androgynmn, where they arise in a 
spherical cluster at the top of the stem. 

The ordinary Moss or Liverwort plant, however, also re- 
produces by sexual means, the sexiial organs developing especially 
in the early spring. Their general character, in the case of 
Liverworts, will be gathered from an examination of Pellia. 
The male organs are found in the region of the midrib, and are 
visible to the naked eye as a number of dark pimples, each of 
which, in a vertical section (Fig. 152, A), is seen to correspond 
to a single more or less spherical antheridiitm. The latter 
(Fig. 152, B) is borne on a very short stalk, and almost fihs a 
flask-shaped depression in the thallus which communicates with 
the exterior by a narrow pore. 

The antheridium possesses a wall (Fig. 152, B, ic.) which is 
composed of a single layer of ceUs, and at maturity encloses 
numerous small, colourless, closely packed spermatozoid mother- 
cells, each producing a single spermatozoid (Fig. 152, D). 
Through vigorous absorption of water, in wet weather, by the 
cells of the wall, the apex of the antheridium is ruptured, and 
the mucilaginous mass of mother-cells is discharged into the 
surrounding moisture. Here occurs the final liberation of the 
spermatozoids , each of which possesses a spirally coiled body 
(formed mainly from the nucleus of the mother-cell), bearing 
two long cilia at the shghtly tapering front end (Fig. 152, C) ; 
a small vesicle, representing the remaining cytoplasm of the 
mother-ceU, is generally seen at the opposite end, but is shed 
during movement. 




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ANTHERIDIA OF MOSSES 279 

In some Liverworts {e.g. Fegatella) the groups of antheridia 
are embedded in slightly thickened patches on the upper side 
of the thaUus (Fig. 146, D, a.). A rather exceptional condition 
obtains in Marchantia, wliere the thallus-lobe, in which the male 
organs are sunk, is raised above the general surface on a definite 
stalk (Fig. 146, F). Amongst the leafy forms the antheridia 
are situated, either singly or in groups, in the axils'of the leaves. 




Fig. 153. — Vertical section through the antheridial head of the JIoss 
Fiinaria, showing the antheridia (a.) and interspersed sterile hairs (p.). 

The antheridia of IVIosses show essentially the same structuie 
as those of Liverworts, except that they are elongated and 
possess a broader stalk (Fig. 153, a.). They occur m groups, 
interspersed with multicellular sterile hairs (p.), at the top of 
the stem or its branches. Each antheridial group is surrounded 
by a number of spreading protective leaves {involucre), which 
are not infrequently arranged to form a kind of cup, and usually 
differ in shape and size, and sometimes also in colour, from the 
ordinary leaves. The male plants of Mosses are thus easily 



28o ARCHEGONIA OF LIVERWORTS AND MOSSES 

recognised (Fig. 150, B, E). Tlie spermatozoids sliow tlie same 
general structure as do those of Liverworts, and are liberated 
in much the same way during wet weather, the mass of mother- 
sells often being ejected to some slight distance. 

The female organs, or archegonia , which are essentially similar 
in the two groups, differ markedly from those of Thallophyta. 
They are flask-shaped structures (Fig. 154, B), composed of a 
lower swollen part, or venter (v.), and a long neck («.), which, in 
Mosses, is often twisted. The neck is a tube consisting of a 
single layer of elongated cells surrounding a narrow canal. The 
latter is occupied by a row of naked neck-canal cells which 
ultimately become mucilaginous, and, in swelling, push apart 
four lid-cells which, till then, close the upper end of the neck- 
canal. The venter possesses a wall (one-layered in Liverworts, 
Fig. 154, B, two-layered in Mosses), enclosing a large naked 
ovum (e.), with a small ventral canal cell {v.) at the base of the 
neck. The venter is borne on a short stalk which is rather 
more massive in Mosses than in Liverworts. 

In Mosses, where they are interspersed with sterile hairs, 
and in most leafy Liverworts, the archegonia occur in groups 
at the top of the stem or its branches. They are enveloped by 
a number of " invohtcral " leaves which generally differ in size, 
and sometimes also in shape and colour, from the ordinary 
leaves ; in most Mosses, however, such archegonial groups are 
scarcely distinguishable from the ordinary leaf-buds. In some 
leafy Liverworts (e.g. Lophocolea) the leaves of the involucre are 
fused together, almost to their tips, to form a cup. 

In some of the thalloid Liverworts [e.g. Pellia) the archegonia 
are found at the front end of the thallus, where they occupy 
depressions roofed over by a scale-like involucre (Fig. 154, A, i.). 
In other cases (e.g. Marchantia, Kg. I4('), G) they are situated 
on special stalked star-hke upgrowths which are di\-ided into 
4"io loljes ; the archegonia in these cases form groups on tlie 
lower surface, between the lobes, each group being enclosed 
between a pair of involucral scales. 

The diverse forms of involucre surrounding the archegonia 
not only protect them from desiccation, but form capillary 
chambers tending to retain water. This facilitates the move- 
ment of the sperms in fertilisation, which onlj- takes place when 



FERTILISATION 281 

the thallus is covered with a film of moisture. In forms like 
]\Iarchantia, the stalk bearing the lobed archegonial disc does 
not lengthen till after fertilisation, so that the necks of the 
archegonia remain in close contact with the moist upper surface 
of the thallus. In the same way the branches bearing the 
archegonial groups of Mosses are at first usually short and 
adjacent to the soil, so that they are readily covered with moisture 
during rain. 

After escaping from the antheridia, the spermatozoids, swim- 
ming in the surface films of moisture, are chemotactically 
attracted to the open necks of the archegonia by substances 




Fig. 154. — A, Longitudinal section through the apex of the thallus of Pellia, 
showing a group of archegonia {ar.) and the involucre (i.). r., rhizoid. 
B, Single archegonium of Marchantia, on a larger scale, e., egg ; 
«., neck ; v., ventral canal cell. 

(cane-sugar in Mosses) contained in the mucilage formed by the 
breaking down of the canal cells. The spermatozoid passes 
down the neck-canal and enters the egg, after which nuclear 
fusion occurs. 

The fertilised ovum secretes a cell-wall and, without any 
resting-stage, commences to divide and grow into a new Moss 
or Liverwort plant. This, however, differs fundamentally from 
the parent, especially in being parasitic upon the latter, and at 
maturity constitutes an organism (the sporogonium, Fig. 150 ; 
Fig. 155, A) that is almost solely concerned in the production 
and dispersal of spores. This simpUcity of structure can possibly 
be related to the parasitic habit, thus affording an interesting 



282 SPOROGONIUM OF LIVERWORTS 

analogy to parasitic Angiosperms. As the embryo enlarges, the 
venter of the archegonium stretches and grows into a protective 
covering, the calyptra, which can often be recognised around 
the young sporogonium (Fig. 155, B ; and Fig, 156, I, c). 

In Liverworts the dividing ovum first forms a spherical mass 
of cells which later elongates and graduallj' becomes differen- 
tiated into three regions. The end adjacent to the thallus 
penetrates into the underlying tissue as a cone-shaped sucker, 
the foot (Fig. 155, B,/.). This consists of parenchymatous cells, 
and serves to absorb and transfer food to the developing sporo- 
gonium. The opposite end swells into a, usually, spherical 
capsule (ca.) with a wall of one or more layers enclosing a mass 
(the archcsporiiim) of spore-forming cells. Up to a certain stage 
all the latter are alike, but the archesporium ultimately differ- 
entiates into two kinds of cells, which at hrst are often arranged 
in radiating series. One type is rounded, and, since each of 
these cells divides to form four spores (Fig. 155, C, sp.), they are 
known as spore-mother cells. The other cells are elongated and 
at first thin-walled, when they assist in the translocation of 
food-materuils to the developing spores, but subsequentl}/ thev 
acquire spiral thickening bands, and form so-called clatcrs 
(Fig. 155, C, e. ; see also Fig. 147, D). 

The region between capsule and foot, consisting of small 
cells, often densely crowded with starch-grains, constitutes the 
seta or stalk (Fig. 155, B, s.), which always remains short until 
the spores are mature. At this stage, ho«-e\'er, m PelUa and 
most leafy Liverworts, the stalk elongates rapidly, owing to a 
great increase in the length of its cells at the expense of the 
starch-reserves, and bursts through the cah'ptra, which can be 
recognised as a torn sheath around its base (Fig. 155, A, c.). 
The capsule is thus raised abo\'e the damp soil into the less 
humid air, where dispersal of the spores by air-currents is more 
readily attained. WLere, as in Marchantia. the archegonia are 
borne on a special receptacle, the stalk of the latter elongates 
after fertilisation (cf. abo^•e), and scr\-es the same purpose as the 
seta of other Liverworts, whilst the sporogonium retains the 
short stalk of the embryonic phase. 

In Pellid and most leafy Li\-erworts the capsule-wall splits 
into four petal-like valves (Fig. 147, C), although m Marchantia. 



SPOROGONIUM OF LIVERWORTS 



283 



the apex ruptures irregularl)'. Owing to their unequal thick- 
ening, the elaters tend to coil and bend as the contents of the 
capsule become drier. This movement is, however, prevented 
until rupture of the capsule takes place, when the tensions set 




Fig. 155. — The sporo,5onium of Pcllia. A, Small part of a thallus, showing 
a mature sporogonium, with the ruptured calyptra (c.) at the base 
(after Leunis). B, Longitudinal section through an almost mature 
sporogonium, still enclosed within the calyptra (c.) ; the elaters are 
shown as dark lines among the much larger spores. ca., wall of 
capsule ; /., foot ; ;'., involucre : ».. neck of archegonium ; ;-.. rhizoid ; 
s., seta; sp., spore. C, A group of spores (s/>.) and elaters (e.). 



up in the elaters are manifested in a wriggling movement which 
loosens the mass of spores and flicks them into the air. Not 
uncommonly tufts of elaters remain adhering to the top of the 
stalk [e.g. Pcllia) or to the valves (Fig. 147, C). 



284 SPOROGONIUM OF MOSSES 

In a few Liverworts belonging to the genus Riccia, the 
sporogonium, which is here embedded in the thallus, is of much 
simpler construction, consisting merely of a spherical capsule 
within whose one-layered wall only spores are produced. 

In Mosses the embryo elongates considerably at an early 
stage, and soon acquires the shape of a rod tapering at either 
end (Fig. 156, H and I). The lower extremity penetrates into 
the tissue of the Moss-stem and forms the absorptive foot (/.), 
which is, however, much less distinctly outlined than in Liver- 
worts. Soon after, a swelling, the apophysis, appears on the 
rod-shaped embryo, a little way above its middle, separating 
the future seta and capsule. The apophysis, which plays a 
part in the nutrition of the growing sporogonium, is a local 
assimilatory region composed of cells rich in chloroplasts and 
provided with abundant intercellular spaces (Fig. 156, A, ap.). 
The epidermis here contains stomata (Fig. 156, F) like those of 
higher plants, except that the septa between the guard-cells 
break down at maturity, so that the latter form an oval canal 
surrounding the pore (Fig. 156, G). 

The upper part of the embrj'o enlarges progressively, during 
the subsequent development, to form the capsule, so that at 
maturity the latter is usually far more prominent than the 
apophysis, which, though it always remains recognisable, does 
not grow much after its first formation (cf. Fig. 156, A). The 
mature capsule is here also raised aloft by a considerable elonga- 
tion of the slender stalk (Fig. 150), and again this causes rupture 
of the calyptra. In Mosses, however, this takes place in such a 
way that the upper part of the calyptra is carried up on the 
capsule as a hood (Fig. 150, A, ca.). When this is removed, the 
apex of the capsule is seen to be separated from the rest bj' a 
slight constriction, and it is this part that becomes detached as 
a lid to allow of dispersal of the spores. 

The internal structure is best studied in a longitudinal section 
of a nearly ripe capsule (Fig. 156, A). Here the axis is occupied 
by a rather broad column of thin-walled parenchyma (the 
columella, Co.) passing below into the tissue of the apophysis [ap.) 
and above into that of the lid (/.). Surrounding the columella 
are two concentric cylinders of cells, separated by the granular 
archcsporiitm {a.), each mother-cell of which, as m Liverworts, 



an. 




Fig. 156. — Structure and development of the lloss-sporogonium (all figures, 
except F, represent Fiinaria hygrometrica). A, Longitudinal section 
through young capsule, apophysis (ap.), and the top of the stalk. 
B, Part of the capsule in the region of the lid, much enlarged. C, Small 
part of archesporium and spore sack, enlarged. D, Two pairs of 
peristome-teeth (inner and outer). F, Aperture of dehisced capsule, 
showing arrangement of peristome-teeth. F, Section of small part 
of apophysis of Bryum argenteum. G, Single stoma, in surface view. 
H and I, Stages in development of the sporogonium. a., archesporium ; 
an., annulus ; c, calyptra ; Co., columella ; d., diaphragm ; ep., epi- 
dermis ; /., foot ; i., inner peristome tooth ; I., lid ; «., neck of arche- 
gonium ; 0., outer peristome tooth ; p., peristome-layer ; S., air-space. 
(D after Ruhland ; F after Braithwaite ; F and G after Haberlandt ; 
the remainder after Sachs.) 

2S5 



286 SPOROGONIUM OF MOSSES 

divides to form four spores (Fig. 156, C), Between the spore- 
sack so formed and the capsule-wall is a conspicuous air-space (S.) 
bridged by occasional fine threads of green cells ; in a few 
Mosses {e.g. Polytrichtm) a similar air-space separates the spore- 
sack from the columella. The capsule is protected by a thick- 
walled epidermis {ep.) beneath which are several layers of cells 
containing chloroplasts. The features just described are equally 
recognisable in a transverse section through the middle . of the 
capsule. 

Just below the constriction (Fig. 156, A and B) marking the 
commencement of the lid, a diaphragm (d.) composed of thickened 
cells, and having the form of a circular ledge perforated by the 
central thin-walled tissue, stretches inwards from the epider- 
mis {ep.) of the capsule. Arising from the inner edge of this 
diaphragm is a dome-shaped layer of cells (the peristome, p.) 
characterised by marked thickening of the tangential {i.e. inner 
and outer) walls {i. and 0.), and of the adjoining portions of 
the horizontal walls, although the radial {i.e. vertical) walls 
remain thin. At the lower edge of the lid the thick-walled 
epidermis is interrupted by one or two rings of larger thin-walled 
cells which constitute the so-called annulns {an.). Dehiscence 
eventually takes place along this line, as a result of the drying 
of the capsule, and soon after the lid is blown off. The epidermal 
cells above the annulus are usually markedly flattened, and form 
the lower edge of the lid (Fig. 156, B). 

At maturity all the thin-walled tissues of the capsule shrivel, 
leaving, apart from the spores, only the epidermis, the diaphragm, 
and the thickened walls of the peristome. Each of the rows of 
cells forming the latter necessarily tapers from base to apex, 
and, as a result of the breaking down of the thin radial {i.e. ver- 
tical) walls between some or all of the rows, a number of separate 
plate-like pcristoine-teeth (Fig. 156, D), attached below to the 
edge of the diaphragm, are formed. When both inner and 
outer tangential walls of the peristome are thickened, a double 
set of teeth {0. and i.) of course arises as the thin portions 
of the horizontal walls shrivel. The peristome varies greatly 
in different Mosses, and, together with the characters of the 
leaves, constitutes one of the chief means for distinguishing the 
different genera and species. 



PROTONEMA 



287 



The peristome-teeth are extremely sensitive to changes in 
the moisture-content of the air, curhng outwards when tire latter 
is dry, and inwards when it is damp. In dry weather, therefore, 
the powdery mass of spores inside the capsule is readilj' dispersed, 
whilst in wet weather the dome is reconstituted (Fig. 156, E), 
and the aperture, left by the shedding of the lid, closed. The 
actual dispersal, in which the elasticitj' of the seta plays a 
considerable part, is effected by a catapult-meclianism similar 
to that of the Cowslip and Poppy fruits. 




Fig. 157. — Moss-protonema. A, Development of young Fimaria- plant (6.) 
from the protonema (p.) (after Campbell). B, Stages in germination 
of spores, and C, protonema of Dicranella heieromella (after Servettaz). 
^., gemma on protonema ; r., rhizoids ; sp., spore-membrane. D, Single 
cell of overground part of protonema. 



The spores ultimately give rise to a new Liverwort or Moss, 
but the adult plant is not produced directly on germination. 
This is especially obvious in Mosses, where the spores in most 
cases develop into a branched multicellular filamentous structure, 
the protonema, which exists partly above and partly below 
ground (Fig. 157, C). The overground portion appears as a 
green weft on the surface of the soil, and its cells contain numerous 
discoid chloroplasts (Fig. 157, D), whilst the underground portion 
closely resembles the rhizoids of the ordinarj? Moss (Fig. 157, C, r.). 



288 PROTONEMA 

In fact, the latter readily grow out into protonema, if exposed 
to the light, and can thus bring about the development of new 
plants. 

Vegetative multiplication of the protonema, by the de- 
tachment of branches or of small terminal groups of cells 
(Fig. 157, C, g.), is often prolific, and may set in at a comparativelj? 
early stage. Sooner or later, however, some of the short lateral 
branches become pear-shaped and undergo segmentation by three 
oblique walls, which cut out the characteristic tetrahedral apical 
cell of the stem of the adult Moss (Fig. 157, A, b.). By its 
continued divisions a small mass of cells is formed from which 
leaves soon arise, and it is not long before the new Moss attains 
recognisable dimensions. This formation of Moss-plants takes 
place at many points on the protonema, and, as the latter usually 
dies away soon after, the individuals become independent. A 
protonema readily develops from most parts of the adult Moss, 
in fact, in all cases of vegetative propagation from fragments 
of the thallus (cf. p. 277), a protonema is first formed, and from 
this the new plants only arise secondarily. 

The protonema of the foliose Liverworts is similar in appear- 
ance to that of Mosses, but in most of the thalloid species the 
early stages are not sharply marked off from the adult. 

The life-history of the Brj'ophyta thus includes two distinct 
phases : the one, concerned in the production of the sexual 
organs, is relatively highly organised and self-supporting, whilst 
the other, concerned solely with the production of asexual spores, 
is always dependent, and relies for its sustenance, either entirely 
(most Liverworts) or partly (Mosses), upon food obtained from 
its host. These two phases normally alternate with one another 
and arise from one another, and a similar alternation is encoun- 
tered in all the higher groups of plants. For convenience of 
designation the spore-producing phase is spoken of as the sporo- 
phyte, and the sexual phase as the gameioplivte. 



CHAPTER XX 

Ferns 

lugh exhibiting a marked advance in organ- 

TiiE Bryophyta, thd with the Thallophyta, differ from the 

isation as compare plants, comprised in the third class of the 

remaining flowerlef;^ or Pteridophyta, in lacking roots and true 

Vegetable Kingd'-In the Pteridophyta, moreover, the gameto- 

vascular tissue.- thalloid in form and short-lived, whilst the 

phyte is usual phase in the life-history is far more conspicuous 

spore-produciranent, and is quite independent of the gametophyte 

and more per very earliest stages. In the sexual process and 

except in i structure of the sexual organs, however, there is 

the generic correspondence between the two classes, the Pteri- 

consideralike the Bryophyta, possessing motile male cells, and 

dophytaending on the presence of hquid water at the time of 

thus deion. The Pteridophyta, as a matter of fact, in general 

fertilismoist habitats, although rpite a considerable number 

favourw under relatively dry conditions. Included in this class 

can g'e Ferns (Filicales), the Horsetails (Equisetalcs) , and the 

are tosses (Lycopodiales), of which the first-named will be 

Club/.ered in the present chapter. 

consi many common Ferns the stem is an underground rhizome, 
1 is relatively insignificant in comparison with the con- 
whious, usually deeply divided, pinnate fronds which alone 
spicir above the surface ; a notable exception is, however, 
appted by the Tree Ferns of subtropical regions, which have 
affonarked trunks. The subterranean stem is either horizontal 
walk the Bracken, Pteris aquilina, Fig. 158, B), or compact 
(as nore or less erect [e.g. Lady Fern, Athyrium filix-famina ; 
andl Fern, Osmunda regalis). It rarely shows branching, 
Ro3;h this occurs at intervals where the rhizome is elongated 
tho-ig 2S9 



290 



RHIZOME 



[e.g. Bracken). In the creeping forms, t|he leaves usually arise in 
a row, on either side, and are separated hiy well-marked internodes 
(Fig. 158, B). In short-stemn-ied species, however, whether the 
rhizome be erect or inclined [e.g. Male Shield, Fern, Nephrodiumfilix- 
mas 1) , the leaves are densely crowded, and^ show an obvious spiral 
arrangement. In the more compact formSj a conspicuous feature 




Fig. 158. — The Bracken (Pleris aqiiilina). A, Diagram of transverse seiotion 
of rhizome. Scl., sclcrencliyma ; St., steles. B, Rhizome shewing 
apex with growing point (g.p.), successively older leaves (I, II, ^III), 
and the bases of leaves of former seasons. C, Yoimg frond. 1 

of the rhizome, which adds appreciably to its apparent thickn.ess, 
are the adhering bases of the leaves, which persist after lanidna 
and petiole have died away. All the roots of the adult Fern 
are adventitious (Fig. 158, B) ; they usually arise in the neigh- 
bourhood of the leaf-bases, and are gcnerall}- black in colour 
and forked. 

Fhe young leaf is mostl}- more or less densely clothed with 
' y\lso kn(jwn as Aspi dtiiDi fili.x-iiias and LastrcFa filir-iiias. 



LEAF 



291 



brown scaly hairs or ramenta, many of which are shed as the 
blade unfolds ; they usually persist, however, upon the petioles 
(Fig. 159, A, r.) and on the leaf-bases clothing the older parts 
of the stem. As the leaf-rudiment produced behind the growing 
point gradually develops, the axis of the lamina becomes coiled 
after the manner of a watch-spring, and, in the case of compound 
lea\-es, the individual portions become inrolled in a similar way 
(Fig. 15S, C). Each seg- 
ment of the blade grows 
by means of an apical cell, 
which thus occupies a pro- 
tected position within the 
spiral formed by the ma- 
turer parts. These features 
are readily observed in a 
young Bracken-frond, as it 
emerges from the soil in 
spring, and the prolonged 
growth of its tip is evi- 
denced b}- the retention of 
the coiled character in the 
uppermost portion, long 
after the older basal por- 
tion is fully expanded. 
The development of Fern- 
lea\-es is usually ^'ery 
slow, the rudiment being 
produced and undergoing 
gradual enlargement for 
two or more years before 
the frond appears above 
II, HI). 

Not all Fern-leaves are as deeply compound as in the Bracken 
(Pteris aquilina) 01 Lady Fern {Athyriuui filix-fcEiiiina). Those of 
the Poh-pody [Polypodiitm viilgare, Fig. 159, B) and Hard Fern 
(Blechmtm spicant, Fig. 172), for instance, are simply pinnate, 
whilst those of the Hart's Tongue Fern {Scolopendriiim vulgare, 
Fig. 159, A) are undivided. The venation is exceedingly char- 
acteristic, the midribs of the leaves or pinnae, as the case may 




Fig. 159. — Entire fronds of A, Hart's 
Tongue Fern [Scolopendriiim vul- 
gare) ; B, Polypody [Polypodiitm 
vulgare) ; and C, Maidenhair Fern 
[Adianiiim). r., ramenta ; s., son. 
(All three considerably reduced.) 

the surface (cf. Fig. 158, B, I, 



292 STRUCTURE OF FERN-STEM 

be, bearing numerous forked lateral veins which are not con- 
nected by cross-branches (Fig. 163, A, B). 

A general idea of the more characteristic features in the 
internal structure of the stem can be gathered from a study of 
the Bracken rhizome. In transverse section (Fig. 158, A) this 
is seen to be traversed by two rings of vascular strands or 
steles {St.). Separating the rings are two bands of dark brown 
sclercnchyma {Set.), and the same tissue also forms a layer 
beneath the epidermis ; its elements (Fig. ifji, Scl.) differ from 




Fig. 160. — Part of a stele from the rhizome of tlie Bracken (Plcris aquilina) 
in transverse section. En., cndodcrmis ; P., pericycle ; P.p.. phloem- 
parencliyma ; P.xy., proto.xylem ; .S"., sieve-tube ; X.p., xylem- 
parencliyma ; A3'., xylem. 

the fibres of higher plants in being short and relati\-cly thin- 
wallcd. The remaining ground tissue is parenchymatous, and 
contains an abundance of starch. 

The vascular strands are concentric (Fig. 160), with the 
component tissues more or less S3'mmetricall_y arranged. The 
term stele is customarily applied to these concentric strands of 
Ferns, but this does not neccssarilj? imply that the}' are com- 
parable to the entire vascular cylinder of a higher plant. Each 
stele is bounded by an endodermis {En.), with dark brown mem- 
branes, and a pericycle (P.), composed of rather larger thin- 



STRUCTURE OF FERN-STEM 



293 



walled cells having comparatively few contents. Next follow one 
or two layers of much smaller cells, constituting the so-called 
phloem-parenchyma (P.p.). Immediately rfithin is a conspicuous 
zone consisting chiefly of large, empty-looking, thin-walled 
elements, the sieve-tubes (5.), which are often absent towards 
the ends of the strands, and which are separated from the central 
xylem by a zone of small-ceUed wood-parenchjma [X.p.). The 
bulk of the xylem consists of large tracheids (Xy.), but near the 
ends and towards 
the centre of the 
strand, small ele- 
ments, representing 
the protoxylem 
(P.xy.), can usually 
be recognised. 
There is generally a 
small central mass of 
wood-parenchyma. 

In longitudinal 
sections, cut so as 
to pass radially 
through a stele 
(Fig. 161), the sieve- 
tiihes (s.) appear as 
elongated structures 
with tapering ends, 
and bearing the 
sieve areas on their 
sloping radial faces. 




E-J.S. 



Fig. 161. 



-On the left, part of a radial longi- 
tudinal section through one of the steles of 
the Bracken-rhizome, e., endodermis ; g.t., 
ground tissue; p., pericycle ; p.p., phloem- 
parenchyma ; s.. sieve-tube ; t., tracheid of 
the metaxylem ; x.p., xylem-parenchyma. 
On the right, a little of the sclerenchyma 
(Scl.) in longitudinal section. 
Under the high 

power these sieve areas exhibit a rather irregular ladder-like 
(scalariform) thickening of shining bars, with intervening darker 
zones exhibiting a fine dotting and bearing a number of highly 
refractive adhering granules. The longitudinal walls of the 
metaxylem tracheids [t.) exhibit several rows of closely arranged 
oblong bordered pits, producing an exceedingly characteristic 
type of scalariform thickening, whilst the protox5dem tracheids 
are spirally thickened in the usual way. 

The number of vascular strands observed in a transverse 



294 



STRUCTURE OF FERN-STEM 



section of the stem \'aries considerably in different Ferns. Some 
{e.g. Gleichenia, the Bracken of the Tropics) possess but a single 
central stele of comparatively large size, whilst many of the 
Tree Ferns exhibit a complex system of concentric rings of steles. 
In the Male Fern {Nephrodiiiin filix-nias) , again, the strands form 
but a single ring (Fig. 162, B, .Si.). The arrangement and histo- 
logical structure of the vascular tissues within each stele is, 
however, essentiallj.' the same in all cases. The 3''oung Fern 





Ftg. 162. — Ncplirodiiiin filix-uias. A, Skeleton of vascular system showing 
the cylindrical network of steles, with numerous leaf-trace strands 
arising from the edges of the " leaf-gaps " (original). B, Transverse 
section of the rhizome (after De Bary). l.h., leaf-bases ; /./., leaf- 
trace strands ; Scl., sclerenchyma ; St., stcm-stcles. 



seedling always possesses but a single central strand which, as 
the plants become older, enlarges and, in forms like Ncphrodinm 
and Picris, gradually breaks up into the separate steles of the 
adult. 

The relations 1)etween the \'ascular system of stem and leaf 
are particularly plain in the Male Fern [Ncplwodiuni) . and are 
easily decii)hered, if its rhizome be allowed to rot, so that all 
but the hard vascular tissues decay. In the resulting skeleton 
(Fig. 1(12, .\) the steles form a cylindrical network, in which each 



STRUCTURE OF LEAF 295 

mesh or leaf-gap corresponds in position to the attachment 
of a leaf. The latter is supplied loy the numerous fine strands 
passing out from the margin of the mesh. These leaf-traces can 
be recognised, in a transverse section of the rhizome (Fig. 
162, B), as arcs of small strands (l.t.) occupying the lobes that 
represent the leaf-bases {l.h.) In the case of the Bracken (Plerts) 
a series of transverse sections, cut in the region of the node, 
shows that the leaf-base is supplied by several strands passing 
off from the outer ring. The gap thereby created is fiUed by 
steles from the inner series, which pass out, through a break 
in the sclerenchyma-ring, on the side towards the leaf-trace. 
The petiole in both these Ferns is thus traversed by a large 
number of strands, normally forming an arch, but in certain 
genera {e.g. Gleichenia) the leaf-stalk, like the stem, contains but 
a single stele. 

In its general structure the leaf conforms to the ordinary 
dorsiventral type. Since many Ferns grow in shady situations, 
the epidermis often contains chloroplasts, and the palisade tissue 
is not uncommonly poorlj? developed or the mesophyll even 
spongy throughout (cf. Fig. 165). The ultimate branches of the 
concentric strands traversing the petioles become collateral 
within the pinnje, owing to development of the phloem only on 
the lower side. 

The roots of most Ferns are diarch, and owe their frequent 
wiry character to the development of an exceedingly thick- 
walled sclerenchymatous cortex, but otherwise do not differ 
appreciably from those of higher plants. The tetrahedral apical 
cell, by whose divisions the tissues of stem and root are formed, 
has already been described on p. 18. 

In the ordinary course of events the P'ern-plant sooner or 
later commences to produce spores. These are developed within 
small, stalked, usually brown-coloured sporangia, almost in- 
variably borne on the under-surfaces of the fronds, which in 
some few cases are completely covered by them {e.g. Rusty- 
Back). Most commonly, however, they are arranged in nimierous 
separate groups, or sort, each usually comprising a considerable 
number of sporangia arising from a slight central swelling of 
the leaf-tissue, known as the placenta (Fig. 165, pi.). 

Such sori are well seen in the Common Polypody {Polypodium 



296 



SORI 



vtilgare, Fig. 159, B, s.), where they appear as small circular 
brown patches situated over the lateral veins of the pinna;. A 
similar arrangement obtains in the Male Fern {Nephrodium, 
Fig. 163, B), but here each sorus is protected by a kidnev-shaped 
outgrowth (indusitim, In.) of the placenta, which withers when 
the sporangia are mature. In the Maidenhair Fern (Adiantum, 
Fig. 159, C, s.) and the Wall Rue {Asplcniuin riita miiraria) the 
soi'i are near the edges of the pinnules, and are covered hy 
indusia taking the form of little flaps projecting inwards from 

the margin, whilst in the 
Bracken (Ptcris, Fig. 163, 
D), where the sporangia 
form a continuous fringe 
all round the margin of 
the pinnule, the incurved 
edge of the latter functions 
as an inclusium. 

A peculiar tA'pe of sorus 
is foimd in the Film}' 
Ferns (Hymcnoph3'llacec'E) , 
which are so-called be- 
cause of their delicate 
fronds, consisting of but a 
single layer of cells. 
Though mainly encoun- 
tered in the Tropics, they 
are represented in Britain 
by one or two species [e.g. 
HymcnophvUnm tiiiibn'dgeiisc. Fig. 164) occurring in damp ca^'es and 
other humid situations in rocky districts. Here the sporangia arise 
from rod-like placentae (s.) projecting from the leaf-margin, and 
each sorus is enveloped at its Ijase in a cup-shaped indusium (;'.). 
In transverse sections through a fertile pinnule of NcpJirodiiDii 
filix-mas (Fig. 165), the bulging placenta {pi.) on the underside 
is seen to be supplied Ijy tracheids (/.) from the o\'erlying vascular 
bundle. The indusium (/.) appears as an \uubrella-shapcd out- 
growth, one kn'cr of cells thick, arising from the top of the 
placenta. On the sides of the latter are l)ornc numerous 
sporangia in \'arious stages of development, 




Fig. 163. — Sori of various Ferns. A, 
Hart's Tongue Fern [ScoJopendriuyn). 
B, Male Fern (Nephrodium). C, As- 
plenium. T), Bracken (Pleris). In., 
indusium. 



STRUCTURE AND DEVELOPMENT OF SPORANGIA 297 



A mature sporangium (cf. also Fig. 166, B) consists of a stalk 
of usuall}' three rows of elongated cells, terminated by a biconvex 
capsule which is more or less oval in side-^'iew, and encloses 
the spores within a wall of a single la\'er of cells. The cells of 
the wall fit firmly together, and most of them are thin-walled 
and \'ery flat, appearing more or less polygonal in surface-view, 
but tabular in optical section. The edge of the capsule, how- 
e^•cr, is occupied by a single 
row of speciallv differentiated 
cells. The greater part of this 
band, from the base of the 
capsule on one side to half-waj' 
down the other edge, is com- 
posed of cells which have all 
but their outer walls strongly 
thickened, and which constitute 
the annuliis {a. in Fig. 165 and 



an. in Fig. 166, B). The re- 




mainder, forming the so-called' 
siomium, are much broader and 
thin-walled throughout (Fig. 
166, B, s.), and it is here that 
the mature capsule ruptures. 

Each sporangium arises as 
a papillate outgrowth from a 
single surface cell of the placenta, 
which is cut off bj' a cross- 
wall, and undergoes division by- 
three oblique septa to produce 
a tetrahedral apical cell (b^ig. 

166, C, I, II). The three rows of segments, primaril)- cut off 
parallel to the three flat faces of the latter (III), elongate and 
become the stalk. Next, walls are formed parallel to all four 
faces of the apical cell (IV), and give rise to the one-la3-ered 
capsule-wall (Fig. 166, D and E, u\), an inner nutritive laj-er, 
or tapdiim {(.), and a central cell, the archesporiimi {a.). The 
last-named divides to form the spoix mother-cells, each of ^^hich 
gives rise to four spores (cf. p. 309). Their development takes 
place at the expense of the food-materials contained in the 



Fig. 164. — On the left part of a 
Filmy Fern {Hviuenophvlliiin 
tiiiibyidgeiise), bearing sori (s.). 
The two smaller figures on the 
right show a sorus, entire and 
in longitudinal section respec- 
tivcl)'. !., indusium; s., sorus. 




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308 



STRUCTURE AND DEVELOPMENT OF SPORANGIA 299 

g-ranular thin-walled cells of the tapetum, which thus disorganises 
before the sporangium matures. 

Witli the ripening of the sorus the indusium dries and shrivels, 
exposing the sporangia, which likewise begin to lose moisture. 
As evaporation proceeds, the thin outer walls of the annulus 




Fig. 166. — Nephrodium fiUx-mas. A, Mature spores, greatly magnified. 

B, A single sporangium, showing the annulus [an.) and stomium (s.). 

C, Early stages (I— IV) in the development of the sporangium (after 
Miillcr). D and E, Successively later stages. a., archesporium ; 
ap., apical cell ; g., gland ; St., cells of stalk ; t., tapetum ; iv., wall. 
F, Germinating spore {sp.). p., beginning of prothallus ; r., rhizoid. 



commence to bulge inwards (cf. Figs. 165 and 166, B), owing 
to the gradual reduction in the volume of their sap. Thus an 
increasing tension is set up in the annulus,^ which e^•entually 

1 It will be realised that the side-walls of the annulus are much less 
thickened than the horizontal walls, so that the contraction is entirely in 
the longitudinal direction. 



300 DEVELOPMENT OF PROTHALLUS 

leads to rupture of the capsule-wall along the plane of greatest 
weakness, viz. the junction between the transversely extended 
cells of the stomium (Fig. i66, B). On the sudden release of 
the tension the annulus, carrying with it a great part of the 
capsule-wall and many of the contained spores, flies back bej'ond 
the position of equilibrium, and, with the subsequent recoil, 
the spores are shot out as from a sling ; they are further dispersed 
1)V air-currents, and may thus be carried to a considerable 
distance. The dehiscence of ripe sporangia ^can be observed 
under the microscope by adding glycerine. 

Apart from the difference of arrangement, to which reference 
has already been made (p. 296), the sporangia of the Bracken, 
and most other British Ferns, agree in all essential respects with 
those of the Male Fern. Other tj'pes occur, however, as, for 
instance, in the Royal Fern {Osmiinda regalis). Here the spo- 
rangia are short-stalked and pear-shaped, and lack the charac- 
teristic annulus and stomium of the other forms. Dehiscence 
takes place by a vertical split starting from a group of thick- 
walled cells, a little below the summit of the sporangium, and 
extending over the top of the latter and some way down the 
opposite face (Fig. 173, B). 

The spores of most Ferns retain their capacity for germination 
for some time. In their thick walls three layers can often be 
distinguished, of which the outermost, usually dark-coloured 
and opaque (Fig. 166, A), is cuticularised, and constitutes the 
principal protective covering, whilst the innermost is thin and 
clastic. In germination (Fig. 166, F) the outer coats (Sp.) are 
burst, and the innermost is stretched to form a long, colourless, 
tubular outgrowth, which becomes separated off from the main 
body of the spore by a transverse wall, and penetrates the 
soil as the first rhizoid (r.) of the future ])lant. The remainder 
of the spore-contents, enveloped by tjic nmcrmost kn'cr of the 
\\-all, lengthen into a short horizontal filament (p.) whose few 
cells contain chloroplasts. \^'ithin the terminal cell t\\o inter- 
secting walls soon arise, and the apical cell thus formed cuts off 
segments on either side, so that the filament widens into a flat 
plate. 

As the front margin broadens, two or more of its cells become 
nic'risternatic (Fig. 167, £,'./>.) and de^'elopmcnt proceeds rapidh' ; 



PROTHALLUS 



301 



at the same time horizontal divisions in the middle region of 
the plate form the cushion, which is several cells thick. The 
green thalloid plant thus established is known as a prothallus, 
and when mature is usually heart-shaped, sometimes attaining 
a diameter of over a centimetre (Fig. 167). AU the cells are 
alike and contain chloroplasts. From the underside, especially 




Fig. 1G7. — Fern-prothallus (magnified about thirty-five times), from the 
under-surface. A considerable number of autheridia (an.) are seen, 
on the older part, and the projecting necks of twelve archegonia (ar.), 
in the region of the cushion, from which also numerous rhizoids (r.) 
arise, g-p-, growing point. 



in the region of the cushion, scattered superficial cells grow out 
as colourless rhizoids (r.), which serve for anchorage and the 
absorption of moisture. 

When the prothalli grow densely crowded, they often remain 
more or less filamentous, a condition which is normal for most 
of the Filmy Ferns. 

The prothallus is the Fern-plant (gamdophyte) that bears 
the se.xual organs. These are of the same general type as in 



302 ANTHERIDIA AND ARCHEGONIA 

Bryophyta, and are produced by the outgrowth of single surface- 
cells. They are situated on the lower surface, and both kinds 
usually occur on the same prothallus (Fig. 167). The antheridia 
{an.), found mainlv on the thinner marginal wings and the less 
robust prothalli, are almost spherical structures (Fig. 168, g.) 
with a small number of relativel)' large spermatozoid mother- 
cells (s.). The enveloping wall {w.) is peculiar in being composed 
only of two superposed ring-shaped cells surmounted by a 
dome-shaped cap-cell (Fig. 168, c). The spermatozoids (Fig. 
168, d), which are developed in the same way as in the Bryophyta 
(of. p. 277), have a spirally coiled, spindle-shaped body bearing 
numerous cilia near the pointed end. Dehiscence of the an- 
theridium, by the breaking away or rupture of the cap-cell 
(Fig. 168, e and /), takes place in wet weather, and the liberated 
spermatozoids swim in the film of water retained by capillarity 
between the lower surface of the prothallus and the soil. 

The archegonia are restricted to the region of the cushion, 
in which the}-' are partially embedded, the necks alone projecting 
(Fig. 167, ar. ; Fig. 168, a and h). The necks are all curved 
towards the pointed end of the prothallus, and differ from those 
of a Bryophyte archegonium in being short and composed of 
only four longitudnial rows of cells (Fig. 168, c), whilst there 
is but a single canal cell. The \'enter, containing the egg (0.) 
and the ventral canal cell {v.c), appears as a mere cavity in the 
tissue of the prothallus. At maturit}- (Fig. 1O8, h) the four cells 
at the top of the neck are forced apart by a mucilage containing 
malic acid, formed during the disorganisation of the canal cells, 
and an open passage is thus left leading down to the egg. It 
is apparently the malic acid that attracts the spermatozoids to 
the archegonia. 

After fertilisation the ovum becomes enveloped by a thin 
membrane, and divides b\- three successive walls into octants 
of a sphere (hig. 169, A). Their further segmentation leads to 
the differentiation of four apical cells, which are situated one 
m each quadrant, and \\'hich rcspecti\-ely gi\'e rise to the root, 
the stem, the lirst leaf, and the " foot " of the oiibryo. The 
foot develops as a large parenchymatous sucker which becomes 
firmly lodged in the tissue of the cushion, from whose cells it 
absorbs nourishment for the }oung h'ern (Fig. lOy, B,/). 



EMBRYOLOGY 



303 



By this means rapid growth of the root (Fig. 169, B, ;■.) takes 
place, and it soon pierces the prothallus and penetrates into the 
soil. Simultaneously the first leaf (/.), carrying with it the still 
rudimentary stem (s.), emerges on the lower side, and, arching 
up through the notch at the front end of the heart-shaped pro- 
thallus, exposes its green blade to the light. The stem now 
grows more vigorously, giving rise to adventitious roots which 




Fig. 168. — a-c, archcgonia, e,/, and^, antheridia, and d, single spermatozoid 
of the Fern. a, immature, and b, mature, archegonium in longitu- 
dinal section ; c, neck in surface view. 0., egg ; v.c, ventral canal 
cell, g, almost mature antheridium in optical section, showing the 
wall {w.) and the spermatozoid mother-cells (s.) ; e, dehiscing antheri- 
dium with the escaping spermatozoids ; /, dehisced antheridium from 
above, [a after Goebel ; the remainder after Knv.) 



replace the short-lived primarj' root, and it is not long before 
further leaves develop (Fig. i6g, C). The latter, however, show 
but a very gradual increase in complexity, from the first, often 
almost undivided leaf, to the normal compound foliage of the 
adult Fern, which is frequently only attained after the lapse of 
several years. Production of sporangia is generally deferred till 
this stage is reached. 

The young Fern, like the sporophyte of Liverworts and 
Mosses, is thus for a time dependent for nourishment on the 



304 



EMBRYOLOGY 



prothallus. But, as soon as the root has become established in 
the soil, and the first leaf has spread out its lamina to the light, 
this dependence ceases, and soon after that the prothallus withers 
away (cf. Fig. 169, C). As compared with Bryophj'ta, the rela- 
tive importance of the two phases in the hfe-history is therefore 
reversed. The freediving sporophyte usually attains large dimen- 







Fig. 169. — Embryology of the Fern. A, Longitudinal section through an 
archegonium, showing the octant-stage in the division of the fertilised 
egg (after Sadebeck). B, Young embryo escaping from the pro- 
thallus {p.) (after Hofmcister). /., foot ; /., hrst leaf ; r., hrst root ; 
s., stem. C, Young plant, with the remains of the prothallus {p.) at 
the base, four unfolded leaves of increasing complexity, and one 
leaf not yet unfolded (original). 

sions and exhibits a complexity of structure which is only 
approached by the perennial gametophytes of some few Mosses. 
On the other hand, the sexual generation is as a general rule 
shortdived, though in a few Ferns (('.,i,'. the Royal Fern, Osiiiuiida) 
it persists for some years. Normally the two generations alter- 
nate regularly with one another, but occasional abnormalities 
are encountered. 



APOSPORY AND APOGAMY 



305 



Thus, in certain Ferns the prothaUi may arise by direct 
budding from the lea\es or sporangia, without the formation 
of spores (apospoiy), a condition that has been experimentally 
induced in a \-ariety of the Lady I-'ern {Atliyn'iiin filix-fccmina) 
by pinning detached segments of the fronds on damp sand. 
In other cases the sporophj'te develops vegetati\'el\' from the 
prothallus without the intervention of sexual organs (apogiitnv)} 




Fig. 170. — Small part of a frond of Asplenium biilbiferum, showing the 
vegetative production of new plants (about half natural size). 

Vegetati\'e multiplication of the sporophyte is not infrequent, 
new plants arising from buds formed on the surface of the fronds, 
as in the commonlv culti\"ated Asplenium hitlbifcrnm (Fig. 170). 
In the normal life-cycle, of Bryophyta and Pteridophyta 
alike, the spore is the starting-point of the sexual, and the 
oospore of the asexual, generation. The spore mother-cells 
almost in\-ariably give rise to four spores, after undergoing two 
successive nuclear di\-isions. The nuclear changes involved 
differ, however, in se\'eral important respects, from those observed 
in the ordinary vegetati^•e divisions of the plant (cf. p. 21 et seq.). 

1 This is analogous to the loss of sexuality observed among Fungi 
(cf. pp. 236, 245). 
20 



3o6 REDUCTION— DIVISION 

Prior to the first division the chromatin reticulum concentrates 
into its constituent chromosomes, but these soon appear joined 
togetlier to form a thread. The latter thereupon contracts in 
a tangled manner around the nucleolus, towards one side of the 
nuclear cavity (Fig- 171, A) ; this stage, which is termed synapsis, 
has in other groups been recognised in living material, and is 
not therefore a contraction due to methods of fixation. 

As the thread spreads out again, on emerging from this 
condition, the nucleolus is seen to have disappeared, its chromatic 
material having presumably been absorbed into the chromatin 
thread. At the same time the latter becomes thrown into a 
number of loops (Fig. 171, A), each formed of two, more or less 
closely approximated, chromosomes, joined end to end. \'\'hen 
segmentation takes place (Fig. 171, B), the number of separate 
portions produced is only half that observed in the vegetative 
divisions of the same plant, each segment being composed of 
the two chromosomes forming one of the above-mentioned loops 
(Fig. 171, C). 

The nuclear spindle is established in the ordinary way, and 
the double chromosomes become grouped at its equator 
(Fig. 171, D). Thereupon each chromosome splits transversely 
into two, one half (actually a icJiole chromosome) passing to either 
pole. Each of the daughter-nuclei thus contains half the number 
of chromosomes characteristic of the vegetative cells, and, for 
this reason, this nuclear division is often spoken of as the 
" reduction " division. The second nuclear division follows almost 
immediately on the first, and does not differ essentially from 
that described for the ordinary vegetative cells.' 

Each of the four resulting spores (Fig. 171, E) thus possesses 
the halved number of chromosomes, and so do all the cells of 
the gametophyte arising from a spore. The normal number of 
chromosomes is restored in the fertilised egg, in consequence of 
the fusion of the male and female nuclei. It will be realised that 
the reduction division differs chiefly from the normal t^'pe in 
the occurrence of synapsis and in the passage of entire chromo- 
somes to the respective poles of the nuclear spindle. Moreover, 
there is good reason for believing that the associated chromosomes 

1 The phenomena exhibited during the two nuclear divisions in the spore 
mother-cells are often referred to as meiosis. 



REDUCTION— DIVISION— SPORE-FORMATION 



307 



of each separating pair respectively represent the chromatic 
material of the two parents from which the given organism arose 
(cf. p. 390). 

The importance of this phenomenon is also emphasised by 
the occurrence of an essentially similar method of reduction in 
animals. Reduction occurs in the formation of spores, not only 
in all Bryophyta and Pteridophyta, but also in the higher plants 
(Phanerogams). In the case of the Thallophj'ta the actual reduc- 
tion has onl}' been observed in relatively few cases, but it cannot 




Fig. 171. — Nuclear divisions in spore mother-cells of Nephrodium (after 
Yamanouchi). A, Late synapsis. B, Segmentation of chromosomes. 
C, Association of chromosomes. D, Equatorial plate. E, Part of a 
tetrad. 



be doubted that it takes place in all forms exhibiting a sexual 
process. It would appear, however, that it may be effected at 
various stages in the life-history, sometimes (as in Spirogyra) 
during the first division in the zygospore, in other cases {e.g. 
Fiicus) during the formation of the sexual cells. It may be 
noted that the latter is the customarj' stage for the occurrence 
of the reduction division amongst animals. 

The four nuclei, produced in the spore mother-cell, usually 
become grouped so as to form a tetrahedral figure. Each, 
together with part of the cytoplasm, becomes surrounded by a 
separate wall, the whole of the contents of the mother-cell being 



^,o8 



SPOROPHYLLS 



thus used up to form four spores (Fig. 171, E). Their faces of 
contact are more or less flattened and triangular, while the outer 
walls are convex, so that each spore, at its inception, has the 
form of a tetrahedron with a rounded base. In many cases, 
however, the spores round off after the mother-cell membrane 
has broken down. 

In most Ferns there is only one kind of frond which fulfils 
the ordinary functions of a leaf, as well as those connected with 




Fig. 172. — The Hard Fern (Blechiuim spicant), showing foliage leaves and 
sporophylls. [Photo E. J. S.] 



spore-production. In a few species, however, there is division 
of labour, as in the Hard Fern (Blcchiium spicant, Fig. 172). 
Here some of the leaves, spreading out near the surface of the 
ground, have broad pinn;e and are purely vegetative, whilst 
others, which assume an erect position, have narrow lobes and 
are almost entirely concerned in the production of sporangia. 
Such fertile leaves are called sporophylls. 

A less marked specialisation is seen in the Royal Fern 
{Osmitnda regalis), where the lower part of tlie huge frond is 
sterile, whilst the upper pinnules, bearing the sporangia on both 



SPOROPHYLLS 



309 



sides, exhibit practically no lamina (Fig. 173). In this connec- 
tion it may be noted that in some tropical Ferns (g.^. Platycerium) , 
which grow aloft on the branches of trees (a habit commonly seen 




Fig. 173. — A, Small part of a frond of the Royal Fern (Osmtiiida regalis), 
showing sterile pinnules (below) and fertile pinnules (above). (About 
half natural size.) (Original.) B, Single sporangium (after Luersen). 



in the Polypody in this country), the lowest leaves are modified 
to form an oblique bracket by which humus is retained. 

A greater contrast between leaves and sporophylls is met 
with in other groups of Pteridophyta, some of which are con- 
sidered in the next chapter. 



CHAPTER XXI 

Horsetails and Clubmosses 

The Ferns {Filicalcs) alone of the three existing groups of 
Pteridophyta are \\'idely represented at the present day. Not 
only are they almost ubiquitous in their distribution, but they 
comprise a large number of families and genera. The Bracken 
in North Temperate zones, and Glcichcnia in the Tropics, illus- 
trate, moreover, the important role played bj' P'erns in many 
t^'pes of vegetation. 

The Horsetails {Eguisctalcs) and Clubmosses {Lycopodialcs), 
on the other hand, \^'hich, like the Ferns, have been traced 
back in the fossil state to very early periods of the earth's history, 
are now onlj' represented by a few vexy distinct genera. These 
groups flourished vigorously, however, at the period when the 
Coal Measures were laid down ; then they comprised •\\-oodj' 
plants which, in great part, attained to the dimensions of trees 
(Fig. 177), at least sixty feet high in the case of Calamitcs. These 
features have been lost by the living forms of the present day, 
which are herbaceous and of small dimensions. It is, indeed, 
probably correct to regard the remote past as the age of trees 
and the present rather as that of herbs. 

By contrast with I-'crns the lca\'es of Horsetails and Club- 
mosses are remarkably small and simple in form, so that the 
habit of the plant is here determined mainly' by the character 
and extent of branching of the stem (Fig. 174, B and C ; 
Fig. 180, A). The Horsetails {Eqiiisctiim) are switch-plants 
(Fig. 174, B and C) whose green, longitudinally furrowed, stems 
bear ^^•horls of brownish scalc-lea\'es fused to form a toothed 
sheath {SI.) around each node. The branches are likewise 
whorled (Fig. 174, B), and, since they arise relatively late, have 
to pierce their way through the bases of the fused lea\es. The 

310 



STRUCTURE OF EOUISETUM 



311 



erect shoots are the upturned ends of the branches of horizontal 
underground rhizomes (Fig. 174, C), \\'hich bear similar whorls 
of scale-leaves, as well as numerous adventitious roots, at each 
node, the primary root dj'ing away at an early stage, as in Ferns. 
The rhizome of the Field Horsetail (Eqnisetum arvense) bears, 
in addition, tuber-like storage-organs formed from modified 
branches. In the ancient Horsetails [Calamitcs) of the Palfeozoic, 
leaves (cf. Fig, 175, B) and branches were likewise whorled. 




Fig. 174. — A, Section of stem of Equisetum palnslre (after Pfitzer), showing 
tlie numerous air-spaces (shaded) and the ring of vascular strands (v.). 

B, Upright, fertile shoot of E. pahistre, bearing a terminal cone (Sp.). 

C, Habit of E. limosum, showing the subterranean rhizome, with 
numerous adventitious roots arising from the nodes and t^vo aerial 
shoots [a.]. SI., whorls of scale-leaves. (B and C original.) 

The anatomy of the stem superficially resembles that of a 
Dicotyledon in the arrangement of the vascular tissues, and 
invariabty shows great reduction of the xylem and a conspicuous 
system of air-canals in the cortex and pith (Fig. 174, A). These 
aquatic characteristics are to be expected in the case of partially 
submerged species like Eqitisetiim liniositm and E. paliisfre, but 
the fact that others (e.g. E. arvense), growing in relatively dry 
situations, nevertheless exhibit these features indicates that the 



312 



REPRODUCTION OF EOUISETUM 



genus as a whole is derived from aquatic ancestors. Indeed 
there is good e\idence that the Calamites of the Coal Measures, 
which exhibited similar aquatic features combined with pro- 
nounced secondary thickening, were inhabitants of swamps. 

The sporangia of Horsetails are borne on little mushroom- 
shaped sporophylls (Fig. 176, B), differing widely from the 




Fig. 175. — Common forms of prescr\'ation of the fossil Horsetails and 
Clubmcsses. A, Pith-cast of Caltniules, B, Annularia sphenophyl- 
loides. the foliage of Calamiles. C, Small part of the surface of the 
stemof a Ic/>)rforf(;»c'ro», showing the characteristic Icaf-cnshions [I.e.) ; 
I.S., leat-scar. 

ordinary lca\'es, and collected together at the ends of the stems 
to form cones or strohili (Fig. 174, ]i ; Fig. 176, A). These 
usually terminate the ordinary vegetative shoots, though in the 
Common Field Horsetail (E. arvcnsc) they are found on special 
fertile shoots, produced earlier in the season, and distinguished 
by their brown colour and the absence of branches. Each 
peltate sporophyll stands out at right-angles to the main axis, 



REPRODUCTION OF EOUISETUM 



313 



and bears on the inner face of its hexagonal lamina a ring of 
5 to 10 sporangia encircling the stalk (Fig. 176, B). In the }'oung 
cone the heads of the sporopln'lls fit closely together, thus 
forming a compact protection for the sporangia ; lint as the 
latter mature the sporoph5'lls separate, through elongation of 
the axis, and permit escape of the green spores. The individual 
sporangia are rather larger than those of Ferns, and have a 
several-layered wall. Many of the fossil Horsetails possessed 
cones of more elaborate 
structure. 

At maturity the 
outermost coat of each 
spore is seen to consist 
of four extremely hygro- 
scopic spiral strips (Fig. 
176, C, D) which only 
remain attached at one 
point. Groups of spores 
consequentl}' tend to 
cling together when the 
contents of a sporangium 
are scattered by the 
wind, and this may be 
of importance, since the 
archegonia and anthe- 
ridia are usually pro- 
duced on distinct 
prothaUi. The spores 
are, however, all alike, 
the sex of the resulting 

prothaUi depending on the conditions of nutrition, those poorl}' 
nourished becoming male, '\\hilst those well nourished become 
female. Both kinds are more or less richl)' branched, but the 
male are m.uch smaller than the female. The sexual organs 
borne upon them do not differ essentiallj^ in construction from 
those of Ferns. It may be noted that, when Fern prothaUi 
grow densely crowded, they often bear antheridia only, although 
unisexuality is here the exception rather than the rule. 

The present-da}' Clubmosses are represented mainly bj- the 




Fig 



>. — A, Entire cone, and B, single 
sporophyll of Eqiiiseliim maximinn (ori- 
ginal). C and D, Mature spores, showing 
the splitting of the outer coat (C after 
Sachs; D after Dodel-I'ortl. 



314 



LYCOPODIUM 



genera Lycopodinni (Fig. 179) and Selaginella (Fig. 180). Tlie 
Quilhvort (Isoetes lacitsiris, Fig. 178), which occurs submerged 
in mountain tarns, is a pecuhar member of this group, many of 
whose features recall those of the fossil Clubmosses (Lcpido- 
dendron and Sigillaria, Fig. 177). Fragments of the stems of 

the ffH'mer with, or often 
without, the leaves, are 
very conunon in the Coal 
Measures and are readily 
identified bj- their charac- 
teristic markings (Fig. 

17.5. g. 

The small spirally ar- 
ranged leaves of Lyco- 
podiinn densely clothe the 
stems, \\-hich are either 
erect, as in the Fir Club- 
moss [L. sdago), or pros- 
trate, except for the 
cone-bearing shoots, as in 
the Bear's Foot [L. clavatum , 
Fig. 179, A) The stems, 
as well as the occasional 
adventitious roots, exhibit 
forked branching, and 
never contain more than a 
single stele, 1 whose detailed 
structure is often some- 
what complex. The sessile 
leaves are attached bj' a 
broad cushion-like base, 
and are traversed by but 
a single median vein (big. 179, B) ; the same was the case in 
Lcpidodcndron, the persistent leaf-cushions affording the dis- 
tinctive markings above referred to. The British species of 
Lvcopodium are found in nmist ujiland pasture, except for 
/,. imittdcdiim, which occiu's in lowland bogs. 

1 A .single stele was likewise found in LcpidoJeiidro}! where, however, it 
became enveloped by a broad zone of seeondary wood and phloem. 







Fig. 177. — Kestoration of various fossil 
Clubnrosses (Lepidodeiidroii and 
Sigillaria). (.'\fter Grand Kury.) 



REPRODUCTION OF LYCOPODIUM 



315 



The spoi'ophylls (Fig. 179, B) are similar to the foliage-leaves, 
and are commonly in whorls ; they are readily recognised by the 
single large, somewhat kidney-shaped, sporangium which each 
bears on its upper surface. In most species the sporophylls are 
aggregated in cones, as in Eqtiisetuni, although in L. selago, for ex- 
ample, the reproductive region is not clearly marked, zones of sporo- 
phylls usually alternating with 
vegetative leaves. Another pecu- 
liarity' of this species is the 
development of large hulbih in the 
axils of the uppermost leaves ; 
these structures, which serve for 
vegetative reproduction, must 
not be mistaken for fertile 
leaves. 

The cones, when present, oc- 
cupy the ends of erect branches, 
which are of the ordinary type, ex- 
cept in L. davahim (Fig. 179, A); 
here they bear minute leaves, 
at rather wide intervals, con- 
trastingmarkedlywith the densely 
arranged sporophjdls of the cone. 
Each sporangium contains nu- 
merous spores (Fig. 179, D) 
which, after being shed, give 
rise to peculiar fleshy proUialli 
(Fig. 179, C). In most species 
these grow underground as sapro- 
phytes, obtaining their food with 
the aid of a mycorrhiza from 
the humus in which they are 

embedded. Both kinds of sexual organs are borne on the same 
prothallus. 

Although Selaginella is represented in Britain only by 
5. spinosa (Fig. 180, C), which is found in similar habitats 
to those frequented by our species of Lycopodtum, several 
members of the genus are ccmmonly cultivated in greenhouses. 
The general habit is like that of Lycopcdium , but in most 




Fig. 178. — The Quillwort {Isoetes 
laciislris), somewhat reduced. 



3i6 



STRUCTURE OF SELAGINELLA 



species the leaves are arranged in four rows, two comprised of small 
leaves situated on the upper side of the stem, and two rows of 
large leaves towards the lower side (Fig. i8o, A and B) ; at 

each node there 
is one large and 
one small leaf. 
The British 5. 
s p ino s a, and 
other species in 
which the habit 
is erect, however, 
possess leaves 
that are all ahke 
(Fig. i8o, C). 

In many Ss- 
lagi iiellas the 
method of root- 
ing is peculiar, 
the roots arising 
from special 
leafless branches 
kno\'\-n as rhizo- 
phores (Fig. iSo, 
A, rh.). These 
are formed in 
pairs at thepoinis 
of forking of the 
stems, but usu- 
ally only one 
member of each 
pair develops. 
This gro\\'s down- 
wards, generally 
forking r e - 
originate from the 




Fig. 179. — Lycopodiuin. A, Small part of a plant 
of L. davatum, with a fertile shoot bearing two 
cones. B, Single sporophyll of same, with a 
dehiscent sporangium. C, Prothallus of L. 
clavalntii. D, Longitudinal section of sporophyll 
with sporangium. E, Spore. (B after Stras- 
burger ; C after Bruchman ; the rest original.) 



peatedly, and, on reaching the soil, root 
swollen tips of the ultimate branches. 

The stem is traversed b}' one or few steles,- essentiall)' like 
those of Ferns, except that each is surrounded by an air-space. 

' One in 5. spiiipsn. tbroc in 5. Itiniissiaiia. 



STRUCTURE OF SELAGINELLA 



317 



which is bridged either by strands of cortical cells, or by radially 
elongated endodermal cells. The roots and rhizophores are 
peculiar in exhibiting but a single protoxylem group. 

The cones, again situated at the ends of usually erect branches 
(Fig. 180, C), exhibit four rows of sporophylls, all of the same 




Fig. 180. — A and B, Selaginelia niarlensii (original). A, Part of a plant 
showing the leafy shoots, the rhizophores (yh.), and the roots arising 
from their ends. B, Small part of a branch enlarged, to show the two 
kinds of leaves. C, Plant of S. spinosa (after Wettstein), showing two 
cones. D, Sporophyll, and E, foliage leaf of S. spinosa (after Hie- 
ronymus). .S^., sporangium. 



size and shape, and each with an axillarj' sporangium borne on a 
short stalk (Fig. 180, D, Sp. ; Fig. 181, A, E). The sporangia 
are of two kinds : the one kind (microsporangia) , generally found 
towards the apex of the cone, are fiUed with numerous small 
microspores (Fig. 181, A, mi.) ; the other kind (megasporangia) 
each contain only four large megaspores (Fig. 181, A, mg.). 



3ib 



SPORES OF SELAGINELLA 



These features are readily observed in longitudinal sections 
through the cones, which also show the presence of a 
small outgrowth (the ligule, Fig. i8i, /.) from the upper 
surface of each sporophyll, between its upturned tip and the 
sporangium. Such ligules, though most conspicuous on the 

sporophylls, oc- 
cur also on all 
the vegetative 
1 e a ^f e s , but 
their function 
is altogether 
obscure. A 
ligule is not 
met with in the 
genus Lycopo- 
diiiin, but ap- 
pears to ha\'e 
been character- 
istic of most of 
the extinct re- 
presentativesof 
the f a m i 1 J' , 
which also pos- 
sessed t^vo 
kinds of spores. 
The 3'oung 
sporangia of 
Selagindlahd^ye 
a several-lay- 
ered wall , en- 
closing a large 
numberofspore 
mother-cells, and, up to this stage, all are alike. In the nncro- 
sporangia each mother-cell gives rise to four small spores, but 
in the megasporangia only one develops further, enlarging 
rapidly at the expense of the others, and dividing to form the 
single tetrad (Fig. i8i, A). An American species (S. rupestris) 
exhibits an even greater reduction, since occasionally only one 
of the four megaspores reaches maturity, and, in this and 




Fig 



i8i. — Selaginella tittibrosa. A, Longitudinal 
section tlirough part of a cone, sliowing micro- 
sporangia (mi.) above and megasporangia [mg.) 
below. E, Small part of megasporangium en- 
larged to show the stalk and wall. I., ligule. 
B, Single megaspore enlarged. C, Tetrad of 
microspores on the same scale of magnification 
as B. L), Tetrad of micrcisporcs enlarged. 



PROTHALLI AND EMBRYO OF SELAGINELLA 319 

certain other cases, the spores are retained within the spor- 
angium until after the fertihsation of the archegonia produced 
in the resuhing prothaUi. Both kinds of sporangia dehisce by 
a wide slit, the spores ordinarily maturing their prothalli on the 
ground. 

The contents of the microspore divide to form a few-celled, 
exceedingly reduced male proUiaUiis, developing neither chloro- 
plasts nor rhizoids, and consisting in the main of a small number 
of spermatozoid mother-cells (Fig, 182, f). The resulting biciliate 
spermatozoids (Fig. 182, d) are liberated, during wet weather, 
by the rupture of the coats of the microspores. 

The female prothallus begins to develop within the megaspore 
long before the latter is shed, and, like the male, exhibits con- 
siderable reduction and usually remains colourless. The early 
stages of its formation are characterised by repeated division of 
the megaspore-nucleus, separating walls onl}' arising much later. 
When ready for fertilisation the prothallus consists of a small- 
celled tissue, situated opposite the apex of the tetrahedral spore 
and exposed by the rupture of its coats at this point (Fig. 182, 
a and i, p.), while the bulk of the spore-contents, still enclosed 
in the megaspore-wall, are occupied by large cells laden with 
food-material (/). The few archegonia, which have very short, 
scarcely projecting necks (Fig. 182, e), are embedded in the 
small-celled apical region (Fig. 1S2, a, b, ar.). 

The fertilised egg divides transversely, the inner (lower) half 
giving rise to the embryo proper. The outer (upper) half, forming 
the so-called suspcnsor (Fig. 182, b and c, S.), divides a few times 
and elongates considerably, so that the developing embryo is 
pushed down into the large-celled nutritive tissue (/.) below. 
The suspensor is a device characteristic of the higher seed-plants, 
but in other essential respects the embryology resembles that 
of Ferns. Absorption of the stored food takes place by means 
of a sucker or foot (Fig. 182, c,ft.), but the 3'oung plant acquires 
independence at an early stage (Fig. 182, g). 

Regarded as a whole, the Pteridophyta exhibit a great 
variety of vegetative structure, and considerable specialisation 
in their reproductive processes. They oi^er a marked contrast 
to the Bryophyta in the relative importance of the spore- 
producing phase, which is an independent plant highly adapted 



320 



SELAGINELLA 



to a terrestrial existence. Associated with the last-named feature 
we see the differentiation of proper conducting tissue and of 
true roots. Tliere is, moreover, a marked tendencj'' towards 




Fig. iS^. — Sehi^iiwlhi, pruthallia and embryulogy. a. front view, and 
b. vertical section of mature megaspore witlr female jirothallus ; c, older 
embryo ; d, spermatozoid ; e, archegonium in longitudinal section ; 
/, microspore with contained male prothallus ; i;. voung plant still 
attached to the megas])ore (sp.). ar.. archegonui ; c, embryo (in b) ; 
/., large-celled nutritive tissue (in h) ; //., foot (in c) ; /.. leaves of 
embryo ; o., egg (in c) ; p. prothallus ; r., radicle ; .V., suspensor ; Sp., 
coat of megaspore. (a, c, and ^i,' after l-iruchman ; d after BelajeH ; 
remainder after Pfciter.) 



division of lalsour, on the one hand lictwccn \-cgctative and 
reproductive leaves, on the other lietween the prothalli. In 
the h'crns all the latter are alike and Ijear hoth kiiids of sexual 
organs, although there are occasional exceptions when the con- 



HOMOSPORY AND HETEROSPORY 3^1 

ditions of nutrition are abnormal (cf. p. 313). This tendency 
leads to tlie development of definite unisexual prothalli in the 
Horsetails, which, however, like the Ferns, have only one kind 
of spore, i.e. are honiosporons. Lycopodinm resembles the Ferns 
in these respects, but in Selaginella, not only are the prothalli 
unisexual, but they are produced from two kinds of spores, 
i.e. this genus is hcterosporotis. 

Although heterospory involves the risk of the two sexes of 
prothalli not germinating in sufficientl}' close proximity to one 
another, certain advantages obviously accrue from it. The 
most conspicuous of these is similar to that which ra^y have led 
to the evolution of oogamy (cf, p. 223), viz. the provision by the 
mother-plant of an abundant store of food for the development 
of the new sporophyte. These reserves are laid down in the 
megaspore before it is shed, and, as a consequence, the resulting 
prothallus can dispense with rhizoids and an assimilatory me- 
chanism. Moreover, the embryo acquires additional protection 
during the early stages of its development from the coats of the 
megaspore, within which the greater part of the prothallus 
remains enclosed (cf. Fig. 182, h.). A further step would 
obviously be the retention of the megaspore within the spo- 
rangium until after fertihsation and during the development of 
the embryo, and this we shall find realised in the Phanerogams, 
the last great class of the Vegetable Ivingdora. The microspores, 
requiring no appreciable amount of food-reserves, and being 
consequently of small dimensions, can be produced in large 
numbers. This affords an increased power of dispersal whereb}' 
the association of the two prothalli is rendered more probable. 



21 



CHAPTER Xxn 

The Cycads and Certain Extinct Seed-bearing Plants 

A GROUP may now be considered which, although portrajdng 
many of the characteristics of Flowering Plants, nevertheless 
shows several features reminiscent of Ferns. The plants in 
question are the Cycads, which have a wide distribution in the 
tropical regions of America, South Africa, Eastern Asia, and 
Australia, though most of the genera have a very restricted 
range. Fossil records show the group to be a very ancient one 
that played a particularly important part during the Mesozoic 
period, the present-day representatives appearing merely as 
relics. The Sago-palm {Cycas revolnta) is in most respects 
typical of the living forms. 

Most Cycads (Fig. 183) have the appearance of Palms or 
Tree Ferns, the often huge pinnate leaves forming a crown at 
the top of the partly subterranean and tuberous, or overground 
and columnar, stems ; in the latter case the stems may attain 
a height of sixty feet. The trunk is rarely branched, and, in 
the older portion, its entire surface is covered with an armour 
formed by the persistent bases of the leaves of previous seasons. 
Fresh crops of leaves are produced at intervals, the outer ones 
being modified to form protective bud-scales. In some of the 
genera the unfolding leaves exhibit a spiral inrolling of the 
pinnae [e.g. Cycas, Fig. 184), or of the midrib, similar to that 
characteristic of Ferns. Other resemblances to this group are 
to be found in the forked veining of the blades (Fig. 185, E) 
and in the structure of the petiole. The numerous vascular 
strands of the latter have the protoxylem embedded within the 
metaxylem, a feature especially characteristic of Ferns (cf. p. 293 
and Fig. 160, P.xy.), whereas in the vast majority of the Seed- 

322 



CYCADS 



323 



plants the^ protoxylem is immediately adjacent to the pith 
(cf. p. 79). 

Cycads often attain a great age and their stems exhibit 
secondary growth, with the same general plan of construction 
as in a woody Dicotyledon, except for the absence of annual 
rings. The pith is very large, and, in the Sago-palm, contains 




i 



Fig. 183. — Young plant of Cycas circinalis. [Photo : E. J. S.] 
Fig. 184. — Young leaf of Cycas, showing spiral inroUing of the pinnse. 
[Photo: E. J. S.] 



the stores of starch which are one of the sources of the sago 
of commerce. 

The sporophylls are of two kinds, microsporophylls and mega- 
sporophylls, and are arranged in distinct male and female cones, 
borne on separate individuals. The microsporophylls are thick 
scales vi'hich are spirally arranged (Fig. 185, A), and bear, on 
their under-surface, numerous sporangia (Fig. 185, D), often 



324 



CONES AND SPOROPHYLLS 



collected together in small groups ; they dehisce by a wide split. 
In most cases the megasporophylls are of a similar nature, although 
they produce only two megasporangia, usually placed one on 
either side of the stalk-like lower part (Fig. i86, B) ; the mega- 
sporangia are of a peculiar tj'pe and are known as ovules. In 
Cycas, however, the megasporophjdls are very leaf-like, although 
relatively small, hair}', and brown in colour, and often bear 




Fig. 185, — A, Entire male cone of Bowenia speciahilis, and B, part of the 
same in longitudinal section, showing the microsporangia (s/>.) on the 
lower surfaces of the sporophylls. C, Upper, and D, lower surface of 
a microsporophyll of Cycas. sp., microsporangia. E, Part of a pinna 
of the leaf of Eiicephalartos to show the veining. 



more than two of these ovules (Fig. 1S6, A). The female plant 
of Cycas can, as a matter of fact, be profitably compared with 
such a Fern as Blechnnin (cf. p. 308), since in both cases the 
sporangia occur on fronds which are but little modified. An 
examination of the megasporophylls of different Cycads shows 
aU stages in the reduction of the lamina to a condition in which 
the leafdikc character is almost entirely obscured [e.g. EnecpJm- 
lartos, Fig. 186, B). 

The ov2iles (inegasporatigia) (Fig. 1S6, C and D) arc of con- 



OVULES OF CYCADS 



325 



siderable size, but contain only a single large megaspore {p.) 
within the several-laj'ered parenchymatous wall or niicdlus (».). 
The delicate texture of the latter can be related to the presence 
of a thick fleshy protective covering (0. and i.). This integument 




Fig. 1 85. — A, Megasporophyll of Cycas revoliUa with four ovules (about 
half natural size). B, Mature megasporophyll of Encephalartos hil- 
denbrandtii, bearing two seeds (about half natural size). D, Diagram 
of longitudinal section of ovule of Boivenia speciabilis, and C, front end 
of same enlarged. ar., archegonia ; «., inner fleshy layer of integu- 
ment ; i.b., vascular bundle of inner series ; mi., micropyle ; «., 
nucellus ; 0., outer fleshy layer of integument ; o.b., bundle of outer 
series; ;^., female prothallus within megaspore; p.c, pollen chamber; 
p.t., pollen tube ; s., stony laj'er of integument. (A and B original ; 
C and D after Kershaw.) 



complete^ envelops the nucellus, and is indeed fused with it 
except at its extreme apex. In this region the integument is 
pierced b}? a narrow canal, the micropyle (»n'. ), leading down to the 
tip of the nucellus, in which a conical cavity, the pollen chamber 
{p.c), is formed b}' the breaking down of the tissue. The ovule is 



326 POLLINATION AND FERTILISATION 

supplied b}' two vascular strands from the sporoph}'!! These 
fork at its base to form two systems of branches, extending 
almost to the extreme tip: those of the outer series (o.b.) traverse 
the peiipheral layers of the integument, whilst those of tlie 
inner (i.b.) run close to the line of junction of the latter and 
the nucellus. 

The wind- or insect-borne microspores {pollen grains) are 
drawn into the pollen chamber by the drying up and contraction 
of the mucilaginous fluid (fonned by the disintegrating cells of 
the nucellus) which exudes from the micropyle at the time of 
pollination. Within the pollen chamber germination ensues 
(Fig. i86, . C, and 1S7, C), and a short branching sucker-like 
pollen tube (p.t.) grows into the adjacent tissue of the nucellus.^ 
Subsequent!}.' two spermatozoids, each with a spiral band of 
cilia 2 (Fig. 187, B, s.), are developed within the main body of 
the microspore. In the meantime the large megaspore has 
become filled with a uniform tissue, the female prothallus (Figs. 
186, C, and 187, C, p.) which produces a, commonly small, number 
of archegonia (ar.), with very minute necks (».) and large eggs (0.), 
at the end adjacent to the micropyle. 

The nucellar tissue between the pollen chamber and the 
female prothallus breaks down (Fig. 187, C), with the formation 
of a slimy fluid in which the liberated spermatozoids (s.) swim 
to the archegonia (0.), and in this way fertilisation is accom- 
plished. The nucleus of the fertilised egg divides repeatedl}' 
to form numerous nuclei which, at least in the lower part of 
the oospore, become separated by cell-walls. It is this region 
alone that gives rise to the embr3'0 (Fig. 187, D, e.), whilst the 
remainder ser\'cs for nutrition. 

In each ovule only one onhryo ultimately develops, its growth 
taking place at the expense of the surrounding prothallus, into 
the centre of which it is carried b}' the marked elongation of a 
suspensor (Fig. 187, D, sp.). Upon reaching a certain stage, 
however, in which two cotj'ledons, plumule, and radicle can be 
distinguished, the embryo becomes dormant, the residue of the 

• The sequence of events in the germination of the microspores of 
Cycads is very similar to that in Conifers (cf. p. 348). 

2 Apart from the Cycads, the Maiden-hair Tree (Ginkgo biloba) is the 
only seed-plant which has cihated, free-swimming, sperms. 



SEEDS OF CYCADS— PTERIDOSPERMS 



327 



female prothallus around forming a nutritive tissue, the endo- 
sperm. The whole is enveloped by the thick integument, now 
differentiated into three layers, which can even be recognised 
in an immature form in the young ovule. These layers comprise 
an inner (Fig. 1S6, C, »'.), and a much thicker outer (0.), flesh, 
with an inter- 
vening very 
hard stony 
layer (s.). This 
product of 
megasporan - 
gium, female 
prothallus, and 
embryo is 
known as a 
seed, and is a 
plum-like 
structure many 
times the size 
of the original 
ovule. 

The Cycads 
obviously show 
many super- 
ficial resem- 
blances to 
Ferns, but 
these are even 
more pro- 
nounced in 
another group 
of plants 

[Pteridospcrms) which, like the Calamites and Lepidodendrons 
of the Coal Measures, are known only as fossils. The members 
of this group, though closely resembling the Ferns in habit, 
show analogies with Cycads in their anatomy, their mode 
of reproduction, and especially in the possession of seeds. 

A complete knowledge of such fossil plants is only acquired 
gradually and as a result of prolonged research. At first the 




Fig. 187. — A, Germinating microspore of Cycas, 
showing vegetative cell {v.c), antheridial cell (a.c), 
and tube cell (t.c). B, Pollen tube of same with 
the two spermatozoids (s.). C, Diagram of longi- 
tudinal section through apex of nucellus (nil.) and 
female prothallus (p.) of Dioon edule, showing 
pollen tubes [p.i.) and pollen grains in various 
stages of development, spermatozoids (s.), and 
archegonia with eggs (0.) and necks (n.). D, Two 
proembryos of Dioon edule, the left-hand one 
younger than the right-hand one. e-, embryo; 
sp., suspensor. (A and B after Ikeno ; C and D 
from Chamberlain,) 



328 VEGETATIVE ORGANS OF LYGINOPTERIS 

separate fragments of stem, root, leaves, etc., are studied as 
unrelated structures, but subsequently patient toil pieces them 
together till a more or less complete picture of the whole is 
obtained. The external appearance of a fossil plant is best 
appreciated from casts or impressions preserved in consolidated 
mud, sand, etc. Not infrequentl}', however, the tissues have 
been impregnated with silica, calcium carbonate, etc., so that 
the internal structure is recognisable. Such petrified portions 
are cut up into thin slices, and these are ground down until 
sections, comparable to those prepared from a living plant, are 
obtained. 

Lyginoptcris (Lyginodendron) oldhamia, one of the commonest 
of the Coal iVleasure fossils, has been pieced together in this way 
until it is known with a degree of completeness only shared by a 
few li\'ing plants. Lyginopteris was probablj' a wood)' scrambler, 
with relatively slender stems and large compound Fern-like 
leaves (Fig. i88), the whole surface being beset with spines and 
large glandular hairs. The leaves were separated by long inter- 
nodes and produced occasional axillary branches, whilst the 
stem was attached to the soil by a number of adventitious roots 
arising near its base. The microsporangia and megasporangia 
(ovules) were borne on the ultimate ramifications of the ordinary 
foliage-leaves (cf. Fig. 190). 

The stem underwent considerable secondarj? thickening. In 
transverse sections (Fig. 189) the parenchymatous pith, which 
included patches of sclerotic tissue, is seen to ha^'e been sur- 
rounded by five or more groups of primarj' wood (A'.), some of 
which occurred in pairs ; in each such strand of primary X3'lem 
the protoxylem occupied a more or less central position. Beyond 
was a prominent zone of secondary wood [Sec), with wide paren- 
cln-matous rays, through which the leaf-trace bundles emerged 
{Bs.). The delicate cells of the cambium and phloem, external 
to the wood, are rarely well preserved. A layer of cork-like 
tissue {periderm) was usually developed in the pericyclic region, 
whilst the thin-walled inner cortex contained numerous secretory 
cells. A sharp contrast is afforded by the outer cortex, with 
its s}'stem of radial sclerenchj/matous plates [S.), anasto- 
mosing at frequent intervals and giving mechanical support to 
the stem. 



VEGETATIVE ORGANS OF LYGINOPTERIS 329 

Each leaf was supplied by one of the primary xylem strands 
which, as it passed outwards, became associated with phloem 




Fig. 188. — The foliage {formerly known as Sphenopteris hccninghausii) of 
Lyginopteris oldhamia. (From a drawing, after Potonie, lent_^by 
Prof. F. W. Oliver, F.R.S.) 



and, on entering the pericycle, divided into two. The double 
bundle (Fig. 189, Bs.) traversed several internodes before bending 



33° 



MICROSPORANGIA 



out into the leaf, whose detailed structure was much like 
that of many of the simpler Ferns. The root was similar to 
that of recent plants, possessing from two to eight xylem- 
groups. 

The microsporangia of Lyginopterh were borne on the under- 
side of pinnte, with much reduced laminee, and were more or 
less fused in pairs. The pinna, with its pendant elongated 



'<.V^ 




Fig. 189. — Photograph of transverse section of the stem of Lygiitopleris 
oldhamia (reproduced by the courtesy of Prof. F. W. Oliver, F.R.S.). 
Bs., leaf traces ; S., sclcrenchyma plates in outer cortex ; Sec, secondary 
wood ; A'., primary xylcm strand. 



sporangia, had somewhat of the appearance of an epaulet 
(Fig. 190). The sporangia produced numerous small spores, 
which were presumably convej/ed by the wind to the ovules, 
where they became lodged within the pollen chamber. Of their 
further fate nothing is known, but it is probable that they 
developed a small male prothallus, giving rise to spermatozoids, 
somewhat like those of Cycads. 

The ovules arose singly from the ends of truncated branches 
of the fronds, and showed the same plan of construction as in 



OVULES AND SEEDS 



^M 



Cycads (Fig. 191B), except that each possessed, in addition to 
the integument, a second protective covering. This took the 
form of a lobed cup-hke structure {Cii.), comparable with the 
cupule of a Hazel-nut, and beset with the same glands as occur 
on the vegetative organs of this plant (cf. Fig. igiA). The 
robust integument (Fig. 19IB, /.) was fused with the nucellus 
(sporangium-wall, n.), except for the apical portion, which was 
pierced by the narrow canal- 
like micropyle. 

The tip of the nucellus 
was produced into a flask- 
shaped pollen chamber (Pc.), 
whose neck projected very 
slightly be3'ond the micropyle 
(cf. Fig. 191A), so that the 
microspores reached the nu- 
cellus direct, a point of con- 
trast to Cycads and other 
recent plants. The central 
portion of the pollen chamber 
was occupied by a dome of 
parenchymatous tissue, but 
between it and the surround- 
ing wall was a narrow chink- 
like space (Fig. 191B, Pc.) in 
which the pollen presumably 
germinated and liberation of 
the sperms ensued. The single 
huge megaspore (in.) has been 
found filled with a uniform 
thin-waUed prothallus, which 
doubtless bore the archegonia near its apex. The seed was 
■ supplied by a single vascular strand, which gave off branches 
to both envelopes. Those traversing the cupule (Fig. 191B, Vb.) 
extended into its lobes, whilst those running in the integument 
(I.b.) penetrated to the neighbourhood of the micropyle, where 
the integument became free from the nucellus. 

The plants described in this chapter are especially charac- 
terised b}' the method of fertihsation and the possession of seeds. 




Fig 1 10 — Ijltimate pinnules of the 
foliage of Lyginopteris oldhamia, 
with microsporangia. (From a 
photograph by Mr. R. Kidston, 
F.R.S.) 



332 



THE SEED-HABIT 



As regards the former feature, the only essential difference from 
Selaginella lies in the germination of the microspores in close 
proximity to the megaspore, for which special devices such as 
the mucilaginous poUination drop and the pollen chamber are 
created. Moreover, with the help of the pollen tube some nourish- 
ment is obtained from the adjacent nucellus. 

The seed may be looked upon as an extreme development 
of heterospory. Even in Selaginella it was noted that in some 
species there may be reduction to a single megaspore, which is 
retained within the sporangium till after fertilisation (cf. p. 319). 




Fig. 191A. — Restoration of the seed (Lagenostoma lomaxi) of Lyginopteris 
oldhamia, in its glandular cupula. (After Oliver.) 



Such reduction and retention are eminently characteristic of the 
seed-habit, where dehiscence of the megasporangium is dispensed 
with. The added protection, furnished by the integument, 
admits of a change in the character of the sporangium-wall, 
which becomes a thin-walled tissue ser\-ing as intermediary 
between the \-ascular sj'stem of the ovule and the developing 
megaspore and female prothallus. 

The possession of a vascular system, another characteristic 
of the ovule, facilitates nutrition and the storing up of food- 
reserves for the young embryo. It is probably in consequence 
of the increased food-supply that o\'ules and seeds in general 
attain so large a size as compared with the sporangia of lower 



THE SEED-IIABIT 



333 



plants. As a general rule {e.g. most Conifers and Flowering 
Plants), however, the vascular system of the ovule stops short 
at its base. The elaborate vascular supply of the early seed- 
types, as contrasted with the more modern ones, may perhaps 
be related to the possession of motile spermatozoids, and to 
the relative degree of exposure and size of the ovules. 

One important aspect of the permanent retention of the 




Fig. igiB. — Reconstrviction of a longitudinal section through the seed of 
Lyginopteris oldhamia (after Oliver). Cu., cupule ; /., integument; 
I.b., bundle traversing integument ; m., megaspore ; «., nucellus ; 
Pc, pollen chamber ; Vb., bundle traversing cupule. 



megaspore, involving considerable economy, is the possibiUty of 
postponing the accumulation of food-reserves to a relatively late 
stage, when fertilisation has taken place and the embryo has 
begun its development. The embrj-o is, moreover, more ade- 
quately protected, during its early phases, than is possible in 
Pteridophyta and Bryophyta, and, after attaining a certain 
stage, remains dormant and securel}' shielded until conditions 
arise suitable for its further growth. 



CHAPTER XXIII 

The Conifers 

All seed-plants, with the exception of certain fossil forms 
{e.g. Pteridosperms and some fossil Clubmosses), are grouped as 
Phanerogamia, the fourth and highest class of the Vegetable 
Kingdom. The further subdivision of this class into Gymno- 
sperms and Angiosperms is based on the manner in which the 
ovules are borne ; those of the former are freely exposed, as in 
the Cycads already considered, whilst those of the latter, the 
Flowering Plants proper, are enclosed in a structure called the 
ovary. There are, however, several other points of contrast, 
which will become apparent later. The Phanerogams as a whole 
are further distinguished by a higher differentiation of the strobili 
than is found among Pteridophyta. 

The Gynmosperms include, apart from the Cycads, the Maiden- 
hair Tree {Ginkgo hiloba, cf. p. 326), the important group of the 
Conifers, and other forms known only as fossils. The Conifers, to 
which belong such familiar plants as the Scotch Fir {Pimis sylvestris, 
Fig. 200), Tarch {Larix europcea, Fig. 193), Yew {Taxiis baccata, 
Fig. 201), etc., are essentially characteristic of temperate zones, 
some being the chief forest trees of colder regions (cf. p. 391). 
They furnish some of the most important sources of timber 
(p. 129), turpentine, resin, etc. (p. 63). There are some 350 known 
species, of which more than a fifth belong to the genus Piiitts. 

All the Conifers are woody, and the majoritj^ are trees. The 
regular habit, so marked in the Spruce Fir {Picea excelsa) and 
Cypress {Cupressus) , characterises the group and is an outcome of 
the monopodial branching.^ The feature which gives them their 
most distinctive stamp, however, is the fohage, which in general 
consists of small needle-like leaves (Figs. 192, 193, and 201). 
1 Cf. F. & S., p. 75. 
334 



HABIT AND FOLIAGE 



335 



These usually persist for several years, although the Larch, for 
instance, is deciduous. 

The needles are either borne on the ordinary long shoots 
{e.g. Yew, Fig. 201, a, and Silver Fir, Fig. 193, A), or, as in several 
common genera {Pinus, Larix, Cednis, all members of the 
Abietineje), are restricted to special dwarf-shoots. These arise 
in the axils of scale-leaves on the long shoots, and bear a few 
dark-coloured scales below and a variable number of foliage 
leaves above. In the 
Pines the number of the 
latter is limited (five in 
Pinus strohiis, two in P. 
sylvestris, Fig. 200, and 
only one in P. mono- 
phylla), and no further 
leaves are produced after 
the first season, the whole 
dwarf-shoot being shed at 
the end of two or three 
years. On the other 
hand, in the Larch (Fig. 
193, B) and Cedar 
(Cedncs) , where the dwarf- 
shoots are larger and bear 
a tuft of needles, quite a 
considerable number of 
new leaves is formed 
annuaUy, although after 
some years this production 
ceases and the dwarf-shoots die away. These two Conifers also 
differ from the Pines in the fact that the needles are not entirely 
confined to the dwarf-shoots, since, during the first season's 
growth, they occur on the long shoots also. It will be realised 
that only the normal shoots contribute to the permanent branch- 
system of the trees. 

In most cases the leaves are sessile, although those of the 
Yew have a short stalk (Fig. 201, a). Their bases are often 
fused with the stem for a short distance, and persist after the 




Fig. 192. — Branches of the Spruce Fir 
(Picea exceha) with three ripe cones. 
[Photo: E. J. S.] 



leaves have fallen, leaving characteristic scars 



Spruce Fir, 



336 HABlt AND FOLIAGE 

Picea) ; iii the Scotch Fir similar scars are left by the decurrent 
bases of the dwarf-shoots. 

The pecuhar appearance of the Arbor Vita [Thuja, Fig. 194) 
and the Cypress {Cupressus) , both belonging to the Cupressineae, 
is due to the presence of minute leaves arranged in decussate 
pairs, and almost fused with the stem upon which they are 




Fig. 193. — A, Branch of Silver Fir (Abies) Viith-vaaXe cones. B, Branch 
of the Larch (Lafix), showing several dwarf-shoots, one of them 
bearing a young female cone. (Both about natural size.) 

borne. Moreover, owing to the larger size of the lateral leaves, 
and the restriction of most of the branches to their axils, the 
shoot as a whole acquires a flattened appearance. In the com- 
mon Juniper {Juniperus), which is likewise a member of the 
Cupressinex, three leaves of the usual needle type arise at each 
node, and such a whorled arrangement is characteristic of this 
whole family. Large flattened leaves are seen in Araitcaria, 



SEEDLINGS 



17 



and a similar type of foliage is found in Podocarpiis, which is 
allied to the Yew. Podocarpus is the most important genus of 
Conifers in the Southern Hemisphere, comprising some sixty 
species, of which several furnish valuable timber. 

In spite of the considerable diversity in the mature structure, 
the seedlings; of most Conifers exhil)it great uniformity, the 
young stem bearing ordinary needle-leaves for some little distance 
above the cotyledonary node (Fig. 195). The dwarf-shoots of 





Fig. 194. — Arbor Vit.-c {Thuja). A, Branch with ripe female cones (about 
natural size). B, Small part of a branch enlarged to show the leaf- 
arrangement. 



Finns and other Abietinea2, as well as the scale-like leaves found 
in the Cypress, etc., only appear at a later stage and as a 
secondary development. By appropriate means the juvenile 
foliage can be made to persist, even in the adult condition, as 
in some cultivated varieties of Thuja and Ciipressus (the Retino- 
spora of nurserymen). Even the deciduous habit of the Larch 
appears as a secondary acquisition, since in the seedling the 
leaves persist for some time. It is not always that the varied 
specialisation which a group has undergone, in evolving from a 
22 



.l-,8 



STEM-STRUCTURE 



common ancestral type, is as plainly decipherable as in the 
Conifers. 

The appearance of the stem in transverse section is very 
similar to that of a Dicotyledon having secondary growth. The 
Scotch Fir affords a typical example (Fig. 196, A). Here the 





Fig. 195. — Seedlings of various Conifers. A, Scotch Fir (P(»»s sj'/rcsAr;s). 

B, Cypress {Cupressus), showing tlic two cotyledons and plumule. 

C, Larch (Larix), older seedling which has already produced several 
dwarf-shoots. Cot., cot3'ledons ; d.s., dwarf-shoot ; /;., hjrpocotyl. 

irregular outlme is due to the adherent bases of the scale-leaves 
which are occupied by a large-celled tissue (Fig. 196, B), on 
whose inner side the cork-cambium {Ca.) arises. With the pro- 
duction of cork, therefore, the epidermis [e.) and the dwarf-shoots 
become exfoliated, so that the older branches have a relatively 
smooth surface. The narrow cortex is rather lacunar and con- 



STEM-STRUCTURE 339 

tains schizogcnous rcsin-canals (r.), whose structure has ah'eady 
been described (cf. p. 152). These resin-canals are particularly 
characteristic of Conifers, and are found in all parts of the 
plant ; the}' are entirely absent only from the Yew. 

The primary and secondary vascular tissues are not easily 
distinguished from one another, particularly in the case of the 
phloem, the broad zone of which is chiefly secondary in origin. 







Fig. 196. — Structure of the young stem of the Scotch Fir (Pinus sylvestris) 
in transverse section. A, Entire section (diagrammatic). B, Small part 
of the pcriplieral tissues enlarged. C, Single resin-canal enlarged, 
showing the epithelium (op.). Ca., cork cambium ; e., epidermis ; 
Ph., phloem ; r., resin-canals ; Xy., xylcm. 



This tissue (Ph.) is strikingly uniform, its elements, which have 
thick white shining walls, exhibiting a radial arrangement. The 
rows of more or less empty-looking sieve-tubes, which have no 
companion cells, are interspersed with a smaller number of 
irregular files of phloem-parenchjina, whose cells contain dense 
contents. In longitudinal sections the sieve-tubes appear as 
long tapering elements bearing sieve-areas (Fig. 197, E, si.) upon 
their sloping radial walls. 

The secondary wood (Fig. 196, Xy.), internal to the narrow 



340 STEM-STRUCTURE 

cambial zone, also exhibits a very uniform, radially seriated 
structure. Except for the narrow medullary raj'S and occasional 
resin-canals, it consists entirel}? of fibre-like tracheids. which are 
differentiated among themselves only in respect of the distinctions 
between spring- (Fig. 197, A, Sp.) and autumn-wood (An.) ' (see 
p. 125). The tracheids bear a sin,gle row of large circular bor- 
dered pits on their radial walls, as can be recognised in trans- 
verse, but more readily in radial longitudinal (Fig. 197, C, h.), 
sections, when the pits themseh-es are seen in surface view ; in 
the autumn-wood the tangential walls are also pitted. The 
groups of primarv xvlem, composed of spiral tracheids, project 
into the small pith and are separated from one another by the 
primar\-' rays. 

The structure just noted for the Scotch Fir is that tj'pical 
of most Conifers, but resin-canals are absent from the wood in 
certain genera (being often replaced by resin-cells), whilst in 
the Araucarias, and occasionally in other members of the group 
{e.g. Pi mis pahistris), the tracheids bear two or more rows of 
bordered pits. 

Radial and tangential longitudinal sections exhibit the same 
arrangement of the medullary rays as in Dicotyledons (Fig. 197). 
In some Conifers certain rays, which are relatively wide, are 
traversed by resin-canals connecting those of the pith and cortex. 
As a general rule the rays consist of uniform cells, whose walls 
often bear simple pits in Pinits and its allies, although elsewhere 
usually smooth. Several AbietinccX', including the Scotch Fir, 
show a complex differentiation of the rays, best seen in radial 
longitudinal sections. In the region of the wood the cells of 
the middle rows, which bear simple pits of exceptionally large 
size, are more particularly concerned with storage, and contain 
copious starch (Fig. 197, C, s.) ; on the other hand, the dead and 
empty cells of the marginal rows (/.), which liear small bordered 
pits and often exhibit peg-like ingrowths of the walls, have a 
conducting function. Where the rays tra^•crse the phloem, all 
the cells have thin waWi and dense cytoplasm, but those at the 
margin (Fig. 197, E, a.) are often drawn out into linger-like 
processes which are insinuated between the sieve-tubes. 

' Annual ruigs arc, however, absent fri)ni ,sunie .lyaiicdiia^, and from 
most of the fossil representatives of thi.s ,L,'roiip. 



LEAF-STRUCTURE 



34^ 



The root, which is gcncrall}' cliarch, shows quite the 
structure, although characterised by the presence of a 
resin-canal within each protoxylem-group. 

The leaves of most Conifers are tra^x'rsed by a single vein 



usual 
large 




Fig. 197. — Structure of the stem of the Scotch Fir (Pmus sylvesiris.) 
A, small part of the secondary wood in transverse section, showing 
spring- (Sp.) and autumn- {-!«) wood; B, the same in tangential 
longitudinal section showing a medullary ray ; C, the same in radial 
longitudinal section, with part of a medullary ray ; D, single bor- 
dered pit in section (on the left) and from the surface (on the right) ; 

E, small part of the secondary phloem in radial longitudinal section ; 

F, single sieve-tube in tangential longitudinal section, a, albuminoid 
cells of medullary ray of phloem ; b, bordered pits ; s, storage cells 
of medullary rays of wood ; Si., sieve-plates ; t, tracheidal cells of 
medullary rays of wood. 



only, but, apart from that, their structure is somewhat varied. 
Its range can, however, be gauged from a consideration of the 
relatively simple dorsiventral leaf of the Yew {Taxus, Fig. 198) 
and the more complex centric one of the Scotch Fir {Pimis 
sylvesiris, Fig. 199). In the former case the epidermis (Fig. 



342 



LEAF OF TAXUS 



198, ep.) shows the thick cuticle and sunken stomata (Fig. 198, B) 
associated with transpiration-reduction ; the pronounced papillfe 






F.E.r 



Fig. 198. — A-D, Structure of the leaf of the Yew [Taxiis baccata) in transverse 
section. A, Diagram of whole section ; B, single stoma enlarged ; 
C, central part of transverse section ; D, a few cells of the transfusion 
tissue. 6., bundle ; ep., epidermis ; p., palisade tissue ; ph., phloem ; 
s., spongy tissue ; St., stomata ; 1., transfusion tissue ; xy., xylem. 
E, Tangential longitudinal section of leaf of Piinis, showing a resin - 
canal (r.c.) and the transverse lamclhx of assimilating cells. 



on the lower surface (Fig. 19S, B, C) are a special peculiarity. 
The mesophyll, comprising two layers of palisa,de cells {p.) and 



LEAF OF PINUS 



343 



spongy tissue (s.), is traversed by the single bundle (b.), which 
exhibits the usual dorsiventral structure. On either side of the 
xylem, however, are occasional spirally or reticulately thickened 
elements (i.), which sometimes bear bordered pits. These dead 
cells constitute what is called transfusion tissue, and may serve 
both for the lateral conduction of water, thus compensating for 
the absence of side- veins, and as water-reservoirs. 

The epidermal cells of the P«ms-leaf (Fig. 199, A-D) also 
have a very pronounced cuticle {Cu.), and, in this case, are so 
strongly thickened that their cavity is often reduced to a mere 
dot (ep.) ; beneath the epidermis is a strongly thickened hypo- 
derm (A.). The deeply sunken stomata (St.) occur at intervals 
all round the leaf, as seen in transverse section, but, if the leaf 
be examined whole with a lens, they are seen to be arranged in 
longitudinal rows. The mesophyll contains several prominent 
resin-canals (r.) and consists of more or less isodiametric cells, 
characterised by the presence of inwardly projecting folds of 
the wall (»'.), which increase the surface for absorption of carbon 
dioxide (cf. p. no). It will be noticed that, apart from the 
respiratory cavities of the stomata, there are few intercellular 
spaces between the cells of the mesophyU. This tissue, how- 
ever, consists of successive transverse lamellae, and these are 
separated from one another by narrow air-spaces (Fig. 198, E). 
The twin-bundles (in some species of Finns, as well as in 
Picea and Larix, there is only a single one) are connected 
by a group of thick-walled mechanical cells [Scl.) and are 
embedded in an extensive mass of tissue bounded by a well- 
marked bundle-sheath (5.). This tissue consists, in large part, 
of ordinary living parenchymatous cells containing starch- 
grains. Scattered among these are the dead and empty cells 
of the transfusion tissue (cf. especially Fig. 199, C, tr.), in this 
case bearing small bordered pits (&.), which are seen both in 
section and from the surface. At the outer edge of the phloem 
of each bundle is a group of cells, whose dark contents are 
rich in proteins (a.), and which probably assist in the con- 
duction and storage of elaborated food-material. 

^It may be pointed out that, whilst many of the features of 
Coniferous leaves are those usually found in plants with reduced 
transpiration, others (such as the limited vascular supply and 



344 



MALE CONES 



the transfusion tissue) are special peculiarities of the group. 
The necessity for the reduction of transpiration may weU be 
connected with the relative inefficiency of the tracheids as water- 
conducting elements.! The relation between the vascular system 
of stem and leaf in Conifers is similar to that which obtains in 
Flowering Plants (cf. p. 114). 




Fig. 199. — Structure of the leaf of Piinis in transverse section. A, Diagram 
of wliole section. B, Small part of the superficial tissues, enlarged. 
C, A few cells of the transfusion tissue, etc., from the central vascular 
cylinder, greatly enlarged. D, About half the central vascular cylinder. 
a., albuminous cells ; b., bordered pit on transfusion cell ; Cii., cuticle ; 
ep., epidermis ; h., hypoderm ; i., folds on walls of mesophyll-cells ; 
ph., phloem ; ^., resin-canal ; S., bundle-sheath ; Sc/., sclercnchyma ; 
St., stomata ; Ir., elements of transfusion tissue ; xy., xylem. 

The sporophylls of Conifers, like those of Cycads, are grouped 
in cones (Fig. 200), the two kinds being most commonly found 
on the same plant, though Taxits (Fig. 201), for instance, fur- 
nishes an exception. The male cones, which often occur in 
clusters (Fig. 200, on the right), are yellow oval structures of 
diverse size, and frequently arise in the a.xils of the foliage- 
[e.g. Taxiis, Fig. 201, c ; Abies, Fig. 193, A) or scale-leaves 

' Cf. F. and S., p. 1 1^. 



MALE CONES AND MICROSPORANGIA 



345 



w^<^ 



(e.g. Scotch Fir and other species of Pinus). In Pinits the 
male cones replace the dwarf -shoots (Fig, 200), but in other 
cases (e.g. Larch) the latter bear leaves below the male cones. 

The axis of the male cone (Fig. 202, A) supports large numbers 
of densely crowded inicrosporophylls (stamens), which mostly 
have the form of 
flattened scales, 
whose upturned 
overlapping 
tips are alone 
visible at the 
surface. They 
usually bear two 
large micro- 
sporangia (pollen 
sacs) on their 
under-surface 
(Fig. 202,'&, ps.) 
although there 
are sometimes 
several (e.g. Cii- 
pr es siis). In 
Taxus the sta- 
mens are peltate 
in form, with 
several (5 to 9) 
pollen sacs pen- 
dant from the 
lower side of the 
lamina (Fig. 201, 
d). In Arau- 
caria also the 
microsporophylls bear numerous (10 to 20) pollen sacs. 

The young microsporangia are provided with a wall of several 
layers which encloses large numbers of spore mother-ceUs, each 
dividing in the customary manner to form four microspores 
(pollen grains). The pollen sacs usually dehisce by means of a 
broad split, and the pollen is distributed bj^ the wind. In Pinus 
and its allies the microspores develop a pair of blister-lU^e en- 




FiG. 200. — The Scotch Fir (Pinus sylvestris). On the 
right a branch bearing two clusters of male cones ; 
on the left a branch with two fertilised female 
cones above, a mature cone from which the seeds 
have been shed below, and a young, just pollinated, 
cone at the extreme apex. On both branches the 
dwarf -shoots of the previous season areseen below, 
and young dwarf -shoots of the current year's 
growth above. (From a photo by W. B. Johnson.) 



346 



FEMALE CONES 



largements, between the cuticle and tlie inner laj'cr of tlie 
membrane ; these contain water, which subsequently evaporates, 
thus leaving two bladders which act as wings (Fig. 204, A, w.). 
The ovules (megasporangia) of the Yew (Taxus) usually occur 
singly within small buds, which arise in the axils of the leaves 
and are generally spoken of as female cones ; they closely re- 
semble the ordinary vegetati\'e buds. Each such cone bears 
several minute scales, the single ovule occupving a terminal 
position with the micropjde facing outwards. The ovule is 
peculiar in possessing, apart from the usual integument, a second 



(XT. 




Fig. 201. — The Yew (Taxus baccata). a, branch bearing two ripe -seeds ; 
b, a ripe seed in longitudinal section, showing the aril (ar.) ; c, single 
male cone, enlarged ; d, single microsporophyll ; e, branch with male 
cones, [d after Eichlcr ; the rest original.) 



protective covering termed the aril , which remains inconspicuous 
till after fertilisation. It then de\-elops into a bright-coloured 
fleshy cup in\'esting the seed (Fig. 201, a, b). 

In most Conifers, however, more or less numerous ovules 
are found within each of the fe)iialc cones, and these in the Monke}^ 
Puzzles [Araucaria) , for instance, may attain considerable dimen- 
sions. The cones themselves occupy diverse positions. Thus, in 
the Scotch Fir (Fig. 200, on the left) they first appear as small 
reddish structures, situated at the ends of short stalks, just 
beneath the apical bud of the current 3-ear's growth. In the 
Larch, where they are larger and of a bright crimson colour, 
they terminate some of the dwarf-shoots, with a rosette of green 



MEGASPOROI^HYLLS AND OVULES 



347 



needles at their base (Fig. 193, B). The small cones of the 
Cj'press and Arbor Vita (Fig, 194) occupy a similar position. 

The scale-like sporophylls generally show a spiral arrange- 
ment, although in Cnpressus and Thuja they are decussate, like 
the foliage-leaves. In Piniis and other Abietine;e (Larix, Abies, 
etc.) the axis of the cone bears two different t3'pes of scales 
which are quite distinct from one another, and occur in super- 
posed pairs (Fig. 202, D). The lower or bract scale {bs.) of each 
pair is smaller than the upper or ovulif erous scale (Os.). The latter 
appears to arise in the axil of the former, and bears, on its upper 
surface, two ovules (Ov.) whose micropyles face towards the axis 




Fig. 202. — A-B, Structure of male cone of Pinus sylvestris. A, Upper 
part of cone, in longitudinal section ; B, Single sporophyll. ps., pollen 
sacs. C, Pollen grain. D, Longitudinal section througli female cone 
of Pinus. E, Single megasporophyll of Piuiis, from below. F, Part 
of D, enlarged, bs., bract scale; c, megaspore ; /, integument; 
m., micropyle ; «., nucellus ; Os., ovuliferous scale ; Ov., ovule. 

of the cone (Fig. 202, E). The difference in size becomes more pro- 
nounced as the cone grows older, the ovuliferous scale enlarging to 
a much greater extent than the bract scale. In the Wellingtonia 
{Sequoia, Fig. 205, D) and Cupressinefe bract and ovuliferous 
scales are almost completely joined, whilst in some Conifers {e.g. 
Cryptomeria) the line of fusion is plainly recognisable. The scales 
in the Cupressineae usually bear more than two ovules, situated 
with the micropyles directed outwards ; not infrequently the upper- 
most scales are sterile {e.g. Thuja). The Monkey Puzzles are 
peculiar in having but a single ovule on each megasporophyll. 

The ovules in all Conifers, apart from those of the Yew-group, 
possess a single thick integument (Fig. 202, F, I ; Fig. 203, ?'.). This 



348 POLLINATION 

is partially fused on one side with the ovuhferous scale (Fig. 202, 
F), and, except in a few genera, the central nucellus is only free 
from the integument in the region of the micropyle (Fig. 203, 
B, mi.). A single megaspore {p.) is usually differentiated within 
the nucellus of the young ovule by the division of a mother-cell 
which arises h^'podermall}?. This forms a linear tetrad of 
potential spores of which onlv the innermost survives. The 
functioning spore rapidly enlarges and eventually displaces 
the greater part of the nucellus, though a pronounced cap 
of tissue still remains in the free portion beneath the micro- 
pyle (Fig. 203, n.). The thin-walled parench)-matous female 
prothalhis (/>.), which is richlv supplied with food-material, 
as a rule arises onl}' after pollination has occurred. The 
archegonia (ar.), which are usuallv few m number, are generally 
differentiated at the micropylar end. Each consists of a rela- 
tively huge ovum (0.) embedded in the tissue of the prothalhis, a 
minute ventral canal cell {v.c), and a very short inconspicuous 
neck [a.) composed of one or more tiers of cells. ^^Tlen the 
archegonia are numerous (as in most Cupressinea;) they are 
usually collected together in groups, opening into a common 
shallow depression [archcgonial chcunber), at the front end of the 
prothallus. 

When the mature pollen is being scattered broadcast by the 
wind, the axis of the female cone elongates slightly, so that the 
cone-scales spread apart, thus giving access to the ovules. At 
this time each of the latter secretes a mucilaginous drop which 
oozes out from the micropyle, and serves to catch up the micro- 
spores as they are blown between the scales. B\- the gradual 
shrinking of the mucilage the pollen grains are sucked through 
the micropyle on to the surface of the nucellar cap, and, soon 
after this, the scales of the female cone enlarge and once again 
fit tightly together. This process of pollination must be sharply 
distinguished from that of fertilisation which occurs at a later 
stage — often only after a prolonged inter\-al (see below). 

When shed, the contents of the microspores ha\'e already under- 
gone division, the cell, or cells, thus cut off from the general body 
lying against one side of the grain (Fig. 204, B) ; in most cases 
this comprises the single antlieridial cell {a.c.) onl\-, but in Finns 
and related forms its formation is preceded by the cutting off 



GERMINATION OF POLLEN 



349 



of two very flat cells, which soon break down (Fig. 204, B, v.c). 
The remaining and larger portion of the pollen grain forms the 
so-called tube cell (/.».). On reaching the nucellus the outer 
membrane of the microspore is ruptured, and the tube cell 
grows out to form the pollen tube (Fig. 203, A, p.t.), into whose 



mi. 




uc. 



Fig. 203. — Structure of the Ovule of Piniis. A, Diagrammatic longitudinal 
section of mature ovule (after Co^ilter and Chamberlain). B, Front 
portion of same greatly enlarged, showing a pollen tube penetrating 
the nucellus (after Strasburgcr). C, Young archegonium showing the 
layer of nutritive cells investing the egg (after Ferguson), a., neck 
of archegonium; ar., archegonium; i., integument; mi., micropyle ; 
n., nucellus ; 0., egg ; p., female prothallus ; p.t., pollen tube ; s., stony 
layer of integument ; v.c, ventral canal cell. 

tip its nucleus wanders, whilst simultaneously the antheridial 
cell divides into two, the inner half {body cell) subsequently 
forming the two male cells. 

The growth of the pollen tube into the nucellus soon ceases, 
and is not resumed till a short time before fertilisation. This 
interval between pollination and fertilisation is comparatively 



350 FERTILISATION AND EMBRYOLOGY 

short (four to six weeks) where the seeds mature in the same 
year as that in which poUination occurs {e.g. Thuja, Picea), but 
is very prolonged (twelve to thirteen months) in the Pines, where 
pollination ensues in May or early June, whilst the seeds are not 
shed till the autumn of the following 3'ear. In the latter case 
there is very pronounced growth of the female cones in the 
interval between pollination and fertilisation (cf. Fig. 200). 

With the resumption of growth the pollen tube penetrates 
deeper into the nucellus (Fig. 203, p.t.), frequently exhibiting 
slight lobing, and the two naked male cells (often merely nuclei), 
resulting from the di\-ision of the body cell, pass down into its 
apex (Fig. 204, C, o). On reaching an archegonium the neck 
is crushed, and the pollen tube, breaking open at its tip, discharges 
its contents into the ovum. Subsequently one of the two male 
nuclei fuses with the egg-nucleus. In Pin lis the second male 
nucleus is smaller (Fig. 204, C) and aborts, though in some genera 
{e.g. of Cupressinea2), where the archegonia occur in groups and 
the pollen tube discharges into the common archegonial chamber, 
both the male nuclei, which are here of equal size, may function. 

The fertilised egg, becoming enveloped in a delicate membrane, 
almost immediately exhibits two successive divisions of its 
nucleus (Fig. 204, D). The four nuclei thus formed wander to 
the end of the egg remote from the micropyle, where they become 
grouped in a single plane. Further division, accompanied by 
the formation of separating walls, ultimately results in the 
development of three or four superposed tiers (Fig. 204, E), each 
usually consisting of four cells and occupying only a small part 
of the oospore. In this prooiibryo the tier furthest from the 
micropyle mostly gives rise to the new plant, ^ whilst the ceUs 
of the adjacent tier elongate very considerabl}- (Fig. 204, F, S.), 
and form a sitspcnsor (cf. Sclagiiuila, p. 319), wliich carries the 
developing embryo {em.) down into the middle of the prothallus. 

The cells of the embryonic tier divide repeatedly to form 
an extensive mass of tissue. At the end away from the micro- 
pyle a number of lobes soon grow out and form the cotyledons 
(Fig. 204, G, cot.). These surround the developing plumule {p.), 

' In Pinus and some other genera the cells of the proembryo commonly 
separate, and as many as eight embryos may be formed, but, as in other 
conifers, one only reaches maturity. 



SEEDS 



351 



whilst the radicle {r.) arises as a pointed structure at the opposite 
end, adjacent to the suspensor (S.). The number of cotyledons 
is very variable ; there are only two in Taxits and CupressinecB 
(Fig. 195, B), whilst in Pinus (Fig. 195, A) there are from three 
to seventeen, according to the species. 

As the embryo enlarges it absorbs the food-reserves stored 




Fig. 204. — Pollen and embryology of Pinus. A, Mature pollen grain 
in which the nucleus has not yet divided ; B, pollen grain at the 
time of pollination ; C, lower end of pollen tube a little before fertilisa- 
tion, a.c, antheridial cell ; s.c, stalk cell ; t.n., tube nucleus ; v.c, 
vegetative cell; w., wing of pollen grain; t^ (in C), the two male 
nuclei. D-G, Embryology. D, Fertilised ovum, with the nucleus 
divided into four : E, lower end of oospore, showing the young pro- 
embryo ; F, much older proembryo ; G, almost mature embryo. 
cot., cotyledon ; em., embryo ; p.. plumule ; r., radicle; S., suspensor. 
(A-D after Ferguson ; the remainder after Strasburger.) 



up in the prothallus and gradualty displaces it. At the time 
when the seed is fully mature, however, a considerable part of 
this tissue still persists unaltered around the embryo and con- 
stitutes the endosperm (Fig. 205, A, e) ; the seed of Conifers is thus 
albuminous. The food-reserve is largely of the nature of oil, and 



352 



SEEDS 



is present in sufficient quantity to render the kernels of some 
species of Pines of nutritive value. The method of germination 
of the seeds is practically the same as that of a Castor Oil, the 
seedlings (Fig. 195) possessing long hypocotjds (/;.) with epigeal 
cotyledons {coi.). 

During the development of the embrj'o the entire ovule 
enlarges considerably, and at the same time the integument hardens 
to form the seed-coat, or testa (s.). The greater part of the 




Fig. 205. — Seeds and cones of various Conifers. A, longitudinal section 
of ripe seed of the Stone Pine {Pi)iiis pinca), sfiowing the testa (s), 
theperisperm (n, cf. p. 371), endosperm (c) , and the embryo with radicle 
(c) and numerous cotyledons (Co) ; H, young female cone of Juniper 
[Juniperus) and C, ripe cone of same ; D, outer edge of ripe cone- 
scale of WcUingtonia (Sequoia) ; E, single cone-scale of Piints bearing 
the two seeds with wings (w) ; F, ripe cone-scales of Douglas Fir 
(Psciidotsiign). b.s., bract scale ; o.s., ovuliferous scale. (B, after 
Antoine.) 

latter comes to consist of very thick-walled tissue, foreshadowed 
as a dark zone in the integument of a young P/;;//s-ovule 
(Fig. 203, A, s.), and constituting the hard la\'er which is so 
conspicuous a feature in the seed of the Stone Pine (Pi 11 lis pinca). 
The micropyle appears on the testa as a minute opening, and 
it will be realised, from the foregoing description, that the tip 
of the radicle lies just beneath this. In Piinis and some of its 
allies, a thin membranous flake liecoivics detached from the 



RfPE CONES 3,^j 

Ovuliferous scale and adheres to the ripe seed, forming a wing 
which aids in dispersal by the wind (Fig. 205, E, iv). In 
Cephalotaxus and a few other genera the ripe seeds are plum-hke, 
resembhng those of Cycads. 

The cones also undergo considerable enlargement in ripening, 
often becoming many times larger than they were prior to 
fertilisation (Fig. 200). The ripe cone-scales are usually woody, 
and gape apart to set free the seeds, although in the Cedar, for 
instance, they are shed with the latter. At this stage the bract- 
scales in Pinus are no longer recognisable, though in some of the 
related forms [e.g. the Douglas Fir, Pseiidotsitga donglasii, Fig. 
205, F), they are conspicuous even in the mature cone. The 
Juniper is peculiar in that the cone-scales become fleshy, forming 
a berry-like structure (Fig. 205, C) in which the ripe seeds are 
embedded ; the seeds are in this case distributed by birds. 

In the life-history of the Conifers the method of fertilisation 
shows a considerable advance on that of Cycads, since the pollen 
tube, which is there but an organ for the absorption of nourish- 
ment from the nucellus (a function which it may also fulfil to 
a slight extent in the Conifers), is here utilised to convey the 
motionless male cells to the egg. Thereby the necessity for 
the presence of liquid water at the time of fertilisation is dis- 
pensed with, and we may see herein the loss of the last traces 
of the probable aquatic ancestry of the Vegetable Kingdom. 



23 



CHAPTER XXIV 

The Angiosperms 

The majority of the Angiosperms, the second subdivision of 
the Phanerogamia, are easily distinguished from the Gymno- 
sperms by tlie possession of flowers, which are reaUy highly 
specialised fertile shoots. In some cases, however, the flowers 
are relativelj' inconspicuous (e.g. Grasses and other wind- 
pollinated plants), and scarcely conform to the popular notion 
of these structures. As a matter of fact, the Angiosperms are 
much more markedly characterised by the enclosure of their 
ovules in a protective ovary, as well as by the possession of true 
vessels and of sieve-tubes of a special type (cf. p. 28). 

The ordinary flower ^ comprises stamens (microsporophylls) 
and carpels (megasporophylls) surrounded by one or two series 
of modified leaves, serving either for protection (calyx) or for 
the attraction of pollinating insects (corolla). These " non- 
essential " organs are not uncommonlj' reduced [e.g. apetalous 
flowers) or absent (naked flowers, e.g. Hazel). Though the 
majority of Angiosperms have hermaphrodite flowers, uni- 
sexuality is not uncommon, the two sexes occurring either on 
the same (moncecious, e.g. Hazel) or on different plants (dioecious, 
e.g. Willow). 

In those flowers, which for many reasons are regarded as 
relatively unspecialised [e.g. Ranunculacese and its allies), the 
floral axis is elongated, bearing sepals, petals, stamens, and 
carpels at successivelj' higher levels (hypogj-nous flowers), and 
these parts are wholly or in part spirally arranged ; the stamens 
and carpels, moreover, arc usuallj' numerous, and not joined in 

1 The reader is advised to familiarise himself fully with (he broad 
features of iloral structure (cf. F. and S., chapters x\iii, xix) before passinc; 
on to the subsequent matter. 

354 



THE FLOWER 355 

any way. The further evolution of the flower has brought about 
increased efficiency and precision in pollination, and appears to 
have involved a gradual shortening of the floral axis, with the 
result that the different sets of organs became whorled and 
reduced in number. At the same time fusion of parts took 
place to a more or less marked extent. Thus, the commonest 
type of Dicotyledonous or Monocotyledonous flower possesses 
two whorls of stamens (3 to 5 in each whorl), and a syncarpous 
ovary formed by the fusion of a small number of carpels. 
Another development appears to have been concerned with the 
better protection of the ovary, the receptacle gradually becoming 
more and more concave (progressive stages can be found in the 
perigynous Rosacea), until the ovary is completely enveloped 
(epig5aious flowers). Certain it is that the most highly specialised 
families (Umbelliferse, Composite, Orchidacese) all possess such 
epigynous flowers, with the different members in whorls. The 
irregular (zygomorphic) flowers found in many families are no 
doubt a special development connected with the perfection of 
mechanisms for insect pollination (cf. p. 373). 

The classification of Angiosperms into Monocotyledo7is and 
Dicotyledons is based on many morphological and anatomical 
features (leaf-form, number of cotyledons in the embryo, stem- 
and root-structure, and number of floral parts), the Monocoty- 
ledons being an essentially herbaceous group exhibiting a high 
perfection of means for perennation and hibernation {e.g. bulbs, 
corms, etc.). The evidence that the Dicotyledons and Mono- 
cotyledons were derived from a common stock is almost over- 
whelming. This fact is plainly indicated, for example, by the 
widespread traces of the presence of a cambium in Monocoty- 
ledons, and the extremely few characters, either of development 
or structure, that are entirely restricted to the one or the other 
group (cf. also p. 367). 

The Angiosperms are the predominant vascular plants on 
the earth's surface at the present day, and comprise some 125 000 
distinct species. In Britain this preponderance is especially 
marked. The native Angiosperms here number about 2,000 
species and the Gymnosperms four species ; amongst the 
Cryptogams, there are about 60 Pteridophyta, 600 Mosses, 250 
Liverworts, several thousand species of Alga;, and some 5,000 



356 OVARY AND CARPELS 

species of Fungi. The Angiosperms first appear in the geological 
succession in the Cretaceous period, but the study of these and 
of the abundant Tertiarj? fossils, though affording much that is 
of interest, has so far shed no light whatsoever on the origin 
of the group, which is merged in obscurity. The details of the 
life-history betra}-- a very high degree of specialisation, but afford 
practically no evidence as to how this has been attained. 

The ovary of the Angiosperm consists of one or more carpels 
or megasporophylls, whose nature is most obvious where they 
are distinct from one another, as in the Marsh Marigold and 
Pea. Here dissection shows the carpel to have a leaf-like struc- 
ture, the midrib being marked by a well-defined vascular bundle, 
whilst the somewhat enlarged edges are joined and form two 
placentce, each bearing a row of ovules. Except, therefore, for 
the inrolling, such a carpel might be compared to the megasporo- 
phyll of Cycas (cf. p. 324). 

Since the ovules are enclosed, the pollen cannot reach the 
micropyle direct as in G^niinosperms, but is received by the 
tip of the carpel, which is modified to form a special receptive 
surface, the stigma. Upon this germination of the pollen takes 
place. The stigma bears numerous papilla-like hairs, whose 
secretion may nourish the deA'cloping pollen. Very commonly 
the upper part of the carpel is produced into a stalk-like pro- 
longation, termed the style, which serves to raise the stigma to 
a position more suitable for the reception of the pollen. 

When, as is usually the case, several carpels are fused together 
to form a syncarpous ovary, the number of component carpels 
may be indicated by the number of loculi or by the number of 
lobes exhibited by the stigma. In most syncarpous ovaries the 
ovules retain their position on the margins of the carpels (axile 
or parietal placentation), though other arrangements obtain in 
some cases. Thus, in the Primulacea and Dock-family (Poly- 
gonaceae) the ovules are borne on a prolongation of the floral 
axis (free central placenta of the former), whilst in the Flowering 
Rush [Biitomus] and the \Mnte \\'ater Lily {Nyinpluca) they 
occur on the entn^c inner surface of the carpels. In many Angio- 
sperms the ovary contains numerous o\-ules, but the number is 
not uncommonly reduced to one in each loculus {e.g. Lhnbellif eras) , 
or the whole ovary produces but a single o\'ule (t-.^. Gramineae, 



OVULES 



357 



e.s. 



Composite). This last condition is often characteristic of some 
of the most advanced types of Flowering Plants. 

The ovules of Angiosperms (Fig. 206), which in their general 
construction are hke those of Gymnosperms, usually possess 
two integuments (inner, I.i., and outer, O.i.). A single one is 
the rule amongst the 
gamopetalous Dicotyle- 
dons (Sympetalse) and a 
few polypetalous families 
{e.g. UmbeUiferffi, Salica- 
ceae.etc). In these latter, 
however, the single in- 
tegument is generally re- 
latively robust, a fact 
which suggests its possible 
origin from the fusion of 
two ; a partial union of 
this character is, indeed, 
seen in some members 
of the Ranunculaceffi.^ 
The nucellus («.), with 
very few exceptions {e.g. 
Bog Myrtle, Myrica) , is 
only clearly separated 
from the integument at 
the apex, where it forms 
a cap of tissue extending 
between the large mega- 
spore (embryo sac, e.s., 
see p. 365) and the micro- 
pyle {m.). The part of 
the nucellus in contact 
with the integument can often be distinguished from the latter, 
in sections of the ovule, by the dense contents of its cells, ^^'hich 
form a nutritive layer around the developing embryo sac. 

By far the commonest tj'pe of ovule is the anatropous one 
(Figs. 206 and 207, C), in which the main body of the ovule is 




Fig. 20G. — Diagrammatic longitudinal 
section through an anatropous ovule, 
greatly magnified. a., antipodal 
cells ; Ch., chalaza ; e., egg ; e.s., 
embryo sac; /., funicle ; I.i., inner 
integument; m., micropyle ; «., 
nucellus ; O.i., outer integument ; 
^., polarnuclei ; r., raphe; s., syner- 
gida; ; V .b., vascular bundle of raphe. 



' In a few Rosacea; [e.g. Geiiiii) there is only a single integument due 
to failure of the inner one to develop. 



358 



OVULES 



bent down against its stalk (fiiniclc, f.), so that the micropyle {ni.) 
is directed towards the placenta. The greater part of the funicle 
is in this case fused with the adjacent portion of the outer in- 
tegument, which is in consequence not easily distinguished on 
this side. The fused region, known as the raphe {r.), is traversed 
by the vascular bundle {V.b.) which enters the funicle from the 
placenta, and which extends as far as the base of the nucellus 
(a region known as the chalaza, Ch.). 




Fig. 207. — Ovules and stages in their development (after Le Maout and 
Decaisne). A, Young ovule, before the integuments appear, showing 
nucellus (Nit.) and dividing archesporium (ar.). B, Orthotropous ovule 
of Polygoniini. C, Anatropous ovule in longitudinal section, showing 
funicle (/.) and raphe (r.). D, I-IV (in the direction of the arrow), 
successive stages in the development of the anatropous ovule of 
Chelidonium. E, Successive stages (in the direction of the arrow) in 
the development of the campylotropous ovule of the Mallow [Malva). 
i., inner integument ; n., nucellus ; 0., outer integument. 

Erect or orthotropous ovules, in which the micropyle faces 
away from the placenta, and lies at the opposite end to the 
usualljr short funicle (Fig. 207, B), are much less frequent, but 
are found, for example, in the Knotweeds [Polygonum). Another 
rare type, the campylotropous ovule (Fig. 207, E), seen in many 
Caryophy]lacc;e, is bent in such a wa)' that the stalk appears 
to be attached midway between the chalaza and micropyle. 



DEVELOPMENT OF OVULES 



359 




All xA.ngiospermous ovules are supplied by a vascular bundle 
which runs through the funicle, but terminates at the chalaza, 
except in those rare instances (e.g. Myrica) where the nncellus 
is not in intimate contact with the integument and hence 
provided with a special vascular supply. 

The ovule invariably arises on the placenta as a small mound 
of thin-walled cells which represents the future nucellus (Fig. 

207, A and D, I). Around the base of this the integuments 
subsequently develop as ring-like up- 
growths, the inner being formed before 
the outer (II-IV). If a third integument, 
or aril, be present (as in the Spindle-tree, 
Euonymus), this is the last to arise, and 
only becomes conspicuous after fertilisa- 
tion. 

All ovules are at first erect. The 
anatropous form, characteristic of the 
maiorit3^ is due to excessive growth on 
one side of the chalazal region of the 
rudiment, so that the latter gradually 
becomes curved till the micropj'le points 
towards the placenta (cf. Fig. 207, D). 
In the case of campylotropous ovules 
the mature form is due to even more 
extreme asymmetrical growth of the 
chalazal region (Fig. 207, E). Stages in 
the development of anatropous ovules 
can be readily studied in transverse 
sections through young ovaries of the 
Popp3'. 

In the case of the stamens or niicrosporophylls the leaf-like 
character is not so obvious as that of the carpels. The lamina 
is represented b}' the connective which bears the microsporangia 
(pollen sacs) on its margin, i.e. in the same position as the 
ovules. Clearer evidence of the foliar nature of the stamen is 
afforded by cases in which the connective is developed as a flat 
petal-like structure (as in the Pondweeds, Potamogeton, Fig. 

208, B, Co.), or in which stipule-like outgrowths arise from the 
bases of the filaments (as in the Onions, Allium, Fig. 208, C). 



Fig. 208. — Various forms 
of stamens. A, Tulip 
(ordinary form). B, 
Pondweed (Potamo- 
geton), showing the 
enlarged connective 
(Co.). C, Onion (Al- 
liitin), with stipular 
outgrowths from the 
base of the filament. 
a., anthers ; /., fila- 
ments. 



36o 



STAMENS 



A single vascular bundle traverses filament and connective. 
In the vast majority of cases the anther bears four pollen sacs, 
two on cither side of the connective (Fig. 209, A), although 
the staniens of the MaUow-family (Malvace»), for instance, have 
but two pollen sacs. In transverse sections through verj' J'oung 
immature flower-buds, the anthers are seen to consist of uniform 








Fig. 20(j. — Development of the anther. A, Section of rather more than 
half the anther of a young stamen, showing the jjollen-sacs at the 
mother-cell stage (the mother-cells are seen in various phases of 
division). B, Section of young anther before the archesporia are 
differentiated. C and D, Later stages, a., dividing archcsporial cells ; 
£p., epiderniis of anther ; /., fibrous layer ; Jil., filament ; 111. 1., middle 
layer ; I., tapetum ; v.b., vascular bundle of connective. (A original, 
the remainder after Warming.) 



thin-walled tissue bounded by a distinct epidermis, whilst a 
slight lobing foreshadows the future pollen sacs (Fig. 200, B). At 
a slightly later stage four subepidermal strips of \'ar)'ing width 
(often of only a single cell) become distinguishable, one in each 
lobe ; these strips consist of cells with prominent nuclei and 
dense contents and constitute the archesporia (Fig. 209, C. a.), each 
of which extends as a narrow band of tissue for almost the %\'hoJe 



STAMENS 36T 

length of the anther. Each archesporial cell divides tangentially 
(Fig. 209, C), and, from the inner halves (a.) thus formed, the 
actual pollen-producing tissue arises, so that the pollen mother- 
cells are really subepidermal in origin. The outer half of each 
archesporial cell divides to form the wall of the pollen sac 
(Fig. 209, D), which comes to consist of several layers. 

In transverse sections through older anthers (Fig. 209, A ; 
Frontispiece, Fig. 210), the mass of spore mother-cells in the 
centre of each pollen sac is seen to be surrounded by a nutritive 
layer {tapehtm, t.). This is composed of large, often palisade- 
like, cells with prominent nuclei and dense contents. Between 
the tapetum [t.) and the epidermis {ep.) of the anther are two 
or more layers, the outermost of which generally consists of 
rather large, subsequently thickened cells {fibrous layer, f.), 
whilst the others (;;;./.) are usuallj/ very much flattened. 

Each pollen mother-cell, as in the groups previously studied, 
undergoes two divisions (cf. Fig. 209, A), the first of which is 
the reduction division. The four pollen grains, thus formed 
within each mother-cell, generally fall apart as the membrane 
of the latter becomes dissolved. But in a few cases the members 
of each tetrad remain in connection, when the pollen is shed 
[e.g. Typha, Neottia, Fig. 211, D, a-c ; Ericaceae, Fig. 211, D, d.), 
and a more extreme condition is seen in Orchids, where all the 
pollen grains formed b}^ the single stamen cohere as two masses 
or pollinia (Fig. 211, F). Such cohesion is, however, only met 
with in insect-pollinated flowers. During the development of 
the poUen grains the tapetum undergoes gradual disorganisation 
(Fig. 212, f.), prior to which the cells often become bi- or 
multi-nucleate. 

The mature pollen grain possesses a wall of two layers, the 
outer of which is thick and cuticularised, whilst the inner is 
delicate. \\'here the pollen is wind-borne, its surface is com- 
monly smooth, 1 but when convej'cd by insects it is usually 
sculptured in various ways (Fig. 211, B, G). The outer layer 

^ Similar smooth pollen is characteristic of plants in which pollination 
is effected under water (e.g. Hornwort). Such water-borne pollen is some- 
times highly specialised, that of the Sea-Grass (Zostera), for example, being 
tubular, of the same specific gravity as sep.-'water, and without any cuticu- 
Jarised layer. 



362 



POLLEN 



is perforated, or rendered thin, by one or more pores or pits 
(Fig. 211, C, 0.), through one of which the pollen tube is sub- 
sequently protruded. In this respect the pollen of Angiosperms 
is more specialised than that of Gymnosperms, where the outer 
layer of the membrane is merely burst open in germination. 
As the pollen matures the waUs of the cells composing the 




~^^ G. 



Fig. 



211. — Diverse types of pollen. A, Willow-herb {Epilobiuni), with 
three thin spots in the outer coat. B, Hollyhock (AlthcBa), with 
numerous pores. C, Small part of same enlarged, showing the pores (0.). 
D, Pollen-tetrads, a-c, of Bird's Nest Orchid (Neottia nidus avis), 
d, of Bilberrj' {Vaccinium inyrlillus). E, ^ixisV (Mimnlus moschatiis). 
F, PoUinium of Orchis niorio. G, Pollen grain of Gourd (Cucurbita 
pepo), showing the embossed lids {/.) of the outer coat, which become 
pushed off by the growing pollen tubes. (A-D after Sachs ; d and F 
after Wettstein ; E after Mohl ; G after Schacht.) 



fibrous layer become strengthened hy a network of thickening 
bars, and simnltaneousljf the cells loose their living contents 
(Fig. 212, en.) ; many of the cells in the region of the connective, 
moreover, develop similar thickenings. At this stage the two 
pollen sacs, in each half of the anther, become confluent through 
the dr\''ing up of the intervening thin-walled septa, and it is this 
same process of desiccation that ultimately leads to the setting 



DEHISCENCE OF STAMENS 



363 



free of the pollen. Drying is usually due to direct loss of moisture 
to the air, but may result from the withdrawal of water by 
neighbouring cells of high osmotic content, belonging either to 
nectaries (e.g. Stcllaria) or to sugar-containing tissue {e.g. Fox- 
glove, Digitalis) ; in the latter case dehiscence of the stamens 
ensues, even in clamp air. 

Actual splitting usually takes place longitudinally, along the 
middle line of each anther- 
lobe ; the line of weakness is 
sometimes bounded on either 
side by a spindle-shaped group 
of enlarged epidermal cells {e.g. 
Lily). The radial thickenings 
of the cells of the fibrous layer 
(Fig. 212, en.) prevent con- 
traction in this direction, so 
that the shrinkage during dry- 
ing is mainly tangential, and it 
is probably the tension thus set 
up that finally results in rup- 
ture. In a few flowers dehis- 
cence takes place in other ways, 
as, for instance, by apical pores 
(Ericaceee), or by the forma- 
tion of subapical valves which 
hinge backwards to allow the 
shedding of the pollen {e.g. Bar- 
berry) . 

Before the pollen grain is 
shed its nucleus divides into two 
(Fig. 215, B). One of these 
becomes surrounded by a small 
envelope of denser cytoplasm {generative cell, g.c), whilst the 
other {iiibe nucleus, t.n.) lies freely in the general body of the 
grain. In this condition the latter is transferred, in one way 
or another, to the stigma of the same or of a different flower,^ 
and here germination takes place (Fig. 215, F). The tube nucleus 
passes into the tip of the pollen tube, and the naked generative 
1 Cf. F. i^nd S,, chapters xx, xxi. 




Fig. 212. — Transverse section 
through part of a pollen sac 
of a ripe anther, en., fibrous 
layer; m,, middle layers ; p.g., 
pollen grams; t., disorganised 
tapetum. 



364 



GERMINATION OF POLLEN 



cell sooner or later follows suit (Fig. 215, C). The former usually 
remains undivided, whilst the latter ultimately produces two 
male nuclei (cf. below). 

In its downward growth the pollen hibe is sometimes nourished 
by the secretion of papillae forming a continuous lining to the 
canal or canals traversing the style and extending uninterruptedly 
into the loculi of the ovary {e.g. Tulip and Violet). In many 
cases, however, the style is solid, the pollen tube penetrating 
through the axial core of thin-walled tissue, nourished by the 
sugary sap which the latter contains. In some cleistogamic 




Fig. 213. — Diagrams illustrating normal fertilisation (porogamy, A) and 
chalazogamy (B). e.s., embryo sac ; m., micropyle ; ii., nucellus ; 
pi., pollen tube. 



flowers {e.g. Purple Deadnettle) the pollen grains germinate in 
the anthers, and the pollen tubes grow from there to the stigma. 
The period of time occupied by the pollen tube in growing 
down to the ovules varies greatly in different species, and bears 
no relation to the distance to be traversed. In the Crocus, 
where the style is 6-10 cm. long, the interval is only from one 
to three days, whilst in the Meadow Saftron {Colchicmn), with 
a style of about the same length, it is six months (from autumn, 
when polhnation occurs, to the following spring). A similar 
prolonged interval not uncommonly obtains in woody plants 
{e.g. Hazel), though the distance traversed is short, and in some 
Oaks the ovules do not even commence development until after 



EMBRYO SAC (MEGASPORE) 365 

pollination. In most spring-flowering plants the anthers develop 
as far as the mother-cell stage in the previous autumn, although 
in the Hazel ripe pollen can be found in the male catkins during 
midwinter. 

As a general rule the pollen tube traverses the cavity of the 
ovary, and thus reaches the micropyle of one of the ovules 
(porogamy. Fig. 213, A). But, in certain trees and shrubs {e.g. 
Elm, Beech, Hazel), it grows through the placental tissue and 
enters the ovule near the chalazal end (chalazogamy, Fig. 213, B). 
The growth of the pollen tube may therefore be compared to that 
of a fungal hypha, and its power to penetrate tissues, and eventu- 
ally the megaspore membrane, is doubtless due to an analogous 
secretion of digestive enzymes. 

The downward growth of the pollen tube, from the stigma 
into the ovary, is mainly determined by a chemotropic stimulus 
due to substances contained in the ovules or in the ovary- 
Wall. This fact can be experimentally demonstrated by sowing 
pollen grains of the Wild Hyacinth (Scilla) in a 5 per cent., 
or of Echeveria retiisa in a 15 per cent., solution of cane-sugar 
around a fragment of the ovary, taking care to avoid the inclusion 
of air-bubbles. Observed under the microscope, the pollen tubes 
tend to grow in the direction of the piece of ovary. 

The first steps towards the development of the megaspore 
are to be found in very young ovules, usually before the integu- 
ments have become plainly differentiated (Fig. 207, A). As in 
the case of the stamens, the archesporiuni {ar.) arises subepidermally, 
but here it usually comprises but a single cell situated near the 
apex of the nuceUus. This divides tangentiaUy (Fig. 207, A, ar.), 
the inner half generally constituting the mother-cell, which as 
a rule, by two successive divisions, of which the first is the 
reduction division, gives rise to a row of four spores, forming a 
tetrad within the nucellus. It is usually the innermost member 
of the tetrad that becomes the functioning megaspore (generally 
known as the embryo sac), this cell subsequently increasing 
greatly in size so as to occupy the greater part of the nucellus 
(Fig. 206, e.s.). In the Mistletoe-family (Loranthacete) no proper 
ovules are differentiated ; a parenchymatous outgrowth, which 
arises from the base of the ovary and almost fiUs the latter, 
develops a number of archesporia, from each of which an embryo 



',66 



EMBRYO SAC 



sac results. This affords another instance of the reduction 
characteristic of parasites (cf. pp. 179, 231). 

Until it has reached a considerable size the embryo sac con- 
tains but a single nucleus, which usually lies in the middle of a 
strand of cytoplasm running from end to end of the sac and 
bridging the large central vacuole. Sooner or later, however, 
a nuclear division occurs, and the two resulting nuclei wander 
towards opposite ends of the embryo sac, where each again 

divides twice. Three of the four 
nuclei thus formed, at the end 
remote from the micropyle, 
usually become separated by cell- 
walls and constitute the antipodal 
cells (Figs. 206, 214, 215, D, a.), 
which probabl)^ assist in the 
nourishment of the young em- 
bryo. They frequently enlarge 
after fertilisation, and, in some 
few cases [e.g. Burr-reed Spar- 
ganiimi), they may even divide 
v-s J~wm«J.!i«N:ir¥«KXK4'(V^ ^° ioTva an antipodal tissue. 
\'r''iivT3^^^!ft6^^^ Of the four nuclei at the 

* " ' ' micropjdar end of the embryo 

sac, three become surrounded b}' 
an envelope of specialised cyto- 




FiG. 214. — Longitudinal section of 
ovule of Marsh Jlarigold {Cal 



tha), showing the structure of plasm, and constitute a group of 



the mature embryo sac. 
antipodal cells ; e., egg ; m., 
micropyle ; p., primary endo- 
sperm nucleus. 



naked cells known as the egg- 
apparatus (Fig. 206). This con- 
sists of the egg [e.) and two 
syncrgidce (s.), the latter, which 
usuaUy lie in front of the former, being supposed to aid in 
the passage of the male nucleus to the female cell. The 
remaining nuclei (one at each end, one of them the sister- 
nucleus of the egg) pass back to the middle of the embryo sac, 
where these two polar nuclei (Figs. 206, p. ; 215, I),p.n.) meet 
and ultimately fuse to form the primary endosperm nucleus 
(fig. 214, p.). This is the stage reached by the embryo sac at 
the time of fertilisation. 

The sequence of events during this development and the 



EMHRVO SAC 



367 



resulting structure of the embryo sac are practically identical in 
the vast majorit}' of the Angiosperms (both Monocotyledons and 
Dicotyledons) which have been investigated, and this uniformity 
constitutes one of the strongest arguments for the origin of the 
group from a common ancestry. Tubular sucker-like outgrowths 




Fig, 215. — Germination of pollen and fertilisation. A-D, Lily [Liliuni) 
(after Guignard). E-F, Tulip (Tulipa) (after Ernst). A, Mature 
pollen grain, showing the single nucleus. B and E, Later stage, showing 
generative cell {g.c.) and tube nucleus (tji.). C, Tip of pollen tube 
with tube nucleus (t.n.) and two male cells (rj). D, Embryo sac at 
the moment of fertilisation, showing antipodals (a.), polar nuclei (p-n.), 
ovum (0.), one synergid (5.), and the two vermiform male cells (S) 
discharged from the tip of the pollen tube (p-t-). F, Early stage in 
formation of pollen tube. 



from the chalazal or both ends of the embryo sac are met with in 
the Beech, Hornbeam, and some S3.Tnpetalae (Plantago lanceolata, 
Scrophulariaceje) , and probably aid in the transference of food- 
material to the developing embryo. The same end is served 
by the haustorial outgrowths from the later-arising endosperm 
(see below) which occur in some genera. 

Having passed through the micropyle, the pollen tube pene- 



368 FERTILISATION AND EMBRYOLOGY 

trates the overlying nucellar cap, and thus reaches the embfyo 
sac. Meanwhile the generative cell has divided to form two 
male nuclei (Fig. 215, C, o), which are extruded from the tip 
of the pollen tube, and, entering the embryo sac, fuse respec- 
tively 1 (a) with the egg, and (h) with the two polar nuclei 
(cf. Fig. 215, D), or with the nucleus formed by the fusion of 
the latter. The former fusion results, in the usual way, in the 
production of an embryo, but the latter also leads to abundant 
division, whereby a nutritive tissue, the ciidospenn, is formed. 
In the utilisation of both male nuclei for these different purposes, 
the Angiosperm exhibits a characteristic difference from the 
Gymnosperm. 

The product of the second fusion, which is really built up of 
three nuclei (one from either end of the embryo sac and one 
male nucleus), divides repeatedly, till the sac becomes filled 
with numerous free nuclei distributed uniformly throughout its 
protoplasmic content. Formation of separating walls now takes 
place almost simultaneousl\^ so that the sac becomes occupied 
by a continuous thin-walled tissue, the endosperm, which stores 
food for the developing embyro. It thus fulfils the same function 
as in Gymnosperms, where, however, it is formed already prior 
to fertilisation. 

During the nuclear divisions leading to endosperm-formation, 
the fertilised egg, now provided with a cell-wall, enlarges con- 
siderably and commences to segment. The first division is 
unequal, a smaller cell situated at the end away from the 
micropyle being cut oft; from the large remaining portion. The 
latter forms the sac-like basal cell (Fig. 216, I-VI, b.) and pla3.'s 
no further part in the development of the embryo. The smaller 
segment undergoes successive transverse divisions, so as to gi^•e 
rise to a short elongating siispciisor (Fig. 21(1, S.). Hereby the 
shghtly larger hemispherical terminal cell ((■.), \\-hich subse- 
quently produces the embryo proper, is carried down into the 
heart of the developing endosperm. Young procmbryos at this 
stage of development, with a suspcnsor composed of a number 
of flat cells, are readily squeezed out of ver\' young seeds of 
the Shepherd's Purse, such as can be remo\-ed from the o^•aries 
of flowers which have not yet faded ; the seeds are best mounted 

• 'J'his process is often spoken of as " double fertilisation." 



EMBRYOLOGY OF DICOTYLEDONS 



3^>9 



in water, and gentle pressure applied by tapping the cover- 
glass. 

More advanced stages can be obtained in the same way from 
progressively older seeds. These show rather longer suspcnsors 




Fig. 2i6. — Embryology of the Shepherd's Purse {Capsella bursa-pasloris). 
The successive stages are numbered I-IX. In VII and VIII only the 
embryo and a small part of the adjacent suspcnsor are shown, IX is 
a longitudinal section of a mature seed, showing the coiled embryo. 
b., basal cell; Co., cotyledons ; e., embryo ; p., plumule ; r., radicle ; 
S., suspcnsor. 



and an increasing number of divisions in tire enlarging embryonal 
cell (Fig. 216, IV-VI). The latter soon segments into octants, 
by three walls at right-angles to one another (III, IV), and sub- 
sequently dermatogen, periblem, and plerome are successively 
delimited by walls parallel to the surface (V, VI). Mean- 
24 



370 EMBRYOLOGY 

while the embryo assumes a more or less flattened form, and, 
at its wider free end, the futm'e cotyledons appear as two 
lobes (VII, Co.) between which the plunmle (p.) develops. The 
attached end of the embryo invariably becomes the radicle 
(VIII, r.). which thus faces towards the micropvle. Such em- 
bryos are to be found in seeds from almost ripe fruits (cf, IX). 
The general method of embryo-development just described 
is that characteristic of a large number of Dicotjdedons, although 
there are numerous differences in detail which are be\'ond the scope 
of this book. In Monocotyledons (Fig. 217, A-E), where the 
suspensor is often a more bulky structure (cf. especially Fig. 
217, G), the single cotj'ledon (Co.) occupies the terminal position, 
whilst the plumule (p.) arises towards one side. It will be 
noticed that the embryos of Gymnosperms and Angiosperms do 
not develop a special aljsorptive organ (foot) , such as charac- 
terises those of Brj'ophyta and Pteridophyta, although in a few 
cases haustorial outgrowths arise from the suspensor. 

The stimulating effect of fertilisation is not confined to the 
embryo, but also affects the ovule as a whole and the enveloping 
ovary, whilst in some cases even adjacent parts of the flower 
undergo considerable enlargement and change [e.g. the perianth 
in the Mulberry, the receptacle in the Strawberry). The seeds 
are often many times the size of the ovules from which they 
sprang [e.g. Pea, Bean, etc.). The enlarged ovary-wall becomes 
the pericarp, and the integuments, or integument, form the 
testa, whose final texture differs greath', according to the nature 
of the fruit (dehiscent or indehisccnt) and the means of dispersal. 
The character of the testa varies enormousl}', both as to its 
surface, which is often sufficiently distinctive in its sculpturing 
[e.g. Corn Cockle, Agrostcmma) to ser\-e for purposes of identi- 
fication, and as to its durabilitv and thickness. The latter 
feature is probably responsible for the slow germination of some 
seeds. It may l)e noted in this connection that the seeds of 
the Hawthf)rn and others, which are normally swallowed and 
pass through the alimentary tract of animals, germinate much 
more readily after having been acted upon by the gastric juices. 
The embryo may absorb all the food-materials in the endo- 
sperm before entering upon its resting-stage {c.xalbiiminous seeds, 
as in the Shejiherd's I'urse, Pea, etc.), or part of the endosperm 



EMfekYo 



m 



may persist up to germination {albuminous seeds, e.g. Buckwheat, 
Castor Oil, most Monocotyledons). In a few plants {e.g. many 
CaryophyllaceEe) the nucellus is not entirely displaced by the 
endosperm, a thin layer {perisperm) remaining, even in the ripe 




Fig. 217. — Embryology of various plants. A-D, Successive stages in the 
development of the embryo of the Arrowhead (Sagittaria) (after 
Schaffner). E, Longitudinal section of ripe achene of the Water 
Plantain (Alisma) (original). F, Formation of several embryos (em.) 
from a massive suspensor, by budding, in Eryihroniurn americanum 
(Liliacea;) (after Jeffrey). G, Proembryo, with massive suspensor, of 
Liliitm (after Coulter). H, Embryo of Tway blade (Listera ovata) at 
time when seed is shed (after Pfitzer). I, Proembryo of Spariium 
junceum (Leguminosse) (after Guignard). J, Longitudinal section of 
Orange pip [Citrus) (with two embryos) (after Wettstein). b., basal 
cell ; Co., cotyledon ; /., funicle ; p., plumule ; pc, pericarp ; r., 
radicle ; S., suspensor ; t., testa. 

seed, and functioning for the storage of food. The degree of 
differentiation attained by the embryo, at the time when it enters 
upon its resting-stage, varies considerably. Thus, in the Runner 
Bean {Phaseolus), even the venation of the first pair of plumular 



372 APOGAMY, ETC. 

leaves is distinguishable, in the Castor Oil the plumule is merely 
a peg-like structure, whilst in the Lesser Celandine [Ficaria) the 
entire embryo is an undifferentiated mass of cells (see also 
Fig. 217, H), a condition likewise encountered in the embryos of 
most parasites. In some plants, moreover, there is no resting 
period {e.g. in the tropical Mangroves), whilst in many trees the 
seeds germinate most readily if sown immediately on reaching 
maturity. 

Exceptions to the normal sequence of events described in 
the foregoing pages are by no means uncommon. Apogamy 
(cf. p. 305), for example, has been recorded in quite a large 
number of Compositce (e.g. Dandelion, Hawkweed), as well as 
in some species of Lady's Mantle {Alclicmilla). In such cases 
the reduction di\-ision does not appear to occur, and the 
embryo arises from an unfertilised cell of the embryo sac having 
the normal number of chromosomes. Such apogamy is of course 
akin to vegetati\'e propagation, but gains the advantages afforded 
by the mechanism for seed-dispersal. Despite the non-occurrence 
of a sexual process in such apogamous forms, pollination some- 
times appears to furnish a necessary stimulus for embryo- 
formation. More rarely it is an ordinarj' cell of the nuceUus 
that divides to form the embryo (e.g. Orange, Citrus), a case 
analogous to the apospory described among Ferns (p. 305). In 
the Hawkweeds all three conditions — apogamy, apospory, and 
normal fertilisation — have been observed. 

The occasional presence of more than one embryo within a 
seed ma}' be due to several causes. Sometimes more than one 
member of the tetrad, formed by the megaspore mother-cell, 
de^'elops into an embr3'o sac, so that several embryos are present 
from the first, ]-jut more frequently accessory embryos arise by 
vegetative budchng from the procmbryo (Fig. 217, F). Orange 
pips frequently contain se^-cral embryos, of which one is the 
outcome of a se.xual fusion, whilst the others are derived from 
nucellar cells which are presumably stimulated to growth as a 
result of fertilisation (Fig. 217, J). 

The general course of the life-history in Angiosperms is 
obviously very similar to that of Gymnosperms. In both cases 
the young emijrj-o lives, as a parasite, M'ithin the ovule, which 
forms a protecti\e cn\'elope around it until the time of germina- 



POLLINATION 373 

tion. As with the young Fern, however, its independence is 
soon establislied. In contrast to Gymnosperms, the most stril-cing 
features are connected witli tlie very efficient arrangements for 
the protection and nourishment of the developing embryos and 
seeds, and tlie highly perfected mechanism for pollination by 
virtue of which tlie most intimate relation often obtains between 
the flower and the pollinating agent. Not all flowers, however, 
are equally highh* adapted, and almost all grades of specialisation 
can be found in relation to pollination by the agency of insects, 
wind, or water. The more advanced of the insect-polhnated 
flowers usually exhibit a fusion of both sepals and petals, a 
zj'gomorphic corolla, and a reduced output of pollen. In the 
most extreme cases only one type of insect is effective in pollina- 
tion, or is most efficient in the pollination of that particular 
species. Wind-pollinated flowers are usually inconspicuous, pro- 
duce an abundance of pollen, and often possess richly branched 
stigmas. Water-pollinated plants, which like wind-pollinated 
ones are commonly unisexual, exhibit iioating devices cither in 
the female flowers or pollen. 

The pollen grains and embrj-o sacs of Angiosperms are ob- 
viously comparable to the microspores and megaspores of Gj'm- 
nosperms and Sdaginella, and within them divisions take place 
which lead to the formation of the male and female gametes 
respectively. The contents of the embrj'o sac may therefore be 
regarded as a female prothallus, and the contents of the micro- 
spore as a still more reduced male prothallus. By those who 
find such comparisons profitable, an alternation of generations, 
analogous to that of Pteridophyta and Bryophyta, is thus 
recognised. 



CHAPTER XXV 

Heredity and Evolutiox 

It will be a matter of common knowledge that the offspring of 
either plants or animals resemble their parents ver\' closely. 
This fact, though so obvious, really in\'olves the fundamental 
principle of inheritance , i.e. the progeny inherit the characteristics 
of their parents. Thus, if we sow the seeds obtained from a 
self-pollinated flower of the Foxglove, the numerous resulting 
seedlings obviously inherit the same general characters. Closer 
observation, however, shows that there are many minute points 
of difference which may in the main be related to the fact that 
the conditions in the seed-bed are not uniform (cf. also Fig. 218). 
The features presented by any particular indi^-idual may, as a 
matter of fact, be regarded as the outcome or resultant of two 
sets of factors, being either inherited or due to the effect of the 
environment. Under environment we understand all the external 
influences — physical, chemical, and biological — to which the or- 
ganism is subjected. In the case of plants, and still more in 
that of animals, the conditions of the cn\-ironnient are not 
constant throughout the life of the individual, but in general 
it is those experienced in the early stages of de\-elopmcnt which 
are most potent in moulding the organism. 

Individual variations, though probably in the main correlated 
with differences in the environment, may wx-ll also result from 
changes in the internal conditions \\hich are more difficult to 
analyse. Such variations may be of two kinds. ThcN' are 
qualitative or substantive, when, for instance, they concern the 
shape or size of the entire plant (Fig. 218) or of any of its parts, 
the degree of hairiness, the type of colouration (Fig. 219), etc. 
On the other hand, they are quantitative or vicrisfic, when they 

374 



VARIATION 



375 



involve differences in the number of constituent parts, such as 
perianth-segments, leaflets in a compound leaf, etc. 

It is easiest to study variation if some character is selected 
which is capable of exact measurement, as, for instance, the 
length of the leaf in the Privet, the length of the Runner Bean 
seed, or the number of 
ray-florets in the Daisy. 
Taking the first of these 
cases as an example, it 
will be found that if, say, 
a thousand leaves are 
measured, the difference 
between the length of the 
shortest and the longest 
is quite considerable, and 
that the majority of the 
leaves are of an average 
size. If all the thousand 
leaves be classified ac- 
cording to their lengths 
into separate groups, 
differing b)? increments of 
one millimetre, and the 
number of leaves in each 
group counted, those 
containing the smallest 
number will be found to 
be situated at the two 
extremes. Between these 
points the number of 
leaves of each particular 
length will be found to 
increase with considerable regularity, as the size of its individuals 
approaches that exhibited by the majority. This most frequent 
size is termed the mode, and often corresponds very closely to 
the arithmetical mean of all the measurements. 

By plotting a curve, in which the ordinates represent the 
number of individuals in each group, and the abscissa: tha re- 
spective lengths, the variation can be graphically represented 




Fig. 2i8. — Three equal-aged individuals of 
the Marsh Cudweed (Gnaphalium 
iiliginostim) from the same locality, 
showing individual variation. All 
three are in fruit. (Natural size.) 



376 



VARIATION 



(Fig. 220). The larger the number of individuals taken into 
account, the smoother the outline of the curve. Such variation 
curves are most commonly symmetrical (Fig. 220, left), but they 
may be one-sided or asymmetrical (Fig. 220, right), as in the case 
of the meristic variation of the corolla-segments of many flowers, 
where there are often relatively few examples with less than the 
normal number of parts. 



The 
closely 
chance, 



lal svmmetrical curve of variation agrees very 



with that 
a feature 




representing variation depending on pure 

which in itself suggests that the manifold 

differences in the conditions of 

; the environment are involved. 

The classical example of such 
chance variation is afforded by 
the repeated tossing up of two 
coins, the combination of one 
head and one tail being most 
frequent (forming about 50 per 
cent, of the cases), whilst the 
combinations two heads or two 
tails occur with about equal in- 
frequency. A more iUustrative 
curve of chance variation would 
be obtained if ten coins were 
tossed simultaneously for a large 
number of times in succession. 

In cases of meristic variation 
it will be noticed that the differ- 
ence between the extremes is much greater when the mode 
corresponds to a large number than when it coincides with a 
small one. Thus, if variation due to fission of parts (saj- of the 
corolla of a flower) be equal in two species, the one having a 
mode of five {i.e. usually five petals) and the other ha^•ing a 
mode of ten {i.e. usually ten petals), there would be just double 
the chances of fission occurring in the latter as in the former. 
The extreme condition [i.e. 10 and 20 petals) would be attained 
where all the petals imderwent fission. That is, the range would 
be greater in the one case than in the other, although the actual 
frequency of fission of the individual segments was the same 



Fig. 2ig. — Seeds of different varie- 
ties of Castor Oil {Ricimis), to 
illustrate variation witliin a 
species. [Photo. E. J. S.] 



INHERITANCE IN PURE LINES 



377 



4-000. 



.,75,000 



2000 _ 



in the two species. Hence a comparison of variation in two 
cases, where the modes are dissimilar, can only be made by 
taking such dissimilarity into consideration, a comparison of 
the curves alone being misleading. The position of the mode 
can often be to some extent changed by modifying the environ- 
mental conditions, 
although the range of 
variation remains 
practically unaltered. 
It appears that 
individual variations 
are not inherited, and 
that in a pure line 
of descent the average 
of the race is main- 
tained. ^ This has 
been established by 
sowing seeds from a 
single individual of 
pure descent which 
has been self-ferti- 
lised, the seed pro- 
duced by the result- 
ing plants (likewise 
self-fertilised) being 
sown in separate 
groups, and the same 
procedure followed 
for several successive 
generations. It was 
thus found that the 
average size of in- 
dividuals derived from small parents is practically identical 
with that of individuals growai from tall parents ; also that 
heavy seeds do not beget heavier-seeded offspring than those 
derived from light seeds. The following data, which serve 




J 



4 56 7 5^10 
Perianth se&? 

(HlMEMOlME) 
G. U. YULE 



Fig 



10 \5 20 

RPvYS OF STIG^AR 

(PnPRVEFk) K.TEHRSON 



2 20. — Variation curves, symmetrical oa 
the left and asymmetrical on the right. 



' By a pure line is understood a pure-bred strain produced by self- 
fertilisation from a single individual. 



378 SELECTION AND THE STRUGGLE FOR EXISTENCE 

to illustrate this point, arc taken from Johansen's experiments 
with Beans which were self -pollinated : 



Weight of seeds 


Average wei 


ght of seeds 


of parents. 


of ofispring. 


350-400 mg. 


• 572 


mg. 


450-500 ,, 


■ 535 


,, 


500-550 „ 


• 570 


,, 


550-600 ,, 


• 565 


, . 


600-650 ,, 


■ 566 


,, 


650-700 ,, 


■ 555 


, , 



When dealing with a population consisting of members 
possessing varied hereditary constitution {i.e. not a pure line), 
a pure strain can often be obtained by selecting individuals 
which show a particular desired character and breeding from 
these, with due precautions against cross-pollination. In this 
way it might, for example, be possible by artificial selection to 
obtain pure lines, exhibiting a tall or dwarf habit. Lhider natural 
conditions the environment may often exercise such a selective 
influence (so-called natural selection). The possession of a par- 
ticular character is sometimes decisive in determining which 
plant shall survive in competition with others, against adverse 
conditions of climate, etc. Even in a pure strain, some selective 
action might lead to the elimination of all but the heaviest, 
shortest, etc., individuals in each successive generation. This 
was the essence of Darwin's theory of the origin of new species. 

The majorit}/ of plants produce such an abundance of spores 
or seeds, as the case may be, that even with the most efficient 
dispersal it would usually be impossible for all the offspring to 
become established. Many a common plant would, indeed, if 
the means of dispersal were adequate, and all the progeny of 
successive generations survived, rapidl)' co\'er a large surface 
of the globe. That this does not happen is due to that ceaseless 
competition of living organisms with one another which is known 
as the struggle for existence. A Mullein plant, for example, may 
produce as many as 700,000 seeds. The resulting seedlings are 
not of equal vigour, and those first to succumb in the struggle 
for existence are obviously the weakest. The latter, applying 



ADi\PTATION 



379 



the term in its widest sense, owe their lack of vigour partly to 
adverse environmental conditions and partly to inherited char- 
acteristics. Just as a human being with a " weak constitution " 
may have inherited his defect or owe it to the surroundings in 
which he grows up. 

Unlike most animals, plants cannot choose their place of 
habitation. The seeds or spores are carried passively to a 
variety of situations, and their chances of developing into mature 




Fig. 221. — Alpine {A.) and lowland (L.) forms of the Rock Rose [Helicm- 
theinum vulgare). (After Bonnier.) 



plants depend upon their power of accommodating themselves 
to the environment in which they may be placed. But many 
plants exhibit this power of adaptation to a very marked degree. 
as is well illustrated by those aquatics which can grow either 
totally submerged or on exposed mud (cf. p. 175), by the sun- 
and-shade-forms of woodland plants (p. 168), and by the Alpine 
forms of lowland plants (p. 170, Fig. 221). 

It has long been a matter of dispute as to whether or no 
the changes, impressed upon an organism by the environment, 
bring about any corresponding internal modification, by means 



38o MUTATION 

of which the new characters can be transmitted to the offspring. 
So far all attempts to demonstrate satisfactorily the inheritance 
of such acquired characters have proved a failure. It is indeed 
difficult to conceive of any plausible means by which characters, 
acquired by the plant in the course of its lifetime, could affect and 
modify the hereditary mechanism which must obviously be con- 
tained within the fusing gametes. On the other hand, the marked 
adaptations of plants in the wild state to their normal environ- 
ment, features often retained when the organism is transferred 
to another habitat, naturally suggest the possibility of the 
unstable acquired characters becoming ultimately stabilised. 
This view has led to the conception of the origin of new species, 
etc., through cumulative selection and hereditary transmission 
of favourable variations (or mutations, cf. below), tending 
towards more complete harmony between the plant and its 
environment. Another point, upon which there is much differ- 
ence of opinion, is the actual influence of the environment in 
moulding the structure of a plant. Whilst some consider that 
adaptation to the environment is due to selection, others believe 
in a direct response to changed surroundings. 

Seeing that the individual variations above considered are 
all encompassed within the apparently fixed range of the species 
(cf. p. 377) , they could scarcely seem to have led to the evolution 
of new forms. But in carefully selected and self-fertilised 
cultures of the higher plants, and even in pure cultures of lower 
organisms, it has been found that slight or pronounced departures 
from the mode occasionally arise which breed true to their new 
characters from the very first, i.e. these are hereditarily trans- 
mitted. Such mutations, which are sometimes far more pronounced 
than the individual variations, and consequently obvious even 
to superficial observation (Fig. 222), may well be responsible 
for the origin of new species. As to the causes of mutation 
we are, however, in complete ignorance, though it is tempting 
to assume that the external environment is the stimulus that 
brings about the internal change. If this could be experi- 
mentally proved, many of the divergent views at present held 
could be harmonised. 

A familiar instance of a mutation is afforded by the Irish 
Yew [Taxus baccata, van fasiigiaia), which differs from the 



MUTATION 381 

Common Yew, from whence it arose, in its darker foliage and 
cypress-like habit. Detailed studies of mutations were first 
made by De Vries on a species of Evening Primrose {CEnoihera 
lamarckiana) , which occurred as an escape in a field near Amster- 
dam, where it was found producing a considerable number of 




Fig. 222. — A new form or mutant which arose from a pure strain of 
Duke of Albany Pea and which differs in the narrow lanceolate sti- 
pules. The type on the right, the mutant on the left. [By per- 
mission of Prof. Bateson, F.R.S., and IVIiss Pellehcw.] 



new forms, which subsequently bred true. For instance, one 
form was distinguished by the possession of broad leaves, another 
by red-veined leaves, others by dwarf- or giant-habit, and 
so on, as the following epitome of some of De Vries' cultures 
shows. 



HYBRIDS 



Mutation tn (JLnothera lamarckiana. 

(The horizontal lines show successive generations, invariably obtained 
onljf by sowing seeds of the ordmary form.) 





Giant 




l^oL-in. 


I 


— 


II 


— 


III 


. — 


IV 


I 


V 


. — 


VI 


— 


VII 


— 



White- 
leaved 
Form. 


Oljlong- 
leaved 
Form. 


Rcd- 
vcined 
Form. 


Xormal 
Form. 


Form. 


Broad- 
Icavc-d 
Form. 


— 


— 





9 





— 


— 


— 


— 


15,000 


5 


5 


— 





I 


10,000 


3 


3 


15 


176 


8 


14,000 


60 


73 


-5 


135 


20 


8,000 


49 


142 


II 


29 


3 


1,800 


9 


5 


— 


9 


— 


3,000 


II 


— 



Since it has been snggestcd that De Vries' mutants sprang 
from an original!}- h}-brid stock (cf. below), it may be mentioned 
that other instances of mutation have since been described in 
which the pure-bred character of the original strain appears to 
be beyond question. Moreover, mutations do not occur only in 
plants raised from seed, where as a consequence two gametes 
are involved. The propagation of vegetative mutations, or sports, 
which first appear only on a few branches of the plant, has 
given rise to the Copper Beech, and to the cut-leaved varieties 
of Alder, Beech (Fig. 223), etc. 

Another mode of origin of new forms is due to the recom- 
bination of characters which takes place when two different 
races, varieties, or species are employed as the respective parents. 
In such cases the fertilised eggs are, of course, produced by the 
participation of both parents, one furnishing the female and the 
other the male sexual cell. Such " crossing " is often a failure, 
no seed being set, but when successful the next generation is 
found to show a mixture of the characters of the two parents, 
some belonging to the one and some to the other. 

In respect to any single contrasting feature {e.g. height, hairi- 
ness, etc.), however, the character of one or other parent fre- 
quently altogether predominates {i.e. the offspring all show the 
one feature), although sometimes the hvbrid is intermediate 
between the two (cf. p. j&y). Since the characteristics of both 
parents must luux been inherited, those of the one must, in 
the former case, be supposed to remain dormant, or, as is 
usually said, latent. As a matter of fact, if such hybrid plants 



HYBRIDS 



3% 



are self-fertilised, the latent character reappears in a certain 
number of the individuals arising from the resulting seeds, a 
fact which proves that it must have been present, although not 
outwardly manifest. 

An exact study of the phenomena of hybridisation was first 
undertaken by Mendel, an Austrian monk, by considering the 
behaviour of single pairs of characters only. In one of his 
earliest experiments he crossed tall and dwarf Peas, the whole 
of the resulting 
offspring (first 
generation) be- 
ing tall. The 
seeds produced 
from these, by 
self-fertilisation, 
afforded two 
classes of in- 
dividuals, three- 
quarters being 
tall and one- 
quarter dwarf. 
But of these 
tall Peas of the 
second genera- 
tion (if again 
self-fertilised) 
only one-third 
bred true, the 
remainder be- 
having just like 

the original hybrids of the first generation, and giving rise 
to tails and dwarfs in the proportion of three to one. The 
dwarfs in every case bred true. In the second generation of 
such a cross, then, as regards any one particular pair of charac- 
ters, half the offspring are pure (one quarter resembling one 
parent, another quarter the other parent), whilst the other half 
are impure and bear the characters of both. Representing the 
tall Pea by T and the dwarf Pea by d, these results can be 
graphically expressed as follows : 





Fig. 223. — Twigs of A, ordinary Beccli, and B, cut- 
leaved Beeeli, illustrating a vegetative mutation. 



384 HYBRIDS— ALLELOMORPHS 

First generation : Td (all tall). 

Second generation: iT : id: 2Td (one tall, one dwarf, two 
hybrids). 

A considerable number of other pairs of characters have been 
stuched in the same way, with identical results, though there 
are a number of exceptions, some of which are at present 
inexplicable. 

The latent character is usually spoken of as the recessive 
and the other as the dominant. In the experiment with tall 
and dwarf Peas above described, where the difference is one of 
size, it is believed by many that the dwarf habit is due rather 
to the absence of a character causing talluess than to the presence 
of a special character for dwarfness. And it may well be that 
in all cases the recessive character is caused b}' the absence of 
something which determines the dominant {e.g. a glabrous form 
of a particular plant may be due to the absence of a character 
for hairiness, etc.). Pairs of characters, which thus combine in 
the hrst hybrid generation and segregate in the subsequent 
generations, are termed allelomorphs. The following are further 
examples, the dominant character in each case being that first 
named. 

Yello'w and green, round and wrinkled seeds in Peas. 
Prickly and smooth fruits (Thornapple, Pdeld Buttercup). 
Susceptibility and resistance to Rust in Wheat. 
Starchy and sugary endosperm in Maize. 
Hard (glutenous) and soft (starchy) endosperm in Wdieat. 

A feature which may be of great significance is that almost all 
the mutants so far studied behave as rcccssi\-es to the parent 
stocks from which they sprang. It mav be added that recent 
research has shown that many apparently' simple characters can 
be analysed into a number of subordinate ones, which, however, 
are very commonly inherited together. 

The most important principle, demonstrated by the experi- 
ments of Mendel and subsequent workers in this field, is that 
the characters introduced by either parent do not become in- 
extricably intermingled in the hybrid offspring, but become 
separated out again in the gametes ; that is to say, each of 



HYBRIDS, SEGREGATION 



385 



the latter bears only one allelomorph. It is only on this basis 
that the reappearance of a latent character in the second genera- 
tion can be explained. If the dominant and recessive characters 
of an aUelomorphic pair are represented by A and a respectively, 
then the fertilised eggs produced by crossing will all have the 
constitution Aa, and the resulting plants will all show the 
dominant character only ; such individuals are spoken of as 
heterozygotes. It is believed that in these hybrids, during the" 
formation of each tetrad of spores, and in conjunction with the 
reduction division (see p. 306), the characters A, a become 
separated, so that each spore, and consequently each of the 




Fig. 224. — Geum rivals (A), Geuni urbanum (C), and the hybrid between 
them, Geum intermedium (B), In each case the receptacle and calyx 
arc shown, together with an enlargement of a single carpel. 

gametes, to which it ultimately gives rise, bear either A or a. 
It is probable, moreover, that the determinants for these charac- 
ters are contained within the nuclei of the sexual cells (p. 390). 
During self-fertilisation, and assuming the gametes to meet 
according to the laws of chance, there are four possible com- 
binations : viz. (i) a male gamete bearing A and a female bearing 
A; (2) a male bearing A and a female bearing a; (3) a male 
bearing a and a female bearing A ; and (4) a male bearing a and 
a female bearing a. That is to say, among each four fertilised 
eggs that result, there will probably be i AA, 2 Aa, and i aa. 
Seeing that A is invariably dominant, there will in the second 
generation be three individuals with the dominant to one indi-' 
25 



386 HYBRIDS, SEGREGATION 

vidual showing the recessive character ; of the former, however, 
two will be heterozygotes, which would exhibit segregation 
according to the same principle in the ensuing generation, whilst 
the other, as well as the individuals possessing the recessive 
character, are homozygotes. 

Further evidence, in support of this hypothesis, is furnished 
by the result of crossing the hybrid of the first generation 
with one or other parent (i.e. Aa x AA or Aa x aa). In 
this case only two kinds of combinations will be obtained, 
viz., taking the case in which the hybrid is crossed with the 
parent-form having the dominant character, AA and Aa, and 
there are obviously equal chances for either combination to 
occur. The second generation of such a cross does, as a matter 
of fact, afford individuals half of which are hybrid and half 
pure. Moreover, when it is recalled that the endosperm of 
Angiosperms develops as the result of a nuclear fusion (p. 368), 
it is of interest to note that, if varieties of Maize possessing 
different types of endosperm (i.e. variously coloured or con- 
taining sugar and starch respectively) are crossed, that of the 
resulting seeds exhibits evidence of its hybrid origin. 

If two pairs of characters are considered, it has been found 
experimentally that, whilst the first generation consists only of 
hybrids showing both dominants, the second comprises a number 
of distinct individuals occurring in the following proportions : 
9 with both dominant characters, 3 with one dominant and one 
recessive, 3 with the other dominant and the other recessive, 
and I showing both recessives. The relations wiU be plain if 
the diagram in Fig. 225 is studied. Thus, if yellow round Peas 
are crossed with green angular ones, the first generation all bear 
yellow round Peas ; in the second generation there will be 
9 yellow round, 3 yellow angular, 3 green round, I green 
angular. Of the nine individuals showing both dominant char- 
acters one only breeds true, as also does the one bearing both 
recessive characters. The remainder, on being self-fertilised, 
exhibit segregation according to the particular characters which 
they contain. 

The dominance of a character is only important for the 
elucidation of the observed facts in the many cases where 
dominance occurs. But the principle of segregation is equally 



HYBRIDS, SEGREGATION 



387 



applicable where the heterozygote is intermediate between the 
two parents, or, as in some cases, even differs from both. 
A hybrid intermediate between the two parents is obtained, 
for instance, when the Star and Chinese Primroses are 




Fig. 225. — Scheme to illustrate the progeny of the second generation 
when two pairs of characters {A a, Bb) are taken into consideration. 
The individuals of the first generation will all have the constitution 
AaBb. When segregation occurs, the resulting gametes will have 
the four possible constitutions AB, Ab, aB, ah} and the scheme shows 
the possible methods of combination of such gametes. When both 
dominants are present (in nine out of the sixteen cases) the squares 
are cross-hatched, when the dominant A is present the shading slopes 
from left to right, when B is present from right to left ; when neither 
dominant occurs the square is left unshaded. 



crossed. The petals in the first generation are intermediate In 
character between those of the two parents (Fig. 226) ; in the 
second generation segregation takes place in the usual way, 
the hybrid-individuals (one-half) stiU exhibiting flowers of the 

' The combinations Aa and Bb do not arise, since dominant and reces- 
sive characters are segregated. 



388 



HYBRIDS, SEGREGATION 



intermediate type, whilst the remainder consist of Star and 
Chinese Primroses in equal numbers. 

Hybridisation experiments not only teach us the principle of 
segregation of characters, but also emphasise the fact that 
external appearance is no certain guide to the internal charac- 
teristics. This is perhaps most strikingly illustrated by the 
effect of crossing certain pure-bred strains of white-flowered 
Sweet Peas. We should naturally expect all the offspring to 




F2.. 



Fig. 22C. — Hybrids between Chinese [Primula sinensis) and Star Primroses 
(P. stellata). At the top the two parents, the Chinese (C) with large, 
rather wavy, much crenated petals, and the Star (5.) with smaller flat 
petals exhibiting only a notch. The Fi generation is intermediate 
between the two in these respects. In the F2 segregation takes place 
in the usual way. (After Punnett.) 



be white-flowered too, but in reality all have coloured flowers, 
with a purple standard and blue wings. Here, clearly, the parents 
possess characters which in combination lead to the production 
of a coloured flower, though when separated they produce no 
visible effect. By breeding plants the limited knowledge of the 
constitution of any organism, which is obtained by mere exami- 
nation of external or internal structure, can thus be to some 
extent supplemented. 

When two varieties cross in nature, the appearance of the 
hybrid is of course influenced by all the characters present in 



GRAFT-HYBRIDS 389 

the parents. Whether it resembles one or other parent, or is 
roughly intermediate between them, will depend on whether 
one parent contributes a greater proportion of dominant, or 
more conspicuous, characters than the other. Common examples 
of such hybrids in the wild state are afforded by various Willows, 
Getim intermedium (Fig. 224, B), Quercus intermedia, etc. 

It has long been familiar to gardeners that shoots, which show 
a mingling of the characters of scion and stock, are sometimes 
produced as a result of grafting. The intermediate nature of 
these so-called graft-hybrids appears to be due to the fact that 
both scion and stock contribute to their development, the tissues 
of the one forming a skin over those of the other. The character 
of the seeds, and of the resulting offspring, is determined by the 
plant responsible for the formation of the subepidermal layer 
from which the archesporial tissue arises. This explains the 
ffact that the seeds of graft-hybrids always breed true to the 
characters of either scion or stock. The commonest example of 
such a graft-hybrid is that known as Cytisus adami, which is due 
to the grafting of the Purple Broom [Cytisus pttrpureus) on the 
Laburnum (C. laburnum). 

It has been repeatedly noted that the fusion of the nuclei 
of the gametes appears to be the most important step in sexual 
reproduction. This is supported by the fact that, in all the 
higher plants, the male cell consists of little else than the nucleus 
(cf. pp. 350, 368), and that, in hybrid-experiments, it is im- 
material whether the one or the other parent is employed as 
the male.' The nucleus has also been seen to play a very im- 
portant part in the activity of the cell, and, when dividing, to 
pass through a very complex series of changes. These have as 
their outcome an equal distribution of the chromatic material, 
which indeed is the only part of the nucleus that remains 
recognisable throughout all the phases of division. It may 
therefore be reasonably supposed that, in some way or other, 
the chromatin is the carrier of the hereditary qualities of the 
organism. 

Sexual fusion may then be regarded as operating in two 
ways, firstly as a stimulus leading to further development, 

1 Some forms of CEnothera and Epilobium appear to constitute aq 
exception to this generalisation, 



390 SEXUAL REPRODUCTION 

secondly as a possible means of introducing new characters or 
of rearranging those already present in the two parents. The 
former result may be attained by other stimuh. Amongst 
animals, for instance, the eggs of the Frog have been caused 
to develop bj^ mere pricking with a glass needle covered with 
blood-serum, and those of Sea-urchins by treatment with solu- 
tions having a higher osmotic pressure than sea-water. Amongst 
plants the polyembryonic seeds of the Orange (cf. p. 372) furnish 
an analogous example ; here certain cells, apart from the ferti- 
lised egg, have developed into embryos, but there is no evidence 
that the apogamously produced seedlings are any less vigorous 
than those resulting from sexual fusion. It seems probable, 
therefore, that the chief advantage of sexual reproduction lies 
in the possibility of producing organisms, with a slightly different 
hereditary constitution, such as may survive under conditions 
that would be unfavourable to the pure parent strain. In 
other words, sexual reproduction provides material upon which 
natural selection can operate. 

The segregation of characters above referred to is most prob- 
ably effected during the reduction division in the spore mother- 
cells. In this process the chromosomes, instead of splitting 
longitudinally, as in the vegetative divisions, separate in their 
entirety into two sets. These pass to the respective daughter- 
nuclei, so that, of the four resulting spores, two possess the 
characters borne by one set, and two those borne by the other 
set. If this hypothesis be true, the two separate sets of chromo- 
somes probably represent the paternal and maternal chromatin 
respectively. In the case of an allelomorphic pair, it is assumed 
that the dominant character is present in one set and the re- 
cessive in the other. In the reduction division, therefore, the 
allelomorphs will become separated, so that pure recessives and 
pure dominants can be bred. Such a theoretical conception is 
incapable of proof, and it is only warranted because it tallies 
with the observed facts. 

In vegetative propagation the offspring normall}^ exhibit no 
change of character, as compared with the parent, and new 
forms can only arise bj' mutation. Mutations in vegetatively 
produced offspring, and even in certain branches of an individual, 
have indeed been occasionally observed (cf. p. 3S2), and may 



GEOGRAPHICAL DISTRIBUTION 391 

be the means of maintaining the race in harmony with its 
environment. Although vegetative mutation appears to be 
comparatively infrequent, it should be borne in mind that groups 
like Bacteria and Cyanophycese, which multiply exclusively by 
vegetative means, often do so with great rapidity. An excep- 
tionally large number of generations is therefore formed in a 
short time, and so presumably the opportunities for mutation 
are proportionately great. 



Intimately bound up with the subject of the origin of new 
forms, considered in the preceding pages, is that of the geo- 
graphical distribution of plants. This is mainly related to differ- 
ences of climate, the distribution of a species often depending 
on its tolerance of cold or heat, dryness or moisture, etc. Under 
more or less uniform climatic conditions, such as obtain in our 
own country, soil differences play a large part in determining 
the regional distribution of plants. Most frequently it is the 
physical properties of the soil that are important in thus modi- 
fying the effects of climate. Of two areas receiving the same 
annual rainfall, for instance, that with sandy soil may be a 
desert, whilst that with peaty soil forms a bog. 

The effect of climate is weU illustrated by the distribution 
of forest, grassland, and desert over the earth's surface. In 
general, forest occupies the regions of highest, and desert those 
of lowest, rainfall, whilst grassland occurs in areas of medium, 
rather uniformly distributed, rainfaU ; the influence of man or 
of winds may, however, profoundly modify the applicability of 
this generalisation. In northern latitudes forests are composed 
of such trees as Birch, Pine, and Spruce, whilst in temperate 
zones deciduous trees, such as Beech, Oak, and Ash, predominate. 
On the slopes of high mountains, like those of Switzerland, the 
same relative distribution occurs, with deciduous forests below 
and Coniferous ones above. 

But, apart from such cases in which the range of species is 
limited by their inability to withstand competition under adverse 
climatic conditions, certain species, genera, or even families are 
found to be restricted to particular areas of the earth's surface, 
though others are equally well suited to their existence. This 



392 



GEOGRAPHICAL DISTRIBUTION 



fact is constantly being illustrated by the successful spread of 
species after their first introduction into a country. For example, 
the Canadian Pondweed {Elodea canadensis), which was brought 




K.J.S. 



BeXaiLim iHoXlani 

1560 
&evmau>^ 1565-67 



Fig. 227. — Map showing tlic spread of the Canadian Pondweed {Elodea 
canadensis) after its appearance in England in 1S47. Tlic broken 
lines connect identical years. 

to England from America about 1847, now occupies almost every 
waterway in this country (cf. Fig. 227). A similar phenomenon, 
on land, is presented liy the Prickly Pear in Australia. In 



GEOGRAPHICAL DISTRIBUTION ,393 

isuch cases it must be supposed that the ordinary agencies of 
dispersal have not sufficed to bring the species to all regions in 
;which it can thrive successfully. 

The natural barriers to the spread of plants are oceans, 
mountain ranges, deserts, etc., and it is significant that isolated 
islands {e.g. New Zealand) are peculiarly the home of species, 
iso-called endemics, which are found there and there alone. There 
is, moreover, good reason for believing that, in a region devoid 
of natural barriers, the area over which a species is distributed 
is proportional to the age of the species or the time that has 
elapsed since its introduction. The capacity of a species for 
extending its geographical range, when the new territory is 
attained, depends on the efficiency of its seed-dispersal mechanism 
and the rapidity of its spread by this or by vegetative means. 

The theory of evolution explains the resemblances between 
the members of a genus or family as the necessary consequence 
of their origin from a common ancestor or from closely related 
forms. It has already been seen how new types can arise as a 
result of mutation or hj'bridisation. Since these show a great 
resemblance to their known parents, it is reasonable to regard 
the many features in common, between species of a genus or 
between the genera of a family, as indications of a natural affinity 
between them. What has already been said, then, with regard 
to the distribution of species should also apply in a general way 
to genera and families, if these really comprise groups of forms 
with natural affinities. In many cases, indeed, the same prin- 
ciples are applicable ; for example, the genus Commidendron (a 
member of the Composita;), with three species, is restricted to 
St. Helena, and whole families are sometimes largely confined 
to definite areas, as the Epacridacea (which are closely allied to 
the Heather-family) to Australia and Tasmania (cf. also Fig. 228). 

Geological research has shown that oceans and continents 
have undergone manifold changes, even during the period of 
existence of many living species, and these secular changes 
probably afford the clue to the discontimwus distribution of many 
species and families. For certain groups, now represented only 
in widely separated areas over the earth's surface {e.g. the Cycads), 
are known to be ancient, and may well have attained their wide 
distribution before the present barriers were as pronounced or 



394 



EVOLUTION 



had even become established. A similar instance is furnished 
by the occurrence of the same Arctic species on the tops of many 
high mountains, although the present-day climate of the plains 
offers an insurmountable barrier to their dispersal from one 
chain to the other. These plants may, however, well represent 
the last remnants of a widespread flora of glacial times which, 
when the ice-sheet melted, found suitable conditions only on 
the mountain summits. 

The conception of evolution regards the organic world, as we 
find it to-day, as consisting not of a number of immutable forms, 
but as presenting one phase in an everchanging series. The 




Fig. 228. — Outline map of the world, showing the distribution of Caly- 
canthaceae (indicated by shading), Proteaces (black), and Resedacea; 
(dotted). 



organisms of the present are the offspring of those of the past, 
and will themselves, in turn, give rise to the organisms of the 
future. Those animals or plants which have become extinct 
must be supposed to have failed to " make good " in the com- 
petitive struggle. From the fossil records it is known that 
whole floras and faunas have thus perished, leaving no living 
representatives or only much modified descendants (cf. Chapters 
XXI, XXII). Such disappearance may well be an outcome of 
the secular, but none the less profound, changes that have 
marked the history of the earth's surface since life first appeared. 
Organisms, unsuited to the new conditions, would inevitably 



EVOLUTION 



393 



perish as a result of being handicapped in competition against, 
either their more adaptable contemporaries, or new forms which 
were better equipped to withstand the changed environment. 
The evolutionist conceives of all life as having developed 




Fig. 229. — Seedling of an Acacia, showing the gradual differentiation of 
phyllodes, and suppression of the laminae, in leaves of successive ages. 

from relatively simple undifferentiated unicellular organisms, 
many of whose descendants, through the ages, have gradually 
acquired an increasing complexity of structure in relation to 
progressive division of labour. On this hypothesis, if our know- 
ledge of all the extinct plants of former eras were complete, we 



-396 EVOLUTION 

could reconstruct the genealogical history (phylogeny) of the 
present-day forms. Those of the past would furnish the links 
connecting genus with genus, and family with family, till all 
were connected up to the primitive simple organisms from which 
they sprang when life first developed. The fact that many fossil 
plants and animals do combine characters, that now serve to 
distinguish separate groups, is one of the strongest pieces of 
evidence for such a conception. But, further, the geographical 
distribution of living plants and animals, showing, as already 
noted, restriction of similar species to definite areas, is most 
readily explained as due to their origin, in that area, from common 
or closely related parents. 

The features characteristic of the various members of a genus 
or family are often only fully apparent in the adult state, whilst 
the earlier the stage of development of an organism, the more 
difficult does the determination of its identity become. The his- 
tory of the individual may be regarded as, to a limited extent, 
recapitulating the history of the race ; in this connection it may be 
noted that in the ordinary course of reproduction every individual 
commences life as a single cell. Such an interpretation also 
explains the frequent occurrence of rudimentary structures {e.g. 
the gill-slits in the embryo of the Chick, or the trefoil leaves in 
the seedling stages of species of Acacia possessing phjdlodes. 
Fig. 229), which often perform no function, or are even com- 
pletely lost, in the adult. Even amongst living organisms a 
graduated series, as has been seen in Chapters XIII to XXIV, 
can be recognised. The simplest members of this series are 
doubtless relics, though almost certainly modified, of the earliest 
flora and fauna, which have found a place in the economy of 
nature even under existing conditions. 

The highest efficiency is only attained bv great specialisation 
which proportionately diminishes the capacity for adaptation 
to a new environment. But the world of living things is a world 
of never-ceasing change, and hence the past history' of the organic 
universe is the history of extinction of specialised races and 
individuals. The future of a group is thus seen to be dependent 
upon its less specialised, and thus at the moment less successful, 
members. But as in time, so too in space, the spread of a species 
may be handicapped by its lack of plasticity. . - 



EVOLUTION 397 

The brief review of the groups here given is sufficient to 
indicate that the extinct Clubmosses and Horsetails, in the era 
of their success, were more specialised and more complex than 
their present descendants, which play so subordinate a role, and 
probably owe their survival to features which characterised 
the less successful members of these groups in the past. Such 
considerations lead one to suspect that the subordinate groups 
and individuals of any one age are the most likely starting-points 
for the dominant vegetation of the next, and so we can understand 
why the fossil record presents us with abundant examples of 
clearly defined groups {i.e. of the prevalent successful forms) and; 
comparatively few representatives of groups " in the making."'. 
Indeed, the fossil plants of past ages and the living organisms' 
of to-day combine to emphasise the rarity of the " missing link " 
which, like the thinker in advance of his age, is not sufficiently 
in harmony with the environment to command success, buti 
yet marks the beginnings of the fades of the future. It is not, 
therefore, surprising that our progress in the reconstruction. 
of the genealogical tree of the Vegetable Kingdom is slow, and , 
that many of the groups remain in striking isolation from one 
another. 



APPENDIX 

I. The Compound Microscope (Fig. 230). — This consists essenti- 
ally of a stand which has for its purpose the appropriate support 
and adjustment of the optical parts. The latter comprise two 
systems of lenses, known respectively as ocular [Oc.) and objective 
[Ob.), whilst accessory structures are constituted b}' a mirror (M.) 
for reflecting light, and, in better instruments, a condenser {Co.) 
for concentrating light upon the object. 

The stand consists of a heavy /oo^ or base {Fa.), bearing 
a rigid upright pillar (L.). To the latter the remaining por- 
tions of the microscope are hinged in such a way that the whole 
can be employed either in a vertical or in an inclined position. 
The part actually hinged to the piUar is known as the limb, 
and to this the flat stage (S.) is attached, at right-angles, at the 
lower end, and the body tube (T.) towards the upper end. The 
stage, which is usually square, is perforated by a central aperture 
through which the light from the mirror reaches the object. In 
transferring the microscope from place to place, it should always 
be carried by the non-movable parts. 

The body tube is a hollow cylinder, usually of brass, and is 
adapted to take the ocular or eyepiece [Oc.) at its upper, and 
the objectives (Ob.) at its lower, end. In order to secure rapid 
change of magnification, a nosepiece {N.), bearing two or three 
objectives, is screwed into the lower end of the body tube ; by 
simply turning this a different objective can be brought to bear 
on the object. The distance between eyepiece and objective 
can be increased or decreased by pulling out or pushing in the 
draw-tube {D.t.), which is fitted into the upper end of the body 
tube. In order to focus the object clearly, the entire system of 
lenses can be moved nearer to, or farther from, the stage by means 
either of the coarse or fine adjustments. The coarse adjustment 

398 



THE COMPOUND MICROSCOPE 



m 



usually consists of two large milled heads (C.) on either side of 
the limb, and by turning either of these, which actuate a rack 
and pinion mechanism, a relatively large movement is brought 
about. The fine ad- 
justment is generally 
operated by a single 
milled head (F.), 
situated at the top 
of the limb, and by 
this means a very 
slight movement is 
effected, enabling 
great accuracy of 
focussing to be at- 
tained. 

Below the stage 
is fixed an adjust- 
able aperture, the 
diaphragm (D.), by 
means of which the 
amount of light 
reaching the object 
on the stage can 
be regulated. If a 
condenser is pre- 
sent, it is placed 
between the dia- 
phragm and the 
stage, and, in the 
best instruments, its 
distance below the 
stage is adjustable 
by means of another 
milled head (H.). 
The mirror (Af.), 
which is concave on 
one surface and fiat on the other, is either attached to the 
underside of the stage or (as in the type illustrated) forms part 
of the adjustable system bearing the condenser. 




Fig. 230. — Diagrammatic representation of a 
compound microscope, C, Coarse adjust- 
ment; Co., condenser; D., diaphragm; D.t., 
draw-tube ; F., tine adjustment ; Fo., foot ; 
H., milled head controlling substage adjust- 
ment for condenser; L., pillar ; M., mirror ; 
N., nosepiece ; Ob., objective ; Oc, ocular or 
eyepiece ; S., stage ; T., body tube. 



400' THE COMPOUND MICROSCOPE 

' The objectives most commonly in use are 5 in. and ^ in./ 
which designations imply that, when focussed upon the object, 
they are approximately two-thirds and one-sixth inch respec- 
tively from the latter. These distances are the focal lengths, and 
the smaller they are the greater is the magnifying capacity of the 
lens. As a consequence, the longer focus lens is often spoken of 
as the low power, and the short focus lens as the high power. The 
image formed by the objective is projected on to the eyepiece, 
where it becomes further magnified. The amount of magnifi- 
cation of the eyepiece is commonly indicated by a number 
engraved upon it. 

The object to be examined, mounted on a glass-slip in water 
or some other appropriate fluid and covered with a cover-glass, 
is placed on the stage, and light is projected on to it from below 
by means of the mirror. If a condenser is present, the flat side 
of the mirror is employed, but if not the concave side. To 
focus the object, gradually lower the tube by means of the coarse 
adjustment till the image becomes clear, and then turn the milled 
head to and fro until the image appears most distinct. Proceed 
in the same way when using the high power, but exercise the 
greatest care not to bring the objective in contact with the 
cover-glass, and immediately the image begins to appear use the 
fine adjustment only. The aperture of the diaphragm should 
be diminished tiU the maximum amount of detail is visible, 
whilst by adjusting the condenser the light reflected from the 
mirror can be accurately focussed upon the object. 

In working with the microscope, it is best to accustom oneself 
to employ either eye. When drawing, view the object with the 
left, and sketch with the aid of the right, eye. As a first exercise 
in microscopic observation, it is well to examine the small 
e^ir-bubbles almost invariably present in large numbers in a 
drop of water. Under (the low power these appear as black 
dots or bright patches with broad dark margins ; this dark border 
is due tp refraction. Adjust the shde so that one of the smaller 
bubbles is in the centre of the field of view, and turn the nose- 
piece so as to view the bubble with the high-power lens. Using 
the fine adjustment, it will be noted that at a high focus the 

1 For the study of Bacteria and other minute organisms, higher powers 
are required, such as /.j in. objectives. 



REAGENTS 4ot 

curved surface is seen and the outline appears shadowy, whilst 
at a lower focus only the circular equatorial portion (the optical 
section) is visible, and the outline becomes well defined. 

II. Reagents. — Details as to the mode of preparation of the 
principal reagents mentioned in this book are given in the fol- 
lowing : 

Ammoniatcd Copper Oxide [Cuprammonia). — This reagent must 
be freshly prepared. Add ammonium chloride, and subsequently 
excess of sodium hydrate, to a solution of copper sulphate. The 
blue precipitate produced is filtered and washed thoroughly, 
and then dissolved in a small quantity of strong ammonia. 

Eaii de Javelle (mainly potassium hypochlorite). — Mix 20 
parts of chloride of lime with 100 parts of water. Allow to 
stand, and then add a solution of 15 parts of caustic potash in 
100 parts of water. Filter after some hours and use the filtrate. 

Fehling's Solution (an alkaline solution of cupric oxide). — ■ 
According to Haas, this is best obtained by mixing equal quan- 
tities of a solution containing 69 '28 grams of pure crystallised 
copper sulphate in i litre, and of a solution containing 350 grams 
of Rochelle salt (potassic sodic tartrate) and 100 grams of 
sodium hydrate in i litre. The resulting solution is of a clear 
dark blue colour. Ten cubic centimetres of this solution are 
reduced by 0'05 gram of glucose. 

Iodine Solution. — This is made by dissolving crystals of iodine 
in a strong solution of potassium iodide. For use this is diluted 
to a light brown colour. 

Millon's Reagent. — This is a mixture of mercuric nitrate and 
nitrite. It can be prepared by dissolving 15 grams of mercury 
in 30 grams of cold nitric acid (sp. gr. i'42), which operation 
should be performed in a fume cupboard. Dilute with twice 
the volume of distilled water, and filter after two hours. This 
reagent can also be bought ready made from the usual dealers 
in chemicals. 

Phenylhydrazine Hydrochloride (after Mangham). — Prepare 
separate solutions, in ten times their weight of glycerine, of phenyl- 
hydrazine hydrochloride and sodium acetate respectively'. Place 
the material to be investigated in equal drops of these two solu- 
tions, thoroughly mixed, and, after covering with a cover-glass, 
heat for one to several hours in an oven. 
26 



^02 REAGENTS 

Phlorogliicin. — Prepare a saturated solution in alcohol. Treat 
material with this for a short time, and then mount in strong 
hydrochloric acid. 

Scharlack Red. — Prepare a saturated solution in a mixture of 
70 parts absolute alcohol and 30 parts water by volume. 
Filter and keep well stoppered. 

SiilpJiiiric Acid. — For cellulose tests it is usual to employ the 
concentrated acid. Great care must he exercised in its use, and 
strong ammonia should be at hand to neutralise any drops that 
may be spilled. 

in. Artificial Sea-water. — For this purpose Tidman's sea-salt, 
dissolved in distilled water in appropriate concentrations, can 
be employed ; or a solution can be made according to the following 
formula given by Osterhout : 

1,000 parts sodium chloride (gram-molecular solution). 
78 ,, magnesium chloride (gram-molecular solution). 
38 ,, magnesium sulphate (gram-molecular solution). 
22 ,, potassium chloride (gram-molecular solution). 
10 ,, calcium chloride (gram-molecular solution). 

This solution has an osmotic pressure of about 22^4 atmo- 
spheres ; when diluted with an equal volume of distilled water, 
the osmotic pressure is halved, when diluted with twice its 
bulk of distilled water the osmotic pressure is one-third of that 
of the undiluted solution, and so on. 

IV. Artificial Semipermeable Membranes. — For this purpose 
the membranous precipitate of copper ferrocyanide, formed when 
solutions of copper sulphate and potassium ferrocyanide meet, 
is often employed. The precipitate is customarily deposited 
in the wall of a small pot of unglazed porous porcelain, which 
is thoroughly washed so that it is impregnated with water. 
It is then nearly filled with a dilute solution of copper sulphate 
(2-5 grams per litre) and stood in a solution of potassium ferro- 
cyanide (2-1 grams per litre), where it is left for some time. 
Ultimately it is thoroughly washed and soaked in water. It is 
best to prepare several pots in this way, as some are sure to be 
faulty. To overcome this difficulty, Philip recommends depositing 
the copper ferrocyanide precipitate in a film of gelatine, formed 



ARTIFICIAL SEMIPERMEABLE MEMBRANES 403 

over one end of a glass tube by dipping it in 20 per cent, gelatine 
to which a little potassium bichromate has been added ; the 
latter has the effect of rendering the gelatine insoluble, if it is 
allowed to set in the light. After this the tube is again filled 
with the copper sulphate and the closed end allowed to dip into 
the potassium ferrocyanide solution until the gelatine has acquired 
the brown colour of the precipitate. 

Still another method consists in allowing the copper ferro- 
cyanide precipitate to form in a membrane of celloidin. This 
can be obtained by pouring a celloidin solution on a clean mercury 
surface contained in a Petri dish and allowing the solvent to 
evaporate away. The membrane which remains is then fitted 
over the open end of a thistle funnel, the overlapping portion 
being tied securely round the flange of the bulb. In drying, the 
membrane contracts slightly and becomes stretched taut. A 
very strong combination is obtained if two membranes of this 
kind are fitted over one another. 

A piece of pig's bladder, which should be thoroughly dried 
before use, or a piece of parchment, stretched over the end of a 
thistle funnel, is often quite effective as a semipermeable mem- 
brane. In all cases the solution whose osmotic pressure is to 
be determined is placed in one of these osmometers, and the whole 
is immersed in water in such a way that diffusion between the 
two solutions can only take place by way of the semipermeable 
membrane. The pressure can then be calculated from the 
height to which the solution rises within a glass tube fitted on 
to the osmometer, or by means of a suitable manometer. 

V. Equivalent Osmotic Pressures of Sea-water. — For the fol- 
lowing data, which refer to strengths of Tidman's sea-salt, we 
are indebted to Mr. F. M. Haines, B.Sc. ; 



Concentration of solution Osmotic pressure 


Concentration of solution 


Osmotic presaur 


in grams per loo 


c.c. in atmosplieres. 


in grams per roo c.c. 


in atmospheres 


O-OI 


o-o8 


o-og 


072 


O-02 


o-i6 


o-i 


0-8 


0-03 


0-24 


0-2 


1-6 


0-04 


0-32 


0-3 


2-4 


0-05 


0-40 


0-4 


3-2 


o-o6 


0-48 


0-5 


4-0 


0-07 


0-56 


0-6 


4-8 


o-o8 


0-64 


07 


5-6 



404 



PRESERVING AND STAINING 



Concentration of solntion 


Osmotic pressure 


Concentration of solution 


Osmotic pressure 


in j,aaras per loo c.c. 


in atmospheres. 


in grams per loo c.c. 


in atmosplreres. 


0-8 


6-4 


1-5 


1 1 -9 


0-9 


7-15 


1-6 


12-7 


i-o 


7-95 


1-7 


13-5 


i-i 


8-7 


1-8 


14-3 


1-2 


9-6 


1-9 


I5-I 


1-3 


IO-3 


2-0 


15-9 


1-4 


Ii-i 







The osmotic pressure for concentrations between those given in this 
table are proportional to the values between which the particular con- 
centration lies. 



VI. Preserving and Staining. — To preserve material for ana- 
tomical investigation, ordinary methylated spirit will usually 
serve, provided there is at least four times the volume of liquid 
as of material. For showing nuclear structure, however, other 
fixatives are employed (cf. Chamberlain, Metliods in Plant His- 
tology), e.g. acetic alcohol, made by adding one part of glacial 
acetic acid to four parts of alcohol. After remaining in this for 
a few minutes up to several hours, according to the texture of 
the material, the latter is transferred to ordinary spirit. 

For staining, the thinnest sections (cf. VIII below) should be 
placed in a few drops of safranin,' on a slide, for from five to 
fifteen minutes, more safranin being added at intervals to replace 
that lost by evaporation. The excess of the stain is now removed 
by washing the sections with spirit, and then a few drops of 
Kleinenberg's hcematoxylin are allowed to act for half a minute. 
After this the sections are washed with spirit, and permanent 
preparations are made in the following way : 

The spirit is changed several times, and finalh' replaced by 
a few drops of absolute alcohol. In this way dehvdration 
{i.e. removal of water) is effected. To the alcohol a drop or two 
of clove oil is then added, and this mixture is in turn replaced 
by pure clove oil. The sections should now become transparent, 
and, if this fails to occur, they have not been sufhciently de- 
hydrated. After two to three minutes the oil is poured off, 
and Canada balsam, dissolved in xylol, added. A cover-glass 
is then carefully let down on to the sections, and the slide placed 

' Or methyl blue can be used, the sections being left in this for about 
half a minute. 



PRESERVING AND STAINING 405 

on one side till the balsam sets. Throughout all these processes 
the greatest care should be taken that the sections are never 
without a covering of liquid. 

If permanent preparations are not required, such stains as 
phloroglucin, aniline chloride, etc., can be employed to differ- 
entiate the tissues, and the sections are usually mounted in 
glycerine diluted with an equal volume of water. Preparations 
can also be mounted in glycerine jelly, such mounts being much 
more rapidly and easily prepared than those with Canada balsam, 
but they often perish after a few years. 

The liquid stains most commonly employed are prepared as 
follows : 

Aniline Blue. — Saturated solution in alcohol or water, with 
a trace of acetic acid. 

Bismarck Brown. — Dissolve 2 grams in 100 c.c. of 70% alcohol. 
Stain for about i^ minutes. 

Eosin. — 1% solution in either water or alcohol. Stain for 
3 to 5 minutes. 

Gentian Violet. — I % solution in water. Stain for from 10 
to 15 minutes, transfer to alcohol, and quickly counterstain with 
Bismarck brown. 

Hematoxylin. — Best bought prepared ready for use. 

Methyl Blue. — Saturated aqueous solution. For live staining 
this is greatly diluted. 

Sa/ranin.—z% solution in 50% alcohol. 

VII. Measurement under the Microscope. — -This is accomplished 
by means of an eyepiece micrometer and a stage micrometer 
(both obtainable from the usual dealers in microscopic requisites) . 
The stage micrometer is an ordinary shde on which is mounted 
a millimetre scale, divided into tenths and hundredths, and 
obtained by photographic reduction. The eyepiece micrometer 
fits into the ocular, and consists of a scale that is usually 
divided into a hundred equal parts. For each objective the 
value of a division of the eyepiece micrometer is determined in 
terms of the divisions of the stage micrometer {i.e. in hundredths 
of a millimetre). If subsequently any object is measured with 
the eyepiece scale, its actual size can be calculated. 

VIII. Section-cutting.— Dm'mg this operation both razor and 
material should be kept moist with either water (for fresh 



4o6 SECTION-CUTTING 

material) or spirit (for preserved material). Hold the object 
between the thumb and first finger of the left hand, and arrange 
the tips of the remaining fingers so as to form a rest on which 
the razor blade can be glided backwards and forwards through 
the material. Note that the razor must be gently drawn through 
the object, and not pressed, as in ordinary cutting. The greatest 
care should be taken that the axis of the object is either at right- 
angles (for transverse sections) or parallel to the razor blade 
(for longitudinal sections). For the latter only a very short 
length of the stem, etc., should be used. When very thin objects, 
such as leaves, are to be cut transversely, small rectangular pieces, 
including a vein, are embedded in a vertical incision made in a 
short length of Elder-pith. Sections are then cut of the pith, 
as well as of the embedded object. 

After cutting, transfer the sections to a shde, on which a 
drop of water or dilute glycerine has previously been placed, 
by means of a well-moistened brush, and reject all but the two 
ihmnest. Complete sections are in most cases quite unessential, 
whilst the small fragments will usually be the thinnest. Oblique 
sections, even if thin, are quite valueless. The razor should be 
carefully cleaned by wiping it from the back towards the edge. 
IX. The following is an epitome of the main divisions of the 
Vegetable Kingdom : 
I. Cryptogamia. 
I. Thallophyta. 

Flagellata (organisms related to Algje, Fungi, and 

Protozoa). 
Bacteria. 
Peridineee (motile organisms of fresh and salt water, 

possibly related to Diatomacese). 
Algffi. 

[a) Cyanophyceas or Myxophycea (Blue-green Algs). 
[h) Chlorophyceje (Green Algje). 
(c) Phasophycea; (Brown Algfe). 
{d) Rhodophyceffi (Red Algiu). 
[e) Diatomacea: or Bacillaricce (Diatoms). 
Charales (Stoneworts). 
Myxomycetes (Slime Fungi). 
Fungi. 



CLASSIFICATION OF PLANTS 407 

, V -n, , /Zygomycetes (incl. Mticor). 

n Phycomj^cetes - „-^*' \ ,. \ -n ,i ■ i 

lOomycetes (mcl. Pythiitm). 

[b) Basidiom3'cetes. 

(i) Uredineffi (Rust Fungi). 

(2) Hemibasidii (Smuts and Bunt). 

'Hymenomycetes (Toadstools 



(3) Eubasidii 



and Mushrooms). 
Gasteromycetes (Puffballs, 

Stinkhorns, etc.). 

/Discomycetes (incl. Peziza, etc.). 

(c) Ascomycetes ■ Pyrenomycetes (incl. Clavkeps , 

I Erysiphaceae, etc.). 

[d) Lichenes. 
2. Archegoniatas. 

A. Bryoph3'ta. 

[a) Hepaticffi (Liverworts). 

(i) Marchantiales (incl. Rice' a, Marchantia, 
etc.). 

(2) Jungermanniales (incl. Cephalozia, Pellia, 

etc.). 

(3) Anthocerotales. 

(6) Musci (Mosses). 

(i) Sphagnales (Bog-mosses). 

(2) Andrseales. 

(3) Bryales (incl. most of the Mosses). 

B. Pteridophyta. 

Filicales (Ferns). 

Psilophytales (only living genera Psiloliim and 

Tmesipteris) . 
Sphenophyllales (Fossil). 
Equisetales (Horsetail "Ferns" and Fossil 

Calamites). 
Lycopodiales [incl. Lycopodium, Selaginella, Lep'uio-, 

dendron (Fossil), and Isoetes]. 
CycadofiUces and Pteridospermae, 



4o8 CLASSIFICATION OF PLANTS 

II. Phanerogamia. 

1. Gymnospermse. 

Cycadales. 

Bennettitales (Fossil only). 
Cordaitales (Fossil only). 
Ginkgoales (Maidenhair Tree). 
Coniferales (Fir Trees, Yew, etc.). 

2. Angiospermas. 

Monocotyledones (incl. Liliacese, Juncaceae, Graminefe, 

Araceje, etc.). 
Dicotyledones. 

((7) Archichlamydese. 

(i) Sympetate (an artificial assemblage of the more 
specialised families). 

The following Hst of works, deahng with some of the more 
advanced aspects of botanical science, may prove useful to those 
who desire to extend their reading or for purposes of reference : 

(a) Anatomy 

Haberlandt, " Physiological Plant Anatomy." (English transla- 
tion by M. Drummond.) Macmillan & Co. 

{b) Biochemistry 

Haas and Hill, " An Introduction to the Chemistry of Plant 

Products." Longmans, Green & Co. 
Monographs on Plant Chemistry, Longmans, Green & Co. : 

Bayliss, " The Nature of Enzyme Action." 

Osborne, " The Vegetable Proteins." 

Armstrong, " The Simple Carbohydrates and the Glucosides." 

Russell, " Soil Conditions and Plant-growth." 

(c) Economic Botany 

Freeman and Chandler, " The World's Commercial Products." 

Pitman & Sons. 
Freeman, " Current Investigations in Economic Botany." New 

Phytologist, vols, iv and v, 1905-6. 
Common Commodities of Commerce Series. Pitman & Sons, 



BIBLIOGRAPHY 409 

{d) Floras 

West, " The British Freshwater Algae." Cambridge University 

Press. 
Macvicar and Jameson, " The Student's Handbook of British 

Hepatics." John Wheldon & Co. 
Dixon and Jameson, " The Student's Handbook of British 

Mosses." John Wheldon & Co. 
Hooker, "The Student's Flora of the British Isles." Macmillan 

& Co. 

{e) Fossil Botany 

Seward, " Fossil Plants." 4 vols. Cambridge University Press. 

Scott, " Studies in Fossil Botany." A. & C. Black. 

Scott, " The Present Position of Palaeozoic Botany." Progressus 

Rei Botanicae, i, 1907, pp. 139 et seq. 
Laurent, " Les Progres de la paleobotanique angiospermique 

dans la dernicre decade." Ihid., i, 1907, pp. 319 et seq. 



(/) Heredity, etc. 

Darwin, " Origin of Species." 

Punnett, " MendeUsm." Macmillan & Co. 

Bateson, " Mendel's Principles of Heredity." Cambridge Uni- 
versity Press. 

Bateson, " The Progress of Genetics since the Rediscovery of 
Mendel's Papers." Progressus Rei Botanies, i, 1907, 
pp. 368 et seq. 

Lock, " Variation, Heredity, and Evolution." John Murray. 

{g) Morphology 

West, " Algae." Cambridge Botanical Handbooks. Cambridge 

University Press. 
Butler, " Fungi and Disease in Plants." Thacker, Spink & Co. 
Harshburger, " A Textbook of Mycology and Plant Pathology." 

Churchill. 
Ellis, " Outhnes of Bacteriology." Longmans, Green & Co. 



410 BIBLIOGRAPHY 

Cavers, " The Interrelationships of the Bryophyta." New 

Phytologist, vol. ix, 1910. 
Bower, " The Origin of a Land-flora." Macmillan & Co. 
Coulter and Chamberlain, " Morphology of Gymnosperms. " 

D. Appleton & Co., New York. 
Coulter and Chamberlain , " Morphology of Angiosperms." 

D. Appleton & Co., New York. 
Willis, " Flowering Plants and Ferns." Cambridge University 

Press. 

[h) CEcoLOGY OF Plants 

Tansley, " Types of British Vegetation." Cambridge University 

Press. 
War^ning, " CEcology of Plants." (Enghsh translation by 

P. Groom.) Clarendon Press. 
Schimper, " Plant Geography." Clarendon Press. 

(?) Physiology 

Pfeffer, " Physiology of Plants." 3 vols. (English translation 

by A. J. Ewart.) Clarendon Press. 
Darwin, " Lectures on the Physiology of Movement in Plants." 

New Phytologist, vol. vi, 1907. 

(j) Technique 

Chamberlain, " Methods in Plant Histology." D. Appleton & 
Co., New York. 

[k] Microscopy 
Spitta, " Microscopy." John Murray. 



INDEX 



The principal references are printed in heavier type. Illustrations 
are indicated by (fig.) after the paginal reference. 



Abies, 335, 336 (fig.), 344. 347 ; 
A . pectinata, 129. 

Abietinea;, 335, 337, 340, 347. 

Abnormal growth, 142, 143 (fig.), 
252, 254 (fig.), 267. 

Abscission-layer, 141, 142 (fig.). 

Absence-hypothesis, in hybridisa- 
tion, 384, 

Absorption of carbon dioxide, 88, 
109, 259, 343. 
of food by, cotyledons, 150 ; em- 
bryos, 282, 284, 302, 320, 351, 
370 ; Fungi, 230, 232, 236, 252 ; 
insectivorous plants, 150 ; 
parasitic Angiosperms, 179 ; 
plant-cells, 13, 15. 
of mineral salts, 10, 13, 15, 72, 

270. 
of organic material, 72, 252, 
of water, 147, 158 ; by aerial 
roots, 73, 74 ; by aquatics, 93, 
171 ; by Fern-prothalli, 301 ; 
by Lichens, 259 ; by Liver- 
worts, 270 ; by Mosses, 274, 
276 ; by mucilage, 38 ; by 
plant-cells, 10, 26 ; by root- 
hairs, 67, 72. 

Absorptive hairs, 150. 

Acacia catechu, 39 ; A. Senegal, 39. 

Acacia, False (Robinia pseudacacia) , 
115, 126. 

Acacia, seedlings of, 395 (fig.), 396. 

Accumulation of food-substances, 
13, 40. 

Acer, 130; A. pseudoplatanus , 10, 
95, 121, 128 (fig.), 142 (fig.) ; 
A. saccharinum, 45, 124; A. 
siriatiun, 135. 

Acetic acid, 265 ; — alcohol, 404. 

Achene, 371 (fig.). 

Achlya, 231. 

Acids, organic, 49, 53, 54, 60, 265, 
302. 

Aconitine, 61. 



Aconitum napellus, 61. 

Acorn (Quercus), 60. 

Acorus calamus, stem-structure 

85 (fig.), 86- 

Acquired characters, inheritance of, 
380. 

Activators, 57. 

Active hydathodes, 147, 

Active principle of. Aniseed (Pim- 
pinella anisum), 62 ; Bitter 
Almond (Prunus amygdaliis), 
48 ; Cinchona, 61 ; Cinnamon 
(Cinnamomumzeylanicu7n), 62 ; 
Cloves (Eugenia caryophyllata), 
62 ; Cruciferaj, 48 ; Deadly 
Nightshade (A tropa belladonna) , 
61 ; Ergot (Claviceps pur- 
purea), 255 ; Erythroxylon coca, 
61 ; Eucalyptus, 62 ; Foxglove 
(Digitalis), 48 ; Ginger (Ziyi- 
giber officinale), 63 ; Hemlock 
(Conium maculatum) , 61 ; Hen- 
bane (Hyoscyamus niger), 61 ; 
Hop (Humulus lupulus), 63 ; 
Indigo-plant (Indigofera), 48 ; 
Ipecacuanha (Psychotria ipeca- 
cuanha), 61 ; Lavender (Laven- 
dula), 62 ; Monkshood (Aconi- 
tuin napellus), 61 ; Opium 
(Papaver somniferitm) , 61, 155 ; 
Pepper (Piper), 63 ; Pepper- 
mint (Mentha piperita), 62 ; 
Quillaia-haik, 48 ; Soapwort 
(Saponaria), 48 ; Solanacese, 
61 ; Strychnos mix vomica, 61 ; 
Thornapple (Datura stra^no- 
niuni), 61 ; Tobacco (Nicotiana 
tabacum), 61 ; Vanilla, 48 ; 
Woad (I satis tinctoria), 48. 

Adaptation, 379 (fig.), 380; to 
habitat, 158-79 (figs.), 210, 
236, 238, 321 ; to insect- 
pollination, 373. 

Adiantum, 291 (fig.), 296. 



411 



412 



INDEX 



Adoxa moschatellina, epidermis, 95, 

96 (fig.). 
Adsorption, 56. 
Advantage of, heterospory, 321 ; 

oogamy, 223 ; seed-habit, 332, 

333. 
Adventitious roots of. Ferns, 290, 
303 ; Horsetails (Eqidsetum), 
311; Lvcopodium, 315; Ly- 
ginopleris, 328 ; Sclaginelia, 

317- 
.flicidiospores, 247. 
iEcidium, 246 (fig.), 247. 
JEgopodium podagraria, petiole, 

113 (fig.). 
Aerating system, 26, 88, 96, 107, 

108 (fig.), 109 (fig.), no, 139, 

170 ; of aquatics, 175, 176, 

177 ; of Lichens, 259 ; of 
marsh-plants, 174, 175 (fig.), 

178 ; of Moss-sporogonium, 
284 ; of secondary wood, 124. 

Aerenchyma, 174, 175 (fig.). 
Aerial roots, structure of, 73, 74, 

75 (fig.)- 
Aerobic Bacteria, 263, 266. 
Mscuhis, 52, 59 (fig.) ; hairs of, 

102 ; stem-structure of, 120, 

121 (fig.), 123 (fig.), 127. 
Afiinity between species, genera, 

etc., 393- 
African rubber (Landolphia), 157. 
Agar-agar, source of, 204 ; use of, 

264. 
Agaricus, 245 ; A. campesiris, 248, 

249 (fig). 
Age and geographical distribution, 

393- 

Agrostemma, no, 370. 
Air-bladders (of Alga?), 201 (fig.), 

204 (fig.). 
Air-bubbles, appearance of, under 

microscope, 400. 
Air-canals of, aquatics, 174 (fig.), 

175. ^7*^ i bog-plants, 178 ; 

Horsetails (Eq\tisetum), 311. 
Air-spaces. See Intercellular spaces. 
Albumen, 11, 231. 
Albuminoid cells (of Conifers), 341 

(fig.). 344 (fig.)- 

Albuminous seeds, 150, 351, 371. 

Alehemilla, 372. 

Alcohol, and protoplasmic move- 
ment, 4 ; use of, in determining 
width of stomatal aperture, 
99, 100. 

Alcoholic fermentation, 53, 255, 
256, 257. 



I Alder (Alnus), 60 ; cut-leaved, 
I 382; root-nodules of, 268; 

vessels of, 28, 35 (fig.). 

Aleurone grains, 51 (fig.). 

Aleurone layer (of Grasses), 52 (fig.). 

AlgEe, 7, 13, 39, 181-229 (figs.), 231, 
236, 355 ; of Lichens, 257, 
259, 260 (fig.). 

Algae, chloroplasts of, 181, 207-10 
(figs.) ; classification of, 198, 
199,406; economic importance 
of, 203, 204 ; sexual repro- 
duction of, 184-6 (fig.), 213, 
214 (fig.), 220-28 (figs.) ; struc- 
ture of, 180-207 (figs). ; vege- 
tative reproduction of, 212, 
212- 



14 
fruit of, 

155. 255 : 



2i5 ; zoospores of 
(figs.), 216-19 (figs.). 
Alistna plaiitago, 100, 146 

371 (fig.)- 
Alkaloids, 60, 61, 62, 64 

reactions of, 5i. 
Allelomorphs, 384, 385, 390. 
Allium cepa, 97 (fig.), 359 (fig.). 
Almond, Bitter [Prujiiis amvgdahis] 

48. 
Alnus, 28, 35 (fig.), 60, 268. 
Alpine plants, 63, 170, 379 (fig.), 

394- 
Alternation of generations, 288, 

304. 305, 373- 

Althcea, epidermis of, 95 ; pollen of, 
362 (fig.). 

Amanita muscaria, 255; A. phaU 
loides, 255. 

Amanitopsis vaginaia, Plate II. 

Amides, 50. 

Amino-acids, 52, 53. 

Amitosis, 21. 

Ammonia, conversion to nitrates, 
266. 

Ammoniated copper oxide, 401 ; 
and callose, 82 ; and cellulose, 
32 ; and cork, 137 ; and 
cuticle, 92 ; and lignified walls, 
32. 

Amj'gdalin, 48. 

AnabcBna, 206 (fig.). 

Anaerobic Bacteria, 263, 267 ; — 
respiration, 257. 

Anaesthetics, 61 ; and motile lower 
organisms, 183 ; and proto- 
plasmic movement, 4. 

Anatomy, and habitat, 15S-79 ; 
of Alpine plants, 170 ; of 
Angiosperms, 65-143 (figs.) ; 
of Ayaucaria, 340; of bog- 
plants, 17S ; of Coniferales, 



INDEX 



413 



338-44 (figs.) ; of Cycadalcs, 
322, 323; of Equisetum, 311 
(fig.) ; of Filicales, 292-5 
(figs.) ; of Fiicus, 202, 225 
(fig.) ; of Gleichenia, 294, 295 ; 
of HepaticEe, 272, 273 (fig.) ; 
of Laininaria, 202 ; of Larix, 
343 ; of Leucobryum, 277 ; of 
Lichens, 259, 260 (fig.) ; of 
Lycopodium, 315 ; of Lyginop- 
teris, 328-30 (fig.) ; of Mar- 
chantia, 273 (fig.) ; of Mnium, 
-75 (fig') ; of Musci, 275 
(fig.), 276 ; of Nephrodiutn, 
294 (fig.) ; of parasitic Angio- 
sperms, 178 (fig.), 179 ; of 
Pellia, 272, 278 (fig.) ; of 
Picea, 343 ; of Finns, 338-41 

(fig.), 342 (fig.), 343. 344 (fig) : 
of Polytrichiim, 276 ; of Pteris, 
290 (fig.), 292-4 (fig.) ; of 
Selaginella, 317, 318 ; of 
Sphagnum, 275 (fig.), 276 ; of 
Taxus, 339. 341, 342 (fig.) ; of 
water-plants, 170-77 (figs.) ; 
of xerophytes, 158-68 (figs.). 
See also Leaf-, Root-, and Stem- 
structure. 
Anatropous ovules, 357 (fig.), 358 

(fig). 359. 

Andrsealcs, 407. 

Anemone nemorosa, 148, 248. 

Angiospcrms, 180, 334, 354-73 
(figs.) ; classification of, 355, 
408 ; embryo sac of, 366 (fig.), 
367 (fig.) ; embryology of, 
368-72 (figs.) ; fertilisation 
of, 368 ; flowers of, 354, 355, 
373 ; fossil, 356 ; leaf of, 91- 
116 (figs.) ; ovary of, 356 ; 
ovules of, 357-9 (fig.) ; pollen 
sacs of, 360-63 (figs.) ; root 
of, 65-75 (figs.), 131 (fig), 
132 ; seeds of, 370-72 ; sta- 
mens of, 359 (fig.) ; stem of, 
76-90 (figs.), 117-30 (figs.), 

135-41 (figs.). 

Animal galls, 143 (fig.). 

Animals, compared with plants, 
14, 25, 181, 1S8, 193, 194, 
247, 307 ; differences between 
plants and, 188, 193, 194 ; 
diseases of, due to plants, 230, 
231, 236, 239, 252, 255 ; pro- 
tection against, 39, 48, 58, 64, 
104, 106, 126, 157. 

Aniline blue, 405 ; and callose, 82 ; 
and mucilaginous walls, 38. 



Aniline salts, and lignified walls, 

32. 
Aniseed (Pimpinella anisiim), 62. 
Aniseed oil, 62. 

Anisogamy, 185 (fig.), 186, 220. 
Annual rings of Conifers, 340 ; of 

Dicotyledons, 119 (fig.), 123 

(fig.), 125, 132. 
Annular vessels, 35 (fig.), 36. 
Annularia sphenophvlloides, 312 

(fig.). 
Annulus of Fern-sporangium, 297, 

298 (fig.), 299 (fig.) ; of Moss- 

sporogonium, 285 (fig.), 286 ; 

of Mushrooms, 248, 249 (fig.). 
Anomalous secondary thickening, 

132-4 (fig.). 
Antagonism, 14, 15. 
Antheridial cell of. Conifers, 348, 

351 .(fig.) ; Cycads, 327 (fig.). 
Antheridial groups of. Liverworts, 

270 (fig.), 279 ; Mosses, 274 

(fig.), 279 (fig.). 
Antheridium, 222 ; of Algae, 221 

etseq. (fig.) ; of Ferns, 301 (fig.), 

302, 303 (fig.) ; of Fucus, 221 

(fig), 225 ; of Fungi, 233 (fig.) , 

234, 236 ; of Liverworts, 277, 

278 (fig.) ; of Mosses, 279 (fig.) ; 

of (Edogonium, 221 (fig.), 222 ; 

of Vaucheria, 221 (fig.), 224. 
Anthers, 359 (fig.) ; dehiscence of, 

363 ; structure of, 360-63 

(figs.). 
Anihoceros, 260. 
Anthocerotales, 407. 
Anthocyanins, 63. 
Antipodal cells, 357 (fig.), 366 (fig.), 

367 (fig.). 

Antipyretics, 61. 

Antiseptics, 62, 155 ; in cork, 138 ; 
in heart-wood, 126. 

Apctalous flowers, 354. 

Apical cells, 16-18 (figs.) ; of 
Alga3, 16 (fig.), 17 (fig.), 195, 
201 ; of Dictyota, 17 (fig.) ; of 
Equisetum, 18 (fig.) ; of Fili- 
cales, 18, 291, 302 ; of 
Hepatica;, 270, 272 ; of Musci, 
18, 276, 288 ; of Sphacelaria, 
16 (fig.), 17 ; tetrahedral, 18 

(fig.). 

Apocarpous ovarj-, 354. 

Apocynaceaj, 157. 

Apogamy in, Algae, 214 ; Anglo- 
sperms, 372, 390 ; Ferns, 305 ; 
Fungi, 236. 

Apophysis, 284, 285 (fig.). 



4H 



INDEX 



Apospory in, Angiosperms, 273 ; 
Ferns, 305. 

Apothecium, 239 (fig.), 240 (fig,), 
241, 258 (fig.), 259. 

Apple (Pyntsmalus), 31, 55, 60, 137. 

Aquatic Fungi, 231, 236. 

Aquatics (excl. Algje), 69, 72, 146, 
170-77 (figs.), 379 ; air- 
canals of, 174 (fig.), 175, 176 ; 
epidermis of, 91, 92, 93, 172 ; 
land- and water-forms of, 175-7 
(figs.) ; leaf of, 172, 177 
(fig.) ; mechanics of, 88, 170, 
171 ; pollen of, 361 ; stem of, 
170-74 (figs.) ; stomata of, 

lOI. 

Aqueous tissue, 94 (fig.), 159 (fig.), 

160, 166, 167, 168 (fig.). 
Araceffi, 408. 
Arachis hypogcea, 49. 
Araucaria, 336, 340, 345, 346, 347. 
Arbor-Vita; (Thuja), 336, 337 (fig.), 

347. 

Archegonial chamber (of Cupres- 
sineae), 348, 350 ; — discs (of 
Liverworts), 270 (fig.), 280. 

Archegoniat^, 407. 

Archegonium, 280 ; of Coniferales, 
348, 349 (fig.) ; of Cycadales, 
325 (fig.), 326, 327 (fig.) ; of 
Filicalcs, 301 (fig.), 302, 303 
(fig.) ; of Hepatica;, 280, 281 
(fig.) ; of Musci, 280 ; of 
Selaginella, 319, 320 (fig.). 

Archesporium, 282, 389 ; of Coni- 
ferales, 348 ; of Filicales, 297, 
299 (fig.) ; of Hepatica2, 282 ; 
of Musci, 284, 285 (fig.) ; of 
ovules of Angiosperms, 358 
(fig.), 365 ; oil stamens, 360 

(fig.), 361. 
Archichlamydea;, 408. 
Arctic plants, 63, 394. 
Arctium, coUenchyma, 31 (fig.). 
Arenaria peploides, 94. 
Aril of Angiosperms, 359 ; of 

Taxus, 346 (fig.). 
Ayistolochia, 120. 
Arm-palisade, no, iii (fig.), 343, 

344 (fig.)- 
Armillaria mellca, Plate II. 
Arrowhead (Sagittaria), embryology 

of, 371 (fig.). 

Artichoke, Jerusalem (Helianthits 
tuberosits), 44. 

Artificial nectary, 149 ; sea-water, 
402 ; semi-permeable mem- 
branes, 402, 403 ; stoma, 99. 



Artificial selection, 378. 

Arum maculaium, 112. 

Ascomycetes, 231, 238-45 (figs.), 
259, 408. 

Ascospores, 240 ; development of, 
245 ; of Ergot (Claviceps), 241 
(fig.), 242 ; of Erysiphaceae, 
243, 244 (fig.) ; of Enrotium, 
243 (fig.) ; of Lichens, 260 ; 
of Pesiza, 240 (fig.). 

Ascus, 240 (fig.), 241 (fig.), 242, 243 
(fig.), 244 (fig.), 245. 

Asexual reproduction, 212, 219, 
229, 288 ; of Algje, 212-14 
(fig.), 216-20 (figs.) ; of As- 
comycetes, 238 et seq. ; of 
Basidiomycetes, 245 et seq. ; 
of Chlamydomonas, 182-4 
(fig.), 188; of Coniferales, 344-8 
(figs.) ; of Conjugate, 216; 



of 
of 
of 
of 
of 

of Hepa- 

(fig.) ; of 

of Lycopo- 

of Musci, 

of CEdo- 

(fig.) : of 

of Phv- 

; of 
Sela- 



scq. 
of 



319 ; 

(fig.) 



of Cyanophyceae, 216 
Cycadales, 323-5 (figs.) ; 
Ectocarpits, 218, 219 (fig.) 
Equisetum, 312, 313 (fig.) 
Filicales, 295-300 (figs.) 
Fungi, 231 et seq. ; 
ticas. 277, 282-4 
Lichens, 260 (fig.) ; 
dium, 315, 316 (fig.) 
277, 284-7 (figs.) ; 

gonium, 216, 217 

Phsophyceae, 218 ; 

comycetes, 232 et 

Rhodophyceae, 228 

ginella, 318 (fig.) 

Ulothrix, 212-14 

Vaiicheria, 217, 218 (fig.). 
Ash {Fraxinus excelsior), 120, 122, 

128, 129, 391. 
Asparagin, 50. 
Aspergillus, 242, 243 (fig.). 
Aspidium. See Nephrodiiim. 
Asplenium, 296 (fig.); A. bulbi- 

ferum, 305 (fig.) ; A. ritta 

mttraria, 296. 
Assimilation. See Carbon dioxide 

assimilation. 
Assimilatory stems, 88, 162 (fig.), 

167, 178, 310. 
Assimilatory tissues of, aquatics, 

202 (fig.) ; centric leaves, 165 

(fig.), 166 ; cladodes, 159 (fig.) ; 

Conifers, 342 (fig.), 343, 344 

(fig.) ; dorsiventral leaves, 

107-11 (figs.) ; Ferns, 295. 

298 (fig.) ; Fhcus, 202, 225 

(fig.) ; Liverworts, 273 (fig.), 

-78 (fig.) ; Mosses, 276, 284, 



INDEX 



415 



285';(fig.) ; stems, 88, 162 (fig.), 

167 ; sun- and shade-leaves, 

168 (fig.), 169 (fig.). 
Asterionella, 193 (fig.). 
Astragalus, 39. 

Alhyrium filix-fosmina, 289, 291, 

305- 

Alropa belladonna, 61. 

Atropine, 61, 62. 

Attaching organs of, Chloro- 
phycex, 197, 228 ; Cladophora, 
196 (fig.) ; Fucus, 200 ; Lamin- 
aria, 199 (fig.) ; CEdogonium, 

217 (fig.) ; Pelveiia, 203 (fig.) ; 
Ulothrix, 213 (fig.) ; Vaucheria, 

218 (fig.). 

Attractive mechanisms, 58, 149, 

354- 
Aulacommon androgynmn, 277. 
Autunin-wood of Conifers, 340, 

341 (fig.) ; of Dicotyledons, 119 

(fig.), 122, 123 (fig,), 125. 
Axile placentation, 356. 
Axillary buds, vascular supply of, 

114. 
Azotobacter, 266, 267 (fig.). 

Bacillarieae, 206, 207 (fig.), 406. 

Bacillus, 262, 267 (fig.) ; B. amylo- 
bacter, 263 ; B. anthracis, 263 
(fig.) ; B. carotovorus, 265 ; 
B. osdamitis, 267 (fig.) ; 
B. radicicola, 267 (fig.) ; B. siib- 
iilis, 263 ; B. tetani, 263 (fig.), 
264, 267 (fig.) ; B. typhosus, 
263 (fig.). 

Bacteria, 61, 252, 261-8 (figs.), 
406 ; cultures of, 264 ; phy- 
siology of, 261, 263, 264-7 ; 
reproduction of, 262, 263, 267 
(fig-)' 39^ ' structure of, 262, 
263 (fig.). 

Bacterium, 262 ; B. aceticum, 265 ; 
B. denitrificans, 266. 

Balanced solutions, 15, 

Bamboo (Bambusa), 88. 

Barberry (Berberis vulgaris), cork 
and lenticels of, 137, 139 (fig.) ; 
rust of (Pucciiiia graminis), 
246 (fig), 247 ; stamens of, 363. 

Bark, 119 (fig.), 120, 140, 141; 
chemical substances in, 60, 
61, 62. 

Barley (Hordeum viilgare). 46, 257. 

Barriers to plant-dispersal, 393. 

Bartsia, 144. 

Basal cell (of Angiosperm proem- 
bryo), 368, 369 (fig.), 371 (fig.). 



Basidiomycctcs, 231, 240, 245-51 

(figs.), 252, 255, 407. 
Basidiospores, 246 (fig.), 247, 249 

(fig.). 

Basidium, 249 (fig.). 

Bast, 28, 34, 65. See Phloem 

Bay Laurel (Laurus nobilis), 151. 

Bean, Broad (Vicia faba), 53, 148; 
Runner — - (Phaseolus multi- 
florus),93. 94 (fig.), 115, 144, 145 

(fig.). 147. 371. 
Bean, protein of, 52 ; root of, 71 

(fig.), 72 ; starch of, 42. 
Bear's Foot (Lycopodium clavatiim), 

314. 

Bedstraw (Galium), 98. 

Beech {Fagus sylvatica), 72, 129, 
140, 170, 365, 367, 391 ; cut- 
leaved — ,382, 383 (fig.) ; wood 
of, 121, 122, 126. 

Beer, 256. 

Beetroot (Beta), chemical substances 
in, 31, 45, 50, 63 ; secondary 
thickening of, 133. 

Beet-sugar, 45. 

Beggiatoa, 268. 

Belladonna (Airopa belladonna), 61. 

Bending strains, resistance to, 33, 
87, 115, 172. 

Bennettitales, 407. 

Berberis. 137, 139 (fig.), 247, 363. 

Bergamot oil, 62 ; — orange (Citrus 
auraiHium, var. bergamia), 62. 

Beta, 133. 

Betula, 72, 122, 129, 137, 139, 141. 

Betulin, 137. 

BicoUateral bundles, 81 (fig.), 82. 

Bilberry (Vaccinium myrtillus), 
pollen of, 362 (fig.) ; stem of, 
167, 168 (fig.). 

Biolo,gic strains of. Bacteria, 268 ; 
Erysiphacese, 253 ; Uredineje, 
248, 253. 

Birch (Betula), 72, 129, 391 ; bark 
of. 137. 139. 141 I wood of , 122. 

Bird's-Nest Orchid (Neottia nidus- 
avis), 179 ; mycorrhiza of, 72, 
73 (fig.) ; pollen of, 361, 362 

(fig.). 

Bishop's-weed (/Fgopodium poda- 
graria), petiole of, 113 (fig.). 

Bismarck brown, 405 ; and cell- 
walls, 32. 

Bitter Almond (Prunus amvgdahts) , 
48. 

Black Bindweed (Polygonum con- 
volvulus), 113 (fig.), 148; 
— Bryony (Tamus communis) , 



4i6 



INDEX 



85, 87 (fig.), 112 ; — Currant 
(Ribes nigrum), 105 ; — Mould 
(Mucor), 231 ; — Walnut (Jug- 
lans nigra), 129. 

Blackberry-rust (Phragmidium bul- 
bosum), 248. 

Bladder-hairs, 94 (fig.). 

Bladderwort (Utricularia), 150. 

Bladderwrack (Fucits vesiculosiis), 
38, 200-202 (figs,), 225 (fig.). 

Blechnunt spicant, 291, 295, 308 

(fig.). 324. 

Bleeding of plants, 124. 

Blue-green Algse (Cyanophyces), 
192, 210, 257, 268, 406 ; re- 
production of, 206 (fig.), 216, 
220 ; structure of, 204-6 

(fig-). 
Blue Gum (Eucalvptns globulus), 
129 ; — Mould [Peincillium), 

239. 

Body cell (of Conifers), 349, 350. 

Bog-moss (Sphagnum), i^^. 275 
(fig.), 276, 407 : — Myrtle 
[Myrica gale), 268, 357, 359. 

Bog-plants, structure of, 177, 178. 

Boletus, 245, 250, 251 (fig.). 

Borage (Borage), hairs of, 104. 

BoraginaccEe, 102, 104. 

Bordeaux mixture, 254. 

Bordered pits, 35 (fig.), 36, 37 (fig.), 
68, 120, 340, 341 (fig.), 343; 
of Ferns, 293 ; of wood-fibres, 
33, 121 ; one-sided, 122 ; role 

of. 37. 
Bowenia spectabilis, 324 (fig.), 325 

(fig.). 

Box (Biixus), 86, 127. 

Bracken (Pteris aquiliua), extra- 
floral nectaries of, 148, 149 
(fig.) ; habit of, 289, 290 (fig.) ; 
leaf of, 60, 112, 169, 291, 295; 
rhizome of, 290 (fi.g.), 292-4 
(fi,gs.) ; sori of, 296 (fig.). 

Bracket-Fungus (Pulvporus squa- 
mosus), 250 (fig.). 

Bract-scale (of Conifers), 347 (fig.), 

352 (fig). 353. 

Bramble (Jiubus spp.), prickles of, 
104. 

Branched hairs, 101,102 (fig.), 103 ; 
in Alga;, 221 (fig.), 225. 

Branching, forked, 200, 201 (fig.), 
269, 275, 290, 315, 317; 
monopodial, 334 ; of Algae, 
195-8 (figs.) ; of Conifers, 
334. 335 i of Cycads, 322 ; of 
Equisctuw, 310, 311 (lig.) ; of 



Ferns, 289, 290 ; of Liver- 
worts, 269, 270 (fig.) ; of 
Lvcopodium, 315 ; of Lyginop- 
tcris, 328 ; of Mosses, 275 ; of 
rhizoids, 274 ; of roots, 65, 71 
(fig.) ; of Selagiuella, 317 (fig.) ; 
of stems, 72, 90. 

Brandy, 257. 

Brazil Nut (Berthollciia excelsa), 
food-substances of, 49, 51. 

Bread, 52. 

Breathing, 178, 201, 257; and 
protoplasmic movement, 4 ; 
of fat-containing seeds, 50. 

Breeding, 378, 382, 388. 

Bristle hairs, 104. 

Broad Bean (Vicia faba), 53 ; ex- 
trafloral nectaries of, 148. 

Brooklime (Veronica beccabimga), 
stoma of, 160 (fig.). 

Broom (Cytisus scoparius), stem of, 
88, 167. 

Brown Algse (Phaeophycex-), 406 ; 
reproduction of, 216, 218, 219 
(fig.), 221 (fig.), 224-6 (fig.); 
structure of, 198-204" (figs.), 
208, 210. 

Brownian movement, 210, 262. 

Brussels Sprouts, " Finger and 
Toe " disease of, 254 (iig.). 

Bryales, 407. 

Bryony, Black (Tamils commitnis), 
IT2; stem- structure of 80 87 

(fig.). 
Bryophyta, 269-88 (figs.), 289, 

y>5. 307. 321, 333. "370, 373. 
407 ; antheridia of, 277-80 
(fi.gs.) ; archegonia of, 280, 
281 (fig.) ; habit of. 269-72 
(fi.gs.), 274 (fig.) ; habitat of, 
269 ; sporogonia of. 281-7 
(figs.) ; structure of, 272, 273 
(fig.), 275 (fig.), 276, 278 (fig.) ; 
vegetative reproduction of, 
277, 287 (fig.), 288. 

Bryuni argenteuin, 285 (fig.). 

Buckthorn. Sea (Hippophea rham- 
)ioides), hairs of, 103 (fig.). 

Buckwheat (Fagopyruiii), 371. 

Bud-scales, iii, 322. 

Budding of suspensor, 371 (fig.), 
372 ; of Yeast (Saccliaromvces), 
-50 (fig.), 257. 

Bulbils (of Lycopodimn sclago), 315. 

Bulbs, 40, 44, 45, 355. 

Bulrush (Scirpus), 178. 

Bundles, 76 ; bicoUateral, 81 (fig.), 
82 ; coUatcral, 76, 78 (fit? ), 



INDEX 



417 



79, 85 (fig.), 112 ; concentric, 
85 (fig.), 86, 133 (fig.), 134, 292 
(fig.) ; cortical, 86 ; leaf-trace, 
114. 173, 294 (fig.), 295; 
medullary, 86; secondary, 133 

(fig-), 134. 
Bundle-ends, 113, 145, 146, 147 

(fig.)- 
Bundle-sheath, of Conifers, 343, 

344 (fig.) ; of leaves, 107, 108 

(fig.). 112, 113 ; of stems, 85 

(fig.), 86. 
Bunt, 407. 
Burdock {Arcliiiin), collenchyma, 

31 (fig.). 

Burr-reed (Sparganiiim), 366. 

Butcher's Broom (Ruscus aculeatus), 
cladode-structure, 159 (fig.), 
166. 

Butormts, 356. 

Butter, cause of rancidity of, 265. 

Buttercup [Ranunculus spp.), 384 ; 
leaf of, 177 (fig.) ; nectaries of, 
148 ; root-structure of, 66-9 
(figs.) ; stem-structure of, 86, 

87. 
Butterwort [Pinguicula), glands of, 

149, 1,50 (fig.). 
Buttress-roots, 125. 
Butyric acid, 265, 267. 
Biixus sempervirens, 86, 127. 
By-products of plants, 3, 40, 58- 

64, 138, 144, 156. 

Cabbage, " Finger and Toe " dis- 
ease of, 251, 254 ; — Mildew 
(Peronospora parasitica). 243. 

Cabinet-work, timber used in, 126, 

130. 
Cactacese, 178. 
Cactus, 167. 
Caffein, 5i ; and tannins, 13, 60 ; 

permeability of protoplasm to, 

13- 

Calamites, 310, 311, 312 (fig.), 407. 

Calcareous Alga;, 204. 

Calcified hairs, 104; — walls, 39, 

204. 
Calcium oxalate, 58, 59 (fig.), 130, 

237 ; reactions of, 59. 
Callithamnion, 204. 
Callitriche stagiialis, anatomy of, 

175-7 (figs.). 
Callose, 82, 231 ; reactions of, 82. 
Callnna, 72 ; leaf of, 162, 163 (fig.). 
Callus, 142. 
Caltha, 72, 356 ; hydathodes of, 

146 ; ovule of, 366 (fig.). 

27 



Calycanthaceae, geographical distri- 
bution of, 394 (fig.). 

Calyptra, 282 ; of Liverworts, 282, 
283 (fig.) ; of Mosses, 274 (fig.), 
284, 285 (fig,). 

Calyptrogen, 20. 

Calyx, 354, 

Cambium, 29 (fig.), 90, 117, 118 
(fig.) ; of aquatics, 173 ; of 
cork, 135, 136 (fig.), 137, 138 
(fig.), 139 (fig.), 141 ; of leaves, 
112; of lenticels, 139, 140 
(fig.) ; of Monocotyledons, 117, 
133 (fig.), 3.55 ; o£ roots, 66 
(fig.), 131 (fig.) ; of Coniferous 
stems, 328 ; of Dicotyledonous 
stems, 79, 80 (fig.), 81 (fig.), 
83 (fig.), 117, 118 (1-ig.), 119 
(fig.), 121 (fig.). 

Campanulaces, 44, 153, 154. 

Camphor, 63. 

Campion (Lychnis), 88. 

Campylotropous ovules, 358 (fig.), 

359. 

Canada Balsam, use of, 404, 405. 

Canadian Pondweed (Elodea cana- 
densis], cell-structure, 5, 6 (fig.) ; 
spread of, in British Isles, 392 

(fig.). 

Cane-sugar, 45, 46, 53, 54, 281 ; 
hydrolysis of, 46, 53, 55 ; 
osmotic pressure of, 11; re- 
actions of, 47. 

Cannabis saiiva, 34, 265. 

Cantharellus, 251 (fig.). 

Caoutchouc, 155, 157. 

Caps (of CEdogonium), 195, 217 (fig.). 

Capsella bitysa-pastoris. embryo- 
logy of, 368-70 (fig.) ; hairs 
of.' 102 (fig.) ; White I^ust 
of. 231, 233 (fig.). 

Capsule of. Fern-sporangium, 297- 
300 (figs.) ; Liverworts, 271 
(fig.), 282, 283 (fig.) ; Mosses, 
100, 274 (fig.), 284-7 (fig). 

Carbohydrates, 31, 40-48. 50, 82, 
266, 267 ; reactions of, 41, 44, 
47; translocation of, 88, no. 

Carbon dioxide, absorption of, 88, 
109, 259, 343 ; circulation of, 
265, 266 ; formation of, in 
alcoholic fermentation, 53, 256. 

Carbon dioxide assimilation, 40, 41, 
107 ; and light, 96 ; by Alga;, 
181, 183, 202, 207 ; by roots, 
73, 74 ; by stems, 88, 162, 167 ; 
removal of products of, 41, 
no; sugars formed in, 46. 



4TC 



INDEX 



Carbonate of lime, deposition of, 
on cell-walls, 39, 204 ; in 
vessels, 126. ■ 

Carotin, 64, 109. 

Carpels, 354, 355, 35O. 

Carpiims, 128, 367. 

Carpospores (of Red AlE;a;), 205 
(fig.). 228. 

Carrageen (Cbondriis crispus). 204, 

■^05 (fig.)- 
Carrot (Daiiciis camla), 64 ; soft 
rot of (BacHhis cayotovonis), 

Cayteria, 186, 187 (fi.t;-.), 188. 

Caryn, 130. 

Caryophyllacca;, 358, 371. 

Cassava {Manihot idiUssiina), 43. 

Castanea saliva, 130. 

CastiUoa clasiica, 157. 

Castor oil, 49. 

Castor oil plant (Ricinus communis) , 
aleurone grains of, 50, 51 (fig.) ; 
fat of, 49; hypocotyl of, 118 
(fig.) ; seeds of, 371, 372, 376 

(fig.). 
Casitarina, stem-structure of, 162 

(fig.). 
Catalytic action of enzymes, 55, 56. 
Catechu, 39. 
Caterpillars, fungal disease of, 239, 

-55. 
Catharinea widulata, leaves of, 276. 
Caustic potash and, callose, 82 ; 

cork, 136 ; fats, 40. 
Ceara-rubber (Manihut glaziovii), 

157. 

Cedar [Cedriis). 335, 353. 

Cedar wood (Cedrela spp.), 129, 

Ccdriis, 335, 353. 

Celandine, Greater {Chelidouiiim 
■inajtis), laticiferous vessels of, 
153. 154 ; ovule of, 358 (hg.). 

Celandine, Lesser (Ficaria veriia), 
embryo of, 372 ; hydathodcs 
of, 145, 146 ^fig), 147 (fig.). 

Celery (Apiiim), 27. 

Cell-contents, living, 1-8 (figs.) ; 
non-living, 40-64 (figs.). 

Cell-division. 21-5 (figs.) ; effect 
of wounding on, 2O, 142 ; in 
reproductive cells, 18^, i\\ 
(fig.), 214, 305-8 (fig.).~ 

Cell-plate, 24, 25 (fig.). 

Cell-sap, 3, 6, 10, 26, do, 63 ; osmotic 
pressure of, 8, 10, 11, 12, ih. 

Cell-wall, 1, 4, 9, 24, 29-39 (figs.) ; 
euticularisation of. 91 ; forma- 
tion of, 24, loO (fig.) ; growth 



of, 29 ; incrustation of, 39 ; 
lignification of, 32 ; mucila- 
ginous, 38, 95, 184, 189 (fig.), 
202, 214, 262 ; of Bacteria, 
262 ; of Diatoms, 206, 207 
(fig.) ; of Fungi, 231 ; of hairs, 
loi ; stratification of, 24, 29, 
33 ; suberisation of, 136 ; 
thickening of, 29-37 (figs.), 47- 

Cells, apical, 16-18 (figs.) ; aqueous, 
166 ; assimilatory, 107-11 
(figs.) ; division of, 21-5 
(figs.), 305-8 (fig.) ; epider- 
mal, 91-6 (figs.) ; glandular, 
148, 149 (fig.) ; growth of, 28, 
29 ; mechanical, 31-4 (figs.) ; 
meristematic, 18 ; parenchy- 
matous, 27 ; phj'siology of, 
8-15 ; plasmolysis of. 5 
(fig.), 8; prosenchymatous, 27; 
sexual, 184, 214 ; structure of, 
1-8 (figs.) ; turgescence of. 9. 

Cellulose, 4, 24, 31, 91, loi, 103 ; 
decomposition of, 265 ; re- 
action of, 4, 31, 32 ; reserve — , 

48. 53. 
Central cylinder of, aquatics, 170- 

74 (ifg.) ; root, 66 (fig.), 68 ; 

stem, 78. 
Centric leaves. 116, 165 (fig.), 166, 

172 ; of Pinus, 343, 344 (fig.). 
Centric starch-grains, 42 (fig.). 
Cephaelinc, 61 . 
Cephalotaxiis, 353. 
Ceplialozia, 272, 407 ; C. bicuspi- 

data, 271 (fig.). 
Ceyamiam, 204, 205 (fig.). 
Ceyasliuni, vascular system of, 113, 

114 (fig.). 
Ceyatriphvllitm de^iicvsiini, anatomy 

of, 170, 171 (fig.). 
Cereals, 248 ; starch of, 43. 
Cclyaria islavdica, 25S, 261 (fig.). 
Chalaza, 357 (fig,). 338, 359, 365, 

367. 
Chalazogamy, 364 (fig.). 3G3. 
Chance variation, 376. 
Cliara, 39, 198, 
Charales, 406. 
Cheese, production of, 263. 
Clu'Iidomifii/ unijas, laticiferotlS 

vessels of, 15^, 1 S4 ; ovule of, 

33S (fig.). 
Chemical changes in cell-wall, 29, 

i2. 38, 91, 130. 
Chemistry of plants. 43-64. 
Chemotaxis, 222, 224, 232, 2S1, 

302. 



INDEX 



419 



Chemotropism, 232, 365. 

ChenopodiacejE, secondary thicken- 
ing in, 133. 

Cherry {Primus cerasus), 141. 

Cliestnut (Castanea saliva), 130. 

Chickweed, Mouse-ear (Ceras/ium), 
vascular system of, 113, 114 

(fig-)- 

Chile saltpetre, origin of, 266. 

Chinese Primrose {Primula sinensis) 
glandular hairs of, 105 (fig.). 

Chitni, 231. 

Chlamydomonas, asexual reproduc- 
tion of, 182-4 (figs-). i«« ; 
movements of, 182, 183 ; Pal- 
■inella-st'dge of, 182 (fig.), 184 ; 
sexual reproduction of, 184-6 
(fig.), 220 ; structure of, 181, 
182 (fig.), 207. 

Chlamydomonas angulosa, 182 (fig.) : 
C. media, 182 (fig.) ; C. mona- 
dina, 182 (fig.), 185 (fig.), 186, 
220 ; C. pertyi, 185 (fig.) ; C. 
reinhardii, 182 (fig.), 185 (fig.). 

Chlamydospores, 23S, 248. 

Chlor-zinc-iodide, and callo.se, 82 ; 
and cellulose, 32 ; and cork, 
136 ; and lignificd walls, 32. 

Chlorella, 260. 

Chloroform, and enzyme-action, 
56 ; and protoplasmic move- 
ment, 4. 

Chlorophycea;, 198, 406 ; chloro- 
plasts of, 207-10 (fig.) ; colonial, 
188—92 (fig.) ; filamentous, 195-8 
(figs.) ; sexual reproduction 
of, 184, 185 (fig.), 213 (fig.), 
214, 220-24 (fig-). ^26-8 (fig.); 
unicellular, i8i-8 (figs.), 192 
(fig.), 208, 209 (fig.) ; zoospores 
of, 212-14 (lig.), 216-1S (figs.) ; 
vegetative reproduction of, 
212, 216, 217 (fig.). 

Chlorophyll, 4, 6, 109, 194. 198 ; 
and cork, 136 ; and light, 169 ; 
and starch -formation, 40, 41 ; 
formation of, 41 ; in roots, 73 ; 
in stems, 88, 162, 167, 178 ; 
in variegated leaves, iii; 
screening of, 102, no, igg. 

Chloroplasts, 5 ; division of, 25 ; 
movement of, 112 ; of Algae, 
7 (fig.), 8, 198, 207-10 (fig.) ; 
of Bryophyta, 272, 278 (fig.) ; 
of Chlamydomonas, 181, 182 
(fig.) ; of Cladophora, 196 (fig.), 
208 ; of Desmids, 209 (fig.) ; 
of Diatoms, 208 ; of Ectocarpiis, 



219 (fig.) ; of epidermis, 91, 
169 ; of Hcematococcus, 187 
(fig.) ; of higher plants, 5, 6 
(fig.), 8, 88, 92 (fig.), 96, 107, 
108 (fig.), 109 (fig.) ; of Hor- 
midium, 215 (fig.) ; of Qido- 
goniitm, 208 (fig.), 217; of 
Phseophyceae, 208 ; of Pleura- 
coccus, 190, 192 (fig.) ; of 
KhodophyccEe, 208 ; of Spiro- 
gyra, 7 (fig.) ; of Ulva, 215 
(fig.) ; of Zygnema, 208, 227 
(fig.) ; starch-formation in, 6 
(fig.), 40, 181, 207. 

Chloyoxylon swietenia, 130. 

Chondrus crispus, 204, 205 (fig.). 

Chromatin network, 22, 24, 306, 
389. 

Chromoplasts, 64. 

Chromosomes, 21-5 (figs.), 306, 
307 (fig.), 372 ; segregation of, 
306, 390. 

Chroococcus, 205, 206 (fig.). 

Cilia, 182 (fig.) ; of motile Alga;, 
182 (fig.), 183, 187 (fig.), 189 
(fig.) ; of spermatozoids, 221 
(fig.), 224, 277, 278 (fig.), 302, 
.303 (fig.), 320 (fig.), 326, 327 
(fig.) ; of zoospores, 213 (fig.), 
216, 217 (fig.), 218 (fig.), 219 
(fig.), 232, 233 (fig.). 

Cinchona, 6 1 . 

Cinnamomiim camphora, 63 ; C. 
zeylanicnm, 62. 

Cinnamon {Ciniianioinum ' zeylaiii- 
cuni), 62. 

Circcsa luletiana, 58, 59 (fig.) 146 
168. 

Circulation of carbon dioxide, 265, 
266 ; of nitrogen, 263, 2O6, 
267. 

Citric acid, 60. 

Citrus, 371 (fig.), 372 ; C. aurantium, 
var. bergatnia, 62. 

Cladode, structure of, 159 (fig.). 

Cladonia, 258 (fig.), 259, 260 (fig.). 

Cladophora, chloroplast of, 196 
(fig.), 208 ; habitat of, 195, 
197 ; sexual reproduction of, 
215, 220 ; structure of, 195-7 
(fig.) ; vegetative reproduction 
of, 2i6; zoospores of, 196 (fig.), 
215- 

Cladophora glomerata, 196 (fig.). 

Classification of Vegetable King- 
dom, 180, 181, 230, 231, 258, 
269, 289, 334, 355, 408-8. 

Clavaria, 250 ; C. cinerea, 251 (fig.). 



420 



INDEX 



Ciaviceps purpuyea, 49, 239, 255, 
407 ; life-history of, 241 (fig.). 
242. 

Cleistogamic flowers, 364. 

Climate and plant-distribution, 391, 

394- 
Climbing hairs, 104, 
Close-grained wood, 127. 
Closing of stomata, 98-100 (fig.). 
Closterimn, 209 (fig.). 
Clostyidiuni pasteuriaHum, 267 (fig.). 
Clove [Eugenia caryophyllata), 62 ; 

secretory cavities of, 151, 152 

(fig-). 
Clubmoss (Lycopodiiim), 180, 289, 

310, 314-16 (fig.), 318. 321. 
Cluster crystals, 58, 59 (fig.). 
Cluster-cups (secidia), 246 (fig.), 

247. 
Coal Measures, plants of, 310, 312 

(fig-). 327-33 (figs.). 
Coarse adjustment (of microscope), 

398, 399- 
Coarse-grained wood, 127. 
Cocaine, 61 . 
Coccws-forms (of Bacteria), 262, 

263 (fig.). 
Cocoa {Theobroma cacao), 49, 53, 

61 ; — butter, 49. 
Coconut oil, 49 ; — Palm [Cocos 

nucifeya), 49. 
Coffee (Coffea spp.), 47, 6i. 
Cola acuminata, 61. 
ColcJiicum, 364. 
Collateral bundles, 76, 78 (fig.), 79, 

85 (fig,), 112. 
Collecting cells, no, in (fig.), 161 

(fig.). 
Collema, 258, 260, 
CoUenchyma, 31 (fig.) ; of leaf, 108 

(fig.), 112 ; of stem. 31 (fig.), 

34 (fig.). 77 (fig). 79, 81 (fig.). 

83 (fig,), 87, 88; mechanical 

value of, 33. 
Colloids, 43, 55, 56. 
Colonial AlgK, 188 (fig,), 189 (fig.), 

192, 205, 206 (fig,). 
Colonisation, 206, 257, 259. 
Colouration of water by Alga;, 181, 

188, 192. 
Colours of flowers, 63. 
Coltsfoot (Tiissilago fayfaya), 102. 
Columella (of Moss-capsule), 284, 

28,'; (fig.), 286. 
Comfrey (Symphytum) , 104. 
Commidendyon, 393. 
Common bundles, 114 (fig.), 115. 
Companion cells, 28, 29 (fig.), t,6 



(fig.), 68, 78 (fig.), 79, 80 (fig.), 
82, 83, 84, 85 (fig.), 121 (fig.), 

130- 
Competition among plants, 378, 

394- 
Complementary tissue (of lenticels), 

139. 
Composite;, 44, 153, 154, 355, 356, 

372, 393. 

Compound microscope, 398-400 
(fig.) ; sieve-plates, 130; starch- 
grains, 42 (fig.), 43. 

Concentric bundles, of Angio- 
sperms, 85 (fig.), 86, 133 (fig.), 
134 ; of Ferns, 292 (fig.). 

Conceptacles (of Fitcus), 201 (fig.), 
202, 224, 225 (fig.). 

Condenser (of microscope), 398, 

399. 400- 

Conducting elements and tissues, 
27, 28, 29 (fig.), 35 (fig.) ; of 
Conifers, 339-41 (fig.) ; of 
Ferns, 292-5 (figs.) ; of 
leaves, 108 (fig.), Ii2-i4(fig,); 
of Mosses, 275 (fig.), 276 ; of 
roots, 66 (fig.), 68, 70 (fig,), 
132 ; of stems, 78-86 (figs.), 
119-26 (figs.). 

Conduction of elaborated food- 
material in, Angiosperms, 28, 
65, 88, no, in, 112 ; aquatics, 
171 ; Conifers, 343 ; Mosses, 
276 ; by laticiferous elements, 
156 ; by the xylem, 124. 

Conduction of water and mineral 
salts in, Angiosperms, 27, 35, 37, 
65, 88, 112, 121, 125 ; Conifers, 

343. 344 ; Mosses, 276. 
Cones of, Abies. 336 (fig.), 344, 

347 ; Ayaucayia. 345, 346, 347 ; 
Cedrus, 353 ; Conifers, 344-7 
(figs.), 352 (fig.) ; Cupres- 
sinese, 337 (fig.), 345, 347 ; 
Cycads, 323, 324 (fig.) ; 
Equisetum, 311 (fig.), 312. 313 
(fig.) ; Jumpeyus, 352 (fig.), 
353; Layix, 336 (fig.), 345, 
346, 347 ; Lycopodium, 314, 
315, 316 (fig'.) ; Punts, 345 

(fig.). 346. 347 (fig.). 352 ; 

Sclaginella, 317 (fig.), 318 (fig,); 
Sequoia, 347, 352 (fig.) ; Taxus, 

344. 34.'5. 34t> (fig.) ; Thuja, 
.337 (fig.). 347- 

Coniferous timber, 127, 129. 

Conifers (Coniferales), 180, 333, 
334-,S3 (figs.). 408 ; arche- 
gonia of, 348, 349 (fig.) ; 



INDEX 



421 



embryology of, 350-52 (fig.) ; 
female cones of, 336 (fig.), 337 
(fig-). 345-7 (figs.), 352 (fig.) ; 
fertilisation of, 349, 350, 353 ; 
habit of, 334-6 (figs.) ; leaf 
of. 161, 335-7, 341-4 (figs.) ; 
male cones of, 336 (fig.), 
344-6 (figs.), 347 (fig") ; 
ovules of. 333. 348, 349 
(fig.) ; pollen sacs (micro- 
sporangia) of. 345, 347 (fig.) ; 
pollination of, 348 ; root of, 
70, 341 ; seedlings of, 337, 
338 (fig.) : seeds of, 351, 352 
(fig.); stem of, 117, 335, 338- 
41 (figs.) ; wood of, 37 (fig.), 

127, 340. 341 (fig). 
Confine, 61. 
Conium maculahtm, 61. 
ConjugataE) 208 ; reproduction of, 

216, 226-8 (fig.) ; structure 

of, 208-10 (fig.). 
Conjugation, 226, 227 (fig.), 237 

(fig.). 
Connective, 359 (fig.), 360 (fig.), 

362. 
Contractile vacuoles, 181, 182 (fig.), 

187 (fig), 193, 213. 
Conversion of sewage, 265. 
Convolvulus sepium, 153. 
Copper Beech, 382. 
Copra, 49. 
Copyinits, 245. 
Coral-Fungus (Clavaria), 250, 251 

(fig.). 
Coral reefs and Alga;, 204. 
Coral-spot disease (Nectna), 253. 
Corallina, 204, 205 (fig.). 
Corchorus, 34. 
Cordaitales, 407. 
Cordyceps, 239, 255. 
Cork, 135-42 (figs.) ; and leaf-fall, 

141, 142 (fig.) ; and wounds, 

'' 142 ; commercial sources of, 

"' 140 ; formation of, 131 (fig.). 

£3 T35-8 (figs.) ; functions of, 

137. 138 ; properties of, 137. 

138. 
Cork-cambium (phellogen), 135-42 

(figs.) ; and aerenchyma, 175 ; 

of Conifers, 338, 339 (fig.). 
Cork Elm, 141 ; — Maple, 141 ; — 

Oak (Qtierciis suber), 140. 
Cork- wings, 141. 
Corms, 355. 
Corn Cockle [Agrostenima gHhago). 

no, 370. 
Cornus sanguinea. galls of , 143 (fig.). 



of aquatics, 

(fig.), 175 ; 

of pulvinus. 

et scq., 295 

43. 76 et 



171 
of 

115; 
of 

seq., 



Corolla, 354, 373. 
Cortex, 65, 76 ; 

(fig.). 173 
petiole, 114; 
of root, 65 
stem, 1 9, 30 

134. 338. 
Cortical bundles, 86. 
Cosmarinm, 209 (fig.), 210 ; C. 

meneghini, 2og (fig.). 
Cotton, 103 ; — plant (Gossvpiiiin 

spp.), 102, 103. 
Cotton-grass (Eriophorum), 178. 
Cotton-seed oil, 49. 
Cotyledons, digestive action of, 

150 ; food-reserves of, 40, 41, 

44, 50 ; of Angiosperms, 355. 

369 (fig.), 370. 371 (fig.) ; oi 

Conifers, 338 (fig.), 350, 351 
(fig). 352 (fig.) ; of Cycads, 
326 ; structural features of, 30, 

153. 
Covering hairs, 102 (fig.). 
Cow-wheat (Melampyrum). 179. 
Crassulaces, 178. 
Creeping Buttercup {Rainiiwiilus 

repens), root-structure of, 66-9 

(figs.). 
Crenoihrix, 268. 
Cress (Lepidimn) , 48 ; " damping 

off" disease of, 235 (fig.). 
Crocus, 364 ; C. sativus, 64. 
Crops, fungal diseases of, 230, 

234. 235 (fig). 241 (fig.), 

(fig.). 245, 246 (fig.), 247, 

252-4. 
Crossing (hybridisation), 382- 
Crowberry (Empetrum nigrum) 

of, 163 (fig.). 
Cruciferae, 48, 63, 69, 103, 104, 
Cryptogamia, 355, 406. 
Cryptomeria, 347, 
Crystal-sand, 58. 
Crystalloid, 50, 51 (fig.). 
Crystals of, inulin, 44, 

osazones, 47 (fig.) ; 

lime, 58, 59 (fig 

pigments, 64. 
Cuckoo- pint (Arum niaciilnlum), 112. 
Cucumber- family (Cucurbitaccje), 

81 ; — mildew (Erysiphe poly- 

goi'i), 243. 
Ciicitrbita, hairs of, I02 (fig.) ; 

phloem of, 29 (fig.), 82, 130 ; 

stem-structure of, 80-83 (figs), 

87 ; tyloses of, 127 (fig.) ; 

vessels of, 35 (fig.), 83. 
Cucurbita pepo, pollen of, 362 (fig.). 



231, 
243 



9. 
leaf 

231. 



45 (ng.) ; 
o.xalate of 

:■), 130, 237; 



422 



INDEX 



Cultures (of Bacteria), 264. 
Cup-Fungus (Peziza), 239 (fig.), 

240 (fig.). 
Cuprammonia. See Ammoniated 

copper oxide. 
Cupressinejc, 336, 347, 348, 350, 

351. 

Citpressus, cones of, 345, 347 ; 
habit of, 334, 336, 337 ; seed- 
ling of , 337, 338 (fig,). 

Cupule (of Lvgiiioptens), 331, 332 

{fi.?-), 333 (fig.). 
Ctivcuma louga, 64. 
Curing of rubber, 157 ; of tobacco, 

265. 
Currant (Ribes). 105, 115; cork of. 

137, 138 (fig.). 
Curves of variation, 375-7 (fig.). 
Cuscuta, 178 (fig.), 179. 
Cushion (of Fern-protlrallus), 301, 

302. 
Cut-leaved Beech, 382, 383 (fig.). 
Cuticle, 91-3 (fig.), 9.5 ; function 

of, 92, 93 ; of aquatics, 172 ; 

of Coniferous leaves, 342, 343, 

344 (fig.) ; of plants of dry 

habitats, 158, 159, 160 ; of 

shade-leaves, 169. 
Cuticular transpiration, 93, 161. 
Cuticularisation, 91, 92, 300, 361. 
Cuticulariscd layers (of epidermis), 

91. 

Cuttings, callus-formation in, 142. 

Cj'anophycea?, 192, 210, 237, 268, 
406 ; reproduction of, 206 
(fig.), 216, 220, 391 ; structure 
of, 204-6 (fig.). 

Cycadofilices, 407. 

Cj'cads (Cycadales), 322-7 (figs.), 
393, 407 ; anatomy of, 323 ; 
cones of, 323, 324 (fig.) ; em- 
bryology of, 326, 327 (fig.) ; 
fertilisation of, 326, 327 (fig.), 
353; habit of, 322, ^2', (fig.); 

ovules of, 324, 323 {''^^ri) : 

pollination of, 326 ; seeds of, 
327 ; spcrmatozoids of, 320, 

327 (fig,), 
Cyceis, 324 (fig.), 327 (fig.), 3,36; 

C. circincdh, 323 (fig.) ; C. 

revohita, 322, 325 (fig). 
Cylindrocystis, 209 (fig.), 
Cymbella, 207 (fig,). 
Cynoglossum, hairs of, 102 (fig.), 
Cyperacea?, stoma of, 100. 
Cypress (Cupressiis), cones of, 343, 

347 : habit of, 334, 33<), 337 ; 

se<'dling of, 337, 338 (fig.). 



Cystococcus, 259, 

Cystoliths, 38 (fig. ), 39. 

Cysiopus, 231-4 (fig.) ; C. candidus, 

233 (fig), 

Cytase, 53. 

Cytisus adaiiii, 3S9 ; C. scnparius, 
167. 

Cytoplasm, i -7 ; composition of, 
2, 50 ; growth of, 25 ; nu)vc- 
ment of, 4, 0, 104 ; permea- 
bility of, 9, 12, 13, 15 ; pro- 
perties of, 3 ; structure of, 3. 4. 

C5ftoplasmic connections, 24, 30, 
190, 

DcBdalea queiriiui, 250. 

Dahlia, inulin of, 44, 45 (fig.). 

Damp habitats, structural features 
of plants of, 96, 103, 160, 166, 
177, 276, 296. 

" Damping off " Fungus (Pythium), 
231, 232, 253 ; structure and 
life-history of, 234, 235 (fig.), 
236. 

Dandelion (Tarnxaeuin), 154. 170, 
372 ; flowers of, Plate I 
(Frontispiece). 

Darwin's " Origin of Species," 378. 

Date {Plicenix dactvlifera), endo- 
sperm of, 30 (fig.), 47. 

Datura sli^amoiniiiii, 61, 62 (fig), 

384, 
Daughter-cells, 17, 24, 183; — 

colonies, 188 (fig.), 189 (fig.). 

190 ; — nuclei, 21, 2;}, 24, 2^ 

(fig.), 390. 
Deal, Red (Piinis sylvestyis). 120 ; 

White (Pieea excelsa). 129. 
Dead-nettle (Laminm), 4, 103, 364 ; 

coUenchyma of, 31 (fig.l, 34 

(fig.) ; leaf of, 168, 169 (fig.). 
Deadlv Nightshade (Alropa hclhi- 

di'iuia), 01. 
Death Cap (Amanita phalloidcs), 

Decay clue to Bacteria, 232, 2i)^, 
263, 2()ii ; due to Fungi, 230, 
232 ; prevention of, 12b. 

Dehiscence of, antheridia, 277, 280, 
302, 303 (fig.) ; Fern-sporangia, 
298 (fig.), 299, 300 ; Liverwort- 
sporogonium, 271 (fig.), 282 ; 
mierosporangia of Conifers and 
C\-cads, 324, 343 : Moss-spc^ro- 
gonium, 283 (fig), 2S0, 287 ; 
stamens, 31)3. 

Dcndyobiti))!. aerial roots of, 73 (fig.), 

llcnitrification, 2 06, 



INDEX 



423 



Dentifrices, 207. 

Depression of stomata, 96, 158, 160 
(fig.), 165. 170. 

Dermatogen. 19, 20, 74, oj, 369. 

Desert, distribution of, 391. 

Desert-plants, 178. 

Desmidium, log (fig.). 

Desmids (Desmidiacea;), 192, 2ir; 
reproduction of, 209 (lig.), 
216, 220, 228 ; structure of, 
2o8-:o (fig.). 

Deiitzia, hairs of, 102 (tig.), 103. 

Development of, anther, 360 (fig.), 
361 ; ascospores, 240 (fig.), 
245 ; cambium, 117, 118 (fig.) ; 
cork-cambium, 135, 136 (fig.), 
137 ; Fern-leaf, 290 (fig.), 291 ; 
Fern-prothallus, 299 (fig.), 
300 ; Fern-sporangium, 291-9 
(fig.) ; gametes, 184 (fig.), 213 
(fig.), 214; .gonidia, 232, 243 
(fig.) ; laticiferous cells, 153 ; 
laticiferous vessels. 154 ; 
lateral branches of root, 65, 71 
(fig.) ; lateral branches of 
stem, 90 ; leaves, 19 (fig.), 90 ; 
lenticels, 139 ; Moss-plant, 2S7 
(fi,g.), 288 ; Mushroom {Agai'i- 
cus), 248, 249 (fig.) ; ovules of 
Angiosperms, 358 (fig.), 359 ; 
pollen grains, 360 (fig.), 361 ; 
procambial strands, 89 ; root- 
cap, 18, 20 (fig.), 55 ; secre- 
tory cavities, 151 ; sieve-tubes, 
28 ; spermatozoids, -77 ; 

spores, 305-8 (fig.) ; sporo- 
gonium of Liverworts, 282 ; 
sporogonium of Mosses, 284, 
285 (fig.) ; stamens, 360 (fig.), 
361 ; starch - grains, 41 ; 
stomata, 98; tyloses, 126; 
vascular tissues, 89, 90; vessels, 
28 ; zoospores. 212, 213 (fig.), 
216, 217 (fig.), 218 (fig.), 219 
(fig.). See also under Embryo- 
logy. 

De Vries, 381. 

Dextrin, 11, 46, 47, 53. 

Dextrose, 45. See also Glucose. 

Dialysis, 57. 

Diaphragm, of aquatics, 175 ; of 
microscope, 399, 400 ; of Moss- 
capsule, 2S5 (fig.), 286. 

Diarch roots, 69, 70, 131 (fig.), 295, 

341- 
Diastase, 46, 53, 56 ; extraction of, 

53, 54- 
Diatomaceous earth, 207. 



Diatoms (Diatomacese), 39, 192 
193 (fig.), 206, 207 (fig.), 20t 
211, 406 ; reproduction of, 2 it 

219, 228. 

Dicotyledonous timbers, 129, 130. 

Dicotyledons, 355, 367, 408 ; cm- 
bryology of, 368-71 (fig.) ; 
epidermis of, 93 ; leaf of. 93, 
98, 107-14 (figs.) : root of, 
05-71 (figs.), 131 (fig.), 132; 
stem of, 76-84 (figs.), 80, 117-3C 

(figs.), 135-41 (figs.). 
Dtcyanella heleromella, protonema of, 

287 (fig.). 
Dietyota, 17 (fig.). 
Differentiation of sex, 186, 2r2 

220, 227, 228. 

Diffusion in plant-cells, 6, 12, 13, 

5-- 
Digestive glands, 144, 149, 15c 

(fi.g). ^ 

Digitalin, 48. 

Digitalis, 48, 363. 

Direcious, 223, 225, 334. 

Dioon ediile, 327 (fig.). 

Diospvros, 126. 

Direct nuclear division, 21. 

Disaccliarides, 44. 45. 

Disc of Umbellifera;', 148. 

Discomycetes, 407. 

Diseases of plants, and immunity 
253 ; control of, 247, 253, 254 ; 
due to animals, 143 ; due tc 
Bacteria, 265 ; due to Fungi 
230. ^31. ^35. ^36. 242, 243, 
^45. ^48, 251, 252, 253. 

Dispersal. 219, 392 (fig), 393; barrier; 
to, 393 ; of gonidia, 232, 242 ; 
of microspores, 326, 330, 345 ; 
of oospores, 21}, ; of seeds, 46 
50, 103, 353, 372 ; of soredia 
of Lichens, 261 ; of spores, 
238, 242, 245, 249. 1-,}, 282, 
287, 300, 313, 321 ; of zygo- 
spores, 186. 

Dissipation of energy, 265. 

Distribution of plants, 391-3. 

Division of, cells, 16, 21-5 (figs.) ; 
chloroplasts, 25; nuclei, 20-2; 
(figs.) ; spore mother-cells 
306-8 (fi,g.). 

Division of labour, 395 ; in Algae, 

190. 197, 198, 199. 210. 22^ ; 

in Ferns, 30S ; in he; ero- 

sporous plants, 321, 
Dock (Rumex), 113. 
Dock-family (Polygonacea;), 356. 
Dodder (Cuscuta), 178 (fig.), 179. 



424 



INDEX 



Dog's Mercury (Mercurialis peren- 
nis), leaf of, 37 (fig.), 59 (fig.), 
168, 169; petiole of, 113 (fig.). 

Dogwood [Covnus sauguijiea), galls 

of, 143 (fig-)- 
Dominant characters, 384 et seq. 
Dormant characters (of hybrids), 

382. 
Dorsiventral leaves, structure of, 

qS. 107-11 (figs.), 116, 295, 

341. 342 (fig.). 
Double chromosomes. 306. 
Double fertilisation, 368. 
Dou,glas Fir (Pseudotsuga douglasii), 

128, 129; cone of, 352 (fig.), 

353- 
Dragon Tree {Dractzua). secondary 

thickening of, 133 (fig.), 134. 
Drapariialdia, 197 (fig.). 
Drosera, tentacles of, 150. 
Drought, resistance to, 186, 191, 

205, 215, 269, 271. 
Dr)^ habitats, structural features of 

plants of, 38, 88, 92, 94, 96, 

III, 158-66. 
" Dry rot " of timber (MeruUus 

lacyvmans) , 2^2. 
Duckweed (Lemna), 112, 260. 
Dunes, 163. 
Dwarf-males (of CEdogoninm), 221 

(fig.), 223. 
Dwarf-shoots (of Conifers), 335, 

336 (fig.), 337, 338. 
Dyes, vegetable, 64, 126, 261. 
Dynamite and Diatoms, 207. 

Eau de Cologne, 62. 

Eau de Javellc, 401 ; and lignified 

walls, 32. 
Ebony (Diospyros), 126. 
Echeveria retusa. 365. 
Economic plants, 34, 35, 39, 43, 45, 



28. 



49. 53. 



103, 124, 120, 



140. 155. 157. -°3. -04. -07. 
255, 256. 257, 261, 276, 323, 
334. 337. 35-!. 
Economic utilisation of. Alga?, 203, 
204 ; alkaloids, 61 ; Bacteria, 
265 ; Bog-moss {Spliagiuiiii), 
276; Conifers, 127, 129, 334, 
337. 35^ ; cork, 136, 140 ; 
Diatoms, 207 ; ethereal oils, 
62 ; fats, 49 ; fibres, 34. 
35; Fungi, 255, 256, 257; 
glucosides, 48 ; gums, 39 ; 
hairs, 103 ; latex, 155, 157 ; 
Lichens, 261 ; starches, 43 ; 
.sugars, 45, 124; timber, 126, 



127-30 ; vegetable pigments, 
64, 126, 

Economy of water, 158-66. 

Ectocarpiis. 199 ; habitat of, 195 ; 
sexual reproduction of , 212, 219 
(fig.), 220 ; structure of, 198, 
219 (fig.) ; zoospores of, 218, 
219 (fig.). 

Ectocarpus httoraJis. 219 (fig.). 

Ectotrophic mycorrhiza, 72. 

Edible plants, 43, 44. 45, 48, 49, 
52. 53. 60. 203, 204, 255, 261. 
352. 

Egg (ovum). 220 ; of Alga?. 220, 
221 (fig.), 222, 224, 226 ; of 
Angiosperms, 357 (fig.), 366 
(fig.), 367 (fig.), 368; of Coni- 
fers. 348, 349 (fig.) ; of Cycads, 
326, 327 (fig.) ; of Ferns, 302, 
303 (fig.) ; of Fungi, 233 (fig.), 
234. 236 ; of Liverworts, 280, 
281 (fig.) ; of Mosses. 280; of 
Sclagitiella, 319, 320 (fig.). 

Egg-apparatus (of Angiosperms), 
366 (fig.). 

Elaborated food-material, conduc- 
tion of, in Angiosperms, 28, 65, 
88, 110. Ill, 112; aquatics. 
171 ; Conifers. 343 ; Mosses, 
276 ; by laticiferous elements, 
156 ; by the x\-lem. 124. 

EIrEagiuts, 103. 

Elasticity, of coUenchyma, 31, 33 ; 
of cork. 137 : of fibres, 33 ; of 
wood. 127, 128. 

Elaters (of Liverworts) 271 (fig) 
282. 283 (fig.). 

Elder {Sambucus), cork of. 1^6 
(fig.) ; leaf of. no, in (fig.) ; 
lenticels of, 140 (fig.) ; pith of, 

30 (fig). 151. 

Elm (Ulmiis), 130, 139. 141. 365 : 
Cork — . 141 ; secondarv wnod 
of. 120, 126. 

Elodca canadensis, cell-structure. 
5, <) (fig.) ; spread of. in British 
Isles. 392 (fig.). 

Elongation, zone of. in root, 65. 

Embryo of, Angiosperms, 367, 368- 
72 (figs.) ; Conifers, 350-52 
(fig), 370 ; Cycads, 326. 327 
(fig-). 332. 333 ; Dicotyledons. 
368-71 (figs.) ; Ferns, 294. 
302-4 (fig.) ; Fiiais, 221 
(fig.), 22U; Li\-crworts, 2S2 ; 
Monocotyledons. 370, 371 (fig.); 
Mosses, 284, 285 (fig.) ; para- 
sitic Angiosperms, 179, 372; 



INDEX 



425 



Selaginella, 319, 320 (fig.), 
321. 

Embryo sac, 357 (fig.), 364 (fig.), 
367 (fig.), 372, 373 ; develop- 
ment of, 365, 366 (fig.). 

Emetics, 61. 

Empetrum nigrum , leaf of, 163 (fig.). 

Empusa musccs, 231, 238. 

Emiilsin, 48, 33. 

Encephalarios, 324 (fig.) ; 11. hildcii- 
brandlii, 325 (fig.). 

Enchanter's Nightshade (Circcea 
lulctiaiin), 58, 50 (fig.), 146, 
168. 

Endemics, 393. 

Endodermis, 68, 69, 150 ; function 
of,. 69 ; of aquatics, 69, 171 
(fig,), 173 (fig.), 174 (fig.) ; of 
Ferns, 292 (fig.) ; of roots, 66 
(fig.), 68, 69, 70 (fig.), 71, 75 
(fig.) ; of Selaginella, 318. 

Endogenous, 71, 72. 

Endosperm, 327, 351, 384 ; food- 
reserves of, 30 (fig.), 40, 44, 50, 
51 (fig.). 52, 150 ; hybrid, 386 ; 
of Angiosperms, 367, 368, 370, 
371, 386 ; of Conifers, 351, 352 
(fig.) ; of Cycads, 327. 

Endotrophic m5'Corrhiza, 72, 73 

(fig.). 

Energy, hberation of, in fermenta- 
tion, 257, 265, 266, 268. 

Environment, 374. 395 ; adapta- 
tion to, 158-79, 203, 210, 236, 
238, 321 ; influence of, 16S, 
169, 175-7. 374. 376, 377, 
378, 379. 380. 

Enzymes, 53-7, 63 ; digestive, 52, 
149, 150, 363; extraction of, 
53, 54 ; fermenting, 55, 256, 
257 ; mode of action of, 55, 
56, 256 ; of Fungi, 55, 230 ; 
oxidising, 55, 155 ; proteoly- 
tic, 52, 53. 5-t. 149. 

Eosin, 43, 405 ; and sieve-plates, 
82. 

Epacridaceff, geographical distri- 
bution of, 393. 

Epidermis, 65, gi-io6 (figs.), 158- 
63 (figs.) ; functions of, 65, 
77, 95 : glandular, 148, 150 ; 
modification of, 94, 95 ; of 
anthers, 360 (fig.), 363 ; of 
aquatics, 172, 176 (fig.), 177; 
of leaf, I, 2 (fig.), 4, 90, 91-101 
(figs.), 107, 108 (fig.), Ill (fig.), 
115, 158-63 (figs.), 169, 295, 
298 (fig), 341. 342 (fig.). 343. 



344 (fig.) ; of Moss-capsule, 
285 (fig.), 28C ; of root, 66, 74 ; 
of stem, 19 (fig.), 76, 77, 79, 
81 (fig.), 83 (figO, 93, 94 (fig.), 
98, 136 (fig.), 137. 1,59 (fig.). 

Epigynous flowers, 355. 

Epilobium, 389 ; pollen of, 362 
(fig.) ; E. hirsiititm, 174. 

Epiphytes, 73, 272 (fig,), 309 ; 
among Alga?, 206. 

Epithelium, J51, 152 (fig.1, 339 

(fig.). 
Epithem, 146, 147 (fig.), 148. 
Equimolecular solutions, 11. 
Equisetales, 289, 310-13 (figs.), 407. 
Equisetum, 39, 180, 289, 310-13 

(figs.), 321 ; growing point of, 

18 (fig.). 
Equisetum arvense, 311, 312; E. 

limosum, 311 (fig.) ; E. maxi- 
mum, 18 (fig.), 313 (fig.) ; E. 

palustre, 311 (fig.). 
Equivalent osmotic pressures of 

sea- water, 403. 
Erect ovules, 358 (fig.). 
Erepsin, 52. 
Ergot (Claviceps purpurea), 49, 239, 

255 ; life-history of, 241 (fig.), 

242. 
Ericaceas, 178, 361, 363. 
Eriodendron anfracluosuni, 103. 
Eriophorum, 178. 
Ervngium niaritimum-, 93 ; petiole 

of, 113 (fig,). 
Erysiphaceae, 230, 239, 2^1, 253, 

407 ; structure and life-history 

of, 242-4 (fig.). 
Erysiphe graminis, 243 ; E. poly- 

goni, 243, 
Erythronium americanum, 371 (fig.). 
Erythroxylon coca, 61. 
Esters, 49. 
Ethereal oils, 62, 63, 64, 105, 106, 

126, 151. 
Ethyl alcohol, 256. 
Euasti'um, 209 (fig.). 
Eubasidii, 407. 
Eucalyptus, 62 ; E. globulus, 129 ; 

E. marginata, 130. 
Eudorina elegans, 188 (fig.), 192, 
Eugenia caryophyllata, 62 ; secre- 
tory cavities of, 151, 152 (fig.). 
Euonymus, 139, 359. 
Euphorbia, laticiferous cells of, 133, 

154 (fig.), 155 ; leaf of, no. 
Euphorbia amygdaloides, no, 153. 
Euphorbiaceae, 153, 157. 
Eurotium, 242, 243 (fig.). 



426 



INDEX 



Evening Primrose (CEnothera), 
mntation in, 381, 382. 

Evergreen leaves, 112, 335. 

Evolution, 210, 337, 338, 355, 367, 
393-6 ; of flowers, 3,55 ; of 
new species, etc., 3^8, 380, 

381 (fig.)- 
Exalbuminous seeds, 370. 
Excentric starch-grains, 41, 42 

(fif?.). 
Exodcrmis, 67 (fig.), 09, 72, 74, 75 

(fig.). 
Exogenous, 90. 
Extinct plants, 310, 312 (fig.), 313, 

314 (fig.). 319, 322, '3-27-33 

(figs.), 334, 340, 356, 394. 407. 
Extraction of, enzymes, 53, 54 ; 

fibres, 34 ; oils, 49 ; rubber, 

157 ; sugars, 45, 46. 
Extrafloral nectaries, 144, 148, 

149 (fig.). 

Exudation of water, 144-8, 149. 

Eyepiece (of microscope), 398. 

Eye-spot, 181, 193 ; of Chlamy- 
domonas, l8l, 182 (fig.) ; of 
Heematococcus (Sphaerella). 187 
(fig.) ; of zoospores, 213 (fig.), 
219 (fig.). 



Fagtis sylvatica, 72, 129, 140, 170, 

365, 367. 382, 383 (fig.) i wood 

of, 121, 122, 126. 
Fall of leaves, etc., 141, 142 (fig.). 
False Acacia (Robinia pseiidacacia). 

115, 126. 
False plasmolysis, 13-15 ; — tissues, 

239, 241, 248, 249 (fig.). 
Fats, 49, 50, 53, 263 ; formation of, 

50 ; reactions of, 49. 
Fatty acids, 49, 53, 54 ; — bodies, 91, 

92, 136, 223. 
Favus, cause of, 253. 
Febrifuges, 61. 
Fegatella, 270 (fig.), 279. 
Fehling's solution, 401 ; and sugars, 

47, 401 ; and tannins, bo. 
Female sexual cells, 186, 220, 221 

(fig), 233 (fig.), 234, 244 (fig.). 

See also Egg. 
Fennel (Fceniculmn). 61. 
Fennel-leaved Pondweed (Potamo- 

geton pecliiiatus), stem of, 172. 
Fermentation, alcoholic, 255, 25!) ; 

due to Bacteria, 265. 
Ferments. See Enzymes. 
Ferns (Filicales), 180, 193, 289 309 

(figs.), 310, 321, 322, 407; 



antheridia of, 30: (fig.), 302, 
303 (fig.) ; archegonia of, 301 
(fig.), 302, 303 (fig.) ; embryo 
of, 294, 302-4 (fig.), 373 ; 
fertilisation of, 302 ; growing 
point of, 18, 295 ; habit of, 
289-91 ; leaf of, 290-92 
(figs.), 295, 303 : petiole of, 
20.5 : prothallns of, 300-302 
(fig.), 304, 305, 313 ; root of, 
290, 295 ; sporangia of, 295- 
300 (figs.), 303, 308, 309 (fig.) ; 
stem of, 289, 290 (fig.), 292-5 
(figs.) ; vegetative reproduction 

of, 305 (fig). 

Ferrugineous waters, 268. 

Fertilisation, 222, 223, 289, 306, 
332, 382 : m Alga-, 221 (fig.), 
222, 223, 226: in Angiosperms, 
364 (fig.), 366, 367 (fig.), 368, 
372 ; m Conifers, 349, 350, 
353 ; in Cycads, 326, 327 (fig.) ; 
in Ferns, 302 ; in Fungi, 233 
(fig.), 234 ; in Liverworts, 280, 
281 ; in Mosses. 281 ; in 
Selaghiella, 319. 

Fibre-yielding plants, 34, 35. 

Fibres; 32-4 (fig.), 73, 78 (fig.), 86, 
115, 167, 168 (fig.) ; bordored- 
pitted, 33, 37, 121 ; mechanical 
value of. 33 ; of Ferns, 292, 
293 (fig.) ; of secondary 
phloem, 130 ; of secondary 
wood, 33, 121, 123 (fig.). 

Fibres of nuclear spindle, 23. 

Fibrous layer (of anthers), 360 (fig.), 
361, 362, 363 (fig.). 

Fibrous tracheids, 37 (fig.), 134, 

340, 341 (fig.). 
Ficaria veriia. embryo of, 372 ; 
hydathodes of, 145. 146 (fig.), 

147 (fig). 
Ficits. 125 ; F. elaslica. 39. 95, no, 

157. 
Field Horsetail (Eqiiiscliini arccnsc), 

311, 312. 
Fig [J-'iciis). 39. 

" Fi.gurc " (of wood), 127, 128. 
Filaments of, Algic, 7, 195-8 (fig.). 

205, 206 (fig.) : Bacteria, 2(/)2. 

263 (fig.). 268 : Fungi. 230, 

2^1 ; stamens, ^^9 (fig.). ^00 

(fig'.). 
I'ilicales, 310, 407. .S'l'c herns, 
h'ilniy l'\Tns (Hynienoph\"llacca'), 

290, 297 (fig.), 301. 
Fine adjustment (of microscope), 

399. 



INDEX 



42; 



" Finger and Toe" disease (Plas- 
modiophora brassicce), 143, 251, 

254 (fig.)- 
Fir Clubmoss (Lycopod'mm sclago), 

314. 

Fir, Douglas (Pseudolsuga doiig- 
lasii), 128, 129, 352 (fig.), 353; 
Scotcli — {Piniis sylvestris), 37, 
141. 152, 166, 334 <?Z seq. (figs.); 
Silver —(.4 6/fs), 335, 336 (fig.), 
344, 347; Spruce — (Picca 
cxcelsn). 63, 334, 335 (fig.). 

Fixation of nitrogen, 266, 267. 

Flagellata, 406. 

Flax (Linum usitalissimiim), 34, 
265 ; New Zealand — (Phor- 
mium tenax). 34. 

Fleshy roots, structure of. 132, 133, 

154, 155 (fig.)- 
Flexibility (of petiole), 115. 
Floating Alga? (Plankton), 192, 193 

(fig.). 207, 210. 
Floating leaves, structure of, loi, 

174. 177. 
Floating Pondweed {Potamogeton 
nata)is), stem-structure of, 172, 

174 (fig)- 

Floral axis, 354, 355, 370; — nec- 
taries, 144, 148, 149, 363. 

Flowering Plants. See Angiosperms. 

Flowering Rush. (Bittomus), 356. 

Flowers, 354-6, 373 ; cleisto- 
gamic, 364 ; colour of, 63 ; 
detachment of, 142 ; nectaries 
of, 144, 148, 149, 363 : odour 
of. 62 ; zygomorphic, 355, 

373- 
Flowers of tan {Fiiligo), 251. 
Flowing water, plants of, 19,5, 197. 
Fly-disease (Empnsa nuisccB), 231, 

238- 
Fly Toadstool (Amanita muscaria), 

-55- 
Focusing under the microscope, 

400. 

Foliose Liverworts (Jungerman- 
niales), 271 ; antheridia of, 
279 ; archegonia of, 280 ; pro- 
tonema of, 288 ; sporogonia of, 
271 (fig-), 282 ; structure of, 
271 (fig.), 272 ; vegetative 
reproduction of, 277. 

Food-content of plant-products, 53. 

Food-plants, 43. 44, 45, 48, 49, 
52, 53, 60, 203, 204, 255. 261, 

323. 352- 
Food-reserves, 40-52. 242, 319 ; 
accumulation of, 13 ; in seeds, 



40. 44. 45, 47- 4'J, 50. 52, 33; 
351 ; in storage-organs othe 
than seeds, 44, 45, 50 ; nitre 
genous, 50 ; storage of, 40. 6^ 
88, 132 ; translocation of, t 

41, 46, 52, 124, 132, 382. 
Food-storage, 13 ; in Fungi, 231 

in Liverworts, 273; in oospores 

223 ; in roots, 132 ; in seed; 

40. 44. 45, 47. 49. 50, 5- 

333. 351 ; in spores, 319, 321 

in zygospores, 185. 
Foot, 282, 370 ; of Ferns, 302 ; c 

Liverworts, 282, 283 (fig.) ; c 

Mosses, 284, 285 (fig.) ; c 

Selaginella^ 320 (fig.). 
Forests, distribution of, 334, 391. 
Forget-me-not (Myosotis), 102 

— mildew (Oidium), 243. 
Forked branching of, Algse, 200, 20 

(fig.) ; Ferns, 290 ; Lvcopc 

diiim, 315 ; Mosses, 269, 275 

Selagiuella, 315. 
Forked venation, 292, 322. 
Fossil plants, 310, 312 (fig.), 31; 

314 (fig), 319, 322, 327-3 

(figs-), 334. 340, 356, 394. 40; 
Foxglove (Digitalis), 48, 363. 
Fragmentation, in Algs, 212, 21 

(fig.), 216; in Bryophyta, 27; 
Fraxinus excelsior, 120, 122, iii 

129. 
Free central placenta, 356; — m: 
clear division, 245, 319, 368. 
Fronds of. Brown Alga2, 199-20 

(figs.) : Ferns, 289-91 (fig. 

297 (fig). 304 (fig-), 305 (fig-^ 

308 (fig.), 309 (fig.). 
Fructose (fruit-sugar), 45, 46, 5; 

55 ; reactions of, 47. 
Fruit-bodies of, Ascomycetes, 23 

(fig), 240 (fig.), 242, 243 (fig. 

244 (fig.) ; Basidiomycete: 

248, 249 (fig.), 250 (fig.), 25 



(fig 
260. 



Lichens 



58 (fig.: 



Fruits, 31, 33, 46, 61, 63, 93, 10; 
370 ; unripe, 60. 

Frnllania taniarisciiii, 272 (fig.). 

Fuchsia, hydathodes of, 146 ; lea: 
structure of, 107-10 (figs.). 

F'liciis, conceptacles of. 201 (fig. 
202, 224, 225 (fig.) ; habita 
of, 200 (fig.), 203 ; sexu; 
reproduction of, 220, 221 (fig. 
224-6 (fig.), 307 ; thallusof,2oo 
202 (figs.), 225 (fig.) ; veget£ 
tive reproduction of, 216. 



428 



INDEX 



Fucus plalvcarpus, 224 ; F. serratus, 
201 (fig.), 202, 224 ; F. vesi- 
culosiis, 38, 200-202 (figs.), 225 

(fig.). 
Funaria, anthcridia of, 279 (fig.) ; 
development of, 287 (fig.) ; 
habit of, 274 : leaf of, 276 ; 
sporogonium of. 285 (fig.). \ 

Function of, aerenchyma, 17,5 ; i 
alkaloids, 64 ; antipodal cells, 
366 ; bordered pits, 37 ; cam- 
bium, 117; chromoplasts, 64; 
chromosomes, 307, 390 ; cilia, 
182 ; collenchyma, 31 ; con- 
tractile vacuoles, 181 ; cork, 
135. 137, 13S ; cuticle, 92 ; 
elaters of Liverworts, 282 ; 
endodermis, 69 ; epidermis, 
65, 77, 95 ; ethereal oils, 64 ; 
exodermis, 67 ; extrafloral 
nectaries, 149 ; eye-spot, 18: : 
glucosides, 48 ; hairs, loi, 102, 
104, 106, 165 ; hydathodes, 
147; latex, 155, 156; len- 
ticels, 139 ; leucoplasts, 41 ; 
medullary rays, 124 ; mucilage, 
38 ; nectaries, 149, 363 ; 
nuclei, 3, 389 ; palisade tissue, 
107 ; phloem, 88 ; resins, 64 ; 
root-hairs, 72 ; sclerenchyma, 
33; spongy tissue, 107, no; 
stomata, 100 ; s^-nergidje, 366 ; 
tannins, 64 ; transfusion tissue 
of Conifers, 343 ; veins of leaf, 
112; velamen, 74; vestibule 
of stomata, 96, 160 ; xylem, 
88 ; relation of structure and, 
72-4. 87, 88, 109, no, 112, 
115, 144 et scq. 

Fungi, '180, 230-57 (figs.); classi- 
fication of, 231, 406, 407 ; 
economic importance of, 255, 
-5^', -.57 : physiology of, 53, 
72, 100, 252-7 ; protection 
against attacks of. 142, 254 ; 
reproduction of, 232-. 51 (figs.); 
structure of, 230, 231 ; zoo- 
spores of, 232, 233 (fig.), 235 
(fig.), 236. 

Fungicides, 254. 

Funicle, 357 (fig.), 358 (fig.), 359. 

Furniture, timber used for, 129. i ^o. 

Fusion of, cells, 28, 142, 154 ; floral 
members, 355, 373 ; gametes, 
184, 185 (fig.), 213 (fig.), 214, 

^19 (fig), -^37 (fig.). 389 (see 
also Fertilisation) ; integu- 
ments, 357. 



Galactose, 53. 

Galium aparine. hairs of, 104. 

Galls, due to Fungi, 254 (fig.) ; due 
to insects, 60, 142, 143 (fig.). 

Gamctangium, 219 (fig.), 220. 

Gametes of, Chlatnydomonas, 184, 
185 (fig.), 186, 220; Clado- 
phora, 215, 220 ; Ectocarpus, 
219 (fig.), 220 ; Mucor, 237 
(fig.), 238 ; Pleurococcus, igi ; 
Ulothrix, 213 (fig.), 214. See 
also Egg and Spermatozoids. 

Gametophyte, 288, 289, 301, 306. 

Gamopetalous Dicotyledons, 357. 

Gaseous exchange, mechanism of, 
in Angiosperms, 67, 88, 96-101 
(figs.), 107, 108, 139, 140 (fig.), 
175, 178 ; in Conifers, 343 ; in 
Liverworts, 273. 

Gasteromycetes, 407. 

Gelatinous Lichens, 258, 

Gemmae of. Liverworts,. 270 (fig.), 
273 (fig.), 277 ; Mosses, 277, 
287 (fig.). 

Generative cell (of Angiosperms), 
363, 367 (fig.), 368, 

Gentian violet, 405 ; and lignified 
walls, 32; and starch-grains, 43. 

Geoglossum, 239 (fig.), 240. 

Geo,graphical distribution of plants, 
310, 322. 334, 337, 391-3. 

Geolo.gical changes and plant-dis- 
tribution, 393. 

Geotropic stimuli, perception of, 

75. 78- 

Geranium, Garden {Pelargoni'tim), 
hairs of, 105. 

Germination of, Fuciis, 221 (fig.), 
226 ; oospores, 223, 224, 233 
(fig.) ; Pelveiia, 226 ; pollen, 
356, 362, 363, 367 (fig.) ; seeds, 
352, 370, 372 ; spores, 238, 246 
(fig.).' 247, 287 (fig.), 299 (fig.), 
300, ^26, 348 ; zoospores, 213 
(fig.),'2i4,'2i7 (fig.), 218 (fig.): 
232; zygospores, 186, 21^ 
(fig.), 214, 228. 

Gciini, ovules of, 357 ; hybridisa- 
tion in, 385 (fig.). 

Geiini iiitcrmediinn, 385 (fig.), 389. 

Gills (of Mushrooms), 248, 249 (lig.). 

Ginger (Zingiber officinale), 63. 

Gingerbccr, 265. 

Ginkgo biloba, 326, 334. 

Ginkgoalcs, 408. 

Gipsywort (Lycopus europaiis), 174. 

Glacial period, flora of, 394. 

Glands, digestive, 149, 150 (fig.) ; 



INDEX 



429 



oil-secreting, 105 (fig.), 151-3 
(fig.) ; sugar-secreting. 148, 149 
(fig.) ; water-secreting (hyda- 
thodes), 144-8 (figs.). 

Glandular cells, 148, 149 (fig.). 

Glandular hairs, 105 (fig.), 106 298 
(fig.), 299 (fig.), ;,28. 

Gleichenia, 294, 295, 310. 

Globe animalcule (V'olvox), i8g 
(fig,), 190. 

Globoid (of aleurone grains) 51 

(fig.). 

Glceocapsa, 205, 206 (fig.). 

Glucose, 45, 46, 48, 53, 55, 63 ; 
osmotic pressure of, 1 1 ; re- 
actions of, 47. 

Glucosides, 48, 63, 64 ; hydrolysis 
of. 48, 53- 

Glutamin, 50. 

Glycerine, 49, 53. 

Glycerine jelly, use of, 405. 

Glycine, 49. 

Glycogen, 231, 255, 262 ; reactions 
of. 231. 

Gnaphaliiim iiliginosuin, 375 (fig.). 

Gonidia of, Claviceps purpurea, 
241 (fig.), 242 ; Cystopus, 232, 
233 (fig.) ; Erysiphaceas, 243, 
244 (fig.) ; Eurotium [Asper- 
gillus), 243 (fig.) ; Penicillium, 
243 (fig.) ; Phytophthora, 234, 
235 (fig.) ; Pythium, 234, 235 
(fig.), 236 ; Rust-Fungi, 247. 

Gonidiophore, 243 (fig.). 

Gonium, 189, 191 (fig.). 

Gooseberry (Ribes grussularia), 31 ; 
— Mildew (Sphaerotheca niors- 
uvce), 230, 242. 

Goosegrass (Galium aparine), hairs 
of, 104. 

Goosefoot, Silver (Obione portula- 
coides), hairs of, 94 (fig.) ; 
secondary thickening of, 133. 

Gorse (Ulex), 167 ; epidermis of, 
92, 160 (fig.). 

Gossypiuin, 49, 102. 

Gourd {Cucurbita pepo), pollen of, 
362 (fig.). 

Graft-hybrids, 389. 

Grafting, 142, 389. 

Grain (of wood), 127. 

Gramineae, 39, 44, 80, 88, 117, 163, 
354. 356, 408 ; diseases of, 
241, 243, 245 ; grains of, 52 
(fig.), 150 ; hydathodes of, 
145 ; intercalary meristems of, 
21 ; stomata of, 97 (fig.), 100, 

lOI. 



Graminin, 44. 

Grape (Vitis vinifcra), 60, 257; 
— mildew [Plasmopora viticola), 

243- 
Grape sugar. See Glucose. 
Grasses. See Graminea;. 
Grassland, distribution of. 391. 
Grasswrack (Zoster a), pollen of, 

361 ; stem-structure of, 172. 
Gravity, perception of, 75, 78. 
Greater Celandine (Chelidonium 

majus), laticiferous vessels of, 

153, 154; ovule of, 358 (fig.). 
Green Algje, 406. See Chlorophyceae. 
" Green cells " of animals, 260. 
Green manuring, 267. 
Green stems, structure of, 88, 102 

(fig.), 167, 178, 310. 
Ground Ivy (Nepela glechoma), 168 ; 

epidermis of, 94 (fig.). 
Ground tissue, 65, 88 ; of aquatics, 

171 (fig.), 173 (fig.), 175 ; of 

leaf, 107-11 (figs.) ; of petiole, 
114 ; of root, 65, 67 ; of stem. 



19-30. 33. 



76, 



338: 



secondary, 133. 134, 137. 

Growing point, 16-20 (figs.), 26, 
89, 90; intercalary. 21 ; of 
Alga;, 16 (fig,), 17 "(fig.), 195, 
201 ; of Diciyoia, 17 (fig.) ; of 
Equisetum, 18 (fig.) ; of Fern- 
prothallus, 300, 301 (fig.) ; of 
Ferns, 18, 291, 302 ; of leaf, 
18 (fig.), 291 ; of Liverworts, 
27'b, 272 ; of Mosses, 18, 276 
288 ; of root, 18, 20 (fig.). 65 ; 
of Sphacelaria, 16 (fig.), 17; 
of stem, 18 (fig.). 19 (fig.) ; uni- 
cellular, 16-18 (figs.). 

Growth, 16-20, 26, 31, 36, 40, 86 ; 
of Algs, 195, 198, 199, 201 ; of 
cells, 25, 26, 28, 29, 80, 93, 
117, 153. 183. 191 ; of pollen 
tubes, 350, 364, 365 ; second- 
ary, iij et seq. 

Guaiacum officinale, 130. 

Guard cells, 96-101 (figs.) ; change 
of shape of, 98-100 (fig.) ; 
development of, 98 ; raised, 
96 ; sunken, 96, 160. 

Guelder Rose (Viburnum opulus), 
extrafloral nectaries of, 148. 

Gum arable, 39 ; — tragacanth, 39. 

Gums, 38, 39, 155. 

Gutta percha, 157. 

Gymnosperms. 180, 322-7 (figs.), 
334-53 (figs.), 354, 355, 356, 
362, 368, 370, 372, 373, 407. 



430 



INDEX 



Habit of, Abies, 335, 336 (fig.) ; 
Atliyyium fiUx-fcemina, 289, 
291 ; Blechnum spicant, 291, 
308 (fig.) ; Cedrus, 33.5 ; Cephal- 
ozia, 271 (fig.). 272 ; Cctraria 
islandica, 261 (fig.) ; Chondnis 
cyisptis, 205 (fig.) ; Cladonia, 
258 (fig.) ; Cladophora, 196 
(fig.) ; Conifers, 334-6 (figs.) ; 
Cupressiis, 336 ; Cycacls, 322, 
323 (fig.) ; Drapanuildia, 197 
(fig.) ; Ectocarpus, 219 (fig.) ; 
Equiseiuni, 310, 311 (fig.); 
Fegatella, 270 (fig.) ; Frullania, 
272 (fig.) ; Fiicus, 200-202 
(figs.) ; Funaria, 274 ; Hepa- 
ticae, 268-72 (figs.) ; Hymeno- 
phyllaces, 296, 297 (fig.) ; 
Hypnum, 274 (fig.), 275 ; 
Isoetes, 315 (fig.) ; J uiiiperiis, 
336 ; Laminaria, 199 (fig.) ; 
Larix, 335, 336 (fig.) ; Lepi- 
dodendron, 314 (fig.) ; Lichens, 
258 (fig.) ; Lycopodhim, 314, 
316 (fig.) ; Lyginopteris, 328, 
329 (fig.) ; Marchaiitia, 270 
(fig.) ; hinium, 274 (fig.) ; 
Mosses, 274 (fig.) ; Nepfiro- 
dium filix-mas, 290 ; Osmiinda 
regalis, 289, 309 (fig.) ; Par- 
melia physodrs, 258 (fig.> ; 
Pellia. 270 (fig.), 283 (fig.) ; Pel- 
I'elia, 203 (fig.) ; Pli;eophyceEe, 
199-204 (figs.) ; Picca, 334, 
335 (fig.) ; Ptnus, 33,'?, 345 
(fig.) ; Polypodium. 291 (fig.) ; 
Polytrichum, 274 (fig.) ; Pteris 
aquilina, 290 (fig.), 291 ; 
Rhizocarpon confervoides, 258 
(fig.) ; Rhodophycea?, 205 
(fig.) ; Sargassum, 204 (fig.) ; 
Scolopendrium. 291 (fig.) ; 
Selagiiwlla, 316, 317 (fiig.) ; 
Sphagnum, 275 (fig.) ; Taxus, 
335, 346 (fig.) ; Thuja, 336, 
337 (*i§-) ', Tortilla muralis, 
274 (fig.) ; Tree Ferns, 289 ; 
Ulva, 198 ; Usnea harbata, 
258 (fig.) ; Xanihoria parie- 
tina, 258 (fig.). 

Habitat:, adaptation to,i68, 169 (fig.) , 
170. 175-7 (figs.). 379 (fig.), 380. 

Hajmatochrome, 188. 

H<smatococcus, 186, 187 (fig.) ; H. 
droebakensis, 187 (tig.) ; H. 
phivialis, 187 (fig.). 

Hjematoxylin, 404, 405 ; and rell- 
walls, 32 ; source of. 120. 



Hcematoxyloti campechiamnn, 126, 
Hair-moss (Polytrichum), 269, 274 

(fig.), 276, 286. 
Hairs (trichomes), 4, 5 (fig.), 93, 
101-6 (figs.), 158, 162, 163, 
169, 178; bristle-. 104; climb- 
ing, 104; covering, 102 (fig.); 
function of, loi, 102, 104, 106, 
165 ; glandular, 63, 105 (fig.), 
106 ; peltate, 103 (fig.), 105 ; 
root -, 65, 06, 67 (fig.) : stel- 
late, 102 (fig.), 103 ; stinging, 
104, 105 (fig.) ; water-storing, 
94 (fig.) ; water-secreting, 144-6 

(fig). 
Hairs of, Alga?, 202, 219 (fig.), 221 
(fig.), 224, 225 (fig.) ; Bartsia, 
144 ; Black Currant, 105 ; 
Bladderwort, 150 ; Boragin- 
acese, 102, 104 ; Brambles, 104; 
Broad Bean, 148 ; Butterwort, 
149, 150 (fig.) ; Chinese Prim- 
rose, 105 (fig.) ; Coltsfoot, 
102 ; Cotton-plant, 102, 103 ; 
Crucifera?, 103, 104 ; Cyno- 
glossum, 102 (fig.) ; Dciitzin, 102 
(fig.), 103 ; Elceagnus, 103 ; 
Ferns, 291 (fig.) ; Fungi, 240 
(fig.), 244 ; Garden Geranium, 
105 ; Goosegrass, 104 ; Hop, 

102 (fig.), 104, 105 ; Horse 
Chestnut, 102 ; Ice-plant, 94 ; 
Labiata?, 103 ; Malvaceae, 103 ; 
Mosses, 2 79 '(fig.), 280 ; Mullein, 

103 ; Oleander, 161 (fig.), 162 ; 
Pitcher-plant, 149 ; Plane, 
103 ; Rose, 104 ; Runner 
Bean, 144, 145 (fig). 147 ; Sea 
Buckthorn, 103 (fig.) ; semi- 
parasites, 144, 143 (fig.) ; 
Shepherd's Purse, 102 (fig.) ; 
Silk-cotton tree, 103 ; Silver 
Goosefoot, 94 (fig.) ; Silver- 
weed, 102 ; Stinging Nettle, 
104. loj (fig.) ; Sundew, 150 ; 
Thistle, 102 ; Tiliacea?, 103 ; 
Toothwort, 143 (fig.) ; Vege- 
table Marrow, 102 (fig.) ; 
Yellow Rattle, 144, 145 (fig.). 

lialica, leaf-structure of, 106. 
Hard Fern (Blechnum spicant), 291, 

^95. 308 (fig.), 3-4- 
Hard wood, 127. 
Harebell family (Campanulacca^), 

44, 153, 154. 
Hart's-tongue Fern (Scolopeiidninn 

viilgarc), 291 (tig.), 290 (fig.). 
Haustoria o(, embryo sac, 367; 



INDEX 



431 



243 ; 
178 



269, 277. 

; leaf of, 



Fungi, 232, 233 (fig.) 
parasitic Angiosperm: 

(fig.). 179. 
Hawkweed (Hieracinm), 372. 
Hawthorn (CratcEgus). 137, 370. 
Hay, fermentation of, 261. 
Hazel (Corylus), 354 ; pollen tube 

of. 364. 365. 
Heart-wood, 125, 126. 
Heat, and enzyme-action, 56 ; and 

protoplasmic movement, 4 ; 

evolution of, in fermentation, 

261, 265. 
Heat-rays, protection against, 64, 

95, 102, 138. 
Heath-plants. 178, 257, 
Heather {Calluna), 72 

162, 163 (fig,). 
Heather-family (Ericaceje), 178, 

361, 363.' 

Hedera helix, 10 ; secretory canals 
of, 152 (fig.). 

Helianthemiim vulgare, 379 (fig.). 

Helianthits, 49, 113 ; stem-struc- 
ture of, 76-80 (figs.), 87. 

Helleboriis fcetidus, cuticle of, 91. 

Hemibasidii, 407. 

Hemlock [Coniitm maculatum), 61. 

Hemp {Cannabis saiiva), 34, 265. 

Henbane [Hyoscvamus niger). 61. 

Hepatics, 180, 193, 269-73 (figs.), 
277, 278, 355, 407 ; antheridia 
of, 277, 278 (fig.), 279 ; arche- 
gonia of, 280, 281 (fig.) ; 
embryo of, 282 ; fertilisation 
in, 280, 281 ; .gemmae of, 270 
(fig.), 273 (fig.), 277 ; growing 
point of, 270 , 272 ; habitat of, 
269, 272 ; sporogonia of, 271 
(fig.), 282-4 (fig.) ; thallus of, 
269-73 (figs.). 278 (fi.g.). 

Herbaceous plants, stabilitv of, 9, 

87. 
Heredity, 374-91 ; nucleus and, 389. 
Hermaphrodite flowers, 354. 
Heterocj-sts (of Blue-green .A.lga;), 

206 (fig,). 
Heterospory, 321, 332. 
Heterozygo'te, 385. 
Hevea brasiliensis, 156 (fig.), 157. 
Hickory (Carya), 130. 
High power (of microscope), 400. 
Hilum (of starch-grains), 41, 42 

(fig.). 43. 
Hinge-cells (of Grasses), 163, 164 

(fig.). 
Hippopha rhamnoides, hairs of, 103 

(fig.). 



Hippuris, growing point of, 19 
(fig.); leaf of, 177: stem- 
structure of, 172, 173 (fig.). 

Hogweed (Heracleum). 31 ; nectary 
of, 148. 

Hokiis mollis, 112. 

Holdfasts. See Attachnig organs. 

Hollow stems, 80, 81 (fig.), 126. 

Holly (Ilex aquifoliuin), 127 ; leaf- 
structure of, 92 (fig.), 95, III, 
113, 160 (fig,). 

Hollyhock {Altlicea), epidermis of, 
9.5 ; pollen of, 362 (fig,) ; rust 
of {Piiecinia malvacearum), 
248. 253. 

Homospory, 321. 

Plomozygote, 386. 

Honey, 46, 148. 

Hop (Humiilus lupiilus), 63, 257 ; 
fibres of, 34 (fig.) ; hairs of, 
102 (fig.), 104, 105 ; Mildew of 
(Sphaerotheca castagnei). 244 

(fig.). 
Honmdiiim, 208, 213 (fig.), 216. 
Hornbeam (Carpinits betulus,) 128, 

367. 

Hornwort (Ceratophylhim), ana- 
tomy of, 170, 171 (fig.) ; pollen 
of, 361. 

Horse Chestnut {.Esciilus hippo- 
castanum), 52, 59 (fig.) ; hairs 
of, 102 ; stem-structure of, 
120, 121 (fig.), 123 (fig.), 127. 

Horse Radish (Coc/i/facja armoracia), 
48. 

Horsetail (Equisetum), 39, 180, 
289, 310-13 (figs.), 321, 407; 
growing point of, 18 (fig.). 

Host, 179, 230, 232, 245, 247, 248, 
252, 253, 264. 

Hot-beds, 261. 

Hot-springs, organisms of, 204, 
261. 

House-leek (Sempervivitm tectorum), 
98. III. 

Humidity, influence of, on plant- 
structure, 168, 170. 

Humiilus lupuliis, 63, 257 ; fibres 
of, 34 (fig.) ; hairs of, 102 (fig.), 
104, 105 ; Mildew of (Sphaero- 
theca casiagnei], 244 (fig.). 

Humus, 239, 248, 252, 266 ; and 
mycorrhiza, 72, 2^2, 316. 

Hyacinth [Sell la), 44, 365 ; grow- 
ing point of root of, 20 (fig.). 

Hybrid endosperm, 386. 

Hybrids, 253, 382-9 ; graft — , 
389 ; intermediate between 



432 



INDEX 



parents, 382, 387, 388 (fig.) ; 
natural, 385 (fig.), 388, 389; 
result of crossing with parent, 
385 ; segregation of characters 
in, 384-8, 390. 

Hydathodes. 144-8 (figs.), 149, 
179 ; function of, 147 ; me- 
chanism of, 147. 

Hvdniim. 250; H. repandiini, 2^1 

(fig.). 
Hvdra viridis, " green cells " of, 

260. 
Hydrocarbons, 62. 
Hydrocotyle, 115. 
Hydrolysis, 46, 54, 55. 
Hylocoiuium, leaf of, 275 (fig.). 
Hymenium of, Ascomycetes, 240 

(fig.), 242 ; Basidiomycetes, 

248, 249 (fig.), 250 ; Cystopus. 

232 ; Rust-Fungi, 247. 
Hymenomycetes, 407. 
Hymenophyllaceaj, 296, 301. 
Hymenophyllum tunbridgense, 296, 

297 (fig.). 
Hyoscyamine, 61, 62. 
Hyoscyamus niger, 61. 
Hypericum perforatum, secretory 

cavities of, 151, 
Hypertrophy, 232. 
Hypha: (of Fungi), 230, 231, 232, 

^33 (fig.), 241, 243 (fig.), 248. 
Hypnuin, 274 (fig.), 275 (fig.), 276. 
Hypocotyl, structure of, 89 (fig.) 

118 (fig.). 
Hj'poderm, 92 (fig.), 95, iii, 162, 

343, 344 (fig.). 
Hypogynous flowers, 354. 



Iceland Moss (Cetraria islandica), 

258. 261 (fig.). 
Ice-plant [Mesembryanthemutn crys- 

talliynmi), 94, 167. 
Ilex aqiiifolium, 127 ; leaf-structure 

of, 92 (fig.), 95, III, 113, 160 

(fig.). 
Illumination. Sec Light. 
Imbibition water, 10, 26, 42. 
Immunity, 253, 384. 
Incrustation of cell-walls, 39, 204, 

206. 
Indiarubber, 157. 
Indiarubber-plant (Ficiis elaslica), 

157 ; leaf of, 39, 95, no. 
Indican, 48. 

Indigo-plant {I ndigofera) , 48. 
Indirect nuclear division (mitosis), 

22-5 (figs.). 



Individual variations, 374-8 (figs.), 

380. 
Indusium (of Ferns), 296 (fig.), 297 

(fig.), -98 (fig.), 299. 
Infection, 232, 234, 235, 242, 246, 

247. 253, 267. 
Inheritance, 374, 379; in pure 

lines, 377, 378 ; of acquired 

characters, 380. 
Injury, effects of, 26, 48, 116, :42, 

216, 252. 
Ink, 60. 

Insect-galls, 60, 142, 143 (fig.). 
Insect-pollination, 148, 326, 354, 

355, 3t'i. 373. 
Insectivorous plants, 144 ; digestive 

glands of, 149, 150 (fig.). 
Integument of, Angiosperms, 357 

(fig.), 358 (fig.), 359, 370; 
Conifers, 346, 347, 349 (fig.) ; 
Cycads, 325 (fig.), 326, 327 ; 
Lygmopteris. 331, 332, 333 

(fig.). 

Intercalary meristems, 21. 

Intercellular spaces, development 
of, 26 ; of aerench}-ma, 174, 
^75 (fig) ; of aquatics, 171 

(fig.), 175, 176 (fig.), 177 ; of 

bog-plants, 178; of leaves, 
96, 107 et seq. (figs.), 170, 
343 ; of Lichens, 259 ; of 
marsh-plants, 174, 175 (fig.) ; 
of Moss-sporogonium, 284 ; of 
root, 66 (fig.), 67 ; of second- 
ary wood, 124; of stems. 88; 
secretory — . 151, 152 (fig.). 

Interfascicular cambium, iiS (fig.), 
120. 

Interval between pollination and 
fertilisation, in Angiosperms, 
364 ; in Conifers, 350, 

Intrafascicular cambium, 118 (fig.). 

Introduction of new forms, 392, 

393- 

Inulase, 46, 53. 

Iiniliii, II, 44, 43, 53; hydrolysis 
of. 46, 53. 55 ; reactions of, 
44, 47 ; sphere-crystals of, 
4 4. 45 (fig.). 

Invert sugar, 46. 

Invertase, 46, 33, 55, 256 ; extrac- 
tion of, 54. 

Involucre, of Liverworts 2 So 28 1 
(fig.), 283 (fig.) ; of Mosses, 
279, 280, 

Iodine, in Seaweeds, 15. 

Iodine-solution, 401 ; and alka- 
loids, 61 ; and cellulose, 31 ; 



INDEX 



433 



and cuticle, 92 ; and dextrin, 
46; and glycogen, 231; and 
' inulin, 44 ; and lignified walls, 
32 ; and proteins, 3, 50 ; and 
starch, 41 ; and tannins, 60. 

Ions, 14, 15. 

Ipecacuanha (Psychotria ipecacu- 
anha), 61. 

Iris, 44 ; leaf-structure of, 96, 97 
(fig.), 99, 115, 116, 160 ; root- 
structure of, 70 (fig.). 

Iris gcrmanica, 10. 97 (fig.). 

Irish Yew (Taxus baccata, var. fasti- 
giala), 380. 

Irisin, 44. 

Iron-Bacteria, 268. 

Irregular flowers, 355, 373. 

Irritability, of guard-cells, 100 ; of 
sexual cells, 222, 224, 281, 302 ; 
of zoospores, 214, 232. 

Isa'.is iinctoria, 48. 

Island floras, 393. 

Isobilateral leaves, structure of, 97, 
116. 

Isoeles laciistris, 314, 315 (fig.), 407. 

Isogamy, in Alg;e, 18,5 (fig.), 186, 
213 (fig.), 214, 220, 227; in 
Fungi, 237 (fig.), 23S. 

Ivy (Hedera helix), 10 ; secretory 
canals of, 152 (fig.). 

Jarrah {Eucalyptus margiuala), 130. 
Jerusalem Artichoke (Helianthus 

tuberosus), 44. 
Johansen, 378. 
Joinery, timber used in, 129. 
Jointed Rush (Juncus articulatus), 

leaf-structure of, 165 (fig), 

166. 
Juglans nigra, 129. 
Juncacese, 408. 
Juncus articulatus. leaf-structure 

of. 165 (fig.), 166. 
Jungermanniales, 407. 
Juniper (Juuipcrus), 336, 352 (fig.), 

353- 
Jute (Corchorus), 34, 2O5. 
Juvenile forms, of An,giosperms, 

395 (fig.), 396 ; of Conifers, 

337. 338 (%■)■ 

Kapok (Eriodendron anfracluosum) , 

103. 
Karyokinesis, 22. 
Knots (in timber), 129. 
Knotweed (Polvgonum). ovules of, 

358 (fig.). " 
Kola {Cola acuminata) , 61. 

28 



Labiatic, 21, 61, 80, 105. 
Laburnum {Cvtisus lahiirniivn). 137, 

389. 

Lacquer-tree {Rhus vermicifera), 155. 
Lactic acid Bacteria, 265. 
Lactose, 265. 
Ladder-like perforations (of vessels), 

35 (fig.). 
Lady Fern {Athyriunt filix-fcemina), 

289, 291, 305. 
Lady's Mantle (Alchemilla), ^-ji. 
Lagenostoma lomaxi, 332 (fig.). 
Laminaria, habitat of, 199, 203 ; 

reproduction of, 218 ; thallus 

of, 199 (fig.), 200, 202. 
Laminaria digitata, 199 (fig.) ; L. 

saccharitta, 199. 
Lamium, 4, 105, 364 ; coUenchyma 

of, 31 (fig-), 34 (fig-)- 

Lanuum galeobdolon, sun- and shade- 
leaves of, 168, 169 (fig.). 

Land-forms of aquatics, 93, 175-7 
(figs.). 

Landolphia, 157. 

Larch (Larix europcea), 129, 334 ; 
cones of, 336 (fig.), 345, 346, 
347 ; habit of, 335, 336 (fig.) ; 
leaf of, 343 ; seedlings of, 337, 
338 (fig.). 

Lastrcea filix-mas. See Nephrodium 
filix-mas. 

Latent characters (of hj-brids), 
382, 383, 384, 385. 

Lateral branches, of roots, 65. 71 
(fig.) ; of stems, 72, 129. 

Lateral conjugation, 227 (fig.). 

Latex, 153-7. 

Lathrcsa, hydathodes of, 145 (fig.). 

Laticiferous cells, 153. 154 (fig.) ; 
— vessels, 154, 155 (fig), 157. 

Laurel {Laurus nobilis), 151. 

Lavender {Lavendula), 61, 62, 106; 
oil of, 62. 

Leaf, anatomy of, 91-116 (figs.) ; 
connection with stem, 114 
(fig.), 294 (fig.), 295, 344 ; 
development of, 90 ; effect of 
environment on, 168-70 (fig.), 
177 (fig.); evergreen, 112, 
335 ; growing point of , 18 (fig.); 
movements of, 115, 116; posi- 
tion in relation to light of, 
112; reduced, iii; reduction 
r.if, 15S. 165, 178; rolled, 
163-4 (figs.) ; shedding of, 
141, 142 (fig.) ; succulent, 
166; support of, 115; varie- 
gated, III. 



434 



INDEX 



Leaf of, Conifers, 334-7 (figs.) ; 
Cycads, 322, 323 (lig.), 324 
(fig.); Eqiiisctiiiii, 310, 311 
(fig.) ; Ferns, 290-92 (figs.), 
303 ; foliose Liverworts, 271 
(fig.), 272 (fig.) ; Lycopodium, 
314. 315, 316 (fig.) ; Lygin- 
npieris, 328. 329 (fig.) ; Mosses, 
-74- -75 (fig-) ; S('lagii/e!h(, 
316, 317 (fig.j; 318,- 
Leaf-fali, 59, 141, 142 (fig.). 
Leaf'gaps (of Ferns), 294 (fig.), 295. 
Leaf-scars, 142. 
Leaf-stalk. Sec Petiole. 
Leaf-structnre of, Alpines, 170; 
aquatics, 91, 92, 93, loi, 172, 
177 (fig-): Bracken (Pteris). 
148, 149 (fig.), 169 ; Brooklime 
{Veronica bcccabitnga), 160 (fig.); 
Caihavinea iiuditlaia, 276 ; 
Clubnioss (I.ycopodium), 315 ; 
Conifers, 341-4 (figs.) ; Corn 
Cockle [Agrostemma githago), 
no; Crowberry (Empetrum 
■lugniui), 163 (fig.) ; Cycads, 
322 ; Dicotyledons, 93, 98, 
107 ei scq. (figs.) ; Elder 
(Samhucns), no, in (fig.); 
Enchanter's Nightshn de 
(Circisa). 146 ; Eiipliorbia amyg- 
daJoides. no; evergreens, 
112, 33,s ; Ferns, 295, 298 
(fig.) ; Eiciis elailica, 95, no; 
Fuchsia^ 107-10 (figs.), 146 ; 
Ground Ivy [Nepeta gleclioma), 
94 (fig.) ; Hakea, 166 ; I-lcather 
(Calluna). 162, 163 (fig.) ; Holly 
{Hex aquifolium), 92 (fig.), 95 
in, 113, 160 (fig.); Horn- 
wort {CeratophvUitm), 172; 
Ilouse-leek {Sem pcrviv iiiii), 
III; Ice-plant {lilcscmbryaii- 
tlienuim). 167 ; Iris, 96, 97 
(fig.), 113, 116, 160; Jointed 
Rush {Jnuciis aidicu/ahts), 165 
(fig.), nil); leafy Liverworts, 
273 ; Lesser Celandine {Ficarin 
verna), 145, 146 (fig.), 147 (fig.) ; 
Lvgiiiuplcris, 329, 330 ; Male 
Fern {K^ephrodium), 298 (fig.) ; 
Marsh' Marigold {CaUha), 146; 
Monocotyledons, 1)3, 98, 112; 
Mosses, '112, 275 (fig.), 276; 
Myrtle {Mvrliis).' 151 ; Nettle 
{Urtica), 38 (fig.) ; Oleander 
{Nei'iinii (dcaiidcr). no, lOi 
(fig) ; Union (,\Uium), 94 
(fiy)- 'Jl (lig-) ; I'onis. 152, 



166, 34^, 344 (fig) ; Prickly 
Saltwort {Salsola), 166; 
Psamma arenaria, 92, 163, 
164 (fig.) ; Ranunculus aitri- 
comus, 92 (fig.) ; R-ne {Ruia), 
151 ; St. John's Wort {Hyperi- 
cum). 151 ; Sea-blite [Siiceda), 
1()6 ; SelagineUa, 318 ; shade- 
plants, 91, 95, 168, 169 (fig.) ; 
Silver Goose'foot {Obione por- 
tnlacoides), 94 (fig.) ; Spliag- 
num, 275 (fig.), 276; Stone- 
crop (Scrff/iH), 166; succulents, 
iiO, 166; Wallflower (Cheir- 
anthus cheiri), in ; Water 
Buttercup {Ranuiicuhts aqiia- 
tilis), 177 (fi,g.) ; Water Star- 
Avort (Calliinche), 177 ; Wild 
Strawberry (Fragaria). 146; 
Yellow Dead-nettle {Lamium 
galcobdolon), 16S, 169 (fig.) ; 
Yellow Pimpernel (Lysimachia 
nemoriitn) , 160 (fig.) ; Yew 
{Taxiis), 341, 342 (fig.). 

Leaf-traces of, Angiosperms, 86, 
114; aquatics, 173; F'erns, 
294 (fig.), 295 ; Lyginopteris, 
329, 330 (fig.). 

Leafy Liverworts ( J ungermanniales) . 
See Foliose I,iverworts. 

Leathery leaves, structure of, 160. 

Leguminosa;, 30, 72, 371 ; root- 
nodules of, 267, 268. 

Lemna. 112. 

Lemon {Citrus lunonum), 60. 

Lenticcls, 77, 139, 140 (fig.), 141. 

Lepidodendroii, 312 (fig.), 314 (fig.), 

315, 407- 

Leptollinx, 268. 

Lesser Celandine {Ficaria verna), 
embryo of, 372 ; hydathodes 
of, 145, 146 (fig.), 147 (fig.). 

Lettuce {Latuca), 33. 

Leucin, 32. 

Leucobrvuin , 277. 

Leucocytes and Bacteria, 263. 

Leucoplasts, 41, 43. 

Levulose, 43. 

Lianes, structure of, 134. 

Lichens, 407 ; habitat of, 237, 
238 ; reproduction of, 23S 
(fig.), 260 (fig.), 261 ; structure 
of, 239. 260 (fig.) ; thallus of, 

-38 (fig.)- 
Lid (of JNloss-capsule), 284, 283 

(fig.). 286. 
Life-histories. See Reproduction. 
Light and, anthocvanin, 63 ; chloro- 



INDEX 



435 



phyll-formatio n, 41, 74; 
chloroplasts, no, in, 112; 
enzyme-action, 56 ; leal-struc- 
ture, III, 168-70, 172 ; posi- 
tion of leaves, 112; sleep- 
movements, 115; stomatal 
mechanism, 100 ; zoospores, 
etc., 183, 214. 

Light, concentration of, 06 (li.t;.) ; 
perception ol'. 1)5, 181 ; pro- 
tection against, 95, 102, no, 
III, 112, 199. 

Lignification, 32. 

Lignified walls, 32, 33, 68 ; re- 
actions of, 32. 

Lignin, 32. 

Lignum vit;c (Guaiacuin ufficinale), 
130. 

Ligule {ol Selagini'lhi). 318 (fig,). 

Liguliflone, 153. 

Liliaccce, 371, 408. 

Lily (Lilittm), no; embryo of. 371 
(fig.) ; pollen and embryo sac 
of, 367 (fig.) ; stamen of, 363. 

Lime (Tilia), secondary phloem of, 
130 ; secondary wood of, 28, 
120, 127. 

Lime-family (Tiliacea;) , 103. 

Limestone rocks, role of Alga? in 
formation of, 204. 

Linaceas, 61, 

Linseed oil, 49. 

Liniim usitatissimuiii, 34, 49. 

Lipase, 49, 53 ; extraction of, 54. 

Listera ovaia, embryo of, 371 (iig.). 

Litmus, 261. 

Liverworts, 180, 193, 269, 355, 407. 
See Hepaticje. 

Lloyd's alcohol method (for deter- 
mining stomatal aperture) , 
99, 100. 

Lockjaw Bacillus {Bacillus tetani), 
263 (fig.). 

Loculi (of ovary). 356. 364. 

Loganiacece, 61. 

Logwood {Heemato.\ylo)i campe- 
chianum), 126. 

Lopjiocolea, 272, 280. 

Loranthaceae, ovary of, 365. 

Loss of sexuaUty, 214, 236, 245, 

305. 372- 
Low power (of microscope), 400. 
Lowland forms of Alpine plants, 379. 
Lupine (Lnpinus), 30, 47, 52. 
Lvcoperdon, 245, 250, 251 (tig.). 
Lycopodiales, 289, 310, 313-20 

(figs.), 407. 
Lycopodiitm, 180, 289, 310, 314, 



lO (fig.). 318, 321, 407 ; L. 
clavaiiiin. 314, 315, 316 (fig.); 
/-. iiiitiidatuni, 315; L. sclago, 

314. 315. 

Lycoptis europaius, 174. 

Lyginopteris {Lyginodi'iidron) old- 
hamia, 328-33 (figs.) ; ana- 
tomy of, 328-30 (fig.) ; micro- 
sporophylls of, 330, 331 (fig.) ; 
ovules and seeds of, 331-3 
(figs.) ; vegetative organs of, 
328, 329 (fig.). 

Lyiigbya, 205, 206 (fig.). 

lA'sigenous secretory cavities, 151, 
,1.52 (fig.). 

Lvsunachia iiemoriitn stoma of, i6o 

LyUu'uni salicat'ia, 174. 



]\Iacyocvstis, 200. 

Madder {Rubia pevegyina), stoma of, 

97 (fig.). 98. 

Mahogany (Sivietenia mahagoni), 
126, 130. 

Maiden Hair Fern {Adiuiituiii), sori 
of, 291 (fig.), 296. 

Maiden Hair Tree (Ginkgo biloba), 
326, 334, 408. 

Maize {Zea mais), disease of, 248 ; 
grains of, 45, 52. 53. 384 ; 
hybrid-endosperm of, 386 ; leaf 
of, 117; root-structure of, 73, 
74 (fig.) ; stem-structure of, 
84-6 (figs.). 

Male Fern {Nephrodiiim fihx-mas), 
sorus of, 296 (fig.), 298 (fig.) ; 
sporangium of , 299 (fig.) ; spore- 
formation in, 307 (fig.) ; stem 
of, 290, 294 (fig.). 

Male sexual cells, 186, 220, 389; of 
Angiosperms, 364, 367 (fig.), 
368 : of Conifers, 349, 350, 351 
(fig). 353 ; of Fungi, 234, 236, 
244 (fig.). Sec also Spermato- 
zoids. 

Malic acid, 60, 302. 

Mallow (Malva), 130, 152- ovule 
of, 358 (fig.). 

Mallow-family (Malvacea;), hairs 
of, 102, 103 ; stamens of, 360. 

Malt, 46, 256, 257 ; — sugar, 45". 

Maltase, 47, 53. 

Maltose, 45, 46, 53, 56 ; hydrolvsis 
of. 47. 53 i reactions of. 47. 

Malva, 130, 132 ; ovule of, 358 (fig.). 

Malvaceje, hairs of, 102, 103 ; 
stamens of, 360. 



436 



INDEX 



Alandioc {Manihot ulilisslma), 43. 

Mangrove (Rhizophora) , 372, 

Manihot glaziovii. 157 ; M. iitilis- 
sima, 43. 

Mannose, 53. 

Manure, fermentation of, 261 ; use 
of Algse for, 203. 

Maple (Acer rampeslre) , 28, 130, 
133 ; Cork — , 141. 

Maple sugar, 124, 

Marchantia, 407 ; antheridia of, 
279 ; archegonia of, 280, 281 
(lig.) : gemma; of, 270 (fig.), 
27^ (fig.), 277 ; fertilisation of, 
281 ; habit of, 270 (fig.) ; 
sporogonia of, 281, 282 ; struc- 
ture of, 271, 273 (fig.). 

Marchantiales, 407. 

Mare's Tail (Hippiins), growing 
point of, ig (fig.) ; leaf of, 
177; stem-structure of 172, 

173 (fig^). 
Margarine, 49. 

Marine Alg^e. See Seaweeds. 
Marram-grass {Psamma avenaria), 

leaf-structnre of, qj, 163, 164 

(fig.)- 

Marrow (Cucurbita), hairs of, 102 
(fig.) ; phloem of, 29 (fig.), 82, 
130 ; stem-structure of, 80-83 
(figs.) ; tyloses of, 127 (fig.) ; 
vessels of, 35 (fig.), 83. 

Marsh Cudweed [Gnaphalium uli- 
ginosum), 375 (fig.) ; — Marigold 
(Caltha palustris), 72, 146, 356, 
366 (fig.) ; — Pennywort {Hy- 
dfocotylc), 115 ; — Samphire 
(Salicornia), 175 (fig.). 

Marsh-plants, structure of, 72, 100, 

174, 175 (fig.)- 

Meadow Rue (Thalictyum), stem- 
structure of, 86. 

Meadow Saffron [Colchicuin), 364. 

Meadowsweet (Spiraa itlmayia), 
169. 

Measurement under the microscope, 
403. 

Mechanical elements and tissues, 
27, 31-6 (figs.) ; of Ferns, 
2()2, 293 (fig.) ; <it Fiiciis, 202 ; 
of leaves, 112, 114, 115, 166; 
of Lyginopteris, 329, 330 (fig.) ; 
of Mosses, 276 ; of roots, 6g, 
72, 73 ; of secondarily thick- 
ened stems, 121, 123 (fig.), 125, 
137 ; of young stems, 87, 88. 

Mechanics of, aquatics, 170, 171, 
172; leaves, 92, 95, 112, 115, 



n6 : petioles, 115; roots, 
69, 72, 73 ; secondarily thick- 
ened branches, 125, 137 ; 3-oung 
■stems, 87, 88. 

Medicinal plants, 48, 49, 60, 61, 155, 
255. 276. 

Medulla (of Alga;), 202, 225 (fig.). 

Medullary bundles, 86. 

Medullary rays, of Conifers, 340, 341 
(fig.) ; of Lyginopteris, 328, 
330 (fig) ; of root, 132 ; of 
secondary phloem, 130; of 
secondary wood ,119 (fig.), 120, 
122-4 (fig.). 128 (fig.) ; primary 
— , 76. 77 (fig). 78 (fig.), 118. 

Medullary sheath, 79, 124. 

Megasporangium, 318 (fig.), 319, 

324. 325 (fig.), 328, 346. 

Mcgaspore, 321, 332 ; of Angio- 
sperms, 357, 365, 373 ; of 
Conifers, 348, 349 (fig.) ; of 
Cycads. 325 (fig.), 326, 327 
(fig.) ; of Lyginopteris, 331, 
333 (fig.) ', tif Selagiiiella, 318 
(fig). 319. 

Mcgasporophylls, of Angiosperms, 
3.54. 356; of Conifers, 347 (fig.); 
of Cycads, 324, 325 (fig.). 

Meiosis, 305-7 (fig.). 

Melampyrum, 179. 

Melosira, 193 (fig.), 207 (fig.). 

Membrane, cell-, see Cell-wall ; 
semi-permeable — , 8, 9. 

Mendel's law, 383-7. 

Mentha piperita, 62. 

Menthol, 62. 

Mercurialis perennis, leaf of, 37 
(fig.), 59 (fig.), 168, 169 ; petiole 
of, 113 (fig). 

Meristems, intercalary, 21, 199 ; 
primary, 18-21 (figs.), 65, 79, 
89; secondary. 117, n8, 135, 

137- 

Mcristic variations, 374, 376. 

Merulius lacrymans, 252. 

Meseinhryant'hemuin crvstalliiiinu. 
94, 167. 

Mesophyll of, Angiosperms, 107- 
12 (figs.); aquatics, 172, 177 
(fig.) ; Conifers, 342 (fig.). 343. 
344 (fig) ; Ferns. 295. 298 
(fig.) ; shade-leaves, 169 (fig.), 
170. 

Mesozoic period, plants of, 322. 

Metabolism, 58. " 

Metaxylem, 68, 69 ; of Ferns, 292 
(fig.), 293 (fig.) ; of roots, 68, 
70 (hg.) ; of stems, 78 (fig.), 79, 



INDEX 



437 



So (%), 8: (fig.), 83 (fig,), 85 
(fig.), 85, 87 (fig.). 

Methyl blue, 404, 405 ; and cellu- 
lose, 32 ; penetration of, into 
living plant-cells, 12. 

Metroxylon, 44. 

Micrasterias, 209 (fig.), 210. 

Micrococcus, 262, 264. 

Micrometers, 405. 

Micropyle of Angiosperms, 357 
(fig.), 364 (fig.) ; of Conifers, 

347 (fig.). 348. 349 (fiS.) ; of 
Cycads, 325 (fig.), 326; of 
Lyginopteris, 331. 

Microscope, construction of, 398- 
400 (fig.). 

Microspora comma, 263 (fig.). 

Microsporangium, 318 (fig.), 319, 
324 (fig), 328, 330, 331 (fig). 
3.59. 

Microspores, 321, 332 ; of Angio- 
sperms, 373 ; of Conifers, 345, 
347 (%■). 348, 349 ; of Cycads, 
326, 327 (fig.) ; oi Lyginopteris, 
330 ; of Selaginella, 318 (fig.), 
319. 

Microsporophylls of, Angiosperms, 
354. 359 ; Conifers, 345, 346 
(fig.). 347 (fig.) ; Cycads, 323, 
324 (fig.) ; Lxginopleris, 331 

(fig.). 

Middle lamella, 24. 27, 30 (fig.) 31 
(fig.). 36, 37 (fig.). 68, 83, 141. 

Middle layer (of anther-wall). 360 
(fig.). 363 (fig.). 

Midrib of, Algje, 200, 201 (fig.) ; 
carpels, 356; leaves. 31, 108 
(fig.), 113, 115 ; Liverworts, 
270 (fig.), 272 ; Mosses, 274, 
275 (fig.), 276. 

Mildews (Erysiphacea?). 230, 239, 
252, 253 ; structure and life- 
history of, 242-4 (fig.). 

Milk, souring of, 265. 

Millet-grass (Millium effusiim), stoma. 
of, 97 (fig.). 

Millon's reagent, 401 ; and pro- 
teins, 50. 

Mimosa pudica, movements of, 115, 
116. 

Mimulus moschatus, pollen of, 362 

(fig.). 
Mineral deposits in cell-walls, 39. 
Mineral salts, absorption of, 10, 

13. 15. 72. 270 ; conduction of, 
65, 88, 112 ; toxic effect of, 

14. 15. 

Mint {Mentha), 61. 



Mistletoe (Visciim album). 30. 
Mistletoe -family (Loranthacca), 

ovary of, 365. 
Mitosis, in spore-mother cells, 

305-7 (fig.) ; in vegetative 

cells, 20-25 (figs.). 
Mnium, 274 (fig.), 275 (fig.). 276. 
Mnium hornum, stem-structure of, 

275 (fig.). 
Mode (in variation), 375, 376, 377, 

380. 
Molassine meal, 276. 
Molecular solutions, 11. 
Monarch roots, 318. 
Monkey Puzzle (Aruiicariu), 34O, 

347- 

Monkshood {Aconiluin), 61. 

Monocotyledons, 44, 58, 355, 367, 
408 ; embryology of, 370, 371 
(fig.) ; epidermis of, 93 ; flowers 
of, 355 ; leaf of, 93, 98, 112, 
114. 116, 146; root of. 67, 70 
(fig.), 71 ; secondary thicken- 
ing of, 133 (fig.). 134 : stem of, 
36, 82, 84-6 (figs.), 87, 117; 
seeds of, 371. 

Monoecious, 223, 354. 

Monopodial branching, 334. 

Monosaccharides, 44, 45, 46, ^'^. 

Morel [Morchella), 239 (fig.), 240, 255. 

Morphine, 61, 155. 

Moschatel (Adoxa moschatellina), 
epidermis of, 95, 96 (fig.). 

Mosses (see Musci), 180, 193, 257, 

269, 355. 407. 
Mother-cells of, megaspores, 319, 
348. 365. 372 ; pollen grains, 
345. 360 (fig.) I spermatozoids, 
277. 302, 303 (fig.) ; spores. 
282, 284, 285 (fig.), 297, 305-7 

(fig.). 
Mother - of - Thousands {Saxifraga 

sarmeiitosa), 8, 63. 
Moulds, 61, 230, 231, 237 (fig.). 239, 

242, 243 (fig.), 252. 
Jlountains, distribution of forest 

on, 391. 
Mounting of sections, etc., 400, 404. 
Mouse-ear Chickweed {Cerastium), 

vascular svstem of, 113, 114 

(fig.). 
Jlovements of, chloroplasts, 112 ; 
cytoplasm, 4, 6, 104 ; guard- 
cells, 98, 99 (fig.) ; motile Algae. 
181, 182, 183, 189, 190, 193 ; 
spermatozoids, 222, 226, 280, 
302, 326; zoospores, 214, 217, 
3l8, 232 ; .sleep — , 115. 



43S 



INDEX 



Mucilaj^'e, 38, 58, 149, 130, 167, 281, 
30J ; of Algje, 188, 20j, 213. 
217, 222, 225. 

Mucilage-cavities, 152 ; — cells, 273 
(fig) ; — glands, 150 (fig.). 

Mucilaginous walls, 38, 95, 184, 189 
(fig.), 202, 214, 262. 

Mucoy, 231, 252, 257, 406 ; structure 
and life-history of, 236-8 (fig.). 

ilfjifoi'-yeast, 257. 

Mulberry (Morns), 370. 

Mullein (Vcybascum), 103, 378. 

Multicellular, i. 

Multiciliate zoospores, 218 (lig.). 

Multinucleate structures, i-^^, ig8, 
208, 218 (fig.), 224, 234, 326,- 
361, 368. 

Musci, 269, 274-7 (figs.), 304. 
407 ; antheridia of, 279 (fig.) ; 
archegonia of, 280 ; branching 
of, 274, 275; embryo of. 284, 
285 (fig.) ; fertilisation in, 
281 ; growing point of, 18, 
276; habitat of, 269, 277; 
leaves of, 112, 274, 275 (fig.) ; 
protonema of. 287 (fig.), 288 ; 
rhizoids of, 274 ; sporogonia 
of, 284-7 (fi.§-) ; stem of, 
275 (fig.), 276 ; stomata of, 
100, 284, 285 (fig.) ; vegetative 
reproduction of, 277, 287 (fig.), 
288. 

Mushroom [Agariciis], 245, 255, 
407 ; structure and reproduc- 
tion of, 248, 249 (fig.). 

Musk {Mimulus moschaius), pollen 
of, 362 (fig.). 

Mustard (Sinapis), 69. 

Mustard oil, 48, 63. 

Mutants, 381 (fig.), 382, 384. 

Mutation, 380-82 (fig.) ; vegeta- 
tive, 382, 383 (fig.), 390. 

Mycelium, 230, 236, 237 (fig.), 239, 
243, 248. 

Mycorrhiza, 72, 252, 31b; of Bird's- 
Nest Orchid (Ncultia), -ji, 73 

(fig.). 
Myrica gale, 268 ; ovule of, 357, 

359. 

Mvriophyllum , 171. 

Myronate of potash, 48. 

Myrosin, 48, ,=i3. 

Myrtacea', (12. 

Myrtle {Mvi'liia cniiniiiniis). secre- 
tory cavities ot, 131. 

Myxomycetes, 250, 231, 40I). 

jMyxophyeea;, 406. Sec Cyano- 
phycejp 



Naked cells, 184, 186, 213, 250, 

366 ; — flowers, 354. 
Narcotics, 61 
Nasturtium, Garden (Tropceoliiin), 

63. 1 1-5. 
Natural hybrids, 385 (fig.), 388, 

389. 

Natural selection, 378, 390. 

Navi cilia, 207 (fig.). 

Neck of, archegonium of, Conifers, 
348. 349 (fig.) ; Cyeads, 326, 
327 (fig.) ; Ferns, 302, 303 
(fig.) ; Liverworts, 280, 281 
(fig.); Mosses, 280; Sclcigiiiclla, 
319, 320 (fig.). 

Neck-eanal cells, 280, 302. 

Nectaries, 144, 363 ; mechanism 
of, 149 ; structure of, 148, 

149 (fig.). 
Nech-ia, 239, 242, 233 
Needle-shaped crystals, 58, 59 (fig.), 

^37. 
Negative pressure, 35, 69. 
Neottia nidits-avis, 179; mycorrhiza 

°f. 72, 73 (fig.) : pollen of, 361, 

362 (fig.). 
Xepeta glechoma, 1O8 ; epidermis 

of, 94 (fig.). 

Nepenthes, glands of, 149. 

Nephrodium fiUx-mas, sorus of, 296 
(fig.), 298 (fig.) ; sporangium 
of, 299 (fig.) ; spore-formation 
fi"'. 3°7 (fig.) ; stem of, 290, 
,.294 (fig). 

Neriuni oleander, leaf-structure of, 
no, 161 (fig.), 162. 

Nettle (Urtiea), hairs of, 104. 103 
(fig.) ; leaf-structure of. 38 
(fig.). 39 ; root of. 132. 

New species, origin of, 378, 3S0, 
381, 382, 393. 

New Zealand Flax (Plinniiniiii 
lenax), 34. 

Nicotiana tabaciim, 01. 

Nicotine, 61. 

Nightshade, Deadly {Atropa bella- 
donna), 61 ; Fnchanter's — 
{Circa-a hitetiatui), 38, 39 (fig) 
146, 168. 

Nitrate-Bacteria (Nitrobaclcr), 266. 

Nitric acid, and proteins, 50. 

Nitrite-Bacteria (Nitroso)no)!as), 266. 

N itntluictcr, 200. 

Nitrogen-fixation, 266, 267 

Nitrogen-fixing Iiacteria, 266 267 
(ti,?.). 

Nitrogeuous lood-reser\cs, 30-32. 

Nih'osomonas, 266. 



INDEX 



439 



Nitzschia, 207 (li.t;.). 

Nodules ol Lcguminosx', etc, 267 

268. 
Non-essential organs of flowers, 

354- 
Non-reducing sugars, 47. 
Non-vascular plants, 180 et set/. 
Nosepiece (of microscope), 398. 
Nostoc, 205, 206 (fig.). 259. 
NucoUus of, Angiosperms, 357 (lig.), 

358 (fig.), 3.59, 365, 371, 37-i ; 

Conifers, 348, 349 (fig.), 350, 

353 ; Cycads. 325 (fig.), 320. 

327 (lig.) ; Lvgiiioptcns, 331, 

333 (fig.). 

Nuclear division, direct, 21 ; in- 
direct, 20-25 (figs.) ; in .spore- 
mother cells, 305-7 (fig.), 385; 
in vegetative cells, 24, 25. 

Nuclear membrane, 3, 22, 24. 

Nuclear sap, 22. 

Nuclear spindle, 21 (lig.), 2^ (fig.), 
306, 307 (fig.). 

Nuclei, 2 (fig.), 3, 4. 5 (hg.), 7 (fig.), 
50 ; function of, 3, 385, 389 ; 
fusion of, 184, 21Z, 234, 238, 

^45, ^47, -^Si, 30'J, 350. 3'''', 
368, 389 ; and heredity, 389, 
390. 
Nuclei of, ascus, 245 ; Bacteria, 
2O2 ; Cladophoya, 196 (fig.), 
208 ; Cyanophyceae, 205, 210 ; 
Fungi, 231 ; laticiferous ele- 
ments, 153 ; CEdogonuiin, 208, 
217 (fig.) ; Rust Fungi, 247 ; 
spcrmatozoids, 277, 278 (fig.) ; 
Vaiicheria, 198, 218 (fig.) ; 
Yeast (Saccharomvces), 255, 

256 (fig.). 
Nucleolus, 2 (fig.), 3, 21 (fig.), 11, 

24. 306. 

Nutrition, 40, 183, 188, 220 ; and 
sex of Equisctum - prothalli, 
313 ; in plants and animals 
compared, 194 ; of embryos, 
302, 320, 321, 320. 332, 351, 
3C'7. 37°, 373 : °t Moss-sporo- 
gonium, 284 ; of secondary 
wood, 124, 

Nvtiiphcea, a 01, 356. 

Oak (Qiierciis), 130, 3G4, 391 ; bark 
of, 60, 141 ; cork of. 140 ; 
wood of, 28, 35 (fig.), 122, 128 

(fig). 
Oat (Avena), 53 ; fungal diseases 

of, 241, 248 ; starch of, 43. 
Obelia, 193. 



OhiDiic pnytulacoid':^,^ hairs of, 94 
(fig.) ; secondary thickening 

of, 133. 

Objective, 398, 400. 

Ocular, 398. 

CEdogoiiimn, habitat of, 195 ; 
sexual reproduction of, 217 
(fig.), 220-23 (fig), ^^'i; 
structure of, i()5, 197, 208, 217 
(fig.) ; zoospores of, 216, 217 

(fig.). 

(Edogoiiium concateiinluin, 217 (fig.), 
221 (fig.) ; O. diplaiidrum, 221 
(fig.) ; O. laiuhboruiighi, 221 
(fig.); O. pachvaiidrium, 217 
(fig.), 221 (fig.).^ 

CEttaiithe crocaia, stem-structure of, 
86. 

Oinothcra, 389 ; 0. lainarchicnia, 
mutation in, 381, 382. 

Oidium, 243. 

Oil-cake, 50. 

Oils, 40, 49, 50, 53, 155, 351 ; in 
Fungi, 231 ; in oospores, 223; 
in Vaucheria, 208 ; in zygo- 
porcs. 185 ; reactions of, 49. 

Oils, volatile or ethereal, 62, 63, 64, 
105, 106, 126, 151. 

Old Man's Beard (Usnea), 258 (fig.). 

Olea europiBa, 49. 

Oleander (Nerium oleander), leaf- 
structure of , no, 161 (fig.), 162. 

Oleic acid, 49. 

Olive (Olea eiiropaa), 49. 

Olive oil, 49. 

One-sided bordered pits, 122. 

Onion {Allium ecpa), 45 ; cell- 
structure of. I, 2 (fig.), 3, 
26; epidermis of, 94 (fig.), 97 
(fig.), 99 ; stamens of, 359 

(fig). 
Oogamy, 220, 221 (fig.), 223, 226, 

234, 238. 
Oogonium of, Cystopits, 233 (fig.), 
234 ; Fiiciis, 221 (fig.), 224, 

225 (fig.) ; (Edogoiiium, 217 
(fig.), 221 (fig.), 222 ; Pelvetia, 

226 ; Vattcheria, 221 (fig.), 224. 
Oomycetes, 231-6 (figs.), 238, 406. 
Oospore of, Algs, 221 (fig.), 223, 

224, 226 ; Angiosperms, 368 ; 
Conifers, 350 ; Cycads, 326 ; 
Ferns, 302 ; Fun,gi, 233 (fig.), 
234, 230 ; I^iverworts and 
Mosses, 281 ; Scliigiiiella, 319. 

Open grain of wood, 127. 

Opening of stomata, 98-100 (fig.). 

Opium, 61. 



440 



INDEX 



Opium Vn-ppY [Pii paver soiiiiiif cm III), 

6i, 155- 
Optical section, 6 (fig.), 401. 
Orange (Citrus aurantium), 62, 151 ; 

polyembryony of, 371 (fig.), 

372, 390. 
Orchids (Orchidacecs), 355 ; aerial 

roots of, 73-5 (fig.) ; pollen of, 

361. 
Orchis morio, pollen of, 362 (fig.). 
Oregon Pine (Pseudotsnga douglasii), 

129. 
Organic acids, 60. 
Organic material, absorption of, by 

Fungi, 230. 232, 236, 237, 259 ; 

by insectivorous plants, 149, 

150; by mycorrhiza, 72, 232; 

by parasitic Angiosperms, 179. 
Origin of new species, 378, 380, 381, 

382, 393- 
Orthotropous ovules, 358 (fig.). 
Oryza saliva, 4^, 53, 257 ; starch of, 

42 (fig.), 43. 

Osazones, 47 (fig.). 

Oscillatoria, 205. 

Osmic acid, and fats, 49. 

Osmometers, 402, 403. 

Osmosis, 8. 

Osmotic pressure, 8, 10 ; deter- 
mination of, 10, II, 12, 15 ; of 
different strengths of sea-water, 
402, 403 ; of .guard-cells, 100 ; 
of food-reserves, 11, 44, 45 ; 
role of, in growth of plant-cells, 
26. 

O&iintnda rcgalis, habit of, 289, 309 
(tig.) ; prothallus of, 304 ; 
sporangia of, 300, 309 (fig.). 

Osterhout, 13. 402. 

Ovary (of Angiosperms), 334, 354-G, 

364. 365. 

Ovules of, Angiosperms, 354, ^56, 
357-9 (figs.), 365, 306 (fig.) ; 
Conifers, 334, 336, 347 (fig.), 
348, 349 (fig.) ; Cycads, 324, 
325 (fig.) ; Lygiiioptcris. 328, 
330, 331 ; parasites, 179, 
365. 

Ovuliferous scale (of Conifers), 347 

(fig.). 348, 352 (fig.). 353. 
Ovum. See Egg. 
Oxalate of lime, 58, 59, 130, 237 ; 

reactions of, 59. 
Oxalic acid, 58. 
Oxidases (oxidising enzymes), 55, 

155. 
Oxygen and Bacteria, 263 ; and 
protoplasmic movement, 4. 



Pieony {PcEoiiia), 8. 

Pateozoic period, plants oi, 311. 

Pahsade tissue, 38 (fig.), 107, 108 
(fig.), 109, 110, 161 (fig.); arm 
— , no, III ; function of, 107, 
109 ; of Alpines, 170 ; of 
centric leaves, 116, 166; of 
stems, 167 ; of sun- and shade- 
leaves. 169 (fig.) ; of Taxns- 
leaf, 342 (fig.). 

Palmella-stage of, Chlamydoinonas, 
182 (fig), 184; Uloihrix, 214, 

215 (fig). 
Palmitic acid, 49. 
Palms, 44, 117. 

Papaver somniferuin . 61, 153. 
Papaveraccae, 133, 154. 
Paper, 35, 129. 
Papilla;, 103, 148, 342 (fig.) ; stig- 

matic, 336, 364. 
Para-rubber {Hcvea brasiliciisis), 

156. 157.. 

Parallel venation, 112. 

Parasites, protection again.st, 138, 
142, 

Parasitic Angiosperms, 178 (fig.), 
179. 231, 365, 372 ; Bacteria, 
263; Fungi, 220 et seq., 252-5. 

Parasitism, 231, 247, 248, 252, 239, 
264, 2S1, 288, 372 ; and im- 
munity, 233. 

Parenchyma, 27, 33. 69. 

Parenchyma-sheath, 112, X13. 

Parenchymatous cells, 27, ^2, 65, 
76. 

Parietal placentation, 356. 

Parmelia physodes, 258 (fig.). 

Passage-cells of, endodermis, 70 
(fig.), 71, 75 (fig.) ; cxodermis, 
57, 74. 75 (fig.). 

Passive hydathodes, 148. 

Pathogenic Bacteria, 263, 2 04. 

Pea (Pisuiii), 336. 370 ; hybrids of, 
383, 384, 380; mutation in, 
381 (lig.) ; protein of, 30, 52 : 
root of, 72. 74 ; starch of, 42 

(fig.). 
Pea-mildew (Erysiphc polygoiii), 243. 
Pea-nut [Arachis hypogcsa), 40, 33. 
Pear (Pyrus), 60 ; stone-cells of, 

30 (figs.), 32, 33. 
Pe.'tic substances, 31, 
Pectose, 231. 

Pediaslrmn, 191 (fig.), 192. 
Pelargonium, hairs of, 105. 
Pellia, 407 ; anatomy of, 272, 278 

(fig.) ; antheridia of, 277, 278 

(fig.) ; archegonia of, 2S0, 281 



INDEX 



441 



(fig,) ; habit of, J70 (fig.) ; 

sporogonium ol, 282, 283 (fig.). 

Pcllionia, starch-formation in, 41 

4^ (fig.)- 
Peltate hairs, 103 (fig.), 105; — 

leaves, 115. 
Peltigera, 259. 
Pelvetia canaUculata, conceptacles 

of, 226, 269 ; habitat of, 202, 

203 ; thallus of, 203 (fig.). 
Penicilliiim, 239 ; gonidia of, 242, 

243 (fig-). 

Pentarch roots, 69, 71 (fig.). 

Pepper (Piper iiigriiin), 63. 

Peppermint (Mentha piperita), 62 ; 
— oil, 62. 

Pepsin, 52, 54, 57. 

Peptones, 52, ^},. 

Perception of, gravity, 75, 78 ; 
light, 95, 181. 

Perforations of vessels, 28, 35 (fig.), 
83, 121. 

Perfumes, 62, 106. 

Perianth, 370. 

Periblem, 19, 20, 90, 95, 369. 

Pericarp, 370, 371 (fig.). 

Pericycle, 69, 78, 137 ; of aquatics, 
171 ; of Ferns, 292 (fig.) ; 
of Lvginopteris. 328, 330 ; 
of roots, 66 (fig.), 68, 69, 70 
(fig.), 71 (fig.), 131 ; of stems, 
78, 118. 

Pericyclic fibres, 78 (fig.), 79, 80 
(fig.), 87, 118 (fig.), 121 (fig.). 

Periderm (of Lvginopteris), 328. 

Peridinea^, 406. 

Perigynous flowers, 335. 

Periodicity of Alga?, 192, 211. 

Perisperm of Angiosperms, 371 ; 
of Conifers, 352 (fig.). 

Peristome of Mosses, 285 (fig.), 
286 ; mechanism of, 287. 

Perithecium, 241 (fig.), 242. 

Permanent mounting, 404, 40,5. 

Permeability of cell-walls, 9, 29, 
31, 32, 36, 67, 71, 91, 93. 136. 

Permeability of cytoplasm, 9, 12, 
13, 15 ; and guard-cells, 100 ; 
and leaf-movements, 115; de- 
monstration of, 12, 13. 

Peronospora parasitica, 243. 

Petals, 63, 103, 354, 373. 

Petiole, and leaf-fall, 141 ; move- 
ment of, 115, 116; nectaries 
on, 148; of Angiosperms, 113 
(fig.),;_ii5; of Cycads, 322 ; of 
Ferns, 295. 

Petrifactions, 328. 



Peziza. 231, 239 (fig.), 240 (fig.), 

407. 
Phajophycea;, 406 ; reproduction 

of, 216, 218, 219 (fig.), 221 

(fig.), 224-6 (fig.) ; structure 
of, 198-204 (figs.), 208, 210. 

Phanerogamia, 307, 321, iZi-lZ 
(figs.), 407. 

Phaseolus muttijiorus, 115, 371; 
epidermis of stem of, 93, 94 
(fig.) ; hydathodcs of, 144, 

145 (fig-), 147- 
PhcUoderm, 135, 136 (fig), 137, 

138 (fig.), 139- 

Phellogcn, 135-42 (figs.) ; and 
aerenchyma, 175 ; of Conifers, 
338, 339 (fig.). 

Phenylhydrazine hydrochloride, 47, 
401. 

Phloem, 28, 29 (fig.). 82 ; function 
of, 28, 65, 88 ; secondary, 
118, 119 (fig.), 120, 121 (fig.), 
130, 134 

Phloem of, aquatics, 171 (fig.), 172, 
173 (fig.); Conifers, 339 (fig.), 
340, 341. (fig.) ; Dicotyledons, 
28, 29 (fig.), 65, 82 ; Ferns, 
292 (fig.), 293 (fig.) ; leave.s, 
107, 112, 113; Monocotyle- 
dons, 84, 85 (fig.), 86 ; parasi- 
tic Angiosperms, 179; petioles, 
113 (fig.) ; roots, 66 (fig.), 68, 
69, 70 (fig.) ; stems, 76, 77 
(fig.), 79, 81 (fig.), 82, 84 (fig.), 
85 (fig.), 86, 118 (fig.). 

Phloem-bundles, 86. 

Phloem-parenchyma, 29 (fig.), 78 
(fig.), 79, 82, 83, 84, 86, 88, 
130, 132 ; of Conifers, 339 ; 
of Ferns, 292 (fig.), 293 

(fig.). 

Phloroglucin, 402 ; and lignified 
walls, 32. 

Phormiuni tenax, 34. 

Phosphorescence of meat, 265. 

Photosynthesis. See Carbon dioxide 
assimilation. 

Phototaxis, 183, 214. 

Phragmidiinn biilbosiiin, 248. 

Phycomycetes, 231-8 (figs.), 240, 
243, 406. 

Phylogeny, 210, 396. 

Physiology of, cells, 8-15 ; Bac- 
teria, 261, 263, 264-7 i Fungi, 
55, 72, 100, 252-7. 

Phytophthora infestans, 230, 231, 
253. 254 ; structure and repro- 
duction of, 234, 235 (fig.), 236, 



442 



INDEX 



f'3. 1-9. 350 ; habit 
335 (fiS-) ; leaf of. 



Picea cxcclsa 
of. 334. 
343- 

Pigments, 60, 64, 150 ; of Bacteria, 
264 ; of chloroplasts, 109 ; of 
corii, 137 ; of flowers, 63 ; of 
Fungi, 231 ; of Hizmatococcus, 
188; of heart-wood, 125, 126; 
of Lichens, 261 ; of Seaweeds, 
198, 199, 

Pihferous layer, 66, 67 (lig.), 69, 

1^. 77- 

Pimpernel, Yellow {Lvsiiiiachia 
nemorum), stoma of, ii>o (li,;;.). 

Pimpinella anisiim, 62. 

Pine, Oregon (Pseudotsugii ilmtg- 
lasii), 129 ; Pitch — {Piuiis 
palustris), 129; Stone — (Piinis 
Pinea), 352 (fig.). 

PiHgiiicula, glands of, 149, 150 (fig.). 

Pinus, archegonia of, 348, 349 (fig.) ; 
economic importance of, 63, 
129; embryology of, 350, 351 
(fig.) ; female cones of, 345 
(fig.). 346. 347 (fig.) ; fertilisa- 
tion of, 350 ; habit of, 334, 
335; leaf-structure of, 152, 166, 
342-4(figs.) ; malecones of, 344, 
345 (fig.), 347 (fig.) ; ovules of, 
348, 349 (fig.) ; pollen of, 345, 
347 (fig-). 351 (fig) ; polhna- 
tion of, 348, 350 ; seeds of, 
351. 35^ (fig.) ; seedlings of. 
337. 338 (fig.) ; stem-structure 
of. 338-41 (figs.) ; wood of, 
37 (fig.). 340. 341 (fig)- 

Pintis mojwphvlla. 335 ; P. palus- 
tyis, 129, 340; P. pinaster, 
63 ; P. pinea, 332 (fig.) ; P. 
strohus, 129, 335 ; P. sylvestris, 

37 (fig-). 1-^9. 15^. 166. 334 et 

seq. (figs.), 391. 
Piper nigrum, 63. 
Pit, simple, 30 (fig.), a, 121, T22, 

276; bordered, 33, 35 (fig.), 

36, 37, 68, 120, 293, 340. 341 

(fig.). 

Pit-canals, 33. 

Pit-membrane, 30 (fig.), 36, 

Pitch Pine {Piniis palustris) 

Pitcher-plant [Nepenthes), 
of, 149. 

Pith of, petiole, 114; root 
(lig.), 72, 00 ; stem 
43, 4.5, 76, 77 (lig.). So 
172, 173 (fig.), 323, 340. 

Pith-cavity, 80, 81 (fig.). 

Pitted vessels, 35 (fig.), 36, 



120. 
129. 
glands 



69, 
19, 



30, 



Placenta of, Angiospcrms, 350, 3,59, 
365 : Ferns, 295, 296, 298 (fig.). 

Plane (Plataiius), bark of, 141 ; 
hairs of, 102, 103. 

Plankton, 192, 193 (fig.), 207, 210. 

Plantago laneeolata, 367. 

Plants, compared with animals, 
14, 25, 181, 188, 193, 194, 
-47. 307 : differences between 
animals and, 188, 193, 194. 

Plasmatic membrane, 3, 4, 8, 9, 11, 
12, 14. 

Plasmodiophura brassiecE, 251, 254 

(fi.g-). 
Plasmodium (ol Slime-Fungi), 250. 
Plasmolysis, false, 13, 14, 13 ; 

nature of, 9 ; true, 5 (fig), 8, 

II, 15- 
Plasinnpora vitieoia, 243. 
Plastids, 4, 25, 41, 50, 64. 
Platvceriuni, 309. 
Plerome, 19, 20, 89, 369. 
Plenrococcus, i, 190-92 (fig.). 
Pleurotcenium. 209 (fig.). 
Pleurotus ostrealns, Plate II. 
Plimmer, 54. 
Plocamium, 204. 
Plum (Prnniis), 93 ; Sih-cr-lcaf 

disease of [Stereitm purpuyeiim), 

250, 252. 
Plumule of, Angiosperms, 369 (fig.), 

370. 371 (fig.). 37,^ ; Conifers, 

338 (fig.). 350. 351 (fig.) ; 

Cycads, 326. 

Pneumococcus, 263 (fig.). 

Podoearpus, 337. 

Poisonous plants, 48, 60, 61, 255. 

Poisons, and protoplasmic move- 
ment, 4. 

Polar nuclei (of embrvo sac) 357 
(fig.), 366, 367 (fig."), 36S. 

Polarised light and starch-grains, 

43. 

Poles (ol nuclear spindle), z^. 

PfjUarded trees, 142. 

Pollen mother-cells, 360 (fig.), 361. 

Pollen of, Angiosperms 350 361 
362 (fig.): 307 (fi,g.), 373 ; 
Conifers, 345, 348, 349, 351 
(fig.) ; Cycads, 326 ; water- 
plants, 301. 

Pollen-chamber, of Cvcads, 325 
(fig.). ^2i^; of lA'guiol^teyis, 3^0, 

331. 33^. 333 (fig.)- 
Pollen-sacs i^\ , Aii,L;i()SiH'rms t^^.i, 
360-03 (tigs); Conifers, 343. 
347 (fig) : Cycads, :^-^, "324 
(fig-)- 



INDEX 



44i 



Pollen tube of, Angiospcrms, 3(33, 
364 (lig.). 365, 3C7 (fig.) ; 
Conifers, 349 dig.), 350, 351 
(fij?.), 353 ; Cycads, 325 (fig,), 
326, 3^7 ffig-). 332. 

Pollination, by insects, 46, 361, 
373 ; by water, 361 ; by wind, 
3t>i, 373 ; of Angiosperms, 
355,356,363,37^; of Conifers, 
345, 348, 349, 350 ; of Cycads, 
326; oi Lyginopteris, 330. 331. 

Pollination-drop, 332 ; of Conifers, 
348 ; of Cycads, 326. 

PoUinia (of Orchids). 361, 362 (tig). 

Polyarch roots, 69, 70 (fig). 

Polyembryony, 371 (fig.), 372, 390. 

PolygonaceEE, 35O. 

Polygoimm, ovule of, 358 (fig.). 

Polvgoninn convolvulus, extrafloral 
nectaries of, 148 ; petiole of, 

113 (fig-)- 
Polypetalous Dicotyledons, 337. 
Polypody {Polvpodium), 309 ; sori 

of, 291 (fig.), 293. 
Polvporus, 2j^ ; P. sqiunuosus, 250 

' (fig-)- 
Polysaccharides, 31, 43, 44, 48, 

231 ; hydrolysis of, 46, 48. 
Polystictis versicolor. Plate fl. 
Poiytrichum, 274 (fig.), 276, 2S6 ; 

P. juniperhimn, 269. 
Pond-scums (Algs), 195 et scq. 
Pond v;eed{Potaniogetoii) , stamen of, 

359 (fig-) i stem-structure of, 

17^, 174 (fig-)- 
Pondweed - family (Potamogeton- 

aceae), 178. 
Poplar (Popnlus). 130, 141. 142 ; 

vessels of, 28, 35 (fig.). 
Poppy (Papaver), 61, 155, 359. 
Poppy-family (Papaveracefe), 153, 

i'54- 
Populations, inheritance among, 

378. 
Populus, 130, 141, 142 ; vessels of, 

^8, 35 (fig-)- 
Pores of. Liverworts, 273 (fig.) ; 

pollen-grains of Angiosperms, 

362 (fig.) ; sieve-plates, 28 ; 

stomata, 96, 98, 99 (fig.), 100. 
Porogamy, 364 (Ag-)- 365- 
Porometer, 99, 100. 
Potamogeton, 359 (fig.) ; P. lucens, 

172 ; P. nataiis, 172, 174 (fig.) ; 

P. pectinalus, 172. 
Potamogetonacea?, 178. 
Potash, and callose, 82 ; and cork, 

136 ; and fats, 49. 



Potato {Solituiiiji titbcrnsitni), food- 
content of, 41, 42 (fig.), 43, 50, 
52, 53 ; starch of, 41, 42 (fig.). 

Potato-blight (Phyiophlhora iii- 
festans). 230. 231, 253. 254; 
structure and reproduction of, 
234. 235 (fig.), 236. 

Potato-family (Solanacea:'), 58, 60, 
61, 81. 

Preservation of fossil-plants, 312, 
328 ; of plant-material, 404. 

Prickles, 104. 

Pricklv Pear, 392. 

Prickly Saltwort {Salsola kali), leaf 
of, 94, 166. 

Primary endosperm nucleus (of 
Angiosperms), 366 (fig.). 

Primary medullary rays, 76. 78 
(fig.), 118; of Conifers, 340; 
of secondarily thickened stems, 
119 (fig.), 120, 122, 124; of 
.secondarily thickened roots, 

132- 
Primrose (Primula), hairs of, 105 
(fig.) ; hybrids of, 387, 388 

(fig-)- 
Primulacea?, 356. 
Procambial strands, 89, 90, 117, 

171. 
Proembryo of, Angiosperms, 368, 

369 (fig.), 371 (fig-). 372 ; 

Conifers, 350, 351 (fig.) ; 
Cycads, 327 (fig.). 

Prop-roots, structure of , 73, 74 (lig.). 

Prosenchymatous cells, 27, 32, 37. 

Proteaces, distribution of, 394 (fig.)- 

Proteases, 53. 

Protection against, animals, 39, 48, 
58, 64, :o4, 106, 126, 157 ; 
excessive transpiration, 64, 91, 
92, 106, 137, 158-65, 203 ; 
heat-rays, 64, 95, 102, 138 ; 
injury, 92, 115, 142, 155; 
intense light, 95, 102, no, in, 
112, 199; parasites. 138, 142. 

Protection of embryos, 321. 332, 
333, 373 ; of lateral roots, 72. 

Protective tissues, gi et scq.. 135 
et seq. 

Proteins, 2, 7, 14, 40, 50-52, 53, 
143, 262, 343 ; decomposition 
oi. b^' Bacteria, 263 ; hvdrolv- 
sis of, =,1 ; in latex. 135 ; re- 
actions of, 3, 49 ; S()lul)le, 50 ; 
translocation cif, 52, 88. 
Proteolytic enzj-mes. 52, 33, 54, 

149. 
Prothalhis of, Angiosperms, 373 ; 



444 



INDEX 



Conifers, 348, 349 (fig.) ; 
Cycads, 325 (fig.), 326, 327 
(fig.) ; Equisetum, 313 ; Ferns, 
299 (fi,g.), 300^302 (fig.), 304, 
3°5. 313 i Lycopodiiim, 316 
(fig.) ; Lyginopteris, 330, 331 ; 
Selagliiella, 319, 320 (fig.), 321. 

Protococcus (Hcematococcus), 186. 

Protonema, 287 (fig.), 288. 

Protophloem, 90. 

Protoplasm, 2, 3 ; composition of, 
2, 50 ; growtli of. 25 ; move- 
ment of, 4, 6 ; permeability 
of, 9, 12, 13, 15 ; properties 
and structure of, 3, 4. 

Protoplasmic connections, 24, 30, 
190. 

Protoplast, 2, 3. 

Protoxylem, 36, 68, 89 ; of Cycads, 
322 ; of Ferns, 292 (fig.). 293 ; 
of leaf, 112, 114 ; of Lvgiiiop- 
teris, 328 ; of root, 66 (fig.), 68, 
69, 70 (fig.), 318, 341 ; of stem, 
78 (fig.), 79, 80 (fig.). 81 (fig.), 
83 (fig.), 84 (fig.), 85 (fig.), 86, 
118 (fig.). 

Protoxylem canal. 85 (fig.), 86. 

Prussic acid. 48 ; and enz3.-mc- 
action, 56. 

Psamina arenayia, leaf-structure of, 
92, 163, 164 (fig.). 

Pseudomonas, 262, 263 (fig.). 

Pseudotsuga douglasii, 128, 
cone of. 352 (fig), 353. 

Psilopfiytales, 407. 

Psiloium, 407. 

Psychotria ipecacuanha, 61. 

Pteridophyta, 289-321 (figs.; 

334. 355, 370, 373, 407- 
Pteridosperms, 327-33 (figs.) 

407. 
Pteris aquiliiia, extrafloral 

taries of, 148, 149 (fi 

of, 289, 290 (fig.) 

112, 169, 291, 295 

290 (fig.), 292-4 

of, 296 (fig.). 
Ptomaines, hi. 
Pnccinia anemones, 247, 248 ; P. 

graminis, 245-7 (^g,) '• P' "'"'- 

vacearum, 248, 253. 
Puff-ball (Lycoperdon), 245, 250. 

251 (fig.), 407. 
PuUing-strains, resistance to, in 

aquatics, 88, 170 ; in roots, 72. 
Pulses, 43. 
Pulvinus, structure of, 115, 116 

CI?.)- 



129; 



333, 
334, 

nec- 

habit 

leaf of. 60, 

rhizome of, 

(figs,) ; sori 



Pure cultures (of Bacteria), 264 ; 
-~ Unes, 377, 378. 

Purine-dcrivatives, 61. 

Purple Dead-nettle {Lamium pur- 
piireum), 364 ; — Loosestrife 
(Lythntm salicaria), ij^. 

Putrefaction, 265. 

Pvrenoids. 7 (fig.), 8, 181, 182 (fig.), 
187 (fig.). 196 (fig.), 207, 208, 
209 (fig.), 217 (fig,), 227 (fig.). 

Pyrenomycetes, 407. 

PvtJiium debaryanum, 231, 252, 
253, 406 ; structure and life- 
history of, 234, 235 (fig,), 236. 

Qualitative variations, 374, 375 

(fig.), 376 (tig.). 
Quantitative variations, 374, 
Querciis, 130, 364 ; bark of. 60, 

141 ; cork of, 140 ; wood of, 

28, 35 (fig,), 122, 128 (fig.). 
Quei'cus intermedia, 389 ; Q. suber, 

140. 
OuiHaia-\>cir\i, 48. 
Quillwort [Isoeies Jacustris), 314, 

315 (fig.). 
Quinine, 61. 

Radiation, prevention of, 138. 

Radicle of, Angiosperms, 369 (fig.), 
37°, 37^ (fig.) ; Conifers, 350, 
351 (fig.). 35^ (fig.) ; Cycads, 
326 ; Selaginella, 320 (fig,). 

Raffia-tape, 34. 

Rainfall and plant-distribution. 391. 

Raising of stomata, go, 160 (fig,), 
161, 162, 170. 

Ramentum (of Ferns), 291 (fig.). 

Rancidity of butter, cause of, 265. 

Ranunculacefe, 60, 354, 357. 

Ranunculus aqnatilis, 177 (fig.) ; 
R. auyicomus, 92 (fig.) ; R. 
repens, 66. 

Raphe (of anatropous ovules), 357 
(fi.?.). 35S (fi,g.). 

Rapliio, 3t. 

Raphides, 58, 39 (fig.). 

Rattle, Yellow' (Riiinauthus), hyd- 
athodes of. 144, 145 (fig.). 

Rays, of Conifers. 340, 341 (fig.) ; 
of Lyginopteris, 328, 330 (fig.) ; 
of root. 132 ; of secondary 
phloem, 130 ; of sccondarv 
wood, 119 (fig,), 120, 122-4 
(fig,), 128 (fig,) ; primary — , 
76, 77 (fig.), 78 (fig.), iis: 

Reactions of, alkaloids, 61 ; cal- 
lose, 82 ; cellulose, 4, 31. 32 ; 



INDEX 



445 



cuticle, gz ; dextrin, 46, 47; 
ethereal oils, 63 ; fats, 49 ; 
glycogen, 231 ; inulin, 44, 47 ; 
ligniiied walls, 32 ; mucilage, 
38, 95 ; oils, 49 ; oxalate of 
lime, 59 ; proteins, 3, 49, 50 ; 
starch, 8, 41, 43 ; suberised 
walls, 136, 137 ; sugars, 47 ; 
tannins, 49. 60 ; terpenes, 63. 

Reagents, microscopic, 401, 402, 

Receptacle of flower, 354, 355, 370. 

lieception of pollen, in Angiospcrms, 
3.56 ; in Gymnosperms, 326, 
348; in Lyginopteris, 331. 

Receptive spot, 221 (fig.), 222, 224. 

Recessive characters, 384 et seq. 

Red Alga; (Rhodophycea;), 203, 
406 ; reproduction of. 205 
(fig.), 228 ; structure of, 203, 
204, 205 (fig.), 208. 

Red Deal (Pinus sylvesiris), 129 ; 
— Currant (Ribes), 115. 

lied Snow, 188. 

Reducing sugars, 47. 

Reduction of, flowers, 354, 355 ; 
leaf-surface, 158, 178 ; num- 
ber of ovules, 356 ; number of 
spores, 319, 332 ; poUen-out- 
put, 373 ; vascular system, 

171-3. 3"- 
Reduction division, 305-7 (fig.), 

361, 365, 372 ; and segregation 

in hybrids, 385, 390. 
Remedies against parasitic Fungi, 

254- 
Reproduction, 190, 193, 195, 197, 

207, 211, 212, 215, 216, 228, 

229, 320, 321. 
Reproduction of, Abies, 336 (fig.) 

344, 347; Adiantum, 296 
Agaricus, 248, 249 (fig.) ; Algae 
212-29 (figs.) ; Angiosperms 
357-73 (figs.) : Araucana 

345, 346, 347 ; Ascomycetes 
238-45 (figs.) : Asplenium 

2y|j (fig.), 305 (fig.) ; ^^'I'y- 

rium, 305; Aulacomnion, 277 
Bacteria, 262, 267 (fig.) ; Basi 
diomycetes, 245-51 (figs.) 
Blechnum, 295 ; Boletus, 250, 
251 (fig.) ; Bryophyta, 277- 
88 (figs.) ; Cednis, 353 ; 
Cephalozia, 271 (fig.) ; Chlamy- 
domonas, 182-6 (figs.), 188, 
220; Cladophora, 196 (fig.), 
197, 212, 215, 216, 220; 
Clavaria, 250, 251 (fig.) ; 
Claviceps purpurea, 241, 242 



(fig.) ; Conifers, 344-53 (fig.) ; 
Conjugatae, 216, 226-8 (fig.) ; 
Cryplomeria, 347 ; Cupressineae, 
347, 348, 350 ; Cupressus, 345, 
347 (fig.) ; Cyanophyceae, 206 
(fig.), 216, 220; Cycads, 
3^3-7 (figs.) ; Cystopus, 231-4 
(fig.) ; Desmids, 209 (fig.), 
216, 220, 228 ; Diatoms, 216, 
219, 228; Ectocarpus, 212, 
218, 219 (fig.), 220; Empusa, 

238 ; Equisetum, 312, 313 
(fig.) ; Ery.siphaceaE, 242-4 
(fig.) ; Eudorina, 188 (fig.), 
i8g ; Eurotium (Aspergillus), 
242, 243 (fig.) ; Fegaiella, 270, 
(fig), 279 ; Ferns, 291 (fig.), 
295-309 (figs.) ; Funis, 20: 
(fig.), 216, 220, 221 (fig.), 
224-6 (fig.) ; Funayia, 279, 285 
(iig.), 287 (fig.) ; Geoglossum, 

239 (fig.), 240 ; HiBmatococcus 
(Sphaerella), 188 ; Hepatica;, 
277-9 (fig.), 280, 281 (fig.), 
282-4 (fig.), 288 ; Hormi- 
dium, 215 (fig.), 216 ; Hvdnum, 
250, 251 (fig.) ; Hymenophyl- 
lacea;, 296, 297 (fig.), 301 ; 
Juniperus, 352 (fig.), 353 ; 
Laniinaria, 218 ; Layix, 336 
(fig.), 345, 346, 347 ; Leafy 
Liverworts, 277, 279, 280, 282, 
288 ; Lichens, 259, 260 (fig.), 
261 ; Lycoperdon, 250, 251 
(fig.) ; Lycopodium, 315, 316 
(fig.) ; Lyginopteris, 330-33 
(figs.) ; Marehanlia, 270 (fig.), 
273 (fig.). 277, 279, 280, 281 
(fig.), 282 ; Morchella, 239 
(fig.), 240 ; Mucor, 236-8 
(fig.) ; Musci, 277, 279 (fig.), 
280, 284-8 (figs.) ; Myxo- 
mycetes, 251 ; Neelria, 242 ; 
Nephrodium filix-mas, 296 
(fig), 297, 298 (fig.), 299 (fig.), 
300; CEdogoniuin, 216, 217 
(fig.), 220-23 (fig.) ; Oomy- 
cetes, 231-6 (figs.), 238 ; 
Osmunda, 300, 304, 308, 309 
(fig.) ; parasitic Angiosperms, 
179, 365 ; Pediastrum, 192 ; 
Pellia, 270 (fig.), 277, 278 
(fig.), 280, 281 (fig.), 282, 283 
(fi.g.) ; Pelvetia, 226 ; Penicil- 
liuin, 242, 243 (fig.) ; Peziza, 
239 (fig.), 240 (fig.) ; Phyco- 
mycetes, 231-8 (figs.) ; Phy- 
tophthora, 234, 235 (fig.), 236; 



446 



INDEX 



Picea,i50;Pi)iits. 344. 345(fig-), 
346, 347 (fis). 348. 349 (fig.). 
350. 351 (fig). 3.5^ (fig.). 353 ; 
Pleurococcus, i<)i, 192 (fig.) ; 
Polypodimn, 296 ; Polvporiis, 
250 (fig.) ; PoIyU-icliiitn, 274, 
(fig.), 286 ; Pteris, 2'>t> (fig.), 
300 ; Piicciiiia gyainims, 245-7 
(fig.) ; Pylliiiim, 234, 235 (fig.), 
236; Khodophyceje, 205 (fig.), 
228 ; Riccia, 284 ; Saccharo- 
myces, 256 (fig.) ; Saprohgnia, 
235 (fig.), 236 ; Scenedesinus, 
192 ; Sclerotinia, 239 (fig.), 
241 ; Scolopendrium , 296 (fig.) ; 
Selaginella, 318-20 (fig) ; 
Sequoia, 347, 352 (fig.) ; Sphag- 
num. 275 (fig.) ; Spirogvra , 
216, 226-8 (fig.) ; Taxus, 

344. 34.5. 346 (fig.). 347 I 
riiuja, 337 (fig.), 347, 330; 
Tubcy, 240, 241 ; iUothyix, 212- 
15 (figs.), 220; UrcdineEE, 
245 -.S (fig); Ustilaginere, 
248; VaUL'heria, 217, 218 
(fig.), 220, 221 (fig.), 223, 
224; \olvox, 190; Xylaria, 
239 (fig.), 242; Zyguema. 22b-8 
(fig,) ; Zygomycetes, 236-8 (fig.). 

Reproductive cells, asexual, 212, 
219. See Gonidia, Spores, and 
Zoospores. 

Reproductive cells, sexual, 184, 
214, 220. See Egg, Gametes, 
and Spcrmatozoids. 

Resedaceae, distribution of, 394 (fig.). 

Reserve-cellulose, 48, 53 ; hydroly- 
sis of, 53. 

Reserve-substances, 40-52, 242, 
319 ; accumulation of, 13 ; in 
seeds, 40, 44, 45, 47, 49, 50, 
5-. 333. 351 ; in storage-organs 
other than seeds, 44, 45, 50 ; 
nitrogenous, 50 ; .storage of, 
40, 88, 132 ; translocation of, 

<^. 41, 46, 5-:. 124. 13-^, -8^. 
Resin-cells, 340. 
Resln-passages, 63, 339 (fig.), 340, 

341, 34^ (fig.), 343, 344 (fig.)- 
Rcsms, 63, 64, 125, 151, 153, 334. 
Resistance to, bending, J15, 172; 
disease, 263, 384 ; drought, 
186, igi, 205, 215; pulling 
strains, 72, 88, 170; tearing, 

93. 115. 
Jiespiration, 178, 201, 257 ; and 
protoplasmic movement, 4 ; 
of fat-containing seeds, 50. 



Respiratory cavitj-, 97, 108 (fig.;, 
no, 164 (fig.), 166, 343. 

1-iesting nucleus, 21 (fig.), 22. 

Resting stages of, Chlamydomonas, 
188; Cladophora, 216; Fungi, 
241 (fig.) ; HcEmatococcus 
(Sphacrella). 187 (fig.), 188; 
Mucor, 238 ; Ulothrix, 215 
(fig.) ; Yeast, 256 (fig.). See 
also Oospores and Zygospores, 

Retention of megaspores, 321, 332, 

333- 
Reticulate tracheids, 113 ; — veins, 
112 ; — vessels, 35 (fig.), 36, 83 

(fi,?.). 
Retuiosporci. 337. 
Retting of fibres, 265. 
Rheitm rhaponticum, 113. 
Rhinanthus. hydathodes of, 144, 

145 (fig). 

Rhizocarpoyi confervoides, 258 (fig.). 

l^hizoids of, Fern-prothallus, 299 
(fig.). 300. 301 (fig.) ; Liver- 
worts, 270 (fig.), 271. 272, 273 
(fig.). ^78 (fig.) ; Mosses, 274, 
287 (fig.), 288. 

Rhizomes, 82, 88. in ; of Ferns, 
289, 290 (fig.) ; of Horsetails 
(Equisetum), 311 (fig.); of 
Mosses, 274. 

Rhizophores (of Selaginella), 317 
(fig.), 318. 

Rluzopus stolonijey, 237. 

Khodophjxea;, 203, 400 ; reproduc- 
tion of, 205 (fig.), 228; struc- 
ture of, 203, 204, 205 (fig.), 208. 

Rhubarb {Rheum rhaponticuy}}), 113. 

J{hus veymicifera, 155. 

Jiibes. 105, 115; cork of, 137 138 

(fig.). 
Riccia, 284. 407. 
Rice [Oryza saliva), 43, 33, 237 ; 

starch of, 42 (fig.), 43. 
Ricinus coiiiiiiiniis, aleurone grains 

of, 30, 31 (fig) ; fat of, 49 ; 

hypocotyl of, 118 (fig.) ; seeds 

of, 371, 372. 376 (fig.), 
king-bark, 141. 
Ringworm, cause of, 233. 
Rivularia, 206 (li,g.). 
Robuiia pseudacacia, 113, 126. 
Rocc-eUa, 2(n. 
Rock-pools, plants of, 193, 203, 

204. 
Rock Rose {Hclianlhenuim viilgayc), 

379 (fig.). 
Rolled leaves, 17S; structure of, 

163 (fig.), 104 (fig.). 



INDEX 



447 



Root-cap, i8. ::o (fig.), 65, 71 (fig.), 

74- 

Root-hairs, 3, 10, 13, 65, 66, 67 
(fig.) : function of, 72. 

Root-nodules ol Leguminosa;, 267, 
268. 

Roots, aerial, 73, 74. 75 (fig.) ; 
anomalous thickening of, 133 ; 
cork-formation in, 131 (fig.), 
137 ; fleshy — , 132, 133, 154, 
155 (fig.) ; growing point of, 
16, 18, 20 (fig.) ; lateral — , 65, 
71 (fig.) ; of Ferns, 290, 303 ; 
of Horsetails (Eqiiisettim). 311 ; 
of Lycopodiiim , 315 ; of Lv- 
ghiopteris, 328 ; of Selaginella, 
317 (fig); prop—, 73, 74 (fig.); 
relation of structure and func- 
tion in, 72 ; .secondary thicken- 
ing of, 131 (fig.), 132 ; transi- 
tion from stem to, 8g (fig.). 

Root-structure of, Bean [Phaseohts], 
71 (fig.) ; Beet (Beta). 133 ; 
Conifers, 341 ; Creeping Butter- 
cup [Rayiunculus repens), 66-9 
(figs.) ; Dandelion (Taraxa- 
ciim), 154 ; Dendrobimn, 75 
(fig.) ; Dicotyledons, 65-70 
(figs.) ; Ferns, 295 ; Ivis, 70 
(figs.) ; Leguminosa;, 72 ; Ly- 
ginopteris, 330 ; Maize {Zea 
mats), 73, 74 (fig.) ; Monocoty- 
ledons, 70 (fig.),, 71 ; Nettle 
(Uriica), 132 ; Salsify (Trago- 
pogon), 44, 132, 154, 155 (fig.) ; 
Selaginella, 318 ; Wallflower 
(Cheiranthus cheiri), 70 ; White 
Mustard (Sinapis alba), 69. 

Rope, 34. 

Rosacese, 137, 355, 357. 

Rose (Rosa), 104, 137, 389 ; — Mil- 
dew (Sphaerotheca paiiiiosa), 

^43- 
Rosette-habit of Alpines, 170. 
Rosin, 63. 

Rotation of crops, 248, 267. 
Royal Fern (Osmunda regalis), 

habit of, 289, 309 (fig.) ; pro- 

thallus of, 304 ; sporangia of, 

300, 309 (fig.). 
Rubia peregrina, stoma of, 97 (fig.), 

98. 
Rubiaceje, 61. 
Rubber, 157. 
Rue (Ruta graveolens), secretory 

cavities of, 151. 
Riimex, 113. 
Runner Bean (Plmseoliis), 115, 



371 ; epidermis of stem of, 

93, 94 (fig-) i hydathodes of, 

144. H5 (fig-). 147- 
Ruscus aculeatus, cladode-structure 

of, 159 (fig.), 166. 
Rush (Juncus), 178 ; leaf-structnre 

of, 163 (fig-), 166. 
Rust-Fungi (Uredinese), 143, 245-8 

(fig.), 252, 2.53, 407. 
Rust of Wheat (Piicemia gramivis), 

384 ; structure and life-history 

of, 245-7 (fig.). 
Ruta graveolens, secretory cavities 

of, 151. 
Rye (Secale), fungal diseases of, 

239, 241. 

Saecharomvces, physiology of, 53, 
54, 56, 57, 256, 257, 265 ; 
structure and reproduction of, 
255, 256 (fig.). 

Saccharose, 45. 

Saccharum officinarum, 45, 46. 

Saffron, 64. 

Safranin, 404, 405. 

Sagittaria, embryology of, 371 (fig.). 

Sago, 43, 323. 

Sago-Palm (Cycas revoluta), 322, 

3^3- 

St. John's Wort (Hypericum), secre- 
tory organs of, 151, 153. 

Sake, 257. 

Salicacea;, 357. 

Salicase, 48, 

Salicin, 48. 

Salicornia, aerenchyma of, 175 

(fig.)- 

Saligenin, 48. 

Salmon-disease (Saprolegnia ferox), 
230, 236. 

Salsify (Tragopogon), root-structure 
of, 44, 132, 154, 155 (fig.). 

Salsola kali, leaf of, 04, i56. 

Saltmarsh plants, 178, 216; root- 
hairs of, 10. 

Sambiiciis, cork of, 136 (fig.) ; leaf 
of, no. III (fig.) ; lenticels of, 
140 (fig.) ; pith of, 30 (fig ), 
151. 

Samphire, Marsh (Salicornia), 
aerenchyma of, 175 (fig). 

Sand-dune plants, 163. 

Sandalwood (Santalum album), 130. 

Sap, cell, 3, 6, 10, 26, 60, 63 ; 
nuclear — , 22. 

Sap-wood, 125, 126. 

Saponaria, 48. 

Saponification of fats, 49. 



448 



INDEX 



207 ; 
S. 



cork, 
and 



Saponins, 48. 

Sapotacese, laticiferons elements of, 

157. 
Saprolegiiia, 231, 235 (fig.) ; S. 

ferox, 236. 
Saprophytic Algas, 206 ; — Angio- 

sperms, 178, 179 ; — Fungi, 

230 et seq., 252, 253. 
Sarcina, 262. 
Sargasso Sea, 203. 
Sargassum baccife/um, 203, 204 

, (fig)- 
Satinwood [Chloroxvlou swieteiiia], 

130. 

Saxifraga sannentosa, 8, 63. 
Saxifrages {Saxifraga), chalk-glands 

of, 147. 
Scalariform tracheids, 293 (fig.). 
Scale-bark, 141. 
Scale-leaves, structure of, iii. 
Scales of Liverworts, 271, 272, 273 

(fig.) ; of Mosses, 274. 
Scars, 141, 142. 
Scenedesnnis, 191 (fig.), 192 

S. obliquus, 191 (fig.^ 

quadricauda, 191 (fig.). 
Scharlack Red, 402 ; and 

136; and cuticle, 92 

fats, 49. 
Schizogenous secretory cavities, 

151, 152 (fig,). 
Schultze's solution. See Chlor-zinc- 

iodide. 
Scilla, 44, 365 ; growing point of 

root of, 20 (fig.) ; 5. nulans, 44. 
Scion, 142, 389. 
Scirpus, 178. 
Sclerenchyma, 32-4 (fig) ; of 

assimilatory stems, 162 (fig.) ; 

of Ferns, 290 (fig.), 292, 293 

(fig.), 294 (fig,), 295 ; oiHakea^ 

leaf, 166; of leaf, 112; of 

Lyginopteris, 328, 330 (fig.) ; 

of secretory canals, 152 (fig.) ; 

of stem, 80, 81 (fig.), 83 (fig.), 

87 (fig,), 139 (fig.). 
Scleroderma vulgave, 251 (fig.). 
ScleroHnia, 239 (fig.), 241. 
Sclerotium, 239 (fig.), 241 (fig.), 

24^. 2.55- 

Scolopendrium vulgare, leaf and 
sorus of, 291 (fig.), 296 (fig.). 

Scotch Fir (Pinus sylvestris), arche- 
gonia of, 348, 349 (fig.) ; em- 
bryology of, 350, 351 (fig,) ; 
female cones of, 345 (fig.), 346, 
347 (fig-) ; fertilisation of, 
350 ; habit of, 334, 335 ; leaf- 



structure of, 152, 166, 342-4 
(figs.) ; male cones of, 344, 
34,5 (fig.). 347 (fig.) ; ovules of, 
348. 349 (fig.) ; pollen of, 
345, 347 (fig). 351 (fig.) ; 
pollination of, 348, 350 ; seeds 
of, 351, 33- ; seedlings of, 

337. 338 (fig.) ; stem-structure 
of. 338-41 (figs.) ; wood of, 
37 (fig.). 340, 341 (fig). 

Scrophulariacea;, 60, 367. 

Scutellum (of Grasses), epidermis 
of, 150. 

Sea Elite (Siiada- maritima). i65 ; 
— Buckthorn [Hippophiz rham- 
iioides), 103 (fig.) ; — Grass 
(Zostera). 172, 361 ; — Holly 
[Ervngliiin maritimum) , 93, 113 
(fig') ; — Lettuce (Ulva), 198, 
215 (fig,) ; — Purslane {Arenaria 
peploides), 94. 

Seasoning of timber, 128, 129. 

Seawater, artificial, 402 ; equivalent 
osmotic pressures of, 403 ; use 
of, in determining osmotic pres- 
sure, II, 15 ; in plasmolysis, 
8, 15. 

Seaweeds, 38, 180, 195, 198-205 
(figs.), 211 ; ash of, 15. 

Secondary bundles, 133 (fig.), 134. 

Secondary medullary rays of. Coni- 
fers, 340, 341 (fig.) ; Dicoty- 
ledons, 119 (fig.), 120, 122-4 
(fig.), 128 (fig.), 130 ; roots, 132. 

Secondary phloem of. Conifers, 
339. 341 (fig.) ; roots, 131, 
132; stems, 118, 119 (fig.), 
120, 121 (fig,), 130. 

Secondary thickening, 117-26 
(figs.), 130-43 (figs,) ; anoma- 
lous — , 132-4 (fig,) ; effects of, 
iig, 120, 135, 137; of Cheno- 
podiacea3, 133 ; of Conifers, 

338. 339 (fig.) ; of Cycads, ^li ; 
of Dicotyledonous stems, 117- 
26 (figs,) ; of DraciBna, 133 
(fig.), 134 ; of fossil Pterido- 
pliyta, 312, 315 ; of leaf, 112 ; 
of 'Lyginopteris. 328, 330 (fig.) ; 
of JVIonocotyledons, 133 (fig.), 
134 ; of roots, 131 (fig.), 132. 

Secondary wood of. Conifers, 127, 
339-41 (figs.) ; Cycads, 323 ; 
Lyginopteris. 328, 330 (fig,) ; 
root, 131, 132; stem, 118, 
119-26 (figs.). 

Secretion, 38, 144-57 ; in glan- 
dular hairs, 104-6; of 



INDEX 



449 



digestive juices, 149, 150 ; of 
sugar, 1,48, 149 : of water, 
144-8. 
Secretory canals, 152, 339 (fig.) 
340, 341, 34-; (fig-), 344 (ftg.) 

— cavities, 63, 151, 152 (fig.) 

— cells, 63, 130, 151. 328, 340 

; — hairs, 105 (fig.), 106, 
144, 145 (fig.), 149, 150 

(fig.)- 
Section-cutting, 405, 406. 
Sedatives, 61. 
Sedge (Carex), 88. 
Sedge-family (Cyperaceee), stoma 

of, 100. 
Sedtiin, leaf of, 116, 166 ; stoma of, 

97 (fig-). 98. 
Seduni album, 116; 5. spcctahile, 

97 (fig-)- 

Seed-coat, 352 (fig.), 370, 371 (fig.). 

Seed-plants, 322-73 (figs.). 

Seedlings, diseases of, 231, 235 
(fig.) ; of Conifers, 337, 338 
(fig.), 352: of Ferns, 294, 304 
(fig.) ; of Selaginella, 320 (fig.). 

Seeds, 332, 333 ; dispersal of, 46, 
50. 103. 353, 372 ; food- 
reserves of, 40, 44, 45, 47, 49, 
50, 150 ; of Angiosperms, 371-3; 
of Conifers, 351, 352 (fig.) ; of 
Cycads, 325 (fig.), 327 ; of 
graft-hybrids, 389 ; of Lygiiiop- 
ieris, 331-3 (figs.) ; of parasites, 
179, 372 ; of Taxus, 346 (fig.). 

Segments (of growing point), 16 
(fig.), 17 (fig.), 18, 195. 

Segregation of characters in hybrids 
384-8, 390. 

Selaginella, 314, 321, 332, 373, 407; 
anatomy of, 317, 318 ; arche- 
gonia of, 319, 320 (fig.) ; cones 
of, 318 (fig.) ; embryology of, 
319, 320 (fig.) ; habit of, 316, 

317 (fig.) ; prothalli of, 319, 
320 (fig.) ; spermatozoids of, 
319, 320 (fig.) ; sporangia of, 

318 (fig.), 319.. 

Selaginella kraussiana, 317 ; S. 
martensii, 317 (fig.) ; S. riipes- 
tris, 319; S. spiiiosa, 316, 
317 'fig)' S. umbrosa, 318 

(fig-)-' 

Selection, 378, 380, 390. 

Self-fertihsation, 224, 385 ; — pol- 
lination, 374; — pruning, 142. 

Semiparasites, I4.t, 147. 

Semipermeable membranes, 8, 9 , 
artificial — , 402, 403. 

29 



Sensitive Plant [Mimosa pudica), 
pulvini of, 115, 116 (fig.). 

Sepals, 354, 373. 

Septa (of vessels), 28, 83, 120, 121. 

Septate wood-fibres, 121. 

Sequoia, cone of, 347, 352 (fig.). 

Serrated Wrack [Fucits sevratus), 
201 (fig.), 202. 

Seta (of sporogonium) of Liver- 
worts, 282, 283 (fig.) ; of 
Mosses, 284, 285 (fig.). 

Sewage, conversion of, 26.5. 

Sexual cells. See Egg, Gametes, 
Spermatozoids. 

Sexual differentiation, 186, 212, 
220, 227, 228. 

Sexual fusion, 184, 185 (fig.), 213 
(fig.), 214, 219 (fig.), 237 (fig.). 
See also Fertilisation. 

Sexual organs and reproduction, of 
Algje, 184-6 (fig.), 188, 213, 
214 (fig.), 220-28 (figs.), 269 
Angiosperms, 365, 366, 368 
Chlamydomonas, 184-6 (fig.) 
Conifers, 348-51 (figs.) 
Cycads, 326, 327 (fig.) ; Cys- 
topus, 233 (fig.), 234; Equi- 
setiini, 313 ; Ferns, 301-3 
(figs.) ; FucHs, 221 (fig.), 224-6 
(fig.) ; Fungi, 233 (fig.), 234, 
236, 237 (fig.), 238, 242, 
244 (fig.), 245 ; Liverworts, 
277 -9 (fig.) ; Miicor, 237 
(fig.), 238; Mosses, 279 (fig.) ; 
CEdogonium, 221-3 (fig-) ; 
Selaginella, 319, 320 (fig.) ; 
Ulollinx, 213 (fig.), 214; 
Vaiicheria, 221 (fig.), 223, 224. 

Sexual reproduction, 288, 289, 306, 
307, 389, 390. 

Shade-leaves, 112; structure of, 91, 
95, 96, 100, 168-70 (fig.), 295, 

379. 

She-Oak (Casuarina), stem-struc- 
ture of, 162 (fig.). 

Sheath, bundle-, of Conifers, 343, 
344 (fig.) ; of leaves, 107, 108 
(iig), 112, 113; of stems, 85 
(fig.), 86. 

Sheath of Blue-green Algje, 206 

(fig.). 
Shepherd's Purse (Capsella bitrsa- 

pastoris), embryology of, 368- 

70 (fig.) ; hairs of, 102 (fig.) ; 

White Rust of, 231, 233 (fig.). 
Shingle beach. Lichens of, 258 (fig.). 
Sieve plates, compound, 130 ; of 

Angiosperms, 28, 29 (fig.), 78 



450 



INDEX 



(fig-). 79. 82 ; of Brown Algae, 
202; of Conifers, 341 (fig.). 

Sieve tubes, function of, 28, 88 ; of 
Angiosperms, 28, 29 (fig.), 80 
(fig.), 82, 85 (fig.) ; of aquatics, 
171, 174 (fig.) ; of Conifers, 
339, 340. 341 (fig-) ; of Ferns, 
292 (fig.), 293 (fig.) ; of second- 
ary phloem of Dicotyledons, 
121 (fig.), 130. 

Sigillaria, 314 (fig.). 

Siliceous deposits due to Diatoms, 
207. 

Silicification of cell-wall, 39, 206. 

Silicified hairs, 104. 

Silk-Cotton Tree (Eriodendron an- 
fractuosiim) . 103. 

Silkworm disease, 230. 

Silver Goosefoot (Obioiie porlula- 
coides), 94 (fig.), 133 ; — Fir 
{Abies). 335, 336 (fig.), 344, 347. 

Silver I^eaf Disease of Plum (Stereum 
purpureum), 250, 252. 

Silverweed [Poteulilla aiiserina), 102. 

Simple pits, 30 (fig.), 33, 121, 122, 
276. 

Sinigrin, 48. 

Skeletal tissues, 31-6 (figs.), 72, 

87. 115- 
Skin-diseases due to Fungi, 253. 
Sleep-movements, 115. 
Slime-Fungi (Myxomycetes), 250, 

251, 406. 

Smuts (Ustilagineas), 230, 245, 248, 

252, 407- 
Soap, 50. 

Soapwort (Saponaria), 48. 

Soft rot of Carrots (Bacillus caro- 
tovorus), 265. 

Soft rubber, 157 ; — wood, 127. 

Soil Algae, 198, 204, 208, 216 ; 
— Bacteria, 266, 267 (fig.). 

Soil-differences and plant-distribu- 
tion, 391. 

Soja-beans (Glycine), 49. 

Solanaceae, 58, 60, 61, 81. 

Solitary crystals, 58, 59 (fig.). 

Soredia (of Lichens), 260 (fig.), 261. 

Sorrel, Wood- (Oxalis acetosella). 95. 

Sori (of Ferns), 291 (fig.), 295, 296 
(fig.), 397 (fig-)- 

" Souring ' of milk, 265. 

Space-parasites, 260. 

Spartium junceum, proembryo of, 

371 (fig-)- 
Spawn (of Mushrooms), 248. 
Spermatozoid mother-cells, 277, 278 

(fig.). 302, 303 (fig-). 319' 



Spermatozoids, 220, 236, 330, 333 ; 

of Algffi, 220, 221 (fig-), 222, 

224, 225 ; of Cycads, 326, 327 

(fig.) ; of Ferns, 302, 303 (fig.) ; 

of Liverworts, 277, 278 (fig.) ; 

of Mosses, 280 ; of Selaginella, 

319, 320 (fig.). 
Spermogonia (of Rusts), 246 (fig.), 

247. 
Sphacelaria, growing point of, 16 

(fig-). 17- 
Sphaei-ella (HcBmaiococcus), i85, 187 

(fig.)- 
Sphaerotheca castagnei, 244 (fig.) ; 
S. morS'iivcB, 243 ; S. pannosa, 

243- 
Sphagnales, 407. 
Sphagnum, 274, 275 (fig.), 276. 
Sphenophjdlales, 407. 
Sphenopteris hceni-nghausii, 329 (fig.). 
Sphere-crystals (of inulin), 44, 45 

(fig-)- 

Spices, 62. 

Spiderwort (Tradescantia), cell- 
structure of, 4, 5 (fig.). 

Spinach (Spinacia oleracea), stem 
of, 86. 

Spinach-family (Chenopodiacea?) , 
secondary thickening in, 133. 

Spindle-fibres, 23 (fig.), 24, 25 (fig.) 

Spindle-tree (Euonxmns earopcBus), 

139, 359- 

SpircBa ulmaria, 169, 

Spiral fiowers, 354 ; — tracheids, 37 
(fig.), 113, 293, 340 ; — vessels, 
35 (fig.). 36,' 68, 80 (fig.), 83 
(fig.), 89, 172. 

Spireme, 21 (fig.), 22. 

Spirillum, 262, 263 (fig.) ; S. 
rubruni, 263 (fig.). 

Spirogyra, 16, 195, 307 ; cell- 
structure of, 7 (fig.), 208; 
filaments of, 195, 197, 210 ; 
fragmentation of, 216 ; sexual 
reproduction (conjugation) of 
226-8 (fig.). 

Spirogvrabellis, 227 (fig.) ; S. webcri 
227 (fig.). 

Sponges, " green cells " of, 260. 

Spongy parenchyma, 38 (fig.), 92 
(fig.), 107, 108 (fig.), 109 (fig.), 
110; function of. 107, iio; of 
shade-leaves, 170. 

Sporangia of, Algae, 218, 228 ; 
Conifers, 345 - 8 (figs.) ; 
Cycads, 323-6 (figs.), Eqitise- 
turn, 312, 313 (fig.) ; Ferns, 
295-300 (figs.), 303, 308, 309 



INDEX 



451 



(fig.) ; Fungi, 235 (fig.), 236 
237 (fig.), 238, 240 (fig.) 
Lycopodiitm, 315, 316 (fig.) 
Lvginopteris, 328, 330-33 
(figs.) ; Selaginella, 317 (fig.), 
318 (fig.), 319 ; Slime-Fungi, 
251. 

Spore-mother cells, 282, 390. 

Spore-mother cells of, Angiosperms, 
361, 365 ; Conifers, 345, 348 ; 
Ferns, 297, 305, 307, 308 ; 
Liverworts, 282 ; Mosses, 284, 
285 (fig.) ; Selaginella, 319. 

Spores of, Algje, 206 (fig.), 213, 
215-20 (figs.), 228; Bacteria, 
262, 263, 267 (fig.) ; Conifers, 
345, 348 ; Cycads, 326, 327 ; 
Equisetmn, 313 (fig.) ; Ferns, 
299 (fig.), 300, 305, 306, 307 
(fig), 308 ; Fungi, 237 (fig.), 
238, 240 (fig.), 245, 246 (fig.), 
248, 249 (fig.), 253; Lycopo- 
dium, 316 (fig.) ; Mosses, 286, 
287 (fig.) ; Selaginella, 318 

(fig-), 319. ■ 
Sporogonium, 281 ; of Liverworts 

270 (fig.), 271 (fig.), 282-4 (fig.); 

of Mosses, 274 (fig.), 275 

(fig.), 284-7 (fig.). 
Sporophylls of, Angiosperms, 354, 

356, 359 ; Conifers, 344-7 

(figs.) ; Cycads, 323-5 (figs.) ; 

Equisetmn, 312, 313 (fig.) ; 

Ferns, 308 (fig.) ; Lycopodium, 

315, 316 (fig.) ; Selaginella, 317 

(fig.), 318 (fig.). 
Sporophyte, 288, 289, 303, 321. 
Sports, 382. 
Spraying, 254. 

Spread of species, 392 (fig.). 
Spring-wood, of Conifers, 340, 341 

(fig.) ; of Dicotyledons, 119 

(fig.), 123 (fig,), 125. 
Spruce Fir {Picea excelsa), 63, 129, 

350, 391 ; habit of, 334, 335 

(fig.) ; leaf of, 343. 
Spurge (Euphorbia), laticiferous 

cells of, 153, 154 (fig.), 155 ; 

leaf of, no. 
Spurge-family (Euphorbiaces), 153, 

157, 
Stability of herbaceous plants. 9, 87. 
Stag's Horn Fungus (Xylaria), 

239 (fig.), 242. 
Staining of sections, 404. 
Stains, 405 ; for cellulose, 32 ; for 

lignified walls, 32 ; for sieve 

plates, 82 ; for starch 43. 



Stalk-cell (of Conifers), 351 (fig.). 
Stamen, of An,^iosperms, 354, 355, 
359-63 (figs.) ; of Conifers, 345, 

347 (fig-)- 
Staphyloeoccus, 262 ; S. aureus, 263 

(fig-)- 
Starch, 13, 40-44 ; economic im- 
portance of, 43, 53 ; hydroly- 
sis of, 46, 53 ; reactions of, 8, 

41- 

Starch-grains, 40-43 (figs.), 66 (fig.), 
67 (fig.), 96 ; and pyrenoids, 7 
(fig.), 8, 181, 207 ; centric,. 42 
(fig.) ; compound, 42 (fig.), 
43 ; development of, 41, 42 
(fig.), 43 ; excentric, 41, 42 
(fig.) ; in secondary wood, 
124 ; stains for, 43 ; transi- 
tory, 6 (fig.), 40, 41, 42 (fig.) ; 
under polarised li.ght, 43. 

Starch-grains of. Cereals. 43 ; Legu- 
minosse, 42 (fig.) ; Oat (Aveiia), 
43 ; Pellionia, 41, 42 (fig.) ; 
Potato (Solanitm), 41, 42 (fig.), 
43 ; Rice (Oryza), 42 (fig.),- 43 ; 
root-cap, 74 ; Spurge (EnpJior- 
bia), 154, 155 ; starch-sheath 
77, 80 (fig.). 

Starch-sheath, 78, 79, 80 (fig.), 118. 

(fig-)- 

Starch-solution, 43, 46. 

Starwort, Water (CalUtriche), ana- 
tomy of, 175-7 (figs.). 

Staurastrum, 209 (fig.). 

Stele of, Angiosperms, 78, 84 ; 
Ferns, 290 (fig.), 292-5 (figs.) ; 
Lycopodiales, 315, 317. 

SieUaria, 363. 

Stellate hairs, 102 (fig.), 103. 

Stem, anatomy of unthickened, 
76 - 88 (figs.) ; anomalous 
secondary thickening of, 132, 
133 (fig.) ; assimilatory — , 162 
(fig.), 167, 178, 310; connection 
with leaf, 114 (fig.), 294 (fig.), 
295 ; cork-formation in, 135- 
42 (figs.) ; epidermis of, 76, 
93 ; growing point of, 16, 18 
(fig.), 19 (fig.) ; lateral branch- 
ing of, 72 ; relation of structure 
and function, 87, 88 ; second- 
ary thickening of, 117-27 
(figs.) ; succulent, 167 ; transi- 
tion from root to, 89 (fig.). 

Stem of, Calamites, 310, 311 ; 
Conifers, 334, 335 ; Cycads, 
322, 323 ; Eqtiisehim, 310, 31: 
(fig.) ; Ferns, 289, 290 (fig.) ; 



452 



INDEX 



Lycopodium, 314, 316 (fig.) ; 
Lyginopteris, 328 ; Mosses, 274 
(fig), 275 ; Selaginella, 317 

(fig-)- 
Stem-structure of, aquatics, 88, 
170-74 (figs.); Aristolochia, 
120; Ash [Fraxiims], 122; 
Bamboo [Bambusa), 88 ; Bar- 
berry (Berberis), 137, 139 (fig.) ; 
Beech (Fagiis), 121, 122, 140 ; 
Birch (Betiila). 122, 137, 139, 
141 ; ijlack Bryony (Tamus), 
86, 87 (fig.) ; Box (Buxus), 
86, 127 ; Bracken (Pteris), 
292-5 ( fi,gs.) ; Broom (Cytisiis), 
88, 167 ; Butcher's Broom 
(RuscHs), 159 (fig.) ; Buttercup 
(Ranunculus), 86, 87 ; Cactus, 
167 ; Campion (Lvchnis), 88 ; 
Castor Oil Plant (Ricinus), 118 
(fig.) ; ChenopodiaceeC, 133 ; 
Cherry (Prunus), 141 ; Coni- 
fers, 338-41 (figs.) ; Cucurbi- 
taceje, 81 ; Currant (Ribes), 
137. 138 (fig.) : Cycads, 323 ; 
Dicotyledons, 76-84 (figs.), 86, 
117 et seq. (figs.) ; Dodder 
(Cuscuta), 178 (fig.), 179 ; 
Draccena, 133 (fig.), 134 ; Elder 
(Sambucus), 136 (fig.), 140 
(fig.), 151 ; Elm (Ulmus), 120, 

126, 139, 141 ; Equisetum, 
311 (fig.); Ferns, 292-5 
(figs.) ; Gleichenia, 294 ; Gorse 
(Ulex), 167 ; Grasses (Grami- 
neee), So, 88 ; Grasswrack 
(Zostera), 172; Holly [Ilex), 
127 ; Hornwort (Ccratophyl- 
luin), 170, 171 (fig.); Horse 
Chestnut [JEsculus), 120, 121 
(fig.), 123 (fig.), 127 ; LabiatEe, 
80; Laburnum (Cytisus), i},"] ; 
lianes, 134; Lime (Tilia), 120, 

127, 130 ; Lycopodium, 315 ; 
Lyginopteris, 328-30 (fig.) ; 
iVIaize (Zea), 84-6 (figs.) ; 
Male Fern [Nephrodiiim), 294 
(fig.), 295; Mallow (Malva), 130 ; 
Maple (.-leer), 135, 141; Mare's 
Tail (Hippuris), 172, 173 (fig.) ; 
Marrow (Cucurbita), 80-83 
(figs.), 87, 126, 127 (fig.), 130; 
Meadow Rue [Thalictniin), 86; 
Monocotyledons, 84-6 (figs.), 
87; Oak (Quercus), 122, r28 
(fig.), 140, 141 ; Plane (Pla- 
taiius), 141 ; Pondwccd (Pota- 
mogeton). 172, 174 (fig.) ; Pop- 



lar [Populus), 141 ; Rosaceae, 
137 ; Scotch Fir (Finns), 141, 
338-41 (figs.) ; Sedge (Carex), 
88 ; Selaginella, 317, 318 ; 
She-Oak (Casnarina) 162 
(fig.) ; Silver Goosefoot (Obione), 
133; Spinach (Spinacia), 86; 
Spindle Tree (Euonymus), 
139 ; Spurge (Euphorbia) , 153, 
154 (fig.) ; Solanaceae, 81 ; 
succulents, 167 ; Sunflower 
(Helianthus), 76-80 (figs.), 87 ; 
Sweet Flag (Acorus), 85 (fig.) ; 
Sycamore (Acer), 121, 128 
(fig.) ; Tree Ferns, 294 ; Um- 
belliferK, 80, 88 ; Water Drop- 
wort (Oenanthe), 86; Water 
Milfoil (Myriophyllum) . 171 ; 
Water Starwort (Callitriche), 
175, 176 (fig.) ; Whortleberry 
(Vaccinium), 167, 168 (fig.); 
Willow (Salix), 121, 124, 126, 

137. 139. 
Stentor, 3. 

Stereum purpureum, 250, 252. 
Sterilisation, 264. 
Stigma (eye-spot), 181, 193. See 

also Eye-spot. 
Stigma (of Angiosperms), 64, 356, 

364, 365. 373- 
Stimulants, 48, 61. 
Stimulus of environment, 380 ; 

of sexual fusion, 222, 223, 370, 

389, 390. 

Stinging Nettle (Urfica), hairs of, 
104, 105 (fig.) ; leaf-structure 
of, 38 (fig.), 39 ; root of, 132. 

Stinkhorn (Phallus), 407. 

Stipules, nectaries on, 148 ; on 
stamens, 359 (fig.) ; vascular 
supply of, 90, 114. 

Stock (Matthiola), 104. 

Stomata, 94 (fig.), 96-101 (figs.), 
107, no, 298 (fig.) ; artificial, 
99 ; development of, 98 ; in- 
fluence of external conditions 
on, 99, 100 : mechanism of, 
98-100 (fig.) ; plugging of, 
161 ; raised, 96, 160 (fig.), 161, 
162, 170; sunken, 96, 158, 160 
(fig.), 165, 170. 

Stomata of, Brooklime (i'cromca), 
160 (fig.) ; Butcher's Broom 
(Rnscus), 159 (fig.) ; Conifers, 
342 (fig.), 343, 344 (fig.) ; 
floating leaves, loi, 174 ; Gorse 
(Ulcx), 160 (fig.) ; Graminea2 
and Cyperacea", 97 (fig.), 100, 



INDEX 



433 



loi ; Holly {Ilex), i6o (fig.) ; 
Iris, 96, 97 (fig.) ; land-forms 
of aquatics, 177 ; Madder 
(Rubia), 97 (fig.), 98; Mosses, 
100, 284, 285 (fig.) : Stonecrop 
(Sedum), 97 (fig.), 98 ; Yellow 
Pimpernel (Lysiinachia) , i5o 

(fig.). 
Stomatal chambers, 161-3 (figs.) ; 

— grooves, 162 (fig.). 
Stomium (of Fern-sporangium), 

297, 299 (fig.), 300. 
Stone-cells, 30 (fig.), 32, 33. 
Stone-crop (Sedum), leaf of, 11 5, 

166 ; stoma of, 97 (fig), 98. 
Stonecrop - family (Crassulaceje), 

178. 
Stone Pine (Pinus pinea), seed of, 

352 (fig.). 
Stonewort [Chara), 39, 198, .406. 
Storage of food-reserves, 13, 40, 

44, 48, 50, 64, 88, 132, 155, 231, 

273, 276, 343 ; of water, 38. 94 

(fig.),95. 158,166, 167, 272, 343. 
Storage-organs and tissues, 88, 132, 

133 ; in Algae, 202, 225 (fig.) ; 

in Fungi, 242 ; in Mosses, 276. 
Stratification, of cell-wall, 29, 33, 

208 ; of starch-grains, 41, 42 

(fig.). 
Strawberry [Fragavia], 370 ; hy- 

dathodes of, 146. 
Streaming of cytoplasm, 4, 6, 104. 
Streptococcus. 262 ; S. pyogenes, 

263 (fig.). 
Striation of cuticle, 92. 
Strobilus, 312. 334. See Cones. 
Structure. See Anatomy, Leaf, 

Root, Stem. 
Struggle for existence, 378, 394. 
Strychnine, 61. 
Strychnos niix-votnica, 61. 
Style, 356, 364. 
Snisda, 166. 
Suberisation, 136, 137. 
Suberised walls of, cork, 136, 137, 

13S, 142 ; endodermis, 68, 69, 

T71 ; exodermis, 67. 
Sub-hymenium, 24.0 (fig.), 249 (fig.). 
Submerged aquatics, structure of, 

9:, 92, 10:, 170-77 (figs.). 
Subsidiary cells, 97 (fig.), 98. loi. 
Substantive variations, 374, 375 

(fig.), 376 (fig.), 
Subterranean parasites, 2^4. 
Succulent fruits. 31, 46 ; — plants, 

116, 166, 167, 178; — storage- 
organs, 44. 50. 



Sucrose, 45, 46 ; hydrolysis of, 46 ; 

reactions of, 47. 
Sugar Beet (Beta), 45, 
Sugar Cane (Saccharimi offlciuannji), 

45, 46 ; — Maple (Acer saccha- 
rimtm), 45. 

Sugars, 13, 40, 41, 44-7, 155, 364 ; 
fermentation of, 256, 257 ; 
hydrolysis of, 46 ; in foliage- 
leaves, 46; in wood, 124; 
microchemical tests for, 47 ; 
non-reducing, 47 ; of nectaries, 

46, 149 ; reactions of, 47 ; 
reducing, 47. 

Sulphur-Bacteria, 268. 

Sulphuretted hydrogen, 268 ; in- 
fluence on enzyme-action, 56. 

Sulphuric acid and cell-walls, 3, 4, 
32, 68, 92, 137, 402. 

Sundew (Drosera). tentacles of, 
1.50. 

Sunflower (Helianthiis), 49, 113; 
stem-structure of, 76-80 (figs.), 

87. 
Sun-leaves, iii, 169 (fig.), 170. 
Support of plant, 73, 170, 
Susceptibility to disease, 253, 384. 
Suspensor of, Angiosperms, 368, 

369 (fig.), 370, 371 (fig.) 
Conifers, 350, 351 (fig.) 
Cycads, 326, 327 (fig.) ; Sela- 
ginella. 319, 320 (fig.). 

Sweet Corn (Zea), 45. 

Sweet Flag (Acorus calmyiiis), stem- 
structure of, 85 (fig.). 86. 

Sweet Pea (Lathyrus), hybrids of, 
. 388. 

Swietenia uiahogoui, 130. 

Sj'camore (Acer pseudoplatanus), 
epidermis of, 95 ; leaf-fall in, 
142 (fig.) ; secondary wood of, 
121, 128 (fig.). 

Symbiosis, 257-60. 

Sympetalas, 357, 367, 408. 

Synapsis, 306, 307 (fig.). 

Syncarpous ovary, 355, 356. 

Synergidae, 357 (fig.), 366, 367 (fig.). 



TeBiiiophyllum, 74. 

Tamils communis. 112 ; stem- 
structure of, 86, 87 (fig.). 

Tannic acid and alkaloids, 61. 

Tannin, 60, 64, 125, 137, 155; 
reactions of, 12, 13, 49, 60. 

Tannin-sacs, 130, 151, 166. 

Tanning, 39, 5o. 

Tap-root, 71. 



454 



INDEX 



Tapetum, of Angiosperms, 360 
(fig.), 361, 363 (fig.) ; of Ferns, 
297, 299 (fig.). 

Tapioca, 43. 

Taraxacum, 154, 170, 372 ; flowers 
of, Plate I (Frontispiece). 

Tartaric acid, 60. 

Taxus baccata, 128, 334 ; female 
cones of, 346 (fig.) ; habit of, 
335. 346 (fig.) ; leaf-structure 
of, 34 T, 342 (fig.) ; male cones 
of, 344. 345. 346 (fig.) ; ovules 
of, 346 : seeds of, 346 (fig.), 
351 ; stem of, 339. 

Taxus baccata, var. fastigiata, 380. 

Tea (Thea sinensis), 61, 63. 

Teak (Tectona grandis), 126, 130, 

Tearing, protection against, 93, 115. 

Tectona grandis, lib, 130. 

Teleutospores, 246 (fig.), 248. 

Tension, resistance to, 72, 88, 170. 

Terpenes, 62, 63. 

Terrestrial Algse, 190, 198, 204, 205, 
208. 

Tertiary fossil-plants, 356. 

Testa, 352 (fig.), 370, 371 (fig.). 

Tetrads, of pollen grains, 361, 362 
(fig.) ; of spores, 307 (iig.), 318 
(fig.), 319, 348, 365, 372- 

Tetrahedral apical cells, 18 (fig.), 
288, 297. 

Tetrarch roots, 66 (fig.), 69, 131 

(fi,?.). 
Tetraspora, 190, 191 (fig.). 
Tetrasporcs of Red Algse, 205 (fig.), 

228. 
Textiles, 34, 103. 

Thalictrum, stem-structure of, 86. 
Thallophyta, 180-268 (figs.), 307, 

406. See AlgEe and Fungi, 
Thallus of, Algs, 180, 195-207 

(fi.gs.), 223 (fig.) ; Fungi, 230, 

231 ; Lichens, 258-61 (figs.) ; 

Liverworts, 269-73 (^gS-). 
Tlieobroma cacao, 49. 
Theobromine, 61. 
Thermophilic Bacteria, 261. 
Thickening of cell-walls, 29-37 

(fi.gs.) ; annular, 35 (fig.), 36 ; 

reticulate, 35 (fig.), 36, 74, 83 

(fig.), 120, 273 ; spiral. 35 

(fig.), 36, 37 (fig-). 74. 80 (fig.), 

83 (fig.), 120. 
Thistle, 102. 
Thornapple (Datura stramonium), 

61, 62 (fig.l, 384. 
Thread-Bacteria, 268. 
Thuidium, 276. 



Thuja, 347, 350 ; habit of, 336, 337 

(fig.). 

Tilia, secondary phloem of, 130 ; 
secondary wood of, 28, 120, 
127. 

Tiliacece, 103. 

Timber, 127-30 ; commercial 

sources of, 129, 130, 162, 334, 
337 ; " Dry Rot " of (MenUius 
lacrymans), 252. 

Tissues, I, 27, 65 ; epidermal, 91- 
106 (fi,gs.) ; false, 239 : glan- 
dular, 148. 149 ; ground, 65, 
88, 107-11 ; mechanical, 31-6 
(figs.) ; meristematic, 18, 117, 
135 ; secondary, 120-42 (figs.) ; 
secretory, 151-5 (figs.) ; storage, 
88, 132 ; vascular, 67-70 
(figs.), 78-86 (figs.), IT2, 113. 

Tissue-tensions, 12, 87. 

Tmesipteris, 407. 

Toadstools, 230, 245, 255, 407. 

Tobacco (Nicotiana tabacitm), 61, 
265. 

Tolypothrix, 205, 206 (fig.). 

Tomato (Solanum Ivcopersicum), 

63. 
Toothwort (Lathrcea), hydathodes 

of. 145 (fig-). 
Tortula, 274 ; T. muralis, 269, 274 

(fig.). 
Torus (of bordered pits), 36, 37, 

341 (fig.). 
Toxic effect, of Bacteria, 265 ; 

of mineral salts, 14, 15. 
Tracheidal cells (of Conifers), 341 

(fi,?.). 
Tracheids, 37 (fig.), 179 ; of Conifers, 

37 (fig.). 1^7. 340. 341 (fig.). 
344 ; of Ferns. 293 (fig.), 296, 
298 (fig.) ; of foliage-leaves, 37 
(fig.), 113. 146; of secondary 
wood of Dicotyledons, 120, 
121, 134. 
Tradescantia, cell-structure of, 4, 5 

(fig.). 
Tragopogon, root-structure of, 44, 

13^. 154. 155 (fig.). 
Transfusion tissue, 342 (fig.) 343 

344 (fig.). 
Transition from stem to root 89 

(fi.g-). 
j Transitory starch, 6 (fig.), 40, 41, 

4- (Iig.). 40. 

Transmission of acquired charac- 
ters, 380. 

Transpiration, 35, 158, 203 ; and 
anthocyanins, 03 ; and hyda- 



INDEX 



455 



thodes, 147 ; control of, 92, 
95, 100, 137; cuticular — , 93, 
161; of Alpines, 170; of bog- 
plants, 177 ; of rolled leaves, 
163; of shade-leaves, 170; of 
succulents, 167 ; of woodland 
plants, 178 ; reduction of, 
58, 64, 93, 95, loi, 106, 158- 
65 (figs.), 342, 343, 344. 

Tree Ferns, 289, 294. 

Triarch roots, 69. 

TropcBolinn, 63, 115. 

Tropical plants, 73, 75, 103, 125, 
153. 157, -04. 289, 294, 296, 
309, 372. 

Truffle (Tuber). 240, 241, 255. 

Tube-cell of, Angiosperms, 363. 367 
(fig.) ; Conifers, 349, 351 (fig.) ; 
Cycads, 327 (fig.). 

Tubers, 40, 43, 44, 50, 88 ; of 
Eqiiisetum, 311. 

Tulip [Tulipa), pollen and stamen 

of, 359 (fig.). 364, 367 (fig.). 

Turgescence, 9, 10, 87, 98, 100, 149, 
152, 198 ; and sleep - move- 
ments, 115, 116. 

Turmeric, 64. 

Turnip, " Finger and Toe " disease 
of (Plasmodiophora), 254. 

Turpentine, 63, 334. 

Twaj'blade (Listera ovata), embryo 
of, 371 (fig.). 

Tyloses, 126, 127 (fig.), 142. 

Tyndall, 64. 

Typha, 361. 

Tyrosin, 52. 



Ulex europaius, 167 ; epidermis of, 
92, 160 (fig.). 

XJhnus, 130, 139, 141, 365 ; second- 
ary wood of, 120, 126. 

Ulothrix, 212-15 (figs.) ; filaments 
of, 197, 207, 208, 210, 213 
(fig.) ; habitat of, 195 ; Pal- 
mella-stages of, 214, 215 (fig.) ; 
sexual reproduction of, 213 
(fig.). 214, 220 ; vegetative 
reproduction of, 212, 215 (fig.) ; 
zoospores of, 212-14 (fig.). 

Ulothrix zonata, 213 (fig.). 

Ulva. 198, 215 (fig.). 

Umbellifera;, 60, 61, 80, 88, 148, 
153. 355. 356. 357- 

Underground parasites, 254. 

Undulation of epidermal walls, 93. 

Unicellular Algje, i, 181-92 (figs.), 
795, 205, 206 (fig.), 20S-10 



(figs.) ; — Fungi, 255, 256 

(fig.) ; — growing points. 16-18 

(figs.) ; — hairs, loi, 103, 104. 
Unisexual flowers, 354, 373 ; pro- 

thaUi, 313, 319, 321. 
Uredineae, 143, 245-8 (fig.), 252, 253, 

407. 
Uredospores, 245, 246 (fig.), 247. 
Urtica, hairs of, 104, 105 (fig.) ; 

leaf-structure of, 38 (fig.), 39 ; 

root of, 132. 
Usnea barbata, 258 (fig.). 
Ustilagineas, 230, 245, 248, 252. 
Ustilago longissima, 251 (fig.). 
Uiricularia, 150. 



Vaccinium myrtilliis, pollen of, 
362 (fig.) ; stem of, 167, 168 

(fig.). 

Vacuoles, 2 (fig.), 3, 5 (fig.), 6 (fig.), 
26, 51; contractile — , 181, 182 
(fig.), 187 (fig.), 193, 213 ; of 
Desmids, 209; of Fungi, 231 ; 
of Yeast, 255. 

Vanilla planifolia, 48. 

Vanillin, 48. 

Variation, 374-80 (figs.) ; measure- 
ment of, 375-7 (fig.). 

Variation-curves, 376, 377 (fig.). 

Variegated leaves, iii. 

Varnish, 155. 

Vascular bundles, 76 ; bicoUateral, 
81 (fig.), 82 ; collateral, 76, 78 
(fig.), 79, 84, 85 (fig.), 112; 
concentric, 85 (fig.), 86, 133 
(fig.), 134, 292 (fig.) ; cortical, 
86 ; medullary, 86 ; second- 
ary, 133 (fig.). 134. 

Vascular cylinder, of aquatics, 
i70-74(figs.) ; of root, 66 (fig.), 
68 ; of stem, 78. 

Vascular system, 65 ; differentia- 
tion of, 89. 

Vascular system of, aquatics, 170- 
74 (figs.), 176 (fig.) ; cladodes, 
160 ; Conifers, 339-41 (figs.), 
343 ; Cycads, 322 ; Eqiiisetum, 
311 (fig.) ; Ferns, 292-5 
(figs.) ; leaves, 90, 107, 112- 
14 (fig.) ; Lycopodium, 315 ; 
Lyginopteris, 328-30 (fig.) ; 
Mosses, 276 ; ovules, 325 (fig.), 
326, 331. 332, 333 (fig.). 357. 
358, 359 ; parasites, 179 ; 
petioles, 113 (fig.) ; pulvini, 
115, it6 (fig.) ; roots, 66 (fig.), 
68, 70 (fig.) ; Selaginella, 317 ; 



456 



INDEX 



stamens, 360 (fig.) ; stems, 
77-86 (figs,), 117 et seq. '■ 
stipules, 90, 114. 

-Vaucheria, 49 ; habitat of, 198 ; 
sexual reprodxiction of, 220, 
221 (fig.), 223, 224, 226 : thallus 
of, 198, 208, 218 (fig.) ; zoo- 
spores of, 217, 218 (fig.). 

Vaucheria sessilis, 221 (fig.). 

Vegetable Caterpillar (Cordvceps), 
239. 255. 

Vegetable Kingdom, classification 
of, 180, i8i, 230, 231, 238, 269, 
289, 3.^4. 3.55. 406-8 ; com- 
pared with Animal Kingdom, 
14, 25, 181, 1S8 ,193, 194, 247, 

307- 
Vegetable Marrow (Cucurhita), hairs 
of, 102 (fig.) ; phloem of, 29 
(fig.), 82, 130 ; stem-structure 
of, 80-83 (figi^-). 87 ; tyloses of, 
127 (fig.) ; vessels of, 35 (fig.), 

83. 

Vegetative cell of pollen of Conifers, 
349. 351 (fig-); of Cycads, 327 
(fi.§.). 

Vegetative mutations, 382, 383 
(fi.g), 390. 

Vegetative reproduction, 183, 390. 

Vegetative reproduction of. Algae, 
182 (fig.), 183, 184, 188 (fig.), 
191, 192 (fig.), 212, 216 ; Ferns, 
305 (fig.) ; Hormidium, 215 
(fig.), 216; Liverworts, 277; 
Lvcopodium selago, 315 ; May- 
chantia, 270 (fig.), 277 ; Mosses, 
277, 287 (fig.), 288 ; Yeast! 
256 (fig.). 

Veins, 107, no, in, 112, 115. 

Velamen. 74, 75 (fig.). 

Venation of, Angiosperms, 112 ; 
Conifers, 341 ; Cycads, 322, 
324 (fig.) ; Ferns, 291. 292. 

Venter (of archegonium) , 280, 282, 
302, 319. 

Ventral canal cell of Bryophyta, . 
280, 281 (fig.) ; of Conifers, 348, 
349 (fig.) ; of Ferns, 302, 303 

(fig-)- 
Veronica beccabuiiga, stoma of 160 

(fi.g-). 
Vessels, 27, 28, 35-7 (fig.), 354 ; 
of metaxvlem, 36, 66 (fig.), 68 
78 (fig.): 79, 80 (fig.) ; of 
protoxylem, 36, 66 (fig), 68, 
79, 80 (fig.) ; of secondary' 
wood. 120, 121, 123 (fig.), 125, 
126, 127, 12S (fig.), 132. 



Vessels, laticiferous, 154, 155 (fig), 

157- 

Vestibule, 96, 159, 160 (fig.). 

Viburnum opiilus. extrafloral nec- 
taries of, 148. 

Vicia faba, 53 ; extrafloral nectaries 
of. 148. 

Vine (Vitis vinifera), 121. 

Vinegar, 265. 

Violet (Viola), style of, 364. 

Volatile oils, 62,' 63, 64, 10,5, 106, 
126, 151. 

Volutin, 255, 256 (fig.), 262. 

Volvox, i8g (fig.), 190 ; 1'. aureus, 
189 (fig.) ; V.globator, 189 (fig.). 

Vulcanisation of rubber, 157. 

Vulcanite, 157. 

Wall Moss (Torlula muralii), 269, 
274 (fig.) : — Rue lAsplenimn 
rtita-mitraria) , 296. 

Wall of cell, see Cell-wall ; of hairs, 
103, 10.). 

Wallflower {Cheiranthiis clieiri). leaf 
of. III ; root of, 70. 

Walnut (Juglans). 126. 129. 

Warmth, and enzyme-action, 56 ; 
and protoplasmic movement, 4. 

Wart-disease of Potato, 254. 

Waste-products of plants, 3, 40, 
58-64, 138, 144, 136. 

Water, absorption of, 10, 26, 38, 67, 
72, 73, 74, 93, 147. 158, 171. 
259. 270, 276, 301 ; conduction 
of. 27, 35, 37, 65, 88, 112, 121, 
125, 276, 343, 344; exudation 
of, 144-8, 149; storage of, 
38, 94 (fig.), 95, 158, 166, 167, 

272. 343-' 

Water-bloom, cause of, 192. 

Water Buttercup {Raiiiniculu;. 
aquatilis), 177 (fig.) : — Drop- 
wort {Oenanlhe crocata). 86 

— (Lily (NvmphcBa), loi, 356 

— Milfoil {Mvriophvlhiin), 171 

— Plantain (Alisina. plantago), 
100, 146, 371 (fig.) ; — Starwort 
(Callitriche), 173-7 (fi.?-)- 

Water-containing hairs. 04 (fig.). 
Water-cultures, absorption from, 

10. 
Water-forms (of aquatics), 03 175-7 

(fi.gs.). 
Water-plants. See Aquatics. 
Water-pollinated plants, 361, 373. 
Water-pores, 145-8 (figs.). 
Water-secreting hairs (hydathodes), 

144. 145 (fig.). 



INDEX 



457 



Wax, 93, loi, i6i, 174. 

Wellingtonia (Sequoia), cone of, 
347, 352 (fig.). 

Wetting of leaf-surface, prevention 
of, 93, loi, 103. 

Wheat (Triiicuin), grains of, 52 
(fig). 53, 384 ; — mildew {Ery 
siphe graminis) , 243 ; — rust 
(Puccinia graminis), 245-7 
(fig-), 2.53, 384 ; — smut (Us- 
tilago) , 230. 248. 

Whisky, 257. 

White Convolvulus (C. sepium), 
153 ; — Dead Nettle (Lamium 
album), 4, 105 ; — Deal (Picea 
excelsa), 129 ; — Lily (Lilium), 
no; — Mustard (Sinapis alba), 
69; — Pine (Abies pectinata), 
129; — Rust of Cruciferas (Cys- 
topus candidiis) , 231-4 (fig.) ; 

— Stonecrop (Sediim album), 
116. 

Wholemeal bread, 52. 

Whorled branching, 310, 311 (fig.) ; 

— flowers, 355. 
Whortleberry ( Vaceinium myrtillus) , 

pollen of, 362 (fig.) ; stem of, 

167, 168 (fig.). 
Wild Hyacinth (Scilla nutans), 44, 

365 ; — Strawberry (Fragaria 

vesca), 146. 
Willow (Salix), 48, 103, 137, 139, 

354, 389 ; wood of, 121, 124, 

126, 128. 

Willow-herb (Epilobium), 103, 174; 

pollen of, 362 (fig.). 
Wilting and stomata, 100. 
Wind-pollinated plants, 326, 330, 

345. 348, 354. 361, 373- 
Wine, 256. 
Winged stems, 167. 
Woad (I satis tinctoria), 48. 
Wood. See Xylem. 
Wood Anemone (A. nemorosa), 148, 

248; — Sorrel (Oxalis acetosella), 

95 ; — Spurge (Euphorbia amyg- 

daloides), 153. 
Wood-fibres, 33, 121, 123 (fig), 125, 

127, 128 (fig.). 
Wood-parenchyma, 79, 83, 86 ; of 

Ferns, 292 (fig.), 293 (fig.) ; of 
leaf, 112, 113; of secondary 
wood, 121, 122, 123 (fig.), 124, 
126, 132. 

Wood-pulp, 35. 

Wood-vessels. See Vessels. 

Woodland plants, structure of, 
168, 169, 178, 379. 



I Woody perennials, 40, 364 ; phloem 
of, 82 ; root of, 70 ; secondary 

I growth of, 117 et seq. 

' Wound parasites, 252, 253 

' Wounds, effect of, 26, 252 ; healing 
of, 64, 1«, 142, 155. 



Xanthophyll, 109. 
Xanthoprotein reaction, 50. 
Xanthoria parietina 258 (fig ) 260 

(fig.). 
Xerophytes, 158. See also Dry 

habitats, plants of. 
Xylaria hypoxylon, 239 (fig.). 
X5dem, 27, 32, 33, 36, 37, 65 ; 

function of, 65, 88, 112 ; 

primary, 66 et seq., 78 ei seq., 

112 et seq. ; secondary 33 
T18, 119-26 (figs.). 

Xylem of, aquatics, 171 (fig), 172, 

173 (fig.). 174 (fig.). 176 (fig.) ; 

Conifers, 37 (fig.), 127, 339 
(fig.) ; Ferns, 292-4 (figs.) ; 
leaf of Angiosperms, 107, 112, 

113 ; parasites and saprophytes, 
178, 179; petiole, 113 (fig.); 
root of Angiosperms, 66 et seq. 
(figs.) ; stem of Angiosperms, 
78 et seq. (figs.). 

Xylem-canal of aquatics, 171 (fig.), 
172, 174 (fig.) ; Monocotyle- 
dons, 36, 85 (fig.), 86. 



Yeast (Saccharomvces), physiology 
of, 53, 54. 56, 57. 256, 257, 
265 ; structure and reproduc- 
tion of, 2.55, 256 (fig.). 

Yellow Dead-nettle (Lamium gale- 
obdolon), 168, 169 (fig.) ; — Pim- 
pernel (Lysimachia vemornm), 
160 (fig.) ; — Pine (Pinus stro- 
biis), 129; — Rattle (Rhinan- 
thits), 144, 145 (fig.). 

Yew (Taxus baccata), 128, 334, 
408 ; female cone of, 346 (fig.) ; 
habit of, 335, 346 (fig.) ; leaf- 
structure of, 341, 342 (fig.) ; 
male cones of, 344, 345, 346 
(fig.) ; ovules of, 346 ; seeds 
of, 346 (fig.), 351 ; stem of, 

339. 
Yorkshire Fog (Holctis mollis). 112. 

Zea mats, disease of, 248 ; grains 
of, 45, 52, 53, 384 ; hybrid- 
endosperm of, 386 ; leaf of, 



458 



INDEX 



117; root-structure of, 73, 74 
(fig.) ; stem-structure of, 84-6 
(figs.). 

Zingiber officinale, 63. 

Zonation of Seaweeds, 199, 200, 
202, 203. 

Zone of elongation of root, 65. 

Zooglrea-stages (of Bacteria), 262. 

Zoosporangia of Algae ; 196 (fig.) ; 
Fungi, 218 (fig.), 219 (fig.). 

Zoospores, 214, 219, 238. 

Zoospores of, Cladophora, 215 ; 
Cystopus, 232, 233 (fig.) ; Ecto- 
carpiis, 218, 219 (fig.) ; CEdo- 
gonium, 216, 217 (fig.) ; Pleuro- 
cocctts, 191 ; Saprolegnia, 235 
(iig.), 236; Ulothrix, 213 (fig.), 
214 ; Vaiicheria, 217, 218 
(fig-). 



Zostera, pollen of, 361 ; stem- 
structure of, 172. 

Zygnema, cell-structure of, 208 ; 

sexual reproduction of, 226-8 

(fig.). 

Zvgnema pectinatum, 227 (fig.). 

Zygomorphic flowers, 355, 373. 

Zygomycetes, 236-8 (fig.), 406. 

Zygospores, 186, 307 ; dispersal of, 
186, 214, 228. 

Zygospores of, Chlamydomonas 
(fig.), 186 ; Conjugatffi, 
(fig.), 228 ; Desmids, 20- 
228 ; Ectocarptis, 219 
220 ; Mucor, 237 (fig.) 



220 ; mucor, 237 (ng.j 

Ulothrix, 213 (fig.), 214. 

Zygote, 184, 185 ''fipl t8 



i»5 
227 

:09 (fig.), 
(fig-), 
238; 



(fig.), 214- 
Zymase, 53, 57 



(fig 



213 



256. 



Printed hy Hazell, Watson & Viney, Lit.. London and Aylesbury. 



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