c

i)

Gornell University Library
Ithaca, Nem Pork

BOUGHT WITH THE INCOME OF THE

SAGE ENDOWMENT FUND

HENRY W. SAGE

1891

imum

9

Cornell University

Library

The original of this book is in
the Cornell University Library.

There are no known copyright restrictions in
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ELEMENTS OF PLANT BIOLOGY

ELEMENTS OF PLANT
BIOLOGY

A. G. TANSLEY, M.A., F.R.S.

University Lecturer in Botany and Examiner in Elementary
Biology to the University of Cambridge

LONDON: GEORGE ALLEN & UNWIN LTD.
RUSKIN HOUSE, 40 MUSEUM STREET, W.C.1
NEW YORK: DODD, MEAD & COMPANY

Aue

PREFACE

Tu1s book is intended primarily for medical students
and others who do not necessarily intend to continue
the study of botany, but who desire or are obliged to
obtain some elementary knowledge of plants, particularly
in relation to general biology. It may perhaps also
be useful in the highest forms of schools where biology
is taught, as well as in training colleges. But it
is unsuitable for pupils under seventeen.

The book is based on the first portion, which deals
mainly with plants, of the course in Elementary
Biology for the Preliminary Examination in Science
and the First Examination for the M.B. degree at
Cambridge. This part of the course has been framed,
in complete freedom as to-.the material chosen and
its treatment, but in co-operation with the Professor
of Zoology, to serve as an introduction to Biology
suitable for freshmen, many of whom know nothing
whatever of the subject. This first portion of the
course, which is covered by the present work, occupies
one term, and comprises twenty-four lectures and
forty-eight hours of practical work in the laboratory.
The rest of the course, which deals mainly with
animals, comprises about thirty-six lectures and
seventy-two hours of practical work, and includes
some elementary treatment of heredity and evolution.

No claim is made to ideal pedagogic method. The

problem we are faced with at Cambridge is a strictly
7

ELEMENTS OF PLANT BIOLOGY

practical one, conditioned by the fact that the students
include some who know nothing of biology and are
only beginning to learn elementary chemistry and
physics, and by the further fact that the classes are
large, varying from 120 to 250 students, and have to
be taught on the old-fashioned academic system of
an hour’s lecture followed by two hours’ practical
work, a system which has serious educational draw-
backs, though also obvious conveniences.

The problem, then, is to provide, under these
conditions, a course of teaching which shall be as
interesting as possible and shall serve to introduce
the student to the fundamental facts and principles
of biology, both as part of his training for life and
more particularly as an introduction to the study
of medicine, which, as has often been said, is really
a specialised branch of applied biology.

The time is past when we could hope to give the
budding medical student a thorough training in
elementary botany and zoology, and with the ever
increasing complexity of the medical curriculum
proper there is constant pressure to shorten and
simplify the preparatory parts. It seems to me that
biologists should yield to this pressure to a reasonable
extent, and frankly ask themselves how far the things
that have been traditionally taught really serve as the
best introduction to biology for medical students under
existing conditions.

At the same time the provision of a good intro-
duction to general biology in the early part of the
student’s course is certainly of as much importance
now as ever it was. If the foundations are not broad
enough, the student’s outlook will necessarily suffer
when he comes to his more specialised work; and

PREFACE )

some serious knowledge of the specific structures,
activities and evolution of plants is an essential part
of a broad biological foundation. There are things
about life as a whole that we can only learn from
plants, and there are others which some knowledge
of plants helps us to understand far more completely.
The ideal course in elementary biology would un-
doubtedly be given by a teacher dealing with both
plants and animals. Unfortunately, the differentiation
of botany and zoology has gone much too far to
make it possible to find or to employ such a teacher
in most existing British universities. The Cambridge
course attempts a compromise which approaches,
though it cannot attain, this ideal, for the introductory
lectures deal with animals as well as with plants,
while plants as well as animals are freely used as
illustrations in the concluding lectures of the zoological
portion, which are concerned with heredity and
evolution.

If all school children were taught some biology,
as they certainly should be, and this were on well
considered and recognised lines, the task of the
university teacher would be more straightforward
and more profitable: he could give his students a
far better training in the time at his disposal. As it
is he is bound to assume that they know nothing
of biology and very little of chemistry and physics.
A rudimentary knowledge of inorganic chemistry
is, however, assumed in this book, and this is now
fairly common among students entering the university.

The chief points in which this book differs from most
works of similar scope are the following. First of
all much more space is devoted to biological facts

Io ELEMENTS OF PLANT BIOLOGY

of general significance which can be best illustrated
by the lower forms of plant life. The vascular plants
are briefly treated: an attempt is made to outline
the structure and life history of the seed plant,
but much of the morphology is left on one side as
of no great interest or importance to the student
of general biology. Secondly, after an introductory
lecture on the general characters and differences
of animals and plants, the student is at once intro-
duced to the most important organic substances
which make up the body of the organism, and then
to a brief consideration of some of the physical
characters of organic substances and of protoplasm.
This is followed by an account of amceba, and of the
chief functions of organisms in general, and this again by
a general account of the cell (Chapter VI). This line of
approach, entered upon before the student knows
anything worth mentioning of the detailed structure
of organisms, certainly has drawbacks, but in a
necessarily short course it has also great advantages,
because it enables the structures and activities
described later on to be followed with some under-
standing of their significance in the light of the funda-
mental properties and functions of organic and living
substance.

The Green Plant cell, as the most fundamental
unit in the plant world, is dealt with in Chapter VII,
and then the plant cell without chlorophyll, illustrated
(as functioning organisms) by Yeast and Bacteria.
The Bacteria are considered at greater length than
is usual in elementary books of this scope, because
of their enormous importance in the general economy
of the world, and because, as it seems to the author,
it is specially desirable for medical students to get

PREFACE Il

a- fair general notion of their activities before they
enter, later in their medical course, upon the highly
specialised study of the disease-producing forms—
which are, after all, from the general biological
standpoint, a relatively unimportant fraction of the
group. The saprophytic and parasitic fungi are next
dealt with, and then the more detailed study of
the chlorophyll-bearing plants is entered upon, and
occupies the rest of the book. Here the evolutionary
series is roughly followed, for it seems to the author
that the interest and educational advantage of this
method of treatment are decisively in its favour.

The Chlamydomonas-Volvox series is dealt with
in much more detail than is usual, because of the
unique interest of this series as illustrating the origin
of sex and of the soma. Fucus is taken as the best
example of primitive tissue differentiation in a relatively
large plant, Pellia as a simple land plant. The
archegoniate plants are treated very briefly indeed,
detailed descriptions of structure and development
being deliberately omitted, though an attempt is
made to give, in very brief outline, the fundamental
facts of the biological significance of vascular plants
and of heterospory.

The rest of the book deals with the Seed Plants,
first with their external form and methods of vegetative
propagation, then with the architectural elements of
which they are composed—a course which has been
found very useful in practical teaching because it
relieves the later descriptions of the structure of the
plant organs from being overloaded with accounts
of the tissue: elements themselves. The vegetative
organs of the plant—root, leaf and stem—follow,
and the account of the vegetative structure of the

12 ELEMENTS OF PLANT BIOLOGY

seed plant closes with the woody stem. Finally,
the flower, fruit, seed and seedling are dealt with
in order, thus completing the sketch of the structure,
economy and life history of the seed plant.

Throughout the book the effort has been made
to treat the material from the point of view of general
biology rather than with the narrower outlook of
pure botany.

The schedules of practical work follow pretty
closely those in use in the Cambridge course; in a
few cases alternative material is suggested. Though
it is not suggested that the selection of material
cannot be improved, nor to be supposed that any
teacher will desire to follow the schedules in every
detail, it was thought that the interests of teachers and
students would best be served by presenting a perfectly
definite selection of material which has been proved
by experience to be workable and instructive. Section
B on p. 34 and (7) on p. 47 were suggested to me by
Dr. M. C. Rayner.

The practical work suggested at the end of each
chapter occupies from 2 to 24 hours. It is not con-
templated that the students should cut their own
sections in that time. It is found by experience
that so large a proportion of the time available in a
short course is spent in learning to cut sections, and
the sections cut are often so inferior, that the
result is wholly unsatisfactory. Much better results
are obtained by providing students with good
sections and then insisting that they shall examine
and draw them properly. It is not of course suggested
that it is not good for students to do as much of the
necessary manipulation of their material as possible,
but the particular technique of cutting hand-sections

PREFACE 13

with a razor is of no further use to anyone unless
he becomes a professional botanist (of no very great
use indeed to many professional botanists), and to
acquire any useful proficiency occupies far more time
than it is worth, when the student’s hours are strictly
limited.

With a small class and ample time the case is quite
otherwise, and not only should students cut many
of their own sections under these conditions, but
they may be taught to prepare material and experi-
ments with great advantage to themselves and to their
ultimate grip of the subject. Such conditions, however,
simply do not exist in big university classes with
every hour of the student’s working day filled up.

In the earlier schedules, especially, fairly precise
instructions are given to the student, and some general
hints are prefixed to the book (p. 17), suggested by
experience of the mistakes students most frequently
make in doing their practical work. They are of
course intended only for those who have no previous
experience of microscopic work in the laboratory.

It is believed that the actual measurement of
microscopic objects, though not usual in elementary
classes, is a valuable help to the student in relating
the structures he sees under the microscope to the
objects he sees with the naked eye, giving the former
a greater objective reality in his mind. It is therefore
suggested that the student should learn early in
the course to calibrate a micrometer eyepiece with a
micrometer slide, and that he should frequently
measure the diameters of microscopic objects.

I am greatly indebted to my daughter Margaret,
who has drawn the whole of the illustrations. The

14 ELEMENTS OF PLANT BIOLOGY

majority are copied, with or without modification, from
published figures. They are intentionally limited in
number, no attempt having been made to illustrate
everything that is described when the details of
structure can quite easily be made out on the objects
seen in the laboratory. Accurate pictures of the
material actually studied provide too great a tempta-
tion to copy the picture rather than the object, and
it is, of course, impossible to lay too great stress
on the value of making careful drawings from the
objects themselves. For this reason a good deal of
the structure of the higher plants is illustrated by
diagrams and generalised pictures which are not
faithful copies of nature.

Professor A. E. Boycott, of University College,
London, has been good enough to read the chapter
on Bacteria, and has given me the help of his very
valuable criticisms. My colleague, Mr. F. T. Brooks,
has kindly performed a similar service in reading
Chapters X and XI, on the Fungi. Mr. S. M.
Wadham, the Senior Demonstrator in Botany at
Cambridge, has given ungrudging help in organising
the Practical Work and suggesting improvements.
Finally, I am deeply indebted to my friend Dr. F. F.
Blackman, who has placed at my disposal his critical
knowledge of plant physiology and biochemistry by
advising in detail on Chapters II, III and VII, as well
as on other smaller sections of the book.

A, G. T.
CAMBRIDGE,

July, 1922.

CONTENTS

PAGE

PREFACE . s ‘ - . 5 : 3 . . i 5

HINTS TO STUDENTS ON PRACTICAL WoRK : . s TZ
CHAPTER

I. Inrropuctory. PLanrs AND ANIMALS . . 2r

II. OrGANIc SUBSTANCES AND THEIR CHEMICAL
CHARACIERS . . eg me - 36

III. Somr PHYSICAL CHARACTERS OF ORGANIC SuUB-

STANCES Sf . : 5 - + « 48

IV. PRoTopLasM AND THE AMGBA. PrRoTococcus . 61
V. THE ViTAL FUNCTIONS . oe Bo © JO
VI. THECELL . ‘i - se el Sl ly i oe 92
VII. THE GREEN PLANT CELL . : ‘i 3 * » E52

VIII. THE CoLtourLtEss PLANT CELL. THE YEAST
PLANT . ‘ r é 2 é é 5 + 127

IX. BacTERIA . . 1 ee eee 8

X. SapropHytic Funct. Mucor AnD PENICILLIUM. 157

XI. Parasitic Fune1 i 2 © © 8 « 4& 69
XII. Origin oF SEX AND OF THE SoMA. THE
GREEN ALIGE . rr i:

XIII. DIFFERENTIATION OF Tissues. Fucus: Tuer

SEA WRACK Co em a ek BT
15

16

CHAPTER

XIV.

XV.
XVI.
XVII.
XVIII.
XIX.
XX.
XXI.
XXII.

XXII.

KXiV,

CONTENTS

THE SimpLest LAND PLANTS—LIVERWORTS
AND Mosses. THE PTERIDOPHYTA

THE SEED PLants: Forms AND Lire HISTORIES
Tue TIssuE ELEMENTS OF SEED PLANTS

THE Root . : ‘ : 3 : : ‘

THe FoiiaGE LEAF

THE PRIMARY STEM

Tue Woopy STEM

THE FLOWER

THE FRvuIt . . 7 5 é

THE SEED AND ITS GERMINATION

CoNCLUSION : s F . F r . 7

INDEX ‘ F F $ : . A

PAGE
231
254
270
288
302
315
328
344
361
375
389

405

HINTS TO STUDENTS ON PRACTICAL WORK

Use of the Microscope.

(1) Remember that the microscope is a delicately
adjusted scientific instrument, and must not be treated
roughly. Do not lift it by the tube, but always by
the solid part of the stand.

(2) Take great care not to get liquids of any kind
on to the lenses. Ifa lens is dusty, wipe it with a soft
handkerchief.

(3) Always see that the slide and coverslip are
perfectly clean before use.

(4) If you cannot then get a clear view of an object,
look first to the surfaces of the lenses, especially of the
objectives. If there is glycerine on the surface of the
lens, it must be carefully and repeatedly wiped with
a soft handkerchief. Breathing on the lens between
wipings will often help. Take care never to scratch
the lenses. This is often done by rubbing with a harsh
cloth.

(5) In examining an object or section under the
microscope look at it first with the naked eye, then
with a hand lens, then with the low power, and finally
with the high power. This procedure enables you to
get a preliminary idea of the general form of the object
or section, before you look at its details, and thus to
relate the details to the larger features of its structure.
Remember that the world you see under the micro-
scope is not really a different world from that which

2 -

18 HINTS TO STUDENTS ON PRACTICAL WORK

you see with the naked eye. The microscope is merely
an instrument which extends the power of the eye
so that it can detect the fine details of structure.

(6) Never put on the high power unless the object
is covered with a coverslip, or you will be almost certain
to get the mounting medium on to the lens of the
objective.

(7) Never focus the high power carelessly, or you
will jam it on to the slide, spoil the object, and very
likely injure the objective. When you have put on
the high power, note its distance from the coverslip
by looking from the side (you will soon learn to judge
about the distance at which it is in focus). Correct
the distance with the coarse adjustment carefully, till
you judge it is about right, and then look through the
microscope and focus accurately with the fine adjust-
ment,

Observation and Drawing of Structure.

Careful observation, followed by accurate drawing
of the structures of plants and animals, whether seen
with the naked eye, with a hand lens or under the
microscope, is an indispensable means of acquiring a
sound first-hand knowledge of biology. It is also one of
the best methods of training the mind to form and retain
clear mental pictures, and this power is most valuable
m almost every occupation of life, most notably in the
everyday work of the medical man. It is therefore of
great importance that the student should seriously
bend his mind from the start to learning how to
represent what he sees in drawings and diagrams.

The power of making good representations on paper
of what one sees varies very much in different people
—some find it comparatively easy, others very difficult.

DRAWING 19

But those students who find it difficult should not be
discouraged, for everyone can improve this power by
practice, and it is the effort to improve which is specially
valuable as training of the mind.

The following points should always be kept in
view :—

(x) Outlines should be represented by clear firm
lines on smooth paper with a pencil of medium hardness
sharpened to a good point. Avoid all “‘ muzziness ”’
in drawings: it always indicates a “‘muzzy’”’ mental
picture, which is useless, Do not put in “ shading ”’
in drawings of microscopic objects, and never ‘‘ shade ”’
any drawing till you are quite satisfied that the outlines
are accurate. Shading is very seldom necessary.

(2) Draw only what you can actually see. Never
pretend to see what you do not see. But remember
that practice in observation will enable you to see
things that you cannot see at first. So do not jump
to the conclusion that things cannot be seen which
the demonstrator says can be seen, because you cannot
see them at once.

(3) Always draw on a large scale, so that all details
can be clearly shown.

(4) Always write the names of the parts of what
you draw at the side of the drawing, either in full or
with unmistakable abbreviations, connecting each name
with the corresponding part by a straight line, so that
the drawing is at once intelligible.

(5) When you have to represent a complicated
structure as seen under the microscope, for instance
a section of an organ of a higher plant showing various
tissues, make (a) a diagram, including the outlines
only of the ¢isswes, under the low power, to show the
distribution of the tissues; and (0) detailed drawings

20 HINTS TO STUDENTS ON PRACTICAL WORK

of small samples of each tissue under the high power,
to show the structure of the different cells.

Do not make a drawing of the whole section and
try to put in all the cells. To do it accurately would
take a very long time, and the time spent would be
largely wasted. To do it carelessly and inaccurately
is worse than waste of time: it is training in slovenly
work.

Elements of Plant Biology

CHAPTER

INTRODUCTORY. PLANTS AND ANIMALS

Botany is that part of biology specially concerned
with those living organisms we call plants. Plants
form the basis of all life as it is lived upon the earth,
because they alone have the power of making new
supplies of what are called organic substances—the
only substances on which animals can feed—out of
inorganic substances. And the study of plants teaches
us some things about life which we cannot learn,
and others which we cannot so easily learn, from the
study of animals. It is therefore necessary not only
for all those who want to acquire some knowledge of
biology as a whole, but also for those who will be daily
concerned in their profession with the most highly
developed and most complex of all living organisms
—man—to learn something about plants if they are
to acquire a firm foundation for their later studies.
It is not necessary for them to gain the detailed and
comprehensive knowledge that the botanist has to
acquire, and their studies should be directed rather
to what plants can teach them about life as a whole
than to a knowledge of plants for their own sakes.
The title of this book, which is designed especially |
for the medical student who knows nothing of biology,

22 INTRODUCTORY. PLANTS AND ANIMALS

and whose time is strictly limited, is intended to empha-
sise this point of view. But such a student must under-
stand that if he is to acquire a foundation which shall
be of real use to him in his later work he must give
himself freely to the labour of acquiring a firm elemen-
tary knowledge of the nature and constitution of the
bodies of plants, of the outline of their structures and
of their various activities, not only from books and
lectures, but also at first-hand in the laboratory.

What is a Plant? Differences between Animals and
Plants.—The animals and plants with which we are
most familiar are the higher, i.e. the more complex
ones. They mostly live on the land, and are on the
whole, though by no means invariably, bigger than
the lower, less complex, forms. Certain outstanding
differences between the familiar higher animals, on
the one hand, and the familiar higher plants, on the
other, are very obvious. The animals move about,
the plants are rooted in one place: the animals are
compact, the plants are branching, in their habit of
body: the animals are variously coloured, the plants
are green, or at least have green leaves. The most
important functional difference is that animals consume
solid food, while plants do not; and this is really the
voot of all the other differences.

The locomotion of animals is related to the fact
that they have to move about to find their food, which
must be organic, i.e. must consist of special complex
chemical substances forming part of, or produced by,
other animals or plants, while the food of plants consists
of liquid and gaseous inorganic substances which are
found everywhere in the earth and air.

The compact form of animals is connected with the
necessity of locomotion and with the fact that the

DIFFERENCES BETWEEN ANIMALS AND PLANTS) 23

body is organised round the gut (alimentary canal),
which digests the organic food, i.e. changes it into
forms which can directly nourish the body. The
branching habit of plants, on the other hand, exposes
the greatest possible surface to the soil and to the air,
from which the plant directly absorbs the liquid and
gaseous inorganic substances which form its food.

Plants are green or have green parts because of
the green colouring matter (chlorophyll) which enables
them to build up their bodies from simple inorganic
substances—or, in other words, to form organic
substances from inorganic.

And the many other differences of structure and
organisation which distinguish the higher animals and
the higher plants are all related, directly or indirectly,
to the fundamental difference in their foods.

There are, however, cases in which both animals
and plants depart, in one or other respect, from these
general characters. There are parasitic animals which:
do not move about, but remain fixed on or in the
“host,’”’ and which consume only liquid food, though
this is always organic, and derived from the body of
thehost. There are plants which are partly carnivorous,
with special organs which digest solid animal food
and absorb the products of digestion. There are
branching animals and compact plants. There are
plants which live largely on organic (liquid) food.

Furthermore, there are simple minute microscopic
organisms living in water and other liquids which
have some animal and some plant characters, so that
biologists have sometimes considered them as animals
and sometimes as plants. These last are more or less
changed representatives of some of the organisms

1 “ Leaf-green,” from Greek yAwpdc, green, and gvAdop, a leaf.

24 INTRODUCTORY. PLANTS AND ANIMALS

which were produced early in the history of life, and
they have not become differentiated like the higher
organisms, the unmistakable familiar animals and
plants. Thus there is a minute oval organism called
Chlamydomonas (Fig. 20, p. 185) which lives in pools.
It is green and lives exclusively on liquid and gaseous
food like a plant, but during most of its life it swims
actively about like an aquatic animal. It must be
reckoned as a plant, because, as we saw, the mode of
feeding is the basal character of difference, but it still
retains the animal character of free locomotion. And
there are other minute organisms which feed in both
ways, partly on liquid and gaseous inorganic food,
partly on solid organic food, and these escape the
meshes of any definition of an animal or of a plant.
We cannot, in fact, frame any comprehensive definition
which shall sharply separate all living organisms into
animals and plants. This conclusion illustrates a
truth that the student will realise more and more fully
as his biological studies progress, namely, that it is
useless to expect the facts of nature exactly to fit our
definitions, however carefully framed. There are so
many different ways in which living substances may
take shape, so many varieties of function, i.e. of ways
in which it may work, and so many possible combina-
tions of both of these, that there are certain to be excep-
tions to every rule we try to lay down. But that does
not mean that we have to give up the task of analysing
and classifying form and function, or the science of
biology would be impossible. We can always recognise
types of structure and types of function to which organisms
conform, more or less closely, because their physical
and chemical constitution forces them, so to speak,
along certain lines of differentiation and behaviour.

RELATION TO ENERGY 25

Only we must not expect to draw sharp lines. The
forms included in any groups we make will always
tend to shade off into forms belonging to other groups.

Thus we can say that the essential animal character
depends upon the habit of consuming solid organic
food ; that the essential plant character depends upon
the habit of absorbing liquid and gaseous inorganic
food; that a man is indubitably an animal, while
an oak tree is as indubitably a plant; that with the
man we can group a host of other different kinds of
organism as animals, and with the oak tree a host of
other organisms as plants; but that when we come
to minute microscopic organisms we find the differ-
ences which are so clear and sharp among the higher
forms becoming blurred, till finally we arrive at forms
of which it is impossible to assert that they are definitely
animals or definitely plants.

Relation of Animals and Plants to Energy.—The
essential difference between animals and plants in
their relation to food determines also their character-
istic difference in relation to energy. All living organisms
may be regarded as machines transforming energy
from one form into another, for instance, from the
potential energy locked up in the molecules of organic
food to the kinetic energy seen in motion of the body
and in the production of heat (animal), or from the
“radiant ”’ (kinetic) energy of sunlight to the potential
energy of organic substances formed in the body
(green plant). This subject will be dealt with.more
in detail in later chapters.

Meanwhile we note the broad difference between
animals and plants in their relation to energy is that
animals consume organic food and spend the energy it
contains in heat and motion, while plants build organic

26 INTRODUCTORY. PLANTS AND ANIMALS

substance and store energy. And since life consists in the
expenditure of energy, animals live much more intensely
than plants, and correspondingly they are sensitive to
much more varied stimuli, i.e. to the influences which
lead to their expending energy in definite ways. But
because they are alive plants do spend energy and are
sensitive to stimuli. In the course of the life of a tree,
for instance, a large amount of energy is expended in
lifting the branches high into the air and in pushing
the root tips through the soil, though these things
are done so slowly, measured by the standard of the
rate of motion in animals, that we are not at once
impressed by them. The shoots of a plant also bend
towards the light, and the roots towards supplies of
water, i.e. they respond to different stimuli in definite
ways. Also animals do store energy in various forms
of organic food, e.g. glycogen (a form of starch) in the
liver and the muscles, and thus have a reserve supply
of potential energy which enables them to carry on
their active life for a time without consuming fresh food.

The Naming and Classification of Plants and Animals.
—The immense multitude of individual plants and
animals existing on the earth are exceedingly varied
in form and structure, and in order to get any under-
standing of plant or animal life we must classify them
in some way. We recognise at once—mankind has
recognised from the earliest times—that there are
many different kinds, but of these some resemble each
other so closely that they are difficult to distinguish,
and can only be separated by those who have made
a special study of the forms in question, while others
are very distinct indeed. For instance, a blackberry
is obviously different from a raspberry in the colour
and taste of the fruit, and in the fact that the former

CLASSIFICATION 27

has hard prickles which can tear the flesh, while the
latter has soft prickles which cannot. But there are
many different kinds of blackberry, some of which
differ from one another in such small and variable
characters that even specialists who have spent a large
part of their lives in the study of the blackberries do
not agree as to exactly how they should be grouped.

We distinguish the kinds or groups of individuals
which resemble one another more or less closely, and
which interbreed freely, as species, but authorities
differ as to what they consider species, and there is
not yet agreement as to whether the conception of a
species can be made at all a precise conception. Species
which are most like each other are grouped into genera,
and genera which are most like each other into families.
Thus the raspberry and the blackberry are different
species of one genus, the sweet briar and the dog rose
of another genus belonging to the same family. The
hare and the rabbit are different species of one genus ;
the dog, the wolf and the fox of another belonging to
quite a different family, though to the same large
group, the mammals, which include all animals that
suckle their young, comprising such diverse types as
rats and mice, elephants, whales and men.

The conventional nomenclature, which is used in
naming the different species we recognise, and enables
us to record and systematise them, gives each genus
a Latin name—a noun—and adds a qualifying adjec-
tive for the species. Thus the genus to which both
the blackberry and raspberry belong is called Rubus,
the former (if we lump all the blackberries together as
one species) being Rubus fruticosus, the latter Rubus
ideus. The genus Rosa includes Rosa rubiginosa, the
sweet briar, and Rosa canina, the dog rose, as well

28 INTRODUCTORY. PLANTS AND ANIMALS

as many other species. Both these genera, along with
many others, belong to one family, the Rosacee, or
Rose family, because they have certain characters of
the flower and fruit in common. The hare is Lepus
timidus, the rabbit Lepus caniculus: the dog, the
European wolf and the fox respectively, Canis familiarts,
C. lupus, and C. vulpes. In the last two cases it will
be seen that the specific name is a noun (the Latin
name of the animal in question), though it is used in
place of an adjective. Very many species, of course,
have no common names in any language because they
have not impressed their existence on man by their
usefulness or harmfulness, or conspicuousness—they
have not attracted his attention in any way, until
botanists and zoologists began to study the different
forms for their own sakes, The Latin nomenclature
has the indispensable advantage of being international.
It is in fact a relic of the time when Latin was the
universal language of learned men.

Genera, as has been said, are grouped into families,
families into orders, orders into still larger groups,
each successively higher grade of groupings containing
forms which are less and less like those belonging to
other groupings of the same grade.

Range of Form and Structure in the Plant World.
—Starting with the higher forms, including the trees,
shrubs and herbs with which we are most familiar,
the following are the larger groups of plants:

(t) The SEED PLANTS (SPERMOPHYTES), including
(i) the Angiosperms, or true Flowering Plants, which
produce seeds completely enclosed in bag- or box-like
structures, and comprise nearly all the herbs, grasses,
etc., and the shrubs and broad-leaved trees, and (ii)
the Gymnosperms, with seeds not so enclosed, but

THE LARGER GROUPS OF PLANTS 29

often borne in cones, as in the Coniferz, mostly needle-
leaved trees (pines, firs, etc.). The Seed Plants have
the most complicated internal structure of any plants.

(2) The PTERIDOPHYTES, plants of the same grade
of organisation as the Ferns, and possessing, like the
Seed Plants, stems, roots and leaves, and a well-devel-
oped internal water-conducting system, but reproducing
themselves not by seeds, but by very minute bodies
called spores. These include the Ferns, Clubmosses;
and Horsetails. The living forms of the two last-
named groups represent a very small proportion of
the plants belonging to these groups, which flourished
in certain past ages, and which, often growing to the
size of trees, are preserved as fossils in the coal measures
and other rocks. In the same rocks are preserved
forms which are intermediate between the Pteridophytes
and the Seed Plants. These formed seeds, but seeds
which were different in structure from the seeds of
existing plants.

(3) The BrYopHYTEs, including the Mosses and the
Liverworts, also reproducing themselves by spores, but
much smaller on the whole than the Pteridophytes,
and with a less well-developed water-conducting system,
or none at all. The Mosses and some of the Liverworts
have distinct stems and leaves, but other liverworts
have none, the plant body consisting of a flat thallus,
usually a branched ribbon-shaped structure, rather
like an indefinitely growing leaf. These forms are
mostly only able to live in relatively damp places,
though the majority are land plants.

(4) The Atc#, plants living mostly in water, and
able, like the previous groups, to form organic sub-
stances from inorganic. They all contain chlorophyll.
but some of them, the red and brown seaweeds, for

30 INTRODUCTORY. PLANTS AND ANIMALS

instance, are not green in colour, because the colour
of the-chlorophyll is modified by the presence of other
pigments. They include the seaweeds, many of which
are large plants, with a considerable degree of internal
organisation, and the algz that live in freshwater,
most of which are pure green in colour, and consist of
simple filaments or single microscopic cells (see below).

(5) The Funa1, plants of about the same grade of
organisation as the alge, and, like them, comprising
a great range of size and complexity of organisation,
from the bulky mushrooms and toadstools to simple
moulds and mildews and minute microscopic forms.
They are distinguished by not possessing chlorophyll,
and they depend on organic food.

(6) The LicHENs, peculiar compound plants, each
consisting of an alga and a fungus associated together
in one plant body, or thallus, which resembles in a
general way the liverwort thallus referred to above,
though it often branches in a complicated way. The
thallus of lichens is variously coloured: sometimes
the green colour of the included alga can be seen through
the fungal investment. Lichens are especially found
on rocks, the trunks of trees, or the bare soil.

Many of the alge and fungi are microscopic, and some
of them are very minute indeed. Among these micro-
scopic forms, which live in water or some other liquid,
we come to the groups of organisms referred to on
pp. 23-4, which are on the borderline between plants
and animals, and are sometimes classed together as

(7) The Protista. Among these are the BAcTERIA,
the smallest of all visible organisms, which are on the
whole plants, but many have the animal character of
locomotion. They are of enormous importance to
mankind, as we shall see in a later chapter. The

CELLULAR STRUCTURE 31

Protista include many other groups of simple organisms
of very various characters.

As we descend the series outlined above, starting
with the higher (most complex) plants, we find that
the forms of plant life (with the exception of the fungi
and lichens) are more and more dependent on external
water, most of the algz living all their life immersed
in water. It is established biological doctrine that
the higher plants, which are able to live on dry land,
though their roots must in fact always be able to
obtain some water from the soil, have gradually arisen,
during the history of the world, from forms like these
lower plants which are confined to water. This is
the doctrine of organic evolution, as applied to plants,
a doctrine of whose truth Darwin first succeeded in
convincing the world.

Cellular Structure of Plants.—When we come to
examine their structure with the microscope, we find
that the bodies of nearly all plants and animals are
composed of what are called cells and the products
of cells. In the case of plants the cells are closed
spaces, surrounded by a cell wall of non-living substance,
and containing living substance called protoplasm, in
which all the ultimate life processes occur. This proto-
plasm is the only part of the bodies of living beings
which is actually alive. A large part of the body
consists of non-living organic substances formed by
the protoplasm. Thus in plants the body is composed
of a “skeleton’’ of cell walls, with living substance
in each cell cavity. The protoplasm is, however,
generally continuous from cell to cell by means of
exceedingly thin filaments of protoplasm (which cannot
be seen by ordinary microscopic observation) passing

32 INTRODUCTORY. PLANTS AND ANIMALS

through the cell walls, so that all, or nearly all, the
protoplasm of the plant really forms a single connected
structure. ,

The protoplasm of many of the cells of the bodies
of the higher plants disappears during the life of the
plant, so that only the dead cell walls remain, for in-
stance in the outer bark and the heartwood of tree
trunks, which are thus dead, though integral parts of
the living plant body, just as are the nails, for instance;
in the case of man.

Green Colour of Plants. Chlorophyll.—The great
majority of plants, from the simplest alge to the
flowering plants, are green, or have green parts (fungi
and bacteria are an exception). It has already been
mentioned that this green colour is due to the presence
of a pigment called chlorophyll, whose presence, owing
to the light which it absorbs, enables the protoplasm
of the plant cell containing it to build up organic
substances out of simple inorganic substances which
the cell absorbs. The chlorophyll is contained in
definite protoplasmic bodies called chloroplasts, usually
spherical, oval or disc shaped, within the general
protoplasm of the cell. This process of the formation
of organic substances from inorganic by the activity
of the chloroplasts under the action of light is called
photosynthesis. It 1s upon photosynthesis that the con-
tinuance of life in the world ultimately depends, because
it is the only natural process which makes new organic
substance on a sufficiently large scale.

Protoplasm, as has been said, is the seat of all the
essential life processes. Before we can begin to analyse
the nature and relations of these it is essential to learn
something of the nature of protopiasm itself. Though
iG i. A putting together with the help of light,’”’ from Greek ddc,

abhian tanwnthna«

PRACTICAL WORK 33

we are far from understanding the whole secret of life,
we do know that many of the manifestations of life
depend upon the chemical nature and physical proper-
ties of protoplasm and of the substances derived from
it. The next two chapters will therefore be devoted to
these subjects.

PRACTICAL WORK.

A. USE oF THE MicROSCOPE,

(1) Examine the microscope, especially the mirror, substage
diaphragm, low and high power objectives, eyepieces, coarse and
fine focussing adjustments. [The demonstrator should show the
students how to handle a microscope, explain the outlines of its
working, and give the necessary cautions.]

(2) Put a small drop of water on the middle of a slide, and
place in it a few grains of sand. Cover with a coverslip so as
to include in the water some bubbles of air.

Examine first with the low power and then with the high
power

(a) a sand grain, noting the shape, colour and translucency ;
(0) an air bubble, noting its appearance in different focal
planes.

(3) Mount a single leaf from a moss plant in a drop of
water. Cover with a coverslip so as not to include bubbles
of air.

Examine first with the low and then with the high power.
Note that the leaf consists of a single layer of box-like compart-
ments (cells), with transparent walls, through which can be seen
the green chloroplasts containing the pigment chlorophyll. The
cells of the moss leaf are a good type of green plant cell. _

(4) Determine the apparent length in w* in the field of the
microscope (with fixed tube length) of a division of the eyepiece
micrometer scale, (a) with the high and'(b) with the low power
objective, by determining the correspondence of the eyepiece
scale with the divisions of the measured scale engraved on the
micrometer slide. Make a note of these values for future use.
Now measure the length and breadth of a cell of the moss leaf and
the diameter of a chloroplast under the high power,

' w= '0or mm.

3

34 INTRODUCTORY. PLANTS AND ANIMALS

B. Types oF THE PLant KINGDOM.

A series of specimens will be given out by the demonstrators
to be handed round. These illustrate types of all the great
groups of the plant kingdom, from the highest (flowering) plants.
Ask the demonstrator any questions that occur to you about
them.

Examine also the fully labelled demonstration series.

C. DIFFERENCES BETWEEN SPECIES OF THE SAME GENUS.

Examine carefully fresh specimens of two or three species
of flowering plant belonging to the same genus. Write down the
differences which strike you, and get the demonstrator to correct
your notes and to point out the characteristic differences in case
you have failed to observe them. [The species used must depend
upon the time of year and upon what can be obtained. In
the autumn Quercus rvobur, Q. sessiliflora and Q. ilex are suitable ;
in April Veronica agrestis, V. hedevifolia and V. tournefortii ;
in May or June Ranunculus acer, R. vepens and R. bulbosus.]

D. EXCEPTIONS TO THE CHARACTERISTIC FORMS AND STRUCTURES
oF PLANTS AND ANIMALS.

(Demonstration Specimens.)

(1) A Branching Animal.—Fresh or museum specimens of a
hydroid polyp or a bryozoon “ colony.” Each individual has a
mouth surrounded by tentacles and a gut. It consumes solid
food consisting of tiny organisms living in the sea, and is therefore
an animal in spite of the facts that it is fixed to one spot and
that the whole “ colony ”’ has a branching form like a plant.

(2) A Compact Plant.—Mamillaria, or other cactus from
subtropical America, shows no obvious division of its shoot
into stem and leaves. It has a branching root in the soil like
an ordinary plant, but its shoot consists of a compact fleshy
green stem bearing spines or bristles which represent the leaves.
The compact form decreases the evaporating surface and at the
same time stores water which is held by the abundant mucilage
the plant contains, so that the cactus is able to live in a hot, dry
climate.

(3) An Animal with no Mouth.—The tapeworm is a parasitic
animal which absorbs its liquid organic food from the intestines
of the animal in which it lives. It has no mouth or gut, having
lost these by degeneration in correspondence with its habit of
life.

PRACTICAL WORK 35

(4) A Plant which consumes Solid Organic Food.—The sundew
(Drosera) and the pitcher plant (Nepenthes) are insectivorous
plants which digest and absorb the products of insects that fall
on to the leaf or into the pitcher (a modified part of the leaf).
This is a character not possessed by most plants, but it serves
to demonstrate the common powers of plant and animal proto-
plasm. In other respects the sundew and the pitcher plant are
ordinary flowering plants.

CHAPTER II

ORGANIC SUBSTANCES AND THEIR
CHEMICAL CHARACTERS

ALL the activities of life depend upon protoplasm,
which is therefore the essential part of every living
organism. But by no means all parts of an organism
consist of or contain living protoplasm. For instance,
the heartwood and outer bark of a tree, the hairs,
feathers, nails or hoofs of .a warm-blooded animal are
destitute of protoplasm. These lifeless parts of an
organism are, however, all formed from or by proto-
plasm, and they consist mainly of complex chemical
compounds, containing carbon, hydrogen and oxygen,
often also nitrogen and sulphur, as well as other elements,
which are called ‘ organic’ compounds because they
are associated with organisms. There is no sharp
distinction between organic and inorganic compounds
in chemistry, and many of the simpler organic compounds
formed by organisms can also be made synthetically
in the laboratory.

All the organic compounds contain the element
carbon, organic chemistry being sometimes known as
“the chemistry of the carbon compounds,” though
many substances containing carbon (for instance, cal-
cium carbonate) are not specially associated with
organisms. Of the immense number of organic com-
pounds known certain classes are specially important

in the structure and activities of living organisms, and
36

WATER. CARBOHYDRATES 37

we must have some knowledge of the nature and proper-
ties of these before we can understand the nature and
functions of protoplasm.

We must, of course, be careful to distinguish between
an organic substance which is a mixture of chemical
compounds in various proportions, and a chemical
compound which has a perfectly definite chemical
composition, i.e. its molecule or ultimate unit of
structure consists of a definite number of atoms of
different elements arranged in a definite way. Milk,
for instance, is an organic substance which has a
varying composition, being a mixture in various propor-
tions of the chemical compounds water, milk sugar,
various definite fats, proteins, salts, etc. Such organic
substances as wood, horn, hair, etc., are, similarly,
mixtures of chemical compounds.

Of the chemical compounds which enter into the
composition of the body of an organism, water (H,O)
is of first importance, as we shall see in detail
later. It is essential to the structure of living proto-
plasm, and is found in greater or less proportion in
all parts of the body. More than go per cent. of the
weight of a herbaceous plant is water, as can easily be
shown by weighing the plant and then heating it at
100° C. till it loses no more water and weighing again.

The three classes of chemical compounds which play
the leading part in organisms are the carbohydrates,
the fats, and the proteins, the two former consisting
of carbon, hydrogen and oxygen, the latter with nitrogen,
sulphur, and sometimes phosphorus in addition.

THE CARBOHYDRATES.

These are a class of carbon compounds in whose
molecules the atoms of hydrogen and oxygen are

38 ORGANIC SUBSTANCES : CHEMICAL CHARACTERS

generally present in the proportion of two to one,
as in water.

The Sugars.

The most important carbohydrates are the sugars, of
which there are many different kinds. Glucose (grape
sugar), C,H,.O,, levulose, C,5H,,O4, sucrose (cane sugar),
C,,H..0,,, and maltose, C,.H,.0,,, are the only ones
we need mention here. ‘It will be seen that the two
former have the same number of atoms in the molecule,
and so have the two latter. They differ, however, in
the way in which the atoms are grouped, and this
leads to differences in crystalline form, and to differences
in reaction with other compounds.

The sugars are readily soluble in water ; moreover, they
easily move about the body by liquid diffusion, and their
molecules possess a large amount of potential (chemical)
energy, so that when the molecule is oxidised and broken
up a great deal of energy is set free in the kinetic form.
The kinetic energy which appears in organisms as heat and
movement is mainly derived from the oxidation and breaking
up of sugars. For these reasons the sugars are extremely
important in the sum total of the chemical changes,
called metabolism, which are perpetually occurring in.
the living protoplasm of plants and animals.

Glucose (grape sugar), C,H,.O,, is the most prominent
of the sugars in the economy of organisms. While
glucose is a relatively stable substance in the test tube,
it takes up oxygen more easily than other sugars, and
thus acts as a moderately strong ‘‘ reducing agent,”
i.e. it can take oxygen from (or ‘“‘ reduce ’’) certain other
substances. An example of this action is the behaviour
of glucose solution when mixed with a cupric salt
(i.e. a salt of copper containing much oxygen) in alkaline

SUGARS 39

solution—such as Fehling’s solution—and warmed.
Some of the oxygen is taken from the cupric salt, and
red cuprous oxide is formed. This is the ordinary test
for sugars that behave in this way—the “reducing
sugars ’’ as they are called. Maltose is also a reducing
sugar, though to a less extent than glucose. On the
other hand, sucrose (cane sugar) does not reduce
Fehling’s solution at all.

In the living cell glucose very readily takes up oxygen
and breaks up its molecule into carbon dioxide and
water. This is the basis of the process of respiration,
which is of fundamental importance in the economy of
living organisms :—

C,H,,0, + 60, = 6CO, + 6H,O
glucose oxygen carbon water
dicxide

Sucrose or saccharose (cane sugar), C,.H,.0,,, is
the most important sugar commercially. It is the
main product of the sugar cane and the sugar beet,
and is the principal sugar we use as food. It is very
much sweeter than glucose. The importance of sugar
as a food for animals depends on the characters already
mentioned, solubility and high potential energy. It is
thus easily absorbed through the wall of the alimentary
canal into the blood, and the readiness with which it
is oxidised places large amounts of energy at the dis-
posal of the muscles, where the energy appears in the
kinetic form, as motion and heat. Sugar is, in fact,
used in this way by all living cells, but most energetically
by the muscles, which are the great energy spenders
of the animal body.

Sucrose is not a reducing sugar, and, in the living
cell, is converted into glucose and levulose before being
oxidised. This process of the conversion of sucrose

40 ORGANIC SUBSTANCES : CHEMICAL CHARACTERS

into glucose and levulose is an example of hydrolysis,
i.e. the splitting of a molecule by reaction with water.
CypH990,, + HO = C,Hj,0, + C.H,20,
sucrose water glucose levulose
Various sugars are found mixed in different proportions

in living cells and are constantly being converted from
one form into another.

The Polysaccharides.

These are another important class of carbohydrates.
They derive their name from the fact that they are
formed by the putting together of many molecules of
sugar to form one molecule of polysaccharide, a process
known as condensation. The most important poly-
saccharides of plants are the starches and the celluloses.
There are many different kinds of each, but for our
purpose we may treat them in each case as one.

Starch is formed as solid grains in the protoplasm
of the living cells of plants. The substance of each
grain is laid down in concentric layers round a centre
called the hilum of the starch grain. Starch is formed
by the union of many molecules of glucose with the
elimination of one molecule of water from each :—

NC gH 504 = (CH 95), + #H,O

The value of 7 has not been exactly determined, but
it is probably somewhere about 100. Thus the starch
molecule is far more complex than the glucose mole-
cule. Starch has a very characteristic blue reaction
with watery solution of iodine in potassium iodide.
Most starches are quite insoluble in cold water; they
are hydrolysed in the presence of dilute mineral acids
into various simpler carbohydrates, and eventually

POLYSACCHARIDES. FATS 41

into glucose. Hydrolysis of starch also takes place in
plant. cells (see below). Starch is a very common and
widespread substance in plant cells. It bulks largely
in the economy of plants, though it stands second to
the sugars in this respect. It also forms a very valu-
able food for animals, being hydrolysed into sugars
(or rather its hydrolysis is catalysed) by the action of
the digestive juices of the mouth and of the small
intestine.

Cellulose is a substance of even more complex composi-
tion than starch. Its empirical formula is the same,
i.e. it has the same proportion of carbon, hydrogen and
oxygen atoms in the molecule, though a much higher total
number. Thus, if starch be represented by (C,H,)0,),
we may represent cellulose by (C,H,)0,),,.

Cellulose is of immense importance because it forms
the main substance of the cell walls of most plants,
and thus the “skeleton” of the plant body. Linen
and cotton are nearly pure cellulose, and paper contains
a varying amount according to the plant substance
from which it is made. Cellulose is formed in a similar
manner to starch by the condensation of many mole-
cules of sugar. It is not coloured blue with iodine,
but it can be altered, e.g. by the action of strong
sulphuric acid into a substance which resembles starch
in so far as it is so coloured with iodine.

THE Fats.

These, like the carbohydrates, consist of carbon,
hydrogen and oxygen, but the fat molecule is much
larger than the sugar molecule, and contains a much
smaller proportion of oxygen. Fats are widely dis-
tributed in plant cells, and form a characteristic store
of non-nitrogenous food in many seeds, e.g. linseed,

42 ORGANIC SUBSTANCES : CHEMICAL CHARACTERS

cottonseed, coconut, castor oil seed. They contain an
even larger store of potential energy per molecule than
the carbohydrates, and thus require more oxygen for
complete oxidation. Of numerous kinds two examples
are palmitin (C;,H,,O,) and olein (C;,H,),0,). Fats
occur in plant cells as complex mixtures of different
individual fats. They are usually found in the liquid
state, and are then often spoken of as “ oils.”

Fats are even more widespread and important in the
animal than in the plant body, forming a very valuable
reserve store of energy.

THE PROTEINS.

These are the most important of the nitrogen-
containing organic substances. The molecule consists
of carbon, hydrogen, oxygen, nitrogen and sulphur,
with sometimes phosphorus in addition. The protein
molecules are the largest and most complex of any
chemical compounds, each molecule consisting of many
hundreds, or even thousands, of atoms. The different
groups of atoms composing these huge molecules have,
like the atoms composing the molecules of all chemical
compounds, a perfectly definite position in regard to
one another. The more numerous the atoms the greater
will be the number of different relative positions possi-
ble. Hence there are an enormous number of distinct
protein molecules with comparatively small differences
between them.

Now protein molecules form the essential basis
of protoplasm. Protoplasm, as we shall see in
succeeding chapters, has the same general character-
istics and powers in all living organisms, but it is
obvious that the protoplasm of every species differs

.in some way from that of every other species, or it

PROTEINS 43

could not build up a different body. It is probable
that differences between species ultimately depend
upon comparatively slight differences in structure
between some of the proteins present, or between the
relative amounts of the different proteins present, or
perhaps between the ways in which different protein
molecules are built up into more complex protoplasmic
structures, of which we actually know practically
nothing. Such biochemical differences between even
closely allied species have recently been shown to hold
for other important organic substances of the body,
for instance between the starches of different species
of plants and between the hemoglobins (the red pig-
ment of the blood) of different species of animals. It
has been shown that the starch of every species (out
of many hundreds examined) is in some respect different
from the starch of every other species; and the same
is true of hemoglobin. The proteins are much more
difficult to investigate, but when we consider that the
molecules of many of them are much more complicated,
it becomes obvious that the possibilities of slight but
perfectly definite differences are much more numerous.

Proteins as a class show certain colour changes
when acted upon by certain substances; for instance
a yellow colour, when after boiling with nitric acid
ammonia is added; a violet colour, with a drop of
dilute copper sulphate followed by excess of caustic
soda. A dilute solution of iodine in potassium iodide,
which colours starch blue, stains proteins yellow. In
such ways proteins, as a class, can be distinguished
from other classes of organic compounds. Into the
characters of the different proteins we shall not attempt
to enter here.

Apart from their universal occurrence in living proto-

44 ORGANIC SUBSTANCES: CHEMICAL CHARACTERS

plasm, as the essential part of its structure, proteins
occur in more or less solid aggregations as grains (called
“aleurone grains’) of non-living substance in the
cells of certain parts of plants, such as seeds. Here
they represent a store of protein substance used as
food by the young seedling, and are comparable in
this respect with starch grains or oil (fat) globules,
which similarly form stores of non-nitrogenous sub-
stance.

There are of course many other classes of organic
substances besides the three mentioned. In plants, for
instance, the so-called ‘‘ aromatic substances ’”’ occur
universally. They contain carbon, hydrogen and
oxygen, and are founded on the structure known as
the benzene ring, or a similar ring, but our knowledge
of their réle in the economy of plants is still obscure.
The three classes of organic substances—the carbo-
hydrates, fats and proteins—briefly described comprise
the most inportant organic substances entering into
the composition of organisms, and they can all be
used as foods by animals.

Enzymes.—These are a very important class of
substances, whose exact chemical composition is still
unknown, but many of them can easily be isolated.
Their importance consists in the fact that they act as
catalysts to many of the chemical changes occurring
in living protoplasm. A catalyst is a substance that
facilitates a chemical change without itself entering
into the final products of the change. It probably
works by temporarily entering into combination with
the substance or substances in reaction, causing a
disruption of their molecules, and ultimately separating
from the products. Owing to the fact that the enzyme
itself is not involved in the final product, a very small

ENZYMES 45

amount of enzyme is sufficient to carry out the change
in large quantities of substance.

Enzymes in great variety exist in the living body,
each kind acting upon a particular class of substance.
Thus the lipases facilitate the hydrolysis of fats,
which are decomposed into simpler substances; the
proteases have a similar action upon proteins. Of the
enzymes which work upon carbohydrates, diastase,
which converts starch into sugar, is the best known.
The first stage of the reaction produces dextrin (a
soluble polysaccharide of the same empirical formula
as starch) and maltose :—

(CgH 1505), + H,0 —>(C,Hy995), + Cy2H2204;

starch water dextrin maltose
The molecule of maltose is hydrolysed (its hydrolysis
being catalysed by another enzyme, maliase) into two
molecules of glucose :—

CypHy20,, + H,0 = 2C,H.0,
maltose water glucose

Invertase similarly catalyses the hydrolysis of sucrose
into one molecule of glucose and one molecule of the
closely allied sugar levulose :—

CipH20,, + HO = CyHy.0, + CpHy20¢

sucrose water glucose levulose

Cytases facilitate the hydrolysis of the celluloses into
simpler soluble carbohydrates.

Many, if not all, of the enzymes have what is called
a reversible action, i.e. they may bring about a change
in etther direction, e.g. of starch into sugar, or of sugar
intostarch. The direction in which the reaction actually
proceeds at any given moment depends upon the rela-

t The uniform termination -ase is used in the naming of enzymes,

46 ORGANIC SUBSTANCES : CHEMICAL CHARACTERS

tive concentration of the soluble end products. Thus,
if the sugar concentration is low, starch will be converted
into sugar, but when it rises above a certain point the
sugar will be converted into starch. We shall see in
the sequel that these reversible reactions are very
‘important in the living organism. .

In nature organic compounds always occur as
constituents or products of living organisms, but
chemists can now make many of them out of simpler
substances in the laboratory, for instance the sugars,
and even some of the simpler proteins.

PRACTICAL WORK.
CARBOHYDRATES, FATS AND PROYEINS.

(rt) Compare the samples of solid glucose and of solid sucrose
provided. Note that the first is a crystalline powder, while the
second forms large well-defined crystals. Compare their sweet-
ness by tasting. Dissolve the samples in two separate test
tubes each containing a little water, and note that each dissolves
completely. Test a little of each with very dilute solution of
iodine in potassium iodide ' and note that there is no coloration.
Add a few drops of Fehling’s solution (copper sulphate made
strongly alkaline) to each test tube, and heat. Note that one
solution turns red owing to the formation of red cuprous oxide,
while' the other does not.

(2) Examinea thin slice of potato under the low and high powers
of the microscope. Draw one or two of the starch grains under
the high power. Measure their long and short diameters with
the micrometer eyepiece and express the values in yz (see Practical
Work I (4) p. 33). Treat the section with very dilute iodine
solution and examine again.

(3) The tube provided contains very dilute starch paste.
Pour a little into a watchglass, and add one drop of very dilute
iodine solution. Now add a little taka-diastase (a commercial
preparation of diastase also containing maltase, see p. 45) to the
contents of the tube, and shake the tube vigorously at intervals.
After a minute pour a little of the mixture into another watch-

t This solution will be alluded to in future simply as “iodine
solution.”

PRACTICAL WORK 47

glass and add a drop of very dilute iodine. Shake the tube
vigorously again at intervals and after another two minutes
pour a little into a third watchglass, adding a drop cf very dilute
iodine. Note the colours in the three cases. The pinkish colour
with iodine is due to the formation of dextrin, a carbohydrate
intermediate between starch and sugar. On adding a few drops
of Fehling’s solution to the contents of the test tube after half
an hour, and heating, a slight reducing effect should be obtained.
This is due to the glucose present.

(4) Test with iodine solution a piece of good writing paper
(nearly pure cellulose) across which strong sulphuric acid has
been streaked.

(5) Examine the demonstration specimen of paper boiled in
strong sulphuric acid, neutralized with alkali and then treated with
Fehling’s solution to demonstrate the formation of a reducing
sugar (glucose) ; also of the cut date stones (the very thick
walls of whose cells consist of cellulose) similarly treated.

(6) Note that desiccated coconut ‘meat’ (food material
used by the embryo growing into a seedling) is oily to the touch.
Place some in a test tube with water and boil: the oil rises to
the surface. Add a few drops of Sudan 3 (an aniline dye) and
allow to stand: the oil is coloured red. Add a few drops of
Sudan 3 to olive oil in a test tube: the oil is coloured red. Pour
half the red oil into another test tube. Add a little alcohol to
one and a little water to the other. Note the position taken up
by the water and alcohol respectively in relation to the oil:
also the coloration resulting.

(7) The two substances provided in separate test tubes are
(a) white of egg shaken up with water, which is a typical protein,
(®) bean meal shaken up with salt solution. Divide each into
three portions and subject each set of three io the following
tests, noting the coloration in each case :—

(i) Add a few drops of strong nitric acid. Boil and then
cool thoroughly. Add a few drops of ammonia solution) xantho-
proteic reaction).

(ii) Add one drop of dilute copper sulphate solution. Now
add excess of caustic soda or potash (biuret reaction).

(iii) Add a few drops of iodine solution.

CHAPTER IIT

SOME PHYSICAL CHARACTERS OF ORGANIC
SUBSTANCES

Crystalloids and Colloids.\—A solid substance, when
placed in water, may behave in one of several ways.
First, it may react chemically with the water, as for
instance. sodium, which breaks up the molecule of
water, combining with the oxygen and _ liberating
hydrogen. Secondly, it may dissolve in the water to
form a true solution, as for instance common salt
(sodium chloride), the salt seeming to disappear in
the solution (where it is called the solute, the dis-
solving substance being the solvent), Thirdly, it may
remain practically unaltered, as for instance quartz
sand (silica). If, however, such a substance as the
coarse grains of silica are ground in a very finely
grinding mill into finer and finer particles, a point of
fineness is ultimately reached at which the particles
stay suspended in the water, without settling, to
form what is known as a colloid! solution, or sol, which
differs from a true solution in several important
respects. This fourth kind of behaviour with water
is a very characteristic feature of many organic
compounds.

Broadly speaking, the substances which form true
solutions with water, or some other liquid solvent,
have small or comparatively small molecules, such for
instance as simple inorganic salts, and many of the

t Greek xdAda, glue, from the jelly-like nature of the most familiar
colloids (gels). See p. 53.
48

CRYSTALLOIDS AND COLLOIDS 49

simpler organic compounds, such as the sugars. On
evaporation of the solvent they reappear, often in the
form of crystals. Substances which behave in this
way are known as crystalloids. The substances which
when mixed with water become dispersed in very fine
particles through the water, each particle consisting of
a number of molecules aggregated together, are called
colloids. It is to be noted that many simple substances
—even elements such as gold and platinum—may be
prepared in such a form that they behave as colloids
with water, i.e. they become dispersed in minute particles
through the liquid. We now know that we cannot sharply.
divide substances into the two categories, crystalloids
and colloids, but speak of a substance, in a given
condition, behaving as “‘a crystalloid or as a colloid ”’
with water, or some other solvent. Nevertheless the
organic compounds with large molecules, e.g. poly-
saccharides, proteins, etc., commonly do behave as,
and are therefore commonly spoken of as, colloids.
The aggregate of the dispersed particles of a colloid
is known as the disperse phase, the continuous liquid
medium in which they are dispersed as the continuous
phase of the colloid sol. The disperse phase does not
necessarily consist of solid particles, it may consist of
very minute liquid droplets, and then the sol is called
an emulsion sol, or an emulsoid, while if the disperse
phase is solid we have a suspension sol, or a suspensoid.
Coarse particles or droplets, suspended in a liquid,
fortin ordinary suspensions or emulsions, and tend to
sink slowly to the bottom, or rise to the top, as they
are heavier or lighter than the liquid. Thus the fat
globules suspended in milk slowly rise to the surface
as cream. If we shake up a handful of soil in a beaker
of water, the coarser and heavier particles sink to the

4

50 PHYSICAL CHARACTERS OF ORGANIC SUBSTANCES

bottom at once, others sink more slowly, but the very
fine colloidal particles (clay) remain suspended indefi-
nitely, forming a colloid sol with the water.

Microscopic and Ultramicroscopic Particles.—The
larger of these very fine colloid particles, such as those
of an Indian ink sol, may be clearly seen under the
high power of the microscope. When they are quite
freely suspended, that is not adhering to the glass
slide or coverslip, they may be seen to be in continuous
oscillation, or intermittent jerky movement. This is
due to the continuous bombardment of the disperse
particles by the molecules of the liquid, which are of
course themselves in constant motion. At any given
moment the probability is that this bombardment will
be unequal on different sides of the suspended particle,
and this will therefore move away from the side on
which it is most heavily bombarded. If the bombard-
ment happens to be momentarily equal on all sides
the particle will remain motionless. This kind of
movement is called Brownian movement, from its dis-
coverer, the botanist Robert Brown.

The disperse particles of the finer sols, such for
instance as a Congo Red sol, are too small to be seen
under the highest powers of the microscope, and the
sol appears clear. With the highest powers and the
best definition the smallest particles that can be clearly
defined under the microscope are those not less than
about ‘15 #1 = ‘00015 mm. in diameter. But by
means of an instrument called the ultramicroscope—
which consists essentially of a high power microscope
with Jateral instead of vertical illumination—when
lighting from the side and viewing against a dark

t w is one-thousandth part of a millimeter and is the ordinary unit
of microscopic measurement.

ULTRAMICROSCOPIC BODIES 51

background, smaller objects than these can be detected,
though they cannot be clearly defined. They are seen
as tiny blurs of light owing to diffraction of the rays
round the ultramicroscopic particles. By this method
particles of about ‘005 w(=5 wy)! may be detected,
which we know from calculations based on other data
is about the size of the very large molecules of soluble
starch.2 The disperse particles of very fine sols run
down to perhaps 2°5 pp, the size of the molecules of
hemoglobin, the red pigment of the blood. The
molecules and ions (single atoms, or small groups of
atoms, into which a fine solute may be dissociated)
of true (crystalloid) solutions are much smaller than
this, from °5 wp (alcohol molecule) down to perhaps
‘I pp, which is about the diameter of a hydrogen ion
(separate atom of hydrogen).

To give some more easily comprehensible notion of
the relative sizes of these exceedingly minute objects:
if we imagine the hydrogen ion magnified a million
times, so that it was just visible to the naked eye as
the smallest possible ink dot on a sheet of paper, then
a very large organic molecule (such as might just be
detected with the ultramicroscope), if it were magnified
to the same extent, would be about the size of a small
pea, and a very small bacterium (the smallest organisms
that are just visible with the microscope) would be
twice the diameter of a football. Represented on this
scale a real pea would appear more than three miles
in diameter.

Internal Surface of Colloids.—The very large number
of particles or droplets forming the disperse phase of

* pp is one-thousandth of 1 yw, and is the unit of ultramicroscopic

measurement.
2 Soluble starch is a simpler form of starch produced on boiling

with dilute hydrochloric acid and forming a colloid sol with water.

52 PHYSICAL CHARACTERS OF ORGANIC SUBSTANCES

a colloid soil present collectively an enormous surface.
Ii we take a solid spherical object, such as an orange,
and cut it into halves, it is clear that we have greatly
increased the exposed surface, for to the original external
surface we have added the cut surfaces of the two
halves. If we now cut the halves into quarters the
total surface is still further increased, and the more
we divide the orange the greater the surface exposed.
Thus the collective surfaces of all the disperse particles
in a drop of a colloid sol are immensely greater than
the surface of the sphere which would be formed if
all the particles in the drop were aggregated in a
single mass.

Surface Energy, Surface Tension and Adsorption.—
Now, where a surface exists, i.e. where solid or liquid
matter is in contact with other matter, that surface
is the seat of free energy, because of the break in
action of molecular forces at the surface, and this
is expressed in what is called surface tension. The
surface of a drop of liquid in air, for instance, tends
to contract, and that is why a free drop of liquid assumes
the spherical shape, in which the surface has contracted
as much as possible, and has the smallest possible
area in relation to the mass of the liquid. Liquids
with a high surface tension form drops which do not
easily spread upon a solid surface, because their high
surface tension tends to keep the drop spherical and
prevent it spreading. Mercury, water and alcohol
form a series of liquids with decreasing surface tension,
and a corresponding increasing readiness to “ wet,”
i.e. spread upon, a clean sheet of glass.

The enormous collective surface, i.e. the sum of all
the surfaces, of the disperse particles or droplets of
a sol involves a corresponding amount of surface energy

ADSORPTION. GEL FORMATION 53

of the particles or droplets. This is of very great
importance in determining the behaviour of the colloid
towards liquids, solutes and other sols, which may be
attracted to the surfaces of the disperse particles or
droplets, and held there by the surface energy, at the
same time decreasing the surface tension of the dis-
perse phase. This process is called adsorption. The
living protoplasm of the cell is believed to be a colloid
sol of which the continuous phase is a solution of various
crystalloids, and the disperse phase consists of protein
and fat droplets or particles. These take up and hold
by adsorption the molecules and ions of solutes which
can diffuse into the cell and come within the range
of their surface energy. In this way large quantities
of various substances may be taken in and held in the
living cell. Dyes are taken up in the same way by
protoplasm and by the other organic colloids of the
plant or animal, and on this process largely depends
the staining of tissues employed in making microscopic
preparations.

Gel Formation.—When hydrosols (sols with water
as the continuous phase) lose water, for instance by
evaporation, their disperse particles get closer and
closer together. In the case of certain sols, including
those whose disperse phases consist of large organic
molecules, such as those of gelatine for example, the
molecules or aggregates of molecules tend, on loss of
water, to run together in chains. As more water is
lost, we picture these chains becoming more and more
entangled with one another, the sol becomes more and
more viscous (treacle-like), till ultimately a jelly—
technically called a gel—is formed. On further loss
of water the gel shrinks, and eventually becomes solid.
If water is again added it is absorbed, and the substance

54 PHYSICAL CHARACTERS OF ORGANIC SUBSTANCES

gradually swells till it passes back into the condition
of a sol.

A gel which can thus pass back into a sol is called
a reversible gel. A piece of solid gelatine, for instance,
will absorb water till it passes into the gel, and finally
into the sol condition, in which the original gelatine has
disappeared from sight. Allowed to evaporate the sol
will lose water till the gel reappears and the gelatine
finally becomes solid again; and these opposite pro-
cesses can be carried out an indefinite number of
times.

Many organic colloids of the living body are in the
gel condition, e.g. cell walls, starch grains, gums,
mucilages, etc., and some of these, by the absorption
and loss of water, can pass into sols and back again.
Protoplasm itself may exist either as a sol or as a gel.
The internal surface of a gel, though less than that of
the disperse phase of a sol, is nevertheless very great,
and gels accordingly show those characters, such as
the power of adsorption, which depends upon internal
surface. ;

Membrane Formation.—Gels may be formed from
sols in other ways than by loss of water. For instance,
when a sol is in contact with some other substance,
a film or membrane of gel structure is often formed
on the plane of contact. The protoplasmic sol always
forms a membrane or film on its free surface, and these
protoplasmic gel membranes play a vital part in the
economy of the living cell.

Diffusion.—A very important difference between
crystalloids and colloids, a difference used by Thomas
Graham, who first investigated the subject in the
middle of last century, to distinguish between them
is their relative diffusibility,

DIFFUSION 55

If a strong (concentrated) solution of a crystalloid
is brought into contact with a weak (dilute) solution
of the same crystalloid in the same solvent, diffusion
proceeds until the solute is equally distributed through
the whole of the liquid. This is an expression of
the general physical law that all systems tend towards
eqguilibvium. The vate of diffusion varies directly with
the difference in concentration of the two solutions,
and with the temperature, inversely with the size of
the molecule of the solute. If now the two solutions
of different concentrations are separated by a colloidal
membrane of the nature of a gel, such as vegetable parch-
ment, crystalloid solutes will diffuse through the
membrane as they would diffuse into a liquid with
which they were directly in contact, though less
rapidly. The rate of this diffusion through a membrane
depends partly on the nature of the membrane, and
partly on the size of the molecule of the solute, partly
again on the chemical relation between the solute and
the membrane.

The disperse phase of a colloid sol, on the other hand,
does not in general pass through a gel membrane at
all, or only does so with extreme slowness, and this
again depends on the size of the particles, on the nature
of the membrane, and on the chemical relations
between them. It was this difference which Graham
used to distinguish colloids from crystalloids, and he
showed that a colloid and a crystalloid in mixed solution
could be separated by placing the solution in a parch-
ment bag and plunging the bag into water, when the
whole of the crystalloid would eventually escape through
the membrane, leaving the colloid pure behind. This
process is called dialysis.

We may say, broadly, that crystalloids in solution

56 PHYSICAL CHARACTERS OF ORGANIC SUBSTANCES

will dialyse, and that colloids will not. But different
crystalloid solutes pass through a given membrane at
very different rates. The membrane may be likened
to a sieve with meshes of definite size, though varying
within limits. These meshes will let through molecules
up to that size, but not larger ones. The disperse
particles of a colloid sol are in general too large to pass
through the membrane, but in some cases they may
be just small enough to pass in small numbers. The
matter is further complicated, as we have seen, by the
reaction in some cases between the molecules of the
solute or disperse phase and those of the membrane.
A membrane which will allow a solute to pass through
it is said to be permeable to the solute. But a mem-
brane which is permeable to the solvent and not to
the solute is said to be semi-permeable. Different
colloidal membranes show very different degrees of
permeability to different crystalloid solutes, some
letting through a large variety of solutes, others being
impermeable to many, while others are impermeable
to practically all solutes, though they allow water to
pass. This fact has a great importance in the living
cell because the gel membrane or film on the surface of the
layer of protoplasm which lines the cell wall is @ semi-
permeable membrane, allowing certain solutes to pass
quickly, others slowly, and others, again, not at all.
Osmosis.—If we, place some sugar solution in a
parchment bag (i.e. a colloidal membrane) open at
the top, and immerse the bag in water up to the level
of the sugar solution within the bag, we find that the
level of the liquid; within the bag gradually rises owing
to the passage of water from the beaker through the
membrane, while the sugar does not pass out into the
beaker. The membrane is said to be ‘‘ semi-permeable

OSMOSIS 57

to sugar solution,” the molecules of the solute being
unable to pass through it, while the molecules of water
can. If common salt solution, instead of sugar, is
within the bag, the molecules of salt can pass out
(though at first the liquid rises in the bag because
the water passes in more quickly than the salt can
pass out), since the membrane is permeable to them.
The solution within the bag becomes less concentrated
owing to the entrance of water and the escape of salt,
while the liquid outside becomes a salt solution of
increasing strength till the two are of equal concentration
and equilibrium is attained. But in the case of the
sugar equilibrium cannot be attained in this way
because the sugar molecules cannot escape through
the membrane, and so the water continuously enters,
rising in the bag if it is open at the top, distending its
extensible wall if the bag is closed. This process of
the passage of a solvent through a semi-permeable
membrane which will not allow the solute to pass is
called osmosis, and the pressure developed on the wall
of the membrane is known as osmotic pressure.

Various solutes, among which sugars are prominent,
to which the protoplasmic membranes bounding the
cells appear to be practically impermeable, are formed
within the living cells of plants and animals. These
are known as osmotic substances, and many of the pro-
cesses of absorption and movement of water within
the organism depend on the passage of water through
membranes on the other side of which such solutes
exist, in accordance with the universal tendency to
establish equilibrium.

We now begin to see the importance of the fact that
the structure of organisms is so largely built up of

58 PHYSICAL CHARACTERS OF ORGANIC SUBSTANCES

colloidal substances. On the one hand we have the
power of the colloid gel to absorb water, and the power
of both gels and sols to adsorb a great variety of
substances as a result of their immense internal surfaces.
On the other we have the fact that the gel membrane,
especially the protoplasmic membrane, is semi-perme-
able, so that it does not allow of the escape of osmotic
substances within the cell, while it does allow water
to enter freely, and other substances, such as salts,
at various rates.

The colloidal nature of complex organic substances
largely depends on the great size of their molecules,
and this is most conspicuously seen in the proteins
which form the basis of protoplasm. The great size
of these complex molecules is often associated with
instability of particular arrangements of the atoms
which compose it. This makes possible the existence
of very numerous chemical compounds of distinct
though closely similar molecular structure. A given
group of atoms within the molecule is replaced by
another of somewhat different arrangement. This is
the cause of the extraordinary richness and variety
of the chemical and physical changes within the cell
which are the basis of the varied manifestations of
life and of organic form.

Though we are still very far from understanding
exactly how what we call living protoplasm actually
carries out all the complex processes which take place
within it, we can already trace the direct dependance
of many of them on the chemical and physical
structure and relations of the substances of which
protoplasm is composed.

CRYSTALLOIDS AND COLLOIDS 59

PRACTICAL WORK,

A. BEHAVIOUR OF CRYSTALLOIDS AND COLLOIDS WITH WATER.

(1) Place some crystals of potassium bichromate at the bottom
of a test tube and just cover with hot water. The crystals
quickly dissolve and disappear and there is no expansion on
solution. Examine a drop of the solution under the microscope—
it is clear, no particles are to be seen. This is a tvue (crystalloid)
solution. Leave the tube of solution and examine later: as the
solution cools and the solvent evaporates, crystals of the salt
reappear.

(2) Warm some powdered gluein a test tube with a little water :
the glue disappears gradually, taking up water and forming a
colloid sol. On cooling and standing a solid jelly (gel) is formed.

(3) Place a rectangular strip of dry gelatine on a sheet of glass
over squared paper. Mark the edges of the gelatine with Indian
Ink opposite the divisions of the paper. Allow the ink to dry,
then dip the gelatine in water and float it on the glass in a little
water. Note the gradual expansion of the gelatine as it absorbs
water. After a time measure the increase in area of the gelatine
strip by again placing the glass over squared paper. Now
transfer the gelatine to a piece of fine muslin and hang it up to
dry. It contracts, and gradually decreases in area owing to loss
of water. The gelatine is a colloid gel.

(4) Put one drop of Indian \Ink in a test tube and dilute with
water till you can see through the mixture. Filter, and examine
a drop of the filtrate under the high power of the microscope.
It is full of fine particles. This is a coarse suspensoid sol. The
particles show Brownian movement. The particles do not settle
if the tube is left standing.

(5) Place a very little solid Congo Red in water. It apparently
dissolves, the solution looks clear and the microscope reveals
no particles. This is a fine suspensoid sol of which the disperse
particles are ultramicroscopic.

(6) Examine the demonstration in which the three liquids
(a) potassium bichromate solution, (b) Congo Red sol, (c) Indian
Ink sol are illuminated laterally by a strong beam (Tyndall’s
beam). (a) is clear, (6) and (c) cloudy; (6) looks more cloudy
when thus lighted than when looked at against the light.

B. DIFFERENCE IN THE RATES OF DIFFUSION OF CRYSTALLOIDS
AND COLLOIDs.

(7) The two test tubes provided contain at the bottom orange

and red jellies respectively. The orange jelly is impregnated

60 PHYSICAL CHARACTERS OF ORGANIC SUBSTANCES

with potassium bichromate, the red with Congo Red. Halt
fill the tubes with water and view against a sheet of white paper.
The potassium bichromate diffuses into the water quickly, the
Congo Red very slowly.

(8) Another test tube contains jelly, the bottom layer of which
contains sodium carbonate, while the top layer does not. Poura
little hydrochloric acid on the top of the jelly, it diffuses down
through the substance. When the layer containing sodium
carbonate is reached by the hydrochloric acid a reaction takes
Place, bubbles of carbon dioxide appearing in the jelly. [Note.—
The jelly must be made up immediately before the class, or the
sodium carbonate will diffuse up into the top layer of jelly, and
the reaction will take place immediately the hydrochloric acid
enters the jelly.]

C. COAGULATION OF PROTEIN COLLOID By Hear.

(9) Place small portions of fresh white of egg in two test tubes.
Add a little water to one: the water is absorbed by the colloid.
Slowly heat the other: the white of egg sets to a solid. Cool
and add a little water. The coagulated protein no longer absorbs
water, having lost its colloid structure.

D. ForRMATION OF CoLLOID MEMBRANE.

(10) Carefully drop a crystal of copper acetate into a test
tube containing a 2 per cent. solution of potassium ferrocyanide.
A double decomposition occurs as the two salts meet and the
copper ferrocyanide is precipitated to form a colloidal membrane
around the crystal. This membrane enlarges by breaking and
patching so that it seems to grow.

E. SWELLING OF VEGETABLE COLLOIDS IN WATER.

(rr) Cover with water on a sheet of glass the rectangular piece
of seaweed [Chondrus is very suitable] provided, being careful
to wet both sides. Note the expansion as with the sheet of
gelatin in (3).

(12) Add water to a dried section of seaweed under the micro-
scope and note the expansion that takes place.

(13) Compare the size and shape of dry peas with those of
peas that have been soaked in water for 24 hours. Place six
soaked and six unsoaked peas in rows touching one another:
measure the aggregate diameter of the six in each case and
determine the average absolute and percentage increase in the

soaked peas.

CHAPTER IV

PROTOPLASM AND THE AMGBA.
PROTOCOCCUS

Constitution and Structure of Protoplasm.—The vis-
cous but semi-liquid mobile protoplasm that forms
the bulk of many animal and plant cells is to be regarded
as a mixed colloid sol. The continuous phase is water
in which various organic and inorganic substances
are dissolved. These solutes may vary to a consider-
able extent according to circumstances. The disperse
phase we believe to consist essentially of ultramicro-
scopic particles of protein, which may possibly include
separate protein molecules, as well as aggregations of
molecules. Fats or fat-like bodies may also exist as
part of the disperse phase, and these may be closely
associated with the protein droplets. In this sol
numerous chemical and physical processes are con-
stantly taking place, and it is the sum of these that
give rise to the chemical phenomena of life.

Besides the ultramicroscopic disperse particles, which,
together with the continuous watery phase, form, so
far as we can tell, the ultimate structural basis of living
protoplasm as a form of matter, the protoplasm ordinarily
contains much larger microscopically visible granules
and droplets of very various size and very various com-
position. The presence of these last, like that of the
solutes, depends upon the state of the cell in regard to

nutrition, etc. The sum total of the chemical processes
6x

62 PROTOPLASM AND THE AM@BA. PROTOCOCCUS

going on in the living protoplasm is called metabolism,
and these larger granules and droplets are metabolites,
i.e. products of metabolism. They must be carefully
distinguished from the ultramicroscopic disperse particles
which we cannot see, but whose existence we confidently
infer from different lines of evidence, and which form
the permanent structure of protoplasm.

In the clear protoplasm of the pseudopodium of the
amoeba (see Fig. 1), in which there are no coarse granules,
very numerous fine particles can be seen in Brownian
movement. This movement is suddenly brought to
an end by electrical stimulation. When the stimula-
tion ceases the Brownian movement recommences. The
mobile sol has been momentarily converted into a
reversible gel. When protoplasm is killed by certain
‘poisons ’’ (what are called in biology “‘ fixing reagents,”
such as acetic acid, chromic acid or dilute iodine solu-
tion, etc.) the sol is converted into an irreversible gel.
If protoplasm is heated to 60°C., however, chemical
changes occur in the proteins, and the whole. colloid
structure is destroyed. The proteins are then said to
be “‘ coagulated.”

The protein particles forming the disperse phase of
the protoplasmic sol very readily run together to form
a membrane (gel) in any plane on the two sides of which
they are subjected to different physico-chemical action.
This occurs for instance on the outer surface of a
detached unit of protoplasm such as a cell, and this is
accordingly covered with a semi-permeable protoplasmic
membrane, the ectoplasm, which is relatively fixed and
sharply distinguished from the rest of the cell proto-
plasm (endoplasm). Similar gel membranes are formed
round the vacuoles (segregated drops of liquid enclosed

! Greek pevafody, a changing over.

CHANGES IN AGGREGATION OF PROTOPLASM 63

in the endoplasm), and also round the cell xucleus.
It will be noted that all these phenomena of the living
cell are strictly in accordance with what we learned of
the behaviour of non-living colloids in the last chapter.

The special characters of protoplasm are largely
dependent on the fact that it has the power of readily
passing from the sol to the gel condition and back again.
Normally it may be said to exist somewhere near the
margin of the sol and the gel conditions. About 0° C. it
gels and becomes inactive, though as we shall see in
later chapters it is not “ killed,” i.e. its essential struc-
ture is not destroyed, at far lower temperatures. As
the temperature rises it becomes more labile and more
active, till above 40° C. changes begin to occur which
eventually destroy its structure altogether. Many other
causes besides changes of temperature and the loss or
absorption of water send the protoplasm from the sol to
the gel condition and back again, and it is on the changes
in aggregation of the protein molecules and groups of
molecules, the association and dissociation of these with
one another, and with molecules of fats and salts, that
what we call the vital activities of the protoplasm
depend. The more aggregation occurs, the more gel-
like and relatively stable the structure of the protoplasm
becomes, the less free the motion of particles and
molecules from one part of the living cell to another.
Consequently different chemical and physical processes
can go on in different parts of the cell (as is often
the case) without interfering with one another. Dis-
aggregation leads to the opposite effect. Many of the
more specialised animal cells have their protoplasm
more or less permanently in the gel condition, and the
specialised character of their activities depends largely
on this fact. But on the whole the life activities

64 PROTOPLASM AND THE AMCEBA, PROTOCOCCUS

depend essentially on the maintenance of the proto-
plasm within a comparatively narrow range of mole-
cular aggregation, and on its power of passing backwards
and forwards within this range.

Since the watery continuous phase of protoplasm is
an essential part of its structure, the equilibrium of
the protoplasmic water must be maintained or the
structure of protoplasm would be destroyed. In
other words, the forces tending to drive water out of
the cell and those tending to draw water into the cell
must be balanced. This condition of balance is main-
tained by primitive organisms living in water and
consisting of naked protoplasm, when they are sur-
rounded by water. If such an organism is transferred
to air, evaporation from the surface into the air at once
destroys the water balance, the body loses water and
the organism dies. This is why terrestrial organisms
which live surrounded by air must have some means
of checking the loss of water by evaporation and of
renewing it as it is lost. We shall see in later chapters
that the history of evolution of land plants is very
largely a history of the appearance and development
of structures which have these results.

The Amceba.—The most suitable organism for pre-
liminary study is the simple minute animal called the
amoeba. There are various species which live in fresh-
water pools and slow rivers, where they creep about
upon the surface of the mud or upon the water plants.

Each individual consists of a minute irregular mass
of naked protoplasm varying in different species from
about 100 » to about 250 w in diameter.t The surface
layer of the body (ectoplasm) is clear under the micro-

t Le. from about 75 to 4 of a millimeter, or about 45 to zdo of
an inch.

AMGBA 65

scope, the rest of the protoplasm is filled with granules
visible under the microscope and very variable in size
and nature. In the ectoplasm, but bulging into the
endoplasm, is situated a spherical space containing
liquid, and called the contractile vacuole, because it can

Fic. 1.—Ameba proteus. Two successive views of the same individual,
The arrows show the direction of movement of the pseudopodia,
which in A are adhering to the glassslide. ps., pseudopodium ;
ect., ectoplasm (clear); end., endoplasm (granular) ; », nucleus;
c.v., contractile vacuole; d, diatoms (unicellular plants), which
have been taken into the endoplasm as food. The larger granules
are the undigested remains of food. After Cash. x about 320
(the organism is just visible to the naked eye).

be observed to contract till it disappears completely

or almost completely, and then quickly appears again

in the same spot. The contractile vacuole is generally

regarded as excretory in function, i.e. as forcing out of

the body, whenit contracts, waste products of metabolism

including urea (a waste product containing nitrogen),
5

66 PROTOPLASM AND THE AMCEBA. PROTOCOCCUS

and perhaps also carbon dioxide. Thus its function is
equivalent to that of the kidneys and bladder of the
higher animals, and partly perhaps to that of the
lungs.

In the endoplasm is a rounded body called the
nucleus, which, as we shall see presently, is an all
important part of the protoplasm. The rest of the
protoplasm (ectoplasm and endoplasm together) is
called cytoplasm. Nucleus and cytoplasm together form
the structural unit of living organisation in the vast
majority of plants and animals. Such a unit is called
a cell, and the amceba is commonly called a unicellular
animal because it consists of one such unit, in contrast
with multicellular animals and plants which consist of
many, in the largest forms of many millions.

The nucleus is covered with a nuclear membrane (gel
membrane) and contains a substance chromatin, which
can usually be seen under the microscope in the form
of granules. The chromatin: is distinguished by its
power of taking up and becoming coloured by dyes
(stains), particularly certain stains (‘‘ nuclear stains ’’),
with great readiness. This probably takes place by
the adsorption of the particles of the dye by the dis-
perse phase of the gel of which the chromatin con-
sists. Chemically the chromatin is distinguished by the
presence in it of complex proteins containing phosphorus.

The function of the nucleus in the life of the cell
can be inferred from its behaviour and from the be-
haviour of the cytoplasm in relation to it under various
conditions. Certain large Protozoa (unicellular animals)
can be cut into pieces under the microscope without
killing them. When this is done and one part of the
divided cell contains the nucleus while the others do

* Greek yp@pa, colour.

FUNCTION OF THE NUCLEUS 67

not, the part containing the nucleus regenerates, i.e.
it grows again into a complete cell. The parts not
containing the nucleus remain for a time sensitive and
motile, but they cannot assimilate food, and hence
they ultimately die. In living plant cells, when the
protoplasm is in a state of great activity and metabolism
is proceeding rapidly, the nucleus is large and con-
spicuous (Fig. 43, A, B, C), but when the cell is com-
paratively inactive the nucleus is usually small and
inconspicuous. Again, when local activity is going on
in a cell the nucleus commonly moves to the spot in
which work is being carried out. For instance, if a
plant cell is thickening its cell wall in one spot the
nucleus moves to the spot where the thickening is
going on and remains there while this activity is in
progress (Fig. 43, E). It appears to be “ superintending
operations.”

From facts like these we infer that the nucleus is
essential in directing the metabolic and formative cell
processes. Exactly what it does we do not know, but
very likely it sends out into the cytoplasm chemical
substances, perhaps enzymes, which are essential in
carrying out the work of the protoplasm.

The chromatin of the nucleus is certainly different
in every species of organism, and even in those smaller
groups of very uniform individuals within the species
which are called ‘“‘ pure strains.” The differences
probably depend on the existence in the chromatin of
slightly different proteins (cf. p. 43). The cytoplasm
of different species and strains also probably differs in
the same sort of way, but we have very good evidence
that the differences of the chromatin are the most
important. The peculiar formative powers of each
species, the powers that enable it to reproduce the

68 PROTOPLASM AND THE AMGBA. PROTOCOCCUS

exact specific form of body and function, are certainly
carried mainly, if not entirely, by the chromatin in
reproduction, in which the nuclei of the reproductive
cells play the leading part. Often indeed the nucleus
of one of the parent cells of the new organism is prac-
tically the only thing contributed by that parent
to the new organism, which may nevertheless exhibit
some of the characteristics of that parent in a perfectly
pure form.

Life Processes of Amcoeba.—The most obvious thing
the amceba does is to move. When the animal is active
its shape continually changes, the clear ectoplasm
thrusts out from the body and is followed by the stream-
ing granules of the endoplasm. The protoplasm of
the projection so formed (pseudopodium) glides along
the surface to which the amceba adheres, and may be
followed by the streaming forward of the rest of the
body, which thus gradually glides away from the
position it occupied at first. Thus the animal con-
stantly changes its shape: and at the same time its
position. Locomotion (movement from place to place)
follows as an immediate result of the streaming move-
ment of the protoplasm in a constant direction. The
protrusion of pseudopodia is not, however, necessarily
followed by locomotion: several pseudopodia may be
put out at different spots on the surface of the body
and again withdrawn, so that no continuous streaming
in one direction occurs. Different species of amceba
vary very considerably in the shape and dimensions
of their pseudopodia.

The streaming movement of the pseudopodia is also
one method by which the amoeba feeds. When a
pseudopodium touches another living organism much

s The name ameeba is derived from Greek dyoufy, change.

INGESTION AND DEFACATION 69

smaller than the amceba, for instance a unicellular
green plant (alga) or a colony of bacteria, the proto-
plasm flows round it or draws it into the body, so as
completely to enclose the prey. With the prey a drop
of water may be ingested, forming a food vacuole. The
food is then acted upon by enzymes formed by the
amceba’s protoplasm, and the digestible parts digested,
i.e. broken down into soluble organic substances,
probably proteins, carbohydrates and fats, which can
be assimilated by the ameeba, i.e. built up again into
new “‘ amceba-protoplasm,”’ while the indigestible parts
are ejected from the body as feces. This process of
defecation is sometimes loosely called ‘‘ excretion,”
but it must be carefully distinguished from the excretion
of urea and carbon dioxide which are formed by the
chemical breaking down of the proteins and carbo-
hydrates of the body The ingestion of the prey when
the pseudopodium has once come into contact with it,
and also the ejection of faeces, can be explained by the
purely physical force of surface tension, for it has been
shown that a drop of chloroform (representing the
amoeba) placed in contact with a minute fibre of glass
covered with a varnish of shellac will ingest the fibre,
remove the coating, and then eject the fibre (see Fig. 2).
A drop of chloroform will also coil up a flexible fibre
of shellac within itself just as an amoeba will coil up
a flexible algal thread (Fig. 3).

By the formation of new protoplasm the amceba
naturally adds to its bulk and thus grows in size, unless
of course the breaking down processes (katabolism)
which result in excretion of the katabolites balance or
exceed the building up or assimilative processes.

Ameeba is also sensitive to external stimuli, i.e. its
motion is directed in relation to such stimuli. For

70 PROTOPLASM AND THE AMGBA. PROTOCOCCUS

instance, its protoplasm streams away from a source
of heat when the temperature rises above 35°C. At
o° C. and at 35°C., or in the absence of sufficient dis-

Fic. 2.—Spreading of a drop of chloroform over a shellac-covered
glass fibre. Contact with the shellac lowers the surface tension
of the drop of chloroform, which spreads over and draws the
fibre into itself, removing the shellac. When the glass fibre is
bare the chloroform recovers its surface tension, reassumes the
spherical form and thus expels the fibre, which it cannot ‘ wet.’”’
The arrows in a, ¢ and d@ show direction of movement of the
fibre; in 6 the movement of the chloroform; S, shellac in the
chloroform. After Rhumbler.

SENSITIVENESS OF AMCEBA 71

solved oxygen in the water, the pseudopodia are with-
drawn and the animal becomes spherical and motion-
less. Ameeba is also sensitive to the presence of food,
for it does not normally ingest objects which it cannot
digest, and this probably means that chemical substances

Fic. 3.—A, ingestion and coiling up of a thread of Oscillavia (an
alga) by Ameba verrucosa. B, ingestion and coiling up of a
thread of shellac by a drop of chloroform. After Rhumbler.

diffusing out from the organisms which form its prey
come into contact with the ectoplasm of the amceba,
and this stimulus leads to the envelopment of the
prey.

Reproduction of Amceba.—When the growth of the
individual amceba has reached a certain limit the animal

72, PROTOPLASM AND THE AMG@BA. PROTOCOCCUS

divides into two. The nucleus first divides into two
equal parts, and then a furrow appears in the cytoplasm
and rapidly deepens till the two halves become entirely
separate. This simple method of multiplication is
called binary fission. Each of the halves is a complete
amoeba in every respect, and each proceeds to lead its
own independent life and ultimately grows to the size
of the parent amoeba, when it divides again in the
same way. [The ameeba has other modes of reproduction
which will not be dealt with here.]

Protococcus.—On the bark of trees, on old palings,

Fic, 4.—Cells of Protococcus viridis. Xx about.800. Each is bounded
by a colourless cell wall (c.w.). The shaded portion is the green
chloroplast which frequently occupies most of the space within
the wall (band c); pyv., pyrenoid. Inaandd the cells have recently
divided and are remaining together.

etc., especially on the north side, which does not en-
counter the direct rays of the sun and thus tends to
remain damp, one often finds the surface covered with
a thin green crust, which crumbles in dry weather
under the point of a knife. If a little of this green
crust is scraped off into a drop of water on a slide and
examined first with the low and then with the high
power, it is seen to consist of numerous very small
green cells, which often hang together in groups of
‘two, four, or more. This is a minute unicellular alga

STRUCTURE AND DIVISION OF PROTOCOCCUS 73

called Protococcus vulgaris, which inhabits the surface
of the damp tree bark, and feeds by absorbing rain
water that falls on the tree and trickles down over
the surface. This rain water dissolves carbon dioxide
from the air and small quantities of mineral salts, and
thus it contains all the elements necessary for the
food of the green alga.

Under the high power of the microscope (Fig. 4),
each cell of Protococcus is seen to be bounded by a
colourless cell wall of firm consistency. Inside this is
the essential living part of the organism—the protoplasm
of the cell. With the low power this appears uniformly
green, but under the high power it can be observed
that the green protoplasm does not fill the whole of
the space within the cell wall, but takes the form of
a curved plate (chloroplast) in which can often be
observed a central shining body (pyrenoid),: the rest
of the cell being occupied by colourless cytoplasm.
By appropriate staining the cell nucleus can also be
distinguished, but it is generally hidden by the massive
chloroplast.

Protococcus reproduces itself by cell division, the
nucleus dividing first and then the chloroplast; a
delicate wall is afterwards formed across the cell,
separating it into two halves (Fig. 4, a, left-hand cell).
The dividing cell wall thickens and the cells may
separate, but very frequently they remain together
‘in little groups. All stages of cell division and separa-
tion can frequently be seen.

If we compare Ameba and Protococcus as a repre-
sentative simple animal and a representative simple
green plant, we note first that they resemble one another

«A protein crystal-like body found in the chloroplasts of many
green alge and acting as a centre of starch formation.

74 PROTOPLASM AND THE AM@BA, PROTOCOCCUS

in both being isolated, independent, free-living organ-
isms, each consisting essentially of cytoplasm with
nucleus. But they show the following differences :—

(1) Ameba has no cell wall, while Protococcus has.

(2) Ameba moves from place to place, while Pro-
tococcus is stationary.

(3) Ameba feeds on living prey, while Protococcus
does not, having no means of ingesting solid bodies.

On the other hand

(4) Protococcus has part of its cell protoplasm coloured
green (chloroplast), which enables it to form organic
substance, and eventually protoplasm, from liquid and
gaseous inorganic substances which it can absorb
through its cell wall. Ama@ba has no such power
because it has no chloroplast.

PRACTICAL WORK.

A. AMCGBA AS A TYPE OF SIMPLE ANIMAL,

(1) Place a drop of water containing the scrapings of the
surface mud of a pond containing amcebe (together with other
organisms and organic débris) in the centre of a slide and cover
with a coverslip. Examine first with the low power, and pick
out the amcebe, which appear as irregular blobs of greyish or
brownish appearance. They will gradually creep out from the
débris along the surface of the slide or coverslip.

Examine a specimen with the high power and notice that it
is constantly changing its shape, sometimes slowly and sometimes
rapidly. Distinguish the clear ectoplasm from the granular
endoplasm, and note that the granules of the latter are very
various in size and shape. The endoplasm may include quite
large remains of ingested prey, e.g. the cell walls and disintegrated
contents of unicellular plants. Note alsothe transient projections
of the body (pseudopodia).

Identify the contvactile vacuole, and watch it contracting and
quickly reappearing at full size. Identify also the rounded,
granular, light-refracting nucleus. Measure the average diameter
of the amoeba and the diameter of the nucleus with the eye-piece
micrometer.

PRACTICAL WORK 75

(2) Draw an active specimen in successive stages of motion
to show the change of shape depending on the protrusion and
withdrawal of pseudopodia. See if you can observe an amceba
in process of ingesting another organism as food.

(3) Kill the amceba by running a drop of iodine or acetic methyl
green under the coverslip and note that the nucleus is stained more
deeply than the cytoplasm.

(4) Examine a demonstration specimen of amceba which has
been fixed, stained and permanently mounted, showing the
deeply stained nucleus.

B, Protococcus sas A TYPE OF SIMPLE PLANT.

(1) Scrape off a little of the green crust from the bark of a
tree into a drop of water and cover with a coverslip. Examine
with the low power and note the numerous green specks and
clumps in the field of the microscope. Put on the high power
and distinguish cell wall, chloroplast, and colourless cytoplasm.
The chloroplast generally contains a bright granule, the pyrenoid.
More than one chloroplast may exist in the cell. The nucleus
is usually difficult or impossible to distinguish. It lies in the
colourless cytoplasm, often in the concavity of the curved
chloroplast.

(2) Examine stages in ‘the division of the cells (only found
when the Protococcus is kept damp) (a) when the new cell
wall is not yet formed but the chloroplasts of the daughter cells
are distinct, and (6) after the cross wall is formed but the two
daughter cells are still clearly derived by division of the parent
cell. Add a drop of very dilute iodine solution and observe the
effect.

CHAPTER V

THE VITAL FUNCTIONS

THE activities of a living organism which together make
up what we call its “life”? may be conveniently con-
sidered under separate heads which are often called
“the vital functions.’’ These divisions are to some
extent artificial, for one process passes into another,
and others are the results of the interaction of more
than one. We must always remember that what we call
the different life processes in reality form a continuous
whole. They are all expressions of the activity of
protoplasm in maintaining its physico-chemical equi- ©
librium in relation to its surroundings, though these
expressions are strikingly different in different organisms.
The ‘‘ vital functions’’ are those expressions which
we can seize upon and give names to, and which are
of essentially the same nature in all living organisms,
whether animals or plants, simple or complex.

We must carefully distinguish between the activity
of the organism as a whole and the activity of the
protoplasm itself, though the former depends of course
directly upon the latter, since protoplasm is the only
living part of an organism. This distinction is least °
evident when the whole body of the organism, as in the
ameeba, consists solely of a naked mass of protoplasm.
In multicellular organisms, the relations of the whole
body to its surroundings are necessarily different from

the relations of the protoplasm of each living cell to
76

FEEDING 77

its surroundings. This we can illustrate in the case of
each of the “ vital functions.”

(1) Feeding.—In the case of amceba, what we call
the food of the animal is the living prey which it engulfs.
The food of the protoplasm, however, consists of the
products of digestion of the prey which can be directly
incorporated in the protoplasm, or can be broken down
to set free energy without such incorporation. Similarly,
in the case of a higher animal, the food of the animal
is the animal or plant, or the part or product of the
animal or plant, which the animal eats; the food of
the protoplasm of its living cells consists of the digested
products of the original food which have passed into
the blood and can be absorbed and incorporated into
their structure by the living cells, or broken down
into simpler substances so as to liberate energy.

In a green plant the food of the plant as a whole
consists of the water and inorganic salts absorbed by
the roots from the soil, and of the carbon dioxide
absorbed by the leaves from the air. The food of the
living protoplasm of the plant cells, on the other hand,
consists of the sugars and proteins built up from these
simple substances taken in by the plant. In the case
of a germinating seed the food is obtained directly from
reserve stores—starch or fats, which are converted into
sugar, and proteins which are converted into a soluble .
form—that are packed away in the seed. Here, it will
be noted, the foods of the young plant are of exactly
the same chemical nature as the foods of an animal,
and they can be used, and are in fact largely used, as
food by animals, as when we eat bread made from the
reserve stores of the wheat seed, or beans, peas or nuts,
which are also seeds with large stores of food.

Thus, the food of the organism as @ whole ts very

7 8 THE VITAL’ FUNCTIONS

different in different kinds of organism, though it must
always contain the necessary chemical elements, but
the food of protoplasm is always of the same nature.

Four main types of feeding are often distinguished :
holozoic, in which the organism ingests solid organic
food (characteristic of animals) ; holophytic, in which
the food of the organism consists wholly of inorganic
substances, including carbon dioxide (characteristic of
green plants); saprophytic, in which organic food is
obtained from the dead body or from the products of
some other organism in a liquid form; and parasitic,
in which organic food is obtained direct from the living
body of some other organism, but this “ host ’’ organism
is not immediately killed. The mode of nutrition of
saprophytic and parasitic organisms is not essentially
different. It is of no essential importance whether
the liquid organic substances, provided they are suitable
as food, come from a living or from a dead body or
product ; and many species of fungi, for instance, may
be both saprophytic and parasitic (see Chapter XI),
though in many cases species are closely adapted to
one or the other mode of life, Animal parasites may
have mouths, and suck or devour organic liquids or
tissues of the living body of the host, or they may,
like the tapeworm, have no mouths, and absorb liquid
organic food through their body wall.

(2) Assimilation..—This word literally means ‘‘ making
like,’ and in its strict sense is applied to the process
of incorporation of the fina] foodstuffs in the specific
protoplasm of the organism. It is, in fact, the actual
feeding of the protoplasm. We know practically
nothing of how this is done The incorporation of the
carbon derived from the carbon dioxide of the air in

« Latin similis, like.

ANABOLISM AND KATABOLISM—GROWTH 79

the molecule of sugar, the first stage in the formation
of the protein molecule that goes to form the structure
of protoplasm, is often called ‘‘ carbon assimilation.”

Anabolism and Katabolism—All the constructive
chemical processes which take place in the protoplasm
are classed together as anabolic * processes or anabolism,
as opposed to the destructive or katabolic processes
(katabolism). Thus the formation of sugar from carbon
dioxide and water in a green plant is an anabolic process,
the breaking down of sugar, and, in the animal body,
the breaking down of proteins into urea and uric acid,
are katabolic processes. Assimilation in the strict
sense, i.e. the incorporation of foodstuff in the proto-
plasm, is the ultimate anabolic process. ~

(3) Growth.—In its simplest form growth follows
directly from assimilation, from the incorporation of
new material which increases the mass of the protoplasm.
Unless this new protoplasm is destroyed as quickly
as or more quickly than it is formed, increase in bulk
must result. In amoeba the animal simply increases in
size till it reaches a certain limit, when division takes
place, and the two daughter amcebe go on feeding and
growing independently. In more complex organisms
of definite shape growth takes very various forms in
different parts of the body, but it always leads to
permanent increase in the bulk of the organism—a.
temporary inflation of part of the body, for example,
is not a process of growth.

The higher plants differ widely from the higher
animals, in showing localised growth, in most cases
at the ends of the branches of the shoot and of the
root, leading to increase in length of the branches,
and this may be more or less continuous throughout

* Greek dvd, up, 2 Greek xatd, down.

‘

80 THE VITAL FUNCTIONS

the life of the plant. In addition to this increase in
length of the branches, definite layers of cells inside
the plant may also grow, increasing the thickness of
the stem and root, as happens in the case of trees.

Most animals have a-definite growth phase in youth,
which involves the whole of the body, but is different
in nature, amount and duration in the different organs.
Thus a child’s. trunk and limbs grow more, from birth
to adult life, than its head, and similar differences are
shown in the growth of the different internal organs.

Both in animals and plants it is clear that the growth
of an individual living cell is a different thing from the
growth of the organism as a whole, though the latter
depends upon the growth and division of the cells.

(4) Differentiation.—A multicellular organism arises,
during its individual lifetime, from a single cell. This
cell divides, and the daughter cells, instead of separating
from one another, as they do after cell division in
amoeba, remain together, and again divide, the new
daughter cells behaving in the same way. In this
manner the adult multicellular body is produced. But
the cells of this body are not all alike ; sets of them are
grouped together into tzsswes and organs, and the cells
of one tissue differ widely from those of another in |
form, structure and function. The process of becoming
different is called differentiation, and accompanies the
growth of the bodies of all multicellular organisms.
Thus growth depends upon the two processes, cell
division and cell differentiation.

The protoplasm of a single cell may be differentiated.
Thus in the Protococcus cell the chloroplast is differentiated,
from the colourless cytoplasm, whereas in amcba
there is no such differentiation. In some unicellular
animals the differentiation reaches a high pitch, the
structure of the single cell being very complicated.

RESPIRATION 81

But such intracellular differentiation! has limits, because
a cell is unworkable if it exceeds a certain size, and the
elaborate differentiation of the higher organisms depends
upon their multicellular structure.

(5) Respiration.—The functions hitherto considered
are all concerned with building up the protoplasm of
the organism. We have now to consider processes con-
nected with the expenditure of energy by the organism
and with the breaking down of organic substances.

When the amceba moves it does work and spends
energy in doing it. This energy is derived from the
energy locked up (potential energy) in the molecules of
organic substance taken in as food—largely in, the
carbohydrates. The molecules of these substances are
broken down into simpler substances and largely
oxidised in the process, the energy being set free and
appearing in the kinetic form, i.e. as mass motion—
for instance, the streaming of the amceba cytoplasm—
and heat. Free oxygen is necessary to carry out this
process, and that is why free oxygen is required by
very nearly all living beings. Aquatic organisms get
their oxygen from that which is dissolved in the water
they live in, terrestrial forms direct from the air. In
its absence the activity of the organism comes to an end.

The release of energy is obtained almost entirely by
the oxidation of sugar (glucose) with the formation of
carbon dioxide and water.

The generalised equation is :—

C,H,,0, + 60, = 6CO, + 6H,O

glucose oxygen carbon water
dioxide
potential energy — > kinetic energy

(mass motion and heat)

This process is what is called in biology respiration.

1 Differentiation within the cell, from Latin éxtva, within,

82 THE VITAL FUNCTIONS

The same chemical process (oxidation) takes place
when we burn sugar in the air, the same products,
carbon dioxide and: water, and kinetic energy in the
form of heat, resulting. When we burn coal or petrol
in an engine a similar process takes place. Here the
kinetic energy is partly used to drive a machine, for
instance a steam locomotive or a motor car, and this
mass motion is comparable with the mass motion of
protoplasm. In both cases some of the energy appears
as heat, raising the temperature of the cell or of the
engine, as the case may be.

The living protoplasm of all plants requires free
oxygen for respiration just like that of animals, though
plants do not use it so quickly as active animals do.
In green plants in light respiration is masked, because,
as we shall see in a later chapter, the opposite process—
the formation of sugar from carbon dioxide and water,
with production of free oxygen—is carried out at a
greater rate than respiration. In plants (and in parts
of plants) that are not green and are growing quickly,
ie. expending much energy, for example in germinating
seeds and opening flower buds, a great quantity of
oxygen is used, and has to be taken from the air, and
a great deal of carbon dioxide and heat are produced.

“Breathing’’ in the higher animals, also called
“respiration,” ! consists of the rhythmically alternating
processes of “‘inspiration’? and “‘expiration.”” The
former is the taking of air into the lungs so that oxygen
may be absorbed through the walls of the lung tissue
by the hemoglobin of the red blood corpuscles, and
carried by them in the blood stream to all the tissues
of all the organs of the body, where respiration in the

t This of course is the original meaning of the term (from Latin
spiro, breathe).

RESPIRATION, KATABOLISM AND EXCRETION 83

general biological sense takes place in the living cells.
The return blood stream comes back to the lungs laden
with carbon dioxide, the result of the tissue respiration,
and this gas, diffusing into the air passages, passes out
of the body in the “expired” air. Here again we
get the contrast between the function of the organism
as a whole, in this case carried out by special organs—
the lungs—and the function of the individual cells of
which it is composed. It is especially in the muscles
that sugar is most actively broken down in the bodies
of the higher animals, and accordingly it is here that
the greatest amount of kinetic energy is set free, as we
see in the vigorous contractions of our muscles.

Plants have no “ organs of respiration ’’ comparable
with the lungs of the higher vertebrate animals. The
oxygen used in respiration by the living cells of plants
diffuses into the plant from the air through the system
of air spaces between the cells (intercellular spaces),
and so through the water saturating the wet cell walls
into the protoplasm. The oxygen actually used by the
living cells in respiration, both in animals and plants,
is always oxygen dissolved in liquid, though it is
ultimately derived from the air.

(6) Katabolism and (7) Excretion.—Respiration is
essentially a katabolic process, involving the breaking
down of an organic substance (sugar) in the living cell.
But animals have, in addition, a nitrogenous katabolism
involving the breaking down of proteins. This process
goes on largely, though not wholly, in the liver, and the
comparatively simple nitrogenous substances formed in
the liver pass into the blood and are excreted through
the kidneys as urea (CH,N,O) and uric acid (C;H,N,O,),
and pass out of the body in the urine. In the amceba
these nitrogenous excretions are probably expelled

”

84 THE VITAL FUNCTIONS

through the contractile vacuole. It will be noted
that these substances are not nearly so fully oxidised
as CO,, i.e. oxygen forms a much smaller proportion of
the molecule, and the animal derives a_ negligible
quantity of its energy from the breaking down of
proteins. Plants have no comparable nitrogenous kata-
bolism. If katabolites are formed from the breaking
down of proteins they are not systematically excreted,
but are probably often used again for the formation of
proteins. Plants are, in fact, much more economical of
their nitrogen than are animals. Various substances,
some of them containing nitrogen, may be cast off
by plants, as in the bark of trees, while carbon dioxide
regularly diffuses out of the tissues that are not green,
and from all tissues in darkness, but there is in plants
no specialised excretory system.

(8) Movement.—This is: the most obvious of all the
expressions of life. We can again distinguish between
the movement of the pxotoplasm of living cells such as
we see in the streaming of the protoplasm in amceba
and the movement of paris (for instance the limbs,
involving whole organs and systems of tissues in the
higher animals), or the movement of the whole organism
from place to place (locomotion). In amceba we can
observe under the microscope how the first leads directly
to the second (formation of pseudopodia), and this
again to the third. In a higher animal the same con-
nexion occurs, but the chain of causation is much longer.
The movements of parts, for instance the limbs, depend
on the contraction of the living substance of highly
specialised tissues, the muscles; through the definite
relationship of these with the bones the movements
of several muscles, co-ordinated through the agency of
the central nervous system, leads to the movement of

MOVEMENT OF PROTOPLASM 85

a limb; while the co-ordinated movements of the
limbs may result in the movement of the body from
place to place.

In the cells of plants the rapid streaming movement
of the cytoplasm, which in certain cases may be observed
under the microscope to stream round the cell, is perhaps
always the result of an injury or other shock: there is
no evidence that it occurs normally in the untouched
plant. Nevertheless it provides conspicuous proof of
the power of plant protoplasm to move comparatively
rapidly. There is also plenty of evidence of the power
of rapid movement of plant cells under normal conditions,
e.g. in the motility of many of the lower unicellular
plants and of the reproductive cells in many fixed
plants, though these move by means of the contraction
of special delicate cytoplasmic processes projecting from
the cell body (cilia and flagella) whose rapid beating
pulls or pushes the organism through the surrounding
liquid. Many unicellular animals also move in this way.

The commonest form of movement of the protoplasm
of plant cells is, however, a slow streaming movement,
usually too slow to observe directly, but evidenced by
the frequent changes in place of the constituents of the
living cell body, such as the nucleus, and in green cells,
the chloroplasts. The growth of a plant clearly involves
the result of a multitude of slow movements, and all
of these, like the much quicker movements of animals,
depend upon a constant supply of energy which is
derived from the release of potential energy by the
breaking down of organic molecules through oxidation.
A moment’s reflection will convince us of the large
amount of energy involved in the building up and
raising of the branches of a tree, eventually weighing
perhaps many tons, high into the air against the force

86 THE VITAL FUNCTIONS

of gravity, and in the burrowing of the roots far into
the substance of resistant soil.

(9) Response to Stimuli.— All living protoplasm responds
to various stimuli in various ways, or in other words it
is. sensitive to the forces acting upon it from its sur-
roundings, This response depends upon the setting
up of processes within the protoplasm which result in
some change im the activity of the latter. The most
conspicuous expression of response to stimulus is a
movement. It is necessary to be clear as to the differ-
ence between this kind of movement and the merely
passive movement which results from the application
of an external force. For instance, if one man pushes
another over the edge of a cliff, the falling of the victim
is a passive movement, resulting from the force of the
push and the force of gravity. But if the intended
victim, after receiving the push, saves himself by a
sudden leap to one side, that movement is a response
to the stimulus of the push. It involves a change in
the activity of his nerve and muscle cells. This is a
very complicated response in a highly complex organ-
ism, though it is not nearly so complicated as the
responses we carry out every minute in the course of
our daily lives.

The very simplest organisms, however, exhibit re-
sponses to various stimuli, and these are specific, i.e.
they vary quite definitely according to the stimulus.
It is probable that they can all be explained as con-
sisting of a series of purely physical and chemical pro-
cesses within the protoplasm, set in motion by the
stimulus ; as for instance in the case of the ingestion
of food by the protoplasm of the amceba, which, as we
saw, is closely paralleled by the ingestion and ejection
of a glass splinter coated with shellac by a drop of

TROPISMS. REPRODUCTION 87

chloroform. But in most cases we do not as yet know
nearly enough of the chemistry and physics of the living
cell to furnish a detailed explanation of the causes and
course of the particular response in every case.

When the whole organism moves in response to an
external stimulus, the motion is called a faxis.1 When,
as in a fixed plant, an organ bends in response to a
stimulus, the motion is called a tvopism.2 For instance,
motile unicellular green plants (e.g. Chlamydomonas,
Chapter XII) are Positively phototactic3 in light of
weak or medium intensity coming from one side
i.e. they move towards the source of such light. Motile
unicellular organisms in general are chemotactic, either
positively or negatively, i.e. they move towards or away
from the source of various chemical substances diffusing
from a given source, according to the nature of the
substance. The tips of the branches of the stem and
root of a higher plant are phototropic, geotropic4 and
hydrotropic5 in different cases, bending towards or away
from a source of light, the direction in which gravity
is acting, or a source of moisture. Thus the tips of
roots are positively geotropic and hydrotropic, but
negatively phototropic, and so on. Of the detailed
effects of other stimuli, e.g. electrical stimuli, on the
protoplasm of cells we do not know enough to speak
confidently, though we know that such effects exist.

(x0) Reproduction may be defined as the production
of a new individual or new individuals from pre-existing
ones. It has been usefully defined as discontinuous
growth, since the formation of a new individual always
involves processes of growth, and this growth is
discontinuous, for it begins in the parent organism

t Greek rdéic, a disposition. * Greek tpdzoc, a turn.
3 Greek dc, dutdc, light. 4 yj, the earth. §% ddwp, water.

88 THE VITAL FUNCTIONS

and is carried on in the separate new organism, or
organisms, which is, or are, the offspring.

The simplest form of reproduction is binary fission
such as we saw in the ameeba, the division of the body
of the parent into the bodies of the offspring. We
do not completely understand the forces which set this
process at work. All we can say is that it seems to
be ultimately determined by a size limit corresponding
with a limit of the surface/bulk ratio, which decreases
as the organism grows in size. We know that the
surface of a sphere increases as the square of the radius,
while its volume increases as the cube of the radius.
Thus a unicellular organism, as it grows in size, will
have a smaller and smaller surface in proportion to
its volume, and this will have an important effect on
the processes going on within the protoplasm. The
supply of oxygen, for instance, which has to pass through
the surface from the surrounding water will be pro-
gressively less for each unit volume of protoplasm
the larger the organism becomes. It is probable that
some effect of this nature initiates the process of division.

A large number of unicellular organisms are reproduced
by binary fission or by some simple modification of it.
But multicellular organisms produce special reproduc-
tive cells which are separated from the bodies of the
parents and produce new organisms (offspring) by
growth and differentiation, which may begin and be
carried on for some distance {in the highest organisms
to the greatest extent) before complete separation
occurs. In Chapter XII we shall consider the relation
of these special reproductive cells to the individual
unicellular organism which reproduces itself by binary .
fission.

Reproduction may also take place, as in most plants

PRACTICAL WORK 89

and in some of the lower multicellular animals, by the
separation of parts of the body which are not special
reproductive cells, e.g. by the separation of some
segments of the body in worms or by the rooting of
cuttings and “‘layers”’ of plants. This formation of
new individuals from non-specialised or only slightly
specialised parts of the ordinary body of the organism
is called vegetative reproduction. It does not occur in
the highest animals.

PRACTICAL WORK.

A. RESPIRATION AND PRODUCTION OF HzEaAT IN LIVING SEEDS
AND FLOWER BuDs.

(1) The test tube provided contains living barley grains
(seeds) which have been wetted and placed in the test tube,
which was corked on the previous day. They absorb water and
swell as a preliminary to sprouting. They also absorb oxygen
and give off carbon dioxide in considerable quantity as a result
-of the rapid respiration of their living cells, which resume active
life as a result of absorption of water and oxygen. Test for
the carbon dioxide by dipping the rod into lime water, carefully
uncorking the test tube and inserting the tip of the rod close to
the grains without touching them. After thirty seconds or so look
at the drop of lime water on the rod against a dark background.

(2) Examine the demonstration consisting of a U-tube con-
taining coloured water and connecting with two corked flasks,
one containing wetted living and the other wetted dead barley
grains. Fixed to the cork inside each flask is a lump of caustic
potash which readily absorbs carbon dioxide. In the flask con-
taining living grains oxygen has been absorbed by the grains
while the corresponding carbon dioxide produced is absorbed
by the caustic potash. In the flask containing dead grains no
such change has occurred. Hence the total amount of free
gas in the two flasks changes and the pressure on the surface
of the liquid in the two limbs of the U-tube becomes unequal.

(3) Examine the demonstration showing that the temperature
rises in the middle of a mass of wetted living barley grains and
not in the corresponding mass of dead ones.

(4) Examine the demonstration showing the rise of tempera-

go THE VITAL FUNCTIONS

ture of chrysanthemum (or other suitable) buds in a vacuum
(thermos) flask. Here the evolution of heat is “ cumulative ”’
for a time because the heat cannot escape, and the rising
temperature accelerates the process of respiration. |

B. LocaLisATION OF GROWTH IN PLANTS. RESPONSE TO
STIMULUS.

(1) Take two beans which have germinated and produced
roots from one to two inches long. If the surface of the root is
wet, dry it carefully with a piece of torn filter paper, taking great
cave not to break ov injure the voot. With a carefully sharpened
stick (or lead pencil) dipped in Indian ink make marks on the
root at intervals of one-tenth of an inch from the top to the seed.
Now push a large pin carefully through each bean, and fasten
to the under side of the cork of a wide-mouthed bottle, so that
when the cork is replaced the root of one points vertically down-
wards while that of the other is horizontal. Pour a little water
into the bottom of the bottle, replace the cork with the beans
pinned to it and turn the bottle upside down for a few seconds,
so as to wet the beans and provide water for their growth. Leave
the bottles till next time.

(2) Examine the demonstration showing the aggregation of
the green motile unicellular plants (Euglena) on the glass side of
a jar. The Euglenz were scattered through the water in the
jar, which was covered with black paper. The only light reaching
the Euglenze came through the stencil holes in the paper, and
they moved towards it, eventually adhering to the glass under
the openings.

C. ORGANIC FOODSTUFFS OF PLANTS AND SOME OF THEIR
CoLouR REACTIONS.

Representatives of the three great classes of organic food-
stuffis—proteins, carbohydrates and fats—are stored in seeds and
other storage organs such as tubers, and are used to feed the
young plant arising from the seed or tuber before it is able to
feed itself. These foodstuffs can be identified under the micro-
scope, partly by their appearance, partly by the use of different
stains and reagents :—

(1) With watery solution of iodine in potassium todide
(commonly called ‘‘ iodine solution ”’) grains of proteins
turn yellow or yellow brown, stavch blue, cellulose
and fats remain uncoloured.

PRACTICAL WORK gr

(2) With solution of zinc chloride in potassium iodide
(‘‘ Schulze’s solution ‘’) proteins turn yellow as before,
starch blue, cellulose blue or purple, fats remain
uncoloured.

(3) With Sudan 3 (an anilin dye) proteins, starch and fats
remain uncoloured, fats turn orange-red.

Stain an example of each of the three sections of different
seeds : (a) bean, (6) castor oil plant (Ricinus), (c) lupin, in watch-
glasses containing the reagents (1), (2), (3) above. Wash in
water and mount in dilute glycerine. Draw a single suitable
cell of each section under the high power, and determine the
classes of food substance present in each.

CHAPTER VI

THE CELL

WE have now become acquainted with two simple
living organisms, Ame@eba and Protococcus, with their
structure, behaviour and life histories; and we have
seen that the “ vital functions ’’ which they exhibit are
expressions of the activity of the protoplasm of which
their bodies are composed, and that these same “ vital
functions ” are exhibited by all living organisms, however
large and complex, however different in structure and
appearance—by trees and human beings, just as by
Protococcus and Ameba. But we have also seen that
the exact ways in which the various functions are
performed by the organism as a whole varies with its
structure, and before we can understand these variations
in any detail we must become acquainted with the
organisation and variations of the unit of which organic
structure is built up—the ceil.

Unicellular and Multicellular Organisms. Differen-
tiation and Division of Labour. Tissues.—We have
already seen that the whole body of an ameceba is a
unit of living protoplasm consisting of cytoplasm and
nucleus: the same is true of Protococcus, which has,
however, a special green protoplasmic body called the
chloroplast within the cell which enables the organism
to make carbohydrate out of carbon dioxide and water,
and also a non-living cell wall of cellulose (a complex
carbohydrate) which is formed by the protoplasm.

92

DIFFERENTIATION 93

These organisms are called unicellular, because they
consist of one such unit, in contrast with the great
majority of plants and animals which are composed
of many such units, each with its own cytoplasm and
nucleus—which are in fact multicellular.

The unicellular organism may show a considerable
differentiation of parts within the cell. The primary
differentiation into nucleus and cytoplasm is found in
all (except, perhaps, the bacteria, see Chapter IX),
but in Pvotococcus we find the differentiation, charac-
teristic of all green plant cells, of the cytoplasm into
the chloroplast and colourless cytoplasm. Many of
the higher Protozoa have quite complicated cells. For
instance the cell may have a firm cortical cytoplasm
(gel structure) which maintains the shape of the body,
a definite opening (mouth and gullet) through this leading
into the central more motile cytoplasm (sol), as well
aS numerous exceedingly fine threads of cytoplasm
(cilia) which project from the surface of the body, and
by their rapid beating pull the animal quickly through
the water. Thus we have different parts of the cell,
differentiated and performing different functions for the
whole, e.g. the mouth and gullet for feeding, the
cilia for motion, etc. Other kinds of Protozoa have
skeletons of non-living substance, enclosing the
protoplasm which projects through openings in the
skeleton.

Though this differentiation is often carried much
further in a unicellular organism like a Protozoon than
in the individual cells of a multicellular organism, the
differentiation between different cells which is possible
in a multicellular organism is much greater, because
there is no limit to the size of the organism. In the
simpler multicellular plants (e.g. Green Algz) there is

94 THE CELL

little or no differentiation between different cells, but
in the higher plants it is considerable.

A set of similar cells which perform one special
function or set of functions in the organism is called a
tissue.t Thus in the higher plants there are absorbing
tissues, conducting tissues, sugar-forming tissues, pro-
tective tissues, etc. Specialisation of different cells for
different functions reaches a far higher pitch in the
higher animals, where different tissues are associated
in the different organs, and work together to perform
the special function of the organ, e.g. lungs, heart,
liver, kidneys, etc.

However specialised the structure and work of a
cell may be, it must necessarily be able to carry out
the vital functions described in the last chapter, or it
would not be alive. It depends, however, upon its
position in the body and its relation to other cells and
tissues for the essentials of its continued existence—
it cannot lead a free independent life like the cell of
a Protozoon and obtain its food and oxygen directly
from outside the organism. Thus the muscle cells
depend for their food and oxygen on the circulation
of the blood which carries these netessary substances
to them. Each highly differentiated cell or tissue
specialises on one function of protoplasm, which repre-
sents its particular work for the organism as a whole.
Thus the fibres of the muscle tissue (Fig. 5, F) show
the power of contracting—a specialised form of move-
ment—in a very high degree, and by means of the
co-ordination of the contractions of many muscles the
organism moves. The cells of the glands attached to the
alimentary canal specialise in the production of enzymes
which they pour out into the cavity of the gut for

t From the analogy of its texture with that of a textile fabric.

SPECIALISATION OF CELLS 95

purposes of digestion of the food of the whole organism.
Some of the gland cells lining the air passages are
ciliated and others produce mucus (Fig. 5, B).

Fic. 5.—Cells from different tissues of a higher vertebrate animal.
A, blood corpuscles; 7, red corpuscles—these are not complete
cells and have no nuclei; w, white corpuscle, whose structure
and movements are very similar to those of amcba. B, cells
of the ciliated glandular epithelium lining the trachea (air tube
to lungs); ~, nucleus; m, mucus being expelled from the cell;
c, cilia. C, cells of cartilage recently divided, embedded in
cartilaginous matrix secreted by the cells; , nucleus. D, cells
and fibres of connective tissue (which forms the “ packing ”’ of the
organs of the body); ”, nucleus. E, fat-forming cells ; f, globules
of fat; », nucleus, F, Part of striped muscle fibre. Here the
protoplasm is highly modified into special contractile substance ;
n, nucleus: many nuclei in each fibre, which is composed of many
fused cells. The nuclei are situated just below the very thin
outer membrane. .

96 THE CELL

The white blood corpuscles, on the other hand, are
unspecialised cells, very much like amcebe (Fig. 5,
A, w). Highly specialised cells lose the power of
multiplying by division as soon as the tissue to which
they belong is fully differentiated. In general the
power of reproduction by division is the function of
an unspecialised cell.

The Cell Doctrine.— Every multicellular organism begins
life as a single cell, which is separated from the body
of the parent and by cell division, growth, and (in all
but the simplest forms) by differentiation of the products
of division gives rise to the body of the adult offspring.
In this process of cell division the daughter cells do not
separate from one another, as in the binary fission
of a unicellular organism, but remain together, and in
all the higher forms undergo differentiation into tissues
and organs to form the different. parts of the body.

This is the basis of the generalisation—known as the
cell doctrine—that organisms consist of cells, which are
the structural units of the organism, and that the
functions of the organism as a whole are made up of
the sum of the functions of all its cells. Broadly
speaking this is true, but certain qualifications have to
be made.

In the first place, in some of the lower forms the
body does not consist of distinct cells, though it contains
a great many nuclei. Thus in some alge and fungi
among the plants the body is composed of a branched
tube of cellulose (or similar substance) enclosing a
continuous mass of cytoplasm in which are scattered
numerous nuclei. As the plant feeds and grows the
protoplasm at the tips of the branches increases and
the nuclei embedded in it increase in number by division,
the wall covering the tip continually having new

THE CELL DOCTRINE 97

layers added to it by the protoplasm within, and being
pushed out as the tube grows in length. Here there
is no cell structure, and such plants are called non-
cellular. Also there are parts of animals, and phases
in the development of animals, where there is no clear
cell structure, but for instance a network of cytoplasm
with a nucleus at each “‘ node” of the network, i.e.
at each thickening of the cytoplasm where several
branches of the network meet. This kind of structure
does not however differ essentially so far as its working
is concerned from ordinary cell structure. We have
already seen (p. 66) that the nucleus is essential to the
carrying out of the vital functions of the protoplasm ;
and in these cases of -non-cellular or incomplete cellular
structure we must suppose that each nucleus governs,
so to speak, a certain sphere of cytoplasm around
it, though this sphere is not sharply separated, as it
is in ordinary cellular structure, from the neighbouring
spheres or units. Such a working unit of nucleus and
cytoplasm has been called an energid. An ordinary
living cell represents one such energid, but in. non-
cellular living structures the energids are not separated
by clear boundaries.

The second qualification of the cell doctrine that
has to be made relates to the unity or individuality of
the multicellular organism. To some degree in all such
organisms, and very notably in the higher animals
with a central nervous system, the activities of each
cell are modified by its connexion with the other cells
of the body, so that the organism works asa whole. In
this sense, as we saw in the last chapter, the organism
as a whole has functions which are something distinct
from the functions of the individual cells or energids
of which it is composed, though these are built up as

Z

98 THE CELL

a result of the sum total of the functions of all its cells
or energids.

Cell Walls and Intercellular Substance.—In animals
the substance of the body does not necessarily consist
entirely of the actual bodies of living cells, though the
whole of the substance is formed by the activities of
these. The cells themselves frequently lie in a
“‘matrix”’ of intercellular substance which has been
formed by the cells which were originally close together
but eventually lie singly or in groups scattered in this
matrix. This is very clearly seen in the case of cartilage
(Fig. 5, C) and bone (which is developed from cartilage
by changes in the matrix), In the case of connective
tissue, the tissue which forms the “‘ packing”’ of the
organs of the body, the matrix is differentiated into
two kinds of fibres which run through it, singly or in
bundles, independently of the connective tissue cells
(Fig. 5, D). In the case of muscle the actual cytoplasm
of the original cells is modified into a peculiar substance
which shows no cell structure, but is composed of fibres
that have an enormous power of contractility. The
cell nuclei still remain lying below the membrane
enclosing the muscle fibre (Fig. 5, F). Some animal
tissues have a distinct cell wall surrounding the cyto-
plasm of each cell. This may be composed of modified
cytoplasm or of simpler organic substances, usually
containing nitrogen.

Plants, on the other hand, very rarely have ‘“‘ inter-
cellular substance.”” So much of the substance of the
plant body as is not actually composed of the living
bodies of cells is made up of actual cell walls, which
in the adult state are still clearly seen to have been
formed by the cells, and the basis of these walls is
generally cellulose. The cell walls of a plant form the

THE PLANT CELL 99

skeleton or framework of the plant body, and in the
higher plants a considerable part of the structure (in
trees the great bulk of the structure) is composed of
the walls of cells which have died, so that only the
wall is left. These walls are often very thick and
hard, growth in thickness and the deposition of sub-
stances other than cellulose having taken place before
the protoplasm of the cell died and disappeared.
Another feature of the tissues of the higher plants
(land plants) is the existence of a system of air spaces
(intercellular spaces) between the cells. These are
made by the separation of the walls of certain adjacent
cells from one another, and penetrate the plant, com-
municating with the outer air and forming the channels
by which gases, such as water vapour, carbon dioxide
and oxygen, produced or used by the living cells, diffuse
from these to the air outside, or vice versd.

THE PLANT CELL.

The first cells of organisms to be described were those
of the cork tissue of plants, which were seen by the
Englishman Hooke in 1667 with the compound micro-
scope that was devised about the middle of the seven-
teenth century. Hooke compared these cork cells with
the ‘“‘cells’’ of honeycomb. Thus it was the walls
enclosing cavities to which the term was first applied.
Little accurate knowledge of the contents of these cavities
was obtained for nearly two centuries. But in 1831
the English botanist, Robert Brown, discovered the
cell nucleus, and in 1846 the German botanist, von
Mohl, identified the semi-transparent viscous substance
formed within many of the “cells”’ of the adult higher
plant as the actual living substance of the organism,

100 THE CELL

Fic. 6.—A, embryonic cells from the meristem (growing point) of
the root. B, beginning of vacuolation. C, cells from elonga-
ting region with larger vacuoles; c.w., cell wall; cyt., cytoplasm ;
n, nucleus; vo., nucleolus; v, vacuole. The top right-hand cell
in C has been cut and water admitted to the cavity; the nucleus
has absorbed water and burst, a new gel membrane (m) being
formed on the surface of the escaped contents in contact with
the water. x 660 (after Sachs).

STRUCTURE OF PLANT CELL Iot

and called it protoplasm! This term was then extended
to the essentially similar substance found in animals.
The parallelism between the units of protoplasm, each
with its nucleus, in animals and plants was soon recog-
nised, and the term cell gradually came to be used in
its modern sense as applying primarily to these living
units, which may or may not have definite cell walls.
In plants, however, as we have seen, they have walls
in the vast majority of cases, and the term cell is still
applied not only to the living cytoplast-nucleus unit,
but also to the wall and cavity after the protoplasm
has disappeared.

The adult living plant cell is generally characterised
not only by the possession of a cell wall, but also by
a large central vacuole or space filled with a watery
liquid (cell sap), enclosed within the cytoplasm (Fig. 6, C).
So large is this in proportion to the whole space within
the cell. that the cytoplasm. is often a mere thin layer
lining the wall. The nucleus, which alfays remains
surrounded by cytoplasm, is embedded in this film
and bulges it out into the vacuole (Fig. 6, C). Vacuoles
are by no means the monopoly of plant cells—they
frequently exist in those of animals—but animal cells
as a class are not characterised by the possession of
a large central vacuole, the formation of which depends
on the mode of nutrition and growth of the typical
plant cell. The layer of cytoplasm next the vacuole
“vacuole wall’’), like the external layer (ectoplasm ?)
next the cell wall, is an exceedingly thin film—a gel
membrane, such as we saw is usually formed at the
boundary of a colloid sol (Fig. 7).

Embryonic (Meristematic) Cells.—In the higher plants

1 Greek mpétoc, first, and mAdoya, thing formed or moulded, as

from clay or wax.
2 éxtdc, outside.

102 THE CELL

new cells are mainly produced by division from pre-
existing cells at and near the tips of the branches of
the root and shoot. These regions of active cell division
are called meristems. The cells in this region are all
young because they have all recently arisen from cell
division. The characteristic features of such a meris-
tematic or embryonic cell (Fig. 6, A) are that the
cell wall is relatively thin, the cell cavity is completely
filled with protoplasm and the nucleus is large, its
diameter commonly being as much as two-thirds to three-
quarters that of the whole cell. The granular endoplasm

Fic. 7.—Enlarged diagram of the portion X of Fig. 6. C: c.w., cell wall;
ect., ectoplasm ; endo, endoplasm; v.w., vacuole wall; pi., plastid.
Supposed to be multiplied 6,600 times. __

contains various granules and droplets (metabolites)
of different sizes, It also contains small refringent
bodies called flastids, which may under certain cir-
cumstances form starch, and if exposed to light develop
chlorophyll and become chloroplasts. The endoplasm
is bounded on the outside by the very thin layer of
ectoplasm, which is, however, only just visible under
the highest powers of the microscope and with the
best optical definition, since it is only a fraction of
I p» in thickness (Fig. 7).

The large and conspicuous nucleus (Fig. 6) is a

t Greek weptotyc, divider. 2 doy, within.

KARYOKINESIS 103

spherical body bounded by the nuclear membrane and
containing a network or detached granules of chromatin
bathed by a liquid, the nucleay sap. The nuclear mem-
brane and chromatin are doubtless of gel structure, the
nuclear sap a sol. There is also present in most cases
a conspicuous spherical deeply staining body (sometimes
more than one) called the nucleolus. This seems to
be of the nature of reserve material which is used to
“feed”? the chromatin when division occurs. The
nucleus in this condition is often called the “ resting
nucleus,” because it is not dividing, but} the term is
rather misleading, for the nucleus is really carrying out
its main function, that of “ directing’’ the nutrition
and growth of the cell. ,

Division of the Nucleus (Karyokinesis) and of the
Cell.—Meristematic cells constantly divide, and between
two successive divisions there is a period of growth
(increase in bulk) of the protoplasm till each daughter
cell attains about the size of the mother cell before
division, when each divides again. The division of
the cell is always preceded by division of the nucleus,
which takes place in a remarkable, complicated fashion,
the process being known as karyokinesis! or mitosis 2
(Fig. 7, G). Except in the nuclear divisions preceding
the formation of the reproductive cells, this process
of karyokinesis shows substantially the same features
in every nuclear division preceding cell division in
practically all animals and plants, the differences met
with being of quite minor importance.

The chromatin granules (or chromatin network
[Fig. 8, a]) of the ‘‘ resting nucleus ” become arranged in

1 Greek xdpvoy, a nut (nucleus), xfyjoic, motion.
2 From Greek yutdw, to stretch the warp in the loom, because of
the thread structures appearing in the process,

104 THE CELL

bent chromatin rods of equal length, called chromosomes
(Fig. 8, 5), the nucleolus and nuclear membrane?! dis-
appearing. A spindle-shaped structure, consisting of
exceedingly delicate protoplasmic threads, appears in
the cell, frequently stretching across almost its entire
width (Fig. 8, c). This spindle is called the achromatic
spindle, because its threads do not take up stains and
become deeply coloured as do the chromosomes. The
threads converge on two spots (poles). In some plant
cells and in most animal cells each pole is occupied by
a staining granule called a centrosome to which the
threads are attached. The chromosomes now become
arranged on the spindle at its equator. Each chromo-
some commonly has the form of.a rod bent in the middle
rather like a hairpin, the free ends projecting outwards,
perpendicular to the long axis of the spindle, the bend
directed towards the centre of the spindle (Fig. 8, 4).
At this time each chromosome is seen to have a double
structure, the bent rod being split longitudinally, but
this splitting may occur earlier, so that the double
structure is visible as soon as the chromosomes appear
from the resting nucleus (Fig. 8, c). Spindle fibres
running from the poles towards the equator (and distinct
from the fibres on which the chromosomes rest, which
run continuously from pole to pole) now become
attached to the chromosomes at or near the inwardly
directed bend (Fig. 8, A). These fibres contract and
thus pull the longitudinal halves of the chromosomes
(daughter chromosomes) apart, the daughter chromo-
somes remaining in contact longest at the free ends
(Fig. 8, e and B). By continued contraction of the

t In some animal cells the nuclear membrane remains intact
during karyokinesis, the whole of the processes described taking place
inside it,

KARYOKINESIS 105

achromatic threads the daughter chromosomes are
completely separated and pulled along the guiding

Fic. 8.—Karyokinesis in a meristematic cell (see text).
Modified from Strasburger.

spindle threads (Fig. 8, f) to the neighbourhood of the
poles, where the chromosomes lose their separateness and
individuality (Fig. 8, g),each group becomes surrounded

106 THE CELL

by a nuclear membrane, and two daughter nuclei are
thus constituted (Fig. 8, h).

At the same time (in the tissue cells of plants),
thickenings of the achromatic spindle fibres appear in
the equatorial plane (Fig. 8, g), ie. midway between
the two daughter nuclei, and grow till they fuse laterally,
forming a plate or membrane across the cell (Fig. 8, h),
which becomes joined to the lateral cell walls. This
membrane then becomes converted into cell wall sub- —
stance, forming a new cell wall, which divides the original
cell into two. The achromatic spindle then disappears,
all except the equatorial portion on which the thicken-
ings which initiated the new cell wall were formed.
This portion of the threads, penetrating the new cell
wall, remain, and thus maintain continuity between
the protoplasm of the two sister cells. This continuity
can be demonstrated as existing between the protoplasm
of adult living cells.

It will be seen that the process of karyokinesis has
the effect of dividing the chromatin of the nucleus
accurately into halves between the two daughter cells.
It is believed that each chromosome carries the organic
basis of certain characteristics of the cells of the par-
ticular species of organism, and that when each is
divided longitudinally into halves, these characteristics
are equally shared by the two halves (daughter chromo-
somes), and thus each daughter cell comes to have the
same characteristics as the mother cell.

Development of the Adult Cell from the Embryonic
(Meristematic) Cell (cf. Fig. 6)—So long as cells remain
meristematic they do not cease to pass through the
rhythm of growth and division, but the products
of cell division on the side towards the body of the
root or shoot gradually pass out of the meristematic

VACUOLATION AND GROWTH OF CELLS 107

state and assume the characters of permanent tissue
cells. The power of cell division does not necessarily
cease, but divisions become less frequent and in some
cases do not recur.

The permanent tissue cells commonly attain a size
far greater than that of the meristematic cells—many
of them grow especially in the direction of the axis
of the root or stem, and become several times longer
than they are broad. This great increase in the size
of the cell does not result, however, from increase in
bulk of the protoplasm, but from the formation and increase
in size of vacuoles. Drops of liquid (cell sap) appear
in the endoplasm, increase in size (Fig. 6, B), and
eventually run together, forming one large vacuole,
in the midst of which the nucleus, always surrounded
by a layer of cytoplasm, is suspended by strands of
cytoplasm (bridles), which run across the vacuole to
the layer lining the cell wall. When the vacuole
increases still further in size the bridles are thinned
out till they collapse, and the whole of the cytoplasm
then forms a layer on the cell wall, the centre of the
cell being completely occupied by the large vacuole
(Fig. 6, C, left-hand bottom cell).

During the process just described the appearance
and increase in size of the vacuoles distends the ex-
tensible cell wall, which would be thinned out to the
point of rupture were it not thai new cellulose is
continually added to it by the protoplasm lining its
inner surface. When the cell has reached its definitive
size this process of addition of cellulose often con-
tinues, and now the cell wall is actually thickened,
the successive layers added to its inner surface being
often clearly visible on each side of the original thin
wall (middle lamella). This thickening is not, however,

108 THE CELL

always quite continuous over the whole surface. At
certain spots the wall may not be thickened, so that
channels or fits are left, leading from the middle lamella
to the cavity of thecell. The pits of adjacent cells are
practically always formed opposite to one another. This
arrangement greatly facilitates the passage of water
and solutes from cell to cell.

Osmotic Pressure and Turgor.—What is the force
which causes the distension of the cell during its growth
from the embryonic to the adult condition ? It is the
pressure (often called ‘‘ osmotic pressure ’’) caused by
the ‘‘ attraction” of osmotic substances within the cell
for water, which is drawn into the cell in the effort
to establish equilibrium (cf. p. 57), and develops a liquid
pressure against the wall. We must suppose that the
vacuoles are initiated in the protoplasm by the local
concentration of an osmotic substance, e.g. sugar, in
certain spots, perhaps within plastids... Water is
attracted to these spots and more water enters the cell
from without to replace what is thus removed from the
general cytoplasm. As the pressure in the vacuoles
increases the cytoplasm is pressed against the cell
wall, which is itself progressively distended, just as the
inner tube of an inflated bicycle tyre is pressed against
the outer cover. If the cell were freely suspended in
water this process would continue till the wall burst
or till the elastic reaction of the wall counterbalanced
the distending force due to the attraction of the osmotic
substance for water. But when the cell is surrounded
by other cells similarly distended they all press against
one another and give rigidity to the whole tissue.
This condition of a tissue is called turgor or turgidity,

«1 ln any case a semi-permeable membrane is formed round each
vacuole, whether derived from a plastid or formed in the endoplasm.
N

WILTING AND PLASMOLYSIS 10g

and herbaceous plants depend upon it for maintain-
ing their stiffness and the erect position of their
shoots.

Wilting and Plasmolysis.—If a plant cell loses water
faster than it can obtain it, turgidity cannot be main-
tained, the cell becomes soft and limp, and the tissue
loses its rigidity. This is what happens on a very
hot day when plants lose water by evaporation to the
air faster than they can replenish their supply by root
absorption. The plant is said to wilt. The loss of
turgidity takes place even more rapidly when a plant
is pulled up by the roots and left in the sun.

Turgidity can also be lost in another way. If the
cell or tissue be placed in a sugar or salt solution of
osmotic strength greater than that of the cell sap,
water is drawn out of the cell against the pull of the
cell sap. The protoplasm is then no longer held tight
against the wall by the pressure from within, and it
contracts away from the wall as the liquid pressure
within diminishes by the withdrawal of water. This
process is called plasmolysis.: The osmotic substances
of the cell sap do not, however, pass out through the
semi-permeable membranes represented by the vacuole
wall and ectoplasm, and if the cell or tissue be placed
again in water this re-enters the cell, whose turgidity
is re-established.

PRACTICAL WORK,

(1) Examine the growth of the bean roots set up in the corked
bottle last time. They show (a) local growth, (b) curvature
from the horizontal to the vertical plane, i.e. the bending of an
organ in response to an external force. Sketch diagrammatically
these results alongside the sketches of the roots as originally
set up.

1 Greek Avatc, loosening of the plasma from the wall,

IIo THE CELL

THE PLANT CELL.

(2) Strip off the surface layer of cells from the piece of onion
scale provided. Examine and draw carefully under the high
power two or three of the cells, marking the parts.

(3): Tease out in a drop of water a little of the pulp of the
Snowberry, taking care that it is kept well wetted. Note that
the tissue is Joose, with many air spaces between the cells, many
of which are spherical. Examine the cells under the high power.
Note that the proportion of cell sap to protoplasm in each cell is
very high. Look for the nucleus. Staining with dilute iodine
solution will often help in case of difficulty.

(4) Examine the stained longitudinal section of the tip of a
beanroot. Note that the actual tip is formed by the “ root-cap,”’
The area just behind this is the ‘‘ growing point” or primary
meristem, and consists of small mostly square cells densely filled
with protoplasm and with large conspicuous densely staining
nuclei. Draw carefully under the high power (a) examples of
meristematic cells with ‘‘ resting nuclei,” (6) different stages of
nuclear division? (compare with demonstration slides), (c)
different stages of vacuolation of the cells behind the meristem.

Measure the diameter of a meristematic cell and of its nucleus,
also the length of one of the longest cells in the section.

CELLS oF ANIMAL TISSUES.

(5) Examine the demonstration specimens of stained and
mounted animal tissues.3 Note cytoplasm, nucleus and absence
of cell wall: also, e.g. in cartilage, the intercellular substance
or “ matrix.”

TURGIDITY AND PLASMOLYSIS.

(6) Place one of the turgid bean roots (a) in 10 per cent. CaCl,
solution,4 leaving the other (b) on the table. After a few
minutes note that (a) has become quite limp. Place it in water,
and it gradually regains its turgidity; (b) gradually wilts as it
loses water by evaporation to the air.

1 This may be omitted if the Practical Work is too long.

2 Demonstration slides under immersion objectives showing stages
in karyokinesis should be available if possible.

3 Three or four slides will suffice: columnar epithelium, cartilage,
connective tissue and a preparation of muscle showing nuclei are
suggested as suitable.

4 NaCl solution is liable to injure the protoplasmic membranes.

PRACTICAL WORK IIt

(7) With a knife split a length of fresh bean stem longitudinally
into quarters. Note that the ends of the strips curve outwards.
The harder tissues of the outside of the stem are in a state of
elastic tension in the intact stem, i.e. they are tending to contract
in the longitudinal direction, but cannot do so against the swelling
force of the turgid cells of the centre (pith) of the stem, which
are similarly prevented from actually swelling. When the
stem is divided longitudinally the inner cells swell and the outer
ones contract, so that each strip of stem bends outwards.

(8) Strip off a fragment of the coloured surface layer of
Tvadescantia leaf as free as possible from the green tissue below.
Mount in water and examine under the high power. The vacuoles
of the purple cells are filled with coloured cell sap. Place a drop
of ro per cent. CaCl, solution on the slide just touching the edge
of the coverslip, and draw the solution under the coverslip by
holding a fragment of blotting paper against the opposite edge.
Watch the effect on the cells—plasmolysis. Reverse the action
by running in a drop of water in the same way and watch the
recovery of turgor.

CHAPTER VII

THE GREEN PLANT CELL

Chloroplasts and Chlorophyll.— The majority of
plants we are familiar with have green stems and
leaves, or green leaves alone. The green colour is
due to the mixed pigment chlorophyll present in the
chloroplasts (which are definite, oval, spherical or disc
shaped, protoplasmic bodies lying in the cytoplasm of
certain of the livingcells of the leaf or stem), and in
these alone. The lower green plants (for instance the
Mosses and Liverworts) possess chloroplasts of the
same nature as those of the seed plants, but in the
Green Alge the chloroplasts are very various in shape
and size (e.g. spiral bands, Fig.9). The chloroplasts are
derived from the plastids (which multiply by simple
division) found in the meristematic cells, and are not
formed afresh in the protoplasm.

Chlorophyll is a mixture of four different pigments
held in the protoplasm of the chloroplast in a colloid
state: chlorophyll a (C;;H,.O;N,Mg) giving a blue-
green solution, chlorophyll B (C;;H,.O,N,Mg) giving
a pure green solution, cavotin (Cy H;.) giving orange
crystals, and xanthophyll (C,H,;,0,) giving yellow
crystals. These are mixed in the chloroplasts in the
proportion of six molecules of the two green pigments
to two of the two orange and yellow ones.

1 Carotin also occurs in other parts of various plants. It is, for
instance, the red colouring matter of carrots, of many red and orange
coloured flowers, etc.; also of the “ eyespot” in Chlamydomonas
(see p. 186),

12

SPECTRUM OF CHLOROPHYLL II3

Photosynthesis.—The spectrum of chlorophyll shows
definite absorption bands.t These absorption bands
represent the rays most strongly absorbed by the
chlorophyll, and thus the radiant energy available
for the work which the chloroplast carries on. That
these are the rays actually used in photosynthesis,
or at least in one or more of the processes that go to
make up photosynthesis, can be shown by a very pretty
experiment first devised by the German botanist
Engelmann. A thread of green alga living and carry-
ing on its work in water is illuminated by a solar spec-
trum so that different parts of the thread are illuminated
by different coloured rays. A culture of a certain

Red Orange ‘Yellow Green Blue Violet

77

Fic. 9.—Diagram illustrating Engelmann’s experiment, to show by
the aggregation of a motile bacterium (b) highly sensitive to oxygen
the regions of maximum evolution of oxygen by a green alga (a~a)
illuminated by a solar spectrum. The most active rays correspond
with the red and orange.

bacterium (Bacterium termo) which is very sensitive
to oxygen, so that its cells move towards any source

1 When a beam of white light is split up by passing through a prism
into a band or spectrum of differently coloured rays, owing to the
different angles through which the different rays are bent by the
prism, and is allowed to fall on a white surface, the spectrum so formed
(solar spectrum) is continuous. But if the beam has passed through
a translucent coloured substance, the spectrum formed shows dark
bands (absorption bands) corresponding inversely with the colour of
the translucent substance, the colour of the substance being caused
by the combined effect on the eye of the coloured rays that have
passed through it. The absorption bands are caused by the absence
of the rays that have been stopped or absorbed by the coloured
substance. Thus a pure red substance shows absorption in the other
colours of the spectrum—blue, green, etc., a blue substance in the
red, orange, etc., and so on.

8

II4 THE GREEN PLANT CELL

of oxygen diffusing into the water, is then introduced
in the neighbourhood of the illuminated algal thread.
The minute cells of the bacteria swarm towards and
cluster round those portions of the thread illuminated
by the coloured rays corresponding with the chief
absorption band of chlorophyll, especially in the red
(Fig. 9).

This behaviour of the bacteria shows that free oxygen
(one of the products of the process of photosynthesis)
is being given off most actively at those points; and
consequently that those particular rays are the ones
most active in photosynthesis.

Sugar, probably glucose, is the first product of the
process of photosynthesis that can be definitely recog-
nised in the green cells of the leaf, free oxygen being
liberated at the same time. ‘These substances are only
formed in the presence of a certain intensity of the proper
rays of the spectrum (pure white sunlight contains all
the rays), and in the presence of carbon dioxide and
water. We may represent the general equation of
photosynthesis thus :—

6CO, + 6H,O = C,H,,.0, + 60,
It is likely, however, that formaldehyde is first formed :—
CO, + H,O = CH,O + 0,
and that the formaldehyde is then condensed to form
glucose :—
6CH,O = C,H,,0,

but this is uncertain, since formaldehyde has not been
demonstrated in the leaf. Nevertheless the ‘“ reduc-
tion ” of CO, in-presence of water to form formaldehyde
and also the condensation of formaldehyde to form
glucose are both operations that can be performed in

STRUCTURE OF MESOPHYLL II5

the chemical laboratory, For the first process consider-
able energy is required, and it is this that certainly
requires, in the chloroplast, the radiant energy of
light. This first process may then be designated as
photolysis, the second process, that of condensation to
form sugar being a process of chemosynthesis. For
ordinary biological purposes, however, we may consider
them together as photosynthesis.

Structure and Functions of the Green Plant Cell.—
The green tissue (mesophyll *) which forms the bulk of
an ordinary foliage leaf (see Fig. 10) consists of cells
with thin cellulose walls lined inside by a layer of
cytoplasm enclosing the nucleus and numerous chloro-
plasts, and with a large central vacuole. These cells
are, in fact, “‘ adult ’’ living cells (see p. IoI) containing
chloroplasts (Fig. 11).

The mesophyll tissue is covered by a layer of colour-
less living cells (epidermis) on the surface of the leaf,
and the outer walls of these cells, in contact with the
external air, has a waterproof layer of cell wall sub-
stance (cuticle) which prevents the cells of the leaf
drying up by evaporation to the air (Fig. 10, st.) The
epidermis (and cuticle) is, however, pierced by a number
of holes (stomata?) which lead from the external air
to the air-containing intercellular spaces between the
mesophyll cells. These intercellular spaces are inter-
communicating, and are so arranged that every meso-
phyll cell has some part of the outer surface of its wall
in contact with the internal atmosphere of the leaf.

The internal atmosphere of the leaf, being in com-
munication with the external air through the stomata,
has at first the same composition as the latter,
but this composition is continually being modified by

t Greek péoos, middle, and diAdgv, a leaf. 2 otéua, mouth.

116 THE GREEN PLANT CELL

the interchange of gases between it and the mesophyll
cells, and equilibrium between it and the external
air is constantly tending to be re-established by the
diffusion of gases through the stomata, into or out of
the leaf. Thus if the mesophyll cells absorb carbon
dioxide, as they do when illuminated, the pressure of
this gas decreases in the intercellular spaces and fresh
carbon dioxide from the air streams in through the

Fic, 10.—Diagram of transverse section of a leaf to show the relation
of the mesophyll (mes.) to the system of intercellular spaces (int.)
and the stomata (st.). The white arrows represent the evapora-
tion of water from the mesophyll cells into the intercellular
spaces, and its diffusion through the stomata to the outer air.

stomata to restore the equilibrium. The oxygen pro-
duced in the mesophyll cells under the same conditions
increases the pressure of oxygen in the intercellular.
spaces, and it escapes through the stomata into the
external air, The air in the intercellular spaces is
normally nearly saturated with water vapour, owing
to the constant evaporation of water from the wet
mesophyll cells into the air in contact with them,
and this water vapour is always escaping through the

FUNCTION OF CHLOROPLAST 117

stomata if the outer air is drier, as it usually is, than
the air inside the leaf (Fig. 10, white arrows).

Let us consider now what happens in the chloroplast
of a mesophyll cell which is suddenly well illuminated.
The light falling upon it is robbed of those rays of the
spectruin corresponding with the absorption bands

Fic, 11,—Two mesophyll (palisade) celis. Fic. 12.—A, diagram of chlo-

The one on the leftis seenin optical roplast of mesophyll cell
section, the one on the right insur- to showits activity when
face view. Above are two cells of illuminated ; v, vacuole ;
the upper epidermis. chl., chloro- int., intercellular space
plast; m, nucleus; c.w., cell wall; B, starch grains formed
ep., epidermal cell; int., intercellular in chloroplast.

space.

of chlorophyll. The energy so obtained is used to
split up the molecules of carbon dioxide provided by
the respiration of the protoplasm, and the carbon is
united with the elements of water (always present in
the cell) with the ultimate formation of sugar, oxygen
at the same time being liberated (Fig. 12, A). Some
of the sugar and oxygen so formed is doubtless used

118 THE GREEN PLANT CELL

in the respiration of the colourless protoplasm, but,
under favourable conditions, the rate of photosynthesis
far exceeds the rate of respiration in a green cell. This
means in the first place that far more carbon dioxide
is used than can be obtained from the process of respira-
tion, the pressure of the gas in the cell decreases and
fresh supplies enter from the air of the intercellular
spaces. This in turn causes an inflow of carbon dioxide
through the stomata.

The oxygen formed as a by-product of photosynthesis
increases the pressure of this gas in the cell, and is given
off at the cell surface into the intercellular spaces and
thence diffuses through the stomata into the open air,
so that well illuminated leaves are constantly giving
off free oxygen. The sugar continuously produced
by the chloroplasts diffuses into the cell sap. Some of
it passes into the bundles (veins) of the leaf and along
the bundles to other parts of the plant. But more
sugar is produced than can be removed from the meso-
phyll by this means, and the concentration of sugar in
the cell continuously rises during the day.

Formation of Starch in the Chloroplasts.—In most
green plants, though by no means in all, the excess of
sugar after a certain degree of concentration is reached,
is converted into starch by condensation, and this
forms small grains in the substance of the chloroplast :—

nCgH,,0, = (C.H,,0;), + “HO

A foliage leaf of a starch-forming plant at the close
of a bright day when large amounts of sugar have
been formed shows its chloroplasts packed with starch
grains (Fig. 12, B), as can easily be seen by examining
a section of the leaf under the high power. The starch
grains appear as bright granules in the substance of

‘

STARCH FORMATION. 119

the chloroplast. The mass of starch formed in the
leaf, and the dependence of this process upon illumina-
tion, can be strikingly demonstrated to the naked eye
by covering part of the leaf during the day with tinfoil
while still on the plant and leaving the rest exposed.
If in the afternoon the leaf is cut off, killed with boiling
water, the colour taken out with alcohol, and the leaf
is then placed in a dish and flooded with iodine solution,
the part of the leaf that was exposed to light will
quickly turn blue-black owing to the staining of the
starch grains as the iodine gradually penetrates the
tissues, while that part of the leaf protected from
light remains colourless.

The function of the chloroplasts as starch formers
is quite distinct from their function as sugar producers.
It does not depend directly on the photosynthesis of
sugars or on the green colour of the chloroplasts, but
on the actual concentration of sugar in the leaf. The
leaves of many plants do not form starch at all, and
the concentration of sugar in the leaf necessary to lead
to starch formation varies considerably in different
species. The power of forming starch from sugar is
shared by the colourless plastids (leucoplasts) found
in the colourless cells of the plant, particularly in seeds,
tubers, etc., but also in stems and roots, where large
quantities of starch are formed from the sugar which
comes to these cells from the leaves. Chloroplasts
themselves can form starch from sugar which they
have not made, as can be proved by the experiment
of floating detached living ledves on sugar solution in
the dark, when starch will appear in the chloroplasts.

As the illumination decreases the rate of sugar
formation likewise decreases, and when it falls below a
certain intensity—in natural illumination about sunset—

20 THE GREEN PLANT CELL

the photosynthesis of sugar becomes negligibly small.
Since a certain amount of sugar is always being
removed from the cell through the veins, the concentra-
tion of sugar in the mesophyll cells falls, and the starch
grains in the chloroplasts are gradually reconverted
into sugar during the-night, and this sugar is likewise
conveyed away by the veins. Thus both during the
day and during the night there is a constant stream of
sugar away from the leaf to other parts of the plant
where it is used in respiration, in the formation of new
protoplasm and new cell walls, or is stored as starch, or
sometimes remains as sugar (beetroot, sweet fruits,
etc.), or as some other carbohydrate such as inulin
(Jerusalem artichoke, etc.), or is converted into fats.

The formation of glucose from starch and of starch
from glucose may be represented by the general
reversible equation :—

condensation
MCgHy20, == (CoH 005), + “H,O
hydrolysis

The reaction is catalysed by the enzymes diastase
and maltase, probably in either direction (see p. 45
for the intermediate substances formed).

Photosynthesis in a Green Cell living in Water.—In
a green cell living in water the processes of sugar and
starch formation are essentially the same as in the
mesophyll cell of a leaf. The only difference is that
the carbon dioxide is derived directly from that which
is dissolved in the water instead of being absorbed
from the air, and that the sugar does not leave the cell.
Starch (or in some species fat) is formed when the
concentration of sugar passes a certain point. The sugar
is used for respiration, to form the cellulose for cell
wall substance, and also (together with the elements of

RESPIRATION AND PHOTOSYNTHESIS 12r

mineral salts entering the cell from the surrounding water),
to make the proteins which form the basis of the new
protoplasm produced as the cell grows and divides.

Respiration and Photosynthesis.—It will be noted
that the ‘general equation representing respiration is
the exact reverse of that representing photosynthesis.
The two equations may be combined thus :—

RESPIRATION
potential energy —~» liberation of energy —> kinetic energy
(oxidation)
CgH 1206 + 60, =a 6COg + 6H,0
(reduction)
potential energy <— locking up of energy <— kinetic energy
PHOTOSYNTHESIS

In the mesophyll cell of a well-lighted leaf the photo-
synthetic process very greatly exceeds respiration,
so that carbon dioxide and water are continuously
used up, sugar and free oxygen produced. When
illumination falls to a certain low level of intensity
the two processes will exactly balance, and the system
represented by the green cell will be in equilibrium in
respect of these processes. In still weaker light or
in the dark photosynthesis stops altogether and sugar
and oxygen are continuouly used up, carbon dioxide
and water produced. Thus it is true that green plants
give off carbon dioxide in the dark. But the popular
notion that it is ‘‘ unhealthy ”’ to keep plants in a bed-
room at night is nevertheless unfounded—the amount
of the gas given off as a result of the respiration of
the plant cells is far too small to raise appreciably
the carbon dioxide content of the air.

Synthesis of Proteins.—The formation of sugars in
the green cells, their condensation into starch, and the

122 THE GREEN PLANT CELL

hydrolysis of this to sugars again, form a very important
part of the metabolism of green plants. Sugar is not
only used by the plant in respiration, but it is also the
material from which the cell walls, the skeleton of the
plant, are formed. For these reasons the carbohydrate
metabolism of the plant makes up much the greater
part of the whole. of its metabolic processes. But the
synthesis of proteins to form the basis of the new
protoplasm which is constantly being formed in meri-
stematic cells is clearly also of essential importance.
This appears to take place by the interaction of soluble
carbohydrates with the nitrogen and sulphur derived
from the nitrates and sulphates absorbed by the roots.
It seems that comparatively simple organic nitrogenous
substances called amino-acids are thus formed, and
that these are synthesised by successive condensations
(analogous to the condensations of sugar to starch or
cellulose) into more complex nitrogenous substances,
and eventually into proteins. These anabolic processes
probably occur in the mesophyll cells of the leaf, whence
the nitrogenous substances are conveyed, in relatively
simple form, to growing organs and storage organs.
At least the final synthesis of the protoplasmic proteins
must also take place in the meristematic cells where
new protoplasm is actually being formed. It is not
possible to enter here into the details of these processes,
some of which are still largely obscure and others
unknown. Some of the simpler proteins—the foly-
peptides—can be made synthetically in the laboratory.

The Raw Materials of Plant Food.—The chemical
elements which are constituents of the molecules of the
most important organic substances involved in the
structure of organisms have been enumerated on page 37.
Of these the green plant obtains carbon from the carbon

RAW MATERIALS OF FOOD 123

dioxide of the air, hydrogen and oxygen largely from
water, nitrogen, sulphur and phosphorus from nitrates,
sulphates and phosphates dissolved in the water of the
soil. The remaining elements essential to the nutrition
of plants are potassium, magnesium, calcium and iron,
and these are obtained from the same mineral salts.
Other elements, such for instance as sodium, silicon,
etc., are habitually absorbed by the plant, but they are
not essential as materials of food. This can be proved
by growing the plant notin soil but in a mixed solution
of various salts. Such water cultures, as they are called,
show that a plant can grow and flourish when it is pro-
vided with a selection of soluble salts in dilute watery
solution containing only the elements named. A good
“complete” water culture solution is the following :—

Calcium nitrate a és .. 4 grams
‘Potassium nitrate .. a .. I gram
Magnesium sulphate ss -. I gram
Potassium phosphate a .. I gram
Distilled water ea 2s +. 50 CC.

This is a stock solution. 2 or 3 c.c. should be used
for 1 litre of culture solution, distilled water being
added to make up the litre, and a drop of iron chloride
added. Other soluble salts can be substituted provided
they contain the same elements, for instance potassium
nitrate, calcium phosphate, calcium sulphate, magnesium
chloride.

But if one of the elements represented in the salts
is omitted from the solution the plant does not flourish,
and eventually dies. The most rapid failure results
from the omission of nitrogen, and this is clearly because
nitrogen is an essential constituent of the protein
‘molecules. When the store of combined nitrogen in
the seed has been used up by the growing plant, no

124 THE GREEN PLANT CELL

4 3 I 2 5
Fic. 13.—Water cultures of Buckwheat (after Nobbe). 1, plant
grown in complete solution; 2, without potassium; 3, with
sodium instead of potassium; 4, without calcium; 5, without
nitrogen.

PRACTICAL WORK 125

new protoplasm can be formed, and growth necessarily
ceases unless fresh supplies of combined nitrogen in
a suitable form, i.e. as nitrate, can be obtained. The
ordinary green plant cannot directly use the free gaseous
nitrogen of the air to build up its proteins. And the
plant cannot develop successfully, and dies sooner or
later, in the absence of any one of the other essential
elements.

PRACTICAL WORK.

(t) Soak the leaf provided in alcohol and note that a green
pigment is gradually extracted from the leaf. After an hour’s
soaking pour some of the green solution into a test tube and
examine with the hand spectroscope. Compare with (6).

(2) Strip off part of the colourless surface layer of cells (epi-
dermis) of the leaf of Gladiolus provided. Below you will see
colourless ribs (veins) with strips of green tissue (mesophyll)
between. Cut out a small portion of this green tissue with the
point of a sharp scalpel or penknife and mount in a drop of
water, taking care to wet the tissue thoroughly. Put ona cover-
slip and examine under the low power. Note the small cells with
green dots, and the air spaces (now partly filled with water) between
them. Examine with the high power and carefully draw one
of the cells of which you can get a clear view, marking the cell
wall, the cytoplasm lining the wall with chlovoplasts embedded in
it, the central vacuole, and the small shining spherical nucleus.
Stain a portion of the green tissue with hematoxylin for ten
minutes and note that the nucleus especially absorbs and becomes
coloured by the stain. 7

(3) Examine under the microscope in a drop of water a whole
leaf of the water weed Elodea. This is a ‘‘ water leaf,” wholly
submerged in nature and of very simple structure, consisting,
except for the veins, of only two layers of cells, all containing
chloroplasts. The structure of each cell is essentially the same
as that of the mesophyll cell of Gladiolus, though the cells are
much larger and oblonginshape. Thenucleusis generally difficult
to detect: it is often hidden in a clump of chloroplasts. When
visible it is seen as a rather large, pale, often faintly granular oval
body. Notethat all the chloroplasts are embedded in cytoplasm :
none of them lie in the large central vacuole. The cytoplasm
may sometimes be seen streaming actively round the cells, carrying
the chloroplasts with it. (Cf. p. 85.)

126 THE GREEN PLANT -CELL

(4) Mount a cross section of the fresh stem of Pellionia in a
drop of water. Notice under the high power the chloroplasts
in the cells near the surface of the section, with starch grains (bright
colourless granules) inside them. In cells further from the
surface note that the starch grains are larger and have burst
out of the chloroplasts. The chloroplast which has formed the
grain can often be seen attached to one end of the grain. The
growth of these deeper lying grains is continuous and is pro-
bably the result of a continuous supply of sugar, while the
sugar supply of the superficial cells probably depends, as in a
mesophyll cell, directly upon photosynthesis and therefore upon
light, so that the starch grain ceases to grow and is converted
back into sugar every night. Draw examples under the high
power of different relations of starch grain and chloroplast, and
then test with iodine.

(5) With the hand spectroscope compare the spectrum of the
alcoholic solution of chlorophyll (1) with that of the translucent
green leaf and with the spectrum of sky light. [Compare also
with the demonstration spectra.] Note the absorption bands of
the chlorophyll spectrum.

(6) Examine the demonstration water cultuves, and observe
the effects of leaving out each of the essential elements in the
mixed solution of salts used to feed the plant.

CHAPTER VIII

THE COLOURLESS PLANT CELL. THE YEAST
PLANT

WE have already seen that many of the living cells
of the higher green plants do not contain chlorophyll,
e.g. the meristematic cells, all the cells of the root,
many of those in the interior of the stem, and also
those which form the surface layer (epidermis) of her-
baceous stems and leaves. The cytoplasm of many of
these cells, however, contain plastids, which may turn
green on exposure to bright light, and thus become
chloroplasts. All these colourless cells have to be fed
with sugar, ultimately coming from the green cells,
to repair the waste of respiration; and the meriste-
matic cells, which are actively dividing, not only
require large quantities of sugar, but must also be
supplied with nitrogenous substances to make new
protoplasm.

There are also plants which have uo green cells:
a few saprophytes and parasites ! among theseed plants
which live upon the organic substances of humus
(decaying vegetable substance such as leaf mould)
or upon the organic substances of the bodies of living
green plants; the great group of Funcr; and finally
the group of unicellular plants known as BacTERtia,
which, as we shall see in the next chapter, are of extra-
ordinary importance in the economy of the life of the

1 See P. 300

128 THE COLOURLESS PLANT CELL. THE YEAST PLANT

world. All of these colourless plants, with the excep-
tion of a few kinds of bacteria, obtain the materials
for the formation of new protoplasm and the energy
for carrying out their life processes by absorbing liquid
organic substances from outside.

Yeast (Saccharomyces ').—A simple organism with
which it is convenient to begin the study of colourless
plants is the unicellular yeast plant. There are many
different kinds or species of yeast, some of them differ-
ing markedly in the appearance and size of the cell,
but mainly distinguished by the difference of their
activities. There is convincing evidence that the
yeasts are derived from a certain group of the higher
fungi, but they live and maintain themselves indefinitely
as unicellular plants. :

Structure (Fig. 14).—The single yeast cell is spherical
or oval in shape, about 8 to 12 p in diameter—a small
cell on the scale of the tissue cells of the higher plants,
but about the same size as a cell of Protococcus—sur-
rounded by a cellulose cell wall. The cytoplasm is
more or less granular, and there is a central oval vacuole,
though the diameter of this in proportion to that of
the whole cell is not so great as in the ordinary adult
tissue cell of the higher plant—in other words, the
cytoplasmic layer lining the cell wall is relatively
thick (Fig. 14, C). The central vacuole is really intra-
nuclear—fine threads of chromatin applied to the
inside of nuclear membrane extending around the
vacuole from an aggregation of chromatin at one end
(Fig. 14 C, chr.). This aggregation of chromatin can
sometimes be seen as a granule in the living cell.
According to the metabolic condition of the cell, other
small vacuoles or droplets of liquid, and granules of

t “ Sugar-fungus.””

STRUCTURE OF YEAST PLANT 129
various substances, may appear in the surrounding
cytoplasm (Fig. 14, C).

Conditions of Life.—‘‘ Wild” yeasts are found in

Fic. 14,—Yeast plant (Saccharomyces). A, yeast cells budding and
forming chains. x 200. c.w., cell wall; ppm., protoplasm; vac.,
vacuole. B, two cells forming buds. The black granules are
volutin, a complex organic substance formed by the yeast cell.
C, diagram of the structure of a yeast cell made up from informa-
tion about its structure obtained from treating and staining the
cell with various reagents; .v., nuclear vacuole; xJ., nucleolus ;
chy., chromatin; gl., glycogen (after Wager and Penistone).
D, formation of four spores within a cell.

exposed sugary secretions or exudations of plants,
for instance in the “nectar” of flowers, in the drops
9

I30 THE COLOURLESS PLANT CELL. THE YEAST PLANT

of juice appearing on the surface of a fruit with ruptured
skin, and the like. But the yeasts, like the wheat plant
or the apple tree, are best known in the ‘‘ domesticated ”’
or cultivated condition. There are two main uses to
which yeast is put by man—the fermentation of sugary
plant juices to make alcoholic drinks, such as beer,
wine, or cider, and the “ raising”’ of bread by means
of the carbon dioxide given off by active yeast.

“ Brewers’ yeast’ is a frothy, viscous, cream-
coloured liquid with an “alcoholic”? smell. ‘‘ Bakers’
yeast’ is a similarly coloured putty-like substance.
A little of either placed in a drop of water under the
microscope is seen to consist of myriads of yeast cells,
but while in bakers’ yeast these are mostly single, in
brewers’ yeast they are united in chains which are often
branched, the result of rapid reproduction by budding
(Fig. 14, A and B).

Nutrition. Yeast can live in a culture solution of
salts containing the essential elements of the food of
protoplasm, of which the most complex is ammonium
tartrate (NH,),.C,H,O,, an organic salt containing
carbon, hydrogen and oxygen as well as nitrogen. The
other elements can be obtained from simple inorganic
salts such as potassium phosphate, K,PO,, calcium
phosphate, Ca,(PO,),,and magnesium sulphate, MgSO,.
With these alone, therefore, it can construct proteins.
But it grows very slowly in such a medium, for the
amount of energy it can obtain from ammonium
tartrate is small. If sugar be added to the solution
it grows and multiplies much faster, for from the sugar
it can obtain a large amount of energy.

Reproduction.— Yeast reproduces itself primarily by
budding (Fig. 14, A and B). A tiny area of the cell wall,
usually at one end of the oval cell, is thrust out by the

at

BUDDING AND SPORE FORMATION I3I

osmotic pressure within the cell, cytoplasm passing
into the bud. The bud grows until it reaches the size
of the mother cell, the nucleus of which meanwhile
divides, one of the daughter nuclei passing into the
new cell. The daughter cell may then be cut off and
become a new unicellular yeast plant. So actively,
however, does budding take place in a solution contain-
ing sugar (with the other necessary elements present
in a suitable form) that the daughter cell produces
another bud before separation has taken place, and
this again another, and thus the chains of yeast cells
are formed. A cell may even bud in two or three
places. at once, so that the chains are branched.

Spore Formation.—Another method of reproduction
may occur, however, if the yeast is starved, e.g. by being
‘left on a slab of plaster of Paris with a little water.
Under these circumstances the nucleus divides into
two and then into four, the cytoplasm withdraws from
the wall and becomes aggregated round the four neclei
in four tiny spherical masses, each of which secretes a
comparatively thick wall (Fig. 14, D). Each of the
spores so formed is about 3 win diameter. If now the
yeast culture dries up, the original cell wall collapses,
and the spores can be carried away and float with
the dust in the air, the living contents remaining in
a dormant condition for a long time. Falling into a
suitable liquid medium, the dormant protoplasm of
the spore absorbs the liquid and becomes active, burst-
ing the spore wall, pushing out, and covering itself
with a new ordinary cell wall, thus giving rise to a new
vegetative yeast cell.

The spores of yeast are therefore a vesting stage in the
life history, able to withstand desiccation for some time.
This is true of most of the spores formed by plants.

I32 THE COLOURLESS PLANT CELE. THE YEAST PLANT

Alcoholic Fermentation.—The yeast plant is of
special interest both biologically and practically, because
it is able for a time, in a suitable nutritive medium
containing sugar, to carry on a process which we must
regard as a modified kind of respiration, in place of
ordinary respiration. The yeast cells produce an
enzyme (zymase), by the catalytic activity of which
the molecule of sugar (glucose) is split into two mole-
cules of ethyl alcohol and two of carbon dioxide :-—

C,H,,0, = 2C,H,O + 2C0,
glucose ethyl alcohol carbon
dioxide
thus liberating energy from the sugar molecule
without using free oxygen. This process in the living
cell is called anaerobic respiration, as opposed to the
ordinary aerobic: respiration, in which free oxygen is
used. Of the sugar actually absorbed by the yeast
cells some is split up in this way and the energy is
used in growth, while some is doubtless used, together
with nitrogenous substances, for the formation of new
protoplasm. None of the zymase formed by the
cell passes out into the surrounding sugar solution,
but quantities of. sugar pass into the yeast cells, and
are split up into alcohol and carbon dioxide. The
carbon dioxide forms bubbles in the liquid, which froths,
while the alcohol accumulates, and the energy set
free raises the temperature of the liquid. Meantime
the yeast cells grow and bud with great rapidity, enor-
mously increasing their number. This is the process
known as alcoholic fermentation, and is the foundation
of the making of beer and wine.

Brewing.—The liquid fermented in brewing beer is

'.called wort, and is a hot water infusion of malt, i.e. sprout-
: “Living in air” from Greek dyjp, air, and Bloc, life.

MALTING BARLEY 133

ing barley grains. Water is added to the fresh barley
grains and kept standing on them about three days,
being changed two or three times during that period.
The wet grains are then spread out on the floor of the
malting house, and occasionally raked over so as to
allow the air free access to them. The temperature
of the grains rises considerably during the week or so
they are left on the floor. This is due to the production
of heat by the respiration of the grains, which absorb
oxygen from the air. The starch stored in the grains
is converted into sugar, and much of this sugar is split
up in respiration. At the end of the “‘ malting,” however,
the sprouting grains are still full of sugar and of soluble
nitrogenous organic substances—the reserve stores of
the seed are mobilised for the growth of the young plant.

The malt is then removed to kilns and dried for
one or two days at 70° or 100° C., according to the kind
of beer that is to be produced.t The kilned malt is
removed to vats to which water is added at 50° C., the
temperature being afterwards raised to 76°C. This is
the process of infusion of the malt, the liquid produced
being the wort, which is now filtered off, the malt
residue, which still contains a considerable amount of
nutritive substance, being used as cattle food. The wort
is boiled with hops for two or three hours to extract the
bitter aromatic substances from the hops, and then
sieved off and run into underground marble vats, where
it is cooled to 8° C., and the yeast—a pure culture of a
particular strain of yeast?—is added. Here it stands

t The details of malting and brewing procedure given are those
followed at the famous Carlsberg Brewery at Copenhagen, where
‘* Pilsner” and ‘‘ Lager” light beers are the products. The malt is
kilned at 70° C. for the former and at 100° C. for the latter.

2 In the case of ‘‘ Pilsner’ and ‘‘ Lager’ beer a so-called ‘‘ bottom

yeast,’ which sinks to the bottom of the wort. For brewing the
heavier English beers ‘“‘ top yeasts *? are used.

134. THE COLOURLESS PLANT CELL. THE YEAST PLANT

for twelve to fourteen days while the process of fermenta-
tion takes place. The carbon dioxide produced rises to
the top, so that the surface of the fermenting wort
becomes covered with a thick layer of froth. Much of
the carbon dioxide escapes, but being heavier than air
lies on the surface of the vat, so that workmen have
sometimes been asphyxiated by approaching the vats
incautiously. The temperature of the wort rises, owing
to the great liberation of energy in fermentation, but
is kept down to 12° C. by currents of cold air directed
on to the vats. When the fermentation has reached
the desired point the beer is run into specially lined
storage casks or tanks and kept at 1° C, for three months,
when it is ready for bottling.

The whole of the installation is kept scrupulously
clean, or infection of the wort with ‘ wild” yeasts and
bacteria would occur, undesired fermentations would
take place, and the beer spoiled.

The kind and quality of beer produced depends not
only on the kind of yeast used and the time during
which fermentation proceeds, but also on the kind and
quality of the malt and hops (and these again on the
strains of barley and hops, and on the soil and climate
in which they are grown), for beer is a complex solution
of many substances derived from different bodies con-
tained in the malt and hops, besides the sugar, which
is the main substance fermented.

Wine -Making.—In wine-making the fermentable
liquid (must) is grape juice, which is pressed from the
overripe grapes in a wine-press. The yeasts which
cause the fermentation in the must are always present
on the overripe grapes, and do not have to be added
as is done in the making of beer. As in the case of
beer, but to a far higher degree, the kind and quality

VINTAGE WINES. FERMENTATION AND RESPIRATION 135

of wine depends not only on the way it is made, but
on the kind of grape used, the climate and soil in which
the vines are grown, and the weather during the season
in which the grapes ripen. Thus not only are claret,
port, sherry and hock, for instance, totally different
wines, because they are made in different ways, but
particular vineyards in each wine-growing district
produce particular qualities of wine, and finally the
different years. or “‘ vintages’ from the same vineyard
have different characteristics owing to the weather
during the year. Sometimes the characters so pro-
duced actually override the differences due to the
different vineyards, so that a connoisseur can recognise
a particular vintage. A fine wine is an immensely
complex solution, and the chemistry of many of the
organic substances it contains is still very imperfectly
understood. Slow chemical changes continue in the
wine while it is in cask and while it is in bottle. Up
to a certain point the wine constantly ‘‘ improves,”
and after that point deteriorates.

In making sparkling wines the later stages of fer-
mentation are allowed to take place in the bottles, and
the carbon dioxide, which develops a considerable pres-
sure, is thus retained. Fermentation is, however, always
stopped before it is complete and the yeast removed.

Relation of Fermentation to Respiration. — The
alcoholic fermentation of sugar by the yeast plant, or
rather by the enzyme zymase which it produces, is, as
has been said, to be regarded as a modified kind of
respiration into which free oxygen does not enter
(anaerobic respiration). But yeast cannot go on fer-
menting sugar indefinitely. After a time it must have
free oxygen and respire aerobically. It is interesting
to note that ordinary (aerobic) respiration begins with

136 THE COLOURLESS PLANT CELL. THE YEAST PLANT

a splitting of the sugar molecule without oxidation,
free oxygen only taking part in the reaction towards
the end of the process, resulting in the formation of
carbon dioxide and water. Alcohol may actually be
formed in ordinary plant cells if free oxygen is ex-
cluded. Thus alcoholic fermentation by yeast repre-
sents the first stage of the ordinary respiratory process
continued for a long lime and very energetically.

Alcoholic fermentation of sugar by yeast is only
one kind of fermentation. Sugar can be split up into
other substances than ethyl alcohol and carbon dioxide,
and a great variety of other organic substances can be
split up in a similar way. These varied fermentations
are mainly carried out by various members of the great
class of unicellular plants, the Bactevia, with which we
shall deal in the next chapter.

PRACTICAL WORK.

A. YEAST,

(1) The corked bottle contains yeast fermenting a sugar solu-
tion. Dip a glass rod into lime-water and test the air in the
bottle for carbon dioxide in the same way as with the respiring
barley (p. 89). Repeat the test at the end of the practical work.

(2) Place a drop of brewers’ yeast on a slide, cover, and examine
with the high power. Measure the length and breadth of several
of the isolated cells and determine their average size. Draw two
or three on a large scale, marking cell wall, cytoplasm and central
(nuclear) vacuole. A refractive mass (chromatin) can often be
seen at one end of the vacuole. Note also the granules and
small vacuoles that can often be seen in the substance of the
cytoplasm. Stain with Schulze’s solution. The cell wall stains
blue (cellulose).

(3) Trace the method of budding by which the yeast cells
multiply and draw examples of different stages.

(4) Smear a drop of yeast culture thinly over a perfectly
clean coverslip and dry the smear over the flame of a lighted
match. Place five drops of methylene blue in a clean watch-
glass and fill it up with water, Immerse the coverslip in the

PRACTICAL WORK 137

stain and leave it there for one minute. Take it out and wash
by gently moving the coverslip in water. Dry both sides of
the coverslip with blotting paper. Find out on which side the
smear is, and place the coverslip, smear downwards, on a small
drop of dilute glycerine on a slide. Examine with the high
power.

B. Tuick CELLULOSE AND MuciILacinous CELL WALLS.

(1) Examine in water sections of the date-stone (seed of the
date-palm) cut so as to show the elongated cells in transverse
section. Note the very thick cell walls. These are penetrated
by pits running from each cell cavity and ending at the middle
lamella of the cell wall (the middle lamella was the original
thin wall separating two adjacent cells before thickening took
place). The pits of adjacent cells are always exactly opposite
one another.

Mount the section in a drop of Schulze’s solution, and leave
for ten minutes. Then note that the mass of the cell wall is
stained purple (cellulose), while the middle lamella remains
unstained or only faintly stained. The mass of cellulose is
converted into sugar when the seed germinates and the sugar is
used for the respiration and growth of the young plant.

(2) Examine a dry seed of the flax plant (linseed). Note
the sharp outline of the opaque seed. Now put the seed in a
watchglass with a little water. The outline of the seed is soon
surrounded by a transparent fringe. Compare the feel of the
dry and the wet seed between the fingers.

Now examine a thin section of the seed dry under a coverslip,
and, running inadrop of water, watch the swelling of the surface
layer of cells. The middle lamelle of the walls separating the
cells can be distinguished from the swollen mucilaginous mass
of the walls. Note also the cuticle bounding the walls on the
outside. Methylene blue will stain the mucilage and render it
more easily visible.

CHAPTER IX

BACTERIA

BACTERIA are the smallest of known organisms, but
they are of quite extraordinary importance in the
economy of nature, an importance only exceeded by
the green plants. While the latter form the basis of
all life by building up living substance from inorganic
materials, the former destroy dead organic matter,
breaking it down into simpler forms and eventually
into inorganic substances which are available for the
food of green plants. Bacteria are also of direct prac-
tical importance to men, not only because some of them
cause various deadly diseases, but because others are
used by him, as yeast is used, to carry out fermentations
—not alcoholic fermentations, but processes of the same
general nature, such as the “ripening ”’ of cheese, the
“curing ”’ of tobacco, and so on. There are a large
number of different species of bacteria known, and
certainly far more exist than have yet been recognised.

Size, Shape and Structure.—The diameter of an
average bacterial cell is only about 1p, far less than
that of other unicellular organisms. There certainly
exist, however, living organisms which may probably
belong to the Bacteria and which are too small to recog-
nise under the highest powers of the microscope.t Thus
we know that yellow fever, and also swine fever, are

t It will be remembered that an object less than about -15 yin
diameter cannot be clearly defined under the highest powers of the
microscope (p. 50). R

13

FORM TYPES OF BACTERIA 139

caused by different specific living organisms which are
invisible and which will pass through the meshes of a
porcelain filter that excludes any visible bacterium. It
is at least probable that many different kinds of such
ultramicroscopic organisms exist, for there is room
between the lower limit of microscopic visibility and
the size of a complex protein molecule for a collection
of such molecules to be associated into a system which
would form an ultramicroscopic protoplasmic unit with
specific characters.

The visible bacteria may be classed in two groups—
a group of simpler forms which are the more numerous,
and a more highly developed group with which we need
not concern ourselves here. The simpler forms may
be roughly classed according to their shape into three
types: (1) the coccus, which is spherical, on the average
Ip in diameter ; (2) the bacillus, a straight cylindrical
rod, rp or less in diameter and up to about 10 p long ;
and (3) the spivillum, a curved rod which may be spirally
coiled like a corkscrew. Some bacteria, however, are
not of these shapes, but may be oval in outline, etc.
(see Fig. 15). When bacteria were first beginning to
be systematically studied, sixty or seventy years ago,
these ‘‘ form types’’ were largely used as generic names,
the different species being distinguished according
to the habitat or activity, and some of these species
are still recognised, e.g. Bacillus coli (which lives in
the human intestine), Bacillus anthracis (which causes
the disease anthrax), and so on. It is not, however, the
form of the bacterial cell but its specific activity
that is its most important feature, and many new
genera have been established.

Each bacterial cell consists of a minute mass of
protoplasm in which a separate nucleus cannot be

140

BACTERIA

detected. The whole of the protoplasm stains strongly
with the ordinary nuclear stains, and it is likely that
what corresponds with the chromatin of the ordinary

Fic.

15.—Various forms of Bacteria. x 1,000. A, Staphylococci
(groups) and Streptococci (curved chains) from pus (the large
spherical body is a pus corpuscle). B, Nitrosomonas, a nitrifying
bacterium from soil, with single flagellum. C, Bacillus typhosus
(typhoid), showing flagella. D. Bacillus anthvacis (anthrax),
showing spores (one in the centre of each cell). E, Bacillus
tetani (tetanus). Note the spherical spore at one end of and
much broader than the cell. F, Cholera spirillum, with a flagellum
at each end of the cell. G, Spivochete pallida (syphilis spirillum).
After Muir and Ritchie.

cell nucleus exists in the bacterial cell equally distri-
buted through the cytoplasm. The protoplasm is
surrounded by a membrane of gel structure (as is always

MOVEMENT 141

the case on the surface of a protoplasmic unit). This
is not composed of cellulose, and is not to be compared
with the cell wall of an ordinary plant cell. In some
kinds of bacteria the colloid membrane swells very
greatly in the liquid medium inhabited by the organism,
and thus the individual cell, or group or colony of
cells, may become surrounded by a thick mucilaginous
investment, making the whole colony quite visible to
the naked eye and slimy to the touch. A colony of
this kind is called a zooglea.

Flagella or cilia (extremely delicate threads of proto-
plasm arising from the surface of the cell, singly or in
groups, and projecting into the surrounding medium)
may be detected in many species by special methods
of staining (Fig. 15, B, C, F).

Movement and Response to Stimulii—Many kinds
of bacteria swim about actively in a liquid medium,
in most cases no doubt by the beating of their flagella,
for these have never been detected in the non-motile
forms. Some bacteria, however, especially the spirilla,
appear to move by wriggling their bodies (rather as
an eel moves), i.e. by the contractility of the cell proto-
plasm as a whole.

Motile bacteria move, as a rule, in response to chemical
stimuli, i.e. towards some substances and away from
others (positive and negative chemotaxis). A good
example is Bacterium termo, mentioned on p. II3 as
specially sensitive to free oxygen and used in Engel-
mann’s experiment to pick out the particular rays of
the spectrum in which the chloroplasts liberate most
oxygen.

Nutrition.—The foods of bacteria are very various
both in kind and in chemical nature, and their habitats
naturally correspond with these different foods. Some

142 BACTERIA

species feed directly on the proteins of living animals
or plants, some on various solid or liquid organic sub-
stances in the living body or outside it. Some can
get their nitrogen from comparatively simple organic
salts like ammonium tartrate, just as yeast can;
others get it from even simpler salts ; others, again, can
use the free nitrogen of the air. A few can form
their protoplasm from inorganic substances alone, as
green plants can, though they do it in quite a different
way, using the energy liberated in the oxidation process
instead of the energy of sunlight. The great majority,
however, feed on more or less complex organic sub-
stances, from which they obtain energy for growth and
multiplication.

The actual form in which the food is absorbed by
the bacterial cell must of course be liquid or gaseous
—in the great majority of cases it is liquid. That is
why we class bacteria as plants, and, so far as their
nutrition is concerned, with the great group of colour-
less plants known as the fungi. But both in structure
and affinities (by which we mean descent) the bacteria
are very different from and most probably have no
connexion with the fungi proper. Bacteria are most
probably more or less directly derived from the most
primitive forms of life, of which we have no direct
knowledge. We must, however, conceive of primitive
organisms as consisting of very minute forms living in
water, and obtaining their energy in different ways—
from light, from chemical energy such as that liberated
in the oxidation process, or from ingesting other minute
organisms. From this undifferentiated group the green
plants developed along one line ; the animals, with their
habit of feeding on other organisms, along another ;
while the bacteria as we know them to-day are a highly

REPRODUCTION AND SPORE FORMATION 143

specialised group, most of which feed on liquid organic
substance, while retaining an exceedingly simple struc-
ture. Bacteria in the same forms shown by existing
species have been found preserved in: fossils many
millions of years old.

Reproduction.—The simpler bacteria multiply by
simple division of the cell and separation of the daughter
cells so formed. Sometimes the cell grows to a larger
size than the normal before division; in other cases
the cell divides and each daughter cell subsequently
grows tothe normal size. The daughter cells of a bacillus
frequently remain together for a time after division, so
that lines of cells having the form of jointed rods are
produced.

In most bacteria growth and multiplication proceed
with great rapidity. Under very favourable condi-
tions a bacterium may reach maturity and divide in
twenty to thirty minutes. If division takes place only
once an hour, seventeen million individuals will have
arisen from a single cell in twenty-four hours. The chief
factors which control the rate of multiplication are
temperature and food supply.

Spore Formation.—tIn certain species of the simpler
bacteria, chiefly bacilli, spores are produced under
certain conditions. These spores are formed in the
same sort of way as yeast spores, by the aggregation
of the protoplasm at a given spot in the cell, the proto-
plasm of the spore becoming surrounded by a dense wall.
As a rule only one spore is formed in each cell, either
in the centre or close to one end. When the spore
comes into conditions suitable for growth, the spore wall
is split, and a new vegetative cell protrudes, assuming
the form characteristic of the species.

Bacterial spores are by far the most resistant form

144 BACTERIA

of living protoplasm known. Many can withstand
for some time exposure to dry heat of considerably
over 100°C, while most vegetative forms are quickly
killed at 50°C. In some cases spores can even with-
stand immersion in boiling water for several minutes,
while they may remain alive in a dry and inactive
condition for years. The great difference between the
powers of resistance of vegetative cells and spores is
illustrated by the anthrax bacillus (B. anthracis).
The active vegetative cell is killed by two minutes’
exposure to I per cent. carbolic acid, while spores
resist immersion in the same strength in some cases
for fifteen days. Vegetative forms can, however, with-
stand extreme cold for a long time, for instance the
temperature of liquid air (— 190°C.) for twelve
months.

Dispersal.—Spore formation is not a means of muilti-
plication in bacteria any more than it is in the case of
yeast. Only one spore is normally formed in the bac-
terial cell, and the growth and division of the organism
is completely arrested until the spore germinates.
The spore condition is the vesting stage of the bacterium,
enabling it to withstand conditions unfavourable for
vegetative growth and multiplication. Spores are, of
course, an important means of dispersal for the forms
which produce them, since they are passively carried
about in currents of air without injury, whereas vegeta-
tive bacterial cells will sooner or later be killed by drying
up. But only comparatively few species of bacteria
are known to produce spores, most kinds being dis-
persed entirely by means of the ordinary cells. Bac-
teria often get into the air by detachment of minute
particles from the surface on which they are growing.
Such particles form part of the dust of the air, and prac-

HABITATS AND LIFE CONDITIONS 145

tically all air at low levels, except over mid-ocean,
contains more or less dust. Dust is carried about in
currents of air, and is often blown by the wind for very
long distances. When the air becomes comparatively
quiet, the particles drift slowly down and settle, Any
living cells which it contains, vegetative bacterial
cells, bacterial spores or fungal spores, will grow if
they find themselves in suitable conditions of moisture,
food supply and temperature. In such conditions
spores will germinate, and such vegetative cells as have
not been killed by drying up will grow and divide.

Habitats and Conditions of Life.—This is why all
dead bodies exposed to air putrefy and eventually fall
to pieces, provided they remain moist and not too
cold ; and why all milk, beer or wine similarly exposed
turns sour. These processes all depend on the activity
of the living protoplasm of bacteria. Each is carried
out by particular species only, and cells or spores of
some of these are always found in the dust of the air,
so that they are certain to fall sooner or later on to the
surface of the dead body or into the organic liquid.
These processes go on more rapidly at fairly high
temperatures, simply because such temperatures are
the most favourable for bacterial growth and meta-
bolism; and they are suspended at o°C. or below
(though the bacteria are not killed), because protoplasm
cannot work at the freezing-point of water.

In order to feed, live and multiply, each species of
bacterium must have the appropriate conditions for
that species, the right sort of food, the presence (or
absence) of oxygen, a sufficiency of water in the medium,
and a suitable temperature. The temperature relations
of bacteria are remarkably wide and various. Most
species live at the ordinary temperatures which prevail

10

146 BACTERIA

in the part of the world where they are. In temperate
climates few species will grow at temperatures above
30° C., though some occur in manure heaps, etc., whose
optimum? is as muchas 70°C. Species which are para-
sitic on warm-blooded animals live best at 37°C., and
some of them die at temperatures less than 20° C.

Sterilisation.—If all bacteria and their spores are
to be destroyed in a given enclosed space, the air in
that space may be raised to a temperature which they
cannot survive. This can usually be effected by dry
heating to 160° C. for half an hour or to 180° C. for ten
minutes. If a liquid is to be similarly “ sterilised,”
it may be boiled in a flask for several minutes, with a
cotton wool plug inserted in the neck of the flask.
The contents of the flask will then be thoroughly cleared
of active cells. The plug acts as a filter to prevent the
entrance of cells or spores during cooling when air is
drawn into the flask through contraction of the cooling
air inside. Spores in the liquid may, however, survive
the boiling. Heating with steam at high pressure
(which raises the temperature of the steam) at 115°C.
for ten minutes, or 120° for five minutes, will kill any-
thing, even the most resistant spores.

A flask containing a fermentable or putrescible
liquid, i.e. a liquid in which bacteria can grow and
bring about changes in the chemical composition,
thus plugged and sterilised, can be kept for any length
of time without change. If the plug is removed at
any time fermentation will at once begin and the charac-
teristic bacteria will be found in the liquid. This is
a clinching proof, originally due to the great French
chemist and biologist Pasteur, that all such changes
are due to the activity of micro-organisms.

t Most favourable temperature.

STERILISATION AND PURE CULTURE 147

An alternative method of sterilising a liquid which
avoids the use of heat, and has also the advantage of
excluding the bodies of bacteria, is by filtering through
unglazed earthenware. The liquid to be sterilised is
forced under pressure through the filter, whose pores
must be so small that they will not allow the smallest
bacterium to pass. To be efficient such a filter must
be made of the finest ‘‘ china clay.” But-in practice
sterilisation by filtration is uncertain, because of in-
equalities in the size of the pores and imperfections in
the filter. Such filters are often attached to domestic
water taps to make sure that drinking water contains
no harmful bacteria.

Pure Cultures.—In the early days of the study of
bacteria it was impossible for the observer to be sure
that he was not dealing with a mixture of different
kinds. The cells in a living culture of bacteria might
show various forms, or bring about various effects
on the culture medium from time to time. This might
be due to the powers of a single kind of bacterium,
or on the other hand to the gradual disappearance of
one species and the multiplication of another (which
was also present) as the chemical constitution of the
medium was altered owing to the effect of the bacteria
upon it. No increase of accurate knowledge of the
different kinds of bacteria was possible under these
conditions. :

The discovery of the possibility of complete sterilisa-
tion of a medium by Pasteur not only proved that all
changes of the nature of putrefaction and fermentation
were brought about by living organisms, but it enabled
perfectly pure cultures of a single kind of bacterium to
be made and kept. The making of pure cultures for
the study of bacteria was introduced about the year

148 BACTERIA

1880 by the great German bacteriologist, Koch, and has
been an indispensable means of developing the vast
modern science of bacteriology. The various technical
methods of making pure cultures are based on the fact
that if a small quantity of a liquid containing bacteria
is diluted with a large excess of sterile water and the
bacteria evenly distributed through it, each drop of
the mixture will contain only a few or even a single
organism. Such a drop added to a sterile culture
medium will probably give rise to a growth consisting
of one sort of bacillus only. In practice it is more
convenient to immobilise the bacteria in the diluted
mixture by making it with a warm gelatine or agar
jelly, which will set when it is cold. The individuals
then multiply and give rise to visible ‘ colonies,”
each originating from one organism, which can be trans-
ferred to other sterile media, and the forms of the cells
and the effects of their growth on the medium can be
studied at leisure.

Genera, Species and ‘Strains’? of Bacteria.—The
original ‘‘ genera’’ of bacteria, based on form names
such as bacillus (rod), coccus (sphere), spirillum (spirally
curved rod), are still to some extent retained, but have ’
been considerably added to as knowledge has increased,
and the present classification is partly based on form
and partly on activity. Thus rod forms are still called
Bacterium and Bacillus, the genera of cocci are dis-
tinguished by prefixes (Micrococcus, Stveptococcus—
cells arranged in chains, Staphylococcus—cells arranged
like bunches of grapes, etc.). Many of the simpler
spiral forms are placed in Vibrio and Spirtillum, while
the long spiral forms which move by contractility of
the body protoplasm and have no flagella are placed
in Spivochete, Other genera are Sarcina (a coccus

ee

SPECIES AND ‘‘ STRAINS ” 149

form growing in rectangular masses), Nitrobacter (oxi-
dising nitrates in soil), etc.

The species of bacteria are often called after the name
of the discoverer, or from their form, colour, activity
or habitat: in the case of the pathogenic (disease
producing) forms, the species is often called after the
name of the disease, e.g. Bacillus tuberculosis, the form
causing phthisis and other forms of tuberculosis. Some
species are so closely alike in form that they can only
be distinguished by their effect upon the medium in
which they are cultivated, or upon the animal in which
they live. Thus Bacillus typhosus of typhoid fever
resembles so closely B. coli communis, a regular inhabi-
tant of the human intestine, that they can only be
distinguished by the wide difference in their actions
(for instance whether they can or cannot ferment
various kinds of sugars), the former producing a dan-
gerous disease through the substances it excretes.

So far as we know the species of bacteria are as
constant as those of other organisms, like always pro-
ducing like, but different “‘strains’’ belonging to the
same species may differ rather widely in their activities ;
for instance one may be specially virulent as a disease
producer, while another may give rise to quite mild
symptoms, so that, as in the higher plants, we have
what are called “ aggregate species’ and “‘ elementary
species’ (pure strains). How far strains of widely
different activities are produced solely by the effect
of changed conditions, we do not certainly know.
Changed conditions undoubtedly do produce great
differences ; thus by growing Bacillus coli in a culture
medium containing a trace of carbolic acid it is easy
to obtain a race which is much more resistant to the
disinfectant action of the phenol than is the normal

I50 BACTERIA

species. Among the enormous number of kinds of
bacteria, a few have become parasitic on the higher
animals and cause disease. Hence, as in the past,
new diseases may make their appearance when suit-
able circumstances occur, by the adaptation of a pre-
viously harmless form to new conditions in an animal
body, where it produces poisonous substances, giving
rise to the symptoms of the disease.

Respiration and Fermentation.—Many bacteria, like
the vast majority of animals and plants, can only
live in the presence of free oxygen. These are called
aerobic * forms (cf. p. 132). Some, on the other hand,
like yeast, can do without free oxygen for a time, and
others, again, cannot live in its presence (anaerobic
forms), and obtain their energy by splitting up organic
substances without oxidation.

Bacteria which live on complex organic substances,
as most of them do, nearly always cause profound
changes in the medium they inhabit. They break
down the organic substances of the medium (pre-
sumably by the excretion of enzymes) just as yeast
breaks down sugar. The term fermentation is technically
applied to all such processes, and is not confined to
alcoholic fermentation as in popular usage. Different
kinds of bacteria ferment different substances. Thus
certain bacteria found in milk (Bacterium acidi lactici
and many others) ferment milk sugar (lactose), splitting
one molecule of lactose (with a molecule of water)
into four molecules of lactic acid :—

C,5H2.0,, + H,O = 4C,H,O;
lactose water lactic acid
and the accumulation of the lactic acid in the milk
turns it sour. Alcohol itself is split up by certain
t Living in air, from Greek dip, air, and Blog, life.

VARIOUS FERMENTATIONS I5I

bacteria, especially the ‘‘ vinegar plant,” Bacterium
aceticum, in the presence of oxygen, into acetic acid
and water :—

C,H,O + O, = C,H,0, + H,O

ethyl alcohol - acetic acid

Their activity is the cause of the souring of wine or
beer if left standing in the air, and is used in the making
of vinegar. The “ripening ”’ of cheeses, the “ curing ”’
of tobacco, the “ retting”’ of flax (the isolation of the
fibres of the flax plant when the stems are placed in
water), and many similar processes useful to man
depend on fermentations carried out by different
bacteria; and so do many others which are not so
welcome, such as the turning rancid of butter and the
turning bitter of sweet fruit such as a cut melon, as well
as the putrefying of meat, etc.

Putrefaction and Subsequent Changes in Nitrogenous
Organic Substances.—-Putrefaction is the general name
given to various fermentations of the highly complex
proteins which form the basis of protoplasm. These
fermentations result in the formation of a great number
of compounds of varying degrees of complexity contain-
ing nitrogen and sulphur, among which are the evil-
smelling gases so characteristic of the putrefactive
processes. Putrefaction is carried out by various
bacteria which are very widely distributed and many of
which form spores, so that practically no dead body
on the earth’s surface can escape them. The process
takes place in all dead substances rich in proteins which
remain moist and not too hot or too cold to allow
of the activity of the putrefactive bacteria. Freezing
arrests their action, and so do the antiseptic substances
formed in peat bogs. That is why meat is kept in cold

152 BACTERIA

storage, and the bodies of animals which have been
engulfed in bogs are often found perfectly preserved
after many years.

After the first putrefactive decompositions other
kinds of bacteria reduce the products of putrefaction
to simpler substances and so on until ammonium
(NH,) compounds are produced, together with carbon
dioxide and water. This process is noticed in the
strong smell of ammonia (NH,) arising from a heap of
stable manure at a certain stage of decomposition.
The débris of plants—fallen leaves, twigs, etc.—undergo
similar decompositions, but since a smaller proportion
of their substance is formed of proteins and there is
a much larger proportion of carbohydrates (cellulose,
etc.), the result is more carbon dioxide and water and
less ammonia than in the case of animal substances.
Also the early stages of decomposition of plant remains
are largely carried out by fungi, though bacteria also
play a part. The decaying plant débris, together
with any decaying animal bodies or other animal sub-
stances present, form the humus of the soil. Humus is
always in process of reduction to the simple chemical
substances named above, together with various mineral
salts, and is as constantly replenished by fresh organic
débris.

When the organic substances have been brought
into the state of simple salts containing nitrogen,
other kinds of bacteria take up the work, some (Nitro-
somonas) converting the ammonium salts into nitrites,
i.e. salts formed from nitrous acid (HNO,) and bases
present in the soil, others again (Nitrobacter) carrying
out a further oxidation and producing nitrates, formed
from bases and nitric acid (HNO,). This. process of
the oxidation of ammonium compounds to nitrates,

CIRCULATION OF NITROGEN 153

which takes place in two stages, each effected by a special
set of bacteria, is called nitrification.

Nitrates are the form in which green plants mostly
absorb their nitrogen supply through their roots, so
that the whole series of bacterial activities described
results in providing green plants with exactly the kind
of food they require, and at the same time gradually
remove dead bodies and organic débris from the surface
of the earth and make room for fresh life.

During the progress of conversion of complex nitro-
genous compounds into simple ones a great deal of
nitrogen is lost to the air in the form of the free gas,
but certain soil bacteria (e.g. Azotobacter, Clostridium)
are able to fix this and incorporate it into the substance
of their bodies. Other bacteria (Pseudomonas) which
live in tubercles that are formed on the roots of certain
plants such as those of the pea family (Leguminosa) are
also able to absorb free nitrogen from the air, and the
green plant in which the bacterium lives gets the benefit
of this, eventually breaking up and absorbing the
products of the dead bodies of these tubercle bacteria.
Thus a certain amount of the nitrogen lost to the air
during the process of decomposition is again brought
into the form of living protoplasm in these ways.

Circulation of Nitrogen and Carbon in Nature.—In
all these ways the nitrogen contained in the proteins
of the protoplasm and other nitrogenous substances of
plants and animals is converted during their decay
into a form in which it can be used again by green
plants, so that a constant circulation of nitrogen is always
going on in nature, through the agency on the one hand
of various kinds of bacteria which break down the
complex substances into simple ones, and on the
other of green plants which build them up again into

154 BACTERIA

complex ones. In this circulation the animals play
a comparatively small part, for they merely convert
the plant proteins into the proteins of their own bodies.

Besides the circulation of nitrogen there is also a
circulation of carbon, which appears in the form of
carbon dioxide, not only as the result of the respiration
of plants and animals, but also during decomposition
of the complex carbon compounds of animal and plant
bodies, notably the carbohydrates found in the cellulose
of plants, but also the fats and proteins. Distinct bac-
teria carry out the disintegration of the cellulose of
which humus is largely composed, and this process
ultimately results in the production of carbon dioxide
and water (as well as hydrogen and marsh gas). The
carbon dioxide is then, as we know, used by the green
plant to build up its body.

Pathogenic (Disease-producing) Bacteria.—A small
proportion of the total number of species are partly
or wholly parasitic on plants and animals in which
they cause disease. Partial parasites (such as Bacillus
tetani) live naturally in soil containing organic matter
and only occasionally gain entrance to the animal
body. Complete parasites (such as the spirochete of
syphilis, or the coccus of cerebro-spinal fever) have no
life outside the body, and their existence depends on their
being passed on directly from one person to another.

Bacteria often invade living animal bodies by way
of wounds, and may be a great danger to life, for in-
stance on battlefields, owing to the poisons or toxins
which they produce. The bacteria are carried into
the wound either by the instrument causing the wound,
or from the clothes, or by dirt afterwards getting on
to the wounded surface. The most important are the
species of Streptococcus and Staphylococcus ; and these

DISEASE BACTERIA 155

may get into the blood stream through the wound
and thus be carried through the body, bringing about
septicemia (blood poisoning).

The immense progress of modern surgery has been
made possible, first by the use of antiseptics, i.e. by treat-
ing the wound with a substance (such as weak carbolic
acid) which destroys the bacteria. The introduction
of the use of these in surgery was due to Joseph Lister
(afterwards Lord Lister), and resulted in a great decrease
in the death-rate of surgical patients from blood poison-
ing. Later on aseptic methods of surgery were intro-
duced, in which the surgeon’s hands, instruments and
dressings used are carefully sterilised, and the wound
thus kept free from all bacteria. :

Besides the Streptococci and Staphylococci, other
kinds of bacteria may be introduced into wounds con-
taminated with soil rich in dung, and cause such specific
diseases as tetanus (Bacillus ‘tetani) and gas-gangrene
(Bacillus weilchit), so called because of the large amount
of gas produced in the wound by the action of the
bacillus on the tissues. These are both spore-producing
bacilli which inhabit the intestines of horses and cows,
where they do no harm. The spores persist for a long
time in the manured soil, and germinate when they
enter the wound, producing active cells which form
dangerous toxins that often lead to death.

In the case of most “infectious ’’ diseases, human
beings directly or indirectly infect one another with
spores or active cells of various pathogenic species,
and these give rise in the body to different specific
toxins which cause the symptoms of the corresponding
disease. Some of the most serious among them are
tuberculosis in its various forms (Bacillus tuberculosis),
syphilis (Spivochete pallida), typhoid or enteric fever

156 BACTERIA

(Bacillus typhosus), epidemic meningitis or ‘‘ spotted
fever’’ (Meningococcus), plague, cholera, diphtheria,
influenza, etc.

PRACTICAL WORK.

BAcTERIA.

Note that all bacteria are extremely small, and very careful
focussing with the high power is necessary to find them.

(1) Bacteria in Potato Extract—Mount a drop of the turbid
liquid from the surface of water in which a slice of potato has
been soaking, put on a coverslip and observe the numerous
rod-shaped aerobic bacteria (Bacillus mesentericus) in motion ;
also from the deeper layer of water the rod shaped cells, tapering
at the ends of the anaerobic Clostridium butyricum. Note the
different staining of the two with iodine.

(2) Note the elongated form and the rapid spiral movements
(like those of eels) of the Spiviila from horse dung which has been
kept in water for a week. Kill with iodine solution and again
observe.

(3) Bacillus subtilis (hay bacillus)—(a) Mount a drop of hot
water infusion of hay, and observe the delicate rod-shaped
bacilli in motion. (b) Examine a preparation showing the same
species fixed and stained. Compare their size with that of a
yeast cell by means of the micrometer eyepiece.

(4) Stain a drop of each of the bacterial cultures (2) and (34)
by the coverslip method used for yeast (p. 136 (4)).

(5) Examine the sections of a nodule of the root of lupin (or
other leguminous plant), and note the ‘' bacteroids’ (dead
bacteria) in the central tissue.

(6) Remove the cover from the dish of sterilised gelatine or
agar jelly provided, thus allowing the dust from the air to settle on
it. Replace the cover after twenty minutes, and label the dish
with your name. The dishes should be examined again after
a week’s interval.

Demonstration Specimens.—Examine carefully the demonstra-
tion slides, tubes and plates of various bacteria.

[A few representative types of culture and stained preparations
of various pathogenic bacteria should be obtained for exhibition.
They can usually be borrowed from a pathological laboratory.
Cultures and slides of Bacillus tetani and B. anthracis, showing
spores of B. tuberculosis, and of Staphylococcus and Streptococcus
are suggested as suitable.]

The student should be warned not to handle the cultures.

CHAPTER X

SAPROPHYTIC FUNGI

Mucor AND PENICILLIUM.

Tue Fungi differ from the Bacteria in being plants
whose bodies are composed, not of single cells and
cell colonies, but of delicate branching threads which
grow in liquid or solid “ substrata’ containing organic
substances which they can absorb as food, and often
sending branches into the air. This system of branch-
ing threads is called the mycelium, and the individual
branches are called hyphae. The mycelium of many
species is large and conspicuous. As a general rule
fungi can only grow actively in damp situations, and
they are commonest when and where the air contains
a large amount of water vapour.

Fungi may be either saprophytic or parasitic (see
p. 78), ie. they may either take their food exclusively
from the tissues of a living organism (plant or animal)
or they may take it exclusively from a non-living
organic substance, e.g. the dead bodies of organisms,
dung, milk, cheese, jam, leather, etc., or from humus
(see p. 154). But this distinction is not absolute. For
instance a parasitic fungus may begin its life in the
living body of an animal or plant and continue to
grow after the “host”’ is dead. Or a fungus which

is ordinarily a saprophyte may, on occasion, attack
187

158 SAPROPHYTIC FUNGI

the living body of an organism. A fungus which is
strictly confined to parasitism is called an obligate
parasite, one which can live either saprophytically or
parasitically is a facultative parasite.

Moulds.—This name is commonly given to saprophytic
fungi which grow on organic substances, provided they
contain enough water, such as bread, jam, cheese,
leather, tobacco, etc., on which they produce whitish,
yellowish, greenish or bluish mycelia. They flourish
particularly when the air is very damp and warm.
In damp tropical climates any such substance will
become covered with mould in a day, if it is eapesee
to the air.

Mucor.—One of the commonest genera of white
moulds is Mucor, which forms a white, fluffy feltwork,
rather like cotton wool in appearance, in and on the
surface of such substances as those mentioned. If a
piece of this feltwork is examined under the micro-
scope it is seen to consist of delicate branched threads,
forming the mycelium or body of the fungus. The
mycelium of Mucor is a branched non-cellular tube,
the wall enclosing the cytoplasm, which contains
numerous very minute nuclei, though these cannot be
seen except by special methods of staining. The
cytoplasm encloses many vacuoles, which are smaller
towards the growing tips of the hyphe. The actual
tip of each hypha is rounded and the protoplasm at
the apex is without vacuoles. As the hypha grows
in length the nuclei at the tip constantly divide.

The mycelium branches profusely, many of the
branches penetrating the substratum on which the
mould is growing, and absorbing liquid food. In this
way it acts like the root of a higher plant, except that
here the food is largely organic. The essential ele-

VEGETATIVE GROWTH 159

ment carbon is taken by the fungus mainly in the
form of a soluble carbohydrate such as_ sugar.
Nitrogen, sulphur and the other necessary elements
are absorbed in the form of various salts, together
with water, although nitrogen is sometimes absorbed in
the form of very complex compounds. The mycelium
also branches in the air above the substratum, but
its aerial growth is necessarily strictly dependent on
the extent of the growth, and therefore of the absorb-
ing surface, in the substratum.

Here for the first time among the types we have
studied we meet with the fixed and branching habit
characteristic of plants. The “food” of Mucor exists
all round it in the bread or jam or other organic sub-
stances which forms its substratum, and the branch-
ing habit of its “‘ root’ puts it in touch with as much
food as possible.

Oxygen for respiration is obtained from the air,
and the necessity of free oxygen is illustrated by the
fact that the mould forms a cushion only on and in
the top layer of jam in the jam-pot. It is probable
that the mycelium cannot normally penetrate into the
deeper layers of jam because of the deficiency of free
oxygen away from the air. Various species can,
however, like yeast, live anaerobically for a time and
can produce alcohol and carbon dioxide by splitting
sugar molecules without oxidation.

So long as the fungus can get “food, water and
oxygen it continues to grow, i.e. the nuclei divide
and the protoplasm increases in amount at the tips
of the hyphe. The wall covering the tip is con-
tinuously pushed out and new wall substance is con-
tinuously added to make good the thinning, just as
in the growth in size of the embryonic cell orale

160 SAPROPHYTIC FUNGI

higher plant (p.107) Every now and then the grow-
ing hypha branches, i.e. a new branch of the tube is
pushed out not far from the tip, and a new growing
hypha is thus produced. This continuous indefinite
growth and branching of the plant body is very
characteristic of typical plants as opposed to the
limtied compact growth of the typical animal (see
Chapter I, p. 22). From what is known in the case of
other fungi the growing tips of the hyphe are
probably sensitive to the stimuli of food substances
and of free oxygen, i.e. they will turn and grow
towards regions whence these substances are diffusing
towards the hypha.

Reproduction.—(1) Spove formation. In place of
the simple cell division ot Ama@ba, Protococcus and
bacteria, and the simple budding of yeast, the main
method by which Mucor multiplies and produces
new individuals is the formation of spores, produced
in special chambers (sporangia) cut off from the general
mycelium. Special hyphe (sporvangiophores) arise as
branches of the mycelium and typically grow straight
up into the air. These sporangiophores are positively
phototropic, ie. they grow towards brighter illumi-
nation. This generally involves growing away from
the substratum, so that the sporangium formed at the
end of the sporangiophore is placed in the best position
for free distribution of its spores into the air.

The sporangium itself is formed by the swelling up
of the tip of the hypha (Fig. 16, a), so that under a
hand lens the sporangiophore looks like a minute pin
with a globular head (the sporangium). A cross wall
is formed, cutting off the head from the sporangiophore,
and the protoplasm of the sporangium breaks up into
a sfimber of-tiny multinucleate cells, each of which

“ae

SPORE FORMATION 161

secretes a wall upon its surface. Directly the proto-
plasm of the sporangium breaks up, its turgidity, which
has been maintained by the osmotic pressure in the
vacuoles against the semi-permeable cytoplasm sur-
rounding them, is destroyed, and owing to the osmotic
pressure being still maintained in the sporangiophore
on the other side of the cross wall, this is pushed into
the cavity of the sporangium and pressed against and
into the mass of spores (Fig. 16,6). This pressure
squeezes the spores against the wall of the sporangium ;
and eventually, with the progressive drying of the
wall by evaporation (since it is no longer in contact
with the protoplasmic water), the wall becomes brittle
and is burst, the spores being scattered into the
air as a fine dust (Fig. 16, c).

The spores float in the air and are easily carried
about by slight currents. The air of rooms and of
towns generally contains innumerable spores of Mucor
and other moulds, as well as bacterial cells and spores,
and these gradually settle on all objects when the air is
quiet. Thus any suitable organic substance, provided it
is moist, will develop moulds or colonies of bacteria,
or both, because the spores will germinate. (or the
bacterial cells grow) on such a substance from which
they can get suitable food. The protoplasm of the
spore absorbs water and food, the wall of the spore
bursts, and the protoplasm, covered by a new delicate
wall (Fig. 16, d), is pushed out, grows and branches to
form a new Mucor plant.

Only the air of regions very remote from human
dwellings, and from the substances on which moulds
and bacteria thrive, is comparatively free from spores,
and is in that sense pure. The purity of the air in
this sense can be measured by the number of mould

II

162 SAPROPHYTIC FUNGI

and bacterial colonies starting on a given surface of
sterilised nutritive agar or gelatine which has been
exposed to the air for a given time.

(2) Chlamydospore formation.—In some species of
Mucor the protoplasm may contract into oval masses
at short intervals along the length of a hypha (for
instance a sporangiophore) the short diameter of the
oval corresponding with or slightly exceeding the
width of the hypha. Each oval mass becomes clothed
with an independent thick wall, which often bulges
out the walls of the parent hypha. The chlamydo-
spores so produced become free by the breaking
up of the wall of the now empty hypha, and are
resistant resting spores which germinate to produce
new plants under favourable conditions. Similar
chlamydospore formation is not uncommon among
fungi.

(3) Conjugation.—Quite a different sort of repro-
ductive process sometimes occurs in Mucor. The tips
of two hyphe arising from the same or from different
branches of the mycelium approach one another, and
come into contact (Fig. 16, e), each swelling into the
form of a club. A cross wall is formed towards the
base of each club-shaped structure, cutting off a com-
partment at the tip (Fig. 16, f). One of these is fre-
quently larger and more swollen than the other. The
walls at the tips which are in contact are now
absorbed and the cytoplasm of the two compart-
ments mingles. The nuclei divide actively and then
conjugate in pairs. The walls now swell up, forming
a nearly spherical structure, and become dark-coloured,
often black, with projecting excrescences on the sur-
face (Fig. 16, h). Eventually the zygote (product of
conjugation so formed) separates from the hyphe on

ZYGOTES OF MUCOR 163

each side. The zygotes are 70 to 80 pw in diameter
and just visible to the naked eye. Inside the dark,
rough outer wall are two layers of inner wall.

The zygote, in the protoplasm of which a consider-
able amount of organic food material, largely. fats, is
accumulated, is capable of remaining quiescent for a
considerable time, the thick outer wall resisting desic-
cation. In a suitable medium it germinates, the
thick outer wall bursting and the thin inner wall being
pushed out by the protoplasm and growing into a
new mycelium which often forms a single sporangium
at once (Fig. 16, 2).

The zygote of Mucor is a resting stage in the life
history, preserving the plant through conditions
unfavourable to active growth in the same way as
the spores of yeast and bacteria; while the spores of
Mucor, though also a resting stage in the sense that
growth of their protoplasm is temporarily suspended,
are primarily a means of multiplying and dispersing
the individuals of the species, being formed in im-
mense numbers. and much less resistant than the
spores of bacteria.

The process of conjugation, with the formation of a
zygote, or product of conjugation, in Mucor is the
first example of,this process we have met with. As
we shall see in later chapters, this process is almost
universal in the organic world, though in some of the
lower forms it is apparently absent. In Mucor the
two conjugating masses of protoplasm are often
exactly alike, though in some cases one hypha is
larger than the other. The conjugation of two equal
protoplasts is called tsogamy.t

In some kinds of Mucor hyphe from the same

1 Greck dooc, equal, and yayéw, marry.

164 SAPROPHYTIC FUNGI

mycelium (even from the same branch) will conjugate,
and in these zygotes are commonly found. In other
kinds zygotes are much rarer, since conjugation will
only occur between hyphe of two different “ strains,”
one of which grows more vigorously than the other.
Here we have a functional differentiation between the
conjugating hypheze which is not quite on a par with,
but shows an interesting parallel to, the ordinary sex-
differentiation, the essential characters of which we
shall have to consider in Chapter XII.

Penicillium (Blue Mould).—This is a common form
growing on substances similar to those on which
Mucor occurs. It forms the well-known mould of
Stilton and Gorgonzola cheese. The mycelium differs
from that of Mucor in being septate, i.e. there are
cross walls at intervals, dividing the mycelial tube
into multinucleate compartments. The characteristic
blue-green pigment is distributed through the cyto-
plasm, and has nothing in common with the chlorophyll
of a green plant.

Spore Formation: Conidia.—Upright, aerial hyphe
arise from the mycelium as in Mucor, but instead of
forming sporangia, each forms several branches at
the same level, the branches continuing the same
general direction of growth but diverging slightly
from one another. At a certain height each of these
branches again in the same way, so that a sort of
compound pencil of hyphe is formed with the tips
of their ultimate branchlets close together on the same
level. From the tips of these branchlets minute
spores called conidia are budded off, and before each
is quite detached another is budded off behind it, so
that a series of parallel or nearly parallel chains of
conidia are produced (Fig. 16, 7). These are very

PENICILLIUM 165

lightly connected, so that a slight jar or a current of
air is sufficient to detach them, and they float off into

Fic. 16.—Reproduction of Mucor. (1) Spore formation (a-d):
a, young sporangium (sp) ; 6, optical section of mature sporangium
(col., columella) ; c, remains of sporangium wall after bursting,
with a few spores still adhering ; d, germinating spore with two germ
tubes. (2) e-h, Conjugation: e, conjugating hyphz in contact ;
f, separation of tips of conjugating hyphe; g, later stage; h,
mature zygote; 7, germination of zygote, germ tube forming a
single sporangium.

Penicillium: j, part of branched conidiophore budding off
chains of conidia.

the air, where they behave as spores, germinating on
any favourable substratum on which they may happen
to settle.

166 SAPROPHYTIC FUNGI

There is also a complicated method of conjugation
which is comparatively rarely met with and will not
be described here.

Other Saprophytic Fungi.—There are numerous other
forms of saprophytic fungi, including many with com-
plicated reproductive bodies (“‘ fruit bodies ’’), of which
the common mushroom (Agaricus campestris) is a
good example. The mycelium lives in the humus of
the soil, particularly in well manured pastures, and
resembles that of Mucor in a very general way, except
that it is septate, as are all the higher fungi. The
mushroom itself is the fruit body, which arises first as
a small weft of hyphe forming a portion of the fila-
mentous mycelium. This increases in size by constant
branching and grows into a small very compact nodule,
which increases in size and complexity, becoming
differentiated into a short stalk and an arched top.
Finally it grows up above the soil by the rapid elonga-
tion of the hyphe forming the stalk and expands the
umbrella-shaped pileus t or topofthe mushroom, From
the tips of the hyphe which run at right angles to
the surfaces of the platelike “ gills’ on the underside
of the pileus there are budded off spores, -four or two
from each hypha tip, each formed on a small projecting
point. The enormous number of spores produced by a
single mushroom may be gauged by placing a ripe
pileus, detached from its stalk, underside downwards
on a piece of white paper and leaving it for a day or
two. On gently removing the pileus a pattern of the
gills will be found designed in streaks of microscopic
spores on the paper.

= ** A woven hat.”

MUCOR AND PENICILLIUM 167

PRACTICAL WORK.
Mucor.

(I) Examine with a hand lens the young mycelium of Mucor
growing in gelatine containing some nutritive substance such as
raisin-extract. Cut out a very small portion of the gelatine
containing the mycelium near its edge, and place it in a large
drop of water on a slide till it becomes nearly liquid. Then
put on a coverslip: squeeze it gently and look for hyphe, first
with the low and then with the high power of the microscope.
Draw a portion of a hypha, including its tip, under the high power,
showing wall, protoplasm containing vacuoles, which are smaller
towards the tip. Note the absence of cross walls.

(2) Examine an older plant (if possible growing in a watchglass
of gelatine) which has produced sporvangia—first with a hand
lens. Then place the watchglass on the stage of the microscope
and focus slowly down with the low power. In this way an
excellent idea of the habit of growth of Mucor is obtained. Note
the spherical sporangia of different ages borne on the summits
of the sporangiophores, and the branching mycelium below.

Mount some of the mycelium, carefully removed in a drop
of methylated spirit to get rid of air, and add a drop of dilute
glycerine. Find and draw various stages in the development
of the sporangia.

(3) Examine a still older plant with mature sporangia. Mount
some as in (2) and draw stages before and after bursting. Note
the columella. Draw also a few spores.

(4) Examine a culture or a prepared slide showing zygotes.
Look for and draw stages in conjugation and the formation of
the zygote.

PENICILLIUM.

(5) Examine Penicillium (Blue Mould) in the same way as
Mucoy—frst in situ with the branched conidiophores rising into
the air. Examine the brushes of radiating conidiophores under
the low power. Examine part of the vegetative mycelium in
gelatine squeezed out in water, and note the segmented mycelium
with pale blue-green cytoplasm. Draw two or three segments
under the high power.

Carefully remove some conidiophores, dip them in a drop of
spirit, and mount in dilute glycerine; examine under the high
power the chains of conidia and their mode of formation by
budding from the tips of the conidiophore branches,

168 SAPROPHYTIC FUNGI

DEMONSTRATION SPECIMENS.

(6) Examine demonstration specimens of representative higher
saprophytic fungi, such as the Mushroom, Puffball, etc. Note
that the conspicuous part of the fungus above ground is a compli-
cated spore-bearing fruit body. The mycelium lives below the
surface in a soil rich in organic matter.

Examine (if available) stages showing the development of
the mushroom from a small nodule on the mycelium.

CHAPTER XI

PARASITIC FUNGI

A PARASITE, as we have seen (p. 78) is an organism
which gets its food directly from another living
organism. Bacteria living in the intestines are scarcely
parasitic in the strict sense, though they actually live
within the body of another organism. Many of them
are harmless saprophytes living on the partly digested
food of the animal, but some cause diseases because
they produce toxins. Other bacteria, however, live in
the tissues, which they break down by means of the
enzymes they secrete, absorbing some of the products :
these are parasites in the strictest sense.

Similarly with the fungi. Many are saprophytes:
some of these, however, are able to attack and break
down the tissues of living organisms (facultative para-
sites); others are exclusively parasites, and some are
strictly confined to one species of host.

Fungi Parasitic on Animals.—Though the great
majority of “zymotic”’ (germ) diseases of man and
the higher animals are caused by bacteria or by pro-
tozoa (unicellular animals), e.g. malaria, sleeping sick-
ness, dysentery, some—especially skin diseases—are
caused by. genuine fungi which form a mycelium.

The skin diseases generally known as ringworm are
among the commonest of these. These are caused by
several different kinds of fungi. Trichophyton, whose

mycelium consists of chains of oval or rectangular
169

170 PARASITIC FUNGI

cells 5 to 8y in diameter, lives in and feeds upon the
layers of living skin cells. These living skin cells are
not on the actual skin surface, which consists of dead
cells constantly produced by division of the living
cells below the surface and rubbed off. The growth of
the fungus often causes local inflammation, and the
raising of the skin in circular pustules corresponding
with the centres of infection. The mycelium grows
outward from these centres, the central parts dying
off and thus producing the ring-shaped structures,
characteristic of the disease.

Ring-shaped structures formed in this way are pro-
duced by many fungi, both parasites and saprophytes,
for instance the “ fairy rings ’’’ often seen in meadows,
and also sometimes by the higher plants, for instance
grasses, which similarly spread outwards from a
centre by vegetative growth, the central parts dying
off. The cause of the dying off of the older central
parts of the plant may be either the exhaustion of the
food supply, or the production of substances which
act as a poison to the plant itself, in the same way
that the accumulation of alcohol and carbon dioxide
in fermenting wort eventually stops the growth of the
yeast plant.

Another form of ringworm is caused by Microsporon,
which attacks the living bases of the hairs of the skin,
in which it spreads, forming a mycelium of rectangular
cells, and causing the hair to break off short. The
mycelium then breaks out to the surface of the hair,
where it forms abundant spores (2 » in diameter) by
thickening of its cell walls. These spores are easily
brushed off and render the disease extremely con-
tagious. A number of allied species live on dogs,
cats, horses, etc., and these are contagious to

FUNGAL PARASITES ON ANIMALS 14t

man, generally infecting the hairs of the scalp in
children.

Other fungal diseases of the skin or mucous mem-
branes are Pityriasis, due to Microsporon furfur, which
forms a spore-producing mycelium on the skin,
especially in phthisical patients ; and “‘ thrush,’ caused
by a fungus which has a septate mycelium forming
white patches on the tongue and on the mucous mem-
brance of the throat, mostly in children, and also
producing spores, round or oval in shape.

“ Blastomycosis ’’ of the lung or kidney is caused
by single cells, which form endogenous spores, like
yeast, and may also produce a mycelium in artificial
cultures. This kind of fungus is also found on the
skin, often following a slight wound, and causing
suppuration (pus formation). Another fungus which
inhabits the lung cavities is Aspergillus fumigatus,
which forms a spore-bearing mycelium and spreads
into the bronchioles (small branching air tubes of the
lungs), This form is generally, but not always, found
in company with disease-producing bacteria. Asper-
gillus also occurs in the external ear.

Saprolegnia and Fish Disease.—A good example of a
facultative parasite is Saprolegnia, which lives sapro-
phytically on the dead bodies or parts of animals and
plants that have fallen into or died in the water, but
which also attacks living aquatic animals, such as
insects, fish and amphibia. A spore germinating on
the body of an aquatic insect, for instance, sends out
a tube which penetrates the tissues of the insect, and
branches to form a mycelium which ramifies through-
out the body. This, of course, causes the death of
the insect, and the fungus lives saprophytically for a
time on the dead tissues. Eventually thick branches

172 PARASITIC FUNGI

of the mycelium are put out into the water, and each
is cut off by a cross wall, forming a sporangium. The
protoplasm of the sporangium then divides to form a
number of spores. These are motile spores called
zoospores,* oval in shape and with flagella attached to
the pointed end. They are set free by the bursting of
the sporangium wall, swim about in the water, and
germinate, producing a new mycelium if they find a
suitable organic substratum, living or dead. Sexual
organs are also formed on the mycelium. These: will
not be described here, but it may be noted that, as
is often *the case in fungi which form sexual organs,
conjugation may not take place, the female cell (egg)
developing into a new plant without fertilisation.
This behaviour is called parthenogenesis.

Saprolegnia sometimes attacks fish, particularly carp
and gold-fish in stagnant ponds, entering by the gills,
which it often blocks up, causing the death of the
fish. According to some authorities it is a species of
Saprolegnia which causes salmon disease, attacking
the gills of the salmon, and sometimes killing them in
great numbers, thus causing extensive damage to
salmon fisheries. Other authorities, however, believe
that bacteria are the primary cause of this disease in the
salmon, which are attacked by Saprolegnia only when
the fish are already weakened by the bacterial attack.

Fungal Parasites of Plants.—Fungi parasitic on the
higher plants are far more numerous in species and far
commoner than those which attack animals. They often
cause widespread and serious damage to crop plants.

Pythium de Baryanum (causing the ‘‘ damping off”’
of seedlings).—Closely sown young seedlings in a
moist atmosphere, such as that of a greenhouse, some-

1 Greek EGov, animal, from the motility of the spores.

““ DAMPING-OFF ”’ OF SEEDLINGS 173

times topple over owing to the dying of the stem
just where it emerges from the soil (Fig. 17,G). This
is called ‘‘ damping off,’ and is due to the attack of
a fungus (Pythium) which has formed an extensive
branching mycelium in the tissues of the seedling.
The mycelium itself ramifies in the air spaces (inter-
cellular spaces) between the living cells of the host
(Fig. 17, A, B), but it pierces the living cells where it
touches them (Fig. 17, B), breaking down their proto-
plasm. The seedling is soon killed and the fungus
lives on for a time in the decaying stem. Eventually
the ends of some of the hyphe swell up to form a
conidium (Fig. 17, A, sp., and C), which is cut off from
the hypha by a cross wall, and becomes detached.
In damp air this may grow out at once to form a new
mycelium which penetrates a fresh seedling, but in
water it puts out a short tube at the end of which
a similar body is formed (Fig. 17, D). This is a
zoosporangium (cf. Saprolegnia) whose protoplasm
divides to form zoospores (z), which on being set free
by the bursting of the sporangium wall swim about
in the water. The zoospores germinate (z,) to form
new mycelia which penetrate fresh seedlings. Zoospores
are never produced except in water.

Sexual organs are also produced. The female organ
is a spherical structure formed by the swelling of the
end of a hypha which is cut off from the rest by a
cross wall. In this spherical structure the protoplasm
contracts to form a smaller sphere (egg). The male
ergan is club-shaped, and is also cut off from the
hypha by a cross wall (Fig. 17, E).

The male organ applies itself closely to the female
organ (a), and a short tube from the former penetrates
the latter (0). Through this the male protoplasm

L

PARASITIC FUNGI

Fic.

17.—Pythium. A, hyphe in tissue of seedling; sp., young
sporangium. 8B, ditto, more highly magnified. The hyphe (A)
have penetrated the cell walls (c.w.) through narrow holes which
the growing tips have bored out. C, branched hypha emerged
into the air, forming sporangia (sp.). D, germination of
sporangium in water. The contents have divided to form a
number of zoospores which are enclosed in a bladder pushed
through a hole in the wall; 2, free zoospore; 4 germinating
zoospore. E, sexual reproduction (conjugation); @, young
female organ (spherical) and male organ (club-shaped) in contact ;
b, penetration of male organ ; ¢, fertilised egg (zygote) surrounded
by thick wall, male organ empty. F, germination of zygote:
in water (similar to D). G, cress seedling attacked by Pythium
just above the soil surface. (After Marshall Ward, etc.)

POTATO BLIGHT 175

passes to coalesce with that of the female. The fertilised
egg (zygote) now secretes a wall (c), and remains for a
time in a resting condition. Eventually it germinates,
forming zoospores in water (F), which germinate, and
if the hyphe of this find a suitable seedling they pene-
trate it through a stoma just above the ground level.

The hyphe of Pythium can only grow in air which
is nearly saturated with water vapour, but these con-
ditions are often realised between the stems of the
seedlings growing in a seed-box which is frequently
watered. When a seedling has been killed the hyphe
grow out from it and easily bridge the space to another
seedling, which they enter and kill, and so on till the
whole crop quickly becomes a rotting mass. The
hyphe of the mycelium are easily visible as very
delicate white filaments, rather like cobweb, stretching
between the seedlings.

Potato Blight (Phytophthora infestans)—This is a
fungus closely allied to Pythium, and often causes
enormous damage to the potato crop. Potato blight
first appeared in Europe between 1845 and 1850 and
ruined the Irish crop, causing serious and widespread
famine. Since then it has always been with us, but
its incidence is much more severe in some years than
in others, according to the weather.

The mycelium ramifies in the tissues of the leaf,
and the diseased patches turn brown as the tissues
die. Under conditions favourable for the spread of
the disease the whole potato haulm is soon reduced to a
rotting mass. If the under side of a potato leaf bear-
ing a patch of the disease is examined with a hand
lens, the edge of the patch is seen to be covered with
very delicate white glistening threads (the hyphe)
which have grown out through the stomata of the

176 PARASITIC FUNGI

leaf (Fig. 18, F). Minute white bodies can sometimes
be seen on the threads. These are the conidia, like
those of Pythium, cut off from the ends of the hyphe
(Fig. 18, E,F, A). They are formed in great abundance
in damp air and are easily carried by the wind from
one plant to another. In water, for instance in a
raindrop or dewdrop on a leaf, the contents of the
conidium break up into zoospores (Fig. 18, B, C), as
in Pythiuvm, and each of these eventually germinates
to form a hypha (D), which infects the leaf by
growing in through a stoma or by piercing the cuticle.
The conidia may also be carried down by rain into
the soil, where they infect the young potato tubers
which are growing beneath the surface. Whether
the fungus can survive the winter in the soil and rein-
fect the seed tubers in the spring is not certainly
known. One means of transmission of the disease
from season to season is by mycelium carried over in
certain of the tubers used for “‘seed.’’ Such tubers
are not sufficiently diseased to be detected, but in
the early summer some of them may give rise to
diseased shoots on which air-borne conidia are formed.
These act as centres of infection to surrounding plants.

Phytophthora appears on the leaves of the potato
plant in the damp south-west, for instance in southern
Ireland, Cornwall and the Isle of Wight, as early as
the end of May. From these regions it develops suc-
cessively north-eastwards across the country. It usually
appears in Cambridgeshire and Lincolnshire for instance
—great potato-growing counties—about the end of
July or the beginning of August, but its spread and
the severity of the disease depend very largely on
the weather, A warm, nearly saturated atmosphere,
with constant south-westerly winds, are the conditions

POTATO BLIGHT 177

most favourable to the spread of the fungus, for in
such conditions the conidia are formed, scattered by

the

Fia.

wind, and germinate, with the greatest rapidity.

18.—Conidia and zoospores of Potato Blight (Phytophthora
infestans). A, conidium cut off from the end of a hypha.
xX 500. B, conidium evacuating zoospores. xX 500. C. two
zoospores. xX 500. D, zoospore germinating. E, branched
conidiophore. The conidia have fallen from the tips of three
branches, one is shown free and one still attached. The branch
on the right is just forming two conidia. x 120. F, young
conidiophore protruding through a stoma and bearing a young
conidium at its tip. x 200. (After Frank.)

12

178 PARASITIC FUNGI

In a dry summer the effects of the disease are usually
negligible.

Potato blight cannot be cured once it has got a
hold on the plant, but its spread can be very largely
checked by spraying the leaves with ‘ Bordeaux
mixture.” This is a ‘solution’ of copper sulphate
and lime; it dries on the leaves, and when wetted
again by rain or dew it forms a poisonous copper
solution which kills the zoospores, germ tubes and
young hyphe. In this way the crop can be very
efficiently protected against the onset of the disease.
Even if the disease is only moderately severe spraying
will increase, or more strictly will prevent a diminution
of, the potato crop. By destroying the zoospores and
young hyphe it enables the leaves to go on making sugar
and proteins, essential for the growth and stocking with
starch of the potato tubers, during August for instance,
at a time when they might otherwise be injured and
their chlorophyll largely destroyed by the fungus, even
if the shoot of the plant were not entirely killed.

+ Rust of Wheat (Puccinia graminis and P. glumarum).
-—Puccinia graminis is an example of a highly speci-
alised parasite with a much more complicated life
history than that of Pythium or of Phytophthora. On
the leaf or stem of the wheat plant straw-coloured or
reddish streaks may appear in June or July. These
are masses of uvedospores, a special kind of conidia
(Fig. 19, A), which are formed on the ends of hyphe that
have burst through the surface of the leaf or stem
from a mycelium in the tissues below. Detached and
blown about by the wind, the uredospores germinate

Fic. 19.—Life history of Rust Fungus (Puccinia). A, isolated
uredospores of P. graminis (black rust). Xx 475. B, germination
or uredospore showing penetration of the germ tube through a
stoma into the wheat leaf. Note the futile attempts at branching

LIFE HISTORY OF RUST FUNGI 179

O36 © _
a 1e) ne

Dinfey a

on the surface, and the aggregation of protoplasm in the branched
apex. xX 475. C, single teleutospore. x 200. D, germination
of teleutospore of another rust fungus and formation of sporidia.
X 475. E, ecidium cup of another form in vertical section
(cross-section of leaf); myc., mycelium of the fungus; @, zcidio-
spores; /t., leaf tissue; ep., epidermis of leaf. (After De Bary.)
x 150. F, germination of zcidiospore on a grass leaf, penetration
of a stoma by and branching of the germ tube, x 475. (Allexcept
E after Plowright.)

x80 PARASITIC FUNGI

in a drop of rain or dew on the surfaces of fresh wheat
leaves, and the germ hyphe enter the stomata
(Fig. 19, B), branch within the leaf, and penetrate the
living cells. Hyphe from this mycelium burst out
from the surface of the leaf again, forming fresh masses
of uredospores; and so the process is repeated, and
the fungus spreads during the latter part of the grow-
ing season from one wheat field to another.

The uredospores formed about the time the crop is
harvested can survive the winter and are capable of
infecting the young wheat plants in the following
spring. But another kind of spore, the éeleutospore,
two-celled and with thick dark walls (Fig. 19, C), are
also formed in numbers towards the end of the season
by the mycelium of Puccinia graminis (the Black
Rust). The teleutospore hibernates, and in the spring,
instead of germinating directly to form a new mycelium,
sends out short thin hyphe which cut off small thin
walled conidia called sporidia (Fig.19, D), and these do
not infect the wheat plant, but instead the leaves of
the barberry (Berberis vulgaris), a not uncommon
shrub of hedgerows. The germ tubes from the sporidia
form mycelia within the barberry leaf, and these form
spores of a fourth kind (@cidiospores) in cup-shaped
structures (@cidia) on the surface of the leaf (Fig. 19, E).
At the base of each cup-shaped structure there is a
mass of parallel hyphe at right angles to the surface
of the leaf, and from the end of each a chain of
zcidiospores is budded off in much the same way that
the conidia of Penicillium are budded off from the
end of the conidiophores (p. 164). These zcidiospores
are detached, and if they germinate on a wheat leaf

1 Greek teAevtyj, end, because they are formed at the end of the
season of active growth of the fungus.

PLANT DISEASES 181

the germ tube penetrates a stoma (Fig. 19, F) and in-
fects the wheat plant again, producing a mycelium which
forms uredospores, the type with which we started.

Thus the fungus may live on alternate hosts (cf. the
bacilli of tetanus and gas-gangrene, p. 155), the wheat
and the barberry; though it may also, by wintering
in the uredospore condition, live exclusively on the
wheat plant. Long before this complicated life his-
tory was worked out, farmers had noticed that if
there were barberry bushes in their hedges their wheat
was particularly liable to ‘“ rust,’ and in some countries
it is a legal offence to allow the barberry on a farm.
Wheat rust often causes losses to the wheat crop of
the world represented by millions sterling.

Puccinia graminis (the Black Rust) is rare on
wheat in this country. Puccinia glumarum (the Yellow
Rust) is much commoner, but, so far as is known,
only two spore stages occur in the life-history of this
species—uredo- and teleuto-spores.

There are thousands of different kinds of parasitic
fungi known which infest wild and cultivated plants,
some living exclusively on one species of host, some,
like black rust of wheat, living on alternate hosts, and
others being less exclusive in their habits. They
often do severe and widespread damage to important
crops and cause heavy losses in money. In the
seventies of the last century, for instance, the coffee-
planting industry of Ceylon was literally destroyed
by a fungus (Hemileia vastatrix) parasitic on the coffee
plant. The study of the life histories and habits of
these parasitic fungi forms one of the principal branches
of plant pathology, which has become within recent
years a study of great practical importance. For it
is only by a careful and thorough study of the life

182 PARASITIC FUNGI

histories and necessary conditions of life of the different
fungal parasites that we can get the knowledge neces-
sary to enable us to devise methods of preventing or
mitigating their attacks. The cure of plant diseases
caused by fungi is seldom possible; but preventive
measures, taken after a thorough knowledge has been
obtained of the parasite and of the conditions under
which the disease spreads, are often very successful.
Just as in the case of animal diseases, “‘ prevention is
better than cure,’’ but with plants it is almost the
only method which is of any use at all.

Bacteria are the cause of some plant diseases, just
‘aS we have seen that mycelial fungi cause certain
animal diseases. But the latter are far more wide-
spread as pathogenic plant parasites, just as bacteria
are by far the most widespread and destructive of
the parasites of animals. This difference is mainly
due to two causes. First the spores of fungi are pro-
duced and carried about mainly in air, while many
bacteria are largely carried about in the blood-stream
which is absent from plants; and with this the second
cause is connected. The life processes of fungi are
adjusted to ordinary air temperatures, and the tempera-
tures of the bodies of plants exceed these by very
little. On the other hand, the life processes of patho-
genic bacteria go on much more rapidly at higher
temperatures, such as that which is maintained in the
body of a warm-blooded animal (about 37° C.).

PRACTICAL WORK.
“WHITE Rust” (Albugo).

(1) The white pustules on the shoots of the Shepherd’s Purse
(Capsella) are caused by a parasitic fungus (Al/bugo) which often
deforms the inflorescence. The mycelium of the fungus sends
out hyphe which bud off chains of conidia just below the surface
of the host in such numbers that they burst off the surface

PRACTICAL WORK 183

layer of tissue. Scrape off a small portion of the pustule,
mount in a drop of water and examine with the high power.
Draw some of the spherical conidia formed by the club-shaped
hyphe (conidiophores).

(z) In cross-sections through a pustule (sections should be
cut in alcohol and mounted in dilute glycerine) note the layer
of closely packed chains of conidia perpendicular to the surface,
and the mycelium in the tissue of the host.

(3) In sections through older pustules note the oogonia, each
containing a fertilised egg (zygote) in the interior of the tissue
of the host.

Funct PaRAsITIc ON ANIMALS.

(4) From the dead fly or ant’s ‘“‘egg’”’ (pupa) infested by
Saprolegnia * (which attacks both living and dead aquatic animals)
scrape off a small portion of the surface, mount in a drop of water,
and cover. Under the high power note the hyphz, and the
long club-shaped sporangia containing spores (which afterwards
escape and swim in the water as zoospores, settling and germinat-
ing on other living or dead aquatic animals).

(5) If available, examine demonstration specimens of other
parasitic fungi growing in the bodies of fish or of aerial insects
(flies, caterpillars, etc).

(6) Examine slides showing the spores of ringworm (Microsporon)
on the bases of human hairs.

“ TRUE ” Rust Funct,

(7) The wheat (or other grass) leaf provided bears orange or
reddish pustules produced by the rust fungus Puccinia. Examine
first with a hand lens and then place the leaf dry on a slide and
examine with the low power. Note that the surface layer of
the leaf is burst along the line of the pustule, which is formed of
masses of spores (uvedospores). Scrape off some of these and
examine under the high power in a drop of water. Draw one or
two spores and look for thin places in the wall through which
germination will take place.

(8) Examine similarly the leaf-bearing ‘“‘ ecidium cups”
formed by close-set chains of spores (ecidiospores) cut off from
‘ends of hyphz in the leaf. This is another stage of a rust fungus
(Puccinia).

(9) Examine any fresh or museum specimens of other parasitic
fungi that may be available.

t Easily obtained by keeping dead flies or ants’ ‘‘ eggs’ in water
brought in from a stagnant pond containing much organic débris.

CHAPTER XII

ORIGIN OF SEX AND OF THE SOMA. THE
GREEN ALG#

LirE probably began in the sea, and some at least of
the earliest organisms were probably minute free-
floating forms which were able to absorb and use
light-energy to build up their bodies from simple
inorganic substances, and must therefore be classed as
plants. Of these some developed flagella and became
actively free swimming, and their descendants are
still represented by yellow, brown or green flagellate
unicellular plants (alge) which live in the sea or in
fresh water. All these free-floating and free-swimming
organisms, whether animals or plants, are collectively
called plankton. One series of green flagellate plankton
alge, nearly all of which are confined to freshwater,
illustrates very beautifully the way in which the
differentiation of sex among conjugating cells came
into existence, and the same series also illustrates the
origin of what is called the soma,? that is the body
of an organism as opposed to its reproductive cells.
“Immortality ’’ of Unicellular Organisms.—The body
of a unicellular form, such as an amceba, a bacterium
or a yeast plant, not only feeds and grows, it also
divides (or buds) and produces new individuals of the
species. This production of new individuals, or repro-
duction as it is called, is in origin simply an extension

' Greek wAayxtéc, wandering. 2 odma, body.
184

““SoMA”’ AND ‘‘ GERM CELLS” 185

of the growth process: it is discontinuous growth, as
we saw in Chapter V, because it is growth conditioned
by the separation of a part or parts of the individual
to form new individuals. The protoplasm of the
individual organism continues to exist and to increase
in bulk, but it can only increase beyond a certain
limit of size if it separates to form two or more new
individuals. Under favourable conditions of life this
process continues indefinitely, so that the protoplasm
of which the organism is composed is immortal in the
sense that it need never die so long as the conditions
remain favourable to its continued life, though it
dies of course as soon as the conditions become suffi-
ciently unfavourable. But in the higher organisms—
in the great majority of multicellular animals and
plants—the functions of nutrition and growth are, as
we know very well, separated from the function of
reproduction. The feeding and growing body does
in most cases regularly die whether the general condi-
tions of life continue favourable or not. It dies, as
we say, of old age: it is a soma or “ mortal” body.
On the other hand, the reproductive cells (germ cells)
which it produces grow, under certain conditions, into
new individuals of the species. The organisms we are
now going to consider will help us to understand how
this separation of functions came to arise.
Chlamydomonas.—The organisms belonging to this
genus are very common in pools, rain-water tanks,
etc. Each is a minute green cell (Fig. 20, a), oval or
sometimes rather oblong in shape, with a basin-shaped
chloroplast occupying the hinder end of the cell, con-
taining usually one conspicuous pyrvenoid, and enclosing
central colourless cytoplasm containing the spherical
nucleus. The front end of the cell also consists of

186 ORIGIN OF SEX AND OF THE SOMA

colourless cytoplasm to which are attached two long
delicate flagella that protrude into the water in which
the organism lives: by the rhythmical lashing of the
flagella Chlamydomonas swims actively about, con-
tinually rotating on its long axis as it moves forward,
like a rifle bullet in flight. The protoplasm of the
cell is closely covered with a cellulose cell wall which
thins away at the front end to which the flagella are
attached. A red eyespot on the surface of the proto-
plasm, containing carotin (see p. 112), which renders

Fic. 20.—Chlamydomonas: a, diagram of swimming individual
(vegetative cell); b-d, stages of division to form four new
swimming individuals; c.w., cell wall; chl., chloroplast; pyr.,
pyrenoid; c.c., central cytoplasm; n, nucleus; c.v., contractile
vacuoles; st., eyespot; i, flagella.

the organism sensitive to the direction of light, and
two very small contractile vacuoles just below the
insertion of the flagella, complete the equipment of
the cell.

The nutrition of Chlamydomonas resembles that of
Protococcus (p. 73). It can live and flourish in a
weak solution of the proper inorganic salts (such as
that given on p. 123), which provide it with the neces-
sary elements for building up its protoplasm ; and it
makes sugar and starch (laid down round the pyre-

REPRODUCTION OF CHLAMYDOMONAS 187

noid) from the elements of carbon dioxide and water.
It is in fact in its manner of feeding essentially a
green plant, its constant active movement notwith-
standing. We have every reason to believe that the
‘main series of green plants are derived from similar
motile forms.

Reproduction of Chlamydomonas. — (a) Vegetative
Division.—In this, the commonest process of repro-
duction, the cell comes to rest, the protoplasm with-
draws from the wall, the nucleus divides into two,
and this process is followed by the division of the
cytoplasm, including the chloroplast, a furrow appear-
ing on the surface and extending inwards till separation
is complete (Fig. 20, 0). Sometimes the process of
division stops there, and each new cell produces
flagella and secretes a cell wall on its surface. The
two daughter individuals begin to move actively, by
lashing their flagella, within the cell wall of the mother,
which they burst and then swim out into the water,
leaving the empty shell of the mother cell wall behind.
Each grows to the size of the mother individual. We
see that this process is essentially the same as the
division of an amceba into two new amcebe, except
that here the dead rigid cell wall is left behind. Under
favourable conditions of life division takes place once
every day, towards evening, and is complete in a few
hours.

Very often, however, division does not stop with the
first bipartition, but each daughter cell divides again
(Fig. 20, c), so that four daughter individuals are
formed instead of two (Fig. 20, d). These of course
are correspondingly smaller, but on escape from the
mother wall they quickly grow to the full size.

(5) Formation of Gametes and Conjugation.—Some-

188 ORIGIN OF SEX AND OF THE SOMA

times the process of division. continues further, so
that by repeated bipartitions from eight to sixty-four
cells are formed, their size varying inversely as the
number produced. In some species they secrete cell
walls, in some their bodies are naked, but in other

Fic, 21.—Conjugation of Chlamydomonas: a, gametes approachin
one another; 6 and ¢, stages of fusion; d, zygote. (After Dill.)

respects they are identical with the ordinary indi-
viduals. When these larger numbers of small daughter
cells are produced, they do not, as a rule, at once
grow into full-sized individuals, but conjugate in pairs,
and are hence called gametes.1 In the process of con-

1 Greek yayéw, marry.

CONJUGATION IN CHLAMYDOMONAS 189

jugation two -gametes swim towards each other
(Fig. 21, a), their front ends come into contact, and
their bodies gradually fuse completely (Fig. 2r, ,
Fig. 22, a, b), cytoplasm with cytoplasm, and nucleus
with nucleus. When the gametes have cell walls the
protoplasmic bodies slip out of them, leaving the

closely allied to Chlamydomonas: a, beginning of fusion; 8,
advanced stage; ¢, zygote with eight flagella; d, zygote with
thick wall after loss of flagella. (After Dill.)

empty shells behind (Fig. 21, c). The single cell formed
by the union of the two gametes (Fig. 21, d) is called
the zygote (cf. Mucor, p. 162; Pythium, p. 175). In some
cases the zygote continues to swim for a time with
its four flagella, two derived from each gamete
(Fig. 22, c—this is drawn from a form in which the

go ORIGIN OF SEX AND OF THE SOMA

single cell has four flagella and the zygote therefore
eight), but eventually it loses the flagella, becomes
perfectly spherical and secretes a thick cell wall
(Fig. 22, d), going into the vesting state, in which con-
dition it can resist a certain amount of desiccation
(cf. Mucor, Pythium, etc.). On germination the proto-
plasmic contents of the zygote divide to form two or
four ordinary (vegetative) individuals.

(c) Differentiation of Sex.—When the gametes are all
equal in size, as they are in several species of Chlamy-
domonas, they are called isogametes.1 But in certain
species they are of two sizes, the larger derived from
fewer, the smaller from a greater number of divisions
of the mother cell. Such gametes are called hetero-
gametes.2, Conjugation then, so far as has been
observed, takes place, only between a large and a
small gamete. In one species (C. monadina), in which
the details of the process have been carefully observed,
and in which all the gametes have cell walls, the
anterior ends of the two come into close contact, as
in the case of isogametes, and the cell walls of the
two become fused together (Fig. 23, a). The proto-
plasm of the small gamete separates from its wall
(sometimes secreting a new wall round its hinder
end, Fig. 23, B), and slips through the channel
formed by the fusion of the two into the cavity of
the large gamete, where it eventually fuses completely
with the body of the larger, nucleus with nucleus and
cytoplasm with cytoplasm—the chloroplasts with their,
pyrenoids remaining distinct longest (Fig. 23, c).
The mass of protoplasm (zygote) so formed contracts,
becomes spherical, and secretes a thick cell wall within
the cell cavity of the large gamete (Fig. 23, c).

1 Greek joog, equal, and yapéw. 2 étegoc, other (different).

SEXUAL DIFFERENTIATION Igt

Now, here we have two leading characters of the
sexual differentiation of gametes, a difference in size
and a difference in activity. Though both gametes
begin life as free swimming cells of identical structure,
one is more active than the other in the actual pro-
cess of conjugation. This difference in activity seems
here to be a mere mechanical result of the difference
in size. If we suppose the protoplasm of each gamete

Fic. 23.—Conjugation of Chlamydomonas monadina. a, Beginning
of fusion of the small (male) and large (female) gamete. (Note
that each has the normal structure of a vegetative Chlamy-
domonas-cell.) 6, Further stage of fusion (drawn on a larger
scale). Note the central area of colourless cytoplasm, derived
from the front ends of the two gametes, and now containing
the two nuclei (equal in size) in contact but not yet fused.
c, Spherical zygote formed within the cell wall of the female
gamete and itself clothed with a cell wall. Note that the two
gamete nuclei have now fused to form the single zygote nucleus,
but the chloroplasts are still separate. (After Goroschankin.)

to be attracted equally strongly towards the other,
the body of the small one would be more easily drawn
through the comparatively narrow canal formed between
them ; and this conclusion is supported by the unusual
cases figured in Fig. 24. Here the gametes have come
into contact obliquely, or have swung round after con-
tact, so that no canal is formed, but the protoplasm of
both slips out of its cell wall and the two form a zygote

192 ORIGIN OF SEX AND OF THE SOMA

outside, just as in the case of the walled isogametes
in Fig. 22.

In forms which show a more complete differentiation
of sex, as we shall presently see, not only the size
but also the structure of the two conjugating gametes
is very markedly different, so that the large one
(female) is necessarily quite passive, and the small
(male) is alone active in the process.

We now have to consider a series of forms which
do not live singly like Chlamydomonas, but in which
the cells produced by ordinary division, each of which

Fic. 24.~-Unusual type of conjugation in the same species. The
gametes have met obliquely, and the protoplasm of both has
emerged from the cell walls to form a spherical zygote outside.

has the same essential structure as a Chlamydomonas
cell, remain together, surrounded by a common (muci-
laginous) envelope, through which the flagella pro-
trude. The colony or cenobium* of cells so formed
behaves like a single organism, moving through the
water by the co-ordinated beating of the flagella of
all the cells. The first form we shall consider is
Pandorina, which is a spherical coenobium of (usually)
16 cells pressed closely together, so that each cell is
somewhat wedge-shaped (Fig. 25, A). In reproduction
each cell of the coenobium divides by 4 closely follow-
ing bipartitions to produce 16 cells, and each group
of 16 cells forms a new ccenobium (Fig. 25, B), the

* Greek xowds, common, and flog, life.

PANDORINA 193

Fic. 25.—Pandorina. A, colony (ccenobium) of 16 cells in the
vegetative state. B. reproductive condition—each -cell of the
coeenobium has divided to form a new daughter ccenobium. C,
free-swimming gametes of various sizes; a, b, conjugation of
gametes of equal size ; c, of unequal size; d, zygote still retaining
the flagella of the gametes which formed it; e, conjugation
between small (male) gametes which have originated elsewhere
and large sluggish (female) gametes which have remained where
they were formed by division; f, walled zygote; g, escape of
contents of zygote as a free-swimming zoospore which will form
a new ccenobium by division.

13

194 ORIGIN OF SEX AND OF THE SOMA

16 young coenobia escaping from the mother envelope
into the water and then growing to the full size. The
formation of gametes takes place by similar divisions,
but this proceeds to various extents, thus producing
gametes of different sizes. The individual gametes so
formed become loosened from the mother envelope,
all but the largest escaping singly and swimming about
individually in the water (Fig. 25, C). Gametes are pro-
duced at the same time from a number of ccenobia
lying close together, so that the thin mucilage becomes
full of swimming gametes of all sizes. These con-
jugate in pairs without reference to size (a, b, c), except
that the largest, which have remained where they
were formed, can only conjugate with the small active
gametes, which seek them out (Fig. 25, e). Spherical
zygotes are formed (d), which become covered with a
cell wall (f), from which the protoplasm eventually
escapes (g) as a flagellated cell, and this divides to
form a new ccenobium.

Here then we have an even earlier stage in the
evolution of sex than in Chlamydomonas monadina—
an intermediate condition between isogamy and hetero-
gamy—for there is no sharp division into two sizes,
the largest gametes alone, which do not move from
their places, representing the female condition, the
smaller either acting as isogametes (a) or as males (e).

Eudorina and Pleodorina.—These are forms with
spherical coenobia, larger than Pandorina, and with all
the cells (32 to 128) spherical and separate, forming
a single layer on the surface of the ccenobium. Each
cell is of the Chlamydomonas type of structure. In
Eudorina all the cells of the coenobium are alike and
all take part in division to form new ccenobia, which
takes place just as in Pandorina. The gametes are,

EUDORINA AND PLEODORINA 195

however, strongly differentiated. The female gametes
are very much like the ordinary vegetative cells except
that they soon lose their flagella. They are not set
free from the colony in which they are formed, which
scarcely differs in appearance from a vegetative colony.
The male gametes, on the other hand, are formed by
division of each cell of a vegetative colony into 64
cells (male gametes) forming a plate, and each of
these is long and narrow, tapering at the end, the
green chloroplast being represented only by a yellow
coloration at the hinder end.t The plate of male
gametes swarms out as a whole, like a coenobium,
but soon breaks up into its constituent gametes, each
of which is capable of seeking out and conjugating with
a female gamete, the process resulting in the forma-
tion of a zygote. Here we have the third character
of sex differentiation—a difference in structure between
the two gametes, as well as the difference in size and
activity seen in Chlamydomonas monadina and to
some extent in Pandorina. It is to be noted that the
male gamete has suffered reduction in its nutritive
equipment, i.e. its chloroplast, as well as in size.

In Pleodorina illinoiensis (Fig. 26, A) the ccenobium is
almost identical with that of Eudorina, except that
(usually) four of the cells at the front end of the
ceenobium (which is often elliptical in shape) are
smaller than the others. When division occurs to
form new coenobia these four smaller cells do not
divide like the others; and they remain behind,
eventually dying, when the daughter cenobia escape.
Here we have the first indication, in this series of
organisms, of the appearance of a soma or mortal

1 The male gametes of Endorina are very much like those of Volyox
(see p. 200 and Fig. 28, D).

196 ORIGIN OF SEX AND OF THE SOMA

body which takes no part in reproduction. It is
difficult to say why these particular cells should be
incapable of division. Perhaps it may be connected
with a more highly developed function for perceiving
the direction of light. It is generally true that speciali-
sation of vegetative function in a cell carries with
it loss of reproductive power. It is noteworthy that
all transitions are found between the Eudorina condi-
tion, in which all the cells are equal and capable of

Fic. 26.—Pleodorina. A, P. illinoiensis, Coenobium consisting of
28 large cells capable of division to form new ccenobia and 4 small
cells at the front end of the colony which do not divide and die
when the new ccenobia formed by the large cells are liberated.
B, P. californica. Sterile (somatic) cells more numerous. The
arrow indicates the direction of locomotion in both cases.

division, and the occurrence of these sferile cells, which
may vary both in size and in number from one to
twelve, and that the capacity for division depends upon
the size of the cell. Pleodorina illinotensts is in fact con-
sidered to be a state of Eudorina, and thus we have
the appearance of the soma, a fundamentally im-
portant step in evolution, first arising as a fluctuating
condition within the limits of a species. .
A larger form of Pleodorina (P. californica) has

PLEODORINA AND VOLVOX 197

been described (Fig. 26, B), typically consisting of 128
cells, of which only about one-half, situated in the
hinder part of the coenobium, are capable of division,
while the remaining cells in the front part of the

Fic. 27. Volvox aureus. x 180. (After Klein.) A ccoenobium which has
produced all three kinds of germ cells. The three large (walled)
spherical cells are fertilised eggs, the small groups of cells are
derived from androgonidia, which will divide further to form
sperms (cf. Fig. 28, D), the largest spheres are young ccenobia
derived from parthenogonidia; division in these last is complete,
but the cells have not yet separated. Note that about a quarter
of the mother ccenobium (the front quarter) is free from repro-
ductive cells. The arrow shows direction of locomotion. Compare
Fig. 26, A and B.

coeenobium: are somatic, i.e. sterile and incapable of
division. This marks a further step in the restriction
of the reproductive (germ) cells.

Volvox.—In Volvox, the largest and most highly
differentiated member of this series of forms, the
ceenobium consisting of many hundreds of cells, the

198 ORIGIN OF SEX AND OF THE SOMA

development of the soma has been carried much
further. Here all the cells of the conobium are
somatic (Fig. 27), i.e. purely vegetative in function and
incapable of division, with the exception of a: limited
number of purely reproductive or germ cells, These
germ cells are of three kinds: (1) so called partheno-
gonidia (Fig. 28, A), each of which divides to form a
new coenobium (Fig. 28, B), and thus corresponds in its
reproductive function with the ordinary vegetative cells
of a Pandorina or of a Eudorina; (2) gynogonidia
(Fig. 28, C), which develop directly into passive (female)
gametes (eggs) without flagella, these eggs being very
large cells highly stored with food; and (3) andro-
gonidia, each of which divides, as in the case of
Eudorina, already described, to form a plate (Fig. 28, D)
or a sphere of male gametes (sperms) * which have the
same general characters as those of Eudorina. These
three types of reproductive cell may be distributed
in any combination. Thus a coenobium may reproduce
entirely by means of parthenogonidia, or it may pro-
duce eggs only, or colonies of sperms only, or any
two, or all three forms of germ cell (Fig. 27).

There are two species of Volvox found in Britain, V.
aureus (Figs. 27, 28, A-D) and V. globator. The former
has rounded somatic cells which are separated from one
another byconsiderable spaces, which are bridged by deli-

* Greek onépua, seed. This is a useful term applied to all male

gametes both in animals and plants; the male fertilising element has
often been called the ‘‘seed ’’ in common language.

Fic. 28. Volvox. A, portion of the surface of a coenobium showing
vegetative cells joined to one another by from one to three proto-
plasmic threads: also a parthenogonidium (non-sexual germ
cell). x 550. B, partly grown daughter ccenobium derived
from the division of a parthenogonidium. x 550. C, egg
joined to neighbouring vegetative cells by bundles of protoplasmic
threads, x about 700. D, isolated “‘ plate’’ of male gametes
(sperms) derived by division of an androgonidium. x 700.

VOLVOX 199

E, isolated sperms of V. aureus (note chloroplast, eyespot and
flagella). xX 825. F, smaller sperms of V. globatoy on same
scale (chloroplast degenerate). G, egg of V. globatoy surrounded
by sperms. x 4oo. (After Klein and Cohn.) A~E, V. aureus.
F, G, V. globator.

200 ORIGIN OF SEX AND OF THE SOMA

cate threads of cytoplasm (Figs. 27, 28, A-C). V. globator,
which is larger and less variable in size, has its cells
much closer together and joined to one another by stout
angular projections from the cytoplasm of the cell
bodies (Fig. 29). It isinteresting to note that V. aureus
has larger green male gametes (Fig. 28, E), while
V. globator has smaller very thin sperms, with the hinder
end yellow and the two flagella usually attached about
the middle of the body (Fig. 28, F), thus departing
much further than the sperms of V. aureus from the
structure of the primitive Chlamydomonadine cell,
though still showing clear traces of derivation from
that structure. The sperms of V. globator are nearly

Fic. 29.—Vegetative cells of V. globatoy seen in profile of surface of
cenobium. Note cell walls and broad protoplasmic connexions.
The cell on the left is a young parthenogonidium. x 1,600.

as much reduced and highly specialised as those of

green plants very much higher in the scale of vege-

tative structure. The extreme contrast between the
male and female gametes in size, shape and structure
is very obvious (Fig. 28, G), though each may be
clearly derived from the Chlamydomonadine cell
through the stages of differentiation we have traced.
Nature and Significance of the Differentiation between

Male and Female Gametes.—The wide difference in

structure and function between the male and female

gametes of Volvox globator is repeated in the sexual
differentiation of gametes in the vast majority of
organisms, including.all the higher forms of life, both

CHARACTERS OF MALE AND FEMALE GAMETES 201

animals and plants, and in nearly all the higher organisms
—Seed Plants and a few other are exceptions—the
male gamete or sperm is a small motile free swim-
ming cell, while the female gamete or egg is a large
passive spherical cell. The sperm is exceedingly sensi-
tive, at least in a large number of cases, to chemical
substances diffusing out from the female cell. Sperms
are produced in immense numbers and only a minute
proportion succeed in conjugating with eggs. The body
of the spermcell is reduced to the smallest sizecompatible
with its function, which is to carry the paternal nucleus
to the egg. Its chromatin contribution to the zygote
nucleus is exactly equal to that of the egg nucleus
(see Fig. 35, F, G). The hereditary characters are
carried by the conjugating nuclei, and the paternal
and maternal chromatin are equal in bulk and equiva-
lent in effect on the characters of the offspring.

The egg, on the other hand, provides in the first
instance the store of organic food substance with
which the new individual produced from the zygote
starts its life, and the size and passivity of the female
gamete are correlated with this function. The fact
that far fewer eggs than sperms are produced is also
a result of this difference in size. The first beginnings
of the differentiation we saw in Chlamydomonas and
in Pandorina. The comparatively slight difference
between the gametes in these forms appears as the
result of what may be regarded as an accidental differ-
ence in the rapidity and duration of the process of
division of the mother cell. The divisions of the
mother cell of the smaller (male) gametes are more
rapid and continue longer, so that smaller cells are
produced, while the divisions of the mother cell of
the female gametes is slower, so that larger cells are

202 ORIGIN OF SEX AND OF THE SOMA

produced, the structure of the two being identical.
The individual sluggishness of the larger gametes in
Pandorina is probably merely a result of their larger
size, but since they tend to remain passive and also
contribute more cytoplasm to the zygote, we see here
the foundations laid of the specific characters—the
femaleness—of the egg in the higher forms. In these
the differentiation is carried further and has become
fixed—the smaller (male) gametes are no longer capable
of conjugating with one another. They are short-
lived cells, incapable of nourishing themselves, but
extremely active and sensitive, their sole function
being to get to the egg as quickly as possible and
contribute their quota of nuclear material to the
zygote. The female gamete has taken over entirely.
the function of feeding the new individual, and the
amount of food that can be stored in it is not limited
by the need of motility. This is a much more efficient
arrangement for giving the new individual a good
start in life than the conjugation of isogametes, both
of which are motile and neither of which can con-
tribute much food to the zygote.

The origin of sex is an excellent example of the
origin of a differentiation which is at first, as it were,
accidental, i.e. appearing without reference to its ulti-
mate use, but is later fixed and further developed into
an extremely efficient working mechanism. The more
we learn of the evolution of structure and function in
organisms the more we find that something like this
is the history of the evolution of new characters.

In the higher forms the egg is often, as we shall see
in the sequel, much reduced in size because parts of
the parent organism take over from the egg the
function of providing food for the new individual
produced from the zygote.

FILAMENTOUS GREEN ALG# 203

Besides the motile green alge, some of which have
been described in the preceding pages, there are many
immotile unicellular forms, such for instance as Proto-
coccus (Chapter IV). Some of these live on damp
earth, tree-trunks, or in similar damp situations,
while many float in water. Some are ccenobiate, like
the Volvocines, and others form irregular loose colonies
with an indefinite number of cells.

But there are also a large number of filamentous
(thread-like) forms, the body commonly consisting of
simple (Fig. 30, a) or branched (g) threads composed of
cylindrical cells placed end to end and containing one
or more chloroplasts of various shapes. Many of the
filamentous forms which live in water are reproduced
by the division of the contents of their cells into
motile flagellate cells called zoospores (Fig. 30, 6), of the
same type of structure as the Chlamydomonas cell,
but smaller and without a cell wall—cells in fact
closely similar to Chlamydomonas gametes. These
zoospores escape from the mother cell in which they
were formed, swim about for a tmie (c), and then settle
down on some solid object and germinate, secreting a
cell wall (d), growing in length (e) and dividing to
form a thread of cells of the type characteristic of
the species (Fig. 30, f). We may consider that in this
form of reproduction the plant reverts to the condi-
tion of a Ghlamydomonas-like ancestor for the purpose
of reproduction : or to put the matter another way a
Chlamydomonas-like ancestor settled down and divided
without separation of the daughter cells from the
cell wall, and then by growth and further division a
thread of cells was produced, any of which at a later
stage can produce a brood of Chlamydomonas-like
cells which escape, and each of which reproduces the
filament.

204 ORIGIN OF SEX AND OF THE SOMA

The zoospores of such a filamentous alga may in
certain cases conjugate instead of germinating at once,
thus acting as isogametes. Such zoospores are called
facultative gametes. But in most cases the gametes are

Fic. 30.—Filamentous green alge: a, vegetative thread of Ulothrix
with band-shaped chloroplasts (chl.); 6, thread whose cells
are forming zoospores (like the individuals of Chlamydomonas
but without walls) ; c, Free-swimming zoospore; d, germination
of zoospore, which has lost its flagella, is attached by the front
-end to a solid object and has secreted a wall; e, the cell has
grown in length; /, young filament derived from e by further
growth in length and transverse cell division (this grows into
a filament like a); g, branched filamentous alga (Stigeoclonium),
some of whose cells are vegetative, others have formed zoospores ;
h, free zoospores.

smaller than the zoospores, though quite similar in
structure, i.e. more of the gametes, often double as
many, are produced from the mother cell. Some

EVOLUTION OF THE SOMA 205

filamentous forms have sexually differentiated gametes,
and these in most cases show a wide differentiation
between sperms and eggs, comparable with that seen
in Volvox.

Sexual differentiation of gametes is found in various
different groups among the alge, evidently inde-
pendently evolved along separate lines of descent.
One may say that there is an inevitable tendency
towards sexual differentiation, and towards the fixa-
tion of the extreme form in which the sperms are
most widely different from the eggs. This high
specialisation of the gametes, as already shown, is
certainly the most efficient mechanism for the pro-
duction of vigorous new individuals through conjugation.

The “Soma” in Plants and Animals.—The evolu-
tion of a soma or mortal body is beautifully illustrated
in the Pandorina-Eudorina-Pleodorina-Volvox series.
This evolution consists essentially in the separation of
the vegetative and reproductive functions. In the
lower forms of the series all the cells of the body dis-
charge both functions, in the higher some cells dis-
charge the vegetative, others the reproductive func-
tion. Directly we have any cells limited to the vege-
tative functions, we have a soma or mortal ‘“ body ”
by the very fact that these cells can no longer reproduce
the species.

It must be noted, however, that this particular series
of organisms is not in the direct line of evolution of
any of the higher organisms. Volvox is the culmi-
nation of its own line of descent. It is probable that
no further increase in size or complication is possible
to the motile coenobiate form of organism. The soma
has been evolved on many other lines of descent’ from
unicellular organisms. This particular line is chosen

206 ORIGIN OF SEX AND OF THE SOMA

for illustration because of the existence of several forms
which make up a closely connected series; and mid-
way in this series Pleodorina shows the actual first
appearance of the soma, P. illinoiensis being a very
slight modification of the Eudorina type.

The vast majority of multicellular animals have a
well-marked soma, i.e. a body consisting of tissues whose
cells‘ are not germ-cells and do not reproduce the
species by spore or gamete formation; but in many
of the lower invertebrates new individuals are produced
by budding of these somatic tissues which are not too
highly specialised for particular vegetative functions.
In the higher animals this process falls into abeyance,
and the life of the somatic tissues is strictly limited to
the service of the individual and comes to an end
with the life of the individual.

In plants, however, the power of reproducing the
species is much more often retained by the cells of
the vegetative body. In the filamentous green alge,
for instance, referred to in the preceding section, all,
or nearly all, the cells of the multicellular thread of
which the body is composed retain the power of form-
ing zoospores and gametes, and thus may become
“germ cells.” In the bulky alge (seaweeds) with
massive tissues, as well as in all the higher plants, this
power is lost by the general body cells, but the plant
may be reproduced “ vegetatively’ by budding, as in
many of the lower invertebrate animals, new plants
being thus formed apart from the germ cells proper.
This power of ‘“‘ vegetative reproduction ”’ is retained
by some at least of the body cells of practically all
plants, even the highest and most complicated forms,
as we shall see in later chapters.

Taken together these facts show us that the dis-

SPIROGYRA 207

tinction between somatic and germ cells is not an
absolute one. Though the germ cells (spores and
gametes) are the specialised reproductive cells, whose
sole function is to reproduce the species, this power
may be retained by the body cells to a varying extent
in different organisms. The body cells of plants retain
it far more generally than those of animals, and this
is undoubtedly connected with the fact that plant
cells are, in general, far less highly modified for the
performance of special functions than is the case with
the cells of the higher animals. .
Spirogyra.—This is a filamentous green alga belong-
ing to a group which is distinguished by producing
gametes that are not flagellated cells, conjugation
taking place entirely within the mother cell walls. The
body consists of an unbranched thread composed of
cylindrical cells placed end to end. Each cell has a
large central vacuole and a thin layer of cytoplasm
lining the wall: in this is embedded a single chloro-
plast in the form of a green band which winds
spirally round the cell from end to end (Fig. 31, A).
In the chloroplast is a row of pyrenoids. The nucleus
is suspended in the centre of the cell. In other species
several chloroplasts are present, running parallel with
one another. The crossing lattice structure seen when
the cell is looked through from the side is due
to the fact that the parts of the chloroplasts which
run round the further side of the cell are seen at the
same time as the parts on the side towards the observer,
so that’ the former appear to cross the latter. In a
wide cell containing several chloroplasts, when the
near. side of the cell can be focussed alone, the far
side being out’ of view, it is clear that there is no actual
crossing (Fig. 31, B). In such a large cell the nucleus

208 ORIGIN OF SEX AND OF THE SOMA

Y

chL c v
aa

Fic. 31.—Spivogyra. A, one complete cell with parts of two adjacent
cells of the filament of a species with one chloroplast in each cell ;
cy., cytoplasm lining the wall; v, vacuole; , nucleus suspended
by a group of bridles; chl., spiral chloroplast; p, pyrenoid. B,
surface view of cell of large species with several chloroplasts,
showing nucleus (nz) lying below, connected with surface by
bridles (by.) which run to pyrenoids (p). x about 550. C, zygote
formed by conjugation of gamete formed in neighbouring cell
(marked g¢ = male) of the same filament with gamete formed
in cell marked 9—female. D, five filaments which have
taken part in conjugation. On the extreme left filament which
has acted as female, and all of whose cells contain zygotes. Next
is a filament all of whose cells have acted as male, alternate cells,
conjugating with those of the filament on the left and on the
right. On the extreme right are two filaments in process of con-
jugation—successive stages from above downwards (left, male;
right, female). . At the top of the right-hand filament a cell (v.c.)
which has remained vegetative. (Modified from G. S, West.

SPIROGYRA 209

is suspended by long thin bridles (dr.), each of which
runs into a chloroplast opposite a pyrenoid (f). This
is probably connected with the function of the nucleus
in controlling nutrition (see p. 67), since it is round the
pyrenoids that starch is laid down.

The cells of Spirogyra divide, after karyokinetic
division of the nucleus, exclusively in the plane per-
pendicular to the long axis and halfway between the
end walls, the new cell wall, secreted and constantly
covered by cytoplasm, growing out in the form of a
ring from the cylindrical wall, the effect being like
the gradual closing of an iris diaphragm, till the two
daughter cell cavities are completely separated. The
cells then grow in length till they reach the length of
the standard cell of the species. Under certain con-
ditions the thread breaks up into lengths, or even into
single cells, by the splitting apart of adjacent cells ;
and in this way the number of individual threads
increases. Spivogyra is sometimes said to be “ physio-
logically unicellular,” because each cell functions as
a self-contained unit—it is immaterial to its life
whether it is isolated or whether it forms part of a
thread.

Gamete Formation and Conjugation.—Under certain
conditions two threads lying side by side form gametes,
one from each cell. The cell walls of one of the threads
on the side towards the other thread are thrust out
in blunt projections, one from each cell, and the cell
body of each cell begins to leave the wall. Almost
immediately similar projections are thrust out from
the cells of the other thread opposite those of the
first set, the pairs of projections meet between the
threads, the parts of the cell walls in contact are
absorbed, and an open conjugation canal is thus formed

14

210 ORIGIN OF SEX AND OF THE SOMA

between the two cell cavities. The cell bodies of the
thread which began the process (male gametes) now slip
through the canals and fuse with the bodies of the
opposite cells of the other thread (female gametes),
which have meanwhile contracted away from the cell
wall, and a zygote is thus formed in the cavity of
each female cell (Fig. 31, D). The zygote becomes
covered by a thick wall and ultimately germinates to
form a new Spirogyra thread by splitting of the thick
outer wall and protrusion of the. inner thin wall
(Fig. 32, C). Cf. the germination of the zygote of
Mucor (p. 163,and Fig 16, 2). In some species, however,
adjacent cells of the same thread may conjugate
(Fig. 31, C), one acting as the male the other as the
female cell, and in other cases, again, parthenospores may
be formed, quite similar to zygotes in appearance and
germination, but each produced from the contents of a
single cell without conjugation.

The sexual differentiation of the gametes of
Spirogyra is mainly seen in the active and passive
roles of the two conjugating cells: it does not involve
a difference of structure or even of size. But in one
species at least there is an interesting change in the
structure of the male gamete after it becomes part of
the zygote, its chloroplast degenerating and disin-
tegrating in the zygote (Fig. 32, A, B), so that only the
chloroplast derived from the female gamete remains
and gives rise to the chloroplasts of the new thread
formed on germination. This is a late occurring
degeneration of the nutritive equipment of the male
gamete, which may be compared with the ‘reduction
or disappearance of the chloroplast in the ordinary
sperm before the formation of the gamete (cf. p. 200).
Other slight indications of “‘ maleness”’” may be found

DIFFERENTIATION OF GAMETES 2Il

in some species, for instance the cells forming male
gametes are sometimes shorter than those forming the

Fic. 32.—A and B, two stages of development of the zygote of a
species of Spivogyra in which the wall is transparent and the
chloroplast of the male gamete is degenerating and breaking
up. The chloroplast of the female gamete alone produces the
chloroplasts of the cells of the new individual. x 800 (after
Chmielevsky). C, germination of zygote: two cells of the new
individual are produced. X 395.

females, and in other cases the conjugation canal is
mainly or even wholly formed by the male conjugating
cell, the female cell merely swelling up to meet it.

212 ORIGIN OF SEX AND OF THE SOMA

PRACTICAL WORK.:

(1) Inasample of Chlamydomonas note, under the low power,
the moving green dots. Under the high power note in a specimen
at rest (the flagella often get stuck to the slide or coverslip,
and the cell thus anchored oscillates to and fro) the cell wall,
the basin shaped chloroplast with pyrenoid, the clear front end
of the body with the base of the flagella, and the red eyespot.
Look out for stages of division. Run a drop of dilute iodine
solution under the coverslip and observe again. .

(2) In a living demonstration specimen under an apochro-
matic immersion lens' observe the finer details of cell structure,
including the nucleus situated in the colourless central protoplasm
in the hollow of the chloroplast. ,

(3) Examine if possible Pandorina and Eudorina (preserved
material if fresh cannot be obtained), noting the construction of
the ccenobium and the fact that the structure of each cell is
of the Chlamydomonas type. Examine also stages of division
to form daughter ccenobia if these are available.

(4) Volvox. In V. aureus (on the whole the commonest
species) note the large spherical ccenobia, each consisting of
several hundred spherical cells. Compare several specimens,
and trace the development of the daughter coenobia from the
large cells (parthenogonidia) of the mother. If sexual colonies
are available note the development of the sperms by repeated
division of special cells (androgonidia) and the large gynogonidia,
each of which becomes an egg.

(5) Examine a single vegetative cell under the high power
and note that it is of the Chlamydomonas type. Look for the
flagella. Run in a drop of iodine and look for the threads of
cytoplasm connecting the cells.

(6) Compare the structure of V. globator, and examine any
demonstration slides of stages of development and details of
cell structure that may be available.

t The material for this practical work may be varied according
to what can be obtained. Chlamydomonas is generally available
during the warmer months, and can in any case be kept in the labora-
tory without much difficulty. The ccenobiate forms are not so easy
to keep in cultivation, and fresh material is by no. means always
available. For this reason it is advisable to keep a stock of material
preserved in formalin, illustrating at least the vegetative structure
of Pandovina, Eudorina, Pleodorina, and Volvox, and the formation
of daughter coenobia in these forms. Pleodorina can often be found
by careful searching through samples of Eudorina. It is desirable
also to prepare slides showing the formation of gametes, etc.

PRACTICAL WORK 213

SPIROGYRA.

(7) Draw a single cell ' on a large scale under the high power,
showing cell wall, cytoplasm, chloroplast with pyrenoids, vacuole,
nucleus (with nucleolus) and bridles (if present).

(8) Compare the pyrenoids of two preserved and decolorised
samples, one of which, previous to killing has been well
illuminated, the other kept for a day or two in the dark. Now
stain each with iodine and observe again. Note that the pyrenoid
itself stains brown with iodine (it is a protein crystalloid) as
contrasted with the starch formed round -it which stains dark
blue (almost black).

(9) Examine the conjugation of Spirogyra, fresh if possible, if
not in preserved material, and draw as many stages in the process
of conjugation as you ean find.

(10) Plasmolyse vegetative Spirogyra + with 5 per cent, calcium
chloride solution (ordinary salt solution injures the cells) and draw
stages in plasmolysis. Recover by placing in water.

1 A species with a single chloroplast or two chloroplasts in each
cell is the easiest to draw in the first instance. This should be com-
pared with a large species such as S. crassa containing seven or eight

chloroplasts, and showing bridles running into the pyrenoids.
2 If time permits.

CHAPTER XIII

DIFFERENTIATION OF TISSUES. FUCUS: THE
SEA-WRACK

BESIDES the Green Algz, which are mostly unicellular or
filamentous fresh-water forms (the “ sea lettuce’ is an
example of a green alga which is marine and has a large
thin body or thallus, which is soft and membranous,
and coinposed of two layers of green cells), there are
two other large groups of alge, commonly called the
Red and the Brown Seaweeds, because they are
respectively red and brown in colour and they live in
the sea, mostly in the intertidal zone or not very far
below low-water mark. The plastids of the cells of these
seaweeds contain the chlorophyll pigments, but in a
different proportion to that in which they exist in
green chlorophyll, and also other pigments in addi-
tion. For instance in the Brown Seaweeds there is a
smaller proportion of the pure green pigments (chloro-
phyll a and f) and a larger proportion of the yellow
and orange constituents (xanthophyll and carotin),
and, in addition, a special orange pigment, fucoxanthin
(C,9H;,0,), peculiar to the Brown Seaweeds. This
‘combination gives the ph@oplasts,! as they are called,
a brown or olive green colour.

The Brown Alge vary from unicellular forms, through
a considerable series of filamentous types, either con-

1 Greek gaidc, greyish.
ar4

EXTERNAL STRUCTURE OF FUCUS 215

sisting of single rows of cells or of stouter threads
several cells thick, to large forms! with quite bulky
bodies consisting of more or less differentiated tissues.
One of these latter forms we shall now study in some
detail, because it illustrates very well the fundamental
principle of differentiation of function and corresponding
differentiation of structure leading to the origin of
distinct tissues.

Several different species of the genus Fucus live
attached to rocks, mainly between tide marks, on
the coasts of the cooler countries of the northern
hemisphere. They are the commonest seaweeds on the
rocky British coasts, and frequently cover the rocks so
thickly as to make them very slippery to walk upon
just after the tide has ebbed, leaving the mucilaginous
surface of the seaweed wet and slimy. ‘

The individual plants of Fucus vary from a few
inches to several feet in length. The body or thallus
consists of a cylindrical stalk or stipe attached to the
rock substratum by a more or less branched spread-
ing holdfast which cements itself firmly by means of
the gluelike mucilaginous walls of the surface cells
to the rock or stone on which the plant grows. Above,
the stipe passes into the flat-forked frond by the gradual
appearance of thin wings distinct from the thickened
midrib, which is a direct continuation of the stipe. At
the end of each branch of the frond is a groove or
depression, often containing hairs consisting of single
chains of cells, and it is by division of the cells at the
base of the groove (Fig. 33, A) that the branches grow.

The end portions of some of the branches are
swollen, and thickly set in the swollen frond are slightly

t Some of the Brown Seaweeds, for instance Macrocystes and
Nereocystis, living in the Antarctic and Pacific Oceans, are immense
plants, hundreds of feet long.

216 DIFFERENTIATION OF TISSUES. FUCUS

raised papillz, each of which is the projecting top of
a spherical structure, hollow within, called a con-
ceptacle. A minute hole leads through the tip of the
papilla into the hollow of the conceptacle, which con-
tains the sexual organs bearing the highly differentiated
gametes. There are also (often) smaller, sterile con-
ceptacles from which hairs protrude through the
opening to the outside.

Thus in Fucus we have an external differentiation
of parts or organs, though a very simple one: the
holdfast which fixes the plant below, the stipe which
is specially tough, and the frond which plays the chief
part in photosynthesis and in growth; while the
sexual reproductive organs are represented by groups of
cells arising on what is really the surface of the frond,
though the surface locally dips down, so to speak, to
form the internal surface of the hollow conceptacle.

Microscopic Structure of the Thallus.—The minute
structure of the different organs of bulky plants is
best studied by examining under the microscope
sections of the organ thin enough to be translucent
when mounted in a liquid medium. To obtain com-
plete information as to the structure of such an organ
these sections have to be cut in different directions. The
most instructive section is that taken at right angles
to the axis of symmetry of the organ (transverse or
cross section), for this displays the distribution of
tissuer about that axis. But the knowledge we gain
from a transverse section must be supplemented by
the examination of sections taken through and parallel
with the axis (longitudinal sections), in order to study
the structure of the tissues in longitudinal extension.

(r) Frond.—A cross-section of the middle of the
frond shows three clearly marked regions: (a) the

INTERNAL STRUCTURE 217

surface layer of cells (palisade or photosynthetic layer),
(b) the larger isodiametric cells lying below (cortex), and
(c) the central region of cells (medulla) the bodies of
which are separated from one another by more than
the thickness of an ordinary cell wall.

(a) Palisade Layer.—This consists of a single layer of
cells (Fig. 33, A, #) whose long axes are perpendicular to
the surface of the frond. Each cell of this layer
essentially resembles a mesophyll cell (especially a
palisade cell) of the leaf of a higher plant (Figs. ro, r1).
It contains a central vacuole and a peripheral layer
of cytoplasm containing the nucleus and packed with
pheoplasts. In this layer (as in the palisade layer of
the mesophyll of a typical leaf) the greater part of
the work of photosynthesis is carried on. These cells
contain the greatest mass of pheoplasts, and the raw
materials of the process (water, dissolved carbon
dioxide and mineral salts) have direct access to them
when the plant is covered by the sea at high tide.

(b) Cortex.—These cells (Fig. 33, A, c) are larger than
those of the palisade layer, having larger vacuoles and
fewer phoplasts per unit bulk. The nucleus is often
suspended in the vacuole by cytoplasmic bridles.

(c) Medulla.—The central region of the frond is
occupied by cells (Fig. 33, A, me.) most of which are
apparently isolated from one another. They are not,
however, separated by air spaces like many of the
cells of the tissues of a higher plant, but by a mucila-
ginous substance (mu.), which is really formed by the
swelling of the middle layer of the joint wall between
two adjacent cells. When these cells are first formed
in development, the cell bodies are separated by thin
walls, but the walls gradually increase in thickness
and the middle layer becomes mucilaginous, takes up

218 DIFFERENTIATION OF TISSUES. FUCUS

water and swells, forcing the cells apart and —
the thickness of the thallus, The layers of wall on each
side of this swollen middle layer, i.e. in direct contact
with the cell bodies, also increase in thickness, more
or less, but remain of firmer consistency, so that in
the adult condition the cells appear isolated, each
covered by a wall of its own (which may be thin or
thick) and separated from its neighbours by a mucila-
ginous matrix. The medullary cells form chains or
strands (like: the threads of a filamentous alga) of
cylindrical cells placed end to end. It is these chains
which are separated from one another by the mucila-
ginous matrix derived from the middle layer of the
original cell wall (Fig. 33, A). The cross walls separating
the successive cells of a medullary strand remain thin.
The structure of the body of a medullary cell is not
different in essentials from that of a cortical cell, that
is to say there is a central vacuole and peripheral
cytoplasm with nucleus and phezoplasts, but these
last are often very sparsely scattered.

The direction of the medullary strands differs as
between the wings and the midrib: in the former they
run horizontally or obliquely, in the latter longitudin-
ally, so that they appear in cross-section as circles.
Groups of cells at the outer edge of the medulla of
the midrib, just below the cortex, have especially
thick tough walls, and these may be called fibres, by
analogy with the somewhat similar longitudinally
running thick walled cells of the higher plants. Like
them they increase the toughness of the thallus. The
main function of the medullary cells is probably that
of conduction of.organic food substances tormed by
the photosynthetic layer to the regions of growth at
the apex_of the frond. i

STRUCTURE OF STIPE AND APEX 219

In longitudinal sections of the frond the palisade
and cortical cells appear very much the same as they
do in transverse sections, but the medullary cells of
course look different, those of the midrib being now
cut in longitudinal section, i.e. showing the length of
the chains, instead of as single cells cut transversely.

(2) Stipe.—The stipe consists of cells similar to those
of the frond, but with some notable differences. There
is no single layer of surface cells overlying a distinct
cortex, but several layers of cells in radial rows, the
cells of each row separated by thin walls, indicating
division parallel to the surface of the frond (tangential
division), The surface itself is rough owing to the
knocking about the stipe suffers from the waves
against the rocks, and often shows radial splits. The
medulla, which forms much the greatest bulk of the
stipe, consists largely of fibres, with a few: wide cells
having dense vacuolated contents, including a number
of pheoplasts and fairly thick walls. These may
perhaps serve to conduct organic food from the frond
down to the holdfast, which increases in size, sending
out fresh short branches as the plant grows. As the
base of the stipe is approached the thick-walled fibres
increase and the holdfast itself consists exclusively of
them.

Apical Growth and Nutrition.—The basis of growth
in Fucus, as in the higher plants, is active cell divi-
sion at the tips of the branches. At the bottom of
the apical groove of each branch there is situated a
comparatively large six-sided, box-like cell (Fig. 33, A, a),
with one side forming part of the surface of the base
of the groove. This apical cell gives rise by its divi-
sions to the cells of all the tissues of the branch. The
apical cell is constantly dividing unequally, cutting off

220 DIFFERENTIATION OF TISSUES. FUCUS

a flattish oblong cell from each of the four sides and
from the base (towards the centre of the frond) in
turn. After each division it grows to its original
size. The cells cut off divide further and form an
almost homogeneous small-celled thin-walled tissue,
densely filled with pheoplasts, in the whole region of
the branch apex. As one passes away from the apex
the different tissue regions quickly differentiate
(Fig. 33, A). The surface cells do not change their
character much and form the photosynthetic layer.
The cells lying next below increase in size by swelling
of the vacuole, but do not increase their cytoplasm.
These cells become the cortex. The cells towards the
centre of the frond grow in length very considerably
and separate laterally by the swelling of the middle
lamelle of their lateral walls, as already described,
thus forming the medulla. The strands of medullary
cells are also passively stretched owing to the elonga-
tion of the frond by the continued division of the
surface cells at right angles to the surface plane.

It is clear that at the apices of the branches there
is constantly going on a great increase in the bulk of
protoplasm and of cell wall substance, and this requires
a continuous supply of soluble carbohydrates and
proteins or other nitrogenous organic substances.
Some of this organic food will no doubt be supplied
by the synthesis of these substances from carbon
dioxide, water and salts absorbed by the surface layers
of cells close to the apex, which contain numerous
pheoplasts. But in an actively growing plant the
consumption will be much greater than this supply,
and the balance required must come mainly from the
palisade layer of the mature parts of the frond. The
easiest channels for this flow of substances are neces-

Fic.

TISSUE STRUCTURE 221

RE es

a)

33.—A, longitudinal section through the apex of frond of Fucus
(the cells are in outline, the intercellular mucilage dotted); a, apical
cell surrounded by the cells produced from its previous divisions;
p, photosynthetic layer; c, cortex; me., medullary cells (mu.,
mucilage formed from their walls). Further from the apex the
cortical cells increase in size and the medullary cells inlength. B,
young plant of Fucus in the cylindrical stage. Apical groove just
developing. Medullary cells beginning to lengthen and to separate;
vh., first rhizoids which develop into the “‘ holdfast.”’ C, longitu-
dinal section of part of the bristle-like thread of Stictyosiphon
(6 cells thick) showing dense protoplasmic contents with phzoplasts
of surface cells (photosynthetic layer), which are half the
length of the next layer (cortex). These, again, are half the
length of the central cells (medulla), which are slightly separated
longitudinally. This arrangment is due to the more numerous
divisions of the surface cells, Compare B.

222 DIFFERENTIATION OF TISSUES, FUCUS

sarily the strands of medullary cells, because these
form the shortest paths, and also, owing to the length
of these cells, there are fewer walls to pass in a given
distance. The medullary cells are connected at, one
end with the cortex, and through it with the palisade
layer of the mature frond; at the other with the mass
of embryonic cells lying behind the apical cell.
Hence these medullary cells may be called conducting
cells. The diffusion of substances from a region of
higher to a region of lower concentration, in plant
and animal tissues for instance from a region of
constant production to a region of consumption (i.e. con-
version into other substances), is a necessary conse-
quence of the physical laws of diffusion, though the
vate of diffusion and the actual paths it takes will
depend on a number of variable factors, such as the
nature of the substances and the obstacles to diffusion.
The cortical cells will act to a certain extent as a
storage tissue, since if the stream of soluble organic
food from the photosynthetic layer through the
cortex and medulla to the growing points is checked,
owing to the supply being temporarily greater than
the demand, some of the surplus food will be arrested
and will tend to accumulate in the cortical cells as
well as in the medullary cells.

Sexual Reproduction.—Unlike the plants hitherto
considered Fucus reproduces itself exclusively by
means of gametes, and these show high sexual dif-
ferentiation, comparable with that of the gametes of
Volvox (cf. Figs. 28,G, and 35, D). The two kinds of
gametes are produced in the cells of the sexual organs
arising on the inner surfaces of the conceptacles. In
some species the male and female organs are formed
in the same conceptacle in others in different concep-

SEXUAL ORGANS 223

tacles which may be produced on different individual
plants.

The female organ (oogonium) arises from a single
cell on the surface of the conceptacle. This grows
up to form a papilla which is cut off by a cross wall
at the base, and then divides transversely to form a
stalk cell and a body cell (Fig. 35, A). The latter
becomes large and spherical and its nucleus divides
into eight by successive bipartitions. These eight nuclei
have different fates in different species. In some the
cytoplasm divides correspondingly and eight eggs are
formed in the oogonium (Fig. 35, B), but in others
four or six of the nuclei degenerate and disappear
and the whole of the cytoplasm forms four or
only two eggs. In others, again, seven of the
eight nuclei degenerate and only one egg is
formed from the whole of the protoplasm of the
oogonium.! The oogonium wall has two layers: the
outer bursts, setting free the inner wall as a bladder
enclosing the eggs (Fig. 35, C).

The male organs (antheridia) are club-shaped cells
(Fig. 34, A, a), which are branches of a hair that arises,
like the oogonium, from a surface cell of the con-
ceptacle. The nucleus of the antheridial cell divides by
repeated bipartitions to form 64 nuclei (Fig. 34, C, n),
and the cytoplasm divides correspondingly so that
64 sperms are formed (D). Each of these is a minute
biflagellate pear-shaped cell containing besides the
nucleus (Fig. 34, D, E, ) a single orange pheoplast.
The mass of 64 sperms is freed from _ the

« This process is essentially the same as the formation of the so-
called ‘‘ polar bodies ’’ of animal eggs. In the animal a brood of four
gametes is formed from the mother cell, of which three degenerate
(polar bodies), leaving a single egg. In Fucus and allied genera a
- brood of eight gametes is formed, of which all are fertile eggs, or in

some species four, six or seven degenerate.

224 DIFFERENTIATION OF TISSUES, FUCUS

antheridial hair enclosed in the inner wall of the
antheridium.
The bladders containing the eggs and sperms are

Fic. 34.—A, branched hair from conceptacle of Fucus showing

antheridia (a) and “‘sterile’’ cells (sé.). B, bladder, formed.

by inner wall of antheridium, bursting and setting free the sperms
(male gametes). Cand D, stages in development of antheridium.
-E, free male gamete with two flagella ; », nucleus; ph., pheoplast.

forced through the pore forming the mouth of the
conceptacle by the pressure of the swollen walls of
the hairs which fill up most of the space within its

&

FERTILISATION AND GERMINATION OF ZYGOTE 225

cavity. They are expelled from the conceptacle at
low tide when the plants are exposed to the air, and
lie in masses in the film of water adhering to the slimy
surface of the frond. The masses of sperm bladders,
where these are separate from the eggs, can be easily
distinguished with the naked eye by their bright
orange colour due to the orange phzoplasts of the
sperms. The bladders enclosing the sperms and eggs
then burst (Figs. 34, B, 35, C), setting the gametes free
in the film of water.

The spherical egg contains much food material and
is many times the diameter of the sperm. It secretes
a substance of unknown nature which attracts the
sperms, and these, usually in considerable numbers,
cluster round it (Fig. 35, D), and by their constant
oscillation frequently set up a vortex in which the egg
rotates. One of the sperms eventually penetrates
the egg and the sperm nucleus travels across the
cytoplasm and fuses with the egg nucleus (Fig. 35, E,
F,G). Though the sperm nucleus is much smaller than
that of the egg, the amount of chromatin it contains is
seen to be equivalent in bulk, as is always the case.

Development of the Young Plant.—The fertilised
egg (zygote) germinates at once. A cell wall is
secreted, the zygote lengthens, one end becoming
pointed, and the nucleus divides by a cross wall; the
cell corresponding with the pointed end forming the
first attaching organ or rhizoid,t which sticks to any
solid substratum with the help ‘of its mucilaginous
wall. The upper cell divides repeatedly, and this part
of the plant body elongates into a club-shaped form
(Fig. 33, B), fresh rhizoids meanwhile growing out at
the base. Very soon an apical cell is established on

: Root-like organ, Greek piZa, root.
15

226 DIFFERENTIATION OF TISSUES. FUCUS

DIFFERENTIATION OF SURFACE ANDINTERNALCELLS 227

the surface in the centre of the upper end, and by the
more rapid growth and division of the neighbouring
surface cells the apical cell shortly becomes sunk in
a hollow. Up to this point the young plant is nearly
cylindical in form, but now the upper part grows more
quickly in one longitudinal plane than in the plane
perpendicular to it, and thus the wings of the frond
are started.

A marked difference is soon apparent between the
growth of the surface cells and those occupying the
centre of the thallus. The former remain small,
divide actively and form the palisade (surface) layer.
The cells lying immediately below do not divide so
rapidly and form the cortex. The central cells are still
more sluggish and become passively stretched in the
longitudinal direction as the surface increases in
extent by active growth and division of the outer
cells (Fig. 33, A). At the same time the middle layers
of their walls become mucilaginous and separate the
bodies, central cells from one another laterally, thus
giving rise to the characteristic structure of the
medulla?

This same contrast between outer and inner cells
can also be seen in other Brown Seaweeds (not at all
closely allied to Fucus) whose thallus consists of stout

Fic. 35.—A, developing oogonia from surface of conceptacle with
accompanying hairs. B, body cell of oogonium containing eight
young eggs (female gametes). C, freeing of the eight eggs by
bursting of the bladder surrounding them. D, single egg (female
gamete) surrounded by swarming male gametes. E, fertilisation.
The nucleus of a sperm (male gamete) (sp.v.) has penetrated the
egg and is approaching its nucleus. F, sperm nucleus in contact
with wall of egg nucleus. G, sperm nucleus inside egg nucleus.

t The hypha-like chains of thick-walled cells (fibres) which form
the whole of the holdfast, the greatest part of the medulla of the
stipe, and also appear in the midribs of the fronds, begin to grow out
from the cortical and the medullary cells as soon as these separate,
and the “ hyphz’? grow independently in the mucilaginous matrix.

228 DIFFERENTIATION OF TISSUES. FUCUS

cylindrical threads from six to twenty cells thick. The
surface cells are small and densely filled with phzo-
plasts, the central cells are larger and especially
longer, and possess many fewer pheoplasts per unit
volume (Fig. 33, C).

We can only relate this difference between surface
and central cells, which is quite a general feature of
bulky alge, to the much more favourable conditions
for active growth and division in which the surface
cells are placed. They get more oxygen and more
dissolved salts, for all these are obtained directly from
the surrounding water; they are also better illumi-
nated. Thus it appears that the differentiation of
tissues in these algee depends directly on the different
conditions under which the cells develop. The dif-
ferent functions of the tissues in the adult alga are
determined by their structure and position in relation
to the source of food and oxygen and to the growth
of the thallus as a whole. Since growth is localised
at the tips. of the branches, the medullary cells, as
explained on p. 220, owing to their structure and posi-
tion, are the natural channels of conduction of the
organic foodstuffs elaborated by the photosynthetic
cells.

This is a case in which the origin of differentiation
in organisms appears to be directly due to differences
in the conditions in which different cells develop,
i.e. to physical and chemical differences of environ-
ment. The differentiation so initiated is, if i results
in @ workable mechanism, fixed and further specialiect
in higher forms.

In the Laminariacee, the group of Brown Sea-
weeds which have the longest bodies, including the
gigantic Macrocystis and also the big “tangles”

STRUCTURE OF FUCUS 229

(Laminaria) of our own coasts, some of the medullary
cells are very highly specialised for conduction of
organic substances, and are strikingly like, even in
small details, the sieve tubes of the higher plants,
described in Chapter XVI.

PRACTICAL WORK.

Fucus.
Vegetative Structure.

(t) Make a sketch of samples of the thallus of Fucus, showing
the holdfast, the stipe, the branching, the positions of the apical
grooves, and the conceptacles. Note that the stipe is extremely
tough and cannot be broken by pulling with the hands, whereas
the frond can. If the frond is broken the hair-like fibres may
be seen projecting from the broken surfaces.

(2) Examine a cross-section of the frond under the low power
and draw a diagyam"™ of the general plan of distribution of the
tissues, marking :—

(a) the palisade (photosynthetic) layer on the surface,

(0) the cortex of large isodiametric cells, with fewer phzo-
plasts per unit volume,

(c) the medulla of elongated cells running in strand in various
directions and separated by a matrix of cell wall sub-
stance.

Note that the extra thickness of the midrib is caused by the
greater thickness of the medulla in that region.

(3) Examine a cross-section of the frond under the high power,
and make careful drawings * of small samples of the various tissues,
including the thick-walled fibres (‘‘ hyphz ’’), which are mainly
localised just below the cortex of the midrib, on the edge of
the medulla. Note that the conducting cells run horizontally
or obliquely in the wings, longitudinally (so that they are cut
transversely) in the midrib.

(4) Examine two longitudinal sections of the frond (a) cut
through the midrib at right angles to the surface of the frond,
(0) cut parallel to the surface of the frond through the centre.
Identify the various tissues already seen in transverse section,
and from a comparison of the appearance of the cells in the

1 See p. 19. 2 See p. 20.

230 DIFFERENTIATION OF TISSUES. FUCUS

two views deduce their shapes. Draw under the high power
samples of the cells which are not identical in appearance in
transverse and longitudinal sections. Note the thin transverse
walls of the conducting cells.

(5) Examine transverse and longitudinal sections of the stipe,
noting the differences between its structure and that of the frond,
especially (a) the worn surface with radial splits between the
surface cells; (b) the absence of a distinct palisade layer and
the radial rows of cortical cells with relatively thin tangential
walls, indicating that this layer has been largely formed by
secondary cell division parallel to the surface; (c) the mass of
fibres (‘‘ hyphz ’’) mostly cut transversely, forming the medulla,
interspersed with large isolated cells (the original medullary cells).
Make drawings to illustrate these points.

In the longitudinal section of the stipe identify the tissues
seen in transverse section.

(6) Examine a longitudinal section through the apex of the
frond, showing the apical cell and the origin of the adult
tissues.

Sexual Organs.

(7) Examine a section across the reproductive region of the
thallus, first under the low power. Note that the vegetative
tissues have the same general characters as in the purely vegetative
part of the frond, but that the medullary cells here form a net-
work, the cells being cut in various directions : this is the result
of the increase in thickness of this part of the frond, the strands
of cells being drawn out in all, not only in one direction.

(8) Examine the development and structure of the antheridia
and oogonia in the same or in different conceptacles, and draw
as many stages of development as you can distinguish. Note
also the “‘ sterile ’’ hairs in the conceptacle.

(9) Examine fresh material in which eggs and sperms have
been liberated, and appear as little masses on the outside of the
frond. In species with the sexes on separate plants the sperms
can be distinguished by their bright orange colour. Mix some
eggs and sperms in a drop of sea-water or in salt solution of about
the same concentration. Draw under the high power an egg
and some sperms and watch the movements of the latter. The
early stages of fertilisation (conjugation) can often be seen.

CHAPTER XIV

THE SIMPLEST LAND PLANTS: LIVERWORTS
AND MOSSES. THE PTERIDOPHYTA.

HITHERTO we have been dealing entirely (except in
the case of the Fungi) with plants which live in water,
and get the whole of the raw materials of their food
supply from water and the substances dissolved in it.
But very far back in the history of the earth some
plants emerged from the water and established them-
selves on the land, henceforward getting their carbon
dioxide and their oxygen for respiration direct from
the air. These terrestrial plants have, in course of
time, dominated the surface of the earth, and, like
the terrestrial animals, have developed the most
complex structures. The most highly developed group
are the seed plants or flowering plants which are the
most completely adapted to land life. The history
of the evolution of the plant kingdom, beyond the
stage of the earliest land plants, is mainly a history
of increasing adjustment to terrestrial conditions. We
know nothing of how plants succeeded in first emerging
from the water, nor can we follow in detail the
course of their subsequent evolution. But by study-
ing the simplest land plants now existing, plants
which have become stabilised, so to speak, at an early
stage of adjustment to terrestrial conditions, we can

231

232 THE SIMPLEST LAND PLANTS

get some idea of how these conditions affected the
land migrants, and of the adjustments in structure
and function that have taken place. It must be
clearly understood, however, that the simplest forms
of land plants now existing certainly do not represent
the actual stages in the evolution of land vegetation.
They represent rather side lines of the evolutionary
tree which have become stabilised, and are probably
incapable of much further evolution.

The plants known as Liverworts and Mosses are
two groups of green plants, all comparatively small
and with tissues of comparatively simple organisation.
They resemble one another in the structure of their
reproductive organs and in their life histories, but
are distinctly different in the form and structure of
the plant body. In the last respect the Mosses are
decidedly more highly developed than the Liverworts.
Both represent comparatively low stages in adapta-
tion of the green plant to terrestrial life. Most of
them live in situations where the soil and the air are
constantly moist, so that they are in little danger of
losing water by evaporation so quickly that they dry
up. They are not nearly so completely protected as
the seed plants against this risk by having an almost
impermeable waterproof covering, though many of
them are protected to some extent. And they can
absorb water over much or over the whole of their
surface, which the higher plants cannot do. Some
species actually live in places (e.g. rock surfaces, walls,
etc.) where they are liable to dry up, and these can
remain alive, though dormant, in the dry condition ;
when wetted they quickly absorb water and resume
active life.

Liverworts : Pellia.—This is a common _ liverwort

THALLUS OF PELLIA 233

growing on soil among damp herbage, or in marshes.
Pella has no differentiation into distinct stem and
leaves, any more than Fucus has. The body con-
sists of a flat green thallus, which may, be elongated

Fic. 36.—Pellia. A, plant (natural size) growing in bright light
with crowded branches; spn., sporogonia of various ages, the
oldest with long stalks and open capsules. B, part of thallus
on a larger scale, showing forking at the tip; rvh., rhizoids arising
from lower side; u, antheridia; spv., sporogonium with opened
spore capsule showing bunch of hair-like elaters (e/.).

and band-shaped, or may (especially in bright light)
grow slowly in length and branch freely so that the
plant is tufted (Fig. 36). The margins of the thallus
(wings) are often “‘crisped”’ because they have grown
quicker than the thicker central part, or midrib. The
latter bears on its under surface brown rhizoids
(Fig. 36, B, vh.), which enter the soil and absorb from
it water and dissolved salts.

A thin cross-section of the thallus shows that all
the cells are living and may contain chloroplasts,
though these are mainly concentrated in the upper
(sometimes also in the lower) surface layer. This
represents an incipient differentiation of photosynthetic

234 THE SIMPLEST LAND PLANTS

tissue, but there is little differentiation in the structure
of the cells. In the surface cells starch grains may be
detected enclosed in the chloroplasts if the plant has
been exposed to fairly bright light; in the central
cells there are large starch grains which have been
formed by chloroplasts, out of which they have burst,
and which may sometimes be still detected as green
smears on the surfaces of the grains (cf. Pellionia,
p. 126). The rhizoids are seen to be tubular outgrowths
of the cells on the lower surface of the midrib. The
midrib passes gradually into the wings, which at their
outer edges are only one cell thick. Pellia, like Fucus,
grows by division of apical cells at the tips of the
branches of the thallus. The cells cut off from these
divide further to form the whole of the tissue of the
thallus.

Reproduction and Life History.—Pellia, like Fucus,
and also like all the higher plants, is reproduced by
means of sexually differentiated gametes; and as in
Fucus these are formed in sexual organs arising from
surface cells of the thallus. There is an important
difference, however: in Pellia (as in all the Liverworts
and Mosses and in the higher group called Pteridophyta)
the sexual organ, both male and female, is covered in
the mature state by a@ wall consisting of a layer of
cells (Fig. 37, A, C, w), instead of by a cell wall only
as in Fucus. This we may perhaps relate to the
greater protection from evaporation required by sub-
aerial life. The antheridia (Fig. 37, A) are spherical
structures formed on the upper surface of the thallus,
each in a little cavity due to the arrest of cell divi-
sion in the thallus tissue’ just below the spot where
the antheridium is formed and the growing out of
the thallus cells to roof in the antheridium on each

SEXUAL ORGANS OF PELLIA 235

side. The sperms, very many of which are produced
in each antheridium, are not unlike those of Volvox
in shape, and like them have two flagella, but there
is no trace of a chloroplast, and the long, narrow,

Fic. 37.—Pellia. A, part of cross-section of thallus, showing ripe
antheridium; w, wall of antheridium; spe., sperms (male) gametes
coiled up. x 80. B, single male gamete (after Guignard,
x 122). C, four archegoniain various stages of development. On
the left the archegonium is nearly ripe; w, wall of venter con-
taining egg; », neck, which will shortly open at the top. D, group
of multicellular spores (spo.) with spirally thickened elaters (¢/.).

236 THE SIMPLEST LAND PLANTS

spirally coiled body of the sperm consists almost
wholly of nucleus, with a little colourless cytoplasm
at the front end to which the flagella are attached
(Fig. 37, B). The sperm cell is here reduced to its
lowest limits—the paternal nucleus and the flagella,
which are the means of carrying the nucleus to the egg.

The female organs are formed in groups, overhung
by a membrane which grows out from the thallus
behind them. They are flask-shaped, and in each
there is a single egg contained in the body (venter 1)
of the flask, which it practically fills (Fig. 37, C). The
neck of the flask is composed of a single layer of cells
continuous with those forming the wall of the venter.
There is open communication from the egg to the
exterior through the channel of the neck, which is
filled with mucilage. This flask-shaped type of female
organ, which is characteristic of the Liverworts and
Mosses and also of the lower vascular plants (Pteri-
dophyta), is called an archegonium.

When a film of water (rain or dew) is present on
the surface of the thallus the ripe antheridium opens
by the swelling and bursting of its wall and thus sets
free the sperms, which are attracted to the mouth of
the archegonium by the secretion of a substance, prob-
ably cane sugar, produced by the latter and diffusing
out into the water. On reaching the mouth the
sperms become entangled in the mucilage, wriggle
down the neck and one of them fuses with the egg,
which then becomes a zygote and secretes a wall.

The egg of Pellia (Fig. 37, C) is much smaller than
that of Fucus, and this is related to the fact that it is
not cast out from the thallus, and does not, therefore,
have to depend on its own resources during germi-

1 Belly.

SPOROGONIUM OF PELLIA 237

nation. The egg of Pellia is retained, after fertili-
sation, in the female organ, and germinates in situ,
so that the embryo is protected by and can draw
food from the thallus. This is the first example we
have met with of the protection and feeding of the
embryo by the mother organism, a condition which is
of great importance, and is carried much further in
the higher organisms, both plants and animals.
Sporogonium.—The embryo of Pellia, and indeed
of all the Mosses and Liverworts, does not develop into
a completely independent plant, but into a spore-pro-
ducing structure called a sporogonium (Fig. 36, spn.),
which remains attached to the thallus; and the spores
produced by this germinate to produce new Pella plants.
The mature sporogonium produced from the fertilised
egg consists of (1) a foot embedded in and drawing
organic food from the thallus, (2) a long stalk, and
(3) a spherical head or spore capsule which is at first
green. The stalk remains short for some two or three
months in the spring while the spores develop, but
when these are nearly ripe it quickly lengthens
(Fig. 36, A), and in two or three days attains a length
of 2 to 3inches. The spore capsule is now dark, almost
black, and in dry air it opens, four splits running down
from the centre of its upper surface to the base where
it joins the stalk. The four strips of wall thus
separated fold back as four separate flaps, which form
a cross-shaped structure standing out from the top
of the stalk and exposing the mass of spores within
(Fig. 36, A). If the opened capsule be examined with
a hand lens there can be seen, interspersed with the
dust-like spores, a number of delicate threads, the
so-called elaters (Fig. 36, B, el.). These are long cells
with spirally wound thickenings inside their walls

238 THE SIMPLEST LAND PLANTS

(Fig. 37, D, el.). A large bunch of them arises from
the base of the capsule. The elaters are very hygro-
scopic, i.e. very sensitive to small changes in the
amount of water vapour in the air, and as they take
up or lose water from the air they twist about, dis-
turbing and scattering the mass of spores, which float
off into the air.

The spores are large and multicellular (Fig. 37, D,
spo.), i.e. the original spore cell divides into several
cells on ripening, and these contain chloroplasts. The
spores do not form a resting stage in the life history,
they die unless they quickly fall upon damp soil where
they can germinate. One of the cells grows out and
gives rise to the apical cell of the new Pellia plant,
and the first rhizoids grow from the ends of the spore.

Besides the thalloid forms, of which Pellza is one,
there are many Liverworts whose shoots have distinct
stems and leaves, the leaf being a thin membranous
structure consisting of a single layer of cells containing
chloroplasts. The cells of the stem are longer and
often thick-walled, but there is very little tissue
differentiation. These leafy Liverworts live largely on
rocks or tree-trunks in wet climates, and absorb water
through the whole surface of the plant, especially
the leaves. They are attached by rhizoids to the
substratum on which they grow, but the rhizoids are
of no importance in the absorption of water, as those
of Pella are.

Mosses.—The Mosses always have their shoots differ.
entiated into stem and leaf, and most of them have
a certain amount of tissue differentiation, considerably
more than the Liverworts. They are on the whole
larger plants than the Liverworts, and depend more
upon absorption of water from the soil, though a

MOSSES 239

number of species of Mosses are small plants growing
on rocks and tree-trunks which can survive desiccation.

The rhizoids are branched multicellular cell threads
which arise from the base of the stem and enter the
soil, from which they absorb water. The stem has a
central strand of long narrow thin-walled cells from
which the protoplasm has disappeared, and these
form a water channel from the absorbing region
(rhizoids) to the evaporating region (leaves). The
cells of the outer layers of the stem (cortex) have
thick brown walls. The leaves ordinarily consist of a
single layer of cells containing chloroplasts, but there
is often a midrib, several cells thick, consisting of
several cell-layers and possessing thin-walled water-
conducting cells like those of the centre of the stem.
The shoot grows by the division of a single apical cell
at its tip, as in the Liverworts and in Fucus. Each
segment cut off from this apical cell gives rise by
division to the tissue of a single leaf and to the segment
of the stem which bears it.

The Moss plant is a stage further on than the
Liverwort in adaptation to land life. Though they
can absorb water through their leaves, many Mosses
have a regular water current from rhizoids to leaves.
The existence of a definite water-conducting tissue
corresponds with this localisation of absorption in one
part of the body (rhizoids) and of evaporation in another
(leaves), which is the mark of a terrestrial plant, and
which is absent in Fucus and almost absent in Pellia.
This feature is, as we shall see, carried to a much
higher level of development in the Vascular Plants.

Sexual Organs and Sporogonium of Mosses.—The
sexual organs and gametes of the Mosses have a
general resemblance to those of Liverworts, though

240 THE SIMPLEST LAND PLANTS

they differ in certain details of structure. The sexual
organs, accompanied by hairs and surrounded by
leaves, which are sometimes coloured yellow or red,
are borne at the tips of upright shoots that have
stopped growing.

The fertilised egg cell, like that of the Liverwort,
germinates in situ within the archegonium, and
the embryo grows into a sporogonium which, as in the
Liverwort, remains attached to the parent plant. The
Moss sporogonium, however, is of much more compli-
cated structure than that of the Liverwort. It has a
foot, stalk and spore capsule, but the stalk elongates
much earlier and possesses a central water-conducting
strand like that of the leafy stem. Surrounding this
there is sometimes a layer of elongated living cells
which conduct organic food to the developing spore
capsule. These are comparable in function with the
medullary cells of Fucus. The spore capsule itself is
quite an elaborate structure. Its wall consists of
several layers of cells containing chloroplasts and is
covered by an epidermis with stomata, possessing in
fact a structure like that of the leaf of a higher plant.
“Thus the sporogonium, which, during development, is
supplied with organic food by the leafy parent plant, is
able when nearly mature to make some of its own food
from the carbon dioxide of the air and the water and
salts brought up from the leafy parent plant through
the foot and stalk. When the spores are ripe a lid
is detached from the top of the capsule, and distri-
bution of the spores is often assisted by the move-
ments of hygroscopic teeth set round the edge of the
opening, which thus perform the same function as the
elaters of the Liverwort capsule. The spores germi-
nate on damp soil to form an alga-like growth (proto-

VASCULAR PLANTS 241

nema) of branching green cell threads, which spreads
and often persists for a long time on damp soil.
From the protonema leafy moss plants arise by budding.

Vegetative Reproduction.—Both Mosses and Liver-
worts spread largely by vegetative reproduction. The
thalli of Liverworts branch freely as they creep on
the soil, and the older parts, from which the branches
have arisen, gradually die off, so that the branches,
which have sent out rhizoids from their under surfaces,
become detached independent plants. In the Mosses
branching often takes place towards the base of the
shoot, and the lateral branches, creeping on or in the
soil, become attached by rhizoids, their tips growing
up into the air. The decay of the older part of the
shoot makes the new shoots independent plants.
Very many Mosses branch at the base in this way,
and the aerial shoots so produced grow up in close
neighbourhood, their tips forming the surface of the
characteristic moss tufts or cushions.

Vascular Plants (Pteridophytes and Seed Plants).—
The plants above the Mosses in the scale of adaptation
to terrestrial life are marked by several great steps in
advance. First they have true voots instead of rhizoids.
These are branches of the plant body many cells thick
which enter the soil, branch and form very efficient
organs for fixing the plant and absorbing water and
salts from the soil. The branches of the root are
comparable with the branches of the shoot, and like
them consist of complicated tissue structures, but
they differ from them in several important respects.
We shall have to consider these differences in detail
in later chapters. Here we need only note that roots
are colourless, and that they often bear voot hairs,
which are tubular outgrowths of the surface cells

16

242 THE SIMPLEST LAND PLANTS

like the rhizoids of Pellia, increasing the absorptive
surface.

Secondly, there is a highly differentiated double
conducting tissue system—the vascular system—run-
ning throughout the plant, and consisting of a water-
conducting and an organic food-conducting portion.
We have already seen that something of the kind,
though not very highly differentiated, exists in the
stalk of the sporogonium in Mosses. This double
conducting system is a necessity in bulky plants in
which the regions of water absorption, evaporation,
photosynthesis and growth are all localised.

Thirdly, the photosynthetic organs, the foliage leaves,
are typically plates of tissue several cells thick, in
which the photosynthetic tissue (mesophyll) is inter-
penetrated by intercellular air spaces and is covered
externally—in common with the whole shoot—by a layer
of cells (epidermis) without chloroplasts, whose outer
walls, in contact with the air, develop a waterproof
layer, the cuticle. The epidermis and cuticle are pierced
by pores (stomata), each surrounded by a pair of special
cells (guard cells) containing chloroplasts which regulate
the size of the pore (Figs. 50, 51).

+ Most of these characters are foreshadowed in the
Mosses. In the sporogonial wall, for instance, we
met with the essential characters of the mesophyll,
epidermis and stomata of the foliage leaf; and a
cuticle is developed more or less strongly both on the
outer surface of the sporogonium and elsewhere. But
the cuticle is neither so universal nor (usually) so
strongly developed in Mosses and Liverworts as in
the Vascular Plants. Again, we saw that rudiments
of the double conducting system are found in the
stalk of the sporogonium, which is in all respects the

THE PTERIDOPHYTA 243

structure most highly adapted to subaerial life below
the level of the Vascular Plants themselves. And
finally, in one of the highest families of Mosses we
have underground shoots which have many of the
characters of true roots. But in their totality the
features described above are characteristic of and—
apart from a few aberrant forms adapted to special
conditions of life—are universally found only in the
Vascular Planis, which include the Pteridophytes and
the Seed Plants.

The Pteridophyta (Ferns, Clubmosses and Horse-
tails)—Important as these plants are to the botanist,
it is beyond the scope of this book to devote to them
any special study. All the vegetative characters of
the Vascular Plants can for our purposes be more
conveniently considered in connexion with the Seed
Plants. But some knowledge of the outline of the
life histories of Pteridophytes is essential to under-
standing the later phases of adaptation to terrestrial
life which are characteristic of the Seed Plants.

In the Mosses and Liverworts we have two kinds of
reproduction which regularly alternate with one another
in the life history—the sexual process of fertilisation,
which can only be carried out in water owing to the
fact that the sperms are motile swimming cells, and
the process of spore formation and dispersal, which
takes place in air. This emphasises the incomplete-
ness of adjustment of these plants to terrestrial (sub-
aerial) life. In the Pteridophyta, which include three
or more divergent stocks or phyla of plants originating
so far back in early geological time (certainly before
the Devonian rocks were laid down) that we know
nothing of their actual origin, we still find these two
alternating types of reproduction, in spite of the

244 THE PTERIDOPHYTA

development of the structural characters of the vege-
tative body described in the last section, which adapt
them for terrestrial life.

In the Ferns we find a thalloid structure called the
prothallus (Fig. 38, A), resembling in a general way the
thallus of Pellia, which can only live in damp places,
which, like Pella, forms rhizoids (rh.) on its lower
surface, and which bears sexual organs (Fig. 38, A, an,
a, B and D) very much like those of Pellia (but only
on its lower surface). The spetms are formed and
liberated in much the same way, though they differ
in having numerous flagella (Fig. 38, C) instead of
only two, and fertilisation takes place in a film of
water on the surface of the prothallus. The fertilised
egg (zygote) germinates im situ, producing an embryo
within the archegonium wall. The embryo soon sends
a sucker (the foot) into the tissue of the prothallus
(Fig. 38, E). But now comes a wide divergence. Instead
of developing into a sporogonium which remains
attached to the thallus during its whole life, the
embryo of the Fern soon develops a voot which pene-
trates the soil, and a leaf which rises above the soil and
begins to carry on photosynthesis (Fig. 38, F, 7 and J).
The development of the stem is slow, but a second
larger leaf and a second larger root are shortly pro-
duced. During this time the young plant has been
drawing food from the prothallus, but after a while it
becomes quite independent (the prothallus ultimately
dying off), and grows constantly bigger, successive

Fic. 38.—Life history of Fern. A, prothallus from below; rh.
thizoids; av., antheridia; a@, archegonia. B, antheridium con-
taining coiled sperms. C, single sperm with numerous flagella.
D, archegonium; e, egg; », neck. E, section through embryo
drawn in outline, showing prothailus, foot, beginnings of first
root, first leaf, and position of growing point of stem, not yet
developed; vh., rhizoids of prothallus. F, young plant still

LIFE HISTORY OF FERN 245

attached to prothallus with well-developed first leaf and root;
st., stem (still small). G, pinnule of fern leaf from below showing
seven sori (so.) and ripe sporangia (spg.) protruding from below
membrane of sorus. H, single sporangium opening to set free
spores (sp.). I, spore (sp.) germinated to form prothallus; rh.,
rhizoids; a, apical cell.

t

246 THE PTERIDOPHYTA

leaves becoming larger and larger, and more and more
complex in structure till they approximate to the size
and structure of the big compound leaves characteristic
of most kinds of adult ferns. The internal structure
of the body also increases in complexity, developing
the characteristic double vascular system.

Our common ferns are mostly very much larger
than Liverworts or Mosses, and the Tree Ferns of
the tropics are often 20 or 30 feet high. This great
increase in stature is dependent on the development
of the vegetative characters described in the last
section, which provide a greatly improved equipment
for terrestrial life, and enable the fern to grow indefi-
nitely, constantly putting out new leaves and new
roots. The older parts of the plant body tend to
die off. .

The adult fern leaves (fronds) form spores similar to
the spores of Mosses and Liverworts. These are pro-
duced on the under sides of the fronds in little bag-like
structures (sporangia), which are formed in groups
(sori) usually covered with a membrane (Fig. 38, G).
The sporangia can just be seen with the naked eye.
When the spores are ripe the walls of the sporangia
split open and set the spores free to float in the air
(Fig. 38, H). On damp soil the spore germinates to
form the prothallus, which bears the sexual organs.

Here it may be tseful to summarise the life histories
of the Liverwort, the Moss and the Fern in the form
of a table (see opposite page).

The plant bearing sexual organs is called the
gametophyte, the spore-bearing generation (free living
only in the case of the Ferns and other Pteri-
dophytes) the sporophyte. In the Moss the game-
tophyte is more highly adapted to terrestrial life than

LIFE HISTORIES OF LIVERWORT, MOSS AND FERN 247

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248 THE PTERIDOPHYTA

in the other two, and the sporogonium of the Moss
is also much more highly fitted for life in com-
paratively dry air. But it is only in the Fern that
the spore-bearing generation is free living, completely
adapted to terrestrial life, while the prothallus is on
the same level of development as the thallus of Pellia,
and like it must have damp soil on which to grow
and a film of liquid water in which the sperms can
swim for fertilisation. The Fern, therefore, though a
terrestrial plant, is still tied, so far as its sexual repro-
duction is concerned, to semiaquatic conditions, un-
doubtedly a heritage from the aquatic alge from which
it is descended. ,

The majority of Ferns live in damp shady places,
where they are more likely to find suitable conditions
for the production of prothalli from their spores and
the occasional free water necessary for the liberation
and swimming of the sperms. But some depend for
their propagation almost entirely on vegetative repro-
duction, and these can live in comparatively dry
habitats. The common bracken fern (Pteridium aqui-
linum) is a good example. It flourishes on rather
dry sandy soil, its underground stems spreading far
and wide and continually sending up new fronds,
while the older parts gradually die off. It can and
does produce prothalli from its spores when they fall
in a suitable damp spot, but this is in many localities
a rare possibility, and the great bulk of this vigorous,
aggressive, rapidly spreading plant is produced vege-
tatively.

The Small-Leaved Cone-Bearing Pteridophytes.—The
Clubmosses (Lycopods) and Horsetails (Equiseta) differ
widely from the Ferns in general appearance, though
the broad outlines of their life history are essentially

HORSETAILS AND LYCOPODS. HETEROSPORY 249

the same. Instead of large compound fronds they
have small scale-like leaves, and in the Horsetails
these are so small that the great bulk of the photo-
synthesis is done by the surface tissues of the stem.
Another point in which they differ from the Ferns is
that their sporangia are borne in cones, i.e. in con-
nexion with small crowded leaves, mostly differing more
or less from foliage leaves, at the ends of some of the
shoots (Fig. 39, A). The prothalli also are unlike those
of Ferns, being cylindrical or thin, branched structures
instead of flat. In many of the Lycopods the pro-
thalli are not green, but live saprophytically in humus
or humous soil. These prothalli grow very slowly,
and the sporophytes depend very largely on vegetative
reproduction. The British species of Lycopods live
mostly on the hills of the north and west where the
\ soil is humous, i.e. contains a large amount of plant
débris, which decays very slowly indeed, owing to the
cool damp climate. The Horsetails mostly live in
marshes, by pond sides and in damp places generally,
but one species, the Field Horsetail (Equisetum arvense),
grows very commonly on hedgebanks and the edges of
fields, though on stiff, moisture-retaining soils. It is
the only British Pteridophyte occurring in such situ-
ations and holding its own with the dominant seed
plants—the common hedgerow plants. Like the other
Horsetails, it has long underground stems which spread
below the soil and send up the green aerial shoots at
intervals.

Heterospory.—In the great majority of existing
Pteridophytes the spores are all alike, but in a
few two kinds of spores are produced, large mega-
spores, much larger than ordinary spores, only one or
four being produced from a single sporangium, and

250 THE PTERIDOPHYTA

small microspores, about the same size as ordinary
spores (Fig. 39, C). These are formed in separate
sporangia (Fig. 39, B). In these heterosporous species the
ordinary kind of free-living prothallus, bearing both
male and female sexual organs, is not produced. The
megaspore on germination produces a small mass of
cells at its apex, the bottom of the spore being some-
times afterwards filled with cells (Fig. 39, G), and on
this small prothallus one or more archegonia are
formed. The microspore produces a prothallus of even
fewer cells, in most cases inside the microspore wall
(Fig. 39, D), and with the exception of one or two, the
whole of these form a'single antheridium (Fig. 39, D).
The spore absorbs water and the wall bursts, setting
free the sperms (E, F). Fertilisation takes place as in
the ordinary (homosporous) Pteridophytes. The ferti-
lised egg obtains the food which enables it to grow
into a self-supporting plant mainly or entirely from
the organic food supplies stored in the megaspore (G).

The far-reaching reduction of the prothallus and the
suppression of its powers of independent growth and
nutrition that we see in these heterosporous Pteri-
dophytes involves the direct dependence of the young
sporophyte produced from the fertilised egg not only on
the gametophyte but on the spore (megaspore) which
produced it, instead of on a free-living sexual genera-
tion, as in the homosporous Pteridophyte. This opens
the door, so to speak, to a further adaptation to land
life, in which the megaspore is retained in the
sporangium, and the sexual generation need no longer

Fic. 39.—Selaginella, a heterosporous cone-bearing Pteridophyte.
A, end of leafy branch, with three cones (natural size). B, portion
of stem of cone with megasporangium (left) opened showing
megaspores, and (right) microsporangium setting free microspores.
C, microspore (mi.) and megaspore (me.) showing thick wall

HETEROSPORY 251

and cell inside (prv.) from which the prothallus will be formed. D,
microspore germinated to form antheridium which completely
fills the spore cavity. E, microspore opened with sperms em-
bedded in mucilage formed from breakdown of walls in water.
F, two free flagellated sperms. G, megaspore opened and
germinated to form prothallus with three archegonia (ar.), the
eggs in two of which have developed into embryos at the expense
of the prothallial tissue, while the third (a73) is unfertilised
and still contains the egg.

252 THE PTERIDOPHYTA

be produced under the conditions of moist soil and
air where it can live as a semiaquatic and find the
free water necessary to the process of fertilisation by
motile free swimming male gametes. The freedom
from this necessity is not however attained by the
existing heterosporous Pteridophytes, in which the
microspores still germinate only on damp soil and
the sperms still have to swim to the archegonia in a
water film. The necessary step to complete freedom
from the semiaquatic habitat is the retention of the
megaspore in the megasporangium, where it may
germinate and produce the eggs in a sheltered position,
withdrawn from the danger of desiccation, and the
bringing of the microspores to the megasporangium
or its immediate neighbourhood, so that the male
gametes . produced from them may reach the egg
without the presence of external water. This is the
step which, as we shall see in the sequel, has been
taken by the Seed Plants.

PRACTICAL WORK.

(1) Examine plants of Peilia, if possible growing plants. Sketch
the form of the band-shaped or crisped green branching thallus,
with thick midrib attached to the soil by rhizoids and passing
gradually into the thinner wings at the edges.

(2) Examine a transverse section of the fresh thallus, showing
the uniform thin-walled tissue. Note that the chloroplasts are
mostly concentrated in the upper (sometimes also in the lower)
surface layer of cells. In these small starch grains can be seen
if the plant has been well illuminated. The central cells contain
large starch grains on the surface of which can sometimes be
seen the remains of a chloroplast. From the lower surface of
the midrib rhizoids arise, each as a tubular branch of a single
cell.

Draw under the high power samples of the cells of the upper
and lower surfaces with rhizoids and of the central cells.

3) Place some of the contents of the ripe spore capsule of
Peilia in a drop of dilute glycerine and examine the spores and

PRACTICAL WORK 253

elateys under the high power. Note that each spore is divided
into several cells packed with chloroplasts.

(4) Examine a prepared longitudinal section through the centre
of a spore capsule showing stalk, capsule wall, spores and elaters.

(5) Make a sketch of a single plant of Funaria (or other
moss) with the naked eye or under a hand lens. In Funaria
note the short axis (stem) with a tuft of brown rhizoids at its
base, and bearing broad delicate green leaves. The leafy axis
may bear a slender stalk terminating in a nodding pear-shaped
spore capsule.

(6) Detach a single leaf, mount in a drop of water and examine
under the high power. Observe that the leaf consists of a single
layer of cells more or less alike, except the midrib, which is com-
posed of elongated cells. Note the leaf cells have thin walls:
each is lined with a layer of cytoplasm containing chloroplast
and enclosing a large central vacuole.

(7) Examine some of the brown rhizoids in dilute glycerine,
and note that they are branching threads of different thicknesses
composed of long cylindrical cells with oblique end walls.

(8) Examine a prepared transverse section of a moss stem
showing the outer cortex of thick-walled cells, the inner cortex
of thin-walled cells, and the strand of narrow thin-walled water~
conducting cells in the centre.

CHAPTER XV

THE SEED PLANTS: FORMS AND LIFE
HISTORIES

TAKEN as a whole the Seed Plants represent the most
complete adaptation to terrestrial life that we find
within the plant kingdom; and, correspondingly,
they are the plants which now dominate the face of
the earth. On the vegetative side this adaptation is
represented by, the broad features of organ and tissue
development described on pp. 241-2, which are common
to all the vascular plants, but are carried to a higher
pitch of specialisation in the Seed Plants than in
the Pteridophyta. On the reproductive side it is
represented primarily by the seed itself and by the
structures and processes leading up to and accompanying
its development. The seed is a direct metamorphosis
of the ovule: in other words the ovule changes directly
into the seed. And the ovule is simply a megaspor-
angium containing a single megaspore, which has
germinated, not by producing any structure external
to itself, but by producing the female gamete or egg
inside the megaspore as a result of the division of
the megaspore nucleus. The male gamete, produced, as
in the heterosporous Pteridophytes, inside the micro-
spore, is brought to the neighbourhood of the mega-
sporangium still contained in the microspore, and then,
bythe growth of a germ tube put out from the microspore,

to the megaspore and to the egg itself. The process
254

WOODY PLANTS AND HERBACEOUS PLANTS 255

of conjugation of the gametes. is thus rendered quite
independent of external water, and the gametes are
withdrawn from the danger of desiccation. The
fertilised egg also germinates inside the megaspore,
where it produces the embryo, which develops up to
a certain point and then enters upon a resting stage.
The ovule (megasporangium) containing the resting
embryo, together with a store of food providing for
the development of the embryo into the free-living
plant, is called the seed. Here then not only the sexual
generation, but also the early stages of the new individual
developed from the fertilised egg ave produced within
the body and nourished at the expense of the parent
sporophyte.

The hundreds of thousands of existing species of
Seed Plants are exceedingly various in form and life
history. We can only consider a few of the general
types into which these can be grouped.

First, a broad distinction, though by no means an
absolute distinction, can be drawn between woody
plants and herbaceous planis. The subaerial shoots
of woody plants (trees and shrubs) persist from
year to year, and constantly form new portions of
the shoot system which are continuations of the portions
already formed, At the same time the parts already
formed in most cases grow continuously in thickness,
and the new layers so added are mainly composed
of hard woody tissue. Thus the whole plant body
constantly increases in size.

In the herbaceous plant, on the other hand, the
subaerial shoot consists mainly of soft tissue and is
short-lived, generally dying down after one growing
season. The whole vegetative plant may die after
one season’s growth, when its seeds have ripened

256 THE SEED PLANTS: FORMS AND LIFE HISTORIES

(annual plants), or the aerial shoots alone may die off,
leaving persistent underground shoots (perennial plants).
The underground shoot may be woody and persist
indefinitely, constantly increasing in thickness as
well as in length like a woody subaerial shoot, or while
growing at the apex from year to year it may con-
tinuously die off behind. In this chapter we shall
consider some of the leading herbaceous types of seed
plant body.

Erect Herbaceous Annual.—This is the simplest
type of herbaceous plant, but is not to be considered
the most primitive, the form from which the other
types are derived. On the contrary, it is believed by
botanists, on very good grounds, that woody perennial
plants are the most primitive forms of the higher plants,
and that the herbaceous, and finally the annual plants,
have been derived from these.

The body of an annual (Fig. 40, A) consists of a
descending portion, the primary root or taproot, and its
branches. These penetrate the soil, fix the plant, and
absorb water and inorganic salts from the soil; and the
green shoot, which ascends into the air and consists of
the axes (stems) and typically flat plate-like organs borne
on them—the foliage leaves.

The first leaves borne by the stem—those nearest
the root—are typically a pair already formed on the
embryo in the seed, and generally simple in form:
these are called the cotyledons. The part of the stem
between the cotyledons and the top of the root is
called the hypocotyl. In the young seedling plant

Fic. 40.—A, diagram of annual plant: 47., taproot; hyp., hypocotyl ;
cot., cotyledouns ; /, foliage leaf ; a¥., axillary bud; 7.b., terminal bud ;
fi.b., flower bud. B, part of underground stem (rhizome) of Coral
root (Dentaria) showing fleshy scale leaves (sc.l.) of adventitious
roots (7), lateral bud (/.0.), aerial shoot (a.s.), arisen from the ter-

HERBACEOUS TYPES 257

minal bud. The rhizome is branching from two lateral buds. C,
base ofa young plant of the Yellow Loosestrife (Lysimachia vulgaris),
with lateral shoots forming rhizomes. cot, cotyledon; #7., tap-
root; a.s;, aerial shoot from epicotyl; a.s., from axil of coty-
ledon; rh., rhizomes.

17

258 THE SEED PLANTS: FORMS AND LIFE HISTORIES

the terminal bud is produced between the two cotyledons.
This consists of an apical (primary) shoot meristem
bearing the rudiments of leaves on its sides. It grows
out into the main shoot of the seedling (epicotyl), the
stem elongating and the leaves developing and unfolding.
The levels of the stem from which a leaf (or a pair or
whorl of leaves) actually arise are called nodes, and
the bare stretches of stem between the nodes are inter-
nodes. Branches of the shoot are developed from
lateral buds, which are nearly always developed in the
angle between a leaf and the internode above the leaf
(axil of the leaf), and are hence called axillary buds.
Each axillary bud repeats the structure and develop-
ment of the terminal bud. A bud is maintained at the
tip of the main axis and at the tip of each branch
(terminal buds of the branches), the meristem being
constantly renewed by cell division as its products
develop into mature tissue.

Eventually flowers are produced. These are parts
of the shoot or separate shoots (i.e. the whole product
of a lateral bud) bearing specialised leaves (floral
leaves), some of which produce the gametes. Sometimes
one flower is produced from the terminal bud, generally
a larger number from various axillary buds. A bud
which is forming floral leaves is called a flower bud.
After the flowers have opened, conjugation of the
gametes produced by the floral leaves has taken place,
and the zygotes have grown into embryos within the
seeds, which are contained in the modified remains of
the flower called the fruit, the whole of the vegetative
part of an annual plant—the entire root and shoot—
dies, the life of the species being continued solely in
the seeds, which under favourable conditions in due
course germinate to form new plants.

RHIZOMES 259

Rhizome-forming Plants.—Many herbaceous plants,
indeed the great majority of species, do not merely
form a single aerial shoot system which dies at the
end of the growing season, but, in addition to aerial
shoots, an underground or surface shoot or shoot system
which survives from one season to the next, and is
called a rhizome. The rhizome may be formed by the
whole epicotyl of the seedling plunging into the earth,
and growing horizontally instead of vertically upwards
to form an aerial shoot ; but in most cases the rhizome
is produced by an axillary bud near the base of the
aerial shoot growing out horizontally. Usually the
rhizome bears scale leaves (Figs. 40, B, 41, A, C, sc.l.)
which remain small and do not turn green, but some-
times it bears foliage leaves which grow up into the
air on long stalks. From the rhizome one or more
aerial shoots, usually bearing both foliage leaves and
flower.buds, grow up into the air. Very often the
terminal bud of the rhizome turns up and produces
a single aerial shoot (Figs. 40, B, 41, B, a.s.), the growth
of the rhizome itself being continued by the growth
of one or more axillary buds. Aerial shoots also often
arise directly from axillary buds.

At the end of the growing season the aerial shoots
with their foliage leaves die down, but the rhizome
remains alive in a dormant condition till the beginning
of the next growing season, when growth is renewed
from terminal and lateral buds and a fresh portion
of the rhizome and new aerial shoots are produced. The
older parts of rhizomes commonly die off and decay. In
countries like England, with comparatively mild winters,
the growth of the rhizome of many species continues
throughout the winter except during periods of severe
frost, and new leaves and buds are_slowly produced,

260 THE SEED PLANTS : FORMS AND LIFE HISTORIES

Fic. 41.—A, young plant of the Field Bindweed (Convolvulus arvensis),
showing the thin white underground shoots (rh.) which ramify deeply
in the soil and make the plant a troublesome weed. B, hori-
zontally growing rhizome of a grass. C, plant of the Chickweed
Wintergreen (Tvienialis) which has arisen from the tuber (#1). The
new tuber (#2) has arisen at theend of the rhizome. rh., rhizomes;
y, adventitious roots ; sc.l., scale leaves; a.s., aerialshoots. (After
Warming.)

RHIZOMES 261

in preparation for the more active growth and the
formation of the aerial shoots in the next spring.

Roots are always produced on ‘rhizomes, either gene-
rally distributed on the surface or localized at the bases
of the aerial shoots (Fig. 41, B), or at the bases of foliage
leaves arising directly from the rhizome. Such roots,
arising directly from a stem, are sometimes called
adventitious roots as opposed to the taproot and its
branches, which in rhizome-growing plants are short-
lived. Organic food materials (starch or oil—sometimes
sugar—and proteins) are stored in rhizomes during the
winter, and are ‘‘ mobilised,” i.e. converted into soluble
forms when new growth takes place.

Rhizomes may be short and compact, the axis of the
rhizome shoot being then usually vertical or oblique
and growth being very slow (‘‘ male fern,’ dandelion,
plantain) ; or they may be elongated and horizontal,
often running for considerable distances below the
surface of the soil (Figs. 40, B, 41). It is this type of
rhizome, with brown or dingy coloured surface, from
which the name (meaning “ root-like organ ’’) is taken.
But rhizomes are quite sharply distinguishable from
roots because they bear leaf structures and have the
anatomical construction of stems, not of roots. The
spread of many plants, such as sedges, rushes, some
grasses (e.g. couchgrass or “ twitch”’), is due to this type
of rhizome, which grows and branches freely, sending
up aerial shoots from the upward turning terminal,
or from lateral buds. Stolons or runners are very
thin, horizontal, quickly growing shoots, which run
usually upon the surface of the soil (e.g. strawberry
runners), and do not bear roots. The terminal bud
eventually turns up, thickens, produces a rosette
of foliage leaves, strikes roots down into the soil,

262 THE SEED PLANTS: FORMS AND LIFE HISTORIES

and becomes an independent plant. These offsets
are regularly used by gardeners to propagate the
plant. :

Some horizontal rhizomes, on the other hand, are
thick, fleshy and comparatively slow growing (Solomon’s
Seal), and these store considerable quantities of organic
food reserves during the winter.

Tubers.— When the food is localised in a definite
portion of an underground stem, the rest being thin,
the swollen portion is called a tuber (Fig. 41, C, 2).
The beginnings of tuber formation are well seen in the
Chinese Artichoke (Stachys tuberifera). Many species of
Stachys have uniform rhizomes, but in the Chinese
Artichoke several successive internodes are swollen,
the nodes being somewhat constricted by comparison
with the internodes and bearing alternating pairs of
triangular scales. A thinner portion of the rhizome
follows, the terminal bud turning up to produce the
aerial shoot. The lateral buds which continue the
growth and branching of the rhizome grow out to form
several thin internodes, followed by the thick tuberous
portion in each case, so that the tubers are connected
together in branching chains.

In the Potato plant (Solanum tuberosum) the main
shoot is erect, but thin lateral shoots are formed
from buds in the axils of basal leaves, and each of these
grows horizontally or obliquely downwards and swells
at its apex into a potato tuber. When very young
(about the size of a pea) the tuber is seen to bear minute
scale leaves, but these do not grow as the body of the
tuber swells, and when the latter is full grown their
scars appear, with the bud in the axil of each, as the
“eyes” of the potato. The “rose”’ end of the potato
is the apex of the tuber where the “ eyes ”’ are crowded.

CORMS 263

When fully grown the stalk separates from the tuber.
If left in the ground over the winter, the eyes grow out
next year into the aerial “‘haulms’”’ of new potato
plants. Roots (‘‘ adventitious roots’’) are produced
from the base of the haulm just above the point at
which it springs from the tuber.

Corms.—This name is given to a local thickening at
the base of the erect aerial shoot, which carries the life
of the plant from one growing season to the next. At
the end of the growing season the aerial shoot dies
and falls off, leaving a scar on the top of the corm,
which remains dormant during the autumn and winter,
and produces new aerial shoots from axillary buds in
the next growing season. The garden Crocus is an
excellent example.

In the autumn, when the corm is “ ripe”’ and ready
for planting out, it is a rounded structure flattened
at top and bottom and covered with thin brown
membranous scales, which are inserted along horizontal
lines encircling the corm and appearing as brown
circular scars when the scales are stripped off. In a
large vigorous corm there are two or three stout buds
covered with white scales and projecting through the
brown covering at the top of the corm. These can
be seen to spring from the axils of the brown scales
around the apical scar left by the detachment of the
aerial shoot. Sometimes there are smaller buds on the
sides of the corm in the axils of lower scales.

When the corm is planted, a circle of roots, the be-
ginnings of which are already laid down in the ripe
corm, first grow out, and growth soon begins in the
axillary buds, which produce the green shoots that
push up through the soil in February and flower in
March. The rudiments of the foliage leaves and flowers

»

264 THE SEED PLANTS: FORMS AND LIFE HISTORIES

can be clearly seen in longitudinal section of a bud
taken in the autumn. The food which supplies the
material for growth is stored, largely in the form of
starch, in the tissues of the corm. When the aerial
shoots are fully developed the base of each begins to
swell by the growth in thickness of the stem tissue,
and starch, made from the sugar produced by the
foliage leaves, is deposited in this tissue. Thus a new
corm is formed at the base of each aerial shoot, and
each new corm goes on growing so long as the foliage
leaves are alive (May or June). During the summer
or autumn the new corms, covered by the scale leaves
which were developed at the base of the aerial stem,
and which gradually became dry and brown, become
detached from the old corm, which is now finished and
shrivelled, while the upper part of the aerial shoot,
also dead, is detached from the top of the corm. Mean-
while axillary buds develop in the axils of the scale
leaves of the new corms, swell and produce the rudi-
ments of next year’s foliage leaves and flowers, so that
by October the new corms are “ ripe.”

If the Crocuses are left in the ground, the new corms
develop where they are, and after a few years become
very crowded, so that the new corms are apt to be
small and feeble, often possessing only one new axillary
bud which is strong enough to flower. That is why
a better crop of flowers is produced if corms are taken
up as soon as the leaves die, and only the most vigorous
selected for replanting.

Other examples of plants which go through the
winter in the form of corms are the Meadow Saffron
(Colchicum autumnale)—found in meadows and woods
especially in the west of England—which produces its
new corm in the same way, but at the side of the old

BULBS. PERENNATION 265

one; the bulbous buttercup (Ranunculus bulbosus) ;
common in dry pastures; and Cyclamen.

Bulbs.—The life history of a plant which perennates
by means of bulbs is very similar in general features
to that of the corm-forming plants, the distinction being
that in the bulb the store of food is contained in the
scale leaves (bulb scales) of the winter shoot, the stem
being represented by a disc at the base of the bulb.
On this are inserted the outer membranous brown
protective scales, and the massive bulb scales which
form the greater part of the body of the bulb.

In the Tulip, which is a suitable type for study,
there is however another difference. The aerial flowering
shoot is produced from the terminal bud of the bulb,
not, as in Crocus, from axillary buds. The new bulbs
are formed, not by the swelling of part of the aerial
shoot, but by the formation of buds in the axils of the
bulb scales. These buds gradually swell during the
growing season, as the old bulb scales become emptied,
and thus produce the new bulbs inside the old one.
It is even more important to dig up and remove tulip
bulbs from the-soil at the end of the growing season
if the crop is to be maintained.

Some bulb-forming plants (onions, lilies, etc.) also
produce under certain conditions small bulbs called
bulbils in place of all or some of the flowers. These
become detached, fall to the ground, and in suitable
soil strike root and reproduce the plant.

Perennation, Multiplication and Spreading. — The
types of more or less specialised underground shoots
described—rhizomes, tubers, corms and bulbs—all serve
to carry the plant on in the vegetative form from one
growing season to the next, for they can exist unharmed
in a dormant condition in the soil through the winter,

266 THE SEED PLANTS: FORMS AND LIFE HISTORIES

when the aerial shoots have died; and they store food
from which the next season’s growth is started.
Primarily these underground shoots are thus a means
of perennation. Many plants which flower in early
spring have bulbs, corms or rhizomes, and are able to
make new growth quickly because of the great store
of organic food at their immediate disposal.

When a rhizome branches and the old parts die off
so that the branches are separated, the plant is pro-
pagated vegetatively, i.e. the number of individuals is
multiplied by purely vegetative means, just as we
saw happened in Mosses and Liverworts, and as happens
also in Spirogyra when the cell thread, after increasing
in length by cell division and growth, splits into lengths,
and produces a corresponding number of new plants.

It is essentially the same process when a plant
produces many tubers, or a Crocus corm several aerial
shoots, at the base of each of which a new corm is
formed, or when a bulb produces several new lateral
bulbs. Though the plant is thus multiplied, the new
individuals are developed close together, and compete
for the same space, light and food—the species is not
dispersed. But dispersal occurs, though gradually,
if the underground stems spread widely, producing
new roots and new aerial shoots from their tips. The
advantage of the bulb or corm is that it is better pro-
tected from desiccation than many rhizomes, and is
thus particularly well addpted to plants living in
climates with a very dry season. South Africa, for
instance, and the Mediterranean region have a large
number of bulb and corm producing species. The
British climate has no very dry season, and there are
many more rhizomatous plants which are able to
stand our comparatively mild winters perfectly well,

SHOOTS AS POTENTIAL INDIVIDUALS 267

though the rhizomes are not specially well protected,
and often grow slowly throughout the winter.

The methods of vegetative propagation in the higher
plants show that any shoot which has the power of
rooting at its base may be regarded as a potential
individual. In artificial propagation by means of
cuttings, a branch shoot is cut off and the cut end
stuck into damp soil. Roots are produced from this
surface, and the detached shoot becomes a new plant.
This extensive power of producing new individuals
vegetatively is a character which separates the highest
plants very sharply from the highest animals. The
bodies of the latter are very highly integrated, i.e.
they form very closely knit wholes, so that none of their
parts can live independently, while the bodies of plants,
with their indefinite growth and number of branches,
constantly repeating the structure of the original
shoot, and able under suitable conditions to live inde-
pendently, do not form nearly such highly integrated
individuals. This is largely connected with the fact
that the bulk of their bodies is composed of com-
paratively undifferentiated living cells which are not
so far removed from germ cells as those of the more
highly specialised tissues of the higher animals. (Cf.

pp. 206-7.)
PRACTICAL WORK.

Annual Herbaceous Plant.

(1) Examine a small herbaceous plant with a single main shoot
and ataproot. Makeasketch of the plant showing the following
parts : taproot, branch roots, hypocotyl, cotyledons, nodes, internodes,
leaves, leaf veins, terminal buds, axillary buds, flowers or flower buds.

[Any small quickly developing annual is suitable if it retains
its cotyledons till it produces flowers. Chickweed (Siellaria
media) or one of the annual Veronicas—V. agrestis, Tournefortii
or hederefolia—does very well.]

268 THE SEED PLANTS: FORMS AND LIFE HISTORIES

Rhizomes.

(2) Examine and sketch a portion of the Couchgrass or
“Twitch ’’ (Agropyrum repens) or any grass of similar habit.
The main stem of the plant is the rhizome, bearing scale leaves
and roots. The aerial leafy shoots arise from buds in the axils
of the rhizome scales. Branching of the rhizome is secured by
the outgrowth of other axillary buds which grow horizontally.
Note the abundant production of roots at the bases of the aerial
shoots, and the sharp points formed by the folded scales covering
the terminal buds of the rhizome branches. These can penetrate
stiff clay and often bore through objects such as potatoes which
they may encounter. i

(3) Compare the stout, more slowly growing rhizome of Solo-
mon’s Seal (Polygonatum). The scales which cover the terminal
bud fall off, leaving only brown circular scale scars on the surface
of the rhizome. The single aerial shoot is formed each year by
the turning up of the terminal bud. The continuation and
branching of the rhizome are secured by other buds formed in
the spring. Compare the early spring condition with the summer
condition (museum or herbarium specimens).

Tubers.

(4) Sketch the tuberously thickened branching rhizomes
of the Chinese Artichoke (Stachys tuberifera). Note the thin
portions of rhizome preceding and following the tubers, the
constricted nodes, with triangular scales and axillary buds in
opposite pairs, alternating on succeeding nodes, and the up-
wardly turned terminal bud.

(5) Examine potato tubers—a seed potato ready for planting,
and a sprouted potato are most instructive. Mark on your
drawings the scar of the tuber stalk, the eyes, the terminal bud,
and in the sprouts the scale leaves, developing foliage leaves and
adventitious roots. Examine a specimen showing early stages
in the formation of tubers with the stalk arising from the axil of
a leaf on the parent plant. and the minute scales on the very
young tuber.

Corm.

(6) Crocus, autumn stage. Carefully peel off the dry mem-
branous scales and note that they are inserted one above the
other, the lower ones overlapping the upper on the side of the
corm, leaving brown circular scars when detached. Identify
the stem scars at the top and bottom of the corm. The former
is the scar of last spring’s flowering and leafy shoot, of which

PRACTICAL WORK 269

the present corm is the swollen base. The scar at the bottom is
the attachment of the present to last year’s corm, which has
shrivelled and fallen off. Note the circle of young roots ready
to grow out round this scar.

With a sharp knife cut longitudinally through the corm, taking
care to pass through the terminal scar and through one of the buds
at the top of the corm, and note in the bud (a) the external scale
leaves, (b) the rudiments of the foliage leaves, (c) one or more flower
buds. Make a drawing of the cut face showing the vascular
cylinders passing to the top scar and to the buds. Test the cut
surface for starch.

(7) Compare the flowering stage of the Crocus plant, and-note
the swelling at the bases of the aerial shoots which will become
the new corms.

Bulb.

(8) Tulip, autumn stage. Cut longitudinally exactly through
the centre of the bulb. Note the disc-shaped stem at the base
with point of attachment to the old bulb, rudiments of roots,
membranous protective scales, fleshy bulb scales, terminal bud
of aerial shoot with rudiments of foliage leaves and flower.
Look for a small bud or buds in the axils of the bulb scales.
Test the cut bulb scales with iodine.

(9) Examine the flowering stage and note the roots grown out
from the edge of the bulb stem, and the flowering shoot developed
from the summit of the bulb. Cut longitudinally through the
bulb and note the flabby depleted bulb scales, and the new bulb
or bulbs arisen from the axillary bud or buds seen in (8).

(10) Examine any other examples of underground shoots or
of vegetative propagation that may be available.

ey ca XVI

THE TISSUE ELEMENTS OF SEED PLANTS

BEFORE we study the microscopic structure of the vege-
tative organs of the Seed Plants it is desirable to have
some knowledge of the different elements of which
the tissues are built up, i.e. of the different kinds of
more or less specialised cells of which the bodies of the
higher plants are composed. If we can recognise the
elements of the different tissues we meet with in the
structure of root, stem or leaf we shall the more readily
grasp the way in which these tissues are arranged and
interrelated in these organs.

All the various elements that make up the tissues of
a vascular plant are derived from embryonic cells such as
are found in the primary meristems at the tips of the
branches of the root and shoot (see Chapter VI, p. 102).
Most of the body of a herbaceous plant, the “‘ soft ”’
tissues, are composed of living cells with cellulose wall
of moderate thickness, and a large central vacuole,
so that the cytoplasm forms a layer on the cell wail.
The development of such cells from the embryonic
cells was described in detail on p. 107. This sort of
tissue is called parenchyma, and it forms most of the
‘“‘eround tissue ’’ of the plant body; it is also found
associated in masses, strands or single cells with the
more specialised tissues. The mesophyll of the leaf
(pp. I15) is composed of this sort of cell in which the
numerous plastids embedded in the cytoplasm have

270

LIVING PARENCHYMA 271

become chloroplasts. Certain adult living cells, how-
ever, differ in one way or another from this typical
structure.

Fic. 42.—Development of adult liying cells (parenchyma) from
embryonic cells. A, embryonic (meristematic) cells. B, begin-
ning of vacuolation. C, fully grown cells (here much elongated)
with characteristic large central vacuole; c.w., cell wall; cyt.,
cytoplasm; x», nucleus; vo., nucleolus; v, vacuole.

272 TISSUE ELEMENTS OF SEED PLANTS

Cells rich in Protoplasm. Secretory Cells : Protein
Cells.—Some cells remain densely filled with protoplasm
even when adult, or at least contain much more
cytoplasm in proportion to cell sap than the average
tissue cell. The nucleus is large and conspicuous.
Prominent among these are the cells (gland cells or
secretory cells) whose special function is the secretion
of a definite substance, such for instance as the cells
of the nectaries of flowers which secrete a sugary fluid—
nectar; as also the cells which produce a particular
enzyme, such as are found in seeds where large quantities
of stored food material are brought into a soluble
form in a short time, and diastase, cytase, or protease
is produced in comparatively large quantities. The
glands of insectivorous plants, also, secrete proteolytic
enzymes which bring the proteins of their victims’
bodies into soluble forms. Secretory cells, when active,
have large conspicuous nuclei, and are often destitute
of a vacuole. They closely resemble in structure and
appearance the cells of the glandular epithelium of
animals (Fig. 5, B) whose function is the same.

A kind of cell which resembles these in appearance
and structure, and may indeed be said to belong to

1 It is to be understood that the production of such substances
is by no means the monopoly of “secretory cells.” They may be
produced and are produced in the most various living cells. The cells
called secretory are those which are specialised in this direction, and
continuously produce large quantities of the substances in question.

Fic. 43.—Living tissue elements of the Seed Plant. A, secreting
(glandular) hair arisen from epidermis of leaf. The four cells
of the hair are richly protoplasmic and have conspicuous nuclei.
Compare with living cells of epidermis and mesophyll below
which contain much less protoplasm; s.h., secretory hair; 1,
nucleus; v, vacuole; ep., epidermis; mes, mesophyll; ch.,chloroplast.
B, secreting cells from seed of Rye (Secale), with dense granular
cytoplasm and conspicuous nucleus (#). C, part of sieve tube
in longitudinal section showing two sieve plates (covered with
callose) and funnel-shaped slimy cell contents (contracted away
from wall). On the right two companion cells (protein cells)
with dense cytoplasm and conspicuous nuclei. On the left an

ACTIVE LIVING CELLS 273

ordinary parenchyma cell. D, sieve plate and companion cell
as seen in cross-section. E, epidermal cell with large vacuole,
but large conspicuous nucleus in contact with local thickening
of outer cell wall which is being formed; , pit in lateral wall.
F, large water bladder representing a single much swollen cell
of the epidermis, with little cytoplasm but large nucleus; ep.,
ordinary epidermal cells; mes., mesophyll.

18

274 TISSUE ELEMENTS OF SEED PLANTS

the secretory cell type, is the protein cell (Fig. 43, C),
which is constantly associated with the cells (sieve tubes)
that conduct organic food substances. The exact
functions of these protein cells are not understood :
perhaps they secrete enzymes which act on the substances
passing along the sieve tubes. Protein cells have
large conspicuous nuclei, and are very rich in protein
contents.

Sieve Tubes are the main channels of transport of
organic substances from one part of the plant to another.
Each sieve tube consists of a row or chain of elongated
cells (Fig. 43, C), comparable with the medullary cell
chains of Fucus. The cross walls separating the
successive cells of the chain early become perforated by
holes—eaten out as it were by the action of an enzyme
—which in extreme cases may be as much as 5 p in
diameter, and through which pass relatively coarse
“slime strands ’’ connecting each cell with the next
cell of the tube. The perforated cross wall is called
a sieve plate (Fig. 43, D). The vacuoles of the sieve
tube cells become filled with a highly nitrogenous
thin mucilage. The layer of cytoplasm lining the walls
remains, but the nucleus disappears, and the cells can
hardly be considered “living ’’ in the full sense. On
the sieve plate there is eventually formed a mass of
a carbohydrate substance called callose which blocks
the holes in the plate (Fig. 43, C). Callose refracts
light strongly and takes a deep colour with certain
stains. The blocking of the holes by the callose
apparently interrupts. the transport of substances
along the tube; later the callose is dissolved, and
ultimately the sieve tube becomes empty and no longer
functional, for instance in the parts of woody plants
which are several years old.

WATER TISSUE 275

Each sieve tube cell has one or more companion cells
alongside of it (Fig. 43, C). These are typical “ protein
cells” (see above). In most Seed Plants they are cut
off by division from the mother cell of each sieve tube
segment at an early stage of its development, before
the sieve tube cell acquires its special character.

The sieve tubes are certainly the main conducting
channels of organic nitrogenous substances (amino-acid
compounds, amides and perhaps the simpler proteins)
and of sugars, but we know very little indeed of the
manner in which the transport takes place.

Living Cells poor in Protoplasm. Water Tissue.—At
the other extreme from secretory and protein cells are
water cells, which have a layer of cytoplasm lining the
wall very thin in comparison with the size of the cell
(Fig. 43, E, F), the vacuole being filled with a very
watery sap, i.e. an extremely dilute solution. These
- cells are, however, only an extreme form of the typical
thin-walled parenchymatous cell. The bodies of
succulent plants consist mainly of such thin-walled
water cells, but in the typical succulents (Cactz) the
cells contain a large quantity of pentosans (substances
having a similar relation to pentose sugars that the
celluloses have to the hexose sugars) which attract
water strongly. The active cells, for instance the
photosynthetic cells near the surface of the succulent
stem or leaf, can draw upon the store of water in the
water tissue.

The epidermal cells act as water cells in an ordinary
leaf and provide a small supply of water which is drawn
upon by the mesophyll cells if they are losing water
to the air more quickly than they can be supplied
from the water-conducting system. In some cases
isolated epidermal cells are many times the size of

276 TISSUE ELEMENTS OF SEED PLANTS

their neighbours, and act as special water bladders °
(Fig. 43, F). And the leaves of certain plants growing
in places where they are specially liable on occasion to
lose water quicker than it can be supplied have a
many layered thin-walled epidermis, and this massive
water tissue loses water to the mesophyll and partially
collapses, bellows fashion, under such conditions. Thick
leaves often possess a central water tissue.

Thick-Walled Cells—Pits.—Many of the living cells of
a plant thicken their walls considerably after the cell
has grown to its definitive size. This thickening of the
wall is often general or uniform over the whole wall
surface, and may be carried so far that the cell cavity is
greatly reduced in volume (Fig. 44, C, D, E). Successive
layers of cellulose are laid down by the cytoplasm, and
the layering (stratification) of the wall can often be
clearly seen under the microscope (Fig. 44, B, C).
This appearance is supposed to depend upon the smaller
or greater amount of water contained in successive
layers of wall, making the cellulose more or less solid
in consistency.

The continuity of the layers is, however, generally
interrupted at certain spots, where the original wall,
formed at cell division (middle lamella), is not added to.
These thin places in the wall are called fits. Pits are
always formed opposite to one another in adjacent
cells of a thick-walled tissue (Fig. 43, E, £), so that
the cell cavities are only separated by the thickness
of the middle lamella (pit membrane). A pit is very
commonly circular in section, so that it appears as a
lighter circular area when the wall is seen in surface view
under the microscope. The walls of a pit are commonly
perpendicular to the middle lamella, so that the pit is
of equal diameter from top to bottom (“simple pits ”).

BORDERED PITS 297

But in some cells, particularly the vessels and tracheids
of the water-conducting system (see below), the pit is
narrower where it opens into the general cell cavity
than at the bottom on the pit membrane. The walls
thus overhang the pit, which is then said to be bordered

on NN

Fic. 44.—Mechanical tissue elements of the Seed Plant. A, collen-
chymatous cells in longitudinal section, showing great thickening
of parts of the walls forming longitudinal “ pillars’’ of wall
substance. B, the same in cross-section; w, thickened walls.
C, thick walled “‘stche cell’’ with branching pits. D, part
of three fibres in longitudinal section. E, fibres in cross-section.

(Fig. 45, I). Some bordered pits are not included in
the general thickness of the cell wall, but are built out,
so to speak, into the cell cavity in the form of a flattish
dome, with the pit opening at its summit. The two

278 TISSUE ELEMENTS OF SEED PLANTS

opposite pits of adjacent cells then form a lens-shaped
structure on the cell wall. The extreme example of
this type of bordered pit is seen in the tracheids (water-
conducting cells) of conifers, forinstance the commonpine.

The fine threads of cytoplasm, originally the achro-
matic spindle threads (see p. 106), connecting the cyto-
plasm of adjacent cells, are sometimes, though not always,
confined to the- pit membranes of thick-walled cells.
The existence of pits greatly facilitates the passage
of substances in solution from one thick-walled cell to
another, and through the pits, no doubt, the sugar
which supplies the carbohydrate material of which the
thick cell wall is made mainly enters the cell. We do
not know what determines the formation of a pit at
any given spot on the cell wall, though in some cases
it is clearly connected with the presence of a bundle
of cytoplasmic threads passing through the middle
lamella which in some way must arrest the deposition
of cellulose at that spot. The formation of thick walls
in general is an expression of excess of soluble carbo-
hydrate substance in the cell, which is condensed and
added to the wall as cellulose. In many cases, indeed,
thick-walled tissue is of no particular use to the plant,
but is simply formed as a result of carbohydrate excess,
when photosynthesis is unchecked, but there is not a
sufficient supply of salts to form proteins for the
manufacture of new protoplasm. Thus most plants
living in dry climates form great masses of thick-walled
tissue. But thick-walled cells may be of great use to
the plant, since they lend rigidity and toughness to
its body (mechanical tissue).

Local Thickening of Cell Walls.—Sometimes the
thickening of a cell wall is not uniform over the whole
surface, but local: for instance in a box-shaped cell

MECHANICAL OR SUPPORTING CELLS 279

one wall only may be thickened. The external walls
of the epidermal cells covering the shoot are generally
considerably thicker than the lateral and inner walls,
and sometimes the outer wall is very thick in the centre
and lens-shaped (Fig. 43, E). In collenchyma, the sup-
porting tissue of elongated living cells commonly met
with just below the epidermis in herbaceous stems and
in the midribs of leaves, the thickening is confined to
the corners of the cells, so that longitudinally running
pillars of cellulose are formed, connected with the
neighbouring pillars by thin membranes (Fig. 44, A, B).
Thus there is a certain amount of “give’’ under
horizontal strain in this tissue. In many cells ribs or
other local projections of various shapes are laid down
on the inner surface of the wall. The spiral thickenings
on the walls of the elaters of Pellia, and various similar
thickenings in the cells of Seed Plants (Fig. 45, A-G),
are cases in point.

Dead Cells.—In many cells of the plant body the
protoplasm dies after the development of the cell
is completed, the wall alone remaining. While these
dead cells can no longer take an active part in the life
of the plant, they often form an integral part of its
structure, and many of them are of essential importance.
The two chief functions they carry out are those of
mechanically strengthening the plant body and of
water conduction. The chief mechanical or supporting
tissues are composed of thick-walled elongated cells
with long tapering ends, which fit between one another,
so that the tissue is very tough and not easily broken
by strains. Such cells are called fibres (Fig. 44, D, E).
The wall of the fibre is sometimes very thick, almost
obliterating the cell cavity. It is usually penetrated
by narrow pits, which serve to bring the sugars used

280 TISSUE ELEMENTS OF SEED PLANTS

for thickening into the cell. The protoplasm generally
dies when the thickening is complete. The fibres are
commonly found in bands or masses not very far from
the surface of a herbaceous stem and ina similar position
in the midrib and larger veins of the leaf, but they
also form a large part of the wood of many woody
plants, for instance the oak, to which they give great
hardness and toughness. This depends partly on
changes in the cell wall (see below). Fibrous tissue
is used for making linen, canvas and paper, in which
the fibres are woven together to make the textile fabric.
The walls of the fibres in linen and the best paper are
nearly pure cellulose.

Other thick-walled cells are box-shaped, spherical
or oval, and when the walls are very hard they are
called stone cells, which form the substance of the stone
in stone fruits (plum, cherry, etc.), and isolated cells
or nests of cells in the flesh of a gritty pear. Stone
cells frequently have beautiful branched pits (Fig. 44, C),
formed by the meeting and coalescence of several
different pits as the inner surface of the cell is diminished
by the progressive general thickening of the wall.

Masses of thick-walled cells near the surface of an
organ may also have an important function in checking
loss of water by evaporation.

The water-conducting cells of a plant are called
tracheids and vessels (Fig: 45). These are dead cells
with lignified walls (see below), which form continuous
water-conducting channels throughout the plant parallel
with the sieve tubes A ¢évacheid is usually an elongated
cell whose wall may be thickened in various ways.

(a) Spiral tracheids have one or more lignified bands
of wall substance (Fig. 46) laid down on the inner
surface of the wall and running round and round the

SPIRAL AND ANNULAR TRACHEIDS 281.

cell through its entire length. The successive turns of
the spiral are at first in close lateral contact (Fig. 45, A).
The wall itself, apart from the thickening, usually
remains thin and cellulose.

poe

y I

Fic. 45.—Water-conducting tissue elements. A, spiral tracheid, not
yet elongated, surface view. B, elongation beginning, separating
coils of spiral thickening. C, portion of B more highly magni-
fied, showing nucleus and cytoplasm still alive. D, part of
tracheid with coils of spiral further separated. E, ditto of
annular tracheid. FF, spiral “tracheid with cavity nearly
obliterated owing to pulling out of spiral thickening till it no
longer supports the thin wall. G, ditto in annular tracheid.
H, part of scalariform vessel. I, bordered pits: in (a) sectional
view, (b) surface view (corresponding points joined by horizontal
lines). J, part of pitted vessel. K, part of reticulate vessel.

(6) Annular tracheids (Fig. 45, E, G) are similar, but
have the thickening in the form of separate rings of
lignified substance, set one above the other, at first
in close contact,

282 TISSUE ELEMENTS OF SEED PLANTS

Both these types are formed in tissue which is still
elongating, and the stretching of the thin cell wall
separates the coils of the spiral or the annuli, as the
case may be (Fig. 45, B—G).

(c) Pitted tracheids have general thickening of the
cell wall, but with numerous pits often set so close
together (Fig. 45, J) that only ribs of thickening are
formed between them. The pits are generally bordered,
often with greatly overhanging walls (Fig. 45, I). When
the pits are elongated and horizontal in direction, as
seen in surface view, running across one face of a poly-

(

Fic. 46.—Part of a spiral tracheid very highly magnified and seen
in longitudinal section, showing attachment of the spiral thickening
band to the thin wall.

gonal tracheid, so that there are horizontal bars of
thickening between successive pits, like the rungs of a
ladder, the tracheid is called scalariform (Fig. 45, H).
When the pits are angular and variable in outline, so
that there is a network of thickening between them,
we have the reticulate type (K). The pits are, however, ,
very often circular, oval or polygonal in outline and
very closely set (J).

A vessel has its walls lignified and thickened just
like a tracheid, but it consists of a row or chain of

_

LIGNIFICATION 283

cells with open communication between them, owing to
the cross walls having been dissolved wholly or partially
by the protoplasm before the cells were fully developed.
A vessel is thus comparable with a sieve tube in that
both are conducting structures formed of a chain of
cells in continuity with one another. Vessels may be
pitted, scalariform or reticulate, and have exactly the
same function as chains of tracheids. They are, however,
more efficient than chains of tracheids because, owing
to the central perforation or complete disappearance
of the cross walls, they offer less resistance to the
passage of water. The wider water-conducting elements
are always vessels. The widest tracheids are not more
that 100 p» in diameter, usually much less, while vessels
commonly reach a width of 300 p, and some measure
as much as 700 p across.

Alteration of the Substance of the Cell Wall.—The
walls of the living cells of the plant body commonly
remain cellulose, but in many cell walls important
changes affecting the functions of the cell.takes place
in the wall, usually while the cell is still alive, but
sometimes after death.

One of the most important modifications is lignification,
Lignification occurs in its extreme form in the walls of
the tracheids, vessels, wood fibres, and often in the paren-
chyma of the wood. It also often affects other fibres to
a greater or lesser degree. It depends on the deposition
in the wall of lignocellulose, consisting of cellulose and
two other constituents (an aromatic substance and a
pentosan) often classed together as lignin. This gives
characteristic colour reactions with various chemical
reagents, such as phloroglucin and hydrochloric acid
(magenta red), pyrogallol and hydrochloric acid (blue-
green), aniline chloride or aniline sulphate (golden

284 TISSUE ELEMENTS OF SEED PLANTS

yellow). Lignified walls also take up the aniline dyes
strongly, so that the walls of the tracheids and vessels
and often also of the fibres are deeply coloured in
permanent microscopic preparations of plant tissues
that have been stained with these dyes. Lignification
hardens the wall considerably, but does not decrease
its permeability to water.

In contrast to lignification, in which lignin is laid
down in a cellulose matrix, is the formation of parts
of certain cell walls, not by cellulose, but by one of
two aggregate substances known as cutin and suberin.
These are substances which differ chemically from one
another, but are closely allied and have the same
physical property of being impermeable to water.
They are not true fats, but they are allied to fats and
show some of the same reactions, for instance staining
with Sudan 3. Cutin is the substance of the cuticle
covering the epidermis of the shoot, and suberin forms
one of the layers of the wall in cork cells. Hence
both the outer walls of the epidermis of the herbaceous
shoot and also cork tissue (which is formed in the bark
of woody plants, see Chapter XX) are practically
impermeable to water, and thus prevent the drying
up of the shoot by evaporation. The impermeability
of cork to water accounts of course for the use of the
bark of the cork oak (Quercus suber), which is very
pure cork, to make bottle corks.

The contrast between cutin and cellulose can be
demonstrated by treating a section through the epidermis
with Schulze’s solution (solution of iodine in zinc
chloride) which stains the cellulose blue, owing to the
formation of “‘ amyloid,’ a substance that gives, like
starch, a blue colour with iodine, but the cutin of the
cuticle yellow.

MUCILAGE. INTERCELLULAR SPACES 285

Another important substance of the cell wall is
pectin, of which the middle lamella (the original thin
wall formed at cell division) is often composed. Pectin
is also formed in the cell walls (and cell contents) of
succulent fruits. The pentosans (see p. 275), which,
as we have seen, are characteristic of certain succulent
plants, also occur in lignified cell walls, being the
chief constituents of the ‘‘ wood gums.” The presence
of these and other carbohydrates which readily take up
water and become mucilaginous are the cause of the
ready formation of slimy substances from many cell walls
and cell contents on addition of water, for instance the
swelling of the middle lamelle of the medullary cells
of Fucus, the swelling of the surface cells of linseed
(p. 137), the breakdown into mucilage of the external
cells of the root cap (p. 290), and many other cases.
The process is the absorption of water by a carbo-
hydrate gel, quite comparable with the swelling of
gelatine considered in Chapter III (p. 53).

Intercellular Spaces.—Most of the living tissues of
the higher plants are interpenetrated by a system of air-
containing spaces (intercellular spaces) initiated by the
breaking apart of the cells along the plane of the middle
lamella. The air in these spaces is often called the
‘internal atmosphere ” of the plant. Its composition
agrees more or less closely with ordinary atmospheric
air, but except in tissues like the mesophyll of the
leaf (Fig. 10), which are constantly setting free oxygen
as the result of the photolysis of carbon dioxide, it
contains a smaller proportion of oxygen and a larger
proportion of carbon dioxide than atmospheric air,
owing to the respiration of the living cells, which use
up oxygen and set free carbon dioxide. The air in the
intercellular spaces is normally saturated or nearly

286 TISSUE ELEMENTS OF SEED PLANTS

saturated with water vapour, because it is surrounded
by living cells saturated with water. The exchange of
gases between the living cells in the interior of the
plant and the external air takes place by diffusion
through this system of intercellular spaces.

PRACTICAL WORK.

(1) Ordinary living parenchyma. Several examples of this,
with and without chloroplasts, have already been seen.

(2) Secretory (gland) cells, Examine demonstration specimens
of sections through the enzyme secreting glands on the inner
surface of the “pitcher” of the Pitcher Plant (Nepenthes)
and also of sections through a nectary. Note in both cases that
the cells are densely filled with cytoplasm and have conspicuous
nuclei.

(3) Thick-walled cells, local thickening of cell walls, secvetory
cells and protein cells in transverse section of leaf of pine (a per-
manently stained and mounted section of a large-leaved species,
such as P. pinaster, is best).

Examine first the thick-walled cells in the corner of the leaf.
Those just below the surface layer have abundant cytoplasm
and conspicuous nucleus—they are still active. Note the
stratification of the walls, the middle lamella, and the pits.
The cavities of the epidermal cells are almost obliterated. Note
the thick layer of cuticle on their outer surfaces.

The mesophyll (photosynthetic) cells below the thick-walled’
tissue have numerous choloroplasts and plate-like projections
of the walls (local thickenings) into the cavities.

Note the vesin canals at intervals (one in each corner of the
leaf). These are intercellular channels surrounded by secretory
cells, and (outside these) thick-walled cells. Theresinisexpelled by
the secretory cells and accumulates in the canal.

Examine the protein cells on the outer flanks of the central
pairofbundles. These are richin cytoplasm and have conspicuous
nuclei. They adjoin the sieve tubes of the bundles,

Note also the lens-shaped bordered pits on the walls of some
of the cells around the bundles.

(4) Stone cells. Tease out ona slide in a drop of dilute glycerine
a little of the flesh of a pear, and note the thick-walled pitted
stone cells, singly and in groups, among the thin-walled paren-
chyma. When the stone cells are numerous the pear is gritty
between the teeth.

PRACTICAL WORK 287

(5) Steve tubes and companion cells. Examine the demonstra-
tion preparations of sieve tubes and companion cells in trans-
verse and longitudinal section of the stem of Cucurbita. Note
the thick perforated sieve plates (cross walls of sieve tubes),
the callose, the cytoplasm contracted from the side walls of the
tube but not from the plates, and the narrow companion cells
(protein cells) with granular contents and conspicuous nuclei.
Measure with the micrometer eyepiece the diameters of a sieve
tube, of a companion cell and of the pores of the sieve plate.

(6) Vessels, fibves and wood-parenchyma. Examine a piece
of Cucurbita stem and identify the longitudinally running
strands (bundles) with large openings (vessels) visible to the naked
eye. Tease a little of a previously macerated bundle from the
stem of Cucurbita in a drop of dilute glycerine, and observe the
narrow spiral and annular vessels and pieces of the broad reticu-
late vessels. Measure their diameters. Thin-walled parenchyma
cells are associated with the vessels. Treat similarly a little
macerated willow wood and note that it consists mainly of
fibres—long narrow cells with thick walls and tapering ends—
and of a few vessels bearing close-set bordered pits where they
abut on one another. Measure the diameter of a vessel. Note
also the thick-walled oblong cells with simple pits running through
the wood in plates (medullary rays).

(7) Water tissue. Note the massive thin-walled tissue forming
the bulk of the succulent leaf of Kleinia (or other ‘‘leaf succulent ’’).
Contrast this with the layer of tissue containing chloroplasts near
the surface. The green cells draw on the water tissue when
they lose water more rapidly than it can be supplied through
the bundles from the roots.

Examine transverse sections of the fresh Begonia leaf and note
the many layers of thin-walled colourless cells on the upper side.
This is derived from the upper epidermis by divisions parallel
with the surface. These cells are living, and the very thin cyto-
plasmic lining of the cell walls can be seen here and there. The
water tissue on the lower side contains chloroplasts, and is not
derived from the epidermis but from the mesophyll tissue. It
is, however, very distinct from the main photosynthetic layer
in the centre, which is densely packed with chloroplasts.

CHAPTER XVII

THE ROOT

HAVING now obtained some knowledge of the appearance
and structure of the elements of which the tissues of
the bodies of the higher plants are composed, we pass
on to consider the nature, structure and functions of the
main vegetative organs of the plant—the root and the
shoot (stem and leaves)—aggregations of these tissues.

Roots are colourless axes of the plant which
typically descend into and grow in the soil, fixing
the plant and absorbing water and dissolved salts
from the soil. It is pretty certain that they are to
be regarded as specialised underground branches of the
thallus which formed the body of remote ancestors of
the higher plants. According to their origin in the
life of the individual seed plant we distinguish the
primary root system, i.e. the taproot or main descending
axis of the plant (already laid down in the embryo)
and its branches, from the adventitious roots which
arise from some part of the shoot, e.g. from a rhizome
or from the lower part of an erect stem (see Chapter XV,
p. 261). These two kinds of roots do not, however,
differ structurally in any essential respect.

Tropisms of Roots.—Some of the most important
physiological characters of roots are their distinctive
tropisms (see Chapter V, p. 87). The taproot is in
the great majority of cases positively geotropic, i.e. it

grows towards the centre of the earth, and if it is placed
288

TROPISMS 289

in a horizontal position the region just behind the tip
bends so as to bring the apex pointing vertically down-
wards. This bending is carried out by the cells on the
upper side of the elongating region growing faster than
those on the lower side, but how the root perceives
the stimulus of gravity is not by any means fully
understood. The branches of the taproot do not
grow vertically downwards but obliquely, making some
angle less than a right angle with the continuation of
the main root.

Many roots are also negatively phototropic, i.e. they
tend to grow away from a source of bright light. The
strongest tropism of roots is, however, positive hydro-
tropism, or the tendency to grow towards a region of
greater moisture. In the case of a taproot growing
in ordinary moist soil these three tropisms will clearly
all act in the same direction, they will all take the root |
directly downwards, for not only is this the direction
of the pull of gravity, it is the direction away from the
light, and it is also the direction of greater moisture,
for the surface layers of soil will be drier than the deeper
ones as a result of evaporation to the air. But if the
soil is rather dry and the source of moisture is to one
side, then the roots will bend in that direction, the
hydrotropic tendency overcoming the others.

Growing roots require a supply of free oxygen for
respiration, and the roots of most ordinary land plants
flourish best and develop root hairs (their absorbing
organs) most abundantly in well aerated soil, in whichthe
air is saturated or nearly saturated with water vapour.

Structure.—The apical (primary) meristem of the
root is covered by a root cap (Fig. 47, A, 7.c.), thickest
at the apex and gradually thinning at the sides till it
comes to an end a short distance behind the tip. The

19

290 THE ROOT

root cap is composed of living cells derived from the
primary meristem (f.m.) from which they are continually
renewed by active cell division, while those on the surface
of the cap break down into mucilage and are rubbed
off as the root tip is pushed through the soil. Behind
the root tip (Fig. 47, a, A) there is a bare stretch of
varying length, often about 5 to 10 mm. long, where
the root is actively growing in length: this is the
elongating region (E). Behind this again is the ,oot-
hairy region (R), thickly covered under favourable
conditions of soil moisture and aeration, with root
hairs, each of which is a tubular outgrowth from a
single cell of the surface layer (piliferous layer) (Fig. 48).
The root hairs come into intimate contact with moist
soil particles and absorb water owing to the greater
osmotic strength of the cell sap than of the dilute
solution outside, and also salts in solution. It should
be understood that the entrance of the salts into the
root hair is independent of the entrance of water, and
depends primarily on the difference of concentration of
the various solutés inside and outside the root-hair cell.
The ectoplasm of the root hairs, which is impermeable
to the “osmotic substances’ in the cell, must of
course be permeable to these salts. New root hairs are
formed in front of the root hair region by the growing
out of the cells of the piliferous layer into papillz which
elongate to form the hairs (Fig. 49, A). The hairs
at the back usually die, but in some cases they are
persistent and clothe the root for a long distance even
to the point of origin of a root several inches long.
But ordinarily the life of a root hair is short, and behind
this region the root is again bare.

Growth of the Root Tip.—As new cells are formed by
division of the meristem the ones already formed pass

STRUCTURE 291

into the elongating region, increase actively in length
by the formation and enlargement of the vacuole, and

per Ph end

[* ph. mxX
px

Fic. 47.—Diagrams illustrating the structure of the root. a, in
longitudinal section. A, apical region; E, elongating region; R,
root-hairregion; 7.c., root cap; p.m., primary meristem ; v.c., vascular
cylinder; px., protoxylem ; c, cortex; p./., piliferous layer; 7.h., root
hairs. b, part of vascular cylinder (the conjunctive tissue ot
the cylinder is supposed to be transparent, the endodermis opaque).
ec, the same with endodermis removed; end., endodermis; pey.,
pericycle; ph., phloem; px., protoxylem; mz.,metaxylem. d, Dia~-
gram of cross-section of cylinder through origin of branch root ;
cors, ends, %, p.ms, ¥.cs, cortex, endodermis, xylem, primary
meristem and root cap of branch root. On the left the primary
meristem of the opposite branch is just formed in the pericycle.
The root in b—d is tetrarch, i.e. with 4 xylem and 4 phloem strands.

gradually become differentiated into the various tissues.
The force of elongation of the cells in this region exerts

292 THE ROOT

a push up and down the root. The region behind the
elongating region is held fast in the soil by the ro
hairs, but the root apex, covered by the root cap, is

Fic. 48.—Cross-section of a small root taken in the root-hair region :
v.h., root hairs; p.1., piliferous layer; cor., cortex; e, endodermis ;
per., pericycle; px., protoxylem ; mx., metaxylem; 'ph,. phloem (s.t.,
sieve tube; c.c., companion cell). This root is diarch, i.e. with
2 xylem strands, joined to form a plate, and 2 phloem strands.

pushed between the particles of soil, its passage being
lubricated by the mucilaginous surface of the cap.
Internal Structure—A cross-section of the root
taken in the middle of the root-hair region gives the
best idea of the primary plan of construction of the
root. On the surface is the piliferous layer (Fig. 48, p.1.),

ENDODERMIS AND VASCULAR CYLINDER 293

each cell of which is capable under favourable conditions
of growing out to form a root hair (7.4.). Within
is the cortex (cor.), composed of thin-walled parenchyma
interpenetrated by intercellular spaces. In the centre
is the vascular cylinder, containing the conducting
(vascular) system.

The vascular cylinder is separated from the cortex
by a single layer of cells, the endodermis: (Fig. 48, e).
The lateral and transverse cell walls, i.e. those walls which
are common to adjacent cells of this layer, or the central
strips of these walls (Fig. 49, B), are composed of cutin
(see p. 284) : the outer and inner walls of the endodermis,
i.e. those abutting on the cortex and the tissues of the
cylinder respectively, are cellulose. Thus the endodermis
is stiffened and united into a more or less rigid layer
whose cells cannot be separated (Fig. 49, C), and there
is a free passage for water and solutes from cortex
to cylinder only through the inner and outer walls and
through the bodies of the endodermal cells.

Immediately below the endodermis is a layer of
parenchymatous cells, the pericycle (Fig. 48, per.),?
generally only one cell thick. Below and immediately
abutting on the pericycle come the vascular (conducting)
elements proper, arranged in alternating strands of
tracheids and vessels—xylem3 and sieve tubes (s.t.)
with companion cells (c.c..—phloem.4 The xylem and
phloem strands are separated from one another by

t Greek éydov, within, dépya, skin, i.e. the inner skin, the skin of
the vascular cylinder. This is not to be verbally confused, as is often
done by beginners in biology, with the ‘‘ endoderm,’’ the lining of
the primitive gut of animals. The terms are both derived from the
same Greek words, but have a very different technical meaning. It
is therefore important to keep the distinctive ending.

? Greek vepi, around, and xdKdAog, a ring, ‘‘ the cell layer surround-
ing the cylinder.”

' 3 Greek &¥Aov, wood—the wood ofa tree is secondary xylem, see p. 331.

4 Greek, ¢Aoidg, bark—the inner bark of a tree is secondary phloem

(P- 340).

Fic.

THE ROOT

49.—A, three cells of the piliferous layer at the beginning of
the root-hair region in longitudinal section, showing the early
stages of the development of root hairs. Note the accumulation
of cytoplasm and the presence of the large nucleus (which passes
into the hair) at the point of origin of the root hair. B, diagram
of single endodermal cell, showing the cutinised band (black)
round the radial and horizontal walls. The tangential walls
(front and back) are supposed to be transparent. C, diagram
of portion of endodermis, supposed to be seen from within the
cylinder, showing the way the cells are fitted together and
“cemented ”’ by the cutinised bands. D, small portion of edge
of cylinder in an old and large root, showing “ horse-shoe”’
thickening of endodermis, and ‘‘ passage cell’’ opposite proto-
xylem. E, diagram of cross-section of vascular cylinder in
which secondary thickening has begun, the secondary tissue
at present formed only opposite the primary phloems. 4X,,
primary xylem; X,, secondary xylem; P;, primary phlom; C,
cambium.

PROTOXYLEM AND METAXYLEM 295

parenchymatous rays which are continuous with the
pericycle. In slender cylinders, where the number of
alternating strands of xylem and phloem is small the
centre of the cylinder is filled up with large xylem
vessels in contact with the strands of xylem which
abut on the pericycle (Fig. 48). In massive cylinders
where the number of xylem and phloem strands is
large the centre is occupied by parenchyma (pith).

The outermost tracheids, abutting on the pericycle,
are narrow, and spiral or annular. They are the first
xylem elements to be formed, and are hence called
protoxylem (Fig. 48, px.). Within these are larger pitted
vessels (metaxylem) (Fig. 48, mx.), which, in a narrow
cylinder, fill up the centre (Fig. 48). The spiral or
annular thickenings of the outermost tracheids are
laid down and lignified in the elongating region of the
root, the protoplasm dying after the thickenings are
complete. The thin cellulose wall of the tracheid is
passively stretched by the active elongation of the
surrounding parenchymatous tissue, and the successive
turns of the spiral thickening (or the annuli) are pulled
apart (Fig. 45, B—G). As long as they are not too
widely separated, the internal thickenings of the tracheid
prevent the obliteration of the cavity by the pressure
of the surrounding turgid tissue, and so this first
formed water-conducting channel is kept open.

Thus the spiral and annular tracheids are very
beautifully adapted to perform their function under
the conditions in which they are formed, for they both
admit of longitudinal extension to keep pace with
the growth in length of the tissue, and are protected
against obliteration by the surrounding turgid tissue.
In organs which elongate a great deal, the spiral
tracheids first formed are ultimately stretched so much

296 THE ROOT

that the thickening no longer supports the wall: and
the tracheids are crushed (Fig. 45, F).

A transverse section across the elongating region |
shows the protoxylem elements already lignified, but
the cells which will develop into metaxylem vessels
still thin walled and living. In the root-hair region
the walls of these gradually thicken and lignify, and
various stages in their development can be traced.

Absorptive Function of the Root.—Owing to the
existence of substances such as sugar, which attract
water, in the cell sap of the root hair, water is drawn
in from the soil outside. As the solution within the
vacuole becomes more dilute, the absorptive power: of
the root-hair cell decreases, and when the dilution
has passed a certain point the absorptive power of the
adjoining cortical cells, whose cell sap will then be more
concentrated than that of the root-hair cell, will cause
water to be drawn into them from the root-hair cell.
In the same way water will be drawn across the cortex
from cell to cell through the endodermis and pericycle
to the xylem tracheids and vessels. The mechanism
of the passage of water from the living cells of the
cylinder into the xylem tracheids and vessels is not
fully understood, but it seems probable that osmotically
active substances exist in considerable quantities in
these also. When they lose their protoplasm after
differentiation a considerable amount of organic
substance must be left in their cavities.

It must be clearly understood that the passage of
solutes, for instance the mineral salts—nitrates, sulphates
and phosphates—absorbed from the soil and ultimately
passing into the xylem, do not necessarily travel across
the cortex at the same rate as the water. Their passage
depends on the state of equilibrium between the different

BRANCHING 297

cells in regard to each salt, and on the relative perme-
ability of the cytoplasmic membranes to the different
salts.

Structure of the Older Parts of the Root.—In ex-
ceptional cases, as has already been said, root hairs
may persist indefinitely during the life of the root,
but normally their life is quite short, and they die
and peel off, with the piliferous layer, behind the
limited root-hair region. The surface of the root is
then formed by the outer cells of the cortex, and the
outer walls of these cells, now in contact with the soil,
are cutinised and no longer absorb water from the soil.
Meanwhile the metaxylem vessels of the vascular
cylinder are now completely lignified, and the conduct-
ing capacity of the primary structure has reached its
fullest extent. Secondary changes may now occur :—

(x) Branching. —Branch roots arise in this region, by
the division of the cells of the pericycle, usually opposite
a .protoxylem strand. A new apical meristem is
established, covered by a root cap, and the tip of
the new root grows out through the endodermis and
cortex of the mother root (Fig. 47, d), partly by digesting
the cells, partly by mechanical pressure, and emerges
into the soil. When a root is branching freely a
fairly close-set row of branches may grow out from
opposite each protoxylem of the mother root, so that
if the mother root is tetrarch, four such rows of branches
will appear. The vascular tissues of the new root
establish connexions with the corresponding’ tissues of
the mother root; thus the xylem strands connect
with the xylem opposite which the new root has arisen
by the differentiation of tracheids in the intervening
tissue, the phloems by the formation of sieve tubes
connecting with the sieve tubes of the two phloem

298 THE ROOT |

strands on each side of the point of origin. In this |
way the conducting systems of the mother root and of;
the branch roots become part of one system. The
cortex of the branch, however, never has any con-
nexion with the cortex of the mother root.

(2) Secondary Thickening.—In perennial plants which
have a persistent primary root system, and also in
many annuals, the primary tissues are added to by
the activity of a secondary meristem. In other words,
certain living cells of the primary tissue begin to divide,
and the products of their division become differentiated
to form new permanent tissues, which are called
secondary tissues, especially new (secondary) xylem and
phloem. This process takes place in the roots and
stems of all the woody plants, and to a certain extent
in most herbaceous plants belonging to the Dicoty-
ledons (the largest group of seed plants) and to the
Conifers (pines, firs, etc.). The secondary vascular
meristem is called the cambium, and its activity results
in the great and continual increase in thickness which
occurs in the stems and roots of trees. We shall have
to consider it again in connexion with the woody stem,
but we may here note the beginning of the process
in persistent roots.

The formation of the cambium starts by the tangential
division of the conjunctive cells just inside and on the
flanks of the primary phloem strands. The cells cut
off on the inside, i.e. towards the centre of the root,
become new (secondary) xylem, the strands so formed
alternating with the primary xylem strands (Fig. 49, E).
A few sieve tubes are cut off on the outside of the
cambium and added to the primary phloem. The
cambium now extends round the outside of the primary
xylem strands so as to form a complete layer round

SECONDARY TISSUES. CORK FORMATION 299

the cylinder. This layer forms a wavy line as seen
in cross section, sweeping out round the primary
xylem and in round each primary phloem (Fig. 49, E, c).
Opposite the primary xylems it sometimes forms nothing
but parenchymatous tissue (secondary rays), but in
other species it forms xylem to the inside and phloem
to the outside, just as it does opposite the primary
phloems. Owing to its greater activity at first opposite
the primary phloems and to the formation of hard
masses of secondary xylem in these regions the cambium
soon straightens out and becomes circular in cross-
section. In the root of a tree it continues its activity
year by year, and eventually forms a bulky cylinder
of secondary wood, and a much thinner cylinder of
secondary phloem.

(3) Cork Formation.—Another secondary meristem
besides the cambium also arises in woody roots, usually
in the pericycle. This is called\the cork cambium or
phellogen, because it cuts off cells to the outside, certain
layers of whose walls are formed of suberin. These are
called cork cells, and owing to the impermeability of the
corky walls to water and solutes, the cortex and endo-
dermis are cut off from any functional connexion with
the vascular cylinder and soon scale off. This formation
of “ bark ” on woody roots is quite parallel to that which
takes place on tree-trunks. On the inside the phellogen
often forms parenchymatous tissue, which is sometimes
called the ‘‘ secondary cortex.”

Modified Roots.—In some species the roots, or some
of them, depart more or less from the typical root
structure and behaviour. In some plants (climbers
and epiphytes) they may turn green and carry on
photosynthesis for the plant. Later they may grow
down to and enter the soil, then assuming typical

300 THE ROOT

root characters. In parasitic seed plants organs called
haustoria of very variable structure bore into the tissues
of the host and absorb food from it. Sometimes these
haustoria (Thesium, Rhinanthus) are just modified
branches of ordinary soil roots, which fasten on and bore
into the roots of other plants. In other cases (Mistletoe)
the parasite never has any connexion with the soil at
all, but the seed germinates on a tree, and its root-
like haustoria bore into the tissues of the tree, obtaining
water and mineral salts from the xylem. In other
cases, again (dodder), the haustoria are strands of tissue
(in which sieve tubes may be developed) that tap
the sieve tubes of the host. And finally, in the most
extreme cases (Rafflesia, a tropical parasite), the whole
body of the parasite consists simply of a branching
mass of strands of tissue, like the hyphe of a fungus,
which live entirely in the tissues of the host, occasionally
coming to the surface and producing immense flowers.

In some plants (carrot, turnip, parsnip) the main
taproot is greatly swollen, and acts as a food storage
organ through the winter in the same way as a corm
or tuber. The great mass of fleshy tissue is produced
by the cambium, and may be reckoned as parenchy-
matous secondary xylem and phloem, in which a few
strands of conducting elements are formed here and
there.

PRACTICAL WORK.

(1) Draw a single mustard seedling that has been germinated
in moist air (being careful to keep the root moist), and note
the long densely crowded root hairs, not yet full grown towards
the tip of the root, and the bare elongating region between the
root hairs and the apex. Distinguish root from hypocotyl.
Compare with seedlings grown in soil, and in the latter note the
clinging of the root hairs to soil particles.

(2) Transfer a seedling to a slide, just covering the end of the
root with water under a coverslip, and examine under the micro-

PRACTICAL WORK 301

scope. Note especially the region of short (young) root hairs,
each of which arises from a surface cell. Note also the region
of primary meristem covered by the root cap, the bare elongating
region and the vascular cylinder seen through the semi-transparent
cortex.

(3) Examine the fresh taproot, 2 or 3 inches long, of a
Bean seedling. Observe the root cap covering the growing
point and the four or more longitudinal rows of lateral roots.
Cut the root longitudinally through the centre in the region of
origin of the branch roots, and make out with a lens that these
arise from the surface of the central cylinder.

In the older part of the root the cortex can generally be separ-
ated from the cylinder by twisting the root so as to break the
cortex and then pulling it off.

(4) Make a low power diagram of a transverse section of a
buttercup root taken through the root hair region, and showing
piliferous layer, cortex and cylinder, and in the cylinder the cross-
shaped xylem with four arms, the ends of which are formed by
the narrow tracheids of the protoxylem. Between the arms of
the xylem are the four strands of phloem. Put in the boundaries
of the tissue regions, but not individual cells.

With the high power examine the different tissues in detail,
and draw a portion of the cylinder, including a segment of endo-
dermis and pericycle, protoxylem, large vessels of the metaxylem (in
the centre these still have thin walls—cytoplasm and nucleus
can sometimes be seen in them), also sieve tubes and companion
cells of the phloem, and conjunctive tissue. Starch grains can
be seen in the cortex.

(5) Examine the cross-section of a young Bean root, noting
that the outline of the cylinder is not so easy to define as in the
Buttercup, the endodermis being less sharply distinguished from
neighbouring cells, but on careful examination with the high
power the cutin strips of the radial walls can be made out.
The xylem strands are not joined in the centre, which is occupied
by pith. The thicker walled cells forming the bulk of the phloem
are fibres: a few sieve tubes can be seen outside these. In
older sections cambium (formed early here) and secondary tissues
can be made out.

(6) Examine longitudinal sections through the centre of the
tips of Maize and Bean roots. Note in each root cap, primary
meristem, and the growth in size and elongation of the cells as the
apex is left. In the Maize especially note the great increase
in width of the rows of cells which will form the large vessels,
and the stages of development of these.

CHAPTER XVIII

THE FOLIAGE LEAF

THE leaves and stems of the higher plants are
differentiated organs of the shoot. In most of the
lower chlorophyll-containing plants (Fucus, Pellia) the
shoot does not show this differentiation, but it is
established in all the vascular plants, except a few
peculiar forms which have lost it. The foliage leaf is
essentially the organ of photosynthesis, its principal
tissue, the mesophyll, containing the great majority of
the chloroplasts of the plant, though the parenchy-
matous tissue of the stem generally contain a certain
number.

Essential Structure.—The typical foliage leaf (Fig. 50)
is essentially a thin plate of mesophyll, thin enough to
allow light of sufficient intensity to reach all the cells,
with air spaces between the cells, communicating with
the outer air, and thus permitting free diffusion of gases
between the cells and the air.. The mesophyll is inter-
penetrated by vascular bundles (veins) which carry water
and salts coming from the root (through the vessels and
tracheids of the xylem) to the mesophyll, and formed
organic substances away from it (through the sieve tubes
of the phloem) to other regions of the plant, principally
to growing regions and storage organs. The mesophyll
is protected by a layer of cells (the epidermis), typically
not containing chlorophyll and acting as a water tissue
(see p. 275), and the continuous outer layer of the

302

CUTICLE AND STOMATA 303

external walls of these cells, in contact with the outer
air, consists of a waterproof covering, the cuticle, com-
posed of cutin (see p. 284). If the cuticle were perfectly
continuous over the whole surface of the leaf it would
prevent the interchange of gases between the mesophyll
and the outer air, for instance the supply of carbon
dioxide which the chloroplasts use as part of the raw

Fic. 50.—Part of transverse section of dorsiventral foliage leaf: cut.,
cuticle ; u.ep.,upper epidermis ; mes.,mesophy]ll ; pal., palisade tissue ;
sp., spongy tissue ; znt., intercellular space’, v.b., vascular bundle
(in longitudinal section, passing out of the plane of section to
the right—further to the right a bundle is seen cut in transverse
section) ; sh., bundle sheath; , xylem; ph., phloem; /.ep., lower
epidermis; st., stoma. The black arrows indicate the passage
of liquid water from the xylem tracheids through the sheath
into the mesophyll cells and epidermal cells. The white arrows
the evaporation of water from the mesophyll cells into the inter-
cellular spaces and diffusion out through the stomata.

material of photosynthesis. The epidermis, with its
cuticle, is in fact pierced by minute holes (stomata),
each enclosed by two specialised epidermal cells (guard
cells), and these pores lead into the system of intercellular
space of the mesophyll (Fig. 50).

_Transpiration.—The pores of the stomata not only
allow of the diffusion of carbon dioxide from the air

304 THE FOLIAGE LEAF

into the leaf as it is used up by the chloroplasts of the
mesophyll cells, but also, of course, allow the water
vapour evaporated from the saturated mesophyll cells
into the intercellular spaces to diffuse out to the
(usually) drier air outside the leaf, This process 1s
called transpiration.

Large quantities of water are lost by the plant in
this way. Ifa plant growing in a flower-pot is weighed,
and after a certain interval weighed again, the loss in
weight represents almost entirely transpired water, for
the gain or loss in weight from photosynthesis or respira-
tion is negligible by comparison with the loss by trans-
piration. The pot itself and the soil must be carefully
covered with a metal or rubber covering waxed to the
stem round the hole through which the stem passes, to
prevent loss of water by direct evaporation from pot
and soil. If desired, transpiration from the surface of
the stem through the stomata it bears can also be
prevented by waxing its'surface. In this way it can be
shown that it is through the leaves that the loss of water
in transpiration mainly occurs. A sunflower plant whose
total leaf area was 5,616 square inches lost 14 pints
of water in 12 hours; and it has been calculated that
the trees in an acre of beech forest transpire about
1,400 tons of water during the summer.

This constant loss of water from the leaf is probably
useful to the plant because it helps to set up a current
(transpiration current) through the xylem vessels and
thus to bring up dissolved salts from the root more
rapidly. That active transpiration is not necessary to
the plant, or at least not to some plants, is shown by
the fact that growth continues in tobacco plants, for
instance, which are kept in air saturated with water
vapour, so that transpiration takes place very slowly

STRUCTURE OF STOMATA 305

indeed. Evaporation from the mesophyll cells does
however, take place even in saturated air, because the
living cells, owing to the process of respiration, which
is always going on and which liberates heat, are kept
at a somewhat higher temperature than the surrounding
air.

Structure and Mechanism of Stomata.—We have seen
that if the leaf were covered by a perfectly continuous
layer of cuticle it would lose little or no water, but it
could not obtain carbon dioxide from the air. The
stomata allow of the carbon dioxide getting in, but they
also allow of water vapour getting out, and though this
may be useful to the plant it introduces the danger of
water loss so rapid that it cannot be covered by a
corresponding supply from the root. This danger is
partly met by the automatic closing of the stomatal
pore when there is a deficiency of water in the tissues of
the leaf. The guard cells of the stoma which surround
the pore, unlike the other epidermal cells, contain
chloroplasts (Fig. 51, C, E), and are thus able to make
sugar and maintain a high osmotic pressure and power
of absorbing water. The guard cells are firmly joined
to one another at the two ends, but in the centre there
is a split (the stomatal pore) between them, while the
side walls away from the pore abut on, and can be
pushed into the cavities of, the adjacent epidermal cells.
The cavity of the guard cells is often confined to the
middle part of the cell as seen in vertical section : above
(next the outer air) and below (next the intercellular
space into which the pore leads) the wall is thick and
cutinised.

When the guard cells become turgid the cell cavities
increase in size, and since the ends and the top and
bottom are firmly held, the side walls increase in length,

20

306 THE FOLIAGE LEAF

the outer ones bulging into the adjacent epidermal
cells, the pressure of whose cell sap is lower, and the

ss...

AU

G

4,
2D
2
>,
=
S

Fic, 51.—A, diagram of stoma seen in vertical section. The thin
lines represent the position of the walls of the guard cells when
the stoma is closed, the thick lines when it is open, the outer
lateral walls bulging into the adjacent epidermal cells. B, the
same in surface view (Half the stoma only shown). C, stoma
of Iris leaf in vertical section, showing the guard cells sunk below
the surface of the epidermis at the bottom of the vestibule (v).
D, the same in surface view: epidermal cells focussed, the
outlines of the guard cells seen faintly below. E, the guard

cells focussed. F, ending of a fine vein in the mesophyll: one
line of tracheids surrounded by sheath cells with few chloroplasts.

MECHANISM OF STOMATA 307

inner ones curving away from each other and thus opening
the pore (Fig. 51, A, B, heavy lines). This is the normal
condition in bright light and when there is plenty of
water in the leaf. If the air outside becomes very dry
water vapour diffuses out through the pores very rapidly,
and thus the vapour pressure in the intercellular spaces
of the mesophyll is reduced. This in turn increases
the rate of evaporation from the mesophyll cells into
the intercellular spaces and depletes the cells of water.
Water is drawn into the mesophyll cells from the
epidermal cells and eventually into these from the guard
cells of the stomata. The guard cells in consequence
lose their turgor and become flaccid, closing the pore by
the straightening of the extensible portion. of the walls
(Fig. 51, A, B, thin lines).t At night also, owing to the
cessation of sugar formation by the chloroplasts of the
guard cells, the osmotic pressure falls and the pore closes.

This automatic mechanism closing the stomata when
the supply of water is depleted does not always completely
stop the loss of water from the plant, as may be seen
towards the close of a hot dry day in summer, when the
leaves of some plants become limp and droop owing to
the continued loss of water to the air, a loss which they
are unable to make good from the root quickly enough
to maintain the turgor of the cells. This may be due
partly to the incomplete closure of the pores, and partly
to the fact that the cuticle is not completely impermeable
to diffusion of water vapour. A drooping plant will
gradually recover if a bell jar be placed overit. The jar
prevents the removal by currents of air of the vapour

t This mechanism can be imitated by cutting off a piece of the inner
tubing of a bicycle tyre 10 inches long (5 inches on each side of the
valve), tying up the two ends tightly and fixing them to a straight
piece of wood so that the concave side of the tube lies along the wood.
If air is pumped into the valve the tube curves away from the wood
as it becomes inflated, and straightens as it is deflated again.

308 THE FOLIAGE LEAF

escaping from the leaf, and the accumulation of this
round the leaves checks further diffusion from their
intercellular spaces, and thus enables the turgor of the
leaf cells to be recovered by the retention in the leaf
of the water coming from the root.

Most land plants possess structures which tend to
produce the same sort of effect as the bell jar placed over
the plant, i.e. the prevention of the rapid removal by
air currents of the water vapour escaping through the
stomata. The commonest of these is the sinking of the
stoma in a pit (vestibule), the bottom of which, occupied
by the guard cells and pore, is sunk below the general
level of the cuticle (Fig. 51, C). Sometimes there is a
deep groove or cavity in the leaf containing several
stomata. In other cases the leaf rolls or folds up in dry
weather, thus protecting the surface bearing the stomata.
In other cases, again, there is a thick covering of hairs
either over the whole surface bearing stomata or confined
to the cavities in which the stomata are sunk. All
these structures serve the same purpose—to keep the
air just outside the stomata relatively still and thus
prevent the rapid removal of water vapour by currents
of air, which is a very much quicker process than the
slow diffusion, which continues in any case. The drying
effect of wind is well known, and the shoots of plants
exposed to strong, and especially of course to dry,
winds, very quickly wilt and even die (through loss of
water greater than they can recover), unless they are very
well protected in one of the ways described. It is
among plants living in very dry climates, or frequently
exposed to strong winds during the growing season, or
growing in dry soil, that the structures described are
most developed, for it is only such plants (xevophilous *

t Greek Enpdc, dry; ¢édos, friend.

STRUCTURE OF MESOPHYLL 309

or xeromorphic plants), thoroughly well protected against
water loss, that can survive in these habitats.

Structure of the Mesophyll.—The commonest type
of foliage leaf is the dorsiventral leaf, in which the upper
and lower sides are distinctly differentiated. In the
first place the stomata are usually present in much
greater numbers on, or are even confined to, the lower
surface of dorsiventral leaves. Secondly, the mesophyll
is differentiated into two strata—the palisade and the
spongy tissue. The palisade tissue lies immediately
below the upper epidermis.t It consists of from one to
several layers of cylindrical or prism-shaped cells with
their long axes perpendicular to the surface of the leaf,
and with narrow intercellular spaces between them.?
The cytoplasm lining the side walls of the palisade cells
is packed with chloroplasts (Figs. 11, 50), the palisade
tissue being the main photosynthetic tissue of the leaf.
The spongy tissue occupies the space between the palisade
and the lower epidermis, and consists of rounded or
irregularly shaped cells with large intercellular spaces
between them. Under each stoma there is usually a
particularly large intercellular space. The spongy cells
contain chloroplasts, but not nearly so many as the
palisade cells. The spongy tissue is the main transpiring
tissue of the leaf.

The more a plant is exposed to sun and dry air the
greater the development of palisade tissue, and in some
plants which live in dry sunny climates the whole of the

t Sometimes separated from it by a layer of hypoderm (im6, below,
dépua, skin), which often acts as a supplementary water tissue, and
is often thick walled and helps to strengthen the mechanical structure
of the leaf.

a The palisade cells are generally in close lateral contact, so that
on a cross-section of the leaf no intercellular spaces may be visible.
But if a “‘ flat’ section parallel to the leaf surface be made through
this tissue, the narrow intercellular spaces can always be seen between
the cells.

310 THE FOLIAGE LEAF

mesophyll is palisade, thus reducing the rate of transpira-
tion. Leaves held on the plant in a position in which
both sides are equally illuminated frequently have the
two surfaces alike, palisade being developed under each
epidermis. Inversely, in plants growing in damp and
shady situations the palisade tissue takes on the
characters of spongy tissue, the cells are short and are
separated by large intercellular spaces ; and in extreme
“shade leaves’ the whole mesophyll is of the spongy
type. The deeper the shade and the damper air
(conditions generally found together in nature) the
greater the total volume of intercellular spaces in
proportion to the total volume of the mesophyll cells.
Water Tissue.—The epidermis of a typical foliage leaf
acts, as we have already remarked (p. 275), as a water
store for the mesophyll cells ; and, as we have also seen
(p. 276), some leaves have a many layered epidermis
which greatly increases this water store. In extreme
cases the epidermal water tissue may form the greater
part of the thickness of the leaf. Part of the mesophyll
may also be very poor in, or even destitute of, chloro-
plasts, e.g. the central tissue of isolateral leaves, and this
also acts as a water store for the photosynthetic tissue.
Vascular Bundles (Veins).— Water and salts are brought
to the mesophyll of the leaf through the vessels and
tracheids of the xylem; while sugars and organic
nitrogenous substances are conducted away from the
mesophyll through the sieve tubes of the phloem. A
strand of xylem and a strand of phloem are associated
together to form a vascular bundle, the xylem being
towards the upper, the phloem towards the lower face
of the leaf. In the leaves of dicotyledons (the greater
number of seed plants) there is a large central vein,
the midrib, running up the centre of the leaf, and this

VASCULAR BUNDLES 311

usually projects from the lower surface (sometimes from
the upper surface also) and contains several vascular
bundles. The outer tissue of the midrib below the
epidermis commonly consists of collenchyma (p. 279).
From the midrib large secondary veins branch off, and
these also are usually thicker than the general thickness
of the leaf, so that they project on the lower surface.
From the large secondary veins smaller ones branch,
and from these smaller ones still, and the branches join
again (anastomose), so that the whole substance of the
leafisinterpenetrated by a network of vascular bundles.
The smaller veins are embedded in the general thickness
of the leaf. Each of these smaller bundles is surrounded
by a sheath of living cells (bundle sheath), which may or
may not contain chloroplasts (Fig. 50, sh.). The finest
branches of the bundle network are destitute of phloem
and each ends blindly in the mesophyll as a strand of
tracheids covered by the bundle sheath (Fig. 51, F).

From these terminal tracheids the water coming from
the roots through the continuous xylem channels is drawn
by osmosis into the mesophyll cells, and the dissolved
salts also diffuse, though independently of the water
(cf. pp. 290, 296), into the same cells, which are constantly
using them up to form complex organic substances.
The sugars and amino-compounds, formed as the result
of photosynthesis, diffuse out of the mesophyll cells into
the bundle sheath, and thence into the sieve tubes, along
which they pass into those of the larger veins, and so into
the midrib and out of the leaf.

Fibres.—The larger leaf bundles nearly always have
a strand of fibres, often crescentic in cross-section, on
the outer surface of the phloem, i.e. the surface turned
towards the lower face of the leaf, and there is sometimes
another similar strand on the upper surface of the xylem,

312 THE FOLIAGE LEAF

i.e. turned towards the upper face. In the larger veins
and in the midrib these fibre strands are frequently.
massive, and they help, sometimes in very notable degree,
to stiffen the mechanical structure of the leaf. The veins
of the leaf then act rather like the ribs of an umbrella.

The rigidity of the leaf is partly maintained by the
turgor of the living cells and partly by the veins, and these
two factors vary in their relative importance in different
species. Leaves in which the veins are sufficient by
themselves to keep the leaf rigid do not droop even
when their living cells are seriously depleted of water.
In other cases the base and centre of the leaf is kept rigid
by the larger veins, while the less supported tip and
edges are drooping. The edges and tip also suffer first
from loss of water by evaporation because they are
farthest from the source of supply.

Other Forms of Foliage Leaf.—While the great
majority of foliage leaves have the form of thin plates,
exposing a great surface in proportion to their bulk to
the light and air, there are many leaves which differ
from this type. Besides the succulent leaves (see p. 275),
which may be thick and flat, or oval or even circular in
cross-section, the commonest types are the needle-shaped
leaves (pine) and the long bristle-shaped leaves of many
grasses, especially those which grow in dry places. The
narrow leaves of other grasses often fold on the
midrib, the stomata being confined to the approximated
surfaces, or roll up, in dry weather. These characters
all tend to reduce or to protect the transpiring surfaces,
and are generally related to diminished water supply or
to diminished water-conducting capacity of the xylem.

t A thick and rigid cuticle and a thick-walled hypoderm may also
be important in maintaining the rigidity of leaves under severe water
loss. This is one reason why evergreen leaves of leathery texture such
as those of laurels do not readily droop.

SCALE-LIKE LEAVES 313

. Sometimes the leaves are small and scale-like, though
still green (cypress), and in such cases the stem as well
as the leaves may have a well-developed photosynthetic
tissue. Finally, the leaves may be reduced to minute
functionless scales, photosynthesis and transpiration
being carried out entirely by the green stem, to which
the photosynthetic tissue, resembling in all respects
the mesophyll of a typical leaf, is then confined.

Occasionally in such cases the leaves are represented
by bristles or spines instead of scales.

PRACTICAL WORK.

(1) Make a clean cut under water across the base of the leaf
stalk of a geranium (Pelargonium) leaf, and place it in a glass
with the cut end dipping below the surface of aqueous eosin
solution. After a time observe that the eosin has been sucked up
the xylems of the vascular bundles and eventually appears in
the leaf veins.

(2) Examine in a drop of dilute glycerine the transverse section
of the typical dorsiventral leaf blade (Hellebore) provided. Make
a drawing under the high power of (a) the midrib showing xylem,
phloem, fibres, collenchyma and epidermis, (b) of atypical segment
of the blade showing the upper epidermis with cuticle, palisade
cells, spongy tissue, with fewer chloroplasts and large intercellular
spaces, and the lower epidermis with a stoma in section.

(3) To another section add a drop of Schulze’s solution, and
note the different staining of the cuticle and of the wall substance
below it.

(4) Strip off a piece of the epidermis from the surface of the
leaf of Ivis, taking care that you get part of it at least free from
adherent mesophyll cells. Mount the strip in dilute glycerine
with the outer surface uppermost. Find and draw a stoma with
adjacent epidermal cells carefully under the high power. In
focussing down, the sides of the rectangular vestibule first come
into view, and below this the two guard cells with stomatal
pore between them.

(5) Now examine a transverse section of the same leaf. Find
and draw very carefully a stoma, which is now seen in sectional
view. Note especially the cuticular thickenings of the sides of

314 THE FOLIAGE LEAF

the vestibule and of the walls of the guard cells, and that the
latter bulge into the adjacent epidermal cells. Note the inter-
cellular spaces and the mesophyll cells below the stoma.

Examine also in the same section and draw a vascular
bundle in cross-section, distinguishing (a) xylem with vessels and
tvacheids, including the narrow protoxylem elements at the
extremity of the bundle, (b) phloem with sieve tubes and companion
cells, (c) fibres. Treat with aniline chloride or aniline sulphate.
The lignified tissues stain bright yellow.

(6) Cut the end of the leaf stalk of the floating leaf of Limnocharis
across with'a sharp knife, place the blade in water, and suck the
end of the stalk vigorously. Dark patches appear in the leaf
blade owing to water being sucked in through the stomata and
filling the large intercellular spaces which communicate with
those of the leaf stalk.

(7) Examine the shoots of Gorse (Ulex). Note the arrangement:
of leaves, branches and buds. The leaves are narrow and
spinous, but not so rigid as the branch spines.

(8) Examine any other examples of needle-shaped, bristle-
like, folded or rolled leaves that may be available, both on the
plants and in section.

CHAPTER XIX

THE PRIMARY STEM

THE shoot or subaerial part of the body of the higher
plant consists, as we have seen, of stem and leaves
together. The stem forms the branching axis of the
shoot system and bears the leaves, which grow out from
its sides. It thus (a) supports and displays the foliage
leaves in the light and air, (6) acts as the channel of
communication between root and leaves, and (c) supports
and displays the flowers, in which the reproductive cells
are formed and in which conjugation of the gametes
takes place.

Tropisms.—The erect aerial shoot is negatively geo-
tropic, growing away from the centre of the earth, just
as the taproot is positively geotropic, growing towards
it. This can be seen in plants kept in the dark which
grow straight upwards. If the plant is laid on its side
in the dark, the actively elongating region of the shoot
bends so as to bring the apex into the vertical position
again. The shoot is also positively phototropic, growing
towards the source of maximum illumination. These
two tropisms normally act in the same direction and
cause the shoot to grow straight upwards as the opposite
ones cause the root to grow downwards. But if illumina-
tion is one-sided, the positive phototropism of the shoot
overcomes its negative geotropism, so that it bends
towards the source of light. This can be seen in the

case of many herbaceous plants grown in pots in windows
315

316 THE PRIMARY STEM

which bend so that their shoot apices point towards
the open sky, and in plants growing on the edge of a
wood which behave in the same way. The leaves tend
to set themselves at right angles to the source of light
(diaphototropism). As in the case of roots, the bending
occurs by the cells on one side of the elongating region
growing faster than those on the other, but the apex
alone perceives the light, as can be seen by covering the
apices of some of a crop of seedlings (e.g. of canary
grass) with tinfoil caps and illuminating from one side,
when bending no longer takes place in those which
are so covered.

Etiolation—When a normally erect aerial shoot is
grown in the dark, it does not turn green, the plastids
becoming yellowish, the internodes grow enormously
in length and the leaves remain small and scale-like,
while the tissues are not properly differentiated. Such
a shoot is said to be etiolated.

Structure of the Aerial Stem.—The general structure
of the stem depends a good deal upon the fact that it
is essentially a leaf-bearing organ. We distinguish
the nodes or levels of insertion of the leaves (which may
arise from the stem singly or in pairs or circles, called
whorls), from the internodes or bare stretches of stem
between these levels. The stem terminates in a bud,
which is simply the developing apex of the shoot bearing
the developing leaves on its sides, and usually covered
by the incurving of the partly grown leaves, which arch
over and serve to protect the very delicate apical
meristem from desiccation.

The minute structure of the outer layers of the stem
resembles that of the foliage leaf in more than one
respect. Thus it is covered by an epidermis which has
the same character as the leaf epidermis, and possesses

STRUCTURE 317

stomata identical with leaf stomata, though they are
usually much fewer in number. Below this there is in
the green aerial stems of the great majority of plants
a living tissue (cortex) containing chlorophyll, though
generally not highly specialised like the mesophyll of a
foliage leaf. Very frequently, especially in the primary
shoots of woody plants, the outer cortex is collen-
chymatous. In some few cases, especially when the
leaves are reduced and functionless, the outer cortex,
or portions of it, may be differentiated as palisade tissue.

Below the cortex, as in the root, the vascular cylinder
occupies the centre of the stem. But the vascular
cylinder of the stem is very different in character from
that of the root. In the first place it is generally very
much wider (the cortex being correspondingly narrower)
in proportion to the whole width of the axis. Secondly,
the vascular tissue is arranged like that of the leaf in
bundles, each consisting of a strand of xylem and a
strand of phloem. Each of these bundles has the
xylem towards the centre of the stem, and the phloem
outside, towards the cortex; and all the bundles (in
the plants called dicotyledons) are arranged side by side,
separated by strips of parenchymatous tissue, the rays,
and forming a hollow cylinder enclosing the usually
parenchymatous pith, which fills up the centre of the
stem. Each bundle, like those of the larger leaf veins,
nearly always possesses a strand of fibres outside the
phloem (Fig. 52, A, B, per.) These fibrous strands may
be separated laterally by parenchyma, or they may be
joined laterally to form a complete fibrous cylinder.
This fibrous cylinder, or the fibrous strands together
with the intervening parenchyma, forms the pericycle
or outermost layer of the vascular cylinder, and
immediately outside it comes the endodermis, whose

318 THE PRIMARY STEM

lateral and horizontal walls do not always (as in the
root) possess a central band of cutin (see p. 293), though

Fic. 52.—Structure of the primary stem. A, diagram of cross-section
of stem showing vascular bundles arranged in a cylinder; X,
xylem; p%., protoxylem; P, phloem. B, single bundle in cross-
section ; coy., cortex; end., endodermis; per., pericycle (fibrous
opposite bundle). C, diagram of segment of stem seen obliquely
fromabove. The cortex and conjunctive tissue of the cylinder are
supposed to be transparent, the epidermis and endodermis opaque.
D, diagram of the course of the leaf trace and stem bundles
at a node bearing two opposite leaves in Clematis. The bundles
towards the observer are black, those away from the observer
shaded. Three bundles enter the stem from each leaf. The
laterals (J) at once join bundles from the internode above (two
of which fork), the central] bundle (c) of each trace continues alone
into the internode below (joining with others at a lower node).

sometimes they do. The stem endodermis often differs,
however, in other ways from the cells of the cortex

COURSE OF VASCULAR BUNDLES 319

outside it, e.g. by possessing specially large and
conspicuous starch grains, when it is called the “ starch
sheath.”

The vascular cylinder of the stem thus differs in many
ways from that of the root, and this is related to its very
different conditions of life and to the fact that its
bundles are primarily the direct downward continuation
of those of the leaves (Fig. 52, D). They are identical
in structure with the leaf bundles, and their orientation
is the same, the outward position of the phloem in the
stem clearly corresponding with its lower position in
the leaf.

The stem bundles do not pursue a completely in-
dependent course. A certain number come in from each
leaf, they often branch, and they always sooner or later
join on to neighbouring bundles which have come
down through the internode above from higher leaves
(Fig. 52, D). Traced downwards through the internode
the bundles fuse with one another laterally at various
levels, so that they leave room for those entering the
cylinder at the next node below. As the base of the
stem is approached more fusions take place, so that
comparatively few bundles enter the hypocotyl, and
these usually fuse with the cotyledon traces, so that
only the latter are directly continuous with the vascular
cylinder of the root. In the hypocotyl or at the top of
the primary root the cylinder narrows, the pith dis-
appears, the pericyclic fibres die out, and the xylem and
phloem strands change their relative positions, so that
they come to be alternate, i.e. situated on different radial
planes. At the same time the cortex changes its character
and the shoot epidermis is replaced by the piliferous layer.

Maintenance of the Erect Position.—The herbaceous
stem is maintained in an upright position partly by the

320 THE PRIMARY STEM

turgidity of its living cells, This is shown by the
drooping of many stems, or of their young upper portions,
when they are wilted. This source of rigidity is, however,
supplemented in many plants by the collenchyma of
the outer cortex, which is differentiated early, that is
fairly close behind the apical growing point, and stiffens
the surface layers of the stem. This surface stiffening
resists slight bending strains to which the upper part of
the shoot is subjected by the wind. Soon the lignifica-
tion of the primary xylem elements increases the support
afforded by the collenchyma. But the most important
mechanical support of most primary herbaceous stems
is contributed by the fibres of the pericycle. These are
thickened later, i.e. further from the growing point,
than the collenchyma, after growth in length has ceased.
Owing to the relative narrowness of the cortex compared
with the width of the stem, the fibres of the pericycle,
which often have very thick walls, form a rigid continuous
or interrupted cylinder not far from the surface, and
enable the stem to support the weight of leaves and
branches and to resist the bending strains imposed by
the wind.

Apical Meristem.—A longitudinal section through the
tip of the shoot shows the stem tip composed of meriste-
matic (embryonic) cells, and bearing on its sides the
first beginnings of the leaves, which arise as projections
on the surface caused by locally increased cell division.
As the young leaves increase in size they usually grow
faster on the lower than on the upper surface, with the
result that they curve over the growing tip of the stem
with its younger and as yet smaller leaves (Fig. 53),
thus serving to protect these and the meristem itself,
which are not as yet covered by a well-developed cuticle
from drying up and other injury. This curving of the

DEVELOPMENT BEHIND THE APEX 321

partly developed leaves over the shoot tip gives the
characteristic “‘ bud” structure.

The surface layer of the meristem is quite separate
from the underlying meristematic tissue, i.e. it forms new
cell walls only in the direction perpendicular to the
surface, and thus gives rise only to the epidermis of
the shoot. Below this surface layer of the meristem the
cell divisions give rise to the young tissues of the cortex
and of the vascular cylinder.

Development of the Shoot immediately behind the
Apex.—As in the case of the root, the products of division
of the meristematic cells away from the apex increase
in size by the development of vacuoles, and at the same
time divide less frequently.

Pith.—The cells in the centre of the stem, which will
give rise to the pith, are generally the first to lose their
meristematic activity. They often increase greatly in
size, partly by their own growth, and partly because
they are passively stretched by the active growth of the
surrounding tissues. This very frequently results in
the pith cells becoming the largest parenchymatous cells
in the stem. Sometimes the energetic growth of the
surrounding tissues, especially in length, after the
growth of the pith cells has ceased, leads to the breaking
and ultimate death of the pith. This is the cause of
the frequent hollowness of herbaceous stems in which
the whole of the pith or its central portion has disappeared
in the adult stem.

Zone of the Vascular Bundles, etc.—Just outside the
pith, in the zone which gives rise to the vascular bundles,
the rays and the pericycle, very many of the cells soon
cease to divide by horizontal walls, divisions continuing
only parallel to the axis of the stem. Growth in length
of these cells continues, so that they become elongated.

alr

322 THE PRIMARY STEM

The strands of such elongated cells which will eventually
form the vascular bundles are known as the desmogen
strands (Fig. 53, desm.), and these soon become distinct
from the surrounding tissue. The outlines of the tissues
which will be formed later from these often become clear
at an early stage of development, and the large cells
which will become the vessels of the xylem become
apparent. About the same time, or even earlier,
the limit between the vascular cylinder and the cortex
becomes evident, owing to differentiation between peri-
cycle and cortex, the cells of the latter continuing to
grow in width.

Development of Leaves and Axillary Buds.—While
these changes have been taking place in the stem tissue,
the leaves, which first appear very close to the stem
apex as small papilla, or, where the adult leaves have
a broad insertion on the stem, as curved ridges of
meristematic tissue, have been growing in length and
breadth. In the axil of each leaf there arises, sooner or
later, another papilla of meristematic cells, the rudiment
of the axillary bud (Fig. 53) ; and upon the sides of this
there arise the rudiments of the first leaves of the lateral
shoot into which the bud may develop. The bud may
grow out at once to form a branch, not much behind the
main shoot in development. On the other hand, it may
remain a bud for a long time, even for many years ; and
in some cases it never develops further at all. Its fate
depends on various external and internal conditions.

Differentiation of the Tissues.—The first tissue to
attain the adult form is the protoxylem of the vascular
bundles. Close to the inner limit of the desmogen strand
certain elongated cells become spirally thickened (see
p. 280), and the thickening band rapidly becomes lignified,
the protoplasm then dying. This happens at first at

DEVELOPMENT BEHIND THE APEX 323

or just below the node of a leaf, in the largest central
bundle of the leaf trace, and the differentiation of the
protoxylem progresses in both directions, downwards

Fic. 53.—Diagrams of the differentiation of a stem behind the apex.
On the left is a diagram of a longitudinal section, on the right
of transverse sections at the levels A, Band C: J, leaf; azx.bud.,
axillary buds; desm., desmogen strand; px., protoxylem. For
further explanation see text.

into the internode below and upwards into the leaf
(Fig. 53, px.). In other words, the elongated cells ina line
with the first formed spiral tracheids likewise become
converted into spiral tracheids, so that a strand of proto-

324 THE PRIMARY: STEM

xylem is formed which, so to speak, grows in length at
each end, extending up into the leaf and down into the
stem, and these strands form the first water channels
of the shoot. Later, protoxylem is formed in the smaller
bundles, and the strands extend till they meet those of
other bundles, and thus form a continuous water-
conducting system.

Elongating Region of the Stem.—The formation of the
protoxylem takes place at the beginning of the elongating
region of the stem, which is much longer than that of
the root, and often extends over several centimetres or
even severalinches. It is the internodes which elongate,
separating the successive leaves from one another, the
nodal regions growing mainly in diameter.

Meanwhile the other tissues of the shoot are rapidly
differentiating. The cuticle as it is exposed to the air
by elongation of the internodes becomes thicker; the
outer layers of the cortex frequently become collenchy-
matous by the thickening of the longitudinal walls at
the corners of the cells; chlorophyll develops in the
plastids ; and intercellular spaces between many of the
cortical cells.

In the vascular cylinder the fibres of the pericycle
are clearly marked at an early stage, but their walls
are not thickened till relatively late, generally after
elongation has ceased. Narrow sieve tubes (protophloem)
usually appear early just below the pericyclic fibres and
opposite the protoxylem of each bundle. When growth
in length has ceased, the development of the primary
vascular bundles continues by the centripetal forma-
tion of the larger sieve tubes -and companion’ cells
(metaphloem), and of metaxylem, consisting of larger
scalariform or some other type of pitted vessels. Differ-
entiation progresses outwards (centrifugally) in the

MODIFIED FORMS OF SHOOT 325

primary xylem, the pitted vessels following on the
spiral tracheids of the protoxylem. A band of tissue
between the xylem and phloem remains undifferentiated,
and this is the seat of the cambium or secondary vascular
meristem in stems which undergo secondary thickening.

The tissues of the leaf keep pace in differentiation
with those of the stem, the leaf blade expanding, while
the epidermis, veins and mesophyll acquire their adult
characters.

Modified Forms of Shoot.—While the typical aerial
shoot stands erect in the air, other forms do not develop
the mechanical tissues required to maintain the erect
position. Some of these trail on the ground, others
twine round any support (such as the erect stem of an
upright plant, the side of the stem away from the support
growing faster than theside touching it). Other shoots,
as we have already seen (Chapter XV), grow below the
surface of the soil and do not bear foliage leaves, but
only scale leaves. Others, again, grow very little in
length but increase in diameter (‘‘ rootstocks,’’ corms,
tubers). All these types of shoot have the same essential
plan of construction as the typical erect aerial stem,
but they differ very widely in details. Thus the cortex
may be relatively very broad, and it may contain
vascular bundles in addition to those of the cylinder
In some cases the vascular cylinder early loses its
identity altogether, so that the bundles are scattered
through the stem. Chlorophyll is not developed in
subterranean shoots, nor do they, asa rule, bear stomata,
Adventitious roots are practically always produced on
subterranean shoots, and very often also on creeping
shoots, especially at the nodes, as well as from the
bases of erect aerial shoots below the surface of
the soil.

326 THE PRIMARY STEM

PRACTICAL WORK

(1) Sketch the piece of herbaceous shoot provided, marking
leaf blade, leaf stalk, axillary buds, nodes, internodes. Note and
indicate the arrangement of the vascular bundles as seen on
the cut surfaces of the stem and leaf stalk: also the epidermis,
cortex, pericycle, vays and pith. [A piece of the mature shoot
of almost any large herbaceous plant is suitable, e.g. sunflower,
vegetable marrow, bean.] Examine also the preparation of a
piece of shoot made transparent in canada balsam so that the
course of the vascular bundles can be followed.

(2) Pull off the outer leaves of the Brussels sprout provided.
This is a bud whose outer leaves become mature with very little
elongation of internodes. With a sharp knife or a razor cut a
longitudinal section as accurately as possible through the tip, and
make a diagrammatic sketch showing (a) growing point (primary
meristem), (b) the youngest leaves, (c) older leaves with buds in
their axils, (d) young vascular bundles. Pull off the remaining
leaves and note their insertions and their axillary buds.

(3) Examine the prepared slide of the same for microscopic
details. Note especially the origin of the vascular bundles
(desmogen strands), the first spiral tracheids of the protoxylem
and the general appearance of the pith cells.

(4) Examine in dilute glycerine a cross-section of the stem of
the sunflower, at first with the naked eye and with a hand lens;
and then draw a diagram with the help of the low power. Mark
the outlines of the following tissues and tissue-systems : epidermis,
outer cortex, inner cortex, endodermis (starch sheath) ; vascular
cylinder, consisting of pericycle (composed of fibres opposite the
bundles, parenchymatous between), vays, pith, and vascular
bundles, showing xylem, phloem and beginning of cambium.

Put a section in a drop of Schulze’s solution for a few minutes,
and note the colour reactions of the various tissues, showing the
distinction between cellulose and lignified tissues.

(5) Now make careful high-power drawings of a few cells of
each of the following tissues : (a) epidermis with cuticle ; (b) outer
cortex (collenchymatous) ; (¢) inner cortex (parenchymatous) ;
(d) endodermis ; (e) pericycle fibres; (f) phloem, including sieve
tubes, companion cells and parenchyma; (g) cambium ; (h) xylem,
including large (pitted) vessels, parenchyma and xylem fibres ;
and () protoxylem; (7) cells of the medullary vays which are
beginning to form interfasciculay cambium.

Examine a prepared stained section of the same to verify
details and compare the staining.

PRACTICAL WORK 327

(6) Examine the radial longitudinal section, and draw under
the high power a sample of the following tissues: (a) phloem,
showing sieve tube (with sieve plate) and companion cell;
(0) metaxylem, showing part of a pitted vessel with adjoining
parenchyma and fibres; (c) protoxylem, showing spiral vessels
and adjoining thin-walled parenchyma. [It is necessary that
the section should pass radially through a bundle in order that
the above tissues should appear in it.]

(7) Examine the demonstration showing positive phototro-
pism and the fact that the apex of the shoot is the light-perceiving
organ [capped Setavia seedlings are suitable]: also examples
of etiolated shoots to compare with examples grown in light
{bean plants are suitable].

CHAPTER XX

THE WOODY STEM

General Characters of Woody Plants.—We saw in
Chapter XV that perennial herbaceous plants maintain
their vegetative bodies from year to year by means of a
persistent underground shoot (rhizome, tuber, corm or
bulb), which throws up new aerial shoots at the beginning
of the next growing season. New portions of the rhizome
are formed every year, new tubers, corms or bulbs are
produced, the older parts of the rhizome, the old tubers,
corms and bulbs dying off. The aerial shoots of woody
plants, on the other hand (trees and shrubs), do not die
down every year, leaving only the rhizome or other
form of underground shoot to continue growth the
next year, but themselves continue growing from year
to year, the terminal (or lateral) buds of the branches
forming a fresh portion of shoot in each growing season,
this being a direct (or indirect) continuation of the portion
formed the year before. At the same time the portions
of stem formed in earlier years grow in thickness each
year, and are thus able both to support and to conduct
water and salts from the root to the constantly increasing
shoot system above. This process of growth in thickness
is called secondary thickening, and is brought about by
the activity of the cambiwm or secondary (vascular)
meristem, whose beginnings we have already noted.
In most climates, also, there is a definite season of the

year unfavourable or impossible for growth (either a
328

WINTER BUDS 329

cold winter or a dry season), and the shoots of a woody
perennial must be protected from drying up during
that period, and this is effected by another secondary
meristem, the cork-cambium.

The main structural features in which a woody
perennial differs from a herbaceous plant are therefore
three. First, it produces secondary vascular and
supporting tissue. Secondly, it has means of protection
(bark) of its general shoot surface during the unfavour-
able season. Thirdly, its aerial buds (winter buds)
are protected during the same period.

Winter Buds.—At the tips and on the sides of the
younger portions of the branches of a tree or shrub
buds are to be seen during the winter covered with
brown scales. These bud scales are modified leaves
which were formed at the end of the last growing season,
and they completely cover in the delicate tissue (apical
meristem) in the interior of the bud. The winter bud
scales are largely composed of cells with corky (water-
proof) walls, and other (gland) cells are often present
which secrete a resin or gum that glues the scales
together and thus renders the bud additionally water-
tight. This arrangement not only prevents the tissues
within from drying up, but also stops rain from soaking
in and rotting the tissues.

Within the bud are young, partly developed foliage
leaves, and sometimes, in addition, a group of ready
formed flower buds (inflorescence).1 In the case of
fruit trees, such as the apple and pear, those winter buds
which contain the young flowers are called fruit buds,
because, of course, it is the potential production of fruit
from the flowers which interests the fruit gardener.

* Compare the buds on the crocus corm and in the tulip bulb, which
are also winter buds, though borne on an underground shoot.

330 THE WOODY STEM

They can be distinguished from the “‘ leaf buds ”’ (buds
of shoots which will produce leaves, but no flowers) at
any time during the winter in such fruit trees as the
pear, and very easily in early spring when they have
begun to swell, by their more rounded form.

In the spring the tissue of the axis begins to grow,
and this growth eventually bursts the, bud and pushes
out the foliage leaves, which unfold and grow to their
full size, while the internodes of the stem elongate.
If there are young flowers in the bud, these also grow
and open, The winter bud scales fall off, leaving a
zone of scars on the stem, and these will clearly mark
the junction of last year’s stem and the stem formed
during the present growing season. At the close of
the growing season the foliage leaves fall off, and the
terminal bud (or sometimes a lateral which takes the
place of the terminal) once more passes into the winter
condition by forming winter bud scales which close it
in. These again will fall off and leave another zone of
scars in the next spring. In this way successive zones of
bud scale scars are left on the stem, and the interval
between two of these zones represents a year’s growth
of the branch. By noting the positions of these zones
the history of a woody branch can be read for some years
back. Eventually the scars are destroyed by the scaling
off of the bark.

Some of the axillary buds formed on the current
year’s shoot may grow out during the growing season
to form branches. The rest form winter buds, and of
these some will grow out next spring, but others
remain small (dormant buds), and will only grow out later
on, under some special stimulus, such as may result, for
instance, from the destruction of the terminal shoot.

Secondary Meristems.—Growth in thickness of a

SECONDARY MERISTEMS 331

woody stem is carried on by the activity of a secondary
(vascular) meristem (cambium) which arises in the un-
differentiated tissue between the xylem and the phloem
of each primary bundle, and extends across the rays
to form a continuous ring! of meristematic (actively
dividing) tissue which runs round the stem. By far the
greatest part of the mass of a tree-trunk is formed of
secondary xylem, which is the wood of the trunk, by the
activity of the cambium.

Another secondary meristem, the cork cambium or
phellogen, arises in the stems and roots of most woody
plants, external to the vascular tissues. In stems it
arises most often in the outermost layer of the cortex,
immediately below the epidermis, but sometimes in the
epidermis itself, in the pericycle, or in a deep layer of the
cortex. This forms the tissue (cork) of which the outer
bark of a tree or shrub is largely or wholly composed.

The Cambium and Secondary Thickening.—The cam-
bium consists of active meristematic cells, typically
elongated in the direction of the axis of the stem, and
also tangentially, while they are often narrow in the
radial direction. The top and bottom walls of each
cell are commonly inclined like a roof or penthouse,
the slopes being directed tangentially (Fig. 54, A).
Properly speaking, the term cambium is confined to a
single layer of cells, which continually divide by new
tangential walls (Fig. 54, A—-C), one of the daughter cells
at each division becoming a tissue mother cell, giving
rise to vascular, fibrous or parenchymatous tissue
elements of the secondary xylem or phloem, according
to the side on which it is cut off, the other growing again
to form the cambial cell. The term is often, however,

1 As seen in cross-section: it is really, of course, a cylinder.
2 Greek geAddg, cork, and yervdw, produce,

332 THE WOODY STEM

loosely applied not only to the cambial (initial) layer
itself, but also to the undifferentiated tissue mother
cells on each side of it. When cell division is rapid
relatively to the rate of differentiation, there are several
layers of this undifferentiated tissue between the fully
formed secondary xylem and phloem (Fig. 54, E).

The cambial layer in the bundle itself (interfascicular
cambium) is really a layer of the desmogen strand which
has remained meristematic, and the process of cell
division and differentiation of the products (tissue
mother cells) into secondary xylem and phloem elements
is a direct continuation of the cell division and differentia-
tion, which has already taken place in the desmogen
strand, producing primary xylem and phloem, and
proceeding outwards from the protoxylem and inwards
from the protophloem. In the case of the xylem
especially the process often goes straight on with no
pause, the elements of the secondary xylem continuing
in the same radial rows as those of the primary xylem,
so that it is impossible to say where the one ends and the
other begins. In some cases, however, there is a distinct
pause between the completion of the primary and the
beginning of secondary development.

Each tissue mother cell cut off from a cambial cell
on the inside, ic. towards the primary xylem, may
become either (a) a tracheid, (b) asegment of a vessel,
(c) a xylem fibre, or (d) by horizontal division a vertical
file of parenchyma cells (Fig. 54, D). The cells undergo
the corresponding modifications of the cell wall, the
protoplasm dying in all except the last case. Each
tissue mother cell cut off from a cambial cell on the
outside, i.e. towards the primary phloem, may become
either (a) a segment of a sieve tube with one or more
companion cells, (b) a phloem fibre, or (c) by horizontal

ACTIVITY OF THE CAMBIUM , 333

B
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1 c 1 1 £ 1
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Fic. 54.—Activity of the cambium. A-D, diagrams of cambial
cell and its divisions (inner and outer—tangential—walls supposed
to be transparent, lateral—radial—walls opaque). A, single
cambial cell seen obliquely from within. B, first tangential
division (1). C, second tangential division (2); c, cambial cell;
#, xylem tissue mother cell; », phloem tissue mother cell. D,
diagram of tissue mother cell which has divided horizontally
to form a vertical file of short parenchyma cells. E, part of
cambium and tissues it has produced as seen in transverse section
under high power; c, cambial cells; v, xylem vessels; ¢, tracheid ;
m.y., medullary ray cells; s.t., young sieve tube; .p., phloem

parenchyma; w.p., wood parenchyma; /f, fibres.

Ss

or
a

2

Wal
erga

CG

334 THE WOODY STEM

division a vertical file of phloem parenchyma cells.
The xylem fibres are usually more strongly lignified
than the phloem fibres, and the xylem parenchyma is
often, though not always, thick walled and lignified.
The tracheids and fibres commonly elongate very much
in developing from the tissue mother cells, and their
pointed ends slide past one another, so as to overlap
very considerably.

Secondary Rays.—Certain cambial cells, instead of
forming any of the above-mentioned elements, cut off
cells on both sides, which then divide horizontally,
each cell of the file so formed sometimes elongating
radially, so that it corresponds on each side with
several elements of the secondary xylem or phloem.
Several cambial cells adjacent in the vertical direction
(sometimes also several adjacent in the tangential
direction) behave simultaneously in this way, and once
a set of cambial cells has begun to form them, it does
not produce anything but this kind of cell. In
this way vertical plates of tissue called secondary rays
are formed, running through the secondary vascular
tissue in a radial direction, several or many cells deep
(ie. in the vertical direction) and one or several cells
broad (i.e. in the tangential direction), As the secondary
xylem increases in bulk it also of course increases in
circumference, and the cambium increases in circum-
ference with it. This it does by the vadial division of
its cells, i.e. by putting new cells into its circumference.
Every year fresh secondary rays are begun, while those
already begun are continued, so that in proceeding from
the centre to the circumference of the secondary xylem
of a woody stem several years old, the number of rays
cut by successively larger circles continually increases,
and the secondary tissue, as seen in cross-section, is

SPRING WOOD AND AUTUMN WOOD 335

divided up into radial wedges by the rays. The rays
are generally joined by tangential bands of xylem
parenchyma, so that the wood is divided into relatively
small blocks of tracheids and vessels, each in contact
with living parenchyma cells. It is probable that this
fact is related to the mechanism of conduction of water
and other substances through the wood, which is not
fully understood. Accompanying the rays are hori-
zontally running intercellular channels which com-
municate with the cortical system of intercellular spaces,
and thus serve to aerate the living cells of the secondary
xylem. Broad rays (principal rays) are often formed
as continuations of the primary rays (separating the
primary bundles) when these are narrow, and can be
distinguished from the narrow subordinate rays formed
opposite the primary bundles (Fig. 55, C).

Annual Rings.—The well-known concentric rings seen
on the cut stump of a felled tree are the expression of the
fact that the cambium does not form perfectly uniform
wood throughout the year. In the early summer, when
its activity begins, it forms numerous and often large
vessels with accompanying parenchyma (“‘ spring wood’).
These provide new conducting channels for taking water
and salts up to the new foliage. In the late summer,
after these channels have been provided and the foliage
is no longer increasing, the vessels are smaller and fewer
in number, and the bulk of the wood produced is often
composed of fibres, the water-conducting elements being
sometimes quite absent (‘‘ autumn wood”’), Thecontrast
between the alternating zones of spring and autumn
wood gives the rings which can be counted with the naked
eye on the surface of a tree-stump. A check to growth,
for instance a severe drought at midsummer or the
wholesale destruction of the foliage by caterpillars,

336 THE WOODY STEM

Fic. 55.—Diagrams of secondary thickening in the stem. A, cross-
section of stem with bundles separated by broad primary rays
in the first year of thickening; secondary xylem and phloem
are formed all round the cylinder, bridging the broad primary
rays. B, ditto, with bundles separated by narrow primary rays;
the cambium continues the primary rays as secondary (principal)
rays; s.t., secondary tissue. C, section of stem after six years
of thickening; the primary rays are continued as principal rays,
and new narrower subordinate rays are formed in the wedges of
secondary tissue formed opposite the primary bundles.

SAPWOOD AND HEARTWOOD 337

followed by the outgrowth of buds and the unfolding of
fresh leaves, will give rise to a double ring in one year,
so that the estimation of the age of a tree by counting
the rings is not always quite accurate. Sometimes a
similar annual alternation of bands of sieve tubes (spring)
and fibres (autumn) can be detected under the micro-
cope in the secondary phloem.

Sapwood and Heartwood.—When a tree-trunk has
reached a certain age, which varies in different species
of tree, the wood nearest the centre undergoes certain
changes, which gradually spread outwards as increase
in thickness continues. The living cells (rays and
xylem parenchyma) die and the walls of the lignified
cells lose water, increase in hardness and usually change
in colour, often becoming darker (oak), and sometimes
producing a pigment which stains them a distinct colour
(e.g. black in ebony, yellow in satinwood and yellow-
wood, purple in logwood); this harder internal wood
is called heavtwood. It no longer functions as a water-
conducting tissue, and it is the wood most suitable for
use in building, furniture making, etc., because it does
not warp by losing much water through evaporation on
drying. The wood nearest the cambium which is still
conducting water and still contains living cells is called
sapwood. The sapwood forms a belt of approximately
constant width, continually added to on the outside by
the cambium, and continually converted into heartwood
on the inside.

Some trees have no heartwood, the dead wood in the
centre remaining soft and often decaying after it dies, as
in the willow. This is frequently the cause of hollow
trunks, though hard heartwood too sometimes decays.

Cork Formation.—The phellogen or cork cambium
arises in most cases from the outermost layer of cortical

22

338 THE WOODY STEM

cells (Fig. 56, a), i.e. immediately below the epidermis,
sometimes from the epidermis itself, occasionally in
a deeper layer of the cortex, and fairly often in the
pericycle. In roots the phellogen is nearly always
formed in the pericycle.

Each phellogen cell arises as the result of two rapidly
following divisions of the living tissue cell in which it
is formed, the two new parallel walls being tangentially

Fic. 56.—Cork formation: a, b, Beginning of phellogen in the outer-
most layer of cortex; ep., epidermis; phel., phellogen. c, Seven
layers of cork formed by phellogen below epidermis. d, Diagram
of two cork cells showing the three layers of the wall (see text)..

directed and cutting out the phellogen cell between them

(Fig. 56, 0, phel.). The phellogen cell then continues to

divide,cutting off a series of daughter cells on the outside,

i.e. towards the surface of the stem, forming a regular

radial row, and the walls of these are thickened, and be-

come partially converted into corky substance (Fig. 56,c).

The middle lamelle remain unchanged, the next layers

CORK AND LENTICELS 339

become corky (a complex substance, suberin, of which
fatty acids are constituents, being deposited in them),
and acellulose layer is laid down next the cell cavity
(Fig. 56, da). The cork cell then dies, for the corky cell
wall is waterproof and prevents the diffusion of water
and solutes into and out of the cell necessary to the
continued life of the protoplasm. Brown colouring
substances are usually formed both in the walls and in
the cavities of the cork cells. The thick layer of cork
cells with waterproof walls forms a very effective barrier
between the living tissues of the plant and the outer
world. It is effective not only in checking evaporation
and gaseous interchange in general, but also in stopping
the attacks of minute insects and the germ tubes of
fungal spores. Like the epidermis, however, which it
supersedes, it is interrupted at certain spots by openings,
and these are called lenticels.

Lenticels are formed in the following way: Cork
formation typically begins below the stomata at a
somewhat deeper level of the cortex than over the rest
of the surface of the stem, and thence it spreads to the
outermost cortical layer (or to the epidermis itself) over
the general surface. The vigorous cell division below
the stomata raises the epidermis at these points and
eventually bursts it. The cork cells formed below the
stoma are loose, with abundant intercellular spaces
between them, and this loose brown tissue is pushed out
through the opening made by the bursting of the stoma,
and forms a purulent projection on the surface. The
whole structure is called a lenticel, and through the
air spaces between the cells of the lenticellar cork,
which communicate with the intercellular spaces of the
cortex below, diffusion of gases takes place between the
living cells of the stem and the outside air.

340 THE WOODY STEM

Bark.—The original phellogen does not, as a rule,
remain active indefinitely. In most cases it stops
dividing after some years, and a new phellogen is formed
in a deeper layer of the cortex, producing a second layer
of cork, separated from the first by some layers of cortical
cells. This second layer is succeeded by a third, after
another interval, at a still deeper level, and so on till
the new phellogen come to be formed in the parenchyma
cells of the secondary phloem. The later formed
phellogens cut deeper and deeper into the secondary
phloem, the older parts of which, together with the
pericycle and cortex (the whole mass being called the
outer bark), are thus killed, being cut off from the living
and growing tissues of the stem on each side of the
cambium. The zone of functional living phloem (inner
bark) thus remains approximately constant in width,
being continually reduced on the outside by the forma-
tion of deeper and deeper layers of cork, and continually
added to on the inside by the cambium. This is a
parallel phenomenon to the formation of heartwood
from the older secondary xylem. In an old tree-trunk
only a comparatively narrow cylinder on each side of
the cambium is alive and functional in conduction.
The great mass of the tissue of the trunk is dead (heart-
wood and outer bark), though it may be useful to the
tree—the heartwood giving extra support, the outer
bark efficient protection.

The bark of trees, as is well known, varies very much
in appearance. Some, like the plane tree and the
“ paper birch,” have smooth bark, which scales off in
uniform flat thin layers. This is because the phellogens,
and consequently the layers of cork which they produce,
are uniform and parallel. Others, such as the common
oak, the elm and the pine, are rugged, the surface being

USES OF CORK 341

very uneven because the bark scales off in chunks.
This is because the courses of the phellogens are irregu-
lar and curved, intersecting one another. The actual
nature of the cork formed also varies in different trees,
sometimes being hard and woody, while in other cases
the corky substances of the cell walls is very pure,
giving an elastic character to the bark.

Uses of Cork.—The “ cork ”’ of ordinary life is obtained
from the bark of the cork oak (Quercus suber), an ever-
green oak native in the Mediterranean region. The first-
formed bark is rough, but after this has been removed the
bark subsequently formed is very uniform in structure,
and the corky substance of the cell walls very pure.
This bark is removed in sheets at intervals of several
years, boiled in vats to soften it, pressed flat, and then
cut up as required. The dark streaks seen in cork are
the lenticles, composed of loose powdery cells, which
are continuously formed by the phellogen, so that they
penetrate the whole thickness of the bark. Bottle
corks are always cut at right angles to the course of
the lenticels, i.e. parallel to the surface of the bark,
so that solid cork intervenes between the lenticels in
the length of the cork. If they were cut the other way
so that the lenticels ran through them from end to end,
the cork would not be gas and water tight. Bungs,
on the other hand, which are not intended to be gas-
tight, are stamped straight out of the cork sheet,
so that the lenticels run from top to bottom of the
bung.

Cork tissue is not only gas and water tight, it is
also a very slow conductor of heat. Thus a jacket of
cork prevents the living tissues in the interior of a
tree-trunk from being heated or cooled too quickly
with external changes of temperature. This character,

342 THE WOODY STEM

together with its lightness, makes it useful to man for
a great variety of purposes.

PRACTICAL WORK.

(1) Make a sketch of a winter branch of horse-chestnut (or
beech) showing several years’ growth. Mark terminal buds,
lateral buds (distinguishing dormant buds) covered with winter bud
scales, leaf scars, bud-scale scars, lenticels ; also (in horse-chestnut),
if present, the scar of a fallen inflorescence. Mark also the
parts of the branch originally formed in each year, beginning with
last season and working backwards. Dissect a large terminal
bud.

(2) Make a diagrammatic sketch showing the structure of any
sprouting bud (sycamore is very suitable, but almost anything
will do) after the first foliage leaves are clear of the bud scales,
marking winter bud scales, foliage leaves, elongating internodes
and tervminal bud. [Opening buds preserved in spirit should be
provided if fresh material is unavailable owing to the season.]

(3) Examine the transverse section of a woody twig about
three years old, first with a hand lens, then with the low power
of the microscope. (Sycamore is again suitable, but elm or
lime will do equally well.] Draw a diagram under the low power,
showing—(a) the cork ; (b) the outer cortex ; (c) the inner cortex ;
(ad) the pericycle ; (e) the secondary phloem ; (f) the cambium ;
(g) the secondary xylem with the annual rings, and in each the
spring and autumn wood ; (h) the principal secondary rays ; (i) the
primary xylems ; (7) the pith.

(4) Examine the branch of Portugal Laurel that has been
kept with its end dipping into red ink, or a watery solution of
eosin. Split it longitudinally and note that the red solution
has eosin in the wood only.

(5) Make diagrammatic sketches of the three faces (transverse,
radial and tangential) of the segment of oak trunk provided,
showing—

On the transverse section the annual rings, the large spring
vessels which appear as small holes in the wood, the masses of
jibves making up the great bulk of the wood, the broad secondary
vays crossing the cambium and extending through the functional
living phloem of the inner bark (yellow brown) into the dead
phloem (red brown) cut off between the dark layers of cork of
the outer bark.

On the radial face the spring vessels appear as grooves in the

PRACTICAL WORK 343

‘

wood and the rays as “silver grain,’’ because their course is
sinuous. On the tangential face the rays are seen in their lens-
shaped transverse section, and their vertical extent can be
determined.

Note on the transverse and radial faces the distinction between
heartwood and sapwood.

(6) Draw under the high power a small portion of a cross-
section of the outer tissue of the stem of a woody plant during
its first summer’s growth to show the origin of the phellogen
and the development of cork [Ailanthus is suitable]. Compare
the remaining tissues of the stem with (3).

(7) Examine the piece of willow branch provided. Observe
the lenticels on the surface. Peel the branch and note that the
bark separates along the line of the cambium.

CHAPTER XXI

THE FLOWER

A FLOWER is a shoot, or the termination of a shoot,
whose leaves (floval leaves) are specially modified in
different ways. The upper (inner) leaves produce the
reproductive cells, the lower (outer) ones serve to
protect the flower bud and (usually) to render the flower
conspicuous. The apical meristem of the flower bud
ceases its activity with the production of the floral
leaves, so that terminal growth in length of the floral
shoot is at an end,

The seed plants are heterosporous, like Selaginella
(p. 251), and while one set of floral leaves bears micro-
sporangia containing microspores, another set bears
megasporangia, each megasporangium normally con-
taining one megaspore. The prothallus produced by
each is greatly reduced and is formed together with the
gametes inside the spore. One of the essential characters
of seed plants is the means by which the gametes
are brought together. The microspores are carried by
external agency, usually insects or the wind, to a
special part of the floral leaf bearing the megasporangia.
There the microspores germinate, each putting out a
germ tube which, carrying the male gametes, grows
towards and penetrates the megasporangium, setting
free the male gametes inside ,the megaspore, in the
immediate neighbourhood of the female gamete or egg.

Conjugation thus takes place inside the megaspore.
344

PARTS OF THE FLOWER 345

The flowers of some kinds of seed plants are of two
kinds, the one producing microsporangia alone ie male ”’
flowers), the other megasporangia alone (‘‘ female ”’
flowers) ; but most seed plants have flowers producing
both kinds of sporangia, though on separate floral leaves,
and these are called ‘‘ hermaphrodite ”’ flowers.

Parts of the Flower.—The flower is a condensed
shoot in which the internodes have elongated very little, if
at all, and the crowded nodes on which the floral leaves
are borne are together called the receptacle (Fig. 57).
The lower part of the same axis continuous with the
receptacle is often bare of leaves, and is called the
peduncle if the flowers are borne singly arising from
vegetative shoots, the pedicel if it belongs to a system
of branches (inflorescence), each bearing a flower. Leaves
in the axils of which flowers are borne or themselves
borne on peduncle or pedicel and somewhat different
from the foliage leaves are called bracts.

Perianth.—The floral leaves themselves are arranged
in whorls (a whorl is a circle of leaves arising at the
same level on the axis) one above the other. The
outer (lowest) leaves form the perianth, which very
commonly consists of two whorls, the calyx, whose
leaves, the sepals, are often green and rather like small
simple foliage leaves, and the corolla, whose leaves, the
petals, are generally larger and more or less brightly
coloured. Sometimes, however, the perianth consists
of one whorl only, or of two whorls whose leaves are
alike. In the flower bud the perianth leaves are curved
inwards, enclosing the two inner (upper) whorls of
floral leaves which bear the two kinds of sporangia.

Stamens and Carpels.—The first (lower) of these is
called the andrecium,! the individual leaves the stamens.

t Greek dvijp, dvdpdc (anér, andros), a man, because the male gametes
are formed in the microspores produced by the stamens.

346 THE FLOWER

Each stamen has a’ stalk, the filament, and bears
(usually two) pairs of microsporangia, which are
known as pollen sacs, together forming the anther or
head of the stamen. When ripe the pollen sacs are full

connective anther
pollen Sac

we.

lL
pecs e

pedicel

Fic. 57.—Diagram showing the parts of a flower.

of pollen grains (microspores) (Fig. 58, A). The
uppermost whorl of floral leaves, occupying the centre
of the flower, is called the gynecium,? and its leaves the
carpels.» A carpel is a folded leaf with its margins

t Greek, yur}, yuvarxds (guné, gunaikos), a woman, because the
female gametes are formed in the megaspores.

2 Latin carpellum, a little fruit, from Greek xapmde, a fruit, because
the carpels later on grow into the fruit.

ANTHER, POLLEN AND POLLEN TUBES 347

Fic. 58.—Development of pollen (microspores). A, cross-section of

anther showing four pollen sacs (microsporangia), one full of
pollen grains (microspores) ; also fibrous layer of wall and vascular
bundle. B, cross section of opened anther, with the walls of
the pollen sacs turned back. The dotted lines show their original
position and the arrows the direction of movement when they
break open. C, pollen grain with nucleus (right) and “‘ genera-
tion cell,’”’ which represents the ‘‘ male prothallus”’ (left). D,
germination of pollen grain. The nucleus has entered the tube
and is followed by the “‘ generative cell.’”’ E, growing apex of
pollen tube with nucleus and two male gametes into which the
generative cell has divided. F, part of stigma with germinating
pollen grains, their tubes pushing down between the loose stig-
matic cells.

348 THE FLOWER

coherent so as to form a closed bag-like structure, the
ovary (Figs. 57,59, A). The free end of the leaf forms
a structure, which, when mature, lacks the characteristic
shoot epidermis, bearing on its surface papille or hairs
which secrete a sugary solution. This is called
the stigma (Fig. 57), and is the organ which receives the
pollen grains. The stigma is often raised above
the ovary on a more or less hollow stalk, the style. On
the thickened margins of the carpellary leaf (placenta)
inside the cavity of the ovary are the ovules (Fig. 59, A).
Each ovule (Fig. 59, B) is a megasporangium covered
by two coats, each composed of one or more layers of
cells, and with a body (nucellus) consisting of an ovoid
mass of cells, of which one, the megaspore, early becomes
much larger than the rest, and when mature fills much
of the space within the nucellus, one end lying close
to the surface of the free end of the nucellus, just below
a pore (micropyle) left by the incomplete closure of the
coats of the ovule. During development the ovule
generally turns completely round upon itself, i.e. through
an angle of 180 degrees, so that the free end comes to
point towards the placenta on which the ovule is inserted.
In a few cases the ovule does not turn, and the free end
points away from the placenta.

Development of Gametes.—The pollen grain or
microspore when ripe consists of a cell with a thick
cutinised outer wall, often covered with projections
so that the surface is rough, and a thinner internal
wall consisting of cellulose. One or more interruptions
in the thick outer wall leave thin places covered only
by the thin inner wall. The cell is densely filled with
cytoplasm and contains a large conspicuous nucleus.
This nucleus divides into two: one daughter nucleus
is the nucleus of the ripe grain; the other is somewhat

DEVELOPMENT OF GAMETES 349

elongated and is surrounded by a little cytoplasm, so
that it forms a little naked cell within the pollen grain

oc. ne ue.
syn.
Pn
oon i 2 TNUC
fy
ant Li
B vb.

Fic. 59.—The ovule and fertilisation. A, cross-section of a single

free carpel containing two rows of ovules; m.v., midrib of car-
pellary leaf; p/., placenta (thickened margin of carpellary leaf)
bearing row of ovules. B, ripe ovule (megasporangium) in
longitudinal section; v.b., vascular bundle; muc., nucellus (body
of ovule = wall of megasporangium); o.c., outer coat; i.c., inner
coat; micry., micropyle. The embryo sac (megaspore) contains
in the centre the secondary nucleus (sec.n.) ; at the micropylar
end the two synergide (syv.) and the egg, and at the opposite
end the three antipodal cells (amt.). C, development of embryo
sac; a-d, division of primary nucleus to form eight nuclei, four
at each end; ¢é., formation of egg apparatus, group of antipodal
cells and secondary nucleus. D, embryo sac at the time of
fertilisation; p.t., pollen tube, which has entered the upper end
of the sac and evacuated the two male gametes, one of which
is seen in contact with the nucleus of the egg (female gamete)
and the other with the two polar nuclei, which in this case have
not yet fused to form the secondary nucleus of the sac. Contact
will be followed by fusion, in the former case giving the zygote,
in the latter the mother nucleus of the endosperm.

cell (Fig. 58, C). This independent cell later divides

to

form the two male gametes (E). Thus the male

350 THE FLOWER

prothallus is reduced to the lowest possible terms, for:
it consists of nothing but the single mother cell of the
two gametes and the nucleus of the grain which later
functions in forming the pollen tube.

The nucleus of the megaspore (embryo sac) divides
by three successive divisions to form eight nuclei
(Fig. 59, C, a—-e), a group of four at each end of the
sac. Two of these nuclei, one from each group of four,
fuse together in the centre to form the secondary nucleus
of the sac, the other three of each group remain together
and accumulate cytoplasm, so that two groups of three
cells are formed one at each end of the sac. The
three cells at the micropylar end of the sac form the
egg apparatus, the cell farthest from the micropyle
being the egg (female gamete), the other two the
synergids.* The three cells at the opposite end of the
sac are called the antipodal cells (Fig. 59, D, ant.).

It is to be noticed that while one cell only of the
eight normally functions as a female gamete, its nucleus
is the sister of a nucleus which fuses in the centre of
the sac with a corresponding nucleus from the antipodal
group. The secondary nucleus so formed undergoes,
as we shall see presently, a further development.
As an abnormality, also, one of the synergidz or one
of the antipodal cells may fuse with a male gamete,
thus itself acting as a gamete. We must, therefore,
probably: regard the two groups of cells as broods of
four gametes, only one of the eight being normally
functional as a sexual gamete, i.e. fusing with a male
gamete, though two others normally behave in a gamete-
like way. This recalls the brood of eight eggs in

t Synergide (Greek ody, with, and épyov, work) = “‘ co-operators,”
because these two cells are supposed to assist in directing the pollen
tube to the egg.

POLLINATION AND FERTILISATION 351

Fucus, of which, it will be remembered, all are func-
tional in some species, while in other species four,
two, or only one are functional eggs.

Pollination.—When the pollen is ripe the anthers
open in various ways. Very commonly a longitudinal
split runs down the side of the anther between the two
pollen sacs whose walls break through along this line
and fold back, exposing the pollen grains (Fig. 58, B).
The split is actually caused by the contraction on
drying up of the wall of the sac, whose cell walls have
fibrous thickenings, the tension which develops eventually
tearing apart the weak unthickened tissue between the
two sacs on each side of the anther.

The pollen grains are conveyed to the stigma in
various .ways. Sometimes the stigma touches the
openedyanther, rubbing off the pollen (self-pollination) ;
but @ry often the pollen is conveyed, generally
by insects or the wind, to the stigma of another
flower of the same species (cross-pollination). The
stigma is papillose and sticky, or hairy, so that
the grains, which are often rough coated, readily
stick to it.

Germination of the Pollen Grains. Fertilisation.—
The cells of the stigma secrete a sugary solution which
is absorbed by the grains lying among them, and these
germinate, a tube (pollen tube) being pushed out from
the thin spot on the wall. Into this tube pass first the
pollen grain nucleus, and then the independent cell,
which divides to form the two male gametes during
the growth of the tube (Fig. 58, D, E). The tube
grows into the stigma (Fig. 58, F) and down through
the loose tissue of the style till its tip reaches the cavity
of the ovary. The growing tip is positively chemotropic
to sugar, or some other substance secreted by the tissue

352 THE FLOWER

in the region of the micropyle, which the tube enters,
then pushing between the cells of the nucellus and
entering the embryo sac. Here the tip bursts, setting
free the two male gametes. These are elongated and
often curved or slightly twisted spirally, recalling the
spirally twisted male gametes of the lower plants.
They appear to wriggle actively in the sac, one fusing
with the egg, the other with the secondary nucleus of
the sac (Fig. 59, D).

The zygote or “fertilised egg’ gives rise to the
embryo of the new plant, the secondary nucleus of
‘the sac, to which three nuclet have now contributed,
divides to form a new tissue, the endosperm, which fills
the sac, and at the expense of which the embryo grows.
The endosperm nucleus is of the nature of a zygote
nucleus, being formed by the fusion of (a) the sister
nucleus of the functional female gamete, (b) a corre-
sponding antipodal nucleus, (c) a male gamete. This
“ triple fusion ”’ is most unusual, and leads to irregulari-
ties in division of the endosperm “‘ zygote’ nucleus.
The endosperm may thus be regarded as an “‘ abnorm-
ally ’’ formed embryo, which is sacrificed to the feeding
of the normal embryo.

Varieties of Floral Form and Structure.—The variety
of form and structure among flowers is exceedingly ©
great, and seed plants are classified largely by means
of these differences. Thus the floral leaves of each °
kind, particularly the stamens and carpels, vary from |
a large and indefinite number (buttercup) to a small
and definite number (apple, cherry, corncockle). The
floral leaves in each whorl may be quite free and separate
from one another! (buttercup and Fig. 60, A) or they

t This is expressed by the prefix apo- (Greek dad, away from).
Thus the buttercup flower is apocarpous.

INSERTION OF FLORAL LEAVES ON RECEPTACLE 353

may be joined to one another laterally! (calyx of
corncockle, carpels in Fig. 60, B). The successive
whorls of floral leaves may arise from the sides of a
conical receptacle (Fig. 60, A), or they may arise from

vy
YY

_ Fic. 60.—Diagrammatic vertical sections illustrating different rela-
tions of the receptacle (black throughout) to the floral whorls.
A, conical receptacle bearing the successive whorls on its sides
(hypogynous type), carpels separate. B, flat receptacle, carpels
united (syncarpous gynecium). C, basin-shaped receptacle
with two free carpels at the base, the remaining whorls on the
edge of the basin (perigynous type). D, receptacle fused to the

sides of the ovaries (epigynous type), which are then said to be
“inferior.”

a flat receptacle (Fig. 60, B), or the receptacle may
be cup-shaped (Fig. 60, C, and cherry), with the sepals,
petals and stamens arising from the edge of the cup,
and the carpels from its bottom. Finally, the sides of

« Expressed by the prefix syn- (Greek ody, together). The corn-
cockle flower is synsepalous. The gynecium in Fig. 60, B, is syn-

carpous.
23

354 THE FLOWER

the cup-shaped receptacle may be fused with the walls
of the carpels (Fig. 60, D: apple, pear). These
different relations of the receptacle to the various whorls
of the flower are expressed by the terms hypogynous,
perigynous and epigynous,t the other whorls of the
flower being below the gynecium in the first case,
vound it (i.e. borne on the edges of the receptacular cup)
in the second, and above or on it in the third, where
the sides of the receptacle are fused with and close over
the walls of the carpels.

On the whole, in the evolution of the great group
of flowering plants, the flowers with separate and numer-
ous floral leaves are more primitive, those with few and
joined floral leaves the more advanced (the stamens,
having thin stalks, generally remain separate throughout, —
though they are joined to one another in a few cases).
And the perigynous and epigynous types of flower
are similarly more advanced than the hypogynous.
Especially in the case of the carpels, as we pass from more
primitive to more advanced flowers, there is a tendency
for the carpels (a) to decrease in number, (6) to become
fused, and (c) to become enclosed in the receptacle.
The ovary is said to be inferior in this last (epigynous)
type of flower (apple, narcissus).

Besides these general tendencies in the evolution of
flowers there are many other differences depending
on the relative size, shape and colour of the different
floral leaves. These affect particularly the mode of
pollination. A flower has to be regarded as a whole,
as an organ which not only produces and _ protects
the spores and gametes but is also so constructed as
to bring about the conjugation of the gametes through
the preliminary process of pollination. In many cases

x Greek ind, wepi and émt, below, round and on.

CROSS POLLINATION 355

the flower is a very wonderful and perfect mechanism
for securing this process.

Cross-Pollination by Insects.—A very large number
of flowers, probably the great majority, are adapted to
cross-pollination by insects. Insects of different kinds
(mainly bees, flies, butterflies and moths) visit flowers
to feed on the nectar or on the pollen, or on both, or
to collect them (bees) for their young. While in the

axule free central
a “By “(By
Cc
° ® : saa
~=e A :

Fic. 61.—A, ground plan of a flower (floral diagram) showing five
free sepals (black), five alternating free petals, ten stamens in
two alternating whorls of five each, and three free (apocarpous)
carpels. B, cross-sections of syncarpous ovaries of three carpels
each (a, b, c), showing different types of placentation: avile
(note correspondence of placentz with those of the carpels in
A), free central (the infolded carpel walls have disappeared, leaving
one central placental column), and parietal (carpels not infolded,
margins joined to form placentz on inside of outer wall).

flower they brush against the ripe anthers, and the

pollen grains stick to their hairy bodies. On visiting

another flower of the same species—and bees especially
often keep to one kind of flower on one journey—the
insect may brush against the stigma and rub off the
grains. The positions of the ripe anthers and ripe
stigmas in the flower are generally such that this probably
or even inevitably happens.

The petals of insect-pollinated flowers are often

large, brightly coloured and conspicuous, so that the

350 THE FLOWER

larger insects can see them from a distance. The
flower often has nectaries, little masses of gland cells
which secrete a sugary solution, and this nectar is
the main attraction of many flowers to nectar-eating
insects. In the case of butterflies and moths it is the
sole attraction. The ripe anthers and ripe stigmas are
commonly held in such a position that the insect has
to brush past them in reaching the nectary.

A very common arrangement is that in which the
anthers ripen first, and the filaments bend so as to
bring the anthers into the path of the insect, while
later on the stigmas ripen and bend into the same
position. Such a flower is called protandyvous.t An
insect visiting the flower in the first stage will carry
away pollen, and this will be rubbed on to the stigmas
directly it visits a flower in the second stage, since
the insect will brush the anthers and stigmas in the
two flowers with the same part of its body.

Thus in the buttercup the nectaries are on the inner
sides of the petals near their-bases. When the petals
first open the centre of the flower is occupied by a mass
of anthers concealing the undeveloped stigmas. The
first anthers to open are those on the outside, next
the petals, and the ripening of the anthers progresses
gradually inwards to the innermost ones, the ripe
anthers bending outwards. This is the “ male stage ”’
of the flower. Ifa large insect visits the flower it alights
in the centre on the mass of anthers and pushes its
head down between these and the petals to get at the
nectar. In doing so pollen will adhere to the lower
side of its head or body. By the time the last anthers
have opened the carpels have grown up in the middle

1 Greek mpdtoc, first, and dvjp, avdpdc, man, because the male
elements (pollen) ripen first.

CROSS POLLINATION 357

of the flower and their stigmas are ripe (incomplete
protandry). The same insect alighting on the centre
of the flower in this (the “female ”’) stage will brush
off on the stigmas any pollen that may be adhering to
the lower side of its body from a previous visit to a
flower in the male stage, thus effecting cross-pollina-
tion. A small insect, on the other hand, may alight
anywhere, on the petals or among the anthers, and
will crawl down to the nectary or wander about eating
pollen, and it will only touch the stigmas in the female
stage by chance, if at all. The majority of large con-
spicuous flowers are pollinated mainly by large insects.
In the absence of insect visitors, which do not visit
flowers in cold dull weather, the stigmas of the butter-
cup eventually curl out far enough to come into
contact with the innermost anthers, which still probably
have some pollen adhering to them, and thus effect
self-pollination.

In the corncockle (Lychnis githago) and the pinks
(Dianthus) protandry is complete, so that self-pollina-
tion is impossible. The anthers lie on the platform
provided by the flat limbs of the petals on which the
insect must alight. At this time the unripe stigmas
are concealed in the narrow tube of the flower. Then
the anthers fall off and the stigmas grow up and lie
on the platform in exactly the same position that the
anthers previously occupied.

In other flowers which are not protandrous the stigmas
are generally held well in advance of the anthers, so
that they are the first objects the insect meets in entering
the flower, and any pollen it already bears will tend to
be rubbed off on them. On pushing further into
the flower the anthers are encountered.

Some flowers are so nicely adjusted to the structure

358 THE FLOWER

and habits of particular insects that they are only cross-
pollinated when they receive visits from these insects,
and remain sterile, setting no seed, in their absence.
Inconspicuous flowers are often adapted for wind-
pollination (grasses, sedges, most catkin-bearing trees,
pines and firs, etc.). Wind-pollinated flowers produce
pollen in large quantities, and this is smooth and dust-
like, not rough or sticky, like the pollen grains usually
conveyed by insects. A great deal is wasted because
its carriage to the stigmas of another flower of the
same species is a matter of pure chance. The stigmatic
surfaces are generally large, and this of course increases
the chance of some of the right pollen being caught.
Darwin showed that the offspring of crossing are,
on the whole, and with certain exceptions, more vigorous
than the offspring of self-pollination, when individuals
arising from seed produced in the two ways are grown
side by side. Thus variations in the flower tending
to secure crossing will tend to be fixed and perpetuated
because the offspring of flowers with such variations
will tend to survive more often than those in which
self-pollination occurred. It seems to be only in this
way that we can explain the origin and fixation of the
various beautiful and often astonishingly accurate
mechanisms which bring about crossing in flowers.
Many inconspicuous flowers (chickweed, the small
field speedwells, etc.) are, however, habitually self-
pollinated, and these perpetuate themselves indefinitely
with perfect success. Many garden and field crops
(green peas, wheat) are in the same position. There
is no evidence that continued “ self-fertilisation ’’ is
in any way harmful to the race, though a chance cross
in an habitually self-fertilised species will often result
in a distinct increase in the vigour of the offspring.

THE FLOWER 359

PRACTICAL WORK.

(rt) Cut a flower of buttercup+ (Ranunculus) longitudinally
exactly down the middle with a razor or very sharp knife, and
draw the cut surface on an enlarged scale, showing (a) veceptacle,
(b) calyx of sepals, (c) corolla of petals, with (d) nectaries, (e) stamens,
(f) carpels.

(2) Draw under a lens a single stamen showing (a) the filament,
(b) the anther with two pairs of pollen sacs, (c) the connective
(continuation of the filament between the pairs of pollen sacs),
(d) the line of dehiscence of the anthers. Draw also a side view
of a carpel showing ovary and stigma. Mount a stamen and a
carpel in a drop of dilute glycerine on a slide and examine with
the low power.

(3) Make a diagram of a section through a mature anther,
showing the four pollen sacs, in some of which two nuclei may
be seen, the connective traversed by a vascular bundle, and the
“fibrous layer ”’ of cells between the epidermis and the cavity
of each pollen sac. The cells of this bear rib thickenings on their
walls, and it is the tension developed in this layer that splits
the anther open.

Compare the section through a dehisced anther.

.(4) Remove a carpel from the old flower or young fruit of
Caltha (Marsh Marigold), draw the side view showing ovary and
stigma, and then split the ovary open so as to show the two
rows of ovules attached to the inner edges (towards the centre
of the flower), which are the joined margins (placentz) of the
carpellary leaf.

(5) Under a low power draw a diagram of the transverse section
of the carpel of Caliha or Aquilegia (Columbine), showing the
closed ovary of the carpeliary leaf with its midrib, and, on the
opposite side, its joined margins (placentz) to which the ovules
are attached. The section should pass longitudinally through
the centre of one ovule, and the side of the next belonging
to the other row, since the ovules of the two rows alternate.

(6) In a section passing through the centre of an ovule identify
and draw carefully under the high power (a) the coats of the ovule
(identify if possible the micropyle, which is very narrow and may
not be traversed), (b) the nucellus, (c) the embryosac with vacuolated
cytoplasm and conspicuous secondary nucleus, (d) the antipodal

1 If the buttercup cannot be obtained fresh, it is well to examine
first any fairly large fresh flower that can be obtained, draw a median
section through it, and then compare with the preserved buttercup
flower.

360 THE FLOWER

cells (also very conspicuous), (e) the egg cell and the two synergide.
Note that the body of the ovule is turned round so that the
micropylar end faces towards the placenta. Notealso the vascular
bundle running from the placenta up the stalk of the ovule
to the base of the nucellus.

(7) Compare the “‘ male”’ and ‘‘ female ”’ stages of the corn-
cockle (Lychnis), pink (Dianthus), or other strongly protandrous
flower. Make careful drawings of longitudinal sections through
the flowerineachstage. [In Lychnis or Dianthus show the tubular
calyx (synsepalous), the separate petals, each with “ claw ’”’ and
“limb ” (the limbs forming the alighting platform for insects},
the ten stamens, and the (five) carpels joined (syncarpous)
to form a single ovary with free central placenta, and five
stigmas.]

(8) Examine a flower of the cherry, and draw a median
longitudinal section through it, noting especially the cup-shaped
receptacle with sepals, petals and stamens borne on its edge, and
the single carpel at the base of the cup, with a long style, which
projects above the opening and bears a flat stigma at its summit.

”

CHAPTER XXII

THE FRUIT

Just as the development of the flower culminates with
the ripening of the spores and the production of the
gametes followed by fertilization, so the development
of the fruit culminates with the ripening of the seeds,
which are the ovules after the zygote (fertilised egg)
has developed into the embryo of the new plant.

Development of Seed from Ovule.—After fertilisation
the zygote divides and forms a (usually spherical)
embryonal cell towards the centre of the embryo sac,
and a stalk (suspensorv) connecting this with a basal
cell, which remains attached to the micropylar end of
the sac (Fig. 62, A). The cells of the suspensor divide
at right angles to its length and elongate, pushing the
embryonal cell down into the endosperm ! tissue which
is formed by the rapid division of the secondary nucleus,
food substances being poured into the sac through the
vascular bundle. Between the numerous nuclei which
arise from this, cell walls appear, thus filling the sac
with parenchymatous tissue.

The embryonal cell now divides, and the mass of
cells to which it gives rise forms the embryo proper, the
suspensor ceasing to grow (Fig. 62, B). In a dicotyle-
dinous plant (the majority of the flowering plants)
two rounded projections arise on the free end of the
embryo (Fig. 62, C), and these develop into the two

1 From Greek éydov and ovépya, ‘inside the seed.’
361

362

Fic.

THE FRUIT

62.—Development of embryo. A, embryo sac shortly after
fertilisation. The secondary nucleus has divided many times
and the sac is rapidly filling with the endosperm tissue; food
is coming in through the vascular bundle of the ovule (arrows). The
fertilised egg (zygote) has divided to form the suspensor and the
embryonal cell. B, sac full of endosperm, into which the embryo
is being pushed by the elongation of the suspensor. C, growth
of embryo (cells not shown) : the free end develops two lobes (the
rudiments of the cotyledons). D, older embryo ; cot., cotyledons ;
hyp., hypocotyl; 7, radicle; susp., suspensor. E, ripe endospermic
seed showing embryo with two thin cotyledons closely appressed
with epicotyledonary bud (epic.) between, hypocotyl (hyp.),
tadicle (vad.); also endosperm, perisperm (per.), testa, and
micropyle. Note B-E are drawn on progressively smaller scales,
the size of the seed and embryo having very greatly increased.

DEVELOPMENT OF EMBRYO 363

cotyledons, which are the first two leaves of the plant
(Fig. 62,D,E). Between them the terminal bud of the
primary shoot axis (epicotyl) is formed. At the other
end of the embryo, where it joins the suspensor, the
primary root is formed, the apical meristem just
inside the surface layer of cells which form the first
root cap.

In the monocotyledons! the free end of the embryo
forms the single cotyledon, and the epicotyledonary
bud is developed laterally. The part of the primary
axis between the cotyledons and the primary root is
the hypocotyl.

The embryos of different species vary very much in
the stage of development they have reached by the
time the seed is ripe. In some cases, particularly in
small seeds, the embryo remains in a very rudimentary
stage of development, surrounded by the endosperm.
In others it develops within the seed, not only large
leaf-like cotyledons, but an epicotyledonary bud with
the rudiments of several additional leaves. In such
large well-developed embryos the vascular system of
cotyledons, hypocotyl and primary root is all “‘ blocked
out,”’ so that the various tissues can be clearly recognised
in the embryo, though the cells remain small and without
any thickenings on their walls till germination. Very
often the seed grows to many times the size of the
ovule at fertilisation, as for instance in the bean. This
extensive development of the embryo requires of course
an ample supply of food, which is brought up through
the vascular bundles of the placenta and of the ovule
stalk to the developing seed. The embryo itself
grows at the expense of food absorbed from the endo-

t Monocotyledons include the grasses, sedges, lilies, orchids, palms,
etc.

364 THE FRUIT

sperm, and this is supplied from the bundle which
terminates at the base of the nucellus.

When the embryo is ripe it may fill the whole of the
space within the embryo sac, or it may still be sur-
rounded by a mass of endosperm (Fig. 62, E) (see p. 362).
The nucellus of the ovule is sometimes represented in
the seed by a thin layer of tissue which may be stored
with food substance (perisperm). The coat (or coats)
of the ovule becomes differentiated in various ways,
their cell walls generally thickened and cutinised or
lignified, to form the seed coat or desta.

It will be useful here to summarise the corresponding
structures in ovule and seed :—

The ovule becomes the seed.

The coat (or coats) of the ovule becomes the testa.

The nucellus may become the perisperm.

The tissue formed by the division of the secondary
nucleus of the embryo sac forms the endosperm.

The zygote becomes the embryo (together with the
suspensor—a transitory structure).

Development of Fruit trom Carpels, ete.—While the
changes described above are taking place in the ovule,
others are proceeding in the rest of the flower. Thepetals
and stamens usually fall off very soon after fertilisation,
while the calyx is often, though not always, persistent,
and sometimes grows considerably in size. The stigma
and style fall off or wither away, but the walls of the
ovary typically develop into the walls of the fruit
(pericarp), keeping pace with, or even outstripping, the
growth of the seeds. The pericarp, into which the
ovary wall develops, differs very much in different
species, being sometimes thin and membranous (pea),
sometimes thick and woody (hazel nut), sometimes
soft and succulent (raspberry, tomato).

STRUCTURE OF FRUITS 365

In popular language a fruit is a fleshy envelope which
can be eaten enclosing seeds, but the edible part is not
necessarily formed from the pericarp. For instance,
the fleshy part of an apple, a fig, or a pineapple, is not
pericarp at all. The botanical conception of a “ true ”’
fruit (i.e. the structure formed from the carpels alone
by the time the seeds are ripe) is both wider and narrower
than the popular conception. Thus it includes the
bean pod and the coconut, while it excludes all but
the “ pips” of a fig, and the core of an apple. We
may, however, conveniently include in the general term
“fruit” all the structures enclosing the ripe seeds,
whether derived from pericarp or not.

The only way to understand the nature of the parts
of a fruit in this wide sense is to follow their develop-
ment from the flower. It is especially the receptacle
that very frequently takes part in the structure of the
fruit. This is necessarily the case in all fruits formed
from “inferior ” ovaries (p. 354), because the wall of the
receptacle is here fused with the wall of the carpels.
The wall of the receptacle and the wall of the carpel
may together form quite a thin membrane, but, on the
other hand, one or other or both may swell up and
become fleshy in the fruit.

The cherry, the rose and the apple belong to the
same family (Rosacez, the rose family), and they illus-
trate these differences very well. The cherry, as we
have already seen, has a cup-shaped receptacle with
a single free carpel at the base of the cup. It is the
ovary of the carpel alone which forms the cherry. The
receptacle does not grow after fertilisation, and is soon
flattened out by the great growth of the young fruit—
it can still be seen as a little disc at the base of the cherry
where the stalk joins the fruit. The wall of the carpel

366 THE FRUIT

becomes differentiated into three layers—the ‘‘ skin,”
the “ flesh’’ and the “stone”: the ‘“ kernel’’ is the
seed.

In the rose the flower is perigynous as in the cherry,
but there are many separate carpels, each containing
a single seed, within the urn-shaped receptacle, beyond
the mouth of which the stigmas project. In the develop-
ment of the fruit the carpels themselves do not increase
very much in size, but the wall of the receptacle grows
and becomes fleshy, forming the well-known red “ hip”
or rose fruit.

In the apple and pear the receptacle in the flower
has very much the same shape as in the rose, thoughit
is less elongated ; but it is fused with the walls of the
five carpels enclosed within it. Both receptacle and
ovaries increase greatly in size after fertilisation, the
former becoming the ‘‘flesh”’ of the apple and the
latter the ‘“‘core.” The “pips” are the seeds. A
cross-section of an apple shows that the ovaries are
separate from one another as they are in the rose, though
they are embedded in the flesh of the receptacle.

In the strawberry the receptacle, instead of being
hollow and covering the carpels, is convex, and in the
development of the fruit it swells and becomes succulent
separating the one-seeded carpels (“pips”) by its
great increase in surface, over which the carpels are
distributed. The style of each carpel may still be
seen attached to each “ pip.” The cinquefoils, which
are close allies of the strawberry, have exactly similar
carpels, but the receptacle does not become fleshy,
so that the carpels are still crowded together in fruit.

In the mulberry, which is derived from aninflorescence
(group of flowers), it is the perianth leaves (two pairs)
which become succulent, enclosing the pip-like ovary.

INDEHISCENT AND DEHISCENT FRUITS 367

Sometimes a whole inflorescence develops into a
single fruit (often called an “‘ aggregate fruit’’). In
the fig, for instance, the axis of the inflorescence is
concave, bearing the crowded flowers on its inner surface.
In fruit the axis (common receptacle of the flowers)
becomes succulent, the ovaries (‘ pips’’) remaining
hard. In the pineapple the whole of the flowers of
the inflorescence (including perianth and stamens),
as well as the bases of the bracts, become succulent.
The hard tips of the bracts are exposed on the surface
of this ‘aggregate ’’ fruit.

Indehiscent and Dehiscent Fruits.—Sometimes the
change in form and appearance of the carpels in the
passage from flower to fruit is very slight, as in
the buttercup, rose, strawberry. In the buttercup, for
instance, the carpels, which are one-seeded, scarcely
grow at all, but their walls become dry and membranous,
changing in colour from green to brown. When ripe
they are easily detached from the receptacle and are
shaken off by the wind. The pericarp softens and
decays in damp soil, and when the seed inside germinates
the young plant pushes through its remains. This
is called an indehiscent fruit, because the pericarp does
not open when the seeds are ripe. A dry one-seeded
membranous walled fruit of this kind is called an
achene. It isa common type, not only in the butter-
cup family (Ranunculacez), as in buttercup, anemone,
clematis, but also in the rose family (Rosacez), where
it is found in cinquefoil (Potentilla), herb-bennet (Geum)
and others ; and it may, as we have seen, be associated
with a succulent receptacle (rose, strawberry). Much
the same type of fruit is found in the great family
Compositz, in which the flowers are aggregated in close
heads (dandelion, thistle), and the outer ones are

368 THE FRUIT

frequently sterile, with a conspicuous one-sided corolla, ,
so that the whole flower-head looks like a single flower
(daisy). Here the gynecium of each flower consists
of two united carpels, the ovary being inferior. Only
one carpel, however, with one seed, comes to maturity.
The wall of the ovary really consists of receptacle
and pericarp fused, but since it is membranous and
one-seeded the mature fruit is for practical purposes
anachene, though it is derived froma bicarpellary inferior
ovary.

A nut is a one-seeded fruit with a woody pericarp.
Here again it may be derived from a bicarpellary inferior
ovary, as in the common hazel nut (Corylus).

Dry fruits which contain many seeds are practically
always dehiscent, i.e. they open to let out the ripe seeds.
A simple example is a pea or bean pod. This consists
of a single elongated carpel held horizontally in the
flower, and generally more or less flattened laterally.
The lower edge is the midrib of the carpellary leaf,
the upper edge the joined margins (placentz) bearing
the seeds. _The seeds and the ovary grow very con-
siderably before ripening, multiplying their original
size many times over. When fully grown the walls
dry and split along both edges, the seeds being easily
detached and falling to the ground. Practically all
the pea family (Papilionacee) have this type of fruit
(legume). The marsh marigold, hellebore, columbine,
larkspur and monkshood—all members of the butter-
cup family (Ranunculacee)—have similar fruits, but
they have several separate carpels in each flower, and
the fruit splits along the placental margin only (follicle).

Many dry dehiscent fruits are formed from syncarpous
gynecia, with axile, free central or parietal placentation
—Fig. 61 (foxglove. corncockle. violet). These all open by

DISPERSAL OF FRUITS AND SEEDS 369

the longitudinal splitting of the dry capsule, as this kind
of fruit is called. In other cases the top of the capsule
comes off along a horizontal line (pimpernel). Sometimes
the capsule opens by pores in the wall (poppy), out of
which the very small seeds are shaken when the stem
is swayed by the wind, by which they may be carried
a considerable distance from the parent plant.

Methods of Fruit and Seed Dispersal. (1) Dispersal
by Wind.—This is a very common means of dispersal.
Seeds and dry one-seeded fruits vary very much in the
distance they may be blown. The smaller the seed
or fruit the further it will be carried, if the shape and
the specific gravity are the same, because the smaller
the seed the greater the ratio of surface to bulk. Many
small seeds are carried for some distance in this way.
On the other hand, a large seed like the broad bean
is scarcely affected by the wind and simply drops out
of the pod. If a seed is flat instead of being round,
its surface will be greatly increased, and it will blow
much further. Many seeds and one-seeded fruits
bear outgrowths from the testa or pericarp, which greatly
increase the surface on which the wind can act. These
are of two main kinds—wings and hairs (plumes).

Winged fruits are pretty common. A _ well-known
example is the sycamore fruit. There are two joined
carpels in the flower, and from the free side of
each a curved wing grows out. When the fruit is
ripe the two carpels split apart, and the two are often
detached from the tree separately. The shape of the
wing causes the one-seeded “ mericarp”’ (separated
part of the whole fruit) to spin as it falls, and it is thus
more likely to be carried away by the wind. Other
examples of winged one-seeded fruits are those of the ash
(‘‘ keys’), the birch, and the hornbeam. In the last two

24

370 THE FRUIT

the wings are formed by adherent bracts. Examples
of winged seeds are those of the pine and of the tropical
genus Zanonia.

Examples of plumed fruits are those of the “old
man’s beard” (the wild clematis), where the styles are
persistent in the fruit and are clothed with long silky
hairs, and those of the dandelion, groundsel, thistle
(‘thistle down”), and of many other members of
the Composite. Here there is a special plumed structure,
the pappus, surmounting the indehiscent one-seeded
fruit. Examples of plumed seeds are those of the
willows, and of the various species of willowherb (the
riverside ‘‘ codlins and cream,” the “ rosebay ”’ willow-
herb of open woods, and the other species). Here the
capsule opens and sets free the crowds of tiny seeds
which drift about, floated in the air by their very fine
silky hairs.

(2) Dispersal by Animals.—Some indehiscent fruits
bear hooks which catch on passing animals and stick to
their coats, the fruits being readily pulled off the plant.
Brushed off later on, the seed may germinate at a distance
from the parent plant. The hooks are sometimes out-
growths of the pericarp (“ cleavers,”’ Galium aparine),
or the hook is formed from the style (herb-bennet,
Geum urbanum). In the burdock (Arctium), a member
of the Composite, the bracts surrounding the flower
head are hooked, and the head when in fruit is easily
detached, so that it is carried off as a whole, and the
dry one-seeded fruits drop off. All these fruits very
readily stick to the trousers or skirt as the wearer
brushes against the plants.

Animals are also constantly distributing dry fruits
and seeds even if these have no special means of adhering
to them—carrying them about in the crevices of their

SUCCULENT FRUITS 371

bodies, in the mud adhering to their feet and so on.
An almost incredible number of different seeds and
small dry fruits may often be found in the pockets and
seams of the clothes as well as on the boots of men who
spend much time in the open country.

In the case of succulent fruits which are sought and
eaten by birds and mammals, the seeds, which are
protected by an indigestible covering, are often swallowed
and voided at a distance. The attractive succulent
part of the fruit may be the inner layers of the pericarp
(melon, raspberry, date, etc.) or special ingrowths from
the pericarp (orange), or it may be the perianth leaves
(mulberry), or the receptacle of the flower which has
become fleshy in the fruit (strawberry, apple), or of
the whole inflorescence. The seeds of such fruits are
nearly always protected by a layer of hard-walled cells,
which resist the digestive enzymes secreted in the
alimentary canal of the animal and thus prevent
the substance of the seed from being digested. In
most cases this is the testa of the seed, but when the
succulent part of the fruit is the receptacle or perianth,
it is the whole pericarp (strawberry, apple, fig, mulberry),
or where the outer layers of the pericarp alone are
succulent, it is the inner layer, ic. the “‘stone’”’ of
“stone fruits,” the seed itself (the kernel) being digestible
once the stone is removed. : ;

Very often the seeds of succulent fruits stick to the
beaks of birds pecking at the fruit, because they are
surrounded by a sticky pulp, and are later rubbed off
or fall off when the pulp dries. The seeds of mistletoe
are distributed in this way. The birds peck at the
mistletoe berries and the seeds stick to their bills.
They clean their bills by rubbing them on the bark of
trees, and the seeds catch in the crevices, where they

372 THE FRUIT

germinate. The mistletoe is a parasite on trees, and
the modified roots (haustoria) produced by the seedling
penetrate the bark and into the wood of the tree.

(3) Dispersal by Water Carriage.—Many seeds and
fruits float when they drop into water, and may thus
be carried considerable distances by rivers, germinating
if they are stranded in a suitable spot. Many seeds
become waterlogged and sink after a short time, but
others float for long periods, or indefinitely. The seeds
of many riverside plants are surrounded by a tissue
containing much air, and are thus specially equipped
for floating.

The seeds and fruits of a large number of species of
tropical seashore plants—both trees and herbs—have
such a floating tissue and are able to stand long soaking
in salt water without harm. They are constantly
carried about in shore currents and stranded on the
coast, where they germinate and produce fresh vegeta-
tion. Newly formed islands of coral or volcanic material
are largely populated by plants in this way, and this
feature of tropical coast plants is also largely responsible
for the great uniformity of this vegetation over very
wide areas. The coconut is the best-known example,
the very thick woody pericarp being very light in
proportion to its bulk and causing the fruit to float
indefinitely.

It has been shown that this character of coastal
plants has been developed in different species from
other causes not related to the capacity for floating.
But it is clear that the species whose seeds and fruits
can float will have a much greater chance of successful
propagation, and especially of wide dispersal, than if
they had seeds or fruits which sank at once or could
not long resist soaking in salt water. There is thus a

PRACTICAL WORK 373

strong selective effect which will inevitably lead to the
prevalence of these species in this particular habitat.

The same general consideration holds of most of
these so-called “ adaptive’’ characters. They have not
in the first instance appeared in relation to the conditions
in which they are of special use to the organism, but
the existence of the character may favour the spread
of the species when these conditions arise, and may
even in extreme cases secure its continued existence
in circumstances which would lead to the extinction
of a species not so equipped.

PRACTICAL WORK.

Development of Seed and Embryo.

(1) Slit open the fresh ovaries of Shepherd’s Purse (Capsella),
choosing those from which the petals have just fallen, and older
stages up to the full-sized capsules. Draw under the low power
seeds of different stages of development.

Squeeze the ovules of different ages under a coverslip so as to
press out the embryos, and draw various stages in the develop-
ment of the embryo. Note the suspensor and the spherical
embryo in the young stage, and the subsequent development
of the two cotyledons.

Dehiscent Fruits.

(2) Examine the young fruit of the Sweet Pea (Lathyrus)
projecting from the remains of the flower. Split the ovary
down and note the two rows of young seeds attached to the
upper edge. Note the similarity of the single carpel here with
the carpel of Caltha (p. 359). In the ripe fruit note the mature
seeds and the mode of dehiscence.

(3) Examine any available examples of syncarpous, dry
dehiscent fruits (capsules).

Indehiscent Dry One-Seeded Fruits.

(4) Cut the young “‘ aggregate fruits ’ of Herb-bennet (Geum)
longitudinally in half so as to show the insertion of the individual
achenes on the receptacle. Note that a hook is developed in
the style just below the stigma, which falls off.

374 THE FRUIT

(5) Draw a single fruit of Dandelion (Taraxacum), showing
the stalked pappus surmounting the one-seeded, dry indehiscent
ovary.

(6) Draw the fruit of sycamore (Acer), consisting of two dry
one-seeded carpels. The pericarp of each has grown out in the
flower to form a long curved wing. Note that the two carpels
easily separate. Dissect one of the seeds, and note the thin
testa and the green coiled embryo with long strap-shaped cotyledons.

Examine the flower of the sycamore and note the developing
wings of the ovary, and that each carpel originally contained two
ovules.

Succulent Fruits.

(7) Inthe young cherry (Pyvunus) note the suture (joined margins)
of the single carpel, the scay where the style has fallen off, and the
flattened disc-like (once basin-shaped) receptacle. Cut a longi-
tudinal section, and distinguish the three layers of the pericarp
(skin, flesh and stone). In the young seed distinguish the esta
nucellus and gelatinous endosperm. Look for the young embryo
at one end.

Compare the ripe cherry. Break the stone and dissect the
seed (kernel). The embryo with two cotyledons now occupies
the whole space.

(8) Draw a median longitudinal section of an Apple or Pear
flower (Pyrus) from which the petals have fallen and in which
the receptacle is distinctly swollen. Note the tubular receptacle,
with the sepals attached to its rim, and the remains of the stamens
just inside it. The ovaries of the carpels are buried at the base
of the tube, and the styles pass up through the centre.

(9) Draw transverse and longitudinal sections of a partly
grown apple or pear (about three-quarters of an inch in diameter)
showing the relation of the carpels (core) to the fleshy receptacle.

(10) Draw similar sections of a ripe apple or pear, identifying
the structures visible with those seen in the earlier stages. Note
the withered styles still lying in the central tube, the remains
of the stamen still often present within the sepals: the vascular
cylinder running from the top of the stalk to the base of the carpels
and its continuation through the central column serving the
placentze; also the bundles running through the flesh of the
receptacle.

CHAPTER XXIII

THE SEED AND ITS GERMINATION

WE have already seen (p. 363) that different seeds vary
very much in the size attained by the embryo when the
seed is ripe, apart from the dimensions of the seed
itself, which varies from the microscopic seeds of orchids
to the enormous seed of the coconut. During develop-
ment the embryo absorbs more or less of the endosperm,
but in some cases it is still surrounded by endosperm
in the ripe seed, while in others it absorbs the whole
of the endosperm and comes to occupy the entire space
within the testa. Seeds in which endosperm is still
present at maturity are called endospermic seeds,
and those which have no endosperm left when they
are ripe are non-endospermic.» Examples of the former
are wheat, maize (and the cereals generally), castor
oil, date, etc.: of the latter, bean, pea, acorn, etc. In
non-endospermic seeds the embryo, and especially the
cotyledons, are often large and swollen, their cells packed
with the food material which in the endospermic seeds
would remain in the endosperm till the time of germina-
tion. This food material takes the forms with which
we have already become familiar. The nitrogenous
organic substance is in the form of solid grains of protein
material, the non-nitrogenous, either as carbohydrate—
starch grains, or more rarely as thick cellulose walls

t The ie name is “ albuminous seeds,’’ from the analogy of the

“‘ white ”’ “albumen ”’ of a bird’s egg, which, like the a aad
is a store os food for the developing embryo.

+ Or “ exalbuminous.”’
375

376 THE SEED AND ITS GERMINATION

e.g. date)—or as fats, in the form of oil drops, plant fats
being liquid at ordinary temperatures: the cytoplasm
of the cells of the endosperm (and perisperm if present)
or of the embryo itself being densely packed with these
substances. The proportion of water in a seed is very
much less than in the actively growing vegetative parts
of the plant.

Conditions of Germination.—So long as the ripe seed
is kept dry the protoplasm of its cells remains in a
dormant condition. Seeds may remain alive in this
condition for many years, and though the germination
of ‘“‘“mummy wheat” has not been verified, such an
occurrence is not by any means out of the question.
Seeds have been known to germinate after keeping
dry for a century, though some seeds die within a
comparatively short time.

The first requisite for germination is liquid water
Some ripe seeds (e.g. acorns) contain enough water to
germinate at once if evaporation is checked by keeping
them in saturated air, but once they have lost a certain
proportion of their water they must have more supplied
to them from outside in order to germinate. The
second requisite is free oxygen. The living cells of the -
embryo must be able to vespive in order to liberate
the energy necessary for growth; and both water and
oxygen are necessary to the chemical changes which
precede and accompany germination. The third is
a suitable temperature. At or below 0° C. protoplasm is
inactive, and germination cannot occur any more than
can the growth processes of an ordinary growing plant.
Above this temperature the chemical changes on which
germination depends takes place more and more quickly,
so that the higher the temperature, up to a certain point,
the more rapid germination will be. But above a

PROCESSES PREPARATORY TO GERMINATION 377

certain temperature the working of the protoplasmic
‘“machine ”’ is thrown out of gear and death results.

Enzyme Formation and Activity——One of the first
processes preparatory to germination is the production
of enzymes in the cells of the endosperm and embryo,
by the activity of which the various reserve foods are
converted into soluble forms. The starch is acted
upon by diastase, the cellulose (as in the date seed)
by cytase, the fats by the lipases, the proteins by
various proteases (see p. 45). The carbohydrates and
fats thus give rise to great quantities of sugars, and
these are largely used for respiration, i.e. for the libera-
tion of energy which is seen in the active growth of the
embryo: part’ of this energy takes the form of heat.
Thus the main interchanges between the germinating
seed and its surroundings are first of all the active
absorption of water resulting in the swelling of the seed
(cf. the experiment with peas, p. 60), then the absorption
of free oxygen, and the evolution of large quantities
of carbon dioxide and heat (p. 89). Some of the sugar
is used for forming new cell walls, while the soluble
nitrogenous substances into which the solid proteins
are converted by the activity of proteolytic enzymes
are worked up again to form the basis of the new
protoplasm produced in active cell division.

Structural Changes in Germination. Epigeal and
Hypogeal Cotyledons.—Most of the cells of the embryo,
especially if it is small and relatively slightly developed
in the resting stage, actively divide during the first
stage of growth, but cell division soon becomes mainly
concentrated in the apical meristems. The actual
growth of the embryo into the seedling takes place
very largely by vacuolation and consequent increase
in size of the cells,

378 THE SEED AND ITS GERMINATION

The first external change is the pushing out of the
radicle into the soil, sometimes through the micropyle,
sometimes through a break in the testa, into the soil,
where it bends straight downwards to become the tap-
root, and often grows to a considerable length before
the shoot appears. In plants with a “ fibrous ’”’ root
system the development of the taproot soon stops, and
other roots grow out from the base.

In the development of the shoot we have to distinguish
two types of germination. In the first, by far the com-
monest type, the hypocotyl increases enormously in
length, thrusting, or sometimes pulling, the cotyledons,
with the epicotyledonary bud lying between them, out
of the testa (Fig. 63, A). The cotyledons are said to
be epigeal ! (raised above the soil). The cotyledons are
often held with a certain firmness by the testa, and
the base of the hypocotyl being fixed by the primary
root in the soil, the hypocotyl curves in elongating,
setting up a strain and eventually hauling the cotyledons
out of the testa. As soon as these are free the hypocotyl
straightens out, carrying the cotyledons on its summit,
and these expand and turn green (if they are not green
already), standing out on each side in a horizontal
or upwardly inclined position.

This is the end of the first stage of growth of the
seedling, which thus becomes an autonomous green
plant, no longer dependent on the food in the endosperm,
which has been mostly absorbed by the time the
hypocotyl elongates. Many epigeal cotyledons are
thin, like the later formed foliage leaves, but in some
cases the fleshy cotyledons of non-endospermic seeds
are raised above the ground (French bean).

Sometimes the cotyledons are not at once successfully

Greek émt, on, and v7, earth,

TYPES OF GERMINATION 379

freed from the testa, which is carried up, sticking on to
one or both. In most cases it is soon shaken off by
their further growth, but occasionally seedlings are

Fic, 63.—Types of germination. A, endospermic seed with epigeal
cotyledons. B, non-endospermic seed with hypogeal cotyledons.
C, endospermic seed with hypogeal cotyledons, which remains
embedded in the endosperm and acts as asucker ; end., endosperm ;
cot., cotyledon; ep., epicotyl; rad., radicle.

handicapped, or even killed from this cause. In the
marrow a projection of tissue is developed from the
base of the hypocotyl which holds down one side of
the testa (the seed being here flattened and the testa

380 THE SEED AND ITS GERMINATION

consisting of two valves which grip the cotyledons
rather like the two shells of an oyster) and acts as a
fulcrum on which the elongating hypocotyl works in
pulling the cotyledons out of the testa.

In the second type of germination the cotyledons
remain within the seed (hypogeal'). The hypocotyl
does not elongate, but the epicotyledonary bud at
once grows out, at the expense of the food stored in
the cotyledons, when these are large and swollen as
in non-endospermic seeds (Fig. 63, B), (bean, acorn) ;
while in endospermic seeds the cotyledon acts as a
“sucker,” remaining embedded in the endosperm,
absorbing and transmitting the soluble foodstuff from
the endosperm to the growing parts of the seedling
(Fig. 63, C). This is well seen in the monocotyledonous
date seed in which the peg-like cotyledon embedded in
the endosperm produces cytase, which attacks the solid
cellulose of the endosperm and absorbs the sugar pro-
duced. In the cereals a special organ, the scuéellum,
lies between the axis of the embryo and the endosperm,
and its surface cells secrete diastase which passes into
the starchy endosperm, attacks and hydrolyses the
starch grains, the sugar produced being absorbed by
the scutellum. In all cases the endosperm or cotyledon
cells contain protein grains, the nitrogenous material
represented by which is of vital importance in the forma-
tion of new protoplasm, though much less in amount
than the carbohydrate material. Much of the sugar
derived from this latter is broken up in the intense
respiration which occurs in the germinating seed.

The plant may in fact be regarded as living at its
highest intensity during germination. At no other time
is there so extensive and rapid a mobilisation of food

t §nd and yf, below the earth.

USE OF SEEDS BY MAN 381

reserves for purposes of growth, whether on the side
of the formation of new protoplasm and cell walls, or
on the side of the expenditure of energy. A comparable
situation occurs at the resumption of growth in a perennial
plant after the winter rest, when the winter buds
on the rhizome or woody stem grow out to form new
shoots; but it is not so intense or concentrated as in
the seedling.

Use of Seeds by Man.—Owing to the concentrated
supplies of organic food they contain seeds form by
far the most important vegetable foodstuffs of man.
The great bulk of his cultivated crops are grown for
the seed. First in importance among these come the
cereals : wheat, rice, maize, millet, barley, oats and rye,
which all belong to the great family of grasses (Graminee).
These are all starchy seeds, but with a considerable
proportion of protein. The ‘protein varies in different
cereals, for instance wheat, the standard food of the
white man, contains considerably more than rice, the
standard food of the natives of large parts of the tropics.
Barley is grown in central and north Europe mainly
for malting, the seeds being germinated (malt) and the
reserves rendered soluble, so that they can be dissolved
out in hot water to form the basis (wort) for the making
of beer (p. 132). But in other parts of the world
barley is grown for bread-making.

Next in importance to the cereals come the pulses
(beans, peas, lentils, etc.), which belong to the legume
family, the fruit consisting of a single dehiscent carpel
(p. 368). Most of these are also rich in starch, but
contain a particularly high proportion of proteins.
The pulses are also widely used as foodstuffs, both for
men and cattle, but not so extensively as the cereals.

Finally, there are the fat (oil) containing seeds.

382 THE SEED AND ITS GERMINATION

Of these certain species of palm are among the chief.
The West African oil-palm (El@is guineensis) and
the coconut palm (Cocos nucifera) are the two leading
species. The former is a constituent of most of the
native food in West Africa, and it has been extensively
planted in other parts of the continent. In comparatively
recent years a great export trade in ‘‘ palm kernels ” has
been developed, and from these the oil is extracted
for various purposes, prominent among which is the
manufacture of margarine. The coconut is used very
extensively by the natives throughout the damp tropics
for food, lighting and other things, and the oily endosperm
is very extensively sun-dried and exported as copra,
which is used for margarine, soap-making, etc.

Other important oil-bearing seeds are the “‘ soy bean ”
(a leguminous plant), linseed (the seed of the flax),
and cottonseed (the seed of the cotton plant), the oils of
which are all used in making food and for a variety
of industrial purposes. After most of the oil is
extracted the residue is utilised as cattle food (‘‘ cake’’).

Relation of the Size of the Seed to Dispersal and to
the Chances of Life of the Seedling.—A large seed con-
taining a large amount of food material naturally enables
a large seedling to be produced in a comparatively short
time, just as a considerable shoot system can be quickly
produced from a bulb or tuber: whereas a seedling
produced from a small seed uses up its reserves very
quickly, and has the laborious work of slowly building
up new organic substance from inorganic, with a root
and leaf surface which is at first of very limited extent.
Provided it finds a suitable spot for germination and
growth, the seedling derived from a large seed thus
has a great initial advantage in competing with other
plants for space and light, because it is less likely to be

DEATH RATE OF SEEDLINGS 383

smothered and to have its light cut off by the vege-
tation around it

Plants producing small seeds, on the other hand,
have two great advantages. The seeds can be produced
in far greater numbers with an equal supply of food
from the parent plant, and they are much more readily
dispersed, so that there is more chance of many of
them reaching comparatively distant spots where they
can germinate and establish themselves, and thus
the species has a better chance of wide distribution
and ultimate survival. The two most widely distributed
and numerous families of flowering plants, the grasses
(Graminez) and the composites (Compositz), both have
small one-seeded fruits, and distribution is often facili-
tated in the latter case by the pappus (p. 370).

Death Rate and Competition.—The death rate of
seeds and seedlings in nature, like that of all young
organisms, is enormous. Besides the large number of
seeds which fall in places where they cannot germinate
and the large number that are eaten by animals, many
seeds which do begin to germinate are killed at an early
stage by finding no suitable soil in which they can root,
by being smothered or cut off from light by other plants,
or by the attacks of fungi or small insects. At a rather
later stage very many are eaten off by rodents or by
browsing animals. No seedling of a woody plant can
survive, for instance, in heavily pastured grassland.
Perennial herbaceous plants like the grasses survive
in such land because of their underground and surface
shoot systems, which possess buds that grow out as soon
as the upper shoots are eaten off.

If we suppose an annual plant to produce only ten
seedlings a year, and all of these survive and them-
selves produce seed, we should have in the twelfth

384 THE SEED AND ITS GERMINATION

year a million million individuals of that species, the
offspring, in the twelfth generation, of a single plant.
Many species at the present time are actively spreading
and increasing their territory, though the increase is
never more than a minute fraction of that which would
occur if all offspring survived. Many others are decreas-
ing, not holding their own in the struggle for space,
light and water. Probably a much larger number of
species are more or less holding their own. It is by
competition between individuals of the same and different
species that this approximate balance is maintained.
Significance of the Seed in the Evolution of the Plant
World.—The ovule, and the seed into which the ovule
develops, represent the culmination of the adaptation
of the sexual reproductive processes to the conditions
of plant life on land, where an external supply of water
on the surface of the soil is no longer available. We
saw (in Chapter XIV) the preparatory stages, which
necessarily preceded the appearance of seeds, and which
developed many millions of years ago, during the Palzo-
zoic age, in the ancestors of our modern seed plants.
These preparatory stages are still represented in the
few existing heterosporous Pteridophytes. Instead of
uniform spores which germinate to form green free-
living plants—the prothalli of such plants as ferns—
which produce the sexual organs, the heterosporous
forms produce two kinds of spores, the large megaspores
and the small microspores, which germinate to form
microscopic prothalli, dependent on the spores, and
respectively producing the male and the female gametes.
The process of conjugation still depends, howéver,
on external liquid water in which the flagellate
male gametes can swim, and this dependence severely
restricts the habitats in which such plants can live.

SIGNIFICANCE OF THE SEED IN EVOLUTION 385

The progress to the development of the ovule depends
on the retention of the megaspore within the megasporan-
gium and the appearance of some means other than
their own locomotion in water of bringing the male
gametes to the female. We do not know all the steps
in the evolutionary process, because we have not
discovered the fossil forms which were the actual
ancestors of our modern seed plants. But the essential
new features are two, first the appearance of a wall of
cells to the megasporangium (nucellus of the ovule) and
of one or two coats of cells covering the nucellus, and
secondly of the germination of the microspore (pollen
grain) to form a germ tube (pollen tube) which can
grow to the neighbourhood of the egg, carrying the male
gametes with it. A further feature is the closure of the
leaf (carpel) round the megasporangia (ovules). This is
not found in the most primitive existing seed plants, such
as the pines and firs, which are called ‘“‘ Gymnosperms,’’ !
because their ovules are not enclosed in carpels.

The pollen grains are brought to the ovule in Gym-
nosperms, and to a special receptive organ of the carpel
(stigma) in most flowering plants, by the wind or by
insects, and the perfection of the method of their
transport has undoubtedly increased the chances of
fertilisation in a great variety of species. From the
stigma to the ovule the growth of the pollen tube and
the life of the male gametes it encloses are protected by
the fact that it takes place inside the tissue of the carpel.
The female gamete is well protected by nucellus, ovule
coats and carpel wall. All these arrangements enable
the process of fertilisation to be carried out in flowers
exposed to dry air in which exposed gametes would
at once be killed.

t Greek yuyrdc, naked, and omépya, seed.
25

386 THE SEED AND ITS GERMINATION

But the withdrawal of the process of fertilisation to
the interior of the well-protected ovule has another great
advantage—it enables the zygote, the result of fertilisa-
tion, to be developed into an embryo in a protected
position, and to be supplied by the parent plant with
the food necessary to its development in that position.
In this way the young plant is able to establish itself
in a short time after the seed germinates—it has a
chance of penetrating quickly into the damp soil below
the surface because it has a store of food at its disposal
for rapid growth.

In the fern plant the sexual generation fs a free-
living green plant produced by the germination of the
spore on the soil. The young sporophyte is “ parasitic ”
on the prothallus during the first stage of its growth.
In the seed plant the prothallus is reduced to its lowest
terms and is “ parasitic’ on, or rather in, the mother
sporophyte (the “ vegetative’ plant). But not only so:
the new sporophyte produced from the zygote is also
parasitic on (i.e. it derives its food from) the mother
sporophyte, and is protected by it in the altered
megasporagium (seed) up to a certain stage of its
development, when it can, on the germination of the
seed, rapidly become free living.

Thus the seed plant has successfully solved the problem
of sexual reproduction, and of giving the new generation
a good start in life under conditions which the lower
plants are quite unable to meet. The principle of the
mating of the delicate and easily destroyed gametes
inside the body of the parent, and of the protection of
the embryo during the early stages of its life, is the
same that we see successful in the higher animals,
though the details of the mechanism are so widely
different. Taken together with the power of maintain-

PRACTICAL WORK 387

ing and increasing the aerial shoot from year to year,
thus permitting of that great increase in the bulk of
the body which has given us the trees, the dominant
plants over a large part of the earth’s surface, the
production of ovules and seeds has enabled the higher
plants almost to cover the face of the globe.

PRACTICAL WORK,

Seeds and Seedlings,

A. THE BroaD Bean (Vicia Faba): a seed without endosperm
and with hypogeal cotyledons,

(1) Make drawings of the side and front views of the seed
showing the long scar of attachment to the pod and the position
of the micropyle at one end of the scar. [By squeezing a bean
which has been thoroughly soaked a drop of water can be forced
out through the micropyle.] Pull the testa from a soaked bean
to show the embryo, which completely fills the seed. Detach
carefully one of the massive cotyledons, and draw the internal
surface of the other with the curved epicotyl and the straight
tadicle at one side.

(2) Sketch a series of germinating seeds and seedling in different
stages, marking cotyledons, epicotyl, foliage leaves (the lowest
are rudimentary), axillary buds, terminal bud, primary root,
lateral voots. The hypocotyl remains short.

B. THE VEGETABLE Marrow (Cucurbita): a seed with perisperm
and thick epigeal cotyledons.

(3) Note the flatness of the seed, the scay of attachment (pit-
like) and the adjacent micropyle. Split the seed lengthwise
and observe the greenish layer of perisperm (nucellus), two rather
thick cotyledons (though much thinner than in the bean), with
very minute epicotyledonary bud between, and radicle.

(4) Make a series of sketches from stages illustrating the
elongation of the hypocotyl, the pulling of the cotyledons from
the testa, the straightening of the hypocotyl and the expansion
and greening of the cotyledons. Mark the various organs of
the seedling, the epicotyledonary leaves differing in shape from
the cotyledons and the ‘‘ peg’’ or ‘heel’ at the base of the
hypocotyl, which holds down one valve of the testa while the
cotyledons are extracted.

388 THE SEED AND ITS GERMINATION
C. Tue Castor-O1r Prant (Ricinus): A seed with oily endosperm
and epigeal cotyledons.

(5) Remove the mottled brittle testa (with its wart-like swelling
at one end) from the seed, exposing the white mass of oily endo-
sperm: split this open very carefully and observe the embryo
in the middle. The very thin membranous cotyledons are
extremely delicate and easily tear. They extend almost the
whole length of the seed and have a complete system of veins
already developed. Draw the embryo, noting the short cylindrical
hypocotyl, continuous with the radicle, and the epicotyledonary
bud.

(6) Sketch a series of stages in the development of the seedling,
showing the gradual straightening and elongation of the hypocotyl
(which may reach a length of 8 to 12 inches and a considerable thick-
ness), the large flat cotyledons and the development of the epicoty]-
edonary shoot. Note the buds in the axils of the cotyledons and
the well-developed root system.

(7) If available, compare maize, wheat and date seedlings,
showing endosperm and ‘‘sucking’’ organs: also the very small
seedlings with epigeal cotyledons of some of our common weeds
and garden crops.

CHAPTER XXIV

CONCLUSION

In the foregoing chapters we have become acquainted
with some facts about living beings, especially plants,
and we may now briefly consider the chief lessons
about life that we learn from them,

In the earlier chapters we saw that life everywhere
depends upon a substance we call protoplasm—that
this is present wherever we find life—in all organisms
from amceba to man, and from bacteria to the seed
plants—and that it is not present in lifeless objects.
This protoplasm, as we saw, has a more or less definite
chemical constitution. It is not itself a chemical
compound, but a mixture made up in all organisms of
the same classes of compounds. In physical structure,
too, it is everywhere similar, belonging to a kind of
substance—or rather a special condition of matter
known as the colloid condition—which consists of very
minute particles or droplets of one substance (disperse
phase) dispersed through a continuous medium (con-
tinuous phase) of another. When the continuous phase
of a colloid is a liquid such as water the colloid is
called a sol, and certain sols as the result of the loss
of water, or of other causes, pass into a jelly-like
conditon known as a gel, in which the disperse particles
are more aggregated, though there is no sharp limit
between the sol and the gel conditions.

Protoplasm, as we saw, is a mixed colloid sol or
389

390 CONCLUSION

gel, often fluctuating between the two conditions, with
particles of protein, or loose combinations of proteins
and fats or proteins and salts as the disperse phase,
and water, with various substances in solution, as the
continuous phase; and it is on this definite chemical
nature and physical structure that the definite forms
of activity—the “ vital functions ’’-—-which are dis-
played by all living beings, depend. So much is certain,
though we are still far from understanding in detail
exactly how all the processes which take place in the
protoplasmic sol or gel result in the characteristic life
phenomena. In some cases we can already partly
understand how this happens—as for instance in the
case of the amoeba taking certain objects into its body
and casting out others (p. 69)—and as the study of
the constitution and activity of protoplasm progresses
we are steadily getting to understand more and more
of these details.

Perhaps the two most fundamental properties or
powers of protoplasm are first its power of maintaining
the equilibrium of its physical structure within a certain
range of, external conditions (temperature, vapour,
pressure, etc.), and secondly its power of assimilation,
i.e. of taking into and making part of its own structure
certain chemical substances that contain the elements
of which protoplasm is itself composed. The first of
these properties is the basis of the maintenance of an
organism, of its continued life under varying conditions,
provided these do not pass beyond certain limits ; the
second is the basis of growth and reproduction, of the
increase in size and number of existing organisms.
Neither of these powers is in itself unique in nature.
There are many cases in which a portion of matter
retains its equilibrium as a system for a longer or

CHARACTERS OF PROTOPLASM 391

shorter time,.and many others in which a portion of
matter may be said to assimilate and grow, for instance
the simple case of a crystal growing in a solution of
the salt of which it is composed by adding to its own
structure molecules of the salt from the solution. But
there is no other case in which so complicated and
delicately adjusted a system as a unit of protoplasm
(a free living cell) can do both these things indefinitely.
It is the combination of complexity of structure and
instability of its essential chemical constituents with
the power of maintaining equilibrium of the system as
a whole and of adding to the system by the assimilation
of fresh material from outside that makes protoplasm
a unique substance in nature, and what we call life
a unique phenomenon.

To these two powers—of self-maintenance and of
assimilation and growth—we must add the power of
respiration, the power, that is, of oxidising organic
substances, principally sugars, within the protoplasmic
complex, and thus of setting free their potential energy,
which is available for the production of mass movement
and heat.

Though protoplasm has everywhere the same general
chemical composition and physical structure, it differs
in certain details im every different kind of animal and
plant. If we accept the conclusion that all the pheno-
mena of life depend upon the chemical and physical
composition and structure of protoplasm, we must
believe that the differences in the manifestations of
life shown by different kinds of organisms depend on
differences of composition and structure, just as the
essential general uniformity of life depends on the
essential general uniformity in the composition and
structure of the protoplasm of all organisms, And this

392 CONCLUSION

belief is obtaining more and more detailed support
from recent research into the biochemistry of different
species (p. 43).

The differences between closely allied but distinct
kinds of animals and plants probably depend mainly
on comparatively small differences in some of the
proteins of which their protoplasm is composed, or on
differences in the forms of aggregation of their protein
molecules. When life first appeared upon the earth,
or rather in the water, there were probably different
kinds of very simple organisms having such differ-
ences, which affected their behaviour and consequently
their mode of life. As to the details of such differences
we know of course nothing, but it is evident that they
must have gradually led to the adoption of such very
different modes of life as we see to-day distinguishing,
for instance, the animals from the plants. There are
still existing many different kinds of minute unicellular
organisms (Protista) which are neither unmistakable
animals nor unmistakable plants, but which show a
mixture of animal and plant characters. Yet the vast
majority of organisms are either distinctively animals
or distinctively plants.

The main characteristic of animals—on which all
their other features are based—is that they can only
take the nitrogen which all organisms must take in
to make their proteins from ready made proteins, and
since proteins are only found in nature in the bodies
or as products of the bodies of organisms, animals must
consequently feed on other organisms or on organic
products. Plants, on the other hand, can take their
nitrogen in simpler forms—as simple salts, or even in
certain cases as free nitrogen. The colourless plants,
which in the matter of nutrition are intermediate

STRUCTURAL CHARACTERS OF ANIMALS 393

between animals on the one hand and green plants on
the other, vary very much in respect of the form in
which they can assimilate nitrogen. Most bacteria
take it in complex compounds, but some in simpler ones,
while others can fix free nitrogen. The fungi proper
show a‘similar but more restricted variability. The
green plants, on the whole, take their nitrogen in the
form of simple salts, such as nitrates, but even they
show variability. Some of the lowest forms can take
their nitrogen as complex organic compounds, and even
some of the seed plants (which as a class only use
nitrates) can feed on organic nitrogenous compounds.
This variability doubtless arose very early in the history
of life, if it did not exist from the very beginning, and
was the origin of the great differences of structure and
mode of life in existing organisms.

The animals are the most specialised class. Organisms
which can only take nitrogen in the form of proteins,
and thus from organic sources, were forced, so to speak,
into certain structures and habits, or they could not
survive. They must either have a naked protoplasmic
surface, as amoeba has, through which solid food can
be directly ingested, or alternatively they must have a
special opening in the surface (mouth) through which
food can be taken in. A further development, found
in the great majority of animals is the existence of
a cavity in the body (gu?) into which the mouth leads
and where the food is digested—broken up and rendered
soluble—so that it can be absorbed by the living proto-
plasm; and an exit from this cavity (anus) through
which the indigestible residue (feces) can be discharged.
A still further development is a system of tubes
(circulatory system, vascular system) which carries the
soluble products of digestion to the living cells of

394 CONCLUSION

all parts of the body where it is actually assimilated
A further consequence of the habit of living on soli
organic food is that most animals—at least most of
the higher animals—have to seek it, since it is very
unevenly distributed through their surroundings, and
thus have to move actively about, both for this purpose
and in order to escape from being themselves devoured.
They must move in definite directions and capture
and devour their prey in definite ways. Therefore
they must be sensitive to a great variety of external
impressions of definite kinds, and hence the gradual
development of the muscular system, the nervous
system and the sense organs.

The higher animals are, in fact, brought into relation
through their specific life-needs with a much greater
range of their environment than is the case with plants,
and this fact has called forth the very high differentiation
and specialisation of their bodies. The: necessities of
locomotion have kept the bodies of active animals
compact, with the different specialised organs and tissues
very closely dependent on one another, so that no con-
siderable part can be detached without crippling or
killing the whole. This characteristic is known as integ-
vation (the making of ‘a whole individual, the parts of
which are closely interdependent) and is a feature of the
highest (vertebrate) animals as well as of such highly
developed specialised invertebrates as the insects and
crustacea. Integration becomes greater and greater as
we ascend the animal scale. The lower animals are
much less integrated—the worms for instance, which
can be cut into pieces without being killed—and when
we come down to such forms as the hydroid polyps we
find that they share many of the characters of plants.
They are fixed branching forms of indefinite continuous

EVOLUTION OF PLANTS 395

growth, and little or no integration of the body as a
whole, though they are true animals, living exclusively
on organic food,

The powers of rapid locomotion and extreme sensi-
tiveness to varied stimuli, together with the compactness
and high integration of the body, are thus seen to be
not necessarily associated with the fundamental animal
character of living on solid organic food, but to be
merely a possible development of animal organisation.

The early organisms which had the power of living
on liquid foods, whether simple mineral salts and
carbon dioxide dissolved in water, or more complex
solutes such as sugars and nitrogenous substances
derived from other organisms, were not forced to retain
or develop the means of ingesting solid food, and it
is these forms which have given rise to the class of
organisms we call plants. Those which could intercept
light energy by means of a pigment such as chlorophyll
were able to use the simplest forms of raw food material,
carbon dioxide and simple salts, and it is these forms
which have given rise to the green plants—the main
line of plant evolution.

The green plants, as the result of their greenness,
have the unique power of building up protoplasm from
simple inorganic constituents—they can assimilate
carbon and nitrogen from carbon dioxide and nitrates.
These they find everywhere around them, the latter
in water and soil, the former in the air as well. Since
they are able to find their food wherever there is enough
water to dissolve it and to maintain their protoplasmic
structure, they are the only possible forerunners of
the extension of life upon the earth, living where other
organisms cannot live, and themselves providing organic
food for animals and for colourless plants.

396 - CONCLUSION

But the power which green plants have of construct-,
ing carbohydrates from carbon dioxide and water led,
broadly speaking, to the formation of more carbo-
hydrate material (sugars) within the cell than could
be used for the formation of new protoplasm, since the
available nitrogen in the form of nitrates, though
widely distributed, is nothing like so abundant as the
carbon dioxide and water. This excess carbohydrate
material is condensed in the form of the polysaccharides
starch and cellulose, the latter covering the protoplast
in the form of the characteristic cell wall, which is such
an important feature of the structure of plants and
very largely determines the configuration and structure
of their bodies and the direction of their evolution.

Under most conditions of life green plants make new
protoplasm slowly, and their bodies contain a far smaller
proportion of protoplasm and proteins, and a far larger
proportion of carbohydrates, than those of animals.
They grow slowly, continuously, and more or less
indefinitely, and correspondingly their bodies are much
less highly integrated than the bodies of the higher
animals. An “individual plant’ is much less of an
‘individual’ than an individual higher animal, such
as an insect or a vertebrate. Large parts of it can be
cut off with little or no injury to the rest of the body,
and any sufficient portion (in some cases very small
fragments indeed) can grow into a new plant under
suitable conditions.

These features are shared by the colourless plants—
the fungi—which have not the power of making carbo-
hydrates from carbon dioxide and water. But fungi
must live where they can obtain at least their carbo-
hydrate food ready made, and thus they are not
pioneers, like the green plants, but camp followers,

EFFECT OF THE APPEARANCE OF CHLOROPHYLL 397

like the animals. Their nutrition and metabolism are
intermediate between the two, since they can use
simpler nitrogenous foods than animals can, and though
they grow much more quickly than green plants they
tend, like the latter, to form an excess of carbohydrates.
Some authorities believe that the fungi are all derived
in evolution from green plants through the loss of
chlorophyll, but we have no sufficient grounds for
certainty. The fungi may have been derived, like the
bacteria, directly from primitive forms which had a
type of nutrition intermediate between that of animals
and that of green plants.

We have thus seen that the starting points of the.
great primary differentiation of living beings into
animals and plants must have depended upon differences
in the constitution of the protoplasm of the earliest
forms of life, just as the differences between species in
existing organisms must ultimately depend upon differ-
ences between their protoplasms. Some organisms
happened to produce the complex pigment chlorophyll
(p. 112), which by the absorption of light enabled the
protoplasm to use carbon dioxide as food—to assimilate
carbon in that simple form. Such organisms differed
from those which did not produce chlorophyll perhaps
in no other respect, and yet so small a difference has
eventually led, in the course of long ages, to the differ-
ence between a man and an oak tree. And this is the
story of all evolutionary development. The widest
differentiations have their origin in such small differ-
ences, of the order of specific differences, which are
just sufficient to give the necessary bias to wholly

t We know existing unicellular organisms which differ from one
another in just this way, and consequently, while almost iden-

tical in form and structure, have different habitats and modes of
feeding. . .

398 CONCLUSION

divergent lines of development ; though the converse is
by no means true—only a few specific differences
actually lead to wide divergence in subsequent evolution.
We are still very ignorant as to the causes of the first
appearance of such small differences. From the point
of view of their effects they are chance occurrences.
We have seen that the activities of living substance
which are common to all organisms—the vital functions
—are functions of the particular, the unique physico-
chemical complex which we call protoplasm, and that
we can partly explain how these functions or activities
result from the physico-chemical structure. We have
also seen reason to believe that the great primary
differentiation between animals and plants arose from
small differences in the protoplasm of different primitive
organisms, and that similar differences exist now
between closely allied species. There is evidence that
such differences are still frequently occurring between
different individuals or groups of individuals of the
same species, and that in this way new species originate.
In the higher forms of life, animals and plants with
complicated bodies of definite structure, such slight
changes in the chemical composition of the protoplasm
may be expressed in various definite ways, in slight
but definite alterations in the form and structure, or
in the colour, of the body or of particular parts of the
body, such as we know is actually characteristic of the
differences between species. In some cases such
changes, leading to the origin of new species, can be
shown with fair probability to be the result of the
effect of some change in the surroundings, for instance
increased dryness or wetness, or the presence of some
chemical substance which is absorbed by the body.
But in the greater number they are probably the result

SURVIVAL OR DISAPPEARANCE OF NEW FORMS 399

of changes in the protoplasm due to internal causes
which we cannot yet trace.

Another and probably very important cause of the
appearance of new species is the crossing of distinct
but closely allied species, the gametes contributing to
the zygote protoplasm (chromatin) of slightly different
chemical composition or physico-chemical structure, and
thus producing an individual of a new type.

Not all new forms which come into existence in one
of these ways succeed in surviving and maintaining
themselves. Some of the changes that may occur
may make the protoplasmic “machine” unworkable, and
in that case the new organism dies and no new species
appears. In other cases the change may lead to a
modification of structure or function which is badly
out of harmony with the conditions in which the new
organism finds itself. For instance, to take a purely
hypothetical case, a seed plant living in a climate where
it was occasionally exposed to very dry air might
undergo a protoplasmic change which led to the cells
of the epidermis being unable to form a good cuticle,
so that the plant would rapidly lose water by evapora-
tion from its epidermal cells. Such a plant would dry
up and die, and the new form would never establish
itself.

But in some cases the change may actually lead to
the new form being bettev adapted to the life conditions,
and in that case the new form will not only survive,
but will have an advantage in the struggle for existence.
We saw in the last chapter that a very small proportion
of the seeds produced by a seed plant actually find
suitable conditions for germination, and that a very
small proportion of the seedlings produced by the
seeds which do germinate grow into adult plants which

400 CONCLUSION

themselves set seed. It is clear that.those which are
best equipped for. succeeding under the existing con-
ditions will be most likely to survive in the struggle
to establish themselves. For instance, suppose the
place where the seeds fall to the ground is an open
spot where heavy rain falls at intervals of several
weeks, but that there are very dry periods between:
suppose further that the surface layers of soil, say to
a depth of 2 or 3 inches, are very permeable to
water and easily dry by evaporation, but that the
deeper layers are permanently moist. If the radicles
of seeds germinating during the rain period grow slowly,
so that they have not penetrated more than an inch
when the dry period begins, the root will be starved of
water and the seedling will die. But if a change occurs
in the constitution of the seedling so that the radicle
grows quicker and the root gets down to the perma-
nently moist layers of soil before the dry period sets
in, the seedlings will establish themselves and grow
into. adult plants.

To take another case: suppose the seeds to be
those of a plant growing on the floor of a forest where
the soil is at all times moist, but the light penetrating
the foliage of the trees is not more than sufficient for
the needs of the seedlings. Suppose the seeds of
various species fall on the soil in great numbers,
and, the conditions for gérmination being favourable,
nearly all germinate. Here there is no difficulty about
the roots—there is plenty of moisture in the soil for
all. But the seedlings will grow up very crowded and
will cut off the light from one another. Those which
grow most rapidly will rise above the others, will get
all the available light, and will establish themselves ;
the slower growing seedlings will be badly shaded and

NATURAL SELECTION AND ADAPTATION 401

will eventually die. Any change in the constitution
of the seedling of a species under these conditions
which enables its shoot to grow more quickly will give
it a decisive advantage in the struggle for light.

In the former case the seedlings were struggling with
their inorganic environment, in the latter competing with
other organisms. The world of organisms is the scene
of both these kinds of struggle at every stage of the
life history of individual species, but the struggle is
most severe in the early stages of life, when the indi-
viduals are most numerous and have not yet established
themselves. This struggle for existence necessarily leads
to the success of the forms which are best equipped to
succeed. There is a selection of such forms by the
elimination of those which do not succeed, and it is
this which Darwin called natural selection. —

One of the first things that must strike the student
of biology is the marvellous adaptation of the different
kinds of plants and animals to the conditions in which
they live, and this adaptation is in large part brought
about by the success of those forms which have changed
in directions advantageous to them. Since they have
succeeded they aré the forms we actually meet with—
the failures have disappeared. But there are many
degrees of success. Some species are tremendously
successful, spreading widely and constantly establish-
ing themselves in new places—carrying, so to speak,
all before them. Others hold their own, but no more,
succeeding perhaps in some places, failing in others.
Again, there are species which are not now holding their
own, but are slowly, or rapidly, dying out.

In the course of evolution different species have
become closely adapted to different kinds of life con-
ditions—habitats as they are called—for instance hot

26

402 CONCLUSION

moist climates, hot dry climates, cold climates, and
so on; and again to seashores, marshes, forests, as
weeds of cultivated land, and to many other special
habitats. They continue to grow in these places and
no others, because their economy is adjusted to these
particular conditions of life.

In all this we see varied special cases of the great
universal law of equilibrium, which governs all the
processes of which we have any knowledge, from the
movements of the planets to those of molecules, atoms
and electrons, from the activity of protoplasm to the
vagaries of the human mind. All things which exist
are constantly tending towards positions of balance or
equilibrium, i.e. of relative repose, and all activity, all
motion, represents some phase of this universal process.
The universe consists of the most varied kinds of
systems in relatively stable or unstable equilibrium,
and every fresh disturbance of equilibrium from out-
side any system leads to fresh activity in the system
which tends towards the establishment of a new
equilibrium.

A free-living unit of protoplasm®represents a very
striking special case of a system of particles which can
maintain a moving equilibrium, by assimilation of fresh
particles of special kinds of matter from without, and
the breaking down (katabolism, respiration) of certain
substances within. This double process it can only
maintain within a certain range of external conditions.
Outside these the equilibrium is destroyed and the
organism dies, the matter of which it is. composed
breaking up into simpler systems of dead matter which
return to simpler states of equilibrium. By assimila-
ting faster than it katabolises the protoplasmic unit

EXTERNAL AND INTERNAL EQUILIBRIUM 403

grows, till its equilibrium can no longer be maintained
as a single system, and it divides, breaking up into fresh
units like itself. When these units separate reproduction
takes place, when they remain together a multicellular
body arises. The different cells of this, because they are
differently placed in regard to each other and to the
environment, are differently affected by each other and
by the environment, and differentiation begins between
the different cells. A new equilibrium tends to be
established, and we have a characteristic form and
structure developed. This must be in general harmony
with (i.e. in equilibrium with) the external conditions,
or the organism will die and disappear. Only those
forms which are in harmony with their surroundings
will survive, and it is only those which we see around
us. But the possible kinds of harmony are very varied,
for in the first place the protoplasm of different kinds
of organism is different, owing to the perpetual changes
which go on in the protein complexes that form the
basis of protoplasm, the perpetual readjustments of
internal equilibrium; and in the second place the
external conditions are different for different individuals.
The constant interactions of one and the other are as
constantly bringing about fresh adjustments, new
positions of the total equilibrium of the whole organism
—in other words, different species of organisms.
These are the causes of the immense variety in the
forms and structures of the organisms that we see
around us, and of the fact that they are in general,
and very often in the minutest particulars, wonderfully
well adapted to their surroundings. Otherwise they
could not come into existence. But alongside of,
coexisting with, the features of structure and function
that are thus adapted, we see many others which are

404 CONCLUSION

not. There is no bar to the appearance of characters
which are of no use to the organism, nor even of char-
acters which are disadvantageous to it, provided they
do not handicap the organism sufficiently to destroy tts
chances of continued existence. —

Proof that a structural or functional character is of
use to an organism is no explanation of the origin of
the character in question. A useful character, even
when the organism could not survive without it, is
merely an expression of a partial equilibrium with the
environment. The origin, that is the causation of a
character, is not to be sought along these lines, but by
studying the play of forces at work. We have con-
sidered several cases, for instance the origin of the
sexual differentiation of algal gametes (pp. 200-202),
the causes 01 the differentiation between the external
and internal cells in the Brown Seaweeds (p. 228), the
ability of the fruits or seeds of many seashore and
riverside plants to float in water for a long time with-
out injury (p. 372), in which it is clear that the
causation of these characters: is distinct trom their
advantageousness to their possessors. Their usefulness
may explain their maintenance, but not their origin.

All characters alike, essential, useful, useless, or
harmful, are the inevitable products of an organism’s
constitution—in the last analysis of the constitution of
its protoplasm—reacting according to the laws of
physics and chemistry to the conditions of its environ-
ment. The science of biology, like every other science,
is concerned with causation, the fixed relations between
phenomena, and it is only by the study of the
causation of the characters of living organisms that
our knowledge of biology can be advanced.

INDEX

achromatic spindle, 104-106, 278

adaptation, 401

adsorption, 53

adventitious roots, 261, 288, 235

zcidiospores, 179-181

aerial (subaerial) shoot, 255-261,
263, 264, 266

aerobic respiration, 132, 135, 150

aggregation of.protein molecules,
6

3

albuminous seed, 375

alcoholic fermentation, 132-136

anabolism, 79

anaerobic respiration, 132, 135, 150

Alge, 29, 72, 73, 96, 184, 203,
206, 207, 214

alternate hosts, 181

amino-acids, 122

Ameba, 64-72, 74, 79, 81, 83, 84,
92, 96, 393

andrecium, 345

Angiosperms, 28

animals and plants, differences
between, 22-26, 206, 207, 392-

397

annual plant, 256, 257

annual rings, 335, 336

anther, 346, 347, 351, 355-357

antheridium, 223, 224, 234-236,
244, 245, 250, 251

antipodal cells, 349, 350

apical cell (of Fucus), 219

apical (primary) meristem, 289,
291, 297, 320, 363, 377

apocarpous, 352

archegonium, 235, 236, 244, 245,
250, 251

“ aromatic substances,” 44

assimilation, 69, 78, 390, 402

autumn wood, 335, 336

axil, 258

axillary bud, 258, 259, 262-265,
322, 323, 326, 330

405

bacillus, 139, 148, \149, 154-156

Bacteria, 30, 51, 113, 127, 136,
188-156, 393

bacteriology, 148

bark, 299, 329, 840, 341

binary fission, 72, 88

bone, 98

Bordeaux mixture, 178

bordered pits, 276-278

bracts, 345

brewing, 132-134

Brownian movement, 50, 62

Bryophytes, 29

bud scale scars, 330, 342

bud scales, 329, 330, 342

buds, 256-259, 261-266, 316, ‘322,
223, 326, 328-330, 378, 381

bulb scales, 265

bulbils, 265

bulbs, 265, 266

callose, 274

calyx, 345, 353» 364

cambium, 298, 325, 328, 381, 386

carbohydrates, 37-41, 81, 90, 92,
120, 122, 152, 154, 159, 220,
274, 278, 285, 375, 377, 380,
396, 397

carotin, 112, 186, 214

carpels, 346, 352, 353) 354, 355)
365-368, 369

cartilage, 95, 98

catalysts, 44

cell, 31, 32, 54, 56, 58, 66, 80,
92-109, 391

cell doctrine, 96-98

cell sap, 101, 107, 109, III, 118,
290, 296

cell wall, 31, 72, 73, 98, 99-101,
39

cellular structure, 31

cellulose, 41, 45, 47, 92, 96, 98,
107, 122, 128, 136, 141, 152,

406

154, 186, 270, 275, 276, 278~
281, 284, 377, 380

cereals, 381

chemosynthesis, 115

chemotaxis, 87

chemotropism, 351

Chlamydomonas, 24, 87, 185-192,
194, 195, 201, 203

chlamydospores, 162

chlorophyll, 23, 32, 112-114, 117,
178, 214, 302, 317, 395, 397

chloroplasts, 32, 72, 73, 74, 80,
Ioz, I12, I13, 115-120, 185,
190, 233, 234, 236, 242, 302,

303

chromatin, 66-68, 103, 106, 128,
129, 140, 399

chromosomes, 104-106

cilia, 85, 93, 141

classification, 26-28

Clubmosses, 29, 248, 249

coagulation of proteins, 62

coccus, 139, 148, 154

ceenobium, 192-199

coffee disease, 181

collenchyma, 279, 311, 317, 320

colloids, 48-60, 61, 62, 141, 389

companion cells, 272, 273, 275, 293,
324, 332

competition, 384, 401

conceptacle, 216, 222-225

conducting cells (of Fucus), 222

conidia, 164, 173, 176, 177

connective tissue, 95, 98

continuous phase, 49, 61, 390

contractile vacuole, 65, 84, 186

cork, 284, 299, 331, 887-842

corms, 268, 264, 265, 266, 325

corolla, 345

cortex, 291-298, 296, 297, 301
317, 320, 321, 322, 326

cortex (of Fucus), 217, 220, 222

cotyledons, 256, 257, 362, 363,
378-380

crystalloids, 49, 51, 55

cuticle, 115, 242, 284, 303, 307,
308

cutin, 284, 293, 294, 297, 303, 305

cytase, 45, 272, 377, 380

cytoplasm, 66, 67, 73, 74, 93, 96—
98, 100, IOI, 107, 108, 112, 115,
127, 158, 161, 164, 185, 186,
189, 190, 200, 202, 207-209,
217, 218, 223, 236, 270, 274,
276, 278, 286, 348, 350, 376

INDEX

“* damping-off,” 172-3

death rate of seedlings, 383
dialysis, 55

diastase, 45, 120, 272, 377, 380
differentiation, 80, 93, 94, 215, 238
differentiation, intracellular, 81,

93
diffusion, 54, 55, 99, 114, 116, 118,
222, 339
disease bacteria, 154-156_
disperse phase, 49, 55, 61, 62, 390
dormant buds, 330

ectoplasm, 62, 101, 102°
egg apparatus, 350
elaters, 235, 287, 240, 279
embryo, 237, 240, 244, 245, 251,
255, 288, 361, 362, 363, 364,
374-377) 386-388
embryonal cell, 361, 362
embryonic cells, 100-102, 270, 320
emulsoid, 49
endodermis, 291, 293, 294, 296,
297; 317-319
endoplasm, 62, 102
endosperm, 349, 352, 861, 362,
364, 374-380, 388
endospermic seed, 375, 379, 380
energid, 97, 98
energy, 25, 26, 38, 42, 77, 81, 82,
84, 85, 121, 130, 132, 142, 381
enzymes, 44-46, 67, 94, 120, 132,
_ 150, 272, 274, 377
epicotyl, 258, 362, 378-380
epidermis, 115, 242, 272, 273, 275,
278, 284, 302, 303, 305-307,
309, 310, 316, 321, 338, 339
epigeal cotyledons, 378, 379, 387,
388
epigynous, 353, 354
equilibrium, 55, 57, 108, 404
equilibrium, law of, 55, 402, 403
ethyl alcohol, 132
etiolation, 316
Eudorina, 194, 195
exalbuminous seed, 375
eyespot, 186

facultative gametes, 204

facultative parasites, 158, 169

families of plants, 27

fat-cells, 95

fats, 41, 42, 61, 63, 77, 91, 95,
120, 154, 376, 377, 381, 382,;
390, 392, 396

INDEX

feeding, 68, 77, 78

female gamete, 191, 195, 200, 202,

210, 344, 359-352
fermentation, 130, 132-136, 138,
150, 151
Ferns, 29, 244-248
fibres, 218, 279, 280, 311
filament, 346
filamentous green algz, 203
flagella, 85, £41, 172, 186, 192, 193,

198-200, 235, 236, 244, 245
floral leaves, 258, 344-346, 352-

354

flower, 258, 344-360

flower bud, 257, 258

Flowering Plants, 28

foliage leaf, 22, 242, 244, 246, 256,
802-314, 315, 325

food vacuoles, 69

foot, 237, 244, 245

formaldehyde, 114

frond, 215, 216, 220

fruit, 258, 361-374

fruit buds, 329

fucoxanthin, 214

Fucus, 214-2380, 240, 351

Fungi, 30, 96,127, 157-183, 231,393

gametes, 188-192, 195, 198, 204,
205, 209, 222, 234, 239, 250,
252, 344, 349, 350, 352

gelatine, 54, 59, 285

gel membrane, 54-56, 58, 62, 66,
IOI, 103

gels, 58, 54, 58, 59, 62, 63, 66, 103,

° 285

genera, 27, 139, 148

geotropism, 87, 288, 315

germ cells, 185, 197, 198, 206, 207,
267

gland cells, 94, 95, 272

glucose, 88, 39, 81, 114, 120

glycogen, 26, 129

growth, 79, 87, 88, 90, 106, 160,

390
guard cells, 242, 303, 805-308
Gymnvsperms, 28, 385
gynecium, 346, 353, 354

habitats, 401

hemoglobin, 43, 82

haustoria, 300, 372

heartwood, 337

herbaceous plants, 255-258, 267,
328, 329

407

heterogametes, 190

heterospory, 249-252

holdfast, 215, 219

holophytic, 78

holozoic, 78

Horsetails, 29, 249

humus, 125, 152, 166
hydrotropism, 87, 289

hyphe, 157, 164

hypocotyl, 256, 257, 362, 378-380
hypogeal cotyledons, 379, 380, 387
hypogynous, 353, 354

“immortality " of unicellular
organisms, 184, 185

inferior ovary, 353, 354, 368

inflorescence, 329, 345

integration, 394, 395

intercellular spaces, 99, 115, 285,

_ 293, 304, 309, 310, 335, 339

intercellular substance, 98

“internal atmosphere,” 115, 285

internodes, 258, 316, 345

invertase, 45

irreversible gel, 62

isogametes, 190, 204

karyokinesis, 103-106, 209

katabolism, 79, 83, 84, 402

kinetic energy, 25, 38, 81-83, 85,
121

levulose, 38, 39, 45
lenticels, 339, 341, 343
leucoplasts, 119
Lichens, 30
lignification, 283, 284
lignin, 283
lignocellulose, 283
lipases, 45, 377.

liver, 26, 83
Liverworts, 29, 232-238
Lycopods, 248, 249

male gametes, 191, 195, 200, 202,
210, 250, 252, 344, 347, 359,
352

maltase, 45, 120

malting, 133, 381

maltose, 38, 45

mechanical tissue, 278, 279, 312,
320

medulla, 217

megasporangium, 249-252, 254,
255, 344 345, 347, 348, 385

408

megaspore, 249-252, 344, 348-350,
384, 385

ineristem, 100, 102, 258, 270, 289,
320, 321, 377

meristem, secondary, 298, 299,
328, 330, 331

meristematic cells, 101-108, 127,
320, 321

mesophyll, 115-117, 242, 270,

302-305, 307, 809-311, 313
metabolism, 62, 65, 122
metabolites, 62
metaphloem, 324
metaxylem, 295, 296, 324
mmicrayy®, 848, 349, 350, 352, 378,

397

microsporangium, 249-252, 344,
, 3457347 ;

microspore, 249-252, 344, 346,
347, 348, 384, 385

middle lamella, 107, 276, 278,

285, 338
midrib, 215, 233, 234, 310
Mosses, 29, 232, 288-241
movement, 22-24, 68, 84-87, 93
Mucor, 158-164
multicellular organism, 66, 80, 93,
96, 206, 403
muscle tissue, 94, 95, 98
must, 134
mycelium, 157, 164

natural selection, 401

nectary, 272, 356, 357

nitrification, 153

nodes, 258, 316, 345

nomenclature, 27

non-cellular plants, 97, 158

non-endospermic seed, 375, 380

nucellus, 348, 349, 364, 385

nuclear membrane, 66, 103, 104,
106

nucleolus, 100, 103

nucleus, 62, 66-68, 73, 95, 97,
100, 102, 103-106, 107, II7,
125, 139, 158, 162, 185, 187,
189, I9I, 201, 207, 217, 218,
223-225, 226, 236, 272-274,
347-350, 352, 361

obligate parasite, 157

oils, 42, 376, 381, 382
oogonium, 223

“ organic.” compounds, 36, 38
osmosis, 67, 311

INDEX

osmotic pressure, 57, 108, 131, 305
osmotic substances, 57, 108
pate 848, 351, 353, 354, 365-

ovale, 254, 255, 848, 349, 359, 360,

palisade layer (of Fucus), 217,
222

Pandorina, 192-194

pappus, 370, 374, 383

parasites, 78, 154, 158, 169-183,
300

Parasitic, 23, 78, 157, 300

parenchyma, 270, 271

parthenogenesis, 172

pectin, 285

pedicel, 345

peduncle, 345

Pellia, 282-288

Pellionia, 126, 234

Penicillium, 164-166

pentosans, 275, 285

perennation, 265

perennial, 256

perianth, 345

pericarp, 864, 365, 368, 369-371

pericycle, 298, 317, 318, 320, 338,
340.

perigynous, 353, 354 366

perisperm, 364, 387

petals, 345, 346, 353, 355

pheoplasts, 214, 217, 218, 223,
225, 228

phellogen, 299, 331, 887-841

phloem, 292, 298, 295, 297-299,
310, 317, 318

phloem fibres, 332, 334, 336, 337

photolysis, 115, 285

photosynthesis, 32, 118-115, 118,
I2I, 220, 302, 303, 309

phototaxis, 87

phototropism, 87, 160, 289, 315

Phytophthora, 175-177

piliferous layer, 290, 292, 297

pith, 295, 317, 319, 321

pits, 108, 276-278, 279-282

placenta, 348, 349, 355, 368

placentation, 355, 368

plankton, 184

plant pathology, 181

plasmolysis, 109, 111, 213

plastids, 102, 108, 119, 127,-270

Pleodornia, 195-197, 206

pollen, 347, 355-358 x

INDEX

pollen grains, 346-348, 351, 355,
358, 385

pollen sac, 346, 347, 351

ae tube, 347, 349, 350, 851,
395

pollination, 351, 354

pollination, cross, 351, 355, 358

pollination, self, 351, 358

polysaccharides, 40, 41, 49, 396

Potato blight, 175-177

potential energy, 25, 38, 42, 81,
85, 121

principal rays, 335, 336

protandrous, $56, 357

proteans, 45, 272, 377

protein cells, 272, 274

proteins, 42-44, 49, 61-63, 73, 77,
122, 130, 142, 15%, 154, 272,
278, 375, 377, 381, 390, 392

prothallus, 244, 245, 248-251

Protista, 30, 392

Protococcus, 72-74, 80, 92, 93

protophloem, 324, 332

protoplasm, 31, 32, 33, 36, 56, 58,
61-64, 64-70, 76, 77, 82, 84,
100-101, 389, 404

protonema, 240

protoxylem, 295, 296, 297, 322-
325

pseudopodium, 62, 65, 68, 71

Pteridophytes, 29, 243-251, 384

Puccinia, 178-181

pulses, 381

pure culture, 147, 148

“ pure strains,” 67, 149

putrefaction, 151

pyrenoid, 72, 78, 185, 186,
207, 208, 209

Pythium, 172-175

190,

rays, 295
receptacle, 345, 353, 354, 365-368
reproduction, 71, 73, 87, 184

Tespiration, 39, 81-88, 118, 121,
132, 135, 136, 150, 159, 289,
305, 376, 377, 391

reversible gel, 54, 62

rhizoids, 221, ae 233, 238, 239,
241, 244, 2

, 257) 359-262, 265, 266,
381

ringworm, 169

root cap, 285, 289, 290-292

root hairs, 241, 289, 290, 294, 296
root, the, 241, 288-301

409

runners, 261
Rust of wheat, 178-181

Saccharomyces, 128, 136

salts, 73, 77, 123, 130, 142, 152,
159, 186, 217, 220, 240, 241,
290, 296, 300, 304, 31I, 392,
393, 395

Saprolegnia, 171, 172

saprophytes, 127, 157-168

saprophytic, 78, 157, 158

sapwood, 337

scale leaves, 259, 262, 325

secondary meristem, 298, 328, 331

secondary rays, 884-336

secondary thickening, 255, 298,
328

secondary tissue, 298, 329

secretory cells, 272

seed, 254, 255, 258, 361-364,
365, 368, 369-372, 375-388

Seed Plants, 28, 243, 252, 254, 255,
344, 384-386

semipermeable membrane, 56, 62,
108, 109

sepals, 845, 346, 353

sexual differentiation, 184, 190,
200, 205, 210 211, 234°

sieve plate, 274

sieve tubes, 229, 274, 293, 297,
311, 324, 332, 333, 337

sol, a 53, 54-56, 61-63, ror,
389

solutes, 48, 55-57, 61, 108, 293,
296, 299

solution, colloid, 48

solution, crystalloid, 48, 55, 59, 391

solution, ‘‘ true,’’ 48, 59

solvents, 48, 55, 56

soma, 184, 185, 195, 198, 205, 206

species, 27, 42, 43, 67, 148, 149,
398, 401, 403

spectrum (of chlorophyll), 113

spermophytes, see seed plants

sperms (= male gametes) 198, 199-
201, 212, 223-225, 230, 235,
236, 244, 245, 248, 250, 251

spirillum, 139, 148

spirochete, 148, 154, 155

sporangiophore, 160

sporangium, 160, 161

spore capsule, 237, 240

spores, I31, 143-145,
155, 160-162

sporidia, 179, 180

146, 151,

410

sporogonium, 237, 240

spring wood, 335, 336

stamens, 845, 346, 352, 353, 364

starch, 40, 43, 45, 77, 90, 118-120,
122, 178, 186, 209, 376, 377,
381, 396

starch grains, 40, 46, 117, 118,
126, 234, 301, 319, 375, 380

sterilisation, 146, 147

stigma, 848, 351, 355. 356, 357,

_ 364, 385

stimulus, 26, 86

stipe, 215, 216, 219

stolons, 261

stoma, I15, 242, 303, 305-308,
317, 339

stone cells, 280

storage tissue (of Fucus), 222

struggle for existence, 401

style, 346, 848, 351, 364, 370

subaerial shoot, see aerial

suberin, 284, 299, 339

subordinate rays, 335, 336

sucrose, 38, 39, 45

sugars, 38, 39, 45, 56, 57, 77, 81-
83, 114, 117-122, 149, 159, 305,
310, 380, 395, 396

supporting tissue, 279, 320

surface energy, 52

surface tension, 52, 69, 70

suspensoid, 49, 59

suspensor, 361-364

synergide, 349, 350

taproot, 256, 257, 267, 288

taxis, 87

teleutospores, 179-181

terminal bud, 256, 258, 261, 262,
265, 316, 320, 321, 328

testa, 364, 369, 371, 378-380

thallus, 214, 233

tissue mother cell, 331-334

tissues, 80, 94, 95, 98, 106, 107,
217, 238, 241, 242, 270

tracheids, 280-283

tracheids, annular, 281, 295

tracheids, pitted, 282

tracheids, reticulate, 282

tracheids, scalariform, 282

tracheids, spiral, 280-282, 295

transpiration, 304, 310

tropisms, 87, 288, 315

tubers, 119, 262, 265, 266, 325

turgor (turgidity), 108, 109, 110,
395, 307

INDEX

ultra-microscope, 50

ultra-microscopic particles, 50, 51,
61

unicellular organism, 66, 93, 96,
184, 185

urea, 65, 83

uredospores, 178-181

uric acid, 83

vacuole, 62, 100, I0I, 107-109,
III, 115, 117, 125, 128, 158,
207, 217, 218, 270, 273, 274,
275, 296

vacuole wall, 101, 102

vascular bundles, 302, 310,
318, 321, 324, 325

vascular cylinder, 291, 293, 294,
295, 297, 317, 318, 319, 322,
324, 325

Vascular Plants, 241-243

vascular system, 242, 246

vascular tissue, 317

vegetative reproduction, 206, 241,
248, 266, 267

veins, 302, 310-312, 325

317,

vessels, 282, 288, 293, 296, 297,
324, 325, 332, 335

vestibule, 306, 308

“vital functions,” 76, 92, 97,

390
Volvox, 197-200

water cultures, 123

water, essential importance of, 37
water tissue, 275, 276, 310
wilting, 109, 308, 320

wind pollination, 358
wine-making, 134

wings, 215, 216, 218

winter buds, 329, 330

woody plants, 255, 328

wort, 132-134, 381

xanthophyll, 112, 214

xylem, 298, 295, 298, 302, 304,
310, 311, 317, 318, 331-334

xylem (wood) fibre, 332-335

yeast plant, 128-136, 142,
150 2

143,

zoogloea, 141

zoospores, 172, 173, 175, ‘176, 203

zygote, 162-164, 175, 189, 208-
210, 225, 236, 244, 352, 361

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“ We doubt whether the leading facts in the structure and life-history
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Elements of Vital Statistics

By Sir ARTHUR NEWSHOLME, K.C.B., M.D.

Late Principal Medical Officer of the Local Government Board (England) and
Lecturer in Public Health in Johns Hopkins University, U.S.A.

Demy 8vo. 215. net,

This work was first published in 1889 and re-written in 1899, since
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The subject divides itself naturally into consideration of the data of
vital statistics, their compilation, and their interpretation; and under
each of these headings the needs of physicians, public health officers and
social wotkers will be’ carefully met, and numerous illustrations given of
statistical errors and the methods for ‘avoiding them.

A wide interpretation is given to the scope of Vital Statistics, which
are regarded as including not only the measurement of population, of
natality and mortality and of sickness, but also some of the chief
problems of epidemiology, of poverty, of crime and of genetics.

The Evolution of Continuity
in the Natural World

By DAVID RUSSELL, M.D.
Demy 800. Illustrated 165. et.

This book throws a new light on the problem of Natural Evolution,
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still governs, the process. Its chief concern, however, is with the
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unrecognized “Fundamental Species of Continuity,” within which
evolved the various animal and vegetable ‘‘Sub-species.” In the light
of present-day opinion the views of the writer are in many respects
revolutionary, and their acceptance must necessarily involve changes in
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The New Psychology : and
its Relation to Life

By A. G. TANSLEY

Demy 8v0, Sixth Impression 105, 6d. net:

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Woodland Creatures

By FRANCES PITT
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Demy 800. Illustrated 12s, 6d. net.

In this book the author gives an intimate description of the lives, habits,
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The Call of the Wildflower

By HENRY S&S. SALT

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Cr, 8v0. 6s. net.

This book will appeal strongly to flower-lovers. It is a study of wild:
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Egypt—Old and New

By PERCY F. MARTIN, F.R.GS.

Author of “The Sudan in Evolution”
Frap. 8v0. With 45 Coloured and many other Illustrations 215, net.

This is a popular description of perhaps the most fascinating country
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lands.

Its Nature, Develop-

Language ment, and Origin
Demy 8v0. By OTTO JESPERSEN 185, net.

“Chief among Professor Jespersen’s many qualities we would place
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Studies in Ancient Art
By JEAN CAPART

Keeper of the Royal Museum Cinquantenaire, Brussels
Transtarep By WARREN R. DAWSON
Demy 8v0. Profusely Illustrated 165, net. .
This book is a translatidn of a part of Professor Capart’s monumental
work “Lecons sur L’Art Egyptien,” published at Liége in 1920. To

those interested in the ancient arts of Egypt M. Capart’s name is sufficient
indication of the interest and importance of this book.

Some Aspects of Art

Education
Cr. 80. Foreworp by Sir JAMES YOXALL 55. neh.

Published under the auspices of the National Society of Art Masters.

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